Space Laser Research Articles

星载激光遥感林业应用发展研究

Forestry observation is an important direction of the development of space-borne LiDAR. So far, foreign space laser remote sensing payloads have been developed. The experimental LiDAR carried by Chinese ZY-3(02) satellite is a big breakthrough in earth observation of space-borne LiDAR in China, and the oncoming multi-beam LiDAR of Chinese CE-1 satellite will be applied to forestry remote sensing. The vertical structure of forest was acquired by ICESAT-1/GLAS waveform, while point cloud of forest was obtained by ICESAT-2/ATLAS photons. The space-borne LiDAR forestry remote sensing gradually entered the practical stage, beyond the experimental stage. During this period, technical system, configuration, and techniques had improved. In this paper, the applications and technique developments of main forestry remote sensing space-borne LiDAR in use were analyzed, and the characteristics and development trend were summarized, based on the study of detection principle, technical system, observation condition, etc. Space-borne forestry observation LiDAR payloads should be designed according to scientific mission, application requirements and technical characteristics, using the advantages of space-borne LiDAR, and based on which the development trend and research focus of space-borne forestry observation technology and application were prospected.

Progress and Prospect of Space Laser Communication Technology

Progress and Prospect of Space Laser Communication Technology

空间激光测高技术发展及展望

Space-based laser altimetry, which measures the planetary surface topography based on time-of-flight ranging, is an important technology for earth and deep space exploration. The development history and current status were sorted out systematically and development in three directions was proposed, including multi-beam observation, underwater terrain observation of offshore and islands and laser active multispectral detection. It was suggested that some key technologies should be developed, including infrared single photon detection, ultra-narrowband optical filtering, multiphoton resolution detection, single-photon data on-board preprocessing and high conversion efficiency laser and nonlinear optical frequency conversion, to promote the further development of Chinese space laser altimetry technology.

Model analysis and resonance suppression of wide-bandwidth inertial reference system

In the fields of earth observation, deep space detection, laser communication, and directional energy weapon, the target needs to be observed and pointed at accurately. Acquisition, tracking, and pointing (ATP) systems are usually designed to stabilize the line of sight (LOS) within sub-micro radian levels. In the case of an ATP system mounted on a mobile platform, angular disturbances experienced by the mobile platform will seriously affect the LOS. To overcome the problem that the sampling frequency of detectors is usually limited and achieving several hundreds of hertz is difficult, the wide-bandwidth inertial reference system (WBIRS) and fast steering mirror are usually integrated into ATP systems to mitigate these angular disturbances. To reduce the structural stress, a flexible support providing two rotational degrees of freedom is usually adopted for the system. However, the occurrence of resonant points within the bandwidth will be inevitable. Measurements have to be taken to compensate these low-frequency resonant points to realize a wide bandwidth and high precision. In this paper, the low-frequency resonant points of a system were simulated using finite element analysis and tested by a system identification method. The results show that the first-order resonance happened at 34.5 Hz with a gain of 28 dB. An improved double-T notch filter was designed and applied in a real-time system to suppress the resonance at 34.5 Hz. The experimental results show that the resonance was significantly suppressed. In particular, the resonance peak was reduced by 79.37%. In addition, the closed-loop system settling time was reduced by 36.2%.

High Speed Laser Information Transmission Technology of Deep Space Between Moon and Earth

The application of light wave for deep space information transmission will have the advantages of smaller terminal volume,mass and low power consumption under the same data rate,and the receiving sensitivity is high during the transmission process. Because of the narrow beam width,it can make the energy centralized and reach a long transmission distance. Using light wave as the information carrier of deep space exploration will have the advantages that the microwave band does not have. The feasibility of technology is described, and the research status of the moon earth laser information transmission technology is summarized,providing a reference for the development and engineering application of deep space laser communication in China.

Carrier-envelope phase stabilization of an Er:Yb:glass laser via a feed-forward technique.

Few-cycle pulsed laser technology highlights the need for control and stabilization of the carrier-envelope phase (CEP) for applications requiring shot-to-shot timing and phase consistency. This general requirement has been achieved successfully in a number of free-space and fiber lasers via feedback and feed-forward (FF) methods. Expanding on existing results, we demonstrate CEP stabilization through the FF method applied to a SESAM mode-locked Er:Yb:glass laser at 1.55μm with a measured ultralow timing jitter of 2.9 as (1-3MHz) and long-term stabilization over a duration of 8h. Single-digit attosecond stabilization at telecom wavelengths opens a new direction in applications requiring ultra-stable frequency and time precision such as high-resolution spectroscopy and fiber timing networks.

