Introduction:

LASER stands for Light Amplification by Stimulated Emission of Radiation. Laser technology started with Albert Einstein in 1917, he has given theoretical basis for the development of Laser. The technology further evolved in 1960 when the very first laser called Ruby Laser was built at Hughes Research Laboratoriesby T.H. Mainmann.

Characteristics of Laser Radiation:

The laser beam has the following properties that distinguish it from an ordinary beam of light:

Types of emission and absorption

Stimulated Absorption or Induced Absorption

An atom in the lower energy level or ground state energy level (E1) absorbs the incident photon and goes to excited state (E2) as shown in the figure below. This process is called induced or stimulated absorption.

Induced absorption
Rate of induced absorption will be proportional to number of atoms in ground state (N1) and density of radiation ( \( \rho \)). $$\frac{dN_1}{dt} \propto \rho\;N_1 $$ or, $$\frac{dN_1}{dt} = B_1\; \rho\;N_1 $$ where this proportionality constant B1 is called Einstein's coefficient of induced absorption.

Spontaneous Emission:

An atom in the excited state spontaneously emits a photon and goes to the lower energy level. This process is called spontaneous emission. The energy of the emitted photon is equal to the energy difference between the two states. The spontaneous emission process is shown in the figure below.

Spontanous emission
Rate of spontanous emission will be proportional to number of atoms in excited state (N2). $$\frac{dN_2}{dt} \propto N_1 $$ or, $$\frac{dN_2}{dt} = A_2\; N_2 $$ where this proportionality constant A2 is called Einstein's coefficient of spontaneous emission.

Stimulated Emission:

If an external photon of energy hν interacts with an atom in the excited state (E1), the atom gets de-excited and goes to the lower energy level emitting a photon. This process is called stimulated emission. The energy of the emitted photon is equal to the energy difference between the two states. The stimulated emission process is shown in the figure below.

Stimulated emission
Rate of stimulated emission will be proportional to number of atoms in excited state (N2) and density of radiation ( \( \rho \)). $$\frac{dN_2}{dt} \propto \rho\;N_2 $$ or, $$\frac{dN_2}{dt} = A_2\; \rho\;N_2 $$ where this proportionality constant A2 is called Einstein's coefficient of stimulated emission.

Population

Number of atoms per unit volume of material with a specific energy or number of atoms per unit volume in an energy level is called population. If N1 is the number of atoms in energy level E1, then population of E1 is N1.

Population inversion

Normally atoms tends to remain in ground state. The lasing action ( stimulated emission ) happens when only there is enough atoms in higher energy state. So we need to invert the population, or should increase the population in higher energy states. So population inversion is process in which makes a more atoms in an excited state than in the ground state.

Population inversion

Pumping in laser

The process of supplying energy to a material for achieving population inversion is called pumping.

Pumping can be done by various methods,

  • Optical pumping

    When we use light for achieving population inversion or to raise the atoms of a system from one energy level to another,such type of pumping is called optical pumping.

    Optical Pumping
  • Chemical Pumping

    Chemical pumping in a laser is a method of creating population inversion by using chemical reactions to pump energy into the active medium of the laser. This can be done by introducing a reactant gas into the laser cavity, or by using a chemical reaction on the surface of the active medium. The excited atoms or molecules can then release energy in the form of light through stimulated emission, which is the principle behind the operation of a laser.

  • Electric Pumping

    Electrical pumping in a laser is a method of creating population inversion by using an electric current to pump energy into the active medium of the laser. This can be done by passing an electric current through a solid-state active medium, such as a crystal or a semiconductor, or by using an electrical discharge to excite a gas or a liquid active medium.

  • Thermal Pumping

    Thermal pumping in a laser is a method of creating population inversion by heating the active medium of the laser. This is done by passing an electrical current through a wire or by using an external heat source, such as a lamp or a flame, to heat the active medium. As the active medium is heated, its atoms or molecules become excited and enter an excited state, resulting in a population inversion.

