Development of Self-aligned Ti Optical Transition-Edge Sensors at 1550 nm

We report on self-aligned Ti TES single-photon detectors embedded in a 1550 nm optical cavity. The circular TES chip is shaped with a dry-etch process from the backside to protect the TES and wiring from possible damage and aligned to a single-mode fiber using a standard fiber ferrule and a matching sleeve. The critical temperature of Ti film is about 300 mK, resulting in a relatively short time constant of 1.3 μs. By choosing an active area of 15 × 15 μm 2 , our optical Ti TES can distinguish single photons at 1550 nm and reaches a system photon detection efficiency of 55% and an energy resolution of about 0.7 eV.