Kilowatts class diode-pumped Cs vapor laser (DPCL) has been realized and this kind of lasers have military applications potentially for its high output power with high efficiency. Pumped by a fiber coupled laser diode, the key operating parameters of a DPCL are studied, including the spot size of focused pumping light, pressure ratio of buffer gases, vapor cell length, temperature of Cs vapor and reflectivity of output coupler. The spot size is properly chosen in the consideration of both the intensity scalability and mode matching. Pressure ratio is optimized under a modest pressure of mixed gases of helium and ethane. Under the optimized pressure ratio, the Cs vapor can absorb the pumping energy and convert it into laser energy efficiently. Besides, the temperature and reflectivity are also optimized to operate the DPCL in optimum state. The results have significant instructions for the experimental design of DPCL.
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.
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.
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.
The modified Bridgman method with heat field rotation was used to grow ε-polytype single crystals of pure and 1, 2 and 10 mass % S-doped GaSe or solid solution crystals GaSe1-xSx, x = 0.002, 0.091, 0.412. The interaction of ultrashort laser pulses of ∼ 100 fs duration at 800 nm and 2 μm with the grown crystals was studied at room temperature. Up to 3.4-fold advantage of S-doped crystals in limit pump intensity (no decrease in the transmission) was found under 800 nm pump at S-content increase up to 10 mass %. The advantage became a half less at 2 μm pump due to a decrease of two-photon absorption in pure GaSe crystals. The spectral dependence of transient absorption is recorded with 37 fs resolution and interpreted. It was ascertained that first observable damage of high quality crystals is caused by dissociation of submicrometer thick surface layer to initial elements and do not influence the frequency conversion efficiency until alloying of dissociated Ga. Local microdefects, multiphoton absorption and transient transmission processes are identified as key factors responsible for damage threshold.
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.
Damage threshold of non-linear GaSe crystals under IR fs (Ti:Sapphiere 800 nm laser and 1.1-2.9μm OPG) and ns (2. 79
Er3+:YSGG and 10.6μm CO2 laser) pulse pumping is studded in details. Local micro defects and field induced effects (GaSe dissociation, multiphoton absorptions and transient transparency origin effects) are identified as responsible for damage threshold in this case. Local (including nano scaled) defects and thermal effects are identified as reason of damage threshold under ns pulse pumping.
In order to lead the laser beam transmit in the atmosphere convergently, an experiment of laser focus at the distance of
450m and 300m has been operated in the outdoor place. The actual manipulations are as follows: Firstly, the laser was
collimated by a beam expander, then the near-parallel laser beam was transmitted with a Galileo telescope system, and
the distance between the concave lens and the convex lens can be tuned through a precise displacement platform, so the
focus of the system changed due to the tiny displacement of the concave lens. Secondly, the average power of the laser
spot can be measured using power meter, the power is 47.67mW and the standard deviation is 0.67mW while the focal
length is 450m. Thirdly, the energy distribution was found through the laser beam analyzer. The spot images were saved
using the beam analyzer, then the saved image can be processed with Matlab software afterwards. The function named
EDGE and Sobel operator was used in the pre-processing of the saved image, then method of median filter was used in
the course of image de-noising and 53H filter was adopted in the signal analysis. The diameter of laser spot was obtained
by the method above, the diameter is 5.56mm and the standard deviation is 0.24mm. The spot center excursion is
0.56mm, it is 10.43% of the total diameter of the laser spot. At last, the key factors of the energy dissipation in the
focusing system can be summarized as follows: restriction of the diffraction limit, attenuation in the atmosphere,
geometrical aberration of optical system, and the diffraction limit and the geometrical aberration are significant in the
three factors above, so we can reduce the impact of the both factors during the design of optical system. The reliable
referenced data of the system design can be acquired through the primary experiment research.
Two-axis optoelectronic tracking equipments generally use such mode that azimuth and pitch system are independently
controlled. It leads to the two structures completely uncoupled, and reduces the state space dimension. But the main
problem is that the rotational inertia of azimuth is much larger than that of pitch, which leads its dynamic tracking
performance is worse. So to find a more suitable mechanics and more effective control strategy is necessary. Considering
the tracking ability, this paper designs BMC (bandwidth mutual compensation) to uniformly correct the error of both the
azimuth and pitch system in real time, it is realized by cross coupling control to the azimuth and pitch. The simulation
result verified that compared with classical uncoupling system BMS not only guarantees the state space dimension but
improves systematic tracking performance about 2" therefore, it is an effective control mode to the two-axis photoelectric
Based on the analysis of principle of tracking and aiming system, some important factors to design the structure of
tracking-aiming system and the layout of optical system are discussed. Besides, the paper gives the present developing
situation of fast-steering mirror at home and abroad, analyzes the advantages and disadvantages of FSM with axis, and
presents a novel design of flexible axis FSM. The main axis of composite axis system is tracked by motor to drive the
frame, and the sub-axis is tracked by voice coil motor (VCM) to drive FSM. The structure of FSM and designing
principle of VCM are introduced, and the emulation analyses of inherent frequency and deformation under load of the
FSM with software COSMOS are also given.
