Vibration Isolation in Optical Test Systems
- Created: Thursday, 01 September 2011
In the world of precision motion control there is a continuous pursuit of higher performance, whether it’s resolution, speed, stability, accuracy or step size. Even if a specified device appears on paper to be capable of achieving excellent results, the design of the mechanics and the platform stability play an integral role in achieving peak performance.
There is certainly an aspect of “art” in
optical system design as these unique
inventors look for new, innovative techniques
to shape, bend, and manipulate
light, either through the creation of
new optical elements or by combining
multiple elements. Equally important
parts of the design are the mechanical
elements that support, move, or control
each component in the optical path.
Each of these mechanical elements contributes
to the stability (or instability) of
the output beam and can be a hidden
source of error if not properly designed
Vibration ControlDynamic PerformanceOptical Delay Lines
A common application where vibration platforms, optical components, and precision motion control must be carefully combined is the optical delay line (ODL). Optical delay lines are used to make minute changes in the total path length photons travel before reaching their destination. Common applications for delay lines include optical coherence tomography and pumpprobe investigations such as two-dimensional infrared and transient-absorption spectroscopy. In these applications it is critical not only to be able to achieve very small incremental motions but also to maintain beam position along the total path of travel. In some applications data is taken in a “step-and-settle” process but other applications require data “on-the-fly,” which requires more rigid components and careful attention to excitation frequencies of all components involved.
Newport manufactures an ODL kit (Figure 4) that allows users to select various levels of performance ranging from 100nm stages that provide 0.67 femtosecond (fs) delays, up to 1250nm stages that provide 8.33-fs delays. The ODL kit has been designed to provide the performance and stability required for critical applications. Notice in the Newport ODL the common use of 1.0- in. diameter posts and their minimal height since this provides the highest level of stability for optical mounts as shown in the testing results.
In the post testing results presented at the beginning of this article it was shown that the 0.5-in. diameter post exhibited 33.1 micro-in/lb displacement which translates to 0.840mm or 840μm motion when experiencing a 1- lb force. A 1-lb force is rather large for a typical optical experience but even at 1/1000th of that amount, it would translate to an 840nm movement, which would not allow users to achieve better than a 5.6-fs delay. Using a 1.0- in. diameter post in this same set-up would permit reaching a 0.72-fs delay. Although these displacement approximations represent a 4-in. post height, and shorter heights would enable better performance, consideration of the structural design factors that affect optical stability, including component resonance and stiffness, is critical in all applications that involve optical elements and precision motion control. These applications include micromachining, sensor characterization and calibration, laser imaging or optical material characterization.
In sensitive applications, susceptibility to vibration and the realization of precision motion is not only a function of the optical platform, but also of the selection and installation of the optical components. It is a function of how all of these elements interact with the motorized positioners that is essential to achieving the desired step size of position sensitivity.
This article was written by Vyacheslav M.
“Slava” Ryaboy Ph.D., D. Sc., Principal
Mechanical Engineer, and James Fisher, Sr.
Director, Newport Vibration Control Division
(Irvine, CA). For more information, contact Dr.