OFC2025|Colorless Detection Technology for Uncooled Coherent Optical Systems in WDM Superchannels

Introduction

The continuous demand for higher data transmission rates in optical communication systems has driven technological innovation, particularly in data center applications. A recent advancement in this area is the colorless detection of wavelength-division multiplexing (WDM) signals, which eliminates the need for laser wavelength control and demultiplexers (deMUX). This paper explores this innovative approach, which enables uncooled coherent optical systems with significant tolerance to laser frequency drift [1].

Overview of Colorless Detection Technology

Amid ongoing discussions regarding the use of intensity-modulation direct detection (IM-DD) versus coherent detection in data center applications, coherent detection provides higher per-wavelength data rates. However, traditional coherent detection has a fundamental limitation: the local oscillator (LO) must emit a frequency close to the transmitted signal to correctly downconvert it. This requirement mandates precise temperature control, which is power-hungry in uncooled environments.

Conventional “colorless” receivers typically utilize tunable lasers, which are not truly colorless since they still need to be fixed at specific channel wavelengths. Moreover, simultaneous detection of multiple WDM channels requires multiple cooled lasers locked to the WDM grid.

The innovative method described in this paper replaces single-frequency LOs with a multi-line frequency comb to cover potential drift ranges. Although comb-based receivers have been proposed for optical access networks, these designs are not suitable for wideband WDM signals due to spectral aliasing issues when the signal bandwidth exceeds the comb’s free spectral range (FSR).

Operating Principle of Colorless Detection

The system considers a gapless WDM signal (superchannel) consisting of N subcarriers with a total optical bandwidth of B Hz. The colorless coherent receiver includes K balanced coherent receivers (BCRs) and a comb LO with an FSR of B/K.

Figure 1: Schematic of a comb-based SCM superchannel colorless coherent receiver, extendable to any number of BCRs.

The comb bandwidth is designed to exceed the relative frequency drift between the transmitter and receiver. Both the signal and LO are power-split and fed to all BCRs. Due to the multi-wavelength LO, each BCR downconverts the superchannel into multiple replicas at different frequencies, with the BCR bandwidth acting as a spectral slicer.

To introduce orthogonality between BCR outputs, the system borrows from optical time-division multiplexing (OTDM) techniques. By adding a series of time delays to the comb-based LO, the system evenly partitions the repetition period T = 1/Δf, allowing separation of overlapping subcarriers.

Three key configurations maximize receiver bandwidth efficiency:

1. The subcarrier spacing (Wc) is significantly smaller than the comb FSR.

2. The comb FSR is an integer multiple (m) of the subcarrier spacing (Δf = mWc).

3. The BCR bandwidth covers (m+1) subcarriers.

With these settings, even under arbitrary frequency drift, all subcarriers will find complete replicas, and overlapping subcarriers are aligned in frequency, allowing digital signal processing (DSP) to independently process each subcarrier group.

Experimental Setup and Results

A proof-of-concept experimental was conducted using two BCRs (K=2) as shown in the figure below.

Figure 2: Experimental setup including modulators, polarization components, and signal processing workflow.

The combs at both the transmitter and receiver were generated using a high-power RF tone of 33.936 GHz to drive phase modulators. At the transmitter, a wavelength-selective switch picked comb lines with an FSR of 135.744 GHz. Three lines were selected and sent to an I/Q modulator, which was driven by an 8-SCM signal. Each 16.8-GBd subcarrier was modulated with probabilistically shaped 64-QAM symbols.

To simulate polarization-division multiplexing (PDM), the signal was combined with a decorrelated copy via a 10-meter delay. The signal was transmitted over 81 km of standard single-mode fiber (SSMF). The receiver comb spectrum cascaded three phase modulators, and an FSR of 203.616 GHz was selected. Seven comb lines were selected, covering ~1.2 THz of bandwidth, allowing tolerance of up to ~1-THz frequency drift for a 400-GHz WDM superchannel.

To verify colorless detection, the LO seed laser was fixed at 193.4 THz while the transmitter laser frequency was swept over a 1.02 THz range. The figure below shows the normalized generalized mutual information (NGMI) measured across this range.

Figure 3: NGMI values at various frequency offsets, including received spectrum and constellation diagrams.

NGMI remained stable and above the forward error correction (FEC) threshold across the full frequency offset range, demonstrating successful WDM transmission over 81 km SSMF even with frequency drift up to 1.02 THz. A net data rate of 3.35 Tb/s was achieved.

Advantages and Applications

This colorless detection system offers several advantages:

1. Eliminates the need for power-hungry wavelength control, making it ideal for power-sensitive applications.

2. Requires hardware and bandwidth comparable to traditional WDM receivers (excluding the comb LO).

3. Utilizes a generic multiple-input multiple-output (MIMO) equalizer for simultaneous polarization and subcarrier demultiplexing.

4. Exhibits strong tolerance to laser frequency drift (~1 THz), enabling true uncooled operation.

This approach is particularly suited for:

• Highly parallel data center interconnects where energy efficiency is critical

• Point-to-multipoint networks with uncooled lasers

• Applications demanding cost-effective coherent detection with minimal temperature control

Conclusion

The demonstrated colorless coherent receiver for multi-Tb/s WDM superchannels represents a significant advancement in optical communications. By eliminating the need for wavelength control and demultiplexers, the technology addresses key challenges of deploying coherent detection in data centers and other uncooled environments. Experimental validation confirms stable performance over a wide frequency drift range, affirming the viability of this approach for high-speed, energy-efficient optical interconnects.

Reference

[1] D. Che, M. Mazur, and N. K. Fontaine, "Colorless Detection of a 3.2-Tb/s-Class WDM Superchannel Aiming for Uncooled Coherent Optics," in OFC 2025, Optica Publishing Group, 2025, Paper Th1D.2.