涡旋光束在不同盐度的水下湍流中的传输特性的实验研究
卢腾飞 刘永欣 吴志军
Experimental investigation on propagation characteristics of vortex beams in underwater turbulence with different salinity
LU Teng-fei, LIU Yong-xin, WU Zhi-jun
引用本文:
卢腾飞，刘永欣，吴志军. 涡旋光束在不同盐度的水下湍流中的传输特性的实验研究[J]. 中国光学, 2022, 15(1): 111-118. doi: 10.37188/CO.EN.2021-0001
LU Teng-fei, LIU Yong-xin, WU Zhi-jun. Experimental investigation on propagation characteristics of vortex beams in underwater turbulence with different salinity[J]. Chine Optics, 2022, 15(1): 111-118.
doi: 10.37188/CO.EN.2021-0001
在线阅读 View online: doi/10.37188/CO.EN.2021-0001
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文章编号 2095-1531（2022）01-0111-08
Experimental investigation on propagation characteristics of vortex
beams in underwater turbulence with different salinity
梦到大火LU Teng-fei ，LIU Yong-xin *，WU Zhi-jun
（Fujian Provincial Key Laboratory of Light Propagation and Transformation , College of
Information Science and Engineering , Huaqiao University , Xiamen 361021）
* Corresponding author ，E-mail : yongxin@hqu.edu
Abstract : It is very important to study the propagation characteristics of light beams in ocean turbulence. In order to get clor to the actual situation, we build a device which can control both the salinity and the intens-ity of underwater turbulence to study the propagation characteristics of vortex beams and a Gaussian beam in underwater turbulence. The results show that compared with the underwater turbulence without a salt, the light spot will be more diffu and the light intensity will be weaker in the underwater turbulence with a salt. When the topological charge m is 2, the scintil
lation index of the vortex beam in the underwater turbu-lence with salinity of 4.35‰ is larger than that in the underwater turbulence with salinity of 2.42‰, no mat-ter it is strong turbulence or weak turbulence. When the vortex beam with m =2 propagates to the same dis-tance, the scintillation index increas with the increment of the salinity and the intensity of underwater tur-bulence. Under different salinity conditions, the radial scintillation index of the vortex beam with m =2 de-creas firstly and then increas with the increa of the radial distance. In addition, we t up another exper-imental device which can transmit a longer distance. The scintillation index of the vortex beam with m =2 is much higher than that of the Gaussian beam in the underwater turbulence within 20 m propagation distance,and the scintillation indices of both the vortex beam with m =2 and the Gaussian beam increa with the in-crea of the propagation distance.
Key words : vortex beam; underwater turbulence; scintillation index; salinity; propagation
涡旋光束在不同盐度的水下湍流中的
传输特性的实验研究
卢腾飞，刘永欣*，吴志军
（华侨大学 信息科学与工程学院 福建省光传输与变换重点实验室，福建 厦门 361021）
重新装系统摘要：研究光束在海洋湍流中的传输特性尤为重要。为了更贴近实际情况，人工搭建了能控制水下湍流强度和盐度的装
收稿日期：2021-01-19；修订日期：2021-03-01
基金项目：国家自然科学基金资助项目（No. 61505059，No. 61575070，No. 61275203）；华侨大学研究生科研创新基金
Supported by National National Science Foundation of China (No. 61505059, No. 61575070, No. 61275203);Postgraduates’ Innovative Fund in Scientific Rearch of Huaqiao University
第 15 卷 第 1 期中国光学Vol. 15 No. 1
2022年1月
Chine Optics
Jan. 2022
置来研究涡旋光束和高斯光束在水下湍流中的传输特性。结果表明：相比于未添加海盐的水下湍流，
光束在增添海盐的水下湍流中传输光斑会更加弥散，光强会更弱。无论是强湍流还是弱湍流，m=2的涡旋光束在盐度为4.35‰的水下湍流中的闪烁因子都大于其在盐度为2.42‰的水下湍流中所对应的闪烁因子。另外，m=2的涡旋光束传输到相同的距离时，其闪烁因子随着水下湍流的盐度和强度的增大而增大。不同盐度条件下，m=2的涡旋光束的径向闪烁因子随径向距离的增大呈先减小后增大的变化趋势。另外，搭建了传输距离更长的实验装置，在20米的传输距离内，拓扑电荷m=2的涡旋光束的闪烁因子远高于高斯光束所对应的闪烁因子，且m=2的涡旋光束和高斯光束的闪烁因子都随着传输距离的增大而增大。
关键词：涡旋光束；水下湍流；闪烁因子；盐度；传输
中图分类号：O436 文献标志码：A doi：10.37188/CO.EN.2021-0001
1 Introduction
Turbulence caus intensity fluctuations when the lar beam propagates in the random media, which is called scintillation[1-2]. The intensity fluctu-ation (scintillation) can reduces the signal-to-noi ratio and increas the bit error rate. Investigations of scintillation of lar beams in ocean turbulence become more and more important becau of their wide applications in underwater optical communica-tion and imaging[3-8]. In recent years, people have studied the scintillation index
