Performance of quantum-dash mode-locked lasers (QD-MLLDs) for high-capacity coherent optical communications.

We investigate the capabilities and limitations of quantum-dash mode-locked lasers (QD-MLLDs) as optical frequency comb sources in coherent optical communication systems. We demonstrate that QD-MLLDs are on par with conventional single-wavelength narrow linewidth laser sources and can support high symbol rates and modulation formats. We manage to transmit 64 quadrature amplitude modulation (QAM) signals up to 80 GBd over 80 km of standard single-mode fiber (SSMF), which highlights the distinctive phase noise performance of the QD-MLLD. Using a 38.5 GHz (6 dB bandwidth) silicon photonic (SiP) modulator, we achieve a maximum symbol rate of 104 GBd with 16QAM signaling and a maximum net rate of 416 Gb/s per carrier in a single polarization setup and after 80 km-SSMF transmission. We also compare QD-MLLD performance with commercial narrow-linewidth integrable tunable laser assemblies (ITLAs) and explore their potential for use as local oscillators (LOs) and signal carriers. The QD-MLLD has 45 comb lines usable for transmission at a frequency spacing of 25 GHz, and an RF linewidth of 35 kHz.

Efficient coupling between free electrons and the supermode of a silicon slot waveguide.

Laser light can modulate the kinetic energy spectrum of free electrons and induce extremely high acceleration gradients, which are instrumental to electron microscopy and electron acceleration, respectively. We present a design scheme for a silicon photonic slot waveguide which hosts a supermode to interact with free electrons. The efficiency of this interaction relies on the coupling strength per photon along the interaction length. We predict an optimum value of 0.4266, resulting in the maximum energy gain of 28.27 keV for an optical pulse energy of only 0.22 nJ and duration 1 ps. The acceleration gradient is 1.05 GeV/m, which is lower than the maximum imposed by the damage threshold of Si waveguides. Our scheme shows how the coupling efficiency and energy gain can be maximized without maximizing the acceleration gradient. It highlights the potential of silicon photonics technology in hosting electron-photon interactions with direct applications in free-electron acceleration, radiation sources, and quantum information science.

Silicon photonic broadband polarization-insensitive switch based on polarization-mode diversity conversion.

We present a 2 × 2 polarization-insensitive switch on a 220-nm silicon-on-insulator platform, employing a balanced Mach-Zehnder interferometer (MZI) structure. This design incorporates polarization-insensitive adiabatic couplers, polarization rotators based on mode hybridization and evolution, and thermo-optic mode-insensitive phase shifters with wide waveguides. The switch exhibits broadband polarization-insensitive characteristics, with extinction ratios larger than 15 dB, insertion losses less than 2.3 dB, and polarization-dependent losses less than 1 dB for wavelengths ranging from 1500 nm to 1600 nm. The power consumption required for simultaneously switching the fundamental transverse electric (TE0) and transverse magnetic (TM0) polarized modes is 29.1 mW. These results highlight the potential of the switch as a building block for on-chip polarization-division-multiplexed optical interconnects.

AL-aided AMC in a multi-user white-light OFDMA VLC system over a light-diffusing fiber loop.

To develop an adaptive modulation scheme for flexible high-speed multi-user visible light communication (VLC), automatic modulation classification (AMC) is adopted for monitoring the modulation formats of different subcarrier groups. An AMC scheme based on a joint convolutional neural network (CNN), active learning (AL), and data augmentation (DA) is demonstrated over an orthogonal frequency division multiplexing access (OFDMA) VLC system. The configuration of the diffuse white-light VLC system is combined with a pair integrated transceiver module, a light-diffusing fiber (LDF), and a wireless channel, which can provide white-light illumination and ubiquitous access. Within the forward error correction (FEC) threshold, the data rates of the white-light VLC links can reach 325.5 Mbps with a bit error rate (BER) of 2.163 × 10-3. An experiment with two-user access via the proposed VLC link with an unequal bandwidth allocation was demonstrated. The performance of the AL-aided CNN AMC scheme also shows a classification accuracy rate of 95.48% for the constellation diagrams of different subcarriers of the OFDMA signal over 240 training samples and faster convergence than a CNN-based AMC.

