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The Optical Communications Demonstrator (OCD) is a laboratory based lasercom terminal that was developed to validate several key technologies such as precision beam pointing, high bandwidth beacon tracking and beacon acquisition.
Mountain-top to mountain-top optical link experiments have been initiated at JPL, in order to perform a systems level evaluation of optical communications. Progress made so far is reported. ne NASA, JPL developed optical communications demonstrator (OCD) is used to transmit a laser signal from Strawberry Peak (SP), located in the San Bernadino mountains of California. This laser beam is received by a 0.6 m aperture telescope at JPL's Table Mountain Facility (TMF), located in Wrightwood, California. The optical link is bi-directional with the TMF telescope transmitting a continuous 4-wave (cw) 780 run beacon and the OCD sending back a 840 nm, 100 - 500 Mbps pseudo noise (PN) modulated, laser beam. The optical link path is at an average altitude of 2 km above sea level, covers a range of 46.8 km and provides an atmospheric channel equivalent to approx. 4 air masses. Average received power measured at either end fall well within the uncertainties predicted by link analysis. The reduction in normalized intensity variance (sigma(sup 2, sub I)) for the 4-beam beacon, compared to each individual beam, at SP, was from approx. 0.68 to 0.22. With some allowance for intra-beam mis-alignment, this is consistent with incoherent averaging. The sigma(sup2, sub I) measured at TMF approx. 0.43 +/- 0.22 exceeded the expected aperture averaged value of less than 0.1, probably because of beam wander. The focused spot sizes of approx. 162 +/- 6 microns at the TMF Coude and approx. 64 +/- 3 microns on the OCD compare to the predicted size range of 52 - 172 microns and 57 - 93 microns, respectively. This is consistent with 4 - 5 arcsec of atmospheric "seeing". The preliminary evaluation of OCD's fine tracking indicates that the uncompensated tracking error is approx. 3.3 micro rad compared to approx. 1.7 micro rad observed in the laboratory. Fine tracking performance was intermittent, primarily due to beacon fades on the OCD tracking sensor. The best bit error rates observed while tracking worked were 1E-5 to 1E-6.
A mountain-top-to-mountain-top optical communications demonstration was conducted between the JPL Table Mountain Facility (TMF), Wrightwood, California, and Strawberry Peak (SP), Lake Arrowhead, California, during the months of August and September of 2000. The link was nearly horizontal at an altitude of 2 km and spanned a range of 46.8 km. A 780-nm multibeam beacon broadcast from TMF was received by the JPL Optical Communications Demonstrator (OCD) located at SP. The received beacon was utilized as a pointing reference to retransmit an 852-nm communications laser beam, modulated at 400 Mb/s by a PN7 pseudo-random bit stream (PRBS) sequence. The long atmospheric path resulted in atmospheric-turbulence-induced angle-of-arrival fluctuations of the beacon at the OCD aperture. A .ne-steering control loop was used to track the resulting beacon centroid motion and update the pointing of the communications laser beam transmitted from SP to TMF. Fried parameters, or r0, inferred from focal spot sizes received at SP were 4 to 5 cm whereas, using the spot sizes received at TMF, they were 2 to 3 cm. In both cases, theory predicts larger r0 values. The predicted angle-of-arrival fluctuations compare well with measured rms displacements exhibited by the focal spots at either end of the link. An uncompensated error of ~1.1 rad in the x-axis and ~2 rad in the y-axis was obtained using centroid data logged by the OCD. Average bit-error rates of 10-5 were recorded for extended periods of time. An atmospheric coherence length r0 of 3 to 5 cm was inferred using the focal-plane spot size measured on the CCD tracking sensor and compared to a predicted value of 5 to 7 cm using reasonable atmospheric models. The irradiance bounds required for the CCD tracking sensor to perform centroiding was found to range from 2000 to 3000 integrated pixel counts, although a more reliable range was 600 to 3000, indicating a dynamic range of 6 to 11 dB. The motion of the spot on the focal plane was also recorded and yielded rms angle-of-arrival-induced focal-plane displacement of 9 to 11 m, compared to a predicted value of 7.8 to 11 m. The irradiance measurements made with the OCD clearly indicate that best tracking performance is obtained when the mean received signal is 2000 to 2200 counts. The best tracking performance resulted in an rms uncompensated error of 1.1 rad. The uncompensated error appeared to increase with either an increase or decrease in mean beacon-signal level. The rms uncompensated error determined by deriving the beacon displacement power spectral density from the beacon centroid-versustime data and the rejection function of the control loop yielded a higher value of 3.4 rad.
