We report a method for locking the output wavelength and reducing the spectral linewidth of diode lasers by feeding back light to the emitters from a wavelength selective external optical cavity. Ten years ago our team developed a stepped-mirror that allowed a single external cavity to lock the wavelength of a stack of diode array bars by equalizing path lengths between each emitter and the grating. Here we report combining one such step-mirror external cavity with an array of power dividers, each sending a portion of this feedback power to a separate diode array bar stack.
External cavity diode laser systems are well-suited for diode pumped alkali laser (DPAL) systems due to their high power efficiency and excellent wavelength control under changing thermal loads. By conditioning the characteristics of feedback power, external cavities can narrow the spectral bandwidth and limit transverse modes of diode laser bars. Existing configurations typically use low-efficiency diffraction gratings at the Littrow angle to send back to the diodes a small fraction of the power, while directing the majority of the power forward in the output beam. We previously reported that a stepped mirror allows a single external cavity to condition the output beams of a stack of diode array bars. In this report, we describe a new approach that could use a single external cavity to condition the output beams of several hundred diode array bars. A high efficiency grating is used to feedback essentially all the power in the external cavity, and power splitters then distribute the power to multiple diode array stacks. A 384 bar module capable of 20 kW power output into a modelimited slowly diverging beam with a spectral width below 0.050 nm has been designed and proposed for use in a DPAL. A 50 bar 3 kW prototype is currently being assembled.
The Smart, White-Light Dazzler (SWLD) is a nonlethal weapon designed to aim and deliver a dazzling and disabling light flash of maximum eye-safe energy to a selected target. The two key features of the SWLD technology are its self-aiming and power-adjusting capabilities; optical barriers, such as dark glasses, rifle scopes, binoculars, etc., and iris aperture, whether the eyes are light or dark adapted, are automatically taken into account by using a low-power infrared (IR) laser to probe and return a glint from the eye(s) of the target. Using the retro-reflected glint the dazzle pulse is adjusted and directed to arrive at the target with maximum allowable nonlethal energy at any range from 1 m to 100 m.
The collateral risk of this technology is very small. If the weapon is misaimed dramatically, the returned glint may come from an unintended person who will then be dazzled. Although this person will be incapacitated for 2-3 minutes, he will suffer no long-term effects. We assume all persons in dangerous situations would rather be accidentally, temporarily dazzled than suffer more serious consequences.
The SWLD adds an important tool to the spectrum of nonlethal responses available for use by military and law enforcement personnel. Applications include dispersing persons in crowd control and disabling terrorists in hijacking situations. The dazzle process may be repeated, choosing the next most susceptible target until a crowd is subdued. One important application in counter-terrorism is onboard planes where a pilot can fire a SWLD through a cockpit-door window and dazzle a hijacker with no damage to passengers.
This paper describes a quantitative schlieren technique called Calibrated Color Schlieren (CCS) that is capable of measuring the light deflection angle in both spatial directions simultaneously and hence is able to extract the projected density gradient of a two-dimensional flow. CCS makes use of a graded color filter in combination with a square source of size whose size may be varied to change the sensitivity. A calibration polynomial is used to obtain the deflection angle from color ratios at each pixel. The technique’s performance was assessed in terms of repeatability, sensitivity and accuracy using the Prandtl-Meyer expansion fan at the wedge-plate shoulder in a supersonic flow. From the measured deflection angles the density gradient and the density are computed. The density information agrees well with Prandtl-Meyer theory. The technique is also applied to a more complex wake flow, which required the use of a color correction based on a shadowgraph image.
