Endface Geometry and Connector Reliability in the Outside Plant

WHITE PAPER PON Deployment of Angle Polished Connectors: Endface Geometry and Connector Reliability in the Outside Plant PON Deployment of Angle Polished Connectors: Endface Geometry and Connector Reliability in the Outside Plant By Steven C Zimmel Abstract: Recent wide-scale deployment of APC connectors in the outside plant for FTTH initiatives has forced connector manufacturers to consider reliability issues of these connectors Angled connectors exhibit two phenomena not found in non-angled connectors: First, the lower ferrule endface radius of angled connectors makes the connector more susceptible to permanent fiber withdrawal at elevated temperatures Second, any rotation of the ferrule in angled connectors will increase the apex offset of the connector These phenomena may result in loss of physical contact between the fibers of a mated connector pair This air gap may increase reflectance and insertion loss, reducing system reliability The air gap may also allow contaminants to migrate on the fibers Ferrule rotation and permanent fiber withdrawal need to be minimized for acceptable connector performance in the outside plant ©2005 Optical Society of America OCIS codes: (060.2340) Fiber optic components Introduction Angled physical contact (APC) fiber optic connectors with zirconia ferrules have become the de facto standard in PON networks currently being deployed for FTTH initiatives The angle on the fiber in APC connectors produces low back reflections when the connector is not mated to another connector: Reflected light is directed into the fiber cladding because the endface angle is greater than the acceptance angle of the fiber The low reflection of an unmated APC connector provides an advantage for PON networks: A connector in the PON will usually remain unmated until service is required from that circuit This results in unused connectors emitting their signal into air, which can produce Frensel reflections as high as –14dB for connectors without an angle on the endface Service providers need to minimize system reflections so it won’t interfere with OLT card operation One solution is to use non-angled connectors and mate all unused connectors to a terminator This solution adds cost because terminators are an additional component Using APC connectors provides a cost-effective method to greatly reduce reflection issues caused by unmated connectors in the PON Using APC connectors eliminates the need for terminators because the reflectance of unmated APC connectors is less than -50dB Endface Geometry and Connector Reliability in the Outside Plant FTTH deployments are expected to install millions of APC connectors into outside plant environments over the next several years Up to this point, APC connectors have been primarily used for central office and testing applications The upcoming large-scale deployment of APC connectors in the outside plant forces us to think about some of the issues that are unique to APC connectors in extreme environmental conditions This paper will consider two of these issues First, it is necessary to understand the importance of connector ferrule endface geometry The parameters used to describe the endface of an APC connector are radius, apex offset, fiber height, and endface angle These parameters are defined in the left side of Figure Fiber height is the distance the fiber protrudes or recesses from the ferrule If the fiber is recessed too far into the ferrule, obvious air gaps will occur Apex offset of an APC connector is the distance from the apex of the polished ferrule radius to the center of the fiber core when the endface is viewed at and an angle of 8° (viewed perpendicular to the plane of the Apex offset Apex of the Radius when viewed at 8˚ Optical Fiber Endface Radius Fiber Height Air Gap Fig Endface Geometry of an APC Connector and of Two Mated APC Connectors APC Endface Geometry and Outside Plant Reliability APC connectors provide two benefits First, APC connectors produce fiber-to-fiber contact in mated connector pairs Physical contact between fibers in mated connectors is needed because it prevents air gaps from forming between optical fibers Air gaps cause increased reflectance and inconsistent insertion losses that will reduce system performance and reliability In addition, physical contact prevents contamination from migrating onto the fiber cores The second benefit of APC connectors is that they minimize reflectance If the radius is polished at an angle such that the angle of the fiber is greater than the fiber’s angle