WAVELENGTH SELECTIVE OPTICAL DEVICE
Technical field
The present invention relates to an optical device for manipulating an optical signal propagating in a waveguide, such as an optical fibre. More particularly, the present invention relates to a highly versatile device capable of manipulating individual channels within a wavelength division multiplexed optical signal.
Technical background In optical communications based on wavelength division multiplexing (WDM) , information is sent through an optical waveguide, such as an optical fibre, on a multiplicity of wavelength channels simultaneously. Each wavelength channel propagate through the fibre independ- ently of all other wavelength channels. By using WDM, very large data rates can be achieved.
However, as the number of wavelength channels increases, it becomes increasingly important to manipulate each individual channel separately. Therefore, there is a need for devices and methods for manipulating individual channels within a wavelength division multiplexed optical signal.
Furthermore, the optical signals may propagate in either direction through the fibre, for which reason the operation of such devices and methods preferably should be invariant to the propagation direction of the signal.
Summary of the invention
It is an object of the present invention to provide an optical device for manipulating an optical signal propagating in a waveguide, such as an optical fibre, which optical device exhibits operational inversion symmetry with respect to the .propagation direction of said
optical signal in said waveguide. In other words, it is an object of the present invention to provide a device for manipulating an optical signal, wherein the operation of the device is the same for both possible propagation directions of the optical signal through the waveguide. The above-mentioned object is met by an optical device of the kind set forth in the appended claims .
An optical device according to the present invention for manipulating an optical signal propagating in a wave- guide comprises a first and a second tilted reflector, which are provided in said waveguide. The first and the second reflectors are superimposed upon each other within the waveguide, and arranged to deflect light out from the waveguide into two beams having symmetrical propagation directions with respect to the waveguide (i.e. with respect to the propagation of light within the waveguide) . In other words, the reflectors deflect light out from the waveguide in two different directions, which have the same angle with respect to the waveguide, i.e. light is deflected out from the waveguide symmetrically. One advantage of an optical device according to the present invention is that spectrally selective manipulation of the optical signal is allowed, while at the same time operational inversion symmetry of the device is maintained.
Furthermore, an optical device according to the present invention can be used for manipulating individual wavelength channels within a wavelength division multiplexed optical signal . Another advantage of an optical device according to the present invention is that it provides an optical device for manipulating an optical signal, which device is inherently cascadeable. In other words, any number of optical devices according to the present invention can be arranged in series in order to provide a cascaded structure for manipulating any number of wavelength channels simultaneously.
In the context of the present application, a tilted reflector is a reflector which has an inclination with respect to the propagation direction of light in the waveguide. Consequently, light incident upon the tilted reflector from the waveguide will have a non-normal angle of incidence, and will therefore be deflected away from its original propagation direction. Preferably, the inclination of the tilted reflector is such that light is deflected out from the waveguide. In a preferred embodi- ment, the angle of inclination of the tilted reflectors with respect to the propagation direction of light within the waveguide is close to 45 degrees, so that light is deflected out from the waveguide in a substantially normal direction (i.e. substantially perpendicularly out from the waveguide) . Preferably, the tilted reflectors are comprised of blazed Bragg gratings. It is to be noted that the angle of deflection from a blazed Bragg grating is determined by the angle of the blaze and of the period of the grating. Perpendicular deflection can be obtained even if the tilt angle of the grating is different from 45 degrees with respect to the waveguide if the appropriate period is chosen.
In connection with the present invention, it is preferred that enhanced wavelength selectivity is provided by means of an external Fabry-Perot type resonator, the reflectors in the waveguide being operative to deflect at least some light into a resonant mode in said external resonator.
The Fabry-Perot type resonator is typically defined by at least two resonator mirrors. It is to be understood that said resonator mirrors may be comprised of any suitable structure for providing optical reflection, such as a metal layer, a dielectric stack or a distributed Bragg structure . In one aspect, the present invention provides an optical device for selectively transmitting, in a forward-propagating direction, or reflecting back, in a
backwards-propagating direction, individual wavelength channels within a wavelength division multiplexed optical signal propagating in an optical fibre.
In another aspect, the present invention provides an optical device for manipulating individual channels within a wavelength division multiplexed optical signal, which device is inherently cascadeable. In other words, any number of devices can be arranged in series (in cascade) and thereby provide means for manipulating any num- ber of channels simultaneously. In this aspect, the present invention provides a channel manipulation element for manipulation of individual channels within a wavelength division multiplexed optical signal.
