The laser diode (LD) is a light source characterized by small size, light weight and high performance. Osaka University has been actively conducting research and development to precisely collect and directly process infrared LD light. Violet laser diode (VLD) light can be focused in a smaller spot because its wavelength is shorter than that of infrared LD light.
We report herein the development of a unit to precisely focus VLD light and the use of this unit, together with dyes and pigments, to create color micro-markings on a plastic plate. Concretely, we show corrections made to ensure that the single-mode VLD light can be changed to collimated light via a collimator lens and its wavefront aberration can be measured for precise light collection. The corrected light was precisely focused and used to irradiate a dye or pigment that was applied to a plastic surface. This resulted in markings on the part exposed to irradiation despite the fact that no absorption agent was used. We also achieved microscopic markings on a polyester (PET) plate by optimizing irradiation conditions, as well as 100-µm and 200-µm color markings.

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Research progress on laser nano-fabrication with the combination of AFM, NSOM and transparent particles mask is reviewed. With the combination of other advanced processing tools, laser irradiation can push the processing feature size down to ~ 20 nm. However, laser nano-fabrication with single optics brings about the technical challenge of slow processing speed. Parallel laser nano-patterning was recently developed to achieve large area and high speed nano-fabrication with laser irradiation through a micro-lens array. Laser interference lithography is also studied to fabricate 100 nm functional periodic nanostructures on substrate surfaces.

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In the automotive industry laser applications have been expanding from small machined parts of engines and transmissions to larger pressed parts of car body. One reason behind the increase of laser applications in the automotive industry is that the high performance and flexible solid state lasers have been developed. Another reason behind is that laser technology offers the potential to meet new vehicle requirements originating from environmental issues and societal demands, which ordinary processing techniques are unable to satisfy. This paper describes recent laser applications as typical examples of the use of lasers by the Japanese car industry to meet the requirement for lighter vehicles and improved fuel economy, which is one of the most critical vehicle issues facing car companies today.

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Tailored blank welding system by using high power fiber laser has developed and installed in the production line. The developed system is consisted of the panel load/unload unit, the welding unit, the system controller and the fiber laser oscillator. The quality inspection method has also developed by using the in-process monitoring system, iL-Viewer^TM. The system has achieved the high reliability and the excellent process performance in the production line.

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The authors have developed underwater laser processing, namely welding and peening, to enhance surface properties of metallic components. The process developed could significantly reduce radiation dose to workers in nuclear facilities due to the shielding effect of water from radiation as well as due to remote operation attainable by laser technologies. Laser welding or cladding makes corrosion-resistant layers on the surface of components underwater, which reduces stress corrosion cracking (SCC) susceptibility. Various metallurgical and mechanical tests were performed to prove that the laser cladding made sound layers even if the surface had been degraded. Laser peening imparts compressive residual stress in near-surface layers by irradiating intense laser pulses without any surface preparation. High pressure plasma generated through ablative interaction underwater impinges on the surface and induces plastic strain to make compressive residual stress. Both processes are used to enhance surface properties, such as resistance to SCC, fatigue or wear of various metal materials.

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We succeeded in producing Mg from MgO without reducing agent using high power cw laser. Mg in turn can react with water to produce hydrogen, heat and MgO, thus natural resources like solar power can be stored as chemical energy. In this study, a focused CO₂ laser irradiated MgO target in vacuum, high temperature is exerted in tiny spot thus Mg production in equilibrium is achievable. The resultant Mg vapor is deposited on Cu plate and analyzed by EPMA (electron probe micro analysis); the results showed points of more than 30% mol fraction of Mg deposited, with an average of 8 mol%. So far bulk analysis results showed that 6 mol% of Mg is obtained as solid deposit, which can be translated to energy conversion efficiency of 0.35 mg/kJ. These experiments showed that our scheme can be used for magnesium production with higher efficiency and large throughput.

