Integrated Silicon-on-Insulator AWG Spectrometer with Single Pixel Readout for 2.3 um Spectroscopy Applications (Student Paper) Anton Vasiliev 1 , Muhammad Muneeb 1 , Jeroen Allaert 1 , Roel Baets 1 and G¨ unther Roelkens 1 1 Photonics Research Group, Ghent University-imec and Center for Nano- and Biophotonics, Technologiepark-Zwijnaarde 15 - 9052 Ghent - Belgium Tel: +3293314815, e-mail: anton.vasiliev@ugent.be ABSTRACT A compact and cheap mid-infrared spectrometer is realized by integrating a Silicon-on-Insulator (SOI) Arrayed Waveguide Grating (AWG) spectrometer operating in the 2.3 μm wavelength range with a high performance photodiode. The AWG has twelve output channels with a spacing of 225 GHz (4 nm) and a free spectral range (FSR) of 3150 GHz (56 nm), which are simultaneously collected by a single, transistor outline (TO)-packaged extended InGaAs PIN photodiode. The response of each AWG channel is discerned by time-sequentially modulating the optical power in each output channel using integrated Mach-Zehnder based (MZI) thermo-optic modulators with a π-phase shift power consumption of 50 mW. The photonic chip is interfaced using off-the-shelf electronic components and a standard 9/125 single-mode fiber. The response of the AWG is limited to one FSR using a 50 nm Full Width Half-Maximum (FWHM) bandpass interference filter. Using 31 μW optical power in the fiber, the absorption spectrum of a 0.5 mm thick polydimethylsiloxane sheet (PDMS) is sampled and compared to a benchtop spectrometer to good agreement. Keywords: integrated spectrometer, silicon photonics, arrayed waveguide grating, mid-infrared spectroscopy 1 INTRODUCTION Silicon photonics is expanding its application areas beyond optical communication. In particular, it is possible to access longer wavelengths in the same Silicon-on-Insulator (SOI) waveguide platform for novel compact sensing applications while still leveraging the well-established CMOS fabrication technology [1]. The mid-infrared wavelength range from 2-4 μm is of interest for spectroscopic detection of various substances in solid, liquid or gas phase. There are several approaches towards the realization of a compact and cheap spectroscopic system. For spectroscopy of gases, a high resolution spectrum in a narrow wavelength range is typically required. This can be realized with mid-infrared distributed feedback (DFB) laser sources which can be thermally tuned over a narrow wavelength range [2]. Spectroscopy of liquids and solids involves broad absorption features and an integrated dispersive spectrometer together with detectors and a broadband source would be a more cost-efficient solution. An integrated wavelength (de)multiplexer such as an arrayed waveguide grating (AWG) is needed. The free spectral range (FSR) and channel spacing can be tailored to fit the application. It is possible to heteregeneously integrate III-V-on-Silicon detectors on the photonic IC with the spectrometer [3]. However, existing packaged (cooled) mid-infrared photodetectors currently outperform the on-chip solutions. We present an approach for integrating a single standard TO-can packaged InGaAs PIN photodiode with a twelve channel AWG with 225 GHz (4 nm) channel spacing. The AWG output arms are modulated using on-chip balanced Mach-Zehnder interferometers (MZIs) by thermal tuning [4]. As an example, the absorption spectrum of polydimethylsiloxane (PDMS) in the 2.3 μm wavelength range is recovered. 2 EXPERIMENTS The AWG is fabricated on a 200 mm SOI wafer with a 400 nm thick crystalline Si device layer and 2 μm buried oxide layer thickness. Rib waveguides and grating couplers are defined with a 180 nm deep etch and are cladded with SiO 2 and planarized down to the silicon device layer. A thin 0.9 μm layer of SiO x is deposited on top. 100/10 nm thick Ti/Au resistors are defined on the arms of the modulators. The resistors measure 200x2 μm 2 and realize a π-phase shift with ≈ 50 mW of power dissipation. As a final step, the chip is passivated with a thin layer of BCB. The circuit is designed for 2.3 μm wavelength and TE polarized light. The AWG has twelve channels with 225 GHz (4 nm) channel spacing and an FSR of 3150 GHz (56 nm). The MZI modulators of the photonic chip are wirebonded to a printed circuit board (PCB) and are addressed by a home-built 16-bit current source through USB. A Hamamatsu G12183-010K PIN photodiode is fixed to a pre-amplifier PCB with a variable gain up to 10 6 V/A. The PD is manually positioned and fixed 0.5 mm above the output grating coupler array of the AWG, see Fig. 1(a). The output grating array covers an area of 300x140 A. Vasiliev thanks Research Foundation - Flanders (FWO) for a PhD fellowship.