This paper reports the result of follow-on study about the propagation characteristics of direct single mode fiber (SMF)
coupled free-space optical (FSO) signals which have been obtained during recent outdoor propagation experiments over
100-500 m link distance. We observed good agreements of the surge height distribution of SMF-coupled optical signal
intensity with those of the lognormal prediction. However, the same comparison for the fade depth of SMF-coupled
signal indicated 2-3 times larger results. These statistical analyses have validated the method used in this paper to
compare atmospheric turbulence model with real experimental data and predict the SMF-coupled signal quality based on
the scintillation index determined by Cn2 profiles.
KEYWORDS: Free space optics, Mirrors, Single mode fibers, Sensors, Servomechanisms, Scintillation, High power lasers, CCD cameras, Optical tracking, Fiber couplers
This paper describes the design of a wide field-of-view single-mode-fiber coupled free space laser communication terminal which has mutual beacon tracking system using a commercially available fast steering mirror. Using the terminals, we tried a low power laser beam transmission experiment over 500-m distance using a 1064 nm wavelength. This paper also reports the experimental results of terminal performance as well as propagation characteristics.
This paper reports propagation characteristic of direct single-mode-fiber (SMF) coupled free-space-optical signal.
Accumulated data at recent indoor/outdoor demonstration experiments over 30-500 m link distance under weak to
medium atmospheric turbulence conditions are analyzed and discussed. The results show asymmetric surge/fade ratios
and long lower tail distribution in SMF-coupled signal intensities due to phase interference at the SMF receiving
aperture, which is different from the result in the normal irradiance distribution, such as Gamma-Gamma distribution or
log-normal distribution. This asymmetry indicates that reliable FSO links should be operated under sufficiently weak
atmospheric turbulence environments where scintillation index is less than 0.1. This requirement is also valid to maintain
mutual beacon tracking to assure precise tracking/pointing accuracy required for the direct SMF coupling.
The author describes the key design feature of a newly developed compact free-space optical terminal, which can achieve robust direct single-mode-fiber-coupling with advanced bidirectional beacon tracking system and excellent operational performance under various adverse weather conditions, such as strong atmospheric turbulences and rain/snow falls. The author found that there is a good correlation between the scintillation index of beacon/signal receiving intensities and the signal fading depth. Therefore, the signal link quality can be predicted from the beacon scintillation index using the classical Rytov theory. According to the outdoor experimental results, it can be concluded that the scintillation index of the beacon beam should be less than 0.1 for stable and reliable communication links.
KEYWORDS: Free space optics, Near field optics, Telescopes, Collimation, Near field, Free space optical communications, Signal attenuation, Optical tracking, Single mode fibers, Fiber couplers
This paper reports a new beam setting for single-mode-fiber coupled free-space laser communication terminals with
bidirectional beacon tracking. To maintain stable and robust laser beam transmission over 0.2-2km terrestrial horizontal
link, optimum beam setting is found to be not a collimated beam but a symmetric confocal beam between the two
terminals. The paper also describes the detail of optics, such as optical antenna and fiber coupling lens used in the
compact free-space optical terminals.
This paper discusses the operational condition for direct single-mode-fiber-coupling FSO terminals under the various
adverse weather conditions, such as strong atmospheric turbulences and rain falls. A good correlation between the
scintillation index of the intensities of beacon receiving power and the signal fading depth has been observed, which
allows us to predict the signal link quality based on the beacon scintillation index provided by the classical scintillation
theory and concludes that the scintillation index for the beacon beam should be less than 0.1. This paper also reports the
effect of performance enhancements provided by the new adaptive controller for the stable and robust terminal operation.
This paper presents the results of the technology demonstration of a short distance free-space optical (FSO)
communication link with 2.5 Gbps data rate. Each terminal consists of a gimbaled telescope, an acquisition and coarse
tracking CCD and digital control system, a fine tracking system, a fiber-optic interface, transceivers, beacon lasers, and a
bit error test set. Two different fine-tracking subsystems were set-up on these two terminals: the first one is based on a
micro mechanical steering mirror developed at NICT, Japan; while the second is based on liquid crystal (nonlinear
spatial light modulation mechanism), developed at CSA, Canada. Performance of the two systems, their compatibility
and the future work are discussed.