Feature issue introduction: ultra-wideband optical communications.

This Feature Issue covers the important aspects to develop ultra-wideband optical communication systems including optoelectronics, impairment modeling and compensation, optical amplification, superchannel and multi-band transmission and control, and so forth. This Introduction provides a summary of the articles on these topics in this Feature Issue.

Broadly tunable and ultra-highly selective detection of radio frequency signals enabled by a silicon photonic chip.

Precise and agile detection of radio frequency (RF) signals over an ultra-wide frequency range is a key functionality in modern communication, radar, and surveillance systems, as well as for radio astronomy and laboratory testing. However, current microwave solutions are inadequate for achieving the needed high performance in a chip-scale format, with the desired reduced cost, size, weight, and power. Photonics-based technologies have been identified as a potential solution but the need to compensate for the inherent noise of the involved laser sources have prevented on-chip realization of wideband RF signal detection systems. Here, we report an approach for ultra-wide range, highly-accurate detection of RF signals using a conceptually novel feed-forward laser's noise cancelling architecture integrated on chip. The technique is applied to realization of an RF scanning receiver as well as a complete radar transceiver integrated on a CMOS-compatible silicon-photonics chip, offering an unprecedented selectivity > 80 dB, spectral resolution < 1 kHz, and tunability in the full 0.5-35 GHz range. The reported work represents a significant step towards the development of integrated system-on-chip platforms for signal detection, analysis and processing in cognitive communication and radar network applications.

Electrically reconfigurable waveguide Bragg grating filters.

We propose and demonstrate an electrically reconfigurable waveguide Bragg grating filters in silicon-on-insulator using a multiple-contact heater element. There are six electrical pads connected to the heater element in an equidistant manner. These electrical pads allow to create different heat, and corresponding refractive index, distributions across the grating so that the local Bragg wavelength corresponding to the heated segments can be controlled. In turn, this control over the heat distribution allows the device to be reconfigured to implement different filter spectral responses. These filters are applicable for both wavelength division multiplexing systems and optical signal processing applications. As a verification, we demonstrate the generation of two (or more) separate filter bands with a spacing up to 35 nm or a Fabry-Pérot cavity with a 1.6 nm free-spectral range. Moreover, we explain a firm and accurate simulation framework of the proposed device based on COMSOL Multiphysics and the transfer matrix method, which is in excellent agreement with our experimental measurements.

All-silicon multi-band TM-pass polarizer on a 220 nm SOI enabled by multiplexing grating regimes.

We propose an all-silicon design of a multi-band transverse-magnetic-pass (TM-pass) polarizer. The device is based on one-dimensional gratings that work under different regimes that depend on the polarization. With a tapered structure, it is revealed that the operation bandwidth can be extended by multiplexing the diffraction in O-band and the reflection in S-, C-, and L-bands for the transverse-electric (TE) mode. By simulation, we achieve a 343 nm device bandwidth with insertion loss (IL) < 0.4 dB and polarization extinction ratio (PER) > 20 dB. The operation wavelength range covers commonly-used optical telecommunication bands including the O-, S-, C-, and L- bands. Experimental results also show IL < 1.6 dB and PER > 20 dB from 1265 nm to 1360 nm corresponding to the O-band, and from 1500 nm to 1617 nm that corresponds to the C-band. The device is a single-etched design on the standard 220 nm silicon-on-insulator (SOI) with silicon oxide cladding. Such a simple and compatible design paves the way for developing practical multi-band silicon photonic integrated circuits.

Standalone, CMOS-based Faraday rotation in a silicon photonic waveguide.

