Abstract:
An attenuator for use in a wavelength division multiplexer (WDM) uses an opaque knife-edge as a light attenuator. The attenuator is moved along a single axis for controllably blocking the light output of an optical fiber whose light output is to be attenuated. By selectively moving the edge of the attenuator in front of the optical fiber, the attenuator can block any amount of the light output. Multiple attenuators are incorporated in a WDM, each attenuator being used in a channel associated with a particular narrow band of wavelengths.

Description:
FIELD OF THE INVENTION 
     This invention relates to fiber optics communications and, in particular, to an attenuator for optical signals in an optical fiber. 
     BACKGROUND 
     The relatively wide bandwidth of light that may be transmitted through a conventional optical fiber enables multiple light signals, each at a different wavelength, to be multiplexed and transmitted simultaneously over the same optical fiber. Such a technique is called wavelength division multiplexing (WDM). It is common for a single optical fiber to simultaneously transmit  16  or more multiplexed channels for any form of communication, including telephone communications and cable television. 
     In WDM, the signals (either electrical or optical) to be conveyed on each channel are converted into light signals within a narrow band of wavelengths (e.g., 2 nanometers) associated with a particular channel. A 16 channel WDM would use a total bandwidth of about 32 nanometers. A common center wavelength is on the order of 1500-1600 nanometers. 
     Converting an electrical or optical signal into a particular narrow band of wavelengths is well known. For example, an electrical signal may be applied to a particular type of laser diode which generates wavelengths within a particular bandwidth. Other techniques may include converting the electrical signal into a light signal and eliminating unwanted wavelengths. Some devices for extracting a specific narrow band of wavelengths from an optical signal include: 1) a tuned waveguide; 2) a diffraction grading; 3) a taper filter; and 4) other types of filters, such as a coated silica substrate where certain wavelengths are refracted and other wavelengths are reflected. 
     The process of causing the optical signals to be within a particular narrow bandwidth also typically causes the optical intensities to differ for each channel. As a result, after the optical signals for the channels have been limited to their respective optical bandwidths, such as shown in FIG. 1, each of these optical signals must be attenuated so that the light intensity transmitted is equal for each channel and is of a predetermined level. This is so that the transmission performance for each channel is predictable. Such attenuators for each of the three channels ( 1 ,  2 , and n) shown in FIG. 1 include attenuators  12 ,  13 , and  14  for attenuating the optical signals in optical fibers  16 ,  17 , and  18 , respectively. Similar attenuators reside in a demultiplexer  19 . 
     FIG. 2 illustrates the intensity levels of the optical signals in each of the three channels, each optical signal being within a different narrow bandwidth of light. As seen, the intensity of the optical signal in channel  1  prior to attenuation is greater than that of the optical signals in channels  2  and  3 , and the optical signal in channel  3  is greater than the intensity of the optical signal in channel  2 . Attenuators  12 ,  13 , and  14  serve to equalize the intensity levels of the three channels by selectively lowering the overall intensity of the higher intensity signals to equal that of the lowest intensity signal. One such attenuator will be discussed later with respect to FIGS. 3,  4 A, and  4 B. 
     The light outputs from the attenuators  12 - 14  are then applied to optical fibers  20 ,  21 , and  22  and combined into a single optical fiber  24  so as to multiplex the n channels onto a single optical fiber. Hence, the device of FIG. 1 acts as a multiplexer to simultaneously transmit multiple channels, each at a different light bandwidth, along the same optical fiber. Additional multiplexers may be employed to multiplex additional channels on other optical fibers. The optical fibers may then be bundled in a cable for transmitting many optical signals. 
     Ultimately, the signals on the optical fiber  24  are demultiplexed by a demultiplexer  19  to separate out the various wavelengths of light into separate channels using well known means. These separate channels are then attenuated to have equal, predetermined intensities and converted into electrical signals, if required, for various applications such as by using photodetectors. Such demultiplexers include detraction gratings and filters which may be tuned to transmit a narrow range of predetermined wavelengths. 
     The attenuation levels in the multiplexer and demultiplexer may be determined empirically. 
     One popular prior art technique for attenuating the intensity of a light output within a narrow band of wavelengths uses a neutral density filter for each of the wavelength bands of interest. Such a filter removes a selected amount of light depending on where the light impinges upon the filter. FIG. 3 illustrates a neutral density filter  30  composed of a silica substrate  32  with a coating  34  composed of material for progressively absorbing the light output of a fiber optic cable  36  as filter  30  is moved in the direction of arrow  38 . The percentage of absorption of light output from cable  36  with respect to each area of filter  30  is identified in FIG.  3 . The light exiting filter  30  is received by a fiber optic cable  40 . It would be understood that additional optics, such as collimators, may be used at the ends of the fiber optic cables  36  and  40  to cause the light between the two cables to be collimated. 
