Germanium is widely used as lens or windows in infrared optical systems, however, germanium optical elements may be damaged by melting under high energy laser irradiation. Therefore, it is necessary to carry out theoretical and experiment research on laser damage threshold of the germanium optical material. In this paper, the effect of laser beam diameter on the damage threshold of germanium was analyzed by numerical calculation. Besides, the difference of damage threshold represented using line power density and area power density was compared. It was found that when the diameter ratio of beam spot to sample was 0.02 to 0.09, the damage threshold decreased by 27.9% and 85.6%, respectively, when using W/cm and W/cm2 as the unit correspondingly. Considering the difference between the size of the beam and the element in optical system, the line power density is more suitable for extrapolation and comparison. In addition, the numerical results were verified by damage threshold experiment under the continuous laser of 1080 nm, which indicates the damage threshold of germanium is 263W/cm, 280W/cm and 290W/cm respectively, and the beam diameter is 0.5mm, 1mm and 2mm correspondingly. It was found that when the diameter ratio of beam spot to sample was 0.021 to 0.083, the damage threshold increased by 10.3% and decreased by 85.6%, respectively, when using W/cm and W/cm2 as the unit correspondingly. These results provide data support for the design and application of germanium optical elements to ensure the reliability of the high energy laser system.
As located in the focal plane of the imaging system, the image sensor will be easily influenced by the huge optical gain, which is brought in the image sensor by the external optical system and microlens on the surface of the device. Laser has a great influence on the image sensor, which is a sensitive link of the anti-laser reinforcement of the imaging system. Improving the performance in extreme light conditions in order to study the vulnerability of image has an important significance of reinforcement. As a typical visible light image sensor, which has advantages of sensitivity, high dynamic, small, light and so on, IT-CCD has been widely used in the fields of reconnaissance, detection and military. An 800nm femtosecond pulse laser was used to carry out experimental research on the laser irradiation effect of IT-CCD. The results shown that the local pixel of IT-CCD was in a state between undamaged and the white point damaged after irradiating by the laser, which was named by gray point. It was shown that the influenced pixels of IT CCD were changed by the laser, but no obvious deformation occurred. Through microscopic detection and analysis, the damage mechanism was expounded, further analysis was done. With focus ion beam (FIB) technique, it was found that there was photosensitive potential well, micro-structure of SiNx filling layer under microlens of the IT-CCD. When the gray point damage occurred, neither the photosensitive potential well at the bottom of the device was damaged, nor was the microlens structure on the surface. It turned out that the SiNx filling layer was influenced by the laser. Through elucidating the mechanism of this damage of the gray point, it lays a foundation for damage mechanism research.
We present a narrow linewidth frequency-doubled Cr:LiSAF laser with a 450- to 460-nm tunability and maximum repetition frequency (RF) of 63 kHz. Under a pump power of ∼900 mW, the fundamental wavelength could be tuned at the range of 883 to 1020 nm, with a maximum output power of 180 mW at 910 nm. The pulsed operation was achieved by using an acousto-optical modulator. An LBO crystal was adopted for intra-cavity frequency doubling and a maximum output power of 44.8 mW was obtained at 455 nm, indicating a slope efficiency of 11.2%. The spectral linewidth was <0.1 nm in the whole tuning range.
The effect of single junction GaAs solar cells irradiated by 808nm, 1070nm and 10.6um CW lasers is investigated. The results show that, as long as under the same laser coupling intensity, the damage modes of solar cells under different irradiation conditions are similar. With the increase of laser coupling intensity, the maximum temperature of solar cells rises, and the maximum power of solar cells shows a ‘stair-step’ decline. The multiple irradiation experiments of triple junction GaAs solar cells by 1070nm CW laser are carried out. The results show that when the laser intensity is more than 12.8W/cm2 , the performance degradation of solar cells will show a significant accumulation effect. In addition, the thermal sensitive damage factors are explored and verified. The results show that the maximum temperature and the duration of high temperature are sensitive factors for laser irradiation damage of solar cells.
A 3D mathematical model was established for the investigation of the thermomechanical behavior of aluminum alloys (Al-7075) under the combined action of tensile loading and laser irradiations. The transient temperature fields and stress-strain field was obtained by using the finite element method. The Johnson-Cook’s constitutive equation is implemented in the FE model to study the failure behaviour of alloy. The effects of various pre-loading and laser power densities on the failure time, temperature distribution and the deformation behavior of aluminum alloys are analyzed. The results indicate the significant reduction in failure time for higher laser power densities and for high preloading values, which implies that preloading may contribute a significant role in the failure of the material at elevated temperature. The numerical result agrees well with our previous experiment results, concluding that the numerical model is reasonable.
