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Cell sheet engineering allows investigators/clinicians to prepare cell-dense three-dimensional (3-D) tissues, and various clinical trials with these fabricated tissues have already been performed for regenerating damaged tissues. Cell sheets are easily manipulated and 3-D tissues can be rapidly fabricated by layering the cell sheets. This study used optical coherence tomography (OCT) to noninvasively analyze the following processes: (1) adhesions between layered cell sheets, and (2) the beating and functional interaction of cardiac cell sheet-tissues for fabricating functional thicker 3-D tissues. The tight adhesions and functional couplings between layered cell sheets could be observed cross-sectionally and in real time. Importantly, the noninvasive and cross-sectional analyses of OCT make possible to fabricate 3-D tissues by confirming the adherence and functional couplings between layered cell sheets. OCT technology would contribute to cell sheet engineering and regenerative medicine., Highlights • Development of an OCT system to noninvasively analyze three-dimensional cell sheet-tissues. • Noninvasive and cross-sectional observation of the adhesions between layered cell sheets in detail. • Noninvasive and cross-sectional observation of beating three-dimensional cardiac tissues.

Cell sheet technology has a history of application in regenerating various tissues, having successfully completed several clinical trials using autologous cell sheets. Tomographic analysis of living cell sheets is an important tool in the field of cell sheet-based regenerative medicine and tissue engineering to analyze the inner structure of layered living cells. Optical coherence tomography (OCT) is commonly used in ophthalmology to noninvasively analyze cross-sections of target tissues at high resolution. This study used OCT to conduct real-time, noninvasive analysis of living cell sheet cross sections. OCT showed the internal structure of cell sheets in tomographic images synthesized with backscatter signals from inside the living cell sheet without invasion or damage. OCT observations were used to analyze the static and dynamic behaviors of living cell sheets in vitro and in vivo including (1) the harvesting process of a C2C12 mouse skeletal myoblast sheet from a temperature-responsive culture surface; (2) cell-sheet adhesion onto various surfaces including a culture surface, a synthetic rubber glove, and the dorsal subcutaneous tissue of rats; and (3) the real-time propagation of beating rat cardiac cells within cardiac cell sheets. This study showed that OCT technology is a powerful tool in the field of cell sheet-based regenerative medicine and tissue engineering. © 2014 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 103B: 1267–1273, 2015.

Highly-efficient fully air-cooled green laser light source with frequency doubling module is reported. As a result, 3.6 watts green laser light generation with about 9% wall-plug efficiency is achieved. By using fully air-cooled RGB lasers including the green SHG (Second Harmonic Generation) laser and illumination optics for speckle noise reduction, a laser projection engine is prototyped. A laser rear-projection system with this laser projection engine shows several merits; wide color gamut (133%NTSC), 61inch large screen size, and low electric power consumption (less than 100W, at 650cd/m2).

Continuous-wave 3 W green-light generation at 542 nm is reported in bulk periodically poled MgO:LiNbO3 using single-pass frequency doubling of a Yb-doped fiber laser operating at 1084 nm. The Yb-doped fiber laser had an output power of 9 W, single polarization, beam quality factor M2~1.05, and a spectrum bandwidth of 0.07 nm. MgO:LiNbO3 samples of 1.0 mm thickness, 25 mm interaction length, and 7.38 µm domain period were fabricated by using a high-voltage multi pulse poling method. A maximum green power of 3 W with a 34% infrared-to-green conversion efficiency and a 6%/W normalized conversion efficiency was obtained. In addition, the generated green light possessed beam quality factor (M2) of less than 1.1 and an output stability of 1.5%.

We report on electric-field poling of MgO:LiNbO3 (MgLN) using patterned electrodes. Investigation of electric properties of MgLN reveals that polarization switching causes reversible resistance change and that this phenomenon can explain the physical mechanism of random domain growth. Based on these results, we propose a domain-control method in MgLN single crystals by suppressing resistance reduction during the poling process. Using this method, short periodic structures consisting of submicron domain geometries have been achieved in X-, Y-, and Z-cut MgLN crystals. High performances of these periodic domain-inverted structures are also demonstrated by evaluating their potentials as second-harmonic generation devices.

