We present a new technique for the correction of atmospheric distortion in telescope images. Most of this distortion arises from a random phase variation of the incoming light across the telescope aperture. This variation limits the resolving power of even large telescopes to about one arc second. If the sharpness of the images is defined in a suitable way, this sharpness is maximized only when the phase distortion of the incoming light is zero. We present computer simulations of a simple feedback system in which active optical elements, set to maximize the sharpness, correct most of the atmospheric distortion. Photon statistics set the limiting magnitude of the object for which a practical feedback system can work. Details in a sixth magnitude object smaller than 0.1 sec of arc should be resolvable. The system can be conveniently employed within existing telescopes.

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Real-time correction of atmospherically degraded telescope images through image sharpening

The theory and observations of the optical propagation effects of the clear turbulent atmosphere are reviewed, with particular attention to those characteristics most important to the designer of an optical communication system. Among the phenomena considered are the variance, probability distribution, spatial covariance, aperture smoothing, and temporal power spectrum of intensity fluctuations, and similar quantities for phase fluctuations and angle of arrival.

A survey of clear-air propagation effects relevant to optical communications

A point-to-multipoint bi-directional wide area telecommunications network employing atmospheric optical communication. The network comprises a primary transceiver unit, a plurality of subscriber transceiver units and an optical router. The primary transceiver unit generates a first light beam on which it modulates first data. The primary transceiver unit atmospherically transmits the first light beam to the optical router which demodulates the first data, modulates the first data on a second light beam and transmits the second light beam to the plurality of subscriber transceiver units in multiplexed manner. The subscriber transceiver units receive the second light beam and demodulate the first data from the second light beam. Conversely, the subscriber transceiver units atmospherically transmit a third light beam on which they modulate second data to the optical router which demodulates the second data, modulates the second data on a fourth light beam and transmits the fourth light beam to the primary transceiver unit. The primary transceiver unit atmospherically receives the fourth light beam and demodulates the respective second data from the fourth light beam. The optical router of the network comprises a secondary transceiver unit, a plurality of transceiver modules and an electronic router for routing data between the secondary transceiver unit and the plurality of transceiver modules to establish communication channels between the primary transceiver unit and the plurality of subscriber transceiver units. The secondary transceiver unit communicates with the primary transceiver unit and the transceiver modules communicate with the subscriber transceiver units. The transceiver modules comprise an X-Y beam deflector for deflecting the second and third light beams to a portion of the subscriber transceiver units in a time-multiplexed fashion. In an alternate embodiment of the optical router, the first light beam is redirected to the subscriber transceiver units and the third light beam is redirected to the primary transceiver unit by a mirror and lens set assembly rather than being demodulated and modulated in the router. Applications such as telephony, the Internet, teleconferencing, radio broadcast, HDTV, interactive TV, and other television forms are contemplated for employment on the network.

Point-to-multipoint wide area telecommunications network via atmospheric laser transmission through a remote optical router

A hybrid wireless optical and radio frequency (RF) communication link utilizes parallel free-space optical and RF paths for transmitting data and control and status information. The optical link provides the primary path for the data, and the RF link provides a concurrent or backup path for the network data, as well as a reliable and primary path for the control and status information. When atmospheric conditions degrade the optical link to the point at which optical data transmission fails, the hybrid communication link switches to the RF link to maintain availability of data communications. The switch may occur automatically, based on an assessment of the quality of the optical signal communicated through the optical path.

Hybrid wireless optical and radio frequency communication link

A free-space optical communications system for transmitting data between an aircraft computer system (14) and a ground-based computer system (12). The system includes a pair of corresponding optical transmitters (36) and optical receivers (38) that transmit and receive optical signals transmitted between the two computer systems. Included within each optical transmitter is one or more light-emitting diodes (42) that produce optical signals corresponding to the data to be transmitted. A beam-forming prism (44) is bonded directly to the light-emitting diodes to direct the optical signal uniformly over a target area. The optical receiver includes one or more infrared windows (50) to reduce the amount of ambient light received by the optical receiver. A compound parabolic concentrator (64) collects light transmitted from the optical transmitter and directs the light onto an avalanche photodiode (66), which includes thermal bias compensation. An AC network couples the output signal of the photodiode to a transimpedance amplifier (70). An optional optical shroud (34) surrounds the optical transmitters and receivers to further reduce the amount of ambient light that is received by the optical receivers.

Wide-angle, high-speed, free-space optical communications system

During a series of balloon flights in southeastern New Mexico new data was obtained on microthermal atmospheric turbulence structure from the surface to altitudes of 12 to 15 km and the beginning of the tropopause region. It was possible to compute the temperature structure coefficient and the refractive-index-structure coefficient. The major significance of the data is the thermal evidence for turbulence and the strength of this effect in the upper-altitude regions.

Measurements of turbulence profiles in the troposphere.

A satellite system for optical communications such as a multi-access laser transceiver system. Up to six low Earth orbiting satellites send satellite data to a geosynchronous satellite. The data is relayed to a ground station at the Earth's surface. The earth pointing geosynchronous satellite terminal has no gimbal but has a separate tracking mechanism for tracking each low Earth orbiting satellite. The tracking mechanism has a ring assembly rotatable about an axis coaxial with the axis of the field of view of the geosynchronous satellite and a pivotable arm mounted for pivotal movement on the ring assembly. An optical pickup mechanism at the end of each arm is positioned for optical communication with one of the orbiting satellites by rotation of the ring.

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Multi-access laser communications transceiver system

Laser beam scintillation and log-amplitude variance evaluation for wavelengths on digital computer

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Wavelength Dependence of Laser-Beam Scintillation*

The NGST Wavefront Control Testbed (WCT) is a joint technology program managed by the Goddard Space Flight Center (GSFC) and the Jet Propulsion Laboratory (JPL) for the purpose of developing technologies relevant to the NGST optical system. The WCT provides a flexible testing environment that supports the development of wavefront sensing and control algorithms that may be used to align and control a segmented optical system. WCT is a modular system consisting of a Source Module (SM), Telescope Simulator Module (TSM) and an Aft-Optics (AO) bench. The SM incorporates multiple sources, neutral density filters and bandpass filters to provide a customized point source for the TSM. The telescope simulator module contains a flip-in mirror that selects between a small deformable mirror and three actively controlled spherical mirror segments. The TSM is capable of delivering a wide range of aberrated, unaberrated, continuous and segmented wavefronts to the AO optical bench for analysis. The AO bench consists of a series of reflective and transmissive optics that images the exit pupil of the TSM onto a 349 actuator deformable mirror that is used for wavefront correction. A Fast Steering Mirror (FSM) may be inserted into the system (AO bench) to investigate image stability and to compensate for systematic jitter when operated in a closed loop mode. We will describe the optical design and performance of the WCT hardware and discuss the impact of environmental factors on system performance.

Optical design and performance of the NGST wavefront control testbed

Log amplitude variance calculation methods compared in statistics of optical scintillation by application to measurements with laser

Michael W. Fitzmaurice

Papers

Measurements of turbulence profiles in the troposphere.

Multi-access laser communications transceiver system

Wavelength Dependence of Laser-Beam Scintillation*

Optical design and performance of the NGST wavefront control testbed

Measurement of Log-Amplitude Variance