Abstract:
An optical system for high speed transmission of optical data. The system may condition light signals from a source for projection into an optical medium that is to convey the signals with high speed to another place. This conditioning may result in the light having an annular intensity distribution or profile. Much of the intensity of the light is near the periphery of the optical medium. This medium may be an optical fiber. This annular distribution may be attained with an optical element having a slope discontinuity or light from it being defocused to a certain extent at the optical medium. Either of these characteristics or both of them may used in the optical system so it can transmit light signals at very high rates.

Description:
BACKGROUND  
         [0001]    The present invention pertains to optical transmission of signals and more particularly to high speed light signal transmission in optical fibers.  
           [0002]    Achieving a high gigahertz bit per second data rate in an optical fiber system is difficult and requires careful control of intensity distribution of light signals at the input face of the optical fiber in the system.  
         SUMMARY  
         [0003]    A feature of the present invention is attaining a gigahertz bit per second data rate in an optical fiber. Robust compliance with a Telecommunications Industry Association specification is sought. Light distribution of a particular profile at a face of the optical fiber is a factor to achievement of the high speed conveyance of light signals. Optical element design and a certain focus are several elements of a high speed optical system. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]    [0004]FIG. 1 is a diagram of a high speed optical system;  
         [0005]    [0005]FIG. 2 is a graph of an intensity distribution curve of a corrected optic projecting effectively a point source of light;  
         [0006]    [0006]FIG. 3 is a graph of an intensity distribution of an annular projection of light;  
         [0007]    [0007]FIGS. 4 and 5 show cross-sectionals of lenses having slope discontinuities;  
         [0008]    [0008]FIG. 6 reveals several focal adjustments of an optical element;  
         [0009]    [0009]FIG. 7 is a set of spot diagrams of intensity profiles of defocused and focused light.  
         [0010]    [0010]FIG. 8 is a schematic of the optical system having the lens slope discontinuity and defocus features. 
     
    
     DESCRIPTION  
       [0011]    [0011]FIG. 1 shows a layout of an optical system  10  for coupling light signals at very high rates. A laser light source  11 , such as a vertical cavity surface emitting laser (VCSEL) may emit light signals  12  which go through a transfer optical element  13 . From optical element  13 , light rays  14  may impinge on a core or face  15  of an optical fiber  16  which may be a multi-mode fiber. Rays  14  may propagate through fiber  16  and exit fiber  16  at core or face  17  as light rays or signals  18 . The light rays or signals  12  may be conditioned into light rays or signals  14  to make high speed transmission through optical fiber  16  or other like medium possible.  
         [0012]    Achieving, for instance, a ten gigahertz bit per second data rate in fiber  16  with an approximately 2000 megahertz kilometer bandwidth may require careful control of the intensity distribution of light  14  at fiber face  15 , i.e., a launch condition. An industry specification specifies a power distribution at the output fiber face sufficient to achieve a 2000 MHz-Km bandwidth-distance product in a 500 MHz-Km GI fiber. The Telecommunications Industry Association (TIA)/Electronic Industries Alliance (EIA)-492AAAB specification (hereafter “TIA specification”) effectively says that a 2000 MHz-Km bandwidth at 850 nm through 50/125-micron graded index multimode fiber can be achieved if at the end of the fiber the encircled flux within a radius of 4.5 microns is less than or equal to 30 percent of the total and the encircled flux within a radius of 19 microns is equal to or greater than 86 percent of the total encircled flux. An example of the fiber may be Corning&#39;s standard 50/125 multimode fiber which has a core radius of 25 microns and a cladding radius of 75 microns. The core and cladding indexes of refraction are 1.4948 and 1.4800, respectively. The wavelength is 850 nm.  
         [0013]    Various illustrative examples of the present invention may provide the appropriate distribution of power into fiber  16  to achieve the data rate performance of a 10 gigahertz bit per second operation at 850 nm that is compliant with the above-noted TIA specification. The power of light source  11  may be redistributed by optical element  13  from the center to the outskirts of the beam which is projected on to core  15  of fiber  16 . The velocities of the various modes of light are more diverse closer to the center of core  15  than the velocities of the modes of light closer to the perimeter of core  15 . Since the velocities of the modes of light near the circumference of core  15  are close together, a light pulse having its flux or power concentrated more towards the perimeter will come through fiber  16  tighter and more distinguished in shape. This closeness of velocities of the various modes makes possible for very high rates of data transmission. That is at least one reason for the outer concentration of the power of light signals in core  15 .  
