self-starting mode locking (original) (raw)

Definition: mode locking which is quickly achieved after turning on a laser, without external intervention

Categories: laser devices and laser physics, light pulses

• pulse generation
  • mode locking
    * active mode locking
    * passive mode locking
    * fundamental mode locking
    * harmonic mode locking
    * self-starting mode locking
    * additive-pulse mode locking
    * Kerr lens mode locking
    * hard/soft aperture mode locking
    * soliton mode locking
    * nonlinear mirror mode locking
    * regenerative mode locking
    * (more topics)

Related: passive mode lockingsaturable absorbersKerr lens mode lockingsemiconductor saturable absorber mirrorsEasier Self-Starting Passive Mode Locking for Short LasersSelf-starting of Passively Mode-locked Lasers

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DOI: 10.61835/lqd Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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Contents

What is Self-starting Mode Locking?

Some passively mode-locked lasers exhibit difficulties with starting the mode-locking process: after turning on the pump power, they often start in a (possibly noisy) continuous-wave mode, and the generation of ultrashort pulses can be started only with an external intervention, e.g. by knocking on an optical component of the laser resonator or vibrating a mirror (moving mirror technique). Mode-locked lasers which automatically start mode locking without such intervention are called self-starting. This property is obviously important for practical applications.

Self-starting Bulk Lasers

Self-starting mode locking is normally achieved when passive mode locking of a bulk laser is done with a semiconductor saturable absorber mirror (SESAM) or some other type of “slow” saturable absorber. The finite recovery time of such a device is helpful for self-starting mode locking, since it reduces the saturation power for long pulses, as occur during the start-up phase, and thus increases the saturation effect at that time. With a Kerr-lens mode-locked laser, self-starting is significantly more difficult to achieve because the Kerr lens provides a very fast absorber, the effect of which is weak for long pulses. Note that Kerr lens mode locking is sometimes called “self mode-locking” because it does not require a visible mode-locking device, but not in the sense that one would achieve self-starting mode locking that way.

Another factor inhibiting reliable self-starting is the presence of parasitic intracavity reflections. Even if the reflectivities involved are very small, they can be important because coherent coupling can occur in many resonator round trips. Such reflections must therefore always be carefully suppressed in mode-locked lasers, e.g. by avoiding any optical surfaces (even anti-reflection coated ones) perpendicular to the beam. In ring lasers, the effect of parasitic reflections tends to be weaker [7].

An uneven spacing of the resonator mode frequencies can also inhibit self-starting. This can result from chromatic dispersion and from mode pulling effects, which can arise from, e.g., spatial hole burning.

Self-starting is generally more difficult to achieve in mode-locked lasers with long laser resonators, i.e. with a low pulse repetition rate, and for lasers generating very short pulses. This results from the huge ratio of peak power (in the mode-locked state) to average power. The saturable absorber parameters must be adjusted to achieve reasonably strong (i.e. not too strong) absorber saturation under mode-locked conditions, which leads to only very weak effects for the first fluctuations occurring in a continuous-wave state.

Fiber Lasers

In fiber lasers, parasitic reflections are generally more difficult to eliminate. This is particularly the case for setups which also contain bulk-optical elements, where e.g. reflections from fiber ends can cause problems. Angle-polished fiber ends are a way to suppress that effect, if the polishing angle is sufficiently large, but this technique implies additional handling and alignment efforts. Even within a fiber, Rayleigh scattering leads to some level of back-reflections, particularly in rare-earth-doped fibers. Other factors which make self-starting more difficult are the use of fast (artificial) saturable absorbers, e.g. realized using nonlinear polarization rotation, and the typically long laser resonators.

For such reasons, ultrafast fiber lasers often exhibit non-self-starting mode locking, despite the usually large modulation depth of the absorber. It can occur e.g. that after switching the laser on, the pump power first has to be set to a high level where mode locking starts, and then reduced to a level where stable high-quality pulses are generated.

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What does 'self-starting mode locking' mean?

A mode-locked laser is called self-starting if it automatically begins to generate ultrashort pulses after being switched on, without needing any external intervention like knocking on a component.

Why is self-starting mode locking often difficult to achieve?

Self-starting can be inhibited by several factors, including parasitic intracavity reflections, uneven spacing of resonator mode frequencies, and the use of very fast saturable absorbers. It is also more difficult in lasers with long resonators.

Which type of saturable absorber helps with self-starting?

Why are many mode-locked fiber lasers not self-starting?

Fiber lasers often have long resonators and can suffer from parasitic reflections, e.g., from fiber ends or due to Rayleigh scattering. Many also use fast artificial absorbers based on nonlinear polarization rotation, all of which make self-starting more difficult.

Is 'self mode-locking' the same as 'self-starting mode locking'?

No. 'Self mode-locking' is another term for Kerr lens mode locking, which does not use a separate absorber device. However, lasers using Kerr lens mode locking are often difficult to self-start.

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