Nonreciprocity is a fundamental requirement of signal isolation in optical communication systems. However, on chip isolator designs require either post-processing steps or external magnetic biasing, which are impractical for commercial applications. This raises the need for standalone devices which support nonreciprocal functionality using standardized fabrication techniques. Here, we report the first design of an electromagnetic coil surrounding a waveguide which exclusively employed the complementary metal-oxide-semiconductor (CMOS) process flow. The coil supported an electric current up to 14 mA. In simulations, it generated an alternating magnetic flux density up to 1.16 mT inside a strip waveguide and thereby induced a rotation of 50.71 picodegrees for the fundamental transverse-magnetic mode at a wavelength of 1352 nm. Our analysis further revealed methods to increase the rotation by orders of magnitude. It demonstrated the scope of manufacturing processes and serves as a building block for the development of a commercially viable, on-chip optical isolator.

Design, analysis, and characterization of a compact silicon photonic modulator with meandered phase shifters.

We present a C-band Mach-Zehnder modulator with meandered phase shifters and a compact footprint of 432 × 260 µm2 on the silicon-on-insulator platform. Electrode, p-n junction, and optical transit time are considered when performing the electro-optic bandwidth (EO BW) simulations. The simulation results prove that the dominant bandwidth limiting factor for this type of modulator is optical transit time. The insertion loss of the modulator without bias is 2.1 dB. The measured half-wave voltage (V π ) and 3-dB EO BW at -0.5 V bias are 6.4 V and 7.7 GHz, respectively. 53 Gbaud PAM-4 transmission over 2 km of standard single-mode fiber is achieved at a bit error rate (BER) below the 6.7% overhead hard-decision forward error correction BER threshold of 3.8×10 -3.

Net 400-Gbps/λ IMDD transmission using a single-DAC DSP-free transmitter and a thin-film lithium niobate MZM.

The insatiable growth of datacenter traffic mandates increasing the capacity of cost-effective intensity modulation direct detection (IMDD) systems to meet the foreseen demand. This Letter demonstrates the first, to the best of our knowledge, single-digital-to-analog converter (DAC) IMDD system achieving a net 400-Gbps transmission using a thin-film lithium niobate (TFLN) Mach-Zehnder modulator (MZM). Employing a driver-less DAC channel (128 GSa/s, 800 mVpp) with neither pulse-shaping nor pre-emphasis filtering, we transmit (1) 128-Gbaud PAM16 below the 25% overhead soft-decision forward error correction (SD-FEC) bit error rate (BER) threshold and (2) 128-Gbaud probabilistically shaped (PS)-PAM16 under the 20% overhead SD-FEC threshold, which respectively correspond to record net rates of 410 and 400 Gbps for single-DAC operation. Our results highlight the promise of operating 400-Gbps IMDD links with reduced digital signal processing (DSP) complexity and driving swing requirements.

Time-variant entropy regulated multiple access for flexible coherent PON.

To further increase the data rate and rate flexibility of next-generation passive optical networks (PONs), we propose a scheme jointing probabilistic constellation shaping (PCS), constellation diagram identification, and coherent detection. The rate of the coherent PON is dynamically regulated with time-variant entropy, and monitored by the image classifier in real-time. The transmitted cascade frames allocated to different optical network units (ONUs) from the optical line terminal (OLT) can be synchronized, identified, and distinguished by the labeled entropy values. The proposed graphic monitoring scheme is experimentally demonstrated based on a dual-polarized coherent optical transmission system. A 0.5 interval value of entropy is precisely tuned to achieve a 96.13% constellation identification accuracy for entropy/rate by a fast normalized cross correlation coefficient (NCC)-based image classifier. A real-time data rate from 350 to 550 Gb/s with PCS-64-quadrature amplitude modulation (64-QAM) format for a single ONU is achieved which can maximally support five independent ONUs with single wavelength and two polarization states and the entropy varying from 3.5 to 5.5 with 0.5 intervals, respectively.

Reconfigurable microwave photonic filter based on a quantum dash mode-locked laser.

We demonstrate a reconfigurable microwave photonic (MWP) filter using a quantum dash (QDash) mode-locked laser (MLL) that can generate an optical frequency comb (OFC) with ∼50 comb lines and a free spectral range of 25 GHz. Thanks to the large number of comb lines, the MWP filter responses can be easily programmed by tailoring the OFC spectrum. We implement MWP filter responses with Gaussian, sinc, flat-top, and multiple peaks, as well as demonstrate that tuning of the central frequency. We achieve a minimum 3 dB bandwidth of ∼100 MHz for a sinc-shaped MWP filter, while the maximum out-of-band rejection can be up to ∼30 dB with Gaussian apodization. Our results show that the QDash-MLL is a promising OFC source for developing integrated and reconfigurable MWP filters.

