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Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 14, Iss. 20 — Oct. 2, 2006
  • pp: 9016–9021
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Determination of the differential mode delay of a multimode fiber using Fourier-domain intermodal interference analysis

J.Y. Lee and D.Y. Kim  »View Author Affiliations


Optics Express, Vol. 14, Issue 20, pp. 9016-9021 (2006)
http://dx.doi.org/10.1364/OE.14.009016


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Abstract

We present a novel differential mode delay (DMD) measurement method for a multimode optical fiber (MMF) based on Fourier-domain low-coherence interferometry (fLCI) for the first time. An optical spectrum analyzer and a mode scrambler are used to obtain the intermodal interference signal in spectral domain after light passes an MMF. The MMF is used as a common path interferometer by itself without using any reference arm in an interferometer. Instead of using an offset launching technique, we present an effective method to obtain all available intermodal interference signals at a single shot by using a mode scrambler. The measured results using the proposed method are compared with those obtained using a conventional time-domain DMD measurement method.

© 2006 Optical Society of America

1. Introduction

There has been much effort to develop a next-generation multimode optical fiber (MMF) for local area network transmission systems as a physical layer for 10-gigabit data transmission links. The differential mode delay (DMD) of an MMF is a fundamental parameter which determines the bandwidth of an MMF [1

1. P. Pepeljugoski, M. J. Hackert, J. S. Abbott, S. E. Swanson, S. E. Golowich, A. J. Ritger, P. Kolesar, Y.C . Chen, and P. Pleunis, “Development of system specification for laser-optimized 50-µm multimode fiber for multigigabit short-wavelength LANs,” J. Lightwave Technol. 20, 1256–1275 (2003). [CrossRef]

]. There have been many reports on the concept and the measurement methods of DMD [2

2. S. E. Mechels, J. B. Schlager, and D.L Franzen, “High-resolution differential-mode delay measurements in optical fibers using a frequency-domain phase-shift technique,” IEEE Photon. Technol. Lett. 9, 794–796, (1997). [CrossRef]

]. The manufacturers of an MMF consider a DMD measurement method to be an important diagnostic tool to optimize the design and the fabrication process of an MMF. This is because DMD is considered to be a standard specification parameter for an MMF to guarantee robust operation over time and a certain bandwidth regardless of a launching condition or a laser property [3

3. P. F. Kolesar and D. J. Mazzarese, “Understanding Multimode Bandwidth and Differential Mode Delay Measurements and Their Applications,” in Proc. of the 51st International Wire and Cable Symposium of IWCS Inc., Lake Buena Vista, FL, 453–460, (2002).

].

Several measurement methods have been developed to determine the DMD of an MMF [4

4. TIA-455-220-A, Differential Mode Delay Measurement of Multimode Fiber in the Time Domain, Telecommunication Industry Association (2003).

7

7. Y. Painchaud, M. A. Duguay, and F. Ouellette, “Interferometric time measurements of intermodal dispersion in optical fibers by using CCD photodetector array,” Opt. Lett. 17, 1423–1425 (1992). [CrossRef] [PubMed]

]. In particular, the time-domain DMD measurement method is the industry standard. In the conventional time-domain DMD measurement method [4

4. TIA-455-220-A, Differential Mode Delay Measurement of Multimode Fiber in the Time Domain, Telecommunication Industry Association (2003).

], a short optical pulse is launched at one end of a test fiber. As each mode has a different propagation speed in an MMF, all modes of the sample MMF are selectively excited with an offset launching method, and the differences in propagation speed are measured. Depending on the different mode propagation speeds, the optical pulse experiences pulse broadening, or spreading. This broadening is measured using a fast detection system. The conventional time-domain DMD measurement system requires complicated and expensive instruments, such as a pulse laser and a fast detection system with a sampling oscilloscope, to measure the broadening of the short optical pulse. Although the recently proposed optical frequency-domain DMD measurement technique based on optical frequency-domain reflectometry (OFDR) improves measurement resolution, this method also uses an expensive tunable laser source (TLS) and a complicated subsidiary interferometer to suppress noise associated with nonlinear frequency sweep in an optical source [5

5. T. -J. Ahn, S. Moon, Y. Youk, Y. Jung, K. Oh, and D. Y. Kim, “New optical frequency domain differential mode delay measurement method for a multimode optical fiber,” Opt. Express 13, 4005–4011, (2005). [CrossRef] [PubMed]

, 6

6. T. -J. Ahn and D. Y. Kim, “High resolution differential mode delay measurement for a multimode optical fiber using a modified optical frequency domain reflectometer,” Opt. Express 13, 8256–8262, (2005). [CrossRef] [PubMed]

].

