The main advantages and disadvantages of unstable resonators versus stable resonators

Advantages of Unstable Resonators

• The mode volume is usually wider and controllable.
• Transverse modes are easily discriminated.
• All mirrors can be perfectly (100%) reflecting, that is particularly attractive in the infrared region, where metallic mirrors can be used.

Disadvantages of Unstable Resonators

• The output beam cross-section is in the form of a ring, i.e. has a dark hole at its center.
• The beam has diffraction rings.
• The laser is more sensitive to the cavity perturbations.

Dear, Parvin

Unstable resonators are often used for the purpose of selection of transverse modes and reduce the angular divergence of the radiation. When large volumes of the active medium, this method of selection is the most promising, since it is not associated with any complication of the laser system, with no significant reduction in the efficiency of the radiator.

The example of the resonator of telescopic type well shows the increased selective properties of the unstable resonators forming light bunches with high degree of spatial coherence. It is known that from the point of view of angular selection it is more favorable to work in the conditions of rather big diffraction losses as diffraction provides generation failure first of all of fashions of high orders.

As for the unstable resonator diffraction (geometrical) losses are always great, at its use actually it isn't required to take any measures for additional selection of cross fashions. The laser with the unstable resonator generates usually only the main cross fashion; thus the diffraction limit of divergence is often reached. We will notice that from the point of view of an orientation of radiation it is desirable to have higher values of coefficient of expansion M.

Very important advantage of lasers with unstable resonators is effective filling of volume of the active environment weed radiations. Even with rather small lengths of resonators the volume occupied by a field of the main fashion is rather great. Unlike steady resonators the light bunch in the unstable resonator has no banner; in this regard it is possible not to be afraid that density of light energy of a bunch can increase in separate points of the active environment to the values corresponding to a threshold of destruction of the medium.

Unstable resonators are perspective from the point of view of creation of lasers with rigidly set spectral and temporary characteristics. As it was already noted, development of process of generation in the laser with the unstable resonator begins in the central (priosevy) region of an active element, and then extends on all its volume. If directly on an optical axis to enter the radiation from the additional generator having accurately certain spectral and temporary characteristics at the small power in the laser resonator, thereby it is possible to exercise effective management of process of formation of radiation and to receive as a result desirable output parameters of a light beam.

It is necessary to notice that the role of priosevy area of an active element in development of process of generation has also the negative side: lasers with unstable resonators are sensitive to different irregularities and defects in the active environment or on the reflecting coverings in close proximity to an optical axis of the resonator parameters of a light beam.

Fast development of process of generation from an axis to the periphery brings in the direction, in particular, to that spatial (in the direction, perpendicular to an axis) development of a huge impulse in the mode of modulation of good quality proceeds in lasers with unstable resonators quicker, than in lasers with steady resonators. As a result duration of the light impulse generated by the laser considerably is reduced. We will emphasize that use of the unstable resonator can significantly affect dynamics of process in the laser.

More information in:

lib.alnam.ru/book_fopt.php?id=115

http://mash-xxl.info/info/185734/

http://msd.com.ua/principy-lazerov/dostoinstva-j-nedostatki-neustojchivyx-rezonatorov-s-rezkoj-granicej-zerkala/

Best Regards, Leonid

The main advantages and disadvantages of unstable resonators versus stable resonators

Advantages of Unstable Resonators

• The mode volume is usually wider and controllable.
• Transverse modes are easily discriminated.
• All mirrors can be perfectly (100%) reflecting, that is particularly attractive in the infrared region, where metallic mirrors can be used.

Disadvantages of Unstable Resonators

• The output beam cross-section is in the form of a ring, i.e. has a dark hole at its center.
• The beam has diffraction rings.
• The laser is more sensitive to the cavity perturbations.

I would strongly recommend to read the excellent book "Lasers" ( Chapters 14- 23) by Anthony Siegman who made a lot in this field and who recently passed away.