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The cutting performance of concrete using a laser-based hybrid technique was investigated. Experimental conditions included 7 kW laser power, 10 mm laser beam diameter, and 1.25-50 mm/s scanning speed. Results of surface scanning tests and 100 mm deep cut tests indicated that the shortest operation time was about 5.5 hr/m² for plain concrete and 7.2 hr/m² for heavy concrete.

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In the past decade, femtosecond (fs) laser microfabrication has received significant attention because of its powerful three-dimensional (3D) integrating and manufacturing capabilities. A series of multifunctional micro-components and micro-devices, including microoptics, microfluidics, micromechanics, microelectrics, microplasmonics, and so on, can be fabricated and integrated in glass chips with true 3D configurations using a unified fs laser microprocessing technique. For this reason, this technology is particularly suitable for fabricating lab-on-a-chip (LOC) devices, which have already created a revolution in the fields of chemical, biological, and medical research. A LOC device, due to its highly integration, standardization and automation, allows for performing chemical and biological analyses with ease of use, low sample and reagent consumption, low waste production, high speed of analysis, and high reproducibility. In this paper, we will review the 3D femtosecond laser fabrication of the multifunctional micro-components in glasses as well as their integration. In addition, we will report the latest progress in this active research field, e.g., the fabrication of a tunable microfluidic waveguide, selective surface metallization on dielectrics, and the formation of surface-enhanced-Raman-scattering (SERS) substrate, etc.

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To study the physical mechanism of femtosecond (fs) laser induced nanostructuring on solid surfaces, we observed the initial stage of morphological change on diamond-like carbon and titanium nitride films in fs laser ablation at low fluence. Using the thin film targets patterned with submicrometer-size stripes, we have found that the nanostructure starts to be formed on the stripe along the direction perpendicular to the laser polarization. The experimental results have shown that nanoscale ablation for the nanostructuring is preferentially initiated with the enhancement of localized electric field on the stripe surface with high curvature.

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Nano-processing using a near-field generated by laser irradiation onto the nano-particle is becoming a new emerging nanofabrication technology, because it can fabricate nano-sale structures even with near-infrared laser. Nano-scale area near the contact point with the dielectric particle on the material surface can be ablated by lens effects and/or Mie scattering depending on the particle size. We report on new phenomena, leading to a new nano-processing technique via surface plasmons, even by the use of dielectric particles.

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High-pressure phases which are Β-Sn, Imma and simple hexagonal structures of silicon are quenched using an intense femtosecond laser-driven shock wave. These high-pressure phases have never synthesized in ambient pressure by hydrostatic and conventional shock compression methods. Femtosecond laser was irradiated to single crystal-silicon with no-dopant. We confirmed the existence of these high pressure phases by analyzing the crystalline structure of the femtosecond laser irradiated silicon using grazing incidence synchrotron x-ray diffraction.

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We demonstrate the fabrication of three-dimensional (3-D) hollow microstructures embedded in photostructurable glass by a femtosecond (fs) laser direct writing. Fs laser direct writing followed by annealing and successive wet etching in dilute hydrofluoric (HF) acid solution resulted in the rapid manufacturing of microchips with 3-D hollow microstructures for the dynamic observation of living microorganisms in fresh water. The embedded microchannel structure enables us to analyze the continuous motion of flagellum movement of Euglena gracilis. Such microchips, referred to as nano-aquariums realize the efficient and highly functional observation of microorganisms.

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Sub-micron periodic gratings inside bulk lithium fluoride (LiF) are holographically fabricated by interference with the second harmonic (390 nm) of a mode-locked Ti:sapphire oscillator-amplifier laser. As one optoelectronic application, pulsed, room temperature distributed feedback (DFB) color center laser action is demonstrated, for the first time, in the green spectral region based on the F₃+ color centers in LiF utilizing both a micro-structure encoded by femtosecond-laser-pulses, and higher gain of the laser-active centers produced by an e-ray irradiation at 200 K. A lasing output with a linewidth of 0.5 nm is obtained at approximately 537 nm, which value reflects the selective laser resonator. Realization of green and red DFB color center lasers based on the F₃+ and F₂ centers in LiF excited by a single wavelength can be expected.

