Fiber-based indoor testing of laser rangefinder extinction ratio

Abstract

Laser rangefinders (LRFs) are one of the most prominent optronic systems used for both civilian and military applications. One way to characterize their performance is by measuring the extinction ratio (ER), defined as the attenuation at which the LRF detects 50% of the transmitted pulses. At Lumibird Photonics Sweden AB, the ER is currently determined using an outdoor over-the-air (OTA) test range, where the laser pulse is fired at a well-defined reference target. To eliminate the weather dependence inherent to outdoor OTA testing, this thesis investigates an indoor laboratory test bench using a single-mode fiber to guide the pulses as an alternative ER measurement method. The work focused on establishing stable free-space-to-fiber coupling, selecting a suitable baseline attenuation before fiber injection, and comparing the ER behavior with an OTA reference measurement. Using a deliberately larger focused beam at the fiber input reduced the influence of pulseto- pulse beam variations on the coupling efficiency. In addition, nonlinear propagation in the fiber was found to be the main limitation on the maximum usable coupled intensity, causing pulse distortion when it was too high. With suitable baseline attenuation, the final test bench produced a clear detection roll-off similar to the OTA reference. The test-bench ER was obtained at ≈ 101 dB of added attenuation, and the measured distance agreed with the expected fiber-delay distance within ≈ 1 %. Including estimated setup-specific losses, the same 50% detection point corresponded to a test-bench total attenuation of ≈ 110 dB, compared with ≈ 130 dB for the OTA reference, with the difference likely arising from idealized assumptions regarding losses in the test bench. Overall, the test bench shows potential as an indoor complement to OTA ER testing, but further characterization of losses, repeatability, and uncertainty is required before it can be used for final performance evaluation.