Abstract:
A fast acting, low insertion loss switch for use in an optical fiber communication system includes a small diameter waist region incorporating a Bragg grating which is precisely tuned by tensioning to be reflective at a selective wavelength band. By a small physical shift of the waist, as by lateral displacement of an attached small magnetic element with a magnetic field, the periodicity of the grating is varied so that the selected wavelength is passed through the waist with virtually no loss. The incremental motion required can be introduced by a variety of devices, selected in accordance with cost, size and response time requirements. Modules having a number of such switches in close juxtaposition offer distinct advantages for wavelength division multiplexed systems.

Description:
FIELD OF THE INVENTION 
     This invention relates to optical waveguide systems and devices, and more particularly to systems and devices which are intended to switch signals of selected optical wavelengths from one waveguide to another. 
     BACKGROUND OF THE INVENTION 
     Systems for communicating or processing data at optical wavelengths are in wide use, and the number of applications for such systems are increasing rapidly. A basic component for such systems is the optical switch, for transferring an input signal to either of two lines (such as optical waveguide fibers). There are a number of corollaries of electromechanical switches that are known, operating on the principle of displacing a conductive element so that it completes an optical circuit with one line or another. However, even though much work has been done on improving alignment and matching techniques, the transition of a signal from one element to another inevitably introduces losses. The techniques for reducing those losses introduce undesirable and sometimes unacceptable increments of cost. 
     Furthermore, modern communication and other systems take advantage wherever economically feasible of the broadband capabilities of optical waveguides, as by the use of wavelength division multiplexing to propagate a number of wavelength separated signals on one optical fiber. In the standards that have evolved for wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM), there are specific, closely spaced channels at designated wavelengths. Wavelength sensitive devices, such as add/drop filters and couplers, are utilized to extract signals in specific channels from, or add signals in specific channels to, the multiplexed signal. In some modern systems, therefore, switching a specific wavelength out of a WDM line requires both a wavelength selective coupler and a conventional switch. The added elements are not only more costly, but the losses introduced where matching or transitions are needed become unacceptable, particularly if a number of switches are to be used in the system. There is therefore a need for a wavelength selective optical switching system that has negligible loss and can provide a high degree of selectivity for a specific channel. 
     SUMMARY OF THE INVENTION 
     Devices and systems in accordance with the invention employ a grating assisted optical coupler having a length of small diameter (e.g. less than 15 microns) fiber that includes a Bragg grating that is reflective at wavelengths of a given periodicity. This length of fiber is held under precise tension, to tune the. periodicity of the refractive index variations to the specific wavelength desired. Under these circumstances, input signals at the selected wavelength are reflected back to a drop port, until the periodicity of the grating is varied. In the present example this is done on demand by stretching the small diameter section a small amount, shifting the effective wavelength to outside the band of the target channel. Thus the signal passes through the grating region to a different output fiber instead of being reflected. In either event, fiber continuity is not affected and losses are very low. By this arrangement, the small diameter section containing the grating is stretched only within its elasticity limits, and only by a small movement, which is imparted very rapidly, achieving desired switch response times. 
     In a more specific example of a wavelength selective optical switch, the unit comprises an add/drop coupler having an input port and drop port on one side of a waist region of small diameter containing the Bragg grating, and also having throughput port and add port terminals on the opposite side of the Bragg grating region. The waist region is held under controlled tension in a temperature compensated structure, and also tuned by tension to selected grating periodicity. This grating assisted coupler includes biconical tapered sections leading from each of the ports to the waist region, at which two greatly reduced diameter fiber elements are fused together. To stretch this reduced diameter waist region rapidly in controlled fashion, a small ferromagnetic element is attached to a tapered section at a distance from the Bragg grating. The ferromagnetic metal element and the fiber are deflected toward an associated electromagnet having a pole tip of the order of 1 mm away, stretching the Bragg region. This is realized within a switching time of less than 10 milliseconds. Thus selected wavelength signals that normally would be reflected back from the Bragg grating region to the drop port are transferred through the waist region to the output port. This coupler further offers the advantage of enabling addition of a signal of the selected wavelength at the add port, even though a multiplexed signal at the same wavelength is concurrently being dropped. Further mechanical stability and switch life are enhanced by incorporating a buffer element between the electromagnetic pole tip and the metal on the tapered fiber section which is deflected during switching. 
