We find that the random fiber laser (RFL) without point-reflectors is a temperature-insensitive distributed lasing system for the first time. Inspired by such thermal stability, we propose the novel concept of utilizing the RFL to achieve long-distance fiber-optic remote sensing, in which the RFL offers high-fidelity and long-distance transmission for the sensing signal. Two 100km fiber Bragg grating (FBG) point-sensing schemes based on RFLs are experimentally demonstrated using the first-order and the second-order random lasing, respectively, to verify the concept. Each sensing scheme can achieve >20dB optical signal-to-noise ratio (OSNR) over 100km distance. It is found that the second-order random lasing scheme has much better OSNR than that of the first-order random lasing scheme due to enhanced lasing efficiency, by incorporating a 1455nm FBG into the lasing cavity.
© 2012 OSA
Since the demonstration of the distributed random feedback within telecom fibers generating stable continuous-wave (CW) lasing output , this kind of random lasers based on fibers have attracted a lot of attention because of their outstanding advantages, such as long-distance signal delivery ability, ultra-low intensity noise, cavity simplicity and high lasing efficiency, etc. Therefore, it was considered to have significant impact on fiber-optic communication and sensing, etc . Valuable work has been made to study such type of random fiber lasers (RFLs) and various unique physical features have been reported [3–10]. However, there are many more features of the random lasing mechanism, in particular, its potential applications for long-distance sensing, yet to be fully explored, in order to essentially achieve the goal of improving the performance of the sensing system, such as extension of sensing distance or enhancement of optical signal-to-noise ratio (OSNR), etc.
In this paper, we present a substantial work to promote the RFL towards practical fiber-optic sensing applications. The combination of the RFL and conventional fiber-optic sensors can form a new type of fiber-optic sensing systems with capability of realizing long-distance sensing. Firstly, the thermal stability of the RFL is investigated, and we find that the RFL with an open-cavity is temperature-insensitive and hence it is quite suitable to act as a stable long-distance transmission platform for delivering sensing signals. Secondly, we demonstrate that the RFL with a half-open cavity, in which the sensing FBG and the pump source are located at each side of the fiber span, can form a long-distance remote-sensing system based on the first-order random lasing, and the sensitivity and the thermal stability of such a system is evaluated; Finally, we push the aforementioned scheme by utilizing the second-order random lasing, and a much better sensing performance is observed while the thermal stability is kept; The simplicity and stability of the proposed schemes are the key advantages over previous remote point-sensing system [11–14].
2. Thermal stability of the RFL
Figure 1(a) shows the experimental setup of the first-order RFL built as the basis for the 100km remote sensing system. A Raman fiber laser at central wavelength of 1365nm is used as the pump laser. The pump is launched into the fiber span through a 1365/1461nm wavelength division multiplexer (WDM). 100km single mode fiber (SMF) is used as the distributed feedback mirror and the Raman amplification medium. As the pump power is increased, the fiber starts to lase as the distributed Rayleigh backscattering forms a sufficient feedback per round trip. Figure 1(b) clearly shows the linear growth of the generated output power as the pump power is above threshold and goes up. The generated CW radiation around 1455nm corresponded to Raman gain profile is separated out through the 1461nm port of the WDM. A high-resolution optical spectrum analyzer (OSA) is used to observe the optical spectrum, and the output power is monitored simultaneously. The inset in Fig. 1(b) shows a typical spectrum of the RFL. Essentially there are two peaks localized near the Raman gain maxima, the one at the shorter wavelength (~1454nm) is much more pronounced and is with a ~1.1nm bandwidth (BW). It was previous reported that if the pump power is further increased, the peak at the longer wavelength (~1462nm) will be more and more pronounced , then there will be two clear peaks. For the purpose of sensing applications in the next section, only the peak at the shorter wavelength is concerned.
To study the temperature response of the spectrum of the RFL, the 100km SMF is put into a temperature controlled chamber and the remaining components of the RFL setup is kept in room temperature. The pump power is set to be 1.7W. The chamber temperature is adjusted from −40°C to + 50°C with a 10°C step. During each measurement the spectrum is recorded when the in-chamber temperature reaches the set value for at least 25 minutes. Figure 2(a) shows the variation of the central wavelength of the left lasing peak according to the surrounding temperature, and Fig. 2(b) compares the spectra at −40°C and + 50°C . It can be clearly seen that the lasing wavelength is insensitive to the temperature change. It is also noted that the mechanical vibration during the heating procedure will not destabilize the lasing output.
