What is Optomechanical Design?

Optomechanical design is the subdiscipline of optical design that focuses on integrating optical components into the mechanical structures that hold or move them while minimizing the impact of structural, dynamic, and thermal loads on optical performance. It is the intersection of optical design and mechanical design with the goal of producing manufacturable, cost-effective, and robust optical devices.

The resulting design must meet a wide array of requirements, including component cost, manufacturing cost and schedule, assembly cost and schedule, mechanical reliability, maintenance effort, size, weight, packaging and shipping, ease of alignment, and optical performance.

Optomechanical design is not the same as designing an optical system on an optical table, where the goal is simply to prototype the optics or explore a new area of the optical sciences. Optomechanics focuses on the mechanical portion of products used outside an optics lab, usually by people who are not optical engineers or researchers. Everything from the camera in your smartphone to the mirrors and lenses in the James Webb Space Telescope requires extensive optomechanics to ensure the entire product meets or exceeds all of its design goals.

Once the light path for the optical device is defined and characterized using a tool like Ansys Zemax OpticStudio optical system design and analysis software, the optomechanical design process can begin, using the optical geometry as a starting point. 

Each optical device will have different requirements and design steps, but most steps fall into one of the following five categories.

1. Material Selection

The first step is to define the material used to manufacture each optical and mechanical part in the system. Lenses can be made from glass or polymers, while mirrors and mechanical components have multiple material options. 

Because differences in the coefficient of thermal expansion can cause alignment, stress, and mechanical fatigue issues, it’s important to choose materials with a similar coefficient of thermal expansion (CTE). Aluminum and stainless steel are popular materials for structural components. Glass or carbon-filled polymers can provide similar properties at a lower weight, and composite materials can offer very high stiffness and a low CTE. Even when using off-the-shelf components, design engineers must know the materials used in those subassemblies.

Once the base materials are defined, engineers then need to specify which post-processing to apply. Material post-processing can be coatings, anodization, surface finish, or heat treating. Each step impacts the mechanical and optical characteristics of the treated parts.

Another consideration in material selection is the material used for adhesives and fasteners in the design. Thermal mismatches in fasteners with the material they attach can lead to significant loading. The wrong adhesives can outgas, which can coat optical surfaces — and if not strong enough for the application, the lenses they hold can become misaligned.

2. Structural Design

For an optical design to perform properly, the components in the light path need to remain in their nominal orientation and location. The optomechanical designer must decide which mechanical components are best to hold each optical component and how the mechanical structure is connected to form an assembly. Tolerancing plays an essential role in this step. 

In addition, if any optical features require controlled movement during operation, actuation mechanisms must be chosen and designed. Standard actuation methods include lead and ball screws, precision threaded interfaces, voice coils, and solenoids. Precision gears, cams, and electric motors can also be part of actuation devices. In adaptive optics, a mirror is deformed by mechanical actuators to change its optical properties, often correcting for optical aberrations. 

Most objects that make up the structural design hold or move optics, but some also shield the optics from contamination, thermal loads, and unwanted external light. Barrels, baffles, and enclosures are some of the typical components used to protect the optical pathway.

Weight and size also play an essential role in this part of the design process. The structural design starts with the location of the optical components and the envelope, including mass, that the device must fit within. Engineers evaluate external loads in the form of forces, acceleration, and temperature changes to see how much each component might move or distort — and to make sure the structure will not permanently deform or break. In addition, they modify the design to keep out unwanted contaminants such as dust, chemicals, moisture, and light. 

Another part of the structural design is thermal management. Light sources like lasers often generate heat, and sensors usually have very specific operating temperature ranges. Both must be kept within allowable temperature limits, and passive, active, and cryogenic cooling are sometimes required.

3. Lens-to-Mount Interface Design

Once the design teams decide how to hold or position the optics, they must define how to connect each lens to the structure. Optical lens mount design is a unique mechanical problem addressed with proven methods. Capture devices like retaining rings, snap rings, spacer rings, ring flanges, and edge mounts each have advantages and disadvantages. Engineers must understand the loading, costs, and optical tolerances of each approach to choose the right one. 

