Subwavelength grating waveguide devices in siliconon-insulators for integrated microwave photonics

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1 Suwvelength grting wveguide devices in siliconon-insultors for integrted microwve photonics (Invited Pper) Lwrence R. Chen* Deprtment of Electricl nd Computer Engineering, McGill University, Montrel, QC H3A E9, Cnd *Corresponding uthor: Received August 26, 216; ccepted Octoer 14, 216; posted online Novemer 21, 216 We provide n overview of our recent work on developing suwvelength grting (SWG) wveguide devices s n enling technology for integrted microwve photonics. First, we descrie wvelength-selective SWG wveguide filters, including ring resontors, Brgg grtings, nd contrdirectionl couplers. Second, we discuss the development of n index vrile opticl true time dely line tht exploits sptil diversity in n equl-length wveguide rry. These SWG wveguide components re fundmentl uilding locks for relizing more complex structures for dvnced microwve photonic signl processing. OCIS codes: 5.277, , , doi: /COL Microwve photonic (MWP) systems exploit the dvntges of photonics, especilly with regrds to ultrrod ndwidth, dptility, nd prllelism, chrcteristics tht re significntly more chllenging to implement in the electronic domin [1 3]. Thus, MWP systems cn e used to relize numer of microwve signl processing functions, including, wveform genertion, Hilert trnsformtion, nd time dely/phse shifting. Significnt efforts re currently devoted towrd developing integrted photonic technologies to relize MWP signl processing functions [3 7]. This hs een motivted in prt y the need to ddress issues ssocited with conventionl MWP systems nd signl processing engines sed on either fier or ulk optics, prticulrly with regrd to the lck of compctness, stility, nd reliility. A numer of MWP integrtion pltforms hve een explored (see Ref. [4] for review) nd ech pltform hs strengths/dvntges nd/or disdvntges in terms of relizing pssive functionlity, e.g., opticl signl routing or filtering, nd ctive functionlity, e.g., O/E, E/O, nd integrtion with RF electronics. Recently, there hs een growing interest in relizing integrted opticl components sed on suwvelength grting (SWG) structures [8 17]. As illustrted in Fig. 1(), SWGs re formed y periodic rrngement (with period Λ) of high refrctive index mteril (e.g., silicon) with thickness surrounded y low refrctive index mteril (e.g., silic); the duty cycle of the SWG is defined s D ¼ Λ. Light cn propgte in the direction perpendiculr to the suwvelength structure, i.e., crosswise opertion, s well s prllel to the xis of the structure, i.e., lengthwise opertion. In this pper, we consider the ltter cse, wherey light cn propgte in the SWG structure in the sme wy s in conventionl wveguide [8]. To crete n SWG wveguide, finite trnsverse dimensions, e.g., height h nd width W, re pplied to mteril of high refrctive index, s shown in Fig. 1(). The crrier frequency of the incident light signl determines the operting regime of the SWG wveguide. We re prticulrly interested in wht is known s the suwvelength regime, wherey the SWG wveguide cn e modeled s conventionl wveguide hving the sme trnsverse dimensions nd uniform refrctive (effective) index long the direction of propgtion. The effective index of the SWG wveguide depends on the duty cycle D. Note tht tpers re used to convert wveguide mode propgting in conventionl wveguide (e.g., silicon nnowire wveguide) into Bloch mode tht propgtes in the SWG wveguide. These (SWG) tpers, depicted in Fig. 1(c), re sed on linerly chirped grting structure tht hs uniform period P nd where the wveguide width is nrrowed in liner mnner from width W 1 to W 2 over length L tper [12]. In this pper, we review recent work on relizing sic uilding locks sed on SWG wveguides in SOI tht cn Fig. 1. SWG wveguide in SOI: () sic structure, () wveguide cross-section, nd (c) top view of m SWG tper used to couple light etween n SWG wveguide nd conventionl wveguide /216/14(5) Chinese Optics Letters

