Infrared wire-grid polarizers were fabricated consisting of a 500-nm pitch Al grating on a low toxic chalcogenide glass (Sb-Ge-Sn-S system) using the direct imprinting of subwavelength grating followed by a deposition of Al metal by thermal evaporation. To fabricate the subwavelength grating on a chalcogenide glass more easily, the sharp grating was formed on the mold surface. The fabricated polarizer with Al thickness of 130 nm exhibited a polarization function with a transverse magnetic transmittance greater than 60% in the 5–9-μm wavelength range, and an extinction ratio greater than 20 dB in the 4–11-μm wavelength range. The polarizer can be fabricated at lower costs and simpler fabrication processes compared to conventional infrared polarizers.
An optical system was designed and fabricated to achieve signal wavelength conversion. Although a rare-earth doped
phosphor was useful to achieve the infrared-to-visible conversion, its long-lasting phosphorescence prevented high-frequency
modulation. This problem was solved by using a time-space conversion method, in which a phosphorescent
disk was rotated to attain the fast-response wavelength conversion. When an infrared pulse train with 500-ns duration
and 1-MHz repetition rate was focused on the rotating disk, phosphorescent dots were created along the disk periphery.
By detecting the phosphorescence at a downstream position of the dot trajectory, a visible signal of 1 MHz was
observed.
Polydimethylsiloxane (PDMS) contains a large, flexible free space between weakly-bonded molecules, which allows
notable molecular diffusion. A toluene solution of diarylethene (photochromic dye) was mixed with a PDMS oil, and
then the mixture was cured in a glass vessel by adding a curing agent. Violet laser (405 nm wavelength) irradiation
induced an absorption band at around 530 nm, and consequently, the irradiated portion exhibited a red color. The colored
portion gradually expanded to the entire sample because of diffusion of the dye molecules. This diffusion characteristic
was used for improving an organic dye durability against a photo-induced degradation.
Polyethylene-glycol (PEG) exhibited bistable transmission characteristics (transparent or opaque) in heating (melting)
and cooling (freezing) processes. The bistable temperature range depended on the molecular weight and extended when different types of PEGs were mixed. For example, when PEGs with molecular weights of 300 and 2000 were mixed, the bistability took place in the 2–38 °C range. This phenomenon was used to fabricate a bistable optical fiber switch in which the mixed PEG was put in the gap between the ends of two polymer fibers. The bistable transmission property was demonstrated by controlling the device temperature with a Peltier element.
We fabricated a subwavelength-grating structure on the Y2O3 ceramic substrate, which has higher transparency than
silicon in the mid-infrared range. After coating a photoresist on this substrate, we formed a grating pattern of 350-nm
pitch by the two-beam interference of the He-Cd laser (325-nm wavelength). By using this photoresist grating as a mask,
WSi was etched with reactive SF6 ions. The transmittance of the transverse magnetic (TM) polarization was greater than
70% in the 3-7-μm wavelength range without antireflection films and the extinction ratio was over 20 dB in the
2.5-5-μm wavelength range. In addition, we also fabricated near-infrared wire-grid polarizer consisting of a 230-nm
pitch WSi grating on a SiO2 substrate. The TM polarization transmittance of the fabricated polarizer exceeded 80% in the
1000-1600-nm wavelength range. The extinction ratio was higher than 20 dB in the 650-1500-nm wavelength range.
A polarization beam splitter and tunable retarders were combined into a single element by using Si pentaprisms and
liquid crystal (LC). An LC layer that was sandwiched between the two pentaprisms acted as a polarization beam splitter
that reflected s-polarized light and transmitted p-polarized light. The other two LC layers that were attached to the outer
surfaces of the pentaprisms acted as tunable retarders; i.e., they changed the polarization states of the reflected and
transmitted light beams. When these beams were reflected back to the former LC layer (the beam splitter), wavelength
selection took place owing to the wavelength dependence of the polarization state. This performance was confirmed by
optical experiments in the infrared region.