Analysis of Photoelectric Conversion Efficiency of Space Laser Energy Transfer Based on Temperature Rise Effect

Temperature is an important factor affecting the efficiency of laser energy transmission. In this paper, aiming at the application of laser energy transmission in space, the existing spatial laser energy transmission photoelectric conversion model is improved and the mathematical model of photocell temperature rise is established. The model is used to analyze the influence of photoelectric cell temperature change on photoelectric conversion efficiency. The model takes into account various factors such as temperature, photoelectric cell material, space thermal radiation, laser space transmission characteristics, and improves the accuracy of the current model. On this basis, the maximum photoelectric conversion efficiency under different conditions and the corresponding photoelectric cell temperature are calculated. The conclusion shows: when other conditions remain unchanged, the photoelectric conversion efficiency first increases and then decreases with the increase of temperature. In addition, the stability value of the temperature of the photocell is generally different from the corresponding temperature value under the maximum efficiency under different conditions. So it is necessary to adjust the temperature according to the specific situation. It provides a theoretical basis for the practical application and temperature control of laser energy transfer in space.

Shipborne Acquisition, Tracking, and Pointing Experimental Verifications towards Satellite-to-Sea Laser Communication

Acquisition, tracking, and pointing (ATP) is a key technology in free space laser communication that has a characteristically high precision. In this paper, we report the acquisition and tracking of low-Earth-orbit satellites using shipborne ATP and verify the feasibility of establishing optical links between laser communication satellites and ships in the future. In particular, we developed a shipborne ATP system for satellite-to-sea applications in laser communications. We also designed an acquisition strategy for satellite-to-sea laser communication. In addition, a method was proposed for improving shipborne ATP pointing error. We tracked some stars at sea, achieving a pointing accuracy of less than 180μrad.We then acquired and tracked some low-Earth-orbit satellites at sea, achieving a tracking accuracy of about 20μrad. The results achieved in this work experimentally demonstrate the feasibility of ATP in satellite-to-sea laser communications.

Optical adaptive filtering for Doppler shift simulator

Based on free space laser communication, this article describes the working principle of electro-optical frequency shifting, designs an optical adaptive filtering module, and builds the core module of the dynamic optical Doppler shifting simulator for laser channel. It is expected to be applied to the heaven-ground integrated communication link. In this article, we adopt the electro-optical frequency shifting technique combined with the microwave-light wave. In the 1 550 nm band, the negative feedback algorithm is used to complete the adaptive filtering, which realizes optical Doppler frequency shifting and high-precision locking. The frequency shift range reaches +5.5–+32 GHz, and the analog precision is better than 645 Hz. When the microwave frequency is greater than 13.5 GHz, the signal-to-noise ratio (SNR) of the output optical power reaches 20 dB, which lays the foundation for the next stage space laser communication.

An electric field measurement method based on electro-optical modulation for corona discharge in air.

To study corona discharge in air, it is important to measure the electric field in the discharge space, which is directly related to the corona discharge process. In this paper, we present a method for measuring the electric field with electro-optical modulation. This method uses space laser modulation based on Bi4Ge3O12 (BGO) crystals, and we suggest using the dual optical path difference method to improve the measurement performance. The measurement error is analyzed from theory, including the measurement error of a highly inhomogeneous electric field and the error due to theoretical approximation. In system calibration, the system noise and the dual optical path difference method were analyzed. The dynamic linear range is determined, and measurement effectiveness in the severely nonuniform electric field is verified by comparison with a finite element numerical simulation. We measured the electric field in Trichel pulses under negative dc voltage and found electric field pulses. These measurement results show that the characteristics of Trichel pulses are directly related to the electric field. Therefore, this method can provide strong support for the study of corona discharges.

