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Compact SOI arrayed waveguide grating demultiplexer with broad spectral response
Optics Communications 258 (2006) 155–158 www.elsevier.com/locate/optcom
Compact SOI arrayed waveguide grating demultiplexer with broad spectral response Qing Fang *, Fang Li, Yuliang Liu Research and Development Center for Optoelectronics, Institute of Semiconductors, The Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, P.R. China Received 6 June 2005; accepted 25 July 2005
Abstract A compact eight-channel flat spectral response arrayed waveguide grating (AWG) multiplexer based on siliconon-insulator (SOI) materials has been fabricated on the planar lightwave circuit (PLC). The 1-dB bandwidth of 48 GHz and 3-dB bandwidth of 69 GHz are obtained for the 100 GHz channel spacing. Not only non-adjacent crosstalk but also adjacent crosstalk are less than 25 dB. The on-chip propagation loss range is from 3.5 to 3.9 dB, and the total device size is 1.5 · 1.0 cm2. 2005 Elsevier B.V. All rights reserved. Keywords: Silicon-on-insulator; Arrayed waveguide grating; Broad spectral response; Compact
1. Introduction The performance of wavelength division multiplexing (WDM) optical networks greatly depends on the spectral characteristics of their components [1]. One key component of WDM networks is the arrayed waveguide grating (AWG) [2,3]. In order to allow the concatenation of many such devices and reduce the need for accurate wavelength control, the filter deviceÕs response must approximate *
Corresponding author. Tel.: +860182304119; +860182304016. E-mail address: [email protected] (Q. Fang).
fax:
a broad spectral response. For an AWG multiplexer/demultiplexer of Gaussian type, the ratio of the 1-dB bandwidth to channel spacing is small. Some methods have been reported to broaden the spectral response, such as asymmetric MZ filter method [4], multiple Roland circles method [5] and input MMI coupler method [6] in silica and InP based AWG, but never in the SOI based AWG. Silicon-on-insulator (SOI) technology has shown to be a promising technology for guided wave photonic devices operating in the infrared. A number of SOI guided wave optical devices and circuits with high performance have already been demonstrated. SOI technology offers tremendous
0030-4018/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2005.07.058
156
Q. Fang et al. / Optics Communications 258 (2006) 155–158
potential for cost-effective monolithic integration including the variable optical attenuator, wavelength demultiplexer, photodetectors and electronic circuitry [7–9]. In this paper, we consider an optimal design of MMI input waveguides and exponential multimode taper output waveguides for broadening the spectral response of an AWG demultiplexer based on SOI materials. The measured results show that the 3 dB bandwidth is about 70% of 100 GHz channel spacing. The insertion loss uniformity is 0.5 dB with FSR of 15 nm. Compared to the silica AWG, The bend radius of the AWG based on SOI is smaller because of the high relative index difference. The device size is only 1.5 · 1.0 cm2.
2. Theory and design
the epitaxial layer of 2.5 lm-thickness is chosen. In order to broaden the spectral response, an MMI coupler is connected at the end of the input waveguide and exponential multimode waveguides is chosen as the output waveguides at the end of the second free propagation region (FPR). A twofold image can be obtained at the end of the MMI coupler if the MMI coupler is designed reasonably. According to the design theory of AWG, the 1:1 image of the twofold image can be produced at the focal line of second FPR, which connects the exponential multimode output waveguides. The spectral response of AWG is determined by the overlap between the 1:1 focused field of twofold image and the output guide-mode. The process of field overlap is illustrated schematically in Fig. 2. The spectral response of AWG can be given Z 2 T ðDf Þ ¼ uimage ðy Y Þuo ðyÞ dy ; ð1Þ
With the increase of the etching depth on the SOI chip, the refractive index difference becomes high. High refractive index difference can reduce the AWG chip size remarkably because of not only reduction of the minimum bend radius but also reduction of the slab focal length by the smaller lateral spacing of waveguides connected to the slabs. The different epitaxial layer thickness has different maximal etching depth for a single guided mode. That is to say, the epitaxial layer thickness is an important factor for the AWG chip size, shown in Fig. 1. In our design, the SOI wafer with
where uimage is the 1:1 focused field of the twofold image; uo is the output guide-mode field and Y is the peak separations of the twofold image. The output guide-mode field uo and the input guidemode field ui can be approximated by the following Gaussian distribution:
Fig. 1. AWG chip size vs. epitaxial layer thickness.
