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We present the design, fabrication, and characterization of integrated Bragg gratings in silicon-on-insulator slot waveguides. The Bragg gratings are formed with sidewall corrugations, either on the inside or on the outside of the waveguide. We demonstrate resonators implemented using phase-shifted Bragg gratings in slot waveguides, showing quality factors up to 3 × 104. Due to the strong optical confinement in the slot, these devices are promising for optical sensing applications. The devices were fabricated using a CMOS-compatible process, facilitating high-volume and low-cost production.
© 2013 Optical Society of America
1. Introduction
Bragg gratings are critical components for numerous applications in optical communication and sensing systems. Recent advances in silicon photonics have led to the integration of Bragg gratings on the silicon-on-insulator (SOI) platform [1]. Typically, the modulation of the refractive index (RI) is achieved by physically corrugating a silicon waveguide, and various structures using this approach have been developed in the last decade [1–11]. One of the promising applications of silicon photonic Bragg gratings is biological/chemical sensing [12,13]. The sensitivity is generally defined as the slope of the wavelength shift versus the RI change of the surrounding medium, and is determined by the overlap between the electric field and the surrounding medium [13, 14]. Most conventional silicon photonic Bragg gratings are based on strip or rib waveguides, where the electric field is concentrated in the silicon so that only weak evanescent field tails are available for the sensing. For example, we have demonstrated a strip waveguide grating sensor with a high quality (Q) factor of 27600, but the sensitivity is only 59 nm/RIU, which leads to an intrinsic limit of detection (LOD) of 9.3×10−4 RIU [13], similar to that of a strip waveguide ring resonator (e.g., 1.1×10−3 RIU in [15]). To improve the sensitivity, we recently demonstrated Bragg gratings in a slot waveguide [16], in which the electric field is concentrated in the low-index slot region [17]. This unique property makes the slot waveguide very sensitive to the RI change of the cladding. Our experimental result has shown a high sensitivity of 340 nm/RIU at around 1550 nm, thanks to the enhanced light-matter interaction. By introducing a phase shift into the gratings and thus constructing a cavity, we obtained a strong resonance with a high Q factor of 1.5×104 in water. This Q factor is much higher than most reported values for slot waveguide ring resonators [13,18,19]. The first use of slot waveguide ring resonator for biosensing applications was demonstrated by Barrios et al. [18] on a Si3N4–SiO2 platform, showing a sensitivity of 212 nm/RIU and a Q factor of 1800. Claes et al. developed a slot waveguide ring resonator on a SOI platform [19], showing a sensitivity of 298 nm/RIU and a low Q factor of 330. Compared with these results, our device in [16] shows a significant improvement in the Q factor because it does not suffer from the high bending loss and mode mismatch loss in slot rings. By combining the high sensitivity of the slot with the high Q factor of the gratings, we obtained an intrinsic LOD of about 3×10−4 RIU, which is, to our knowledge, the best experimental result for slot-based biosensors [16]. From a broader perspective, this new type of device combines both the advantages of slot waveguides (e.g., strong light-matter interaction, increased nonlinearity especially in a silicon-organic hybrid approach [20]) and Bragg gratings (e.g. flexible spectral response, and widespread applications). For example, it is promising to develop efficient modulators by utilizing its high sensitivity to refractive index changes.
In this paper, we present a comprehensive study on slot waveguide Bragg gratings, including both uniform and phase-shifted gratings. The gratings are formed with sidewall corrugations either on the inside or outside of the slot waveguide. In Section 2, we describe the details of the designs with numerical simulations. In Section 3, we present the experimental results of our devices fabricated in a CMOS foundry.
2. Design and fabrication
In this work, we designed and fabricated our devices using the imec standard passive silicon photonic technology, accessed through the ePIXfab foundry service. It uses a 200 mm SOI wafer that consists of a thin silicon layer of 220 nm on top of a 2 μm buried oxide layer. The fabrication process is based on 193 nm optical lithography and dry etching [21]. We chose this technology because it is capable of making slot waveguides with reasonably narrow slot widths (down to 100 nm [19, 22]), as well as waveguide Bragg gratings with small sidewall corrugations [6, 8, 23]. More importantly, it is compatible with commercial CMOS fabrication technology, thus allowing for high-volume and low-cost production.
