December 15, 2014 / Vol. 39, No. 24 / OPTICS LETTERS
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Silicon hybrid demultiplexer with 64 channels for wavelength/mode-division multiplexed on-chip optical interconnects Jian Wang, Sitao Chen, and Daoxin Dai* Centre for Optical and Electromagnetic Research, State Key Laboratory for Modern Optical Instrumentation, Zhejiang Provincial Key Laboratory for Sensing Technologies, Zhejiang University, Zijingang Campus, Hangzhou 310058, China. *Corresponding author: [email protected] Received October 13, 2014; revised November 18, 2014; accepted November 19, 2014; posted November 20, 2014 (Doc. ID 224844); published December 15, 2014 A monolithically integrated 64-channel hybrid demultiplexer on silicon is demonstrated experimentally to enable wavelength-division-multiplexing and mode-division-multiplexing simultaneously for realizing an ultra-large capacity optical-interconnect link. The present hybrid demultiplexer consists of a four-channel mode multiplexer realized with three cascaded asymmetrical directional-couplers and four identical arrayed-waveguide gratings (AWGs) with 16 channels. For the fabricated hybrid multiplexer, the excess loss and the crosstalk are about −7 and −10 dB, respectively. Better performances can be achieved by minimizing the imperfections (particularly in AWGs) in the fabrication processes. The present hybrid demultiplexer is scalable to have more channels by utilizing more wavelengths, modes, and polarizations. © 2014 Optical Society of America OCIS codes: (130.3120) Integrated optics devices; (030.4070) Modes; (060.4230) Multiplexing. http://dx.doi.org/10.1364/OL.39.006993
As the demand for bandwidth increases rapidly to satisfy more and more data services, optical interconnects with ultra-high capacity are desired and advanced multiplexing technologies have been explored to expand the link capacity by introducing more channels [1]. Particularly, wavelength-division-multiplexing (WDM) is one of the most successful multiplexing technologies realized by utilizing multiple wavelengths. However, when many wavelength-channels are involved, lots of laser sources are needed accordingly and the wavelength management becomes complicated and expensive. In recent years, the multimode space-division-multiplexing (SDM) technology utilizing multiple mode channels has been paid much attention in terms of increasing the capacity of a singlewavelength carrier not only for long-haul fiber optical communications but also for on-chip optical interconnects. For a multimode SDM optical-interconnect link, one of the essential key components is mode demultiplexer to demultiplex different mode-channels, which can be realized by using multimode interference (MMI) couplers [2,3], adiabatic mode-evolution couplers [4], asymmetrical Y-junction [5–7], as well as asymmetrical directional couplers (ADCs) [5–16]. Among them, the ADC-based mode demultiplexer has advantages, including the broad wavelength band, the ultra-compact footprint, and the flexible scalability for realizing more mode channels. Previously, a four-channel TM mode multiplexer with a low loss and crosstalk has been demonstrated with cascaded ADCs in our group [11]. To greatly increase the capacity of an opticalinterconnect link to even Peta-bit/s, hybrid multiplexing technology has been proposed [10,11] by making several multiplexing technologies combined together compatibly. For example, an eight-channel hybrid multiplexing system enabling the polarization-division-multiplexing (PDM) and mode-division-multiplexing (MDM) simultaneously has been proposed and demonstrated [14,15]. As the WDM are mature, it is of interest to develop a hybrid multiplexing technology to enable the MDM 0146-9592/14/246993-04$15.00/0
and WDM simultaneously. Recently some MDM-WDM links with a few modes and wavelengths have been demonstrated with various types of mode demultiplexers, as shown in Table 1. For example, in [16] the mode demultiplexer based on ADC-assisted microrings was used to realize a 2-mode × 3-wavelength MDM-WDM link with a wavelength channel spacing Δλch as large as 15–16 nm. In [17], a mode multiplexer based on an asymmetrical Y-junction was used to realize a 2-mode × 3-wavelength MDM-WDM optical link with Δλch 8–10 nm. More recently a SiN mode demultiplexer based on ADC-assisted microrings was demonstrated for realizing an MDMWDM link [18]. However, the total channel number is very limited and the channel spacing in these experiments is large (∼10 nm; see Table 1). In this Letter, we propose and demonstrate a silicon 64-channel hybrid demultiplexer by combining a mode demultiplexer and four arrayed-waveguide gratings (AWGs) for the first time to the best of our knowledge. For the present case, we use our 1 × 4 mode demultiplexer based on cascaded ADCs, which is broadband and thus WDM-compatible. The four AWGs have a channel spacing as small as 3.2 nm. These AWGs are designed identically to have the same central wavelength for the wavelength-channels (λ1 ; λ2 ; …; λN ). As an example, our silicon hybrid Table 1. WDM-MDM Hybrid Demultiplexing Technology Reference No.