Fig. 2. Spectral response profile by overlap integral.
ui ðyÞ ¼ C 1 expðy 2 =w2i Þ; uo ðyÞ ¼ C 2 expðy
2
=w2o Þ;
ð2Þ ð3Þ
where C1 and C2 are constant; wi and wo are the width of input waveguide and output waveguide, respectively. The twofold image field uimage is described by the following sum of two Gaussian distributions:
Q. Fang et al. / Optics Communications 258 (2006) 155–158
(
"
#
ðy Y =2Þ2 w2i " #) 2 ðy þ Y =2Þ þ exp . w2i
3. Fabrication and results
uimage ðy Y Þ ¼ C exp
Substituting (3) and (4) into (1), then ( " # 2 ðy Y =2Þ T ðDf Þ / exp w2i " #) ðy þ Y =2Þ2 þ exp . w2i
ð4Þ
ð5Þ
At the same time T ðDf Þ / expðy 2 =w2o Þ.
157
ð6Þ
So the spectral response of AWG is affiliated with the peak separations Y of the twofold image and the width of the input/output waveguide. With the increase of the width of output waveguide, the top of the spectral response becomes flat. By optimizing the values of Y and wo, the ideal flat spectral response of AWG can be obtained. We have designed an eight-channel 100-GHz arrayed waveguide grating (AWG1) with MMI input waveguide and exponential multimode taper output waveguides. The width of each array waveguide with minimum radius of 1000 lm is 2.0 lm. The device is designed to operate at the grating order of 150, with a path length difference of 67.38 lm, and the FPR focal length of 2763 lm. To reduce the insertion loss uniformity of the device, the free spectral range of 15 nm is chosen. According to the self-image theory, the width and length of MMI which connects input single mode waveguide with first FPR are 5 and 35 lm, respectively. The peak separation of the twofold image is about 3.0 lm. The exponential multimode taper is chosen as output waveguide. The beginning width and end width of the taper are 4 and 2 lm, respectively. In order to obtain the ideal spectral response, the value of the exponential is chosen as 3. In order to compare the flat spectral response with that of convertional AWG, another normal arrayed waveguide grating (AWG2) having the same device parameters is allocated in the chip.
The AWGs were fabricated on SOI wafer having a 2.5 lm-thick Si on the top of a 0.4 lm-thick SiO2 layer, shown in Fig. 3. After photolithography and pattern formation, the devices were etched a rib height of 1.2 lm by the inductive coupled plasma (ICP) etching technology. In order to protect the rib waveguides from being destroyed, a 2.0 lm-thick SiO2 layer was grown by PECVD after ICP etching process. The AWG devices were measured by the combination of an amplified spontaneous emission (ASE) resource and an optical spectrum analyzer. Light was coupled into the input waveguide of the AWG chip using the cone-type optical fiber for reduce the coupling loss. The spectral responses of the device were shown in Figs. 4 and 5. The Fig. 4 shows the spectral response of conventional AWG2 with the propagation loss of 1.6 dB and the crosstalk of 28 dB. At the same time, the 1-dB bandwidth is 19 GHz and 3-dB bandwidth is 30 GHz. Compared to the spectral response of convertional AWG2, the Fig. 5 shows the flat spectral response of AWG1 with the 1-dB bandwidth of 48 GHz and 3-dB bandwidth of 69 GHz. Crosstalk to neighboring and all other channels of AWG1 are less than 25 dB. The on-chip propagation loss range is from 3.5 to 3.9 dB. Those results display that AWG1 has a more than 100% of 1- and 3-dB bandwidth ratio, compared to
Fig. 3. The input waveguide of the fabricated SOI AWG.
158
Q. Fang et al. / Optics Communications 258 (2006) 155–158
based on silicon-on-insulator materials has been reported. The total chip size is only 1.5 · 1.0 cm2. The 1-dB bandwidth of 48 GHz and 3dB bandwidth of 69 GHz are obtained for the 100 GHz channel spacing. Compared to the conventional AWG, the flat spectral response of the AWG with MMI input waveguide and exponential multimode taper output waveguides has a more than 100% of 1- and 3-dB bandwidth ratio. Crosstalk to neighboring is less than 25 dB; and the on-chip propagation loss range is from 3.5 to 3.9 dB. Fig. 4. Measured spectral response of the conventional AWG.
