Abstract:
This invention relates to a method of making optical fiber having low polarization dependence and an acousto-optical filter with low PDL. A section of the fiber is heated and then allowed to cool. At least the heating is controlled to reduce stresses in a cladding layer surrounding a core of the interaction length after the interaction length is allowed to cool to reduce polarization dependence of the cladding layer. Preferably, at least time and temperature of heating is controlled.

Description:
CROSS-REFERENCES TO RELATED CASES  
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 09/738,282, filed on Dec. 14, 2000, which is a continuation of Ser. No. 09/426,060, filed Oct. 22, 1999, now U.S. Pat. No. 6,266,462, which is a continuation-in-part of Ser. No. 09/022,413, filed Feb. 12, 1998, now U.S. Pat. No. 6,021,237, which claims priority to Korean Application No. 97-24796, filed Jun. 6, 1997. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention generally relates to a method of preparing optical fiber with low polarization dependence for use in acousto-optic applications, and, more particularly, to an acousto-optic filter employing such a fiber to reduce polarization-dependent loss (PDL) in the filter.  
           [0004]    2. Discussion of Related Art  
           [0005]    As an optical signal traverses an optical network, the signal is subject to losses and nonlinear effects that result in signal attenuation and distortion. Amplifiers, such as erbium-doped fiber amplifiers (“EDFA&#39;s”), are typically placed approximately every 80 kilometers along an optical fiber to boost signal strength. However, such amplifiers impose their own distortions on the signal power spectral distribution (as a function of wavelength). One of the major distortions is caused by the non-uniform gain profile (as a function of wavelength) of the amplifiers, which imposes a non-uniform spectral distribution on the amplified signals. It is especially important in wavelength division multiplexed (“WDM”) networks to maintain a uniform spectral distribution across all channels.  
           [0006]    Static filters are often used to attenuate the signal power as a function of wavelength to achieve a substantially uniform power distribution. Static filters, however, cannot adapt to dynamically changing conditions such as amplifier aging, temperature variations, channel add/drop, fiber loss and other changes in components along the transmission line. Moreover, the required filter shape is dependent upon system configuration e.g. the spacing between amplifiers. Static filter characteristics cannot be modified to compensate for these changes without replacing the filter itself.  
           [0007]    To overcome these problems, it is known in the art to employ dynamic wavelength tunable filters to flatten or equalize the signal spectrum, as well as to obtain any desired spectral shape. One such filter is an all-fiber acousto-optic tunable filter (“AOTF”) described in U.S. Pat. No. 6,233,379, entitled “Acousto-optic filter,” which is assigned to the assignee of the present invention and incorporated by reference herein. As described in the patent, the all-fiber AOTF is a multiple notch filter, with a transfer function characterized by notch depth and center frequency (or wavelength).  
           [0008]    One problem with the all-fiber AOTF is that the effect of the filter on light in the fiber is polarization dependent. For example, although the filter may attempt to place a notch at one desired center frequency, the notch will effectively be placed at a different center frequency for each polarization splitting one notch into two. The relative frequency shift between the polarization-dependent notches causes a difference between the transmissions of the different polarizations through the filter as a function of frequency, which results in a polarization-dependent loss in the filter. It is desired to reduce the polarization dependence of light in optical fiber, and to thereby reduce PDL in an all-fiber AOTF.  
         SUMMARY OF THE INVENTION  
         [0009]    This invention relates to a method of making optical fiber having low polarization dependence and an acousto-optical filter, generally of the kind described in U.S. Pat. No. 6,266,462, with low PDL. A section of the fiber is heated and then allowed to cool. At least the heating is controlled to reduce stresses in a cladding layer surrounding a core of the interaction length after the interaction length is allowed to cool to reduce polarization dependence of the cladding layer. At least time and temperature of heating may be controlled.  