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Vertically aligned ZnO nanowires have been successfully synthesized on annealed c-cut sapphire substrates by a catalyst-free nanoparticle-assisted pulsed-laser deposition (NAPLD) in Ar and N2 background gases. In NAPLD, the nanoparticles formed in the background gas by laser ablation are used as a starting material for the growth of the nanowires. The surface density of the nanowires can be controlled by varying the density of nanoparticles, which are accomplished by changing the energy of the ablation laser, the repetition rate of the laser and so on. When single ZnO nanowire synthesized in a N₂ background gas was excited by 355 nm laser-pulse with a pulse-width of 8 ns, stimulated emission was clearly observed, indicating high quality of the nanowire. These nanowires were used as building blocks for an ultraviolet photo-sensor, field emitters and an ultraviolet light emitting diode with a structure of n-ZnO/ZnO nanowire/p-GaN.

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ZnO nanorod arrays have attracted considerable attention as a candidate for fabricating gas sensors, dye-sensitized solar cells, and light emitting diodes (LEDs) in the last few years. Controlling the growth of ZnO nanorods is of importance in terms of reproducibility for the practical applications. In this paper, we have grown ZnO nanorod arrays on Si (100) substrates by two-step growth. First, ZnO powder dispersed on a Si substrate is thermally annealed in air. Next, after annealing for nucleation, ZnO was deposited on the seed layer for one hour duration. As a result, ZnO nanorod arrays were observed by field emission scanning electron microscopy (FE-SEM). The diameter of the thinnest ZnO nanorods was 30 nm, and the length ranged from 500 nm to 10,000 nm. Furthermore, we investigated their surface enhanced Raman scattering (SERS) properties by using a structure known as a nanoshell, and coating gold (Au) around ZnO nanorods.

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We employed epi-GaN substrates for ZnO film growth, and studied the deposition and post-annealing effects. ZnO films were grown by pulsed laser deposition (PLD) method. The as-grown films were annealed for one hour under atmospheric pressure air. We investigated ZnO morphology after annealing. The post-annealed ZnO films grown at T_g = 700 °C have very smooth surfaces and the rms roughness is about 0.5 nm. Finally, we inserted ZnO post-annealed buffer layer between ZnO epi-layer and GaN/sapphire substrates. It is confirmed by AFM that growth temperature of 700 °C helps the films grow in step-flow growth mode. It is observed by cathode luminescence spectrum that the ZnO film grown at 700 °C has very low visible luminescence, indicating the decrease of the deep level defects. It is also revealed by Hall measurements that carrier concentration is decreased by increasing the growth temperatures. Thermal treatment and post-annealed ZnO buffer layer can be used to fabricate ZnO hetero-epitaxy.

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Device-quality ITO by pulsed Nd:YAG laser, successfully used for Organic Light Emitting Device (OLED), was deposited at room temperature and 250 °C. Although the optical transmittance of > 90% is achieved in the visible range, the ITO film resistivity is too high to enable an efficient OLED, as compared to that reported for KrF laser deposited ITO. At 250 °C and laser wavelength of 355 nm, the ITO film resistivity decreased by 10x to 2 × 10^-4 Ωcm while its optical transmittance was > 90 %. For PET, the heating temperature was limited to 150 °C and hence the ITO resistivity could only be decreased by a factor of 2 to 5 × 10^-4 Ωcm. The thermally induced crystallization of ITO, which has a preferred <111> directional orientation texture, largely accounts for lowering of its film resistivity. The OLED was based on simple device structure of ITO/(PVK+TPD+Alq₃)/Al, which output brightness was compared to that fabricated on the commercial ITO. Effet of an ultra-thin diamond-like carbon (DLC) layer in the OLED was investigated.

