Love-Electronics: Laser, action and einstein’s theory of laser,type,its applications in industry and magnetic field.

Laser, action and einstein’s theory of laser,type,its applications in industry and magnetic field.

Contents

1.introduction

2.what is laser

3.types of laser

4.overview

5.characteristic of laser

6.operation

7.laser devices

8. Laser components

9.einstein theory of laser

10.application

11. reference

Introduction

The word Laser is an acronym for light amplification by stimulated emission of radiation.A laser is a device that creates and amplifies a narrow, intense beam of coherent light. Atoms emit radiation,we see it every day when the "excited" neon atoms in a neon sign emit light. Normally, they radiate their light in random directions at random times. The result is incoherent light. The trick in generating coherent light of a single or just a few frequencies going in one precise direction is to find the right atoms with the right internal storage mechanisms and create an environment in which they can all cooperate to give up their light at the right time and all in the same directi

The ruby laser, a simple and common type, has a rod-shaped cavity made of a mixture of solid aluminum oxide and chromium. The output is in pulses that last approximately 500 microseconds each. Pumping is done by means of a helical flash tube wrapped around the rod. The output is in the red visible range.

The helium-neon laser is another popular type, favored by electronics hobbyists because of its moderate cost. As its name implies, it has a cavity filled with helium and neon gases. The output of the device is bright crimson. Other gases can be used instead of helium and neon, producing beams of different wavelengths. Argon produces a laser with blue visible output. A mixture of nitrogen, carbon dioxide, and helium produces IR output.

Lasers are one of the most significant inventions developed during the 20th century. They have found a tremendous variety of uses in electronics, computer hardware, medicine, and experimental science.

Overview

Wavelengths of commercially available lasers. Laser types with distinct laser lines are shown above the wavelength bar, while below are shown lasers that can emit in a wavelength range. The height of the lines and bars gives an indication of the maximal power/pulse energy commercially available, while the color codifies the type of laser material (see the figure description for details). Most of the data comes from Weber's book Handbook of laser wavelengths [1], with newer data in particular for the semiconductor lasers.

Types of Laser

· According to their sources:

1.Gas Lasers

2. Crystal Lasers

3.Semiconductors Lasers

4.Liquid Lasers

· According to the nature of emission:

1.Continuous Wave

2.Pulsed Laser

· According to their wavelength:

1. Visible Region

2. Infrared Region

3. Ultraviolet Region

4.Microwave Region

5.X-Ray Region

Characteristics of Laser

Highly Monochromatic:

* Laser ray is highly pure beam of light with respect to the wavelength and the frequency of the photons forming it.

Highly Directional

* laser beam is highly intense and very narrow beam this is because its divergence is very small.

* Laser beam transfers in straight lines approximately parallel to each other.

Highly Coherent

* The laser photons are coherent,in phase and have the same direction.

Fundamental of Laser Operation

1.Excitation of the atom:

* The Ground level (E1): It is the nearest energy level to the nucleus. e.g. the ground level for Hydrogen atom is (K) level while that for (cr) atom is (N).

* The excited levels: They are levels whose energies are higher than the ground levels.

1) Transition of an atom from lower energy level (E1) to the higher energy level (E2) requires absorption of a quantized amount of energy equals the difference in energy of these two levels.

2) Transition of the atom from the higher energy level (E2) to the lower energy (E1) is accompanied by the emission of an amount of energy E= E2-E1 in the form of light quantum (photon) of energy (h.v)

E=h.v=E2-E1

v=E/h=(E2-E1)/h

where (v) is the frequency of the emitted radiation.

In 1917 , Einstein showed that emission can exist by one of the two following ways:

1) Spontaneous emission:

It takes place when the atom moves of its own from the higher energy level (E2) to the lower energy level (E1) emitting a photon of energy (h.v). Therefore the spontaneous transition occurs by its own without control i.e.there is no relation between the incident and emitted photons with respect to their direction or phase. so the emitted light photons are incoherent. The spontaneous emission takes place in all traditional sources e.g. neon lamp, sodium lamp-----etc.

2) Stimulated emission:

It takes place when a photon of energy (h.v) passes by an excited atom so the atom is stimulated to emit a photon having the same energy , frequency and phase of the incident photon. Therefore we obtain two identical photons which are in the same direction. And these two identical photons stumuli another two excited atoms thus we obtain (4) coherent photons and so on.

The stimulated emission is characterized by:

The emission of new photon in addition of the initial one.

The photon in the stimulated emission , has the same energy of the initial photon , consequently it must have the same frequency and wavelength.

The waves associating the two photons initial and that produced in the stimulated emission are in phase.

The conditions needed to obtain stimulated emission(Laser):

Exciting a large number of atoms to exist in the higher energy level.

Doing enough arrangements to enable most of the emitted photons to emerge in the same direction.

Laser Devices

This section discusses the historical evolution from microwave lasers to optical lasers and finally to xray lasers.

Microwave Laser

Optical Laser

Gas Dynamic Laser

X-ray Laser

Plasma Laser .

