The wavefront reconstruction with interaction matrix composed of high-order Zernike polynomials as basis may be unintendedly less accurate when the wavefront manipulator of huge number of elements such as Microelectromechanical systems (MEMS) mirrors and Liquid Crystal on Silicon (LCOS) is used. One of the reasons of the lack of the accuracy in reconstruction comes from the mismatch between the rectangular elements of the LCOS in the orthogonal arrangement and the projected patterns obtained by the Zernike polynomials defined in the polar coordinates. To improve the accuracy of the wavefront reconstruction by the LCOS, the use of the random phase patterns is proposed with presumption to be appropriate for the orthogonally arranged high number of elements. The residual fitting errors of reconstructed wavefront are evaluated by numerical simulation to show the potential of the use of the random phase patterns instead of the use of the Zernike polynomials. It is found by the Monte-Carlo simulation of the Kolmogorov model that the more random phase patterns one uses, the more accurate one achieves to reconstruct the wavefront compared to the use of the Zernike patterns. Additionally, the comparison of the Strehl ratio of the AO system obtained with the Zernike patterns and that of the random patterns is performed.
Recently, the free space optical (FSO) communications have been widely studied as an alternative for large capacity communications and its possible implementation in satellite and terrestrial laser links. In satellite communications, clouds can strongly attenuate the laser signal that would lead to high bit-error rates or temporal unavailability of the link. To overcome the cloud coverage effects, often site diversity technique is implemented. When using multiple ground stations though, simplified optical system is required to allow the usage of more flexible approaches. In terrestrial laser communications, several methods for optical system simplification by using a multimode fiber (MMF) have been proposed.
KEYWORDS: Optical communications, Free space optical communications, Optical networks, Data communications, Satellites, Free space optics, Satellite communications, Telecommunications, Data centers, Control systems
A terrestrial free-space optical communications network facility, named IN-orbit and Networked Optical ground stations
experimental Verification Advanced testbed (INNOVA) is introduced. Many demonstrations have been conducted to
verify the usability of sophisticated optical communications equipment in orbit. However, the influence of terrestrial
weather conditions remains as an issue to be solved. One potential solution is site diversity, where several ground
stations are used. In such systems, implementing direct high-speed optical communications links for transmission of data
from satellites to terrestrial sites requires that links can be established even in the presence of clouds and rain. NICT is
developing a terrestrial free-space optical communications network called INNOVA for future airborne and satellitebased
optical communications projects. Several ground stations and environmental monitoring stations around Japan are
being used to explore the site diversity concept. This paper describes the terrestrial free-space optical communications
network facility, the monitoring stations around Japan for free-space laser communications, and potential research at
NICT.
KEYWORDS: Polarization, Digital signal processing, Receivers, Signal detection, Telecommunications, Free space optical communications, Oscillators, Free space optics, Modulation, Data communications
Single polarization high-speed optical transmission is important for bidirectional free-space optical communication
system in order to have enough isolation up-link and down-link by signal discrimination using orthogonal polarization
states. At recent advance in digital coherent technology, polarization re-combining in combination with polarization
diversity receiver is widely used to suppress performance degradation when polarization states of signal and local
oscillator are misaligned by system vibration or shocks. However, in order to implement the re-combining function in
digital signal processing, appropriate algorithm is required for realizing the system stability. In this paper, we
demonstrate a new algorithm implementation for single polarization receiver with maximal-ratio-combining (MRC)
technique. First we exhibited the problem for state-of-the-art polarization re-combining in instability due to singularity
condition at 45-degree-azimuth elliptic polarization. In order to overcome this problem, we proposed a newly MRC
algorithm added splitting ratio dependent phase correction coefficients and achieved stable re-combining at 45-degreeazimuth
elliptic polarization signal. And we successfully demonstrated the stable receiving for 50-Gb/s single
polarization QPSK signal with all polarization states by our digital coherent receiver platforms added the newly MRC
algorithm, compared with previous-proposed MRC algorithm.
