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1、 TIA DOCUMENT FOTP-199 In-line Polarization Crosstalk Measurement Method for Polarization- Maintaining Optical Fibers, Components, and Systems TIA-455-199 DECEMBER 2002 TELECOMMUNICATIONS INDUSTRY ASSOCIATION The Telecommunications Industry Association represents the communications sector of Copyrig
2、ht Telecommunications Industry Association Provided by IHS under license with EIALicensee=IHS Employees/1111111001, User=Wing, Bernie Not for Resale, 03/29/2007 21:38:44 MDTNo reproduction or networking permitted without license from IHS -,-,- NOTICE TIA Engineering Standards and Publications are de
3、signed 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 their particular need. The exi
4、stence of such Publications shall not in any respect preclude any member or non-member of TIA from manufacturing or selling products not conforming to such Publications. Neither shall the existence of such Documents preclude their voluntary use by non-TIA members, either domestically or internationa
5、lly. TIA DOCUMENTS TIA Documents contain information deemed to be of technical value to the industry, and are published at the request of the originating Committee without necessarily following the rigorous public review and resolution of comments which is a procedural part of the development of a A
6、merican National Standard (ANS). TIA documents shall be reviewed on a five year cycle by the formulating Committee and a decision made on whether to reaffirm, revise, withdraw, or proceed to develop an American National Standard on this subject. Suggestions for revision should be directed to: Standa
7、rds is the line-width of the light source at the 3 dB intensity level. Beat length. The length of fiber needed to create a 2 (360-degree) phase difference between orthogonal polarization states. It can be expressed by the following relation: BLB/= (2) where is the center wavelength of the source; B
8、is the birefringence of the fiber given as (ne no); ne is the extraordinary refractive index of the sample; no is the ordinary refractive index of the sample. h-parameter. A length normalized form of polarization crosstalk. The attribute h has units of inverse meters and is given by the equation ()
9、maxmin 1 /tanh/1PPLh = (3) where L is the fiber length; Pmax and Pmin are the portions of the optical power with polarization plane parallel and perpendicular, respectively, to the excited axis. The plane of polarization of the light launched into the fiber is parallel to either principal optical ax
10、is. The function tanh-1 is the inverse hyperbolic tangent. For Pmin/Pmax less than 0.1, a sufficient approximation to the above equation is ()LPPh/ maxmin = . (4) Copyright Telecommunications Industry Association Provided by IHS under license with EIALicensee=IHS Employees/1111111001, User=Wing, Ber
11、nie Not for Resale, 03/29/2007 21:38:44 MDTNo reproduction or networking permitted without license from IHS -,-,- 4 2 Normative references Test or inspection requirements may include, but are not limited to, the following references: FOTP-57 (EIA-455-57), Optical Fiber End Preparation and Examinatio
12、n FOTP-80 (EIA-455-80), Cutoff Wavelength of Uncabled Single-mode Fiber by Transmitted Power FOTP-193 (EIA-455-57), Polarization Crosstalk Method for Polarization- Maintaining Optical Fiber and Components Copyright Telecommunications Industry Association Provided by IHS under license with EIALicense
13、e=IHS Employees/1111111001, User=Wing, Bernie Not for Resale, 03/29/2007 21:38:44 MDTNo reproduction or networking permitted without license from IHS -,-,- 5 3 Apparatus Figure 1 provides an example of the use of this method to measure the polarization crosstalk along a section of PM fiber that is i
14、lluminated through a rotatable polarizer. In this case, light passes through a polarizer and is coupled by a lens into the sample. (Many other configurations of the optical launch are possible. For example, light may be coupled directly from a laser chip to the PM fiber). Figure 1 Simplified block d
15、iagram of the polarization crosstalk method. 3.1 Narrowband light source Use a single-line laser (DFB or external cavity) or other narrow-band source with wavelength appropriate to the fiber under test. The source must be sufficiently coherent to avoid depolarization of the light by the PM fiber und
16、er test. In order for this to be true, the coherence length must satisfy a constraint based on the beat length of the PM fiber. Most PM fibers have beat lengths on the order of a few millimeters. See Annex A for details of the source coherence length constraint. 3.2 Polarizer Provide a polarizer wit
