Photodetectors’ behavior and mechanism of transient response are still not understood very well, especially under high photon injection. Most of the researches on this topic were carried out with ultra-short laser pulse, whose pulse width ranged from femtosecond scale to picosecond scale. However, in many applications the durations of incident light are in nanosecond order and the light intensities are strong. To investigate the transient response characteristics and mechanisms of narrow-bandgap photovoltaic detectors under short laser irradiation, we performed an experiment on HgCdTe photodiodes. The n<sup>+</sup>-on-p type HgCdTe photodiodes in the experiment were designed to work in spectrum from 1.0μm to 3.0μm, with conditions of zero bias and room temperature. They were exposed to in-band short laser pulses with dwell time of 20 nanosecond. When the intensity of incident laser beam rose to 0.1J/cm<sup>2</sup> order, the photodiodes’ response characteristics turned to be bipolar from unipolar. A much longer negative response with duration of about 10μs to 100μs followed the positive light response. The amplitude of the negative response increased with the laser intensity, while the dwell time of positive response decreased with the laser intensity. Considering the response characteristics and the device structure, it is proposed that the negative response was caused by space charge effect at the electrodes. Under intense laser irradiation, a temperature gradient formed in the HgCdTe material. Due to the temperature gradient, the majority carriers diffused away from upper surface and left space charge at the electrodes. Then negative response voltage could be measured in the external circuit. With higher incident laser intensity, the degree of the space charge effect would become higher, and then the negative response would come earlier and show larger amplitude.
The far field beam profile is of significant importance to the analysis of the atmospheric propagation effect and evaluation of the beam control capability, tracking and aiming precision of laser system. In the paper, technology of laser beam measurement such as mid-infrared laser detection at wide temperature range, power density attenuation, photoelectric and calorimetric compound method for laser measurement, synchronous detecting of multi-channel pulsed signal are introduced. A series of instrumented target with detector array are developed for laser beam power density distribution measurement at far field. The power in the bucket, strehl ratio, centroid and jitter of beam can be calculated from the measured results.
The discoloration and optical characteristics of the gold plating film under long-time high power laser irradiation are investigated. The fabrication process of gold plating on nickel underplate on rough surface of copper and aluminum alloy substrates is introduced. The measurement results of the diffuse reflectivity for the samples with different surface roughness indicate that roughness of the gold layer surface should be 4μm to obtain the maximum value of diffuse reflectivity. The discoloration and variation of diffuse reflectivity are experimentally studied under 2000W irradiation. The research results show that the discoloration and degrading of reflectivity are caused by the diffusion of Ni to the gold plating surface and forming NiO thin film due to the porosity of the gold film and high temperature treatment. A change of diffuse reflectivity related mechanism is described. Several plating solution recipes are used to eliminate the discoloration and mitigate the degrading of the reflectivity on gold surface.
The output power of a narrow line-width laser is usually limited by the Stimulated Brillouin Scattering effect. In Master Oscillator Power Amplifier structures, multi-point pump could rearrange the gain distribution along the fiber, leading to the suppression of the Stimulated Brillouin Scattering effect with maintained amplification efficiency. A theoretical model concerning 100W-level fiber amplifiers is proposed. Stimulation is performed to analyze the amplification process of the laser signal and Stimulated Brillouin Scattering. The results demonstrate that the power of scattering light decreases from 3.2W to 6.8mW (with two-point pump) indicating the effectiveness of this new technology in Stimulated Brillouin Scattering suppression.
Modeling of Tm-doped fiber lasers pumped with 793 nm, 1.6 μm and 1.9 μm is presented and compared. Output performance of three different pump schemes with active fiber length, pump power and output reflectivity is investigated. Numerical simulation shows that, with 793 nm pump, the cross relaxation process is of vital importance for high efficient operation of Tm-doped fiber laser. And, 1.9 μm pump scheme is more likely to offer even higher output compared with 793 nm pump and 1.6 μm pump.