Advantages of optical links like small, lightweight and power efficient terminals are practical for high data rate
services of disaster preparedness and environmental research. In this paper, we demonstrate experimental results of 40-Gbit/s optical free space transmission using single-polarization quadrature phase shift keying (SP-QPSK) modulation
format and digital coherent detection. The digital coherent detection enabled a high sensitivity and a tolerance to
transmission impairments, which have attractive features for free space transmission system. We developed a 50-Gbit/s
SP-QPSK transmitter and offline-receiver with the optical antenna system. SP-QPSK optical modulation signal with a
line rate of 50-Gbit/s including 20% FEC is employed for high receiver sensitivity. A cascade by EDFAs consisting of a
low noise pre-EDFA and an optical level controlled EDFA is developed to compensate for level fluctuation without
degrading receiver sensitivity. Maximal ratio combining algorithms and carrier phase estimation algorithms are used at
the offline-receiver for QPSK signal detection. We succeeded 4-meter indoor free space transmission having same
performance as that with fiber connection using the developed system. The optical received power was -42 dBm at bit
error rate of 10-3. While for outdoor 50-meter transmission, we confirmed the received bit error rate larger than FEC
limit.
Current development status of the small optical transponder (SOTA) to be installed into a small satellite is described,
where the breadboard model, the engineering model and the protoflight model are respectively introduced. The tracking
performance is estimated to show that the angular error is low enough in comparison to the divergence angle of the
transmitted beam.
Since the optical inter-satellite communication has attractive advantages such as high-speed transmission with high
confidence, almost no electronic magnetic interference, and low power consumption, it has been activity investigated.
However, directivity control of the laser beams requires a bulky and complicated system in satellite mobile
communications. A more flexible and high accurate system with small and simple mechanism has been desired. In this
study, we propose a new method of optical inter-satellite communication with a dynamically reconfigurable optical
directional device in which diffraction gratings are automatically rewritten and reorganized in response of incident
conditions by moving satellites. For realizing such a device, we have developed Sn2P2S6 crystals which have a high
sensitive photorefractivity and dynamic reconfigurable property. Furthermore, this crystal has hundreds times faster
response than conventional photorefractive materials such as BaTiO3. These features are extremely advantageous to
construct a high-speed and flexible communication system with a large tolerance to displacement of moving satellites.
To investigate the possibility of the dynamically reconfigurable optical inter-satellite communication system, we
experimentally evaluate the temporal and spatial characteristics of Sn2P2S6 crystals for the variation of the beam incident
angle. Moreover, the diffraction beam from the crystal has phase conjugate wavefronts of the beam entering from the
counter direction. We try to utilize this behavior to suppress the beam spread and to reduce the background light such as
sunlight with a spatial filtering technique that has sensitivity in wavefront differences of the signal and background light.
Angular microvibrations of platform jitter on the optical inter-orbit communications engineering test satellite are measured in space during ground-to-satellite laser communication links. The microaccelerations are measured by the onboard accelerometers at a sampling rate of 2048 Hz. The angular microvibrations are estimated from the measured microaccelerations using the tracking characteristics of the laser communications terminal and the conversion factor on the basis of microvibration data obtained from ground-based tests. The power spectral density (PSD) of the satellite microvibrations is analyzed by using the fast Fourier transform analysis and the data length is examined according to the frequency resolution of the PSD. The in-orbit measurements of the PSDs are compared with those obtained from the ground test. The angular microvibrational base motion is estimated and a PSD up to 1024 Hz is additionally provided as a database of the real measurement results with previously obtained in-orbit measurements. The measured results will contribute to the angular jitter estimation and the design of a tracking control loop for space laser communication systems in the future.
Experiments involving laser communications between an optical ground station and a low earth orbit (LEO) satellite
were successfully conducted from 2006 to 2009. The optical ground station is located in Koganei, Tokyo, and was
developed by the National Institute of Information and Communications Technology (NICT), Japan. Four laser beams
were transmitted from the optical ground station to the LEO satellite in order to reduce the intensity of the fluctuation of
the optical signal caused by atmospheric turbulence. The frequency characteristics of the downlink are evaluated on the
basis of theory and the measurements, where the sampling rate was 20 kHz. The speckle patterns were averaged and the
frequency response of the received optical signal was filtered by the telescope aperture. The basis of the temporal
aperture filtering function is derived and compared with the measured results.
A prototype carbon nanotube actuator operated with low voltage is studied to achieve a function to tilt a mirror angle.
The process to fabricate the actuator is introduced and the moving characteristics are measured. The response time of this
actuator should be improved to be much faster, but the result indicates a potential of the actuator to control the tilt mirror
angle.
