Patent Application
20110299183
1. Field of the Invention
This invention relates generally to the field of adaptive optics, and in particular to a plate-type deformable mirror suitable for use in a wide range of adaptive optics applications.
2. Background of the Invention
Adaptive optics is a technique for controlling the spatial phase of light that has been under development for several decades. In a general adaptive optics system, light is reflected from a deformable mirror and a small fraction is split off to illuminate a sensor. The sensor provides feedback to a control computer that adjusts the deformable mirror to change some property of the beam of light. Astronomers have used adaptive optics systems to remove the distortions induced by the atmosphere and achieve higher quality images from large telescopes. Adaptive optics systems have also been used on lasers to improve the beam quality and to shape the intensity profile.
For many years, plate-type deformable mirrors have been favored for both imaging and laser applications. U.S. Pat. No. 3,904,274 describes one of the first embodiments of a plate-type deformable mirror in which a thin flexible plate is attached to an array of piezoelectric actuators. U.S. Pat. Nos. 4,657,358 and 4,674,848 both describe an important variation of the plate-type deformable mirror in which the mirror surface is cooled for high power laser operation. In the past, cooling was necessary on these deformable mirrors because the even the best coatings had significant absorption. The heating of the deformable mirror caused warping of the mirror surface, a change in the response of the actuators to voltage, and exposed the device to potential damage, primarily at the sites where the actuators were bonded to the face sheet.
Plate-type deformable mirrors have only changed slightly in the more recent years. Today, these mirrors typically consist of lead manganese niobate (PMN) actuators to reduce the actuator hysteresis and reduce the operating voltage. They also use specially design plates with sculpted cross-sections so that the actuators attach to stiffer pillars that extend down from the back of the flexible plate.
There has been some work in recent years on applying micromachining technology to the fabrication of deformable mirrors. In U.S. Pat. No. 5,619,059, Li et al. describe a deformable mirror device with a multi-layer dielectric coating over sections of the mirror surface. U.S. Pat. No. 6,108,121 describes a continuous surface MEMS deformable mirror with a high reflectivity coating. While the modifications described in these patents make it possible for the MEMS mirrors to be applied to some high-power lasers, they have not demonstrated applicability to the lasers being considered for high-power laser weapons.
As laser power and system design power density has increased, traditional plate-type deformable mirrors have proved to be less effective. The PMN actuators change their response to voltage by about 3% per degree Celsius, so any significant heating causes the actuator response to vary. The most commonly used deformable mirror fabrication technique requires the actuators to be bonded to the sculpted faceplate before the faceplate is polished and coated. Many of the best coating techniques used today for high-power laser applications, like Ion-Beam Sputtering (IBS), are deposited with a significant amount of coating stress. When a traditional deformable mirror is coated using these high-stress coating techniques, the resulting mirror surface can be so significantly warped that the mirror surface is of very limited utility.
Mansell recently presented a modification to an older deformable mirror fabrication technique leveraging an older coating technique in an attempt to increase mirror reflectivity in 2007 (MANSELL et al., “Development of a Deformable Mirror for High-Power Lasers”, SPIE's Mirror Technology Days, Aug. 1, 2007). This presentation showed a device with lead zirconate titanate (PZT) actuators instead of the traditional PMN actuators because of the large reduction in voltage response variation with respect to temperature. In his presentation, Dr. Mansell described two different deformable mirrors. One deformable mirror was coated on only one surface. The second deformable mirror had a high reflectivity coating applied to both surfaces of the faceplate before the faceplate was bonded to the actuators. The deformable mirror coated on both surfaces exhibited less laser-illumination-induced distortion, but the technique was risky because the faceplate with a coating on both surfaces effectively created an optical etalon. In the right circumstances (certain angles of incidence, certain faceplate thicknesses relative to the wavelength, etc.), the effective reflectivity of the faceplate etalon can decrease dramatically causing much of the laser light to be transmitted through the faceplate to the underlying structure and causing damage to the deformable mirror. Despite the potential danger of the etalon transmission, the deformable mirror was a success with all of the actuators of both mirrors firing and a very low thermally-induced distortion.