Acknowledgment This work was supported by National Natural Science Foundation of China under Grant No. 90104003.
References
Fig. 5. Measured spectral response of the AWG with MMI input waveguide and exponential multimode taper output waveguides.
AWG2. But the propagation loss of AWG1 is bigger 2.0 dB than AWG2, because the MMI coupler and the exponential multimode taper output waveguides are used in the AWG1. 4. Conclusion A compact eight-channel flat spectral response arrayed waveguide grating (AWG) multiplexer
[1] M.K. Smit, Cor van Dam, IEEE Journal of Selected Topics in Quantum Electronics 2 (1996) 236. [2] M.K. Smit, Electronics Letters 24 (1988) 385. [3] J.H. den Besten, M.P. Dessens, C.G.P. Herben, X.J.M. Leijtens, F.H. Groen, M.R. Leys, M.K. Smit, IEEE Photonics Technology Letters 14 (2002) 62. [4] M.R. Amersfoort, J.B.D. Soole, H.P. Leblanc, N.C. Andrendakis, A. Rajhel, C. Caneau, Electronics Letters 32 (1996) 449. [5] K. Okamoto, K. Takiguchi, Y. Ohmori, IEEE Photonics Technology Letters 8 (1996) 373. [6] Y.P. Ho, H. Li, Y.J. Chen, IEEE Photonics Technology Letters 9 (1997) 342. [7] S. Janz, A. Balakrishnan, S. Charbonneau, P. Cheben, M. Cloutier, A. Delage, K. Dosson, L. Erickson, M. Gao, P.A. Krug, B. Lamontagne, M. Packirisamy, M. Pearson, D.-X. Xu, IEEE Photonics Technology Letters 16 (2004) 503. [8] T. Bakke, C.P. Tiggws, C.T. Sullivan, Electronics Letters 38 (2002) 177. [9] P.D. Trinh, S. Yegnanarayanan, F. Coppinger, B. Jalali, IEEE Photonics Technology Letters 9 (1997) 940.
Compact SOI arrayed waveguide grating demultiplexer with broad spectral response Qing Fang *, Fang Li, Yuliang Liu Research and Development Center for Optoelectronics, Institute of Semiconductors, The Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, P.R. China Received 6 June 2005; accepted 25 July 2005
Abstract A compact eight-channel flat spectral response arrayed waveguide grating (AWG) multiplexer based on siliconon-insulator (SOI) materials has been fabricated on the planar lightwave circuit (PLC). The 1-dB bandwidth of 48 GHz and 3-dB bandwidth of 69 GHz are obtained for the 100 GHz channel spacing. Not only non-adjacent crosstalk but also adjacent crosstalk are less than 25 dB. The on-chip propagation loss range is from 3.5 to 3.9 dB, and the total device size is 1.5 · 1.0 cm2. 2005 Elsevier B.V. All rights reserved. Keywords: Silicon-on-insulator; Arrayed waveguide grating; Broad spectral response; Compact
1. Introduction The performance of wavelength division multiplexing (WDM) optical networks greatly depends on the spectral characteristics of their components [1]. One key component of WDM networks is the arrayed waveguide grating (AWG) [2,3]. In order to allow the concatenation of many such devices and reduce the need for accurate wavelength control, the filter deviceÕs response must approximate *
Corresponding author. Tel.: +860182304119; +860182304016. E-mail address: [email protected] (Q. Fang).
