Basic Principle of LASER, Construction and Working of a Ruby Laser and Uses of Laser.

Explain the basic principle of LASER. Describe with neat sketch, the construction and working of a Ruby Laser. Give the uses of Laser.

A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation".The first laser was built in 1960 by Theodore H. Maiman at Hughes Research Laboratories, based on theoretical work by Charles Hard Townes and Arthur Leonard Schawlow.

A laser differs from other sources of light in that it emits light which is coherent. Spatial coherence allows a laser to be focused to a tight spot, enabling applications such as laser cutting and lithography. Spatial coherence also allows a laser beam to stay narrow over great distances (collimation), enabling applications such as laser pointers and lidar. Lasers can also have high temporal coherence, which allows them to emit light with a very narrow spectrum. Alternatively, temporal coherence can be used to produce ultrashort pulses of light with a broad spectrum but durations as short as a femtosecond.

Basic Principles of Lasers
 To explain the process of light amplification in a laser requires an understanding of the energy transition phenomena in the atoms of its active medium. They include:  spontaneous emission, stimulated emission/absorption and non-radiative decay.
The theory of quantum mechanics states that the electrons of atoms can take different energy states, E1, E2, E3, for example, with E1<E2<E3.

Spontaneous Emission
By quantum mechanics the lower energy level is more stable than higher energy levels, so electrons tend to occupy the lower level. Those electrons in higher energy levels decay into lower levels, with the emission of EM radiation. This process is called spontaneous emission. The radiation emitted is equal to the energy difference between the two levels.

 E2 - E1 = hn0
  • Where E2 is the upper energy level
    E1 is the lower energy level
    h is Plank’s constant
    n0 is frequency of the radiated EM wave.

Stimulated Emission
 This is crucial if lasing is to occur. Suppose the atoms of the active medium are initially in E2. If external EM waves with frequency n0 that is near the transition frequency between E2 and E1 is incident on the medium, then there is a finite probability that the incident waves will force the atoms to undergo a transition E2 to E1. Every E2-E1 transition gives out an EM wave in the form of a photon. We call this stimulated emission since the process is caused by an external excitation. The emitted photon is in phase with the incident photon, has the same wavelength as it and travels in the same direction as the incident photon.

Stimulated Absorption
 If the atom is initially in the ground level E1, the atom will remain in this level until it gets excited. When an EM wave of frequency n0 is incident on the material, there is a finite probability that the atom will absorb the incident energy and jump to energy level E2. This process is called Stimulated Absorption.

Non-Radiative Decay
 Note that the energy difference between the two levels can decay by non-radiative decay. The energy difference can change into kinetic energy or internal energy through collisions with surrounding atoms, molecules or walls.

Population Inversion
 Normally the population of the lower energy levels is larger than that of the higher levels. The processes of stimulated radiation/absorption and spontaneous emission are going on in the same time, yet even if we ignore the decay factors, stimulated absorption still dominates over stimulated radiation. This means that the incident EM wave cannot be amplified in this case.

Amplification of incident wave is only possible when the population of the upper level is greater than that of the lower level. This case is called Population Inversion. This is a mechanism by which we can add more atoms to the metastable level and hold them there long enough for them to store energy, thereby allowing the production of great numbers of stimulated photons.

To do this, we pump atoms into the metastable level at a rate that exceeds the rate at which they leave. A large number of atoms are therefore excited to and held in this level, leaving an almost empty level below it. The atoms stay in this metastable level without de-exciting while the population builds up, giving rise to a population inversion.

In practise laser action cannot be achieved for only two levels, as described above. Three and four level systems work however. An analysis of these systems follows, followed by a description of the pumping schemes for each system.     

(Note: A metastable level is one that has a long lifetime and the for which the probability of spontaneous emission is low. This favours conditions for stimulated emission. If an atom is excited to a metastable state it can remain there long enough for a photon of the correct frequency to arrive. This photon will then stimulate the emission of a second photon.)