Lasing in Two, Three, and Four-level Atoms

For the sake of our studies, let's first consider a laser medium whose atoms have only two energy states: a ground state and one excited state. In such an idealized atom the only possible transitions are excitation from the ground state to the excited state, and de-excitation from the excited state back into ground state. Could such an atom be used to make a laser?

There are several important conditions that our laser must satisfy. First of all, the light that it produces must be coherent. That is to say, it must emit photons that are in-phase with one another. Secondly, it should emit monochromatic light, i.e. photons of the same frequency (or wavelength). Thirdly, it would be desirable if our laser's output were collimated, producing a sharply defined "pencil-like" beam of light (this is not crucial, but clearly a desirable condition). Lastly, it would also be desirable for our laser to be efficient, i.e. the higher the ratio of output energy - to - input energy, the better.

Let us begin by examining the requirements for our first condition for lasing, coherence. This condition is satisfied only when the lasing transition occurs through stimulated emission. As we have already seen, stimulated emission produces identical photons that are of equal energy and phase and travel in the same direction. But for stimulated emission to take place a "passer-by" photon whose energy is just equal to the de-excitation energy must approach the excited atom before it de-excites via spontaneous emission. Typically, a photon emitted by the spontaneous emission serves as the seed to trigger a collection of stimulated emissions. Still, if the lifetime of the excited state is too short, then there will not be enough excited atoms around to undergo stimulated emission. So, the first criteria that we need to satisfy is that the upper lasing state must have a relatively long lifetime, otherwise known as a meta-stable state, with typical lifetimes in the milliseconds range. In addition to the requirement of a long lifetime, we need to ensure that the likelihood of absorption of the "passer-by" photons is minimized. This likelihood is directly related to the ratio of the atoms in their ground state versus those in the excited state. The smaller this ratio, the more likely that the "passer-by" photon will cause a stimulated emission rather than get absorbed. So, to satisfy this requirement, we need to produce a population inversion: create more atoms in the excited state than those in the ground state.

Achieving population inversion in a two-level atom is not very practical. Such a task would require a very strong pumping transition that would send any decaying atom back into its excited state. This would be similar to reversing the flow of water in a water fall. It can be done, but is very energy costly and inefficient. In a sense, the pumping transition would have to work against the lasing transition.

It is clear, from the above diagram, that in the two-level atom the pump is, in a way, the laser itself! Such a two-level laser would work only in jolts. That is to say, once the population inversion is achieved the laser would lase. But immediately it would end up with more atoms in the lower level. Such two-level lasers involve a more complicated process. We will see, in later material, examples of these in the context of excimer lasers, which are pulsed lasers. For a continuous laser action we need to consider other possibilities, such as a three-level atom. In fact, the first laser that was demonstrated to operate was a three-level laser, Maiman's ruby laser.

In the above diagram of a three level laser the pump causes an excitation from the ground state to the second excited state. This state is a rather short-lived state, so that the atom quickly decays into the first excited level. [Decays back to the ground state also occur, but these atoms can be pumped back to the second excited state again.] The first excited state is a long-lived (i.e. metastable) state which allows the atom to "wait" for the "passer-by" photon while building up a large population of atoms in this state. The lasing transition, in this laser, is due to the decay of the atom from this first excited metastable state to the ground state. If the number of atoms in the ground state exceeds the number of atoms that are pumped into the excited state, then there is a high likelihood that the "lasing photon" will be absorbed and we will not get sustained laser light. The fact that the lower lasing transition is the ground state makes it rather difficult to achieve efficient population inversion. In a ruby laser this task is accomplished by providing the ruby crystal with a very strong pulsating light source, called a flash lamp. The flash lamp produces a very strong pulse of light that is designed to excite the atoms from their ground state into any short-lived upper level. In this way the ground state is depopulated and population inversion is achieved until a pulse of laser light is emitted. In the ruby laser the flash lamp light lasts for about 1/1000 of a second (1 ms) and can be repeated about every second. The duration of the laser pulse is shorter than this, typically 0.1 ms. In some pulsed lasers the pulse duration can be tailored using special methods to be much shorter than this, down to about 10 fs (where 1 fs = 10-15 s or one thousandth of a millionth of a millionth of a second). So, the output of a three-level laser is not continuous, but consists of pulses of laser light. To achieve a continuous beam of laser light a four-level laser is required.

Here, the lower laser level is not the ground state. As a result, even a pump that may not be very efficient could produce population inversion, so long as the upper level of the laser transition is longer lived than the lower level. Of course, all attempts are made to design a pump that maximizes the number of excited atoms. A typical four-level laser is the helium-neon (He-Ne) gas laser. In these lasers electric pumping excites helium atoms to an excited state whose energy is roughly the same as the upper short-lived state in the neon atom. The sole purpose of the helium atoms is to exchange energy with neon atoms via collisional excitation. As it turns out, this is a very efficient way of getting neon atoms to lase.

