LASER THEORY

LASER THEORY

INTRODUCTION

1. The use of Lasers both commercially and by the military is very wide-ranging. Commercial applications find Lasers in industrial and medical cutting equipment, CD players and Laser pointers to name only a few. In military use, Lasers are used for:
a. Range finding.
b. Target designation / illumination.
c. Missile guidance.
d. Laser radar.
e. Directed Laser Energy Weapons.
2. As the enemy is highly likely to employ an array of Laser systems against you, there is a need to understand and be able to exploit and counter such systems. Indeed one of the fastest growing areas of industry is in Laser Warning Receivers as many countries try to give their platforms some protection against such systems. The word Laser comes from Light Amplification by the Stimulated Emission of Radiation. A Laser is a device that generates and amplifies coherent light radiation in the Ultra Violet, Visible and InfraRed parts of the EO spectrum. Lasers operate at selected discrete wavelengths dependant on the selection of material used as the Lasing medium.
OBJECTIVE
3. The objective for this section is to explain the basics of Laser theory, explain how a simple system operates and different types of laser have different properties. The following areas will be covered:
a. Basic theory.
(1) Electron States.
(2) Excitation.
(3) Coherency.
(4) Divergence.
b. The requirements of a simple Laser system
c. Types of Laser system.
d. Laser propagation
BASIC LASER THEORY

4. To understand how a laser operates you need to have an understanding of molecular theory. Energy is stored in electrons that exist in differing bands around the nucleus of an atom, as shown in figure below.



Movement of electrons between the bands or levels can occur, and when an electron drops from a higher level (further away from the nucleus) to a lower level, a photon of light is given off. It is the harnessing of these photons that forms the basis of all Laser systems.

5. In its normal state the electrons of a particular substance are situated in specific bands around its nucleus. In order for the electrons to move to higher levels they must either collide with another moving particle or absorb a photon of radiated energy. This process is known as Excitation and the device that performs this in a Lasing system is called a ‘Pump’. Figure shows an electron decaying and emitting a photon of light energy.



The wavelengths of the energy given off in this manner cover from IR through Visual to the UV wavelengths. The wavelength is directly proportional to the difference between energy levels. This process does not have to be man-made and occurs naturally in some materials where electrons decay back to their ground state emitting the light in a process known as spontaneous decay. An example of this can be found in naturally fluorescent materials.

6. Another property of these materials is that, if you stimulate these materials with a stream of photons and raise the electron states to higher levels, as the electrons at a higher state decay they emit another photons with exactly the same properties as the incident photon. These then combine together and increase the strength of the light. The figure below illustrates this process for a single electron. Note how the resultant photon has the same wavelength and is in phase with the original.




7. To create a large enough flow of energy a population inversion has to be created where electrons in a higher state exceed the thermal Equilibrium State. Stimulated emissions then exceed spontaneous emissions. In a practical laser more than two energy levels are used with three or four being common. The figure below





8. shows electron energy levels E1 and E2 for a normal disposition and also an example of a population inversion.




9. The process of flooding the Lasing material with photons to create the population inversion is known as ‘pumping’. There are three methods of pumping Optical, Electrical or Chemical. Optical pumping uses a tube containing a quartz iodine compound and a Nd/YAG crystal is excited by a light from a high energy lamp or low power Laser. Electrical pumping applies a charge between two electrodes in a gaseous or semiconductor material, while in Chemical pumping a chemical reaction between two materials raises electrons to a higher state. The reaction between Hydrogen and Fluorine produces excited 2HF molecules while Carbon Monoxide and Oxygen produce excited CO2 molecules.

REQUIREMENTS FOR A SIMPLE LASER SYSTEM
10. A Laser system requires the following components:

a. A medium with a suitable energy level system.

b. A source of energy to produce the required population inversion.

c. An optical resonator to amplify the signal and produce the output beam.

11. Making the emitted photons travel parallel to an optical axis within a device called a Cavity Resonator can create a Laser beam. The parallel photons are then made to travel back and forth as they are reflected by two mirrors, one of which is only 50% reflecting, at either end of an optical cavity. The beam continues to build in strength until it is of sufficient to pass through the 50% mirror and thus is emitted as the Laser beam as shown in figure below.


