CA1278714C - Optical branching filter - Google Patents
Optical branching filterInfo
- Publication number
- CA1278714C CA1278714C CA000529045A CA529045A CA1278714C CA 1278714 C CA1278714 C CA 1278714C CA 000529045 A CA000529045 A CA 000529045A CA 529045 A CA529045 A CA 529045A CA 1278714 C CA1278714 C CA 1278714C
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- CA
- Canada
- Prior art keywords
- input
- optical
- light
- output
- lens
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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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
- G02B6/24—Coupling light guides
- G02B6/26—Optical coupling means
- G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
- G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
- G02B6/29379—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means characterised by the function or use of the complete device
- G02B6/2938—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means characterised by the function or use of the complete device for multiplexing or demultiplexing, i.e. combining or separating wavelengths, e.g. 1xN, NxM
-
- 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
- G02B6/24—Coupling light guides
- G02B6/26—Optical coupling means
- G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
- G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
- G02B6/29346—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means operating by wave or beam interference
- G02B6/29361—Interference filters, e.g. multilayer coatings, thin film filters, dichroic splitters or mirrors based on multilayers, WDM filters
- G02B6/29362—Serial cascade of filters or filtering operations, e.g. for a large number of channels
- G02B6/29365—Serial cascade of filters or filtering operations, e.g. for a large number of channels in a multireflection configuration, i.e. beam following a zigzag path between filters or filtering operations
- G02B6/29367—Zigzag path within a transparent optical block, e.g. filter deposited on an etalon, glass plate, wedge acting as a stable spacer
-
- 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
- G02B6/24—Coupling light guides
- G02B6/26—Optical coupling means
- G02B6/32—Optical coupling means having lens focusing means positioned between opposed fibre ends
Abstract
ABSTRACT OF THE DISCLOSURE
An optical branching filter branches or mixes light of n different wavelengths. A first input/output part for light consisting of the n wavelengths mixed together is provided at one of two surfaces of a transparent block. Various second input/output parts for forming optical filters each allowing only light having a respective predetermined one of the n wave-lengths to pass therethrough and reflecting light having other wavelengths, are provided at any one of two surfaces. A first input/output port and n second input/output ports, respectively, is formed by an optical waveguide and a lens which optically connects the optical waveguide and first input/output part or a second input/output part. The positions of each second input/
output port are selected so that the length of the optical paths formed between the lenses of the first input/output port and the second input/output port are in the reverse proportion to the lengths of wavelengths corresponding to the second input/
output portions.
An optical branching filter branches or mixes light of n different wavelengths. A first input/output part for light consisting of the n wavelengths mixed together is provided at one of two surfaces of a transparent block. Various second input/output parts for forming optical filters each allowing only light having a respective predetermined one of the n wave-lengths to pass therethrough and reflecting light having other wavelengths, are provided at any one of two surfaces. A first input/output port and n second input/output ports, respectively, is formed by an optical waveguide and a lens which optically connects the optical waveguide and first input/output part or a second input/output part. The positions of each second input/
output port are selected so that the length of the optical paths formed between the lenses of the first input/output port and the second input/output port are in the reverse proportion to the lengths of wavelengths corresponding to the second input/
output portions.
Description
~'~787~L4 The present invention relates to an optical branching filter which mixes or branches light of different wavelengths.
An optical branching filter is a basic device for a wavelength division multiplexing optical communication system.
For example, it is possible using mixed light of n different ~1' ~2' ' ~n to transmit n times the amount of information as with a single wavelength ~0 over a single line of optical fiber.
Furthermore, n-channel bothway simultaneous optical communication can be realized by allocating light of different wavelengths for transmission and reception, namely wavelengths ~ 3~ ' ~2n-1 are allocated for transmission and wavelengths ~2' ~4' ' ~2n are allocated for reception. In order to realize such optical communication systems, an optical mixing filter which mixes the light of different wavelengths and inputs the light to a single optical fiber line is arranged at the sending side and an optical branching filter which branches or separates out the different wavelengths from the mixed wavelength light is arranged at the receiving side.
The optical mixing filter and optical branching filter currently put into practical use generally mix and branch the light by employing a dielectric material filter composed of a thin di-electric multilayer film laminated in a plurality of layers such as SiO2 and TiO2 as a bandpass filter which allows transmission of light having particular wavelength but reflects light having other wavelengths.
The optical mixing filter can also be used as an optical ~'~787~4 branching filter by inverting the incoming and outgoing directions of light to/from the dielectric material filter. These mixing and branching filters have the same structure and usually both types are referred to as optical branching filters. To realize this optical branching filter, various optical components in addi-tion to the dielectric material filter are required. For example, there are required optical lenses which cause the light sent from the optical fiber to be directed effectively into each dielectric material filter and also the light passed through the dielectric material filter to be directed effectively into the optical fiber.
