## Design of SiON-based grating-assisted vertical directional couplers

Optics Express, Vol. 14, Issue 3, pp. 1055-1063 (2006)

http://dx.doi.org/10.1364/OE.14.001055

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### Abstract

In this paper, we present the design of a SiON-based grating-assisted codirectional coupler for dissimilar waveguides, based on an accurate numerical method. All the design parameters, including coupling length and coupling efficiency, have been calculated at the free-space optical wavelength of λ =1.54 μm for both TE and TM polarisations as a function of grating height, thickness of the coupled slabs, gap thickness between the two coupled planar waveguides. It is shown that efficient (up to 70%) and polarisation independent vertical couplers can be realised.

© 2006 Optical Society of America

## 1. Introduction

1. G. L. Bona, R. Germann, and B. J. Offrein, “SiON high-refractive-index waveguide and planar lightwave circuits,” IBM J. Res. Dev. **47**, 239 (2003). [CrossRef]

2. A. Melloni, R. Costa, P. Monguzzi, and M. Martinelli, “Ring resonator filters in silicon oxynitride technology for dense wavelength-division multiplexed systems,” Opt. Lett. **28**, 1567 (2003). [CrossRef] [PubMed]

3. L. L. Buhl, R. C. Alferness, U. Koren, B. I. Miller, M. G. Young, T. L. Koch, C. A. Burrus, and G. Raybon, “Grating assisted vertical coupler/filter for extended tuning range,” Electron. Lett. **29**, 81 (1993). [CrossRef]

4. G. Z. Masanovic, G. T. Reed, W. Headley, B. Timotijevic, V. M. N. Passaro, R. Atta, G. Ensell, and A. G. R. Evans, “A high efficiency input/output coupler for small silicon photonic devices,” Opt. Express **13**, 7374 (2005), http://www.opticexpress.org/abstract.cfm?URI=OPEX-13-19-7374. [CrossRef] [PubMed]

5. V. M. N. Passaro, “Optimal design of grating-assisted directional couplers,” J. Lightwave Technol. **18**, 973 (2000). [CrossRef]

6. W. Huang and J. Hong, “Transfer matrix approach based on local normal modes for coupled waveguides with periodic perturbations,” J. Lightwave Technol. **10**, 1367 (1992). [CrossRef]

_{2}buffer layer over the Si substrate have been considered as a technological solution because of their high versatility, full compatibility with optical fibre technology and low propagation loss (0.2 dB/cm) [7

7. K. Worhoff, A. Driessen, P. V. Lambeck, L. T. H. Hilderink, P. W. C. Linders, and Th. J. A. Popma, “Plasma enhanced chemical vapor deposition silicon oxynitride optimized for application in integrated optics,” Sens. Actuators A **74**, 9 (1999). [CrossRef]

## 2. GADC structure

*d*and refractive index

_{1}*n*. The analogous parameters for the upper slab are

_{1}*d*and

_{2}*n*. In all calculations, we have used

_{2}*n*= 1.47 (allowing low insertion loss for coupling with optical fibres),

_{1}*d*= 2 μm, and

_{2}*n*= 1.51. The mono-modal condition [8

_{2}8. G. T. Reed and A. P. Knights, *Silicon photonics: an introduction* (Wiley, Chichester, 2004). [CrossRef]

*d*< 5.37 μm and

_{1}*d*< 2.08 μm at the wavelength of 1.54 μm. The other layers are all formed by SiO

_{2}_{2}with the refractive index of

*n*= 1.463. We have already shown that the refractive index of SiON can be accurately tuned within this range using PECVD [9

9. G. Z. Masanovic, G. T. Reed, V. M. N. Passaro, W. Headley, M. R. Josey, G. J. Ensell, R. M. H. Atta, and A. G. R. Evans, “A grating based coupler for fibre to silicon waveguide excitation,” in *Integrated Optics and Photonic Integrated Circuits*,
G. C. Righini and S. Honkanen, eds., Proc. SPIE5451, 381 (2004). [CrossRef]

