IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 24, NO. 22, NOVEMBER 15, 2012 2005

Time Division Multiplexing Optical Time DomainReflectometry Based on Dual Frequency Probe

Xuping Zhang, Yuejiang Song, and Lidong Lu

Abstract— A new frequency coding method is proposed andexperimentally demonstrated that employs a phase modulatordriven by sequential radio frequency and a synchronouslycontrolled acoustooptic modulator to generate time divisionmultiplexing probe pulses for coherent optical time domainreflectometry (C-OTDR). Each probe pulse contains two probefrequencies that are symmetrical regarding the frequency of localoscillator (LO). Heterodyne between the backscattered Rayleighlight of the dual frequency probe pulse and LO generates thesame intermediate frequency. Experimental results show that thenew C-OTDR, using ten frequency-coded probe pulses, brings an8.0-dB dynamic range enhancement, compared with conventionalC-OTDR.

Index Terms— Coherent detection, frequency-coded probepulse, optical time domain reflectometry, time divisionmultiplexing.

I. INTRODUCTION

COHERENT optical time domain reflectometry(C-OTDR) is an important instrument for fiber

characterization and fault location for long haul multi-spanundersea optical transmission line monitoring [1]. Themeasurement DR is strongly dependent on averaging theresults of a large number of measurements [2]–[6]. Forquickly increasing measurement number, an M-ary frequencyshift keying (FSK) probe method was proposed [3]. In thismethod, probe pulses with the same pulse width but differentfrequencies forming frequency coded probe pulse sequenceare launched into a transmission line and coherent detectiongenerates different IFs with relative time delay. Then theseIFs are simultaneously processed and finally synthesized tobe one OTDR trace. Experimental results showed that it couldmultiple measurement efficiency, but it is difficult to controlthe distributed Bragg reflector (DBR) laser to get frequencyprecisely coded probe pulse sequence, so that the bandwidthof the band pass filter (BPF) in its signal processing unit isset to 50 MHz, which will lead degradation of signal to noiseratio (SNR).

Manuscript received June 8, 2012; revised August 22, 2012; acceptedSeptember 3, 2012. Date of publication September 6, 2012; date of currentversion October 31, 2012. This work was supported in part by the NationalBasic Research Program of China under Grant 2010CB327803, in part by theNational Natural Science Foundation of China under Grant 60907022, Grant61027017, and Grant 60644001, and in part by the Jiangsu Provincial NaturalScience Foundation of China under Grant BK2010379.

The authors are with the Institute of Optical Communication Engineering,Nanjing University, Nanjing 210093, China (e-mail: yjsong@nju.edu.cn).

Color versions of one or more of the figures in this letter are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LPT.2012.2217737

Fig. 1. Experimental setup of the TDM-OTDR.

As an amelioration, a phase modulator (PM) controlledby step changing RF was employed to generate frequencyprecisely coded probe pulse sequence. The PM converts sin-gle frequency laser to multi-frequency light, but only the−1st order frequency is adopted as the probe [4]. For eachfrequency in the probe pulse sequence, coherent detectionbetween the backscattered Rayleigh light and LO generatescorresponding IF, each being related to an OTDR trace. Thenthese IFs are processed by parallel computing method andsynthesized to be one final OTDR trace. Experiment using tensequential probe frequencies was conducted and experimentalresults showed a 5.0 dB DR improvement. The measurementspeed was near 10 times faster than that of conventionalC-OTDR [4], however, by this method only one sideband ofthe multi-frequency light from the PM was used, which wastedmuch of the light power, and the DR enhancement was onlydependent on increasing measurement number to reduce noiselevel [5], [6].

In this letter, a new time division multiplexing OTDR(TDM-OTDR) using dual frequency probe is proposed to fur-ther enhance the performance of C-OTDR, which improves themeasurement DR by doubling the signal power and increasingmeasurement number to reduce noise level.

II. PRINCIPLE OF THE TDM-OTDR

The schematic diagram of the TDM-OTDR is shown inFig. 1. External cavity laser diode (ECLD) generates light withwavelength of 1561.42 nm and linewidth of 3.7 kHz. The laserfrom ECLD is split into two paths by a 90/10 coupler. Theone with higher power is used for probe light and another is

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2006 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 24, NO. 22, NOVEMBER 15, 2012

used as LO. A PM driven by sinusoidal RF converts singlefrequency laser to multi-frequency light. The multi-frequencylight output from PM can be described as [5],

E P M = √P0

∞∑

n=−∞Jn(β) cos[2π( f0 + n fm)t] (1)

where P0 is the total output light power from PM, Jn is thenth order Bessel function, f0 is the frequency from ECLD,β and fm are modulation depth and modulation frequencyof PM respectively. The state of polarization of the probelight is adjusted through a polarization controller (PC) to makeinsertion loss of PM minimal. The modulation depth of PMis fixed at 1.81, so according to equation (1) the output powerof PM concentrates on two dominant frequencies—±1st orderfrequencies, each sharing 34% of the total output power, asshown in Fig. 2. A variable optical attenuator (VOA) adjuststhe input light power of AOM1, which up-shifts the frequencyof the input light by 40 MHz and generates probe pulses. Thenthe probe pulses are polarization scrambled by a polarizationscrambler (PS) with scrambling rate of 1000 Hz before beinglaunched into the fiber under test (FUT). The FUT is connectedby two spools of fiber with length of 25.09 km and 49.36 kmrespectively.

