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Abstract
We experimentally demonstrated a one-to-two-point free-space optical communication (FSO) system based on non-mechanical beam servo device in the laboratory. After the initial pointing, two sets of liquid crystal variable retarder cascaded polarization gratings perform non-mechanical beam servo and realized switching or working simultaneously of two communication links. The non-mechanical beam steerer had four diffraction fields; each can achieve beam steering with a 3.72° field and 30.77 µrad resolution, and the system emission efficiency was higher than 77%. The corresponding switching times of links at 2, 4, and 10 Hz were 46.7, 43.8, and 42.1 ms, respectively. In the quasistatic condition, the sensitivities of the two links under the data rate of 10.3125 Gbps were -23.18 and -23.01 dBm, respectively, indicating the service transmission capability of the multi-node beam control system.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
With the continuous development of big data, cloud storage, high-definition 4 k media, and the Internet of Things, radio frequency communications are increasingly difficult to meet the exponential growth of the demand for data traffic. Free-space optical communication (FSO) has developed rapidly in recent years, because of its large bandwidth and long transmission characteristics [1–3]. At present, FSO has been widely used in Space Laser Communication [4,5], and trying to provide technical support for the construction of the Internet of Things and smart cities construction [6].
It is required to design the near-diffraction limit optical system and high-precision beam tracking system to achieve the long-distance FSO link establishment [7,8]. A conventional FSO systems require complex and precise mechanical rotating elements, which usually divided into coarse tracking and fine tracking [9,10]. The servo table for coarse tracking can achieve large angle beam deflection and hundred microarc metric level tracking accuracy. The precision tracking system usually adopts electro-magnetic vibrator or piezoelectric-ceramic vibrator to compensate for the residual error of the coarse tracking system and realize microarc measurement tracking. In the context of new applications such as satellite Internet and urban combat LAN coverage, the development of FSO is constantly breaking through in the direction of networking. Multi-satellite interconnection usually uses single satellite multi-load mode. In recent years, there have also been reports of special antennas, such as paraboloid of revolution antenna [11]. However, it did not avoid the complex and precise mechanical structure, which challenged to the miniaturization of the FSO system and limited its application in optical networking.
Liquid crystal has the characteristics of electricity changing the crystalline phase [12], which has made important research progress in biomedicine [13,14], pressure and temperature sensing [15,16]. When the laser passes through the liquid crystal optical device, the anisotropic liquid crystal molecules can regulate the laser field under the action of the applied electric field. In 1999, Gori theoretically analyzed that polarization gratings (PG) based on liquid crystal materials have a role on the periodic modulation of the incident laser polarization state [17]. Subsequently, Oh et al. performed a complete numerical analysis of the PG and investigated the diffraction properties of the linear and circular birefringence PGs [18]. In 2011, Kim experimentally demonstrated a non-mechanical deflection system of large angle beams with cascade waveplates and PGs [19]. The experiments verified the ultra-high diffraction efficiency (∼100%) of the polarization grating and the large beam steering field (∼44°) of the system. It indicated the potential of PGs for large-angle beam deflection control. By 2012, Boulder Nonlinear Systems (BNS) had made liquid crystal polarization gratings into commercial products [20]. With the continuous breakthrough of the response time, diffraction efficiency and angle range of liquid crystal polarization grating [21,22], it has been applied in the liquid crystal laser [23], wide viewing angle holographic 3D display system and virtual reality (VR) [24,25]. In recent years, researchers were committed to developing liquid crystal polarization grating (LCPG) in high resolution 2D beam steering [26], non-mechanical self-alignment system [27,28], which provided a lot of reliable research basis for the application of LCPG in FSO. With its small size, fast response and non-mechanical beam deflection, the PG is very attractive to the FSO networking system.
In this paper, we designed a non-mechanical beam steering system based on liquid crystal variable retarder (LCVR) cascade PG. The wavelength tunable linear polarized laser pass through blazed grating to realize beam steering having a 3.52° field of regard with 30.77 µrad resolution at C band. The beam steering angle was further diffracted to the four angle ranges after passing through a two-stage LCVR cascade PG. Based on this beam emitting device, we built an indoor one-to-two-point FSO system. Two laser communication links can be switched or work simultaneously by controlling the driven voltage of LCVR. The system’s total efficiency was higher than 73% and the switching time of the system was less than 50 ms. The sensitivity of each communication link is better than -23 dBm at a 10.3125 Gbps data transmission rate, indicating the potential of the system in the field of FSO networking.