1. Alternately, instead of mirrors, some systems use the polished ends of a crystal (Ruby Laser), the two faces of a semiconductor or rotating rather than fixed mirrors. The use of a rotating mirror to replace the 50% transmission mirror allows for the laser signal to be pulsed.

LASER PROPAGATION

12. One of the main features of Lasers is that they produce a narrow parallel beam with very little divergence or spreading out of the shape. This is known as a collimated beam. If this beam travels thorough Space or a vacuum then there is very little if any divergence and absorption of the beam.

13. However this is not the case through the Earth’s atmosphere where the beam will be absorbed and scattered. The main losses are caused by:

a. Absorption and Scattering.

b. Beam Jitter caused by inconsistent output and vibration from servo-controlled tracking systems.

c. Atmospheric Turbulence produces variable refractive indexes that cause beam steering problems.

d. Beam Divergence (small).

TYPES OF LASER
14. There are five main types of Laser:

a. Solid state.

b. Gas.

c. Semiconductor or diode.

d. Liquid.

e. Chemical.

15. Solid state lasers use a solid rod made up of a crystal or a special glass that contains atoms of the lasing medium. Characteristics of the laser depend on the active material used as well as the substrate or host material. Materials include chromium, erbium, titanium and neodymium.

16. Nd-YAG has widespread applications including range finders and target designators. They can be battery powered, small and light. They are rugged and resistant to temperature variations.

17. Gas Lasers use a pure gas or a mixture of gasses as the lasing medium. to produce the different wavelengths as shown in the table below. CO2 used for research in HEL systems. Helium-neon lasers are small, cheap, simple and can be used for long periods. They are used by the military for ring gyros. They can use the IR, UV and visible bands as shown below.

GAS COLOUR WAVELENTH
Helium / Neon Visible Red 0.628 μm
Argon Visible Green 0.55 μm
Carbon Dioxide IR 10.6 μm

19. Liquid Lasers use an organic dye solvents as the lasing medium. These low powered lasers are widely used in medical research. They can be tuned to produce Lasers with wavelengths from 0.34 to 1.17 µm.

20. Semi-conductor/Diode lasers. An example of a Semiconductor Laser is one made from Gallium Arsenide. The beam produced is rectangular and diverges and expands rapidly making it ideal for use with fibre optic communications. Also used in CD players and DVDs.

21. Chemical lasers. Energy required provided by a chemical reaction between 2 elements. Suitable for use in HELs. Elements commonly used Hydrogen fluoride ( 2.5 – 3.0 ), Deuterium fluoride ( 3.5 –4.0 ) and Oxygen iodide ( 1.315 )

22. When selecting a Laser for a particular task, there are many factors that have to be considered such as platform size, cost and complexity. However the main constraint is that as the power rises, the efficiency of the system decreases. The table below shows various different types of Laser and their power and efficiency. The ultimate challenge in Laser technology is to produce high-power high-efficiency systems.


Type of Laser Power Efficiency
Gas Watts to Kilowatts 20%
Liquid Megawatts 25%
Solid Ruby Gigawatts 1%
Solid Nd/YAG Kilowatts 2%
Solid Semiconductor Low 2%


SUMMARY

23. In order to produce a Laser you output you require to have a suitable Lasing material that can produce a satisfactory population inversion of electrons. There must be more atoms, ions or molecules in the excited state than there are in the lower ground stable state. This permits the emission of large numbers of light photons that are combined to produce the Laser beam.
24. Some form of a ‘Pump’ is normally required to achieve this population inversion and amplification of the emissions requires a cavity or optical resonator.
25. Most Lasers are inefficient and require large amounts of power. Depending upon the requirement of producing equipment for a particular role or mission, the system designer can select from Gas, Liquid or Solid- state Lasers. Generally Gas and Liquid Lasers are more efficient than Solid-state Lasers but tend to be larger and more expensive. Solid-state lasers are inefficient but tend to be cheaper and smaller, making them very suitable for low power devices.
26. The main property of a Laser is that being Collimated and Coherent, high levels of accuracy can be achieved. However the beams travel will be altered by bending and propagation losses through the Earth’s atmosphere.