Photochromic polymer composites were fabricated by encapsulating dye solution in a polycarbonate membrane. The
membrane contained through holes of 50 nm diameter. These nanoholes provided a sufficient free volume for the dye
molecules to change their structure in the photochromic isomerization process. A polymer composite containing a
toluene solution of diarylethene exhibited red color when it was irradiated with violet laser, and returned to the
transparent state by green laser irradiation. Another polymer composite containing spiropyran turned to blue by
ultraviolet lamp irradiation and returned to the transparent state by green laser irradiation. A nonlinear input-output
characteristic and a rewritable-grating function were demonstrated by using these photochromic polymers.
Alkaline etching of silicon surfaces was studied to make anisotropic microstructures. An aqueous solution of potassium
hydroxide was used as an etchant. The etching rate of silicon was heavily dependent on crystal orientation and
temperature; i.e., the etching rate for the (100) surface was four times larger than that for the (111) surface, and they both
increased by ten times as temperature rose from 25 to 60 °C. A laser beam was irradiated to a silicon surface to create a
temperature distribution that realized selective etching. A pulsed green laser (532 nm) of 5 ns duration was used as a
light source to enhance temperature difference between irradiated and nonirradiated portions. By passing through a
photomask and an imaging lens system, the laser beam created an optical power distribution on a silicon plate dipped in
an etchant. Depending upon the mask pattern, a groove array or a two-dimensional pit array was created on the silicon
surface. These pits took a rectangular shape on the silicon (100) plate, while they took a triangular or hexagonal shape on
the (111) plate.
Fluorescent droplet cavities were created in elastomer by using an ink-jet method. A solution for creating droplets was
composed of fluorescent rhodamine, alcohol, and surfactant. Polysiloxane was used as a matrix, since its elasticity
allowed droplet deformation that caused resonance-wavelength shift. The injected dye solution self-formed a sphere in
the raw liquid of polysiloxane due to surface tension. The polysiloxane matrix solidified in 8 h after adding a curing
agent. The droplet exhibited whispering-gallery-mode emission when it was excited by a frequency-doubled Nd:YAG
laser pulse. The resonance peaks shifted to either short or long wavelengths as the droplet deformed by pressing the
elastomer.
Upconversion characteristics of rare-earth cations were utilized for emitting or controlling visible light with infrared
light. A fluorescent glass rod was fabricated by using durable AlF3-based glass that contained high-concentration Er3+
cations. This glass rod acted as a two-way wavelength converter; i.e., visible light (~500 nm) was converted to infrared
light (~800 nm) as it passed through the glass, and infrared light that propagated in the opposite direction was converted
to visible light. An infrared-responsive photochromic compound was fabricated by dispersing spirobenzopyran and
upconversion powder (Gd2O2S:Yb3+Er 3+) in photocurable acrylate. When this compound was exposed to ultraviolet light
(~370 nm), a strong absorption band appeared in the visible spectral region due to photochromic isomerization of
spirobenzopyran. This absorption band disappeared by irradiation of a 940-nm laser beam, since the upconversion
powder emitted green light that caused bleaching of colored spirobenzopyran.
We fabricated an infrared wire-grid polarizer that was made of a tungsten silicide (WSi) grating on a Si substrate. The
photolithography by the use of the two-beam interference was conducted for generating the short-period grating
structure. This photoresist pattern was used as a mask for the reactive ion etching of the WSi coating and the Si substrate.
Consequently, we could fabricate the WSi/Si grating with 400-nm period and 550-nm depth that acted as a wire-grid
polarizer. The transmittance of TM polarization was 58% at 4-μm wavelength, which exceeded the theoretical
transmittance of Si (54%). This enhancement of the transmittance was caused by the reduction in the reflectance due to
the subwavelength-grating structure. The extinction ratio at 2.7-μm wavelength was 20 dB. We also measured the
extinction coefficient κ of WSi, and verified that WSi was a suitable polarizing material in the mid-infrared range.
We fabricated infrared Fabry-Perot filters by stacking two wet-etched Si plates. When an electric voltage was applied between the plates, the spacing between the plates changed due to an electrostatic force, which caused a shift of interference peaks. The Si plates were etched in a KOH solution to 34-μm thickness in order to reduce the driving voltage. When the voltage was raised from 0 to 20 V, an interference peak shifted from 7.9- to 5.5-μm wavelength, corresponding to the decrease in the spacing from 7.9 to 5.5 μm. The peak transmittance increased to 91% by an antireflection coating on the outer surface of the filter. This coating was effective to suppress the interference inside the Si plates that created a complicated spectrum.
