1、TIA/EIA TELECOMMUNICATIO SYSTEMS BULLETIN VS ITM-23 Measurement of the Nonlinear Coefficient of Single-Mode Fibers TSB62-23 SEPTEMBER 2001 TELECOMMUNICATIONS INDUSTRY ASSOCIATION The Tekrnrnmicatiom Indiwtry Auwdation represents the mmunicationy secior of Elsctronic Industries Alliance NOTICE TIA/EI
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6、 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 that may be acceptab
7、le 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 to revise and up
8、date this Bulletin from time to time as may be occasioned by changes in technology, industry practice, or government regulations, or for other appropriate reasons. (From Project No. 3-4654, formulated under the cognizance of the TIA FO-6.6 Subcommittee on Optical Fibers .) Published by TELECOMMUNICA
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10、rnational (303-397-7956) All rights reserved Printed in U.S.A. TSB-62-23 ITM - 23 Measurement of the nonlinear coefficient of single-mode fibers Con tents 1 I n trod uction 2 Normative references 3 Apparatus 4 Sampling and specimens 5 Procedure 6 7 Documentation Calculations and interpretation of re
11、sults Annex A Continuous wave dual-frequency method (CW) 10 Annex C Table of acronyms 21 Annex D Bibliography 22 Comparison between this ITM and IEC or ITU Standards 22 Annex B Pulsed single-frequency method (PM) 16 Annex E Foreword . ii Measurement of the nonlinear coefficient of single-mode fibers
12、 . 1 ITM - 23 1 i TSB-62-23 ITM - 23 Measurement of the nonlinear coefficient of single-mode fibers Foreword This Telecommunications Standards Manual comes from TIA (Telecommunications Industry Association) Project No. 4654, and was formulated under the cognizance of TIA FO-6.6 Subcommittee on Optic
13、al Fibers and Materials. This ITM is part of the series of test procedures included within Recommended Standard TIAEIA-TSB-62. There are five annexes, all of them informative. Key words: nonlinear coefficient, single-mode optical fiber. ii TSB-62-23 ITM - 23 Measurement of the nonlinear coefficient
14、of single-mode fibers 1 Introduction 1.1 Intent This informative test method describes two methods for uniform measurement of the nonlinear coefficient of single-mode fibers in the 1550 nm region. 1.2 Scope The nonlinear coefficient (nLc) is the ratio of the Kerr nonlinear refractive index, n2, and
15、the effective area, A,fi (see FOTP-132), as: n2 nLc = - The nonlinear coefficient is related to the following nonlinear optical distortion effects as a combined parameter: Self-phase modulation (SPM) Cross-phase modulation (XPM) Four-wave mixing (FWM) Other fiber attributes, such as chromatic disper
16、sion, also influence the transmission. Two methods are given, with details specific to each in normative annexes. They are: Method A Continuous-wave dual-frequency Method B Pulsed single-frequency Both methods require injecting very high powers (5 dBm or more) into the fiber, measurement of this pow
17、er (absolute), and measurement of the output spectrum - which is modified by nonlinear effects. Both methods use calculations that combine these measured results with those derived from other measurements such as attenuation (FOTP-61 or FOTP-78) and chromatic dispersion (FOTP- 168, FOTP-169, or FOTP
18、-175). Both methods have limitations on the length of fiber that can be measured - in a relationship with the chromatic dispersion at the wavelength being measured. 1 TSB-62-23 Method A I requires injecting the light of two wavelengths into the fiber. The light of both wavelengths is constant at var
19、ious power levels. At higher power, the lights beat due to the nonlinear effect and produce an output spectrum that is spread. The relationship of the power level to a particular metric of spectrum spreading is used to calculate the nonlinear coefficient. Method B 2, 31 requires injecting pulsed lig
20、ht at a single wavelength. The pulses must be of a duration substantially less than 1 ns and the input peak power of these pulses must be measured and related to the nonlinear spreading of the output spectrum. 1.3 Applicability Measurements of the nonlinear coefficient are used to characterize speci
21、fic single-mode fiber designs for the purpose of system design relative to power levels and distortion or noise effects derived from the nonlinear optical behavior. This test method applies to a parameter that is not specifiable, at least at the present time. Therefore, this test method shall not be
