Abstract:
An improved polarization control method and apparatus is provided. The described invention uses super-polished squeezing surfaces to apply pressure against a polyimide coated fiber thereby minimizing micro-bending effects that cause losses in the fiber. Special control circuitry has also been designed to maintain a driving source of piezo-electrics that control the squeezing surfaces at a resonant frequency, thereby minimizing the voltages needed to drive the piezo-electrics.

Description:
BACKGROUND OF THE INVENTION 
     Fiber optic network equipment and test equipment often require careful control of the polarization of light propagating in a fiber optic cable. One method of obtaining such polarization control is to insert a series of wave plates in the light path of the propagating light. However, utilizing wave plates typically involves directing light out of the fiber and redirecting the polarization corrected light back into an optical fiber. Such techniques are complicated and require careful alignment. Redirecting the polarization corrected light back into the optical fiber also results in back reflections and insertion losses. 
     In order to control the polarization of light in a fiber without the losses and alignment problems associated with wave plate systems, several patents describe a technique to control the polarization of light propagating in a fiber by applying pressure to an optical fiber. These references include U.S. Pat. No. 4,988,169 entitled “Optical Signal Control Method and Apparatus” issued to Neigle G. Walker; U.S. Pat. No. 4,753,507 entitled “Piezoelectric Loading Housing and Method” issued to Ramon P. DePaula et al; and U.S. Pat. No. 5,903,684 entitled “Independent Control of Normally Interdependent Light Transmission Characteristics of Optical Fiber” issued to Robert M. Payton. All three patents are hereby incorporated by reference. 
     Each of the three references describes a polarization compensation system that utilizes a plurality of fiber squeezers. Each fiber squeezer squeezes a different segment of the optical fiber. It is known that applying a transverse compressive force to a length of optical fiber changes the refractive index of the fiber via a photoelastic effect and introduces a stress induced birefringence. By applying transverse pressure along different directions, each fiber squeezer rotates the polarization of light propagating in the optical fiber about orthogonal axes on a Poincare sphere. 
     Although the principles of using pressure on a fiber to control polarization are well documented, one problem with building such systems is high signal losses caused by fiber squeezing. Typical activation induced losses in such systems are in the 0.5 dB range. The activation-induced loss measures the addition insertion loss caused by the activation of the device and is defined as the difference of the maximum and minimum insertion loss of the device at all activation conditions. This specification is particularly important because all polarization-impairment compensation schemes involve a feedback signal to activate the polarization controller. The activation-induced loss causes errors in the feedback signal and directly degrades the performance of the compensation apparatus. When a polarization controller is used in an instrument for measuring the polarization dependent loss (PDL) of optical components, the activation-induced loss limits the resolution and accuracy of the measurement. Controller PDL also contributes to error in the feedback system for PDL measurements and complicates the design of compensation hardware and software. 
     Current pressure based polarization controllers also suffer reliability problems because the applied pressure causes fiber fracturing and breakage. For example, the DePaula reference (507 patent) states that at room temperature, fiber fracturing begins when the fiber is deformed by only 1 percent. For example, a 125 micrometer glass fiber begins fracturing when the deformation is only 1.25 micrometers. To control fiber breakage and minimize losses, Shimizu reference describes coating the fiber with metal prior to the application of pressure. However, uniform metal coatings are not easily reproducible in production. 
     Another problem with prior art fiber squeezing systems is that high voltages are needed to drive the piezoelectric actuators that move the squeezers. Thus the driver circuits of the piezoelectric actuators require large power supplies and transformers to “step up” the voltages. These additional components increase the size and cost of the polarization controllers. 
     Thus an improved system for minimizing activation losses, minimizing fiber breakage and reducing the power needed to drive the piezoelectric is needed. 
