Method for characterizing mode group properties of multimodal light traveling through optical components

Abstract

The invention concerns a method for characterizing mode group properties of multimodal light traveling through an optical component, comprising: providing a Mode Group Separating optical fiber in an optical path between a light source and said optical component; launching reference pulses of light with a wavelength t from said light source through said Mode Group Separating optical fiber into said optical component at discrete intervals between a core center and a core radius of said fiber.
The Mode Group Separating optical fiber is a multimode fiber with an -profile graded index core with an -value chosen such that said fiber satisfies the following criterion at the wavelength t: $\frac{.Math. .Math. .Math. L}{T_{REF}} > 4$
where: is a time delay difference between consecutive mode groups; L is a length of said fiber; T.sub.REF is a Full Width at Quarter Maximum of said reference pulses.

Claims

1. A method for characterizing mode group properties of multimodal light traveling through an optical component, wherein said method comprises the steps of: providing a Mode Group Separating optical fiber (5) in an optical path between a light source (1) and said optical component (8); launching reference pulses of light with a wavelength .sub.t from said light source (1) through said Mode Group Separating optical fiber (5) into said optical component (8) at radial offsets between a core center and a core radius of said Mode Group Separating optical fiber; detecting a light signal output by said optical component (8); wherein said Mode Group Separating optical fiber is a multimode fiber with an -profile graded index core with an -value chosen such that said Mode Group Separating optical fiber satisfies the following criterion at the wavelength .sub.t: $\frac{.Math. .Math. .Math. L}{T_{REF}} > 4$ where: is a time delay difference between consecutive mode groups expressed in ps/m; L is a length of said Mode Group Separating optical fiber expressed in m; T.sub.REF is a Full Width at Quarter Maximum of said reference pulses expressed in ps, where the Full Width at Quarter Maximum of a reference pulse is the difference between the two time values at which the power of the reference pulse is equal to quarter of its maximum value.

2. The method according to claim 1, comprising: measuring a Dispersion Modal delay profile for said Mode Group Separating optical fiber, called a reference DMD profile; measuring a Dispersion Modal delay profile at the output of said optical component, called a resulting DMD profile; comparing said reference DMD profile and said resulting DMD profile in order to characterize the mode group properties of multimodal light traveling through said optical component.

3. The method according to claim 1, wherein said Mode Group Separating optical fiber satisfies the following criterion at the wavelength .sub.t: $\frac{.Math. .Math. .Math. L}{T_{REF}} > 5.$

4. The method according to claim 1, wherein said wavelength .sub.t is between 800 nm and 1500 nm.

5. The method according to claim 1, wherein said Mode Group Separating optical fiber has a core diameter of 50 m 2.5 m and a numerical aperture NA=0.2 0.015, and wherein:
(,)=p00+p10*+p01* +p11**+p02* .sup.2 with: p00 between 1.461 and 1.116 p10 between 0.001516 and 0.00128 p01 between 1.061 and 1.317 p11 between 0.0006583 and 0.0007706, and p02 between 0.3125 and 0.2581.

6. The method according to claim 5, wherein: p00 =1.288 p10 =0.001398 p01 =1.189 p11 =0.0007145 p02 =0.2853.

7. The method according to claim 1, wherein said Mode Group Separating optical fiber is a Few Mode optical fiber comprising an optical core having a radius R.sub.1 and a maximal refractive index n.sub.0 and an optical cladding surrounding the optical core, said optical cladding having at its outer edge a refractive index n.sub.Cl, wherein said optical cladding comprises an inner cladding layer directly surrounding said optical core, with an inner radius R.sub.1 and an outer radius R.sub.2 R.sub.1, said inner cladding layer having a constant refractive index n.sub.2=n.sub.Cl, and a region of depressed refractive index n.sub.trench,called a trench, surrounding said inner cladding layer, said trench having an inner radius R.sub.2, with R.sub.2 R.sub.1, and an outer radius R.sub.3, with R.sub.3 >R.sub.2.

8. The method according to claim 7, wherein: $R_{1} = 14 .Math.m 0.5 .Math.m$ $_{1} = \frac{(n_{0}^{2} - n_{Cl}^{2})}{2 n_{0}^{2}} = 0.705 % 0.04 % R_{2} = 15.3 .Math.m 0.5 .Math.m$ $R_{3} = 21.4 .Math.m 0.5 .Math.m$ $n_{3} = n_{trench} - n_{Cl} = - 5 10^{- 3} 0.5 10^{- 3}$ and wherein :
(,) =p00 +p10*+p01*+p11**+p02*.sup.2 with : p00 between 1.947 and 1.604 p10 between 0.001977 and 0.001743 p01 between 1.389 and 1.644 p11 between 0.0009262 and 0.001038, and p02 between 0.3776 and 0.3235.

