Free-space coherent optical communications have a potential application to offer last mile bottleneck solution in future local area networks (LAN) because of their information carrier, information security and license-free status. Coherent optical communication systems using orthogonal frequency division multiplexing (OFDM) digital modulation are successfully demonstrated in a long-haul tens Giga bits via optical fiber, but they are not yet available in free space due to atmospheric turbulence-induced channel fading. Adaptive optics is recognized as a promising technology to mitigate the effects of atmospheric turbulence in free-space optics. In this paper, a free-space coherent optical communication system using an OFDM digital modulation scheme and adaptive optics (FSO OFDM AO) is proposed, a Gamma-Gamma distribution statistical channel fading model for the FSO OFDM AO system is examined, and FSO OFDM AO system performance is evaluated in terms of bit error rate (BER) versus various propagation distances.
Free-space laser communication has been demonstrated with application potential in many areas such as line-of-sight communications, satellite communications and the last mile solution in a fiber optics networking. Both 0.8 and 1.5 micron wavelengths are currently used in state-of-the-art free space laser communication systems; unfortunately the system performance is imposed by atmospheric turbulence. To reduce the atmospheric effect in free-space laser communication systems, several techniques have been used, such as adaptive optics, aperture averaging and multiple transmitters; however, significant improvement has not been achieved. Theoretically, the seeing effect may be released using a longer wavelength. In this paper, we present a 3.5 micron free-space laser communication system model and its system performance evaluation. A 3.5 micron propagation model based on MODTRAN simulation results in different weather patterns is presented first, and a propagation link budget system model is described after that. The propagation channel performance evaluation results are presented by means of bit error rate versus various propagation distances.
When compared to a Shack-Hartmann sensor, a CMOS image sharpness sensor has the advantage of reduced complexity in a closed-loop adaptive optics system. It also has the potential to be implemented as a smart sensor using VLSI technology. In this paper, we present a novel adaptive optics testbed that uses a CMOS sharpness imager built in the New Mexico State University (NMSU) Electro-Optics Research Laboratory (EORL). The adaptive optics testbed, which includes a CMOS image quality metric sensor and a 37-channel deformable mirror, has the capability to rapidly compensate higher-order phase aberrations. An experimental performance comparison of the pinhole image sharpness feedback method and the CMOS imager is presented. The experimental data shows that the CMOS sharpness imager works well in a closed-loop adaptive optics system. Its overall performance is better than that of the pinhole method, and it has a fast response time.
The production of atmospheric-like turbulence is important to evaluate adaptive optics performance. Several techniques are commonly used to produce atmospheric-like turbulence in the laboratory, such as heating airflow, using diffractive and refractive optical components, and the atmospheric turbulence phase plate; however, the atmospheric turbulence phase plate has the advantage of low cost and easy control of the turbulence property. The properties of phase plates have been studied by many authors; however, a practical methodology to characterize the turbulence strength of the phase plate has not been discussed. The purpose of this paper is to propose a practical methodology to characterize the turbulence strength of the phase plate. In order to develop the methodology, a theoretical model of the optical system with the phase plate is derived first and the turbulence strength metric, which is in terms of the incident beam size versus D/r0, is obtained based on a comparison of theoretical and experimental data.
The ground-based optical interferometer with large apertures is a potential research tool for the study of stellar astrophysics and the synthesis of high-resolution stellar images. However, atmospheric turbulence can impose a significant limitation on the interferometer's performance. In order to reduce those degrading effects, we investigate the effectiveness of high-order adaptive optics in ground-based optical interferometry. The purposes of this paper are (1) to evaluate the performance with and without using high-order adaptive optics in a ground-based optical interferometer with large-aperture telescopes, and (2) to investigate the possibility of using the Strehl ratio to estimate visibility. The theoretical methodology and computer simulation results used to evaluate the performance of a ground-based stellar interferometer with high-order adaptive optics are presented, and a numerical computational method that uses the Strehl ratio to estimate the mean squared atmospheric coherence loss factor is developed.
