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Abstract
Fully integrated monolithic, multi-channel InP-based coherent receiver PICs and transceiver modules with extended C-band tunability are described. These PICs operate at 33 and 44 Gbaud per channel under dual polarization (DP) 16-QAM modulation. Fourteen-channel monolithic InP receiver PICs show integration and data rate scaling capability to operate at 44 Gbaud under DP 16-QAM modulation for combined 4.9 Tb/s total capacity. Six channel simultaneous operation of a commercial transceiver module at 33 Gbaud is demonstrated for a variety of modulation formats including DP 16-QAM for >1.2Tbit/s aggregate data capacity.
© 2017 Optical Society of America
1. Introduction
Fully integrated optical system-on-chip (SOC) photonic integrated circuits (PICs) directly benefit the performance, economics, power consumption, density and reliability of high capacity dense wavelength division multiplexed (DWDM) systems. Photonic integration minimizes the number of required optical and electrical assembly processes and testing requirements, resulting in significant cost reductions. Photonic integration also enables creation of super-channels that increase DWDM line card capacity and reduce operational complexity. Additionally, PICs improve system reliability by minimizing the number of optical couplings compared to discrete component systems [1].
Decades ago, significant progress was made developing coherent optics for telecommunications [2], including the first reported submarine trial using a coherent receiver in 1990 [3]. However, the market need for coherent optics was delayed by the development of the erbium doped fiber amplifier (EDFA) and DWDM systems. These advances greatly expanded reach and fiber capacity of optical systems, leading to a rapid increase in the data capacity in the network. Small scale integrated, single-channel optical chips in the form of electro-absorption modulators integrated with distributed feedback (DFB) lasers were first realized in 1986 [4] and later commercialized in the mid-1990’s. These first commercial InP PICs enabled data rates up to 10 Gb/s per wavelength using on-off-keying (OOK). Fully integrated, optical SOC 10-channel InP photonic integrated circuits (PICs) were first developed and commercialized in 2004 [5]. These PICs, the first multi-channel, fully integrated DWDM SOC, condensed >60 formerly discrete optical components and hundreds of optical couplings into a pair of PICs. In 2008, the first systems utilizing coherent modulation were deployed in telecom networks, enabled by the use of digital signal processor (DSP) chips in coherent receivers [6]. These first coherent receivers were built from discrete optical components and operated at 40 Gb/s data rates. The first fully integrated multi-channel coherent Tx and Rx PICs were developed in 2011 with 500 Gb/s PM-QPSK superchannel capability [7–9]. The photonic ICs monolithically integrated over 600 optical functions into a pair of InP chips and were the first fully integrated coherent Tx and Rx PICs and the first multi-channel coherent PICs.
Recently, 4.9 Tb/s total capacity, InP extended C-band tunable coherent Tx PICs integrating 14 channels while operating up to 44 Gbaud have been reported [10,11]. In this paper, we report on the DC and RF characterization of the extended C-Band tunable coherent Rx PICs operating at 33 and 44 Gbaud. Commercial transceiver modules integrating 6-channel Rx and Tx PICs are shown to be capable of 1.2 Tb/s of data transmission using 33 Gbaud, DP 16-QAM modulation. The monolithically integrated, extended C-Band tunable InP Rx PIC has been developed by building on multiple generations of InP transmitter and receiver PIC technology [5,7–16]. The devices presented in this paper demonstrate state-of-the-art integration of widely-tunable lasers in multi-channel InP coherent receiver PICs, realizing multi-Tb/s links as a result of the unprecedented monolithic integration level of widely tunable lasers with the requisite suite of coherent optical functions.
2. PIC Design
The receiver PIC architecture is shown in Fig. 1. Photonic IC chips with both N = 6 and N = 14 channels are described herein. The monolithic InP receiver PIC accepts two inputs polarized to be TE on chip from free space optics that rotate one signal path polarization from TM to TE. Spot size convertors are employed to ensure coupling loss from PIC-to-fiber is <2dB. Each polarization signal path is split with a 1 × N multi-mode interference coupler (MMI)-based power splitter to feed N 90° optical hybrid MMIs. The optical hybrids mix the incoming signals with one of the independently controlled, integrated widely tunable local oscillator laser outputs. MMI-based optical taps on the laser output paths are used to couple light out from the chip to measure wavelength over the extended C-band (>40 nm) with a wavemeter for characterization during development and manufacturing. Wavelengths are calibrated by sweeping the bias of mirrors and phase sections of widely tunable lasers while measuring the tapped light. Whereas earlier generations of PICs employed distributed feedback (DFB) laser sources with limited wavelength tuning range [5,7–9,12–16], the PICs described herein enable each channel on the multi-channel PIC to span the entire extended C-band (40 nm), maximizing optical channel utilization and capacity as well as enabling reconfigurability on a super-channel or on a per-channel basis. These capabilities substantially reduce network cost while improving network agility. Each local oscillator is an independently controlled, widely-tunable laser (WTL), based on a four-section DBR type design, incorporating differentially tuned grating mirrors consisting of reflection combs of different spectral spacing to achieve continuous tuning over the entire extended C-band [17]. The gratings for each mirror are patterned along the mirror lengths to produce a ~5nm reflection peak spacing, and separate gain and phase-tuning sections are provided between the mirrors. Single-ended, high speed photodiodes detect I1, I2, Q1, and Q2 data streams from each of the two hybrid MMIs corresponding to the two polarizations. The Rx PIC is designed to operate at up to 44 Gbaud data rates and support BPSK, 3QAM, QPSK, 8-QAM, and 16-QAM modulation formats. Flexible baud rates and modulation formats allow the optical transmission system to maximize data throughput for any given optical span.
