Distance measurement based on two-photon absorption process in Si-avalanche photodiode with pulsed reference light

Distance measurement using two-photon absorption (TPA) process in a Si-avalanche photodiode (Si-APD) can provide a wide measurement range and high precision. The principle is based on a characteristic of TPA photocurrent that is proportional to the mean square of the optical intensity. Thanks to this square characteristic, the intensity correlation between the probe and reference lights sinusoidally modulated at the same frequency is obtained without using a complicated electrical circuit, and then the distance to the target is measured. The TPA-based distance measurement can also be applied to a multi-point fiber Bragg grating (FBG) sensor and a multicore FBG curvature sensor, where multiple FBGs with the same reflection spectrum is discriminated from the distance to each FBG. In these applications, FBGs with low reflectance should be used to suppress the interference noise due to multiple reflection between the FBGs. However, it also weakens the reflected light from the FBG and deteriorates the signal-to-noise ratio (SNR) of the detected signal. This problem cannot be solved by simply increasing the probe and pump power, because it can damage the optical components. In this study, we propose using pulsed reference light with a high peak power and synchronously sampling the signal with the pulsed light. The principle is confirmed by a distance measurement of a 5-km-long optical fiber.