or Val 25, No 8117 Ape 2017 [OPTICS EXPRESS AOS Bae CMOS-compatible broadband co-propagative stationary Fourier transform spectrometer integrated on a silicon nitride photonics platform XIAOMIN NIE," RoeL Baets"? " Eva RYCKEBOER,"? GUNTHER ROELKENS,"? AND. "Photonics Research Group, Department of Information Technology, Ghent Universit-imec, Ghent 5-900, Belgium Center far Nano and Biophotones(NB-Photonic). Ghen University, Ghent B-9000, Belgium mall XaominNedugene Abstract: We demonstrate a novel type of Fourier Transform Spectrometer (FTS) that can be realized with CMOS compatible fabrication techniques. This FTS contains no moving components and is based on the direct detection of the interferogram generated by the interference of the evanescent fields of two co-propagating waveguide modes. The theoretical analysis indicates that this type of FTS inherently has a large bandwidth (>100 nm). The fist prototype that is integrated on a Six waveguide platform is demonstrated and hasan extremely small size (0.1 mm*). We introduce the operation principle and report on the preliminary experiments, The results show a moderately high resolution (6 nm) which is in good agreement with the theoretical prediction, © 2017 Optical Society of America (OCS caer: (30.5300) Spectroscopy, Fourier tansforms, (30.5190) Specmetrs, (130-3120) Itgrated opis devices References and links 1. PLR, Grits and J. A. De Hath, Fourier Prunsorm Infrared Spectrometry John Wiley aS 2. C.P. Bacon, ¥. Mates, and R. DeFiece, "Minature specuoscope instunen chemi" Rev. Sei Insum. 71), 1-16 2008), 3. T Sandner, A. Kenda, C. Drabe, H, Schenk, and W. Scher, “Miniruized FTIR-pesrometer based opsical [MEMS transitory actual” n SOEMS-MEMS 2007 Micro and Nanofariction (ISOP, 2007, pape 645502. 4. K.Yo,D Lee, U Krishnamoorthy, N. Pack, and O, Solan, "Micromachined Fourier inform specter on silica opial bench platform: Ses, Action A: Phys. 130, $23-530 2006), L. Wu, A. Pus SR. Sumuslson, . Guo, and H. Xi, “A mirartk-insensitve Fowier tansfonm spectomster based on a lage vertical displacement micromir with dual elective surface” in TRANSDUCERS 2009-2009 ‘ternational Solid-State Sensors (IEEE, 2003, pp. 2090-2093 6, E.Le Conte, Bsize, ?. Henech, | Siefanon, A. Morea, G. Léonds, G, Leblond,P. Ker, J. M, Fel and Reyer," Wavelengtvscale stationary-wave integrated Foue transform spectrometry” Nat. Photonics 1(8}, 477= $78 2007) M. Flot, P.Chebes, Sav, A. Scot B. Solheim, and D.X. Xu, “Planar waveguide spatial heterodyne spectrometer” Pre, SPIE 6796, 679631 2007). 8. S-P-Davis, MC. Abrams, and JW. Brault, Fourer Transform Spectrometry (Academe Press, 2001) 9. X. Nie E.Ryckeboer, 6, Rolkens, and R.Baes, “Nove concep fora troadband co-popagativ tationay Fourie cansform spectrometer integrated on a SIIN$ waveguide platform.” in Confrence on Lasers and Eleto-Optc, (OSA Tesineal Digest online) (Optica Society of Amerie, 2016), paper IW2A.120, K. Solehmainen, M. Kapulanen, M. Harjnne, nd T- Aalto, “Adiabatic and mulimode interference couplers on silicon on-iasulato” IEEE Photonics Tech. Let. 18(21), 2287-2289 (2008) 11. W.Shi, X Wang, W. Zhang, L-Cheostonski and N-A..acger,“Contadicetonal coupler ia siion-om-iasulator rib waveguides” Opt. Let 3620), 399-4001 (2011 12, AZ. Sabramanian,P Nestens, A Dhaka, R Jansen, T. Clies,X. Rotenberg. F. Peyskens, §, Seba, Belin, B. Da Bos, and K, Leysens, "Lowes singlemode PECVD silicon nde holon wire waveguides for 532-900 1m wavelength window fabricated within s CMOS pl line” IEEE Photon J. 56), 2202809 (2013) 13, Mc Fie, E, Lambert, 8 Pathak, B. Maes, P. Bientman, W. Bogaert, and P Dumon, “Improving the design ele for nanophotonis components" Compu. Sei (5), 313-324 2013). 