Publications and Presentations by Scientists at RAN Science & Technology, LLC
Russell Kurtz, Ph.D., Chief Scientist:
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Improving the Accuracy of Surface Metrology (2011)
There is a constant search for more accurate measurement, which generally leads to higher cost, greater complexity, or devices that do not lend themselves to manufacturing environments. We present a method of using statistical sampling to improve metrological accuracy without these undesirable effects. For metrology of flat surfaces and steps between flat surfaces, this method demonstrated precision improvement up to a factor of 55, and accuracy increase of at least a factor of 10. The corresponding precision and accuracy improvements on a spherical surface were both factors of eight. Since this accuracy improvement can be implemented in software, it does not affect the speed of measurement or the complexity of the hardware, and it can be used to improve the accuracy of a wide range of metrology systems
Photorefractive Amplification at High Frequencies (2011)
Photorefractive optical amplification, while useful, is a slow process. Under some circumstances, however, it amplifies optical signals effectively even when one is modulated at a relatively high frequency. We determine the reasons for this capability (what we have called the “Fast Photorefractive Effect”) and analyze its enhanced bandwidth, improvements over standard photorefractivity, and limitations. View Power Point Presentation: Photorefractive Amplification at High Frequencies – 0r- Some Vibrations Are Harder to Measure Than Others
Directed Acoustic Shearography (2010)
Modern vehicles use modern materials, including multiple metallic layers, composites, and ceramics. This has led to significant improvements in quality, reliability, and lifetime, at the cost of significantly increased complexity. It is particularly difficult to test these modern materials for buried defects such as internal corrosion, glue/weld failures, and disbonds, yet these defects can lead to damage and even failure of the part. As one tool in the array of nondestructive evaluation (NDE) technologies, we report on Directed Acoustic Shearography (DAS), which combines the sensitivity of shearography with the speed of ultrasonic imaging, and adds improved depth resolution. We show that DAS is particularly useful in detecting buried defects in modern materials, how it lends itself to automation, and present early tests of DAS detecting buried defects as small as 1/32 inch in a multilayer aluminum structure. View Power Point Presentation: Directed Acoustic Shearography – or – What You Can’t See CAN Hurt You
The Fast Photorefractive Effect and its Application to Vibrometry (2009)
(As published in the Journal of Holography and Speckle)
We previously reported on what we describe as the “fast photorefractive effect,” photorefractive signal amplification at much greater frequencies than predicted by its grating formation speed. In this paper we explain the effect and its potential for application to vibrometry. We demonstrated photorefractive amplification in Cu:KNSBN (whose grating formation speed is <5 Hz), matching the standard model with CW illumination. We then demonstrated photorefractive amplification of vibrometric signals at frequencies up to 4 MHz. Our theory of the fast photorefractive effect indicates that the amplification bandwidth of Cu:KNSBN at 488 nm illumination could exceed 800 GHz.
High Speed Nano-Optical Photodetector for Free Space Communication (2007)
(As published in Micro (MEMS) and Nanotechnologies for Defense and Security)
An inexpensive, easily integrated, sensitive photoreceiver operating in the communications band with a 50-GHz bandwidth would revolutionize the free-space communication industry. While generation of 50-GHz carrier AM or FM signals is not difficult, its reception and heterodyning require specific, known technologies, generally based on silicon semiconductors. We present a 50 GHz photoreceiver that exceeds the capabilities of current devices. The proposed photoreceiver is based on a technology we call Nanodust. This new technology enables nano-optical photodetectors to be directly embedded in silicon matrices, or into CMOS reception/heterodyning circuits. Photoreceivers based on Nanodust technology can be designed to operate in any spectral region, the most important to date being the telecommunications band near 1.55 micrometers. Unlike current photodetectors that operate in this spectral region, Nanodust photodetectors can be directly integrated with standard CMOS and silicon-based circuitry. Nanodust technology lends itself well to normal-incidence signal reception, significantly increasing the reception area without compromising the bandwidth. Preliminary experiments have demonstrated a free-space responsivity of 50 µA/(W/cm2), nearly an order of magnitude greater than that offered by current 50-GHz detectors. We expect to increase the Nanodust responsivity significantly in upcoming experiments.
