Traveling-wave Mach-Zehnder Modulator Research Articles

RF crosstalk mitigation via floating shields in parallel silicon traveling-wave modulators

High-density integration of electro-optic (E-O) elements in photonic integrated circuits (PICs) offers compact, low-cost high-capacity transceiver modules. Tighter component spacing increases compactness, but leads to crosstalk that can degrade performance. We investigate and characterize the crosstalk between silicon traveling-wave Mach-Zehnder modulators (TW-MZMs) laid out in parallel in PICs. We propose the use of floating shield strips to reduce E-O crosstalk between these TW-MZMs. We show via simulation and experiment that these strips can increase shoreline bandwidth density by nearly 50%. The improvement in E-O crosstalk covers frequencies as high as 60 GHz.

Monolithically integrated 112 Gbps PAM4 optical transmitter and receiver in a 45 nm CMOS-silicon photonics process.

We demonstrate a transmitter and receiver in a silicon photonics platform for O-band optical communication that monolithically incorporates a modulator driver, traveling-wave Mach-Zehnder modulator, control circuitry, photodetector, and transimpedance amplifier (TIA) in the GlobalFoundries Fotonix (45SPCLO) platform. The transmitter and receiver show an open 112 Gbps PAM4 eye at a 4.3 pJ/bit energy efficiency, not including the laser. Extensive use of gain-peaking enables our modulator driver and TIA to achieve the high bandwidths needed in the 45 nm CMOS-silicon photonics process. Our results suggest an alternative to the frequent approach of bump-bonding BiCMOS drivers and TIAs to silicon photonics.

Traveling-Wave Mach-Zehnder Modulator Integrated With Electro-Optic Frequency-Domain Equalizer for Broadband Modulation

Broadband modulator is a key component of future high-capacity and high-baud-rate optical fiber communication. An electro-optic (EO) frequency-domain equalizer is proposed and investigated to increase the 3-dB bandwidth of optical modulators. The equalizer can be integrated with conventional traveling-wave modulators using a crossing waveguide, U-turn waveguide, or ferroelectric-domain-inversion. The 3-dB bandwidth of the modulator can be doubled by integrating the equalizer. As a theoretical consideration, we modeled several types of EO equalizers and investigated the function and parameter dependency of the equalizers in detail. For the experimental demonstration, a titanium-diffused lithium niobate optical waveguide modulator integrated with the EO equalizer was fabricated. The optical loss including fiber-coupling was as low as 5.4 dB, and the half-wave voltage was 3.7 V. The measured electro-optic response at 110 GHz is –0.4 dB, and that of the 3-dB bandwidth is larger than 110 GHz, which is the upper frequency limit of our measurement equipment. The 3-dB bandwidth of the modulator estimated using the fitting curve obtained from the measured electrical propagation loss of the modulator electrode was approximately 200 GHz.

Tunable V-Cavity Laser Monolithically Integrated With Traveling Wave Mach-Zehnder Modulator

We present the monolithic integrated widely tunable V-cavity laser and traveling wave Mach-Zehnder modulator operating in the C-band. A wide wavelength tuning of 64 channels from 1513 nm to 1565 nm is obtained with SMSRs mostly over 40 dB. The <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{V}_{\pi }$ </tex-math></inline-formula> is 5.5 V and the static extinction ratio is 14 dB. The 3-dB electro-optic bandwidth is measured to be 14.2 GHz. By optimizing the device structure and thickness of intrinsic region on a semi-insulating substrate wafer, a theoretical bandwidth over 40 GHz can be achieved.

