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Abstract
We report, for the first time, a single state polarization (TE) dual-band grating fiber-chip coupler on a 700 nm thick silicon nitride platform that couples to both O and C band channels. Dual-band coupling with a single grating coupler is achieved using cross mode k-space equalization. By phase matching the first-order vertical TE01 mode and the fundamental TE00 mode in two distinct bands, we observe simultaneous co-directional dual-band coupling to a single waveguide. Experimental peak efficiency per coupler is measured to be −7.3 dB in the O-band and −8.2 dB in the C-band and the combined 1 dB bandwidth is observed to be 82 nm.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Surface grating fiber-chip couplers are a versatile device to couple light. They enable compact device footprint, are compatible with wafer-scale testing and can be easily fabricated using standard foundry processes. However, a critical limiting factor has been their intrinsic operational bandwidth. Compared to in-plane edge coupling, surface gratings have lower bandwidth, and for that reason most grating couplers have been primarily restricted to operate either near the C or O or S-bands with compromised bandwidth. To address this bandwidth limitation, several multi-band grating schemes were demonstrated on Silicon-on-insulator (SOI) platform. A dual-band splitting 1D grating was proposed [1] in which Silicon overlays were used to provide suitable phase matching for duplexed wavelengths at O and S-bands. Similarly, a bi-wavelength 2D chirp grating coupler was demonstrated [2] for narrowly spaced S and C bands respectively. A polarization splitting dual-band coupler where C and O-band signals of TE and TM polarization states are coupled co-directionally, has also been reported [3]. More recently, a new kind of dual wavelength band coupler based on subwavelength gratings has been demonstrated for S and L bands [4]. These schemes are primarily targeted towards developing integrated transceivers for fiber-to-the-home network (FTTH) services where upstream and downstream signals are channelled through separate bands. On platforms like Silicon Nitride (SiN) however, such couplers can be implemented for a host of non-linear applications like generation of broadband frequency combs, cross-phase modulation, supercontinuum generation and multiwavelength parametric oscillators [5–11]. Other possible applications can be chip-integrated amplifiers like the one reported in [12] where SiN resonators with rare earth nanoparticles was pumped in O-band to amplify C-band signal.
Here we report for the first time a dual-band grating coupler in SiN using a novel cross-mode, k-space equalization, that can be used to couple both O and C-bands using a single grating. In particular, we use a 700 nm thick SiN, which has better phase matching for multi-wavelength non-linear applications [8,10]. Such thick SiN films have been shown to exhibit low propagation losses of upto 1 dB/cm [13]. With careful tuning of grating parameters, we numerically calculate a coupling efficiency of 40 % (−4 dB) in O-band and 34 % (−4.7 dB) in C-band, which can even further be boosted to over 60 % (−2.2 dB) in each band with the addition of bottom reflectors.
2. Phase matching
To couple two wavelength bands, it is essential to study the mode dispersion characteristics. Figure 1(a) shows the variation of different transverse electric (TE) modes supported by a planar SiN waveguide as a function of thickness. For a thickness beyond 600 nm and 750 nm, the slab waveguide supports higher-order vertical modes at O-band and C-band respectively. Figure 1(b) depicts the propagation constants of the fundamental and first-order vertical mode at 1550 nm and 1310 nm. It can be observed that for a thickness of around 700 nm, there is a phase-crossing between the propagation constant of the fundamental TE00 mode in C-band and the higher-order vertical TE01 mode in O-band. The phase matching necessarily means that one could design a single grating fiber-chip coupler that could simultaneously couple 1550 and 1310 nm, however, exciting TE00 and TE01 modes respectively. As per the phase matching condition, the period for the first-order diffraction of a uniform grating coupler is expressed as,
where θ is the fiber inclination angle, nc is the top cladding index which in this case is SiO2, λ0 is the free-space wavelength and is the average effective index of the Bloch mode propagating in the grating region which can be approximated [14] to the following linear expression, where dc is the duty cycle and and denote the effective indices of the corresponding order of TE modes in the slab and etched regions respectively. For our design, we choose SiN thickness of 700 nm for two reasons, the first being the matching propagation constants (7.6 μm−1) of the two orthogonal modes at 1310 and 1550 nm, and the second being the suitability of building waveguides for non-linear applications. With the thickness fixed, a dual-band grating coupler was designed and optimized for efficient coupling to both the wavelength bands.
Fig. 1 (a) Effective indices of fundamental and higher-order vertical TE modes at 1310 and 1550 nm as a function of semi-infinite SiN slab thickness. (b) Propagation constant β of fundamental and first-order TE mode at 1310 and 1550 nm as a function of semi-infinite SiN slab thickness.