Mutual Injection Locking: A New Architecture for High-Power Solid-State Laser Arrays (2005)
(As published in IEEE Journal of Selected Topics in Quantum Electronics)
Bidirectional (mutual) injection locking was demonstrated with solid-state lasers, producing significant improvements over traditional single-direction injection locking. Each laser element shares part of its output with other elements in bidirectional locking, distinct from single-direction (traditional) injection locking where one master laser provides the locking signal for a number of slaves. In a phase-locked array, the individual laser outputs add coherently, and the brightness of the entire array scales with the square of the number of elements, as if the active material diameter were increasing. Benefits of bidirectional locking, when compared to traditional injection locking, include reduced laser threshold, better output beam quality, and improved scaling capability. Experiments using two Nd:YVO4 lasers confirmed that mutual injection locking reduced lasing threshold by a factor of at least two and increased the output beam quality significantly. The injection locking effects began with 0.03% coupling between lasers and full-phase locking for coupling exceeding 0.5%. The 0.5% requirement for full phase-locking is significantly lower than the requirement for traditional injection locking. The large coupling requirement limits traditional injection-locked arrays to fewer than 20 elements, while mutually injection-locked arrays have no such limit. Mutual injection locking of an array of lasers can lead to a new architecture for high-power laser systems.
Image Tiling for a High-Resolution Helmet-Mounted Display (2005)
Head-mounted or helmet-mounted displays (HMDs) have long proven invaluable for many military applications. Integrated with head position, orientation, and/or eye-tracking sensors, HMDs can be powerful tools for training. For such training applications as flight simulation, HMDs need to be lightweight and compact with good center-of-gravity characteristics, and must display realistic full-color imagery with eye-limited resolution and large field-of-view (FOV) so that the pilot sees a truly realistic out-the-window scene. Under bright illumination, the resolution of the eye is ~300 µr (1 arc-min), setting the minimum HMD resolution. There are several methods of achieving this resolution, including increasing the number of individual pixels on a CRT or LCD display, thereby increasing the size, weight, and complexity of the HMD; dithering the image to provide an apparent resolution increase at the cost of reduced frame rate; and tiling normal resolution subimages into a single, larger high-resolution image. Physical Optics Corporation (POC) is developing a 5120 x 4096 pixel HMD covering 1500 x 1200 mr with resolution of 300 µr by tiling 20 subimages, each of which has a resolution of 1024 x 1024 pixels, in a 5 x 4 array. We present theory and results of our preliminary development of this HMD, resulting in a 4k x 1k image tiled from 16 subimages, each with resolution 512 x 512, in an 8 x 2 array.
Reflection Shearography for Non-Destructive Evaluation (2004)
Conventional nondestructive evaluation (NDE) techniques include visual inspection, eddy current scanning, ultrasonics, and fluorescent dye penetration. These techniques are limited to local evaluation, often miss small buried defects, and are useful on polished surfaces only. Advanced NDE techniques include laser ultrasonics, holographic interferometry, structural integrity monitoring, shearography, and thermography. A variation of shearography, employing reflective shearographic interferometry, has been developed. This new shearographic interferometer is presented, together with models to optimize its performance and experiments demonstrating its use in NDE.
Long-Range Phase-Conjugate Interferometry (2004)
The most accurate method of measuring distance and motion is interferometry. This method of motion measurement correlates change in distance to change in phase of an optical signal. As one mirror in the interferometer moves, the resulting phase variation is visualized as motion of interferometric fringes. While traditional optical interferometry can easily be used to measure distance variation as small as 10 nm, it is not a viable method for measuring distance to, or motion of, an object more than half the illumination coherence length distant. This typically limits interferometry to measurements of objects within <1 km of the interferometer. We present a new interferometer based on phase conjugation, which greatly increases the maximum distance between the illumination laser and the movable target. This method is as accurate as traditional interferometry, but is less sensitive to laser pointing error and operates over a longer path. Experiments demonstrated measurement accuracy <15 nm with a laser-target separation of 50x the laser coherence length.
