Investigation of a strongly gain coupled DFB laser cascade for simultaneous multigas sensing

Simultaneous detection of hydrogen sulphide and carbon dioxide by dual wavelength operation of a strongly gain coupled (SGC) three-section distributed feedback (DFB) laser cascade operating in the 1575-1585 nm region is reported. The device consisted of three cascaded sections (S₁ , S₂ and S₃) and an optical amplifier. Dual wavelength operation was achieved by biasing S₁ and S₂ above threshold. By precise dual independent wavelength tuning of the laser diode, both gases were simultaneously detected using wavelength modulation spectroscopy. The use of a SGC DFB laser in a dual wavelength mode has the potential to eliminate the need for multiple laser devices to simultaneously monitor more than one gas. However, when operating the SGC in a dual wavelength emission mode, thermal cross-talk between the lasing sections complicates somewhat the setting of the emission wavelength of S₁ and S₂ . Stabilisation techniques to minimise the effect of this cross-talk have been investigated.