[16]
[17]
[18]
Mode-channel 2 2 3 number λ-channel number 3 3 1 15, 16 8, 10 / Δλch (nm) λ-channel / / / crosstalk (dB) Mode-channel −22, −18, ∼ − 30 −10, crosstalk (dB) −12 −15, −24 Excess loss (dB) 3–16 / / © 2014 Optical Society of America
This work 4 16 ∼3.2 −10 ∼ 14 ∼ − 20 ∼5
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Fig. 1. Configuration of the hybrid demultiplexer, including a four-channel mode demultiplexer and four 16-channel AWGs. Here a mode multiplexer is also included for characterization.
demultiplexer has a mode multiplexer with four modechannels and four AWGs with 16 wavelength-channels so that one has 64 channels to be demultiplexed in total (see Fig. 1). A silicon-on-insulator (SOI) platform with a 220-nm-thick silicon core layer on a 2-μm-thick buried oxide layer is chosen here and the nanowire waveguides have a 310-nm-thick PMMA upper-cladding for protection. Here the 4-channel mode demultiplexer is designed for TM polarization and realized by three cascaded ADCs consisting of a single-mode narrow waveguide and a wide bus waveguide. As demonstrated before [11,14], here the width of the narrow waveguide are chosen as w 400 nm to be single-mode while the width of the wide waveguide are chosen optimally according to the phase matching condition, i.e., neff 10 neff 2i , where neff 10 is the effective index of the fundamental mode for the narrow waveguide, neff 2i is the effective index of the ith higher-order mode for the wide waveguide. Since the effective indices of the guided modes in the wide bus waveguide are discrete, the phase-matching condition can be satisfied for only one mode. Consequently mode-selective coupling process is obtained, which is the fundamental mechanism for the design of ADC-based mode demultiplexers. More details can be found in Ref. [11]. In order to demultiplex the wavelength-channels, we use ultrasmall SOI-nanowire AWG, which is preferred for the case with more channels in comparison with other wavelength-division-multiplexer, like microring resonators, Mach–Zehnder lattice filters, etc. [19]. For the SOI-nanowire AWGs, which are also designed to work with TM polarization, there are 16 channels (N ch 16) and the channel spacing is Δλch 3.2 nm (400 GHz). The diffraction order is chosen as m 13 to make the free spectral range (FSR) be larger than the product N ch Δλch 51.2 nm so that 16 channels are available. The path difference between the adjacent arrayed waveguides is ΔL 11.55 μm, calculated with the formula ΔL mλc ∕neff , where λc is the central wavelength, and neff is the effective index of the fundamental mode in the arrayed waveguide. The gap between the adjacent arrayed waveguides at the end connecting with the free propagation region is chosen as 500 nm to avoid the lag effect of the etching process. The end separation between the output waveguides is about 2.43 μm and the length of the free propagation regions (FPRs) is 100 μm. In our design, the waveguide for the waveguide-array has a cross-section 500 nm × 220 nm to be single-mode and
Fig. 2. Optical microscope image of the fabricated hybrid demultiplexer. Here a mode multiplexer is also included for characterization.
these arrayed waveguides have adiabatic tapers at both ends connecting to the input/output FPRs. The end-width and the length for these tapers are 1.0 and 10 μm, respectively. The AWGs are designed to have three input ports and the central input port is connected with one of the output ports for the four-channel mode demultiplexer, while the other two are idle to be used as the input port for characterizing the AWG demultiplexers. For the fabrication, the following processes are included: (1) an E-beam lithography for patterning the waveguides and the grating couplers; (2) an ICP etching process to etch the top silicon layer down to buried oxide layer; and (3) thin PMMA film spanned to cover the wafer as the upper-cladding. Figure 2 shows the microscope image of the fabricated chip, which includes a 4 × 1 mode multiplexer, a 2.36-μm-wide multimode bus waveguide, and a hybrid demultiplexer consisting of a 1 × 4 mode demultiplexer and four 16-channel AWG demultiplexers
Fig. 3. Measured responses of 1–16 channel of AWG #i (i 1–4) normalized with the transmission response of a straight waveguide on the same chip: (a) i 1, (b) i 2, (c) i 3, and (d) i 4.