In another aspect, the present invention can be used as an interferometer, since the present invention allows selective control of spectral resonance and phase relation.
In another aspect, the present invention can be used as a variable and spectrally selective optical atten- uator. For example, any degree of transmission along the fibre can be achieved by appropriately altering the characteristics of the external Fabry-Perot type resonator.
In another aspect, the present invention can be used as a digital modulator for modulating individual channels within a WDM signal. Such modulation is rendered possible by the present invention at high speeds and with low dispersion. Further advantages are obtained by virtue of the device being wavelength tuneable. The device may also be used as a modulator for lasers.
In another aspect, the present invention can serve as an add/drop filter. Individual WDM channels may conveniently be added or dropped by a respective channel manipulation element . Any channel can be made either to be reflected back from the element or to be transmitted through the element along the fibre.
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guide can be controlled. Consequently, coupling strength or attenuation of the optical device is controlled.
Yet another way of tuning the optical device according to the present invention includes tilting at least one of the resonator mirrors. There are two different ways of tilting the resonator mirror. The mirror can either be tilted parallel to the waveguide, or perpendicularly to the waveguide. When the mirror is tilted parallel to the waveguide, the resonant wavelength is shifted to a slightly different wavelength, and the resonance is broadened. When, on the other hand, the mirror is tilted perpendicular to the waveguide, the main effect is to reduce the coupling between the waveguide and the external resonator. By sufficiently tilting one of the resonator mirrors perpendicularly to the waveguide, the resonance of the resonator will be removed. Hence, the resonator will no longer be resonant and, consequently, manipulation of the corresponding channel within the optical signal in the waveguide is interrupted. Another possibility of tuning the device is to tilt both of the resonator mirrors, while keeping them essentially parallel to each other, i.e. while keeping the resonance in the external resonator. Light of a different wavelength will in this case be coupled into the resonant mode of the external resonator. It is to be noted that a slight adjustment of the separation between the mirrors might be necessary in this case.
Broad-band spectral selectivity of the device can be achieved by designing the reflectors inside the optical fibre to have a wavelength dependent reflectivity and/or by designing the mirrors of the Fabry-Perot type resonator to have wavelength dependent reflectivity. High- precision, narrow-band spectral selectivity is achieved by the etalon-effect of the Fabry-Perot type resonator. The combined effect of said broad-band and said narrowband selectivity provides a highly versatile and spectrally selective device for manipulating individual chan-
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Brief description of the drawings Further features and advantages of the present invention will be apparent from the following detailed description of some preferred embodiments thereof. In the detailed description, reference is made to the accompanying drawings, on which: Figure 1 is a schematic illustration of an optical device according to the present invention having deflecting reflectors and an external Fabry-Perot type resonator,
Figure 2 is a schematic illustration of another optical device according to the present invention,
Figure 3 is a schematic illustration of the core of an optical fibre provided with reflectors in the form of superimposed, blazed Bragg gratings,
Figure 4 is a schematic illustration of two paired optical devices in transmitting mode, according to the present invention,
Figure 5 is a schematic illustration of two paired optical devices in reflecting mode, according to the present invention, Figure 6 is a schematic illustration of a plurality of optical devices arranged in cascade for individually manipulating a plurality of wavelength channels simultaneously,
Figure 7 is a schematic illustration of a wavelength selective, dynamic attenuator according to the present invention,
Figure 8 is a schematic illustration of one method of tuning by tilting a resonator mirror,
Figure 9 is a schematic illustration of another method of tuning by tilting both resonator mirrors,
Figure 10 shows the filter characteristics of one type of tuning, and
Figure 11 shows the filter characteristics of another type of tuning.
On the drawings, like parts are designated like reference numerals.
Detailed description of preferred embodiments
A preferred embodiment of an optical device 1 according to the present invention is schematically shown in figure 1. In the figure, a cross section of a piece of optical fibre 10 is shown, the light guiding core 11 of which is schematically indicated by a broken line centrally in the piece of fibre 10. In the core 11 of the fibre 10, there is provided two superimposed reflectors 12 and 13. The two reflectors are oriented at right an- gles with respect to each other, in order to deflect light impinging upon the two superimposed reflectors into two anti-parallel beams. The device shown in the figure further comprises two external mirrors 14 and 15, forming an external Fabry-Perot type resonator. The resonator is positioned so that the deflecting reflectors 12, 13 are enclosed within the resonator. Furthermore, the deflecting reflectors and the external Fabry-Perot type resonator have such mutual orientations that at least some of the light deflected out from the core of the fibre enters a resonant mode in the resonator. By consequence, at least some of the light in any resonant mode in the resonator will also be deflected back into the core 11 of the fibre 10 by the crossed reflectors 12, 13. The device shown in figure 1 is sometimes referred to, in this specification, as a channel manipulation element.
