Self-referencing techniques in photonics sensors and multiplexing Carmen Vázquez, Julio Montalvo, David S. Montero, Pedro C. Lallana Displays and Photonics Applications Group, Dpto. Tecnología Electrónica, Escuela Politécnica Superior, Carlos III University of Madrid, Avda. Universidad 30, 28911 Leganés, Madrid, Spain ABSTRACT A short review of self-reference techniques for remote fiber-optic intensity sensors and possible integration in multiplexing sensor networks is reported. Special focus is given to developments on radio-frequency (RF) source modulation techniques in interferometric configurations operating under incoherent regime. Experimental results on ring resonator (RR) configurations in transmission and reflection modes are included. Sensitivity, optimum insertion losses and robustness to intensity error fluctuations are reported. Sensors are interrogated at two sub carrier frequencies having a high rejection of interference from laser source intensity fluctuations and loss in the fiber lead. Dependence on source coherence is also analysed. Scalable self-referencing sensor networks with low insertion losses implemented in Coarse Wavelength Division Multiplexing (CWDM) technology are reported. The possibility of remote self-referenced measurements using a full- duplex fiber down-lead tenths of kilometers long with no need for optical amplification is also described. Fiber Bragg gratings (FBG) are used in the reflection configuration, thus increasing the sensitivity of the optical transducers. Low- cost off-the-shelf devices in CWDM and DWDM technology can be used to implement and scale the network. Applications to specific photonic sensors are also envisaged and these techniques can be used in networks of microfiber loop resonators, being the microfiber loop the sensing element itself. Keywords: remote sensing, self-referencing, fiber-optic sensor, ring resonator, micro-fiber loops 1. INTRODUCTION Intensity based sensors are inherently less sensitive than their interferometric counterparts as they rely on amplitude instead of optical-phase. In addition, the transducing mechanism used for converting physical or chemical measurands into a modulation of light intensity sometimes shows poor linearity. However, these limitations do not prevent intensity modulated sensors from having a large potential. They still benefit from the intrinsic advantages of optical detection such as remote sensing, electromagnetic immunity and security in hazardous environment; they can use available standard fibers and sources; the transducers are simple and can easily be made selective to specific measurands; no sophisticated detection system is needed, absolute measurements with high bandwidth can be taken and multiplexing of a number of sensors on the same fiber lead can be made. Because of that there is a wide variety of intrinsically safe photonics sensors in which the external parameter of interest modulates the attenuation of an optical signal [1]. A serious drawback of loss modulation is interference from variation in loss due to long term aging of optical source as long as short-term fluctuations of optical power loss in the fiber lead used to operate the remote sensor, which can occur for example in fiber bends. It is needed a reference signal for calibration out of the sensor response, which undergoes all other losses in the system, and which is multiplexed in the same fiber lead as the measurement signal. Spatial referencing is the simplest to implement with the reference and measurand signals being located within two separated optical fibers [2]; but it is difficult to ensure similar common-mode variations in two separated fibers. Time division [3], wavelength normalization [4], spectral splitting [5], frequency-based self-referencing methods on differential amplitude [6-9] or using amplitude to phase conversion [10], and others using counter propagating signals [11] or electronic means [12] have been reported to overcome this problem. In the frequency-based self-referencing techniques, the input light is RF modulated and an amplitude ratio measurement are performed at the reception [6-9] or intensity modulations are converted into electrical phase modulation [10, 13]. Different interferometric schemes such as Mach-Zehnder, Michelson [7] and RR topologies [6, 8-9] are considered in the Invited Paper Photonic Materials, Devices, and Applications II, edited by Ali Serpengüzel,Gonçal Badenes, Giancarlo Righini Proc. of SPIE Vol. 6593, 65931X, (2007) · 0277-786X/07/$18 · doi: 10.1117/12.723743 Proc. of SPIE Vol. 6593 65931X-1