Asymmetric self-coherent detection.

The bandwidth upgrade required in short-reach optical communications has prompted the need for detection schemes that combine field reconstruction with a cost-effective subsystem architecture. Here we propose an asymmetric self-coherent detection (ASCD) scheme for the field reconstruction of self-coherent (SC) complex double-sideband (DSB) signals based on a direct-detection (DD) receiver with two reception paths. Each reception path consists of a photodiode (PD) and an analog-to-digital converter for the detection of a part of the received optical signal that experiences a different optical transfer function via the configuration of an optical filter. We derive an analytical solution to reconstructing the signal field and show the optimal filter response in optimizing the signal SNR. Further, we numerically characterize the theoretical performance of a specific ASCD scheme based on a chromatic dispersion filter and validate the principle of the ASCD scheme in a proof-of-concept experiment. The ASCD scheme approaches the electrical spectral efficiency of coherent detection with a cost-effective DD receiver, which shows the potential for high-speed short-reach links required by edge cloud communications and mobile X-haul systems.

23-dB average isolation using a silicon photonic Mach-Zehnder modulator.

We demonstrate an optical time-gate isolator entirely fabricated on the silicon-on-insulator (SOI) platform based on a conventional traveling-wave Mach-Zehnder modulator (TW-MZM) design. The device achieves 18.2 dB (22.7 dB) time-averaged isolation when driven with 2.0-Vpp (7.1-Vpp) differential clock signals at 6.8 GHz and biased at null. Under these conditions, the isolator blocks backward light at all time regardless of driver amplitude, but produces periodic modulation in the forward direction. Moreover, we embed our isolator in a digital communication link and measure a signal-to-noise ratio (SNR) penalty of only 0.5 dB due to the isolator at 13.6 Gbaud PAM-4 data rate. Our device can be integrated in larger circuits to protect laser sources or mitigate interference.

Asymmetric direct detection of orthogonal offset carriers assisted polarization multiplexed single-sideband signals.

We propose and demonstrate the asymmetric direct detection (ADD) of polarization division multiplexed single-sideband (PDM-SSB) signals with orthogonal offset carriers. ADD exploits the photocurrent difference to eliminate the Y-Pol interference in the X-Pol, and the X-Pol signal intensity to eliminate the X-Pol interference in the Y-Pol without resorting to iterative algorithms. This enables not only low-complexity signal linearization but also a simplified receiver front-end composed of a single optical filter, two single-ended photodiodes and two analog-to-digital converters (ADC). In the experiment, we first perform a parametric study of the proposed scheme at 40 Gbaud in the back-to-back configuration (B2B) to evaluate the performance impact of different system parameters including the carrier to signal power ratio (CSPR), the matched filter roll-off, and the filter guard band. Next, we demonstrate the transmission of 416 Gbit/s PDM 16-QAM signal over 80 km single-mode fiber (SMF) below the soft-decision forward error correction (SD-FEC) threshold of 2×10-2. We also numerically study the effectiveness of a 2×2 multiple-input-multiple-output MIMO equalizer in alleviating the inter-polarization linear crosstalk resulting from the non-orthogonal PDM-SSB signals due to polarization-dependent loss (PDL), which is not negligible for potential on-chip implementation of ADD.

Asymmetric direct detection of twin-SSB signals.

We propose the asymmetric direct detection (ADD) of twin-single sideband (SSB) signals based on a simple receiver front-end composed of one optical filter and two photodiodes. ADD exploits the photocurrent difference between a filtered and unfiltered signal pair to reconstruct and linearize the received twin-SSB signal with a high electrical spectral efficiency (ESE). We evaluate the performance impact of the critical system parameters on ADD and demonstrate 231 Gb/s net rate 16-QAM twin-SSB transmission with 6.03 b/s/Hz ESE over an 80 km standard single-mode fiber below the ${1} \times {{10}^{ - 2}}$1×10-2 hard-decision forward error correction threshold. We also found that the bit error rate performance of ADD is robust against the relative center wavelength drifting of the optical filter.

