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Abstract
Optical power splitters (OPSs) have been widely used in photonic integrated circuits, but an OPS with a large fabrication tolerance and free choice of power splitting ratio (PSR) is still highly desired for thin-film lithium niobate (TFLN) platform. Here, we propose and experimentally demonstrate several 1 × 2 OPSs with PSRs from 50:50 to 5:95 using TFLN platform. The proposed devices are built by multimode interference structure to achieve a broad bandwidth and large fabrication tolerance. Various PSRs can be obtained by adjusting the geometry structure of the multimode interference region. All of our fabricated devices feature an insertion loss lower than 0.3 dB at the wavelength of 1550 nm, and a PSR variation less than 3% in the range of 1520 nm to 1590 nm.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Thin-film lithium niobite (TFLN) [1–6] have recently emerged as a promising platform for future on-chip optical communication systems owing to its wide optical transparency window, excellent electro-optical (EO) property, and large nonlinear optical coefficient. Various high-performance photonic devices have been realized, including EO modulators [7–9], microring resonators [10–12], frequency combs [13–16], polarization management devices [17–19]. However, optical power splitters (OPSs) featuring free choice of power splitting ratio (PSR) and excellent fabrication tolerance based on TFLN platform still need to be studied.
Normally, there are two types of methods to realize OPSs. One is based on directional coupler (DC). DC [20,21] is one of the most common OPSs and can obtain an arbitrary PSR. However, it usually suffers from wavelength sensitivity and has tight fabrication tolerance, which strongly limit its applications. The other is multimode interference coupler (MMI). OPSs based on MMI [22–30] have been widely studied for their large bandwidth and good fabrication tolerance. A wide range of MMIs based on TFLN platform have been reported [9,31–34], but most of them are 3-dB MMI, which separates the optical power uniformly into two outputs. In practical applications, OPSs with different PSRs attract more interest. For example, only a small amount of light is required for on-chip signal feedback or monitoring [35,36]. Therefore, MMIs with free-choice PSRs are highly desired. Although the arbitrary-ratio OPSs using asymmetric MMI have been reported for silicon-on-insulator platform [22], such OPSs for TFLN platform remain elusive.
In this paper, we proposed and demonstrated broadband and low-loss 1 × 2 OPSs with arbitrary PSRs using TFLN platform. The proposed devices possess asymmetric multimode interference region. The PSR of the devices can theoretically cover from 50:50 to 0:100. Both simulated and experimental results show that the insertion loss (IL) of the devices is less than 0.3 dB at the wavelength of 1550 nm and the PSR variation is less than 3% in the range of 1520 nm to 1590 nm. Furthermore, the proposed devices have good fabrication tolerance and are compatible with the fabrication processes of popular high-performance TFLN photonic devices.
2. Architecture and design
The schematic diagram of the proposed device is shown in Fig. 1(a). The device is designed on an X-cut TFLN with a 360-nm-thick LN layer and a 4.7-µm-thick buried oxide layer. The waveguides are ridge-type with a rib height of 180 nm and a slab thickness of 180 nm. The upper cladding is SiO2 with a thickness of 1 um to protect the device. The proposed device can be divided into three parts: input single mode region (I), asymmetric multimode interference region (II), and output single mode region (III). Figure 1(b) shows the corresponding top-view version. The proposed device features an asymmetric multimode interference region, which is obtained by removing the top left corner of the multimode interference region. The removed part is rectangular with the length and width of Lr and Wr, respectively. Here, we denote P1 and P2 as the optical power output from Output1 and Output2, respectively, and the PSR is given by
In the case of Lr = 0 µm, the proposed device is symmetric, and it is equivalent to a 3-dB 1 × 2 MMI. According to the self-imaging principle [37], two-fold images of the input TE0 mode will occur when the length of the multimode interference region (L) satisfies the following condition
Fig. 1. (a) Schematic diagram of the proposed device. I: input single mode region. II: asymmetric multimode interference region. III: output single mode region. (b) Top-view version of the proposed device. Input, the input port; Output1 and Output2, the output ports; L, W, Lr, Wr, and Wgap are the geometrical parameters of the device. (c)-(d) Simulated electric field intensity distributions of the proposed device for the cases that Lr = 0 µm and Lr ≠ 0 µm, respectively. 1.0, 0.5, 0.0, normalized electric field intensity. The white outline represents the boundary of the device.