Metastable states

Excited states in atoms have a short life of some nanoseconds ( about \(10^{-9} \) sec). So atoms reached in this higher energy will spontanously move to ground state by losing energy. For laser to work, excited states with higher lifetime (\(10^{-3}\;sec \)) is available, which is called metastable states. in short a metastable state is a relatively long-lived energy state

Metastable states

Components of laser

Components of laser

  • Energy source ( Pump source )

    The energy source in a laser is used to pump energy into the active medium, typically through optical pumping, electrical pumping, or thermal pumping. This energy is used to excite atoms or molecules in the active medium and create population inversion.

  • Active medium ( Lasing medium )

    The active medium is the material that is used to create the laser beam. It is typically a solid, liquid or gas, and can be made of various materials such as crystals, glasses, gases, or semiconductors. The atoms or molecules in the active medium are excited by the energy source, and then release energy in the form of light through stimulated emission.

  • Optical resonator

    The optical resonator is a cavity that contains the active medium and reflects light back and forth through it. It is made up of two or more mirrors that are placed at either end of the active medium. The mirrors are designed to be highly reflective, so that most of the light is reflected back through the active medium, amplifying it with each pass. This creates a stable, intense beam of light, which is the laser beam.

Ruby Laser

A ruby laser is a type of solid-state laser that uses a synthetic ruby crystal as the active medium. The active laser medium is a synthetic ruby rod, which is typically a few centimeters long and a few millimeters in diameter.

Construction

The ruby laser consists of three main parts: the ruby rod, the pumping source ( flash lamp), and the optical resonator. The ruby rod is made of a synthetic ruby crystal, which is a type of aluminum oxide (Al2O3) that has been doped with chromium ions (Cr3+).

Ruby laser

The Xenon flash lamp is used to excite the chromium ions in the ruby rod, causing them to emit light.

The optical resonator is used to create a standing wave inside the ruby rod. This is done by reflecting the light from the ruby rod back and forth between two mirrors, one of which is partially reflective. The partially reflective mirror allows some of the light to pass through and escape the resonator, creating the laser beam.

Working

When the flash lamp is activated, it pumps energy into the ruby rod, exciting the chromium ions. The energy from the flash lamp causes the atoms jump to a higher energy levels E3 and E4.

Energy level diagram of ruby laser

The excited atoms from energy levels E4 and E3 falls to a metastable by a non-radiative transition. From this metastable state, atoms jumps to ground state by stimulated emission. This stimulated emission results in laser emission of wavelength 694 nm.

Applications

The ruby laser can be used for a variety of applications, including materials processing, scientific research, and medical treatment. Due to its high power and the ability to produce pulses of light, it is often used for cutting and welding materials, as well as for medical treatments such as skin resurfacing.

He-Ne Laser

A He-Ne laser (short for Helium-Neon laser) is a type of gas laser that uses a mixture of helium and neon gases as the active medium. The He-Ne laser is one of the most common types of gas laser, known for its stability and ease of use.

Construction

The He-Ne laser consists of three main parts: the gas discharge tube, the pumping source, and the optical resonator.

He-Ne Laser
The gas discharge tube contains a mixture of helium and neon gases at low pressures, typically around a few torrs. The tube is sealed and contains two electrodes at either end.

The pumping source is used to excite the atoms in the gas mixture, causing them to emit light. This is typically done using a high voltage electrical discharge, which is passed through the gas discharge tube between the two electrodes. The electrical discharge causes the atoms in the gas mixture to become excited and emit light.

The optical resonator is used to create a standing wave inside the gas discharge tube. This is done by reflecting the light from the tube back and forth between two mirrors, one of which is partially reflective. The partially reflective mirror allows some of the light to pass through and escape the resonator, creating the laser beam.

Working

When the electrical discharge is applied to the gas discharge tube, it excites Helium the atoms in the gas mixture. The energy from the electrical discharge causes the helium atoms to jump to higher energy levels F2 and F3 .
Energy level digram of He-Ne laser.

