Infrared cavity ringdown and integrated cavity output spectroscopy for trace species monitoring

Although the ability of high finesse optical cavities to provide effective absorption path-lengths exceeding 10 km. has been known for quite some time, attempts to utilize this property for the purposes of high-resolution spectroscopy have often resulted in extremely complex experimental systems. Here, we demonstrate how off-axis optical paths through such cavities can be employed to produce relatively simple spectrometers capable of ultrasensitive absorption measurements. A proof-of-concept study using visible diode lasers has achieved a normalized absorption sensitivity of 1.8*10^-10 cm^-1Hz^-1/2. Additionally, quantum cascade lasers have been employed to extend this method into the mid-infrared region, where sensitivities of 1.2*10^-9 cm^-1Hz^-1/2 have been obtained.