CA2349045A1 - Method of low loss polarisation compensation in arrayed waveguide gratings based devices by overlayer deposition or etching of asymmetric compensating regions in the slab waveguides - Google Patents
Method of low loss polarisation compensation in arrayed waveguide gratings based devices by overlayer deposition or etching of asymmetric compensating regions in the slab waveguides Download PDFInfo
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- 238000000034 method Methods 0.000 title claims abstract description 15
- 238000005530 etching Methods 0.000 title claims description 11
- 230000008021 deposition Effects 0.000 title claims description 6
- 230000010287 polarization Effects 0.000 claims abstract description 27
- 230000003287 optical effect Effects 0.000 claims abstract description 10
- 230000001419 dependent effect Effects 0.000 claims description 11
- 238000001228 spectrum Methods 0.000 description 7
- 239000012212 insulator Substances 0.000 description 3
- 239000000463 material Substances 0.000 description 3
- VYPSYNLAJGMNEJ-UHFFFAOYSA-N Silicium dioxide Chemical compound O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 description 2
- 239000006185 dispersion Substances 0.000 description 2
- 238000003780 insertion Methods 0.000 description 2
- 230000037431 insertion Effects 0.000 description 2
- 238000004519 manufacturing process Methods 0.000 description 2
- 238000002310 reflectometry Methods 0.000 description 2
- 229910052581 Si3N4 Inorganic materials 0.000 description 1
- 238000013459 approach Methods 0.000 description 1
- 230000015556 catabolic process Effects 0.000 description 1
- 229910052681 coesite Inorganic materials 0.000 description 1
- 230000008878 coupling Effects 0.000 description 1
- 238000010168 coupling process Methods 0.000 description 1
- 238000005859 coupling reaction Methods 0.000 description 1
- 229910052906 cristobalite Inorganic materials 0.000 description 1
- 238000006731 degradation reaction Methods 0.000 description 1
- 230000000593 degrading effect Effects 0.000 description 1
- 230000010354 integration Effects 0.000 description 1
- 238000000206 photolithography Methods 0.000 description 1
- 238000001878 scanning electron micrograph Methods 0.000 description 1
- 239000004065 semiconductor Substances 0.000 description 1
- 239000000377 silicon dioxide Substances 0.000 description 1
- 235000012239 silicon dioxide Nutrition 0.000 description 1
- HQVNEWCFYHHQES-UHFFFAOYSA-N silicon nitride Chemical compound N12[Si]34N5[Si]62N3[Si]51N64 HQVNEWCFYHHQES-UHFFFAOYSA-N 0.000 description 1
- 229910052682 stishovite Inorganic materials 0.000 description 1
- 239000000758 substrate Substances 0.000 description 1
- 229910052905 tridymite Inorganic materials 0.000 description 1
- 238000009966 trimming Methods 0.000 description 1
Classifications
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- 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
- G02B6/12007—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 forming wavelength selective elements, e.g. multiplexer, demultiplexer
- G02B6/12009—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 forming wavelength selective elements, e.g. multiplexer, demultiplexer comprising arrayed waveguide grating [AWG] devices, i.e. with a phased array of waveguides
- G02B6/12014—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 forming wavelength selective elements, e.g. multiplexer, demultiplexer comprising arrayed waveguide grating [AWG] devices, i.e. with a phased array of waveguides characterised by the wavefront splitting or combining section, e.g. grooves or optical elements in a slab waveguide
-
- 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
- G02B6/12007—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 forming wavelength selective elements, e.g. multiplexer, demultiplexer
- G02B6/12009—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 forming wavelength selective elements, e.g. multiplexer, demultiplexer comprising arrayed waveguide grating [AWG] devices, i.e. with a phased array of waveguides
- G02B6/12023—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 forming wavelength selective elements, e.g. multiplexer, demultiplexer comprising arrayed waveguide grating [AWG] devices, i.e. with a phased array of waveguides characterised by means for reducing the polarisation dependence, e.g. reduced birefringence
-
- 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
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- Physics & Mathematics (AREA)
- Engineering & Computer Science (AREA)
- Microelectronics & Electronic Packaging (AREA)
- General Physics & Mathematics (AREA)
- Optics & Photonics (AREA)
- Optical Integrated Circuits (AREA)
Abstract
A method of providing polarization compensation in an optical device is disclosed wherein two asymmetric prism-shaped compensating regions are provided in input and output couplers of the device
Description
Method of low loss polarisation compensation in arrayed waveguide gratings based devices by overlayer deposition or etching of asymmetric compensating regions in the slab waveguides Background of the Invention 1. Field of the Invention This invention relates to the field photonics, and in ~aarticular to a method of Iow loss polarisation compensation in arrayed waveguide.
