Abstract:
A system is provided to obtain loss optimized output optical power by way of feedback control and stabilization in an optical signal switching or routing system. The optical signal switching or routing system includes at least two input optical fibers and at least two output optical fibers, a controllable mechanism for directing an optical beam from one of the input optical fibers to one of the output optical fibers, and a mechanism for measuring the optical power applied to output optical fiber. The measuring mechanism provides a measure of the output optical power through a signal processing apparatus to a control apparatus. Possible other inputs to the signal processing apparatus include the input optical power, test optical power, etc. The inputs to the signal processing apparatus are compared and the signal processing apparatus outputs a signal to the control apparatus to provide optimized output power.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of fiber optics, and more particularly to the field of optical signal switching, routing, and monitoring. 
     2. Description of Related Art 
     FIG. 1 is an illustration of an exemplary optical switching system  10  for practicing the invention. For example, optical switching system  10  may represent a 3-dimensional optical switching system. A 3-dimensional optical switching system allows for optical coupling between input fibers and output fibers in different planes using lens arrays and mirror arrays. The lens arrays and mirror arrays provide proper angle and position of light beams traveling from input fibers to output fibers. That is, a light beam must leave and enter a fiber in a direct, beam path. 
     Referring to FIG. 1, a generalized optical switching system  10  includes input fiber array  12 , first lens array  14 , first beam steering apparatus (e.g., Micro-Electro-Mechanical-System, or MEMS, mirror array)  16 , second beam steering apparatus (e.g., MEMS mirror array)  18 , second lens array  20 , and output fiber array  22 . System  10  might also include additional or different elements, such as apparatus  24  and  26  for controlling the mirror arrays  16  and  18 , respectively. 
     Input fiber array  12  provides a plurality of input optical fibers  28  for forming light beams  30  transmitted to (and through) first lens array  14 . First lens array  14  includes a plurality of optical lenses  32 , which are used to focus beams of light from each input optical fiber  28  to individual mirror devices  34  on mirror array  16 . Mirror devices  34  may be electronically, magnetically, or otherwise individually movable to control the beam path of each beam formed by the input optical fibers  28 . 
     Mirror device  34  may be a gimbaled mirror device having a rectangular, elliptical, circular, or other appropriate shape. The plurality of mirror devices  34  for mirror array  16  can pivot a reflective component thereof (not specifically shown in FIG. 1) to redirect or reflect light to varying mirror devices on second mirror array  18 . Second mirror array  18  also includes a plurality of mirror devices, similar to those described with regard to first mirror array  16 , which are used to redirect and reflect light beams to varying lenses  36  on second lens array  20 . second lens array  20  focuses beams of light from second mirror array  18  t o individual output fibers  38  of output fiber array  22 . 
     Optical switching system  10  allows light beams from any input fiber  28  of input fiber array  12  to be redirected t o any output fiber  38  of output fiber array  22 . The above arrangement, including mirror arrays  16 ,  18  may also be used in scanning systems, printing systems, display systems, and other systems that require redirecting beams  20  of light. 
     It should be noted that for each input optical fiber  28  there is an associated mirror device (such as mirror de vice  34 ) on mirror array  16 , and for each output optical fiber  38  there is an associated mirror device on mirror array  18 . In general, there will be a minimum of two input optical fibers and two output optical fibers, and correspondingly two mirrors on each of arrays  16  and  18 . There need not be an identical number of input and output optical fibers, although this is typically the case. Also, there will typically be more than two such input and output optical fibers. 
     In general, many types of beam steering arrangements will lend themselves to use with the present invention. For example, two mirror arrays  16 ,  18  are shown in FIG.  1 . Good coupling of a beam into an output optical fiber typically requires controlling fiber position and angle in two dimensions, in addition to the two dimensions of mirror rotation. So, four degrees of freedom are required. However, in some instances, few or greater degrees of freedom may suffice. For example, a one-mirror array shown FIG. 2, where a single mirror array  42  controlled by apparatus  44  is used for directing the beams, or a three or more mirror array arrangement (not shown) may be appropriate. Also, while the above description has been in terms of reflective beam steering, other types of beam steering, such as refractive or diffractive beam steering may employ the present invention with equal advantage (although not otherwise discussed in detail herein). 