We propose a novel method for controlling the shape of a short period domain-inverted structure utilizing a multi-pulse poling method. Using this technique, periodically domain-inverted structures with periods of 1.4-2.4 μm were fabricated for over 10 mm interaction length in 2 mm thick Z-cut MgO: LiNbO3 crystal. We have demonstrated highly efficient ultraviolet light generation (340-390 nm) using bulk QPM-SHG devices with these structures. Continuous wave 72 mW at wavelength of 372 nm was generated with conversion efficiency of 17 %.

Quasi-phase matched (QPM) second-harmonic generation (SHG) device offers various application for blue and ultra-violet (UV) radiations. Advanced poling technology made it possible to realize micron and sub-micron poling geometry in efficient nonlinear materials. Using these techniques, first-order nonlinear gratings have been realized for high power blue and UV SHG. Based on these devices, high power short wavelength laser sources were demonstrated for practical use. Recent progress in this area is reviewed with emphasis on research work of the author's group using bulk and waveguide QPM devices.

Cascaded quadratic nonlinearities in periodically poled lithium tantalate are studied with femtosecond pulses by using the spectrally resolved two-beam coupling technique. The nonlinear phase shifts show obvious dependence on the phase mismatch conditions. Relatively large intrinsic nonlinear refraction and its anisotropy are also measured.

We demonstrate highly efficient frequency doubling of a 160 mW-AlGaAs three-section tunable distributed Bragg reflector (DBR) laser diode with real refractive-index-guided structure in a quasi-phase-matched second harmonic generation (QPM-SHG) ridge-type waveguide device. We realized high coupling efficiency of the DBR laser diode and the QPM-SHG waveguide device using a planar-type butt-coupling configuration. High-power SHG blue violet laser was fabricated, whose volume is 0.3 cc. This SHG blue-violet laser satisfied the several specifications for optical disk systems, and the pulsed blue-violet light (410 nm) power of 62 mW was generated.

Cell sheet technology has a history of application in regenerating various tissues, having successfully completed several clinical trials using autologous cell sheets. Tomographic analysis of living cell sheets is an important tool in the field of cell sheet-based regenerative medicine and tissue engineering to analyze the inner structure of layered living cells. Optical coherence tomography (OCT) is commonly used in ophthalmology to noninvasively analyze cross-sections of target tissues at high resolution. This study used OCT to conduct real-time, noninvasive analysis of living cell sheet cross sections. OCT showed the internal structure of cell sheets in tomographic images synthesized with backscatter signals from inside the living cell sheet without invasion or damage. OCT observations were used to analyze the static and dynamic behaviors of living cell sheets in vitro and in vivo including (1) the harvesting process of a C2C12 mouse skeletal myoblast sheet from a temperature-responsive culture surface; (2) cell-sheet adhesion onto various surfaces including a culture surface, a synthetic rubber glove, and the dorsal subcutaneous tissue of rats; and (3) the real-time propagation of beating rat cardiac cells within cardiac cell sheets. This study showed that OCT technology is a powerful tool in the field of cell sheet-based regenerative medicine and tissue engineering.

Quasi-phase-matched second-harmonic generation (QPM-SHG) devices using LiNbO3 optical waveguides show great potential as low noise and high beam quality blue-violet light sources applicable for high-density optical disks. Optical waveguides with a step index profile are desired for achieving strong confinement and sufficient overlap between the propagation modes in order to realize a high conversion efficiency. Liquid phase epitaxy of LiNbO3 is a novel technique which yields a step index thin-film waveguide structure by doping ZnO in the film. Ultraprecision machining is a powerful technique for processing three-dimensional optical waveguides. The maximum SHG power of 50 mW at 425 nm wavelength was obtained using a ridgetype waveguide device which was fabricated from a periodically domain-inverted MgO: LiNbO3 crystal by ultraprecision machining.