9. The method according to claim 8, wherein: p00=1.776 p10=0.00186 p01=1.517 p11=0.000982 p02=0.3505.

10. The method according to claim 1, wherein L=550 m and T.sub.REF=40 ps.

11. The method according to claim 1, wherein said optical component comprises: a Variable Optical Attenuator (VOA); an optical fiber; a coupler; or a detector.

12. The method according to claim 1, comprising: collecting light output by said optical component into a second Mode Group Separating optical fiber; and wherein said second Mode Group Separating optical fiber is a multimode fiber with an -profile graded index core with an -value chosen such that said Mode Group Separating optical fiber satisfies the following criterion at the wavelength .sub.t: $\frac{.Math. .Math. .Math. L}{T_{REF}} > 4$ where: is a time delay difference between consecutive mode groups expressed in ps/m; L is a length of said Mode Group Separating optical fiber expressed in m; T.sub.REF is a Full Width Quarter Maximum of said reference pulses expressed in ps.

Description

5. BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention can be better understood with reference to the following description and drawings, given by way of example and not limiting the scope of protection, and in which:

(2) FIGS. 1 to 6 describe the features of a Mode Group Separating optical fiber according to embodiments of the invention, among which:

(3) FIG. 1 illustrates the parameters and T.sub.REF used in the criterion

(4) 0 $\frac{.Math. .Math. .Math. L}{T_{REF}} > X$
for a multimode optical fiber according to an embodiment of the invention, for two consecutive mode groups with normalized pulse power;

(5) FIG. 2 shows an approximation of the time-delay difference as a function of the wavelength t and of the parameter;

(6) FIG. 3 shows an abacus of the criterion

(7) $\frac{.Math. .Math. .Math. L}{T_{REF}} > X$
for a multimode optical fiber according to an embodiment of the invention with a length L=550 m and a FWQM of the reference pulse T.sub.REF=40 ps;

(8) FIG. 4 depicts the refractive index profile of a Few Mode Fiber according to an embodiment of the invention;

(9) FIG. 5 shows the approximation of FIG. 2 for the FMF of FIG. 4;

(10) FIG. 6 shows the abacus of FIG. 3 for the FMF of FIG. 4.

(11) FIGS. 7 to 13 describe several features of an embodiment of the method and system according to the invention using a specific Mode Group Separating optical fiber according to FIGS. 1 to 6, among which:

(12) FIG. 7 shows the DMD profile of a MGS optical fiber according to an embodiment of the invention;

(13) FIG. 8 shows the DMD profile measured thanks to the system of FIG. 13;

(14) FIG. 9 shows the power of the pulses (pulse trains) as a function of the launch position expressed in lam plotted for both DMD measurements of FIGS. 7 and 8;

(15) FIG. 10 shows the Mode Power Distributions of the light entering the VOA of FIG. 13;

(16) FIG. 11 shows the Mode Power Distributions of the light leaving the VOA of FIG. 13;

(17) FIG. 12 shows the loss per Mode Group Number inside the VOA of FIG. 13;

(18) FIG. 13 shows an embodiment of a system used for characterizing the behavior of Mode Groups traveling through an optical component, such as a Variable Optical Attenuator.

(19) The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

6. DETAILED DESCRIPTION

(20) The general principle of the invention relies on the use of a specifically designed Mode Group Separating optical fiber, which is inserted on the optical path between a light source and an optical component under test, and which has the ability to separate the Mode Groups in time domain before or after these are launched into the optical component under test. As a consequence, the behavior of these Mode Groups while passing through the optical component may be studied individually from each other.

(21) Embodiments of the invention thus provide a simple and valuable method for characterizing Mode Groups properties of multimodal light traveling through optical components. The experimental results thus achieved may be used for improving the design of multimode and few-mode optical fibers, as well as the design of optical components.

(22) FIGS. 1 to 6 describe the features of a Mode Group Separating optical fiber according to embodiments of the invention.

