CA2068439C - Mode converter - Google Patents
Mode converterInfo
- Publication number
- CA2068439C CA2068439C CA002068439A CA2068439A CA2068439C CA 2068439 C CA2068439 C CA 2068439C CA 002068439 A CA002068439 A CA 002068439A CA 2068439 A CA2068439 A CA 2068439A CA 2068439 C CA2068439 C CA 2068439C
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- Prior art keywords
- optical
- mode
- waveguide
- subsections
- converter according
- Prior art date
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Classifications
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
- G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
- G02B6/126—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind using polarisation effects
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
- G02B6/14—Mode converters
Abstract
Mode converter for converting a fraction of one guided mode of an optical signal in an incoming optical waveguide section (A) into another guided mode in an outgoing wave-guiding section (C) by means of a periodic coupling between both guided modes in an intermediate optical waveguide section (B).
The intermediate section (B) has a periodic geometrical structure as a result ofan N-fold periodic sequence of two light-guiding subsections (P,Q) within a period length (LP + LQ). The sequence can be obtained by arranging for the waveguide profiles of the subsections to differ from one another, preferably as a result of differences in width. The sequence can also be obtained by offset joining of the two subsections with the same waveguide profiles. Advantages are:the high degree of integrability, the ability to co-integrate a laser light source ID
an optical section of a coherent optical receiver and the achievement of a new integrated design of such an optical section, which design is free of metallizedelements.
The intermediate section (B) has a periodic geometrical structure as a result ofan N-fold periodic sequence of two light-guiding subsections (P,Q) within a period length (LP + LQ). The sequence can be obtained by arranging for the waveguide profiles of the subsections to differ from one another, preferably as a result of differences in width. The sequence can also be obtained by offset joining of the two subsections with the same waveguide profiles. Advantages are:the high degree of integrability, the ability to co-integrate a laser light source ID
an optical section of a coherent optical receiver and the achievement of a new integrated design of such an optical section, which design is free of metallizedelements.
Description
20~
Title: Mode converter A. Ba.k~ ' of the invention 1. Field of the invention The invention is in the field of the cc,..~. of guided modes of light waves in integrated optical c~ More particularly, the invention relates to a mode converter based on the principle of periodic coupling between guided modes of a light wave ~lull..6dlh~ in a channel-type optical ~ juidc. In 5 addition, the invention relates to an optical input section for a coherent optical receiver in which such a mode converter is used.
Title: Mode converter A. Ba.k~ ' of the invention 1. Field of the invention The invention is in the field of the cc,..~. of guided modes of light waves in integrated optical c~ More particularly, the invention relates to a mode converter based on the principle of periodic coupling between guided modes of a light wave ~lull..6dlh~ in a channel-type optical ~ juidc. In 5 addition, the invention relates to an optical input section for a coherent optical receiver in which such a mode converter is used.
2. Prior art In a coherent optical receiver such as can be used in a coherent optical 10 network, a laser is usually ill~ull~ul~t~l as local oscillator. The light from said laser is mixed with an optical signal received from such a network by the receiver. Since the light I through the network is generally not pOI~ e.Vil.g, the p-' of the optical signal received is ' ' - ' The optical signal received is therefore first split into two 15 ~ . TE and TM, which are then processed separately. This is done by mixing with the light of the local oscillator either directly before or directly after splitting. This technique is known by the term p~
diversity'. This means, however, that the light of the local oscillator must also contain both ~ ' - Cullll in order to have mixing ~ which 20 correspond in ~ with the two p~l-.;,-~;.. -- c .. l.. 1~ of the optical signal received. A laser which is standard in this rrmn~,~' and has a h of the emitted light in the near infrared transmits, however, only TE-polarized light. To obtain the other ~ ' - CUIII~UII..~
could be given to tilting the laser through a suitable angle. However, in an 2~S~43~
integrated design of the coherent optical receiver, in which the laser is co-intPgrat~-rl tilting the laser is tro~ if not imrrp,~ti~ ^ It is therefore first necessary to convert a portion of said TE-polarized light into TM-polarized light with the aid of a ~ converter or rotator. A p~ '; ' J~ converter is l~r~ as meaning a device with which a known portion of one Vll.l 1, TE or TM, in the optical signal at the input of said device is converted into the other p~ c r ~ ~ TM or TE
.ly, at the output, with a well-defined phase with respect to the one pvl~ - c ~ 1 . A p. ,l~ rotator is such a type of device in which, 10 however, a phase shift is uncontrolled. Such TE/TM p~ .la and rotators are known per se, for example from references [1], [2] and [3] (see under D.). Reference [1] discloses a pol~l converter for optical waves which is able to convert any input p~ .. into any desired output p ' This known converter comprises a ~,- ' ~ rotator sandwiched between two phase 15 shifters. Both the phase shifters and the ~ l rotator are based on electro- optical ~ sti~n of the ~lul of the TE ~VIIIpV.~III and the TM
~VII.I The actual cu..~..;,;o~ of a fraction of the one ~ into the other i , ~ (TE - TM) with identical intensity takes place in the ~ l - rotator. In this r~mr.~:: , use is made of a periodic electrode 20 structure provided over a suitably chosen length on top of an optical waveguide in order to bring about a periodic coupling between the two p.
using suitably chosen, adjustable control voltages. As a ~
of a repeated coupling of this type, it is possible, A~prnflin~ on the chosen control voltage, cycle length and number of couplings, to convert a desired 25 portion of the one . into the other. The ~l ~ ;.... cv..~..[..~ known from the references [2] and [3] also make use of the principle of periodic coupling between the two p~ . - in an optical waveguide on 20~8~
the basis of electro-optical effects with the aid of a periodic electrode structure.
Reference 9 discloses a fiber-optical analog of the r~ ; converter according to reference 2, based on l,;l~f~ due to ~ i~al stress effects. A periodic coupling is achieved in said analog by bringing about a 5 periodic ' I pressure in the Inn~itl.~li...l direction on a -' ' b;.erlil.6c.ll fiber or a bimodal fiber using a comb-type pressure device, the pressure exerted by the latter on the fiber being capable of being controlled~ u~lc~ lly.Theseknown~u..~.t~.~havethegreatadvantageof electrical controllability, and they are consequently widely arrli~a~'-, even in10 the case indicated above. However, they have the drawback that in arrlir~tin-~in which a fixed fraction always has to be converted, such a controllability is in fact superfluous, and therefore makes a circuit such as the a~uv .~ d coherent optical receiver, ~ J c.. .~ and makes the int~grahility thereof difficult.
B. Summarv of the invention The object of the invention is to overcome the a~uv, - -d drawback. In this; - it makes use of the fact, known from the theory relating to glass fiber splices, such as, for example, from reference 14], that if, 20 in a ~ ;uide~ an abrupt ~ ;----;ly occurs in the ~ ;ùid~ profile of the guide, it is possible for coupling to take place between a guided mode in the guide upstream of the .1;~ y and any possible guided mode du..llall~
thereof. A coupling of this type is, however, generally weak and the fraction of ;ù~ of one mode to a desired one is therefore small. This fraction 25 increases, however, if said ~ y is repeated in the ~ ., uidc with a cycle which is selective for the pair of guided modes involved in the ~
Making use of this, the invention provides a mode converter for the ~,, 2~8~3~
of a fraction of a signal ~vllll - of an optical signal ~lu~6~.lh.g according toa first guided mode into a signal Cvlll~ ~lu~..6~1il.g according to a second guided mode, c~ .e a channel-type ~ 6uide in which a periodic coupling between the two guided modes of an optical signal ~lv~ ,dthlg in the ~ 6uid~ takes place, which ~ .6.,id~ comprises an incoming wave-guiding section, an wave-guiding section and an outgoing wave-guiding section, .k .,. (~ ;~ d in that the ;..t.. ,..~ wave-guiding section has a periodic B ~ l structure consisting of a periodic sequence of two wave-guiding s~lhCP t within a period length, the lengths of the 5llhs~p~t and the 10 number of periods being matched to the desired ~ a;Ou fraction. The invention makes it possible to model a specific converter having a desired conversion fraction for every specific pair of guided modes, that is to say an incoming and an outgoing mode, by suitable choice of the waveguide profiles of the s~)hs~pcti~mc of the lengths and the number of ~ liliol~s of the s~
15 and the manner in which the s--hs~Pcti~nc join one another. The Cvll~, -principle of the invention is ~ .l611l s~ liv~, the scl~ livily increasing as the number of ,e~liliv~s increases. The invention is very suitable for ;..- in integrated ~ ~, It makes it possible to co-integrate a local light source in a simple way into an integrated design of an optical input section 20 of a coherent optical receiver based on ~ diversity.
Reference [8] discloses another specific ~ ' - rotator based on a ~ .6uide of the rib type in which a periodic ~ ,lliC vialuli is provided by means of a periodic ..~IIIII.~Ili.dl loading of the rib over a certain coupling length.
Optical input sections of a coherent optical receiver based on ~, ' -diversity are disclosed per se, for example, by reference [5] and [6]. By applying mode Cvll~.~lt~.la according to the invention, a design of an optical section of this æ 06~ 7 type is possible in which the use of metallized elements can be avoided. Such elements are usually necessary in the polarization splitters used in optical sections of this type.
The invention therefore also relates to an optical input section for a coherent optical receiver based on polarization diversity, comprising mixing/splitting means for the polarization-independent equal mixing of a first optical signal received via an incoming optical channel and containing polarization modes (TE, TM), and a second optical signal originating from a local light source and containing polarization modes (TE, TM) and then splitting equally in terms of power into first and second mixed signals having two polarization modes, first and second splitting means for the respective splitting up of the two polarization modes present in the first and second mixed signals into separate optical signals, each having one of the two polarization modes, for presentation to equally as many outgoing optical channels, characterized in that first and second conversion means are furthermore provided for converting at least a fraction of one of the two polarization modes into a guided mode different in order from that in which both the polarization modes mentioned propagate in the first optical signal and the second optical signal, respectively, in that the mixing/splitting means comprise a multimode power coupler, and in that the first and second splitting means are mode splitters.