5s, 2007), applications to biology and suns nepsoiorg/16.1364/08.25.008409 Journal © 2017| Received 16 Jan 2017 revised 21 Mar 2017; accepted 30 Mar 2017; published 7 Ape 2017or Vol 25. No.8 17 Ae 2017 | OPTICS EXPRESS ALTO Bae 14, F.Thomas,B. Matin, C. Dashemin,R. Puget, F. Morin, C, Bonneville, T: Gontiz,P. Renech and. Le Coaret, “Msjor advances in dovslopments and algonthms of he nationary-wavsitegratd Fouricrtansorm tchnologs” i ih Ene on te Eminent OSA Tec Dt (oie) (Opel Seca Aer, 2016), eer 15, A. Dhaial P. Wuytens, F,Peykens, AZ. Subramanian A Skinach, N Le Thos, and R. Bass, “Nanophotonic Lab-Os-AChip Raman Sensors: sensitivity comparison wih confocal Raman microscope” ia Proceedings of IEEE Conference on BioPhotoncs (EEE, 2015), pp I 16. E. Ryckebocr, R Bockstacl, M, Vrslembrovck, and R. Baes, "Glucose sensing by wavegide-based absorption spectroscopy ona licon chip.” Biomed. Opt, Express (8), 1636-1688 (2013), 1. Introduction Optical spectrometers have become an indispensable tool in various fields that involve optical spectrum analysis, Its application ranges from biochemical sensing to food quality control and even to astronomical radiation analysis. Among all types of modem specirometers, the Fourier transform inftared spectrometer (FTIR or FTS) offers important advantages such as a high ‘throughput and the multiplex advantages (1, In a standard FTIR, all wavelengths are measured simultaneously in a large series of subsequent increments of the optical path difference (OPD) ina Michelson interferometer. The spectral content of the signal is encoded inthe interferogram, created by the interference between an optical signal and its delayed version, and can be decoded. by applying a Fourier transform. Typically, the longer the interferogram an FIS can measure, the higher the spectral resolution. ‘The high demand for optical spectrometers and the growing preference for portable and robust devices have together created a trend towards the miniaturization and integration of the FTS [2]. Although there exist various miniaturized Fourier transform spectrometers that still involve moving components [3-5], it would be better to have an FTS without any moving. components, Currently, integrated FTS that are implemented without any moving components can be divided mainly into two categories: the stationary wave integrated FTS (SWIFTS) [6] and the spatial heterodyne spectrometer (SHS) [7] In the scheme of SHS, an array of unbalanced Mach-Zehnder interferometers (MZIs) is used to generate a spatially varying interference pattern. To increase the resolution fora given spectral bandwidth, a larger number of MZIs is required [7], implying a rapidly growing size of the device when sealing the spectral resolution. However, for applications that have Tess constraints ‘on the footprint, SHS can be particularly suitable since the underlying principle allows SHS to achieve high resolution in a narrow spectral band without degrading the signal to noise ratio, On the other hand, SWIFTs-based devices ean achieve high resolution within a small footprint, In such devices, the interferogram is the standing wave pattern generated by the interference of two counter-propagating beams inside a waveguide, On top of the waveguide, nano-seatterers are carefully positioned to couple this interference pattern to a detector array. ‘As introduced in the work of Etienne le Coarer et al [6], the pitch of the interference pattem can be expressed as 4/(2neys) which is much smaller than the pitch size of state-of-the-art photodiode arrays. As a result, the interferogram is subsampled, leading to a limitation on the spectral bandwidth of the FTS. In this paper, we propose a new type of stationary wave integrated PTS design. Our proposed design, as a waveguide based stationary FTS, is compact and stable. Moreover, in our design, the subsampling is avoided by generating an interferogram that is spatially stretched compared to the original SWIFTs concept. This effectively helps to lift the restriction on the operation bandwidth. Our spectrometer is designed to target on-chip Raman or absorption spectroscopy applications. The envisioned bandwidth of about 100 nm is large enough to accommodate these applications. Compared to a SWIFTS, for a given device length, a lower resolution is obtained in cur case. However, we demonstrate that a spectral resolution of 6 nm can be realized, which is sufficient for most applications of Raman spectroscopy and infrared absorption spectroscopy ofticle ‘Val 25. No.8 17 Apr 2047 | OPTIGS EXPRESS NETH ne liquids and solids. Since in on-chip Raman or infrared absorption spectroscopy the signals are collected in a single mode waveguide, the use of single mode waveguides in the spectrometer 4does not