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We have investigated a one-step method to fabricate a microstructure on a silica glass plate by using laser-induced backside wet etching (LIBWE) that consists of excimer laser mask projection system and diode-pumped solid state (DPSS) laser beam scanning system. Well-defined deep microtrenches without crack formations on a fused silica glass plate were fabricated by LIBWE method with the UV lasers. We have demonstrated the micro- fabrication of pits-array and patterned grating on the surface of silica glass plates by LIBWE method at 248 nm and 266 nm with dye solutions. The two new systems allow us to use rapid prototyping of high precision surface microfabrication of silica glass as laser direct-write processing.

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Small vibration sensors with the laser interferometer have been developed. However, there are some disadvantages, for example, weakness to external vibration, difficulty in optical alignment, and so on. These disadvantages are improved by making use of a self-coupling effect of a semiconductor laser, because our sensor is composed of only a laser diode and a lens. When a small vibration, whose amplitude is less than half-wavelength of the laser, is measured, a reference signal is needed. The reference signal is generated by moving the sensor head quickly. And also an output signal is needed at maximum sensitivity. This output signal is generated by moving the sensor head slowly. The signal is controlled by a microcomputer. The vibration amplitude is evaluated from the ratio of the output signal to the reference one by the microcomputer. This sensor can detect a small vibration with minimum amplitude of about 10nm, and a small vibration is able to be measured irrespective of the frequency in the range between 500 Hz and 3000 Hz.

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Emerging possibilities of using ultrashort pulse lasers as a new light source for advanced optical metrology are addressed in comparison to traditional sources such as CW lasers and white light. Emphasis is on explaining various principles of absolute distance interferometry that become practical by exploiting the unique nature that a train of ultrashort pulses is a phase-locked combination of a large number of monochromatic laser lines evenly spaced over a wide spectral range. In that context, adopting a single ultrashort pulse laser offers many new opportunities of performing absolute distance measurements with sub-wavelength resolutions over extensive ranges. It is also discussed that the new light source allows one to improve the measuring performances of other fields of optical metrology such as large-scale surface profiling.

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We have been developing a detection system using lasers for foreign materials in some foods such as raisins, tomatoes, and grapes. Here we describe experimental results required for constructing the detection system. We have observed that some foreign materials mixed in raisins could be discriminated by taking laser emission reflectance ratios at appropriately chosen two laser wavelengths. Based on the experimental results, a prototype of the foreign food detection system has been produced.

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An optical method of temperature measurement to analyze CO₂ laser active medium based on near-infrared absorption spectroscopy with a 1.6-µm distributed-feedback (DFB) semi- conductor laser is described in this paper. The method was used to measure gas temperatures in the range 400 K to 530 K for discharge electrode distances of 5 and 7.5 mm. Real-time temperature monitoring of CO₂ was achieved by using the DFB semiconductor laser. Such near-infrared absorption spectroscopy has the potential to be a practical method for diagnostic of the active medium of a CO₂ laser.

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We report on a selective modal oscillation dynamics of Random Lasing (RL) by using a simple two-dimensional RL model. This model consists of two terms; two-dimensional scattering model, and rate equations. Using this model, we show how lasing modes behave as a scattering state of photons inside the system changes. Calculation results indicate that closed loop shaped light path and strong feedback mechanisms produced by scatterers around the loop are necessary for RL's modal oscillation. Background scattering events efficiently support the modal oscillation on the closed loop path.

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China loess attracts attention in East Asia. In the previous research of our group, an elemental detection method with LIBS has been developed. In this work, we tried to develop the element detection method of China loess with a LD pumped passively Q-switched Nd:YAG laser. We developed the system which can duplicate appearance of China loess in the air. The breakdown thereshold of China loess blown by the air was estimated. And the required sensitivity in the system was also estimated.

