Working Wavelength Range Research Articles

Standing Wave Field Distribution in Graded-Index Antireflection Coatings

Standing wave field distributions in three classic types of graded-index antireflection coatings are studied. These graded-index antireflection coatings are designed at wavelengths from 200 nm to 1200 nm, which is the working wavelength range of high energy laser system for inertial-fusion research. The standing wave field distributions in these coatings are obtained by the numerical calculation of electromagnetic wave equation. We find that standing wave field distributions in these three graded-index anti-reflection coatings are quite different. For the coating with linear index distribution, intensity of standing wave field decreases periodically from surface to substrate with narrow oscillation range and the period is proportional to the incident wavelength. For the coating with exponential index distribution, intensity of standing wave field decreases periodically from surface to substrate with large oscillation range and the period is also proportional to the incident wavelength. Finally, for the coating with polynomial index, intensity of standing wave field is quickly falling down from surface to substrate without an obvious oscillation. We find that the intensity of standing wave field in the interface between coating and substrate for linear index, exponential index and polynomial index are about 0.7, 0.9 and 0.7, respectively. Our results indicate that the distributions of standing wave field in linear index coating and polynomial index coating are better than that in exponential index coating for the application in high energy laser system. Moreover, we find that the transmittance of linear index coating and polynomial index coating are also better than exponential index coating at the designed wavelength range. Present simulation results are useful for the design and application of graded-index antireflection coating in high energy laser system.

Comparison of Integrated Optical Phase Shifters Designed in Different Regime

A comparative study, in aspects of both wavelength dependence and fabrication tolerance, is carried out between silica-based phase shifters designed in two different regime, namely length difference regime and refractive index difference regime. Results show that in the wavelength range of 1500-1600 nm, phase shifter designed in refractive index difference regime has a working wavelength range 2.8~3.1 times wide as that designed in length difference regime; while in the aspect of fabrication tolerance, phase designed in length difference regime is advantageous, with respect to waveguide core dimension error, and waveguide core refractive index error as well.

Design of ITER divertor VUV spectrometer and prototype test at KSTAR tokamak

Design and development of the ITER divertor VUV spectrometer have been performed from the year 1998, and it is planned to be installed in the year 2027. Currently, the design of the ITER divertor VUV spectrometer is in the phase of detail design. It is optimized for monitoring of chord-integrated VUV signals from divertor plasmas, chosen to contain representative lines emission from the tungsten as the divertor material, and other impurities. Impurity emission from overall divertor plasmas is collimated through the relay optics onto the entrance slit of a VUV spectrometer with working wavelength range of 14.6–32 nm. To validate the design of the ITER divertor VUV spectrometer, two sets of VUV spectrometers have been developed and tested at KSTAR tokamak. One set of spectrometer without the field mirror employs a survey spectrometer with the wavelength ranging from 14.6 nm to 32 nm, and it provides the same optical specification as the spectrometer part of the ITER divertor VUV spectrometer system. The other spectrometer with the wavelength range of 5–25 nm consists of a commercial spectrometer with a concave grating, and the relay mirrors with the same geometry as the relay mirrors of the ITER divertor VUV spectrometer. From test of these prototypes, alignment method using backward laser illumination could be verified. To validate the feasibility of tungsten emission measurement, furthermore, the tungsten powder was injected in KSTAR plasmas, and the preliminary result could be obtained successfully with regard to the evaluation of photon throughput.

Laser absorption precursor research: microstructures in the subsurface of mono-crystalline silicon substrate.

In high-energy laser systems, the energy absorption coefficient of silicon optical elements is one of the most critical performance indicators. The absorption coefficient of substrate limits the absorption of the overall elements. Since mono-crystalline silicon is transparent in working wavelength range, the subsurface absorption precursors also influence the entire absorption dramatically. In this paper, the subsurface of a super-polished silicon substrate is exposed by ion beam etching (IBE) as deep as 4.6μm. In different depth layers, morphology and energy absorption are measured with an atom force microscope and photothermal instrument, respectively. In the 100nm layer, microstructures are found, and their heights decrease while widths increase with IBE. Finally, structures are diminished below the 1.12μm layer. Absorption increases with the structures' appearance. When the structures are fully exposed, absorption reaches the peak value, 327.5% of the unremoved surface. Once structures are removed, the absorption value falls down to the lowest point, 67.5%, which verifies that structures influence the absorption significantly. According to the structure depth and energy dispersive spectrometer results, the structures are most likely the densificated micro zones, generated by fabrication processes. In practical fabrication, a subsurface layer of 1.12μm thick needs to be removed by stress-less processes, to obtain a low-absorption element.