KEYWORDS: Satellites, Laser communications, Satellite communications, Aerospace engineering, Communication and information technologies, Communication engineering, Data communications, Atmospheric optics, Sensors, Meteorological satellites
The restarted OICETS-ground laser communications experiments are introduced. The events are sequentially
summarized from the launch of OICETS to the end-of-life, where the reopened experiments started from October 2008.
In the period, the satellite-ground laser communications campaign with the four optical ground stations of DLR, ESA,
JPL and NICT are conducted from April 2009 to September 2009. The open pointing characteristics of OICETS
measured in those trials show that the performance remains almost the same as before in 2006. The average rate of the
link establishments through the whole period is about 0.6 due to the weather conditions. The viewable periods of
OICETS from the four ground stations are analyzed as an example. The result indicates that the satellite could be
accessible once an hour from at least one of the four ground stations, which implies a possibility of a LEO satellite-ground
quasi-continuous connection.
JPL in collaboration with JAXA and NICT demonstrated a 50Mb/s downlink and 2Mb/s uplink bi-directional link with the LEO OICETS satellite. The experiments were conducted in May and June over a variety of atmospheric conditions. Bit error rates of 10-1 to less than 10-6 were measured on the downlink. This paper describes the preparations, precursor experiments, and operations for the link. It also presents the analyzed downlink data results.
A free-space quantum key distribution system is being developed by the National Institute of Information and
Communications Technology (NICT) in Koganei, Japan. Quantum cryptography is a new technique for transmitting
information where the security is guaranteed by the laws of physics. In such systems, a single photon is used for the
quantum information. However, since the transmission distance in optical fibers is limited by the absorption of photons
by the fiber, the maximum demonstrated range has been limited to about 100 km. Free-space quantum cryptography
between an optical ground station and a satellite is a possible solution to extend the distance for a quantum network
beyond the limits of optical fibers. At NICT, a laser communication demonstration between the NICT optical ground
station and a low earth orbit satellite was successfully conducted in 2006. The use of free-space quantum key
distribution for such space communication links is considered an important future application. This paper presents
conceptual designs for the onboard transceivers for satellite quantum cryptography
The R&D activities and current status in NICT on space laser communications are reported, where it is shown the goal
and scenario originating from the satellite-ground laser communication demonstrations with ETS-VI since 1994 for two
decades. The experiences obtained in the demonstrations have been inherited to the experiments with OICETS in 2006.
The experiments using the satellite are on going in 2008. Among these demonstrations, a laser terminal with
combination of key technologies was experimentally produced. For the next space laser communication system, we
have started the next version development technologies, on which trial manufactures are currently in progress. They have
been (and will be) implemented and tested at the ground station.
Quantum cryptography is a new technique for transmitting quantum information. The information is securely transmitted due to the laws of physics. In such systems, the vehicle that transfers quantum information is a single photon. The problem with using photons is that the transmission distance is limited by the absorption of the photons by the optical fiber along which they pass. The maximum demonstrated range so far is approximately 100 km. Using free-space
quantum cryptography between a ground station and a satellite is a possible way of sending quantum information farther than is possible with optical fibers. This is because there is no birefringence effect in the atmosphere. However, there is a complication in that the directions of the polarization basis between the transmitter and the receiver must coincide with each other. This polarization changes because the mobile terminals for free-space transmission continuously change their attitudes. If the transmission protocol is based on polarization, it is necessary to compensate for the change in attitude between the mobile terminals. We are developing a scheme to track the polarization basis between the transceivers. The preliminary result is presented.
The experimental results of an inter-orbit laser communication performed under an atmospheric influence is presented.
The demonstration was planned so that the optical link was supposed to graze the earth's rim because of the satellite
revolution around the earth. The trial was successfully carried out on 5th April, 2006. The measured experimental data
are introduced to show the temporal behavior of the OICETS's optical terminal. The atmospheric influence on the
optical link is calculated with a theoretical model to obtain a probability density of normalized intensity as a predictive
value. The probability density is also estimated from the experimentally measured data. The comparison shows that the
theoretical prediction well describes the experimental results.