           [0010]    The optical fiber may be used for constructing an acousto-optical filter. The filter includes a support, and first and seconds mounts at spaced locations on the support. The optical fiber has first and second mounted portions secured to the first and second mounts respectively. An exposed section of the fiber is heated and cooled between the first and second mounted portions. A signal generator is operable to generate a periodic signal. An electro-acoustic transducer has a terminal connected to the signal generator and an actuating portion, the electric signal causing vibration of the actuating portion, and the actuating portion being connected to the interaction length so that the vibration generates a transverse wave traveling along the interaction length. Such a filter has the ability to reduce an amplitude of one or more selected wavelengths of light as the light travels through the interaction length.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The invention is further described by way of example with reference to the accompanying drawings wherein:  
         [0012]    [0012]FIG. 1 is a side view illustrating manufacturing of optical fiber;  
         [0013]    [0013]FIG. 2 is a side view illustrating severing of a length of optical fiber manufactured according to the process shown in FIG. 1;  
         [0014]    [0014]FIG. 3 is a cross-sectional side view of an interaction length of the severed length of the optical fiber of FIG. 2;  
         [0015]    [0015]FIG. 4 is a view similar to FIG. 3 after a section of a jacket of the optical fiber is stripped;  
         [0016]    [0016]FIG. 5A is a cross-sectional end view on  5 - 5  in FIG. 4 illustrating stresses in a cladding layer of the interaction length;  
         [0017]    [0017]FIG. 5B is a cross-sectional plan view through a section of the optical fiber;  
         [0018]    [0018]FIG. 6 is a side view illustrating apparatus that is used to anneal the cladding layer of the interaction length;  
         [0019]    [0019]FIG. 7 is a cross-sectional side view of an acousto-optical tunable filter according to an embodiment of the invention;  
         [0020]    [0020]FIG. 8 is a side view illustrating functioning of the filter;  
         [0021]    [0021]FIG. 9 is a view similar to FIG. 7 illustrating coupling of x-polarized light into the cladding layer;  
         [0022]    [0022]FIG. 10 is a view similar to FIG. 9 illustrating coupling of y-polarized light into the cladding layer;  
         [0023]    [0023]FIG. 11 is a graph illustrating transmission of x-polarized light and y-polarized light through a core of the fiber, both before and after annealing; and  
         [0024]    [0024]FIG. 12 is a graph illustrating PDL before and after annealing.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]    Optical fiber fabrication typically consists of two major steps: preform fabrication and fiber drawing. There are a number of different methods for preform fabrication, such as modified chemical vapor deposition (MCVD), outside vapor deposition (OVD), and vapor-phase axial deposition (VAD). FIG. 1 shows a preform that is manufactured utilizing the MCVD technique. The process is initiated with a silica tube  12 , which eventually forms an outer cladding layer of a fiber. An inner cladding material  14  is deposited on an inner surface of the silica tube  12 , and eventually becomes an inner cladding layer of the fiber. A core material  16  is deposited on the inner cladding material  14 .  
         [0026]    A heat source  18  is located near an end of the preform  10 . The heat source  18  heats the end of the preform  10  to approximately 2000° C. to melt it. Rollers  20  engage with material melted out of the end of the preform  10 . The rollers  20  rotate, thereby drawing an optical fiber  24  out of the preform  10 . As the fiber is drawn, a polymer jacket material (not shown) is coated on the fiber. The optical fiber  24  shown here is a single-mode fiber which is composed of the materials  12 ,  14 , and  16 , and is rolled into a roll  26 .  
         [0027]    In FIG. 2, a length  28  of the fiber  24  is paid out from the roll  26  and cut from the remainder of the fiber on the roll  26  for the purpose of constructing an optical filter according to the invention. FIG. 3 illustrates in cross section a portion of the severed length  28 . The optical fiber includes a glass core  30  made of the core material  16 , a cladding layer  32  surrounding the core  30 , wherein the cladding layer itself may include an inner cladding layer  32 A surrounding the glass core  30 , and an outer cladding layer  32 B surrounding the inner cladding layer  32 A. A jacket  34  surrounds the cladding material  32 B.  
