Abstract:
A microstructured optical waveguide is formed to include a periodic sequence of “plugs” of optically active material within the inner cladding air tunnels. The plugs are utilized as a grating structure for generating resonant and periodic structures. The waveguide (in one embodiment, an optical fiber) is tunable by changing the spacing of the plugs (e.g., heating the structure, changing the pressure within the structure, etc.), or by modifying the initial spacing of the plugs during the formation of the microstructured optical waveguide (i.e., by modifying the “dipping frequency” of the waveguide into a reservoir of optically active material). In general, any number of different types of optically active material may be used to form the plugs, where two or more different materials may be used in the same structure, and introduced in an alternating fashion.

Description:
FIELD OF THE INVENTION 
   The present invention relates to microstructured optical waveguide elements and, more particularly, to the inclusion of periodic “plugs” of optically active material in the cladding structure of an optical waveguide, such as a fiber, to provide tunable periodic and resonant structures. 
   BACKGROUND OF THE INVENTION 
   Optical devices that modify the properties of optical signals include devices such as tunable filters, attenuators, switches, polarization rotators and the like. Such devices use various means to periodically vary the refractive index of one or more regions of the device to change the phase/amplitude of a signal propagating through the device. Conventional devices of this kind include structures such as Bragg gratings and/or long period gratings to introduce the desired periodicity. Typically, conventional gratings are periodic perturbations in the photosensitive refractive index of the core of the optical fiber or waveguide. These gratings are created by UV exposure and are thus permanent in nature. Tuning of the applicable wavelength range may be achieved, for example, by introducing physical strain variations in the grating, temperature variations, magnetic field variations, or other environmental methods of inducing physical changes in the grating. 
   In a different scheme, it is desirable to have available an all-fiber device in which a periodic structure of a certain desired material (fluid/polymer/microspheres) is introduced into the fiber without the need for hydrogen loading or a photosensitive core. The introduction of the optically active material thus eliminates the need to use UV laser sources to write the grating structure, which is considered to save a significant amount of time and effort. Moreover, the choice of the active material&#39;s refractive index gives an additional degree of freedom in determining the difference in the refractive index perturbation, which is usually desired to be as large as possible so that coupling between different modes or different polarizations of one mode can be achieved in a relatively short coupling length. 
   SUMMARY OF THE INVENTION 
   The needs remaining in the prior art are addressed by the present invention, which relates to microstructured optical waveguide elements and, more particularly, to the inclusion of periodic “plugs” of optically active material in the cladding structure of an optical waveguide to provide tunable periodic and resonant structures. 
   In accordance with the present invention, selected air channels within a microstructured optical waveguide, such as an optical fiber, are periodically filled with optically active material so as to manipulate the evanescent fields (propagation constants, polarization, etc) of light propagating along the guide/fiber. In a particular embodiment, the air channels are introduced in the cladding region so as to surround the core region and extend in the axial direction along the length of an optical fiber. Optically active material is then infused in one or more of the cladding layer air channels to change the optical properties of a propagating optical signal, where the active material is infused using a “periodic” process so as to create separate, periodic “plugs” of optically active material disposed along the length of the air channel. The active material is infused so as to exhibit a period A, similar to well-known grating structures. The periodicity may be used in accordance with the present invention to provide coupling between the different polarizations of the propagating mode and create a polarization rotator. 
   Tunability of the transmission properties within the microstructured optical fiber can be achieved by changing the periodicity of the optically active material, such as by heating the air in the channels on both ends of a tapered microstructure fiber section, so as to induce pressure on both sides of the periodic structure, which results in compressing the air between the plugs and changing the period of the microfluidic structure. As an alternative to an air/active material periodicity in the cladding, two different optically active materials may be infused periodically, with each material exhibiting different optical properties (e.g., one material with a dn/dT&gt;0, and another material with a dn/dT &lt;0). 
   In accordance with the operation of the present invention, the application of, e.g., temperature, light or an electric or magnetic field will vary the optical properties such as refractive index, loss, scattering, or birefringence of the active material, which in turn varies or affects the propagation properties of optical signals in the device. 
   Other and further aspects and embodiments of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the drawings, 
       FIG. 1  contains a diagram of an exemplary microstructured optical fiber including periodically infused active material in accordance with the present invention; 
       FIG. 2  illustrates an exemplary arrangement useful in forming the periodic microstructured optical fiber of the present invention; 
       FIG. 3  illustrates a tapered section of microstructured optical fiber including periodic optically active plugs, the taper used to provide coupling of the input optical signal into the cladding layer including the plugs; 
       FIG. 4  contains the transmission spectra for both a microstructured optical fiber including periodic plugs, as formed in accordance with the present invention, and a microstructured optical fiber including a continuous-filled cladding structure; 
       FIG. 5  contains the transmission spectra for a set of three microstructured optical fibers including periodic plugs of optically active material, each spectrum associated with a different “dipping frequency” (i.e., a different resultant periodicity of the plugs); 
       FIG. 6  contains the transmission spectra for an exemplary microstructured optical fiber including periodic plugs in accordance with the present invention, each spectrum associated with a different ambient temperature for the fiber; and 
       FIG. 7  contains a series of transmission spectra for an exemplary microstructured optical fiber including periodic plugs in accordance with the present invention, each associated with a different temperature for the tapered fiber section including the plugs. 
   

