Abstract:
An embodiment of the invention relates to a single photon emission system having a proximal end, a distal end, and a single photon emitter located between the proximal end and the distal end; wherein the single photon emission system is adapted to guide optical pump radiation, which is inputted at the proximal end to optically excite the single photon emitter, along a predefined direction that runs from the proximal end to the distal end; and wherein single photons emitted by said single photon emitter, are guided along said predefined direction to the distal end.

Description:
[0001]    The invention relates to a single photon emission system. Hereinafter, the terms “optical radiation” and “light” refer to any sort of electromagnetic radiation of any wavelength, whether visible or not. 
       BACKGROUND OF THE INVENTION 
       [0002]    A single photon emission system is described in the International Patent Application WO 2009/105814 A1. A pump source generates optical pump radiation which is directed to a single photon emitter by a microscope objective. The optical pump radiation optically excites the single photon emitter to emit single photons which are transmitted to the microscope objective. The propagation direction of the single photons is opposite to the propagation direction of the optical pump radiation. A beam splitter is used to separate the back-travelling single photons from the optical pump radiation. 
       OBJECTIVE OF THE PRESENT INVENTION 
       [0003]    An objective of the present invention is to provide a single photon emission system which may be realized more compact compared to former single photon emission systems. 
         [0004]    A further objective of the present invention is to provide a single photon emission system which achieves a high efficiency with regard to the emission of single photons. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    An embodiment of the present invention relates to a single photon emission system having a proximal end, a distal end, and a single photon emitter located between the proximal end and the distal end; wherein the single photon emission system is adapted to guide optical pump radiation, which is inputted at the proximal end to optically excite the single photon emitter, along a predefined direction that runs from the proximal end to the distal end; and wherein single photons emitted by said single photon emitter, are guided along said predefined direction to the distal end. 
         [0006]    According to this embodiment the propagation direction of the emitted single photons corresponds to the propagation direction of the optical pump radiation. This allows to provide two separate radiation paths: A first radiation path may be optimized for providing pump radiation to the single photon emitter; and a second radiation path may be optimized for guiding the single photons separately to a separate output of the single photon emission system. Thus, the “pump radiation” path and the “single photon” path may be optimized individually. 
         [0007]    Furthermore, very compact single photon emission systems may be fabricated since external devices like beam splitters for separating an ingoing optical pump radiation beam from outgoing single photons are not necessary. 
         [0008]    Preferably, the photon emission system comprises a section where the refractive index increases along the predefined direction. If the single photon emitter is positioned in this section, the majority of photons will be emitted along the predefined direction and reach the distal end. 
         [0009]    In a preferred embodiment a first material and a second material are arranged adjacent to the single photon emitter, wherein the first material is closer to the proximal end than the second material, and has a lower refractive index than the second material. 
         [0010]    In order to achieve a compact system, the single photon emitter may be arranged at a surface of a lens. E.g., the lens may have a flat (or planar) surface, and the single photon emitter may be arranged at the flat (or planar) surface of the lens. 
         [0011]    According to further preferred embodiments, the lens is a solid immersion lens or a gradient index lens. 
         [0012]    The single photon emission system may comprise a first subsystem connected to the proximal end and configured to guide the optical pump radiation along the predefined direction, and a second subsystem optically coupled to the first subsystem and configured to guide the single photons emitted by the single photon emitter along the predefined direction to the distal end. In order to enable an easy alignment of the pump radiation towards the single photon emitter, the first and second subsystems may be pre-assembled, however, this is not mandatory. 
         [0013]    The single photon emitter is preferably arranged at or adjacent to the interface between the first and second subsystems. 
         [0014]    The first subsystem may comprise a filter adapted to attenuate fluorescence radiation generated inside the first subsystem in response to the optical pump radiation. 
         [0015]    The second subsystem may comprise a filter adapted to attenuate the optical pump radiation. 
         [0016]    The single photon emission system preferably comprises a filter adapted to reflect the optical pump radiation and create an optical pump radiation field maximum at the location of the single photon emitter. 
