Spectral peaking in an ultrashort-pulse fiber laser oscillator with a molecular gas cell.

Here we report the demonstration of a spectral peaking phenomenon in a fiber laser oscillator. An HCN gas cell was inserted in an ultrashort-pulse Er-doped fiber laser with single-wall carbon nanotubes. Sech2-shaped ultrashort pulses with intense multiple sharp spectral peaks were stably generated. When the generated pulses were coupled into highly nonlinear fiber, enhanced multiple spectral peaks were generated by periodical spectral peaking in the optical fiber. The characteristics and physical mechanism of spectral peaking in the fiber laser were investigated via numerical simulations. As the magnitude of absorption was increased, the magnitude of the generated spectral peaks increased almost exponentially. It was clarified that the spectral peaks were generated through the accumulation of filtering components generated in each round trip.

Switching dynamics of silicon waveguide optical modulator driven by photothermally induced metal-insulator transition of vanadium dioxide cladding layer.

We investigated the switching dynamics of optical modulators consisting of a Si waveguide with a VO2 cladding layer by utilizing the photothermal effect, which induces a metal-insulator transition in VO2. The devices exhibited stable optical switching with a high extinction ratio exceeding 16 dB. The switching time of the insulator-to-metal transition (heating process) ranged from tens of nanoseconds to microseconds depending on the incident light power, and that of the metal-to-insulator transition (cooling process) was several microseconds regardless of the incident light power. The heat transfer in the devices was numerically simulated to reproduce the switching characteristics and revealed that the temperature change in the first few micrometers of the VO2/Si waveguide governed the switching time. The thermal structural design of the device is thus of key importance to improve the switching speed of the device.

Initial alignment control technique using on-chip groove arrays for liquid crystal hybrid silicon optical phase shifters.

Flexible and localized initial alignment control of liquid crystal (LC) is important to enhance the performance and functionality of LC hybrid silicon photonic devices. This work proposes an initial LC alignment control technique based on integration of a nanometer-scale groove array in the buried oxide layer near a Si waveguide. We achieved control of the initial angle of LC director around the Si waveguide by selecting the required integrated groove direction and reduced the driving voltage by introducing a vertical groove array into the phase shifter. We then used the local and flexible LC initial alignment controllability to develop a Mach-Zehnder optical switch and ring-resonator wavelength filter. This approach will be helpful when integrating LC-loaded devices with various characteristics and functionalities into optical integrated circuits.

Silicon waveguide optical modulator driven by metal-insulator transition of vanadium dioxide cladding layer.

We have fabricated compact optical modulators consisting of a Si waveguide with a VO2 cladding layer. These devices showed a sharp decrease in transmittance at around 60 °C, which is attributable to the metal-insulator transition of the VO2 cladding layer. By systematically varying the length of the device, we evaluated the transmission losses per unit length of the device to be 1.27 dB/µm, when the VO2 cladding layer was in the insulating (ON) state and 4.55 dB/µm when it was in the metallic (OFF) state. Furthermore, we found that the device showed an additional loss in the OFF state, which is attributable to a structural effect. As a result, an 8-µm-long device showed a large extinction ratio of more than 33 dB.

Broad-band surface optical coupler based on a SiO2-capped vertically curved silicon waveguide.

A chip-surface optical coupler based on a vertically curved Si waveguide was demonstrated for coupling with high-numerical-aperture single-mode optical fibers with a mode-field diameter of 5 µm. This device features a dome-like SiO2 coupler cap, which acts as collimation lens. We succeeded in fabricating this structure using an isotropic SiO2 deposition technique employing plasma-enhanced chemical vapor deposition and obtained a light output that approximates that of a 5-µm-waist Gaussian beam. The fabricated coupler showed a coupling loss of less than 4.2 dB and a 0.5-dB-loss bandwidth above 150 nm for TE-polarized light.

Characterization of a novel allergenic protein from the octocoral Scleronephthya gracillima (Kuekenthal) that corresponds to a new GFP-like family named Akane.

Certain marine organisms have been known to cause allergic reactions among occupational fishermen. We have previously reported that bronchial asthma among the workers engaged in spiny lobster fishing in Japan was caused by octocorals such as Dendronephthya sp. and Scleronephthya gracillima (previously named Alcyonium gracillimum). Now we have found another octocoral, Scleronephthya gracillima (Kuekenthal), which causes the allergic disease in fishermen. The octocoral was characterized as a new green fluorescent protein (GFP)-like family. The new allergen has a molecular mass of 27 kDa in 1D and 2D SDS-PAGE under reduced conditions. The 27 kDa component was determined to be an allergen by western blotting, ECL immune staining method and absorption of patient sera with the antigen. Furthermore, the combination of analysis with LC-ESI-MS/MS and MASCOT search in the NCBInr database concluded the 27 kDa component had the sequence YPADI/LPDYFK, and that the 22 kDa component had the sequence QSFPEGFSWER, which both matched a GFP-like protein in Acropora aculeus and in Montastraea annularis. Further analysis by MALDI-TOF/MS/MS and MASCOT search in the NCBInr database of all 27 kDa eight spot components from 2D SDS-PAGE indicated that the sequence QSFPEGFSWER also matched as GFP-like protein in Lobophyllia hemprichii and Scleractinia sp. To our knowledge, this is the first report of the new allergenic protein that corresponds to a new GFP-like protein named Akane, and which has fluorescent emissions in the red and green part of the spectra at 628 nm and 508 nm, respectively.

