In this study we propose and demonstrate a novel technique for measuring distance with high definition three-dimensional imaging. To meet the stringent requirements of various missions, spatial resolution and range precision are important properties for flash LIDAR systems. The proposed LIDAR system employs a polarization modulator and a CCD. When a laser pulse is emitted from the laser, it triggers the polarization modulator. The laser pulse is scattered by the target and is reflected back to the LIDAR system while the polarization modulator is rotating. Its polarization state is a function of time. The laser-return pulse passes through the polarization modulator in a certain polarization state, and the polarization state is calculated using the intensities of the laser pulses measured by the CCD. Because the function of the time and the polarization state is already known, the polarization state can be converted to time-of-flight. By adopting a polarization modulator and a CCD and only measuring the energy of a laser pulse to obtain range, a high resolution three-dimensional image can be acquired by the proposed three-dimensional imaging LIDAR system. Since this system only measures the energy of the laser pulse, a high bandwidth detector and a high resolution TDC are not required for high range precision. The proposed method is expected to be an alternative method for many three-dimensional imaging LIDAR system applications that require high resolution.
In KAIST, Kumgang laser is being developed for demonstration of the kW level coherent beam combination using
stimulated Brillouin scattering phase conjugation mirrors. It will combine 4 modules of DPSSL rod amplifier which
produces 1 kW output power. It is composed of the single frequency front-end, pre-amplifier module, and main amplifier.
The output powers of the pre-amp and main amplifier module are 200 W (20 mJ @ 10 kHz / 10 ns) and 1.07kW (107 mJ
@ 10 kHz / 10 ns), respectively.
Optical design of an image magnifier based on a lens array is presented. The proposed image magnifier does not have a limit for object size due to off-axis aberration because the object is divided into segments and is kept apart from the nearest neighbor segment with the period of the lens array. Several images can be sequentially generated by magnifying sequentially specific objects among mosaic objects, which are composed of multi-images. The designed lens for the image magnifier corresponds to a lens array with an aperture of 2 mm and a magnification ratio of 3. Furthermore, an application for a display system of the designed image magnifier is also presented.
A refractive beam shaper is designed, which transforms a Gaussian beam profile into a diverging uniform line beam profile, exactly, an elongated super-Gaussian profile. The advantage of our optical system is that the area of uniform illumination can be changed by simply shifting the position of the observation plane without using an additional optical element. Whereas previous refractive beam shapers have been designed to have a specific intensity distribution at a certain position, our refractive beam shaper has been designed to generate a desired intensity and wavefront simultaneously, so that it gives a desired beam profile during propagation. The designed refractive beam shaper generates a uniform line beam with 4 mm beam width at half maximum intensity and a diverging angle of 13.3 deg. Furthermore, we have checked the utility and the stability of the output beam by calculating the changes in the size, the uniformity, and the efficiency of the line beam when it propagates a distance of 960 mm.
This paper describes about the new design method for hyper-spectral Imaging spectrometers utilizing convex grating. Hyper-spectral imaging systems are power tools in the field of remote sensing. HSI systems collect at least 100 spectral bands of 10~20 nm width. Because the spectral signature is different and induced unique for each material, it should be possible to discriminate between one material and another based on difference in spectral signature of material.
I mathematically analyzed parameters for the intellectual initial design. Main concept of this is the derivative of "ring of minimum aberration without vignetting". This work is a kind of analytical design of an Offner imaging spectrometer.
Also, several experiment methods will be contrived to evaluate the performance of imaging spectrometer.
We introduce a design of an Offner imaging spectrograph with its performance and tolerancing results. It is a traditional Offner spectrograph employing two concave mirrors and one convex reflective grating for dispersing light in the SWIR band (900~1700 nm). The optical system uses 25um-pitch pixels for the detector and the goal spectral sampling is 3.2nm. Its performance is analyzed in terms of MTFs, spot diagrams, and distortions – keystone and smile. This design focuses on the yaw(beta-tilt) sensitivity of the tertiary mirror as the compensator hence is expected to act as a performance-improving breakthrough for the entire system as the inverse sensitivity confirms it is the most sensitive component. The procedure of the inverse sensitivity evaluation is explained, and then budgeting the tolerances for each element for the practical production is described.
