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Abstract
We propose and demonstrate on-chip power splitters based on adiabatic rib waveguide enabling arbitrary splitting ratios on a monolithic silicon photonic platform. The devices are elaborately engineered based on adiabatic directional couplers with a trapezoid-structure in the longitudinal direction in the mode evolution region. The measurement results indicate that the proposed devices can achieve over 150 nm bandwidth for arbitrary splitting ratios of 50%:50%, 70%:30% and 90%:10%. The mode evolution footprint is greatly narrowed to below 79 µm with an insertion loss of less than 0.22 dB. The demonstrated arbitrary ratio power splitters offer a promising application prospect in high-density photonic integrated circuits.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The silicon-on-insulator (SOI) is an intriguing platform for large-scale integration owing to its considerable optical field confinement and complementary-metal- oxide-semiconductor (CMOS) compatibility [1,2]. Optical power splitters with arbitrary splitting ratios (ASRs) are indispensable building block in photonic integrated circuits (PICs) [3,4], widely used for power distribution and the elements of more complex optical devices and systems, such as wavelength-division-(de) multiplexer (WDM) [5,6], optical phased arrays (OPA) [7–9], modulators [10–12] and optical switches [13–15]. The most popular types of power splitters on silicon photonic platform mainly contain Y-branches, multimode interference (MMI) couplers and directional couplers (DCs). The Y-branch coupler, recognized for its compact size and expansive operating bandwidth, can nonetheless suffer from mode mismatch loss given insufficiently small junction angles [16,17]. Employing the self-imaging principle, MMI have elicited substantial interest due to their wavelength insensitivity and significant fabrication tolerance, but it has poor transmission efficiency for asymmetric power splitting [18,19]. While structural optimization can enable MMI splitters to achieve arbitrary splitting ratios, they tend to exhibit higher insertion loss (IL) compared to DCs. DCs mainly work based on evanescent field coupling, which is typically explained by coupled-mode theory. Despite their compact footprint and low loss, their power splitting ratio is highly prone to dispersion, thereby limits their operating bandwidth [20]. There are many schemes in the literature to improve the operating bandwidth of DCs. Sub-wavelength gratings (SWGs) can significantly mitigate wavelength dependence of the DC due to their modulation mode dispersion. These devices are capable of stable power splitting, boasting a vast operating bandwidth and compact footprint, while necessitating great process accuracy [21,22]. Bent directional couplers modify the mode coupling region into a curved shape, thereby extending the operational bandwidth. Nevertheless, these studies predominantly focus on a 3-dB power distribution ratio, with scant elaboration on ASRs [23]. When an asymmetric arm is incorporated into the coupling region of the DC, the refractive index can be adjusted, facilitating a broadband operation. While offering a significant bandwidth and low loss, these devices are limited for further extending their operational bandwidth [24]. By virtue of the mode evolution theory, the adiabatic directional couplers (ADCs) introduce a trapezoid structure into the coupling region, thereby achieving a wide operating bandwidth and large fabrication tolerance [25–27]. However, they require a sufficient size to ensure perfect mode evolution, posing an issue for high-density PICs. The pursuit for a high-performance power splitter that concurrently delivers broadband, small footprint, low-loss and robustness against fabrication tolerance, remains an ongoing challenge.
In this paper, we propose and experimentally demonstrate broadband and compact arbitrary ratio power splitters. The devices based on rib waveguide consist of a trapezoid structure in an adiabatic transition region. We present a complete design methodology based on adiabatic rib waveguides on a silicon photonics platform, which can achieve the characteristics of large operating bandwidth, small size, low loss and large process tolerance. Measurement results show that our proposed device can accomplish various power splitting ratios from 50%:50% to 90%:10% in the wavelength range from 1450 nm to 1600 nm. With adiabatic rib waveguide and elaborate design, the device footprint can be reduced to less than 79 µm, with an IL of less than 0.22 dB.