In this paper, a series of effects of CCD interaction with laser are taken together in consideration. These effects divide the light intensity axis into three sections named respectively as ‘normal’, ‘dazzle’ and ‘damage’, along the positive direction. For the effects on first two sections, a general model is proposed to describe them, which reflect the performance jump charateristics of CCD under laser irradiation. In fact, the model contains the jump functions of three performance parameters, which are response efficiency, charge transfer inefficiency and leakage current. Thereinto, the first is used to describe the pixel itself, and the remaining two are used to describe the influence between pixels. The leakage current parameters include a variety of situations, such as the leakage current between pixels, the leakage current between channels and even the leakage current between subarrays in a large array. When all three kinds of parameters don’t jump, the CCD works normal. When anyone of them jumps, the CCD is dazzled by light. Of course, the parameter jump in a dazzled CCD can return to normal when light intensity decreases. However, the damage section on light intensity axis is temporarily not described in this paper. After all, the damaged CCD is not a CCD again.
In order to study the mechanisms of thermal damage during laser machining in GaInP/GaAs/Ge tandem solar cells (TSCs), the spatial electroluminescence (EL) characterization on sub-cells pre and post laser irradiation was carried out. Results showed that post laser irradiation, the EL of GaAs middle cell increased to saturation in the damage zone, but decreased to zero at the rest part. A theory was put up to explain this phenomenon by using two-unit equivalent circuit model, and then verified through GaInP top cell spatial EL analysis. Conclusion was drawn that current redistribution induced by local shunt resistance decreasing in GaInP top cell was the main cause for the EL enhancement in GaAs middle cell.
The CCD is widely used to detect laser signal in many industrial vision and automation systems. When the charge coupled device(CCD) is irradiated by intense laser, the image quality may decrease and reversible dazzling effects occur such as single pixel saturation, crosstalk, and full saturation. Understanding the laser dazzling phenomena, on the one hand, it can help to choose the suitable laser sources for the measuring systems, on the other hand, it can optimize the designs of the CCD structures. In order to effectively utilize the dazzling effects, the laser saturation threshold of the CCD must be known. Due to the different internal structure and working mode of CCD, the corresponding laser saturation threshold is also different. In this paper, the dazzling effects on the array CCD induced by a 532nm wavelength pulsed laser is investigated by finite element method. A physical model is established base on drift-diffusion equation according to the working process of CCD and the principle of laser dazzling effects. The working state of CCD under different laser power, charge density, electron concentration and potential curve are presented. This model is found to be useful for the analysis of laser saturation threshold.
Optical components in large aperture optical imaging system are susceptive to intense laser radiation. Optical band pass filter for spectra selection and nonlinear filter for intense laser blocking, are usually focal or near focal elements in imaging system, will more likely to be permanently damaged by high intense unwanted laser irradiating. In order to relate the optical imaging performance degradation to laser irradiating parameters, characteristics of the damage sites and so on, a test facility consisting of scaled electro-optical imaging system and standard imager evaluation system was constructed, and the linear spread function, modulation transfer function, signal transfer function, and noise power spectrum were obtained. Experimental imaging performance degradation law of damaged fused silica and colored glass filters were analyzed. Furthermore, some brief discussions on the imaging performance degradation mechanisms are also presented.
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+-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/cm2 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.
Three types of laser irradiating experiments on single junction GaAs solar cells with the same laser energy coupling intensity were carried out, which were irradiated by in-band (808 nm) and out-of-band (1.07 μm) continuous wave lasers respectively and simultaneously. On the basis of the changes of current-voltage characteristic curves of irradiated solar cells, the damage degrees could be divided into three stages which were gently, seriously and thoroughly damaged stages. The damage mechanism was studied from two aspects: output changes of solar cell equivalent circuit under different configuration settings, thermal analysis model. The results show that damage degrees of gently and thoroughly damaged stages is insensitive to irradiation intensity. However, the damage degree of seriously damaged stage is sensitive to irradiation intensity and this is regarded to be related to thermal decomposition of GaAs. Moreover, the increase of PN junction defects leads to performance degradation of irradiated solar cells. In conclusion, the thermal damage leads to the increase of PN junction defects, thus results in the performance degradation of cells.
Besides the excessive saturation effect, a new mechanism of temporary black screen of CCD camera induced by laser is found. The final output of CCD camera is the difference between the signals in effective pixels and optical black pixels. As the effective pixels of CCD camera are being irradiated by intense laser, many carriers induced by photo overflow from effective pixels into the optical black pixels. As a result, both the effective and optical black pixels will reach saturation, which makes the final output of CCD become zero. This effect has the same black screen phenomenon as the excessive saturation, and is named as pseudo-excessive saturation effect in this article.
KEYWORDS: Sensors, Detector arrays, Laser applications, Optical testing, Signal attenuation, Signal detection, Pulsed laser operation, Mid-IR, Temperature metrology, Laser energy
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.
