Our team has conducted theoretical and experimental studies on the effects of atmospheric environment on wireless optical communications, and established simulation software system. By studying adaptive optical system without wavefront sensing and partially coherent light communications, the performance of optical communications has been improved.

1. Atmospheric Laser Communication Simulation Software

Atmospheric attenuation and atmospheric turbulence effects have great influence on the optical communication. Our team has established and perfected algorithm and simulation software of atmospheric laser communication based on relevant principles of optical communication technologies. The software mainly includes the impact of atmospheric environment, analysis of communication, and calculations for various communication platforms.

2. Experimental research on partially coherent optical communication

Since the partially coherent light is less affected by turbulence than full coherent light, reducing the coherence of the light source can effectively suppress the turbulence effect to a certain extent and improve the communication performance. The team uses rotating glass and fiber array beams to generate partially coherent light and carries out experimental research on communication in simulated atmosphere turbulence.

(1) Partially coherent light atmosphere turbulence transmission experiment using ground glassFig. 1 Schematic diagram of experiment on atmosphere turbulence transmission characteristics of partially coherent beam by ground glass generation method

The experimental system is shown in Fig. 2. From the test results, it can be concluded that partial coherent light can effectively suppress the effect of turbulence and improve atmospheric laser communication performance. Fig. 2 Rotating frost glass communication experiment systems Fig. 3 Comparison of BER with the same SNR

(2) Optical fiber array partial coherent optical communication experiment

Based on the partially coherent light generation method with adjustable coherence, a partially coherent optical communication experiment system is built, and one communication experiment of 155 Mbit/s pseudo-random data under OOK modulation was performed in a hot- air- type atmospheric turbulence simulation pool. The schematic diagram of the system is shown in Fig. 4. By applying different amplitude random voltage modulation to the multi-channel lithium niobate phase modulator, partial coherent light source with different coherence can be generated, and the transmitted signals can be analyzed by using an eye diagram analyzer.

Fig. 4 Schematic diagram of optical fiber array partially coherent communication experiment

Fig. 5 Physical map of optical fiber array partially coherent communication experiment

The experimental results show that the quality of the eye pattern of the array beams loaded with the random phase modulation is better, and that the influence of the atmosphere turbulence effect is suppressed. Furtherly, the partially coherent light has the ability to improve the atmospheric laser communication quality of the long-distance near-earth.

 (a) Without random phase modulation                       (b) With loading a random phase modulation voltage

Fig. 6 Comparison of eye signals after turbulent atmospheric signal

3. Experiment Research on Adaptive Optical Technology without Wavefront Sensing

The adaptive optical technology without wavefront sensing has no limitation on the distortion conditions, such as the flicker effect. The controling signal required by the wavefront corrector is used as an optimization paramete, and the system performance index of the imaging resolution and the received light energy is used as an objective function of the optimization algorithm. Compared with conventional adaptive optical technology, the complexity of this new technology is greatly reduced and adaptive optical technology without wavefront sensing is more suitable for applications such as atmospheric optical communication with serious scintillation effect.

Fig. 7 No wavefront sensing adaptive optical system principle structure diagram

The system principle is shown in Fig. 7 which mainly includes three modules: light intensity acquisition, wavefront control, and wavefront correction. The workflow: one beam with wavefront distortion arrives at the wavefront corrector, and the reflected light beam is reflected to the lens and focused to the CCD for light intensity detection. The light intensity distribution is calculated by the performance optimization module and the correction phase is calculated by the wavefront corrector. The intensity of light received by the CCD is improved so that the light intensity distribution of the CCD receiving light beam meets the requirements of the optical receiving system.

Fig. 8 No wavefront sensing adaptive optical experimental light path diagram

(a) Initial far-field distribution                                  (b) Optimized far-field distribution

Fig. 9 Comparison of far field distribution before and after optimization