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Abstract
On-chip Bragg gratings with high reflectivities have been found to have widespread applications in filters, resonators, and semiconductor lasers. However, achieving strong Bragg reflections with flat response across a broad bandwidth on the popular 220 nm silicon-on-insulator (SOI) platform still remains a challenge. In this paper, such a high performance device is proposed and fabricated, which is based on a slot waveguide with gratings etched on the inner sidewalls of the slot. By manipulating the local field in the slot region using a chirped and tapered grating-based mode transition, the device achieves a flat response with ultra-high reflection and low transmission for the TE mode across a broad operating bandwidth. Leveraging the ultra-high birefringence of the SOI waveguide, the device functions both as a TE slot waveguide reflector and a TM pass polarizer. Simulation results demonstrate that the device exhibits an ultra-high rejection of more than 50 dB and a reflectivity exceeding 0.99 for the TE mode across a 91 nm wavelength range, while maintaining a high transmittance of larger than 0.98 for the TM mode. Experimental results validate that the device performance is consistent with the simulation results. A fabricated device based on such a gratings exhibits a low insertion loss (<0.8 dB) and high polarization extinction ratio (>30 dB) over 100 nm bandwidth (1484 nm–1584 nm), demonstrating that the performance of the present design is competitive with that of the state-of-the-art SOI Bragg gratings.
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1. Introduction
Bragg gratings have found widespread use in telecommunication, sensing, spectroscopy and fiber lasers [1–4]. Recently, there has been significant interest in realizing Bragg grating devices in integrated waveguides, particularly on the silicon-on-insulator (SOI) platform, which is considered a promising platform for the ultra-dense on-chip integration of photonic and electronic circuitry due to its compatibility with CMOS fabrication.
Strong waveguide Bragg gratings on the SOI platform have become increasingly popular due to their ability to provide small footprint, large bandwidth, high reflectivity. These properties are crucial for a wide range of applications, including filters [5–8], resonators [9,10], and semiconductor lasers [11,12]. However, it is still challenging to realize a strong Bragg gratings on a SOI wafers with thin silicon device layer, e.g., the commonly used 220 nm SOI wafer [13]. In conventional strip or rib waveguides, the electromagnetic field concentrates in the silicon core region, resulting in weak fields near the sidewalls. This limitation makes it difficult to simultaneously achieve strong Bragg gratings with both broad bandwidth and high reflectivity.
Slot waveguides [14–19] have gained considerable traction on the SOI platform due to their robust field enhancement in the nanoscale slot region, making them an ideal choice for constructing strong Bragg gratings. The additional sidewall provided by the slot region allows for more flexibility in grating etching, and the localized field enhancement facilitates a robust interaction between the grating and the electromagnetic field. Despite several proposed devices based on slot waveguide Bragg gratings [20–24], only a few have been successfully fabricated and experimentally demonstrated [20]. In a notable study [20], Wang et al. experimentally demonstrated a resonator deploying phase-shifted Bragg gratings in slot waveguides, revealing a quality factor of up to 3 × 104. However, owing to the shallow sidewall etching process, these gratings have smaller reflection coefficients. Consequently, a larger number of periods are required to achieve strong reflection, resulting in limited bandwidth [13]. Therefore, there is a significant opportunity to fully leverage the advantages of slot waveguides in constructing strong Bragg gratings with smaller sizes, high reflectivity, and a preferably flat response across a broad operating bandwidth.
In this study, we present and experimentally validate a broadband Bragg grating with a flat response on a 220 nm SOI platform. The device utilizes a slot waveguide Bragg grating with etchings on the inner sidewall of the slot. By strategically manipulating the local field in the slot region through a chirped and tapered grating-based mode transition, the device achieves a flat response with ultra-high reflectivity and low transmission for the TE mode over a broad operating bandwidth. Leveraging the SOI waveguide's ultra-high polarization mode birefringence, the device serves as both a TE slot waveguide reflector and a TM pass polarizer. Simulation results indicate that the device exhibits an ultra-high rejection >50 dB and reflectivity >0.99 for the TE mode within a 91 nm wavelength range, while maintaining high transmittance >0.98 for the TM mode. Experimental validation confirms the device's performance consistency with simulation results. As a high-performance TM-pass polarizer, the fabricated device demonstrates low insertion loss (IL) (<0.8 dB) and a high polarization extinction ratio (PER) (>30 dB) over a 100 nm bandwidth (1484 nm–1584 nm). In comparison to state-of-the-art TM-pass polarizers [25–30], this device offers advantages of a high PER, low IL, and high robustness within its operating bandwidth.
