What is a laser?

L - Light

A – Amplification

(by the)

S - Stimulated

E – Emission

(of)

R – Radiaition

 

A laser - 'Light Amplification by Stimulated Emission of Radiation' - produces an intense narrow beam of light. The light waves in a laser beam are:

• a) the same frequency
• b) in phase with each other

Laser light is very different from normal light given out by a light bulb. The light released is monochromatic ie. it is made up of just one wavelength which means the light appears as just one colour.

400 – 450 nm dark purple - violet.

450 – 500 nm blue.

500 – 580 nm blue – green.

580 – 600 nm yellow.

600 - 650 nm orange.

650 – 700 nm red

Properties of laser light

Monochromatic – all the light comes one or a very narrow band of wavelengths

Cherent – all the light waves are in phase with each other

Directional – has a very tight beam which is very strong and concentrated. Most normal light sources eg. a torch, the light diverges rapidly. Lasers diverge very slowly.

 

How a laser works – Part 1

A laser consists of a gain medium which is a material with properties that allow it to amplify light by stimulated emission such as ruby crystal or a dye material.  This is placed inside a highly reflective optical cavity which consists of two mirrors arranged so that light can bounce backwards and forwards whilst passing through the gain medium. Usually one of the mirrors is partially transparent or may have a shutter mechanism through which the output laser beam passes.

Energy is required for this amplification by using an ‘energy pump’ this can be electrical current or light at a different wavelength. Generally this light will be provided by a flashlamp.  Lasers used in clinics will also have a cooling system, beam shutter and as well as elements that affect properties such as the wavelength of the emitted light, shape of the beam and beam delivery mechanism.

 

The elements of a laser typically include..

The Energy Source – or energy pump

Gas Lasers can be ’pumped’ with  electrical current

Dye Lasers use a flashlamp or another laser

Solid State Lasers can use diode lasers or flashlamps

The Active Medium

The active medium determines the characteristics of the emitted laser beam

Example

A solid state laser uses a synthetic crystal rod of Yttrium Aluminium Garnet (YAG) as the active medium with the impurity Neodymium (Nd) acting as the lasing dopant. This creates a wavelength of 1064nm (near IR) and is used for laser hair removal.

Beam Delivery System

Delivery systems depend on:-

Wavelength: fibre optic materials absorb longer wavelengths

Energy and power characteristics ie. CO2 and Q switching where energies are very high.

1. A hollow articulated arm with angled mirrors at the joints are used for CO2 or Q switched lasers.
2. Fibre Optics use reflectors to trap the light inside. Light rays travel along plastic or glass fibres.

How a laser works – Part 2

The gain medium mentioned above absorbs energy from the intense flashes of light or electrical discharges from the energy pump or source. This energy is transferred to the atoms in the lasing material raising some of them into an ‘excited’ state with higher energy electrons. The more atoms that become excited the more efficient the lasing material.

The term population inversion is when the number of atoms in the excited state exceeds the number of atoms in the ground state.

The ‘excited’ electrons have greater energy than electrons which are ‘relaxed’. The electron which absorbs energy to reach this excited level, can also release this energy. The emitted energy is emitted in the form of light energy called photons. The photon emitted has a very specific wavelength or colour that depends on the state of the electron's energy when the photon is released.

In this way atoms interact with light by absorbing or emitting photons. Emission of photons can be stimulated or spontaneous.  If it is stimulated the photon is emitted in the same direction as the passing light therefore amplifying the light.

The optical cavity containing a couple of mirrors, at either end of the lasing medium, reflect the photons backwards and forwards through the lasing medium. This stimulates other electrons to create many more photons at the same wavelength and phase by making the electron release energy.

The laser light is then emitted through a shutter in one of the mirrors or the mirror could be half silvered, only allowing laser light through.

 

How a laser works – Part 3

An atom is made up of a nucleus which contains protons and neutrons surrounded by orbiting electrons.

Spontaneous Emmission

Energy is input to the atom

The electron is excited to a higher state

The exited electron returns to a lower relaxed state and emits a photon.

Stimulated emission

Energy is input to the atom

The Electron is excited to a higher state

The exited electron returns to a lower relaxed state and emits a photon which interacts with another atom with excited electrons stimulating that electron to drop to a lower level and emitting another photon. The photons created are of the same frequency and are in phase ie. coherent. This effect cascades through the optical chamber creating more and more coherent photons