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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 11 — May. 23, 2011
  • pp: 10613–10618
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Bandwidth measurement of multimode fibers using system level bit error rate testing

Xin Chen, Jason Hurley, and Ming-Jun Li  »View Author Affiliations


Optics Express, Vol. 19, Issue 11, pp. 10613-10618 (2011)
http://dx.doi.org/10.1364/OE.19.010613


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Abstract

We propose and demonstrate a novel bandwidth measurement method for multimode fibers through measuring the bit error rate and power penalty associated with the testing system. The relationship between system performance and bandwidth limitation is established through the use of well characterized electric filters. With the calibration information, bandwidths of actual fibers were measured. The results were compared with those from other methods. The benefit of the BER based bandwidth measurement method is discussed.

© 2011 OSA

1. Introduction

Multimode fibers (MMFs) have seen increased interest and use in recent years in short distance communications, such as in data centers, and enterprise networks. A number of standards cover the area, which include 10G Ethernet standards, Fiber Channel, 40-100Gb Ethernet and Infiniband. The grade of multimode fiber is mainly determined by the bandwidth. There exist many methods to measure the bandwidth of multimode fiber. One type of measurement is the time domain method based on differential mode delay (DMD) measurement as described in FOTP-220 standard. The DMD measurement data can be converted into fiber bandwidth following standard procedures. Another type of the measurement method is the frequency domain method [1

1. S. Yang and R. L. Gallawa, “Fiber bandwidth measurement using pulse spectrum analysis,” Appl. Opt. 25(7), 1069–1071 (1986). [CrossRef] [PubMed]

,2

2. M. Sauer, A. Kobyakov, and J. George, “Radio over fiber for picocellular network architectures,” J. Lightwave Technol. 25(11), 3301–3320 (2007). [CrossRef]

].

2. Experimental calibration

As shown in Fig. 1
Fig. 1 shows the schematic layout of the experimental setup that we used to obtain the calibration information.
, a 10G transmitter (Tx) with an 850nm VCSEL is driven by a pattern generator. The optical signal passes through a short multimode fiber and a variable optical attenuator (VOA) before reaching the receiver. The multimode fiber in this setup is short with length of around 1m and is considered as a back to back condition in contrast to the situation where a longer fiber is used in application length. The VOA is used to control the optical power to the receiver so that we can generate a BER performance curve. The receiver is a linear photo-detector connected with a linear transimpedance amplifier (TIA). In choosing the receiver, care is taken so that the receiver will have a linear response to the signal and no signal conditioning circuit including data recovery and electronic dispersion compensation chip is used. A number of electric filters with Bessel-Thompson 4th spectrum shape (obtained from Picosecond Pulse Labs) and varying cutoff frequency are inserted between the transmitter and the pattern generator. Note that in order for the bandwidth limited signal to be faithfully converted into optical signal by the transmitter, the transmitter has to be analog in nature, which means there is no regulating circuit to distort the electrical signals that is used to drive the transmitter. A BER performance curve is obtained for each electric filter, which emulates a multimode fiber with a particular bandwidth. Since the filter has a slight attenuation and the spectrum limitation alters the signal shape, the peak to peak magnitude of the signal reaching the Tx is different slightly for each filter. To ensure the consistency between measurements, we adjust the peak to peak driving voltage of the pattern generator at the output of each electrical filter to be the same. The same adjustment is also performed when we take out the electric filter and replace the short fiber with a longer fiber under test in actual measurements.

The BER performance curves for a number of electrical filter cutoff frequencies are shown in Fig. 2
Fig. 2 BER versus received power curves for different filter frequencies.
. The power penalty is determined from the increased receiver power, relative to the BER performance curve in back to back condition, for BER equal to 10−9. The power penalty as a function of the filter cutoff frequency is shown in Fig. 3
Fig. 3 The Power penalty as a function of the filter cutoff frequency at 10−9 BER level.
. The electrical filter frequency is specified at the electrical 3dB level using 20log10()operator. However, in the optical domain, the bandwidth is defined at the optical 3dB level using 10log10(). As a result 3 dB in the optical domain is equivalent to the electrical 6dB level. For a Bessel-Thompson 4th order filter, the electrical 6dB point is greater than the electrical 3dB point by a factor of 1.3654. With an added axis, Fig. 3 also illustrates the relation between the power penalty and the optical bandwidth. The experimental data of power penalty and filter cutoff frequency is fitted using a functional form of P=axb. The best-fit values for ‘a’ and ‘b’ are found to be 30.37 and −1.66 respectively.