Also needed are fixing mechanisms which fix the end face of op-tical fiber to the focus point of the lenses.
However, the optical branching filter deals with light having various wavelengths as the refractive index of a lens generally changes in accordance with the wavelength, it follows that the focus point of lens depends on the wavelength of the in-cident light.
Therefore, since the fixing mechanisms require adjust-ment at focus points for each wavelength, the manufacturing yield of the optical blanching filter deteriorates. This deterioration becomes distinctive with increase in number of wavelengths to be mixed or branched. In particular, various kinds of semiconductor laser or light emitting diodes for long to short wavelengths have been developed and the number of wavelengths dealt with an optical branching filter tends to increase and therefore the problem of adjustment at focus points is considered very serious.
1'C ~787~4 It is accordingly an object of the present invention to provide an optical branching filter which is capable of improving the manufacturing yield by realizing common adjustment of the distance between the lens and end face of the optical fiber ir-respective of wavelength.
According to the present invention, there is provided an optical branching filter for branching or mixing light of n different wavelengths, comprising; a block which is optically transparent and has two parallel surfaces, a first input/output part for light consisting of the n wavelengths mixed together provided at one of said two surfaces, second input/output parts, provided at any one of said two surfaces for forming optical filters each allowing only light having a respective predetermined one of the n wavelengths to pass therethrough and reflecting light having other wavelengths, respectively, a first input/output port and n second input/output ports, respectively formed by an optical waveguide and a lens which optically connects said optical wave-guide and said first input/output part of a second input/output port, wherein the positions of each second input/output port are selected so that the lengths of the optical paths formed between the lenses of said first input/output port and each said second input/output port are in the reverse proportion to the lengths of wavelengths corresponding to said second input/output ports.
The invention will now be described in greater detail with reference to the accompanying drawings, in which:
Figures l(a) and (b) are schematic plan views of an 7E~7~4 optical branching filter of the present invention;
Figure 2 is a pictorial presentation for explaining the distance between lenses and the distance between a lens and an optical fiber end face;
Figure 3 is a sectional view of the assembled fixing mechanism of an optical fiber end face and lens;
Figure 4(a) is a plan view of a practical optical branch-ing filter;
Figure 4(b) is a perspective view of a practical optical branching filter;
Figure 5 is a graphical presentation for explaining the relation of the distance between lenses and the distance between a lens and an optical fiber end face;
Figure 6 is a graphical presentation for explaining wavelength characteristics of dielectric material filters; and Figure 7 is a graphical presentation for explaining the relation of distance between lens and optical fiber end face and the loss increase.
Figures l(a),(b) are schematic plan views illustrating a structure of optical branching filter of the present invention.
In Figures l(a),(b), four different wavelengths ~ 2~ ~3~ ~4 are mixed and branched. In a glass block 1 consisting of an opti-cally transparent substance such as BK7 having a pair of parallel surfaces, the parallel surfaces thereof are provided with a band-pass filter 13 which allows only the light of wavelength ~1 to pass, a bandpass filter 15 which allows only the light of wave ': .
787~4 length ~2 to pass and a bandpass filter 17 which allows only the light of wavelength ~3 to pass. Mixed light of four different wavelengths ~ 2~ ~3~ ~4 is emitted from the end face of optical flber 3, converted to parallel light by a spherical lens 2, and is input to the glass block 1 through an anti-reflection film 12 provided for eliminating reflection. Among this mixed light, only the light of wavelength ~1 is able to pass the band-pass-filter 13 provided to the opposite surface of glass block 1, is output to the outside of glass block 1 through the anti-reflection film 14, is focused by spherical lens 4 and is then input to the optical fiber 5 effectively. The other wavelengths A2, ~3, ~4 are re-flected within block 1. Moreover, only the light of wavelength ~2 is able to pass the bandpass filter 15, is output to the out-side of glass block 1 through the anti-reflection film 16, is focused by spherical lens 6 and is then input to the optical fiber 7. The wavelengths ~3, ~4 are reflected. In the same way, the light of wavelength ~3 is able to pass the bandpass filter 17 and is then input to the optical fiber 9 by the spherical lens 8 through the anti-reflection film 18. Here, the light reflected by the bandpass filter 17 is only the light of wavelength ~4 and therefore the light of the wavelength ~4 is input to the optical fiber 11 by the spherical lens 10 via only the anti-reflection film 19.
The bandpass filters and anti-reflection films are form-ed by the dielectric material multilayer film of SiO2, TiO2 obtained by vacuum deposition.
~7~q~4 In the example explained above, each wavelength is branched from the light consisting of four mixed wavelengths. On the other hand, the light consisting of four mixed wavelengths can be input to the optical fiber 3 by reversing the travelling direction of the light of wavelengths ~ 2~ ~3~ ~4 emitted from the optical fibers 5, 7, 9, 11.