   DETAILED DESCRIPTION 
   The principle of coherent microfluids resonance can best be understood by reference to the microstructure optical fiber  10  of  FIG. 1 . As shown, fiber  10  comprises a core region  12  surrounded by an inner cladding layer  14  and an outer cladding layer  16 . As will be described in detail below, inner cladding layer  14  comprises a periodic disposition of an optically active material  18 , active material  18  being in this example disposed within a plurality of separate air channels  20  axially disposed so as to surround core region  12  and extend along the length of fiber  10 . An important factor in the tenability of the structure of the present invention is the periodic disposition of active material  18  within channels  20 , where separate “bubbles” or “plugs” of active material  18  (see photograph associated with  FIG. 1 ) are formed to exhibit a period of Λ. As will be discussed in detail below, the presence of periodically-spaced active material plugs  18  causes phase matching between the propagating fundamental mode and higher order modes. To achieve coupling between co-propagating waveguide modes, phase matching needs to satisfy the following relationship:
 
β fun −β high =2π/Λ,
 
where β fun  and β high  are the propagation constants of the fundamental and higher order modes, respectively, and Λ is the period of the active material sections  18 , as illustrated in  FIG. 1 . In microstructured optical fiber  10 , the evanescent field of the fundamental mode overlaps with channels  20  of inner cladding  14 , as shown in  FIG. 1 , and therefore passes through active material  18  which functions to provide coupling to the higher order mode. When the two modes are phase matched, optical power is exchanged between them. The amount of light transferred into the higher order mode at certain wavelengths is related to the coupling coefficient.
 
   The coupling coefficient depends on the index difference in the periodic structure and the overlap between the mode fields E fun  and E high  through the following:
 
κ=∫∫ωε 0 /2(Δ n ) 2   E   fun   {circle around (x)}E   high   dA 
 
As will be described below, the refractive index of one exemplary optically active material (trifluorotoluence) is 1.405, so that the index difference between the fluid and the air in each channel  20  gives rise to an index difference Δn=0.405. Although the index difference is much larger than the index changes obtained in photosensitive optical fiber gratings, the coupling coefficient κ is related to the overlap of the fundamental and higher order mode. The calculated coupling coefficient κ for a conventional fiber with similar dimensions as that inner cladding of the fiber as described below is on the order of approximately 1×10 −5 /μm, which is approximately the same order of magnitude as in a conventional long period grating. It is to be noted that while the exemplary embodiment discussed herein utilizes a microstructured optical fiber, the principles of the present invention are equally applicable for use with any microstructured optical waveguiding arrangement, such as a slab optical waveguide.
 
