This talk will cover my personal recollections about the early years of lasers and the laser damage situation in the 60s: the persons involved, the lasers used in damage experiments, the companies involved, funding agencies involved, personal interactions in those days, the nature of the papers in 1968, and the unique style of management and conduct of the first and later meetings by Art Guenther and Alex Glass, a co-invited speaker with me at this 50th anniversary meeting.
Epitaxial single-crystal chemical-vapor-deposited diamond with (100) crystal orientation is obtained from Element Six (Ascot, United Kingdom) and Apollo Diamond (Boston, Massachusetts). Both companies supply 5×5-mm squares with thicknesses of 0.35 to 1.74 mm. Element Six also provides disks with a state of the art diameter of 10 to 11 mm and a thickness of 1.0 mm. The absorption coefficient measured by laser calorimetry at 1.064 µm is 0.003 cm−1 for squares from Element Six and 0.07 cm−1 for squares from Apollo. One Apollo specimen has an absorption coefficient near those of the Element Six material. Absorption coefficients of Element Six disks are 0.008 to 0.03 cm−1. Each square specimen can be rotated between orientations that produce minimum or maximum loss of polarization of a 1.064-µm laser beam transmitted through the diamond. Minimum loss is in the range 0 to 11% (mean=5%) and maximum loss is 8 to 27% (mean=17%). Element Six disks produce a loss of polarization in the range 0 to 4%, depending on the angle of rotation of the disk. Part of the 0.04 to 0.6% total integrated optical scatter in the forward hemisphere at 1.064 µm can be attributed to surface roughness.
Epitaxial single-crystal chemical-vapor-deposited diamond was obtained from Element Six Ltd. (Ascot, UK) and from
Apollo Diamond (Boston, MA). Both companies provided 5 x 5 mm squares with thicknesses ranging from 0.5 to 1.5
mm. In addition, Element Six provided 10-mm-diameter disks with a thickness of 1.0 mm. The absorptance of all
specimens at 1064 nm was measured by laser calorimetry, with good agreement between independent measurements at
the University of Central Florida and at QinetiQ (Malvern, UK). Depolarization at 1064 nm and ultraviolet absorption
properties are also reported.
Absorption coefficients of highly transparent optical materials (glass) have been measured using a z-scan technique. Absorption coefficients as low as 10−5 cm−1 and lower can be measured by scanning a substrate of a transparent optical material in the focal region of a lens. Glass substrates with relatively high absorption coefficients are used for calibration and for verifying the results of measurements. The sensitivity of this technique proves to be high enough to reveal differences between absorption coefficients of different substrates made of the same material (BK7). We have suggested and used a novel procedure, scanning nonlinear lens profiling, for characterization of homogeneity of glass substrates and other transparent optical materials. Optical nonlinearity of glass substrates allows fast and high-precision measurement of the power and power density of high-power near-IR laser beams without large temperature increases and related problems.
We present a summary view of the DARPA SHEDS and ADHELS programs. The goal of these programs is development of technology of a future compact, field-deployable high energy laser (HEL) system.
We have applied z-scan technique for measuring absorption coefficients of highly transparent optical materials (glass) and have used those for characterizing the power and power density of high power near IR laser beams. Self-phase modulation due to thermal indexing is the process underlying this technique. Glass plates with appreciable absorption coefficients are used for calibration purposes and for verifying the results of measurements. Fundamentally, as low absorption coefficients as 10-5 cm-1 can be measured by scanning a plate of a transparent optical material in the focal region of a lens. The sensitivity of this technique proved to be high enough to reveal strong variation between the absorption coefficients of optical windows made of the same material BK7 but obtained from different sources. We have suggested and used a novel procedure, scanning nonlinear lens profiling, for characterization of homogeneity of optical glasses and other transparent optical materials. Most importantly, the technique can be used for fast and high precision measurement of power of high power laser beams without evoking large temperature increase and related problems.
We present a summary view of the DARPA Super High Efficiency Diode Sources (SHEDS) and Architecture for Diode High Energy Laser Systems (ADHELS) programs. The goal of these programs is development of technology of a future compact, field-deployable high energy laser (HEL) system.
We present a summary view of the DARPA MIATA program. This program is an effort to push the technology of mmW focal plane arrays towards smaller, lighter, and less expensive implementations, thereby increasing the military utility of this technology.
