Airborne LiDAR has become a kind of indispensable measurement device in the current field of remote sensing. However, target extraction using traditional airborne LiDAR based on single-point scanning requires filtering and point cloud segmentation operations, which are complicated and time consuming. Although some researchers have studied streak tube imaging LiDAR (STIL) before, there are few reports in which it is used as an airborne LiDAR for ground measurement in large-scale field. We propose a method of ground target extraction using STIL. Taking advantage of the structural properties of the STIL, complex filtering and point cloud segmentation algorithms are avoided in the target extraction method. The purpose of this article is to verify the feasibility of airborne STIL in ground target extraction. We analyzed the raw streak signal image collected by field experiment and used morphology and intensity information to extract features. After that, we employed the decision tree classifier to classify the four kinds of targets and evaluated the extraction results. The results show that the target extraction achieved satisfactory consequences under an acceptable level. That demonstrates that ground target extraction using STIL is feasible in the field of large-scale remote sensing.
A laser diode end-pumped passively Q-switched YAG/Nd:YAG/Cr4+:YAG burst mode laser at 1064 nm is reported. The maximum burst energy of 13.2 mJ at a repetition rate of 100 Hz is got in a duration of 1 ms. The pulse repetition frequency increases linearly and peaks at 31.1 kHz in the pulse burst, while the pulse width keeps nearly constant around ~2 ns. The highest peak power of 373.7 kW is achieved at 10.2 kHz. The beam quality of passively Qswitched 1064 nm laser is also investigated.
The streak tube imaging LiDAR has promising application prospect due to the ability of full waveform sampling and high sensitivity. This kind of LiDAR generates massive point cloud data with high efficiency. However, the distribution of laser foot points is found usually irregular due to the scanning mode of this kind of LiDAR. This paper focuses on the interpolation of ideal points to realize uniform distribution of the point cloud using interpolation techniques, including nearest neighbor, arithmetic mean and inverse distance weighted interpolation. Specifically, we propose a new homogenization method in which the inverse distance weighted interpolation is improved. The suitability of homogenization methods for point cloud generated by streak tube imaging LiDAR is tested. The results show that the nearest neighbor method has better restoration of buildings with abrupt elevation values and inverse distance weighted interpolation outperforms other selected methods when processing flatland. It has been proven that the new method we propose possesses the advantages of both nearest neighbor and inverse distance weighted techniques.
An all-diode-pumped Nd:YAG burst-mode laser was demonstrated. A diode-laser-side-pumped Nd:YAG laser with Qswitcher, as laser oscillator, was employed to produce burst pulse directly. When the diode-laser worked at 10Hz and Qswitcher was operated at 10kHz, a maximum burst energy of 456mJ was obtained in the master oscillator with the highest optical efficiency of ~30%. Each burst included 19 pulses in the pumping duration of 2ms. A master oscillator and power amplifier architecture was adapted to scale the burst energy to meet PLIF system applications. The burst energy of 456mJ obtained from laser oscillator was amplified to 2.2J by use of three diode-laser-side-pumped Nd:YAG modules as laser amplifiers. The energy extraction efficiency of each stage was achieved to ~13%, ~22% and ~24%, respectively. The single pulse energy of 1064nm laser at 10 kHz reached to ~116 mJ with pulse width of 9.8 ns and a peak power of ~11.8 MW. The performances of pulse-burst laser we constructed can be better and improved greatly by use of more amplifiers.
Planar laser induced fluorescence (PLIF) diagnosis technology has been widely applied in flow field study and combustion diagnosis. Due to the complexity of the experimental environment of practical PLIF applications, The PLIF diagnostic system needs a good environment adaptation. In this paper, we reported a high energy Nd:YAG MOPA laser with repetition rate of 500Hz, which was applied in PLIF diagnostic system. A diode-laser side-pumped Nd:YAG module, which was pumped from five directions and optimized for better gain distribution, was employed to build a EO Q-switched Nd:YAG oscillator. A stable structure design of oscillator resonant cavity was used to improve the environmental adaptability of Nd:YAG laser. In our environmental adaptation experimentation, the laser oscillator has an energy fluctuation of <5% in the temperature range from 5°C to 45°C. In order to scale the pulse energy to meet the PLIF system requirements, we employed three 2500W diode-laser five-direction-side-pumped Nd:YAG modules as laser amplifiers to build MOPA system. Finally, the laser pulse energy of ~18mJ was amplified to 68mJ at 1064nm. Using KTP crystal as a frequency doubling crystal, we obtained a pulse energy of 35.6mJ at 532nm. The developed Nd:YAG laser has been used in our 500Hz-PLIF diagnostic system successfully.
