Abstract:
An optical communication device includes a first photon source emitting a first beam modulated with information. The first beam intersects a thin metal film and engenders a first surface plasmon wave thereon. Part of the first beam reflects from the metal film to form a reflected beam. A polarization structure rotates the polarization of the reflected beam. A reflecting structure reflects the reflected beam to form a second beam propagating back toward the film, which beam passes through the polarization structure again. On the metal film, the second beam engenders a second surface plasmon wave. Interaction between the first and second surface plasmon waves creates a surface plasmon standing wave. A second source provides a third beam intersecting the first and second beams at the metal film. Interaction between the third beam and the surface plasmon standing wave modulates the third beam as it passes through the metal film.

Description:
FIELD 
     The present invention relates to an optical communication or modulation device. In particular, the present invention relates to an optical communication device in which information borne by a first beam of photons is controllably passed to a second beam of photons that is thence distributed to a set of photon receptor elements. Further, the present invention relates to spectral information in the second beam of photons that is diffracted into multiple parts by surface plasmons to provide spatial separation of different wavelengths in the second beam of photons. Further, the invention relates to amplification of an optical communication signal. 
     BACKGROUND 
     Photon signal processing has an increasing importance in telecommunications and data processing. In optical communication systems, it is often necessary to analyze, energy-amplify, process, or distribute photon signals that are particularly brief or rapidly changing. In such systems, it is desirable to minimize processing and distribution time. It is also desirable to spatially separate and distribute information content in a rapid and controllably varying manner. In many applications, processing or distribution elements of an optical communication system should provide response times of the order of femtoseconds, and have passbands extending up into the petahertz range. Fiber optic transmission networks which incorporate opto-electronic, electro-optic, opto-mechanical, or other devices involving physical electronics or electrodes have response times that are generally too slow and passbands that are generally too narrow. 
     What is needed, therefore, is a system having a faster response time and broader bandwidth than is currently available for modulating, amplifying, and/or distributing brief and rapidly varying photon signals. 
     SUMMARY 
     The above and other needs are met by an optical communication device having a first photon source, such as an infrared laser, for providing a first beam of photons modulated with information. The device includes an intersection plane for intersecting the first beam of photons at a first angle of incidence and for reflecting a first portion of the first beam of photons to form a reflected beam of photons propagating at a second angle of incidence. A polarization rotating structure is provided to rotate the polarization of the reflected beam of photons. A reflecting structure reflects the reflected beam of photons to form a second beam of photons. The second beam is passed through the polarization rotating structure to rotate the polarization of the second beam of photons. The intersection plane receives the second beam of photons at the second incidence angle, and intersects the second beam of photons with the first beam of photons. In a most preferred embodiment of the invention, a film layer at the intersection plane supports a first surface plasmon wave formed by the first beam of photons and a second surface plasmon wave formed by the second beam of photons. Interaction between the first and second surface plasmon waves on the film layer forms a surface plasmon standing wave. A second photon source, such as an ultraviolet laser, provides a third beam of photons which passes through the film layer at the intersection of the first and second beams of photons. As the third beam of photons passes through the film layer, the surface plasmon standing wave modulates the third beam with the information carried by the first beam. 
     Also in a preferred embodiment, the surface plasmon standing wave on the film layer is operable to scatter the third beam of photons in a diffraction pattern having a central peak portion and side peak portions spatially disposed on either side of the central peak portion. Some preferred embodiments include a first photon collection device, such as a photodetector, which is operable to receive the first peak portion, and at least one second photon collection device operable to receive at least one of the side peak portions. 
     In an alternative embodiment, the invention provides an optical communication device that includes a first photon source, such as a laser, for emitting a first beam of photons in a first direction, where the first beam of photons is modulated with information. A beam splitter receives the first beam and divides the first beam into second and third beams of photons. A beam directing structure directs the second beam in a second direction, and directs the third beam in a third direction which is different from the second direction. The second and third beams intersect at an intersection plane, there being a second incidence angle between the second beam and the intersection plane, and a third incidence angle between the third beam and the intersection plane. The device has a second photon source, such as a laser, for emitting a fourth beam of photons that intersects the second and third beams at the intersection plane. The device further includes a film layer, such as a metal film, disposed at the intersection plane which is transmissive to the fourth beam of photons. Based on interaction between the second, third, and fourth beams on the film layer, the fourth beam is modulated with the information as the fourth beam passes through the film layer. 
