Controlled Wavelength Tuning

Although fixed-wavelength lasers have proved their importance for many spectroscopic applications (see Sect.6.3 and Chap.5,8 and 9), it was the development of continuously tunable lasers which really revolutionized the whole field of spectroscopy. This is demonstrated by the avalanche of publications on.tunable lasers and their applications (see, e.g., [1.10]). We shall therefore treat in this section some basic techniques of controlled tuning of single-mode lasers, while the next chapter gives a survey on tunable coherent sources, developed in various spectral regions. 

Since the laser wavelength X of a single-mode laser is determined by the optical path length nd between the resonator mirrors, 

Discontinuous tuning of lasers, (a) Part of the Ne spectrum excited by a single-mode dye laser with Doppler-limited resolution, which hides the cavity mode hops of the laser, (b) Excitation of Na2 lines by a single-mode argon laser in a weakly collimated molecular beam. The intracavity etalon was continuously tilted but the resonator length was kept constant 

For many applications in high-resolution spectroscopy, it is desirable that the fluctuations of the laser wavelength X around the programmed tunable value x(t) are kept as small as possible. This can be achieved by stabilizing X to the reference wavelength of a stable external F.P.I. 

Tuning of the etalon transmission by tilting the etalons has the disadvantage that the reflection losses increase drastically with increasing 0. This results from the finite diameter of the laser beam, which causes an 

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Controlled Wavelength Tuning of Single-Mode Lasers... 

Controlled Wavelength Tuning of Single-Mode Lasers ------------------6--------------... 

Turn the monochromator wavelength setting to 285 nm using the coarse adjustment, then use the fine wavelength control to tune in to the line maximum at 285.2 nm. 

Ihnable laser A laser, such as a dye laser, capable of continuously tuning the wavelength (or frequency) of its output radiation Continuous tuning means the smooth change of wavelength across the spectrum according to the controlled physical tuning of the oscillator. 

Inter-pulse spectroscopy uses the QCL in pulsed mode (normally at or close to room temperature). A short current-pulse to the laser (in the range 0.1 -10 ps) is superimposed on a slowly varying current or temperature ramp as a result, controlled wavelength scanning is achieved. The current pulse needs to be tailored carefully in its amplitude and duration for optimum performance, since the pulsing introduces a frequency chirp, which may adversely affect the laser line width. The typical tuning range for this technique is on the order of 1 -2 cm with repetition... 

This reaction has been carried out with a carbon dioxide laser line tuned to the wavelength of 10.61 p.m, which corresponds to the spacing of the lowest few states of the SF ladder. The laser is a high power TEA laser with pulse duration around 100 ns, so that there is no time for energy transfer by coUisions. This example shows the potential for breakup of individual molecules by a tuned laser. As with other laser chemistry, there is interest in driving the dissociation reaction in selected directions, to produce breakup in specific controllable reaction channels. 

Tuni 7g The interferometer is tuned by electrode voltage control. The band-pass center wavelength is displaced accordingly. The FWHM of the transmission pass band is approximately 70 nm at the 4.2 pm wavelength. The stable tuning range of L is... 

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