Long-wave infrared (LWIR) lasers based on high-voltage pulsed discharges in the high-pressure CO2 gas have reached multi-TW peak power. As optical pumping appears to be a more viable pathway toward ultrahigh power and high repetition rate, we investigated multiple apparatus including direct and indirect optical pumping. Indirect optical pumping through stimulated Raman scattering in N2 gas could be efficient, and is relatively insensitive to the pump wavelength. On the other hand, laser technologies for direct optical pumping have higher maturity levels at wavelengths of ~ 1.4 μm and ~ 2.0 μm among multiple excitation bands in the mid-IR wavelength.
Ultra-short pulse lasers are dominated by solid-state technology, which typically operates in the near-infrared. Efforts to extend this technology to longer wavelengths are meeting with some success, but the trend remains that longer wavelengths correlate with greatly reduced power. The carbon dioxide (CO2) laser is capable of delivering high energy, 10 micron wavelength pulses, but the gain structure makes operating in the ultra-short pulse regime difficult. The Naval Research Laboratory and Air Force Research Laboratory are developing a novel CO2 laser designed to deliver ~1 Joule, ~1 picosecond pulses, from a compact gain volume (~2x2x80 cm). The design is based on injection seeding an unstable resonator, in order to achieve high energy extraction efficiency, and to take advantage of power broadening. The unstable resonator is seeded by a solid state front end, pumped by a custom built titanium sapphire laser matched to the CO2 laser bandwidth. In order to access a broader range of mid infrared wavelengths using CO2 lasers, one must consider nonlinear frequency multiplication, which is non-trivial due to the bandwidth of the 10 micron radiation.
In our diamond-cooled approach, thin disks of laser gain material, e.g., Nd:YAG, are alternated between thin disks of single crystal synthetic diamond whose heat conductivity is over 2000 W/m-°K. The gain medium is face-pumped (along the optical axis) by the output of laser diode arrays. This optical configuration produces heat transfer from Nd:YAG to the diamond, in the direction of the optical axis, and then heat is rapidly conducted radially outward through the diamond to the cooling fluid circulating at the circumference of the diamond/YAG assembly. This geometry effectively removes the heat from the gain material in a manner that permits the attainment of high power output with excellent beam quality.
KEYWORDS: Pulsed laser operation, Laser systems engineering, Laser applications, Laser propulsion, Gas lasers, Rockets, High power lasers, Solid state lasers, Laser development, Control systems
Laser-powered lightcraft systems that deliver microsatellites to low earth orbit have been studied for the Air Force Research Laboratory. One result of this study has been discovery of the significant influence of laser wavelength on the power lost during laser beam propagation through Earth’s atmosphere and in space. Here, energy and power losses in the laser beam are extremely sensitive to wavelength for earth-to-orbit missions. And this significantly affects the amount of mass that can be placed into orbit for a given maximum amount of radiated power from a ground-based laser.
This paper reviews the basic concepts of laser propulsion and summarizes work done to date using a 10 kW device. The paper describes a candidate megawatt class CO2 laser system which can be scaled relatively near-term to multi-megawatt power levels using demonstrated technology. Such a system would potentially be capable of launching micro-satellites into low earth orbits (LEO) at relatively low cost. Our projections indicate that payloads of about 1kg/megawatt are achievable. The long wavelength of a CO2 laser will require the use of a large aperture telescope and/or large effective beam capture area for the lift vehicle. We believe that these limitations, not withstanding, rep-pulsed CO2 in a blow-down configuration lasting 200-300 seconds could achieve the desired propulsion objectives. The laser would use a helium-free, nitrogen/carbon dioxide mixture to provide a very cost effective fuel.
In this paper, we summarize the performance of a diamond cooled diode pumped solid state (DPSS) Nd:YAG laser developed by Textron Systems Corporation (TCS). Over 50% intrinsic extraction efficiency at 50-Watts average power, equivalent to volumetric power extraction of 2000 W/cc, has been experimentally demonstrated. Beam quality (BQ) of less than 1.1 XDL has been measured in an oscillator configuration with the laser operating in TEM00 mode. With a simple passive thermal lens correction, BQ of less than 1.5 XDL was obtained from an oscillator amplifier arrangement at specific pump rates up to 1600 W/cc and 600 W/cc, respectively. By using an intra-cavity acousto-optic modulator, Q-switched laser pulses as short as 6 ns were obtained from the oscillator. This concept is amenable to scaling up to very high power levels. A conceptual design, using our validated database, for a 100-kW class laser is also presented.
This paper reviews work done at Trex Enterprises Corporation over the past 18 years on electro-optic surveillance and tracking systems. The range of objects that can be detected and tracked cover awide range of brightness and velocities, from slower moving mortars to fast moving bullets.
