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Abstract
We propose and demonstrate an electrically reconfigurable waveguide Bragg grating filters in silicon-on-insulator using a multiple-contact heater element. There are six electrical pads connected to the heater element in an equidistant manner. These electrical pads allow to create different heat, and corresponding refractive index, distributions across the grating so that the local Bragg wavelength corresponding to the heated segments can be controlled. In turn, this control over the heat distribution allows the device to be reconfigured to implement different filter spectral responses. These filters are applicable for both wavelength division multiplexing systems and optical signal processing applications. As a verification, we demonstrate the generation of two (or more) separate filter bands with a spacing up to 35 nm or a Fabry-Pérot cavity with a 1.6 nm free-spectral range. Moreover, we explain a firm and accurate simulation framework of the proposed device based on COMSOL Multiphysics and the transfer matrix method, which is in excellent agreement with our experimental measurements.
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1. Introduction
Over the last two decades, the integration of silicon photonic devices on the silicon-on-insulator (SOI) platform has attracted much attention thanks to the compatibility with the complementary metal-oxide-semiconductor (CMOS) process, low fabrication costs, and high fabrication yield [1]. Photonic filtering devices are basic building blocks that exist in almost every photonics signal processing system due to their capability of providing a user-defined response based on the target application [2]. On-chip waveguide Bragg gratings (WBGs) offer promising solutions to enable filtering capabilities and are especially appealing because of their simple operating principle [3]. The long-established limitation in conventional WBGs is that their spectral characteristics are fixed once the gratings are fabricated. Thus, to create multiple filter responses, special designs of multiple WBGs are required for a user-defined task. Recently, silicon-based WBGs have been proposed with tunable index modulation profiles. Achievements include tuning the center wavelength [4] or the amount of dispersion for chirped structures [5–9] or tuning resonance features [10], as in a phase-shifted structure [11].
Considering wavelength-division multiplexing (WDM) systems, it may be required to filter multiple wavelengths at the same time [12]. Most of the reported electrically tunable WBGs are limited to filtering one wavelength at a time. The purpose of this work is to develop an electrically tunable WBG that can be used to create different spectral responses, such as multiple tunable reflection bands or a Fabry-Pérot (FP) filter. Tunable multi-band filters can enable different processing functions in optical communications and microwave photonics. For example, a tunable multi-band filter, i.e., where the number of wavelength bands can be varied as well as the specific wavelengths of operation, can be used for reconfigurable optical add-drop multiplexing (of course, the spectral response needs to be optimized for this specific application). As a second example, FP-like filters with tunable FSR can be used for reconfigurable generation of chirped microwave waveforms based on spectral shaping and wavelength-to-time mapping (here the FP filter has a uniform FSR and needs to be combined with a dispersive medium providing a nonlinear wavelength-to-time mapping).
Specifically, we propose and demonstrate electrically tunable WBGs on the SOI platform using a multiple-contact heater element. The heater element is uniform, and there are six electrical pads located equidistantly along the structure (∼ 50 µm spacing) to control different heat distributions over multiple segments of the grating. We showcase that such a device can be used to create multiple tunable filters based on which grating segment is being heated; moreover, with proper tuning, FP resonances can be created.
2. Operating principle of the proposed device
A WBG is a wavelength selective filter that can be implemented in SOI by etching sidewall corrugations into a strip waveguide. The Bragg wavelength ${\lambda _B}$ is defined by the relationship ${\lambda _B} = 2{n_{\textrm{eff}}}\mathrm{\Lambda }$, where ${n_{\textrm{eff}}}$ is the effective refractive index of the waveguide and $\mathrm{\Lambda }$ is the grating period. The main parameters of the WBG, including the grating period ($\mathrm{\Lambda }$), (average) width of the waveguide, corrugation width (dW), and the number of grating periods (N), determine the corresponding spectral response. For example, the average waveguide width and corrugation width determine in part the effective indices which, in conjunction with the grating period, set the Bragg wavelength $({\lambda _B}$) while the corrugation width and the number of grating periods determine the bandwidth of the filter and the peak reflectivity (or extinction ratio).
