1. Introduction

For decades, silicon photonics has been extensively researched for its attractive advantages of being a low-cost material and CMOS compatibility. Moreover, high-density optical integration can be achieved because the index contrast between Si core and SiO₂ cladding is large. Through development of various active and passive devices, today some Si-photonics modules and sub-systems have begun to be commercialized.

However, there is still a challenge ahead in the full-scale commercialization of Si-photonics integrated circuits used in datacenter and telecommunications networks. This includes the development of high-performance silicon optical couplers for optical assembly, categorized mainly as either chip-surface or chip-edge couplers. Chip-surface optical couplers possess superiorities in optical density and layout flexibility in the optical circuits compared with chip-edge couplers. Nevertheless, the grating couplers that have been dominantly used as chip-surface optical couplers are apt to sacrifice performance in coupling efficiency and bandwidth because they use diffractive effects to change the direction of light propagation to the chip-surface. To overcome these drawbacks of grating couplers, various techniques have been reported, such as multi-stage grating [1], thick Si waveguide grating [2–4], multi-layered grating [5,6], and reflector integration [7,8].

As another approach, chip-surface couplers based on vertically bended Si waveguides have been reported [9,10]. We have also proposed and demonstrated a coupler with a radius of curvature of a few micrometers by introducing an ion-implantation bending (IIB) method as shown in Fig. 1(a), calling it an elephant coupler in an analogy to a raised elephant trunk [10,11]. The fabricated elephant coupler achieves high-efficiency and broad-band coupling when coupled to a 2-µm-spot tip-lensed single-mode optical fiber (SMF).

 

Fig. 1 Elephant couplers (a) SEM images of vertically curved Si waveguides deformed by IIB method (b) Propagating electric-field profile of the 5-µm-beam-spot elephant coupler.

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Recently, we designed a coupler for coupling with a high-numerical-aperture (HNA) SMF with a mode-field diameter (MFD) of about 5 µm using 3D-FDTD method [12]. The HNA-SMFs are promising optical counterparts because they exhibit some tolerance to fiber alignment errors as well as robustness in fiber bending because of a large contrast in refractive index between core and cladding material. The designed coupler in Fig. 1(b) features a dome-like SiO₂ coupler cap as well as a coaxial structure of Si core and SiO₂ cladding. This coupler cap acts as collecting lens for the radiative light from a Si inversed taper with a length as short as 6 µm. Consequently, high-efficiency optical coupling is realized in a small device compared with the well-known couplers exploiting the adiabatic mode-field expansion technique with the taper length of more than tens micrometers [13–20]. The designed coupler numerically shows a coupling loss of 0.8 dB and broad-band coupling with a 0.5-dB-loss bandwidth of 420 nm between a TE-polarized 5-µm-beam-waist Gaussian beam assuming butt-joint coupling with HNA SMFs.

We have therefore a coupler with good coupling properties. However, such a three-dimensional structured device has not been realized as yet. In particular, forming the dome-like SiO₂ coupler cap, which has an important role in controlling the output beam divergence, has proved challenging. In this work, we describe its fabrication applying isotropic SiO₂ deposition to a vertically curved waveguide (VCW) of Si using plasma-enhanced chemical vapor deposition (PECVD) employing tetraethyl orthosilicate (TEOS). The measured far-field pattern (FFP) of the output beam was well-fitted with a 5-µm-waist Gaussian beam, and thereby we confirmed that the dome-like coupler cap truly behaves as a collimation lens. The fabricated device had a coupling loss of less than 4.2 dB for TE polarization and broad-band coupling with a 0.5-dB-loss bandwidth that is wider than 150 nm.

2. Device design

First, we describe the design of the elephant coupler [12], which was configured with a VCW segment and a spot-size-converter (SSC) segment as shown in Fig. 1(b). The height and width of the VCW segment are 220 nm and 430 nm, respectively, to confine the propagating light strongly in the Si core. Therefore, the light is directed toward the chip-surface along a bend of a few micrometers. Next, in the SSC segment, the width is exponentially tapered from 430 nm to 50 nm with a taper-length of 6 µm. Such a short taper drastically leaks propagating light into the surrounding SiO₂ cladding, thus enlarging the optical field quite efficiently. However, its large divergence angle leads to a large coupling loss between HNA-SMFs due to mismatch in the numerical aperture. Therefore, to reduce the divergence angle, a dome-like SiO₂ structure with a diameter of 5.4 µm was introduced at the coupler cap to act as collimation lens. Consequently, the 5-µm-beam-spot light output with a quasi-flat phase-plane was obtained. The designed coupler numerically shows high-efficiency coupling with losses of 0.8 dB and a 0.5-dB-loss bandwidth of 420 nm for TE polarization. In this loss calculation, a 5-µm-waist Gaussian beam was focused on the top of the device from free-space, and the loss at the position P in Fig. 1(b) was calculated from the overlap integration between the coupled light and the TE-0th mode of the Si waveguide. Therefore, the coupling loss includes reflection loss at the air-coupler interface and the bending loss at the vertically curved waveguide, meanwhile the absorption loss resulting from ion implantation and reflection loss at the fiber-air interface were not included. (The fiber-air reflection loss was estimated to be 0.18 dB when the effective index of HNA-SMFs is 1.5.)