*n*= 1.470 are: SiH

_{4}/N

_{2}gas flow = 155 sccm (standard cubic centimetre), NH

_{3}gas flow = 50 sccm, N

_{2}O gas flow = 800 sccm, p = 1 Torr, T = 300 °C, P = 15 W. The geometrical and optical parameters of the structure are chosen, without any lack of generality, such to have only two leaky guided modes. This choice is similar to the TMM approach, in which only two local normal modes are considered at each grating section [6

6. W. Huang and J. Hong, “Transfer matrix approach based on local normal modes for coupled waveguides with periodic perturbations,” J. Lightwave Technol. **10**, 1367 (1992). [CrossRef]

*A, B*) which exchange optical power along direction of propagation. It can be calculated by the FBT under the resonance condition as:

*β*,

_{A}*β*are the propagation constants of the two modes, calculated in the presence of the grating. A good estimation of Λ can be also found by CMT only if the grating depth is small with respect to the slab thicknesses, as:

_{B}*n*and

_{eff}^{(even)}*n*are the effective indices of the even and odd modes of the unperturbed structure, respectively. A number of calculations in this paper have been carried out by considering FBT approach by using 23 space harmonics and double precision complex algorithms. The effective index difference between the perturbed GADC structure and the unperturbed structure (SiON layers without grating) was always <0.25 % giving very low theoretical insertion loss (<0.07 dB for TE and <0.06 for TM) at the interface between these two structures (i.e. structures with and without gratings). No significant difference has been found by comparing the FBT results with TMM predictions, as the grating depth is rather small (0.05 ÷ 0.15 μm). The rectangular grating profile with a 50% duty cycle has been considered. The bandwidth of the GADC, working as a filter, has been evaluated by CMT as:

_{eff}^{(odd)}*L*is the coupling length,

*ΔN*is the effective index change and the index dispersion has not been taken into account because it is negligible.

## 3. Numerical results

*d*= 3 μm. It can be seen that the best coupling condition occurs at mid range depths (0.1 μm) for gap thickness ≈ 2.5 μm.

_{1}*Λ*≈ 56.4 μm and

_{TE}*Λ*≈ 57.8 μm for TE and TM polarisation, respectively. It is important to note that the dependence on grating depth is not significant. For the average grating period of ≈ (

_{TM}*Λ*+

_{TE}*Λ*)/2 it would be so possible to obtain polarisation independent devices at the expense of some reduction of the coupling efficiency, however less than 10%.

_{TM}*d*) has also been investigated (Fig. 7). Assuming a grating 0.15 μm deep, the best efficiencies are obtained by using large slab 1 thickness and reduced gap thickness (2 μm). It is important to note the behavior of TE and TM polarisations is very similar in these SiON-based structures, although usually any grating-based component is strongly polarization-dependent.

_{1}*n*) changes (up to 0.2%) have a strong influence on both the efficiency (up to 20%) and length (up to 26%), while slab 2 (upper slab with grating) index has a more moderate influence (up to 15% and 13%, respectively). However, we have already shown [9

_{1}9. G. Z. Masanovic, G. T. Reed, V. M. N. Passaro, W. Headley, M. R. Josey, G. J. Ensell, R. M. H. Atta, and A. G. R. Evans, “A grating based coupler for fibre to silicon waveguide excitation,” in *Integrated Optics and Photonic Integrated Circuits*,
G. C. Righini and S. Honkanen, eds., Proc. SPIE5451, 381 (2004). [CrossRef]

## 4. Conclusion

## References and links

1. | G. L. Bona, R. Germann, and B. J. Offrein, “SiON high-refractive-index waveguide and planar lightwave circuits,” IBM J. Res. Dev. |

2. | A. Melloni, R. Costa, P. Monguzzi, and M. Martinelli, “Ring resonator filters in silicon oxynitride technology for dense wavelength-division multiplexed systems,” Opt. Lett. |

3. | L. L. Buhl, R. C. Alferness, U. Koren, B. I. Miller, M. G. Young, T. L. Koch, C. A. Burrus, and G. Raybon, “Grating assisted vertical coupler/filter for extended tuning range,” Electron. Lett. |

4. | G. Z. Masanovic, G. T. Reed, W. Headley, B. Timotijevic, V. M. N. Passaro, R. Atta, G. Ensell, and A. G. R. Evans, “A high efficiency input/output coupler for small silicon photonic devices,” Opt. Express |