2.1. Slot waveguide
Figure 1(a) illustrates the cross section of an SOI slot waveguide. It consists of two silicon arms separated by a low-index slot region. The geometric parameters include the arm width (Warm) and the slot width (Wslot ), both of which are defined by the lithography, and the waveguide height (H), which is fixed at 220 nm. The cladding material can be air, water, glass, or other low-index materials. In this work, we use silicon oxide as the cladding material. The simulated mode profile shows that the quasi-transverse-electric (quasi-TE or TE) mode has a high optical intensity inside the slot region. Figure 1(b) shows a scanning electron microscope (SEM) image of the cross section of a fabricated slot waveguide. Note that the etched sidewalls are not perfectly vertical, which will slightly change the mode profile.
Fig. 1 Cross section of a slot waveguide. (a) Schematic diagram and the simulated mode profile for the fundamental quasi-TE mode. Warm = 270 nm, Wslot = 150 nm, H = 220 nm, and the upper cladding material is silicon oxide. (b) SEM image of the focused ion beam (FIB) milled cross section of a fabricated device (the small hole in the centre was due to the incomplete coating of platinum deposited to protect the waveguides during the FIB milling).
2.2. Uniform gratings
The gratings are formed with periodic sidewall corrugations either on the inside or the outside of the slot waveguide, as shown in Fig. 2. Both configurations can modulate the effective index of the optical mode. Here, we present our designs of uniform gratings in two different waveguides (WG1 and WG2) with various corrugation widths (ΔWin or ΔWout ). Table 1 lists all of our design variations. All devices have 1000 grating periods. The effective index (neff) of each waveguide was calculated at 1550 nm. The grating period (Λ) was chosen to obtain a Bragg wavelength close to 1550 nm while being compatible with the foundry design rules.
Fig. 2 Schematic diagrams (not to scale) of the slot waveguide Bragg gratings with corrugations (a) inside and (b) outside.
Table 1. Design variations for uniform slot waveguide Bragg gratings.
Figure 3 shows the top view SEM images of two fabricated devices. Note that we used square sidewall corrugations in the mask layout, as illustrated in Fig. 2. However, as is clearly seen in Fig. 3, the fabricated corrugations were rounded and resemble sinusoidal shapes due to lithographic effects. This is not surprising since such fabrication imperfections are common for waveguide Bragg gratings with sidewall corrugations, especially when using optical lithography [6, 8, 24], and they can be predicted by lithography simulations [25].
Fig. 3 Top view SEM images of the fabricated slot waveguide Bragg gratings: (a) corrugation inside for Wslot = 150 nm, Warm = 270 nm, and ΔWin = 20 nm. (b) corrugations outside for Wslot = 150 nm, Warm = 270 nm, and ΔWout = 40 nm.
2.3. Phase-shifted gratings
Figure 4(a) shows the schematic of a phase-shifted Bragg grating using corrugations on the outside of the slot waveguide. The length of the phase shift is equal to one grating period. On each side of the phase shift, there are N grating periods that function as distributed Bragg reflectors (DBR). Thus, a cavity is created by the phase shift, and, as a result, a resonant peak will appear at the centre of the stop-band of the transmission spectrum [9, 26–28]. Figure 4(b) shows a top view SEM image of a fabricated device, with the phase shift highlighted in the dashed box.
Fig. 4 Phase-shifted Bragg gratings with corrugations on the outside of the slot waveguide: (a) schematic diagram (not to scale), (b) SEM image showing the phase shift region of a fabricated device, (c) transmission spectrum simulated by FDTD with the following geometric parameters: WG1, ΔWout = 40 nm, and N = 50.
In order to better understand this structure, we also performed 3D finite-difference time-domain (FDTD) simulations. Figure 4(c) shows a simulated transmission spectrum featuring a sharp resonant peak at the centre of the stop-band. Figure 5 shows the electric field distributions for the on-resonance state and an off-resonance state. Again, we can see that the electric field in strongly confined in the slot region, whether the wavelength is on- or off-resonance. In Fig. 5(a), the wavelength is 1543 nm, located in the left valley of the stop-band in Fig. 4(c). The incoming light is mostly reflected back by the first DBR mirror on the left. As the wavelength shifts to the resonant peak at 1550 nm, the cavity starts to resonate and light is concentrated around the phase shift, as shown in Fig. 5(b).