         [0028]    As part of the process of constructing a filter according to the invention, a portion of the jacket  34  is removed to expose a section  36  of the fiber. A number of techniques may be employed to remove the jacket, including mechanical stripping and exposure to hot sulfuric acid, among others. FIG. 4 illustrates the severed length  28  after a portion of the jacket is stripped from the section  36 . First and second portions  38 A and  38 B of the jacket remain on the cladding layer  32 . The portions  38 A and  38 B are located on opposing sides of the stripped section  36 .  
         [0029]    In order to achieve guiding characteristics, the core  30  is designed to have a higher refractive index than the cladding region by adding impurities such as GeO 2  and P 2 O 5  to the SiO 2  basis of the core material  16 . Such impurities in the core  30  not only create the required refractive index difference with respect to the cladding  32 A and  32 B, but also make the coefficient of the thermal expansion (CTE) and the melting temperature different from that of the cladding. Therefore, when the preform is fabricated in a high temperature of approximately 2000° C. and cooled down to room temperature, a significant amount of stress is generated in the core  30  and the cladding  32 A and  32 B. This inherent stress is called “thermally-induced stress”.  
         [0030]    Moreover, when the preform  10  is pulled to the optical fiber  28  at the drawing tower, the optical fiber is exposed to a drawing tension of typically 100-1,000 N, and this stress becomes frozen in the optical fiber  28  while the optical fiber  28  is cooled down to room temperature. Therefore, an additional stress field is created in the optical fiber, which is called “mechanically-induced stress.” 
         [0031]    [0031]FIGS. 5A and 5B show the combined stress profile. The core is under axial, radial and tangential tensile stress. The cladding  32 A and  32 B is under radial tensile stress and under axial and tangential compressive stress. There is thus a discontinuity of the stress field in the core-cladding interface.  
         [0032]    According to the invention, the cladding layer  32  of the stripped section  36  is then annealed utilizing an apparatus  50 , as shown in FIG. 6. The apparatus  50  may employ, for example, a modified CW-200 Fused Coupler/WDM workstation sold by Lightel Technologies, Inc. of Kent, Wash. The apparatus  50  includes a support structure  52 , first and second attachment formations  54  and  56  respectively, a flame nozzle  58 , and a hydrogen source  60 .  
         [0033]    The attachment formation  54  is rigidly secured to the support structure  52 . The attachment formation  56  is movably secured to the support structure  52 . The hydrogen source  60  is connected to the flame nozzle  58 . The flame nozzle  58  is secured to the support structure  52  for movement between the attachment formations  54  and  56 .  
         [0034]    In use, the portions  38 A and  38 B of the jacket  34  are attached to the first and second attachment formations  54  and  56 , respectively. A force F is applied, which tends to move the attachment formation  56  away from the attachment formation  54 , thereby creating a tension in the stripped section  36 .  
         [0035]    Hydrogen from the hydrogen source  60  flows to the flow nozzle  58  and is lit at an exit from the flame nozzle  58  to create a flame  64 . The nozzle  58  and the flame  64  are located above the stripped section  36  so as to heat the stripped section  36  from above. Hydrogen may be preferred to any other source of fuel because hydrogen combustion does not produce carbon or hydrocarbon byproducts that may deposit on the cladding layer  32 . Those skilled in the art will recognize that electro-resistive and other heating sources may be employed in the present invention instead of the hydrogen flame described in this example.  
         [0036]    The nozzle  58  moves in a direction  66  parallel to the longitudinal axis of the stripped section  36 . The advancing flame  64  heats areas of the stripped section  36  as those areas are exposed to the flame  64 . Heating of the stripped section  36  is primarily due to radiation from the flame  64 . Regions of the stripped section  36  trailing the flame  64  are allowed to cool. Cooling of the stripped section  36  is primarily due to convection of the heat to ambient air. The force F compensates for heat-induced elongation of the stripped section  36  by moving opposing ends of the stripped section  36  apart. The fiber is heated and cooled without the core  30  expanding by more than 20%.  
