The imaging spectrometer monitoring the CO2 content in the atmosphere is mainly divided into the weak CO2 band and the strong CO2 band. Compared with the strong CO2 band, the weak CO2 band has a relatively clean spectrum, less interference from water vapor and other gases, and weaker CO2 absorption, and the instrument can receive stronger groundreflected solar radiation signals. However, the concentration of CO2 in the atmosphere is low. To use this band to detect changes in CO2 concentration, it is necessary to ensure that the detection instrument has a sufficiently high spectral resolution. Using the immersion grating as the dispersive element in the spectrometer can greatly reduce the size and weight of the instrument while achieving high spectral resolution. In this paper, the quartz immersion grating used in the weak CO2 detector is optimized and designed, the groove parameters of the grating are designed and optimized according to the requirements of the weak CO2 detector used, and the rectangle and different bottom angles are designed and optimized in consideration of the actual production error. In the 1.595-1.625 μm band, the duty cycle of the rectangular groove is 0.3-0.35, the groove depth is 560-630 nm, and the TiO2 thickness is 110-120 nm, the diffraction efficiency of the grating is higher than 80%; the duty ratio is 0.4-0.45, When the groove depth is 590-660 nm and the TiO2 thickness is 100-105 nm, the diffraction efficiency of the grating is higher than 80%. When the duty ratio of the trapezoidal groove is 0.3-0.35, the bottom angle of the trapezoid is 82°-86°, the groove depth is 590-630 nm, and the TiO2 thickness is 120-125 nm, the diffraction efficiency of the grating is higher than 80%. When the duty ratio of the trapezoidal groove is 0.4-0.45, the bottom angle of the trapezoid is 82°-86°, the groove depth is 620-670 nm, and the thickness of TiO2 is 105 nm, the diffraction efficiency of the grating is higher than 80%.
Methane is the second largest greenhouse gas after carbon dioxide, and the measurement and monitoring of methane is essential for regional and city-scale emission reduction strategies. Atmospheric methane concentration observations are also important for predicting regional emission trends. Spectrometers are important optical instruments for monitoring the content of greenhouse gases in the atmosphere, and most spectrometers choose a grating as the spectroscopic element. Due to the higher resolution of the immersion grating, it reduces its volume while improving the performance of the spectrometer and reducing the weight of the whole machine, so it is of great significance to study the immersion grating for the practical application of the spectrometer. In this paper, the quartz immersed grating was designed for weak CH4 band. The rectangular groove structure with high refractive index dielectric film was designed in consideration of possible fabrication errors. In the 2.275-2.325μm band, the duty cycle of the rectangular groove is 0.3~0.45, the thickness of TiO2 is 165~170nm and the groove depth is in the 800~980nm region, the diffraction efficiency of the grating is higher than 70%. When the thickness of TiO2 is 170nm and the groove depth is in the 800~950 nm region, the diffraction efficiency of the grating is higher than 80%. When the TiO2 thickness is 175~180 nm, the overall polarization degree is less than 0.1. For trapezoidal grooves, when the duty cycle is 0.3~0.35, the bottom angle of the trapezoid is 80°~86°, the thickness of TiO2 is 195 nm, the depth of the grating groove is 820~900nm, and the diffraction efficiency can be above 80%. When the thickness of TiO2 is 177~195nm, the polarization degree is less than 0.1. The TiO2 film will be deposited by atomic layer deposition.
Imaging spectrometer is an important optical instrument for monitoring the content of carbon dioxide (CO2) in the atmosphere. Using an immersion grating as the dispersive element in the spectrometer can significantly reduce the size of the grating while maintaining the same spectral resolution. The immersion grating with dielectric film is one of the key elements in greenhouse gases imaging spectrometer. In this paper, the quartz immersed grating was designed for weak CO2 band. The rectangular groove structure with high refractive index dielectric film was designed in consideration of possible fabrication errors. In the 1.595 ~ 1.625 μm band, the duty cycle of the rectangular groove is 0.3 to 0.35, the thickness of TiO2 is 110 to 125 nm, and the grating groove depth is 560 to 640 nm, the diffraction efficiency can reach more than 80%. When the duty cycle is 0.4~0.45, the TiO2 thickness is 95~105 nm, and the grating groove depth is 570~700 nm, the diffraction efficiency can reach more than 80%. For the trapezoidal groove, when the duty cycle is 0.3~0.35, the bottom angle of the trapezoid is 82°~86°, the thickness of TiO2 is 120~125 nm, and the grating groove depth is 565~620 nm, the diffraction efficiency can reach more than 80%. When the thickness of TiO2 is 115~120 nm, the overall degree of polarization is less than 0.1.The immersed grating with the period of 697nm in a quartz substrate of 245 mm × 138mm will be fabricated by holographic lithography - ion beam etching in late 2021. The TiO2 film will be deposited by atomic layer deposition.
