Clamping

With the exception of lasers with very short resonators (very small), for most systems, the typical pulse width obtained from a Q-switched laser is around 10–20ns. However, after using cavity emptying technology, the minimum pulse width can be shortened to 1~2ns. The limiting factor here is the length of the cavity, i.e. the length of the cavity determines the width of the pulse. With mode locking, ultrashort pulses with pulse widths in the picosecond or femtosecond range are obtained from solid-state lasers. With this technique of locking the longitudinal mode of the laser output, the pulse width is inversely proportional to the bandwidth of the laser.

The output of the laser oscillator is affected by strong fluctuations from interference from the longitudinal resonator mode in relation to the random phase. By establishing a fixed phase relationship between the longitudinal modes, it is possible to cycle through the resonator to generate a single pulse with regular phase and high power. Pattern locking requires a mechanism that achieves lower losses for radiation peaks that are denser than the average intensity of the resonator.

Passive clamping

The radiation itself binds to the saturable absorber, producing periodic modulation, which results in a fixed phase relationship in the longitudinal mode. A true absorber is a material with a limited number of absorption centers, such as organic dyes or semiconductors. Effective saturable absorbers are mechanisms that utilize the nonlinear refractive index and spatial loss of optical materials.

Active clamping

The acousto-optic modulator provides phase or frequency modulation precisely at the frequency intervals of adjacent modes, which results in a higher gain of mode-locked pulse trains compared to CW operation.

History

Mid-60s of the 20th century Mode-locked from solid-state lasers, which used organic dyes as saturable absorbers, were used for about 10 years or so, and flash lamp-pumped ruby, chin glass, and Nd:YAG laser systems were used to obtain picosecond pulses, all of which used saturable absorbers. The main drawback of using a dye cell as a mode-locking element is the poor repeatability of emission.

In pulsed solid-state lasers, the saturable dye absorber not only produces mode-locking, but also Q-switching. Each pump pulse emitted by a flash produces a series of mode-locked pulses with a pulse duration of tens of nanoseconds typical of a typical Q-switched pulse. Each flash pulse builds up a mode-locked pulse from the noise, and since the statistics of this process are random, the laser output varies greatly. The instability of the dye solution exacerbates the problem of poor output reproducibility as it deteriorates over time and decomposes by light.

Due to the difficulty of obtaining reliable and consistent mode-locked pulsed outputs, the focus has shifted from pulsed mode-locked solid-state lasers to organic dye mode-locked lasers. The large product of gain and bandwidth of dye lasers, combined with the new mode-locking technique, enables the generation of pulses as short as tens of femtoseconds. Therefore, despite the disadvantages of handling and maintaining dye solutions, dye lasers are still the main object of study.

Meanwhile,Tunable solid-state laser materials have also been developed. Chin sapphire is a well-known crystal with a thousand tunable lasers, and its product of gain and bandwidth is equal to or greater than that of organic dyes, so it is an ideal material for generating femtosecond pulses. Driven by effective broadband lasers, new passive mode-locking techniques have been developed, such as the addition of pulsed mode-locking and Kerr lens mode-locking.

In particular, argon-pumped titanium-sapphire lasers modulated by passive mode-locking via KLM lenses have become the standard for femtosecond research. In the all-solid-state laser version, the argon laser is replaced by a frequency-doubling Nd:YAG laser. Pattern locking with semiconductor saturable absorbers has now become a very important technology. For example, diode-pumped solid-state lasers, such as Cr:Li lasers, such as Cr:LiSAF, use semiconductor saturable absorbers for passive mode locking. The result is a compact and robust system with pulses in the femtosecond order.

The development of active-mode-locked solid-state lasers has also been rapid, initially by inserting an electro-optical or acousto-optic modulator into a resonator to clamp a continuously pumped Nd:YAG laser from a krypton arc lamp or aluminum lamp. The method of phase or amplitude modulation has not changed much over the years compared to passive mode-locking, but bulky and inefficient lamp-pumped solid-state lasers have been replaced by extremely compact and efficient diode-pumped lasers. These lasers can produce very stable and reliable mode-locked pulse trains. The latter is capable of producing a very reliable and stable mode-locked pulse train, and the pulses output by these CW mode-locked lasers can be selected as seed pulses for further amplification.

Today,Laser diode end-pumped Nd:YLF or Nd:YAG lasers are actively mode-locked by an acousto-optic modulator and can provide output pulses with pulse widths of 10-20ps. This kind of system has high reliability and good long-term reproducibility of energy generation.