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Abstract
Free-space point-to-point optical communication often suffers from atmospheric turbulence and device vibration where the environment is harsh. In this paper, by introducing an adaptive system composed of turbulence compensation and fast auto-alignment installation, we propose and experimentally demonstrate an optical communication system that is effective against turbulence and vibration. Turbulence compensation can increase the coupling efficiency by at least 3dB, while fast auto-alignment can reduce the spatial range of beam vibration caused by device vibration by 72.22%. Since the photodiode detector (PD) is sensitive to optical power, reducing the loss of the link improves the communication quality of the system. Bit-error rate (BER) of 10-Gbaud 16-ary quadrature amplitude modulation (16-QAM) signal transmission in the link is also measured under different transmitted power, having ∼8dB power penalty improvement with the adaptive system. In addition, turbulence compensation for higher-order modes such as optical vortex (OV) beams is also implemented, showing a promising prospect in space-division multiplexing (SDM) applications.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In recent years, there is a growing interest in free-space optical (FSO) communication links for scientific, commercial, and military applications [1–4]. Compared to traditional radio-frequency (RF) technology, FSO communication not only offers an orders-of-magnitude increase in transmission capacity over a large, unregulated and license-free frequency spectrum, but also reduces the size of the antenna. Moreover, compared to traditional RF technology, the extremely narrow divergence of the laser beam enables higher interference-properties and immunity against eavesdropping [5]. In addition, many technologies have been proposed to further improve the transmission capacity of an optical communication link, such as multi-level modulation formats [6], wavelength-division multiplexing (WDM) [7], and space-division multiplexing (SDM) [8,9], providing an excellent prospect for the development of FSO communication.
Unfortunately, such FSO communication link often suffers from atmospheric turbulence and device vibration [10]. Atmospheric turbulence distorts the wavefront, resulting in both amplitude and phase errors at the receiver, which causes deterioration of the coupling efficiency [11,12]. Different kinds of methods to combat atmospheric turbulence have been investigated including arrayed incoherent receivers [13], pulse-position modulation signaling with coherent arrayed receivers [14], and digital coherent arrays with electronic wavefront correction [15]. However, the current state of an FSO communication technology is dominated by the use of adaptive optics (AO) to correct wavefront distortions caused by atmospheric turbulence [16–20]. AO turbulence compensation systems are usually divided into two categories: one is a turbulence compensation system based on a wavefront sensor (WFS) [17,21], and the other is a WFS-less turbulence compensation system [22,23]. Due to the high cost of the WFS, the WFS-less system employing the beam intensity profiles captured by the camera has practical application value, rather than the beam wavefront measured by the WFS. The other disruptive factor of FSO systems is device vibration, leading to the coupling misalignment problem at the receiver. Therefore, the auto-alignment system used to achieve optical path correction, automatic coupling and coupling efficiency optimization at the receiver is proposed as another kind of AO system, solving the device vibration problem [24,25,26]. Such an auto-alignment system is indispensable in applications such as ground-to-aircraft optical communication, underwater optical communication, and optical communication in harsh environment [27–30]. It is general a closed loop system, which compensates for the angle and the displacement of the beam in real time when the position of devices shifts due to vibration, ensuring high quality of received signal at the receiver.
In this paper, we propose and experimentally demonstrate a free-space AO communication link against atmospheric turbulence and device vibration. As mentioned above, natural phenomena such as the vertical thermal gradient and the slight wind will cause the atmospheric turbulence and device vibration, which will greatly reduce the transmission performance of an FSO communication system. Thus, a WFS-less system supported by Zernike Polynomials based Stochastic Parallel Gradient Descent (SPGD) algorithm [31] and a fast auto-alignment system are introduced for the FSO communication link in our work. The transmission performance using 16-ary quadrature amplitude modulation (16-QAM) signals in the case of adaptive system assistance is studied, showing the power penalty improvement of 8dB with the adaptive system. Additionally, turbulence compensation for optical vortex (OV) beams is demonstrated, showing its application prospect in SDM applications.