     Coupling a light beam from an input optical fiber to an output optical fiber requires that the mirrors of mirror arrays  16 ,  18  be angularly controlled, and that the mirror angles be precise. Such precision, and a degree of control thereover, is needed to optimize output optical power. To obtain such precise positioning, one can either employ or not employ knowledge of actual mirror angle. One method to determine the mirror angles is to add structures to the mirrors that indicate the angle of mirror rotation. One example of this is torsion sensors attached to suspension elements associated with each mirror. See, for example, U.S. Pat. No. 6,044,705. This approach has several disadvantages, including a relatively large number of required interconnections, added processing and manufacturing steps, greater risk of yield losses, etc. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention recognizes the limitations of direct measurement of mirror position, and presents an alternative technique for allowing optimization of output optical power. Our approach is to use an optical signal incident on each mirror to determine the mirror&#39;s angle. According to one embodiment, the output optical power is measured, for example at each output optical fiber, and compared with either the input optical power or a reference value, and based on the results of the comparison, a mirror or mirrors are rotated, and output power measured again. This process is referred to herein as feedback stabilization control of mirror position. According to another embodiment, this may be an iterative process, to allow narrowing in on a desired mirror angle for loss optimized output optical power. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The features and advantages of the present invention are described and will be apparent to those skilled in art from the following detailed description taken together with the accompanying figures, in which like reference numerals in the various figures denote like elements. 
     FIG. 1 is an illustration of a prior art optical switching apparatus employing multiple reflective devices according to the prior art. 
     FIG. 2 is an illustration of a prior art optical switching apparatus employing a single reflective device according to the prior art. 
     FIG. 3 is an illustration of an optical system according to the present invention. 
     FIG. 4 is an illustration of an optical power measurement apparatus as employed by one embodiment of the present invention. 
     FIG. 5 is an illustration of an optical system according to an alternative embodiment of the present invention. 
     FIG. 6 is an illustration of an official power is computed by two analog to digital converter. 
     FIG. 7 is an illustration of a single beam steering device. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will now be described in detail with reference to examples thereof. However, such examples are for illustrative purposes, and should not be read as limiting the present invention, the scope of which being defined by the claims hereof. 
     With reference to FIG. 3, there is shown therein a system  46  suitable for implementation of one embodiment of the present invention. For illustration purposes only, there is shown only two input and output optical fibers, although it will be readily understood that the present description applies equally to a virtually arbitrary greater number of such fibers. In one embodiment lenses  32  may be formed from an ACT Microdevices 8-fiber collimator array (ACT Microdevices, Radford, Va.). System  46 , in addition to those elements discussed with reference to FIG. 1, further includes input optical power measurement apparatus  48  associated with each input optical fiber  28  and output optical power measurement apparatus  50  associated with each output optical fiber  38 , each optical power measurement apparatus providing output signals representative of measured optical power. System  46  further including a signal processing apparatus  52 , connected to receive as inputs the output signals provided by optical power measurement devices  48  and  50 . Signal processing apparatus  52  provides output signals which are representative of a calculated comparison of the input optical power measured by optical power measurement apparatus  48  and output optical power measured by optical power measurement apparatus  50 . These signals are used by mirror control apparatus  24 ,  26  to, for example, generate voltages which applied to electrodes such as  49 ,  51 , to control the angular position of mirror  34 ,  48 . In one embodiment, mirrors  34 ,  48  may be composed of CMS Mikrosysteme Mikro-scanner 1.2 mirrors (CMS Mikrosysteme GmbH, Chemnitz, Germany). 
     As will be further discussed, the calculated comparison may be one of a large number of different types of analytical comparisons. In addition, control apparatus  24  and  26  are configured such that they receive as inputs the output signals from signal processing apparatus  52 . System  46  is thereby configured such that an analytical comparison of the input and output optical powers may be made, and the angular position of either mirror  34 , mirror  48 , or both may be adjusted in response to that analytical comparison in order to optimize output optical power. 
     The aforementioned analytical comparison is in part a function of the number of degrees of freedom of the system. For example, in the simplest case, mirror  34  might rotate in only one plane, so the mirror control apparatus  24  and/or  26  would have only one output signal for each mirror. Techniques for maximizing or minimizing the output of a single variable are well known, for example using a steepest decent method (see, e.g., E. Kreyszig, ‘Advanced Engineering Mathematics’, John Wiley &amp; Sons 1993). However, in other systems two variable mirror planes are required for loss optimization, so the mirror controller  24 ,  26  will have two outputs. This optimization of a function of multiple variables can be done with a variety of techniques, such as random search techniques and gradient search techniques (see, e.g., E. Kreyszig, ‘Advanced Engineering Mathematics’, John Wiley &amp; Sons 1993). Optimization techniques typically start with a random or periodic search to find a good optimization starting point, then a gradient search to find the local optimum. 
