1、 TIA-455-199 December 2002 (r 01/2012) FOTP-199 In-Line Polarization Crosstalk Measurements Method for Polarization-Maintaining Optical Fibers, Components, and Systems NOTICE TIA Engineering Standards and Publications are designed to serve the public interest through eliminating misunderstandings be
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22、 THE CONTENTS HEREOF, AND THESE CONTENTS WOULD NOT BE PUBLISHED BY TIA WITHOUT SUCH LIMITATIONS. TIA-455-199 i FOTP-199 In-line polarization crosstalk measurement method for polarization-maintaining optical fibers, components and systems Contents Foreword .iii 1 Introduction.1 2 Normative references
23、 4 3 Apparatus.5 4 Preparation of the sample6 5 Procedure.7 6 Calculations or interpretation of results8 7 Documentation10 8 Specification information 10 Annex A Source coherence length criterion. 11 Annex B Mathematical basis of the method .12 Annex C Example measurement applications. .14 Annex D C
24、omparison between this standard and IEC or ITU standards 19 TIA-455-199 ii This page left blank. TIA-455-199 iii FOTP-199 In-line polarization crosstalk measurement method for polarization-maintaining optical fibers, components and systems Foreword From TIA Project No. 3783, formulated under the cog
25、nizance of TIA FO-6.9, Subcommittee on Fiber Optic Sensors. This FOTP is part of the series of test procedures included within Recommended Standard TIA/EIA-455. This Standard describes a new test method. There is one normative annex and three informative annexes. Key words: polarization crosstalk, p
26、olarization-maintaining optical fiber, pm component, pm fiber, PMF. TIA-455-199 iv This page left blank. 1 1 Introduction 1.1 Intent This Standard specifies an in-line method for measuring the accumulated polarization crosstalk at a particular physical location along a span of polarization-maintaini
27、ng (PM) fiber. 1.2 Scope This Standard is applicable to single sections of PM fiber, to cascaded PM fibers, and to PM fibers interconnected with optical devices. It is also applicable to polarization-maintaining components that lack PM fiber pigtails, in which case the measurement is performed on a
28、PM fiber jumper connected to the output of the component. The method requires gently stretching or heating approximately 0.1-0.3 meters of PM fiber in order to generate at least a fraction of a cycle of phase shift between the fast and slow waves. Measurement applications are described in Annex C. I
29、n all applications of this method, the measured crosstalk value represents the accumulated crosstalk at the location along the PM fiber where the stretching or heating is applied, irrespective of the optical fibers and components that follow it. Hereafter the test device is referred to as the “sampl
30、e”. The sample must include a section of PM fiber along which the polarization crosstalk is to be measured. This Standard complements FOTP-193, which describes the measurement of polarization crosstalk at a plane beyond the output interface of the sample. The polarization crosstalk of PM itself, wit
31、hout connectors, tends to be proportional to fiber length. In such cases the proportionality constant, commonly referred to as the “h-parameter,” is another measure of PM fiber performance. A test method for h-parameter is the subject of FOTP-192. This method is restricted to wavelengths greater tha
32、n or equal to that at which the fiber is effectively single-mode. The cutoff wavelength of an uncabled fiber may be determined by FOTP-80. 2 1.3 Background In theory, if perfectly linearly polarized light is launched into a PM fiber with its plane of polarization exactly aligned to either of the dev
33、ices principal optical axes, then the light radiated at the distal end will also be perfectly linearly polarized. In practice, some light may be coupled to the opposite polarization mode by the sample itself or by connectors attached to its input end, so that a small, unintended, component is presen
34、t. This Standard describes a method for measuring this imperfection in-line along a section of PM fiber. The method depends upon the fact that monochromatic light waves propagating in the linear polarization modes of the PM fiber undergo a relative phase shift when the PM fiber is stretched or heate
35、d (the nominal refractive index remaining nearly constant). The phase change causes the output polarization to trace a circular arc on the Poincar sphere. The crosstalk is calculated from the radius of this arc. 1.4 Definitions For the purposes of this standard, the following definitions apply. Orth
36、ogonality. A polarization relationship between two differently polarized waves in which the superimposed waves do not interfere. In the case of linearly polarized waves, the waves are orthogonal if the electric field directions are perpendicular. Birefringent material. A material in which orthogonal
37、 polarizations propagate at different speeds. A linearly birefringent material is one in which the orthogonal polarizations are linear. Materials may also be circularly or elliptically birefringent. Principal optical axes. In a linearly birefringent fiber, two radial directions that define the elect
38、ric field planes of the fast and slow waves. Along these two directions the index of refraction is minimum and maximum, respectively. Polarization modes. In the context of this procedure, two linearly polarized optical waves having electric fields aligned with the principal optical axes, commonly re
39、ferred to as the fast and slow polarization modes. Polarizer. Optical element designed to transmit light of a single polarization state most commonly, a linear state. Extinction, or extinction ratio, of a polarizer. The ratio of optical power in the transmitted polarization state to the power passed
40、 in the orthogonal polarization state. The extinction of a polarizing element is measured with unpolarized light incident on the device. 3 = /lc 2Polarization crosstalk. The ratio of the power in the undesired to the desired principal optical axes. Coherence length. The coherence length, lc, of an o
41、ptical light source shall be defined by the following relation: (1) where is the center wavelength of the light source; is the line-width of the light source at the 3 dB intensity level. Beat length. The length of fiber needed to create a 2pi (360-degree) phase difference between orthogonal polariza
42、tion states. It can be expressed by the following relation: BLB /l= (2) where is the center wavelength of the source; B 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
43、 normalized form of polarization crosstalk. The attribute h has units of inverse meters and is given by the equation ( )maxmin1 /tanh/1 PPLh = (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
44、 excited axis. The plane of polarization of the light launched into the fiber is parallel to either principal optical axis. 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) 4 2 Normative r
45、eferences Test or inspection requirements may include, but are not limited to, the following references: FOTP-57 (EIA-455-57), Optical Fiber End Preparation and Examination FOTP-80 (EIA-455-80), Cutoff Wavelength of Uncabled Single-mode Fiber by Transmitted Power FOTP-193 (EIA-455-57), Polarization
46、Crosstalk Method for Polarization-Maintaining Optical Fiber and Components 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 illuminated through a rotatable polarizer. In this case, light passes through a po
47、larizer 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 diagram of the polarization crosstalk method. 3.1 Narrowband light source Use a si
48、ngle-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 under test. In order for this to be true, the coherence length must satisfy a constr
49、aint 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 with extinction ratio at least 20 dB better than the polarization crosstalk value to be measured. Rotatable polarizerHeat.or stretchPMFunder testPolarization adjusterNarrowband light sourcePolarimeterData trace6 3.3 Input optics Use an optical lens to couple linearly polarized light into the test fiber. The lens sh
