JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 30, NO. 8, APRIL 15, 2012 1173 L-Band Multiwavelength Single-Longitudinal Mode Fiber Laser for Sensing Applications R. A. Perez-Herrera, A. Ullan, D. Leandro, M. Fernandez-Vallejo, M. A. Quintela, A. Loayssa, Member, IEEE, J. M. Lopez-Higuera, Senior Member, IEEE, and M. Lopez-Amo, Senior Member, IEEE (Invited Paper) Abstract—In this work, a novel single-longitudinal-mode (SLM) four-wavelength laser configuration for sensing applications in the L-band is proposed and experimentally demonstrated. The sensor system presented here is based on ring resonators, and employs fiber Bragg gratings to select the operation wavelengths. The stable SLM operation is guaranteed when all the lasing channels present similar output powers. It is also experimentally demonstrated that when a SLM behavior is achieved, lower output power fluctuations are obtained. Characterization of the lasing structure for temper- ature sensing is also shown. Index Terms—Erbium-doped fiber (EDF), fiber Bragg grating (FBG), multiwavelength laser, optical fiber amplifier, optical fiber ring laser, single-longitudinal mode (SLM). I. INTRODUCTION T HE utilization of fiber lasers for fiber optic sensors multi- plexing has attracted much attention since the first demon- strations in the early 90’s [1]. These lasers may be the optical source of a multiplexing network [2], or the laser itself may be the multiplexing structure [1], [3]. This last option offers high signal to noise ratios and it is particularly appealing for remote sensing applications [4]–[6]. Long-wavelength band (L-band) operation (from 1565 to 1625 nm) in fiber optic sensing is limited by the higher price of the utilized optical components in comparison with the C-band (from 1530 to 1565 nm) ones and because of a slightly higher attenuation in the optical fiber. However, this band is especially interesting for gas sensing applications, among others. In addition to this, L-band EDFA is one of the key devices for the WDM transmission networks, which is combined with the conventional-wavelength-band (C-band) EDFA between 1525 and 1565 nm to produce very wide broadband amplification. The advantage of the utilization of L-band EDFAs is the reduc- tion of four-wave mixing (FWM) problem in dispersion-shifted Manuscript received June 14, 2011; revised September 21, 2011; accepted October 24, 2011. Date of publication October 28, 2011; date of current version March 16, 2012. This work was supported in part by the Spanish Comisión Interministerial de Ciencia y Tecnología within project TEC2010-20224-C02. R. A. Perez-Herrera, D. Leandro, M. Fernandez-Vallejo, A. Loayssa, and M. Lopez-Amo are with the Department of Electric and Electronic Engi- neering, Universidad Pública de Navarra, E-31006 Pamplona, Spain (e-mail: rosa.perez@unavarra.es; dani.leandro.glez@gmail.com; montserrat.fer- nandez@unavarra.es; alayn.loayssa@unavarra.es; mla@unavarra.es). A. Ullan, M. A. Quintela, and J. M. Lopez-Higuera are with the Photonics Engineering Group, University of Cantabria, Cantabria, Spain (e-mail: angel. ullan@unican.es; quintelm@unican.es; higuera@teisa.unican.es). Digital Object Identifier 10.1109/JLT.2011.2174138 fibers (DSF), and it can also easily achieve flat gain even without a gain-flattening filter (GFF) rather than the C-band EDFA. Re- cently many researches are striving to develop L-band EDFAs. To this goal, some aspects such as EDF length, pump configu- ration, pump wavelength, and with non-silica based EDF fibers are being explored [7]. L-band tunable lasers are also important for testing and mea- suring L-band devices for WDM transmission systems. The de- sign of L-band EDFAs is essential for developing L-band tun- able erbium-doped fiber lasers (EDFLs) [8], [9]. The advantage of extending the range of wavelength opera- tion of fiber lasers into the L-band expands the limits of present day tunable sources and hence the detection of gases with ab- sorption lines in the L-band [10]. In this region there are several gases of interest such us carbon dioxide (CO ) and hydrogen sulphide (H S) [11]. Detection of these trace gases such as methane, carbon monoxide and carbon dioxide is extremely important for both pollution monitoring and safety reasons in the oil and gas industries, in water treatment plants, in landfill sites and in commercial or domestic environments, where methane gas may filter up through the ground and create an explosion hazard (the lower explosive limit, LEL, for methane is 5% by volume methane gas) [12]. Fiber optical sensors based on direct absorption spectroscopy, operating in the 1–2 m spectral region, allow safe, remote location of sensors combined with the availability of low cost fiber components, connectors and compact gas cells. Several demonstrations of the application of the L-band hollow-core photonic bandgap fibers (HC-PBF) to the detection of methane have been reported in the literature [13]. It is therefore of commercial interest to develop portable gas sensors, based on optical techniques [14]. Optical sensors have the potential advantages of: (i) intrinsically safe, (ii) ability to detect a specific gas by selection of appropriate wave- lengths, (iii) able to operate in zero-oxygen environment (e.g., for purging of pipe lines), and (iv) low cost of maintenance, since the gas-detection principle is a physical process (not a chemical reaction), and therefore, poisoning of the sensor is not an issue (although dirt/contamination on the optics needs to be considered in the sensor design). Regarding the light source for these optical techniques, it is worth noticing that stable single-longitudinal-mode (SLM) erbium-doped fiber lasers (EDFLs) have attracted great in- terest recently, because they can be potentially applied in fiber sensing systems, dense wavelength division multiplexed 0733-8724/$26.00 © 2011 IEEE