The N is a nonnegative integer and Lπ is the beat length of the two lowest-order modes in the multimode interference region. Optimizing the length (L) and width (W) of the multimode interference region, an 1 × 2 OPS with PSR of 50:50 and low IL can be obtained. As shown in Fig. 1(c), in this case, the electric field intensity distribution of the device is symmetrical. To achieve a device with other PSR, we need to break the symmetry of the structure. Properly removing the top left corner of the multimode interference region (see Fig. 1(b)), i.e., Lr ≠ 0 µm, is a good solution. Though the removed corner contains little optical field, the resulting asymmetric structure will cause a significant change in the field distribution [22,37]. As shown in Fig. 1(d), when Lr ≠ 0 µm, the electric field intensity distribution is asymmetrical and clearly different from that in Fig. 1(c). Since the top left corner of the multimode interference region is removed here, the optical power of Output1 is lower than that of Output2. To ensure low-loss devices and maintain mode matching from the single mode region to the multimode interference region, two specific values of Wr, i.e., 1.5 µm and 2 µm, were chosen in combination with the electric field intensity distribution (see Fig. 1(c)). As Lr increases, the asymmetry within the multimode interference region becomes stronger for a given Wr, resulting in a more uneven power splitting. Hence, by performing a straightforward sweep of Lr based on this rule, it is possible to achieve OPSs with any desired PSRs while maintaining the desired characteristics.
Therefore, we can obtain devices with low IL and arbitrary PSR by simultaneously optimize Lr, Wr and L. The Lumerical 3D finite-difference time-domain (FDTD) solver is used to perform rigorous optimization of the structure. As shown in Fig. 1(b), the input and output waveguides are adiabatic tapers of the same dimension. In order to ensure low transmission loss, the length of the tapers is 80 µm, and the width is changed from 4.5 µm to 0.8 µm. The waveguide with 0.8 um width here supports only single-mode transmission. In addition, W is set to 9.6 µm to support the desired high-order modes. For the gap between the two output waveguides (Wgap), it is set between 0.3 µm and 0.6 µm to allow more optical power transmitted to the output waveguides.
Overall, by changing the values of Wr, Lr, and L, we can obtain 1 × 2 OPSs with arbitrary PSRs between 50:50 and 0:100. Here, the devices with PSRs of 50:50, 40:60, 30:70, 20:80, 10:90, and 5:95 are experimentally demonstrated, and their corresponding values of Wr, Lr, and L are listed in Table 1. It is important to note that the proposed device in this work is specifically designed for the TE0 mode and is not intended for operation in the TM0 mode.
Table 1. The values of Wr, Lr, and L used in the simulation and fabricated devices.
3. Fabrication and measurement
The proposed devices were fabricated on a commercial X-cut lithium niobate-on-insulator (LNOI) wafer using electron beam lithography (EBL) and inductively coupled plasma (ICP) dry etching. The scanning electron microscopy (SEM) images of the devices with PSRs of 50:50, 40:60, 30:70, 20:80, 10:90, and 5:95 are shown in Fig. 2(a)-(f), respectively.
Fig. 2. Scanning electron microscopy (SEM) images of the proposed devices. (a)-(f) correspond to devices with power splitting ratios of 50:50, 40:60, 30:70, 20:80, 10:90, and 5:95, respectively. Lr represents the length of the removed region. (g) Schematic of the design layout for test. In, the input port; Out1 and Out2, the output ports.
