## Optical reflectometry based on correlation detection and its application to the in-service monitoring of WDM passive optical network

Optics Express, Vol. 15, Issue 9, pp. 5318-5326 (2007)

http://dx.doi.org/10.1364/OE.15.005318

Acrobat PDF (477 KB)

### Abstract

We propose and demonstrate a novel technique for measuring the distribution of the reflectivity along an optical fiber transmission line. Unlike the conventional optical time-domain reflectometer (OTDR), the proposed technique utilizes the data-modulated transmitter itself instead of the optical short-pulse source, and monitors the distribution of the back-reflected light by calculating the cross-correlation of the transmitted and back-reflected signals. In this paper, we describe the operating principle of the proposed technique and discuss its potential limitation on the dynamic range. We also show that this limitation can be mitigated by using the discrete-component elimination algorithm. In addition, we experimentally demonstrate that the proposed technique can be used for the in-service monitoring of the transmission fibers in a wavelength-division multiplexed passive optical network (WDM PON).

© 2007 Optical Society of America

## 1. Introduction

1. N. Tomita, H. Takasugi, N. Atobe, I. Nakamura, F. Takaesu, and S. Takashima, “Design and performance of a novel
automatic fiber line testing system with OTDR for optical subscriber
loops,” J. Lightwave Technol. **12**, 717–726
(1994). [CrossRef]

3. K. C. Reichmann, N. J. Frigo, P. P. Iannone, X. Zhou, M. Leblanc, and S. Chabot, “In-service OTDR limitations in CWDM
systems caused by spontaneous Stokes and anti-Stokes Raman
scattering,” IEEE Photon. Technol. Lett. **16**, 1787–1789
(2004). [CrossRef]

4. N. J. Frigo, P. P. Iannone, K. C. Reichmann, X. Zhou, and M. W. Stodden, “Centralized in-service OTDR testing
in a CWDM business access network,” J.
Lightwave Technol. **22**, 2641–2652
(2004). [CrossRef]

6. K. Tanaka, H. Izumita, N. Tomita, and Y. Inoue, “In-service individual line
monitoring and a method for compensating for the temperature-dependent
channel drift of a WDM-PON containing an AWGR using a 1.6 mm tunable
OTDR,” in Proceedings of *European Conference
on Optical Communication*, 3, paper 448, pp.
295–298
(1997).

7. K. W. Lim, E. S. Son, K. H. Han, and Y. C. Chung, “Fault localization in WDM passive
optical network by reusing downstream light
sources,” IEEE Photon. Technol. Lett. **17**, 2691–2693
(2005). [CrossRef]

8. N. Takeuchi, N. Sugimoto, H. Baba, and K. Sakurai, “Random modulation cw
lidar,” Appl. Opt. **22**, 1382–1386
(1983). [CrossRef] [PubMed]

9. M. Nazarathy, S. A. Newton, R. P. Giffard, D. S. Moberly, F. Sischika, W. R. Trutna Jr., and S. Foster, “Real-time long range complementary
correlation optical time domain reflectometer,”
J. Lightwave Technol. **7**, 24–38
(1989). [CrossRef]

## 2. Operating principle

8. N. Takeuchi, N. Sugimoto, H. Baba, and K. Sakurai, “Random modulation cw
lidar,” Appl. Opt. **22**, 1382–1386
(1983). [CrossRef] [PubMed]

### 2.1 Basic principle

*B*and launch the signal light into the transmission fiber. Let

*s*(

*t*) be the transmitted binary signal having a value of +1 or -1 and

*T*the bit duration of the signal (=1/

*B*). The input optical power is expressed by

*P*(

_{in}*t*)=

*P*(1+

_{a}*s*(

*t*)), where

*P*is the average input power. If we denote the distribution of the reflectivity including the round-trip loss as

_{a}*R*(

*z*), the power of the light back-reflected to the transmitter can be expressed as

