We experimentally demonstrate a high-quality phase shift keying demodulator based on a silicon photonic wire waveguide. Since the birefringence of the waveguide generates extremely huge differential group delay, an ultra-compact and high-extinction-ratio delay line interferometer is devised in TE and TM modes. We firstly calculated and simulated the requirements for propagation length and waveguide’s dimensions. Then, we measured the interference spectrum, eye pattern, bit error rate, and temperature dependence to ascertain its feasibility for DPSK demodulation. For a 2.8 cm-long wire waveguide, a free spectral range of 9.6 GHz and an error-free DPSK demodulation around 10 Gb/s are obtained.
© 2012 OSA
Various advanced modulation formats, such as multilevel phase shift keying (M-PSK) [1,2], multilevel quadrature amplitude modulation (M-QAM) , and orthogonal frequency division multiplexing (OFDM) [4,5], have captured attention in optical telecommunications fields, not only for high-capacity transmission applications but also for middle- and short- reach applications [6,7]. These formats promise great improvements of bandwidth utilization efficiency and the optical signal to noise ratio (OSNR) in comparison with the traditional on-off-keying (OOK) format. Particularly, quadrature phase shift keying (QPSK) and dual-polarization QPSK (DP-QPSK) have attracted a great deal of attention, because standardization and many major development projects for practical applications are underway. Methods for detecting PSK signals can generally be separated into two types: coherent detection and direct detection. Coherent detection provides a great improvement of the OSNR by mixing a local oscillator light into the transmitted signal light and very effective compensations for signal degradations due to chromatic dispersion (CD) and polarization mode dispersion (PMD) on the transmission line. However, a digital signal processing (DSP) unit is necessary to exploit the full potential of coherent detection. This makes these systems costly and complex, and also results in high power consumption and overheating. Thus, direct detection with delay line interferometers (DLIs) is a reasonable method for achieving M-PSK transmission, especially in metro and access networks. Previously reported DLIs have mostly been constructed from free-space optics with a Michelson interferometer , and a low-Δ silica planar lightwave circuit (PLC) or optical fibers with a Mach-Zehnder interferometer [9–12]. Since all of these devices are constructed on the basis of controlling the propagation length between the two arms, the total area is not easy to reduce due to the different direction of the arms. For example, we investigated the areas of commercial differential phase-shift keying (DPSK) demodulators for a bit rate of 10 Gb/s, which are ~5500 and ~500 mm2/ch for free-space-optics and silica PLC-based DLIs, respectively. In recent years, silicon photonics technology has become widely known as a very effective approach to resolve the issues, because the high refractive index contrast enables strong optical confinement and small bend radii with negligible excess loss. As a result, silicon rib-waveguide-based DLIs  and silicon wire waveguide-based dual-bus micro-ring resonator [14–17], have been shown to successfully work as PSK demodulators. Here we propose a polarization DLI constructed from only a single silicon photonic wire waveguide, the basic idea of which is presented in Ref , which shows potential as a simple, compact and high-quality demodulator. In this paper, we develop the concept to fully understand the basic and advanced characteristics of the device in detail for future telecommunication applications.
2. Principle and numerical estimation
First, we describe the conventional demodulation principle with free-space optics, a PLC, and other structures, for PSK formats using DLI(s). A 1-bit time slot of the modulation rate ΔΤ is expressed as
In the device design, we calculated the group refractive indices using the film mode matching method . To obtain larger B value, the cross-sectional dimension of the waveguide should be designed with a relatively high aspect ratio, such as ~2:1. When the designed core width and core thickness are 460 and 200 nm and the BOX thickness is 3 μm, B is expected to be 1.274. Figure 1 shows the calculated mode profiles, where the group refractive indices are 4.218 for the TE mode and 2.944 for the TM mode. According to the Eq. (2), the required length for 10- and 40-Gb/s DPSK demodulation is estimated to be approximately 2.4 and 0.6 cm, respectively.
3. Basic device characterization
The silicon waveguide and spot size converters (SSCs) with an inverse taper  were fabricated from a conventional SOI wafer. SiO2/SiOx was deposited by electron cyclotron resonance-chemical vapor deposition (ECR-CVD), and the structure was defined by reactive ion etching (RIE) of the SiO2/SiOx and by ECR-RIE of silicon. With our recent improvement and optimization of the fabrication processes, the propagation loss of TE and TM mode can be suppressed to around 1 dB/cm in a silicon waveguide, even if the dimensions are 400 nm × 200 nm. The fabrication process is described in more detail in Refs [21,22].
