A cavity-resonator-integrated guided-mode-resonance filter (CRIGF) consisting of a grating coupler (GC) and a pair of
distributed-Bragg-reflectors (DBRs) on a thin-film dielectric waveguide is reviewed. The CRIGF has been recently
proposed by the authors to provide a narrow-band reflection spectrum for an incident wave of a small beam width from the free space. A newly developed analysis model for device design with performance simulation is introduced. Curved gratings are utilized to construct a resonator for a small-aperture CRIGF. Design, fabrication and characterization of CRIGFs of 10 μm aperture are described with a resonance wavelength of 850 nm. A Ge:SiO2 guiding core layer was deposited on a SiO2 glass substrate, and GC and DBRs were formed by the electron-beam direct writing lithography. A normal polarization-dependent CRIGF is shown with a obtained narrowband reflection spectrum of 0.2 nm full width at half maximum. A crossed-CRIGF is also discussed to eliminate the polarization dependence. It is successfully demonstrated that measured reflection spectra for TE and TM incident beams were well coincident with each other.
Fast and low power consumption optical switches are required for photonic networks. To this end, we proposed the
optical gate switch using phase change material (PCM) and silicon waveguides. This switch had low power consumption
because it consumed power only when the state was changed. Furthermore, the chip size is very small due to the large
refractive index change of PCM.
In this paper, we studied the phase-change characteristics of various kinds of thin GST film on SOI (silicon-oninsulator).
A laser diode (LD) with a wavelength of 660 nm was used to irradiate the material. We compared the optical
responses by laser pulse irradiation on GST films, and concluded GST-147 was the most suitable material for the optical
switch because it had the lowest phase change threshold.
The phase-change characteristics of GST-225 films with thickness of 25 nm, 50 nm and 75 nm were also examined.
Thicker GST films had lower phase change thresholds. However, thermal simulations showed that the phase of the
bottom part of thicker films may not be changed. Therefore, we concluded that GST films with thicknesses between 25
nm to 50 nm are the most suitable for optical switches.
Integrated-optic devices enabling intra-board high-density optical interconnection with tens terabit-per-second
transmission bandwidth from two-dimensional (2D) array of VCSELs to 2D array of photodiodes are reviewed. Design
strategy and WDM signal transmission are discussed. A cavity-resonator-integrated grating input/output coupler and a
different-guided-mode-coupling DBR are integrated to provide free-space-wave optical add/drop multiplexing function
for a future high-performance signal-processing system in package using WDM optical interconnection. Design example
of eight-wavelength multiplexing system within 20-nm wavelength range is presented and theoretically simulated
performances are shown.
One-dimensional surface-relief gratings were fabricated using a direct imprinting process with a glassy carbon (GC) mold at the softening temperatures of oxide glasses. The maximum grating height attained in this study was 730 nm when the grating period was 500 nm, which could be formed by the pressing at the softening temperature of glass under constant pressure of 0.4 kN/cm2. A large area glass imprinting was attempted using a GC mold with a periodic patterned area of 6 mm x 6 mm, which has the period of 500 nm and groove depth of 350 nm, respectively. Phase retardation of 0.1 λ was recognized between TE-polarized and TM-polarized lights at 600 nm wavelength. The measured values were in excellent agreement with those calculated using a rigorous coupled wave analysis.
A hydrofluoric acid (HF) bonding method has been applied for the thin film filters integration. The quality of bonding was examined by the evaluation of bonding strength and the observation of bonding interface. The bonding strength depends on the applied pressure and temperature during bonding. The maximum bonding strength was more than 10 MPa. High quality bonding interface was observed by scanning electron microscope and transmission electron microscope. A color filer array of red, green and blue (RGB) was designed to separate the white light. The filters were integrated into a glass body successfully by HF bonding. Its transmittance spectra were unchanged after heating at 450oC for 20 hours. In addition, integration of filters was carried out to fabricate an optical demutiplexer for coarse wavelength division multiplexing system. It was confirmed that the integrated filter array acted as a demutiplexer.
