Beam Properties, Alignment, and Polarization
Beam Properties, Alignment, and Polarization
Thermal and Mechanical Mounting of Shutters
User Built Circuits
Each laser shutter has an optical rating that must be observed to avoid damage. Factors include rated wavelength (for dielectric mirrors) or range of wavelengths (for metal mirrors), CW maximum power, peak energy density (fluence), recommended beam diameter, polarization vector, and alignment control.
Laser shutters are designed for typical beams that use 50-80% of the aperture diameter. This allows good thermal dissipation on metal mirrors and keeps the damage threshold at a high level. When a small beam is to be used in a larger aperture, and the fluence or CW power is high, consult our sales engineer for estimated thresholds of optical damage; we have the data.
Beams need to be aligned entering the input aperture to ensure the dumped energy is reflected to the proper location. Dielectric optics requires good alignment practice to provide the highest reflectivity and damage threshold. As a rule, the entering beam should be orthogonal to the input face within about 5 degrees.
Some advantage can be achieved in exposure properties by locating a smaller beam in a particular location within the aperture, on the axis of mirror movement. This can enhance the open or closing speed, by a small amount, typically 20%. NEVER direct a beam into the output aperture. Materials at this point are not intended to be exposed to radiation.
Some laser shutters use near-grazing incidence to achieve greater reflection. If the laser is polarized, align the polarization vector with the label on the input aperture. This provides lower operating temperatures on the mirror and less sensitivity to contamination. Damage thresholds are increased. At lower power levels, polarization alignment is not required.
Laser Shutters must be rigidly mounted to keep the beam aligned in the aperture. Heat generated inside the shutter must be conducted through with the mounting. Both can be achieved with simple mounts, to bases, chassis, chiller plates, etc.
There are three basic principles to adhere to: the mount must be made of high thermal conductivity material, there must be a large cross section of material from the laser shutter base all the way to the “infinite” large mass, and the mount or hardware in close proximity must not be ferromagnetic. This usually leads everyone to use aluminum.
The laser shutters deliver internal heat to the base-plate, so you must mount here to achieve good thermal flow. We don’t recommend any pads or grease, since the surfaces are flat and large area. If an “infinite mass” such as an optical table or frame is not available, consider a water chiller plate. Many small units are available commercially at low cost.
The mass you conduct to will become a thermal capacitor, so a small block will heat up fast and then the laser shutter will overheat. Depending on the duty cycle of the laser shutter, solid mounting may not be mandated, but one should plan for a worst case scenario where the shutter is left on.
When laser shutters exceed about 50 degrees C, performance is degraded, and above 80 degrees C most shutters will fail to open, signaling a thermal overload. We use very low out-gassing materials, but out-gassing is accelerated by temperature, so keep your shutter temperature in control. Temperatures exceeding 110 degrees C will cause irreversible damage to the polymers.
Keep away from magnetic materials and devices. Farady rotators and other magnetic devices can modify the laser shutter magnetic field. Mounting brackets must never be ferromagnetic, which is also a poor heat conductor.
In situations where you want to isolate the vibration and shock from any experimental hardware, consider a water chiller plate, or in some cases, air cooling. Then you can “suspend” the laser shutter mechanically while still receiving cooling. Sufficient air cooling for small packages is not simple, so check with our staff before attempting air cooling. Some models have very low power dissipation, simplifying mounting.
In vacuum applications, the common mount is an aluminum bracket from the laser shutter base to the vacuum vessel wall, or an internally welded shelve.
Many of our laser shutter models can be driven from simple DC circuits. Cost savings can be realized by designing and building simple circuits for your specific application. This is common for our OEM customers, who usually have a DC power supply in their main electronic system.
Our laser safety shutter models were designed to be driven easily from simple DC circuits. The high speed laser shutters require sophisticated circuits, which are difficult to design.
