## 1 Introduction  
Gases are used in a broad range of applications and therefore it is important to have sensors capable of measuring concentration with high precision. In particular, acetylene is a highly flammable gas that is used in several industrial applications. Its lower explosive limit (LEL) of 2.5% by volume and its upper explosive limit (UEL) of 100% by volume make this gas extremely hazardous. Therefore, sensors capable to detect its concentration with high sensitivity and low cross sensitivity to other gases are necessary. In general there are different sensor configurations, for instance, non-dispersive sensors (NDS) and those based on the tunable laser absorption spectroscopy (TLAS) technique. In NDS, basically all wavelengths from a broadband source arrive together to an optical detector. Different configurations of this type of sensors can be implemented by using basic components such as a broadband source, a band pass filter and a detector. In order to either increase the sensitivity, to minimize the cross sensitivity of the sensor to other molecules, or to minimize some type of errors, authors have proposed designs that use either two detectors, two sources, two cells (one for reference and one for measurement), or a combination of these components [1; 2; 3]. In sensors based on the TLAS technique, ideally only one wavelength arrives at the optical detector at a time. Hence, to implement this sensing setup it is necessary to have a laser emission that can be tuned within the range where ro-vibrational absorption lines of the target molecule occurs [4]. Commonly, TLAS can be performed in two different modes: Analytical Tunable Laser Absorption Spectroscopy (ATLAS) or Line Locked Tunable Laser Absorption Spectroscopy (LL-TLAS) [5]. In ATLAS mode, the laser line emission is scanned to resolve with high resolution one or more absorption lines of the target molecule [6; 7; 8; 9; 10]. In LL-TLAS mode, the emission is ideally matched with the central wavelength of the absorption line, thereby the overall laser intensity observed by the detector will be affected as the gas concentration is varied [5]. Diode lasers have been used as TLAS sources since a long time ago [7; 8; 9; 10]. Hence, diode lasers, as single-mode distributed feedback laser diodes (DFB), are quite popular for this application. Another light source option for LL-TLAS sensing in the near infrared (NIR) region are fiber lasers, since these can provide narrow line width emissions and are relatively low cost because these can be implemented with standard communication components [4]. These lasers can be used to detect gases such as acetylene (C\({}_{2}\)H\({}_{2}\)), hydrogen iodide (HI), ammonia (NH\({}_{3}\)), carbon monoxide (CO) and carbon dioxide (CO\({}_{2}\)) [11; 12; 13]. Lasers that will be used in TLAS applications must be able to be continuously tuned over the spectral range where the ro-vibrational absorption lines occur. Hence not all optical fiber lasers can be finely tuned due to the mode hopping effect [12; 13; 14; 15; 10], however some optical fibers with continuous tuning have previously been achieved [13; 14; 15]. Moreover, some authors have proposed fiber lasers for gas sensing [12; 16]. A practical limitation of LL-TLAS is the difficulty to maintain fixed the wavelength of the laser at the absorption line center over time due to different factors, such as laser temperature fluctuation and oscillation current drift [17]. Acetylene sensors operating in the NIR region and based on optical fibers can be found in literature. Some of these sensors have achieved sensitivities in the order of 700 ppm by using path lengths of around 25 cm [18]. Higher sensitivities have been reported by using cells with lengths in the order of meters [19; 20; 21]. For instance, it has been reported a sensor with sensitivity of 100 ppm in which 1 m cell was used [19]. Another example is the sensor that achieved a sensitivity of 50 ppm by using a cell length of 27 m [21].