To extend the operation wavelength range of dual-polarization optical hybrids (DPOH), we propose a highly symmetrical interferometer design for a polarization beam splitter and an optical hybrid to reduce temperature and wavelength dependence. The design successfully decreases this dependence, and a fabricated DPOH with silica-based planar lightwave circuits provides temperature-insensitive performance with a polarization extinction ratio of over 25 dB and phase errors of less than 3 degrees over the entire C- and L-bands.
© 2011 OSA
Advanced modulation formats using polarization multiplexing and multi-level phase modulation, such as a dual-polarization quadrature phase shift keying (DP-QPSK) and 16 QAM, are widely investigated, and they are considered a very promising technology for providing large capacity photonic transport networks. In such systems, integrated coherent receivers are key devices for intradyne detection with local oscillators. These receivers are composed of an optical passive circuit, photodiodes and trans-impedance amplifiers [1–4]. The optical passive circuits are key components and include a polarization beam splitter (PBS) to divide an input optical signal into two orthogonal polarization components, and 90-degree optical hybrids (OH) to mix the two polarization components with the local oscillator light, and they must be integrated in one chip to realize a compact receiver. Such an integrated circuit is called a dual-polarization optical hybrid (DPOH) and it determines several important characteristics of the integrated coherent receiver, including the polarization extinction ratio (PER), the phase error of optical hybrids, the insertion loss and the common mode rejection ratio (CMRR).
Several materials including InP and silicon have been investigated for use in DPOHs. A silica-based planar lightwave circuit (PLC) can provide excellent optical passive performance characteristics such as a low loss, good stability, and high reliability, and enable us to realize a DPOH [5,6]. We have reported a silica-based PBS with a high PER [7,8] and an optical hybrid with a very low phase error . The silica-based DPOH is considered a promising candidate for the optical passive circuit of an integrated coherent receiver.
To further improve the characteristics of the silica-based DPOH, in this paper we propose new designs for both a polarization beam splitter and an optical hybrid. The proposed designs are based on highly symmetrical interferometers, and can eliminate the temperature and wavelength dependence of the PER and phase error. Reducing this dependence allows us to investigate the extension of the operational wavelength range of DPOH from the C-band to the entire CL-band. We report fabrication results for a newly designed silica-based DPOH for an integrated receiver [10–12].
Silica-based PLCs are suitable for realizing various interferometers, and a PBS can also be realized by employing a Mach-Zehnder interferometer (MZI) with a birefringence adjuster such as a stress release groove [5,7] (Fig. 1 (a) ) and a wide core [6, 8]. This adjuster provides light with a 180-degree phase shift between the TE and TM modes, and this leads to polarization dependent performance. However, such adjusters change not only the birefringence but also the mean refractive index of the waveguide. As a result, additional phase tuning is needed for PBS operation. To eliminate the phase offset caused by such an index change, the length difference between the waveguides arms has to be tuned. Accordingly, the MZI becomes an asymmetric interferometer. Therefore, owing to the temperature dependence of the refractive index of waveguides, the PER of the previously reported PBS is temperature dependent and the operational range of the PBS is limited to the C-band.
Figure 1(b) shows our proposed configuration for a silica-based PBS. It is a symmetrical interferometer with a Y-branch and a 2 x 2 multimode interference (MMI) coupler, where the interferometer arms each incorporate a polyimide quarter waveplate, one tilted at 0 degrees and the other tilted at 90 degrees. The two quarter waveplates make the optical path difference (OPD) −90 and 90 degrees for the TE and TM inputs, respectively, in a push-pull manner without adding any phase bias. As the two outputs from the Y-branch are in-phase, the OPD becomes 0 and 180 degrees for TE and TM light, respectively, as a result of the 90-degree shift at the 2 x 2 coupler. Therefore, the proposed symmetrical configuration can split an input into TE and TM components. In such a symmetric MZI, the phase change owing to the temperature dependence of the waveguides and waveplates can be eliminated between the arms, so the temperature dependence of a PER can be reduced. It should be noted that the proposed configuration does not require precise control of the waveguide birefringence, and is applicable to a wide variety of waveguides.
An optical hybrid [5,6,10] can also be realized with a symmetrical configuration. Figure 2 (a) and (b) show the configurations of a conventional and our proposed optical hybrid, respectively. The conventional optical hybrid consists of four 2 x 2 couplers and a quarter-wavelength delay. As the quarter-wavelength delay provides a 90-degree phase shift, the phase error is wavelength dependent. We have reported that this wavelength dependent phase error can be suppressed by selecting the input ports as shown in Fig. 2(a). However, the configuration requires very precise effective refractive index control to set the phase shift at 90 degrees. In contrast, our proposed configuration replaces one of the 2 x 2 couplers with a 1 x 2 Y-branch and eliminates the quarter-wavelength delay. This configuration gives a 90-degree phase shift via phase rotation at the 2 x 2 couplers in a similar manner to our proposed PBS configuration. Therefore, the phase error is very tolerant as regards fabrication parameters, and is stable over wide wavelength and temperature ranges.