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so that the characterization for the mode-demultiplexing can be processed. Here we use fully etched focus grating couplers to achieve a convenient and efficient fiber-chip coupling for TM polarization. The grating couplers are etched as deeply as the waveguides so that single step etching process is needed. The grating coupler has a period of 1.01 μm, the duty-circle of about 0.8, and the length of 40 μm. The measured fiber-chip coupling loss is around 7 dB at 1550 nm.
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For the characterization setup, a tunable laser (Agilent 81940A) and a power meter (Agilent 8163A) as the light source and receiver, were used. The tuned wavelength range of the laser is from 1520 to 1610 nm. The polarization state of the input light is controlled by a polarization controller, and the single-mode fibers for the input/output are tilted with an angle of ∼10°. Figures 3(a)–3(d) show the measured spectral responses from all 16 output ports of AWG #1, #2, #3,
Fig. 4. Normalized measured response of the photonic integrated circuit (for the hybrid demultiplexer), including a four-channel mode-multiplexer, four-channel mode-demultiplexer, and four 16-channel AWGs with the light input at Ii : (a) i 1, (b) i 2, (c) i 3, and (d) i 4.
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and #4, respectively, when light is launched from the idle input port (see Fig. 2) of these AWGs in the hybrid demultiplexer. These responses, shown in Figs. 3(a)– 3(d), are normalized with the transmission response of a straight waveguide on the same chip. According to the property of an AWG, these measured spectral responses should be similar to the results measured when light is launched from the central input port (which is the case for the hybrid demultiplexer). The main difference between them is that there is a wavelength shift (∼3.2 nm). Because the wavelength for the central channel shifts to 1541 nm because of the fabrication deviation, the wavelengths for the edge channels (#15 and #16) are expected to be 1519.8 and 1516.6 nm, which are out from the range of the tunable laser used (1520–1610 nm). Therefore, only the 1–14th channels are shown in the designed free spectral region (FSR) while the fifteenth and sixteenth channels with central wavelengths around 1575 nm are shown in another FSR. We also note that the performances of four AWGs with identical designs have some differences caused by the fabrication deviation. The excess losses of the central wavelengths for the four AWGs are about 6.2, 5.8, 6.4, and 6.5 dB. The channel crosstalks for them are around 9.5, 14, 11, and 9.5 dB. The AWG’s performances can be improved by perfecting the fabrication as well as the designs [20]. For the characterization of the fabricated hybrid demultiplexer, the signals with different wavelengths are launched from the input port I 1 , I 2 , I 3 , and I 4 of the 4 × 1 ADC-based mode multiplexer one by one and then coupled to the TM2 , TM0 , TM1 , and TM3 modechannel in the 2.36-μm-wide multimode bus waveguide, respectively. After propagating through a 200-μm-long multimode bus waveguide, light is first demultiplexed to four groups of signals by the 1 × 4 mode demultiplexer of the present hybrid demultiplexer. Here the signals (with different wavelengths) carried by the same guided-mode are dropped to the same output port by the mode multiplexer and then demultiplexed by the followed AWG demultiplexer. The signals with different wavelengths are output from the output ports of the AWG demultiplexer separately and coupled to the fiber. Figures 4(a)–4(d) show the measured normalized spectral responses of the fabricated hybrid demultiplexer when light is launched at input port I 1 , I 2 , I 3 , and I 4 , respectively. From these figures, it can be seen