Experimental analysis of atmospheric channel model with misalignment fading for GEO satellite-to-ground optical link using "LUCAS" onboard optical data relay satellite.

This paper describes experimental analysis of atmospheric channel model of Geostationary Earth Orbit (GEO) satellite-to-ground optical link by utilizing the Laser Utilizing Communication Systems (LUCAS) onboard the optical data relay GEO satellite. Our research work examines the effect of misalignment fading and various atmospheric turbulence conditions. These analytical results clarify that atmospheric channel model is well fitted to theoretical distributions with misalignment fading under various turbulence regimes. We also evaluate several atmospheric channel characteristics, including coherence time, power spectral density and probability of fade, in various turbulence conditions.

Experimental verification of fiber coupling characteristics for FSO downlinks from the International Space Station.

Free-space optical (FSO) systems are compulsory to realize high capacity and interference-free communication links from low-Earth orbit (LEO) satellite constellations as well as spacecraft and space stations to the Earth. To be integrated with high-capacity ground networks, the collected portion of the incident beam should be coupled into an optical fiber. To accurately evaluate the signal-to-noise ratio (SNR) and bit-error rate (BER) performance metrics, the probability density function (PDF) of fiber coupling efficiency (CE) must be determined. Previous studies have experimentally verified the CE PDF for a single-mode fiber, however, there is no such investigation for the CE PDF of a multi-mode fiber (MMF) in a LEO-to-ground FSO downlink. In this paper, for the first time, the CE PDF for a 200-µm MMF is experimentally investigated using data from an FSO downlink from the Small Optical Link for International Space Station (SOLISS) terminal to a 40-cm sub-aperture optical ground station (OGS) supported by a fine-tracking system. An average CE of 5.45 dB was also achieved given that the alignment between SOLISS and OGS was not optimal. In addition, using the angle-of-arrival (AoA) and received power data, the statistical characteristics such as channel coherence time, power spectral density, spectrogram, and PDFs of AoA, beam misalignments, and atmospheric turbulence-induced fluctuations are revealed and compared with the state-of-the-art theoretical background.

Free-space optical wiretap channel and experimental secret key agreement in 7.8 km terrestrial link.

Secret key agreement using physical properties of a wireless channel is becoming a promising scheme to establish a secret key between two users, especially in short-distance radio frequency (RF) communications. In this scheme, the existence of codes or key distillation that can make the leaked information to an eavesdropper arbitrarily small can be derived in an information theoretical way, given a priori knowledge on the channel linking a sender (Alice), a legitimate receiver (Bob), and an eavesdropper (Eve), which is called the wiretap channel. In practice, however, it is often difficult for Alice and Bob to get sufficient knowledge on Eve. In this study, we implement a free-space optical wiretap channel in a 7.8 km-terrestrial link and study how to estimate Eve's tapping ability, demonstrating high speed secret key agreement in the optical domain under a certain restricted condition of line-of-sight.

Transmission analysis for OFDM signals over hybrid RF-optical high-throughput satellite.

In this paper, a theoretical investigation of the performance of a communication scenario where a geostationary-orbit satellite provides radio-frequency broadband access to the users through orthogonal-frequency-division multiplexing technology and has an optical feeder link is presented. The interface between the radio frequency and the optical parts is achieved by using radio-on-fiber technology for optical-electro and electro-optical conversion onboard and no further signal processing is required. The proposed scheme has significant potential, but presents limitations related to the noise. The noise in both forward and reverse links is described, and the system performance for an example scenario with 1280 MHz bandwidth for QPSK, 16QAM, and 64QAM subcarrier modulation is estimated. The obtained results show that under certain conditions regarding link budget and components choice, the proposed solution is feasible.

Quantum photonic network and physical layer security.

Quantum communication and quantum cryptography are expected to enhance the transmission rate and the security (confidentiality of data transmission), respectively. We study a new scheme which can potentially bridge an intermediate region covered by these two schemes, which is referred to as quantum photonic network. The basic framework is information theoretically secure communications in a free space optical (FSO) wiretap channel, in which an eavesdropper has physically limited access to the main channel between the legitimate sender and receiver. We first review a theoretical framework to quantify the optimal balance of the transmission efficiency and the security level under power constraint and at finite code length. We then present experimental results on channel characterization based on 10 MHz on-off keying transmission in a 7.8 km terrestrial FSO wiretap channel.This article is part of the themed issue 'Quantum technology for the 21st century'.

LEO-to-ground polarization measurements aiming for space QKD using Small Optical TrAnsponder (SOTA).

Quantum communication, and more specifically Quantum Key Distribution (QKD), enables the transmission of information in a theoretically secure way, guaranteed by the laws of quantum physics. Although fiber-based QKD has been readily available since several years ago, a global quantum communication network will require the development of space links, which remains to be demonstrated. NICT launched a LEO satellite in 2014 carrying a lasercom terminal (SOTA), designed for in-orbit technological demonstrations. In this paper, we present the results of the campaign to measure the polarization characteristics of the SOTA laser sources after propagating from LEO to ground. The most-widely used property for encoding information in free-space QKD is the polarization, and especially the linear polarization. Therefore, studying its behavior in a realistic link is a fundamental step for proving the feasibility of space quantum communications. The results of the polarization preservation of two highly-polarized lasers are presented here, including the first-time measurement of a linearly-polarized source at λ = 976 nm and a circularly-polarized source at λ = 1549 nm from space using a realistic QKD-like receiver, installed in the Optical Ground Station at the NICT Headquarters, in Tokyo, Japan.

Free-space optical channel estimation for physical layer security.