2. Design principle and analysis
The schematic diagram of the proposed rib-waveguide-based ADC with arbitrary ratio is exhibited in Fig. 1(a). The device is based on SOI platform comprised of a 220 nm core silicon layer, a 2 µm BOX and a 2 µm SiO2 cladding. In the adiabatic transition region, it is mainly composed of two tapered waveguides with constant gap to complete the perfect mode evolution. Compared to conventional DCs, ADCs offer inherent wavelength insensitivity and considerable fabrication tolerance. In terms of the conventional DCs, the device generally includes two parallel strip waveguides, mainly involving mode interference. As the odd and even modes interfere as they propagate along the waveguide, power oscillations occur at the output port [28]. However, ADCs exhibit different operation principles. There is only single mode permitted to propagate in the adiabatic transition region. And the desired mode will undergo an evolution with the assurance of absolute single-mode propagation [29]. With perfect adiabatic evolution, the refractive index distribution at the terminal end of the adiabatic waveguide governs the optical power distribution. This operating principle enables arbitrary power splitting ratios based on the properties of adiabatic couplers. In order to better illustrate the principle and structure of our device, we have divided the device into five parts as shown in Fig. 1(b).
Fig. 1. (a) Schematic view of the proposed 3-dB power splitter based on adiabatic rib waveguide on SOI platform. (b) The top view of the schematic and the geometric parameters.
Region I is the input of the device and consists of two straight rib waveguides with a length of 10 µm. To avoid crosstalk, the gap G1 between the waveguides at the input needs to be large enough of around 3 µm. The waveguide width W is set to 350 nm to ensure that there is only a single mode propagating in region I. Region II is composed of a pair of Bessel-curve waveguides to connect the input and the adiabatic transition region with a length of 5 µm in the transverse direction. The gap between the waveguides is narrowed from 3 µm to 100 nm G2. The Bessel-curve waveguide is adopted to further reduce the scattering loss of the waveguide by virtue of the smooth transitions of the Bessel curves. Region III consists of a pair of taper waveguides with 100 nm gap to achieve perfect mode evolution. We initially set W1, W2, W3, and W4 to 470 nm, 230 nm, 350 nm, and 350 nm, respectively, and further optimize the waveguide widths to achieve arbitrary ratios. In addition, the region needs to ensure single-mode operation and the desired mode will not be converted into other higher-order modes. Based on the adiabatic principle, the mode evolution length does not need to be precisely defined, thus we can further optimize its dimensions for arbitrary ratios. Region IV comprises a pair of Bessel-curve waveguides with a transverse length of 5 µm, increasing the waveguide gap from 100 nm to 3 µm. Region V is the output of the device composed of a pair of taper waveguides with widths varying from W3 and W4 to 350 nm, allowing an integration with the rest of the device. The fundamental structure parameters are exhibited in Table 1.
Table 1. Device Parameters and Corresponding Values
Based on the above operating principle, we define δW1 = W3-W4 to seek the relationship between power splitting ratio and the width difference of the adiabatic waveguides. In the simulation process, we mainly use 3D FDTD to calculate the spectral response. We mainly use the TE fundamental mode as the incident mode, and the simulation wavelength covers from 1400 nm to 1600 nm. After the calculation of the relationship between δW1 and the power splitting ratio as shown in Fig. 2, we can achieve the power splitting ratios of 50%:50%, 70%:30%, and 90%:10% with δW1 of 0 nm, 18.2 nm, and 56.5 nm, respectively. Based on the results obtained above, we simulate and optimize the ADCs for the above power division ratios, and the adiabatic dimension can be reduced from 100 µm to 78 µm, 70 µm and 79 µm, respectively. The transmission curves at different power splitting ratios are shown in Fig. 3 (a-c). It can be seen that our proposed device is capable of achieving stable ASRs in the desired band with transmission deviations from average expected value of 0.499 ± 0.026, 0.296 ± 0.033, 0.098 ± 0.018 for 50%:50%, 70%:30% and 90%:10% splitting ratios, respectively.
Fig. 2. Simulated function between δW1 and transmission at a wavelength of 1500 nm with an adiabatic waveguide length L of 100 µm.
Fig. 3. (a) Transmission curve for a power splitting ratio of 50%:50% with an adiabatic waveguide length of 78 µm. (b) Transmission curve for a power splitting ratio of 70%:30% with an adiabatic waveguide length of 70 µm. (c) Transmission curve for a power splitting ratio of 90%:10% with an adiabatic waveguide length of 79 µm.
Figure 4 shows the optical field profile at wavelengths of 1500 nm with TE0 mode input. It can be clearly found that the optical power at the end of the adiabatic waveguide gradually concentrates towards the upper waveguide as δW1 varies. Afterwards, the light field leaves the output thus leading to different splitting ratios.