The principle of action of the optical device 1 shown in figure 1 will now be described. In the figure, propagation direction of light is indicated by arrows. Note that the arrows have been displaced for clarity. Assume that an optical signal is incident from the left in the figure (indicated by an arrow 5) . Said signal will impinge upon the two superimposed reflectors 12, 13,
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mode there must be a constant constructive interference within the resonator, i.e. the resonator must support a standing wave at the resonant wavelength. Consequently, as long as odO is held constant, the Fabry-Perot type resonator will remain resonant to the same wavelength.
Recall now that, according to the present invention, light is coupled out from the external resonator both when, co ing from the upper portion of the resonator and when coming from the lower portion of the resonator. Therefore, in each propagation direction in the waveguide (left and right) , there will be a superposition of light from the upper and the lower portion of the resonator. Whether constructive or destructive interference is obtained is determined by the phase relation between light from the upper portion and light from the lower portion. The phase relation, in turn, is determined by the optical path lengths odl and od2 of each respective portion of the resonator. Consequently, constructive interference is any desired direction to any desired degree can be achieved by appropriately controlling the optical path lengths odl and od2 , allowing any ratio between transmission and reflection at the optical manipulation element .
It is to be noted that, if the degree of trans- mission (reflection) of the element is to be adjusted without changing the wavelength to which the resonator is resonant, odl and od2 must be changed while keeping odO constant (i.e. a lateral translation of the external resonator) . Lateral translation means a translation perpendicularly to the longitudinal direction of the waveguide .
In figure 2 , another embodiment of an optical device according to the present invention is shown. In this case, light is not deflected out from the waveguide perpendicularly, but at another angle. Note, however, that the deflected beams are still symmetrical with respect to the longitudinal direction of the waveguide.
In the shown embodiment, light cannot be coupled back in the waveguide in the counter propagating direction. However, the embodiment shown in figure 2 is preferred when a channel within a WDM signal is to be manipulated for attenuation or modulation.
The deflecting reflectors shown in figure 2 are comprised of crossed Bragg gratings which have equal periods and equal but opposite blaze angles. In this way, light is deflected symmetrically out from the waveguide. As mentioned above, the deflection angle from a blazed grating is determined by both the blaze angle and the period. Hence, a predetermined deflection angle can be achieved by appropriate selections of period and blaze angle . A preferred embodiment of the tilted reflectors is schematically shown in figure 3. Here, the reflectors are comprised of blazed Bragg gratings 21, 22. Each blazed Bragg grating is comprised of a periodic structure of refractive index variations, which are inclined with re- spect to the propagation direction of light within the waveguide (i.e. the grating is blazed). Consequently, the blazed gratings are, in fact, tilted reflectors. The characteristics of blazed Bragg gratings are commonly known in the art. Each of the blazed gratings 21 and 22 shown in figure 3 are arranged to deflect light of a predetermined wavelength out from the waveguide perpendicularly. Hence, it can be said that the blazed gratings are arranged at right angles with respect to each other. In effect, light from the waveguide, which light is incident upon the two blazed gratings, will be perpendicularly deflected out from the waveguide in two opposite directions, i.e. in two beams that are symmetrical with respect to the blazed gratings. In this case, the deflected beams are essentially anti-parallel to each other and perpendicular to the waveguide.
