Many scientific endeavors are benefitted by the development of increasingly high energy laser sources for lidar applications. Space-based applications for lidar require compact, efficient and high energy sources, and we have designed a novel gain head that is compatible with these requirements. The gain medium for the novel design consists of a composite Nd:YAG/Sm:YAG slab, wherein the Sm:YAG portion absorbs any parasitic 1064 nm oscillations that might occur in the main pump axis. A pump cavity is built around the slab, consisting of angled gold-coated reflectors which allow for five pump passes from each of the four pumping locations around the slab. Pumping is performed with off-axis diode bars, allowing for highly compact conductively cooled design. Optical and thermal modeling of this design was done to verify and predict its performance. In order to ultimately achieve 50 W average power at a repetition rate of 500 Hz, three heads of this design will be used in a MOPA configuration with two stages of amplification. To demonstrate the pump head we built it into a 1064 nm laser cavity and performed initial amplification experiments. Modeling and design of the system is presented along with the initial oscillator and amplifier results. The greatest pulse energy produced from the seeded q-switched linear oscillator was an output of 25 mJ at 500 Hz. With an input of 25 mJ and two planned stages of amplification, we expect to readily reach 100 mJ or more per pulse.
Fibertek has demonstrated a dual-wavelength narrow linewidth UV laser transmitter for NASA airborne ozone DIAL remote sensing application. The application requires two narrow linewidth lasers in the UV region between 300 nm and 320 nm with at least 12 nm separation between the two wavelengths. Each UV laser was based on a novel ring structure incorporating an optical parametric oscillator (OPO) and a sum frequency generator (SFG). The fundamental pump source of the UV laser was a single frequency 532 nm laser, which was frequency-doubled from a diode-pumped, injection-seeded single frequency Nd:YAG laser operating at 1064 nm and 50 Hz repetition rate. The ring frequency converters generated UV wavelengths at 304 nm and 316 nm respectively. The demonstrated output energies were 2.6 mJ for 304 nm and 2.3 mJ for 316 nm UV lines, with room to potentially achieve more energy for each laser. Linewidth narrowing was achieved using a volume Bragg grating as the output coupler of the OPO in each ring oscillator. We obtained spectral linewidths (FWHM) of 0.12 nm for the 304 nm line and 0.1 nm for the 316 nm line, and the UV energy conversion efficiencies of 12.2% and 9.1%. Fibertek built an airborne DIAL transmitter based on the reported demonstration, which was a single optical module with dual-wavelength output at the demonstrated wavelengths. NASA plans to field the UV laser transmitter as a key component of the High Spectral Resolution Lidar–II (HSRL–II) high altitude airborne instrument to perform autonomous global ozone DIAL remote sensing field campaigns.
Motivated by the growing need for more efficient, high output power laser transmitters, we demonstrate a multi-wavelength laser system for lidar-based applications. The demonstration is performed in two stages, proving energy scaling and nonlinear conversion independently for later combination. Energy scaling is demonstrated using a 1064 nm MOPA system which employs two novel ceramic Nd:YAG slab amplifiers, the structure of which is designed to improve the amplifier’s thermal performance and energy extraction via three progressive doping stages. This structure improved the extraction efficiency by 19% over previous single-stage dopant designs. A maximum energy of 34 mJ was produced at 500 Hz with a 10.8 ns pulse duration. High efficiency non-linear conversion from 1064 nm to 452 nm is demonstrated using a KTP ring OPO with a BBO intra-cavity doubler pumped with 50 Hz, 16 ns 1064 nm pulses. The OPO generates 1571 nm signal which is frequency doubled to 756 nm by the BBO. Output 786 nm pulses are mixed with the 1064 nm pump pulses to generate 452 nm. A conversion efficiency of 17.1% was achieved, generating 3 mJ of 452 nm pulses of 7.8 ns duration. Pump power was limited by intra-cavity damage thresholds, and in future experiments we anticipate >20% conversion efficiency.
Fibertek has demonstrated a single frequency, wavelength stabilized near infrared laser transmitter for NASA airborne water vapor DIAL application. The application required a single-frequency laser transmitter operating at 935 nm near infrared (NIR) region of the water vapor absorption spectrum, capable of being wavelength seeded and locked to a reference laser source and being tuned at least 100 pm across the water absorption spectrum for DIAL on/off measurements. Fibertek is building a laser transmitter system based on the demonstrated results. The laser system will be deployed in a high altitude aircraft (ER-2 or UAV) to autonomously perform remote, long duration and high altitude water vapor measurements.
