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Abstract
A high-performance optical filter is proposed and realized with multimode waveguide grating (MWG) and two-mode multiplexers on the x-cut lithium-niobate-on-insulator (LNOI) platform for the first time, to the best of our knowledge. The present optical filter is designed appropriately to avoid material anisotropy as well as mode hybridness, and has a low excess loss of 0.05 dB and a high sidelobe suppression ratio (SLSR) of 32 dB in theory with Gaussian apodization. The fabricated filters show a box-like response with 1-dB bandwidth of 6–23 nm, excess loss of ∼0.15 dB, sidelobe suppression ratio of >26 dB. The device performance is further improved with a sidelobe suppression ratio as high as 48 dB and a low excess loss of ∼0.25 dB by cascading two identical MWGs.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Lithium niobate (LN) occupies a very important role in the field of integrated photonics due to its excellent electro-optic effect (∼34 pm/V), ultra-wide transparency window (350–5000 nm) and superb nonlinear effect among numerous photonic integrated circuits (PICs) platforms. In particular, with the breakthrough of thin-film LN (TFLN) preparation technology and the development of LN etching technology, a series of notable photonic devices has been reported on the TFLN platform [1], including ultra-high-performance modulators [2–5], broadband frequency comb sources [6–9], record-breaking wavelength converters [10–12], and micro-cavity resonators [13–15], as well as polarization-handling devices [16–18]. However, there is still an absence of practical, high-performance passive photonic devices on the TFLN, especially for optical filters, which will pose challenges for realizing scalable and complex lithium niobate PICs.
As it is well known, an optical filter is one of the most crucial passive devices in various optical systems, including wavelength-division-multiplexing (WDM) systems to expand the bandwidth of existing fiber networks [19]. Generally speaking, on-chip optical filters can be realized by using various promising waveguide structures, such as micro-ring resonators (MRRs) [20,21], arrayed-waveguide gratings (AWGs) [22,23], cascaded Mach-Zehnder interferometers (MZIs) [24], waveguide Bragg gratings [25–28]. Recently, optical filters based on subwavelength gratings (SWG) waveguides have been demonstrated with impressive performances [29–31]. As mentioned above, people have proposed and realized various silicon photonic filters with excellent performances. In contrast, the situation becomes very different when using LN-on-insulator (LNOI) waveguides instead of silicon photonic waveguides because of the material anisotropy. As it might be noticed, the x-cut LNOI is commonly used for applications in high-speed modulation and quasi-phase matched nonlinear interactions. For the x-cut LNOI, there might be some significant polarization crosstalk related to the mode hybridness when the light goes through a waveguide bend [13,32]. As a result, it is usually difficult to implement optical filters which usually consist of curved structures, such as AWGs [22,23], cascaded MZIs [24] and MRRs [20,21]. A possible solution is avoiding introducing any curved structures. For example, a coarse wavelength-division multiplexing (CWDM) optical filter based on an angled multimode interferometer (AMMI) on LNOI has been demonstrated successfully [33]. However, this AMMI-based optical filter operates without a flat-top spectral response, the sidelobe suppression ratio is only 18 dB and has a large footprint, which is hard to scalable. More importantly, its performance is difficult to be further improved, which makes it very limited in practical applications. As an alternative, several waveguide Bragg grating filters have been proposed with LNOI optical waveguides [34,35]. However, they are unavailable for light adding-dropping because there are only two ports, and thus it is hard to be cascaded for realizing multi-channel WDM applications.
Recently, silicon multimode waveguide gratings (MWGs) have been attracting a lot of interests due to the flexible wavelength selectivity, ultra-large free spectral ranges (FSRs) and box-like spectral responses [25]. In addition, with the help of mode (de)multiplexers, no circulator is required to separate the incident and reflected signals, which is totally different from those conventional singlemode Bragg gratings (SMG) and provides a very attractive option for optical filtering. Unfortunately, when using LNOI waveguides, the situation becomes very different because of the significant LN anisotropy and the possible strong mode hybridness in LNOI waveguides with angled sidewalls caused by the fabrication technology [36], which is totally different from silicon photonic waveguides. Consequently, the idea of designing silicon MWGs can not be copied to the LNOI case directly and one has to make the design very carefully.