The first bi-directional laser communications demonstration between the optical ground station developed by the
National Institute of Information and Communications Technology (NICT) located in Koganei, Tokyo and the Optical
Inter-orbit Communication Engineering Test Satellite (OICETS) was successfully conducted in March, May, and
September, 2006. The Kirari Optical communication Demonstration Experiments with the NICT optical ground station
(KODEN) were jointly conducted by the Japan Aerospace Exploration Agency (JAXA) and NICT. Data from the uplink
and downlink optical communication links were analyzed. For the downlink, the scintillation index agreed well with the
theoretical results calculated based on the strong fluctuation theory. The aperture averaging effect was the dominant
factor in reducing the variation of the downlink signals. The probability density functions as a function of elevation
angles were measured and compared with the theoretical model, showing good agreement. For the uplink, the
scintillation index disagreed with the calculated results based on the strong fluctuation theory. The multiple beam effect
of the uplink transmission with large beams will have an additional reduction factor, which will help to establish
ground-to-satellite laser communication links in the future. Four laser beams transmitted from the optical ground station
to the OICETS satellite also helped to reduce the optical signal's intensity fluctuation due to atmospheric turbulence.
We present a novel splicing method for photonic crystal fibres (PCFs) with a double phase-conjugate mirror (DPCM). The DPCM is an optical device with photorefractive crystal (PRC) which generates phase-conjugate beams easily. In this report, we experimentally measure the splice losses of the DPCM for transverse PCF offset. We numerically estimate the splice losses in the case that butt coupled PCFs without DPCM. Comparing the experimental and numerical values of the splice loss of PCFs, we discuss the tolerance of the DPCM for the PCF displacement. Also, we discuss the causes of loss inside the DPCM module.
We propose a new LD-fiber-coupling-method for bidirectionally-pumped erbium-doped fiber amplifier (EDFA) using
two mutually-pumped phase-conjugate mirrors (MPPCMs). MPPCM is formed by two mutually incoherent beams in
photorefractive crystal (PRC) and can convert each incident beam into each phase-conjugate beam of them. Owing to
mode conversion by phase-conjugate characteristics, the coupling loss due to mode mismatching between a pump beam
and a fiber is improved. Therefore, precision lens and lensed fiber for mode matching are not required. Additionally,
because photorefractive index gratings that compose a MPPCM are rewritable in real time, MPPCM can dynamically
adapt to a variety of system error factors, such as misalignment of components, mode conversion generated by LDs or
fibers with defects. In our system, the overall coupling efficiency of pump beams is mainly determined by diffraction
efficiency of MPPCMs that depends on the photorefractive coupling strength and the intensity ratio of two incident
beams. However, achievable diffraction efficiency in this system is restricted by optical loss between two MPPCMs.
This is because the transmitted pump beams passing through the EDF are reused for forming the opposite MPPCM. To
achieve a high signal amplification gain, absorption loss of pump beam powers in the EDF is unavoidable. Therefore,
we analytically and experimentally evaluate the acceptable loss between two PRCs to form both MPPCMs.
The first bi-directional laser communication demonstration between an optical ground station and the Optical Inter-orbit
Communication Engineering Test Satellite (OICETS) was successfully conducted in March, May, and September, 2006,
with an uplink of 2 Mbps and a downlink of 50 Mbps. The optical ground station, located in Koganei, Tokyo, Japan, is
operated by the National Institute of Information and Communications Technology (NICT), Japan. Four laser beams
were transmitted from the optical ground station to the OICETS satellite in order to reduce the optical signal's intensity
fluctuation due to atmospheric turbulence. The optical scintillation as a function of the number of beams and the
frequency response were measured, and the uplink and downlink laser transmission results were obtained.
The tracking and pointing performance of the Laser Utilizing Communications Equipment (LUCE) equipped on the
Optical Inter-orbit Communications Engineering Test Satellite (OICETS) is presented. The operation characteristics of
LUCE observed in the ground-satellite communication demonstrations are focused on. Since the angular movement of LUCE's optical antenna required in the ground-satellite trials exceeds the specification demanded in the inter-satellite communications, the marginal performance for proper tracking and pointing can be observed. During the ground-satellite communication trials, the ground stations found periodical discontinuances in the optical link. By looking at the
LUCE's telemetries, the cause of the repeated breaks is revealed.
Optical LEO downlinks from the Japanese OICETS to the optical ground station built by the German Aerospace Center
(DLR) near Munich have been performed. This was the first optical LEO downlink on European grounds. The ground
station received a 50-Mbit/s OOK signal at 847 nm on its 40-cm Cassegrain telescope and sent two spatially displaced
beacon beams towards OICETS. Five out of eight trials could be performed successfully while the other three were
hindered by cloud blockage. A BER of 10-6 has been reached. The elevation angle above the horizon ranged between 2°
and 45°. The Fried parameter and the scintillation were measured with instruments inside the ground station. The beacon
power received by the LUCE Terminal onboard OICETS has also been recorded. This paper describes the setup of the
experiment and highlights the results of the measurement trials.
