Laser Fundamentals

History:

Laser stands for Light Amplification by Stimulated Emission of Radiation. Albert Einstein (1916) theoretically proved the process of stimulated emission (that is, when more atoms occupy a higher energy state than a lower one, it is possible to force them to return to an unexcited state by stimulating them with the same energy as would be emitted naturally). German Physicist Rudolf Walther Ladenburg observed the stimulated emission in 1928. In 1951, Charles H.Townes from Columbia University demonstrated a working device that focused excited ammonia molecules in a resonant microwave cavity, where they emitted a pure microwave frequency. Townes named the device a MASER for Microwave Amplification by Stimulated Emission of Radiation. 

On May 16, 1960, Theodore H. Maiman (Hughes Research Laboratories) fired bright pulses from a photographer’s flash lamp to excite chromium atoms in a crystal of synthetic ruby and produced red pulses from a Ruby rod about the size of a fingertip. 
In December 1960, Ali Javan and his co-workers built the first gas laser, which generated a continuous infrared beam using a mixture of helium and neon. In 1961, lasers began to appear on commercial markets.

Components of a typical laser:

1. Gain medium
2. Laser pumping energy
3. High reflector
4. Output coupler
5. Laser beam

October 1961: Operation on the first neodymium glass laser.
 In 1962, Robert N Hall and his team made the first semiconductor laser at the General Electric Research and Development Center. Development of the first yttrium aluminum garnet laser was in June 1962. The invention of Gallium arsenide phosphide laser (the basis of today’s LED) was in October 1962.
In 1963, two researchers from the University of Michigan used lasers to make the first three-dimensional holograms.
In 1964, Kumar Patel at Bell Labs invented a carbon dioxide laser. The most powerful continuously operating laser of its time. It is now used worldwide as a cutting tool in surgery and industry. Again this year, Joseph E. Geusic and Richard G. Smith at Bells Lab invented the Nd: YAG laser and proved it was ideal for cosmetic applications. 
In 1965: The first chemical laser at the University of California.
In 1966, Mary L. Spaeth of Hughes Research Labs invented the tunable dye laser pumped by a Ruby laser. Another important discovery this year was a breakthrough in fiber optics. Charles K. Kao received the Nobel Prize in Physics for this work.
Basov et al. developed an excimer laser at P. N. Lebedev Physical Institute in 1970.
Izuo Hayashi et al. designed a semiconductor laser that operated continuously at room temperature (1971).
In 1976, John M. J. Maday and his team at Stanford University in California demonstrated the first free electron laser.
Peter F. Moulton of MIT’s Lincoln Labs developed the titanium-sapphire laser to generate short pulses (1982).
In 1994, Jerome Faist et al. invented the first quantum cascade laser at Bell Labs.
 In 1996, the invention of the first pulsed atom laser using matter instead of Light at MIT by Wolfgang Ketterle.
In 1997, Shuji Nakamura et al. developed the Gallium Nitride laser at the University of California. 
2004: Ozdal Boyraz et al. invented the first silicon Raman laser at the University of California.
In 2009: The highest-energy laser in the world, the National Ignition Facility at Lawrence National Labs.
Researchers at ETH Zurich developed a vertical External Cavity Surface-emitting laser in 2011.
A team from Yale University created a random laser in 2012. Researchers made a white laser in 2015. In that laser, light modulation was by a synthetic nanosheet of zinc, cadmium, sulfur, and selenium. They can emit red, green, and blue light in varying proportions. In 2017, researchers at the Deft University demonstrated an AC Josephson junction microwave laser. In 2017, researchers from the Physikalisch Technische Bundesanstalt established a new 

world record by developing an erbium-doped fiber laser with a line width of only ten millihertz.

Red (660 & 635 nm), green (532 & 520 nm), and blue-violet (445 & 405 nm) lasers (3)

Essential requirements of laser action:

• Lasing medium
• Population inversion
• Pumping
• Resonance cavity

Lasing Medium

The medium in which the lasing transaction takes place is known as the lasing medium, the heart of the laser system, a combination of host material and laser-active material. 

Host material provides sufficient particles for excitation, leading to population inversion. 

Active material alone participates in the lasing transactions. 

The selection of the active material depends on the structure of their energy levels.

Population inversion:

The number of atoms in the energy level is known as the population of that level.

Let N₁ and N₂. be the number of atoms of ground level of energy E₁ and excited level of energy E_2.

The number of particles at the excited level is greater than that of the ground level (excited energy level is densely populated), the state of population inversion (N₂>N₁).

It is a non-equilibrium condition.

The number of atoms at the excited level, N_2 = N₁e^-(E2-E1)/KT

K is the Boltzmann’s constant. T is the absolute temperature. The above equation shows that at temperature T,(N1>N2).

Pumping techniques:

Pumping is for supplying sufficient energy for excitation with the help of an external source. 

