Compact and high-resolution wavelength detectors for read-out of optical sensors
An extremely compact wavelength detector is presented which is capable to resolve wavelength changes down to about 1 pm. A photosensor array or position detector element is coated with a linear variable bandpass filter. The filter converts the wavelength information of the incident light into a spatial intensity distribution. The centroid of the spatial distribution is determined by a differential read-out of two adjacent elements of the photosensor array or the position detector. The centroid of the distribution is shifting when the wavelength distribution of the incident light is changing. This device can be used to read-out an optical sensor which displays a wavelength shift upon a stimulus. In particular, variable reflection properties of one and two-dimensional photonic crystals have been detected. The strength of this interrogation method will be demonstrated for the case of Fiber Bragg grating (FBG) sensors, which are frequently used for various applications. A FBG is formed by a periodic modulation of the refractive index along the core of the fiber. This pattern reflects a certain narrow wavelength spectrum determined by the period of refractive index profile. An external stimulus (e.g., temperature or strain) changes the periodicity of the grating and therefore will alter the reflected wavelength spectrum. The resulting wavelength shift is a direct measure of the stimulus. The two- and three-dimensional analogues to FBG sensors are photonic crystal sensors. In these crystals a periodic modulation of the refractive index is realized in more than one direction. The crystals may be filled with or attached to a species under test. Alterations of the species can be detected by monitoring the optical reflection and transmission properties.
Kiesel, P. ; Schmidt, O. ; Mohta, S. ; Johnson, N. M. Compact and high-resolution wavelength detectors for read-out of optical sensors. Optics East 2006, Session PT105: Photonic Sensing Technologies; 2006 October 1-4; Boston; MA; USA.