3. Design and simulation
The 2D dual-band grating coupler was designed using a commercial finite difference time domain (FDTD) solver from Lumerical Inc. Grating parameters involving period Λ, etch depth tEt, duty cycle dc, inclination θ, and the buried oxide (BOX) thickness were optimised to achieved maximum coupling. A 3D schematic of device design is illustrated in Fig. 2. The device is embedded in a 2 μm SiO2 top cladding. A Gaussian source of beam radii 4.6 μm and 5.2 μm is considered for 1310 and 1550 nm regions respectively. The source is placed a few microns above the top-cladding in air. The coupling efficiency is calculated through a power monitor placed across the slab waveguide region, a few tens of microns away from the gratings. For both bands, the number of grating period lines is optimized to 15. Figure 3 depicts a contour map of coupling efficiency as a function of the period and etch depth. It is observed from the simulations that the coupling efficiency in C-band is more sensitive to etch depth than the O-band. Based on the contour, an etch depth of 350 nm and periods 920–960 nm is optimal for dual-band operation and is used for further simulation and analysis. Figure 4 depicts the spectral response as a function of the grating period. Although the peak periods of O and C-bands may not exactly converge, we determine 930 nm to be optimal for dual-band operation. For this period a peak coupling of 40 % is observed at 1290 nm, and 34 % is observed at a wavelength of 1562 nm. The corresponding 1 dB bandwidths are 47 nm and 49 nm for O and C-bands respectively.
Fig. 2 Schematic of an SiN grating device used in simulations. λo,c are the corresponding free-space wavelengths in each band. Inset shows the fiber in air, inclined at an angle θ to the grating. The top-cladding nc is a 2 μm thick layer of SiO2. tSiN is the thickness of SiN film which is 700 nm here and tEt is the grating etch depth. W is the patch width.
Fig. 3 Simulated fiber-to-chip coupling efficiency as a function of grating period Λ and etch depth tEt at 6° fiber inclination and 50% duty cycle at (a) 1310 nm and (b) 1550 nm.
Fig. 4 Simulated spectral response as a function of grating period at 6° fiber inclination, 50% duty cycle and at an etch depth of 350 nm for (a) O-band and (b) C-band.
Figure 5 shows the inclination angle dependence of grating spectrum for a 930 nm grating period. Increase in inclination angle leads to a blue shift in the peak wavelength. Peak coupling angle for O-band is observed at 7° and 6° for C-band. The difference in peak efficiency between 6° and 7° in O-band is only <1 %, so 6° is chosen as the incident angle for coupling. The dependence of period duty cycles on coupling efficiency is plotted in Fig. 6. Maximum coupling of 40 % (O-band) and 34 % (C-band) is observed for a duty cycle of 50 %. Based on these plots, 50 % duty cycle is determined to be optimum for providing a good coupling within each band.
Fig. 5 Coupling efficiencies in (a) O-band and (b) C-band at different fiber inclination angle θ for a 930 nm grating period and 50% duty cycle.
Fig. 6 Coupling efficiencies at different duty cycles for (a) O-band and (b) C-band at 6° fiber inclination, and 930 nm grating period.
Figure 7 shows the effect of BOX thickness on the coupling efficiency. To phase match both the bands, an optimal oxide thickness of 1.7 μm is needed to give a power coupling of 33 % at 1310 and 1550 nm. Furthermore, it has to be noted that the uniformity of the oxide layer is essential to achieve maximum coupling in both the bands, as deviation would result in preferential efficiency in any one of the bands.
Fig. 7 Coupling efficiency as a function of buried oxide (BOX) thickness at 930 nm period for both the bands.
The coupling efficiency can be further improved by incorporating a bottom reflector [15]. A generic surface grating coupler primarily suffers from two loss mechanisms, namely, the field overlap with fiber mode and leakage to the substrate. Suppressing the latter is critical to achieving a high directionality. It is well known that by adding a distributed Bragg reflector (DBR) underneath the grating, directionality can be substantially improved, resulting in higher-coupling efficiency. Figure 8 depicts the improvement in the coupling efficiency with a bottom DBR (2 layers of Si/SiO2 of 110/270 nm). With the DBR, a coupling efficiency of over 60 % for both O-band and C-band at peak wavelengths of 1.295 and 1.57 μm is achieved with a 1 dB bandwidth being 50 nm in each band.
Fig. 8 Coupling comparison, for gratings with a bare Si substrate and those with a bottom DBR stack at period 930 nm, 6° inclination angle and 1.7 μm BOX thickness for (a) O-band and (b) C-band.
