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15. Pulse laser

Pulse laser is a heart of the pulse holographic setup. In comparison with gas lasers its structure is much more complex. The most modern achievements of quantum electronics are realized in a pulse laser in order to get a powerful pulse of coherent light of extremely short duration. Let us consider a typical scheme of a pulse laser, see fig.
The master oscillator is the first device in the line of the light pulse formation. Generation of the original low-power pulse occurs in it and also the main pulse parameters - duration, mode composition, polarization and coherence are set in this oscillator. The master oscillator has a ring resonator - the light beam runs by a "circular" trajectory. In reality its trajectory is more similar to a triangle in the vertexes of which mirrors are situated, one of them (the output mirror) has a small transmission factor, see fig. Active element is the rod of high-clean monocrystal of the double fluoride of yttrium-lithium activated by neodymium (LiYF4:Nd, abbreviation - YLF) is located in the one resonator arm.
The rod is located in one of focus of the longitudinal elliptic reflector, see fig. below. The rod pulse flash-lamp which serves for pulse pumping the active element is situated in the second focus. The module comprising the body, elliptic reflector, flash-lamp and the laser rod is called a quantron. During pumping the neodymium ions become excited and go to the higher energetic levels - inversion population takes place.
For creation of the ultra-short generation pulse (about 20 ns) the Q-switch modulator - a light shutter of special construction - is located in the resonator. It doesn't pass light in an ordinary state. But when energy density of radiation from the active element exceeds a certain critical level the shutter opens abruptly and radiation is "splashing" into the ring resonator. Conditions of radiation formation in the master oscillator are such that the radiation wave length belongs to the infra-red region and is equal to 1.053 micron. This radiation is invisible that's why adjustment of the master oscillator is very complex especially because very high adjustment accuracy is needed.
One more optical element is located before the output mirror and its appearance is similar to a simple glass cube with thoroughly polished faces. In fact it's a Fabry-Perot interferometer. It selects a frequency spectrum of the master oscillator and narrows it to such extent that the radiation coherence length rises up to several meters.
The beam at the master oscillator output is sufficiently narrow and isn't fully uniform in its section. In order to increase uniformity of energy distribution over the beam section (to remove transversal modes of higher orders) and to adjust its diameter with diameter of the amplifier rod the beam passes through a system of lenses and prisms (spatial filter).
The amplifier construction is analogous to the oscillator construction. The active element in the form of the rod of optical glass with addition of neodymium is also located in the quantron in one of the focuses of the elliptic reflector. A powerful flash lamp is located in another focus and this flash-lamp makes pumping of the active element. The moments of starting the flash-lamps of the master oscillator and of the amplifier are strictly synchronized. Only in this case the light pulse from the oscillator will be amplified by passing through the active element of the amplifier. For removal of heat discharged by the flash-lamps water cooling of quantrons is used.
When the laser beam passes through the active element of the amplifier and through the coordinating optical elements the wave front uniformity is broken because of aberrations and diffraction effects. In order to increase uniformity of the beam one more unusual optical element - the SBS sell (Stimulated Brillouin Scattering) - a so called cell of the wave front inversion, is located behind the amplifier as the reflecting mirror. Its principle of operation is based on the effects of non-linear optics and is very complex. The cell reflects the radiation falling on it as if in antiphase and so the beam non-uniformities are to a great extent compensated by reverse passing through the amplifier. As a result of double passing through the amplifier we obtain a powerful and pure beam.
For protection of the main oscillator from the reverse penetration of the amplified laser beam which can simply destruct a crystal of the main oscillator a polarizer is set before the amplifier and a quarter-wave plate is set between the amplifier and the SBS sell.
Polarizer orientated in horizontal plane passes radiation from the oscillator to the amplifier which is oriented in the same plane. The quarter-wave plate turns polarization into circular polarization and by return pass of the reflected beam polarization is turned by 90 degrees. Hence the amplified beam won't pass through the polarizer to the master oscillator now but will be reflected from it and will come to the frequency doubler.
This device is also a "representative" of non-linear optics working on the basis of the KDP-crystal (KDP - DihydroPhosphate of Potassium) and it transforms input radiation into a harmonic with doubled frequency. At input of the KDP-crystal radiation has a wave length half as big as the wave length of original radiation that is 0.53 micron. This is already a visible green light. For effective transformation of frequency the quarter-wave plate is put before the KDP-crystal and this plate transforms linear polarization of the beam into circular polarization. One more quarter-wave plate which finally forms a linearly polarized laser beam is put behind the KDP-crystal.
The last optical element of the laser - a light filter which lets pass only green light and stops the rests of infra-red light. A fragment of pulse laser is shown in the photo. You can see lenses and prisms - elements of the spatial filter. The module in white body is a thermostat in which the frequency doubler is situated. The light filter is fixed on the face plane of the thermostat.

The pulse laser is characterized not by power (as a continuous laser) but by pulse energy. Laser of the described structure can create pulse energy of 5 Joules and higher. It's a very high energy which allows to record holograms with dimensions more than 1x1 m. Energy of 1 Joule is enough for recording holograms with dimensions of 28x40 cm. Coherence length of radiation exceeds 2 m. Pulse duration of 20 ns removes all restrictions relating to the object rigidity and to the holographic setup as a whole. With the help of the pulse laser it's possible to record portraits of people and domestic animals. You can simply hold objects in your hands and also record puffs of smoke, drops of water and so on. Advantages of pulse laser significantly broaden possibilities for creation of art scenes.

Note: Author expresses his gratitude to Kornev Alexey for assistance in writing this lesson.

1. O. Svelto, Principles of Lasers, New York, Plenum Press, 1989.