Our recommended low-cost circuit for the process and safety laser shutter models is the Capacitor Discharge Circuit shown on our Electronic Shutter Controller Technology page. The benefits are lowest possible cost, easy layout, and low part count, enhancing MTBF potential. The disadvantage is power dissipation in the voltage dropping resistor, and the associated heat. If adequate heat sink surfaces are present, and the electrical power budget is not near maximum, this circuit should be given high consideration, for rapid implementation.
For an electromechanical device such as a laser shutter, this circuit provides a smooth, adiabatic transition from the higher “Boost” voltage down to the long term “hold” voltage. The exponential voltage and current decay curve is also the “pulling force” curve of the electromagnet. This force decays as the mechanical flexing spring force increases, yielding a softer opening stop and very little mechanical bounce recoil, as the flexure arrives at the open state.
Alternative circuit approaches usually apply a “Boost” voltage for a timed period, then quickly drop to a lower voltage for holding long term. This is more efficient. The lower voltage is achieved with Pulse Width Modulation techniques, either using a separate inductor, or the laser shutter electromagnetic winding itself as the switching inductor.
There are some practical limits, since the laser shutter can be of high inductance, and most PWM are designed to operate above audio frequencies (~30KHz). A variant is to use two DC supplies, typically 24 V and 5 V, and switch between the two for Boost and Hold voltages. Many OEM systems have 24 and 5 V available.
When a laser shutter is ordered without a controller, it is assumed the user will build their own circuit.
Below are common issues that affect electronic performance:
a. Thermal mounting insufficient. The laser shutter electromagnet heats up and cannot pull the flexure open. For safety laser shutters, which dissipate from 1-10 W of electrical power, the temperature rise is usually from the optical load. DO NOT attempt to increase the hold voltage significantly to compensate for poor mounting. Stay away from magnetic sources or ferromagnetic material.
b. Laser Shutters must receive specified voltage/current. The forces are non-linear, so it is very important to make sure both boost and hold voltage are of proper value. If not, inconsistent opening will occur, much like a temperature problem. Check that your DC supply does not load down, the switch element such as bipolar or FET transistor are not a large voltage drop, and cable lengths should be of proper gauge to reduce voltage drops. On the capacitor discharge circuit you can increase the capacitor for more opening assurance margin.
c. Heat, vibration and shock from the external environment the laser shutter is mounted to will all affect performance. Use more holding voltage in higher shock and vibration environments. This also true when the ambient temperature is high.
d. Watch tolerances on electrical circuits. It’s real easy to watch a 24 V supply drop down to 20 V through connectors, cables, switches, and semiconductors. Use low loss elements.
e. Two-level circuits must be calibrated correctly for proper hold. If the boost signal is too short the laser shutter will not fully open. If marginal, the fast acceleration of the flexure and resulting velocity may cause a recoil instead of a latch to the open state. Of course a very long boost period will eventually settle, regardless of recoil. The ideal circuit switches from boost level (typically 24V) to hold level (typically 5 V) when the flexure is about 75% through it’s excursion to open. There should be no significant dropout in voltage between boost and hold, unless properly accounted for.
f. Advanced circuits can apply a “Brake” impulse just before the shutter closing to damp the impact. A “Current Ramp” on opening can damp the open impact. Our CX4000B PCB achieves all of these features.
Laser shutter lifetime is measured by mechanical failure and optical failure. When operated correctly, the optical elements can have near infinite life if kept clean. Extreme engineering of materials has yielded minimum particle generation. Purge ports are available for the most demanding semiconductor applications.
Some models allow for cover removal and optics cleaning procedures. Mechanical lifetime is controlled by elastomer and polymer bonding techniques, and stresses from impacts, both from opening and closing the laser shutter. This can be controlled by the electronic controller waveform damping feature.
We manufacture a wide variety of products, drawing compromises for intended markets. The Safety and Process laser shutters are designed to provide over 100 million cycles with recommended control circuits, and some are designed for over 1 billion cycles. We have tens of thousands of products produced that demonstrate the lifetime performance. We routinely run large batch life tests on revisions and new products.