High energy femtosecond laser micromachining with hollow core photonic crystal fiber delivery

In this paper, we demonstrate a new way to deliver the high energy femtosecond laser with Kagome-type hollow core photonic crystal fiber for micromachining compared to the traditional free space laser delivery. To achieve the high coupling efficiency and overcome the nonlinear effect in the delivery, the coupling system is optimized and the fiber is vacuum pumped to less than 5 mbar. High delivery efficiency more than 90% is achieved. To our best knowledge, this is the highest efficiency of femtosecond laser delivered by Kagome type hollow core fiber. With vacuum pump, we lowered the nonlinear effect and make the laser beam have good beam profile and slightly pulse duration change even in the delivery of laser with the pulse energy of 100 μJ, average power of 20 W, and pulse duration of 234 fs. The whole system is also used to make micromachining of aluminum and stainless steel with good processing quality.

Beam pointing analysis and a novel coarse pointing assembly design in space laser communication

Nowadays, Small satellites take with high precision and lightweight laser communication terminals become a hot research direction. In this work, factors affecting beam pointing performance have been studied and a novel coarse pointing assembly (CPA) is designed based on flexure technology. The beam pointing performance is influenced by three aspects: Rayleigh distribution pointing error composed of static bias error and dynamic jitter, coupled motion between optical terminal and satellite and point ahead angle. Based on beam pointing requirements, a novel 2-D spatial flexure parallel coarse pointing assembly is proposed. Firstly, the type synthesis of this mechanism is performed and the freedom is verified by screw method. Then the detailed structural design is given. The pivotal component-elliptic arc flexure hinge is optimized by genetic algorithm and its rotation accuracy is less than 10 um. A flexure rod is used in actuating legs which is made rigid by spatial pseudo-rigid body method in kinematic analysis process. Finally, both of forward kinematics and inverse kinematics have been mathematically analyzed and the virtual prototype simulation is performed in Adams. The rotational accuracy can reach to 3 μrad. The position, velocity and acceleration motion curves shown that the platform moves stable which means it’s easy to control. This work provides a new idea for future design of space optical communication terminal.

Enhanced radiation resistant properties of Nd:GSAG laser crystal by co-doping of Cr3+

With the further exploration of outer space by human beings, the demand for outer space laser is increasing. Therefore, developing laser materials that can resist the space irradiation become increasingly important. In this study, high quality Nd, Cr:GSAG crystal was grown successfully by Czochralski method. The X-ray diffraction results indicate that the crystal structure remains unchanged after the irradiation with 60Co gamma-ray. In addition, Raman, absorption and fluorescence spectra of the as-grown crystal are almost unaffected by the irradiation, suggesting the excellent radiation resistant ability of the crystal. Compared with the Nd:GSAG crystal, the co-doping of Cr3+ can enhance the radiation resistant ability and luminescent efficiency apparently. Importantly, LD pumped 1.06 μm laser results indicate the Nd, Cr:GSAG crystal can be considered as a promising high efficiency laser material for the application in harsh radiation environment, such as in outer space.

Nonmechanical two-user tracking method of space-polarization division using the liquid crystal optical phased array

We propose a two-user tracking method called space-polarization division to enable simultaneously multiple terminals tracking in the field of free space laser communications. This method is based on the nonmechanical beam steering device of liquid crystal optical phased array, which is capable of continuously laser beam pointing and multiple beams generating. An optical architecture of two-user access is presented to show a one-to-two communication link between the relay satellite terminal and user terminals. The two-user tracking strategy is modeled numerically and experimentally verified to show the feasible and agile two-user tracking with a good accuracy less than 50 microradians which meet the requirements of coarse tracking.

Effect of AlPO4 join concentration on optical properties and radiation hardening performance of Yb-doped Al2O3-P2O5-SiO2 glass

A series of Yb-doped silica glasses containing equimolar amounts of P2O5 and Al2O3 were prepared using a sol–gel method combined with high-temperature sintering. Raman spectroscopy confirmed the formation of AlPO4 join with a P/Al ratio of 1. However, for a P/Al ratio slightly larger than 1, both the AlPO4 join and P=O bond are formed. Based on the absorption, emission spectra, and electron paramagnetic resonance measurements, it was determined that the increase of the AlPO4 join concentration in Yb-doped silica glass leads to (i) a decrease in the refractive index with a factor of 1.1005 × 10−4 (which allows for a low numerical aperture facilitating single-mode laser), (ii) the sustained spectroscopic properties of Yb3+, (iii) a decrease in Yb cluster concentration, and (iv) an improved radiation hardening performance (beneficial to high-power space lasers). These results suggest that Yb-doped Al2O3-P2O5-SiO2 glass with an appropriate concentration of AlPO4 join has potential applications in high-power space fiber lasers.