     BRIEF SUMMARY 
     The present invention describes an improved polarization control system for controlling the polarization of light in an optical fiber. The systems uses various mechanisms such as piezoelectric drivers to control a fiber squeezing surface that applies a transverse compressive force to a segment of optical fiber. In one embodiment of the invention, the piezoelectric driver is driven at a resonance frequency to minimize the voltages needed to operate the piezoelectrics. The fiber-squeezing surface are preferably super polished to reduce irregularities below 100 microns. To minimize the possibility of optical fiber breakage, a poly-imide coating is maintained between a core of the fiber and the surface applying the transverse compressive force. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 shows a plurality of fiber squeezers oriented to vary the polarization of light transmitted through an optical fiber. 
     FIG. 2 shows an expanded cross sectional view of one embodiment of a fiber squeezer coupled to a power source. 
     FIGS. 3A-3B show a stack of piezo-electrics used in an embodiment of the fiber squeezer. 
     FIG. 4 shows a control system for maintaining the frequency output of a power supply at the resonant frequency of the piezo-electric. 
     FIG. 5 shows a graph of current output by the power supply versus frequency used to drive a piezo-electric. 
     FIGS. 6A -6B show an expanded view of a squeezing surface of the fiber squeezer. 
     FIG. 7 is a graph that shows the output when pressing a super-polished fiber squeezer versus the output when pressing with a regular polished fiber squeezer. 
     FIG. 8 shows a cross sectional view of a coated fiber for use in one embodiment of the invention. 
     FIGS. 9A and 9B show embodiments of systems to measure polarization dependent loss using fiber squeezer polarization controllers. 
     FIG. 9C shows one embodiment of a system to stabilize the state of polarization of light passing through a component with polarization dependent loss using fiber squeezer polarization controllers. 
     FIGS. 10A-10B show flow charts that describe a method of using fiber squeezer polarization controllers in the system of FIG.  9 B. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a series of fiber squeezers  104 ,  108 ,  112 ,  116  positioned along an optical fiber  120 . Each fiber squeezer is typically oriented such that the transverse compressive force applied in a first direction  124  by a first fiber squeezer  104  forms an angle  128  with the transverse compressive force applied in a second direction  132  by an adjacent second fiber squeezer  108 . Both transverse compressive forces are applied in planes that are orthogonal to a propagating direction of light in optical fiber  120 . 
     When a fiber squeezer applies pressure to a fiber, a linear birefringence is induced in the fiber. The slow axis of the birefringence is oriented in the direction of the applied pressure and typically increases linearly with the applied force. The pressure-induced birefringence can vary from 0 to pi/2. The applied force also changes the optical path length and induces a phase change in the light propagating in the fiber. More specifically, the retardation of light with a polarization oriented along the slow axis of the birefringence may be retarded from 0 to 2pi with respect to light with a polarization oriented perpendicular to the slow axis. 
     By adjusting the pressure of each fiber squeezer  104 ,  108 ,  112 ,  116 , the polarization of the light propagating in the fiber can be rotated along a Poincare sphere as described in several prior art references such as the previously incorporated by reference &#39;169 patent. A suitable control system (not show) may monitor the polarization input to and output by the polarization controller  100  to regulate the amount of pressure applied by each fiber squeezer  104 ,  108 ,  112 ,  116 . In general only two fiber squeezer such as fiber squeezer  104 ,  108  would be sufficient to control the polarization of the optical signal guided in the fiber, however, in practice additional fiber squeezers are used to make the polarization controller “reset” free. Resetting the polarization controller causes temporary disruption of the output polarization state and is undesirable in systems where continuous polarization tracking is required In prior art implementations, the relative orientation angle between two adjacent fiber squeezers is typically set to 45 degrees. However, in the present invention, other orientations may be used. 
     FIG. 2 illustrates a cross section of a fiber squeezer such as fiber squeezer  104 . In FIG. 2, a fiber holder  204  includes an ultra smooth flat surface  208  that supports a first side, typically a polyimide coating  212  surrounding a cladding  216  of optical fiber  220 . A pressure block  224  including a second ultra smooth surface  228  presses against an opposite side of the poly-imide coated optical fiber  220 . 