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This paper reports a detailed experimental investigation of bubbles with various diameters generated by laser-induced breakdown in water, which is an important issue in the field of laser medical care. The bubbles were generated by a pulsed Nd:YAG laser at 266-1064 nm, and the sizes of the bubbles were measured by a CCD camera. A time-resolved measurement with a gated ICCD camera revealed the process of bubble formation after laser breakdown. We were able to achieve manipulation of single bubble generation with desired bubble diameter ranging from 10 to 200 µm. The bubble size could be stably controlled as a function of the injected laser energy into water, which varied between 0.5 mJ and 12 mJ. More interestingly, the injected energy can form either a single bubble or multiple bubbles, with an energy more than 8 mJ, probably due to excess heat injection into the water. These results show the potential for generating nano-bubbles or micro-bubbles of arbitrary size.

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We developed a compact trace gas sensor, which incorporates a cylindrical thin-disk multi-pass cell. This cell is useful because the optical path length can be selected easily in a small absorption volume across the beam path of 25 cm³. We studied the optical characteristics of this cell and demonstrated some gas measurements with this system. This paper discusses the performance of the gas detection system which incorporates a cylindrical cell.

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This study aims to develop a new concept lidar for lightining strike prediction. In cloud-to-cloud discharges, the electromagnetic pulse rotates the polarization plane of a beam propagating beam through an ionized atmosphere due to the Faraday effect. Although this effect is small for visible light, it can be detected by a lidar system with an extinction ratio of polarization of >30 dB. In this study, the rotation of the polarization plane of a propagating beam due to the Faraday effect was detected in a high-voltage discharge experiment and verified by the numerical analysis results of the cloud-to-ground returning strike model. In the experiment, the shock wave due to the discharge was detected in addition to the rotation of the polarization plane. Based on these results, a scenario for cloud-to-cloud discharge detection by lidar measurement is proposed, and is reflected in lidar development.

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To know the air quality of the urban atmosphere, precise measurement of key species is essentially important such as NOx and VOCs because they are very reactive and some of them are toxic. NOx indicates combination of NO and NO₂ molecules. Exhaust of combustion mainly contains NO but it is easily oxidized by either ozone or peroxyl radicals in the atmosphere into NO₂. Resultant NO₂ is again converted to NO in the photolysis of solar UV. Thus, NO and NO₂ achieve chemical equilibrium within a couple of minutes in daytime atmosphere. Selective measurement of NO₂ is suffered from chemical interference when conversion technique to NO is applied. Therefore, direct measurement of NO₂ is needed such as optical absorption and laser induced fluorescence (LIF) techniques. Although optical absorption technique does not need calibration process but sensitivity is not enough for lower concentration. Thus, we applied LIF technique for detecting NOx family molecules such as NO, NO₂, NO₃, and N₂O₅. We present herein instrumentation for both NO₂ and NO₃ sensors using pulsed laser excitation. We used an SHG of DPSS Nd:YVO₄ laser and tunable dye laser pumped by it to measure N₂ and NO₃, respectively. The detection limits of radical sensors are 2 ppt for NO₂ and 7 ppt for NO₃ under some practical conditions. Using this instrument we measured N₂O₅ in sub-urban air of Tokyo. Using chemical model analysis we assessed heterogeneous loss processes of N₂O₅ on the aerosol surface and discussed chemical fate of NOx emitted in urban atmosphere. Photochemical oxidant formation is becoming great concern as revival issue from 1970's. To investigate photochemical ozone formation process in the atmosphere, HOx radicals, OH, HO₂, and RO₂ radicals, investigation is important as well as NOx measurements and great efforts are paid for detection of such radicals using either chemical ionization mass spectrometry or spectroscopic methods such as LIF. In addition to the information of HOx radicals, reactive VOCs measurements are also necessary for evaluate air quality. However, kind of VOCs existing in the urban atmosphere is believed to be more than 500. Individual measurements of chemical species are not practical to cover such a large number of molecules. Thus, we proposed to measure OH reactivity in the atmosphere and assess the air quality using OH reactivity as an index of pollution or photochemical activity. We present here the developed technique how to measure OH reactivity in the real atmosphere. We conducted a couple of field expedition using this technique and show some of the results.

