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.
Liquid crystal polarization gratings (LCPGs) represent a relatively new technology capable of nonmechanically and efficiently steering light over a large field-of-regard in discrete steps. Due to their reliance on thin liquid crystal cells instead of mechanical moving parts, LCPG beam steering systems are attractive options for steering both active and passive optical sensors, especially in size, weight, and power (SWaP)-constrained platforms. This paper describes recent developments in large-aperture LCPG steering systems and summarizes the performance being achieved.
A number of scientific, defense, and civilian/commercial applications of coherent laser radar require correction and/or deliberate generation of very large offset frequencies between the local oscillator (LO) reference frequency and the pulsed transmitter laser frequency. An example of this system requirement is the need for agile, stable, multi-GHz offset control between master and local oscillator (MO/LO) sources in space-based lidar applications, where platform motion must be compensated for in order to perform efficient heterodyne detection of much smaller Doppler shifts due to atmospheric winds, and to lower system signal processing bandwidth demands. Another example is generation of few-GHz MO/LO offsets to accurately resolve atmospheric absorption spectra and measure gas species concentration in coherent differential absorption lidar (DIAL) applications. In this paper, we describe the development of two generations of eyesafe, diode-pumped MO/LO laser technology and actively phase-locked control electronics, specific to the space-based Doppler platform compensation problem. The lasers are based on CTI's METEOR single frequency laser technology, using Tm,Ho:YLF (2.05 μm wavelength). Fast, programmable offset-locking of the two single-frequency lasers to as much as ± 10.0 GHz was consistently demonstrated using a custom-fabricated wideband (~ 4 GHz @ 3 dB down) 2 μm-sensitive heterodyne photoreceiver as the servo detection element. Offset-frequency step settling time, control accuracy, and the phase-sensitive servo system will be described in detail. Application of the technique and technology in atmospheric CO2 DIAL measurement applications currently under way at CTI will also be discussed.
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.
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.
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.
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.
This paper discusses the design and current development status of the coherent lidar transceiver being developed by Coherent Technologies Inc for the NASA program, SPARCLE (SPAce Readiness Coherent Lidar Experiment). SPARCLE is intended as a precursor mission to a fully operational satellite system, measuring global wind profiles from the Space Shuttle, with a planned launch date in March 2001.
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.
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.