Light and general radiation laws. Coherence and incoherence. Emission, absorption and amplification of radiation. Units and physical constants

Fig. 3.1. Schemes of levels of laser radiation: (a) three-level scheme of ruby laser radiation: E₁-main level, E₂-metastable level; wavy arrows-pumping of bands E₃ and E`₃ with the help of green (з) and blue (c) light and laser transition (л); direct arrows - nonradiative transitions; b) four-level scheme: E₁ - the main level, E₂ - the lower working level, E₃ - the metastable level, E₄ - the absorption band.
The first ruby ​​laser belongs to the class of solid. In this class, in addition to ruby, other dielectric crystals and glasses containing ions of rare-earth metals are used as an active medium. Among them, we call yttrium-aluminum grained Y₃Al₅O₁₂ (Nd³⁺), nickel NiNb0₃ (Nd³⁺), lithium-yttrium fluoride LiYF₄ (Nd³⁺) activated by Nd^{3 +} ions. At the present time, a laser effect has been detected in several hundred dielectric crystals with active ions introduced as impurities or embedded in a crystal lattice. The second method is more perfect, since in this case the active ions are distributed more uniformly over the crystal glass, which makes it possible to increase their dosage to ~ 20% (in the impurity version it is ~ 5%) and, accordingly, to increase the radiation power without overheating the crystal. Incidentally, in order to avoid overheating of the crystal (and also the difficulties of obtaining a powerful optical pumping system operating in a continuous mode), most solid-state lasers operate in a pulsed mode, although they are known to operate in a continuous mode.

The pulsed mode of operation described above, with an example of a ruby ​​laser, in which the total duration of coherent radiation is equal to the pump duration (~ 10^-3 s), is called the free-running mode. Recall that in this case the generation pulse consists of a series of successive short spikes of duration ~ 10^-7 s each, separated by microsecond intervals, and the total pulse duration is 10^-3 s and that, under this regime, the peak power is approximately 10 kW. But there is another pulsed mode of operation of the solid-state laser, in which the duration of the total generation pulse can be made equal to the duration of one spike in the case of free generation (and even shorter), which allows a sharp increase in the laser peak power.

Such a mode of operation is achieved by the so-called method of modulation of the Q-factor of the resonator, the essence of which is the forced delay in the onset of the process of laser radiation. To this end, the design of the resonator is complemented by a special rapid electronic-optical gate, which can open or close the input of radiation to the resonator. Pumping in this mode of operation is performed with a closed shutter, which makes it possible to accumulate the maximum number of excited ions at a metastable level (since with a non-working cavity, generation can not begin even when the inverse population of levels is reached). Then, quickly opening the shutter, a resonator is included, so that the energy stored in the active substance instantaneously (for 10^-7 - 10^-8 s) is released in the form of a powerful light pulse.

There are other methods for obtaining even shorter generation pulses and their corresponding peak powers. These include the method based on synchronization, i.e. constructive interference of several resonant modes of the resonator. This method also uses a shutter, but a special, so to speak, self-correcting design. It is a cuvette placed in the resonator with a special dye, which, due to opacity, breaks the feedback, which leads to an increase in the inverse population of the active substance. This process continues until there is a superluminescence, the strongest pulse of which illuminates the gate fluid and thereby includes a resonator providing generation. In order to ensure synchronous operation of all modes of the resonator, a mode of the amplitude or phase modulation of the resonator is used, consisting either in modulating the transmission of the output mirror or modulating the distance between the mirrors L and the frequency of intermode beats . The resulting impulse has a duration of 10^-9 - 10^-10 s and a very high power ~ 10¹⁰ W. An even larger peak power is obtained using solid-state lasers on glass with impurity Nd³⁺, which generate ultrafast (for that time) pulses with a duration of 10^-11-10^-12 s.

We note that at a huge peak power the energy of these pulses is relatively small. But if necessary, it can be increased by adding to the laser-generator one or several lasers-amplifiers. In this case, the peak power will also increase, reaching values ​​of the order of 10¹³ - 10¹⁴ W.

In addition to pulsed for some crystals (for example, for the above-mentioned garnet, as well as glass with Nd^{3 +}), a continuous mode of generation is possible. A necessary condition for this is the four-level scheme of operation of the active substance shown in Fig. 3.1, b. The pre-property of this scheme before the three-level scheme is that the laser transition occurs between the third and second levels ( ), where E₂ is not basic, has a smaller population than E₁. In this connection, the inverse population of the metastable level E₃ is much easier to obtain in a four-level scheme (with less pumping) than in a three-level one (for this it is not necessary to "lift" from the E1 level more than 50% of the particles). Both non-intermittent lasers (garnet and glass-based) belong to the same type of neodymium lasers, since the generation of laser radiation in them occurs due to quantum transitions between the energy states of the trivalent Nd^{3 +} ions. The same type also includes lasers with Nd^{3 +} ions placed in another condensed medium, for example, a semiconductor, organometallic or organic liquids.

The real scheme of the Nd^{3 +} levels is more complicated than that shown in Fig. 3.1, b (there are several pumping bands and lower intermediate levels), but in principle it works as a four-level scheme, which most likely generates infrared radiation with a wavelength of μm. Note that neodymium lasers operate not only in the continuous generation mode, but also in all the above-mentioned pulse modes (continuous generation, modulated Q-switching, mode locking). And the continuous mode of operation of the laser itself is essentially impulsive, since it represents a continuous sequence of pulses of duration ~ 10^-9 s, which follow one after another with the frequency mentioned above . We note that, according to experts, modern solid-state lasers with LED pumping have an efficiency reaching 30%, an output power of up to 10 kW, high frequency stability (comparable to the stability of microwave generators) and a narrow line (about 1 Hz).