Fig. 1 Multi-channel, widely tunable coherent Rx PIC architecture. The architecture includes two polarization waveguide inputs and power splitters, and each of N channels (N = 6 or 14) comprise a widely tunable local oscillator with tap, two 90° hybrid MMIs, and eight high speed photodiodes. The TM signal path is rotated by free space optics (not shown) to be polarized TE on chip.
The devices presented in this paper are fabricated via a large scale (LS) PIC integration platform as described previously [5]. Epitaxial layer structures are grown on InP substrates using metal-organic vapor phase epitaxy (MOVPE) in multi-wafer reactors. The active elements consist of multi-quantum well (MQW) active regions whereas the passive regions (waveguides and MMIs) consist of bulk double heterostructure (DH) waveguides. Conventional etch and regrowth techniques (e.g., as described in [5,7]) are utilized to monolithically integrate the multiple epitaxial layers for active devices and passive routing. Subsequently, the PIC wafers are fabricated in a back-end wafer fabrication process sequence similar in complexity to that used to form heterostructure bipolar transistor (HBT) integrated circuits [18], and the process is also analogous in complexity and similar in number of mask levels to 90 / 130nm CMOS integrated circuit processes [19,20]. The back-end wafer fabrication is performed to define the optical waveguides, form electrical isolation, contact p and n regions of the active devices, form bondpads, and facilitate passivation of the devices. The precise control of critical dimensions (e.g., waveguides) is realized via dry-etching to enable a high degree of control of such parameters as waveguide width, etch depth, sidewall roughness, etc., that are required to achieve sufficient performance over extended C-band while simultaneously maintaining low loss of all elements. Finally, conventional metallization and dielectric deposition techniques are utilized in the fabrication processes for the widely tunable PIC devices described in this paper.
3. 14-channel coherent receiver PIC performance
The performance and capability of the integrated widely tunable local oscillators on the 14-channel receiver PIC chip are obtained using lensed fibers to couple light into the edge of the chip. The lasers are calibrated by measuring a tapped portion of the light, mapping the output power and wavelength as a function of the bias conditions on the mirror sections, while keeping a constant bias on the gain section. An independent phase tuning section is used to align the laser cavity mode with the grating reflection peaks in order to achieve stable operation across the entire tuning range. Figure 2 provides a detailed view of the tunable laser operating wavelength maps on all channels of a 14-channel Rx PIC. Each color on the plot indicates the lasing wavelength as a function of estimated change in optical index of the mirrors. This wavelength color map is parameterized as a function of estimated refractive index change on each of the two mirrors,
where is the waveguide group effective index, dλ is the wavelength shift on the mirror and λc is the center wavelength over the tuning range. Each mirror on the tunable laser requires less than 120 mW of power in order to achieve continuous tuning across the entire extended C-band (>4.8 THz). In addition, the gain and phase sections require 200 mW and 50 mW of maximum power respectively during operation, for a total maximum operating power of less than 500 mW for each laser.Fig. 2 Wavelength tuning maps for all extended C-Band tunable local oscillators on a 14-channel coherent receiver PIC. Each map shows the laser wavelength as a function of the estimated effective index change on the two mirror sections. The laser gain section was biased 3x above threshold, and no bias was applied to the phase tuning sections.
Figure 3 shows the local oscillator wavelengths (measured from the integrated MMI-tap) for all 14 channels on the Rx PIC at the various set points, demonstrating the ability to tune continuously across the extended C-band without any gaps. Measurements on standalone, widely tunable test laser chips fabricated with the same laser design and process as used on the fully integrated PICs show that the lasers are capable of >16 dBm total optical output power (per laser, on chip) under nominal bias conditions, without SOA or other form of amplification.
Fig. 3 Wavelength tuning range on all 14 channels of widely tunable local oscillators on the multi-channel coherent Rx PIC. The extended C-band is the range indicated by the gray shaded area.