     In another aspect, the invention provides a method for modulating a carrier beam of photons with information carried by a first beam of photons. The method includes steps of propagating a first beam of photons in a first direction, and reflecting a portion of the first beam to form a reflected beam propagating in a second direction. The reflected beam is reflected to form a second beam which is substantially collinear with the reflected beam and which propagates in a third direction opposite the second direction. The polarization of the second beam is rotated, and the second beam is intersected with the first beam at an intersection plane. The method further includes propagating a carrier beam of photons toward the intersection plane, and intersecting the carrier beam with the first and second beams at the intersection plane. Based on interactions between the first, second, and carrier beams at the intersection plane, the carrier beam is modulated with the information as the carrier beam passes through the intersection plane. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further advantages of the invention are apparent by reference to the detailed description when considered in conjunction with the figures, which are not to scale so as to more clearly show the details, wherein like reference numbers indicate like elements throughout the several views, and wherein: 
     FIG. 1 depicts an optical communication system according to a preferred embodiment of the invention; 
     FIGS. 2A-B depict a method of optical communication according to a preferred embodiment of the invention; 
     FIG. 3 depicts an optical communication system according to an alternative embodiment of the invention; 
     FIGS. 4A-B depict a method of optical communication according to an alternative embodiment of the invention; 
     FIG. 5 depicts an optical communication system according to another alternative embodiment of the invention; and 
     FIG. 6 depicts an optical communication system according to yet another alternative embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     Depicted in FIGS.  1  and  2 A-B is a preferred embodiment of an optical communication, amplification, and distribution system  10 . The system  10  includes a first photon emission source  12 , which is most preferably an infrared diode laser and modulator. The wavelength of infrared radiation emitted by the source  12  is preferably about 1.55 μm. The first photon source  12  emits a first beam of photons  14   a  that is modulated with information (step  100  in FIG. 2A) based on an information signal  13 . The modulated first beam of photons  14   a  may also represent an incoming signal in an optical communication system that is to be repeated, amplified, and/or distributed by the system  10 . 
     For purposes of this description, modulation includes any form of controlled variation of the direction, amplitude, or frequency of a beam of photons with time, where the controlled variation preferably contains the information. Elements of the information may be represented directly by instantaneous or time-interval values of the spatial directions of propagation, by the variations in amplitude, or by variation in frequency of a photon beam. Additionally, the modulation may convert a beam of photons into a succession of shaped pulses, in which case the elements of the information is represented by the shapes, the positions, the frequency content, or the intervals between the shaped pulses. 
     The first beam of photons  14   a  impinges upon an entrance element  17   a  which is coupled to the convex surface of a plano-convex cylindrical lens  24  using an index-matching gel. The entrance element  17   a  and the lens  24  are preferably fabricated from high-purity quartz, and the curvature of the inner surface of the entrance element  17   a  and the curvature of the outer surface of the lens  24  are preferably matched. The outer surface of the entrance element  17   a  that receives the beam  14   a  is preferably flat, and is normal to the direction of propagation of the beam  14   a . Thus, the entrance element  17   a  serves to prevent focusing or divergence of the first beam  14   a  as it enters the lens  24 , thereby substantially maintaining the beam cross-section. 
     On the planar surface of the lens  24  is a thin film  26  of a material having a negative permittivity, such as a thin film of evaporated gold. Although gold is the most preferred material for the film  26 , other materials could be used, such as aluminum, silver, lithium, indium tin oxide, tin oxide, or refractory metal silicides. The film  26  is thin enough to be optically transparent, so that optical energy may be transmitted through the film  26 . For a gold film  26 , the preferred thickness is about five nm. The planar surface at which the film  26  resides is also referred to herein as the intersection plane. 
     The beam  14   a  impinges upon the film  26  at the intersection plane at an angle of incidence β 1  relative to a normal to the surface of the film  26 . The angle of incidence β 1  is selected such that the energy and momentum of photons in the beam  14   a  matches that of surface plasmons in the film  26 . For purposes of this description, surface plasmons are the quanta associated with collective electron motion comprising a longitudinal surface plasmon wave in the film  26 . 
     Generally, the momentum of a surface plasmon in the film  26  is expressed as 
     
       
         h/2π×k, 
       
     
     where h is Planck&#39;s constant and k is the vacuum wave vector. The surface plasmon wave vector K is related to the vacuum wave vector k of the incident photons by 
     
       
           K=n×k× sin β,  (1) 
       
     
     where n is the index of refraction of the medium supporting the film  26 , which in the preferred embodiment is the quartz lens  24 . This relation of equation (1) expresses the conservation of energy of the lateral component of the plasmon momentum. Generally, the frequency, and thus the energy, of the surface plasmon should equal that of the photons in the beam  14   a  in order to satisfy the conservation of energy relationship. The degree of excitation of a surface plasmon is then dependent upon the complex index of refraction of the layer  26 , and is determined by application of Maxwell&#39;s equations and Cartesian boundary conditions. 