For the basic optical characterizations, we used 3-μm thermally expanded core (TEC) fiber to couple a C-band amplified spontaneous emission (ASE) light source into the silicon waveguide with SSCs. The propagation loss and coupling loss were measured as 1.2 dB/cm and 0.75 dB/facet for the TE mode and 0.7 dB/cm and 0.7 dB/facet for the TM mode. The extinction ratio (ER) of the polarization interference calculated from these measurement results is over 20 dB even in a 2.4-cm-long waveguide for 10-Gb/s demodulation.
Figures 2(a) -2(c) show the CAD layout, photographs of the fiber pigtail module and fabricated device, and a schematic diagram of the setup for measuring the interference spectra. The module contains total 75 silicon photonic wire waveguides (5 parameters of length × 5 waveguides × 3 blocks), and we choose four of them to connect with in/output fiber array blocks by epoxy adhesive. Since group delay comes from the difference between TE and TM modes, the polarization angle of input light should be adjusted to around 45 degrees with reference to the waveguide substrate by a polarization controller (PC). At the output of the module, a second PC is needed to control the polarization in order to match it with the principal axis of the polarization beam splitter (PBS), and then the constructive and destructive signal paths are separated as each polarization. When we aligned the polarization angle of in/output lights, we used optical spectrum analyzer (OSA) to make the ER maximum value. Therefore, the accurate angle would be slightly shifted from just 45 degrees, because the insertion loss for each polarization mode has difference as shown in Table 1 . Using the loss parameters in Table 1, we calculated the theoretical polarization angles of incident light and the tolerance for keeping the ER above 20 dB. For a 2.4 cm-long device (e.g. theoretical length for 10-Gb/s demodulation), the results are 41 degrees and +/−5.5 degrees, respectively. In the experiment, we used 2.8-, 1.6- and 1.15-cm-long silicon photonic wire waveguides to characterize the interference spectra from the constructive port of the PBS. The measurement results for each length around the wavelength of 1550 nm (~193.5 THz) are shown in Fig. 3 , where free spectral ranges (FSRs) of 9.6, 17.0 and 23.8 GHz and ERs of 12, 16 and 19 dB are obtained, respectively. Note that the factor limiting the observed ER is the measurement equipment: the OSA’s minimum resolution is approximately ~2 GHz. Hence, we expect the actual ER for each DLI is higher than the measured value, especially for the ER of 12 dB with the FSR of 9.6 GHz.
Calculated and measured FSRs for each device length are shown in Fig. 4 . The difference between the calculation and measurement is most likely due to structural error. We analyzed the FSR dependence of the fluctuations by simulation to discuss which parameter is dominant. In our fabrication processes, the fluctuations of both the width and thickness are lower than +/−5 nm. As a result, with a 2.4 cm-long device, for example, the FSR shift for +/−5 nm width offset is only + 2 ~-2%, while that for the thickness offset is + 11 ~-14%. The thickness of the SOI layer on a four-inch wafer actually fluctuates a little, which can be observed using a commercial laser interferometer for measuring film thickness. Therefore, the FSR difference between the calculation and measurement could mainly come from the thickness fluctuation. Another possibility is that the SSC on each side which also could cause relatively weak polarization dependence of group velocities, and they are not included in the device length when conducting the calculation.
Temperature sensitivity and controllability are important for an interferometer, especially, toward future practical use in actual systems. Although the main cause of thermal shift is the thermo-optic effect in silicon, known as a coefficient (Δn/ΔT) of 1.9 × 10−4 1/°C , the both indices of the two polarization modes are changed along the positive direction in this DLI. Thus, we have to measure the dependence of frequency shift on temperature detuning using a digital controlled heater. We chose a 2.8 cm-long silicon photonic wire waveguide for testing, because described in the next section was performed for the demodulation experiment around 10 Gb/s, which is equivalents to that length. Figure 5 shows plots with a linear fitting line, where the coefficient of 20.0 GHz/°C was measured at a wavelength of 1550 nm. When we calculated as a thermo-optic effect coefficient, we obtained 5.8 × 10−5 1/°C, which corresponds approximately to one-third compared with that of bulk silicon material. We suppose that the dominant factor is the confinement area difference between the TE and TM modes due to the structural asymmetry of the designed waveguide.
