Abstract:
A bidirectional fibre optic probe comprises an optical in/out coupler and a single fibre or a bundle of fibres, each fibre having a proximal end and a distal end and a numerical aperture NA=sin θ. The numerical aperture NA describes the range of angles over which the optical fibre&#39;s proximal end can accept or emit light. The numerical aperture depends on the refractive index n of the fibre core and is given by NA=n sin* θ. θ is the acceptance angle being defined as the half angle of the acceptance cone of the fibre at its proximal end.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority under 35 USC §119 to European Patent Application No. EP 09153808.2 which was filed on Feb. 26, 2009. 
       TECHNICAL FIELD 
       [0002]    The invention is directed to a fibre optic probe for use in spectroscopy. The probe comprises one or more fibres and an optical in/out coupler for coupling in and coupling out light into and from, respectively, the same optical fibre. The optical coupler is arranged at one end of the optical fibre or the bundle of fibres that hereinafter is called the proximal end. 
       BACKGROUND OF THE INVENTION 
       [0003]    Fibre optic probes for spectroscopy are generally known in the art. In general such fibre optic probe comprises a first fibre or a first bundle of fibres to guide light from the proximal end of the probe to the distal end of the probe, and a second fiber or second bundle of fibers are used to guidelight back to the proximal end of the probe. An optical sensor element such as attenuated total reflectance sensor element or transmission/reflection (Trans-Reflex) element is arranged at the distal end of the fibre probe. The optical sensor element generally is arranged and adapted in a way that it interacts with a sample for determining the sample&#39;s spectroscopic properties so that light entering the optical sensor element is modified by the sample and is reflected back into the optical fibre of the bundle of optical fibres so it eventually is emitted out of the proximal end of the fibre or the bundle of fibres so they can be analyzed by an analyzing detector element such as spectrometer based on diffraction grating, Fourier Transform interferometer or spectral filter with a related photo element or array of detectors. 
         [0004]    Examples for such probe are disclosed in U.S. Pat. No. 5,754,722. 
       SUMMARY OF THE INVENTION 
       [0005]    It is an object of the invention to provide an improved spectroscopy probe that provides a high efficiency, small diameter and high flexibility. 
         [0006]    According to the invention, this object is achieved by a bidirectional fibre-optic spectroscopic probe with an optical in/out coupler. The probe comprises a single fibre or a bundle of fibres, each fibre having a proximal end and a distal end and a numerical aperture NA=sin θ. The numerical aperture NA describes the range of angles over which the optical fibre&#39;s proximal end can accept or emit light. The numerical aperture depends on the refractive index n of the fibre core and is given by NA=n sin* θ. θ is the acceptance angle being defined as the half angle of the acceptance cone of the fibre at its proximal end. 
         [0007]    The optical coupler is an in/out coupler arranged adjacent or near the proximal end for coupling light into said proximal end and for collecting light emerging from said proximal end of the fibre or the bundle of fibres. The bidirectional fibre optic spectroscopic probe further comprises an optical sensor element arranged adjacent or near said distal end of the fibre optic probe. The optical sensor element is adapted to sense a spectroscopic property of a sample to be brought into contact or in proximity to said sensor element. 
         [0008]    The in/out coupler is arranged and adapted to couple higher mode light rays into the proximal end of the fibre and to let lower mode light rays pass to an analyzing sensor element. The optical in/out coupler comprises incoupling means that are adapted and arranged to couple light into said proximal end of said fibre or said bundle of fibres with an incidence angle between θ and θ/x, wherein x is in the range between 1 and 10. The optical in/out coupler further comprises outcoupling means that are adapted to collect light emerging from the proximal end of the fibre or bundled fibres with angles smaller than θ/x. Thus, the in/out optical coupler is adapted to couple higher-order modes of light into said fibre or bundle of fibres and to collect a relatively smaller-order mode of light emerging from said proximal end of said fibre or said bundle of fibres. 
         [0009]    The fibre or each fibre of the bundle of fibres is adapted to operate as a converter that converts light modes of light propagating along the fibre from relatively higher-order modes to relatively lower-order modes. θ/x is the in/out separation angle. Each fibre can be seen as a spatial filter that attenuates said relatively higher order modes of light and redirect their power at least partly into said relatively lower order modes of light while the light propagates along said fibre and reflects from the distal end and said optical sensing element. 