Here, it will become a problem that the spherical lenses deal with the lights in the different wavelengths. Namely, the refractive index of a lens changes in accordance with the wave-length and therefore the focal points of spherical lenses 4, 6, 8, 10 are also different in the input/output ports A, B, C, D.
In order to have the lights of wavelengths ~ 2~ ~3~4 input most effectively to the optical fiber 5, 6, 9, 11, the end faces of optical fiber 5, 6, 9, 11 must be provided at the focal points of the spherical lenses 4, 6, 8, 10 at respective input/output ports.
Therefore, since the end faces of each optical fiber must be arranged at focal point at each input/output port A, B, C, D, it is required to adjust the distances between the spherical lenses and optical fiber end faces, Ql' Q2' Q3, Q4 respectively.
The input/output ports A, B, C, D and the mixing light input/output port COM (optical fiber 3 and spherical lens 2) are assembled as shown in Figure 3.
~L~787~L4 Fig. 3 is a sectional view of the structure of the assembly of spherical lens and optical fiber at the respective input/output ports A, B, C, D, COM. The outer end part of the optical fiber cord 23, consisting for example, of NYLON,is removed and a ferrule is formed by the optical fiber 24(core and cladding)and a sleeve 22 in which the optical fiber 24 is inserted. An assembled holder 20 consisting of stainless provides a cylindrical hole for fixing such ferrule a,nd a cylindrical hole for fixing a drum lens 21, which is fixed to the assembled holder 20 by glass solder 25, in'different dia-meters. Moreover the'ferruie~i~s fixed to the .
assembled holder 20 by laser welding of the part 26.
The drum lens 21 is formed by grinding a spherical lens.
The assembled'input/out-put ports A, B, C, D, COM are fixed on a substrate 27 via an oblique spacer 28 as shown in Figs. ~(a),(b).
Moreover, the glass block 1 providing bandpass filters is fixed onto the substrate'b~ means of a fixing member 29 and screws. Here, the input/output ports A, B, C, D, COM are fixed through the oblilque spacer 28 in order to make matching of the optical axis'easier.
However, as mentioned above,although it is possible to fix the optical components easily with good accuracy onto ~278'^~
the substrate 27 such that the spherical lens and optical fiber, which form respective input/output ports, are assembled to be a unit, it is very troublesome to adjust the distances between the optical fiber end and lens, shown in Fig. 3, corresponding to respective input/output ports A, s, C, D, COM (corresponding to wavelength).
Accordingly, it is apparent that the manufacturing yield can be very much improved by making the distances between the optical fiber end and lens,Ql,Q2,Q3,Q4, equal without depending on wavelength.
A method for making the distances between optical fiber end and lens~ 2,~3,4, equal will now be explained hereunder.
Fig. 2 is a pictorial presentation for explaining O a Ll bet~ween lenses in the input/output port COM and input/output port A in Fig. 1 and the distance ~= ~1 between the optical fiber end and lens at the input/output port A.
When a focal point of lens 4 for light of wavelengthj~O is considered as fO when in the relation of7~0 < ~ , the focal point of lens 4 for light of wavelength 7~1 becomes fl. Namely, since the refraction index decreases when wavelength becomes longer, the 87~4 focal point of lens 4 shifts in such a dirçction further away from lens 4.
An example in which sK7 is used as the lens 4 is further explained in detail.
The refraction index of sK7 is 1.5145 for a He-Ne laser beam of wavelength ~0 = 0.63 ~m and when the radius r of the spherical lens is 2.5 mm, the focal dis-tance fO is 1.18 mm.
Fig. 5 shows the relation between the lens-to-lens distance L and increment of focal distance for each wavelength ~ = 1.3 ,um, 1.2 ym, 0.89 ~m and 0.81 ~m with reference to fO = 1.18 ym. ~
As is apparent from Fig. 5, when the lens-to-lens distance L increases, increment ~ of focal distance also increases.
Moreover, it can also be unders~ood that the rate of increase is larger as the wavelength becomes longer. In the case where the light emitted from the optical fiber end is converted to perfectly parallel light by the sphe-rical lens 2, the increment ~ only depends on the wavelength without depending on the lens-to-lens distance L. However, the spherical lens 2 cannot perfectly convert the light to per~ectly parallel light and converts it actually to light which is a _ g _ lZ787~
little diverging. Accordingly, when the lens-to-lens distance L increases,the increment also increases.