The immersion grating is one of the key components of the imaging spectrometer due to its high spectral resolution. The silicon immersion grating is conducive to the miniaturization and weight reduction of related satellite-borne instruments to reduce the launch cost of satellite payloads. This paper introduces the application of silicon immersion grating at home and abroad, its working principle and manufacturing methods. The symmetrical trapezoidal groove silicon immersion gratings with groove density of 200 lp/mm and 400 lp/mm were designed. The grating diffraction efficiency and polarization were analyzed by finite element analysis. The photoresist mask was fabricated by UV exposure and holographic exposure, and then the uniform arrays of V-shaped grating on a 2-inch Si substrate were achieved by reactive ion etching and wet etching. The measured results of groove parameters met the design requirements.
Due to the addition of immersion medium, when the diffraction happens in high refractive index medium, the optical path difference and angular dispersion will increase proportionally. Therefore, when immersed grating is used in imaging spectrometer, it will have higher resolution, higher angular dispersion and more compact instrument structure. In this sense the immersed grating is one of the key elements in greenhouse gas hyperspectral spectrometers. In this paper, we have designed the immersed grating with high diffraction efficiency for the atmosphere methane spectral band (the shortwave infrared). The diffraction characteristics of the immersed grating have been investigated by using rigorous coupled wave theory. The simulation showed: the groove depth and duty cycle of the immersed grating must be controlled within the range of 1050nm-1100nm and 0.3-0.36 respectively, in order to guarantee the 1 order diffraction efficiency would be over 80% at wavelength 2300nm. The immersed grating with the period of 1319nm in a silica substrate of 120 mm x 95mm will be fabricated by holographic lithography - ion beam etching in late 2020.
Asteroid exploration has become one of the important ways to understand the origin of the universe, exploit the space mineral resources and protect the earth from the asteroid impact. The imaging spectrometer, which can acquire the spatial and spectral information simultaneously, has become one of the most important payloads for the asteroid exploration. In this paper, the optical design of a broadband Offner imaging spectrometer for the asteroid exploration is presented. It covers from 0.4μm to 3μm, and the F/# is 6 and 3 for the VNIR band (0.4μm-1.0μm) and the SWIR band (1.0μm -3.0μm), respectively. The convex grating in the imaging spectrometer is divided into two regions of different groove spacing, and each region is optimized with different blazed angles to improve the spectral response of this imaging spectrometer. The diffraction efficiency is analyzed with the Comsol software and the result is greater than 0.36 in the wavelength range of 0.4μm to 3μm. After optimized with the optical design software, the diameter of the imaging spectrometer's spot diagram is less than one pixel, and the lowest MTF is 0.460 at the Nyquist frequency. The smile and keystone is less than 10% of the pixel.