         [0010]    In other words, the claimed invention uses the effect that an optical fibre can concentrate and convert higher-order light modes into lower-order light modes while light travels along the fibres. This allows to couple in light with higher-order modes with relatively high energy and to regain a high-energy signal with lower light modes. Using the spatial filter characteristics allows for incoupling more light and thus a higher energy of light into said fibre while benefiting from the advantages of single fibre use to deliver light to the optical sensor element and thereafter reflected back to the probe&#39;s proximal end. All in all, the signal to noise ratio can be improved without the need to have two fibers, at least—as they are also used in common probes: one fiber to deliver light and the other to collect light back from the sensing element. Thus the need in two or more fibers in mixed fiber bundles is eliminated together with the need to use large size optical elements (like expensive Diamond ATR-elements). In result the probe diameter can be substantially reduced together with the price, while probe will be more flexible and can be used in mini-reactors, for human endoscopy and even inside blood vessels. Thus, the efficiency in the signal-to-noise ratio of the probe is improved while all the other probe parameters are substantially improved as well. 
         [0011]    Further advantages result from the ability to use only a single fiber to deliver light to and collect light from the distal detector element for a single channel spectrometers. This invention is also applicable for multichannel spectrometers where each fiber is used as described above. 
         [0012]    In a preferred embodiment, the fibre optical spectroscopic probe comprises a single fibre for guiding light coupled into said proximal end of said single fibre to said optical sensor element at its distal end of said single fibre and back from said optical sensor element to said proximal end of said single fibre. The single fibre is adapted to act as a propagating light mode converter from higher-order modes of light into lower-order modes of light, thus separating by means of the optical coupler the light coupled into said fibre from the light emerging from said fibre. The light coupled into said fibre is coming from a light source. The light emerging from said fibre has passed to the sensing element at the distal end of the fibre. Thus, the typical outcoupling efficiency of a fibre optic probe can be exceeded—in contrast to what can be achieved when using beam-splitters at the proximal fiber end or Y-fiber splitters, which provide a maximum efficiency that can not exceed 50% in the best case (see FIG. 5 to DE 10034220A1) 
         [0013]    It is further preferred that the fibre optic spectroscopic probe comprises two or more fibres, wherein one fibre is used to record a reference spectrum and the other fibre is used for signal spectrum registration for spectral comparison, while each of two or more fibres is used in both guiding directions as described above. Such arrangement is preferred whenever a reference or background spectrum variation is possible and where a comparison of spectra between the signal spectrum and the background is required. Both of the at least two fibres operate bidirectional, while a dual channel spectrometer eliminates the need to measure, at first, background spectra and only then to measure signal spectra—which always requires calculation of these spectra ratio afterwords Dual (or multi) channel spectrometer with two or more bidirectional fiber probes enables the unique possibility of remote control for any process with background and signal spectra to be measured under the same conditions and in the same time preventing any drift and artifacts in control of important process parameters. 
         [0014]    It is further preferred that the optical in/out coupler comprises a light guiding element or a light directing element that is adapted and arranged to let light emerging from said proximal end of said fibre in an angle between θ and θ/x, directly pass to an analyzing detector and to direct light emerging from a light source to be coupled into said fibre, so it enters the proximal end of the fibre in an angle between θ/x and θ. 
         [0015]    A preferred embodiment of the in/out coupler comprises a light guiding element that is adapted and arranged to let light emitted by the proximal end of the fibre directly pass to an analysing detector element whereas light emitted from a light source is redirected so that it is directed to said proximal end of that fibre. In other words, the analysing detector element may be arranged inline with the fibre at its proximal end whereas the light source is arranged off the direct light path to the fibre proximal end and thus needs to be redirected in order to eventually hit the fibre proximal end. The light guiding element may comprise a mirror or even consist of mirror. Such arrangement allows feeding of all low order light modes emitted by the proximal fibre end to an analyzing detector element thereby avoiding unnecessary losses. Thus as much as possible of the light that is modified by the sample is used. On the other hand, incoupling efficiency can be lower since excess light can easily be provided by a common light source. In addition, means can be provided to collect more light from a large size light source and incouple the light into the fiber using a bundle of light incoupling fibers or fiber tapers designed for collection of light from a large size light spots as is pointed out in more detail with respect to  FIGS. 6 and 7  below. 