3. Stable remote point-sensing with the RFL
The temperature insensitivity enables the RFL to be a good candidate for remote temperature point-sensing. A FBG with a central wavelength at 1454 nm and 98% reflectivity is spliced to the end of the 100km fiber span, and it acts a point mirror. Due to the dominant mode-selection effect introduced by the FBG, the output spectrum of the RFL exhibits single highly pronounced peak, as shown in Fig. 3(a) . The OSNR of the peak is ~20dB, and the bandwidth is ~0.27nm. To study the temperature response of the whole sensing system, two measurements are carried out: 1) at first only the FBG is put inside the temperature controlled chamber; 2) then the 100km fiber is put into the chamber as well. As the central Bragg wavelength of the FBG is shifted under the temperature variation, the central wavelength of the lasing peak is shifted accordingly. Figure 3(b) compares the temperature response of the central lasing wavelength in both measurements. It can be seen that the two data sets are well matched. In this regime the 100km fiber span is both a distributed lasing medium and part of the sensing system, within which only the FBG is temperature sensitive while the distributed lasing cavity is intrinsic temperature-insensitive. The thermal-stability and the ultra-long power delivery ability made the RFL an accurate remote temperature sensing system. It should be noted that with current fiber length a high OSNR of 20dB is maintained, therefore the ultimate limit of such a sensing system can be significantly beyond 100km.
4. Remote sensing utilizing the second-order random lasing and discussions
Figure 4 shows the experimental setup of the RFL utilizing the second-order random lasing for the long-distance point-sensing. The 1365nm pump source is launched into a 100km fiber span through a 1365/1461nm wavelength division multiplexer (WDM) and a 1455nm FBG, and the 1455nm FBG is used as a point mirror with high-reflectivity. The light-waves propagating towards the pump side is tapped out through a 1:99 coupler for spectral analysis, and those arriving at another end (far-end) of the fiber span are also monitored. A 1560nm FBG and a 1563nm FBG will be successively attached at the far-end of the fiber span as sensing heads, which will be discussed next.
As the pump power is increased, the system starts to lase as the distributed Rayleigh backscattering along the fiber span together with the 1455nm FBG forms a sufficient feedback per round trip. As shown in Fig. 5(a) , the generated first-order random lasing is located at 1455nm, corresponding to the FBG wavelength and within the Raman gain curve of the 1365nm pump laser. As the pump power is increased further, the second-order random lasing located at the C-band appears. When the 1455nm lasing power is near the threshold level for the generation of the C-band lasing, the spectrum of the second-order lasing shows random spikes and dips, as shown in Fig. 5(b). As the 1455nm lasing power is well above the threshold level, the optical spectrum becomes stabilized. As indicated in Fig. 5(c), a C-band continuous-wave (CW) radiation is generated as the stable second-order random lasing. It should be noted that the direct pump source of the C-band CW is the 1455nm light-wave (i. e., the first-order random lasing), and the lasing cavity for the C-band CW is fully distributed, as the 1455nm can be considered transparent for the C-band light-wave.
To demonstrate the remote-sensing ability of the system, a FBG with a central wavelength at 1560 nm is spliced to the end of the 100km fiber span, and it acts as a sensing head. Due to the dominant wavelength-selection effect introduced by the 1560nm FBG, the spectrum of the second-order lasing exhibits highly pronounced peak, as shown in Fig. 6(a) . The OSNR of the peak is more than 35dB, and the bandwidth is ~0.26nm. To study the temperature response of the spectrum of the 1560nm spike, the 1560nm FBG is put into a temperature controlled chamber and the remaining components of the RFL setup is kept in room temperature. The chamber temperature is adjusted from −20°C to + 50°C with a 10°C step. Again, during each measurement the spectrum is recorded when the in-chamber temperature reached the set value for at least 25 minutes. Figure 6(b) shows the variation of the central wavelength of the lasing spike according to the chamber temperature. It can be clearly seen that the change of the lasing wavelength is linear to the temperature change.
