There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

The coherent transmission technology using digital signal processing and advanced modulation formats, is bringing networks closer to the theoretical capacity limit of optical fibres, the Shannon limit. The in-phase/quadrature electro-optic modulator that encodes information on both the amplitude and the phase of light, is one of the underpinning devices for the coherent transmission technology. Ideally, such modulator should feature a low loss, low drive voltage, large bandwidth, low chirp and compact footprint. However, these requirements have been only met on separate occasions. Here, we demonstrate integrated thin-film lithium niobate in-phase/quadrature modulators that fulfil these requirements simultaneously. The presented devices exhibit greatly improved overall performance (half-wave voltage, bandwidth and optical loss) over traditional lithium niobate counterparts, and support modulation data rate up to 320 Gbit s−1. Our devices pave new routes for future high-speed, energy-efficient, and cost-effective communication networks. In-phase/quadrature (IQ) electro-optic modulators are underpinning devices for coherent transmission technology. Here the authors present IQ modulators in the lithium-niobate-on-insulator platform, which provide improved overall performance and advanced modulation formats for future coherent transmission systems.

/pdf/high-performance-coherent-optical-modulators-based-on-thin-3bgx2fadbn.pdf

High-performance coherent optical modulators based on thin-film lithium niobate platform

Coherent detection offers high spectral efficiency and receiver sensitivity, but digital signal processing (DSP)-based coherent receivers may be prohibitively power hungry for data centers even when optimized for short-reach applications, where fiber propagation impairments are less severe. We propose and evaluate low-power DSP-free homodyne coherent receiver architectures for dual-polarization quadrature phase shift keying (DP-QPSK) for inter- and intradata center links. We propose a novel optical polarization demultiplexing technique, for DP-QPSK and higher-order modulation formats, with three cascaded phase shifters driven by marker tone detection circuitry. We consider carrier recovery based on either optical or electrical phase-locked loops (PLLs). We propose a novel multiplier-free phase detector based on XOR gates, which exhibits less than 0.5 dB power penalty relative to a conventional Costas loop phase detector. We also study the relative performance of homodyne DP-differential QPSK, for which carrier phase recovery is unnecessary. Our proposed DSP-free architectures exhibit ∼1 dB power penalty at small chromatic dispersion compared to their DSP-based counterparts. We estimate conservatively that the high-speed analog electronics of an electrical PLL-based coherent receiver consume nearly 4 W for 200 Gbit/s DP-QPSK, assuming a 90-nm complementary metal-oxide semiconductor process.

Design of Low-Power DSP-Free Coherent Receivers for Data Center Links

Client-side optics are facing an ever-increasing upgrading pace, driven by upcoming 5G related services and datacenter applications. The demand for a single lane data rate is soon approaching 200 Gbps. To meet such high-speed requirement, all segments of traditional intensity modulation direct detection (IM/DD) technologies are being challenged. The characteristics of electrical and optoelectronic components and the performance of modulation, coding, and digital signal processing (DSP) techniques are being stretched to their limits. In this context, we witnessed technological breakthroughs in several aspects, including development of broadband devices, novel modulation formats and coding, and high-performance DSP algorithms for the past few years. A great momentum has been accumulated to overcome the aforementioned challenges. In this article, we focus on IM/DD transmissions, and provide an overview of recent research and development efforts on key enabling technologies for 200 Gbps per lane and beyond. Our recent demonstrations of 200 Gbps short-reach transmissions with 4-level pulse amplitude modulation (PAM) and discrete multitone signals are also presented as examples to show the system requirements in terms of device characteristics and DSP performance. Apart from digital coherent technologies and advanced direct detection systems, such as Stokes–vector and Kramers–Kronig schemes, we expect high-speed IM/DD systems will remain advantageous in terms of system cost, power consumption, and footprint for short reach applications in the short- to mid- term perspective.