of acceptance, reflected light will be directed into the cladding Using this method, one can produce connectors with reflectance in the –50dB range when unmated and as low as -80dB or better when mated Physical contact is realized by polishing a radius onto the ferrule containing the potted optical fiber and by polishing the fiber such that it is close to level with the ferrule surface The key for physical contact to occur is to center the apex of the radius over the optical fiber If the apex is approximately centered on the fiber, physical contact is guaranteed because the fibers will be the first objects to contact each other when connectors are mated Page angled surface) If the apex offset becomes too large, an air gap will develop because the high point of the ferrule on the radiused endface will be too far away from the fiber core per the right side of Figure The endface radius is applied to the ferrule by polishing the connector on a compliant surface If the radius becomes too large, the ferrule will become effectively flat This situation could cause local concave features that will introduce a gap between the fiber The radius is usually applied at an 8° angle The angle is in a plane that is 8° from perpendicular to a plane that intersects the connector key 9° connectors are used, but are rare APC endface geometry parameters are specified such that physical contact will be maintained over a large range of operating conditions For example, at high temperatures, pressure on ferrule endfaces caused by the spring in the connector may cause the fibers to permanently withdraw into the ferrule if the epoxy holding the fibers creep If the initial fiber height is too recessed into the ferrule, the fiber may creep too far back into the ferrule causing loss of physical contact Proper specification of these endface parameters is necessary to guarantee physical contact in the outside plant Endface Geometry and Connector Reliability in the Outside Plant Several industry standards exist that address APC connector endface geometry The purpose of these specifications is to guarantee that physical contact between different vendor’s connectors will not be lost The most used standards are IEC-60874-14-6 and Telcordia GR-326, Issue These documents require apex offset to be less than 50 microns, radius to be from 5mm to 12mm, and fiber height to be ±100nm These three requirements will allow physical contact to be maintained at temperatures as high as 85°C with proper epoxy selection The IEC is in process of updating endface requirements for APC connectors The new requirements will reflect current ferrule material properties and will tie all three properties together into one function The new requirements are expected to be published in 2005 The preceding discussion shows that proper ferrule endface geometry is required for good system performance Properly specified endface geometries guarantee physical contact between fibers in mated connectors Physical contact is needed so air gaps don’t form between fibers in connectors, which will cause increases in reflectance and insertion loss, reducing system performance Now that endface geometry and the importance of physical contact have been defined, we can discuss issues concerning these properties in APC connectors in the outside plant The effort to maintain physical contact in APC connectors deployed in the outside plant has two unique challenges not present in non-angled connectors: First, APC connectors are much more prone to large permanent fiber withdrawals when exposed to high temperatures If the fiber withdraws too much, physical contact can be lost Second, ferrule rotation can create air gaps because of the angle of the endface These phenomena must be accounted for to guarantee proper connector performance and reliability in the outside plant Permanent Fiber Withdrawal, Radius, and Ferrule Cleanliness: One manner that fiber height can change, causing fibers to lose physical contact, is if the fiber permanently withdrawals into the ferrule This phenomenon occurs when mated connectors are exposed to elevated temperatures The force on mated ferrule endfaces transmits pressure to the fiber that is held in place with an epoxy This pressure may cause the epoxy and fiber to creep back into the ferrule at elevated temperatures If the pressure is greater than the epoxy bond strength, the creep will not recover and the fiber/epoxy will permanently withdraw into the ferrule inside diameter Figure shows an electron microscope scan of an connector before (left) and after (right) GR-326 environmental testing