Optical pumping: using the light energy to raise the atoms of a system from one energy level to another, used in solid-state laser, glass laser, etc.

Gas lasers use electrical pumping. A powerful electric discharge converts gas into plasma. Atoms in the active region collide with ionized gas and share energy to an excitation transition.

Chemical pumping is used in chemical lasers. Exothermic reactions with active material release energy for excitation.

Heat pumping is used in gas dynamic lasers. The active material is heated to a high temperature and rapidly cooled to release sufficient energy.

Electron beam pumping is used in semiconductor lasers. A beam of fast-moving electrons passes through a semiconductor chip. Electrons in the valence band of the chip get raised to the conduction band, producing population inversion.

Excitation and Emission of Radiation

In the atom model, negatively charged electrons revolve around the nucleus.

Each orbit has fixed energy values. The lowest energy level for an individual atom is the orbit near the nucleus, named the ground state. The orbits away from the center are called excited levels. 

Interaction between particles never happens when electrons revolve around the nucleus in specified orbits having specific energy values(7).

That means no absorption or release of energy.

To make an atom emit radiation, excitation of an atom (by absorbing energy) is very essential.

Pumping is a method to supply energy for excitation with the help of an external source.

Several pumping techniques are there.

When electrons absorb energy corresponding to a particular energy level transition, they move to the excited level, called the excitation transition, resulting in population inversion.  

Excited levels are unstable (lifetime 10-8 seconds).

Hence, electrons drop from a higher energy to a lower energy level by emitting photons.

The energy of the emitted photon is equal to the difference in energy between the two levels.

Emission or absorption of energy is in the form of photons (E₂ -E₁= h√).

There are two ways of releasing photons during downward transfer: spontaneous and stimulated emission. 

Fundamentals

The photon emitted by stimulated emission is identical to the photon that triggered its emission, and both photons can go on to trigger stimulated emission in other atoms, creating the possibility of a chain reaction. For this to happen, many of the atoms or molecules must be in the proper excited state so that the photons can trigger them. In most materials, atoms or molecules drop out of excited states rapidly, making it difficult or impossible to produce a chain reaction. The materials chosen for lasers are the ones that have metastable states, which stay excited for a relatively long time. In laser physics, such a material is called an active laser medium. The energy source continues to pump energy into the lasing medium. The result is to have enough atoms or molecules in an excited state for a chain reaction to develop. (3)
Lasers are distinguished from other light sources by their coherence. Laser beams can be focused on tiny spots, achieving a very high irradiance, or they can have a shallow divergence to concentrate their power at a great distance. Temporal (or longitudinal) coherence implies a polarized wave at a single frequency whose phase has a correlation over a relatively great distance (the coherence length) along the beam. (8) A beam produced by a thermal or incoherent light source has an instantaneous amplitude and phase that vary randomly concerning time and position, thus having a short coherence length.
Laser characterization is according to their wavelength in a vacuum. Single-wavelength lasers produce radiation in several modes with slightly different wavelengths. Temporal coherence implies monochromaticity to some extent. Some lasers emit a broad spectrum of light with different wavelengths. Some lasers are not in single spatial mode and have light beams that diverge more than required. All such devices are in the class of lasers based on producing light by stimulated emission. 

Metastable level:

The intermediate energy level between the ground and the excited level is known as the metastable state. It has a comparatively large lifetime. Atoms will reside at this level for a long time and help to achieve population inversion. 

Spontaneous emission:

Spontaneous emission is a natural de-excitation process by emitting radiation in different directions without any order. It is an uncontrollable process.

Features of spontaneous emission:

• Natural de-excitation process
• Emitted radiations are in different PΥ states (p-momentum and Υ-state of vibration or polarization)
• Order less
• Uncontrollable process
• Different phase
• Incoherent radiation
• 
  

Stimulated emission:

Stimulated emission is the basic principle of laser. It is a de-excitation transition process with the help of stimulation. It is a fully controllable process by emitting radiation in multiple numbers. The rate of emission is proportional to the population at the excited level. In stimulated emission, the emitted light will be coherent with the incoming wave. In laser action, the stimulating emission triggers a chain reaction in which the radiation from one atom stimulates another in succession until all the excited atoms in the system have returned to normalcy. In doing so, coherent monochromatic light is emitted. 

Features of stimulated emission:

• De-excitation by stimulation
• Emitted radiations are in the same PΥ states.
• Emitted radiations are in a particular order (2ⁿ). When n=1, the output is two radiations. When n=2, four radiations, and so on (avalanche model).
• fully controllable process
• Emitted radiations are in the same phase of
• Coherent radiations

The triggering pulse for stimulated emission need not come from outside. Anyone spontaneously emitting photons from within can stimulate and trigger the emission process(7).