2. Experiment and results

Figure 1 shows a schematic diagram of our experimental arrangement used for the DMD measurements of an MMF. It is based on a common path intermodal interferometer. An amplified spontaneous emission (ASE) light source made of an Erbium doped fiber was used as an optical source, and an optical spectrum analyzer (OSA, Ando 6324B) was used for optical signal detection. The ASE had a center wavelength of 1530 nm. An OSA placed at the output of the test optical fiber measures a spectral interferogram, which contains the intermodal delay information of an MMF. A 15 m long MMF (InfiniCor SX+50/125, Corning Inc.) with a core diameter of 50 µm was prepared as a test sample. As we need to measure the temporal delay between the fastest and the slowest modes in a sample MMF, it is quite important to excite all available modes in an MMF. Conventional DMD measurement methods use the scanning offset launching method to selectively excite all available modes in an MMF [4

4. TIA-455-220-A, Differential Mode Delay Measurement of Multimode Fiber in the Time Domain, Telecommunication Industry Association (2003).

]. However, this offset launching method takes a long time to selectively excite individual mode group in an MMF. To overcome this problem, we used two mode scramblers (Newport FM-1) at the beginning and the end of a sample fiber. The first mode scrambler is to couple light from a single mode fiber into all the available modes in an MMF. On the other hand, the second one at the end of the sample MMF is to make lights from all of the guided modes in the MMF which are to be coupled back into a single mode fiber connected to the OSA. This generates spectral intermodal interference signals on the OSA between all of the guided modes in the MMF. A short length of an MMF is sandwiched between two corrugated metal surfaces with a gentle pressure controlled with a precision translation stage in a mode scrambler. Microbending in an MMF generated by the device helps make sufficient mode coupling among guided modes in the fiber such that the distribution of power among the guided modes becomes independent of the launch conditions of light. The wavelength span in the OSA was only about 10 nm from 1545 nm to 1555 nm in order to reduce the chromatic dispersion effect in the measurement.

Fig. 1. Experimental setup of our DMD measurement method for a multimode optical fiber.

The measured spectral interferograms of the test fiber for several different corrugation pitch sizes of the mode scrambler are shown in Fig. 2(a). There are various frequency components in the measured spectral interferograms, and each frequency component corresponds to a relative group delay between two modes in the test fiber. The corrugation pitch of the mode scrambler is changed from 12.5 µm to 75.0 µm. The pressures of the first and the second mode scramblers were set equal and kept constant for the entire measurements.

Fig. 2. DMD measurement results for several different corrugation pitch sizes of a mode scrambler: (a) measured spectral interferograms by an OSA, (b) calculated relative time delay graphs obtained from spectral interferograms in Fig. 2(a) by Fourier transformation, and (c) calculated relative time delay graphs in log scale (10dB/div.)

We also measured the DMD of the same optical fiber by using a time-domain impulse response method to confirm our measured results [4

4. TIA-455-220-A, Differential Mode Delay Measurement of Multimode Fiber in the Time Domain, Telecommunication Industry Association (2003).

]. A gain-switched semiconductor laser (OPG-1500, Optune Inc.) was used as the input pulse source. It has a 28 ps pulse width at 1550 nm wavelength, and its repetition rate is 50 MHz. A sampling oscilloscope was used for short optical pulse width detection (86100A, Agilent Inc.). The length of the test fiber was increased to 450 m for this measurement due to the measurement resolution of the time-domain technique. The measured results are shown in Fig. 3. Meanwhile, Fig. 3(a) shows the measured DMD profile using the conventional scanning offset launching method. In this method, a single mode probe fiber was scanned from one end of the core to the other end with an offset value of 1 µm in order to excite all the modes from the lowest mode group to the highest order mode group. The total mode group was divided into eight mode groups according to the mode excitation condition. It shows that the temporal delay between the leading peak and the trailing peak or the DMD of the fiber is about 1.45 ps/m.