*P*(

_{ret}*t*) =

*P*(

_{in}*t*)⊗

*R*(

*ν*/2) , where ⊗ stands for the convolution operation, and

_{c}t*ν*is the group velocity of light in the fiber. Then, we detect the back-reflected light by using a photo detector. The signal voltage after the detection

_{c}*ν*(

_{det}*t*) can be expressed by the sum of dc- and ac-components as follows:

*η*is the conversion efficiency of the detector,

*ν*

_{0}≡

*ηP*and

_{a}*ν*′≡

_{det}*ηP*

_{a}*s*(

*t*)⊗

*R*(

*ν*/2) are the dc- and ac-components of

_{c}t*ν*(

_{det}*t*), respectively. Hereafter, we use the prime (′) to express the ac-component.

*ν*(

_{ref}*t*) (i.e., reference signal) can be expressed by using

*s*(

*t*) as follows:

*ν*

_{ref0}and

*ν*′(

_{ref}*t*) are the dc- and ac-components of

*ν*(

_{ref}*t*), respectively. The cross-correlation function

*q*(

*τ*) between

*ν*′(

_{det}*t*) and

*ν*′(

_{ref}*t*) is given by

*ϕ*(

_{s}*τ*) is the autocorrelation function of

*s*(

*t*). (

*ϕ*(

_{s}*τ*) = 1-|

*τ*|/

*T*for |

*τ*|<

*T*and

*ϕ*(

_{s}*τ*) = 0 for |

*τ*|≥

*T*). Since

*T*is sufficiently short,

*ϕ*(

_{s}*τ*) can be well approximated by using a delta function as

*Tδ*(

*τ*) . Then, Eq. (3) becomes

*q*(

*τ*) =

*η*

*P*

_{a}*ν*

_{ref0}

*TR*(

*ν*

_{c}

*τ*/2). Thus, the reflectivity distribution can be derived by using the cross-correlation function

*q*(

*τ*) as

*R*(

*z*)∝

*q*(2

*z*/

*ν*).

_{c}*ν*′(

_{det}*t*) and

*ν*′(

_{ref}*t*). In order to cope with this difficulty, we filter out the low-frequency components of

*ν*′(

_{ref}*t*) and

*ν*′(

_{det}*t*) by using a first-order low-pass filter with a cut-off frequency of

*f*(≪

_{c}*B*). Here, let

*ν*

_{ref,f}′(

*t*) and

*ν*

_{det,f}′(

*t*) denote the ac-components of the reference and returned signals after filtering, respectively. Then, the cross-correlation function between

*ν*

_{ref,f}′(

*t*) and

*ν*

_{det,f}′(

*t*) is given by

*ϕ*

_{s,f}(

*τ*) is the autocorrelation function of

*s*(

_{f}*t*), which is the signal derived by low-pass filtering of

*s*(

*t*). Since

*f*≪

_{c}*B*,

*ϕ*

_{s,f}(

*τ*) can be expressed by

*πf*exp(-2

_{c}T*πf*|

_{c}*τ*|). Thus, the time width of the autocorrelation function is broadened due to this filtering. The dashed curve in Fig. 2(a) shows an example of

*ϕ*

_{s,f}(

*τ*) calculated for

*f*=3 MHz. It has a single peak with a finite time width and quickly converges to zero as |

_{c}*τ*| increases. Since the cross-correlation obtained in Eq. (4) represents the convolution of this broadened autocorrelation function

*ϕ*

_{s,f}(

*τ*) and the distribution of the reflectivity, the spatial resolution is limited by the time width of

*ϕ*/

_{s,f}(*τ*). The spatial resolution is given as 0.22*ν*_{c}*f*when we define the spatial resolution by using the full-width at half maximum (FWHM) of

_{c}*ϕ*

_{s,f}(

*τ*). For example, when

*f*=3 MHz, the spatial resolution becomes 15.0 m.

