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Abstract
Slot waveguide has attracted a lot of attention due to its ability to confine light in the low refractive index region, while strip waveguide acts as the basic component of guiding light due to its relatively low optical loss. In the multifunctional photonic integrated chips, it is critical to achieve the low loss transition between the strip waveguide and the slot waveguide. In this work, a silicon nitride strip-slot mode converter with high efficiency, large bandwidth, and large fabrication tolerance are proposed and demonstrated through the numerical investigation and experiments. The coupling efficiency of the mode converter is up to - 0.1 dB (97.7%), which enables the extremely low transition loss between the strip waveguide and the slot waveguide. Moreover, the fabrication process of silicon nitride photonic devices with high performance is introduced, which is fully compatible with the CMOS technology. Photonic devices based on silicon nitride with the characteristics of the low optical loss and the temperature insensitivity represent a new paradigm in realizing silicon-based photonic multifunctional chips.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Most of the silicon-based photonic devices are based on strip waveguides, which confine the light in the high refractive index region and perform low optical propagation loss at the communication wavelength [1–5]. However, the characteristics of the strip waveguide limit the performance of some functional photonic devices, such as optical sensors [6–10] and nonlinear modulators [11, 12]. Conversely, slot waveguide confines light in the low refractive index regions leading to a large portion of the light interacts with the surrounding materials, which consists of two high refractive index regions separated by a subwavelength width of a low refractive index region [13, 14]. This remarkable characteristic can be applied to enhance performances of those special functional devices. Nevertheless, compared with the optical loss of silicon strip waveguide (∼1.5 dB/cm), the high optical propagation loss of silicon slot waveguide (∼10 dB/cm) is adverse to high-density on-chip integration. Therefore, normally the slot waveguide conducts the optical sensing/modulating functions and etc., while the low-loss strip waveguide carries out the on-chip optical transmission. However, the optical mode in the slot waveguide is a non-Gaussian-like mode [15], whereas the light mode in the strip waveguide is a Gaussian-like mode. The mode mismatch between the two kinds of waveguide leads to the low optical transition efficiency utilizing the conventional strip-slot butt-joints [16], which directly results in the performance degradation of the integrated photonic chip. Therefore, a compact and efficient strip-slot mode converter is extremely desired to connect the slot waveguide and strip waveguide. Recently several silicon strip-slot mode converters on the silicon-on-insulator (SOI) platform have been studied through introducing a sharp tip at the joint [17] or 2-fold-image multi-mode interferometer (MMI) [16].
Besides the silicon waveguide devices, silicon nitride emerges as another complementary metal-oxide-semiconductor transistor (CMOS)-compatible material with the wide spectral transparency and the low optical propagation loss [18, 19]. Moreover, silicon nitride has the advantages of a low thermal-optical coefficient (∼2.45 × 10−5 /℃) and a large fabrication tolerance compared with silicon, which has attracted considerable attention for the temperature-insensitive functional devices based on the slot waveguide, especially optical sensors [3] and nonlinear micro-ring modulators [13]. Thus, it is also crucial to achieve an efficient strip-slot mode converter on the silicon nitride platform, which is not reported yet.
In this work, we proposed and demonstrated an MMI-based strip-slot mode converter on silicon nitride platform numerically and experimentally. Firstly, the working principles of the proposed mode converter are explained thoroughly and the numerical analysis is performed. Then the fabrication process and testing of the fabricated devices is described in details. The test results show that the mode converter achieves a coupling efficiency of up to -0.1dB (97.7%), which enables the optical transition with an almost negligible loss between strip waveguides and slot waveguides. By utilizing the standard CMOS processes, this work carries forward the multi-materials and multifunctional integration of silicon-based photonic chips.