     It is often desired to optimize a system such as system  46  for minimum loss. In such a case, the analytical comparison will be made such that the loss optimized optical power is the greatest possible optical output power. In such a case, the signal processing apparatus  52  may include a ratio circuit, and serve to calculate the ratio of output optical power to input optical power. The greater the ratio, the greater the output optical power. For example, a ratio of better than one-half would indicate a loss of less than 3 dB. However, it may be desired to have every channel (i.e., the signal on each output optical fiber) of the system have approximately the same optical loss. In this case channels with loss lower than some threshold (e.g., average) might be loss optimized to have less than minimum loss to equalize power to other channels. In other applications, it might be desired to have the same optical output power for every channel. In this case, channels with strong input signals might be loss optimized to have higher loss. It may alternatively be desirable to maintain an individual channel at a preset or calculated power level, for example when the input optical power is subject to power variations. In such a case, the optical power stabilization may be achieved by monitoring fluctuations at the input optical power measure and controlling attenuation so as to stabilize output optical power. 
     In one embodiment  54  of the present invention shown in FIG. 4, the power coupled into an output optical fiber  38  is determined by sampling the power in the fiber with a fiberoptic power splitter  56  proximate the output optical fiber, and a photodetector  58  to convert this sampled power to an electrical level, which may be output at output  60 . In one embodiment, the fiberoptic power splitter is a ThorLabs #1020A-10 10/90 optical power splitter (ThorLabs, Inc., Newton, N.J.) and the photodetector is an Epitaxx ETX500T8 (Epitaxx Optoelectronic Devices, West Trenton, N.J.). 
     This measurement of the optical output power is not a direct measurement of optical loss of the system, and fluctuations of input optical power would make it difficult to provide a mirror angle corresponding to an optimized output optical power, whether is be minimum loss or some other optimization. Therefore, in embodiment  46  shown in FIG. 3, the optical loss of the system is measured by sampling the power at the input of the fiber  28 , for example by using an optical power splitter  56  and photodetector  58 , as shown and described in FIG. 4, at the input optical fiber as well. The optical loss may be found, for example, from the ratio of the output power to the ratio of the input power. 
     In the embodiment of FIG. 3, the feedback stabilization relies on the input optical signal delivered by the input optical fiber  28 . However, in general, the output optical power is compared to a reference optical power. According to another embodiment  62  of the present invention, the output optical power may be compared to a reference optical power, other than input optical power, and control performed as a function of that comparison. For example, as shown in FIG. 5, an external optical source  64  (such as a Fujitsu FLD148G3NL-B 1.475 nm laser with an optical power splitter to allow one laser to be used with more than one channel, from Fujitsu Compound Semiconductor, Inc., San Jose, Calif.) under separate control  66  is used, for example, through a wavelength division multiplexer fiber coupler  68  (such as model DiCon PCC-14-15 available from DiCon Fiberoptics, Inc., Berkeley, Calif.) as a reference optical power source, having a known or measured optical power. This known or measured optical power may be used as an input to signal processing apparatus  52 , which performs the previously discussed analytical comparison to thereby provide control signals to mirror control apparatus  24 ,  26 . Embodiment  62  may serve several different purposes, such as system calibration, etc. 
     According to an embodiment  70  of the present invention shown in FIG. 6, the ratio of the input to output optical power is computed by two analog to digital (A/D) converters  72 ,  74  to digitize the input power level and output power level, and a microprocessor  76  to perform the analytical comparison function, such as division. Microprocessor  76  calculates the voltages needed to set the desired mirror angles, and converts this to an analog voltage with multiple digital to analog (D/A) converters  78 . In one implementation, the microprocessor is a Microchip PIC16C770 (Microchip Corp., Ariz.) with 6 integrated A/D converters. The microprocessor takes the ratio of the input and output signal level, and computes the new mirror control voltages. The new voltages are sent to a Linear Technology LTC1257 D/A converter (Digi-Key, Thief River Falls, Minn.), with a maximum output voltage of approximately 10 V. The mirrors typically require high voltage to cause mirror deflection. High voltage drivers to increase the voltage for the mirrors can be built using high voltage MOSFET devices using standard commercial practice. 
     While the above describes the invention with reference to several specific exemplary embodiments, numerous variations and alternate embodiments are contemplated and will be appreciated by one skilled in the art. For example, with reference to FIG. 7, there is shown therein a simple embodiment  80  in which only a single beam steering apparatus  82  is shown. In embodiment  80 , beam steering apparatus  82  consists of two independently controllable beam steering sub-apparatus  84 ,  86 . Signal processing and control are as otherwise previously described. Likewise, an arrangement with more than two beam steering apparatus, although not shown, is merely an extension of the previous description. Thus, the spirit and scope of the present invention will be defined by the appended claims, with no intended limitation thereof by the preceding description.