         [0017]    The single photon emission system may further comprise a first and second fiber each having an end facet, wherein the single photon emitter is arranged between the end facets of the first and second fibers. 
         [0018]    A filter may be arranged at the end facet of the first fiber. This filter preferably attenuates fluorescence radiation generated inside the first fiber in response to the optical pump radiation. 
         [0019]    Furthermore, a filter may be arranged at the end facet of the second fiber for attenuating the optical pump radiation. 
         [0020]    The end facet of the first and/or second fiber may have a at least partly curved surface. The single photon emitter is preferably arranged between the curved surfaces of the end facets of the first and second fiber. 
         [0021]    According to another preferred embodiment, a first subsystem may comprise a first fiber having an end facet arranged at the interface between the first subsystem and a second subsystem. The second subsystem may comprise a second fiber having an end facet also arranged at the interface between the first and second system. A first filter may be arranged at the end facet of the first fiber, and may attenuate fluorescence radiation generated inside the first fiber in response to the optical pump radiation and reflect the single photons emitted by the single photon emitter. Further, a second filter may be arranged at the end facet of the second fiber, and may attenuate the optical pump radiation and reflect the single photons emitted by the single photon emitter. Preferably, the reflection of the second filter is lower than that of the first filter. 
         [0022]    According to a further preferred embodiment, a light shade may be arranged in the radiation path. The light shade may attenuate an inner section of the radiation beam of the optical pump radiation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    In order that the manner in which the above-recited and other advantages of the invention are obtained will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended figures. Understanding that these figures depict only typical embodiments of the invention and are therefore not to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail by the use of the accompanying drawings in which 
           [0024]      FIGS. 1-7  show exemplary embodiments of a single photon emission system according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0025]    The preferred embodiments of the present invention will be best understood by reference to the drawings, wherein identical or comparable parts are designated by the same reference signs throughout. 
         [0026]    It will be readily understood that the present invention, as generally described herein, could vary in a wide range. Thus, the following more detailed description of the exemplary embodiments of the present invention, is not intended to limit the scope of the invention, as claimed, but is merely representative of presently preferred embodiments of the invention. 
         [0027]      FIG. 1  shows a first exemplary embodiment of a single photon emission system  10 . The single photon emission system  10  comprises a proximal end  11 , and a distal end  12 . The proximal end  11  of the single photon emission system  10  is optically coupled to an external pump laser  20  which generates an optical pump radiation (pump light) P. The optical pump radiation P is inputted into the proximal end  11  of the single photon emission system  10 . 
         [0028]    The single photon emission system  10  comprises a first subsystem  30  and a second subsystem  40  which is optically coupled to the first subsystem  30 . 
         [0029]    The first subsystem  30  is connected to the proximal end  11  and guides the optical pump radiation P along a predefined direction which is indicated by an arrow D in  FIG. 1 . The first subsystem  30  comprises a first waveguide  31 , a first lens  32 , a first filter  33 , and a second lens  34 . The first waveguide  31  guides the optical pump radiation P to the first lens  32  which inputs the optical pump radiation P into the first filter  33 . 
         [0030]    The first filter  33  attenuates fluorescence radiation that is generated inside the first waveguide  31  in response to the optical pump radiation P. The attenuation of the fluorescence radiation may be based on absorption and/or reflection of the fluorescence radiation. As can be seen in  FIG. 1 , the first filter  33  is preferably arranged on a flat (or planar) surface  321  of the first lens  32 . The first filter  33  may consist of one or more layers of material deposited on the flat surface  321 . 
         [0031]    The second lens  34  focuses the filtered optical pump radiation P′ onto a single photon emitter  41  which is arranged on the outer surface of a lens  42  and thus at the interface between the first and second subsystems  30  and  40 . 