This paper reviews the basic phenomena and technologies associated with design of repetitively pulsed CO2 lasers operating at power levels of ~ 100 kW and pulse repetition rates of ~ 100 Hz and includes some simple diagnostic techniques. Such systems are of potential interest for materials processing investigations and as potential drivers for higher power systems for launching of small payloads to space.
This paper reviews the basic phenomena and technologies associated with design of repetitively pulsed CO2 lasers operating at power levels of approximately 100 kW and pulse repetition rates of approximately 100 Hz. Such systems are of potential interest for ablative propulsion investigations and as potential drivers for higher power systems for launching of small payloads to space.
This paper reviews the performance of a conventional direct detection CO2 Differential Absorption Lidar (DIAL) system with the coherent spread spectrum approach developed, validated and patented by Textron. The analysis shows that the coherent approach is far superior in terms of maximum attainable standoff range at a specified transmitter average power and substantially reduced system power and associated size and weight at a predetermined range. The requirements on local oscillator stability are fairly relaxed and the spread spectrum/coherent DIAL concept is fairly easy to implement. Performance parameter maps are presented for ground-based, low-altitude and high-altitude airborne systems with a range of aperture sizes and pulse formats.
Many luminous sources provide continuous or quasi-continuous radiation at near IR and longer wavelengths. The radiation continuum serves as a source of background photons, which can be used for discrete line-of-site absorption measurements by-known atmospheric constituents. The intensity ratio is uniquely determined by the absorption coefficient and range, is independent of broadband attenuations and scattering. The absorption coefficients are known and/or can be accurately calculated for a wide range of practical viewing conditions (i.e., sensor height, viewing angle, etc.). Hence, the intensity ratio and/or integrated intensity ratio can be used to uniquely derive the range of the radiating source. Fabry-Perot interferometers can provide the high throughputs and resolving powers required in compact packages. The measurements and analyses show that ranging accuracies representing down to 1 - 2% of the total range should be achievable at stand-off ranges of upto hundreds of kilometers depending on the size of the collection optics, brightness of the source and available observation times. The paper will provide an overview of the patented Textron concepts, trade-offs associated with instrument resolving powers and hardware implementation issues.
This paper compares the performance of a conventional direct detection CO2 Differential Absorption Lidar (DIAL) system with the coherent spread spectrum approach developed and patented by Textron. The analysis shows that the coherent approach is far superior in terms of maximum attainable standoff range at a specified transmitter average power and substantially reduced system power and associated size and weight at a predetermined range. The requirements on local oscillator stability are fairly relaxed and the spread spectrum/coherent DIAL concept is fairly easy to implement. Some comparative validation data are provided.
This report briefly reviews the development, capabilities, and current status of pulsed high-power coherent CO2 laser radar systems at the Maui Space Surveillance System (MSSS), HI, for acquisition, tracking, and sizing of orbiting objects. There are two HICLASS systems, one integrated to the 0.6 m Laser Beam Director and one just integrated Summer 2000 to the 3.7 m Advanced E-O System (AEOS). This new system takes full advantage of the large AEOS aperture to substantially improve the ladar range and sensitivity. These improvements make the AEOS HICLASS system potentially suitable for tracking and characterization experiments of small < 30 cm objects in low-earth-orbits.
This paper reviews the basic discharge physics and procedures that have been used to pulse excite gas lasers. A variety of different discharge schemes have been implemented over the past thirty years to volumetrically excite high pressure discharges with cross sections ranging from 102 iO cm2 and lengths of 102 lO3cm. The associated energy outputs derived from these systems range, in the case of C02 lasers, from lO — 106 J/pulse. The techniques use transverse excitation and have been successfully applied to a wide variety of gases and mixture with lasing outputs extending from the vacuum ultraviolet to the long-wave infrared. The various schemes have been adapted to provide effective discharge excitations lasting from iO —iO seconds. The short excitations have been applied to miniature and traveling wave preionization stabilized discharges and the long pulses to large cross section high energy devices. The paper will focus exclusively on discharge excitation and stability issues and relevant discharge related kinetics. It will not specifically address issues relating to laser extraction, repetitively pulsed operation and the associated problems of thermal control, flow management and acoustics dumping. The review provides a somewhat parochial perspective in that it reflects my own personal contributions to this field and my direct association with lasers covering this size distribution and associated energy range. The parochialism is reflected mainly in the representative devices I have selected for the paper. The paper is presented in six parts. This introduction is followed by a brief review of the relevant discharge physics of high pressure discharges. The next three sections separately address and provide specific properties and examples of each category. The paper concludes with a brief summary.
