Patent Description:
Plasmonic light-waves are electromagnetic waves propagating on metal surfaces coupled with surface electron oscillations. The coupling to electron oscillations enables extreme modifications to the propagating light, but this comes at a price of enhanced attenuation. However, careful design of complex metal-optic structures is a key-enabler for many ground breaking technologies. Merging plasmonics and optical fiber technologies has been previously explored, but primarily for sensing applications.

<NPL>, relates to a coupling system for side-coupling pump light into double-clad fiber. The light of a collimated high-power diode is focused through the fiber on a binary gold reflection grating placed behind the fiber at a flat outer surface portion of the fiber's D-shaped pump core.

In one aspect, the invention relates to an optical fiber waveguide system including:.

In another aspect, the invention relates to a method for forming an optical fiber waveguide, including:.

Further embodiments are set out in the dependent claims.

Referring to <FIG>, a system <NUM> that may be used in one embodiment of the present disclosure is shown. In this example the system <NUM> comprises a length of D-shaped optical fiber <NUM> having a cladding 12a with a flat surface portion 12b, and a core-guide 12c encased within the cladding 12a. The core-guide 12c is a glass fiber that enables a core-guide mode when an optical input signal <NUM> is input into the core-guide 12c. A relatively thin metal layer <NUM> is secured or formed thereon on the flat surface portion 12b of the cladding 12a. The thin metal layer <NUM> forms a plasmonic device (hereinafter "plasmonic device <NUM>"), which supports a plasmonic mode when the optical signal <NUM> is travelling through the core-guide 12c.

In the D-shaped optical fiber <NUM> such shown in <FIG>, the core-to-fiber surface distance is reduced in a specific region, as indicated by dimension D1 in the side cross sectional view of <FIG>, enabling the enhanced coupling to the plasmonic device <NUM>. The thin metal layer which forms the plasmonic device <NUM> may be, for example, constructed from, for example and without limitation, gold (Au), silver (Ag) or copper (Cu), and be on the order of between about tens of nanometers to hundreds of nanometers in thickness, or possibly even thicker. In this example the plasmonic device <NUM> forms a lattice-like structure having a plurality of spaced apart strips 14a which form grooves 14b therebetween. The grooves 14b in this example are formed normal to a longitudinal axis of the core-guide, indicated by line "A" in <FIG>. The strips 14a in this example may be spaced apart by a distance of up to hundreds of micrometers, or possibly even greater, and each strip has a width that will be selected based on the specific application, in one example up to hundreds of micrometers in width, or possibly even greater. In some applications, the thin metal layer forming the plasmonic device <NUM> may have no grooves at all. The plasmonic device <NUM> may be created/applied through any suitable, well-known process, for example, by masked evaporation, sputtering, lithography or even controlled plating. However, it will be appreciated that the precise construction of the plasmonic device <NUM> may be tailored to a specific application to best achieve specific desired performance or results. The plasmonic device <NUM> supports a plasmonic local mode (or alternatively a lossy waveguide made with absorbing medium). By modifying the structural properties of the metal layer forming the plasmonic device <NUM>, in connection with the cross sectional D-shape of the optical fiber <NUM> core structure, and the core spacing (i.e., D1 in <FIG>), the coupling of optical energy into the plasmonic device <NUM> from the core-guide 12c can be precisely tuned as the input optical signal <NUM> travels through the core-guide 12c. In this manner the energy level of the optical signal <NUM> travelling through the core-guide 12c can be controlled (and will usually depend on the optical frequency), such as, for example, to create a controlled spectral transmission. In one example the controlled spectral transmission may be used to controllably attenuate the optical signal travelling through the core-guide 12c.

