We report on the design and fabrication of TE and TM polarization selective Bragg grating filters in the form of sinusoidal perturbations on the waveguide sidewall and etched holes on the top of the waveguide, respectively. Combining the two geometries on a silicon-on-insulator waveguide resulted in Bragg grating filters with high extinction ratios of approximately 60 dB

Klitis, Charalambos

Cantarella, Giuseppe

Strain, Michael J.

Sorel, Marc

English

EEG data (.mbi & .mb2 files) and E-Prime response data (.txt) files

Monica Truelove-Hill (3208266)

The Francis Crick Institute

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      Klitis, C., Cantarella, G., Strain, M. J. and Sorel, M. (2017) High-extinction-ratio TE/TM selective Bragg grating filters on silicon-on-insulator. Optics Letters, 42(15), pp. 3040-3043.     There may be differences between this version and the published version. You are advised to consult the publisher’s version if you wish to cite from it.    http://eprints.gla.ac.uk/146478/            Deposited on: 28 August 2017                       Enlighten – Research publications by members of the University of Glasgow http://eprints.gla.ac.uk 1High-Extinction-ratio TE/TM selective Bragg gratingfilters on silicon-on-insulatorCHARALAMBOS KLITIS1,*, GIUSEPPE CANTARELLA2, MICHAEL J. STRAIN2, AND MARC SOREL11School of Engineering, University of Glasgow, Glasgo G12 8LT, UK2Institute of Photonics, University of Strathclyde, Glasgow G1 1RD, UK*Corresponding author: c.klitis.1@research.gla.ac.ukCompiled August 28, 2017We report on the design and fabrication of TE and TMpolarisation selective Bragg gratings filters in the formof sinusoidal perturbations on the waveguide sidewalland etched holes on the top of the waveguide respec-tively. Combining the two geometries on a silicon-on-insulator waveguide resulted in Bragg grating filterswith high extinction ratios of approximately 60 dB. ©2017 Optical Society of AmericaOCIS codes: (130.0130)Integrated optics; (130.5440) Polarization-selective devices;(230.1480) Bragg reflectors;(350.2770) Gratings.Integrated photonics devices are extremely popular and ver-satile components for telecommunication systems [1], quantumphotonics [2, 3],hybrid III-V on silicon lasers [4, 5] and signalprocessing [6, 7]. In these applications, the high index contrastoffered by the silicon-on-insulator (SOI) photonic platform isof great benefit to component miniaturisation and dense inte-gration [8]. However, the main drawback of highly confiningwaveguides is a stronger scattering that still represents the mainsource of propagation losses. Also, scattering from non-idealsurfaces causes coupling to counter-propagating guided modesand polarisation rotation effects that severely degrades the per-formance of several integrated circuits [9]. One such example isthe popular sidewall Bragg grating that offers a simple techno-logical geometry to design TE polarisation filters with apodisedand chirped profiles [10–12]. The strong polarisation scatter-ing limits the maximum extinction ratio (ER) of these filtersto approximately 30 dB [10], which prevents their exploitationin applications requiring a high ER such as quantum circuits[13–15]. Filters with higher ER values can be obtained withmore complex designs such as Bragg gratings integrated withMach-Zehnder interferometers [16] or coupled ring resonators[15, 17].In this paper we present the design and evaluation of an al-ternative grating geometry consisting of etched holes on the topsurface of the waveguide. Contrary to sidewall gratings, thissolution is mostly interacting with the TM mode and thereforeprovides an appealing solution to selectively filter the TM po-larisation only. Not only does the combination of both gratinggeometries greatly improves the performance of Bragg gratingfilters but also offers an interesting solution for on-chip polar-isation manipulation. The theoretical design of the filters willbe discussed in section II, followed by device fabrication andcharacterisation in section III, discussion of results in section IVand conclusions in section V.Fig. 1. Simulated optical spectra of sidewall gratings for theTE polarization on a SOI single-mode waveguide as a functionof (a) grating length with a fixed grating amplitude of 5 nmand (b) grating amplitude with a fixed grating length 50 µm[18]In order to improve the extinction ratio of Bragg filters, onecan either increase the length or the strength of the grating.Figure 1 shows the simulated spectra of a sinusoidal sidewallgrating for the TE mode as a function of its length and amplitudeon a single mode SOI waveguide with 220 nm height and 500nm width. For all the simulations described in this work unless2stated otherwise, an eigenmode expansion (EME) [19] solverunder the commercial software Mode solutions from Lumericalwas used [18].Acting on both the length and amplitude of the gratingsoffers the ability to accurately control the ER and the stop bandwidth of the transmission spectrum. However, sidewall gratingshave almost no effect on the TM polarisation as shown by thesimulated optical spectra of Figure 2. In fact, only very strongand long gratings have any measurable ER, which is howeverseveral orders of magnitude lower than the figures obtained onthe TE polarisation with identical grating parameters. LargerERs for the TM polarisation can be theoretically achieved iflonger and stronger gratings are