An improved 8-channel silicon mode demultiplexer is realized with TE-type and TM-type grating polarizers at the output ends, and these gratings serve as fiber-chip couplers simultaneously. The present 8-channel silicon mode demultiplexer includes a three-waveguide PBS (for separating the TE0 and TM0 modes) and six cascaded ADCs (for demultiplexing the high-order modes of both polarizations). The grating polarizers with high extinction ratios are used to filter out the polarization crosstalk in the 8-channel hybrid multiplexer efficiently and the measured crosstalk for all the mode-channels of the improved 8-channel mode multiplexer is reduced greatly to ~−20dB in a ~100nm bandwidth.
© 2014 Optical Society of America
In order to satisfy the increasing demand for optical interconnect capacity, various advanced multiplexing techniques have been developed and applied for optical fiber communication networks. It is well known that wavelength-division-multiplexing (WDM) is one of the most successful multiplexing techniques. In recent years, the capacity of WDM-based optical communication is going to be saturated and the spatial-division-multiplexing (SDM) technique  is re-activated by introducing multi-core fibers [1–3] as well as few-mode fibers [4–7]. The SDM channels carrying different data share the same wavelength and thus only one laser diode (LD) with a fixed wavelength is needed. When combining the WDM and SDM technologies, the total channel numbers can be increased significantly to improve the link capacity. The SDM technique also helps to reduce the system cost because fewer laser diodes are needed. The SDM technology is also becoming attractive for optical interconnects in data centers which are bandwidth hungry and cost sensitive. It might be more convenient to upgrade an existing network or install a new network with the SDM technology for short-distance data communications in comparison with long-distance optical networks because the installation of new fiber infrastructure over a long distance is very complicated . Particularly, for photonic networks-on-chip, optical signals propagate along planar optical waveguides and the control of light propagation / conversion are not difficult with some specific on-chip waveguide structures, which makes it be promising to introduce on-chip SDM technology and one of the potential applications is the interconnect for the multi-processor or interchips. In comparison to the multi-core SDM, the multimode SDM  provides a way to be more concise and compact photonic integrated circuits (PICs) as only one multimode bus waveguide is included and the guided-modes carrying different data are overlapped spatially in the multimode bus waveguide. The conciseness of the PICs will help make the circuit design easy and convenient. It is even possible to realize a promising approach of combining the multi-core and multimode multiplexing technologies in the future in order to achieve ultra-high link capacity.
In a multimode SDM system, low-loss and low-crosstalk mode (de)multiplexer is one of the most important devices. Various structures have been proposed to realize mode (de)multiplexer [8–15]. In , a two-channel mode multiplexer is proposed by using a complicated structure with multimode interference couplers and phase shifters. However, this design is inconvenient and inflexible to be extended for more mode-channels. A mode multiplexer with more flexibility is using adiabatic mode-evolution couplers and the popular structures include adiabatic Y-branches  as well as adiabatic directional couplers . With this design, one can achieve low-loss and low-crosstalk mode (de)multiplexer at the expense of large footprints. Alternatively, the mode (de)multiplexers based on cascaded asymmetrical directional couplers (ADCs) [8, 12] have a small footprint and high scalability for more mode channels. In our previous paper, a 4-channel mode multiplexer with low loss and low crosstalk has been demonstrated theoretically and experimentally for TM polarization . Grating-assisted ADCs have also been used to realize mode multiplexers , which however has a relatively large footprint due to the weak coupling of grating structures. When combining ADCs with microring resonators, a WDM-compatible multimode SDM technology has been realized . More recently, we have proposed and realized an 8-channel hybrid demultiplexer enabling the multimode SDM and polarization-division-multiplexing (PDM) technologies simultaneously by utilizing ADCs and a polarization beam splitter (PBS) . According to the experimental results demonstrated in , we note that the dominant crosstalks for the TM0, TM1, TM2 and TE3 mode-channels are from some orthogonal polarization modes. In this paper, we give a comprehensive analysis for the crosstalk in the 8-channel hybrid demultiplexer and propose a solution to significantly reduce the polarization crosstalk by introducing optical polarizers at the end of each port. An improved 8-channel hybrid demultiplexer with low crosstalk is demonstrated by introducing TE-type or TM-type grating polarizer at the output ends. The grating polarizer also serves as an efficient fiber-chip coupler. Our experimental results show that the maximum crosstalk of the demonstrated 8-channel hybrid multiplexer is reduced greatly from −11dB to be ~−20dB in a ~100nm wavelength bandwidth.