Further investigation will be made with respect to Fig.5. A
longer wavelength generally results in a larger incre-ment~e. On the other hand, a shorter wavelength results in a shorter increment~. Therefore, when the lens-to-lens distance L of shorter wavelength is set larger and the lens-to-lens distance L of longer wave-length is set smaller, the difference between the distances ~ 2~Q3~4 can be smaller. Considering these points,the difference of distance between lens and optical fiber end resulting from difference of wavelengths can be alleviated to such a degree as an aberration by setting the lens-to-lens distance in reverse proportion to the length of wavelength. More specifically, the lens-to-lens distance between khe lens 2 of the mi~ing light input/output port COM and the lenses ~, 6, 8, 10 of the in~ut/output ports A,B,C,D of respective wave-lengths can be set in such a manner that it is reversely proportional to the wavelength by setting the wavelengths of the input/output ports A,B,C,D in such a manner as ~1> J~2> ~3 >~4 g In the following explanation, practical values are given. The wavelengths are set as follow; ~1 = 1.3 ~m,7~2 = 1.2 ~m, ~3 = 0.89 ,um,~4 = 0.81 ym and the wavelength characteristics of filters for each waveleng~h are set to that of bandpass filters shown in Fig. 6, a bandpass filter for the wavelength ~4 being unnecessary. In the case where the incoming angle l of the light at the input/output port COM is set to 23,the outgoing angle 02 is set to 15 ,the refractive index n for the reference light of glass block 1 (He-Ne laser beam wavelength = 0.63 lum) is 1.5154,the distance between the lens and glass block at each input/output port COM, A, 0 Ll L2 = L3 = L4 = 5 mm, the distance between the lens 2 of the input/output port COM and the lens 4 of input/output port A is e~pressed as follow;
Lo + Ll + La x l/(COS15) x ~l/n) - 21.7 mm In the same way, the distance between the lens 2 of the input/output port COM and the lens 6 of input/
output port B is expressed as follows;
Lo + L2 + La x 2~COS15 ) x ~l/n) -. 33.5 mm Concerning the input/output port C, Lo + L3 + La x 3 x 1/(cosl5) x (l/n) - 45.2 mm Concerning the input/output port D, Lo + L4 + La x 4 x l/(coslso) x (l/n) - 56.9 mm Where l/coslso is used for converting La to the ~278714 distance in air.
In this case,the deviation ~Q of focal distance at each input/output port can be obtained from Fig. 5 as follow;
Input/output port A; ~ = 38 jum Input/output port B; ~e = 46 ,um Input/output port C; d~ = 32 jum Input/output port D; a~ = 34 ~m Therefore, since the focal distance fO of a spheri-cal lens for the reference light is 1.18 mm, the optimum distances ~ 2'Q3'~4 between the optical fiber end and lens at each input/output port A, B, C, D are indicated as follow;
At the input/output port A, ~1 = 1.18 + 0.038 (mm) At the input/output port B, ~2 ~ 1.18 + 0.046 (mm) At the input/output port C, ~3 = 1.18 + 0.032 (mm) At the input/output port D, ~A ~ 1.18 + 0.034 (mm) A mean value of the maximum and minimum values for all input/output ports can be obtained as follow;
(0.046 + 0.032)/2 = n . 040 Therefore,in the case where a common deviation AQ at each input/output-port is set to 0.040, the following can be obtained.
An optical branching filter is a basic device for a wavelength division multiplexing optical communication system.
For example, it is possible using mixed light of n different ~1' ~2' ' ~n to transmit n times the amount of information as with a single wavelength ~0 over a single line of optical fiber.
Furthermore, n-channel bothway simultaneous optical communication can be realized by allocating light of different wavelengths for transmission and reception, namely wavelengths ~ 3~ ' ~2n-1 are allocated for transmission and wavelengths ~2' ~4' ' ~2n are allocated for reception. In order to realize such optical communication systems, an optical mixing filter which mixes the light of different wavelengths and inputs the light to a single optical fiber line is arranged at the sending side and an optical branching filter which branches or separates out the different wavelengths from the mixed wavelength light is arranged at the receiving side.
The optical mixing filter and optical branching filter currently put into practical use generally mix and branch the light by employing a dielectric material filter composed of a thin di-electric multilayer film laminated in a plurality of layers such as SiO2 and TiO2 as a bandpass filter which allows transmission of light having particular wavelength but reflects light having other wavelengths.
The optical mixing filter can also be used as an optical ~'~787~4 branching filter by inverting the incoming and outgoing directions of light to/from the dielectric material filter. These mixing and branching filters have the same structure and usually both types are referred to as optical branching filters. To realize this optical branching filter, various optical components in addi-tion to the dielectric material filter are required. For example, there are required optical lenses which cause the light sent from the optical fiber to be directed effectively into each dielectric material filter and also the light passed through the dielectric material filter to be directed effectively into the optical fiber.
Also needed are fixing mechanisms which fix the end face of op-tical fiber to the focus point of the lenses.
However, the optical branching filter deals with light having various wavelengths as the refractive index of a lens generally changes in accordance with the wavelength, it follows that the focus point of lens depends on the wavelength of the in-cident light.