17、h extinction ratio at least 20 dB better than the polarization crosstalk value to be measured. Rotatable polarizer Heat. or stretch PMF under test Polarization adjuster Narrowband light source Polarimeter Data trace Copyright Telecommunications Industry Association Provided by IHS under license with
18、 EIALicensee=IHS Employees/1111111001, User=Wing, Bernie Not for Resale, 03/29/2007 21:38:44 MDTNo reproduction or networking permitted without license from IHS -,-,- 6 3.3 Input optics Use an optical lens to couple linearly polarized light into the test fiber. The lens should be of the low-birefrin
19、gence (strain-free) type to avoid transforming the launch polarization state from linear to elliptical polarization. 3.4 Connector support fixture For Samples with connectorized fiber pigtails, provide a fixture to align the key of the test fiber connector with the pass axis of the polarizer. 3.5 Ou
20、tput optics Connect the output of the test fiber directly to the polarimeter, or couple via a lens if the polarimeter has an open beam input port. 3.6 Polarimeter Use a fast polarimeter with Poincar sphere display to measure and record the output state of polarization of the test fiber. The polarime
21、ter may be of any type that measures the Stokes parameters and displays the measured polarization on the Poincar sphere. The wavelength range of the polarimeter must include the wavelength produced by the light source. 3.7 Heating and stretching fixtures Stretching and heating should be gently appli
22、ed to avoid damage to the fiber and its coating and, in the case of stretching, to avoid locally bending the fiber, allowing light to couple between the driven and the non-driven polarization mode. If the fiber is mechanically gripped for stretching, the gripping process should maintain the fiber in
23、 a straight, unbent trajectory. If the fiber is heated, the heat source should deliver heat comparable to that of a common hair dryer set at low to medium heat. 4 Preparation of the sample 4.1 The connector key of the fiber under test shall be nominally aligned with one of the principal axes, typica
24、lly the slow axis. 4.2 Support the test fiber in some manner with essentially zero tension (typically less than 15 gm). Copyright Telecommunications Industry Association Provided by IHS under license with EIALicensee=IHS Employees/1111111001, User=Wing, Bernie Not for Resale, 03/29/2007 21:38:44 MDT
25、No reproduction or networking permitted without license from IHS -,-,- 7 5 Procedure 5.1 Couple the input and output ports of the Sample to the polarized light source and to the polarimeter, respectively, as shown in figure 1. See Annex C for other measurement examples. 5.2 Align the polarizer pass
26、axis with the desired physical axis of the sample. This usually requires a user-provided fixture which may include an optical connector to interface with the sample. By industry convention, PM fiber applications usually transmit light in the slow polarization mode and connectors are installed with t
27、he key in alignment with the slow axis of the sample. If the polarizer is coupled to the sample without the use of a connector interface, i.e., in open beam fashion, alignment is simplified by mechanically marking, in advance, the orientation of the desired birefringent axis of the sample. An exampl
28、e is the use of capillary tubes with a D-flat on the outer surface to signify orientation. 5.3 Adjust the polarization adjuster to maximize transmission through the polarizer (that is, maximize the power received at the polarimeter). 5.4 Choose an approximately 0.3 to 0.5 meter region of the PM fibe
29、r at which the crosstalk is to be measured. Gently heat or cool this region to produce at least a 90-degree arc on the Poincar sphere display. Overheating, in particular, can damage buffer coatings. Alternatively, the arc may be generated by gently stretching the selected region of the PM fiber. Avo
30、id sharply stressing or bending the test fiber at the clamps while stretching it, as this can couple light from the desired to the undesired polarization modes. Expand the Poincar sphere display as needed to allow inspection of the displayed trace. Copyright Telecommunications Industry Association P
31、rovided by IHS under license with EIALicensee=IHS Employees/1111111001, User=Wing, Bernie Not for Resale, 03/29/2007 21:38:44 MDTNo reproduction or networking permitted without license from IHS -,-,- 8 6 Calculations 6.1 Superimpose a circle on the displayed data as shown in figure 2(a). 1 r Data tr
32、ace Fitted circle 1 r r BA Poincare sphere (a)(b) Figure 2 - Poincar sphere representations; a) front view of data arc and fitted circle, and b) 90-degree-rotated cutaway view defining the geometrical relationships. Points A and B represent the polarization modes (eigenmodes) of the fiber. 6.2 Calcu
