Coherent Optical Communication

Posted on | In

Citation

[@liRecentAdvances2009] : 很好的 coherent optical communication review paper.

[@kikuchiFundamentalsCoherent2016] : 比較老派的 coherent optical communication review paper.

[@liSiliconMicroring2016] : 新的 micro-ring tunable laser for coherent optical communication.

[@parkCoherentKnocking2022] : coherent is knocking on the data center door

Introduction

30 年前做的 coherent optical communication 前幾年又火紅起來。最近看了幾篇 review papers, 感覺還蠻能 catch up.

大致整理一下 30 年前的技術和這幾年的不同。

簡單說 30 年前 coherent optical communication 主要賣點是 sensitivity at receiver. All optical Er-doped fiber amplifer 可以 distributedly amplify optical signal, 可以讓簡單的 intensity modulation + direct detection optical communcation system kill coherent communication.

因此 IM-DD transmitter/receiver + all optical Er-doped fiber amplifer is the king! 做掉 coherent optical communication.

之後由於 WDM 和 DWDM 興起。Channel capacity 變成 key issue. 第一步是 narrow channel spacing, 第二步是增加 spectral efficiency (modulation efficiency). IM-DD + optical filter 系統只能做到 WDM (>100GHz); coherent optical communcation 因為可以結合 electrical filter (50GHz or smaller) 以及 better modulation efficiency (8PSK, 16QAM) 又復活！

不過重點是高速 DSP 能夠減輕之前 coherent optical communcation 的系統複雜度。像是 polarization, frequency drift, modulation/demodulation, FEC encode/decode, etc. 更進一步 DSP 可以做 dispersion equalization, fiber nonlinearity mitigation (how?) 讓系統 performance 更勝於 IM-DD 系統。

Issues 1992 Coherent Comm. 2022 Coherent Comm. Comment
Sensitivity (Electrical) LO gain at Rx only
Lose to (distributed) Er-doped fiber amplifier		 (Optical) Er-doped fiber amplifier + LO gain at Rx  
Laser phase noise (Electrical) Phase diversity reciver
Lose to IM-DD (Intensity modulation-direct detection)		 (Optical) Narrow linewidth laser (for phase modulation) of 100’s kHz FWHB Micro-ring laser
WDM wavelength accuracy Bulky tunable wavelength laser + auto-frequency control loop (AFC) Silicon-integrated tunable wavelength laser + auto-frequency control loop Micro-ring laser
(D)WDM close wavelength spacing (Optical) tunable wavelength laser + optical filter
Only WDM, not DWDM		 (Electrical) coherent + RF filter
ITU G694.1defines 100GHz (0.8nm) for 40 channels, or 50GHz (04nm) for 80 channels		  
SE (Spectral Efficiency) (Optical) IM-DD of low SE (Optical) PSK, QAM due to better laser + (electrical) DSP for phase lock  
Polarization (Optical) Polarization control (Electrical) Polarization diversity receiver using DSP Polarization shift keying?
Dispersion (Optical) Dispersion compensation fiber (Elecrical) Equalization using DSP  
Fiber nonlinearity (Optical) lower power, soliton, wavelength spacing (Electrical) nonlinearity mitigation using DSP  

還有 optical laser / modulator 可以 integrated on silicon. 提供更 compact, precise, and better quality tunable laser and modulator/demodulator TX/RX.

New Concept

Silicon Photonics on SOI + DSP

Silicon Photonics selling point

  • compact
  • potentially integrated with silicon driver and detection

One way to tackle cost and footprint is optical and electronic integration. Optical integration reduces cost by minimizing assembly touch points, minimizing mechanical adjustments, minimizing test apparatuses, and minimizing the amount of material required. Choices for optical integration platforms include Si, InP, GaAs, silica, and polymer. Si photonics (SiPh) is attractive because of the following: 1) all the waveguides are formed from the original crystal boule instead of epitaxy, except for the Ge photodetectors. This results in nearly zero defects. 2) Si has an ideal oxide, silica, which is easily formed and has excellent properties including very low optical loss, high electrical isolation, large refractive index contrast, hermetic sealing, and wafer bondability–leading to silicon-on-insulator (SOI) technology 3) Because of SOI technology, Si waveguides enjoy high index contrast both horizontally and vertically, resulting in extremely high confinement and hence tight bends for compactness. 4) SiPh modulators use carrier movement rather than band-edge effects, making them less sensitive to temperature [1]. 5) Si is a very low cost and strong material allowing for large 300-mm-diameter wafers.

DSP Selling Point

  • (Low speed) control of laser power/wavelength, wavelength tuning,
  • (High speed) Data path
    • TX (modulator) / RX (demod)
    • FEC
    • Data mitigation: polarization division multiplexing, dispersion, nonlinearitys
  • Low power
  1. Polarization Division/Diversity Multiplexing (PDM) vs. MIMO: 可以視為 MIMO.

image-20221205235238540

  1. Micro-ring chip (narrow-line width tunable laser in a butterfuly package)

    image-20221205235609178

Company

Inphi -> Clarify -> Marvell

Alphawave -> Bania Labs

Acacia -> Cisco

​ 400Gbsps/QPSK

Application

Data center

Long haul

Metro

Access

Reference