Methane is a potent greenhouse gas and on a per molecule basis has a warming influence 72 times that of carbon dioxide over a 20 year horizon. Therefore, it is important to look at near term radiative effects due to methane to develop mitigation strategies to counteract global warming trends via ground and airborne based measurements systems. These systems require the development of a time-resolved DIAL capability using a narrow-line laser source allowing observation of atmospheric methane on local, regional and global scales. In this work, a demonstrated and efficient nonlinear conversion scheme meeting the performance requirements of a deployable methane DIAL system is presented. By combining a single frequency 1064 nm pump source and a seeded KTP OPO more than 5 mJ of 1.6 μm pulse energy is generated with conversion efficiencies in excess of 20%. Even without active cavity control instrument limited linewidths (50 pm) were achieved with an estimated spectral purity of ~95%. Tunable operation over 400 pm (limited by the tuning range of the seed laser) was also demonstrated. This source demonstrated the critical needs for a methane DIAL system motivating additional development of the technology.
5W peak power at 911 nm is demonstrated with a pulsed Neodymium (Nd) doped fiber master oscillator power amplifier (MOPA). This result is the first reported high gain (16dB) fiber amplifier operation at 911nm. Pulse repetition frequency (PRF) and duty-cycle dependence of the all fiber system is characterized. Negligible performance degreadation is observed down to 1% duty cycle and 10 kHz PRF, where 2.5μJ of pulse energy is achieved. Continuous wave (CW) MOPA experiments achieved 55mW average power and 9dB gain with 15% optical to optical (o-o) efficiency. Excellent agreement is established between dynammic fiber MOPA simulation tool and experimental results in predicting output amplified spontaneous emission (ase) and signal pulse shapes. Using the simulation tool robust Stimulated Brillion Scattering (SBS) free operation is predicted out of a two stage all fiber system that generates over 10W's of peak power with 500 MHz line-width. An all fiber 911 nm pulsed laser source with >10W of peak power is expected to increase reliability and reduce complexity of high energy 455 nm laser system based on optical parametric amplification for udnerwater applications. The views expressed are thos of the author and do not reflect the official policy or position of the Department of Defense or the U.S. Government.
The increasing use of lidar remote sensing systems in the limited power environments of unmanned aerial vehicles and
satellites is motivating laser engineers and designers to put a high premium on the overall efficiency of the laser
transmitters needed for these systems. Two particular examples upon which we have been focused are the lasers for the
ICESat-2 mission and for the Laser Vegetation Imaging Sensor-Global Hawk (LVIS-GH) system. We have recently
developed an environmentally hardened engineering unit for the ICESat-2 laser that has achieved over 9 W of 532 nm
output at 10 kHz with a wall plug efficiency to 532 nm of over 5%. The laser has a pulse width of <1.5 ns and an M<sup>2</sup> of
<1.5. For the LVIS-GH lidar, we recently delivered a 4.2 W, 2.5 kHz, 1064 nm laser transmitter that achieved a wall
plug efficiency of 8.4%. The laser has a pulse width of 5 ns and an M<sup>2</sup> of 1.1 We provide an overview of the design and
environmental testing of these laser transmitters.
KEYWORDS: Signal to noise ratio, Fiber amplifiers, Optical filters, Optical amplifiers, Phase modulation, Digital filtering, Interference (communication), Lab on a chip, Signal detection, Bandpass filters
We report on a novel fiber based coherent detection system employing an optical preamplifier, a spectrum bandpass filter, and a time-domain filter. The time-domain filter, a synchronous time gate, reduces the in-band Amplified Spontaneous Emission (ASE ) beat noise, which cannot be achieved by the spectrum bandpass filter alone. In preliminary experiments with a 100 GHz bandpass filter, no degradation is observed from the optically preamplified coherent detection compared to pure coherent detection. With a 10 ns pulse width, 500 kHz repetition rate, and 10 pW input power, 2.78 dB and 1 dB signal-to-noise (SNR) improvement has been achieved, respectively, when 5% and 50% time gating duty cycle is used.
