Algorithms and principles for wavelength measurement for laser communications

The rapid expansion of free-space optical communication systems will serve demand for global data exchange, yet existing technology gaps in frequency control continue to limit interoperability. One important challenge is precise wavelength measurement, which is critical for coherent laser communication systems that require tight frequency alignment to maintain signal-local oscillator alignment. While high-precision wavelength meters and optical spectrum analyzers exist, many are too large, complex, or power-hungry for use in constrained environments such as field-deployed terminals. Without wavelength monitoring, systems may struggle to maintain performance over time, especially in dynamic or thermally varying conditions that cause laser drift. To address this challenge, we developed a wavelength measurement system that leverages the stability of hydrogen cyanide (HCN) absorption lines as a wavelength reference. Our system uses a simplified architecture and algorithm to estimate an unknown wavelength in a resource-constrained environment. The algorithm involves capturing spectra with known HCN R-P molecular transitions and the unknown laser, fitting the absorption lines and laser to determine their relative positions, and using those fits to create an estimate of the unknown laser’s wavelength. Our experimental results demonstrate a significant step forward in wavelength measurement technology, with a frequency error of < 1 GHz across the C band. This achievement paves the way for the development of more accurate and reliable wavelength measurement systems, enabling the creation of flexible networks and advancing the field of coherent laser communication.