What are the main Characteristics of Laser?

A laser is an electronic device that emits electromagnetic radiation. The term LASER is an acronym for Light Amplification by Stimulated Emission of Radiation. The most significant characteristics of laser are :

• Directionality.
• High intensity.
• Extraordinary monochromaticity.
• The high degree of coherence as compared to the light from the ordinary sources of light like incandescent bulbs. 

These properties of lasers make them different from other conventional sources of light.

Characteristics of Laser Light 

Directionality

The ordinary source of light (say an electric bulb or the sun) emit light in all direction. The light emitted by these two separates out considerably as the light propagates through space.

That is, the beam divergence of the ordinary sources is very large and hence useful for lighting homes and workplaces. On the other hand, laser beam Travels only in One Direction.

The Beam divergence of the laser near the source is almost zero, but the laser beam shows little (negligible) divergence after it travels a considerable distance from the source.

The property of the laser that it travels in One Direction Only (without separating) is known as directionality. The negligible divergence of the laser beam makes the laser useful in range finders, remote sensing, surveying, etc.

The distance from the laser source over which the light rays remain parallel is known as the Rayleigh range and is given by 

                                                Rayleigh range  =  d² /λ           …..(1)

Where d is the diameter of the laser beam and λ is the wavelength of light. 

The laser is so little divergence beyond the Rayleigh range because of diffraction phenomena. The divergence of the laser beam beyond the Rayleigh range is given by 

                                                        θ= λ/d.          …..(2)

Where d is the aperture or diameter of the laser beam. Equation (2) gives the angular spread of the laser beam.

Beam Diameter

                                  tan θ = r/L = D/2L

Where D is the beam diameter of the laser beam at a distance L from the laser source. Since  θ is very small then, tan θ= θ

                                    Θ = D/2L

Hence, beam diameter,    D = 2  θL

Here θ is the angle between the centre (or axis) of the laser dean and edge of the laser beam. This angle ( θ ) Is known as the divergence angle.

The edge of the beam is defined as the point where the intensity (power/unit) of the laser beam drops to 1/e² times the maximum value of the intensity of the laser beam. 

Linear spread of the laser beam = Lθ

If the laser is focussed with a lens of focal length f, then L = f. 

Hence, the liner spread of the laser beam = fθ.

The areal spread of laser beam = (liner spread)²

                                                   = (fθ)²

Intensity of Laser Light

The intensity of a wave is defined as the energy per unit time per unit area placed at a right angle to the direction of propagation of the wave.

That is, intensity, I = E/t/A = P/A ( watt/m² ).

Since laser gives out light into a narrow beam and the light energy is concentrated in a region of a small area, therefore, the intensity of the laser beam is very high.

The intensity of a 1mW laser is found to be a thousand times the intensity of a 100W incandescent bulb (i.e. ordinary electric bulb). For a typical laser, the intensity of the order of 10¹³ Wm^-2. Therefore, laser due to its very high intensity is widely used for cutting and welding metal and alloys.

Monochromaticity of laser

Monochromaticity is defined as the degree of monochromator character ( i.e. single wavelength or single frequency) of light. In other words, Monochromaticity is a property of light containing only one wavelength (colour).

No source of light can emit monochromatic light, that is light of a single wavelength or colour. But we can compare the monochromatic character of the laser beam and the light emitted by the ordinary source of light. The lights emitted by an ordinary source of light has a wide range of wavelength or frequency card bandwidth.

However, laser light has a small range of wavelength or frequency that is small bandwidth. The bandwidth of an ordinary source of light is about 10¹⁰ Hz. However, the bandwidth of good quality laser is about 500Hz.

The lights emitted by an ordinary source of light spread over a wide frequency range is ( called bandwidth, Δv = v₂ – v₁) around the central frequency v₀.

However, light emitted by a laser source spreads over a small frequency range ( Δv^/ = v^/₂ – v^/₁ ) around the central frequency v₀. The ratio Δv/v₀ measures the degree of monochromaticity of the light.

 That is, the degree of non-monochromaticity of a light wave is defined as ε = Δv/v₀

Where Δv is the bandwidth and v₀is the central Frequency of the light beam. A light beam will be perfectly monochromatic if its degree of non-chromaticity is zero. This is possible if Δv = 0. As Δv cannot be 0 for any type of light beam, so no light beam can be perfectly monochromatic.

However, Δv is very small in the case of the laser beam, so the laser beam is approximately Monochromatic in character. On the other hand, Δv is very large in the case of light emitted by an ordinary source, so the light emitted by such a source of light is highly non-monochromatic.

Coherence of laser light

An ordinary source of light (say an electric bulb) emits light in all directions randomly. The light waves emitted by an ordinary source of light spread in all directions and are not in phase with each other as shown in fig.

Such a beam of light whose component waves are not in phase with each is called an incoherent beam of light. If incoherent waves combine, then the amplitude of any of the combining waves.

However, light waves emitted by laser are always in phase with each other at every point in the space. Hence, a laser light beam is known as a coherent beam of light. If coherent waves combine, then the amplitude of the resultant wave is always greater than the amplitude of any of the combining waves.

Coherence is the term used to describe the phase cracks correlation phenomena between two waves with respect to time or space. Thus, coherence is of two types number. 

• Temporal Coherence.
• Spatial Coherence.

Temporal Coherence

Temporal coherence is the correlation between the electric field at a point in the space at a time t₁ and the electric field at the same point at time t₂. A wave is said to have a temporal coherence if the phase difference between the electric fields at a point at times t₁  and t₂ is constant during the time interval Δτ = t₂.- t₂ 

Spatial coherence

Spatial coherence refers to the phase relationship between waves travelling side by side at a certain distance from each other.

A beam of light possesses a spatial coherence if the phase difference between the light waves (constituting the beam) crossing two points on a plane perpendicular to the direction of propagation of the beam is time-independent.

For this reason, spatial coherence is also known as transverse coherence or lateral coherence. 

Consider a beam of light travelling along the X-axis. Let abcd be a plane perpendicular to the direction of propagation of the Beam (i.e. X-axis). Let P and Q be the two points on the plane within the beam of light.

If the phase difference between the Waves crossing P and Q at any instant is constant, then the beam of a light constituted by these waves is said to have spatial coherence. These are the main four characteristics of lasers that useful in industrial applications and in the medical field.

Types of Laser and their wavelength