Since the energy levels of the He, F1 & F2 states are parallel ( or have same energy) to metastable states of neon ( which are E6 & E4), collisions between these excited helium atoms and ground-state neon atoms results in a selective and efficient transfer of excitation energy from the helium to neon. That means due to collision with He atoms, Ne atoms will jump to higher energy levels E6 & E4.

The stimulated emission from E6 to E3 results in laser emission of 633 nm which is red.

Other laser emission include IR emission due to transition from E6 to E5 ( 3.39 micrometer) and E4 to E3 with a wavelength of 1.15 micrometer.

Applications

  • Alignment
  • Interferometry
  • Spectroscopy
  • Metrology
  • Medical treatment (photocoagulation)

Advantages

  • Stable and long lifetime
  • Easy to use and low maintenance
  • Inexpensive
  • Highly-coherent beam of light at a specific wavelength
  • Safe to use and no harmful radiation produced

Semiconductor Laser

A semiconductor laser, also known as a diode laser, is a type of laser that uses a semiconductor as the active medium. It is a relatively small, low-powered laser that is widely used in a variety of applications due to its compact size and low cost.

Construction

The semiconductor laser consists of two main parts: the active region and the p-n junction. The active region is made up of a thin layer of semiconductor material, typically made of gallium arsenide (GaAs) or indium phosphide (InP). This material is doped with impurities to create a p-type semiconductor on one side and an n-type semiconductor on the other side.

Semiconductor Laser

The p-n junction is created where the p-type and n-type semiconductors meet. When a forward-bias voltage is applied to the p-n junction, it creates a depletion region that acts as a waveguide for the laser light. The active region is also surrounded by two mirrors, one of which is partially reflective, which forms the optical resonator.

Working

The p-n junction of a semiconductor laser acts as a light-emitting diode. When a forward-bias voltage is applied to the p-n junction, it creates a depletion region that acts as a waveguide for the laser light. Electrons in the n-type semiconductor recombine with holes in the p-type semiconductor, releasing energy in the form of photons. These photons are then amplified and made coherent by the optical resonator, resulting in a laser beam.

Semiconductor Laser (Energy levels)

The semiconductor laser can be operated in both continuous wave and pulsed mode, depending on the application. The wavelength of the laser light emitted by a semiconductor laser is determined by the properties of the semiconductor material used in the active region.

Applications

  • Optical storage devices (CD, DVD, Blu-ray)
  • Telecommunications
  • Barcode scanning
  • Laser printing and engraving
  • Medical treatment (laser therapy)

Advantages

  • Compact size and low cost
  • High efficiency and low power consumption
  • Wide range of wavelength options
  • High reliability and long lifetime
  • Easy to control and modulate the output

Applications of Lasers

  • Surgery (ophthalmology, dermatology, etc.): Lasers are used in surgery to make precise cuts and to cauterize blood vessels. They are particularly useful in ophthalmology for correcting vision problems, and in dermatology for removing skin lesions and tattoos.

  • Industry (cutting, welding, marking, etc.): Lasers are used in industry for cutting and welding a variety of materials, including metals, plastics, and ceramics. They can also be used for marking and engraving on surfaces, and in micro-machining to create small and precise parts.

  • Communications (fiber optics, satellite, etc.): Lasers are used in fiber optic communications to transmit data over long distances, and in satellite communications to increase the capacity of signals sent to and from satellites. They are also used in LiDAR technology for self-driving cars and other applications.

  • Measurement and sensing (ranging, lidar, etc.): Lasers are used in a variety of measurement and sensing applications, including in LIDAR systems which use laser light to measure distances and create 3D images. They are also used in ranging and alignment systems, as well as in industrial process control.

  • Scientific research (spectroscopy, microscopy, etc.): Lasers are used in scientific research for a variety of purposes, including in spectroscopy to study the properties of matter, and in microscopy to study small samples at high resolution. They are also used in particle acceleration, and in measuring the properties of the atmosphere.

  • Entertainment (laser light shows, etc.): Lasers are used in entertainment to create spectacular light shows and laser displays. They are also used in theme parks and other attractions to create special effects and in projection mapping.