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In this paper, we propose and demonstrate a millimeter-resolution long-range optical frequency domain reflectometry (OFDR) using an ultra-linearly 100-GHz swept optical source realized by injection-locking technique and cascaded four-wave-mixing (FWM) process. The ultra-linear sweep is realized using an external modulation method with a linearly swept radio frequency (RF) signal. The RF signal sweeps from 16 GHz to 19.3 GHz, and the slave laser is injection-locked to the 8th-order sideband of the master laser, achieving a frequency sweeping span of ~25 GHz. By using the injection-locked frequency-swept laser as the optical source of OFDR, we obtain a spatial resolution of 4.2 mm over 10-km measurement range. A polarization beat length of 10.5 cm is measured benefiting from the high spatial resolution. To improve the spatial resolution further, FWM process is used to broaden the frequency sweeping span. Frequency sweeping span of ~100 GHz is achieved with cascaded FWM. We demonstrate a 1.1-mm spatial resolution over 2-km measurement range with the proposed ultra-linearly swept optical source.
© 2017 Optical Society of America
1. Introduction
Optical frequency domain reflectometry (OFDR) is now attracting considerable attention for non-invasive diagnoses of fiber networks [1], and for distributed optical fiber sensing because of its high spatial resolution [2–4]. Especially, for the fiber to the home (FTTH) access networks, as bend insensitive fiber is widely installed in FTTH networks, the bending in optical fiber links is very difficult to locate by using reflectometry because of the low bending loss, which shortens the service lifetime of the optical fiber if the bending is ignored and just as it is. In order to locate the bending position, a reflectometry with both high spatial resolution and relatively long measurement range is required. Usually, a millimeter spatial resolution is necessary to locate the bending, and a measurement range of several kilometers is required for the FTTH applications.
OFDR provides high spatial resolution, which is inversely proportional to the frequency sweeping span of the optical source. For this technique, a broader span helps to obtain a higher spatial resolution [5]. Currently, the realization of optical frequency sweeping can be achieved by inner-cavity tuning of a tunable laser, or external modulation of a continuous wave (CW) lightwave. A spatial resolution of 22 micrometer has been reported via a wavelength tunable laser with a wide sweeping span of 40 nm [6]. However, its measurement range is limited since the inner-cavity tuning brings a broad spectral linewidth and a bad sweeping linearity. On the other hand, one can use external modulation with frequency-swept radio frequency (RF) signal to sweep the optical frequency ultra-linearly, and several centimeters spatial resolution over 5 km has been obtained by using this method [7]. A synthesized RF frequency sweeper with a wide sweeping span is essential to realize high spatial resolution in this method. However, it is very difficult to obtain such a continuous frequency sweeper since there are always many breaks during a wideband sweeping. High-order sideband modulation technique has been proposed to broaden the optical frequency sweeping span [8], and a centimeter-level resolution is obtained with an originally narrow RF sweeping span. However, the span is still limited because the high-order sidebands overlap with each other when it exceeds the frequency interval between two adjacent sidebands, making millimeter-resolution very difficult to be realized for long-range application.
In this paper, we introduce the injection-locking technique to extract the frequency-swept lightwave from the overlapped high-order sidebands. Usually, an optical filter is required to follow the sweep, which is very difficult to realize. The injection-locking technique cannot only provide a same filtering effect using a synchronous coarse current adjustment to follow, but also provide an amplification effect [9, 10]. The RF signal sweeps from 16 GHz to 19.3 GHz, and the slave laser is injection-locked to the 8th-order sideband of the master laser. We have obtained a frequency sweeping span of ~25 GHz, which corresponds to a spatial resolution of 4 mm in OFDR theoretically. In this way, we realize a spatial resolution of 4.2 mm, which is near 4 times higher than the results obtained in Ref. 7. Moreover, phase-noise-compensation (PNC) technique is introduced to mitigate the effect of phase noise which is magnified by high-order sideband modulation [11, 12], and the measurement range is extended to 10 km. To our best knowledge, this is the highest spatial resolution in long-range OFDR over 10 km. By using this technique, a polarization beat length of 10.5 cm is measured benefiting from the high spatial resolution. To further break the limit of electronic bottlenecks imposed by present electronics and commonly used modulators [13], a cascaded four-wave-mixing (FWM) process is introduced to broaden the span to ~100 GHz [14, 15]. A spatial resolution of about 1 mm over 2-km measurement range is firstly reported.