A high-speed, eight-wavelength visible light-infrared pyrometer for shock physics experiments

An eight-channel, high speed pyrometer for precise temperature measurement is designed and realized in this work. The addition of longer-wavelength channels sensitive at lower temperatures highly expands the measured temperature range, which covers the temperature of interest in shock physics from 1500K-10000K. The working wavelength range is 400-1700nm from visible light to near-infrared (NIR). Semiconductor detectors of Si and InGaAs are used as photoelectric devices, whose bandwidths are 50MHz and 150MHz respectively. Benefitting from the high responsivity and high speed of detectors, the time resolution of the pyrometer can be smaller than 10ns. By combining the high-transmittance beam-splitters and narrow-bandwidth filters, the peak spectrum transmissivity of each channel can be higher than 60%. The gray-body temperatures of NaI crystal under shock-loading are successfully measured by this pyrometer.

Widely wavelength tunable fast intensity-modulated light source for biophotonic applications.

High modulation depth, fast (megahertz to gigahertz), intensity-modulated light sources of various wavelengths within the 0.7-1.35μm bio-penetration window are highly desirable for many biophotonic diagnosis systems. In this Letter, we present a novel scheme of a wavelength tunable, ultra-broadband light source which simply consists of a pump laser, a nonlinear fiber, and demodulation circuits. The working wavelength range of the light source is from 0.7 to 1.35μm which covers a vast part of the bio-penetration window, and its modulation frequencies extends from tens of megahertz to gigahertz. The performances of the proposed light source in either working wavelength range or modulation frequency bandwidth are much superior to any typical laser diodes or solid state lasers currently employed in the frequency-domain or other biophotonic utilization. The wide applicability of this novel light source in diverse biophotonic applications can be observed from our carefully designed diffused optical spectroscopy phantom measurement.

Design of Plasmonic Modulators With Vanadium Dioxide on Silicon-on-Insulator

We present design of plasmonic modulators using vanadium dioxide (VO2) as modulating material realized on silicon-on-insulator (SOI) wafer with only 200 nm × 140 nm modulating section within 1 μ m × 3 μ m device footprint. By utilising the large refractive index contrast between the metallic and semiconductor phases of VO2, the modulator can achieve a broad working wavelength range from 1100 to 1800 nm around C-band, with a high modulation depth of 21.5 dB/ μ m. We also analyse effects of using seed layer of different dielectric materials for growing VO2 on modulation index by exploring the mixed combination of VO2 and different dielectric materials. Our device geometries can have potential applications in the development of next-generation miniaturised high-frequency optical modulators in silicon photonics for optical communications.

Ultra-compact visible chiral spectrometer with meta-lenses

Conventional compact spectrometers have a fixed spectral resolution and cannot resolve the polarization properties of light without additional optical elements, while their larger counterparts are bulky and costly. Here, we demonstrate multiple off-axis meta-lenses in the visible integrated on a single planar substrate. They possess both focusing and strongly dispersive properties and are designed to provide different spectral resolutions as well as working wavelength ranges on the same chip. We realize a compact spectrometer using only these meta-lenses and a CMOS camera and achieve detector-limited spectral resolutions as small as 0.3 nm and a total working wavelength range exceeding 170 nm for a beam propagation length of only a few cm. In addition, this spectrometer has the capability to resolve different helicities of light in a single measurement. This chip-camera setup represents the most compact configuration so far achieved for a spectrometer with similar performance and functionality, and its compatibility with large-scale fabrication processes makes it broadly applicable.

Integrated High-Performance Two-Stage Ratiometric Wavelength Monitors on Silicon

We design and fabricate an integrated high-performance two-stage ratiometric wavelength monitors (RMWMs) on silicon. The first stage is a coarse wavelength monitor with a wide working range based on two unbalanced Mach–Zehnder interferometers acting as edge filters. The second stage is a fine wavelength monitor with picometer wavelength resolution utilizing two tunable microrings acting as edgefilters. The highest measured wavelength resolution can reach 1.6 pm in a 0.7-nm-wide wavelength range for the fine wavelength monitor. The working wavelength range can reach about 55 nm with a resolution of 0.1 nm for the coarse wavelength monitor. By thermally tuning the resonant wavelengths of microrings, we can obtain high resolution on the whole 9.6-nm free spectral range of the microrings. The influence of perturbation of temperature is also discussed. This letter can solve the problem that the measurable wavelength range and resolution limit each other in a conventional RMWM.

Development of New Sensitive Broadband Elements of Sensors Based on Carbon Nanotubes

A new type of sensitive elements based on carbon nanotubes (SECNT) was developed for photoelectric detectors of optical radiation. The optical parameters of the SECNT were tested. The working wavelength range was found to be 500-8000 nm. The maximum photosensitivity of the SECNT reached 0.7 mA/W in the near-IR range of the spectrum. The time resolution of the sensitive element was no worse than 30 μsec.