2. Structure and analysis
Figure 1 shows the 8-channel silicon hybrid demultiplexer including a three-waveguide PBS and six cascaded ADCs . The PBS is used to separate the fundamental modes for TE and TM polarizations (TE0 and TM0) while the six ADCs are designed to demultiplex the high-order modes of both polarizations (i.e., the TE1, TE2, TE3, TM1, TM2, and TM3 modes). According to the phase matching condition, the parameters for the PBS and the ADCs are chosen optimally, as shown in Ref. . The TE3, TE2, TE1, and TE0 modes are dominantly output from ports O1~O4 respectively while the TM0, TM1, TM2, and TM3 modes are dominantly output from ports O5~O8, respectively.
Figures 2(a)–2(h) show the calculated transmission responses at the eight output ports (O1~O8) of the designed hybrid demultiplexer, respectively when all of the modes (TE3, TE2, TE1, TE0, TM0, TM1, TM2, and TM3 modes) are launched from the input port of the bus waveguide at the left, respectively. Here a three-dimensional finite-difference time-domain (3D-FDTD) method is used for the calculation. One should note that some higher-order modes might be cut-off as the bus waveguide is tapered down. Therefore, the crosstalk from the cut-off higher-order modes to the following output ports will be negligible. For example, when the TM3 mode is launched from the input port of the bus waveguide, it will be dominantly dropped by the first ADC (which is designed for the TM3 mode) and received by the O8 port (as shown in Fig. 2(h)) while there is a small part of the TM3 mode power resident in the bus waveguide (due to the fabrication deviations). This resident power is still carried by the TM3 mode and propagates forward. When it goes through the following adiabatic taper, the bus waveguide will become too narrow to support the TM3 mode and the resident power carried by the TM3 mode becomes radiated. In this case, little crosstalk will be introduced to the following ports. Therefore, here the transmissions to some output ports are too low to be shown in Figs. 2(a)–2(h).
From Figs. 2(a)–2(h), it can be seen that the crosstalk from orthogonal polarization modes is dominant for some output ports. For example, the dominant crosstalk for the TM2 mode-channel (port O7) comes from the TE3 mode-channel and the polarization crosstalk is ~−11dB @1550nm, as shown in Fig. 2(g). The reason is that the second-stage ADC designed for the TM2 mode does not have a significant phase-mismatch between the TE3 mode in the bus waveguide and the TE0 mode of the access waveguide. Consequently, some part of power carried by the TE3 mode is dropped to port O7 by the second-stage ADC designed for the TM2 mode. From Fig. 2(a) we also note that the crosstalk from the launched TM2 mode in the bus waveguide to port O1 (which is for the TE3 mode channel) is much lower (<−20dB @1550nm) because most power of the TM2 mode has been dropped by the second-stage ADC before it arrives at the third ADC working for the TE3 mode-channel. Consequently very little power carried by the TM2 mode-channel arrives at the third ADC and thus little crosstalk is introduced to the TE3-mode channel.
