Fourier-transform spectrometer has been offering powerful analytical tool for broadband spectroscopic LIDAR over other infrared spectrometer for optical remote sensing method for studies of atmospheric compositions, climate changes, and transport of pollutions. However, it has limited performance due to the instrumental limit of mechanical scanning in the interferometer. So, we propose a new approach to Fourier-transform spectroscopic technique by sweeping repetition rate of the femtosecond laser pulses referenced to an atomic Rb clock with a less than 10-12 stability instead of the obsolete mechanical scanning, which allows direct measurement without correction process. The proposed method enables a longer scanning range amplified by the ratio between the propagation time through the delay line and the average cavity round-trip time than conventional FTS for high-precision and presents accurate optical sampling less than 0.02 fs sampling jitter using homodyne interferometer with immunity to environmental disturbance less than 8 nm. We present a repetition rate sweeping based Fourier-transform spectrometer using fiber femtosecond laser and it comprise all-fiber based Mach-Zender interferometer with a differential arm length of 60 m. Here, we demonstrate a high-precision broadband LIDAR by resolving P-branch absorption lines of HCN gas cell spanning 70 nm while the repetition rate is scanned over 1 MHz covering scanning interval of 60 cm, which leads to a spectral resolution of 500 MHz. Consequently, we may achieve efficient spectroscopic LIDAR system with unprecedented performance by alleviating the prevalent difficulties in overcoming the performance limitation of the Fourier-transform spectrometer based on mechanical scanning in remote sensing application.
기존 FTS 의 긴 기계적 이송 과정에서 발생하는 low accuracy 와 resolution, 그리고 complex system design 의 단점을 향상시키기 위하여, 우리는 안정된 펨토초 레이저 펄스의 반복률을 sweeping 함으로써 Broadband high resolution spectroscopy 가 가능한 새로운 방식의 FTS 기술을 제안한다. 이때 펨토초 레이저 펄스 반복률을 Dual-servo 를 이용하여 루비듐(Rb) 기준 시계에 10-12 이하의 안정도로 위상 잠금하여 수백 kHz 의 영역에서 sub-mHz 이하의 안정도로 Sweeping 함으로써 interferogram 의 정확한 sampling 이 가능하다. 또한 Unbalanced arm의 긴 OPD 를 호모다인 간섭계를 이용하여 8 nm 이하로 안정화함으로써 진동이나 온도변화 등의 환경변화에 둔감한 간섭계를 구현하여 결국 0.02 fs 이하의 sampling accuracy 를 갖는 정밀한 interferogram 을 얻었다. 다음 분광기술의 성능 검증을 위하여 P-branch lines of a HCN reference cell 을 대상으로 실험을 실시하였으며, ~60 m 의 긴 OPD 를 갖는 Mach-Zehnder 간섭계에서 ~1 MHz fr-sweeping 으로 20 의 scanning range 증폭 효과를 통해 ~8 THz (70 nm) 의 펨토초 광원의 주파수 대역에서 500 MHz 의 분해능으로 HCN absorption lines 를 측정하였다.