     FIG. 2  illustrates a particular apparatus that may be used to form a periodic microstructure optical fiber, such as fiber  10  of  FIG. 1 . As shown, apparatus  30  comprises a fluid reservoir  32  filled with optically active material  18 . A motor  34  is coupled to a clamping apparatus  36 , where clamping apparatus  36  is attached (as shown) to an outer housing surrounding fiber  10 . The motion of motor  34  is redirected through clamping apparatus  36  to provide periodic linear dipping (illustrates as ±y in  FIG. 2 ) of endface  22  of fiber  10  into fluid reservoir  32 . A vacuum pump  38  is attached to opposing endface  24  of fiber  10  such that as a vacuum is applied via endface  24 , fluid plugs  18  and air (alternating) are drawn into each channel  20  of fiber  10 . The period Λ of fluid plugs  18  is controlled, in this particular example, by adjusting the rotational frequency ω of motor  34 . The ratio of fluid to air can also be controlled by adjusting the displacement of fiber  10  into fluid  18 , as compared to the displacement of fiber in air during one cycle. A microstructured optical slab waveguide may be similarly clamped within such apparatus to form a periodic disposition of active material within the cladding layers of the waveguide structure. 
   In the particular embodiment of fiber  10  as described thus far, fiber  10  comprises a set of six approximately cylindrical channels (in this case, “tubes”)  20  within inner cladding layer  14 . In a fiber that comprises an 8 μm, germanium-doped core region  12 , the propagating light signal will not interact with active plugs  18  in cladding layer  14 . In order to achieve interaction between light propagating in the fiber and active plugs  18 , fiber  10  needs to be tapered and stretched, as shown in  FIG. 3 , to create a tapered region  40 . Within tapered region  40 , the mode field expands into cladding layer  14  and thus becomes affected by active plugs  18 . 
   As shown in  FIG. 3 , fiber  10  has been tapered in region  40  to a waist outer diameter of approximately 30 μm (inner diameter of approximately 8 μm) over a length of 7 cm. Periodic microfluidic plugs  18 , in this example trifluorotoluence, are spaced with a period Λ of approximately 460 μm. As discussed above, light propagating in tapered region  40  will spread out of core region  12  and interact with periodic plugs  18 . Coherent coupling is thus achieved between the fundamental mode (LP 01 ) and the higher order mode (LP 02 ) by virtue of the presence of periodic plugs  18 . The generated higher order mode then propagates through the adiabatic up-tapered section  42  and transforms undisturbed into the section  44  of inner cladding layer  14 . Although the higher order mode is guided in inner cladding  14 , it will be attenuated when it reaches a splice S with a section of conventional single mode fiber  50 . The resonant coupling to the higher order mode thus manifests itself in a resonant loss peak centered at a wavelength governed by the period and the propagation constants of the respective modes. In order to obtain coupling at a desired wavelength, the required period of the perturbation is given by the first above-described equation and is based on the knowledge of the effective indices of the respective core and higher order modes; the latter can be calculated using conventional beam propagation methods. The calculated difference between the effective indices of the LP 01  and LP 02  modes (Δn eff   01-02 )=0.0045, assuming that no fluid is present in the air gaps, and suggests a period of 435 μm for resonant coupling around 1.5 μm. It is to be noted that the effective indices are affected by the presence of plugs  18  in channels  20  by virtue of the interaction of the evanescent fields with the optically active material of plugs  18 . 
     FIG. 4  illustrates the transmission spectra for both a fiber with periodic microfluidic plugs (such as plugs  18 ), as well as for a continuous fluid-filled inner cladding region. Curve A illustrates the transmission over a wavelength range of 1530 nm to 1600 nm for a continuous filled fiber, showing very little difference in transmission as a function of wavelength. In contrast, curve B illustrates the transmission for a structure with a periodic plug structure, where in this case, a significant loss (filtering effect) is introduced at a wavelength of approximately 1565 nm. 
