Laser
Ruby Laser
The ruby laser is the first type of laser actually constructed, first demonstrated in 1960 by T. H.
Maiman. The ruby mineral (corundum) is aluminum oxide (Al2O3 ) with a small amount (about
0.05%) of chromium ions (Cr3+) which gives it its characteristic pink or red color by absorbing
green and blue light.
The ruby laser is used as a pulsed laser, producing red light at 6943Å. After receiving a pumping
flash from the flash tube, the laser light emerges for as long as the excited atoms persist in the
ruby rod, which is typically about a millisecond.
The ruby laser has following main parts:
1. The working substance (active medium)- is in the form of a rod of ruby crystal (10 cm in
length, 0.8 cm in diameter) in which Cr3+ are active centers while Al and O2 are inert.
2. The resonance cavity- is made by silvering and polishing the ends of ruby rod. Fully reflecting
plates at the left and a partially reflecting plate at the right, both optically plane and accurately
parallel.
3. The optical pumping system -consists of a helical xenon discharge tube. It produces flash of
few milliseconds.
Ruby laser uses a three level pumping scheme. The xenon discharge generates a flash of white
light for few milliseconds. The Cr+3 ions are excited to the E3 level by absorbing green and blue
component of white light. From there the Cr+3 ions undergo non-radiative transitions and
quickly drop to the metastable level E2. The metastable state has greater life time than E3.
Therefore Cr+3 ions accumulate at E2. When more than half ions are accumulated at E2 the
population inversion is established between E2 and E1. A chance spontaneous emission leads to
chain stimulated emission. Red light (of wavelength 6943 Å) emerges from the front face.