The experiment results on the inter-orbit laser communications between OICETS and a geostationary satellite and the
results of two kinds of orbit-to-ground laser communications between OICETS and ground stations are summarized.
The geostationary satellite for the inter-orbit demonstrations is the European Space Agency's geostationary satellite,
ARTEMIS, and the ground stations for the orbit-to-ground demonstrations are of the National Institute of Information,
and Communications Technology (NICT) in Japan and the German Aerospace Center (DLR), respectively. The
descriptions of those experiments contain some statistically analyzed results as well as data samples measured during
the demonstrations. The authors present the overview of these demonstration progresses and discuss on the results.
Ground-to-satellite laser communication experiments between the optical ground station located in Koganei of downtown Tokyo and a low earth orbit (LEO) satellite, the Optical Inter-orbit Communications Engineering Test Satellite (OICETS) called "Kirari", were successfully performed in March and May, 2006. The optical communication demonstration experiment at the optical ground station was conducted in cooperation between the Japan Aerospace Exploration Agency (JAXA) and the National Institute of Information and Communications Technology (NICT). Ten trials over the course of two months were conducted during the test campaign. Acquisition and tracking of the satellite were successful on seven days out of the assigned test days. The tests were unsuccessful during three out of the four days when it was cloudy or rainy, but they were successful on all six of the days when partly clear skies were predominant. For the uplink, the fluctuation of the received signal power was well minimized by using multi-beam laser transmissions. The bit error ratio (BER) on the downlink was measured to be as low as 10-5. The applicability of the onboard optical terminal was demonstrated, aiming not only for geostationary earth orbit (GEO)-LEO links but also for ground-to-LEO optical links.
In the bidirectional pumped Erbium Doped Fiber Amplifier (EDFA), we propose the new method with two Mutually Pumped Phase Conjugate Mirrors (MPPCMs) to couple the pump beam into the fiber. This method makes it possible to couple the pump beam into the propagation mode of the fiber without high precision adjustment by the characteristic of MPPCM. Additionally, high precision readjustment is not required when a LD, fiber or lens is exchanged due to failures. In our method, it is important to obtain high diffraction efficiency of MPPCM for high coupling efficiency between the pump beam and the fiber. Diffraction efficiency of MPPCM depends on the intensity ratio of two incident beam intensities inside the PRC. We analyze the coupling efficiency for the intensity ratio of two pump beams, and show the optimum intensity ratio of two pump beams. Then, we perform an experiment to couple the pump beam (Ar+ laser 514.5nm) to the fiber (multimode fiber of 62.5μm core diameter). In the experiment, coupling efficiency higher than 35% was confirmed. Moreover, in our method, the time to complete the coupling depends on the beam intensities in the early stage of the process. Therefore, we also propose the 4f imaging system to achieve a faster coupling at the early stage, and design the optical system to improve the time to complete the coupling in our method.
We propose a new optical intersatellite communications system with a phase conjugate mirror (PCM) in formation flying (FF). In conventional optical intersatellite communications, high-accurate target acquisition and tracking are required for both the transmitter and the receiver. In our system with a PCM, when a control beam from the receiver is captured by a PCM in the transmitter, the signal beam from the transmitter introduced back to the receiver as its phase-conjugate replica. Thus, it is not necessary for the transmitter to target the receiver. Another advantage of using a PCM is that we can utilize spatial filtering. Background noise by sunlight with the laser wavelength can also be efficiently suppressed by a spatial phase modulation/demodulation and filtering processes using phase compensation by the PCM, which leads to the improvement of the signal-to-noise ratio (SNR) and hence provides high data transmission rates in the system. In order to efficiently filter out the background noise, a large beam propagation angle is required in spatial filtering. We spatially modulate the background noise by the diffuser and reduce the beam diameter by the expansion/downscale optical system as a method to enlarge the beam propagation angle. In this paper, we show that our system can separate the noise from the signal by using the expansion/downscale optical system even under spatial phase modulation. In the analysis, the SNR is 32.6[dB] at scale=8.0×104, when a spatial phase modulation by the diffuser is θ=1.5×10-5[rad].
