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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 20 — Sep. 26, 2011
  • pp: 19122–19128
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Centimeter-level spatial resolution over 40 km realized by bandwidth-division phase-noise-compensated OFDR

Xinyu Fan, Yusuke Koshikiya, and Fumihiko Ito  »View Author Affiliations


Optics Express, Vol. 19, Issue 20, pp. 19122-19128 (2011)
http://dx.doi.org/10.1364/OE.19.019122


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Abstract

We present a bandwidth-division phase-noise-compensated optical frequency domain reflectometry (PNC-OFDR) technique, which permits a fast sweep of the optical source frequency. This method makes it possible to reduce the influence of environmental perturbation, which is the dominant factor degrading the spatial resolution of frequency-domain reflectometry at a long measurement range after compensation of the optical source phase noise. By using this approach, we realize a sub-cm spatial resolution over 40 km in a normal laboratory environment, and a 5 cm spatial resolution at 39.2 km in a field trial.

© 2011 OSA

1. Introduction

Optical reflectometry with a narrow spatial resolution and a long measurement range is becoming increasingly important as a tool for optical fiber network maintenance. Optical time domain reflectometry (OTDR) [1

1. M. K. Barnoski and S. M. Jensen, “Fiber waveguides: a novel technique for investigating attenuation characteristics,” Appl. Opt. 15(9), 2112–2115 (1976). [CrossRef] [PubMed]

] can provide a long measurement range but it suffers from a tradeoff between spatial resolution and sensitivity resulting in a resolution of greater than one meter. For applications such as the distributed measurement of polarization mode dispersion (PMD) [2

2. B. Huttner, J. Reecht, N. Gisin, R. Passy, and J. P. von der Weid, “Local birefringence measurements in single-mode fibers with coherent optical frequency-domain reflectometry,” IEEE Photon. Technol. Lett. 10(10), 1458–1460 (1998). [CrossRef]

] in previously installed fibers, the beat length caused by a high PMD section may be much less than one meter, making it nearly impossible to measure with OTDR.

In this paper, we present a bandwidth-division PNC-OFDR technique capable of reducing the influence of environmental perturbation via a fast sweep of the optical source frequency, thus decreasing the ratio of the acoustic noise band (~several kHz) to beat frequency. The bandwidth-division scheme lets us deal with high-frequency signals even when we employ a fast sweep of the optical source frequency at 3 THz/s. Meanwhile, the signal bandwidth is greatly reduced by using this scheme, enabling us to sample and process more data (corresponding to a full frequency sweep of 15 GHz), thus helping us to improve the spatial resolution with the sacrifice of the measurement time. By using this technique, we realize a sub-cm spatial resolution over 40 km in a normal laboratory environment and a 5 cm spatial resolution at 39.2 km in a field trial.

2. Principle

The theory of PNC-OFDR has already been described in detail [8

8. X. Fan, Y. Koshikiya, and F. Ito, “Phase-noise-compensated optical frequency domain reflectometry with measurement range beyond laser coherence length realized using concatenative reference method,” Opt. Lett. 32(22), 3227–3229 (2007). [CrossRef] [PubMed]

], and we provide just a brief introduction here. In PNC-OFDR, an auxiliary interferometer is used to provide phase noise information about the optical source allowing us to resample the temporal signals of the measurement. The phase noise of the measurement signal is then compensated when the reflection events occur around the best compensation positions, and is totally eliminated at those positions. Figure 1
Fig. 1 Reference signals used in each section of FUT.
shows schematically that the FUT could be divided into N sections for compensation using different concatenation-generated phases (CGPs) XN(t), which are calculated using the following expression:

XN(t)=n=0N1X1(tnτref),
(1)

where X1(t) is a phase term obtained from an auxiliary interferometer with a delay time τref in one arm. The phase noise term Φ(t) is compensated from θ(t) – θ (t-τFUT) to the following term:

Φ(t)=[θ(t)θ(tτFUT)]τFUTNτref[θ(t)θ(tNτref)],
(2)

where τFUT is the time needed for a round-trip in the FUT. When τFUT is equal to ref, the phase noise term is canceled out and the optimum compensation can be achieved.