_{c}### 2.2 Limitation on the dynamic range and its improvement by the discrete component elimination algorithm

*ν*

_{ref,f}′(

*t*) and

*ν*

_{detf}′(

*t*) are digitally processed after sampling and the data length is finite. Therefore, the ensemble average in Eq. (4) can be replaced with the average by using the sampled data with a finite length. In order to take this into account, we denote the sampling interval and the total number of sampling points by Δ

*t*and

*N*, respectively, and rewrite Eq. (4) as follows,

*k*is an integer, and the convolution is carried out in the discrete time domain. In Eq. (5),

*ϕ*

_{s, f}(

*k*Δ

*t*) is the autocorrelation function of

*s*(

_{f}*i*Δ

*t*) given by

*ϕ*

_{s, f}(

*k*Δ

*t*) has a sharp peak at

*k*Δ

*t*= 0, but it accompanies with the noise background due to the finite sampling length. As an example, the solid curve in Fig. 2(a) shows

*ϕ*

_{s, f}(

*k*Δ

*t*) simulated for

*N*=4096, Δ

*t*=100 nsec (sampling rate

*f*=10 MHz), and

_{s}*f*=3 MHz. In order to evaluate this background noise, we define the background noise suppression ratio (BNSR) by the ratio of the peak amplitude to the standard deviation (i.e., root-mean square) of the background noise of

_{c}*ϕ*

_{s,f}. By assuming that

*s*(

*t*) is a stationary random variable, we can derive the BNSR analytically after some algebraic calculations as follows:

*N*for

*f*/

_{c}*f*=0.1 and 0.3, respectively. When

_{s}*f*is chosen to be larger than 0.3

_{C}*f*, the BNSR can be well approximated by

_{s}*N*

^{1/2}. For example, when

*N*=4096, the BNSR becomes 18 dB. Dots and open circles in Fig. 2(b) show the BNSR values obtained by the numerical simulation. They agree well with the theoretically calculated lines using Eq. (7).

8. N. Takeuchi, N. Sugimoto, H. Baba, and K. Sakurai, “Random modulation cw
lidar,” Appl. Opt. **22**, 1382–1386
(1983). [CrossRef] [PubMed]

9. M. Nazarathy, S. A. Newton, R. P. Giffard, D. S. Moberly, F. Sischika, W. R. Trutna Jr., and S. Foster, “Real-time long range complementary
correlation optical time domain reflectometer,”
J. Lightwave Technol. **7**, 24–38
(1989). [CrossRef]

*ν*′(

_{detf}*t*) is known, (

*ϕ*

_{s,f}can be easily calculated. That is, we can calculate the background noises generated from the discrete reflection points precisely. Thus, we can cancel out the background noise by the following steps: (a) calculate the cross-correlation using Eq. (5) directly, (b) find the highest discrete reflection point in the cross-correlation trace, (c) calculate

*ϕ*

_{s,f}from the measured reference signal

*ν*

_{ref,f}′(

*t*) and estimate the background noise components, and (d) subtract it from the original cross-correlation function. As an example, we applied this discrete component elimination algorithm to the cross-correlation trace shown in Fig. 3(b). The solid curve in Fig. 3(c) shows the result when we subtracted the background noise generated by the peak at 10 km by the proposed algorithm. The results show that the background noise level is drastically reduced and the small peak component located at 5 km can be clearly observed. The recursive use of this algorithm can substantially improve the BNSR even when there are many reflection points in the transmission line.

*ξ*(

*t*) is the impulse response of the HPF. Usually,

*ξ*(

*t*) is an S-shape function having a negative tail and it makes the dead-zone after the discrete reflection point depending on the cut-off frequency. Also it becomes impossible to measure the distributed reflection like Rayleigh scattering.