A second generation optical communications demonstrator (OCD-2) intended for airborne applications like air-to-ground and air-to-air optical links is under development at JPL. This development provides the capability for unidirectional high data rate (2.5-Gbps) transmission at 1550-nm, with the ability to receive an 810-nm beacon to aid acquisition pointing and tracking. The transmitted beam width is nominally 200-(micro)rad. A 3x3 degree coarse field-of-view (FOV) acquisition sensor with a much smaller ~3-mrad FOV tracking sensor is incorporated. The OCD-2 optical head will be integrated to a high performance gimbal turret assembly capable of providing pointing stability of 5- microradians from an airborne platform. Other parts of OCD-2 include a cable harness, connecting the optical head in the gimbal turret assembly to a rugged electronics box. The electronics box will house: command and control processors, laser transmitter, data-generation-electronics, power conversion/distribution hardware and state-of-health monitors. The entire assembly will be integrated and laboratory tested prior to a planned flight demonstrations.
The Optical Communications Demonstrator (OCD) is a laboratory-based lasercom demonstration terminal designed to validate several key technologies, including beacon acquisition, high bandwidth tracking, precision bearn pointing, and point-ahead compensation functions. It has been under active development over the past few years. The instrument uses a CCD array detector for both spatial acquisition and high-bandwidth tracking, and a fiber coupled laser transmitter. The array detector tracking concept provides wide field-of-view acquisition and permits effective platform jitter compensation and point-ahead control using only one steering mirror. This paper describes the detailed design and characterization of the digital control loop system which includes the Fast Steering Mirror (FSM), the CCD image tracker, and the associated electronics. The objective is to improve the overall system performance using laboratory measured data. The. design of the digital control loop is based on a linear time invariant open loop model. The closed loop performance is predicted using the theoretical model. With the digital filter programmed into the OCD control software, data is collected to verify the predictions. This paper presents the results of the, system modeling and performance analysis. It has been shown that measurement data closely matches theoretical predictions. An important part of the laser communication experiment is the ability of FSM to track the laser beacon within the. required tolerances. The pointing must be maintained to an accuracy that is much smaller than the transmit signal beamwidth. For an earth orbit distance, the system must be able to track the receiving station to within a few microradians. The failure. to do so will result in a severely degraded system performance.
A bidirectional horizontal-path optical link was demonstrated between Strawberry Peak (SP), Lake Arrowhead, California, and the JPL Table Mountain Facility (TMF), Wrightwood, California, during June and November of 1998. The 0.6-m telescope at TMF was used to broadcast a 4-beam 780-nm beacon to SP. The JPL-patented Optical Communications Demonstrator (OCD) at SP received the beacon, performed ne tracking to compensate for the atmosphere-induced beacon motion and retransmitted a 844-nm communications laser beam modulated at 40 to 500 Mb/s back to TMF. Characteristics of the horizontal-path atmospheric channel as well as performance of the optical communications link were evaluated. The normalized variance of the irradiance fluctuations or scintillation index delta2/I at either end was determined. At TMF where a single 844-nm beam was received by a 0.6-m aperture, the measured delta2/I covered a wide range from 0.07 to 1.08. A single 780-nm beam delta2/I measured at SP using a 0.09-m aperture yielded values ranging from 0.66 to 1.03, while a combination of four beams reduced the scintillation index due to incoherent averaging to 0.22 to 0.40. This reduction reduced the dynamic range of the fluctuations from 17 to 21 dB to 13 to 14 dB as compared with the OCD tracking sensor dynamic range of 10 dB. Predictions of these values also were made based on existing theories and are compared. Generally speaking, the theoretical bounds were reasonable. Discussions on the probability density function (PDF) of the intensity fluctuations are presented and compared with the measurements made. The lognormal PDF was found to agree for the weak scintillation regime as expected. The present measurements support evidence presented by earlier measurements made using the same horizontal path, which suggests that the aperture averaging effect is better than theoretically predicted.