(23) As stated previously in this document, in an optical fiber, a certain number of modes can propagate. The lowest-order LP mode is LP.sub.01, also called the fundamental mode. Each mode is associated with a particular propagation constant. In typical multimode fibers, these modes can be grouped together in sets of modes with the same or very similar propagation constant, the principal mode groups. The modes within a certain principal mode group have very similar properties. Therefore, modes are often treated in terms of mode groups instead of individual modes. The number of modes increases with mode group number m: while the lowest-order mode groups only consist of one mode, the higher-order mode groups will contain several modes. In principal modes groups with modes of identical propagation constants, the mode delay will be generally the same for each mode. This is the reason why they are often treated as one mode group with a single mode delay for all modes.

(24) The difference of time of flight between consecutive mode groups is, at first order, function of the -value, the numerical aperture (or delta of the core), the core diameter and the wavelength of operation. At second order, this difference of time of flight between consecutive mode groups is function of the dopant content within the fiber core (depending on whether the core is fully doped with Germanium, fully doped with fluorine or whether it exhibits a germanium and fluorine co-doping).

(25) According to an embodiment of the invention, these parameters are adapted so that the mode group can be separated in time at DMD measurements. This condition can be expressed as follows:

(26) $\frac{.Math. .Math. .Math. L}{T_{REF}} > X$

(27) where is the time delays difference between consecutive mode groups in ps/m, L is the minimum fiber length to be used in the DMD measurements, expressed in m, T.sub.REF is the FWQM (Full Width Quarter Maximum) of the reference pulse used in the DMD measurements, expressed in ps, and X is a threshold that is greater than 4 and more preferably larger than 5.

(28) Throughout this document, the terms DMD measurements designate measurements of the delay due to the modal dispersion, known under the acronym DMD for Dispersion Modal Delay graphical representation. The DMD measurement procedure has been the subject of standardization (IEC 60793-1-49 and FOTP-220, each of which is hereby incorporated by reference in its entirety) and is also specified in Telecommunications Industry Association Document no. TIA-455-220-A, which is hereby incorporated by reference in its entirety. The DMD metric is expressed in units of picoseconds per meter (ps/m) so that the total delay is normalized by fiber length.

(29) A DMD graphical representation is obtained by injecting a light pulse having a given wavelength .sub.0 at the center of the fiber and by measuring the pulse delay after a given fiber length L; the introduction of the light pulse of given wavelength .sub.0 being radially offset to cover the entire core of the multimode fiber.

(30) FIG. 1 illustrates the parameters and T.sub.REF used in the criterion

(31) $\frac{.Math. .Math. .Math. L}{T_{REF}} > X$
for a multimode optical fiber according to the invention, for two consecutive mode groups with normalized pulse power. On FIG. 1, the X-axis corresponds to the time, expressed in ps, and the Y-axis corresponds to the normalized power.

(32) By solving the scalar wave equation (as described in High-Speed Transmission in Multimode Fibers, by Ronald E. Freund et al., Journal of Lightwave Technology, Vol. 28, No. 4, Feb. 15, 2010, which is hereby incorporated by reference in its entirety) through numerical simulation, one can approximate as follows, for a 50 m MMF with a numerical aperture NA=0.200:
(,)=p00+p10*+p01*+p11**+p02*.sup.2 with: p00=1.288 (1.461, 1.116) p10=0.001398 (0.001516, 0.00128) p01=1.189 (1.061, 1.317) p11=0.0007145 (0.0006583, 0.0007706) p02=0.2853 (0.3125, 0.2581),
and where the values between brackets ( ) correspond to the 95% confidence bounds.

(33) Such an approximation may be graphically displayed as shown on FIG. 2, where the X-axis corresponds to the wavelength expressed in nanometers, the Y-axis corresponds to the -value of the MMF fiber, and the Z-axis corresponds to the time delays difference between consecutive mode groups in ps/m. Each black dot on FIG. 2 corresponds to a value calculated from the above model equation for (, ), while the gray-shaded surface corresponds to the surface which may be approximated on the basis of the computed dots.

(34) Making the assumption that the length of MGS optical fiber used is L=550 m, with a FWQM of the reference pulse T.sub.REF=40 ps, one may derive the abacus displayed on FIG. 3, where the X-axis corresponds to the wavelength expressed in nanometers, and where the Y-axis corresponds to the -value of the MMF fiber. The different curves drawn on the abacus of FIG. 3 correspond to the first part

(35) $\frac{.Math. .Math. .Math. L}{T_{REF}}$
of the criterion described above. The number set on each curve indicates the value of

(36) $\frac{.Math. .Math. .Math. L}{T_{REF}},$
ranging from 4 to 10.