Therefore, since the fixing mechanisms require adjust-ment at focus points for each wavelength, the manufacturing yield of the optical blanching filter deteriorates. This deterioration becomes distinctive with increase in number of wavelengths to be mixed or branched. In particular, various kinds of semiconductor laser or light emitting diodes for long to short wavelengths have been developed and the number of wavelengths dealt with an optical branching filter tends to increase and therefore the problem of adjustment at focus points is considered very serious.
1'C ~787~4 It is accordingly an object of the present invention to provide an optical branching filter which is capable of improving the manufacturing yield by realizing common adjustment of the distance between the lens and end face of the optical fiber ir-respective of wavelength.
According to the present invention, there is provided an optical branching filter for branching or mixing light of n different wavelengths, comprising; a block which is optically transparent and has two parallel surfaces, a first input/output part for light consisting of the n wavelengths mixed together provided at one of said two surfaces, second input/output parts, provided at any one of said two surfaces for forming optical filters each allowing only light having a respective predetermined one of the n wavelengths to pass therethrough and reflecting light having other wavelengths, respectively, a first input/output port and n second input/output ports, respectively formed by an optical waveguide and a lens which optically connects said optical wave-guide and said first input/output part of a second input/output port, wherein the positions of each second input/output port are selected so that the lengths of the optical paths formed between the lenses of said first input/output port and each said second input/output port are in the reverse proportion to the lengths of wavelengths corresponding to said second input/output ports.
The invention will now be described in greater detail with reference to the accompanying drawings, in which:
Figures l(a) and (b) are schematic plan views of an 7E~7~4 optical branching filter of the present invention;
Figure 2 is a pictorial presentation for explaining the distance between lenses and the distance between a lens and an optical fiber end face;
Figure 3 is a sectional view of the assembled fixing mechanism of an optical fiber end face and lens;
Figure 4(a) is a plan view of a practical optical branch-ing filter;
Figure 4(b) is a perspective view of a practical optical branching filter;
Figure 5 is a graphical presentation for explaining the relation of the distance between lenses and the distance between a lens and an optical fiber end face;
Figure 6 is a graphical presentation for explaining wavelength characteristics of dielectric material filters; and Figure 7 is a graphical presentation for explaining the relation of distance between lens and optical fiber end face and the loss increase.
Figures l(a),(b) are schematic plan views illustrating a structure of optical branching filter of the present invention.
In Figures l(a),(b), four different wavelengths ~ 2~ ~3~ ~4 are mixed and branched. In a glass block 1 consisting of an opti-cally transparent substance such as BK7 having a pair of parallel surfaces, the parallel surfaces thereof are provided with a band-pass filter 13 which allows only the light of wavelength ~1 to pass, a bandpass filter 15 which allows only the light of wave ': .
787~4 length ~2 to pass and a bandpass filter 17 which allows only the light of wavelength ~3 to pass. Mixed light of four different wavelengths ~ 2~ ~3~ ~4 is emitted from the end face of optical flber 3, converted to parallel light by a spherical lens 2, and is input to the glass block 1 through an anti-reflection film 12 provided for eliminating reflection. Among this mixed light, only the light of wavelength ~1 is able to pass the band-pass-filter 13 provided to the opposite surface of glass block 1, is output to the outside of glass block 1 through the anti-reflection film 14, is focused by spherical lens 4 and is then input to the optical fiber 5 effectively. The other wavelengths A2, ~3, ~4 are re-flected within block 1. Moreover, only the light of wavelength ~2 is able to pass the bandpass filter 15, is output to the out-side of glass block 1 through the anti-reflection film 16, is focused by spherical lens 6 and is then input to the optical fiber 7. The wavelengths ~3, ~4 are reflected. In the same way, the light of wavelength ~3 is able to pass the bandpass filter 17 and is then input to the optical fiber 9 by the spherical lens 8 through the anti-reflection film 18. Here, the light reflected by the bandpass filter 17 is only the light of wavelength ~4 and therefore the light of the wavelength ~4 is input to the optical fiber 11 by the spherical lens 10 via only the anti-reflection film 19.
The bandpass filters and anti-reflection films are form-ed by the dielectric material multilayer film of SiO2, TiO2 obtained by vacuum deposition.
~7~q~4 In the example explained above, each wavelength is branched from the light consisting of four mixed wavelengths. On the other hand, the light consisting of four mixed wavelengths can be input to the optical fiber 3 by reversing the travelling direction of the light of wavelengths ~ 2~ ~3~ ~4 emitted from the optical fibers 5, 7, 9, 11.
Here, it will become a problem that the spherical lenses deal with the lights in the different wavelengths. Namely, the refractive index of a lens changes in accordance with the wave-length and therefore the focal points of spherical lenses 4, 6, 8, 10 are also different in the input/output ports A, B, C, D.
