Modal loss mechanism of micro-structured VCSELs studied using full vector FDTD method.

Modal properties of vertical cavity surface-emitting lasers (VCSELs) with holey structures are studied using a finite difference time domain (FDTD) method. We investigate loss behavior with respect to the variation of structural parameters, and explain the loss mechanism of VCSELs. We also propose an effective method to estimate the modal loss based on mode profiles obtained using FDTD simulation. Our results could provide an important guideline for optimization of the microstructures of high-power single-mode VCSELs.

Design of waveguide grating with ultrafast tunable index contrast.

A long-period waveguide grating (LPWG) with a tunable index contrast is proposed. The design features a simple configuration that consists of a two-mode waveguide formed on periodically poled lithium niobate with an angle with respect to its domain wall and a traveling-wave electrode. In the design, the electrical traveling wave introduces a periodic change in the refractive index of waveguide, which functions as a long-period waveguide grating that couples between symmetric and anti-symmetric core modes. The index contrast of grating can be controlled by the traveling-wave intensity. For application to ultrafast device, structural parameters satisfying velocity and impedance matching conditions are numerically calculated.

Ultralow bending loss fibers with higher-order mode strippers.

We propose and demonstrate bend-insensitive fibers equipped with higher-order mode strippers. The mode stripper is realized by filling a section of air holes with epoxy to attenuate any higher-order modes that are excited at fiber junctions and are confined by the air holes surrounding the core. We found that the higher-order modes are well suppressed with 5 cm-long epoxy columns. An ultralow bending loss of 0.025 dB/turn at a bend diameter of 10 mm, together with single-modeness, is experimentally demonstrated in a bend-insensitive fiber with six air holes 16 µm in diameter.

Sidelobe suppression in all-fiber acousto-optic tunable filter using torsional acoustic wave.

We propose two techniques to suppress intrinsic sidelobe spectra in all-fiber acousto-optic tunable filter using torsional acoustic wave. The techniques are based on either double-pass filter configuration or axial tailoring of mode coupling strength along an acousto-optic interaction region in a highly birefringent optical fiber. The sidelobe peak in the filter spectrum is experimentally suppressed from -8.3 dB to -16.4 dB by employing double-pass configuration. Axial modulation of acousto-optic coupling strength is proposed using axial variation of the fiber diameter, and the simulation results show that the maximum side peak of -9.3 dB can be reduced to -22.2dB. We also discuss the possibility of further spectral shaping of the filter based on the axial tailoring of acousto-optic coupling strength.

Analyses of unintentional intensity modulation in all-fiber acousto-optic tunable filters.

We theoretically and experimentally analyze unintentional intensity modulation phenomena in two types of all-fiber acousto-optic tunable filters utilizing flexural and torsional acoustic waves. Output filter signal at a resonant wavelength shows time-varying oscillations with even- and odd-order harmonics of applied acoustic frequency, which are explained by two factors of static mode coupling and acoustic back reflection. The magnitudes of static coupling and acoustic reflection in our devices are estimated from the measured first and second harmonic modulation powers.

Experimental excitation and characterization of cladding modes in photonic crystal fiber.

We extend the previous theoretical study on the effect of outer silica cladding to the analysis on real field profiles in a practical PCF. Clear field profiles for the first higher-order cladding modes are presented and discussed. The observed mode fields are reproduced by numerical calculations, and it turns out that they correspond to LP(16) mode groups. Optical properties of the observed modes such as lobe direction and polarization are also investigated. The results of this study will be useful in the design of the PCF-based optical devices utilizing cladding mode coupling.

Tunable gratings in a hollow-core photonic bandgap fiber based on acousto-optic interaction.

We demonstrate a coherent intermodal coupling in a hollow-core photonic bandgap fibers (PBGF) using an acoustic grating. A theoretical study on the properties of acoustic and optical wave propagating the PBGF is performed indicating that the applied frequency higher than 10 MHz is required to obtain the acousto-optic interaction at the wavelength around 1550 nm. Wavelength-tunable coupling from fundamental mode to higher-order core modes is achieved with more than 90% coupling efficiency at the applied frequency of 11.54 MHz. The low dispersive properties and the mode filed distributions of the higher-order guided modes of the air-core PBGF in the experiment show reasonable agreement with the simulation results.

Polarization-coupling all-fiber acousto-optic tunable filter insensitive to fiber bend and physical contact.

We show that an all-fiber acousto-optic tunable filter based on polarization mode coupling using torsional acoustic wave is immune to the fiber bend and physical contact in the acousto-optic interaction region. We also propose and demonstrate a novel strain-free and size-reduced tunable filter with a 4-m-long fiber acousto-optic interaction region looped into a 5- cm-diameter coil.

Axial strain dependence of all-fiber acousto-optic tunable filters.

We report the axial strain dependence of two types of all-fiber acousto-optic tunable filters based on flexural and torsional acoustic waves. Experimental observation of the resonant wavelength shift under applied axial strain could be explained by theoretical consideration of the combination of acoustic and optical effects. We discuss the possibility of suppressing the strain effect in the filters, or conversely, the possibility of using the strain dependence for wavelength tuning or strain sensors.

Combined effects of optical and acoustic birefringence on acousto-optic mode coupling in photonic crystal fiber.