2. Device structure and principle
Figure 1 depicts a schematic diagram of the proposed slot waveguide Bragg gratings, providing a three-dimensional (3D) view and two enlarged views of the grating sections. The device consists of input/output slot waveguides, a central uniform grating section, and two identical tapered mode transitions connecting them. To enhance reflection, gratings are positioned on the inner sidewall of the slot waveguide. Linear chirped gratings are adopted in the transition sections to reduce mode transition loss. The grating parameters of the mode transitions and the uniform grating section are detailed in Fig. 1(b) and Fig. 1(c), where Λ0 and Λ1 denote the start and stop pitch width of the linear chirped grating, a0/Λ0 and a1/Λ1 represent the duty cycle, N1 and N2 indicate the number of periods of the uniform grating and chirped grating. The input and output waveguides are commonly used slot waveguides with an outer width of W0 and a slot width of g1. The minimum and maximum spacing between adjacent grating teeth in this tapered chirped grating are denoted as g2 and g3 respectively, as illustrated in Fig. 1(c). The variations in the adjacent grating teeth in the chirped gratings are described by the equations g2i = g2i-1+Δg1 and g3i = g3i-1+Δg2. The periodic variation of the chirp grating is expressed by the formula Λi=Λ0 + i × (Λ1-Λ0)/N2, where i represents the i-th period.
Fig. 1. Three-dimensional schematic (a), chirped gratings top view (b), and central uniform gratings top view (c) of the device.
When light travels through such a periodic structure, depending on certain conditions, its behavior can be described using three propagation regions: Bragg reflection, subwavelength guided wave propagation, and radiation [31,32]. By regulating the structural parameters of the grating, we can utilize the high birefringence of the SOI waveguide to promote TE mode presence within the Bragg reflection region and TM mode presence within the propagation region. Hence, the device functions dually as a TE slot waveguide reflector and a TM pass polarizer. The pitch width of the gratings must satisfy specific conditions outlined in [25,26].
Additionally, based on the coupled mode theory [33], we can mathematically relate the rejection (R) and bandwidth (Δλ) of a Bragg grating-based reflector to the grating parameters and filter length (L) using the equation below:
The device is designed on a standard 220 nm SOI wafer with a 2 µm buried oxide layer. To streamline the design, several default parameters are set as follows: the maximum waveguide width of the central uniform grating section is W1 = 800 nm, the minimum grating teeth spacing is g2 = 60 nm, the maximum grating teeth spacing is g3 = 300 nm, the duty ratio of the grating (a1/Λ1 and a0/Λ0) is set to 0.5, and the minimum pitch width is Λ0 = 120 nm. Additionally, the input and output slot waveguide parameters are set as W0 = 500 nm and g1 = 100 nm. Modal and transmission analyses are conducted using the finite difference frequency-domain (FDFD) and 3D finite difference time-domain (FDTD) methods, respectively.
