Explain Ruby Laser under following heads: Construction – working with Diagram.
(i) Active medium:
It is a solid-state laser, in which a rod of synthetic ruby crystal is used as active medium. The ruby crystal is obtained by doping small amount (about 0.05% by weight) of chromium oxide (Cr2O3) in Aluminum oxide (Al2O3), so that some of the aluminium ions (Al3+) are replaced by chromium ions (Cr3+). These chromium ions give the crystal a pink or red color depending upon the doping concentration. Al2O3 only acts as the host while the chromium ions act as active centers in ruby crystal and responsible for the laser action. The length of ruby rod is usually 2 cm to 30 cm and diameter is 0.5 cm to 2 cm.
(ii) Optical resonator:
To construct the optical resonator cavity, the ends of the rods are polished such that they become flat and parallel to each other. Now the one of the ends is coated with silver completely while the other one is partially silvered. Thus, the two silver-coated ends of the rod act as optical resonator system.
(iii) Pumping system:
Ruby is a three energy level laser system. After absorbing light photons of wavelength 5500 Å from xenon flash lamp, some of the Cr3+ ions at ground energy level E1 get excited to higher energy level E3. At this energy level, they are unstable and by losing a part of their energy to the crystal lattice, they fall to the metastable energy level E2, whose lifetime is much longer (about 10-3 s). Therefore, the number of Cr3+ ions goes on increasing in E2 state while the number of these ions in ground state E2 goes on decreasing due to pumping by flash lamp and soon the population inversion is achieved between states E2 and E1.
Now some of the Cr3+ ions will decay spontaneously to the ground state E1 by emitting photons of wavelength 6943 Å. The photons those are moving parallel to the axis of the rod will reflect back and forth by the silvered ends of the rod and stimulate other excited Cr3+ ions to radiate another photon with same phase. Thus, due to successive reflections of these photons at the ends of the rod the number of photons multiplies. After a few microsecond a monochromatic, intense and collimated beam of red light of wavelength 6943 Å emerges through the partially silvered end of the rod. The Ruby laser is a pulsed laser that emits light in the form of very short pulses.