A single-fiber subassembly includes a first photodiode for receiving incident light, a laser diode for transmitting emergent light, and a same-wavelength optical splitter having a positive direction. The splitter includes a first birefringent plate, a half-wave plate, a 45° Faraday rotator, and a second birefringent plate arranged in sequence along and vertical to the positive direction. An included angle between an optical axis of the first plate and the positive direction is α, where 0°