A laser radar (LADAR) system with a Geiger mode avalanche photodiode (GAPD) is used extensively due to its high
detection sensitivity. However, this system requires a certain amount of time to receive subsequent signals after detecting
the previous one. This dead time, usually 10 ns to 10 μs, is determined by the material composition of the detector and
the design of the quenching circuits. Therefore, when we measure objects in close proximity to other objects along the
optical axis using the LADAR system with GAPD, it is difficult to separate them clearly owing to the dead time problem.
One example for that is a case of hidden objects behind partially transparent blinds. In this paper, we suggested a
modified LADAR system with GAPD to remove the dead time problem by adopting an additional linear mode avalanche
photodiode (LAPD) as a complementary detector. Because the LAPD does not have dead time while still maintaining
relatively low detection sensitivity, the proposed system can measure an object placed within the dead time with high
detection sensitivity. Light is emitted from the pulsed laser of a light source and is delivered into a fast photodiode to
generate a start signal. Most of laser pulses are directed onto the target and scattered from the surfaces of targets. The
scattered light in the field-of-view of the system is divided by a polarizing beam splitter, after which it becomes incident
to two different types of APDs, the GAPD and the LAPD. The GAPD receives the signals from the target with high
sensitivity, and the signals scattered in the dead time zone are then detected by the LAPD. The obtained signals are
analyzed at the same time. In this way, the signals scattered from objects placed within the dead time can be
distinguished clearly.
In this paper, a new method to improve the SNR by temporal filtering method in LADAR system using two Geiger-mode
avalanche photodiodes (GmAPDs) is proposed. The new method is implemented by using two GmAPDs with
beam splitter and employing AND process to their ends. Then, timing circuitry receives the electrical signals only if each
GmAPDs generates the electrical signals simultaneously. Though this method decreases the energy of a laser-return
pulse scattered from the target, it is highly effective in reducing the false-alarm probability because of the randomly
distributed noise on the time domain. Then it needs not any image processing steps. The experiments are performed to
prove the advantage of the new method proposed with varying the time bin size. The experimental results represent that
the improvement of SNR.
A method to determine correct focus in direct detection laser radar system using Geiger-mode avalanche photodiode
focal plane array (GmAPD-FPA) is proposed. It is implemented by laser pulses with controlled beam diameter and
energy on a distant target. And the time-of-flight (TOF) of laser pulses are obtained for each pixel in GmAPD-FPA.
With multiple laser pulses, time correlated single photon counting (TCSPC) is carried out to obtain target detection
probability. Using target detection probabilities of each pixel, the photon distribution on GmAPD-FPA is acquired. The
condition to determine correct focus is the minimum photon distribution in GmAPD-FPA. In theory part, the range of
laser pulse energy is decided. The experiments are carried out with commercial 1x8 pixel GmAPD-FPA. The
experimental results show that the focus position is founded using this method and a spatial resolution of a laser radar
system is improved where the 1x8 pixel GmAPD-FPA is located in focus position.
An Fourier Transform Infra-Red(FTIR) spectrometry has played an significant role in a variety of fields in recent years.
In particular, FTIR spectrometer technology has been adopted in passive remote sensing system to predict detection
probabilities of stand-off hazardous compounds. There are three steps to detect hazardous compounds. Firstly, MODerate
spectral resolution atmospheric TRANsmittance(MODTRAN) algorithm is used to calculate spectral radiances of
background and atmosphere affected by hazardous compounds. It transfers a difference of spectral radiance between
background and hazardous compounds existing region to FTIR spectroscopy system. Secondly, FTIR spectrometry
system collects an interferogram which represents spectral radiances respective to given time intervals (reciprocal of
wavenumber) and sends it to signal processing part. Lastly, the signal processing part performs Fourier Transformation
on the interferogram and identifies the spectral radiance with reference data from gas library by using Spectral Angle
Mapper(SAM) algorithm which results in visualizing the hazardous gases.
However, there are some noises which affect the interferogram and the spectrum in practice. Specifically, there are two
main noises which have critical effects on the interferogram and the spectrum by reducing its Signal to Noise Ratio(SNR)
such as detector noise, jitter of moving mirror and optics misalignment.
