Quasi-distributed and wavelength selective addressing of optical micro-resonators based on long period fiber gratings.

A novel all-in-fiber method for coupling light to high-Q silica whispering gallery mode (WGM) optical micro-resonators is presented, which is based on a pair of long period fiber gratings (LPGs) written in the same silica fiber, along with a thick fiber taper (15-18 µm in waist) in between the LPGs. The proposed coupling structure is robust and can be replicated many times along the same fiber simply cascading LPGs with different bands. Typical Q-factors of the order of 10(8) and total coupling efficiency up to 60% were measured collecting the resonances of microspheres or microbubbles at the fiber end. This approach uniquely allows quasi-distributed and wavelength selective addressing of different micro-resonators along the same fiber.

Long period grating-based fiber coupler to whispering gallery mode resonators.

We present a new method for coupling light to high-Q silica whispering gallery mode resonators (WGMs) that is based on long period fiber gratings (LPGs) written in silica fibers. An LPG allows selective excitation of high-order azimuthally symmetric cladding modes in a fiber. Coupling of these cladding modes to WGMs in silica resonators is possible when partial tapering of the fiber is also implemented in order to reduce the optical field size and increase its external evanescent portion. Importantly, the taper size is about one order of magnitude larger than that of a standard fiber taper coupler. The suggested approach is therefore much more robust and useful especially for practical applications. We demonstrate coupling to high-Q silica microspheres and microbubbles detecting the transmission dip at the fiber output when crossing a resonance. An additional feature of this approach is that by cascading LPGs with different periods, a wavelength selective addressing of different resonators along the same fiber is also possible.

Optical frequency conversion in silica-whispering-gallery-mode microspherical resonators.

High quality factor whispering-gallery-mode microresonators are ideally suited for nonlinear optical interactions. We analyze, experimentally and theoretically, a variety of χ((3)) nonlinear interactions in silica microspheres, consisting of third harmonic generation and Raman assisted third order sum-frequency generation in the visible. A tunable, room temperature, cw multicolor emission in silica microspherical whispering-gallery-mode microresonators has been achieved by controlling the cavity mode dispersion and exciting nonequatorial modes for efficient frequency conversion.

Mid-range structure of niobium-sodium-phosphate electro-optic glasses.

The mid-range structure of glasses of the glass-forming system xNb(2)O(5)-(60 - x)P(2)O(5)-40Na(2)O, with x varying from 0 to 40, has been investigated by Raman spectroscopy and electro-optical Kerr measurements. It was found that the mid-range inhomogeneities in the glasses under study vary with x and their compositions and structures dramatically differ from the gross glass composition. It is shown that inhomogeneities with the 8NaNbO(3) + (Nb(2)O(5)·P(2)O(5)) composition are responsible for the electro-optical sensitivity of the glasses. The electro-optical structural elements of the inhomogeneities of this type are assumed to be fragments of the quasicubic lattice of crystalline NaNbO(3) demonstrating a rather high electro-optic Kerr coefficient. The obtained results are discussed in terms of the concepts of constant stoichiometric groupings and crystal motifs.

High Q silica microbubble resonators fabricated by arc discharge.

Microbubble resonators combine the unique properties of whispering gallery mode resonators with the intrinsic capability of integrated microfluidics. Here an improved fabrication method of microbubble resonators is presented, based on the heating of a slightly pressurized capillary by a rotating arc discharge. Rotation of the electrodes, moved out of a fiber splicer, ensures a homogeneous distribution of the heat all over the capillary surface. The demonstrated microbubble resonators have Q factors up to 6×10(7) at 1550 nm. Microbubbles were filled with water and aqueous solutions of ethanol in order to test the refractive index sensing capabilities of such resonators, which also show a good temporal stability. The limit of detection of our microbubble resonator sensor is 10(-6) RIU.

Planar coupling to high-Q lithium niobate disk resonators.

