Properties of the Laser Radiation

The main features of the laser radiation reflect the features of stimulated emitted photons.

(i)

The laser light can be very monochromatic. In principle, the laser linewidth depends on the active medium and on the laser design. The best results have been obtained for the low pressure gas lasers (Hansch was able to reach a ratio $\Delta\lambda/\lambda \simeq 10^{-15}$). Therefore, lasers can be effectively used for high resolution spectroscopy.

(ii)

The laser beam can be of very low divergent (spatial coherence), which means that its diameter is increased only slowly in space. In principle, with laser beams it is possible to reach the diffraction limit:

$$\Theta \approx \frac{2\lambda}{\pi d}$$ (28)

This means that using a lens, it is possible to focus a laser beam on a spot of the diameter:

$$d \approx \frac{2\lambda}{\pi}\frac{f}{D}$$ (29)

where, $f$ denotes the focal distance of the lens.

As, for the best lenses, the ratio of focal lens to diameter is about 0.8, it is a rule of thumb that the half of the light wavelength is approximately equal to the lower limit of the diameter of the spot.

(iii)

Short laser pulses can be produced. Using specific techniques for generating the laser pulses (Q-switching, modes locking, pulse compression), the extremely short pulse duration can be achieved. As short as several hundreds of atoseconds ($10^{-18}$) pulse duration has recently been obtained! The pulse duration in the nanosecond ($10^{-9}$), picosecond ($10^{-12}$), and femtosecond ($10^{-15}$) range are now available commercially. Particularly, femtosecond laser pulses are of great importance, because they allow to investigate chemical reactions in the real-time domain.

(iv)

High power output. The continuous $CO_2$ lasers are now can produce the high power output up to 100 kW. These technological lasers are now widely used in industry and for military aims. The pulsed lasers, especially those operating in the pico- and femtosecond time-domain can have extremely high pick power. Particularly, the pick powers from $10^9$ to $10^{12}$ watts are now available. High power pulsed lasers are widely used for investigation of multiple photon processes.

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