KEYWORDS: Free space optics, Signal attenuation, Telescopes, Sensors, Optical tracking, Single mode fibers, Near field, Space telescopes, Near field optics, Scintillation
This paper discusses a new concept of free-space optical (FSO) communication terminals, characterized as the mutual
tracking of near field Gaussian beams and the direct single mode fiber coupling to achieve Tera-bit/seconds (Tbps) class
FSO link in short range outdoor environments. It first shows the required optical configurations including beacon
transmitters, tracking sensors and fast steering mirrors to construct the mutual tracking system and then discusses their
capabilities and performance for this specific near field operation in which conventional tracking system is not enough to
assure stable and reliable link performance. It also presents one of the simplest realizations of the new FSO terminals and
their test results.
KEYWORDS: Free space optics, Single mode fibers, Mirrors, Signal attenuation, Sensors, Servomechanisms, Telescopes, Scintillation, Atmospheric turbulence, Space telescopes
This paper shows the design and the performance of a new free-space optical communication terminal including the
results of the indoor and outdoor demonstration experiments in a short link distance. To provide flexible and high-speed
connectivity to the terrestrial free-space optical communications, a new compact laser communication terminal has been
developed at NICT. The terminal has a feature to connect the free-space laser beam directly to single mode fiber by
using a special fiber coupler to focus the free-space laser beam and couple it into the single mode fiber, fast and accurate
fine tracking system and a small refractive-type telescope with diffraction limited performance. The bandwidth of the
fine tracking system is more than 5 kHz using an off-the-shelf miniature Galvano mirror actuator and an analog PID
controller.
It is envisioned that the next generation of ultrahigh-speed laser communication systems will utilize compact optical antennas equipped with advanced beam tracking and effective fiber coupling mechanisms. Such laser communication systems will be used not only for space communications but also to provide optical links for long-distance terrestrial communications. We present the design of a high-speed laser communication system developed utilizing compact optical antennas with off-axis free-form surface (FFS) mirrors. We describe FFS optical devices and their design contribution in realizing compact optical antennas. Furthermore, an innovative fiber coupling device made from a glass ferrule and fiber is introduced, and with this device it is possible to couple the laser beam seamlessly to a single-mode fiber. We also present a fine tracking mechanism that uses a miniature fine pointing mirror (FPM) incorporated in the antenna. The machanism functions by feeding back the incident angle of the signal detected by a quadrant detector (QD) to the FPM. The achievable actuator response frequency for tracking is approximately 2 kHz, and it has been demonstrated to effectively mitigate the effects of laser beam angle-of-arrival fluctuation as a result of atmospheric turbulence.
Free-space optical communication has emerged as a competitive and viable technology for offering high data
rates, improved capacity, cost-effective and an easy to deploy solution for providing connectivity between two
points which are up to a few kilometers apart. In this paper we present experimental work which demonstrates
the practicality of next generation free-space optical (FSO) communication systems suitable for short-haul,
high-speed and robust data links. This experimental system is placed between two buildings in the Waseda
University campus area for a communication link spanning a distance of 1 km.
We outline the design of the optical antenna which uses 1550 nm wavelength and directly coupling a freespace
optical beam to a single-mode fiber without the need for OE/EO conversion, to offer a communication
link with data rates from 2.5 Gbps to 10 Gbps. The antenna is capable of overcoming most common limitations
inherent in FSO communication systems, such as atmospheric induced beam wander and scintillation effects. A
high-speed tracking mechanism which utilizes a fine positioning mirror (FPM) capable of tracking and controlling
the received beam and focusing/steering most of beam power into the fiber is presented. This FPM is capable
of suppressing the frequent power fluctuations caused by beam angle-of-arrival (AOA) variations.
This paper presents experimental results of the FSO communication system capable offering stable performance
in terms of measured bit-error-rate (BER). Performance results showing increasing the systems data rate
from 2.5 Gbps to 10 Gpbs are also presented.