In this paper, a theory of the effects of the detector noise and the jitter of moving mirror on the interferogram and its
demonstration through simulation are presented.
Direct-detection three-dimensional imaging laser radar system using Geiger-mode av.alanche photodiode (GAPD) is
investigated in order to acquire three-dimensional images of objects at a long distance (more than 100m). Due to
extremely high sensitivity of the GAPD, a laser radar system using GAPD is not only advantageous in terms of ranging a
distant object but also in detecting a target screened by a sparse obstacle located in front of it. Both laser radar systems
using a single-pixel GAPD and 1x8-pixel GAPD focal plane array as detectors are built up and analyzed. Passively Q-switched
microchip laser is used as a laser source and a compact peripheral component interconnect system, which
includes a time-to-digital converter (TDC), is set up. With both the GAPD having short dead-time (45ns) and the TDC
functioning multi-stop acquisition, the system operates in a multi-hit mode. Three-dimensional images taken by the laser
radar systems are shown. Both the single-shot precision and the dependence of the precision on the effective number of
laser pulses are shown. Range walk reduction and autofocus techniques are proposed and demonstrated experimentally;
they improve the accuracy and transverse spatial resolution of the laser radar system, respectively.
A new technique to improve the accuracy of direct detection
time-of-flight (TOF) laser radar using a Geiger-mode
avalanche photodiode (APD) by reducing range walk error is presented. The range walk error refers to the change of the
measured distance as a function of the waveform and energy of the laser-return pulse scattered from the target. The
technique is based on the theoretical model, established with Poisson statistics, of the system and it enables to overcome
the inherent limitation of Geiger-mode APD so that the mean number of photons of the laser-return pulse is expectable
within the range in which the target detection probability varies. The theoretical model for the detection probability of
each time bin in a TOF histogram is derived after it is experimentally proved that the creation of primary electrons in the
Geiger-mode APD is Poisson-distributed. It is shown that the range walk error depends on the energy of the laser-return
pulse at the Geiger-mode APD with the theoretical model regarded as the TOF histogram of multiple laser pulses in a
single-hit case. The method which reduces the range walk error with the center of mass detection in the TOF histogram
is then proposed, and the experiment for its proof is carried out. The experimental results show that the theoretical model
is appropriate and the range walk error is reducible within the range in which the detection probability varies.
The beam combination method using stimulated Brillouin scattering phase conjugate mirrors is a promising technique
for solid state lasers of high power/energy operating with high repetition rate. The key technology of this method is the
phase control of the SBS waves. In the previous works, the principle of this phase control technique was demonstrated experimentally. As a next step, in this work, amplifiers have been added to the beam combination system. Inserting the amplifiers, a stabilized phase difference has been obtained with a fluctuation less than λ/50 at 44 mJ total output energy and 10 Hz repetition rate.
Arbitrarily complex 3-D polymeric structures with a resolution of under 100 nm are fabricated by two-photon
absorption polymerization (TPAP) by an ultra-short pulsed laser. This method can be applied to many scientific and
engineering fields such as micro/nano-optics, MEMS, microfluidic system, and so on. Many 3-D structures by TPA
fabrication have been made. However the structures made with an acrylate-based prepolymer material have seriously
structural problems, such as shrinkage, collapse, distortion, etc. These problems make the fabrication of a large and fine
3-D structure difficult. Using an epoxy-based material like SU-8, which is widely used in the conventional lithography,
the problems above can be prevented. Although SU-8 is designed for the UV lithography, a two-photon absorbing dye
and proton acid generator can make it a base prepolymer material for the two-photon absorption polymerization. We
studied the size of voxel or the resolution of fabrication from the SU-8 structures under the various fabrication
conditions such as the laser output power and the exposure time. We demonstrated 3-D micro/nano structures with SU-8
and compared them with same structure with SCR-500.
