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Proposed unstable-resonator diode lasers produce solid, collimated output beams without external collimating optics. Simpler to construct, used as pumps for optical-fiber amplifiers in telecommunications, for other solid-state lasers, and sources of light for free-space communications.
Semiconductor laser diodes emit coherent light by simulated emission generated inside the cavity formed by the cleaved end facets of a slab of semiconductor that is typically less than a millimeter in any dimension for single emitters. The diode is pumped by current injection in the p-n junction through the metallic contacts. Laser diodes emitting in the range of 0.8 micron to 1.06 micron have a wide variety of applications from pumping erbium doped fiber amplifiers, dual-clad fiber lasers, solid-state lasers used in telecom, aerospace, military, medical purposes and all the way to CD players, laser printers and other consumer and industrial products. Laser diode bars have many single emitters side by side and spaced approximately .5 mm on a single slab of semiconductor material approximately .5 mm x 10 mm. The individual emitters are connected in parallel maintaining the voltage at -2V but increasing the current to ~50-100A/bar. Stacking these laser diode bars in multiple layers, 2 to 20+ high, yields high power laser diode arrays capable of emitting several hundreds of Watts. Electrically the bars are wired in series increasing the voltage by 2V/bar but maintaining the total current at ~50-100A. These arrays are one of the enabling technologies for efficient, high power solid-state lasers. Traditionally these arrays are operated in QCW (Quasi CW) mode with pulse widths ~10-200 (mu)s and with repetition rates of ~10-200Hz. In QCW mode the wavelength and the output power of the laser reaches steady-state but the temperature does not. The advantage is a substantially higher output power than in CW mode, where the output power would be limited by the internal heating and hence the thermal and heat sinking properties of the device. The down side is a much higher thermal induced mechanical stress caused by the constant heating and cooling cycle inherent to the QCW mode.
A ground based differential absorption lidar is under development at NASA's Goddard Space Flight Center to make range resolved measurements of CO2 within the planetary boundary layer. This is a direct detection lidar designed for both photon counting and analog use. Technology being developed for this instrument will be discussed including efforts in fiber lasers, optical parametric amplifiers and both InGaAs and HgCdTe solid-state detectors. The capabilities of this system are investigated and preliminary results presented.
A high energy double-pulsed Ho:Tm:YLF 2-micrometer laser amplifier has been demonstrated. 600 mJ per pulse pair under Q-switch operation is achieved with the gain of 4.4. This solid-state laser source can be used as lidar transmitter for multiple lidar applications such as coherent wind and carbon dioxide measurements.
A high energy double-pulsed Ho:Tm:YLF 2-micron laser amplifier has been demonstrated. 600 mJ per pulse pair under Q-switch operation is achieved with the gain of 4.4. This solid-state laser source can be used as lidar transmitter for multiple lidar applications such as coherent wind and carbon dioxide measurements.
Fabricated as single-chip integrated circuit. Device based partly on concept proved in commercial solid-state lasers: using unstable-resonator oscillator to define electromagnetic mode and, following oscillator, traveling-wave amplifier to generate high power. Mode-definition and power-amplification functions optimized separately. Hyperbolic-grating, unstable-resonator oscillator/amplifier diode laser produces single-longitudinal-mode, broad, laterally coherent, diffraction-limited, high-power beam.
