New design philosophies have allowed
monolithic optics solutions for photonics
applications to enter the mainstream.
Discrete optics that are individually mounted and manually aligned in optomechanical assemblies work well in a laboratory setup, where technicians can maintain alignment and adjust for environmental factors. However, for commercial implementations that require reliable performance and long-term system stability, the traditional architecture is more of an encumbrance.
Building on several decades of optics technology, Agilent Technologies Inc., Santa Clara, Calif., has created a new optomechanical design principle called complex monolithic optics (CMO). They have also launched a development program in support of this technology to help customers take advantage of integrated optics design. Potential savings include size, cost, and upkeep. Two years ago, the company decided to enter CMO production based on inquiries from customers. The decision reflects an idea whose time has come in the optics world.
Piecing together an optics puzzle
Monolithic architecture allows Agilent to use tighter tolerances to build a more compact interferometer than a similar metal-encased device. Image: Agilent Technologies Inc.
The emergence of better optical and fabrication technologies has allowed monolithic optics to become a reality for even conventional research purposes. Computer-enabled design enables tighter tolerances from start to finish and is now available to designers using glass, crystal, and ceramics as their building blocks. When Agilent decided to commercialize CMO, R. Todd Belt, optomechanical engineer at Agilent, helped develop a list of design considerations. He recognized the need to take manufacturability into account at every step of the design process.
When developing a CMO, residual stresses and degrees of freedom must be reduced, as well as the mismatch in coefficients of thermal expansion. These come about by way of random relative movement, bulk geometry, birefringence, and wavefronts. CMO technology eliminates these hurdles.
“You have to have good thin-film coating expertise because all of the coatings end up being immersed,” says Vince Barich, operations manager for Precision Optics and Assemblies at Agilent. The idea for CMOs emerged, in fact, from refinement of coating techniques and the introduction of discrete optics with multiple coatings and optical features on the same face. Researchers realized the next step could be combining the optical elements together without the use of casings.
A significant design point is failure analysis. Stress analysis is difficult because critical failures in optics come from random local dislocations. These so-called brittle critical failures mean that monolithic optics fabrication is a delicate process that should minimize handling and maximize inspection. Optics materials are expensive—two to three times more than stainless steel—which means high-precision machinery will be desirable during assembly to reduce costly waste.
Agilent employs a nondestructive sub-surface damage testing method. The polishing process, for example, may cause damage. Hot spots or optical perturbations can be detected through metrology and the process requirements adjusted to eliminate defects.
Another important consideration is how adhesion bonding will be used. Vulcanizing sealants, epoxies, solders, and fritting are all options which are affected by cure time, thickness, surface finish, outgassing, and thermal expansion issues.
Monolithic: smaller and more precise
By matching radii of the optics, two materials with different indices of refraction can be bonded monolithically. Elimination of parts has obvious advantages, including lower weight, smaller size, and tighter tolerances. In addition, Belt cited higher natural frequency, lower thermal drift and drift hysteresis, lower outgassing, and fewer degrees of freedom when using CMO technology. All this adds up to precision in a smaller package, perfect for defense and aerospace industry applications.
“For the customers we work with it’s not only smaller, it’s lighter. If you take a very tight system with a lot of equipment, CMO can let you put more stuff in the package,” says Barich.
As an example, a monolithic multi-axis interferometer designed to the same size as a traditional glass-and-metal device will give the same beam pattern but would be smaller in size. A monolithic setup can incorporate elements such as an optical fiber coupler, corner cubes, a polarized beamsplitter cube, reference mirrors, waveplates, and displacement beamsplitters.
Mainstreaming monoliths
CMOs have been thought of as high-end solutions for specific design requirements, but Agilent has realized they could fulfill specific customer needs without estimating first. They could go to the customer, bring clients into the optics design process, and, using Agilent’s “kit bag” of optical devices, develop a new device. The company’s optical scientists, engineers, and in-house production team develop the solution in cooperation with the customer. Monolithic designs, incorporating proprietary optical fabrication techniques, thin-film coating expertise, and sub-surface damage control, are created using deterministic manufacturing controls and verified with metrology. The process, says Barich, can mean a production line time savings of months.
“We can turn around a CMO in four to six months. For traditional optics, it might be a year,” he says.
The practicality of CMO technology for a company depends heavily on system requirements. Low-cost applications don’t always merit work in designing monolithic components. There are issues with alignment, performance, part count, contamination, and vibration.
“CMO is a little agnostic to what the marketplace requires,” says Barich, because it’s intended to fulfill specific process needs. Alignment and contamination issues are reduced or eliminated during the CMO construction process and downtime in minimized. System installation is also easier, which helps lower ownership costs. Agilent’s datum faceplate can also be used to drop in a replacement monolithic device.
Currently, Agilent’s major CMO customers are in the aerospace and defense industry, but the company reports the potential to expand significantly to other industries.
“The future of cost-effective optical system design will depend on smaller, lighter, more reliable optical assemblies,” says Barich.