Classification of pork soft tissues, including skin, fat, loin, tenderloin and ham muscles, was achieved using combination of laser-induced breakdown spectroscopy, principal component analysis and k nearest neighbors classification.
A high-efficiency, high-repetition-rate burst-mode 1064 nm laser under pulsed 878.6 nm laser diode pumping is demonstrated. Pulses at repetition rates ranging from 10 kHz to 100 kHz are produced during the time period of 1 ms pumping duration by using an acousto-optical Q-switch. The maximum pulse burst energy of 44 mJ at 10 kHz is obtained at the incident pump power of 108.5 mJ, yielding an optical-to-optical efficiency of 40.5%. The shortest pulse width at 10 kHz is 9.4 ns at the maximum pump energy of 108.5 mJ. The peak powers are estimated to be ~468.1 kW and 30.1 kW at 10 kHz and 100 kHz in the burst-mode oscillator.
Waveform sampling LiDAR is a hot topic in LiDAR technique due to its high precision geodesy and multi-layer target detection ability. Especially, the LiDAR systems applying streak tube have encouraging application due to their special properties about high-sensitivity and full waveform sampling ability. This paper describes a kind of LiDAR system applying the full waveform sampling stripe principle array. Basing on the planar fitting of square object, the elevation error of points cloud got from flight experiment is analyzed. The statistical properties of elevation error are got.
Streak tube imaging lidar (STIL) is an active imaging system that has a high depth resolution with the use of a pulsed laser transmitter and streak tube receiver to produce three-dimensional (3-D) range images. This work investigates the optimal signal width of the lidar system, which is helpful to improve the depth resolution based on the centroid algorithm. Theoretical analysis indicates that the signal width has a significant effect on the depth resolution and the optimal signal width can be determined for a given STIL system, which is verified by both the simulation and experimental results. An indoor experiment with a planar target was carried out to validate the relation that the range error decreases first and then increases with the signal width, resulting in an optimal signal width of 8.6 pixels. Finer 3-D range images of a cartoon model were acquired by using the optimal signal width and a minimum range error of 5.5 mm was achieved in a daylight environment.
We present a system to measure objective backscattering properties at 2.52 terahertz (THz). The optical setup combining 90° off-axis parabolic mirrors with 15° off-axis parabolic mirror decreases the size of the system and then realizes its compact structure. The calibration object, a conducting sphere with a diameter of 50 mm, was introduced to eliminate the influence of the instability of THz radiation and the background noise on measurement results. The lock-in amplifier was adopted to enhance the signal-to-noise ratio (SNR) and then make it possible to observe delicate backscattering behaviors on the surface of the object. Backscattering properties of four scale models were measured in this paper. Experimental results indicate that the maximal error of our system is less than 1 dB, paving the way for practical measurements of objective backscattering properties at THz frequencies.
Dye-doped distributed feedback (DFB) structure is an essential structure for DFB laser. This paper presents an operative method to design a kind of dye-doped distributed feedback laser based on dye-doped holographic polymer dispersed liquid crystal (HPDLC) matrix. The results show that we have processed DFB structure of 10μm period, with a relatively low period deviation of less than 1%. Furthermore, we gain output characteristics of DFB structure through experimental methods, which show good characteristics for wide tuning range, narrow linewidth laser output production.
Calculating radar cross section (RCS) of complex conductive targets is of great significance to design highly precise radar system, recognize targets and so on. This paper simulates complex objects with a periodic array of cylinders, and mainly focuses researches on the impacts on RCS by number, spacing and size of cylinders. The experimental results show that the biggest backward scattering RCSs of two-body to five-body cylinders were 0.0334sm, 0.0750sm, 0.1334sm and 0.2084 sm.
