This paper presents an improved wavelength coded time-domain reflectometry based on the 2 × 1 optical switch. In this scheme, in order to improve the signal-noise-ratio (SNR) of the beat signal, the improved system used an optical switch to obtain wavelength-stable, low-noise and narrow optical pulses for probe and reference. Experiments were set up to demonstrate a spatial resolution of 2.5m within a range of 70km and obtain the beat signal with line width narrower than 15MHz within a range of 50km in fiber break detection. A system for wavelength-division-multiplexing passive optical network (WDM-PON) monitoring was also constructed to detect the fiber break of different channels by tuning the current applied on the gating section of the distributed Bragg reflector (DBR) laser.
© 2014 Optical Society of America
Distributed optical fiber sensing has been extensively investigated in past decades. And the most widely used approaches are based on the optical time domain reflectometry (OTDR) [1–7]. It has been a very important diagnostic tool for the testing of fiber transmission systems and components since it was first demonstrated by Barnoski and Jensen . Nevertheless, in order to improve the performance of OTDR, many techniques are still proposed, such as polarization OTDR [1,5,8,9], phase-sensitive OTDR , photon-counting OTDR [11–15] and correlation OTDR [16,17].
Besides the new technologies mentioned above, the wavelength coded time-domain reflectometry (OTDR) is a new kind of optical heterodyne technique which is recently developed. The probe and reference optical pulses have different wavelengths to enable the optical heterodyne detection, which can significantly improve the performance of the system. But since the spatial resolution of the system is related with the width of the probe and the reference optical pulse, the width of the probe and reference pulses is usually reduced to 200ns or even shorter . As a result, the distributed Bragg reflector (DBR) laser should be tuned from one wavelength to another wavelength and be stable within 200ns or even shorter. Because of this strict requirement for the DBR laser , the application of the wavelength coded OTDR is limited. Furthermore, the line width of the beat signal is also affected by the instability of the probe pulse and reference optical pulse wavelength.
In this paper, an improved wavelength coded OTDR based on the 2 × 1 optical switch was proposed. In this scheme, the current applied on the phase section of the DBR laser is modulated by a fixed periodic electrical signal. And one optical switch is adopted to get the narrow probe and reference optical pulses. In every detecting period, the output light of the DBR laser can pass through the optical switch after the DBR laser has established the needed wavelength and has been stable. Therefore, the tuning speed of the DBR laser has no relationship with the line width of the beat signal. Then the system has no requirement for the tuning speed of the DBR laser, and the line width of the beat signal is narrow. Finally, the experiments were set up to demonstrate the ability of the wavelength coded OTDR system. In the first experiment, the fiber breaks at the fiber distance of 50km and 70km at the corresponding to the FC/PC connector at the end of the fiber were detected by the system. In the second experiment, the wavelength coded system was used to monitor the wavelength-division-multiplexing passive optical network (WDM-PON) and the fiber breaks at the channels and the COM port of the arrayed waveguide grating (AWG) were successfully detected.
2. Dynamic characteristic of the DBR laser
2.1 Experiment architecture
Preparing for the improved wavelength coded OTDR system, an experimental system was setup to demonstrate the dynamic characteristic of the DBR laser used in the system, as shown in Fig. 1. A square waveform voltage was applied to the phase section of the DBR laser. Corresponding to the high voltage and the low voltage, the laser generates two different light waves at different time. A 50resistor was used between the waveform generator and the laser to limit the current of the phase section. Coupler 1 was employed to separate the light from the DBR laser into two paths, and delayed fiber was inserted into one path, then the light of the two paths was coupled into the photo-detector by coupler 2. The period of the square waveform was set as:
2.2 Measurement for dynamic characteristic of DBR laser
Since the frequency of the beat signal varies with time, it is necessary to observe the dynamic characteristic of the laser in the time-frequency domain. At present, short time Fourier transform and wavelets analysis are the main methods for the time-frequency domain analysis. Comparing with wavelets analysis, the frequency resolution of STFT is fixed and the arithmetic of STFT is simple , so it is more specifically suited to our application.
Figure 2 shows the STFT result of the beat signal using a 1024 point Hamming window. The overlap was set as 768 points. From Fig. 2, it can be seen that, the wavelength of the output laser slowly increases after the high voltage has been applied on the phase section of the DBR laser, and with the increasing of the wavelength, the line width of the beat signal and the noise level decreases together. The output light is stable after 80, so the DBR laser cannot be used in the classical wavelength coded OTDR which uses 200 ns or even shorter optical pulse as the probe and reference signal in the experiment . Therefore, the improved system proposed can avoid the restrict requirement of the laser and expand the application range of the wavelength coded OTDR.
