A modified low-cost unimorph deformable mirror (DM) driven only by positive voltages for atmospheric turbulence compensation is presented. The 214 patterned inner actuators generate convex deformations for aberration correction, while one outer ring actuator generates an overall concave bias. To evaluate the aberration correction capability of the proposed DM, the iterative reconstruction of Zernike aberrations and correction were performed in an adaptive optics test system. The experimental results indicate that the fabricated DM has an excellent aberration correction capability, particularly matching the first 20 term Zernike aberrations with the normalized residual root-mean-square (RMS) error <5%. Furthermore, the random atmospheric turbulence aberrations were simulated based on Karhunen–Loève coefficients and reconstructed using the fabricated DM. The simulative and experimental results show that the atmospheric turbulence aberrations can be steadily compensated with λ/40 (λ=2.2 μm) RMS residual error, indicating the prospect for atmospheric applications.
Once a laser beam suffers from wavefront aberrations, the intensity of the focal spot degrades and the shape of
the focus spot distorts. The same problem also exists in femtosecond laser fabrication system. The aberrations in the
femtosecond laser fabrication system contain two main components: system aberrations and aberrations from the
refractive index mismatch problem. Recently, adaptive optics (AO) has been used for laser beam aberrations correction
to improve the light beam quality. In this paper, we introduce an adaptive optics system with double drive modes
unimorph deformable mirror (DM) into the femtosecond laser fabrication system. In the experiments, the hill-climbing
algorithm based on Zernike modes is used to control the DM to correct the aberrations in the close-loop manner. After
correction for system aberrations and the refractive index mismatch aberrations, the shape and maximum intensity of the
focal laser spot is much improved.
Unimorph deformable mirrors are attractive in adaptive optics system due to their advantages of simplicity, compact, low cost and large stroke. In this paper, a double drive modes unimorph deformable mirror is presented, which comprises a 200 μm thick PZT layer and a 400 μm thick silicon layer. This deformable has 214 inner actuators in the 50-mm active aperture, which are for the aberration correction and a outer ring actuator for generating an overall defocus bias. An analytical model based on the theory of plates and shells is built to predict the behavior of the deformable mirror. The stroke of the deformable mirror is tested in the experiments. In order to test the performance for aberration correction, the deformable mirror is used to correct the aberration from its imperfect initial mirror surface in the close-loop manner. The root-mean-square value of the mirror surface after the close-loop correction for ten iterations is about λ/40, which indicates this deformable mirror has a good aberration correction performance. This DM has the potential to be used for astronomical adaptive optics.
Unimorph deformable mirrors are attractive in adaptive optics system due to their advantages of simplicity, compact, low
cost and large stroke. In this paper, a double drive modes unimorph deformable mirror is discussed, which comprises a
100 μm thick PZT layer and a 200 μm thick silicon layer. This deformable mirror (DM) can achieve two different
directions deformation of concave and convex driven by only positive voltage. The dual direction maximum defocus
deformations are -14.3 μm and 14.9 μm. The close-loop performance of this DM is also tested in an experimental
adaptive optics system based on Hartman-Shack wavefront sensor. In experiments, the DM is controlled by the steepest
descent algorithm (SD) to corrected the aberrations in a close-loop manner. The ability of this DM of correction for the
system aberration and reconstruction for the low order Zernike mode aberration is tested. The root mean square (rms)
value of the system aberration after close-loop correction is about 28 nm. The reconstruction results for most low order
Zernike mode aberrations have a relative error less than 10%.
This paper presents a low-cost silicon unimorph deformable mirror (DM) that will be used for astronomical adaptive optics. The device has a simple construction consisting of a 400-μm-thick silicon wafer and a 200-μm-thick lead zirconate titanate film, with 214 actuators and 50-mm active aperture. The actuator displacement of the fabricated device is about 1 μm at 50 V. And the resonance frequency is 550 Hz. The simulation and experimental generation of Zernike mode shapes indicate that the fabricated DM has an excellent correction performance for both low order and high order aberrations.
