Laser Mirrors

Definition: high-quality mirrors used in laser resonators and other optical setups

The resonator of a laser contains mirrors, which must meet a number of criteria, in particular

• low reflection losses (for highly reflecting mirrors), or (for output couplers) a well-defined transmission in a certain wavelength range
• high optical quality (good flatness over large area, low microscopic roughness) of the surface (avoiding wavefront distortions which could deteriorate the beam quality)
• good resistance against high optical intensities to avoid damage (particularly in Q-switched lasers)

In almost all cases, dielectric mirrors based on multilayer structures (mostly quarter-wave mirrors) are used as laser mirrors. (The article on dielectric coatings presents some details on the fabrication of dielectric laser mirrors.) Normally, one of the mirrors, which is used as output coupler, has a significant transmission for the laser radiation, whereas all other mirrors are highly reflecting (e.g. with > 99.8% reflectivity). Some mirrors can also be made as dichroic mirrors, allowing the injection of pump light into the gain medium of an end-pumped laser. (For some quasi-three-level lasers, the requirements on such dichroic mirrors can be demanding due to a close proximity of pump and laser wavelength.)

Of course, laser mirrors can also be used to reflect light outside the laser resonator. For example, it is common to use a pair of steering mirrors, each deflecting the beam by ≈ 90°, for sending a laser beam into some apparatus. The mirror mounts of the steering mirrors typically have two or three adjustment screws, which make it possible to adjust the virtual origin and direction of the beam.

Residual Transmission

Even highly reflecting laser mirrors have some residual transmission. Particularly in solid-state lasers, this can lead to additional output beams with substantial powers, which are sometimes used for diagnostic purposes, such as for monitoring the laser power without using a part of the output beam. However, problems can arise from the nonuniformity of the residual transmission, which can be strong for highly reflecting mirrors.

Mirror Substrates

Usually, laser mirrors are fabricated based on glass substrates (e.g. BK7 or fused silica), but it is also possible to deposit mirror coatings directly on a laser crystal (or glass), e.g. for monolithic lasers. Typical mirror substrates are of cylindrical shape, with a diameter of e.g. 1 inch (≈ 25.4 mm) or 0.5 inch, and a thickness of e.g. 6 mm. Even for highly reflecting mirrors, some substrate properties can be important, in particular the surface quality, but also high stiffness, a low thermal expansion coefficient and/or a high thermal conductivity (to avoid thermal bulging in high-power lasers). For partially transmitting mirrors, it can also be important to have a high optical homogeneity (to avoid beam distortions for the transmitted light) as well as low absorption and scattering losses.

Mirror substrates may have curved surfaces, leading to focusing or defocusing laser mirrors. The effective focal length is one half the curvature radius, assuming normal incidence. For strong curvature, e.g. with a radius of curvature well below 10 mm, it can be difficult to obtain high-quality mirror coatings. Some specialists can make good mirrors with radii of the order of 1 mm.

Special Mirror Types

Special types of dielectric mirrors such as chirped mirrors can also provide a suitable amount of chromatic dispersion in the resonator of a mode-locked laser. It may then be possible to avoid the use of, e.g., a prism pair and thus to construct fairly compact femtosecond lasers.

There are also supermirrors with extremely low reflection losses, but these are rarely used in laser resonators.

Metallic mirrors, such as silver mirrors, are normally not suitable for laser resonators, because they have much higher reflection losses and are also not suitable as output couplers. Also, they tend to oxidate on the surface and thus to lose in surface quality and reflectivity.

See also: dielectric mirrors, dielectric coatings, dichroic mirrors, laser resonators, output couplers, supermirrors, Spotlight article 2006-10-22

Categories: lasers, photonic devices