that one has the dominant responses from the ports of AWG #i when light is launched from port I i (i 1 ∼ 4). The crosstalk observed at the output ports of AWG #j (j ≠ i) is caused by the undesired mode coupling in the 1 × 4 mode demultiplexer. It can be seen that the crosstalk between the mode-channels is about −16 to − 25 dB, which is similar to that for a single 1 × 4 mode demultiplexer reported in [11]. The spectral responses of the wavelengthchannels are similar to those of a discrete AWG demultiplexer, which benefits from the broad band response of the ADC-based mode demultiplexer. The excess losses of the central wavelength-channels for the hybrid demultiplexer are around 7 dB, which is comparable to that of the discrete AWGs. Therefore, it can be induced that the AWG loss is the dominant loss source for the present
hybrid demultiplexer. It is also shown that the ADCbased mode demultiplexer and the AWG demultiplexers work together very well with excellent compatibility. In summary, a hybrid demultiplexer consisting of a 1 × M ADC-based mode demultiplexer and M AWG demultiplexers with N channels has been proposed to enable WDM and MDM simultaneously for realizing an ultralarge capacity optical-interconnect link. As an example, we have demonstrated a 64-channel hybrid demultiplexer based on SOI nanowires by combining a 1 × 4 mode demultiplexer based ADCs and four identical AWGs with 16 channels. We have shown that the excess loss and the crosstalk for the fabricated hybrid multiplexer are about 7 dB, and −10 dB, respectively, which can be reduced by improving the fabrication processes. The present hybrid demultiplexer is scalable to have more channels by utilizing more wavelengths, modes, and even polarizations in the future. Other WDM filters (such as planar concave gratings and microring resonators) can also be applied for the hybrid demultiplexer. This project was partially supported by an 863 project (no. 2011AA010301), the Natural Science Foundation of China (nos. 6141101056, 11374263, and 61422510), Zhejiang provincial grant (2011C11024). References 1. D. Dai and J. E. Bowers, Nanophotonics 3, 283 (2014). 2. Y. Kawaguchi and K. Tsutsumi, Electron. Lett. 38, 1701 (2002). 3. T. Uematsu, Y. Ishizaka, Y. Kawaguchi, K. Saitoh, and M. Koshiba, J. Lightwave Technol. 30, 2421 (2012). 4. J. Xing, Z. Li, X. Xiao, J. Yu, and Y. Yu, Opt. Lett. 38, 3468 (2013). 5. N. Riesen and J. D. Love, Appl. Opt. 51, 2778 (2012). 6. J. D. Love, R. W. C. Vance, and A. Joblin, Opt. Quantum Electron. 28, 353 (1996). 7. W. Chen, P. Wang, and J. Yang, Opt. Express 21, 25113 (2013). 8. M. Greenberg and M. Orenstein, Opt. Express 13, 9381 (2005). 9. S. Bagheri and W. M. J. Green, in Proceedings of IEEE Group IV Photonics Conference (IEEE, 2009), pp. 166–168. 10. D. Dai, in Asia Communications and Photonics Conference, (IEEE, 2012), paper ATh3B.3. 11. D. Dai, J. Wang, and Y. Shi, Opt. Lett. 38, 1422 (2013). 12. Y. Ding, J. Xu, F. Da Ros, B. Huang, H. Ou, and C. Peucheret, Opt. Express 21, 10376 (2013). 13. H. Qiu, H. Yu, T. Hu, G. Jiang, H. Shao, P. Yu, J. Yang, and X. Jiang, Opt. Express 21, 17904 (2013). 14. J. Wang, S. He, and D. Dai, Laser Photon. Rev. 8, L1 (2014). 15. J. Wang, P. Chen, S. Chen, Y. Shi, and D. Dai, Opt. Express 22, 12799 (2014). 16. L.-W. Luo, N. Ophir, C. P. Chen, L. H. Gabrielli, C. B. Poitras, K. Bergmen, and M. Lipson, Nat. Commun. 5, 3069 (2014). 17. J. Driscoll, C. Chen, R. Grote, B. Souhan, J. Dadap, A. Stein, M. Lu, K. Bergman, and R. Osgood, Opt. Express 22, 18543 (2014). 18. Y. Yang, Y. Li, Y. Huang, and A. Poon, Opt. Express 22, 22172 (2014). 19. W. Bogaerts, P. Dumon, D. Van Thourhout, D. Taillaert, P. Jaenen, J. Wouters, S. Beckx, V. Wiaux, and R. Baets, IEEE J. Sel. Top. Quantum Electron. 12, 1394 (2006). 20. S. Pathak, D. Van Thourhout, and W. Bogaerts, Opt. Lett. 38, 2961 (2013).