2. Working principle and design
The structure of the proposed strip-slot mode converter is shown in Fig. 1, which mainly consists of three parts: strip taper, 1 × 2 MMI region, and slot tapers. Wstrip and Wtaper are the width of the strip waveguide and the end-width of the strip taper, respectively; Wslot represents the width of the slot which is the low refractive index region located at the center of the slot waveguides and slot tapers; Wsl is the width of the rail waveguide of high-index; WMMI and LMMI are the width and length of the multimode region of MMI, while L denotes the total length of the mode converter. The whole device is based on a 400 nm-thick Si3N4 waveguide layer with a 3 μm-thick of thermal oxide layer beneath the waveguide layer and 3 μm -thick upper cladding SiO2 layer above. The thermal oxide layer is used to isolate the waveguide layer and the substrate layer to reduce the waveguide optical loss due to the light leakage from the substrate, while the upper cladding layer is used to protect the devices. And the refractive indices of Si3N4 and SiO2 are 1.98 and 1.445 at the wavelength of 1550 nm, respectively.
The strip taper structure of the input is employed to reduce the additional loss of the device. MMI is the transition region of strip taper and slot taper, which converts Gaussian-like mode light from input strip taper to a 2-fold image. Then, the slot tapers are utilized to convert a 2-fold image to the non-Gaussian-like light mode of the slot waveguide. The basic working principle of MMI is the self-imaging effect, namely an input field profile is reproduced in single or multiple images at periodic intervals along the propagation direction of the waveguide. The number of optical modes is related to the width of the MMI. Herein, the number of optical modes can be limited by adjusting the width of the MMI. According to the imaging theory, the wider width of the MMI, the more modes, leading to the higher qualify of the image, which means lower insert loss [20]. In order to ensure high quality of the 2-fold image in the core of the MMI, WMMI is chosen to be 3.5 μm through the self-image calculations. In addition, the parameters of the structure are Ltaper = 10 μm, Wstrip = 0.8 μm, Wtaper = 1.8 μm. The optical field distribution of transverse electric (TE) and transverse magnetic (TM) mode in the MMI region is shown in Figs. 2(a) and 2(b), showing that the incident optical field is reproduced and a 2-fold image is formed at the mid-way from the self-imaging length. A-A’ cut locates at the center of first 2- fold image of the MMI region. The optical field distribution of the slot waveguide with Wslot = 180 nm and the total width Wslot + Wsl + Wsl = WMMI = 3.5 μm is also performed. Figures 2(c) and 2(d) show electric field (E), magnetic field (H), and optical power (P) distributions of TE and TM mode at the first 2-fold image (A-A’ cut) of the multimode waveguide and the fundamental mode in the slot waveguide, respectively. It can be seen clearly that the power-intensity distributions of the 2-fold image and the fundamental eigenmode of a slot waveguide with the same waveguide width and mode polarization are highly similar.
Fig. 2. Optical power flow density (P) of the MMI region (WMMI = 3.5 μm) (a) TE, and (b)TM; electric field (E), magnetic field (H), and optical power distributions of cross-section of the first 2-fold image in MMI region and the eigenmodes in slot waveguide (c) TE and (d)TM.
Mode overlap radio (Γ) is utilized to quantitatively describe the similarity of optical field distribution in the MMI region and the slot waveguide, which is defined by the following formula [21, 22]:
Fig. 3. Mode overlap radio (Γ) versus the change of the width of the slot; Inset, schematic configuration of the MMI region and the slot waveguide with the same waveguide width.
The coupling efficiency (η) of the strip-slot mode converter is not only related to the mode overlap ratio discussed above, but also to the length and width of the MMI, the width of the slot, and the length of the slot taper (Lst). Typically, coupling efficiency (η) is expressed as η = 10lg (Pout/Pin). Considering the mode field distribution characteristics and general application scenarios of slot waveguide, only the strip-slot mode converter working in TE mode is performed in this numerical simulation. To evaluate the performance, the coupling efficiencies of the mode converters with different dimensions are calculated. The design of the MMI is mainly contain the width and length of the multimode region and the position of the taper. The number of optical modes and position of the 2-fold image are mainly affected by the width and length of the multimode region. Figure 4(a) shows the coupling efficiencies with the varied WMMI. When WMMI is 3.5 μm and LMMI is in the range of 6 μm and 10 μm, the coupling efficiency almost constant, which is above -0.17 dB (96%). If we narrow the width of the MMI region, the activated mode will decrease, leading to the lower quality of the 2-fold image. And if we wide the width of the MMI region, the length of the MMI region should also be lengthened to maintain the high coupling efficiency, which will enlarge the footprint of the mode converter. In addition, the coupling efficiencies with the varied slot taper lengths are simulated. As shown in Fig. 4(b), the longer the slot taper, the higher the coupling efficiency. The E intensity distribution in the strip-slot mode converter is presented in Fig. 4(c). As a small part of the E-field distributes in the middle of the slot tapers, the slot tapers do not introduce much additional loss. Thus, the long slot tapers can transform the light from the MMI region to the slot. Based on the analysis above, the dimensions of the strip-slot mode converter are ultimately determined: Ltaper = 10 μm, WMMI = 3.5 μm, LMMI = 8 μm, Lst = 20 μm.