Topics covered include: Fiber-Optic Sensor Would Monitor Growth of Polymer Film; Sensors for Pointing Moving Instruments Toward Each Other; Pd/CeO2/SiC Chemical Sensors; Microparticle Flow Sensor; Scattering-Type Surface-Plasmon-Resonance Biosensors; Diode-Laser-Based Spectrometer for Sensing Gases; Improved Cathode Structure for a Direct Methanol Fuel Cell; X-Band, 17-Watt Solid-State Power Amplifier; Improved Anode for a Direct Methanol Fuel Cell; Tools for Designing and Analyzing Structures; Interactive Display of Scenes with Annotations; Solving Common Mathematical Problems; Tools for Basic Statistical Analysis; Program Calculates Forces in Bolted Structural Joints; Integrated Structural Analysis and Test Program; Molybdate Coatings for Protecting Aluminum Against Corrosion; Synthesizing Diamond from Liquid Feedstock; Modifying Silicates for Better Dispersion in Nanocomposites; Powder-Collection System for Ultrasonic/Sonic Drill/Corer; Semiautomated, Reproducible Batch Processing of Soy; Hydrogen Peroxide Enhances Removal of NOx from Flue Gases; Subsurface Ice Probe; Real-Time Simulation of Aeroheating of the Hyper-X Airplane; Using Laser-Induced Incandescence To Measure Soot in Exhaust; Method of Real-Time Principal-Component Analysis; Insect-Inspired Flight Control for Unmanned Aerial Vehicles; Domain Compilation for Embedded Real-Time Planning; Semantic Metrics for Analysis of Software; Simulation of Laser Cooling and Trapping in Engineering Applications; Large Fluvial Fans and Exploration for Hydrocarbons; Doping-Induced Interband Gain in InAs/AlSb Quantum Wells; Development of Software for a Lidar-Altimeter Processor; Upgrading the Space Shuttle Caution and Warning System; and Fractal Reference Signals in Pulse-Width Modulation.
Four technologies were evaluated to determine their effect on Satellite Power System concepts. Two of these technologies, solid-state power amplifiers and magnetrons, are replacements for the Klystrons used for dc to RF conversion on the satellite. A third technology, laser power transmission, transmits the energy at laser frequencies rather than microwave frequencies. The fourth technology, multibandgap solar cells, has the promise of significantly increased solar to dc conversion efficienty as compared to the reference-concept silicon and gallium arsenide solar cells. The design characteristics of concepts resulting from application of these technologies are summarized.
The use of nonlinear crystals to extend the frequency range of solid-state laser sources is proposed. The harmonic generation of high-average-power laser sources and CW-laser-sources nonlinear crystals is considered. The development of Nd:YAG pumped parametric oscillators and optical parametric amplifiers using LiNbO3 or AgGaS2 is studied. The LiNbO3 oscillator has tunable output over the 1.4-4.0 micron range and is applicable for remote sensing measurements of molecules and of humidity and temperature; AgGaS2 oscillators provide the potential for 3-15 micron infrared generation. Advances in material synthesis techniques related to the design and synthesis of nonlinear media are discussed. Various procedures for the synthesis of nonlinear crystals are described.
A solid-state laser source of tunable and narrow-bandwidth UV light is disclosed. The system relies on light from a diode laser that preferably generates light at infrared frequencies. The light from the seed diode laser is pulse amplified in a light amplifier, and converted into the ultraviolet by frequency tripling, quadrupling, or quintupling the infrared light. The narrow bandwidth, or relatively pure light, of the seed laser is preserved, and the pulse amplifier generates high peak light powers to increase the efficiency of the nonlinear crystals in the frequency conversion stage. Higher output powers may be obtained by adding a fiber amplifier to power amplify the pulsed laser light prior to conversion.
Laser infrared radar (lidar) undergoing development harmless to human eyes, consists almost entirely of solid-state components, and offers high range resolution. Operates at wavelength of about 2 micrometers. If radiation from such device strikes eye, almost completely absorbed by cornea without causing damage, even if aimed directly at eye. Continuous-wave light from laser oscillator amplified and modulated for transmission from telescope. Small portion of output of oscillator fed to single-mode fiber coupler, where mixed with return pulses. Intended for remote Doppler measurements of winds and differential-absorption measurements of concentrations of gases in atmosphere.
The overall objective of this Phase 2 effort was to develop and deliver to NASA a high repetition rate laser-diode-pumped solid-state pulsed laser system with output in the green portion of the spectrum. The laser is for use in data communications, and high efficiency, short pulses, and low timing jitter are important features. A short-pulse 1 micron laser oscillator, a new multi-pass amplifier to boost the infrared power, and a frequency doubler to take the amplified infrared pulsed laser light into the green. This produced 1.5 W of light in the visible at a pulse repetition rate of 20 kHz in the laboratory. The pulses have a full-width at half maximum of near 1 ns. The results of this program are being commercialized.
