Wavelength modulation, laser spectroscopy

Presently, grating monochromators are used every time a sample must be illuminated with quasi-monochromatic radiations that are tunable in a broad spectral range or for experiments in the visible-UV range. Another interest of dispersive monochromators is the possibility of wavelength modulation of the output of these monochromators in order to get the first derivative of the transmission spectrum. This has the advantage of increasing the sensitivity, and this technique is also used in laser spectroscopy. 

Wavelength modulation spectroscopy involves modulating the laser current at a frequency, /, fypically... 

F.S. Pavone, M. Inguscio, Frequency- and wavelength-modulation spectroscopy Comparison of experimental methods, using an AlGaAs diode laser. Appl. Phys. B 56, 118 (1993)... 

Fig. 10.21. Arrangements for frequency modulation spectroscopy with diode lasers. In (a) low frequencies are employed and the term wavelength modulation spectroscopy pertains. In (b) high frequencies are used and discrete components replace the lock-in amplifier [10.84]...

Oh, D. B. Paige, M. E. Bomse, D. S., Frequency Modulation Multiplexing for Simultaneous Detection of Multiple Gases By Use of Wavelength Modulation Spectroscopy With Diode Lasers , Appl. Opt. 1998, 37, 2499-2501. 

These experiments demonstrate that tunable diode laser absorption spectroscopy is well suited for in situ measurements of species concentrations in combustion flows, when a line-of-sight technique is appropriate, and for accurate measurements of spectroscopic parameters needed to characterize high-temperature absorption lines. The technique is sensitive, species specific and applicable to a large number of important combustion species including reactive intermediates, and hence it should prove to be a useful tool in future studies of combustion chemistry. The potential of tunable laser absorption spectroscopy in particleladen flows should also be noted (12), in that modulation of the laser wavelength on and off an absorption line allows simple discrimination against continuum extinction by particles. 

We have developed a novel ultrasensitive detection method, thermal lens microscopy (TLM), for nonfluorescent species [13]. TLM is photothermal spectroscopy under an optical microscope. Our thermal lens microscope (TLM) has a dual-beam configuration excitation and probe beams [13]. The wavelength of the excitation beam is selected to coincide with an absorption band of the target molecule and that of the probe beam is chosen to be where the sample solution (both solvent and solute) has no absorption. For example, in determination of methyl red dye in water, cyclohexane, and n-octanol, a 514-nm emission line of an argon-ion laser and a 633-nm emission line of a helium-neon laser were used as excitation and probe beams, respectively [21], Figure 4 shows the configuration and principle of TLM [13]. The excitation beam was modulated at 1 kHz by an optical chopper. After the beam diameters were expanded, the excitation and probe beams were made coaxial by a dichroic mirror just before they were introduced into an objective lens whose magnification and numerical aper-... 

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