According to a third broad aspect, the present invention provides combined mode converter/splitter for æ OG~
converting a fraction of a first signal component of an incoming first optical signal propagating in a first polarization mode (TE or TM) into a second signal component propagating in a second polarization mode (TM or TE), and the emission of second and third optical signals respectively via outgoing second and third monomodal optical waveguides, the second optical signal essentially containing the second signal component and the third optical signal essentially containing the unconverted part of the first optical signal, which converter/splitter consecutively comprises between the first optical waveguide and the second and third optical waveguides:
- a tapered piece for the transition of a monomodal to a bimodal optical waveguide, - conversion means for converting at least a fraction of the first signal component into a guided mode differing in order from that in which the first signal component propagates in the first optical signal on leaving the tapered piece, and - a mode splitter.
C. References [1] GB-A-2090992;
[2] H.-P. Nolting et al., "TE-TM Polarization Transformer With Reset-free Optical Operation For Monolithic Integrated Optics", Proc . ECIO' 87 Glasgow, pp. 115-118;
diversity'. This means, however, that the light of the local oscillator must also contain both ~ ' - Cullll in order to have mixing ~ which 20 correspond in ~ with the two p~l-.;,-~;.. -- c .. l.. 1~ of the optical signal received. A laser which is standard in this rrmn~,~' and has a h of the emitted light in the near infrared transmits, however, only TE-polarized light. To obtain the other ~ ' - CUIII~UII..~
could be given to tilting the laser through a suitable angle. However, in an 2~S~43~
integrated design of the coherent optical receiver, in which the laser is co-intPgrat~-rl tilting the laser is tro~ if not imrrp,~ti~ ^ It is therefore first necessary to convert a portion of said TE-polarized light into TM-polarized light with the aid of a ~ converter or rotator. A p~ '; ' J~ converter is l~r~ as meaning a device with which a known portion of one Vll.l 1, TE or TM, in the optical signal at the input of said device is converted into the other p~ c r ~ ~ TM or TE
.ly, at the output, with a well-defined phase with respect to the one pvl~ - c ~ 1 . A p. ,l~ rotator is such a type of device in which, 10 however, a phase shift is uncontrolled. Such TE/TM p~ .la and rotators are known per se, for example from references [1], [2] and [3] (see under D.). Reference [1] discloses a pol~l converter for optical waves which is able to convert any input p~ .. into any desired output p ' This known converter comprises a ~,- ' ~ rotator sandwiched between two phase 15 shifters. Both the phase shifters and the ~ l rotator are based on electro- optical ~ sti~n of the ~lul of the TE ~VIIIpV.~III and the TM
~VII.I The actual cu..~..;,;o~ of a fraction of the one ~ into the other i , ~ (TE - TM) with identical intensity takes place in the ~ l - rotator. In this r~mr.~:: , use is made of a periodic electrode 20 structure provided over a suitably chosen length on top of an optical waveguide in order to bring about a periodic coupling between the two p.
using suitably chosen, adjustable control voltages. As a ~
of a repeated coupling of this type, it is possible, A~prnflin~ on the chosen control voltage, cycle length and number of couplings, to convert a desired 25 portion of the one . into the other. The ~l ~ ;.... cv..~..[..~ known from the references [2] and [3] also make use of the principle of periodic coupling between the two p~ . - in an optical waveguide on 20~8~
the basis of electro-optical effects with the aid of a periodic electrode structure.
Reference 9 discloses a fiber-optical analog of the r~ ; converter according to reference 2, based on l,;l~f~ due to ~ i~al stress effects. A periodic coupling is achieved in said analog by bringing about a 5 periodic ' I pressure in the Inn~itl.~li...l direction on a -' ' b;.erlil.6c.ll fiber or a bimodal fiber using a comb-type pressure device, the pressure exerted by the latter on the fiber being capable of being controlled~ u~lc~ lly.Theseknown~u..~.t~.~havethegreatadvantageof electrical controllability, and they are consequently widely arrli~a~'-, even in10 the case indicated above. However, they have the drawback that in arrlir~tin-~in which a fixed fraction always has to be converted, such a controllability is in fact superfluous, and therefore makes a circuit such as the a~uv .~ d coherent optical receiver, ~ J c.. .~ and makes the int~grahility thereof difficult.
B. Summarv of the invention The object of the invention is to overcome the a~uv, - -d drawback. In this; - it makes use of the fact, known from the theory relating to glass fiber splices, such as, for example, from reference 14], that if, 20 in a ~ ;uide~ an abrupt ~ ;----;ly occurs in the ~ ;ùid~ profile of the guide, it is possible for coupling to take place between a guided mode in the guide upstream of the .1;~ y and any possible guided mode du..llall~
thereof. A coupling of this type is, however, generally weak and the fraction of ;ù~ of one mode to a desired one is therefore small. This fraction 25 increases, however, if said ~ y is repeated in the ~ ., uidc with a cycle which is selective for the pair of guided modes involved in the ~
Making use of this, the invention provides a mode converter for the ~,, 2~8~3~
of a fraction of a signal ~vllll - of an optical signal ~lu~6~.lh.g according toa first guided mode into a signal Cvlll~ ~lu~..6~1il.g according to a second guided mode, c~ .e a channel-type ~ 6uide in which a periodic coupling between the two guided modes of an optical signal ~lv~ ,dthlg in the ~ 6uid~ takes place, which ~ .6.,id~ comprises an incoming wave-guiding section, an wave-guiding section and an outgoing wave-guiding section, .k .,. (~ ;~ d in that the ;..t.. ,..~ wave-guiding section has a periodic B ~ l structure consisting of a periodic sequence of two wave-guiding s~lhCP t within a period length, the lengths of the 5llhs~p~t and the 10 number of periods being matched to the desired ~ a;Ou fraction. The invention makes it possible to model a specific converter having a desired conversion fraction for every specific pair of guided modes, that is to say an incoming and an outgoing mode, by suitable choice of the waveguide profiles of the s~)hs~pcti~mc of the lengths and the number of ~ liliol~s of the s~
15 and the manner in which the s--hs~Pcti~nc join one another. The Cvll~, -principle of the invention is ~ .l611l s~ liv~, the scl~ livily increasing as the number of ,e~liliv~s increases. The invention is very suitable for ;..- in integrated ~ ~, It makes it possible to co-integrate a local light source in a simple way into an integrated design of an optical input section 20 of a coherent optical receiver based on ~ diversity.
Reference [8] discloses another specific ~ ' - rotator based on a ~ .6uide of the rib type in which a periodic ~ ,lliC vialuli is provided by means of a periodic ..~IIIII.~Ili.dl loading of the rib over a certain coupling length.
Optical input sections of a coherent optical receiver based on ~, ' -diversity are disclosed per se, for example, by reference [5] and [6]. By applying mode Cvll~.~lt~.la according to the invention, a design of an optical section of this æ 06~ 7 type is possible in which the use of metallized elements can be avoided. Such elements are usually necessary in the polarization splitters used in optical sections of this type.
The invention therefore also relates to an optical input section for a coherent optical receiver based on polarization diversity, comprising mixing/splitting means for the polarization-independent equal mixing of a first optical signal received via an incoming optical channel and containing polarization modes (TE, TM), and a second optical signal originating from a local light source and containing polarization modes (TE, TM) and then splitting equally in terms of power into first and second mixed signals having two polarization modes, first and second splitting means for the respective splitting up of the two polarization modes present in the first and second mixed signals into separate optical signals, each having one of the two polarization modes, for presentation to equally as many outgoing optical channels, characterized in that first and second conversion means are furthermore provided for converting at least a fraction of one of the two polarization modes into a guided mode different in order from that in which both the polarization modes mentioned propagate in the first optical signal and the second optical signal, respectively, in that the mixing/splitting means comprise a multimode power coupler, and in that the first and second splitting means are mode splitters.
According to a third broad aspect, the present invention provides combined mode converter/splitter for æ OG~
converting a fraction of a first signal component of an incoming first optical signal propagating in a first polarization mode (TE or TM) into a second signal component propagating in a second polarization mode (TM or TE), and the emission of second and third optical signals respectively via outgoing second and third monomodal optical waveguides, the second optical signal essentially containing the second signal component and the third optical signal essentially containing the unconverted part of the first optical signal, which converter/splitter consecutively comprises between the first optical waveguide and the second and third optical waveguides:
- a tapered piece for the transition of a monomodal to a bimodal optical waveguide, - conversion means for converting at least a fraction of the first signal component into a guided mode differing in order from that in which the first signal component propagates in the first optical signal on leaving the tapered piece, and - a mode splitter.
C. References [1] GB-A-2090992;
[2] H.-P. Nolting et al., "TE-TM Polarization Transformer With Reset-free Optical Operation For Monolithic Integrated Optics", Proc . ECIO' 87 Glasgow, pp. 115-118;
[3] R.C. Alferness and L.L. Buhl, "Electro-optic waveguide TE-TM mode convertor with low drive voltage ", OPT . Letters, vol . 5, No .
- 5a -2~S843~
Il, Nov. 1980, pp. 473-475;
- 5a -2~S843~
Il, Nov. 1980, pp. 473-475;
[4] H.-G. Unger 'Planar optical waveguides and fibres', Clarendon Press, Oxford 1980, cpt 8 'Fibre junctions and i ', section 8.1 'Analysis of fibre mode excitation', pp. 700-709;
15] T. Okoshi et al., "r~ iL.,li.,......... -diversity receiver for h. t.,~ /coherent optical fiber ..~ ;...., IOOC '83, June 1983, Paper 30C3-2, pp. 386-387;
[6] C. Duchet and N. Fldd~ .g, "New TE/TM p~' splitter made in Ti:LiNbO3 using x-cut and z-axis ~1U~.6dti~U", Electronics Letters, 5th July 1990, Vol. 26, No. 14, pp. 995-997;
[7] W.K. Burns and A.F. Milton, "Mode .-,nv~l in planar-dielectric separating waveguides", IEEE J. QUANT, ELECTR., Vol. QE-ll, No. 1, January 1975, pp. 32-39;
[8] Y. Shani et al.: "P~ .. rotation in .... ,.~ liC periodic loaded rib ~ .6~id~ ;,", Trttf-gr~t~ d Photonics Research, April 9-11, 1991, paper ThH3, Proc. IPR 1991, pp. 122-123.