A tunable laser (New focus TLB-8800) and a photodetector (Thorlabs PM100D) were used to characterize the performance of the fabricated devices. In our study, grating couplers (GCs) were employed as optical input and output interfaces for all the devices. Additionally, we prepared multiple pairs of back-to-back GCs to calibrate the coupling-induced loss. To accurately determine the PSR and IL of the devices, we cascaded multiple devices in series. This approach allowed us to obtain precise and reliable measurements. As shown in Fig. 2(g), several devices cascaded together along each output of the proposed OPSs (i.e. weaker-to-weaker and stronger-to-stronger concatenation). The number of cascaded devices, denoted as n, plays a crucial role in improving the measurement accuracy by compensating for fabrication and measurement errors. However, it is important to appropriately adjust n, especially for weaker outputs, to ensure that the output power falls within the measurement range of the photodetector. For the cascade of outputs with higher optical power (Output2), we chose a fixed value of n = 8 for all PSRs. This selection allows for accurate measurements while maintaining sufficient output power. For the cascade of outputs with lower optical power (Output1), different values of n were employed for different PSRs. Specifically, we used n = 2 for a PSR of 5:95, n = 3 for a PSR of 10:90, n = 4 for a PSR of 20:80, and n = 5 for PSRs of 30:70 and 40:60. For the device with a 50:50 PSR, only one cascade with n = 8 was required due to its symmetrical structure. Inverse tapers are added to prevent reflection from the ports without output. Moreover, to make sure the experimental results are reliable, several identical cascade devices were fabricated for each PSR and the average of their measured results were taken as the final results. The following results are all obtained by measuring on a same wafer.
Figure 3(a) presents the PSRs of the proposed devices varying with Lr at the wavelength of 1550 nm. The dashed black line and solid red line are the simulated and measured results, respectively. Increasing Lr, the PSR of the devices changes from 50:50 to 5:95. It can be observed that the simulated and measured results of the proposed devices are in good agreement. The slight difference between the two results is mainly caused by the fabrication imperfection. The first two-fold image gradually moves further away while Lr increases [22,37], i.e., the total length (L) of the multimode interference region becomes larger, as shown in Table 1. This is also confirmed by the simulated electric field intensity distributions of devices with PSRs of 50:50, 30:70, and 5:95 plotted in Fig. 3(b)-(d).
Fig. 3. (a) Simulated (dashed black line) and measured (solid red line) power splitting ratios (PSRs) varied with Lr at the wavelength of 1550 nm. Lr represents the length of the removed region. (b)-(d) Simulated electric field intensity distributions for devices with PSRs of 50:50, 30:70, and 5:95, respectively. 1.0, 0.5, 0.0, normalized electric field intensity. The white outline represents the boundary of the device.
Figure. 4(a) shows the PSRs of the proposed devices as functions of wavelength from 1520 nm to 1590 nm. The ripple of the PSR is caused by the measurement setup, such as vibration. In the wavelength range of 70 nm, the variation of the PSRs is less than 3% in both simulated (dashed lines) and measured (solid lines) results, indicating that the proposed devices can work well over a wide wavelength range. For the proposed structure, the extra loss caused by removal is small since the energy distribution in the removed region is weak. The IL of the proposed devices with different PSRs in the range of 1520 nm to 1590 nm is presented in Fig. 4(b). Although the IL of the devices with different PSRs are smaller than 0.25 dB in the simulation, the IL of the experimental results has a higher value because of the measurement error. To achieve further reduction in IL for the devices, one possible approach is to rotate the output waveguide of Output1 in a counterclockwise direction. This rotation allows for capturing more optical power from the multimode interference region, thereby reducing IL. However, it is important to note that this rotation may introduce additional fabrication tolerances and complexities.