Subwavelength beam focusing via multiple-metal slits arranged along a triangle surface

A compact plasmonic structure is proposed to actualize the subwavelength beam focusing, through a metal slit array arranged along a triangular or trapezium surface profile. The incident light passes through the metal slits in the form of surface plasmon polaritons (SPPs) and then scattered into radiation fields. The constructive interference of radiation fields from individual slits with different depths and widths gives rise to beam focusing. The advantages of the proposed plasmonic lens are having a much smaller lateral dimension and broad working wavelength range. This is of importance for realizing densely integrated photonic circuits. The finite-difference time-domain (FDTD) method is employed to verify the proposed design. The simulation results indicate that the focal spot is beyond the diffraction limit.

MWIR/LWIR filter based on Liquid–Crystal Fabry–Perot structure for tunable spectral imaging detection

An electrically tunable medium-wave infrared (MWIR)/long-wave infrared (LWIR) filter based on the key structure of Liquid–Crystal (LC) Fabry–Perot (FP), which works in the wavelength range from 2.5μm to 12μm, is designed and fabricated successfully in this paper. According to the optical interference principle of the FP cavity and electrically controlled birefringence of nematic LC molecules, the particular functions including spectral selection and spectral staring and spectral adjustment, can be realized by the developed MWIR/LWIR filter driven and controlled electrically. As to the LC-FP filter, both planar reflective mirrors are shaped by depositing a layer of aluminum (Al) film (∼60nm) over one side of double-side polished Zinc Selenide (ZnSe) wafer (∼1mm), and then polyimide (PI) layer with the thickness of ∼100nm is coated directly on Al film. With typical sandwich architecture, the depth of the cavity with nematic LC molecules sealed in is ∼7.5μm. To make sure the LC molecules parallel aligned and twist regularly under voltage driving signal applied on Al film, which also acts as electrode, the V-grooves are formed in PI layer with the depth of ∼90nm and the width of ∼350nm at average by strong rubbing. The typical transmission spectrum in MWIR&LWIR wavelength range and several spectral images in MWIR wavelength range based on the fabricated LC-FP filter, have been obtained through applying a voltage driving-signal with different root-means-square (RMS) value over the electrodes of LC-FP filter in the selected voltage range from 0VRMS to 19.8VRMS. The testing result demonstrates a prospect of realization smart spectral imaging and further integrating the LC-FP filter with infrared focal plane arrays (FPAs) to achieve the purpose infrared multispectral imaging. The developed MWIR&LWIR LC-FP filters show some obvious advantages such as wide working wavelength range, electrically tunable spectral selection, ultra-compact, low cost, being compatible with standard microelectronic technology.

用于深空探测的宽谱段大视场小型光学系统设计

A light optical system with wide bands,large field of view and lightweight was designed.The focal length is 35 mm,F-number is 7.5,working wavelength range is 400~900nm,and a view angle is60°.A complex double-Gauss construction was applied in the designed system,and spherical surfaces were used in all lens,so that the total length could be 115 mm and minimum modulation transfer function could be greater than 0.45 at 70lp/mm.Barrel distortion and aberration vignetting were used to improve illuminance distribution homogeneity,marking the margin field illuminance reach 80% and the image plane illumination uniformity be 8%.This optical system has a good thermo-optical properties,can achieve fine image quality at the working temperature range of 0~40Celsius,which is suitable for the deep space exploration.

Study and analysis of light infrared detection system with dual spectrum and wide temperature range

Because harmonic diffractive optics elements has special achromatism, athermalization and arbitrary phase modulation characteristics, an infrared detection system method with dual spectrum and wide temperature range was presented based on the most advanced infrared dual color detector which had a format of 320 X 240 and the pixel pitch of 30 microm. A hybrid refractive/ harmonic diffractive infrared detection system with dual spectrum and wide temperature range was designed. The working wavelength range was 3. 8 - 4. 2 and 8. 8 -11. 2 microm. The system was only consisted of three lenses, including one aspheric surfaces and a harmonic diffraction surface, which made the system have compact structure and light weight. In the temperature range -120 -200 degreesC, the RMS radius of spot diagram in 3. 8 approximately 4. 2 and 8. 8-11. 2 pm was 19. 07 and 17. 75 microm respectively, which is less than the pixel size of infrared detector with 30 m, the enclosed energy in 30 microm, the enclosed energy in 3. 8-4. 2 and 8. 8-11. 2 microm is 88. 7% and 82. 4% in two pixel size. The method and structure was convenient and predominant. It was proved that the design was feasible.