[9] R.C. Youngquist et al., "All-fibre . .. ~ using periodic coupling", IEE Plu~e~ , Vol. 132, Pt.J., No. 5, October 1985, pp.
277-286.
D. Short d~clil,i of the drawin~
The invention will be explained in greater detail by means of the ~ c- - of a number of ~ ly embo~' in which reference is made to a drawing wherein:
FIG. 1: .l;~ lly shows a mode converter according to the invention in a lnn~it~ l section;
2~ 3~3~
FIG. 2~ ; YIIY shows an optical ~ uid~ of the rib type in cross section suitable for a mode converter according to FIG. I;
FIG. 3: shows a block diagram of a known optical input section of a coherent optical receiver in which mixing precedes splitting;
15] T. Okoshi et al., "r~ iL.,li.,......... -diversity receiver for h. t.,~ /coherent optical fiber ..~ ;...., IOOC '83, June 1983, Paper 30C3-2, pp. 386-387;
[6] C. Duchet and N. Fldd~ .g, "New TE/TM p~' splitter made in Ti:LiNbO3 using x-cut and z-axis ~1U~.6dti~U", Electronics Letters, 5th July 1990, Vol. 26, No. 14, pp. 995-997;
[7] W.K. Burns and A.F. Milton, "Mode .-,nv~l in planar-dielectric separating waveguides", IEEE J. QUANT, ELECTR., Vol. QE-ll, No. 1, January 1975, pp. 32-39;
[8] Y. Shani et al.: "P~ .. rotation in .... ,.~ liC periodic loaded rib ~ .6~id~ ;,", Trttf-gr~t~ d Photonics Research, April 9-11, 1991, paper ThH3, Proc. IPR 1991, pp. 122-123.
[9] R.C. Youngquist et al., "All-fibre . .. ~ using periodic coupling", IEE Plu~e~ , Vol. 132, Pt.J., No. 5, October 1985, pp.
277-286.
D. Short d~clil,i of the drawin~
The invention will be explained in greater detail by means of the ~ c- - of a number of ~ ly embo~' in which reference is made to a drawing wherein:
FIG. 1: .l;~ lly shows a mode converter according to the invention in a lnn~it~ l section;
2~ 3~3~
FIG. 2~ ; YIIY shows an optical ~ uid~ of the rib type in cross section suitable for a mode converter according to FIG. I;
FIG. 3: shows a block diagram of a known optical input section of a coherent optical receiver in which mixing precedes splitting;
5 FIG. 4: shows the same as FIG. 3, wherein splitting precedes mixing;
FIG. 5: shows a combined mode converter/splitter according to the invention in Irngitl litsl section;
FIG. 6: shows a block diagram of an optical input section according to the invention.
10 FIG. 7: shows the same as FIG. 4.
E. De~ ,l of ~ vLllY embodiments Two polarized modes are able to propagate in a ' I channel-type (optical) ~ Euide in an isotropic medium, such as, for example, in InP or in 15 a standard - ' ' optical fiber. These modes can be referred to as TE
(Ll~u~ electric) and TM (transverse magnetic). This t~rtn ~~ O,y is in fact ; e since said modes cannot be described by a single electric or magnetic field colll~~ . In a 1 ~ l; of these guided modes, all three electric and all three magnetic field vector c - .~ must always, after all, be included.
20 ~ .li' ' , it is the case that, with a choice of an Oll' ~ ' acial system which is standard in integrated optics, the TE mode is .' its~d by the Ey - and the Hx .ulllr t, and the TM mode by the Ex CC~ and the Hy c~ . .l In this c~ ., the z-acis indicates the IJlU~ ,ClliU..
direction, and the x-axis is usually chosen perp~-n~lir~ t to the slab-type 25 substrate. Waveguides of this type are, moreover, usually symmetrical with the xz plane as the plane of symmetry as a result of the nature of the knûwn A~ '` techniques- For a channel-type waveguide in a standard optical fiber, g aDy plane through the z-axis is a plane of symmetry. This symmetry manifests itself, in even (+) or odd (-) form, in the field vector .uu,l of the guided modes. For the various modes, this even or odd symmetry is shown in TABLE
1.
mode TEoo TEol TMoo TMo field vector 10 component s + +
z -- + +
I S .~ + +
Waveguide profile of a channel-type waveguide is llnrl~r~t~d as meaning the geometry of the section of the guide, including the optical properties of the wave-guiding medium and its :IU~ ling~ From the theory relating to optical fiber splices it is known, for example from reference 14], that 25 if an abrupt transition from one waveguide profile to another ~ .6uid~ profile occurs in a ~ .6uide, it is possible for coupling to take place between a guidedmode in the guide upstream of the transition and any possible guided mode in the conductor dv..ll~ ..lll of the transition. In this cAnn~ - coupling with radiation modes is in principle also possible. However, it is assumed that the 30 ~ ions referred to in this . - are such that the coupling to radiation modes can be neglected and can therefore be left out of c, .c;~ ;.. here. The degree of coupling, that is to say, the fraction of the power of a guided mode upstream of the transition which is converted into one or more guided modes d~,..l,~ll~..l,. of the transition, can be calculated by means of the integral of the 35 scalar product of the (modal) field vectors upstream and d.,.. u~ .lll of thetransition (see equation (8.6) in reference [4]). This theory is generally 2~43~
applicable to any transition from one channel-type conductor into another in a sequence of two or more channel-type ~ 6uid~s having different modal field profiles. But not every coupling between guided modes upstream and du..,..,ll~..u, of the transition is readily possible. From TABLE I it follows, for 5 example, that, in a - of two a~uu~ LI;C ~ ' ' ~ .c,uid.;., no >;Un of the TEUo mode to TMoU mode or vice versa can take place since these two modes have a different symmetry. In a sequence of a ~ ,;.al waveguide with an ~ -' ' one, or of two different .c,u;d~s~ however, coupling will in fact take place 10 between said TE mode and TM mode since the symmetry of the modal field vector c- ~ is destroyed. In a sequence of two different bimodal u;d~i a good coupling can take place at the transition between the TEoU and the TMo1 mode, or between the TEU1 mode and the TMoo mode and vice versa since, according to TABLE 1, the modes for each pair - ' 15 have the same field symmetry. The couplings - ' between the various TE and TM modes at a waveguide transition ûf this type are, however, weak and the . \ .I:>;UU fraction is therefore low. For ~rrli~ti,~-~c such as, for example, in a coherent optical receiver, however, larger Cull\~ ;ull fractions are necessary, in this case ~ lu~ t~ly 50%, than can be obtained with a single 20 transition. Larger conversion fractions of this type can be obtained by making use of a periodic structure in which the desired coupling is able to repeat itself S-- rr~ y often for the desired .ol~-. fraction to be obtained. Since the ~)IU~ dt;UU constants of the different modes differ to some extent in the same ;uidc, the distance between two _ ~. couplings can be chosen in 25 such a way that a s--hs~q--~nt coupling takes place whenever the two modes to be coupled have become 180 out of phase since the previous coupling. In that case a positive interference always occurs between the C ~ c of the same 2~ 3~
desired mode generated at the . ~ .. couplings and the ~ b~ of the ~ couplings will reinforce each other. With given ~..v~E;uid~ profiles of the ~ uidc upstream and du .. ll~ ull of a transition, the distances between ~ couplings and the number of ~ ,Liliul~s for each mode pair 5 are A~trrt~ in order to obtain a desired co~ fraction from one mode into a certain other mode. The present coupling ",~ is therefore a selective - ' -FIG. I shows l~;AV.;.. ~;.-lly in a IL^n^itlltlit^^l section a mode converter according to the invention made up of channel-type wave-guiding lO sections, viz. an input section A, an ^~i^te section B and an output section C. The ;..~ ;7llr section B consists of an N-fold repetition of two s~q~ nti^lly arranged wave-guiding S~ P and Q having different modal field profiles. Let the sllhc~cti~A~nc P and Q have mode-~ pPn~ nt ~llr constants ~Pm and ~Qm~ c~ , where the index m may have the values I
l5 and 2. In this - , m=l indicates the mode of which a fraction has to be converted and m=2 indicates the mode in which said .u.l~ iùll results. The lengths Lp and LQ of the s~lbs~cti~^nc p and Q are ~ t~rtn ~~ by Lp = ~ p1 - ~P2 I ~ and LQ = ~ Q1 ~ I~Q2 1 (l) the number of l~liliulls N is ~1Pt~rmin~d by f12 = sin2 (2C12 * N) (2) 5 where: f12 is the fraction of the intensity of the mode l at the transition from the section A to the first s~hs~.- P, which fraction is converted into mode 2 after N couplings at the transition from the N-th s~ Q to the section C;
C1 2 = the coupling factor of the modes I and 2 at each P-Q and Q-P transition.
5m=l m-2 type P Q A,C
TEoo TEol bimod a6ym sym (a)sym n n n Sym asym n n n n asym asym n 10TEoo TMoo monomod asym sym (a)sym n n n 6ym asym "
n n n aBym asym TEoo TMol bimod sym sym (a)sym TEOl TMoo bimod sym sym (a)sym 15TEol TMol bimod asym sym (a)sym n n n sym asym "
Il n n asym asym "
TMoo TMOl bimod asym sym (a)sym Il n n sym asym "
20 ~ n n asym asym ll m=2 m-l type P Q A,C
TABLE 2 shows which mode CO.I\,, can be achieved with a channel-type ~.m,~;uid~ structure as shown in FIG. 1. The ditto mark " indicatesthat the same item is meant as in the row above. Each row in the table is 30 i..t..~ t~d in the following way. A guided mode entering via the input section A referred to or ditto-marked in the first column under m=l, is converted into the guided mode referred to or ditto-marked in the second column next thereto under m=2 if the wave-guiding sections P, Q, A and C are of the type ' in the third column next thereto and are ~y , ~ or asymmet-20~39 rical in accul.l"uce with the ~ in the corresponding columns. Thus,for example, the seventh row means that the mode TEUû in a bimodal channel-type waveguide (indicated by bimod) having s~ I subs~ctir~nc P and Q
(indicated by sym) can be converted into the TMUl mode, it being possible for 5 the section A and the section C to be ~, ' or ..~ ic~l (indicated by (a)sym). Fu.i' ~, the 4th, 5th and 6th rûw, read in . hitv~tir~n, indicate that in a - ' ' v~v~6u;dc (indicated by monomod), at least one of whose s~lbse-~ P and Q is ,~,.~,llllll~lli.,ll, the mode TEUû can be converted into the mode TMUU.