In subsequent work, MANSELL et al. demonstrated the ability to put two different coatings on the two surfaces of the plate to both compensate stress and achieve high-power operation (MANSELL et al., “Novel Plate-Type Deformable Mirrors and Adaptive Optics Systems for High Power Lasers”, DEPS Directed Energy Systems Symposium, Apr. 8, 2009 and Provisional Patent Application Filed Sep. 12, 2008). Unfortunately, the use of the double-side coated plate-type DM makes the practice of polishing the faceplate after the actuators are bonded to the surface difficult.
One challenge with using a plate with coatings on both surfaces is that the deformable mirror faceplate needs to be bonded to actuators after the coatings have been applied, thus eliminating the possibility of polishing out any bond-induced distortion without removing the coating.
The primary object of this invention is to enable the bonding of actuators at discrete points to a deformable mirror faceplate such that the bonding stress is minimized and the faceplate surface distortion is minimized.
In the manufacture of a typical plate-type deformable mirror, actuators must be put in contact with a faceplate. In the process under consideration here, the faceplate is manufactured and coated prior to the bonding to actuators. Bonding can take place with a variety of methods, but in the preferred process we describe here, we use epoxy to attach the top of the actuators to the faceplate. We have found through experimentation that the majority of the faceplate distortion is induced when epoxy makes a connection from the sides of the actuator to the faceplate. This effect typically occurs when epoxy overflows from the top of the actuator to the sides. This bond is typically referred to as a fillet (YODER, Paul R., Mounting Optics in Optical Instruments, 2nd Edition, SPIE Press Book (4 Aug. 2008), ISBN: 9780819471291.). To verify that this fillet effect was responsible for the bond-induced distortion, we manufactured a prototype deformable mirror with just a few actuators and allowed the fillet effect to occur. We were easily able to see the bond-induced distortion on an optical interferometer. Then we carefully used a sharp steel blade to scrape away the majority of the epoxy in the fillet after it had cured and found that the bond-induced distortion was greatly reduced.
We have found that the literature cites volume control as the primary method of suppressing fillets. Unfortunately, volume control over the small actuator-to-faceplate bonds is extremely difficult, so an alternative solution was sought. Furthermore, the amount of epoxy varies from actuator to actuator because the faceplate is typically not perfectly flat and the surface formed by the actuator tops is not perfectly flat. The epoxy must bridge this gap.
We invented a solution in which the fillet bond force vector is broken so that it cannot induce distortion onto the deformable mirror faceplate. We present here different embodiments of this invention.
In the preferred embodiment, we apply a thin piece of tape cut into a square with a square section cut from the center that is smaller than the actuator tops to form a frame shape at the desired contact point of each of the actuators. Then a precise amount of epoxy is used to fill the center of the tape frame. Since the tape is thicker than most natural epoxy bonds, conventional pipetters can be used for this dosing. Finally, the actuators are brought in contact with the tape such that the hole in the tape frame is on the top of the actuator. Epoxy that overflows from the center of the frame can still form a fillet between the side of the actuators and the tape, but the tape to faceplate bond is weak enough to provide flexibility and greatly reduce fillet-induced distortions.
An alternative embodiment involves replacing the tape with an alternative flexible material like photoresist. Photoresist can be deposited and patterned into the same frame shapes using conventional lithographic techniques. Then the same process described above can be used to attach the actuators. As a final optional step, the photoresist can be removed after the epoxy has cured with a solvent like acetone.
In one more alternative embodiment, the sides of the actuators themselves are coated with a material to inhibit adhesion of the epoxy. Some such materials include silicone rubber, Teflon, and photoresist, which can be dissolved after the manufacture.
In this description, we referred to bonding the actuators to the faceplate, but the techniques described here are equally applicable when an interface material exists between the actuator tip and the faceplate.
A wide variety of different actuators can be used to warp the faceplate. In the preferred embodiment, we use PZT actuators. Other actuation methods include electrostricitive, magnetostrictive, MEMS, hydrostatic (fluid pressure), electrostatic, and other pizeoelectric actuators.
A wide variety of different bonding mechanisms can be used in this invention. We use epoxy in the preferred embodiment, but there are many different liquid adhesives that can be used as alternatives including acrylic bonding.