## 3. Experiments

^{31}-1. The signal light was launched into the fiber through an optical circulator. The average input power launched into the fiber was 0.8 dBm. To detect the back-reflected light, we used a conventional ac-coupled avalanche photo diode (APD), which was designed for a 155 Mb/s synchronous digital hierarchy (SDH) receiver. The data and the detected electric signals were filtered out by 3-MHz LPF’s, and digitized by a 9-bit A/D converter at a sampling rate of 10 Ms/s. Then, we calculated their cross-correlation by using a personal computer. In this experiment, the spatial resolution was determined by the bandwidth of the LPF to be 15 m. The maximum number of sampling points of the A/D converter was 4096, and the measurement range was 40.9 km. The inset in Fig. 4 shows an example of the sampled waveform. Since the cut-off frequency of the LPF was much lower than the bit-rate, the waveforms of the reference and detected back-reflected signals looked like noises having continuous probability distribution functions. The period of the 2

^{31}-1 PRBS was 1.72 sec and it was much longer than the sampling duration (=

*N*Δ

*t*) of 4 msec. Therefore, there was no need to take into account the periodicity of the PRBS during the sampling of

*N*-point data, and we can assume the transmission data was random.

*N*=4096. The reflection at the fiber end was clearly observed even without averaging, in spite of the background noise. We evaluated the BNSR by calculating the ratio of the peak of the cross-correlation to the root-mean square of the background noise. The closed circles in Fig. 5(c) represent the BNSR measured as a function of the number of sampling points. The solid line indicates the theoretically calculated value of BNSR by using Eq. (7). The square-root dependence on the number of sampling points

*N*is clearly seen. The small discrepancy from the theoretically calculated line was attributed to the noises generated by the other impairment factors.

*ν*

_{det,f}′(

*t*) is not kept high (e.g., that the back-scattering level is too low to be measured), the BNSR improvement by the proposed algorithm becomes small. In this case, the BNSR can be simply determined by the S/N ratio of

*ν*

_{det,f}′(

*t*).

9. M. Nazarathy, S. A. Newton, R. P. Giffard, D. S. Moberly, F. Sischika, W. R. Trutna Jr., and S. Foster, “Real-time long range complementary
correlation optical time domain reflectometer,”
J. Lightwave Technol. **7**, 24–38
(1989). [CrossRef]

## 4. Summary

## References and links

1. | N. Tomita, H. Takasugi, N. Atobe, I. Nakamura, F. Takaesu, and S. Takashima, “Design and performance of a novel
automatic fiber line testing system with OTDR for optical subscriber
loops,” J. Lightwave Technol. |

2. | F. Yamamoto and T. Horiguchi, “Allowable received OTDR light power
for in-service measurement in lightwave SCM
systems,” J. Lightwave Technol. |

3. | K. C. Reichmann, N. J. Frigo, P. P. Iannone, X. Zhou, M. Leblanc, and S. Chabot, “In-service OTDR limitations in CWDM
systems caused by spontaneous Stokes and anti-Stokes Raman
scattering,” IEEE Photon. Technol. Lett. |

4. | N. J. Frigo, P. P. Iannone, K. C. Reichmann, X. Zhou, and M. W. Stodden, “Centralized in-service OTDR testing
in a CWDM business access network,” J.
Lightwave Technol. |

5. | U. Hilbk, M. Burmeister, B. Hoen, T. Hermes, J. Saniter, and F. J. Westphal, “Selective OTDR measurements at the
central office of individual fiber link in a
PON,” in |

6. | K. Tanaka, H. Izumita, N. Tomita, and Y. Inoue, “In-service individual line
monitoring and a method for compensating for the temperature-dependent
channel drift of a WDM-PON containing an AWGR using a 1.6 mm tunable
OTDR,” in Proceedings of |

7. | K. W. Lim, E. S. Son, K. H. Han, and Y. C. Chung, “Fault localization in WDM passive
optical network by reusing downstream light
sources,” IEEE Photon. Technol. Lett. |

8. | N. Takeuchi, N. Sugimoto, H. Baba, and K. Sakurai, “Random modulation cw
lidar,” Appl. Opt. |

9. | M. Nazarathy, S. A. Newton, R. P. Giffard, D. S. Moberly, F. Sischika, W. R. Trutna Jr., and S. Foster, “Real-time long range complementary
correlation optical time domain reflectometer,”
J. Lightwave Technol. |