     The filter  30  is adjusted in the direction of arrow  38  using a micrometer to select the desired amount of attenuation. 
     FIG. 4A illustrates the ideal light energy versus time for a number of pulses of the attenuated light received by fiber optic cable  40 . In reality, however, this light signal contains ripples and other distortions, as shown in FIG. 4B, due to reflections at the interface of filter  30  causing constructive and destructive interference. Further, an inherent property of the silica  32  and the coating  34  is that there is always some attenuation even at the minimum attenuation level of filter  30 . 
     What is needed is a light attenuator for a WDM system which is inexpensive, reliable, and does not suffer from the performance drawbacks of the prior art attenuators. 
     SUMMARY 
     In one embodiment, an attenuator for use in a wavelength division multiplexer (WDM) uses an opaque (e.g., metal) wedge-shaped device, referred to as a knife-edge, having a substantially triangular face which controllably blocks the light output of an optical fiber whose light output is to be attenuated. By selectively moving the knife-edge of the triangular face in front of the optical fiber, the attenuator can block any amount of the light output. The position of the attenuator in one embodiment is adjusted by means of a fine screw (e.g., a micrometer) which acts as a potentiometer control. 
     The use of such an attenuator instead of a neutral density filter includes the advantages of: 1) no noise (ripple) due to reflections and interference; 2) no residual attenuation so that the attenuation can be zero; 3) a wide dynamic range (0%-100%); 4) high stability; and 5) compact size. 
     A preferred embodiment attenuator includes a wedge-shaped knife-edge attenuator where the substantially triangular face has a beveled light blocking portion so as not to be directly orthogonal to the light output. Any reflections of light from the beveled portion do not reflect back into the impinging light so as to avoid any interference between the impinging and reflected light. 
     To minimize reflections, the knife-edge attenuator is essentially a black color, such as anodized aluminum. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a prior art wavelength division multiplexing and demultiplexing system. 
     FIG. 2 illustrates the level of intensity of the optical signals in each of the three channels of FIG. 1 prior to attenuation. 
     FIG. 3 illustrates the use of a neutral density filter as an attenuator in the device of FIG.  1 . 
     FIGS. 4A and 4B illustrate the ideal and actual light outputs, respectively, for various wavelength bands from the neutral density filter of FIG.  3 . 
     FIG. 5 illustrates the wedge-shaped attenuator variably inserted in the path of an optical beam between an output optical fiber and an input optical fiber. 
     FIG. 6 illustrates the impinging light beam on the attenuator of FIG. 5 as the attenuator is moved along the direction of the arrow. 
     FIG. 7A illustrates a preferred embodiment of the attenuator having a beveled edge for selectively blocking light from a light source to minimize reflections back into the impinging beam. 
     FIG. 7B is a top down view of the attenuator of FIG. 7A illustrating the reflection of light from the beveled edge. 
     FIG. 8 illustrates the attenuator of FIG. 7A in various positions relative to an impinging light beam showing the various degrees of attenuation. 
     FIG. 9 illustrates one embodiment of the attenuator apparatus including terminations for incoming and outgoing optical fibers as well as an adjustment for the knife-edge attenuator to select the amount of attenuation of the light between the source optical fiber and the receiving optical fiber. 
     FIG. 10 is a top down view of the device of FIG. 9 showing a screw adjustment for controlling the light attenuation. 
     FIG. 11 illustrates the attenuator portion removed from the device of FIG.  9 . 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     FIG. 5 illustrates an attenuation system which includes a transmitting optical fiber  50 , a collimator  52  for collimating the light output of fiber  50 , a wedge-shaped movable attenuator  54  (also referred to as a knife-edge), a receiving collimator  56 , and a receiving optical fiber  58 . Optical fibers  50  and  58  are conventional as are collimators  52  and  56 . Collimators  52  and  56  may be any commercial collimator. Collimators may be in the form of a single lens, a compound lens, or a gradient index (GRIN) lens. Such lenses are well known and described in Optics Guide 5, 1990, by Melles Griot Inc., incorporated by reference. Collimator  52  collimates the light output from fiber  50 , as shown in the dashed-line light output  60  of collimator  52 . Without collimator  52 , the light output from fiber  50  would scatter and diffuse. 
     Attenuator  54  is formed of a material which is opaque to the wavelength of interest. In one embodiment, attenuator  54  is aluminum or any other metal and is anodized so as to be black to reflect very little of the impinging light from collimator  52 . In one embodiment, the length of attenuator  54  is on the order of 8 mm. Attenuator  54  may have any shape which allows an edge to be selectively positioned in the optical path to attenuate the light. 
