1、TIA/EIA TELECOMMUNICATIONS SYSTEMS BULLETIN Polarization Maintaining Fiber in Telecommunications: Applications and Challenges TSBl20 SEPTEMBER 2000 TELECOMMUNICATIONS INDUSTRY ASSOCIATION Representing the telecommunications indushy in association with the Electronic Industries Aiance Elsctronic indu
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6、gineering data or information useful to the technical community, and represent approaches to good engineering practices that are suggested by the formulating committee. This Bulletin is not intended to preclude or discourage other approaches that similarly represent good engineering practice, or tha
7、t may be acceptable to, or have been accepted by, appropriate bodies. Parties who wish to bring other approaches to the attention of the formulating committee to be considered for inclusion in future revisions of this Bulletin are encouraged to do so. It is the intention of the formulating committee
8、 to revise and update this Bulletin rom time to time as may be occasioned by changes in technology, industry practice, or government regulations, or for other appropriate reasons. (From Project No. 4252, formulated under the cognizance of the TIA FO-6.9 Subcommittee on Polarization Maintaining Fiber
9、s, Connectors, and Components.) Published by TELECOMMUNICATIONS INDUSTRY ASSOCIATION 2000 Standards and Technology Department 2500 Wilson Boulevard Arlington, VA 2220 1 PRICE: Please refer to the current Catalog of ELECTRONIC INDUSTRIES ALLIANCE STANDARDS and ENGINEERING PUBLICATIONS or call Global
10、Engineering Documents, USA and Canada (1-800-854-7179) International (303-397-7956) All rights reserved Printed in U.S.A. TSB 120 TSB-120 Polarization Maintaining Fiber in Telecommunications: Applications and challenges Purpose The purpose of this bulletin is to provide a brief introduction to polar
11、ization maintaining fibers and to draw attention to several application-related issues. The bulletin is intended for both users and manufacturers of polarization maintaining fiber and was prepared by TIA subcommittee 6.9 - PM Fiber, Connectors and Components. What is polarization maintaining fiber?
12、Polarization maintaining (PM) fiber is distinguished fiom ordinary single-mode fiber by the degree and uniformity of its birefringence, the difference in index of refraction between orthogonal axes of the fiber. Linear birefringence causes the separation of a launched wave into fast and slow linearl
13、y polarized components. It is the ability of PM fiber to guide a linearly polarized light wave that accounts for the majority of PM fiber applications. Figure 1 illustrates the importance of careful alignment with a principal axis. Launch on-axis Polarization main ta ned I Polarization off-axis unst
14、able Figure 1 - Polarization-maintaining fiber preserves a linear state of polarization if the input electric field is aligned with a principal axis. All single-mode fibers exhibit some degree of birefringence. In ordinary transmission fibers, birefringence is a weak effect that arises fiom imperfec
15、tions in the fiber drawing and packaging processes and is modified by the mechanical stresses of the installation environment. Long fibers can be modeled as the concatenation of a large number of randomly oriented local birefiingent elements - lumped retarders, or waveplates. In contrast, PM fiber i
16、s designed to exhibit a consistently high value of linear birefiingence. -1- TSB 120 Birefringence is inversely related to beat length, the propagation distance over which the fast and slow waves experience a 360-degree change of phase. Ordinary fiber has a beat length in the tens of meters, whereas
17、 the beat length of PM fiber, the beat length is typically a few millimeters. The large and uniform linear birefringence of PM fiber is achieved by placing the fiber core in a transverse mechanical stress field. Several widely used implementations are shown in Figure 2. To achieve the robust polariz
18、ation-maintaining ability that is key to most PM fiber applications, the built-in stress is designed to be much higher than stresses produced by typical fiber environments. Slow axis O (key) Panda A 3Q Bow tie mis Oval core a oval stress region Figwe 2 - Examples of polarization maintaining fiber co
19、nstruction. PM fiber applications In optical fiber communication applications, PM fiber is typically used to guide linearly polarized light to a device that requires a particular orientation of linearly polarized light for proper operation. For example, undersea lightwave telecommunication systems d
20、o not tolerate the wavelength “chirp” that results from directly modulating the current of DFB laser diode. The solution is to externally modulate, often using a Mach-Zehnder implemented in lithium niobate. In order to provide the modulator with linearly polarized light at the orientation that produ