3. Bandwidth measurement for actual fibers

With the calibration curve, we were able to test the bandwidth measurement method on real fibers. The experimental setup used for actual fiber testing as shown in Fig. 4
Fig. 4 Experimental Setup used for actual fiber bandwidth testing.
is similar to that in Fig. 1 but with some changes. The electrical filter connected to the transmitter is removed. The short jumper fiber is replaced by actual fiber under test.

In another experiment, we measured a few cabled multimode fibers with higher bandwidth and compared the results with those obtained from the frequency domain method [1

1. S. Yang and R. L. Gallawa, “Fiber bandwidth measurement using pulse spectrum analysis,” Appl. Opt. 25(7), 1069–1071 (1986). [CrossRef] [PubMed]

,2

2. M. Sauer, A. Kobyakov, and J. George, “Radio over fiber for picocellular network architectures,” J. Lightwave Technol. 25(11), 3301–3320 (2007). [CrossRef]

]. The results of frequency domain method were obtained using a network analyzer which includes a frequency sweeping source to modulate an 850nm VCSEL and an analyzer that processes the signals from the photo-detector. The transmitter is the same analog transmitter used in Fig. 1. The measurement results are shown in Table 1

Table 1. The bandwidth measurement results from three cabled fibers.

table-icon
View This Table
. The data from the frequency domain method is processed in two different ways by either retrieving the optical 3dB bandwidth from measured spectrum data or retrieving from the 3dB of the best Gaussian fit of the measured data. It can be found that the results from different methods agree reasonably well, but in some cases (for example, Fiber #2) the difference is significant. In general, the bandwidth obtained from BER method can be above or below those from the frequency domain method. We believe the difference is largely due to the deviation of the measured frequency response from ideal Gaussian shape. Note that although the Gaussian model is widely used in the MMF field, it may deviate from the actual fibers frequency response more or less. When such deviation occurs, the optical 3dB bandwidth is a less accurate measure of the bandwidth and becomes less well tracked with the system performance. The BER bandwidth for Fiber #2 is substantially lower than the 3dB bandwidth. This is understandable, as shown in Fig. 6
Fig. 6 The frequency response of Fiber #2.
, in most parts of the frequency response, the frequency response curve falls below the ideal Gaussian shape, which is causing more severe bandwidth limitation than an ideal Gaussian type spectrum and with the same optical 3dB bandwidth. We believe the BER testing provide a more accurate measure as it has taken into account the contribution for each portion of the spectrum response. The obtained bandwidth matches that from an ideal Gaussian fiber that yields the same system performance. One may consider the bandwidth obtained from the BER method to be a new bandwidth slightly different from the optical 3dB bandwidth. The benefit is that the bandwidth reported from the BER testing is truly ranked with the system performance.

4. Conclusion

References and links

1.

S. Yang and R. L. Gallawa, “Fiber bandwidth measurement using pulse spectrum analysis,” Appl. Opt. 25(7), 1069–1071 (1986). [CrossRef] [PubMed]

2.

M. Sauer, A. Kobyakov, and J. George, “Radio over fiber for picocellular network architectures,” J. Lightwave Technol. 25(11), 3301–3320 (2007). [CrossRef]

3.

J. B. Schlager, M. J. Hackert, P. Pepeljugoski, and J. Gwinn, “Measurements for enhanced bandwidth performance over 62.5-mm multimode fiber in short-wavelength local area networks,” J. Lightwave Technol. 21(5), 1276–1285 (2003). [CrossRef]

4.

S. Bottacchi, Multi-Gigabit Transmission Over Multimode Optical Fibre: Theory and Design Methods for 10 GbE Systems (Wiley, 2006).

5.

A. Gholami, D. Molin, and P. Sillard, “Compensation of chromatic dispersion by modal dispersion in MMF- and VCSEL- based gigabit ethernet transmissions,” IEEE Photon. Technol. Lett. 21(10), 645–647 (2009). [CrossRef]

OCIS Codes
(060.2270) Fiber optics and optical communications : Fiber characterization
(060.2300) Fiber optics and optical communications : Fiber measurements

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: April 21, 2011
Revised Manuscript: May 9, 2011
Manuscript Accepted: May 12, 2011
Published: May 13, 2011

Citation
Xin Chen, Jason Hurley, and Ming-Jun Li, "Bandwidth measurement of multimode fibers using system level bit error rate testing," Opt. Express 19, 10613-10618 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-11-10613


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