Recent years, millisecond laser become a research hotspot. Avalanche photodiode (APD) based on silicon structure has excellent characteristics such as low noise and high-sensitivity. It is key components in receives for long-haul high-bit-rate optical communication system. The failure mechanism of silicon APD remains quite unknown, although some silicon p-i-n photodiode failure modes have been speculated. The COMSOL Multiphysics finite element analysis software was utilized in this paper. And the 2D model, which based on heat conduction equation, was established to simulate the temperature field of the silicon avalanche photodiode irradiated by millisecond laser. The model presented in the following section is a work which considers only melting of silicon by a millisecond laser pulse. The temperature dependences of material properties are taken into account, which has a great influence on the temperature fields indicated by the numerical results. The pulsed laser-induced transient temperature fields in silicon avalanche photodiode are obtained, which will be useful in the research on the mechanism of interactions between millisecond laser and photodiode. The evolution of temperature at the central point of the top surface, the temperature distribution along the radial direction in the end of laser irradiation and the temperature distribution along the axial direction in the end of laser irradiation were considered. Meanwhile, the fluence threshold value was obtained through the model. The conclusions had a reference value for revealing the mechanism of interactions between millisecond laser and the silicon avalanche photodiode.
The photoelectric detector is a very significance part in laser and its application system, but when photoelectric detector irradiated by high energy laser, the laser may cause thermal damage to the photoelectric detector, when the temperature more than its melting point and vaporization point, there will be a permanent damage in PIN photodetector, leading to the failure of photoelectric detector. In order to study the photodiode damage mechanism by millisecond pulse laser irradiation, a set of experimental system has been built, choosing appropriate pulsed laser parameters to irradiate silicon-based PIN photodiode and monitoring the surface temperature in the process of irradiation, until the PIN photodiode complete failure. The measurement results of real-time temperature, responsivity change and damage morphology were analyzed to conclude the failure reason of the PIN photodiode. The results showed that with the increase of laser energy, the PIN photodiode surface temperature would be also increased accordingly. Before the laser irradiation, the responsivity of PIN photodiode was the same. But after the laser irradiation, the responsivity of the PIN photodiode would be changed and with the increase of laser energy, the decline extent of responsivity would be also increased. Judging from the ablation, crack and fold zone on the surface of PIN photodiode after the laser irradiation, the damage was for thermal stress effect. The continuity of material confined its free expansion. Therefore, the uneven thermal expansion induced the great thermal stress. At the same time, the silicon transited from brittle to ductile and the yield strength dramatically decreased. Once the maximum thermal stress exceeded the critical stress, the plastic deformation and the brittle cracks of silicon would be generated. With the increase of laser energy, the thermal stress damage extent of PIN photodiode would be also increased accordingly and the black area of laser ablation would be also larger. In this paper, the damage mechanism of silicon-based PIN photodiode irradiated by millisecond pulse laser is that the thermal stress causes the phenomena of ablation, fold and responsivity change. The conclusions have a vital significance in improving the performance of PIN photodiode in the field of laser application.
In the process of laser propagation, the wavefront distortion and wave breaking of target laser may be caused by the
impact of transmission environment, resulting in that the actual target spot appears the change of spatial and temporal
distribution. It is built a numerical simulation model of millisecond pulse laser interaction with material by COMSOL
Multiphysics software and a contrastive analysis is made on the temperature distribution differences of alumina under the
effect of actual target laser and ideal Gaussian laser respectively. And the study shows that the temperature of alumina
raises with the increase of energy and obvious deposition on the surface of temperature by laser irradiation. Moreover,
temperature difference exists inside laser irradiation area due to the gradient difference appearing in the spatial
distribution of laser energy during the interaction between laser and alumina. As a result, the energy transfer from
high-temperature area to low-temperature area, following by the temperatures of two adjacent areas to affect each other.
When analyzing the actual target laser, the spatial distribution of temperature field is consistent with the stray spots in
general though, they are slightly different with in local area. Such local difference is determined by the heat transfer
coefficient of alumina materials and energy distribution gradient of beam spot together. When the energy of actual target
laser is greater, the difference between temperature field and the spatial distribution is more obvious. Since the energy of
actual target laser has a significant gradient difference with that of Gaussian laser, its interaction with alumina also shows
an obvious difference in temperature distribution. Therefore, not all incident lasers can be idealized as the standard beam
spot. The results have guiding significance in improving the application of laser processing quality in terms of
In order to improve the capacity of beam collimation for laser beam expander, it is necessary to design a more reasonable
and feasible structure of beam expander system. Laser beam expander is used to compress the laser divergence angle, in
order to reduce the energy losing in long distance scanning acquisition system. This paper introduces the working
principle and design idea of the laser beam expander, the collimating multiplying power focal length and the collimated
magnification formula of expander main, secondary mirror. According to the third-order aberration theory, Considering
the spherical aberration, sine difference and divergence angle, the reasonable analysis of optical path, ZEMAX optical
design software was used to design large-diameter laser beam expander and analysis and optimize, And given the actual
design data and results. Display the maximum optical path difference is ±0.01λ of the main light ray and each light ray.
To combination the rear- group objective lens of Galileo and Kepler beam expander, a large-diameter(1.475m) laser
beam expander was designed with 0.2m in the diameter, 1/2m in the relative caliber. In the objective lens System, a
high-order aspherical was used to the aberration of extra-axial point. we can see that the image quality is close to the
diffraction limit from the curves of wavefront. In addition to improve image quality effectively, the system has the
characteristics of simple structure, less costly and less design difficulty to compare with the other beam expanding
system. And make the output beam's divergence angle smaller, energy density higher, and the beam quality has been
greatly improved. The results show that the beam expander is fully meet the design requirements, the use effect is good.
Design and research of laser beam expanding system not only improves the quality of the laser beam in the laser system,
but also enlarge the application field of laser technology in photoelectric system.