Nonlinear materials for optical limiting applications have drawn great attention because of its special features such as high linear transmission, strong nonlinear absorption and ultrafast response time. Optical limiting properties of two organic liquid crystal materials ( ILC and L34 ) are tested and analyzed in this paper. Firstly, Optical limiting test platform of ILC and L34 materials is set up by Nd:YAG laser with a pulse width of 5ns and the wavelength of 532nm. Then the transmittance of ILC and L34 materials irradiated by different laser energy density is measured. Finally, optical limiting mechanism of ILC and L34 materials is analyzed basing on two photon absorption (TPA) properties. The research results show that ILC and L34 materials have good nonlinear optical limiting properties to the 5ns, 532nm laser. The transmittance of ILC and L34 materials gradually drop with the increase of laser energy density. When laser energy density is less than 0.2J/cm2, the transmittance of ILC material is roughly 80%. While once laser energy density is more than 2 J/cm2, the transmittance decreases to 40%. In addition, nonlinear optical limiting property of L34 material is better than ILC material owing to high TPA properties of L34 material, and Optical limiting threshold of ILC material is higher than L34 material. Optical limiting threshold of ILC and L34 materials are 3J/cm2 and 1.4J/cm2 respectively. The conclusions have a reference value for laser protection on ILC and L34 materials.
In order to study deeply damage mechanism of HgCdTe crystal irradiated by multi-pulsed CO2 laser and obtain the
characteristics of surface morphological and chemical composition changes. Firstly, Irradiation effect experiment is
conducted on the Hg0.826Cd0.174Te crystal by pulsed CO2 laser, which has a pulse width of 200ns and repetition frequency
ranges from 1 Hz to 100 kHz. Then morphological and chemical composition changes of Hg0.826Cd0.174Te crystal is
measured by field emission scanning electron microscope (FESEM) and damage threshold is obtained by morphology
method. Finally, the impact of laser power density on morphological and chemical composition changes is analyzed. The
research results show that: damage threshold of Hg0.826Cd0.174Te crystal which is irradiated by multi-pulsed CO2 laser is
950 W/cm2. The crystal surface melting phenomenon is very obvious, the obvious crack which is caused by thermal
stress is not found in the surface, and a large number of bulges and pits are taken shape in the laser ablation zone.
Chemical composition changes of the crystal are obvious, and a lot of O element is found in the laser ablation zone. With
the increase of laser irradiation power, the content of Hg element decrease rapidly, the content of Cd, Te and O element
raise by degrees, and chemical composition changes of the crystal are more and more obvious. When the irradiation
power density is 1.8kW/cm2, the surface becomes smooth in the ablation zone due to the impact of laser impulse force,
and the content of the chemical compositions is that Hg accounts for 0.23%, Cd accounts for 21.38%, Te accounts for
26.27%, and O accounts for 52.12%. The conclusions of the study have a reference value for the Hg0.826Cd0.174Tecrystal
in the application of making infrared detector and pulsed CO2 laser in the aspect of laser processing.
High peak power picosecond laser ablation of silicon draws great attention in solar cell manufacture,laser optoelectric countermeasure applications, eta. This paper reports the damage process of ultrafast lasers interaction with silicon,which is based on Two-Temperature Model(TTM) and 1-on-1 damage threshold test method. Pulsed laser caused damage manifests in several ways, such as heat damage, mechanical effect and even eletrical effect. In this paper, a modified Two Temperature Model is applied in ultrashort laser interaction with silicon.The traditional Two-Temperature Model methods is proposed by Anismov in 1970s to calculate the interaction between ultrafast laser with metals, which is composed of free electrons and lattice. Beyond the carrier and lattice temperture model, an additional excited term and Auger recombination term of carriers is taken into account in this modified Two-Temperature Model model to reflect the characteristics in semicondutors. Under the same pulse-duration condition, the damage threshold is found to be 161 mJ/cm2 and a characteritic double-peak shape shows up. As the pulse energy density rises from 50mJ/cm2 to 161 mJ/cm2, the difference between carrier and lattice temperature steps down proportionally.Also,a detailed interaction process between photon-electron and electron-phonon is discussed. Electron and lattice temperature evolutes distinctly different, while the former is much higher than the latter until heat tranfer finished at 200 picoseconds. Two-peak feature of electron temperature is also identified. As the pulse duration increases from 20 picosecond to 60 picosecond, the he difference between carrier and lattice temperature steps down significantly. The calculated damage threshold does not change fundamentally, remaining approximately 0.16J/cm2. Also, the damage mechanism is found to be thermal heating with the pulse width between 20 and 60 picoseconds at threshold fluences which is identical to experiment test result. This research is valuable to laser applications and/or laser shielding applications.
