1. Introduction

Arrayed waveguide grating (AWG) is one of the important building blocks in a wavelength division multiplexing (WDM) system for expanding the capacity of optical data communication and interconnection systems and has been demonstrated on various photonic platforms with III–V, Group IV, and polymer material systems [1–16]. Especially with the progress of the silicon photonics technology, silicon (Si) AWG has received much attention. The Si AWG on a silicon-on-insulator (SOI) platform can have a compact device size an order of magnitude smaller than that of silica AWGs due to the high-refractive-index contrast between the Si core and the ${\mathrm{SiO}}_2$ cladding. However, in a Si AWG, tightly confining silicon waveguides (WGs) inherently induce larger phase error and scattering/transition losses as the optical signal propagates across the device, which results in a degraded performance of cross talk and insertion loss. The abrupt transition at the boundary between the slab region and the arrayed WGs causes multimode generation due to an optical field mismatch as well as the scattering, which results in additional transition losses. Si AWGs fabricated with a single-etch technique for channel-type arrayed WGs were reported with $\sim 3\mathrm{dB}$ insertion loss [7,8]. For improving the insertion loss, Si AWGs with a double-etch technique have been widely investigated and successfully demonstrated with down to $\sim 1.5\mathrm{dB}$ insertion loss [9–16]. However, further improvement of insertion loss in an AWG is desired for the application of a densely integrated system with photonic components such as a modulator, photodiode, WDM device, optical coupler/splitter, and polarization device. A transition loss, which is the major contribution to the insertion loss in an AWG, depends on the etch depth and the gaps between the arrayed WGs at a star coupler boundary. Most of the reported Si AWGs fabricated with a double-etch technique used a 70 nm shallow etched AWG structure on SOI of a 220 nm top silicon, due to the difficulty in reducing the shallow etch depth below 70 nm at the boundary of a star coupler without a light coupling between adjacent WGs.

In this paper, we demonstrate eight-channel Si AWGs on a 6 in. SOI wafer with a low transition loss by using an ultrashallow etch structure near the boundary region of the star coupler for suppressing multimode generation and the scattering due to the optical field mismatching effect. In order to minimize the transition loss, the boundary region between a star coupler and the AWGs is systematically designed with various conditions of shallow etch depths and distances between an ultrashallow etched structure and a deep etched structure.

2. Design and Fabrication

When AWGs are coupled to a slab structure in an AWG, the multimode excitation in the boundary region depends on the allowed eigenmodes and the mode symmetry of the slab and AWGs. Figure 1 shows the calculated single-mode coupling condition in the coupled structure of a slab and a rib WG, as a function of the rib WG width ($W$) and etch depth ($D$). The calculations are performed for the transverse electric (TE) polarization and wavelength range of 1.5–1.6 μm by Fimmprop simulator [17]. The dimension of the slab structure used in the simulation is 10 μm wide and 220 nm thick, and the plane-wave like slab mode is coupled to the rib WG. As the $D$ decreases, the $W$ that satisfies the single-mode coupling condition increases due to weak confinement of light in the rib WG. $W$s have a variation of only $\sim 6\%$ for a wavelength shift of 100 nm. The calculated results show that the single-mode coupling condition can be satisfied for the wide wavelength range. If higher order modes are excited in arrayed WGs of an AWG, these contribute to deterioration of cross talk with a phase error. In an AWG, high order modes do not survive through the arrayed WGs, contributing to the increase of insertion loss. Therefore, in the design of AWGs, the calculated widths, which satisfy the single-mode coupling condition, are adopted for an aperture size of arrayed WGs to suppress higher order modes at the boundary of star coupler.

 

Fig. 1. Calculated single-mode coupling conditions in the coupled structure of a slab and a rib WG as a function of $W$ and $D$. Inset shows a schematic diagram of a simulation for a light propagation from a slab to a rib WG.