Optical Transmitter Equalization With Tunable Mismatched Terminations in a Silicon Modulator

We demonstrate a 2.5 mm silicon photonics traveling-wave Mach-Zehnder modulator with on-chip tunable termination to study termination mismatching as a technique for optimizing modulator performance by leveraging the monolithic integration of electronic and photonic devices. Time domain reflectometry measurements demonstrate the tunability of the termination impedance with respect to the modulator transmission line Z <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> . Electro-optic frequency response measurements show the bandwidth enhancement effect of the termination mismatch, whereby a 3 dB bandwidth above 40 GHz was achieved. Optical link time-domain measurements were performed to optimize the termination mismatch for maximum eye-diagram opening at 50 Gb/s. Our measurements demonstrate that active on-chip termination can be utilized to optimize the modulator performance and mitigate the risk incurred from fabrication variations which can result in a sub-optimal termination impedance.

A 50-GHz Bandwidth Traveling-Wave Mach-Zehnder Modulator With Built-In Feedback Equalization

A 50–GHz bandwidth traveling-wave Mach-Zehnder modulator (TWMZM) was designed with a new equalization technique, which takes advantage of the intrinsic delay in a traveling-wave electrode (TWE) and a local negative-feedback circuit. With this idea, given the availability of flip-chip bonding, a long TWE can be equalized as a single stage or be segmented into a few stages, each being equalized separately to optimize the performance of the entire device. The detailed analysis of this equalization technique is presented. As a demonstration, a 1.6–mm long equalized TWMZM was designed using a commercial silicon photonic process and a 0.13 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu \mathrm{m}$</tex-math></inline-formula> SiGe process. A 2–stage cascaded design with equal equalization was adopted and optimized for high bandwidth and high extinction ratio. The circuit was designed using the multi-physics electronic-photonic integrated circuit (EPIC) design flow we developed recently, wherein both electronic and photonic parts were modeled and the EPIC circuit was simulated for system level performance evaluation. Simulation results showed that the equalization technique increased the 3-dB EO bandwidth of the TWMZM from 28 to 50.4 GHz, an 80% increase. Large signal performance with non-return to zero signals upto 100 Gb/s was evaluated and the results showed significant improvements in the eye quality with equalization. The performance evaluation with pulse amplitude modulation (PAM)-4 signal was also carried out and the results showed successful operation at 106 Gbaud PAM-4 for the equalized TWMZM, makes it a promising candidate for future 200 Gbps-per-lambda optical transceivers.

High Baud Rate Transmission With Silicon Photonic Modulators

We review high baud rate operation of silicon photonic modulators with emphasis on short-reach and metro-haul applications. The current status of both devices and digital signal processing are discussed. The basic design of a typical carrier-depletion-based dual-drive traveling-wave Mach-Zehnder modulator (TW-MZM) is studied by simulation. Based on an all-silicon TW-MZM, we report the transmission experiments of high baud rate single sideband 4-ary pulse amplitude modulation (PAM-4) signals with linear equalization and artificial neural network based equalization, respectively. Our work indicates all-silicon modulators can address the requirements of future optical communication systems operating at 100 Gbaud.

Net 220 Gbps/λ IM/DD Transmssion in O-Band and C-Band With Silicon Photonic Traveling-Wave MZM

We present the design and characterization of O-band and C-band silicon photonic (SiP) traveling wave Mach-Zehnder modulators (TW-MZM) allowing 220 Gbps/λ net rate operation. The designed modulators show over 45 GHz 3-dB E-O bandwidth with a single-segment design. In the O-band, with simple linear feed forward equalization, we transmit net 203 (200) Gbps signal over 2 km (10 km) of single-mode fiber (SMF) below the hard-decision forward error correction (HD-FEC) BER threshold of 3.8 × 10−3. With the aid of nonlinear Volterra equalizer and one 2.3Vpp driving signal, we transmit net 225 (216) Gbps PAM8 signals assuming 20% overhead soft-decision FEC with a normalized general mutual information (NGMI) threshold of 0.8798 over 2 km (10 km) of SMF. The C-band design enables net 220 Gbps in B2B and net 215 Gbps over 500 m of SMF above the specified NGMI threshold. These results are the highest reported net rate for SiP MZM in an intensity modulation direct-detection (IM/DD) system, fabricated entirely in a commercial foundry.