The design of the diode-pumped gain medium is critical to the successful deployment of lasers in space-based missions. We have developed a number of diode-pumped, conductively cooled zigzag slab designs for this application. These designs include both one-sided and two-side pumped and cooled designs. In one of the one-sided pumped and cooled amplifier designs we optimized the efficiency by maximizing the overlap between the extracting beam and the diode pumps at the total internal reflection (TIR) surface, a so-called “pump on bounce” approach. With this approach we achieved an electrical to optical efficiency from the amplifier of over 11% with an output beam M<sup>2</sup> of approximately 3. By reducing the size of the extracting beam to reduce diffraction effects in the slab the beam quality could be improved to an M<sup>2</sup> of 1.5 but the amplifier electrical to optical efficiency dropped to 6.7%. The other one-sided approach we have investigated is a near Brewster angle slab that incorporates beam propagation parallel to the slab axis and achieves good efficiency by a high overall volume fill factor. In a high beam quality oscillator (M<sup>2</sup> = 1.2) we achieved over 6% electrical to optical efficiency with a Brewster angle head design. Modeling of the thermal effects in both approaches has been performed and will be reported on. The final design approach we have investigated is based on two-sided pumping and cooling. Both modeling and preliminary experimental results indicate that this approach will allow scaling to higher average powers while still maintaining beam qualities and extraction efficiencies at least as good as those obtained with the one-sided pumped and cooled approaches. From the results of these tests and analyses, we have developed a design for a space-qualifiable 1 J, 100 Hz laser operating at 1064 nm.
We have designed and built two versions of a space-qualifiable, single-frequency Nd:YAG laser. Our approach to frequency stabilization of the seeded oscillator is a variation of the “ramp and fire” technique. In this design, the length of the pulsed laser cavity is periodically varied until a resonance with the seed laser is optically detected. At that point the pulsed laser is fired, ensuring that it is in resonance with the seed laser. For one of the lasers the resulting single frequency pulses are amplified and frequency tripled. This system operates at 50 Hz and provides over 50 mJ/pulse of single-frequency 355 nm output. It has been integrated into the GLOW (Goddard Lidar Observatory for Winds) mobile Doppler lidar system for field testing. The second laser is a 20o Hz oscillator only system that is frequency doubled for use in the High Spectral Resolution Lidar (HSRL) system being built at NASA Langley Research Center. It provides 4 mJ of single-frequency 532 nm output that has a spectral purity of >10,000. In this paper we describe the design details, environmental testing, and integration of these lasers into their respective lidar systems.
Various military lidar applications such as underwater mine detection, obstacle avoidance, IRCM, and 3 D lidar incorporate high repetition rate solid-state lasers to accomplish the mission. The recent advances and demonstrations in high power Ytterbium (Yb) fiber lasers/amplifiers make the fiber media a viable alternative to bulk lasers for these applications. The fiber laser geometry maximizes the pump absorption and mode matching for overall high efficiency, (factor-of-two over bulk laser sources) while minimizing thermal effects. In this presentation we will show experimental and modeling results on various master oscillator Yb doped polarization maintaining (PM) fiber amplifiers being developed for high repetition rate applications. We have demonstrated >20 W of average output power with M<sup>2</sup> <1.3, repetition rates up to 75 kHz and pulse widths ranging from <1 ns to 250 ns. Results of a pulsed PM MOFA efficiently pumping a Periodically Poled Lithium Niobate (PPLN) optical parametric oscillator (OPO) and KTP doubler will also be presented.
We have developed an advanced angle-angle-range-intensity laser radar system designed for long-range imaging and target discrimination. Brassboard hardware has been built, and performance testing has demonstrated the capability of direct-detection imaging ladar systems to significantly improve probability of target discrimination over passive-only sensors. In this paper we discuss the most recent generation of short-pulse ladar hardware and the results of recent range testing of this hardware against accurate target models. Progress in the development of the next-generation short-pulse laser transmitter and GHz digitizing receiver is also described.
