Farzin Amzajerdian

Design and fabrication of a compact lidar telescope

A prototype compact off-axis reflective lidar telescope has been designed and fabricated for remote sensing of atmospheric winds from space and airborne platforms. The 250 mm aperture telescope consists of two mirrors and a collimating lens to achieve a very compact size, without any central obscuration.It has no internal focal point to prevent air breakdown, and the pupil relay optics has also been eliminated. This paper presents the results of optical design and sensitivity analysis along with the predicted performance. The major design issues for lidar systems, particularly the one that utilizes coherent detection for higher sensitivity and Doppler frequency extraction, are the wavefront quality, polarization purity, and a minimum backscattering off the reflective surfaces. These design issues along with the other optical characteristics of this lidar telescope are presented. The effect on the wavefront quality of the tilt, decentration and axial spacing tolerances for the mirrors, collimating lens and quarter wave plate is discussed.

Design and analysis of a spaceborne lidar telescope

A spaceborne telescope has been designed and analyzed for a 2-micron solid state coherent lidar system operating on a satellite. The optical system consists of a large off-axis reflective telescope, a large-aperture diffractive scanner, an image derotator and a lag angle compensator. Due to the orbiting motion of the satellite and scanning, the boresight of the telescope shifts during the round trip travel time of the laser pulses to the target. In a coherent lidar system utilizing optical heterodyne detection, the relative alignment of the received signal with respect to the local oscillator beam is particularly critical. Two compensators have been designed to correct the boresight errors as well as the wavefront errors caused by beam wandering due to the boresight changes. Several design approaches for the compensators have been investigated. The optical and optomechanical design issues for such a system are discussed. The results of optical performance, modeling, and tolerance analysis for the telescope are also presented.

Lightweight lidar telescopes for space applications

NASA is intent on exploiting the unique perspective of space-based remote optical instruments to observe and study largescale environmental processes. Emphasis on smaller and more affordable missions continues to force the remote sensing instruments to find innovative ways to reduce the size, weight, and cost of the sensor package. This is a challenge because many of the proposed instruments incorporate a high quality meter-class telescope that can be a significant driver of total instrument costs. While various methods for telescope weight reduction have been achieved, many of the current approaches rely on exotic materials and specialized manufacturing techniques that limit availability or substantially increase costs. A competitive lightweight telescope technology that is especially well suited to space-based coherent Doppler wind lidar has been developed through a collaborative effort involving NASA Marshall Space Flight Center (MSFC) through the Global Hydrology and Climate Center (GHCC) and the University of Alabama in Huntsville (UAH) at the Center for Applied Optics (CAO). The new lightweight optics using metal alloy shells and surfaces (LOMASS) fabrication approach is suitable for high quality metal mirrors and meter-class telescopes. Compared to alternative materials and fabrication methods the new approach promises to reduce the areal density of a meter-class telescope to less than 15-kg/m2; deliver a minimum ?/1O-RMS surface optical quality; while using commercial materials and equipment to lower procurement costs. The final optical figure and finish is put into the mirrors through conventional diamond turning and polishing techniques. This approach is especially advantageous for a coherent lidar instrument because the reduced telescope weight permits the rotation of the telescope to scan the beam without requiring heavy wedges or additional large mirrors. Ongoing investigations and preliminary results show promise for the LOMASS approach to be successful in demonstrating a novel alternative approach to fabricating lightweight mirrors with performance parameters comparable with the Space Readiness Coherent Lidar Experiment (SPARCLE). Development and process characterization is continuing with the design and fabrication of mirrors for a 25-cm telescope suitable for a lidar instrument.

Metrology of optical beam expander for space readiness coherent lidar experiment

Stringent requirements of this telescope demands testing of its individual elements through each step of fabrication and complete metrology of the integrated telescope both at Helium-Neon and the operational two micron wavelengths.

Primary mirror manufacturing considerations for a space-based coherent lidar

The measurement of winds from a space borne platform is of significant scientific importance to both weather prediction and climate research. One of the key technologies embodied in coherent detection of winds from space is the use of large aperture, compact, lightweight, high-quality wavefront, photon-efficiency optics. This paper discusses the optical design, the mechanical design, material preference, diamond turning issues, polishing requirements, and coating selections for the primary mirror of a 25X afocal beam expander intended for use in space-based coherent lidar systems.

Thermal considerations for the SPARCLE optical system

The SPAce Readiness Coherent Lidar Experiment (SPARCLE) is the first demonstration of a coherent Doppler wind lidar in space. Coherent lidars can accurately measure the wind velocity by extracting the Doppler frequency shift in the back-scattered signal from the atmosphere through optical heterodyne (coherent) detection. Coherent detection is therefore highly sensitive to aberrations in the signal phase front, and to relative alignment between the signal and the local oscillator beams. The telescope and scanning optics consist of an off-axis Mersenne telescope followed by a rotating wedge of silicon and a window of fused silica. The wedge is in very close proximity to the experiment window, and is essentially in contact with the scanner motor/encoder system. The can environment temperature is nominally 20 degrees Celsius, the window ranges from -20 degrees Celsius to 0 degrees Celsius, and the scanner motor/encoder system alone could generate temperatures as high as 35 degrees Celsius. This thermal environment, coupled with the relatively large sensitivity of silicons refractie index to temperature, has required careful thermal design and compensation techniques. This paper discusses the optical issues of these thermal effects and a variety of methods used to ameliorate them.

Characterization of an optical subsystem for 2-μm coherent lidars

This paper presents the test results on a compact, off-axis telescope which is the precursor projector/receiver for a NASA Shuttle-based coherent lidar system operating at a wavelength of 2 microns to measure atmospheric wind profiles. The afocal telescope has an entrance pupil diameter of 25 cm, and an angular magnification of 25x. To determine the transmitted and returned optical wavefront quality, the telescope was tested in a Twyman-Green configuration at the operational wavelength. Interferograms were obtained via an infrared camera, and analyzed using a digitizing tablet and WYKO WISP software. Interferograms were obtained with and without an 11.7 degree wedged silicon window located in the entrance pupil. This window, which rotates orthogonal to the telescope optical axis, serves as the lidar system scanner. The measured wavefront information from the interferometer was used in a GLAD heterodyne receiver model to predict the effect of the optical system on the lidar performance. The experimental setup and procedures will be described, and the measurement results of the coherent lidar optical subsystem will be presented in this paper.

Optomechanical design of a multi-axis stage for the SPARCLE telescope

A critical component in the 2-micrometer coherent spaced-based lidar system (SPARCLE) is the compact, off-axis, 25-cm aperture telescope. The stressing optical performance demanded from this telescope coupled with the difficulty associated with aligning such a fast, off-axis system; has created the need for a multiple-axis alignment stage for the secondary mirror. Precision micrometer kinematic mounts were used in the laboratory to demonstrate the ability to successfully align the telescope. For the flight configuration, a more robust and considerably smaller stage (both in size and weight) had to be designed in order to fit within the space shuttle packaging constraints. The new stage operates with multiple degrees of freedom of motion to achieve micrometer precision alignment and then uses a mechanical multiple point support to lock-in the alignment and provide stability. The optomechanical design of the flight stage is described.