Figure 1 (a-b) illustrate the schematic diagram and side view of the proposed reconfigurable WBG in SOI. The grating is uniform and incorporates an independent heater element located 2 µm above the waveguide. The heater element is made of TiN; its thickness is 120 nm, and it is 2.5 µm wide. There are six electrical pads distributed in an equidistant manner along the length of the grating; the separation between each electrical pad is 50 µm. The WBG is designed such that the average width of the waveguide is 500 nm, and the height is 220 nm to satisfy the single TE mode criteria. To maintain a transmission response in the C-band, we set Λ = 318 nm, the number of grating periods (N) is 787, and the corrugation width (dW) is 50 nm. The total length of the grating is N × Λ ≈ 250 µm. The device was fabricated via a multi-project wafer run at the Advanced Micro Foundry (AMF) in a (CMOS)-compatible process using 193 nm ultraviolet (UV) lithography. Figure 1 (c) shows the measured transmission response of the WBG: the 3 dB bandwidth is 9 nm and the extinction ratio at the Bragg wavelength is > 30 dB corresponding to a peak reflectivity > 99.9%. Figure 1 (d) shows the chip under test with a DC probe connected to the electrical pads, and a fiber ribbon array coupled into the vertical grating couplers for input and output coupling. Figure 1 (e) shows the layout design of the proposed device.
Fig. 1. (a-b) Schematic diagram and sideview of the proposed device. (c) Measured transmission response of the WBG without any applied voltage. (d) Picture of the chip under test. (e) Layout design of the proposed device.
Figure 2 shows examples of how reconfiguration is achieved. An applied voltage on one segment of the grating creates a localized thermal change which, in turn, changes the effective refractive index and hence corresponding Bragg wavelength. In other words, by applying a bias voltage to a pair of electrical pads, the refractive index of the corresponding grating segment can be tuned thanks to the large thermo-optic coefficient of silicon. The thermal tuning coefficient can be expressed through the following [13]:
where T is the temperature, $\lambda $ is the operating wavelength, and ${n_g}$ is the group index. The thermo-optic coefficient in silicon is $dn/dT = ({1.86 \pm 0.08} )\times {10^{ - 4}}K$ [13]. Thus, the entire index profile of the grating can be electrically reconfigured by applying voltages to multiple pairs, and with different voltages, creating a wide range of temperature distributions and hence, the possibility to obtain diverse spectral characteristics. We simulated the device using the COMSOL Multiphysics software to investigate the thermal distribution on the surface of the WBG. The COMSOL simulation incorporates the TiN electric heaters, Si waveguide, and a sufficiently large $\textrm{Si}{\textrm{O}_\textrm{2}}$ cladding layer. COMSOL simulates electromagnetic heating (resistive heating) by solving the heat equation assuming the electric current as a heat source, which requires coupling the electric current simulation and the heat transfer simulation. In electric current simulations, we apply the voltage difference directly to the desired electrical pads (TiN), whereas the rest of the metal and $\textrm{Si}{\textrm{O}_\textrm{2}}\; $ boundaries are electrically insulated (no reflections from the boundaries). For the heat transfer, we set the background ambient temperature of the $\textrm{Si}{\textrm{O}_\textrm{2}}\; $ cladding layer to room temperature (293.15 [K]). The coupling between the electric current and heat transfer modules simulates the increases in TiN, Si, and $\textrm{Si}{\textrm{O}_\textrm{2}}$ temperatures based on their respective thermal conductivities and heat capacities at the different electric currents. Table 1 shows the material properties used in the simulation and the thickness of each layer according to the fabrication process standards. The simulation procedure can be summarized as follows: using COMSOL thermal model in steady-state, we sweep the voltage applied to different electrical pads and record the temperature profile along the WBG structure. Then, we map that into the changes of the refractive index profile to recalculate the effective indices for each sweep. Lastly, we use these effective refractive indices in determining the necessary parameters (e.g., propagation constant, coupling coefficient) to simulate the WBG response using the transfer matrix method (TMM) [13].Fig. 2. Schematic diagrams to illustrate how reconfigurations (modes of operations) are performed. Applying voltage (heat) on particular segments creates a localized thermal change which in turn changes the effective refractive index and hence corresponding Bragg wavelength.
Table 1. Thermal and electromagnetic properties of the materials used in COMSOL simulations
Figure 3 shows the simulated thermal distribution for the different cases when voltage is applied on a pair (or multiple pairs) of electrical pads (the temperatures shown correspond to those on the surface of the waveguide though we have confirmed that there is negligible temperature difference between the surface and core of the waveguide).
Fig. 3. Thermal distributions in the WBG when voltage is applied on a pair or multiple pairs of electrical pads. Plots are labelled for each case (a-h). See numbers mapping in Fig. 1 (a).