Fig. 5 Electric field distributions for light incident from the left at (a) 1543 nm and (b) 1550 nm. The simulation parameters are the same as in Fig. 4(c). The field was recorded at the middle of the silicon waveguide in the vertical direction (i.e., corresponding to the plane of y=110 nm in Fig. 1(a)).
For a phase-shifted Bragg grating, the Q factor is determined by two loss mechanisms: coupling loss and waveguide loss [27]. The coupling loss depends on the grating length and the grating coupling coefficient, both of which can be adjusted through design. The waveguide loss, by contrast, is inherent in the fabrication and it arises primarily from the waveguide roughness. Therefore, the intrinsic Q factor is limited by the waveguide loss [14]:
where ng is the group index, and α is the waveguide propagation loss in dB/cm. Compared with conventional low-loss waveguides on silicon (i.e., strip and rib waveguides [29]), slot waveguides exhibit relatively high losses. Typical reported loss values for slot waveguides are on the order of 10 dB/cm [30–32]. Assuming that the loss is 10 dB/cm and ng = 3.35 (simulated value for WG1 in Table 1), the calculated QI is about 5.89 × 104 at 1550 nm. If this resonator is critically coupled [14], the total Q is reduced by a factor of two and becomes about 2.95 × 104. As will be further discussed in Section 3, these approximations give values comparable to the maximum Q values that we have observed experimentally.2.4. Device layout
The device layout schematic is shown in Fig. 6(a). It consists of one input port and two output ports (transmission and reflection). Each port is an integrated waveguide-to-fiber grating coupler designed for TE polarization. We use a Y-branch splitter, as shown in Fig. 6(b), to collect the reflected light. In order to minimize the total footprint and bending losses, we use 500 nm wide strip waveguides for the Y-branch design, as well as for the routing. The coupling between the strip and slot waveguides is realized through mode converters [33]. As shown in Fig. 6(c), one arm of the slot waveguide expands linearly in the coupling region and eventually becomes the strip waveguide. The other arm is slightly tilted in the coupling region, with its width and distance from the first arm unchanged; then after the coupling region, it bends away and tails off. The coupling length is 5 μm to ensure that the coupling loss is negligible [32–34].
Fig. 6 (a) Schematic diagram of the device layout (not to scale, inset shows an SEM image of a fabricated slot waveguide Bragg grating), and SEM images of (b) the Y-branch and (c) the mode converter.
3. Measurement results
The fabricated devices were characterized using an automated measurement setup, which consists of a tunable laser source that has a wide tuning range (Agilent 81600B opt. 201, 1455 nm – 1640 nm), a dual optical power sensor (Agilent 81635A), a four-channel fiber array (PLC Connections PM fiber array with 127 μm pitch), and an automated micro-positioning stage. The resolution of the spectral measurements is 1 pm. Note that the tunable laser has two outputs: one with a high output power and the other with a low source spontaneous emission (SSE). As will be shown, our Bragg gratings exhibit high extinction ratios. Therefore, it is necessary to choose the low SSE output of the tunable laser, which offers a high signal-to-SSE ratio and the large dynamic range needed to completely characterize the devices [35].
3.1. Uniform gratings
Figure 7 shows the measured raw spectral responses of a uniform grating. The transmission spectrum shows a deep notch with an extinction ratio of more than 40 dB. The centre wavelength is about 1550 nm, in good agreement with the design value. Accordingly, a peak is seen in the reflection spectrum. The peak power is about 3 dB below the transmission level, corresponding to the extra loss of the Y-branch. The Y-branch also has weak parasitic back-reflection due to the abrupt waveguide discontinuity, which limits the noise floor of the measured reflec-tion spectrum.
Figure 8 shows the measured transmission spectra for the devices based on WG1. The spectra were normalized using the method described in [6] to subtract the insertion loss, i.e., the envelope of the transmission spectrum in Figure 7. We can see that all the devices exhibit high extinction ratios of about 40 dB. As the corrugation width is increased, the stop-band becomes broader due to the increased grating coupling coefficient. The bandwidths for all design variations are also plotted versus corrugation width in Fig. 9. We can see that the bandwidth ranges from 2 nm to more than 20 nm. In Fig. 9, the green curve is well above the other three curves, and the device using WG1 with 20 nm inside corrugations have the largest grating coupling coefficient. An intuitive explanation is that the optical field is strongly confined in the slot, as shown in Fig. 1(a), and a small corrugation can have a large impact on the field. Increasing the slot width reduces the optical confinement in the slot, and, therefore, the effect of a particular corrugation inside the slot of WG2 is smaller than it is in WG1, which is why the red curve is below the green curve. When the corrugations are placed on the outside of the slot waveguide, in both WG1 and WG2, the bandwidths are narrower and very similar to each other, due to the fact that the evanescent field tails are relatively weak.
