What is weak coherent light?

Coherence describes the correlation between waves and waves themselves and between waves and other waves for some intrinsic physical quantity.

When two waves interfere with each other, constructive interference or destructive interference will occur because of the phase difference. If the phase difference between two sine waves is constant, then the frequencies of the two waves must be the same, which is called "complete coherence". Two "completely unrelated" waves, such as incandescent lamp or light from the sun, cannot be clearly observed because of the unstable interference pattern. There is a "partially coherent" wave between these two extremes.

Coherence can be roughly divided into temporal coherence and spatial coherence. Time coherence is related to the bandwidth of the wave; Spatial coherence is related to the finite size of wave source.

The coherence between waves can be measured by the degree of coherence. Interference visibility is the irradiance comparison of interference patterns between waves, and coherence can be calculated from interference visibility.

-from Wikipedia Coherence

From the definition of surface dryness, we can know that waves are all coherent, but not all manifestations of coherence can be directly observed due to the influence of the characteristics of waves themselves. Sunlight and incandescent lamps are also related, but they are called "completely irrelevant" because of the limitation of observation means and the lack of understanding of light by predecessors. Note that the irrelevance in this part is quoted, which shows that its coherence can still be used by everyone, although this part is very weak. In China, the "ghost imaging" realized by Han Shensheng's research group of Shanghai Institute of Optics and Mechanics with sunlight is an application of the correlation and coherence characteristics of sunlight.

The monochromaticity of laser is very good, that is, the spectrum is very narrow and the corresponding coherence is very good, which can be understood as strong coherent light. Weak coherent light, I understand it as light with weak or low coherence, such as sunlight, mercury lamp, incandescent lamp, ASE and LED lamp.

Generally speaking, the wider the spectrum of light, the worse the coherence of light, and the coherence of light is inversely proportional to the spectrum width of light. . . The spectrum of sunlight and LED lamps is very wide. Generally speaking, the wider the spectrum of light, the greater the influence in the experiment, and the collimation of sunlight is not particularly good. Therefore, in general optical experiments, people prefer to use laser, but it is not absolute. For example, I study the optical characteristics of LED lamps and apply its advantages of wide spectrum. Is the purpose of femtosecond laser to achieve time compression by broadening the spectral width? There is also a kind of light called chirped light, which has a wider spectrum and better time effect. ) Its time pulse width reaches the order of femtosecond, which seems to take advantage of time and has good measurement accuracy, but notice that the pulse broadening of femtosecond laser is very serious and it is easy to be stopped in optical fiber. The following is a schematic diagram of femtosecond pulse laser in time domain and frequency domain.

As can be seen from the above figure, the frequency spectrum of femtosecond pulse laser is very wide, reaching 300nm, while the general LED lamp is only about 40nm. Therefore, its coherence in time is weak. So it can be considered as weakly coherent light. . . .

Optical tomography

Wikipedia: integrated circuit resonance imaging. IEEE Journal of Medical and Biological Engineering, 2000, 19: 34.

2. Bourne and E. Wolf. Optical principle: the electromagnetic theory of light propagation, interference and diffraction. Cambridge university press. 2000.

3. Faucher; Mengedoht used partially coherent light interferometry to measure eye length. Optical letters. 1988, 13 (3): 186.

2. Femtosecond pulse laser is weakly coherent light.

3. Advantages, except those mentioned in Wikipedia.

The main advantages of optical coherence tomography are

The resolution of imaging living tissue can reach micron level.

Fast direct imaging of tissue morphology

No sample preparation is required.

Ion radiation is not required.

I think there are the following points:

A. Easy to obtain, white light can be used for tomography, and the light source is cheap.

As shown below: doi:10.1038/nphoton.2013.350.

DOI: 10. 1038/n photon . 20 13.27 1

B. The signal-to-noise ratio is relatively high. If monochromatic laser is used, there will be interference of irrelevant information.

C. the damage to the organization is very small. If the same scanning effect is obtained, the power of strong coherent light is greater.

I am a novice in quantum optics, an undergraduate in electronics and a graduate student in quantum optical measurement. I can't guarantee the correctness of this article. Just talk about your own understanding. The contents quoted in this article are all from the Internet. Please contact and delete the infringing part.