Learning-based digital back propagation to compensate for fiber nonlinearity considering self-phase and cross-phase modulation for wavelength-division multiplexed systems.

We propose a learning-based digital back propagation (LDBP) technique that addresses self-phase modulation (SPM) and cross-phase modulation (XPM) of a wavelength-division multiplexed (WDM) optical signal. The LDBP has a structure defined for each of the individual channels of a WDM signal, and connections between the channels address the XPM to effectively compensate for nonlinear waveform distortion of the signal in long-distance optical transmission systems. We attempt to optimize the limited number of parameters used in the structure such as the dispersion, nonlinear coefficient, and walkoff parameter to compensate for the nonlinear phase shift induced by the XPM. We derive equations to update the parameters through an iterative process based on the stochastic gradient descent algorithm. We verify the effectiveness of the proposed LDBP technique through a transmission experiment that uses an 11-channel WDM, 32-Gbaud, dual-polarization 16 quadrature amplitude modulation (QAM), and probabilistically shaped (PS) 64QAM signals. With the focus on the LDBP with a 1-step/span configuration, we operate the learning process using the received 16QAM signal in the experiment. We confirmed the successful convergence of particular parameters of the model of the transmission line after the developed learning procedure. We apply the LDBP with fixed optimized parameters to the received waveforms of the PS-64QAM signals and compare the performance with some DBPs. We observed that the proposed LDBP technique that considers XPM with 1-step/span configuration exhibits the best performance in compensating for nonlinear waveform distortion. Additionally, the learning process is effective for the case considering both SPM and XPM compared with the case of SPM only. Finally, we investigate the computational complexity of the LDBP and reveal that the total calculation cost is of the same order as that of a conventional DBP considering only SPM with a 2-step/span configuration.

Polarization-insensitive local-oscillator-carrier loopback modulation for a cost-effective and high-port-count wavelength-routing optical switch.

A wavelength-routing optical switch uses a wavelength-tunable laser at each input port, and this transmitter implements output port selection by tuning the wavelength that is associated with each output port. With coherent transmission, loopback modulation of a local oscillator (LO) carrier generated at the output port can eliminate the need for a wavelength-tunable laser. However, loopback modulation can be unstable since the power fluctuates because fiber traversal by the light creates polarization rotation. Here, we propose a simple polarization-alignment circuit and verify its effectiveness in creating a high-port-count optical switch system. The proposed circuit consists of passive components and aligns the polarization state of the supplied LO carrier to be linearly polarized along the x-direction of a TE-input dual-polarization (DP) IQ modulator. The circuit is shown to yield stable modulation with Q-variation of less than 0.8 dB, regardless of any birefringence along the transmission path. The proposal's effectiveness is verified in optical switch system experiments with DP-QPSK signals; 1,856 × 1,856 switch scale is achieved with loopback modulation.

Compensation of SOA-induced nonlinear phase distortions by optical phase conjugation.

To answer the question: "Is optical phase conjugation (OPC) capable of compensating nonlinear distortions caused by not only Kerr effect of optical fibre, but also the carrier dynamics of semiconductor optical amplifiers (SOAs)?", we investigate the effectiveness of OPC-based nonlinear compensation for SOAs amplifying a few-channel WDM signal modulated with m-QAM. We use a pair of SOAs with an OPC stage sandwiched between the two so that the combination works as a low-distortion amplifier. Symbol-period longer than the gain recovery time is chosen in our experiments to avoid bit-pattern effects introduced by the SOA. We amplify a 12Gbaud, 16QAM modulated three-channel WDM signal with this technique in the back-to-back configuration which remarkably outperforms a single SOA in the nonlinear regime of operation with an average Q2 improvement better than 4 dB for an output power of 4 dBm. We further demonstrate the practical advantage of the low distortion higher output power capability of the SOA shown in the back-to-back result by carrying out a transmission of the amplified signal through a 160-km fibre, where relatively high launch power is desirable. We also study the case of 64QAM signals and show that approximately a 3 dB Q2 factor improvement can be obtained over single SOA, while without nonlinear phase distortion compensation, the demodulation is nearly impracticable.

Design and verification of a LO bank enabled by fixed-wavelength lasers and fast tunable silicon ring filters for creating large scale optical switches.