As an ultrafast laser has recently been developed, this leads to the innovative nanotechnology, the 3-D fabrication of
the two-photon absorbed (TPA) photo-polymerization. The 3-D micro/nano structure by this method has a resolution of
sub-hundred nm which is much smaller than the diffraction limit. Usually the 3-D polymer micro/nano structure by this
method is made by stacking many of a unit polymer structure, so called 'voxel'. The size of the voxel is considered as
the fabrication resolution. The size of a voxel, or the fabricating resolution is determined by several fabricating
conditions such as the laser output power, the exposure time, the N.A. of the focusing lens, the types of polymerizing
material, and the pulse-width. The voxel size due to power, exposure time and NA has been done by many research
groups. Although the pulse-width is a very important condition for two-photon absorption, the study of influence on
fabricating resolution by the pulse-width has not been done before. Therefore we studied the voxel size under the
condition of increasing the pulse-width of the laser. To stretch the pulse-width, a single mode fiber (SMF) has been
used. We demonstrated that the voxel lateral size decreased as pulse-width stretched to several picoseconds.
We have found that it is possible to preserve the temporal waveform of the reflected wave generated from stimulated
Brillouin scattering (SBS) by using a prepulse technique. In this work, the fundamental research has been carried out to
preserve the deformed pulse shape reflected from SBS medium. It is well known that the reflected SBS wave has a steep
rising edge. If one employs SBS cells in series, the rising edge of the pulse shape becomes steeper every time it reflects
at every SBS cell. This deformation of the SBS wave can cause the undesirable effects when we employ several SBS
cells in series, such as an optical breakdown in the optical components and the lower reflectivity and lower fidelity of the
phase conjugated wave in the SBS medium. A prepulse energy of 5 mJ and a time delay of 5 ns have been measured to
be the optimum values under this experimental condition. This prepulse method is useful in developing a multistage
system employing several SBS cells in series for high-power laser applications.
The beam combination technique using stimulated Brillouin scattering phase conjugate mirrors (SBS-PCMs) is one of
the most promising technology to realize high energy/ high power/ high repetition rate. The beam combination technique
using SBS-PCM can compensate any optical distortions occurred in the amplifier chain because it gives the phase
conjugated wave for the good beam quality. In this paper we will introduce the cross type amplifier as a basic unit of the
proposed beam combination system and show essential technology for realization of the beam combination system, such
as the new SBS phase control technique proposed by the authors. These new techniques are the most simple among the
phase locking techniques developed previously, and furthermore it is possible not only to lock but also to control the
phases of the SBS waves very accurately.
We have introduced the additional prepulse with main pulse to generate the stimulated Brillouin scattering (SBS) and
investigated the effect of this technique. In general, temporal pulse shape deformation takes place when the pulse is
reflected from the medium breeding SBS. This deformation of the SBS wave can cause optical breakdown in the optical
components and consequently it leads to low reflectivity and low fidelity of the phase conjugated wave in the SBS
medium. It has been shown that there is optimum prepulse time delay and minimum energy for preserving the SBS
waveform. This method is so simple that it can be applied to other systems and utilized in many applications easily, such
as high-power laser and optical isolator applications employing several SBS cells.
The beam combination technique using stimulated Brillouin scattering (SBS) phase conjugate mirrors (PCMs)
proposed by one of the authors, H. J. Kong, is a promising one for realization of high energy/power laser system with
high repetition rate. However, phase controlling of the SBS waves is essentially required for beam combination system,
since the SBS-PCM generates the random phase. Recently, we have achieved successful results for phase locking by the
self-generated density modulation method. But it showed a long-term phase fluctuation due to the long-term fluctuation
of the density of the liquid SBS medium. To compensate this long-term phase fluctuation, we have designed new phase
stabilization system. In this paper, we will introduce this system and show successful experimental results.
By numerical simulations, it has been shown that a conventional cylindrical rod can be used as a hollow conic beam generator by illuminating a parallel laser beam inclined to the axis of the rod. Half of the conic beam is formed by the reflection at the surface of the cylindrical rod, and the opposite side of the conic beam by its transmission. We discuss the parameters to determine the size of the conic beam and the effect of the dielectric multilayer coating on the intensity distribution of the conic beam. The line beams of the shapes such as circle, ellipse, parabola, or hyperbola can be generated by this hollow conic beam generator, depending on the position and orientation of the observing plane.
