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Lasers & Sources

Designing for Stability

A monolithic mount for laser cavity mirrors eliminates long-term slip and creep.

From oemagazine February 2004
31 February 2004, SPIE Newsroom. DOI: 10.1117/2.5200402.0007

In most scientific and industrial lasers, intracavity laser mirrors are typically held in adjustable mounts that can contain an average of 12 separate metal components. These provide the requisite θX and θY adjustment through a combination of fine-pitched screws and some type of flexure or spring mechanism.

For some lasers, the mechanical complexity of this approach introduces some limitations. Performance requirements dictate fine threads for precision, with substantial mechanical stability. The problem is that no one has ever managed to create a perfect locking mechanism that does not affect the alignment. Even tightening a simple locking screw, for example, can be sufficient to misalign the cavity. The inability to permanently lock these mounts can lead to long-term degradation in laser performance if the precision mount is not assembled correctly.

Over the past several years, we have been working on an alternative mirror mount design that could eliminate these issues without adding additional cost to the laser. This work has culminated in a new type of monolithic, deformable design that can be fabricated from a single piece of metal.

There are three distinct parts of this monolithic structure: the base, the stem, and the upper (mounting) section. As with other cavity mounts, the optic is held in place by a non-outgassing optical cement. To adjust, the user inserts a fork-shaped adjustment tool into holes on either side of the upper mount. Permanent θX and θY adjustments can be made independently by either twisting or bending the mount with this tool, thereby slightly deforming the stem section. Although the design is simple in principle, successful implementation required careful design, analysis, and testing.

The first step was to choose a suitable material that could be deformed, yet still provide long-term rigidity. A plot of the theoretical relationship between stress and deformation in metal shows a linear portion that represents the elastic region. The applied force must push beyond this into the plastic region in order to achieve permanent deformation. Analysis indicated that fully annealed aluminum was the ideal metal to permit this deformation at realistic stem diameters.

We then went through several iterations in mount shape. A conventional square-mount section seemed intuitively a good place to start; however, tests on prototypes proved that adjustment could loosen the optic. Finite-element analysis showed that this occurred because the stress was not all confined to the stem but was spreading into the mounting interface. We solved this problem in the final design by switching to an inverted T shape, which blocked the stress from reaching the optic interface.

The mount base is bolted to the laser baseplate. In initial designs we used a single mounting screw but thermal shock testing indicated this did not provide sufficient stability. In the final design, we switched to three mounting screws set in a D-shaped base. The use of three screws also provides precise registration, eliminating the need for overly large adjustments that could push the metal past the rupture point.

This mount is now being used in commercial lasers. In production, it takes only minimal practice for a technician to become proficient at adjusting the mount. The lasers are tested by thermal shock before shipment. Long-term studies have indicated that the biggest environmental impact on laser stability and performance is the thermal cycling due to daily on/off operation. We have found that these long-term effects can be accurately simulated by exposure to ultracold (-40 °C) temperatures for a few hours. Tests on lasers with these new mounts show virtually no change in laser performance after this grueling test. oe

Jim Clark, Pascal Champagne
Jim Clark is director of reliability and optics engineering, and Pascal Champagne is an optical engineer at Spectra-Physics, Mountain View, CA.