We are developing a family of fast, widely–tunable cw diode-pumped single frequency solid-state lasers, called Swift. The Swift laser architecture is compatible with operation using many different solid-state laser crystals for operation at various emission lines between 1 and 2.1 micron. The initial prototype Swift laser using a Tm,Ho:YLF laser crystal near 2.05 micron wavelength achieved over 100 mW of single frequency cw output power, up to 50 GHz-wide, fast, mode-hop-free piezoelectric tunability, and ~ 100 kHz/ms frequency stability. For the Tm,Ho:YLF laser material, the fast 50 GHz tuning range can be centered at any wavelength from 2047-2059 nm using appropriate intracavity spectral filters. The frequency stability and power are sufficient to serve as the local oscillator (LO) laser in long-range coherent wind-measuring lidar systems, as well as a frequency-agile master oscillator (MO) or injection-seed source for larger pulsed transmitter lasers. The rapid and wide frequency tunablity meets the requirements for integrated-path or range-resolved differential absorption lidar or applications where targets with significantly different line of sight velocities (Doppler shifts) must be tracked. Initial demonstration of an even more compact version of the Swift is also described which requires less prime power and produces less waste heat.
We report an eyesafe diffraction-limited single-frequency 1617 nm Er:YAG laser transmitter for coherent laser radar applications. The transmitter utilizes a master oscillator/power amplifier architecture, enabling the production of high peak power output. The pulsed oscillator is Q-switched and cavity-dumped, resulting in a 1.1 ns pulsewidth. The pulsed oscillator is injection-seeded by a commercial 1617 nm CW distributed feedback laser diode, resulting in single longitudinal mode output. The oscillator and amplifier are directly pumped into the Er:YAG laser upper state by commercial diode-pumped CW 1533 nm Yb,Er-doped fiber lasers. The injection-seeded pulsed oscillator produces an average output power of 2.2 W at 10 kHz pulse repetition frequency (PRF) with a pulsewidth of 1.1 ns (0.20 MW peak power) with a beam quality 1.1 times the diffraction limit. The oscillator has a slope efficiency of 47% in the CW mode, and a conversion efficiency of 85% from CW mode to injection-seeded pulsed mode. The power amplifier produces 20 W in the CW mode with an optical-to-optical conversion efficiency of 34% and a beam quality 1.1 times the diffraction limit, and 6.5 W in the pulsed mode at 10 kHz PRF with 1.1 ns pulsewidth (0.59 MW peak power).
Numerous coherent lidar systems have shown the ability to make measurements of atmospheric winds over the past three decades. During the past decade a 2-micron-wavelength coherent lidar system for remote wind and aerosol backscatter measurements has been advanced from initial breadboard demonstration units to a turnkey coherent lidar product, the WindTracer"R" that can operate autonomously and reliably. In this paper, the instrument is described and recent examples of wind measurement capability are provided.
The sensitivity of a mDoppler sensor is proportional to the velocity noise PSD (Power Spectral Density (m/s)2/Hz). In long-range applications, LO (Local Oscillator) laser frequency noise can be the dominant velocity noise source. In this paper we develop the relationship between the LO laser frequency PSD (Hz2/Hz) and the measured velocity noise PSD. The integrated velocity PSD or velocity variance is shown to depend upon the LO noise PSD shape and amplitude, the target round-trip time, and the measurement. The performance of a frequency stabilized and unstabilized LO laser, which exhibit a white and 1/f2 frequency noise spectrum respectively, is then predicted from this transfer function theory.
Detection statistics for a coherent laser radar are substantially different from those of a direct detection laser radar. Direct detection ladar detection statistics vary depending upon the detection mode. Speckle noise also impacts the detection statistics. For a single-pixel single- frequency single-polarization coherent detection transceiver, speckle noise can only be suppressed through temporal averaging. Some degree of speckle averaging can also be achieved in coherent detection systems by using a multiple frequencies or dual polarizations. In addition to these, a direct detection receiver can exploit spatial diversity to suppress the effects of speckle. This paper develops example performance comparisons. We show that a photon-counting direct detection receiver can exploit spatial diversity to suppress the effects of speckle. This paper develops theory useful for describing the performance of these three receiver architectures against diffuse and glint targets and provides example performance comparisons. We show that a photon-counting direct detection receiver can, in principle, provide superior performance, however practical limitations of current detection technology particularly in the near IR spectral region reduces the performance margin and for many applications a coherent detection receiver provides superior performance.
This paper discusses the design and development of a 2J, 10Hz coherent Doppler wind lidar transmitter for global wind sensing from the International Space Station. This work is being performed in support of a proposal to operate such a lidar from the Japanese Experimental Module. The conceptual lidar transmitter design is complete and risk reduction measurements are currently underway to demonstrate the 2J, 10Hz operation using a 2 micron laser transmitter with a MOPA (master oscillator – power amplifier) configuration. The paper discusses the lidar performance requirements for global wind sensing from the Space Station, general design characteristics of the two micron lidar transmitter, and the current status of the risk reduction measurements.
