Optical microspectrometers are potentially important components for improving the functionality of lab-on-a-chip systems used for detection and identification of cells and other biological material. For disposable and portable applications, monolithic integration of the microfluidic channels, optical waveguides and diffraction grating is desirable. This paper presents the design and simulation of a transmission grating that can be integrated with microfluidic channels for on-chip fluorescence detection. The grating design features two stigmatic points and large facet sizes that can be easily fabricated in low-cost polymers. Scalar simulations predict grating efficiencies greater than 74% for wavelengths from 500 nm to 700 nm in the -2 diffraction order.
Integrated planar concave gratings are promising candidates for Wavelength Division Multiplexing (WDM) devices. However, device insertion losses reported to date are higher than other options such as Arrayed Waveguide Gratings (AWGs) mainly due to the strict requirement for a deep vertical etch of grating facets. We present a novel planar concave grating demultiplexer design based on total internal reflection and the focusing properties of ellipses which simplifies the fabrication of the device by avoiding grating facet metallization and improves grating efficiency by reducing its sensitivity to non-vertical etching of the facets. As a proof of concept of the grating design, a 4-channel coarse WDM demultiplexer working near 850nm with a channel wavelength spacing of 20nm has been fabricated in PMMA polymer using the LIGA process, a deep x-ray lithography technique using synchrotron radiation. Smooth and extremely vertical sidewalls 220 microns deep have been obtained. Test results for this preliminary device show a -7dB on-chip loss for the best channel. Based on the test data, a –3dB loss can be reasonably expected for a device with optimized material processing. In the future, the LIGA process can be used to fabricate a metal stamp that can massively produce devices in polymers with plastic embossing or molding techniques.