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We have developed a novel optical fiber ring laser using a semiconductor optical amplifier (SOA) as the gain medium, and taking advantage of polarization anisotropy of its gain. Since the frequency difference of the bi-directional laser is proportional to the introduced birefringence, the laser in the present study is applicable to a frequency-domain fiber sensor with the birefringent material. For the application the accuracy and stability of the beat signal frequency is important, but it was not sufficient because the polarization of the propagating light was not constant in the single-mode fiber (SMF) which was incorporated into the ring laser cavity. In order to reduce this ambiguity, we have improved the fiber ring laser by using polarization-maintaining fiber (PMF) and a PMF-coupler instead of SMF and a SMF-coupler. The linewidth of the beat signal has been narrowed and the accuracy of the beat signal frequency has been enhanced.

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In the present work, Poly(3-hexylthiophene)/Polymethylmethacrylate [P3HT/PMMA] composite thin film was prepared using spin coating method on the top of triangular prism as composing a prism coupling waveguide. The optical bistable characteristics of prism coupling waveguide comprising with P3HT/PMMA composite film was measured for different input laser power intensity using optical bistable measuring equipment. Also the effect of organic gas treatment on the optical bistable behavior of P3HT/PMMA quasi-waveguide has been investigated. The measured optical bistabilities have excellent stability and hysteresis characteristics. In the input laser power dependence of optical bistability, the on-off position shifts with the increase of input laser power intensity. A Nd:YAG laser with a wavelength of 1064 nm, a pulse width of 5 ns and a repetition frequency of 10 Hz was used for this measurement.

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Regioregular poly(3-hexylthiophene) [RR-P3HT] thin films were prepared on fused quartz glasses by spin-coating and drop-casting from the chloroform solutions. The effect of surface modification of quartz glass by hexamethyldisilazane (HMDS) on the RR-P3HT film structures was investigated. Film structures and morphologies were characterized by UV-visible absorption spectra, out-of-plane X-ray diffraction (XRD) and atomic force microscopy (AFM). The third-order nonlinear optical (NLO) susceptibilities Χ⁽³⁾(-3ω; ω, ω, ω) of the RR-P3HT films were determined by the third harmonic generation (THG) technique with an incident wavelength of 1064 nm. The drop-cast film shows enhanced Χ⁽³⁾ of about three times of that of the spin-coated one, and HMDS treatment can increase the Χ⁽³⁾ further to about 30 percent higher than the untreated one, keeping all the film thicknesses around 140 nm These results revealed the deposition method and surface modification effects on the RR-P3HT film structures, and the importance of higher ordering (enhanced inter-molecular interactions through π-π stacking) and increased crystallinity to the enhancement of the Χ⁽³⁾ of the polymeric films for their applications in NLO devices.

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Antireflection coatings (AR), beam splitter coatings (BS) and high-reflectance coatings (HR) were prepared to study the effect of material properties and stack structures on the laser induced damage behaviors of coatings at a wavelength of 248 nm. All the coatings were deposited on fused silica substrates by electron beam reactive evaporation technique with the materials of aluminum oxide and silicon dioxide. Laser-induced damage thresholds (LIDTs), damage morphologies and profiles of damage sites of multilayer coatings were characterized to investigate the damage mechanism of coatings irradiated by the excimer laser. Besides, the temperature fields were calculated to better understand the damage mechanism with our programmed software. The results indicate that the absorption of defects and the electric field distribution of thin film greatly contribute to LIDTs of thin films.

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The optical properties of Nd³⁺:LaF₃ and Nd³⁺:(La_1-x,Ba_x)F_3-x (x=0.1) crystals, which were efficiently grown by the micro-pulling down method, were investigated using a vacuum ultraviolet spectrum-resolved streak camera system. The fluorescence of the Nd³⁺:LaF₃ crystal is centered at 174 nm with a full width at half maximum of 8 nm and a lifetime of 8.9 ns. On the other hand, the fluorescence of the Nd³⁺:(La_1-x,Ba_x)F_3-x (x=0.1) crystal is centered at 180 nm with a full width at half maximum of 12 nm and a lifetime of 6.1 ns. Since the (La_1-x,Ba_x)F_3-x (x=0.1) host is transparent in the vacuum ultraviolet region having a transmission edge at 160 nm, it is proposed that Nd³⁺:(La_1-x,Ba_x)F_3-x (x=0.1) is suitable as a vacuum ultraviolet scintillator and as a potential laser material.

