Showing papers on "Adaptive optics published in 1977"

Bandwidth specification for adaptive optics systems

Abstract: A simplified expression for the bandwidth of an adaptive optics system is found to depend on a weighted path integral of the turbulence strength, where the weighting is transverse wind velocity to the 5/3 power. The wave-front corrector is conservatively assumed to match the phase perfectly, at least spatially, if not temporally. For the case of astronomical imaging from a mountaintop observatory, the necessary bandwidth is found to be less than 200 Hz.

Real-time atmospheric compensation

Abstract: We have developed an active optical imaging system capable of correcting optical wave-front errors in real time at frequencies in the kilohertz range. Wave-front errors due to atmospheric turbulence in the propagation path, as well as optical figure aberrations and wave-front errors due to mechanical and thermal changes may be compensated. The system used an ac shearing interferometer, a parallel analog data processor, and a monolithic piezoelectric active mirror arranged in a closed-loop configuration. This compensation system can be applied to apertures of any size; experimental results are shown for a system with 21 correction zones.

The multidither principle in adaptive optics

Abstract: Coherent optical adaptive techniques (COAT) offer promise for overcoming the deleterious effects of phase distortions experienced by optical beams propagating through distorting optics or via a turbulent and absorbing atmosphere. The theory of four classes of such systems, which employ similar multidither principles, is explored. Many modes of operating these systems are briefly reviewed and a detailed analysis of the most widely employed—a glint referencing system with sinusoidal dithers—is developed. A servo signal-to-noise analysis indicates how the optimum choice of dither magnitude depends on the system noise.

Experimental studies of a deformable-mirror adaptive optical system*

Abstract: The measured characteristics of two different deformable mirror designs are presented. Each mirror is uncooled and has 37 piezoelectric actuators. The mirrors have comparable frequency response usable to over 20 kHz, but possess quite different actuator influence functions. One exhibits an exp(−αr1.5) characteristic and the other has an exp(−γr2.5) characteristic. The latter mirror is used in an 18-channel multidither adaptive optical system to perform both phase dither and phase correction functions. The measured characteristics of this system’s performance include a 2 ms convergence time and effective turbulence compensation. Evidence is presented of “2Nπ” servo ambiguities, a behavior which reduces an adaptive system’s overall phase compensation performance. The performance of the deformable mirror adaptive system is compared to an equivalent segmented (piston) mirror system.

Optical resolution through a turbulent medium with adaptive phase compensations

Abstract: The theory of short-term average modulation transfer function developed by Fried is extended by considering the correlation between the turbulence-induced phase distortion and the residual phase aberrations after adaptive phase compensations. Calculation of the optical resolution shows that the original approximation is valid for the normalized lens diameter D/r0 less than 4.0; conversely, for D/r0 greater than 4.0 the optical resolution becomes arbitrarily large because the MTF is unrealistically overcorrected at high spatial frequencies. The extended formulation is then applied to the cases with higher-order phase compensations such as focus and astigmatism. The peak optical resolution occurs at larger values of D/r0 when the higher-order corrections are included.

Theory of multidither adaptive optical systems operating with zonal control of deformable mirrors

Abstract: A theory is developed for multidither adaptive optical systems that employ N discrete actuators driving a deformable control and dither mirror with the same shape of influence function. The major emphasis is on mirrors with a Gaussian influence function. It is shown, as a special case, that single-actuator displacements or errors of such systems yield error signals that can be described in closed form, via sine integral functions of the actuator displacement. Closed-form expressions for both the first- and second-harmonic content in the dither signal outputs are developed. Expressions for servo cross coupling (from adjacent actuator deformations) are developed and it is shown how the coupling is typically larger than the mechanical (actuator center) coupling. Effective modulation index comparisons are established between Gaussian and piston mirrors, for piston and Gaussian phasing error components. It is shown how a potential secondary maxima lock-up condition (the 2nπ problem) can occur when operating hill-climbing adaptive optical systems with deformable mirrors. In some cases a major loss in system Strehl ratio may result.

Turbulence-degraded beam quality: improvement obtained with a tilt-correcting aperture

Abstract: In this paper, we have calculated the degradation in the focal-plane irradiance distribution due to atmospheric turbulence and the potential improvement realizable by employing a wavefront filt-correcting aperture. It has been shown that, when the aperture diameter is of the order of the outer scale of turbulence, virtually no improvement is realized relative to the uncompensated case. For the case when the long-term coherence length is small compared with the diameter, there is a several-decibel improvement over the longterm case; however, for a full phase-compensating aperture (e.g., COAT), peak intensity can be increased an additional several decibels. When the coherence length is not much smaller than the diameter, close to diffraction- limited performance can be expected. Comparisons are also made of the reduction of on-axis intensity with no compensation, tilt-correction, and a full phase-compensating aperture. It is shown that the effective coherence length of the compensated aperture due to the residual amplitude fluctuations is greater than the long-term coherence length by a factor proportional to the square root of the Fresnel number of the aperture. Approximate formulas are also presented for the tilt-corrected MTF for arbitrary aperture irradiance distributions.

Holographic grating scanners with aberration corrections.

Abstract: Optically recorded Fresnel zone plates have proper spatial frequency characteristics for use as a laser scanner. Such grating scanners can easily be made and can scan over large cone angles with, however, severe aberrations of the diffracted laser beam at large deflection angles. Optically recorded Fresnel zone plates for laser beam scanning are investigated, and their aberration properties are analyzed. A method for removing the wavefront error caused by this type of zone plate is discussed, and the feasibility of the method is demonstrated.