The region on the left is the optical fiber, the region on the right is the ferrule, and the thin section in the middle is the epoxy Notice how the fiber is lower than the ferrule in the right photo This is the phenomenon of permanent fiber withdrawal It has been observed that permanent fiber withdrawal is significantly larger in APC connectors than non-angled connectors that are subjected to GR-326, Issue environmental testing We routinely observed APC connectors start a GR-326 test with a protruding fiber and finish with fibers recessed to a point that it no longer meets the –100nm requirement However, we rarely see this occur in non-angled connectors The question we must ask ourselves is why APC connectors permanently withdraw so much more than non-angled connectors? Before we answer that question, we need to determine which of the GR-326 tests causes the withdrawal An experiment was performed to determine which GR326 environment causes the most permanent fiber withdrawal The experiment consisted of six groups of 12 mated connector pairs (24 APC/SC connectors mated in 12 receptacles) Each group was subjected to either one week of one of the four GR-326 environmental tests, a –40°C cold age test, or an ambient age as a control group The connectors were measured for endface geometry using an interferometer, mated together in an adapter, and subjected to one week of one of the tests listed above After week the connectors were removed from the chambers and allowed to rest at room temperature for one day Next the connectors were uncoupled and the endface geometry was measured The average, maximum, and minimum permanent changes in fiber height are shown in Table Page Endface Geometry and Connector Reliability in the Outside Plant Table 1: Permanent Fiber Height Withdrawal for Various GR-326 Environments Group 1: Thermal Age Group 2: Thermal Cycle Group 3: Humidity Age Group 4: Condensation Cycle Group 5: Cold Temp Age Group 6: Ambient (Control) Average -109.9 nm -71.0 nm -164.3 nm -129.8 nm -42.8 nm -0.2 nm Maximum -61.2 nm -14.6 nm -108.1 nm -31.7 nm -34.7 nm -3.7 nm Minimum -104.9 nm -15.7 nm -291.3 nm -23.6 nm -62.6 nm -4.5 nm Table shows that thermal age and humidity age induced the most average withdrawal and produced the highest extremes Interestingly, the sample that went through thermal cycle saw less change than thermal age Even more interesting is that –40° age showed very little change Prolonged exposure to elevated temperatures induces the most fiber height change This is evident from the fact that the tests that spend the most time at high temperatures (thermal and humidity age) exhibit the most change Thermal and condensation cycles are exposed to high temperatures, but only for 1/8 of the test cycle However, even though the cycling tests show far less change, there is still a significant amount of withdrawal This indicates that even short exposures to elevated temperatures may cause the fiber to withdraw in APC connectors Experiments were then conducted to determine what specifically about APC connectors induces more permanent fiber withdrawal than non-angled connectors Non-angled (UPC) and angled connectors (APC) differ in two major ways: First, APC connectors have an endface radius of 5mm to 12 mm and UPC connectors have an endface radius of 10mm to 25mm These endface radii are defined in IEC and Telcordia standards The smaller radius in APC connectors will cause the pressure on the fiber to be higher because the spring force is spread out over a smaller contact area This higher pressure may cause the fiber to creep more during GR-326 testing Second, many APC connector manufacturers perform a secondary grinding operation of the ferrule to change the chamfer to geometries that aid in achieving low apex offsets This operation is done before the ferrule is potted and may introduce contaminants to the ferrule I.D The experiment looked at two factors: Radius and ferrule I.D cleanliness APC connectors were made with radii of 5-7 mm, 9-11mm, and 18-22 mm Ferrule inside diameters were either used as provided or cleaned with a steam bath and acetone The sample allocations for the experiment are shown in Table Groups are not equally sized because the data represents a summation of three separate experiments evaluating the same two parameters Page Fig Above: SEM Scan of a Non-Withdrawn APC Fiber Below: SEM Scan of a Permanently Withdrawn APC Fiber Endface Geometry and Connector Reliability in the Outside Plant We observe that the primary cause of withdrawal in APC connectors is that a smaller endface radius increases the