Abstract:
Apparatus and methods for scrambling optical modes in multimode fibers to achieve uniform light distribution in guided multi-mode light for various applications.

Description:
TECHNICAL FIELD 
       [0001]    This application relates to fibers and fiber devices. 
       BACKGROUND 
       [0002]    Optical fibers are optical waveguides and are used to guide light. Optical fibers can be single-mode fibers that support only one traverse waveguide mode or multi-mode fibers that support two or more traverse modes. A multimode fiber (MMF) can be used to direct light in multiple modes for various applications such as applications requiring guiding light at high optical power levels. 
         [0003]    Some applications require a spatially uniform distribution of light over the cross section of a multimode fiber. Examples of such applications include, illumination devices in lighting applications, optical local-area-networks (LANs) over MMF and image encryptions, fiber-based genetic analysis and functional genomics such as Massively Parallel Signature Sequencing (MPSS) and molecular interactions. 
         [0004]    Mode scrambling has been a well-known technique to improve the uniformity in multi-mode fibers, different approaches have been demonstrated: (i) the modification of multi-mode fiber itself, such as through extremely high temperature annealing, or special fabrication procedure in the cladding; (ii) twisting plastic optical fiber instead of traditional MMF; (iii) selecting one special mode through mode coupling. These approaches can suffer from one or more disadvantages such as uncertainty or extreme requirements during manufacturing, increased cost of fiber, considerable optical loss caused by various factors in these techniques. 
       SUMMARY 
       [0005]    Implementations and examples are provided for apparatus and methods for scrambling optical modes in multimode fibers to achieve uniform light distribution in guided multi-mode light for various applications. The described apparatus and methods can be used to provide practical solutions with a longer life time and reliable performance for a mode scrambling device engaged to a multimode fiber. 
         [0006]    In one aspect, an example for a multimode optical fiber device includes a multimode fiber that supports light in two or more optical waveguide modes; and fiber squeezers engaged to the multimode fiber at different locations along a lengthwise direction of the multimode fiber to squeeze the multimode fiber along two or more different squeezing directions perpendicular to the lengthwise direction. The fiber squeezers are responsive to respective control signals to modulate a degree of squeezing at selected squeezing frequencies, respectively, to cause mixing of light in the two or more optical waveguide modes inside the multimode fiber to increase a spatial uniformity of optical power across a cross section of the multimode fiber. This example includes a control unit that produces the respective control signals to the fiber squeezers, respectively, and controls the fiber squeezers to produce either two or more different degrees of squeezing or to squeeze at two or more different selected squeezing frequencies. 
         [0007]    In another aspect, an example for a method for scrambling light in different optical waveguide modes in a multimode optical fiber includes providing a multimode fiber that supports light in two or more different optical waveguide modes; engaging fiber squeezers to the multimode fiber at different locations along a lengthwise direction of the multimode fiber to squeeze the multimode fiber along two or more different squeezing directions perpendicular to the lengthwise direction; and controlling the fiber squeezers to modulate a degree of squeezing at selected squeezing frequencies, respectively, to cause mixing of light in the two or more optical waveguide modes inside the multimode fiber to increase a spatial uniformity of optical power across a cross section of the multimode fiber. 
         [0008]    In yet another aspect, an example for a multimode optical fiber device includes a linear actuator responsive to a control signal to cause a dimensional change along a straight line; and a fiber stretcher frame comprising at least one expandable slot that is engaged to the actuator so that the dimensional change along the straight line is across the expandable slot to change a width of the expandable slot. The fiber stretcher frame is structured to amplify the dimensional change of the actuator to produce an amplified change in a circumference of an exterior surface of the fiber stretcher frame. This example includes a multimode fiber that supports light in two or more optical waveguide modes and wraps around the exterior surface of the fiber stretcher frame to form a fiber loop; and a control unit that produces a control signal to the linear actuator and controls the linear actuator to modulate a degree of stretching of the fiber loop at a selected frequency to cause mixing of light in the two or more optical waveguide modes inside the multimode fiber to increase a spatial uniformity of optical power across a cross section of the multimode fiber. 