Modulation and switching of semiconductor lasers are important for laser-based information technology. Typically the speed of modulation and switching is limited by interband processes such as stimulated and spontaneous recombinations which occur on a nanosecond time scale. This is why the diode laser modulation has been restricted to tens of GHz. Modulation at higher speed is highly desirable as the information technology enters into the so-called tera-era. In this paper, we study the possibility of utilizing THz-field-induced plasma heating to modulate quantum-well lasers. This is a timely study since, with the advancement of THz solid-state sources and free-electron lasers, THz physics and related technology is currently coming out of its infancy. The investigation of interplaying THz and optical fields is also of intruiging fundamental interest. First, we introduce theoretical plasma heating results for the quantum-well optical amplifier in the presense of an intense half-cycle THz pulse. The heated carrier distributions are then utilized to calculate the THz-pulse-induced change in refractive index and gain profile. Since the electron-hole-plasma is heated using intraband transitions, we circumvent the usual complications due to an overall change in density, and the nonlinear recovery is governed solely by the carrier-LO-phonon interactions, typically 5 ps for a complete recovery. This procedure implies THz and sub-THz switching and recovery rates, respectively; using either gain modulation or index modulation. Plasma heating via steady-state THz fields is also studied. Finally, numerical simulation of a coupled set of equations to investigate the THz modulation based on a simplified model for quantum-well lasers is presented. Our results show that a semiconductor laser can be modulated at up to 1 THz with little distortion with a THz field amplitude at the order of a few kV/cm. Laser responses to a change in THz frequency will be shown. Constraints, practicalities, and applications will be discussed.
Solid-state 2-microns laser has been receiving considerable interest because of its eye-safe property and efficient diode pump operation, It has potential for multiple lidar applications to detect water vapor. carbon dioxide and winds. In this paper, we describe a 2-microns double pulsed Ho:Tm:YLF laser and end-pumped amplifier system. A comprehensive theoretical model has been developed to aid the design and optimization of the laser performance. In a single Q-switched pulse operation the residual energy stored in the Tm atoms will be wasted. However, in a double pulses operation mode, the residual energy stored in the Tm atoms will repopulate the Ho atoms that were depleted by the extraction of the first Q-switched pulse. Thus. the Tin sensitized Ho:YLF laser provides a unique advantage in applications that require double pulse operation, such as Differential Absorption Lidar (DIAL). A total output energy of 146 mJ per pulse pair under Q-switch operation is achieved with as high as 4.8% optical to optical efficiency. Compared to a single pulse laser, 70% higher laser efficiency is realized. To obtain high energy while maintaining the high beam quality, a master-oscillator-power-amplifier 2-microns system is designed. We developed an end-pumped Ho:Tm:YLF disk amplifier. This amplifier uses two diode arrays as pump source. A non-imaging lens duct is used to couple the radiation from the laser diode arrays to the laser disk. Preliminary result shows that the efficiency of this laser can be as high as 3%, a factor of three increases over side-pump configuration. This high energy, highly efficient and high beam quality laser is a promising candidate for use in an efficient, multiple lidar applications.
A solid-state source of long pulse length, sodium-resonance radiation was developed for the purpose of generating an artificial star in the earth's mesospheric sodium layer. This radiation is generated by sum-frequency mixing the output of a 1.064 micron Nd:YAG laser with the output of a 1.319-micron Nd:YAG laser. By operating these lasers at wavelengths very close to the peak of their tuning curves, it is possible to match the wavelength of the sum-frequency radiation to that of the sodium D2 adsorption wavelength. Two pulsed laser systems were constructed, one producing as much as 0.6 J of sodium resonance radiation at a 10-Hz repetition rate and another producing as much as 24 mJ at a 840-Hz repetition rate. In both laser systems, the 1.06-micron and 1.32-micron Nd:YAG lasers are configured as mode-locked master oscillators followed by power amplifiers. Other aspects of this project are presented.