         [0037]    The effect of heating and cooling the stripped section  36  is that the cladding  32 A and  32 B is annealed. Fiber formed by modified chemical vapor deposition has stress characteristics that are particularly conducive to the beneficial effects of this process.  
         [0038]    The flame  64  may be in the range 1-20 mm wide as measured along the stripped section  36 . The flame  64  may be held at a distance of 0.1-5 mm, or, or more particular 0.5-5 mm from the stripped section  36 . Movement of the flame in the direction  66  may be at a speed of 1-50 mm per second, or, more particularly, 1-10 mm/s. The stripped section  36  may be heated to a temperature between 500-1300° C., and, more particularly, to between 800-1000° C. The force F may be in the range 0.05-0.5 N, or, more particularly, 0.05-0.15 N, maintained substantially constant.  
         [0039]    [0039]FIG. 7 of the accompanying drawings illustrates an acousto-optic filter  120  constructed according to an embodiment of the invention. The filter  120  is of the kind described in U.S. Pat. No. 6,266,462, issued Jul. 24, 2001, which is incorporated herein by reference. The filter  120  includes a mounting construction  122 , the severed length  28  of the optical fiber, and an electrical signal generator  130 .  
         [0040]    The mounting construction  122  includes a heat sink  132 , an acoustic wave generator, such as a piezo-electric transducer  134 , an acoustic wave propagation member  136 , such as an aluminum horn, an outer tube arrangement  138 , and an end plug  140 .  
         [0041]    Gold terminals are sputtered on opposing surfaces of the piezo-electric transducer  134 . One terminal is located against the heat sink  132  and attached to the heat sink  132 . The base of the acoustic wave propagation member  136  is then attached to an opposing terminal of the piezo-electric transducer  134 .  
         [0042]    Openings are made in the heat sink  132 , piezo-electric transducer  134 , and acoustic wave propagation member  136  to form a continuous passage. The end of the severed length  28  having the portion  38 A of the jacket is inserted through the opening of the acoustic wave propagation member  136 , whereafter it is inserted through the openings in the piezo-electric transducer  134  and the heat sink  132 .  
         [0043]    The portion  38 B of the jacket is then located in a groove in the end plug  140 . A resin is then placed in the groove and allowed to cure, thereby securing the portion  38 B of the jacket to the end plug  140 .  
         [0044]    Resin is also applied to the interaction length  37  where it protrudes from a tip  150  of the acoustic wave propagation member  136 , and flows into the tip  150  of the acoustic wave propagation member  136 . The resin then cures and secures the interaction length  37  to the tip  150  of the acoustic wave propagation member  136 .  
         [0045]    A damper  152  is located on the optical fiber  142 . The damper  152  is coaxially disposed on the stripped section  36  adjacent the portion  38 B of the jacket. The length of exposed fiber from the tip  150  to the end of the damper  152  nearest the tip  150  is the “interaction length”  37  of the filter. Generally, the interaction length or “interaction region” is the length of fiber in which light is coupled from one mode to another, and, more particularly in this case, the portion of the exposed section  36  not covered by the damper  152 .  
         [0046]    An end  154  of the outer tube arrangement  138  is then located over the portion  38 B of the jacket and moved over the end plug  140  until it contacts a surface of the heat sink  132 . A second, opposing end  156  of the outer tube arrangement  138  is located over the end plug  140 . The positioning of the end plug  140  is then adjusted within the end  156 . By adjusting the positioning of the end plug  140 , the interaction length  37  of the optical fiber  142  is tensioned by about 0.2 N to eliminate slack. When a predetermined tension in the interaction length  37  is reached, a resin is applied to an interface between the end plug  140  and the end  156 . The resin is allowed to cure, thereby securing the end plug  140  stationarily within the end  156 . The tension of the interaction length  37  is thereby set.  
