Compact, Temperature-Tuned OFDR Laser

Because OFDR-based fiber interrogation systems rely upon interferometry between sensors with respect to a unique reference length, the excitation source (laser) must lase at a single longitudinal mode (SLM). If the excitation source contains multiple modes, the resulting beat frequency becomes a super-position of the multiple frequencies caused by the modes; as a result, the sensor cannot be accurately defined in the Fourier domain. For OFDR systems with high sensing ranges, a continuous wavelength tunable laser must be used to accommodate the resonant wavelength shift of the fiber sensors due to environmental changes. External cavity lasers (ECLs) have been used due to their narrow linewidth and ability to lase at a SLM with no mode-hopping between steps. However, the mechanical complexity associated with tuning, susceptibility to vibration and shock, and high price point leave much to be desired. To overcome the limitations of OFDR-based FOSS systems resulting from non-ideal excitation sources, NASA has developed a narrow linewidth solid-state laser based on the Distributed Feedback (DFB) laser. NASAs laser is continuously tuned by manipulating the laser cavitys temperature via a thermal-electric cooler feedback system. This continuous wavelength tuning generates a clean clock signal within an auxiliary interferometer, while the laser simultaneously interrogates multiple FBGs to produce a clean sensing interferometer. A Fourier domain spectrograph is used to show the unique frequency (i.e., location) of each FBG. While NASAs excitation source provides several performance advantages over conventional lasers used in OFDR, it is also highly compact and one eighth the cost of the ECLs traditionally used as excitation sources in OFDR-based systems. The laser has no moving parts, which also substantially improves system reliability. Originally developed to demonstrate a low-cost interrogator for liquid level sensing in oil tanks, NASAs compact, temperature-tuned OFDR laser can be applied wherever OFDR-based fiber optic sensing is desirable. Additional applications may include temperature distribution sensing, strain sensing, pressure sensing, and more. NASA AFRC has strong subject matter expertise in fiber optic sensing systems, and has developed several patented technologies that are available for commercial licensing. For more information about the full portfolio of FOSS technologies, visit: https://technology-afrc.ndc.nasa.gov/featurestory/fiber-optic-sensing