Low-cost interrogation unit for Fiber Bragg Grating sensors

Abstract

We have developed a low-cost compact and fast wavelength detector suitable for the read-out of many kinds of optical sensors that are based on optical wavelength shifts caused by an external stimulus, e.g., temperature, strain, stress, acceleration, bio-coating, or chemical environment. Examples are Fiber Bragg Grating (FBG) sensors, laser cavity sensors, micro-sphere or micro-ring based sensors as well as photonic crystal sensors. In comparison to their electronic counterparts, these optical sensors have many advantages. They are very sensitive, allow for remote and distributed sensing, can be used in harsh environments, and are immune to electromagnetic interferences. The main disadvantage of optical sensors is their comparatively high cost. One way to address this is to reduce the cost of the sensor component itself, e.g. by using plastic optical fibers instead of glass or by utilizing less expensive fabrication methods, e.g. nano-imprint lithography, UV lithography, FBG fabrication with fs-lasers, and fabrication of FBG during the fiber drawing process. However, interrogation systems that are capable of resolving the small wavelength shifts (sub pm to few nm) produced by these sensors are still bulky and expensive. The compact and fast wavelength monitor we present is capable of resolving pm wavelength changes. This is realized with a photosensor array or position detector element, coated with a linear variable filter, which converts the wavelength information of the incident light into a spatial intensity distribution on the detector. Differential read-out of two adjacent elements of the photosensor array or the position detector is used to determine the centroid of this distribution. A wavelength change of the incident light is detected as a shift of the centroid of the distribution. The performance of this wavelength detector was tested with a wavelength tunable light source. We have demonstrated that our device is capable of detecting wavelength changes as small as ~0.1 pm. The wavelength monitor can be used as read-out unit for any optical sensor that produces a wavelength shift in response to a stimulus. In particular, changes in the reflection properties of one and two-dimensional photonic crystals can been detected. The performance of this interrogation method has been tested for the case of temperature and strain sensors based on FBGs.