Some of our high speed laser shutters operate at very high velocities (6 m/s) and are designed with finite lifetimes in the 100-500 million cycle range. Controllers can be set to compromise speed vs. lifetime. The standard settings represent the general market’s desired level of compromise.
Optical damage can be avoided if kept clean and if operated at proper wavelengths. Severe optical damage can affect mechanical elements. Most optics can be replaced if damaged (LIDT).
Electrical damage can occur if thermal management is poor. This is irreversible damage, but does not occur under proper operation. Our wet-wound electromagnets have the highest reliability and thermal conductivity to the mounting surface.
This technology represents the highest reliability in a mechanical laser shutter product. In applications where lifetime is much more important then speed, consult our sales engineers to assure your user-built control product is delivering the ideal waveform to the shutter.
Many of the optical and laser shutter models are routinely used beyond 1 billion cycles in OEM equipment, primarily high speed processing. The inherent nature of this technology allows us to demonstrate such performance over the course of just a month or two of life testing.
We are able to offer life test results to OEM customers once the electrical drive has been chosen and the laser shutter model, with all options, is chosen. They are a pair, and the life time performance is dependent on each other.
We use low out-gassing materials, so make sure not to add any contaminants to your optical system. Both temperature related out-gassing and photochemical reactions can create films that will degrade optics.
Product enhancements and revisions over the years have resulted in the lowest possible particulate generation over hundreds of millions of cycles. This protects the mirrors and absorbers internally and mitigates any ejected material from the shutter apertures. Additional features are added to the most critical applications, semiconductor clean rooms, where purge ports are added to the shutter for negative pressure flow.
We do not recommend silicone grease for mounting or the use of any tapes on the laser shutter or beam containment tubes. In manufacturing and processing environments, take measures to keep the laser shutter clean from ejected materials while cutting or welding.
Never use compressed air to clean laser shutter; it is contaminated with water and oil vapor. Use either a clean room vacuum or dry, laboratory dusting gas, such as nitrogen. Large particles can inhibit laser shutter flexure motion. This can occur if not properly packaged when shipping, or if ferromagnetic debris is pulled into the aperture, including small nuts and screws!
ALWAYS bag and seal the laser shutter in plastic before shipping to avoid contamination into apertures.
Jitter is not a hard spec on our laser shutter models, since it varies with controller electronics, user-built circuits.
Jitter and/or “Bounce” is influenced by three factors in our designs: Changes in Force, Stiction, and Friction.
The primary factor is changes in Force, which is 100% controlled by the current in the electromagnet. Temperature changes alters the copper wire resistance resulting in a current change – but only for voltage-controlled electronics. Using current-controlled electronics such as our CX4000B PCB, the forces do not change. Repeatable forces allow smooth, jitter-free operation up to the maximum rated shutter repetition rate.
Friction and stiction refer to the physics of the open and closed resting surfaces, and are virtually zero influence in our flexure designs. The use of advanced properties polymers yields virtually zero wear, friction and stiction even at elevated operating temperatures.
Using a current-controlled drive requires that the thermal mounting is proper for the power dissipation load, electrical and optical dumping. Current-control does not recognize heat build-up and thermal run-away can occur if the shutter is un-mounted or poorly mounted. We recommend our CX4000B PCB for the absolute fastest speed, jitter-free motion, and power dissipation efficiency.
The maximum repetition rate that the high speed laser shutters are rated for is generated partly due to increased jitter. Above the maximum repetition rate, the jitter can start to become large compared to the exposure length. Centering the beam in the aperture will reduce jitter.
There are some dynamic processes in the flexure movement that create some very low jitter “sweet spots” at high rep rates, but in general one should evaluate jitter for the application. At low rep rates, the high speed laser shutters yield about 5 microseconds of jitter, increasing to about 50 microseconds in the 200 Hz range.
It is somewhat controllable with the controller waveform, so if critical to your application, let our staff know. An alternate model or carefully calibrated controller may be a solution.