2 e used to implement more complex, integrted MWP signl processing engines. This includes SWG filters such s microring resontors (MRRs), Brgg grtings (BGs), nd rodnd contrdirectionl couplers (CDCs), s well s index-vrile opticl true time dely lines (OTTDLs). Vrious MRR-sed integrted circuits hve een implemented in SOI s well s silicon nitride [5]. Recently, we demonstrted oth conventionl MRR nd rcetrck ring resontors sed on SWG wveguides in SOI [17]. First, we relized conventionl MRRs with dd drop ports sed on two SWG us wveguides nd n SWG ring wveguide, s shown in Fig. 2(). The devices (s well s others descried in this pper) were fricted using e-em lithogrphy nd full etch. The wveguide dimensions re h ¼ 22 nm W ¼ 5 nm. The wveguides sit on 3 μm thick uried oxide (BOX) lyer nd re covered y n index-mtched cldding lyer of thickness 2 μm (this lso pplies to ll other devices descried in this pper). Here, the SWG grting period is Λ ¼ 3 nm nd the duty cycle is D ¼ 5%. A gp length g ¼ 64 nm is used for coupling etween the ring nd us wveguides. The SWG tper hs P ¼ 2 nm, nd the wveguide width is reduced from 5 to 2 nm over length L tper ¼ 15 μm. Verticl grting couplers (VGCs) optimized for TE trnsmission serve to couple light into nd out of the device; conventionl nnowire wveguides in SOI connect the VGCs to the SWG wveguides (vi the SWG tpers). The typicl fier-to-fier loss of n MRR is 16 db; sources of loss include coupling loss due to the VGCs used for input nd output coupling, propgtion loss in the SWG wveguide (including ending loss), nd insertion loss from the SWG tpers used for coupling etween conventionl nnowire nd SWG wveguides (due to mode mismtch). Figures 2(c) 2(e) show the mesured through responses for SWG MRRs with different ring rdii. Our MRRs exhiit Q-fctors of severl hundred to 12; higher vlues up to 6 with n ir cldding or 4 with fluidic cldding were lso otined [14]. We ttriute irregulrities in the mesured responses of the MRRs (e.g., in the shpe of the resonnces) to friction nd processing imperfections. To reduce further the fier-to-fier loss, the SWG tper cn e optimized to reduce the mode mismtch loss (see Ref. [12]). A 9 ending loss of 1.5 ð1þ db ws demonstrted in Ref. [12] for end rdius of 1 (3) μm. These losses cn e reduced even further y utilizing trpezoidl silicon segments (s opposed to rectngulr silicon segments) in the SWG wveguide ends [16]. Reducing totl ending nd scttering losses further will e necessry so tht SWG MRRs hve comprle chrcteristics (e.g., Q-fctor nd/or extinction rtio) to their conventionl silicon (or silicon nitride) wveguide counterprts. Due to the longer coupling region etween the resontor nd us wveguides, rcetrck designs my e preferred. Figure 3 shows the lyout nd zoom of rcetrck resontor sed on SWG wveguides. The length of the directionl coupling section L long with the gp g determine the coupling efficiency nd operting condition (under-coupled, criticlly coupled, or over-coupled) of the resontor. The curved section hs rdius r, nd the totl perimeter of the rcetrck tht determines the free spectrl rnge (FSR) is 2πr þ 2L. For n experimentl implementtion, we considered the sme SWG wveguide prmeters s for the MRRs s well s r ¼ 2 μm, L ¼ 4 μm, nd g ¼ 6 nm. Figure 3(c) shows the mesured through nd drop responses; the fier-to-fier loss is lso 16 db. An extinction rtio s high s 33 db ws otined t wvelength of nm; the corresponding Q-fctor is 153. On the other hnd, the limited extinction rtio in the drop response my e due to the directionl coupling/operting condition. For some filtering pplictions, the periodic nture of MRRs is undesirle. In this context, BGs re more suitle. BGs cn e relized y inducing periodic vritions in the refrctive index long the direction of propgtion s well s physicl corrugtions or vritions in the wveguide geometry. We cn crete periodic vrition in the effective index long the direction of propgtion y interleving two SWG wveguides hving different Drop x y z Input g Through c d e wvelength 1625 [nm] wvelength 1625 [nm] μm wvelength 1625 [nm] 165 Fig. 2. Conventionl MRR sed on SWG wveguides in SOI: () schemtic nd () SEM prior to top oxide cldding deposition, nd mesured through responses (normlized) for ring rdius of (c) 15, (d) 2, nd (c) 25 μm. Fig. 3. SWG rcetrck resontor: () lyout, () detiled view, nd (c) mesured through (lue) nd drop (red) responses (normlized). See text for device prmeters. 14-2