To solve the local optimum problem, a modified hill-climbing algorithm based on Zernike modes is presented for wavefront sensorless adaptive optics. This algorithm adopts the Zernike mode coefficients, instead of the actuators’ voltages in a traditional hill-climbing algorithm, as the adjustable variables to optimize the object function. First, the principle of the algorithm is described. Then the efficiency of the modified and traditional hill-climbing algorithms are analyzed numerically and experimentally using a He-Ne laser beam shaping system with a 37-element unimorph deformable mirror. The results of both simulations and experiments demonstrate that the modified hill-climbing algorithm can eliminate the local optimum problem with a fast speed of about 100 iterations.
As a new type of grating, the freestanding blazed transmission grating combines the advantages of traditional transmission gratings (low mass, relaxed alignment and flatness figure insensitivity) with those of reflection gratings (high broadband diffraction efficiency, high spectral resolution). A freestanding blazed transmission grating with period of 1μm was successfully fabricated by holographic lithography and anisotropic wet etching of silicon. The duty cycle is about 0.13. The aspect ratio of a single grating bar is about 77. The thickness is 10μm and the open area fraction is about 58.8%. The size of a single die is 15mm×15mm divided into four 5mm ×5mm windows. The diffraction efficiency of the grating was measured at the National Synchrotron Radiation Laboratory in the wavelength region of 5-50nm. The results show a strong blazing effect in the direction of specular reflection from mirror-like grating sidewalls, as expected.
Unimorph DMs are attractive for laser beam shaping due to their advantages of simplicity, compact, low cost and large
stroke. This paper reports a novel unimorph deformable mirror used for laser beam shaping. The developed DM consists of
both an inner actuator array and an outer ring actuator. The inner actuator array is used for aberration correction, while the
outer ring actuator is used to generate an overall defocus bias. A laser beam shaping experimental setup with a He-Ne laser
was constructed. The correction capability of the DM is demonstrated by simulation and experiment.
The adaptive optics system (AO) without a wavefront sensor produces an alternative tool to correct the aberrations where the wavefront distortions are hard to be measured directly. This kind of adaptive optics system optimizes an evaluation parameter directly relevant to the wavefront quality to achieve the wavefront correction, and the optimization algorithms determines the efficiency of wavefront sensor-less AO system. In this paper, several frequently-used algorithms covering hill climbing algorithm, genetic algorithm, simulated annealing algorithm and two hybrid algorithms, known as hybrid genetic-hill climbing algorithm and hybrid simulated annealing-hill climbing algorithm (SAHC), have been investigated to improve the correction efficiency. The correction speeds and correction effects of the five kinds of algorithms have been compared in simulations and experiments for an adaptive optics system with 19-elements unimorph deformable mirror. The results show that hill climbing algorithm has the fastest correction speed, but suffered from the local optimum problem. The other four kinds of algorithms can get good correction results and have similar correction effects. Considering the correction effect and correction speed comprehensively, SAHC is the most efficient optimization algorithm in the five kinds of optimization algorithms.
A low-cost silicon unimorph deformable mirror (DM) with 214 actuators is presented. The finite element simulation
indicates that the designed DM has an excellent correction performance for both low order and high order aberrations. The
experimental actuator deflection is about 2μm at 100V. This DM has the potential to be used for astronomical adaptive
optics.
As an important optical element, beam sampling grating (BSG) is used in the terminal of inertial confinement fusion
(ICF) drivers. It can provide a very slight sampling beam for the precision diagnosing of laser energy and wavefront
distortion. However, in practice, its non-uniform diffraction efficiency seriously influences the accurate signal of
sampling beam, and finally affects diagnostic ability. BSG is usually fabricated by holographic ion beam etched (HIBE)
process. In this paper, a mechanical polishing processing technology was used to improve uniformity of the diffraction
efficiency of BSG after HIBE. In the processing, cerium oxide (CeO2) was used to polish the local areas of grating
where exhibit higher diffraction efficiency with the purpose of changing the depth of grating profile, and then they have
similar efficiency with the surrounding areas. By iteration of the above process, BSG finally achieve the improved
uniformity of diffraction efficiency over the area of a 430 x 430 mm2. The RMS of diffraction efficiency of BSG after
mechanical polishing shows great reduction down to 4.8% as compared with that of the as-polished RMS of 21%. The
effects of this processing on laser damage was characterized by the measuring the LIDT for the laser radiations of
355nm.