When a free-space optical beam propagates through the atmosphere it experiences deterioration and deformation of its wave-front caused from small scale, randomly localized changes in the atmospheric index of refraction. This results in beam wander and scintillation effects which can reduce the link availability and may introduce burst errors. This paper outlines experimental work on a free-space optical (FSO) communication system which connects an optical beam directly to a single-mode fiber (SMF) without any optical-to-electrical (O-E) conversion. In order to effectively couple the 1550 nm transmitted optical beam to a SMF it is necessary to be able to track and control the beam angle-of-arrival (AOA) changes. To achieve this, we have developed an optical antenna which uses a fine positioning mirror (FPM) capable of performing high-speed beam tracking and steering thus reducing to a great extent the optical power fluctuations of the received beam coupled to the SMF. This optical power fluctuation is partly a result of beam angle-of-arrival fluctuations caused by atmospheric turbulence. In our experiments we have tried to measure and quantify the magnitude of atmospheric turbulence experienced by an optical beam propagating through the atmosphere. First we demonstrate the relation between the AOA fluctuations and the frequency characteristics of the scintillation effects for a free-space optical beam propagating through a turbulent atmosphere. We use this information to determine the optimum antenna FPM tracking speed for improved performance and error free transmission. The results in the improved fiber received power as well as continuous error free transmission are presented.
Next generation laser communication systems will require small optical antennas and fine tracking system. Such optical communication systems might be applied not only to space communications such as optical feeder links, intersatellite links and stratospheric platforms but also optical links for long distance communication on the ground. We have developed a high-speed laser communication system including a small optical antenna which utilizes off-axis free form surface optical system, and a fine tracking system which feeds back an incident angle signal detected by
quadrant detector to a small fine positioning mirror. In particular, the available mirror actuator response frequency for tracking is approximately 2kHz. Accordingly, even though the received 1.5μm laser beam experiences angle-of-arrival fluctuation as a result of atmospheric turbulence the antenna with this fine tracking function has a capability coupling the laser beam directly to the single mode fiber.
The paper presents the results of experimental study of an optical-beam tracking concept involving two systems based on different principles. One is all-optical tracking, which utilizes a nonlinear optical material providing automatic fine-tracking feature. Another is traditional opto-mechanical technology using a quadrant avalanche detector, a voice coil-mirror actuator, control electronics, and computer interface. The possibility of establishing automatic mutual tracking between two communicating parties without involving computer-aided beam addressing has been experimentally proven. Capabilities and limitations of both systems are described. The all-optical system performs better than the traditional one when it tracking laser beam angular disturbances of magnitude of a few mrad and the jitter frequency is high (≥100 Hz). The traditional opto-mechanical system shows higher efficiency at lower jitter frequencies. A combination of an all-optical fine-tracking module and an opto-mechanical coarse tracking module is suggested for applications where using our originally proposed all-optical approach for both coarse and fine beam steering / tracking would be less efficient.
We show that a photonic crystal can be designed as a passive field-of-view expander of optical receivers. In other words, light received at different angle within a given angular range is adjusted to propagate within a smaller angular range when exiting the device and illuminating the actual detector. We plan to employ that function in receivers for optical free-space communications and propose an approach for suppressing beam wander and scintillation of the focal spot on the detector due to the wavefront distortions. Since the alignment of the propagation direction is performed in a passive way, the beam adjustment to stabilize the coupling efficiency of the detector is delay-free. We describe here the idea of this approach in order to show how to utilize effectively the anomalous dispersion characteristics of photonic crystals. In our approach, the photonic crystal behaves like a homogeneous medium with a refractive index less than 1. We discuss the design of photonic crystal structure with such an optical characteristic, and first predict the propagation angle of the beam after passing through the crystal using the dispersion characteristic. Then this prediction is confirmed by electromagnetic analysis using the FDTD (finite-difference time-domain) method. Finally we present a simplified optical setup for the receiver.