Lens-to-mount interface design is often an interactive process between the lens designer and the mechanical engineer. This is because many mounting schemes depend on the curvature and polished precision optical surface of the lens to fix the position of the lens axially and to keep it from rotating off of the optical axis. 

The high precision of each surface allows for accurate positioning. The lower tolerances on the ground rim or bevel are looser and make them less ideal for holding lenses in place. In some designs, elastomers or adhesives serve well as an interface between the lens and the supporting hardware.

4. Other Optical Component Interface Design

An effective design also includes defining the optomechanical interface for components, aside from the lenses. Optical sources and detectors are an important part of the light path, and their position relative to other components is critical. They are often mounted on a printed circuit board (PCB) or are in their own enclosure, so engineers need to understand their mounting requirements and adjust the design accordingly. 

Where lenses are thin cylinders, mirrors and prisms can come in a variety of shapes that then drive the options engineers have to hold them in place. Mirrors are especially sensitive to distortion, so mirror mounting schemes are used to avoid bending the mirror, and prisms can often be bulky and are very sensitive to the angle of their optical surfaces to the light axis. Clamps and screws are common mounting schemes for these types of components, as are adhesives or elastomers. 

5. Designing for Cost, Manufacturability, Assembly, and Optical Alignment

The final category of design tasks looks at how much various design solutions cost, how they impact the manufacturability of the optical system, the assembly process, and how to align the optical components. All of these factors impact the overall commercial viability of the product using the optical system. 

The team should work with manufacturing and quality engineers to not only reduce the cost of each part in the assembly, but also to create preliminary processes for cleaning, assembling, aligning, and fixing the location of the optical components in an automated and repeatable manner. 

For larger projects, teams of mechanical, optical, and optomechanical engineers work together to integrate optomechanics into the design process. In smaller teams, engineers must take a multidisciplinary approach and understand optical and mechanical behavior. 

The typical design flow of an optical system, including optomechanics, can be broken into the following steps:

1. Optical Design

   The first step is to optimize the optical components in the system, such as lenses, mirrors, prisms, sources, and detectors. In this step, engineers define each optical component’s properties, shape, location, and relative position. They then calculate the optical performance, predict how light changes as it moves through the optical elements, and vary the geometry and positions until the optical performance meets the design requirements.

2. Optomechanical Systems Design

   The overall optomechanical design focuses on designing the structure to hold those components, control their mechanical motion if they need to be actuated during operation, or protect them from the outside environment and stray light. The optomechanical design team also works to calculate and minimize cost, maximize manufacturability, and consider assembly and alignment needs.

3. Optomechanical Loading and Response

   Engineers then determine and apply environmental loads such as gravity, temperature change, vibration, acceleration, and force during assembly and operation. They then calculate how the mechanical structure deflects and how the optical components are distorted or moved from their nominal position.

4. Assess Impact on Optical Design

   The optical performance is then re-evaluated with the distorted or displaced optical components to determine if the performance is still within an acceptable range.

5. Iterate Between the Optical and Optomechanical Design

   If the resulting optical performance falls outside the allowables, engineers iterate back and forth on the optical and optomechanical design until the cost and optical performance are acceptable. Accurate and timely simulations, meaningful test data, and clear communications between the different disciplines drive the effectiveness and efficiency of the iterations.

The monetary and scheduling impact of each design option must be minimized while still keeping the optical system performance within acceptable values. Achieving this goal is the fundamental challenge of optomechanics. 

Optical design considerations dominate in a lab situation where a breadboard can be created and modified by hand. However, when placed into a product, engineers must consider conflicting requirements when driving the design to an optimal solution. Successful teams use a robust design process combined with simulation to overcome these challenges. 

A Cooperative, Multidisciplinary, and Iterative Design Process

The industry developed the subdiscipline of optomechanics to address the need for a more iterative and multidisciplinary design process. Before its introduction, optical engineers developed an optical design and sent it to a mechanical engineering team to figure out how to hold, move, and protect the optics. This disjointed approach would often result in a design that didn’t meet the optical specifications or costly fixes late in the design process. 

To solve this problem, companies form multidisciplinary teams with engineers who understand the unique mechanical aspects of optical systems and the basics of optics to consider both areas when making design decisions. Clear, frequent, and concise communication is fundamental to the success of any multidisciplinary team. 