We present a method for controlling the shape of the domain-inverted structure in an off-cut MgO: LiNbO3 crystal utilizing two-dimensional high-voltage application. Using this technique, a periodically domain-inverted structure with a period of 3.2 Jim and thickness of 2.0 μm was fabricated over a 10-mminteraction length. This structure has made possible sufficient overlaps between propagation modes and domain inversion in the waveguide. Using this structure, we demonstrated a cw blue-violet second-harmonic generation of 17.3 mW at the wavelength of 426 nm with a single pass of 55 mW of cw AlGaAs laser diode input, which corresponded to a 31 % power conversion efficiency.

ZnO-doped LiNbO3 (LN) liquid-phase epitaxial thin films grown on LN substrates were investigated as modified LN substrates for optical waveguide devices. The potential for optical use as substrates compatible with conventional ones was examined in quasi-phase-matched second-harmonic generation (SHG) devices from the viewpoint of the domain inversion process and optical damage properties. The formation of a periodically inverted-domain structure and the increased optical damage resistance in SHG were demonstrated.

The resistance to photorefractive damage is investigated for several nonlinear crystals. A quasi-phase-matched (QPM) second harmonic generation (SHG) waveguide device is fabricated in 5 mol% Mg-doped LiNbO3 which has high resistance to optical damage. The SHG blue laser of the QPM-SHG waveguide device and a tunable distributed Bragg reflector (DBR) laser diode is demonstrated, wherein output stability of continuous blue light is measured. The SHG blue laser, using the QPM-SHG waveguide device with broadened flat matching response, shows good modulation characteristics. The pulsed peak power of 23 mW with rectangular modulated waveform is generated. The SHG blue laser is installed to an optical head, and good recording and readout characteristics are demonstrated. Moreover, butt-coupled SHG blue laser is examined to gain a miniature module with volume of 0·8 cc.

A compact and high power green-light source has been developed in diode-pumped solid-state laser based on periodically poled Mg:LiNbO3 (PPMgLN). Optimization of the PPMgLN length in Nd:YVO4 intracvity can achieve wide operation temperature range over 30 °C with maintaining high conversion efficiency. Based on this result, we demonstrated 3 cm3 laser module which generated continuous-wave 0.7-W green light with wall-plug efficiency as high as 11.8%.

A novel multi-laser diodes (LDs) condenser/homogenizer which maintains the LDs' polarization plane and is suitable for use with a liquid crystal light bulb is proposed. With the condenser/homogenizer, high power and uniform illumination is successfully obtained. A prototyped laser projection system with the condenser/homogenizer showed superb wide color gamut.

A compact, high power SHG green light source with wide operation temperature range over 30 degrees Celsius has been developed by using a wavelength-locked laser diode and a 0.5mm-long periodically poled Mg:LiNbO3(PPMgLN). A continuous green light of 1.7W was generated by intracavity frequency doubling of a Nd:YVO4 laser with the PPMgLN. The wall-plug efficiency was as high as 18.9%. The volume of the green light source was 2cc.

A high power SHG blue laser with direct modulation capability at high frequency has been demonstrated usinga tunable distributed Bragg reflector (DBR) laser diode and a waveguide-type quasi-phase-matched secondharmonic generation (QPM-SHG) device in x-cut Mg doped LiNbO3. Continuous blue light power of 12 mWand pulsed peak power of 26 mW were generated. The SHG blue laser was installed to an optical head, andreadout of ROM disk and recording on phase change optical disk were performed. A butt-coupling-type 0.8cc SHG blue laser was also fabricated. It was indicated that SHG blue laser is applicable to high-density opticaldisk system.

Continuous-wave UV radiation at 354nm, the third harmonic of a Nd:GdVO4 laser operating at 1063nm, is reported from single-pass cascaded, first-order periodically poled MgO:LiNbO3 (PPMgLN) crystals. A high-voltage multipulse poling method was used in order to realize PPMgLN with uniform domain period (Λ) of micron-order width and 50% duty cycle along the entire crystal length. Initially, second-harmonic generation of the 1063nm Nd:GdVO4 laser at 2.1%∕W conversion efficiency, producing 1.15W at 531nm, is obtained from a 10-mm-long PPMgLN with Λ=6.95μm. Sum frequency mixing of this 531nm light with the residual 1063nm fundamental radiation in a 3.6-mm-long PPMgLN with Λ=1.78μm produces 47mW of UV with 3.9%∕W∕cm normalized conversion efficiency.