10. | ITU-T Recommendation G. 983.1, |

11. | ITU-T Recommendation G. 984.2, |

12. | IEEE Standard 802.3ah, |

13. | D. Derickson, |

**OCIS Codes**

(060.2330) Fiber optics and optical communications : Fiber optics communications

(060.2360) Fiber optics and optical communications : Fiber optics links and subsystems

(120.1840) Instrumentation, measurement, and metrology : Densitometers, reflectometers

**ToC Category:**

Fiber Optics and Optical Communications

**History**

Original Manuscript: March 14, 2007

Revised Manuscript: April 12, 2007

Manuscript Accepted: April 12, 2007

Published: April 16, 2007

**Citation**

Y. Takushima and Y. C. Chung, "Optical reflectometry based on correlation detection and its application to the in-service monitoring of WDM passive optical network," Opt. Express **15**, 5318-5326 (2007)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-9-5318

Sort: Year | Journal | Reset

### References

- N. Tomita, H. Takasugi, N. Atobe, I. Nakamura, F. Takaesu, and S. Takashima, "Design and performance of a novel automatic fiber line testing system with OTDR for optical subscriber loops," J. Lightwave Technol. 12, 717-726 (1994). [CrossRef]
- F. Yamamoto and T. Horiguchi, "Allowable received OTDR light power for in-service measurement in lightwave SCM systems," J. Lightwave Technol. 18, 286-294 (2000). [CrossRef]
- K. C. Reichmann, N. J. Frigo, P. P. Iannone, X. Zhou, M. Leblanc, and S. Chabot, "In-service OTDR limitations in CWDM systems caused by spontaneous Stokes and anti-Stokes Raman scattering," IEEE Photon. Technol. Lett. 16, 1787-1789 (2004). [CrossRef]
- N. J. Frigo, P. P. Iannone, K. C. Reichmann, X. Zhou, and M. W. Stodden, "Centralized in-service OTDR testing in a CWDM business access network," J. Lightwave Technol. 22, 2641-2652 (2004). [CrossRef]
- U. Hilbk, M. Burmeister, B. Hoen, T. Hermes, J. Saniter, and F. J. Westphal, "Selective OTDR measurements at the central office of individual fiber link in a PON," in Optical Fiber Communication Conference and Exhibit, Technical Digest (Optical Society of America, 1997), paper Tuk3.
- K. Tanaka, H. Izumita, N. Tomita, and Y. Inoue, "In-service individual line monitoring and a method for compensating for the temperature-dependent channel drift of a WDM-PON containing an AWGR using a 1.6 mm tunable OTDR," in Proceedings of European Conference on Optical Communication, 3, paper 448, pp. 295-298 (1997).
- K. W. Lim, E. S. Son, K. H. Han, and Y. C. Chung, "Fault localization in WDM passive optical network by reusing downstream light sources," IEEE Photon. Technol. Lett. 17, 2691-2693 (2005). [CrossRef]
- N. Takeuchi, N. Sugimoto, H. Baba, and K. Sakurai, "Random modulation cw lidar," Appl. Opt. 22, 1382-1386 (1983). [CrossRef] [PubMed]
- M. Nazarathy, S. A. Newton, R. P. Giffard, D. S. Moberly, F. Sischika, W. R. Trutna, Jr., and S. Foster, "Real-time long range complementary correlation optical time domain reflectometer," J. Lightwave Technol. 7, 24-38 (1989). [CrossRef]
- ITU-T Recommendation G. 983.1, Broadband Optical Access Systems Based on Passive Optical Networks (2005).
- ITU-T Recommendation G. 984.2, Gigabit-capable Passive Optical Networks (GPON): Physical Media Dependent (PMD) layer specification (2003).
- IEEE Standard 802.3ah, Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications (2004).
- D. Derickson, Fiber Optic Test and Measurement, ch. 11 (Prentice-Hall, New Jersey, 1998).

## Cited By |
Alert me when this paper is cited |

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.

« Previous Article | Next Article »

OSA is a member of CrossRef.