Tunable single-frequency sources in the 2-4 micron wavelength region are useful for remote DIAL measurements of chemicals and pollutants. We are developing tunable single-frequency transmitters and receivers for both direct and coherent detection lidar measurement applications. We have demonstrated a direct-diode-pumped PPLN-based OPO that operates single frequency, produces greater than 10 mW cw and is tunable over the 2.5 —3.9 micron wavelength region. This laser has been used to injection seed a pulsed PPLN OPO, pumped by a 1.064 micron Nd:YAG laser, producing 50-100 microJoule single-frequency pulses at 100 Hz PRF near 3.6 micron wavelength. In addition, we have demonstrated a cw Cr:ZnSe laser that is tunable over the 2.1 —2.8 micron wavelength region. This laser is pumped by a cw diode-pumped Tm:YALO laser and has produced over 1.8 W cw. Tm- and Tm,Ho-doped single-frequency solid-state lasers that produce over 50 mW cw and are tunable over approximately 10 nm in the 2 —2.1 micron band with fast PZT tuning have also been demonstrated. A fast PZT-tunable Tm,Ho:YLF laser was used for a direct-detection column content DIAL measurement of atmospheric CO2. Modeling shows that that all these cw and pulsed sources are useful for column-content coherent DIAL measurements at several km range using topographic targets.
Advances in coherent lidar using eyesafe solid state lasers in recent years have driven the development of increasingly compact, high performance single frequency CW lasers for use as master oscillator and local oscillator sources. In addition to highly stable single-frequency operation for coherent detection, many applications require agile frequency tuning capability. Examples include space-based coherent lidar where the local oscillator must be tunable in order to compensate for the fast platform motion. For a 45 degree conical scan about nadir the 7.5 km/s platform velocity introduces a ± 5.3 GHz Doppler shift. We have recently developed a Tm;Ho:YLF master oscillator producing over 50mW of single frequency power that can quickly tune over 25 GHz in frequency using a PZT. Over 50 GHz of continuous mode-hop-free single frequency tuning has been demonstrated by temperature tuning. In this paper we review the status of master/local oscillator work at CTI. We also describe the application of this 2.05 ?m laser to column content measurements of atmospheric CO2 and water vapor using a direct detection colunm content DIAL technique.
Lidar remote sensing of micro-Doppler signals is important for a large number of civilian and military applications. The single most important performance metric of these sensors is their velocity measurement precision. The velocity precision of a micro-Doppler lidar is limited by any one of various noise sources, which include shot-noise, local-oscillator frequency noise, speckle decorrelation noise, refractive turbulence advection noise and pointing jitter. In this paper, we present a theory, which describes these noise sources and their wavelength dependence. For example, it will be shown that the turbulence advection noise is wavelength independent while speckle decorrelation noise is proportional to the illumination wavelength and that the noise sources are, to a first-order, independent of the interrogation waveform classification (i.e., pulsed or CW). The results from recent field measurements using a doublet-pulse lidar will be compared with theory.
Coherent laser radar and other demanding applications require extremely stable, environmentally immune cw laser sources to act as single frequency references in the Doppler measurement process. Powerful new techniques such as micro-Doppler remote vibration sensing place even greater demands on these reference oscillators, which require sub-kHz absolute reference frequency stability over several milliseconds to resolve sub-mm/sec vibration signatures at many ten's of kilometers range. We report on progress toward super-stable (1 - 100 Hz relative stability over relevant lidar times of flight) cw eyesafe master oscillators for such applications, incorporating active frequency stabilization techniques. We also describe wide band agile frequency offset locking between two frequency-stable oscillators, relevant to the problem of compensating for large platform-induced Doppler shifts in space-borne coherent lidar applications. In these experiments, two cw lasers were programmably offset locked across a +/- 4.5 GHz span, to an accuracy of 5 kHz.
Solid-state coherent Doppler lidar sensors operating at eyesafe wavelengths have broad application to a variety of wind measurement scenarios. We have developed a modular, autonomous, high PRF, diode-pumped coherent lidar sensor that is appropriate for wind profiling at high temporal resolution. This paper describes the design of the sensor, provides examples of high-resolution wind data, and compares the performance with modeling.
A novel high time-bandwidth product waveform lidar has been developed. The lidar operates at the eyesafe 2 micrometers wavelength and produces a sequence of two or more cavity- dumped pulselets with a controllable intra-pulse spacing. The number of and spacing for the individual pulselets is adjusted to match the target and atmospheric characteristics. This waveform agility enables the sensor to operate at very long stand-off ranges. Performance predictions and results from recent field demonstrations are described.