impair the light collection efficiency of the spectrometer. We have developed a first prototype that is fabricated on a CMOS compatible SisNs ‘waveguide platform. This photonic integrated circuit (PIC) technology allows to fabricate spectrometers cost-effectively at a wafer scale with high yield, Moreover, the FTS prototype can be further integrated with a CMOS photodiode array to create a very compact, portable device. In the following sections, we will fist introduce the theoretical principles of our new device and discuss the design and simulation results. After that, we will give the first experimental results from our prototype on-chip FTS. We will discuss these prefiminary experimental results and analyze the potential of our device 2. Principle of the co-propagative stationary FTS All types of Fourier transform spectrometers are based on the creation of an optical interferogram. In the co-propagative Stationary FTS that is proposed here, the interferogram js generated by the interference between two waveguide modes that propagate in the same direction but with a slightly different phase velocity, This is the origin of the name co propagative stationary FTS. A 3D representation of the structure we propose is shown in Fig. I Fig. 1. Conceptual drawing ofthe co-propagatve stationary FTS. 9] ‘The signal to be analyzed is first injected into the input port of a Multimode Interference (MM coupler which then splits the light into two parallel waveguides with different width, One can also understand this as the excitation of two supermodes in a coupled waveguide system, These two modes are predominantly confined in the left and right waveguide core and are orthogonal to each other so that there will be no energy exchange related to mode conversion along the propagation, The excited waveguide modes that propagate in the waveguides with different widths will have different phase velocities. We space the parallel waveguides in such ‘away that the evanescent tails of the two waveguide modes slightly overlap. In this way, the interferogram is created in the region between the two parallel waveguides, At the location ‘where the interferogram has the best contrast, a well-designed grating with proper period is positioned in order to diffrat the interferogram upwards onto the detection system, for example,Vol 25. No.8 1 Ap 2017 | OPTICS EXPRESS ACT2 1 photodiode array as shown in Fig. 1 ‘As an example, we consider the injection of a monochromatic signal with a wavelength A. ‘The intensity distribution of the interferogram as function of propagation distance x can be related to the optical intensity in each waveguide as I(x) oly + + 2VTihh cos (AB), a ‘with 1) the intensity of the individual waveguide mode and Af the propagation constant difference, which is in approximation proportional to the difference in the effective indices of the two waveguide modes Arey, 2 As a result, we can express the period of the interferogram as a AS @ Brey If one compares this expression to what is found in the original SWIFTs design [6], one can find that we are actually stretching the interferogram by a factor of 2eyy/A/eyy, which makes the period as lage as several tens of micrometers. Consequently, itis now easy to find photodiode arrays that are commercially available and that have a pixel pitch small enough to avoid subsampling ofthe interferogram. One ofthe Key charactristies of the FTS is the spectral resolution. In our co-propagative stationary FTS, the spectral resolution is determined by the length of the single-sided interferogram that we capture, To give the expression of the resolution, we consider a single wavelength injection. The fact that the interferogram we ean measure only covers a finite Jength L will broaden the delta-shaped spectrum in the Fourier domain into a sine line shape, sine[e(f. ~ 1/A)L], where fis the spatial frequency. We define the spectral resolution, which equals to the Full Width Half Maximum (FWHM) of the sine shape spectrum, as o In principle, one can obtain higher spectral resolution through additional data processing [8] However, as this topic is out of scope for this paper, we will continue to use the resolution as defined by Eq. (4) in the following