Chip-size wavelength detectors
Chip-size wavelength detectors will be described that are capable of resolving small wavelength changes over a broad spectral range. These devices can be configured either as compact spectrometers or wavelength shift detectors. Applications that are anticipated to benefit from the spectrometers include reagentless optical detection for class identification of biomolecules and chemicals in fluidic and aerosol samples. The wavelength shift detectors are applicable for any optical sensor in which a stimulus (e.g., temperature, strain, PH-value, etc.) results in a wavelength shift of an optical output signal, such as the interrogation system for Fiber Bragg Grating sensors, photonic crystal sensors, Fabry-Perot type sensors, and sensors in which an analyte influences a laser cavity and thereby its emission wavelength. Key features of our spectrometer include compactness (chip-size), no mechanical parts, customized spectral resolution, fast and suitable for broad wavelength range (UV to FIR), simultaneous spectral and spatial imaging is possible, and monolithic integration is feasible. Our concept is based on combining a light sensitive element, which would normally be used to provide spatial information (e.g., detector arrays and CCD chips), with a mirror or optical cavity that incorporates controlled inhomogeneous (spatially dependent) transmission/reflection properties. The combination of an optical coating, with laterally varying transmission/reflection properties, and the light-sensing element defines a correlation between position and wavelength, that is, the spatially dependent signal of the detector contains analyzable information on the incident spectrum. Thus, the output signal of a coated detector array yields the wavelength spectrum of the incident light. One can select from a large variety of light sensing elements and design coatings covering a broad spectral range from the deep UV to the far-IR. Systems demanding either very high wavelength resolution (e.g., for absorption spectroscopy or Raman spectroscopy) or a wide spectral range (e.g., fluorescence spectroscopy) can be accommodated by choosing the appropriate coating. The size of the spectrometer can be only slightly larger than that of a detector array, and fabrication requires only conventional, readily available processing technology, which should enable a cost effective manufacturing process. Figure 1 provides a proof-of-principle demonstration of the spectrometer concept. A prototype spectrometer with a CCD chip was used to record the emission spectrum from a helium arc lamp over the spectral range from 400 to 700 nm. The spectrometer was calibrated with a tunable monochromatic light source, and the spectral resolution (FWHM) was better than 6nm. The spectrum also illustrates good stray light suppression of about three orders of magnitude even for this non-optimized configuration. Equipped with a low-cost position sensor or detector arrays our device functions as a wavelength shift (ƒ´ƒÜ) detector with high spectral resolution (<1 pm). This is illustrated in Fig. 2 with the response of the detector to changes in the wavelength of the incident light. The monochromatic light was provided by a tunable vertical-cavity-surface-emitting laser diode. The ƒ´ƒÜ detection limit in this example approaches 0.1 pm. Key features of the ƒ´ƒÜ detector include low cost, compactness, no mechanical parts, 10-4 nm (0.1pm) spectral resolution, independent of incident light intensity, fast and suitable for broad wavelength range, precise absolute wavelength sensing by referencing, and suitable for distributed sensing. Applications include (Laser) wavelength monitor, BioChem sensing, power grid monitoring, environmental monitoring, and read-out for fiber sensors.
Schmidt, O. ; Kiesel, P. ; Bassler, M. ; Johnson, N. M. Chip-size wavelength detectors. ISSSR 2006 (International Symposium on Spectral Sensing Research); 2006 May 29 - Jun 2; Bar Harbor; ME; USA.