A DF cw laser system is used an an example to illustrate the requirements for HEL diagnostics. The laser device performance parameters are identified, with primary diagnostics being near field power, near field irradiance, wavelength, angular beam jitter, and beam quality. A variety of component/subsystem diagnostics are reviewed, for the gain generator, optical resonator, diffuser/ejector and fluid supply subsystems. Methods of characterizing these subsystems, as well as approaches to the primary diagnostics, provide an adequate data base for understanding the high energy laser system.
Aerodynamic windows utilize gas flows to provide pressure and gas species isolation interfaces which are transparent to laser radiation. For CW chemical and gasdynamic lasers, a continuously operating window is required. An axial type window is described which is capable of sustaining a large pressure ratio (of at least up to 100) and is characterized by excellent optical quality. In this window the laser beam is extracted from the low pressure environment through an optical duct and enters into the supersonic uniform flow region of a Laval nozzle via an oblique opening in the duct. The beam crosses the oblique shock formed by the wedge surface of the opening and passes along the nozzle centerline through the nozzle throat, its subsonic section and into the atmosphere. Dry nitrogen flows through the supersonic nozzle opposite to the laser beam direction past the oblique duct opening into the aerowindow diffuser. Two windows of this type have been designed, built and operated over the past 9 and 7 years, respectively. The feasibility of using a shock tube generated pulsed flow field as a single-shot window for short, high-energy laser pulses was also investigated. Experiments were performed to define operating conditions which cause a minimal degradation in the quality of a laser beam transmitted through the window. In the experiments a scribed diaphragm shock tube with glass endwalls was used to simulate the window, and a laser pulse was transmitted along the tube axis after the diaphragm burst. The optical quality of the nearly one-dimensional pressure wave field and the turbulent contact interface was determined using both direct beam quality ("power in the bucket") and interferometric techniques. It was shown that with proper choice of design variables the beam quality degradation caused by this pulsed aerodynamic window is less than 10%.
A computer-controlled small signal gain measurement system has been developed for use on HF/DF chemical laser systems. The probe laser is an electrically initiated continuous wave (CW) HF/DF laser with active frequency stabilization. The probe is run single-line, single-mode with an automatic system for switching from one lasing line to another. The scanning system is designed to allow rapid, two-dimensional scan patterns to be generated in either Cartesian or polar coordinates. The entire system, including the probe laser, scanning system, data acquisition, and data reduction is controlled by a DEC PDP 11/34 minicomputer with a minimum amount of human intervention. The system is now operational and examples of measurements will be presented.
The Laser Wavefront Analyzer at the Air Force Weapon Laboratory records a 32 x 32 array of intensity and phase information for a 10 cm square input aperture at a rate of 100 frames per second. This device has been used to characterize the single pass gain profile and wavefront phase quality for a 15 cm diameter flowing carbon dioxide amplifier. The results of this experiment will be presented.
We have recently measured the refractive index of Hydrogen Fluoride gas from 2.5 to 2.9 microns, with a wavelength resolution of .004 microns 1. We will describe the theoretical modeling of those results and discuss their implications in this paper. The data of reference 1 are shown in figure 1. It is clear that resonant dispersion dominates the refractive index over much of the measurement region. The solid line is the best theoretical fit to our data. On the basis of our theoretical modeling, we feel that these results are significant because resonant dispersion effects may be an important factor in laser medium quality for certain lines. In order to determine the extent of these effects, further research is required, particularly for Deuterium Fluoride.
A simple automated device has been developed at the Air Force Weapons Laboratory to rapidly measure integrated irradiance profiles for circular symmetric "far field" distributions. The system consists of a computer driven measurement device and interface/timing electronics. IRIS can be used with both pulsed and continuous lasers. The basic concept will be presented along with specific examples.
The HELEN laser system at the Atomic Weapons Research Establishment, is a two beam Nd-glass laser used for the study of laser plasma phenomena relevant to weapons physics, and is capable of generating 100 J pulses of 1TW peak power in each arm. This paper presents an overview of the system with particular reference to recent developments in beam diagnostics. The diagnostics discussed fall into two categories of equal importance. Firstly, the measurement of beam parameters required for the complete analysis of experimental target data, namely laser pulse energy, pulse width, pre-pulse ratio and far-field intensity distribution; and secondly, measurement of parameters used to ensure optimum system performance such as near-field intensity distribution, amplifier gains and passive component transmission.
Experimental research for high energy lasers is currently accomplished with large volume CO2 flowing gain cells and unstable resonator mirror configurations which often include conical optical elements. There are a number of diagnostics available throughout the HEL community. However, there has never been a complete comparison between theoretical results and experiment, because of the limitations in diagnostic equipment and the complexity of current resonator designs. Analytical and numerical models for CO2 gain cells with unstable resonators also have limitations. As experimental configurations and theoretical models increase in complexity, comparison between the two becomes increasingly difficult because the number of variables is too large. In this presentation, we discuss the unknown variables and problems in making a complete comparison. A list of available diagnostic equipment and its limitations is presented. Some results from analytical and numerical models are compared to data. Finally, a summary of requirements for obtaining a reliable and complete comparison between experiment and theory is given.
The diagnostic measurements and accuracy goals required for an operational HEL test system must be derived from top level objectives. In contrast, prediction of system measurement uncertainty must be based on a bottoms-up assessment of state-of-the-art instrumentation capability. In this paper we present a methodology that can be used to define and evaluate an integrated diagnostic system for any HEL test system. Using this methodology we define and evaluate a specific diagnostic system for an example laser test system.
A high performance heterodyne interferometer is described that measures transient phenomena of flow fields. Data are acquired through 64 detectors in parallel with the output from each detector directed into a dedicated phase detector, digitizer, and buffer memory. This system operates at a 100 kHz frame rate with a precision of λ/300 rms.
Several different configurations of wavefront sensing instruments, that are based on the Hartmann sensor concept have been reported in the literature during the past years. The present paper describes experimental results that were obtained by using a novel technique that was first reported last year.1 This technique is aimed at interrogating wavefront deforming characteristics of an optical system under test by injecting a pencil beam of light (typically a diffraction limited, well collimated laser beam), that is translation scanned across the entrance aperture, and analyzing in real time the transmitted or reflected pencil beam. The two most important characteristics of the pencil beam that arrives at the receiver; i.e., the angle of arrival and intensity are both monitored simultaneously. This data is processed and displayed in a choice of formats. The present paper shows experimental results based on this diagnostic technique. The measured data is shown in the form of isometric plots of the angle of arrival. These re-sults are then compared to analytically expected behavior.
An interferometer which provides for the precise figure measurement of optical surfaces through the interference of two pencil beams, reflected off the optical surface. Since reference surfaces are not required, the interferometer is also capable of analysing aspheric optical surfaces like axicons. The accuracy of the figure measurement is ± 2 nm.
Wave optics has been used to study the property of the shadow and its effect on the multipass interferograms. Both analytical work and experimental work have been performed to investigate the shadow effect. For a flow medium or optical surface containing high spatial frequency distortion, the interferometer geometry and mirror pairs must be properly selected in order to reduce the intensity modulation on the interferograms.