Optical Micro-Spectrometer with Sub-Nanometer Resolution Ivan Avrutsky, Kalyani Chaganti, Ildar Salakhutdinov, Gregory Auner Department of Electrical and Computer Engineering Wayne State University, Detroit, MI 48202, avrutsky@eng.wayne.edu ABSTRACT We have developed a super compact optical fluorescence spectrometer. Our innovative design combines advantages of guided wave planar optics and free-space microoptics. This innovation allows for miniaturization that is not achievable with pure planar or pure free-space optics. A prototype device has volume of its optical part below 1cm 3 . The spectrometer covers 450nm-650nm spectral range, and provides spectral resolution of 0.5nm. Next generation prototype, currently under development, will have volume below 10mm 3 , cover the entire visible range from 400-700nm, provide spectral resolution of at least 2nm, and work simultaneously with up to 35 independent optical inputs. The key component of the micro- spectrometer is diffractive optical element. The targeted application is on-chip diagnostic systems, in particular for detection of hazardous materials. Keywords: microspectrometers, diffractive optics, optical sensors, optical microsystems, lab-on-a-chip systems 1 INTRODUCTION Miniature optical spectrometers are required for on-chip systems that use optical spectroscopy as analytical tool for DNA sequencing, microarrays reading, detection of hazardous substances and other applications. A state-of-the- art in optical microspectrometers is reviewed in a recent paper [1]. As a rule, smaller size of the device implies worse spectral resolution. For micro-spectrometers with size of optical components of about 1cm, the best reported resolution in visible range is from 5nm-10nm for both planar integrated optical [2] and free-space microoptical [3] designs. Very recently, a microspectrometer based on sensing a standing wave in front of a mirror [4] has been proposed. It is very compact but its resolution is not better than 6nm. A high-resolution (0.07nm) single input channel microspectrometer based on an arrayed waveguide grating is described in [5]. Better resolution in this case comes in a package with a limited spectral range (14nm) and presumable large footprint to accommodate as many as 250 waveguides in the array. We demonstrate that combining integrated optics and microoptics allows for further miniaturization, which is not achievable with pure planar or pure free-space design. In the concept of a microspectrometer reported here, the optical input is guided by a channel waveguide, and the expansion section as well as the grating is implemented in a planar waveguide. Then light out-coupled to free-space by the grating is spectrally dispersed by the same grating and focused by a microlens on an image sensor. More advanced version of the microspectrometer uses a single diffractive optical element replacing the grating and the microlens. Within this concept, the tradeoff between the microspectrometer size and its spectral resolution is such that a device with optical components size below 1cm provides an order of magnitude better spectral resolution compared to the best microspectrometers described in the literature. Or, alternatively, the optical part of the microspectrometer can be squeezed down to several cubic millimeters, while providing the spectral resolution of single nanometes. Using a properly designed diffractive optical element allows for another significant improvement: the microspectrometer can work in parallel with dozens of independent optical inputs. 2 LENS-BASED PROTOTYPE Spectral resolution as good as 0.5nm in visible range for a micro-spectrometer that has optical components measured by less than 1cm in all dimensions has been verified by experiments. Slightly larger (2cm) version of this device is capable of resolving 0.2nm. The resolution tests are performed using laser sources. Overall functionality of the microspectrometer is demonstrated by measuring spectrum of Rhodamine-575 dye. Figure 1 (Color). The experimental setup (top), the CCD camera (bottom left inset), and the fluorescence spectrum of R-575 dye measured using the microspectrometer (bottom). 0 100 200 300 400 500 600 700 10 20 30 40 ~575nm fluorescence Intensity in a.u. Pixel number 514.5nm pump 75nm 170nm Glass Removable 1mm path cuvette with dye 632.8nm 514.5nm Image plane 328 NSTI-Nanotech 2006, www.nsti.org, ISBN 0-9767985-8-1 Vol. 3, 2006