1、 TIA DOCUMENT FOTP-220 Differential Mode Delay Measurement of Multimode Fiber in the Time Domain TIA-455-220-A (Revision of TIA/EIA-455-220) January 2003 TELECOMMUNICATIONS INDUSTRY ASSOCIATION The Telecommunications Industry Association represents the communications sector of NOTICE TIA Engineering
2、 Standards 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
3、for their particular need. The existence 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, e
4、ither domestically or internationally. 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 procedu
5、ral part of the development of a American National Standard (ANS). Further details of the development process are available in the TIA Engineering Manual, located at http:/www.tiaonline.org/standards/sfg/engineering_manual.cfm TIA Documents shall be reviewed on a five year cycle by the formulating C
6、ommittee 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: Standards Annex D is informative and is not considered part of this standard (Calculation information) Annex E i
7、s informative and is not considered part of this standard (References) Key words: multimode, differential mode delay, effective modal bandwidth, calculated effective modal bandwidth, DMD, EMB, EMBc iiiTIA-455-220-A This Page Left Blank ivTIA-455-220-A FOTP-220 Differential mode delay measurement of
8、multimode fiber in the time domain 1. Introduction 1.1 Intent This document describes a method for characterizing the modal structure of a graded index multimode fiber. In the procedure described in this standard, the output from a fiber that is single-mode at the test wavelength excites the multimo
9、de fiber under test. The probe spot is scanned across the endface of the fiber under test, and the optical pulse shape and delay are determined at specified offset positions. Two results can be produced from the same data. First, the difference in optical pulse delay time between the fastest and slo
10、west mode groups of the fiber under test can be determined. The user specifies the upper and lower limits of radial offset positions over which the probe fiber is scanned in order to specify desired limits of modal structure. Second, the optical pulse shapes can be combined using specific weights to
11、 determine a calculated effective modal bandwidth (EMBc), and by calculating a sequence of EMBc values with different sets of weights, a minimum EMBc can be calculated, corresponding to a range of transmitters. The test quantifies the effects of interactions of the fiber modal structure and the sour
12、ce modal characteristics excluding the source spectral interactions with fiber chromatic dispersion. Adding the effects of chromatic dispersion and source spectral width will reduce the overall transmission bandwidth, but this is a separate calculation in most transmission models. In this test, the
13、effects of non-zero spectral width are minimized but any residual effects will tend to increase the DMD value and decrease the EMBc value. 1.2 Scope This standard applies only to multimode, graded-index glass-core glass-clad fibers (material class Ia, as listed in TIA/EIA-4920000-B) Note: This test
14、method is commonly used in production and research facilities and is not easily accomplished in the field. 1.3 Definitions The user of this standard specifies either the maximum DMD for the outer (ROUTER) and inner (RINNER) limits of radial offset position over which the probe spot is scanned, or th
15、e minimum EMBc for the EMBc values from a sequence of DMD weights. The estimated difference in optical pulse delay time between the fastest and slowest modes excited for all radial offset positions between and including RINNERand ROUTERwill be called Differential Mode Delay (DMD). The Effective Moda
16、l Bandwidth (EMB) is the bandwidth associated with the transfer function, H(f), of a particular laser/fiber combination. 1TIA-455-220-A 2 Normative references Test, inspection, and safety requirements may include, but are not limited to, the following references: ANSI Z136.1: American National Stand
17、ard for Safe Use of Lasers ANSI Z136.2: American National Standard of the Safe use of Optical Fiber Communications Systems Utilizing Laser Diode and LED Sources TIA/EIA-440-B Fiber Optic Terminology TIA/EIA 4920000-B Generic specification for Optical Fibers FOTP-57 (TIA/EIA-455-57B) Optical fiber En
18、d Preparation and Examination FOTP-127 (TIA/EIA-455-127) Spectral Characterization of Multimode Laser Diodes FOTP-168 (TIA/EIA-455-168A) Chromatic Dispersion Measurement of Multimode Graded-Index and Single-Mode Optical Fibers by Spectral Group Delay Measurement in the Time Domain FOTP-191 (TIA/EIA-
19、455-191) Measurement of Mode Field Diameter of Single-Mode Optical Fiber FOTP-203 (TIA/EIA-455-203) Launched Power Distribution Measurement for Graded Index Multimode Fiber Transmitters FOTP-204 (TIA/EIA-455-204) Measurement of Bandwidth on Multimode Fiber FOTP-230 (to be TIA/EIA-455-230) Effective
20、Bandwidth (EB) Measurement on Multimode Fiber OFSTP-4 (TIA/EIA-526-4) Optical Eye Pattern Measurement Procedure 3 Apparatus 3.1 Light Source Use a light source that introduces short duration, narrow spectral width pulses into the probe fiber, as defined below. The temporal duration of the optical pu
21、lse shall be short enough to measure the intended differential delay time. The maximum duration allowed for the optical pulse, characterized as full width at 25% of maximum amplitude, will depend both on the value of DMD to be determined and the sample length. For example, if the desired limit is 0.
