Abstract:
A light detection apparatus is described. The apparatus includes a photomultiplier tube having a window for receiving light incident thereon. A photocathode is affixed to an inner surface of the window in a known manner. The apparatus further includes an optical fiber and a means for coupling the optical fiber to the window of said photomultiplier tube so that light can be introduced into the window at an angle that results in total internal reflection of the light. The coupling means may be embodied as a fiber optic terminal connector. Alternatively, the coupling means may include a prism affixed to the outside surface of the window.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/886,249, filed Jan. 23, 2007, the entirety of which is incorporated by reference herein. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    In the detection of weak light that is transmitted to a photomultiplier-based detector using fiber optic light guides, the photomultiplier tube loses light because of reflection at the input window and also because of transmission through the light sensitive photocathode surface, especially at longer wavelengths (i.e., the red-end of the visible spectrum). 
         [0003]    It has been shown that, for a narrow beam (“pencil”) of light that is directed toward the input window of a photomultiplier tube at an angle relative to the normal of the window surface, a large increase in sensitivity can be obtained as a result of total internal reflection that occurs in the photomultiplier&#39;s window. A prism is used to adapt the light beam to the window and prevent a large reflection at the first interface. The range of angles is limited. Hitherto, fiber-delivered light was not expected to benefit from this phenomenon because of the large angular spread of such light. 
       SUMMARY OF THE INVENTION 
       [0004]    In accordance with a first aspect of the present invention there is provided an apparatus for the detection of a light signal. The apparatus according to this aspect of the invention includes a photomultiplier tube having a window for receiving light incident thereon. A photocathode is affixed to an inner surface of the window in a known manner. The apparatus further includes an optical fiber and a means for coupling the optical fiber to the window of said photomultiplier tube so that light can be introduced into the window at an angle that results in total internal reflection of the light. The coupling means may be embodied as a fiber optic terminal connector. Alternatively, the coupling means may include a prism affixed to the outside surface of the window. 
         [0005]    In accordance with another aspect of the present invention there is provided an apparatus for the detection of a light signal. The apparatus according to this aspect of the invention includes a photomultiplier tube having a window for receiving light incident thereon and a photocathode affixed to an inner surface of the window. An opaque housing surrounds the photomultiplier tube. The housing has an opening therein that is in alignment with the window. The apparatus also includes an optical fiber and a connector attached to the housing at the opening for coupling the optical fiber to the window of the photomultiplier tube. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0006]    The foregoing summary as well as the following detailed description will be better understood when read with reference to the drawings, wherein: 
           [0007]      FIG. 1  is a perspective view of a light detection apparatus according to the present invention; 
           [0008]      FIG. 2  is a schematic view in partial section of the light detection apparatus shown in  FIG. 1 ; 
           [0009]      FIG. 3  is a schematic view of a window of a photomultiplier tube used in the light detection apparatus shown in  FIG. 1 , in which an optical fiber light guide is coupled to the edge of the window; 
           [0010]      FIG. 4  is a chart of graphs of the cathode spectral response for three light detectors according to the present invention and a comparative light detector; 
           [0011]      FIG. 5  is a schematic view of a window of a photomultiplier tube used in the light detection apparatus shown in  FIG. 1 , in which an optical fiber light guide is coupled to the window with a prism; and 
           [0012]      FIG. 6  is a schematic circuit diagram of a photomultiplier tube used in the light detection apparatus according to this invention. 
       
    
    
     DESCRIPTION OF THE INVENTION 
       [0013]    I have determined that it is possible to obtain a significant increase in photomultiplier sensitivity in the case of fiber-delivered light using the total internal reflection phenomenon. This development makes the photomultiplier a very effective detector for applications such as laser induced fluoresence or cytometry where the light signal is transmitted to the detector by fiber optic means. 
         [0014]    Referring now to the drawings, and in particular to  FIGS. 1 and 2 , there is shown a light detection apparatus  10  for the detection of light according to this invention. The apparatus  10  includes a photomultiplier tube  12  and an optical fiber  16  that is seamlessly connected to the input window of the photomulitplier tube  12 . The photomultiplier tube  12  has a window  20  at one thereof. The window is preferably plano-plano (i.e., planar inner and outer surfaces) in construction to allow the light to maintain a constant angle of internal reflection as the light passes transversely across the window. The photomultiplier  12  is surrounded by an enclosure  14  that is opaque to light. The enclosure  14  has an opening  21  that is aligned with a beveled edge  22  of the window  20 . The optical fiber  16  is coupled to the photomultiplier  12  with a connector  18  that is preferably an SMA connector. The connector  18  is attached to the enclosure  14  over the opening  21  so that an end of the optical fiber  16  is in close proximity to the beveled edge  22 . 
