The collimator is an essential part of the fiber optic rotary joint design. This study proposes the Large-Beam Fiber Collimator (LBFC) with a double collimating lens and a Thermally Expanded Core (TEC) fiber structure. The transmission model is constructed based on the defocusing telescope structure. The effects of TEC fiber’s mode field diameter (MFD) on the coupling loss are investigated by deriving the loss function for the influence of collimator mismatch error and implementing it on a fiber Bragg grating temperature sensing system. The experimental results show that the coupling loss decreases with the increase of the MFD of TEC fiber, while the coupling loss is less than 1 dB when the mode field diameter is greater than 14 µm. TEC fibers can reduce the effect of angular deviation. Considering the coupling efficiency and deviation, the preferred mode field diameter for the collimator is 20 µm. The proposed LBFC enables bidirectional transmission of optical signals for temperature measurement.
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