     Various mechanisms may be used to control the pressure applied by pressure block  224  to optical fiber  220 . In a first embodiment, a spring  232  maintains a constant force on pressure block  224 . The spring  232  typically has a spring constant K such that the force applied by the pressure block is equal to F=K X where X is the distance by which the spring is compressed. 
     The pressure applied by the spring is adjusted by changing the compression of spring  232 . In one embodiment, pressure on spring  232  is controlled by a screw  236 . Threads on screw  236  interlock with threads in holder  204  such that rotation of screw  236  moves the screw in and out of holder  204 . Rotation of screw  236  in a predetermined direction increases the compression of spring  232  and causes pressure block  224  to press harder against optical fiber  220 . The increase in pressure further changes the index of refraction and increases the birefringence of fiber core  216 . 
     In a second embodiment of the invention, a piezo-electric actuator replaces screw  236  in moving pressure block  224 . FIGS. 3A and 3B illustrate a piezo electric stack  300 . A electrical source such as voltages source  304  provides power to stack  300 . The stack includes piezo electric elements  308 ,  312 ,  316 . Altering the voltage applied across stack  300  changes the displacement of stack  300 . Substituting screw  236  with piezo electric stack  300  allows a user to use the output voltage of voltage source  304  to control the force applied by pressure block  224 . 
     One difficulty with piezo electric elements such as elements  308 ,  312 ,  316  is that significant voltages are typically needed to achieve the desired displacement. Often the voltage requirements may exceed 50 volts. Generating these relatively high voltages in solid state systems involves transformers and powerful power supplies. However, by setting and maintaining the driving frequency of voltage source  304  at the resonant frequency of piezo electric stack  300 , the necessary voltage can be significantly reduced. 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 in best case situations, to below 2 volts. These low voltage makes it possible to drive the piezo-electric stack using low cost commercially available integrated circuits. 
     In order to maintain the output frequency of the power source driving the piezoelectric stack at the resonant frequency, a driving circuit current monitor is illustrated in FIG.  4 . In the driving circuit current monitor of FIG. 4, a variable frequency power source  404  is coupled to at least one piezo-electric  408  in polarization scrambler  412  via circuit loop  416 . A control circuit  420  monitors the power output of power source  404 , typically by monitoring either the current and/or the voltage of the output. 
     The current used by piezo-electric  408  typically varies as the frequency output by power source  404  changes. FIG. 5 graphs current output by power source  404  when powering a piezo electric along vertical axis  504  versus frequency of the power supply output along horizontal axis  508 . The minimal point  512  is the resonant frequency of piezo-electric  408 . When the output of power source  404  is at the resonant frequency, the current requirements of piezo-electric  408  are minimized. Thus in one embodiment of the invention, control circuit  420  dithers the frequency of the power signal output by power source  404  to determine the resonant frequency of the piezo-electric and maintains the output frequency of power source  404  at the determined resonant frequency. When the resonant frequency changes due to changing environmental parameters such as temperature, control circuit  420  automatically adjusts the frequency output of power source  404  to track the changing resonant frequency. 
     FIGS. 6A and 6B show an expanded view of a squeezing surface  604  of the fiber squeezer  200 . Traditionally, such fiber squeezing surfaces would have irregularities including protrusions  608  and indentations  610  that deviate from a plane  612  of a smooth surface. Each protrusion has a height  616  that represents the shortest distance from the top  624  of the protrusion to the plane  612  of the smooth surface Likewise, each indentation has a depth  620  from the bottom of the indentation to plane  612 . It has been discovered that such irregularities, and particularly the protrusions, are largely responsible for the activation losses when squeezing surface  604  presses against the fiber. In particular, the protrusions produce microbending in the fiber surface that results in light loss. Trace line  704  of FIG. 7 shows the drop  706  in light intensity propagating in the fiber due to microbending effects when an untreated squeezing surface applies pressure. 
