Gas and Dye Lasers

However, even for lasers with a very sophisticated stabilization system, the residual uncompensated fluctuations of nd cause frequency fluctuations that are large compared with this theoretical lower limit. With moderate efforts, laser linewidths of Avl = 10" —10 Hz have been realized for gas and dye lasers. With very great effort, laser linewidths of a few Hertz or even below 1 Hz [5.81,5.102] can be achieved. However, several proposals have been made how the theoretical lower limit may be approached more closely [5.103,5.104]. 

Perhaps even more than other fields of application in laser spectroscopy, ion physics is likely to benefit rapidly from progress in the U.V. range. Frequency doubled CW gas and dye lasers are now able to provide on a routine basis the few milliwatts of monomode laser power which have been necessary for the N20" photodissociation experiment. Further in the U.V., the Doppler tuning method could take advantage with much benefit of the high spectral brightness of injected excimer amplifiers operated at high repetition rate. 

Modem lasers cover radio, microwave, infrared, visible, ultraviolet, and X-ray spectra. There are following types of lasers gas, chemical, excimer, solid-state, fiber-hosted, photonic crystal, free electron, and dye lasers. Taken in combination, the laser techniques allow one to produce a coherent beam of different frequency, power, and duration up to very short-duration pulses on the order of a few femtoseconds. Laser research has produced a variety of improved and specialized... 

Andrews [9] and others [10] have listed the emission lines of the most commonly available discrete-wavelength lasers (such as ruby, Nd YAG, Er YAG, excimer lasers) over the range 100 nm-10 /u.m. Molecular lasers (HF, CO, CO2, NO) can be tuned to a large number of closely spaced but discrete wavelengths. Continuously tuneable lasers comprise some metal ion vibronic lasers (e.g. alexandrite and Ti sapphire [11]), some diode and excimer lasers, dye and free-electron lasers. Tuneable sources of coherent radiation span the electromagnetic spectrum from 300 nm to 1 mm, with limited tune-ability down to about 200 nm. Wavelength coverages of tuneable lasers have been reported [8]. In operation lasers can be either pulsed (e.g. various metal ion tuneable vibronic lasers, excimer and dye lasers, metal vapour) or continuous wave (major types atomic and ionic gas lasers, dye and solid-state lasers). Most lasers with spectral output in the UV are bulky and expensive devices (especially sub 200 nm) and operate in the pulsed mode. On the contrary, many visible lasers are available which are compact, require low maintenance expenses and operate in continuous-wave (CW) mode. 

The crystalline mineral silicates have been well characterized and their diversity of stmcture thoroughly presented (2). The stmctures of siHcate glasses and solutions can be investigated through potentiometric and dye adsorption studies, chemical derivatization and gas chromatography, and laser Raman, infrared (ftir), and Si Fourier transform nuclear magnetic resonance ( Si ft-nmr) spectroscopy. References 3—6 contain reviews of the general chemical and physical properties of siHcate materials. 

The potential of a tunable dye laser should not be overlooked. A tunable dye laser, employing an organic dye as lasing material allows one to choose any suitable excitation line within a particular region. This is in contrast to the case of a gas ion laser which has a limited number of emission lines at fixed wavelength. Nevertheless, a tunable dye laser has significant drawbacks such as poor resolution imposed by the dye laser linewidth (1.2 cm-1) and a continuous background spectrum which requires the use of a tunable filter 15-18). 

Historically, this has been the most constrained parameter, particularly for confocal laser scanning microscopes that require spatially coherent sources and so have been typically limited to a few discrete excitation wavelengths, traditionally obtained from gas lasers. Convenient tunable continuous wave (c.w.) excitation for wide-held microscopy was widely available from filtered lamp sources but, for time domain FLIM, the only ultrafast light sources covering the visible spectrum were c.w. mode-locked dye lasers before the advent of ultrafast Ti Sapphire lasers. 

Monochromatic light can also be obtained from other types of lasers solid state, gas, ion, dye. Among them argon ion laser with its many lines is an especially valuable light source used in many sensors. However, these types of lasers are expensive, the modulation of the light cannot be done internally and external modulators (e.g. choppers) should be used. Wavelengths emitted by some exemplary lasers are presented in Table 1. 

Figure 4. The sample cell arrangement in the DCSHG experiment, where the sample solution was inserted between two glass slips (lop), and the optical design for the DCSHG dispersion experiment, where the compressed H gas medium was pumped by a tunable pulsed dye laser source for Stokes generation by stimulated Raman scattering (bottom). (E° is the static electric field.) Key beam guiding prisms P, Stokes...

The necessary pump powers can be achieved either by other lasers (e.g. nitrogen lasers, solid-state lasers or even focussed He-Ne- or Ar+-gas lasers) or by flash-lamps. The simplest practical arrangement is a square spectrophotometer cell, polished on all sides, containing the dye solution which is pumped by a nitrogen laser whose beam is focussed into a line parallel to and directly behind one of the cell windows. Then the Fresnel reflection from the two adjacent windows gives enough feedback in most cases, so that no additional resonator mirrors are needed and the dye laser oscillation starts. 

A further improvement and more freedom in the choice of laser wavelengths can be expected with the use of dye vapors. In liquids, the phase-matching concentration is set by the requirement that the anomalous dispersion of the dye compensates for the normal dispersion of the solvent. The latter is a new parameter that can be varied at will in the gas phase by changing the nature and partial pressure of the buffer gas. The broader resonances of dyes as opposed to metal vapors, which are sometimes used for this purpose, is an advantage for tunable frequency tripling of dye lasers. Another advantage results from the possibility of working at much lower temperatures than with metal vapors. 

Gas-discharge lamps are used to optically pump the metastable helium atoms into a higher excited electronic state, which has a dipole-allowed transition to the ground state. Only He (2 S) can be pumped selectively, thereby producing pure He(23S) beams. For the heavier rare gases, both metastable states are equally pumped by gas-discharge lamps. The use of cutoff filters to selectively pump one state is not adequate because of the temperature dependence of the filter transmission and the low / numbers of the pumping transition. Metastable neon can be selectively pumped by a continuous wave (cw) dye laser,60 whereas Ar, Kr, and Xe have so far only been selectively pumped by pulsed dye lasers.61... 

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