4. DPSK demodulation
Figure 6 shows a schematic diagram of the setup for signal pattern observation and bit error rate (BER) measurements for DPSK signal. We used a tunable laser diode (TLD), a pulse pattern generator (PPG), and a zero-chirp LiNbO3 DPSK modulator to generate a non-return-to-zero (NRZ) 10-Gb/s pseudorandom bit stream (PRBS) of length 211-1. The polarization angle of the light was adjusted to around 45 degrees with reference to the waveguide substrate with a polarization controller (PC). To simplify the characterization setup, we carried out this experiment with the fiber pigtail module. A second PC was placed at the output of the module to control the polarization angle, and then a PBS was placed to separate the signals into a constructive or destructive port in accordance with its polarization state. Finally, each signal was coupled into a photo detector (PD) and electrical amplifier to observe the demodulation pattern with a sampling oscilloscope or measure the BER with an error detector. Figures 7(a) - 7(c) show 10-Gb/s PRBS patterns of electrical output from the PPG, DPSK optical output from the LiNbO3 modulator, and demodulated optical output from the PBS, respectively. Figure 7(d) is the eye diagram of the demodulated signal. Clear eye opening is exhibited. Both demodulated signals [Figs. 7(c) and 7(d)] are observed from the constructive port of the PBS in this measurement. The experiment results in Figs. 7(a)-7(d) are measured with a single-ended PD, in Figs. 8 and 9 are measured with balanced PD as shown in Fig. 6.
Next, we placed several optical components, including a variable optical attenuator, band pass filter, and balanced detector, to the measurement setup to measure BER versus OSNR characteristics of DPSK modulation rate around 10.0 Gb/s with 2.8-cm-long device. Error-free operation was estimated by measuring up to a BER of 10−9 at bit rates of 9.6 and 10.0 Gb/s with PRBS length of 231-1. When comparing with the two bit rates, the OSNR penalty was observed as only 0.2 dB. Finally, we confirmed the modulation rates from 8.8 to 10.8 Gb/s versus the OSNR requirement for a BER of 10−9 using the silicon wire waveguide based demodulator (Fig. 9). With our experiment, 1-dB OSNR penalty tolerance of approximately 1.6 Gb/s (8.8 ~10.4 Gb/s) and the lowest OSNR requirement of around 9.6 Gb/s are observed, respectively. The results correspond approximately to the investigation using conventional DLI as in Ref .
As a first trial to demonstrate demodulation in a single silicon waveguide, we used two external PCs and one PBS to obtain perfect demodulation. However, note that these components were already experimentally demonstrated as integrable devices with our previous works [24–26], as a waveguide based polarization rotator (PR) and PBS (e.g., the PC has compatibility with PR). Additionally, they are very compact such as the lengths of ~20 μm and ~100 μm, respectively. Hence, even if a high-bit-rate and large-scale integrated device, such as one with 40 Gb/s × 32 channels, were constructed, we expect the whole area size would be just 48 mm2 (15 mm × 3.2 mm), assuming the total width including separation for each channel is 100 μm, including all attached devices mentioned above. Another discussion is about the polarization dependence of this device, because single polarization operation causes various inconveniences in the actual transmission system. Compared with free-space-optics based DLI, waveguides based DLIs basically require compensation for the polarization dependence due to the asymmetrical refractive indices profile. The problem can be resolved by well-known polarization diversity configuration with insertion of a half-wave plate [27,28] or polarization separation by a PBS at the demodulator input . In the case of silicon wire waveguide, the latter method is suited. Although it requires additional waveguide based PRs and PBSs, the whole area size is estimated within 76.8 mm2 (16 mm × 4.8 mm) with 40 Gb/s × 32 channels.
In addition, one of our future conceptions is that epitaxially grown germanium PDs and transimpedance amplifiers can be integrated with compact optical devices such as WDM filter  and this kind of PSK demodulator.
We proposed a silicon-based DLI for PSK demodulation using a high-aspect-ratio photonic wire waveguide with huge birefringence. In both frequency- and time-domain experiments, a perfect demodulation of 10-Gb/s DPSK signal was demonstrated in a single waveguide. Because the device layout is extremely simple and compact, the device can replace previous demodulators based on free-space optics, a PLC, and other structures. This simple principle and structure using silicon photonic technology can accomplish future M-PSK demodulation of higher bit rates.
The authors thank Mr. Toshifumi Watanabe of NTT Advanced Technology for CAD design and fabrication processes, Mr. Takashi Goh of NTT Photonics Laboratories for BER measurement and helpful discussion. They also thank Dr. Shinji Mino of NTT Photonics Laboratories and Dr. Sei-ichi Itabashi and Mr. Shin’ichiro Mutoh of NTT Microsystem Integration Laboratories for encouragement.
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