A single-photon detector (SPD) is sensitive to incidence of individual quanta of light and has many applications in photonics, such as fluorescence measurements, laser ranging, optical time-domain reflectometer, and quantum optics experiments. Near-infrared SPDs at the telecommunication wavelength of 1550 nm are indispensible for fiber-optic QKD, with choices including cryogenic superconducting nanowire single-photon detectors (SNSPD) and electrically-cooled InGaAs avalanche photodiodes (APDs). Between them, APDs have practical advantages for compactness, low-cost and not requiring ultra-low temperature refrigeration. Under Geiger mode, APD's strong capacitive response to sub-nanosecond gating has to be rejected through purpose-designed readout circuit so as to enable detection of weak photon-induced avalanches. Rapid gating and read-out circuits add challenges to modularization and miniaturisation, but which is a necessary step to serve a wide range of applications.
Research group have recently developed a novel readout circuit that incorporates a surface acoustic wave (SAW) filter into an asymmetric radio-frequency Mach-Zehnder interferometer, referred to as ultra-narrowband interference circuit (UNIC), and realized exceptional performance for narrow-band rejection of the SPD capactive response. Thanks to the long group delay of the SAW filter, the UNIC interferometer can produce an ultra-narrrow band rejection with a manufacturing tolerance easily achievable in the RF track lengths. The UNIC can provide a wide and continuous pass band in the frequency domain and therefore brings little distortion into the avalanche signal. They reported their development of a standalone InGaAs SPD module that fully integrates driving and readout electronics as well as temperature regulation and compensation. Its dimension is measured just 8.8×6×2 cm3 and is nearly a factor of 4 smaller in volume than the existing compactest detector module that uses a monolithically integrated readout circuit. Simultaneously, this size reduction does not bring performance deterioration.
The research team developed a fully-integrated InGaAs/InP detector module of a size of just 8.8×6×2 cm3. They use their previous UNIC techniques for the APD signal readout, but add an automatic temperature compensation to ensure an optimal performance over a wide ambient temperature range. With a 1.25 GHz clock input, the module is characterized to have comparable performance to its counterpaert built with bench-top equipment. The UNIC-SPD exhibits excellent performance with a net detection efficiency of 30 % at an afterpulsing probability of 2.4 % under 3 ns hold-off time. The compact size and state-of-the-art performance allow our UNIC-SPD module a huge potential for single-photon imaging and high-speed quantum key distribution.