Effect of Wafer Tilt During Ion Implantation on the Performance of a Silicon Traveling-Wave Mach-Zehnder Modulator

This paper reports a study of the effect of wafer tilt during dopant implantation on the performance of silicon PN phase shifter and traveling-wave Mach-Zehnder modulator. The PN phase shifter is designed and process simulated to include the effects of different fabrication processes in the device performance. The wafer tilt during implantation is varied from 0° to 3°, 5°, and 7°. The resulting crystal damage during dopant implantation to form the PN junction and the active concentration profile upon annealing, along with the formation of dopant–defect clusters, are discussed. Compared to 0° tilt, 7° tilt results in $1.58\times $ higher phase shift and better modulation efficiency. The overall phase shifter performance is improved using 5° wafer tilt for implantation resulting in $1.23\times $ lower absorption, $1.45\times $ better modulation efficiency, and $3.14\times $ higher 3 dB $RC$ modulation bandwidth for lumped-driven phase shifter. A traveling-wave electrode to enhance the modulation bandwidth is used, and the modulator performance for non-return-to-zero on-off-keying modulation with–2.5 V bias and 2.5 $\text{V}_{pp}$ drive signal across each arm is evaluated using a dual-arm push-pull drive. The sample with 5° tilt shows better traveling-wave and modulator high-speed characteristics compared to the other samples. Among the four samples with different wafer tilts, the sample with 0° tilt shows the worst phase shifter performance, and the sample with 3° tilt shows the worst modulator characteristics. The best overall performance is obtained for the sample with 5° tilt. Compared to the modulator with implantation at 0° tilt, the 5° tilted sample shows $2.3\times $ higher 6.4 dB electrical bandwidth and $1.36\times $ higher 3 dB electro-optic bandwidth at–2.5 V using a traveling-wave electrode with $1.48\times $ lower energy-per-bit for 5 km transmission at the KP4-forward-error-correction bit-error-rate threshold. The comparison of the effect of wafer tilt angles on various device metrics is presented and discussed.

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.

50 GBd PAM4 transmitter with a 55nm SiGe BiCMOS driver and silicon photonic segmented MZM.

We demonstrate an optical transmitter consisting of a limiting SiGe BiCMOS driver co-designed and co-packaged with a silicon photonic segmented traveling-wave Mach-Zehnder modulator (MZM). The MZM is split into two traveling-wave segments to increase the bandwidth and to allow a 2-bit DAC functionality. Two limiting driver channels are used to drive these segments, allowing both NRZ and PAM4 signal generation in the optical domain. The voltage swing as well as the peaking of the driver output are tunable, hence the PAM4 signal levels can be tuned and possible bandwidth limitations of the MZM segments can be partially alleviated. Generation of 50 Gbaud and 53 Gbaud PAM4 yields a TDECQ of 2.8 and 3.8 dB with a power efficiency of 3.9 and 3.6 pJ/bit, respectively; this is the best reported efficiency for co-packaged silicon transmitters for short-reach datacenter interconnects at these data rates. With this work, we show the potential of limiting drivers and segmented traveling-wave modulators in 400G capable short-reach optical interconnects.