2. Principle
In an OFDR system, the spatial resolution is given by
where c and n are the light velocity in vacuum and the effective refractive index of fiber core, and ∆F is the frequency sweeping span. According to Eq. (1), a narrower spatial resolution requires a larger ∆F. By utilizing the high-order sideband modulation, an enlarged span can be obtained. However, it is still limited because of the overlap between two adjacent high-order sidebands as shown in Fig. 1. In order to further broaden the frequency sweeping span, injection-locking technique is introduced to extract the frequency-swept lightwave from the overlapped high-order sidebands.
Fig. 1 The schematic illustration of the spectrum for the frequency sweep with high-order sidebands of external modulation.
The electric field of the optical comb (master laser) which is generated by high-power RF modulation can be decomposed into its Fourier components
where Jn are n-th order Bessel functions of the first kind, β is the frequency modulation index, v0 is the frequency of unmodulated optical carrier, fm is the frequency of the RF signal, and θ(t) is the random phase fluctuations of the optical carrier. When the slave laser is injection-locked to the K-th sideband, its emission fields can be given byEquation (3) means that the desired sideband can be obtained by utilizing injection-locking technique. Different from the conventional optical bandpass filter to extract the necessary wavelength, the center wavelength of the injection-locked slave laser can be changed very quickly by controlling its working current, which means that a synchronous current control to the slave laser may maintain a stable optical injection locking to follow the frequency sweep.
The injection-locking technique has solved the problem of overlap between two adjacent sidebands and broadened the frequency sweeping span. However, the frequency sweeping span is still limited by the electronic bottleneck. An effective solution is to use all-optical processing technique, for example, using optical nonlinear effect to generate new frequency components and further broaden the sweeping span. Let us consider the degenerated FWM process, the generated first-order idler has an electric field Ei which can be described by
where ωp, ϕp and Ap are respectively the frequency, phase and amplitude of the pump. ωs, ϕs and As are respectively the frequency, phase and amplitude of the seed, and C is a constant related to FWM efficiency. After the FWM process, the pump, the seed and the first-order idler (idler-1) will have a frequency relationship given by ωi-1 = 2ωp-ωs. Considering a pump with a frequency sweeping span of ∆ω, the obtained idler-1 will have a frequency of 2(ωp + ∆ω)-ωs. Therefore, the frequency sweeping span of the idler-1 increases to double the pump sweeping span, as shown in Fig. 2(a). With a cascaded configuration to obtain multi-stage FWM, the frequency sweeping span can be further broadened. As shown in Fig. 2(b), the idler-1 generated by the first stage FWM is filtered as the pump for the second stage FWM. Similarly, the idler-1 of the second stage FWM will have a further doubled frequency sweeping span.Fig. 2 The schematic illustration of the principle. (a) Broadening the frequency sweeping using first stage FWM; and (b) using second stage FWM.
Note that the high-order sideband modulation and FWM have a capability of broadening the frequency sweeping span, but it will also magnify the laser phase noise and bring more phase noise if the RF sweeps nonlinearly. In order to mitigate the effect of the phase noise and extend the measurement range, a PNC technique is also introduced in the system [11, 12].
3. Experiments
3.1 Injection-locking scheme
The detailed experimental setup of the proposed injection-locking scheme is shown in Fig. 3. A narrow-linewidth fiber laser (NKT, Adjustik E15, linewidth < 1 kHz) is employed as the optical source and then connected to an intensity modulator (IM), which has a low Vπ. Therefore, with a high-power RF modulation, many sidebands can be produced and a wideband optical comb can be obtained. The optical comb is used as the master laser in and then injected into a distributed feedback (DFB) diode laser (linewidth ≈3 MHz), which does not have an isolator inside and serves as the slave laser. In order to acquire high injection ratio, the polarization state of the injecting lightwave is adjusted properly. As mentioned in Ref. 10, the injection-locked slave laser provides a narrow linewidth (nearly equal to the master laser), which means a long measurement range in OFDR system. Besides, ultra-linearly frequency sweep can be achieved by sweeping the frequency of the RF signal applied on the modulator. In our experiments, the RF signal sweeps from 16 GHz to 19.3 GHz, and the current applied on the slave laser should be coarsely tuned synchronously to make sure its frequency follows in tight with the sideband wish to be locked. Then, the injection-locked frequency-swept lightwave is used as the optical source of OFDR, to be launched into both the main interferometer and the auxiliary interferometer, which is used to compensate the magnified phase noise of the optical source. Since the PNC algorithm is same as what used in Ref. 11 and Ref. 12, we only give a brief introduction here. Two beat signals can be obtained from the main interferometer (scattered signal) and the auxiliary interferometer (reference signal). Considering the scattered signal sampled based on timing that corresponds to certain increments in the phase term of the reference signal, for example π increments, the phase noise term can be given by
where θ(t), τFUT, τREF are the random phase fluctuation caused by phase noise of optical source, the time delays of the main interferometer and the auxiliary interferometer, respectively. It is obvious that the phase noise term Φ(t) is cancelled out when τREF is equal to τFUT. By using this method, the phase noise can be compensated when the measurement distance is shorter than the coherence length of optical source. For reflection events beyond the coherence length, a concatenately generated phase can be used to generate the required reference phase signal for the compensation [11, 12].Fig. 3 Experimental setup of the injection-locking scheme. FL: fiber laser; IM: intensity modulator; VOA: variable optical attenuator; PC: polarization controller; DFB: distributed feedback diode laser; Amp: RF amplifier; AWG: arbitrary waveform generator; FUT: fiber under test; BPD: balanced photodetector; A/D: analog-to-digital converter.