2. Concept
The concept of free-space AO communication link against atmospheric turbulence and device vibration is illustrated in Fig. 1. In the scenario of free-space optical communications, turbulence and device vibration are the main culprits in the degradation of communication quality, especially for the situation shown in Fig. 1. In the atmospheric link from A to B, the atmospheric turbulence is the most serious problem which affects the link. Atmospheric turbulence distorts the wavefront, resulting in both amplitude and phase errors at the receiver, which causes deterioration of the coupling efficiency. Thus, a Zernike Polynomials based turbulence compensation SPGD algorithm is proposed and experimentally verified to compensate for the turbulence in our work. From B to C, base stations and drones suffer from motor vibrations. The vibration of a stable flying drone features high frequency and small displacement. This kind of vibration will lead to misalignment problem at the receiving end, causing a lower received power, which is the most serious issue from B to C. Since the PD is sensitive to received power, the vibration of drones will cause bit errors at the receiving end, even make the link from B to C unable to work. Introducing a fast auto-alignment system, through the real-time closed-loop control, most of the beam is always received by the receiver of the drone. In addition, the fast auto-alignment system may be also efficient to overcome huge and slow vibration caused by base station or wind. The huge and slow vibration can be sliced in the time domain, so it can be thought as consists of many small displacements in the same direction in many moments. With the assistance of the AO system composed of these two systems, high communication efficiency can be ensured in such a communication downlink in the lab.
Fig. 1. Concept of free-space AO communication link against atmospheric turbulence and device vibration.
3. Experimental setup
Figure 2 shows the experimental setup of a free-space AO communication link. An electrical 10-Gbaud signal is generated by an arbitrary waveform generator (AWG) and 16-QAM advanced modulation format is adopted. The electrical signal is applied to the intensity modulator (IM) and then indirectly modulated into an optical signal by the 1550 nm light diode (LD). The signal is amplified by an erbium-doped fiber amplifier (EDFA) and then connected to a collimator (Col.1) so that a fundamental Gaussian mode carrying advanced modulation signal is coupled out from fiber to free space. The output light passes through a turbulence plate to emulate the atmospheric turbulence in free space. The light is reflected by a mirror (M1) and then launched onto a phase-only spatial light modulator (SLM) to compensate for turbulence by loading an effective pattern. Considering that SLM is a polarization-sensitive device, a polarizer (Pol.) is used to adjust the polarization state to be aligned to the optimal working direction of SLM for efficient phase modulation. The beam is then split into two parts through the beam splitter (BS), one part is received by a charge-coupled device (CCD1, monitoring intensity profiles of light through turbulence in order to determine the pattern loaded onto the SLM), and the other part is coupled into a single-mode fiber (SMF) through Col.2. The SMF is connected to Col.3 to produce Gaussian light in free space. As shown by the red light in Fig. 2, the beam is reflected by M2 and then passes through a fast auto-alignment system. The fast auto-alignment system consists of two alignment stages. Each alignment stage consists of one quadrant detector, one position sensing detector (PSD) auto aligner, two piezo controllers, one BS, and one piezo mirror mount (PMM). The displacement of an incident beam relative to the calibrated center is sent from quadrant detector to PSD auto aligner to calculate the drive voltage given by two piezo controllers. The piezo mirror mount is driven by the drive voltage to center the beam on the detector. Two lenses (L1, f1=400 mm; L2, f2=100 mm) are placed into the system to narrow the beam and a motor is connected to M2 to simulate the vibration conditions. Another 1550 nm CCD2 with a flip mirror is used to capture the beam transmitted in the link. The light is reflected by M3 and then coupled into SMF through the objective lens (OL). A fiber photodiode detector (PD) transmits the signal to an oscilloscope (OSC) for bit-error rate (BER) performance measurement.
Fig. 2. Experimental setup of free-space AO communication link against atmospheric turbulence and device vibration. AWG: arbitrary waveform generator; LD: light diode; IM: intensity modulator; EDFA: erbium-doped fiber amplifier; SLM: spatial light modulator; Col.: collimator; Pol.: polarizer; BS: beam splitter; PMM: piezo mirror mount; SMF: single-mode fiber; PSD: position sensing detector; FM: flip mirror; OL: objective lens; OSC: oscilloscope.