In addition, the design process must be iterative, allowing the evaluation of design changes in both areas. Tools must be in place that enable geometry and tolerance information to move back and forth between disciplines. Optical system design usually follows the standard conceptual, preliminary, and final design phases, with iterations taking place at each step. Effective teams leverage simulation, prototyping, testing, design reviews, and proper engineering documentation to find and resolve problems early in the design process. 

Simulation-Driven Optomechanical Design

Both optical and mechanical simulation play an important role in meeting and overcoming the challenges of optomechanics. Creating a virtual representation of the design allows engineers to quickly understand the performance of the design from an optical and mechanical perspective, as well as how the two interact. 

A typical simulation workflow for optomechanical design takes the geometry derived from an optical simulation and passes it to mechanical design tools where the mounting and enclosure design is specified. 

Engineers use structural, kinematic, computation fluid dynamics (CFD), and thermal simulation packages like Ansys Mechanical structural finite element analysis software that leverage the finite element analysis (FEA) method to simulate various aspects of the mechanical design. They then apply environmental loads such as forces, acceleration, shock, vibration, and temperature changes and calculate how the assembly responds.

Once the simulation has obtained an estimate of how the system behaves under load, engineers pass the resulting physical distortion and calculated tolerances back to the optical simulation tool to be run and checked by optical engineers to see if the optical performance is within acceptable bounds.

A more efficient simulation workflow uses a tool like Zemax OpticStudio for component-level design, which integrates directly with Mechanical CAD with a workflow that includes a growing number of optomechanical design and simulation capabilities within the optical design software itself. Zemax OpticStudio Enterprise takes this workflow to the next level with integrated multiphysics loading, fitting, and visualization tools.

Engineers can also leverage system-level optical design and validation tools like Ansys Speos CAD integrated optical and lighting simulation software to evaluate other optomechanical considerations. Speos software allows for the evaluation ofstray light reflecting off of mechanical components, blockage of light by optomechanical components, or vignetting, which is the dimming of saturation or brightness on the periphery of the beam path. A system-level validation can also look at the quality and shape of the focus and spot size at the detector. 

Optomechanics has rapidly evolved in recent years to address the increased use of optical systems in a wide range of industries. These industries count on several cameras and other sensors for:

• Consumer products
• Medical devices
• Photography
• Metrology
• Optical communication
• Manufacturing automation
• The Internet of Things (IoT)
• Earth observation
• Aerospace and defense applications
• Automotive sensors
• Autonomous systems lidar and optical cameras
• Scientific instruments
• Astronomy

New advancements in manufacturing methods, improvements in material science, miniaturization, and greater computing resources — that can handle the processing and storage of optical information — drive this evolving list of applications.

All of these changes are driving the need for improvements in optomechanics. 

Here are a few trends that engineers should be aware of and prepare for: 

Continued Miniaturization

Improvements in materials and manufacturing processes are pushing the size of optical assemblies and the optomechanics supporting them to smaller and smaller sizes. As parts get smaller, the complexity and precision of structural components must improve.

Smaller designs are also more sensitive to temperature changes. Miniaturization also makes physical testing harder, increasing the need for simulation to virtually prototype optomechanical designs.

Evolution of Adaptive Optics

Actively changing the shape and, therefore, the optical properties of lenses and mirrors is a promising way to compensate for distortions caused by mechanical and thermal loads. These real-time adjustments require outstanding control software coupled with fast and accurate electromechanical actuation.

Properly designing effective and affordable adaptive optics requires a proven optical design process that includes a strong optomechanics workflow.

Additive Manufacturing

The use of additive manufacturing (AM), also called 3D printing, gives optomechanical engineers new design freedoms for creating complex geometries that can drastically improve mechanical robustness and thermal management.

AM enables the creation of complex assemblies as single parts or the integration of cooling into the structure. Modern AM systems can create highly accurate metal, polymer, and carbon-filled polymer parts. 

More Demanding Operating Environments

The growth of optical system applications also means operating optical instruments in harsher environments. Temperature variations and loads are increasing since the devices are no longer inside a controlled environment.

A great example of this is optics applications in autonomous vehicles. Automotive designers are adding more cameras and lidar sensors, which see harsh vibrations and extreme temperatures. 

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