Journal of Directed Energy, Volume 5, Number 4

Jitter on the target of a high energy laser system can have a significant effect on energy transfer during an engagement with mortars or other artillery. Jitter in an actual target engagement can be caused by turbulence of the projectile, the atmosphere, inconsistencies within the tracking system of the laser, and jitter within the laser system itself. In this paper we compare the effect of jitter on heat transfer with the effects of stretching the stationary spot size in one direction with a cylindrical lens. Results show that stationary spot size of a certain horizontal beam radius cannot be equated with sinusoidal jitter of equal amplitude.

Measurements of the electric and magnetic fields within a rail gun can provide critical electromagnetic details that can be used for diagnostic purposes as well as to support modeling and simulation efforts. Fiber optic probes based on electro-optic (EO) and magneto-optic (MO) materials are ideal for such measurements because of their small sizes and nonmetallic structure. The output signals from the sensors are carried by fiber optic cables and monitored on an oscilloscope, whose signal voltages follow a simple proportional relationship to respective electric and magnetic fields. The main technical challenge in this effort is to overcome the extreme mechanical, electromagnetic, and acoustic environment within the rail gun bore, which can generate significant background noise in the sensors. Overcoming this challenge is particularly critical in EO sensors, since the magnetic field signature of the rail gun is large enough to yield acceptable signal-tonoise ratios in our MO sensors. In this report, we describe our EO and MO sensors, modifications necessary to improve their robustness and resilience in rail gun use, and various electric and magnetic field data from the NRL rail gun.

All dielectric field probes based on electro-optic (EO) and magneto-optic (MO) crystals are becoming attractive alternatives to conventional D-dot and B-dot sensors. Conventional sensors are limited by their size, frequency response, metallic structures, and inability to directly measure true waveforms in real time. A single EO sensor can measure electric fields from dc to 20 GHz, and an MO sensor can measure magnetic fields from dc to 2 GHz. Both sensors are small in size, minimally intrusive, and can be combined to form an integrated sensor that measures both electric and magnetic fields. EO and MO sensors provide an instantaneous signal output that precisely replicates the external field in real time, without the need of intervening signal processing. In this report, we discuss recent advancements in improving sensitivity and robustness of the sensors, as well as an evaluation of their use at higher frequencies (up to 20 GHz).

This paper introduces a method for measuring high-voltage discharges into an RG218/U coaxial cable. Specifically, a slab-coupled optical sensor (SCOS) is inserted into an RG218/U coaxial cable via a 3-mm diameter hole that is drilled and then filled with epoxy. This paper explains the method for inserting the fiber optical sensor into the coaxial cable as well as considerations that must be taken into account when inserting the fiber optical sensor into the coaxial cable, such as the dielectric breakdown of the cable, as well as reflections caused by the hole in the cable used to insert the sensor. This paper shows and discusses SCOS measurements from a series of discharged pulses ranging from 9.2 kV to 54 kV, simulating a high power-microwave weapon voltage discharge.

Stochastic parallel gradient descent (SPGD) is a metric-based approach for adaptive optics. This approach optimizes a scalar metric by taking random perturbative steps for many actuators simultaneously. Conventional adaptive optics uses a wavefront sensor to compute the conjugate phase for the deformable mirror, requiring high levels of computational resources. In contrast, the SPGD metric may be determined using a small region of a single charge-coupled device camera by evaluating the power-in-the-bucket, or one of several other metrics available. The focus of this research was to evaluate the performance of metric adaptive optics for compensating atmospheric effects on laser beam propagation. The SPGD algorithm was modified to use an adaptive gain to increase the convergence rate of the algorithm while maintaining a low steady-state error. Perturbations that follow Kolmogorov statistics were determined in Zernike space and shown to provide faster convergence than simply perturbing actuator voltages. Results of laboratory experiments to evaluate the performance of the SPGD algorithm using phase plates to simulate atmospheric turbulence are presented.

It is well known that coherently illuminated objects will result in degraded images and degraded track centroids, compared to an incoherently illuminated object. These degradations from temporal coherence are manifested by increased intensity modulation in an image caused by coherent constructive and destructive interference of reflections from different parts of the object. To reduce such coherent interference, techniques for reduction of temporal coherence are of interest to reduce interference from different depth cells of the object. It is also well known that propagation of a spatially coherent laser through the atmosphere and focused on a target can cause scintillation. This scintillation can result in non-uniform illumination of a target that can result in a spatially modulated active image and/or an erroneous active-track centroid. Hence one might also consider techniques for spatial coherence reduction of a transmitted beam as a means for mitigating such scintillation. In this effort, the impact of both spatial and temporal coherence reduction are explored in parallel, addressing the possibility of reduction of coherent artifacts described above, within the practical limits of existing spatial and temporal modulation devices. In this preliminary study of the use of partially coherent sources for directed-energy applications, simulation results show a factor of 2 or more reduction in atmospheric scintillation on target using reasonable levels of spatial modulation to reduce spatial coherence. On the other hand, little reduction in laser speckle was found in simulated received imagery, using reasonable reductions of temporal coherence of the transmitted beam.

Risk assessment for exposure to the output from a high-power laser system involves consideration of the range of directions over which laser energy can be emitted, and the damage that the incident laser energy could cause to an unprotected observer or sensor. A deterministic laser hazard zone (LHZ) based on nominal hazard distances derived from applicable maximum permissible exposure (MPE) limits, and incorporating all possible directions in which laser energy could be fired, can be severely restrictive for testing and training operation purposes with high-power laser systems on outdoor ranges. An alternative approach based on the use of probabilistic risk assessment (PRA) techniques can result in more practical, less onerous, clearance restrictions due to quantification of the extant risk and introduction of the acceptable risk level concept. The probabilistic laser hazard evaluation technique is based on an application of the formal risk assessment process developed since the early 1980s, and which is also shown to underpin the deterministic hazard assessment technique. This discussion is an extended version of a paper presented at the 2015 International Laser Safety Conference (ILSC 2015).