A new material for optics is being developed that promises to be far more robust than alternative materials. It is a photo-thermo-refractive (PTR) glass in which Bragg gratings (holograms) can be written in the interior (not the surface) of the glass. The gratings are permanent as they are not removed by illuminating them with light at other wavelengths or by heating unless the temperature exceeds 400 degree(s)C. This technology can be used to make diffractive elements such as spatial filters, attenuators, switches, modulators, beam splitters, beam samplers, beam deflectors, selectors of particular wavelengths (notch filters, add/drop elements), spectral shape formers (gain equalizers), spectral sensors, angular sensors, Bragg spectrometers, and transverse and longitudinal mode selectors in a laser resonator. The PTR Bragg grating has been exposed to a 100 W, 1096 nm beam focused to 100 kW/cm2 spot for 10 minutes without exhibiting any temperature rise. The pulsed laser damage threshold has been measured to be within 30% of that of the best silica glass used in high power 1064 nm systems. The useful spectral range of this glass is from 350 nm to 2.8 microns.
Using a recent theory of scintillation, including the development of a probability density function (PDF) for the irradiance, a theoretical analysis is carried out on the system performance of a coherent array receiver in terms of the scintillation index, the implied carrier-to-noise (CNR), and signal fade statistics. We also describe a field experiment in which measured data from a wave reflected from a rough (Lambertian) target located 1000 m from the transceiver was used to calculate the scintillation index, CNR, and probability of fade. The results of this study support the theoretical models.
The detection and processing of laser communication signals are drastically affected by the fading induced onto these signals by atmospheric turbulence. One method of reducing this fading is to use an array of detectors in which each of the detector outputs are added together coherently. This requires measuring the phase difference between each of the receivers and co-phasing each of the detector outputs. This paper presents experimental verification at the Innovative Science and Technology Experimentation Facility over an outdoor range of a 1.06 micron eight element coherent receiver used to mitigate the effects of fading. The system is composed of a 60 mw Nd:Yag laser used as the transmitter and a 27 MHz AO modulator used to frequency shift the transmitted beam. The receiver is composed of eight 1 cm lenses launching the eight received optical signals into eight signal mode optical fibers. Phase compensation between each of the eight receivers is accomplished using single mode fibers wrapped around PZT cylinders that are controlled by phase compensating electronics. The carrier-to-noise (CNR) ratio was measured on a single channel and was then compared with the CNR obtained from the coherent sum of the eight channels. The improvement of the CNR for the coherent sum as compared to a single channel was then compared against theoretical predictions.
We investigated a space diversity optical heterodyne array receiver for optimal coherent detection of finite transverse coherence length fields. In such an array receiver, each of the independent IF signals are appropriately phase-shifted and summed to generate a single IF signal with improved statistical qualities. In this paper we show that the advantages offered by a coherent array are both an increased average carrier-to-noise ratio and a reduction in carrier fading. We also show that the equal gain diversity reception technique performs almost as well as the maximal ratio technique with significantly less hardware complexity. Finally, we present the experimental results from a two element, one micron, laboratory breadboard ladar system.
A theoretical model is presented for the mutual coherence function that describes the power spectrum of the intermediate frequency (IF) signal resulting from coherent laser radar detection of a fully illuminated rough rotating cylinder. Preliminary calculations for the correlation length and bandwidths of the coherently detected dynamic speckle fields show that Doppler effects on speckle dynamics are on the same order of magnitude as speckle translation effects, and that scatterer exchange effects are negligible. We show that for a fully diffuse object, the shape of the spectrum (and autocorrelation function) is Gaussian. The spectrum appears to be the sum of two Gaussian functions when the scattering object has a near-specular component, one due to the specularly reflected field and the other due to the diffusely reflected field. While the IF BW of the signal resulting from the diffusely scattered field depends upon the cylinder's radius, the IF BW of the specularly reflected field depends upon the width of a window function, which is wavelength dependent.
This paper summarizes progress toward developing coherent ladar technology using arrays of subapertures for coping with target-induced speckle which can reduce the carrier-to-noise ratio for ladar systems. We have selected an optimum receiver design, developed algorithms for determining how to co-phase the IF signals from the different sub-apertures/receivers, and have begun to test these on a two-aperture, coherent 1064 nm ladar. Computer simulations of the ladar are being developed including partially developed speckle effects from the target. Target speckle has been measured from different materials and at two different wavelengths for bench-marking the computer simulations. We have measured and are developing the theory for the time-dependent optical phase shifts observed from a rough, rotating target. The signal enhancement has been calculated that might be expected for a rough target illuminated by a coherent, 1064 nm laser. Finally, we have designed and are beginning to implement an experiment to show that imaging through a turbulent medium can be achieved.
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