pressure on the ferrule/fiber/epoxy bond because of the smaller contact area The higher pressure causes the fiber to permanently creep into the ferrule at elevated temperatures Contamination within the ferrule adds to the amount of withdrawal by compromising the epoxy bond strength Table 2: Design of Experiments Sample Allocation Table Ferrule not Cleaned Radius Ferrule Cleaned 5-7 mm 36 APC connectors 60 APC connectors 9-11mm 12 APC connectors 36 APC connectors 18-22 mm 24 APC connectors 12 APC connectors The only physical difference between angled and nonangled connectors is the angle at which the connector is polished They are otherwise identical The reason that non-angled connectors withdraw less than angled connectors is that non-angled connectors are usually polished with a radius of about 18mm This is because standards specify radius for non-angled connectors to be 10mm to 25mm The same creep and withdrawal mechanisms occur in non-angled connectors However, because angled connector ferrules may have more contamination, and because angled connectors have smaller radii, we see more withdrawal in APC connectors All connectors were mated to another connector within an adapter The connectors were then subjected to three days of +85°C thermal age followed by three days of +75°C/95% RH humidity age in order to induce the most fiber withdrawal The endface geometry was measured before and after with a interferometer Figure and Table illustrates the results of the experiment The experiment produced three conclusions: First, radius had the largest affect on permanent fiber withdrawal Second, cleanliness has a smaller, but still significant affect Lastly, there is little or no interaction between cleanliness and radius P ermanent Wi thdrawal (nm) Permanent Fiber Withdrawal Experiment Results 170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 Ferrule Not Cleaned Ferrule Cleaned 11 13 15 17 19 21 23 25 Fig APC Connector Fiber Height Withdrawal Test Results Page Endface Geometry and Connector Reliability in the Outside Plant Table 3: Design of Experiment Results: Radius Ferrule not Cleaned Ferrule Cleaned 5-7 mm 128.6 nm 100.9 nm 9-11mm 65.2 nm 51.36 nm 18-22 mm 17.5 nm 16.1 nm The results of this experiment suggest that APC connectors should be manufactured with higher endface radii Endface radius is controlled by IEC document 6075414-6 and GR-326, Issue These documents specify a range of 5mm to 12mm It is common for connector manufacturers to polish APC connectors at the low end of the radius specification However, as the experiment shows, shifting the average to the upper end Average Permanent Fiber of the 5mm to 12mm range will significantly reduce the amount of permanent fiber withdrawal, reducing the risk of air gaps forming The results also suggest that ferrule cleaning before potting will reduce fiber withdrawal and the possibility of air gaps Page Endface Geometry and Connector Reliability in the Outside Plant Ferrule Rotation and Apex Offset: We now examine the issue of ferrule rotation within APC connectors and the affect this rotation has on apex offset and insertion loss The rotation is about the ferrule axis as shown in figure le rru ation R ot Directi on of fe Fig View of an APC/SC Connector from the Front Ferrule (Axis is perpendicular to Apex offset is the distance between the center of the optical fiber and the apex of the polished ferrule endface radius when the ferrule is viewed at 8° If the distance between the apex and the fiber center is large, physical contact may be lost The air gap created by the large apex offset will increase insertion loss and reflectance Therefore it’s important to keep apex offsets low so physical contact of the optical fibers is maintained Apex offset will change if the ferrule is rotated about its axis because the endface is at an 8° angle Physical contact results from the 8° planes of two mated connectors being parallel to each other If one rotates about the ferrule axis, the planes will no longer be parallel This results in movement of the apex, which will increase apex offset The change in apex offset as a function of ferrule rotation can be determined using coordinate system transformations Figure shows the endface of the APC connector as it would be viewed by an interferometer along the z-axis That is, an interferometer looks at the radius normal to the x-y plane The ferrule axis is tilted q degrees from normal to the x-y plane and is in the y-z plane q is also the polish angle, usually 8° To simulate rotation about the ferrule axis, we rotate the points on the sphere q about the