4. Fabrication and characterization
To demonstrate the dual-band grating coupler, the device is built on a Si wafer. Initially, a bare-Si wafer was taken as a substrate on which 1.7 μm BOX layer was deposited using plasma enhanced chemical vapour deposition (PECVD) followed by a 700 nm SiN layer deposition also using PECVD. This was followed by a two step patterning. In the first step, grating couplers were patterned on patches of width W 15 μm and length 300 μm, using electron-beam lithography (Raith E-line) with MaN 2403 resist. The patterns were subsequently etched for 350 nm using Inductively Coupled Reactive Ion Etching (ICP-RIE) through Flourine chemistry. After grating pattering, the patch waveguide was defined by etching the full 700 nm thickness of SiN till the BOX layer. The sample is then deposited with a 2 μm thick SiO2 to form top-cladding. Figure 9 shows a scanning electron microscope (SEM) image of a grating after first layer pattering and etching.
The fabricated devices were optically characterized for the coupling efficiency. Figure 10 shows the experimental setup used for the characterization. Light from two superluminescent laser diodes (one each from O and C-bands), was passed through polarization controllers and onto a 3 dB combiner. This combiner was connected to a goniometric stage with single mode fibers cleaved at one end, facing the device. Fiber from the output goniometric arm was connected to an optical spectrum analyzer (OSA). The controller arms for each source were varied to ensure maximal polarization alignment at the cleaved fiber output so as to couple exclusively TE modes. An angle sweep was performed on the stage mount to determine optimum coupling. Device spectrum extracted from the OSA was normalized by the reference source spectrum to determine fiber to grating coupling performance. Figure 11 and 12 depict the measured coupling efficiency of a dual-band grating coupler for various grating periods and fiber inclination angles respectively. A peak coupling efficiency of −7.3 dB at 1289 nm and −8.2 dB at 1551 nm is achieved with a grating period of 930 nm with 50 % duty cycle at an inclination angle of 5°. A 1 dB bandwidth of 34 nm and 48 nm is achieved in O and C band respectively. Overall, the losses are about 3 dB higher than simulated values and can be attributed to process induced defects. It maybe pointed out that we have implemented the device on a low temperature PECVD SiN, although a low loss stoichiometric Silicon Nitride platform can also be considered. In O-band, we observe periodic non-uniform fluctuations in the device spectrum. These oscillations are thought to be originating due to intermodal beating between TE01 and TE11 modes in the multimode patch waveguide. Such beating effects have been previously reported in few mode fibers [16, 17]. It is however expected, that no beating would occur in wire waveguides which are most likely to be single mode in lateral dimensions and where most of dual-band applications are targeted.
Fig. 10 Schematic of experimental characterization setup of SiN dual-band coupler device under test (DUT).
Fig. 11 Measured fiber-to-chip coupling efficiency (CE) of SiN couplers in (a) O-band and (b) C-band for different grating periods with 50% duty cycle and 5° fiber angle.
Fig. 12 Measured fiber-to-chip coupling efficiency (CE) in (a) O-band and (b) C-band at period 930 nm and 50% duty cycle with different fiber inclination angles.
5. Conclusion
In summary, we have proposed, designed and demonstrated a unidirectional dual-band TE grating coupler on 700 nm thick Silicon Nitride on oxide platform. The dual-band coupling is achieved by virtue of the grating acting as a first-order TE01 coupler in O-band and a zero-order TE00 coupler in C-band. Experimental peak coupling efficiencies of −7.3 dB and −8.2 dB are obtained in C and O-band respectively. A combined 1 dB bandwidth of 82 nm, i.e. 34 nm and 48 nm is achieved in O and C band respectively which is considerably higher than those reported on SOI platforms. Moreover, improvements in coupling performance to −2.2 dB in both bands can be realised with minimal change in bandwidth, by incorporating a bottom reflector. The demonstrated grating can potentially pave way for multi-band non-linear on-chip signal processing in Silicon Nitride.
Funding
DST-SERB; MHRD through NIEIN project; from MeitY and DST through NNetRA; MeitY-Sir Visvesvaraya Young Faculty Research Fellowship.
Acknowledgments
We thank DST-SERB for funding this research. We also acknowledge funding support from MHRD through NIEIN project, from MeitY and DST through NNetRA. SKS thanks MeitY-Sir Visvesvaraya Young Faculty Research Fellowship.