fax:
a broad spectral response. For an AWG multiplexer/demultiplexer of Gaussian type, the ratio of the 1-dB bandwidth to channel spacing is small. Some methods have been reported to broaden the spectral response, such as asymmetric MZ filter method [4], multiple Roland circles method [5] and input MMI coupler method [6] in silica and InP based AWG, but never in the SOI based AWG. Silicon-on-insulator (SOI) technology has shown to be a promising technology for guided wave photonic devices operating in the infrared. A number of SOI guided wave optical devices and circuits with high performance have already been demonstrated. SOI technology offers tremendous
0030-4018/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2005.07.058
156
Q. Fang et al. / Optics Communications 258 (2006) 155–158
potential for cost-effective monolithic integration including the variable optical attenuator, wavelength demultiplexer, photodetectors and electronic circuitry [7–9]. In this paper, we consider an optimal design of MMI input waveguides and exponential multimode taper output waveguides for broadening the spectral response of an AWG demultiplexer based on SOI materials. The measured results show that the 3 dB bandwidth is about 70% of 100 GHz channel spacing. The insertion loss uniformity is 0.5 dB with FSR of 15 nm. Compared to the silica AWG, The bend radius of the AWG based on SOI is smaller because of the high relative index difference. The device size is only 1.5 · 1.0 cm2.
2. Theory and design
the epitaxial layer of 2.5 lm-thickness is chosen. In order to broaden the spectral response, an MMI coupler is connected at the end of the input waveguide and exponential multimode waveguides is chosen as the output waveguides at the end of the second free propagation region (FPR). A twofold image can be obtained at the end of the MMI coupler if the MMI coupler is designed reasonably. According to the design theory of AWG, the 1:1 image of the twofold image can be produced at the focal line of second FPR, which connects the exponential multimode output waveguides. The spectral response of AWG is determined by the overlap between the 1:1 focused field of twofold image and the output guide-mode. The process of field overlap is illustrated schematically in Fig. 2. The spectral response of AWG can be given Z 2 T ðDf Þ ¼ uimage ðy Y Þuo ðyÞ dy ; ð1Þ
With the increase of the etching depth on the SOI chip, the refractive index difference becomes high. High refractive index difference can reduce the AWG chip size remarkably because of not only reduction of the minimum bend radius but also reduction of the slab focal length by the smaller lateral spacing of waveguides connected to the slabs. The different epitaxial layer thickness has different maximal etching depth for a single guided mode. That is to say, the epitaxial layer thickness is an important factor for the AWG chip size, shown in Fig. 1. In our design, the SOI wafer with
where uimage is the 1:1 focused field of the twofold image; uo is the output guide-mode field and Y is the peak separations of the twofold image. The output guide-mode field uo and the input guidemode field ui can be approximated by the following Gaussian distribution:
Fig. 1. AWG chip size vs. epitaxial layer thickness.
Fig. 2. Spectral response profile by overlap integral.
ui ðyÞ ¼ C 1 expðy 2 =w2i Þ; uo ðyÞ ¼ C 2 expðy
2
=w2o Þ;
ð2Þ ð3Þ
where C1 and C2 are constant; wi and wo are the width of input waveguide and output waveguide, respectively. The twofold image field uimage is described by the following sum of two Gaussian distributions:
Q. Fang et al. / Optics Communications 258 (2006) 155–158
(
"
#
ðy Y =2Þ2 w2i " #) 2 ðy þ Y =2Þ þ exp . w2i
3. Fabrication and results
uimage ðy Y Þ ¼ C exp
Substituting (3) and (4) into (1), then ( " # 2 ðy Y =2Þ T ðDf Þ / exp w2i " #) ðy þ Y =2Þ2 þ exp . w2i
ð4Þ
ð5Þ
At the same time T ðDf Þ / expðy 2 =w2o Þ.
157
ð6Þ
So the spectral response of AWG is affiliated with the peak separations Y of the twofold image and the width of the input/output waveguide. With the increase of the width of output waveguide, the top of the spectral response becomes flat. By optimizing the values of Y and wo, the ideal flat spectral response of AWG can be obtained. We have designed an eight-channel 100-GHz arrayed waveguide grating (AWG1) with MMI input waveguide and exponential multimode taper output waveguides. The width of each array waveguide with minimum radius of 1000 lm is 2.0 lm. The device is designed to operate at the grating order of 150, with a path length difference of 67.38 lm, and the FPR focal length of 2763 lm. To reduce the insertion loss uniformity of the device, the free spectral range of 15 nm is chosen. According to the self-image theory, the width and length of MMI which connects input single mode waveguide with first FPR are 5 and 35 lm, respectively. The peak separation of the twofold image is about 3.0 lm. The exponential multimode taper is chosen as output waveguide. The beginning width and end width of the taper are 4 and 2 lm, respectively. In order to obtain the ideal spectral response, the value of the exponential is chosen as 3. In order to compare the flat spectral response with that of convertional AWG, another normal arrayed waveguide grating (AWG2) having the same device parameters is allocated in the chip.
