Gain lasers

Light pulses with halfwidths of 10" sec have been generated mainly by Q-switched solid-state lasers but can be obtained principally with all high-gain laser transitions, as for instance CO2 lasers " ), nitrogen lasers ) (X = 3300 A), or dye lasers ). 

Walling JC, Peterson OG (1980) High-gain laser performance in Alexandrite. IEEE J Quantum Electron 16 119-120... 

Figure 3.10 Output from a free-running high-gain Laser, pumped by a pulsed excitation Light source (flash Lamp or Laser)...

Valdmanis J A and Fork R L 1986 Design considerations for a femtosecond pulse laser balancing self phase modulation, group velocity dispersion, saturable absorption, and saturable gain IEEE J. Quantum. Electron. 22 112-18... 

The importance of laser light, in brief, is tliat its base characteristics, coherence, spectral and polarization purity, and high brilliance allow us to manipulate its properties. Gain switching [i, 10] and mode locking [16] are prime examples of our ability to very specifically control tire laser output. It is easy to see why lasers are tire ideal sources for optoelectronic applications. 

The most useful direct modulation teclmique is the current gain switching of semiconductor laser devices. This technique is unique to semiconductor sources, nearly all other lasers are modulated externally. In tliese devices tire excitation current of tire laser is modulated, resulting in modulated gain and tlierefore modulated output power. A detailed analysis of tliis process is found in [27]. Simply put, an oscillating current of tire fonn... 

In practice the laser can operate only when n, in Equation (9.2), takes values such that the corresponding resonant frequency v lies within the line width of the transition between the two energy levels involved. If the active medium is a gas this line width may be the Doppler line width (see Section 2.3.2). Figure 9.3 shows a case where there are twelve axial modes within the Doppler profile. The number of modes in the actual laser beam depends on how much radiation is allowed to leak out of the cavity. In the example in Figure 9.3 the output level has been adjusted so that the so-called threshold condition allows six axial modes in the beam. The gain, or the degree of amplification, achieved in the laser is a measure of the intensity. 

Unless the cavity is tuned to a particular wavelength the vibration-rotation transition with the highest gain is the P-branch transition involving the rotational level which has the highest population in the 3 state. This is P(22), with J" = 22 and J = 21, at normal laser temperatures. The reason why this P-branch line is so dominant is that thermal redistribution of rotational level populations is faster than the population depletion due to emission. 

One of the first applications of this technique was to the enrichment of and "B isotopes, present as 18.7 and 81.3 per cent, respectively, in natural abundance. Boron trichloride, BCI3, dissociates when irradiated with a pulsed CO2 laser in the 3g vibrational band at 958 cm (vj is an e vibration of the planar, D j, molecule). One of the products of dissociation was detected by reaction with O2 to form BO which then produced chemiluminescence (emission of radiation as a result of energy gained by chemical reaction) in the visible region due to A U — fluorescence. Irradiation in the 3g band of BCls or "BCI3 resulted in °BO or BO chemiluminescence. The fluorescence of °BO is easily resolved from that of "BO. 

The efficiency of a helium—neon laser is improved by substituting helium-3 for helium-4, and its maximum gain curve can be shifted by varying the neon isotopic concentrations (4). More than 80 wavelengths have been reported for pulsed lasers and 24 for continuous-wave lasers using argon, krypton, and xenon lasing media (111) (see Lasers). 

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