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JTu2E.16.pdf CLEO 2020 © OSA 2020
Free-Space Adaptive Optical Communication Systems
Against Atmospheric Turbulence and Device Vibrations
Yize Liang
+
, Xinzhou Su
+
, Lulu Wang, and Jian Wang*
Wuhan National Laboratory for Optoelectronics and School of Optical and Electronic Information,
Huazhong University of Science and Technology, Wuhan 430074, Hubei, China.
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These authors contributed equally to this work.
*
Corresponding author: jwang@hust.edu.cn
Abstract: We experimentally demonstrate an adaptive free-space optical (FSO) communication
system against atmospheric turbulence and device vibrations. BER of 10-Gbaud 16-QAM signal
transmission is measured under different transmitted power, showing ~8dB penalty improvement
with the adaptive system.
1. Introduction
In recent years, there is a growing interest in free-space optical (FSO) communication links for scientific, commercial,
and military applications
[1]
. Unfortunately, such a FSO communication system often suffers from atmospheric
turbulence and device vibrations. Atmospheric turbulence distorts the wavefront, resulting in both amplitude and phase
errors at the receiver
[2]
. The current state of FSO communication technology is dominated by the use of adaptive
optics (AO) to correct wavefront distortions
[3]
. AO turbulence compensation systems are usually divided into two
categories: one is a compensation system based on a wavefront sensor (WFS), and the other is a WFS-less
compensation system.
Due to the high cost of the WFS, the WFS-less system employing the beam intensity profiles
captured by the camera instead of the wavefront measured by the WFS has practical application value. Device
vibrations cause 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 device vibrations problem
[4]
.
In this paper, we demonstrate a free-space optical link with AO system against atmospheric turbulence and device
vibrations. As mentioned above, natural phenomena such as the vertical thermal gradient and the slight wind will cause
the atmospheric turbulence and devices vibration, which will greatly reduce the performance of a free-space optical
communication system. Thus a WFS-less system supported by Zernike Polynomials based Stochastic Parallel Gradient
Descent (SPGD) algorithm and a fast auto-alignment system are introduced for the free space optical link in our work.
The transmission of 10-Gbaud 16-Quadrature Amplitude Modulation (16-QAM) signals in the case of adaptive system
assistance is studied, showing the power penalty improvement of 8dB with this adaptive system.
2. Concept and Experimental Setup
The concept of free-space AO system is illustrated in fig.1(a). In the atmospheric link from A to B, there is often
atmospheric turbulence, so a SPGD algorithm is proposed and experimentally verified in our work. From B to C, as
we all know, base stations and drones suffer from wind and motor vibrations.
Introducing a fast auto-alignment system,
most of the beam is always received by the receiver of the drone.
Fig.1(b) shows the experimental setup of the AO system. An electrical 10-Gbaud 16-QAM signal is generated by
an arbitrary waveform generator (AWG). The electrical signal is received by the intensity modulator (IM) and then
indirectly modulated into an optical signal by a 1550nm light diode (LD). The signal is amplified by an Er-doped fiber
amplifier (EDFA) and then connected to a collimator (Col.1). The output red light passes through a turbulence plate
to simulate the turbulence. The beam is reflected by a mirror (M1) and then launched onto a spatial light modulator
Fig.1 (a)Concept and (b) Experimental setup of free-space AO link against atmospheric turbulence and vibrations.
© 2020 The Author(s)
JTu2E.16.pdf CLEO 2020 © OSA 2020
(SLM) to compensate for turbulence by loading an effective pattern. The beam is then split into two parts through the
beam splitter, one is received by 1550nm camera1(monitor intensity profiles of beam through turbulence in order to
determine the pattern loaded onto the SLM) and the other is coupled into a collimator (Col.2) and received by a single
mode fiber (SMF). The SMF is connected to another collimator (Col.3) to produce Gaussian light in free space.
Then
the beam passes through a mirror(M2) and 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). Two lenses are placed to narrow the
beam and a motor(196Hz) is connected to M2 to simulate the vibration conditions.
Another 1550nm camera (1550nm
camera2) with a flip mirror is used to record the beam transmitted in the link. The beam is reflected by a mirror(M3)
and then coupled into SMF through the objective lens (OL). A fiber photodiode detector (PD) transmits the signal to
an oscilloscope (OSC) for BER performance measurement.
3. Experimental Results and Conclusion
Fig.2 (a) shows the intensity profiles recorded by 1550nm camera1 under 6 different turbulence. The compensation
algorithm not only compensates for the shape of the beam, but also changes its position. The above factors together
make the coupling efficiency after turbulence compensation improve compared to conditions without compensation,
as shown in fig.2 (b). We continuously record the beam drift at the 1550camera2 with an interval of 1/10 second for
500 pictures and make scatter plots of the displacements of the spot. As shown in fig.3 (a), without an auto-alignment
system, the beam exhibits a distribution range of 0.36 mm under the influence of motor vibration. Note that the beam
is only about 0.51mm, vibration range of 0.36mm will greatly reduce the power at the receiving end. Due to the
introduction of the fast auto-alignment system, as shown in fig.3(b), the beam vibration range is controlled to be 0.1
mm. Fig.3 (c) displays the measured BER performance under five different conditions, with constellation diagrams of
the first point of five curves illustrated beside BER curves. In the case of the adaptive system is not introduced to the
optical link, the BER curve is far from the curve with the adaptive system, having ~about 8 dB penalty to the reference
curve (with the adaptive system) at the 7% hard-decision (HD) forward-error-correction (FEC) threshold.
In summary, we demonstrate a free-space AO system. Turbulence compensation can increase the coupling
efficiency by at least 3dB, while fast auto-alignment reduces the spatial range of beam vibrations caused by device
vibrations by 72.22% in our system. BER of 10-Gbaud 16-QAM signal transmission in the link is measured under
different transmitted power, showing ~8dB penalty improvement with the adaptive system.
4. Acknowledgements
This work was supported by the National Natural Science Foundation of China (NSFC) (11774116), the Natural Science Foundation of Hubei
Province of China (2018CFA048), the Key R&D Program of Guangdong Province (2018B030325002), the Open Fund of IPOC (BUPT)
(IPOC2018A002), the open program from State Key Laboratory of Advanced Optical Communication Systems and Networks (2020GZKF009),
the Program for HUST Academic Frontier Youth Team (2016QYTD05), and the Fundamental Research Funds for the Central Universities
(2019kfyRCPY037).
5. References
[1] Khalighi M A. Communications Surveys&Tutorials,2014.
[2] Dikmelik Y ,et al. Applied Optics, 2005, 44(23):4946-52.
[3] Ren Y, et al. Optics Letters, 2014, 39(10):2845-8.
[4] Lu Z , et al. Optics Express, 2017, 25(15):17971-.
Fig. 2 (a) Intensity profiles recorded by 1550nm camera1 under three different conditions: without turbulence, with turbulence and without
compensation, with turbulence and with compensation (b)Measured coupling efficiency with or without compensation.
Fig. 3 (a) Measured beam displacements without fast auto-alignment system (b) Measured beam displacements with fast auto-alignment system
(c) Measured BER performance under five different situations；W/: with; W/O: without.