Cost-minimizing distribution matching supporting net 800 Gbit/s PS-PAM transmission over 2 km using a 4-λ EML TOSA.

Probabilistic shaping (PS) allows tunable spectral efficiency that is suitable for realizing high throughput intra-data center transceivers. In this Letter, we integrate the cost-minimizing distribution matching (CMDM) in the probability amplitude shaping scheme to generate PS-PAM signals with ultra-short symbol block lengths for reduced serial processing delay. We detail the principle of CMDM and present two different methods of implementation. We demonstrate that CMDM enables the transmission of single wavelength net 200 Gbit/s PS-PAM-8 over 2 km of single-mode fiber (SMF). We show that similar performance is achievable using a constant composition distribution matcher, yet requiring 10 times longer symbol block lengths. We also report, to the best of our knowledge, the first demonstration of net 800 Gbit/s transmission over 2 km of SMF using a packaged 4-λ electro-absorption modulated laser transmitter optical sub-assembly (TOSA).

Optimization of thermo-optic phase-shifter design and mitigation of thermal crosstalk on the SOI platform.

We first optimize the design and compare the performance of thermo-optic phase-shifters based on TiN metal and N++ doped silicon, in the same SOI process. The designs don't require special material processing, show negligible loss, and have very stable power consumption. The optimum TiN design has a switching powerPπ=21.4 mW and a time constantτ=5.6 µs, whereasPπ=22.8 mW andτ=2.2 µs for the best N++ Si design, enabling 2.5x faster switching compared to the metal heater. Doped-Si-based heaters are therefore the most practical and efficient on standard SOI. In addition, to optimize the layout density of highly integrated dies, we experimentally characterize internal and external thermal crosstalk for tunable Mach-Zehnder interferometers (MZIs) based on both heater designs for various power, distances, and etching patterns. Deep trenches are the best structures not involving special fabrication techniques to mitigate heat leakage affecting phase-sensitive devices close to heaters. Given the numerous applications of thermal tuners, this work is relevant to almost all silicon photonics designers.

400 Gb/s O-band silicon photonic transmitter for intra-datacenter optical interconnects.

We present and experimentally demonstrate a silicon photonic (SiP)-based four-lane 400 Gb/s transmitter for fiber-rich intra-datacenter optical interconnects. Four parallel SiP series push-pull traveling wave Mach-Zehnder modulators (MZMs) operating in the O-band are used in the transmitter. The MZMs have an average electro-optic (EO) bandwidth of approximately 30 GHz at 3 V reverse bias voltage. To assess the parallel operation, we measure the EO crosstalk between the four MZMs, where the EO crosstalk between the closest MZMs is below -17 dB over 50 GHz bandwidth. Then, we use a four-channel digital-to-analog converter (DAC) to simultaneously drive the MZMs and characterize the performance of the transmitter versus various parameters. Results reveal that 53 Gbaud pulse amplitude modulation over 4-levels (PAM4), i.e., 100 Gb/s net rate, per lane can be received at a bit error rate (BER) below the KP4- forward error correction (KP4-FEC) threshold of 2.4×10-4 using only a 5-tap feed-forward equalizer (FFE) at the receiver. In addition, we show that 53 Gbaud and 64 Gbaud PAM4 per lane can be received at a BER below the KP4-FEC and 7% hard decision FEC (HD-FEC), respectively, using a driving voltage swing below 1.8 Vpp. To the best of our knowledge, these are the best results for 100 Gb/s PAM4 using a single electrode SiP TWMZM with a lateral PN junction in a multi-project wafer process. Finally, we show that the BER is still below the KP4-FEC at maximum crosstalk for all lanes, and an aggregate rate of 400 Gb/s can be achieved at an average BER of approximately 1×10-4.