The CO2 laser DIAL concept has been applied successfully to the detection of common industrial byproducts and a variety of toxins and simulants (see Figures 1 and 2). Work to date has focused almost exclusively on detection of these compounds at low concentrations and at relatively short standoff ranges. The current techniques use large optics, multimode lasers and direct detection. The large optics partially offset the reduced sensitivity of the direct detection receiver. The standoff range can be enhanced by increasing the pulse energy but will still be severely limited by the inherently high noise equivalent power (NEP) oflong wavelength infrared (LWIR) detectors. It has long been recognized that range performance can be enhanced considerably in a photon counting sense by using a fully coherent system (see Figure 3 for state-of-the-art NEP estimates for direct and coherent detectors); good speckle averaging, however, should be retained in coherent sensors to achieve high-sensitivity and this will have a profound influence on the measurement technique. Standoff ranges of 100-200 km can be achieved with high altitude platforms (ER-2 or balloons) and coherent transmit/receive (shared) 30 cm aperture systems with pulse energies of< 1 .0 Joules/pulse (J/p) and repetition rates of 30-50 Hz. A key feature of the high sensitivity measurement approach is to use the coherent mode-locked pulse burst waveform as a wide bandwidth source to provide both speckle averaging (within each pulse) and coherent detection. There is also considerable synergism between the systems required for long-range imaging laser radar and remote sensing; both sensors will use coherent CO2 lasers and similar pulse formats. The laser radar/remote sensing functions can be combined into a single sensor suite. The key functional upgrade is to incorporate tunability in the mode-locked transmitter to produce, on demand, a selection of different CO2 laser wavelengths within the 9-1 1 pm atmosphere window and thus accommodate the remote sensing capability; a matched frequency agile local oscillator is also required for the shot noise limited coherent receiver, to provide 'photon counting' capabilities at all wavelengths of interest.
The talk will review the measurement capabilities of the FLD system and highlight key features of some major subsystems developed under the program which currently reflect the state-of-the-art.
The pulse separation and associated range ambiguity of a CO2 laser radar using a pulse burst waveform consisting of 1.3 ns mode-locked pulses spaced 40 ns apart was successfully doubled to 80 ns and 12 m respectively by suppressing every alternate pulse. This was done by sending the mode-locked laser output beam with 40 ns pulse spacing through a resonant Raman-Nath acousto-optic modulator driven at 18.75 MHz which is 1.5 times the drive frequency used to intracavity mode-lock the CO2 laser transmitted. The spatial filtering of the diffracted pulses results in the suppression of the alternate pulses to better than 15 dB. The throughputs for the unsuppressed pulses were greater than 95%. The laser radar capability in detecting large targets,
The axial separation distance between an intracavity resonant Raman-Nath acousto-optic modulator and the secondary mirror of an unstable resonator of a TE CO2 laser has strong effects on the mode-locking efficiency. Experimental measurements showed that the mode-locker laser pulse width was a periodic function of the separation distance with a period of about 18 mm. Preliminary simple analyses using a two-beam interference model yield a periodic sine-squared acousto-optic diffraction intensity distribution as a function of separation distance with the mode-locked pulse width varying as the inverse of sine to the fourth power of the diffraction intensity. The observed pulse width was in good agreement with the calculated value using the above interference theory.
The Field Ladar Demonstration (FLD) program (also known as the Hi-Class-High Performance CO2 Ladar Surveillance Sensor) which is building a high power, coherent, carbon- dioxide, multi-function laser radar at the Maui Space Surveillance Site (MSSS) facility on Mt. Haleakala, Maui has completed its Phase 2 testing. The Phase 2 effort has the primary objective of demonstrating that a subset of the final system components (oscillator, receiver, processor and beam director) provide the capability required for an operational hardbody and remote chemical detection sensor. The final major component, the power amplifier, will be added in Phase 3. The objectives for this phase include real-time satellite metric determination and demonstrations of range-Doppler imaging and long range, remote chemical species detection (lidar). Specifically, the Phase 2 testing will demonstrate: real-time (30 Hz) generation of satellite range and range rate; detection of return signals from 'uncooperative' satellites (having no retro-reflectors); wideband imaging; and measurement of path-integrated chemical species absorption at long range (18 km) using a coherent DIAL technique and the FLD, wavelength agile transceiver.
A cw carbon-dioxide laser capable of being switched at a 30- Hz rate to different laser lines in the 9 - 11 micrometer spectral region has been developed as a single mode local oscillator for high-detection-sensitivity coherent lidar applications. Over sixty 12C16O2 laser lines (fifty lines for a 13C16O2 laser) with output power varying from 0.5 W to 5 W were obtained using a compact galvanometer controlled scanning mirror/fixed grating configuration; with a specially designed galvanometer-input voltage waveform digitially controlled through a 16-bit D/A converter, the laser frequency fluctuation was found using a heterodyne technique to have settled to about 0.3 MHz peak-to-peak in 25 ms following each line switching, which is adequate for our coherent lidar atmospheric sensing applications.