The fast and widely tunable wavelength bank is a key enabler in creating wavelength-routing optical switches that do not use fast wavelength tunable lasers. A cost-effective design criterion needs to be developed before it can be applied to intra data center networks. In this paper, we develop a systematic method for designing a wavelength bank that yields high port-count and fast wavelength-routing optical switches for intra data center application. The wavelength bank is created with fixed-wavelength laser sources and wavelength-tunable filters with rapid wavelength selectivity. To optimize the optical switching system that uses the wavelength bank for supplying local oscillator (LO) lights for coherent detection, various parameters are analyzed, including effective bandwidth, laser output power, loss distribution, splitter port count, and optical amplifier gain. We carry out numerical simulations for optimizing the tradeoff between system performance and cost. To verify the designed wavelength bank, a silicon ring filter is newly fabricated with an average fiber-to-fiber insertion loss of 5.3 dB over a 22-nm bandwidth. Using 256-Gb/s DP-QPSK signals, experiments demonstrate a 1,024×1,024 optical switch that uses a fabricated silicon ring filter. The effectiveness of the scalable and fast-tunable LO bank is verified by achieving 262.1-Tb/s switch throughput with switching time under 18 µs.

Simple and fully CMOS-compatible low-loss fiber coupling structure for a silicon photonics platform.

A simple low-loss fiber coupling structure consisting of a Si inverted-taper waveguide and a 435 nm wide and 290 nm thick SiN waveguide was fabricated with fully complementary metal-oxide semiconductor (CMOS)-compatible processes. The small SiN waveguide can expand to the optical field corresponding to a fiber with a mode-field diameter of 4.1 µm. The fiber-to-chip coupling losses were 0.25 and 0.51 dB/facet for quasi-TE and quasi-TM modes, respectively, at a 1550 nm wavelength. Polarization-dependent losses of the conversion in the Si-to-SiN waveguide transition and the fiber-to-chip coupling were less than 0.3 and 0.5 dB, respectively, in the wavelength range of 1520-1580 nm.

Ultra-compact silicon photonics switch with high-density thermo-optic heaters.

Miniaturization of silicon photonics switches is essential for both dense integration and low-loss operation. However, it has remained unclear how small the switches can be made while using thermo-optic (TO) element switches. In this paper, the minimum possible distance between adjacent TO phase shifter arms was first examined. Next, the architecture for a switch matrix for the high-density arrangement of TO switches that includes multi-layer electrical wirings for compact electrical wire-out was proposed and demonstrated. As a result, we achieved 1/23 miniaturization of an 8 × 8 silicon photonics switch for the PIC part when compared with our previous design.

SiN/Si double-layer platform for ultralow-crosstalk multiport optical switches.

We experimentally demonstrate a double-layer platform of silicon nitride and silicon for ultralow-crosstalk multiport optical switches. By using a silicon nitride overpass with a large gap of 1.5 µm, we achieve a crosstalk of less than -50 dB and -45 dB almost entirely in the C-band for 4 × 4 and 16 × 16 switches, respectively. To demonstrate the scalability of the platform, we also measured a 32 × 32 passive test device and show that a worst-case crosstalk of less than -50 dB is feasible with appropriate gate switches.

Integrated silicon photonic wavelength-selective switch using wavefront control waveguides.

A wavelength selective switch (WSS) can route optical signals into any of output ports by wavelength, and is a key component of the reconfigurable optical add/drop multiplexer. We propose a wavefront control type WSS using silicon photonics technology. This consists of several arrayed waveguide gratings sharing a large slab waveguide, wavefront control waveguides and distributed Bragg reflectors. The structure, design method, operating principle, and scalability of the WSS are described and discussed. We designed and fabricated a 1 × 2 wavefront control type WSS using silicon waveguides. This has 16 channels with a channel spacing of 200 GHz. The chip size is 5 mm × 10 mm. The switching operation was achieved by shifting the phase of the light propagating in each wavefront control waveguide, and by controlling the propagation direction in the shared large slab waveguide. Our WSS has no crossing waveguide, so the loss and the variation in loss between channels were small compared to conventional waveguide type WSSs. The heater power required for switching was 183 mW per channel, and the average extinction ratios routed to Output#1 and Output#2 were 9.8 dB and 10.2 dB, respectively.

Non-duplicate polarization-diversity 8 × 8 Si-wire PILOSS switch integrated with polarization splitter-rotators.

We demonstrate a fully integrated polarization-diversity 8 × 8 thermo-optic Si-wire switch that uses only a single path-independent insertion loss (PILOSS) switch matrix. All input/output ports of the PILOSS switch matrix are uniquely assigned for polarization diversity without switch duplication. To integrate polarization splitter-rotators on a chip, we propose a compact path-length-equalized polarization-diversity switch configuration. Polarization-dependent loss (PDL) and differential group delay (DGD) are minimized. The 8 × 8 switch is fabricated by the CMOS-compatible fabrication process on 300-mm diameter wafer and additional etching of upper cladding after dicing. The chip size is 7 × 10.5 mm2. A PDL of 2 dB and a DGD of 1.5 ps are achieved. The crosstalk in the worst-case scenario is -20 dB in the full C-band.

