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We propose a novel fiber-based free-space optical (FSO) coherent receiver for inter-satellite communication. The receiver takes advantage of established fiber-optic components and utilizes the fine-pointing subsystem installed in FSO terminals to minimize the influence of satellite platform vibrations. The received beam is coupled to a single-mode fiber, and the coupling efficiency of the system is investigated both analytically and experimentally. A receiving sensitivity of −38 dBm is obtained at the forward error correction limit with a transmission rate of 22.4 Gbit/s. The proposed receiver is shown to be a promising component for inter-satellite optical communication.
© 2013 Optical Society of America
1. Introduction
Free-space optical (FSO) communications offer a potentially huge bandwidth and very high speeds, making them extremely attractive for meeting the ever-increasing demand for broadband services. These desirable characteristics have prompted extensive research, and several in-orbit FSO links have been performed. In November 2001, the European Space Agency (ESA) performed the first semiconductor-laser inter-satellite link experiment (SILEX) between its data-relay satellite ARTEMIS and the Earth observation satellite SPOT-4 [1]. Coherent detection was introduced into the FSO system when the German Space Agency (DLR) verified their homodyne binary phase-shift keying (BPSK) laser communication terminals in February 2008 [2]. The receiver built by the DLR uses a space-optical hybrid to achieve coherent detection [3], which requires the support of a complicated optical-mechanical structure [4].
Currently, the most common type of optical communication systems are based on optical fibers that can achieve very high capacities. Recent research has tended to focus on designing fiber-based coherent FSO systems, which take advantage of the established components of fiber-optic communication systems. Such schemes offer a number of advantages: erbium-doped fiber amplifiers (EDFAs) can be used to amplify the received signals to improve the sensitivity; high-speed digital signal processing (DSP) technology can be introduced to implement coherent detection with the high sensitivity associated with homodyne detection but without phase locking the local oscillator (LO) laser [5]; optical polarization multiple-input-multiple-output (MIMO) systems can be applied, and polarization estimation and tracking of the received signal by DSP algorithms mean that no optical dynamic polarization control is required at the receiver [6].
On-ground fiber-based coherent FSO links have recently been discussed. A polarization-multiplexed optical wireless transmission with coherent detection has been reported [7]. Another report presents a fiber-based noncoherent transmission system with closed-loop compensation mechanism [8]. However, these reports have overlooked the coupling efficiency, which is critical in the case of inter-satellite FSO links where the received power is as low as hundreds of nanowatts, and the system is more vulnerable to coupling loss.
In transparent free-space links, coupling efficiency is limited by pointing errors, which causes the focal spot at the fiber tip to be laterally displaced. These errors are usually introduced by vibrations of the optical platform of the satellite. In our proposed receivers, fine-pointing subsystems are utilized to compensate for this displacement. This closed-loop compensation mechanism is widely employed in conventional FSO communication terminals, so it will not lead to increased system complexity.
In this paper, we propose a novel fiber-based FSO coherent receiver for inter-satellite communication. The vibration compensation is realized by a fine-pointing subsystem installed in the FSO terminals. The effect of the residual pointing errors is evaluated, and the performance of the system is confirmed by on-ground experiments.
2. System structure
The structure of the fiber-based FSO coherent receiver is shown in Fig. 1. Received light is first coupled into a single-mode fiber via the fine-pointing subsystem. This subsystem includes a position sensor in the focal plane, a steering mirror, and closed-loop control units. Upon receiving the pointing error signal detected by a position detector, the control units implement a particular control algorithm, such as proportional-integral-derivative (PID) controller calculation algorithm. Then the control units output a voltage signal to the steering mirror to change the direction of incoming light so that the fluctuation is compensated. The control units are typically based on digital signal processor and field programmable array [1]. After the coupling, the signal is detected coherently by a fiber-based receiver, which consists of an EDFA, optical filter, optical hybrid, LO laser, balance detectors, and DSP device. Note that the fine-pointing subsystem is limited by the sensitivity of the position sensor and the bandwidth of the loop. The coupling efficiency with residual errors will be discussed next.
