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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 7 — Apr. 8, 2013
  • pp: 7868–7874
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CMOS-compatible high efficiency double-etched apodized waveguide grating coupler

Chao Li, Huijuan Zhang, Mingbin Yu, and G. Q. Lo  »View Author Affiliations


Optics Express, Vol. 21, Issue 7, pp. 7868-7874 (2013)
http://dx.doi.org/10.1364/OE.21.007868


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Abstract

We present a high efficiency double-etched apodized fiber-to-waveguide grating coupler on a silicon-on-insulator substrate, which can be fabricated using deep UV photolithography. The fabricated grating coupler yields a coupling loss of −1.5 dB with 3-dB bandwidth of 54 nm at a wavelength of 1560 nm. Measurements and simulations show that the double-etched apodized grating coupler design is robust and tolerant to fabrication process variations.

© 2013 OSA

1. Introduction

There are two main factors that have limited the coupling efficiency of grating couplers. One is the directionality (defined as the portion of the light power coupled upwards normalized with respect to the total out-coupled optical power). Different structures such as slanted gratings [4

4. B. Wang, J. H. Jiang, and G. P. Nordin, “Embedded, slanted grating for vertical coupling between fibers and silicon-on-insulator planar waveguides,” IEEE Photon. Technol. Lett. 17(9), 1884–1886 (2005). [CrossRef]

], a substrate mirror [5

5. F. Van Laere, G. Roelkens, M. Ayre, J. Schrauwen, D. Taillaert, D. Van Thourhout, T. F. Krauss, and R. Baets, “Compact and highly efficient grating couplers between optical fiber and nanophotonic waveguides,” J. Lightwave Technol. 25(1), 151–156 (2007). [CrossRef]

] or a silicon overlay [6

6. D. Vermeulen, S. Selvaraja, P. Verheyen, G. Lepage, W. Bogaerts, P. Absil, D. Van Thourhout, and G. Roelkens, “High-efficiency fiber-to-chip grating couplers realized using an advanced CMOS-compatible Silicon-On-Insulator platform,” Opt. Express 18(17), 18278–18283 (2010). [CrossRef] [PubMed]

] were demonstrated to diffract more light towards the fiber. However these approaches have their drawbacks. The fabrication of the slanted gratings or substrate mirror is not CMOS compatible and very complex. By applying an overlay on top of the device layer to enhance the directionality, a coupling efficiency of −1.6 dB has been reported [6

6. D. Vermeulen, S. Selvaraja, P. Verheyen, G. Lepage, W. Bogaerts, P. Absil, D. Van Thourhout, and G. Roelkens, “High-efficiency fiber-to-chip grating couplers realized using an advanced CMOS-compatible Silicon-On-Insulator platform,” Opt. Express 18(17), 18278–18283 (2010). [CrossRef] [PubMed]

], but the use of amorphous silicon limits the thermal budget. Another factor that limits the coupling efficiency is the mode mismatch between the fiber mode and the field profiles of diffracted light from waveguide grating. The coupling efficiency can be optimized by engineering the coupling strength of the grating structure to achieve better mode match [7

7. D. Taillaert, P. Bienstman, and R. Baets, “Compact efficient broadband grating coupler for silicon-on-insulator waveguides,” Opt. Lett. 29(23), 2749–2751 (2004). [CrossRef] [PubMed]

9

9. M. Antelius, K. B. Gylfason, and H. Sohlström, “An apodized SOI waveguide-to-fiber surface grating coupler for single lithography silicon photonics,” Opt. Express 19(4), 3592–3598 (2011). [CrossRef] [PubMed]

, 12

12. A. Mekis, S. Gloeckner, G. Masini, A. Narasimha, T. Pinguet, S. Sahni, and P. De Dobbelaere, “A Grating-Coupler-Enabled CMOS Photonics Platform,” IEEE J. Sel. Top. Quantum Electron. 17(3), 597–608 (2011). [CrossRef]