Figure 3 shows the calculated wavelength dependence of phase shifters based on the 2 x 2 MMI and the delay line. Both are designed for the 90-degree phase shifter. The delay line is a simple phase shifter, but its phase changes linearly along the wavelength. So the phase error of the delay line is about 6 degrees over 100 nm around a wavelength of 1575 nm. On the other hand, the 2 x 2 MMI has a much smaller phase error of about 0.4 degrees. This error is 10 times smaller than that of the delay line. Therefore, the MMI is more suitable for the 90-degree phase shifter, and is also more tolerant against fabrication errors, such as refractive index and waveguide width errors. By using this MMI as a phase shifter, our proposed PBS and optical hybrid can expand the operational wavelength range.
We used our proposed PBS and optical hybrid to design a DPOH for an integrated receiver . The layout of the designed DPOH is shown in Fig. 4 . The signal input is divided into X (TM) and Y (TE) polarization components at the PBS with the symmetrical configuration. The X polarization component is then converted from TM light to TE light at the polyimide half waveplate. The local input is divided into two at the beam splitter (BS). The signal and local light are mixed at the symmetrical 90-degree optical hybrids and the interference signals exit from the chip. In the coherent receiver, the input waveguides are directly connected to optical fibers, and the output waveguides are coupled to photodiodes via a lens. Therefore, we set the respective widths of the input and output waveguides at 11 μm to suppress the fiber coupling loss and at 5 μm to relax the tolerance of the optical lens systems. The distance between polarization outputs X and Y was set at 10 mm to match the RF interface of the coherent receiver.
3. Fabrication results
We fabricated the designed DPOH using a silica-based PLC with a relative index difference of 1.5%. We inserted three kinds of waveplates in a groove formed across the waveguides (Fig. 5 ).
First, we evaluated the wavelength dependence of the DPOH performance from a signal input to outputs. Our proposed PBS successfully exhibited a high PER of better than 25 dB over the C- and L-bands (1530-1620 nm) (Fig. 6(a) ). The loss of the DPOH was less than 9 dB including a 6 dB intrinsic loss at the optical hybrid without SMF fiber coupling loss (Fig. 6(b)). The differences between the losses of the positive and negative ports from the output coupler were sufficiently small at less than 0.2 dB, and this helped the integrated coherent receiver to realize an excellent CMRR.
Figure 6 (c) shows the phase characteristics of the optical hybrid. To measure the phase error, we attached an extra delay line circuit and a Y-branch splitter in front of the input ports, and estimated the phase error from the transmission spectra . The estimated phase difference was almost 90 degrees, and the phase error over the C- and L-bands was less than 1 degree. Note that this OPOH did not undergo any phase tuning, such as UV trimming.
Next, we focused on the temperature dependence of our DPOH. Figure 7 (a) shows the temperature dependence of the PER. For comparison, we also measured that of a conventional PBS with a stress release groove. The conventional PBS based on the asymmetrical MZI had a large temperature dependence. On the other hand, our proposed PBS based on the symmetrical MZI maintained a high PER of more than 25 dB over a wide temperature range. Such stable characteristics contributed to the low temperature dependence of the loss. The conventional temperature-sensitive PBS changes both its PER and its loss, so the total loss of the DPOH was also temperature sensitive. Figure 7(b) shows the temperature dependence of the loss. Our DPOH maintained a low loss over a wide temperature range, and its deviation was less than 0.2 dB. We also investigated the phase difference at the 90-degree OH. Figure 7 (c) shows the phase deviation from that at a temperature of 30 degrees, and the estimated phase deviation was less than 2 degrees. Our symmetrical design for the OH contributed to both the fabrication tolerance and the temperature-independent performance. From these experimental results, our proposed DPOH can maintain a high performance level over a wide wavelength and temperature range.
We proposed highly symmetrical designs for a PBS and a 90-degree optical hybrid, and these designs successfully reduced the temperature dependence of a DPOH using a silica-based PLC. The PER exceeded 25 dB, and the loss was less than 9 dB. Our optical hybrid can maintain a low phase error of less than 3 degrees. These excellent operational characteristics can be achieved over the C- and L-bands over wide temperature range. We believe that our results will contribute greatly to the realization of a cost-effective and compact coherent receiver with ultra-wideband operation.
This work is supported by the R&D project on “High-speed Optical Edge Node Technologies” of the Ministry of Internal Affairs and Communications (MIC) of Japan.
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