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Characterization of Encircled Flux Source For Multimode Fiber Measuremets Jing Zhang*a, Eric Jun Hao Cheungb, Nigel Guohong Terc, Ravi Doddavaramd a National Metrology Centre (NMC), Agency for Science, Technology and Research (A*STAR), 1 Science Park Drive, Singapore 118221 b Engineering Science Programme, Faculty of Engineering, National University of Singapore c School of Electrical and Electronic Engineering, Nanyang Technological University d Psiber Data Pte. Ltd., 3 Science Park Drive, Singapore 118223 *Corresponding author: [email protected] Abstract: The encircled flux (EF) measurement was discussed. The measurement result of an EF compliant source was demonstrated. Insertion loss of a multimode fiber was evaluated with the EF compliant source. The measurement uncertainty was discussed. OCIS codes: (060.2270) Fiber Characterization; (060.2300) Fiber Measurements 1. Introduction With increasing adoption of high speed fiber networks, qualifying the infrastructure becomes imperative before deployment. Be it Ethernet standards or the Fiber Channel standards, the primary performance metric has always been the insertion loss (attenuation) of the channel. The performance specifications are usually defined by standards bodies like IEEE, ISO, TIA etc. But with higher speeds, the requirement has become very stringent and the accuracy/uncertainty of loss measurements plays a vital part in qualifying the channel meaningfully. For instance, the insertion loss requirement for an OM3 Fiber for a 40GBase-SR4 standard is about 1.9 dB/Km. However, measurements of loss and bandwidth in multimode fibers highly depend upon the launching conditions of the light source being used for the measurement. By simply using a different light source the loss measurement for multimode fiber can be very much different. With such tight limits, the launch conditions of the light source play a key role in determining the accuracy of measurement. Encircled flux (EF), defines the integral of power output of the fiber over the radius of the fiber. It is being adopted as a more precise method of defining mode fill for bandwidth simulation and loss testing and has become part of several new multimode fiber testing standards. It sets limits for the amount of optical power included within a specified radius of the fiber core. It is intended to create a more reproducible modal condition for multimode fiber testing. Since EF is new, testing sources for EF are not easily available. EF measurement has yet been demonstrated to be well correlated between labs or manufacturers. G. He et al. presented a near-field scanning system for evaluating conformity to the encircled flux (EF) standard of a measurement source used for MMF loss/attenuation characterization [1]. In this paper, we studied CCD based EF measurement and analyzed the uncertainty in the EF measurement. The insertion loss of multimode fiber was measured by using an EF compliant source. The uncertainty in MMF loss measurement was discussed. 2. EF measurement CCD OD filter We set up a near field imaging system (Fig. 1) to have a magnified image of the near field beam profile. A 20X objective lens and a 6.5X zoom lens were combined in the imaging system to obtain the near field image with enough magnification. A near infrared CCD camera (laser profiler) was used to capture the magnified image. A stage micrometer was used to measure the dimension of the near field beam profile by replacing the multimode fiber in the imaging system at the same setting to obtain the image of the scale in the same field of viewing as the image of near field beam profile. Co-axial illumination was at the same wavelength as the light source under test to avoid the alignment error caused by the chromatic dispersion of the imaging system. Zoom Lens Objective Lens Multimode fiber light source Co-axial illumination Stage micrometer Fig 1. Near field measurement setup 4000 3500 Intensity A.U. Intensity A.U. 3000 Cross-section along x-axis 2500 2000 1500 Cross-section along y-axis 1000 500 Pixels along y-axis Pixels along x-axis 0 -300 -200 -100 0 100 200 300 Position of pixels (a) (b) Fig 2. Near field image of a LED based EF compliant MMF source. (c) A sample of LED based EF compliant 50/125 μm MMF source was measured. Figure 2(a) shows the near field optical profile of the MMF light source. Figure 2(b) is the plot of the near field image with the intensity (absolute unit) of each pixel. The centre of the beam was then determined by our program. The cross-sections at the centre of the beam along a-axis and y-axis are shown in Fig. 2(c). The two cross-sections matched with each other. The length of 300 pixels in the image corresponded to 29.66 µm physical length on the light source. The intensity at the crosssection is a function ( I(r) ) of radius ( r ) away from the optical centre of the core. The EF function is expressed as 𝑟 𝑅 𝐸𝐹(𝑟) = ∫0 𝑥𝐼(𝑥)𝑑𝑥/ ∫0 𝑥𝐼(𝑥)𝑑𝑥 (1) where the integration limit R is defined in IEC-61280-1-4 [2] and TIA-455-203 [3] as equal to 1.15 times of the nominal core radius of the MMF-under-test. Figure 3(a) shows the calculated EF of the sample plotted with EF template specified by IEC 61280-4-1 [4] for 850 nm and 50/125 μm fiber. Repeated measurement results are plotted in Fig. 3(b). 