]. Most recently, a silicon apodized grating coupler was demonstrated with a coupling loss of −1.2dB for a waveguide thickness of 340 nm and a deep waveguide etch depth of 200 nm [8

8. X. Chen, C. Li, C. K. Y. Fung, S. M. G. Lo, and H. K. Tsang, “Apodized waveguide grating couplers for efficient coupling to optical fibers,” IEEE Photon. Technol. Lett. 22(15), 1156–1158 (2010). [CrossRef]

]. However, such apodized grating structure requires feature sizes (minimum feature size of 50 nm) that are beyond the limit of current deep UV lithography. This becomes an obstacle for the actual deployment of such device in mass production.

Here we report simulations and experimental results for a double-etched apodized waveguide grating coupler on silicon-on-insulator (SOI). By introducing the double-etched structure into the apodized grating design, the inherent tight feature sizes’ requirement to achieve high coupling efficiency is relaxed so that the current deep UV photolithography is capable of the device fabrication.

2. Device designs and principle

In order to reach high coupling efficiency between fiber and waveguide, a highly directional grating is required whilst the grating coupling strength α needs to be optimized to obtain a Gaussian-shaped field profile in order to match the fiber mode. Therefore, in our design, we first choose the silicon waveguide thickness of 340 nm but not the conventional thickness of 220 nm to improve the directionality of the grating. Furthermore, the apodized grating coupler design with engineered coupling strength for each grating period was employed to enhance the coupling efficiency.

The optical power confined in the grating may be described by P(z) = P0exp(−2az) for uniform grating, where α is the coupling strength. In order to obtain a Gaussian-shaped output field profile G(z), which is defined as G2(z) =dP(z)/dz, the coupling strength along z-axis is given by

α(z)=G2(z)2[10zG2(t)dt]
(1)

The coupling strength can be varied by modifying the fill factor for each grating period. In a single etched apodized grating coupler on 340-nm-thick SOI, the optimized design requires the fill factor f (defined as the ratio of the etched groove width g to the grating period Λ) ranging from 0.08 to 0.4 to be applied along z-axis [8

8. X. Chen, C. Li, C. K. Y. Fung, S. M. G. Lo, and H. K. Tsang, “Apodized waveguide grating couplers for efficient coupling to optical fibers,” IEEE Photon. Technol. Lett. 22(15), 1156–1158 (2010). [CrossRef]

]. The grating period Λ satisfies the phase matching condition: qλ = Λ(neff - nc sinθ), where λ is the center wavelength, neff is the effective index of the grating, nc is the refractive index of the cladding material, θ is the off-vertical tilt angle of the fiber, q is the diffraction order (equal to 1 for most grating couplers). The average effective index neff and the grating period would also change with f. According to Eq. (1) and the phase matching condition, the grating period Λ is determined to be gradually changing from 548 to 610 nm along z-axis with f changing from 0.08 to 0.4 for the central wavelength around 1550 nm. The etched groove width for each period is thus given by g = f Λ. Then the first few grating periods with small fill factor (~0.1) would result in that the groove widths (minimum feature size of ~50 nm) are beyond the limit of current deep UV lithography [8

8. X. Chen, C. Li, C. K. Y. Fung, S. M. G. Lo, and H. K. Tsang, “Apodized waveguide grating couplers for efficient coupling to optical fibers,” IEEE Photon. Technol. Lett. 22(15), 1156–1158 (2010). [CrossRef]

].