     Other aspects of this combination reside in the fact that the mounting structure which holds the ends of the span containing the Bragg grating compensates for temperature variations that would otherwise affect wavelength sensitivity. The change in periodicity, when the switch is activated, does not affect passband characteristics, so that there is no waste of spectrum or interference. 
     Only a small deviation in length is needed to change the reflection characteristic of the Bragg grating, and so a number of other expedients can be employed to effect switching in accordance with the invention. For example, a small deflection movement can be introduced even more rapidly by employing a leverage arm whose free end shifts the waist region laterally with a small angle movement of a rotary actuator at the opposite end. Other actuating forces may alternatively be generated using electronic, electromagnetic or electrostatic effects. Extremely high rate movements through limited distances are possible with capacitive devices, cantilevered elements, Hall effect devices and piezoelectric devices, for example. Furthermore, a small voice coil mechanism might alternatively be used to provide higher speed movement and faster switching. 
     Methods in accordance with the invention include the steps of establishing a narrow band of wavelength selectivity, using tension to maintain the selectivity and then varying the responsiveness by mechanically stretching until the periodicity has been shifted outside the selected wavelength band. Further, the methods employ tuning and temperature stabilization to enhance the precision and long life properties of the device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the invention may be had by reference to the following description, taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a perspective view, partially broken away, of a switch for optical waveguide systems; 
     FIG. 2 is an enlarged fragmentary view of a part of the switch system of FIG. 1, showing further details of a Bragg grating device in a mounting package; 
     FIG. 3 is a perspective view, partially broken away, of a switch enclosed in a housing; 
     FIG. 4 is an enlarged side view of the device of FIGS. 1 to  3 , showing the relationship of different parts of the waveguides incorporating the switch, but exaggerated in size for clarity; 
     FIG. 5 is a graph of wavelength response versus signal wavelengths for a Bragg grating switch in accordance with the invention showing response variations in undeflected (solid line) and deflected (dotted line) states; 
     FIG. 6 is a graph of signal wavelength in nanometers versus grating length in nanometers for a Bragg grating switch in accordance with the invention; 
     FIG. 7 is a flow diagram of the control steps used in switching optical signals in accordance with the invention; 
     FIG. 8 is a diagram of the control mechanism portion of an alternative fiber optic switch in accordance with the invention; and 
     FIG. 9 is a diagrammatic view of a different type of in-fiber switching device in accordance with the invention. 
    
    
     DETAILED DESCRIPTION 
     A wavelength selective optical switch  10  is shown in FIGS. 1 to  4 , to which reference is now made. It utilizes a fused fiber coupler configured as a WDM add/drop filter of the type described in U.S. Pat. No. 5,805,751. Namely, a small diameter (e.g. &lt;15 microns) waist region  12  of two fused, asymmetric optical waveguides  13 ,  14  extending at each end into separate adiabatically tapered diverging lengths which at their outer ends are of typical 95 to 125 micron constant diameter. A Bragg grating is written in the waist  12  of the coupler using UV exposure. The Bragg grating reflects a WDM channel supplied from an input port  20  back onto a drop port at a wavelength determined by the period of the grating  16  during fabrication and the tensioning of the waist  12 . All other wavelengths are transmitted on to a through port  24  without reflection. An add port  26  on the same side as the through port  24  is available for insertion of signals at the chosen wavelength, by reflection off the grating  16  so that they reach the through port  24 . 
