Mode Locking Devices

For passive mode locking, one uses a saturable absorber in the laser resonator, which is in most cases a reflective device inserted as an end mirror of the resonator. The essential functions of that absorber are the following:

• It needs to initiate the mode locking. Usually, immediately after turn-on the laser will be in continuous-wave operation with some more or less substantial intensity noise. The saturable absorber will always favor the most intense part of the circulating radiation and eventually let only a single pulse “survive”, typically only after thousands of resonator round trips.
• In the simpler case of a “fast” absorber, having a recovery time well below the pulse duration, both the leading and trailing temporal wing of the pulse are attenuated, leading to a pulse shortening effect similar to that of an active modulator. However, the loss modulation is much faster, since it is driven by the pulse itself. Therefore, passive mode locking typically leads to much shorter pulse durations.
• Mode locking can also work well with “slow” absorbers, where the recovery time may be more than 10 times longer than the pulse duration. One may expect stability problems due to positive net gain “behind” the circulating pulse, but in fact stability is given if the recovery time is not too long [7].
• An absorber also suppresses light circulating at other times, which (as long as it is weak enough) cannot saturate the absorption and therefore experiences higher power losses.

In most cases, one uses a so-called semiconductor saturable absorber mirror (SESAM) [6] for passive mode locking. Such devices can be tailored for a wide range of operation parameters, and are therefore successfully used for a wide range of mode-locked lasers concerning pulse durations, pulse repetition rates, optical wavelength, output power etc. SESAMs can be used both in solid-state bulk lasers just as end mirrors, and in mode-locked fiber lasers. In the latter case, it is preferable to use a fiber-coupled SESAM, which may either contain a fiber which is simply butted to the semiconductor structure, or a fiber collimator for operation with larger mode area.

Essential parameters of a SESAM for mode locking are the following:

• Operation wavelength: both the contained Bragg mirror and the absorber layer(s) must be suitable for the intended operation wavelengths, so that these devices usually work only within a wavelength range with a width of a few tens of nanometers. For operation in new wavelength regions, a substantial new device development may be necessary, involving new materials, adapted design parameters etc.
• Modulation depth: This is the maximum change of reflectivity due to the absorber saturation. It should be large enough to safely initiate the mode locking and produce short pulses, but not too large, since otherwise one may obtain a reduced power efficiency, absorber damage or Q-switching instabilities.
• Recovery time: This is the time for the saturated absorption to recover to its normal state. Often, it is substantially longer than the pulse duration, but if it is too long, instabilities will result. The obtained pulse duration will often have little dependence on the absorber recovery time.
• Saturation fluence: this together with the beam area determines the saturation energy, which should usually be of the order of a quarter of the circulating pulse energy, or sometimes less.

Some other types of saturable absorbers are also sometimes used for mode locking, e.g. absorbers based on carbon nanotubes [8, 9, 11] or graphene.

There are also various kinds of artificial saturable absorbers. These are devices which exhibit decreasing optical losses for higher intensities, but not based on real light absorption. Such devices can be based on e.g.

• Kerr lensing combined with some kind of aperture (→ Kerr lens mode locking) [4]
• a nonlinear mirror device containing a frequency-doubling crystal, as sometimes used for passive mode locking of solid-state bulk lasers [1, 12]
• a nonlinear fiber within an auxiliary resonator (→ additive-pulse mode locking) [2]
• nonlinear polarization rotation in a fiber, combined with a polarizing element, often used for passive mode locking of fiber lasers [5]
• a nonlinear fiber loop mirror, also used for mode locking of fiber lasers [3]
• an array of waveguides, exhibiting nonlinear coupling [10]

The optimum parameters of a saturable absorber strongly depend on the type of laser and the intended operation regime. The choice of absorber parameters should be based on a comprehensive quantitative understanding of the system, because otherwise non-ideal performance parameters or even instabilities can result, in other cases premature absorber degradation or instant destruction. Ideally, one should study the intended operation based on a numerical pulse propagation model.

Here you can submit questions and comments. As far as they get accepted by the author, they will appear above this paragraph together with the author’s answer. The author will decide on acceptance based on certain criteria. Essentially, the issue must be of sufficiently broad interest.

Please do not enter personal data here; we would otherwise delete it soon. (See also our privacy declaration.) If you wish to receive personal feedback or consultancy from the author, please contact him e.g. via e-mail.