If the mode converter is one which converts a guided mode of the zero-th order into a guided mode of the first order, the wave-guiding section A can be - ' ~, while the sectiûn P and Q are bimodal. Preferably, a tapered piece is then provided between section A and the first section P, which tapered piece forms a gradual transition from - ' ' to bimodal without a coupling 15 such as that in the transition between the sections P and Q being able to occur.
In view of the reciprocal nature of the coupling ...~ -.. u~d..I~,;l.g the mode .~,IIV~ TABLE 2 remains . ' '~r valid if the items of the columns m=l and m=2 are illl...ll,~l.6~. This is indicated in the last line of the table by m=2 and m=l, I~..liv.l~ r1~ ' the first and second column.
In FIG. 1, the various wave-guiding sections A, P, Q and C are shown with different cross section. This is purely symbolic in order to indicate that their waveguide profiles may differ. Although differences of this type can ofteneasily be achieved by such differences in cross section, they may also be obtained in other ways. In addition, if one of the s~hser~tir~nc is s~ l, the waveguide profile of said s~hse.- and the vv~v~6uide profiles of the sections A and C can be chosen to be identical.
Each converter according to TABLE 2 having a structure according to 2~ 3~
FIG. 1 can easily be im. 1 ~ ' in integrated form, for example on the basis of InP. FIG. 2 shows a cross section of a channel-type waveguide having a rib structure. A substrate I of InP having a refractive index n1 supports a light-guiding film 2 of InGaAsP having a refractive index n2 somewhat higher than 5 n1, and a buffer layer 3 of InP thereon, again having a refractive index n1 Said buffer layer 3 is provided with a rib 4 having a recungular cross section, height h and width d of the same material for example obtained from the buffer layer by recessing with the aid of etching techniques. The waveguide formed under a rib having a rectangular cross section of this type in an isotropic medium is a 10 ~ one. The waveguide becomes an a~ ;C one by d~ . said symmetry, for example by removing a small corner 5 in the right upper corner from the rectangular cross section over the length of the ~O~;uide, for example by an additional etching operation. By providing, in the same but mirror-image way, the ....~ l ~ in the cross section, instead of at the right-hand side, at the 15 left-hand side, that is to say, in this example by removing an equally large small corner 6, an ..~ li.al ~a~l;uid~ is also obtained, but with an opposite symmetry. If the same but mutually mirror-image symmetry is provided in the cross section both on the right and on the left, that is to say by removing both a small corner 5 and a small corner 6, a ~ àl waveguide is again obtained, 20 but with a wave-guide profile different from the waveguide having the original rectangular cross section. Instead of removing material, the same effects can, of course, be obtained by growing on material. As a result of a suitable choice of the width d, the waveguide becomes .- ~ or bimodal. Different al ~a~. ~;uid~s can also be obtained by a small variation in the width 25 d, in which case the mode type of the waveguide does not change. An a~lllll.~ll~ tO be provided must also be such that the mode type of the waveguide does not change, but this is not, however, critical.
2 ~ 9 At a transition in a sequence of wave-guiding sections, however, the concept of ~ is relative. A transition between two s~qu"nti~lIy arranged ' sections, of which the plane of symmetry of the section is offset du....,l.~ ll of the transition with respect to the plane of symmetry of the 5 section upstream thereof is - .~. ' '~ a transition from a ~ al to an ' guiding section for a guided mode. This applies both to .~ ' sections having identical ~ uidc profiles and to ,~l..lll~lli...l sections having different ~ .E;uid~ profiles. This means that a ~llllll~lli.al wave-guiding section having an ....~ull,.~.lli~àl ll~...u..il,~ or widening with10 respect to a ~UII~ l wave-guiding section preceding it also provides a symmetry/ asymmetry transition. This cul~ ullls, however, to an ...,~ .~
obtained, I~;,U. ~ ly, by the removal or the growth of a small corner 5' having the same height h as the rib 4. A separate etching operation is, however, no longer necessary for a removal of this type.
All the .. ~.. liri.. l;. required for a specific mode converter can be provided simply and with the required accuracy on a waveguide of this type having a rib structure with existing etching l~cl.~iu,. by suitable choice of the masks to be used in the process. Of course, other ~ uide structures standard in integrated optics can also be used for this purpose. More generally still, any 20 mode convener from TABLE 2 can be produced by simple . -~ ;r;- -l ;- - ~ to any single channel-type waveguide with the aid of known int~'ll " t: I ._' Example 1:
According to TABLE 2, a TEûo~TMoo convener can be produced with 25 the aid of - ' ' waveguide sections. For a rib-type waveguide as described above on an InP substrate, n1=3.209, and a film of InGaAsP, n2=3.325, film thickness 0.50~m, buffer layer thickness O.lO~m, rib height (above the buffer 2~ 3~
layer 3) 0.45~m, rib width must be chosen d=2.0,~1m ( ~ ' "), Lp _ LQ =
~,U~UlU~iLUdt~ly 80 ,~m. The refractive indexes n1 and n2 and the lengths Lp andLQ of the S~lbF- -' apply to optical signals having a ~ .1..l6lh of 1.3~m. The sections A and C are ~ - ' and have the same ~vcl;uidc profile. If one 5 of the two subs~ctir~nc is ..~ for example the section P, as a result of removing a small corner 5 having a height of 0.23~m and a square cross section, and the other ~ - l, for example having the same modal field profile as the sections A and C, the calculated coupling factor is Cl2 = 3.4*10~3 for the coupling between the modes TEoo and TMoo. To obtain a . ~ ;Oil of 50%, 10 the fraction fl2 must be = Yz. This is achieved, according to equation (2) if2C~2*N = K7~, that is to say if the number of periodic .~.liliol~s of the coupling N = 116. The total length of the section B is then ..~,UlU~ill.dt~ly 18.5 mm. If the 5"1 .-' Q is also made ~ . ;- -l with an ~ Ullll~ ' equal to but the mirror image of that of the s~hs~ctinn P, the coupling factor doubles asa result, so that the number of couplings, and consequently the length of the section B, can be reduced to half. For a 100% Cullv~ ' , the number N has to be doubled.
Example 2 According to TABLE 2, a TMoo-TMol converter can be COil.lll ' with the aid of bimodal waveguide sections. This type of mode converter has been designed with the aid of a ~ ti~n method known under the name Effective Index Method. For a rib-type waveguide as described above on an InP
substrate, nl = 3.1754, and a film of InGaAsP, n2 = 3,4116, film thickness 0.473~m, buffer layer thickness 0.304 ~m, rib height (above the buffer layer 3) 0.200 ,um, rib width d=8.5 ,um (bimodal!), Lp must be chosen = LQ = 387 ~m.
The refractive indexes nl and n2, and the lengths Lp and LQ of the sllhs~cti-mc ~6~4~
are given here for optical signals having a ~ of 1.5 ~m. The two s~!hs~:- are ~ ' and have the same ~ Euid~ profile. The su~s- ~ P and Q are connected in sequence in the I ~ Ain~l direction d~ t.ly offset to the left and to the right with respect to one another, the offset always being 0.56 ~m. The calculated coupling factor C12 = 0.131 for the coupling between the TMoo and TMol modes. To obtain a ~.. .;OI~ of 100%, at least with sufficient accuracy, a total of 12 sections are s~ffi~ipnt The -- occurring in the process is calculated to be < O.ldB. The total length of the converter is ~pp~ .y 4.7 mm. If the section A is .- ~ 1 at 10 least for the TM ~ ;"," for example having a rib width of 4.3 ~m, a tapered piece must be included between the section A and the first subsection P of the ~ - ' section B to achieve a gradual transition from a - ' I to a bimodal waveguide. The section C may be a direct of the last s--~-- ~ P or Q.
With the aid of FIG. 3 to 6 inclusive, some applications will be explained below of the mode . ~ described above in two types of optical input sections, known per se, for a coherent optical receiver operating on the basis of p~ diversity.
FIG. 3 shows a block diagram of a first type of the known optical input 20 sections in which mixing precedes splitting. This section comprises a mixer 11 having an input optical channel a for any light signal to be detected, that is to say with an unknown TE/TM ~1 ~;~ l;"" li" " _ ' , and an input optical channel b for a light signal having a 50% TE/TM rl- ;~"i. di~ ,n ...;c;. -~;.~ from a local light source 12. The mixer 11 dictril _t~s a signal it has 25 mixed equally in terms of power over two optical channels c and d. Then each of the signals obtained on these outputs is split with the aid of TE/TM
~' splitters 13 and 14 known per se, and the signals split in this way are 206g~3~
presented at outgoing optical ehannels e, f, g and h of said splitters for further ;..g. All the optical channels are in principle - ' ' A 3dB power coupler is known as mixer. The local light source 12 is preferably co-integratedin an integrated form of an optical input section of this type. If said light source 5 12 is a laser, it can only provide one sute in which a light signal it emits via a optical channel j eontains only one ~ c--~ r Thus, the light signal of a co ~ " - ' laser standard in ~~O ~ on the basis of InP and having light in the near infrared eontains only the TE pnl ,;,~l;".. ~ , ' This means that a mode eonverter has to be il~v~ t~d between the output of the 10 light souree 12 and the input optical channel b of the mixer 11 for a panial mode . ~/~la;ull~ in this case 50%. Since both the optical channel j and the optical channel b are ,.. n.1~l a 50% TEoo~TMoo mode converter 15 according to the invention can be chosen for this purpose.