One unclaimed application of the system <NUM> may be as a frequency notch filter. The propagation in an optical fiber could be simplistically viewed in a ray optics description as a ray zig-zag bouncing inside the fiber due to total internal refractions. At each frequency the ray propagation angle is different (representative of the waveguide modal wavenumber, k-vector). The curve that details the k-vector of the propagating mode as a function of the frequency is the dispersion curve characterizing the waveguide. The metal layer forming the plasmonic device <NUM> (i.e., being a plasmonic waveguide) has a different dispersion curve than that of the fiber core (i.e., the core-guide mode 12c). When the two waveguides (i.e., plasmonic device <NUM> and core-guide 12c) are put close together, the coupling between their modes is created, and at certain frequencies the angle of propagation of the two matches better, which results in enhanced coupling (i.e., more optical energy transferred to the plasmonic device <NUM>, as indicated by waveform 16a in <FIG>). At the frequency of enhanced coupling to the plasmonic mode (i.e., the plasmonic device <NUM>), the attenuation of the propagating mode will therefore be enhanced, resulting in a notch filter. Since the notch location depends on structural properties, this sets the basis for a pulse spectrum reshaping scheme by cascading two or more filters. By tuning the difference between the wave-vector resonance of the plasmonic waveguide formed by the plasmonic device <NUM> and that of the fiber waveguide formed by the core-guide mode 12c, and setting the nonlinearity in the coupling of the two such that there is less coupling (thus less loss) when the intensity increases, fast pedestal suppression function (at the speed of the nonlinear effect) can be achieved. In this regard it will be appreciated that the "pedestal" referred to represents the lower intensity parts of the pulse away from its central peak. The fast pedestal suppression is shown 3A. <FIG> also shows that at high intensity, indicated by point <NUM>, for the coupling of <FIG>, fast pedestal suppression is achieved. As shown in <FIG>, to achieve pedestal suppression function, the coupling at low intensity is set to be high, resulting in a low transmission from the core-guide mode 12c to the plasmonic device <NUM>. As the intensity increases, the coupling, "α", is designed to be reduced by the change in the nonlinear index between the two waveguides - resulting in increased transmission, as shown in <FIG> by the curve <NUM> and the difference between the low intensity point <NUM> and the high intensity point <NUM>. The system <NUM> is thus implementing a pedestal suppression function, and the details of the function are being set by the nonlinear optical coupling scheme between the two waveguides formed by the core-guide 12c and the plasmonic device <NUM>. Such a "passive optical valve" component can improve the contrast between the "ON" state and the noise level. The nonlinearity would result from the deposited nonlinear material layer making up the plasmonic device <NUM> or from the metal nonlinearity itself. This concept of designing a nonlinear optical transmission function to an optical fiber segment could be further explored to obtain other complex transmission shapes, for example by depositing plasmonic cavities onto the flat surface of the D-shaped fiber instead of a uniform thickness metal layer, which adds dispersion curve resonances. An additional example is a saturable absorber function resulting in a flat top shaped pulse.

The above teachings for designing the nonlinear transmission could be further extended to affecting the accumulated phase. Similar to how spatially modifying the index (e.g., lens), and thus spatially the accumulated phase, could reshape the light spatially, temporal reshaping of the phase could reshape the pulse in time. The coupling between the two waveguides formed by the core-guide 12c and the plasmonic device <NUM> modifies the intensity profile and, due to the optical nonlinearity, results in a modified refractive index. The net modal index (related to the phase accumulation of the propagating mode) could be estimated as the overlap integral of the modified refractive index and the field distribution shape. Therefore to obtain a negative b-integral, more energy should be guided at lower refractive index parts of the waveguide (clad) at higher intensities. This is a non-typical material response that could be designed into the system <NUM> using the structural approach developed above. In this scheme, the coupling and the nonlinearity in the plasmonic device <NUM> (i.e., effectively the plasmonic 'cladding') may be designed such that at high intensity, more power is wave-guided at lower net index, which results in a negative Kerr effect and allows for a b-integral compensator for a laser system's front-end. The main existing solutions for front-end pulse shaping with sufficiently fast response presently suffer from being based on bulk components and limited by properties of a given set of available materials. For example, a b-integral compensator could be implemented using KTP crystal near the phase matching angle (through a cascaded khi-<NUM> nonlinearity). The system <NUM>, modified as described above, would have the advantage of being an in-fiber integrated device, and have the wavelength configurability based on the design.

Still another function that could be tailored using the system <NUM>, which forms plasmonic fibers, is unique fiber dispersion. As shown in <FIG>, this optional configuration could be achieved by forming resonant cavities <NUM> deposited onto the flat surface 12b of the D-shaped fiber <NUM>, and could be even further enhanced by strong coupling of plasmonic structures and cavities with excitons in dye molecules and quantum dots. In one example, the dye molecules are lossy at a given narrow band wavelength, and could replace the metal material as the lossy media, or alternatively could be added to it, to thus reshape the response.