In an optical communications system based on transmission of signals through optical fibres, there is an
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> β a TJ β β 0 rtf ft ε CQ rtf rH Φ Φ 0 4J rd rtf - Φ Dl X β ϋ a • TJ 0 Dl -H rtf Φ 1 SH ftf -H β £ rH Φ φ a CQ TJ X 4-1 rd CQ Dl -H ϋ rH φ o Φ rβ β UH s SH SH 0 rd 0 J φ rH rtf ft a 0 φ Φ ϋ 0 rH rH β rH ftf rH φ φ D) ra rtf
Φ Φ ϋ ϋ β •H • ra Φ φ -H ε SH UH -H 4J rtf rtf -H rQ -H rQ rd Φ β rβ SH φ UH 4-1 U Ss 4J ra SH φ T 4J β β f a Φ o rd β J 4-J β Ss 4-> Φ -U SH rd Φ CJ O ε UH β rtf 0 (t 0 rd CO β a rtf φ 4J 4-J rd Ss D) -H ftf Φ Φ SH a SH rd X rd -H a φ rd D) 0 UH 4-J a rd 4-1 ε φ β X 4J φ Φ -H 4J SH rd rH Φ rQ Φ UH D) SH &H Φ CD
CQ TJ 0 TJ SH ra ftf SH 0 -H rQ rd •H fH O TJ CQ β Φ Ss -H SH rd 0 -H rtf Φ
CJ Φ -H ft SH J 4-1 a 4J 4J rH β ε ε 0 4J SH Φ a fH Φ Φ Ss UH . fH SH
Φ 0 rH rtf . 0 rtf φ (tf β CQ CQ Φ φ 0 0 ϋ 0 Dl 0 • ftf rd Dl 4J Φ φ 0 rQ
Λ ■5 0 TJ ft rH fH U rβ -H SH β β > CQ rtf SH Dl β -H φ 3 4J β «. β β SH UH -H
4J 4J Ss β rH ft (tf 4-1 4-J -H 0 β rtf φ UH Φ d β -H β • ■<H rtf ra Φ -H rd rQ UH
4J rtf D ftf rQ rH β UH CJ (tf rβ SH φ rd -H -H 4J >, φ -H ^ SH SH ε -H SH
4-1 UH rβ β J Φ ft TJ 0 . rβ CQ 4J 4J TJ ftf rH rd a CQ 0 • -H β rH UH Φ 4-1
(d 0 β -H -H rβ TJ CQ 0 CJ rtf 4-J CJ TJ Φ -H rd SH -. Dl rH rS .. 4-1 rβ 4J J TJ Ss X SH rβ -H CQ -U 4J Φ -H 0 β rH rH rβ SH D 0 rtf Φ rtf rtf Lfl CD β 4-J J φ 0 0 CQ rH Φ 0
4-1 β -H 0 ftf -U TJ 4J Φ β Φ Φ Φ a 4-J ft -H 4-J Dl fH ft -H • -H φ Di Φ TJ rβ 4-> rd rH ft
0 • rH ε ^ rtf CQ β -H ε > β 0 ε rH β Φ 0 0 4J ra Φ ε d rH -H g CQ β υ ft CD
Φ -H Φ β & Φ in ϋ β SH o Φ Φ -H 0 Ss . 0 -H UH SH β rH fH Φ φ Φ UH 0 Φ Φ -H -H d
CJ CQ • 0 ε rd -H Φ Φ SH rH 4J Φ J -. Φ ft Φ ftf
CQ -H -H a a rH rH Φ > , ε 4J 4-J rtf β Φ u TJ TJ Φ TJ UH Φ Φ CJ φ > Φ X . SH 1 CQ d Dl CD Φ φ SH ftf SH rH Φ ft rH SH rtf -H 4-> CQ β d rQ d o rβ Φ rH > 0 TJ ra CJ 4J Φ ra Di -H CQ > Φ rH rH 0 a 4J rβ ε - ftf ι rtf -H a Ss CQ a β 0 rQ rH rtf rβ -H r Φ φ -H UH -H TJ rtf Dl ra rd ftf Φ J CQ β a ε rQ CQ φ TJ a CO 0 rQ rtf a ϋ rQ a EH 4-1 CQ Φ Ss β -H 4-J υ UH 4-J β O 4J rtf <ψ Φ > Φ φ o φ rtf 0 Dl β 0 d -U SH -H O ■H β O ra a β
CQ rd -H -H β SH rtf rQ CQ Di a SH UH TJ CJ 4-J TJ -H • a Ss TJ -H ftf -H TJ 4-J β 4-1 0 0 Φ t~~ a rH 4J CQ 0 Φ rtf rβ -H β β H Φ Φ ftf Φ Q Di 0 4-1 -H rtf -H ϋ ■H < rtf -H rd SH SH o 4-J ϋ -H ra fH 0 SH -H -H rβ rQ fH rd H sε d J (tf d Φ a rtf Φ rtf ε -P 4J TJ Φ
Ci -H Φ CQ 4J 4-J a CQ ra 4-1 ft 4-J 4J SH 4-J -H 0 4-J ft rtf O Φ CO Ss Dl CO rH d (tf Di \, UH
O > SH SH -H β β Dl rH Ss ε rtf β ■ SH 4J a rH 0 CQ 0 β Φ UH Di rβ rH β TJ UH
^ rQ 0 -H rβ Φ φ -H rtf 0 CQ o SH O rtf 0 CJ rtf d 0 β Φ S -H UH rd rtf rβ UH Φ -H fH 0 a φ T -H
O 0 UH TJ E-i > ε UH d SH -H J Dl ϋ ft 4-1 CQ d -H υ ftf rQ 4-1 rH 0 CQ fH 4-J 0 TJ ra CQ a rH rtf TJ o
shown multiplexer can be incorporated into an optical fibre-to-fibre router.
