Towards Raman spectroscopy on a microchip

In numerous applications, Raman spectroscopy is utilized to monitor unique Raman scattering in order to identify specific molecules or structures. We aim at developing a low-cost, compact, hand-held apparatus for Raman spectroscopy of the skin and tooth. A critical function of this system is spectral separation of the Raman-scattered signals. In our case, the core element of such a device is an arrayed-waveguide grating (AWG). Two silicon oxynitride single-mode channel waveguide geometries are designed for Raman spectroscopy of the skin and tooth. For tooth measurements we design the AWG to have a central wavelength of 901 nm and a free spectral range of ~22 nm, with a resolution of 0.2 nm. The skin application requires the AWG to be polarization insensitive, to have a central wavelength 881 nm, and to have a minimum resolution of 5.5 nm. For the Raman measurements light from a Ti:Sapphire laser at 785 nm, after passing through a laser-line filter, is reflected from a dichroic mirror and focused onto a sample with a microscope objective. The backscattered light from the sample is collected by the same optics; the Rayleigh component is again reflected by the dichroic mirror, which transmits the Raman wavelengths. An edge filter removes the residual light at the laser wavelength, and the light is focused into the input channel of the integrated spectrometer using a x50 microscope objective. The output channels of the AWG are imaged onto an electron-multiplying CCD through a camera lens. The set-up is tested by measuring the Raman spectra of silicon and cyclohexane. The preliminary dental measurements show excellent agreement between the spectra of healthy and carious tooth enamel measured with our integrated device and spectra recorded using a conventional Raman spectrometer.