Since the dominant crosstalk is from the orthogonal polarization mode, we realize that such polarization crosstalk can be filtered out by introducing polarization-selective devices (such as polarizers and PBSs) at the end of the output ports of the mode (de)multiplexer. As it well known, various polarizers [16–20] and PBSs [21–24] have been realized on SOI platform. In this paper, we use high extinction-ratio TE-type and TM-type polarizers based on grating, which also serve as fiber-chip couplers simultaneously. According to the design given in Ref. , the gratings have a duty cycle of 0.5 and an etching depth of 70nm. And the optimized grating periods are 0.63μm and 1.0μm for the TE- and TM-type polarizers, respectively. Figures 3(a) and 3(b) show the calculated transmissions of the designed TE-type and TM-type grating polarizers when the TE0 or TM0 mode is launched with a 10° incident angle, respectively. For this calculation, a 3D-FDTD simulation is used. From Figs. 3(a) and 3(b), it can be seen that the designed TE-type and TM-type grating polarizers have high extinction ratios over a broad band. The extinction ratio at the central wavelength (~1550nm) is higher than 25dB and 30dB for the TE-type and TM-type polarizers respectively. This is favorable to filter out the polarization crosstalk and significantly improve the performance of the 8-channel hybrid demultiplexer. The performance of the grating polarizers can be even improved further by some specific designs as demonstrated in Refs. [26, 27].
3. Fabrication and characterization
In order to characterize the 8-channel hybrid multiplexer with grating polarizers, we designed and fabricated a PIC including the 8-channel hybrid multiplexer, a 100μm-long multimode waveguide and a hybrid demultiplexer on a silicon-on-insulator wafer which has a 220nm-thick top-silicon upon a 2μm-thick buried oxide layer. The fabrication processes include: (1) An E-beam lithography patterning for the waveguides; (2) An ICP etching process to etch the top silicon layer down to buried oxide layer; (3) A second E-beam lithography patterning for the grating polarizers (couplers); (4) A shallow-etching process with 70nm etching-depth for the grating patterns; (5) PECVD deposition for the 2.3μm-thick SiO2 upper-cladding. Figure 4 shows the microscope image of the fabricated hybrid (de)multiplexer with grating polarizers. Each single-mode access waveguide has a TE or TM grating polarizer at the end, and a 300μm-long adiabatic taper is used to connect the 10μm-wide grating and the 500nm-wide single-mode access waveguide.
Generally speaking, the lithography process is one of the most important steps for the fabrication of any photonic integrated device to control the waveguide width within the fabrication tolerance to achieve good performances as designed. For the 8-channel hybrid multiplexer, the simulation results given in Ref.  show that the 8-channel hybrid multiplexer without polarizers has a fabrication tolerance of ± 5~10nm for the waveguide width when the crosstalk is required to be lower than −10dB. In our experiment, the deviation of the patterning linewidth is less than ± 5nm by utilizing the E-beam lithography patterning process carefully. Furthermore, the improved 8-channel mode demultiplexer demonstrated here has a larger fabrication tolerance because of the help from the grating polarizers. For the grating polarizer, the second shallow-etching process to achieve a 70nm etching-depth is a key step. Fortunately, the extinction ratio of the grating polarizer is not very sensitive to the etching depth. For example, the grating polarizer has an extinction ratio of >20dB when the etching depth varies from 60nm to 80nm. In experiment, the etching depth can be controlled accurately by slowing down the etching rate. In our fabrication process, the etching rate is slowed down to ~2nm/s and the deviation of the etching depth is less than ± 6nm in our lab.
For the measurement, a tunable laser (Agilent 81940A) is used as the light source and a powermeter (Agilent 8163A) is used at the terminal to monitor the output. Single-mode fibers are aligned with a 10° incident angle to couple light to/from the chip. The polarization state of input light is adjusted by a polarization controller. When measuring the 8-channel hybrid multiplexer, the light is launched from an input port Ii (i = 1,…, 8) and the transmission responses at the output ports (O1~O8) are measured one by one. Figures 5(a)–5(h) show the measured transmission responsesat a fixed output port Oi (i = 1,…, 8), respectively, when light is launched from any of the input ports (I1~I8). Here these transmission responses are normalized by the transmission of a straight bus waveguide (w = 2.363μm) with TE-type or TM-type grating polarizers at both ends on the same chip. Since the output powers at some non-major output ports are beyond the power range of our powermeter, the corresponding transmission responses are not shown in Figs. 5(a)–5(h). As expected, Figs. 5(a)–5(d) show that the TE(4-i) mode in the bus waveguide is excited dominantly and dropped to output port Oi when TE-polarized light is launched from input port Ii (i = 1, 2, 3, 4). Similarly, from Figs. 5(e)–5(h), it can be seen that the TM(i−5) mode in the bus waveguide will be excited dominantly and dropped to output port Oi when TM-polarized light is launched from input port Ii (i = 5, 6, 7, 8). The measured excess losses around 1560nm are about 3.1dB, 2.2dB, 3.5dB, 0.2dB, 0.7dB, 2.1dB, 1.5dB, and 1.4dB for the TE3, TE2, TE1, TE0, TM0, TM1, TM2, and TM3 mode channels, respectively. The loss is mainly from the insufficient cross-coupling due to the fabrication deviation.