         [0009]    In yet another aspect, an example for a method for scrambling light in different optical waveguide modes in a multimode optical fiber includes providing a multimode fiber that supports light in two or more different optical waveguide modes; and modulating a dimension of at least one linear transducer to cause spatial disturbances at multiple locations in the multimode fiber to cause mixing of light in the two or more optical waveguide modes inside the multimode fiber to increase a spatial uniformity of optical power across a cross section of the multimode fiber. 
         [0010]    These and other examples and implementations are described in detail in the drawings, the detailed description and the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIGS. 1 and 2  show two examples of multimode fiber mode scramblers. 
           [0012]      FIG. 3  shows a testing system for measuring performance of a multimode fiber mode scrambler. 
           [0013]      FIGS. 4A ,  4 B and  4 C show examples of measured beam patterns using the system in  FIG. 3 . 
           [0014]      FIG. 5  shows an example of a piezo-electric fiber squeezer. 
           [0015]      FIG. 6  shows one exemplary design of a fiber stretcher device using an actuator to control a dimension of an expandable slot in a stretcher frame. 
           [0016]      FIGS. 7A and 7B  show one implementation of the design in  FIG. 1  using a PZT linear actuator. 
           [0017]      FIGS. 8A and 8B  show one implementation of the design in  FIG. 1  using a PZT linear actuator to control dimensions of two expandable slots in a stretcher frame. 
           [0018]      FIGS. 9A and 9B  show one implementation of the design in  FIG. 1  using two or more PZT linear actuators to control dimensions of two expandable slots in a stretcher frame. 
           [0019]      FIGS. 10A and 10B  show one exemplary design a fiber stretcher device using multiple actuators arranged in a radial configuration to control a radial dimension of a stretcher frame. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    A multimode fiber has a waveguide structure with a fiber core and a fiber cladding to support light in two or more different optical waveguide modes. The radial profile of the refractive index of the multimode fiber can be step-indexed to have a high-index fiber core and a low-index fiber cladding outside the fiber core, or be configured to have a graded index profile that decreases in value along the radial direction from the center of the fiber. Disturbance can be applied to different locations along the lengthwise direction of the multimode fiber to cause mixing of different modes. A proper disturbance can be implemented by modulating a dimension of at least one linear transducer to cause spatial disturbances at multiple locations in the multimode fiber to cause mixing of light in two or more optical waveguide modes inside the multimode fiber to increase a spatial uniformity of optical power across a cross section of the multimode fiber. The present mode scrambling techniques can be used with various multimode fibers without requiring special fibers or modification to fibers and thus are versatile. This mode scrambling can be implemented in various configurations. 
         [0021]      FIG. 1  illustrates one example of a multimode fiber mode scrambler  10  based on the above scrambling mechanism. A multimode fiber (MMF)  1  is provided to guide light in two or more waveguide modes supported by the MMF  1 . A light source, such as a laser or an LED, can be used to produce the light which is coupled into the MMF  1 . Multiple fiber squeezers  2 ,  3 ,  4 ,  5 ,  6  and  6  are engaged to the MMF 1  at different locations along a lengthwise direction of the MMF  1  to squeeze the MMF 1  along two or more different squeezing directions perpendicular to the lengthwise direction. This squeezing at different locations along the MMF  1  causes spatial disturbances to the optical modes which, in turn, cause mixing of light in the different modes at these different locations. The mode mixing increases the spatial uniformity of optical power across the cross section of the MMF  1 . 
         [0022]    At least two fiber squeezers are needed and four or more fiber squeezers are preferred to produce sufficient mode mixing based on tests. Each fiber squeezer can include a linear actuator or transducer to cause a motion that squeezes the MMF  1 . Piezo-electric transducers can be used. Two adjacent fiber squeezers, e.g.,  3  and  4 , can be oriented to squeeze along two different directions. In the illustrated example, the squeezing directions of two adjacent fiber squeezers are at 45 degrees with respect to each other. The fiber squeezers are responsive to respective control signals to modulate a degree of squeezing at selected squeezing frequencies, respectively. Two different fiber squeezers can be controlled to produce different squeezing on the MMF  1  in terms of the degree of the squeezing or squeezing amplitude, the squeezing frequencies, and a combination of both. 
