Distributed Raman amplification (DRA) based on ultra-long fiber laser (UL-FL) pumping with a ring cavity is promising for repeaterless transmission and sensing. In this work, the characteristics (including gain, nonlinear impairment and noise figure) for forward and backward pumping of the ring-cavity based DRA scheme are fully investigated. Furthermore, as a typical application of the proposed configuration, ultra-long-distance distributed sensing with Brillouin optical time-domain analysis (BOTDA) over 142.2km fiber with 5m spatial resolution and ± 1.5°C temperature uncertainty is achieved, without any repeater. The key point for the significant performance improvement is the system could offer both of uniform gain distribution and considerably suppressed pump-probe relative intensity noise (RIN) transfer, by optimized design of system structure and parameters.
© 2013 OSA
In optical fiber communication and sensing, Erbium-doped fiber amplifier (EDFA) and distributed Raman amplification (DRA) are two crucial signal amplification schemes [1–3]. EDFA can counteract the intrinsic loss of the transmission fiber, but at the cost of reduced signal-to-noise ratio (SNR). DRA can provide much lower noise figure (NF) than EDFA, as it is based on spatially distributed gain along the fiber . Moreover, DRA could realize wide-band and controllable gain profile through multi-wavelength pumping .
To obtain a better trade-off between the nonlinear impairment and amplified spontaneous emission (ASE) noise accumulation , high-order DRA using multiple pumps was proposed since it has more constant gain distribution [5,6]. In recently years, an alternative 2nd-order DRA based on an ultra-long fiber laser (UL-FL) pumping with a linear cavity formed by a pair of fiber Bragg gratings (FBGs) at 1455nm was proposed and paid considerable attentions [7–10]. The combined facet-end feedback and Rayleigh backscattering play significant roles for lasing [7–14]. Compared with conventional 2nd-order DRA using externally injected 2nd-order pump [5, 6], this approach provides a simplified and cost-effective system design. If one (two) FBG is removed to form a linear half-open (open) cavity, the random fiber laser (RFL) could be generated through pure Rayleigh backscattering [15–17]. For RFL, photons propagating in a long fiber are scattered by random refractive index inhomogeneities defined by material and fabrication imperfections. Complying with Rayleigh’s law, the partial reflections are random in space both for amplitude and phase, interfering coherently with each other, thus leading to the random distributed feedback . Experimental investigations on the RFL-based DRA  and sensing applications [19–21] have been performed by the authors and A. M. R. Pinto et al..
The 2nd-order pumping can also be formed by a ring cavity configuration, firstly proposed and demonstrated for long-distance optical transmission by A. J. Stentz and T. N. Nielsen et al. [22, 23]. In this paper, the gain, nonlinear impairment, and noise figure characteristics for the ring cavity scheme were explored. As a typical sensing application, the approach was applied to signal enhancement in ultra-long-distance distributed sensing using the Brillouin optical time-domain analysis (BOTDA) [24–31]. 142.2km sensing range with 5m spatial resolution and ± 1.5°C temperature uncertainty was demonstrated, without using any repeater, for the first time. Compared with our recent experimental results of BOTDA using RFL-based DRA with a linear cavity , ~20km distance extension was achieved, while keeping the same magnitude order of spatial resolution and temperature uncertainty. Such a substantial upgrade is mainly attributed to the optimized ring cavity design, thereby the gain is pushed inside the fiber more deeply and the pump-probe relative intensity noise (RIN) transfer is reduced. These improvements are beneficial for achieving higher SNR and measurement accuracy towards the far end of fiber.
2. DRA based on UL-FL with a ring cavity
2.1 System structure
The experimental arrangement of the system for the DRA using UL-FL with a ring cavity is shown in Fig. 1. The 1454/1462nm laser was produced in a ring cavity by splicing the port of wavelength-division-multiplexers (WDMs) with transmission wavelength centered at 1455nm. The feedback for lasing arises from the ultra-long ring cavity and Rayleigh back-scattering. Amplification of the laser is provided by a 1366nm primary pump (high-power fiber Raman laser). The 1454/1462nm laser acts as the 2nd-order pump for 1550nm signal. The two wavelength generation results from the intrinsic dual-peak structure of Raman gain spectrum [3, 32].
The 1550nm signal from a distributed feedback (DFB) laser was injected into the standard single-mode fiber (SMF) through WDM. 142.2km fiber was spliced to the common (COM) port of WDMs. Two types of amplification scheme (forward and backward pumping) were tested. For backward pumping, the 1550nm signal was counter-propagating with the 1366nm primary pump. An optical time-domain reflectometry (OTDR) was used to measure the gain distribution. Finally, an optical spectrum analyzer (OSA) was used to record the lasing spectrum, on-off gain, and noise figure (defined as the degradation in SNR due to ASE noise [3, 18]).