Nowadays, the monitoring of CO2 concentration has gradually become the focus of scientists all over the world. In order to study the effects of CO2 on climate change and global ecosystems, hyperspectral and high spatial resolution CO2 detectors are necessary. Holographic grating is the core element of that CO2 detectors, and the immersion holographic grating can greatly improve the resolution of grating and reduce the volume of spectrometer. Therefore, it is necessary to take research on immersion grating. In this paper, we have designed and optimized a quartz immersion grating used in the CO2 detectors. We have designed and optimized the parameters of the grating, such as the width ratio and groove depth, according to the requirements of the spectrometer used and the actual fabrication errors, we designed and optimized rectangular and trapezoidal groove with different bottom angles to obtain high efficiency and low polarization-dependent immersion gratings. In the 2.04-2.08 μm band, with rectangular groove, the groove depth of the quartz immersion grating is 950 nm, the duty cycle is 0.7, the -1 order diffraction efficiency is over 82%, and the degree of polarization is below 12%. When the groove is trapezoidal and the bottom angle is 85 degrees, the -1 order diffraction efficiency is over 79% and the degree of polarization is below 10% at duty cycle of 0.63 and groove depth of 1050 nm. Then under the trapezoidal groove with a bottom angle of 80 degrees, when the duty cycle is 0.62 and the groove depth is 1100 nm, the -1 order diffraction efficiency is over 81% and the degree of polarization is less than 9%. Finally, we will fabricate a sample of immersion grating with a period of 1117 nm on a quartz substrate by holographic ion beam etching in late 2019.
With the rapid development of augmented reality technology, people can clearly see the superposition of virtual and real world images at the same time. The diffractive optical waveguide has obvious advantages over the geometric optical waveguide because of its high diffraction efficiency, light weight and difficulty in generating ghost images. Therefore, the use of holographic gratings as coupling elements for planar waveguides has been widely used in head-mounted display systems. Because high efficiency grating is required as coupling element in coupling, we use slanted trapezoidal surface relief grating as coupling element of the planar waveguide. In this paper, a slanted trapezoidal surface relief grating has been designed as a high-efficiency coupling element with a grating period of 520 nm and a material of BaK3 glass with a refractive index of 1.54. By optimizing the structure of the slanted grating, the relationship between the groove parameters and the diffraction efficiency of the slanted rectangular grating and the slanted trapezoidal grating is analyzed in detail. The results show that under the normal incidence of light at 630 nm, the groove depth is 450 nm, and the slant angle θ1 is 40°, when the -1 order diffraction efficiency of the TE polarized light is higher than 80%, the ranges of the slant angle θ2 and of the maximum diffraction efficiency value would be obtained. This can greatly improve the coupling efficiency of the holographic planar waveguide.
The depolarization grating is needed particularly for use in imaging spectrometers used in sensing the atmosphere weak CO2 spectral band (1595nm - 1625 nm) at spectral resolution in the order of 0.1 nm whilst ensuring a high efficiency for both TE and TM polarizations. The diffraction characteristics of the depolarization grating have been investigated by using rigorous coupled wave theory. The simulation results show: the groove depth and duty cycle of the depolarization grating must be controlled within the range of 730nm-780nm and 0.3-0.37 respectively, in order to guarantee the 1 order diffraction efficiency is over 70% for both TE and TM polarizations at the atmosphere weak CO2 spectral band. The depolarization grating with the period of 869nm in a fused-silica substrate of 120 mm × 97mm will be fabricated by holographic lithography - ion beam etching in late 2018.
Hyperspectral imaging spectrometer is an important measurement and analysis instrument, which combines imaging with spectrum technology. Nowadays, it has been widely used in research fields of atmosphere, ocean, geology, ecology, astronomy and so on. Convex blazed grating is a key component in the hyperspectral imaging spectrometer. At present, holographic-ion beam etching is an important method to fabricate convex blazed gratings. The common way of holographic-ion beam etching is that to etch the photoresist grating mask directly. However it is difficult to control the groove of photoresist mask accurately. This paper proposes a method of fabricating convex blazed grating by native substrate grating mask, ion beam etching and reactive ion beam etching are used to fabricate a native substrate grating mask based on photoresist grating. The diffraction efficiency of the convex blazed grating is investigated by FDTD (Finite Difference Time Domain) theory. The first-order diffraction efficiency can be over 45% within visible to near-infrared waveband through controlling the blaze angle from 6.4° to 7.2°. Furthermore, a convex blazed grating has been fabricated with the period of 2.45um, the blaze angle of 6.8° and the anti-blaze angle of 60°, theoretical analysis shows that the first-order diffraction efficiency is more than 50% within visible to near-infrared waveband.