Narrow laser linewidth is required for coherent optical transmitters and receivers. In addition to linewidth, a high side-mode-suppression-ratio (SMSR) is important to avoid coherent crosstalk. In Fig. 4(a), we demonstrate that the 14-channel PIC has narrow linewidth on all channels (<200 kHz). The linewidth is taken at the center of the C-Band (1548 nm) and calculated from phase noise power spectral density (PSD) measurements over the range of 10-100 MHz. PSD is measured by heterodyne beating of light from a tapped exit port for each tunable laser with a ~10 kHz linewidth reference laser. The linewidth performance at the center of the C-band is consistent with the performance of the widely-tunable lasers at the edges of the C-Band as well. Data on all six widely tunable lasers for product parts (as described in section 4 below) show a linewidth performance <200 kHz at the center and edge of the C-band (data not shown). Furthermore, the lasers all operate with >54 dB SMSR for all channels, as seen in Fig. 4(b), sufficient for a negligible coherent crosstalk penalty.
Fig. 4 Data from all 14 channels from a fully integrated coherent receiver PIC with per-channel extended C-band widely tunable lasers. Mid-band linewidth is calculated from power spectral density measured over 10-100 MHz (a), and SMSR (b) is measured near the center of C-band, 1548 nm. Cumulative distributions are plotted for the 14-channel PIC linewidths and SMSR.
Signal path loss in the receiver PIC and photodiode response are characterized by coupling an external tunable laser source to the input of the PIC, sweeping its wavelength across the extended C-band and measuring PD photocurrent. Figure 5(a) shows the normalized photocurrent distribution for all 14 channels of the Rx PIC in the form of a cumulative distribution function, wherein each data point represents the photocurrent averaged for each channel and polarization. The data are normalized to a specification limit required to achieve long-haul transmission as described below. In order to measure path loss from the local oscillator to the photodiodes, the on-chip PIC local oscillator is swept across the extended C-band, and photocurrent is averaged across the four photodiodes of each polarization and plotted in the CDF as shown in Fig. 5(b). Nearly the full distribution of parts show wide clearance to the 6-channel PIC product spec line for both fiber-coupled responsivity and local oscillator photocurrent specs.
Fig. 5 Cumulative distribution functions of fiber-coupled facet responsivity (a), and local oscillator photocurrent (b) measured on-chip with integrated high speed detectors, and plotted normalized to the 6-channel spec limit for both polarizations of the 14-channel PIC.
The high speed, integrated waveguide photodetector electro-optic response is plotted in Fig. 6 for all 4 TM side photodiodes of all 14 channels (56 monolithically integrated waveguide photodetectors). We note that the TM and TE side photodiodes are all designed and fabricated the identically; however, in the figure, only TM data are measured for clarity. The 3 dB bandwidth is >22 GHz (is measured with a 50 ohm probe), and supports 44 Gbaud, 16 QAM operation.
Fig. 6 Normalized electro-optic response (S21 vs. frequency) for all 56 TM photodetectors monolithically integrated on a 14-channel coherent receiver PIC. Bandwidth (at −3 dB) is ~22 GHz (at −5V photodetector bias).
4. 1.2 Tb/s extended C-band tunable transceiver module performance
The commercial 6-channel transmitter and receiver PICs are co-packaged in a low temperature co-fired ceramic (LTCC) module with ASICs for Mach-Zehnder modulator (MZM) control, a multi-channel MZM driver, and a multi-channel trans-impedance amplifier as shown in Fig. 7. The MZM driver and TIA ASICs are based on SiGe BiCMOS technology and each integrate 24 high-speed data streams that are hybrid integrated with the Tx and Rx PICs. In addition, the transceiver integrates free-space optics to implement polarization combining / splitting and rotation. The transceiver modules have been designed to operate at 33 Gaud and with dual polarization coherent modulation. RF measurements for these modules were taken using fully packaged modules, and both DC and RF signals were provided on a custom circuit board. Digital-to-Analog converter (DAC) pattern memory from an on-board commercial chip was used generate the RF signals. Figure 7 shows 33 Gbaud constellation diagrams using several polarization-multiplexed coherent formats (QPSK, 8-QAM, and 16-QAM) for one of the channels of the transceiver module. The tests were performed in loopback mode, with the transmitter and receiver PICs in a single module transmitting and receiving the signal.
Fig. 7 Extended C-band tunable Tx and Rx PICs integrated in a transceiver module (left) and recovered constellations at 33 Gbaud in loopback mode (right). Performance of the transceiver module is sufficient to enable data transmission >8000 km (QPSK), 4200 km (8-QAM), and 1750 km (16-QAM), in a commercial DWDM system.
