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Laser Shutters
Definition Of The Laser Shutter
The laser shutter is an optical and mechanical physical apparatus mounted at or near the output of a laser. It allows a beam to pass through unobstructed when commanded to open and safely terminates the specular beam with minimal backscatter when commanded to close. Closure takes place at the end of an operation or if the system experiences a safety breach. Well-engineered units provide a fail-safe closure on power loss or control signal loss. The beam is terminated by optical absorption inside the enclosure. A lightweight moving mirror system is used to steer the beam to the absorber, or beam dump, when in the closed position. The mirror and absorber optical properties must be rated for the demands of the laser irradiance. This separates a laser shutter from imaging-type shutters in cameras and vision systems and from rotary choppers. High reflectivity, high-damage-threshold mirrors, and high-absorption, ultra-low outgassing beam dumps are required in most laser shutters.
The goal of 100% throughput and 100% termination for open and closed states sets the laser safety shutter apart from any of the modulation-type devices, including electro-optic, acousto-optic, and liquid crystals. Rapid closing speed is a desired feature, attainable through careful design. A position-sensing circuit, using various switch types, is a common feature added to the laser shutter to independently monitor the state of the aperture regardless of the open/close command signal and power.
Role Of The Laser Shutter
The primary role of the laser shutter is safety. This falls in two categories: general safety of personnel and living organisms and economical safety protection of materials and lost time.
Of utmost importance is the protection of the laser operator, staff, and visitors in any part of the potential exposure zone. Second, research animals, living tissue cultures, and microbes under irradiation and study must have protection in the case of an irradiation problem. Irreversible damage or destruction of living organisms usually carries much more than just a material loss, especially in advanced biological research.
Exposure of personnel or other living organisms can come from many sources. The primary source is usually the laser beam itself or excessive scatter after incidence on other items in the beam path. Additional concerns come from the laser beam producing photochemical-induced toxic vapors, toxins in tissues, thermal burns, cell nucleus damage, and ignition of materials resulting in fire and potentially toxic smoke.
A laser shutter needs to react quickly to any signal commanding it to close so threats mentioned do not have significant time to develop. As the laser power levels increase, closing speed of the shutter becomes more important. A well-designed laser shutter can often close more quickly than an electrical power-down command to the actual laser can perform. Stored charge in capacitors and energy in inductors in the laser power supplies can sometimes delay the shutdown, whereas a typical shutter may close in under 10 ms.
The economic impact of a safety breach usually results in lost time or materials in production processing environments and research applications. If something goes wrong in a long train of elements, such as an optic burn, then the laser safety shutter will quickly block the laser and protect the rest of the system. Although there are many means to protect personnel, such as enclosures, curtains, and the like, the laser shutter is usually the final element to protect the system from component failure escalation and the loss of valuable processed materials and tools. The shutter allows the laser to keep running with full thermal equilibrium and stable modes, permitting beam exit on command. This sharply cuts time lost waiting for laser stabilization after a safety breach has occurred. The breach is investigated and acknowledged and exposure is restored without the long wait, usually measured in minutes; a long time in processing systems. Typical types of safety breaches seen in the economic sense are x–y table movement errors, processed material surface irregularities, and foreign objects entering the work area, such as material remnants or slugs. In each case, the system would detect a safety problem and close the shutter; the operator would quickly rectify the problem and the process would continue. A minute of time may have been consumed.
Another safety aspect of the laser shutter is the use in extremely important applications in which multiple shutters are used redundantly. Three shutters may be placed in series, opened one by one, to facilitate critical operations such as pyroinitiation of rocket propellants, explosives, or one-shot state-of-the-art research. Here, the economic impact can be quite severe, and the mechanical laser shutter is a layer of safety along with many sensor systems and controls.
Required Features And Performance
Because the laser shutter needs to absorb all of the energy entering it when closed and needs to reduce any backscatter to a very low level, the optical and thermal systems in the design must scale with the laser power. Fluence and power density become important issues to avoid mirror damage and absorber damage. The shutter needs to be a drop-in, clean element that does not generate significant particles or outgassing contamination. Usually, there are constraints on the shutter switching speed, aperture, physical envelope size, and available control power (electrical or pneumatic). A designated lifetime, usually measured in cycles open/closed, is often specified. Most designs will experience some mechanical wear, fatigue, and particulates over time. Because the laser is nearby and optics trains typically follow the shutter, particulates and outgassing are a major design criterion.