2. Description of Related Art As the most widely used optical fibres do not prese~°ve polarization, it is important that optical components used with optical fibres are polarization independent.
In phasar-based devices, polarization independence is achieved if both TE and TM fundamental modes propagate in the arrayed waveguide section with the same propagation constants, and thus the wavelengths of the corresponding modes (measured in the waveguides) are identical. .A difference in propagation constant arising from the waveguide birefringence results in a frequency shift Df between TE and TM spectra of a demultiplexer, according to:
Q~ ~ ~ Nte - NtnT
a ' Nte where f is the central frequency, Nte and N~ are the effective waveguide indices for TE and TM polarization, and N a is the group index of the waveguide TE
mode.
In practical phasar-based devices, the condition for polarization independent operation Of ~ 0 is rarely satisfied due to material and waveguide birefringence.
Polarization compensation techniques are thus required to achieve polarization insensitive operation.
In some devices such as compact silicon-on-insulator (SOI) arrayed waveguide gratings (AWGs), the frequency shift can be as large as a few hundred GHz, depending on thickness of Si layer and on the waveguide cross-section geometry.
The waveguide dimensions are particularly important for devices that use ridge waveguides, because of different boundary conditions for TE and TM polarized fields. This asymmetry in the boundary conditions can be eliminated in buried and raised-strip waveguides with square cross-section, yielding low birefringence. However, this is at the expense of increased loss due to scattering on roughness in the vertical side-walls arising from imperfections in photolithography and etching. The side-wall scattering is a major concern in SOI
waveguides, where the high index contrast leads to strong coupling of the waveguide mode with side-wall imperfections. This can result in a prohibitively large scattering when the waveguide width is reduced to less than a few microns.
By using ridge waveguides and the appropriate choice of the waveguide cross-section geometry, the birefringence can be removed and single-mode operation achieved at the same time. In practice, it is though difficult to reproducibly achieve birefringence-free operation only by tailoring the waveguide cross-section. The difficulties are due to limitations on the achievable fabrication tolerances as it becomes increasingly difficult to fab~°icate waveguides of desired cross-section as the waveguide width shrinks and approaches the order of a micron. Waveguides of such small dimensions can lbe fabricated in platforms with a large refractive index step such as silicon-on-insulator, silicon nitride or III-V semiconductors. Reducing feature size is indis~~ensable for next generation of compact and integrated optical and photonics devices, allowing for integrations of a number of functionalities on a single die such as multiplexers, demultiplexers, lasers, photodetectors, optical switches, attenuators and other passive and active optical components as well as electronic circuitry.
Several techniques can be used to reduce the polari2;ation dependent wavelength shift. These include insertion of a half-wave plate in the middle of the waveguide array (H. Takahashi et al., Opt. Lett. Vol.17, 499,1992), dispersion matching with adjacent diffraction orders (M. Zirngibl et al., Electron. Lett. Vol. 29, 201,1992), special layer structure with low birefringence (H. Bissessur et al., Electron.
Lett.