Using holographic methods to fabricate antireflection coatings is attractive because arbitrarily small reflectivities may be achieved at a single wavelength with a simple photographic process. In this paper, we discuss the theoretical aspects of these holograms and the technical aspects of fabricating them. We present numerical simulations that predict the spectral response of the coating as a function of the bandwidth of the recording source and the thickness and modulation of the emulsion. These results are confirmed with spectrophotometer measurements of actual coatings. Recording holographic antireflection coatings requires a single collimated light source. The hologram-air interface is sued to create the second wave; this allows the coating to automatically correct for variation in the surface of the hologram. However, this method requires a post recording coating procedure to produce a diffracted wave with the correct phase shift. After the hologram is recorded and processed, a thin layer of SiO2 or MgF2 is applied to the hologram to adjust the phase shift between the diffracted wave and the surface reflection. Additionally this coating procedure enhances the durability of the coating and, if MgF2 is used, lowers the refractive index modulation required for complete cancellation of the surface reflection.
The study of a self-excited compressible jet using single and double-pulsed, phase-shifted interferometry in conjunction with a 9 beam tomographic illumination system is described. A plane wave holographic interferometer using a pulsed ruby laser has been adapted to provide multiple illumination directions of a volume that is approximately 4 centimeters on a side. This set-up is being used to study the transient behavior of compressible jets and may be operated using double-exposure holographic interferometry to study the instantaneous behavior of the flow; alternatively, the system may ge operated in a double-pulse mode to study the fluctuations in the flow. The tomographic reconstructions are made using a Fourier-Bessel expansion. To illustrate the performance of the system, an oscillating pipe-collar nozzle flow producing a wavy flow pattern was studied. The instantaneous measurements show the flow to be oscillating in one plane, whereas from the differential results it is found that this plane is rotating during the oscillation period.
Singular value decomposition has served as a diagnostic tool in optical computed tomography by using its capability to provide insight into the condition of ill-posed inverse problems. Various tomographic geometries are compared to one another through the singular value spectrum of their weight matrices. The number of significant singular values in the singular value spectrum of a weight matrix is a quantitative measure of the condition of the system of linear equations defined by a tomographic geometery. The analysis involves variation of the following five parameters, characterizing a tomographic geometry: 1) the spatial resolution of the reconstruction domain, 2) the number of views, 3) the number of projection rays per view, 4) the total observation angle spanned by the views, and 5) the selected basis function. Five local basis functions are considered: the square pulse, the triangle, the cubic B-spline, the Hanning window, and the Gaussian distribution. Also items like the presence of noise in the views, the coding accuracy of the weight matrix, as well as the accuracy of the accuracy of the singular value decomposition procedure itself are assessed.
The study of an underexpanded compressible jet using pulsed, phase- shifted interferometry in conjunction with a 9 beam tomographic illumination system is described. A plane wave holographic interferometer using a pulsed ruby laser has been adapted to provide multiple illumination directions of a volume that is approximately 4 centimeters on a side. This set-up is being used to study the transient behavior of compressible jets and may be operated using double-exposure holographic interferometry to study the instantaneous behavior of the flow; alternatively, the system may be operated in a double-pulse mode to study the fluctuations in the flow. The tomographic reconstructions are made using a Fourier-Bassel expansion. To illustrate the performance of the system, an underexpanded nozzle flow producing a series of compression-rarefaction 'diamonds' was studied. The images show sharp reconstructions of the compression-rarefaction diamond pattern as well as some interesting secondary flow structures.
Simultaneous quantitative measurements of both the temperature and velocity fields for 2D transient natural convection in an enclosure are made using calibrated multichannel electronic interferometry and digital particle image velocimetry. Calibrated multichannel electronic interferometry, a technique for quantitative flow visualization of transient phenomena, is discussed. This technique uses an interferometer combined with diffraction gratings to generate the three phase shifted interferograms simultaneously. The optical system is calibrated using standard piezoelectric phase shifting. The alignment is determined using spatial crosscorrelation. The calibration and alignment routines account for errors due to the separation of the phase shifted interferograms. Digital particle image velocimetry is implemented simultaneously with the interferometric measurements using a separate video channel. The system is used to examine the development of transient natural convection in enclosures at three angles of inclination.