         [0032]    The lens  42  forms a third lens of the single photon emission system  10  and is positioned at the second subsystem  40 . The second subsystem  40  further comprises a filter  43  that forms a second filter of the single photon emission system  10 , a lens  44  that forms a fourth lens of the single photon emission system  10 , and a waveguide  45  that forms a second waveguide of the single photon emission system  10 . 
         [0033]    The filtered optical pump radiation P′, which is focused on the single photon emitter  41  by the second lens  34 , optically excites the single photon emitter  41  to emit single photons S. 
         [0034]    As mentioned above, the single photon emitter  41  is arranged at the outer surface of the third lens  42 . Preferably, the third lens  42  consists of a material having a higher refractive index than the material  46  adjacent thereto. E.g., the third lens  42  may consist of glass, and the adjacent material  46  may be air or a polymer material. As such, two different materials, namely the material of the third lens  42  and the material  46 , are arranged adjacent to the single photon emitter  41 , wherein the refractive index increases along the predefined direction D. Due to the increase of the refractive index, single photons S emitted by the single photon emitter  41  will primarily be directed to the right hand side (versus the second systems  40 ) in  FIG. 1  and thus in direction of the distal end  12 . Only a minority of single photons will be emitted to the other direction (i.e. opposite to the predefined direction D) into the first subsystem  30  and head towards the proximal end  11 . 
         [0035]    The single photons S emitted by the single photon emitter  41  as well as the residual filtered optical pump radiation P′ are directed to the second filter  43  which attenuates the optical pump radiation P′. The single photons S however pass the second filter  43  and are coupled into the second waveguide  45  by the fourth lens  44 . The second waveguide  45  guides the single photons S along the direction D to the distal end  12  where they leave the single photon emission system  10 . 
         [0036]    The second filter  43  is preferably arranged on a flat (or planar) surface  421  of the third lens  42 . The second filter  43  may consist of one or more layers of material deposited on the flat surface  421 . 
         [0037]    The first subsystem  30  and the second subsystem  40  are preferably preassembled such that both systems  30  and  40  are movable relative to each other in order to allow for a precise alignment between the focus of the second lens  34  positioned at the first subsystem  30 , and the single photon emitter  41  positioned at the second subsystem  40 . 
         [0038]    The first, second, third, and/or fourth lens  32 ,  34 ,  42 , and  44  are preferably gradient index (GRIN) lenses. 
         [0039]      FIG. 2  shows a second exemplary embodiment of a single photon emission system  100 . The single photon emission system  100  comprises a first waveguide  110  connected to a proximal end  101  of the single photon emission system  100 , a first lens  115 , a first filter  120 , a second lens  125 , a single photon emitter  130 , a third lens  135 , a fourth lens  140 , a second filter  145 , a fifth lens  150 , and a second waveguide  155  connected to a distal end  102  of the single photon emission system  100 . 
         [0040]    The third lens  135  preferably is a solid immersion lens. The single photon emitter  130  may be arranged on a flat (or planar) surface  160  of the third lens  135 . Preferably, the third lens  135  consists of a material having a higher refractive index than the material  165  adjacent thereto. E.g., the third lens  135  may consist of glass, and the adjacent material  165  may be air or a polymer material. As such, two different materials, namely the material of the third lens  135  and the material  165 , are arranged adjacent to the single photon emitter  130 , wherein the refractive index increases along the predefined direction D. Due to the increase of the refractive index, single photons S emitted by the single photon emitter  130  will primarily be directed to the right hand side and thus in direction of the distal end  102 . Only a minority of single photons will be emitted to the other direction (i.e. opposite to the predefined direction D) and head towards the proximal end  101 . 
         [0041]    The first filter  120  attenuates fluorescence radiation that is generated inside the first waveguide  110  in response to the optical pump radiation P. The attenuation of the fluorescence radiation may be based on absorption and/or reflection of the fluorescence radiation. 
         [0042]    The second lens  125  focuses the filtered optical pump radiation P′ onto the single photon emitter  130  which is arranged on the outer surface of the third lens  135 . The filtered optical pump radiation P′ optically excites the single photon emitter  130  to emit the single photons S. 