3. Simulation results

We employed commercial two-dimensional (2D) finite-difference time-domain (FDTD) numerical simulation tool (RSoft) for such grating structures. A Gaussian waveform with 1/e full width of 10.4 μm was employed to represent the single mode fiber input. Coupling efficiency between the fiber and waveguide was calculated using the power in the waveguide mode coming out from the front end of the grating coupler. During simulations, the fiber position which is the horizontal distance from the center of fiber core to the front-end of the grating coupler was adjusted to maximize the coupler efficiency. Normally in the fabrication of the grating coupler, deviations from the designed parameters is expected. The influence of all grating groove widths variation (Δg) was thus numerically studied by assuming the same Λ for each grating period. The simulated spectra with Δg of −10, 0, and 10 nm are shown in Fig. 2
Fig. 2 Simulated coupling loss for the double-etched apodized waveguide grating coupler as a function of wavelength with groove widths variation Δg of −10 (circles), 0 (squares), and 10 nm (triangles). (i) Inset shows the simulated minimum coupling loss as a function of groove widths variation Δg.
. A minimum coupling loss of −0.97 dB (corresponding to 80% coupling efficiency) are predicted. The 1-dB and 3-dB bandwidth are 31 and 53 nm respectively. The grating reflects ~1% of the light back into the waveguide. The coupling loss deviation is within 0.15 dB with the variations of +/− 10 nm as shown in Fig. 2. Smaller groove width results in a reduction of the effective index of the grating and therfore a wavelength red-shift of the peak position, according to the phase matching condition. Inset shows the simulated coupling loss as a function of Δg. The coupling efficiency drops with positive Δg (larger groove widths).

The influence of the grating etch depths variation was also numerically studied as shown in Fig. 3
Fig. 3 Simulated coupling loss for the double-etched apodized grating coupler as a function of etch depth by changing shallow etch depth (squares) and deep etch depth (circles).
. The simulations were done by varying one etch depth while fixing the other. The deep etch depth ed variation has more impact on the coupling efficiency, while the double-etched grating coupler shows much larger fabrication tolerance to the shallow etch depth es. Especially when es is within 50 – 120 nm range, only less than 0.3 dB coupling loss difference is expected. The coupling efficiency drops more rapidly with deeper etch depth for both shallow etch depth es and deep etch depth ed.

4. Device fabrication and characterization

The double-etched apodized waveguide grating coupler was fabricated on a commercial 8-inch 340-nm-thick SOI wafer with 2-μm-thick buried oxide layer using CMOS-compatible technology. Firstly, a 70-nm-thick oxide layer was deposited as hard mask. The grating structure was defined by deep UV photolithography and transferred onto the hard mask layer by dry etching. Secondly, a shallow dry etching was performed through silicon device layer with etch depth es. After that, the first four periods of the grating coupler was covered by the patterned photoresist using photolithography. Then another silicon dry etching process with etch depth (ed - es) was performed in order to reach the target deep etch depth ed for other grating periods. Finally, the waveguide was patterned and etched with a remained slab height of ~100 nm. We remark that our design has inherent large alignment tolerance with double-etched process because the misalignment specification of the deep UV photolithography facility is within ± 60 nm, which is far less than the grating slit width of ~400 nm.

Figure 4(a)
Fig. 4 (a) Top-view scanning electron micrograph (SEM) of the fabricated grating coupler. (b), (c) Zoom-in view SEM images of the device.
shows the top-view scanning electron micrograph (SEM) of the fabricated double-etched waveguide grating coupler on SOI. Figure 4(b) and 4(c) show the zoom-in view SEM images of the device. We can clearly observe the double etched structure as shown in Fig. 4(c). The measured shallow etch depth es is ~85 nm and the deep etch depth ed is ~210 nm. The fill factor and the period are purposely designed to achieve engineered coupling strength for each grating period (shown in Fig. 4(a)). The minimum feature size is ~135 nm.

We then characterized the fabricated double-etched grating couplers by measuring the insertion loss of a waveguide with grating couplers at both ends using cleaved single mode fibers tilted at 10° as optical input and output. A pair of adiabatic linear tapers, each with a length of 500 μm, is used to connect the 10-μm-wide grating region and a 3-μm-wide waveguide section with a length of 550 μm. Another pair of 200-μm-long adiabatic tapers is then used to connect the 3-μm-wide waveguide to a 500-nm-wide waveguide. The total distance between the two grating couplers is 2.9 mm. A tunable laser (Agilent 8164B) was employed as the input light source. The resolution is set to be 0.02 nm. A polarization controller was used to adjust the input light in TE-polarization for the waveguide grating coupler. We remark that there is no index-matching liquid applied between the optical fiber and waveguide grating coupler.