2. Experimental setup and principles
The non-mechanical beam steering system based on LCVR cascade PG is shown in Fig. 1. LCVR is made by antiparallel liquid crystal polymer boxes based on nematic liquid crystal and ITO glass material. Under the friction orientation of the polyimide layer, the fast axis orientation of all liquid crystal molecules was consistent in the whole element plane, as shown by the arrow on the LCVR in Fig. 1. The delay amount δ of LCVR can be expressed as
where d is the liquid crystal layer thickness, λ is the operating wavelength, Δn is the birefringence difference of the liquid crystal material, varying with the driving voltage of the LCVR. PG is of a liquid crystal polymer birefringent material and an N-BK 7 glass substrate, which can realize the adjustable ± 1 order beam diffractive according to the different polarization states of the incident laser.The Jones matrix of LCVR can be expressed as:
According to Eq. (2) and (3), the Jones matrix of the LCVR cascade PG could be expressed as
Equation (4) shows that the system can adjust the polarization state of the incident laser by controlling the driven voltage of LCVR and regulating the intensity distribution of the ± 1 level diffraction beam.
The experimental setup is illustrated in Fig. 2. A linearly polarized wavelength tunable laser (TL) diode was modulated by Mach-Zehnder intensity modulator (MZM), which was driven by a high-speed digital signal generator (DSG). The output power could be amplified to 1 W and transmitted to a reflective blazed grating (BG) via the collimator. The reflection angle was highly precision controllable by tuning the laser wavelength. Two-stage LCVR cascade PG system expanded the beam steering range through a voltage controller (VC). The diffraction beam was monitored by a near-infrared charge-coupled device (CCD) and calibrated the diffraction and divergence angles. The system operating at 10.3125 Gbps was transmitted to two optical terminals (OT) through the non-mechanical beam servo system. The received optical signal was detected by a small form pluggable (SFP) and processed by FPGA. The received power and bit error rate (BER) were obtained by calculating the data from FPGA feedback using LabVIEW.
Fig. 2. Experimental setup of one-to-two-point FSO system based on LCVR cascade PG. TL: tunable laser; MZM: Mach-Zehnder modulator; DSG: digital signal generator; EDFA: erbium-doped fiber amplifier; BG: blazed grating; LCVR: liquid crystal variable retarder; PG: polarization grating; VC: voltage controller; CCD: charge-coupled device; DSP: digital signal processor.
3. Diffraction characteristics of non-mechanical beam servo system
The non-mechanical beam servo system first used a blazed grating for small-range high-precision beam deflection, and then passes through a two-stage LCVR cascade PG for large field beam steering. To establish later FSO links, we adjusted the wavelength of the TL to the C band (1525 to 1565 nm). The grating line was 1000/nm, and the incident angle was fixed to 50.6°. We defined the diffraction angle of 1545 nm as 0°, and the diffraction angle scans from -1.76° to 1.76° when the wavelength of TL was tuned from 1565.62 nm to 1525.22 nm, as shown in Fig. 3. The focal plane array size of the near-infrared CCD was 640 × 512, and the pixel size was 15 µm. The field of view of the CCD was set as 2°. Therefore, the single-pixel detection resolution was 68.53 µrad. We experimentally measured the repeated localization accuracy at a single wavelength and found that at the high-resolution wavelength tuning of the system, this measurement outperformed the resolution of the system.
The beam steering characteristics of the blazed grating is shown in Fig. 4. Figure 4(a) illustrates the numerical simulation results of the resolution and angle control accuracy of the blazed grating in the C-band. With the wavelength tuning from 1565 nm to 1525 nm, the resolution improved from 0.1385 to 0.1350, and the angle control accuracy improved from 30.77 µrad to 29.24 µrad. The numerical simulation results verified that the system angle control accuracy was lost in the measurement error and difficult to express. The resolution of the system could be further optimized by reducing the grating constant and increasing the number of grating lines. The diffraction efficiency measurements of the discrete 40 wavelengths in the C-band are shown in Fig. 4(b). The blazed grating wavelength was 1560 nm, corresponding to the highest diffraction efficiency of 96.86%. All diffraction efficiency measurements were above 92.50%, and the average diffraction efficiency could reach 94.24% with a standard deviation of 0.011, indicating the uniformity of the method within the C-band.