x-axis Then we rotate the sphere j degrees about the z-axis to simulate ferrule rotation Lastly we rotate the sphere back by –q about the x-axis The origin remains at all times on the ferrule axis at the point where the optical fiber exits the ferrule Z-Axis (Negative) Axis of Ferrule Rotation Apex offset X-Axis Apex when viewed at 8˚ Y-Axis Z-Axis (Positive) Fig Coordinate System for Calculating Ferule Rotation Page Endface Geometry and Connector Reliability in the Outside Plant Apex Change vs Rotation for Variouse Radii 300 Change in Apex Offset (microns) 250 12mm 200 150 9mm 100 5mm 50 Ferrule Rotation (degrees) 0 10 Fig Apex Offset as a Function of Ferrule Rotation for Various Radii Generally, the new location for any point rotated about the x-axis as defined in Figure is given by [1]: x“ = x’ (1) y” = y’ cosθ - z0 sinθ (2) z” = y’ sinθ + z0 cosθ (3) where θ is the polish angle θ is also the angle that the ferrule axis is from the z axis Generally, the new location for any point rotated about the z-axis is given by [1]: x“ = x’ cosϕ − y0 sinϕ (4) y” = x’ cosϕ + y0 sinϕ (5) z” = z (6) where j is the angle of ferrule rotation about the ferrule axis Combining the above sets of equations for the three rotations described, we get the following: x = x0 cosϕ − (y0 cosθ − z0 sinθ) sinθ (7) y” = (x0 sinϕ +(y0 cosθ − z0 sinθ) cosϕ) cosθ + (y0 sinθ + z0 cosθ) sinθ (8) z” = − (x sinϕ +(y cosθ − z sinθ) cosϕ) sinθ + ° ° ° (9) (y0 sinθ + z0 cosθ) cosθ x, y, and z are the location of the new apex after rotation xo , yo , and zo are the location of the new apex before rotation R is the ferrule endface radius Aox and Aoy are the x and y components of the original apex before rotation These can be easily found using the apex and bearing data provided by most interferometers: A0x = ApexOffset • cosβ (11) A0y = ApexOffset • sinβ (12) where β is the bearing of the apex in degrees, 0° corresponding to the direction of the connector key Equations (7), (8), and (9) represent any point of the endface radius after rotation given its initial position To find the new apex after rotation we, need to recognize that the new apex will correspond to the values of x0 and y0 that minimize z in (9) after the rotation If we solve for x0 and y0 in (9) such that (9) is minimized, the resulting x0 and y0 will be the original position of the new apex We can then use these values of x0 and y0 to calculate the new apex location, x and y, by inserting these values into (7) and (8) Now that we have the new apex location, we can determine the change in the apex: 2 ∆apex [(x - A0x ) + (y - A0x ) + (z - A0z ) ] ⁄2 and the new apex offset is simply: ApexOffset Afterrotation = [ x + y +z 2] ⁄2 where 2 z0 − R − ( R − (x0 − A0x ) − y0 − Ay0 ) ) ⁄2 Page (10) (13) (14) Endface Geometry and Connector Reliability in the Outside Plant IL vs Ferrule Rotation (Apex=25um, R=12mm, h=0nm) 0.8 0.7 0.6 I L (dB ) 0.5 0.4 0.3 0.2 0.1 0.0 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 10 Ferrule Rotation (degrees) Fig Example of Insertion Loss as a Function of Ferrule Rotation We found the minimum value of z by varying x0 and y0 using the Microsoft Excel solver tool Figure shows how apex offset will change as the ferrule is rotated about the z-axis for a variety of radii The increase in apex offset caused by ferrule rotation may induce an air gap between two mated connectors, which will increase reflectance and insertion loss For 2.5mm ferrules made of Yttria stabilized zirconia, the allowable fiber undercut as a function of apex offset and radius is [2]: h = 1988R • 10 - 50 -.795 - R • 10 +√R • 10 - ApexOffset 6 15) where R is the endface radius in millimeters, apex offset is in microns, and h is in nanometers The last term accounts for thermal effects A negative value of h in (15) indicates a protruding fiber and a positive h designates a recessed fiber Equation 15 shows us that as the apex offset gets larger, the allowable fiber height gets more negative That is, as the apex gets large, the optical fiber must protrude out of the ferrule more to compensate for the gap the apex will create If the actual fiber height of the connector is larger than the value given by equation 15 after ferrule rotation, an air gap will occur This air gap is: gap = if h0 - h < gap = h0 - h if h0 - h > (16) where ho is the actual fiber height and h is the value of (15) using the new apex offset after rotation, equation (14) As mentioned before, air gaps in APC connectors may disrupt service because it will make insertion loss vary sinusoidally for air gaps that are on the order of magnitude of the wavelength