_Article_  
Comparison between Capacitive and Microstructured Optical Fiber Soil Moisture Sensors  
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Optical fiber sensors have been used to measure many parameters, such as temperature, curvature, vibration, displacement, pressure, refractive index, electric field, magnetic field, relative humidity, and gases due to their ability to work as transducers [13]. There exist many different types of fibers that are suitable for sensing applications: silica optical fibers, plastic fibers, or photonic crystal fibers are some common ones.  
Microstructured optical fibers (MOFs) present great sensing potential and they have shown improved properties over traditional optical fiber sensors overcoming some of their handicaps [14; 15; 16]. For example, several pure silica suspended-core fibers have been used in temperature and curvature sensing [17], gas sensing [18], or relative humidity (RH) sensing [19].  
Fiber based optical Fabry-Perot (FP) interferometers are a quite popular optical fiber sensor configuration due to their compactness, simple configuration, flexibility in tuning sensitivity, and dynamic range. This structure presents robustness and multiplexing capability. The FP cavity transfer function consists of an interference pattern that depends on the cavity length and of the refractive index of the cavity, or more precisely, the effective indices of the different modes that are supported by the fiber interferometer. Actually, FP cavities fabricated with MOFs are structures that are commonly used for sensing: a hybrid structure that used a MOF, a hollow-core fiber, and a single mode fiber (SMF) was developed for high-temperature sensing [20], whereas a chitosan FP cavity was fabricated for relative humidity (RH) measurements [21] and also compact FP cavities for humidity between 40% and 95% were reported [22].  
Nanocoated based sensors have experienced a great development during the last decades [23]. Furthermore, new chemical deposition techniques, such as sputtering [24], enable precisely customizing the morphology and thickness of the deposited thin film coatings, and as a consequence, the final properties (sensitivity, kinetics) of the sensor. Sputtering depositions are commonly used to create a homogeneous layer on the surface of a pure silica optical fiber [25; 26].  
Finally, not many works have been reported up to date measuring real soil moisture with optical fibers: cascade fiber Bragg gratings (FBG) sensors were employed to monitor temperature and soil moisture simultaneously [27], hetero-core spliced optical fiber SPR sensor [28], and a summary of the most common techniques employed in humidity sensing [29] were reported.  
In this work, an optical fiber sensor based on a sputtering deposition on a MOF is presented for real moisture measurements and compared simultaneously with a commercial capacitive sensor. The operation principle of the humidity optical sensor has been previously reported by authors [19] showing in detail the construction process of an optimized sensing head and its performance towards humidity variations. In the present paper, the authors propose, for the first time, a sensor that has been adapted and used for measurements in real soil conditions. Moreover, the authors also present a comparison between the performance of microstructured optical fiber sensors, a commercial capacitive sensor, and a comparison of both devices against gravimetric measurements, showing the pros and cons of every type of sensors (and remarking the advantages of fiber optic against commercial capacitive ones). It is worth remarking that the aim of this work is not to present the optical sensing head in detail, as it is fully described in [19], but to show a real comparison between commercial capacitive soil moisture sensors and the application of the optical sensing head for the same purpose.