Resonant, heterodyne laser interferometer for state density measurements in atoms and ions

Abstract

A resonant, two-wavelength heterodyne interferometer has been developed to measure state densities of specific atomic populations. Using a tunable diode laser, one of the wavelengths is set near an absorption line and is sensitive to resonant enhancement of the refractive index. The other wavelength is at least several linewidths away and is used to measure nonresonant effects. The subtraction of the phase shifts of the two interferometers allows one to measure the population density in the lower state of the line being investigated. The interferometer system has been tested using a plasma, created by an inverse pinch plasma source, to measure the state density of Hn=2 by making resonant observations near the Hα (6563Å) transition. Average line-of-sight densities of 5.4×1011cm−3 were measured with a minimum resolvable density of 7.3×109cm−3. The transverse spatial resolution was 1–2mm. The technique described is applicable for measuring lower state densities in any atom or ion where the oscillator strengths are known for a given transition within the tuning range of the laser.

Molecular dispersion spectroscopy for chemical sensing using chirped mid-infrared quantum cascade laser

“…Provided that the beating frequency lies within the electrical bandwidth of the photodetector, RF analysis of the photocurrent measures both amplitude and phase information associated with the respective absorption and dispersion produced by the sample. When the laser optical frequency and the AOM excitation frequency are both constant, the dispersion experienced by either wave results in a phase shift in the RF carrier at frequency Ω [9]. The technique described here additionally utilizes a fast frequencychirp of laser radiation that enhances the magnitude of the measured dispersion signal.…”

“…The first demonstration using a two-beam interferometer using one fixed-frequency and one tunable laser was performed by Crance et al [8]. Recently similar developments using a tunable diode laser and a heterodyne detection scheme were demonstrated [9]. Other implementations include a laser-based version of the "hook" method in a Mach-Zehnder interferometer configuration for fringe displacement measurements [10,11], the use of a high finesse Fabry Perot resonator [12] or techniques based on intra-resonator measurements [13].…”

Investigation on transient process of dense hydrogen microdischarge by dual-wavelength laser interferometry

“…Several parameters are needed to give the neutral particle density from the phase signal by the formula (4). Considering the high gas temperature and low pressure characteristics of the inverse pinch plasma, Moschella et al directly estimated several fitting parameters, which is the prerequisite for obtaining the H (n = 2) state density [14], and then the number density of the first excited state of hydrogen atoms is determined according to the measured phase-shift dispersion curve.…”

“…By subtracting the phase shifts of the two laser beams, the population density at the lower position of the measured line can be given. Recently, this method has been used to measure the n = 2 state density of atomic hydrogen in an inverse pinch hydrogen plasma [14]. This interferometric measurement records phase information, which is not affected by background emission from the plasma, nor intensity fluctuations from the laser or other optical components.…”

Molecular dispersion spectroscopy – new capabilities in laser chemical sensing

“…Therefore a new sensing scheme that is capable of measuring molecular dispersion with relatively simple and robust setup has a potential to overcome some of these drawbacks and provide advantages of dispersion spectroscopy. Such attempts have been made in the past, but only a small amount of progress has been made in adoption of proposed techniques to sensitive trace‐gas detection in nonlaboratory conditions 15,19 , 32–37 . Recently we have introduced a new chirped laser dispersion spectroscopy (CLaDS) technique that relies on molecular dispersion sensing in the gas sample, 38 and the system itself can be easily adopted for field measurements 39 .…”