Continuous-wave blue light generation at 456 nm is reported in bulk periodically poled MgO:LiNbO3 by single-pass frequency doubling of a Nd:GdVO4 laser operating at 912 nm. The Nd:GdVO4 laser had a maximum output power of 2.6 W for 15.7 W pump power, with a beam M2 factor of 1.4. MgO:LiNbO3 samples of 1.0-mm thickness and 4.2 µm domain period were fabricated by using a high-voltage multipulse poling method. A maximum blue power of 167 mW with 8.3% infrared-to-blue conversion efficiency and 4.2%/W normalized conversion efficiency was obtained. To the best of our knowledge, this is the first report on continuous-wave deep blue (below 460 nm) generation by quasi-phase matching in periodically poled MgO:LiNbO3.

Continuous-wave power of 0.89 W at 531 nm with 14.3% conversion efficiency is reported at room temperature from a 2-mm-thick periodically polled uncoated MgO:LiNbO3 by single-pass frequency doubling of a diode end-pumped Nd:GdVO4 laser; the corresponding internal green power and conversion efficiency were 1.03 W and 16.5%, respectively. The MgO:LiNbO3 device of 6.95-µm domain period was fabricated by using a high-voltage multi-pulse poling method; uniform periodicity and 50% duty cycle were obtained on the entire 10-mm-long crystal. The sample exhibited an effective nonlinear coefficient of 14 pm/V (~88% of the ideal nonlinear coefficient), resulting in a measured normalized conversion efficiency of 2.7%/W. To the authors best knowledge this is the highest power ever obtained by quasi-phase matching process at room temperature.

We report on fabrication of a first‐order periodically domain‐inverted structure in −Z‐cut LiTaO3 using proton exchange followed by heat treatment. The expansion of the domain inversion width can be suppressed within the first‐order intervals by shortening the heat‐treatment time. To perform this suppression, we propose quick heat treatment, which features rapid rising rate and short soak time. This technique was used to study the formation process and the expansion characteristics of domain inversion during heat treatment. This produced a periodically domain‐inverted structure with interval of 4 μm and depth of 1.8 μm over a 10 mm length in the LiTaO3. The potential for this structure was evaluated by examining the characteristics of a first‐order quasi‐phase‐matched second‐harmonic generation device with a proton‐exchanged waveguide. We also discuss the nucleation and expansion of domain inversion, based on experimental studies.

A periodically domain‐inverted structure in LiTaO3 has been fabricated for quasiphase‐matched (QPM) second‐harmonic generation (SHG). Uniform proton exchange is achieved with pyrophosphoric acid. Using this technique with a Ta mask followed by heat treatment, the periodically domain‐inverted structure for quasiphase matching is realized over large areas. The effect of fabrication conditions on the domain‐inverted profile is investigated to form a periodic structure having a considerable depth and a short inversion period. Consequently, a third‐order QPM‐SHG device can be fabricated by using this periodically domain‐inverted structure and proton‐exchanged waveguide. To investigate period regularity of the domain‐inverted structure and nonlinearly of the inverted region, phasematching characteristics and conversion efficiency of this device are examined. The formation mechanism of the domain inversion is also discussed.

Frequency doubling of a 680-nm laser diode in periodically poled LiTaO(3) is presented. By selective proton exchange followed by high-voltage pulse application, a second-order periodic domain inversion having uniform periodicity and optimum duty cycle was fabricated over a 10-mm interaction length in a 200-microm-thick LiTaO(3) substrate. A 340-nm wavelength of harmonic ultraviolet light was generated in a single pass through the domain-inverted structure.

We report highly efficient blue-light generation by frequency doubling of a high-frequency superimposed laser diode, stabilized by a grating feedback technique, in a periodically domain-inverted LiTaO(3) waveguide. To increase second-harmonic generation conversion efficiency, we enhanced the peak power of the laser diode by using a gain-switching method. Locking the oscillation wavelength of the laser diode by the grating feedback technique, we obtained 4.5 mW of average blue-light power with 13% conversion efficiency.

