What is spectral width? Is there a certain frequency distribution?
What is the pulse width?
Why do people always use how many ns to describe lasers?
Laser is a Gaussian beam, so it has a divergence angle. But one of the characteristics of laser is its poor directivity? Is it not contradictory to divergence?
Ps, reward the club to people who describe it in their own language so that I can understand it, instead of copying a bunch of explanations. . . Question supplement: Does the pulse width mean the time corresponding to a pulse wavelength plus the blank time between two pulses?
Why "if your instantaneous laser power is 1 MW and the pulse width is 1ms, and my instantaneous laser power is 3 MW and the pulse width is 0. 1 ms, the result is that you shoot down my plane first." The pulse width is not small, okay?
Let's talk about longitudinal mode first.
We know that stimulated radiation is not an absolute single wavelength, but a very narrow bandwidth (although the energy level of electrons is a constant value, it will be very wide due to various reasons such as thermal motion).
When the laser working substance is excited and emits stimulated radiation light, there are photons of various wavelengths in this bandwidth range, and the number is normal distribution with the central frequency as the symmetry axis. These photons of all wavelengths try to resonate in the resonant cavity and become the main wavelength.
If the resonant cavity is short enough, which is only an integer multiple of a specific wavelength among all these wavelengths, then only photons with this specific wavelength can resonate to become the main wavelength, and the laser will output true monochromatic light, which is a single longitudinal mode.
But the actual resonant cavity is usually very long, which may be an integer multiple of several wavelengths in the wavelength range of stimulated radiation, so there will be several wavelengths resonating, and such a laser will output several wavelengths of light (because the bandwidth of stimulated radiation itself is very narrow, and these wavelengths are also very close), which is multi-longitudinal mode.
Generally speaking, the more longitudinal modes, the worse the monochromaticity and coherence. The shorter the resonant cavity is, the fewer the longitudinal modes are, so when high monochromaticity is required, the length of the resonant cavity should be reduced as much as possible.
Second, the horizontal mode.
If the two reflecting surfaces of the laser resonator and the end face of the working substance are ideal planes, there will be no other transverse mode output except the fundamental mode. In this case, there is only one fundamental mode output with the diameter of the working substance. Because only the light of the fundamental mode can form the condition of multiple reflection resonance at this time.
However, in fact, the reflecting surface and the end face can't be ideal planes, especially in solid-state lasers, when the working substance is heated, convex lens effect will occur, which will lead to that some light passing through the working substance in the cavity and slightly different from the fundamental mode direction may also meet the resonance conditions of multiple reflections, so the laser will output several beams in different directions. (The difference in this direction is usually small)
Multi-transverse modes destroy the good directivity of laser output, which is very unfavorable to focusing. When perfect focusing is needed, transverse modes should be reduced as much as possible.
The main ways to reduce the transverse mode are: 1, to improve the equivalent flatness of the optical path formed by the resonator mirror and the end face of the working material, and to compensate as much as possible if the convex lens effect occurs; 2. Reduce the diameter of the resonant cavity and the working substance.
gravedigger