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Abstract
Lithium niobate on insulator (LNOI) is a new photonic integrated platform that provides high optical confinement and retains the inherent excellent properties of lithium niobate (LN). Tunable filters are one of the indispensable devices for integrated optics. Here we design and fabricate a thermo-optic (TO) tunable optical filter using two cascaded racetrack microring resonators (MRRs) based on LNOI. The filter shows a narrow and flat top passband with intra band ripple less than 0.3 dB, 3 dB bandwidth of 4.8 GHz and out-of-band rejection of about 35 dB. The insertion loss of the filter is about −14 dB, including grating coupling loss about −6.5 dB and on-chip loss less than −1 dB. The heating power for center wavelength shift of the filter is about 89.4 mW per free spectral range (FSR). Relevant applications of such filters include optical information processing and microwave photonics.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
In recent years, thin-film lithium niobate on insulator (LNOI) is emerging as a promising photonic integrated platform which provides high optical confinement and inherits the attractive material properties from LN, such as excellent electro-optic, nonlinear-optic and acousto-optic properties, as well as large transparency window. With LNOI wafers becoming commercially available, significant effort and investment has been put for integrated optical applications. Currently, a series of high performance integrated functional devices have been demonstrated on LNOI [1–7], showing great potential applications in optical communications, nonlinear optics and microwave photonics. Among them, tunable optical filters are one of the indispensable devices for optical information processing. With the advance of optical communication system and the vigorous development of microwave photonics, higher requirements for the filtering of narrowband signals have been put forward. For example, in a reconfigurable optical add and drop multiplexer, the narrow bandwidth optical filters on the order of GHz or even MHz are required for high resolution [8]. Moreover, narrow bandwidth optical filters are indispensable core devices in the microwave photonic filtering system which is designed with the aim of solving problems that are difficult to solve in electric domain and bring supplementary advantages inherent to photonics such as small volume, low loss, high bandwidth and strong electromagnetic immunity [9].
The response of an ideal bandpass filter is box-like shaped with a flat top passband, steep band edges and a large out-of-band rejection ratio. Recent advances in narrowband filters have adopted the structures of Bragg gratings [10–13]. Optical filters based on high-order microring resonators (MRRs) have been investigated for decades due to their box-like response, compact size and wide range of applications [14–17]. Currently, high performance narrowband filters based on the structure of MRRs are still lacking. However, the filtering performance of high order MRRs filters is susceptible to fabrication errors, which usually results in misalignment of resonant frequencies and degradation of the passband performance. Previous demonstrations have shown that thermal tuning is a common method for tuning silicon MRRs to eliminate frequency mismatch [18,19]. By utilization of electro-optic (EO) or thermo-optic (TO) effect of LN, actively tuning of LNOI MRRs have been successfully demonstrated [20,21]. In addition, the phase change induced by TO effect is more stable when compared to EO effect according to [2].
In this paper, we demonstrate a TO tunable optical filter using two cascaded racetrack MRRs on X-cut LNOI platform. The realignment of resonant wavelength and central wavelength tunability of the filter are demonstrated utilizing the TO properties of LN. The on-chip loss of the filter is less than 1 dB. The bandpass filter response with low intra band ripple less than 0.3 dB, narrow 3 dB bandwidth of 4.8 GHz and out-of-band extinction ratio of 34.73 dB is demonstrated. The tuning efficiency of the filter is 89.4 mW per FSR. To the best of our knowledge, this is the first demonstration of TO tunable LNOI-based MRRs filter with GHz-bandwidth and flat top passband. The demonstrated tunable filter will be an important part of LNOI versatile photonic integrated platform.
2. Design and simulation
The schematic diagram of the LNOI TO tunable filter configuration is shown in Fig. 1(a). It consists of two cascaded identical racetrack MRRs, metal micro-heaters and two bus waveguides for the through and drop ports. The symmetrical-coupled structure is adopted to maximize the output of the drop port at the resonant wavelength, i.e., the two MRRs have the same distances to its adjacent bus waveguides. To achieve the tunability of the filters, the metal micro-heaters composed of Titanium (Ti) and gold (Au) are placed on top of the uncoupling straight waveguides of the racetrack MRRs. All the designs and simulations performed in this paper are for the fundamental mode of TE polarization (TE0). The demonstrated filter is designed on the X-cut LNOI with buried silicon oxide layer thickness of 4.7 µm and top LN film thickness of 500 nm. As shown in Fig. 1(b), the etching depth hr of LN ridge waveguide is 260 nm and the width of the ridge waveguide Wr is 1.2 µm with a 72° sidewall angle of the cross section based on our previous work [22] to ensure single-mode transmission. The top cladding thickness Hc is 1 µm to realize the optical isolation between the MRR waveguides and the micro-heaters and prevent the introduction of additional optical loss. The thickness of micro-heater Ti Hh is 150 nm and the width Wh is 2.5 µm.
Fig. 1. (a) Schematic diagram of the LNOI TO tunable optical filter. (b) Cross section view of the waveguide and heater. (The schematics are not drawn to scale).