(37) As may be observed on FIG. 3, the blank zone corresponding to values roughly comprised between 1.7 and 2.3 is a forbidden zone, in which the criterion set above cannot be fulfilled by the multimode optical fiber. Such a criterion may either by satisfied by choosing values below 1.6 or above 2.7 at a wavelength =850 nm or above 2.3 at a wavelength =1500 nm.

(38) According to other embodiments of the invention, the Mode Group Separating optical fiber may also be a Few Mode Fiber, also known as FMF.

(39) FIG. 4 shows the refractive index profile of such a FMF, which describes the relationship between the refractive index value n and the distance r from the center of the optical fiber. The x-axis represents radial position with x=0 representing the center of the core region, and the y-axis represents refractive index, expressed as an index difference Dn unless otherwise stated.

(40) In the embodiment of FIG. 4, the optical fiber has an optical core having a refractive index profile n(r) defined as follows:

(41) $n (r) = n_{0} .Math. \sqrt{1 - 2 {(\frac{r}{r_{1}})}^{}} for r r_{1}$

(42) where: r is a variable representative of the radius of the optical fiber, r.sub.1 is the optical core radius, is the normalized refractive index difference, with

(43) $= \frac{n_{0}^{2} - n_{1}^{2}}{2 n_{0}^{2}}$ n.sub.1 is the minimal refractive index of the optical core, n.sub.0 is the maximal refractive index of the optical core, is a non-dimensional parameter that defines the index profile shape of the optical core.

(44) The optical core is directly surrounded by an optical cladding, which comprises a depressed-index ring, also called a trench, with inner radius r.sub.2 and outer radius r.sub.3, and an outer cladding layer with inner radius r.sub.3. In some embodiments such an outer cladding layer comprises pure silica glass (SiO.sub.2) and its refractive index n.sub.Cl is hence that of silica glass. This trench has a negative refractive index difference dn.sub.3=n.sub.trenchn.sub.Cl with respect to the refractive index of the outer cladding.

(45) The cladding also includes an inner cladding layer, with inner radius r.sub.1 and outer radius r.sub.2. The trench is hence spaced apart from the core by the inner cladding layer. The inner cladding layer has a constant refractive index n.sub.2, such that n.sub.2=n.sub.Cl.

(46) In an exemplary embodiment of the invention, such a FMF presents the following features:

(47) $R_{1} = 14 .Math.m 0.5 .Math.m$ $_{1} = \frac{(n_{0}^{2} - n_{Cl}^{2})}{2 n_{0}^{2}} = 0.705 % 0.04 % R_{2} = 15.3 .Math.m 0.5 .Math.m$ $R_{3} = 21.4 .Math.m 0.5 .Math.m$ $n_{3} = n_{trench} - n_{Cl} = - 5 10^{- 3} 0.5 10^{- 3}$

(48) Like previously described in relation to FIGS. 2 and 3, by solving the scalar wave equation through numerical simulation, one can approximate as follows, for such a FMF fiber:
(,)=p00+p10*+p01*+p11**+p02*.sup.2 with: p00=1.776 (1.947, 1.604) p10=0.00186 (0.001977, 0.001743) p01=1.517 (1.389, 1.644) p11=0.000982 (0.0009262, 0.001038) p02=0.3505 (0.3776, 0.3235)
and where the values between brackets ( ) correspond to the 95% confidence bounds.

(49) Such an approximation may be graphically displayed as shown on FIG. 5, where the X-axis corresponds to the wavelength expressed in nanometers, the Y-axis corresponds to the -value of the FMF fiber, and the Z-axis corresponds to the time delays difference between consecutive mode groups in ps/m. Each black dot on FIG. 5 corresponds to a value calculated from the above model equation for (, ), while the gray-shaded surface corresponds to the surface which may be approximated on the basis of the computed dots.

(50) Making the assumption that the length of MGS optical fiber used is L=550 m, with a FWQM of the reference pulse T.sub.REF=40 ps, one may derive the abacus displayed on FIG. 6, where the X-axis corresponds to the wavelength expressed in nanometers, and where the Y-axis corresponds to the -value of the FMF fiber. The different curves drawn on the abacus of FIG. 6 correspond to the first part

(51) $\frac{.Math. .Math. .Math. L}{T_{REF}}$
of the criterion described above. The number set on each curve indicates the value of

(52) 0 $\frac{.Math. .Math. .Math. L}{T_{REF}},$
ranging from 4 to 10.