FIG. 4 shows a block diagram of an optical input section in which 15 splitting precedes mixing. A light signal received via the input channel a is now first presented to a TE/TM ~ splitter 21. Signals split as a result in pol..l- -- mode, TE and TM, are presented via optical channels k and I to different 3dB power couplers 22 and 23, I~ uc~ ly, for mixing with light signals, ~UII~JUIIdil.g in ~,1 . ;,~ mode, presented via optical channels m 20 and n and ;~ e from the local co-integrated light source 12. Between the output optical channel j of the light source 12 and the optical channels m and na combined mode converter/ splitter 25 has been included for this purpose. All the optical channels a, e to h inclusive, and j to n inclusive are again ~....---....n.l l More generally, a combined mode converter/splitter 25 has the function of 25 sending a convened signal fraction which has been split off from the l~.I...illil.g . t.d signal to a separate output. A mode converter/splitter of this kind is shown in detail in FIG. 5 and is made up of three sections, viz.:
20~$~
- a tapered piece 25.1 for ~o~ hlg the - ' ' optieal channel j into a bimodal optical channel, - a TXoo-TYo1 mode converter 25.2 according to TABLE 2, TX and TY each ~c~ h.b one of the two ~,t' ~ modes TE and TM, and S - a mode splitter 25.3; for this purpose, a splitter ean be used which is based on a - ' ' ....~ ' branehing of a bimodal ~ buidc~ that is to say, with a - - in two ' ' branehes having different ,UlU,U~,dtiUII eonstants, such as is disclosed, for example, by reference [7] (more particularly, Fig. 2(a)). In a splitter of such a type, a first-order guided mode 10 upstream of the branching is .. ' ~ 'y converted into a zero-order guided mode of the braneh having the lowest ,ulv,u..~tiol~ eonstant, while the zero-order guided mode upstream of the branch ~lol~sb~lt~;> in the branch having the highest ~1~ r L. ' constant. A mode splitter of this type can be used here because the converter 25.2 preceding it delivers an optical signal in which the TX
15 p~l-.;,-l;-- . mode ~IUU..b~ exclusively as a zero-order guided mode and the TY ~.~ ;.. mode ,u~u,u~6at~i. exclusively as a first-order guided mode. The advantage of this type of mode splitter is that it does not contain any '1i7Pd waveguide(s), this being in contrast to the p~ ' splitters usually used. Use of '1i7Pd elements in an integrated optieal design requires, after all, 0 additional measures to prevent interfering effeet on aull- ~in~ optieal - If a 100% TXoU~TXo1 eonverter is chosen as mode converter 25.2 in a combined mode converter/splitter of this type, TX again standing for one of the two l:,t ' - modes, a p, ' - splitter is obtained which also has no ~Pt~lli7Pd elements. The combined mode eonverter/splitter 25 may be used 25 in the optieal input seetion aeeording FIG. 4 if the mode eonverter 25.2 ul~uul~ d therein is a 50% TEoo-TMU1 mode eonverter"ulu.cclil.g from the that again only the TE p.l -. ;~ mode is presented at the optieal 2~6~43~
channel j. FIG. 6 shows a block diagram of an optical input signal which can be , ' ~ i . ' ~ '.~l without such 11i7~d elements. Just as in the optical input section according to the block diagram of FIG. 3, the mixing takes place here prior to splitting. The essential difference is, however, that the mixing S takes place at tnl-l- ' ' level with the aid of a mixer 31 of the Itin~ l 3dB
power coupler type having bimodal input channels p and q and bimodal output channels r and s. To d;;~li..6uiL them from the - ' ' optical channels, said bimodal optical channels are shown thickened in the figure. As splitting means, mode splitters 32 and 33 of the same type as the mode splitters 25.3 (see10 FIG. 5) may be used if it is ensured that, in the optical channels r and s, and therefore also in the optical channels p and q, the two different ~ ' TE
and TM exclusively propagate in mutually different orders of guided mode, on an equal basis in each of the optical channels p to s inclusive. For this purpose, a 100% TMoo-TMo1 converter 34 is L..c/~ i between the - ' ' 15 input channel a and the bimodal input channel p, and a 50% TEoo-TMo1 converter 36 is ill~Ul~JUl.~t~i between the - ' ' optical channel j for guiding the optical signal ;~ e from the light source 12 and the bimodal input optical channel q of the mixer 31. Both Cc~ 34 and 36 have again been chosen in accol,' with TABLE 2; and each of said cc,ll~ should 20 also be preceded by a tapered piece such as 25.1 from FIG. 5. The outgoing optical channels e to h inclusive are identical to those in FIG. 3 and are therefore provided with C(ll~_~ ling, letters.
FIG. 7 shows a block diagram of an optical input section in which splitting precedes mixing, said input section being a variant of and being 25 capable of being l~ ul-d by the same block diagram as the input section according to FIG. 4. An optical signal received via the input channel a is now first presented to a combined mode converter/splitter 41 having outgoing 2~ 3~
' ' optical channels k' and 1' of the same type as the mode converter/splitter 25 described above (see FIG. 5), in which the mode converter (25.2 in FIG. 5) is a 100% TMoo~TEol mode converter. As a result, signals having the ~~ mode TE are presented via both the optical channels k' and 1' to different 3dB power couplers 42 and 43, one of said signals Cu~ in~ to the converted TM . of the signal received via the input channel a. The optical signal from the light source 12, which also has the~ ; mode TE, is now fed via the output optical channel j of the light source 12 to the input of a a~ ll;...l Y splitter 44 and is presented to the 10 power couplers 42 and 43 after being ~ ul~d in terms of power over outgoing ' I optical channels m' and n'. In this variant, signals exclusively having the p~ ' mode TE are presented for further ah~6 on the outgoing optical channels e', f', g' and h'. The advantages of this variant are that optimi7~titm is required only for one ~ ; - - mode on 15 " - and that, just as in the input section according to FIG. 6, no l; splitters provided with metal elements are again needed in this case.
In addition, a further advantage is that the Y splitter 44 is much easier to construct than the p~' cc.ll~ il.6 and splitting ~ which are necessary i.~ Iy d~..llall~ of the local oscillator 12 in the other variants 20 according to FIG. 3, FIG. 4 and FIG. 6.
FIG. 5: shows a combined mode converter/splitter according to the invention in Irngitl litsl section;
FIG. 6: shows a block diagram of an optical input section according to the invention.
10 FIG. 7: shows the same as FIG. 4.
E. De~ ,l of ~ vLllY embodiments Two polarized modes are able to propagate in a ' I channel-type (optical) ~ Euide in an isotropic medium, such as, for example, in InP or in 15 a standard - ' ' optical fiber. These modes can be referred to as TE
(Ll~u~ electric) and TM (transverse magnetic). This t~rtn ~~ O,y is in fact ; e since said modes cannot be described by a single electric or magnetic field colll~~ . In a 1 ~ l; of these guided modes, all three electric and all three magnetic field vector c - .~ must always, after all, be included.
20 ~ .li' ' , it is the case that, with a choice of an Oll' ~ ' acial system which is standard in integrated optics, the TE mode is .' its~d by the Ey - and the Hx .ulllr t, and the TM mode by the Ex CC~ and the Hy c~ . .l In this c~ ., the z-acis indicates the IJlU~ ,ClliU..
direction, and the x-axis is usually chosen perp~-n~lir~ t to the slab-type 25 substrate. Waveguides of this type are, moreover, usually symmetrical with the xz plane as the plane of symmetry as a result of the nature of the knûwn A~ '` techniques- For a channel-type waveguide in a standard optical fiber, g aDy plane through the z-axis is a plane of symmetry. This symmetry manifests itself, in even (+) or odd (-) form, in the field vector .uu,l of the guided modes. For the various modes, this even or odd symmetry is shown in TABLE
1.
mode TEoo TEol TMoo TMo field vector 10 component s + +
z -- + +
I S .~ + +
Waveguide profile of a channel-type waveguide is llnrl~r~t~d as meaning the geometry of the section of the guide, including the optical properties of the wave-guiding medium and its :IU~ ling~ From the theory relating to optical fiber splices it is known, for example from reference 14], that 25 if an abrupt transition from one waveguide profile to another ~ .6uid~ profile occurs in a ~ .6uide, it is possible for coupling to take place between a guidedmode in the guide upstream of the transition and any possible guided mode in the conductor dv..ll~ ..lll of the transition. In this cAnn~ - coupling with radiation modes is in principle also possible. However, it is assumed that the 30 ~ ions referred to in this . - are such that the coupling to radiation modes can be neglected and can therefore be left out of c, .c;~ ;.. here. The degree of coupling, that is to say, the fraction of the power of a guided mode upstream of the transition which is converted into one or more guided modes d~,..l,~ll~..l,. of the transition, can be calculated by means of the integral of the 35 scalar product of the (modal) field vectors upstream and d.,.. u~ .lll of thetransition (see equation (8.6) in reference [4]). This theory is generally 2~43~
applicable to any transition from one channel-type conductor into another in a sequence of two or more channel-type ~ 6uid~s having different modal field profiles. But not every coupling between guided modes upstream and du..,..,ll~..u, of the transition is readily possible. From TABLE I it follows, for 5 example, that, in a - of two a~uu~ LI;C ~ ' ' ~ .c,uid.;., no >;Un of the TEUo mode to TMoU mode or vice versa can take place since these two modes have a different symmetry. In a sequence of a ~ ,;.al waveguide with an ~ -' ' one, or of two different .c,u;d~s~ however, coupling will in fact take place 10 between said TE mode and TM mode since the symmetry of the modal field vector c- ~ is destroyed. In a sequence of two different bimodal u;d~i a good coupling can take place at the transition between the TEoU and the TMo1 mode, or between the TEU1 mode and the TMoo mode and vice versa since, according to TABLE 1, the modes for each pair - ' 15 have the same field symmetry. The couplings - ' between the various TE and TM modes at a waveguide transition ûf this type are, however, weak and the . \ .I:>;UU fraction is therefore low. For ~rrli~ti,~-~c such as, for example, in a coherent optical receiver, however, larger Cull\~ ;ull fractions are necessary, in this case ~ lu~ t~ly 50%, than can be obtained with a single 20 transition. Larger conversion fractions of this type can be obtained by making use of a periodic structure in which the desired coupling is able to repeat itself S-- rr~ y often for the desired .ol~-. fraction to be obtained. Since the ~)IU~ dt;UU constants of the different modes differ to some extent in the same ;uidc, the distance between two _ ~. couplings can be chosen in 25 such a way that a s--hs~q--~nt coupling takes place whenever the two modes to be coupled have become 180 out of phase since the previous coupling. In that case a positive interference always occurs between the C ~ c of the same 2~ 3~
desired mode generated at the . ~ .. couplings and the ~ b~ of the ~ couplings will reinforce each other. With given ~..v~E;uid~ profiles of the ~ uidc upstream and du .. ll~ ull of a transition, the distances between ~ couplings and the number of ~ ,Liliul~s for each mode pair 5 are A~trrt~ in order to obtain a desired co~ fraction from one mode into a certain other mode. The present coupling ",~ is therefore a selective - ' -FIG. I shows l~;AV.;.. ~;.-lly in a IL^n^itlltlit^^l section a mode converter according to the invention made up of channel-type wave-guiding lO sections, viz. an input section A, an ^~i^te section B and an output section C. The ;..~ ;7llr section B consists of an N-fold repetition of two s~q~ nti^lly arranged wave-guiding S~ P and Q having different modal field profiles. Let the sllhc~cti~A~nc P and Q have mode-~ pPn~ nt ~llr constants ~Pm and ~Qm~ c~ , where the index m may have the values I
l5 and 2. In this - , m=l indicates the mode of which a fraction has to be converted and m=2 indicates the mode in which said .u.l~ iùll results. The lengths Lp and LQ of the s~lbs~cti~^nc p and Q are ~ t~rtn ~~ by Lp = ~ p1 - ~P2 I ~ and LQ = ~ Q1 ~ I~Q2 1 (l) the number of l~liliulls N is ~1Pt~rmin~d by f12 = sin2 (2C12 * N) (2) 5 where: f12 is the fraction of the intensity of the mode l at the transition from the section A to the first s~hs~.- P, which fraction is converted into mode 2 after N couplings at the transition from the N-th s~ Q to the section C;
C1 2 = the coupling factor of the modes I and 2 at each P-Q and Q-P transition.
