The propagation of high peak-power beams in the atmosphere has been observed in field trials with visible-near infrared (VNIR). Longer infrared (IR) wavelengths beams have some propagation characteristics not tested in the VNIR field experiments. We identify some unique characteristics of IR ultrashort- ulse air propagation: greater transmission, much lower dispersion-induced chirp, lower sensitivity to atmospheric turbulence, and much larger critical power. We summarize the results of self-focusing theory applied to IR ultrashort pulse characteristics, apply the theory to predict the IR self-focusing distance, and show the theory is in close agreement with detailed numerical simulations including extinction and turbulence.
Efficient propagation of light through the atmosphere requires the compensation of phase distortions induced by the atmosphere, thermal lensing, thermal blooming, or aero-optics. We have developed a deformable mirror engineered to compensate these aberrations while reflecting high power radiation with minimal heating and thermally induced distortion. We present the results of a multi-year effort to address many of the challenges with existing state-of-the-art designs while reducing cost, complexity, and manufacturing time.
Polymer membrane deformable mirrors offer a low-cost alternative to conventional technology in a wide variety of
adaptive optics or laser beam shaping applications. In this paper we evaluate the suitability of two different kinds of
polymer membrane deformable mirrors for laser machining. We present results showing that a 12.5-mm diameter
nitrocellulose membrane fails near 400 microns of motion. We present results from a demonstration of a high peak
power beam shaping and show a new compact laser beam shaping system using a polymer membrane deformable
mirror. We evaluate the effect of Q-switched laser radiation on polymer membranes at 355nm and 1060nm.
Effective application of membrane deformable mirrors requires understanding of the operating
characteristics of these devices. Using custom developed hardware and software tools, we were able to
quantify the temporal and spatial response characteristics of a membrane deformable mirror. Temporal
characteristics were analyzed using a frequency sweep stimulus while measuring the DM response on a
feedback photodiode. Spatial characteristics of the DM were analyzed in terms of its ability to reproduce
Zernike polynomials of increasing order using a variety of actuator patterns. We present here both the
techniques for performing these measurements and the results from simulation and the laboratory.
Metric adaptive optics systems search over a set of wavefront modes or commands to actuators to
optimize a system performance metric like Strehl ratio or brightness. These systems have been explored
for many decades and have been thought to be unreliable due to local minima in the metric space. It has
been shown that some modes match well with no local minima to a given metric, but they rely on the
ability of a mirror to create reliable replicas of the search modes. We present here a study of the most
common implementation of metric adaptive optics that involves searching over the actuator command
space while evaluating an intensity-based metric. We map an error space relating a common metric to
actuator commands and statistically analyze the error function to determine the quantity and location of the
The performance of laser machining systems can often be improved by adjusting the intensity profile of the
beam on the target. Shaping a laser intensity profile can be efficiently accomplished by adjusting the
spatial phase of the beam before propagating the beam a distance to the target. Beam shaping can be
accomplished with passive diffractive elements, but this technique is only capable of creating a single
intensity profile and is usually very sensitive to the input beam characteristics. Beam shaping with active
optical elements like deformable mirrors can enable the system to achieve multiple shapes and compensate
for non-ideal input beams, but can be very expensive. We present here a demonstration of laser beam
shaping with low-cost membrane deformable mirrors.
The application of adaptive optics has been hindered by the cost, size, and complexity of the systems. We describe here
progress we have made toward creating low-cost compact turn-key adaptive optics systems. We describe our new low-cost
deformable mirror technology developed using polymer membranes, the associated USB interface drive
electronics, and different ways that this technology can be configured into a low-cost compact adaptive optics system.
We also present results of a parametric study of the stochastic parallel gradient descent (SPGD) control algorithm.
Electro-static membrane mirrors have been in use for over ten years in active and adaptive optics
applications in industrial, academic, and research systems. We introduce a deformable membrane mirror
produced from commercial off the shelf (COTS) pellicle membranes. This greatly reduces production costs
of the device. The device can be produced in up to 3 inch formats without large static aberrations. The
mirror is capable of closed loop bandwidth in excess of several hundred Hz. Measurements of the device
influence functions will be presented along with results of closed loop real time control performance
In prior work we introduced a method of choosing mesh parameters for a single wave-optics propagation between two
effective apertures. Unfortunately, most systems that require wave-optics modeling, like modeling laser resonators with
gain media, propagations through the atmosphere, and imaging systems with internal limiting apertures, have multiple
apertures and phase screens that induce diffraction. We begin here by augmenting the single propagation theory to
include diffraction from both apertures and phase aberrations. We then introduce a technique for analyzing complex
systems of simple optics to determine the appropriate wave-optics mesh parameters.