Characteristics and improvement of wideband wavelength-tunable narrow-linewidth source by spectral compression in quasi-dispersion-increasing comb-profile fiber.

We investigated the characteristics and behavior of spectral compression in a quasi-dispersion-increasing comb-profile fiber (CPF). A periodical breathing behavior and sidelobe emission process in the CPF were observed in numerical analysis. Then, taking account of the numerical results, we developed an improved CPF in which the sidelobe suppression was dramatically improved to -24.2 dB while keeping a narrow spectral width of ~0.6 nm. As a seed pulse source, we developed a high-repetition-rate Er-doped ultrashort-pulse fiber laser with single-wall carbon nanotubes and used the improved CPF to realize a high-power, narrow-linewidth source with wide wavelength tunability in the 1.62-1.90 µm band.

Vertical silicon waveguide coupler bent by ion implantation.

We propose and demonstrate that vertically curved waveguides (VCWs) enable vertical coupling between silicon wire waveguides and optical fibers with low wavelength dependence and polarization dependence for wide telecommunication wavelength band light. To bend these VCWs, we implanted silicon ions into silicon wire cantilevers from the vertical direction. The internal stress distribution that was induced by ion implantation drove the bending force, and we achieved vertical bending of the waveguides, with curvature radii ranging from 3 to 25 µm. At a radius of curvature of 6 µm, we obtained a coupling loss of 3 dB using a lens fiber.

Spot-size converter with a SiO(2) spacer layer between tapered Si and SiON waveguides for fiber-to-chip coupling.

We experimentally demonstrate low-loss and polarization-insensitive fiber-to-chip coupling spot-size converters (SSCs) comprised of a three dimensionally tapered Si wire waveguide, a SiON secondary waveguide, and a SiO(2) spacer inserted between them. Fabricated SSCs with the SiO(2) spacer exhibit fiber-to-chip coupling loss of 1.5 dB/facet for both the quasi-TE and TM modes and a small wavelength dependence in the C- and L-band regions. The SiON secondary waveguide is present only around the SSC region, which significantly suppresses the influence of the well-known N-H absorption of plasma-deposited SiON at around 1510 nm.

Low-loss and low wavelength-dependence vertical interlayer transition for 3D silicon photonics.

This paper presents optimized design and measurement results for a low-loss broadband vertical interlayer transition (VIT) device located between lower and upper Si nano-photonic waveguides. The device comprises the lower c-Si taper, the upper a-Si:H taper, and a wide and thin SiON secondary core with a 0.6-µm-thick SiO2 interlayer. The device structure facilitates the low loss VIT, giving insertion losses of 0.87 and 0.79 dB for quasi-TE and TM modes, respectively, at 1550 nm. Also, the evanescent coupling nature of the VIT device renders it wavelength- and polarization-insensitive, leading to loss variation of within 0.5 dB in the C-band.

Sub-1 dB/cm submicrometer-scale amorphous silicon waveguide for backend on-chip optical interconnect.

We demonstrate a submicrometer-scale hydrogenated amorphous silicon (a-Si:H) waveguide with a record low propagation loss of 0.60 ± 0.02 dB/cm because of the very low infrared optical absorption of our low defect a-Si:H film, the optimized waveguide structure and the fabrication process. The waveguide has a core with a thickness of 440 nm and a width of 780 nm that underlies a 100-nm-thick ridge structure, and is fabricated by low-cost i-line stepper photolithography and with low-temperature processing at less than 350°C, making it compatible with the backend process of complementary metal oxide semiconductor (CMOS) fabrication.

Pattern-effect-free all-optical wavelength conversion using a hydrogenated amorphous silicon waveguide with ultra-fast carrier decay.

Ultra-fast carrier decay, recently discovered in a hydrogenated amorphous silicon waveguide, can be exploited for pattern-effect-free all-optical signal processing based on optical Kerr nonlinearity. In this study, we utilized a 10 Gbit/s RZ-OOK data stream as a pump for degenerate four-wave mixing in a low-loss hydrogenated amorphous silicon waveguide. The propagation loss of the waveguide used was 1.0±0.2 dB/cm at 1550 nm. Unlike crystalline silicon waveguides, no noticeable difference was observed in the BER characteristics between the cases of PRBS 2(7)-1 and 2(31)-1.

Ultrafast nonlinear effects in hydrogenated amorphous silicon wire waveguide.

We, for the first time, present the ultrafast optical nonlinear response of a hydrogenated amorphous silicon (a-Si:H) wire waveguide using femtosecond pulses. We show cross-phase and cross-absorption modulations measured using the heterodyne pump-probe method and estimate the optical Kerr coefficient and two-photon absorption coefficient for the amorphous silicon waveguide. The pumping energy of 0.8 eV is slightly lower than that required to achieve two-photon excitation at the band gap of a-Si:H (approximately 1.7 eV). An ultrafast response of less than 100 fs is observed, which indicates that the free-carrier effect is suppressed by the localized states in the band gap.