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Silicon hybrid demultiplexer with 64 channels for wavelength/mode-division multiplexed on-chip optical interconnects Jian Wang, Sitao Chen, and Daoxin Dai* Centre for Optical and Electromagnetic Research, State Key Laboratory for Modern Optical Instrumentation, Zhejiang Provincial Key Laboratory for Sensing Technologies, Zhejiang University, Zijingang Campus, Hangzhou 310058, China. *Corresponding author: [email protected] Received October 13, 2014; revised November 18, 2014; accepted November 19, 2014; posted November 20, 2014 (Doc. ID 224844); published December 15, 2014 A monolithically integrated 64-channel hybrid demultiplexer on silicon is demonstrated experimentally to enable wavelength-division-multiplexing and mode-division-multiplexing simultaneously for realizing an ultra-large capacity optical-interconnect link. The present hybrid demultiplexer consists of a four-channel mode multiplexer realized with three cascaded asymmetrical directional-couplers and four identical arrayed-waveguide gratings (AWGs) with 16 channels. For the fabricated hybrid multiplexer, the excess loss and the crosstalk are about −7 and −10 dB, respectively. Better performances can be achieved by minimizing the imperfections (particularly in AWGs) in the fabrication processes. The present hybrid demultiplexer is scalable to have more channels by utilizing more wavelengths, modes, and polarizations. © 2014 Optical Society of America OCIS codes: (130.3120) Integrated optics devices; (030.4070) Modes; (060.4230) Multiplexing. http://dx.doi.org/10.1364/OL.39.006993
As the demand for bandwidth increases rapidly to satisfy more and more data services, optical interconnects with ultra-high capacity are desired and advanced multiplexing technologies have been explored to expand the link capacity by introducing more channels [1]. Particularly, wavelength-division-multiplexing (WDM) is one of the most successful multiplexing technologies realized by utilizing multiple wavelengths. However, when many wavelength-channels are involved, lots of laser sources are needed accordingly and the wavelength management becomes complicated and expensive. In recent years, the multimode space-division-multiplexing (SDM) technology utilizing multiple mode channels has been paid much attention in terms of increasing the capacity of a singlewavelength carrier not only for long-haul fiber optical communications but also for on-chip optical interconnects. For a multimode SDM optical-interconnect link, one of the essential key components is mode demultiplexer to demultiplex different mode-channels, which can be realized by using multimode interference (MMI) couplers [2,3], adiabatic mode-evolution couplers [4], asymmetrical Y-junction [5–7], as well as asymmetrical directional couplers (ADCs) [5–16]. Among them, the ADC-based mode demultiplexer has advantages, including the broad wavelength band, the ultra-compact footprint, and the flexible scalability for realizing more mode channels. Previously, a four-channel TM mode multiplexer with a low loss and crosstalk has been demonstrated with cascaded ADCs in our group [11]. To greatly increase the capacity of an opticalinterconnect link to even Peta-bit/s, hybrid multiplexing technology has been proposed [10,11] by making several multiplexing technologies combined together compatibly. For example, an eight-channel hybrid multiplexing system enabling the polarization-division-multiplexing (PDM) and mode-division-multiplexing (MDM) simultaneously has been proposed and demonstrated [14,15]. As the WDM are mature, it is of interest to develop a hybrid multiplexing technology to enable the MDM 0146-9592/14/246993-04$15.00/0
and WDM simultaneously. Recently some MDM-WDM links with a few modes and wavelengths have been demonstrated with various types of mode demultiplexers, as shown in Table 1. For example, in [16] the mode demultiplexer based on ADC-assisted microrings was used to realize a 2-mode × 3-wavelength MDM-WDM link with a wavelength channel spacing Δλch as large as 15–16 nm. In [17], a mode multiplexer based on an asymmetrical Y-junction was used to realize a 2-mode × 3-wavelength MDM-WDM optical link with Δλch 8–10 nm. More recently a SiN mode demultiplexer based on ADC-assisted microrings was demonstrated for realizing an MDMWDM link [18]. However, the total channel number is very limited and the channel spacing in these experiments is large (∼10 nm; see Table 1). In this Letter, we propose and demonstrate a silicon 64-channel hybrid demultiplexer by combining a mode demultiplexer and four arrayed-waveguide gratings (AWGs) for the first time to the best of our knowledge. For the present case, we use our 1 × 4 mode demultiplexer based on cascaded ADCs, which is broadband and thus WDM-compatible. The four AWGs have a channel spacing as small as 3.2 nm. These AWGs are designed identically to have the same central wavelength for the wavelength-channels (λ1 ; λ2 ; …; λN ). As an example, our silicon hybrid Table 1. WDM-MDM Hybrid Demultiplexing Technology Reference No.