3. System ability
3.1 System architecture
The improved wavelength coded OTDR was set up to locate the break of the tested fiber as shown in Fig. 3. A Michelson interferometer was employed in the system. The DBR laser was chosen as the wavelength coded light source. A square waveform provided by waveform generator 1 was applied to the phase section of the DBR laser. The DBR laser was connected to the port 1 of the 2 × 1 optical switch, and a tunable laser was connected to the port 2 of the optical switch. And the optical switch was controlled by waveform generator 2. The output of the optical switch was split into two paths by optical fiber coupler 1. One path was amplified by the EDFA and then launched into coupler 3 as the probe optical signal, and the other path is used as the reference optical pulse. A fraction of the probe signal reflected by the break in the tested fiber was recombined with the reference light by coupler 2, and then launched into a high-speed photo-detector.
The operation principles of the reflectometry are described by the schematic in Fig. 4. The wavelength of the output light from the tunable laser was and the wavelength of the output light from the DBR laser switched between and . The wavelength of DBR laser and the tunable laser were chosen in such a way that the wavelength difference between and was so large that the beat signal was far beyond our observing frequency range and the frequency difference between and was set as at 1.06 GHz. In addition, the optical signal at was used to suppress the noise of EDFA . The delay time between the positive edges of waveform 1 and the controlling signal of optical switch must be set larger than 80, which can enable the DBR to establish the stable wavelength. Using this method, high-speed detection with low tunable speed DBR laser can be achieved. While keeping the period of waveform 1 fixed, vary the delay time between and . If the 1.06 GHz signal appears, then the fiber break is located. It is obvious that the beat signal appears when
3.2 Results of fiber break detection and analysis
Figure 5(a) shows the measured spectra of the beat signal when the break locates at approximately 50 km, in addition, the delay time has been converted to the fiber distance. Figure 5(b) shows the beat signal at 50503.5m from which it can be seen that the line width of the beat signal is 15MHz, narrower than the classical wavelength coded OTDR. And the beat signal with narrower line width can be detected within narrower bandwidth, so the noise level can be reduced and the sensitivity can be improved significantly. Figures 5(c) and 5(d) shows the measured spectra at 70km from which it can be seen that the envelope of the peak values shifted when a 2.5m fiber was inserted at the end of the tested fiber. Therefore a spatial resolution of 2.5m was obtained within the range of 70km, and the 2.5 m is the minimum error value of 70 km fiber in this system. The measurement error mainly results from the fluctuation of the fiber length due to the temperature fluctuation and the optical pulse broadening during the propagation in the 70km tested fiber. In conclusion, comparing with the classical wavelength coded OTDR, both the resolution and the measuring range of the system have been improved .
3.3 WDM-PON monitoring
In this section, an experiment was set up to demonstrate the performance of the WDM-PON monitoring system based on the wavelength coded OTDR. The architecture of the experiment is shown as Fig. 6. In Fig. 6(a), the wavelength coded OTDR was connected to the third channel of the AWG for the detection of the fiber break of the COM port. By adjusting the tuning current of applied on the gating section of the DBR laser, the central wavelength of the output laser can be set as 1557.361nm, which is the central wavelength of Channel 3 of AWG. In Fig. 6(b), the wavelength coded OTDR was connected to the COM port of the AWG. Similarly, by adjusting the current applied on the gating section of the DBR, the output laser with the different wavelength can be obtained, and the fiber break detection of the corresponding channel can be realized. Figure 7 shows the measurement results of the WDM-PON monitoring experiment, and from Fig. 7 it can be seen that the clear peak on the power spectrum at 15 km on Channel 1, at 16 km on Channel 2 and at 25 km on the COM port respectively. The relationship between the wavelength of the laser and the tuning current is shown as Fig. 7(d).
In this paper, an improved wavelength coded optical time domain reflectometry which used a low speed tunable DBR laser to achieve fiber break detection was proposed. In the scheme, 15MHz line width beat signal at 50km and a 2.5m spatial resolution at 70km was obtained by detecting the G605 fiber break. The system can be used to detect the fiber break by observing the beat signal at specified frequency and specified time; therefore it is possible to make further efforts by do some signal processing to improve the ability of the system. And an experiment was also carried to demonstrate the ability of being the tuning-OTDR for WDM-PON monitoring. Furthermore, the paper also provides a method to evaluate the dynamic characteristic of the DBR laser based on STFT as described in section 2.
This work was supported in part by the National Natural Science Foundation of China under Grants 61106069, 61021003, 61090391, 61377070, 61307084, 61275079, 61275078, and by the National Basic Research Program of China under Grants 2014CB340102 and 2012CB315702.
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