From Figs. 5(a)–5(h), it can also be seen that the present 8-channel mode demultiplexer with grating polarizers has low crosstalk. The crosstalk is defined as usual to be the difference between the powers at a fixed output port (Oi) when light is launched from the major input port (Ii) and another input port (Ij, j≠i). The dominant crosstalks (around 1560nm) for the TE3, TE2, TE1, TE0, TM0, TM1, TM2, and TM3 mode channels are about −20.8dB, −20.3dB, −18dB, −29.3dB, −36.5dB, −40.6dB, −17.7dB, and −20.9dB, respectively. The accumulated crosstalk from all the other non-major mode channels are −20.5dB, −20.2dB, −16.6dB, −29dB, −33.1dB, −38.3dB, −16.9dB, and −20.6dB for the TE3, TE2, TE1, TE0, TM0, TM1, TM2, and TM3 mode channels, respectively. And the crosstalk is insensitive to the wavelength over a broad band from 1520nm to 1620nm.
One sees that the present hybrid demultiplexer with grating polarizers has much lower crosstalk than the previous hybrid demultiplexer without grating polarizers demonstrated in . For example, for our previous hybrid demultiplexer without grating polarizers, there is a −11dB crosstalk from the TE3 mode channel to the TM2 mode channel (see Fig. 2(g)). In contrast, for the present hybrid demultiplexer with grating polarizers, the crosstalk from the TE3 mode channel to the TM2 mode channel is not observed. The present improved hybrid demultiplexer with low-crosstalk helps the realization of a mode-multiplexed multi-channel optical interconnect link in the future. It is also possible to use any other on-chip polarizer or PBS [16–24] to filter out the polarization crosstalk of the hybrid multiplexer particularly when the mode (de)multiplexer is integrated monolithically with other functionality elements (e.g., the sources and photodetectors) on the same chip. For the present mode (de)multiplexer with grating polarizers, it might be useful for the case of hybrid integration with some active photonic devices (e.g., photodetectors) bonded on the chip since the grating polarizer can simultaneously serve as an efficient coupler between the passive and active components.
In summary, we have demonstrated an improved 8-channel hybrid demultiplexer with the assistance of TE-type and TM-type grating polarizers (which also serve as fiber-chip couplers). The experimental results have shown that the grating polarizers with high extinction ratios filter out the polarization crosstalk and the crosstalk of the 8-channel hybrid demultiplexer is reduced greatly. The dominant crosstalks (around 1560nm) from the TE3, TE2, TE1, TE0, TM0, TM1, TM2, and TM3 mode channels are about −20.8dB, −20.3dB, −18dB, −29.3dB, −36.5dB, −40.6dB, −17.7dB, and −20.9dB, respectively. And the crosstalk is insensitive to the wavelength from 1520nm to 1620nm. On-chip polarizers or PBSs can also be used to filter out the polarization crosstalk of the hybrid demultiplexer when needed.
This project was partially supported by a 863 project (No. 2011AA010301), the Nature Science Foundation of China (No. 11374263), Zhejiang provincial grant (Z201121938), the Doctoral Fund of Ministry of Education of China (No. 20120101110094).
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