For each wavelength channel Ch.l to Ch.5, there is provided one pair each of elements 1.1 to 1.10, as such shown in figures 4 and 5. Paired elements are utilised' in order to avoid coherent mixing of channels. Five such pairs (one for each respective channel Ch.l to Ch.5) are ■ shown in the figure, the multiplexer as shown thereby being designed for WDM communications on -five channels. However, it is to be understood that any number of paired elements can be cascaded in order to provide a multiplexer for any number of wavelength channels.
Although the channel manipulation elements have been described as having a respective set of crossed de- fleeting reflectors, it is also possible to implement the deflecting reflectors as crossed, blazed gratings without any actual division between the reflectors of different elements. Different portions of the chirped gratings will then act as reflectors for different elements. Consider now a first optical signal comprised of five wavelength channels in the first fibre ring 41 and a second optical signal, also comprised of five wavelength channels, in the second fibre ring 42. Each of said rings is connected to the length of fibre comprising the cascaded and paired elements 1.1 to 1.10 by means of a respective optical circulator 43, 44. In this case, it is required that the manipulation elements operate on the zeroth order of deflected light, i.e. that the deflection of light from the waveguide is performed perpendicularly. Consequently, any light reflected back from the cascaded elements 1.1 to 1.10 will continue to propagate in the original fibre ring (41 or 42, depending on its origin) . Now, an exchange of one channel between the two fibre rings is to be executed. Initially, when no exchange of channels is to be executed, each of the pairs in the cascade operates in reflection mode as shown in figure 5, i.e. acts to reflect the corresponding channel back
towards each respective circulator. In order to exchange any of the five channels between the two fibre rings, the appropriate pair of the cascaded elements is switched to transmission mode. As a result, the channel corresponding to the pair operating in transmission mode is passed on to the other fibre ring - a channel exchange has been performed.
Assume, for example, that channel number 2 (Ch.2) is to be exchanged between the two fibre rings. Then, all pairs of elements are operated in full reflection, i.e. acting to couple light back in a backwards-propagating direction with respect to the incident direction, except the pair corresponding to the channel to be exchanged, which pair operates in transmission mode. Thus, in this example of exchanging Ch.2, elements 1.3 and 1.4 are switched to transmission mode. Consequently, in the signal in the first fibre ring 41, channels Ch.l, Ch.3, Ch.4 and Ch.5 are back-reflected by the cascaded structure towards the optical circulator 43, and are thereby directed to the output port for further propagation in the fibre ring 41. In addition, the wavelength channel Ch.2 is received from the second fibre ring 42, which channel is interleaved with the other channels in ring 41. For the signal in the second fibre ring 42, similar interleaving takes place for channel Ch.2 from the first ring 41.
Furthermore, any number of channels can be exchanged simultaneously by switching the appropriate pairs to transmission mode. In fact, the entire optical signal in each of the fibre rings can be exchanged simultaneously by switching all pairs to transmission mode.
A similar configuration can be operated as a wavelength selective, dynamic attenuator, as shown in figure 7. In this case, however, the elements need not be paired, since light is only propagated in one direction through the fibre at any one instant. For the same reason, an optical circulator 51 is only necessary at the input side.
fH Φ
*o 0 ^ Φ SH ra
4J rH SH rQ a ϋ .
