1、 TIA TELECOMMUNICATIONS SYSTEMS BULLETIN ITM-22 Continuous Wave Method for Measuring the Raman Gain Efficiency of Single-Mode Fibers TSB-62.22-A (Revision of TSB-62.22) September 2005 TELECOMMUNICATIONS INDUSTRY ASSOCIATION The Telecommunications Industry Association represents the communications se
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19、F DAMAGES IS A FUNDAMENTAL ELEMENT OF THE USE OF THE CONTENTS HEREOF, AND THESE CONTENTS WOULD NOT BE PUBLISHED BY TIA WITHOUT SUCH LIMITATIONS. TSB-62-22-A i ITM-22 Continuous wave method for measuring the Raman gain efficiency of single-mode fibers Contents Foreword. ii 1 Introduction 1 2 Referenc
20、es 5 3 Apparatus . 6 4 Sampling and specimens. 8 5 Procedure 9 6 Calculations and interpretation of results. 9 7 Documentation. 10 Annex A Comparison between this ITM to IEC and ITU Standards 12 References 13 TSB-62-22-A iiForeword (This foreword is informative only and is not part of this Standard.
21、) This Telecommunications Standards Manual comes from TIA (Telecom-munications Industry Association) Project Number 3-0006, and was formulated under the cognizance of TIA FO-4.2 Subcommittee on Optical Fibers and Cables, which is part of TIAs Fiber Optic Division. This ITM is part of the series of t
22、est procedures included within Recommended Standard TIA/EIA-TSB-62. There is one informative annex. Key words: Raman gain efficiency, single-mode optical fiber, nonlinear effects, Stimulated Raman Scattering, Stimulated Brillouin Scattering. TSB-62-22-A 1 ITM-22 Continuous wave method for measuring
23、the Raman gain efficiency of single-mode fibers 1 Introduction 1.1 Intent This informative test memorandum describes a continuous wave method for measuring the Raman gain efficiency of a single-mode transmission optical fiber. This parameter is a measure of the fibers efficiency at converting input
24、pump power to information signal power. 1.2 Background When a fiber carries high optical intensities, the optical power can be scattered because of interactions with mechanical vibrations in the fiber. For low power levels, the scattered power is a small fraction of the incident power. However, as t
25、he incident power increases, the scattered power increases at a faster pace, and is said to be stimulated. There are two forms of nonlinear stimulated scatteringBrillouin and Raman. Stimulated Brillouin Scattering (SBS) arises because of an interaction between light and mechanical vibrations that oc
26、cur in the form of a sound wave traveling along the length of the fiber (an “acoustic phonon”). SBS scatters light in the reverse direction. Stimulated Raman Scattering (SRS) is an interaction between light and the fibers molecular vibrations as adjacent atoms vibrate in opposite directions (an “opt
27、ical phonon”). Some of the energy in an optical pump wave pis transferred to the molecules, thereby further increasing the amplitude of their vibrations. If the vibrational amplitudes become large, a threshold is reached at which the local index of refraction changes. These local changes then scatte
28、r light in all directionssimilar to Rayleigh scattering. However, unlike Rayleigh scattering, the wavelength of the Raman scattered light Ris shifted to longer wavelengths by an amount that corresponds to the vibrational frequencies of the molecules. The Raman scattered light amplifies information s
29、ignals s. The magnitude or gain efficiency of this amplification depends on: pump wavelength p, TSB-62-22-A 2 signal wavelength s, fiber effective area Aeff(the larger the area, the smaller the power density), fiber material composition (vibration frequency and amplitude depend on material), fiber a
30、ttenuation coefficient, and fiber length. The Raman gain efficiency for a given fiber under test measured using a specific pump source varies with signal wavelength. The method described in this ITM is used to measure Raman gain efficiency ER(s)*over a range of signal wavelengths. The peak Raman gai
31、n efficiency corresponds to a Stokes downshifted frequency of about 13 THz, which equates to an upshifted wavelength of 110 nm for a 1450 nm pump, and 70 nm for a 1240 nm pump. The Full Width Half Maximum (FWHM) of the gain profile is about 7 THz (55 nm) at 1550 nm. 1.3 Definitions For the purposes
32、of this ITM, the following definitions apply. Effective length (Leff). The fibers effective length accounts for decreasing nonlinear effects as light attenuates along a fibers length, and is defined as: LeeffL=1023023.(1) where is the fiber attenuation coefficient in the units of dB/km, and L is the
33、 fiber length in km. When 0.23L 1, equation (1) simplifies to give Leff 1/(0.23), which is the length at which the power in the fiber has decreased by a factor of 1/e. As an example, Leff = 21.7 km when = 0.20 dB/km. Depolarized light. Light is considered depolarized (unpolarized, randomly polarized