The work on laser varicose treatment carried out in CIOMP, CAS cooperating with The First Clinical Hospital, Jilin
University is summarized. Dozens of animal experiments adopting dog and rabbit samples are made in a long time of
several years. Different lasers are used, including long pulse frequency-doubled Nd:YAG(532nm) and semiconductor
laser(808nm). Dozens of animal experiments show that laser has good efficacy to occlude the vein vessels. It has precise
adjustability and relatively short treatment time only needing outpatient office setting with high cost and effect rate; It
provides minimal invasion, often under local anesthesia and intravenous sedation thereby eliminating the need for
general anesthesia, greatly shortens postoperative recovery term, and it is highly safe with no side effects and no serious
The synchronization control technique is one of the most key techniques in the laser range gated (LRG) imaging system. Conventional control circuits are composed of discrete components and Medium Scale Integration (MSI) and Small Scale Integration (SSI), which have bad reliability and poor ability of anti-interference.
The idea of using the Complex Programmable Logic Device (CPLD) to realize the monostable trigger circuit is introduced. On the basis of analyzing the performance requirement to the synchronization control of the laser range gated imaging system, a new synchronization control circuit based on CPLD is presented. The circuit based on CPLD is very compact and flexible and has high reliability and strong ability of anti-interference. The synchronization control circuit based on CPLD can provide a wide selection of gate widths, pulse widths and delays. The synchronization control circuit is successfully used in our experiment and the expected result is obtained.
Laser treatment represents an attractive option to other methods of vessel diseases especially varicose veins. A long pulse (30~50ms) 532nm laser (Fig.1) is used in our experiments with the pulse duration matching the thermal relaxation time of the vessels and the green laser matching the absorption spectrum peak of the blood. Laser irradiates nude vein vessels directly or exterior skin to finish operation faster and to acquire the practical data for upper enteron varicose vein treatment in several animal experiments performed in vivo. The 5J-energy pulse allows us to finely occlude rabbit or dog’s vein vessels up to 2 mm in diameter when irradiating them off external skin (Fig.2). Blood vessels are occluded at once and later biopsy specimens show the immediate and long-term lasting occlusion effect. While irradiating vessels directly (Fig.3), the vessels are usually irradiated to perforate, detailed causes are still under investigation. Animal experiments show long pulse green laser therapy is a safe and effective solution to the vein’s occlusion, which promises such laser with high energy of each pulse and 30~50 ms duration is an ideal candidate for vessel diseases treatment.
It is important to study the interaction effect between laser and photoelectric detector. In most of the laser detecting systems, laser must irradiate the photoelectric detector. The detector will be saturated or damaged easily if the fluence of the laser is higher than the saturation threshold of the detector. Now, many photoelectric detectors are used in space. To achieve their interaction, "Cat’s eye" effect is applied to find and locate the remote aerial detector by laser. The possibility and mechanism of disturbance to remote aerial detector induced by laser are analyzed, and the factors of influence are also analyzed. The fluence of laser on the photoactive area of remote aerial detector is estimated. With two samples of CCD and TV tracking system, the mechanism of soft damage to CCD and TV tracking system induced by laser are introduced. The saturation of CCD and crosstalk of CCD are the main reasons of disturbance. When the tracking system is used in wave-door tracking mode, it will lose its tracking target if the saturation or crosstalk of CCD can enter into wave-door. At last, the influence to disturbance induced by atmosphere effect is also analyzed.
Concerning the disturbances at low speed, such as friction at zero crossings and the moment fluctuation of the motor, we explore the repeat control method rooted in study law to compensate the low-speed-disturbances, and the system's properties at low speed are improved greatly. Here we present our works. Section 1 is the introduction. The friction and moment fluctuation of the motor at low speed for the optoelectronic system are tested out which are shown in section 2. We introduce a repeat controller to the system to compensate the low-speed disturbances in section 3. Concerning the stability, a filter is added to the repeat controller. The experimental results are discussed in section 4. Not only the repeat control can overcome some low-speed-disturbances, but is more useful when tracking a periodic input signal.
A detailed comparison between conventional image intensifiers and electron bombarded CCD (EBCCD) is conducted. These sensors' advantages and drawbacks are analyzed. EBCCD with transferred electron (TE) photocathode and its application in Laser Illuminated Viewing and Ranging (LIVAR) system are shown in details. The TE Photocathode (TEP) developed by Intevac in 1996 has a demonstrated quantum efficiency of 20% or higher over the spectral range between 0.95 and 1.7 μm. The TEP is coupled directly with a CCD chip in an electron bombarded CCD (EBCCD) configuration. The overall noise figure of the EBCCD camera is close to one, approximately half that of a standard Generation-III image intensifier. The EBCCD eliminates the micro-channel plate (MCP), phosphor screen, and fiber optics, and as a result both improved image quality and increased sensitivity can be obtained in a smaller sized camera. Because of the reduction in the number of image conversions and the significantly greater signal-to-noise performance, an EBCCD has higher contrast and resolution than an ICCD. These characteristics make the EBCCD a good candidate for military, helmet-mounted night-vision systems as well as for covert surveillance applications.
At low speed or zero crossings, the existed nonlinear effects such as friction, the fluctuation of the motor moment will make the system jitter heavily. Our researches focus on the analysis and compensation of the motor moment fluctuation when the optoelectronic tracking system works at low speed. Actually, other nonlinear factors at low speed are also checked. As the fluctuation is always presented in sine orderliness, a repeat controller that bases on the study law is applied to the tracking system. Both simulations and experiments show that with the repeat controller, the tracking precision increase about 3 times comparing to the PID one.
A stable adaptive controller is reported here for a class of system with state dependent parasitic effects such as friction. The methodology is constructive, and ensures stable and convergent performance. The control design is applicable to a class of dynamic systems working at low speed with the cited parasitic effects. The design is verified by simulation and a hardware example case.