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The abrupt structural change at the boundary between the slab region and the arrayed WGs causes the optical field mismatching, which results in the transition loss. In device fabrications, the etch depth of the AWGs can be controlled, whereas the gap between the AWGs is limited due to the resolution limit of photolithography. Here, to control the optical field mismatching effectively in device fabrications, ultrashallow etched structures are employed with deep etched structures, which can lead to reducing scattering effect at the star coupler boundary. Figure 2(a) shows a schematic diagram of a star coupler boundary, which has a 20 nm etched rib WG array coupled with a slab structure and the calculated optical field image of a star coupler with the TE polarization and 1.55 μm wavelength, which illustrates the effect of ultrashallow etched WGs near the slab boundary. The gap of the arrayed WGs is 0.3 μm, and the 2 μm long WGs tapered from the 1.8 to 1.2 μm width are connected to the straight WGs, as shown in Fig. 2(a). The aperture size of 1.8 μm is obtained from a single mode excitation condition in Fig. 1. Inset figure is an optical field image of the 2 μm long taper, which shows an adiabatic transition. The simulations are performed with a Beamprop and Omnisim simulator [17,18]. An optical mode in AWGs is the superposition of a slab mode and rib WG modes due to weak confinement of light and a small gap of the AWGs. Figure 2(b) shows the calculated optical field profiles of Fig. 2(a), at positions of z1, z2, z3, and z4 in the z direction. As light propagates from the slab to the ultrashallow etched AWGs, the slab mode at z1 transfers to the combined optical modes of the slab and the rib WG at z2. The optical mode at z3 becomes to the rib WG mode completely. The simulation shows a mode coupling between rib WGs at z4, which is due to the weak mode confinement of the shallow etched rib WG structure. Here, by introducing deep etched structures near z3 of Fig. 2(a), an adiabatic single-mode coupling into the arrayed WGs can be achieved.

 

Fig. 2. (a) Schematic diagram of a star coupler boundary, which has a 20 nm etched WG array coupled with a slab structure, and the calculated optical field image of a star coupler. Inset figure shows an optical field image of 2 μm long taper. (b) Calculated optical field profiles at positions of z1, z2, z3, and z4 in Fig 2(a).

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Figure 3 shows a schematic diagram of a star coupler structure combined with ultrashallow and deep etched AWGs and the simulational results of the transition loss between the star coupler and AWGs for the TE polarization and 1.55 μm wavelength performed with a Beamprop simulator [18]. In the schematic diagram of Fig. 3(a), the AWGs have a width $W$ at the star coupler boundary with a shallow etch depth $D$ and 0.3 μm gap between AWGs. A 2 μm long WG, tapered from $W$ to $W^{\prime }$, is connected to a straight WG section. To increase the mode confinement, the deep etched structure with a 220 nm depth is introduced at a distance of $L$ from the slab, with incorporated linear tapers for an adiabatic mode conversion. Figure 3(b) depicts an example of a calculated optical field image for a star coupler, which has the structure in Fig. 3(a) with $D$ of 20 nm and optimal $L(L_{\text{opt}})$ of 9 μm. It shows that light propagation without an optical coupling between arrayed WGs is possible by employing both ultrashallow and deep etched structures at the boundary of the star coupler. Figure 3(c) shows a calculated relation between the $W$ and $D$, which satisfies the single-mode coupling conditions of Fig. 1 simultaneously. The red line indicates the boundary between the single mode and the multimode coupling region. Black squares in Fig. 3(d) show the $L_{\text{opt}}$ calculated with the $W$s and $D$s fulfilled for single-mode coupling conditions of the black squares in Fig. 3(c), for achieving minimum transition loss of the structure. The simulation result as shown by the red circle in Fig. 3(d) shows that the transition loss across the star coupler boundary is at a minimum around a $D$ of $\sim 20\mathrm{nm}$. If the $D$ is shallower than $\sim 20\mathrm{nm}$, the optical mode is not completely coupled to the arrayed WGs due to a very weak confinement of light, resulting in an increase of the transition loss. As the $D$ increases beyond $\sim 20\mathrm{nm}$, the larger optical field mismatching occurs at the boundary.

 

Fig. 3. (a) Schematic diagram of a star coupler structure combined with ultrashallow and deep etched AWGs for a calculation of the transition loss. (b) One example of a calculated optical field image for a star coupler that has the structure in Fig. (a) with $D$ of 20 nm and $L_{\text{opt}}$ of 9 μm. (c) Calculated parameters $W$ versus $D$, which satisfies the single-mode coupling conditions of Fig. 1. (d) Black square: calculated parameters $L_{\text{opt}}$ versus $D$, which give minimum transition losses. Red circle: calculated transition losses for a star coupler with the optimal parameters of $W$, $L_{\text{opt}}$ and $D$.