In this article, an overview of laser dazzling effect to buried channel CCD camera is given. The CCDs are sorted into staring and scanning types. The former includes the frame transfer and interline transfer types. The latter includes linear and time delay integration types. All CCDs must perform four primary tasks in generating an image, which are called charge generation, charge collection, charge transfer and charge measurement. In camera, the lenses are needed to input the optical signal to the CCD sensors, in which the techniques for erasing stray light are used. And the electron circuits are needed to process the output signal of CCD, in which many electronic techniques are used. The dazzling effects are the conjunct result of light distribution distortion and charge distribution distortion, which respectively derive from the lens and the sensor. Strictly speaking, in lens, the light distribution is not distorted. In general, the lens are so well designed and fabricated that its stray light can be neglected. But the laser is of much enough intensity to make its stray light obvious. In CCD image sensors, laser can induce a so large electrons generation. Charges transfer inefficiency and charges blooming will cause the distortion of the charge distribution. Commonly, the largest signal outputted from CCD sensor is restricted by capability of the collection well of CCD, and can’t go beyond the dynamic range for the subsequent electron circuits maintaining normal work. So the signal is not distorted in the post-processing circuits. But some techniques in the circuit can make some dazzling effects present different phenomenon in final image.
A special waveform of CCD being irradiated by intense laser is explained and simulated. Its specialty is that reference level is altered and becomes equal with saturated data level, which can answer for CCD’s black video induced by laser and named as excessive saturation effect. Alteration of reference level has been explained by signal charges injection into the measuring well during reference time. In CCD, wells barriers are largely lower than channel stop. After that all transfer wells are crammed, many remained signal charges getting rid of clock’s control can be hold in channel, and move along it in thermal diffusion and self-induced drift. They can fill up the measuring well ahead of clock’s permission and alter reference level to saturated data level. Based on the explanation, the waveform is simulated on an equivalent circuit of CCD’s charge measurement structure, which is built on the platform of Multisim2001. The voltage sources and switches are used to manipulate the charge and discharge of a capacitor, which simulates the charge injection and resetting of measuring well. The clocks controlling switches represent the injection and reset clocks in CCD. To simulate clock’s impact on output, other capacitor is used to connect it to capacitor that represents the measuring well. The equivalent circuit is validated by the simulated normal waveform. Then, altering the clock and charging the capacitor ahead, the excessive saturation waveform is simulated, which validates the explanation to excessive saturation effect.
Two methods were described to quantitatively evaluate the damage of optical filters, which were through detecting the change of transmission coefficient and damaged area of optical filters. Based on the quantitative evaluation results, the laser-induced damage of optical filters was classified five damage degrees, which were undamaged, color changed, slight melt, middle melt and serious melt. The laser-induced damage was uncertain event because there were many uncertain factors to affect the laser-induced damage degree of optical filters. In view of the advantage of the Bayesian network in processing indefinite information, this paper emphatically studied the laser-induced damage degree assessment method of optical filters based on Static Bayesian network. A Bayesian model was constructed to assess the damage degree of filters. Upon our laser-induced damage experiments on the optical filters, the results of the quantitative evaluation were compared with the assessment results of Bayesian network model, which indicated that Bayesian network method was available to assess the laser-induced damage degree of optical filters.
The laser coupling effect of material is a fundamental factor to influence laser interaction with matter. The coupling coefficient, which is the material absorptance of the input laser energy, depends on the surface conditions of materials, such as temperature, incident angle, surface airflow, oxidizing environment, and so on. To measure the laser coupling characteristics of materials, two typical online measuring apparatuses were developed in our laboratory. One is based on a conjugated hemi-ellipsoidal reflectometer, which is suitable to measure the laser coupling coefficients of different temperature in vacuum and air environments. The other is based on an integrating sphere and a simple airflow simulator, which can be applied to online measure the laser absorptance of materials subjected to surface airflow. The laser coupling effects on two types of structural materials, which are alloy steels and composite materials, are given in this paper. With the conjugated ellipsoidal reflectometer, the laser coupling effects on a typical alloy steel are investigated in different temperatures under the vacuum and air environment, and the experimental results are analyzed. According the results, metal oxidization plays a key role in the laser coupling enhancement effects. Especially, when the metal is subjected to high power laser irradiation in the high subsonic airflow, metal oxidization which is an exothermic reaction enhances the laser damage effect and the convective heat loss is negligible. Finally, the laser coupling effects on a typical composite material subjected to airflow are studied by using the integrating sphere with an airflow simulator, and the experimental results of laser absorptance during the laser ablation are presented.
Detector is an important device for the far-field laser spot measuring apparatus in form of photoelectrical detector array,
for it acts as an optical-to-electrical converter in measure. Two working parameters of n-type HgCdTe photoconductor
are discussed in this paper. The fundamental electrical properties of n-type Hg1-xCdxTe material are summarized and
related to device performance parameters. It can be found that the dark resistance Rd and the voltage responsivity Rv are
closely bound up with temperature T and the alloy composition x, and the normalized calculating Rd-T and Rv-T
characteristic curves are in good agreement with experimental results at temperature below 20°C. And then the dynamic
responses of a detector under laser irradiation are studied by utilizing 2-D transient heat transfer model and empirical
formulas. Furthermore, experimental investigation on laser damage in PC-type HgCdTe devices is operated by a means
named 1on1. Detectable change in performance parameters has not been found under the irradiation of in-band laser, at
power density beyond the detector linear response zone, and time of 200s. When the power of irradiation strengthened,
the dark resistance increased, and the responsivity reduced. By observing the surface morphology of HgCdTe wafers,
calculating the compositions x from Rd-T characteristic, the causes for performance changing has been analyzed.
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