Frequency modulation Fourier transform spectroscopy

A new method, FM-FTS, combining Frequency Modulation heterodyne laser spectroscopy and Fourier Transform Spectroscopy is presented. It provides simultaneous sensitive measurement of absorption and dispersion profiles with broadband spectral coverage capabilities. Experimental demonstration is made on the overtone spectrum of C2H2 in the 1.5 μm region. OCIS codes: 120.6200, 300.6300, 300.6380, 300.6360, 300.6310, 300.6390, 120.5060 120.6200 Spectrometers and spectroscopic instrumentation, 300.6300 Spectroscopy, Fourier transforms, 300.6380 Spectroscopy, modulation, 300.6360 Spectroscopy, laser, 300.6310 Spectroscopy, heterodyne, 300.6390 Spectroscopy, molecular, 120.5060 Phase modulation Frequency modulation Fourier transform spectroscopy Mandon, Guelachvili, Picqué, 2007 2 Improving sensitivity is presently one of the major concern of spectroscopists. This may be obtained both from the enhancement of the intrinsic signal, and from the reduction of the background noise. In this latter case, modulation has been one of the most effective approach. In particular, Frequency Modulation (FM) absorption spectroscopy [1] has reached detection sensitivity near to the fundamental quantum noise limit, by shifting the frequency modulation of the measurements to a frequency range where the 1/f noise becomes negligible. Moreover, FM spectroscopy benefits from high-speed detection and simultaneous measurement of absorption and dispersion signals. Since Bjorklund’s first demonstrations [1,2] of the efficiency of FM spectroscopy with a single-mode continuous-wave dye laser, the technique has been widely used as a tunable laser spectroscopic method in fields such as laser stabilization [3], two-photon spectroscopy [4], optical heterodyne saturation spectroscopy [5], trace gas detection [6]. In most schemes, the laser wavelength is scanned across the atomic/molecular resonance to retrieve the line shape. More rarely, the modulation frequency is tuned. However in both cases, the measurements are limited to narrow spectral ranges. This letter reports the first results in FM broadband spectroscopy. This work is motivated by our ongoing effort of implementing a new spectroscopic approach simultaneously delivering sensitivity, resolution, accuracy, broad spectral coverage and rapid acquisition. The basic idea, named FM-FTS, is to associate the advantages of FM spectroscopy and high-resolution Fourier transform spectroscopy (FTS). FTS is able to record at once extended ranges, with no spectral restriction. In particular it gives easy access to the infrared domain. In this letter, a new way of modulating the interferogram is implemented. The key concept is that a radio frequency (RF) modulation is performed. The beat signal at the output port of the Fourier transform spectrometer is modulated at constant RF, which is about 10 times greater than the audio frequency generally delivered by the interferometer optical conversion. Together with the advantage, over classical FTS, of measurements performed at much higher frequency, our approach benefits from the synchronous detection ability and from the simultaneous acquisition of both the absorption and the dispersion of the recorded profiles. The experimental principle is presented in Fig. 1. The light emitted by the broadband source is first passing through the interferometer. The output beam is then phase-modulated by an electro-optic modulator (EOM) before entering the absorption cell and falling on the fast detector. The synchronous detection of the detector signal is realized by the lock-in amplifier at the EOM driver reference frequency fm. Recorded data are finally stored on the computer disk with their corresponding path difference position ∆. Their Fourier transform is the spectrum. In more details, the electric field E at the output of the interferometer may be written as: 0 ( ) ( , ) 1 exp exp( )d c.c. (1) 2 c c c c t i i t c ω ω ω ω ⎡ ⎤ ∆ ⎛ ⎞ ∆ = + − + ⎜ ⎟ ⎢ ⎥ ⎝ ⎠ ⎣ ⎦ ∫ E E where E0 is the electric field amplitude of the source at ωc optical pulsation, c is the velocity of light and c.c the conjugate complex of the preceding expression in Eq. 1. The EOM effect on the beam is assumed to have a low modulation index M. As a consequence, each carrier wave of pulsation ωc, has two weak sidebands located at ± ωm = ± 2π fm. Equation (1) becomes: ( ) { ( ) ( ) } 0 ( ) ( , ) 1 exp exp 2 M exp M exp d c.c. (2) c c c c m c m c t i i t c i t i t ω ω ω ω ω ω ω ω ⎡ ⎤ ∆ ⎛ ⎞ ∆ = + − ⎜ ⎟ ⎢ ⎥ ⎝ ⎠ ⎣ ⎦ ⎡ ⎤ ⎡ ⎤ + + − − + ⎣ ⎦ ⎣ ⎦ ∫ E E When interacting with the gas, the carrier and the sidebands experience attenuation and phaseshift due to absorption and dispersion. Following the notations introduced in [1], this interaction may be written as exp(-δ(ω)i φ(ω)) where δ is the amplitude attenuation and φ is Frequency modulation Fourier transform spectroscopy Mandon, Guelachvili, Picqué, 2007 3 the phase shift. The following convention is adopted: δn and φn denotes for n = 0, ±1 the respective components at ωc and ωc± ωm. Then Eq. 2 may be written: ( ) ( ) { ( ) ( ) ( ) ( ) } 0 0 0 1 1 1 1 ( ) ( , ) 1 exp exp exp 2 M exp exp M exp exp d c.c. (3) c c c c m c m c t i i i t c i i t i i t ω ω δ φ ω δ φ ω ω δ φ ω ω ω + + − − ⎡ ⎤ ∆ ⎛ ⎞ ∆ = + − − − ⎜ ⎟ ⎢ ⎥ ⎝ ⎠ ⎣ ⎦ ⎡ ⎤ ⎡ ⎤ + − − + − − − − + ⎣ ⎦ ⎣ ⎦ ∫ E E The intensity I detected by the fast photodetector is proportional to : * I( , ) ( , ) ( , ). (4) t t t ∆ ∝ ∆ ∆ E E ( ) ( ) ( )