         [0023]    A control unit  8  is provided to include an electrical deriver that produces the respective control signals F 1  through F  6  to the fiber squeezers, respectively. The control unit  8  controls the fiber squeezers to produce either two or more different degrees of squeezing or to squeeze at two or more different selected squeezing frequencies. Each control signal can be an oscillating signal oscillating at a selected squeezing frequency for the respective fiber squeezer. For example, a control signal can be a square wave signal or a sinusoidal wave signal. The squeezing frequency can be from 100 Hz to 10000 Hz and can be from 1000 Hz to 10000 Hz to provide sufficient mode mixing in some applications. A housing can be provided to enclose the fiber squeezers  2 - 7  and the control unit  8 . 
         [0024]      FIG. 2  shows another example of a multimode fiber mode scrambler  20  based on a fiber stretcher frame  21 . The fiber stretch frame  21  is structured and engaged to one or more linear piezo-electric actuators  22  cause fiber stretcher  21  to change its dimension to provide the longitude extension at a selected modulation frequency (typically several KHz), therefore, generating spatial mode scrambling along the multimode fiber. 
         [0025]    The fiber stretcher frame  21  can be structured to include at least one expandable slot that is engaged to the actuators  22  so that the dimensional change along the straight line is across the expandable slot to change a width of the expandable slot. The fiber stretcher frame  21  is structured to amplify the dimensional change of the actuator  22  to produce an amplified change in a circumference of an exterior surface of the fiber stretcher frame  21 . The MMF  1  that supports light in two or more optical waveguide modes wraps around the exterior surface of the fiber stretcher frame  21  to form a fiber loop. A control unit  23  is provided to produce a control signal to the linear actuator  22  and controls the linear actuator  22  to modulate a degree of stretching of the fiber loop at a selected frequency to cause mode mixing. Different from the device in  FIG. 1  which causes disturbances at different location son the MMF  1 , the fiber stretcher frame  21  in  FIG. 2  causes longitudinal disturbances to MMF  1  at all locations in the portion wrapped on the frame  21  to cause the mode mixing. 
         [0026]      FIG. 3  shows a system for evaluating the performance of a multimode fiber mode scrambler  31  based on various designs in this application including the designs in  FIGS. 1 and 2 . A laser  30 , such as a laser at 635 nm, is used to produce light into a test MMF  35  to which the scrambler  31  is engaged. A beam analyzer  33  is used to detect the output light in the MMF downstream from the scrambler  31  under test. A control unit  32  with the electrical driver for the scrambler  31  is used to control the operation of the scrambler  31 . Electrical signals are generated by the control unit  32  and applied on the piezo-driver (generating up to 150-V driving voltages on the piezos with a certain frequencies) in the scrambler  31  to provide either fiber squeezing or stretching effect (six driving signals for the squeezing technology and four signals for the fiber stretching approach). The final beam profile of the output beam from the scrambler  31  in the MMF  35  is recorded in the computer  34 . 
         [0027]      FIGS. 4A ,  4 B and  4 C show beam profiles in conducted tests.  FIG. 4A  is the beam profile of the light in the MMF  35  without mode scrambling and exhibits significant spatial features in the spatial distribution of the optical power.  FIG. 4B  shows the beam profile of the output beam from a scrambler based on the design in  FIG. 2 .  FIG. 4C  shows the beam profile of the output beam from a scrambler based on the design in  FIG. 1 . In both cases, the beam uniformity is significantly improved. 
         [0028]    The following sections describe examples of specific implementations of the devices in  FIGS. 1 and 2 . 
         [0029]      FIG. 5  shows a cross section view of a fiber squeezer that can be used to implement the fiber squeezers in  FIG. 1 . A fiber holder  50  includes a smooth flat surface  51  that supports a first side of a MMF  52  which may be coated with a polyimide coating surrounding the fiber cladding. A pressure block  53  including a second smooth surface  53 A presses against an opposite side of the MMF  52  to squeeze the MMF  52  by changing the position of the pressure block  52  with respect to the fiber holder  50 . A linear actuator  54 , such as a PZT stack, is engaged to the pressure block  53  to change the position of the pressure block  54  in response to a control signal from a driver circuit  304  as part of the control unit for the scrambler. The PZT transducer can be a stack of piezo electric elements. Altering the voltage applied across the stack changes the displacement of stack. The applied voltage is an oscillating voltage signal at a selected squeezing frequency, e.g., several KHz. A screw  55  is provided to set the initial position of the pressure block  53 . 