We shows that the phase of a backward stimulated Brillouin scattering (SBS) wave can be controlled, which is necessary for a beam combination laser using SBS phase conjugate mirrors. The phase control is achieved by a self-generated density modulation without seeding the Stokes beam. Theoretical analysis shows that the phase fluctuation is mainly due to the pump energy fluctuation and is inversely proportional to the pump energy. We have achieved that the relative phase difference between two SBS waves is smaller than λ/4 for all the pump pulses.
We report that the phase of backward Stokes waves can be independently controlled, which is necessary for a beam combination laser using stimulated Brillouin scattering-phase conjugate mirrors (SBS-PCM). The phase control is achieved by a self-generated density modulation without seeding the Stokes beam. Theoretical analysis shows that the phase fluctuation is mainly due to the pump energy fluctuation and is inversely proportional to the pump energy. We have achieved that the relative phase difference between two Stokes waves via SBS is smaller than λ/4 for all the pump pulses.
Multi-photon absorption phenomena induced by ultra fast laser have been considered for many applications of microfabrications such as metal ablation, glass etching and photopolymerization. Among the applications, the photopolymerization by two-photon absorption (TPA) has been regarded as a new microfabricating method. It is possible to be used in photo mask correcting, diffractive optical element and micro machining. The TPA photopolymerization is made possible to fabricate a complicated three dimensional (3D) micro-structure which the conventional photomask technology has not been able to make. In fact, the shape of the voxel (volume pixel: a unit structure of TPA fabrication) is an important factor which could affect the microfabrication process. In this paper, we have reported that 3D micro-structures were fabricated and the generation of voxel shape was analyzed for various optical conditions.
Multi-photon absorption phenomena induced by ultra fast laser have been considered for many applications of microfabrications such as metal ablation, glass etching and photopolymerization. Among the applications, the photopolymerization by two-photon absorption (TPA) has been regarded as a new microfabricating method. It is possible to be used in photo mask correcting, diffractive optical element and micro machining. The TPA photopolymerization is made possible to fabricate a complicated three dimensional structure which the conventional photomask technology has not been able to make. Furthermore the TPA photopolymerization process applied to a two dimensional structure fabrication may take shorter time than the old process since the absence of etching and deposition processes. Recently we have made a simple 3D structure and applied the technique to PDMS(poly-dimethyl siloxane) molding.
As a femtosecond laser has recently been developed, both of high power and high photon density are easily obtained. The high photon density results in photopolymerization of urethane acrylate resin whose absorption spectrum is shorter than that of the femtosecond laser. The stereo-lithography using the two-photon absorption (TPA) makes micro structures with great resolution. We used this phenomenon to make micron-sized structures with sub-micron resolution. Before fabricating 3-D structures, precise 2-D structures were preceded. The TPA photopolymerization was applied of poly-dimethyl siloxane (PDMS) molding. In this paper, we report the recent progress and application of this technology in our laboratory.
This paper describes a newly designed multipoint process monitoring system based on an acousto-optic tunable filter. In order to prove the feasibility of the suggested multipoint monitoring system for use in the NIR spectral region, some experiments were carried out in the visible range. The multipoint process monitoring system consists of an AOTF device for wavelength selecting, a CCD imaging sensor, and a specially designed in-line type of optical fiber probe. Unlike an FTS (Fourier Transform Spectrometry) based monitoring system, an AOTF has no moving parts, and it can be rapidly tuned to any wavelength in its operating range within microseconds. Thus, the AOTF is advantageous in terms of faster spectral imaging capability and rigidity required for industrial monitoring environment. Also, Fourier Transform Spectrometry experiments were conducted for comparison with the AOTF based monitoring system. In the current feasibility evaluation, an enhanced optical fiber probe with 3 monitoring points was used. However, the number of monitoring points can be easily expanded to dozens more points as required.
