Alignment problems are frequently encountered in the manufacture of optical system. Typically, optical components must be aligned with each other with a precision on the order of 1 micron to minimize losses within the system. There are a number of solutions to this problem that are currently being implemented in the industry.
U.S. Pat. No. 6,292,499 B1 reveals a technique that uses resistance heaters on the optical mount to melt solder placed beneath it, allowing movement of the mount for optical alignment. The optical mount is a T-shaped ceramic piece such that the material properties of the ceramic can help damp harmful effects of the optical component due to vibration. The optical component is mounted on the upright portion of the T and the resistance heater is added to the horizontal portion of the T-shaped mount. Solder is added to the bottom of the resistance heater and when melted, it is used to attach the optical mount to the optical bench. Small electrical wires are attached to the resistance heater, providing the path for a voltage drop across the heater when movement of the optical component is needed. Repositioning is straightforward if an alignment mechanism is used. When the optical component is in place, the voltage can be turned off, allowing the solder to cool and locking the mount in place. However, wires and resistance heaters take up valuable space within an optical system. It would be better if the optical components could just be aligned, locked in place and put into the optical system with as few parts as possible.
Another solution involves etching V-grooves in silicon substrates to be used for optical alignment. U.S. Pat. No. 6,344,148 B1 uses the process of chemical etching to produce V-grooves that serve as alignment marks, solder containment regions and for aligning optical fiber. The optical component is placed on the substrate and is optically aligned with a microscope, using the etched V-grooves as markers. Solder is then placed in a second set of grooves to adjust the height of the optical component and to clamp it to the substrate, completing alignment. The alignment is based on human vision and the accuracy of results confirmed by looking through a microscope. This process is tedious and time-consuming, especially if the optical components are aligned passively. It would be more beneficial to use translation stages and an active alignment technique rather than trying to passively align the optical components by “feel”.
The previous techniques all use solder or adhesive to mount optical components once they have been aligned. Solder and adhesives can present their own challenges and there have been few innovative solutions created that make working with these elements easier. For example, flux is always needed when using solder, and problems have come about from using flux in certain applications. U.S. Pat. No. 5,249,733 explains that when trying to mount optical components to a substrate, the flux can bubble up under the heat from the soldering process and cause the optical element to fall out of alignment. The solution to this problem was to create a gaseous formic acid shield around the flux during the alignment operation, to allow the solder to form symmetrical configurations under the optical element that are needed in the process of self-alignment. The formic acid atmosphere eliminates the need for flux and the problems that come along with it.
According to U.S. Pat. No. 6,632,028, a eutectic solder is used that reacts with a component in the bond pads of the optical mount. This chemical reaction causes the solder to harden and lock the optical components in place. Most importantly, damage to optical components and the substrate is eliminated because damaging thermal energy is not needed to melt the solder.
Adhesives are also used for locking optical components in alignment. U.S. Pat. No. 5,644,837 explains a method of using thermoplastic and thermosetting resins to attach optical components to a substrate. Microwave energy is used to cure the resins, speeding up cure time and preventing the substrate and optical components from being exposed to harsh cure environments because the microwave energy can be localized and shielded from other components.
U.S. Pat. No. 4,807,956 reveals a technique for aligning an optical fiber using solder or polymer preforms. The preform is wrapped around the fiber at the location where the fiber is to attach to the substrate. The preform and fiber are set on top of a resistor that is used to heat the preform to a temperature that will soften it and allow movement of the fiber. At this point, the fiber can be aligned with an optical component or laser and the current going to the resistor should be shut off. Cooling of the preform sets the fiber in place. This process uses localized heating to melt the preform and keep heat away from other optical components.
As mentioned above, solder and adhesives have always played a leading role in the alignment process but they have not always been easy to work with, especially in the case of removing an optical component from a mount. It has been necessary to develop techniques for quickly and easily rearranging optical components on mounting brackets to avoid ruining a component or wasting valuable time.
Finally, it is necessary to mention that the general manufacturing process for optical mounting brackets has been precision machining. Machining, however, can be a time-consuming and expensive process and it is a process that many companies are trying to avoid.