22、 called out in any TIA specification document. 2 Normative references The following standards contain provisions which, through reference in this text, constitute provisions of this test method. At the time of publication, the editions indicated were valid. All standards are subject to revision, and
23、 parties to agreements based on this ITM are encouraged to investigate the possibility of applying the most recent editions of the standards indicated below. ANSI and TIA maintain registers of currently valid national standards published by them. ANSI/TIA/EIA 440-B Fiber Optic Terminology ANSI/TIA/E
24、IA 455-B Standard Test Procedure for Fiber Optic Fibers, Cables, Transducers, Sensors, Connecting and Terminating Devices, and Other Fiber Optic Components ANSI/TIA/EIA 4920000-B Generic Specijkation for Optical Fibers FOTP-57 (TMIA-455-57B) Preparation and examination of optical Jber endface for te
25、sting purposes 2 TSB-62-23 FOTP-60 (TIA/EIA-45 5 -60A) FOTP-6 1 (TIA/EIA-455-6 1A) FOTP-7 8 (TIA/EIA-45 5 -7 SA) FOTP-132 (TIA/EIA-455-132) FOTP-133 (TIA/EIA-455-133) FOTP-168 (TIA/EIA-455-168) FOTP- FOTP- 69 (TIA/EIA-455- 169) 75 (TIA/EIA-455-175) IEC 61315 Measurement offlber or cable length using
26、 an OTDR Measurement offlber or cable attenuation using an OTDR Spectral attenuation cutback measurement for single-mode optical flbers Measurement of efective area of single-mode optical flber Length measurement of an opticalflber or cable by the phase-shqt method Chromatic dispersion measurement o
27、f multimode graded-index and single-mode optical flbers by spectral group delay measurement in the time domain Chromatic dispersion measurement of single-mode opticalflbers by the phase-shift method Chromatic dispersion measurement of single-mode Opticalflbers by the diferential phase-shqt method Ca
28、libration of optical power meters 3 Apparatus The following apparatus is common to both measurement methods. Annexes A and B include layout drawings and other equipment requirements for each of the methods, respectively. 3.1 Light source See Annexes A and B for detailed characteristics of the light
29、sources. 3 TSB-62-23 3.2 Input optics The input optics include one or more lasers, amplifiers, variable attenuators, couplers and power meters. Bandpass filters and oscilloscopes may be needed for Method B. See Annexes A and B for specific details. 3.3 Input positioner Provide means of positioning t
30、he input end of the specimen to the light source. Typically, this connection is with a fusion splice to a short (1 m) pigtail of single- mode fiber. 3.4 Cladding mode stripper Use a device that extracts cladding modes. Under some circumstances the fiber coating will perform this function. 3.5 Output
31、 positioner Provide a suitable means for aligning the fiber to the output optics. Typically, this connection is with a fusion splice to a pigtail of BI .I fiber. 3.6 Output optics The output optics include a power meter and optical spectrum analyzer. An oscilloscope may be required for Method B. See
32、 Annexes A and B for details. 3.8 Computer Use a computer to perform operations such as controlling the apparatus, taking intensity measurements, and processing the data to obtain the final results. 4 Samples and specimens A specimen is a known length of single-mode optical fiber. The sample and pig
33、tails must be fixed in position at a nominally constant temperature throughout the measurement. Standard ambient conditions (TIAEIA-455-B) shall be employed unless otherwise specified. End faces for the input and output ends of the test sample must be prepared as appropriate (FOTP-57) to obtain low
34、loss splices. If power levels are sufficient to cause damage to the endfaces, fusion splicing may be necessary. The safety 4 TSB-62-23 procedures described in IEC 60825 should be observed when using high optical powers. The measurement method is limited with regard to the measurable length because o
35、f chromatic dispersion. For this reason, the specimen is normally cut from a longer piece of fiber that has been characterized for attenuation coefficient, adB, and chromatic dispersion, D, at the wavelength of interest (1 550 nm). The length of the fiber after being cut-back is referred to as L. Se