5m=l m-2 type P Q A,C
TEoo TEol bimod a6ym sym (a)sym n n n Sym asym n n n n asym asym n 10TEoo TMoo monomod asym sym (a)sym n n n 6ym asym "
n n n aBym asym TEoo TMol bimod sym sym (a)sym TEOl TMoo bimod sym sym (a)sym 15TEol TMol bimod asym sym (a)sym n n n sym asym "
Il n n asym asym "
TMoo TMOl bimod asym sym (a)sym Il n n sym asym "
20 ~ n n asym asym ll m=2 m-l type P Q A,C
TABLE 2 shows which mode CO.I\,, can be achieved with a channel-type ~.m,~;uid~ structure as shown in FIG. 1. The ditto mark " indicatesthat the same item is meant as in the row above. Each row in the table is 30 i..t..~ t~d in the following way. A guided mode entering via the input section A referred to or ditto-marked in the first column under m=l, is converted into the guided mode referred to or ditto-marked in the second column next thereto under m=2 if the wave-guiding sections P, Q, A and C are of the type ' in the third column next thereto and are ~y , ~ or asymmet-20~39 rical in accul.l"uce with the ~ in the corresponding columns. Thus,for example, the seventh row means that the mode TEUû in a bimodal channel-type waveguide (indicated by bimod) having s~ I subs~ctir~nc P and Q
(indicated by sym) can be converted into the TMUl mode, it being possible for 5 the section A and the section C to be ~, ' or ..~ ic~l (indicated by (a)sym). Fu.i' ~, the 4th, 5th and 6th rûw, read in . hitv~tir~n, indicate that in a - ' ' v~v~6u;dc (indicated by monomod), at least one of whose s~lbse-~ P and Q is ,~,.~,llllll~lli.,ll, the mode TEUû can be converted into the mode TMUU.
If the mode converter is one which converts a guided mode of the zero-th order into a guided mode of the first order, the wave-guiding section A can be - ' ~, while the sectiûn P and Q are bimodal. Preferably, a tapered piece is then provided between section A and the first section P, which tapered piece forms a gradual transition from - ' ' to bimodal without a coupling 15 such as that in the transition between the sections P and Q being able to occur.
In view of the reciprocal nature of the coupling ...~ -.. u~d..I~,;l.g the mode .~,IIV~ TABLE 2 remains . ' '~r valid if the items of the columns m=l and m=2 are illl...ll,~l.6~. This is indicated in the last line of the table by m=2 and m=l, I~..liv.l~ r1~ ' the first and second column.
In FIG. 1, the various wave-guiding sections A, P, Q and C are shown with different cross section. This is purely symbolic in order to indicate that their waveguide profiles may differ. Although differences of this type can ofteneasily be achieved by such differences in cross section, they may also be obtained in other ways. In addition, if one of the s~hser~tir~nc is s~ l, the waveguide profile of said s~hse.- and the vv~v~6uide profiles of the sections A and C can be chosen to be identical.
Each converter according to TABLE 2 having a structure according to 2~ 3~
FIG. 1 can easily be im. 1 ~ ' in integrated form, for example on the basis of InP. FIG. 2 shows a cross section of a channel-type waveguide having a rib structure. A substrate I of InP having a refractive index n1 supports a light-guiding film 2 of InGaAsP having a refractive index n2 somewhat higher than 5 n1, and a buffer layer 3 of InP thereon, again having a refractive index n1 Said buffer layer 3 is provided with a rib 4 having a recungular cross section, height h and width d of the same material for example obtained from the buffer layer by recessing with the aid of etching techniques. The waveguide formed under a rib having a rectangular cross section of this type in an isotropic medium is a 10 ~ one. The waveguide becomes an a~ ;C one by d~ . said symmetry, for example by removing a small corner 5 in the right upper corner from the rectangular cross section over the length of the ~O~;uide, for example by an additional etching operation. By providing, in the same but mirror-image way, the ....~ l ~ in the cross section, instead of at the right-hand side, at the 15 left-hand side, that is to say, in this example by removing an equally large small corner 6, an ..~ li.al ~a~l;uid~ is also obtained, but with an opposite symmetry. If the same but mutually mirror-image symmetry is provided in the cross section both on the right and on the left, that is to say by removing both a small corner 5 and a small corner 6, a ~ àl waveguide is again obtained, 20 but with a wave-guide profile different from the waveguide having the original rectangular cross section. Instead of removing material, the same effects can, of course, be obtained by growing on material. As a result of a suitable choice of the width d, the waveguide becomes .- ~ or bimodal. Different al ~a~. ~;uid~s can also be obtained by a small variation in the width 25 d, in which case the mode type of the waveguide does not change. An a~lllll.~ll~ tO be provided must also be such that the mode type of the waveguide does not change, but this is not, however, critical.
2 ~ 9 At a transition in a sequence of wave-guiding sections, however, the concept of ~ is relative. A transition between two s~qu"nti~lIy arranged ' sections, of which the plane of symmetry of the section is offset du....,l.~ ll of the transition with respect to the plane of symmetry of the 5 section upstream thereof is - .~. ' '~ a transition from a ~ al to an ' guiding section for a guided mode. This applies both to .~ ' sections having identical ~ uidc profiles and to ,~l..lll~lli...l sections having different ~ .E;uid~ profiles. This means that a ~llllll~lli.al wave-guiding section having an ....~ull,.~.lli~àl ll~...u..il,~ or widening with10 respect to a ~UII~ l wave-guiding section preceding it also provides a symmetry/ asymmetry transition. This cul~ ullls, however, to an ...,~ .~
obtained, I~;,U. ~ ly, by the removal or the growth of a small corner 5' having the same height h as the rib 4. A separate etching operation is, however, no longer necessary for a removal of this type.
All the .. ~.. liri.. l;. required for a specific mode converter can be provided simply and with the required accuracy on a waveguide of this type having a rib structure with existing etching l~cl.~iu,. by suitable choice of the masks to be used in the process. Of course, other ~ uide structures standard in integrated optics can also be used for this purpose. More generally still, any 20 mode convener from TABLE 2 can be produced by simple . -~ ;r;- -l ;- - ~ to any single channel-type waveguide with the aid of known int~'ll " t: I ._' Example 1:
According to TABLE 2, a TEûo~TMoo convener can be produced with 25 the aid of - ' ' waveguide sections. For a rib-type waveguide as described above on an InP substrate, n1=3.209, and a film of InGaAsP, n2=3.325, film thickness 0.50~m, buffer layer thickness O.lO~m, rib height (above the buffer 2~ 3~
layer 3) 0.45~m, rib width must be chosen d=2.0,~1m ( ~ ' "), Lp _ LQ =
~,U~UlU~iLUdt~ly 80 ,~m. The refractive indexes n1 and n2 and the lengths Lp andLQ of the S~lbF- -' apply to optical signals having a ~ .1..l6lh of 1.3~m. The sections A and C are ~ - ' and have the same ~vcl;uidc profile. If one 5 of the two subs~ctir~nc is ..~ for example the section P, as a result of removing a small corner 5 having a height of 0.23~m and a square cross section, and the other ~ - l, for example having the same modal field profile as the sections A and C, the calculated coupling factor is Cl2 = 3.4*10~3 for the coupling between the modes TEoo and TMoo. To obtain a . ~ ;Oil of 50%, 10 the fraction fl2 must be = Yz. This is achieved, according to equation (2) if2C~2*N = K7~, that is to say if the number of periodic .~.liliol~s of the coupling N = 116. The total length of the section B is then ..~,UlU~ill.dt~ly 18.5 mm. If the 5"1 .-' Q is also made ~ . ;- -l with an ~ Ullll~ ' equal to but the mirror image of that of the s~hs~ctinn P, the coupling factor doubles asa result, so that the number of couplings, and consequently the length of the section B, can be reduced to half. For a 100% Cullv~ ' , the number N has to be doubled.