We present the results of the on-orbit free-space laser communications between the Optical Inter-orbit Communications
Engineering Test Satellite (OICETS) and the Advanced Relay and Technology Mission (ARTEMIS) geostationary
satellite. We first introduce the history of developing OICETS and add some descriptions on characteristics of OICETS,
where the performance of the optical communication terminal named the Laser Utilizing Communications Equipment
(LUCE) is also stated. We summarize the on-orbit investigation results of LUCE's function and finally make a report on
the inter-orbit laser communication experiment carried out by ARTEMIS and OICETS.
We propose a simple, compact and high-security holographic optical memory system using Ulexite in order to produce random patterns of reference beam. 100 hologram multiplexing was achieved by multiplexing exposure, rotating Ulexite by 0.2 degrees every time with LiNbO3 crystal as a recording medium. Moreover, with this system, animation readout images can play for approximately 8 seconds by continuous rotation of Ulexite. As a natural stone, the exactly same Ulexite is very difficult to be found or replicated. Basic experimental results show that Ulexite can be used as a security key for its image-reproducibility and BER calculations.
We present an unconventional approach to passively expand the field of view of optical receivers for free-space laser communications. The illumination area, given by the movement of a focused light spot due to different angles of reception, is reduced to a smaller area without expanding the propagation angle of the spot. The working principle of this approach is based on combining a photonic crystal's negative refractive behavior with properly bent crystal boundaries. The device's optical behavior is determined by means of rigorous electromagnetic computation, but the overall working principle of illumination area reduction can be well visualized by the ray trace method. In addition, we estimate the influence of light modulation for high-speed data transmission. Since the discrete structure of photonic crystals only approximates a bent boundary with an arbitrary inclination angle, a slight modification is introduced into the crystal's structure to enable a more flexible design. Although such a modification influences the negative refractive behavior, the function of field-of-view expansion is still verified and confirmed by means of electromagnetic computation.
We present a simple technique for hologram multiplexing to handle images imposed on different polarization states simultaneously, where light of randomly distorted wavefront and randomly disarrayed polarization is deliberately exploited as a reference light. Such randomly blurred light is generated in succession by a rotary movement of a bundle of optical multi-mode fibers. We show that the direct use of light passing through the fiber bundle contributes to improve the storage capacity of previous hologram recording systems in which a polarizer has been employed with a fiber bundle in rotary movement. Since the actual wavefront patterns provided by fiber bundles are not completely random but quasi-random, a theoretical investigation on the performance of hologram multiplexing is carried out with numerically created quasi-random patterns prior to an experiment. In the end, we perform the experiment of hologram multiplexing for the validity of our approach.
The purpose of this study is to apply a free-space optical interconnection to a reconfigurable board-to-board connection where the wiring patterns connecting boards are optically formed without electrical-optical conversion. We regard a photorefractive bi-directional connection module (PBCM) based on a mutually pumped phase conjugate mirror as a key device to construct such a connection network and employ PBCMs at input/output interfaces of each board. Although optical behaviors of PBCM are influenced by the exposure conditions, we especially focus on the diameter of beams illuminating photorefractive media placed inside PBCM so as to find some geometrical restrictions in a design of networking system. Through numerical analyses, we show a sample configuration of PBCM for the board-to-board interconnection and present a conceptual design of input/output interface.
An unconventional approach to passively expand the field-of-view of optical receivers for free-space laser communications is presented. Our device exploits the negative refractive behavior exhibited by photonic crystals. The particularity of this approach is seen by placing the device behind a lens, where the illumination area given by the ensemble of the focal spots of light received at different angle within a given angular range is reduced to a smaller area without any expansion in the propagation angle of the light for each individual spot. The fundamental idea of this approach is based on a combination of the crystal's negative refractive behavior and properly bended crystal boundaries interfacing with air. In a sample calculation, the reduction of the illumination area is estimated by the ray trace method and the preservation of propagation angles of the incoming beams is confirmed. Since the discrete structure of photonic crystals only approximates a bended boundary with an aribtrary inclination angle, a slight modification is introduced into the crystal's structure to allow a more flexible design. Although such a modification influences the negative refractive behavior, the function of field-of-view expansion is still verified and confirmed by means of electromagnetic computation.
We propose a narrowly-shaped setup containing two prisms symmetrically placed along a straight line. This method features (1) an easy alignment, (2) the capability of hologram multiplexing using array light sources.