Toward Single Lane 200G Optical Interconnects With Silicon Photonic Modulator

In this work, based on a conventional silicon photonic travelling-wave Mach-Zehnder modulator (MZM) with a 3-dB bandwidth of 22.5 GHz, we experimentally demonstrate ultra-high speed optical interconnects with Nyquist shaped pulse amplitude modulation (PAM) signals. For bandwidth-limited systems, a two-tap digital post filter is employed to suppress the equalization-enhanced high frequency noise. The post filter induced inter-symbol interference (ISI) is subsequently eliminated by maximum likelihood sequence detection (MLSD). Enabled by the post filter and MLSD, we achieve 200 Gb/s (80 Gbaud) PAM-6 signal direct detection (DD) transmission over 1 km standard single-mode fiber (SSMF) with a bit-error rate (BER) below the 20% hard-decision forward error correction (HD-FEC) threshold of 1.5 × 10−2. For PAM-4 format, we successfully transmit 192 Gb/s (96 Gbaud) and 176 Gb/s (88 Gbaud) signals over 1 km and 2 km SSMF, respectively, with BERs lower than the 20% HD-FEC threshold. For PAM-8 format, 192 Gb/s (64 Gbaud) and 168 Gb/s (56 Gbaud) signals are generated at back-to-back (BTB) and transmitted over 1 km SSMF, respectively. To our best knowledge, we achieve the highest single lane bitrates ever reported for single polarization PAM-4/6/8 signal generation and DD transmission with all-silicon MZM. The flexible receiver-side digital signal processing (DSP) can significantly enhance the performance of silicon MZM, which provides a promising solution for future single lane 200 G data-center interconnects.

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.

Silicon traveling-wave Mach-Zehnder modulator under distributed-bias driving.

The silicon traveling-wave (TW) Mach-Zehnder modulator (MZM) is one of the most important devices in silicon photonic transceivers for high-speed optical interconnects. Its phase shifter utilizes carrier depletion of pn diodes for high speed, but suffers low modulation efficiency. Extensive efforts have been made on pre-fabrication optimizations, including waveguides, doping, and electrodes to enhance high-frequency modulation efficiency. Instead, we here propose an adaptive post-fabrication distributed-bias driving method that enables 20%∼30% high-frequency efficiency enhancement at both 10 and 25 Gbps without doing any optimizations for a silicon TW-MZM. This method explores the bias nonlinearity of index modulation which, to the best of our knowledge, is utilized for the first time in driving silicon modulators to improve the efficiency. We demonstrated the viability of this adaptive driving concept to achieve better performance, and this Letter could open new avenues for silicon traveling-wave modulator design and performance trade-off.

200 Gb/s transmission using a dual-polarization O-Band silicon photonic intensity modulator for Stokes vector direct detection applications.

We present a dual-polarization O-band silicon photonic (SiP) transmitter for intra-datacenter optical interconnects. The transmitter is built using two identical O-band traveling wave Mach-Zehnder modulators with an average VπL and a bandwidth at 1.5 V bias voltage of 2.88 V.cm and 24.5 GHz, respectively. We experimentally demonstrate the transmitter in a Stokes vector direct-detection (SV-DD) system for dual-polarization intensity modulated signals with 2-level and 4-level pulse amplitude modulation (DP-PAM2 and DP-PAM4) formats. The direct-detection Stokes vector receiver (DD-SVR) followed by offline digital signal processing (DSP) is implemented for SOP de-rotation. We characterize the performance of the SV-DD system versus number of taps, received signal power, state of polarization (SOP), reach, and bit rate. Results reveal that 112 Gb/s DP-PAM2 can be transmitted over 10 km of single mode fiber (SMF) at a bit error rate (BER) below 10-5 at -1 dBm received signal power irrespective of the SOP. Moreover, a 168 Gb/s (42 Gbaud) DP-PAM4 signal can be transmitted over 2 km and 10 km at a BER below the 7% hard-decision forward error correction (HD-FEC) threshold (i.e., 3.8 × 10-3) at 0 dBm and 2 dBm, respectively. Furthermore, 224 Gb/s and 200 Gb/s DP-PAM4 are successfully received at a BER below the HD-FEC in the back-to-back and 2 km cases, respectively. Finally, we compare the performance of the 6 × 2 multiple-input multiple-output (MIMO) equalization to a simpler 4 × 2 MIMO equalization and explain the superior performance of the 6 × 2 in the presence of SVR imperfections.