(tf Φ Φ Dl -H
© a -H d -H Φ Φ 4-1 fH d -H SH rtf UH ϋ >-. CQ ω Φ 4-1 SH ϋ -H rH rd -H zn A-> β rtf β > rH
H 4-1 Φ ϋ H -H Φ rtf CQ SH Φ U rtf in TJ CJ TJ 0 4-1 α. TJ Φ β -H O 4-J ϋ
Φ Φ > SH Ss rH 4-1 rd rtf r rd 4J ftf 0 0 rtf rtf 4J d SH
4-1 4-1 Ss 4-J r ϋ ε Φ 0 rtf
-H rtf CO -H Φ
' «. ε CO rβ ε ra rH 4-1 X! Φ ϋ φ CO a a 4-1 ft J TJ SH ra β 0 J β 0 ra Φ Φ rd rtf a SH Φ 4-J Φ CJ u SH β -H 3 Φ φ rtf X! β
4J -H ϋ φ rd fH 4J ftf
4J 4-1 Di 4-1 rtf 4J β rtf SH β 4J β CO UH o rd Φ 0 a rtf Di β a 0 CQ
4J rβ CJ ft SH β 0 rH Φ
4J d SH •H -H rH φ SH d -H rtf -H rtf β 4-1 β
H Φ a β 0 Φ
Φ Φ 4J φ Φ rd
. CQ Φ rd rβ > rd 4-J 4J rH -H ϋ 4-J 4-J UH β EH ra rtf d 0 -H rtf SH β 4-J Φ ε . Φ φ
Dl rd m o rQ CO 4J σi rH -P
-H Dl SH J β rH ra -H • UH TJ O Φ TJ 4J rtf rH φ rβ CQ β rtf rH rH 4J 4J 4J Φ ftf 0 rtf 4J Φ a rtf φ SH UH 4J
CJ rtf . ft rH 3 a co 0
-H rd -H d a H
4J -P 4-1 •H TJ φ Φ ra Di Φ ft d 0 > rβ φ β TJ
0 0 Φ CQ 3 -H 4J fH -H SH •
CQ •H 4-1 a 4J 0 Φ s TJ > ftf o Dl rH CJ n TJ Φ 4-J rH β 4-1 -H -H β -H
& φ 4J rtf 4-1 SH UH 4-J -H >
4J ϋ rβ β Φ Dl Φ rtf ftf Φ 4-1 Φ 4-1 d β β TJ fH rH -rH ■ rH -H -H 0 CO t~~ β Φ UH rH β TJ SH Φ
-H ft Φ rd Φ fH β TJ 0 rβ tn rd 0 SH β > O Ss Φ SH 4-1
© rH 4J D) β ϋ o ra SH fH -H CQ SH -H O • CJ rd rd -H UH
O
£ -H O ra υ r rtf CQ rQ ε O o
Referring to figure 8, tuning of the device 1 can be performed by tilting one of the resonator mirrors 14 parallel to the waveguide, such that the two resonator mirrors are no longer parallel to each other. Tilting parallel to the waveguide means that the mirror is pivoted about an axis perpendicular to the waveguide. Consequently, the length of the external resonator will vary along the same. Hence, the shape of the filter function will be broadened and shifted towards longer wavelengths. This adjustment of the filtering is schematically shown in figure 10, in which the initial filter function is illlustrated by a solid line, and the filter function at tilted resonator mirror is illustrated by a broken line. In figure 9, another method of tuning by tilting the resonator mirror 14 is illustrated. In this case, the resonator mirror is tilted perpendicularly to the longitudinal direction of the waveguide (the mirror is pivoted about an axis parallel to the waveguide. Hence, the finesse and the quality of the external Fabry-Perot type resonator will be lowered, since less light is coupled back from the resonator into the waveguide when the resonator mirror is tilted in this manner. The resonant wavelength, however, will remain substantially the same. In effect, the filter function is still broadened, but with a maintained centre wavelength. Effectively, this type of tuning can be used for modulating the amplitude of the channel to which the external resonator is resonant. This adjustment of the filtering is schematically shown in figure 11, in which the initial filter function is illlustrated by a solid line, and the filter function at tilted resonator mirror is illustrated by a broken line.
In conclusion, the present invention provides an optical device for manipulating individual channels within a WDM optical signal, which optical device is
tuneable, controllable and configurable, as well as direction invariant as regards propagation of the signal to be manipulated. Furthermore, the inherent possibility to cascade elements according to the present invention provides a virtually unlimited scalability, and may therefore be utilised even if a very large number of wavelength channels are used in a WDM communications system.
Although specific embodiments are presented in the detailed description above, it is to be understood that the present invention can be implemented differently than described. The detailed description of embodiments is not intended to limit the scope of the invention as defined in the claims.