In the proposed optical systemm, prisms' apex angles are limited for two waves interference because of the total reflection at prism's boundary. Therefore, we should take this obstacle into consideration to design the setup. As a simplified calculation model, we assume that the wavelength of the light source is 780 nm, the refractive index of the prism is 1.51, and the focal length of the collimating lens is 10 mm, the apex angle of the first prism is 120 degrees, and that of the second prism is 90 degrees. If the largest oblique incident angle of the first prism is 10 degrees, the distance from light sources to a recording media becomes 45mm. The validity of our optical setup is confirmed experimentally by writing some holograms.
We present an unconventional approach to counteract the spreading of the illumination area in the focal plane of a lens due to an aberrated optical beam by exploiting the negative refractive behavior of photonic crystals. We design a photonic crystal structure and analyze its dispersion property with respect to its effective refraction index. This result is confirmed by electromagnetic analysis using the FDTD method. As an application of this device, we take a look at free-space optical communications and propose a field-of-view expander for optical receivers. A proof-of-principle calculation of our idea is performed by the ray trace method. Results show that the spreading is reduced by more than 90% without expansion of the propagation beam angles.
In this paper we studied about optical and millimeter radio wave cross-link for formation flight communication satellite system in a geostationary orbit. In formation flight system, since the distance among the satellites becomes short compared with the conventional inter satellite comunication link, the size and the weight of the communication system can be reduced. If the high data rate cross-link among the satellites which is equal to the whole transponder bandwidth can be established, the functionally distributed communications satellite system can be constructed. Then, by exchanging a part of the satellite system without the physical contact, in other words, by exchange some old satellites and new ones, the fully reconfigurable and long lifetime (from the point of the function) satellite communication system which can follow the paradigm shift in the terrestrial communications technology can be realize. On the other hand, however, since the maximum of relative angle error among two satellites is enlarged, the tracking becomes difficult. In this study, it turns the electric power which is gotten from the shortening the distance to making beam width large. Here, we examine communication among two satellites (10Km distance) where the data rate is 1Gbps.
We have studied PRCM as a basic device that could achieve a free-space optical communication with the spatial parallelism using mainly photorefractive crystal and four-wave mixing technique. The asynchronous PRCM, which is newly introduced in this paper, has the diffraction grating generated by two control beams independent of the signal beam. Thus, the signal is transmitted at the velocity of light and can be branched with no delay from the multiple nodes connected on the transmission bus line. We also propose a new two-way communication device, to which double phase conjugator technology was applied, as opposed to the one-directional device of optical signals from the transmitting side to the reception side. We conduct a basic evaluation on the branching and transmission characteristics of signals at the spatial optical switch element.
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.
We have proposed an all-optical memory with authentication and unlawful access detection using photorefractive four-wave mixing. If the data is read out by the reading beam with the improper key, the output intensity is scarcely zero and any output isn't obtained. Therefore we can judge at a glance whether or not the used decoding key is the proper key. We analyze the diffraction efficiency with consideration of the phase mismatching to evaluate the fidelity of the hologram. In this analysis, we examine the property of the crystal that depends on three-dimensional coupling coefficient. We simulate the encoding and decoding process with some image data in this all-optical memory with authentication and unlawful access detection. We show that the encoded image is restored to its original image in case of retrieving with the proper key, on the other hand the retrieved image of which intensity is scarcely zero is obtained in case of retrieving with the improper key.
We describe an optical image restoration by using holographic filtering and photorefractive four-wave mixing. A beam generated by four-wave mixing, that is, the phase conjugate beam, reads a hologram recorded by a blurred image and blur function, where the four-wave mixing process is influenced by an erase beam which has the information of the blur function. We calculate the generation efficiency of the phase conjugate beam and show that the amplitude of the phase conjugate beam can be inversely proportional to the intensity of the erase beam. We also simulate the image restoration by preparing sample images and show that the blur effect can be removed from the distorted image by this method.
We propose an optical demodulator with photorefractive cross-polarized duplex two-wave mixing, in which the phase- modulated signal beam is converted into the intensity- modulated output signal beams. We calculate the intensity variations of the output signal beams when phase-modulated signal beam input to photorefractive cross-polarized duplex two-wave mixing.
We analyze the branching gain for extraction of data and the bus gain of PRCM with cross polarized four-wave mixing in a photorefractive crystal from the point of view of pump ratio optimization. The stationary and temporal responses of two- stage-connected PRCMs composed by BaTiO3 crystals are measured and a light controlled optical image bus system are proposed.
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.