We also varied the widths of grating grooves in the mask layout to investigate the fabrication tolerance. Figure 5 shows the measured coupling loss with different Δg (−10, 0, and 10 nm). The coupling loss deviation remains less than 0.3 dB despite the +/− 10 nm variations in actual groove widths. The measurement results are also consistent with the simulations as shown in Fig. 2.

5. Conclusion

A high efficiency double-etched fiber-to-waveguide grating coupler with engineered periods, which was fabricated by CMOS-compatible technology using deep UV photolithography on SOI platform, was reported. The measured coupling loss is −1.5 dB, which is corresponding to 70.8% coupling efficiency. The 3-dB bandwidth is 54 nm. This result is the highest coupling efficiency demonstrated to date for waveguide grating couplers fabricated in a CMOS line using deep UV photolithography. Measurements show the double-etched grating coupler with engineered periods is tolerant to large fluctuations in groove width in fabrication process. Further improvement may be obtained by optimizing the etch depth.

Acknowledgments

This work was supported by the Science and Engineering Research Council of A*STAR (Agency for Science, Technology and Research), Singapore. The SERC grant number: 0921150116.

References and links

1.

W. Bogaerts, R. Baets, P. Dumon, V. Wiaux, S. Beckx, D. Taillaert, B. Luyssaert, J. Van Campenhout, P. Bienstman, and D. Van Thourhout, “Nanophotonic waveguide in silicon-on-insulator fabricated with CMOS technology,” J. Lightwave Technol. 23(1), 401–412 (2005). [CrossRef]

2.

D. Taillaert, W. Bogaerts, P. Bienstman, T. F. Krauss, P. Van Daele, I. Moerman, S. Verstuyft, K. De Mesel, and R. Baets, “An out-of-plane grating coupler for efficient butt-coupling between compact planar waveguides and single-mode fibers,” IEEE J. Quantum Electron. 38(7), 949–955 (2002). [CrossRef]

3.

D. Taillaert, F. Van Laere, M. Ayre, W. Bogaerts, D. Van Thourhout, P. Bienstman, and R. Baets, “Grating couplers for coupling between optical fibers and nanophotonic waveguides,” Jpn. J. Appl. Phys. 45(8A), 6071–6077 (2006). [CrossRef]

4.

B. Wang, J. H. Jiang, and G. P. Nordin, “Embedded, slanted grating for vertical coupling between fibers and silicon-on-insulator planar waveguides,” IEEE Photon. Technol. Lett. 17(9), 1884–1886 (2005). [CrossRef]

5.

F. Van Laere, G. Roelkens, M. Ayre, J. Schrauwen, D. Taillaert, D. Van Thourhout, T. F. Krauss, and R. Baets, “Compact and highly efficient grating couplers between optical fiber and nanophotonic waveguides,” J. Lightwave Technol. 25(1), 151–156 (2007). [CrossRef]

6.

D. Vermeulen, S. Selvaraja, P. Verheyen, G. Lepage, W. Bogaerts, P. Absil, D. Van Thourhout, and G. Roelkens, “High-efficiency fiber-to-chip grating couplers realized using an advanced CMOS-compatible Silicon-On-Insulator platform,” Opt. Express 18(17), 18278–18283 (2010). [CrossRef] [PubMed]

7.

D. Taillaert, P. Bienstman, and R. Baets, “Compact efficient broadband grating coupler for silicon-on-insulator waveguides,” Opt. Lett. 29(23), 2749–2751 (2004). [CrossRef] [PubMed]

8.

X. Chen, C. Li, C. K. Y. Fung, S. M. G. Lo, and H. K. Tsang, “Apodized waveguide grating couplers for efficient coupling to optical fibers,” IEEE Photon. Technol. Lett. 22(15), 1156–1158 (2010). [CrossRef]

9.