[16]
[17]
[18]
Mode-channel 2 2 3 number λ-channel number 3 3 1 15, 16 8, 10 / Δλch (nm) λ-channel / / / crosstalk (dB) Mode-channel −22, −18, ∼ − 30 −10, crosstalk (dB) −12 −15, −24 Excess loss (dB) 3–16 / / © 2014 Optical Society of America
This work 4 16 ∼3.2 −10 ∼ 14 ∼ − 20 ∼5
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Fig. 1. Configuration of the hybrid demultiplexer, including a four-channel mode demultiplexer and four 16-channel AWGs. Here a mode multiplexer is also included for characterization.
demultiplexer has a mode multiplexer with four modechannels and four AWGs with 16 wavelength-channels so that one has 64 channels to be demultiplexed in total (see Fig. 1). A silicon-on-insulator (SOI) platform with a 220-nm-thick silicon core layer on a 2-μm-thick buried oxide layer is chosen here and the nanowire waveguides have a 310-nm-thick PMMA upper-cladding for protection. Here the 4-channel mode demultiplexer is designed for TM polarization and realized by three cascaded ADCs consisting of a single-mode narrow waveguide and a wide bus waveguide. As demonstrated before [11,14], here the width of the narrow waveguide are chosen as w 400 nm to be single-mode while the width of the wide waveguide are chosen optimally according to the phase matching condition, i.e., neff 10 neff 2i , where neff 10 is the effective index of the fundamental mode for the narrow waveguide, neff 2i is the effective index of the ith higher-order mode for the wide waveguide. Since the effective indices of the guided modes in the wide bus waveguide are discrete, the phase-matching condition can be satisfied for only one mode. Consequently mode-selective coupling process is obtained, which is the fundamental mechanism for the design of ADC-based mode demultiplexers. More details can be found in Ref. [11]. In order to demultiplex the wavelength-channels, we use ultrasmall SOI-nanowire AWG, which is preferred for the case with more channels in comparison with other wavelength-division-multiplexer, like microring resonators, Mach–Zehnder lattice filters, etc. [19]. For the SOI-nanowire AWGs, which are also designed to work with TM polarization, there are 16 channels (N ch 16) and the channel spacing is Δλch 3.2 nm (400 GHz). The diffraction order is chosen as m 13 to make the free spectral range (FSR) be larger than the product N ch Δλch 51.2 nm so that 16 channels are available. The path difference between the adjacent arrayed waveguides is ΔL 11.55 μm, calculated with the formula ΔL mλc ∕neff , where λc is the central wavelength, and neff is the effective index of the fundamental mode in the arrayed waveguide. The gap between the adjacent arrayed waveguides at the end connecting with the free propagation region is chosen as 500 nm to avoid the lag effect of the etching process. The end separation between the output waveguides is about 2.43 μm and the length of the free propagation regions (FPRs) is 100 μm. In our design, the waveguide for the waveguide-array has a cross-section 500 nm × 220 nm to be single-mode and
Fig. 2. Optical microscope image of the fabricated hybrid demultiplexer. Here a mode multiplexer is also included for characterization.
these arrayed waveguides have adiabatic tapers at both ends connecting to the input/output FPRs. The end-width and the length for these tapers are 1.0 and 10 μm, respectively. The AWGs are designed to have three input ports and the central input port is connected with one of the output ports for the four-channel mode demultiplexer, while the other two are idle to be used as the input port for characterizing the AWG demultiplexers. For the fabrication, the following processes are included: (1) an E-beam lithography for patterning the waveguides and the grating couplers; (2) an ICP etching process to etch the top silicon layer down to buried oxide layer; and (3) thin PMMA film spanned to cover the wafer as the upper-cladding. Figure 2 shows the microscope image of the fabricated chip, which includes a 4 × 1 mode multiplexer, a 2.36-μm-wide multimode bus waveguide, and a hybrid demultiplexer consisting of a 1 × 4 mode demultiplexer and four 16-channel AWG demultiplexers
Fig. 3. Measured responses of 1–16 channel of AWG #i (i 1–4) normalized with the transmission response of a straight waveguide on the same chip: (a) i 1, (b) i 2, (c) i 3, and (d) i 4.
