Changming Zhao

High-efficiency solar-pumped laser with a grooved Nd:YAG rod

We test the performances of a Nd:YAG rod with a grooved sidewall in two solar pumped laser setups. In both cases, a Fresnel lens with a surface of 1.03  m2 is used as the primary solar light concentrator. In the first setup, a ceramic conical cavity is used as the secondary concentrator. Maximum output power of 20.3 W is obtained, corresponding to a slope efficiency of laser output power with respect to focused solar power of 8.34%. In the second setup, a water tube lens is added in a copper conical cavity to further increase the solar energy concentration; from this setup, 27 W output power is obtained, the slope efficiency of laser output power with respect to focused solar power is 9%. In both cases, the performances of the grooved rod are compared with those of an unpolished rod. The efficiency and the beam quality with the grooved rod are superior to those of the unpolished rod.

High efficient single-frequency output at 1991 nm from a diode-pumped Tm:YAP coupled cavity

A coupled resonator was developed for high efficient room-temperature single-frequency laser operating near 2 mum optical spectral region. 721 mW stable single-longitudinal-mode oscillation at 1991 nm was obtained when the absorbed pumping power was 2.4 W. The optical-to-optical efficiency was 30%, and the slope efficiency was 46%. 5 nm of frequency tuning range was obtained with stable output power. The beam propagation factors M2 were 1.43 and 1.42 in x and y directions, respectively.

50 W low noise dual-frequency laser fiber power amplifier

A three-stage dual-frequency laser signal amplification system is presented. An output from a narrow-linewidth Nd:YAG nonplanar ring-oscillator (NPRO) is split into two parts, one of them is frequency shifted by an acoustooptic modulator (AOM) then coupled into a single mode optical fiber. The other part is coupled into another single mode fiber then combined with the frequency-shifted beam with a 2 to 1 single mode fiber coupler. The combined beam has a power of 20 mW containing two frequency components with frequency separation of 150 ± 25 MHz. The dual-frequency signal is amplified via a three-stage Yb3+-doped diode pumped fiber power amplifier. The maximum amplified power is 50.3 W corresponding to a slope efficiency of 73.72% of the last stage. The modulation depth and signal to noise ratio (SNR) of the beat signal are well maintained in the amplifying process. The dual-frequency laser fiber power amplifier provides robust optical carried RF signal with high power and low noise.

High power amplification of tunable optically carried RF signals by a diode pumped Yb3+ doped LMA silicon fiber

High power optically carried RF signal amplification is demonstrated. The output from a narrow-linewidth NPRO laser with central wavelength of 1064 nm is split into two parts. The frequency of one part is shifted by 150 MHz with an acoustooptic modulator and combined with the unshifted one. The combined beam has power of 50 mW and contains two frequency components with a frequency difference of 150 MHz. The power of the dual-frequency signal is amplified to 10 W using a diode laser pumped Yb3+-doped silica LMA fiber. The frequency separation as well as the relative power ratio remains the same before and after the amplifications. The tuning range of the beat frequency is from 125 to 165 MHz. The highest beat signal to noise ratio is 59 dB at a frequency of 150 MHz. The frequency and the power of the beat signal are stable after the amplification.

Low threshold and high efficiency solar-pumped laser with Fresnel lens and a grooved Nd:YAG rod

Sunlight is considered as a new efficient source for direct optical-pumped solid state lasers. High-efficiency solar pumped lasers with low threshold power would be more promising than semiconductor lasers with large solar panel in space laser communication. Here we report a significant advance in solar-pumped laser threshold by pumping Nd:YAG rod with a grooved sidewall. Two-solar pumped laser setups are devised. In both cases, a Fresnel lens is used as the primary sunlight concentrator. Gold-plated conical cavity with a liquid light-guide lens is used as the secondary concentrator to further increase the solar energy concentration. In the first setup, solar pumping a 6mm diameter Nd:YAG rod, maximum laser power of 31.0W/m2 cw at 1064nm is produced, which is higher than the reported record, and the slope efficiency is 4.98% with the threshold power on the surface of Fresnel lens is 200 W. In the second setup, a 5 mm diameter laser rod output power is 29.8W/m2 with a slope efficiency of 4.3%. The threshold power of 102W is obtained, which is 49% lower than the former. Meanwhile, the theoretical calculating of the threshold power and slope efficiency of the solar-pumped laser has been established based on the rate-equation of a four-level system. The results of the finite element analysis by simulation software are verified in experiment. The optimization of the conical cavity by TracePro© software and the optimization of the laser resonator by LASCAD© are useful for the design of a miniaturization solar- pumped laser.