In the first stage of laser communications experiment between the International Space Station (ISS) and several ground stations using onboard laser communications demonstration equipment (LCDE), an experiment to track and point geodesic satellites with corner cube reflectors is planned in order to evaluate the pointing/tracking performance and optical/mechanical misalignments of LCDE. As a visibility analysis of existing geodesic satellites from LCDE, we counted the number of the satellite passes which continue more than five minutes in the year 2004 and found that there are 20 passes for EGS and five passes for TOPEX. In order to detect the sunlight reflected from the satellite body and to start the tracking, we need a sensitive tracking sensor whose sensitivity is better than -95 dBm. This sensitivity will be achieved by using a quadrant photo- detector with a GaAsP photo-cathode and optimizing the sensor field-of-view. To evaluate LCDE's pointing performance, a pulsed laser at 1552 nm wavelength will be transmitted to the geodesic satellite, and the light reflected from the corner cube reflector on the satellite will be received by LCDE. The current power budget shows the signal-to-noise ratio of the detection of the returned laser pulse will be about 5 dB.
The introduction of the 'hybrid electrode' has proven to successfully simplify both the electrode design and the requirements on the electrical driver of liquid crystal optical devices. In this paper, we present the design and summarize some experimental results with respect to liquid crystal lenses and lens arrays employing the hybrid electrode structure. We further introduce the design of the circular hybrid electrode structure that brought forth, to our knowledge, the first single-layer liquid crystal spherical lens array with quadratic lens apertures.
We have been studying a high-bit-rate satellite laser communications system between a ground station and a geostationary satellite, named high-speed optical feeder- link system. An adaptive optics (AO) is the best solution to compensate the degradation of the laser beam quality due to atmospheric turbulence and to realize such a high-bit-rate laser communication system.
The first ground-to-satellite laser communication experiments were performed during December 1994 and July 1996 to demonstrate basic technologies for space laser communication systems. It used an optical communication package on-board the engineering test satellite VI and its companion ground optical terminals. A b-directional optical link over 40000 km was demonstrated along with precise transmission control of extremely narrow laser beams at both on-board and ground terminals. It is important to know characteristics of laser beam propagation through turbulent atmosphere and to accumulate experimental data. In the paper an overview of measurement result and discussion on laser beam propagation between the ground and the satellite are presented.
This paper reports the application of an adaptive optics (AO) system to our ground-to-satellite laser communication system. We believe that the AO system is one of the best solutions to compensate the degradation of the laser beam due to an atmospheric turbulence and to realize a high bit-rate laser communication system. In the first stage of developing and designing the AO system, we have measured an atmospheric turbulence at Communications Research Laboratory (CRL), located in the west suburb of Tokyo. It shows that Fried's coherence length is about 5 cm in winter and 9 cm in summer at 0.8 µm observing wavelength. We also simulated the AO performance of the laser communication system using our results of the atmospheric measurement.
The Talbot effect is known as a very effective means to illuminate an array of micro-optical components. In this paper, we analyze the Talbot effect along optical multimode slab waveguides with respect to array illumination and present some experimental results. The analysis is based on (1) the seemingly infinite reproduction of the waveguide aperture in the aperture plane caused by kaleidoscopic total reflections in the waveguide walls, and (2) numerical evaluations of the Fresnel-Kirchhoff diffraction formula. The aperture function is generated by phase elements that are based on Ronchy-type phase gratings with a phase step of 180 degree. The analysis is carried out both for mirror waveguides and dielectric waveguides. The experimental verification of the calculated results was obtained by placing a binary phase grating close to the aperture of a rectangular quartz-glass dielectric waveguide.
The ground-to-orbit Lasercom Demonstration conducted between the ETS-VI spacecraft and the ground station at JPL's Table Mountain Facility, Wrightwood CA was the first ground-to- space two-way optical communications experiment. The demonstration was conducted over a period of seven months and required simultaneous and cooperative operations by team members in Tokyo and California. A key objective was to measure the atmospheric attenuation and seeing during the demonstration to validate the performance of the optical link. The telemetry downlinked from the laser communications equipment provided information on the in-orbit performance of the onboard laser transmitter. Downlinked PN data enabled measurement of bit error rates. BERs as low as 10-4 were measured on the uplink and 10-5 on the downlink. Measured signal powers agreed with theoretical predictions.