3 Fig. 4. SWG BG: () schemtic of SWG BG formed y interleving two SWG wveguides of different duty cycles, () device lyout for experimentl demonstrtion, nd (c) SEM of the fricted SWG BG prior to the oxide cldding deposition. duty cycles. Figure 4() shows the schemtic of such n SWG BG, in which two SWG wveguides with duty cycles D 1 ¼ð 1 Λ 1 Þ nd D 2 ¼ð 2 Λ 2 Þ re interleved; the period of the SWG BG is Λ 1 þ Λ 2. We fricted n SWG BG sed on wveguides with cross-section h ¼ 22 nm W ¼ 5 nm nd periods Λ 1 ¼ Λ 2 ¼ 28 nm. The SWG BG hs 2 periods, for totl length of 1.12 mm. To extrct the reflection response, we use compct Y-rnch sed on conventionl silicon nnowire wveguides. We lso use the sme SWG tper nd VGC designs s for the MRRs. The VGCs re seprted y 127 μm, nd fier rion rry is used for testing. The device occupies footprint of 1.18 mm 254 μm. Figures 4() nd 4(c) show the device lyout nd n SEM imge of n SWG BG prior to the deposition of the top oxide, respectively. Figure 5 compres the trnsmission response of n SWG BG with D 1 ¼ 5% nd D 2 ¼ 48% with tht of simple SWG wveguide. The SWG BG exhiits cler rejection pek t resonnt wvelength of nm nd the trnsmission loss is 12 db, corresponding to pek reflectivity of 9.4%. The 3 db ndwidth is.5 nm nd the totl fier-to-fier loss 15 db. One drwck of two-port BG devices is tht they typiclly operte in reflection, which usully trnsltes into the need for opticl circultors or Y-rnch (splitter/ coupler), which, in turn, induces dditionl opticl loss nd cn impct the gin in n MWP system. On the other hnd, widend BG-defined filtering requirements cn e otined using compct SOI grting-ssisted CDCs, which re intrinsiclly four-port (dd drop) devices [18]. Grting-ssisted CDCs re sed on two closely spced (nd symmetric) wveguides nd some form of periodic refrctive index perturtion long the wveguides. We demonstrte n lternte pproch wherey the symmetric wveguides nd grting mechnism re replced y n SWG wveguide in proximity to continuous wveguide. In SOI, ecuse of the lrge opticl phse mismtch etween the SWG nd nnowire wveguides, undesired codirectionl coupling cn e efficiently suppressed. Moreover, the SWG wveguide provides the required grting mechnism nd enles contrdirectionl coupling. The strong index vrition in the SWG leds to reltively strong coupling coefficient such tht power trnsfers cn e otined using reltively short coupling lengths. The schemtic of the proposed design nd the lyout of fricted device re illustrted in Figs. 6() nd 6(). A conventionl nnowire wveguide is grdully rought to gp distnce g from n SWG wveguide to crete the coupler wist hving length L C. As efore, the cross-section of the SWG nd nnowire wveguides is h ¼ 22 nm W ¼ 5 nm; the SWG hs period Λ ¼ 378 nm nd D ¼ 5%. The SWG tpers hve P ¼ 25 nm nd width vrition from 5 nm down to 15 nm over length of 15 μm, nd input/output coupling is performed with VGCs. Figure 6(c) shows the mesured through nd drop responses for n SWG CDC with L C ¼ 4 μm nd g ¼ 15. The ndwidth of the drop/through responses is 16 nm nd the extinction rtio is >35 db. Due to the lrge degree of symmetry in the effective refrctive indices of the SWG nd conventionl nnowire wveguides, there is lrge wvelength seprtion etween the resonnt (drop) wvelength of the SWG CDC t λ C nd unwnted intr-wveguide ck reflections (t λ R1 nd λ R2 ). Figure 6(d) shows the simulted response for n SWG CDC with Λ ¼ 378 nm, g ¼ 2 nm, nd L C ¼ 1 μm. There is single-nd operting window of nerly 2 nm, which cn exceed tht of previously demonstrted grting-ssisted CDCs in SOI. The MRRs, BGs, nd CDCs re fundmentl uilding locks tht hve immedite