Abstract:
A method and apparatus for improving the sensing of a physical parameter using a distributed optical waveguide and scattering. The optical waveguides have improved scattering efficiency and/or improved light capturing capability provided by multi-cladding layers and a tightly confining core waveguide. The core can be highly doped with a material such as germanium to improve scattering. The cladding layers provide a multi-mode waveguide for capturing scattered light. Such optical waveguides are useful in systems that rely on Rayleigh, Raman and Brillouin scattering.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   Embodiments of the present invention relate to distributed optical waveguide sensors. More specifically, embodiments of the present invention relate to distributed optical waveguide sensors having optical waveguides with multiple cladding layers. 
   2. Description of the Related Art 
   Light propagating in a medium can undergo a variety of scattering events, both linear and non-linear. Three types of light scattering are Rayleigh, Raman and Brillouin. In Rayleigh scattering, incident light is elastically scattered at the same wavelength. In Raman scattering, incident light is scattered by the vibrations of molecules or optical phonons and undergoes relatively large frequency shifts. In Brillouin scattering, incident light is scattered by acoustic vibrations (phonons) and undergoes relatively small frequency shifts. 
   Rayleigh, Raman, and Brillouin scattering can be used in distributed optical waveguide sensors to measure a measurand such as temperature or stress over the length of an optical waveguide. Since optical waveguides can be over 30 kilometers long, distributed optical waveguide sensors are suitable for measuring physical parameters over large distances. Distributed optical waveguide sensors that use Rayleigh, Raman, or Brillouin scattering are typically based on either Optical Time-Domain Reflectometry (OTDR) or optical frequency-domain reflectometry (OFDR). In either case, high intensity laser light is propagated in the core of an optical waveguide. Light scattering occurs within the waveguide, part of which is captured in the backward propagating modes of the waveguide and can be detected by a receiver. By monitoring one or more variations in the captured light a physical parameter can be determined. 
   While useful, distributed optical waveguide sensors based on scattering have problems because scattering produces signals that are much weaker than the light that created them. In optical waveguides, the originating light, referred to as pump radiation, produces a relatively small amount of scattered light, only a portion of which is captured. Because the captured light is weak, a significant integration time is required to produce measurements with suitable resolution and accuracy. 
   Therefore, an optical waveguide with improved scattering efficiency would be useful. An optical waveguide that enables improved capture of scattered light would also be useful. 
   SUMMARY OF THE INVENTION 
   Embodiments of the present invention generally provide for distributed optical waveguide sensors having optical waveguides with improved scattering efficiency and/or improved light capture. 
   Embodiments of the present invention comprise an optical waveguide having multiple cladding layers. Some embodiments have predominantly single-mode cores. Some embodiments have cores that are doped to improve scattering, e.g., highly germanium doped cores. Some embodiments include a first cladding layer and a second cladding layer. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     So that the present invention can be understood in detail, a particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
       FIG. 1  is a schematic depiction of a distributed optical waveguide sensor system that is in accord with the principles of the present invention; 
       FIG. 2  schematically illustrates a section of an optical waveguide that is in accord with the principles of the present invention; 
       FIG. 3  illustrates the refractive indexes of a double-clad optical waveguide according to one embodiment of the present invention; and 
       FIG. 4  illustrates the refractive indexes of a double-clad optical waveguide according to another embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The present invention provides for distributed optical waveguide sensors having optical waveguides with improved scattering efficiency and/or with improved scattered light capture. A distributed optical waveguide that is in accord with the principles of the present invention has multiple cladding layers. In some embodiments a predominantly single-mode core, possibly highly germanium doped, provides improved scattering efficiency. The multiple cladding layers provide for a multiple mode optical waveguide for improved light capture. It should be understood that the principles of the present invention will boost signal levels for systems using either optical time domain reflectometry (OTDR) or optical frequency domain reflectometry (OFDR). 
     FIG. 1  schematically depicts a distributed optical waveguide sensor system  100  that is in accord with the principles of the present invention. As shown, the sensor system  100  includes a distributed optical waveguide  102 . That optical waveguide, which includes a core and multiple cladding layers, is discussed in more detail subsequently. The sensor system  100  includes a transmitter  104  and a receiver  106  that is suitable for use with optical time domain or optical frequency domain reflectometry. It is within the scope of the present invention that receiver  106  may comprise any number of individual components necessary to produce or enhance the performance of the invention as described herein. Such components include by way of example and not by limitation, a photo detector, a data analyzer, an analogue-to-digital converter, an amplifier, and other similar devices known by those skilled in the art to assist in the reception of light and its meaningful interpretation as set forth herein. Similarly, the transmitter  104  may comprise any number of individual components necessary to produce or enhance the performance of the invention as described herein. Such components include by way of example and not by limitation, a laser, a modulator, a controller, and other similar devices known by those skilled in the art to assist in the generation and transmission of light energy as set forth herein. In addition, the transmitter  104  and receiver  106  may be in communication (optically or electrically) as necessary for their operation. 
