Spaceborne lidar for ocean vertical profile detection requires a narrow linewidth pulsed laser source with a wavelength of 486.134 nm, which coincides with the H-β Fraunhofer absorption line. An injection-seeded optical parametric oscillator (OPO) is developed to meet this requirement. The 355 nm ultraviolet (UV) laser obtained from a 1064.4 nm Nd:YAG nonplanar ring oscillator (NPRO) after multi-stage cascade amplification and frequency tripling is used as the pump source for the OPO. The OPO is a single-resonant three-mirror ring cavity structure with a critical phase-matched β-barium borate (BBO) as the nonlinear crystal. A 486.134 nm continuous-wave (CW) single-frequency laser obtained by frequency doubling of a 972.268 nm laser diode (LD) with a linewidth of less than 100 kHz is injected into the OPO as a seed source. With an incident 355 nm pump energy of 58.4 mJ, a signal pulse energy of 23.8 mJ and pulse width of 5.05 ns is obtained, with the pump to signal conversion efficiency of 40.7%. The central wavelength of the signal pulse is 486.134 nm, and the corresponding spectral linewidth is 0.017 nm, with the measured wavelength stability over 30 minutes of less than 11 MHz.
The development of broadband frequency combs has drawn consistent interest in the optics community. Among all the approaches, coherent Supercontinuum Generation (SCG) is one of the most promising techniques for applications such as molecular spectroscopy, fundamental chemistry and combustion analysis. A well-selected combination of pump properties and nonlinear medium is crucial in optimizing the spectrum coverage, power density and coherence of SCG. Coherent SCG can be achieved with a variety of pump sources, with pulse durations ranging from quasi-continuous wave to femtosecond. However, a minimum in-phase frequency component must be maintained to truthfully transfer the seed frequency combs to other frequencies. Nonlinear optical fibers for SCG include photonic crystal fiber, and step-indexed fibers. Being the most conventional fiber platform, step indexed Highly Nonlinear Fibers (HNLF) provide unprecedented environmental robustness with ease of handling and all-fiber capability. In this work, we explore the coherent SCG using a GHz repetition rate frequency comb. The output is dual stage amplified to facilitate the self-phase modulation and coherent soliton fission. To drive an octave-spanning coherent SCG, the amplified pulses were compressed to provide high gain power and in-phase spectrum. By carefully optimizing the pump parameters and the fiber properties, we generate an octave supercontinuum spectrum spanning the wavelength range from 1200 nm to 2400 nm, limited by the measurement range of spectrometer.
Sealing design is an effective way to prevent pollution of high power laser. The thermal stress and installation distortion of sealed optical window seriously affect the quality of laser beam. Based on the design scheme of glueless welding seal, this paper fully considering the material, absorption rate and welding layer interaction of the optical window and the base, combined with the finite element simulation method, the distortion of the sapphire and fused silica optical window surface caused by temperature drift and the installation stress was calculated. through the optimization and simulation of the V-shaped flexible release form of the installation stress, the physical model of the optimal design is obtained. Then, the temperature field, deformation and refractive index distribution of the optical window assembly is analyzed during the TEM00 mode Gaussian beam lasing process and the effect of optical window surface distortion, defocusing amount and beam quality degradation with lasting time is calculated. At last, the influence of the change of the internal and external pressure difference is calculated. The results show that the leakage rate after welding is better than 10-12Pa·m3/s, RMS of the mounting surface accuracy is 0.013 λ. The laser power is 10kw and the light lasting time is 120s, the laser beam quality degradation rate β < 1.1, the relax meets the system requirements. The research results provide an important reference for the design, simulation analysis, laser transmission beam quality control and correction of high-power laser anti-pollution and sealed light window components.
Currently, the construction of low-orbit satellite constellation is booming in the world. To make full use of carrier resources and improve launch efficiency, the extra-vehicular launch envelope size of spaceborne laser communication payload need to be smaller and flatter. Tracking mirror is the decisive factor of launch size because its rotating mechanism of external load and itself cannot be packed in to the cabin. . Therefore, we propose the design architecture of the tracking mirror with eccentric rotation axis and independent temperature control. the scanning space of the short side during the movement is reduced by tilting the rotation axis of the pendulum mirror to one side,. To decrease the height of the support axis for the pendulum mirror and minimize the overall launch envelope, the mirror is positioned horizontally during the launch stage. The design of the eccentric pendulum mirror utilizes rigid body isolation and radiation temperature control. It considers various factors including material properties, adhesive layers, and film layers. Additionally, it implements multi-point flexible installation and lens radiation temperature control techniques, while fine-tuning parameters through finite element analysis. The design achieved a 67.5mm reduction in the size of the transmitting envelope, accounting for 43% of the total height of the original mechanism. In the temperature range of 23~33℃, the RMS value of the pendulum mirror type changes to (1/166λ)/℃, which shows a linear trend. Lab test results show that the surface RMS value changes to (1/161λ)/℃ in the range of 21~32℃, which aligns well with the simulation value. The pendulum mirror has passed the mechanical and thermal vacuum environment test and achieved the design requirements, ensuring the smooth delivery of loads and on-orbit operation. Additionally, it introduces an innovative design optimization method for space eccentric tracking pendulum mirrors.
