There is no single cavity design that is clearly superior to all others. The choice of cavity design and materials, instead, depends upon the application. We’ve worked with the leading labs in the world to provide frequency stabilization solutions for a broad range of applications, and can advise on a customized solution for your needs. Factors to consider in choosing the best cavity design for your application include:

Sphericalaberration and chromatic aberration

Spherical aberration is an axial aberration, affecting the entire field equally, including stars at the center. All telescope designs strive to eliminate or minimize spherical aberration. Normally, spherical aberration should not be visible in an optical system. But it is important to understand how it arises to see how it is eliminated in certain designs. The elimination of spherical aberration is critical to how certain telescopes such as Schmidt-Cassegrains and Newtonians are designed.

Sphericalaberration in a lens

Cylindrical cavities feature a tried-and-true geometry for spectroscopy, offering locked laser linewidths at the 10 Hz level. They are the most common cavity design, but other cavity designs offer distinct advantages for some applications.

Cube cavities are the most compact cavity designs, while still delivering narrow linewidth performance. These rigidly-held spacers can be moved without need for realignment.

Sphericalaberration example

A simple spherical mirror cannot focus light to a single point. As the diagram above shows, light from the edge of the field is focused closer to the mirror along the optical axis than is light from the center of the field. This means it is not possible to find a single point of best focus, only a point where the image is smallest but still not sharp. The simplest way to eliminate coma with a single mirror is to change the shape from spherical to parabolic. A parabolic mirror does not suffer from spherical aberration and can focus all light to a single point.

Note that this is the same principle used in radar and satellite dishes. Radio waves are simply electromagnetic radiation, just like visible light only with much longer wavelengths. Satellite dishes are parabolic in shape. Even the sound-collecting dishes you see along the sidelines of NFL games are parabolic to focus the incoming sound waves onto the microphone located at the focal point of the dish.

Refracting telescopes normally use spherical lenses, due to the extreme difficulty and cost associated with constructing aspherical lenses. A single spherical lens of course suffers from spherical aberration. However, a refractor eliminates spherical aberration by combining two lenses with equal but opposite amounts of spherical aberration. More complex refractor designs may use three or four lenses, but the basic idea is the same. These lenses must also work to eliminate a number of other aberrations, so the design process is tricky, but in the end spherical aberration must be the smallest residual aberration if the telescope is to provide a good image. See the Optical Designs section on refractors for more details.

Coma aberration

On the whole, all systems for narrow linewidths use a vibration isolation stage, and the thermal noise floor of the cavity can be reached with any geometry (including cylindrical). If the value of that noise floor is important, then a notched cavity, being longest, has the lowest thermal noise floor (assuming a good choice of mirrors for large beam spot size).

The careful design and mounting of midplane cavities means low vibration sensitivity in the vertical dimension. These cavities can be relocated while preserving alignment.

Notched cavities offer reduced acceleration sensitivity as compared to a cylindrical design. Long spacer lengths, from 100 mm up to 300 mm, yield the lowest thermal noise floor and narrowest laser linewidths. We recommend a notched cavity geometry when Hz-level linewidths and low frequency drift are paramount.

Every telescope design sets out to eliminate spherical aberration. In the case of the Newtonian telescope this is simply done by making the primary mirror parabolic (see the Optical Designs section on Newtonians for more details). Cassegrain reflectors use a very specific combination of mirror shapes to eliminate spherical aberration, normally either a parabolic mirror combined with a hyperbolic mirror, or a pair of hyperbolic mirrors. Commercial Schmidt-Cassegrain telescopes use spherical mirrors, which would, on their own, create spherical aberration. The Schmidt corrector lens on the front of an SCT eliminates the spherical aberration inherent in the mirror design. Most Maksutov-Cassegrains work the same way.