The reported state-of-the art in adaptive optics technology is reviewed as a basis for projecting future development areas. The potential utility of adaptive optics in lieu of other techniques in several high-power and low-power applications is also discussed.
A discrete actuator deformable surface device, which provides as much as 12 μm of surface deformation using ± 1 KV, has been developed at Itek. The discrete actuator device, made from bonded layers of piezoelectric disks, has been continuously exercised for a period of several hundred hours. No change in surface figure was observed, even using a dilatation level of 6 microns. By employing both optical and electrical techniques, the fundamental mechanical resonance has been measured and found to depend upon the boundary condition in a complicated manner.
A new method for dynamically deforming a thin mirror to correct the phase aberration function for defocus and astigmatism is presented. Three piezo-electric type actuators attached to the back edges and parallel to the front surface of the mirror induce edge moments that bend the mirror to its desired shape for correction of the aberrated wave-front. A three-actuator deformable water-cooled mirror breadboard has been designed and built. Major features, design constraints, and performance expectations of the deformable mirror design are described.
Discrete, bimorph actuators have been designed at Itek for a deformable mirror, which has a dilatation sensitivity of 6 microns at 1.5 KV. The bimorph actuators, mechanically designed to accommodate flexibility while bending, provide surface deformation, which depends upon the actuator length and faceplate stiffness. Mechanical resonances have been measured at frequencies well above bandwidths required for wavefront correction. Life test data indicates that less than λ/5 PP wavefront change occurred after exercising the bimorph device for 100 hours at 6 microns.
A sensor system based on the principle of the heterodyne interferometer can be used to sense modal deformations of optical surfaces. This paper describes significant characteristics of these optical mode sensors and optical configurations appropriate to sense piston, tilts, focus, and higher order modes like astigmatism, coma, and spherical aberration. Electronic processing of sensor signals to extract the optical mode magnitudes is described. Then the system is applied to the problem of real-time removal of dynamic tilts produced by air turbulence and mechanical vibrations in the phase data measured by a broad beam hetero-dyne interferometer.
A 69-channel controller for a multidither adaptive optical system having a supervisory computer that establishes operating mode and monitors system performance has been developed. A hybrid approach was utilized wherein analog circuits whose characteristics are adjustable by computer control are employed to process the detected multidither signal. System structure and operating modes are described. Results of electronic component testing are reported.
A multi-actuator deformable mirror and control system is described that will cancel laser wavefront phase errors caused by the quasi-static aberrations occurring in large aperture metal mirror optics. The design of the differential ball screw driven mirror, its characterization and system performance are detailed. By using a sequential dither focus irradiance sensing/optimization algorithm, the system is shown to be able to correct wavefront aberrations of up to 4 waves p-p at 10.6 µm.
This paper describes the alignment system for the eight-beam CO2 laser, Helios, which is now being used in the study of compression and heating of targets as a step toward determining the feasibility of laser-initiated fusion for commercial power generation. The laser system reached design point output of 10 kJ at powers in excess of 20 TW in June 1978.
The accuracy with which beams from the LASL eight-beam CO2 laser, Helios, can be centered onto a fusion target has been measured. The average pointing error of 34 μm is adequate for initial high-power experiments. The technique is rapid and accurately simulates the conditions of actual target experiments.
At present, two wavefront sensor concepts are being developed which drive the deformable mirror in real-time phase conjugate adaptive optical systems. Analyses show that the Hartmann-type sensor and the shearing interferometer can both approach optimum theoretical performance, and, in fact, shearing interferometer sensors have been built which demonstrate this capability. Until now, however, the optimum Hartmann-type sensor has not been built because proposed implementations do not take advantage of AC detection required for long wave application, and because the simple Hartmann sensor poses severe engineering problems of system alignment. The Integrated Imaging Irradiance (I3) Sensor (U.S.Patent #4,141,652) incorporates several novel features which overcome these difficulties, resulting in an AC Hartmann-type sensor that has demonstrated optimum performance with relaxed engineering requirements.