Fig. 4. Beam steering characteristics of the blazed grating. (a) Resolution and angle control accuracy of the blazed grating at C-band; (b) Diffraction efficiency of blazed grating at C-band.
Non-mechanical beam deflection control based on blazed grating could achieve high precision beam deflection with limited range. The LCVR cascade PG system could further expand the beam deflection field while maintaining the beam control accuracy. We fixed the wavelength of the laser at 1545 nm, and the beam steering characteristics of the LCPG cascade PG system is shown in Fig. 5. In the experiment, d was 12 µm and Δn was 0.102, and the delay amount of LCVR without external electric field was calculated to be 0.79λ. We measured the phase delay-voltage curve of LCVR using the light intensity method based on the Stokes vector [29], as shown in Fig. 5(a) By applying an external AC voltage, a flexible and adjustable delay amount between 0∼0.8λ could be obtained. The threshold voltage of the LCVR is approximately 1.21 V. As the voltage of VC was set to 1.37 V, 1.95 V, 2.73 V and 9.52 V, the corresponding phase delay was 0.75λ, 0.50λ, 0.25λ, and 0. The diffraction field of beam steering characteristics of the LCPG cascade PG system is shown in Fig. 5(b). The linearly polarized laser generated by LD was phase delayed 3λ/4 after LCVR1, and converted into left circularly polarized laser. The PG1 subsequently diffracted all of the laser to -1 order, and converted to right circularly polarized laser. At this point, the phase delay of LCVR2 was set to λ/2, and the rotation direction of the circularly polarized laser was changed to perform the -1-order diffraction again through the PG2. The phase delay amount of LCVR and the diffractive polarization state of the PG are shown in Table 1. The diffraction efficiencies of -5.974°, -1.997°, + 1.998° and +5.967° were 77.91%, 78.15%, 78.31% and 79.08%. The energy decay was mainly caused by the loss of LCVR and the polarization degree of the laser source. The fluctuations in the diffraction efficiency are mainly caused by the polarization perturbations and the measurement accuracy. The two-stage LCVR cascade PG expands the wavelength-tuned beam control system field to ± 7.72°.
Fig. 5. (a) Beam steering characteristics of the LCPG cascade PG system. (a) Phase delay of LCVR under different driven voltage; (b) Diffraction field of the LCPG cascade PG system.
Table 1. Phase delay of LCVRs and diffraction polarization state of PGs
When the incident laser polarized state of the PG was not circularly polarized, the PG diffracted at the ± 1 level, which could realize the simultaneous diffraction of the multipoint beam. We set the polarization state of the laser of the incident polarization grating to the linearly polarized state, and the laser field distribution of four different levels of simultaneous diffraction was obtained, as shown in Fig. 6. The diffraction efficiency was uniform for all diffraction orders by controlling the driving voltage of the LCVRs, and the total diffraction efficiency was better than 77% in all cases. The research of multi-point FSO networking could be carried out based on the above research of non-mechanical beam servo system.
4. Transmission characteristics of one-to-two-point FSO system
In this section, we would like to discuss the transmission characteristics of the non-mechanical beam servo system in the one-to-two-point FSO system. The laser was modulated by MZM to 10.3125 Gbps and amplified to 10 dBm. After transmitting into space through the non-mechanical beam servo system, it could be received by two portable optical terminals. The experimental scene in the laboratory is demonstrated in Fig. 7. The angle between the two optical terminals was 7.98°, and the link distance between terminal 1 and terminal 2 were 12.3 m and 13.8 m, respectively. The antenna diameter of the terminal was 50 mm, and the receiving efficiency was approximately 10% after single-mode fiber coupling. In the initial calibration process, the diffraction direction of the blazed grating was relatively adjusted to that of the PG. Furthermore, we marked the diffraction direction of the non-mechanical beam servo system through the CCD and set the height of the terminals. The position of the two terminals was calibrated and recorded by the joint control of the wavelength and polarization state.