Conventional methods for detecting the release of foreign substances into the atmosphere are often slow, expensive, and only give an actual reading for a small section of the area of interest. The Environmental Interferometer can allow inexpensive, real time monitoring of a large area. The principle behind the Environmental Interferometer is the use of a fringe locked Michelson Interferometer scanning throughout a continuous range of colors at an intermediate bandwidth (50-100 nm). The fringe locking allows a the beam in a test arm to be reflected through a test area for about 1 kilometer of distance, while a reference arm is kept in a controlled environment (perhaps fiber optics or a multiple reflected air path) and retain a suitable interference pattern. The use of intermediate bandwidth light allows the central fringe to be located, and thus allows fast scanning through a continuous range of colors. Sampling at n different colors allows the discrimination of n different sources of optical pathlength change. This allows easy discrimination against moisture content change, air turbulence, ground vibrations, and the like, because of their characteristic pathlength change frequencies. The fringe locking allows for the electronic interpretation of a signal and enhances the accuracy of the instrumentation so that small optical pathlength changes can be easily measured and interpreted. A demonstration unit has been created using a 670 nm laser instead of a filtered white light source. Absolute index measurements of test gases injected into a 3 cm pollution chamber were made with the demonstration unit with errors of less than 1%. The fringe locker used in the demonstration unit was able to keep the fringe pattern stable during table oscillations, moderately fast introduction of test gases, and simulated air turbulence.
A three illumination beam, phase-shifted interferometry system is used to measure 3D surface deformation fields to study the mechanics of cracks loaded in shear. The interferometer, which is mounted to the actuator of a servohydraulic loading machine, is augmented with a piezoelectric phase shifter to obtain a sequance of differential phase maps over small load increments. This avoids problems with speckle decorrelation and provides more detailed information about the phenomena of interest. The displacement fields were accumulated relative to the unloaded state by sampling at appropriate locations in the incremental fields to optimize spatial resolution and compensate for large rigid body motions. Features of the effect of fracture surface interference on a shear-loaded crack inferred from the displacement fields are described.
Tomographic reconstruction from both interferometric and absorption data is a potentially powerful tool for experimental observations of compressible fluid mechanics, combustion, and heat transfer. In many of these cases, both flow field images and ensemble statistics are desired. The use of an ensemble of noisy tomographic data sets to synthesize image statistics and stabilize individual reconstructions using a Maximum A Posteriori (MAP) reconstruction technique is presented. The MAP technique uses the ensemble mean and variances of the source function to constrain individual reconstructions of the ensemble. In this paper, we show that by synthesizing the mean and variances using preliminary algebraic reconstructions, the reconstruction of the individual realizations can be improved. The technique is demonstrated using a group of source images generated with a fractal sum of pulses technique. The paper discusses a fractal model for turbulent mixing field images, the selection of the preliminary reconstruction technique, and the results of MAP and synthetic MAP reconstructions.
The simulation of images generated by thermally-radiating, optically- thick turbulent media are discussed and the time-dependent evolution of these images is modeled. This characteristics of these images are particularly applicable to the atmosphere in the 13-15 mm band and their behavior may have application in detecting aviation hazards. The image is generated by volumetric thermal emission by atmospheric constituents within the field-of-view of the detector. The structure of the turbulent temperature field and the attenuating properties of the atmosphere interact with the field-of-view's geometry to produce a localized region which dominates the optical flow of the image. The simulations discussed in this paper model the time-dependent behavior of images generated by atmospheric flows viewed from an airborne platform. The images ar modelled by (1) generating a random field of temperature fluctuations have the proper spatial structure, (2) adding these fluctuation to the baseline temperature field of the atmospheric event, (3) accumulating the image on the detector from radiation emitted in the imaging volume, (4) allowing the individual radiating points within the imaging volume to move with the local velocity, (5) recalculating the thermal field and generating a new image. This approach was used to simulate the images generated by the temperature and velocity fields of a windshear. The simulation generated pais of images separated by a small time interval. These image paris were analyzed by image cross-correlation. The displacement of the cross-correlation peak was used to infer the velocity at the localized region. The localized region was found to depend weakly on the shape of the velocity profile. Prediction of the localized region, the effects of imaging from a moving platform, alternative image analysis schemes, and possible application to aviation hazards are discussed.
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