As a person who specializes in coherent optical communications, I once asked my senior brother a question during the first year of research: "Brother, I have never been exposed to optical fiber communications before. What knowledge should I add and what should I learn? Coherent light?"
The brother said: "You will know by looking more."
Later, I finally learned what coherent light is. I think back to it. In fact, the brother at that time was not able to give an exact description, so I just perfunctory to see more O(∩_∩)O
First, coherent light is not equivalent to laser light. But the coherent light we use is generally laser. By understanding what a laser is, you can know what is coherent .
Characteristics of laser
Laser is a special kind of light. The difference between it and the sunlight, bulbs, and light-emitting diodes that we usually contact is that it has four characteristics: high directivity, monochromaticity, high brightness, and coherence.
Here is an example of laser cutting, if the cutting machine is not equipped with a laser, but a light bulb. Then the iron plate below will be illuminated by a large area. This is high directivity.
The same picture, the red laser, shows his monochromaticity. Sunlight is white light, because it contains all colors of light, and different colors are actually determined by different wavelengths. In other words, the light emitted by the laser is concentrated around a certain fixed frequency, and this bandwidth is much narrower than that of the red light-emitting diode.
As the name implies, the laser can brighten your dog's eyes. Its unit light power is extremely high, so high that it can penetrate steel plates. Imagine you are holding a flashlight and want to shine through a steel plate...
Coherence
Light is also an electromagnetic wave. Any electromagnetic wave can be regarded as a superposition of sine waves. Coherence means that there is a definite phase relationship between each sub-carrier. If two sine waves meet three conditions: the same vibration direction, the same frequency, and a constant phase difference , then they are coherent.
The so-called coherence means that they have a close relationship . I know the state of one person at a certain position (such as the aspect) at a certain time, and I can know the state of all other people's positions at all times.
It's easy to understand and review the three relevant conditions. Because the phase difference between A and B is constant, they are all sine waves with the same frequency and the same vibration direction. I know the phase of A at a certain time at a certain position, so I can deduce the phase of A at other positions and other moments according to A's sine wave expression. After extending the phase difference, I got the phase of B. This is coherence.
The sunlight and the light from electric lamps are all incoherent light. It has no relevant properties. As you can imagine, there are countless luminous points on the sun. Each luminous point emits a photon. The phase of each photon (electromagnetic wave) at the moment of emission is random. Therefore, there is no constant phase difference. Even if we know the state of one photon, we can't figure out the state of the other photon. In other words, there is no such intimate relationship between sunlight.
Laser is a kind of coherent light, and the motion state ( frequency, phase, polarization state, propagation direction ) of each photon in the laser is the same.
Remember : the so-called coherent light means that if you know a state at any time and any position in the light field, then you can know the state at any time and any position in the entire light field through calculation.
Time coherence
Time coherence describes the phase relationship of points along the direction of beam propagation . It means that if you know the state of a point in space at a certain moment, then you can know the state of other moments. Time coherence usually uses coherence time tc t_ctcTo quantitatively describe . In fact, Coherence is not that defying. It is not a full-picture skill. I know a little state, but in fact, it can only calculate the state within a range around it. While tc t_ctcIt is used to delineate this range.
t c t_c tcDescribes such a thing. After I know the state at a certain moment, the interval at this moment does not exceed tc t_ctcI can figure out the state within a period of time. Outside of the range, I can't count it. That is, the maximum time interval with coherence .
The factor that affects the coherence time is the monochromaticity of the light source. The higher the monochromaticity, the longer the coherence time.
Sometimes the coherence length L c = tc ⋅ c L_c=t_c\cdot cLc=tc⋅c represents the coherence time. That is, after knowing the state of a certain point, the coherence timetc t_ctcHow far has this point spread. That is, the light wavelengths at two different points in the light propagation direction have the maximum spatial separation of coherence. (Be sure to think clearly about this point. The concept of space is involved here, but it is absolutely not spatial coherence. Time coherence only describes the coherence in this one-dimensional space in the propagation direction. As shown in the figure )
Spatial coherence
Spatial coherence describes the phase relationship between points on the wave surface perpendicular to the direction of beam propagation , and refers to the coherence on a two-dimensional plane. At a certain moment, when you find the state of a point in space, how much area can be calculated? Just use the coherence area S = (Δ λ θ) 2 S=(\frac{\Delta\lambda}{\theta})^2S=(θD l)2 to describe.
The spatial coherence is related to the high directivity of the beam. The better the directivity, the better the spatial coherence.
In fact, it is easier to understand the meaning of time coherence and spatial coherence, which are modified into propagation direction coherence and vertical propagation direction coherence. This is just an episode, there is no need to distinguish between the two, we only know the overall concept of coherence.