         [0030]    One difficulty with piezo electric elements is that significant voltages are typically needed to achieve the desired displacement. Often the voltage requirements may exceed 50 volts. Generating such relatively high voltages in solid state systems involves transformers and powerful power supplies. To avoid such high voltages, the driving frequency of voltage source can be at the resonant frequency of piezo electric stack to reduce the voltage needed for the sufficient squeezing. It has been found that by driving the piezoelectrics at a resonant frequency, the minimum driving voltages can be reduced to below 10 volts and even below 2 volts. These low voltage makes it possible to drive the piezo-electric stack using low cost commercially available integrated circuits. 
         [0031]    Different fiber squeezers may be structured to have different resonance frequencies to be operated at different squeezing frequencies in resonance. This design can improve the mode scrambling in the MMF. 
         [0032]    Various tests indicate that the contact surfaces  53 A and the  51  should be smooth to reduce optical loss in the MMF  52 . Various fiber squeezing surfaces are designed to have surface protrusions and indentations that deviate from a plane of a smooth surface. Such surface irregularities, and particularly the protrusions, are largely responsible for the activation losses when squeezing surface presses against the fiber. In particular, the protrusions produce microbending in the fiber surface that results in light loss. In order to reduce microbending effects, the fiber squeezing surface can be treated by one of several techniques to generate a “super smooth” surface. One method of generating such a technique is by superpolishing. Typically such a superpolish is accomplished using a fine grade lapping film or polishing compound made of abrasive particles. Such particles may include diamond, silicon carbide, or aluminum oxide. Alternative methods of superpolishing a surface are also available. For example, an electro-polish technique may be used to obtain mirror-type finished metal surfaces. Yet another method of treating the surface of the fiber squeezer is to use adhesives or similar chemicals to fill in the cracks on the surface. When cured, the adhesive forms a smooth hard coating and thus reduces the roughness of the surface. Such lapping films or compounds are traditionally used to polish mirrors, lenses, and fine finish metals to create a super smooth surface. 
         [0033]    The superpolish reduces the height of the highest protrusion to less than 100 microns and preferably to a height of less than 50 microns. Thus when the super polished fiber squeezing surface is pressed against the fiber, the “roughness” or maximum deformation of the fiber surface from the smooth surface is less than 100 microns. By reducing protrusions to less than 100 microns or 0.1 mm, activation losses can be reduced to 0.01 dB. Further polishing can further reduce activation losses. In the laboratory, super polishing has reduced activation losses below 0.002 dB. 
         [0034]    Fibers including MMF can include microcracks the surface of the fiber cladding and these cracks can cause fiber breakage under stress. In particular, when pressure induced fiber stress occurs, fiber breakage starts from one or more microcracks and propagates across a fiber cross section. In order to strengthen the fiber, a polyimide coating can be applied around the fiber cladding. Alternatively, other chemical coatings may also applied to the fiber surface to seal the microcracks and increase fiber&#39;s strength. Such coating materials include chlorinated polydimethylsiloxane, monometric octadecylsilane, β-chloroethylsilsesquioxane, and methylsilsesquioxane. The coating thickness can range from nanometers to 10 microns. Coating uniformity is important because coating non-uniformity induces microbending in the fiber resulting in high activation losses. A few molecular layer of carbon around the fiber cladding may further increase the fiber durability under stress. A typical thickness of the poly-imide coating is between 10 and 25 micrometers. Such polyimide coated fibers are commercially available from Lucent Technologies of Avon, Conn. and sold under the trade name PYROCOAT. These polyimide coated fibers are typically sold for high temperature applications. 
         [0035]    Referring to  FIG. 2 , the fiber stretcher frame  21  can be implemented in various configurations. Several examples are described below. 