In the experimental setup, the turbulence compensation part corresponds to A to B shown in Fig. 1, while the auto-alignment system corresponds to B to C shown in Fig. 1.
4. Experimental principles and results
We utilize six different turbulence plates to emulate the situations in which the beam is affected by atmospheric turbulence in free space. The six different turbulence plates are created by painting clear acrylic onto six pieces of glass [32]. In order to evaluate the strength of the turbulence, we use the phase-shifting interference method to reconstruct the phase distribution of one piece of the turbulence plates. Thus, turbulence strength of this Lab-made turbulence plate can be as expressed as a D of 7 mm (Col. 1, Thorlabs F810FC-1550 with a 7-mm beam waist diameter) and r0 of 2.73 mm. In the experiment, CCD1 firstly records the intensity profiles without turbulence as a reference and then records the intensity profiles taken with turbulence plates. As shown on the upper right corner of Fig. 2, the degraded intensity profile is optimized by the compensated pattern. Employing this SPGD compensation algorithm based on Zernike Polynomials, the effect of turbulence compensation is shown in Fig. 3. Figure 3(a) displays intensity profiles recorded by CCD1 under three different conditions: without turbulence (the first row), with turbulence and without compensation (the second row), with turbulence and with compensation (the third row). We record the intensity profiles under six different turbulence conditions and find that the compensation algorithm not only compensates for the shape of the beam, but also changes its position. The above factors facilitate the coupling efficiency of the beam after turbulence compensation improving compared to conditions without compensation, as illustrated in Fig. 3(b). Coupling efficiency can be calculated using the following formula:
where $\eta $ corresponds to coupling efficiency, ${P_m}$ corresponds to measured power (with or without compensation), and ${P_0}$ corresponds to measured power without turbulence.Fig. 3. (a) Intensity profiles of Gaussian beam recorded by CCD1 under three different conditions: without turbulence, with turbulence and without compensation, with turbulence and with compensation. (b) Measured coupling efficiency with and without turbulence compensation.
Fig. 4. (a) Block diagram of SPGD algorithm. (b) Intensity profiles of OV beam (l = +3) recorded by CCD1 under three different conditions: without turbulence, with turbulence and without compensation, with turbulence and with compensation.
The diagram of the SPGD algorithm is illustrated in Fig. 4 (a) [33–35]. Firstly, a random perturbation conforming to Bernoulli distribution is generated. Secondly, the random perturbation is positively and negatively added to the current compensation pattern respectively. Then the intensity profiles of the beam after transmitting through the two patterns with different perturbations are recorded by CCD1. The intensity correlations can be obtained by calculating the root mean square (RMS) of the recorder intensity profiles and an ideal beam without turbulence. Finally, the next compensation pattern can be computed. The intensity correlation is utilized as the termination condition of the iteration. In addition, the top 20 Zernike modes are used to substitute the 1920×1080 SLM to simplify the calculation. It is worth mentioning that the compensation pattern is always superimposed with a grating to distinguish the modulated light from the unmodulated light in spatial position.
Recently, SDM technology based on optical vortex (OV) beams has been widely studied [8,17,28]. This algorithm still works when transmitting OV beam in the link as depicted in Fig. 4(b). Figure 4(b) shows intensity profiles of OV beam (topological charge equals to +3) recorded by CCD1 under three different conditions: without turbulence (the first row), with turbulence and without compensation (the second row), with turbulence and with compensation (the third row). Compensation for turbulence makes the intensity profiles of OV beams be more regular, and certainly, closer to the center of the picture. Noting that SMF does not support the OV beam, coupling efficiency is not measured in this work.