         [0015]    In order to achieve sufficient internal reflection of the light in the photomultiplier window, the window should be relatively thin, but should be thick enough to permit the light from the fiber to enter in an unobstructed manner. The angle of the fiber relative to the plane of the window is selected to maximize the response of the photocathode. The preferred range of angles is in the range of about 42 degrees to about 85 degrees relative to an axis  23  that is normal to the planar surface of the window  20 , as shown in  FIG. 3 . 
         [0016]    It is also preferred that the end of the optical fiber be positioned as closely as possible to the photocathode, thereby making a substantially continuous light path through the window and toward the photocathode. This approach minimizes the divergence of the light bundle and maximizes the interaction of the light with the photocathode. 
         [0017]    The edge of the photomultiplier tube window would normally not be considered as an entrance point for the light because of the poor collection of the electrons created there. The efficiency of the design would be lost because of failure of the electrons so close to the edge to contribute to the response signal. However, the device according to this invention provides a way to prevent such loss by use of a mirrored surface  28  to block light at the very edge from being absorbed uselessly by the photocathode  24 . Instead, the light is tipped away by the reflector  28  so that the light can be reflected back to the photocathode  24  at a point further from the edge of the window  20 . 
         [0018]    For glancing rays or for light of such long wavelength that absorption by the photocathode is poor, a second mirrored surface  29  is provided at the far end of the window  20  from the point of entry. This second reflector  29  is formed and disposed for returning such light to permit further internal reflection back toward the point of entry, thereby providing a second pass along the photocathode  24 . 
         [0019]    The thickness of the photocathode itself, and/or an adaptive dielectric layer, is chosen preferably to optimize the performance of the apparatus. In the case of total internal reflection, light loss due to transmission through the photocathode or reflection at the glass/photocathode interface is nearly absent. The cathode thickness is preferably thin enough to enhance electron escape. Additionally, the photocathode  24  can be thinner at the beginning of the interaction region, where blue light is most effectively detected, and thicker further across the input window, where red light is detected. 
         [0020]    In an alternative arrangement as shown in  FIG. 5 , a prism  532  is affixed to the top surface of the photomultiplier window  520 . The prism  532  is positioned in a peripheral region of the window surface and disposed in the light path between the optical fiber  516  and the photomultiplier window  520 . This arrangement is preferred for use where the edge of the photomultiplier input window is not accessible. Such a case may occur with a tube having a metal flange around the input window. In the arrangement shown in  FIG. 5 , the light path is not seamless. However, light loss can be minimized by keeping the distance from the fiber tip to the photocathode surface to a minimum. Additionally, the refractive index of the prism can be selected to minimize the angular spread of the light  526  as it emerges from the optical fiber  516 . 
         [0021]    Referring now to  FIG. 6 , there is shown a preferred circuit  40  for the photomultiplier used in the light detection apparatus according to the present invention. The circuit  40  includes a high voltage supply module that utilizes a hybrid voltage divider network in which sections of the dynode chain of the photomultiplier  42  that require higher current levels are separated from those that have very low current requirements. Such a voltage supply network is described in U.S. Pat. No. 7,005,625, the entirety of which is incorporated by reference. The voltage supply network includes a resistor-based voltage divider circuit  44  for controlling the dynode voltage of the upper dynodes in the photomultiplier tube  42 . An active transistor circuit  46  is provided for the lower stage dynodes to provide voltage stabilization without incurring extra current demand. 
         [0022]    The signal from the photomultiplier anode is preferably processed with a transimpedance amplifier  48 . The gain of the photomultiplier-amplifier combination can be selected to permit the detection of single photoelectrons. However, the circuit  40  can be configured with other gain settings depending on the particular application in which the light detection apparatus will be used. The circuit  40  preferably also includes a in internal reference voltage supply  52  for use in setting the high voltage applied to the photomultiplier cathode, dynodes, and anode. 
         [0023]    The benefit provided by the light detection apparatus according to this invention is clearly shown by reference to  FIG. 4  which is a chart of graphs of photocathode response of a conventional photomultiplier tube and three light detection apparatuses according to the present invention. The graphs illustrate the responses of the device photocathodes to light over a wavelength range of 400 to 850 nm. The responses of light detectors having a prism launch arrangement as shown in  FIG. 5  are designated “Inv. A” and “Inv. B”. The response curve of a known, high performance reflection mode photocathode is designated “Comp.” The graph for a light detector having an edge launch arrangement (Inv. C) according to the present invention is also shown. The graphs of the responses for the prism launch examples (Inv. A and Inv. B) are clearly better than the response of the known reflection mode photomultiplier tube up to about 750 nm. The response for the edge-launch example (Inv. C) is significantly better than the response of all the other examples, even at the longer wavelengths. 
         [0024]    It will be recognized by those skilled in the art that changes or modifications may be made to the above-described embodiments without departing from the broad inventive concepts of the invention. It is understood, therefore, that the invention is not limited to the particular embodiments which are described, but is intended to cover all modifications and changes within the scope and spirit of the invention as described above and set forth in the appended claims.