Minimizing atmospheric dispersion effects in compensated imaging

Abstract: A compensated imaging system that corrects optical path length distortions due to atmospheric turbulence by means of an achromatic corrector will have residual errors caused by the dispersion of the atmosphere. These errors become significant for astronomical objects at large zenith angles, but they may be minimized by special dispersion correctors.

Phase conjugate adaptive optics using perimeter phase measurement

Abstract: We describe a concept for a phase conjugate adaptive optics system which avoids the use of a shared aperture and the attendant difficulties when such a system is used with a high-power laser. The concept consists of measuring the phase of a reference wave around the edge of the aperture and interpolating with a suitable formula to find the phase inside the aperture. We assess the potential of this technique to compensate for atmospheric turbulence when the transmitting aperture is circular or annular. Good correction is indicated, particularly for the case of an annular aperture where perimeter phase measurements can give an estimate of the amount of turbulence-induced misfocus.

Diagnostics Of Turbulence By Holography

Abstract: Methods for applying holographic interferometry in the study of turbulence are described. Examples of the effect of various types of turbulent cells on an optical wavefront are presented. Finally, a relatively new technique in which phase control on the two mixed reconstructed waves is discussed. It is concluded that the technique can ultimately provide an extremely high sensitivity for the viewing of phase objects and can provide the basis of automated data reduction based on heterodyne interferometry.

Analysis and design of a high power laser adaptive phased array transmitter

Abstract: The feasibility of delivering substantial quantities of optical power to a satellite in low earth orbit from a ground based high energy laser (HEL) coupled to an adaptive antenna was investigated Diffraction effects, atmospheric transmission efficiency, adaptive compensation for atmospheric turbulence effects, including the servo bandwidth requirements for this correction, and the adaptive compensation for thermal blooming were examined To evaluate possible HEL sources, atmospheric investigations were performed for the CO2, (C-12)(O-18)2 isotope, CO and DF wavelengths using output antenna locations of both sea level and mountain top Results indicate that both excellent atmospheric and adaption efficiency can be obtained for mountain top operation with a micron isotope laser operating at 91 um, or a CO laser operating single line (P10) at about 50 (C-12)(O-18)2um, which was a close second in the evaluation Four adaptive power transmitter system concepts were generated and evaluated, based on overall system efficiency, reliability, size and weight, advanced technology requirements and potential cost A multiple source phased array was selected for detailed conceptual design The system uses a unique adaption technique of phase locking independent laser oscillators which allows it to be both relatively inexpensive and most reliable with a predicted overall power transfer efficiency of 53%

Application Of Lasers To Measure The Optical, Thermal And Mechanical Properties Of Materials

Abstract: The advent of the laser has had a profound effect in most areas of scientific and technological research. A particularly fertile area of application is in the measurement of the optical, thermal, and mechanical properties of materials. This paper discusses the application of laser measuring techniques in the fields of Raman spectroscopy, optical surface scatter, interferometry and optical testing, dimensional stability measurements, microyield and microcreep studies, thermal expansion coefficient measurements, and transient thermal phenomena in optical materials. In these areas of materials measurement, all of the laser's characteristics-temporal and spatial coherence, frequency tunability, collimation, polarization, and high brightness--have been usefully exploited at wavelengths from the ultraviolet to the far infrared.© (1977) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Nonlinear Optical Phase Conjugation For Laser Systems

Abstract: This paper deals with certain nonlinear effects that,can directly generate a wavefront which is the phase conjugate of the incident field. These phenomena can be exploited in pulsed laser systems to remove aberrations in the optical train as well as aberrations arising from turbulence in the atmosphere. We discuss our measurements of the effectiveness of this conjugation process using a ruby laser and stimulated Brillouin scattering in a CS2 waveguide device. Measurements are made of the divergence angle of the beam after correction as a function of interaction length. The application of conjugate processes realizable in SBS, SRS, parametric down conversion, and four-wave mixing is considered for typical CO2 laser systems. System gain, backscatter limitations and parasitic oscillation are discussed for typical pulsed amplifier systems. Amplified spontaneous emission and unwanted glint returns from optical defects appear to be the most serious limitations.

Hypervelocity Particle Measurements By A Holographic Method

Abstract: The object of this paper is to present a new holographic method that permits three-dimensional imaging of hypervelocity particle fields with pulse lasers generating pulses having relatively long pulse durations. Dense and/or deep scenes can be imaged, since the temporal filtering properties of holograms permit discrimination between the region of the scene which is of interest and the unwanted region. The holographic configuration embodies a pulse laser source, a three-mirror reflector, a rotating mirror and a holocamera**. The rotating mirror compensates for the scene motion during exposure and the holocamera generates a local reference beam by either amplitude/ wavefront division or spatial frequency component separation method . The role of the three-mirror reflector is to provide an illuminating beam having a time invariant direction in the image space of the rotating mirror. It is shown that this method permits recovery of the three-dimensional velocity vector of each particle, even though the magnitude of the average velocity vector of the particles is much larger than that of the individual relative velocity vector of each particle with respect to another one. Substantial gain in accuracy is gained by using holocameras recording dark-field images. A theoretical study of the method and some results obtained by laboratory simulation are presented.© (1977) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Estimation and Control in Multidither Adaptive Optics.

Abstract: This paper develops optimal statistical estimation formulation for multidither adaptive optics control loops which potentially can enhance stable beam control performance in the presence of spurious signal-like noise. A multidither autofocus example is chosen to compare estimated assisted and conventional adaptive optical performance in the presence of speckle-generated noise.