To verify the aforementioned theory, Firstly, the commonly used strip waveguide and slot waveguide grating models are constructed in Fig. 2(a), where the grating teeth length of Δw = 20 nm were set on the outer wall of the strip waveguide and the inner side of the slot waveguide, respectively. Figure 2(b) displays the mode field distributions supported by these two waveguide types at a wavelength of 1550 nm. It is worth mentioning that the refractive indices of SiO2 and Si are 1.445 and 3.455, respectively, at the wavelength of 1550 nm. Additionally, when calculating group refractive indices and spectral responses, we took into account the wavelength dependence of the materials. To ensure smooth dispersion curves, we deployed a Lorentz model [34] to fit the experimental material data obtained from Palik's handbook [35]. It is evident that upon comparing the TE modes of strip waveguide and slot waveguide, the field intensity in slot waveguide is primarily distributed within the lower refractive index SiO2 layer between the silicon waveguide, while the TE mode of strip waveguides is mainly confined within the silicon waveguide, which means that the subtle changes in the grating teeth inside the slot waveguide Δw may have a significant impact on the TE mode distribution. Therefore, the TE mode in the slot waveguide grating has a more significant effective refractive index difference Δneff, as shown in Fig. 2(b). Similar behavior is observed in the field intensity distribution of the TM mode, where both strip waveguides and slot waveguides have TM mode field intensity concentrated in the SiO2 layer surrounding the silicon waveguide. Meanwhile, it can be observed that the modes present in slot waveguide maintain a significantly lower group index (ng∼2.6-2.7) compared to the modes in the strip waveguide at the wavelength of 1550 nm. For comparison, Fig. 2(c) shows the transmission spectra of these two types of gratings, where the grating period is calculated by Eq. (2) to ensure that TE mode operates near the central wavelength of 1550 nm. Here, the duty ratio is set to 0.5, and in the subsequent analysis, unless specified otherwise, the duty ratio for the gratings is consistently maintained at 0.5. The results indicate that with the same width of the grating teeth Δw, the slot waveguide grating has a wider operating bandwidth Δλ due to the larger Δneff (lager κ) and lower ng. This is consistent with the above theoretical speculation.
Fig. 2. Schematic diagrams (a) of the two commonly used waveguide types and the field distribution of the supported fundamental modes (b), as well as corresponding transmission spectra (c).
The device structure is derived from the classical slot waveguide grating. Following the principles of coupled mode theory, the aim is to enhance κ more effectively, thereby broadening the operating bandwidth of the slot waveguide grating. This enhancement is achieved by introducing a stronger grating, perturbing the effective refractive index of waveguide modes. Placing this grating within a slot region optimizes its impact on TE modes, intensifying the perturbation in the effective refractive index and consequently amplifying κ. The specific steps are illustrated in Fig. 3(a) and interpreted as follows:
Fig. 3. Proposed device structure evolution diagram (a) and corresponding simulated band diagram for TE0 mode (b) and TM0 mode (c) when Λ=444 nm.
Step 1. Construct the slot waveguide grating as mentioned above, with a calculated period Λ using Eq. (2).
Step 2. Expand the arm width outward to ensure ample degrees of freedom for the internal grating.
Step 3. Increase the slot width to enhance the refractive index difference of the grating, improving κ while maintaining low mode loss.
Figures 3(b) and 3(c) illustrate the alterations in the simulation band diagram across three sequential steps. Throughout these steps, the grating period remains consistently set at the initially calculated Λ=444 nm to ensure the stability of the light cone and facilitate meaningful comparisons. Clearly, in Fig. 3(b), the band gap of step 1 (190.2-196.2 THz) aligns with the transmission spectral bandwidth of the simulated slot waveguide grating in Fig. 2(c). In step 2, widening the width of the slot waveguide arm to a single-side width sufficient for the presence of TE0 results in the convergence of mode energy from the SiO2 surrounding the waveguide towards the center of the Si waveguide. This directly causes an increase in the group index and a reduction in the photonic bandgap. Moving from step 2 to step 3, a robust slot waveguide grating is established by expanding the slot width, significantly enhancing the refractive index difference at the grating interface, and thereby widening the photonic crystal bandgap, consistent with the theoretical derivation outlined earlier. The effective refractive index of the TM0 mode changes gradually as the waveguide height remains constant at 220 nm. As shown in Fig. 3(c), there is either no bandgap or a minimal bandgap (in step 3) in the TM0 mode across all steps. Consequently, by capitalizing on the grating characteristics and simply adjusting the grating period to operate at 1550 nm, the determination of a broadband mode-filtering gratings based on a slot waveguide for TE0 mode is achieved.