Low noise frequency comb carriers for 64-QAM via a Brillouin comb amplifier.

Optical frequency comb lines with poor carrier to noise ratio (CNR) are significantly improved by Brillouin amplification using its extreme narrow bandwidth gain to suppress out of band noise, enabling higher quality signal modulation. Its application to spectral lines of narrow 10 GHz pitch and poor CNR is shown to suppress the otherwise strong phase distortion caused by poor CNR after encoding with 96 Gb/s DP-64-QAM signals and restore the bit error rate (BER) to below the limit for standard forward error correction (FEC). This is also achieved with the required frequency shifted optical pump for amplification obtained by seeding it from the comb itself, sparing the need for lasers and frequency locking. Simultaneous CNR improvement for 38 comb lines is also achieved with BER restored to below the FEC limit, enabled by a multi-line pump that is pre-dispersed to suppress its spectral distortion from the Kerr effect in the gain medium. Carrier performance at minimum BER shows minimal noise impact from the Brillouin amplifier itself. The results highlight the unique advantage of Brillouin gain for phase sensitive communications in transforming otherwise noisy spectral lines into useful high quality signal carriers.

Broadband silicon photonics 8 × 8 switch based on double-Mach-Zehnder element switches.

We fabricated and characterized a silicon photonics 8 × 8 strictly non-blocking optical switch based on double-Mach-Zehnder (MZ) element switches. The double-MZ switches, each of which consisted of an intersection and two asymmetric MZ switches, enabled the suppression of crosstalk across a wide wavelength range. The 8 × 8 switch exhibited an average fiber-to-fiber insertion loss of 11.2 dB and -20 dB crosstalk in a bandwidth wider than 30 nm. Furthermore, we constructed an 8 × 8 polarization-diversity switch by using two 8 × 8 switches and demonstrated 32-Gbaud dual-polarization, quadrature-phase-shift-keying, four-channel wavelength-division-multiplexed signal transmission without significant signal degradation.

On-chip Brillouin purification for frequency comb-based coherent optical communications.

In this Letter, for the first time, to the best of our knowledge, we harness on-chip Brillouin scattering for narrowband amplification and spectral purification of frequency comb lines for coherent optical communications. A parametrically generated optical frequency comb with a low carrier-to-noise power ratio was filtered through narrowband Brillouin amplification utilizing the same comb as the optical pump. This was achieved on a photonic chip to enable successful transmission of an advanced modulation format signal: 64-level quadrature amplitude modulation. 96 Gb/s data were modulated on two polarizations on multiple comb lines across 1532.9-1557.5 nm, demonstrating the scalability of this concept for operation in wavelength division multiplexing applications. The small form factor of the photonic chip reduces the polarization drifts when compared to optical fibers and paves the way for photonic integration.

Novel polarization diversity without switch duplication of a Si-wire PILOSS optical switch.

We demonstrate the compact polarization diversity based on the bidirectional full-port use of a path-independent-insertion-loss (PILOSS) optical switch. A polarization-diversity 4 × 4 strictly non-blocking optical switch is developed using a single thermooptic PILOSS Si-wire switch and fiber-based polarization beam splitters (PBSs) and combiners (PBCs). We measure characteristics of the switch and confirm that the proposed configuration demonstrates the performance in the insertion loss, polarization-dependent loss (PDL), and differential group delay (DGD) comparable with that of a conventional polarization-diversity 4 × 4 PILOSS switch using double switch elements. On the other hand, higher crosstalk is observed. The crosstalk increase is associated with the backward crosstalk at a waveguide intersection based on a directional coupler. The effect of the backward crosstalk on the total crosstalk is estimated, and future prospects are discussed.

Signal phase regeneration through multiple wave coherent addition enabled by hybrid optical phase squeezer.

A newly proposed concept, which is called hybrid optical phase squeezer (HOPS), achieves multi-level optical phase quantization through coherent addition of two (dual-wave scheme) or three (triple-wave scheme) optical waves exploiting optical parametric processes and electro-optic modulation. The triple-wave scheme enables signal phase regeneration free from phase-to-amplitude noise transfer, which is inevitable in the dual-wave scheme. By using HOPS in the dual-wave scheme, 3-fold phase-noise reduction was achieved for 24-Gb/s QPSK signals with a slight increase of amplitude noise. On the other hand, HOPS in the triple-wave scheme allowed phase regeneration of 12-Gb/s BPSK signal with a suppression of phase-to-amplitude noise transfer.

Ultra-high-extinction-ratio 2 × 2 silicon optical switch with variable splitter.