         [0036]      FIG. 6  shows one exemplary design of a fiber stretcher device using an electrically controlled actuator to control a dimension of a slot in a stretcher frame. The illustrated fiber stretcher device includes a stretcher frame  110  that has a frame slot  112  with a slot opening  116  at one end to separate the frame  110  into two parts  110 A and  110 B that are connected at the other end  114  of the frame slot  112 . The frame slot  112  can be centered in the frame  110  to make two equal frame parts  110 A and  110 B or positioned to make frame parts  110 A and  110 B different in size and shape A linear actuator  120 , which expands or contracts along a straight line in response to a control signal to produce a linear change in the dimension of the actuator along the straight line, can be positioned across the frame slot  112  with one end fixed to one frame part  110 A and the other end fixed to the other frame part  110 B. The linear expansion or contraction of the linear actuator  120  exerts a force across the frame slot  112  on the two frame parts  110 A and  110 B to cause them to pivot relative to each other around the connected end  114  of the frame slot  112 . As the dimension of the actuator  120  changes, the change forces the separation of the two frame parts  110 A and  110 B on two opposite sides of the frame slot  112  to change accordingly. Therefore, the frame slot  112  is an expandable slot and acts like a spring. Notably, this design transforms a linear expansion or contraction of the actuator  120  into a change in the circumferential length of the stretch frame  110  which can be shaped in various shapes. This mechanism can be used to stretch a fiber loop formed by winding optical fiber around the exterior surface  150  of the stretcher frame  110  multiple times under tension. The exterior of the stretcher frame  110  can be configured to various shapes, e.g., having smooth curves on the exterior surface  150  for holding a fiber loop. Examples of the exterior surface include circles, ellipses, squares with round corners, rectangles with round corners, and racetrack shapes. 
         [0037]    Therefore, this and other stretcher frame designs based on the present disclosure allow one or more linear actuators to be used to control a change in the circumferential length of a frame for stretching fibers without using cylindrical PZT actuators. Linear actuators, such as linear PZT actuators, are commercially available in large quantities and at relative low cost and can be easily packaged or integrated in various configurations to provide design flexibility in fiber stretchers based on the disclosure of this application. For example, because each linear PZT actuator has a limited expansion and contraction range, two or more PZT linear actuators can concatenate together as a combined actuator to increase the total expansion and contraction range of the combined actuator. Cylindrical PZT actuators tend to have small circumferences and thus have limited expansion and contraction ranges. It is difficult to combine two or more cylindrical PZT actuators to produce a larger circumferential change beyond the rang of each individual cylindrical PZT actuator. The stretcher frame designs that use one or more linear actuators can be structured to amplify the expansion and contraction ranges of the one or more linear actuators to produce large circumferential changes on the frames. This combination of transformation of a linear expansion or contraction of one or more linear actuators into a circumferential expansion or contraction of a fiber stretcher frame and the amplification of the linear expansion or contraction of one or more linear actuators can be advantageously used in various fiber stretcher frames. 
         [0038]    Referring back to  FIG. 6 , the stretcher frame  110  can be made from various materials. For example, the frame  110  can be machined with a single piece of a solid material, such as a metal (e.g., aluminum or copper), an alloy (e.g., stainless steel), and a synthetic material (e.g., plastics). The slot  112  can be machined by milling, sawing, electro wire-cutting, or other process to create the slot opening  116  at the one end. The thickness of the attachment at the other end  114 , which is the pivot point, depends on the material to be used and may be, e.g., from 0.1 mm to 10 mm. For better mechanical performance, a circular shape is formed around the pivot point b 114  y drilling or other process. Alternatively, the slot  112  can be cut all the way through the completely separate the frame  110  into two separated frame parts  110 A and  110 B and a binder component can be used to connect the two frame parts  110 A and  110 B to form the pivot point  114 . For example, a metal sheet between 0.01 mm to 10 mm can be used to bind two parts  110 A and  110 B together and act as a spring. 