M. Antelius, K. B. Gylfason, and H. Sohlström, “An apodized SOI waveguide-to-fiber surface grating coupler for single lithography silicon photonics,” Opt. Express 19(4), 3592–3598 (2011). [CrossRef] [PubMed]

10.

R. Halir, P. Cheben, S. Janz, D.-X. Xu, I. Molina-Fernández, and J. G. Wangüemert-Pérez, “Waveguide grating coupler with subwavelength microstructures,” Opt. Lett. 34(9), 1408–1410 (2009). [CrossRef] [PubMed]

11.

C. Alonso-Ramos, A. Ortega-Moñux, L. Zavargo-Peche, R. Halir, J. de Oliva-Rubio, Í. Molina-Fernández, P. Cheben, D.-X. Xu, S. Janz, N. Kim, and B. Lamontagne, “Single-etch grating coupler for micrometric silicon rib waveguides,” Opt. Lett. 36(14), 2647–2649 (2011). [CrossRef] [PubMed]

12.

A. Mekis, S. Gloeckner, G. Masini, A. Narasimha, T. Pinguet, S. Sahni, and P. De Dobbelaere, “A Grating-Coupler-Enabled CMOS Photonics Platform,” IEEE J. Sel. Top. Quantum Electron. 17(3), 597–608 (2011). [CrossRef]

13.

T. Shoji, T. Tsuchizawa, T. Watanabe, K. Yamada, and H. Morita, “Low loss mode size converter from 0.3μm square Si wire waveguides to singlemode fibres,” Electron. Lett. 38(25), 1669–1670 (2002). [CrossRef]

14.

V. R. Almeida, R. R. Panepucci, and M. Lipson, “Nanotaper for compact mode conversion,” Opt. Lett. 28(15), 1302–1304 (2003). [CrossRef] [PubMed]

15.

A. Delâge, S. Janz, B. Lamontagne, A. Bogdanov, D. Dalacu, D.-X. Xu, and K. P. Yap, “Monolithically integrated asymmetric graded and step-index couplers for microphotonic waveguides,” Opt. Express 14(1), 148–161 (2006). [CrossRef] [PubMed]

16.

Q. Fang, J. Song, X. Luo, M. Yu, G. Lo, and Y. Liu, “Mode-size converter with high coupling efficiency and broad bandwidth,” Opt. Express 19(22), 21588–21594 (2011). [CrossRef] [PubMed]

OCIS Codes
(050.2770) Diffraction and gratings : Gratings
(130.3120) Integrated optics : Integrated optics devices
(250.5300) Optoelectronics : Photonic integrated circuits

ToC Category:
Integrated Optics

History
Original Manuscript: December 12, 2012
Revised Manuscript: January 11, 2013
Manuscript Accepted: January 15, 2013
Published: March 25, 2013

Citation
Chao Li, Huijuan Zhang, Mingbin Yu, and G. Q. Lo, "CMOS-compatible high efficiency double-etched apodized waveguide grating coupler," Opt. Express 21, 7868-7874 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-7-7868