At a long distance such as 40 km, even after phase noise compensation, environmental perturbation becomes a dominant factor degrading the spatial resolution. Therefore, as mentioned in the introduction, a fast sweep of the optical source frequency is needed to reduce the effect of environmental perturbation. However, a fast sweep generates a high beat frequency, which makes it difficult for later sampling or processing. To deal with this problem, we can adopt a bandwidth-division method. If we assume a maximum frequency of F, we need a sampling card with a sampling rate of greater than 2F. Figure 2
Fig. 2 Concept of bandwidth-division process.
shows that we can divide the frequency into M sections, each with a bandwidth of F/M. If we down-convert the m-th (m = 2, 3,…, M) section so that it is within a frequency of 0 < fbase < F/M, we only need a sampling card with a sampling rate of 2F/M. Meanwhile, the amount of data decreases to 1/M compared with that required when using the conventional sampling method.

We should note that XN(t) is no longer the correct reference to compensate for the phase noise in the down-converted signal. To obtain the correct reference, we should also down-convert the frequency of XN(t) by the same value. This can be realized by changing XN(t) as follows:

XN(m)(t)=XN(t)2πm1MFt.
(3)

Figure 3
Fig. 3 Reference signals used in each section of FUT for different bandwidths.
shows schematically how to compensate for every section of the FUT by using suitable reference signals. Another method for accomplishing the compensation correctly is to rebuild the original measurement signal before down-conversion, and use XN(t) for compensation. Although the two methods are equivalent to each other and can be implemented digitally, we adopt the former because it consumes fewer calculation resources.

3. Experimental setup

For this experiment, we choose an M value of 3 for the purpose of preliminary theoretical confirmation. For an FUT of ~40 km, the maximum frequency F of the signals from the main interferometers is ~1200 MHz. Figure 5
Fig. 5 Electrical processing after the signals having been received by one BPD. The bandwidth of BPF1 and BPF2 is 400–800 MHz and 800–1200 MHz, respectively, and the LPF cutoff bandwidth is 400 MHz.
illustrates the electrical process after one BPD. Two electrical switches are used to select the route with the help of synchronization equipment. When the 1st route is selected, there is no processing within the route. When the 2nd or the 3rd route is selected, the signals are filtered by a band-pass filter (BPF) with a bandwidth of 400–800 MHz (BPF1), or 800–1200 MHz (BPF2). Then the signals are down-converted by using a mixer beat with sinusoidal signals whose frequency is 408 MHz (route 2) or 800 MHz (route 3), generated from an arbitrary waveform generator. For route 2, 400 MHz is not used for mixing since the mixer is not ideal and this signal will still be present after mixing, resulting in a spurious peak. Therefore, it is expected that a larger frequency will be used here since the spurious peak does not influence the original results. The signals are then divided into three sections of 0–408, 408–800, and 800–1200 MHz. After the mixer, the signals are filtered by an LPF with a cut-off frequency of 400 MHz, sampled by using an ADC with a sampling rate of 1 GS/s, and collected by a computer, to undergo a numerical compensation process.

The compensation is performed with a high-end personal computer with an Intel Core i7-980X CPU and a 24 GB cache memory. To reduce Rayleigh speckle noise, we take the average intensity of the results obtained with different probe wavelengths by using a laser tuning range of over 60 GHz. The full measurement time is about 2 min, and this is limited by the laser tuning speed. About 30 min is needed to compensate the data of one bandwidth (0–14.0, 14.0–27.5, or 27.5–41.25 km) with a 50-wavelength average.

4. Experimental results

The FUT is composed of a 40 km fiber spool connected to a 1.25 km fiber spool equipped with an angled physical contact (APC) connector. Figure 6
Fig. 6 Measurement results for the reflectivity of backscattered/reflected light wave.
shows the reflectivity of backscattered/reflected light wave, which is a combination of three different divisions. Due to the abrupt attenuation around the cut-off frequency of the electronic filters, these signals with frequencies outside the bandwidth, such as signals around 400–408 MHz, are also highly attenuated. In fact, a more carefully designed filter should be adopted to alleviate this attenuation. Meanwhile, since the BPD used in our setup has a cut-off frequency of 800 MHz, which is less than the original frequencies (800–1200 MHz) generated at the third division, the results at longer distances are greatly attenuated. This is the widest bandwidth of our available equipment, and in fact a BPD with a wider bandwidth should be adopted to avoid the excess attenuation.