The short wavelength, high brightness and good spatial coherence of soft x-ray lasers make them ideal sources for the
diagnostics of dense plasmas. As a widely used diagnosing tool in the research of laser-produced plasma, interferometry
has many advantages in the accurate measurement of the plasma electron density because it directly gives refraction
index mapping from the interference pattern processing. The electron density gradient and electron density of larger
variety plasma will be probed by the soft X-ray diffraction grating Mach-Zehnder interferometer (DGI). At the
wavelength 13.9nm which our DGI uses, we fabricate a laminar grating used as the splitter grating. Its groove density is
1000g/mm, groove depth is 13nm, duty cycle is 0.4±10%. The zero and first diffracted order beams have an equal
metrical diffraction efficiency about 25 percent at 81.3 degrees, and the maximum of the product is 7.6 percent at 81.2
degrees. The results illustrate the laminar grating is suitable for being used as a new pattern splitter grating at the
wavelength 13.9nm.
It is known that rectangular grooves profile and various duty cycles (line to pitch) are required to attain maximum
efficiency. The gold transmission phase gratings have been fabricated using holography and ion-beam etching
techniques. The latent image monitoring technique and real time end-point detection technique were utilized to improve
control of the shape of grooves and duty cycle during exposure and development, respectively. It is difficult to produce
gratings with larger duty cycle in photoresist on gold for standing wave, nevertheless it is revealed in our work that the
duty cycle can be more than 0.4 for the case for photoresist on gold substrates if development is stopped where the slope
of the monitoring curve begins to drop off, not at the peak of the diffracted signal. In ion-beam milling, the influence of
redeposition on the shape of grooves and duty cycle has been overcome by using a thin mask of Chromium with a
comparatively low ion-etch rate and tilting and rotating substrates beneath the ion beam. Finally, some gold phase
gratings with the duty cycles in the range of 0.25-0.45 have been obtained, whose duty cycle value is about 0.45, and
their grooves profile is trapezoidal profile whose left and right facets are both more than 83 degrees approximately.
A novel process that combines interference lithography and ion beam etching is presented for fabrication of magnetic
submicron structures and nanostructures in this paper. Instead of an antireflective coating, vertical standing wave
patterns were removed using oxygen descumming process. A series of magnetic submicronmeter structures were
fabricated on Co0.9Fe0.1 films by this technique. Fabrication of magnetic nanostructures was performed by using a high
exposure dose and modifications in optimized development conditions. A thin Au film was deposited on the sidewall of
the magnetic nanostructures to avoid the oxidation of Co and Fe. The effect of this method was confirmed by X-ray
photoelectron spectroscopy (XPS). Hysteresis loops measured by a highly sensitive superconducting quantum
interference device (SQUID) technique show the different magnetic properties of the magnetic patterns with different
critical dimensions.
Multilayer dielectric grating plays an important role in the laser inertial confinement fusion, which is fabricated by using
holographic lithography and ion beam etching. In this paper details the reactive ion beam etching of multilayer dielectric
grating with SiO2 as the top layer. The etching of SiO2 was carried out by a radio frequency reactive ion beam etcher
with CHF3 chemistry to increase the etch selectivity and get a high fidelity grating pattern transfer from photoresist into
SiO2 coating. The photoresist gratings with different profiles as masks were used to create SiO2 corrugations with
different profiles separately. The etching results were analyzed in detail, including the facet and redeposition effects. In
conclusion, it is essential that the photoresist grating mask should be high and steep enough to get SiO2 grating with
vertical profiles. The multilayer dielectric grating with SiO2 as the top layer exhibited an efficiency of about 95% in the - 1 order at TE polarization of 1 064 nm light at Littrow mounting.
KEYWORDS: Surface roughness, Atomic force microscopy, Silicon, Spatial frequencies, Glasses, Data acquisition, Probability theory, Spectral data processing, Silica, Semiconducting wafers
This paper presents a comparative studied of surface roughness. Samples were first measured by atomic force microscope (AFM). The acquired height data was then processed to calculate their power spectral density (PSD) and multi-fractal spectrum (MFS). The calculation results indicate that MFS of different samples with same sampling length differs significantly from each other, while the MFS of the same sample with different sampling length or different sampling position is quite similar. The calculation also shows that MFS is very sensitive to particles or scratches appeared on the surface. The PSD of the same data acquired from these samples are also presented for comparison. It is clear from the calculation results that the PSD curves vary with the sampling position and sampling length, thus makes the evaluation uncertain. No quantitative index available from PSD, only qualitative information obtained. Comparatively, MFS is better in description of a surface roughness.