discussions. Now, if we for example assume an effective index difference of 0.05 at the wavelength of $00 nm, which is practical ina Sis Ny rib waveguide platform, and consider 1 em long interferogram, we can expect a spectral resolution of 1.28 nm, According to Eq. (4), the resolution can be improved by simply making the waveguide longer and thus recording a longer interferogram, Although Eq, (4) gives the achievable spectral resolution, other factors can also lower the resolution in practice. An important one is that as we nced to constantly couple power to the detector, an exponential decay will be superimposed on the interferogram, which leads to line broadening and lower resolution. In principle, one can carefully design a grating with increasing strength to partially compensate the power decay and thus approach the resolution expressed by Eq, (4). However, in this frst prototype, a uniform grating is designed which leads to a lower resolution as we will see later in the experimental results. The exponential decay superimposed on the interferogram will also degrade the dynamic range of the system, as the optical intensity of the interferogram at large OPD is reduced. To allow the system to have the same dynamic range as in a standard FTS, we will try to improve the compensation of the exponential decay in future versions of the design.or Vol 25.8 1 Ae 2017 | OPTICS EXPRESS ACTS intensity fu) intensity a.) intensity fa.) Bose aaa ee aa 140, Fig. 2 (a) Simulation results of the interference pattem between the two waveguides inthe ‘ase of single wavelength injection. The postion where ane should position the grating is ‘marked by the blue frame. (b) A zoom in ofthe interferogram inthe grating region. (c) A zoom-in ofthe first several petiods of te intensity oscillation, [9] ‘Another characteristic isthe operational spectral range, Since we stretch the interferogram and make its intensity oscillation period as lage as afew tens of micrometers, we can fulfill Nyguist-Shannon’s sampling theorem and avoid subsampling, Now the only factors that limit the bandwidth are the operation wavelength range of the detectors and the spectral range in Which the waveguides remain single-mode, which is typically a few hundreds of nanometers in the Sis platform. In this design, the bandwidth i limited by the MMT coupler to around 100 rim which is aeady sufficient for some on-chip spectroscopy applications. However, in future versions of the design, we can extend the bandwidth by exploring alternative splitters {19}, Last but not least, it is important to have a proper design of the grating that we want 10 position between the two parallel waveguides, Since the purpose ofthe grating isto diffract the interferogram onto the detector array, it determines the efficiency of the spectrometer. Inthe design ofthe first prototype, we use standard grating, which iffacts only half ofthe power in the grating region upwards, However, for future versions of the design, we have learned fiom our ongoing research program that a carefully designed high directionality grating can diffract more than half of the power upwards and thus lead to a higher eflicieney. Moreover it should have very weak reflections, minimizing contia-directional coupling between the (wo waveguides [11], Ths will help reduce the distortions of the interferogram to be measured. The gating strength and the distances between the waveguides and the grating will together play an important role to determine the quality of the interferogram that we can measure. In Fig.2, wwe show the simulation result of the optical intensity distribution in the region between the two waveguides, assuming the injected signal is monochromatic, The blue fame in Fig. 2a) indicates thatthe grating should be positioned atthe region where the optical intensity of the evanescent fields both waveguide modes have the same strength, which leads to best contrast. 3. Fabrication of the first prototype To realize the first prototype of the co-propagative stationary FTS, we used the CMOS pilot line of IMEC [12]. The fabrication starts with a bare silicon wafer. In the first step, a layer of silicon dioxide is deposited using a high-density plasma (HDP) chemical vapor deposition