The output optical spectrum of the slave laser which is injection-locked to the 8th-order sideband of the master laser is shown in Fig. 4(a), and the inset figure shows the original optical comb after the IM. The suppression ratio between the injection-locked sideband and other sidebands is 30 dB, and the output power is 9 dBm, which is over 30 dB higher than that obtained directly from the IM without using injection-locking technique. When the optical frequency is swept, the relative optical frequency can be obtained by counting the cycles of the sinusoidal beat signal generated from the auxiliary interferometer. As shown in Fig. 4(b), the relative optical frequency of the injection-locked sideband changes as a function of time linearly, and the actual locking area is ~25 GHz, which corresponds to a 4-mm theoretical spatial resolution in OFDR. In order to reduce the optical loss caused by bending, a single mode bend insensitive fiber which is made by YOFC (Wuhan, China) is used as the fiber under test (FUT). The length of the FUT is about 10 km, and the detailed parameters are shown in Table 1. Figure 5(a) shows the measured reflection trace. The spatial resolution of the reflection event at the end of the FUT is 4.2 mm after using PNC algorithm.
Fig. 4 (a) Optical spectrum of the slave laser which is injection locked to the 8th-order sideband of the master laser, and the inset is spectrum of the generated optical comb after IM; (b) Relative optical frequency as a function of time after the injection-locking.
Table 1. The parameters of the FUT used in the system.
Fig. 5 (a). Measured reflection trace; (b) Details of reflection peak around 10 km after using PNC algorithm.
Besides reflection events, the bending of optical fibers is another important issue, since it fatigues the optical fiber and shortens its service lifetime. Our proposed OFDR system is suitable for locating the bending positions in FTTH network because of its high spatial resolution and long measurement range. In our experiment, the far end of the FUT has been wounded on mandrels with radii of 4.2 cm, 1.7 cm, 1.2 cm, and 0.65 cm. In order to mitigate the fading effect caused by interference of Rayleigh signals within spatial resolution, a wavelength shift averaging process is conducted in the system. The central wavelength of the master wavelength is shifted from 1549.5 nm to 1550.5 nm with a step of 0.25 nm, which is wider than the frequency sweeping span of the master laser (25GHz). The wavelength of the slave laser is varied synchronously by changing the control current to maintain stable injection-locking. The traces are averaged 50 times (10 times at each wavelength and 5 wavelengths are used). Figures 6(a)-6(c) show the intensity of Rayleigh backscattered signals when the fiber is kept straight, bent with radii of 1.2 cm and 0.65 cm, respectively. Figure 6(d) shows the frequency spectra of Rayleigh signal when the radii are 4.2 cm, 1.7 cm, 1.2 cm, and 0.65 cm. The polarization beat length can be subsequently extracted from these results. Here, due to the roundtrip of backscattered Rayleigh signal, the beat length is obtained as twice as the observed spatial period of variations.
Fig. 6 Intensity of Rayleigh backscattered signals when the fiber is (a) kept straight, (b) bent with a radius of 1.2 cm, and (c) bent with a radius of 0.65 cm; (d) The frequency spectra of Rayleigh signals when the bending radii are 4.2 cm, 1.7 cm, 1.2 cm, and 0.65 cm; (e) Measured polarization beat length versus the square of mandrel radii.
