A
laser is constructed from three principal parts:
Pump source
The
pump source is the part that provides energy to the laser system. Examples of pump sources include electrical discharges, flashlamps, arc lamps, light from another laser, chemical reactions and even explosive devices. The type of pump source used principally depends on the
gain medium, and this also determines how the energy is transmitted to the medium. A
helium-
neon (HeNe) laser uses an electrical discharge in the helium-neon gas mixture, a Nd:YAG laser uses either light focused from a
xenon flash lamp or
diode lasers, and excimer lasers use a chemical reaction.laser is not only light source
Gain medium / Laser medium
The
gain medium is the major determining factor of the wavelength of operation, and other properties, of the
laser.
Gain media in different materials have linear spectra or wide spectra.
Gain media with wide spectra allow tune frequency of laser.First wide
tunable crystal laser with tunabulity more octave represent on photo 3
http://spie.org/x39922.xml . There are hundreds if thousands of different gain media in which laser operation has been achieved (see
list of laser types for a list of the most important ones). The gain medium is excited by the pump source to produce a
population inversion, and it is in the gain medium that spontaneous and
stimulated emission of photons takes place, leading to the phenomenon of optical gain, or amplification.
Examples of different gain media include:
- Liquids, such as dye lasers. These are usually organic chemical solvents, such as methanol, ethanol or ethylene glycol, to which are added chemical dyes such as coumarin, rhodamine and fluorescein. The exact chemical configuration of the dye molecules determines the operation wavelength of the dye laser.
- Gases, such as carbon dioxide, argon, krypton and mixtures such as helium-neon. These lasers are often pumped by electrical discharge.
- Solids, such as crystals and glasses. The solid host materials are usually doped with an impurity such as chromium, neodymium, erbium or titanium ions. Typical hosts include YAG (yttrium aluminium garnet), YLF (yttrium lithium fluoride), sapphire (aluminium oxide) and various glasses. Examples of solid-state laser media include Nd:YAG, Ti:sapphire, Cr:sapphire (usually known as ruby), Cr:LiSAF (chromium-doped lithium strontium aluminium fluoride), Er:YLF, Nd:glass, and Er:glass. Solid-state lasers are usually pumped by flashlamps or light from another laser.
- Semiconductors, a type of solid, crystal with uniform dopant distrubution or material with differing dopant levels in which the movement of electrons can cause laser action. Semiconductor lasers are typically very small, and can be pumped with a simple electric current, enabling them to be used in consumer devices such as compact disc players. See laser diode.
Optical resonator
The
optical resonator, or
optical cavity, in its simplest form is two parallel mirrors placed around the gain medium which provide
feedback of the light. The mirrors are given
optical coatings which determine their reflective properties. Typically one will be a
high reflector, and the other will be a
partial reflector. The latter is called the
output coupler, because it allows some of the light to leave the cavity to produce the laser's output beam.
Light from the medium, produced by
spontaneous emission, is reflected by the mirrors back into the medium, where it may be amplified by
stimulated emission. The light may reflect from the mirrors and thus pass through the gain medium many hundreds of times before exiting the cavity. In more complex lasers, configurations with four or more mirrors forming the cavity are used. The design and alignment of the mirrors with respect to the medium is crucial to determining the exact operating wavelength and other attributes of the laser system.
Other optical devices, such as spinning mirrors, modulators, filters, and absorbers, may be placed within the optical resonator to produce a variety of effects on the laser output, such as altering the wavelength of operation or the production of pulses of laser light.
Some lasers do not use an optical cavity, but instead rely on very high optical gain to produce significant
amplified spontaneous emission (ASE) without needing feedback of the light back into the gain medium. Such lasers are said to be
superluminescent, and emit light with low
coherence but high
bandwidth. Since they do not use optical feedback, these devices are often not categorized as lasers.
Q-SWITCHING
Q-switching, sometimes known as
giant pulse formation, is a technique by which a
laser can be made to produce a pulsed output beam. The technique allows the production of light pulses with extremely high (
gigawatt) peak
power, much higher than would be produced by the same laser if it were operating in a
continuous wave (constant output) mode. Compared to
modelocking, another technique for pulse generation with lasers, Q-switching leads to much lower pulse repetition rates, much higher pulse energies, and much longer pulse durations. Both techniques are sometimes applied at once.
Principle of Q-switching:
Q-switching is achieved by putting some type of variable
attenuator inside the laser's
optical resonator. When the attenuator is functioning, light which leaves the
gain medium does not return, and lasing cannot begin. This attenuation inside the cavity corresponds to a decrease in the
Q factor or
quality factor of the
optical resonator. A high Q factor corresponds to low resonator losses per roundtrip, and vice versa. The variable attenuator is commonly called a "Q-switch", when used for this purpose.
Initially the laser medium is
pumped while the Q-switch is set to prevent
feedback of light into the gain medium (producing an optical resonator with low Q). This produces a
population inversion, but laser operation cannot yet occur since there is no feedback from the resonator. Since the rate of
stimulated emission is dependent on the amount of light entering the medium, the amount of
energy stored in the gain medium increases as the medium is pumped. Due to losses from
spontaneous emission and other processes, after a certain time the stored energy will reach some maximum level; the medium is said to be
gain saturated. At this point, the Q-switch device is quickly changed from low to high Q, allowing feedback and the process of
optical amplification by stimulated emission to begin. Because of the large amount of energy already stored in the gain medium, the intensity of light in the laser resonator builds up very quickly; this also causes the energy stored in the medium to be depleted almost as quickly. The net result is a short pulse of light output from the laser, known as a
giant pulse, which may have a very high peak intensity.
Types of lasers
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| Energy Diagram for He Ne laser |
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| Ruby laser diagram |
