Lew Goldberg

United States Army, Washington, West Virginia, United States

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Publications (14)15.71 Total impact

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    ABSTRACT: The Monoblock laser has become the laser of choice in long-range, eye-safe laser range finders. It is eye-safe with emission at 1570 nm, high pulse energy, simple construction, and high efficiency when pumped by a laser-diode stack. Although the output beam divergence of a typical Monoblock with a 3 mm×3 mm cross section is relatively large (10-12 mrad), it can be reduced to <1 mrad using a telescope with large magnification. In this paper we present a simple and compact technique for achieving significant reduction in the Monoblock beam divergence using a partial reflector that is placed a short distance from the optical parametric oscillator (OPO). Using a 38 mm long Monoblock with a 10 mm long potassium titanyl phosphate OPO, we achieved a beam divergence of <4 mrad, corresponding to a >2.5× reduction from the unmodified laser. Performance using this technique with various feedback and etalon spacings is presented.
    Applied Optics 03/2014; 53(7):1247-51. · 1.69 Impact Factor
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    ABSTRACT: We have fabricated prototype frequency tripled Nd:YAG lasers using 808nm Vertical Cavity Surface emitting laser (VCSEL) arrays for end-pumping. The passively Q-switched Nd:YAG laser generated 15mJ pulses with a duration of 2-4 ns. Used as a source for third harmonic generation, the laser produced in excess of 2mJ at 355nm. Of particular concern was the impact of temperature variation on conversion efficiency, which included effects for both the source laser and non-linear crystals. Various solutions to the temperature effects were explored to enable operation of the frequency tripled laser over a wide temperature range.
    Proc SPIE 03/2013;
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    ABSTRACT: Laser illumination makes it possible to perform high resolution imaging when ambient light level is insufficient to overcome camera noise. The relatively long coherence length of most lasers, however, causes coherent speckle in the camera image plane, which can result in a significant decrease of the image quality and the maximum achievable target identification range. We characterized several types of NIR and SWIR laser diode illumination sources, with emphasis placed on measuring the properties of coherent speckle observed in the camera image plane. Image plane speckle contrast was measured by illuminating the imaged Lambertian surface with single-mode laser, multi-mode laser, wide-stripe laser with two active junctions and broad-band emission, and NIR and SWIR vertical cavity surface emitting laser (VCSEL) arrays. The impact of various imaging system parameters, including pixel size, imaging lens focal length, F-number, and IFOV on the contrast and characteristic size of the speckle intensity distribution were determined. Speckle contrast dependence on the polarization properties of various reflecting surfaces was measured. The reduction of speckle contrast with increasing source spectral width, and increasing size of spatially incoherent VCSEL emitter arrays will be described. We show that a speckle contrast of 5-10% is achievable for a typical long range SWIR imaging system.
    Proc SPIE 02/2013;
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    ABSTRACT: We have explored using 808nm Vertical Cavity Surface emitting laser (VCSEL) arrays for end-pumping of Nd:YAG lasers. A variety of laser designs were explored including a compact passively Q-switched lasers that produced a 22mJ pulse having a pulse width of <1.5ns, and an actively Q-switched laser that produced a 40mJ pulse having a 7 ns pulse width. The VCSEL pumped actively Q-switched laser was used as a source for sum frequency generation. Using a 2mm type II KTP and 3mm type I LBO, we generated greater than 5mJ at 355nm with a 21% THG conversion efficiency.
    Proc SPIE 02/2012;
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    ABSTRACT: During the past few years the Monoblock laser has become the laser-of-choice for Army laser range-finders. It is eyesafe with emission at 1570 nm, high pulse energy, simple construction, and high efficiency when pumped by a laserdiode stack, providing advantages that are not available with other laser types. Although the divergence of the Monoblock output beam is relatively large, it can be reduced to <1 mR using a telescope with a large magnification. This solution, however, is not acceptable for applications where the laser and telescope size must be kept to a minimum. In this paper we present a simple and compact technique for achieving significant reduction in the Monoblock beam divergence using a partial reflector that is placed a short distance from the optical parametric oscillator (OPO). Using an ultra-compact 38 mm Monoblock with a 10 mm long KTP OPO, we achieved a beam divergence of <4 mR, corresponding to a >2.5 X reduction from the unmodified laser. Performance using this technique with various feedback and etalon spacings will be presented. Laser diode array and VCSEL pumping were both investigated with similar results.