When closed, the shutter sees the full optical load and will experience a temperature rise. The internal beam dump absorber must be designed with excellent thermal conductivity in addition to optical absorption. This heat can then be conducted out to a convenient surface for cooling. Conduction and water convection are the most popular methods. Forced-air cooling is usually not preferred because of air gradients and turbulence around the optical zone. Water cooling is usually implemented above about 50 W. Most systems will absorb power levels lower than this into their system mass. Designs should be for the worst case, where the beam is continuously dumped in the shutter for long periods.
Position sensor systems include mechanical microswitches, opto-interrupters, and magnetic reed switches. The independent audit function serves to validate that the command to open actually occurred, and the shutter is not at some point between open and closed. Potential obstructions in the aperture are more likely to occur as the aperture size increases. Many systems use dual switches in both open and closed states for the added security of redundancy.
User Safety Applications
The most popular use of laser safety shutters is in original equipment manufacturer (OEM) capital equipment. These manufacturers use the shutter primarily for safety shutdown purposes and occasionally for safe processing control and low cycle rates, about 1–2 Hz. Because the shutter is providing dual duty for safety and processing, the demands on the design for high cycle lifetime with minimum wear or change are imperative. Products designed for the OEM market tend to be the most robust by trading off some speed and packaging miniaturization for long life or in some cases practically unlimited lifetime. An OEM laser shutter will be linked to the machine’s main information-processing board for control, and the position sensors are used to monitor the aperture state and decide when to perform the next operation. Panel switches on the instrument usually interlock with the shutter to protect any user from irradiation when opening the instrument while power is on.
Researchers are another primary category of users implementing the laser shutter on their optical table, breadboard, or chassis. It becomes just one of many elements, but is usually the first one, adjacent to the laser. These applications usually implement a control box, commercially supplied by laser shutter manufacturers, that provides interlocking features for laboratory door switches, curtain switches, or pressure mats. Light-emitting diode (LED) displays give the researcher the status of the shutter with a simple glance.
Processing applications have primarily economic concerns or concerns for safety of their precious materials, so the shutter is safeguarding overexposure. This can be in welding, cutting, or photochemical processes such as lithography. Any detection of overexposure can be a direct signal to close the shutter.
Technologies Implemented
Because every application has a different set of features emphasized, the designer can tailor the technical features for the job. For commercial manufacturers, the design is more complicated because they must try to implement as many desirable features as possible to cover a wide range of applications. Currently, there are only a handful of manufacturers offering laser shutters with a majority of the desired features designed in one device.
Mirrors
The primary element, and the first in the optical path, is a lightweight mirror capable of handling high continuous wave (CW) power and surviving the pulse threat at the wavelength range of interest. Because most commercial lasers range from 157 nm to about 11 µm in wavelength, a move to use broadband mirrors cannot be successful at high powers. Usually, metal mirrors are used for CW at lower powers in the ultraviolet (UV)/visible range and for higher powers in the infrared. For pulsed laser applications, the mirror nearly always is mandated to glass substrate, dielectric coated mirrors.
How the mirror is mounted is very important regarding light leakage through the coating. As power levels increase, leakage of light for a 99.5% reflective coating can become significant. Nearly all designs call for a mirror bracket, usually made of metal, to hold the mirror and to absorb, scatter, or redirect the leakage via specular reflection.