An instrumentation system has been designed for the evaluation of thermal blooming compensation. The purpose is to investigate the maximization of 10.6 μm CO2 laser radiation in the target plane through maximization of the return 3-5 μm radiation from the laser induced "hot spot" by using a pinhole sensor and a slow dither algorithm. The adaptive optical component to be used in the evaluation is a three-actuator modal deformable mirror supplied to the USAF by Perkin-Elmer Corporation. This paper deals primarily with the design of a thermal blooming cell that will be used for modeling atmospheric path lengths of several kilometers within a laboratory setup where the total path length is limited to a few meters. The theory required to perform the scaling is currently available in the literature. We present it in a format designed to aid the optical engineer, whose primary concern is the evaluation of an adaptive optical system for thermal blooming compensation. The interplay of the many parameters that may be varied is illustrated in graphical form to facilitate choices. As an example, we included our design for a one-meter cell to be incorporated in the slow dither experiment.
A water-cooled molybdenum faceplate (Pratt and Whitney Aircraft) was bonded to a monolithic piezoelectric mirror (MPM, Itek). The device was characterized in terms of surface deformation with applied voltage, linearity, influence function and mechanical resonances. The purpose of these measurements was to determine the effect that a faceplate, at ambient and at hydrostatic pressure, has upon the well-known monolithic deformable mirror.
Utilizing our real-time heterodyne interferometer, we have been able to efficiently obtain accurate optical path difference maps of objects ten meters distant. The measurements were obtained in the presence of significant optical noise sourced from air turbulence and mechanical vibrations. Problems of long pathlength interferometry are discussed and results of noise source and system accuracy measurements are presented along with an important application.
The ability to minimize the pointing and focusing errors at the focal plane is crucial in many applications involving infrared laser systems. This is particularly the case for systems involving multiple beams reaching the focal plane, as in the case of the LASL CO2 laser fusion systems. For example, the LASL Helios CO2 Laser Fusion System has eight 34-cm diameter beams each with an f number of approximately 2.4 coming to focus, the last element being an off-aperture parabola with a focal length of approximately 77.3 cm. The design tolerance for pointing accuracy is ± 25 microns and for focusing accuracy is ± 50 microns for the Helios system. The Smartt interferometer shows promise of not only evaluating the optical quality of the beam, but it can be used to align the beam to the tolerance levels stated above. This paper describes the procedure, as well as experimental results obtained, which show that pointing accuracies of ±12.5 microns and focusing accuracies of ±25 microns are obtained at the focus of a CO2 laser beam in a setup which duplicates the target region of the Helios CO2 Laser Fusion System. The backlash in the x-y-z stage micrometer used in the experiment is estimated to be 10 microns, though precautions were taken to move in the same direction throughout an experimental run.
A method is described for control of optical path length over a range of many optical wavelengths. The operational concept utilizes phase error signals from a pair of selected wavelengths of a multiline laser to generate unambiguous feedback control signals. A problem encountered in single wavelength phase control results with phase disturbances greater than the optical wavelength. The two wavelength technique provides synthetic long wave length error signals generated by beating a pair of single line error signals. The resulting long wavelength error signals in conjunction with single wavelength error signals are employed for control over an extended phase range of many wavelengths. The theory of two wavelength phase control is first presented for multidither and phase conjugate configurations. The latter is shown to have distinct advantages with regard to ease of implementation and performance potential. An operational two wavelength phase control system employing heterodyne phase measurement is then described along with experimental results of open and closed loop performance. The results verify theoretical predictions and indicate extended range phase control with fast response.
For the coherent optical recombination of high power HF chemical laser beams such as master and slave oscillator array (MASOA) or coherent optical adaptive technique (COAT), a wide bandwidth servo system is required to control the phases of the laser beams. A phase-control servo system using a rate feedback and a laser Doppler sensor was designed. Its performance was simulated on a CDC7600 computer. Laser frequency drift was modeled by a Gaussian Markov random process and the mechanical response of the PZT mirror-driver was modeled by a transfer function. The result indicated that a rms steady-state phase error of less than 3 deg. can be achieved. More detailed discussion on the servo system design and simulation will be given.
Competition of longitudinal and transverse modes in stable and unstable resonators of a cw HF chemical laser was investigated. Experimental results indicate strong competition and asymmetry among the modes. Single longitudinal mode operation was also achieved.
Described is the operation of a nine-zone multidither adaptive optics system. Closed-loop control system data show the interaction of dither frequency spacing with achievable system bandwidth. Performance predictions for a 69-zone system are presented.