22、20 ps/m over a sample of length 500 m, the DMD to be measured is 100 ps, and a pulse of duration less than 110 ps is needed. Testing to the same limit in a 10km length of fiber requires measuring a DMD of 2000 ps, and a 2TIA-455-220-A pulse a wide as 2200 ps may be used. Detailed limits are given in
23、 section 6.1, and may depend on the source spectral width. Chromatic dispersion induced broadening resulting from source spectral width shall be within the limits indicated in Annex A. The requirement on spectral width may be met either by using a spectrally narrow source, or alternatively by the us
24、e of appropriate optical filtering at either the source or detector end. The center wavelength shall be within 10 nm of the nominal specified wavelength. Note:A mode locked Titanium-Sapphire laser is an example of a source usable for this application. 3.2 Launch System 3.2.1 Use a probe fiber betwee
25、n the light source and the test sample that is single-mode at the measurement wavelength. The mode field diameter of the probe fiber shall be (8.7 -2.39) 0.5 m, where is the measurement wavelength in micrometers, and the mode field diameter is determined using FOTP-191. (Note: The above equation int
26、erpolates mode field diameters between commercially available values of 5 m at 850 nm and 9 m at 1310 nm.) 3.2.2 Ensure that the output of the probe fiber is single-mode. One means is to create a mode filter by wrapping the probe fiber three turns around a 25-mm diameter mandrel. 3.2.3 The output be
27、am from the probe fiber shall be scanned across the endface of the test sample with a positional uncertainty not exceeding 0.5 m. 3.2.4 The output beam from the probe fiber shall be perpendicular to the endface of the test sample, to within an angular uncertainty not exceeding 1.0 degree. 3.2.5 The
28、launch system shall be capable of reproducibly centering the output beam from the probe fiber to within 1.0 m. 3.2.6 If directly coupled to the test sample, the gap between the output end of the probe fiber and the endface of the test sample shall not exceed 10 m. 3.2.7 Alternate means may be used t
29、o create the probe beam, provided that the probe beam excites substantially the same modes in the fiber under test, for any given offset, as would a beam coupled directly from the output of a single-mode probe fiber. For example, a system of lenses or mirrors may be used to image the output of a sin
30、gle-mode probe fiber onto the endface of the test sample. 3.2.8 Provide means to remove cladding light from the test sample. Often the fiber coating is sufficient to perform this function. Otherwise, it will be necessary to use cladding mode strippers near both ends of the test sample. The fibers ma
31、y be retained on the cladding mode strippers with small weights and/or velcro straps, but care shall be taken to avoid microbending at these sites. 3TIA-455-220-A 3.3 Detection System 3.3.1 Use an optical detection apparatus suitable for the test wavelength. The detection apparatus shall couple all
32、of the guided modes from the test sample onto the detector active area, such that the detection sensitivity is not significantly mode dependent. The detector, along with any signal preamplifier, shall respond linearly (within 5%) over the range of power detected. An optical attenuator may be used to
33、 control the optical intensity on the detector, but it shall not be significantly mode dependent. The temporal response of the detection apparatus shall not be significantly mode dependent. A specific test of mode dependence is given in section 5.1.4. Alternatively, the detectors temporal response m
34、ay be a function of offset as long as it is stable over the course of the measurement (i.e. TPULSE(r) shall fulfill the 5% requirement of 5.1.4). 3.3.2 Ringing of the detector system shall be limited such that maximum overshoot or undershoot shall be less than 5% of the peak amplitude of the detecte
35、d optical signal as measured on the reference. 3.3.3 A device shall be available to position the specimen output end with sufficient stability and reproducibility to meet the conditions of 3.3.1. 3.3.4 The waveform of the detected optical signal shall be recorded and displayed on a suitable instrume
