Coherent tunable sources

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. 

It is admitted that spectroscopy in the far-infrared suffers from the lack of more powerful sources and more sensitive detectors 2 ). Here Fourier transform spectroscopy has some advantages over conventional spectroscopy, e.g. with a grating instrument, and will probably be the most used method until coherent tunable laser sources take over. 

In several regards laser absorption spectroscopy corresponds to microwave spectroscopy, where klystrons or carcinotrons instead of lasers represent tunable coherent radiation sources. Laser spectroscopy transfers many of the techniques and advantages of microwave spectroscopy to the infrared, visible, and ultraviolet spectral ranges. 

There is another important application of stimulated Raman scattering in the field of Raman lasers. With a tunable pump laser at the frequency col, intense coherent radiation sources at frequencies col =1= < v ( = 1,2, 3,...) can be realized that cover the UV and infrared spectral range if visible pump lasers are used (Vol. 1, Sect. 5.8). 

There are, firstly, the improvement of frequency-doubling techniques in external cavities, the realization of more reliable cw-parametric oscillators with large output power, and the development of tunable narrow-band UV sources, which have expanded the possible applications of coherent light sources in molecular spectroscopy. Furthermore, new sensitive detection techniques for the analysis of small molecular concentrations or for the measurement of weak transitions, such as overtone transitions in molecules, could be realized. Examples are Cavity Ringdown Spectroscopy, which allows the measurement of absolute absorption coefficients with great sensitivity or specific modulation techniques that push the minimum detectable absorption coefficient down to 10 " cm ... 

Now several thousands of laser lines are known which span the whole spectral range from the vacuum-ultraviolet to the far-infrared region. Of particular inters are the continuously tunable lasers which may in many cases replace wavelength-selecting elements, such as spectrometers or interferometers. In combination with optical frequency-mixing techniques such continuously tunable monochromatic coherent light sources are available at nearly any desired wavelength above 100 nm. 

In addition to the diatomic RGHs, triatomic RGH ex-cimers can provide tunable coherent photon sources in the visible to the UV region of the spectrum. Table II shows a list of triatomic RGH excimers. 

Many of the most widely used tunable coherent infrared sources use various semiconductor materials, either directly as the active laser medium (semiconductor lasers) or as the nonlinear mixing device (frequency-difference generation). 

While generation of sum frequencies yields tunable ultraviolet radiation by mixing the output from two lasers in the visible range, the phase-matched generation of difference frequencies allows one to construct tunable coherent infrared sources. One early example is the difference-frequency spectrometer of Pine [5.277], which has proved to be very useful for high-resolution infrared spectroscopy. 

V. Wilke, W. Schmidt Tunable coherent radiation source covering a spectral range from 185 to 880 nm. Appl. Phys. 18, 177 (1979)... 

The experimental realization of these tunable coherent light sources is, of course, determined by the spectral range for which they are to be used. For the particular spectroscopic problem, one has to decide which of the possibilities summarized above represents the optimum choice. The experimental expenditure depends substantially on the desired tuning range, on the achievable output power, and, last but not least, on the realized spectral bandwidth Av. Coherent light sources with bandwidths Av 1 MHz to 30 GHz (3 x 10 -1 cm ), which can be continuously tuned over a larger range, are already commercially available. In the visible... 

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. 

The future prospects for selective chemistry research as an aspect of laser development are not as bright as several years ago. With the exception of the quest for a visible chemical laser, most new laser research is oriented towards systems that are patently nonmolecular. The leading candidate for very high power applications is the free electron laser (PEL). In this device, the interaction of a relativistic electron beam with a periodic magnetic structure produces coherent radiation. Such devices on paper can have substantially higher power and efficiency than electric molecular lasers. For moderate power applications, advances in solid state lasers, nonlinear optical conversion processes, and tunable solid state media offer the prospect of broadly tunable compact sources. At low powers, diode lasers and diode laser arrays are gaining increasing application and hold out the promise, when used with solid state media, of versatile tunable sources. 

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