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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
2、stries Alliance Copyright Telecommunications Industry Association Provided by IHS under license with EIALicensee=IHS Employees/1111111001, User=Wing, Bernie Not for Resale, 03/30/2007 03:09:27 MDTNo reproduction or networking permitted without license from IHS -,-,- NOTICE TWEIA Engineering Standard
3、s and Publications are designed to serve the public interest through eliminating misunderstandings between manufacturers and purchasers, facilitating interchangeability and improvement of products, and assisting the purchaser in selecting and obtaining with minimum delay the proper product for his p
4、articular need. Existence of such Standards and Publications shall not in any respect preclude any member or nonmember of TWEIA rom manufacturing or selling products not conforming to such Standards and Publications, nor shall the existence of such Standards and Publications preclude their voluntary
5、 use by those other than TWEIA members, whether the standard is to be used either domestically or internationally. Standards, Publications and Bulletins are adopted by EIA in accordance with the American National Standards Institute (ANSI) patent policy. By such action, TWEIA does not assume any lia
6、bility to any patent owner, nor does it assume any obligation whatever to parties adopting the Standard, Publication, or Bulletin. Technical Bulletins are distinguished rom TWEIA Standards or Interim Standards, in that they contain a compilation of engineering data or information useful to the techn
7、ical 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 that may be acceptable to, or have been accepted by,
8、 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 to revise and update this Bulletin rom time to t
9、ime 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 Fibers, Connectors, and Components.) Published by TELE
10、COMMUNICATIONS 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 Engineering Documents, USA and Canada (1-800-854-
11、7179) International (303-397-7956) All rights reserved Printed in U.S.A. Copyright Telecommunications Industry Association Provided by IHS under license with EIALicensee=IHS Employees/1111111001, User=Wing, Bernie Not for Resale, 03/30/2007 03:09:27 MDTNo reproduction or networking permitted without
12、 license from IHS -,-,- Copyright Telecommunications Industry Association Provided by IHS under license with EIALicensee=IHS Employees/1111111001, User=Wing, Bernie Not for Resale, 03/30/2007 03:09:27 MDTNo reproduction or networking permitted without license from IHS -,-,- TSB 120 TSB-120 Polarizat
13、ion Maintaining Fiber in Telecommunications: Applications and challenges Purpose The purpose of this bulletin is to provide a brief introduction to polarization maintaining fibers and to draw attention to several application-related issues. The bulletin is intended for both users and manufacturers o
14、f polarization maintaining fiber and was prepared by TIA subcommittee 6.9 - PM Fiber, Connectors and Components. What is polarization maintaining fiber? Polarization maintaining (PM) fiber is distinguished fiom ordinary single-mode fiber by the degree and uniformity of its birefringence, the differe
15、nce in index of refraction between orthogonal axes of the fiber. Linear birefringence causes the separation of a launched wave into fast and slow linearly polarized components. It is the ability of PM fiber to guide a linearly polarized light wave that accounts for the majority of PM fiber applicati
16、ons. Figure 1 illustrates the importance of careful alignment with a principal axis. Launch on-axis Polarization main ta ned I Polarization off-axis unstable Figure 1 - Polarization-maintaining fiber preserves a linear state of polarization if the input electric field is aligned with a principal axi
17、s. All single-mode fibers exhibit some degree of birefringence. In ordinary transmission fibers, birefringence is a weak effect that arises fiom imperfections in the fiber drawing and packaging processes and is modified by the mechanical stresses of the installation environment. Long fibers can be m
18、odeled as the concatenation of a large number of randomly oriented local birefiingent elements - lumped retarders, or waveplates. In contrast, PM fiber is designed to exhibit a consistently high value of linear birefiingence. -1- Copyright Telecommunications Industry Association Provided by IHS unde
19、r license with EIALicensee=IHS Employees/1111111001, User=Wing, Bernie Not for Resale, 03/30/2007 03:09:27 MDTNo reproduction or networking permitted without license from IHS -,-,- TSB 120 Birefringence is inversely related to beat length, the propagation distance over which the fast and slow waves
20、experience a 360-degree change of phase. Ordinary fiber has a beat length in the tens of meters, whereas 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 mechanica
21、l stress field. Several widely used implementations are shown in Figure 2. To achieve the robust polarization-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
22、 A 3Q Bow tie m i s Oval core a oval stress region Figwe 2 - Examples of polarization maintaining fiber construction. 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
23、linearly polarized light for proper operation. For example, undersea lightwave telecommunication systems do 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 lith
24、ium niobate. In order to provide the modulator with linearly polarized light at the orientation that produces 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
25、 is required at the laser, the modulator, and the pigtail interface. PM fiber also finds many applications 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 Copyright Telecommunica
26、tions Industry Association Provided by IHS under license with EIALicensee=IHS Employees/1111111001, User=Wing, Bernie Not for Resale, 03/30/2007 03:09:27 MDTNo reproduction or networking permitted without license from IHS -,-,- TSB 120 In some applications, light is intentionally launched such that
27、the electric field projects equally on the fast and slow 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 mechani
28、cal stress. Changes in output state can be detected to a 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, P
29、M fiber is used to guide linearly polarized light. For best 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
30、 slow axis. The connector attachment and fiber polishing 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
31、) with the slow axis. If sections of PM fiber are cascaded 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.
32、Technical challenges Several technical challenges confiont 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 m
33、ust be performed without significantly modifymg the inherent 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. Illumina
34、te the fiber with highly (linearly) polarized light fiom 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 polar
35、ization adjuster, adjusting the launch polarization to minimize crosstalk. An improved crosstalk value indicates that the polarization adjuster has compensated for a residual birefringence induced by the PM fiber connectorization process. -3- Copyright Telecommunications Industry Association Provide
36、d by IHS under license with EIALicensee=IHS Employees/1111111001, User=Wing, Bernie Not for Resale, 03/30/2007 03:09:27 MDTNo reproduction or networking permitted without license from IHS -,-,- TSB 120 2. Measurement of polarization crosstalk The degree to which optical power is coupled into a singl
37、e axis of 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
38、 TIA/EIA-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 meth
39、od measures 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
40、, In-line 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 a
41、nd components upstream of the measurement. Rotatable polarizer Figure 4 - Measurement of PM fiber crosstalk using the polarimetric method. 4 Copyright Telecommunications Industry Association Provided by IHS under license with EIALicensee=IHS Employees/1111111001, User=Wing, Bernie Not for Resale, 03
42、/30/2007 03:09:27 MDTNo reproduction or networking permitted without license from IHS -,-,- 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. The resulting fiber elongation causes a
43、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 crosstalk, the optical power is confined to a
44、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 interconnecting single-mode filber. The en
45、d-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 polarimetric method also provides insight into t
46、he 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 Figwe 5. Both spans of PM fiber are stre
47、tched 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 the worst-case crosstalk of concatenated P
48、M 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 mounted 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- Copyright Telecommunications Industry Association Provided b