Integrated optical backscattered-light collectors for on-chip spectroscopy

Integrated optical backscattered-light collectors for on-chip spectroscopy

Integrated Optical MicroSystems Group, MESA+ Institute for Nanotechnology, University of Twente, PO Box 217, 7500 AE Enschede, The Netherlands

*_{n.ismail@ewi.utwente.nl}

In this work we investigate and compare different ways to perform on-chip collection of backscattered-light from turbid media. Our goal is to realize a low-cost on-chip light collection system that can be applied to spectroscopic techniques, and in particular to Raman spectroscopy as shown in Fig. 1, where the proposed confocal system is connected to a laser-line suppression filter in cascade with a wavelength selective device.

Fig. 1. Schematic of the proposed device with the three distinctive parts: confocal arrangement of two zero-order (m = 0) AWGs for laser delivery and signal collection; an integrated optical filter for laser suppression; an AWG for wavelength selection.

The first investigated method for the collection of backscattered-light is based on integrated optical waveguide probes that present distinct excitation and collection channels. The probes are compared to dual-fiber probes through simulation. The results show that, in case of thin samples, integrated probes exhibit higher efficiencies compared to large-core multi-mode fiber probes. Besides, their performance exceeds that of small-core multi-mode and single-mode fiber probes for all sample thicknesses. The advantages of integrated probes, such as reproducibility, low cost, and on-chip integration with other components, make them more suitable for applications that require probing of thin samples. In order to validate the simulations we have fabricated waveguide probes in silicon oxynitride technology (SiON) [2], and performed measurements of backscattered light from both weakly scattering and highly scattering media.

We also investigate a novel device for focusing the excitation laser light into the sample and collecting backscattered light and present experimental results on the collection efficiency. The layout comprises two zero-order arrayed waveguide gratings (AWGs) in a confocal arrangement. We show that this method of light collection is more efficient than that which makes use of waveguide probes, and its efficiency increases as the sample is moved further away from the facet of the chip.

The proposed method for sample excitation confocal signal collection opens possibilities for on-chip spectroscopic measurements without the need for external optics, and for applications such as scanning confocal microscopy.

The authors acknowledge financial support from the IOP Photonic Devices supported by the Dutch funding agencies Senter-Novem and STW.

References

1. N. Ismail, B. I. Akca, F. Sun, K. Wörhoff, R. M. d. Ridder, M. Pollnau, and A. Driessen, "An integrated approach to laser delivery and confocal signal detection," Optics Letters 35, 2741-2743 (2010).

2. K. Wörhoff, L. T. H. Hilderink, A. Driessen, and P. V. Lambeck, "Silicon oxynitride: a versatile material for integrated optics application," Journal of the Electrochemical Society 149, 85-91 (2002).