Laser waves: physics, history and usages in acoustics

Laser therapy for tinnitus

Laser therapy was offered as a treatment for tinnitus patients.

Can it eliminate tinnitus?

A medical doctor with specialty in Ear Nose and Throat, who is a manager of out-patient clinic in "Ichilov" Hospital in Israel, tested the "soft laser". He took a soft laser device, and a human skull, and checked the amount of light energy that can penetrate the human skull.

The result was: zero.

This part of the article was written for those tinnitus patients who want to continue the follow up on advances in the field of laser therapy for tinnitus patients.

What is laser?

In the spoken English, a laser is an optic instrument that emits visible and invisible light. In the broad aspect light is electromagnetic radiation.

The beam of radiation in the laser is directed to a system that apply on it several processes that increase the energy of the beam. The result is amplification of the radiation, due to the stimulated discharge of photons. The term "laser" is derived from the initials of "Light Amplification by Stimulated Emission of Radiation".

The emitted laser light

The emitted laser light is known for its intense level of spatial and temporal coherence. Such specific optical output can not be achieved by other technologies.

The typical laser beam is known for it's spatial coherence. In optical variables it means: a narrow beam with low grade diffraction. The light energy of laser beams can be very concentrated and seen as small dots. The beams may be of very low divergence. Such a design enables them to concentrate their energy at a large distance.

Typical laser beam

The typical laser beam inside a vacuum chamber have a temporal (also called longitudinal) coherence that includes a polarized light at a single wavelength whose phase is correlated over a relatively large distance. It is a synonym to the coherence length along the beam.

A non laser output may be thermal energy or other incoherent light sources. Typically it has an amplitude and phase in every point of time, and is changing randomly with respect to time and position. Such a beam have a very short coherence length.

Single wavelength lasers

Single wavelength lasers are, usually, not real single wavelength. They produce radiation in few ways having slight variability in their frequencies. Usually also not in a single polarization. Although temporal coherence should have output of monochromatic light, it is not the rule. 

Broad spectrum and various wavelengths lasers

There are lasers that generate a broad spectrum of light, or produce various wavelengths of light at the same time.

"Decaffeinated coffee" lasers

There are some lasers that can be compared to "decaffeinated coffee". They do not follow the criteria of single spatial mode and at the same time their light beams diverge more than required by the definition of diffraction limit.

All these instruments are called "lasers" because they produce that light by stimulated emission.

Laser utilities

Lasers are utilized in applications where there is demand for specific radiation of spatial or temporal coherence and it could not be achieved by simpler technologies.

Main parts

A typical laser device is made of 3 main parts: (1) A gain substance. (2) A highly reflective optical cavity. (3) Means to provide energy to the gain medium. The gain medium can amplify light by stimulated emission under certain circumstances.

The basic laser is made of a cavity between two mirrors organized in a way that light is reflected from mirror to mirror via the gain medium.

The output mirror is partially transparent. That enables the output laser beam to escape through this mirror. The energy of photons of a specific wavelength that cross the appropriate gain medium is amplified. The surrounding mirrors, increases the probability that most of the photons will make many crossing of the gain medium.

Every time the beam is crossing, it is amplified. Part of the beam that is between the mirrors is not reflected, and continues to move forwards via the partially transparent mirror. This is the laser beam which is a beam of light. The scientific term for the process of providing the energy supply for the amplification is named pumping.

The two forms of energy are electrical current or various wavelengths of light. Such source of light may be generated by a flash lamp or even another laser.

Gain medium

Most lasers devices contain elements that enables them to design a specific beam, such as the wavelength and shape. The gain medium of a laser is a material of controlled: (1) purity. (2) size. (3) concentration. (4) shape.

It enables the beam to gain energy by the process of stimulated emission. It can be of any know physical state: gas, liquid, solid or plasma.

The gain medium absorbs pump energy. The electrons that gained this energy move to a higher-energy quantum states, and are called excited electrons. Elementary particles may interact with photons by either absorbing or emitting them.

Emission of photons

Emission of photons can be spontaneous (in natural sources such as sun, stars, meteors, earth quake light and volcano) or stimulated (light bulbs, explosions). In the latter case, the direction of the emitted photon and the light that is passing by is the same.

When the number of elementary particles in one excited state exceeds the number of elementary particles in some lower-energy state, population inversion is achieved.

The summery of the process is that the total quantity of stimulated emission due to light that passes through is higher than the total quantity of absorption. Since the light is amplified it is called an optical amplifier.