         [0016]    The light guiding element or elements or the light directing element or elements preferably comprise mirrors, lenses or fibres or a combination thereof. 
         [0017]    In a particularly preferred embodiment, the optical in/out coupler comprises two or more mirrors. Alternatively, the light guiding in/out coupler comprises one or more lenses in combination with at least one mirror. In a further alternative embodiment, the light guiding optical in/out coupler comprises two or more mirrors in combination with one or more fibres. 
         [0018]    Further preferred embodiments of the optical in/out coupler are disclosed in the following description of exemplary embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The invention shall now be described based on exemplary embodiments depicted in the Figures. The Figures show in: 
           [0020]      FIG. 1 : An illustration of the inventive principle; 
           [0021]      FIG. 2 : a first exemplary embodiment of an optical in/out coupler according to the invention using mirrors; 
           [0022]      FIG. 3 : an alternative embodiment of an optical in/out coupler using mirrors; 
           [0023]      FIG. 4 : an embodiment of an optical in/out coupler using a combination of mirrors and lenses; 
           [0024]      FIG. 5 : an optical in/out coupler using a combination of lenses and mirrors in an alternative arrangement; 
           [0025]      FIG. 6   a : an optical in/out coupler using a combination of mirrors and fibers; 
           [0026]      FIG. 6   b : an alternative embodiment of an optical in/out coupler using a fibre and a mirror; 
           [0027]      FIG. 6   c/d : embodiments similar to  FIG. 6   b  using a fiber having a conical cladding at its proximal end; 
           [0028]      FIG. 7   a : an optical in/out coupler using a combination of lenses and fibres; and 
           [0029]      FIG. 7   b : another alternative embodiment of an optical in/out coupler using a combination of lenses and fibres. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]      FIG. 1  illustrates the basic principle of the invention. A fibre  10  has an angle of acceptance θ. No light can be collected outside the cone defined by θ due to total reflection. According to the invention light shall be coupled into a proximal end  12  of fibre  10  with an angle greater than θ/2x. The light travels along the length of fibre  10  to its distal end  14 . At the distal end  14 , a spectroscopic sensor element  16  is arranged. The spectroscopic sensor element  16  can be a crystal that provides for an attenuated total reflection (ATR) of light entering the crystal and being reflected by the crystal back into the fibre  10  in a manner known as such. Attenuation of the reflected light in the crystal depends on the media adjacent to the crystal. 
         [0031]    After having passed the spectroscopic sensor element  16 , the light travels back along fibre  10  to its proximal end. 
         [0032]    Fibre  10  comprises a core circumferentially surrounded with a cladding having a comparatively higher index of refraction. Typical core/cladding diameters could be, for example, 600 μm (core) and 700 μm (core incl. cladding) or 900/1000 μm, The numerical aperture (NA=n sin* θ, with θ: the acceptance angle and n: refractive index) could be between 0.22 or 0.3, but is not limited to these parameters. 
         [0033]    Fibre  10  acts on the light travelling along fibre  10  as a spatial filter or mode-converter that converts higher order light modes into lower order light modes. Lower order light modes correspond to a smaller angle of propagation with respect to a central axis of fibre  10 . Due to the mode-converting effect of fibre  10 , a significant part of the light coupled into fibre  10  with a relatively high angle of inclination finally emerges from the proximal end  12  of fibre  10  with a relatively small angle of inclination. 
         [0034]    Diagrams a) to f) of  FIG. 1  illustrate the distribution of light propagation angles within fibre  10  and, more particular, a variation of a respective intensity profile for radiation propagating in optical fiber along its length—from its input to output ends with reflecting sensing element,—and for the radiation propagating back to the input end. The profiles in  FIGS. 1   a ) to  1   f ) illustrate the profiles for the case that the input radiation profile a the proximal end of fiber  10  is a ring profile (but within fiber Numerical Aperture), while the output radiation is redistributed to the profile with the main power in smaller output angle. 
