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We report the design and fabrication of a hybrid-integration-type coherent receiver. We optimize the receiver building blocks, and achieve a −25-dB common-mode rejection ratio and a 20-dB signal input power dynamic range.
©2011 Optical Society of America
1. Introduction
To meet the continuously growing demand for increased optical transport network capacity, the combination of polarization division multiplexed (PDM) multi-level modulation formats, such as quadrature phase shift keying (QPSK) or quadrature amplitude modulation (QAM), and coherent detection with digital signal processing (DSP) has been attracting great interest. The key to receiving multi-level modulation formats is to use a coherent receiver with high performance levels, including a wide dynamic range, an excellent common-mode rejection ratio (CMRR), a high polarization extinction ratio (PER), a high responsivity and a small phase error at optical hybrids. In addition, the coherent receiver needs to be compact for implementation in a transceiver module.
A coherent receiver consists of several functional optical and electronic devices, and there is an optimal material for each device in terms of performance. Silica-based planar lightwave circuits (PLCs) or bulk optics are commonly used for optical passive circuits. Photodiode (PD) arrays are usually realized with InP-based materials, and transimpedance amplifiers (TIAs) are realized with SiGe BiCMOS or InP HBT. Several attempts at the monolithic integration of these devices have been reported [1–5]. Monolithic integration can eliminate the optical alignment process. However, at least with current technology, the hybrid integration of an optimized-material-based device seems to be a promising way of achieving improved performance, and of enjoying the merits of coherent detection. In this paper, we discuss the optimum building block design for achieving a high performance at the receiver level, and report the fabrication of a coherent receiver employing the hybrid integration of these devices.
2. Design
2.1 Overview
Figure 1 shows the schematic configuration of our integrated coherent receiver. We use a silica-based planar lightwave circuit (PLC) for the optical passive circuit. All the functions of the optical passive circuits, including a polarization beam splitter (PBS), a beam splitter (BS), a polarization rotator and 90-degree optical hybrids, are integrated into one chip PLC. As for the O/E conversion parts, four InP/InGaAs PDs are integrated into one chip, and a dual-channel InP-based HBT TIA is integrated into another chip. The PD and TIA chips for each polarization state are hermetically sealed into a chip-scale packaged O/E converter (CSP-OE) [6]. The PLC outputs are coupled to the CSP-OE with bulk lenses. These components are installed in a 27 x 50 x 8 mm non-hermetic package with surface mount type RF leads [7].
2.2 Silica-based PLC
Figure 2 shows the schematic layout of the silica-based PLC chip for the receiver frontend. The input signal light is divided into two orthogonal polarization states by the PBS. The locallight is also divided into two by the BS. The two lights are mixed at the optical hybrids, and the in-phase and out-of-phase components are coupled to the PDs. We have applied “symmetric” designs to both the PBS and the optical hybrids [8], to eliminate wavelength and temperature dependence. The PBS features a Mach Zehnder interferometer with polyimide quarter waveplates tilted at 0- and 90-degrees, respectively, inserted in the two arms. As regards the optical hybrid, we have eliminated the optical path difference, and used 90-degree phase rotation at the MMI couplers. The PLC design is described in detail in [8]. The output field diameter is 7 μm. To make the RF connection simple, the spacing between the two optical hybrids is set at 10 mm. The waveguide layout is folded for compactness, and the chip width and length are 19 and 12.8 mm, respectively
2.3 Photodiodes
For coherent detection, in addition to a wide bandwidth and a high sensitivity, an operation under high input optical power is required for the PDs. In order to address these issues, we employed a composite-field maximized induced current PD (composite field MIC-PD) [9]. As the composite-field MIC-PD has a high electrical field in its depleted region as well as a small junction capacitance comparing with conventional MIC-PD [10], it attains a wide bandwidth with a small bias voltage even with a high input optical power. Figure 3 shows the 3-dB bandwidth of the conventional and composite-field MIC-PDs. The diameter of the PD was set at 19 μm to give a large optical coupling tolerance. The composite-field MIC-PD has a 3-dB bandwidth of more than 35 GHz at a reverse bias voltage of 1.5 V and a photocurrent of 2 mA.
2.4 Transimpedance amplifiers
The dual-channel TIA is fabricated with highly reliable 1-μm InP HBT technology [11], whose current cut-off frequency (fT) is 170 GHz. Its power supply voltage is designed to be +3.3 V, which is commonly used in transponder modules. Figure 4 shows the TIA circuit diagram. The operation mode of the TIA can be switched between auto-gain control (AGC) and manual-gain control (MGC) modes. In addition, the TIA is equipped with an output amplitude adjustment function, an output shutdown function and a peak amplitude monitor.
To achieve a wide dynamic range, we gave the variable gain amplifier in the AGC loop (VGA1 in Fig. 4) a cascaded design. VGA1 consists of a 1st variable gain amplifier, a 1st fixed gain amplifier, a 2nd variable gain amplifier, and a 2nd fixed gain amplifier. By using two variable gain amplifiers we attain a large variable gain range of 40 dB, which enables us to realize a constant output amplitude for a wide input signal range. Furthermore, to maintain good input-output linearity, we set the gain of a following amplifier larger than that of the one preceding it. This gain setting prevents the voltage amplitude from being too large in the cascaded amplifiers, and it provides good input-output linearity.
3. Characteristics of receivers
We fabricated the key building blocks including a silica-based PLC, InP/InGaAs PD arrays and InP HBT TIAs. We then integrated these key building blocks into a package. The effective responsivities of the fabricated module for the signal and local input ports were better than 0.093 A/W including a 6-dB splitting loss and 0.054 A/W including a 9-dB splitting loss, respectively. The phase error was better than 1 degree over the entire C-band. Figure 5 shows the measured temperature dependent effective responsivities when we input X-polarization light into the signal port. Thanks to a PER of better than 30 dB, the photocurrent at the Y-polarization ports is well suppressed and dominated by dark current at ambient temperatures of −5, 25 and 80 °C.
Figure 6 shows the measured CMRR for the signal port as a function of frequency. Thanks to a precisely adjusted coupling ratio at the optical hybrids and an optimized optical lens system design, the CMRR was better than −25 dB at up to 25 GHz. We then evaluated our fabricated coherent receiver with 32 Gbaud PDM-QPSK signals. Figure 7(a) shows the experimental set-up. The four outputs of the coherent receiver (XI, XQ, YI, and YQ) were digitized at 50 GS/s using a digital oscilloscope. Figures 7(b) and (c) show the measured Q-values with the OSNR and input signal power parameters, respectively. Here, we change the signal input power for different OSNRs of 16 and 18 dB in Fig. 7(c). The local oscillator power was set at +13.5 dBm. As shown in the figure, the Q-value was within +/− 0.5 dB for a signal input power of −20 to 0 dBm. Our coherent receiver does not require the optical power of the input signal to be adjusted, and thus can eliminate the need for a variable optical attenuator.
Fig. 7 Measured Q-value. (a) Experimental setup, (b) OSNR dependent Q-value, (c) Q-value as a function of signal input power.
4. Conclusion
We described the design and fabrication of hybrid integration type coherent receiver. We optimized the building blocks of the coherent receiver, including optical passive circuits, PDs, TIAs and packaging, and achieved a good CMRR of better than −25 dB at a frequency as high as 25 GHz, a high PER of better than 30 dB and a wide dynamic range of 20 dB.
Acknowledgements
This work is partly supported by the R&D projects on “High-speed Optical Transport System Technologies” and “High-speed Optical Edge Node Technologies” of the Ministry of Internal Affairs and Communications (MIC) of Japan.
References and links
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