 Amplification of Light
 If population inversion exists, N2>N1, the incident signal will be amplified. The incident signal has energy equal to the number of photons times the photon energy we have
U(x) = nhn. The increase in the signal is given by

Where K is a proportionality constant. The solution is

This means that the signal will increase exponentially when there is population inversion. The exponential increase continues until the population inversion reaches a certain point, then the signal saturates, and reaches the steady state.

Ruby Laser

Ruby Laser Definition - A ruby laser is a solid-state laser that uses the synthetic ruby crystal as its laser medium. Ruby laser is the first successful laser developed by Maiman in 1960. Ruby laser is one of the few solid-state lasers that produce visible light. It emits deep red light of wavelength 694.3 nm.

Construction of Ruby Laser
A ruby laser consists of three important elements: laser medium, the pump source, and the optical resonator.

Laser medium or gain medium in ruby Laser
In a ruby laser, a single crystal of ruby (Al2O3 : Cr3+) in the form of cylinder acts as a laser medium or active medium. The laser medium (ruby) in the ruby laser is made of the host of sapphire (Al2O3) which is doped with small amounts of chromium ions (Cr3+). The ruby has good thermal properties.

Pump source or energy source in Ruby Laser
The pump source is the element of a ruby laser system that provides energy to the laser medium. In a ruby laser, population inversion is required to achieve laser emission. Population inversion is the process of achieving the greater population of higher energy state than the lower energy state. In order to achieve population inversion, we need to supply energy to the laser medium (ruby).

In a ruby laser, we use flashtube as the energy source or pump source. The flashtube supplies energy to the laser medium (ruby). When lower energy state electrons in the laser medium gain sufficient energy from the flashtube, they jump into the higher energy state or excited state.

Optical Resonator
The ends of the cylindrical ruby rod are flat and parallel. The cylindrical ruby rod is placed between two mirrors. The optical coating is applied to both the mirrors. The process of depositing thin layers of metals on glass substrates to make mirror surfaces is called silvering. Each mirror is coated or silvered differently.

At one end of the rod, the mirror is fully silvered whereas, at another end, the mirror is partially silvered.

The fully silvered mirror will completely reflect the light whereas the partially silvered mirror will reflect most part of the light but allows a small portion of light through it to produce output laser light.

Working of Ruby Laser
The ruby laser is a three level solid-state laser. In a ruby laser, optical pumping technique is used to supply energy to the laser medium. Optical pumping is a technique in which light is used as energy source to raise electrons from lower energy level to the higher energy level.

Consider a ruby laser medium consisting of three energy levels E1, E2, E3 with N number of electrons.

We assume that the energy levels will be E1 < E2 < E3. The energy level E1 is known as ground state or lower energy state, the energy level E2 is known as metastable state, and the energy level E3 is known as pump state.

Let us assume that initially most of the electrons are in the lower energy state (E1) and only a tiny number of electrons are in the excited states (E2 and E3)

When light energy is supplied to the laser medium (ruby), the electrons in the lower energy state or ground state (E1) gains enough energy and jumps into the pump state (E3).

The lifetime of pump state E3 is very small (10-8 sec) so the electrons in the pump state do not stay for long period. After a short period, they fall into the metastable state E2 by releasing radiationless energy. The lifetime of metastable state E2 is 10-3 sec which is much greater than the lifetime of pump state E3. Therefore, the electrons reach E2 much faster than they leave E2. This results in an increase in the number of electrons in the metastable state E2 and hence population inversion is achieved.

After some period, the electrons in the metastable state E2 falls into the lower energy state E1 by releasing energy in the form of photons. This is called spontaneous emission of radiation.

When the emitted photon interacts with the electron in the metastable state, it forcefully makes that electron fall into the  ground state E1. As a result, two photons are emitted. This is called stimulated emission of radiation.