     In order to reduce microbending effects, the fiber squeezing surface is 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. 
     The objective of the superpolish is to reduce the height  616  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 plane  612  of the smooth surface is less than 100 microns. Trace line  708  of FIG. 7 shows a drop  710  in light intensity in the fiber when the super polished fiber squeezing surface applies pressure. 
     The difference in loss between drop  710  and drop  706  represents the improvement in activation loss due to super-polishing the squeezing surface. By reducing protrusions to less than 100 microns, 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. 
     FIG. 8 shows a cross sectional view of a coated fiber  800  for use in one embodiment of the invention. Fiber  800  includes a central core  804 , a cladding  808 , and a protective polyimide coating  812  (typical fiber generally has a soft crylite coating). The fiber cladding typically has a diameter of approximately 125 micrometers. Microcracks can often found on the surface of the fiber cladding and these cracks are responsible for 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  812  is 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 range from nonometers to 10 microns. 
     Prior art polyimide coatings are used to increase the operational temperature range of the fiber. However, it has been discovered that the polyimide also effectively seals the micro-cracks on the surface of the fiber. Sealing the microcracks significantly reduces the probability of stress induced breakage. Compared with prior art metal coating, the polyimide coating is thin, uniform, low cost, and environmentally more stable. 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, CT and sold under the trade name PYROCOAT. These polyimide coated fibers are typically sold for high temperature applications. 
     Another advantage of the polyimide coated fiber is its small diameter and hard surface. A regular fiber generally includes a core, a cladding, and a soft protective buffer, with a typical diameter of 250 microns. The soft protective buffer dampens the pressure applied to the fiber and thus reduces the fiber squeezing effect. Squeezing induced birefringence in an optical fiber is inversely proportional to the fiber diameter, the small diameter. Thus the small diameter (typically 160 microns) of the polyimide coated fiber is very sensitive to the squeezing induced birefringence. The high sensitivity reduces the power requirements of the fiber squeezer. In particular, the half-wave voltage of the polyimide enhanced fiber squeezer may be reduced by approximately 36%. 
     A particularly suitable application for the described fiber squeezers is in polarization dependent loss (PDL) measurement equipment. The PDL of an optical component is defined as the difference between the maximum and the minimum insertion losses for all possible input states of polarization (SOP). FIGS. 9A and 9B show simple PDL meters that include a stable laser source  902 , a polarization controller  904 , a photodetector  906 , and a microprocessor or a control circuit  908 . A device under test (DUT)  910  is inserted between polarization controller  904  and photodetector  906 . In a first measurement, control circuit  908  adjusts polarization controller  904  to minimize light loss in DUT  910  and thus maximize the optical power reaching photodetector  906 . In a second measurement, microprocessor  908  adjusts polarization controller  904  to maximize light loss in DUT  910  and thus minimize power reaching photodetector  906 . The PDL of DUT  910  can be calculated as: 
     
       
           PDL= 10 log(first measurement/second measurement) 
       
     
     A major source of PDL measurement error arises from laser source  902  instability. FIG. 9B shows a PDL measurement system that minimizes inaccuracies resulting from laser source instability. The system of FIG. 9B includes a coupler  912  inserted between the laser source and the polarization controller. A second photodetector  914  connects an output port  916  of coupler  912  to monitor the laser power fluctuation. A/D convertors  918 ,  920  digitize the outputs of first photodetector  906  and second photodetector  914 . Control circuit  908  computes the ratio of the two powers using the digitized outputs of photodetectors  914 ,  906 . The computed ratio is independent of the laser power. FIG. 10 describes the process of obtaining the maximum power ratio Rmax and the minimum power ratio Rmin These power ratios are used to compute the PDL using the equation: 
     
       
           PDL= 10 log( R  max/ R  min). 