     To hold the span of the waist region  12  under controlled tension between end points on a temperature compensating support, the fiber ends of the 2×2 fused fiber coupler are epoxied into a slot  32  in a slotted “Kovar” ferrule  30  using EpoTek353ND. The epoxy is initially in contact with bare glass surface of the fibers  13 ,  14  on the inside end of ferrule  30  and in contact with the 250 micron diameter fiber buffer material on the outside end of the ferrule  30 . The ferrules  30  are attached with the fiber under tension to compensating elements  33 ,  34 , of 304 stainless steel using laser welding. The tension applied to the waist  12  and the periodicity of the grating 16 fiber determine the center wavelength of the filter. Before attachment to the compensating elements  33 ,  34  the ferrules  30 , with the fibers secured, are rotated a number of times (here ten) in order to minimize the polarization sensitive characteristics of the device. The compensating elements  33 ,  34  are attached at the outer ends of the span to a base  36  by laser welding. The length of the base  36  is 4.9″ while that of the compensating elements  33 ,  34  is 0.75″, in this example, the base  36  being of low expansion material such as “Invar  36 ″. Temperature compensation of the center wavelength of the filter is achieved due to the thermal expansion mismatch of the materials with this configuration. As the temperature is increased the compensating elements  33 ,  34  expand inward faster than the base  36  expands outward. This results in the fiber-holding ferrules  30  moving toward each other giving a negative thermal expansion coefficient required for temperature compensation of the expansion of the span itself. The temperature compensation depends on the location of the welds because this determines the length of the base and the lengths of the compensating elements which can expand in opposing directions. The center wavelength of the filter depends on the tension of the waist region  12  determined by the spacing of the ferrules  30 . 
     A small ferromagnetic metal piece  38  is attached to the top surface of the coupler span in a tapered region leading from the waist  12 , using epoxy or a UV fiber recoating material such as “Desolite” from Desotech Corporation. The metal piece  38  is preferably a 2 mm section of “Kovar” wire of mass approximately 1 mg. The waist region  12  and adjoining tapers are of bare glass after stretching and writing of the Bragg grating  16 . The rest of the bare glass span, back to but not including the waist region  12  may be recoated with a material such as “Desolite” to provide additional environmental protection of the glass (as indicated in FIG.  4 ). An adjacent electromagnet  41  comprising an electromagnet armature  42  with a silicone sheet attached on the end is placed with its pole face  43  1 mm directly above the metal piece  38 . The silicone sheet  44  provides cushioning when the metal piece  36  is deflected toward the electromagnet pole face  43 , thus into contact with the silicone material and not the pole face. 
     The coil  46  about the armature  42  has electrical leads extending outside the unit to an actuator  48  or driver circuit which, when switched on, energizes the armature  42  to create the attractive magnetic field needed for switch operation. 
     The coupler and electromagnet  41  are packaged inside an aluminum tray  50  (FIG.  3 ). Fiber feedthroughs  52  with strain relief boots are used to feed fiber through the walls of the tray  50 . The, coupler and temperature compensated package are placed on a silicone mold  54  in the tray bottom and attached with RTV or other adhesive. The silicone mold  54  is shaped to receive the base  36  and provides protection from vibrational damage. After the fibers  13 ,  14  are fed through the feedthroughs  52  the holes are sealed with epoxy. The external electrical connections to the electromagnet  41  are connected to an electrical connector on the wall of the tray  50 . The lid  56  is sealed to the tray  50  in a dry nitrogen atmosphere using epoxy to form a water tight seal. 
     When the coupler is stretched under tension, the wavelength of the filter changes by an amount given by: 
     
       
         Δλ/λ=(1−p)Δε  (eqn 1) 
       
     
     where Δλ is the change in wavelength of the filter, Δε is the change in strain, and p is the photoeleastic constant. The strain in tapered fibers is non-uniform and varies inversely with the cross sectional area. The strain therefore is mainly concentrated in the narrow tapered regions in the waist  12 . FIG. 6 shows the change in the center wavelength of a filter as it is stretched. Typical tuning values are +0.036 nm/micron. Because the fibers  13 ,  14  are tapered to such a small waist, typically 10-15 microns, much less force is required to tune the wavelength of a coupler grating as compared to an untapered fiber Bragg grating. 