Fig. 3. Modal delay measurements of an MMF from the time-domain method using a scanning offset launching method (a) and a mode scrambler method (b).

Our measurement results agree well with the time-domain method within 4.6% error. Fig. 3(b) is the measured DMD profile of the sample fiber with a mode scrambler. One mode scrambler to excite the mode was placed at the beginning of the multimode fiber. It shows that the mode scrambler could excite all mode groups in the MMF. The corrugation pitch of the mode scrambler is approximately set to be 75 µm.

3. Discussion

We have shown that our proposed measurement method based on Fourier-domain low coherence interferometry can determine the DMD of an MMF very effectively. We used a mode scrambler to excite all available modes in a sample MMF at the beginning of the MMF. After all the mode groups have traveled through the MMF, another mode scrambler is used at the end of the test fiber to mix all of the mode groups to generate interference signals associated with path length differences between modes. Our proposed configuration is a form of common path interferometer, measuring relative intermodal delays within the test fiber without the need for a reference arm. This common path optical interferometer enhances the stability of an interferometric signal by decreasing the effect of external perturbations, such as air fluctuation and vibration, as compared to a conventional two-arm interferometer.

In our method, it is important to determine the appropriate condition of the mode scrambler to excite all available modes at the beginning of a sample fiber and to mix the modes at the end of the fiber. We have used a commercial mode scrambler where a periodic lateral pressure is applied to a sample fiber by using a pair of corrugated metal plates. Because the mode scrambler perturbs the optical fiber laterally, considerable microbending on a fiber above an appropriate level causes higher-order modes to couple to radiation modes, which in turn causes excessive loss in a fiber. When the pressure in the mode scrambler is too low below the appropriate level, mode mixing efficiency is not enough to excite all available modes in a fiber. Therefore, inappropriate conditions for the mode scrambler decrease the accuracy of DMD measurement. In our case, all modes were fully excited when the pitch of the corrugated metal plate was between 75 µm and 87.5 µm. We stripped the coating off the MMF near the position of the mode scrambler and then immersed it in index-matching gel to eliminate the cladding modes of the MMF and prevent unwanted coupling with the core mode. Although in principle, spectral fringe visibility can be degraded by polarization state mismatch between mode groups, we have experimentally observed that it does not influence measurement results; no changes in peaks in Fig. 2(b) were observed when the input polarization state to the MMF was changed.

Our proposed measurement method has advantages over a conventional time-domain measurement method or an optical frequency-domain OFDR method [4

4. TIA-455-220-A, Differential Mode Delay Measurement of Multimode Fiber in the Time Domain, Telecommunication Industry Association (2003).

, 5

5. T. -J. Ahn, S. Moon, Y. Youk, Y. Jung, K. Oh, and D. Y. Kim, “New optical frequency domain differential mode delay measurement method for a multimode optical fiber,” Opt. Express 13, 4005–4011, (2005). [CrossRef] [PubMed]

]. First, our proposed technique is very simple in principle and cost effective because there is no special optical element or auxiliary instrument except for a mode scrambler. The multimode optical fiber plays an important role as an interferometer by itself. Although we used an OSA for detection in this experiment, it is easily exchanged with a spectrometer. In the conventional time-domain DMD method, an expensive optical short pulse source and a fast detection system, such as a sampling oscilloscope, are needed. In addition, it requires a complicated offset launching system to excite all mode groups. As it detects the broadening of an optical pulse in the time domain, the sensitivity or the signal-to-noise ratio of measurement is quite low; if the DMD of a sample fiber is too large, or if the sample length is too long, then the pulse may spread too much in the time domain to be detected as a pulse. The optical frequency-domain DMD measurement method based on the OFDR technique shows a good sensitivity and resolution as it is basically a self-heterodyne method. However, it also requires an expensive optical source, such as a TLS, and requires a complicated auxiliary interferometer to reduce the nonlinearity of the sweeping laser source [5

5. T. -J. Ahn, S. Moon, Y. Youk, Y. Jung, K. Oh, and D. Y. Kim, “New optical frequency domain differential mode delay measurement method for a multimode optical fiber,” Opt. Express 13, 4005–4011, (2005). [CrossRef] [PubMed]

, 6

6. T. -J. Ahn and D. Y. Kim, “High resolution differential mode delay measurement for a multimode optical fiber using a modified optical frequency domain reflectometer,” Opt. Express 13, 8256–8262, (2005). [CrossRef] [PubMed]

].

