We propose temperature independent fiber bending sensor for catheters, which uses a multicore fiber with a Bragg grating inscribed in each core and spectral measurement technique based on intensity correlation of optical signals based on two-photon absorption process in a silicon avalanche photodiode. Although fiber Bragg gratings are generally sensitive to temperature change, the effect can be suppressed by processing the spectral data of reflected light from all the Bragg gratings, which was confirmed by the proof-of-concept experiment, where we successfully measured the same curvature for different temperature of 20, 25, 30, 35 and 40 degrees Celsius.
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.
We experimentally investigated the intercore crosstalk of a multicore fiber (MCF) with Bragg grating inscribed in each core. The purpose is to clarify the effect on the three-dimensional curvature measurement based on a multicore fiber Bragg grating (FBG) and two-photon absorption process in a Si-APD that we have been investigating. The experiment was performed by injecting laser light only in the center core of the MCF and measuring the reflected light from the FBGs in the side cores for the cases that the grating section of the fiber is straight or bent. This measurement was also performed for different wavelength. The measurement results confirmed that the crosstalk was small enough to affect the accuracy of the curvature measurement. We also performed curvature sensing based on the proposed method.
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