Then the probe pulses are launched into the FUT throughan optical circulator (OC). The backscattered Rayleigh lightfR of the multi-frequency probe pulses propagating in theFUT mix with LO in a 3 dB coupler. Heterodyne betweenthe backscattered Rayleigh light and LO generates many IFsthrough balanced photodetector (BPD). A data acquisition card(DAQ) with sampling rate of 100 MHz conducts analog todigital conversion (ADC) for these IFs. Then the obtaineddigital data are processed by parallel digital signal processing(DSP) method using parallel computing toolbox. The DSPprocess includes three steps: digital band pass filtering (BPF),digital down conversion (DDC) and digital low pass filtering(LPF). At last the final OTDR trace is obtained by synthesizingthe power data of these IFs.

To obtain TDM probe pulses with precise frequency coding,an arbitrary waveform generator (AWG) is employed to gener-ate sequential sinusoidal RFs to drive the PM. Both the AWGand AOM1 are synchronously controlled by the trigger frompulse generator. The trigger signals urge the AWG to generatesequential RFs shown in Fig. 3(a), and then the PM outputssequential probe frequencies. Simultaneously the AOM1 gatesthe sequential probe frequencies to obtain probe pulses withpeak power of 4.0 dBm, as shown in Fig. 3(b). The RFsfrom AWG are step changing with duration time of τ andseparation of � f . The sequential RFs that drive the PM rangefrom 28 MHz to 46 MHz with separation � f of 2 MHz forNyquist sampling and digital filtering of IFs. The duration τ ofeach probe frequency is 1 μs with the pulse period T of 1 ms,so the probe pulse width in Fig. 3(b) is 10 μs. The bandwidthof BPF for each IF is 1 MHz, which is less than the separationof two adjacent probe frequencies to avoid crosstalk.

An AOM2 which has the same frequency shift with AOM1is adopted to make the frequency of LO the same withthe 0 order frequency of the multi-frequency probe pulse

0 1 2 3 4 5 60

0.2

0.4

0.6

0.8

1

Modulation depth /Am

Nor

mal

ized

pow

er

|J0(Am|2

|J-1(Am)|2 or |J+1(Am)|2

|J-2(Am)|2 or |J+2(Am)|2

(1.81,0.34)

Fig. 2. Power spectra output from PM at different modulation depths.

(a)

(b)

Fig. 3. Generation of frequency-coded pulses. (a) Sequential modulationfrequencies. (b) Probe pulses from AOM1.

propagating in FUT, so the heterodyne between the ±1stsidebands and LO respectively generates two IFs with the samefrequency [6]. Then the LO can be written as,

EL O = AL O cos[2π( f0 + fAO M )t] (2)

where AL O is the amplitude of LO and fAO M is frequencyshift by AOM2. The backscattered Rayleigh light of theith frequency of the time division multiplexing probe pulsesequence propagating in FUT can be expressed as,

ER =10∑

i=1

+1∑

n=−1

AnR cos[2π( f0 + n f i

m + fAO M )(t + τ − i · τ )

+ ϕin(t + τ − i · τ )] (3)

where AnR is the amplitude of the nth order frequency of

the multi-frequency Rayleigh light from FUT, f im is the ith

modulation frequency of PM and ϕin(t) is a random phase

term. Then the heterodyne process between the backscatteredRayleigh signals and LO can be written as,

i pc = � ∣∣E+

R EL O∣∣2

(4)

where i pc is photocurrent output from BPD, � is a currenttransfer factor. The interaction between backscattered Rayleighsignals with different frequencies can be omitted for their

ZHANG et al.: TIME DIVISION MULTIPLEXING OTDR BASED ON DUAL FREQUENCY PROBE 2007

Fig. 4. OTDR traces corresponding to ten TDM probe frequencies.

power is much lower than that of LO. So the dominant IFsderiving from equation (2) to (4) can be expressed as,

i I F = 2�10∑

i=1

+1∑

n=−1

∣∣An

R

∣∣ |AL O | cos[2πn f i

m(t + τ − i · τ )

+ϕin(t + τ − i · τ )] = 2� ∣

∣AnR

∣∣ |AL O |

×10∑

i=1

{cos

[2πn f i

m (t + τ − i · τ ) − ϕi−1(t + τ − i · τ )]

+ cos[2πn f im(t + τ − i · τ ) + ϕi+1(t + τ − i · τ )]}.