Magnetic submicron meter structures were fabricated on Co0.9Fe0.1 films by laser interference lithography combined with
ion beam etching. The vertical standing wave was restrained by using proper exposure dose and appropriate development
time. Hysteresis loops of the magnetic film and magnetic patterns were presented by surface magneto-optical Kerr effect.
The difference magnetism between magnetic patterns with different critical sizes and the film was discussed.
Parameters of two spherical waves are firstly optimized to get the exact groove densities of diffractive gratings.
Consequently, the groove density differences between on the plane and on the curved substrates are derived. Therefore,
some experimental results are provided to demonstrate the validity of this method above. Meanwhile, the curvatures of
the substrates are measured three times by using long trace profiler (LTP), which assures the repeatability of the bending
technique for grating substrates. At last, the advantage of this method is exhibited through comparing the errors of
grating groove density fabricated by this technique with only two spherical waves.
The major etching processes of a large-aperture multilayer diffraction grating, including the uniformity of the ion beam
current along the major axis and the on-line measurement of the diffraction intensity distribution are described. A large-aperture
ion beam etcher with radio frequency linear source has been developed to fabricate large-aperture diffractive
optical elements. The length with ±5.1% uniformity of the ion beam current distribution along the major axis is 30 cm. A
series of multilayer diffraction gratings were etched successfully by using this etcher with CHF3 chemistry. Multilayer
diffraction gratings on a 80 mm×150 mm BK7 substrate etched for laser systems are shown. The grating exhibits an
averaged diffraction efficiency about 96% at TE polarization of 1 064 nm light viewed at Littrow angle.
We present new progress of the diffraction grating interferometer being pre-aligned used a double frequency grating. To
measure the parallelism of the double frequency grating to a nicety before being built in the interferometer, a device
based on Diffraction Technique for measuring the parallelism of the double frequency grating is designed. It is built of a
semiconductor laser, a collimator, gratings, a precision turnplate, a beeline workbench, a redressal shelves, a ccd
detector. The system error of the device is analyzed in this paper, and the parallelism of the double frequency grating is
measured by this device. The results demonstrate that the diffraction measuring device suits the parallelism measured the
diffracting grating interferometer based on the double frequency grating of that parallelism can attain a high pre-aligning
precision.
A binary transmission phase grating is designed to operate as a dispersive element used in soft x-ray plasma diagnostic in ICF (inertial confinement fusion). The first and zero orders diffraction efficiency is simulated as a function of wave length and grating thickness. It is shown that the first order diffraction efficiency of a transmission phase grating can be up to 23.3% of the incident light, whereas that is not more than 10% for a conventional amplitude transmission grating. The transmission phase grating is fabricated in gold film supported by polyimide membranes using holography and ion beam etching method. The primary experimental results are presented.
Micro-optical elements are becoming more and more important in both consumer and industrial products. Developments such as the tunable liquid lens and the silicon laser will lead to further development of these technologies. In this paper we shall discuss a method that enables perturbation of the shape of a UV curable liquid droplet using an applied electrostatic field. This method provides a novel method for the control of the pre-cured lens profile and hence the final solid lens optical properties. This method also allows the fabrication of aspheric lenses using the UV-curable technique with the degree of aspericity controlled in real time by varying the applied electric field. The analysis of this fabrication method requires the development of an accurate in-situ lens profile measurement system. A range of techniques can be used to examine the resulting solid lens including mechanical techniques such as Dektak and Talysurf profilometry and optical techniques such as laser profilometry and interferometric techniques. We note that in order to fully characterise this fabrication technique it is necessary to measure the surface profile of the lens both post-curing and also when it is in the liquid state. The method chosen to examine the liquid lens is interferometry. In this study the surface profile of the microlens is examined using a Mach-Zehnder interferometer. The development of this interferometric measurement system and the analysis software are discussed. The application of this system in examining the E-field induced perturbation of the UV curable resin lenses will also be discussed and initial attempts to control the optical properties of the lens by preshaping the lens using the E-Field technique are discussed.