     With the coupler grating filter packaged under tension to a precise wavelength, switching is effected for optical signals by applying controlled additional tension to the fiber grating  16 , to change the signal band which is selected. In the relaxed state the fiber drops a selected wavelength channel, onto the drop port  22 , out of multiple channels in a WDM system. When the electromagnet  41  is energized by applying a current from the actuator  48 , the magnetic field attracts the metal piece  38  on the fibers  12 ,  13  deflecting them slightly but significantly. The deflection of the fiber results in an increase in tension, which tunes the filter to a higher wavelength, typically several nanometers, which is out of the wavelength band for the filter. The channel of selected wavelength is no longer routed back to the drop port  22  but is instead switched onto the through port  24 . FIG. 5 shows the drop spectrum of the filter in the unswitched and switched states. When the electromagnet  41  is de-energized the fiber relaxes to its original state and the selected WDM channel is switched back to the drop port  22 . The fiber remains in its range of elasticity during switching. Since the stretching is confined to the narrow waist region, very little force is required for the tuning. 
     Unlike conventional electromechanical switches the light never leaves the fiber. This results in an extremely low loss device, typically less than 0.2 dB insertion loss. Also there is no need for the complex and high mechanical tolerance single mode fiber coupling process required with conventional switches. Switching speed is limited by the time to energize the coil and to deflect the fiber. This is of the order of 10 msec. Strong gratings with over 60 dB attenuation in the transmitted state can be fabricated, to provide high channel isolation on the through fiber when the filter drops the channel. In the on state where the channel is transmitted on the through fiber, isolation on the drop fiber is determined by the roll off of the filter response and can typically be −30 dB. 
     Methods in accordance with the invention, making reference now to FIG. 7, establish a limited passband within a broader optical signal band by first creating a periodic pattern in a waveguiding element that has some elasticity that alters the periodicity. The waveguiding element is tensioned to respond to the selected optical wavelength band when it is desired to transfer optical signals at that wavelength to a given terminal. When the same optical signals are to be transferred to a different terminal the periodicity of the patterns in the waveguiding element is shifted slightly but adequately by changing the tensioning within the elastic limit of the element. 
     As seen in the example of FIG. 8, the desired incremental shift of characteristics of a tensioned span held between two span supports with temperature compensation can also be achieved very rapidly by using an electrostatic device  65 . Here a deflectable wall comprising a flexible electrode  67  of an electrostatic device  65  is coupled to the tapered section in the tensioned span of a coupler. The flexible electrode  67 , when attracted by the charge on a fixed electrode  72  established by a charging source  74 , moves incrementally to displace the tensioned span a given amount, which may be determined by a stop (not shown). Because the mass and tension being overcome are again very low, extremely high repetition rates are feasible. 
     As long as the optical waveguide containing a grating is sufficiently small in cross section at the grating region to be stretched for an effective distance, relative to grating periodicity, the same principle can be used. FIG. 9 depicts a single fiber device  80  including a grating  82  in a narrowed, tensioned span  84 . The device  80  again is responsive only to signals in a selected passband as predetermined tension is applied. Under this condition multi-band input signals from an input port  86  are directed through a circulator  88  to the narrowed region of the span  84 , where the grating  82  reflects signals of the chosen wavelength back through the circulator to a drop port  90 . Other wavelengths, go through the grating region  87  to a throughput port  92 . If a deflector  94  energized by an actuator  96  further tensions the span  84  and changes the grating  82  periodicity an adequate amount, however, all signals go to the throughput port  92 , thus switching the chosen signals. 
     Although a number of alternatives and variations have been described above, the invention is not limited thereto but encompasses all forms and modifications in accordance with the appended claims.