Download Characterization of Encircled Flux Source For Multimode

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.