Micro-optical elements and related technologies are becoming increasingly important in a number of areas. There
are many different methods of fabricating these micro-optical elements. One such technique to produce microlenses
is the UV-curable resin method. This method involves the application of droplets of UV-curable resin to a substrate,
which can then be cured using UV light. The quality of the lenses produced using this method can vary due to a
number of experimental difficulties. These include the fact that the UV resin shrinks as it is cured. This shrinkage
effect is a difficult problem, as it is a property of the UV material and the substrate. The ability to pre-shape the
liquid droplet may enable the manufacturer to compensate for this shrinkage.
In this paper we propose a method to generate a controlled perturbation of the liquid profile and hence the optical
properties of the final lens. We describe our method of inducing variations in the droplet profile using an applied
electric field. This method enables the fabrication of aspheric lenses. The analysis of this fabrication method
requires the development of an accurate lens profile measurement system. A number of techniques can be used to
examine the resulting lens elements including mechanical techniques such as Dektak profilometry and optical
techniques such as interferometric techniques including laser profiler and white light profiler. In this paper we give
the details of the modified Mach-Zehnder interferometric measurement system used, discuss the optical properties of
the lens and show a demonstration of the perturbation of the lens profile.
It has been shown that complex paraxial optical systems, consisting of various lens and distances of free space propagation, can be described using the Linear Canonical Transform (LCT). Indeed it can be shown that many well know optical transforms such as the Optical Fourier Transform (OFT), Optical Fractional Fourier Transform (OFRT), the effect of a lens or Chirp Modulation Transform (CMT) are all subsets of the more general LCT. Using the ABCD Collins matrix formula it is possible to represent these integral transforms in a simpler form, which facilitates system analysis and design. Speckle Photography (SP) in combination with an optical LCT can be used to measure surface motion of an optically rough body. It has previously been shown that Optical FRT's (OFRT) can be used in speckle based metrology systems to vary the range and sensitivity of a metrology system and also to determine both, the magnitude and direction, of tilting (rotation) and translation motion simultaneously, provided that the motion is captured in two separate OFRT domains. In this paper we extend the OFRT analysis to more general LCT systems and demonstrate how simultaneous tilt and translation measurements can be discerned from the speckle images captured prior to, and after motion. A spherical wavefront can be conveniently described using the Collin's matrix notation. By changing the wavefront of the illuminating light we show that we effectively change the domain of the LCT system without changing the bulk elements in the optical system. Thus the complete motion (in-plane translation and small surface tilting) of a rigid body can be determined using one optical LCT system and illuminating fields of varying curvature.
Diffraction gratings are designed for grazing-incidence operation in the soft x-ray spectral region. The efficiencies for various diffraction orders are simulated as a function of angle of incidence and blazed angles. According to the beat phenomenon, the grating pattern with double-frequency on the same substrate is recorded in a holographic way. The blazed grating will be fabricated using holography and ion beam etching method. The primary experimental results are presented. This kind of blazed grating is a key element used as a beam splitter of soft x-ray Mach-Zehnder interferometer, which is a new tool to diagnose the plasma generated by the laser-based inertial confinement fusion.
The Linear Canonical Transform (LCT) is a general transform which can be used to describe linear lossless quadratic phase systems (QPS). It can be shown that the Optical Fourier Transform (OFT), Optical Fractional Fourier Transform (OFRT) and the effect of a thin lens or Chirp Modulation Transform (CMT), are all special cases of the more general LCT. Using the Collins formula it is possible to represent these transforms as ABCD matrices. By cascading relevant matrices together, quite complicated bulk optical systems can be described in a compact manner. Digital Speckle Photography (DSP) can be used in the analysis of surface motion in combination with an optical LCT. It has previously been shown that Optical FRT's (OFRT) can be used in speckle based metrology systems to vary the range and sensitivity of a metrology system and also to determine both, the magnitude and direction, of tilting (rotation) and translation motion simultaneously, provided that the motion is captured in two separate OFRT domains. In this paper we extend this analysis to more general LCT systems. We demonstrate that a spherical illuminating wavefront can be conveniently described using matrix notation. We show that by changing the sphericity of wavefront we can change the domain of the LCT system. Hence by illuminating a target with a plane wavefront and then a spherical wavefront, we capture the motion in two separate LCT domains and we are thus in a position to fully determine the motion of a rigid body without a priori knowledge.