We placed the FUT in two different laboratory environments to check the spatial resolution since it is influenced by the environment. The total sound pressure was 58.1 dB in a normal laboratory environment and 60.7 dB in a relatively noisy laboratory environment. The sound pressure densities of two different experimental environments are shown in Fig. 7(d)
Fig. 7 Details of reflection peaks in different environments. (a) Reflection peaks occurred around 40 km; (b) Reflection peaks occurred around 41.25 km; (c) Reflection peak measured in the field environment; (d) Sound pressure density of two laboratory environments.
while the details of the reflection peaks at different environments are shown in Figs. 7(a)–(b). For the reflection peaks that occur around 40 km (N × 2.5 km), which is an optimum compensation position, a 3-dB spatial resolution of 8 mm (240 Hz in frequency domain) is obtained in a normal environment and it deteriorates to about 2.5 cm (750 Hz in frequency domain) in a relatively noisy environment. For the reflection peaks that occur around 41.25 km ((N + 0.5) × 2.5 km), which is the border of the 16th compensation section, the spatial resolution is still 8 mm in a normal environment, and it deteriorates to about 5 cm (1500 Hz in frequency domain) in a relatively noisy environment.

We compared the results to those obtained without bandwidth-division process at a sweep rate of 1 THz/s, the reflection peaks almost remain the same in the frequency domain. Therefore, it is obvious that the environmental perturbation caused the added frequency components. By sweeping the optical source frequency faster, we decreased the ratio of these added frequency components to beat frequency, therefore making spatial resolution improved. In the normal laboratory environment, we obtained a spatial resolution of 8 mm at any position within the entire measurement range.

5. Summary

We have presented a bandwidth-division PNC-OFDR technique, capable of reducing the influence of environmental perturbation via a fast sweep of the optical source frequency, by decreasing the ratio of the acoustic noise band to beat frequency. The bandwidth-division scheme permits us to deal with high-frequency signals by adopting a fast sweep of the optical source frequency. Meanwhile, the reduction of the signal bandwidth enables us to sample and process more data, thus helping us to improve the spatial resolution. By using this technique, we realized a sub-cm spatial resolution over 40 km in a normal laboratory environment, and a 5 cm spatial resolution of 39.2 km in a field trial.

References and links

1.

M. K. Barnoski and S. M. Jensen, “Fiber waveguides: a novel technique for investigating attenuation characteristics,” Appl. Opt. 15(9), 2112–2115 (1976). [CrossRef] [PubMed]

2.

B. Huttner, J. Reecht, N. Gisin, R. Passy, and J. P. von der Weid, “Local birefringence measurements in single-mode fibers with coherent optical frequency-domain reflectometry,” IEEE Photon. Technol. Lett. 10(10), 1458–1460 (1998). [CrossRef]

3.

W. Eickhoff and R. Ulrich, “Optical frequency domain reflectometry in single‐mode fiber,” Appl. Phys. Lett. 39(9), 693–695 (1981). [CrossRef]

4.

H. Barfuss and E. Brinkmeyer, “Modified optical frequency domain reflectometry with high spatial resolution for components of integrated optic systems,” J. Lightwave Technol. 7(1), 3–10 (1989). [CrossRef]

5.

G. Mussi, N. Gisin, R. Passy, and J. P. von der Weid, “-152.5 dB sensitivity high dynamic-range optical frequency-domain reflectometry,” Electron. Lett. 32(10), 926–927 (1996). [CrossRef]

6.

D. K. Gifford, M. E. Froggatt, M. S. Wolfe, S. T. Kreger, and B. J. Soller, “Millimeter resolution reflectometry over two kilometers,” in 33rd European Conference and Exhibition on Optical Communication—ECOC 2007 (2007), vol. 2, pp. 85–87, paper Tu.3.6.1.

7.