A far-field pattern of an onboard laser transmitter was measured using a trans-atmospheric optical link over 35,000 km range between a satellite and an optical ground station. The acquisition of the far-field pattern is made in use of a new proposed statistical analysis of downlink irradiance data obtained at the ground station. Tracking/pointing statistical characteristics of the transmitter is taken into account in order to estimate downlink irradiance when there would be no atmospheric scintillation effects. The peak directive gain of the downlink laser beam was 104.3 dB. The beam width was 28.5 X 17.5 (mu) rad. These results were consistent with the results at a laboratory test performed before launch of the satellite.
The first ground-to-satellite laser communication experiments were performed during December 1994 and July 1996 to demonstrate basic technologies for space laser communication systems. It used an optical communication package onboard the Engineering Test Satellite VI and its companion ground optical terminals. A bi-directional optical communication link over 40000 km was demonstrated with precise transmission control of extremely narrow laser beams at both onboard and ground terminals. In the paper, the experimental operations performed in the demonstration are introduced in context with acquisition method for various technical data useful for evaluating the terminal characteristics.
Laser beam scintillation of a ground station to a satellite path are presented. After the up-link laser beam propagated to the satellite, the laser beam intensity distribution was varied by atmospheric turbulence. The variance of the up- link logarithmic-amplitude intensity fluctuation was about 0.5 and the maximum duration for the satellite receiver to be able to receive an optical power more than the communication detector sensitivity was about 50 mili-seconds in weak atmospheric turbulence condition. The maximum down- link time duration was about 0.3 seconds. The main cause of the down-link fluctuation can be considered as a fine pointing mirror angle variation in case of an up-link received power fading.
We propose a satellite laser communication system between a ground station and a geostationary satellite, named high- speed optical feeder link system. It is based on the application of (a) high-speed optical devices, which have been developed for ground-based high-speed fiber-optic communications, and (b) the adaptive optics which compensates wavefront distortions due to atmospheric turbulences using a real time feedback control. A link budget study shows that a system with 10-Gbps bit-rate are available assuming the state-of-the-art device performance of the Er-doped fiber amplifier. We further discuss preliminary measurement results of the atmospheric turbulence at the telescope site in Tokyo, and present current study on the design of the key components for the feeder-link laser transceiver.
Liquid crystal waveplates are known as efficient phase or wavefront modulators for applications requiring the active control of laser beams such as beam steering, focusing or diffraction. In developing such devices researchers have used so far two types of electrode structures: (1) the discrete structure where the liquid crystal is modulated by means of a large number of independent narrow stripe-shaped low-resistive electrodes, and (2) the continuous structure where broad electrodes with areas of different resistivities modulate the liquid crystal via a linear or nonlinear voltage gradient generated in the electrode plane. The former approach has the advantage of high transmission efficiency and unproblematic fabrication of the electrodes. The huge number of electrodes requires however a high expenditure in fabricating the complex LC driver. The latter approach keeps down the complexity of the LC driver, however, a high optical throughput has not yet been reported. In this paper, we present an electrode design that aims at combining the advantages found in both approaches. The usefulness of the novel electrode design was verified for the first time in an experiment demonstrating the nearly diffraction limited performance of an adaptive LC microlens array.
Communications Research Laboratory (CRL) developed laser communication equipment (LCE) onboard the engineering test satellite VI(ETS-VI) and a ground system for establishment of basic technologies in optical intersatellite communications. The experiments using a ground-to-space laser link started on December 1994. In the paper, preliminary evaluation for the performance of LCE is presented based on a part of experimental data. Included in the paper are a brief description of operation and data acquisition system, acquisition, tracking and pointing subsystem performance, and communication subsystem performance.
Experimental results of laser beam transmission from a ground station to the engineering test satellite VI (ETS-VI) are presented. The ETS-VI that was launched in August last year has onboard laser communication equipment (LCE). A very narrow laser beam with less than 60 (mu) rad divergence was continuously transmitted to the LCE from a ground station, using a highly accurate satellite optical tracking technique. The optical power fluctuation detected at the LCE was very large. Its log-amplitude variance was about 0.5 and the maximum duration for the LCE to be able to receive an optical power more than the LCE communication detector sensitivity was about 50 milli-seconds. Theoretical analysis of laser beam fluctuation at a satellite is described and compared with the experimental results.