High-temperature-resistant laser-induced Bragg gratings were formed in Ge-B-SiO2 thin glass films fabricated by the plasma enhanced chemical vapor deposition method. Such gratings were induced by exposure of an interference pattern with KrF excimer laser and subsequently annealing at 600°C. The SEM observation of the grating after HF etching revealed that the origin of the grating was the periodic precipitation of crystalline 20 - 40 nm-diameter Ge nanoparticles. Nanoparticles precipitated predominantly in the unirradiated region after laser-induced refractive index change completely erased after annealing up to 500°C. The maximum value of refractive index modulation Δn was 6.8 x 10-3 at 632.8 nm, which corresponds to the Δn nearly 10 times as large as that before annealing. A channel waveguide with high-temperature-resistant-grating exhibited high diffraction efficiency and excellent thermal stability.
A self-trapped filament of ultrashort laser pulses can induce a several-hundred-micron-long region of refractive-index change in silica glass. The maximum refractive-index change and the diameter of refractive-index change are approximately 0.01 and 2 μm, respectively. The filament is 10 - 500 μm long along the pulse propagation axis and its length depends mainly on the numerical aperture of the focusing lens. In this paper, we present the fabrication experiment of volume gratings induced in silica glass by a self-trapped filament of ultrashort pulses. When the 150-μm-long filament was translated perpendicular to the optical axis by 300 μm, a layer of refractive-index change with the thickness of 2 μm was induced. We stacked the layers with a period of several microns and fabricated volume gratings. We entered a He-Ne laser beam at the wavelength of 632.8 nm to the grating with the Bragg angle to measure the diffraction efficiency. The maximum diffraction efficiency was 74.8% with the grating that had the period of 3 μm, and the thickness of 150 μm.
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.
Ge-SiO2 thin films with extremely high photosensitivity against excimer laser light were fabricated by plasma enhanced chemical vapor deposition method. Direct formation of channel waveguide was successfully confirmed only by irradiation with excimer laser through a Cr mask pattern, which was previously coated on the slab-waveguide by sputtering method. Bragg gratings with high diffraction efficiency were also printed in the waveguide by another laser irradiation through the phase mask. Channel waveguides with Bragg gratings, on the other hand, were fabricated on the glass ceramic substrates with negative thermal expansion coefficient. Doping of B2O3 to Ge-SiO2 glass film was effectively suppressed the temperature drift of the stop band (dλ/dT)of the grating. The dλ/dT attained in this study was 5pm/°C, which was less than ½ of those for commercially available waveguide gratings. The waveguides with gratings using the photosensitive oxide thin films should be a promising candidate for future low cost and reliable optical access network.
We found out that GeO2-B2O3-SiO2 thin films fabricated by plasma enhanced chemical vapor deposition method exhibited not only large photo-induced but also thermo-induced refractive index increases, both of which were above 10-3. The former was observed after irradiation with KrF excimer laser, and the latter was induced by annealing at 600°C. The thermo-induced refractive index increase was closely related to the formation of thermo-induced absorption bands during the annealing, and could be suppressed by the laser irradiation prior to the annealing. Bragg gratings were printed in the films by the laser irradiation through the phase mask without H2 loading. The diffraction efficiency decreased rapidly by the annealing up to 500°C, but drastically increased after the annealing at 600°C. The thermo-induced gratings couldn’t be erased by the repeated heat treatments between room temperature and 600°C at all. Considering the suppression of thermo-induced index increase by the laser irradiation, this grating was expected to have the reverse pattern of refractive index compared to that of the as-printed one, and might be applicable to the highly reliable optical and sensing devices.