In this paper the diffraction characteristics of the blazed grating is investigated by using rigorous coupled-wave analysis, which indicates that within the near infrared broadband, the first-order diffraction efficiency can be over 10% at 1.2μm and over 65% at 2.3μm through controlling the blaze angle of the blazed grating. The blazed grating with the period of 8.33μm, the blaze angle about 7.3 degree, and the antiblaze angle about 41.8 degree has been fabricated by holographic lithography - ion beam etching. Theoretical analysis has showed that the diffraction efficiency of these blazed grating can reach 15% at 1.2μm, 73% at 2.3μm and 75% at blazed wavelength.
The high diffraction efficiency of the convex blazed gratings cannot be realized in the broadband. Therefore, we have designed the convex dual-blazed grating to realize higher and uniform diffraction efficiency in the wavelength from 0.4 μm to 2.5 μm. The diffraction efficiency of the convex dual-blazed grating is investigated by using rigorous coupled wave analysis. The results show: when the two blaze angles of the convex dual-blazed grating, one within the range from 2.7 degree to 3.4 degree and the other one within the range from 8 degree to 9 degree respectively, the first-order diffraction efficiency is more than 25 percent at the visible-near infrared band. The convex dual-blazed grating with the period of 5μm in the center will be fabricated by holographic lithography-segmented ion beam etching in late 2017.
Multiview holographic 3D display based on the nano-grating patterned directional diffractive device can provide 3D images with high resolution and wide viewing angle, which has attracted considerable attention. However, the current directional diffractive device fabricated on the photoresist is vulnerable to damage, which will lead to the short service life of the device. In this paper, we propose a directional diffractive device on glass substrate to increase its service life. In the design process, the period and the orientation of the nano-grating at each pixel are carefully calculated accordingly by the predefined position of the viewing zone, and the groove parameters are designed by analyzing the diffraction efficiency of the nano-grating pixel on glass substrate. In the experiment, a 4-view photoresist directional diffractive device with a full coverage of pixelated nano-grating arrays is efficiently fabricated by using an ultraviolet continuously variable spatial frequency lithography system, and then the nano-grating patterns on the photoresist are transferred to the glass substrate by combining the ion beam etching and the reactive ion beam etching for controlling the groove parameters precisely. The properties of the etched glass device are measured under the illumination of a collimated laser beam with a wavelength of 532nm. The experimental results demonstrate that the light utilization efficiency is improved and optimized in comparison with the photoresist device. Furthermore, the fabricated device on glass substrate is easier to be replicated and of better durability and practicability, which shows great potential in the commercial applications of 3D display terminal.
The wire grid polarizer with a particular degree of polarization is an important reference in polarization calibration field in recent years. In this paper, the wire grid polarizer at a wavelength of 670 nm was analyzed. To obtain five different degrees of polarization, five wire grids profiles were optimized by using rigorous coupled-wave theory. The profile of the wire grid was rectangular surface-relief grating, which was determined by the period, the duty cycle, the groove depth and the metal material. We also studied the effects of the deviations of the wire grid profile from optimized parameters on the degrees of polarization and transmittance. Holographic lithography and ion beam etching will be adopted to fabricate the wire grids in late 2017.
A measurement system for diffraction efficiency of convex gratings is designed. The measurement system mainly
includes four components as a light source, a front system, a dispersing system that contains a convex grating, and a
detector. Based on the definition and measuring principle of diffraction efficiency, the optical scheme of the
measurement system is analyzed and the design result is given. Then, in order to validate the feasibility of the designed
system, the measurement system is set up and the diffraction efficiency of a convex grating with the aperture of 35 mm,
the curvature-radius of 72mm, the blazed angle of 6.4°, the grating period of 2.5μm and the working waveband of
400nm-900nm is tested. Based on GUM (Guide to the Expression of Uncertainty in Measurement), the uncertainties
in the measuring results are evaluated. The measured diffraction efficiency data are compared to the theoretical ones,
which are calculated based on the grating groove parameters got by an atomic force microscope and Rigorous Couple
Wave Analysis, and the reliability of the measurement system is illustrated. Finally, the measurement performance of the
system is analyzed and tested. The results show that, the testing accuracy, the testing stability and the testing
repeatability are 2.5%, 0.085% and 3.5% , respectively.