         [0035]      FIG. 1   a ) is the distribution of light propagation angles of the light entering fibre  10  at its proximal end. FIG  1   b ) illustrates the distribution of light propagation angles of light travelling from the proximal end to the distal end  14  approximately half way to distal end  14 .  FIG. 1   c ) is the distribution of light propagation angles of the light leaving fibre  10  at its distal end  14 .  FIG. 1   d ) is the distribution of light propagation angles of the light re-entering fibre  10  at its distal end  14  after having passed spectroscopic sensor element  16 .  FIG. 1   e ) illustrates the distribution of light propagation angles of light travelling back from distal end  14  to the proximal end  12  approximately half way to distal end  12 .  FIG. 1   f ) finally shows the distribution of light propagation angles of the light emerging from proximal end  12 . The centre of each diagram corresponds to an angle of propagation with respect to the fibre central axis of zero. 
         [0036]    From  FIG. 1  it is apparent that a relatively high energy of light can be coupled into fibre  10  at a relatively high angle of inclination with respect to the fibre&#39;s central axis while still a significant portion of the light emerging from fibre  10  at the distal end  12  can be collected within relatively small angles of inclination. While the incoupling efficiency at higher angles of inclination corresponding to higher order light modes is not as good as the incoupling efficiency at lower angles of inclination corresponding to lower order light modes this no serious drawback since a surplus of light energy entering the fibre can easily provided without affecting the probe&#39;s efficiency and signal-to-noise ratio. 
         [0037]    In order to achieve the effect illustrated in  FIG. 1 , an optical in/out coupler is provided at the proximal end  12  of fibre  10  (not shown in  FIG. 1 ). 
         [0038]    A variety of embodiments of such optical in/out coupler is illustrated in  FIGS. 2 to 7 . 
         [0039]    In  FIG. 2 , an optical in/out coupler  20  composed of mirrors is illustrated schematically. The in/out coupler  20  is based on input beam (parallel) focusing with off-axis parabolic mirror  22  to the fiber proximal end under an angle to the fiber&#39;s longitudinal axis and to refocus an outcoming light beam on detector by means of an off-axis elliptical mirror  24 . Thus, the first parabolic mirror  22  directs light emerging from a light source (not shown) to the proximal end  12  of fibre  10  so the light enters fibre  10  at an angle between θ and θ/x. The elliptic mirror  24  directs light emerging from the distal  12  with an angle between 0 and θ/x to a spectroscopic detector element  26 . 
         [0040]    The light source (not shown) can be an infrared heated black body (Globar) with or without a Fourier transformation interferometer. Alternatively, the light source can be a lamp, e.g. a tungsten lamp, a plasma lamp or another type of lamp. The light source could also be a laser to induce fluorescence or Raman scattering signal to be collected by in/out coupler from the proximal fibre end for its spectrum analysis. Further possible embodiments of the light source are a tunable laser or a light emitting diode (LED). 
         [0041]    The spectroscopic detecting element can be an array of pyrodetectors (PDA), photodiodes, charged coupled devices (CCD) or the like. The spectroscopic detecting element can be provided with a diffraction grating. Further, the spectroscopic detecting element can be part of a Fourier transformation interferometer. 
         [0042]    In  FIG. 3 , a different embodiment of an optical in/out coupler  20 ′ composed of mirrors is illustrated schematically featuring an axicon  30  for providing parallel incoming beam, a spherical ring mirror  32  for focusing this beam onto the proximal fiber end and an off-axis elliptical mirror  34  for refocusing the outcoming beam onto a detector with. Thus, the optical in/out coupler  20 ′ according to  FIG. 3  comprises the axicon mirror  30  arranged around the proximal end  12  of fibre  10  and adapted to reflect light from a light source (not shown) to the first spherical ring mirror  32  that further reflects the light from the light source to the proximal end of fibre  10 . The spherical ring mirror  32  has a central opening so light emerging from the proximal end  12  of fibre  10  can pass to the off-axis elliptical mirror  34  that directs the light emerging from proximal end  12  of fibre  10  to a spectroscopic sensor element  26 ′. The central opening in the spherical ring mirror  32  has a diameter that corresponds to the width of the cone defined by the angle θ/x. The total diameter (outer diameter) of the spherical ring mirror  32  corresponds to the cone defined by θ. 
