Coherent Data Center Links

 J. K. Perin, A. Shastri and J. M. Kahn, "Coherent Data Center Links," in Journal of Lightwave Technology, vol. 39, no. 3, pp. 730-741, 1 Feb.1, 2021, doi: 10.1109/JLT.2020.3043951.

This was a great paper that opened my eyes to the use of coherent optical technology for data centers. What follows is the paper outline with selected sentences.

1. Introduction
• "Conventional coherent detection with polarization multiplexing and a strong local oscillator (LO) recovers phase and magnitude in each polarization, thus utilizing all four degrees of freedom of the optical channel, while maximizing optical power efficiency.
2. Optical Detection Methods
• In coherent detection, the decision variables are the electric field quadratures in each polarization, which can be measured after homodyne (or heterodyne) downconversion or estimated from an intensity measurement, in the case of KK detection.
• KK = Kramers-Kronig
1. LO-Free vs LO-Based Downconversion
• Traditional coherent detection employs homodyne LO-based downconversion, whereby the received signal is mixed with a LO laser whose frequency is approximately equal to the transmitter laser frequency. Note that heterodyne downconversion, whereby the LO and transmitter laser frequencies differ by an intermediate frequency, achieves the same performance as homodyne downconversion in most practical scenarios [7], but requires higher receiver bandwidth, since the downconverted signal is centered around an intermediate frequency.
• Although an LO increases the receiver complexity, LO-based downconversion offers two key advantages compared to LO-free downconversion. First, the LO amplification gain can boost receiver sensitivity by up to 20 dB in unamplified links [12]. This gain is not critical in amplified links, where the receiver is limited by amplified spontaneous emission (ASE) noise rather than shot noise. Second, the LO provides an absolute phase reference, allowing for full recovery of amplitude and phase components of the electric field. 
2. Impact of Co-Packaged Optics
3. Typical Embodiment
1. Direct Detection
• Noncoherent detection of OOK has been traditionally used in data center links, given its low cost and high receiver sensitivity.
• Level spacing optimization improves the receiver sensitivity by roughly 1 dB for APD-based receivers Fig. 1a, while in amplified systems Fig. 1b, it results in ∼3-dB OSNR improvement.
• APD= avalanche photo diodes
• RJA note: I believe that this means that the symbol voltage levels are not symmetric to take advantage of the different noise characteristics of the different symbol levels.
• CD mitigation through linear equalization is only effective when CD is small.
• CD=chromatic dispersion
2. Differentially Coherent Detection
3. DSP-Based LO-Based Coherent Detection
• Coherent detection based on high-speed DSP is a mature technology in long-haul systems, but it may be currently unsuitable for data center links, where cost and power consumption are paramount.
• The most power-hungry operations are CD equalization and polarization demultiplexing with PMD compensation, which together amount to roughly 55% of the receiver power consumption.
• PMD = polarization mode dispersion
• Short-reach links in data centers are subject to lower CD and negligible PMD, hence both CD and PMD filters can be simplified.
4. DSP-Free LO-Based Coherent Detection
• A coherent receiver must perform three basic operations: polarization demultiplexing, carrier recovery, and timing recovery. In traditional long-haul coherent receivers all these operations have been performed digitally.
• Three cascaded phase shifter pairs can perform any arbitrary polarization rotation.
• The performance shown in Fig. 3 for DSP-based coherent receivers is independent of dispersion, as CD is compensated effectively by digital equalization.
• The performance gap between DSP-free and DSP-based receivers can be reduced by leveraging analog equalization as proposed in [4]. 
• Although all-analog equalization does not offer the same flexibility or near-optimal performance of digital equalization, it requires substantially lower power since all the operations are still performed in the analog domain, thus avoiding high-speed ADCs and DSP.
5. Kramers-Kronig Coherent Detection
• The KK receiver performs simple direct detection and digitally recovers the electric field quadratures from a signal intensity measurement
• note: KK utilizes the Hilbert transform to recover the phase from the complex envelope.
• the phase ϕ(t) can only be uniquely recovered as long as the signal E(t) satisfies a minimum-phase condition
• The KK receiver enables coherent transmission without requiring a LO, significantly reducing the complexity of receiver optics. However, the additional optical power of transmitting a carrier signal along with data significantly reduces reach. Higher sampling rates, along with strict optical filter requirements, further increase the complexity of the receiver.
4. Complexity and Power Consumption Consideration
• DSP-free coherent solutions have similar, or more, optical complexity, but may offer lower power consumption by removing power-hungry ADCs and DSP.
• it is likely that that for the next few years, low-power direct detection of OOK and 4-PAM will continue to dominate the shortest links (<2 km) within data centers. 
• At the other extreme, links beyond 40 km will continue to rely on DSP-based coherent detection, as it offers the highest receiver sensitivity, highest throughput, and it is more robust to transmission impairments. 
• The intermediate distances from 2 km to 40 km still require a low-power alternative that enables more degrees of freedom than direct detection, while improving receiver sensitivity. So far, DSP-free coherent detection is the detection method that best meets these requirements. DSP-based coherent detection might become attractive over those distances is reduced by innovative designs and CMOS technology improvements.
5. Conclusion