used, however, the theoreticalpropagation losses for the TE mode as extracted from the EMEsimulations can easily exceed 10 dB/cm if a 30 dB extinctionratio for the TM mode is required.Fig. 2. The simulated optical spectra of sidewalls gratings forthe TM polarisation on a SOI single-mode waveguide as afunction of (a) the length with a fixed grating amplitude of 40nm and (b) the grating amplitude with a fixed grating lengthof 200 µm [18].The reason for such different results between the two polarisa-tions is simply a consequence of their modal profile as depictedin the Finite Difference Eigenmode (FDE) simulations of Figure3. While the TE polarised mode has a strong overlap with thesidewalls of the waveguide, the TM polarisation has only a veryweak interaction and hence it is little influenced by the presenceof sidewall gratings. On the other hand, the simulations indicatethat the TM polarisation has a strong overlap with the top andthe bottom of the waveguide, which suggests that a strong TMBragg grating filter can be designed by placing the gratings onthe top of the waveguide.The shape of the top grating has been chosen in this case to becircular for easier fabrication, and hence the grating performanceFig. 3. Profile of the electric field for the (a) TE polarisationand (b) TM polarisation [18] in a 500-nm-wide and 220-nm-high SOI waveguide.was simulated as a function of the radius (r) and etching depth(h) of holes defined on the top of the waveguide as well as a func-tion of the periodicity (Λ) and length of the grating. In designingthe optimal depth and radius of the holes great attention shouldbe paid to the additional propagation losses introduced by thestructure on both polarisations. Figure 4 shows the simulatedpropagation losses for the (a) TE and (b) TM mode. The theoreti-cal losses were calculated with an Eigenmode Expansion (EME)solver from Lumerical Mode Solutions. To find the additionalcontribution to the losses introduced by the gratings only, thetransmission spectra were normalised to a reference spectrumtaken on a straight waveguide with identical length. Followingthe above simulations, the etching depth of the holes was fixedat 40 nm as this value makes the losses lower than 0.5 dB/cmfor both the TE and TM and yet provides sufficient interactionwith the TM mode.Fig. 4. Theoretical losses of the (a) TE and (b) TM polarisedmode as a function of the grating hole radius and etchingdepth [18].Figure 5 shows the simulated optical spectra for the TM po-larised mode for a 40-nm deep circular top-grating as a functionof the (a) grating length and (b) hole radius. As expected, andsimilarly to the sidewall gratings, an increase of the gratinglength or amplitude of the grating perturbation offer higher ERs.The SOI waveguides investigated in this work consisted of asilicon core with height of 220 nm and width of 500 nm. The TEgratings were defined as sinusoidal perturbation on the waveg-uide sidewall (insert of figure 2(b)) and the TM gratings as etchedholes on the top of the waveguides (insert of figure 5(b)). Thedevice pattern was defined using electron beam lithography onHydrogen Silsequioxane (HSQ) and ZEP520A as the resist lay-ers for the sidewall gratings and the top gratings, respectively.The patterns were transferred to the silicon core with the useof Reactive Ion Etching. In order to reduce the insertion loss,inverse tapers and polymer waveguides were used to couplethe light in and out of the chip. The polymer waveguides werefabricated in SU8, with dimensions of 4.5 µm in width and 3.5µm in height. The devices were coated with a 1 µm thick plasmaenhanced chemical vapour deposition (PECVD) SiO2 as a bufferlayer.3Fig. 5. The optical spectra of the TM polarisation on a singlemode SOI waveguide with circular top gratings as a func-tion of the (a) grating length with a hole radius of 40 nm anddepth of 40 nm and (b) hole radius with a length of 670 µmand depth of 40 nm [18].The devices were measured using a standard end-fire rigtransmission setup. A swept tunable laser (Agilent 8164b) wascoupled through an in-line polarizer (20dB) and a polarizationmaintaining (PM) lensed fibre into the input polymer waveguide.The output signal was coupled into a second PM lensed fibre andcoupled directly to an optical spectrum analyser (OSA, Agilent8614). To ensure accurate measurement of the grating stopband,the OSA was operated in high sensitivity mode (-90dBm noisefloor) with a resolution bandwidth of 0.1nm. An in-line polariserwas used for the input fibre and no polarisation control was usedfor the output signal.A number of gratings with different lengths were designedand fabricated to assess the performance of the top and sidewallgratings independently as well the combination of the two. Side-wall gratings with amplitude of 6 nm and top grating holes withetching depth of 40 nm and radius of 40 nm were chosen as TEand TM filters, respectively. The grating period was designedbased on the Bragg relationship, Λ = λB/2ne f f , where ne f f isthe effective index of the optical mode. As for the design ofthe sidewall gratings, a sinusoidal perturbation was chosen toimprove fabrication robustness [20]. These grating dimensionsprovide negligible propagation losses of less than 0.1 dB/cmand grating kappa values of approximately 100 cm−1 for bothpolarisations, which translates into a theoretical ER approaching60 dB on a 1 mm-long grating. The periodicities for the twoBragg