Tunable Raman Lasers

The tunable Raman laser may be regarded as a parametric oscillator based on stimulated Raman scattering. Since stimulated Raman scattering is discussed in more detail in Sect. 8.3, we here summarize only very briefly the basic concept of these devices. 

The ordinary Raman effect can be described as an inelastic scattering of pump photons ha p by molecules in the energy level E(. The energy loss h(cop — cOs) of the scattered Stokes photons hco is converted into excitation 

Those molecules that are initially in excited vibrational levels can give rise to superelastic scattering of anti-Stokes radiation, which has gained energy (/kDs p) = Ei — Ef) from the deactivation of vibrational energy. 

The Stokes and the anti-Stokes radiation have a constant frequency shift against the pump radiation, which depends on the vibrational eigenfrequen-cies (jOy of the molecules in the active medium. 

If the Stokes or anti-Stokes wave becomes sufficiently strong, it can again produce another Stokes or anti-Stokes wave at = cDp — 2cuv 

If the Stokes or anti-Stokes wave becomes sufficiently strong, it can again produce another Stokes or anti-Stokes wave at = -Wy = Wp-2Wy and (2) = Wp+2Wy. Therefore, several Stokes and anti-Stokes waves are generated at frequencies = Wp-nWy = Wp + nwy (n = 1,2,3. ..) 

Tunable lasers as pumping sources therefore allow one to transfer the tun-ability range (Wp Aw) into other spectral regions (wp Aw nw ). 

An interesting realization of a widely tunable OPO is a fiber OPO pumped by a fiber laser. One example is the sub-1 pm operation of a fiber OPO in a fiber ring resonator, pumped by an all-fiber master oscillator plus power amplifier based on a photonic crystal fiber as gain medium [592], A conversion efficiency of 8 6 % from the pump at 1079nm to the anti-Stokes signal at 715nm. This means a frequency shift of 142 THz between pump and anti-Stokes signal. 

Summary Optical parametric oscillators are coherent devices similar to lasers. There are, however, important differences. While lasers can be pumped by incoherent sources, OPOs require coherent pump sources. Often diode laser-pumped solid state lasers are used. While in lasers coherent amplification can last until the inversion in the active medium has fallen below threshold, in OPO s the time dependence of the coherent output is directly coupled to that of the pump laser. Since the pump photon is split into signal and idler photon with u = u i, the energy of the output equals that of the input i.e. there is no energy, i.e. heat deposited in the active crystal. The spectral tuning range is by far wider than for tunable lasers. Most OPOs operate in the near infrared but can be tuned from the visible region to the far infrared. 

A good survey on different aspects of OPOs can be found in [615]. 

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Nonlinear Optical Mixing Techniques 5.8.9 Tunable Raman Lasers... 

For the visible and near-ultraviolet portions of the spectmm, tunable dye lasers have commonly been used as the light source, although they are being replaced in many appHcation by tunable soHd-state lasers, eg, titanium-doped sapphire. Optical parametric oscillators are also developing as useful spectroscopic sources. In the infrared, tunable laser semiconductor diodes have been employed. The tunable diode lasers which contain lead salts have been employed for remote monitoring of poUutant species. Needs for infrared spectroscopy provide an impetus for continued development of tunable infrared lasers (see Infrared technology and RAMAN spectroscopy). 

We conclude this chapter by presenting several examples of deconvolution of real data. Most of these examples represent deconvolutions of data that were used as part of a spectral analysis rather than generated as deconvolution examples or tests. The examples include high-resolution grating spectra, tunable-diode-laser (TDL) spectra, a Fourier transform infrared spectrum (FTIR), laser Raman spectra, and a high-resolution y-ray spectrum. 

For certain special purposes, e.g., the excitation of resonance Raman spectra, tunable dye lasers and solid state lasers are used, e.g., the chromium doped Alexandrite laser and the titanium doped sapphire laser (Demtroder, 1991). 

Buker JF, Sample JD (1976) Tunable Diode Laser Instruments, ISA Reprint. Pittsburg, pp 76-613 Bulkin BJ (1976) Vibrational spectroscopy of liquid crystals. In Brown GH (ed) Advances in liquid crystals, vol 2. Academic, New York, p 199 Bulkin BJ (1981) Vibrational spectra of liquid crystals. In Clark RJH, Hester RE (eds) Advances in infrared and Raman spectroscopy, vol 8. Heyden, London, p 151 Bunker PR (1979) Molecular Symmetry and Spectroscopy. Academic Press, New York Bunow MR, Levin IW (1977a) Biochim Biophys Acta 464 202 Burch DE, Gryvnak DA (1967) J Chem Phys 47 4930... 

Raman spectroscopy was used to probe high-pressure phase transitions for CF4 hydrate systems.366 High-resolution tunable diode laser spectroscopy for CF3C1 showed that the band origin for v, was at 1108.35587(6) cm-1 (35C1) or... 

A tunable pulsed laser Raman spectrometer for time resolved Raman studies of radiation-chemical processes is described. This apparatus utilizes the state of art optical multichannel detection and a-nalysis techniques for data acquisition and electron pulse radiolysis for initiating the reactions. By using this technique the resonance Raman spectra of intermediates with absorption spectra in the 248-900 nm region, and mean lifetimes > 30 ns can be examined. This apparatus can be used to time resolve the vibrational spectral o-verlap between transients absorbing in the same region, and to follow their decay kinetics by monitoring the well resolved Raman peaks. For kinetic measurements at millisecond time scale, the Raman technique is preferable over optical absorption method where low frequency noise is quite bothersome. A time resolved Raman study of the pulse radiolytic oxidation of aqueous tetrafluoro-hydroquinone and p-methoxyphenol is briefly discussed. 

Fig. 1 shows a schematic of the time resolved resonance Raman apparatus for radiation chemical studies. The essential components of the experimental set up are (a) a pulsed electron radiation source for inducing the reactions, (b) a tunable pulsed laser to probe the Raman scattering, (c) a Spex double monochromator for analyzing the scattered light, and (d) a gated detector for recording the spectra. 

A fluorimeter suitable for two-photon excitation using a tunable dye laser has been described. Spectra of diphenylbutadiene in an EPA glass at 77K were used for illustration. Two-photon excited fluorescence was also observed for 0s04 and UFg with excitation using a CO2 and Raman frequency-shifted dye laser, respectively. Polarization effects on two-photon excitation have been examined. ... 

Lasers that have wavelengths in the UV and visible regions of the spectrum are used for resonance Raman spectroscopy. Tunable dye lasers are often used these lasers can... 

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