Abstract:
A telecentric 1×N optical switch according to the present invention switches between output fibers without the need for active alignment by utilizing a telecentric lens group translated in directions perpendicular to the axis of the input fiber. The telecentric lens group directs the input beam to the selected output fiber by translating to a location associated with the selected fiber.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to apparatus and methods for coupling an input optical fiber selectively to one of a plurality of output fibers. In particular, the present invention is a 1×N fiber switch.  
           [0003]    2. Description of the Prior Art  
           [0004]    Currently, there are a number of ways to implement fiber-to-fiber switches, where an input optical fiber is coupled selectively to one of a plurality of output fibers. A first method involves bringing the cut and polished surface of the input fiber into close proximity to the similarly cut and polished end of the desired output fiber. If the fibers&#39; cores (where the light is guided) are positioned closely and accurately enough, most of the light from the input fiber will enter the core of the output fiber. This kind of switch requires accurate positioning of the fibers to a fraction of a micron, if low losses and repeatability are to be accomplished.  
           [0005]    A second switching method involves collimating the light from the input fiber using a lens. The collimated beam is then reflected into a collimator and hence directed into the desired output fiber using a movable mirror. Each output fiber has its own collimator. This type of switch requires each output fiber-collimator to be aligned to a very small fraction of a degree (a few arc seconds) in order to maintain sufficiently low-loss coupling. In addition, the mirrors must accurately reproduce the same output beam angle for each output fiber.  
           [0006]    A third type of switch involves passing the light from the input fiber through an interferometer with two possible outputs, such as a Mach-Zender interferometer. By manipulating the path length of one arm of the interferometer, the input light is directed to either of the two possible outputs. Free-space or fiber interferometers are expensive and must remain stable to a small fraction of a wavelength. Waveguide interferometers require very accurately positioned couplers in order to efficiently couple light from fibers to the waveguide switch and back to the fiber.  
           [0007]    To summarize, all of the known 1×N switching methods require high precision alignment of a number of their optical components. When such switches are to be used with single mode fibers, as are used in optical networking, the required precision of the switch components exceed the accuracy achieved by normal manufacturing processes. Therefore, expensive and time consuming active alignment processes, whereby a component is adjusted while a metric of the output is monitored, are required for each output fiber, often in several stages. Hence, at least N active alignments for each 1×N switch are required for all currently available switches.  
           [0008]    Many uses of 1×N switches, including optical networks, require that the loss be substantially the same for each of the N outputs, and further that this loss be accurately repeated each time the switch is set to each output. For use in optical networks, it is further required that the switch retain this uniformity and repeatability over a substantial environmental temperature range.  
           [0009]    A need remains in the art for a 1×N optical fiber switch which does not require active alignment steps for each output fiber, and which has uniform and predictable losses.  
         SUMMARY OF THE INVENTION  
         [0010]    An object of the invention is to provide 1×N optical fiber switches which do not require active alignment steps for each output fiber, and which have uniform and predictable losses. A 1×N optical switch according to the present invention switches between output fibers by translating a telecentric optical element in a plane perpendicular to the optical axis of the input fiber. The telecentric optical element is preferably a compound lens group. It collects the light emitted from the input fiber and focuses it on a selected output fiber in the output fiber bundle. The telecentric optical element is telecentric on both the input and the output sides.  
           [0011]    This invention describes a 1×N optical fiber switch whose construction does not require placement accuracies greater than what is common in ordinary manufacturing. Thus, this switch may be built economically using normal pick and place or fixturing techniques common in automated manufacturing. The number of outputs, N, of the switch may be a very large number such as 1000 or even more. In addition, the switch described by this invention can achieve coupling efficiencies and repeatability equaling or exceeding the state of the art in optical switches.  
           [0012]    A telecentric 1×N optical switch for coupling an input beam from an input fiber to a selected one of N output fibers includes a telecentric optical element and means for translating the telecentric optical element to a specific one of a group of predetermined locations to direct the beam from the input fiber to a selected output fiber associated with the specific location. The means for translating translates the telecentric optical element in directions perpendicular to the axis of the input fiber. The means for translating includes a computer for controlling translation and at least one driver for inducing translation responsive to the computer. In general the means for translating comprises a horizontal driver and a vertical driver.  
           [0013]    In the preferred embodiment, the computer further comprises means for determining the predetermined locations in a test configuration and means for storing the predetermined locations.  
           [0014]    Generally, the telecentric optical element comprises a lens group. For example, the lens group might comprise six lenses. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is an isometric cutaway view showing the preferred embodiment of a 1×N telecentric switch according to the present invention.  
         [0016]    [0016]FIG. 2 is a side cutaway view showing the telecentric lens group of FIG. 1.  
         [0017]    [0017]FIG. 3 is a flow diagram illustrating an automatic process for determining locations for the telecentric lens group of FIGS. 1 and 2 to select ouput fibers.  