When an optical amplifier is placed inside a resonator which is the cavity between two mirrors and a gain medium between them (resonant optical cavity), it is a laser.

The beam of light

The beam of light made by stimulated emission is having the main input signal in 3 parameters: (1) wavelength. (2) phase. (3) polarization. The optical cavity is responsible for the coherence of the laser beam, the uniform polarization and in many devices, the monochromatic light.

The optical resonator

The optical resonator is not a synonym for "optical cavity".

The common resonator is "sandwich" made of two mirrors that traps the beam of light. The coherent beam travels in both directions until it acquires enough energy. The trapped light reflects back on itself many times. It is done in order to enable an average photon to cross the gain medium as many times as possible before it is emitted from the output opening or lost to diffraction or absorption.

In a case that the amplification in the resonator exceeds the losses, then the energy of the recirculating beam will rise exponentially. The result of the energetic changes is that every event of stimulated emission reverses the energetic state of an atom from its high level, named excited state to the ground level.

The energy of the gain medium is reduced at the same rate. The effect of increasing the output of light energy on the net gain is a reduction of the gain energy. At the lowest point it is called: the medium is saturated.

Continuous activity of laser

In a continuous activity of laser device, there is balance of 3 components that are responsible for the equilibrium charge of the laser power inside the cavity. These are: (1) Pump power. (2) Gain saturation. (3) Cavity losses.

A small pump power may cause inability of the gain to exceed the resonator losses. As a result of it a laser light will not be produced. The minimum energy for pump power that is necessary to initiate laser action is named the lasing threshold. The gain medium is able to increase the energy of any photon that is crossing it. The amplification is not dependent on the direction of traveling. Repeated crossing of the medium by the photons will happen to those that are in a spatial mode supported by the resonator.

Every round of crossing of the medium gives the photon substantial amplification. The traveling photons in the space between the mirrors and the output radiation of the light as laser beam can be seen more or less as a Gaussian beam.

Environmental conditions

The environmental conditions that are required are: traveling in free space or a homogenous medium. When it travels via waveguides, as in an optical fiber, it does not happen. Such laser beams have the smallest divergence per inch.

Laser resonators

The unstable laser resonators (rare part in lasers) produce fractal shaped beams. Near the beam focal region the waves are almost parallel. The optical term is highly collimated.

Wave-fronts

The wave-fronts can be called: focused at infinity. It means that they are planer, normal to the direction of travelling, with no beam divergence at that point. The radiation of a single transverse mode (Gaussian beam) laser practically spreads at an angle that is dependent, inversely, on the beam diameter, as derived from diffraction formulation.

"Pencil beam"

The laser "pencil beam" produced by a common helium neon laser would diverge to an estimated size of 500 kilometers when travells from earth the moon. The light from a semiconductor laser is finishing the chain of events in a tiny crystal. The final beam exits the crystal with a divergence: up to 50°.

Even such a wide angle spread of a beam can be modified into a collimated beam by optic device. Use of optic lens can direct the components of the beam to a narrow angle output in order to make the beam collimated.

Optic manipulation

This technology is used in a laser pointer whose light is generated by a laser diode. That optic manipulation is possible because the laser is a light of a single spatial mode. This absolute ability of light to respond to optical manipulation belongs to the laser beam, spatial coherence. It cannot be achieved by using standard light sources.

Radiation activating

The chain of events for activating radiation in a laser is based on activated emission, where energy source is dependent on the differences in energetic levels during a transition in an atom or molecule. This is a quantum phenomenon discovered by Albert Einstein. There is one exception to that. Free electron laser does not receive energy from atomic transition.

Barcode scanner (reader)

The new age of using laser technology outside the laboratories for the sake of the public was the barcode scanner, introduced in 1974.

Supermarket laser scanners make use of either an alternating mirror or a revolving prism to scan the bar code on the products. The sales of the first laserdisc player, in 1978, were also the first breakthrough of a customer product that included a laser inside.

Other products

The compact disc player was also the first laser-aided device that flooded the market (1982). The next product was the laser prints.

Medical usages

The medical usages are for general surgery, healing of wounds, cutting of kidney stones, irradiation of the retina in detachment, removal of glasses, focal management of acne, stria reduction, cellulitis, hair removal and some companies claims that it helps to reduce tinnitus (soft laser).

There is no known laser therapy that is available for vertigo or Meniere's disease.