     FIG. 2  schematically illustrates a section of the optical waveguide  102 . It should be understood that the optical waveguide  102  can be very long, with lengths of 1–30 kilometers being fairly common. As shown, the optical waveguide  102  is comprised of a core  202 , an inner cladding layer  204 , and an outer cladding layer  206 . The core  202  is thin, has a high index of refraction (see  FIGS. 3 and 4 ), and often only supports a single transverse optical mode, although multiple modes may also be supported. As laser light  210  from the laser source/transmitter  104  travels down the optical waveguide  102 , the laser light  210  is scattered  212  by the waveguide material. If the interaction  212  of the laser light  210  and the waveguide material produces Rayleigh scattering the incident light is elastically scattered at the same wavelength. If the interaction  212  is with an optical phonon the laser light  210  is Raman scattered with relatively large frequency shifts. If the interaction  212  is with an acoustic vibration (phonons) the laser light  210  is Brillouin scattered with relatively small frequency shifts. In any event, a portion of the scattered laser light  210  having suitable overlap with respect to the propagating modes of the waveguide formed by the core  202 , the inner cladding layer  204  and the outer cladding layer  206  will be recaptured by the optical waveguide  102 . 
   The inner cladding  204  and outer cladding  206  form a multi-mode waveguide that efficiently transports the recaptured scattered light (along with the light recaptured by the core propagating modes) to the receiver  106 . That light is collected and processed to determine a physical parameter of interest using known techniques. A highly multimode waveguide having a large capture cross-section greatly improves the capture of the scattered light. While the optical waveguide  102  is shown with two cladding layers, in some applications more than two claddings may be used. 
   Since distributed optical waveguides  102  operate by light scattering within the core  202 , it is beneficial to produce as much scattering as possible. To that end, the pump radiation  210  should be confined in a mode(s) with a small cross-section(s). This produces a high energy density, which increases the scattering efficiency of the non-linear Raman and Brillouin scattering processes. Additionally, a single, well-confined core mode will generally produce the lowest attenuation and dispersion of the propagating laser light  210 . As the length of a distributed optical waveguide  102  increases a well-confined core mode is particularly useful. Core dopants and dopant concentrations, such as highly doping the core  202  with germanium, or other dopants as is known, including rare-earth dopants, can increase scattering. 
   The refractive indexes of the optical waveguide  102  can be adjusted to improve performance.  FIG. 3  illustrates a refractive index profile of a first embodiment optical waveguide, while  FIG. 4  illustrates a refractive index profile of a second embodiment optical waveguide. In both FIGS., distance is shown on the X-axis ( 300  and  400 ) and the refractive index is shown on the Y-axis ( 302  and  402 ). The maximum refractive index is in the core  202 , shown as peaks  304  and  404 , while the minimum refractive indexes, shown as lines  306  and  406 , are in the outer cladding layer  206 . In the embodiment shown in  FIG. 3 , the refractive index  308  of the inner cladding layer  204  is constant. Thus, the embodiment shown in  FIG. 3  uses a step index. However, in the embodiment shown in  FIG. 4 , the refractive index  408  of the inner cladding layer changes with radial distance. This can produce a better optical waveguide  102  in some applications. 
   More complex waveguide structures such as fibers with multiple rings of different refractive index or asymmetric transverse sections, waveguides of different or multiple materials (e.g. glasses, liquids, gasses), planar waveguides, so-called ‘holey-fibers’ or photonic crystal structures could all be designed to have the properties described in this invention. A wave-guide portion enhances nonlinear scattering through properties such as tight mode confinement, low loss and doping, and a waveguide portion that enhances capture of the scattered light through properties such as large modal overlap with the scattered light and high number of guided modes. 
   The core of the waveguide structure does not necessarily have to be concentric to the waveguide structure and may be positioned to optimize the recapture of scattered radiation. The waveguide structure may even consist of multiple cores, one or more of which guide the pump radiation and one or more of which recapture the scattered radiation in accordance with the principles already outlined. 
   While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.