Example 2 According to TABLE 2, a TMoo-TMol converter can be COil.lll ' with the aid of bimodal waveguide sections. This type of mode converter has been designed with the aid of a ~ ti~n method known under the name Effective Index Method. For a rib-type waveguide as described above on an InP
substrate, nl = 3.1754, and a film of InGaAsP, n2 = 3,4116, film thickness 0.473~m, buffer layer thickness 0.304 ~m, rib height (above the buffer layer 3) 0.200 ,um, rib width d=8.5 ,um (bimodal!), Lp must be chosen = LQ = 387 ~m.
The refractive indexes nl and n2, and the lengths Lp and LQ of the sllhs~cti-mc ~6~4~
are given here for optical signals having a ~ of 1.5 ~m. The two s~!hs~:- are ~ ' and have the same ~ Euid~ profile. The su~s- ~ P and Q are connected in sequence in the I ~ Ain~l direction d~ t.ly offset to the left and to the right with respect to one another, the offset always being 0.56 ~m. The calculated coupling factor C12 = 0.131 for the coupling between the TMoo and TMol modes. To obtain a ~.. .;OI~ of 100%, at least with sufficient accuracy, a total of 12 sections are s~ffi~ipnt The -- occurring in the process is calculated to be < O.ldB. The total length of the converter is ~pp~ .y 4.7 mm. If the section A is .- ~ 1 at 10 least for the TM ~ ;"," for example having a rib width of 4.3 ~m, a tapered piece must be included between the section A and the first subsection P of the ~ - ' section B to achieve a gradual transition from a - ' I to a bimodal waveguide. The section C may be a direct of the last s--~-- ~ P or Q.
With the aid of FIG. 3 to 6 inclusive, some applications will be explained below of the mode . ~ described above in two types of optical input sections, known per se, for a coherent optical receiver operating on the basis of p~ diversity.
FIG. 3 shows a block diagram of a first type of the known optical input 20 sections in which mixing precedes splitting. This section comprises a mixer 11 having an input optical channel a for any light signal to be detected, that is to say with an unknown TE/TM ~1 ~;~ l;"" li" " _ ' , and an input optical channel b for a light signal having a 50% TE/TM rl- ;~"i. di~ ,n ...;c;. -~;.~ from a local light source 12. The mixer 11 dictril _t~s a signal it has 25 mixed equally in terms of power over two optical channels c and d. Then each of the signals obtained on these outputs is split with the aid of TE/TM
~' splitters 13 and 14 known per se, and the signals split in this way are 206g~3~
presented at outgoing optical ehannels e, f, g and h of said splitters for further ;..g. All the optical channels are in principle - ' ' A 3dB power coupler is known as mixer. The local light source 12 is preferably co-integratedin an integrated form of an optical input section of this type. If said light source 5 12 is a laser, it can only provide one sute in which a light signal it emits via a optical channel j eontains only one ~ c--~ r Thus, the light signal of a co ~ " - ' laser standard in ~~O ~ on the basis of InP and having light in the near infrared eontains only the TE pnl ,;,~l;".. ~ , ' This means that a mode eonverter has to be il~v~ t~d between the output of the 10 light souree 12 and the input optical channel b of the mixer 11 for a panial mode . ~/~la;ull~ in this case 50%. Since both the optical channel j and the optical channel b are ,.. n.1~l a 50% TEoo~TMoo mode converter 15 according to the invention can be chosen for this purpose.
FIG. 4 shows a block diagram of an optical input section in which 15 splitting precedes mixing. A light signal received via the input channel a is now first presented to a TE/TM ~ splitter 21. Signals split as a result in pol..l- -- mode, TE and TM, are presented via optical channels k and I to different 3dB power couplers 22 and 23, I~ uc~ ly, for mixing with light signals, ~UII~JUIIdil.g in ~,1 . ;,~ mode, presented via optical channels m 20 and n and ;~ e from the local co-integrated light source 12. Between the output optical channel j of the light source 12 and the optical channels m and na combined mode converter/ splitter 25 has been included for this purpose. All the optical channels a, e to h inclusive, and j to n inclusive are again ~....---....n.l l More generally, a combined mode converter/splitter 25 has the function of 25 sending a convened signal fraction which has been split off from the l~.I...illil.g . t.d signal to a separate output. A mode converter/splitter of this kind is shown in detail in FIG. 5 and is made up of three sections, viz.:
20~$~
- a tapered piece 25.1 for ~o~ hlg the - ' ' optieal channel j into a bimodal optical channel, - a TXoo-TYo1 mode converter 25.2 according to TABLE 2, TX and TY each ~c~ h.b one of the two ~,t' ~ modes TE and TM, and S - a mode splitter 25.3; for this purpose, a splitter ean be used which is based on a - ' ' ....~ ' branehing of a bimodal ~ buidc~ that is to say, with a - - in two ' ' branehes having different ,UlU,U~,dtiUII eonstants, such as is disclosed, for example, by reference [7] (more particularly, Fig. 2(a)). In a splitter of such a type, a first-order guided mode 10 upstream of the branching is .. ' ~ 'y converted into a zero-order guided mode of the braneh having the lowest ,ulv,u..~tiol~ eonstant, while the zero-order guided mode upstream of the branch ~lol~sb~lt~;> in the branch having the highest ~1~ r L. ' constant. A mode splitter of this type can be used here because the converter 25.2 preceding it delivers an optical signal in which the TX
15 p~l-.;,-l;-- . mode ~IUU..b~ exclusively as a zero-order guided mode and the TY ~.~ ;.. mode ,u~u,u~6at~i. exclusively as a first-order guided mode. The advantage of this type of mode splitter is that it does not contain any '1i7Pd waveguide(s), this being in contrast to the p~ ' splitters usually used. Use of '1i7Pd elements in an integrated optieal design requires, after all, 0 additional measures to prevent interfering effeet on aull- ~in~ optieal - If a 100% TXoU~TXo1 eonverter is chosen as mode converter 25.2 in a combined mode converter/splitter of this type, TX again standing for one of the two l:,t ' - modes, a p, ' - splitter is obtained which also has no ~Pt~lli7Pd elements. The combined mode eonverter/splitter 25 may be used 25 in the optieal input seetion aeeording FIG. 4 if the mode eonverter 25.2 ul~uul~ d therein is a 50% TEoo-TMU1 mode eonverter"ulu.cclil.g from the that again only the TE p.l -. ;~ mode is presented at the optieal 2~6~43~
channel j. FIG. 6 shows a block diagram of an optical input signal which can be , ' ~ i . ' ~ '.~l without such 11i7~d elements. Just as in the optical input section according to the block diagram of FIG. 3, the mixing takes place here prior to splitting. The essential difference is, however, that the mixing S takes place at tnl-l- ' ' level with the aid of a mixer 31 of the Itin~ l 3dB
power coupler type having bimodal input channels p and q and bimodal output channels r and s. To d;;~li..6uiL them from the - ' ' optical channels, said bimodal optical channels are shown thickened in the figure. As splitting means, mode splitters 32 and 33 of the same type as the mode splitters 25.3 (see10 FIG. 5) may be used if it is ensured that, in the optical channels r and s, and therefore also in the optical channels p and q, the two different ~ ' TE
and TM exclusively propagate in mutually different orders of guided mode, on an equal basis in each of the optical channels p to s inclusive. For this purpose, a 100% TMoo-TMo1 converter 34 is L..c/~ i between the - ' ' 15 input channel a and the bimodal input channel p, and a 50% TEoo-TMo1 converter 36 is ill~Ul~JUl.~t~i between the - ' ' optical channel j for guiding the optical signal ;~ e from the light source 12 and the bimodal input optical channel q of the mixer 31. Both Cc~ 34 and 36 have again been chosen in accol,' with TABLE 2; and each of said cc,ll~ should 20 also be preceded by a tapered piece such as 25.1 from FIG. 5. The outgoing optical channels e to h inclusive are identical to those in FIG. 3 and are therefore provided with C(ll~_~ ling, letters.
FIG. 7 shows a block diagram of an optical input section in which splitting precedes mixing, said input section being a variant of and being 25 capable of being l~ ul-d by the same block diagram as the input section according to FIG. 4. An optical signal received via the input channel a is now first presented to a combined mode converter/splitter 41 having outgoing 2~ 3~
' ' optical channels k' and 1' of the same type as the mode converter/splitter 25 described above (see FIG. 5), in which the mode converter (25.2 in FIG. 5) is a 100% TMoo~TEol mode converter. As a result, signals having the ~~ mode TE are presented via both the optical channels k' and 1' to different 3dB power couplers 42 and 43, one of said signals Cu~ in~ to the converted TM . of the signal received via the input channel a. The optical signal from the light source 12, which also has the~ ; mode TE, is now fed via the output optical channel j of the light source 12 to the input of a a~ ll;...l Y splitter 44 and is presented to the 10 power couplers 42 and 43 after being ~ ul~d in terms of power over outgoing ' I optical channels m' and n'. In this variant, signals exclusively having the p~ ' mode TE are presented for further ah~6 on the outgoing optical channels e', f', g' and h'. The advantages of this variant are that optimi7~titm is required only for one ~ ; - - mode on 15 " - and that, just as in the input section according to FIG. 6, no l; splitters provided with metal elements are again needed in this case.
In addition, a further advantage is that the Y splitter 44 is much easier to construct than the p~' cc.ll~ il.6 and splitting ~ which are necessary i.~ Iy d~..llall~ of the local oscillator 12 in the other variants 20 according to FIG. 3, FIG. 4 and FIG. 6.
Claims (12)
1. Mode converter for the conversion of a fraction of a signal component of an optical signal propagating according to a first guided mode intoa signal component propagating according to a second guided mode, comprising a channel-type waveguide in which a periodic coupling between the two guided modes of an optical signal propagating in the waveguide takes place, which waveguide comprises an incoming wave-guiding section, an intermediate wave-guiding section and an outgoing wave-guiding section, characterized in that the intermediate wave-guiding section has a periodic geometrical structure consisting of a periodic sequence of two wave-guiding subsections within a period length, the lengths of the subsections and the number of periods being matched to the desired conversion fraction.