(53) As may be observed on FIG. 6, the blank zone corresponding to values roughly comprised between 1.8 and 2.2 is a forbidden zone, in which the criterion set above cannot be satisfied by the multimode optical fiber. Such a criterion may either by satisfied by choosing values below 1.8 or above around 2.45 at a wavelength =850 nm or above around 2.25 at a wavelength =1500 nm.

(54) FIGS. 7 to 13 describe several features of an embodiment of the method and system according to the invention using a specific Mode Group Separating optical fiber as described above in relation to FIGS. 1 to 6.

(55) In the embodiment described hereafter, the Mode Group Separating (MGS) optical fiber is a multimode fiber with an -profile graded index core with an -value much lower than conventional multimode fibers, which generally show an -value close to 2. Actually, the -value of the MGS optical fiber is close to 1.6. Moreover, such a fiber has a core diameter of 50 m2.5 m and a numerical aperture NA=0.2000.015. As shown on the abacus of FIG. 3, such a MGS fiber thus satisfies the criterion

(56) $\frac{.Math. .Math. .Math. L}{T_{REF}} > 4$
for any wavelength between 850 nm and 1500 nm.

(57) FIG. 7 shows the DMD profile of this MGS optical fiber, of length L=550 m.

(58) A Ti:Sapphire laser of wavelength =850 nm is used as a light source for obtaining the DMD profile of the MGS optical fiber. The laser pulse is launched to the input side of the MGS fiber by means of a 5 m launch fiber (i.e. the launch spot size has a 5 m diameter). The launch fiber's position is changed in steps of 1 or 2 m, starting at the center of the MGS optical fiber core, and ending at the outer radius of the MGS optical fiber core. Such a DMD measurement complies with the specifications in IEC 60793-2-10 and is hence fully standardized.

(59) A digital signal analyzer detects the pulses leaving the MGS optical fiber. The plotted pulses are normalized: the outer pulse, launched at 25 m from the core center has maximum noise because the pulse travels at the edge of the core, and is partly launched in the non-guiding cladding of the MGS fiber. As a consequence, less pulse power reaches the detector.

(60) The plot on FIG. 7 clearly shows that the different Mode Groups output by the MGS optical fiber are located at different time locations: the Mode Groups are hence temporally separated by the MGS fiber. The left side of the plot is set to 0.0 ns.

(61) As may be observed, from the first to the 13th Mode Group, all Mode Groups show a constant distance in time position. From the 14th Mode Group (encircled on FIG. 7), the pattern start being irregular to some level, which may be caused by cladding effects and/or contributions from leaky modes.

(62) However, FIG. 7 clearly shows that, thanks to the specific MGS optical fiber described above, the mode groups can be separated in time at DMD measurements, which allows characterizing the behavior of these Mode Groups while traveling through an optical component, as will be described in greater details in relation to FIGS. 8 to 13.

(63) These figures focus on an embodiment of the invention, in which the Mode Group selective attenuation of a Variable Optical Attenuator is investigated.

(64) Variable Optical Attenuators, also known as VOA, are commonly used for purpose of Bit Error Rate (BER) measurement of an optical fiber. However, it is known that VOAs show a Mode Group selective attenuation. Such a Mode Group selective attenuation of the VOA must be known and taken into account to measure BER of an optical fiber. If not, the conclusions drawn on the fiber quality may be altered and mixed with the VOA characteristics.

(65) FIG. 13 shows an embodiment of the system used for characterizing the behavior of Mode Groups traveling through an optical component, such as a VOA.

(66) Such a system comprises a laser 1, which is a Ti:Sapphire laser at a wavelength =850 nm. A coupler 2 achieves the coupling of the laser bundle output by laser 1 to a launch fiber 3. The launch fiber 3 has a 5 m diameter spot and is associated to a scan unit 4, in order to achieve the launching of pulses into the Mode Group Separating optical fiber 5. The launch position is offset by 1 m or 2 m steps, starting from the core center to the core outer radius of the MGS fiber 5.

(67) An optical component 8 under test is disposed on the optical path between the output of the MGS fiber 5 and a detector 10. Two couplers 6 and 7 allow coupling of light into and out of the optical component 8. As will be described in greater detail below, in an exemplary embodiment of the invention such an optical component is a VOA.