With the development of terahertz technology and increasing studies on terahertz target scattering properties, research on terahertz target scattering properties measurements attracts more and more attention. In this paper, to solve problems in the detection process, we design a controlling software for Continuous-Wave (CW) terahertz target scattering properties measurements. The software is designed and programmed based on LabVIEW. The software controls the whole system, involving the switch between the target and the calibration target, the rotation of target, collection, display and storage of the initial data and display, storage of the data after the calibration process. The experimental results show that the software can accomplish the expected requirement, enhance the speed of scattering properties measurements and reduce operation errors.
A high-repetition-rate, high-peak-power burst-mode laser for laser-based measurement applications is presented by using a master oscillator power amplifier structure. An laser diode arrays (LDA) side-pumped Nd:YAG acousto-optical (A-O) Q-switched laser serves as the master oscillator. Under pulsed pumping, pulse trains with 2-25 pulses are obtained when the repetition rate changes from 10 kHz to 100 kHz. The maximum pulse burst energy of 31.2 mJ is achieved in the A-O Q-switched pulse burst laser oscillator at 10 kHz. Two LDA side pumped Nd:YAG modules are employed in the amplification stage. After the amplification, the pulse burst energy at 10 kHz reaches ~170 mJ with a single pulse energy of 85.2 mJ and a pulse width of 14.5 ns, generating a peak power of 6.1 MW. At 100 kHz, the total burst energy reaches 220 mJ with a single pulse energy of 8.8 mJ in the pulse burst laser system.
Femtosecond time-resolved coherent anti-Stokes Raman spectroscopy is utilized to measure the premixed
methane/oxygen/nitrogen flame temperature at atmospheric-pressure. The procedure for fitting theoretical spectra to
experimental spectra is explained. The experimental results show good agreements with theoretical ones and present a
good repeatability. Laser parameters are very important for accurate temperature measurements. The effects of laser
parameters on temperature measurements are discussed. Laser parameters in our discussion are shown as follows. Laser
pulse shape is hyperbolic secant and Lorentz, respectively. The delay time between the pump and Stokes is from -40 fs to
+40 fs. The central wavelength of the pump/probe pulses is from 650 nm to 700 nm. Pulse duration is from 40 fs to 120
fs. In 2000 K, variations of delay time between the pump and Stokes pulses lead to less than 5% error and while
variations of the other three parameters lead to less than 1.5% error. Timing jitter is added to the pump/probe pulses and
Stokes pulses. In 2000 K, the results indicate that timing jitter of 10% lead to less than 2% error for temperature
measurements. In the higher temperature measurement, the impact of laser parameters’ error is greater.
We experimentally study high harmonic generation from elliptically polarized femtosecond laser pulses. The harmonic intensity as a function of laser ellipticity is measured. We show that the atomic ellipticity dependence is stronger than the molecular dependence.
A high temperature infrared spectra measuring equipment connected with a FTIR spectrometer (PE) was designed and manufactured. The measuring temperature can range from room-temperature to 500°C and the infrared spectra of substrates and thin films under different temperature can be real-time measured. The Fourier transform infrared transmission spectra of Si substrate under different working temperature were measured in the wavelength region from 2μm to 20μm using high temperature infrared spectra measuring equipment. The measured temperature ranged from room temperature to 500°C with a step of 50°C. Complex dielectric functions of Si substrate under different temperature condition are calculated from FTIR transmittance spectra by WVASE32 software, and the best fitted method was obtained for calculating optical constants of dielectric materials in the high temperature condition. As the increase of working temperature, the refractive index and extinction coefficient of Si substrate increase, when the working temperature reach 300°C, the various quantity of extinction coefficient sharply increase, so Si substrate can be used in the condition below the temperature of 300°C. Thus, through the exact calculated complex dielectric functions under different working temperature condition, we can design and manufacture different thin films using Si as substrate, and applied in in the high temperature condition.
In the calculation of the rough surface, one of the methods to establish the model is to view the rough surface as a plate with hemispherical bosses. Thus the calculation of a hemispherical boss becomes a part of the simulation of the rough surface. With the development of radar technology, the requirement of the radar cross section (RCS) becomes higher, which makes the measurement and calculation of the terahertz RCS become more important. However, due to the different energy distribution between the Gaussian beam and plane wave, the simulation results would exist some error when assuming that the incident beam is a plane wave. In this paper, the method of images is utilized to calculate the RCS of a hemispherical boss at 2.52 THz when the incident beam is Gaussian beam and the results are compared to that calculated when the incident is plane wave.