Fig. 4. (a) Coupling efficiency with the varied LMMI (WMMI = 3, 3.5, and 4 μm, respectively); (b) coupling efficiency with the varied Lst in the wavelength range of 1500 and 1600 nm; (c) E-intensity distribution in the strip-slot mode converter.
3. Fabrication and characterization
3.1 Fabrication of the strip-slot mode converter
The strip-slot mode converter was designed and fabricated on the silicon nitride platform, using the standard CMOS technologies. Low pressure chemical vapor deposition (LPCVD) is usually adopted to fabricate the high-quality silicon nitride films due to its superiorities: less particle pollution and high uniformity of thickness. Optical waveguide devices made by LPCVD-based thin films have the advantages of lower loss, more durable, and higher temperature stability than that made by plasma-enhanced chemical vapor deposition (PECVD). However, tensile strain is introduced during the direct deposition of the silicon nitride film by LPCVD on the wafer. When the accumulated tensile strain exceeds the strength of the silicon nitride films, it even causes the wafer cracking. In this work, a high-quality 400 nm-thick Si3N4 film has been deposited using a two-step growth method and high-temperature annealing processes to release the accumulated stress of Si3N4. High-temperature annealing provides activation energy to the nitrogen atoms and the silicon atoms, leading to the atomic energy increases, therefore defects like vacancy, interstitial atoms, and dislocation set off the recombination reaction in silicon nitride film, or move to the surface of the silicon nitride film and disappear. Furthermore, the high temperature of annealing causes hydrogen atoms in the film to combine to form hydrogen gas and escape. The combined action of these two aspects makes the internal stress of the film decrease significantly. Details of these processes have been reported by our group before [23].
An 8-inch silicon wafer was prepared and the fabrication started with the 3 μm-thick thermal oxidation of the silicon substrate, as shown in Figs. 5(a) and 5(b). The thermal oxide layer is used to isolate the waveguide layer and the substrate layer to reduce the waveguide optical loss due to the light leakage from the substrate. As shown in Figs. 5(c) and 5(d), LPCVD was adopted to deposit the 200 nm-thick Si3N4 film and high-temperature annealing was operated to reduce the stresses on the film. After annealing, chemical mechanical polish (CMP) was utilized to flatten the film surface and unify the film thickness. Then the 200 nm-thick Si3N4 film was deposited by LPCVD again, followed by the lithography fabrication of photonic devices on Si3N4 film. Deep ultraviolet (DUV) lithography was accomplished to define the waveguide and device patterns on photoresist as a soft mask. Inductively coupled plasma (ICP) etching process was utilized to transfer the patterns from the photoresist layer to silicon nitride to form the designed devices, where SiO2 was used as an etch-stop layer. After the pattern transfer, the second annealing is performed to further reduce the accumulated stress in the film. Finally, 3 μm-thick silicon dioxide was deposited as upper cladding.
The top-view scanning electron microscope (SEM) image of the mode converter with Lst = 20 μm is shown in Fig. 6(a), and the zoomed-in image of the slot is shown in Fig. 6(b). The measured width of the slot is172 nm, which is close to the parameter designed. The small blocks in Fig. 6(a) are dummies, which are used to balance the pattern density to reduce the size deviation caused by over-exposure or under-exposure during the fabrication processes.