In order to bring frequency combs out of well-controlled laboratory environments, a robust mode-locked fiber laser is preferable with enhanced self-starting capability, and reduced size, weight, and power (SWaP). We try to reduce cavity loss as much as possible to lower the self-starting threshold and operation power. A lower concentration doped-fiber could restrain non-radiative transitions and scattering from color centers, which are two main loss sources of active fibers. We performed comparison experiments using two fibers with different rare-earth doping concentrations for better SWaP performance. Experimental results match with our predictions. Pumped by a 980nm semiconductor external-cavity stabilized diode laser, a mode-locking self-starts at about 60mW. The net-dispersion is managed by balancing the lengths of the positively dispersive Er-doped fiber with the negatively dispersive PM1550 fiber. It is worth noting that the passive mode-locking can be maintained with pump power as low as 30mW. The laser central wavelength is 1560nm and 3db bandwidth is about 42nm, corresponding to a transform-limited pulse duration of 61.43fs assuming sech2 pulse shape. The repetition frequency is 47.41MHz with more than 90dB signal-to-noise ration at 1KHz resolution bandwidth. At the minimum pump power, the laser output is at 4.2mW average power.
Standing-wave ultrasonic motors (usm) have the advantages of small volume, light weight, fast response, high-precision position, power-off self-locking, no electromagnetic interference and vacuum adaptability etc. It has broad application prospects in the field of space laser communication. Because the driving force and power of single motor are limited, and in order to meet the requirement of torque margin in engineering applications, ultrasonic motors are usually used in pairs in laser tracking and pointing system. However, due to the preload of the motor, the inconsistency between production and manufacturing, and the matching between the driving circuit and the motor and so on, the resonance frequency among motors varies from hundreds of Hz to several kHz, which makes it difficult to realize synchronous control. In addition, the inherent nonlinear characteristics of piezoelectric ceramic also affect the high-precision control of the ultrasonic motor. In order to further improve the synchronous driving control performance of parallel motor, an ultrasonic motor driving control system based on high-performance Digital Signal Processor(DSP) and Direct Digital Synthesis(DDS) technique is designed in the paper. The frequency, amplitude and phase of the motor driving signal can be tuned independently and precisely in this design, and the open-loop characteristics of the ultrasonic motor are measured.
To solve the small-optical aperture and stroke problem of existing fast steering mirror (FSM), we focus on the design of FSM driven by piezoelectric ceramics in space laser communication and lidar systems. The structure design of the FSM and the theoretical analysis of the piezoelectric actuators are carried out. The special structure and installation process of the mirror assembly are designed to ensure the accuracy of the mirror surface. Theoretical calculation and simulation analysis are conducted to evaluate the mirror’s output angle. The dynamic model of the FSM is established to analyze the stiffness. The operating principle and characteristic analysis results of FSM are verified by experimental tests. The results show that the FSM can provide a mechanical excursion angle of ±2.35 mrad, the closed-loop linearity of X axis and Y axis are 0.71% and 0.67%, respectively, and the closed-loop bandwidth of the FSM is 28 Hz. The surface shape accuracy of the mirror after installation can reach 1 / 50λ.
With the wide application of spaceborne lidar, 2 μm laser with high repetition rate and high energy has become an important candidate for coherent detection lidar. Conductively cooling is recognized as the critical technology for high energy, 2 μm lasers. The structure and thermal design of a totally conductively cooled, diode side-pumped, 2 μm laser amplifier is introduced in the paper. The amplifier consists of a 20-mm-long Tm: Ho: YLF crystal pumped by 2-banks of 3-radially arranged diode lasers (LD). Through the research and analysis of the structure and thermal coupling of the amplifier head, the conductively cooling scheme satisfying the need of the application in the space environment is obtained. The peak power consumption of LD is 200 W and the average heat consumption is 23.76 W at 10 Hz. When the coolant temperature is 17°C, the stable temperature of the crystal center is about 30°C, which achieves the result of 2.6 times of laser energy amplification. The experimental data matches the result very well.