grating geometries were designed to be 318 nm and 448nm for the TE and TM polarisation, respectively, due to the dif-ferent effective index for the modes, so as to provide a stop-bandcentred around a wavelength of 1550 nm.Figure 6 shows the optical spectra for different grating lengthsfor the sidewall grating geometry when a TE polarised light iscoupled to the device. The experimental data agree well withsimulated ER values up to 30 dB, after which the experimentalER value saturates because of polarisation scattering. The ER ofthe filter increases until it reaches a saturation value of approxi-mately 30 dB, which is well below the theoretical predictions. Asimilar trend was recorded with devices having a larger gratingamplitude, which indicate that the 30 dB saturation value isindependent of the grating kL product.Fig. 6. Experimental transmission optical spectra of deviceswith 6 nm-wide sidewall gratings, for a TE polarised inputbeam as a function of the grating length.A similar set of measurements was performed on deviceswith top gratings for a TM polarised input beam. Similarly to thesidewall grating devices, the ER increases with the grating lengthuntil a saturation is reached around 25 dB as shown in figure 7.Because the sidewall (top) grating is ineffective on the TM (TE)polarisation, the saturation value is a consequence of the non-perfect ER of the input polariser, which is approximately 30dB,and polarisation rotation effects occurring inside the device as aconsequence of the strong scattering.Fig. 7. Experimental transmission optical spectra of deviceswith circular top gratings with 40 nm depth and 40 nm radiusfor a TM polarised input beam as a function of the gratinglength.The characterisation of the individual filters, was followedby the measurements of devices combining both filters on thesame waveguide. Figures 8 shows the optical spectra for a TEpolarised input signal for a 670 µm-long device combining bothgrating geometries. Because the top grating stop band is slightly4narrower than the sidewall grating stop band, as the couplingcoefficient for the TM filter was chosen to be smaller than the TEfilter, the effect of combining the two geometries on the gratingperformance is clearly visible. In fact, where only the sidewallgrating is effective (i.e. from approximately 1541 nm to 1546nm), the maximum ER is around 30 dB, similarly to the valuesreported in Figure 6. Where the two stop bands overlay (i.e.from approximately 1537 nm to 1541 nm), the ER substantiallyimproves to a value of approximately 60 dB, which is a valuelimited by the substrate scattering. When the two couplingcoefficients are matched, the same high ER is observed for allthe stop band as figure 9 shows. Similar figures in terms ofmaximum ER were measured when a TM polarised input beamwas coupled to the device. It is worth noting that, althoughchallenging, the 160 nm diameter of the top waveguide holescan be fabricated by DUV lithography [21].Fig. 8. The experimental transmission optical spectra of thecombined sidewall and top grating with 40 nm radius for aTE polarised input signal. An increase of the ER is recorded inthe spectral region where both TE and TM stopbands overlap(labelled ‘TE+TM’ in the figure).Fig. 9. The experimental transmission optical spectra of thecombined sidewall and top grating with 80 nm radius for a TEpolarised input signal. Because the width of the TM stopbandis larger than that of the TE, the benefit produced by filteringthe TM mode is visible over the whole stopband.In this work, silicon waveguide sidewall and top Bragg grat-ing filters were designed, fabricated and characterised as a func-tion of their geometrical parameters. Because of the very dif-ferent modal profile in single mode silicon waveguides, thesidewall geometry is mostly effective on the TE polarised modewhile the top grating mostly interacts with the TM polarisedmode. Combining these two geometries on the same waveguideallows the design of gratings that are effective on both polarisa-tions thus substantially improving the grating ER. Experimentaldata show that the ER increases from 30 dB, a limit given by thecombination of polarisation scattering inside the waveguide andnon-ideal input polarisers, to approximately 60 dB. Such gratinggeometries also offer a solution for independent polarisationmanipulation on-chip.Funding:Engineering and Physical Sciences Research Coun-cil (EPSRC) (EP/L021129/1); H2020 Future and Emerging Tech-nologies (FET) (323734); H2020 Excellent Science (645361).Acknowledgment. The authors wish to thank the staff of theJames Watt Nanofabrication Centre at the University of Glasgow.G. C. and C. K. acknowledge Fraunhofer Centre for AppliedPhotonics and EPSRC for the Ph.D. scholarship funding, respec-tively.REFERENCES1. D. T. H. Tan, K. Ikeda, R. E. Saperstein, B. Slutsky, and Y. Fainman, Opt.Lett. 33, 3013 (2008).2. M. J. Collins, C. Xiong, I. H. Rey, T. D. Vo, J. He, S. Shahnia, C. Rear-don, T. F. Krauss, M. J. Steel, a. S. Clark, and B. J. Eggleton, Naturecommunications 4, 2582 (2013).3. S. Azzini, D. Grassani, M. J. Strain, M. Sorel, L. G. Helt, J. E. Sipe,M. Liscidini, M. Galli, and D. Bajoni, Opt. Express 20, 23100 (2012).4. Y. de Koninck, F. Raineri, A. Bazin, R. Raj, G. 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High-extinction-ratio TE/TM selective Bragg grating filters on silicon-on-insulator

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