Opto-mechanics

Optical breadboards and optical tables are flat platforms containing a regular grid of threaded mounting holes for fixing various types of optical mounts or posts. They are particularly suitable for making optical prototype devices, and even some industrial devices are based on an optical breadboard, possibly integrated in some housing. Optical breadboards and tables should be very stiff and should exhibit well damped mechanical resonances. Optical tables usually rest on legs containing pneumatic vibration dampers.

One may directly fix an optical mount on a breadboard, but frequently one uses a post in between them, which also gives some flexibility concerning the height.

For permanent installations, such as industrial lasers, one may use other solutions – for example, custom-made base plates containing mounting features at specific locations rather than a regular grid of holes. These can be cheaper in mass production and also mechanically more stable, but on the other hand much less flexible.

There are various optical rail and cage systems which allow one to move optical elements only in one direction – typically, along some optical axis. For example, one may mount a laser diode, a beam collimator, additional lenses, an aperture, a sample holder, a polarizer and an image sensor in such a rail system. Ideally, an optical rail system is designed such that one can easily insert additional mounts, even with close distances between them, and at the same time it is guaranteed that the angular orientation of elements is very stable even after movements along the axis. There are some trade-offs, however; for example, a high angular precision may require longer mounts which prohibit close mounting of components. Also, magnetic mounts allow easy mounting without turning screws, but are tentatively less precise. Cage systems can be mechanically more stable than rails, but make it more difficult to insert additional elements.

A linear translation stage allows one to move objects over some distance along a straight line. For example, one may fix such a stage on a breadboard and mount a post and a dual mirror holder on it in order to realize an optical delay line. In other cases, one inserts a second linear translation stage, mounted at 90° against the other one, to allow for positioning in two dimensions. For example, such arrangements are used in fiber launch systems.

There are also rotation stages, which can be used for example for positioning a nonlinear crystal with critical phase-matching, or adjust the orientation of a polarizer.

Instead of micrometers screws for manual actuation, translation stages can contain highly precise motors. For example, there are various types of piezo motors and step motors which can do quite precise movements. Sometimes, the position is further improved with a feedback loop. The maximum range of translation, the speed and accuracy of movement can be critical performance factors.

A large variety of mounts for different kinds of mirrors are available, mostly for those with cylindrically or rectangular shaped mirror substrates of certain standard sizes. For example, cylindrical mirrors with a diameter of 1 inch (≈25.4 mm) are particularly common. In most cases, a mirror mount fully encloses the mirror, but there are also clear edge designs with better access at least on one side. There are also corner mirror mounts which fully surround the mirror, but which on most sides block only a quite small area around it, because most of the holder sits in one corner of the mirror.

There are simple fixed mirror mounts, where one can simply insert a mirror, fix it (e.g. with some nylon-tipped screws) and then screw the mount to an optical breadboard, for example. This approach does not allow for precise alignment. However, it is often not required to have all mirrors in an optical setup precisely aligned.

For some mirrors in an optical setup, mirror mounts need to offer precise adjustment options for the angular orientation. Typically, such a kinematic mount has two or even three precise micrometer screws. Cheaper holders are made of aluminum, which can be processed relatively easily but is not as stiff and precise as stateless steel, for example. Apart from the basic construction material, various design details can be quite relevant for the mechanical performance – for example, concerning thermally induced drifts or mechanical vibrations. The quality of used micrometer screws can also be relevant; low quality micrometer screws do not have a high resolution, may exhibit some degree of hysteresis, and they may no more allow for smooth movements after a couple of years. Overall, a substantial technical expertise is required to fabricate high quality mirror mounts.

Ordinary mirror holders with two micrometer screws normally cause a rotation with a fixed point outside the mirror surface. If this is a problem, one may use a gimbal mount, which can have a fix point at the center of the mirror surface. This allows one to adjust the beam orientation without modifying the position of the reflection point, if that point is at the center.

Various kinds of specialized mounts are also obtainable, for example for mounting two or more mirrors with a fixed relative angle. For example, a mount for two mirrors with 90° relative angle is useful for realizing an optical delay line; it can be more compact, more convenient and cheaper than using two separate standard mirror holders.

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