In this paper, we propose a high-performance optical filter with an MWG and two-mode multiplexers on the x-cut LNOI platform for the first time. Here the present optical filters based on x-cut LNOI are realized successfully without worrying about the material anisotropy by utilizing the fact that an MWG is a kind of straight structure without bending section in the grating region. More importantly, the mode hybridness region due to the angled sidewalls can also be excluded in the MWG structures by carefully selecting the parameters. In particular, the Gaussian apodization profile is used to achieve a high sidelobe suppression ratio. The MWG is optimized by varying its main features which lead to obtain an optimum set of parameters and allows us to achieve the best performance for optical filtering. The mode (de)multiplexers used in this paper is based on an adiabatic dual-core taper, which enables efficient and broadband mode evolution between the TE1 mode in the wide waveguide and the TE0 mode in the narrow drop waveguide, as demonstrated successfully for on-chip mode (de)multiplexers on silicon. For the fabricated devices, the measurement results show that the spectral response at the drop port has a low excess loss of ∼0.15 dB, a large 1 dB-bandwidth of 6-23 nm, as well as a high SLSR of >26 dB. Meanwhile, the spectral response at the through port has a high extinction ratio of >36 dB at the center wavelength, which is realized by increasing the corrugation depth. The SLSR is further improved to as high as 48 dB by cascading two identical MWGs.
2. Design and simulation
Here the waveguide structure is patterned on an x-cut 400-nm-thick LNOI with an etching depth of 200 nm and an air upper-cladding. The sidewall angle is set to be θ = 72° according to the state of the fabrication technology in our lab, as shown in Fig. 1. Figure 1(a) shows the schematic configuration of the present optical filter on x-cut LNOI, which is composed of an asymmetric MWG and a mode (de)multiplexer based on an adiabatic dual-core taper (waveguides A and B). Here, all the structure is designed to be with the transverse-electric (TE) polarization and light propagates along the y-direction in order to be better integrated with high-speed modulators on the same chip in the future [4].
Fig. 1. Schematic configurations. (a) The proposed on-chip optical filter on x-cut LNOI. It consists of an MWG and a mode (de)multiplexers based on an adiabatic dual-core taper coupler. (b) The mode (de)multiplexer based on an adiabatic dual-core taper. (c) The cross-section of the LNOI waveguide with some key parameters labeled. (d) Calculated dispersion curves of an LNOI nanowire with He = 200 nm, HLN = 400 nm, θ = 72° and air cladding when operating wavelength at 1550 nm. The dashed circle indicates the mode hybridness region. (e) Simulated mode profiles of the TE0 and TE1 modes.
When operating at the Bragg wavelength λB, the launched optical signals carried by the TE0 mode is reflected and converted to the TE1 mode by the MWG according to the grating equation, which is given by [19]
where neff0 and neff1 are respectively the effective indices of the TE0 and TE1 modes in the MWG, and Λ is the grating period. Then the mode demultiplexer converts the reflected TE1 mode to the TE0 mode at the drop port finally, as shown in Fig. 1(a). For those wavelengths far from the Bragg wavelength, the grating equation is not satisfied, so that the input TE0 mode passes through the MWG with ultra-low losses. In this way, the MWG works as a pass-band filter with a central wavelength around the Bragg wavelength λB.According to the coupled mode theory, it is known that the reflection R and the bandwidth Δλ of the filter are related to the grating geometry and the grating length L, i.e., [37]
In our design, the core width W is kept being 2 µm to avoid higher-order mode excitation and keep away from the mode hybridness region between the TE1 and TM0 modes according to the dispersion curves of LNOI waveguides shown in Fig. 1(d). The corrugation is achieved by offsetting the core region from the central axis by ±0.5Δδ. Here the parameter Δδ is given by the following Gaussian function [38]
where b is the apodization index, N is the total number of the MWG periods, δ is the maximum offset (when i = N/2) for the MWG. In this way, the coefficient can be modified longitudinally and the sidelobe suppression ratio of the MWG can be improved.Definitely, the performance of the optical filter is determined by the MWG parameters, including the multimode waveguide width W, the grating period Λ, the corrugation width δ and the grating number N. In particular, the corrugation depth δ of the MWG affects the coupling coefficient k, and consequently the corrugation depth δ is mainly related to the bandwidth according to Eq. (2). On the other hand, the center wavelength is mainly decided by the grating period Λ. Here, the three-dimensional finite-difference time-domain (3D-FDTD) method with the boundary condition of perfectly-matched layer (PML) was used for simulating the light propagation in our structures. The mesh type is auto-nonuniform and the mesh accuracy level is three (correspondingly the mesh size at the waveguide interfaces is about 40 nm). Figures 2(a) and 2(b) show the simulated results at the drop and through-port of the MWG, respectively, when choosing different corrugation depths of δ = 400, 600 and 800 nm. Here, the other key parameters are given as N = 500, b = 10 and Λ = 444 nm. It shows that the 1-dB bandwidth of the MWG increases greatly. For example, the simulated 1 dB-bandwidths for the designs with δ = 400, 600 and 800 nm are 6, 18 and 27 nm, respectively. The center wavelength has a blueshift as the corrugation depth δ increases due to the reduction of the effective indices, while the SLSRs are kept to be higher than 30 dB at both sides of the passband. More importantly, the designed MWG has a box-like spectral response with a flat top and sharp roll-off. From Fig. 2(b), it can be seen that the extinction ratio at the through port is improved significantly and the bandwidth of the MWG increases greatly as the corrugation depth δ increases due to the addition of the coupling coefficient k, which agrees very well with the prediction from Eq. (2). For example, the bandwidth is as large as ∼30 nm and the extinction ratio at the through port is about 60 dB at the central wavelength when choosing δ = 800 nm. Figure 2(c) shows the simulation results of the MWG with different grating periods Λ. Here the corrugation depth δ is set as 500 nm. Obviously, the larger the period is, the larger redshifted of the center wavelength will be. For the present case, the center wavelength is about shifted by 3 nm as the grating period is varied by 1 nm, which agrees very well with the prediction from Eq. (1). On the other hand, the 1-dB bandwidth and the SLSR are insensitive to the period variation. For the present design, the 1-dB bandwidth is about 13 nm and the SLSR is about 32 dB.