     Attenuator  54  is moved in the direction of arrow  62 , in this case up or down, so as to block a selected amount (shown as shaded portion  63 ) of the light output  60  of collimator  52  to achieve the desired attenuation. The light that is not blocked procedes to the input of collimator  56 , which focuses the light into the receiving fiber  58 . The light from fiber  58  may then be combined with the attenuated light from other channels, such as shown in FIG. 1, so as to multiplex the various signals at different wavelengths onto a single fiber for long distance transmission. 
     FIG. 6 illustrates attenuator  54  from FIG. 5 at different positions relative to a fixed impinging light spot  64  to achieve various attenuations identified in FIG.  6 . 
     Although the black surface of attenuator  54  reflects little of the impinging light, any reflection back into the impinging light could cause interference and distort the signal. The shape of the knife-edge attenuator  66  of FIG. 7A eliminates such reflection into the impinging light. This is achieved by providing attenuator  66  with a beveled surface portion  68  on which the light output of collimator  52  (FIG. 5) impinges. 
     FIG. 7B shows a top down view of the attenuator  66  and illustrates an impinging light beam  70  having a portion  72  reflected away from the impinging light beam and a second portion  74  passing attenuator  66  for receipt by a receiving optical fiber. 
     In one embodiment, the length of attenuator  66  is 8 mm, the width at the large end of attenuator  66  is 3.2 mm, the width at the narrow end of attenuator  66  is 1.5 mm, the width of the beveled portion  68  is 1 mm, the thickness of the non-beveled portion is 0.7 mm, and the angle of the beveled portion with respect to the flat surface of attenuator  66  is 8°. Other sizes and angles would also suffice. For example, the bevel angle can be anywhere from 1° to in excess of 45° while still providing the benefits of the bevel. 
     In one embodiment, the means for shifting attenuator  66  up and down in the direction of arrow  62  includes a block  78  adhesively fixed to a surface of attenuator  66 , where block  78  includes a threaded screw hole  80  through which an adjustment screw is inserted (forming a micrometer). As the screw is turned, attenuator  66  is moved up and down relative to the screw to control the attenuation. Block  78  may have a height of 3 mm and a width a little larger than the width of attenuator  66  to act as a guide (illustrated in FIG. 9) to limit rotational movement of attenuator  66 . 
     FIG. 8 identifies the attenuation for various positions of the attenuator  66  with respect to the fixed light output  82  of the fiber. The position of attenuator  66  may be linearly adjusted by a screw to provide from 0% to 100% attenuation. 
     FIG. 9 illustrates a light attenuation unit  83  incorporating the attenuator  66  shown in FIG.  7 A. The attenuator unit  83  is provided within a sealed housing  84 . The lid of housing  84  has been removed. An input fiber  85  enters through an opening in housing  84  and is terminated at an input to a collimator  86  using well-known techniques. Collimator  86  is supported by an internal structure  88 . The output of collimator  86  is a collimated beam. 
     An internal support  90  retains adjusting screw  92  and, in conjunction with the threaded block  78 , restricts the rotational movement of the attenuator  66 . Support  90  may be formed of metal or plastic. The adjusting screw  92  extends through threads in block  78  as described with respect to FIG. 7A so that turning screw  92  causes attenuator  66  to move up and down with respect to the support  90  to achieve the desired attenuation of the light beam. 
     The light beam exiting collimator  86  proceeds through a hole  91  formed in support  90 , is selectively attenuated by the edge of attenuator  66 , exits through another hole  92  formed in support  90 , and enters a receiving collimator  96 . 
     Collimator  96  properly focuses the attenuated beam onto a receiving fiber  98  whose light may be output to a combinor (not shown) for combining the signals from multiple fibers into a single fiber for long distance transmission. The device of FIG. 9 may serve as an attenuator  12 ,  13 , or  14  in FIG. 1, or any attenuator in the demultiplexer  19 . In one embodiment, the attenuator is incorporated in a WDM system for a telecommunications network, such as a cable television network. 
     FIG. 10 is a top down view of the structure of FIG. 9 showing the top of screw  92  for adjustment. 
     FIG. 11 shows the attenuator portion removed from unit  83 . 
     It would be understood that there are many types of controllers for adjusting the position of attenuator  66  with respect to the collimated beam. Such transport means may include a motor operated transport, a piezoelectric transducer, or any other known means. Further, the adjustment of the attenuator may be automatic by using a feedback circuit for sensing the intensity levels of the various optical signals from the various channels and adjusting the attenuators  66  until the light levels of the channels are equal. 
     The optical signals may be supplied to the attenuator via a laser beam, waveguides, or other transmission paths, rather than optical fibers. 
     While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from this invention in its broader aspects and, therefore, the appended claims are to encompass within their scope all such changes and modifications as fall within the true spirit and scope of this invention.