We present experimental data on message transmission in a free-space optical (FSO) link at an eye-safe wavelength, using a testbed consisting of one sender and two receiver terminals, where the latter two are a legitimate receiver and an eavesdropper. The testbed allows us to emulate a typical scenario of physical-layer (PHY) security such as satellite-to-ground laser communications. We estimate information-theoretic metrics including secrecy rate, secrecy outage probability, and expected code lengths for given secrecy criteria based on observed channel statistics. We then discuss operation principles of secure message transmission under realistic fading conditions, and provide a guideline on a multi-layer security architecture by combining PHY security and upper-layer (algorithmic) security.

Modified E91 protocol demonstration with hybrid entanglement photon source.

We report on an experimental demonstration of the modified Ekert 91 protocol of quantum key distribution using a hybrid entanglement source with two different degrees of freedoms, a 1550 nm time-bin qubit and 810 nm polarization qubit. The violation of the Clauser-Horne-Shimony-Holt inequality could be demonstrated for the entanglement between the polarization qubit in free space and the time-bin qubit through 20 km fiber transmission. The secure key rate in our system is estimated 70-150 bps.

Experimental verification of fiber-coupling efficiency for satellite-to-ground atmospheric laser downlinks.

Optical communication is a high-capacity method that can handle considerable satellite data. When common-fiber optical devices such as optical fiber amplifiers based on single mode fibers are used in free-space laser communication systems, the laser beam has to be coupled to a single-mode fiber. Under atmospheric turbulence it would be difficult to make the required fiber coupling efficiency in satellite-to-ground laser propagation paths. A fast-steering mirror that can operate at high frequencies under atmospheric turbulence is fabricated, and its tracking performance is verified in real satellite-to-ground laser communication experiments. The measured fiber coupling loss of 10-19 dB in satellite-to-ground laser communication links under atmospheric turbulence shows good agreement with the predicted fiber coupling efficiency of 17 dB.

Atmospheric turbulence-induced fading channel model for space-to-ground laser communications links.

The fading channel model for generating a random time-varying signal based on the atmospheric turbulence spectrum for space-to-ground laser links is discussed. The temporal frequency characteristics of the downlink are theoretically derived based on the von Karman spectrum. The rms wind speed based on the Bufton wind model is used as the transverse wind velocity, which makes the simulation simple. The time-varying signal is generated as functions of the receiver aperture diameter and the rms wind speed. The simulated result of the time-varying signal is presented and compared with the gamma-gamma distribution based on the scintillation theory in a moderate-to-strong-turbulence regime.

Polarization measurements through space-to-ground atmospheric propagation paths by using a highly polarized laser source in space.

The polarization characteristics of an artificial laser source in space were measured through space-to-ground atmospheric transmission paths. An existing Japanese laser communication satellite and optical ground station were used to measure Stokes parameters and the degree of polarization of the laser beam transmitted from the satellite. As a result, the polarization was preserved within an rms error of 1.6 degrees, and the degree of polarization was 99.4+/-4.4% through the space-to-ground atmosphere. These results contribute to the link estimation for quantum key distribution via space and provide the potential for enhancements in quantum cryptography worldwide in the future.

Maximum fiber coupling efficiency and optimum beam size in the presence of random angular jitter for free-space laser systems and their applications.

Free-space laser communication systems use optical-fiber-based technology such as optical amplifiers, receivers, and high-speed modulators. In these systems using single-mode fibers, the fiber coupling efficiency is one of the most significant issues to be solved. Optimum relationships between a focused optical beam and mode field size of the optical fiber in the presence of random angular jitter are discussed in relation to fiber-coupled optical systems. Maximum fiber coupling efficiency is analytically derived with the optimum Airy disk radius normalized by the mode field radius as a function of random angular jitter. The fade level of fiber-coupled signals at desired fade probability is investigated. It is shown that the average bit error ratio significantly degrades with the random angular jitter normalized by the mode field radius larger than about 0.3 when the Airy disk size is optimally selected.

Two-dimensional optical beam deflector operated by wavelength tuning.

A new method based on an optical delay line structure is proposed for two-dimensional raster optical beam steering. For one-dimensional beam steering, the laser beam to be deflected is split into N co-directional sub-beams of equal intensity with the aid of a plane-parallel plate. These sub-beams experience a relative time delay, which translates into a phase difference, thus forming a phased array. When the laser wavelength is tuned, the relative phase varies and the far-field interference footprint can be steered across a receive plane. By employing two plane-parallel plates in series, the described scheme can be extended to produce a two-dimensional N x N array of sub-beams, allowing two-dimensional beam steering via wavelength tuning. In this case, wavelength tuning over a larger range leads to a linear deflection which repeats itself in a raster-like fashion. One direction of deflection repeats itself multiple times as wavelength is scanned over larger range, that is, a raster effect. In this paper, the principle is theoretically derived and formulated, and the preliminary experimental results with four sub-beams are presented.

Optimum divergence angle of a Gaussian beam wave in the presence of random jitter in free-space laser communication systems.

The average bit error rate (BER) of optical communication systems is considered in the presence of random angular jitter. First, the received power and the BER in the absence of jitter are reviewed. Then the average BER is obtained in the presence of circularly symmetric, normally distributed jitter by using the probability density function of the optical signal. By minimizing the power penalty for average BER, the optimum ratio of the divergence angle of the laser beam to the random angular jitter at the desired BER is obtained. An analytic approximation of the optimum ratio is derived as a function of the desired average BER. The results can be used for designing the link budget of optical communication and tracking channels in the presence of jitter.