In this paper, Torrance-Sparrow and Oren-Nayar model is adopt to study diffuse characteristics of laser target board. The model which based on geometric optics, assumes that rough surfaces are made up of a series of symmetric V-groove cavities with different slopes at microscopic level. The distribution of the slopes of the V-grooves are modeled as beckman distribution function, and every microfacet of the V-groove cavity is assumed to behave like a perfect mirror, which means the reflected ray follows Fresnel law at the microfacet. The masking and shadowing effects of rough surface are also taken into account through geometric attenuation factor. Monte Carlo method is used to simulate the diffuse reflectance distribution of the laser target board with different materials and processing technology, and all the calculated results are verified by experiment. It is shown that the profile of bidirectional reflectance distribution curve is lobe-shaped with the maximum lies along the mirror reflection direction. The width of the profile is narrower for a lower roughness value, and broader for a higher roughness value. The refractive index of target material will also influence the intensity and distribution of diffuse reflectance of laser target surface.
Cross relaxation (CR) process in thulium ions is described. Performance of Tm-doped fiber lasers with different dopant concentrations is evaluated numerically with and without CR. Simulation shows that CR process can not only improve the slope efficiency and output of the laser system, but also lower the lasing threshold and extend the growth momentum of the laser performance. Backward LD-clad-pumped Tm-doped fiber lasers are built with Tm-doped fibers of different doping levels. A maximum output of 35.3W around 2μm is obtained with a slope efficiency of 47.2% from the 4.5wt.%- doped fiber laser while a higher slope efficiency of 54.1% was achieved from the 6.8wt.%-doped fiber laser. And, modeling shows that these laser systems are much more efficient than that without CR process.
A compact passively Q-switched and mode-locked erbium-doped fiber laser based on graphene saturable absorber was reported in this paper. A fiber ferrule, which had graphene deposited on the core region, was used as the saturable absorber. The Q-switched operation was initiated with a low pump threshold of about 50 mW at 974 nm and the repetition rate can be widely tuned from 14 kHz to 70 kHz along with the increase of the pump power. Moreover, the mode-locking state working at 1559.7 nm with a 0.4nm spectral bandwidth and about 3 ns pulse duration was also demonstrated in the same ring cavity when the pump power increased to about 150 mW. This is, to the best of our knowledge, the first report of the fiber laser, which had the both Q-switched operation with so wide repetition rate and mode-locking operation in the same ring cavity based on the same graphene saturable absorber.
A method for direct laser beam diagnostics based on fiber array is introduced. The designing principle and configuration of the method are presented. The experimental results show that it can provide laser power density distribution information with a high spatial resolution better than 3mm and with an error of measurement less than 3%. The theoretical analysis by finite element method(FEM) for laser radiation indicates that this method should be used to measure the high energy laser beam with diameter of several hundred millimeters for tens of seconds. In addition, the results of fiber array technique are compared, qualitatively, with the spatial beam profile obtained by a near infrared charge coupled device(NIR-CCD). These two methods yield consistent results.
A system based on detector array is developed to measure the far-field temporal and spatial distribution and absolute
pulse energy density of the laser beam. In this experiment, the duration of the laser pulse is about 15ns, the repetition rate
is 400Hz, and the diameter of the far-field beam is about 60cm. The detector array is composed of 112 Si-PIN
photodiode detectors and arranged to be a disk with spatial sample rate of 0.4cm<sup>-1</sup>. Charge sensitive amplifiers and
baseline restoration circuits are used to collect photocurrent of the detectors, and current-input AD converters with
integrator front-end are used to digitalize the multi channel signals. The far-field laser beam profile is reconstructed with
the spatial sample data using special arithmetic of spatial interpolation. The system is capable for absolutely measuring
far-field energy density distribution of repetitively pulsed laser, with response wavelength between 400nm and 1100nm,
minimal detectable pulse duration of about 10ns, and energy density of 0.1-100μJ/cm<sup>2</sup>.