The polarization characteristics of the metal wire-grid polarizer are investigated by Finite Difference Time Domain method. To achieve a high extinction ratio and transmissivity in visible light, the metal wire-grid profile was optimized. The simulation results show that the period should be less than 200 nm. When the period, the glass substrate etching depth and the Al groove depth of metal wire-grid were at 180nm, 20nm and 120nm respectively, the extinction ratio could reach the maximum 40 dB; the transmissivity could reach the maximum 85%. Herein, considering the high reflection of metal aluminum in the manufacturing process, we propose using the copper as anti-reflection coating. Finally, a prototype of the metal wire-grid polarizer has been fabricated by holographic-immersion lithography and ion beam etching.
Owing to damage, thermal issues, and nonlinear optical effects, the output power of fiber laser has been proven to be limited. Beam combining techniques are the attractive solutions to achieve high-power high-brightness fiber laser output. The spectral beam combining (SBC) is a promising method to achieve high average power output without influencing the beam quality. A polarization independent spectral beam combining grating is one of the key elements in the SBC. In this paper the diffraction efficiency of the grating is investigated by rigorous coupled-wave analysis (RCWA). The theoretical -1st order diffraction efficiency of the grating is more than 95% from 1010nm to 1080nm for both TE and TM polarizations. The fabrication tolerance is analyzed. The polarization independent spectral beam combining grating with the period of 1.04μm has been fabricated by holographic lithography - ion beam etching, which are within the fabrication tolerance.
The convex grating is one of the key elements in Offner imaging spectrometers. In this paper the diffraction efficiency of the convex grating is investigated by using rigorous coupled-wave analysis (RCWA), which indicates that within the visible and near infrared broadband, the first-order diffraction efficiency can be over 40% through controlling the blaze angle of blazed convex grating. The blazed convex grating with the period of 6.17μm in the center, the blaze angle about 2.9 degree, the anti-blaze angle about 21 degree, and the ruled area – a convex substrate with its radius 36.31mm and aperture 23.6mm has been fabricated by holographic lithography - scan ion beam etching. Experimental measurements show that the first-order diffraction efficiency is more than 30%, and the first-order diffraction efficiency at blazed wavelength can reach 62%.
To overcome the disadvantages of high-cost, low-efficiency and the difficulty in the realization of the high aspect ratio structure in the fabrication of nanoimprint mold with deep-subwavelength structures by electron beam lithography (EBL), the holographic immersion lithography - ion beam etching is adopted. There are two major challenges of this method: the holographic immersion lithography and the ion beam etching. The former one is to fabricate the photoresist mask with deep-subwavelength structures; and the latter one is to transfer the photoresist mask to the fused silica. Both the two steps have been optimized to achieve the accurate control of the high aspect ratio of the profile. The experiment indicates that titled rotation of the ion beam etching combined with reactive ion beam etching can achieve the accurate control of the high aspect ratio structure. The nanoimprint mold with the period of 180nm and the groove depth of 233nm has been fabricated.
The blazed convex grating is one of the key elements in Offner imaging spectrometers. In this paper the diffraction characteristics of the blazed convex grating is investigated by using rigorous coupled-wave analysis. The results indicate that within the wavelength from 1 μm to 2.5 μm, the first-order diffraction efficiency can be over 40% through controlling the blazed angle of blazed grating. Furthermore, when the blazed angle is 3.9 degree, the first-order diffraction efficiency is still over 40% with the vertex angles between 120 degree and 150 degree. The blazed convex grating with the spatial frequency of 73 L/mm in the center will be fabricated by holographic lithography-ion beam etching.
In this paper, a sub-wavelength metal grating polarizer at visible region is designed by Finite Difference Time Domain (FDTD) method. We have analyzed the effects of the period, the thickness of metal layer and the duty cycle on the performance of the metal grating polarizer. Simulation results show that in order to increase the metal grating transmissivity and extinction ratio and to reduce the angle sensitivity (±30°), the period should be less than 200 nm; Therefore, the structure with 180nm period and an 120nm Al layer is chosen, and the transmission efficiency and extinction ratio is more than 75% and 25dB respectively. Finally, a prototype of the metal grating has been fabricated by holographic-immersion lithography and ion beam etching.