Fig. 1 Structure of the fiber-based FSO coherent receiver system; the system consists of a telescope, fine-pointing subsystem, and a fiber-based coherent receiver (B/S: beam splitter; DAC: digital-to-analog converter; SMF: single-mode fiber; EDFA: erbium-doped fiber amplifier; LO: local oscillator; PBS: polarization beam splitter; BD: balance detector; ADC: analog-to-digital converter; DSP: digital signal processing).
3. Coupling efficiency
3.1. Coupling model
A equivalent model of the optical system is shown in Fig. 2, where the telescope and collimator in Fig. 1 are described as an equivalent lens. This lens has same pupil diameter and transmittance as the telescope. The equivalent focal length is f = Mfcol, with M denoting the magnification of the telescope and fcol the focal length of the collimator.
The end face of a single-mode fiber is positioned in the focal plane of the equivalent lens. The efficiency is defined as the ratio of the power coupled into the fiber to the power in the front surface of the lens, which is given by [9]
where Ui(r) is the incident optical field, and Um(r) is the normalized fiber-mode profile. Both of these functions can be evaluated in either the front surface of the lens or the focal plane, and for convenience, we select the former.The system is design for inter-satellite situations for which the distance is well larger than Fraunhofer distance (typically tens of kilometers) and turbulence can be ignored, hence the incident optical field in the aperture plane of the receiver is a plane wave described by
where Ui is the amplitude of light. This amplitude is only that part corresponding to the amount on the collimator, since some amount from the telescope is divided into the fine-pointing system. θ = (θx, θy) denotes the pointing error, and A(r) is the pupil transmittance of the telescope defined as with R denoting the pupil radius of the telescope and ε the normalized obscuration radius.The fiber-mode profile is generally described by a Gaussian distribution, which gives the profile propagated to the front surface of the lens as [10]
where wm = 5 μm is the core radius of the single-mode fiber, k is the wave number, and f denotes the equivalent focal length as mentioned above.The coupling efficiency is thus derived from Eqs. (1), (2), and (4):
where and J0(x) is the Bessel function of the first kind:When the optical system is perfectly aligned, or when α = 0, the coupling efficiency η can be derived analytically. A maximum coupling efficiency of 81 % is obtained when β = 1.12 and ε = 0. This value is useful to match the numerical apertures of the optical system and single-mode waveguide [11].
3.2. Efficiency with residual pointing errors
We assume that residual pointing error angles in two different directions are independent and identically Gaussian distributed. The residual pointing error angle is the root sum square of these angles and so is Rayleigh distributed with a probability density of [12]
where σi denotes root mean square (RMS) pointing errors in one direction. The distribution of the normalized pointing error defined in Eq. (6) is then The average coupling efficiency is written as From equations Eqs. (5), (11), and (12), numerical estimates of the average coupling efficiency can be obtained. Figure 3 shows the coupling efficiency as a function of the normalized pointing error for the case in which β = 1.12 and ε = 0.Fig. 3 Coupling efficiency as a function of the RMS normalized residual error with β = 1.12 and ε = 0.
A comparison of the ESA and National Space Development Agency of Japan (NASDA) in-orbit fine-pointing systems is presented in Table 1 [13,14]. At current technology levels, the residual pointing error is not significant, and the average coupling efficiency can reach 70 %. Thus, fine-pointing subsystems in terminals can be utilized to compensate the vibrations, and the proposed receiver can be realized with an acceptable coupling loss for inter-satellite optical communication.
Table 1. Comparison of state-of-the-art fine-pointing systems.