     In the preferred embodiment, the first beam  14   a  is unpolarized. As the surface plasmons are excited in the film  26 , a p-polarized component of the first beam  14   a  is absorbed in the film  26 . As depicted in FIG. 1, an s-polarized component of the first beam  14   a  is reflected from the film  26  to form a reflected beam  14   b  (step  102 ), which forms an angle β 1  relative to the normal to the surface of the film  26 . The reflected beam  14   b  passes through an exit element  17   b , which is preferably identical in design and function to the entrance element  17   a , and then through a polarization rotator  19 , such as a Faraday cell, Kerr cell, or Pockels cell. In this preferred embodiment, the polarization rotator  19  is a Faraday cell, which introduces a rotation of 45 degrees to the polarization of the beam  14   b  (step  104 ). The beam  14   b  impinges upon a mirror  21  which is set substantially normal to the direction of propagation of the beam  14   b . The radiation is reflected from the mirror  21  to form a second beam  14   c  (step  106 ), which is substantially collinear with the beam  14   b , but which propagates in the opposite direction, toward the film  26 . The reflected beam  14   c  passes through the polarization rotator  19  which imparts a polarization rotation of 45 degrees to the beam  14   c  in the same direction as the first rotation (step  108 ). Thus, the polarization of the second beam  14   c  upon exiting the rotator  19  is rotated by 90 degrees with respect to the polarization of the reflected beam  14   b  prior to entering the rotator  19 . Hence, the second beam  14   c  is p-polarized as it passes through the element  17   b  and impinges upon the layer  26 . 
     The first and second beams  14   a  and  14   c  each impinge upon the film  26  at the intersection plane at angles of incidence β 1  relative to a normal to the surface of the film  26 . As depicted in FIG. 1, the beam  14   a  intersects with the beam  14   c  in an intersection area  27  on the surface of the film  26  (step  110 ). As discussed above, the angle of incidence β 1  is selected such that the energy and momentum of photons in the beams  14   a  and  14   c  matches that of surface plasmons in the film  26 . With this preferred geometry, the beam  14   a  forms a first surface plasmon wave in the layer  26  which is correlated in phase to the beam  14   a  (step  112 ), and the beam  14   c  forms a second surface plasmon wave in the layer  26  which is correlated in phase to the beam  14   c  (step  114 ). The two oppositely directed surface plasmon waves, which are in correlated phase, interact to form a standing wave of surface plasmons (step  116 ). 
     Although the operation of the invention is not limited to any particular theory, it is helpful to think of the surface plasmon waves in the layer  26  as collective electronic displacements forming a standing array of dipoles having a dipole moment density ρ. Light scatters from the dipole array in a manner similar to that of light scattering from electrons in a transmission diffraction grating. This “dynamical grating” is present whenever the source  12  is on, and is not present when the source  12  is off. The grating also disappears if the frequency of the infrared radiation from the source  12  is significantly altered, or if the incidence angle β 1  is altered by more than about ±0.25 degrees, since only one frequency at a single corresponding incidence angle β 1  can excite surface plasmons. 
     As shown in FIG. 1, the system  10  includes a second photon emission source  28 , such as a continuous-wave ultraviolet or visible light laser. The second photon source  28  emits a beam of photons  30 , also referred to herein as a carrier beam or amplified beam, toward the area of intersection  27  of the beams  14   a  and  14   c  on the film  26  (step  118  in FIG.  2 B). In this preferred embodiment of the invention, the carrier beam  30  is at a higher frequency, and thus has a higher energy level, than the beams  14   a  and  14   c . The angle α of the carrier beam  30  relative to normal to the film  26  is variable, but is most preferably in the range of 15 to 45 degrees. However, one skilled in the art will appreciate that other values for the angle α could be used, and that the invention is not limited to any particular value of the angle α. 
     Interaction between the electric field vector Ε of the carrier beam of photons  30  and the surface plasmons generates an energy density equal to the negative of the vector dot product, −ρ·Ε. Generally, interaction between the carrier beam  30  and the standing wave of surface plasmons occurs if the dipole moment density ρ is nonzero (step  120 ). If the first beam of photons  14   a  is amplitude modulated, the dipole moment density ρ of the surface plasmons on the film  26  is likewise amplitude modulated, and hence the interaction (−ρ·Ε) is likewise modulated. In this manner, the interaction between the surface plasmons on the film  26  and the electric field of the carrier beam  30  transfers the amplitude modulation of the first beam  14   a  to the carrier beam  30  (step  122 ). 