Fig. 6. (a) The top-view SEM image of the strip-slot mode converter; (b) The zoomed-in image of the slot.
3.2 Characterization of the strip-slot mode converter
A tunable laser source with a polarization controller and an optical spectrum analyzer was applied to characterize the transmission spectrum of the devices in a wavelength range between 1500 nm and 1600 nm. Since the loss of a single strip-slot mode converter is relatively low, cut-back characterization structures were applied to reduce the impact caused by system errors. Pairs of converters (from strip-to-slot and back) were arranged in series, as shown in Fig. 7(a). The laser source was coupled into the chip through the focusing grating coupler, which is shown in Fig. 7(b). Figure 7(c) presented the SEM image of the mode converter pair.
Fig. 7. (a) Layout of the cutback characterization structures (pairs of 0, 6, and 12 mode converters); (b) the SEM image of the focusing grating coupler; (c) the SEM image of the coupler pair.
The coupling efficiencies of strip-slot mode converters with different Lst are presented in Fig. 8. When Lst = 20 μm, the coupling efficiency of the mode converter is higher than - 0.25 dB (94.4%) in the wavelength range from 1500 nm to 1600 nm, and as high as - 0.1 dB (97.7%) at 1580 nm, as shown in the Fig. 8(b). Varying the length of the slot taper leads to changes in the coupling efficiencies of strip-slot mode converters. As a small part of the E-field distributes in the slot of the slot tapers, negligible loss is introduced when the mode converter transforms a strip TE-polarized fundamental mode to a slot TE-polarized fundamental mode smoothly. Therefore, the longer the slot taper, the higher the coupling efficiency, which conforms to the numerical investigation. The slight oscillation of the strip-slot coupling efficiency in Fig. 8 is caused by the slight difference of the grating coupling efficiency due to the fabrication nonuniformity. Furthermore, strip-slot mode converters with different WMMI were fabricated. The test results are shown in Fig. 8(b), indicating that the coupling efficiency of strip-slot mode converter with WMMI = 3.5 μm is higher than that of the one with WMMI = 3.6 μm and the coupling efficiencies of both converters remain above - 0.36 dB (92%) in the range from 1500 nm to 1600 nm. Furthermore, a comparison of experimentally realized mode converters was presented in Table 1. Benefiting from the large fabrication tolerance of MMI and modest refractive index of Si3N4, the proposed strip-slot mode converter has a high coupling efficiency and a large process tolerance.
Fig. 8. (a) Normalized transmission spectra of series of 0, 12, 24 mode converters (pairs of N = 0, 6, and 12 converters); (b) Coupling efficiency of strip-slot mode converters with different (Lst = 10, 15 and 20 μm) in the wavelength range from 1500 to 1600 nm; (c) coupling efficiencies of strip-slot mode converters with different WMMI (WMMI = 3.5 and 3.6 μm).
Table 1. Comparison of experimentally realized mode converter
It is worthwhile to indicate that even though all the numerical investigations and experimental measurements are focusing on TE mode, the proposed strip-slot mode converter is also appropriate for TM mode, which can be inferred from the high similarity of the optical field distribution. And it has been demonstrated on Si platform [16]. Therefore, the proposed design can be used for the devices operating in TE and TM mode. Also, the dimensions of the mode converter can be optimized to make it appliable for the optical communication window of λ = 1310 nm.
4. Conclusions
We have designed, simulated, and fabricated an MMI-based strip-slot mode converter on the silicon nitride platform. Both numerical investigations and experimental results suggest the mode converter can achieve a coupling efficiency of up to -0.1 dB (97.7%). In addition, it has the advantages of wavelength-insensitivity, polarization-insensitivity, and large fabrication tolerance. All these superiorities make the proposed mode converter an ideal solution for coupling light between strip waveguide and slot waveguide, allowing for the extensive employment of slot waveguides in multifunctional applications. Also, the fabrication process of high quality silicon nitride films was presented, which is fully compatible with the existing standard CMOS process platform. The investigation in the silicon nitride waveguide device promotes the multi-materials and multifunctional integration of the silicon-based photonic chips.
Funding
National Natural Science Foundation of China (61904196).
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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