Experimental and Simulation Research on Micro-Doppler Effect in Laser Coherent Detection

Micro-Doppler effect and its mathematical model were discussed. The Micro-Doppler effect of vibration and rotation of moving point target were analyzed. The micro-Doppler frequency shift induced by vibration is simulated and proceeded by means of the time-frequency analysis. A micro-Doppler Solid-state coherent ladar system for detecting micro-motion of target was demonstrated to detect the micro-Doppler frequency shifts induced by vibration, complex micro motion and rotation, and the micro-Doppler frequency shift induced by rotations at 0.2 r/s or vibrations at 0.5Hz were detected. The information of micro-motion was extracted by the arithmetic of time-frequency analysis. These results are helpful for targets feature extraction and identification.

Coherent dual-frequency lidar system design for distance and speed measurements

Lidars have a wide range of applications in military detection and civilian remote sensing. Coherent Dual-Frequency Lidar (CDFL) is a new concept of laser radar that is using electrical coherence instead of optical coherence. It uses laser with two coherent frequency components as transmitting wave. The method is based on the use of an optically-carried radio frequency (RF) signal, which is the frequency difference between the two components, which is specially designed for distance and speed measurements. It not only ensures the system has the characteristics of high spatial resolution, high ranging and velocity precision of laser radar, but also can use mature signal processing technology of microwave radar, and it is a research direction that attracts more concern in recent years. A CDFL detection system is constructed and field experiment is carried out. In the system, a narrow linewidth fiber laser with a wavelength of 1064nm is adopted. The dual-frequency laser with frequency difference of 200MHz and 200.6MHz is obtained by acousto-optic frequency shift and recombination. The maximum output power of dual frequency laser is 200mW. The receiver consists of all-fiber balanced InGaAs photo-detector and homemade analog signal processing board. The experimental results show that the distance resolution and velocity resolution of the system are 0.1m and 0.1m/s separately when the working distance is greater than 200m, and the spatial resolution is 0.5mrad.

An endoscope designed with 3D measurement functions

The endoscopic system is widely used in medical and industrial areas, but how to realize the high-precision three-dimensional measurement in the limited space scale still faces many challenges. A method based on the four-step phase-shifting structured light illumination is proposed in this paper for endoscopic 3D measurements. Structured light of which the adjacent phase shift is 90 degrees is generated by the different parts of the time-sharing lighting stripe grating of the optical fiber bundle; CMOS camera is used to collect four structured light images with the phase shift. Finally, the method of four-step phase-shifting is used to demodulate 3D information from the images, and a relative measurement accuracy of 95% within the range of 15-200mm can be obtained. The endoscope with a field of view of 90 degrees, a image resolution of 1280 * 800 and 3D depth calculation time of 0.2 seconds has advantages of simple structure, large field of view, high accuracy and good real-time measurement.

The influence of laser spot size on the micro-Doppler spectrum

Micro-Doppler effect, which is induced by micro motion of target or any structure on the target, is a frequency modulation that generates sidebands about the target’s Doppler frequency shift, such as mechanical vibration or rotation. When a target’s motions incorporate micro motions, the radar echo signal will contain micro-Doppler characteristics related to these motions. Therefore, the micro-Doppler effect provides a new approach to obtain the dynamic properties of targets, which can be used to accomplish the detection and identification of targets, such as the identification of different types of helicopters. Scattering of the laser spot from a target surface modulates the Doppler signal, causes broadening of the signal spectrum, and, adds uncertainty to the signature analysis. A mathematic model of cone spin, which is a typical micro motion, is built first in this paper. Furthermore, an analyzed equation is deduced to predict the micro-Doppler spectral broadening of acquired medium current signals in situations of different laser spot size. It is found that the beam spot size on the target affects the resulting spectral broadening. Finally, an experiment based on the scaled model is performed to verify the simulation. A narrow-linewidth single frequency fiber laser is employed to detect the cone target at different laser spot size by coherent detection with constant detect distance and laser power. The experimental result shows that the beam spot size on the target affects the resulting spectral broadening caused by speckle, which corresponds to the simulation result. The experimental broadening was consistently greater than the theoretical broadening due to other effects that also contribute to the total broadening.

Evaluation of impacts of laser parameters on laser cell efficiencies

Wireless laser power supply to long-distance devices is drawing more and more interest in recent years. As power receivers, laser cells are adhered on these devices. Relatively high laser cell efficiency could be obtained under a monochromatic illumination. In order to study the most efficient laser illumination conditions to the laser cell, the efficiencies of circular and rectangular cells illuminated by laser with fundamental mode are compared. The simulations show that the cell efficiency increases slowly with the increase of the laser power, and decreases with the increase of the spot size. When the rectangular cell and the circular cell have the same area, and the diameter of the circular cell and that of the laser spot are equal, the efficiency of the circular cell is higher than the rectangular cell.