4. Conclusions

Acknowledgment

This work was supported by Creative Research Initiatives (3D Nano Imaging System Group) of MOST/KOSEF.

References and links

1.

P. Pepeljugoski, M. J. Hackert, J. S. Abbott, S. E. Swanson, S. E. Golowich, A. J. Ritger, P. Kolesar, Y.C . Chen, and P. Pleunis, “Development of system specification for laser-optimized 50-µm multimode fiber for multigigabit short-wavelength LANs,” J. Lightwave Technol. 20, 1256–1275 (2003). [CrossRef]

2.

S. E. Mechels, J. B. Schlager, and D.L Franzen, “High-resolution differential-mode delay measurements in optical fibers using a frequency-domain phase-shift technique,” IEEE Photon. Technol. Lett. 9, 794–796, (1997). [CrossRef]

3.

P. F. Kolesar and D. J. Mazzarese, “Understanding Multimode Bandwidth and Differential Mode Delay Measurements and Their Applications,” in Proc. of the 51st International Wire and Cable Symposium of IWCS Inc., Lake Buena Vista, FL, 453–460, (2002).

4.

TIA-455-220-A, Differential Mode Delay Measurement of Multimode Fiber in the Time Domain, Telecommunication Industry Association (2003).

5.

T. -J. Ahn, S. Moon, Y. Youk, Y. Jung, K. Oh, and D. Y. Kim, “New optical frequency domain differential mode delay measurement method for a multimode optical fiber,” Opt. Express 13, 4005–4011, (2005). [CrossRef] [PubMed]

6.

T. -J. Ahn and D. Y. Kim, “High resolution differential mode delay measurement for a multimode optical fiber using a modified optical frequency domain reflectometer,” Opt. Express 13, 8256–8262, (2005). [CrossRef] [PubMed]

7.

Y. Painchaud, M. A. Duguay, and F. Ouellette, “Interferometric time measurements of intermodal dispersion in optical fibers by using CCD photodetector array,” Opt. Lett. 17, 1423–1425 (1992). [CrossRef] [PubMed]

8.

R. Posey, L. Phillips, D. Diggs, and A. Sharma, “LP01–LP02 interference using a spectrally extended light source: measurement of the non-step-refractive-index profile of optical fibers,” Opt. Lett. 21, 1357–1539 (1996). [CrossRef] [PubMed]

9.

Dan Ostling, Bjornar Langli, and Helge E. Engan, “Intermodal beat lengths in birefringent two-mode fibers,” Opt. Lett. 21, 1553–1555 (1996). [CrossRef] [PubMed]

10.

D. Káčik, I. Turek, I. Martinček, J. Canning, N. Issa, and K. Lyytikäinen, “Intermodal interference in a photonic crystal fibre,” Opt. Express 12, 3465–3470 (2004). [CrossRef] [PubMed]

11.

Adam Wax, Changhuei Yang, and Joseph A. Izatt, “Fourier-domain low-coherence interferometry for light-scattering spectroscopy,” Opt. Lett. 28, 1230–1232, (2003). [CrossRef] [PubMed]

12.

R. A. Leitgeb, W. Drexler, A. Unterhuber, B. Hermann, T. Bajraszewski, T. Le, A. Stingl, and A. F. Fercher, “Ultrahigh resolution Fourier domain optical coherence tomography,” Opt. Express 12, 2156–2165, (2004). [CrossRef] [PubMed]

13.

J.Y. Lee, T-J. Ahn, S. Moon, Y. Jung, K. Oh, and D.Y. Kim. “Differential mode delay analysis for a multimode optical fiber with Fourier-domain low-coherence interferometry,” in Optical Fiber and Communication, Technical Digest (Optical Society of America, 2006) OWI18.

14.