         [0039]    Two anchors  131  and  132  are formed on two frame parts  110 A and  110 B for engaging the two ends of the actuator  120 , respectively. The actuator  120  is designed to change its dimension along the linear direction defined by the two anchors  131  and  132 . The actuator  120  is a linear actuator that expands or contracts along one linear direction. The linear actuator  120  can be in various configurations, such as a piezo-electric actuator, an electro-strictive actuator, a magneto-strictive actuator, a magneto-mechanical actuator, and a linear motor actuator. A control circuit  140  is provided to control the operation of the actuator  120  by producing an actuator control signal  142  to drive the actuator  120 . This control signal  142  can be electrical (e.g., when an PZT actuator is used) or magnetic (e.g., when a magneto-strictive actuator is used). The linear actuator  120  can be a combination of two or more concatenated linear actuators that are connected in series. 
         [0040]    The arrangement in  FIG. 1  provides an amplification mechanism to amplify the displacement produced by the actuator  120 . Referring to the insert illustration in  FIG. 1 , the amount of the change in spacing at the slot opening  116  varies with the position of the actuator  120  along the frame slot  112  between the pivot point  114  and the slot opening  116 . Assuming the actuator  120  is at a position is away from the pivot point  114  by h 1  and the spacing between the pivot point and the slot opening  116  is h 2 , a spacing d 1  of the frame slot  112  at the actuator  120  corresponds to a greater spacing d 2  at the slot opening  116 : d 2 /d 1 =h 2 /h 1 . Therefore, the smaller the h 1  (i.e., the closer the actuator  120  is placed to the pivot point  114  is), the bigger the spacing d 2  at the slot opening  116 , provided that the actuator  120  can generate a sufficient force to overcome the counter force of the stretcher frame  110  and the stretched fiber around the stretcher frame  110 . When the actuator  120  is placed at the center, the amount of amplification is 2. 
         [0041]    The spacing between the actuator anchors  131  and  132  is designed to apply a preload on the linear actuator  120  and to apply an initial stretch on the fiber loop on the stretcher frame  110 . This fiber initial stretch condition on the fiber loop allows the fiber frame  110  to decrease or increase the amount of stretch on the fiber loop as the linear actuator  120  contracts or expands. This initial fiber stretch condition can be achieved via various designs. In one design, for example, at least one screw can be mounted on one actuator anchor  131  or  132  to press against and hold one end of the linear actuator  120 . This screw can be turned to press the linear actuator  120  and to expand the frame slot  112  so as to produce a desired slot opening  116  for the initial stretch condition on the fiber loop. In another design, one actuator anchor  131  or  132  may be an adjustable anchor that is movably engaged to frame  110  and can be moved in position relative to the other actuator anchor to adjust the spacing between the two actuator anchors  131  and  132  to achieve the above initial fiber stretch condition. 
         [0042]      FIGS. 7A and 7B  show one implementation of the design in  FIG. 6  using a PZT linear actuator. In this design, a racetrack shaped stretcher frame  210  is used to support a smoothly curved exterior surface  150  for holding a fiber loop with an input fiber end  201  and an output fiber end  202 . Interior portions of the stretcher frame  210  are removed to form inner openings  221  and  222  to reduce the overall weight while maintaining an interior portion  212  in the center of the frame  210 . The frame slot  112  is formed in the interior portion  212  to split the frame  210  into two parts. The two anchors  131  and  132  are formed in the interior portion  212  for engaging the actuator  120  across the frame slot  112 . The two anchors  131  and  132  may be shaped from the interior portion  212  as an integral part as shown in  FIG. 7B  or separate anchor components that are engaged to the interior portion  212 . Mounting holes  211  can be formed in the interior portion  212  of the stretcher frame  210  for mounting the device to a support structure. 
         [0043]    In this example, the actuator  120  is implemented as a linear piezo-electric actuator made from multiple PZT cells stacked together to achieve a relatively high expansion coefficient and can operate at a relatively low voltage for each PZT cell. For example, a PZT actuator with a total length of 10 mm, a total expansion of more than 10 microns may be achieved with a voltage of 150 volts. Such PZT actuators can be made at a relatively low cost because they are widely used and are commercially available in large quantities. Some commercial actuators are in a linear form with small sizes, with a length around a centimeter and a cross section of a few millimeters. As such, the cross section of these commercial PZT actuators is too small to wind fiber loops as fiber stretchers. The stretcher frame  210  shown in  FIG. 7A  and stretcher frames in other designs in this application can amplify the displacements of such linear PZT actuators to achieve sufficiently large fiber stretching for various optical delay applications including fast variable delay applications. 