     FIG. 5  shows the experimentally measured transmission spectra associated with different dipping frequencies, as discussed above in association with  FIG. 2 . As mentioned above, the period Λ of the optically active material plugs in the fiber cladding channels is determined by the dipping frequency, that is, the number of times (n) the fiber is dipped into a reservoir of optically active material during a time interval (t), as well as the velocity (v) in which the microfluidic plugs travel along the tube. During each time interval t, the fluid fills a certain distance determined by the velocity, which may be (for example) 1 cm/sec, as the fluid is being infused. Thus, for a dipping frequency of 125 Hz, the period Λ of the fluid in the fiber will be approximately 80 μm. In a tapered fiber section (such as section  40  shown in  FIG. 3 ), the period will increase by a predetermined factor, which is the ratio of the tapered fiber diameter to that of the un-tapered section. In the arrangement of  FIG. 3 , the tapered diameter is one-fourth that of the non-tapered fiber, so that the period will increase to about 460 μm. The calculated period inferred from the above equation and that observed may be slightly different since the infusion of the fluid into the fiber is extremely sensitive to any change in the frequency of the motor, or the amount of dipping the fiber into the fluid. Further, the resonance position on the transmission spectrum is very susceptible to the difference in the effective indices. 
   In accordance with the practice of the present invention, the period Λ of plugs  18  can be “tuned” by simultaneously heating the air in channels  20  on both sides of tapered region  40 . The heated air applies pressure on both sides of the periodic microfluidic plugs  18 , causing the air gap between plugs  18  to shrink and thus decrease the separation between adjacent plugs  18  (decreasing the period Λ).  FIG. 6  illustrates the effect of temperature changes on a microstructured optical fiber including periodic plugs in accordance with the present invention. Curve A in  FIG. 6  shows the transmission spectrum for a fiber such as fiber  10  when the end portions are maintained at 25° C. In this case, the transmission spectrum contains a notch at a wavelength just above 1590 nm. By heating both ends of fiber  10  to 125° C., the center wavelength of the device will shift downward to a value of approximately 1583 nm, as a result of the decreased period of plugs  18  after heating. Therefore, by controlling the temperature applied to both ends, wavelength tuning can easily be achieved. 
     FIG. 7  illustrates a set of different transmission spectra measured after directly heating plugs  18  within tapered section  40  of an exemplary fiber  10 . In this case, the optically active material&#39;s refractive index exhibited a decrease with increasing temperature (dn/dT of approximately −10 −4 /° C.). Therefore, the coupling efficiency is reduced since the difference between the refractive indices of the fluid and air (Δn) becomes smaller. Curve A illustrates a sharp resonance at room temperature (25° C.), which becomes weaker at higher temperatures. It is also to be noted that the background loss decreases with temperature. This is to be expected since the average loss over the entire tapered section decreases. The resonance tends to shift toward higher wavelengths as the periodic plugs are heated, since the air between the plugs tends to expand and the increases the period of the grating structure formed by the plugs. 
   In essence, the present invention discloses periodically-spaced microfluidic plugs disposed in channels along the inner cladding layer of a microstructured optical waveguide, such as an optical fiber. Coherent resonance structures can therefore be formed within such a waveguide, where the resonance condition is controlled by adjusting the period of the active material plugs (in one example, by adjusting the “dipping frequency”) or heating the waveguide/fiber on either side of the tapered central region. Moreover, the resonance may be attenuated by heating a fluid whose refractive index varies as a function of temperature. Various and other modifications may be made to the microstructured optical fiber as discussed above, where such modifications are considered to fall within the spirit and scope of the present invention as defined by the claims hereinbelow.