Senior Opto-Mechanical Design Engineer

Opto-Mechanical Design

The information on this page is typical of Opto-Mechanical Design. The scenario below is a design that I have completed and 9 times out of 10 you are presented with the optical design as optical elements only. This was a Zemax output that was saved in SolidWorks. All the optical elements are in their true spatial place in 3D space and unsupported as shown below. Having discussed the functionality with the optical designer, it transpires that some elements are fixed in position, some elements are grouped into sub assemblies in fixed positions and some are grouped into sub assemblies that must move to attain a specific focus. This is the core function and responsibility of the Opto-Mechanical Design Engineer.

The image below shows how one particular lens group was housed and supported.

The lens colours in the sub assembly below can be matched with the same colours in the 1st image above.

Opto-Mechanical design is the sub-discipline of optical engineering in which optics such as lenses and mirrors are integrated into mechanical structures as to form an optical instrument assembly. Making the right choices in opto-mechanical design demands the application of intuition, and experience with unknowns verified through testing. The main driver for the tolerancing of the lenses, optical alignment and collimation of the optical train is of course the "Optical Design Specification", which will no doubt include the environment that it must survive in as well. Team members must make decisions in five basic design areas:

Materials
Structural Design
Lens-to-mount interfaces
Mountings for prisms and mirrors
Assembly alignment

Materials:

Insofar as possible, I match the coefficients of thermal expansion (CTEs) of materials used in connected mechanical and optical parts to minimize differential expansion or contraction in the event of temperature changes. There is a wide choice of materials to choose from. Some expensive, some not so expensive. I as an Opto-Mechanical Design Engineer will always be communicating with the optical designer with regards to tolerancing. Aluminium is always a good start point for choice of housing material but after analysis, titanium or other exotic materials may need to be considered. There are instances where it would be more cost effective to amend the optical design rather than the mechanics. The decision may not be with the Opto-Mechanical Engineer or the Optical Designer. Sometimes the decision has commercial implications but the important thing is to ensure all stake holders are aware of deviations from the design goal / specification at all times.

Structural Design:

An optical system functions properly only as long as the optics remain within allowed tolerances of their nominal locations and orientations and structural deflections caused by gravity, temperature or other external forces do not excessively distort the optical surfaces. Structural designs must be stable enough to control these effects throughout the operating temperature range. The structure must constrain the optics in such a manner that they are not damaged or irreversibly moved when exposed to extreme environmental conditions. Temporary deflections of optics and mechanical parts are acceptable during vibration, shock, or temperature changes beyond the operating ranges as long as the parts survive and come back to their nominal positions after exposure.

The FEA analysis proved that the stress produced in the lens housing by the force exerted on it (see 1st image below) under normal operating conditions was well within the materials elastic limit meaning there is no permanent deformation of the lens housing.

The 2nd FEA analysis (2nd image below) proves there was no deformation to the lens housing that would be detrimental to the optical performance even during operational use.

Lens-to-Mount Interfaces

Lens mounting and positioning is critical to the performance of a refractive optical system. For best results, design spacers and retainers to interface with polished surfaces on lenses rather than with ground rims. You will then use the most accurately made surfaces for lens positioning. I made the lens retainer on the far right of the image below to act as a "flexure" that kept an adequate constant pressure on the lenses but flexed to ensure no damage occurred under high temperature and vibration conditions.

Mountings for Prisms and Mirrors

Reflective components such as mirrors, gratings, and some prisms have their own sets of mounting issues because they are more sensitive to surface distortions than refracting optics. Support prisms and small mirrors semi kinematically, if possible. I positioned the mirror in the image below on 3 domed surfaces to minimise surface contact, and retained by springs that acted as flexures to accommodate deformations due to temperature ensuring the mirror survived.

Assembly and Alignment

For best performance of multiple-lens assemblies I always recommend rotating the lenses around their axes to "phase" residual wedges so they counteract each other. Adjustments should be locked after the optics are aligned; techniques include mechanical clamping, epoxy pinning, and soldering. Seal optical instruments during assembly to protect optics from moisture and particulates. They can be purged with dry N₂ or He and pressurized as appropriate. These procedures depend on whether they are space or land based applications. Good Opto-Mechanical design is key to optical system performance.

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