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We have proposed the cryogenic TGG ceramics based Faraday rotator for High peak and high average power laser. As the first demonstration of Faraday effect in a TGG ceramics, its Verdet constant at 1053 nm is evaluated to be 36.4 rad/Tm at room temperature which is same as that of the single crystal. In addition, the temperature dependence of Verdet constant is obtained experimentally. At liquid helium temperature, it is 87 times greater than that at room temperature. The measured thermal conductivity was corresponding to TGG single crystal.

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Photonic crystal (PhC) nanocavities with ultrahigh Q values are reviewed. The demonstrated Q for a width-modulated PhC nanocavity is extremely high at 1.2×10⁶, which corresponds to a photon lifetime of ~1 ns. In addition, an ultrahigh Q of 3.2×10⁵ is demonstrated using a point defect type hexapole cavity. The extremely long photon lifetime enables us to fabricate devices, such as photonic DRAMs, chip integrated slow light materials, and novel wavelength converters.

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This paper briefly overview recent progress on ultra high capacity optical transmission systems and relevant technologies which make these cutting-edge results happen. Steadily increase of the number of FTTH service subscribers should be one of the driving forces for higher capacity back-bone network. Among various clients, Ethernet should play a dominant role so far and in the future. Currently, 1GbE and 10GbE has widely penetrated for computer networking applications. IEEE is now targeting 100GbE as the next step after 10GbE, and intends to standardize it by 2010. So, inevitably traffic demand for back-bone network should expand further to accept 100GbE. For the time being, 43G DWDM systems have already installed for commercial services. To meet the potential demand, we are trying to develop novel ultra high capacity transmission system which can transport 100GbE clients effectively. I will briefly present some latest results of the effort toward the direction.

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In this paper, we present our researches on the applications of periodically poled lithium niobate (PPLN) in optical communications. Tunable wavelength converter and filter are investigated based on PPLN with unique features which show potential applications in next generation all-optical networks.

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The various communication technologies draw much attention for intelligent transport system (ITS). Especially, the ad hoc network between cars is one of main research topics for ITS and those networks are under standardization for establishing ad hoc communication network, for example, wireless access in vehicular environments (WAVE). Also, optical wireless communication draw much attention for indoor communication system. In this paper, we propose a concept of a car-to-car message delivery system. The proposed system is based on optical wireless communication link employing illumination light-emitting diode (LED) brake lamps.

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To establish an interconnection for power transmission between two moving objects, optical phase conjugation is applied for non-mechanical, all-optical tracking and compensation of atmospheric distortions. The installation of the connection starts in sending a coherent pilot signal by the power receiver towards the transmitter. There, the incident light is collected and directed into a nonlinear medium placed inside a ring cavity. Scattered light enters the resonator and initiates a dynamic hologram writing with the incident pilot signal. Hence, a stable oscillation is generated by diffracted light of the pilot signal.
Out-coupling is performed by a combination of an in-cavity half-wave plate and a polarizing beam splitter. This beam is reflected back into the nonlinear medium to generate the phase conjugated beam by four-wave mixing. Phase and direction of this beam can be manipulated without affecting the dynamic hologram thanks to the previous performed out-coupling. The final power transmission is realized by amplification of the phase conjugated beam.
In this paper, we present initial results of the described setup. The angular range of generating the phase conjugated beam and its fidelity is analyzed.