Vol. 30, 336;1994), inserting a waveguide section with a different birefringence in the phased array (M. Zirngibl et al., Electron. Lett. Vol. 31, 1662, 1995), adding polarization splitter at the input of the AWG (M. K. Smit and C. van Dam, IEEE
Journ. of Select. Top. in Quant. Electr. Vol 5, 236,1996), or etching compensating region in slab waveguides Q. -J. He et at., IEEE Photon. Tech. Lett. Vol. 11, 224, 1999).
The above outlined techniques suffer from drawbacks ranging from fabrication difficulties to limitation to special devices, material; and operating conditions.
The compensator etched in a slab region (J. -J. He et at., IEEE Photon. Tech.
Lett.
Vol. 11, 224, 1999) is a particularly attractive easy-to~-fabricate device, but it requires deep etching for the materials and devices with large polarization dependent wavelength shifts, which in turn degrades device performance due to losses in such deeply etched compensators.
Summary of the Invention The present invention provides a method for compensating polarization dependent wavelength shift in phasar-based devices. The method consists of overlayer deposition or etching of two asymmetric ~~rism-shaped compensating regions in input and output couplers of the device, respectively. The asymmetric compensating regions have effective TE/TM mode refractive indices different from those of the original slab waveguide; and thus provide the polarization dependent properties that cancel the polarization dependent wavelength shift of the original uncompensated device.
The invention is based on the compensation of polarization dependent wavelength shift by two compensating regions located in input a:nd output couplers of a phasar-based device, respectively. The use of an asymmetric compensation scheme eliminating total internal reflection problems is also novel.
In phasar-based devices, polarization independence is achieved if both TE and TM fundamental modes propagate in the arrayed waveguide section with the same propagation constants, and thus the wavelengths of the corresponding modes (measured in the waveguides) are identical. .A difference in propagation constant arising from the waveguide birefringence results in a frequency shift Df between TE and TM spectra of a demultiplexer, according to:
Q~ ~ ~ Nte - NtnT
a ' Nte where f is the central frequency, Nte and N~ are the effective waveguide indices for TE and TM polarization, and N a is the group index of the waveguide TE
mode.
In practical phasar-based devices, the condition for polarization independent operation Of ~ 0 is rarely satisfied due to material and waveguide birefringence.
Polarization compensation techniques are thus required to achieve polarization insensitive operation.
In some devices such as compact silicon-on-insulator (SOI) arrayed waveguide gratings (AWGs), the frequency shift can be as large as a few hundred GHz, depending on thickness of Si layer and on the waveguide cross-section geometry.
The waveguide dimensions are particularly important for devices that use ridge waveguides, because of different boundary conditions for TE and TM polarized fields. This asymmetry in the boundary conditions can be eliminated in buried and raised-strip waveguides with square cross-section, yielding low birefringence. However, this is at the expense of increased loss due to scattering on roughness in the vertical side-walls arising from imperfections in photolithography and etching. The side-wall scattering is a major concern in SOI
waveguides, where the high index contrast leads to strong coupling of the waveguide mode with side-wall imperfections. This can result in a prohibitively large scattering when the waveguide width is reduced to less than a few microns.
By using ridge waveguides and the appropriate choice of the waveguide cross-section geometry, the birefringence can be removed and single-mode operation achieved at the same time. In practice, it is though difficult to reproducibly achieve birefringence-free operation only by tailoring the waveguide cross-section. The difficulties are due to limitations on the achievable fabrication tolerances as it becomes increasingly difficult to fab~°icate waveguides of desired cross-section as the waveguide width shrinks and approaches the order of a micron. Waveguides of such small dimensions can lbe fabricated in platforms with a large refractive index step such as silicon-on-insulator, silicon nitride or III-V semiconductors. Reducing feature size is indis~~ensable for next generation of compact and integrated optical and photonics devices, allowing for integrations of a number of functionalities on a single die such as multiplexers, demultiplexers, lasers, photodetectors, optical switches, attenuators and other passive and active optical components as well as electronic circuitry.
