UV Laser Optics, CaF2 Optics, Excimer Laser, Spherical Lenses, Cylindrical Lenses

PRECISION OPTO-MECHANICAL ASSEMBLIES BENEFIT SYSTEM BUILDERS

By Wayne Pantley and David Collier, Alpine Research Optics

David Collier is President and Wayne Pantley is Sales Manager of Alpine Research Optics, 3180 Sterling Circle, Boulder, CO 80301, 303-444-3420, FAX 303-444-1686, E-mail AROcorp@AROcorp.com, http://www.optics.org/arocorp/

It can be advantageous for system integrators to purchase opto-mechanical sub-assemblies, rather than sourcing loose components and performing the assembly themselves. For example, using such value-added sub-assemblies eliminates the need to have assembly personnel who are expert in opto-mechanics, minimizes the production floorspace that must be devoted to assembly, and also keeps the integrator's vendor list short. This article reviews the motivation behind the outsourcing trend and then examines some important aspects of the technology required to create precision opto-mechanical assemblies.

Outsourcing Advantages

It is now common practice for systems manufacturers in a broad range of industries to outsource the production of entire, finished printed circuit board (PCB) assemblies; indeed, this trend is now even being extended to opto-electronics subassemblies, where vendors are routinely being asked to integrate components such as detectors, amplifiers and signal processing electronics on to a single board (see Laser Focus World May 2000). However, this practice has not become nearly as widespread with regards to optics. Most systems integrators still purchase loose optical components, mechanical parts and mechanical subassemblies from separate suppliers and then perform any required assembly themselves.

This situation is now beginning to change for several reasons. First, laser technology itself is maturing and laser products are becoming more turnkey. While this is especially true of newer laser types, such as diode lasers and diode pumped solid state lasers, even older laser technologies, including helium neon, argon ion and excimer, are achieving new levels of reliability and ease of use. In many applications, the laser starts to be perceived as just another component or subassembly. Users are able to simply drop the laser into their system, and no longer expect that they must maintain a significant amount of in-house optical and electro-optical expertise in order to utilize the technology.

As maturing laser technology reduces the need to keep a staff of experts in optics and opto-mechanical assembly, these OEMs must then carefully examine if it makes more sense to still retain some capability in this area, or if it is better to simply outsource the entire process. Typically, the most important criteria used in this "make or buy" decision are cost and cycle time. Cost includes not only the obvious labor component just mentioned and the costs of assembly tools and test instruments, but also the production floor space that must be devoted to the assembly process. This can be quite a significant expense in more demanding applications, since precision opto-mechanical assembly must typically be performed under cleanroom conditions, and cleanrooms are expensive to build, maintain and operate. Thus, outsourcing can move assembly from being a fixed part of overhead costs to a variable expense.

In the same vein, outsourcing enables a system builder to reduce production cycle time; besides offering a cost advantage, this also improves competitiveness. Utilizing the manufacturing capacity of one or more subassembly vendors in parallel allows production of a given item to be ramped up more quickly and sustained at a higher pace. The need to maintain a large production line headcount internally, or to have a flexible manufacturing force, is greatly reduced.

Another important trend that is increasing the desirability of outsourcing subassemblies is the development of various reliable, high power, ultraviolet (UV) lasers (especially frequency multiplied solid state lasers). Optics for UV lasers are simply more sensitive than their visible and infrared counterparts. Deep UV coatings must be cleaned and handled very carefully, scrupulously avoiding the introduction of any contaminants, to prevent damage when used in high power laser systems. Furthermore, a given flatness specification is much harder to achieve in the UV than in the visible. For example, a l/10 flatness specification at 532 nm is only half the actual surface flatness of l/10 at 266 nm. Thus, maintaining a given surface figure throughout the coating and mounting process becomes ever more difficult at shorter wavelengths.

Opto-mechanical Assembly Requirements

Alpine Research Optics (ARO) has been involved in the production of high damage threshold, deep UV coatings for 9 years. Over the past few years, we've experienced a growing demand for assembled products, and found that offering subassembly services provides a competitive advantage over other optics suppliers that produce just substrates or coatings.

Based on our experience, we've identified several key areas of expertise that must be mastered in order for an optics supplier to successfully produce high precision opto-mechanical subassemblies. First, of course, is the ability to build or buy both optical and mechanical components of the requisite quality level. One important point that should be brought out in this connection is the problem with obtaining substrates from one vendor and having them coated by another. High damage threshold, deep UV coatings in particular place stress on an optic, distorting its surface figure. Thus, in order to produce very flat, coated optics (e.g. l/20), it is necessary to understand the stress characteristics of the coating, so that these can be taken into account during the substrate polishing process. Depending upon whether the coating is compressive or tensile, the part may have to be contoured so that the coating stress will pull it into the desired shape, rather than away from that shape.

Producing precision opto-mechanical products also requires the ability to design any necessary assembly tooling, having the appropriate personnel and facilities for performing the mating (e.g. cleanrooms), as well as the ability to measure total performance (typically autocollimation and interferometry) of the sub-assembly. Finally, packaging that avoids environmental contamination, such as nitrogen backfilling, may even be required.

Precision Assembly Techniques

In order to understand the issues encountered, and the type of equipment and expertise that must be applied in precision assembly, it is useful to examine a couple of representative examples. Perhaps the most basic and commonly encountered opto-mechanical assembly task is placing a round mirror in a mount that retains the optic using a single set screw. This may seem incredibly simple, but it is actually difficult to perform while routinely maintaining a flatness specification of l/10 or better. The problem is that the set screw places an asymmetrical stress on the optic, which can push it out of figure.

In order to perform this type of assembly to high precision, it is necessary to interferometrically monitor the process. This allows the effects of screw tightness on surface flatness to be assessed dynamically. One remedy used to counteract or lessen the screw stress is to shim the optic; another solution is to add a large pad to the end of the screw in order to spread out the force it places on the optic over a larger area.

An alternate approach is to first fasten the optic inside an annular metal collar, typically using cement, and then place this assembly in the mirror mount. The rigidity of the metal collar then largely protects the mirror from the screw force. This approach is particularly valuable for optics that must be periodically replaced in the field, as it allows service personnel to put a new optic in the mount and tighten the retaining screw with relative impunity.

An example of a more complex assembly involves bonding a UV mirror to a galvanometer scanner while maintaining l/20 mirror flatness and arc-second level alignment of the mirror to the mounting post. This type of assembly is found in LASIK instrumentation and various types of semiconductor process equipment, such as memory repair units. The first part of the problem in this case is the effect of the adhesive on mirror flatness. As the adhesive cures, it can change shape slightly, putting stress on the optic and deforming it from the specified flatness. One way to counteract this is to thoroughly characterize the effects of the glue on the optic beforehand. Then, the optics can be purposefully shaped during fabrication so that the stress introduced by the adhesive pulls them into greater flatness. Another possible solution is to put an optical coating on the back side of the mirror; this second coating, which has no optical function, can mechanically pre-stress the optic so that it is prevented from warping out of shape during the bonding process.

The mating of the mirror to the mounting post to arc-second accuracy requires custom tooling that allows the alignment to be monitored during the process by laser autocollimation. Active adjustment of the mirror position may then be required to compensate for any positional shifting that occurs as the cement cures.

Conclusion

Producing precision opto-mechanical subassemblies requires a thorough knowledge of the interaction of optical components and mechanical mounts, as well as an understanding of how the assembly will ultimately be used and required to perform. Thus, successfully engaging in this business requires close partnering with the system builder; ideally, the subassembly vendor should be brought in at the design phase so that their expertise can be utilized to create part specifications that are achievable on a production basis.