References
1. G. Roelkens, D. Van Thourhout, and R. Baets, “Silicon-on-insulator ultra-compact duplexer based on a diffractive grating structure,” Opt. Express 15, 10091–10096 (2007). [CrossRef] [PubMed]
2. L. Xu, X. Chen, C. Li, and H. K. Tsang, “Bi-wavelength two dimensional chirped grating couplers for low cost WDM PON transceivers,” Opt. Commun. 284, 2242–2244 (2011). [CrossRef]
3. M. Streshinsky, R. Shi 1,2, A. Novack, R. T. P. Cher, A. E.-J. Lim, P. G.-Q. Lo, T. Baehr-Jones, and M. Hochberg, “A compact bi-wavelength polarization splitting grating coupler fabricated in a 220 nm SOI platform,” Opt. Express 21, 31019–31028 (2013). [CrossRef]
4. W. Zhou, Z. Cheng, X. Sun, and H. K. Tsang, “Tailorable dual-wavelength-band coupling in a transverse-electric-mode focusing subwavelength grating coupler,” Opt. Lett. 43, 2985–2988 (2018). [CrossRef] [PubMed]
5. D. J. Moss, R. Morandotti, A. L. Gaeta, and M. Lipson, “New CMOS-compatible platforms based on silicon nitride and Hydex for nonlinear optics,” Nat. Photonics 7, 597–607 (2013). [CrossRef]
6. Y. Xuan, Y. Liu, L. Varghese, A. Metcalf, X. Xue, P. Wang, K. Han, J. Jaramillo-Villegas, A. Al Noman, C. Wang, S. Kim, M. Teng, Y. Lee, B. Niu, L. Fan, J. Wang, D. Leaird, A. Weiner, and M. Qi, “High-Q silicon nitride microresonators exhibiting low-power frequency comb initiation,” Optica 3, 1171–1180 (2016). [CrossRef]
7. S. Kim, K. Han, C. Wang, J. A. Jaramillo-Villegas, X. Xue, C. Bao, Yi Xuan, D. E. Leaird, A. M. Weiner, and M. Qi, “Dispersion engineering and frequency comb generation in thin silicon nitride concentric microresonators,” Nat. Commun. 8, 372 (2017). [CrossRef] [PubMed]
8. M. Dizaji, C. Krückel, A. Fülöp, P. Andrekson, V. Torres-Company, and L. Chen, “Silicon-rich nitride waveguides for ultra-broadband nonlinear signal processing,” Opt. Express 25, 12100–12108 (2017). [CrossRef] [PubMed]
9. X. Liu, M. Pu, B. Zhou, C. Krückel, A. Fülöp, V. Torres-Company, and M. Bache, “Octave-spanning supercontinuum generation in a silicon-rich nitride waveguide,” Opt. Lett. 41, 2719–2722 (2016). [CrossRef] [PubMed]
10. J. S. Levy, A. Gondarenko, M. A. Foster, A. C. Turner-Foster, A. L. Gaeta, and M. Lipson, “CMOS-compatible multiple-wavelength oscillator for on-chip optical interconnects,” Nat. Photonics 20, 37–40 (2010). [CrossRef]
11. X. Ji, F. A. S. Barbosa, S. P. Roberts, A. Dutt, J. Cardenas, Y. Okawachi, A. Bryant, A. L. Gaeta, and M. Lipson, “Ultra-low-loss on-chip resonators with sub-milliwatt parametric oscillation threshold,” Optica 4, 619–624 (2017). [CrossRef]
12. P. Xing, G. F. R. Chen, X. Zhao, D. K. T. Ng, M. C. Tan, and D. T. H. Tan, “Silicon rich nitride ring resonators for rare-earth doped telecommunications-band amplifiers pumped at the O-band,” Sci. Rep. 7, 2045–2322 (2017). [CrossRef]
13. C. Krückel, A. Fülöp, T. Klintberg, J. Bengtsson, P. Andrekson, and V. Torres-Company, “Linear and nonlinear characterization of low-stress high-confinement silicon-rich nitride waveguides,” Opt. Express 23, 25827–25837 (2015). [CrossRef] [PubMed]
14. X. Chen and H. K. Tsang, “Polarization-independent grating couplers for silicon-on-insulator nanophotonic waveguides,” Opt. Lett. 36, 796–798 (2011). [CrossRef] [PubMed]
15. H. Zhang, C. Li, X. Tu, J. Song, H. Zhou, X. Luo, Y. Huang, M. Yu, and G. Lo, “Efficient silicon nitride grating coupler with distributed Bragg reflectors,” Opt. Express 22, 21800–21805 (2014). [CrossRef] [PubMed]
16. D. Menashe, M. Tur, and Y. Danziger, “Interferometric technique for measuring dispersion of high order modes in optical fibres,” Electron. Lett. 37, 1439–1440 (2001). [CrossRef]
17. J. Savolainen, L. Grüner-Nielsen, P. Kristensen, and P. Balling, “Measurement of effective refractive-index differences in a few-mode fiber by axial fiber stretching,” Opt. Express 20, 18646–18651 (2012). [CrossRef] [PubMed]