of lar beams in ocean transmission[9-19]. In Ref. [16], the scintilla-tion of optical plane and spherical waves were in-vestigated, and the results show that just like in the atmosphere, in underwater media the plane wave is more affected by turbulence as compared to the spherical wave. In Ref. [17], the aperture-averaged scintillations of plane and spherical waves were cal-culated. It was found that the adoption of the aper-ture-averaging technique in an underwater optical communication system can significantly extend its reliable communication distance. In Ref. [18], the on-axis scintillation index of a Pha-locked Par-tially Coherent Flat-Topped (PCFT) lar array beam in oceanic turbulence was studied, and the res-ults show that in the n of scintillation index re-duction, using the PCFT array lar beams has a considerable benefit in comparison with the single PCFT or Gaussian lar beams and also Gaussian array beams. In Ref. [19], considering the pointing errors caud by the slight incline of underwater platform, the scintillation index of partially coher-ent beams propagating through weak oceanic turbu-lence were calculated. However, the above studies were all theoretical studies of scintillation index, and there were no experiments.
A beam with a spiral pha is called a vortex beam, and each photon of the vortex beam carries orbital angular momentum[20]. The rearch of the vortex beam is an important subject due to its poten-tial applications in area such as optical micro manip-ulation, optical information encoding and t
ransmis-sion and other fields[21-24]. Bad on the Huygens Fresnel principle, the spectral density, the spectral degree of coherence and the spectral degree of po-larization of stochastic electromagnetic vortex beams in ocean turbulence were studied[25]. The propagation properties of the vortex beam in ocean turbulence were studied by using the spatial light modulator to show turbulence[26]. However, it is very difficult to do experiment in a real marine environ-ment. Most of the relevant rearch results are ob-tained by computer numerical simulation. Targeting at above problem, we t up an experimental sys-tem containing underwater turbulence to investigate the scintillation index of the vortex beam in under-water turbulence, and rearch the influence of the different turbulence on the scintillation index[27]. In this paper, our main concern is the effects of salin-ity and propagation distance on the scintillation in-dex and the intensity distribution of the vortex beams. We also establish another experimental equipment to make the beam propagate as far as 20 meters, which is longer than 12.6 meters in refer-ence.
112中国光学第 15 卷
2 Experimental device of vortex be-ams passing through underwater turbulence
The experimental system configuration for generating a vortex beam and measuring its intens-ity f
luctuation in underwater turbulence is shown in Fig. 1. The lar with wavelength of 532 nm is util-ized as a light source. The beam is propagating through a telescope system consisting of two lens (L 1 and L 2), who focal lengths are 5 cm and 15cm, respectively. The expanded beam is converted into a vortex beam by passing through a Spiral Pha Plate (SPP). The topological charge of the vortex beam is determined by the structure of the SPP. The generated the vortex beam then transmits through an underwater turbulence simulation device (called Simulator), where the intensity of underwa-ter turbulence is controlled by the flow rate of circu-lating pump water. Figure 2 shows the practical photo of the experimental device, where Fig. 2 (a) is weak turbulence and Fig. 2 (b) is strong turbulence.Since the length of Simulator is 1.8 meters, in order to enable the beam transmit over the longer dis-tance, the reflectors M 1 and M 2 are ud. A detector for measuring the scintillation index is employed,which contains an opto-electron detector with a round role of 5 mm in diameter. The optical signal is converted electrical signal by opto-electron de-tector, and then the Scintillator is ud to collect the electrical signal to measure the scintillation index.