Once the bandgap is established, the immediate focus shifts to discussing the reflection efficiency of the Bragg grating. In this context, simulation optimization is employed to adjust the uniform grating period to Λ1 = 382 nm, targeting operation around 1550 nm. For this simulation the period number of the gratings is set to be 35. However, as illustrated in Fig. 4, it becomes evident that with increasing wavelength, the refractive index difference in the uniform Bragg grating region diminishes. This leads to a reduction in the coupling coefficient κ, resulting in a decrease in reflection intensity and an increase in radiation loss. To address this challenge and enhance the device's performance as a reflector, a transition grating is strategically connected on both sides of the uniform grating, as depicted in Fig. 4. A similar concept of deploying a tapered grating has also been utilized in Ref. [10] to flatten the reflection spectra of a reflector. In this work, we take advantage of the localized field enhancement effects of the slot waveguide to further enhance this effect. A comparative analysis in Fig. 4 reveals that at longer wavelengths within the reflection bandwidth, the transition grating significantly boosts the reflection intensity (overall r > 0.99). This enhancement demonstrates the transition grating's excellent performance as a reflector, effectively mitigating the decrease in reflection intensity and radiation loss associated with the uniform Bragg grating at longer wavelengths.
Fig. 4. Comparison of reflection spectra of TE mode between uniform grating and transition grating added in the wavelength range of 1450 nm–1650 nm.
3. Numerical optimization and characterization
Utilizing the initially determined structural parameters mentioned above, further optimization involves adjusting several other parameters of the device. A critical parameter in this context is the length (L1) of the central grating region. For a Bragg grating, enhancing the reflection can be achieved by increasing the number of period. However, it is essential to consider that a higher number of gratings also results in increased losses for TM mode light. Given that the device functions both as a TE slot waveguide reflector and a TM pass polarizer, three crucial metrics—TE transmission, TE reflection, and TM transmission—are employed to evaluate its performance. Figure 5 illustrates the normalized transmission for TE (a) and TM (b) modes, as well as the reflection for TE (c) input modes at a wavelength of 1550 nm, with respect to the period number of the Bragg grating. Note that the simulation includes the transitional section of the tapered grating, where the number of periods for the tapered grating is N2 = 35, corresponding to L2 = 8.785 µm. The increment in the number of grating periods is set as ΔN1 = 5, and the length of the polarizing section is calculated as L1 = N1×Λ1. To accurately depict variations, the simulation results are presented in dB for TE transmission and in linear format for TE reflection and TM transmission. In Fig. 5(a), the transmittance of TE mode light gradually decreases as the number of grating period increases. However, after reaching N1 = 30, the suppression of TE mode by the gratings stabilizes. In Fig. 5(b), the transmittance of TM mode exhibits a nearly sinusoidal variation with increasing grating periods while maintaining low overall loss. In Fig. 5(c), we can see that the overall TE reflection increases with increasing period, but the change is not very obvious after the period reaches 15. To ensure optimal device performance while maintaining a compact structure, the grating period number N1 is chosen as 35, resulting in a central grating region length of approximately ∼13 µm. At this juncture, the device achieves a suppression of −50 dB or more for TE mode while maintaining high reflection for TE mode and high transmittance for TM mode.
Fig. 5. Normalized transmission for TE (a) and TM (b) modes, as well as the reflection for TE (c) input modes at a wavelength of 1550 nm, with respect to the period number N1.
