The obstacle warning system for Helicopters HELLAS is presented which is available as off the shelf system. The system generates a 3 D Ladar image based on a 1,5 micrometers Erbium-Fiber Laser. The system is specially designed to detect wires with a high detection probability of >99,5 % within one second. The implications of this requirement on the design are discussed.

A 64x64-pixel Flash (scannerless) lidar system that uses a streak tube to achieve range-resolved images is demonstrated. An array of glass fibers maps light from an area in the focal plane of an imaging lens to multiple rows of fibers on the streak tube's photocathode. The time-resolved backscatter return for all 4096 image pixels is recorded during one integration-time of a CCD camera that is coupled to the streak tube's phosphor screen. Data processing yields 64x64-pixel contrast (intensity) and range images for each laser pulse. Range precision better than 2.5% of the range extent is exhibited for a wide variety of targets and terrains at image rates up to 100Hz. Field test imagery demonstrated the capability of the Flash lidar system for imaging vehicles hidden by a tree canopy as well as for imaging sub-surface mine-like targets in the ocean.

The Army Research Laboratory is developing scannerless ladar systems for smart munition and reconnaissance applications. Here we report on progress attained over the past year related to the construction of a 32x32 pixel ladar. The 32x32 pixel architecture achieves ranging based on a frequency modulation/continuous wave (FM/cw) technique implemented by directly amplitude modulating a near-IR diode laser transmitter with a radio frequency (rf) subcarrier that is linearly frequency modulated. The diode's output is collected and projected to form an illumination field in the downrange image area. The returned signal is focused onto an array of metal-semiconductor-metal (MSM) detectors where it is detected and mixed with a delayed replica of the laser modulation signal that modulates the responsivity of each detector. The output of each detector is an intermediate frequency (IF) signal (a product of the mixing process) whose frequency is proportional to the target range. This IF signal is continuously sampled over each period of the rf modulation. Following this, a N channel signal processor based-on field-programmable gate arrays calculates the discrete Fourier transform over the IF waveform in each pixel to establish the ranges to all the scatterers and their respective amplitudes. Over the past year, we have built one and two-dimensional self-mixing MSM detector arrays at .8 and 1.55 micrometers , designed and built circuit boards for reading data out of a 32x32 pixel array, and designed an N channel FPGA signal processor for high-speed formation of range gates. In this paper we report on the development and performance of these components and the results of system tests conducted in the laboratory.

The U.S. Army Research Laboratory (ARL) is investigating a ladar architecture based on FM/cw radar principles, whereby the range information is contained in the low-frequency mixing product derived by mixing a reference ultra-high frequency (UHF) chirp with a detected, time-delayed UHF chirp. ARL is also investigating the use of unique self-mixing detectors that have the ability to internally detect and down-convert light signals that are amplitude modulated at UHF. When inserted into the ARL FM/cw ladar architecture, the self-mixing detector eliminates the need for wide band transimpedance amplifiers in the ladar receiver thereby reducing both the cost and complexity of the system. ARL has fabricated a 32 element linear array of self-mixing detectors and incorporated it into a breadboard ladar using the ARL FM/cw architecture. This paper discusses the basic theory of detector operation, a description of the breadboard ladar and its components, and presents some fundamental measurements and imagery taken from the ladar using these unique detectors.

The main purpose of the work presented here is to study the potential for an active imaging system for target recognition at long distances. This work is motivated by the fact that there are a number of outdoor imaging needs where conventional passive electro optical (EO) and infrared (IR) imaging systems are limited due to lack of photons, disturbing background, obscurants or bad weather. With a pulsed illuminating source, several of these problems are overcome. Using a laser for target illumination, target recognition at 10's of km can be achieved. Powerful diode pumped lasers and camera tubes with high spatial and time resolution will make this technique an interesting complement to passive EO imaging. Beside military applications, civilian applications of gated viewing for search and rescue, vehicle enhanced vision and other applications are in progress. To study the performance limitations of gated viewing systems due to camera, optics and the atmosphere an experimental system was developed. Measurements up to 10 km were made. The measurements were taken at the wavelength 532 nm. To extrapolate the results to future system performance at an eye safe wavelength, 1.5 micrometers nm, a theoretical performance model was developed. This model takes into account the camera and atmospheric influence on resolution and image quality, measured as a signal-to-noise-ratio, SNR. The result indicates turbulence influence, in agreement with the modeling. Different techniques were tested for image quality improvement and the best results were obtained by applying several processing techniques to the images. Moreover, the tests showed that turbulence seriously limits the resolution for horizontal paths close to the ground. A tactical system at 1.5 micrometers should have better performance than the used 532 nm in atmospheric-limited applications close to ground level. The potential to use existing laser range finders and the eye safety issue motivates the future use of 1.5 micrometers for gated viewing.