In order to have the lights of wavelengths ~ 2~ ~3~4 input most effectively to the optical fiber 5, 6, 9, 11, the end faces of optical fiber 5, 6, 9, 11 must be provided at the focal points of the spherical lenses 4, 6, 8, 10 at respective input/output ports.
Therefore, since the end faces of each optical fiber must be arranged at focal point at each input/output port A, B, C, D, it is required to adjust the distances between the spherical lenses and optical fiber end faces, Ql' Q2' Q3, Q4 respectively.
The input/output ports A, B, C, D and the mixing light input/output port COM (optical fiber 3 and spherical lens 2) are assembled as shown in Figure 3.
~L~787~L4 Fig. 3 is a sectional view of the structure of the assembly of spherical lens and optical fiber at the respective input/output ports A, B, C, D, COM. The outer end part of the optical fiber cord 23, consisting for example, of NYLON,is removed and a ferrule is formed by the optical fiber 24(core and cladding)and a sleeve 22 in which the optical fiber 24 is inserted. An assembled holder 20 consisting of stainless provides a cylindrical hole for fixing such ferrule a,nd a cylindrical hole for fixing a drum lens 21, which is fixed to the assembled holder 20 by glass solder 25, in'different dia-meters. Moreover the'ferruie~i~s fixed to the .
assembled holder 20 by laser welding of the part 26.
The drum lens 21 is formed by grinding a spherical lens.
The assembled'input/out-put ports A, B, C, D, COM are fixed on a substrate 27 via an oblique spacer 28 as shown in Figs. ~(a),(b).
Moreover, the glass block 1 providing bandpass filters is fixed onto the substrate'b~ means of a fixing member 29 and screws. Here, the input/output ports A, B, C, D, COM are fixed through the oblilque spacer 28 in order to make matching of the optical axis'easier.
However, as mentioned above,although it is possible to fix the optical components easily with good accuracy onto ~278'^~
the substrate 27 such that the spherical lens and optical fiber, which form respective input/output ports, are assembled to be a unit, it is very troublesome to adjust the distances between the optical fiber end and lens, shown in Fig. 3, corresponding to respective input/output ports A, s, C, D, COM (corresponding to wavelength).
Accordingly, it is apparent that the manufacturing yield can be very much improved by making the distances between the optical fiber end and lens,Ql,Q2,Q3,Q4, equal without depending on wavelength.
A method for making the distances between optical fiber end and lens~ 2,~3,4, equal will now be explained hereunder.
Fig. 2 is a pictorial presentation for explaining O a Ll bet~ween lenses in the input/output port COM and input/output port A in Fig. 1 and the distance ~= ~1 between the optical fiber end and lens at the input/output port A.
When a focal point of lens 4 for light of wavelengthj~O is considered as fO when in the relation of7~0 < ~ , the focal point of lens 4 for light of wavelength 7~1 becomes fl. Namely, since the refraction index decreases when wavelength becomes longer, the 87~4 focal point of lens 4 shifts in such a dirçction further away from lens 4.
An example in which sK7 is used as the lens 4 is further explained in detail.
The refraction index of sK7 is 1.5145 for a He-Ne laser beam of wavelength ~0 = 0.63 ~m and when the radius r of the spherical lens is 2.5 mm, the focal dis-tance fO is 1.18 mm.
Fig. 5 shows the relation between the lens-to-lens distance L and increment of focal distance for each wavelength ~ = 1.3 ,um, 1.2 ym, 0.89 ~m and 0.81 ~m with reference to fO = 1.18 ym. ~
As is apparent from Fig. 5, when the lens-to-lens distance L increases, increment ~ of focal distance also increases.
Moreover, it can also be unders~ood that the rate of increase is larger as the wavelength becomes longer. In the case where the light emitted from the optical fiber end is converted to perfectly parallel light by the sphe-rical lens 2, the increment ~ only depends on the wavelength without depending on the lens-to-lens distance L. However, the spherical lens 2 cannot perfectly convert the light to per~ectly parallel light and converts it actually to light which is a _ g _ lZ787~
little diverging. Accordingly, when the lens-to-lens distance L increases,the increment also increases.
