Nowadays, optical scientists have generated ultrashort laser pulses in an optical fiber by using a method previously considered as physically impossible to achieve. Pulses can be generated in fiber lasers by a system known as a saturable absorber. When the light intensity is low, the absorber blocks light; when it is high, it lets it through. Since in femtosecond pulses the light intensity is much greater than in a continuous beam, the parameters of the absorber can be adjusted so that it only lets through such pulses. To generate higher energy femtosecond pulses in the optical fiber, the Warsaw physicists decided to improve saturable absorbers of a different type that relies on nonlinear optical effects causing a change in the refractive index of glass.
A nonlinear artificial saturable absorber works as follows. At the input, linearly polarized light is divided into a beam with a low intensity and a beam with a high intensity. The medium of the absorber can be chosen so that both light beams to experience a slightly different refractive index: that is, for them to travel at slightly different (phase) velocities. As a result of the velocity difference, the plane of polarization starts to rotate. At the output of the absorber, a polarization filter only lets through waves oscillating perpendicularly to the plane of polarization of the incoming light. When the laser is operating in continuous mode, the light in the beam is of a relatively low intensity, an optical path difference does not occur, the polarization does not change, and the output filter blocks the light. At a high enough intensity typical for femtosecond pulses, the rotation of polarization causes the pulse to pass through the polarizer.
The key performance figures of femtosecond lasers are the following:
- the pulse duration (which is in some cases tunable in a certain range)
- the pulse repetition rate (which is in most cases fixed, or tunable only within a small range)
- the average output power and pulse energy
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