Several techniques can be used to reduce the polari2;ation dependent wavelength shift. These include insertion of a half-wave plate in the middle of the waveguide array (H. Takahashi et al., Opt. Lett. Vol.17, 499,1992), dispersion matching with adjacent diffraction orders (M. Zirngibl et al., Electron. Lett. Vol. 29, 201,1992), special layer structure with low birefringence (H. Bissessur et al., Electron.
Lett.
Vol. 30, 336;1994), inserting a waveguide section with a different birefringence in the phased array (M. Zirngibl et al., Electron. Lett. Vol. 31, 1662, 1995), adding polarization splitter at the input of the AWG (M. K. Smit and C. van Dam, IEEE
Journ. of Select. Top. in Quant. Electr. Vol 5, 236,1996), or etching compensating region in slab waveguides Q. -J. He et at., IEEE Photon. Tech. Lett. Vol. 11, 224, 1999).
The above outlined techniques suffer from drawbacks ranging from fabrication difficulties to limitation to special devices, material; and operating conditions.
The compensator etched in a slab region (J. -J. He et at., IEEE Photon. Tech.
Lett.
Vol. 11, 224, 1999) is a particularly attractive easy-to~-fabricate device, but it requires deep etching for the materials and devices with large polarization dependent wavelength shifts, which in turn degrades device performance due to losses in such deeply etched compensators.
Summary of the Invention The present invention provides a method for compensating polarization dependent wavelength shift in phasar-based devices. The method consists of overlayer deposition or etching of two asymmetric ~~rism-shaped compensating regions in input and output couplers of the device, respectively. The asymmetric compensating regions have effective TE/TM mode refractive indices different from those of the original slab waveguide; and thus provide the polarization dependent properties that cancel the polarization dependent wavelength shift of the original uncompensated device.
The invention is based on the compensation of polarization dependent wavelength shift by two compensating regions located in input a:nd output couplers of a phasar-based device, respectively. The use of an asymmetric compensation scheme eliminating total internal reflection problems is also novel.
Brief Description of the Drawings The invention will now be described in more detail, by way of example, only with reference to the accompanying drawings, in which:-Figure 1 shows an arrayed waveguide with two asymmetric compensating regions;
Figure 2 shows the reflectivity of the compensator boundary as the light passes from the higher to the lower effective index slab;
Figure 3 shows input wavelengths joining the input coupler slab;
Figure 4 shows arrayed waveguides joining the output coupler;
Figure 5 shows the TM and TE spectra for channel #~~ of the compensated device;
and Figure 6 shows the spectrum of polarization compensated AWG.
Detailed Description of the Invention 'The polarization compensation technique disclosed is this invention is based on creating two asymmetric prism-shaped compensating regions in the input and output coupler slab regions, respectively, with effective index of the compensating regions (n~,te and n~"m for TE and TM modes, respectively) different than the effective indices (n~,te and ns"m) of the original slab wameguide. Such a difference in the effective indices can easily be created by overlayer deposition or etching an appropriately shaped region in the slab waveguide, while the strength of the compensator can be adjusted by selecting the etch depth or the thickness and/or refractive index of the overlayer, and thus the induced change in the effective refractive indices of the compensating regions. By using two compensators etched in both input and output couplers (Fig.1), respectively, etch depth required for eliminating the polarization dependent wavelength shift is significantly reduced, as is the extra insertion loss penalty. The overall loss is reduced as a result of a better mode matching between such shallower compensators and the coupler slab waveguide. The splitting the compensator also reduces Fresnel reflection loss at the effective index step at the compensator/coupler boundary.