         [0043]    The single photons S emitted by the single photon emitter  130  as well as the residual filtered optical pump radiation P′ are guided to the second filter  145  which attenuates the optical pump radiation P′. The single photons S pass the second filter  145  and are coupled into the second waveguide  155  by the fifth lens  150 . The second waveguide  155  guides the single photons S along the direction D to the distal end  102  where they leave the single photon emission system  100 . 
         [0044]    The single photon emission system  100  may consist of two preassembled subsystems  170  and  175 . The first waveguide  110 , the first lens  115 , the first filter  120 , and the second lens  125  may belong to the first preassembled subsystem  170 . The single photon emitter  130 , the third lens  135 , the fourth lens  140 , the second filter  145 , the fifth lens  150 , and the second waveguide  155  may belong to the second preassembled subsystem  175 . Of course, the preassembling of subsystems is not mandatory and just a preferred embodiment. 
         [0045]      FIG. 3  shows a third exemplary embodiment of a single photon emission system. The single photon emission system  200  comprises a first waveguide  210  connected to a proximal end  201  of the single photon emission system  200 , a first filter  215 , a single photon emitter  220 , a second filter  225 , and a second waveguide  230  connected to a distal end  202  of the single photon emission system  200 . 
         [0046]    The first filter  215  is arranged on a plane (or planar) surface  235  of the end facet of the first waveguide  210  and configured to attenuate fluorescence radiation that is generated inside the first waveguide  210  in response to the optical pump radiation P. The attenuation of the fluorescence radiation may be based on absorption and/or reflection of the fluorescence radiation. 
         [0047]    The second filter  225  is arranged on a plane (or planar) surface  240  of the end facet of the second waveguide  230  and configured to attenuate the optical pump radiation. The single photons S however pass the second filter  225  and are coupled into the second waveguide  230 . The second waveguide  230  guides the single photons S along the direction D to the distal end  202  where they leave the single photon emission system  200 . 
         [0048]    Preferably, the second filter  225  consists of a material having a higher refractive index than the adjacent material  245 . As such, two different materials, namely the material of the second filter  225  and the material  245 , are arranged adjacent to the single photon emitter  220 , wherein the refractive index increases along the predefined direction D. Due to the increase of the refractive index, single photons S emitted by the single photon emitter  220  will primarily be directed to the right hand side and thus in direction of the distal end  202 . Only a minority of single photons will be emitted to the other direction (i.e. opposite to the predefined direction D) and head towards the proximal end  201 . 
         [0049]    The first waveguide  210  and the first filter  215  may form a first subsystem  260 . The single photon emitter  220 , the second filter  225 , and the second waveguide  230  may form a second subsystem  265 . 
         [0050]      FIG. 4  shows a fourth exemplary embodiment of a single photon emission system. The single photon emission system  300  comprises a first waveguide  310  connected to a proximal end  301  of the single photon emission system  300 , a first filter  315 , a single photon emitter  320 , a second filter  325 , and a second waveguide  330  connected to a distal end  302  of the single photon emission system  300 . 
         [0051]    The first filter  315  is arranged on a curved, preferably concave, surface  335  of the end facet of the first waveguide  310 . The second filter  325  is arranged on a curved, preferably concave, surface  340  of the end facet of the second waveguide  330 . Filter  315  reflects the fluorescence radiation caused by the pump radiation, and filter  325  filters the pump radiation and creates an optical pump radiation field maximum at the location of the single photon emitter  320 . Filters  315  and  325  further reflect the emitted single photons S. 
         [0052]    The reflection of the second filter  325  for the single photons is lower than that of the first filter  315 . As such, the majority of single photons S leave the cavity (resonator) or section between the first and second filters on the right hand side in  FIG. 4  and reach the distal end  302  of the single photon emission system  300 . 