Based on ABCD ray matrix theory and a random phase screen model for the target surface, analytic expressions are developed for the normalized mutual coherence function (MCF) of a reflected Gaussian-beam wave from a finite target in the presence of atmospheric turbulence. This analysis features both pupil plane and image plane expressions and includes partial and fully developed speckle from the target. The target model is a combination of a thin random phase screen and limiting aperture stop such that a weak screen corresponds to a mildly-rough target and a strong (or deep) random phase screen corresponds to fully developed speckle. From the normalized MCF, estimates are given for the speckle size in the pupil plane and image plane as a function of transmitted beam wave characteristics, size and roughness of the target, and size of the receiver collecting lens.

A summary of our MicroSlab laser (patent pending) development activities will be presented. We demonstrated pulsed 1064 nm laser emissions from a 0.75 x 1.5 x 4 mm Nd,Cr⁴⁺:YAG slab laser. The Nd:YAG gain medium was grown on top of a Cr⁴⁺:YAG substrate by the liquid phase epitaxy growth method, were the Cr⁴⁺:YAG layer acted as the passive Q switch. The slab end faces were coated as an end mirror and an output coupler to minimize cavity length. The gain medium was side pumped with a 0.1 x 1.0 laser diode beam, emitting at 808 nm. These conditions produced the 1064 nm single pulse beam, with a pulse energy of 754 μJ and a pulse width of 0.97 ns. The corresponding peak power is 0.77 MW per pulse. A similar MicroSlab laser, different only in the location of the output coupler, was used to drive an intracavity KTP OPO. The cavity, emitting 1573 nm, achieved pulse energy greater than 200 μJ with a pulse width of 1.1 ns. We continue to optimize the pump source and other wavelength conversion crystals to develop highly energetic compact laser sources.

We established a new diode array pumped Er:Yb:Glass test setup for evaluation of the laser performance and q-switch characteristics of various saturable absorber materials at 1.54micrometers . Pumping distribution and maximum gain was analyzed. Passive q-switched laser operation was demonstrated with both U²⁺:CaF₂ and CO²⁺:MgAl₂O₄. TEMoo Q-switch pulses with energy of 0.5mJ and pulse width of 10ns was obtained.

This paper explores two beam steering systems for Infrared Countermeasures applications in the Mid-Wave Infrared range. One system involves the use of decentered achromatic doublet lenses to steer the beam of interest. The second system involves rotating achromatic prisms. The structure of each system is addressed along with the possible materials that could be used in constructing each system. Each system avoids an internal focus. The two systems are compared against one another in terms of their advantages and disadvantages.

Optical distance measurement, based on the time-of-flight (TOF) principle, suffer from the unfavorable ratio of desired accuracy and the small received signal power, which is reflected by the target. Many receiver concepts are discussed in the past, but only any concepts are putted into practice. The concept of Photonic Mixer Devices (PMD) offers any interesting features for laser ranging systems, like fast photo-detection with inherent mixing, accumulation of a adequate signal power by a following integration, flicker noise suppression, background illumination, and interference rejection of two-channel designs. The combination of PMD and phase-lock techniques additionally enables a continuous tracking of the object distance, his relative velocity to the measurement system, and the restoration of the received optical signal. Different design concepts of the Photonic Mixer Device (PMD) with their characteristics and advantages as the key component in an Electro-Optical Phase-Locked Loop (OE-PLL) and an Electro-Optical Delay-Locked Loop (OE-DLL) will be discussed.

Accommodating the large dynamic range of lidar signals is always a challenge for optical engineers. Signals from low altitudes are much larger than signals from high altitudes because of their inverse-range-squared behavior, as well as atmospheric absorption and scattering. It is well known that the onset of received lidar signals with range can be controlled by adjusting the crossover of the laser beam into the receiver field of view. However, a careful analysis has shown that, in many lidar applications much of the system's dynamic range can be used up before the range where the crossover is complete. In addition, the analysis shows that defocus is the primary contributor to the geometrical overlap function in determining the range dependence of the signal, and that understanding defocus is necessary for the optical designer to optimize system performance. Examples are given to illustrate the improvements in dynamic range that can be achieved by optimizing the focus of a lidar receiver.

This paper describes ongoing research on wind lidar techniques and measurements. Joint work between Utah State University and NASA's Goddard Space Flight Center has led to several field campaigns over the last three years involving the HARLIE holographic lidar and its use to determine vertical profiles of the horizontal wind. New data analysis methods have been developed at the Space Dynamics Laboratory for remote sensing of the wind speed and direction as functions of altitude. The third stage of HARLIE analysis is now automated to the extent that manual curve matching is optional. Automation has also been developed for analyzing the video images of overhead clouds, so that their motions can be compared with wind data from HARLIE and other instruments. Refinements and multi-sensor cross-checks of the data analysis methods will continue, as new campaigns are being planned. The long term goal of this work is to achieve a robust, non-Doppler wind sensing capability for use in the Calibration/Validation program for airborne and space-borne wind lidar instruments.