(68) However, it must be noted that, if there is no optical component in box 8, the system of FIG. 13 may allow characterizing the behavior of the Mode Groups traveling through detector 10, and which are input through detector pigtail 9.

(69) FIG. 8 shows the DMD profile measured thanks to the system of FIG. 13, with a VOA inserted in box 8. The light pulses are launched into MGS optical fiber 5, and travel through the VOA 8, before being detected by detector 10.

(70) The resulting DMD profile can be compared with the DMD profile of the MGS optical fiber 5 shown on FIG. 7. The same time scale is used on both FIGS. 7 and 8. Moreover, on the Y-axis, the plots show the normalized pulses at launch position, expressed in m. Hence, the difference in power per Mode Group cannot directly be seen on the DMD profile plots. It may be noted however, that there is an increase of noise for the outer pulses in the DMD profile measured at the output of the VOA, as compared to the DMD profile of FIG. 7 measured for the MGS optical fiber alone.

(71) FIG. 9 offers another way of plotting the results of the DMD measurements with or without the VOA 8: the power of the pulses (pulse trains) is plotted for both DMD measurements as a function of the launch position expressed in m.

(72) Curve 91 shows the pulse power as a function of the launch position for the MGS optical fiber 5. Curve 92 shows the pulse power as a function of the pulse launch position for the DMD light pulses, which have travelled through both the MGS optical fiber 5 and the VOA 8. The shaded area, which appears between curves 91 and 92, is a measure for the Mode Group selective attenuation of the VOA.

(73) According to an embodiment of the invention, the DMD measurement carried out with use of the MGS optical fiber also allows generating the ratio of powers per individual Mode Group, thanks to the fact that the Mode Groups in the DMD profile are temporally apart from each other.

(74) FIG. 10 shows the Mode Power Distributions (MPD) of the light entering the VOA 8. In other words, the power of the light pulses is shown on the Z-axis, as a function of the offset launch expressed in m on the X-axis and of the mode group number on the Y-axis. Such a Mode Power Distribution is measured at the output of the MGS optical fiber 5, when there is no VOA on the optical path in FIG. 13. Such a three-dimensional representation is obtained by using the information on the DMD profile of FIG. 7, though un-normalized and computing the power within each of the of the individual mode groups at each launch position of FIG. 9.

(75) In the same way, FIG. 11 shows the Mode Power Distributions (MPD) of the light leaving the VOA.

(76) The loss (expressed in dB) per Mode Group Number inside the VOA can be computed from the Mode Power Distributions (MPD) of the light entering and leaving the VOA respectively shown on FIGS. 10 and 11, and is illustrated by FIG. 12. The difference in pulse powers between FIGS. 10 and 11, added for the different offset launch positions for each Mode Group, allows computing the loss induced for each Mode Group by the VOA 8.

(77) As may be observed, the loss per Mode Group on average is 6 dB, but higher order Mode Groups are attenuated more. This could be a typical consequence of non-ideal alignment of the optical components, since Mode Groups that travel near the edge of the core are lost more easily.

(78) In an alternative embodiment of the invention, a second Mode Group Separating MMF may be added to the experimental setup of FIG. 13, at the output of the optical component under test 8.

(79) Such a second MGS could help analyze the modes coupling phenomenon, which takes place within the optical component. New modes, induced by mode coupling, could thus be observed.

(80) In yet another embodiment of the invention, the experimental setup of FIG. 13 could be altered by exchanging positions of the optical component 8 and of the MGS fiber 5. The launching fiber 3 and scan unit 4 need only allow for one given launching condition into the optical component 8. The MGS optical fiber 5 hence serves as a tool to separate the Mode Power Distribution induced by the light source 1 into the optical component 8 from a given launching condition.

(81) It must be noted that, for all the embodiments described above, critical parameters are the accuracy of the refractive index profile of the MGS special fiber, the choice of length of this fiber and the positional accuracies of the launch fiber scan unit and the couplers in the system of FIG. 13. The detector characteristics are also crucial when testing an optical component other than the detector itself.

(82) Although the embodiment described above focuses on testing a VOA used within BER equipment, many other embodiments may be considered, such as for example embodiments allowing the qualification of various optical detectors used with SML DMD qualification, which is a most important measurement to qualify OM4 fibers.

(83) New types of optical fibers may also be qualified using the method according to embodiments of the invention, such as Few Mode fibers and fibers that suffer from Microbend.

Method for characterizing mode group properties of multimodal light traveling through optical components

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