PRESENTED BY:
AMIT RAJ

---

[attachment=12136]
INTRODUCTION
 A laser is a device that emits light (electromagnetic radiation) through a process called stimulated emission.
 The term laser is an acronym for “light amplification by stimulated emission of radiation”.
 Laser light is usually spatially coherent, which means that the light either is emitted in a narrow, low-divergence beam, or can be converted into one with the help of optical components such as lenses.
 They have the same frequencies and identical phase.
 Typically, lasers are thought of as emitting light with a narrow wavelength spectrum ("monochromatic" light). This is not true of all lasers, however some emit light with a broad spectrum, while others emit light at multiple distinct wavelengths simultaneously.
Terminology
 The word light in this phrase is used in the broader sense, referring to electromagnetic radiation of any frequency, not just that in the visible spectrum.
 Hence there are infrared lasers, ultraviolet lasers, X-ray lasers, etc.
 Because the microwave equivalent of the laser, the maser, was developed first, devices that emit microwave and radio frequencies are usually called masers.
 In early literature, particularly from researchers at Bell Telephone Laboratories, the laser was often called the optical maser.
 This usage has since become uncommon, and as of 1998 even Bell Labs uses the term laser.
Design
A laser consists of a gain medium inside a highly reflective optical cavity, as well as means to supply energy to the gain medium.
The gain medium is a material with properties that allow it to amplify light by stimulated emission.
In its simplest form, a cavity consists of two mirrors arranged such that light bounces back and forth, each time passing through the gain medium.
Typically one of the two mirrors, the output coupler, is partially transparent. The output laser beam is emitted through this mirror.
BASIC MODE OF OPERATION
Light of a specific wavelength that passes through the gain medium is amplified the surrounding mirrors ensure that most of the light makes many passes through the gain medium, being amplified repeatedly.
Part of the light that is between the mirrors passes through the partially transparent mirror and escapes as a beam of light.
The process of supplying the energy required for the amplification is called pumping.
The energy is typically supplied as an electrical current or as light at a different wavelength.
GAIN MEDIUM
The gain medium of a laser is a material of controlled purity, size, concentration, and shape, which amplifies the beam by the process of stimulated emission.
It can be of any state: gas, liquid, solid or plasma.
The gain medium absorbs pump energy, which raises some electrons into higher-energy ("excited") quantum states.
When the number of particles in one excited state exceeds the number of particles in some lower-energy state, population inversion is achieved and the amount of stimulated emission due to light that passes through is larger than the amount of absorption. Hence, the light is amplified. By itself, this makes an optical amplifier.
When an optical amplifier is placed inside a resonant optical cavity, one obtains a laser.
Continuous wave operation
In the continuous wave (CW) mode of operation, the output of a laser is relatively constant with respect to time. The population inversion required for lasing is continually maintained by a steady pump source.
Pulsed operation
In the pulsed mode of operation, the output of a laser varies with respect to time, typically taking the form of alternating 'on' and 'off' periods.
In many applications one aims to deposit as much energy as possible at a given place in as short time as possible.
In laser ablation for example, a small volume of material at the surface of a work piece might evaporate if it gets the energy required to heat it up far enough in very short time. If, however, the same energy is spread over a longer time, the heat may have time to disperse into the bulk of the piece, and less material evaporates.
Q-switching
In a Q-switched laser, the population inversion (usually produced in the same way as CW operation) is allowed to build up by making the cavity conditions unfavourable for lasing.
Then, when the pump energy stored in the laser medium is at the desired level, the cavity conditions is adjusted (electro- or acousto-optically) to favourable conditions, releasing the pulse.
This results in release of high peak powers as the average power of the laser (were it running in CW mode) is packed into a shorter time frame.
Mode locking
A mode-locked laser emits extremely short pulses on the order of tens of picoseconds down to less than 10 femtoseconds. These pulses are typically separated by the time that a pulse takes to complete one round trip in the resonator cavity.
A pulse of such short temporal length has a spectrum which contains a wide range of wavelengths. Because of this, the laser medium must have a broad enough gain profile to amplify them all. An example of a suitable material is titanium-doped, artificially grown sapphire.
The mode-locked laser is a most versatile tool for researching processes happening at extremely fast time scales also known as femtosecond physics, femtosecond chemistry and ultrafast science, for maximizing the effect of nonlinearity in optical materials, and in ablation applications.
Again, because of the short timescales involved, these lasers can achieve extremely high powers.
Pulsed pumping
• In this mode of achieving pulsed laser operation is to pump the laser material with a source that is itself pulsed, either through electronic charging in the case of flashlamps, or another laser which is already pulsed.
• Pulsed pumping is required for lasers which disrupt the gain medium so much during the laser process that lasing has to cease for a short period.
• These lasers, such as the excimer laser and the copper vapour laser, can never be operated in CW mode.
• Pulsed pumping was historically used with dye lasers where the inverted population lifetime of a dye molecule was so short that a high energy, fast pump was needed.
History
Foundations:
In 1917 Albert Einstein, in his paper On the Quantum Theory of Radiation, laid the foundation for the invention of the laser and its predecessor, the maser, in a rederivation of Max Planck's law of radiation based on the concepts of probability coefficients (later to be termed 'Einstein coefficients') for the absorption, spontaneous emission, and stimulated emission of electromagnetic radiation.
In 1928, Rudolf W. Ladenburg confirmed the existence of stimulated emission and negative absorption.
In 1939, Valentin A. Fabrikant predicted the use of stimulated emission to amplify "short" waves.
In 1947, Willis E. Lamb and R. C. Retherford found apparent stimulated emission in hydrogen spectra and made the first demonstration of stimulated emission.