To efficiently guide the input mode into the grating region and minimize transition loss, the selection of the length (L2) of the transition sections becomes crucial. As L2 and the number of grating periods are interconnected, our optimization focus is primarily on refining the value of N2. Figure 6 illustrates the normalized transmission for TE (a) and TM (b) modes, as well as the reflection for TE (c) input modes at a wavelength of 1550 nm, with respect to the period number N2. Simulation results for TE mode are expressed in dB, while TM mode results are presented in linear format. The increment in the number of periods is denoted as ΔN2 = 5. Upon scrutinizing the graph, as depicted in Fig. 6(a), the TE mode light transmittance gradually decreases with the growing number of transition grating periods. Upon reaching N2 = 35, the TE mode transmittance stabilizes and falls below −50 dB. Moving on to Fig. 6(b), it becomes evident that with an increase in the number of transition grating periods, the TM mode light transmittance consistently remains above 0.99. In contrast, in Fig. 6(c), we observe that the overall TE reflection increases with the growing period, but the change is not very pronounced after the period reaches 20. Consequently, the transition grating period of N2 = 35 is chosen for the device.
Fig. 6. Normalized transmission for TE (a) and TM (b) modes, as well as the reflection for TE (c) input modes at a wavelength of 1550 nm, with respect to the period number N2.
Building upon the determined parameters, we conducted further simulations to analyze the spectral response of the entire device. The transmission and reflection spectra of TE and TM modes in the wavelength range of 1450 nm to 1650 nm are illustrated in Fig. 7. As depicted in Fig. 7(a), within the wavelength range of 1450 nm to 1650 nm, the transmittance of the TM mode consistently exceeds 0.98 across varying wavelengths. In the operational bandwidth of 1479 nm∼1601 nm, spanning 122 nm, the transmittance of the TE mode remains exceptionally low, dropping below −30 dB. Notably, a very flat response is seen over a large bandwidth of 91 nm (1491 nm to 1582 nm) with TE transmittance below −50 dB. Figure 7(b) demonstrates that the TM mode maintains consistently low overall reflection below −30 dB within the wavelength range of 1450 nm to 1650 nm, with minor fluctuations at shorter wavelengths. This behavior can be attributed to the increase in Bragg grating period as the wavelength rises. In contrast, the TE mode exhibits high reflectance, particularly within a specific operating bandwidth of 131 nm (1476 nm to 1607 nm), where it exceeds 0.96. Overall, the current device demonstrates the capability to maintain ultra-high TE rejection over a broad bandwidth. Moreover, its transmission and reflection spectra exhibit remarkable flatness when compared to similar Bragg grating devices reported in the literature [25–29].
Fig. 7. Transmission (a) and reflection (b) spectra for both TM and TE modes in a wavelength range from 1450 nm to 1650 m.
Figure 8 provides insight into the field evolution of the main electric field components for both polarizations as they traverse the device at different wavelengths. It is evident that at different wavelengths, TM-polarized light traverses the polarizer with minimal loss, while TE polarization is strongly reflected by the Bragg grating. The mode transition section effectively mitigates TM mode loss while enhancing the reflection of TE modes.
Fig. 8. Field evolution of the main electric components for both polarizations along the propagation distance through the polarizer at different wavelengths. (a) TE and (b) TM at 1500 nm; (c) TE and (d) TM at 1525 nm; (e) TE and (f) TM at 1550 nm; (g) TE and (h) TM at 1575 nm.
To assess the device performance, we conducted an analysis of the fabrication tolerances for several key parameters, as grating-based devices are typically sensitive to duty ratio and pitch width. The results are presented in Fig. 9, showcasing the transmission and reflection spectra for both TE mode and TM mode as functions of the duty ratio and the pitch width. In Fig. 9, the duty ratio of the gratings is denoted as a/Λ, with a duty ratio of 1/2 serving as the reference standard. It is evident from Figs. 9(a) and (b) that when “a” fluctuates within the range of (-30 nm, 30 nm), the transmittance of TE mode is consistently lower than −50 dB, the reflectance is higher than 0.997, and both of them exhibit relative stability. Meanwhile, the transmittance and reflectance of TM mode show minor fluctuations, with both values remaining higher than 0.998 and lower than −45 dB, respectively. This indicates a significant manufacturing tolerance for deviations in grating pitch width. Moving on to Figs. 9(c) and (d), pitch width has a similar effect on the transmittance and reflectance of TE and TM modes. It is worth noting that the rejection of TE mode decreases significantly at pitch width deviations of −10 to −15 nm. Therefore, to ensure the device operates as a high PER and low IL polarizer, it is advisable to limit the variation of the grating pitch width in the polarization region to within 20 nm (-10 nm, + 10 nm).
