Recent years have seen rising interest in optical system-on-a-chip sensors for biological applications. Vertical cavity surface emitting lasers (VCSELs) are a natural choice for array-based sensors requiring high power and low noise. However, much of the noise characterization of VCSELs has been performed in frequency ranges on the order of 108 to 1010 Hz, whereas many physiological phenomena occur in frequency bands in the hertz to kilohertz range where 1/f and 1/f2 noise is dominant. In this work we characterize the relative intensity noise (RIN) of commercial VCSEL devices and evaluate their feasibility for use in an integrated semiconductor optical sensor for functional brain imaging using Intrinsic Optical Signals (IOS). Our results show RIN on the order of -196 to -174 dB/Hz at an offset of 10 Hz. This is well below the signal-to-background and dynamic range requirements of 6 dB and 86 dB, respectively, for this application.
Miniaturized and robust sensing modules are required for the development of portable integrated biological analysis systems or micro total analysis systems (μ-TAS). This work uses vertical cavity surface emitting lasers (VCSELs), optical emission filters and PIN photodetectors to realize a monolithically integrated, near-infrared, fluorescence detection system. The integration of these optoelectronic devices with biochips will drastically reduce cost of current systems and increase parallelism and portability. The sensor has been implemented on a micro-fluidic format, and sensitivity was evaluated. A theoretical limit of detection of IR-800 dye in methanol is reported to be 40 nM. The sensor sensitivity is limited by laser background as a result of integrating the optoelectronic elements in such close proximity. Significant reduction in laser background from reflections above the sensor is possible by increasing the distance between the sensor and optical interfaces to greater than 3 mm. Also, for distances greater than 500 microns between the sensor and optical interfaces above the sensor, it is found that background from indirect spontaneous emission is much smaller than the background caused by specular reflections of the laser.
Miniaturized, portable and robust sensing systems are required for the development of integrated biological analysis systems and their application to clinical diagnostics. This work uses vertical cavity surface emitting lasers (VCSELs), optical emission filters and PIN photodetectors to realize monolithically integrated, near infrared, fluorescence detection systems. The integration of these micro technologies with biochip applications will drastically reduce cost and allow for parallel sensing architectures, which is particularly useful for flow channel arrays such as in capillary array electrophoresis. This paper focuses on the fabrication of integrated fluorescence sensors. Fabrication procedures have been developed to realize intracavity contacted VCSELs and low noise photodetectors, such as selective AlGaAs wet etching and via planarization. A reflow process with positive photoresist has been developed to provide via electrical contacts and to optically isolate the photodetector from the light source. Three-dimensional microstructures can be simply made by this reflow technique. Optical simulations predict that a detection sensitivity lower than 10000 molecules per 104μm2 sample area. Single molecule detection may be possible in certain sensing architectures.
As biological analysis systems scale to smaller dimensions, the realization of small and portable biosensors becomes increasingly important. The innovation of integrated fluorescence sensors is now possible due to the development of optoelectronics over the past decade. We present the monolithic integration of vertical cavity surface emitting lasers, PIN photo-detectors and optical emission filters to be used as a fluorescence sensor. The integration will drastically reduce cost and size of fluorescence detection systems. Also, parallel sensing architectures of more than one hundred channels will be possible. The sensor will be utilized for near-IR fluorescence detection. This spectral range is compatible with standard AlGaAs optoelectronic technology and will also reduce background fluorescence from complex bio-fluids such as blood. PIN heterostructure photodetectors have been fabricated and tested. Photodetector experiments show extremely low dark current of less than 500fA/mm, quantum efficiency greater than 85 percent and linear detector response. Optical simulations predict a detection sensitivity lower than 10000 fluorescent molecules in a detection area of 104 micrometers 2.
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