Light tunable laser

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). 

Current instruments use complex and refined optical systems in order to radiate the cuvette with monochromatic light selected from the radiant spectrum of the light source. A radically different approach may be practical when tunable lasers become available at reasonable prices. 

For 2PA or ESA spectral measurements, it is necessary to use tunable laser sources where optical parametric oscillators/amplifiers (OPOs/OPAs) are extensively used for nonlinear optical measurements. An alternative approach, which overcomes the need of expensive and misalignment prone OPO/OPA sources, is the use of an intense femtosecond white-light continuum (WLC) for Z-scan measurements [71,72]. Balu et al. have developed the WLC Z-scan technique by generating a strong WLC in krypton gas, allowing for a rapid characterization of the nonlinear absorption and refraction spectra in the range of 400-800 nm [72]. 

Delbriick et al., (1976) have recently sought to determine whether or not the stimulation of Phycomyces involves the excitation of the lowest triplet state of riboflavin. They determined an action spectrum of light-growth response between 575 and 630 nm using a tunable laser beam and taking advantage of the null method described above. This action spectrum was compared with an action spectrum obtained by computer extrapolation of a phototropic action spectrum covering 445—560 nm. 

Figure 7. Light scattering of a microresonator with a water cladding (a) spectra obtained as response to a tunable laser with clearly visible high finesse resonances (b) CCD camera images of the microresonator obtained off-resonance, (c) idem on-resonance.

The system used to measure the optical fiber signals employs two separate frequency tunable laser light sources operating at about 1320 nm wavelength. One laser acts as a pump laser, whereas the other serves as the probe laser that sends optical pulses down the fiber to interact with the counterpropagating laser light wave pumped into the fiber from its opposite end. 

Fundamentally, the properties of laser light are concomitants of its coherence, which is in turn a consequence of the nature of stimulated emission. Most of these properties, especially brightness, monochromaticity, directionality, polarization, and coherence itself, are useful (for many applications, indis-pensible) in a spectroscopic light source. The spectroscopic potential of lasers was recognized even before they were invented. Actual applications remained very specialized until tunable lasers were devised. 

Tunable coherent light sources can be realized in several ways. One possibility is to make use of lasers that offer a large spectral gain profile. In this case, wavelength-selecting elements inside the laser resonator restrict the laser oscillation to a narrow spectral interval and the laser wavelength may be continuously tuned across the gain profile. Examples of this type of tunable laser are the dye lasers were treated in the previous section. 

In the first group definite states of the molecules to be analyzed are excited by the monochromatic and frequency-tunable laser light, thus inducing selective photochemical reactions. 

Most optical spectral measurements, where the measurement of multiple wavelengths is required, will feature some type of polychromatic or broadband light source. There are a few exceptions here, such as tunable laser sources and source arrays. In such instances, the source is effectively "monochromatic at a given point in time. These sources are covered separately under monochromatic sources. 

Alexandrite, the common name for Cr-doped chrysoberyl, is a laser material capable of continuously tunable laser output in the 700-800 nm region. It was established that alexandrite is an intermediate crystal field matrix, thus the non-phonon emitting state is coupled to the 72 relaxed state and behaves as a storage level for the latter. The laser-emitted light is strongly polarized due to its biaxial structure and is characterized by a decay time of 260 ps (Fabeni et al. 1991 Schepler 1984 Suchoki et al. 2002). Two pairs of sharp i -lines are detected connected with Cr " in two different structural positions the first near 680 nm with a decay time of approximately 330 ps is connected with mirror site fluorescence and the second at 690 nm with a much longer decay of approximately 44 ms is connected with inversion symmetry sites (Powell et al. 1985). The group of narrow lines between 640 and 660 nm was connected with an anti-Stokes vibronic sideband of the mirror site fluorescence. 

Copper in minerals luminescence is usually considered only as an effective quencher. Nevertheless, it is well known that a bright blue luminescence is emitted from Cu ions in inorganic solids by UV light irradiation. It was found that these materials have potential application to tunable lasers. For example, in Ca0-P20s glasses Cu is characterized by a luminescence band at 440 nm with a half-width of 100 nm and an excitation maximum at 260 nm. The decay time of luminescence is approximately 25 ps (Tanaka et al. 1994). Red fluorescence possibly connected with the Cu" pair is also known (Moine et al. 1991). 

As a method to control wavepackets, alternative to the use of ultra-short pulses, I would like to propose use of frequency-modulated light. Since it is very difficult to obtain a well-controlled pulse shape without any chirp, it is even easier to control the frequency by the electro-optic effect and also by appropriate superposition of several continuous-wave tunable laser light beams. 

Three topics related to photochemistry are treated in this chapter. Isotope enrichment takes advantage of the monochromatic nature of a light soun > in exact coincidence with an absorption line of a desired isotopic species. .. mixtures of other species. The recent advancement of tunable lasers in tin visible and ultraviolet regions has extended the possibility of isoiupi. enrichment not only in the atomic system, but also in the molecular system... 

In an extension of tills technology, A. S. Moffat reported in 1992 two additional goals of development (1) the use of lasers to control chemical reactions, and (2) development of tunable lasers that can vary the wavelength of the light they generate. Thus, the light source can be tailored exactly to the vibrational frequency of the bond targeted. 

Fig. 9.1 Diamagnetic structure of Na. (a) Experimental excitation curves for even parity levels, m = 1, ms = i, in the vicinity of n = 28. A tunable laser was scanned across the energy range displayed. The zero of energy is the ionization limit. Signals generated by ionizing the excited atoms appear as horizontal peaks. The horizontal scale is quadratic in field. Calculated levels are overlaid in light lines. Some discrepancies are present due to nonlinearity of the lasers, (b) Calculated excitation curves, displayed linearly in field, (c) Same as (b), but for even parity m = 0 states. Note the large effect on anticrossings due to the presence of the nondegenerate s states (from ref. 7).

A CARS experiment has recently been done to determine the amount of vibrational and rotational excitation that occurs in the O2 (a- -A) molecule when O3 is photodissociated (81,82). Valentini used two lasers, one at a fixed frequency (266 nm) and the other that is tunable at lower frequencies. The 266 nm laser light is used to dissociate O3, and the CARS spectrum of ( (a A), the photolysis product, is generated using both the fixed frequency and tunable lasers. The spectral resolution (0.8 cm l) is sufficient to resolve the rotational structure. Vibrational levels up to v" = 3 are seen. The even J states are more populated than the odd J states by some as yet unknown symmetry restrictions. Using a fixed frequency laser at 532 nm (83) to photolyze O3 and to obtain the products 0(3p) + 02(x3l g), a non-Boltzmann vibrational population up to v" = k (peaked at v" = 0) is observed from the CARS spectrum. The rotational population is also non-Boltzmann peaked at J=33, 35 33, 31 and 25 for v" = 0,1,2,3, and k, respectively. Most of the available energy, 65-67%, appears in translation 15-18% is in rotation and 17-18% is in vibration. A population inversion between v" = 2 and 3 is also observed. 

A diode laser spectrometer. In this case, if the laser is a tunable laser, there are only two critical components the tunable laser and the detector. Typically, the enabling technology is the laser, which in this mode acts as the light source and the wavelength selection device. 

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