Large-angle beam splitter with sensitive adjustable power ratio based on superprism effect

A large-angle beam splitter (BS) with adjustable power ratio based on the superprism effect in a two-dimensional photonic crystal (PC) slab is proposed. The finite-difference time-domain results show that by appropriately truncating the PC termination, two split beams with a large separation angle can be achieved over a relative bandwidth of 13.7%. The power ratio of the two split beams can be sensitively adjusted by slightly changing the incident angle or insensitively adjusted by changing the position of the light source, while there are almost no changes in their propagation directions. Further calculations reveal that the cylinder radius of PC has wide tolerance for the BS, which is favourable for practical fabrication. As an example, we proposed a sensitive double-throw optical switch with working wavelength range from 1432 to 1644 nm based on the proposed structure.

Observation of the Two-Dimensional Distribution of Impurity Line Emissions Using a Space-Resolved EUV Spectrometer in LHD

A space-resolved EUV spectrometer for measuring the one-dimensional distribution of impurity line emissions in Large Helical Device (LHD) has been upgraded to measure two-dimensional distributions of impurity line emissions with an extension of working wavelength range to 30∼650Å. The two-dimensional measurement is performed by scanning the observation chord horizontally. A rectangular plasma region of 520 × 700 mm2 in vertical and horizontal sizes can be observed during a single horizontal scan. The horizontal scan requires a time duration of 5 s at least. The spatial resolution is 10 mm in the vertical direction when a spatial-resolution slit of 0.2 mm in width is adopted. Although a spatial resolution in the toroidal direction is 75 mm, it is a function of CCD exposure time and horizontal scanning speed. Two-dimensional distribution of EUV line emissions from several impurities has been successfully observed for the first time from steady discharges in LHD. In this paper two-dimensional distributions of He II (303.78 Å), C V (40.27 Å), C VI (33.73 Å) and Fe XX (132.85 Å) located at different radial positions are presented with simple analysis on the magnetic field structure of LHD.

Design and Fabrication of an Integrated Multi-Channel Optical Filter Working in Visible and near-Infrared Bands

Multi-channel optical filter can separate the target light so that it can be integrated in the multi-spectral imaging system for collecting the images in different spectrum. However, conventional multi-channel optical filter cannot work in visible band and near-infrared band simultaneously. This paper will present a novel method for fabricating the multi-channel filter which can work for the two bands. The method combines pigment-based colorant photoresist microlithography with traditional multi-film vacuum deposition technologies so that the filter has a high integrated level. The results of test show that the filter can be used for the wavelength range from 400 to1200 nm. Because of its wide working wavelength range, this optical filter has a great prospect of being applied in the fields of day and night surveillance, medical imaging, and so on.

Extreme Ultraviolet (EUV) Emission Measurement from a Coaxially Focused Plasma for Various Pressures of an Argon and Ne-Xe Gas Mixture

Ar and Ne-Xe (30%) plasmas were generated in a dense plasma-focus device with coaxial electrodes extreme ultraviolet (EUV) lithography. The EUV intensity and the electron temperature were investigated. An input voltage of 4.5 kV was applied to a capacitor bank of 1.53 µF, and the diode chamber was filled with Ar and Ne-Xe (30%) gas at a prescribed pressure. The inner surface of the cylindrical cathode was lined with an acetal insulator. The anode was made of tin metal. The EUV emission signal with a wavelength in the range of 6 16 nm was detected by using a photo-detector (AXUV-100 Zr/C, IRD). The visible emission line was detected by using a composite-grating spectrometer with a working wavelength range of 200 1100 nm (HR 4000CG). The electron temperature could be obtained from optical emission spectroscopy (OES) and was determined by using a Boltzmann plot under the assumption of local thermodynamic equilibrium (LTE).

Modeling Quantum Efficiency of Ultraviolet 6H–SiC Photodiodes

The quantum efficiency of p-n junction 6H-SiC ultraviolet (UV) photodiodes has been theoretically modeled for the doping concentration range of 1014 -1020cm-3. The calculations take into account the contribution from the depletion region and the doping dependence of charge carrier transport characteristics. Data on optical and physical properties of 6H-SiC that determine the charge carrier transport and bandgap energy are collected and analyzed. The highest average external efficiency of up to 76%, through a working wavelength range of 200-400 nm, can be achieved at lower dopings that result in a fully depleted top active photoabsorbing layer. This is different from the current technology of commercial higher doped SiC UV photodetectors. The detectivity is shown to be conformal to the quantum efficiency in response to the design variations. The temperature dependence of the device does not change the design tradeoffs that depend on the electrical characteristics. The model presented can be extended to elevated temperatures when optical data become available at these temperatures.

Design of Solar Energy Photon-Thermal Conversion Thin Film System

A high-absorption of thin film system used for solar energy photon-thermal conversion is designed. Which is composed of four function parts with metals and dielectric materials. The result of design show that it has a very high absorption over 95% with wide working wavelength range from 400nm to 1000nm and incident angle from 0oto 60o.