Laser components

All lasers have threeprimary components:

Ø Medium

Ø Pump

Ø Resonant Cavity

The laser medium can be gaseous, liquid, or a solid. These could include atoms, molecules, or collections of atoms that would be involved in a laser transition. Typically, a laser is distinguished by its medium, even though two lasers using different media may have more in common than two which have similar media.

There are three different laser pumps: electromagnetic, optical, and chemical. Most lasers are pumped electro-magnetically, meaning via collisions with either electrons or ions. Dye lasers and many solid state lasers are pumped optically; however, solid state lasers are typically pumped with a broad band (range of wavelengths) flash-lamp type light source, or with a narrow band semiconductor laser. Chemically pumped lasers, using chemical reactions as an energy source, are not very efficient. So far, these lasers have been made to work not so much for their usefulness as for their curious operation.

Up to now in our discussion of laser theory we have not really seen how the beam is generated. We know that photons emitted by stimulated emission travel coherently in the same direction, but what is it that defines the beam direction and what allows the intensity of the laser light to get large? The answer to these two questions is coupled together in the resonant cavity. Laser resonant cavities usually have two flat or concave mirrors, one on either end, that reflect lasing photons back and forth so that stimulated emission continues to build up more and more laser light. The "back" mirror is made as close to 100% reflective as possible, while the "front" mirror typically is made only 95 - 99% reflective so that the rest of the light is transmitted by this mirror and leaks out to make up the actual laser beam outside the laser device.

The resonant cavity thus accounts for the directionality of the beam since only those photons that bounce back and forth between the mirrors lead to amplification of the stimulated emission. Once the beam escapes through the front mirror it continues as a well-directed laser beam. However, as the beam exits the laser it undergoes diffraction and does have some degree of spreading. Typically this beam divergence is as small as 0.05o but even this small amount will be apparent if the beam travels long distances.

Even more, the resonant cavity also accounts for the amplification of the light since the path through the laser medium is elongated by repeated passes back and forth. Typically this amplification grows exponentially, similar to the way compound interest works in a bank. The more money in your bank account, with compound interest, the faster you earn more interest dollars. Similarly, the more photons there are to produce stimulated emission, the larger the rate at which new coherent photons are produced. The term used for laser light is gain, or the number of additional photons produced per unit path length.

The last question to address in this section is: why is the resonant cavity called by that name? What does resonance have to do with having mirrors on either end of a region containing the laser medium? Recall that when we discussed resonance on a string, we spoke about the wave traveling one way along the string (say to the right) interfering with the wave reflected at the end traveling back to the left. At a resonant frequency, there are points at which the two waves exactly add or cancel all the time, leading to a standing wave. At other frequencies the waves will randomly add or cancel and the wave will not have a large amplitude. The case of a light wave traveling back and forth in the resonant cavity is exactly analogous in that only at certain resonant frequencies will the light wave be amplified. The required condition is easy to see. The mirror separation distance, L, must be equal to a multiple of half a wavelength of light, just as we saw in the case of a string. In symbols, we have that L = nl/2, where l is the wavelength of the light and n is some integer. In the case of light, because of the small wavelength n is a very large number, implying that there are a huge number of resonant frequencies. On the other hand, only those resonant frequencies that are amplified by the laser medium will have large amplitudes and so usually there are only a few so-called laser modes or laser resonant frequencies present in the light from a laser, as shown in the figure.

Laser Components

Einstein Theory Of Laser

Albert Einstein in his paper in 1917 gave the principle for the laser based on the coefficient of the absorption, spontaneous emission and stimulated emission.

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Applications of Laser

The laser has contributed to humanity as a powerful scientific tool for expanding human knowledge and in its many applications that help people directly. It has been put to work in a vast range of applications and has assumed many forms.

In communications:

Engineers recognized the potential of the laser to replace electrical transmission over copper wires, but how to transmit the pulses presented enormous problems. In 1960, Some Scientists transmitted pulses of light a distance of 25 miles the laser produced an intense and extremely narrow beam of light that was more than a million times brighter than the sun.

In Search of a medium:

Unfortunately, Laser beam is easily affected by atmospheric conditions, such as rain, fog, low clouds, and objects in the air, like birds. Scientists suggested a number of novel schemes to protect the light from interference, including shielding it in metal tubes and using specially designed mirrors and thermal gas lenses to navigate around bends.

In Telecommunications:

Telecommunications relies today on photons, as tiny semiconductor lasers transmit light pulses carrying billions of bits of information per second over glass fibers. Wavelength division multiplexing technology uses various wavelengths, or colors, of light to transmit trillions of bits simultaneously over a single fiber.

In medicine:

Laser is used in the field of medicine after inventing the carbon dioxide laser, which soon permitted surgeons to perform highly intricate surgery using photons, rather than scalpels, to both operate on and cauterize wounds. Lasers today can be inserted inside the body, performing operations that a few years ago were almost impossible to perform.

Today, lasers are also used in a wide range of applications in medicine, manufacturing, the construction industry, surveying, consumer electronics, scientific instrumentation, and military systems. Literally billions of lasers are at work today. They range in size from tiny semiconductor devices no bigger than a grain of salt to high-power instruments as large as an average living room.