Lew Goldberg

VCSEL End-Pumped Passively Q-Switched Nd:YAG Laser with Adjustable Pulse Energy

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.

Integrated optical half adder circuit

An integrated optical half adder circuit was fabricated on LiNbO(3) substrate. The circuit is composed of several elementary logic gates. Interaction between guided light beams was achieved by utilizing LiNbO(3) electrooptic modulators, which operate by changing the cutoff condition of the lowest order guided mode in an outdiffused LiNbO(3) channel waveguide, and CdS photodetectors. Light signals were injected into an array of channel waveguides by end-fire coupling from an array of four single mode optical fibers arranged in preferentially etched V-grooves in silicon.

Optical triggering of a Q -switched Nd:YAG laser via transverse bleaching of a Cr:YAG saturable absorber

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%.

Compact laser sources for laser designation, ranging and active imaging

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-&lgr; 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 &mgr;m 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 PRFs, 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.

Silicon photodiode for optical channel waveguides

An etched mesa silicon photodiode suitable for detecting light propagating in optical channel waveguides has been fabricated. The diode width of 7 μm was slightly less than that of the channel waveguide, thus minimizing junction capacitance and the effect of detection noise caused by unguided substrate light. The fabrication technique can be applied to produce on a single Si wafer multiple photodiodes electrically isolated from each other. Optical energy was coupled from (out‐diffused)LiNbO3 channel waveguides into the photodiodes by means of evanescent field coupling.

Compact VCSEL pumped Q-switched Nd:YAG lasers

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.

Monostatic all-fiber scanning LADAR system.

A compact scanning LADAR system based on a fiber-coupled, monostatic configuration which transmits (TX) and receives (RX) through the same aperture has been developed. A small piezo-electric stripe actuator was used to resonantly vibrate a fiber cantilever tip and scan the transmitted near-single-mode optical beam and the cladding mode receiving aperture. When compared to conventional bi-static systems with polygon, galvo, or Risley-prism beam scanners, the described system offers several advantages: the inherent alignment of the receiver field-of-view (FOV) relative to the TX beam angle, small size and weight, and power efficiency. Optical alignment of the system was maintained at all ranges since there is no parallax between the TX beam and the receiver FOV. A position-sensing detector (PSD) was used to sense the instantaneous fiber tip position. The Si PSD operated in a two-photon absorption mode to detect the transmitted 1.5 μm pulses. The prototype system collected 50,000 points per second with a 6° full scan angle and a 27 mm clear aperture/40 mm focal length TX/RX lens, had a range precision of 4.7 mm, and was operated at a maximum range of 26 m.

Influence of UV illumination on the cold temperature operation of a LiNbO(3) Q-switched Nd:YAG laser.

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.

Compact MEMS mirror based Q-switch module for pulse-on-demand laser range finders

A highly compact and low power consuming Q-switch module was developed based on a fast single-axis MEMS mirror, for use in eye-safe battery-powered laser range finders The module’s 1.6mm x 1.6mm mirror has <99% reflectance at 1535nm wavelength and can achieve mechanical angle slew rates of over 500 rad/sec when switching the Er/Yb:Glass lasing cavity from pumping to lasing state. The design targeted higher efficiency, smaller size, and lower cost than the traditional Electro-Optical Q-Switch. Because pulse-on-demand capability is required, resonant mirrors cannot be used to achieve the needed performance. Instead, a fast point-to-point analog single-axis tilt actuator was designed with a custom-coated high reflectance (HR) mirror to withstand the high intra-cavity laser fluence levels. The mirror is bonded on top of the MEMS actuator in final assembly. A compact MEMS controller was further implemented with the capability of autonomous on-demand operation based on user-provided digital trigger. The controller is designed to receive an external 3V power supply and a digital trigger and it consumes ~90mW during the short switching cycle and ~10mW in standby mode. Module prototypes were tested in a laser cavity and demonstrated high quality laser pulses with duration of ~20ns and energy of over 3mJ.

UV by the fourth harmonic generation of compact side-pumped Yb:YAG laser emission

We present a compact, side pumped passively Q-switched Yb:YAG laser that was operated in a burst mode with pump durations of 2-4 ms at low duty cycles. Intra-pump pulse Q-switched pulse repetition frequencies varied from 5-20 kHz depending on the transmission of the Cr:YAG saturable absorber, which was varied from 70% to 94%. Pump duration, pulse repetition frequency and output coupler reflectivity were optimized to yield maximum Yb:YAG laser average power and laser efficiency, while providing sufficient peak intensity, typically 0.3-1 MW, to enable efficient forth harmonic generation (FHG). Pulse energies and durations were in ranges of 0.3-1.8 mJ and 1.5-7ns, respectively, dependent on the unbleached transmission of the Cr:YAG saturable absorber. We achieved an optical efficiency of greater than 15% for the Yb:YAG laser. Extra-cavity 515 nm second harmonic generation (SHG) was achieved using a 5mm long KTP crystal. The 515 nm light was then frequency doubled by focusing it into a 7mm long BBO crystal, resulting in a 15% conversion efficiency from 1030nm to 257.5 nm, with an average UV power greater than 100 mW.