Broadly tunable infrared laser sources are of interest for a variety of applications including differential absorption lidar, differential scattering lidar, multi-spectral detection and imaging, hard target identification and discrimination, optical communications in poor visibility conditions, and spectroscopy. For chemical sensing applications, sources are particularly sought in the mid-wave infrared (MWIR) and long-wave infrared (LWIR) spectral regions. A variety of laser and nonlinear optical devices have been demonstrated that access these wavelengths. In particular, CTI is developing novel, tunable, narrow linewidth transmitters for coherent and direct detection lidar measurement applications. An example is a multi-watt Cr:ZnSe laser that is tunable over the 2.1 to 2.8 micrometers wavelength region. This laser has been used to pump-tune optical parametric oscillators (OPOs) that are broadly tunable across the MWIR and LWIR. We are also developing tunable Yb lasers that can be used to pump OPOs that emit signal beams in the eyesafe 1.55 micrometers region while generating idler beams that access the 3 to 4 micrometers MWIR band. This paper describes these sources.

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.

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.

This paper describes the development of a laboratory prototype unattended LIDAR system to measure aerosol profiles to 10km and ozone profiles to 3km. One consideration in an unattended system is a robust, eye-safe optical design that can provide the necessary signal levels and dynamic range to produce profiles at required height, resolution, and accuracy. An equally important consideration is a set of algorithms to compute aerosol and ozone profiles under a range of atmospheric conditions. NEXLASER employs an atmospheric state model to help identify and adapt to the varied conditions it must encounter. The signal-to-noise requirements of the algorithms are demonstrated and related back to hardware design. Performance of the system is demonstrated with simulated atmospheric conditions.

Agnes Scott College and the Georgia Institute of Technology are jointly developing an eye safe atmospheric lidar as a unique hands-on research experience for undergraduates, primarily undergraduate women. Students from both institutions will construct the lidar under the supervision of Agnes Scott and Georgia Tech faculty members. The engineering challenges of making lidar accessible and appropriate for undergraduates are described. The project is intended to serve as a model for other schools.

The speckle pattern in the image plane of the burst illumination imaging lidar system is characterized by the intensity correlation function. This speckle pattern, specific to coherent laser light, has two origins: the target roughness and the turbulence perturbation. We propose a model which takes into account both of them. For a non turbulent atmosphere, we find the classic speckle pattern given by Goodman. With a turbulent atmosphere, the perturbation along the forward and backward paths may be coupled or independent. In the coupled turbulence perturbation case, the collected intensity is amplified and the target scintillation is filtered by the receiver. For the independent turbulence perturbation case, the analysis is restricted here to a diffraction-limited atmosphere-lens system and two limiting cases are distinguished: the far-field and near-field assumptions.

Herein are discussed five straightforward field tests that are appropriate for evaluation of the performance of focal plane array (FPA) based ladar systems capable of generating high-resolution 3D imagery. The tests assess system level performance using traditional imaging targets and ladar specific targets. In addition, the tests allow comparisons to be made between the predicted performance of a ladar system and the actual performance. Analysis of actual field test ladar data is included based on appropriateness and availability of data. In the first test, range resolution is examined when the target is obscured by camouflage; the intent is to provide two pulse returns within the same instantaneous field of view (IFOV) and determine the source of the range report from different pixels within the range image with the emphasis on determining performance based on the pulse detection approach that is implemented. The second series of tests evaluates the lateral and range resolution of the FPA using standard modulation transfer function (MTF) and statistical approaches. The third test (Sect. 3.4) involves a moving target to introduce a dynamic version of the previous spatial frequency dependent tests. The fourth test (Sect. 3.5) assesses the system range performance as a function of received signal, essentially determining the performance of the system as signal-to-noise ratio (SNR) is varied. The fifth test (Sect. 3.6) assesses the uniformity of the range resolution and range accuracy of the FPA.

The progress in development of new solid-state SRS lasers is to a great extent due to the use of the neodymium-doped potassium gadolinium tungstate crystal, Nd³⁺:KGd(WO₄)₂(Nd:KGW). This laser crystal has a sufficiently low SRS conversion threshold, so it is often used as a self-frequency converter. We demonstrate two new miniature self-conversion lasers at 1.538 micrometers for application in eye-safe devices. The main characteristics of these lasers are presented. The features of these characteristics are discussed.