       
     
     A second factor that limits PDL measurement accuracy of the test is the activation loss and the PDL of the polarization controller. The low activation loss and the low PDL of fiber-squeezer based polarization controllers makes such controllers especially attractive for use in PDL measurements. 
     The same configuration can also be used to stabilize the SOP of the light passing through a high PDL component  922 , such as a LiNbO3 modulator or a polarizer, as shown in FIG.  9 C. Microprocessor  908  is then programmed to maintain a maximum power in photodetector  906  or to maintain a maximum power ratio Rmax (power received in  906  over power received in  914 ). 
     Software may be written to program control circuit  908 . One method of programming control circuit  908  to control fiber squeezer polarization controllers is described in the flow charts of FIGS. 10A and 10B. In block  1000 , a fixed mechanical load is applied to the fiber squeezer to induce a birefringence in the fiber. The load induces birefringence even when no voltage is applied to the piezo-electric controlling the fiber squeezer. In block  1004 , an initial voltage equal to half the available or maximum voltage range is applied to each fiber squeezer. 
     To obtain the maximum power ratio, the control circuit, such as a microprocessor, changes the voltage applied to a piezoelectric of first fiber squeezer. In one embodiment, the microprocessor selects the first fiber squeezer in block  1006  and increases the voltage applied in block  1008 . When the increased voltage results in an increased power ratio, the microprocessor continues to increase the applied voltage until the power ratio starts to decrease in block  1012 . When an increase in the applied voltage in block  1008  causes the power ratio to drop in block  1012 , the microprocessor decreases the applied voltage in block  1016  until the power ratio starts to decrease in block  1018 . The power ratio computed is stored and the process repeated on the second fiber squeezer in block  1020  to maximize the received power ratio. When, in block  1028 , the stored power ratio and the recently computed power ratio do not match, the first fiber squeezer is selected in block  1030  and the process of adjusting the first fiber squeezer and subsequently the second fiber squeezer described in blocks  1004  to block  1020  is repeated until the power ratio no longer changes in block  1028 . When no further power changes occur, the maximum received power ratio has been reached. 
     After computing the maximum power ratio, the microprocessor computes a minimum power ratio using an analogous procedure. The microprocessor alters the voltage applied to the first fiber squeezer piezoelectric in block  1032 . For purposes of illustration, an increase in the applied voltage is assumed. When the increased voltage results in a decreasing power ratio in block  1036 , the microprocessor continues to increase the voltage in block  1032  until the power ratio starts to increase  1044 . When an increase in applied voltage in block  1032  causes the power ratio to rise in block  1036 , then the microprocessor decreases the applied voltage in block  1048  until the power ratio starts to increase in block  1052 . After the first fiber squeezer, the second fiber squeezer is selected in block  1056  and the process described in block  1032  to  1052  is repeated on the second fiber squeezer to minimize the received power ratio. In block  1060 , the power ratio from the most recent iteration is compared with the power ratio computed in a previous iteration. When the comparison does not match, process of adjusting the first fiber squeezer and subsequently the second fiber squeezer described in blocks  1032  to block  1056  is repeated until the power ratio no longer changes in block  1060 . When no further power changes occur, the minimum received power ratio has been reached in block  1064 . 
     While the Applicant has described various embodiments of the polarization controller and/or scrambler system other embodiment may be apparent to one of ordinary skill in the art. For example, although Applicant has specified particularly dimensions, such as particularly thicknesses of poly-imide coating, other dimensions and thicknesses may also be used and still fall within the scope of the invention. Uses for the fiber squeezer also should not be limited to the particular systems described. Other uses of the fiber squeezers include, but are not limited to, fiber squeezer phase modulators and fiber squeezer polarization rotators. Thus the invention should not be limited to merely the embodiments described in the preceding specification. Rather, the limitations of the invention should only be limited by the claims which follow and equivalents thereof.