The echelle gratings with the ultra-high resolution are one of the key elements in spectroscopy, optical communications and other fields. Currently, the diamond ruling and the wet etching technique are two primary methods to fabricate echelle gratings. In this paper, we have adopted a new method of the echelle gratings fabrication. Firstly, the holographic lithography is used to form a photoresist grating mask. Then, reactive ion etching is adopted to fabricate the native substrate grating mask to replace the traditional photoresist grating mask, which allows more accurate control of the profile. Finally, the tilted ion-beam etching is used to etch the native substrate grating to ensure the precise control of the blazed angle and anti-blazed angle. A prototype of the echelle gratings with a line density of 80 lp/mm has been fabricated by above method.
The holographic lithography - ion beam etching is adopted to fabricate the nanoimprint template with periodic structures. The accurate control of the high aspect ratio of the profile is achieved by the optimization of the holographic lithography and the choice of the appropriate parameters of ion beam etching. There are two major challenging steps of this method: 1) the holographic exposure and development in the fabrication of the photoresist mask and 2) the ion beam etching to transfer the photoresist mask to the fused silica. The experiment indicates that titled rotation of the ion beam etching combined with reactive ion beam etching can achieve the accurate control of the high aspect ratio structure. Two types of nanoimprint template have been fabricated: the period of 250nm and the groove depth of 380nm; the period of 600nm and groove depth of 1400nm, respectively.
A holographic dual-blazed grating with the period of 833 nm at ultraviolet-visible-near infrared was designed. To
achieve higher and uniform diffraction efficiency, the grating profile was optimized by using rigorous coupled-wave
analysis. The results show: when the two blaze angles of the dual-blazed grating, one within the range from 10 degree to
12 degree and the other one within the range from 26 degree to 32 degree respectively, the first-order diffraction
efficiency is more than 30 percent within the wavelength from 0.25μm to 1μm. The holographic dual-blazed grating, one
half with the blaze angle of 11 degree and the anti-blaze angle of 72 degree; and the other half with the blaze angle of 29
degree and the anti-blaze angle of 56 degree respectively, have been fabricated by improved holographic ion beam
etching.
The Offner-like spectrometer, one most widely used hyperspectral imaging spectrometers, offers some advantages
over other spectrometers used in pushbroom imaging spectrometry: low chromatic aberrations, a compact size with low
optical distortion, and large numerical aperture. The standard Offner spectrometer is made of three spherical concentric
elements-- two concave mirrors and one convex grating. Convex grating is the core part of Offner type hyper-spectral
imager. Considering the difficulties in fabrication of small angles convex gratings by traditional ways, we propose a new
method. The ion-beam-etching holographic grating is adopted to obtain the convex blazed gratings with blazed angle of
4°--5°, whose angle can be further reduced by dipping or spinning coating with hardenable liquids. A highly
reproducible blaze angle reduction to as high as a factor of 3 is achieved by controlling the spinning speed and viscosity
of solution. The precise control of the blaze angles and groove profiles will be further studied focusing on the designing
of rotate dip equipment and optimization of pulling speed and viscosity of solution.
The blazed grating is one of the key elements in a spectrometer. Hence we have adopted a new method of its fabrication. First, the combination of ion beam etching (IBE) and reactive IBE is used to fabricate a native substrate grating mask to replace the traditional photoresist grating mask. This is because the former mask allows for more accurate control of the profile than the latter. Then tilted IBE is used to etch the native substrate grating to ensure the precise control of the blazed angle and antiblazed angle. Therefore, an optimal blazed grating profile is successfully fabricated. Two types of blazed gratings with the same period of 833 nm have been fabricated: one with the blaze angle of 11 deg and the antiblaze angle of 72 deg; and one with the blaze angle of 20 deg and the antiblaze angle of 68.5 deg, respectively.
