Fiber squeezer polarization controller with low activation loss

[0002] 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.

[0003] 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.

[0004] 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.

[0005] 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.

[0006] 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.

[0007] Thus an improved system for minimizing activation losses, minimizing fiber breakage and reducing the power needed to drive the piezoelectric is needed.

[0008] 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 surfaces are preferably super polished to reduce irregularities below 100 microns.

[0009] FIG. 1 shows a plurality of fiber squeezers oriented to vary the polarization of light transmitted through an optical fiber.

[0010] FIG. 2 shows an expanded cross sectional view of one embodiment of a fiber squeezer coupled to a power source.

[0011] FIGS. 3A and 3B show a stack of piezo-electrics used in an embodiment of the fiber squeezer.

[0012] FIG. 4 shows a control system for maintaining the frequency output of a power supply at the resonant frequency of the piezo-electric.

[0013] FIG. 5 shows a graph of current output by the power supply versus frequency used to drive a piezo-electric.

[0014] FIGS. 6A and 6B show an expanded view of a squeezing surface of the fiber squeezer.

[0015] 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.

[0016] FIGS. 8A and 8B show a system to measure Polarization dependent loss using fiber squeezer polarization controllers.

[0017] FIG. 8C shows a polarization stabilization system.

[0018] FIGS. 9A and 9B show a flow chart that describes a method of using fiber squeezer polarization controllers in the system of FIG. 8B.

[0019] 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.

[0020] 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 2 pi with respect to light with a polarization oriented perpendicular to the slow axis.

[0021] 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 '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.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] In order to maintain the output frequency of the power source driving the piezo-electric 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.

[0028] 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.

[0029] 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 protrusion, 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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. 8A and 8B show simple PDL meters that include a stable laser source 802, a polarization controller 804, a photodetector 806, and a microprocessor or a control circuit 808. A device under test (DUT) 810 is inserted between polarization controller 804 and photodetector 806. In a first measurement, control circuit 808 adjusts polarization controller 804 to minimize light loss in DUT 810 and thus maximize the optical power reaching photodetector 806. In a second measurement, microprocessor 808 adjusts polarization controller 804 to maximize light loss in DUT 810 and thus minimize power reaching photodetector 806. The PDL of DUT 810 can be calculated as:

[0034] PDL=10 log(first measurement/second measurement)

[0035] A major source of PDL measurement error arises from laser source 802 instability. FIG. 8B shows a PDL measurement system that minimizes inaccuracies resulting from laser source instability. The system of FIG. 8B includes a coupler 812 inserted between the laser source and the polarization controller. A second photodetector 814 connects an output port 816 of coupler 812 to monitor the laser power fluctuation. A/D convertors 818, 820 digitize the outputs of first photodetector 806 and second photodetector 814. Control circuit 808 computes the ratio of the two powers using the digitized outputs of photodetectors 814, 806. 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:

[0036] PDL=10 log(Rmax/Rmin).

[0037] 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.

[0038] The same configuration can also be used to stabilize the SOP of the light passing through a high PDL component 822, such as a LiNbO3 modulator or a polarizer as shown in FIG. 8C. Microprocessor 808 is then programmed to maintain a maximum power in photodetector 806 or to maintain a maximum power ratio Rmax (power received in 806 over power received in 814).

[0039] Software may be written to program control circuit 808. One method of programming control circuit 808 to control fiber squeezer polarization controllers is described in the flow charts of FIGS. 9A and 9B. In block 900, 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 904, an initial voltage equal to half the available or maximum voltage range is applied to each fiber squeezer.

[0040] 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 906 and increases the voltage applied in block 908. 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 912. When an increase in the applied voltage in block 908 causes the power ratio to drop in block 912, the microprocessor decreases the applied voltage in block 916 until the power ratio starts to decrease in block 918. The power ratio computed is stored and the process repeated on the second fiber squeezer in block 920 to maximize the received power ratio. When, in block 928, the stored power ratio and the recently computed power ratio do not match, the first fiber squeezer is selected in block 930 and the process of adjusting the first fiber squeezer and subsequently the second fiber squeezer described in blocks 904 to block 920 is repeated until the power ratio no longer changes in block 928. When no further power changes occur, the maximum received power ratio has been reached.

[0041] 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 932. For purposes of illustration, an increase in the applied voltage is assumed. When the increased voltage results in a decreasing power ratio in block 936, the microprocessor continues to increase the voltage in block 932 until the power ratio starts to increase 944. When an increase in applied voltage in block 932 causes the power ratio to rise in block 936, then the microprocessor decreases the applied voltage in block 948 until the power ratio starts to increase in block 952. After the first fiber squeezer, the second fiber squeezer is selected in block 956 and the process described in block 932 to 952 is repeated on the second fiber squeezer to minimize the received power ratio. In block 960, 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 932 to block 956 is repeated until the power ratio no longer changes in block 960. When no further power changes occur, the minimum received power ratio has been reached in block 964.

[0042] While the Applicants have 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's have 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.