Observations made during a mountain-top-to-mountain-top horizontal optical link demonstration are described. The optical link spans a range of 45 Km at an average altitude of 2 Km above sea level. A multi-beam beacon comprised of eight laser beams emerging from multi-mode fiber coupled lasers (780 nm) are launched through a 0.6 m diameter telescope located at the JPL Table Mountain Facility (TMF) in Wrightwood, California. The multibeam beacon is received at Strawberry Peak located in the San Bernadino Mountains of California. The NASA, JPL developed optical communications demonstrator (OCD) receives the beacon, senses the atmosphere induced motion and using an upgraded fine steering loop compensates the re-transmission of a communications laser beam (852 nm, 400 Mbps) to TMF. The eight-beam beacon allowed a four reduction in normalized irradiance or scintillation index. This in turn was sufficient to eliminate beacon fades sensed b the OCD and allowed the fine steering loop performance evaluation. The uncompensated tracking error was determined to be 2.2 - 3 prad compared to a model prediction of 3.4 prad. The paper also discusses various auxiliary observations made during the campaign
This paper describes the optical system for the Optical Communication Demonstrator (OCD) instrument. With an aperture of only 4 inches, the OCD instrument is designed to demonstrate the capability of communicating from space to a ground station with a small instrument using optical wavelengths.
The Optical Communications Demonstrator (OCD), under development at the Jet Propulsion Laboratory (JPL), is a laboratory-based lasercomm terminal designed to validate several key technologies, primarily precision beam pointing, high bandwidth tracking, and beacon acquisition.
The Dressler (1980) morphological sample of galaxies in clusters is used in an investigation of bright galaxy spatial distribution, which has established that such galaxies with luminous extended halos as the D or cD lie on significant cluster galaxy distribution peaks irrespective of whether they are the brightest cluster member. The mean distance of bright D and cD galaxies from local density peaks is of the order of their halo scales, and the positions of bright galaxies of other morphological types are consistent with their being drawn at random from each morphological population. It is noted that local density peaks with associated d ocD galaxies have a mean density 2-3 times greater than peaks without a D or cD.
The Optical Communications Demonstrator (OCD) is a laboratory-based lasercom demonstration terminal designed to validate several key technologies, including beacon acquisition, high bandwidth tracking, precision beam pointing, and point-ahead compensation functions.
The Optical Communications Demonstrator (OCD) is a laboratory-based lasercom demonstration terminal designed to validate several key technologies, including beacon acquisition, high bandwidth tracking, precision beam pointing, and point-ahead compensation functions.
A modulated fiber-coupled diode laser assemby has been developed. This module will serve as the laser transmitter assembly for the Optical Communication Demonstrator (OCD) system. The laser is capable of greater than 20mW of output at 100 Mbps Q-PPM modulation.
A mechanical design has been developed for the Telescope Optical Assembly (TOA) of the Optical Communications Demonstrator (OCD).
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The Optical Communications Group at the Jet Propulsion Laboratory will deploy an optical communications transmitter demonstration and facility to the International Space Station in January 2002.
The NASA-funded optical communications program being conducted at JPL is described.