We propose and fabricate a novel circuit that combines two two-dimensional (2D) grating couplers and a microring resonator (MRR). According to the polarization states, one 2D grating coupler first splits the input signals into two orthogonal paths, which co-propagate in the loop and share a common MRR, and then the two paths are combined together by the other 2D grating coupler. The proposed circuit is polarization insensitive and can be used as a polarization insensitive filter. For demonstration, the wavelength division and polarization division multiplexing (WDM-PDM) non return-to-zero differential-phase-shift-keying (NRZ-DPSK) signals can be demodulated successfully. The bit error ratio measurements show an error free operation, reflecting the good performance and the practicability.
© 2013 Optical Society of America
It is urgent to increase the spectral efficiency (SE) in optical transport networks to meet the explosive growth of information exchange all over the world. For this reason, the advanced modulation formats combined with various multiplexing techniques have been employed in nowadays optical fiber communication systems [1–3]. Among all these multiplexing techniques, the polarization division multiplexing (PDM) has attracted more and more interest as it can double the SE by carrying two independent data streams on two orthogonal polarization states . Although many previous reports have proposed and demonstrated the integrated transceivers for the PDM signal modulation and detection [5–8], there is few previous report on the corresponding PDM signals processing using the silicon based integrated devices. Only a few schemes demonstrated the relative all optical PDM processing utilizing the discrete devices [9–11]. On the other hand, the silicon photonics provide a good platform for the all-optical signal processing and integration thanks to the complementary metal oxide semiconductor (CMOS) compatible fabrication technology, the high refractive index contrast, the ultra-compact size and so on. However, the high refractive index contrast also introduces large birefringence , which contributes to the great difficulty for practical application. To achieve a polarization insensitive operation on silicon, the device should be specifically designed to support TE and TM modes simultaneously . Alternatively, the polarization diversity technique can be utilized assisting by the polarization rotator [14–16]. Many existing polarization diversity reports were based on the horizontal coupling. The horizontal coupling benefits from low loss and large bandwidth, while on the other hand requires high alignment accuracy and complex fabrication process. The device facets should be polished and the coupling can only be achieved at the edge of the chip. Furthermore, an additional polarization beam splitter/polarization rotator should be designed and fabricated, in addition to a coupler. By contrasting, the two-dimensional (2D) grating coupler is an outstanding candidate for simultaneous coupling and polarization diversity, which couples the two orthogonal modes from a single-mode fiber into the identical modes but two different paths of the waveguide [17–19], resulting in the polarization insensitive operation.
In this paper, we propose and fabricate a compact silicon based polarization insensitive filter which consists of two 2D grating couplers and a microring resonator (MRR) in a loop configuration. One 2D grating coupler is used as the input coupler and polarization beam splitter (PBS), while the other is used as the polarization beam combiner (PBC) and output coupler. Being different from the previous reports, one MRR connects the two 2D grating couplers functioning as the signal filtering/processing unit, where the clockwise and counter clockwise lights are simultaneously utilized. For demonstration, dual-channel PDM non return-to-zero differential-phase-shift-keying (NRZ-DPSK) signals are demodulated using the proposed scheme successfully. The bit error ratio (BER) measurements show an error free operation. The proposed scheme can also be used for other PDM signal processing functions, such as the format conversion  and the ultra-wideband pulses generation .
2. Principle and device description
The principle of the proposed scheme is illustrated in Fig. 1. The PDM signals with two orthogonal polarizations (X- and Y-pol) are vertically coupled into the 2D grating coupler. By optimizing the input polarization states, the two orthogonal polarizations are coupled into the corresponding two orthogonal waveguides, both in TE mode. The two TE modes, which carry the information from X- and Y-pol, propagate in different paths in the circuit. A MRR is inserted to process the signals clockwise and counter clockwise independently. Since the two signals are both in TE modes and two polarizations share different port of the MRR, the MRR can process the PDM signals identically and simultaneously.