         [0018]    [0018]FIG. 4 is a block diagram illustrating the set up for executing the process of FIG. 3.  
         [0019]    [0019]FIG. 5 is a table of specifications of an example telecentric lens group of FIG. 1. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]    The present invention relates to apparatus and methods for coupling an input optical fiber  102  selectively to one of a plurality of output fibers  104 . A 1×N optical switch  114  according to the present invention switches between output fibers using a telecentric optical element, such as a telecentric lens group  106 .  
         [0021]    As shown in FIG. 1, coupling between an input fiber  102  and a bundle of output fibers  104  is accomplished by translating a compound lens group  106  in a plane perpendicular to the optical axis  112 . Lens group  106  collects the light emitted from input fiber  102  and focuses it on a selected output fiber  104  in the output fiber bundle.  
         [0022]    In order for lens group  106  to efficiently couple the light from input fiber  102  to the selected output fiber  104  (especially if the fibers are single-mode fibers) lens group  106  must be telecentric on both the input and output sides. What this means, as shown in FIG. 2, is that a ray input that is parallel to the optical axis of the lens will always result in an output ray that is also parallel to the optical axis, regardless of whether the input ray is translated away from the axis or not.  
         [0023]    In order to provide coupling to a bundle of fibers, both horizontal and vertical displacement are required. Hence, horizontal motion of lens group  106  is accomplished by applying horizontal stage driver  118  to horizontal translation stage element  108 . Vertical motion of lens group  106  is accomplished by applying vertical stage driver  120  to vertical translation stage element  110 . See FIG. 5 for one possible configuration of lens group  106 .  
         [0024]    In FIG. 2, a cone of rays parallel to the optical axis but input off-axis at input position  3 , results in an output cone of rays that is also parallel to the optical axis at output position  3 .  
         [0025]    Thus, for a single input fiber  102 , the cone of light that is output remains parallel to optical axis  112 , even as lens group  106  is translated perpendicular to optical axis  112 . Furthermore, the output cone is translated perpendicular at a rate twice as fast as the lens group. By means of appropriate translations of lens group  106 , the light from input fiber  102  can be coupled efficiently into any selected output fiber  104 .  
         [0026]    As an example, telecentric lens group  106  might comprise six lenses  202 ,  204 ,  206 ,  208 ,  210 , and  212 .  
         [0027]    In the preferred embodiment, switch  114  is computer controlled (for example by means of an on-board microprocessor). The lens group positions need not be specified in advance—in the preferred embodiment, the lens is calibrated by an automatic process after construction. FIG. 3 is a flow diagram illustrating an example of such a process. Refer also to FIG. 4, which illustrates the alignment configuration. The steps of the process are:  
         [0028]    In step  302 , the sharp focus of lens group  106  is temporarily spoiled, for example by placing a plane-parallel glass plate  408  between lens group  106  and either output bundle  104  or input fiber  102 . This spreads the focal point out so that a coarse search pattern can find the approximate position for coupling to each output. This step is optional. If it is used, steps  308  and  310  may be repeated with the spoiling removed, as indicated by arrow  309 , for very precise results.  
         [0029]    In step  304 , a source  402  is connected to input fiber  102 . In step  306  detectors  404  are placed adjacent to output fibers  104  to detect the amount of light appearing at each output during step  308 . (Alternatively, a single detector can be switched to each of the outputs in turn.)  
         [0030]    In step  308 , computer  406  scans lens group  106  (via control signals  412  and  414 , to translation stage elements  110  and  108 ) in a search pattern to find the best coupling position for each of the outputs. Computer  406  monitors detectors  404  via signals  410  to determine how much light is appearing at each output fiber  104  at the various search pattern locations. As the search pattern is executed, computer  406  stores the locations of best coupling to each of the output fibers in memory in step  310 .  
         [0031]    Step  312  indicates that the positioning may be detuned for some output fibers  104  in order to end up with equivalent loss at each output fiber. These detuned positions would replace the corresponding locations previously stored in step  310 .  
         [0032]    In use, test source  402  is removed and the actual input signal is coupled to input fiber  102 . Detector array  404  is removed and replaced with the appropriate output coupling. Detuning plate  408  has been removed. Computer  406  still controls horizontal and vertical movement of lens group  106 , according to the stored location for each output fiber. When switch  114  is commanded to select a particular output, it goes to the stored location for that output. If desired, a feedback signal may may used to maintain accuracy.  
         [0033]    [0033]FIG. 5 is a table of specifications of an example telecentric lens group of FIGS. 1 and 2. Surface numbers and lens numbers are as labeled in FIG. 2. Thus, for example, lens  1  is lens  202  in FIG. 2, and has first surface  1  and second surface  2 . Telecentric lens groups are known in the art, and the group specified in FIG. 5 is only one example.