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We demonstrate a compact and variable-optical-attenuator (VOA) integrated coherent receiver with a silica-based planar lightwave circuit (PLC). To realize the compact receiver, we integrate a VOA in a single PLC chip with polarization beam splitters and optical 90-degree hybrids, and employ a stable optoelectronic coupling system consisting of micro lens arrays and photodiode (PD) subcarriers with high-speed right-angled signal lines. We integrate a VOA and a coherent receiver in a 27x40x6 mm package, and successfully demodulate a 128-Gbit/s polarization division multiplexed (PDM) quadrature phase shift keying (QPSK) signal with a VOA-assisted wide dynamic range of more than 30 dB.
©2012 Optical Society of America
1. Introduction
Digital coherent optical transmission technology, which enables us to compensate for linear transmission impairments including chromatic dispersion (CD) and polarization mode dispersion (PMD), as well as to utilize spectrally-efficient modulation formats such as phase shift keying (PSK) and quadrature amplitude modulation (QAM), has opened a new era of optical communication systems [1]. Intense efforts are now worldwide to deploy 100-Gbit/s polarization division multiplexed (PDM) quadrature phase shift keying (QPSK) modulation and digital coherent reception. The deployment has started in long haul transmission systems, and will be used in metro networks as well as for datacom applications. The transponder size is important as regards shorter reach applications. The size is currently limited by two factors; the power consumption of electronic devices and the size of optics. The power consumption problem will be overcome by using sophisticated digital signal processing algorithms and an advanced LSI fabrication process. Optoelectronic integration is the key to the optics size problem.
An integrated coherent receiver is a key optoelectronic device in a digital coherent transponder. The receiver has various functional parts including a polarization beam splitter (PBS), a polarization rotator, dual-channel optical 90-degree hybrids, high-speed photodiode (PD) arrays and trans-impedance amplifiers (TIAs). Although there have been several attempts at the monolithic integration of these devices [2–5], the hybrid integration of a device based on optimized material seems to be a promising way of achieving improved performance, and of fully enjoying the merits of coherent detection, at least at the moment [6–9]. We have therefore studied coherent receivers [7,8] consisting of silica-based planar lightwave circuit (PLC) dual-polarization optical hybrids [10], InP/InGaAs photodiode (PD) arrays [11,12] and InP HBT trans-impedance amplifiers (TIAs) [13], as we believe that this combination offers the best performance, including high sensitivity, low noise, low total harmonic distortion, a good common-mode rejection ratio (CMRR), a small phase error and a high polarization extinction ratio (PER).
There are two approaches for reducing the effective size of a coherent receiver, namely reducing the size of the receiver itself and integrating more optical devices in the receiver. The former approach involves using a higher refractive index material system for optical passives. However, this had a detrimental effect on the phase error. Instead, in this paper, we propose the use of a compact optoelectronic coupling system. In contrast to our previously reported two-lens optical coupling systems with micro-lens arrays and mirrors [8], we propose employing one micro-lens optical coupling systems with PD subcarriers that have high-speed right-angled signal lines, which enables us to shrink the optical path and stabilize the optical coupling. In addition to the compact optoelectronic coupling system, in the latter approach, we propose the monolithic integration of a variable optical attenuator (VOA), which is frequently used with a receiver to adjust the signal power for widening the dynamic range of the receiver, in the silica PLC chip. With these two approaches, we successfully integrate a VOA-integrated receiver in a 27 x 40 x 6 mm area, which is based on the Optical Internetworking Forum (OIF) implementation agreement [14].
2. Design of coherent receiver
The schematic configuration of our proposed coherent receiver is shown in Fig. 1 . The receiver consists of a silica-based PLC, 4-ch InP/InGaAs PD arrays on subcarriers and 2-ch InP HBT TIA arrays. The components are hermetically sealed in a surface mount technology (SMT) package. The PLC chip is equipped with a 7% tap coupler for a monitor PD, a VOA based on the thermo-optic effect, a Mach-Zehnder interferometer type PBS, a half waveplate as a polarization rotator, and two 90-degree optical hybrids for X- and Y-polarizations. The “symmetric” designs are applied to both the PBS and the optical hybrids to improve wavelength and temperature dependences [10]. The VOA, featuring a symmetric Mach-Zehnder interferometer as shown in Fig. 2 , is designed to have a normally closed configuration to reduce its polarization dependent loss (PDL). The PLC chip size is 15 x 19 mm. The monitor PD is directly attached to the edge of the PLC chip.