We intentionally selected the 1560nm FBG because its Bragg wavelength corresponds to the dip of the spectral profile of the second-order random lasing, as shown in Fig. 5(c), which means its reflected power is relatively weak. Therefore, it is straightforward to deduce that a FBG corresponds to all other spectral component with higher intensity can work as a remote sensor. Moreover, it is demonstrated that the system can utilize multiplexed FBGs for multi-point sensing. The inset of Fig. 6(a) shows the spectrum when an additional 1563nm FBG was attached after the 1560nm FBG. Two spikes both with more than 35dB OSNR were observed at the near-end, and they can be used as excellent indicators of Bragg wavelength shift of the two remotely located FBGs. The obtained OSNR is 15dB higher than what can be got from the first-order random lasing, and this is attributed to the 1455 FBG at the pump side. Without the FBG, most of the 1455nm lasing power locates at the first half of the 100km fiber and propagates towards the pump side, and the power pass through the WDM has no contribution to the generation of the next order lasing, while with the 1455nm FBG significant portion of the 1455nm power is re-distributed towards the far-end of the fiber span. As an evidence, with the FBG the output 1455nm power at the far-end was ~5.5dB higher than the case without the FBG, while the 1365nm pump power was kept 1.8W in both cases. Therefore, the better distribution of the 1455nm power contributes to the higher OSNR of the C-band light-wave recorded at the near-end.
Since the first-order random lasing originates from the half-open cavity formed by the 1455nm FBG and the 100km SMF, it is important to know how the Bragg wavelength shift of the 1455nm FBG will affect the properties of the system. Therefore, we heated the 1455nm FBG from the room temperature (19°C) up to 79°C, it can be clearly seen from Fig. 7(a) that the first-order lasing wavelength was significantly shifted (there is a dip in each spectrum because the near-end monitoring point is placed after the 1455nm FBG). However, as shown in Fig. 7(b), the second-order lasing wavelength was kept the same. The results show that, although the first-order random lasing cavity involves a FBG, the Bragg wavelength shift of such a FBG will not affect the sensing reliability related to the second-order random lasing, as long as the interpretation of the sensing signal is wavelength-based.
There have been several other schemes proposed for ≥100km remote point-sensing, as reviewed in . Bravo et al., demonstrated a 253km system using a fiber loop as the remote reflector and an optical time-domain reflectometer (OTDR) as the interrogator, and the limitations are the lack of multiplexing capability and the long acquisition time (tens of seconds); Fernandez-Vallejo et al., proposed another 250km system, in which a tunable laser is used to interrogate remote FBGs and the line-width of the laser was set to be 0.6nm in order to suppress Brillouin scattering, therefore the accuracy of wavelength interrogation remains to be investigated. Hu et al., showed that inserting Erbium-doped fiber segments into the fiber path can extend the sensing distance, and this method can also be utilized in our system. Leandro et al., combined Raman, Erbium and Brillouin gain with heterodyne detection, to achieve 155km FBG sensing with 10dB OSNR. Compared with previous works, our scheme simply needs one CW laser, the wavelength accuracy depends on the reflection spectrum of FBGs and the resolution of the spectrum analyzer, and the system response is instantaneous.
We demonstrate that the RFL with fully distributed random feedback has the unique thermal stability compared with traditional fiber lasers. Base on such a merit and the ability of long-distance power delivery, we proposed the novel concept of utilizing the first-order and the second-order random fiber lasers to form long-distance point-sensing systems. Two 100km FBG point-sensing systems are experimentally demonstrated to verify such a concept. The first-order random lasing scheme can observe 20dB OSNR, while the second-order random lasing scheme can observe 35dB OSNR. The observed wavelength shift is solely dependent on the Bragg wavelength shift of the sensing FBG remotely located. Neither the long-span fiber as the lasing cavity, nor the near-end FBG used for enhancing the lasing efficiency, will contribute to the monitored wavelength shift, proving that such a novel long-distance point-sensing system is very stable. This work may open a window for realizing a new generation of simple and reliable long-distance point-sensing systems.
The authors would like to thank Dr. X. F. Chen and Prof. L. Zhang in Aston University for providing the 1454nm FBGs. This work is partially funded by NSFC No. 61106045.
References and links
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