34、) when its electric field vector, described in a plane perpendicular to the direction of propagation, is uniformly distributed in all radial directions. Rotation of a polarizer in a beam of depolarized light reduces its intensity by 50% regardless of the polarizers angular orientation. This test, ho
35、wever, is not sufficient to assess whether the light is depolarized because circularly polarized *The notation “CR” is often used in the technical literature, and is variously referred to as the “Raman gain coefficient”1, the “Raman efficiency”2, and the “Raman gain.”3When the in equation (1) is exp
36、ressed in the units of nepers/km, the two occurrences of “0.23” disappear, and the resultant equation is the form that typically appears in the technical literature. TSB-62-22-A 3light produces the same result. To guard against this possibility, a rotatable quarter wave retarder should be inserted b
37、efore the polarizer. If the output intensity is constant over all independent rotations of the retarder and the polarizer, the input light can be considered depolarized. 1.4 Method The method described in this ITM for measuring Raman gain efficiency uses unmodulated continuous waves generated by two
38、 sourcesa signal source and a pump source. The signal source can be broadband (such as an LED or amplified spontaneous emission (ASE) or narrowband (such as one or more tunable lasers). If using a broadband signal source, a tunable filter might be needed at the sources output so that short signal wa
39、velengths do not pump longer signal wavelengths. To minimize the influence of a noisy pump or one whose output power is not completely depolarized, the measurement described in this ITM is made by injecting light from the signal and pump sources so that they propagate in opposite directions (counter
40、 propagation) in the fiber under test. The fiber has an effective length Leff. A pump source having wavelength pinjects optical power Pp into the fiber under test so as to induce stimulated Raman scattering. The pump power should be chosen to minimize ASE noise and amplified double Rayleigh backscat
41、tered signal power. Section 3.1 gives guidance on how to choose the pump power level and spectral width. The pump-induced SRS in the fiber under test amplifies an input signal having wavelength s, which is launched into the fiber under test in a direction opposite to that of the pump. Section 3.2 gi
42、ves guidance on how to choose the signal power level and spectral width. Figure 1. Typical test set-up for measuring the Raman gain efficiency of a fiber. Figure 1 shows a typical test set-up. The output power Poutis measured in three configurations: pump/signalcombinerbroadbandsource, PinOSA,Poutpu
43、mplaser, Ppresidualpump powerdetectorfiber under testpump monitorTSB-62-22-A 4 P1 signal “on” and pump “off.” This indicates the relative magnitude of the launched signal power diminished by the attenuation of the components. P1includes double Rayleigh backscattered power from the unamplified signal
44、. P2 signal “off” and pump “on.” This measures the ASE. P3 signal “on” and pump “on.” This measures the Raman amplified signal, ASE, and double Rayleigh backscattered power from the amplified signal. These three powers are measured over a range of signal wavelengths s p. The “on/off” gain Gon/off(s)
45、 is then computed at each signal wavelength using: ()123/PPPGsoffon= (2) where the Ps are in linear units, such as W or mW. The dimensionless quantity Gon/off(s) is used to compute the fibers Raman gain efficiency for depolarized light: ()effpsoffonsRLPGE)(ln/ = (3) where Ppis the pump power launche
46、d into the fiber under test and expressed in W. Leffis the fiber effective length in km. ER(s) has the units of 1/(Wkm). Figure 2. Raman gain efficiency of depolarized light for a dispersion-unshifted fiber pumped at 1486 nm.40 20 40 60 80 100 120 140 160New Wavelength is Longer (nm)Raman Gain Effic
47、iency- E (s)R(1/(Wkm)0 2 4 6 8 10 12 14 16 18 20 New Frequency is Lower (THz)0.50.40.30.20.10TSB-62-22-A 5Because ER(s) is obtained for a range of signal wavelengths, ER(s) can be plotted versus = sp, or alternatively, versus f = fp fswhere fp and fsare the optical frequencies of the pump and signal
48、 waves, respectively (see Figure 2), and (nm) f(THz) sp(m)/0.3. 1.5 Laser Safety The safety procedures in IEC 60825-1 and 2 should be observed when using high optical powers. 2 References The following standards are referenced in this document and are needed to follow this test procedure. At the tim
49、e of publication, the editions indicated were valid. All standards are subject to revision, and 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/EIA 455-B Standard Test Procedure for Fiber Optic Fi
