The authors analyze the scaling limits of diode-end-pumped Nd:YAG lasers imposed by pumping geometry, thermal distortion, birefringence, and fracture. A scalable 15-W output power diode-end-pumped laser which achieves a 60% slope efficiency is demonstrated. Thermal distortion in this laser is reduced from 2.6 waves to less than 0.15 waves with an aspheric laser rod. Depolarization is reduced by 40 times to less than 0.2% per pass using optical rotators. Modeling results, which accurately predict thermal distortion and birefringence, imply a thermal fracture-limited output power for short end-pumped rods less than 45 W. A dual rod design for an 80-W device is described. One of the four modules in this device has been assembled and tested, producing a CW output of 22.5 W. >

Scaling CW diode-end-pumped Nd:YAG lasers to high average powers

The essential qualities for a successful airborne lidar bathymeter are accuracy, capability, and cost effectiveness. Over the past twenty five years, developments in lasers, optics, electronics, and computers have made it somewhat easier to construct viable airborne lidar systems with varying purposes, and an increasing number are being constructed. Fewer than ten airborne lidar bathymeters exist in the world today, however, because of limited demand and because of the fact that it is still very difficult to meet the above three requirements, particularly the first. It is not hard to get answers from a system. It takes a great deal of understanding and effort, however, to obtain results that will meet international accuracy standards and the operational requirements of the typical customer. The primary considerations in the design, construction, and operation of an airborne bathymeter must be data quality and depth measurement accuracy. Both the physical environment and system hardware components contribute many error sources that must be overcome. This requires thoughtful hardware and software system design and construction, as well as the prediction, modelling, and application of appropriate correctors. Operational procedures for quality control, calibration, and maintenance must be established and followed. In this paper, we describe the large number of hardware design features, software algorithms, bias correctors, displays, and operational procedures that have been developed to provide the basis for a system which will meet required accuracy standards while maintaining efficiency and cost-effectiveness. The above features have all been incorporated into the U.S. Army Corps of Engineers SHOALS operational airborne lidar bathymetry system. SHOALS has been operated, from both a helicopter and a fixed-wing aircraft, to meet a wide range of survey requirements in categories such as charting, dredging, coastal engineering, resource management, modeling, and reconnaissance. Although SHOALS hardware was designed ten years ago, that design has been demonstrated to be optimal through seven years of successful field operation covering a wide range of sponsors, requirements, locations around the world, and environmental conditions. The SHOALS post-flight data processing software suite has provided high accuracy, performed flawlessly, and been regularly upgraded to improve utility and efficiency. The overall system design has proven to be very flexible, and a number of new features and capabilities have been added in hardware and software in response to customer requirements. In this paper, we will describe hardware and software design philosophy and critical design considerations. We discuss in detail how a large number of potential or realized error sources, inherent to airborne lidar hydrography systems in general and for SHOALS in particular, have been overcome.

Meeting the accuracy challenge in airborne lidar bathymetry

A new concept for compensation of birefringence in solid-state rod lasers is presented. The basic idea of our approach is the simultaneous use of an in-cavity polarization rotator and an imaging optical system between the two passes through the laser rod. The theoretical analysis based on matrix optics shows that perfect compensation for the birefringence and bifocusing in laser rods is possible for a large pumping range. Experimental results with two Nd:YAG rods are also presented. The measured depolarization loss per transit for a test beam at 0.63 μm is less than 5% for pumping power up to 6kW per rod. This would allow efficient generation of high-power TEM00 mode and linearly polarized multimode output power from Nd:YAG oscillators in rod geometry.

A novel approach for compensation of birefringence in cylindrical Nd: YAG rods

Since microwaves do not penetrate water, RADAR, the principal tool for remote sensing of the earth and atmosphere, cannot be used directly for the detection of underwater objects. Currently, aerial light detecting and ranging (LIDAR) systems are therefore preferred for the detection and ranging of objects submerged in the sea. LIDAR provides for large area coverage at high speed, but it lacks coherent detection capability, a shortcoming that severely limits system sensitivity and underwater target contrast. In response to this problem, this paper details the merging of RADAR and LIDAR technologies in the constitution of a hybrid LIDAR-RADAR detection scheme. This new sensor configuration has reduced incoherent backscatter clutter by 17 dB in laboratory experiments and related computer simulations. >

Application of RADAR technology to aerial LIDAR systems for enhancement of shallow underwater target detection

Extension of the dynamic range of liquid crystal tunable Lyot filter is demonstrated by incorporating with it a liquid crystal variable retarder as an eliminator for the third and fourth order peaks. The filter is continuously tunable in the range 500 nm to 900 nm with a nominal width in the range 50nm-100nm. Design procedure is described including the exact solution to the LC director profile and the suitability for biomedical optical imaging applications. Flexibility in the design is proposed by applying different voltages to the different liquid crystal retarders thus compensating for small thickness deviations from the nominal values and obtaining the high dynamic range.

Liquid crystal Lyot tunable filter with extended free spectral range

The Australian laser airborne depth sounder WRELADS II was developed to demonstrate the feasibility of using airborne laser techniques for hydrographic survey. This paper describes the system, the experimental program used for its evaluation, and some results obtained by it. Development of the system over a five-year period involved 550 h of airborne evaluation, including the collection of data during 22 sorties over a calibrated test range. Analysis of these data has permitted development of an empirical depth bias model. It is shown that the system meets the requirements of the Royal Australian Navy Hydrographic Service for depth accuracy and provides an adequate data yield.

Airborne laser hydrography in Australia.

The use of optical rotators and waveplates to compensate for thermally induced birefringence in cylindrical laser rods is reported. The use of these components allows polarization coupled lasers to operate efficiently at high average power levels. Experimentally a Nd:YAG oscillator has been operated in a Q-switched TEM(00) mode giving 300-mJ pulses and 15 W of average power.

Birefringence compensation in polarization coupled lasers.

The design of a six-stage liquid crystal tuned Lyot filter for operation at a wavelength of 532 nm is described. Design options including material type and factors affecting performance such as material quality are discussed. The proposed narrow-passband filter will have a bandwidth of less than 0.1 nm. To achieve the performance required of the filter, high transmittance polarizers have been developed.

James Richards

Papers

Airborne laser hydrography in Australia.

Birefringence compensation in polarization coupled lasers.

Design considerations for a liquid crystal tuned Lyot filter for laser bathymetry