KEYWORDS: Plasma, Optical spheres, Foam, X-rays, Wave propagation, Signal to noise ratio, Information operations, High power lasers, Radiography, Optical simulations
Laboratory simulation of the ejecta-ring collision of Supernova 1987A has been performed by using a high power laser syste. Pure hydrodynamic modeling was applied using the invariance of the Euler equations and the experiment was designed with 1D hydrodynamic simulations. Three laser beams of (lambda) equals 0.53 micrometers , at the intensity I equals 2 X 1014 W/cm2 irradiated a CH foil to generate a strong shock wave and subsequent dense plasma flow in a low density CHO foam including a solid sphere at its center. Shock wave propagation and ma sam flow with vortex ring-like structures were observed with gated x-ray radiography. Overall hydrodynamic behaviors are consistent with 1D and 2D hydrodynamic simulations. The collision dynamics is dominated by a complicated interplay of reflected shock, diffracted shock, transmitted shock waves, and plasma flow behind the main shock passage.
A high-power Nd:glass laser system (Sinmyung I) has been constructed and tested in KAIST[1]. In this system, we used a Nd:YLF laser as a master oscillator, a 4-pass amplifier for pre-amplification[2], Nd-doped phosphate glasses for rod amplifiers, and spatial filtering and image relaying units. The system has demonstrated in excess of 80 J at 40 ps full width at half-maximum (FWHM) pulse duration. Output gains, energies and spatial profiles were measured at each amplification stage. X-ray generation experiments in Cu bulk target was done using this system.
A four-aspherical mirror system with unit magnification is investigated for use in deep ultraviolet (DUV) optical lithography. It is derived from the solution of all zero third-order aberrations for the four-spherical mirror system with unit magnification. We have first examined the holosymmetric four-spherical mirror system in which all third-order aberrations are zero and all orders of coma and distortion are also zero. However, the system does not have any optical design freedom left for the correction of higher order aberrations, so a new solution of nonholosymmetric system is derived. In this system aspherizations on the spherical surfaces are carried out to reduce the residual aberrations. The aspherization is optimized to give near diffraction-limited performance for DUV wavelengths of 0.193 μm (ArF excimer laser line). The final system we have obtained consists of all aspherized mirrors with a numerical aperture of 0.35. This reflective system is compact in size and expected to be useful in optical lithographic applications.
The design for a rotationally symmetric four-mirror optical system with reduction magnification 5 x for deep UV (λ = 248 nm of a KrF excimer laser) submicrometer lithography is developed. Initially by using the paraxial quantities, numerical solutions are found for the system, which is free from the four off-axial third-order aberrations-coma, astigmatism, field curvature, and distortion. Aspherization is carried out to the spherical mirror surfaces to reduce the axial and residual off-axial higher order aberrations. The numerical aperture of the final system is as large as 0.38, which gives a Rayleigh resolution of 0.4 μm and hence is useful in submicrometer lithographic applications.
The validity of inertial-fusion experiments relies on precise and accurate knowledge of the characteristics of small
(<1-mm diam) gas-filled shells, such as the wall thickness and outer diameter of the spherical microballoon and the density of
the gas inside it. In the past, dual-beam interferometry has been utilized to determine these quantities. These techniques
produce interference patterns which have a spatial intensity distribution that is sinusoidal. In contrast, multiple-beam
interferometry produces interferometric patterns which are described by the Airy function, and they can have interference
maxima that are sharp and narrow, depending on the number of interfering wave fronts. This work describes a technique that
uses a scanning, plane Fabry-Perot interferometer to determine inertial-fusion target parameters to high accuracy using the
increased precision of multiple-beam interferometry. The microballoon is inserted between the highly-reflective surfaces of an
air-spaced Fabry-Perot etalon that is illuminated with monochromatic plane waves. As the separation between the end mirrors
is varied continuously, the intensities of the light transmitted through the center of the microballoon and passing external to
it are recorded simultaneously as a function of mirror spacing. From this data, the phase difference between these two wave
fronts is determined and the appropriate inertial-fusion target parameters are calculated. Using a scanning Fabry-Perot
interferometer in conjunction with computerized data acquisition and analysis techniques, phase measurements with precisions
of the order of X/100 can be made. With proper refractive index calibration, this precision results in a wall-thickness
measurement accuracy of about 0.1%; and the density of the gas inside the microballoon is found with an accuracy of about
1% of its measured value.
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