Further investigation will be made with respect to Fig.5. A
longer wavelength generally results in a larger incre-ment~e. On the other hand, a shorter wavelength results in a shorter increment~. Therefore, when the lens-to-lens distance L of shorter wavelength is set larger and the lens-to-lens distance L of longer wave-length is set smaller, the difference between the distances ~ 2~Q3~4 can be smaller. Considering these points,the difference of distance between lens and optical fiber end resulting from difference of wavelengths can be alleviated to such a degree as an aberration by setting the lens-to-lens distance in reverse proportion to the length of wavelength. More specifically, the lens-to-lens distance between khe lens 2 of the mi~ing light input/output port COM and the lenses ~, 6, 8, 10 of the in~ut/output ports A,B,C,D of respective wave-lengths can be set in such a manner that it is reversely proportional to the wavelength by setting the wavelengths of the input/output ports A,B,C,D in such a manner as ~1> J~2> ~3 >~4 g In the following explanation, practical values are given. The wavelengths are set as follow; ~1 = 1.3 ~m,7~2 = 1.2 ~m, ~3 = 0.89 ,um,~4 = 0.81 ym and the wavelength characteristics of filters for each waveleng~h are set to that of bandpass filters shown in Fig. 6, a bandpass filter for the wavelength ~4 being unnecessary. In the case where the incoming angle l of the light at the input/output port COM is set to 23,the outgoing angle 02 is set to 15 ,the refractive index n for the reference light of glass block 1 (He-Ne laser beam wavelength = 0.63 lum) is 1.5154,the distance between the lens and glass block at each input/output port COM, A, 0 Ll L2 = L3 = L4 = 5 mm, the distance between the lens 2 of the input/output port COM and the lens 4 of input/output port A is e~pressed as follow;
Lo + Ll + La x l/(COS15) x ~l/n) - 21.7 mm In the same way, the distance between the lens 2 of the input/output port COM and the lens 6 of input/
output port B is expressed as follows;
Lo + L2 + La x 2~COS15 ) x ~l/n) -. 33.5 mm Concerning the input/output port C, Lo + L3 + La x 3 x 1/(cosl5) x (l/n) - 45.2 mm Concerning the input/output port D, Lo + L4 + La x 4 x l/(coslso) x (l/n) - 56.9 mm Where l/coslso is used for converting La to the ~278714 distance in air.
In this case,the deviation ~Q of focal distance at each input/output port can be obtained from Fig. 5 as follow;
Input/output port A; ~ = 38 jum Input/output port B; ~e = 46 ,um Input/output port C; d~ = 32 jum Input/output port D; a~ = 34 ~m Therefore, since the focal distance fO of a spheri-cal lens for the reference light is 1.18 mm, the optimum distances ~ 2'Q3'~4 between the optical fiber end and lens at each input/output port A, B, C, D are indicated as follow;
At the input/output port A, ~1 = 1.18 + 0.038 (mm) At the input/output port B, ~2 ~ 1.18 + 0.046 (mm) At the input/output port C, ~3 = 1.18 + 0.032 (mm) At the input/output port D, ~A ~ 1.18 + 0.034 (mm) A mean value of the maximum and minimum values for all input/output ports can be obtained as follow;
(0.046 + 0.032)/2 = n . 040 Therefore,in the case where a common deviation AQ at each input/output-port is set to 0.040, the following can be obtained.
2 = Q3 ~ ~4 = 1-18 + 0.04 = 1.22 mm.
~'~78~L4 Namely, a common value = 1.22 mm can be obtained as the distance between the optical fiber end and lens.
Fig. 7 is a graph which indicates the loss increase according to the deviation ~ from the optimum position of lens at an input/output port, corresponding to Fig. 2.
As is obvious $rom Fig. 7, the loss is 0.1 dB or less when ~z < 13 pm.
Therefore, in case ~z < 13 ~m,the influence of the loss is so small as to be negligible.
As explained above, when ~is set to 40 pm, the maximum ~ for respective input/output ports A, B, C, D
is e~ual to 40 - 32 = 8 ~m < 13 lum.
Accordingly, the distance between the optical fiber end and lens at each input/output port can be set in common.
If ~ cannot be determined as small as an aberrat-ion, the lens-to-lenS distance ~l'Q2'~3'~4 between the input/output ports can be changed by adjusting Lo~ Ll, L2, L3, L4 of input/output ports A, B, C, D, COM, in view of the relation betrween L and~.
~'~78~L4 Namely, a common value = 1.22 mm can be obtained as the distance between the optical fiber end and lens.
Fig. 7 is a graph which indicates the loss increase according to the deviation ~ from the optimum position of lens at an input/output port, corresponding to Fig. 2.
As is obvious $rom Fig. 7, the loss is 0.1 dB or less when ~z < 13 pm.
Therefore, in case ~z < 13 ~m,the influence of the loss is so small as to be negligible.
As explained above, when ~is set to 40 pm, the maximum ~ for respective input/output ports A, B, C, D
is e~ual to 40 - 32 = 8 ~m < 13 lum.
Accordingly, the distance between the optical fiber end and lens at each input/output port can be set in common.
If ~ cannot be determined as small as an aberrat-ion, the lens-to-lenS distance ~l'Q2'~3'~4 between the input/output ports can be changed by adjusting Lo~ Ll, L2, L3, L4 of input/output ports A, B, C, D, COM, in view of the relation betrween L and~.