2.2 Gain and distribution
Figure 2(a) shows the measured lasing spectrum for various pump powers. At the onset point of lasing, the lasing is generated at the first Raman gain peak near 1454nm (see red curve). With the increased Raman pump power, the peak of 1454nm lasing is gradually decreased because of Raman pumping from the short-wavelength band to another Raman gain peak near 1462nm. The laser near 1462nm is broadened considerably due to turbulent four-wave mixing (FWM) among a large amount of lasing modes over 142.2km fiber (since multiple waves interact with each other via multiple quasi-degenerate FWM processes in a long cavity, the amplitude and phase of each longitudinal mode evolve with time and space in a stochastic (turbulent) way, for more details, see Refs [12, 13].). Particularly, due to the absence of narrow-band filter in the designed system (the pass-band width for 1455nm port of WDM is ~20nm), the lasing efficiency is immune from the nonlinear spectral broadening, hence high efficiency lasing is ensured. This is quite different with the DRA based on UL-FL in a linear cavity, wherein the narrow-band filters of FBG (<0.5nm typically) would result in the energy leak out of its pass-band, thus leading to considerable lasing efficiency degradation .
Figure 2(b) shows the measured on-off gain as function of input power of the primary pump. In the measurement procedure, the 1550nm signal power coupled into the fiber was −5dBm. The primary pump power was adjusted to obtain the given on-off gain. It is found that, at pump powers larger than the lasing threshold point (~30.9dBm), the on-off gain is linearly increased with the increased primary pump. Because the averaged lasing power near 1462nm is the same for both of forward and backward Raman pumping, the on-off gain is similar for the same input pump power. Considering the loss of 0.193dB/km at 1550nm over the 142.2km fiber, splicing loss of ~1dB in the span, and coupling loss of ~0.5dB for WDMs, the on-off gain for transparency transmission is ~29dB, which is reached at ~34.2dBm input for 1366nm pump.
Figure 3 gives the measured gain distributions of backward and forward pumping. From the comparison of forward and backward pumping for the same pump power, the gain of forward pumping is pushed within the fiber more deeply, this is very important for enhancing the SNR at worst contrast regime (near ~110-130km) for distributed sensing application, leading to a much equilibrant sensing accuracy over the fully fiber. Additionally, the worsened nonlinear effect is shown for forward pumping. The impact of nonlinearity for long-distance optical transmission could be reduced by various approaches. For example, the nonlinearity of FWM could be alleviated by means of non-zero dispersion shift fiber (NZ-DSF).Typically, the group velocity dispersion (GVD) is in the range of 4-8ps/(km-nm) in such fibers to ensure that the FWM-induced crosstalk is minimized . The enhanced stimulated Brillouin scattering (SBS) threshold can be realized by broadened spectrum based on phase modulation with frequency range of 200-400MHz typically . The use of large effective area fiber (LEAF) is also an efficient way to reduce the nonlinear effect .
To give a systematical investigation, numerical simulation was performed based on a set of equations , including the terms of Raman gain, Rayleigh backscattering, fiber loss and ASE. Note that the boundary conditions of lasing light in ring cavity differ with that in linear cavity:17]. Besides, C is ~0.8, and ~1dB splicing loss is considered (occurs at ~94.5km for forward pumping, or 47.7km for backward pumping).
Figure 4(a) depicts the simulated power and gain distributions at transparency transmission for forward pumping. The forward 2nd-order pump (from z = 0 to z = L) firstly experiences an increasing process due to the 1st-order pump. With the depletion and fiber attenuation of the 1st-order pump, the forward 2nd-order pump tends to be reduced. The decreased backward 2nd-order pump (from z = L to z = 0) is caused by fiber loss, except an amplification process near the input end is experienced by the 1st-order pump. The signal gain is determined by the total power of forward and backward 2nd-order pump. There is a good agreement for gain distribution from theoretical and experimental comparison (see curves c1 and c2). Figure 4(b) reports the case of backward pumping. Again, the simulated gain distribution fits the experimental data well.
In the above discussions, the 1st-order pump is provided by primary pump at 1366nm, and the 2nd-order pump refers to lasing light at 1462nm.
2.3 Nonlinear impairment and effective noise figure
For the transmission span with the same signal input power, the accumulated nonlinear effect can be quantified through the ratio RNL of averaged power :3]. The measured result is summarized in Fig. 5(a). It is shown that the nonlinear impairment for both of forward and backward pumping deteriorate with increased on-off gain. However, the impairment is lower for backward pumping by ~0-2dB because of smaller path-averaged power, as shown in Fig. 3.
