Wavelength monitors for optical sensing applications
A low-cost and compact wavelength detector is presented which is capable of resolving wavelength changes in the sub-pm range. A photosensor array or position-sensitive detector element is coated with a linear variable 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 shifts as the wavelength spectra of the incident light changes. The device can be used as the read-out 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 strength of this interrogation method has been tested for the case of Fiber Bragg grating (FBG) sensors. We have already achieved a sensitivity of ~12mV/°C, which corresponds to a temperature resolution of less than 0.1°C.
Kiesel, P. ; Schmidt, O. ; Johnson, N. M. Wavelength monitors for optical sensing applications. Photonics Spectra. 2007 March; 41 (3): 62-70.