DOUBLE-MODULATION TECHNIQUE IN PULSED JET EXPERIMENTS WITH TUNABLE DIODE-LASERS

A double modulation technique was used to achieve higher sensitivity i n absorption measurements with a pulsed supersonic jet. The wavelength of a lead-salt diode laser was modulated at a frequency of 10 kHz and detected at twice the frequency by a lock-in amplifier, reducing sign ificantly the influence of the low frequency excess laser noise. The o utput of this lock-in amplifier was measured by a boxcar integrator, d riven at 80 Hz, the frequency of the pulsed jet. In case the duration of the jet pulse is long enough, a second lock-in amplifier can also b e used instead of the boxcar integrator. The optical path of the diode laser radiation through the jet was increased 16 times by using White type multireflection optics. The achieved value for the minimum detec table absorption of Ar-CO complexes in the jet was 2 x 10(-5) in relat ive absorption, limited by the excess noise of the diode laser.