Fibertek is currently under contract to the US Army Soldier and Biological Chemical Command (SBCCOM) at Aberdeen Proving Ground, MD to develop a multi-wavelength lidar system. Under this effort, Fibertek will deliver a system that is capable of detecting the presence of biological aerosols. The SR-BSDS has successfully demonstrated the ability to detect and track a biological aerosol cloud while discriminating between biological and non-biological aerosols and hard targets. The SR-BSDS is an active standoff detection system with both ultraviolet (UV) and infrared (IR) capability. The UV wavelengths can provide near real time detection and ranging of a particulate cloud with demonstrated discrimination capability. Recent enhancements to the IR capability extended the cloud detection range and acquisition capability as well as providing an autonomous operation mode of operation. The SR-BSDS can be operated in one of two modes, manual or autonomous. In the manual mode the operator selects the desired scan field of view, resolution, wavelength, and degree of pulse coadding, then instructs the system to start scanning. The system will monitor its own performance and display information to the operator to indicate proper operation. The system will monitor cloud data and warn the operator when the sensor is aimed at an aerosol of interest. If a biological cloud of interest is found, an audible alarm will sound, and the operator can examine cloud imagery while the system continues to automatically monitor and track all clouds in the field-of-view. The scanning parameters can also be changed easily upon aerosol detection, if desired. In the autonomous mode, the operator selects the desired scan field of view. The system automatically scans for aerosol clouds with the IR beam. This is accomplished in a rapid, single pulse laser firing mode. Once a cloud with specified characteristics is acquired, the system automatically switches over to an UV beam for discrimination interrogation. System status, data and discrimination interrogation results will be transmitted over a wireless modem to a Command Post. All the above will continue to operate without additional operator intervention. The autonomous operation feature was recently demonstrated at Dugway Proving Ground in July of 1999. Field testing to date, both at Aberdeen Proving Ground, MD and Dugway Proving Ground, UT in 1998 and 1999 successfully demonstrated the system's detection, discrimination, scanning functions and autonomous operation. With the initial field testing and system demonstration testing successfully complete, emphasis is on several areas of enhancements in preparation for additional DPG testing and system delivery for field implementation in 2000.
With the advent of high-power laser pump diodes, significant reductions in solid state laser system weight and size are possible due to the efficient coupling of electrical to optical energy. Typical electrical conversion efficiencies for laser diode arrays are 45% for operation up to 20% duty cycle regime. The substantial saving in system weight and size is not primarily due to reductions in the pump module, but rather in the size of power supplies and cooling equipment. Used in conjunction with a Nd:YAG zig-zag slab configuration, diode array pumping allows for simple heat removal and uniform excitation of the gain region. In this paper we will discuss the performance of a compact zig-zag slab laser designed to operate in rugged military or space environments. The laser is air cooled, battering powered and designed to be man-portable with a total weight of 7.5 kg. The frequency doubled output energy at 532 nm is 270 mJ per pulse at 10 PPS.
In this paper we discuss progress toward the development of kilowatt-class diode-pumped lasers with beam quality approaching the diffraction limit. The development of high- efficiency, high-brightness lasers will open new applications for precision laser machining. The design and performance of diode-pumped Nd:YAG lasers with average powers in the kilowatt range are addressed. Fibertek, Inc. is presently developing laser systems at this power level for industrial applications such as drilling, cutting, and welding. This work has a goal of producing 0.5 to 2.5 kW average power with less than four times diffraction-limited beam quality (2.6 mm - mr, D(Theta) /4). Both rod and slab architectures are under development. This talk covers initial results with diode-pumped rod lasers operating at up to 500 W average power.
Diode-pumped solid-state lasers with average power output of up to 200 W and peak power to 100 MW have been developed for military and commercial applications. This paper discusses engineering issues related to scaling solid-state lasers to the kW level while maintaining high beam quality, high nonlinear conversion efficiency, and avoiding optical damage.
An eyesafe source at 1.61 J.UD with 2.1 % wallplug efficiency, is demonstrated using a Nd:YAG pumped KTP optical parametric oscillator with total peak-power conversion efficiency of 70% and an energy conversion efficiency of 47 % to 1.61 J.UD or 30% to l.54J.UD.
The design and performance of a large diode-pumped multi-stage Nd:YAG laser system for space and airborne applications will be described. The laser operates at a repetition rate of 40 Hz and produces an output either at 1.064 micron or 532 nm with an average power in the Q-switched mode of 30 W at the fundamental and 20 W at the second harmonic wavelength. The output beam is diffraction limited (TEM 00 mode) and can optionally also be operated in a single longitudinal mode. The output energy ranges from 1.25 Joule/pulse in the free lasing mode, 0.75 Joule in a 17 nsec Q-switched pulse, to 0.5 Joules/pulse at 532 nm. The overall electrical efficiency for the Q-switched second harmonic output is 4.
An eyesafe source (1.61 micrometers ) with 1.1% wallplug efficiency, is demonstrated using a Nd:YAG pumped KTP optical parametric oscillator with peak-power conversion efficiencies of 70%. Joule-level scaling, kHz repetition-rates, and ns pulselengths are now accessible using this technology.