We demonstrate optical coupling to high-Q lithium niobate disks from an integrated lithium niobate waveguide. The waveguides are made by proton exchange in X-cut lithium niobate substrate. The disks with diameter of 4.7 mm and thickness of 1 mm are made from commercial Z-cut lithium niobate wafers by polishing the edges into a spheroidal profile. Both resonance linewidth and cavity ringdown measurements were performed to calculate the Q factor of the resonator, which is in excess of 10(8). Planar coupling represents the most promising technique for practical applications of whispering gallery mode resonators.

High-Q polymer-coated microspheres for immunosensing applications.

Homogeneous polymeric thin layers have been used as functionalizing agents on silica microspherical resonators in view of the implementation of an immunosensor. We have characterized the microspheres functionalized with poly-L-lactic acid and Eudragit L100, as an alternative to the commonly used 3-Aminopropyltrimethoxysilane. It is shown that polymeric functionalization does not affect the high quality factor (Q greater than 10(7)) of the silica microspheres, and that the Q factor is about 3 x 10(5) after chemical activation and covalent binding of immunogammaglobulin (IgG). This functionalizing process of the microresonator constitutes a promising step towards the achievement of an ultra sensitive immunosensor.

Direct measurement of birefringence in ion-exchanged planar waveguides.

A direct measurement of the birefringence of a planar waveguide obtained by Na(+) -K(+) ion exchange was performed with a double Lloyd interferometer. The results are compared with those obtained by a roundrobin test involving the same sample. Birefringence of as much as Deltan = (2.0 +/- 0.2) x 10(-3) was measured.

Diffuse nonlinearity with a thermal gradient in waveguide prism coupling.

Prism coupling of radiation inside a linear film over a nonlinear substrate is studied with a numerical method, showing that the change of coupling as a function of the input power in the case of a quasi-continuous operation (thermal nonlinearity) is influenced by a thermal gradient generated in the linear film. A comparison with experimental data confirms these calculations.

Nonlinear coupling in a planar waveguide.

The thermal nonlinear response of a waveguide produced by sputtering a Corning 7059 glass film over a nonlinear substrate(Corning CS2-61) is studied with a pump-probe technique in a quasi-cw regime. The coupling efficiency is found to beafunction of pump power and chopping frequency. The coupling zone is scanned with a probe beam to examine the spatial dependence of the coupling efficiency.

Design of optical-waveguide homogeneous refracting lenses.

A class of multielement homogeneous refracting lenses for optical waveguides is presented, and their design procedure is described. These lenses appear to be particularly suitable for use in integrated optical processors, as they can be fabricated by fully planar photolithography and reliable diffusion processes either in glass or in lithium niobate; their optical characteristics can be very good. For example, the design of an f/3.3 waveguide optical system, constituted by five elements, with no field curvature and very well corrected up to a 20 degrees total field of view, is presented.

Reflection holographic filters for compacting optical processors.

Character recognition experiments, carried out with reflection matched filters, are described. The reflection filters are obtained by means of volume holograms, recorded in photographic emulsion and suitably processed. These filters allow a good discrimination among similar characters and exhibit large positioning tolerances. They turn out to be well comparable with transmission hologram filters. An advantage of reflection filters is that the filtering operation can be performed with one-lens processor, thus reducing by one-half the size and the cost of the processor.

Geodesic lenses for guided optical waves.

Two-dimensional lenses are essential elements in integrated optics. As an alternative to the techniques already suggested and tested to obtain such optical systems, a different method is reported that consists of extending to thin film optics the principle of configuration lenses already studied for application to microwave antennas. The working principle of such lenses is that the propagation occurs along a curved surface in a two-dimensional Riemann space. The rays are the geodesics of the surface. Two optical geodesic lenses having planar input and output have been constructed and tested. They can be easily inserted in planar film circuits. The advantage of these optical systems is that the substrate can be prepared in advance with the accuracy of glass optics. Then the dielectric film can be deposited with the same method used for planar components.