Not all mounts perform optimally for every application.
Damon Kopala, Edmund Optics
When engineers are choosing optics, a common mistake they make is overlooking the integration of optical mounts into their systems. There are a variety of mounts available for holding lenses, prisms, mirrors, filters and other optical components. Some examples include bar-type, gimbal, adjustable kinematic, fixed and jaw clamp mounts, just to name a few. When cost is a deciding factor, simple fixed mounts will be more than adequate. However, for applications that demand precise positioning, adjustable, stable, kinematic mounts are essential to the integrity of an optical system.
A three-dimensional rigid body has six degrees of freedom: X, Y and Z are translational, and Rx, Ry and Rz are rotational. A mount is considered kinematic if all six degrees of freedom are fully constrained. Most laboratory kinematic optical mounts use the classic cone, groove and flat constraint system and two or three adjustment screws. One adjustment screw at the groove and one at the flat can be used to adjust the rotational degrees of freedom.
Because the axis of rotation is behind the optic, there will be a slight translation of the optic when an adjustment is made. A third screw can be placed over the cone to compensate for unwanted translation. The three-screw configuration enables the optic to be rotated as needed, using the first two adjustment screws, then returned to its original position along the Z-axis with the third.
Engineers do not want their mount selection to degrade an optical system’s performance. To illustrate the importance of choosing the proper optical mounts, consider an application that employs a 25-mm cube beamsplitter and a 12-mm-focal-length lens to combine and couple two 3-mm beams into a 0.12-NA fiber. To ensure the highest coupling efficiency, assume that both focused spots must be centered on the fiber to within ±2 μm (Figure 1).
Figure 1. The contributions of individual errors and root sum square (total error) are shown for the fiber-coupling system pictured.
Each component will contribute to the overall positioning error in the system, which can be calculated by the root sum square. Because each element potentially has six degrees of freedom, there are many combinations of movement that can occur. For simplicity, only the following movements will be considered:
• Translation along the optical axis of the lens by 1 μm.
• Rotation of 400 μrad of the focusing lens about a point 4 mm from its nodal point.
• Translation along the optical axis of the beamsplitter by 2 μm.
• Rotation of 150 μrad of the beamsplitter about the optical axis.
• Translation along the optical axis of the fiber by 1 μm.
Clearly, the system in the figure undergoes too much movement for beam two to maintain the ±2 μm required for high coupling efficiency. Mount resolution and instability from thermal effects, vibration and gravity (often referred to as pointing stability) may contribute to misalignment, When selecting optical mounts, engineers should begin by asking themselves two questions: Does this mount offer enough resolution? Is this mount stable enough?
Resolution of kinematic mounts is typically classified into linear and angular resolution. The thread pitch of the adjustment screws determines the linear resolution, whereas the placement and thread pitch of the adjustment screws provide the angular resolution; 80- to 100-threads-per-inch (TPI) adjustment screws are the industry standard. While higher resolution can be obtained by simply using adjustment screws with a larger TPI count, this is not always optimal because finer threads are easily damaged.
Sensitivity is another parameter, often provided by manufacturers, that is related to resolution. Because most kinematic mounts are manually driven, it is helpful to know the minimum obtainable movement of the optic. Generally, fingertips are sensitive enough to resolve a 1° turn of the screw. The movement of the optic that corresponds to a 1° turn defines sensitivity.
Thermal stability indicates how well the mount will perform when subjected to changes in temperature. For minimum deflection from thermal effects, the mount’s coefficient of thermal expansion should be matched to that of the optic. Certain types of stainless steel match up very well to glass and are the preferred choice when thermal stability is of utmost importance. Although aluminum has a higher thermal expansion coefficient, kinematic mounts made from it still perform well in typical laboratory environments.
Along with movement from thermal conditions, the effects of gravity over time will contribute to the overall misalignment error. Pointing stability is the measure of this error, specified as an angular movement, and is defined at a certain temperature, time lapse and applied load.
Vibration also can degrade optical system performance. Misalignment from low-level vibration will lead to blur in the image plane, which will happen when, for example, imaging with a high-power microscope objective without using a vibration-isolation platform. Materials with higher stiffness values have greater fundamental, or natural, frequencies and faster settling times, resulting in less vibration disturbance.
In addition to resolution and stability, there are several other factors to consider when choosing a kinematic mount. Most labs have a mixture of metric and English optics, so it is convenient to have a mount that accepts both, for instance, 1.0 in. (25.4 mm) and 25 mm. And systems often are assembled on an optical bench with hole spacing of either 1 in. or 25.4 mm. The type of bench usually dictates whether metric or English mounting hardware is available.This is why counterbore holes, instead of threaded holes, for post mounting are increasingly popular.
A big drawback of many mounts is the need to drop, rather than place, the optic into position. With the high cost of precision optics, it is highly undesirable to drop them into place, even if it is only from a few millimeters. A mount with finger cuts allows guided placement of the optic, which reduces the chances of chipping and fracture.
The key to successfully choosing an optical mount is to prioritize the surrounding requirements. When critical alignment is of primary importance, the user should choose kinematic mounts that offer high resolution and excellent position stability. Thermal stability is increased when all connecting components are constructed from the same material. In the majority of set ups, this would consist of stainless-steel hardware because most optical benches have stainless-steel tops.
However, a setup constructed entirely from stainless-steel components can prove costly. When cost is more of a concern and alignment is less critical, aluminum is a perfectly suitable alternative. There are a wide variety of aluminum kinematic mounts on the market today and, for optimal results, one should look to suppliers who can provide a complete list of specifications that includes tested performance data.
Meet the author
Damon Kopala is an applications engineer at Edmund Optics in Barrington, N.J.; e-mail: email@example.com.