36、nt, such as a high-speed sampling oscilloscope with calibrated time sweep. The recording system should be capable of averaging the detected waveform for multiple optical pulses. 3.3.5 Use a delay device, such as a digital delay generator, to provide a means of triggering the detection electronics at
37、 the correct time. The delay device may trigger the optical source, or be triggered by it. The delay device may be internal or external to the recording instrument. 3.3.6 The combined effect of timing jitter and noise in the detection system shall be small enough that the difference between successi
38、ve measurements of optical delay times, for any fixed launch used in the measurement, shall be less than 5% of the measured value of DMD. Averaging the detected waveform for multiple optical pulses may be used to reduce the effects of timing jitter and noise. If averaging is used, the same number of
39、 averages shall be used in recording all waveforms. The system shall maintain this level of stability over the course of the measurement. 3.4 Computational Equipment This test method generally requires a computer to store intermediate data, and calculate the test results. 4 Sampling and specimens 4.
40、1 Test sample The test sample shall be a known length of optical fiber or optical fiber cable. 4.2 End Preparation Prepare flat endfaces on the test sample in accordance with FOTP-57. 4TIA-455-220-A 4.3 Test Sample Packaging Support the test sample in a manner that relieves tension and minimizes mic
41、robending. 4.4 Test Sample Positioning Position the input end of the test sample such that it is aligned to the output end of the Launch System as described in section 3.2. Position the output end of the test sample such that it is aligned with the detection system, as described in section 3.3. 5 Pr
42、ocedure 5.1 Adjust and measure system response 5.1.1 Couple the output of the probe fiber directly into the detection apparatus. This may be accomplished by mounting the probe fiber in the detection apparatus, or by using a short (1400 nm: S0 =0.101 ps/(nm2 km); 0 = 1310 nm for nominal MMF with 0.20
43、 NA 13TIA-455-220-A Annex B (informative) Discussion of measurement details B.1 DMD Mask This Standard focuses on determining the difference in delay times between the fastest and slowest mode groups excited for a stated range of excitation conditions, as illustrated in Figure B-1. timeTFASTTSLOWr2r
44、1r3r4Figure B-1: Idealized DMD data. Leading and trailing edge times (25% threshold) are identified with “+“. Traces are offset for different excitation positions. Inset shows TPULSE.TPULSEFor any given offset position, the spot from the single-mode probe excites several different mode groups. The r
45、esulting output waveform, U(r,t), exhibits a complicated time dependence, typically showing multiple peaks, and with no guarantee that the individual mode groups will be clearly resolved. The detection level for the leading and trailing edges is chosen at 25% of peak amplitude of a given waveform. T
46、his accounts for instances when the mode group maximally excited by the probe spot at a given offset is separated in time from the other mode groups excited at the same offset. The 25% level assures detection of the separated mode group even when the other modes all have the same delay time, causing
47、 their combined amplitude to exceed that of the separated mode group. The difference between TSLOWand TFASTwill be greater than the DMD by an amount that depends on the temporal width of the optical pulse, the finite bandwidth of the optical detector, and the broadening of each mode due to the sourc
48、e spectral width and the chromatic dispersion of the fiber under test. 14TIA-455-220-A The temporal width of the optical pulse and the finite detector bandwidth are characterized as TPULSE. In the limit of small TPULSE, and assuming a Gaussian shape for the spectrum of the source, the temporal width
49、 at 25% of maximum of each mode at the output of the fiber under test will be ( ) LDtchrom= )2ln(4 , B.1- where: is the RMS spectral width of the source (in nm, determined using FOTP-127), D() is the chromatic dispersion (in ps/(nmkm) and L is the sample length (in km). The factor )2ln(4 comes from the use of 25% of maximum amplitude as the threshold for evaluating DMD, along with the use of RMS spectral width in characterizing the source. The full width at 25% of each mode at the output of the fiber under test is then (2/122chromPulseRE
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