We demonstrate a record-high extinction-ratio of 50.4 dB in a 2 × 2 silicon Mach-Zehnder switch equipped with a variable splitter as the front 3-dB splitter. The variable splitter is adjusted to compensate for the splitting-ratio mismatch between the front and rear 3-dB splitters. The high extinction ratio does not rely on waveguide crossings and meets a strong demand in applications to multiport circuit switches. Large fabrication tolerance will make the high extinction ratio compatible with a volume production with standard complementary metal-oxide semiconductor fabrication facilities.

Ultra-compact 32 × 32 strictly-non-blocking Si-wire optical switch with fan-out LGA interposer.

We demonstrate a 32 × 32 path-independent-insertion-loss optical path switch that integrates 1024 thermooptic Mach-Zehnder switches and 961 intersections on a small, 11 × 25 mm2 die. The switch is fabricated on a 300-mm-diameter silicon-on-insulator wafer by a complementary metal-oxide semiconductor-compatible process with advanced ArF immersion lithography. For reliable electrical packaging, the switch chip is flip-chip bonded to a ceramic interposer that arranges the electrodes in a 0.5-mm pitch land grid array. The on-chip loss is measured to be 15.8 ± 1.0 dB, and successful switching is demonstrated for digital-coherent 43-Gb/s QPSK signals. The total crosstalk of the switch is estimated to be less than -20 dB at the center wavelength of 1545 nm. The bandwidth narrowing caused by dimensional errors that arise during fabrication is discussed.

Carrier recovery for M-QAM signals based on a block estimation process with Kalman filter.

A novel carrier recovery scheme for demodulating optical M-ary quadrature amplitude modulation (M-QAM) signals is proposed and demonstrated. The proposed scheme treats a certain number of consecutive symbols as a processing block for which linear evolution of the carrier phases in time is assumed. The Kalman filter algorithm is employed to simultaneously estimate the carrier-frequency offset and carrier phases of the symbols in each block from the observation result. Consequently, an optimal carrier recovery operation with minimum mean squared error can be obtained, and large phase errors due to optical noise and large carrier-frequency offsets can be tolerated. We experimentally demonstrate the proposed scheme in demodulating optical 16- and 64-QAM signals, confirming its stable operation for carrier-frequency offsets even larger than 10% of the symbol rate of the signal. We also demonstrate 160-km transmission of a single-channel, single-polarization 64-QAM signal by using the proposed scheme in the demodulation process.

Phase regeneration of phase encoded signals by hybrid optical phase squeezer.

We present a new method to perform phase regeneration of phase encoded signals. In our concept called "hybrid optical phase squeezer (HOPS)," a multilevel phase-quantized signal is synthesized through the coherent addition of a phase-conjugate copy of the signal and a phase harmonic of the signal with a frequency shifter. Unlike the conventional method by phase sensitive amplification, HOPS does not use any optical parametric gain such that only optical elements with low optical nonlinearity are necessary for optical phase quantization. In the proof-of-concept experiment, it is confirmed that a 2-level HOPS can perform quadrature squeezing with an extinction ratio of 40 dB. Simultaneous phase regeneration of two coherent wavelength-division-multiplexed 10.75-Gb/s binary phase-shift keyed signals is successfully demonstrated using a 2-level HOPS based on a semiconductor optical amplifier.

Compact 2 × 2 polarization-diversity Si-wire switch.

A polarization-independent 2 × 2 switch based on silicon-wire waveguides has been realized with a compact size of 600 × 500 µm². Polarization-independent operation was achieved with a polarization-diversity technique which implements polarization splitters, TE-TM intersections, and Mach-Zehnder switches. The extinction ratios of the 2 × 2 switch for TE, TM, and a mixed polarization at a wavelength of 1550 nm were measured to be larger than 30 dB, 25 dB, and 30 dB, respectively. The measured switching powers for the TE and TM polarizations were 25 and 55 mW, respectively. The measured polarization-dependent loss was lower than 1 dB. The differential group delay (DGD) between the TE and TM modes was also evaluated using the Mueller matrix method, which was in good agreement with the values estimated from the path lengths for each mode. A path-length-compensated switch was fabricated, whose DGDs for all paths were indeed as small as ~2 ps, mainly from the access waveguides. The switch could provide an important route to develop ultra-compact polarization-independent integrated circuits based on silicon-wire waveguides.

Ultra-compact 8 × 8 strictly-non-blocking Si-wire PILOSS switch.

We report on a path-independent insertion-loss (PILOSS) 8 × 8 matrix switch based on Si-wire waveguides, which has a record-small footprint of 3.5 × 2.4 mm2. The PILOSS switch consists of 64 thermooptic Mach-Zehnder (MZ) switches and 49 low-crosstalk intersections. Each of the MZ switches and intersections employs directional couplers, which enable the composition of a low loss PILOSS switch. We demonstrate successful switching of digital-coherent 43-Gbps QPSK signal.