1. Introduction

Silicon photonics hold great promise for the creation of highly integrated photonic circuits. The high index contrast between silicon and silica permits strong confinement of light, thus enabling small bending radii and strong light-matter interaction. Furthermore, this material choice allows monolithic integration with silicon microelectronics. However, the high effective index and small mode dimensions of single mode silicon waveguides makes fiber coupling challenging.

Using surface grating couplers, the mode matching problem can be solved by expanding the width of the on-chip silicon waveguide, and etching a grating into the expanded section that diffracts light out of plane into a fiber placed normal to the surface. Coupling efficiencies of 37% have been reported for periodic shallow-etched silicon-on-insulator (SOI) grating couplers [1]. A single etch periodic grating coupler where the shallow etched silicon regions were substituted with photonic crystal structures was recently demonstrated, with a measured coupling efficiency of 42% [2]. By applying reflectors below the gratings to reduce the loss to the substrate, coupling efficiencies of up to 69.5% have been measured [3], but these approaches add significantly to fabrication complexity and require the use of non-standard substrates.

Another approach to increase the coupling efficiency is to tailor the leakage factor of the grating to the mode profile of the fiber. Simulations of fill factor apodization of shallow etched gratings have predicted a coupling efficiency of 61% [4] and experimental results of 64% for an etching depth apodized grating were recently presented [5]. Experimental results for the first fully etched and apodized grating coupler was very recently demonstrated for TM mode coupling [6]. Table 1 summarizes figures of merit for some reported fiber to TE mode silicon waveguide surface grating couplers.

Table 1. A comparison of reported figures of merit for fiber to TE mode silicon waveguide grating couplers at 1550 nm

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Here, we present simulations and experimental results for a through-etched SOI grating with fill factor apodization. The main benefits of this approach are: a low back reflection into the silicon waveguide; the use of standard substrates; a single required lithography step; and a large process latitude, since the grating depth is defined by an oxide etch stop and not a timed etch as in [3] and [5]. Figure 1(b) shows a scanning electron micrograph of the fabricated structure.

 

Fig. 1 (a) The layout of the back-to-back grating-to-fiber coupling test circuit. The ring resonator is used to verify that the transmitted light propagates through the single mode silicon waveguide between the two tapers. (b) An electron micrograph of the apodized through-etched SOI grating. The HSQ resist is left on top of the silicon. The A-A’ labels indicate the position of the cross section shown in Fig. 2(a).

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2. Grating coupler design optimization

For simulating and optimizing the grating coupler performance we used the eigenmode expansion technique with perfectly matched boundary layers, as implemented in the open-source software package CAMFR [14]. We described the coupling structure with a two-dimensional cross-section along the waveguide propagation axis, as shown in Fig. 2. This 2D simplification has been shown to overestimate the coupled power by 3% for structures similar to ours [4].

 

Fig. 2 (a) The calculated E_y field distribution in cross section A-A’ shown in Fig. 1(a) for the TE mode propagating from the single mode silicon waveguide on the left and coupling into a single mode fiber at a 10° angle to the surface normal. (b) A similar cross section for the optimal through-etched conventional periodic grating.

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In the model, the fundamental TE mode of a 220 nm thick planar silicon waveguide on a buried oxide (BOX) layer is excited with a wavelength of 1550 nm from the left. The power that diffracts into a 10.4 μm wide Gaussian profile (i.e a single mode fiber) tilted 10° off the x-axis is then calculated as described in [4].

As a starting point, the period and fill factor of an optimal through-etched conventional periodic grating design was found using an exhaustive search. The upper row of Fig. 3 shows, at every period and fill factor combination, the power fraction coupled into the fiber (i.e. the coupling efficiency). In the same manner, the lower row shows the power fraction reflected back into the waveguide. Due to interference with the wave reflected at the BOX-substrate interface below the grating, the coupling efficiency has a strong periodic dependence on the BOX thickness. The standard BOX of 2 μm used at the European ePIXfab silicon photonics foundries is close to the worst case thickness, and thus we designed two devices: one on an optimal BOX of 2.2 μm thickness, and one on the standard 2 μm BOX.

 

Fig. 3 The upper and lower rows show the power fraction coupled into the fiber and reflected back into the waveguide, respectively, at every period and fill factor combination at 1550 nm. The left and right columns correspond to gratings with 2 and 2.2 μm BOX, respectively. The crosses indicate the two starting points selected for the apodization.

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The two designs were then apodized, by varying the width of every bar and gap individually in steps of 10 nm, with a minimum allowed dimension of 120 nm, using a simple genetic algorithm [4]. As shown in Fig. 4(a), this process increased the coupling efficiency of the grating on a 2.2 μm BOX from 51% to 72%, while at the same time the power reflected back into the waveguide reduced from 21% to 0.1%. Table 2 lists the optimal widths of each gap and bar in the apodized gratings. Figure 4 (b) shows the coupling efficiency as a function of the BOX thickness for the apodized grating designed for the optimal 2.2 μm BOX thickness.

 

Fig. 4 (a) The evolution of the power fractions coupled into the fiber (solid line) and reflected back into the silicon waveguide (dotted line) during the algorithmic apodization of the grating on a 2.2 μm BOX. (b) The calculated coupling efficiency from silicon waveguide to fiber as a function of the BOX thickness for the apodized grating designed for the optimal 2.2 μm BOX thickness.