Fig. 9. Transmission (a) and reflection (b) spectra for both TE mode and TM mode as functions of the duty ratio; transmission (c) and reflection (d) as functions of the pitch width.
4. Fabrication and measurement
The device outlined in this paper is fabricated on a 220 nm-SOI wafer featuring a 2 µm thick SiO2 buffer layer. The manufacturing process comprises several key steps. Initially, electron beam lithography is employed to define patterns on a photoresist. Subsequently, inductively coupled plasma etching is used to completely etch the top silicon. This streamlined, one-step etching process simplifies fabrication challenges. Finally, a 2.2 µm thick SiO2 layer is deposited across the entire chip using plasma-enhanced chemical vapor deposition methodology. To realize the coupling between the fiber optic and the chip, polarization-selective grating couplers (GCs) are fabricated at the input and output ports of the device. Additionally, TE-mode and TM-mode reference waveguides of with strip to slot converters are manufactured for normalization of the test results, as depicted in Fig. 10(a). In Fig. 10(b) and (c), a scanning electron micrograph (SEM) illustrates the mode-converter and the Bragg gratings.
Fig. 10. Microscope and SEM images of the fabricated devices. (a) Microscope of the measurement setup and (b) SEM image of the mode converter and (c) SEM image of the device.
Measurement is conducted using a super-continuum source and an optical spectrum analyzer. However, the grating coupler's limited working bandwidth poses a challenge in obtaining spectra within the wavelength range of 1450 nm∼1650 nm with a single measurement. To overcome this limitation, adjustments to the angle (θ) between the fiber optic probe and the direction perpendicular to the chip are made during the measurement. Initially, the fiber probe tilt angle is set to θ=15°, and devices designated for TE and TM measurements are assessed, yielding spectral responses in the range of 1475 nm to 1548 nm. Subsequently, the fiber probe tilt angle is adjusted to θ=8°, and the same devices for TE and TM measurements are reevaluated, resulting in spectral responses in the range of 1548 nm to 1600 nm. Figure 11 illustrates the transmission spectra of the TE and TM modes for the device. The results demonstrates that the trends in transmittance changes for TE and TM modes closely align with the simulation results across the entire wavelength range. When the TE mode light source is introduced, the polarizer exhibits transmittance below −30 dB in an 83 nm bandwidth range from 1505 nm to 1588 nm, displaying a remarkably stable transmission spectrum within this interval. In TM mode, the polarizer consistently maintains high transmittance above −1 dB throughout the entire wavelength range. Despite minor manufacturing process errors causing a slight shift in the center wavelength of the device, at 1550 nm, the transmittance is −32 dB for the TE mode and −0.025 dB for the TM mode. In the same figure, we also included the measured results of the TE reflection. To reduce the influence of end-face reflections when measuring reflection characteristics, refractive index matching liquid was used between the optical fiber probe and the chip. Upon analyzing the reflection spectrum, we observed that the experimental results generally match well with the simulation results. However, there are some fluctuations in the reflection spectrum which may be caused by disturbances like environmental vibrations affecting the refractive index matching liquid. On average, the reflection spectrum indicates a reflectance of approximately −0.5 dB within the operating bandwidth. In order to achieve better results, it may be necessary to use more efficient coupling techniques for reducing the influence of end face reflections between the optical fiber and the chip.
Fig. 11. Measured transmission and reflection spectra of TE mode and transmission spectra of TM mode in the wavelength range of 1470 nm∼1605 nm.