21、ces best modulation efficiency, both the DFB laser and the modulator are PM fiber pigtailed. The pigtails are joined using special PM fiber adapters or fusion splices. Accurate angular alignment is required at the laser, the modulator, and the pigtail interface. PM fiber also finds many applications
22、 in the field of optical sensors. An excellent example is the high-performance fiber gyro, in which the orthogonal axes of the PM fiber carry counter-propagating waves. 2 TSB 120 In some applications, light is intentionally launched such that the electric field projects equally on the fast and slow
23、axes. The relative phase of these fields at the output end of the PM fiber determines the output polarization. This 5050 launch condition makes the output polarization optimally sensitive to slight changes in wavelength, temperature or mechanical stress. Changes in output state can be detected to a
24、first order by measuring the output light through a polarizer (called an analyzer in this application) that is rotated midway between the fast and slow axes. What limits PM fiber performance? In optical fiber telecommunications applications, PM fiber is used to guide linearly polarized light. For be
25、st results, several conditions must be met. The manufacturer of a PM fiber cable must accurately align the optical connector anti-rotation key with a principal optical axis of the fiber axis: by industry convention, the key is aligned with the slow axis. The connector attachment and fiber polishing
26、processes must be controlled to avoid building up internal stresses that can transform the linearly polarized input light to an elliptical state. The user of PM fiber must ensure that input light is highly polarized and aligned (conventionally) with the slow axis. If sections of PM fiber are cascade
27、d using connectors or splices, rotational alignment of the mating fibers is critical. If all of these requirements are met, it is possible to guide polarized light with 10 to 100,000 times more optical power in the slow axis than in the fast. Technical challenges Several technical challenges confion
28、t the manufacturer and user of PM fiber. They are outlined below. 1. Connectorization effects Connectorization involves attaching the PM fiber to the ferrule and polishing the fiber end. In some cases, the core is actively aligned. Each step must be performed without significantly modifymg the inher
29、ent linear birefringence of the fiber, as described above. To measure the contribution of connectorization effects to PM fiber crosstalk, conduct this simple experiment. Connect a PM fiber jumper to a polarimeter as shown in Figure 4. Illuminate the fiber with highly (linearly) polarized light fiom
30、a coherent (narrow-band) light source. Measure crosstalk with the polarimeter and rotate the polarizer to minimize it. Record the crosstalk value. Repeat the exercise with polarized light applied to the PM fiber through a fiber coil-type polarization adjuster, adjusting the launch polarization to mi
31、nimize crosstalk. An improved crosstalk value indicates that the polarization adjuster has compensated for a residual birefringence induced by the PM fiber connectorization process. -3- TSB 120 2. Measurement of polarization crosstalk The degree to which optical power is coupled into a single axis o
32、f the PM fiber is represented by the polarization crosstalk. The attribute is defined as ten times the log of the ratio of power in the intended and non-intended axes of the fiber. Measurement of crosstalk by the broadband lightlcrossed polarizer method, shown in Figure 3, is standardized as TIA/EIA
33、-455- 193 “FOTP- 193, Polarization crosstalk method for polarization-maintaining optical fiber and components“. Polarization maintaining Rotatable fiber under test Rotatable polarizer Figure 3 - Measurement of PM fiber crosstalk using the broadband source/crossed polarizers method. This method measu
34、res the crosstalk at the plane of the polarizer that is located downstream of the device under test, making the method suitable for measurement of any type of PM fiber or component. A second crosstalk measurement method, illustrated in Figure 4, is standardized in TIA/EIA-455- 199 “FOTP- 199, In-lin
35、e polarization crosstalk measurement method for polarization-maintaining optical fibers, components and systems“. In this method, crosstalk is measured at a specific location (in-line) along a PM fiber. The crosstalk at this location is due to the combined effects of the fibers, interfaces and compo
36、nents upstream of the measurement. Rotatable polarizer Figure 4 - Measurement of PM fiber crosstalk using the polarimetric method. 4 TSB 120 The measurement process requires the user to heat or gently stretch a 0.1 to 0.3m long portion of the PM fiber in the region where crosstalk is to be measured.