The infrared polarizers are widely used in the infrared imaging systems as the core components, such as infrared stealth, target acquisition and mine detection, automobile night-vision instrument and other systems. For the requirements of near-infrared imaging systems, a nanowire-grid is designed by Finite Difference Time Domain (FDTD) method. Herein, considering the high reflection of metal aluminum in the manufacturing process, we propose a structure with aluminum-copper nanowire-grid. FDTD method is adapted to analyze the effects of the thickness of aluminumcopper in different combinations on the TM and TE polarization transmission efficiency as well as the extinction ratio when the grating’s period is 300nm. Numerical results and theoretical analysis show that: the reflection on the substrate is suppressed with the optimal thickness of the Cu layer. Considering the resist-substrate reflectivity and the final performance of the polarizer, the structure with an 120nm Al layer, and a 50nm anti-reflection Cu layer is chosen; and the TM transmission efficiency is more than 71%, and the extinction ratio is more than 25dB. At last we used Holographic lithography and IBE to fabricate a prototype of the nanowire-grid.
This paper designs a sub-wavelength metal polarization gratings array composing two orthogonal micro polarization
gratings as one unit. Effects of different metal materials and grating profile on the TM and TE polarization transmittance
as well as the extinction ratio are analyzed by the Finite Difference Time Domain (FDTD) method. Based on the
requirement of the visible light polarization imaging and the resolution of the holographic lithography, we obtain the best
parameters for the grating: period is 250nm, Al thickness is 260nm, and duty cycle is 0.4, the transmittance is higher than
45% and extinction ratio is higher than 100. Then, the sub-wavelength metal polarization gratings have been fabricated
by two methods: the holographic - ion beam etching - Al oblique deposition or the holographic - reactive ion etch (RIE) -
ion beam etching. Preliminary results indicate the polarization information has been obtained. A prototype metal
polarization gratings array will be fabricated in late 2011.
Polarization imaging is a powerful tool to observe hidden information from an observed object, for instance, degree of
polarization, polarization azimuth and polarization ellipticity. According to the request of the visible light polarization
imaging, a metal sub-wavelength polarization gratings array was designed based on FDTD method. And its principle was
analyzed by effective medium theory. The effects of metal thickness and duty ratio on the TM and TE polarization
transmission efficiency as well as the extinction ratio was analyzed by FDTD method when the grating's period is 200nm
and 250nm, and the metal is Al. When the metal sub-wavelength polarization gratings array works in normal incidence,
its TM transmission efficiency are more than 45%, and the extinction ratios are more than 3.5×103 in all visible light
spectrum. Numerical results and theoretical analysis show that the designed metal sub-wavelength polarization gratings
array is a polarizer with high TM polarization transmission efficiency and high extinction ratios.
A fused-silica polarization grating at a wavelength of 1053 nm was designed. To achieve a high extinction ratio and
efficiency, the grating profile was optimized by using rigorous coupled-wave analysis. The results showed: when the
grating period, the duty cycle and the groove depth of polarization grating were at 650nm, 0.39 and 1340nm respectively,
the extinction ratio could reach the maximum 34700; the efficiencies of the TE-polarized wave in the -1st order and the
TM-polarized wave in the 0th order were 92.2% and 99.5% respectively. We also analyzed the effects of the deviations
of the period and depth from optimized parameters on the extinction ratio and efficiency. The holographic lithography
and the ion beam etching will be applied to fabricate a prototype polarization grating in late 2010.
The convex grating is one of the key elements in hyperspectral imaging spectrometers. In this paper the diffraction
characteristics of the convex blazed grating is investigated by using rigorous coupled-wave theory, which indicates that
within the wavelength from 0.4μm to 0.8μm, the plus first-order diffraction efficiency can be over 35% through
controlling the blaze angle of blazed grating. The convex blazed grating with the period of 5μm in the center, the blaze
angle about 4.3 degree, and the ruled area - a convex substrate with its radius 72mm and aperture 35mm has been
fabricated by holographic- scan ion beam etching. Experimental measurements show that the plus first-order diffraction
efficiency is more than 35%, within the wavelength from 0.4μm to 0.8μm.
To alleviate the difficulty in controlling the profile of the photoresist grating mask, advanced segment motion algorithm
has been applied to simulate and analyze the evolution of the surface contour in ion beam etching. The simulation
indicates that the combination of ion beam etching and reactive ion beam etching can effectively control the duty cycle
of the holographic ion beam etching gratings. This has also been validated by the fabrication of four kinds of gratings
with different periods (6.66μm, 3.33μm, 2μm, 1μm). This finding contributes to the fabrication of the holographic ion
beam etching gratings because it helps to simplify the holographic exposure and development.
