Emiko Omoda

Development of a high power supercontinuum source in the 1.7 μm wavelength region for highly penetrative ultrahigh-resolution optical coherence tomography

We developed a high power supercontinuum source at a center wavelength of 1.7 μm to demonstrate highly penetrative ultrahigh-resolution optical coherence tomography (UHR-OCT). A single-wall carbon nanotube dispersed in polyimide film was used as a transparent saturable absorber in the cavity configuration and a high-repetition-rate ultrashort-pulse fiber laser was realized. The developed SC source had an output power of 60 mW, a bandwidth of 242 nm full-width at half maximum, and a repetition rate of 110 MHz. The average power and repetition rate were approximately twice as large as those of our previous SC source [20]. Using the developed SC source, UHR-OCT imaging was demonstrated. A sensitivity of 105 dB and an axial resolution of 3.2 μm in biological tissue were achieved. We compared the UHR-OCT images of some biological tissue samples measured with the developed SC source, the previous one, and one operating in the 1.3 μm wavelength region. We confirmed that the developed SC source had improved sensitivity and penetration depth for low-water-absorption samples.

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.

Ultranarrow Silicon Inverse Taper Waveguide Fabricated with Double-Patterning Photolithography for Low-Loss Spot-Size Converter

Using a double-patterning process of i-line photolithography that twice performs a pair of photoresist patterning and dry etching processes, we were able to form an ultranarrow silicon inverse taper waveguide with a tip end width that was much narrower than the resolution limit of photolithography. We fabricated a spot-size converter (SSC) consisting of a 50-µm-long silicon taper waveguide with gradually decreasing width from 400 to 50 nm and a polyimide second core. The insertion loss of the SSC was 0.55 dB for the transverse electric-like mode, which was the lowest value for an SSC fabricated using photolithography.

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.

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.

Vertically Curved Si Waveguide Coupler with Low Loss and Flat Wavelength Window

Silicon photonics enabling ultra-small photonic integrated circuits induces a necessity to develop efficient optical couplers that connect silicon waveguides with optical fibers or offchip other optical components. Vertical couplers are promising for both wafer-level testing and the integration of optical components on the chip surface, but so far grating couplers have been the only solution virtually. Very recently, we have developed a few-micron-scale vertically-curved silicon waveguide coupler “elephant coupler” formed by ion implantation method. This paper demonstrates that the coupler has high coupling efficiency, weak wavelength dependence, and weak incident angle dependence of wavelength window, which are not obtained by the grating couplers. The paper also demonstrates that the thermal annealing treatment can reduce optical propagation loss induced by the ion implantation.

Transmission Characteristics of Hydrogenated Microcrystalline Silicon Wire Waveguide at a Wavelength of 1.55 µm

We propose hydrogenated microcrystalline silicon (µc-Si:H) as an alternative to hydrogenated amorphous silicon for optical waveguide applications with higher stability against photoinduced degradation. A 180-nm-thick µc-Si:H film containing silicon nanocrystallites with a grain size of ~20 nm was deposited by plasma-enhanced chemical vapor deposition at 250 °C. The transmission losses of µc-Si:H wire waveguides with widths of 420, 470, and 570 nm were 10.1, 6.7, and 6.5 dB/cm for the transverse electric mode, and 9.6, 5.2, and 4.7 dB/cm for the transverse magnetic mode, respectively. From the measured losses, we estimated material-induced scattering loss of less than ~7 dB/cm.

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.

Elephant coupler: Vertically curved Si waveguide with wide and flat wavelength window insensitive to coupling angle

Few-micron-scale vertical silicon coupler that has high coupling efficiency, weak wavelength dependence, and weak incident angle dependence of wavelength window was achieved with vertically curved silicon waveguide “elephant coupler” formed by ion implantation method compatible with LSI manufacturing technology.

Supercontinuum generation for ultrahigh-resolution optical coherence tomography at wavelength of 0.8 µm using carbon nanotube fiber laser and similariton amplifier

We demonstrated supercontinuum (SC) generation for ultrahigh-resolution optical coherence tomography (UHR-OCT) in the 0.8 µm wavelength region using an ultrashort-pulse fiber laser system. An Er-doped ultrashort-pulse fiber laser with single-wall carbon nanotubes was developed as the seed pulse source. A 46 fs, highest quality, pedestal-free, clean, ultrashort pulse was generated with a similariton amplifier. Then, a 60 fs ideal ultrashort pulse was generated at a wavelength of 0.8 µm with a second-harmonic generation (SHG) crystal, and a Gaussian-like SC was generated in a photonic crystal fiber. UHR-OCT was demonstrated using the generated SC, and precise images of a biological sample were observed.