A 3D-FDTD method  is applied to analyze the performance of the 2D grating coupler. The parameters of the 2D grating coupler had been investigated , and we refer and optimize the parameters according to our fabrication process and application. All boundary conditions are set to be the PML (perfect matching layer). The light is vertically injected into the 2D grating coupler in the simulation. As shown in Fig. 2, the signals with three different polarization states are vertically coupled into the 2D grating coupler. Figures 2(a) and 2(b) show the electric field distributions when the input signals are X- and Y-polarized, respectively. According to the electric field distributions, it can be seen that the input X-pol will be coupled to propagate along the Y axis, which means that it will be changed to Y-pol after propagating through the circuit and coupling back to the output fiber. Similarly, the input Y-pol will be output X-pol. Although the polarizations of two tributaries are exchanged, the polarization orthogonality is preserved, meaning the ability for PDM signals processing. The crosstalk between the two different waveguide is lower than −20 dB from the simulation. Figure 2(c) shows the situation that the input signal is with a polarization oriented 45°. The 2D grating coupler here functions as a 50:50 power splitter to couple the input signals into two paths with equal power. In Fig. 3, the power coupled into the circuit is monitored and normalized when the polarization state changes. The polarization angle is referred to the angle between the polarization axis and X-pol. It can be seen that the total power coupled into the circuit by the 2D grating coupler remains unchanged when the input polarization axis rotating gradually from X-pol to Y-pol. The same conclusion can be found at the output, where another 2D grating coupler is utilized. This further demonstrates the circuit is polarization insensitive. It should be noted that the 1 dB bandwidth of the 2D coupler is usually narrow, due to the inherent characteristics, as many previous paper demonstrated. In the system application, the relatively narrow bandwidth might restrict the total capacity, especially in WDM system with more channels. However, we experimentally measured the bandwidth, and it can be found that the 1 dB bandwidth is about 25 nm, while the 3 dB bandwidth is about 60 nm. Although the bandwidth is narrower compared with other coupling schemes, it is sufficient for the WDM system covering the whole C-band. Many previous papers have demonstrated the 2D photonic crystal coupler based products, including the coherent receivers. The coupling loss between the fiber and 2D grating coupler, which is the main loss of the circuit, is measured to be about 5 dB for each 2D grating coupler. It is inherently larger compared with other coupling schemes, such as the horizontal coupling. It is true that the system performance in terms of the optical signal to noise ratio (OSNR) will be degraded if the loss is quite large. However, the loss in our demonstration is reasonable and the demodulation performance is acceptable, which can be validated by the clear eye diagrams and the BER measurements in the experimental part. On the other hand, it will be possible to significantly reduce the loss by optimizing the parameters of the 2D grating coupler, as demonstrated in Ref , where the coupling loss of the 2D grating coupler is only 2.7 dB. As for the MRR part, the principle and design is well-known and the design rule can be referred to our previous paper .
The layout of the circuit is shown in Fig. 4(a). It was fabricated by the electron beam lithography (EBL) and inductively coupled plasma (ICP) etching. A SOI wafer with top silicon layer of 220 nm and SiO2 layer of 3 µm was used. The Figs. 4(b)-4(e) show the scanning electron micrograph (SEM) top view of the 2D grating coupler and the MRR. The grating coupler is a square array of round holes with an etch depth of 90 nm. The diameter of the holes are 270 nm and the lattice period is 580 nm. For the other parts, the top silicon layer has been etched until the oxide layer. As to the MRR, the free spectral range (FSR) is designed to be 100 GHz, so that the radius can be calculated as 100 µm. The gap between the straight and bended waveguides for the coupling region is 250 nm. In order to reduce the footprint of the circuit, four S-bends are designed to connect 2D grating coupler and MRR. For a better fiber coupling efficiency, the 2D grating coupler were designed to be with a large area (12µm by 12µm holes array), resulting in the large width of the waveguide connected to the 2D grating coupler. In contrast, the width of the SOI waveguide is about 450 nm to ensure a single TE mode condition. As a result, the waveguide should be tapered from 12 µm to 450 nm, and a relatively long waveguide is induced. On the other hand, the radius of the MRR is 100 µm, and cannot match the distance between the two output waveguides from the 2D grating coupler. Hence, the waveguide should be bent over to the MRR. Considering the path uniformity of the clockwise and counter clockwise signals and the bending loss, the four S-bends with such a shape were designed and utilized in our design. A broadband light source is applied to measure the transmission spectrum of the proposed filter. The transmission spectrum remains almost the same with three different input polarization states, which coincides with the results of simulation, as shown in Fig. 5. Furthermore, multiple channels operation can be possible by properly choosing the input channel spacing, due to the periodic transmission characteristics of the MRR. To verify the performance of the fabricated circuit, the WDM-PDM NRZ-DPSK demodulation at a total rate of 100 Gb/s based on the available experimental facility is demonstrated in the following session.