In this proof-of-concept experiment, static turbulence plates are utilized. However, the realistic atmospheric turbulence varies dynamically. Thus, improving the speed of turbulence compensation is very important. In fact, the potential limitation of such a turbulence-compensation system is determined by the hardware utilized in the system, that is, the refresh frequency of SLM. A 60-Hz SLM (PLUTO-TELCO-013) is applied in this turbulence compensation system. Considering that dozens of iterations may be needed to compensate for turbulence, the potential compensation frequency of this proof-of-concept system may be several Hz. Yet, the potential frequency of the WFS-less turbulence-compensation system can be improved by other hardware with high refresh frequency, such as digital micromirror devices (DMD). The DMD can achieve a tens-of-thousands frequency. Using a DMD device, the potential frequency of the turbulence-compensation system may be several-hundred Hz or even kHz.
In addition, amplitude and phase distortion effects caused by turbulence are mainly considered in this proof-of-concept experiment. However, in the realistic FSO communication system, beam wander and beam divergence caused by atmospheric turbulence in a long-haul link should be taken into consideration. Hence, in the actual long-haul FSO communication link, especially for the ground-to-air uplink, the optimum divergence angle, the beam radius of the laser and the diameter of both transmitter and receiver may be significant parameters [36].
To emulate the vibration conditions, we connect the mirror (M2) to a motor with a vibration frequency of 196 Hz. Applying such a motorized-mirror, both angular misalignment and lateral displacement are introduced to the system. That is, when the light is incident on the oscillatory mirror, the light would be reflected on a random position and orientation. When the auto-alignment system is turned on, the beam splitter inserted in the optical path sends a part of the beam to the quadrant position sensor to monitor the displacement of the beam relative to the detector's center. The digital signal processor (DSP) inside the PSD auto aligner deals with the difference signals and outputs the corresponding position demand signals which will be used as the inputs to the piezo drivers. When piezo driver is operated together with the PSD auto aligner, high precision closed loop operation is possible using the complete range of feedback. The piezo delivers drive voltage which is proportional to the position demand signals to drive the piezo mirror mount. The piezo mirror mount is driven by the drive voltage to center the beam on the detector. The response frequency of the piezo controller, which determines the frequency limitation of this auto-alignment system, is 244 Hz.
We continuously record the beam drift at the CCD2 with an interval of 1/10 second for 500 pictures. We make scatter plots of the displacements of the beam which represent the vibration degree of the beam center. Red circles are used to mark the spatial extent of the beam distribution. The larger the red circle, the more the beam is affected by the vibration. As seen in Fig. 5(a), in the absence of an auto-alignment system, the beam exhibits a distribution range of 0.36 mm under the influence of motor vibration. Note that the beam in the link is only about 0.51 mm due to two lens, vibration range of 0.36 mm will greatly reduce the power at the receiving end, thus affecting the communication efficiency of the entire link. Due to the introduction of the fast auto-alignment system displayed in Fig. 5(b), the beam vibration range is controlled to be 0.1 mm, reducing the spatial range of beam vibration caused by device vibration by 72.22%. As to the auto-alignment of vortex beam, there may be very little difference between it and a Gaussian beam, due to the DSP in the PSD enables the judgment of the center of all the types of regular beams. Hence, this auto-alignment system might be directly employed in a vortex beam communication link [26].
Fig. 5. (a) Measured beam displacements without fast auto-alignment system. (b) Measured beam displacements with fast auto-alignment system. (c) Voltage received by the PD without fast auto-alignment system. (d) Voltage received by the PD with fast auto-alignment system. (e) Voltage received by the PD with auto-coupling.
In order to further explain the performance of introducing a fast auto-alignment system on the coupling power at the receiver, we measure the receiving voltage of the PD under motor vibration. Figure 5(c) illustrates the voltage received by the PD under 1000 measurement times without fast auto-alignment system, showing great trembling and low average. For comparison, we further record the voltage received by the PD under 1000 measurement times with fast auto-alignment system in Fig. 5(d), which shows an effective improvement compared to the previous results. In our setup, there is also an electronically controlled fiber auto-coupling platform as an assistance to the fast auto-alignment system. Figure 5(e) illustrates the PD receiving voltage change under the operation of the auto-coupling platform. The initial coupling state is deviated, then the coupling power is gradually increased under the action of the automatic coupling platform, and finally reaches the equilibrium state.
