A lightweight high-sensitivity laser spectrometer for the measurement of trace stratospheric species

Summary form only given. 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 10/sup 6/ (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 the agreement between the modelled and the observed spot patterns on the mirror is shown. 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.