         [0043]      FIG. 4  shows an optical in/out coupler  20 ″ similar to the design shown in  FIG. 3  with respect to the incoupling light path same as  FIG. 3 , but using a refocusing lens instead of off-axis elliptical mirror. The incoupling light path comprises an axicon mirror  30  arranged around to the proximal end  12  of fibre  10  reflecting light to be coupled into the fibre to a spherical ring mirror  32  that redirects the light to the proximal end  12  of fibre  10 . Light emerging from the proximal end  12  of fibre  10  is, however, collected by a lens  36  that preferably is arranged in the central opening of the spherical ring mirror  32 . The lens  36  focuses the light emerging from proximal end  12  of fibre  10  to the spectroscopic detector element  26 . 
         [0044]      FIG. 5  illustrates an optical in/out coupler  20 ′″ that is similar to the optical in/out coupler  20 ″ illustrated in  FIG. 4  with respect to the outcoupling light path in that the outcoupling light path comprises a collecting lens  36  that focuses the light emerging from the proximal end  12  of fibre  10  onto the spectroscopic detector element  26 . In the incoupling light path, the axicon mirror is replaced by a diverging lens  38 . The diverging lens  38  directs the collimated light emitted by a light source to the spherical ring mirror  32  surrounding the collecting lens  36 . 
         [0045]    Instead of the diverging lens  38 , an axicon could be provided. 
         [0046]    In  FIGS. 6   a ) to  6   d ) it is illustrated, how an auxiliary fibre  40  can be used to either feed light into proximal end  12  of fibre  10  ( FIG. 6   a ) or to direct light emerging from proximal end  12  of fibre  10  to the spectroscopic sensor element  26 . Elliptical mirrors  42 ,  44  and  46  assist in redirecting and collecting the light. 
         [0047]    According to  FIG. 6   a ) two off-axis mirrors are provided and an additional fiber with flat fiber end or with a distal end shaped as microlens; 
         [0048]    The embodiment according to  FIG. 6   b ) is similar to the embodiment of  FIG. 6   a ), but with a single off-axis mirror and hole for output fiber; 
         [0049]    The embodiment according to  FIG. 6   c ) is similar to the embodiment of  FIG. 6   b ), but using a conical taper made from fiber cladding material to increase light power coupling into the fiber core, while output radiation is refocused by the lens fixed in the central hole of off-axis mirror; 
         [0050]    The embodiment according to  FIG. 6   d ) is similar to the embodiment of  FIGS. 6   b ) and  6   c ), but with a single fiber and conical taper used to collect and launch max power into fiber probe from IR-source and/or a fourier-transformation-interferometer after the source or before the detector. 
         [0051]      FIGS. 7   a  and  7   b  illustrate further alternative embodiments wherein the light to be fed into the proximal end  12  of fibre  10  is guided by a plurality of auxiliary fibres  40 ′ and is fed into proximal end  12  of fibre  10  from a plurality of directions. The auxiliary fibres  40 ′ could be four square PIR-fibers with 1×1 mm cross-section. 
         [0052]    According to  FIG. 7   a  collecting lenses  48  are provided to focus the light to be fed into proximal end  12  of fibre  10  on said proximal end  12 . According to  FIG. 7   b , each distal end of the auxiliary fibres  40 ′ is shaped as microlens thus eliminating the need to use  4  refocusing lenses as in the embodiment according  FIG. 7   a ). 
         [0053]    In  FIGS. 7   a  and  7   b  a collecting lens  50  is arranged in the out coupling light path focussing the light emerging from proximal end  12  of fibre  10  into the spectroscopic detector element  26  similar to the embodiments in  FIG. 4  and  FIG. 5 . 
         [0054]      FIGS. 2 to 7  are examples for optical in/out couplers composed of mirrors, lenses and/or fibres. The man skilled in the out can easily derive further arrangements for achieving a similar effect.