numerous well known instrumental configurations incorporating a qc dfb laser are useful in this regard without limitation such configurations include wavelength modulation direct absorption photoacoustic cell pac and cavity ring down crd configurations published descriptions of spectroscopic instruments incorporating qc dfb lasers include r m williams et al u201c kilohertz linewidth from frequency stabilized mid infrared quantum cascade lasers u201d optics letters 24 1999 1844 1846 b a paldus et al u201c photoacoustic spectroscopy using quantum cascade lasers u201d optics letters 24 1999 178 180 a a kosterev et al u201c methane concentration and isotopic composition measurements with a mid infrared quantum cascade laser u201d optics letters 24 1999 1762 1764 and k namjou et al u201c sensitive absorption spectroscopy with a room temperature distributed feedback quantum cascade laser u201d optics letters 23 1998 219 221 fig7 schematically depicts exemplary point sensing apparatus 70 wherein numeral 71 refers to a qc dfb laser according to the invention for purposes of illustration the qc dfb laser of fig7 is configured for wavelength modulation typically the laser is mounted on a temperature controlled stage not shown for coarse wavelength tuning mid ir radiation 721 from the laser passes through conventional gas cell 73 optionally a multi pass cell with exited radiation 722 impinging on conventional detector 74 the electrical output of the detector is supplied to lock.