Metal mirrors can sometimes be used as a monolithic bracket/mirror. The application of metal mirrors often requires using a high incidence angle to spread the energy over the surface. Damage thresholds are typically an order of magnitude less than a dielectric mirror. Attaching a mirror to a bracket requires attention to shock and vibration so that movement of the mirror in the bracket does not generate particles or allow the mirror to chip and fracture. Spring-loaded concepts and elastomeric bonding are popular. The mirror bracket needs to stay reasonably cool to avoid damage, and also because it is adjacent to the beam when it is pulled out of the way. If it is very hot, temperature gradients occur in the air where the beam passes. This can introduce wave front distortion. Designs often mandate that the mirror bracket be able to withstand a complete failure of the coated optic, or full irradiance of the beam, for a selected period of time. Typically, this is on the order of several seconds, but only in systems that monitor smoke, surface temperature of the mirror bracket, or excessive scatter with independent sensors tied to the safety system. In such a case, the safety system would shut down the power to the laser. When using a moving dielectric mirror, one must maintain the angle of incidence within a few degrees on the coating during all movement. Some relief on this constraint is provided with metal mirrors, but for polarized sources and high angles of incidence one should be aware of the principal angle (analogous to the Brewster angle) of the metal and corresponding absorption for one polarization state. Polished, coated, ferromagnetic metal flexure mirrors represent the lightest weight, fastest moving high-power mirrors. The flexure is pulled by an electromagnet. Although dielectric coatings can be applied to a polished metal surface, the damage threshold is usually much lower than glass due to the surface roughness.
One should also design the moving mirror edge to roll across the beam so that only the coated surface or transparent substrate edge is irradiated. The use of window frame optic brackets is not advisable except at low-power levels. It is possible to capture any optic and leave a path for the extended section of the optic to interrupt the beam and have a metal bracket section, slightly within the edge of the coating, to block the leakage through the coating.
The use of a lightweight, high-integrity mirror allows one to direct the energy to a stationary absorber or beam dump. It is very difficult to cool an absorber that is moving in and out of the beam with any appreciable speed. Thermal cooling umbilical straps, hoses, and the like have significant mass. For this reason, moving mirrors are used almost exclusively. The faster we can close, the less damage we do when an unsafe condition has been detected.
Mirror Movement
Once we have a mounted mirror choice, a system for moving the mirror must be designed. Electromagnetic is by far the most popular approach, with pneumatic systems having special niches. For an electromagnetic system, we need to have the mirror system attached to some ferromagnetic mass to which we can attract an electromagnet. Some simple solutions are readily available, such as partial rotation motors and solenoids. More advanced systems design custom magnetic geometries and commutating ferromagnetic mirror assemblies. Common design approaches to mirror movement are motor shafts, hinges, levers, flexures, pistons (solenoid and pneumatic), and linear slides. For highest reliability, designs with the fewest moving parts are preferred.
Rotating shaft devices, such as motors and galvanometers, have low torsional friction. This is due to bearings. A design emphasis in most bearings is point contact (balls) or line contact (rollers). Both exhibit extremely low cross-sectional area for thermal flow. They are usually lubricated. Sleeve-type bearings are only slightly better for thermal flow and are almost always lubricated or impregnated. If we mount a mirror on a mirror bracket, then attach this to the rotor of such a device, the leakage light for dielectric mirrors or absorption in a metal mirror will manifest itself as heat flowing into the shaft and rotor, warming the lubrication. For higher laser power levels, this approach can lead to outgassing. Enough outgassing on nearby optics is a potential failure mechanism and safety concern. Design emphasis should be placed on getting residual heat out of the moving mirror element. Hinges are another example of a heat flow roadblock. Flexure devices can carry heat down their length to the fixed end. Careful design can allow any of the above techniques to work if intimate contact of the mirror bracket is made with a large mass in the closed position. This lets leakage light heat to take a preferred path out some simple mechanical rest plate.
To arrive at some sort of fail-safe design, independent of gravity, most designs will use some spring loading that the electromagnet or pneumatic system must work against to open the shutter. Then, this stored spring energy should be capable of closing the shutter if the control power is lost in any way. Here, friction can play a role, and designs that show no change over time in required closing time are highly preferred. Bearing wear, lubrication loss or change, gummed rollers, or slides all result in frictional changes, potentially being capable of dominating the stored spring force to close. Stiction of the moving mirror with any surface must be carefully reviewed. This is often a concern with polymer materials and life or temperature changes. A failure of the moving mirror to release from an open position stop due to stiction can render the safety shutter useless. The devices used to open and hold open the mirror in general will dissipate heat, and this heat must be managed. Unless the heat is very small, a thermal path should be designed to allow the joule heating from a magnetic winding to be removed via conduction to a convenient surface for cooling. The fail-safe design is the most popular because many laser systems can be oriented in many directions with respect to gravity. They are also used in space and aerospace applications, where high shock/vibration conditions can exceed gravity forces, requiring a preset, known spring force to be built into the design.