         [0044]    In this example, a set screw  250  is mounted on the anchor  131  and is pressed against to one end of the liner actuator  120  whose the other end is fixed to the anchor  132 . The set screw  250  is turned to push the linear actuator  120  to expand the width of the frame slot  112  to set the fiber stretcher into an initial default stretch condition where a fixed tension is generated by the stretcher frame  210  on the fiber. This set screen  250  can also apply and adjust a pre-load force to the linear PZT actuator  120 . The proper amount of pre-load force to the piezo actuator  120  can affect the performance and operation of the actuator  120 . to have optimum piezo actuator performance. When the actuator control signal, such as a voltage, is applied to the actuator  120 , the actuator  120  expands or contracts to cause the fiber loop wrapped around the race-track stretcher frame  210  to expand or shrink, and thus causing the optical delay to change. 
         [0045]    Stretching can induce optical birefringence in the fiber and this induced birefringence can change the optical polarization of the light. To minimize a change in light polarization of the signal passing through the fiber loop in the fiber stretcher, adhesive  230  can be applied at the end of each straight part of the racetrack stretcher frame  210  to affix the fibers to the mechanical structure so that only a straight fiber section on the stretcher frame  210  undergoes expansion or contraction as the linear actuator  120  expands or contracts while a fiber section that is curved is isolated from the expansion or contraction. The stretching of a straight fiber section does not change the orientation of each principal axis of the fiber and thus does not change the light polarization. For example, adhesive  230  can be applied at four marked locations to fix the fiber as shown in  FIG. 7A . This feature keeps curved or bended fiber portions under a fixed stretch and does not cause curved or bended fiber portions to expand or contract when the linear actuator  120  expands or contracts. The racetrack stretcher frame  210  in  FIGS. 7A and 7B  includes two parallel straight sections and the adhesive  230  is applied at two opposite ends of each straight section. Therefore, the fiber sections in the curved sections of the stretcher frame  210  are mechanically isolated from the stretching action caused by the linear actuator  120  and the action of the linear actuator  120  only applies to the straight section of the fibers to minimize the change in the optical polarization. 
         [0046]    This and other fiber stretcher designs in this application that use a fiber stretcher frame to amplify the displacement of one or more linear PZT actuators can be implemented to allow for small linear PZT actuators with relatively low capacitances to be used to provide high speed tuning in optical delays at a relatively low operating voltage and low power consumption. Because mass-produced commercial PZT actuators can be used in the present designs, fiber stretchers can also be manufactured at a reasonable cost. 
         [0047]      FIGS. 8A and 8B  show one implementation of the design in  FIG. 6  using a PZT linear actuator to control dimensions of two slots in a stretcher frame. The example for this implementation shown uses a racetrack geometry for the stretcher frame  210  similar to the geometry in  FIGS. 7A and 7B . The interior portion  212  are structured to include two frame slots  112 A and  112 B. The first frame slot  112 A has a first slot opening  116 A at one end of the interior portion  212  and a first pivot point  114 A at the other end of the interior portion  212 . The second frame slot  112 B has a second slot opening  116 B at one end of the interior portion  212  and a first pivot point  114 A at the other end of the interior portion  212 . In this particular example, the two frame slots  112 A and  112 B are substantially parallel to each other and share a common actuator  120 . The shared actuator  120  is engaged to across both frame slots  112 A and  112 B to control the slot openings  116 A and  116 B, respectively. The two expandable frame slots  112 A and  112 B divide the interior portion  212  into three sections having three actuator anchors  131 ,  310  and  132 , respectively. The shared actuator  120  is mounted on the three actuator anchors  131 ,  310  and  132  to control the spacing of the first frame slot  112 A based on the displacement of the actuator  120  between the anchors  310  and  132  and the spacing of the second frame slot  112 B based on the displacement of the actuator  120  between the anchors  210  and  131 . Therefore, the two expandable slots  112 A and  112 B allow fibers on both sides of the race-track to be stretched to increase the stretching range in comparison with the fiber stretcher device in  FIGS. 7A and 7B  with a single expandable frame slot  112 . 