In prior work we described a 5x5 ray matrix formalism and how to integrate the effects that are not modeled in wave-optics
with the ray matrix model. In this paper we describe how to complete the integration of the two techniques by
modifying the Siegman ABCD ray matrix decomposition. After removing the separable effects like image rotation and
image inversion, we break the 5x5 ray matrix into two 2x2 sections (a.k.a. the ABCD matrices) that correspond to the
two axes orthogonal to the propagation. We then present a general algorithm that breaks any arbitrary ABCD matrix
into four simple wave-optics steps. The algorithm presented has sufficient generality to handle image planes and focal
planes. This technique allows for rapid and accurate wave-optics modeling of the propagation of light through complex
optical systems comprised of simple optics.
Relay mirrors offer a method of extending the range and efficacy of laser weapons systems by providing a platform
that can relay the high power beam from the source to a target at a lower cost than building more laser weapons.
Most laser weapons' platforms use a Cassegrain aperture telescope to project the high-power beam to a target.
Diffraction causes power in the projected beam to move out beyond the edge of the receiving aperture and into the
central obscuration of the receiving Cassegrain telescope. This paper presents a method of increasing the power
coupling between a projected beam and a receiving telescope by warping the phase of the projected beam and shows
that the spatial frequency and amplitude of the phase aberration is consistent with being integrated with the adaptive
optics control loop on the source beam projector.
Liquid crystal spatial light modulators, lenses, and bandpass filters are becoming increasingly capable as material and electronics development continues to improve device performance and reduce fabrication costs. These devices are being utilized in a number of imaging applications in order to improve the performance and flexibility of the system while simultaneously reducing the size and weight compared to a conventional lens. We will present recent progress at Sandia National Laboratories in developing foveated imaging, active optical (aka nonmechanical) zoom, and enhanced multi-spectral imaging systems using liquid crystal devices.
The relay mirror concept involves deploying a passive optical station at a high altitude for relaying a beam from a laser weapon to a target. Relay mirrors have been proposed as a method of increasing the range of laser weapons that is less costly than deploying a larger number of laser weapons. Relay mirrors will only be effective if the beam spreading and beam quality degradation induced by atmospheric aberrations and thermal blooming can be mitigated. In this paper we present the first phase of a multi-year effort to develop a theoretical and experimental capability at Boeing-SVS to study these problems. A team from MZA and Boeing-SVS has developed a laboratory test-bed consisting of a distributed atmospheric path simulated by three liquid crystal phase screens, a Shack-Hartmann wavefront sensor, and a MEMS membrane deformable mirror. We present results of AO component calibration and evaluation, the system construction, and the system performance.
There are many research efforts around the world to develop adaptive optics technology, but only a few commercial efforts. This paper will investigate some of the boundaries that separate successful laboratory operation from successful commercialization of this technology and report on the progress Intellite has made toward the commercialization of adaptive optics.
Diffractive wavefront control has been demonstrated as a viable technique for high-dynamic-range laser wavefront control. Unfortunately, most conventional programmable diffractive elements, like liquid crystals and segmented mirror arrays, damage when illuminated with high-power laser light. Continuous reflective surfaces coated with multi-layer dielectric stacks have demonstrated high damage thresholds, but are not typically thought of as good diffractive wavefront control elements because of the inability to gneerate rapid spatial phase changes. In this paper, the investigation of a continuous reflective surface as a diffractive wavefront control element is presented.
Micromachined electrostatic membrane mirrors have been in use for several years in active and adaptive optics applications. We introduce a deformable mirror with small static aberrations and resistant to damage during electrostatic snap-down in an architecture designed so that a multi-layer dielectric coatings that can be applied before the mirror is released. We also make laboratory performance measurements and produce a closed loop control algorithm.
High power lasers often suffer from phase aberrations due to spatially non-uniform temperature distributions in optical elements. Adaptive optics has been established to provide engineering control over phase aberrations, but it has been mostly applied to astronomy and has been too costly to use in commercial lasers. The goal of our research has been to develop low-cost adaptive optics systems and components for laser aberration compensation. We present here a summary of our adaptive optics work and our vision of the future of adaptive optics.
We describe the performance of a bulk micromachined deformable mirror coated with a dielectric stack for adaptive optics applications with high power lasers. A reflectance of greater than 99.9% was measured and the mirror had a residual static aberration of less than 90 nm rms primarily in astigmatism. A thermally induced distortion of 71 nm RMS was observed for an incident intensity of 300 W/cm2 and an average power of 57 W. The multi-layer coated deformable mirror survived over half a billion cycles without degradation and survived for 30 hours with 36 W cw 1064 nm laser light. In addition, the thermally induced distortion with 22 W of average laser power (350 W/cm2) was reduced from 88 nm to 31 nm rms.
