Coherent Optical Communication

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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.

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  

Why IM-DD to Coherent?

我們先看目前 IM-DD 的 status 以及問題。

首先 data rate 和距離如下圖：

  • Data rate: 100G(bps) / 400G / 1.6T
  • 距離: Long haul (>100km) / Metro (300km) / Edge (80km) / Campus (10km) / Intra data center (0.5-2km)
  • 直觀說 data rate 和距離成反比

image-20221209222748077

Coherent for Long Haul and Metro

目前 CMOS electrical serdes link 是 25Gbps using NRZ (Non-Return to Zero). 因此 100Gbps 需要 4 wavelength WDM. 如果使用 1.3um wavelength, fiber loss 是 0.5dB/km, 只能到達 20km 就需要 optical or electronic repeater. 如果使用 1.5um wavelegnth, fiber loss 是 0.2dB/km, 距離可以加倍。但是 fiber dispersion 又會限制距離。

因此在 Long haul 和 Metro 已經開始使用 coherent communication.

另外 coherent optical communication 可以使用 QPSK 以及 dual polarization quadruple (x4) per wavelength. 因此原來 100Gbps 的系統可以直接 upgrade 到 400Gbps per wavelength using the same fiber.

Coherent 的兩個好處:

  • 可以用 1.5um wavelength 加上 electronic dispersion compensation 加長 repeater link (40km/repeater).
  • 可以 quadruple link capacity per wavelength

Coherent for Data Center (2022)

目前 (2022) 最熱門的是 1.6T for data center. 目標是 2-10km. 傳統的 IM-DD 系統需要非常多的 wavelegnth.

因爲 IM-DD link 沒有 phase information, 只能在 amplitude 上做文章。因此推出 PAM4 系統增加 spectra efficiency 加倍 data rate per wavelength. 不過會讓距離更短，因爲更容易受到 impairment 的影響。

QSFP-DD (Double Density, PAM4) 的特性：

  • Electrical: 8 links of 25Gbps (NRZ) for 200Gbps; 或是 8 links of 50Gbps (PAM4) for 400Gbps
  • Optical: 8 wavelength?

IEEE 和 Optical Internetworking Forum (OIF) 已經 propose 800Gbps coherent link for campus network (2-10km), 不過 industry 思考 1.6Tbps coherent link.

但是 coherent 的成本相對比較高。因此有所謂的 coherent lite communication 目標是 data center (2-10km) for 1.6Tbps.

幾個節省成本的重點

  • Fixed wavelength 而非 tunable wavelength

  • Low power for lower overall system cost (density, cooling)

  • 106/112 Gbps, 56Gbaud, PAM4

  • retimer, POR switch

  • multiple wavelength lasers: chromatic dispersion laser

TOR (top array switch):

  • chip-to-chip
    • copackage optics: fiber in package
    • inpackage optics: customer (amazon, …)
    • QSFP: opto
  • 2m cable to fiber, TOR switch can be eliminated, no TOR !! eliminated 1-layer. Power and thermal limit
    • switch is lower due to process advantage
    • DSP serdes (Lawrence Loh)
    • Infenaria is too expensse $20,000 module. Infi acadia $2000, integeated
  • chip to module

224G

106/112 Gbps, 56Gbaud, PAM4

retimer, POR switch

Reference