SiO2 is a very important low refractive index material and usually used in combination with high refractive index
material to manufacture the coatings with low optical loss. In this paper, SiO2 films were deposited on Si substrates by an
ion beam sputtering (IBS) technique and SiO2 target with purity of 99.99% was used as SiO2 thin films forming material.
The thickness of SiO2 films grown onto Si substrates are about 900 nm. Aging optical properties of SiO2 films were
investigated as a function of time placed in the air. Spectroscopic ellipsometry was used to measure optical constants of
SiO2 films. The refractive index of SiO2 films change with the increase of placed time. When the placed time reaches
more than 200 days, the value of refractive index tends to be constant and the corresponding variation rate is about 0.5%.
As the placed time increases, the physical thickness and optical thickness also drive to be stabilization. It can be seen that
optical properties of SiO2 films prepared by IBS technique under some process parameters are very stability.
Annealing is an important method to alter the properties of thin films. The effects of thermal treatment in air on optical
properties of SiO2 thin films were investigated. SiO2 thin films were deposited on Si (110) substrates by an ion beam
sputtering (IBS) technique, and then annealed in air under different thermal annealing time of 16 hours, 24 hours, 36
hours, 64 hours and the temperature from 100℃ to 600℃ with 24 hours. Optical properties refractive index and
thickness are studied directly after deposition and after thermal treatment, and they are measured by spectroscopic
ellipsometry. When the thermal annealing temperature was fixed at 300℃, the refractive index of SiO2 films would
reduce with the increase of the thermal annealing time, the optical thickness also reduced but the various quantities are
almost the same. The refractive index of SiO2 films changed with the different thermal annealing temperature. As the
annealing temperature increased, the refractive index of SiO2 films reduced gradually. When the selected annealing
temperature is 500℃, the refractive index of SiO2 films reached minimum. It can be found that the optical properties of
SiO2 thin films can be improved by an adapted annealing procedure.
The influence of pump power on the output characteristics of multiwavelength erbium-doped fiber laser, which employs
the effect of the intensity-dependent loss induced by a power-symmetric nonlinear optical loop mirror, is investigated.
The results of studies show that the multiwavelength output characteristics including the number, the flatness and the
spectral region of output wavelength can be controlled by adjusting the pump power. To optimis the multiwavelength
operation, a suitable pump power must be chosen for this kind of laser.
In the laser systems for measurement and so on, like gyro; in its laser cavity or path, it
makes use of the optical components which need very high performances, like low loss, low
scatter, low reflection, low absorption and high transmittance. In this article, we design and
manufacture the 632.8nm(He-Ne laser) low loss AR (LLAR) on crystal quartz, the results show
this LLAR have the total loss is about 30ppm and have high uniformity, the fluctuations of the
total loss mainly from the surface contamination and the inner defects of crystal material.
With the rapid development of micro optoelectronic device and integration techniques in PMMA fiber, micro-structure
integration in PMMA fiber such as diffraction gratings graved by focusing fslaser inside the PMMA fiber is becoming
popular. However, the white light induced by interaction of femtosecond pulse with PMMA has a property that is similar
to laser beam has been founded, and generated a continuous spectrum from ultraviolet to infrared. Ti:sapphire
femtosecond laser systems with 1kHz, 45fs, 1mJ were used to get the white continuum spectrum in PMMA . The white
continuum spectrum from 440nm to1100nm has been recorded. Also White light filament induced by fslaser at lower
thredhold power inside the PMMA than others , and this white light filaments will damage the PMMA fiber and induce
refractive-index change so as to affect micro optoelectronic device and integration techniques application in PMMA
fiber.
Degenerate four-wave mixing (DFWM) is a nonlinear optical process that has been developed as a
detective tool for making quantitative measurements of gas dynamic properties in the various environments.
This technique can be used to measure temperature and species concentration in both flames and plasma
environments. The resulting coherent signal beam makes DFWM particularly attractive for luminous and
harsh environments, compared to incoherent techniques, such as laser-induced fluorescence (LIF). Forward
DFWM with self-stability of spilt-beam system has been demonstrated in iodine vapor. It's found that there
exists no LIF because of collision quenching at atmospheric pressure and room temperature. But observed
vivid DFWM spectroscopy (554-556nm) of iodine vapor at 0oC and room temperature. Furthermore,
DFWM can probe non-fluorescing species. We describe a novel advanced sensor method for measuring
temperature of gas flows using DFWM. This technique without suffering of severe quenching problems at
atmospheric pressure is of importance to trace atom, molecular and radical in combustion diagnosis.