In theory, the beat length can be calculated as [16, 17]
where λ, R, p, v and r are the wavelength of the light, the radius of the mandrel, the strain-optical coefficient, the Poisson’s ratio, and the radius of the fiber cladding, respectively. Figure 6(e) verifies the linear relationship between LB and R2, and a beat length of 10.5 cm at the distance of 10 km has been measured.3.2 Cascaded FWM scheme
The injection-locking scheme achieves a frequency sweeping span of ~25 GHz, but still limited by the operating bandwidth of the IM and the RF amplifier. In order to further broaden the sweeping span, degenerated FWM is introduced in the proposed system. As shown in Fig. 7, the injection-locked frequency-swept lightwave is amplified by an erbium-doped fiber amplifier (EDFA) and then filtered by a bandpass filter (BPF). Another high power fiber laser, which is divided by a 50:50 optical coupler, is used as the seed of cascaded FWM. The optical power after BPF1 is around 20 mW, and the output power of the seed laser is 180 mW. The optical spectrum of the first stage FWM is shown in Fig. 8(a). The frequency sweeping span of the idler-1 is increased to double the pump sweeping span. Idler-1 is then filtered as the pump for the second stage FWM, and another EDFA is used to boost its power.
Fig. 7 Experimental setup of the cascaded FWM scheme. FL: fiber laser; IM: intensity modulator; VOA: variable optical attenuator; PC: polarization controller; DFB: distributed feedback diode laser; Amp.: RF amplifier; AWG: arbitrary waveform generator; BPF: bandpass filter; HNLF: highly nonlinear fiber; FUT: fiber under test; BPD: balanced photodetector; A/D: analog-to-digital converter.
The optical spectrum of the second stage FWM is shown in Fig. 8(b). The idler-1 of the second stage FWM has a broad frequency sweeping span, which is 4 times magnified compared with the original pump. Then it is filtered by BPF4 and used as the optical source of the OFDR system.
As shown in Fig. 9(a), the relative optical frequency of the idler-1 changes as a function of the time linearly. The frequency sweeping span of the idler-1 is ~50 GHz after the first stage FWM and ~100 GHz after the second stage FWM. Figure 9(b) shows the frequency residual errors after linear fitting, and the standard deviation is 160 kHz for the ~100 GHz sweeping span, which implies a nonlinearity of . Since the injection-locking process does not deteriorate the coherence of the master laser, and the deteriorate factor in FWM process is only 2 for the first-order idler and 3 for the second-order idler, the new source provides a narrow spectral linewidth (< 10 kHz) as we use a narrow linewidth fiber laser as the original light source. For conventional tunable lasers, the linewidth is normally several hundred kHz or even MHz level when it is swept.
Fig. 9 (a) Relative optical frequency changes as a function of time; (b) The frequency residual errors.
In the OFDR system, a 2-km SMF is used as the FUT. Figure 10 shows the measured reflection trace, and we have obtained a spatial resolution of 1.1 mm at the far end of the FUT. For conventional tunable lasers as the optical source of OFDR system, the measurement range may be much limited since PNC algorithm will have more accumulated errors coming from the difference of the measured and the real length of the delay fiber for same distances, limiting the measurement range to a relatively short range. The measurement range of the proposed system is now limited by the memory of the analog-to-digital converter (ADC), which is very promising to be further expanded. We believe that the proposed system is suitable for applications where both high spatial resolution and relatively long measurement range are required.
Fig. 10 Measured reflection trace, and the inset shows the details of reflection peak at the end of the FUT.
4. Summary
In this work, we demonstrated a high spatial resolution long-range OFDR by using an ultra-linearly swept optical source, which is realized by high-order sideband modulation technique and injection-locking technique. A spatial resolution of 4.2 mm over 10-km measurement range is obtained, and a polarization beat length of 10.5 cm is measured benefiting from the high spatial resolution. Moreover, cascaded FWM process is introduced to further broaden the frequency sweeping span. We realized a spatial resolution of 1.1 mm over 2-km measurement range. Comparing with the product (OBR 4600) developed by Luna, which provides a spatial resolution of 3 mm over 2-km measurement range, we improved the spatial resolution about three times. It also provides a possibility to achieve an OFDR system with millimeter-level spatial resolution over a few tens of kilometer measurement range since it is now only limited by the memory of ADC used in our experiment. The proposed high spatial resolution long-range OFDR system has potential applications in many fields, for example, health monitoring of FTTH access network.
Besides, we achieved an ultra-linearly swept laser with frequency sweeping span of ~100 GHz, and the standard deviation of the frequency residual errors is 160 kHz, implying a sweep nonlinearity of . The ultra-linearly 100-GHz swept laser can also be used in imaging laser radar and other applications which require broadband and ultra-linear frequency sweeping.
Funding
This work was supported by the National Natural Science Foundation of China under Grant 61327812, Shanghai STCSM Scientific and Technological Innovation Project under Grant 15511105401.
Acknowledgments
We would like to thank Guangyao Yang and Qingwen Liu for their helps in the experiments.
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