As is well known, the slot waveguide has been widely used because of its distinctive mode characteristics in the TE mode. In above work, we presented a high-performance Bragg grating for the TE mode in the slot waveguide, which also functions as a TM-pass polarizer. Note that such TM polarizers designed for conventional strip waveguides are more commonly used in practical applications. Therefore, to enhance the functionality of the device as a high-performance TM-pass polarizer for the commonly used strip waveguides, we have optimized the mode conversion section to make it more compact. For a reflector, the main concern is its reflection characteristics. However, if it also functions as a TM polarizer, the TM loss and the TE rejection are equally important. The TE rejection determines the polarization extinction ratio (PER) of the polarizer. In this updated device, we have deployed a spliced bi-tapered structure, as illustrated in Fig. 12. The structural parameters of the tapered structure are defined as Wmax = 1.75 µm, La = 5 µm and Lb = 4 µm. To precisely measure the TM mode insertion loss of the device, we employed 10 cascades. The micrograph and SEM image of the device are displayed in Fig. 12. Using similar testing methods as described earlier, we measured the TE and TM mode transmittance, as presented in Fig. 13. It is evident that the trends in the changes of TE and TM mode transmittance across the entire wavelength range closely match the simulation results. When the TE mode light source is introduced, the polarizer exhibits transmittance below −30 dB in a 100 nm bandwidth range spanning from 1484 nm to 1584 nm wavelength. The transmission spectra of the TE mode remain remarkably stable within this bandwidth. For the TM mode, the transmittance is consistently high across the entire wavelength range, surpassing −0.8 dB. Despite slight manufacturing process errors causing a minor shift in the center wavelength of the polarizer, at 1550 nm, the transmittance of TE mode reaches −36 dB, while that of TM mode is −0.32 dB. Hence, the implemented polarizer demonstrates a minimal insertion loss (IL) of less than 0.8 dB and an impressive PER exceeding 30 dB across a 100 nm bandwidth. Comparing with the state-of-the-art TM polarizers on 220 nm SOI in [25–29], the present device has the advantages of low IL, high PER, and flat response.
Fig. 12. Microscope and SEM images of the fabricated devices. (a) Microscope of the measurement setup and (b) SEM image of the single device set and (c) SEM image of the cascaded device set.
Fig. 13. Simulated transmission spectra (a) and measured transmission spectra (b) of TE and TM modes.
Table 1 summarizes the performances of different state-of-the-art reflectors and TM-pass polarizers, as compared with those of our work. From the table, one sees that our work achieves a larger bandwidth and higher reflectance compared to the reflectors in Refs. [10,20,22]. Reference [13] introduces a reflector with maximum bandwidth for strip waveguides, while our work proposes a potential solution for slot waveguides. Compared with recently reported schemes for TM polarizers [25–29], our device has lower insertion loss and higher PER, although the bandwidth is not as large as some of the schemes. Perhaps more importantly, our device shows a flat wavelength response for ensuring more consistent high performance.
Table 1. Comparison of various reported SOI reflectors and TM-pass polarizers.
5. Conclusion
In summary, this study introduces an on-chip Bragg grating on a 220 nm SOI platform, addressing the challenge of achieving strong Bragg reflection with both broad bandwidth and high reflectivity. The proposed device, based on a slot waveguide Bragg grating with inner sidewall gratings, deploys a chirped and tapered grating-based mode transition to manipulate the local field in the slot region, which results in a flat response with ultra-high reflection and low transmission for the TE mode over a broad operating bandwidth. Leveraging the ultra-high birefringence of the SOI waveguide, the device serves dual functionalities both as a TE slot waveguide reflector and a TM pass polarizer. Our simulation results showcase the device's outstanding performance, with an ultra-high rejection >50 dB and reflectivity >0.99 for the TE mode over a 91 nm wavelength range, coupled with a high transmittance >0.98 for the TM mode. The fabricated device demonstrates excellent consistency between the actual performance and the simulation results, which exhibits low insertion loss of less than 0.8 dB and a high polarization extinction ratio of larger than 30 dB over a 100 nm bandwidth between 1484 nm–1584 nm. This excellent performance positions the device as a promising candidate for advanced photonic applications.
Funding
National Natural Science Foundation of China (12004092, 62375050); Natural Science Foundation of Hebei Province for Distinguished Young Scholars (F2023201024); Advanced Talents Program of Hebei University (521000981203).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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