Patent Publication Number: US-2005129367-A1

Title: Method and apparatus for angled fiber optical attenuation

Description:
BACKGROUND OF THE INVENTION  
      The present invention relates generally to fiber optic devices, and more particularly to fiber optic light attenuation when coupling light between a fiber and an active device.  
      Fiber optic data links require a transmitter source to transmit light to a receiver via optical fiber. In order to effectively couple light between a fiber and an active device (laser or light emitting diode (LED)), the end of the fiber is polished to a 45 degree angle. This 45 degree angle allows coupling of light into or out of the fiber at a 90 degree angle to the fiber. The 45 degree angled surface acts as an internal mirror reflecting light into or out of the fiber core. In order for the system to transmit data, the receiver requires optical power within a certain level. If there is more optical power than required the receiver will become saturated. To avoid saturation, manufacturers use three traditional techniques to attenuate optical power: 1) a coated window between fibers and photodiode elements, 2) fiber attenuator or airgap between connectors and 3) coating the fiber ends to attenuate optical power.  
      All of these attenuation techniques increase costs, or are not highly robust and practical. Coating is expensive and increases the risk of damage to the fiber. Fiber air spaced attenuators add to the cost by creating another fiber connection between the transmit (Tx) and receive (Rx), increasing the weight and size of the fiber link. Adding extra fiber to attain the desired attenuation could mean adding an extra kilometer or more of fiber to a 1 meter fiber link, which would add cost and weight and take up valuable space.  
      There is thus a need in the art for systems and methods that couple light between a fiber and an active device and provide practical, robust and inexpensive attenuation of the optical power between the active device and the fiber to avoid saturation of a receiver.  
     SUMMARY OF THE INVENTION  
      The present invention advantageously addresses the needs above as well as other needs by providing an inexpensive, robust and practical method and apparatus for angled fiber optical attenuation to avoid saturation of the receiver.  
      In one embodiment, the invention can be characterized as a fiber optic device comprising an optical fiber with a fiber core having a fiber core cladding interface. The optical fiber has an angled polished end surface that is at an acute angle other than 45 degrees from a side of the fiber.  
      In another embodiment, the invention can be characterized as a fiber optic device comprising a plurality of optical fibers operationally coupled together to form an opto-electronic array module. There is a fiber core within each of the optical fibers, each core having a fiber core cladding interface. Also at least two of the optical fibers has an angled polished end surface. The polished end surface is at an acute angle other than 45 degrees from a side of the fiber.  
      In yet another embodiment, the invention can be characterized as a method for attenuating the amount of light entering an optical fiber comprising the steps of receiving light into an optical fiber and reflecting the light off a polished end surface of the optical fiber into a core of the optical fiber. The polished end surface is at an acute angle other than 45 degrees from a side of the optical fiber, thereby attenuating the light.  
      A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description of the invention and accompanying drawings which set forth an illustrative embodiment in which the principles of the invention are utilized. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other aspects, features and advantages of the present invention will be more apparent from the following more particular description thereof, presented in conjunction with the following drawings wherein:  
       FIG. 1  is a cross sectional side view of a fiber optic device according to an embodiment of the present invention;  
       FIG. 2  is a graph showing fiber angle vs. optical attenuation according to an embodiment of the present invention; and  
       FIG. 3  is a cross sectional side view of a fiber optic device according to an alternative embodiment of the present invention. 
    
    
      Corresponding reference characters indicate corresponding components throughout the several views of the drawings.  
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The following description of the presently contemplated best mode of practicing the invention is not to be taken in a limiting sense, but is made merely for the purpose of describing the general principles of the invention. The scope of the invention should be determined with reference to the claims.  
      Referring to  FIG. 1 , shown is a cross sectional side view of a fiber optic device according to an embodiment of the present invention.  
      Shown is an optical fiber  100 , a fiber core cladding interface  105  of the optical fiber  100  and an active device  110  (a laser or LED, for example). The optical fiber  100  has a polished surface  115 . The internal reflection  120  from the polished surface  115  is shown as well as the optical ray  125  guided by the fiber  100 .  
      The polished surface  115  on the end of the optical fiber  100  is at an angle below 45 degrees. Preferably, the angle is no less than approximately 40 degrees, but may be lower.  
      Light from the active device  110  enters the optical fiber  100  substantially perpendicular to a side of the optical fiber  100  before being reflected  120  by the polished surface  115  into the optical fiber  100 . The optical ray  125  is then guided by the fiber core cladding interface  105  of the optical fiber  100 . Due to the fact that the polished surface is at an angle different than 45 degrees the amount of light entering the fiber  100  is attenuated to eliminate saturation of an opto-electronic receiver (not shown) in a fiber optic transceiver system. The larger the difference of the angle of the polished surface  115  is from 45 degrees, the higher the degree of attenuation.  
      Referring next to  FIG. 2  depicted is a graph showing fiber angle vs. optical attenuation according to an embodiment of the present invention. Such attenuation is useful in opto-electronic array modules that use Vertical Cavity Surface Emitting Laser (VCSEL) and photodiode arrays. During development of 12-channel transceiver arrays, the above attenuation approach was tested and it was found that by attenuating the coupling of light into the fibers  100  by −3 dBm, the eye diagram and bit error rate (BER) showed significant improvement at 2.0 Gb/s data rate for direct, “back to back” transceiver link testing.  
      As shown in the graph of  FIG. 2 , the polished surface  115  of the fiber  100  was angled to 40 degrees to achieve a −3 dBm attenuation change in the light coupling between the fiber and VCSEL array. This technique may also work with data rates above or below 2.0 Gb/s. The angle of the polished surface  115  can be varied according to the graph of  FIG. 2  to achieve a desired attenuation. Typically, a desirable change in attenuation is of approximately 5 degrees or less. In most cases attenuation steps of 1.0 dBm is acceptable.  
      Referring next to  FIG. 3 , shown is a cross sectional side view of a fiber optic device according to an alternative embodiment of the present invention.  
      Shown is an optical fiber  100 , a fiber core cladding interface  105  of the optical fiber  100  and an active device  110  (a laser or LED, for example). The optical fiber  100  has a polished surface  215 . The internal reflection  120  from the polished surface  215  is shown as well as the optical ray  125  guided by the fiber  100 .  
      The polished surface  215  on the end of the optical fiber  100  is at an angle above 45 degrees. Preferably, the angle is no more than approximately 45 degrees, but may be higher.  
      The amount of light entering the fiber  100  having the polished surface  215  above 45 degrees is attenuated in the same fashion as described above for a polished surface at an angle below 45 degrees. Likewise, the larger the difference of the angle of the polished surface  215  is from 45 degrees, the higher the degree of attenuation results.  
      While the invention herein disclosed has been described by means of specific embodiments and applications thereof, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention set forth in the claims.