Nonlinear optical properties of optical materials are important factors. Degenerate four-wave
mixing (DFWM), as a detective tool for yielding effectivex (3) values, is imposed the condition of phase
matching on the incident and generated signal beams. Nowadays DFWM with forward geometry
(FDFWM) has found increasing usage. However, phase-match is automatically achieved in the back
phase-conjugated geometry but in the forward folded boxcars geometry. Also, the efficiency of DFWM
reaches its maximum value when all three input beams are of the same intensity with regardless of the
absorption coefficient and the detuning conditions. Further, weak signal beams under the strong
background of stray light are hardly positioned and distinguished. To solve the problems, a new
optoelectronic technique for detecting forward DFWM spectroscopy on iodine vapor has been performed.
With the help of the detecting system, phase matching can be easily achieved in the optical arrangement.
Finally Real-time detecting the rate of signal to noise so as to timely decrease the stray light with correct
methods. This system makes it feasible that the potential application of FG-DFWM is used as a diagnostic
tool in combustion research and environment monitoring.
KEYWORDS: Image processing, Signal generators, Signal detection, Stray light, CCD cameras, Environmental monitoring, Signal processing, Laser beam propagation, Signal to noise ratio, Phase matching
Degenerate four-wave mixing (DFWM) is a parametric process constrained by conversation of momentum of
the incident and reflected photons, which imposes the condition of phase matching on the incident and generated
signal beams. However, phase-match is not automatically achieved in the forward folded boxcars geometry. Also,
weak signal beams under the strong background of stray light are hardly positioned and distinguished. To solve the
problems, a new image processing system for detecting forward DFWM spectroscopy on iodine vapor is reported.
This system is composed of CCD camera, imaging processing card and the related software. With the help of the
detecting system, phase matching can be easily achieved in the optical arrangement by crossing the two pumps and
the probe as diagonals linking opposite corners of a rectangular box. The signal is generated in the center of the
box and propagates along the fourth diagonal, thus providing good spatial separation from the intense pump beams
and providing a way to position the PhotoMultiplier Tube (PMT). Also it is practical to know the effect of the
pointing stability on the optical path by monitoring facula changing with the laser beam pointing and disturbs of
the environment. Finally Real-time detecting the rate of signal to noise so as to timely decrease the stray light with
correct methods. Steady DFWM signals have been obtained in the experiment. This system makes it feasible that
the potential application of FG-DFWM is used as a diagnostic tool in combustion research and environment
monitoring.
Microstructures develop spontaneously on silicon surface under the cumulative short laser pulses irradiation in different ambient atmospheres. The experimental results suggest that the ambient atmospheres and the laser pulse duration play key roles on the microstructures formation. Only in SF6 ambient, the sharp conical spikes develop. Under the picosecond laser irradiation, silicon surface is melted before the spike arrays formed, while under the femtosecond laser irradiation, the formation of spike array does not pass through the liquid phase. The optical absorption increases remarkably from ultraviolet (~0.25 μm) to the infrared (~19μm) for the microstructured silicon material, which promises for new device applications, such as solar cells, infrared photo-detector.
In some applications of optical phase conjugation in laser beam propagation in the atmosphere, the method of active beam light is used, in which a probe light is transmitted to the target and the reflective light acts as the beacon light. Generally, it is believed that the reflective area on the target must be small enough to be regarded as an unresolved glint, and this limits the application. In this paper, the effect of the reflective area on the optical phase conjugation is discussed, and an experiment by means of nonlinear optical phase conjugation using a ruby laser, Stimulated Brillouin Scattering is an acetone device and a hole to change the reflective area is made. We demonstrate that, the wavefront of light can be restored correctly on the target. In the case of motionless target, the limitation of the reflective area must be satisfied, otherwise the amplified phase conjugated light on the target will return to the probe light source and damage it, whereas, in the case of the remote and high speed moving target, because the reflective place on the target is changed during the period of phase conjugation, the light is only reflected on the target, and so the area may be larger than the limitation.
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