激光 持续性的发光，其振幅不随时间改变。 A single-frequency laser emits light continuously, and its amplitude does not change with time. But how to make a laser pulse? Adding two waves with the same phase and different frequencies will produce the so-called beat frequency, the part of the reinforcing interference will be greatly enhanced, and the part of the destructive interference will cancel each other out. When more waves with the same phase and different frequencies are added, the beat frequency generated is shorter and the intensity of the spikes is greater.
However, if light with different phases is added to each other, there will be no such effect, and only a continuous wave with a disturbance will be generated. The sunlight we see every day is the result of arbitrary mixing and addition of light of various frequencies and phases.
Before talking about femtosecond pulses, you need to understand what femto second (fs) is. 1 femtosecond is equal to 10-15 seconds. If converted into a time concept that is easier to understand, the ratio of 1 second to 1 femtosecond is roughly equivalent to the ratio of 30 million years to 1 second. Therefore, the femtosecond laser is also called super Fast laser.
For a laser to produce such a short pulse, many conditions must be met at the same time. First of all, the laser amplifier itself must have a broadband laser medium, so that it can amplify lasers with various frequencies that meet the aforementioned formula. The latest laser medium is a material called "titanium-doped sapphire crystal", which was only decrypted by the US military in 1988. Because this crystal can accommodate a wide range of frequencies, it can amplify approximately 1 million rays of equal interval frequency in a 1.5 meter long cavity. If these rays can have the same phase, the interference effect can strengthen the light wave to 1 million. Times the intensity, while the length of the pulse is reduced by 1 million times.
Second, there must be a pulse compression mechanism. During the focusing process of the laser line, due to the optical Kerr effect, high-intensity light will further enhance its effect, but other low-intensity parts will be blocked by the aperture of the additional device, losing its effectiveness. Under such a device, the high-intensity part of the laser is amplified more and will become stronger and stronger, and naturally a high-intensity short-pulse laser can be generated.
Finally, it is necessary to have the function of intra-cavity dispersion compensation. As we all know, the speed of light at different wavelengths in the medium is not the same, and the refractive index is also different. The colorful light refracted through a triangular prism is the most obvious example. When the light passes through the medium, it will also have different refraction, but to generate femtosecond pulses, these lights are not allowed to go their own way and go their own way, so these different wavelengths of light must be compensated by the prism through their optical path difference In order to achieve the desired effect of enhanced interference.