    Proc SPIE 02/2012;
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    ABSTRACT: We have explored using UV illumination as a method to mitigate pyroelectric effects, and their associative loss in hold-off for lithium niobate Q-switch materials under cold temperature operation. It has been observed that by illumination of the LiNbO3 Q-switch material from the side, the above bandgap light can provide for an increase in conductivity via an increase in photocarriers. In the presence of strong pyroelectric fields associated with a change in temperature, these carriers can be effectively swept in the direction to eliminate the field. We quantified the improvement in conduction by measuring the decay time for the pyroelectric induced loss in extinction. At negative 20°C, the decay rate for the pyroelectric field in the absence of UV illumination was measured to be 16.7 hours. It was found that by illuminating the LiNbO3 from the side with two UV LEDs operating at 500mA, the decay constant for a built-up pyroelectric charge could be reduced to 1minute. With this technique applied to a LiNbO3 Q-switched laser, the laser was shown to perform over rapid cooling without a degradation in performance.
    Proc SPIE 02/2011;
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    ABSTRACT: The method of optical triggering using a brass board architecture for a Q-switched Nd:YAG laser by direct bleaching of a Cr:YAG saturable absorber was determined to be effective in reducing the pulse-to-pulse timing jitter. A miniaturized triggering setup was employed to enable the brass board operation of the optically triggered laser. A 3mm wide minilaser diode bar (1024nm) with collimated emission was mounted on a compact heat sink and used to bleach the Cr:YAG saturable absorber from a direction orthogonal to the lasing axis. A compact 300A pulse driver, with <0.5 μs rise time and 3-5 μs duration, was developed for pulsing the 3mm diode bar. These components were combined to demonstrate a compact brassboard implementation of the optically triggered passively Q-switched laser.
    Proc SPIE 02/2011;
  • Source
    Lew Goldberg, Chris McIntosh, Brian Cole
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    ABSTRACT: A compact, passively Q-switched Nd:YAG laser utilizing a Cr4+:YAG saturable absorber, is end-pumped by the focused emission from an 804 nm vertical-cavity surface-emitting laser (VCSEL) array. By changing the VCSEL operating current, we demonstrated 2x adjustability in the laser output pulse energy, from 9 mJ to 18 mJ. This energy variation was attributed to changes in the angular distribution of VCSEL emission with drive current, resulting in a change in the pump intensity distribution generated by a pump-light-focusing lens.
    Optics Express 02/2011; 19(5):4261-7. · 3.55 Impact Factor
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    ABSTRACT: UV illumination of a lithium niobate Q-switch was demonstrated as an effective means to eliminate a loss in hold-off and associated prelasing that occurs under cold temperature operation of Q-switched lasers. This degradation occurs due to the pyroelectric effect, where an accumulation of charge on crystal faces results in a reduction in the Q-switch hold-off and a spatially variable loss of the Q-switch in its high-transmission state, both resulting in lowering of the maximum Q-switched pulse energy. With UV illumination, the resulting creation of photo-generated carriers was shown to be effective in eliminating both of these effects. A Q-switched Nd:YAG laser utilizing UV-illuminated LiNbO(3) was shown to operate under cold temperatures without prelasing or spatially variable loss.
    Optics Express 04/2010; 18(9):9622-7. · 3.55 Impact Factor
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    ABSTRACT: To address the issue of pulse-to-pulse timing jitter in a passively Q-switched Cr:YAG/Nd:YAG laser, we have developed a technique for optical triggering, where the energy from a single bar diode was used to bleach a thin sheet within the Cr:YAG saturable absorber from a direction orthogonal to the lasing axis. A strong anisotropy for bleaching effect was observed; with appropriate polarization of the bleaching light the transmission through the saturable absorber was increased from 45% to 63%. This technique was applied to a monolithic Cr:YAG/Nd:YAG laser operating under steady state conditions. By placing the Q-switched pulse at the time corresponding to the steepest slope for change in transmission during bleaching, which occurs ~1mus after the bleaching diode trigger, we measured an 12.5X reduction in the pulse-to-pulse timing jitter, from 100ns for free running operation to 8ns with optical triggering.