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so that the characterization for the mode-demultiplexing can be processed. Here we use fully etched focus grating couplers to achieve a convenient and efficient fiber-chip coupling for TM polarization. The grating couplers are etched as deeply as the waveguides so that single step etching process is needed. The grating coupler has a period of 1.01 μm, the duty-circle of about 0.8, and the length of 40 μm. The measured fiber-chip coupling loss is around 7 dB at 1550 nm.
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For the characterization setup, a tunable laser (Agilent 81940A) and a power meter (Agilent 8163A) as the light source and receiver, were used. The tuned wavelength range of the laser is from 1520 to 1610 nm. The polarization state of the input light is controlled by a polarization controller, and the single-mode fibers for the input/output are tilted with an angle of ∼10°. Figures 3(a)–3(d) show the measured spectral responses from all 16 output ports of AWG #1, #2, #3,
Fig. 4. Normalized measured response of the photonic integrated circuit (for the hybrid demultiplexer), including a four-channel mode-multiplexer, four-channel mode-demultiplexer, and four 16-channel AWGs with the light input at Ii : (a) i 1, (b) i 2, (c) i 3, and (d) i 4.
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OPTICS LETTERS / Vol. 39, No. 24 / December 15, 2014
and #4, respectively, when light is launched from the idle input port (see Fig. 2) of these AWGs in the hybrid demultiplexer. These responses, shown in Figs. 3(a)– 3(d), are normalized with the transmission response of a straight waveguide on the same chip. According to the property of an AWG, these measured spectral responses should be similar to the results measured when light is launched from the central input port (which is the case for the hybrid demultiplexer). The main difference between them is that there is a wavelength shift (∼3.2 nm). Because the wavelength for the central channel shifts to 1541 nm because of the fabrication deviation, the wavelengths for the edge channels (#15 and #16) are expected to be 1519.8 and 1516.6 nm, which are out from the range of the tunable laser used (1520–1610 nm). Therefore, only the 1–14th channels are shown in the designed free spectral region (FSR) while the fifteenth and sixteenth channels with central wavelengths around 1575 nm are shown in another FSR. We also note that the performances of four AWGs with identical designs have some differences caused by the fabrication deviation. The excess losses of the central wavelengths for the four AWGs are about 6.2, 5.8, 6.4, and 6.5 dB. The channel crosstalks for them are around 9.5, 14, 11, and 9.5 dB. The AWG’s performances can be improved by perfecting the fabrication as well as the designs [20]. For the characterization of the fabricated hybrid demultiplexer, the signals with different wavelengths are launched from the input port I 1 , I 2 , I 3 , and I 4 of the 4 × 1 ADC-based mode multiplexer one by one and then coupled to the TM2 , TM0 , TM1 , and TM3 modechannel in the 2.36-μm-wide multimode bus waveguide, respectively. After propagating through a 200-μm-long multimode bus waveguide, light is first demultiplexed to four groups of signals by the 1 × 4 mode demultiplexer of the present hybrid demultiplexer. Here the signals (with different wavelengths) carried by the same guided-mode are dropped to the same output port by the mode multiplexer and then demultiplexed by the followed AWG demultiplexer. The signals with different wavelengths are output from the output ports of the AWG demultiplexer separately and coupled to the fiber. Figures 4(a)–4(d) show the measured normalized spectral responses of the fabricated hybrid demultiplexer when light is launched at input port I 1 , I 2 , I 3 , and I 4 , respectively. From