Pulsed CARS

By taking advantage of the deep penetration depth offered by CARS microscopy with near-infrared laser pulses, CARS tissue imaging has been demonstrated to complement other label-free optical techniques, such as auto-fluorescence that is limited to a few chemical species and optical tomography, second harmonic generation (SHG), and third harmonic generation (THG)... 

The CARS system used to measure temperature and species concentrations in the combustor zone is composed of a single-mode ruby-laser oscillator-amplifier with a repetition rate of 1 Hz and a ruby-pumped, near-infrared broad-band dye laser. The two laser beams are combined collinearly and focused first into a cell containing a nonresonant reference gas and then into the sample volume (approximately 30-u diam. x 2 cm) in the combustion region. The anti-Stokes beams produced in the sample and reference volumes are directed to spatially separated foci on the entrance slit of a spectrometer and detected by separate photomultiplier tubes. An optional means of detection is provided for the sample signal in the form of an optical multichannel analyzer (OMA), which makes it possible to obtain single-pulse CARS spectra. 

Schreiber (6i) pointed out the usefulness of single-pulse CARS for combustion work. The apparatus described above can be easily adapted to perform a number of coherent Raman experiments with single 20 ns (FWHM) laser pulses. Following are examples of the application of single-pulse RIKES and IRS, first to static solutions, then to Xenon-lamp irradiated solutions. 

Figure 3.6-9 Modification of the pulsed CARS set-up shown in Fig. 3.6-8 for backscattering measurements. Shown is only the modified sample area (Weippert et al., 1993).

Structure of the vibrational 0-branch was resolved by pulsed CARS (Beckmann et al., 1981) and by IDSRS (ionization-detected stimulated Raman scattering, Esherick and Owyoung, 1983). 

Ethylene has no dipole moment and a center of symmetry and therefore the Raman spectrum is an important source of structural information. After the early work on the rotational (Dowling and Stoicheff, 1959) and rovibrational Raman spectrum (Feldman et ah, 1956) these spectra were thoroughly studied in a series of publications (Hills and Jones, 1975 Hills et ah, 1977 Foster et ah, 1977). Overtones and combination bands were measured in an intracavity Raman experiment by Knippers et ah (1985). The Q-branch of the U2 band was resolved by pulsed CARS spectroscopy in a molecular beam experiment (Byer et ah, 1981). 

Klick, D., Marko, K. A., and Rimai, L. "Broadband Single-Pulse CARS Spectra in a Fired Internal Combustion Engine." Applied Optics 20, no. 7 (1981) 1178. 

Snelling, D. R., Sawchuck, R. A., and Mueller, R. E. "Single Pulse CARS Noise A Comparison Between Single-Mode and Multimode Pump Lasers." Applied Optics 24, no. 17 (1985) 2771. 

The most common pump system for pulsed CARS experiments are two dye lasers pumped by the same pump laser (N2 laser, excimer laser, or frequency-doubled Nd YAG laser). This system is very flexible because both frequencies co and C02... 

The advantage of CARS compared to infrared absorption ion spectroscopy is the higher sensitivity and the fact that nonpolar molecules, such as N2, can also be studied [459]. With pulsed CARS short-lived transient species produced by photo dissociation in molecular beams can also be investigated [460,461]. 

The most common pump system for pulsed CARS experiments are two dye lasers pumped by the same pump laser (N2 laser, excimer laser, or frequency-doubled Nd YAG laser). This system is very flexible because both frequencies a) and C02 can be varied over large spectral ranges. Since both the frequency and intensity fluctuations of the dye lasers result in strong intensity fluctuations of the CARS signal, the stability of the dye lasers needs particular attention. With compact and stable systems the signal fluctuations can be reduced below 10% [8.59]. 

The unpaired electron in the molecule NO gives rise to two states 113/2 and A Raman transition between these states is allowed and was detected by Rasetti [8] at about 120 cm This electronic Raman effect was rotationally resolved by Fast et al. [22] and Rich and Welsh [160]. The rotational structure of the vibrational Q branch was resolved by pulsed CARS [161] and by IDSRS [108]. Recently, Raman spectra of the dimer (NO)2 were recorded in a molecular beam [162]. 

W Nitsch, W Kiefer. High resolution pulsed CARS spectroscopy of the Q-branches of some simple gases. Opt Commun 23 240-244, 1977. 

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