1.2 EF result 1.0 0.8 Normalized EF Normalized EF 1.2 Upper limit 0.6 0.4 Lower limit 0.2 0 5 10 15 20 EF results 1.0 0.8 Upper limit 0.6 0.4 Lower limit 0.2 25 30 0 5 Core radius (µm) 10 15 20 25 30 Core radius (µm) (a) (b) Fig 3. Normalized EF of the LED based EF compliant MMF source. (a) Plot of one time EF result. (b) Plots of EF results on five times of measurements. 3. EF measurement error analysis The encircled flux measurement involves the intensity measurement and dimension measurement. The errors in the two measurements contribute to the EF calculation result. The sources of dimensional uncertainty are listed in table 1 and the combined standard uncertainty is 0.4%. The expanded uncertainty of the dimensional measurement is 0.9%, estimated at a level of confidence of approximately 95 % with a coverage factor k = 2.2. Table 1: Dimensional uncertainty analysis Source of uncertainty Scale standard uncertainty Uncertainty due to alignment of the stage micrometer Uncertainty due to alignment of fiber source Uncertainty due to scale imaging processing Combined standard uncertainty Expanded uncertainty Ui (%) 1.5E-03 1.0E-03 2.0E-03 2.9E-03 4.0E-03 9E-03 Degree of freedom infinity 5 5 5 14 The uncertainty of the intensity measurement is mainly from the linearity of the CCD camera which is ±1%. As EF is the integral of the pixels within a radius, the error of EF due to intensity error is much less than the intensity error at individual pixels. The error in locating the centre of the beam contributed to the error in EF calculation, which was found to be very small in our cases. 4. Application of LED based EF compliant MMF source in MMF measurement We studied the insertion loss measurement of 50/125 μm fibers by using the EF compliant MMF source. Figures 4(a) and 4(b) show the pair of near field beams before and after the transmission in the 1Km long MMF with the EF compliant MMF source, respectively. The insertion loss of MMF is very modal dependant. The insertion loss was measured on a 200m long fiber with SC connectors. The light source and the standard photodetector had FC connectors. FC – SC fiber patch cords (50/125 μm) were used to connect the light source to MMF, and MMF to standard photodetector. We evaluated the reproducibility of the connectors’ linkage. By using the EF compliant MMF source, the insertion loss of the 200m MMF patch cord was measured to be 0.96dB with 0.008dB standard deviation. The sources of uncertainty of the insertion loss measurement are listed in Table 2. The combined standard uncertainty was 0.042dB. The expanded uncertainty of the 200m MMF patch cord insertion loss measurement was 0.12dB, estimated at a level of confidence of approximately 95 % with a coverage factor k = 2.87. By using a normal VCSEL MMF source, the insertion loss of the same set was measured to be 0.84dB with 0.012dB standard deviation. The combined standard uncertainty was 0.048dB. The expanded uncertainty was 0.12dB, estimated at a level of confidence of approximately 95 % with a coverage factor k = 2.43. The measured insertion loss by using normal VCSEL source was 0.12dB smaller than by using EF compliant source. 73µm (a) (b) Fig 4. Near field image before (a) and after (b) transmission in 1 Km multimode fiber at the same magnifications (testing with EF compliant source). Table 2: Insertion loss uncertainty analysis Source of uncertainty Measurement uncertainty of the R.S. detector Light source power stability Reproducibility of SC/SC fiber connectors’ linkage Repeatability of MMF IL Combined standard uncertainty Expanded uncertainty 5. Ui dB 7.50E-03 3.00E-03 4.00E-02 8.00E-03 4.16E-02 1.19E-01 Degree of freedom infinity 4 4 4 4.66E+00 Conclusion The experiment of encircled flux measurement of 50/125 μm multimode fiber light source at 850 nm wavelength was carried out. The error and uncertainty in the EF measurement were discussed. The insertion loss of a 200m MMF patch cord was measured by using an EF compliant source. The insertion loss was 0.96dB with expanded uncertainty of 0.12dB. 6. [1] [2] [3] [4] References: G. He, et al., “Improved near-field scanning system for encircled flux measurement”, Optoelectronics, IET, Vol. 5 No. 1, pp. 46-49 (2009). IEC 61280-1-4 Ed. 2.0: Fibre-optic communication subsystem test procedures – Part 1-4: General communication subsystems – light source encircled flux measurement method. TIA-455-203-A: Light source encircled flux method. IEC 61280-4-1 Ed. 2.0: Fibre-optic communication subsystem test procedures – Part 4-1: Installed cable plant – multimode attenuation measurement.