عنوان مقاله [English]
Space environment radiation affects the operation and life-time of optical and electronic devices on satellites payloads. LIDAR payloads include electro-optical components such as laser section. LIDAR payloads are usually setup on various platforms. Space-borne remote sensing technology provides global data set of uniform quality and rapid data acquisition and also specially provides key information for evaluating the local, regional of aerosol irradiative forcing in global climate system, cloud properties and precipitation, Air pollution and hydrologic cycle. Also, global coverage collected data is updated in every time range. On the other hand, high expenditures of design, manufacture and using advanced technology are caused an increase in design and manufacture of high reliability LIDAR payloads, the mission duration and its quality in space. Due to space standards of NASA and ESA, one of the effective factors on degradation of space systems is radiation damages. In this research, effect of Total Ionizing Dose (TID) radiation on the operation of LIDAR subsystem orbiting 500 km altitude orbit with a 3-year mission is simulated. Radiation simulations on gain medium (Nd:YAG) and pump (laser diode) are performed by GEANT4 and C++ programming.
Laser subsystem in LIDAR payload includes 3 different parts. These parts are Nd:YAG, mirrors and laser diodes pump. Space radiation sources are classified in three types: Van Allen radiation belts, Galactic cosmic rays (GCR), periodic gradual activities of sun. The sun emits neutralized plasma (almost protons and electrons) to the space. Some energetic particles originated from solar winds and GCRs are trapped in Earth magnetic field. These belts are expanded from 1000km to 65000 km and consist of electrons up to 7Mev and protons up to 300Mev. GCR particles are mainly protons with very high speed originated from out of the galaxy. The most populated particles in 500km altitude are electrons and protons. In order to simulate the radiation effects, Input data such as average flux number of incident particles, total number of particles passing through the sensitive volume are needed. These input data will be entered in GEANT4 toolkit by C++. Most important input data are flux, total number of incident particles during the mission, Energy range, geometry, materials definition and their properties, sensitive volume definition are needed physics and initial position of particles. The most important radiation damages related to lasers and solid state optical devices are total ionizing dose and displacement damage. TID is resulted from passing charged particles through matter, while displacement damage is due to collision of passing particles with atoms of matter. TID effects depend on two factors. The first factor is generation of electron-hole pairs in dielectric layers (such as oxides). The second factor is trapping sites (oxide and interface). This damage results in degradation and possible failure, such as threshold voltage shift, decrease in drive current, switching frequencies, leakage current, noises, etc. In bipolar transistors, hfe degradation, leakage current, offset voltage, changes in offset current, bias current and gain degradation in analog devices are possible effect. In other devices, frequency shift in crystals, mechanical degradations and changes in dielectric parameters are considered.
The results demonstrated that the absorbed dose from different space radiation sources for Nd:YAG and laser diode are important. Also, differences between the absorbed doses in 2 cases were simulated. The first case was absorbed dose with no shield against space radiation and the second one was calculating the absorbed dose with 2 mm Al7075 shielding the sensitive volumes against incoming particles. GEANT4 simulations determined that the absorbed dose in Nd:YAG when no shield was used to protect the laser parts against space radiation, was reached 1951 rad. In order to decrease the absorbed dose, the shield was used and it made considerable changes. The absorbed dose was reduced to 275 rad. Calculation showed that approximately 7×1012 protons would pass through the Nd:YAG, during the mission period. The number of passing particles through the sensitive volume depends on particle flux rate, sensitive volume sizes, mission duration. This value for GCR particles was 4×108 particles in each square centimeter. Results related to simulation of inner Van Allen radiation belt particles effect on Nd:YAG showed that the absorbed doses from the inner belt electrons and protons were about 68 rad and 187.5 rad respectively while these values for GCR protons and alpha particles were about 11 rad and 8.5 rad respectively. These values were about 481 rad, 128.5 rad, 7.5 rad and 6 rad for laser diode respectively. The total values for Nd:YAG and laser diode reached 275 rad and 623 rad. It is estimated that absorbed doses for radiation sensitive parts will be increased about 15 percent and according to the threshold current, the optical power versus threshold current curve will be changed.