Injection Locking Efficiency of Two Independent Lasers (2004)
Bidirectional (mutual) injection locking was demonstrated with solid-state lasers, producing significant improvements over traditional single-direction injection locking. Each laser element shares part of its output with other elements in bidirectional locking, distinct from single-direction (traditional) injection locking where one master laser provides the locking signal for a number of slaves. In a phase-locked array, the individual laser outputs add coherently, and the brightness of the entire array scales with the square of the number of elements, as if the active material diameter were increasing. Benefits of bidirectional locking, when compared to traditional injection locking, include reduced laser threshold, better output beam quality, and improved scaling capability. Experiments using two Nd:YVO4 lasers confirmed that mutual injection locking reduced lasing threshold by a factor of at least two and increased the output beam quality significantly. The injection locking effects began with 0.03% coupling between lasers and full-phase locking for coupling exceeding 0.5%. The 0.5% requirement for full phase-locking limits traditional injection-locked arrays to fewer than 100 elements, while mutually injection-locked arrays have no such limit. Mutual injection locking of an array of lasers can lead to a new architecture for high-power laser systems.
HWIL Lidar Imaging Sensor, 3-D Synthetic and Natural Environment, and Temporal ATR (2002)
In this paper, LIDAR imaging sensors, 3-D synthetic and natural object-centric environment, and temporal (progressive) ATR (Automatic Target Recognition), are discussed in the context of Modeling and Simulation (M&S) and Hardware-in-the loop (HWIL) testing.
Increase Your Laser System’s Lifetime (1992)
When using lasers in an industrial setting, there are many sources of unbudgeted costs. Among these costs are costs of replacing lenses and reflectors damaged in use, the costs of remachining parts which were incorrectly machined due to alignment or laser quality problems, and down time for unexpected repairs. Although none of the sources of unbudgeted costs can be eliminated, many can be ameliorated by a properly implemented routine maintenance program. Such a program prevents many problems by anticipating and eliminating the causes, and reduces the damage of others by detecting blemishes before they turn into disasters. Those companies which have implemented effective routine maintenance programs have mean times between failure of their laser machining systems several times longer, and mean times to repair these systems several times shorter, than companies that have no such program.
This paper presents an example of a routine maintenance program. This program does take some time to implement, probably about five to six hours per month, but this time is likely to be less than the down time prevented by its implementation. When one also recalls the cost reduction due to avoidance of unexpected replacements and remachining, it becomes obvious that implementing a good maintenance program pays for itself in increased efficiency and reduced down time.
Spectroscopic and 3-Microm Lasing Properties of Erbium-Doped Yttrium Aluminum Garnet and the Effects of Holmium Co-Doping (1991, PhD Dissertation)
The spectroscopy and 3-µm lasing behavior of Er:YAG are studied. Er:YAG is shown to lase on two distinct lines near 2.94 µm, specifically 2.9393 and 2.9362 µm. These two laser lines operate simultaneously. Although both begin in level A2 (the second lowest energy Stark sublevel of the 4I11/2 level), the 2.9362-µm line terminates at level Y6 and the 2.9393-µm line at level Y7 (Y7 and Y6 are the highest and second highest energy Stark sublevels, respectively, of the 4I13/2 level). These wavelengths imply that the Y6 level, whose energy was previously unknown, has an energy of 6873.8 cm-1.
The effects of adding Ho3+ ions to Er:YAG is also reported. Energy is transferred from the 4I13/2 level of Er3+ to the 5I7 level of Ho3+. As reported in this dissertation, the value of the coefficient describing this energy transfer is WEH = 9.5±1.0 x 10-20 cm3/s [a later reanalysis of the data finds a value of 4.33 x 10-19 cm3/s, 4.5x greater]. This value suggests that the Er3+–>Ho3+ energy transfer is not strong enongh to unblock the Er:YAG 3-µm transition, but is strong enough to unblock the Er:YLF 3-µm transition. The energy transfer also suggests diode-pumping Er3+ ions, which then transfer their energy to the Ho3+ ions, creating an efficient, diode-pumped 2-µm Ho3+ laser.