C. Dorrer, D. Belabas, J. P. Likforman, and M. Joffre, “Spectral resolution and sampling issues in Fourier-transform spectral interferometry,” J. Opt. Soc. Am. B 17, 1795–1802, (2000). [CrossRef]

OCIS Codes
(060.2300) Fiber optics and optical communications : Fiber measurements
(120.3180) Instrumentation, measurement, and metrology : Interferometry

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: June 16, 2006
Revised Manuscript: August 26, 2006
Manuscript Accepted: September 13, 2006
Published: October 2, 2006

Citation
J. Y. Lee and D. Y. Kim, "Determination of the differential mode delay of a multimode fiber using Fourierdomain intermodal interference analysis," Opt. Express 14, 9016-9021 (2006)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-14-20-9016


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References

  1. P. Pepeljugoski, M. J. Hackert, J. S. Abbott, S. E. Swanson, S. E. Golowich, A. J. Ritger, P. Kolesar, Y.C. Chen, and P. Pleunis, "Development of system specification for laser-optimized 50-μm multimode fiber for multigigabit short-wavelength LANs," J. Lightwave Technol. 20, 1256-1275 (2003). [CrossRef]
  2. S. E. Mechels, J. B. Schlager, and Franzen, D.L , "High-resolution differential-mode delay measurements in optical fibers using a frequency-domain phase-shift technique," IEEE Photon. Technol. Lett. 9,794 -796, (1997). [CrossRef]
  3. P. F. Kolesar and D. J. Mazzarese, "Understanding Multimode Bandwidth and Differential Mode Delay Measurements and Their Applications," in Proc. of the 51st International Wire and Cable Symposium of IWCS Inc., Lake Buena Vista, FL, 453-460, (2002).
  4. <other>. TIA-455-220-A, Differential Mode Delay Measurement of Multimode Fiber in the Time Domain, Telecommunication Industry Association (2003).</other>
  5. T. -J. Ahn, S. Moon, Y. Youk, Y. Jung, K. Oh, and D. Y. Kim, "New optical frequency domain differential mode delay measurement method for a multimode optical fiber," Opt. Express 13,4005-4011, (2005). [CrossRef] [PubMed]
  6. T. -J. Ahn and D. Y. Kim, "High resolution differential mode delay measurement for a multimode optical fiber using a modified optical frequency domain reflectometer," Opt. Express 13,8256-8262, (2005). [CrossRef] [PubMed]
  7. Y. Painchaud, M. A. Duguay, and F. Ouellette, "Interferometric time measurements of intermodal dispersion in optical fibers by using CCD photodetector array," Opt. Lett. 17,1423-1425 (1992). [CrossRef] [PubMed]
  8. R. Posey, L. Phillips, D. Diggs, and A. Sharma, "LP01-LP02 interference using a spectrally extended light source: measurement of the non-step-refractive-index profile of optical fibers," Opt. Lett. 21,1357-1539 (1996). [CrossRef] [PubMed]
  9. Dan Ostling, Bjornar Langli, and Helge E. Engan, "Intermodal beat lengths in birefringent two-mode fibers," Opt. Lett. 21,1553-1555 (1996). [CrossRef] [PubMed]
  10. D. Káèik, I. Turek, I. Martinèek, J. Canning, N. Issa, and K. Lyytikäinen, "Intermodal interference in a photonic crystal fibre," Opt. Express 12,3465-3470 (2004). [CrossRef] [PubMed]
  11. Adam Wax, Changhuei Yang, and Joseph A. Izatt, "Fourier-domain low-coherence interferometry for light-scattering spectroscopy," Opt. Lett. 28,1230-1232, (2003). [CrossRef] [PubMed]
  12. R. A. Leitgeb, W. Drexler, A. Unterhuber, B. Hermann, T. Bajraszewski, T. Le, A. Stingl, and A. F. Fercher, "Ultrahigh resolution Fourier domain optical coherence tomography," Opt. Express 12,2156-2165, (2004). [CrossRef] [PubMed]
  13. J.Y. Lee, T-J. Ahn, S. Moon, Y. Jung, K. Oh, and D.Y. Kim. "Differential mode delay analysis for a multimode optical fiber with Fourier-domain low-coherence interferometry," in Optical Fiber and Communication, Technical Digest (Optical Society of America, 2006) OWI18.
  14. C. Dorrer, D. Belabas, J. P. Likforman, and M. Joffre, "Spectral resolution and sampling issues in Fourier-transform spectral interferometry," J. Opt. Soc. Am. B 17,1795-1802, (2000). [CrossRef]

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