The AWGs were fabricated on SOI wafer having a 2.5 lm-thick Si on the top of a 0.4 lm-thick SiO2 layer, shown in Fig. 3. After photolithography and pattern formation, the devices were etched a rib height of 1.2 lm by the inductive coupled plasma (ICP) etching technology. In order to protect the rib waveguides from being destroyed, a 2.0 lm-thick SiO2 layer was grown by PECVD after ICP etching process. The AWG devices were measured by the combination of an amplified spontaneous emission (ASE) resource and an optical spectrum analyzer. Light was coupled into the input waveguide of the AWG chip using the cone-type optical fiber for reduce the coupling loss. The spectral responses of the device were shown in Figs. 4 and 5. The Fig. 4 shows the spectral response of conventional AWG2 with the propagation loss of 1.6 dB and the crosstalk of 28 dB. At the same time, the 1-dB bandwidth is 19 GHz and 3-dB bandwidth is 30 GHz. Compared to the spectral response of convertional AWG2, the Fig. 5 shows the flat spectral response of AWG1 with the 1-dB bandwidth of 48 GHz and 3-dB bandwidth of 69 GHz. Crosstalk to neighboring and all other channels of AWG1 are less than 25 dB. The on-chip propagation loss range is from 3.5 to 3.9 dB. Those results display that AWG1 has a more than 100% of 1- and 3-dB bandwidth ratio, compared to
Fig. 3. The input waveguide of the fabricated SOI AWG.
158
Q. Fang et al. / Optics Communications 258 (2006) 155–158
based on silicon-on-insulator materials has been reported. The total chip size is only 1.5 · 1.0 cm2. The 1-dB bandwidth of 48 GHz and 3dB bandwidth of 69 GHz are obtained for the 100 GHz channel spacing. Compared to the conventional AWG, the flat spectral response of the AWG with MMI input waveguide and exponential multimode taper output waveguides has a more than 100% of 1- and 3-dB bandwidth ratio. Crosstalk to neighboring is less than 25 dB; and the on-chip propagation loss range is from 3.5 to 3.9 dB. Fig. 4. Measured spectral response of the conventional AWG.
Acknowledgment This work was supported by National Natural Science Foundation of China under Grant No. 90104003.
References
Fig. 5. Measured spectral response of the AWG with MMI input waveguide and exponential multimode taper output waveguides.
AWG2. But the propagation loss of AWG1 is bigger 2.0 dB than AWG2, because the MMI coupler and the exponential multimode taper output waveguides are used in the AWG1. 4. Conclusion A compact eight-channel flat spectral response arrayed waveguide grating (AWG) multiplexer
[1] M.K. Smit, Cor van Dam, IEEE Journal of Selected Topics in Quantum Electronics 2 (1996) 236. [2] M.K. Smit, Electronics Letters 24 (1988) 385. [3] J.H. den Besten, M.P. Dessens, C.G.P. Herben, X.J.M. Leijtens, F.H. Groen, M.R. Leys, M.K. Smit, IEEE Photonics Technology Letters 14 (2002) 62. [4] M.R. Amersfoort, J.B.D. Soole, H.P. Leblanc, N.C. Andrendakis, A. Rajhel, C. Caneau, Electronics Letters 32 (1996) 449. [5] K. Okamoto, K. Takiguchi, Y. Ohmori, IEEE Photonics Technology Letters 8 (1996) 373. [6] Y.P. Ho, H. Li, Y.J. Chen, IEEE Photonics Technology Letters 9 (1997) 342. [7] S. Janz, A. Balakrishnan, S. Charbonneau, P. Cheben, M. Cloutier, A. Delage, K. Dosson, L. Erickson, M. Gao, P.A. Krug, B. Lamontagne, M. Packirisamy, M. Pearson, D.-X. Xu, IEEE Photonics Technology Letters 16 (2004) 503. [8] T. Bakke, C.P. Tiggws, C.T. Sullivan, Electronics Letters 38 (2002) 177. [9] P.D. Trinh, S. Yegnanarayanan, F. Coppinger, B. Jalali, IEEE Photonics Technology Letters 9 (1997) 940.