33、late polarization crosstalk as follows: 2 2 11 11 log10 r r Crosstalk + = (5) where r is the radius of the fitted circle. The largest possible circle is r=1, the radius of the Poincar sphere. Alternatively, crosstalk may be calculated using the following relationship: cos1 cos1 log10 + =Crosstalk (6
34、) Copyright Telecommunications Industry Association Provided by IHS under license with EIALicensee=IHS Employees/1111111001, User=Wing, Bernie Not for Resale, 03/29/2007 21:38:44 MDTNo reproduction or networking permitted without license from IHS -,-,- 9 Annex B provides a derivation of Equations 1
35、and 2. Table 1 lists the crosstalk values for a variety of radii and angles. Table 1 Crosstalk values corresponding to various radii and angles. Crosstalk (degrees)(dB) 80.00-1.5 40.00-8.8 20.00-15.1 10.00-21.2 8.00-23.1 6.00-25.6 4.00-29.1 2.00-35.2 1.00-41.2 0.80-43.1 0.60-45.6 0.40-49.1 0.20-55.2
36、 Radius rCrosstalk (dB) 10.0 0.8-6.0 0.6-9.5 0.4-13.6 0.2-19.9 0.1-26.0 0.08-27.9 0.06-30.4 0.04-34.0 0.02-40.0 0.01-46.0 0.005-52.0 Copyright Telecommunications Industry Association Provided by IHS under license with EIALicensee=IHS Employees/1111111001, User=Wing, Bernie Not for Resale, 03/29/2007
37、 21:38:44 MDTNo reproduction or networking permitted without license from IHS -,-,- 10 7 Documentation 7.1 Report the following information for each test: 7.1.1 Test date 7.1.2 This Standard number 7.1.3 Sample identification 7.1.4 Center wavelength of the optical source 7.1.5 Polarization crosstalk
38、, in decibels (dB) 7.1.6 Sample description, including fiber length and connector type 7.1.7 Identify the principal axis launched into with respect to connector keys 7.2 The following information shall be available upon request: 7.2.1 Description of the spectral width of the optical source 7.2.2 Des
39、cription of the polarimeter system 7.2.3 Description of the launch optics 7.2.4 Value of the extinction ratio of the polarizer 7.2.5 Last calibration date of the polarimeter 7.3 United States military applications require that the following information also be reported for each test. For other (nonm
40、ilitary) applications, this information need not be reported but shall be available for review upon request. 7.3.1 Description of key equipment and components. 7.3.2 Date of most recent calibration of equipment. 8 Specification information The following shall be part of the Detail Specification: 8.1
41、 Sample type 8.2 This FOTP (FOTP-XXX) 8.3 Nominal measurement wavelength 8.4 Failure or acceptance criteria Copyright Telecommunications Industry Association Provided by IHS under license with EIALicensee=IHS Employees/1111111001, User=Wing, Bernie Not for Resale, 03/29/2007 21:38:44 MDTNo reproduct
42、ion or networking permitted without license from IHS -,-,- 11 Annex A (normative) A.1 Source coherence length criterion As discussed in Section 3, the coherence length must exceed a certain characteristic distance in order to avoid depolarization effects. The birefringence of PM fiber is usually exp
43、ressed as the beat length. The coherence constraint can be expressed in terms of the coherence length and the beat length as: B c L L l10 (A-1) where L is the sample length; LB is the fibers beat length; is the source center wavelength in vacuum. EXAMPLE Let the sample length be 10 m, the source cen
44、ter wavelength be 1.5 m, and the fibers beat length be 2 mm at 1.5 m. Then the source coherence length shall be greater than: mm75mm105 . 1 m102 m10 10 3 3 = c l (A-2) Note that the coherence length of Fabry-Perot laser diodes tends to be on the order of tens of millimeters, and may not be acceptabl
45、e for measuring the crosstalk of this example fiber. The preferred sources are distributed feedback (DFB) lasers or external cavity lasers. The source is not required to be tunable. Copyright Telecommunications Industry Association Provided by IHS under license with EIALicensee=IHS Employees/1111111
46、001, User=Wing, Bernie Not for Resale, 03/29/2007 21:38:44 MDTNo reproduction or networking permitted without license from IHS -,-,- 12 Annex B (informative) B.1 Mathematical basis of the method Assume that a polarized narrowband source is coupled into a length of polarization maintaining fiber such
47、 that light propagates in both the fast and slow polarization modes. The output state of polarization is measured using a polarimeter. Stretching or heating the fiber causes a phase shift between the polarization modes, the nominal refractive indices remaining nearly fixed. This phase change produce
48、s a circular arc on the Poincar sphere display of the polarimeter. The circle is concentric with a diameter connecting the two orthogonal states that represent the fast and slow modes. The circle is a great circle when equal amounts of light propagate in the two polarization modes. The circle collapses toward one or the other of these orthogonal states as the