Figure 5(b) shows the measured effective noise figure (ENF) as function of the on-off gain. To enable the direct comparison with EDFA, the ENF is used by eliminating the impact of fiber loss [3, 18]. It is found that, the ENF of forward pumping is lower than that of backward pumping by amount of 2~3dB. The improvement is enhanced slowly with the increased on-off gain. This can be understood by noting that, the ASE generated near the input end experiences losses over the full length of the fiber for forward pumping, whereas it experiences only a fraction of such losses for backward pumping . Mathematically, for the same gain of G(L), G(z) in following formula for ENF  is larger in the forward pumping scheme, resulting in reduced ENF:3,18], α is the fiber loss coefficient for signal.
3. 142.2km BOTDA using UL-FL based DRA with a ring cavity
3.1 Experimental setup
As a typical sensing application, we explored the sensing signal amplification over a long-distance BOTDA shown in Fig. 6. A tunable laser source at 1550nm with 3 KHz line-width and 12dBm output power was split into Brillouin pump and probe lights using a 90:10 coupler. As the peak Brillouin gain is proportional to a factor of 1/(1 + Δνp/ΔνB) (Δνp and ΔνB represent the linewidth of optical source and intrinsic width of Brillouin gain, respectively), the condition ofΔνp<<ΔνB (~35MHz) should be satisfied . The 90% portion was amplified by an Erbium-doped fiber amplifier (EDFA2), and then routed to an acoustic-optic modulator (AOM) with 45dB extinction ratio. Simplex-coding [20, 24, 31] was used to further improve the SNR. The AOM was driven by electrical pulse trains after coding provided by an arbitrary waveform generator (AWG). The return-to-zero (RZ) codes were used to realize the sufficient gain recovery due to ~10ns phonon response time [20, 24, 31].The length of coded pulses was 255 bits, corresponding to ~9.1dB coding gain [24,31]. The coded pulses with 50ns pulse-width and 150ns period were used, corresponding to 5m spatial resolution. A polarization-scrambler (PS) was used to eliminate the polarization-dependent gain fluctuation. Note that the EDFA2 was placed before AOM to avoid the decoding error induced by gain saturation of coded pulse trains. Since the power before EDFA2 remains constant, no gain and power fluctuation due to transient effect is induced. This is the basic requirement for linear decoding process .
The probe beam was intensity-modulated by an electro-optic modulator (EOM) driven by a 10-12GHz microwave generator to create two modulation sidebands, while suppressing the carrier (~30dB) by properly setting the direct current (DC) voltage of the EOM. The probe wave was injected after EDFA1 and acoustic-optic frequency-shifter (AOFS).Two variable optical attenuators (VOA) were used to optimize the Brillouin pump and probe input powers. The output of probe wave from a circulator (CIR1) was firstly pre-amplified by EDFA3 with adjustable gain, and then filtered by a FBG (<0.1nm band-width, to exclude the residual lasing pump, Rayleigh back-scattering and anti-Stokes components).
The Brillouin gain spectrum (BGS) was acquired by a 125MHz photo-detector (PD) after VOA3 (the input power of PD was set at ~-14.5dBm to avoid saturation), by sweeping the microwave generator near Brillouin frequency shift (BFS). The sensing fiber manufactured by Corning with 142.2km composes of three spools with slightly different BFS (~10.875, 10.880, and 10.882GHz, respectively). Note that this BFS difference was well within the width of BGS (~39-56MHz), ensuring the maximal Brillouin interaction. ~5m fiber at 142.152km closing to the lowest SNR due to the noise accumulation was heated inside a temperature controlled chamber. The temperature variation of the hot-spot was 40°C. The sampling rate of analog-to-digital (A/D) converter was 100 MSa/s, corresponding to 1m spatial sampling period. The fast Hadamard transform (FHT) was utilized to obtain the decoded traces.
3.2 Design considerations
For long-distance BOTDA with DRA, the impact factors mainly include: (1) pump-probe RIN transfer [29, 30]; (2) non-local effect caused by Brillouin pump depletion [26–28]; (3) spectral expansion caused by nonlinear effect, such as the self-phase modulation (SPM) and modulation instability (MI) [29, 30]. Hence, parameters optimization should be performed.
