Choosing the right optical mounts and mounting hardware is important when designing optomechanical systems.
Tom Rigney, Newport Corp.
There are many types of mounts that can be used to hold and adjust mirrors, beamsplitters, lenses and other round, thin optics. Companies offer an extensive array of such products, from inexpensive fixed mounts to sophisticated adjustable units.
When designing or selecting an optical mount and mounting hardware for a particular use, temperature stability, vibration immunity and materials selection are important to consider.
For some demanding applications, researchers need an optical mount that will not move after it has been “set” to the desired position. A mount’s thermal stability must be considered for these applications. Thermal stability is a measurement of the optical mount’s sensitivity to changes in temperature that can be caused by the environment or even by the hardware to which the component is fastened. Mounts with low thermal stability can sometimes move, causing angular deflection away from the intended target (Figure 1). As a rule, the better the performance of the mount through temperature fluctuations, the less it will move when subjected to a given change over time.
Figure 1. This graph compares the thermal stability performance of various mirror-mount designs. The choices of material and design geometry will determine how well the mount will perform.
Researchers should consider using mounts that are specifically designed with this property in mind. These solutions typically use advanced thermal finite-element-analysis methods and are verified through extensive testing in tightly controlled thermal chambers.
When choosing mounting hardware, a particular material generally should be mounted to a similar material so that the two coefficients of thermal expansion match. For example, an aluminum mirror mount fastened to a steel post is subject to thermal instability because aluminum moves more than steel during temperature changes. However, a stainless steel mirror mount and a stainless steel post will have matching coefficients and will, therefore, provide more thermal stability (Figure 2).
Figure 2. Mirror mounts with high thermal stability have matching coefficients of thermal expansion between the mount and the post. This stainless steel mount was designed with advanced thermal analysis techniques and verified with extensive testing in a thermal chamber.
Mounts that are more symmetrical also tend to be more thermally stable than those that are not. Surprisingly, some costly “high performance” mounts that claim to be more stable are, in fact, less so because of their asymmetrical design.
Optical mounts that are susceptible to vibration disturbances can adversely affect experimental results or applications. Therefore, a key performance parameter to consider is vibration immunity, or the ability of an optical mount to withstand vibrations from external sources or from its own resonance frequency response. The resonance frequency is the frequency at which a system freely oscillates. For a given material, mounts that have higher resonance frequencies will exhibit quicker settling times and a reduction in vibrational disturbances.
Wary of Myths
One should be wary of a couple of myths when determining whether a mount has immunity to vibration. The first is the “snap,” or “pull on the mount face and let it go,” test. Perpetrators of this myth claim that after snapping the mount, a louder and/or longer ringing of the mount means that it is more vibrationally unstable. This is false. The snap test excites only the springs — not the entire mount. Because of their relatively low mass, springs transmit very little, if any, vibration to the mount.
Another misconception is that a higher mount stiffness means that resonance frequency will be higher. High stiffness means that a large force produces a small displacement for a given mount. In reality, the resonance frequency of a system must consider stiffness, mass and geometry. Steel, for example, is three times stiffer than aluminum but also has about three times the mass. As a result, the resonance frequency for a mount is about the same.
Steel does have an advantage over aluminum with regard to resonance frequency for posts and bases because they can support large “overhanging” masses. With an overhanging mass comparable to, or higher than, the mass of the post, the stiffness of the post will dominate. Therefore, it is better to consider using stainless steel bases (Figure 3), clamps and pedestal posts for mounting optomechanical hardware that is sensitive to vibrational disturbances. In addition, posts with a wider diameter and shorter length generally have higher resonance frequencies than long or narrow posts for a given material.
Figure 3. Various designs of stainless steel bases are available for mounting optomechanical hardware that is sensitive to vibration.
There is no simple test to compare the vibration immunity of two similar mounts, however. A side-by-side vibration characterization with elaborate analytical methods or thorough testing is required.
In the past, companies offered a limited selection of materials for their standard optomechanical mounts. The vast majority, however, used aluminum. Today companies offer more choices, each having its own advantages and disadvantages, and sometimes this makes it more difficult to select the best mount for an application. For example, posts and post holders are offered using a high-strength and thermally stable glass-fiber-reinforced composite material. These low-cost components provide electrical and thermal isolation and chemical resistance that their steel counterparts cannot (Figure 4).
Figure 4. Low-cost composite post holders provide an option for less demanding performance, electrical and thermal isolation, or chemical resistance.
More stainless steel mounts, bases, clamps, etc., are offered at affordable prices using advanced manufacturing techniques. This allows the user to take advantage of the matched coefficients of thermal expansion when mounting hardware to steel optical tables, breadboards or mounting plates. Take care when selecting materials with low coefficients of thermal expansion, such as ceramic or invar, for posts.
If these types of posts are fastened to an aluminum mirror mount, for example, stress builds up at the post-to-component interface during the temperature swings that occur during normal use. The stress can distort the component and thus move the optical element.
Feel vs. stability
Feel is an attribute that is difficult to define and even more difficult to measure. Those who adjust optical mounts regularly develop a sense for the way mounts should “feel” as they are adjusted. Most commonly desired is a smooth rotation of the adjustment knob with little or no side-play and no backlash when the knob is reversed. A rough, stick/slip type of adjustment is usually undesirable — especially when trying to make that last fine adjustment.
However, the feel can sometimes mask a mount that has a lower thermal stability. In lasers that require a stringent “set and forget” type of adjustment whereby the mount must not move after exposure to thermal changes, mount feel is not considered important. Also, in vacuum or laser cavity applications, mounts are often assembled using no lubrication, so screws in this condition will operate with metal-to-metal contact and will have a poor feel.
Some users have developed a perception that stainless steel adjustment screws inserted into brass collets is the best (Figure 5). In this case, we should ask ourselves if it is worth sacrificing thermal performance for an adjustment that feels good. Brass and steel have largely different coefficients of thermal expansion. Therefore, in applications where thermal stability is important, this may be a source of frustration, as the optics move with every temperature fluctuation.
Figure 5. The mount on the left has brass collets, while that on the right does not. The brass collets allow the user to select a variety of drive types. On the other hand, adjustment screws inserted directly into the mount provide the highest thermal stability.
If this is a concern, it may be better to consider optical mounts with stainless steel screws inserted directly into the stainless steel mounts. Certain types of brass collets can be used in the design of a thermally stable optical mount. However, for the optical mount to be truly stable when using brass collets, it is recommended that thermal analysis of the design be performed, followed by extensive thermal cycling tests in a strictly controlled environment.
There are many performance factors to consider when selecting the best optical mount and mounting hardware. Thermal stability, vibration immunity and choice of materials each has a role in performance, depending on the needs of the user. In general, when the budget allows, one should consider matching the materials of each component. If the optical assemblies are fastened to a stainless steel tabletop, it is best to use stainless steel mounts and mounting components, including bases, clamps, posts and pedestals. When the budget is slim or alignment performance is less critical, one can select other materials, such as aluminum, polymer composites and zinc. But these materials may compromise performance.
For best results in system performance and budget, rely on selecting mounts from companies that analyze optical mount designs during product development, verify them through extensive testing and provide performance data.
Meet the author
Tom Rigney is a senior engineering manager at Newport Corp. in Irvine, Calif.; e-mail: firstname.lastname@example.org.