Fig. 4. (a) Power splitting ratio (PSR) as functions of wavelength from 1520 nm to 1590 nm. Lr represents the length of the removed region. The dashed and solid lines represent simulated and measured results, respectively. (b) Insertion loss of the devices with different PSRs in the range of 1520 nm to 1590 nm.
The fabrication tolerance of the proposed OPSs is also considered. A good fabrication tolerance of OPSs is required since fabrication imperfection is unavoidable [33,38,39]. Here, the fabrication tolerance of the devices is numerically analyzed by changing the etching depth (H) and the length of the removed region (Lr). The proposed devices are designed with an etching depth of 180 nm. In the simulation, the total thickness of the LN layer is maintained at 360 nm, while the etching depth is allowed to vary within a range of ±20 nm, in the other word, the slab thickness is also varied within a range of ±20 nm. This variation range accounts for potential fluctuations in the etching process. ${{\partial \textrm{PSR}} / {\partial \textrm{H}}}({{{({5\textrm{nm}} )}^{ - 1}}} )$ represents the variation of the PSR per 5 nm change of the H. The derivative in Fig. 5(a) indicates that the maximum variation of the PSRs is 0.265% when H has a variation of 5 nm. In micro- and nanofabrication processes, fabrication errors are usually less than 50 nm. ${{\partial \textrm{PSR}} / {\partial {\textrm{L}_\textrm{r}}}}({{{({50\textrm{nm}} )}^{ - 1}}} )$ represents the variation of the PSR for a change of 50 nm in Lr. The simulation results in Fig. 5(b) show that the variation of the PSRs is less than 0.15% when Lr has a variation of 50 nm. A fabrication tolerance analysis of Wr is also performed. The simulation results show that the variation of PSR of the device with a PSR of 5:95 is about 0.35% for every 50 nm change of Wr. The above results demonstrate that the proposed devices have good fabrication tolerance under the existing fabrication conditions.
Fig. 5. Fabrication tolerance analysis of the proposed devices with different power splitting ratios (PSRs). (a) Derivative of the PSR versus the etching depth (H) for devices with different PSRs. The inset shows the cross-section of the ridge waveguide. H is the etching depth. The total thickness of the LN layer is fixed at 360 nm. (b) Derivative of the PSR with respect to the length of the removed region (Lr) for devices with different PSRs. The inset illustrates the top view of the proposed device. Lr represents the length of the removed region.
4. Conclusion
In this paper, broadband and low-loss 1 × 2 OPSs with arbitrary PSRs using TFLN platform are experimentally demonstrated. The PSR can be easily controlled by adjusting the geometry structure of the multimode interference region. Both simulated and experimental results show that the IL of the proposed devices is lower than 0.3 dB at the wavelength of 1550 nm. Moreover, the PSR variation is less than 3% in the range of 1520 nm to 1590 nm. Further numerical analysis results indicate that the devices feature good fabrication tolerance. Although our proposed concept does not offer continuous tunability, it is particularly valuable in applications, which only require a fixed PSR, such as optical coherence tomography [40], laser detection and ranging [41], and polarization control [17]. In these applications, OPSs with fixed PSRs are utilized to divide a small amount of light for monitoring, referencing, or feedback purposes. Despite implementing a concept previously reported for the silicon-on-insulator platform [22], the realization of the proposed devices presents significant challenges, stemming from various factors, including the anisotropy of the LN material, a reduced refractive index contrast between the core and cladding, and the implementation of a ridge waveguide. Overall, the proposed devices show promising characteristics, making them good candidates for TFLN platform to implement functions such as on-chip power distribution and signal feedback.
Funding
National Key Research and Development Program of China (2019YFA0705000); National Natural Science Foundation of China (62293523); Innovation Program for Quantum Science and Technology (2021ZD0301500); China youth fund of national natural science foundation projects (52208057); The Guangdong Provincial Natural Science Foundation General Project (2023A1515011191).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
Data underlying the results presented in this paper may be obtained from the authors upon reasonable request.
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