     The modulation of the beam  30  occurs in a time interval determined by the response time of electrons in the film  26  at the top of the Fermi level. The time it takes light to traverse the film  26  is less than the period of the light, and thus the response frequency is actually of the order of 10 15  hertz (petahertz). However, the strength of the dipoles and the coupling to the dipoles may be such that some intensity levels of light are insufficient to engender the novel type of nonlinearity provided by the invention. In such event, the plano-convex lens  24  may be constructed from a highly-polarizable nonlinear medium to augment the effect. Since the field of the surface plasmons is exponential, their electromagnetic energy density is highly concentrated near the surface of the film  26 . This energy density is several orders of magnitude higher than that of the free-space wave, and the polarization is thereby greatly enhanced by the surface plasmons. Thus, the limit on response time is the response time of the electrons of the nonlinear medium over a very short distance. While this is greater than the response time of the electrons of the film  26 , the response frequency is nevertheless many orders of magnitude higher than any prior method of modulation or amplification of photonic signals. 
     Since the dipole moment density ρ is periodic with the wavelength of the surface plasmons, the interaction between the carrier beam  30  and the standing wave of surface plasmons scatters the photons of the carrier beam  30  into a diffraction pattern (step  124 ). As depicted in FIG. 1, the diffraction pattern includes a central diffracted energy peak  32   a  and higher order diffracted peaks  32   b  and  32   c  located spatially symmetrically on either side of the central energy peak. In the preferred embodiment of the invention, the diffracted energy peaks  32   a ,  32   b , and  32   c  are collected by corresponding photon detection devices  34   a ,  34   b , and  34   c  that are spatially arranged at points of maximum intensity of the associated diffracted energy peaks  32   a ,  32   b , and  32   c . The detection devices  34   a ,  34   b , and  34   c  are preferably photosensitive detectors. Alternatively, the detection devices  34   a ,  34   b , and  34   c  are optical systems or waveguides that collect the photons, and direct the collected photons to a photosensitive detector. The detection devices  34   a ,  34   b , and  34   c  may also be systems for converting the photons to a different energy level for purposes of improved propagation in waveguides. 
     Thus, as described above, diffracted photons which are modulated with the information contained in the first beam  14   a  are distributed to an array of detection devices  34   a ,  34   b , and  34   c , such as may form a portion of an optical communications network. Since each detection device  34   a ,  34   b , and  34   c  is assigned a corresponding portion  32   a ,  32   b , and  32   c  of the modulated photons of the carrier beam  30 , the information content is thereby distributed to the detection devices  34   a ,  34   b , and  34   c . Further, since the energy level of the carrier beam  30  of this embodiment is higher than that of the first beam  14   a , this embodiment acts an optical energy amplifier. 
     An alternative embodiment of the invention is depicted in FIGS.  3  and  4 A-B. As shown in FIG. 3, a first beam of photons  14  is emitted from the source  12  in a first direction (step  200  in FIG.  4 A). The first beam  14  impinges upon a beam splitter  16  which divides the first beam  14  into two preferably equal-amplitude beams, referred to herein as second and third beams of photons  18   a  and  18   b  (step  202 ). From the beam splitter  16 , the second and third beams of photons  18   a  and  18   b  impinge upon a beam directing structure  20 , which preferably includes first and second mirrors  21   a - 21   b  and mirror positioners  22   a - 22   b . The first mirror  21   a  redirects the second beam of photons  18   a  in a second direction (step  204 ), and the second mirror  21   b  redirects the second beam of photons  18   b  in a third direction (step  206 ). As shown in FIG. 3, there is preferably an angle of 2β 1  between the first and second beams of photons  18   a - 18   b  after the beams are redirected in the first and second directions. In the preferred embodiment, the distances between the beam splitter  16  and each of the mirrors  21   a - 21   b  are equivalent, thereby providing equivalent phase delays for the two beams  18   a - 18   b  for the paths traveled from the beam splitter  16 . 