3. Device fabrication

We fabricated the coupler using the following procedure. First, Si optical circuits were formed by electron-beam lithography (EBL) and inductively-coupled-plasma reactive-ion-etching (ICP-RIE). The SiO₂ over-cladding was then deposited by TEOS-PECVD, and the termination of the Si waveguides, specifically, the area that becomes the elephant coupler, was cantilevered by partial wet etching of the SiO₂ cladding using a buffered hydrofluoric (BHF) acid solution. Next, the Si cantilever was vertically bent using the IIB method as shown in Fig. 2(a). The degree of bending generally depends on the implanted ion-species, acceleration voltage, beam current, and dose amount [10,11]. For this study, argon ion (Ar⁺) was used. Figure 2(b) shows a SEM image of the Si waveguide after the IIB process with a dose concentration of about 7 × 10¹⁵ cm⁻² under an acceleration voltage of 110 keV and a beam current of 50 µA. The implantation conditions were adjusted to curl the Si-taper tip backward through 90°. Finally a 2.7-µm-thick SiO₂ cladding was deposited again using TEOS-PECVD at a temperature of 350°C as shown in Fig. 3. The dome-like SiO₂ coupler cap and the coaxial VCW with Si core and SiO₂ cladding was formed as predicted by taking effect of the isotropical deposition onto the vertically curved Si waveguide. In a comparison of Figs. 2(b) and 3(b), the angle of bending in the VCW seems to be reduced after the SiO₂ deposition. We previously reported the effect of thermal relaxation on the bent structure through annealing as the Si waveguide is very slender [11].

 

Fig. 2 (a) Schematic of the vertically curved Si waveguide fabricated using the IIB method. (b) SEM image of the curved waveguide with a taper length of 9 µm. Inset shows a top-view of the Si taper before the IIB process.

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Fig. 3 (a) Schematic of the dome-like SiO₂ coupler-top fabricated by applying isotropically SiO₂ deposition using PECVD. (b) SEM image of the fabricated elephant coupler with SiO₂ dome.

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From a scanning ion-conductance microscope (SIM) image of the fabricated coupler after focused ion-beam (FIB) milling, the Si waveguide is confirmed to be buried almost along the centerline of the SiO₂ cladding as shown in Fig. 4. The coupler cap was also found to be almost hemispherical. Moreover, no voids or cracks are evident anywhere in the SiO₂ cladding. Thus, we have succeeded in fabricating a coupler with a unique dome-like SiO₂ coupler cap.

 

Fig. 4 Cross-sectional SIM image of the elephant coupler with a 9-µm-long Si taper. A tungsten sacrifice layer for the FIB milling process was temporarily depositted on the body of the device.

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4. Measurement

FFPs were taken to analyze the output beam profile from the coupler. With the measurement setup shown in Fig. 5(a), we input 1.55-µm-wavelength-band TE-polarized light into the device from the chip-edge using a polarization maintaining fiber (PMF). The light from the elephant coupler was detected using a f-θ lens system. The coordinate axes are defined as indicated in the figure. Figure 5(b) shows the measured FFP profiles of an elephant coupler with a 7-µm-long Si taper; the device is different from that shown in Fig. 4. Relatively smooth intensity profiles with a single peak near 0° were obtained for both axes, and the light is seen to radiate vertically from the coupler. The full-width at half-maximum (FWHM) of themeasured FFP profiles is about 14°, and the profiles are well-fitted with a Gaussian beam with a beam-waist size of 5 µm (gray-dashed line in Fig. 5(b)). We couldn’t catch the beam-waist position along the z-axis due to the measurement system limitation. From this result, we confirmed that the combination of short Si taper and dome-like coupler cap can dictate the optical profile to match with the profile of the HNA-SMF.

 

Fig. 5 Measurement of the FFP:(a) setup and (b) profiles.

