Optical Measurements Leveraging Coherent Fiber Optics Transceivers

Recent advancements in optical technology are invaluable in a variety of fields, extending far beyond high-speed communications. These innovations enable optical sensing, which plays a critical role across diverse applications, from medical diagnostics to infrastructure monitoring and automotive systems. This research focuses on leveraging commercially available coherent optical transceivers to develop novel measurement techniques to extract detailed information about optical fiber characteristics, as well as target information. Through this approach, we aim to enable accurate and fast assessments of fiber performance and integrity, while exploring the potential for utilizing existing optical communication networks to enhance fiber characterization capabilities. This goal is investigated through three distinct projects: (1) fiber type characterization based on intensity-modulated electrostriction response, (2) coherent Light Detection and Ranging (LiDAR) system for target range and velocity detection through different waveform design, including experimental validation of frequency modulation continuous wave (FMCW) implementations and theoretical analysis of orthogonal frequency division multiplexing (OFDM) based approaches and (3) birefringence measurements using a coherent Polarization-sensitive Optical Frequency Domain Reflectometer (P-OFDR) system.

Electrostriction in an optical fiber is introduced by interaction between the forward propagated optical signal and the acoustic standing waves in the radial direction resonating between the center of the core and the cladding circumference of the fiber. The response of electrostriction is dependent on fiber parameters, especially the mode field radius. We demonstrated a novel technique of identifying fiber types through the measurement of intensity modulation induced electrostriction response. As the spectral envelope of electrostriction induced propagation loss is anti-symmetrical, the signal to noise ratio can be significantly increased by subtracting the measured spectrum from its complex conjugate. We show that if the field distribution of the fiber propagation mode is Gaussian, the envelope of the electrostriction-induced loss spectrum closely follows a Maxwellian distribution whose shape can be specified by a single parameter determined by the mode field radius.        

We also present a self-homodyne FMCW LiDAR system based on a coherent receiver. By using the same linearly chirped waveform for both the LiDAR signal and the local oscillator, the self-homodyne coherent receiver performs frequency de-chirping directly in the photodiodes, significantly simplifying signal processing. As a result, the required receiver bandwidth is much lower than the chirping bandwidth of the signal. Simultaneous multi-target of range and velocity detection is demonstrated experimentally. Furthermore, we explore the use of commercially available coherent transceivers for joint communication and sensing using OFDM waveforms.

In addition, we demonstrate a P-OFDR system utilizing a digital coherent optical transceiver to generate a linear frequency chirp via carrier-suppressed single-sideband modulation. This method ensures linearity in chirping and phase continuity of the optical carrier. The coherent homodyne receiver, incorporating both polarization and phase diversity, recovers the state of polarization (SOP) of the backscattered optical signal along the fiber, mixing with an identically chirped local oscillator. With a spatial resolution of approximately 5 mm, a 26 GHz chirping bandwidth, and a 200 us measurement time, this system enables precise birefringence measurements. By employing three mutually orthogonal SOPs of the launched optical signal, we measure relative birefringence vectors along the fiber.