The Ground/Orbiter Lasercomm Demonstration (GOLD) is an optical communications demonstration between the Japanese engineering test satellite (ETS-VI) and an optical ground transmitting and receiving station at the Table Mountain Facility in Wrightwood, California. Laser transmissions to the satellite are performed approximately four hours every third night when the satellite is at apogee above Table Mountain. The experiment required the coordination of resources at CRL, JPL, NASDA's Tsukuba tracking station and NASA's Deep Space Network at Goldstone, Calif. to generate and transmit real-time commands and receive telemetry from the ETS-VI. Transmissions to the ETS-VI began in November 1995 and are scheduled to last into the middle of January 1996 when the satellite is expected to be eclipsed by the Earth's shadow for a major part of its orbit. The eclipse is expected to last for about two months, and during this period there will be limited electrical power available on board the satellite. NASDA plans to restrict experiments with the ETS-VI satellite during this period, and no laser transmissions are planned. Post-eclipse experiments are currently being negotiated. GOLD is a joint NASA-CRL (Communications Research Laboratory) experiment that is being conducted by JPL in coordination with CRL and NASDA.
The optical communication experiment a the Communications Research Laboratory (CRL) in Japan has been performed using the engineering test satellite VI (ETS-VI) which was launched into a high elliptical orbit. The laser communication equipment (LCE) installed on the ETS-VI satellite can measure the radiation power from various areas on the earth like the sea, land and clouds. The LCE charge coupled device (CCD) sensor and quadrant detector (QD) sensor are used to obtain the spectral radiant emittance at a wavelength lambda equals 0.5 micrometers from the earth surface. This paper shows the results of the experiment to measure the background radiation.
This paper shows the most recent result of the laser communication experiment using ETS-VI satellite. The satellite failed into geostationary orbit and it is currently in a high elliptic three- day recurrent orbit, but from December 1994, we have performed laser transmission experiment for both uplink and downlink.
The design concept of simplified gimbals for the purpose of geostationary satellite tracking from low earth orbits is discussed and the preliminary results of a trial fabrication of a tracking system with a 19-cm-diameter telescope and an acquisition/tracking/pointing control system are shown. A new method is introduced for producing light-weight and small equipment by a self-alignment mechanism and its implementation into the optics design is described.
We measured far-field patterns of on-board laser communication equipment (LCE) using a free-space laser transmission simulator. The LCE was developed by Communications Research Laboratories (CRL) for basic optical communications experiments using the Japan's Engineering Test Satellite-VI (ETS-VI), while the free-space simulator is being developed by ATR as an on-ground test system for laser communication terminals. Far-field patterns of an on-board laser communication terminal have measured experimentally by an on-ground test system for the first time. The LCE emitted a laser beam whose peak directive gain was 104.6 dB and whose beamwidth was 31 X 19 (mu) rad in full-width at half-maximum. It was confirmed that the transmitted beam of the LCE met the experimental requirements. Through the measurement, the free-space laser transmission simulator demonstrated its effectiveness in an on-ground measurement of the beam characteristics of a laser communication terminal.
An offset feedback-type point-ahead method is described and its application to the Engineering Test Satellite VI (ETS-VI) laser communication equipment (LCE) is examined. The role in the method of a high-resolution mirror-deflection mechanism driven by a multilayered piezoelectric actuator is pointed out. The pointing resolution using the method is estimated to be less than 1 microrad at the output of the transmitting optics, which is sufficient of the LCE experiment.
The configuration of the acquisition and tracking system of the laser communication system aboard the Japanese Engineering Test Satellite VI (ETS-VI) is described, and the control loop design of the coarse and fine loops are examined. Simulations performed to evaluate the dynamic error for each loop, which showed that the dynamic error satisfies the error budget, are described. The tracking accuracy of each loop was measured with a breadboard model and the resulting data were found to meet the error budget.
An optical satellite communication system is being developed for performing basic optical communications experiments aboard the Japanese ETS-VI satellite to be launched in 1993. This paper describes the experimental plan and the present status of the communication system. The optical communication payload is described, including basic data and block diagrams.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.