Ultra-Low loss and compact coplanar waveguide crossing

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1 Invited Paper Ultra-Low loss and compact coplanar waveguide crossg Amir Hossei a, Yang Zhang b, and Ray T. Chen b a Omega Optics, Inc., Sausalito Dr, Aust, TX 78759, USA b Department of Electrical and Computer Engeerg, the University of Texas at Aust, Aust, TX, 78758, USA ABSTRACT e vestigate the loss mechanism 3-moded multimode-terference couplers that are the buildg blocks of a compact and low-loss waveguide crossg structure. Broadband silicon waveguide crossg arrays with <0.01dB sertion loss per crossg are proposed usg cascaded multimode terference couplers, where lateral subwavelength nanostructures are used to reduce the sertions loss. e design and fabricate a waveguide crossg array with a pitch of 3.08µm. Insertion loss of ~0.02dB per crossg and crosstalk <-40dB at 1550nm operatg wavelength and broad transmission spectrum rangg from 1520 to 1610nm are experimentally demonstrated. Keywords: Silicon photonics, waveguide theory, multimode terference coupler 1. INTRODUCTION Efficient waveguide crossgs are required to materialize full potential of silicon photonics for on-chip optical terconnects. Sgle mode silicon waveguide crossgs with normal tersections result over 1dB loss and ~-10dB cross-talk due to the high dex contrast of the silicon-on-sulator (SOI) platform [1, 2]. This issue has been addressed by several groups over the past decade. Subwavelength gratgs silicon waveguides have been used to lower the effective refractive dex at the crossg resultg sertion loss as low as 0.023dB and <-40dB cross-talk [1]. However, this structure requires ~10µm long adiabatic tapers to gradually reduce the effective refractive dex with near 0.3dB loss per taper. Also, the reduced effective refractive dex (<2) is accompanied by the mode profile extendg several microns laterally, which turn creases the waveguide pitch a cross-grid structure. As another approach, low-q resonator based crossgs suffer from limited optical bandwidth (10-15nm) [3]. Vertically tegrated silicon nitride waveguides over SOI waveguides have been shown to reduce the cross-talk to <-44dB [4], however, the required fabrication process is more complicated than that of sgle layer Photonic Integrated Circuits (PICs). On the other hand, multimode terference (MMI) based crossgs with relatively compact sizes (13x13µm2) have been demonstrated with sertion loss of ~0.2dB [2] [5]. In this type of structures, the self-focusg effect of the MMI is used to form a sgle image of the MMI put waveguide mode profile at the crossg thus mimizg the effect of the crossg waveguide on the mode profile. Recently, usg 2D fite difference time doma (FDTD) simulations it was theoretically shown that a periodic structure formed by cascadg multimode focusg sections can support a low-loss Bloch wave [6]. In addition to the fact that this structure can potentially lower the sertion loss to 0.04dB per crossg, a waveguide pitch of ~3µm also enables compact waveguide crossg arrays. In this paper we show that a compact periodic structure formed by cascadg MMIs with engeered lateral claddg refractive dex can lead to less than 0.01dB loss per crossg allowg tegration of 100s of waveguide crossgs with mimal sertion loss and cross-talk. Optical Interconnects XIV, edited by Henng Schröder, Ray T. Chen, Alexei L. Glebov, Proc. of SPIE Vol. 8991, 89910X 2014 SPIE CCC code: X/14/$18 doi: / Proc. of SPIE Vol X-1

2 x z MMI L s L s L (b) 250nm n c n f SiO 2 y air x MMI Figure 1. A top view schematic of the cascaded multimode terference based waveguide crossgs, (b) a side view schematic of the waveguide structure with lateral claddg dicated, a sgle waveguide crossg structure, (d) a 1 1 MMI. 2. DESIGN AND DISCUSSION (d) MMI The platform is a SOI substrate with 3µm thick buried oxide (BOX) layer and 250nm thick top silicon layer (n f =3.47). A schematic of the waveguide array crossg structure is shown Figure 1(a-b). This arrayed structure may be thought as a cascaded MMI based waveguide crossg shown Figure 1, which, accordg to the self-imagg prciple of multimode waveguides, images of the put field are periodically formed along the multimode waveguide. It has been proposed that the multimode waveguide can be crossed by another one at the pots where sgle-fold images are formed [2]. In order to design a low-loss waveguide crossg array, we first vestigate the loss mechanism a simple 1 1 symmetric MMI structure shown Figure 1(d). Similar to a previously reported design [6], here we assume the multimode waveguide width, MMI =1.2µm, and put/output sgle mode waveguide width, =0.6µm. Note that the MMI region only supports three TE polarized modes. So far, the MMI sertion loss has been explaed by the modal phase errors the multimode waveguide [7-9]. For an ideal self-imagg it is required that, = (+2)/3, where is the propagation constant of mode m, and is the beat length of self-imagg process [10]. Followg the analysis [7] one can write = 1+ (1) where, = (+1)/ is the transverse wave number of mode m, and em is the effective width of the MMI for the m th mode. The modal phase error is given as = = (, ), where is the MMI length. It has been shown that the lateral claddg dex (n c ) [see Fig. 1(b)] can be tuned to mimize for a few number of domant modes. Particularly, at the N-foldg imagg length is given as (/4) () ( ) (2) where, λ 0 is the optical wavelength and P is the number of self-imagg periods. e added the multiplier P/4 to the modal phase error presented [8], sce we have a symmetric terference here (required MMI length is divided by 4 [9]), and P=2 the MMI crossg structure. Also, we note that n f2d =2.9 and n c2d are the effective refractive dices of the fundamental mode of an fite slab waveguide with the same thickness as the MMI (250nm) and core refractive and claddg dices of n f and n c, respectively. hile tung the lateral claddg dex (n c ) is generally applicable to MMIs that support several modes their multimode region, we notice that the multimode waveguide shown Fig. 1(d) only supports 3 modes (0 th, 1 st and 2 nd ), among which the odd 1 st order mode is not excited to due to the symmetry of the structure. That leaves only 2 modes (m=0 and m=2), for which the self-imagg condition is simply reduced to Proc. of SPIE Vol X-2