Initial Pointing Technology of Line of Sight and its Experimental Testing in Dynamic Laser Communication System

The initial pointing unit of the line of sight (LOS) operating in the open-loop mode is an important part of the space laser communication system, which directly determines the size of the field of uncertainty (FOU) and the length of the scan time. In this paper, the composition and working principle of the initial pointing of the LOS in the dynamic laser communication system are studied in detail. The initial pointing angle (IPA) of the LOS is derived by applying the principle of coordinate transformation, and the factors affecting the size of the FOU are analyzed. It is proposed to introduce the extended Kalman filtering technique into the initial pointing to reduce the size of the FOU. In the demonstration experiment of the dynamic laser communication (including airship–ship and aircraft–aircraft), the dual-antenna global positioning system/inertial navigation system composite device is applied to obtain position, attitude, and other parameters in real-time to realize the performance test of the initial pointing system. The size of the FOU is measured with a target-viewing camera, and the filtered and unfiltered results were compared.

Highly efficient coherent conformal projection system based on adaptive fiber optics collimator array

Coherent beam combination (CBC) of fiber laser array, which is a promising approach to overcome the power scaling limitations of the single fiber laser and to achieve high-brightness laser with good beam quality, can be employed in coherent conformal projection system (CCPS) for beam projection and free space laser communication system with enormous potential. To date, all the demonstrated CBC system through several kilometers atmospheric turbulence have employed collimated beams, which significantly decrease the final CBC effect because of the inefficient wavefront overlap. On the other hand, focused CCPS has lots of advantages over collimated projection system since the wave front of the projected beam is closer to that of a convergent spherical wave. In this paper, we design and manufacture focused CCPS based on an adaptive fiber optics collimator array for the first time, which is achieved by introducing controllable spherical aberration through the adaptive fiber optics collimator. A CBC system is setup to evaluate and validate its performance. Results show that compared with the collimated CCPS, the energy portion of the central lobe could be increased by a factor of 44%, which agrees well with the theoretical analysis.

A Novel Approach for Optimization Control of Dynamic Tracking With Platform Vibration in the Ground Test of Satellite Laser Communication Systems

Vibration interference reduces the precision of the control system and leads to the instability of the satellite optical communication link. So how to improve the tracking accuracy of the terminal system under vibration conditions is an important work for the ground test of satellite laser communication systems. In this paper, the tracking characteristic based on the platform vibration is introduced for coarse tracking and the combination of coarse and fine tracking, we propose a structural decoupling approach and iterative learning control scheme using sliding-mode control to investigate the performance of the tracking system in order to achieve the error minimization, and further to improve the system performance. The scheme is introduced to increase the tracking accuracy and the robustness of the system against external disturbances. The experimental results of the steady-state tracking response and the high tracking precision are carried out on a 4.62 km bidirectional free space laser link experiment, and the obtained test results (less than $2~\mu $ rad) demonstrate the effectiveness of the proposed control scheme in suppressing oscillation. This new method is good for the ground test of the satellite laser communication systems.

光强闪烁对空间激光授时抖动的影响

光强闪烁对空间激光授时抖动的影响

Investigation of isolation for free space laser communication in the mono-wavelength optical T/R channels

Favorable isolation performance in optical transmitting-receiving (T/R) channels is key to establishing full-duplex bi-directional laser communication links when designing mono-wavelength laser communication terminals. This study proposes a novel design scheme for T/R channels that use a single laser wavelength. The system includes measures such as the deflection at λ/4 wave plate angles, light trap installation at a polarization beam splitter, and removal of polarization analyzers in optimized systems. A mathematical model describing isolation in the optical T/R channels is presented. Three types of factors that affect the isolation of optical T/R channels were analyzed and simulated in detail. The present simulation results show that the roughness of the λ/4 wave plate and the solid angle of the detector field of view are exponentially related to T/R isolation, which can thus be regarded as the most significant factors affecting isolation. According to the present design scheme, the prototype T/R channel was developed for measuring isolation. The measured value (77.8 dB) agrees well with the result determined using the established mathmatical model (77.3 dB). Conclusively, the present theoretical model can provideguidance for optimizing isolation in a mono-wavelength optical channel. Thus, it can be applied extensively in laser communication systems, including inter-satellite networks.