36、e Annexes A and B for details specific to the length and chromatic dispersion requirements. The fiber may be deployed on a common shipping spool. 5 Procedure 5.1 Deploy the fiber or cable and prepare the ends 5.2 Attach the ends to the input and output optics 5.3 Engage the computer to complete the
37、scans and measurements found in Annexes A and B for the measurement methods. 5.4 Complete documentation 6 Calculations of interpretation of results Unless otherwise specified, the units are in meters, seconds, watts, and radians. The fundamental relationships for the two methods are nearly the same
38、so they are presented here for comparison. 2z n, Method A q = -L,#P e8 2z n, Method B q = -L,P, e8 5 TSB-62-23 where p Nonlinear phase shift (radians) h Wavelength (m) (center of two wavelengths for Method A) Lefi Effective length (m) P Input power (W) (both wavelengths for Method A) Ppeak Peak inpu
39、t power (W) (Method B) If peak power of Method B were equal to twice the power of Method A, the two equations would be identical. The effective length is defined as the following: i - exp(- d) a Le? = Where L = Length (m) a = “natural” attenuation coefficient (nepers/m) Where adB Normal attenuation
40、coefficient (dB/km) The two methods differ in how the phase shift is determined as a function of input power. Once the phase shift and power relationship has been determined, the inverse of equation 2a or 2b, to obtain the nonlinear coefficient is easily computed with the other known quantities. For
41、 Class IVa fiber, the nonlinear coefficient has been measured to be approximately 2.9.10-” W-, provided as an example of the result. 7 Documentation 7.1 Information to be recorded for each measurement 7.1 .I Specimen Identification 7.1.2 Measurement date 7.1.3 Nonlinear coefficient: n2/&ff (W-) 6 TS
42、B-62-23 7.1.4 7.1.5 7.1.6 7.2 7.2.1 7.2.2 7.2.3 7.2.4 7.2.5 Fiber dispersion coefficient (ps/nm.km) Fiber attenuation coefficient (dB/km) Fiber length Information to be available upon request Measurement method used Description of the equipment setup Wavelength(s) of the source Pulse duration (Metho
43、d A only) Typical input power levels 7 TSB-62-23 Annex A (informative) Annex A - Continuous wave dual-frequency method This annex contains requirements specific to Method A. The principle of the method is to inject two continuous wave (cw) optical frequencies, Oa and ab into the specimen at various
44、power levels. The two frequencies beat due to nonlinear effects and create sidebands at frequencies (20, - ab) and (Zab - ma) (see figure AI). The relative intensity of the side bands, II, to the intensity of the main bands, lo, is related to both the phase shift and power injected. -60 I I I I 1562
45、.5 1563.0 1563.5 1564.0 1564.5 1565.0 Wavelength (nm) Figure AI - Output spectral characteristics A.l Apparatus Figure A2 shows a typical arrangement of the test apparatus. 8 Figure A2 - Apparatus for Method A A.l .I Sources Two laser sources, LI and L2 in figure A2, are operated in the cw mode at o
46、ptical frequencies, ma and ab, both within the window of 1550 f 1 O nm. The frequency difference, Ac00 = Coa - ab, corresponds to a wavelength difference, Aho, which places an upper limit on the spectral width of each of the sources: the source spectral widths shall not exceed 0.1 .Aho. The lower li
47、mit on the spectral width is set by the need to avoid Brillouin Scattering (see A.1.3). The wavelength separation lower limit is set by the ability of the optical spectrum analyzer (OSA) to resolve the sidebands and the upper limit is set by the dispersion of the specimen (see A.2). A typical separa
48、tion could be 0.035 THz (0.28 nm), but others are feasible depending on the other details of the setup. The source powers shall be within 0.2 dB of one another. The source power is further conditioned by polarizers, optically amplified, and variably attenuated. The minimum injected power is set by t
49、he limit at which the sidebands are induced. The maximum is set by the need to avoid Brillouin scattering. A.1.2 Optical signal conditioning Polarization controllers, combiners, amplfiers, attenuators and polarizers shall be used in combination so that the light injected into the specimen are in the same polarization state and within 0.2 dB of one another. 9 TSB-62-23 In the example of figure A2, the erbium doped fiber amplifier (EDFA) is used to boost the power to levels sufficient to induce nonlinear effects. This generates amplified spontaneous emission (ASE) which s
