Compensation of Laser Phase Noise Using DSP in Multichannel Fiber-Optic Communications
Doctoral thesis, 2020

One of the main impairments that limit the throughput of fiber-optic communication systems is laser phase noise, where the phase of the laser output drifts with time. This impairment can be highly correlated across channels that share lasers in multichannel fiber-optic systems based on, e.g., wavelength-division multiplexing using frequency combs or space-division multiplexing. In this thesis, potential improvements in the system tolerance to laser phase noise that are obtained through the use of joint-channel digital signal processing are investigated. To accomplish this, a simple multichannel phase-noise model is proposed, in which the phase noise is arbitrarily correlated across the channels. Using this model, high-performance pilot-aided phase-noise compensation and data-detection algorithms are designed for multichannel fiber-optic systems using Bayesian-inference frameworks. Through Monte Carlo simulations of coded transmission in the presence of moderate laser phase noise, it is shown that joint-channel processing can yield close to a 1 dB improvement in power efficiency. It is further shown that the algorithms are highly dependent on the positions of pilots across time and channels. Hence, the problem of identifying effective pilot distributions is studied.
The proposed phase-noise model and algorithms are validated using experimental data based on uncoded space-division multiplexed transmission through a weakly-coupled, homogeneous, single-mode, 3-core fiber. It is found that the performance improvements predicted by simulations based on the model are reasonably close to the experimental results. Moreover, joint-channel processing is found to increase the maximum tolerable transmission distance by up to 10% for practical pilot rates.
Various phenomena decorrelate the laser phase noise between channels in multichannel transmission, reducing the potency of schemes that exploit this correlation. One such phenomenon is intercore skew, where the spatial channels experience different propagation velocities. The effect of intercore skew on the performance of joint-core phase-noise compensation is studied. Assuming that the channels are aligned in the receiver, joint-core processing is found to be beneficial in the presence of skew if the linewidth of the local oscillator is lower than the light-source laser linewidth.
In the case that the laser phase noise is completely uncorrelated across channels in multichannel transmission, it is shown that the system performance can be improved by applying transmitter-side multidimensional signal rotations. This is found by numerically optimizing rotations of four-dimensional signals that are transmitted through two channels. Structured four-dimensional rotations based on Hadamard matrices are found to be near-optimal. Moreover, in the case of high signal-to-noise ratios and high signal dimensionalities, Hadamard-based rotations are found to increase the achievable information rate by up to 0.25 bits per complex symbol for transmission of higher-order modulations.

multichannel transmission


digital signal processing

laser phase noise

Coherent fiber-optic communications



Opponent: Stephan ten Brink


Arni Alfredsson

Chalmers, Electrical Engineering, Communication, Antennas and Optical Networks

Iterative Detection and Phase-Noise Compensation for Coded Multichannel Optical Transmission

IEEE Transactions on Communications,; Vol. 67(2019)p. 5532-5543

Journal article

Pilot Distributions for Joint-Channel Carrier-Phase Estimation in Multichannel Optical Communications

Journal of Lightwave Technology,; Vol. 38(2020)p. 4656-4663

Journal article

Pilot-Aided Joint-Channel Carrier-Phase Estimation in Space-Division Multiplexed Multicore Fiber Transmission

Journal of Lightwave Technology,; Vol. 37(2019)p. 1133-1142

Journal article

On the Performance of Joint-Core Carrier-Phase Estimation in the Presence of Intercore Skew

Journal of Lightwave Technology,; Vol. 37(2019)p. 5291-5298

Journal article

Optimization of Transmitter-Side Signal Rotations in the Presence of Laser Phase Noise

Journal of Lightwave Technology,; Vol. 38(2020)p. 3850-3858

Journal article

Most people use the Internet today to, e.g., read the news, watch movies, and play games. When we use the Internet, we exchange information in the form of data with various places around the world. What allows us to do this? The answer is communication systems, which are used to transfer data between different places. Communication systems that use light and optical fibers are called fiber-optic communication systems and can transfer enormous amounts of data. Today, fiber-optic systems are used all over the globe and form the foundation of the Internet as they connect cities, countries, and even continents together.
The light that is used to carry the data between places is generated using lasers. In an ideal world, the lasers would be able to generate the light without any issues. However, lasers in the real world are not perfect and the light they generate does not have a stable phase. This phenomenon is called laser phase noise. Why is it a problem? Because the data that the light carries is represented by the amplitude and phase of the light. When the phase of the light is instable, it is hard to guess correctly at the receiving end which data was sent. Fortunately, we can compensate for the laser phase noise when we observe the light at the receiving end, which allows us to guess more easily which data was sent.
Fiber-optic communication systems typically contain many channels through which light can travel. This allows us to send many different light signals simultaneously, where each light signal carries different data. This is partly why fiber-optic systems can transfer so much data at once. The act of sending multiple light signals through many channels simultaneously is sometimes called multichannel transmission. This thesis is concerned with the compensation of laser phase noise in multichannel fiber-optic transmission. It turns out that we can compensate for laser phase noise in a powerful way by exploiting certain properties of multichannel fiber-optic systems. The interested reader is referred to page i of the thesis, which includes a more technical summary of the thesis contributions.

Multi-dimensional Signal Processing with Frequency Comb Transceivers

Swedish Research Council (VR) (2018-03701), 2018-12-01 -- 2021-12-31.

MIMOptics: Multi-mode coherent fiber-optical communications

Swedish Research Council (VR) (2013-5642), 2014-01-01 -- 2017-12-31.

Technologies for spatial-division multiplexing: The next frontier in optical communications

Swedish Research Council (VR) (2014-6138), 2015-01-01 -- 2018-12-31.

Unlocking the Full-dimensional Fiber Capacity

Knut and Alice Wallenberg Foundation (KAW 2018.0090), 2019-07-01 -- 2024-06-30.

Subject Categories


Communication Systems



Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4734




Opponent: Stephan ten Brink

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