Scintillator
Detector城市党建
Simulator
Lar智富惠
SPP
M 1
M 2
L 2L 1
Fig. 1 Schematic diagram of vortex beams propagating in
白蔷薇花语
underwater turbulence. L 1, L 2, thin lens; SPP,
spiral pha plate; M 1, M 2, reflectors
Figure 3 shows the scintillation index of the
vortex beam with m =2 transmitted to 3.6 m in weak
underwater turbulence with salinity of 2.42 ‰, the horizontal coordinate is time, and the vertical co-ordinate is the scintillation index measured in real time. The sampling time is t to 1ms during the ex-periment, and the sampling frequency is t to 1 000times.
(a)
(b)
Fig. 2 The practical photos of the experimental device. (a)
Weak turbulence; (b) strong turbulence
Fig. 3 The scintillation of the vortex beam with m =2 trans-mitted to 3.6 meters in weak underwater turbulence with the SA =2.42 ‰
In this study, the scintillation index is meas-ured by a detector with a round role of 5 mm in dia-meter, which means that the scintillation refers to area scintillation index. Recently, the aperture aver-aged scintillation has been propod to study the
第 1 期
LU Teng-fei, et al. : Experimental investigation on propagation characteristics of ......
113
scintillation in a certain region, which is defined as [28]
where
3 Experimental results and analysis
Now we discuss the intensities of the vortex beam with m =2 propagating through different un-derwater turbulences in the experiment. The spot is taken by the beam analyzer. In order to avoid over-exposure during shooting, the attenuator is ud to attenuate the light intensity, resulting in the loss of a little detail of the spot. Figure 4 (Color online)shows the intensity patterns of the vortex beam with m =2 propagating in both the weak and strong under-water turbulences with salinity of 0. As shown in Fig.4, the light intensity is obviously weakened with the increasing transmission distance in both the
weak and strong underwater turbulence. While, the disturbance of the beam in the strong turbulence is significantly higher than that of the beam in the weak turbulence. Figure 5 (Color online) shows the intensity patterns of the vortex beam with m =2propagating in both the weak and strong underwater turbulence with the salinity of 2.42‰. By compar-ing the experimental results in Fig. 4
and Fig. 5, it can be found that in the prence of a salt, the light spots are more diffu and the light intensity is weaker.
(a)
(b)
(c)
(d)
Max
Min
Fig. 4 The intensity patterns of the vortex beam with m =2 propagating in both the weak and strong underwater turbulences
with salinity of 0. (a) z =3.6 m,
weak turbulence; (b) z =3.6 m, strong turbulence; (c) z =5.4 m, weak turbulence;(d) z =5.4 m, strong turbulence
(a)
(b)
(c)
(d)
Max
Min
Fig. 5 The intensity patterns of the vortex beam with m =2 propagating in both the weak and strong underwater turbulences
with the salinity of 2.42 ‰. (a) z =3.6 m, weak turbulence; (b) z =3.6 m, strong turbulence; (c) z =5.4 m, weak turbulence;(d) z =5.4 m, strong turbulence
Figure 6 prents the scintillation index of the vortex beam with m =2 varying with propagation distance in the turbulence with different salinities. It can be found from Fig. 6 that the scintillation index of the vortex beam in the underwater with salinity of 4.35‰ is bigger than that of in the underwater with民办教师补助
salinity of 2.42‰, no matter it is strong turbulence or weak turbulence. This is becau that increasin
g the salinity of underwater is approximately equival-ent to increasing the turbulence of underwater. Fig-ure 7 illustrates the effect of the salinity on scintilla-tion index of the vortex beam with m =2 at 3.6 m
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中国光学
第 15 卷
>红船向未来