Pneumatic systems are popular when used in high magnetic field environments or if any electrical discharge could initiate a safety concern. Sometimes, a magnetic field from another optical device, such as a faraday rotator, can influence a laser shutter using an electromagnet system. A magnetic shield, in the form of a steel aperture plate, is usually sufficient to reroute the field and allow the laser shutter to operate as intended. Another method is to locate the shutter further from the device or vice versa.
Absorbing the Beam
Now that we can move the laser beam with the mirror on demand, we need to focus on the stationary absorber. We want this optical absorber to be stationary so that we can attach it and carry away heat developed as the beam is absorbed. Volume absorbers, such as crystals or doped glass, have limited use at higher power levels because of low thermal conductivity. Surface absorbers in general have the best high-power capability. Water cooling can always be added to their mounting plane. The base material must have significant thermal conductivity, usually dictating copper or aluminum for economic reasons. The focus now becomes how to make this element compact within practical limits, highly absorbing, surviving the pulse threat, and not outgassing or generating particulates over time. We must give special attention to geometry, move away from the concept of a black surface and think about a black path. Trying to achieve all of the desired features with a single incidence absorption is not possible for most lasers, especially as powers increase. We would like to spread the power through the surfaces of the absorber so that we can choose geometries that use high incidence angles and fold the partially reflected or forward-scattered beam as it bounces through the absorption path. The idea here is to achieve somewhat of a progressive absorption flux waveguide. The first incidence does not need to absorb everything; even 50% or less can be effective as long as we continue absorbing and using forward scattering surfaces for each bounce. Clever designs can fit progressive absorbers into the smallest of volumes. We want to avoid surfaces with high potential to backscatter to the mirror and back to the user. This usually favors high angle of incidence paths in the absorber.
With the geometry designed, now the absorber surface morphology and material are chosen. The material is usually a plating. But, because surface morphology, or microscopic roughness, is more important, we need to choose a coating, plating, or evaporated finish that can be manipulated to produce a desired surface morphology. There are not a great deal of choices, but if carefully researched for the band or wavelength of interest, one can find materials that furnish both good atomic or molecular absorption from the material and the potential to be modified, usually by chemical or irradiation techniques, for ideal surface morphology. This ideal morphology would be tall, conical pillars, on the order of tens of wavelengths. Such a surface may only be possible in an attenuated region of the absorber or for lower power levels. High-energy pulsed lasers can destroy such surfaces because the absorbed energy cannot flow as heat fast enough down the pillars to eliminate melting or vaporization. Plating materials should be thin to get the absorbed heat to the main substrate, which was chosen for high thermal conductivity and convenient mounting. Even the highest melting point refractory metals can exhibit damage and oxide smoke when exposed to pulsed lasers, so geometry, surface morphology, and choice of atomic absorption coating are the design hierarchy to avoid such damage.
Cleanliness
The sophistication of a modern laser shutter requires it to be treated as an optical instrument. Particulate generation is given attention for sensitive applications, such as in semiconductor manufacturing. Outgassing properties are sensitive issues to all parties using the shutter with other adjacent optical elements or in sealed instrument environments. The amount of design effort given to these issues can highly influence costs, and a good balance of compromise is usually found for most applications. With the popularity of UV sources for industrial processing, many challenges are present. Replacing convenient materials in the shutter construction with photochemically insensitive ones is a major effort. Clean shutter construction, and designing such that subsequent cleaning can be performed by a user, is a desirable feature of design.
Proper Use Of A Laser Shutter
A first step in using a laser shutter is to mount the device rigidly, with some alignment capability and with good thermal flow to a larger mass. Typically, any shutter dumping over about 5 W should have significant thermal design in the bracket. Post and base configurations are very poor due to near-zero thermal cross section. The thermal bracket should extend, with full cross-sectional area, from the shutter mounting plane all the way to the infinite mass, commonly identified as an optical table or chassis. A water-cooled chiller plate is also viewed as an infinite mass. Because we can experience software control bugs and operator errors, it is recommended to design the long-term thermal capacity of the mounting arrangement for worst-case conditions. Most safety shutters see the energy for only a short period before laser power shutdown, but if the shutter does both safety and processing, it could see long-term closed cycles and require worst-case cooling. Without proper cooling, many shutter designs will lose efficiency in their electromagnetic systems and may open slowly or not at all. The issue of air temperature gradients also emerges without proper cooling. Actual forced-air cooling is not very popular because of flowing air around optics, particulate redistribution, and turbulence air gradients.