The shape of the compensating regions can be determined as follows. In order to eliminate the polarization dependence, it is sufficient to assure that the wave fronts corresponding to both TM and TE slab modes have tlhe same tilt in the output coupler, thus converging to the same position at the focal line. This will be satisfied if the difference between the total optical phase (accumulated as the light propagates from the beginning to the end of the AWG) through the waveguide number i of the phased array and the total phase through the waveguide k of the phased array (i and k being arbitrarily chosen) is identical for both TM and TE
polarizations. It can be shown that this condition of constant phase difference for the two orthogonal polarizations yields the following formula defining the compensator shape:
d. -d = 1 im8~l, ' ° 2 8n$ - 8n~
Here d; is the distance between the end of the array 'Naveguide i and the point where the compensator boundary intersects a line joining the end of waveguide i and the beginning of the central output waveguide. 7:'he locus of these intersection points defines the boundary between the compensating and noncompensating parts of the output coupler; d° = d; (i =0); m is the AWG order; b~, _ ~,,e - ~,~" is the wavelength shift between the TE and TM spectra to k>e compensated; 8ns = ns,te -ns,~, and bn~ = n~,te - n~,~, is the effective index birefringence of the slab waveguide and the compensating region, respectively.
As explained above, this compensator formula assumes splitting the compensator in two parts, one in the input coupler and one in the output coupler. Given the symmetry of the AWG device and the phase conditions assumed when deducing the formula, the two parts of the compensator shoulcL be mirror images of each other along the symmetry axis of the AWG device. )=-I:owever, such symmetric design can result in total internal reflection (TIR) on one of the compensator-slab boundaries, degrading device performance: In the case when the compensating regions are etched, the effective refractive index of the compensator region is lower than that of the non-etched region of the combiner. Such design can result, in the input coupler, in total internal reflection (TIR) for some rays refracted at the slab/compensator boundary, as they pass from a higher effective index (non-etched) to a lower index (etched) regions. It should be noted that in contrast to the situation in the input coupler, in the output coupler light propagates first through the lower index and then through the higher index regions. Consequently, TIR
cannot arise in the output coupler when etched comb>ensators are used. In the case when compensator is created by deposition of an over-layer, similar arguments applies, but because in such case the effective refractive index of the compensating region is higher than that of the original slab wavegu.ide, TIR may occur in the output coupler while TIR cannot arise in the input coupler.
Fig. 2 shows the intensity reflectance versus the angle of incidence assuming 0.4 mm deep etched compensator in 1.5 mm thick Si slab waveguide. It is observed that the reflectivity significantly increases for angles of incidence above 80°, with TIR occurring at 83.1°.
To avoid possible TIR in one of the couplers, the compensator etching or the overlayer depostion are reversed in the coupler in which TIR may occur, so that in the actual device light propagates first through the lower index and then through the higher index region of the latter. To maintain the same polarization compensatian however, the compensator region in that particular coupler must also be reflected about the symmetry axis of the latter. This results in asymmetric compensator regions shown in Fig. 1.
The following example illustrates the effectiveness of the novel technique even when compensating very large polarization dependent wavelength shifts that are typical for compact silicon-on-insulator phasar-based. devices.
AWG demultiplexers were fabricated on SOI substrates with a 1.5 ftrn thick Si layer of (100) orientation on a 1 ~m thick SiO2 layer. Fig. 3 and 4 show SEM images of different parts of the fabricated device. Polarization dependent wavelength shift of a non-compensated device was 8~, _ ~,te - ~,,~, = 2.22 nrn. This. large red shift of the TE spectrum with respect to the TM one was reduced to S~, = 1.78 nm upon the first 0.124 ~.m deep etch step and further reduced to 0.54 run by etching compensator additional 0.218 Vim.
By further trimming, polarization dispersion was reduced down to 0.04 nm, as shown in Fig. 5 where TE and TM spectra are depicted for a channel near the AWG
central wavelength after full-compensation. No crosstalk performance degradation was observed for a fully compensated AWG. It was found that compensator induces a significant red shift of ~5 nm in the peak wavelength. 'This red-shift is expected because of shifting the focus position for a particular wavelength at the output coupler focal line is induced by refraction at tlhe compensator/coupler boundary. An adjustment in AWG design can easily be made to account for this shift. It was also found that overcompensation of the demultiplexer is possible by etching the compensator an additional 0.22 Vim, yielding 8~, _ -3.65 nm. The Spectrum of the compensated device is shown in Fig.. 6.