A Linear Canonical Transform (LCT) is a general integral transform which can be used to describe a whole host of complex paraxial optical systems. It can be shown that Fourier Transform (FT), Fractional Fourier Transform (FRT), Chirp Multiplication Function (CMT), (which is used as a model for a thin lens), and the Fresnel Transform (FST) are all specific cases of LCT's and are particularly important in optics. Using the Collins ABCD matrix formula it is possible to represent the above integral transforms in matrix notation. Furthermore since most bulk optical systems can be built using lenses of different curvatures (CMT) and free space propagation (FST) it becomes straight forward, to describe optical systems using matrix notation, (which is interchangeable with LCT integral notation). Speckle Photography (SP) can be used in the analysis of surface motion in combination with an optical LCT. It has previously been shown that Optical FRT's (OFRT) can be used in speckle based metrology systems to vary the range and sensitivity of the metrology system. Using a novel correlation technique it is possible to determine both, the magnitude and direction, of tilting (rotation) and translation motion simultaneously. In this paper these ideas are extended to more general LCT's, which allow the consideration of more complicated bulk optical systems. Combined with correlation techniques we experimentally demonstrate our ability to determine both, the magnitude and direction, of tilting (rotation) and translation motion of a surface over a greater range and sensitivity than previous OFRT techniques allowed.
A number of current and future optical and optoelectronic devices require the creation of structure in HfO2 film. The reactive ion beam etching of HfO2 film and photoresist mask in Ar/CHF3 gas mixture was examined as a function of ion energy, discharge composition and ion beam incident angle. The details of etch rate have been interpreted in terms of mechanism of etching. The etch rate has shown a square root dependence on ion energy and variation versus incidence angle. The CHF3- plasma produced some chemical enhancement in HfO2 film etching. Compared with pure argon etching, better selectivity and higher fidelity pattern transfer were achieved with Ar/CHF3 for HfO2 film over photoresisit mask. The AFM scans of etched HfO2 film by tapping mode showed good surface quality.
The electromagnetic theory should be applied to determine the diffraction efficiency of structures whose minimum line width is comparable with wavelength or the grooves are too deep, where scalar theory is no longer useful. The coordinate transformation method (the C method) is a very efficient method for obtaining continuous surface-relief grating efficiency for both TE and TM polarization. The local linear grating model (LLGM) models 2-D circular diffractive lens with combination of a series of local linear gratings. We synthesized and analyzed circular diffractive lens with a continuous profile not as previous authors who always use multi-lever structures. The result is compared with that of scalar theory and analysis using LLGM and rigorous coupled-wave theory. This optimization can be used as a complement of the scalar design of diffractive lens.
Ronchi gratings are widely used as beam splitters in interferometry. The phase Ronchi gratings are fabricated in K9 (or BK7) substrates using a photolithographic process and ion beam etching process. The geometry of the gratings was determined within the approximations of the scalar diffraction theory to minimize their diffraction intensities in 0 and 2 orders at 514.5nm. Their diffractive characteristics are measured using Ar+ laser at 514.5 and 496.5nm. The phase Ronchi gratings exhibit the diffraction intensity in 0 or 2 order to be three orders of magnitude less than that in 1 order.
Soft x-ray condenser zone plates are one of the most essential condensing and dispersing elements in the field of soft x-ray microscopy. The zone-plates are used at 3.2nm, whose diameters are 2.8mm. The widths of their outermost zone are 647nm. The zone plate is made of Au. Self-supporting zone plates are fabricated by holography-ion beam etching process to obtain x-ray lithography mask, the x-ray lithography, photolithography and micro electroplating process were used to obtain their self-supporting structure. The self-supporting zone plates are free of membrane absorption and degradation.
Variable Line-Space gratings play an important role in focusing and aberration-reduced. They are widely used in spatial spectrum and synchrotron radiation facilities. However the design and fabrication methods of them are difficult. The problem is to be able to record holographically the expected grooves distribution. The geometric theory of aspheric wave-front recording optics is briefly described. We introduce the genetic algorithm to the parameters optimization of holographic variable line-space gratings. In order to obtain a steady set of recording parameters, we propose that the objective function of the genetic algorithm should consider the effect of the recording parameters errors. The integral expression of the objective function is also derived to improve the efficiency of calculation. Design example of holographic variable line-space plane gratings for a position sensor is given to demonstrate the capability of this method. The line-profiles of variable line-space plane gratings with different recording parameters are also compared in this paper.
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