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Based on the above calculations, eight-channel Si AWGs with 400 GHz channel spacing that have different $D$s have been designed and fabricated on 6 in. SOI wafers with a 220 nm thick top silicon and a 3 μm thick buried oxide layer. The design parameters for the fabricated Si AWGs are summarized in Table 1. In the device groups of AWG-D13s, AWG-D19s, and AWG-D35s, which have different $D$s of 13.7, 19.6, and 35.3 nm, respectively, $L$ is changed from 5 to 16 μm to optimize the single-mode coupling conditions at the star coupler boundary. The $W$s of the AWGs for the phase delay are 1.2 μm for AWG-D13s and AWG-D19s and 1 μm for AWG-D35s with similar sized footprints of $\sim 400\mathrm{\mu m}\times 240\mathrm{\mu m}$. Here, the AWG-D88s group with a conventional double-etch technique that have 88.3 nm shallow etch depth and 220 nm deep etch depth were also designed and fabricated. In the AWG-D88s group, $W$ of the arrayed WGs at the slab boundary is changed from 1.25 to 3.5 μm with the etch depth of 88.3 nm to confirm the effect of the multimode generation on the insertion loss and the cross talk of the Si AWG. AWGs of the AWG-D88s are 88.3 nm etched rib WGs, and bending WGs are channel WGs. Design details of AWG-D88s can be found in [9]. The device patterns were defined by a 365 nm I-line photolithography. A 1 μm thick ${\mathrm{SiO}}_2$ cladding layer was deposited on the top of a device.

Table 1. Summary of Design Parameters for Fabricated Eight-Channel Si AWGs with 400 GHz Channel Spacing

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Figure 4 shows microscopic images and scanning electron microscopy (SEM) images of the fabricated Si AWG devices. A device image is shown with a reference WG to extract the loss from the AWG device. SEM images for a device of AWG-D19s group with $L\sim 7\mathrm{\mu m}$ in Fig. 4(a) show a coupled structure of a 19.6 nm etched WG and a 220 nm etched WG at the boundary between the star coupler and arrayed WGs and a channel WG with a 220 nm etch depth that is inserted for the AWGs and bending WGs. A gap between AWGs is $\sim 0.33\mathrm{\mu m}$ which is limited by I-line photolithography. Figure 4(b) shows SEM images for a device of AWG-D88s group with $W=3.25\mathrm{\mu m}$ which has arrayed WGs with an 88.3 nm etch depth and bending WGs with a 220 nm etch depth. AWG-D13s and AWG-D35s devices have similar structures as Fig. 4(a).

 

Fig. 4. Microscopic images and SEM images of (a) AWG-D19s with $L\sim 7\mathrm{\mu m}$ and a footprint of $400\mathrm{\mu m}\times 260\mathrm{\mu m}$ and (b) AWG-D88s with $W=3.25\mathrm{\mu m}$ and a footprint of $450\mathrm{\mu m}\times 350\mathrm{\mu m}$.

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3. Experimental Results

For device characterization, one-dimensional grating couplers with a $\sim 6\mathrm{dB}/\text{facet}$ coupling loss were used for an optical coupling between cleaved single-mode optical fibers and an AWG device. A broadband light source was coupled into the device input, and the spectral responses of AWGs with the TE polarization were measured with an optical spectrum analyzer, which were normalized with the transmission of the reference WGs. The propagation loss of the channel WG with a 0.5 μm width is 0.35 dB/mm and that of the rib WG with an 88.3 nm etch depth and a 0.5 μm width is 0.15 dB/mm. The loss of the bending WG, as shown in Figs. 4(a) and 4(b), which have a 10 μm radius and a 0.5 μm width, is 0.013 dB/90° and 0.067 dB/90°, respectively.

The AWG-D13s, AWG-D19s, and AWG-D35s with the ultrashallow etched WG structures based on the simulations of Fig. 3 are characterized. Figure 5 depicts the measured insertion losses of the Si AWGs with a $D$ of (a) 13.7 nm, (b) 19.6 nm, and (c) 35.3 nm, as a function of $L$. As shown in the figure, the devices of AWG-D13s, AWG-D19s, and AWG-D35s, with the $W$s that satisfy the single-mode coupling conditions for each $D$, exhibit best performances at the values of $L\sim 12$, 8, and 6.5 μm, respectively, which agree with the simulational results of Fig. 3(c). The measured transmission spectra of the AWGs in the figure correspond to the Si AWGs with best performances in insertion loss and cross talk for each $D$ simultaneously, which are marked with red circles in the insertion-loss graphs. The device with a $L\sim 12\mathrm{\mu m}$ of AWG-D13s group shows an insertion loss of 0.72 dB with a cross talk of less than $-20.8\mathrm{dB}$ and a 2.63 dB nonuniformity. An insertion loss of the device (AWG-D19L8) with $L\sim 8\mathrm{\mu m}$ of AWG-D19s group is 0.5 dB with a cross talk of less than $-23.2\mathrm{dB}$ and a 3.22 dB nonuniformity. The device with $L\sim 6.5\mathrm{\mu m}$ of AWG-D35s group has an insertion loss of 1.1 dB with a cross talk of less than $-21.98\mathrm{dB}$ and a 2.08 dB nonuniformity.