One of the most important performance metrics of an optical communication link is BER, which determines the quality of communication. In the experiment, a random bit sequence with a length of 217 is mapped into a 16-QAM symbol sequence. The real-valued 16QAM IM/DD signals are generated offline by applying a complex conjugate extension satisfied Hermitian symmetry for the inverse fast Fourier transformation (IFFT) input. The real-valued signal is loaded on intensity modulator and detected by a PD at the receiver, then transmitted to an OSC for BER calculation assisted by off-line DSP. At the receiver, the real-valued signal is firstly converted into complex-valued signal by using FFT. Then the signal is recovered by frequency domain equalization. Finally, the restored signal is demapped into bit sequence with a length of 217. BER can be calculated by comparing this bit sequence with initial bit sequence [37,38].
Since the PD is sensitive to optical power, we demonstrate the relationship between BER and transmitted power in the case of 10-Gbaud 16-QAM signal transmission in the link. Figure 6(a) displays the measured BER performance under five different conditions: link without turbulence and vibrations (Ref.), without auto-alignment and without compensation, with auto-alignment and with compensation, without auto-alignment and with compensation, with auto-alignment and without compensation. In the case of the adaptive system not introduced to the optical link, the BER curve is far from the one with the adaptive system, having ∼8 dB penalty to the reference curve (with the adaptive system) at the 7% hard-decision (HD) forward-error-correction (FEC) threshold. The power penalty without any of the fast auto-alignment system or turbulence compensation system in the link can also be calculated from Fig. 6(a). These five lines prove that after the introduction of the fast auto-alignment system and the turbulence compensation system, the loss caused by the coupling of the optical communication link is significantly reduced, thereby reducing the BER. Figures 6(b)-(f) display the constellation diagrams corresponding to the first point of five curves, showing that communication quality is favorable when the receiving power is sufficient.
Fig. 6. (a) Measured BER performance under five different situations: link without turbulence and vibrations, without (W/O) auto-alignment and W/O compensation, with(W/) auto-alignment and W/ compensation, W/O auto-alignment and W/ compensation, W/ auto-alignment and W/O compensation (b) -(f) Constellation diagrams of the first point (the highest point) of five lines.
5. Conclusion and discussion
In summary, we present a free-space AO communication system against atmospheric turbulence and device vibration, which is designed for communication from any point in free space to the base station, and then from the base station to the drone. Zernike Polynomials based turbulence compensation SPGD algorithm and a fast auto-alignment system are introduced for the FSO link in our setup. By means of introducing the entire adaptive system, the loss of the link caused by atmospheric turbulence and device vibration is greatly reduced, reducing the BER of the communication. Transmitting a 10-Gbaud 16-QAM signal, the BER performance for this free-space AO communication link is evaluated and within ∼8dB penalty improvement at the 7% HD FEC threshold.
Noting that all the experimental results are based on an intensity-modulation/direct-detection system, here we figure out how the atmospheric turbulence affects a coherent communication system as a supplement. In a coherent communication system, the coherent detector mixes the data beam with a receiver Gaussian local oscillator (LO) beam. However, atmospheric turbulence causes power coupling of the data beam from the Gaussian mode to higher-order modes. Since data power coupled to orthogonal higher-order modes does not efficiently mix with the Gaussian LO, the atmospheric turbulence degrades the mixing efficiency and induces the deterioration of the coherent FSO link [4].
Funding
National Key Research and Development Program of China (2018YFB2200204); National Natural Science Foundation of China (62125503, 62001182); Key Research and Development Program of Hubei Province of China (2020BAB001); Science, Technology and Innovation Commission of Shenzhen Municipality (JCYJ20200109114018750); Open Fund of IPOC (BUPT) (IPOC2018A002); Open Program from State Key Laboratory of Advanced Optical Communication Systems and Networks (2020GZKF009); Fundamental Research Funds for the Central Universities (2019kfyRCPY037); China Postdoctoral Science Foundation (2020M672334).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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