We report the characterization of a quasi-phase-matched second-harmonic generation device in LiTaO(3) that has a periodically domain-inverted region and a proton-exchanged channel waveguide. A blue-light power of 2.4 mW was obtained at a 424-nm wavelength. The observed temperature bandwidth for FWHM power is approximately 3 degrees C-cm, which is three times wider than that in a similar device in LiNbO(3). It is also shown that diffraction-limited focusing of the generated blue light may be obtained.

Characteristics of waveguides with periodically domain‐inverted regions in LiNbO3 and LiTaO3 for quasi‐phase‐matched (QPM) second harmonic generation are investigated. The domain‐inverted regions in LiNbO3 are formed by heat treatment using selective SiO2 mask and those in LiTaO3 are formed by heat treatment using selective proton exchange. The domain‐inverted region in LiTaO3 is fabricated up to 2.7 μm depth, which is twice as deep as that in LiNbO3. The channel waveguides incorporating the regions are fabricated using proton exchange by pyrophosphoric acid. The propagation loss (0.9 dB/cm) of LiTaO3 waveguide is much lower than that of LiNbO3 waveguide (2.9 dB/cm). The second harmonic power generated in LiTaO3 waveguide by third‐order QPM consequently has three time higher conversion efficiency, compared to that in LiNbO3 waveguide. In domain‐inverted LiNbO3 waveguide, we have observed optical damage. On the other hand, in domain‐inverted LiTaO3 waveguide, stable blue light (421 nm) has been obtained wi...

We report on a theoretical analysis and experiments for bandwidth broadening in quasi-phase-matched (QPM) second-harmonic generation (SHG).We used phase-shifted segments of a periodic grating to obtain a spectrally broadened, nearly flat response simultaneously with high conversion efficiency. We used an x-cut MgO:LiNbO(3) QPM waveguide in our analysis and experiments. The spectral range of the 850-nm fundamental for which SHG conversion exceeded 0.95 of the maximum value broadened from 0.02 to 0.12 nm when a 1-cm-long grating was divided into three segments with optimum phase shift. SHG conversion efficiency was 300%/W for this waveguide. The SHG efficiency and phase-matching characteristics showed good agreement with theoretical results.

Characteristics of polarization selection in Yb fiber laser by use of birefringent fiber- Bragg gratings are investigated. An 8 W, 1064 nm linearly-polarized fiber laser with 87% slope efficiency and

Frequency doubling of a 684 nm laser diode in a first-order periodically poled LiTaO3 is presented. Controlling the side growth of the domain inverted region during high-voltage pulse application, a uniform nonlinear grating with a period of 1.7 μm and 50% duty cycle was fabricated over 10 mm interaction length in a 150 μm thick LiTaO3 substrate. A 342 nm wavelength of harmonic ultraviolet light was generated in a single pass through the domain-inverted structure.

Continuous-wave 456-nm blue light generation is reported from a periodically polled MgO:LiNbO3 crystal by single-pass frequency doubling of a 912-nm Nd:GdVO4 laser. A maximum blue power of 167 mW with 8.3% infrared-to-blue conversion efficiency and 4.2%/W normalized conversion efficiency resulted from a 10-mm-long nonlinear device with 4.19-μm domain period and that operated at 44.5°C temperature.

A half-wave thin film was formed on the facet of a proton-exchanged waveguide by the oblique deposition of Ta2O5. Planar-type butt coupling of the waveguide on the z-cut LiTaO3 substrate to the AlGaAs laser diode in transverse electric mode oscillation was realized with coupling efficiency as high as 53%.

Ultraviolet radiation, the third-harmonic of a Nd:GdVO4 laser operating at 1.06-microns wavelength, is reported from a single-pass cascaded first-order periodically poled MgO:LiNbO3 crystals. Continuous-wave 47 mW at 354 nm with 3.8%/Wcm normalized conversion efficiency resulted.

Continuous-wave power of 1.18 W at 531 nm with 16.8% conversion efficiency is reported at room temperature from a 2-mm-thick, 25-mm long uncoated periodically polled MgO:LiNbO3 by single-pass frequency doubling of a Nd:GdVO4 laser.