 

Fig. 5. Measured insertion losses of the Si AWGs with a $D$ of (a) 13.7 nm, (b) 19.6 nm, and (c) 35.3 nm, as a function of $L$, and measured transmission spectra of the Si AWGs marked with red circles in the insertion loss graphs.

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Figure 6 shows measured insertion losses and cross talks of the Si AWGs with a $D$ of 88.3 nm, as a function of $W$, and the measured transmission spectra of the Si AWGs marked with blue circles in the insertion-loss graph. The performance of AWG-D88s is improved as the $W$ decreases. The measured insertion loss of the AWG device with the $W$ satisfying the single-mode coupling condition is down to 1.43 dB with a cross talk of $-19.3\mathrm{dB}$ at the center wavelength. This experimental result confirms that the device performance can be improved by suppressing the multimode generation of the arrayed WG as the $W$ at the boundary decreases.

 

Fig. 6. (a) Measured insertion losses and cross talks of the fabricated AWG-D88s group at the center wavelength as a function of $W$. (b) Measured normalized transmission spectra of Si AWGs with an aperture size $W$ of 1.25, 1.75, and 3.25 μm, which are marked with blue circles in the insertion loss graph.

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Here, the AWG-D19L8 shows the best performance of insertion loss and cross talk. The nonuniformity of insertion loss can be improved by using a wide FSR AWG while optimizing the roll-off of the far field envelope of a beam profile from the arrayed WGs [11]. In order to improve nonuniformity of the AWG-D19L8, the $8\times 400\mathrm{GHz}$ Si AWG device, which has a wide FSR of 51.2 nm with a target $D$ of $\sim 20\mathrm{nm}$, was designed and fabricated. The actual $D$ of the fabricated device was a little bit overetched to $\sim 23\mathrm{nm}$. The microphotogaphy and measured transmission spectra of the fabricated Si AWG with the wide FSR are shown in Fig. 7. This AWG exhibits a good performance of a 0.63 dB insertion loss with a cross talk of $-23$ to $-25.3\mathrm{dB}$ and an 1 dB nonuniformity, with a small footprint of $430\mathrm{\mu m}\times 350\mathrm{\mu m}$.

 

Fig. 7. Microphotogaphy and measured normalized transmission spectra of the fabricated $8\times 400\mathrm{GHz}$ Si AWG, which has a FSR of 51.2 nm and a $D$ of $\sim 23\mathrm{nm}$.

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This result shows a performance improvement of 0.8 dB insertion loss and $\sim 4\mathrm{dB}$ cross talk compared to the Si AWG fabricated with a conventional double-etch technique. The performance of the Si AWG with an ultrashallow structure can be further improved with a smaller gap between arrayed WGs, which can be realized by using higher resolution photolithography.

Figure 8 shows a summary of the fabricated Si AWGs, which have different $D$s and $W$s at the boundary of a star coupler. The red bar shows a variation of the measured insertion losses and cross talks. AWG devices 1–3 represent the Si-AWGs employing ultrashallow etched WG structures with $D$ of 13.7, 19.6/23, and 35.3 nm, respectively. AWG devices 4–6 represent Si AWGs using a conventional double-etch technique with 88.3 nm shallow etch depth, which have aperture sizes of 1.25, 1.75, and 3.25 μm, respectively. The results of Si-AWGs 4–6 show that the performance improvement can be achieved with decreasing aperture size due to the suppression of multimode generation. The above experimental results show that performance improvement of an AWG can be obtained with ultrashallow etched structure by suppression of scattering and multimode generation near the star coupler boundary.

 

Fig. 8. Summary of the fabricated Si AWGs that have different shallow etch depth $D$ and aperture size $W$ at the boundary of a star coupler. The red bar indicates the variation of the measured insertion losses and cross talks.

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4. Conclusion

We have demonstrated the performance improvement of Si AWGs with low transition loss by suppressing the multimode generation and scattering at a star coupler boundary. The fabricated eight-channel Si AWG based on an ultrashallow structure shows a low insertion loss of 0.63 dB and a low cross talk of $-23$ to $-25.3\mathrm{dB}$. These results show that the performance improvement can be achieved, compared to the Si AWGs based on the conventional double-etched technique, by employing ultrashallow etched WG structure at a star coupler boundary with optimized design parameters.