The applications of Q-switched lasers are well known, for example, laser radar, laser remote sensing, satellite orbit determination, Moon orbit and 'moon quake' determination, satellite laser communication, and many nonlinear optics applications. Most of the applications require additional properties of the Q-switched lasers, such as single-axial and/or single-transverse mode, high repetition rate, stable pulse shape and pulse width, or ultra compact and rugged oscillators. Furthermore, space based and airborne lasers for lidar and laser communication applications require efficient, compact, lightweight, long-lived, and stable-pulsed laser sources. Diode-pumped solid-state lasers (DPSSL) have recently shown the potential for satisfying all of these requirements. We will report on the operating characteristics of a diode-pumped, monolithic, self-Q-switched Cr,Nd:YAG laser where the chromium ions act as a saturable absorber for the laser emission at 1064 nm. The pulse duration is 3.5 ns and the output is highly polarized with an extinction ratio of 700:1. It is further shown that the output is single-longitudinal-mode with transform-limited spectral line width without pulse-to-pulse mode competition. Consequently, the pulse-to-pulse intensity fluctuation is less than the instrument resolution of 0.25 percent. This self-stabilization mechanism is because the lasing mode bleaches the distributed absorber and establishes a gain-loss grating similar to that used in the distributed feedback semiconductor lasers. A repetition rate above 5 KHz has also been demonstrated. For higher power, this laser can be used for injection seeding an amplifier (or amplifier chain) or injection locking of a power oscillator pumped by diode lasers. We will discuss some research directions on the master oscillator for higher output energy per pulse as well as how to scale the output power of the diode-pumped amplifier(s) to multi-kilowatt average power.
A multiple-pass laser amplifier that uses optical focusing between subsequent passes through a single gain medium so that a reproducibly stable beam size is achieved within the gain region. A confocal resonator or White Cell resonator is provided, including two or three curvilinearly shaped mirrors facing each other along a resonator axis and an optical gain medium positioned on the resonator axis between the mirrors (confocal resonator) or adjacent to one of the mirrors (White Cell). In a first embodiment, two mirrors, which may include adjacent lenses, are configured so that a light beam passing through the gain medium and incident on the first mirror is reflected by that mirror toward the second mirror in a direction approximately parallel to the resonator axis. A light beam translator, such as an optical flat of transparent material, is positioned to translate this light beam by a controllable amount toward or away from the resonator axis for each pass of the light beam through the translator. The optical gain medium may be solid-state, liquid or gaseous medium and may be pumped longitudinally or transversely. In a second embodiment, first and second mirrors face a third mirror in a White Cell configuration, and the optical gain medium is positioned at or adjacent to one of the mirrors. Defocusing means and optical gain medium cooling means are optionally provided with either embodiment, to controllably defocus the light beam, to cool the optical gain medium and to suppress thermal lensing in the gain medium.
The ability of solid-state lasers to be tuned in operating frequency at MHz rates by input current modulation, while maintaining a relatively narrow line-width, has made them useful for spectroscopic measurements. Their other advantages include low cost, reliability, durability, compact size, and modest power requirements, making them a good choice for a laser source in micro-gravity experiments in drop-towers and in flight. For their size, they are also very bright. In a filtered Rayleigh scattering (FRS) experiment, a diode laser can be used to scan across an atomic or molecular absorption line, generating large changes in transmission at the resonances for very small changes in frequency. The hyperfine structure components of atomic lines of alkali metal vapors are closely spaced and very strong, which makes such atomic filters excellent candidates for sensitive Doppler shift detection and therefore for high-resolution velocimetry. In the work we describe here we use a Rubidium vapor filter, and work with the strong D(sub 2) transitions at 780 nm that are conveniently accessed by near infrared diode lasers. The low power output of infrared laser diodes is their primary drawback relative to other laser systems commonly used for velocimetry. However, the capability to modulate the laser frequency rapidly and continuously helps mitigate this. Using modulation spectroscopy and a heterodyne detection scheme with a lock-in amplifier, one can extract sub-microvolt signals occurring at a specific frequency from a background that is orders of magnitude stronger. The diode laser modulation is simply achieved by adding a small current modulation to the laser bias current. It may also be swept repetitively in wavelength using an additional lower frequency current ramp.