For example, in Figs. 2(a) and 3(a), voltage is applied over the whole structure, namely, between electrical pads 1 and 6, in that case, perturbations in the refractive index happen along the whole structure, thus, shifting the response of the WBG to the longer wavelengths (see further, Fig. 4). In Figs. 2 (b,c) and 3(b,c), when voltage is applied between electrical pads 1 and 2, the temperature (refractive index) of this particular segment is changed, giving rise to two separate Bragg responses. As another example, in Figs. 2(d) and 3(e), when two voltages sources are used to tune two different segments, namely, between electrical pads 1 and 2, and another between electrical pads 3 and 4, three cascaded WBGs at different wavelengths can be implemented (see further, Fig. 5). In Fig. 3 (f), when two independent voltage sources are used to manipulate the refractive indices of the first and last segments, a FP-like filter can be obtained. Thus, the spectral characteristics of the proposed WBG can be reconfigured according to the specifications of a given application, such as multiple band filters, or FP-like filters. Note that the heater element block extends from pads 1 to 6 and sometimes, depending on which pads are active, there is thermal crosstalk which can increase/decrease the temperature in regions where no voltage is applied. These dips or peaks have a limited impact on the spectral response.
Fig. 4. Measured (left) and simulated (right) WBGs transmission responses with 0.01 nm resolution for voltages applied to different combinations of pads. Voltage(s) are applied between pads (a) 1 and 6. (b) 1 and 2. (c) 1 and 3. (d) 3 and 4.
Fig. 5. Measured (left) and simulated (right) WBGs transmission responses with 0.01 nm resolution for two independent voltages applied to different combinations of pads. Voltage(s) are applied between pads (a) 1 and 2, another between 3 and 4. (b) 1 and 2, another between 5 and 6. (c) 2 and 3, another between 4 and 5. (d) 1 and 2, another between 3 and 4, another between 5 and 6.
3. Experimental results and discussion
Figure 4 shows the measured and corresponding simulated transmission spectra of the reconfigurable WBG filter for different applied voltages and heat distributions. The total fiber-to-fiber loss of the device is -20 to -21 dB over the measured wavelength span; the corresponding fiber-to-fiber loss of a back-to-back vertical grating coupler test structure located near the device and used for alignment is ∼ 15 to 16 dB, giving an insertion loss of ∼ 5 dB. In Fig. 4 (a), a clear redshift of the transmission response of the WBG can be observed when a variable voltage is applied over the whole structure. The total shift is around 2 nm, and the filter bandwidth is 9 nm. Note that the dips shown in the thermal distribution have a negligible impact on the spectral response. In Fig. 4 (b), when a variable voltage is applied between electrical pads 1 and 2 (refer to Fig. 2 (a) for numbers mapping), two Bragg responses can be observed. One response is associated with the grating portion that is heated and the other from the unheated portion. The segment that is heated between electrical pads 1 and 2 is only 50 µm, which is four times shorter than the unheated segment; as such, while it results in a longer spectral response, it is also weaker (the peak reflectivity of the shorter grating about half that of the unheated segment). In Fig. 4 (c), we apply a voltage between electrical pads 1 and 3 which increases the length of the thermally tuned segment to 100 µm. In this case, the two resulting spectral responses for an applied voltage of 8 V have approximately the same peak reflectivity. In Fig. 4 (d), we apply a voltage between electrical pads 3 and 4; the corresponding heated segment shifts to longer wavelengths with increasing voltage, and we also observe an FP cavity formed in the unheated grating segments between pads 1 and 3, as well as 4 and 6. The measured FSR of the formed FP is around 3.6 nm, whereas the simulated value is 4.5 nm.