         [0048]      FIGS. 9A and 9B  show one implementation of the design in  FIG. 6  using two or more PZT linear actuators to control dimensions of two expandable slots in a stretcher frame. Two linear actuators  120 A and  120 B are cascaded in series by actuator anchors  131 ,  411 ,  410 ,  412  and  132  on the interior portion  212 . The first linear actuator  120 A is engaged to anchors  410 ,  412  and  132  to control the spacing of the first expandable frame slot  112 A. A first set screw  250 A is engaged to press the first actuator  120 A to apply a preload. The second linear actuator  120 B is engaged to anchors  410 ,  411  and  131  to control the spacing of the second expandable frame slot  112 B. A second set screw  250 B is engaged to press the second actuator  120 A to apply a preload. The anchor  410  is shared by two actuators  120 A and  120 B. Alternatively, the two actuators  120 A and  120 B can be directly in contact with each other without the middle anchor  410 . 
         [0049]      FIGS. 10A and 10B  show one exemplary design a fiber stretcher device using multiple linear actuators arranged in a radial configuration to control a radial dimension of a stretcher frame. In this example, the fiber stretcher frame includes an inner circular frame  510 , and multiple exterior arc frames  520  circularly arranged around the inner circular frame  510  to form an outer circle concentric with the inner circular frame. Each exterior arc frame  520  is connected to the inner circular frame  510  via a connector  530  so that the inner circular frame  510 , the connectors  530  and the exterior arc frames  520  for an integral structure. Each exterior arc frame  520  can be pulled outward along the radial direction by stretching the connector  530  by different amounts to change its radial position and to stretch the fiber loop on the exterior arc frames  520 . The inner circular frame  510 , the connectors  520  and the exterior arc frames  520  may be made from a metal or a non-metal material. 
         [0050]    In this design, for each exterior arc frame  520 , two linear actuators  520  are symmetrically positioned on opposite sides of the connector  530  and are engaged to the inner side of the exterior arc frame  520  and the outer surface of the inner circular frame  510 . Each linear actuator  520  is oriented along the radial direction and to expand or contract along the radial direction. The linear actuators  520  are engaged to the inner circular frame  510  and arranged to form a circle concentric to the inner circular frame  510 . The dimension and shape of the connector  530  are designed to allow for expansion and contraction along the radial direction to change the spacing between the exterior arc frame  520  and the inner circular frame  510  under action of the two actuators  520 . The exterior surfaces  540  of the exterior arc frames  520  collectively form a circular exterior surface to hold the fiber loop. 
         [0051]    Similar to other designs, the linear actuators  120  can be mounted to apply an initial stretch on the connectors  530  to create an initial fiber stretch condition. Set screws  250  can be installed on the inner circular frame  510  at locations of the linear actuators  120 , respectively, to press each linear actuator  120  to stretch the connector  520  at a preload. A control circuit is used to apply control signals to all actuators  120  to expand or contract the fiber loop that is wrapped around the exterior arc frames  520 . Each linear actuator  120  can be controlled separately for more flexibility in operation. 
         [0052]    The fiber stretchers described in this application can be designed to set their resonant frequencies close to selected frequencies of interest. The resonant frequencies of such a fiber stretcher depend on a number of device parameters, including the mass of the mechanical structure, the actuator&#39;s capacitance, resistance, and resonant frequencies, the Young&#39;s modules of the fiber and the material of the mechanical structure, the number of turns of the fiber wound around the fiber stretcher, and the mounting mechanism. 
         [0053]    While this specification contains many specifics, these should not be construed as limitations on the scope of an invention or of what may be claimed, but rather as descriptions of features specific to particular embodiments of the invention. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or a variation of a subcombination. 
         [0054]    Only a few implementations are disclosed. However, it is understood that variations and enhancements may be made.