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Figure 6 shows the measurement setup and the measured coupling loss of the device shown in Figs. 3(b) and 4. Two elephant couplers were connected with a 5-mm-long Si waveguide. In this measurement, a broad-band TE-polarized light from a super-luminescent diode (SLD) light source was coupled into the device using vertically accessed tip-lensed optical fibers with a 5-µm beam waist. Each fiber was aligned using three-position and three-angle axes manipulation by detecting the peak output power through an optical power meter. Then, the coupling loss was measured using an optical spectrum analyzer (OSA); both measured and 3D-FDTD simulated coupling spectra are presented in Fig. 6(b). The measured loss includes a propagation loss of the ~2.5-mm-long Si waveguide, which was experimentally estimated to be about 0.5 dB. The minimum coupling loss was 4.2 dB near a wavelength of 1600 nm; a broad-band coupling with a 0.5-dB-loss bandwidth of over 150 nm was obtained. However, the experimental result is about 3.5 dB larger than the simulated result including the above-mentioned additional waveguide loss. This may be because the radius of curvature of the Si waveguide in Fig. 4 was larger than the 3-µm radius configured in the design. Consequently, the Si waveguide started to narrow halfway down the bend, and therefore the propagating light started to radiate at the bend. This could be solved using a sophisticated set of conditions in ion implantation. Another reason is the absorption loss due to ion implantation. We experimentally confirmed that the absorption loss may be significantly reduced in post annealing, e.g., the loss was reduced from 0.08 dB/µm to 0.03 dB/µm for the 400-nm-wide waveguide after a 600°C annealing for one hour in a N₂ atmosphere. Furthermore, the deviation of the SiO₂-dome shape from the simulation might have increased the coupling loss to some extent.

 

Fig. 6 Measurement of coupling loss: (a) setup and (b) coupling spectra.

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The measured coupling loss has large wavelength dependence compared to the simulated result. This seems to be also because of the absorption loss. The optical confinement in Si waveguide becomes large for the shorter wavelength, and thus the absorption loss might increase. Another reason is propagation loss of the Si waveguide which is included in the measured result. Generally, it is known that the scattering loss due to Si sidewall roughness is large for the shorter wavelength.

Finally, we describe the fiber angle dependence of the coupling loss in Fig. 7. We measured the three devices on a device chip. The minimum coupling loss was obtained at the angle of 0 degree, and thus the output beam direction was found to be almost vertical. Moreover, the bandwidth for 0.5-dB-loss-increment was as large as 6 degrees, and the on-chip dispersion of the coupling loss was found to be within 0.5 dB.

 

Fig. 7 Alignment sensitivities for coupling loss in fiber angle: (a) fiber angle definition (b) measurement result.

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5. Discussion

For the Si-PICs in the datacenter and telecommunications networks, the return loss and polarization are significant issues to be managed. The reflection loss at the dome-like coupler cap was calculated to be 0.08 dB using 3D-FDTD method, and we believe that the return loss was no matter by introducing anti-reflection coating to the coupler cap. For the polarization issue, this device was designed for a TE-polarized light coupling. The coupling loss for a TM-polarized light was calculated to be 2.4 dB [12], and this loss value includes a bending loss of as large as 1.2 dB at the vertically curved Si waveguide due to the weak optical confinement in Si core. The radiation loss can be reduced by increasing Si waveguide thickness or bending radius.

For the view-point of manufacturing, this fabrication approach can be compatible with planarized back-end-of-line (BEOL) layers because this device can be fabricated after configuration of the standard Si photonic integrated circuits with SiO₂ over-cladding including metallization and surface planarization processes. Moreover, this device doesn’t require the high-temperature process which would affect the metal wires by adjusting the temperature for the post-annealing treatment.

The spot-size conversion mechanism used in this work can be extend to the coupler for standard SMF with the MFD of 10 µm, and we numerically obtained the coupling loss to be 0.6 dB/facet when the Si taper length and SiO₂-dome radius were 20 µm and 6.5 µm, respectively.

6. Conclusion

A chip-surface optical coupler based on a vertically curved Si waveguide for coupling with a HNA-SMF with a MFD of 5 µm was demonstrated. This coupler featured a dome-like SiO₂ coupler cap which reduces the beam divergence angle, and this feature enables the coupler to be configured with a small device footprint. In our study, we succeeded in fabricating such a structure using an isotropic SiO₂ deposition technique employing TEOS-PECVD. The FFP of the fabricated device was well-fitted with the Gaussian beam with a beam-waist size of 5 µm. Moreover, the fabricated coupler showed a coupling loss of 4.2 dB including a 2.5-mm-long Si waveguide loss for TE-polarized light and a broad-band coupling with a 0.5-dB-loss bandwidth of more than 150 nm. The loss could be reduced using sophisticated processing conditions and introducing a post-annealing treatment.