         [0014]    Attaining a power distribution of light on the end face core  15  of fiber  16  may be tried with a conventional, well-corrected, aspheric transfer optics as an optical element  13  that is adjusted to the best focus. This kind of optics may not be sufficient because compliance with the TIA specification could be achieved only for a few special modes, such as mode  2 , 1 of a VCSEL as a light source  11 . Robust compliance for a wide range of modes, for instance, of a VCSEL, and with tolerance of lateral and axial misalignment of the projected light from the optical element to core  15  may be attained with an optical element  16  whose point-source distribution function or point spread function (PSF) at the fiber face of core  15  complies with the TIA specification. PSF refers to a distribution of light on the fiber core face from a point source. The point source may radiate light in a spherical manner but only a cone of the light is captured by the optical element. The outgoing light from the optical element may be converged to a point, for example, with a lens. However, the reality is that the light source is not actually a point, and that diffraction and aberration, among other imperfections, prevent the light from being focused as a point on the fiber face. Even if the source were a point, the diffraction and aberration of the transfer optics or optical element  16  would prevent the projection of a point of light on the fiber  16  end face. A well corrected optic would have distribution curve  19 , as shown in FIG. 2, on the fiber  16  end face. However, curve  19  does not comply with the TIA specification needed to achieve the 10 gigahertz bit per second data rate in the 2000 MHz-Km multimode fiber using 850 nm light. In order to get the power or flux distribution needed by the specification, one may maintain an annular intensity profile on the fiber face after convolving the PSF with the finite light source aperture, apodizing the complex source model amplitude, and including optical magnification. FIG. 3 illustrates an example of an annular intensity profile  20  on a face of core  15  of fiber  16 . The normalized incident field amplitude or intensity of light is shown on the ordinate axis and the cross-sectional distance in microns from the center of the core  17  face of fiber  16  for each amplitude is shown by the abscissa axis.  
         [0015]    Two characteristics of optical element  16 , taken singly or in combination, may produce the light launch profile on fiber face core  15  and maintain robust compliance with the encircled flux conditions of the TIA specification. First, one surface  21  or  22  of optical element  13  may have a slope discontinuity at an optical axis  23  (r=0; r being the distance radially or perpendicularly from the optical axis, from the optical axis). This characteristic provides an axicon function to optical element  13 . The optical prescription for surface  22 , for example, may be a surface of revolution about optical axis  23 . This functionality may be implemented by including it in the surface prescription having an odd power of radius. An axicon function or lens may be used to convert a parallel laser beam into a ring, a doughnut shaped ablation or an annular intensity profile  20  shown in FIG. 3. A surface  22  discontinuity may put into effect the axicon function, phenomenon or lens to produce the annular intensity profile  20  on the face of fiber  16 . An illustration of surface  22  having a slope discontinuity at optical axis  23  is shown in FIG. 4. Line  24  shows the slope of the upper part of surface  22  at optical axis  23  (r=0). Line  25  shows the slope of the lower part of surface  22  at optical axis  23 . As one follows surface  22  across axis  23 , there is a disruptive change of slope from slope  24  to slope  25 . Slope discontinuities may be implemented in various ways. FIG. 5 shows a slope or curvature discontinuity  34  as a small notch-like shape, cusp, indentation or protrusion in surface  22  at area  26  about optical axis  23 . Discontinuity  34  may be sharp, abrupt, rough or smooth. Discontinuity  34  may be of any shape or contour. Elsewhere, the slope may be continuous, such as a function of the distance from optical axis  23  or of the radius, except at optical axis  23 . Discontinuity  34  of slope of surface  23  may appear imperceptible to the eye. Apart from point or area  26 , surface  22  may aspherical or spherical. Surface  21  of optical element  13  may instead have the slope discontinuity.  
         [0016]    An illustrative example of lens surface specifications for optic element  13  may be in the following formulas, constants and variables for each of the surfaces. Surface  1  may be surface  21  and surface  2  may be surface  22  in FIG. 1, or vice versa.  
