The Vertical Dimension Of The Optical Field Helps Expand And Upgrade The Channel

1.

Research background

Free space optical communication is a kind of wireless communication technology with laser as the information carrier, which has the advantages of large capacity, high speed and good safety. It is an indispensable tool for the development of high-speed space communication, and has been widely used in various communication systems, such as passive optical remote sensing, LiDAR, microwave photon radar, etc.

In recent years, with the development of optical field amplitude, frequency, time, polarization and other dimensions, optical communication is once again faced with the challenge of capacity crisis. Therefore, the spatial structure (mode) of the light field is gradually developed to solve the increasingly serious capacity bottleneck problem.

Although the spatial modes obtained by horizontal control of the optical field have fully proved their feasibility in classical and quantum communication, the longitudinal dimension of the optical field, another important spatial dimension of the optical field, has not been applied in the process of information encoding and decoding so far.

2.

Innovative research

In order to solve the above problems, the team of Professor Jianlin Zhao and Professor Peng Li of the School of Physical Science and Technology of Northwestern Polytechnical University proposed a codec method based on the longitudinal control of orbital angular momentum (OAM) mode superposition state and a metassurface to realize the longitudinal control of optical field mode. Based on the geometric phase and transmission phase design of the four-atom structure, the metasurface can realize the complex amplitude control of the transmission field spin-dependent, and then generate 0-15 order OAM mode superposition state, and realize the vertical change of superposition state by "freezing wave" method. After the horizontal mode of vertical change is applied to the information codec, the information codec with modal capacity of 163 is realized in a single channel, which shows that it can increase the modal capacity of the channel exponentially.

The principle of encoding and decoding information in longitudinal dimensions of optical field is shown in Figure 1. The information emitted by Bob at the transmitting end is compiled into ASCII code into multiple OAM mode superposition state, which is superimposed by two OAM modes whose topological charges are l1 and l2 respectively. The light spot presents the shape of | L1-L2 |. These OAM superposition states are loaded into a beam array with longitudinal mode variation for free space transmission using the optical freezing wave principle. When Alice obtains information at the receiving end, it can measure the array optical field modes of different transmission planes such as z1, z2, z3, and obtain information through correct decoding sequence operation.

In order to prove the longitudinal dimension coding ability of this special light field, the encoding information used in the experiment is "Northwestern Polytechnical University", and the ASCII hexadecimal code element is used to encode each letter in the word and the space between the words. Each letter corresponds to two hexadecimal digits, so 74 modes are needed to complete the one-to-one correspondence between the beam angular order and the encoded information.

The experiment adopts a 5×5 array beam, and the longitudinal modulation range L of each frozen wave is divided into three segments, corresponding to 0 ~ 0.4mm, >0.4 ~ 0.8mm, >0.8 ~ 1.2mm. In a single frozen wave channel, the total capacity of the modes that can transmit code in a single channel is 163 because of the longitudinal modulation in 3 segments, each segment has 16 available modes. The third segment of the 25th beam freeze wave is eliminated, and the remaining freeze wave is used to complete the encoding of the corresponding information.

The simulation results at z1= 0.1mm, z2= 0.5mm, and z3= 0.9mm are shown in Figure 2(a), where m represents the number of rows, n represents the number of columns, and the number in the upper left corner of the light field intensity diagram represents the information of angular order. The experimental results are shown in Figure 2(b), and the intensity distribution of the light field measured in the plane z1= 0.1mm, z2= 0.5mm, z3= 0.9mm is given.

As shown in Figure 2, the experimental measurement results are consistent with the numerical simulation results, and the array beams all show a superposition state of OAM mode with changes on demand. Starting from the first line at z1, two hexadecimal digits are decoded in a group in Z-shape order to get the message "Northwestern Polytechnical University".

It should be noted that the number of longitudinal mode changes of the light field in the experiment is only 3, and the method proposed in this paper can achieve higher vertical regulation, so the exponential factor of the channel capacity growth can be further improved.

In order to improve the decoding efficiency, the method of split-plane imaging can also be used to obtain the light field distribution of multiple longitudinal planes at one time. According to the propagation characteristics of light waves, if the complex amplitude information of light field is measured in a single plane, the complex amplitude distribution of other planes can also be obtained by numerical calculation, and then the light field mode of multiple longitudinal planes can be obtained. In addition, by introducing deep learning methods, it is also expected that longitudinally encoded information can be obtained from a single measurement.

3.

Sum up

Based on the metasurface with independent control of polarization state and complex amplitude, the flexible control of OAM mode superposition in the longitudinal dimension of frozen wave array is realized in this paper. By using the light fields of the longitudinal changes of the modes, the power exponential expansion of the channel modes is realized experimentally, and the modal capacity in the channel is effectively increased.