Overview of pulsed lasers
The most direct way to generate laser pulses is to add a modulator to the outside of the continuous laser. This method can produce the fastest picosecond pulse, although simple, but waste light energy and peak power cannot exceed continuous light power. Therefore, a more efficient way to generate laser pulses is to modulate in the laser cavity, storing energy at off-time of the pulse train and releasing it at on-time. The four common techniques used to generate pulses through laser cavity modulation are gain switching, Q-switching (loss switching), cavity emptying, and mode-locking.
The gain switch generates short pulses by modulating the pump power. For example, semiconductor gain-switched lasers can generate pulses from a few nanoseconds to a hundred picoseconds by current modulation. Although the pulse energy is low, this method is very flexible, such as providing adjustable repetition frequency and pulse width. In 2018, researchers at the University of Tokyo reported a femtosecond gain-switched semiconductor laser, representing a breakthrough in a 40-year technical bottleneck.
Strong nanosecond pulses are generally generated by Q-switched lasers, which are emitted in several round trips in the cavity, and the pulse energy is in the range of several millijoules to several joules, depending on the size of the system. Medium energy (generally below 1 μJ) picosecond and femtosecond pulses are mainly generated by mode-locked lasers. There are one or more ultrashort pulses in the laser resonator that cycle continuously. Each intracavity pulse transmits a pulse through the output coupling mirror, and the refrequency is generally between 10 MHz and 100 GHz. The figure below shows a fully normal dispersion (ANDi) dissipative soliton femtosecond fiber laser device, most of which can be built using Thorlabs standard components (fiber, lens, mount and displacement table).
Cavity emptying technique can be used for Q-switched lasers to obtain shorter pulses and mode-locked lasers to increase pulse energy with lower refrequency.
Time domain and frequency domain pulses
The linear shape of the pulse with time is generally relatively simple and can be expressed by Gaussian and sech² functions. Pulse time (also known as pulse width) is most commonly expressed by the half-height width (FWHM) value, that is, the width across which the optical power is at least half the peak power; Q-switched laser generates nanosecond short pulses through
Mode-locked lasers produce ultra-short pulses (USP) in the order of tens of picoseconds to femtoseconds. High-speed electronics can only measure up to tens of picoseconds, and shorter pulses can only be measured with purely optical technologies such as autocorrelators, FROG and SPIDER. While nanosecond or longer pulses hardly change their pulse width as they travel, even over long distances, ultra-short pulses can be affected by a variety of factors:
Dispersion can result in a large pulse broadening, but can be recompressed with the opposite dispersion. The following diagram shows how the Thorlabs femtosecond pulse compressor compensates for microscope dispersion.
Nonlinearity generally does not directly affect the pulse width, but it widens the bandwidth, making the pulse more susceptible to dispersion during propagation. Any type of fiber, including other gain media with limited bandwidth, can affect the shape of the bandwidth or ultra-short pulse, and a decrease in bandwidth can lead to a widening in time; There are also cases where the pulse width of the strongly chirped pulse becomes shorter when the spectrum becomes narrower.