Laser satellite communications (LCS) appear to have potential applications in the future global communication network. However, bidirectional ground-to-space link performance is degraded by atmospheric turbulence. This paper presents the simulations results of laser uplink and downlink propagation through atmospheric turbulence based on turbulence strength and link specification data obtained from the European Space Agency (ESA) ARTEMIS satellite and its associated optical ground station (OGS). The results obtained using this NMSU bidirectional link model indicate that the uplink performance can be improved significantly using tip-tilt error compensation alone while the downlink performance can be improved with higher order adaptive optics compensation.
A ground-based stellar interferometer appears to be a potentially useful research tool in studying stellar astrophysics and synthesizing a high resolution stellar image; however, its short-exposure performance is easily degraded by atmospheric turbulence. Even though adaptive optics has been recognized as a promising technology to improve image quality for a large aperture telescope, the question is often asked: "Is adaptive optics needed in a ground-based stellar interferometer?" In this paper, we develop the appropriate theory and provide simulation results to show why adaptive optics is needed in a ground-based optical interferometer. We also present a novel adaptive optics testbed including a tip-tilt error compensation system and a higher-order phase aberration compensation system to verify our theoretical simulation results.
In this paper, we present a novel adaptive optics testbed and its performance evaluation procedures. The testbed was built in the New Mexico State University (NMSU) Electro-optics Research Laboratory (EORL). NMSU's EORL adaptive optics testbed includes a tip-tilt error compensation system and a higher-order phase aberration compensation system. The tip-tilt error compensation was completed using a fast steering mirror with a quadrant cell detector. The higher-order phase aberration compensation was achieved using a 37-actuator deformable mirror and image sharpness with a stochastic parallel gradient descent algorithm (SPGDA). A metric optimization process was added in the SPGDA to fit an 8-bit deformable mirror control card. The system performance is evaluated using both static and dynamic phase aberration test conditions.
Long baseline stellar interferometers have been considered an essential tool in studying astrophysics; however, fringe visibilities for stellar interferometers with large apertures are often corrupted by atmospheric turbulence. To reduce the atmospheric turbulence effect, adaptive optics may be used to enhance fringe visibility for stellar interferometers with aperture sizes larger than the atmospheric coherence length. Fringe visibility performance evaluation for long baseline stellar interferometers with and without adaptive optics is presented in this paper. The methodologies used in this paper are described as follows: the optical transfer function for stellar interferometers with large apertures is derived first; then, performance metrics, coherence loss factor and Strehl ratio, are defined. Finally, fringe visibility performance with and without adaptive optics for different turbulent strengths is evaluated using computer simulation results. We show that Noll's mean square residual phase error can be used to compare the coherence loss factor of an interferometer with the Strehl ratio of a single telescope.
Long-baseline optical interferometers have become useful tools for obtaining detailed stellar information and high-resolution images in the astronomy community. Several interferometric systems have been implemented successfully without adaptive optics; however, adaptive optical systems may be needed for a new generation of long-baseline interferometers with large telescopes such as those being developed for the Magdalena Ridge Observatory (MRO). This paper introduces the design trade-offs used to investigate the need for adaptive optics for a long-baseline optical interferometer operating in the turbulent atmosphere. Modeling techniques are combined with analytical equations to study the performance of a long-baseline optical interferometer with and without adaptive optics.
Simulation results of a long-baseline optical interferometer with adaptive optics are presented in this paper. Long-baseline optical interferometers have become useful tools for obtaining detailed stellar information and high-resolution images in the astronomy community. Several interferometric systems have been implemented successfully without adaptive optics; however, adaptive optical systems may be needed for a new generation of long-baseline interferometers with large telescopes such as those being developed for the Magdalena Ridge Observatory (MRO). A long-baseline optical interferometer in the turbulent atmosphere is modeled first, then an optical interferometer with an adaptive optics system (AOS) is modeled and the resulting fringe patterns for different input turbulence scales are interpreted. Finally, the performance of a long baseline optical interferometer with and without an AOS is carefully evaluated and recommendations are made for the implementation of adaptive optics in the 1.5-meter MRO telescopes.