Figure 7 shows the coded Brillouin trace before decoding for forward and backward pumping (relative to Brillouin pump) at BFS of 10.880GHz with the same averaged times (1024). A quite noisy quasi-periodic fluctuation (see its spectrum in the inset of (b)) is found for backward-pumping, due to RIN transfer from 1366nm pump. However, the forward pumping is free from RIN transfer. It is well known that the 3dB RIN transfer bandwidth depends on the relative propagation direction between Raman pump and probe lights. For forward pumping, the RIN transfer contribution mainly comes from the backward lasing pump, which is counter-propagating with 1366nm pump (see Fig. 6), thus the larger group velocity mismatch averages out the fast oscillation, leading to much small RIN transfer bandwidth (tens of KHz typically) [3, 6, 10]. Note that the used fiber spools have been re-coiled, resulting in a dip (~47.7km) in Fig. 7(a) caused by additional strain near the fiber ends.
Additionally, the probe input power (~-14dBm) was optimized to simultaneously obtain the higher contrast and the suppressed non-local effect. Moreover, the probe beam was frequency-shifted 200MHz by an AOFS (The value was the same with AOM in the branch of Brillouin pump). Thus, symmetrical side-band for probe wave relative to Brillouin pump was formed. Since the Brillouin pump simultaneously receives the depletion from Stokes signal and amplification from anti-Stokes component, the non-local effect caused by Brillouin pump depletion is suppressed . We replaced the isolator (ISO) in Fig. 6 by a circulator to monitor the pump depletion by PD and oscilloscope. Fig. 8 shows the acquired amplitude at BFS of 10.880GHz (close to peak BFS) and 10.980GHz (beyond the upper-limit of BGS). The depletion factor d is ~1.1% (defined as d = (PB0-PB)/PB0, where PB0 and PB are the peak power of Brillouin pump after sensing fiber without and with maximal Brillouin gain [27, 28]).
Finally, to guarantee the higher contrast and measure accuracy for the far-end of sensing fiber, while avoiding the nonlinear spectral expansion, the power of 1366nm Raman pump has been optimized to ~34.3dBm (closes to the transparency transmission point). The input peak power of Brillouin pump was ~4dBm, which is sufficient for controlling the expansion of width of BGS within a small regime.
3.3 Experimental results
Figure 9(a) shows the 3D plot of decoded BGS around ~5m hot-spot. As a result of the uniform gain distribution and substantially suppressed RIN transfer based on forward pumping (relative to Brillouin pump), a clear Brillouin frequency shift is observed at the hot-spot. Figure 9(b) shows the BGS at different locations (10, 30, 50, 70, 90, 110, 130km) before and after Lorentzian fitting. The full-width at half maximum (FWHM) of BGS shown in the inset of (b) has been controlled within ~56MHz, due to the optimized powers of Raman and Brillouin pumps for weakening the nonlinear spectral expansion. Moreover, no multi-peak structure and BGS distortion are observed over the fully fiber, confirming the eliminated system error of non-local effect caused by pump depletion. Figure 9(c) gives the decoded BGS when the Brillouin pump pulse just enters and leaves from the 5m hot-spot (i.e., at 142.152 and 142.157km, respectively). The peak Brillouin frequency shift (frequency difference between Brillouin pump and probe waves at the maximal Brillouin gain) of 40MHz is shown, consisting with the typical sensitivity (~1MHz/°C ). Additionally, the peak value of BGS at hot-spot is slightly higher than that of neighbouring points. This is also consistent with the previous experimental report .
The retrieved temperature distribution as a function of sensing distance after Lorentzian fitting is shown in Fig. 10(a). By calculating the standard deviation, the worst measurement uncertainty is ~ ± 1.5°C (at the worst SNR regime near ~110-130km). As been expected from the gain distribution in Fig. 4(a), the lower uncertainty is shown in the regime of 0~94.5km where the Raman gain is higher. To show the capability of spatial resolution, Fig. 10(b) gives the zoomed view of temperature distribution around ~5m hot-spot. A clear ~40°C variation is observed, which fits the real temperature variation set at the hot-spot well. By measuring the FWHM of temperature distribution at hot-spot, ~5m spatial resolution is clearly demonstrated.
In summary, the amplification characteristics (including gain, nonlinear impairment and noise figure) of DRA configuration based on UL-FL pumping with a ring cavity have been investigated theoretically and experimentally. Based on the forward pumping (relative to Brillouin pump) scheme, ultra-long-distance BOTDA over 142.2km fiber with 5m spatial resolution and ± 1.5°C temperature uncertainty has been demonstrated, by optimization of system structure and parameters. Such a significant performance improvement mainly arises from both of uniform gain distribution and suppressed pump-signal RIN transfer.
The authors thank the reviewers for their helpful comments. This work is supported by the National Nature Science Foundation of China (NSFC) under grants No.61290312, 61205079, 61205048, and 61106045, the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT), the Construction Plan for Scientific Research Innovation Teams of Universities in Sichuan Province under grant No. 12TD008, and the 251 Talents Program of Sichuan Normal University.
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