Figures 3(a) and 3(b), respectively, show side and top views of the optoelectronic coupling system we used in the receiver. We installed the high-speed 4-ch PD arrays on subcarriers with signal lines for right-angled surface-mounting, and wire-bonded them to the TIAs. In other words, we bent electrical paths instead of bending optical paths with mirrors [7, 8], since it reduces the optical coupling distance between a PLC and a PD to a few hundred micrometers and enables us to use a one-lens system instead of the two-lens system used in [8]. We employed tilted graded-index lens arrays for the collimation as shown in Fig. 3(b), and tilted the collimation path to achieve a high return loss. The lens pitch was precisely controlled and the pitch error was less than 1 micrometer. The micro mirror arrays and PD subcarriers were directly attached to the PLC chip, thus making the proposed coupling system stable against temperature change and stress-induced deformation of the package. The wire-bonding condition was optimized so as not to damage the PD subcarriers-PLC junction.
The package size was 27 x 40 x 6 mm, which is based on the Optical Internetworking Forum (OIF) implementation agreement [14]. It should be noted that the size is basically limited by the DC (1.27-mm pitch, 40 pins) and RF interfaces (1-mm pitch, 20 pins), and could be reduced by using narrower interfaces.
3. Experimental results
We used conventional silica-based PLC technology with a relative index difference of 1.5% for the fabrication. The PER was better than 20 dB over the C-band (1530-1570 nm), and the phase error was less than 5 degrees without any phase trimming, thanks to the “symmetric design” we developed [10]. The loss differences between the positive and negative ports were less than 0.2 dB.
Figures 4(a) and 4(b) show the electrical power dependent attenuation and attenuation dependent PDL, respectively. As we used a normally-off VOA design, the attenuation decreased when we increased the applied electrical power to the thin heater. The power consumption for the full open state was 300 mW. The PDL increased with the attenuation, and was 1 dB at an attenuation of 15 dB. We believe the PDL can be improved to less than 0.5 dB by modifying the circuit layout [15].
Fig. 4 Characteristics of VOA, (a) applied electrical power dependent attenuation, and (b) attenuation dependent PDL.
We fabricated a PLC sub-assembly with two 4-ch PD arrays and measured its frequency response through the signal lines at the top of the subcarrier. The measured 3-dB bandwidth was over 25 GHz, and we confirmed the applicability of the right-angled signal lines on the PD subcarrier to the high-speed electrical signal.
The PLC sub-assembly was then integrated with two dual-channel TIA arrays in the package. The responsivities of the fabricated receiver for the signal and local inputs were 0.09 A/W including a 6-dB splitting loss and 0.07 A/W including a 9-dB splitting loss, respectively. Figure 5 shows the temperature dependent loss deviations for the signal input, where we eliminated the temperature dependence of the PD responsivity. The measured loss deviation was less than 0.3 dB, and we confirmed that our proposed optical coupling systems had good stability. Thanks to the stable optical coupling system and good loss uniformity of PLC-based optical hybrids, the measured CMRR was about 30 dBe, which is applicable to colorless detection without an optical filter. Figure 6 shows the O/E response of the fabricated receiver. The 3-dB bandwidth exceeded 22 GHz, and we observed no degradation caused by the right-angled SMT type subcarriers.
We evaluated the demodulation performance of our fabricated coherent receiver with 128 Gbit/s PDM-QPSK signals. The experimental setup and the measured Q values as a function of signal input power are shown in Fig. 7 , where the OSNR is 16 dB and the local oscillator power is 13.5 dBm. Without the VOA operation, the Q-value deteriorated for signal input powers higher than −5 dBm due to the limited dynamic range of the TIA. On the other hand, by adjusting the signal input power with the VOA, the Q value was successfully maintained within +/− 0.5 dB for a signal input power of −20 to 10 dBm.
4. Conclusion
We proposed a compact and stable optoelectronic coupling system with micro lens arrays and right-angled SMT type PD subcarriers. We fabricated a VOA integrated coherent receiver with the proposed system, and successfully confirmed the expansion of the dynamic range of the receiver up to 30 dB for 128-Gbit/s PDM-QPSK signals.
Acknowledgments
We thank S. Kodama, E. Yoshida, I. Ogawa and M. Yoneyama for helpful discussions and encouragement.
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