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Table 2. The dimensions of two apodized through-etched grating designs with 20 cells

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Figure 2 shows the calculated E_y field distribution for both the apodized and the periodic grating on a 2.2 μm BOX. Figure 2 (a) shows how the apodization matches the radiated field from the silicon grating to the Gaussian mode profile of the fiber, while for the periodic grating in Fig. 2(b) light is lost at the left and right of the fiber.

3. Experimental evaluation

The grating couplers were fabricated on an SOI wafer with a 220 nm silicon device layer and a 2 μm BOX layer. The devices were patterned in hydrogen silsesquioxane (HSQ) by electron beam lithography and then transferred into the device layer by dry etching in a Cl₂/HBr/HeO₂ plasma. For characterization, a refractive index matching oil (Cargille 50350) was used as a top cladding. This oil has a viscosity of only 19 cSt and wets the structures with a very low contact angle, thus we assume complete filling of the grating gaps. A more durable cladding, such as a spin on glass, could be used for production devices.

The efficiency of coupling between a single mode fiber and an on-chip single mode silicon waveguide was determined with the back-to-back grating-to-fiber coupling circuit shown in Fig. 1(a). First, the loss of a SMF-28 patch cord was determined. The fiber was then cleaved and the two ends arranged with a 10° angle and optimum coupling position in the index matching liquid over the gratings. The excess loss through the complete structure was then measured in the wavelength range from 1480 to 1580 nm using an Agilent 86082A Wavelength Domain Component Analyzer and a tunable light source with a 1 pm line width. The polarization in the input fiber was optimized for TE coupling with a “bat ear” fiber polarization controller. The waveguide loss of the straight single mode section was determined by comparing losses through structures with guides of different lengths. The coupling efficiency of each grating coupler is therefore half of the excess loss after subtraction of the waveguide loss.

4. Results

Figure 5(a) shows the power loss of the back-to-back grating coupler structures with different lengths of the single mode waveguide section. The linear fit yields a waveguide loss of −8.2 dB/mm and a coupling efficiency of 35% per grating.

 

Fig. 5 (a) The loss of the back-to-back grating coupler structures with five different lengths of the single mode silicon waveguide section. The linear fit yields a waveguide loss of −8.2 dB/mm and a coupling efficiency of 35% per grating. (b) The solid curve is the measured loss spectrum of the fabricated test structure with apodized gratings on a standard 2 μm BOX. For comparison, the simulated loss spectra of the apodized designs on both 2 μm and 2.2 μm BOX are shown, as well as the simulated spectrum for the grating dimensions obtained in fabrication.

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The solid curve in Fig. 5(b) shows the loss spectrum of one fabricated structure, with the loss of the 140 μm long single mode waveguide section subtracted. The 1 and 3 dB bandwidths of a single grating are 47 and 83 nm, respectively. The dips correspond to the resonance frequencies of the ring resonator, and show that light is successfully coupled to the single mode silicon photonic circuit. The parasitic Fabry-Perot ripple of less than 0.5 dB is higher than predicted by simulation, but consistent with the back-reflection introduced by an error of the fill factor by 5–10% in either direction. For comparison, the conventional periodic through-etched grating coupler reported in [12] exhibited a 2 dB ripple. The maximum coupling efficiency is obtained at a wavelength of 1536 nm.

The dot-dashed curve in Fig. 5(b) shows the simulated spectrum of the apodized design on a 2 μm BOX, with a maximum transmission of 33% per grating at 1550 nm. The 1 and 3 dB bandwidths are 33 and 58 nm, respectively. The dotted curve shows the simulated spectrum of the apodized design on an optimal 2.2 μm BOX with a maximum transmission of 72% at 1550 nm. The 1 and 3 dB bandwidths are 38 and 64 nm, respectively. Additionally, in order to estimate the fabrication tolerance, the simulated effect of a ±5% fill factor error on the coupling efficiency is shown with a lighter dotted line. This shows that narrower gaps in the grating can slightly increase the coupling efficiency. The dashed curve shows the simulated spectrum for the grating dimensions obtained in fabrication, as measured with an SEM calibrated to a laser-interferometer (Raith 150, Raith GmbH, Dortmund, Germany).

The broadening and the wavelength shift of the maximum in the measured transmission spectra, compared to the design values, are likely due to deviations of the smallest gap widths from those designed. These systematic errors could be adjusted by a more careful proximity exposure compensation in the electron beam lithography process. Broadening usually leads to a decrease in peak coupling efficiency, but this can have been counteracted by an increase of the fill factor, as in the case of a +5% fill factor error, or other geometrical imperfections such as slanting of the grating sidewalls.

5. Conclusions

We have presented the design, fabrication, and characterization of a through-etched apodized silicon grating coupler. We measured a coupling efficiency of 35% with a 1 dB bandwidth of 47 nm at 1536 nm on a standard SOI substrate with a 2 μm BOX. Furthermore, we showed by simulation that with an optimized buried oxide thickness, a coupling efficiency of 72% and a 1 dB bandwidth of 38 nm at 1550 nm could be achieved. In particular, simulations show that the apodization process reduces the optical power reflected back into the silicon waveguide from 21% to only 0.1%. Compared to previous eigenmode expansion simulations on single lithography through-etched TE-mode gratings [2], the apodization applied in this work provides an improved coupling efficiency with a sustained high bandwidth and low back-reflection.