3. Experimental setup and results
The experimental setup is shown in Fig. 6. Two CW lights at 1553.3 and 1554.1 nm are coupled and split into two Mach-Zehnder modulators (MZMs) driven by two independent bit pattern generators (BPGs) to obtain two independent NRZ-DPSK signals. The driven data are PRBS 231-1 at 25 Gbaud. The polarization states of the two signals are optimized by the polarization controllers (PC1 and PC2) and combined by a PBC, forming the WDM-PDM NRZ-DPSK signals. Using the vertical fiber coupling, the signals are then coupled into the chip, assisting by another PC (PC3) to match the two orthogonal polarizations in the fiber and the polarization axes of the 2D grating coupler in the silicon waveguide. If the circuit is not used for the PDM signal processing, no matter whether polarization-maintaining fibers (PMFs) are used, the PC3 is not necessary. However, it is necessary to avoid the crosstalk between the two polarization states in the case of PDM processing. An optical isolator is used to eliminate the reflection. At the output 2D grating coupler, the processed signals are amplified by the Erbium-doped fiber amplifier (EDFA) and attenuated by the attenuator (ATT). The output can be detected with the assistance of PC4 and a PBS. All the PCs are necessary to ensure a good PDM signals multiplexing and demultiplexing. A band pass filter (BPF) is used for the wavelength demultiplexing, and an optical spectrum analyzer (OSA) and a communication signal analyzer (CSA) are also used for monitoring. The operation principle for the DPSK demodulation, i.e. the conversion from NRZ-DPSK to pseudo RZ (PRZ), had been analyzed in previous work . One notch of the comb filter aims at the center wavelength of the input signal, resulting a destructive interference to achieve the phase to intensity conversion. Figure 7 shows the spectral evolution of the PDM signal processing. Thanks to the polarization insensitive and periodic filtering characteristics, the dual-channel and dual-polarization signals can be processed identically. The eye diagrams of the input PDM NRZ-DPSK signals, the demultiplexed output X-pol and Y-pol signals at each wavelength are shown in Fig. 8(a). It is shown that the demodulated PRZ in two polarizations show clear and open eyes, despite of a little asymmetry, which attributes to the relative large Q factor of the MRR compared with the conventional delay interferometer demodulator. A larger Q factor will result in a long falling edge and thus an asymmetric eye diagram. This phenomenon had been found in previous demonstrations . If the Q factor of the MRR can be achieved to an optimal value in our application, the demodulation performance can be further improved.
In order to evaluate the performance of the proposed circuit, the BER measurements are performed for the four demodulated signals respectively, by performing the polarization and wavelength demultiplexing. Results are plotted in Fig. 8(b). The error free operation can be obtained for all the signals.
In conclusion, we have proposed and fabricated a polarization insensitive circuit that can be used for all optical WDM-PDM signals processing. For demonstration, the WDM-PDM NRZ-DPSK signals demodulation had been successfully achieved with error free. Arising from this idea, many other polarization insensitive devices for various PDM signal processing functions can be achieved by replacing the MRR with other signal processors, and we believe that these devices can be advantageous in monolithic integrated circuit and the characteristics are sufficient for PDM signal processing in the future optical transport networks.
This work was supported by the National Basic Research Program of China (Grant No. 2011CB301704), National Natural Science Foundation of China (Grant No. 61007042, 61275072 and 61125501), and the Fundamental Research Funds for the Central Universities (Grant No. HUST2012QN104). The authors thank the Center of Micro-Fabrication and Characterization (CMFC) of WNLO for device fabrication.
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