You’re evaluating and designing in a commercial mechanical shutter component for a system only to find the data sheet has less than half the information you need to qualify it. Questions arise, repeated inquiries to the factory, all delaying your effort. Then there’s no data available for a particular spec you require, so it is suggested you try it out, work back and forth with the manufacturer to develop a new spec. If you’ve been down this path you know a technical product that provides complete and thorough specs is a design engineer’s quick guide to performance evaluation and a reliability engineer’s early forecast of the potential to meet an OEM’s quality goals, machine life and periodic maintenance preferences. Best example? Think about data sheets for semiconductors, say a power FET. Everything you need to know is presented, including life data, temp de-ratings and safety precautions. You select with confidence and your project charges forward without delay.
If a mechanical shutter is a new technology to your team, you likely will not know all of the specs you will need as you proceed with the design. This often forces a change of device and delays the project. Long established, quality manufacturers with primarily OEM equipment customers develop complete and thorough specs. Often, specs are revealed that the customer did not even realize was important to project success. Key specs are stated for each product, then intense knowledge of applications and physics allow the application sales engineer to support exotic or unusual cases with near immediate responses. So what are the key specs that you really need to get started? Below are the minimum to evaluate. For lasers, all apply. For optical shutters (low power), such as imaging, irradiation damage specs are not needed.
Aperture Size
You need to get the beam thru it! With Gaussian beams typically the aperture is chosen about 3X the 1/e2 points to avoid diffraction rings from input/output aperture edges. But, when speed is more important, you may tolerate diffraction for a smaller aperture and fast switching.
Package Size
It’s got to fit in your housing. Note the location of the aperture within the body, geometries can be asymmetric. Make sure the depth thru the body can accommodate a diverging beam if you are not collimated. Copper or Aluminum bodies depending on rated thermal load.
Position Sensors
You may not need sensors if you are confident in the reliability of flexure mechanics. Sensors are independent audit circuits, typically 5V logic or micro-switches to monitor the location of beam interrupting elements, completely isolated from the magnetic drive creating the motion.
LIDT
Laser Induced Damage Threshold. Most lasers today are pulsed, so LIDT becomes a mandatory spec to evaluate damage to mirrors, absorbers as well as outgassing and particle generation. Usually given as Fluence, J/cm2 in the low nano-sec range. Extrapolate to other pulse-widths with formulas. An experienced staff can evaluate LIDT at any wavelength or pulse-width.
Optical Power
Either CW, or more commonly pulsed (repetition rate of laser)x(energy per pulse). This is your average power that the shutter needs to absorb cleanly and flow to the baseplate for thermal mounting, heat extraction. Mirrors and absorbers are rated for a wide variety of wavelengths.
Magnetic Power Dissipation, Repetition Rates & Thermal Mounting
If you choose speed, you’ll use more electromagnetic energy to move mass than if you have a safety shutter application. It’s non-linear, so squeezing out faster and faster switching speeds has quickly escalating heat generation. Shutters are rated for continuous rep rates, long term holding open, and closed optical energy absorbing thermal loads. Thermal mounting is much like power semi-conductors. Heat generation (efficiency) is a trade off with switching speeds.
Repetition Rate
How fast can it cycle?? Repetition rate is the fastest continuous cycling rate. Power dissipation increases with rep rates. Most manufacturers limit the rep rate to short bursts due to inadequate magnetic thermal control, which is key for electromechanical device reliability.
Lifetime
Key to reliability is lifetime, either via wear considerations or catastrophic damage. High rep rate, scalable designs allow rapid validation of lifetime specs. Flexure devices with well- designed constraints achieve several Billion cycles lifetime, proved thru large batch, long term high rep rate testing. Particle generation and outgassing is also key to lifetime.
Shock, Vibration & Noise
Custom designs emphasis these parameters if a compromise to speed is allowed. Moving the smallest mass possible is the key, and using critical damping with sophisticated current controlled electronics allows one to almost eliminate these three items. A user can switch between highest speed and quietest, lowest shock and vibration modes using such a controller.
Failsafe Operation
Compare any technology to flexure for reliability and you’ll find a cantilever beam flexure is un-matched in reliability when designed to avoid fracture and stiction (from motion constraints). It is critical when requiring a high SIL safety rating. Gravity is not sufficient for most instruments.
Forward and Backscatter
Geometries, surface morphologies and atomic (molecular) absorption are used to make sure backscatter to source is on the order of 10-6 and diffuse, and forward scatter leakage (far off axis) is even lower. Specs for custom devices can be generated anytime this is a key performance issue. For safety shutters, achieving far below Class 1 limits, and only with scattered photons, is easily achieved.
Wires/Cables/Connectors and Weight
Standard products are a starting point. These commodity options are changed as an OEM’s needs are presented. Even weight, thru unique materials, is customized for avionics.
Jitter and Bounce Recoil
Jitter is created when forces change, stiction is present, and friction occurs. Stiction and friction can be designed out to a third order effect. Force changes typically come from poor thermal management affecting the electromagnet resistance/current. Complete and settled arrival of the moving element in the open and closed position, overcoming bounce (impact recoil) and latching in position is the key to eliminating jitter. Sophisticated current driver electronics achieve this goal up to the switching speed ultimate limits. Jitter control allows the most precise exposures, typically tens of microseconds for small aperture shutters.
Final Note About Specifications
Some specs are easily measured and should appear on every manufacturer’s data sheet. Others are more application specific, difficult to quantify without customer beam and electronic driver data. Manufacturers with intense knowledge and experience in material science and physics can help you address custom specs and understand engineering compromises employed in the inter-dependent properties of advanced mechanical shutters for lasers and optics.
Being a long term OEM supplier, we present the most relevant specs on our product selection guides. But as this article addresses, keep in mind all of the specs your manufacturer should have ready to supply if they know their product well. With hundreds of thousands of products in the field, product application knowledge at NML has resulted in highly refined devices with complete and thorough specs. Designs are driven by many industries and qualified for a majority of Hi-Reliability Capital Equipment machines operating in the 24/7/365 semiconductor industry.