(5)

From equation (5), it can be found that two heterodyne IFs aredirectly synthesized to be one IF with frequency of f i

m . Thisprocess doubles the signal power, compared with conventionalmethod which uses only one probe frequency [5], [6].

The independent OTDR traces corresponding to the ten timedivision multiplexing pulse sequence are as shown in Fig. 4.Then these OTDR traces are synthesized to be one, as shown inFig. 5 (the red line). As noise obeys statistic theory, averagingthe results of a large number of measurements can reducenoise level of OTDR trace, which improves measurement DR[2]–[4]. The DR enhancement can be expressed as,

�DR = 10 lg√

Nd B (6)

where N is a multiple of the primary measurement number.

III. RESULTS AND DISCUSSION

Fig. 5 shows the OTDR traces obtained by conventionalC-OTDR and the TDM-OTDR. As to directly synthesizingthe same IFs, it equals doubling the probe pulse power inconventional method, so the OTDR trace is 3.0 dB higher[5], [6]. Additionally, for the ten time division multiplexingprobe frequencies the measurement number is ten times morethan that by conventional C-OTDR, so from equation (6) thenoise level is 5.0 dB lower [4]. Therefore, in Fig. 5 the DR ofthe TDM-OTDR using ten time division multiplexing probepulses and dual frequency probe is 8.0 dB higher than that ofconventional C-OTDR.

0 10 20 30 40 50 60 70 80 90 95-110

-100

-90

-80

-70

-60

-50

-40

-30

Distance /km

Rel

ativ

e po

wer

/dB

by conventional C-OTDRby TDM-OTDR

5.0dB

3.0dB

Fig. 5. OTDR traces by two C-OTDR schemes.

Fig. 6. Comparison of spatial resolution by two C-OTDR schemes. (a) Firstreflection peak. (b) Second reflection peak.

For comparison of spatial resolution (SR) of the twoC-OTDR schemes, the reflection peaks on the OTDR tracesin Fig. 5 are extracted, as shown in Fig. 6. It can be seenfrom Fig. 6(a) that at position of 25.09 km, both the tworeflection peaks by conventional C-OTDR (blue line) and theTDM-OTDR (red line) are almost the same. For the strong endreflection peaks as shown in Fig. 6(b), both the main reflectionpeaks are nearly overlapping, so by TDM-OTDR the SR is thesame with that by conventional C-OTDR. But in Fig. 6(b) theend reflection peak by TDM-OTDR is broadened, and we inferthat it is due to digital filtering. Filtering IFs with strong powerbrings many sidebands, so sequence adjusting and OTDR tracesynthesizing for each IF broadens the reflection peak.

IV. CONCLUSION

A TDM-OTDR based on dual frequency probe is proposedand experimentally demonstrated. An AWG is employedto generate sequential RF for PM to obtain time divisionmultiplexing probe frequencies. Both the AWG and AOM1are synchronously controlled by the trigger from pulse gen-erator to obtain the time division multiplexing probe pulsesequence. The PM driven by sequential RF outputs multi-frequency light and both the −1st and +1st order frequen-cies are adopted as the probe. In the light path of LO, anAOM2 which has the same frequency shift with AOM1 isused to make the coherent detection between the ±1st orderfrequencies and LO respectively generate two IFs with thesame frequency. Experimental results show that compared

2008 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 24, NO. 22, NOVEMBER 15, 2012

with conventional C-OTDR, the TDM-OTDR using ten timedivision multiplexing probe frequencies can bring an 8.0 dBdynamic range enhancement.

REFERENCES

[1] S. Furukawa, K. Tanaka, Y. Koyamada, and M. Sumida, “Enhancedcoherent OTDR for long span optical transmission lines containingoptical fiber amplifiers,” IEEE Photon. Technol. Lett., vol. 7, no. 5, pp.540–542, May 1995.

[2] J. P. King, D. F. Smith, K. Richards, P. Timson, R. E. Epworth, andS. Wright, “Development of a coherent OTDR instrument,” J. Lightw.Technol., vol. 5, no. 4, pp. 616–624, Apr. 1987.

[3] M. Sumida, “Optical time domain reflectometry using an M-ary FSKprobe and coherent detection,” J. Lightw. Technol., vol. 14, no. 11, pp.2483–2491, Nov. 1996.

[4] H. Iida, Y. Koshikiya, F. Ito, and K. Tanaka, “Ultrahigh dynamicrange coherent optical time domain reflectometry employing frequencydivision multiplexing,” in Proc. 21st Conf. Opt. Fiber Sensors, Ottawa,ON, Canada, 2011, pp. 7753J1–7753J3.

[5] L. Lu, Y. Song, F. Zhu, and X. Zhang, “Dual frequency probe basedcoherent optical time domain reflectometry,” Opt. Commun., vol. 285,no. 10, pp. 2492–2495, 2012.

[6] L. Lu, Y. Song, X. Zhang, and F. Zhu, “Frequency division multiplexingOTDR with fast signal processing,” Opt. Laser Technol., vol. 44, no. 7,pp. 2206–2209, 2012.