Abstract

A tunable diode laser absorption spectrometer using near-infrared InGaAs DFB lasers and detectors has been developed to make balloon-borne measurements of stratospheric tracer species such as methane. The instrument combines second harmonic wavelength modulation spectroscopy with a folded optical path of 101 metres to enable detection of absorptions of less than one part in 10⁶ (with a one second time constant). The long optical path is achieved using a specially designed astigmatic Herriott cell, which gives 202 passes across a base path of 0.5 m. The optical properties of the cell have been modelled, and Fig. 1 shows the agreement between the modelled and the observed spot patterns on the mirror. This design of cell has been shown through modelling and experiment to have a high level of inherent stability against mechanical misalignments and minimal optical fringing.

© 1999 Optical Society of America