     From the mirrors  21   a - 21   b , the second and third beams of photons  18   a - 18   b  pass through the plano-convex cylindrical lens  24 , and impinge upon the film  26  at the intersection plane from the second and third directions at an angle of incidence β 1  relative to a normal to the surface of the film  26 . As depicted in FIG. 3, the second beam  18   a  intersects with the third beam  18   b  in the intersection area  27  on the surface of the film  26  (step  208 ). Thus, the two beams  18   a - 18   b  which were split from the beam  14  are directed opposite one another, but with each incident on the layer  26  at substantially the same angle β 1  within a tolerance of about ±0.25 degrees. With this preferred geometry, the second beam of photons  18   a  forms a first surface plasmon wave in the layer  26  which is correlated in phase to the second beam  18   a  (step  210 ), and the third beam of photons  18   b  forms a second surface plasmon wave in the layer  26  which is correlated in phase to the third beam  18   b  (step  212 ). The two oppositely directed surface plasmon waves, which are correlated in phase, interact to form a standing wave of surface plasmons (step  214 ). The modulation of the carrier beam  30  of this embodiment occurs in substantially the same manner as described above for the preferred embodiment. Thus, steps  216 - 222  in FIG. 4B are performed in substantially the same manner as steps  118 - 124  of FIG. 2B, as described above. 
     Another alternative embodiment of the invention provides for multiple carrier beams operating at different wavelengths. For example, as depicted in FIG. 5, a third photon emission source  36  may be provided, which is most preferably a continuous-wave ultraviolet or visible light laser operating at a wavelength different from that of the second photon emission source  28 . The third photon emission source  36  emits a beam of photons  38 , also referred to herein as a second carrier beam, toward the area of intersection  27  of the beams  14   a  and  14   c  on the film  26 . 
     The second carrier beam  38  interacts with the standing wave of surface plasmons on the film  26  in the same manner as described above, thereby producing a central diffracted energy peak  40   a  and higher order diffracted peaks  40   b  and  40   c  on either side of the central energy peak  40   a . Preferably, the diffracted energy peaks  40   a ,  40   b , and  40   c  are collected by corresponding photon detection devices  42   a ,  42   b , and  42   c . Thus, the information modulated on the first beam of photons  14   a  may be transferred to the diffracted components  40   a ,  40   b , and  40   c  of the second carrier beam  38 . 
     Using the embodiment of FIG. 5, information from the first beam  14   a  may be passed to beams of multiple wavelengths, with photons of each wavelength distributed in different and controllable directions, where the directions are determined by the equations of diffraction. Generally, the equations of diffraction provide that the angular separation between successive orders of diffracted photons may be expressed approximately for small angles by the ratio of their wavelength to the distance between successive diffracting structure maxima. In the case of a standing wave of surface plasmons, the distance between structure maxima is one half of the surface plasmon wavelength. Thus, the angular separation between the diffracted components  40   a ,  40   b , and  40   c  is predictable and controllable, and is different from the angular separation between the diffracted components  32   a ,  32   b , and  32   c . In this manner, the embodiment of FIG. 5 provides a means for controllably distributing information elements that are modulated on carrier beams of different wavelengths to spatially separated detection devices. 
     Yet another embodiment of the invention is depicted in FIG.  6 . This embodiment provides a fourth photon emission source  44 , which is most preferably a diode laser operating at an infrared wavelength different from the wavelength of the first photon emission source  12 . The fourth photon source  44  emits a modulated fourth beam of photons  46   a  toward the film  26  at an optimal angle of incidence β 2  relative to normal to the surface of the film  26 . Since the wavelength of the beam  46   a  is different from the wavelength of the beam  14   a , the optimal incidence angle β 2  for excitation of surface plasmons is different from the incidence angle β 1 . The s-polarization component of the beam  46   a  is reflected from the film  26  to form a beam  46   b , which is reflected from a mirror  52  and polarization-rotated by a polarization rotator  48  to form a p-polarized beam  46   c . The surface plasmon interaction of the beams  46   a  and  46   c  modulates the carrier beam  30  in substantially the same manner as described above. The difference in optimal incidence angles β 1  and β 2  provides for angular, and therefore spatial, separation between the mirror  52  and the mirror  21 . 
     One of the advantages of using multiple emission sources, such as  12  and  44 , is that multiple energy levels can be utilized, such as may occur in optical communication systems that use multiple energy levels. Generally, as long as the photons of the carrier beam  30  have energy below the energy of the photons of the first and fourth beams  14  and  46 , operation of such an embodiment is practicable. It should be appreciated that further modulated beams at other wavelengths and incident on the film  26  from other incidence angles could also be added to the embodiment of FIG.  6 . 
     The foregoing description of preferred embodiments for this invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise form disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiments are chosen and described in an effort to provide the best illustrations of the principles of the invention and its practical application, and to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as is suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.