Double Modulation Technique in Pulsed Jet Experiments with Tunable Diode Lasers

Abstract

A double modulation technique was used to achieve higher sensitivity in absorption measure ments 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 significantly the influence of the low frequency excess laser noise. The output of this lock-in amplifier was measured by a boxcar integrator, driven 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 be 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 detectable absorption of A r-C O complexes in the jet was 2 x 10-5 in relative absorption, limited by the excess noise of the diode laser.

Rotationally resolved infrared absorption spectrum of N+4

“…We have applied this, rather than frequency modulation, in order to avoid etalon fringes that can arise from any reflection between laser diode and detector. Since the background emission caused by impact ionization was relatively low compared to the laser power and was further reduced by a filter in front of the detector, double modulation schemes as used in 24,25 were not necessary.…”

Millimeter-Wave Spectroscopy of Kr-CO and Xe-CO Using a Coaxial Jet Spectrometer

“…The next development was to replace the continuous source with a pulsed supersonic jet. By pulsing the jet, gas consumption is reduced significantly and this additional "modulation" has been shown to improve sensitivity for infrared studies (8). At this time, two systems were built which incorporated this modification (9,10).…”

A combined absorption–wavelength modulation technique for diode laser spectroscopy of van der Waals complexes in a pulsed planar jet

“…21 A signal to noise ratio in the 10 4 -10 5 range was achieved in measurements of the Ar-CO complexes, depending on the mode characteristics and the power of the TDL. For the TDL used in the present study a typical value of (3 -4) ϫ 10 4 for the signal to noise ratio was evaluated from the measured intensities of the methane lines in the supersonic slit jet at a 1 s output time constant of the registration system.…”