The convex grating is one of the key elements in Offner hyperspectral imaging spectrometers. In this paper the design of
the holographic convex grating focuses on the optimization of its recording parameters for aberration correction.
Meantime, the diffraction efficiency of the convex grating is discussed by using rigorous coupled-wave theory. The
results show: within the wavelength range from 0.4μm to 0.78μm, the diffraction efficiency can be over 20% and up to
40% through controlling the groove depth and duty cycle of rectangular grating; the diffraction efficiency can be over
35% through controlling the blaze degree of the blazed grating. The rectangular surface-relief convex grating with the
period of 5μm in the center, and the ruled area - a convex with its radius 100mm and aperture 40mm has been fabricated
by holographic-ion beam etching. A prototype blazed convex grating will be fabricated in late 2010.
Using chirped phase mask to fabricate chirped fiber Bragg gratings (CFBG) is a very important method. In this paper, a
linearly chirped phase mask with the period of 1000nm in the center, ruled area 100×10mm2 and 1nm/mm chirp rate has
been fabricated by a new technique, which combines holographic-ion beam etching and reactive ion beam etching. This
involves two main steps: formation of a grating mask by holographic interference exposure and development, and
transferring it into the substrate to form a permanent phase mask by both ion beam etching and reactive ion beam
etching. Experimental measurements show that the zero order diffraction efficiency is less than 2% and the plus and
minus first-order diffraction efficiency is more than 35%; the nonlinear coefficient is 1.6%. Theoretical analysis also
indicates that these phase masks can be used to fabricate the UV written CFBG.
The optimized design, alignment and experimental results of a compact convex grating hyper-spectral imager with high
fidelity are reported and evaluated. The imager works in visible wavelength range from 0.4 to 0.78 μm. The numerical
aperture of system is 0.2, and the entrance slit images with -1 magnification and linearly dispersed into 7.5mm in width.
In addition, the spectral sample resolution is 0.76nm/pixel with negligible distortion. In order to get good performance
and facilitate alignment, the optical system is both imagery and objective telecentric. The efficiency of the convex
grating can up to about 40 percents by ion-beam etching. The size is 190mm×180mm×90mm and the mass is less than
1kg. The light weighted compact system is portable, and it is feasible in remote sensing.
To alleviate the difficulty of ion beam etching, it is essential to acquire the low gradient holographic photoresist grating
masks as well as the high gradient ones (i.e. the rectangular holographic photoresist grating masks). According to the ion
beam etching experiments, the low gradient photoresist grating masks can effectively eliminate the redeposition. The
control of the profile of holographic photoresist grating masks is investigated in this paper. Considering the effects of
developing time and temperature of the developer on the response of photoresist and adopting the combination of
computer simulation and experiments, a low gradient photoresist grating mask: 30% duty cycle at bottom and 65°
gradient can be fabricated. The study indicates the increase of the developing time causes the diminish of the threshold
volume of exposure while the increase of the slope of the linearity of photoresist response; the rise of the temperature of
the developer leads to both the increase of the threshold volume of exposure and the slope of the linearity of photoresist
response. As a result non-1:1 interferential exposure and low-temperature development are required in the fabrication of
low gradient photoresist grating masks. The 1:7 interferential exposure and 15°C- development have been adopted in this
experiment.
The beam sampling grating (BSG) is one of the important diffractive optical elements used in the final target system of the inertial confinement fusion (ICF). The BSG is designed in this paper. By use of optical holographic theory, achromatic optical system on recording holographic BSG mask is designed. Its aberration is analyzed by using optical design software. Meantime, the diffraction efficiency of the BSG is discussed by using rigorous coupled-wave theory. Maintaining spatial uniformity of the diffraction efficiency across the grating aperture (320mm×320mm) is the most challenging processing requirement. The results shows: there are duty cycle and groove depth in grating parameter, the groove depth is more important to spatial uniformity of diffraction efficiency. Some useful information for practical fabrication of the BSG is given also. A prototype beam sampling grating will be fabricated in late 2005.
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