Inexpensive wavefront sensors and deformable mirrors are essential for addressing potential commercial applications for adaptive optics like laser beam control and ophthalmology. Silicon micromachined deformable mirrors offer the potential for low cost and high actuator density, but there are some problems with the architectures currently available like low mirror quality and high actuator crosstalk. Shack-Hartmann wavefront sensors are still based on traditional charge coupled device (CCD) arrays making them very expensive at high frame rates. To address the need for low cost deformable mirrors, we have implemented a new architecture of silicon deformable mirror designed to be low cost, have low actuator crosstalk, and still maintain good mirror quality. Furthermore, we built and tested a CMOS Shack-Hartmann wavefront sensor to address the needs of the adaptive optics community for high speed wavefront sensing.
The LIGO (laser interferometer gravitational-wave observatory) detector is a complex Fabry-Perot/Michelson interferometer, designed to detect gravitational waves (GW) from astrophysical sources. When a GW strikes the detector, the underlying space will be extended in one direction and contracted in the orthogonal direction. The LIGO detector is designed to detect this space-strain, as a relative change in the lengths of the mutually orthogonal arms. Because this strain is much smaller than the arm length (typically 1 part in 1021), each arm is in the form of an optical resonator, effectively increasing the arm length and, hence, its change for a given strain. The arm-length change is measured as the relative phase shift at the beam splitter. To cope with the tiny phase shift, LIGO detects it as a beat signal between a carrier frequency and a side band frequency at the signal port (also called the dark port) of the Michelson interferometer. The side band is generated by phase-modulating the carrier frequency; the modulation frequency is chosen, so that the side band is far off resonance with the resonators in the arms, while the carrier frequency is on resonance. In this way, the phase shift associated with a relative arm-length change can be detected as amplitude modulation at the modulation frequency.
A new silicon deformable mirror design is presented which provides high reflectivity, an optical quality continuous surface, and high intensity handling capacity. Its features make it useful for a wide range of devices including telescopes, leasers, and photolithographic systems. The mirror architecture is similar to commercial electrostrictive deformable mirrors. A focus corrector built using this architecture exhibited 2.25 microns of actuation at the center through the application of 100v corresponding to a radius of curvature of -2.4m. A 30 micrometers thick 1cm diameter silicon mirrors exhibited its first mechanical resonance at 2.7kHz. A mirror coated with 100nm of gold was shown to be able to withstand 100kW/cm2 of continuous wave 1064nm intensity for 10 minutes without observable degradation. An active mode-matching experiment was performed showing that 99.5 percent of a Nd:YAG beam could be coupled to a finesse 4000 ring cavity.
The flight environment of next-generation theater missile defense interceptors involves hypersonic speeds that place severe aero-thermodynamic loads on missile components including the windows used for optical seekers. These heating effects can lead to significant boresight error and aberration. Ground-based tests are required to characterize these effects. We have developed methods to measure aberrations in seeker windows using a Shack-Hartmann wavefront sensor. Light from a laser or other source with a well known wavefront is passed through the window and falls on the sensor. The sensor uses an array of micro-lenses to generate a grid of focal spots on a CCD detector. The positions of the focal spots provide a measure of the wavefront slope over each micro-lens. The wavefront is reconstructed by integrating the slopes, and analyzed to characterize aberrations. During flight, optical seekers look upstream through a window at 'look angles' angles near 0 degrees relative to the free stream flow. A 0 degree angle corresponds to large angles approaching 90 degrees when measured relative to the normal of the window, and is difficult to simulate using conventional techniques to illuminate the wavefront sensor during wind tunnel tests. For this reason, we developed a technique using laser- induced optical breakdown that allows arbitrary look angles down to 0 degrees.
The performance of an adaptive optical system is strongly dependent upon correctly measuring the wavefront of the arriving light. The most common wavefront measurement techniques used to date are the shearing interferometer and the Shack-Hartmann sensor. Shack-Hartmann sensors rely on the use of lenslet arrays to sample the aperture appropriately. These have traditionally been constructed using MLM or step and repeat technology, and more recently with binary optics technology. Diffractive optics fabrication methodology can be used to remove some of the limitations of the previous technologies and can allow for low-cost production of sophisticated elements. We have investigated several different specialized wavefront sensor configurations using both Shack-Hartmann and shearing interferometer principles. We have taken advantage of the arbitrary nature of these elements to match pupil shapes of detector and telescope aperture and to introduce magnification between the lenslet array and the detector. We have fabricated elements that facilitate matching the sampling to the current atmospheric conditions. The sensors were designed using a far-field diffraction model and a photolithography layout program. They were fabricated using photolithography and RIE etching. Several different designs are presented with some experimental results from a small-scale adaptive optics brass-board.