         [0047]    The signal generator is connected to the transducer  134  through leads  160  and  162 . The lead  160  couples to the heatsink  132 , which is itself electrically coupled to a terminal on one face of the transducer  134 . The lead  162  is electrically connected to the opposing face of the transducer  134 , either directly to the terminal on the opposing face, or indirectly through the acoustic wave propagation member  136 . The heat sink  132  and the acoustic wave propagation member  136  can be made of conductive aluminum so that the terminals on the opposing sides of the piezo-electric transducer  134  are at the voltages of the leads  160  and  162 , respectively. A voltage potential is thereby created across the piezo-electric transducer  134 .  
         [0048]    The signal generator  130  applies across the piezo-electric transducer  134  a voltage at one or more frequencies in the range 0-20 MHz, or more particularly 0-3 MHz. The voltage signal applied across the piezo-electric transducer  134  causes opposing surfaces of the piezo-electric transducer  134  to vibrate relative to one another in a direction transverse to a longitudinal axis of the interaction length  37 . Adjusting the frequency and amplitude of the electrical signal applied to the transducer results in a corresponding change in the frequency and amplitude, respectively, of the mechanical vibration of the transducer. Those skilled in the art will recognize that the invention may employ acoustic wave exciters other than the acoustic wave exciter formed from the combination of the signal generator  130 , acoustic wave generator  134  and acoustic wave propagation member  136  described herein.  
         [0049]    Vibrations of opposing surfaces of the piezo-electric transducer  134  are transferred through the acoustic wave propagation member  136  to the tip  150  thereof. The tip  150  vibrates periodically in response to the change in the voltage. Movement of the tip  150  is transferred to the end of the interaction length  37  closest to the tip  150 .  
         [0050]    [0050]FIG. 8 illustrates how vibration of the tip  150  imposes acoustic waves in the interaction length  37 . In the present example, the waves are y-direction transverse flexural waves that travel along the interaction length  37  from the tip  150  to the damper  152 . The damper  152  is designed to absorb the waves or otherwise minimize reflection of the waves back to the tip  150 . The creation of a standing wave is thereby prevented.  
         [0051]    In use, the filter  120  is inserted into a fiber optic transmission line. A light signal is transmitted through the core  30 . The light signal may be modulated as a WDM signal having many channels, each at a different wavelength. For various reasons, including the non-uniform gain profiles of amplifiers along the fiber optic transmission line, the intensity of light may differ from channel to channel at the point where the light enters the optical fiber  142  of the filter  120 .  
         [0052]    The effect of the acoustic waves in the interaction length  37  is that the intensity of selected wavelengths of light traveling through the interaction length  37  is attenuated by coupling these wavelengths from a mode in the core into one or more modes in the cladding layer  32  of the interaction length  37 . This coupling creates a notch in the transmission spectrum centered at each selected wavelength. By changing the frequency of the applied electrical signal, and thus the frequency of the acoustic waves in the interaction length  37 , the center wavelength of the notch can be altered . Furthermore, by changing the magnitude of the applied voltage (and thereby the magnitude of the acoustic wave), the depth of the notch (representing the amount of light coupled to the other mode) can be changed. By cascading multiple acoustic exciter/interaction length combinations and/or applying multiple acoustic frequencies with each exciter, a combination of notches of different optical center frequencies and depths may be achieved, thereby allowing creation of a desired filter transfer-function as described in Ser. No. 09/738,282. Such a filter may be employed for gain equalization purposes. Those skilled in the art will recognize that, as an alternative to coupling light between core and cladding modes, an AOTF may also couple light between different core modes. Further details of the functioning of the filter  120  are described in U.S. Pat. No. 6,266,462 referenced above.  