The threat of the IR guidance missile is a direct consequence of extensive proliferation of the airborne IR countermeasure. The aim of a countermeasure system is to inject false information into a sensor system to create confusion. Many optical seekers have a single detector that is used to sense the position of its victim in its field of view. A seeker has a spinning reticle in the focal plane of the optical system that collects energy from the thermal scene and focuses it on to the detector. In this paper, the principle of the conical-scan FM reticle is analyzed. Then the effect that different amplitude or frequency modulated mid-infrared laser pulse acts on the reticle system is simulated. When the ratio of jamming energy to target radiation (repression) gradually increases, the azimuth error and the misalignment angle error become larger. The results show that simply increasing the intensity of the jamming light achieves little, but it increases the received signal strength of the FM reticle system ,so that the target will be more easily exposed. A slow variation of amplitude will warp the azimuth information received by the seeker, but the target can’t be completely out of the missile tracking. If the repression and the jamming frequency change at the same time, the jamming effects can be more obvious. When the jamming signal’s angular frequency is twice as large as the carrier frequency of the reticle system, the seeker will can’t receive an accurate signal and the jamming can be achieved. The jamming mechanism of the conical-scan FM IR seeker is described and it is helpful to the airborne IR countermeasure system.
Charged Coupled Devices (CCD) are widely used in military and security applications, such as airborne and ship based surveillance, satellite reconnaissance and so on. Homeland security requires effective means to negate these advanced overseeing systems. Researches show that CCD based EO systems can be significantly dazzled or even damaged by high-repetition rate pulsed lasers. Here, we report femto - second laser interaction with CCD camera, which is probable of great importance in future. Femto - second laser is quite fresh new lasers, which has unique characteristics, such as extremely short pulse width (1 fs = 10-15 s), extremely high peak power (1 TW = 1012W), and especially its unique features when interacting with matters. Researches in femto second laser interaction with materials (metals, dielectrics) clearly indicate non-thermal effect dominates the process, which is of vast difference from that of long pulses interaction with matters. Firstly, the damage threshold test are performed with femto second laser acting on the CCD camera. An 800nm, 500μJ, 100fs laser pulse is used to irradiate interline CCD solid-state image sensor in the experiment. In order to focus laser energy onto tiny CCD active cells, an optical system of F/5.6 is used. A Sony production CCDs are chose as typical targets. The damage threshold is evaluated with multiple test data. Point damage, line damage and full array damage were observed when the irradiated pulse energy continuously increase during the experiment. The point damage threshold is found 151.2 mJ/cm2.The line damage threshold is found 508.2 mJ/cm2.The full-array damage threshold is found to be 5.91 J/cm2. Although the phenomenon is almost the same as that of nano laser interaction with CCD, these damage thresholds are substantially lower than that of data obtained from nano second laser interaction with CCD. Then at the same time, the electric features after different degrees of damage are tested with electronic multi meter. The resistance values between clock signal lines are measured. Contrasting the resistance values of the CCD before and after damage, it is found that the resistances decrease significantly between the vertical transfer clock signal lines values. The same results are found between the vertical transfer clock signal line and the earth electrode (ground).At last, the damage position and the damage mechanism were analyzed with above results and SEM morphological experiments. The point damage results in the laser destroying material, which shows no macro electro influence. The line damage is quite different from that of point damage, which shows deeper material corroding effect. More importantly, short circuits are found between vertical clock lines. The full array damage is even more severe than that of line damage starring with SEM, while no obvious different electrical features than that of line damage are found. Further researches are anticipated in femto second laser caused CCD damage mechanism with more advanced tools. This research is valuable in EO countermeasure and/or laser shielding applications.