         [0053]      FIG. 5  shows a fifth exemplary embodiment of a single photon emission system. The single photon emission system  300  largely corresponds to the fourth embodiment shown in  FIG. 4 . In contrast thereto, the single photon emitter  320  of  FIG. 5  is arranged on the curved surface  340 ′ of the second filter  325 . 
         [0054]      FIG. 6  shows a sixth exemplary embodiment of a single photon emission system. The single photon emission system  400  comprises a first waveguide  410  connected to a proximal end  401  of the single photon emission system  400 , a light shade  415 , a focus system  420 , a single photon emitter  425 , and a second waveguide  430  connected to a distal end  402  of the single photon emission system  400 . 
         [0055]    The light shade  415  is arranged in the radiation path and attenuates an inner section of the radiation beam of the optical pump radiation. As such, only the outer beam rays Po may enter the focus system  420  and reach the single photon emitter  425 . However, the outer beam rays are angled relative to the main axis  460  and therefore cannot efficiently couple into the second waveguide  430 . The amount of optical pump radiation, which enters the second waveguide  430 , is thus very small. 
         [0056]    In order to further attenuate the residual optical pump radiation, which nonetheless couples into the second waveguide  430 , a filter  435  may be arranged in the beam path between the single photon emitter  425  and the distal end  402  of the single photon emission system  400 . 
         [0057]    The single photon emitter  425  may be positioned on the end facet of the second waveguide  430  or at the outer surface of filter  435  as shown in  FIG. 6 . 
         [0058]    The components of the single photon emission system  400  may be arranged in two subsystems  465  and  470  as indicated in  FIG. 6 . 
         [0059]      FIG. 7  shows a seventh exemplary embodiment that largely corresponds to the first embodiment shown in  FIG. 1 . In contrast to the latter, the embodiment of  FIG. 7  comprises an additional filter  500  which is positioned between the single photon emitter  41  and the proximal end  11 , for instance between the single photon emitter  41  and the lens  34 . The filter  500  reflects single photons emitted by the single photon emitter  41  towards the distal end  12 . 
         [0060]    The waveguides  31 ,  45 ,  110 , and  155  as shown in  FIGS. 1 and 2  may be fibers such as singlemode fibers or multimode fibers. 
       REFERENCE SIGNS 
       [0000]    
       
           10  single photon emission system 
           11  proximal end 
           12  distal end 
           20  external pump laser 
           30  first subsystem 
           31  first waveguide 
           32  first lens 
           321  flat surface 
           33  first filter 
           34  second lens 
           40  second subsystem 
           41  single photon emitter 
           42  lens 
           421  flat surface 
           43  filter 
           44  lens 
           45  waveguide 
           46  material 
           100  single photon emission system 
           101  proximal end 
           102  distal end 
           110  first waveguide 
           115  first lens 
           120  first filter 
           125  second lens 
           130  single photon emitter 
           135  third lens 
           140  fourth lens 
           145  second filter 
           150  fifth lens 
           155  second waveguide 
           160  flat surface 
           165  material 
           170  subsystem 
           175  subsystem 
           200  single photon emission system 
           201  proximal end 
           202  distal end 
           210  first waveguide 
           215  first filter 
           220  single photon emitter 
           225  second filter 
           230  second waveguide 
           235  plane surface 
           240  plane surface 
           245  material 
           260  first subsystem 
           265  second subsystem 
           300  single photon emission system 
           301  proximal end 
           302  distal end 
           310  first waveguide 
           315  first filter 
           320  single photon emitter 
           325  second filter 
           330  second waveguide 
           335  curved surface 
           340  curved surface 
           340 ′ curved surface  400  single photon emission system 
           401  proximal end 
           402  distal end 
           410  first waveguide 
           415  light shade 
           420  focus system 
           425  single photon emitter 
           430  second waveguide 
           435  filter 
           460  main axis 
           465  subsystem 
           470  subsystem 
           500  filter 
         D direction 
         P optical pump radiation 
         P′ filtered optical pump radiation 
         S single photons