We have demonstrated a 15 dB lidar receiver detection sensitivity improvement by employing an Er3+-doped fiber preamplifier as the input to an InGaAs P-I-N photodiode. This eye-safe, 1550 nm detection system can significantly improve lidar system performance. It is shown that it is comparatively easier at 1550 nm than at 1064 nm to achieve a higher detection sensitivity for a lidar system using a fiber preamplifier in comparison with that using an avalanche photodiode (APD). The reason is that the Er:fiber preamplifier performs better than a Nd:fiber preamplifier, while the InGaAs APD is less sensitive in comparison with a Si APD. In a lidar receiver using a fiber preamplifier, the beam coupling efficiency from free-space mode to a single- mode fiber is the critical parameter. For moderate target velocities, an automatic front-end alignment system using piezoelectric transducers can be effected to yield a good coupling efficiency. The design and preliminary test results of such a lidar receiver are discussed in terms of optimized optical filter bandwidth, optical preamplifier gain and noise figure, and input saturation level.
A multi-joule, wavelength agile, CO2 transceiver is being assembled in support of a two phase, airborne chemical sensing demonstration employing both direct (Phase I) and coherent (Phase II) detection methods. The Phase II, coherent detection transceiver concept design, and performance are described below.
The High Performance CO2 Ladar Surveillance Sensor system (HI-CLASS) is a state-of-the-art coherent ladar system which will provide precision tracking and high resolution imaging at the Air Force Maui Optical Station (AMOS). System development is occurring in 3 phases representing increasing hardware/software complexity and system capability. The recently-completed Phase I HI-CLASS system employs a compact, pulsed, coherent CO2 oscillator, a heterodyne receiver, and signal recorder coupled to the AMOS 0.6 m Laser Beam Director to demonstrate target (satellite) acquisition and tracking, illumination, return signal detection, signal recording, and off-line processing for range and range rate extraction and range- amplitude imaging. A description of the Phase I satellite ranging and ground-based remote sensing tests verifying the FLD system operating concept will be presented. The cooperative target range and range rate measurements, as well as imaging precursor demonstration, will be discussed. The talk will include a discussion of the 21 km demonstration of remote sensing using natural terrain returns. Results generated on phase I data with the phase II algorithm will also be described.
The paper reviews the basic capabilities and utility of the FLD laser radar system and highlights key features which lead to a combined LADAR/LIDAR capability.
Symmetrical edge and face cooling geometries have been evaluated as a thermal control technique for a high power acousto-optic modulator employed as an intracavity mode locker inside a pulsed TE, 0.5 atm, mode-locked CO2 laser oscillator with a 0.5 - 1 kW average output power. Both cooling methods have proved effective in minimizing transverse thermal gradients generated in a germanium crystal of the acousto-optic modulator operating in the Raman-Nath regime.
Both EO and AO modulators can be used to extend the spectral coverage of CO2 lasers in the 9 - 11 micrometers region. For laser radar local oscillator application, the spectral purity of the frequency shifter output must be high and can be achieved with an AO frequency shifter using a special modulator configuration and a molecular absorption technique. A 500 MHz AO frequency shifter was designed, tested and shown to have a high spectral purity at an output power level of over 50 mW with a conversion efficiency of a few percent.
A large-aperture, high-Q, germanium standing-wave AO modulator for mode locking a pulsed high-energy CO2 laser is described. By operating it in the Raman-Nath regime, optical absorption-induced thermal lensing effects are minimized. Due to a smaller round-trip transit time and desired Bessel function transmission characteristics, only a single set of mode locked pulses are generated.
The High Performance CO2 Lidar Surveillance Sensor system (HI-CLASS) is a state-of- the-art coherent ladar system which will provide precision tracking and high resolution imaging at the Air Force Maui Optical Station (AMOS). System development is occurring in 3 phases representing increasing hardware/software complexity and system capability. The recently-completed Phase 1 HI-CLASS system employs a compact, pulsed, coherent CO2 oscillator, a heterodyne receiver, and signal recorder coupled to the AMOS 0.6 m Laser Beam Director to demonstrate target (satellite) acquisition and tracking, illumination, return signal detection, signal recording, and off-line processing for range and range rate extraction and range-amplitude imaging. This paper will discuss the Phase 1 HI-CLASS hardware configuration, test objectives, and test results to date employing cooperative, retro-equipped satellites and describe the Phase 2 hardware currently undergoing initial testing.
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