Fig. 4. Optical field profile at wavelength of 1500 nm for different power splitting ratios: (a) 50%:50%; (b) 70%:30%; (c) 90%:10%.
In addition, the fabrication tolerance of the device was investigated. We have simulated the spectral response of the arbitrary-ratio power splitters with the introduction of different waveguide width deviations and height deviations. We then calculated the values of the maximum transmission fluctuation in the desired band. The splitting ratio variations for 50%:50%, 70%:30%, and 90%:10% are shown in Fig. 5(a-f) when 50 nm and 10 nm deviations occur in the waveguide width and height, respectively. It is clear that the devices still work stably in the wavelength range from 1400 nm to 1600 nm. Moreover, the shallow etch depth can be greatly controlled under the existing fabrication conditions. The etch depth variations have minor effect on the device performance accordingly.
Fig. 5. (a) Simulated transmission fluctuation with ±50 nm width variation for 50%:50% power splitting ratio. (b) Simulated transmission fluctuation with ±10 nm height variation for 50%:50% power splitting ratio. (c) Simulated transmission fluctuation with ±50 nm width variation for 70%:30% power splitting ratio. (d) Simulated transmission fluctuation with ±10 nm height variation for 70%:30% power splitting ratio. (e) Simulated transmission fluctuation with ±50 nm width variation for 90%:10% power splitting ratio. (f) Simulated transmission fluctuation with ±10 nm height variation for 90%:10% power splitting ratio.
Benefiting from the adiabatic mode-evolution principle, the devices exhibit 200 nm operating bandwidth. With the high coupling strength of the rib waveguide, the device dimension is reduced to less than 79 µm. The overlap between the waveguide sidewalls and the optical field is made smaller, which further reduces the scattering loss. With our elaborate design, the device exhibits excellent properties such as ultra-broadband, compact footprint and low loss, offering a significant prospect for wide-scale applications in large-scale photonic systems.
3. Fabrication and experiment results
We employ electron-beam lithography (EBL) with an accelerating voltage of 100 kv to define the desired waveguide pattern, with a 250 nm positive photoresist of ZEP 520A. Most positive photoresists have better resolution than negative photoresists. EBL allows precise control of the dimensional deviation of the exposed area, thus achieving a better control of gap accuracy. Subsequently, we used inductively coupled plasma to transfer the pattern to the top silicon layer. Our ADCs utilize a rib waveguide with an etch depth of 160 nm, requiring a total of two etches. Firstly, we perform a deep etch with a thickness Hr of 160 nm on the core silicon layer using inductively coupled plasma dry etching. Then we implement a full etch to obtain the corresponding rib and strip waveguides for connecting the adiabatic directional coupler and the grating coupler. TE-mode based grating couplers are fabricated to couple the light from the light source into the designed adiabatic device. Figure 6(a) shows a scanning electron microscope (SEM) image of our fabricated device, which mainly consists of 4-stage cascaded power splitters and grating couplers connecting each port and reference waveguides. A zoomed microscope image of our fabricated ADC is shown in Fig. 6(b). SEM images of the adiabatic transition region and the rib waveguide are shown in Fig. 6(c) and Fig. 6(d), with a waveguide gap of 100 nm. Figure 6(e) shows a microscope image of the grating coupler used in our test circuit. It is highly evident that our fabricated device agrees well with the proposed device.
Fig. 6. (a) Optical micrograph of the cascaded 3-dB power splitter, (b) optical micrograph of the rib waveguide-assisted ADC, (c) SEM image of the center of mode evolution region, (d) SEM image of the fabricated rib silicon waveguide, (e) Optical micrograph of the fabricated grating couplers.
The measurements circuits are established mainly with tunable lasers, power meters, polarization controllers and fiber-chip coupling stages, etc. Figure 7(a) shows the spectral response of the fabricated ADC branches at each level with a length of L = 78 µm. Based on the obtained spectral response, we used linear regression to fit the transmission from 4 cascaded ports at different wavelengths as shown in Fig. 7(b). We normalize the obtained transmission spectra from bar and cross ports as shown in Fig. 7(c) over the wavelength range from 1450 nm to 1600 nm. Figure 7(d) shows the simulated electric filed intensity at wavelengths of 1450 nm, 1500 nm and 1550 nm. Our device exhibits a stable 50%:50% power division with transmission deviation from average expected value of 0.512 ± 0.051, which is in high agreement with the designed power distribution.