In the experiment, the transmission loss under the laboratory can be ignored, and the received power was above -11.2 dBm. The receiving power of the photodetector was controlled at -15 dBm by additional tunable attenuation. After initial calibration, the system wavelength was fixed to 1550 nm, and the driven voltage of the LCVR were switched at the frequencies of 2, 4 and 10 Hz, respectively. The switching characteristics of the two communication links are shown in Fig. 8. When the switching frequency was set to 2 Hz, the average power of terminals 1 and 2 were -15.74 and -15.67 dBm, respectively, with corresponding variance fluctuations ranging from 0.147 to 0.157 and 0.112 to 0.129, respectively. The rising and falling edges of the two terminal power change curves correspond to the rising and falling time of LCVR. The switching time of the system was calculated as 46.7 ms after 100 statistical averaging times. As the link switching frequency was increased to 4 and 10 Hz, the average received power of terminal 1 was -16.06 and -15.63 dBm, and terminal 2 was -15.86 and -16.01 dBm, respectively. The variance fluctuations of the two terminals at 4 Hz varied from 0.145 to 0.154 and 0.124 to 0.134, and the frequency at 10 Hz was 0.177 to 0.180 and 0.163 to 0.169, respectively. The variance changed at different switching frequencies indicated that the power volatility of the link improved with increasing switching frequency. The switching time of the system was also improved, simultaneously. When the switching frequency was increased to 4 Hz and 10 Hz, the system switching time decreased to 43.8 and 42.1 ms. The rising and falling edges of the LCVR showed a strong asymmetry, which could be reflected by the waveform in Fig. 8(a), (c), and (e). As the switching frequency increased, the rising time of the LCVR became the main factor affecting the switching time of the system.
Fig. 8. Receiving power fluctuations and variance of two FSO links in the link switching state. (a),(b) Switching frequency at 2 Hz. (c),(d) Switching frequency at 4 Hz. (e),(f) Switching frequency at 10 Hz.
We also realized the simultaneous communication between the two terminals by setting the corresponding delay amount of LCVR. The link information could be calculated in real-time using data feedback by FPGA via LabVIEW. The BER of two FSO links at a data rate of 10.3125 Gbps were plotted in Fig. 9. The system had an inevitable power fluctuation affected by the environmental vibration. The fitting curves of BER are shown in the red curves in Fig. 9. Under the condition of BER at 1E-6, the receiving sensitivities of link 1 and link 2 were -23.18 and -23.01 dBm, respectively. The difference in the sensitivity of the two links was correlated with the noise properties of the detector.
Fig. 9. BER of one-to-two-point FSO system. (a) BER and fitting curve of terminal 1; (b) BER and fitting curve of terminal 2.
5. Conclusion
In conclusion, we experimentally investigated the transmission characteristics of non-mechanical beam servo system based on LCPG cascade PG. The device diffracted the beam to four discrete angles of ± 5.97° and ± 2°. Combined with the wavelength-dependent diffraction characteristics of the blazed grating, the system can achieve continuous beam steering in four angle range: -7.73° to -4.21°, -3.76° to -0.24°, 0.24° to 3.76° and 4.21° to 7.73°, respectively. The beam steering accuracy was 30.77 µrad in the C band. In addition, we further experimentally demonstrated the capability of this system in one-to-two-point FSO. The weak fluctuation of the receiving power based on the feedback by the two receiving terminals indicated the high repeated positioning accuracy of the system. As the link switching frequency increased from 2 Hz to 10 Hz, the variance fluctuations of two terminals decreased from 0.01 to 0.003 and 0.017 to 0.006, respectively. We realized a one-to-two-point simultaneous laser communication after setting the appropriate phase delay amount. The sensitivity of the two FSO link was -23.18 and -23.01 dBm under the BER of 1E-6. To the best of our knowledge, this could be the first experimental study pertaining to a non-mechanical beam servo system applied in a one-to-two-point FSO system. The experimental results showed the potential of non-mechanical beam control system based on LCVR cascade PG in one-to-multi-point FSO and provided a new method for FSO networking.
Funding
Natural Science Foundation of Jilin Province (222621JC010595193); National Key Research and Development Program of China (2022YFB3902500).
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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