Influence of Specimen Length and Launch Conditions on Multimode Fibre Attenuation Measurement by Cut

 Liyan Zhang, Changfeng Yan, Xinben Zhang, Xufeng Yin, Rui Zhang, Runhan Wang, Jing Li

1 State Key Laboratory of Optical Fiber and Cable Manufacture Technology.

2 Yangtze Optical Fibre and Cable Joint Stock Limited Company. No.9 Optics Valley Avenue, Wuhan, Hubei, China 430073

Author e-mail address:zhangliyan@yofc.com

Abstract: Influence of specimen length and launch conditions on multimode fibre attenuation measurement using cut-back method is studied. The oscillation phenomenon of attenuation spectrum is found in short length fibre and the reason is analyzed.

OCIS codes: (060.2300) Fiber measurements; (060.0060) Fiber optics and optical communications

1. Introduction

For the attenuation test of multimode optical fibre and cable, there are several standards which have established the test requirements [1-3]. For all these standards, cut-back method (or insertion loss method, which in principle, is similar to the cut-back method) is the reference test method (RTM). However, the launch condition requirements are different among these standards, where IEC 60793-1-40 specifies a 70/70 launch[1] while other standards require Encircled Flux(EF) launch[2, 3].The specimen length of multimode fibre and installed cable are varied from 100m to several kilometers. The varied specimen length and launch mode would influence the attenuation result, which the influence however hasn’t systematically been reported before. In this paper, we studied the attenuation measurement under three launch conditions, 70/70 launch, EF launch and overfilled launch (OFL), combined with different specimen length(from 100m to 4.4km). Both traditional graded index 50μm multimode (GI50) fibre and bend-insensitive multimode (BIMM) fibre are involved.

2.  Experimental Setup and Procedure

Figure 1 schematically shows the experimental setup for multimode fibre attenuation test.

Fig. 1. Schematic of the experimental setup for multimode fibre attenuation test

For 70/70 launch condition, limited phase space launch is used to uniformly fill 70% of the test fibre numerical aperture (NA) and 70% of the test fibre core diameter. For overfilled launch, the 70%NA control and 70% launch spot control are removed from the test setup. For EF launch, a mode controller has been used before the fibre sample to give a standard EF mode distribution [3]. The cut-back length is 2 meters as defined in the standard [1].

The specimen length of each conditions are 100m, 150m, 300m, 550m, 1.1km, 2.2km and 4.4km respectively. All of these fibres are coming from the same fibre preform. The attenuation test under each condition has been repeated for at least five times and the average value is reported in this paper. “Attenuation” in this paper represents the meaning of attenuation coefficient (dB/km).

3.  Experimental Results

Figure 2 shows the measured attenuation as a function of specimen length at different launch conditions. It can be seen that, for both GI50 and BIMM fibre, as the specimen length decreases, the attenuation at 850nm increases, especially when the specimen length is less than 1km. The existence of more higher order modes in short fibres results in higher attenuation. As the specimen length gets longer, higher order modes are progressively decayed.

For the same specimen length, different launch conditions result in different attenuation data. The attenuation value under OFL condition is larger than the value under the other two conditions. The reason is that OFL condition will excite all the modes including higher order modes, while EF and 70/70 launch, which are underfilled launch, will excite less higher order modes. For GI50 fibre shown in Fig.2 (a), attenuation under EF launch condition is larger than 70/70 launch condition, while for BIMM fibre shown in Fig.2 (b), attenuation under EF launch condition is smaller than 70/70 launch condition. This may be caused by the different profile design of the two fibres.

Fig. 2 850nm attenuation of three launch conditions as a function specimen length (a) GI50 fibre (b) BIMM fibre

Fig. 3 Attenuation spectrum of different specimen length under different launch conditions, GI50 fibre: (a) 70/70 launch, (b) OFL, (c) EF; BIMM fibre: (d) 70/70 launch, (e) OFL, (f) EF.

Figure 3 shows the attenuation spectrum of different specimen length at different launch conditions for GI50 fibre and BIMM fibre. Oscillation is found in the attenuation spectrum and the amplitude decreased as the specimen length increased. Under each launch condition, we can see the wave peaks and valleys appear at the same wavelength for different fibre length.

For both GI50 fibre and BIMM fibre, OFL condition has the largest oscillation amplitude. However, for GI50 fibre, 70/70 launch condition has the smallest oscillation amplitude while for BIMM fibre, EF launch condition has the smallest oscillation amplitude. The difference may come from the different profile design between GI50 and BIMM fibre.

The principal mode numbers of 850nm and 1300nm have been commuted for GI50fibre [4], see Figure 4. There are 18 and 11effective guide mode groups at 850nm and 1300nm, respectively. There are 7 oscillation waves between 850nm and 1300nm in Figure 3, coinciding with the difference of principal mode numbers between the two wavelengths. The assumption is that oscillation phenomena are caused by the successive mode group cutoff as wavelength increased. However, if we increase the fiber length, the oscillation phenomena can be averaged, and the attenuation spectrum will be smoother.

Fig. 4 Mode power distribution of GI50 fibre: (a) 850nm, (b) 1300nm.

4.  Conclusion

According to the test result we can come to conclusion that, different launch conditions will result in different attenuation for multimode fibre attenuation measurement using cut-back method, especially when the specimen length is shorter than 1.1km. Therefore when testing GI50 or BIMM fibre attenuation using cut-back method, the launch condition should be noticed and a longer specimen length (more than 1.1km) is suggested.

5.  References

[1] IEC 60793-1-40 Optical Fibers-Part 1-40: Measurement Methods and Test Procedures-Attenuation (2001).

[2] TIA-526-14-C Optical Power Loss Measurements of Installed Multimode Fiber Cable Plant; Modification of IEC 61280-4-1 Edition 2, Fiber-Optic Communications Subsystem Test Procedures- Part 4-1: Installed Cable Plant-Multimode Attenuation Measurement (2015).

[3] IEC 61280-4-1 Edition 2.0, Fibre-Optic Communications Subsystem Test Procedure- Part 4-1: Installed Cable Plant- Multimode Attenuation Measurement (2009).

[4] Massimo Olivero, Guido Perrone, and Alberto Vallan, “Near-Field Measurements and Mode Power Distribution of Multimode Optical Fibers,” IEEE Transactions on Instrumentation and Measurement, vol. 59, no. 5, 1382-1388 (2010).