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Wavelength Modulation Spectroscopy (WMS) and Frequency Modulation Spectroscopy (FMS)

To reach higher sensitivity than by using DA spectroscopy, Modulation technique can be included. It provides two main advantages: firstly, it measures a difference signal which is directly proportional to the species concentration and, secondly, it allows to shift a measured signal to a higher frequency region, thereby offering a larger signal-to-noise ratio and thus higher sensitivity.

In practice, this technique is divided into two approaches. Wavelegnth Modulation Spectroscopy (WMS) [1,2], which is devoted to the case where the frequency of modulation is much lower than the absorption linewidth with detection at the modulation frequency (1f) or the next harmonics (2f, etc.). This typically corresponds to modulation frequencies from a few kHz to a few MHz. It has been applied since the early 1970s, with tunable diode lasers (TDL) [2]. From another side Frequency Modulation Spectroscopy (FMS) applies for modulation frequencies which are comparable or greater than the spectral feature of interest (for the moExperimentaly, particular problems or the availability of instrumentation might dictate the choice of the technique.

When the laser is modulated around its center frequency ωL at a frequency ωm with modulation amplitude dw, the instantaneous frequency is ω = ωL + dω cos(ωmt). The intensity of the radiation transmitted through the absorption cell can then be expressed as a Fourier series expansion.

WMS

Wavelength modulation spectroscopy of an absorption peak at ω0. When the modulated laser frequency at ωL probes the absorption feature, part of the wavelength modulation is converted into an amplitude modulation ds, which is subsequently detected with a fast photodetector. Phase-sensitive detection with a lock-in amplifier of the first and second harmonics results in a WMS signal that is proportional to the first and second derivatives, respectively.

Due to ease of the current modulation, WMS mostly was used with diode lasers. In the near-IR, it can be also performed by using commercial external modulators. However, due to increasing needs for trace gas detection, it is advantageous to work in the mid-IR region, where a lot of molecules have strong absorption bands. The goal was achieved by using QCLs [5,6] and OPOs [7,8].

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2. P. Werle, "A review of recent advances in semiconductor laser based gas monitors," Spectrochim. Acta A 54, 197-236 (1998).
3. D. S. Bomse, A. C. Stanton, and J. A. Silver, "Frequency modulation and wavelength modulation spectroscopies: comparison of experimental methods using a lead-salt diode laser," Appl. Opt. 31, 718-731 (1992).
4. F. S. Pavone and M. Inguscio, "Frequency- and wavelength-modulation spectroscopies: comparison of experimental methods using an AlGaAs diode laser" Appl. Phys. B 56, 118-122 (1993).
5. K. Namjou, S. Cai, and E. A. Whittaker, J. Faist, C. Gmachl, F. Capasso, D. L. Sivco, and A. Y. Cho, "Sensitive absorption spectroscopy with a room-temperature distributed-feedback quantum-cascade laser," Opt. Lett. 23, 219-221 (1998).
6. QCL based WMS for NO
7. I. D. Lindsay, P. Groß, C. J. Lee, B. Adhimoolam, and K. -J. Boller, "Mid-infrared wavelength- and frequencymodulation spectroscopy with a pump-modulated singly-resonant optical parametric oscillator," Opt. Express 14, 12341-12346 (2006).
8. D.D. Arslanov, M. Spunei, A.K.Y. Ngai, S.M. Cristescu, I.D. Lindsay, S.T. Persijn, K.J. Boller, and F.J.M. Harren, "Rapid and sensitive trace gas detection with continuous wave Optical Parametric Oscillator-based Wavelength Modulation Spectroscopy," Appl. Phys. B, 103, 223-228 (2011).