A fast-response infrared spectrometer was constructed by using an AgI/Ag-coated hollow fibre and a PtSi CCD sensor array. The infrared transmitting hollow fibre was employed as a gas-flow cell to reduce both sample volume and sampling time. The infrared beam that passed the hollow fibre was diffracted by a grating, and was detected by the sensor array with a 1/60-s frame time. Spectra of flowing CH4 and CO2 gases were measured successfully in the 3-4-μm wavelength range.
Waveguide filters with extremely thermally stabilized KrF laser-induced gratings were fabricated in the highly photosensitive Ge-B-SiO2 thin films. It was discovered that a completely new-type grating with high diffraction efficiency and thermal stability could be formed by annealing a conventional laser-induced grating at 600°C. Such thermally induced gratings couldn't be erased after repeated heat treatment alternating between room temperature and 600°C. We printed a grating in slab waveguide by irradiation with a KrF excimer laser followed by the annealing at 600°C, and then formed the channel in the region of the grating using standard photolithography process. The diffraction peak of 17 dB in depth at 1535.04 nm wavelength was observed after repeated heat treatment alternating between room temperature and 400°C. These thermally stabilized waveguide filters are promising candidate for the highly reliable optical and sensing devices.
Ultrasonic waves exert acoustic pressure on microparticles in liquid. Consequently microparticles are trapped at the nodes of a standing wave that is excited between a pair of ultrasonic transducers. If two orthogonal standing waves are excited by using two pairs of transducers, microparticles are trapped at the intersecting points of the nodes. A larger trapping force is attainable by ultrasonic trapping than by laser trapping that utilizes a weak optical pressure. Therefore ultrasonic trapping is suitable for the manipulation of biological tissues which easily suffer thermal damage by the exposure to a focused laser beam. Microorganisms such as euglena and paramecia were trapped by the ultrasonic waves of approximately 3 MHz. Trapped microorganisms could be transferred to desired positions by changing the ultrasonic frequency. The aggregation of microorganisms was achievable by the cyclic frequency change. This ultrasonic trapping technique was also used to fabricate composite materials with lattice structure; i.e., polymer, glass, or metal particles were ultrasonically arranged in a polymer matrix during the solidification process of the host polymer.
In some areas of fiber optic sensing, one can expect improvement in measurement accuracy and an expansion of the fields of application with the use of infrared transmitting fibers. Unfortunately, sensor development often fails due to a lack of suitable optical designs based on a thorough knowledge of fiber characteristics. This article provides an overview of infrared fiber sensors, describing what infrared fibers can do, which infrared fibers should be selected for what purposes, how the sensor system should be designed. Chronological tables are also shown for infrared fiber thermometry and spectroscopy.
Photosensitive materials, i.e., a photochromic dye and a silver halide, were used for fabricating optical fiber devices. Variable transmiUaixe was demonstmted with a photochmmic liquid-core fiber. A periodic photodarkening pattern was induced in an infrared silver halide fiber.
A coherent fiber bundle for infrared image transmission was prepared by arranging 8400 chalcogenide (AsS) glass fibers. The fiber bundle, 1 m in length, is transmissive in the infrared spectral region of 1 - 6 micrometer. A remote spectroscopic imaging system was constructed with the fiber bundle and an infrared PtSi CCD camera. The system was used for the real-time observation (frame time: 1/60 s) of gas distribution. Infrared light from a SiC heater was delivered to a gas cell through a chalcogenide fiber, and transmitted light was observed through the fiber bundle. A band-pass filter was used for the selection of gas species. A He-Ne laser of 3.4 micrometer wavelength was also used for the observation of hydrocarbon gases. Gases bursting from a nozzle were observed successfully by a remote imaging system.