Pulsed coherent solid-state 2 micron laser radar systems have been developed at Coherent Technologies, Inc. for ground- and airborne-based applications. Ground-based measurements of wind profiles and aerosol backscatter have been performed for several years. Examples of wind and aerosol backscatter coefficient measurements will be presented which cover a variety of weather conditions. Airborne measurements of wind profiles below the aircraft have been performed by Wright Laboratories, operating in a VAD measurement mode and will be reviewed. An engineered flight-worthy coherent lidar system is under development at CTI for flight on the SR-71 aircraft, in support of the High Speed Civil Transport program. Flights will be conducted by NASA-Dryden Flight Research Center at altitudes above 60,000 feet for the measurement of atmospheric turbulence ahead of the aircraft. Efforts are also underway at CTI for the development of high power coherent laser radar systems. Extensive detailed physical optics models of diode-pumped solid-state laser performance have been developed to characterize transient thermo-optic aberrations and the overall efficiency of lasers intended for space-based applications. We are currently developing a 2 micron 0.5 J/pulse transmitter with a 10 Hz PRF and a pulse duration of 400 - 500 ns. The status and expected space-based wind measuring performance for this system will be presented.
Solid-state coherent Doppler lidar sensors operating at the eyesafe 2 micrometers wavelength have experienced rapid development over the past five years. Several ground-based and airborne systems have been successfully demonstrated. CTI is currently making significant strides toward the development of an autonomous, modest-cost wind field sensor for boundary-layer profiling. High spatial resolution 3D coverage for several kilometers around the lidar location will be possible at update rates of about a minute-- depending on the scan volume and grid size. This paper summarizes the results of detailed sensor performance modeling for the boundary layer profiler and discusses preliminary scan concepts and issues.
Under NASA sponsorship, Coherent Technologies, Inc. (CTI) has designed and built the transceiver, and is developing the scanner, for an airborne scanning optical wind sensor. A scanning, single-aperture architecture was chosen for the CTI/NASA Optical Air Data System. Techniques for vector wind estimation form LOS scalar velocity measurements, the choice of scan patterns and wind models for various applications, and various other considerations that led to this decision are discussed within. Estimating wind vectors requires taking multiple scalar velocity projections along non- coplanar lines of slight. THis can be done from several apertures to the same field point, or vice versa, and may involve either fixed or scanned beams. For a scanning, interpolative systems, the choice of scan pattern and wind model are intimately related. Typically, more complicated models require more intricate scans to separate the fit parameters. Vector wind estimation error can arise from a variety of sources. Several effects can contribute to LOS velocity measurement noise, some of which stem form the scan itself. Inaccuracies in the scan deflection vector can also introduce error. Error can enter if the wind field model is not sufficiently sophisticated to account for small-scale turbulence. Finally, a surface-flux measurement technique is introduced, which promises to be less sensitive to noise and turbulence than wind vector estimation.
Coherent lidar systems based on eyesafe solid-state laser technology are rapidly emerging in ruggedized packages. The airport terminal area presents several measurement scenarios appropriate for application of pulsed coherent lidar sensors. This paper briefly reviews the status of coherent lidar technology and presents results produced with a mobile flashlamp- pumped 2.09 micrometers coherent lidar sensor for windshear detection and measurement, wind turbulence estimation, and wake vortex detection and tracking.
The paper describes the design and performance of the Coherent Launch Site Atmospheric Wind Sounder (CLAWS), which is a test and demonstration program designed for monitoring winds with a solid-state lidar in real time for the launch site vehicle guidance and control application. Analyses were conducted to trade off CO2 (9.11- and 10.6-microns), Ho:YAG (2.09 microns), and Nd:YAG (1.06-micron) laser-based lidars. The measurements set a new altitude record (26 km) for coherent wind measurements in the stratosphere.
An account is given of the experimental apparatus and test results of an investigation aimed at the quantification of wavelength-dependent effects on laser radar performance of various atmospheric conditions. Attention is also given to the differences between eye-safe, 2-micron-laser-based ladar system performance and that of the conventional 10.6-micron CO2-laser-based ladars that have been used to date. Attention is given to the results obtained for atmospheric turbulence and path extinction; marked differences emerge between 2-micron and 10.6-micron systems in the case of aerosols and water absorption.
The problems of obtaining and processing information in pulsed laser rangefinders in order to determine ranging object characteristic feature selection and identification under permanent echo conditions are considered, including the influence of target spatial length, radiation beam nonuniformity, and random rangefinder target guidance error. Automatic target selection algorithms and circuits are also considered. The principle of target signal selection and identification based on reflected pulse data is proposed.
A low average power, pulsed, solid-state, 1.06-micron coherent laser radar (CLR) for range and velocity measurements of atmospheric and hard targets has been developed. The system has been operating at a field test site near Boulder, CO since September, 1988. Measurements have been taken on moving targets such as atmospheric aerosol particles, belt sanders, spinning disks, and various stationary targets. The field measurements have shown that this system exhibits excellent velocity measurement performance. A fast-tuning CW Nd:YAG oscillator has also been developed which has a frequency tuning range of greater than 30 GHz (which spans a target radial velocity range of over 16 km/s) and a tuning speed greater than 30 GHz/ms.