Multiple wavelength lasing of (30% Er, 1.5% Ho):YAG is described. In addition to the two wavelengths seen in Er:YAG, (Er, Ho):YAG lases at 2.796 and 2.766 µm during the same pump pulse as the 2.936- and 2.939-µm lines. Both these transitions are attributed to the Er3+ ions. The 3-µm lasing is shown to have a lower gain than the lasing near 2.8 µm, but atmospheric losses at 2.8 µm are sufficient to prevent lasing at these wavelengths. The 2.8-µm lines are seen in (Er, Ho):YAG after the 3-µm lines have self-terminated due to excited-state absorption in the Ho3+.
New Laser Lines of Erbium in Yttrium Aluminum Garnet (1990)
We studied the lasing and spectroscopic properties of erbium in yttrium aluminum garnet, both a a single impurity and when codoped with neodymium or holmium. In all cases, we observed lasing at 2.936 and 2.939 µm; when erbium was codoped with holmium, we also observed lasing at 2.795 and 2.766 µm. [This is in contrast to (Er, Nd):YAlO3, which lased on only one line, 2.73 µm.] By determining the energy-level splitting implied by the four observed laser lines, and combining this with transmission spectroscopy, we were able to assign unambiguous values to the Stark sublevels of the three lowest energy levels of Er3+ in YAG at room temperature.
Mutiple Wavelength Lasing of (Er, Ho): YAG (1989)
We tested a solid state laser material, YAG doped with 30% /(at.) Er3+ ions and 1.5% (at.) Ho3+ ions. The laser levels in both Er3+ and H03+ demonstrated altered lifetimes when compared to equivalently-doped Er:YAG and Ho:YAG, indicating moderate interactions between the Er3+ and Ho3+ ions. When we lased (Er, Ho):YAG, we observed output at three wavelengths: approximately 2.939, 2.936, and 2.796 µm. The first two of these lased simultaneously, while the third appeared later in the same pump pulse. This lasing blueshift may be explained by excited-state absorption (ESA) in the Ho3+ ions.
Operation of High Dopant Density Er: YAG (1986)
Free- running, pulsed, flashlamp- excited operation of 50% and 33% Er doped YAG lasers is reported at 2.94 µm. This laser was described by researchers in the Soviet Union as early as 1975. Since then there have been a number of further reports concerning this material all published by Soviet scientists. This paper represents, to our knowledge, the first publication outside of the Soviet Union about high dopant density Er:YAG laser operation. In addition to confirming some of the performance properties described earlier, this paper presents the unusual temporal waveforms of the Er:YAG, 2.94 µm laser. An outline is given of possible pumping and relaxation processes which may contribute to the laser’s operation. Er:YAG does not lase well at 2.94 µm when the concentration of Er is the usual 1%. However, when larger concentrations are used (generally over 15%) operation at this wavelength can be quite efficient.
A High Efficiency Switching Power Supply (1981, Undergraduate Thesis)
My project was to design a switching power supply which output 5 volts regulated at up to about 5 amps load current, given dc inputs of 16 to 60 volts. The circuit I was originally given, which was not working correctly, used a monolithic “switching power supply regulator” chip. I redesigned the switching power supply so that it would work. The original circuit used the switching regulator mentioned above as most of its control section; I discovered that this version of the switching power supply could not work correctly, and redesigned it so that it would. The new control section was formed of discrete components and a pulse-width modulator (PWM). The specifications forthis new design were: efficiency )96% (to prevent excessive heat losses), small size, about 7 volts unregulated output (i.e., the ripple is unimportant) with up to 5% variation for loads of 0 to 5 amps, inputs of 10.0 to 40.0 volts, and operating temperature range of -25 to +125 degrees centigrade. These specifications were eventually met.
