In this topic we will understand Ruby LASER : It’s working and drawbacks.
This is the first LASER developed in 1960. It is a solid state LASER. It consists of a pink Ruby cylindrical rod whose ends are optically flat and parellel.
One end is fully silvered & the other is partially silvered. Outside the rod, a coiled flash lamp filled with Xenon gas is bounded.
|Fig 1. A Ruby LASER
The ruby rod is a crystal of aluminium oxide (Al2O3) doped with 0.05% chromium oxide (Cr2O3), so that some of the Al+++ ions are replaced by Cr+++ ions.These “impurity” chromium ions give pink colour to the ruby and give rise to the laser action.
A simplified version of the energy-level diagram of chromium ion is shown in figure. It consists of an upper short-lived energy level (rather energy band) E3 above its ground-state energy level E1, the energy difference E3 – E1 corresponding to a wavelength of about 5500 Å. There is an intermediate excited-state level E2 which is metastable having a life-time of 3 x 10-3 s (about 105 times greater than the life-time of E3 which is ~10-8 s).
|Fig 2. Various States in Three-Level LASER
Normally, most of the chromium ions are in the ground state E1. When a flash of light (which lasts only for about a millisecond) falls upon the ruby rod, the 5500-A° radiation photons are absorbed by the chromium ions which are “pumped” (raised) to the excited state E3. The transition 1 is the (optical) pumping transition. The excited ions rapidly give up, by collision, part of their energy to the crystal lattice and decay to the “metastable” state E2.
The corresponding transition 2 is thus a radiationless transition. Since the state 2 has a much longer life-time, the number of ions in this state goes on increasing while due to pumping, the number in the ground state E1 goes on decreasing.
Thus, population inversion is established between the metastable (excited) state E2 and the ground state E1. When an (excited) ion passes spontaneously from the metastable state to the ground state (transition 3), it emits a photon of wavelength 6943 A°.
This photon travels through the ruby rod and, if it is moving parallel to the axis of the crystal, is reflected back and forth by the silvered ends until it stimulates an excited ion and causes it to emit a fresh photon in phase with the stimulating photon.This “stimulated” transition 4 is the laser transition.
This process is repeated again & again because the photon repeats its path due to reflection.
There is a drawback in 3-Level LASER, i.e. the LASER require high pumping power.
In this post we have discussed Ruby LASER, It’s working and drawbacks. You can also go through the following topics to better understand it :