Figure 2 shows the reflectivity of the compensator boundary as the light passes from the higher to the lower effective index slab;
Figure 3 shows input wavelengths joining the input coupler slab;
Figure 4 shows arrayed waveguides joining the output coupler;
Figure 5 shows the TM and TE spectra for channel #~~ of the compensated device;
and Figure 6 shows the spectrum of polarization compensated AWG.
Detailed Description of the Invention 'The polarization compensation technique disclosed is this invention is based on creating two asymmetric prism-shaped compensating regions in the input and output coupler slab regions, respectively, with effective index of the compensating regions (n~,te and n~"m for TE and TM modes, respectively) different than the effective indices (n~,te and ns"m) of the original slab wameguide. Such a difference in the effective indices can easily be created by overlayer deposition or etching an appropriately shaped region in the slab waveguide, while the strength of the compensator can be adjusted by selecting the etch depth or the thickness and/or refractive index of the overlayer, and thus the induced change in the effective refractive indices of the compensating regions. By using two compensators etched in both input and output couplers (Fig.1), respectively, etch depth required for eliminating the polarization dependent wavelength shift is significantly reduced, as is the extra insertion loss penalty. The overall loss is reduced as a result of a better mode matching between such shallower compensators and the coupler slab waveguide. The splitting the compensator also reduces Fresnel reflection loss at the effective index step at the compensator/coupler boundary.
The shape of the compensating regions can be determined as follows. In order to eliminate the polarization dependence, it is sufficient to assure that the wave fronts corresponding to both TM and TE slab modes have tlhe same tilt in the output coupler, thus converging to the same position at the focal line. This will be satisfied if the difference between the total optical phase (accumulated as the light propagates from the beginning to the end of the AWG) through the waveguide number i of the phased array and the total phase through the waveguide k of the phased array (i and k being arbitrarily chosen) is identical for both TM and TE
polarizations. It can be shown that this condition of constant phase difference for the two orthogonal polarizations yields the following formula defining the compensator shape:
d. -d = 1 im8~l, ' ° 2 8n$ - 8n~
Here d; is the distance between the end of the array 'Naveguide i and the point where the compensator boundary intersects a line joining the end of waveguide i and the beginning of the central output waveguide. 7:'he locus of these intersection points defines the boundary between the compensating and noncompensating parts of the output coupler; d° = d; (i =0); m is the AWG order; b~, _ ~,,e - ~,~" is the wavelength shift between the TE and TM spectra to k>e compensated; 8ns = ns,te -ns,~, and bn~ = n~,te - n~,~, is the effective index birefringence of the slab waveguide and the compensating region, respectively.
As explained above, this compensator formula assumes splitting the compensator in two parts, one in the input coupler and one in the output coupler. Given the symmetry of the AWG device and the phase conditions assumed when deducing the formula, the two parts of the compensator shoulcL be mirror images of each other along the symmetry axis of the AWG device. )=-I:owever, such symmetric design can result in total internal reflection (TIR) on one of the compensator-slab boundaries, degrading device performance: In the case when the compensating regions are etched, the effective refractive index of the compensator region is lower than that of the non-etched region of the combiner. Such design can result, in the input coupler, in total internal reflection (TIR) for some rays refracted at the slab/compensator boundary, as they pass from a higher effective index (non-etched) to a lower index (etched) regions. It should be noted that in contrast to the situation in the input coupler, in the output coupler light propagates first through the lower index and then through the higher index regions. Consequently, TIR
cannot arise in the output coupler when etched comb>ensators are used. In the case when compensator is created by deposition of an over-layer, similar arguments applies, but because in such case the effective refractive index of the compensating region is higher than that of the original slab wavegu.ide, TIR may occur in the output coupler while TIR cannot arise in the input coupler.
Fig. 2 shows the intensity reflectance versus the angle of incidence assuming 0.4 mm deep etched compensator in 1.5 mm thick Si slab waveguide. It is observed that the reflectivity significantly increases for angles of incidence above 80°, with TIR occurring at 83.1°.
