Editing Mode locking (section)

===Passive mode locking===

Passive mode-locking techniques are those that do not require a signal external to the laser (such as the driving signal of a modulator) to produce pulses. Rather, they use the light in the cavity to cause a change in some intracavity element, which will then itself produce a change in the intracavity light. A commonly used device to achieve this is a [[saturable absorber]].

A saturable absorber is an optical device that exhibits an intensity-dependent transmission, meaning that the device behaves differently depending on the intensity of the light passing through it. For passive mode locking, ideally a saturable absorber selectively absorbs low-intensity light, but transmits light of sufficiently high intensity. When placed in a laser cavity, a saturable absorber attenuates low-intensity constant-wave light (pulse wings). However, because of the somewhat random intensity fluctuations experienced by an un-mode-locked laser, any random, intense spike is transmitted preferentially by the saturable absorber. As the light in the cavity oscillates, this process repeats, leading to the selective amplification of the high-intensity spikes and the absorption of the low-intensity light. After many round trips, this leads to a train of pulses and mode locking of the laser.

Considering this in the frequency domain, if a mode has optical frequency {{Mvar|&nu;}} and is amplitude-modulated at a frequency {{Math|''nf''}}, then the resulting signal has [[sideband]]s at optical frequencies {{Math|''&nu;'' &minus; ''nf''}} and {{Math|''&nu;'' + ''nf''}} and enables much stronger mode locking for shorter pulses and more stability than active mode locking, but has startup problems.

Saturable absorbers are commonly liquid [[organic chemistry|organic]] dyes, but they can also be made from doped [[crystal]]s and [[semiconductors]]. Semiconductor absorbers tend to exhibit very fast response times (~100&nbsp;fs), which is one of the factors that determines the final duration of the pulses in a passively mode-locked laser. In a ''colliding-pulse mode-locked laser'' the absorber steepens the leading edge, while the [[lasing medium]] steepens the trailing edge of the pulse.

There are also passive mode-locking schemes that do not rely on materials that directly display an intensity-dependent absorption. In these methods, [[nonlinear optics|nonlinear optical]] effects in intracavity components are used to provide a method of selectively amplifying high-intensity light in the cavity and attenuation of low-intensity light. One of the most successful schemes is called [[Kerr-lens mode locking]] (KLM), also sometimes called "self-mode-locking". This uses a nonlinear optical process, the optical [[Kerr effect]], which results in high-intensity light being focused differently from low-intensity light. By careful arrangement of an aperture in the laser cavity, this effect can be exploited to produce the equivalent of an ultra-fast response-time saturable absorber.