of the signal The relationship between air gap and insertion loss as a function of the air gap length and the wavelength is: [ IL + - 10log -2L where L= ( ng - nf ng + nf ) ( 1-cos ( 4πλ n x))] (17) (18) x is the air gap, n is the index of refraction, and λ is the wavelength Figure shows an example of how insertion loss will vary as a function of ferrule rotation for an initial apex offset of 25 microns at a 0° bearing a 12mm radius, and an initial fiber height of 0nm The graph is not centered about zero because the initial apex offset is non-zero Figure shows that small ferrule rotations can cause APC connectors to loose physical contact and see variation in insertion loss In the example shown in Figure 7, a rotation of slightly over 2° will cause the apex offset to become large enough to induce an air gap Different radii, initial apex offset, and initial fiber heights will produce slightly different values It is important that APC connectors used in the OSP be designed such that the ferrule cannot rotate more than a few degrees The actual amount of allowable ferrule rotation will depend on the typical radius and fibre height of a connector Ferule rotation can easily occur in the field if an operator cleans the ferrule or removes the dust cap with a twisting motion If the ferrule doesn’t return to it’s initial polished position, an effective increase in apex offset will result which may lead to higher insertion loss values Page 10 Endface Geometry and Connector Reliability in the Outside Plant Summary: References Connector performance in the outside plant depends largely on the ability of a connector to keep fibers in physical contact over the life of the system If physical contact is lost, insertion and reflectance of connector pairs will increase and may affect system performance Maintaining physical contact in APC connectors has two challenges not present in non-angled connectors First, APC connectors are more susceptible to permanent fiber withdrawal because of the lower radius specifications and to some extent, cleanliness However, trying to manufacture APC connectors with radii on the high end of the 5mm to 12mm industry requirement will reduce this problem The second challenge is reducing ferrule rotation As we’ve shown, small ferrule rotations can effectively increase apex offsets to a level that will separate the ferrule endfaces, creating air gaps Small ferrule rotations in the field are likely to occur simply by removing the dust cap or cleaning the ferrule It’s important for users of APC connectors in the outside plant to select connectors that minimize or eliminate ferrule rotation and minimize permanent fiber withdrawal Both factors will contribute heavily to PON performance and reliability [1] Mary L Boas, Mathematical Methods in the Physical Sciences (John Wiley & Sons, 1983), Chap 10 Page 11 [2] IEC: Manning, R and Gurreri, M., “ PC Single Mode Cylindrical Ferrule End Face Geometry, 3-Mol Percent Yttria Stabilized Tetragonal Zirconia Polycrystal”, IEC SC86B/WG6 October 2003, Montreal WHITE PAPER Web Site: www.adc.com From North America, Call Toll Free: 1-800-366-3891 • Outside of North America: +1-952-938-8080 Fax: +1-952-917-3237 • For a listing of ADC’s global sales office locations, please refer to our web site ADC Telecommunications, Inc., P.O Box 1101, Minneapolis, Minnesota USA 55440-1101 Specifications published here are current as of the date of publication of this document Because we are continuously improving our products, ADC reserves the right to change specifications without prior notice At any time, you may verify product specifications by contacting our headquarters office in Minneapolis ADC Telecommunications, Inc views its patent portfolio as an important corporate asset and vigorously enforces its patents Products or features contained herein may be covered by one or more U.S or foreign patents An Equal Opportunity Employer 1326454 05/05 Revision © 2005 ADC Telecommunications, Inc All Rights Reserved ... testing The region on the left is the optical fiber, the region on the right is the ferrule, and the thin section in the middle is the epoxy Notice how the fiber is lower than the ferrule in the. .. the spring in the connector may cause the fibers to permanently withdraw into the ferrule if the epoxy holding the fibers creep If the initial fiber height is too recessed into the ferrule, the. .. that endface geometry and the importance of physical contact have been defined, we can discuss issues concerning these properties in APC connectors in the outside plant The effort to maintain physical