Ultrasonic waves are useful for arranging small particles in liquid, since the acoustic pressure exerts a sufficient trapping force on the particles. A composite material with layered structure can be fabricated by solidifying a particle suspension during the process of ultrasonic standing wave excitation. Fabrication of a 2D or 3D lattice structure is also possible by simultaneous excitation of two or three orthogonal ultrasonic standing waves. A polysiloxane resin is appropriate as a host material of such composite materials, since it is easily synthesized from a solution and its yields a small-periodicity structure due to its low sound velocity. Acrylic spheres, glass rods, and metal particles have been successfully arranged in polysiloxane resin forming layers or lattice structures. The spacing of particles was approximately 60 micrometers , which was half of the ultrasonic wavelength used. For heavy particles, a sample cell was continually rotated during the solidification process in order to prevent sedimentation.
Inserting an As2S3 optical fiber into a coupling-hole of one of the optical resonator mirrors directly, a pig-tail type CO laser is successfully demonstrated on its fundamental operational characteristics, together with theoretical treatments. First, finding the spatial intensity-distribution on the coupling-hole mirror, its effective transmittance vector t is evaluated in comparison with the measured spatial beam profile. Then, the output power is measured with the various coupling-hole mirrors, obtaining the maximum output power at the coupling-hole diameter of approximately 0.6 mm. These characteristics of the output power vs the coupling-hole diameter are theoretically explained by using the gain parameters G and the internal loss vector ai obtained, being well consistent with the experiments. As a result, the optimum coupling-hole evaluated is quite suitable to compose the pig-tail type scheme.
Optical parallel logic operations were studied using IR light as a signal beam. The optical system was constructed with a planar heat source, a liquid crystal switch array, a chalcogenide glass fiber bundle, and a PtSi CCD sensor array. A variety of logic operations, e.g., AND, NAND, OR, NOR, and XOR, were demonstrated successfully by this optical system.
Nematic liquid crystals were put into the micropores of anodic alumina films. Molecules of a liquid crystal tend to orient along the axis of columnar pores, since the pore diameter is smaller than 0.2 μm. Accordingly, the refractive index of the liquid crystal varies with the polarization of the light that propagates in the alumina film; i.e., the ordinary and extraordinary indices (no and ne) correspond to polarizations that are vertical and horizontal to the pores, respectively. The anisotropy in refractive index causes a prominent polarization function due to the index mismatch between a liquid crystal and alumina (n = 1.64). When a liquid crystal of no = 1.50 and ne = 1.64 is used, the optical loss for vertical polarization is larger than that for horizontal polarization by more than 10 times at 0.85- to 1.30-μm wavelengths. If one coats lecithin on the pore walls, molecules of a liquid crystal orient nonuniformly in the pores. Then the refractive index becomes nonuniform, and consequently, significant light scattering occurs inside the pores. The optical loss tends to decrease at shorter wavelengths and in alumina films with smaller pores.
Wire-grid type polarizers were fabricated by electroplating nickel or copper into the columner pores of anodicalumina films. A film exhibited an extinction ratio larger than 30 dB in the wavelength range of 1-7 pm.
Spectroscopic gas sensing was carried out by using an infrared hollow waveguide as a capillary flow cell. A ZnS-coated Ag hollow waveguide is generally the most suitable selection for use as a flow cell, since it exhibits high transmittance over the wide spectral range around 10 micrometers wavelength. For the corrosive gases that cause serious damage to the ZnS film, SiO2- or GeO2-based glass hollow waveguides are applicable. With these low-loss hollow waveguides, CH4, n-C4H10, NO2, and SO2 gases were measured successfully, and a fast response in gas detection as well as a remarkable reduction of gas consumption was demonstrated.
Various optical fibers were compared theoretically for application to remote radiation thermometry. The highest optical transmission efficiency is attained with a fluoride glass fiber for the temperature range of > 700°C, with a halide crystalline fiber for 120 to 700°C, and with a dielectric-coated metal hollow waveguide for < 120°C. To realize remote sensing of lower temperatures, i.e., from room temperature to ~300°C, a radiometric experiment was carried out using a Ge-coated Ag hollow waveguide. When ambient temperature changed from 22 to 50°C, a serious error arose in the measured temperature, which is attributed to the increase of thermal radiation from the heated waveguide. Since the effect of such noise radiation is inevitable in thermometry using infrared fibers, we propose a measurement method to compensate for the noise level. With the compensation, the effect of ambient temperature was reduced successfully and reliable thermometry was established.
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