Time-multiplexed photon-number-resolving low-level light detection at 850 nm using a Si-APD single photon detector

The photon counting detectors have a wide range of applications in different areas, such as quantum communication, linear-optics quantum computing optical metrology and so on. In this work, a fiber-based 16-channel multiplexer with different time delays was designed based on a construction scheme of 1 (1×4) coupler = 4 (1×4) couplers = 16×optical fibers - 4 (4×1) couplers - 1 (4×1) coupler. The lengths (di, i =1, 2, ...16) of the optical fibers were manufactured to be 0 m, 8.0 m, ... 120.0 m, respectively, with a length difference (Δd) of 8 m. The time delay between the optical fibers (Δt) can then be calculated to be 38.8ns. A pulsed 850 nm laser with a repetition rate of 1 MHz and a pulse duration of 10 ns was adopted to test the time-multiplexing capability of the 16-channel fiber coupler. A 1-GHz Si photodetector and a 1-GHz oscilloscope were used to measure the overall insertion loss and relative power through the 16 different channels. The photoelectric pulse count of the pulsed light passing through the fiber bundle will be measured. According to the loss of optical fiber and the photon detection efficiency of detector, we can roughly the photon detection efficiency of the system. If the fiber bundle with 16 entrances instead of only one, the scheme would be used as a detector array, and for imaging. This is what we're going to do in the future.