5. | V. M. N. Passaro, “Optimal design of grating-assisted directional couplers,” J. Lightwave Technol. |

6. | W. Huang and J. Hong, “Transfer matrix approach based on local normal modes for coupled waveguides with periodic perturbations,” J. Lightwave Technol. |

7. | K. Worhoff, A. Driessen, P. V. Lambeck, L. T. H. Hilderink, P. W. C. Linders, and Th. J. A. Popma, “Plasma enhanced chemical vapor deposition silicon oxynitride optimized for application in integrated optics,” Sens. Actuators A |

8. | G. T. Reed and A. P. Knights, |

9. | G. Z. Masanovic, G. T. Reed, V. M. N. Passaro, W. Headley, M. R. Josey, G. J. Ensell, R. M. H. Atta, and A. G. R. Evans, “A grating based coupler for fibre to silicon waveguide excitation,” in |

**OCIS Codes**

(050.2770) Diffraction and gratings : Gratings

(060.1810) Fiber optics and optical communications : Buffers, couplers, routers, switches, and multiplexers

(130.3120) Integrated optics : Integrated optics devices

(160.3130) Materials : Integrated optics materials

**ToC Category:**

Integrated Optics

**History**

Original Manuscript: November 17, 2005

Revised Manuscript: January 13, 2006

Manuscript Accepted: January 22, 2006

Published: February 6, 2006

**Citation**

Vittorio Passaro and Goran Masanovic, "Design of SiON-based grating-assisted vertical directional couplers," Opt. Express **14**, 1055-1063 (2006)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-14-3-1055

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### References

- G. L. Bona, R. Germann, and B. J. Offrein, “SiON high-refractive-index waveguide and planar lightwave circuits,” IBM J. Res. Dev. 47, 239 (2003). [CrossRef]
- A. Melloni, R. Costa, P. Monguzzi, and M. Martinelli, “Ring resonator filters in silicon oxynitride technology for dense wavelength-division multiplexed systems,” Opt. Lett. 28, 1567 (2003). [CrossRef] [PubMed]
- L. L. Buhl, R. C. Alferness, U. Koren, B. I. Miller, M. G. Young, T. L. Koch, C. A. Burrus, and G. Raybon, “Grating assisted vertical coupler/filter for extended tuning range,” Electron. Lett. 29, 81 (1993). [CrossRef]
- G. Z. Masanovic, G. T. Reed, W. Headley, B. Timotijevic, V. M. N. Passaro, R. Atta, G. Ensell, and A. G. R. Evans, “A high efficiency input/output coupler for small silicon photonic devices,” Opt. Express 13, 7374 (2005), <a href= "http://www.opticexpress.org/abstract.cfm?URI=OPEX-13-19-7374">http://www.opticexpress.org/abstract.cfm?URI=OPEX-13-19-7374</a>. [CrossRef] [PubMed]
- V. M. N. Passaro, “Optimal design of grating-assisted directional couplers,” J. Lightwave Technol. 18, 973 (2000). [CrossRef]
- W. Huang and J. Hong, “Transfer matrix approach based on local normal modes for coupled waveguides with periodic perturbations,” J. Lightwave Technol. 10, 1367 (1992). [CrossRef]
- K. Worhoff, A. Driessen, P. V. Lambeck, L. T. H. Hilderink, P. W. C. Linders, and Th. J. A. Popma, “Plasma enhanced chemical vapor deposition silicon oxynitride optimized for application in integrated optics,” Sens. Actuators A 74, 9 (1999). [CrossRef]
- G. T. Reed and A. P. Knights, Silicon photonics: an introduction (Wiley, Chichester, 2004). [CrossRef]
- G. Z. Masanovic, G. T. Reed, V. M. N. Passaro, W. Headley, M. R. Josey, G. J. Ensell, R. M. H. Atta, and A. G. R. Evans, “A grating based coupler for fibre to silicon waveguide excitation,” in Integrated Optics and Photonic Integrated Circuits, G. C. Righini and S. Honkanen, eds., Proc. SPIE 5451, 381 (2004). [CrossRef]

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