    Proc SPIE 02/2010;
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    ABSTRACT: Optical triggering via direct bleaching of a Cr:YAG saturable absorber was applied to a monolithic Nd:YAG/Cr:YAG laser crystal. The method uses a single laser diode bar to bleach a thin sheet within the saturable absorber from a direction orthogonal to the lasing axis. By placing the Q-switch at the time corresponding to the steepest slope (dT/dt) for change in transmission during bleaching, the pulse-to-pulse timing jitter showed a 13.2x reduction in standard deviation, from 132 ns for free-running operation to 10 ns with optical triggering. We measured that a fluence of 60 kW/cm(2) was sufficient to enable optical triggering, where a diode appropriately sized for the length of the Cr:YAG (approximately 3 mm) would then require only approximately 150 W of optical power over a 1-2 micros duration to enable effective jitter reduction. Additionally, we measured an increase in optical-to-optical efficiency with optical triggering, where the efficiency improved from 12% to 13.5%.
    Applied Optics 11/2009; 48(31):6008-14. · 1.69 Impact Factor
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    ABSTRACT: A method for optical triggering of a Q-switched Nd:YAG laser by direct bleaching of a Cr:YAG saturable absorber is described. This method involves the bleaching of a thin sheet of the saturable absorber from a direction orthogonal to the lasing axis using a single laser diode bar, where the Cr:YAG transmission increased from a non-bleached value of 47% to a bleached value of 63%. For steady state operation of a passively Q-switched laser (PRF=10 Hz), the pulse-to-pulse timing jitter showed approximately 12X reduction in standard deviation, from 241 nsec for free running operation to 20 nsec with optical triggering.
    Optics Express 03/2009; 17(3):1766-71. · 3.55 Impact Factor
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    ABSTRACT: Recent advances in compact solid sate lasers for laser designation, eye-safe range finding and active imaging are described. Wide temperature operation of a compact Nd:YAG laser was achieved by end pumping and the use of multi-λ diode stacks. Such lasers enabled construction of fully operational 4.7 lb laser designator prototypes generating over 50 mJ at 10-20 Hz PRF. Output pulse energy in excess of 100 mJ was demonstrated in a breadboard version of the end-pumped laser. Eye-safe 1.5 μm lasers based on flash-pumped, low PRF, Monoblock lasers have enabled compact STORM laser range finders that have recently been put into production. To achieve higher optical and electrical efficiency needed for higher PRF operation, Monoblock lasers were end-pumped by a laser diode stack. Laser diode end-pumped Monoblock lasers were operated at 10-20 Hz PRF over a wide temperature range (-20 to +50oC). Compared with bulk compact solid state lasers, fiber lasers are characterized by lower pulse energy, higher PRF's, shorter pulses and higher electrical efficiency. An example of fiber lasers suitable for LIDAR, and atmospheric measurement applications is described. Eye-safe, low intensity diode pumped solid state green warning laser developed for US Army checkpoint and convoy applications is also described.
    Proc SPIE 05/2007;
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    ABSTRACT: We describe a next-generation monoblock laser capable of a greater than 10 mJ, 1.5 microm output at 10 pulses/s (pps) over broad ambient temperature extremes with no active temperature control. The transmitter design is based on a Nd:YAG laser with a Cr4+ passive Q switch and intracavity potassium titanyl phosphate optical parametric oscillator. To achieve the repetition rate and efficiency goals of this effort, but still have wide temperature capability, the Nd:YAG slab is end pumped with a 12-bar stack of 100 W (each) diode bars. Different techniques for focusing the pump radiation into the 4.25 mmx4.25 mm end of the slab are compared, including a lensed design, a reflective concentrator, and a lens duct. A wide temperature operation (-20 degrees C to 50 degrees C) for each end-pumped configuration is demonstrated.
    Applied Optics 10/2006; 45(25):6607-15. · 1.69 Impact Factor