         [0017]    Surface  1   
           z={cr   2 /[1+(1−(1 +k ) c   2   r   2 ) 1/2   ]}+A   1   r   1   +A   2   r   2   +A   4   r   4   +A   6   r   6    
         [0018]    c=1/R; R=0.65943 mm  
         [0019]    k=−1.701593  
         [0020]    A 1 =0  
         [0021]    A 2 =0  
         [0022]    A 4 =0.062933  
         [0023]    A 6 =−0.01539  
         [0024]    Surface  2   
           z={cr   2 /[1+(1−(1 +k ) c   2   r   2 ) 1/2   ]}+A   1   r   1   +A   2   r   2   +A   4   r   4   +A   6   r   6    
         [0025]    c=1/R; R=−2.015644 mm  
         [0026]    k=−5.212050  
         [0027]    A 1 =0.025409  
         [0028]    A 2 =0.012167  
         [0029]    A 4 =0  
         [0030]    A 6 =0  
         [0031]    The second characteristic which may be implemented to produce a launch profile having an annular intensity distribution or profile, similar to profile  20  of FIG. 3, is the defocusing of optical element  13  relative to the face of core  15  at a fiber  16  end. This defocusing may result in an intensity profile sufficient to attain compliance with the TIA specification. Optical element  16  is defocused to a region corresponding to approximately ±8λ(f/) 2 . This characteristic may result in the annular or ring-like distribution of light intensity. The area of low or no intensity in the center of the ring or annular distribution may be referred to as the dark spot of Arago in a well-corrected optic. FIG. 6 reveals three focus positions of optical element  13 . Position  27  shows an annular intensity profile of light  14  launched on fiber  16  face of core  15 . The intensity is shown by coordinate I and the distance from optical axis  23  is shown by coordinate R. Position  28  shows a profile having the intensity of light  14  concentrated on optical axis  23 . Position  29  shows an annular intensity profile similar to the profile of position  27 . Position  28  is a focused place for the core  15  face and positions  27  and  29  are defocused places for the face of core  15  to receive launched light  14 . Either position  27  or  29  may be used to achieve the annular distribution of light intensity on the face of core  15 .  
         [0032]    [0032]FIG. 7 reveals an illustrative example of spot diagrams of the intensity profiles as seen on the core  15  end face of fiber  16  to show the defocus characteristic of system  10  for attaining the annular distribution of light intensity so as to comply with the TIA specification. Focus occurs at about 40 microns in FIG. 7 for spot  31 . The best annulus at defocus occurs at 0 microns of adjustment for spot  30 . Scale  32  shows the size of the intensity concentrations for the spots in the diagram.  
         [0033]    Optical system  10  may incorporate both the axicon feature and the annular PSF at defocus, even though either one alone may suffice for attaining compliance with the TIA specification. Incorporating the characteristics or elements is system  10  of FIG. 8. It may be referred to as the “Ringlight” system. Light source  11  may be a VCSEL having about an 8 micron aperture. Between source  11  and optical element  13  is a BK7™ window  33  which may be part of the vacuum sealed package containing the VCSEL. The window is about 0.203 mm thick and its inner surface is about 1.145 mm from the base of the VCSEL. Window  33  is about 0.3 mm from surface  21  of optical element  13 . Optical element  13  is about 2.8 mm long and about 1.7 mm in diameter. Surface  21  may have an even asphere curvature and incorporate a slope discontinuity  26  on axis  23 . Surface  22  may have a SPS type or odd asphere surface. Lens surface  21  or  22  or both surfaces may include a hyperbolic (collimating) surface for receiving and collimating light originating from light source  11 . Also, optical element  13  may be shaped so that light reflected back-toward light source  11  is not focused at a location where light  12  is emitted by source  11 . Optical element  13  may be one piece and be made or molded from Ultem R  1010 which is a General Electric Company plastic. Surface  22  may be about 0.85 mm from the face of core  15  of multimode optical fiber  16 . VCSEL  11  may emit light signals  12  which propagate through window  33 , surface  21  and optical element  13 . The signals may exit element  13  as light  14  that may be launched into core  15  of fiber  16 .  
         [0034]    Although the invention has been described with respect to at least one illustrative embodiment, many variations and modifications will become apparent to those skilled in the art upon reading the present specification. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.