Fig. 2. Simulation results of the optical filter. (a),(b) Simulated spectral responses of the MWG with different corrugation depths δ. TE0 mode is launched into the MWG, (a) is the TE1 mode power monitored at input port, (b) is the TE0 mode power monitored at through port. (c) Simulated spectral responses of the MWG with different periods Λ. TE0 mode is launched into the MWG, (c) is the TE1 mode power monitored at input port. (d) Simulated results for the transmissions of the designed mode (de)multiplexer; Inset: light propagation in the designed adiabatic dual-core taper when operating at wavelengths of 1550 nm.
The mode (de)multiplexer shown in Fig. 1(b) is designed according to the method proposed in our previous work where an adiabatic asymmetric directional coupler (ADC) was used with the principle of adiabatic mode evolution enabling a larger fabrication tolerance and bandwidth [39]. The core widths at the input/output ends of waveguides A and B for the adiabatic couplers are chosen as (wa1, wa2) = (1, 2) µm, (wb1, wb2) = (0.6, 0.2) µm, the taper length (L01, L12, L23) = (50, 100, 50) µm and the gap widths (wg1, wg2, wg3) = (2, 0.25, 2) µm. Figure 2(d) shows the calculated transmission of the backward TE1 mode launched from the right side. It can be seen that the designed mode (de)multiplexer has a low excess loss of 0.05 dB and a high extinction ratio (ER) of more than 20 dB in the wavelength band from 1520 nm to 1600 nm. We also simulate the light propagation in the designed mode (de)multiplexers when operating at the wavelengths of 1550 nm, as shown in the inset in Fig. 2(d). It can be seen that the backward TE1 mode launched from the right side of waveguide A can be converted very efficiently to the backward TE0 mode at the left side of waveguide B.
3. Fabrication and characterization
The designed optical filters based on MWGs were fabricated on an x-cut LNOI wafer from NANOLN, where the nominal thickness of the LN layer is 400 nm and the thickness of the buried-dioxide layer is 3µm. We first patterned the device structure using electron-beam lithography (EBL) and then the LN layer was etched with Ar+ plasma to form the waveguide structure. The microscope image of the fabricated optical filter is shown in Fig. 3(a). Figure 3(b) and Fig. 3(c) exhibit the scanning electron microscopic (SEM) images for the MWG and the grating coupler, respectively. Grating couplers for TE polarization were used to achieve efficient chip-fiber coupling for the convenience of measurement. A broad-band amplified spontaneous emission (ASE) light source and an optical spectrum analyzer (OSA) was applied to measure the output wavelength response. By launching light from the input port and monitoring the transmissions at the through/drop port, the spectral responses of the MWG-based optical filter can be characterized conveniently. The measured results were normalized with respect to the transmission of a 1 µm-wide straight waveguide connected with grating couplers on the same chip.
Fig. 3. (a) Microscope image of the fabricated MWG-based optical filter on LNOI. SEM images of the MWG (b) and the grating coupler (c).
