Types Of Laser
Introduction: Many thousands of kinds of laser are known, but most of them are not used beyond specialized research. The chief types of lasers are solid state lasers, gas lasers and liquid lasers. A solid, liquid, gas or semiconductor can act as the laser medium. Here we discuss only Helium-Neon and semiconductor lasers.
Helium-Neon Laser:
A Helium-Neon laser, usually called a He-Ne laser, is a gas laser of operation wavelength is 632.8 nm, in the red portion of the visible spectrum. This was invented by Ali Javan, William Bennett Jr. and Donald Herriott at Bell Labs. The gain medium of the laser, as suggested by its name, is a mixture of helium and neon gases, in a 5 : 1 to 20 : 1 ratio, contained at low pressure in a glass tube. The energy or pump source of the laser is provided by an electrical discharge of around 1000 V through an anode and cathode at each end of the glass tube. The optical cavity of the laser typically consists of a plane, high-reflecting mirror at one end of the laser tube, and a concave output coupler mirror of approximately 1% transmission at the other end. He-Ne lasers are typically small, with cavity lengths of around 15 cm up to 0.5m, and optical output powers ranging from 1mW to 100mW.
The laser process in a He-Ne laser starts with collision of electrons from the electrical discharge with the helium atoms in the gas. This excites helium from the ground state to the 23S and 21S metastable excited states. Collision of the excited helium atoms with the ground-state neon atoms results in transfer of energy to the neon atoms, exciting neon electrons into the 3s and 2s levels. This is due to a coincidence of energy levels between the helium and neon atoms. This process is given
By the reaction equation:
He electric energy → He∗
He∗ Ne → He Ne∗
where ∗represents an excited state. The number of neon atoms entering the excited states builds up as further collisions between helium and neon atoms occur, causing a population inversion. Spontaneous and stimulated emission between the 3s and 2p states results in emission of 632.82 nm wavelength light.
After this, fast radiative decay occurs from the 2p to the 1s ground state. Because the neon upper level saturates with higher current and the lower level varies linearly with current, the He-Ne laser is restricted to low power operation to maintain population inversion.
With the correct selection of cavity mirrors, other wavelengths of laser emission of the He-Ne laser are possible. There are infrared transitions at 3391.2 nm and 1152.3 nm wavelengths, and a variety of visible transitions. The typical 633 nm wavelength red output of a He-Ne laser actually has a much lower gain compared to other wavelengths such as the 1152.3 nm and 3391.2 nm lines, but these can be suppressed by choosing cavity mirrors with optical coatings that reflect only the desired wavelengths. It is used in interferometry, holography, spectroscopy, barcode scanning, alignment, optical demonstrations.