Second‐harmonic generation in a LiTaO3 periodically poled by electric field is presented. The side growth of the domain‐inverted region during the poling was found to be suppressed using selective proton exchange. Utilizing this technique, a first‐order periodic domain inversion having uniform periodicity was realized over interaction length of 10 mm in a 300‐μm thick LiTaO3 substrate. A 3.8 mW of harmonic blue light was generated for a conversion efficiency of 1.27% in a single pass through the periodically domain‐inverted structure.

Ultraviolet second-harmonic generation in first-order periodically poled MgO:LiNbO3 is presented. Using a high-voltage multipulse application method, we fabricated a first-order nonlinear grating with a period of 1.8 microm and depth of approximately 150 microm over 10-mm interaction length in a 1-mm-thick MgO:LiNbO3 substrate. In a single-pass configuration, continuous-wave 30-mW UV light at 362.5 nm was generated for a fundamental power of 522 mW, corresponding to a normalized conversion efficiency of 11%/W.

We report wavelength stabilization of a laser diode using a highly efficient distributed Bragg reflector (DBR) grating formed on a LiTaO3 quasi-phase-matched second harmonic generation (QPM-SHG) device. Fabricating the second-order DBR on the LiTaO3 waveguide, the reflectivity of 90% and FWHM wavelength bandwidth of 0.2 nm were obtained. By stabilizing the oscillation wavelength of the laser diode, 3.1 mW of blue light was generated in the QPM-SHG device with the monolithic grating.

The fabrication of a periodic domain inversion in LiTaO3 and LiNbO3 using direct ion-beam writing is presented. The polarization of these materials can be reversed at room temperature by irradiating Si2+ ions into +c faces. A first-order periodic domain inversion with a 50% duty cycle, a depth of 300μ, and an interaction length of 1 mm was realized in LiTaO3. Using this structure, 300μW of blue light was generated for a conversion efficiency of 0.25%/W.

Direct frequency doubling of a wavelength locked laser diode with an optical bandpass filter in a confocal optical configuration is demonstrated. The wavelength of the laser diode was locked in single longitudinal mode oscillation and tuned to phase-matching wavelength of a quasi-phase-matched second harmonic generation device based on a LiTaO3 Waveguide. Stable blue light of 4.2 mW was obtained for incident power of 48 mW.

We report high power and stable blue‐light generation by frequency doubling a laser diode, stabilized by a grating feedback technique, in a periodically domain‐inverted LiTaO3 waveguide. Due to the deep domain‐inverted regions and strong waveguide confinement formed by proton‐exchange and a quick heat treatment, high efficiency second harmonic generation (SHG) has been obtained. Locking the oscillation wavelength of a laser diode through the waveguide by a grating feedback technique, 8 mW of stable blue SHG has been demonstrated.

We report on the fabrication of periodic domain inversion on the x face of LiTaO3 by selective proton exchange and quick heat treatment. Observation of domain formation in –c‐cut LiTaO3 indicates an inside field model where electric field caused by proton diffusion reverses the orientation of domains. Based on this model, it has been found that an effective inside field to form domain inversion is also induced in x‐cut LiTaO3 by using selective proton exchange. Accordingly, periodic domain inversion at 10 μm intervals has been successfully formed on the x face of LiTaO3.

We report blue‐light generation using a first‐order quasi‐phase matched second‐harmonic generation (QPMSHG) in LiTaO3 waveguide. A first‐order periodically domain‐inverted structure can be fabricated using a quick heat‐treatment technique. We have examined the dependence of the depth of the fabricated domain‐inverted region on its width, which critically affects the conversion efficiency of SHG, and designed an optimum domain‐inverted structure for the QPMSHG device. As a result, 15 mW of blue light was generated for a conversion efficiency of 10.3%.

We report on fabrication of first‐order periodically domain‐inverted structure in LiTaO3 by using selective proton exchange and heat treatment. Observation of the formation process of domain inversion makes it clear that the shape of the inverted region depends on time and temperature of heat treatment. As a result, enlargement of inversion width in heat treatment is suppressed by shortening the heat treatment time. First‐order periodically domain‐inverted structure, which has 3.5 μm period and 1.5 μm depth, is realized.