The performance of fiber nutation tracking system based on coherent demodulation is constrained by the nonlinearity of devices in the signal coherent demodulation module. With the influence of this non-linear factors, the reconstruction accuracy of intensity envelope fluctuation signal depends on the input optical power. The influence of optical amplifiers and detectors on the signal intensity calculation in the range of our interest input optical power is analyzed theoretically and verified experimentally.
KEYWORDS: Clocks, Receivers, Digital signal processing, Field programmable gate arrays, Transmitters, Modulation, Mechanics, Data communications, Telecommunications, Phase shift keying
Timing synchronization is critical in digital demodulation systems such as intradyne optical receivers. In this paper, a novel timing recovery method for all-digital coherent receivers is proposed. With the help of a parallel architecture, the new method can be implemented on ASIC or FPGA platform, especially when the symbol rate is much higher than the clock rate of FPGAs. Through adjusting the frame structure in the parallel interpolator, the receiver synchronizes its symbol rate with the transmitter. Different from existing parallel timing recovery methods, the proposed method does not adjust the period between adjacent frames of samples, which can be beneficial to the subsequent processing in hardware. The performance is tested by simulation in QPSK modulation. Under relative clock offset between ±50 ppm and jitter noise with 0.3% standard deviation, the proposed method shows almost no degradation compared with its serial equivalent. Combined with different timing error detection algorithms, this method can be used in kinds of modulation formats like MPSK and QAM.
By rotating the half-wave plate, the variable communication bit rate between 5.12Gbps and 2.56Gbps has been verified at satellite-to-ground optical communication linkage, the communication link used Differential Phase Shift Keying(DPSK) modulation format and the wavelength is 1549.731nm. Without error correcting code and adaptive optics, an average bit error rate of 1.9E-9 was achieved while the link distance exceeded 1500km.
A single-mode single frequency eye-safe pulsed all fiber laser based on master oscillator power amplification structure is presented. This laser is composed of a narrow linewidth distributed laser diode seed laser and two-stage cascade amplifiers. 0.8 m longitudinally gradient strained erbium/ytterbium co-doped polarization-maintaining fiber with a core diameter of 10 μm is used as the gain fiber and two acoustic-optics modulators are adopted to enhance pulse extinction ratio. A peak power of 160 W and a pulse width of 200 ns at 10 kHz repetition rate are achieved with transform-limited linewidth and diffraction-limited beam quality. This laser will be employed in a compact short range coherent Doppler wind lidar.
We report a linearly-polarized 1617 nm Er:YAG laser pumped by 1532 nm fiber-coupled laser diodes. An L-shaped resonator was employed incorporating two Er:YAG crystals which were pumped independently. In continuous-wave operation, a maximum output power of 7.73 W was obtained with an optical conversion efficiency of ~15.2% (versus incident pump power). To the best of our knowledge, this was the highest conversion efficiency ever demonstrated for a directly diode-pumped Er:YAG laser. In Q-switched operation, pulses with energy of 7.8 mJ and pulse duration of 80 ns were yielded at a repetition rate of 500 Hz, and the corresponding peak power was ~ 97 kW at 1617 nm.
An all-fiber high modulation speed pseudorandom-coded laser based on master oscillator power amplifier configuration is proposed. We use a high modulation rate distributed feedback laser diode as the seed laser to generate the original pseudorandom pulse train. The modulation rate is 1 Gb/s , which corresponds to a minimum pulse interval of 1 ns. A 1 kHz repetition frequency of 10-order M -sequence pseudorandom pulse train is chosen to balance on-line data processing speed and laser ranging resolution. Then, the pseudorandom pulse train is amplified by two-stage amplifiers to boost the output power. All components used in the amplifiers are built in single mode (SM) fiber, so the final output laser is SM with excellent beam quality. Finally, the peak power of pseudorandom code laser is amplified to 23.6 W from 1.5 mW without wave distortion, corresponding to a gain of 42 dB. The ranging experiment of using the optical fiber delay method indoors shows the transmitter of combining modulated laser diode and multistage fiber amplifiers as a promising solution for developing laser for pseudorandom-coded laser ranging.
Thermal heating is a major limiting factor in scaling the average power of a solid-state laser. In this paper some primary factors which affect the thermal effects of the typical side-pumped and side-cooled slab laser are discussed. The temperature and stress distribution in the cross-section in the laser crystal are calculated by using FEA. Some conclusions which optimize the performance of the side-pumped and side-cooled laser are drawn.
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