When these emitted photons again interacted with the metastable state electrons, then 4 photons are produced. Because of this continuous interaction with the electrons, millions of photons are produced.

In an active medium (ruby), a process called spontaneous emission produces light. The light produced within the laser medium will bounce back and forth between the two mirrors. This stimulates other electrons to fall into the ground state by releasing light energy. This is called stimulated emission. Likewise, millions of electrons are stimulated to emit light. Thus, the light gain is achieved.

The amplified light escapes through the partially reflecting mirror to produce laser light.

The uses of Laser.

When lasers were invented in 1960, they were called "a solution looking for a problem". Since then, they have become ubiquitous, finding utility in thousands of highly varied applications in every section of modern society, including consumer electronics, information technology, science, medicine, industry, law enforcement, entertainment, and the military. Fiber-optic communication using lasers is a key technology in modern communications, allowing services such as the Internet.

The first widely noticeable use of lasers was the supermarket barcode scanner, introduced in 1974. The laserdisc player, introduced in 1978, was the first successful consumer product to include a laser but the compact disc player was the first laser-equipped device to become common, beginning in 1982 followed shortly by laser printers.

Some other uses are:
  • Communications: besides fiber-optic communication, lasers are used for free-space optical communication, including laser communication in space.
  • Medicine: see below.
  • Industry: cutting including converting thin materials, welding, material heat treatment, marking parts (engraving and bonding), additive manufacturing or 3D printing processes such as selective laser sintering and selective laser melting, non-contact measurement of parts and 3D scanning, and laser cleaning.
  • Military: marking targets, guiding munitions, missile defense, electro-optical countermeasures (EOCM), lidar, blinding troops, firearms sight. See below
  • Law enforcement: LIDAR traffic enforcement. Lasers are used for latent fingerprint detection in the forensic identification field[85][86]
  • Research: spectroscopy, laser ablation, laser annealing, laser scattering, laser interferometry, lidar, laser capture microdissection, fluorescence microscopy, metrology, laser cooling.
  • Commercial products: laser printers, barcode scanners, thermometers, laser pointers, holograms, bubblegrams.
  • Entertainment: optical discs, laser lighting displays, laser turntables
In 2004, excluding diode lasers, approximately 131,000 lasers were sold with a value of US$2.19 billion.[87] In the same year, approximately 733 million diode lasers, valued at $3.20 billion, were sold.[88]

Medical Lasers:
Medical lasers can be used as a scalpel.  Since the laser can be controlled and can have such a small contact area it is ideal for fine cutting and depth control.  Medical lasers can also be used to reattach retinas and can be used in conjunction with fiber optics to place the laser beem where it needs to be.  Medical lasers can also be used to stitch up incisions after surgery, by fusing together skin. (LFI)
Laser shows are quite popular and the special effects are amazing.  These use lasers that are in the visible spectrum along with vibrating mirrors to paint images in the air.  Here is an example of a dance with lasers in the background:

You might noticed the fog in the background, that is what allows the laser light to reflect and you to view it.  Another example of laser entertainment is the use of laser signs at trade shows.  Here is an example of a laser Microsoft sign:

Computers and Music:
One popular use of lasers is the reading of CD.  CD's function by having a reflective aluminum layer that has very small pits put in the aluminum.  The pits and the lack of are translated into binary by the computer and then are used for information.  Another use of lasers is in the use of fiber optics.  Since lasers travel very fast they make an ideal way to communicate.  The laser is shot down a fiberglass tube to a receiver.  These wires can be very long with no loss of signal quality.  Also modern multiplexing of the line lets two lasers of different frequencies share the same line. (Serway)
Metal working:
Lasers very accurate point and solid state construction make it ideal or industrial production.  Lasers allow better cuts on metals and the welding of dissimilar metals with out the use of a flux.  Also lasers can be mounted on robotic arms and used in factors.  This is safer then oxygen and acetylene, or arc welding. (Impulse)

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