Claims (4)
1. An optical branching filter for branching or mixing light of n different wavelengths, comprising:
a block which is optically transparent and has two parallel surfaces, a first input/output part for light consisting of the n wavelengths mixed together provided at one of said two surfaces, second input/output parts, provided at any one of said two surfaces for forming optical filters each allowing only light having a respective predetermined one of the n wavelengths to pass therethrough and reflecting light having other wavelengths, respectively, a first input/output port and n second input/output ports, respectively formed by an optical waveguide and a lens which optically connects said optical waveguide and said first input/output part or a second input/
output port, wherein the positions of each second input/output port are selected so that the lengths of the optical paths formed between the lenses of said first input/output port and each said second input/output port are in the reverse proportion to the lengths of wavelengths corresponding to said second input/output ports.
a block which is optically transparent and has two parallel surfaces, a first input/output part for light consisting of the n wavelengths mixed together provided at one of said two surfaces, second input/output parts, provided at any one of said two surfaces for forming optical filters each allowing only light having a respective predetermined one of the n wavelengths to pass therethrough and reflecting light having other wavelengths, respectively, a first input/output port and n second input/output ports, respectively formed by an optical waveguide and a lens which optically connects said optical waveguide and said first input/output part or a second input/
output port, wherein the positions of each second input/output port are selected so that the lengths of the optical paths formed between the lenses of said first input/output port and each said second input/output port are in the reverse proportion to the lengths of wavelengths corresponding to said second input/output ports.
2. An optical branching filter according to claim 1, wherein the distances between said optical waveguides and said lenses are unified to a mean value of the maximum and minimum values among the optimum values of respective second input/
output ports.
output ports.
3. An optical branching filter according to claim 1, wherein said first and second input/output ports each comprises a cylindrical holder which holds unitarily said optical waveguide and said lens therein.
4. An optical branching filter according to claim 2, wherein said optical paths can be changed by adjusting the corresponding distance between each input/output port and said block, whereby said mean value can be set to the desired value.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP2515186 | 1986-02-06 | ||
| JP61-025151 | 1986-02-06 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1278714C true CA1278714C (en) | 1991-01-08 |
Family
ID=12158003
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000529045A Expired - Fee Related CA1278714C (en) | 1986-02-06 | 1987-02-05 | Optical branching filter |
Country Status (5)
| Country | Link |
|---|---|
| US (1) | US4824200A (en) |
| EP (1) | EP0234369B1 (en) |
| JP (1) | JPS6323105A (en) |
| CA (1) | CA1278714C (en) |
| DE (1) | DE3763685D1 (en) |
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| US4274706A (en) * | 1979-08-30 | 1981-06-23 | Hughes Aircraft Company | Wavelength multiplexer/demultiplexer for optical circuits |
| JPS5685701A (en) * | 1979-12-14 | 1981-07-13 | Fujitsu Ltd | Optical branching filter |
| JPS5781227A (en) * | 1980-11-10 | 1982-05-21 | Fujitsu Ltd | Optical branching filter |
| JPS57204507A (en) * | 1981-06-12 | 1982-12-15 | Nec Corp | Interference film type optical multiplexer and demultiplexer |
| JPS58149018A (en) * | 1982-03-01 | 1983-09-05 | Nippon Telegr & Teleph Corp <Ntt> | Optical multiplexer and demultiplexer |
| DE3346365A1 (en) * | 1983-12-22 | 1985-08-29 | Standard Elektrik Lorenz Ag, 7000 Stuttgart | OPTICAL MULTIPLEXER / DEMULTIPLEXER |
| DE3413703A1 (en) * | 1984-04-12 | 1985-10-24 | Standard Elektrik Lorenz Ag, 7000 Stuttgart | OPTICAL MULTIPLEXER / DEMULTIPLEXER |
| US4705349A (en) * | 1985-01-18 | 1987-11-10 | The United States Of America As Represented By The United States Department Of Energy | Optical switch |
-
1987
- 1987-02-05 CA CA000529045A patent/CA1278714C/en not_active Expired - Fee Related
- 1987-02-05 JP JP62025186A patent/JPS6323105A/en active Pending
- 1987-02-06 US US07/011,602 patent/US4824200A/en not_active Expired - Lifetime
- 1987-02-06 EP EP87101668A patent/EP0234369B1/en not_active Expired - Lifetime
- 1987-02-06 DE DE8787101668T patent/DE3763685D1/en not_active Expired - Fee Related
Also Published As
| Publication number | Publication date |
|---|---|
| DE3763685D1 (en) | 1990-08-23 |
| EP0234369B1 (en) | 1990-07-18 |
| EP0234369A1 (en) | 1987-09-02 |
| US4824200A (en) | 1989-04-25 |
| JPS6323105A (en) | 1988-01-30 |
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