2. Mode converter according to Claim 1, characterized in that a period length comprises a first subsection having a first asymmetrical waveguide profile and a second subsection having a second symmetrical waveguide profile.
3. Mode converter according to Claim 2, characterized in that the waveguide profiles of the second subsection and the incoming wave-guiding section are essentially the same.
4. Mode converter according to Claim 3, characterized in that the subsections are monomodal.
5. Mode converter according to Claim 3, characterized in that the subsections are bimodal.
6. Mode converter according to Claim 1, characterized in that a period length comprises two subsections, each having an asymmetrical waveguide profile.
7. Mode converter according to Claim 6, characterized in that the subsections are monomodal.
8. Mode converter according to Claim 6, characterized in that the subsections are bimodal.
9. Mode converter according to Claim 6, characterized in that the waveguide profiles of the two subsections have a mutually opposite asymmetry.
10. Mode converter according to Claim 1, characterized in that a period length comprises two subsections, each having a symmetrical waveguide profile.
11. Mode converter according to one of the preceding claims, characterized in that the periodic geometrical structure is obtained essentially as a result of differences in width between the wave-guiding subsections.
12. Mode converter according to Claim 1, characterized in that the period length comprises two subsections having the same waveguide profile, which subsections join one another in the periodic sequence in an offset manner.13. Optical input section for a coherent optical receiver based on polarization diversity, comprising mixing/splitting means for the polarization-independent equal mixing of a first optical signal received via an incoming optical channel and containing polarization modes (TE, TM), and a second optical signal originating from a local light source and containing polarization modes (TE, TM) and then splittingequally in terms of power into first and second mixed signals, first and second splitting means for the respective splitting up of polarization modes present in the first and second mixed signals into separate optical signals for presentation to equally as many outgoing optical channels, characterized in that first and second conversion means are furthermore provided for converting at least a fraction of one of the polarization modes into a guided mode different in order from that in which the polarization modes mentioned propagate in the first optical signal and the second optical signal, respectively;
in that the mixing/splitting means comprise a multimode power coupler; and in that the first and second splitting means are mode splitters.
14. Optical input section according to Claim 13, characterized in that the local light source is co-integrated, and in that the first conversion means comprise a 100% TM00-TM01 converter according to TABLE 2, and the second conversion means comprise a 50% TE00-TM01 converter according to TABLE
2.
15. Combined mode converter/splitter for converting a fraction of a first signal component of an incoming first optical signal propagating in a first polarization mode (TE or TM) into a second signal component propagating in a second polarization mode (TM or TE), and the emission of second and third optical signals respectively via outgoing second and third monomodal optical waveguides, the second optical signal essentially containing the second signal component and the third optical signal essentially containing the unconverted part of the first optical signal, which converter/splitter consecutively comprises between the first optical waveguide and the second and third optical waveguides:- a tapered piece for the transition of a monomodal to a bimodal optical waveguide, - conversion means for converting at least a fraction of the first signal component into a guided mode differing in order from that in which the first signal component propagates in the first optical signal on leaving the tapered piece, and - a mode splitter.
16. Combined mode converter/splitter according to Claim 15, characterized in that the conversion means comprise a 50% TE00-TM01 converter according to TABLE 2.
17. Combined mode converter/splitter according to Claim 15, characterized in that the conversion means comprise a 100% TM00-TE
converter according to TABLE 2.
18. Combined mode converter/splitter according to Claim 15, characterized in that the conversion means comprise a 100% TX00-TX01 converter according to TABLE 2, TX representing one of the two polarization modes TE or TM.
in that the mixing/splitting means comprise a multimode power coupler; and in that the first and second splitting means are mode splitters.
14. Optical input section according to Claim 13, characterized in that the local light source is co-integrated, and in that the first conversion means comprise a 100% TM00-TM01 converter according to TABLE 2, and the second conversion means comprise a 50% TE00-TM01 converter according to TABLE
2.
15. Combined mode converter/splitter for converting a fraction of a first signal component of an incoming first optical signal propagating in a first polarization mode (TE or TM) into a second signal component propagating in a second polarization mode (TM or TE), and the emission of second and third optical signals respectively via outgoing second and third monomodal optical waveguides, the second optical signal essentially containing the second signal component and the third optical signal essentially containing the unconverted part of the first optical signal, which converter/splitter consecutively comprises between the first optical waveguide and the second and third optical waveguides:- a tapered piece for the transition of a monomodal to a bimodal optical waveguide, - conversion means for converting at least a fraction of the first signal component into a guided mode differing in order from that in which the first signal component propagates in the first optical signal on leaving the tapered piece, and - a mode splitter.
16. Combined mode converter/splitter according to Claim 15, characterized in that the conversion means comprise a 50% TE00-TM01 converter according to TABLE 2.
17. Combined mode converter/splitter according to Claim 15, characterized in that the conversion means comprise a 100% TM00-TE
converter according to TABLE 2.
18. Combined mode converter/splitter according to Claim 15, characterized in that the conversion means comprise a 100% TX00-TX01 converter according to TABLE 2, TX representing one of the two polarization modes TE or TM.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| NL9100852 | 1991-05-16 | ||
| NL9100852A NL9100852A (en) | 1991-05-16 | 1991-05-16 | MODE CONVERTER. |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2068439A1 CA2068439A1 (en) | 1992-11-17 |
| CA2068439C true CA2068439C (en) | 1997-08-26 |
Family
ID=19859255
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002068439A Expired - Lifetime CA2068439C (en) | 1991-05-16 | 1992-05-12 | Mode converter |
Country Status (10)
| Country | Link |
|---|---|
| US (1) | US5185828A (en) |
| EP (2) | EP0640854B1 (en) |
| JP (2) | JP2628258B2 (en) |
| AT (2) | ATE211559T1 (en) |
| CA (1) | CA2068439C (en) |
| DE (2) | DE69203152T2 (en) |
| ES (2) | ES2171433T3 (en) |
| FI (2) | FI111038B (en) |
| NL (1) | NL9100852A (en) |
| NO (1) | NO302916B1 (en) |
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| US3614198A (en) * | 1969-06-23 | 1971-10-19 | Bell Telephone Labor Inc | Thin-film optical devices |
| US3884549A (en) * | 1973-04-30 | 1975-05-20 | Univ California | Two demensional distributed feedback devices and lasers |
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| JPS5949517A (en) * | 1982-09-14 | 1984-03-22 | Nec Corp | Waveguide type electrooptic optical modulator |
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| FR2584825B1 (en) * | 1985-07-11 | 1987-10-09 | Labo Electronique Physique | SEPARATING STRUCTURE, OPTICAL SWITCHING ELEMENT INCLUDING SUCH STRUCTURES, AND OPTICAL SWITCHING MATRIX FORMED FROM SUCH SWITCHING ELEMENTS |
| JPS6217737A (en) * | 1985-07-17 | 1987-01-26 | Nec Corp | Optical heterodyne detecting device |
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-
1991
- 1991-05-16 NL NL9100852A patent/NL9100852A/en not_active Application Discontinuation
-
1992
- 1992-05-05 NO NO921766A patent/NO302916B1/en not_active IP Right Cessation
- 1992-05-08 US US07/880,705 patent/US5185828A/en not_active Expired - Lifetime
- 1992-05-11 AT AT94203130T patent/ATE211559T1/en not_active IP Right Cessation
- 1992-05-11 EP EP94203130A patent/EP0640854B1/en not_active Expired - Lifetime
- 1992-05-11 AT AT92201338T patent/ATE124543T1/en not_active IP Right Cessation
- 1992-05-11 ES ES94203130T patent/ES2171433T3/en not_active Expired - Lifetime
- 1992-05-11 EP EP92201338A patent/EP0513919B1/en not_active Expired - Lifetime
- 1992-05-11 DE DE69203152T patent/DE69203152T2/en not_active Expired - Lifetime
- 1992-05-11 ES ES92201338T patent/ES2075595T3/en not_active Expired - Lifetime
- 1992-05-11 DE DE69232329T patent/DE69232329T2/en not_active Expired - Fee Related
- 1992-05-12 CA CA002068439A patent/CA2068439C/en not_active Expired - Lifetime
- 1992-05-14 JP JP4165226A patent/JP2628258B2/en not_active Expired - Lifetime
- 1992-05-15 FI FI922233A patent/FI111038B/en active
-
1995
- 1995-10-04 JP JP7257411A patent/JP2711654B2/en not_active Expired - Lifetime
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2002
- 2002-06-06 FI FI20021086A patent/FI116003B/en active IP Right Grant
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| ES2075595T3 (en) | 1995-10-01 |
| EP0640854B1 (en) | 2002-01-02 |
| FI922233A (en) | 1992-11-17 |
| EP0513919B1 (en) | 1995-06-28 |
| ES2171433T3 (en) | 2002-09-16 |
| US5185828A (en) | 1993-02-09 |
| FI20021086A (en) | 2002-06-06 |
| DE69232329T2 (en) | 2002-08-22 |
| JP2628258B2 (en) | 1997-07-09 |
| DE69203152T2 (en) | 1995-12-07 |
| EP0640854A3 (en) | 1999-08-18 |
| ATE211559T1 (en) | 2002-01-15 |
| EP0513919A1 (en) | 1992-11-19 |
| JPH08102709A (en) | 1996-04-16 |
| FI116003B (en) | 2005-08-31 |
| JP2711654B2 (en) | 1998-02-10 |
| EP0640854A2 (en) | 1995-03-01 |
| NO921766D0 (en) | 1992-05-05 |
| CA2068439A1 (en) | 1992-11-17 |
| DE69232329D1 (en) | 2002-02-07 |
| NO921766L (en) | 1992-11-17 |
| ATE124543T1 (en) | 1995-07-15 |
| FI922233A0 (en) | 1992-05-15 |
| NO302916B1 (en) | 1998-05-04 |
| FI111038B (en) | 2003-05-15 |
| DE69203152D1 (en) | 1995-08-03 |
| JPH05196830A (en) | 1993-08-06 |
| NL9100852A (en) | 1992-12-16 |
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