We have fabricated silica-based waveguide Bragg grating devices and have investigated temperature sensitivity of the Bragg wavelengths. Temperature sensitivity of Bragg wavelength is caused by temperature dependence of effective refractive index and thermal expansion. We examined boron-codoped germanosilicate glasses as waveguide materials in order to decrease a temperature sensitivity of refractive index. The boron-codoped germanosilicate films were fabricated by a plasma enhanced chemical vapor deposition. We adopted Si, silica, and crystallized glass as substrates in order to control the thermal expansion of the waveguides. Bragg grating with 0.53 μm period was formed by irradiation with a KrF excimer laser light through a phase mask. The Bragg wavelength shift of 9.7pm/°C was obtained in the B-Ge-SiO2 core waveguide on a silica substrate, while the Bragg wavelength shift was 11pm/°C in the with Ge-SiO2 core waveguide on a Si substrate, which was a conventional-type waveguide Bragg grating device. The Bragg wavelength shift was reduced to 7.8pm/°C by using B-Ge-SiO2 core and a crystallized glass substrate with zero thermal expansion coefficient, which was 2/3 of the value of the conventional waveguide Bragg grating device.
GeO2-B2-O3-SiO2 thin films were fabricated by plasma enhanced chemical vapor deposition method. Boron codoping into a GeO2-SiO2 thin film induced large absorption in the vicinity of 240nm, and OH absorption decreased compared to GeO2-SiO2 films. These films of 5 micrometers in thickness exhibited large positive refractive index change without hydrogen loading by irradiation with ArF (193nm) excimer laser pulses. Induced refractive index change was approximately 0.002 which was measured by the prism coupling method. A waveguide was written in this high photosensitive glass film by UV irradiation. The guided mode of the waveguide seems to be single and estimated refractive index change was approximately from 0.003 to 0.004. Three unique phenomena were found in 0.2micrometers thick films on Si substrate. First, these films exhibited large negative refractive index and positive thickness changes by irradiation with ArF laser pulses. Induced negative index change was larger than 0.02 and thickness change was more than 1%. Silica films doped only boron or germanium didn't exhibit such negative index changes. Second, the annealing before laser irradiation decreased the photosensitivity of these films remarkably. Third, these induce refractive index and thickness changes were decreased with time rapidly. These mechanisms were under investigation.
Photosensitive gel films were patterned with a two-beam interference method by use of 325-nm-wavelength He-Cd laser. We fabricated 2D periodic structures on photosensitive ZrO2 gel films. The ZrO2 gel films were prepared from Zr(O-n-C4H9)4 chemically modified by benzoyl acetone. For fabrication of 2D pattern, UV-irradiation process was divided into two pats and the substrate was rotated by 90 degrees between the parts. When total irradiation time was equal to the time for grating fabrication, 2D dotted pattern was obtained. A checked pattern was also obtained with irradiation time of 30 min. We also fabricated surface-relief gratings on photosensitive Pb(Zr, Ti)O3 (PZT) gel films. The PZT gel films were formed by dip coating with sol solution, which was prepared from Zr(O-n-C4H9)4 chemically modified with benzoyl acetone, Ti(O-n-C4H9)4 chemically modified with benzoyl acetone, Pb(CH3COO)2, and alcohol. The gel films could be crystallized by annealing at 700 degrees C for 1 hour. The annealed films had a ferro electric phase and a P-E hysteresis loop was obtained. Uniform gratings with a sub-micrometer period were formed on the gel films by two-UV-beams irradiation and etching with butanol. A maximum first-order diffraction efficiency of 28 percent was obtained under a Littrow mounting condition by use of a linear-polarized 633-nm-wavelength light.
We propose a new technique of correlation filter design for optical correlator. For detection and classification of complex patterns, ability of single optical correlator is not enough. To achieve high performance of classification, a multiple correlator is suitable in respect of flexibility in correlation filter design. We attempt to design a set of correlation filters for use in multiple optical correlators. As the target, we select road signs. In real scene, the scale and aspect ratio of road sign are dependent on the distance and angle of observation. In addition, many kinds of signs are used. Therefore the correlation filter set has to be designed as to have distortion invariance to adapt to the change of aspect and the variation of road signs. We apply the technique of multiple-object correlation filter, such as the synthetic discriminant function, to the design of correlation filter set, in order to obtain the necessary invariance. We calculate 180 filters in order to detect and classify 15 kinds of Japanese road signs in real scene. Computer simulation result shows that the combination of multiple optical correlator with the correlation filter set can indicate high performance of pattern detection and classification.
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