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We present a novel and robust design for a photonic crystal fiber with flattened dispersion, a highly nonlinear coefficient, and low confinement loss for its dual concentric core structure. The proposed fiber has a modest number of design parameters. Analysis results show that the proposed eight-ring photonic crystal fiber is obtained with a nonlinear coefficient greater than 33 W-1 km-1 and a near-zero dispersion slope of -7.828 x [10.sup.-4] ps/[nm.sup.2]/km at 1550 nm. Ultra flat dispersion with a value between -1.380 and +0.9860 ps/nm/km and a superlow-order confinement loss of [10.sup.-4] dB/km are simultaneously obtained ranging from 1400 to 1625 nm. For practical fabrication, the influence of random imperfections of airhole diameters on dispersion and nonlinearity is discussed to verify the robustness of our design. [c] 2010 Optical Society of America OCIS codes: 060.0060, 060.2280.

A new type of photodetector with cascaded waveguide grating filters as a bottom reflection mirror is proposed. A greatly improved spectral response is shown to follow by the integration of a waveguide grating into classical thin-film homogeneous layers. Calculation results for a single grating, a cascaded-double grating, and a cascaded-triple grating structure are demonstrated. An increasing rectangular spectral response is obtained by cascading two or three grating filters. Compared with a traditional photodetector with distributed Bragg reflectors, this new type of photodetector with the same materials requires significantly fewer layers while maintaining narrow flattop response, high peak efficiency, and low sideband reflectance. OCIS codes: 230.5160, 050.2770, 230.4170.

We demonstrate a wavelength-selective photodetector that combines a Fabry--Perot filtering cavity (FPC) with a taper absorption cavity (TAC). The taper cavity shows a nonresonant effect but exhibits an absorption enhancement effect, so that high speed, high quantum efficiency, wide tuning range, and an ultranarrow spectral linewidth can be achieved simultaneously. Device performance was theoretically investigated by including key factors such as taper angle, finite-size diffracting-beam input, and lateral walk-off in the taper cavity. The device was fabricated by bonding a GaAs-based FPC, which can be tuned via thermal-optic effect, with an InP-based TAC. An integrated device with a spectral linewidth of 0.6 nm (FWHM), a wavelength tuning range of 10.2 nm (1518.0-1528.2 nm), a 3 dB bandwidth of 12 GHz, and a quantum efficiency of ~70% was demonstrated, and the absorption layer thickness is only 0.3 [micro]m.

A new, to our knowledge, scheme of all-optical switching is put forward by use of an all-fiber Mach-Zehnder interferometer incorporating ytterbium-doped fiber Bragg gratings. The device utilizes the characteristics of the sharp change of group velocity in transmission with the detuning parameter, [delta] = 2[pi][barn]1/[lambda] - 1/[[lambda].sub.B]). The switching is achieved by changing the Bragg wavelength of the ytterbium-doped arm of the interferometer. A very small shift of the Bragg wavelength can lead to a [pi] phase shift between the two arms, so the power needed to realize complete switching is much lower than that of other schemes. In addition, the device can compensate the dispersion of an optical pulse through the positive group velocity dispersion in transmission provided by the gratings. OCIS codes: 060.1810, 060.4370, 230.1150.

We propose a novel wavelength-selective photodetector with three subcavities, i.e., a filtering cavity, a spacer cavity, and an absorption cavity, for obtaining a narrow spectral response linewidth and a high quantum efficiency simultaneously. A theoretical prediction has been made that a less than 1-nm linewidth and a quantum efficiency as high as 90% are possible. We discuss the effects of the key factors on the performance of this type of photodetector that has been designed and fabricated. A spectral response linewidth of approximately 1.4 nm (FWHM) and an external quantum efficiency higher than 50% have been achieved experimentally. Such devices are promising for wavelength-division multiplexing applications. [C] 2000 Optical Society of America OCIS codes: 230.5160, 060.4080.

We numerically investigate two Fano resonances with high Q-factors based on a permittivity-asymmetric metastructure composed of two pea-shaped cylinders. By employing different materials to break the permittivity-asymmetry, the quasi-bound state of the continuum spectrum (BIC) resonance at 982.87 nm is excited, showing the Q-factor as high as 8183.7. The electromagnetic fields and vectors are analyzed by using the finite-difference time-domain (FDTD) method, and the resonance modes are identified as magnetic dipole (MD) responses and MDs by multipole decomposition in Cartesian coordinates, displaying that the light is confined within a pea-shaped cylinder to achieve localized field enhancement. In addition, the sensing performances of the metastructure are evaluated, and an optical refractive index sensor can be obtained with the sensitivity of 152 nm/RIU and maximum figure of merit (FOM) of 832.6. This proposed structure offers a new, to the best of our knowledge, way to achieve Fano resonant excitation on all-dielectric metastructures and can be used in nonlinear optics, biosensing, optical switches, and lasers.