Optical resonator or Resonance cavity:

An optical resonator is a mechanism to suppress unwanted radiation and to build the desired one.
It consists of a pair of mirrors facing each other and set on an optic axis, which defines the direction of the laser beam.
One of the mirrors is silvered to make it partially reflecting, and the other one is fully silvered to make it fully reflecting. 
The position of the active material is between these mirrors. 
Photons released are in a direction parallel to the optic axis of the resonator.
 Multiple reflections by resonator mirrors help us to increase the distance traveled by the radiations inside the medium.
Using a resonator, we can select the desired wavelength as the output. With the help of end mirrors, the light travels back and forth through the active medium many times. This multiple passes increases the energy density of interacting photons and thus produces sufficiently large amplification. 

Resonator

M1 and M2 are mirrors

R2=100%, fully reflecting 

R1 <100%, partially reflecting 

Characteristics of laser:

The laser beam has unique characteristics that ordinary light does not possess. These features are distinct from other radiations and pave the way for its various applications.

Unique features:

• Highly coherent (Highly coherent because the emitted photons are in the same PΥ state. No difference in phase). 
• Very high directionality (Beam divergence is less)
• High degree of monochromaticity (Energy of emitted photon is h√, h is plank’s constant and √ is the only frequency of all radiations)
• Very high beam intensity (Output is in multiple numbers, avalanche model) 

Uses (included only a few):

• Fibre optic communication using lasers is a key in modern communication, allowing services such as the Internet. The first widely noticeable use of laser was the barcode scanners introduced in 1974.

• Many uses are there in medicine, including laser surgery, laser healing, kidney stone treatment, ophthalmoscopy, cancer treatment, etc.

• Industrial uses are drilling, cutting, and welding. 

• Utilizing the best-focussing property, we can use it for making fine holes. d=fλ/D, where d is the spot diameter, f is the focal length of the lens used, λ is the wavelength of light and D is the diameter of the laser beam.

•  We can do laser welding at room temperature. Need not provide a vacuum around the weld point.

The future of laser:

In 2018: Establishment of LaserNetUS, a network that will help optics researchers across the US form stronger collaborations and improve access to the HERCULES laser and the tabletop particle accelerator experiments it allows. HERCULES/ZEUS laser is a bright future for laser technology and tabletop accelerator research. ZEUS, the Zetewatt-Equivalent Ultrashort pulse laser system, will explore the physics of the quantum universe and is expected to contribute to new technologies in medicine and all other fields.

Recent developments:

An all-silicon laser, a holmium-doped laser on a silicon photonics platform, and a flying microlaser are the latest developments in laser technology. Researchers at Fudan University in Shanghai, China, developed it using silicon nanocrystals with high optical gains. The researchers developed a way of improving the density of silicon nanocrystals in the gain layer to enhance the light intensity. 

A team of researchers from the Massachusetts Institute of Technology (MIT), Harvard University, and other institutions recently demonstrated a holmium-doped distributed feedback laser monolithically integrated on a silicon photonics platform flying laser that can move through optical fiber and deliver light inside the body. Researchers at the Max Planck Institute developed a microlaser.

According to researcher Richard Zeltner, inserting a fiber into the skin, a microlaser emitting at a suitable wavelength could deliver precisely positioned light for use with light-activated drugs. 

Researchers have optically trapped and propelled a particle-based laser for centimeters inside an optical fiber. The new flying microlaser enables high-sensitive temperature measurements along the path. It could offer a novel way to deliver light to remote and inaccessible locations. The flying microlaser is the first demonstration of distributed sensing using a whispering gallery mode resonator. Useful for real-time continuous sensing along an optical fiber. 

Recently, Leonardo Electronics US has installed a new high-energy laser system. They have delivered a new technology for ‘extreme photonics’ research that will allow scientists and engineers to accelerate the development of research projects with potentially world-changing implications. Future applications of the technology developed under this research could revolutionize medical treatment, enable new forms of green energy, and advance imaging capabilities.

Laser safety:

Even a low-power laser with only a few milliwatts of output power can be hazardous to human eyesight when the beam hits the eye directly or after reflection from a shiny surface. At wavelengths in which the cornea and the lens can focus well, the coherence and low divergence of laser light means that it can be absorbed by the eye into a fine spot on the retina, resulting in localized burning and permanent damage in seconds or even less time.

References:

1 Jenkins A. F. and White E. H, Fundamentals of Optics, McGraw-Hill Book Company, 1976.
2 Palanisamy P. K, Engg. Physics, Scitech Pub., India. 
3 En.m.wikipedia.org.
4 News.engin.umich.edu.

5 Anne Fischer, The latest advancements in laser technologies, novuslight.com, 2018.

6 universetoday.com/61210/what-is-static-electricity

7 digilander. Libero.it/marpic/capitoll

8 Conceptual physics, Paul Hewitt, 2002