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An optical processor for modulo operations is estimated. Large scale information processing for modulo operations is effective in an algorithm in prime factorization. This processor is based on a method for modulo multiplication using phase modulation of light wave. And it consists of a Michelson interferometer. Desired interference fringes are obtained by tilting reflection mirrors put at optical arms in the interferometer. Target operations are executed in parallel by measuring fringe patterns with photodetector array. One of the features of the processor is that it achieves parallel data processing for modulo multiplication with only simple operation. In terms of optical implementation, the presented processor seems to give us correct results even though there are some misalignment and noise signals in the processor. Therefore, error tolerance of the presented processor is discussed analytically. Influence of random noises in plane wave is analyzed. As results of analysis, it is verified that the presented processor has robustness against alignment and random errors.

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We have developed an optical tweezers system by on-demand operation. In this work, we achieved real-time displacement of a trapped particle together with a fixed trapped array of particles. The displacement of a trapping beam spot was performed by introducing the phase shift method. The coexistence of the displacing beam spot with the fixed array of beam spots was realized by time-multiplexing operation of a spatial light modulator.

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We propose nonlinear Fourier-transform spectroscopy using a broadband pulse, which can discriminate the origin in Four-wave mixing (FWM) process. By taking account of the response function of a sample in the third-order interferometric autocorrelation (IAC) signal, we theoretically analyze FWM-FTS using a broadband pulse. The discrimination of the origin is achieved by taking advantage of the FWM power spectrum obtained by the Fourier-transform of the IAC signal. In the FWM power spectrum, there are frequency components, which indicate the effects of the two-photon electronic resonance and the Raman resonance. By focusing into the frequency components, the origin in FWM process is identified. Based on the theoretical analysis, we experimentally demonstrate nonlinear FTS employing the tightly focused pulse with a pulse duration of 5 fs and measure the Raman and two-photon electronic spectra in FWM process.

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The change in the cortical blood flow of guinea pigs during auditory stimulation is measured by laser speckle flowgraphy (LSFG). Accuracy of LSFG depends on the number of the frames for the blood flow calculation. The blood-flow maps of rotating disks and cortical tissues are calculated from various numbers of frames of speckle pattern, and the influence of the statistical error and averaging effect on the results is evaluated to optimise the LSFG system. The blood-flow map obtained from successive 24-30 frames can appropriately identify the change in the blood flow caused by auditory stimulation.

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Out of focus fluorescence is a dominant limiting factor in the improvement of imaging depth in two-photon fluorescence microscopy. Here we propose a new two-color two-photon fluorescence microscopy scheme, in which two trains of laser pulses with different wavelengths are concentric and complementary. These two beams are overlapped both in space and time only in the focal area. Monte Carlo simulation demonstrates that this scheme can effectively reject out-of-focus fluorescence excitation, and thus has great potential for providing significant increment of imaging-depth.

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Novel image restoration technique suitable for parallel detection in full-field optical coherence tomography (FF-OCT) is introduced. An en-face information of a sample can be demodulated from only two phase-modulated interference images with simple mathematical Hilbert transform process. With the proposed method, a FF-OCT was implemented and its feasibility was verified by taking tomographic images of a biological sample; the wing of the spilota plagicollis fairmaire ex vivo. The system shows the image resolutions of 1.0 µm for the axial direction and 2.2 µm for the lateral direction.

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Photodynamic Diagnosis (PDD, Fluorescence Diagnosis) and Photodynamic Therapy (PDT) were performed for malignant brain tumor patients. We have treated 250 patients with PDD during surgery and also 63 patients with PDT in our hospital. The rate of total removal of tumor tissues increased by using PDD (fluorescence diagnosis) during surgery and many malignant glioma patients survived for long time by treatment with PDT. PDD and PDT were very effective for malignant brain tumor patients from our experiences.

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Observation of a living plant was performed by a laser speckle microscopy. The merit of this technique is that it is possible to easily obtain information of organisms without any special preparations, such as staining and gene transfection. In this study, a living leaf was observed for half a day at intervals of one hour. To our knowledge, this is the first report of a long-term tissue observation by the laser speckle. It was possible to visualize the fading process of the leaf cut off from the living plant. It was found that the temporal fluctuation of the speckles on the leaf completely ceased within 10 hours after cutting off the leaf from the plant body.

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