Intensity enhancement in off-axis integrated cavity output spectroscopy.

In the field of laser-based absorption spectroscopy, off-axis integrated cavity output spectroscopy is considered to be a sensitive and robust method, employing a simple optical design. However, one of the major drawbacks of non-mode-matched cavities combined with highly reflective mirrors (>99.98%) is its low output intensity. Here, we systematically investigate the increase in cavity output intensity, using a third re-injection mirror before the absorption cavity. The presented design not only enables high transmission power but also retains a long effective path length. To investigate the intensity enhancement, we used a CO absorption line in the near-IR wavelength region at 6240.10  cm. In agreement with our simulation model, we achieved an intensity enhancement factor of 38. We achieved a noise equivalent absorption sensitivity to 1.6×10  cm Hz, which is no longer limited by the detectivity of the detector.