Travelling-Wave Mach-Zehnder Modulator as a Temporal Integrator and a Time-Gate Isolator

This letter demonstrates that a commercial travelling-wave Mach–Zehnder modulator is a versatile device. Next to being a modulator, it is also able to function as an electro-optical temporal integrator with an integration window twice as long as its propagation delay. Using the similar principle and being driven by a periodic RF frequency, the modulator is able to perform as an optical time-gate isolator that blocks any reverse-travelling lightwave, but simultaneously allows forward-travelling periodically-pulsed optical signals to pass. In the experiment, sub-nanosecond forward-travelling time gates and reverse-travelling optical power extinction >20 dB are successfully demonstrated.

Experimental parametric study of 128 Gb/s PAM-4 transmission system using a multi-electrode silicon photonic Mach Zehnder modulator.

We present an experimental study and analysis of a travelling wave series push-pull silicon photonic multi-electrode Mach-Zehnder modulator (ME-MZM) and compare its performance with a single-electrode travelling wave Mach-Zehnder modulator (TWMZM). Utilizing the functionality of the ME-MZM structure plus digital-signal-processing, we report: 1) the C-band transmission of 84 Gb/s OOK modulated data below the KP4 forward error correction threshold with 2 Vpp drive voltage over a distance of 2 km; 2) the transmission of a 128 Gb/s optical 4-level pulse amplitude modulated signal over 1 km of fiber; and 3) the generation of a 168 Gb/s PAM-4 signal using two electrical OOK signals. By comparing the transmission system performance measurements for the ME-MZM with measurements performed using a similar series push-pull TWMZM, we show that the ME-MZM provides a clear advantage in achieving higher baud PAM-4 generation and transmission compared to a TWMZM.

Monolithic Tunable Coupled-Cavity WDM Transmitter in a Generic Foundry Platform

We present the monolithic integration of an extended tuning range coupled-cavity laser with traveling-wave Mach–Zehnder modulators in a low-cost generic photonic foundry platform for 10 Gb/s operation. Using an intra-cavity Michelson interferometer combined with on-chip reflectors, the coupled-cavity laser shows a tuning range of 25 nm. The modulator building blocks with optimized traveling wave electrodes exhibit efficient modulation and 8.5-GHz electro-optic bandwidth, enabling 10 Gb/s ON–OFF keying. We demonstrate error-free operation of the tunable transmitter in a full photonic circuit environment. The laser modulator combination neither requires Bragg grating formation for wavelength tuning nor additional growth steps for multiquantum-well material in the modulator section and is therefore well suited for cost effective foundry platforms specifically for low-cost applications.

Observation of nonreciprocal transmission in binary phase-shift keying modulation using traveling-wave Mach-Zehnder modulators.

In coherent optical transmission, traveling-wave Mach-Zehnder modulators are commonly used to generate various advanced formats where the modulators are biased at the minimum transmission point. Here, we report that an optical isolation effect with lower backward transmission occurs under this condition. This concept is successfully demonstrated to achieve ∼7 dB isolation over a 90-nm wavelength span under binary phase-shift keying modulation using a commercial lithium niobate modulator.

Design Considerations for Traveling-Wave Modulator-Based CMOS Photonic Transmitters

Systematic design and simulation methodology for hybrid optical transmitters that combine CMOS circuits in a 130-nm process and a traveling-wave Mach–Zehnder modulator (TWMZM) in a 130-nm silicon-on-insulator CMOS process is presented. A compact Verilog-A model for the TWMZM is adopted for the electrooptical simulation. A bond wire model using a high-frequency solver is included for accurate package simulation. The transmitter post-layout simulation result exhibits 5.48-dB extinction ratio, 9.6-ps peak-to-peak jitter, and the best power efficiency of 5.81 pJ/bit when operating up to 12.5-Gb/s non-return-to-zero data. A pulse-amplitude modulation four-level transmitter with a detailed linearity design procedure is presented, which has horizontal and vertical eye openings of 49 ps and 203 $\mu$ W when operating at 25 Gb/s, and has power efficiency is 5.09 pJ/bit.