/pdf/200-gbps-lane-im-dd-technologies-for-short-reach-optical-2nuta7bqsd.pdf

200 Gbps/Lane IM/DD Technologies for Short Reach Optical Interconnects

Increasing the modulation speed of semiconductor lasers has attracted much attention from the viewpoint of both physics and the applications of lasers. Here we propose a membrane distributed reflector laser on a low-refractive-index and high-thermal-conductivity silicon carbide substrate that overcomes the modulation bandwidth limit. The laser features a high modulation efficiency because of its large optical confinement in the active region and small differential gain reduction at a high injection current density. We achieve a 42 GHz relaxation oscillation frequency by using a laser with a 50-μm-long active region. The cavity, designed to have a short photon lifetime, suppresses the damping effect while keeping the threshold carrier density low, resulting in a 60 GHz intrinsic 3 dB bandwidth (f3dB). By employing the photon–photon resonance at 95 GHz due to optical feedback from an integrated output waveguide, we achieve an f3dB of 108 GHz and demonstrate 256 Gbit s−1 four-level pulse-amplitude modulations with a 475 fJ bit−1 energy cost of the direct-current electrical input. Directly modulated membrane distributed reflector lasers are fabricated on a silicon carbide platform. The 3 dB bandwidth, four-level pulse-amplitude modulation speed and operating energy for transmitting one bit are 108 GHz, 256 Gbit s−1 and 475 fJ, respectively.

Directly modulated membrane lasers with 108 GHz bandwidth on a high-thermal-conductivity silicon carbide substrate

We demonstrate 100 GBd operation of an optical transmitter module including a distributed feedback laser monolithically integrated with a Mach–Zehnder modulator (DFB-MZM), to the best of our knowledge for the first time. Combining high-speed optics and electronics with digital signal processing (DSP), different schemes for intensity modulation and direct detection at 100, 200, and 300 Gbit/s are analyzed, using electrical signals from a 100 GSa/s BiCMOS DAC. Due to close-to-zero transmitter chirp, 100 Gbit/s NRZ transmission over 1.8 km and 200 Gbit/s PAM4 transmission over 1.2 km at 1550 nm with a bit error rate (BER) below the 7% overhead forward error correction (FEC) threshold (3.8 × 10−3) is achieved. At 200 Gbit/s PAM4, the DFB-MZM module consumes only 0.85 pJ/bit, making it a promising device for attractive dual-lane 400 Gbit/s systems. We also achieve 300 Gbit/s PAM8 transmission over 1.2 km with a BER below the 20% overhead FEC threshold (1.9 × 10−2) by implementing advanced DSP based on a pattern-dependent lookup table to mitigate the electrical and optical device bandwidth limitations.

100 GBd Intensity Modulation and Direct Detection With an InP-Based Monolithic DFB Laser Mach–Zehnder Modulator

We demonstrate, for the first time with a DFB-MZM at 1550 nm, 100-GBd NRZ and PAM4 transmission over up to 1.2 km due to close-to-zero chirp. At 100-GBd PAM4, the DFB-MZM consumed only 0.85 pJ/bit.

100 GBd intensity modulation and direct detection with an InP-based monolithic DFB laser Mach-Zehnder modulator

We present a transmitter optical subassembly consisting of an InP-based travelling wave electrode Mach–Zehnder modulator monolithically integrated with a distributed feedback laser. At 56-Gb/s non-return-to-zero ON/OFF keying (NRZ-OOK), we achieve 3.5-km transmission before breaking the hard decision forward error correction threshold (3.8 ×10−3). 9-km transmission is achieved at 71.6-Gb/s 4-pulse–amplitude modulation (PAM4) and 5 km at 107.4 Gb/s PAM8. The modulation voltage is between 1.5 and 2 V, leading to overall power consumption of 171 mW for the NRZ-OOK case and 206 mW at PAM4 and PAM8.

Low Switching Voltage Mach–Zehnder Modulator Monolithically Integrated With DFB Laser for Data Transmission up to 107.4 Gb/s

An optical transmitter incorporating a monolithically integrated laser and a traveling wave electrode in-phase and quadrature modulator is presented. A co-designed SiGe differential driver was co-packaged for highest quality push-pull modulation. With only 0.1-dB coupling loss between the laser and the modulator, and a low modulator switching voltage the transmitter power consumption was 1.1 W, which fits the CFP4 (analog coherent optics) ACO module overall budget of 6 W. Without DSP for precompensation, FEC or equalization, 32 GBd QPSK with 3 V pp driving voltage and an error vector magnitude of <10% are presented. Operation with an optical signal-to-noise ratio larger than 14 dB is achieved before breaking the hard decision forward error correction threshold (3.8 × 10−3).

Sophie Lange

Papers

100 GBd Intensity Modulation and Direct Detection With an InP-Based Monolithic DFB Laser Mach–Zehnder Modulator

100 GBd intensity modulation and direct detection with an InP-based monolithic DFB laser Mach-Zehnder modulator

Low Switching Voltage Mach–Zehnder Modulator Monolithically Integrated With DFB Laser for Data Transmission up to 107.4 Gb/s

Low Power InP-Based Monolithic DFB-Laser IQ Modulator With SiGe Differential Driver for 32-GBd QPSK Modulation