Fig. 8 Measured transmission spectra of the uniform gratings designed on WG1. (a) corrugations inside, (b) corrugations outside.
3.2. Phase-shifted gratings
We designed a number of phase-shifted gratings based on WG1 and WG2, using both inside-corrugation and outside-corrugation configurations. For each configuration, we used the largest corrugation width listed in Table 1 in order to obtain the largest grating coupling coefficient. Figure 10 shows the measured raw spectral responses of a phase-shifted grating. A sharp resonant peak is clearly seen at the centre of the stop-band in the transmission spectrum and, accordingly, a deep notch appears in the reflection spectrum.
Fig. 10 Measured raw spectra of a phase-shifted grating designed on WG1 with ΔWout = 40 nm and N = 300.
We varied the length of the gratings to study its impact on the Q factor. Figure 11(a) shows a set of transmission spectra for devices based on WG1 with ΔWout = 40 nm and for various lengths. As N is increased from 200 to 500, the stop-band becomes deeper because more light is reflected back. Also, the resonant peak becomes sharper and the Q factor increases from 800 to 1.55×104, although the peak amplitude slightly decreases. The Q factor is also plotted as a function of N for all design variations in Fig. 11(b). We can see that the green curve is well above the other three curves, which verifies that WG1 with ΔWin = 20 nm has the largest grating coupling coefficient. However, when N exceeds 200, the cavity becomes over coupled and the amplitude of the resonant peak drops dramatically so that it cannot be observed. The blue and black curves are on the bottom and overlap with each other, which verifies that WG1 and WG2 with ΔWout = 40 nm have small, and similar, grating coupling coefficients. Finally, the red curve is in between the other three, again in agreement with the result in Fig. 9, and shows the highest Q factor of about 3×104 at N = 500.
Fig. 11 (a) Measured transmission spectra for phase-shifted gratings designed on WG1 with ΔWout = 40 nm and various lengths, (b) Q factor as a function of N.
Based on the discussion in Section 2.3, this is very close to the calculated Q factor under critical coupling condition, assuming that the wavelength propagation loss is 10 dB/cm. In contrast, we have demonstrated phase-shifted gratings using relatively low-loss strip waveguides (2-3 dB/cm) and achieved Q factors up to 1 × 105[28]. Recently, a few approaches have been reported to reduce the loss figures for slot waveguides [32, 36, 37]. For example, Spott et al. reported the lowest loss of 2 dB/cm by using asymmetric slot structures, but at the expense of reduced optical confinement and a relatively complicated fabrication process [37]. However, we expect that further improvements in the lithography and etching processes will reduce the loss of slot waveguides and lead to better performance for slot waveguide Bragg gratings.
4. Conclusions
We have designed, fabricated and characterized Bragg gratings in silicon slot waveguides, including both uniform and phase-shifted gratings. We have investigated a number of design variations for both types of device. The uniform gratings show high extinction ratios of 40 dB and bandwidths ranging from 2 nm to more than 20 nm. The phase-shifted gratings show Q factors up to 3×104, much higher than most reported values for slot waveguide ring resonators. The integration of Bragg gratings in slot waveguides offers many new possibilities. It combines the superior sensing ability of slot waveguides with the flexibility in the design of the spectral response of Bragg gratings. It also has great potential for optical modulation, nonlinear optics and optical signal processing. Finally, the devices were fabricated using a CMOS-compatible process with 193 nm deep UV lithography, which is crucial for low-cost mass production.
Acknowledgments
The authors would like to thank CMC Microsystems for enabling the fabrication of the devices through ePIXfab, Lumerical Solutions, Inc. and Mentor Graphics for the design software and support, the Centre for High-Throughput Phenogenomics at the University of British Columbia for assistance with SEM imaging, and S. A. Schmidt and D. M. Ratner for useful discussions. The authors gratefully acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada, particularly under the CREATE SiEPIC program.
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