         [0053]    [0053]FIGS. 9 and 10 illustrate how light is coupled into the cladding layer  32  after application of an acoustic wave. These figures are for conceptual purposes only, and do not necessarily reflect the actual intensity distribution in the fiber. Light traveling in the core mode in the core  30  couples into both an x-polarized cladding mode including regions  70  and  72  in the cladding  32  (as shown in FIG. 9), and into a y-polarized cladding mode including regions  80  and  82  (as shown in FIG. 10). X-polarized and y-polarized components of light traveling in the core couple preferentially into corresponding x-polarized and y-polarized cladding modes, as shown in FIGS. 9 and 10, respectively. The arrows in FIGS. 9 and 10 indicate the direction and phase differences of the polarization of the light in each mode.  
         [0054]    The center wavelength λ 0  of light coupling into the cladding layer  32  is a function of the index of refraction β of the material of the cladding layer  32 . At different points in the fiber, stress in the cladding layer changes the index of refraction β to an effective index of refraction β eff  which is different from the index of refraction β without any stress in the cladding layer  32 . As a result of this stress-induced change in refractive index, the center wavelength λ 0  shifts, and is thus also recognized as a function of stress in the cladding layer  32 .  
         [0055]    Referring to FIGS. 5A and 5B, there is a larger tensile stress in the x-direction than in the y-direction. The larger tensile stress in the x-direction results in an effective index of refraction in the x-direction β eff-x  which differs from the index of refraction β of the cladding layer  32  with no stress therein. The effective index of refraction in the y-direction β eff-y  is however substantially equal to the index of refraction β of the cladding layer  32  without stresses in the cladding layer  32 . The effective index of refraction in the x-direction β eff-x  is thus different from the effective index of refraction in the y-direction β eff-y  due to the tangential stresses  40 . Light coupling from the core  30  to x and y polarized modes, as shown in FIGS. 9 and 10, will thus be coupled at different center wavelengths, λ 0-x  and λ 0-y .  
         [0056]    [0056]FIG. 11 illustrates how the filter of FIG. 7 filters light when the stresses are not reduced as in FIG. 5. Wavelengths λ are shown on the abscissa and transmission T through the core  30  is shown on the ordinate. It can be seen that there is a relatively large difference between the center notch wavelength of x-polarized light λ 0-x  and the center notch wavelength of y-polarized light λ 0-y .  
         [0057]    Annealing the cladding layer  32 , as discussed with reference to FIGS. 5A and 5B, causes a reduction in tensile stress in the x-direction. A reduction in tensile stress in the x-direction causes a reduction in the stress difference between the x- and y-directions and a corresponding reduction in the difference between the effective index of refraction in the x-direction β eff-x  and effective index of refraction in the y-direction β eff-y . There is also a corresponding reduction in the difference between the center wavelengths of x-polarized light λ 0-x  and y-polarized light λ 0-y , respectively. Referring again to FIG. 11, annealing causes the center notch wavelength of the x-polarized light λ 0-x  to move towards the center notch wavelength of y-polarized light λ 0-y  as indicated by the arrow. This reduction in the difference between the center notch wavelengths indicates a reduction in the polarization dependence of light coupling into the cladding layer, along with a corresponding reduction in the PDL of the filter.  
         [0058]    [0058]FIG. 12 illustrates the extent to which PDL is reduced. The PDL of the filter is defined by the following formula:  
           PDL=|T   x   −T   y |,  
         [0059]    where T x  is transmission of x-polarized light and T y  is transmission of y-polarized light through the core  30 . The PDL before annealing is represented by line  74  and the PDL after annealing is represented by line  76 . The PDL before annealing is as much as 4 decibels (dB) before annealing, and less than 1.0 dB, or, more particularly, less than 0.5 dB, after annealing.  
         [0060]    While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative and not restrictive of the current invention, and that this invention is not restricted to the specific constructions and arrangements shown and described, since modifications may occur to those ordinarily skilled in the art.