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References

  1. W. Bogaerts, R. Baets, P. Dumon, V. Wiaux, S. Beckx, D. Taillaert, B. Luyssaert, J. Van Campenhout, P. Bienstman, and D. Van Thourhout, “Nanophotonic waveguide in silicon-on-insulator fabricated with CMOS technology,” J. Lightwave Technol.23(1), 401–412 (2005). [CrossRef]
  2. D. Taillaert, W. Bogaerts, P. Bienstman, T. F. Krauss, P. Van Daele, I. Moerman, S. Verstuyft, K. De Mesel, and R. Baets, “An out-of-plane grating coupler for efficient butt-coupling between compact planar waveguides and single-mode fibers,” IEEE J. Quantum Electron.38(7), 949–955 (2002). [CrossRef]
  3. D. Taillaert, F. Van Laere, M. Ayre, W. Bogaerts, D. Van Thourhout, P. Bienstman, and R. Baets, “Grating couplers for coupling between optical fibers and nanophotonic waveguides,” Jpn. J. Appl. Phys.45(8A), 6071–6077 (2006). [CrossRef]
  4. B. Wang, J. H. Jiang, and G. P. Nordin, “Embedded, slanted grating for vertical coupling between fibers and silicon-on-insulator planar waveguides,” IEEE Photon. Technol. Lett.17(9), 1884–1886 (2005). [CrossRef]
  5. F. Van Laere, G. Roelkens, M. Ayre, J. Schrauwen, D. Taillaert, D. Van Thourhout, T. F. Krauss, and R. Baets, “Compact and highly efficient grating couplers between optical fiber and nanophotonic waveguides,” J. Lightwave Technol.25(1), 151–156 (2007). [CrossRef]
  6. D. Vermeulen, S. Selvaraja, P. Verheyen, G. Lepage, W. Bogaerts, P. Absil, D. Van Thourhout, and G. Roelkens, “High-efficiency fiber-to-chip grating couplers realized using an advanced CMOS-compatible Silicon-On-Insulator platform,” Opt. Express18(17), 18278–18283 (2010). [CrossRef] [PubMed]
  7. D. Taillaert, P. Bienstman, and R. Baets, “Compact efficient broadband grating coupler for silicon-on-insulator waveguides,” Opt. Lett.29(23), 2749–2751 (2004). [CrossRef] [PubMed]
  8. X. Chen, C. Li, C. K. Y. Fung, S. M. G. Lo, and H. K. Tsang, “Apodized waveguide grating couplers for efficient coupling to optical fibers,” IEEE Photon. Technol. Lett.22(15), 1156–1158 (2010). [CrossRef]
  9. M. Antelius, K. B. Gylfason, and H. Sohlström, “An apodized SOI waveguide-to-fiber surface grating coupler for single lithography silicon photonics,” Opt. Express19(4), 3592–3598 (2011). [CrossRef] [PubMed]
  10. R. Halir, P. Cheben, S. Janz, D.-X. Xu, I. Molina-Fernández, and J. G. Wangüemert-Pérez, “Waveguide grating coupler with subwavelength microstructures,” Opt. Lett.34(9), 1408–1410 (2009). [CrossRef] [PubMed]
  11. C. Alonso-Ramos, A. Ortega-Moñux, L. Zavargo-Peche, R. Halir, J. de Oliva-Rubio, Í. Molina-Fernández, P. Cheben, D.-X. Xu, S. Janz, N. Kim, and B. Lamontagne, “Single-etch grating coupler for micrometric silicon rib waveguides,” Opt. Lett.36(14), 2647–2649 (2011). [CrossRef] [PubMed]
  12. A. Mekis, S. Gloeckner, G. Masini, A. Narasimha, T. Pinguet, S. Sahni, and P. De Dobbelaere, “A Grating-Coupler-Enabled CMOS Photonics Platform,” IEEE J. Sel. Top. Quantum Electron.17(3), 597–608 (2011). [CrossRef]
  13. T. Shoji, T. Tsuchizawa, T. Watanabe, K. Yamada, and H. Morita, “Low loss mode size converter from 0.3μm square Si wire waveguides to singlemode fibres,” Electron. Lett.38(25), 1669–1670 (2002). [CrossRef]
  14. V. R. Almeida, R. R. Panepucci, and M. Lipson, “Nanotaper for compact mode conversion,” Opt. Lett.28(15), 1302–1304 (2003). [CrossRef] [PubMed]
  15. A. Delâge, S. Janz, B. Lamontagne, A. Bogdanov, D. Dalacu, D.-X. Xu, and K. P. Yap, “Monolithically integrated asymmetric graded and step-index couplers for microphotonic waveguides,” Opt. Express14(1), 148–161 (2006). [CrossRef] [PubMed]
  16. Q. Fang, J. Song, X. Luo, M. Yu, G. Lo, and Y. Liu, “Mode-size converter with high coupling efficiency and broad bandwidth,” Opt. Express19(22), 21588–21594 (2011). [CrossRef] [PubMed]

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