Fig. 7. (a) The spectral response of the fabricated ADC branches at each level with a length of L = 78 µm, (b) Linear regression to fit the transmission at different wavelengths of 1450 nm, 1500 nm and 1550 nm, (c) Normalized transmission spectra for 50%:50% splitting ratio, (d) the simulated electric filed intensity at wavelengths of 1450 nm,1500 nm and 1550 nm.
Moreover, we fabricated and tested the devices for different power splitting ratios. Figure 8 (a-f) shows the obtained spectral response, transmission curves, and simulated electric filed intensity for 70%:30% and 90%:10% power splitting ratios, respectively. The experimental result shows that there are deviations from average expected value of 0.281 ± 0.032, 0.109 ± 0.040 for the splitting ratios of 70%:30% and 90%:10%. These devices experimentally show excellent power-division performance over the wavelength range from 1450 nm to 1600 nm. They can theoretically perform an arbitrary ratio power splitting in the operating bandwidth from 1400 nm to 1600 nm, but practical measurements are limited by the laser and the grating coupler spectral response so that the bandwidth of the devices cannot be fully quantified. The wavelength tuning range of our available light source can only cover 1450 nm to 1600 nm, and thus the actual measurements are launched based on this waveband. Besides, Fig. 9 exhibits the measured IL of less than 0.22 dB for arbitrary power splitting ratios of 50%:50%, 70%:30%, and 90%:10% based on the cascaded test circuits. The results are obtained by calibrating off the response of the grating coupler.
Fig. 8. (a) The spectral response of the fabricated ADC branches at each level with a length of L = 70 µm, (b) Normalized transmission spectra for 70%:30% splitting ratio, (c) the simulated electric filed intensity at wavelengths of 1450 nm,1500 nm and 1550 nm for 70%:30% splitting ratio, (d) The spectral response of the fabricated ADC branches at each level with a length of L = 79 µm, (e) Normalized transmission spectra for 90%:10% splitting ratio, (f) the simulated electric filed intensity at wavelengths of 1450 nm,1500 nm and 1550 nm for 90%:10% splitting ratio.
Fig. 9. Measured IL curves corresponding to power splitting ratios of 50%:50%, 70%:30%, and 90%:10% in the wavelength range from 1450 nm to 1600 nm.
Fig. 10. The comparison between the average measured splitting ratios and the simulated splitting ratios.
Fig. 10 shows the comparison between the average measured splitting ratios and the simulated splitting ratios, and it is clear that the experimental results are in great agreement with the simulation results.
Moreover, the comparison of the reported unbalanced coupler and our proposed rib-waveguide based ADC is summarized in Table 2. With the high coupling strength of the rib waveguide and elaborate design, we have simultaneously achieved arbitrary-ratio power splitters with ultra-broadband, compact size, and large fabrication tolerances. These devices offer a great potential for applications in large-scale photonic integrated circuits. In addition, couplers based on rib waveguide are also suitable for TM mode operation. We have realized the tradeoff of compact size and large operating bandwidth through rib waveguides, and we can also consider the double-layer SOIs in the future to further enhance the device performance.
Table 2. Comparison of the arbitrary-ratio power splitters with this work
4. Conclusion
In conclusion, a power splitter with arbitrary ratios based on an adiabatic directional coupler is proposed and experimentally demonstrated by introducing a trapezoid structure in the adiabatic transition. With the elaborate design, we simultaneously achieve an ultra-broadband, compact size, low losses and large fabrication tolerances. By varying the width difference of the adiabatic waveguide ends, ASRs can be achieved. Experimental results demonstrate stable power splitting in the wavelength range from 1450 nm to 1600 nm for power splitting ratios of 50%:50%, 70%:30% and 90%:10%. The device footprint is greatly reduced to below 79 µm for ASRs in adiabatic couplers. Our proposed devices are simultaneously ultra-broadband, compact, low-loss, and easy fabricated, which present high prospects for applications in high-density photonic systems.
Funding
National Key Research and Development Program of China (2022YFB28030100); National Major Scientific Research Instrument Development Project (22127901); National Natural Science Foundation of China (62305367) Shanghai Sailing Program (22YF1456700).
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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