An efficient coupling scheme between the plasmonic mode and free space far-field would result in a side emitting optical fiber <NUM>, as shown in <FIG>. The optical fiber <NUM> includes a cladding 100a having a core-guide mode 100b and an outer surface portion 100c. A carefully designed sub-wavelength metal array <NUM>, having grooves 102a, is known to enable a maximum constructive interference in the far-field, that is, in the direction normal to the optical fiber <NUM> (i.e., indicated by arrows <NUM>), which has been demonstrated for laser diodes facets. Optionally, the angle may be tuned as needed to meet a specific application; in other words the angle does not have to be normal to the optical fiber <NUM>. The side emitting plasmonic section formed by the metal array <NUM> could be repeated along the length of the optical fiber <NUM> to create an array of emitters for emitting optical energy. Furthermore, each emitter could exhibit a different phase in a controllable way, resulting in a phased array controlled beam. <FIG> shows an optical fiber <NUM> coiled with a plurality of aligned, side emitting segments metal segments <NUM> for emitting optical energy, such as shown in <FIG>, which form plasmonic devices. Coiling of the optical fiber <NUM> creates a two dimensional array of coherently added lasers, with total emitter area much larger than the area of the fiber core-guide 12c.

<FIG> shows an embodiment of the present invention in which the optical fiber of <FIG> (shown in simplified side view in <FIG>) is used in connection with a broad area diode and a mirror positioned normal to the axis of propagation of an input wave through the core-guide mode, to enable optical energy to be coupled into the common-mode guide of the optical fiber. Another potential modification may be the addition of a mirror <NUM>, as shown in <FIG>, near the side emitter of the metal array <NUM>. The lens-like shape of the D-shaped fiber cross-section of the optical fiber <NUM> may be used to couple the normal-to-fiber radiation <NUM>, from a pump source <NUM>, for example a broad area diode laser, which is controlled by an electronic controller <NUM>, into a fiber propagating mode (i.e., into
a core-guide mode). Broad-area laser diodes have been recently proposed to set attractive pumping sources for fiber lasers due to their relatively high power (~10W). The fiber coupled broad area diode configuration shown in <FIG> is expected to be of high interest as a pumping scheme since it allows coupling of the pump light from one or more optical pump sources into the longitudinally propagating light <NUM> in the core-guide of the optical fiber <NUM>. This configuration furthermore enables combining the power of several pump sources <NUM> providing optical pump energy 108a at spaced apart locations along the length of an optical fiber <NUM>, as shown in the embodiment of <FIG>. Surface emitting fiber lasers have been suggested in previous work but have typically involved using a complex structure of hollow core fiber filled with a gain medium and radial dielectric multi-layer side walls. The system shown in <FIG> is constructed with significantly fewer component elements and allows for spatial control of the emitted beam.

The various embodiments described herein enable control over optical fiber properties by patterning a plasmonic structure (or plasmonic structures) directly onto optical fibers. Using the strong light-matter interaction of plasmonics allows for the construction of side emitting and pumping of fiber lasers.

It is also to be understood that additional steps may be employed.

Claim 1:
An optical fiber (<NUM>, <NUM>) waveguide system (<NUM>) including:
a first waveguide having a core-guide (12c) and a cladding material (12a, 100a) fully surrounding and fully encasing the core-guide (12c), the cladding material (12a, 100a) having an exposed, flat outer surface portion (12b, 100c) extending along a length of the first waveguide, and the core-guide (12c) enabling a core-guide mode (100b) for an optical signal (<NUM>) travelling through the core-guide (12c);
a second waveguide forming a plasmonic device (<NUM>), and acting as lossy waveguide supported directly on the exposed, flat outer surface portion (12b, 100c) of the cladding material (12a, 100a) of the first waveguide;
the construction of the second waveguide being such as to implement a plasmonic mode waveguide to achieve a desired coupling between the core-guide mode (100b) and the lossy waveguide, using plasmonic coupling, to control an energy level of the optical signal (<NUM>) travelling through the core-guide (12c); and
an optical pump source (<NUM>) configured to inject optical energy (108a) into the core-guide (12c) of the first waveguide from an angle non-parallel to the core-guide (12c), using the plasmonic coupling between the core-guide mode (100b) and the lossy waveguide as the plasmonic device (<NUM>).