| A CCD detector and an optical filter together make up the core module of animaging system. The filter selectively blocks some laser wavelengths to improve theanti-laser-damage ability of the system. In this paper, the damage effects investigationof the module induced by nanosecond, picosecond and femtosecond pulsed laser werecarried out, and some new results were obtained.1. A comparison study of the damage effects of a Sony ICX405AL visible lightCCD induced by nanosecond (1064nm,33ns), picosecond (1064nm,25ps) andfemtosecond (800nm,100fs) single laser pulse was carried out. The investigationmethods and results are listed below:1) Whether exposed to nanosecond, picosecond or femtosecond single laser pulse,the damage processes of the CCD samples all showed the fact that white point damage,white line damage and system failure arose successively in the output pictures with theincrease of laser energy. Under the irradiation of nanosecond single pulse, the thresholdsof these three different damage degrees were150,170and222mJ/cm2, respectively. Inpicosecond regime, the thresholds were15,40and1040mJ/cm2, respectively, in whichthe threshold of failure damage increased sharply. In terms of femtosecond laserillumination, the white point damage threshold was only2.5mJ/cm2, and the white linedamage threshold rose rapidly to216mJ/cm2, finally, even the laser energy density weretuned up to1.96J/cm2, the CCD still could not be failed completely.2) The damage morphologies of the CCD indicated that the ablation of thematerials started from the inside of the device, and expanded gradually to the surface. Atthe beginning of the white point damage, there was not any obvious damage could beobserved on the semiconductor surface. The onset of white line damage was in companywith the ablation depth of4-5μm, and the polysilicon electrode and tungsten lightshield were not damaged. When the device was failed, the ablation depth deepened toabout10μm, and, in the center of damaged area, the structures such as electrodes andlight shield were ablated.3) From the output pictures, damage morphologies and working principles of thedevice, we deduced that the occurrence of the white point was caused by the increasedleakage current as a result of the oxide layer breakdown. The breakdown theory oftransistor oxide layer in the field of integrated circuits was introduced into this paper toinvestigate the white point damage mechanism, in which the laser irradiation effectswere taken into account simultaneously. When the semiconductor and the oxide layerwere all damaged to some extent, numerous of leakage charges entered into the verticaltransferring pathway of the charge, then white line appeared in the pictures. Finally,CCD failed as soon as the MOS capacitors were breakdown. 4) From the aspect of nonlinear absorption of intense laser by Silicon materials, atthe point of failure damage, the peak power of picosecond and femtosecond laser pulsesall exceeded a magnitude of1010W/cm2. In this situation, nonlinear absorption in thematerial was much more significant than linear absorption. As a result, the inducedabsorption depth dramatically reduced, and the ablation depth in the material reducedcorrespondingly. However, the morphology of the damaged CCD indicated that acertain ablation depth was required to fail such device. Therefore, the observed sharpincrease of failure damage thresholds under the irradiation of picosecond andfemtosecond laser pulses should be ascribed to the strong absorption nature thatsuppressed the expansion of ablation depth.2. The coating damage behaviors induced by single laser pulse in nanosecond(1064nm,10ns), picosecond (1064nm,25ps), and femtosecond (800nm,100fs) wereinvestigated. The customized samples contained reflectors, anti-reflectors and visiblelight filters. For the visible light filter, the damage experiment was also carried out usinga wavelength tunable, high repetition rate (80MHz), femtosecond (150fs) pulsed laser.Some obtained disciplinary conclusions are listed as follows:1) The damage behaviors of optical coatings were investigated from the simulationof light field distribution, comprising interference field and light field constructed bydefects. Spectrum broadening appears in femtosecond laser pulses. A method of planewave expansion was used to calculate the interference field in femtosecond regime. Theobtained light field was time dependent and was different from the case ofmonochromatic light incidence. The light fields constructed by defects were modeledconsisting of nodular defects and nanoinclusion defects. The results demonstrated that anodular defect distorted the interference field in a wider spatial range, and that, as fornanoinclusion defects, even in tens of nanometers scale, the local light intensificationshould not be neglected, especially when their absorption coefficients were relativelyhigher.2) The damage behaviors of a coating induced by a shorter laser pulse were moredeterministic, appearing a narrower indirect damaged area and smaller fluctuation ofdamage thresholds. For the same coating, the damage threshold of picosecond laser wasan order of magnitude smaller than of the nanosecond laser, and was in the same levelof that induced by femtosecond pulses.3) At the point of threshold, coating damage was defect induced, whether the pulselengths were in nanosecond, picosecond and even in femtosecond time scales. In thedamaged area of picosecond pulses, damage pits were smaller and shallower than thatbrought about by nanosecond pulses, which was interpreted by means of heat diffusionlength and the simulation of temperature field. When illuminated by femtosecond pulses,local light field intensification caused by defects affected the nonlinear excitation ofmaterials, consequently the laser induced damage behaviors. When the energy density on sample surface was much higher than the threshold, we observed large area ofcoating ablation or peeling off. Under this condition, the effect of interference fieldintensification was stronger than that of defects, and the relevant damage mechanismswere investigated by the calculation of temperature field and plasma density evolution.4) Coating damage induced by single femtosecond pulse is dominant by plasmabreakdown, however, the damage behaviors under the irradiation of wavelength tunable,high repetition rate, femtosecond laser pulses demonstrated that the coating damageprocesses comprised simultaneously the properties of thermal diffusion and the intrinsicionization of coating materials when irradiated by ultrashort laser pulses. Theestablished model explained the trend of thresholds under different wavelengths, whichwas according to the experiment results.5) A phenomenon of coating transmission increase was observed in the experimentof high repetition femtosecond laser pulses damaging the visible light filter, and themain factor should be thermal expansion effects of the coating layers. This wasexperimentally validated by using a1080nm continuous fiber laser to irradiate somecoatings and the theoretical analysis fit in the experiment results.3. The laser irradiation effects of the module system were investigated includingthe cases light filter blocking the incident laser and only decaying the laser, respectively.In the first situation, the CCD could be saturated to some extent only when thefilter was damaged and then the damaged pits scattered the incident laser. As for thesecond case, the saturation area of CCD was derived from the theory of light diffraction.A relatively complete equation was obtained to express the CCD saturation area. Thenthe experiment of CCD saturation effects was accomplished by using a super continuousspectrum source in company with the methods of prismatic decomposition and smallhole filtering. The results showed that, the wavelength that caused the biggest CCDsaturation area was not the wavelength that induced the biggest responsivity of CCDdetectors, but shifted to the direction of longer wavelengths. This was explained by theresults of theory derivations. |
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