Coherent Sources

In quantum physics, coherent sources are a significant and essential topic. This is pretty much an interesting and easy one. Here we discuss and explore this topic and understand it very closely. When we talk about coherent sources, we keep thinking about the word “ coherent.” Before getting started, we had to familiarize ourselves with the term “coherent.”

Coherence is a correlation between phase difference, frequency, and amplitude at different points on a wave. 

As you know, light emits or produces waves. When these waves of light have the same frequencies and waveform and the phase difference between them is constant, they are said to be coherent sources of light. 

Now, you may get confused between the terms used above: phase difference, frequencies, waveforms. 

Here are some basic definitions that help you understand the keywords quickly and effectively to clear your confusion. 

DEFINITIONS 

• Coherent source– When light emits waves that are two or several, having the same frequencies, the same wavelength, and have zero constant phase difference and preferably the same amplitude. 
• Incoherent source – When light emits waves with random frequencies and phase differences. Photons with the same frequency and wavelengths out of phase are not seen in incoherent sources.

How can a coherent source be produced? 

A coherent source can be formed by dividing the amplitude of entering light. This process can be done by the process of partial reflection and refraction. Furthermore, these separated parts further meet with each other to create interference. 

Coherent light is the light when photons are all in step or order, or a change of phase within the beam occurs for all the photons at the same time.

You should know, two separate sources can never be coherent since they keep their phase differences constant.

Two sources of light are said to be coherent when the waves of light have the same 

• phase difference and amplitude 
• wavelength and constant phase
• wavelength and intensity 
• speed and phase 

Types of coherent source

Coherent sources are mainly of two types: Temporal coherence and Spatial coherence.

Each type is explained below in detail.

• Temporal coherence (longitudinal/spectral) –

It describes how successfully a wave may interfere with itself at various times.

It measures the average correlation between the value of a wave and itself delayed by T at any given time. 

It shows us how monochromatic a source is. 

• Spatial coherence – 

It measures the correlation between the phase transverse to the direction of propagation. 

– The spatial coherence of a wavefront indicates how uniform its phase is. 

– The length of spatial coherence is the distance over which the beam wavefront remains flat. 

INTERFERENCE OF LIGHT WAVES 

It is now coming to the next major part of the coherent source that is interference. 

Interference is the addition in the mathematical sense of wave function. 

Here the question arises, How does interference occur? 

When waves of light superimpose and the position of maxima and minima are fixed, an interference pattern is formed. 

In other words, “ interference occurs when two or more waves of light from different coherent sources, are combined, or join each other; the energy distribution due to one wave is disturbed by another. 

This change in the distribution of light energy is called interference of light. 

Type of interference 

There are two types of interference: constructive and the second one is destructive. 

• Constructive interference– 

When waves are at the same place simultaneously, then the amplitude of waves simply collide together. When the two waves pass each other, interference occurs. 

To start exploring the implication of the above statement, let’s consider two waves with the same frequencies traveling in the same direction. 

When we add these two waves, we get the resultant new wave, which is more significant than the original wave and has greater amplitude. This kind of situation is called constructive interference. In simple words, when the two waves pass each other and the maxima of these two waves are added, then the amplitude of the new wave is equal to the sum of the original waves.

• Destructive interference – 

In this, when waves of light collide in such a way that the crest of the first wave collide with the trough of the second wave and the trough of the first wave collide with the crest of the second wave, then the trough and crest of both wave will cancel each other. The produced wave will have an amplitude equal to the difference between the two waves. When the 180 degrees of maxima is out of phase between two waves, they are called destructive interference. The resulting wave has minimum amplitude.

Coherent Source Example

1. Laser light – the light emitted by laser light has the same frequency and phase and constant phase difference. That’s why they are said to be coherent sources. The light has a photon, which will excite and generate a wave. This wave is said to be a coherent wave.
2. Sound wave – the electrical signals from the sound wave have the same frequency and phase. E.g., radio transmitter. As we all know about the radio transmitter, the sound wave generated from this transmitter and that wave have the same frequency level, and the phase difference is also the same. As we are familiar with sound waves, the sound waves create vibration in the air. It is a longitudinal wave. Their directions are parallel to the direction of the wave that is moving.  

Characteristics of coherent sources

1. The waves which are produced have a constant phase difference. 
2.  The waves all have the same frequency.
3. Coherent waves can interfere as they have constant phase relations. 
4. They also have the property which enables waves to have stationary interference.
5. Coherent lights have the same frequency that they have a beam of photons.  

Conclusion

From the above discussion, we can conclude that the waves emitted from light having the same frequency, wavelength, and constant phase difference are coherent sources of light. Temporal coherence is essential for obtaining high bandwidth of communication, and spatial coherence is required for the high efficiency of the optical sources. We need this coherent source to observe the effects of certain optical phenomena such as interference in a lab. Not all the light waves or sound waves are coherent. We can also conclude from this topic that coherent waves have constant phase relations. Coherent sources are necessary to ensure the position of maxima and minima do not change with time as a new intensity of light produces a sustainable interference. And when a light wave is diverging from the point source, the wavefront is spherical. It is also necessary to describe all the relations between a single wave or several waves. Also, the most important thing to keep in mind is that two independent sources will never be coherent because independent sources never have the same frequency nor constant phase differences.