4. Experiment
4.1. Setup
To confirm the performance of the proposed system, we conducted on-ground experiments using the test-bed system shown in Fig. 4. At the transmitter, an arbitrary waveform generator (Tektronix AWG7122B) is used to generate the quadrature phase-shift keying (QPSK) signals at 5.6 Gbaud/s, and an I-Q modulator converts these signals to optical signals. Polarization multiplexing is achieved by dividing the signal into two orthogonal polarizations, offsetting them with optic patch cords of different lengths for decorrelation, and recombining the two branches with a polarization beam combiner (PBC). The data rate per polarization is thus 5.6 Gbaud/s × 2 bits/symbol = 11.2 Gbit/s, which yields a total rate of 22.4 Gbit/s for the polarization-multiplexed QPSK signal.
Fig. 4 Structure of the test-bed system. The same abbreviations as those for figure 1 are used (PSD: position sensitive device; PBC: polarization beam combiner).
To simulate the optical platform vibrations, vibration noise is added to steering mirror 1. The noise has a similar power spectrum density to the ESA Olympus S/C vibration model given by [15]
In practical situations, the light fluctuation is amplified by the telescope before coupling. Since a telescope is not part of our test bed, we amplify the noise same times as telescope magnification. The effective diameter of the simulated telescope can be estimated as the aperture of collimator 2 (8 mm) multiplied by the magnification.The fine-pointing subsystem described in section 2 is employed, and the position sensitive device (PSD) is an InGaAs photodiode with a measurement noise of about 0.6 μm (RMS). The lens has a focal length of 100 mm.
The signal passes through collimator 2 (focal length of 30.6 mm) and a EDFA before entering the optical filter and being detected by the polarization and phase diversity coherent receiver, which is composed of an optical hybrid, a tunable external cavity LO laser with a 120 kHz linewidth, and balance detectors. The sampling and digitization (A/D) in the off-line electronic post-processing is performed by a 4-channel real-time oscilloscope (Tektronix DPO72004B).
4.2. Coupling efficiency
We evaluated the coupling efficiency of the coupled light using an optical power meter. When the fiber is perfectly aligned, a coupling efficiency of 59.4 % was obtained, and Fig. 5 shows the experimental coupling efficiency for various telescope magnifications along with the numerical results from Eq. (12) with β = πwmR/λf = 1.32 and ε = 0. Our proposed fine-pointing system is effective in reducing the effects of vibration to an acceptable level, and the coupling loss of this system is below 4 dB at typical telescope magnifications.
Fig. 5 Coupling efficiency with and without compensation for different telescope magnifications. A magnification of 0 indicates that no vibration is added and that the fiber is perfectly aligned.
4.3. Transmission
Figure 6 plots the experimental time-averaged bit error rate (BER) values of the 22.4 Gbit/s FSO polarization-multiplexed QPSK receiver with coherent detection for various received optical signal powers. This transmission experiment was performed for a magnification of 21, simulating a telescope diameter of about 17 cm (8 mm × 21). The BER performance without vibration (back-to-back) is also shown for comparison. At the forward error correction (FEC) limit (here taken to be BER = 10−3), the receiving sensitivity is approximately −38 dBm.
Fig. 6 BER performance of the receiver, simulating a telescope diameter of about 17cm with β = 1.32 and ε = 0.
5. Conclusion
We have presented a fiber-based FSO coherent receiver that takes advantage of developed fiber-based components. The analytical results showed that the fine-pointing subsystem is effective in reducing the coupling loss to an acceptable level, and this was also verified experimentally. The coupling efficiency of the coupler with the fine-pointing system was about 52 % in the case of a simulated 17-cm diameter telescope. The sensitivity of the receiver was −38 dBm at the FEC limit with a transmission rate of 22.4 Gbit/s. The proposed receiver is a promising component for inter-satellite optical communication.
Acknowledgments
This work was supported by Ministry of Industry and Information Technology and National Defence Technology Basic Research Project during 12th Five-year Plan Period (Grant No. J312012A001). The authors acknowledge the help from Juhao Li.
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