If the shutter mirror and absorber are accessible, the operator should have a maintenance cycle to blow off dust with clean gas or clean them with standard optics solvents. Many designs are closed, requiring clean precautions from initial use throughout the product lifetime other than dry gas blow-offs.
Nearly all laser shutter designs are mechanical. The motion of the mirror system can generate shock and vibration and even some recoil on impact, commonly known as bounce. Mechanical design considerations can eliminate or reduce this action, with added compromises, or it can be addressed with advanced electrical circuitry. Using overdamped or critical damping, the accelerations near stopping points can be gradual, with low shock. Recoil bounce can be eliminated. Depending on the sophistication of the electrical drive and the shutter design, the user should be cautioned that the bounce could expose an edge of the beam, essentially extending the total time to open/close. If position sensors are integral to the product, then the user’s circuitry used to process the sensor signal needs to wait until the mirror has ceased bouncing and settled. Failure to build in settling wait periods for sensors can generate an unsafe condition, either economically in processing or physically for operator safety, even though the time frame may be milliseconds or tens of milliseconds.
Control Basics
Drivers or controllers for the shutter deliver power to create the desired motion, open and closed. They typically accept a low-level signal, such as transistor–transistor logic, and convert this to a current waveform or release of air pressure sent to the shutter. Most arrangements seek some degree of speed and will send a more powerful impulse to open the shutter, then drop to a lower level to hold it open long term. Electrical circuits used are similar to those implemented for other electromechanical systems, such as motors, valves, and solenoids.
Units providing damped motion for the shutter are similar to circuits used in galvanometer drives. These driving techniques can be tailored to achieve low vibration, high speed, or high electrical efficiency, provided the shutter design is intended for the particular feature. The driver and shutter are a mated system, and for highest extraction of a single- or multiple-performance feature, must be viewed as a system from the initial design phase. They function much like a speaker and amplifier matched pair.
The controllers that are commercially available offer many safety features to allow convenient stand-alone use or connection with door interlocking and warning lights. Most offer an interlock connection, a simple make/break contact to run through any series of door, curtain, or pressure pad switches. An armed condition indicator allows the user to know whether the interlocks are closed and the system is ready for actuation. Reset functions allow the user to reset the system after a safety breach. A breach maybe from a door interruption or restoration, the user acknowledgment of this situation is by using a reset function to arm the controller of the shutter. From that point, either external signals from computer systems or manual toggle switches can be used to actuate the shutter open. OEM systems are usually fully automated, with microprocessors handling the decision making and reading position sensor information. Numerous displays are used to indicate status of the shutter, with LEDs the most common. A stand-alone controller instrument for a research application measures about the size of a common brick, whereas an OEM, with limited space, will design the circuitry on their system printed circuit board to be as small as 2 × 2 in.
Guideline For Design Or Commercial Purchase
Whether deciding to embark on a custom design, developing a statement of work for an outside engineering contractor, or making a commercial purchase, a hierarchy should be used to rapidly resolve compromises and extract the maximum benefit of the convolved features. For safety shutters, the recommended progression usually follows this path:
Determine physical package envelope constraints; this is your working space.
Definitely know the full beam size; aperture selection is later.
Get CW maximum power, wavelength, and polarization information.
Pulsed laser parameters, pulse width, repetition rate, energy, wavelength, and polarization must be determined.
Aperture determination depends on alignment, dithering, and potential diffraction rings.
Switching speed is determined; magnetic/pneumatic designs use more space for speed.
Controller circuit decision; whether to design or commercially purchase and the available power.
Determine if position sensors are required or not.
These items cover most applications. In special cases for clean room use or space applications, more design or selection criteria steps are involved.