We investigate in detail the origin of multiple resonance peaks experimentally observed in an all-fiber acousto-optic tunable filter built with a photonic crystal fiber having slightly deformed air-hole structure and non-circular outer cladding. A model for the acousto-optic mode coupling in the PCF is formulated taking into account both the acoustic and the optical birefringence. All experimental results are in good agreement with numerical calculations.

Plasmon-suppressed vertically-standing nanometal structures.

The authors report plasmon-suppressed vertically-standing nanometal-stripe-array structures fabricated by Ar ion sputtering after electron-beam lithography and Ag deposition. When the width of the Ag stripe is comparable to the skin depth of a metal (~ 20 nm), the particle plasmon resonance is strongly suppressed for electric fields oscillating perpendicular to the length of the stripe. This suppression of the particle plasmon excitation is attributed to the limited movement of free electrons localized near the bottom of Ag stripe. This plasmon-suppressed vertically-standing nanometal structures could be used for broad band polarizers.

Bending loss analyses of photonic crystal fibers based on the finite-difference time-domain method.

This is a report on an effective simulation method for the bending loss analyses of photonic crystal fibers. This method is based on the two-dimensional finite-difference time-domain algorithm and a conformal transformation of the refractive index profile. We observed the temporal dynamics of light waves in a bent fiber in a simulation and obtained the bending loss as a function of bend radius and optical wavelength for the commercial photonic crystal fibers. The accuracy of this method was verified by good agreement between the simulation and experimental data.

Analyses of cladding modes in photonic crystal fiber.

Characteristics of cladding modes in a photonic crystal fiber (PCF) are numerically analyzed using the plane wave expansion method. The presence of the outer silica ring in the PCF tends to push the optical fields of the cladding modes toward the rim of the PCF, which creates 'ring modes' whose fields are tightly confined in the outer ring. The dispersion of the cladding modes are determined mainly by the dispersive property of the holey cladding structure. The optical field patterns of the cladding modes and the beatlengths between the fundamental mode and the cladding modes are also investigated.

Reconfigurable microfiber-coupled photonic crystal resonator.

We propose and demonstrate reconfigurable microfiber-coupled photonic crystal (PhC) lasers. In this generic configuration, the position of a PhC resonator can be defined (and redefined) repeatedly by simply relocating a curved microfiber along the linear PhC waveguide. In the proximity of the PhC waveguide in contact with the microfiber, the cutoff frequency (effective index) of the PhC waveguide becomes smaller (larger) than that of a bare PhC waveguide. Accordingly, when a curved microfiber is in contact with the PhC waveguide, a linear PhC resonator having Gaussian-shaped potential well is formed. Experimentally we confirm the formation of the reconfigurable resonator by observing laser operation slightly below three available band edges.

Nonlinear dispersive three-dimensional finite-difference time-domain analysis for photonic-crystal lasers.

The three-dimensional finite-difference time-domain method that can handle dispersive and dynamic nonlinear-gain media is proposed and realized. The effect of carrier diffusion is included through the laser rate equations. Through this three-dimensional nonlinear gain FDTD method, rich laser-dynamics behaviors, such as the lasing threshold, the relaxation oscillation, and the spatial hole burning, are directly observed from a hexapole mode.

Transverse mode control by etch-depth tuning in 1120-nm GaInAs/GaAs photonic crystal vertical-cavity surface-emitting lasers.

Robust and tolerant single-transverse-mode photonic crystal GaInAs vertical-cavity surface-emitting lasers are fabricated and investigated. Triangular lattice patterns of rectangular air holes of various etch-depths are introduced in the top mirror. The stable single-transversemode operation is observed with a large margin of allowance in the etch depth (t = 2.5 +/- 0.6 microm). This stable mode selection mechanism is explained by the mode competition between the two lowest photonic crystal guided modes that are influenced by both the index guiding effect and the etchdepth dependent modal losses.

High birefringence in elliptical hollow optical fiber.

We propose a novel design of highly birefringent optical fiber composed of a central elliptical air hole, a circumferential elliptical ring core, and a circular cladding. The proposed waveguide structure is predicted to produce a linear birefringence higher by an order of magnitude than the solid elliptical core fiber. The large index contrast between the central air and germanosilica elliptical ring core is mainly attributed to the high birefringence and its characteristics are theoretically analyzed in terms of its waveguide parameters.

Square-lattice photonic-crystal vertical-cavity surface-emitting lasers.

The single mode square lattice photonic-crystal vertical-cavity surface-emitting lasers (PC-VCSELs) are proposed and demonstrated. Square-lattice photonic-crystal patterns of various lattice constants are introduced on top mirrors of VCSELs having two different current apertures. The maximum single mode output power of about 1 mW is obtained from the device with lattice constant of 5.0 m and current aperture of 16 m. The PC-VCSEL operates in a single transverse mode in an entire operating current range with a side-mode suppression ratio of over 20 dB. The asymmetric introduction of smaller air holes improves the polarization selectivity.

Birefringence induced by irregular structure in photonic crystal fiber.

The unintentional birefringence induced by the irregular structure in photonic crystal fibers is analyzed numerically using the plane wave expansion method. The statistical correlations between the birefringence and the various irregularities are obtained. The birefringence is found to be largely dependent on the fiber design parameters as well as the degree of the irregularity. And the large pitch and the small air hole make the fiber less sensitive to the structural irregularity, which is successfully explained by the simple perturbation theory. The accuracy of our analyses is confirmed by the detailed investigation of computational errors. This study provides the essential information for the characterization and the design of low birefringence photonic crystal fibers.