The signal quality (Q vs. OSNR) was measured for all six channels of a single transceiver module running simultaneously on a fully assembled linecard, and the pre-FEC constellations shown in Fig. 8 were extracted from the receiver DSP engine. The six wavelengths comprised a superchannel, with each wavelength spaced by 100GHz and centered at about 1547 nm. The data in Fig. 8 only shows the outermost sub-carrier data for each channel/polarization tested, as they show the greatest impairment due to the roll-off in the frequency response of the modulator channel. The use of subcarrier multiplexing has been found to mitigate the effects of fiber non-linearity and increase system reach [21]. Furthermore, the signal quality (Q vs. OSNR) was measured simultaneously for all 6 transceiver module channels. The measured link OSNR margins (to maintain post-FEC error free performance) for the 33 GBaud transceiver modules were sufficient to enable transmitting data over >8000 km (QPSK), 4200 km (8-QAM), and 1750 km (16-QAM), in a commercial DWDM, 50 GHz-spaced, optically amplified link with 20 dB spans. The reach distances include any penalties occurring due to the <200 kHz linewidth of the widely tunable lasers, and other impairments owing to integrated PIC and transceiver module performance. Penalties for non-linear transmission plus 2 dB margin for additional penalties in a real world deployment are also included in the reach distances.
Fig. 8 Simultaneous operation of six channels at 33 Gbaud, DP 16-QAM for a 1.2 Tb/s tunable transceiver module integrating extended C-band tunable, 6-channel Tx and Rx PICs. The data are shown for the outermost subcarrier constellation diagrams.
Although the modules are designed for a 33 Gbaud commercial offering, RF module performance is further assessed by measuring constellations and Q at 44 Gbaud using a reference transmitter and an advanced modulation scope with offline data processing. A 92 GSa/s commercial AWG driving an MZM driver and dual-pol LiNbO3 modulator was used for the reference transmitter in this data set. Outer subcarrier (performance limiting) constellations are shown in Fig. 9, which displays the recovered 44 Gbaud 16-QAM constellations for two polarizations of one of the channels integrated in the 6-channel, widely tunable coherent receiver PIC module. The receiver PIC module performance (Q vs OSNR) was minimally impaired after adjusting for the increase in baud-rate and is capable of supporting 44 Gbaud 16-QAM transmission requirements.
Fig. 9 44 Gbaud, DP 16-QAM constellation diagram (outermost subcarrier) on one channel of a 6-channel, extended C-band tunable, coherent Rx PIC integrated in a transceiver module.
The extended C-Band tunable 1.2 Tb/s transceiver modules (incorporating Tx and Rx PICs with six channels, each operating at 200 Gb/s per channel at 33 Gbaud 16-QAM) have been transitioned into manufacturing. Adequate receiver sensitivity and sufficiently narrow laser linewidth enable the commercial transceiver-based product consistent with reach requirements. The data shown in Fig. 10 were gathered from the manufacturing of widely tunable, coherent Rx PICs produced in 2016 with a sample size of >500 PICs. Minimum fiber-coupled facet responsivity and local oscillator photocurrent cumulative distributions across extended C-band and all six channels are plotted, normalized to the product specifications required for the reach described in Fig. 8. The data show excellent manufacturing capability (>99% yield) to specification. Additionally, the development of this robust design and high yielding process has resulted in >95% yield to specifications for each demanding parameter including minimum responsivity, maximum polarization imbalance, average common-mode-rejection-ratio (CMRR), maximum photodiode dark current, minimum local oscillator photocurrent, and maximum linewidth measured on six channels. For a sample population of chips, I2-I1 and Q2-Q1 responsivity imbalances were less than 2dB, measured across the extended C-band, and the combined response of the estimated AC photocurrent for different polarization average imbalance was less than +/−1dB across the extended C-band. The median CMRR across I and Q data streams was ~29dB for the same population of chips.
Fig. 10 Manufacturing data for extended C-band tunable 1.2 Tb/s coherent Rx PICs for fiber-coupled facet responsivity (a) and local oscillator photocurrent (b). The data show excellent capability (>99% yield) to these specifications needed for good receiver sensitivity.
Extensive use condition and accelerated aging reliability testing of the photodiodes and local oscillators have been conducted. The data have resulted in reliability consistent with 20 year operation and GR-468 requirements for the PICs including measurements of laser power, laser wavelength photodiode leakage current (data not shown.)
5. Conclusion
Extended C-band tunable, monolithic InP coherent receiver PICs operating at 33 Gbaud, DP 16-QAM have been realized and commercialized into 1.2 Tb/s transceiver modules. 14-channel PICs demonstrate scalability of the integration platform and a pathway to multi-Tb/s capacity per chip. The PICs described demonstrate state-of-the-art InP integration and enable multi-Tb/s transmission capacity per chip, with per-channel continuous tunability across the extended C-Band. Early manufacturing data of 6-channel receiver PICs show excellent performance with respect to required specifications.
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