these figures, it can be seen that one has the dominant responses from the ports of AWG #i when light is launched from port I i (i 1 ∼ 4). The crosstalk observed at the output ports of AWG #j (j ≠ i) is caused by the undesired mode coupling in the 1 × 4 mode demultiplexer. It can be seen that the crosstalk between the mode-channels is about −16 to − 25 dB, which is similar to that for a single 1 × 4 mode demultiplexer reported in [11]. The spectral responses of the wavelengthchannels are similar to those of a discrete AWG demultiplexer, which benefits from the broad band response of the ADC-based mode demultiplexer. The excess losses of the central wavelength-channels for the hybrid demultiplexer are around 7 dB, which is comparable to that of the discrete AWGs. Therefore, it can be induced that the AWG loss is the dominant loss source for the present
hybrid demultiplexer. It is also shown that the ADCbased mode demultiplexer and the AWG demultiplexers work together very well with excellent compatibility. In summary, a hybrid demultiplexer consisting of a 1 × M ADC-based mode demultiplexer and M AWG demultiplexers with N channels has been proposed to enable WDM and MDM simultaneously for realizing an ultralarge capacity optical-interconnect link. As an example, we have demonstrated a 64-channel hybrid demultiplexer based on SOI nanowires by combining a 1 × 4 mode demultiplexer based ADCs and four identical AWGs with 16 channels. We have shown that the excess loss and the crosstalk for the fabricated hybrid multiplexer are about 7 dB, and −10 dB, respectively, which can be reduced by improving the fabrication processes. The present hybrid demultiplexer is scalable to have more channels by utilizing more wavelengths, modes, and even polarizations in the future. Other WDM filters (such as planar concave gratings and microring resonators) can also be applied for the hybrid demultiplexer. This project was partially supported by an 863 project (no. 2011AA010301), the Natural Science Foundation of China (nos. 6141101056, 11374263, and 61422510), Zhejiang provincial grant (2011C11024). References 1. D. Dai and J. E. Bowers, Nanophotonics 3, 283 (2014). 2. Y. Kawaguchi and K. Tsutsumi, Electron. Lett. 38, 1701 (2002). 3. T. Uematsu, Y. Ishizaka, Y. Kawaguchi, K. Saitoh, and M. Koshiba, J. Lightwave Technol. 30, 2421 (2012). 4. J. Xing, Z. Li, X. Xiao, J. Yu, and Y. Yu, Opt. Lett. 38, 3468 (2013). 5. N. Riesen and J. D. Love, Appl. Opt. 51, 2778 (2012). 6. J. D. Love, R. W. C. Vance, and A. Joblin, Opt. Quantum Electron. 28, 353 (1996). 7. W. Chen, P. Wang, and J. Yang, Opt. Express 21, 25113 (2013). 8. M. Greenberg and M. Orenstein, Opt. Express 13, 9381 (2005). 9. S. Bagheri and W. M. J. Green, in Proceedings of IEEE Group IV Photonics Conference (IEEE, 2009), pp. 166–168. 10. D. Dai, in Asia Communications and Photonics Conference, (IEEE, 2012), paper ATh3B.3. 11. D. Dai, J. Wang, and Y. Shi, Opt. Lett. 38, 1422 (2013). 12. Y. Ding, J. Xu, F. Da Ros, B. Huang, H. Ou, and C. Peucheret, Opt. Express 21, 10376 (2013). 13. H. Qiu, H. Yu, T. Hu, G. Jiang, H. Shao, P. Yu, J. Yang, and X. Jiang, Opt. Express 21, 17904 (2013). 14. J. Wang, S. He, and D. Dai, Laser Photon. Rev. 8, L1 (2014). 15. J. Wang, P. Chen, S. Chen, Y. Shi, and D. Dai, Opt. Express 22, 12799 (2014). 16. L.-W. Luo, N. Ophir, C. P. Chen, L. H. Gabrielli, C. B. Poitras, K. Bergmen, and M. Lipson, Nat. Commun. 5, 3069 (2014). 17. J. Driscoll, C. Chen, R. Grote, B. Souhan, J. Dadap, A. Stein, M. Lu, K. Bergman, and R. Osgood, Opt. Express 22, 18543 (2014). 18. Y. Yang, Y. Li, Y. Huang, and A. Poon, Opt. Express 22, 22172 (2014). 19. W. Bogaerts, P. Dumon, D. Van Thourhout, D. Taillaert, P. Jaenen, J. Wouters, S. Beckx, V. Wiaux, and R. Baets, IEEE J. Sel. Top. Quantum Electron. 12, 1394 (2006). 20. S. Pathak, D. Van Thourhout, and W. Bogaerts, Opt. Lett. 38, 2961 (2013).