ppliction in MWP signl processing s well s in implementing more complex Trnsmission [2 db/div] SWG wveguide SWG BG db/div]-1 Trnsmission Trnsmission [1 [1 db/div] 12 db -1 Reflection [1 db/div] Reflection [1 db/div] c d Fig. 5. () Mesured trnsmission spectrum of SWG BG nd simple SWG wveguide (see text for prmeters). The difference in trnsmission responses is due in prt to the spectrl response of the Y-rnch (optimized for 155 nm), which is present in the SWG BG only. () Zoom of the trnsmission (lue) nd reflection (green) responses out the resonnt pek t nm. Fig. 6. () Schemtic of the proposed SWG CDC in SOI, () lyout of device with L C ¼ 5 μm, (c) mesured drop (red) nd through (lue) responses (see text for device prmeters), nd (d) 3D FDTD simulted response for SWG CDC with Λ ¼ 378 nm, g ¼ 2 nm, nd L C ¼ 1 μm. 14-3

4 devices nd systems. For exmple, due to their wvelength-selective nture, the mplitude response of MRRs nd BGs cn implement frequency discrimintion filters for phse modultion-to-intensity modultion conversion with pplictions for UWB wveform genertion [2]. MRRs cn e cscded to otin higher-order filter responses or to provide time delys [5]. A seril rry of BGs t different resonnt wvelengths cn lso e used to implement discretely tunle dely line [19]. Cscded MRRs with different resonnt wvelengths (i.e., unique wvelengths within n FSR) or BGs cn e used s spectrl shpers for generting chirped microwve wveforms sed on the principle of spectrl shping followed y wvelengthto-time mpping [7,2,21]. Finlly, the spectrl response of the CDCs cn e tilored y controlling the SWG prmeters or coupling prmeters. In prticulr, phse shifts cn e pplied to relize resonnces within the drop response. Such equivlent phse-shifted grting responses hve pplictions for performing temporl opertions such s differentition nd Hilert trnsformtion [22 24]. It hs een shown tht SWG wveguide crossings exhiit low loss (.23 db crossing) nd crosstlk (<4 db) [25]. Thus, one dvntge of implementing uilding locks sed on SWG wveguides is the potentil for very dense integrtion. A numer of techniques exist to implement n opticl dely pssively, i.e., without gin, nd rodly speking, these cn e divided in two ctegories: (1) length-vrile dely lines wherey the propgtion length (L) of the dely element is vried (i.e., the dely Δt is given y L v g where v g is the propgtion group velocity) nd (2) vrile propgtion velocity dely lines where v g is vried (for some implementtions, this is lso known s wvelengthvrile dely line). These two pproches encompss employing fixed length of wveguide, fier, or free spce s the dely medium, s well s resonnce enhncements in which the physicl pth of the dely medium is enhnced through cvity or y exploiting resonnces where dispersion cn e lrge. There is well-estlished trdeoff etween the mount of resonnce enhncement tht cn e otined (nd hence the mount of opticl dely) nd the operting ndwidth. As such, in this pper, we consider only opticl dely lines (ODLs) tht do not involve resonnce enhncements. An ODL tht provides time delys for pulses or signls t the sme opticl crrier is one form of n OTTDL. The conventionl rchitecture of such n OTTDL employs n rry of wveguides of different lengths wherey the difference in lengths induces the differentil/incrementl dely etween wveguides (lso referred to s tps). For discrete tuning, the different lengths of wveguides cn e cscded using switches or splitters/cominers. To reduce the chip size for integrted implementtions, the wveguides re typiclly rrnged with spirl or curvy/serpentine topologies. Indeed, impressive results of 4-it nd 7-it delys hve een otined in SOI nd silicon nitride [26,27]. To minimize or reduce complexities ssocited with length-vrile OTTDLs is to develop n index-vrile OTTDL, where true time dely control cn e otined through vrition in the group index or propgtion velocity in the wveguides. Gsull nd Cpmny proposed to exploit the prllelism inherent in multicore fiers to implement n index-vrile OTTDL, where the true time dely of ech