K. Tsuji, K. Shimizu, T. Horiguchi, and Y. Koyamada, “Spatial-resolution improvement in long range coherent optical frequency domain reflectometry by frequency-sweep linearisation,” Electron. Lett. 33(5), 408–409 (1997). [CrossRef]

8.

X. Fan, Y. Koshikiya, and F. Ito, “Phase-noise-compensated optical frequency domain reflectometry with measurement range beyond laser coherence length realized using concatenative reference method,” Opt. Lett. 32(22), 3227–3229 (2007). [CrossRef] [PubMed]

9.

X. Fan, Y. Koshikiya, and F. Ito, “Noise of long-range optical frequency domain reflectometry after optical source phase noise compensation,” Proc. SPIE 7503, 75032E, 75032E-4 (2009). [CrossRef]

10.

Y. Koshikiya, X. Fan, and F. Ito, “Influence of acoustic perturbation of fibers in phase-noise-compensated optical-frequency-domain reflectometry,” J. Lightwave Technol. 28, 3323–3328 (2010).

OCIS Codes
(060.2300) Fiber optics and optical communications : Fiber measurements
(120.1840) Instrumentation, measurement, and metrology : Densitometers, reflectometers

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: June 8, 2011
Revised Manuscript: August 23, 2011
Manuscript Accepted: August 23, 2011
Published: September 16, 2011

Citation
Xinyu Fan, Yusuke Koshikiya, and Fumihiko Ito, "Centimeter-level spatial resolution over 40 km realized by bandwidth-division phase-noise-compensated OFDR," Opt. Express 19, 19122-19128 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-20-19122


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References

  1. M. K. Barnoski and S. M. Jensen, “Fiber waveguides: a novel technique for investigating attenuation characteristics,” Appl. Opt.15(9), 2112–2115 (1976). [CrossRef] [PubMed]
  2. B. Huttner, J. Reecht, N. Gisin, R. Passy, and J. P. von der Weid, “Local birefringence measurements in single-mode fibers with coherent optical frequency-domain reflectometry,” IEEE Photon. Technol. Lett.10(10), 1458–1460 (1998). [CrossRef]
  3. W. Eickhoff and R. Ulrich, “Optical frequency domain reflectometry in single‐mode fiber,” Appl. Phys. Lett.39(9), 693–695 (1981). [CrossRef]
  4. H. Barfuss and E. Brinkmeyer, “Modified optical frequency domain reflectometry with high spatial resolution for components of integrated optic systems,” J. Lightwave Technol.7(1), 3–10 (1989). [CrossRef]
  5. G. Mussi, N. Gisin, R. Passy, and J. P. von der Weid, “-152.5 dB sensitivity high dynamic-range optical frequency-domain reflectometry,” Electron. Lett.32(10), 926–927 (1996). [CrossRef]
  6. D. K. Gifford, M. E. Froggatt, M. S. Wolfe, S. T. Kreger, and B. J. Soller, “Millimeter resolution reflectometry over two kilometers,” in 33rd European Conference and Exhibition on Optical Communication—ECOC 2007 (2007), vol. 2, pp. 85–87, paper Tu.3.6.1.
  7. K. Tsuji, K. Shimizu, T. Horiguchi, and Y. Koyamada, “Spatial-resolution improvement in long range coherent optical frequency domain reflectometry by frequency-sweep linearisation,” Electron. Lett.33(5), 408–409 (1997). [CrossRef]
  8. X. Fan, Y. Koshikiya, and F. Ito, “Phase-noise-compensated optical frequency domain reflectometry with measurement range beyond laser coherence length realized using concatenative reference method,” Opt. Lett.32(22), 3227–3229 (2007). [CrossRef] [PubMed]
  9. X. Fan, Y. Koshikiya, and F. Ito, “Noise of long-range optical frequency domain reflectometry after optical source phase noise compensation,” Proc. SPIE7503, 75032E, 75032E-4 (2009). [CrossRef]
  10. Y. Koshikiya, X. Fan, and F. Ito, “Influence of acoustic perturbation of fibers in phase-noise-compensated optical-frequency-domain reflectometry,” J. Lightwave Technol.28, 3323–3328 (2010).

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