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Characteristics of LASER light

The LASER light is «» monochromatic », « directional ', and ' '. consistent ». It is important to understand the meaning of each of these terms, which this page is dedicated


Visible white light is made up of several colourful radiation, that can be separated using a Prism: their respective wavelengths extend from 400 to 700 nanometers (a nanometer is one thousandth of micron, or one-millionth of a millimeter).

You can use a filter to select the red component of white light; the spectral richness of red light obtained thus is less than that of white light, because the filter lets only the wavelengths in the Red (ex: 650 to 700 nm, or light red to dark red).

In comparison to the ability of a filter to restrict the range of the present in a radiation wavelengths, the light emitted by a laser He-do (for Helium Neon) is such that the wavelengths emitted out of the cavity of the laser have almost identical to a nanometer or less!

monochromatic laser light

Left, the light emitted by an electric torch is polychromatic and inconsistent: the white light is made up of several colorful visible radiation. A red filter to select the wavelengths in the Red (ex: 600-650 nm). However, the light emitted by a laser pointer is naturally monochromatic. In addition, the light is coherent: trains of wave emitted between them are "in phase". 

However, despite this selectivity, the light emitted by a laser cannot be truly monochromatic (radiation corresponding to a single wavelength), because beyond practical constraints, this would be in violation of some laws of physics, which impose a certain "range" of wavelength for any emitted radiation. The light emitted by a standard laser is restricted to a narrow range of wavelength.


Directional aspect


Some light sources like those issued by the spotlight on the Eiffel Tower and which illuminate the Parisian night sky seem directional, as they form a "brush" light almost parallel edges that sweep the clouds. In reality these beams diverge after a few hundred meter, and he is happy that the cloud layer that covers the capital frequently intervenes and used screen - there is then the broom of the bright spots formed by the section of the light beams.

If these beams consisted of a laser light, we follow them for several kilometres before you see them diverge. Directional aspect of the laser light, used for some shows sounds and lights, has been popularized in the 1970s by the 'Star Wars' saga where the firing of light laser emitted by the spaceships can cross great distance in maintaining their straight look (it should be pointed out that the needs of staging the speed of this laser light seems slow in these films, and in an empty environment and not spreading as interstellar space, it would be possible to 'see' this light only on his journey,... either by being directly targeted).

The highly directional aspect of LASER light is related to the cavity where is produced and amplified laser light: before emerge, she "resonates", IE performs several back-back between its walls before emerging through a partially transparent mirror. These back and forth are in the axis of the resonance cavity, next to which is positioned the partially transparent mirror: LASER light is emitted according to the direction of this axis which explains his strong 'directionality '.  It is possible to calculate how much "extends" a laser beam with distance depending on features such as its minimum diameter, its wavelength, etc.

It is because of the inevitable diffraction that laser light, yet very directional, always slightly diverges beyond a distance of output. The energy profile Gaussian (bell-shaped profile) of most of the lasers beams made that this divergence is less that of a light whose intensity of the section profile would be constant. The opening angle is given by:

Theta = 1.27 lamda / D

Here D refers to the "width" of the Gaussian beam, and lambda at the wavelength of radiation considered. This size is about two times less than that of the classical diffraction through a circular orifice (2.44 lambda/D). If it emits a laser beam Gaussian through an opening which the diameter is about two or three times larger than D, there were no rings of diffraction at the level of the collected image: in these circumstances, the image by diffraction at infinity of a Gaussian beam remains a Gaussian beam.

This strong directionality (low divergence) is very useful to train focal tasks more compact than a beam that would be more divergent, and focus the light on a limited spatial area.


Consistency and inconsistency

Before we go forward, we must insist on this point: it is because the laser light is coherent action of the laser on the material is so special, and can carry a significant amount of light energy over long distances.

The coherence of the laser light reflects the fact that transported light waves are "in phase". (Stars), or artificial light (bulbs) natural light sources emit a light polychromatic and inconsistent. The emission of photons are carried out in a haphazard manner: same wavelength photons are not in phase.

On the contrary, laser light is coherent: the photons emitted by the laser source are not distinguishable: they have the same phase, same polarization (the angle formed by the vibration of the electric field with the direction of propagation).  The coherence of the laser light is both spatial and temporal.

coherent light

The spatial coherence of the source reflects the fact that different points in a plane perpendicular to the direction of propagation of the laser source are in the same State of phase (the phase difference is zero). Temporal coherence implies that this phase difference is constant in time. The temporal coherence of the laser light is not infinite, but is much longer than that of the light emitted by a non coherent light source. The average duration of coherence of the trains of wave emitted by a source is 10-11 seconds for a source classiquen and 10-8 second for a helium neon laser. The temporal coherence of the laser light is related to its spectral purity.


Spatial coherence:

at any given time, all points in a plane perpendicular to the laser beam are in the same State of phase (same value and orientation of the electromagnetic field).


Temporal coherence 

It reflects the fact that several light waves issued successively by the same point the source remain in phase: this feature and of course closely related to the (nearly) monochromatic appearance of laser light.


Effect of coherence on the LASER light energy

 Addition in incoherent light of N photons whose energy is E produced an energy equal to N x E. If the light is coherent, addition of N photons produced a total energy equal to N2x E. The difference is of exponential nature!

(E) energy or intensity of the emitted light is equal to the square of the amplitude of the light wave (A) associated with the photons that make up; When this light is incoherent, the photons (the same wavelength) have a random phase: there is sometimes constructive, sometimes destructive interference which modulate the amplitude of the resultant wave, resulting in an average amplitude that tends towards √N x has.  Similarly, one can show that a subject moving at random and changing direction at every step through a distance equal to the square root of the total number of not multiplied by the length of a step. Energy (square of this amplitude) is so good in this equal to N x E case.

random direction square root relationship

A Walker performing successive steps of the same length but in a direction to each time random courses a path on average proportional to the square root of the number of not performed. If the Walker steps of one meter, after 100, the distance in a straight line from its point of departure is equal on average to 10 meters. This relationship is similar to that between amplitude and energy: trains of waves not consistent 'add up' as in the random direction no. The coherent wave trains add up as the distance steps which would each be carried out in the same direction.

When waves associated with photons are in phase, the amplitudes are added (constructive interference) and the resulting wave has an amplitude equal to NXA: energy equals N2x E.

energy light wave amplitude relationship

The energy is proportional to the square of the amplitude: in case of "random" phase between the photons, the amplitude of the total wave tends towards the square root of the number of photons present. In phase case identical (laser light), the total wave amplitude is logically equal to the sum of the respective amplitudes of each photon.

2 Responses to "LASER Light peculiarities"

  1. Me says:

    Super interesting, Super clear

  2. Juju says:

    Exactly what I was looking for – > Simple explanations with some concrete examples for essential optics often neglected!
    Thank you

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