3 =2/, n: teger (3) One notes that for any MMI and n c, as long as only the 0 th and the 2 nd modes are excited, there is always an MMI length for which this condition can be perfectly satisfied. In other words, the remag phase error ( ) can be theory completely elimated by tung. In order to confirm this observation, we simulate different 1 1 MMI structures shown Figure 2 (a-c) usg 3D PhotonDesign FIMMPROP, an eigenmode decomposition based simulator. Lear tapers ( =1µm) are used Figure 2 (b-c) for high transmissions as suggested by Chen et al [5]. In each case, we sweep the MMI lengths,, L, L for MMIs shown Figure 2 (a-c), respectively, and fd the maximum transmission. Figure 2 shows optical transmission as a function of n c. As n c creases, the transmission improves all cases. Interestgly, the 1 1 MMI with tapers and without crossg is essentially loss-less and the 1 1 MMI without tapers has the worst performance. Sce the modal phase errors are not applicable any of these cases, we need to consider a different loss mechanism. MMI L MMI L c MMI (b) Transmission (db) (d) Figure 2. Schematics of simulated structures, 1 1 MMI with sgle mode access waveguides, (b) 1 1 MMI with tapered put output transitions, MMI waveguide crossg usg 1 1 MMI with tapered put output transition, (d) simulated transmission versus lateral claddg dex (nc) for the structure (a-c), (d) set shows TE fraction versus nc for the fundamental mode the sgle mode access waveguide (width=0.6µm, solid blue curve) and the second order mode the MMI region (width=1.2µm, dashed green le). 0 (b) F TE n c n c In the structure shown Fig. 2, we note that the fundamental mode the sgle mode access waveguide, and the 2 nd order mode the multimode region are both quasi TE modes with considerable amount of TM polarization (~1.5%) for n c =1 as shown set of Fig. 2 (d). Here, the TE (TM) fraction is the fraction of the Poyntg vector with horizontal (vertical) electric field = (4) As n c creases, both of these modes become essentially completely TE (FTE~100%) and power transmission between the two waveguides improves at the put and output. The tapers help improvg the power transmission by avoidg sharp transitions and by reducg the portion of the power the 2 nd order mode the multimode region [5]. Similarly, when we compare the tapered MMIs with and without waveguide crossgs, we note that the crossg section is much wider compared to the MMI width (L c >>MMI), this section can be thought of as a slab waveguide that supports pure TE modes. hen n c creases from 1 to 2.5, the two excited modes the MMI region also become nearly pure TE and the power transmission between the two section creases. Thus, we conclude that the ma loss the 3-moded MMI structures is due to couplg loss at sharp transitions and not due to modal phase errors. 3. TEST RESULTS e fabricate waveguide arrays normally crossed by other waveguide arrays as shown Fig. 3 usg both conventional MMI crossgs and dex-engeered MMI crossgs. In order to implement n c >1, subwavelength nanostructure (SN) is used to engeer the lateral claddg refractive dex [8]. The SN is periodic along the light propagation direction, and its refractive dex (nsn) can be engeered by tung the fillg factor (ff) of air trench side the SN, which is defed as the ratio between the air trench width () and the SN period (Λ). e use Λ=200nm for the SN to fabricate devices with =30, 40, 50, 60, 70 and 80nm. The width of the SN is 200nm to accommodate the field penetration to the lateral claddg. Proc. of SPIE Vol X-3

Fig. 3. Schematic of a cross-grid MMI based waveguide array crossg. 7 7 cross-grid is shown for simplicity. (b) an optical microscope image of the fabricated 101 101 cross-grid.

3(b) shows an optical microscope image of the fabricated 101 101 cross-grid with dex-engeered MMI crossgs. Figs.