tp cn e controlled through designing proper physicl dimension nd mteril doping concentrtion of ech fier core [28]. The use of SWG wveguides to crete dely or pth mismtch in Mch Zehnder interferometer ws demonstrted recently [29]. We uilt on this ide nd took dvntge of the ility to tune the effective index of n SWG wveguide through control of its duty cycle D to relize n integrted index vrile OTTDL [3]. To investigte the minimum incrementl time dely tht is possile, we fricted 2-rm MZIs incorporting SWG wveguides with different duty cycles in ech rm, i.e., with duty cycle difference of ΔD, s shown in Fig. 7(). The SWG wveguides in ech rm hve the sme length of 8 mm. The SWG wveguides hve cross-section of h ¼ 22 nm W ¼ 5 nm nd period Λ ¼ 25 nm. The SWG wveguides re coupled to conventionl silicon nnowires (used for the Y-rnches) with the sme tper designs s for the SWG MRRs nd BGs. Figure 7() shows the mesured spectrl responses; the FSRs re 5, 2.9, nd 1.7 nm for ΔD ¼ 1%, 2%, nd 3%, respectively, corresponding to dely differences of 1.6, 2.8, nd 4.7 ps etween the MZI rms. The minimum chievle time dely will scle proportionlly with the length of the SWG wveguide, nd su-picosecond time dely resolutions cn e expected with shorter wveguides (<8 mm). Next, we fricted n OTTDL for microwve phse shifting. The structure comprises n rry of 4 seprte SWG wveguides ( microwve signl modulted on n opticl crrier will experience different phse shift when propgting through the different SWG wveguides) of the sme length (8 mm) nd where the duty cycles re vried in 1% increments from D 1 ¼ 6% to D 4 ¼ 3%; reference pth comprising short length of nnowire wveguide tht connects the input nd output VGCs is lso included, see Fig. 8(). The SWG wveguides re Spectrl Response (db) ΔD = 3% ΔD = 2% ΔD = 1% Wvelength (nm) Fig. 7. () Schemtic of MZI incorporting SWG wveguides with different duty cycles D 1 nd D 2 in ech rm. () Mesured spectrl responses of MZIs with ΔD ¼ 1%, 2%, nd 3%. 14-4

5 RF Phse Shift (rd) Reference SWG D3% -6 SWG D4% -8 SWG D5% SWG D6% RF Frequency (GHz) Fig. 8. () Schemtic of OTTDL sed on n rry of 4 SWG wveguides nd experimentl setup for microwve phse shift mesurements. () Mesured RF phse shift vs. modultion frequency for n opticl crrier t 1565 nm. seprted y μm, nd the OTTDL occupies chip size of.51 mm 8.6 mm. The SWG wveguides hve the sme prmeters s for the 2-rm MZIs. The mesured RF phse shift s function of the frequency from the 4 different SWG wveguides is shown in Fig. 8(). The time dely of ech wveguide cn e estimted from the verge slope of the mesured phse shift vs. the frequency response. The incrementl time delys re 8.9, 1.7, nd 7.9 ps etween the SWG wveguides, with D 1 ¼ 6%, D 2 ¼ 5%, D 3 ¼ 4%, nd D 4 ¼ 3%. Note tht while the duty cycles of the SWG wveguides re vried in increments of 1%, this does not trnslte into liner chnge in the incrementl time dely, s the group index of the SWG wveguide is not liner function of the duty cycle. We hve provided n overview of the recent work on developing wvelength-selective SWG wveguide filters sed on MRRs, BGs, nd CDCs, s well s index-vrile OTTDLs. Since SWG wveguides re sed on the sme technology nd wveguide design s silicon nnowires, they re comptile with existing silicon photonic devices, e.g., switches nd modultors. As such, SWG wveguides enhnce the ville component toolox for developing integrted MWP systems in SOI. The technologicl developments descried here, long with others in silicon photonics, point to the fesiility of more complex integrted MWP systems tht cn provide incresed functionlity nd, ultimtely, performnce. This reserch ws supported in prt y the Nturl Sciences nd Engineering Reserch Council of Cnd, the Fonds de Recherche du Quéec Nture et Technologies, nd the Royl Society. The SWG wveguide devices were fricted y R. Bojko t the University of Wshington Nnofriction Fcility, memer of the NSF Ntionl Nnotechnology Infrstructure Network. I thnk Dr. J. Wng, Dr. R. 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