Question

Draw a neat sketch of Ruby Laser. Explain its working with the help of energy level diagram.

Answer 1

The Ruby Laser consists of a single crystal of ruby rod of length $10$cm and $0.8$cm in diameter. A ruby is a crystal of aluminium oxide $A{l}_2{O}_3$, in which some of aluminium ions $\left(A{l}^{3+}\right)$ are replaced by the chromium ions $\left(C{r}^{3+}\right)$. The opposite ends of ruby rod are flat and parallel; one end is fully silvered and the other is partially silvered (i.e.) semi-transparent. The ruby rod is surrounded by a helical xenon flash tube which provides the pumping light to raise the chromium ions to upper energy level(figure). In the xenon flash tube, each flash lasts several milliseconds and in each flash a few thousand joules of energy is consumed.
The simplified energy level diagram of chromium ions in a ruby laser, indicating appropriate excitation and decay is shown in figure.
In normal state, most of the chromium ions are in the ground state $E_1$. When the ruby rod is irradiated by a flash of light, the $5500\stackrel{o}{A}$ radiation (green colour) photons are absorbed by the chromium ions which are pumped to the excited state $E_3$. The excited ion gives up part of its energy to the crystal lattice and decay without giving any radiation to the meta stable state $E_2$. Since, the state $E_2$ has a much longer lifetime $\left({10}^{-3}s\right)$, the number of ions in this state goes on increasing. Thus population inversion is achieved between the states $E_2$ and $E_1$. When the excited ion from the metastable state $E_2$ drops down spontaneously to the ground state $E_1$, it emits a photon of wavelength $6943\stackrel{o}{A}$. This photon travels through the ruby rod and is reflected back and forth by the silvered ends until it stimulates other excited ion and causes it to emit a fresh photon in phase with stimulating photon. Thus the reflections will amount to the additional stimulated emission-the so called amplification by stimulated emission. This stimulated emission is the laser transition. Finally, a pulse of red light of wavelength $6943\stackrel{o}{A}$ emerges through the partially silvered end of the crystal.