4 Fig. 3. Schematic of a cross-grid MMI based waveguide array crossg. 7 7 cross-grid is shown for simplicity. (b) an optical microscope image of the fabricated cross-grid. The designed structures are fabricated on a SOI wafer usg electron beam lithography (EBL) and reactive ion etchg (RIE). MMI crossgs with and without SN are used the cross-grids for comparison. Fig. 3(b) shows an optical microscope image of the fabricated cross-grid with dex-engeered MMI crossgs. Figs. 4(a-b) and (c-d) show scanng electron microscope (SEM) images of the MMI crossgs, with and without SN, respectively. F r L 0 O ri ri (bi s0 0 Fig. 4. SEM images of the fabricated waveguide array crossgs with (a-b) and without (c-d) dex engeerg of the lateral claddgs. Transverse electric (TE) polarized light from a broadband amplified spontaneous emission (ASE) light source is coupled and out of the cross-grid usg SN based gratg couplers [10]. The transmissions obtaed from the cross-grid are normalized to the transmission of a reference waveguide with the same propagation length. e experimentally fd that the dex-engeered MMI crossg with =50nm had the best performance. Fig. 5 shows the normalized transmissions of the 101x101 cross-grids for a horizontal waveguide the middle of the grid (the 51th waveguide) with both the dexengeered (n c =2.5, with SN) and conventional (n c =1, no SN) MMI crossgs. The cross-talk measured from a vertical waveguide the middle of the grid (the 51th waveguide) for dex-engeered structure is also shown Fig. 5. Proc. of SPIE Vol X-4

5 -5 g -30 cn -35 [ Index -engeered cascaded MMI Cross -talk of dex-engeered cascaded MMI Conventional cascaded MMI PO 'f' ji 1\ r n ;klp n ' avelength (nm) Fig. 5. Measured transmission for 101 cascaded MMI crossgs with (black) and without (blue) dex-engeerg, and the cross-talk of dex-engeered MMI crossg. The results show that the conventional MMI crossg has an sertion loss of 0.14dB at 1550nm operatg wavelength, which is comparable to what demonstrated ref. [2]. The dex-engeered MMI crossg has an sertion loss of 0.019dB at 1550nm operatg wavelength. The cross-talk signal is below the noise floor of our testg system, so the exact crosstalk cannot be extracted from the transmission. However, the estimated cross-talk is at least below -40dB over nm wavelength range. Besides the ultra-low sertion loss and low cross-talk, cascadg dex-engeered MMI crossgs enable a waveguide pitch of 3.08µm a cross-grid, which is the most compact footprt for a non-resonant crossg to our knowledge. 4. CONCLUSIONS In conclusion, while the high-dex-contrast of the SOI platform allows small footprts for photonic devices, it also makes excess loss reduction and cross-talk suppression challengg. An ultra-low loss waveguide crossg structure with a waveguide pitch of only 3.08µm has been demonstrated on the SOI platform. The crossg structure, utilizg cascaded dex-engeered MMIs, has an sertion loss of 0.019dB and crosstalk lower than -40dB at 1550 operatg wavelength, and broad transmission spectrum of over more than 90nm bandwidth. 5. REFERENCE 1. P. J. Bock, P. Cheben, J. H. Schmid, J. Lapote, A. Delâge, D.-X. Xu, S. Janz, A. Densmore, and T. J. Hall, Optics express 18, (2010). 2. H. Chen and A.. Poon, Photonics Technology Letters, IEEE 18, (2006). 3. C. Manolatou and H. A. Haus, Passive Components for Dense Optical Integration (Sprger, 2002), pp A. M. Jones, C. T. DeRose, A. L. Lente, D. C. Trotter, A. L. Starbuck, and R. A. Norwood, Optics express 21, (2013). 5. C.-H. Chen and C.-H. Chiu, Quantum Electronics, IEEE Journal of 46, (2010). 6. M. Popovic, E. P. Ippen, and F. Kartner, Lasers and Electro-Optics Society, LEOS The 20th Annual Meetg of the IEEE, (IEEE, 2007), J. Huang, R. Scarmozzo, and R. Osgood Jr, Photonics Technology Letters, IEEE 10, (1998). 8. A. Ortega-Monux, L. Zavargo-Peche, A. Maese-Novo, I. Mola-Fernández, R. Halir, J. anguemert-perez, P. Cheben, and J. Schmid, Photonics Technology Letters, IEEE 23, (2011). 9. A. Hossei, D. N. Kwong, Y. Zhang, H. Subbaraman, X. Xu, and R. T. Chen, Selected Topics Quantum Electronics, IEEE Journal of 17, (2011). 10. X. Xu, H. Subbaraman, J. Covey, D. Kwong, A. Hossei, and R. T. Chen, Applied physics letters 101, (2012). Proc. of SPIE Vol X-5

CHAPTER 2 POLARIZATION SPLITTER- ROTATOR BASED ON A DOUBLE- ETCHED DIRECTIONAL COUPLER

CHAPTER 2 POLARIZATION SPLITTER- ROTATOR BASED ON A DOUBLE- ETCHED DIRECTIONAL COUPLER As we discussed in chapter 1, silicon photonics has received much attention in the last decade. The main reason is