To avoid possible TIR in one of the couplers, the compensator etching or the overlayer depostion are reversed in the coupler in which TIR may occur, so that in the actual device light propagates first through the lower index and then through the higher index region of the latter. To maintain the same polarization compensatian however, the compensator region in that particular coupler must also be reflected about the symmetry axis of the latter. This results in asymmetric compensator regions shown in Fig. 1.
The following example illustrates the effectiveness of the novel technique even when compensating very large polarization dependent wavelength shifts that are typical for compact silicon-on-insulator phasar-based. devices.
AWG demultiplexers were fabricated on SOI substrates with a 1.5 ftrn thick Si layer of (100) orientation on a 1 ~m thick SiO2 layer. Fig. 3 and 4 show SEM images of different parts of the fabricated device. Polarization dependent wavelength shift of a non-compensated device was 8~, _ ~,te - ~,,~, = 2.22 nrn. This. large red shift of the TE spectrum with respect to the TM one was reduced to S~, = 1.78 nm upon the first 0.124 ~.m deep etch step and further reduced to 0.54 run by etching compensator additional 0.218 Vim.
By further trimming, polarization dispersion was reduced down to 0.04 nm, as shown in Fig. 5 where TE and TM spectra are depicted for a channel near the AWG
central wavelength after full-compensation. No crosstalk performance degradation was observed for a fully compensated AWG. It was found that compensator induces a significant red shift of ~5 nm in the peak wavelength. 'This red-shift is expected because of shifting the focus position for a particular wavelength at the output coupler focal line is induced by refraction at tlhe compensator/coupler boundary. An adjustment in AWG design can easily be made to account for this shift. It was also found that overcompensation of the demultiplexer is possible by etching the compensator an additional 0.22 Vim, yielding 8~, _ -3.65 nm. The Spectrum of the compensated device is shown in Fig.. 6.
Claims (4)
1. A method of providing polarization compensation in an optical device, comprising providing two asymmetric prism-shaped compensating regions in input and output couplers of the device.
2. A method as claimed in claim 2, wherein said asymmetric prism-shaped compensating regions are provided by overlayer deposition or etching.
3. A method as claimed in claim 2, wherein the asymmetric compensating regions have effective TE/TM mode refractive indices different from those of the uncompensated device and thus provide the polarization dependent properties that cancel the polarization dependent wavelength shift of the uncompensated device.
4. A method as claimed in claim 3, wherein said device is an optical waveguide.
Priority Applications (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2349045 CA2349045A1 (en) | 2001-05-28 | 2001-05-28 | Method of low loss polarisation compensation in arrayed waveguide gratings based devices by overlayer deposition or etching of asymmetric compensating regions in the slab waveguides |
| PCT/CA2002/000779 WO2002097498A1 (en) | 2001-05-28 | 2002-05-28 | Polarisation compensated optical wdm-device |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2349045 CA2349045A1 (en) | 2001-05-28 | 2001-05-28 | Method of low loss polarisation compensation in arrayed waveguide gratings based devices by overlayer deposition or etching of asymmetric compensating regions in the slab waveguides |
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| CA2349045A1 true CA2349045A1 (en) | 2002-11-28 |
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| CA 2349045 Abandoned CA2349045A1 (en) | 2001-05-28 | 2001-05-28 | Method of low loss polarisation compensation in arrayed waveguide gratings based devices by overlayer deposition or etching of asymmetric compensating regions in the slab waveguides |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CA2233849C (en) * | 1998-03-31 | 2004-05-25 | Jian-Jun He | Integrated polarization compensator for polarization insensitive wavelength multi/demultiplexers based on waveguide diffraction gratings |
-
2001
- 2001-05-28 CA CA 2349045 patent/CA2349045A1/en not_active Abandoned
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2002
- 2002-05-28 WO PCT/CA2002/000779 patent/WO2002097498A1/en not_active Application Discontinuation
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| Publication number | Publication date |
|---|---|
| WO2002097498A1 (en) | 2002-12-05 |
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