A Coherent Detection Method With 106 Higher Intensity Response Sensitivity Than Normal Heterodyne Interferometry

Based on the Nd: YVO
₄
microchip laser frequency-shifted feedback system, we carry out the detection of ultra-weak light signals, and verify that the response limit of the proposed system can reach 0.5 photon per second (corresponding to 2.1 × 10
⁻⁷
photon's energy per frequency modulation cycle of 0.42 μs), with approximately linear response to weak light. Then, the system is compared with the traditional Mach-Zehnder heterodyne interference system, the comparison results confirm the higher intensity response sensitivity of the laser feedback technology (about six orders of magnitude) when detecting both cooperative and non-cooperative targets. On this basis, a verification experiment is carried out on long-distance weak light imaging, and the “THU” pattern made of unpolished aluminum blocks is successfully reconstruct at 60 m distance (with an additional 10
⁻⁷
attenuation provided by the neutral density filter added to the optical path). During the imaging process, the response energy as low as 2 × 10
⁻⁵
photon is achieved in each frequency modulation cycle of 0.42 μs. The successful implementation of the laser frequency-shifted feedback detection enables an anti-disturbance and sensitive coherent method in weak light signals detection.