Figure 4(a) shows the measured spectral responses at the drop port and through port of the fabricated optical filter. Here the device structure is with the following parameters: Λ = 435 nm, δ = 500 nm, and N = 500. From the measured results shown in Fig. 4(a), it can be seen that the box-like spectral response was obtained at the drop port of the MWG. The excess loss of the fabricated optical filter is as low as ∼0.15 dB and the SLSR is as high as ∼26 dB in a very broad band. Moreover, the spectrum shows a very flat-top response with a 1 dB-bandwidth of 13 nm. The spectral response of the through port has a high extinction ratio of 36 dB at the center wavelength. Figure 4(b) shows the measured spectral responses at the drop port of the fabricated filter with different corrugation depths of δ = 400, 600 and 800 nm, respectively. It can be seen that the variation of the corrugation depth influences significantly on the bandwidth of the filter, as predicted theoretically [see Figs. 2(a) and 2(b)]. For example, for the devices with δ = 400, 600 and 800 nm, the measured 1 dB-bandwidths are 6, 18 and 23 nm, respectively. One can also achieve a narrower bandwidth by decreasing the corrugation depth δ properly while increasing the periods number N to maintain a low loss.
Fig. 4. Measurement results of the fabricated optical filter. (a) Measured spectral responses at the drop ports and the through port when light is launched from the input port. Here, the waveguide width W = 2 µm, the period number N = 500, the grating length L = 220µm, the corrugation depth δ = 500 nm and the grating period Λ = 435 nm. (b) Measured spectral responses of the fabricated filter with different corrugation depths δ = 400, 600 and 800 nm. (c) Measured spectral responses at the drop port and (d) the center wavelength of the fabricated filters with different grating periods of Λ = 435, 443 and 450 nm.
In order to realize multi-channel optical filters, a simple approach is setting the MWGs designed with different grating periods Λ. Figure 4(c) shows the measured spectral responses at the drop ports for the fabricated MWGs with Λ = 435, 443 and 450 nm. It can be seen that the center wavelength has a red shift Δλ as the period increases. The wavelength shift is about +42 nm when the grating period Λ increases from 435 nm to 450 nm, which is very similar to the simulation results shown in Fig. 2(c). All three filters can obtain almost uniform spectral responses with an excess loss of ∼0.15 dB, 1 dB-bandwidth of 13 nm and an SLSR of ∼26 dB. One might notice that the SLSR of the third period filter is relatively low (∼20 dB), which is probably due to the random fabrication imperfection.
In order to further improve the performance, we design and demonstrate a device with two identical MWGs cascaded, as shown in Fig. 5(a). Figure 5(b) shows the measured spectral responses at the drop ports. As expected, it can be seen that the SLSR of the filter obviously raises to 48 dB (almost twofold higher than the single optical filter) in a broad wavelength band from 1530 nm to 1605 nm. Meanwhile, the excess loss is still as low as 0.25 dB in passband. These experimental results indicate that this MWG-based optical filter could been very flexible to extend to multi-stage and large-scale architecture, while keep operating in high performance. Although cascading more MWGs will improve the SLSR obviously, the excess loss will increase slightly and the footprint will be large. In most cases, 30 dB SLSR is well enough, one should make their choices according to the situation.
Fig. 5. LNOI optical filter based on two MWGs in cascade. (a) Schematic configurations. (b) Measured spectral responses at the drop port.
Table 1 gives a summary of the state-of-the-art optical filters on LNOI. Among the results reported, it can be seen that the present MWG-based LNOI optical filter have the highest SLSR (∼48 dB) and lowest excess losses (∼0.25 dB). It is also worth noting that, the present MWG-based LNOI optical filter does not require any circulator to separate the reflected and incident light, which is convenient to be integrated with any other photonic devices on a single chip.
Table 1. State-of-the-art optical filters based on LNOI platform
4. Conclusion
In summary, we have designed, fabricated, and characterized the MWG-based optical filter on x-cut LNOI for the first time. The present optical filter shows a broad 1dB-bandwidth and a high SLSR in theory and in experiments. For the fabricated optical filters, the spectral response at the drop port is box-like with a low excess loss of ∼0.15 dB, a large 1 dB bandwidth of 6∼23 nm, and a high SLSR of ∼26 dB. The SLSR can be improved to 48 dB by cascading two identical MWGs at the drop port while the excess loss is about 0.25 dB. The bandwidth can be designed very flexibly as desired, which makes it available widely in many applications, including coarse WDM systems and on-chip spectrometers. Such a high-performance optical filter is expected to provide an indispensable building block for the realization of multi-functional and large-scale PICs on LNOI, which paves the way to practical applications of LNOI photonics in the future.
Funding
National Major Research and Development Program (2018YFB2200200, 2018YFB2200201); National Science Fund for Distinguished Young Scholars (61725503); National Natural Science Foundation of China (61961146003, 62005239, 62135010, 91950205); Natural Science Foundation of Zhejiang Province (LD19F050001, LQ21F050006, LZ18F050001); Fundamental Research Funds for the Central Universities.
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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