Next, we investigate the spectral characteristics of the WBG when multiple voltages are applied to tune the refractive indices of multiple segments simultaneously. In Fig. 5 (a), the corresponding heat distribution considered is shown in Fig. 3 (e): two voltages are applied, one between electrical pads 1 and 2 and another between pads 3 and 4. In this case, three WBG responses can be observed. Since the two segments have the same length (50 µm), their responses will be similar. Thus, we apply a larger voltage (10 V) to one of the segments (between pads 1 and 2) so that its response experiences a greater red-shift and can be clearly distinguished from that of the second segment, where the applied voltage is 8 V. In addition to the responses from the two thermally tuned segments, we have a third response corresponding to the remaining part of the grating that is not subjected to heating. The section between pads 2 and 3 is not heated, but there is some thermal leakage. The section between pads 4 to 6 is not heated at all. So overall, there is a small difference in temperature and hence effective index between these sections. This causes a weak FP (note that the ‘mirror’ defined by the grating between pads 2 and 3 is shorter and thus weaker in reflectivity and broader in bandwidth compared to that between pads 4 and 6 which is longer and thus stronger in reflection and narrower in bandwidth). The wavelength response of the grating between pads 2 and 3 is expected to be redshifted a bit as well. All of this can create some irregularity in the response that is observed. In Fig. 5 (b), the corresponding heat distribution considered is shown in Fig. 3 (f), we investigate further the formation of an FP cavity by applying the same voltage to two segments, one at the beginning of the grating (between pads 1 and 2) and the second at the end of the grating (between pads 5 and 6). In this case, the two segments at the beginning and end of the structure form a FP cavity. The resonances are clear and the measured free spectral range (FSR) is ∼ 1.6 nm, which agrees with the simulation results as well as the theoretical expression for the FSR for an FP cavity as derived in [18]. Figure 5 (c-d) illustrates the capability of the proposed device to create different FP-like cavities. In all cases, the simulated responses show an excellent agreement with the measured responses. Note that we adjusted the effective refractive indices and the corrugation width (e.g., reducing the value from the design) of the WBG to account for variations/errors in fabrication and processing to match the simulated and measured responses.
4. Discussion and summary
By increasing the grating length, number of electrical pads, and using independent heating elements, we can induce a greater range of perturbations in the refractive index and correspondingly, have greater control over shaping the spectral response of the device. It is also possible to improve thermal isolation by using an unheated segment between two neighboring heated segments [11]. For example, the proposed structure in Fig. 6 (a) can be used as a means for improving the heat distribution shown in Fig. 3 (e) [whose corresponding spectral response is shown in Fig. 5 (a)]. Instead of using a one-block heater element, the proposed structure is designed such that each segment is controlled by an independent heater element; moreover, the unheated segment can be used to reduce thermal crosstalk. We perform a simulation to validate that the discontinuity in the heater element can block heat dissipation to other segments as shown in Fig. 6 (b) (blue curves). As a result, the FP resonances and different reflection bands are more clear. Simulations show that the necessary length of the unheated segment to maintain thermal isolation while still ensuring spectral results similar to Fig. 6 (b) (red curves) is approximately 30 µm, see Fig. 6 (c) (red vs. blue curves). The yellow curves are the same as Fig. 5 (a) for comparison. However, there is a trade-off between achieving thermal isolation and the amount of reconfigurability. In other words, if the two segments are isolated as shown in Fig. 6 (a), electrical pads 1, 3 can no longer be activated. Thus, a new design is required to achieve the case in Fig. 4 (c). Ultimately, the target application decides on whether a single block heater or multiple independent heater blocks should be used.
Fig. 6. (a) The simulated schematic diagram when there is no thermal crosstalk between electrical pads 2,3, namely, the heater element is divided into two independent blocks. (b) Thermal distributions for three different cases. (c) The corresponding spectral responses.
In summary, we proposed a WBG with programmable spectral responses enabling multi-band filters and FP-like filters. There are alternative ways to implement multi-band or FP filters. In terms of multi-band filters, it is possible to have multiple tunable bands using multiple gratings, each with its own heating element. In terms of FP filters, it is also possible to use a pair of gratings separated by a length of waveguide with heating elements: heating the gratings allows for tuning of the wavelength of operation while heating the waveguide allows for control of the optical cavity length and thus, tuning of the FSR. We believe that our proposed structure, for which we have demonstrated a proof-of-principle, offers possible benefits in terms of simplicity and ease of reconfiguration (e.g., using fewer electrical drive signals). Simulations show an excellent agreement with the measured responses. We believe such a device can be further optimized to be used in integrated microwave photonics, or optical signal processing applications, or to create chirped or phase-shifted grating structures.
Funding
Natural Sciences and Engineering Research Council of Canada.
Acknowledgments
We thank the CMC microsystems for help with chip fabrication. This research was supported in part by the Natural Sciences and Engineering Research Council (Canada).
Disclosures
The authors declare no conflict of interest.
Data availability
Data underlying the results presented in this paper may be obtained from the authors upon reasonable request.
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