37、 The resulting fiber elongation causes a phase shift between waves in the fast and slow axes, which produces a circular arc on the Poincare sphere. The crosstalk value is calculated rom the diameter of this circle. A great circle indicates that fast and slow axes carry equal power. At very low cross
38、talk, the optical power is confined to a single polarization mode (fast or slow) and the circular trace collapses toward a point. This point represents the fast or slow linear polarization mode of the PM fiber, transformed to an arbitrary location on the Poincare sphere by the birefringence of the i
39、nterconnecting single-mode filber. The end-to-end crosstalk of a PM fiber cable or PM component - including the output optical connector - can be found by measuring the crosstalk on a following segment of PM fiber. The result is valid to within the uncertainty of the connector interface. The polarim
40、etric method also provides insight into the performance of concatenations of PM fibers and components. As the number of fiber interfaces increases, the output polarization state becomes more strongly dependent upon environment (temperature and mechanical stresses). An example application is shown in
41、 Figwe 5. Both spans of PM fiber are stretched randomly (and independently) and the worst-case crosstalk is computed rom the outer diameter of the resulting torroidal pattern. Ultimate application: I Mach-Zehnder I I modulator I source Interface Interface Polarimeter #I #2 Figwe 5 - Measurement of t
42、he worst-case crosstalk of concatenated PM fibers. 3. The need for a high-performance, polarizing launch fixture Characterization of the polarization crosstalk of a connectorized PM fiber can be limited by the quality of the launch. In one commonly performed measurement, the input PM connector is mo
43、unted in a mating “reference” connector that is optimally aligned with the input polarizer. The crosstalk is measured under this condition. The value obtained is attributed to the fiber under test, when in fact part of the crosstalk is due to imperfect fit -5- TSB 120 FOCIS-4A (FC) K VALUE to the re
44、ference connector. The PM fiber industry would benefit from development of a standardized, high-quality polarizing launch fixture. Such a device would have a polarizer with at least 100,000 extinction ratio, near-zero keyway and ferrule clearance, and excellent alignment of the keyway with the polar
45、izer. Any optics placed between the polarizer and the fiber would be of the zero-birefringence type. This standardized polarizing launch would find application in manufacturing, incoming inspection, and research. PLUG (Key Width Dimension) Standardization challenges 2 1. Nomenclature To distinguish
46、PMF from non-PMF (that is, standard SM and MM fiber) connectorization, it is recommended that the end-user add a “PM suffix to the normal connector callout. For example, an FC-PC (PC-polished FC) connector terminated onto PM fiber should be designated “FC-PCPM. An 8” angle-polished FC connector woul
47、d be designated “FC-APCPM. Where key options exist, such as applies to FC connectors per FOCIS-4A, the key option “k=n” should also be called out. For example, the wide key k=4 version of FC-PCPM connector would more officially be designated “FC- PCPM, H. Popular PM connector designations that exist
48、ed at the time of publication are: FC-PCPM, k=n (where n=2,3 or 4) FC-APCPM, k=4 (where n=2,3 or 4) SC-PCPM SC-APCPM applications 2.09-2.14mm 2.15-2.2Omm Wide Kev Ontion 1 2. Connector Intermatability and Key Size Issues At the time of publication, the most popular PM connector is the FC (defined by
49、 TIA/EIA-604-4A, a.k.a. FOCIS-4A) due to the tolerance control capable with the metal body and ease of “tuning” during the termination process. Physical intermateability and achievement of common performance of various manufacturers FCPM terminations is affected by the various anti-rotation key (AR-Key) and adapter slot options available, designated by “k” value and listed in Table 1. Because performance of PM connectors is closely tied to the alignment of the polarization axes relative to the AR key and associated tolerances, the loose tolerance k= 1 (original FC-PC c