Our target is to design a bandpass filter with narrow bandwidth (3 dB bandwidth < 5 GHz), flat top passband (intra band ripple less than 0.5 dB), high out-of-band rejection ratio (>30 dB) and small rectangular factor (30 dB bandwidth/ 3 dB bandwidth <7). For the implementation of such filter, the key requirement is the reasonable design of coupling strength of the coupling region. In this paper, the coupled racetrack MRRs are adopted so that arbitrarily coupling strength can be obtained by changing the length and gap of the coupling region. Besides, the introduced on-chip loss is negligible due to the evanescent coupling between the bus waveguides and MRRs. As shown in Fig. 2(a), the length of coupling region is L1 = 102 µm. With such a relatively large coupling lengths, it is possible to achieve the same desired coupling strength even when the coupling gap is relatively large, which means relatively low requirement of lithography resolution and thus reduce the fabrication difficulty. The radius (R) is 130 µm to make bending loss negligible. The remaining uncoupling straight waveguides with length of L2 are used to place the electrodes. The length of L2 is about 450 µm, which can be adjusted for FSR consideration. The length of heating electrode is about 300 µm. The power coupling coefficient between the resonator and input/output waveguide is designed to be k02 and the power coupling coefficient between the two resonators is designed to be k12. The transmission curve of the MRRs filter is calculated using transfer matrix method (TMM) [23]. The waveguide transmission loss used in the calculation is 0.62 dB/cm as a result of our fabrication process, which is discussed in detail in Supplement 1. The effective refractive index of TE0 used in the calculation is obtained by using a finite-difference method (FDE) mode solver, which is about 1.867 at 1550 nm. The optimal response of the designed MRRs filter is obtained when the k02 and k12 are chosen to be 0.25 and 0.0256. As shown in Fig. 2(b), the calculation shows a flat top passband filter response with on-chip loss of 0.54 dB, 3 dB bandwidth of 4.6 GHz and out-of-band rejection over 35 dB. The rectangular factor is about 6. After the coupling coefficients are determined, the coupling gaps are obtained according to 3D finite-difference time-domain (FDTD) simulation of the coupling region. As shown in Fig. 2(c), the coupling coefficients can be adjusted by changing the coupling gaps. The gaps between the input/output bus waveguide and MRRs, as well as that between the MRRs are about 0.99 µm and 1.45 µm to achieve the power coupling coefficients of 0.25 and 0.0256, respectively. Besides, relatively large power coupling coefficients can still be achieved even when the gap is relatively large, which shows the relatively low requirement of lithography resolution. For example, when the power coupling coefficient is 0.44, the corresponding gap is 0.85 µm.
Fig. 2. (a) Definition of design parameters of the second-order racetrack MRRs filter. (b) Transmission curve of drop port and through port given by TMM with transmission loss of 0.62 dB/cm when k02 is 0.25 and k12 is 0.0256. (c) Simulated power coupling ratio as the coupling gap increases when choosing R = 130 µm and L1 = 102 µm. The illustration defines the coupling gap.
In practice, the power coupling coefficients deviation and dimensional changes caused by fabrication errors have detrimental impacts on the performance of high order MRRs filters. Here, the impacts of these two aspects are discussed. Firstly, the impact on the performance of filters when k02 or k12 deviates from the design value is discussed. As shown in Fig. 3(a)-(d), either k12 or k02 is fixed to design value and the remaining k02 or k12 changes. The on-chip loss of drop port increases slightly and the passband performance degradation is observed. As is shown in Fig. 3(b), when k02 is larger than the design value, a passband splitting is observed. When k02 is smaller than the design value, a non-flat passband is observed. While in Fig. 3(d), it can be seen that the passband performance is opposite when there is a deviation of k12. The 3 dB bandwidth is almost unchanged when k02 varies, whereas obvious change of 3 dB bandwidth is observed when k12 varies. Secondly, the impact on the performance of filters when the radii of the two MRRs are slightly different is analyzed. The radius of the ring can be affected by fabrication errors [24]. As shown in Fig. 3(e)-(f), the passband develops two side peaks when assuming there is a radius deviation. The peaks spacing becomes larger when radius deviation increases. This behavior can be classified to a misalignment of resonance frequencies. The above analysis shows that strict fabrication process and post-fabrication tuning is necessary.
Fig. 3. The impacts of power coupling coefficients deviation and dimensional changes on the performance of the designed filter. (a), (c) Calculated transmission spectra at drop port of the second-order MRRs filter when the coupling coefficients deviate from the design values, (b), (d) are zoom-in views of the part enclosed by the dotted line in (a), (c), respectively. (e), (f) Calculated transmission spectra at drop port of a cascaded second-order MRRs filter when the radii of the two MRRs are slightly different.
3. Device fabrication and measurement
The proposed filter is fabricated on a 500-nm-thick X-cut single-crystalline LN thin film sitting on a 4.7-µm-thick buried silicon dioxide layer (purchased from NANOLN). The patterns are defined using EBL (Vistec EPBG5000+) and transferred onto 180 nm chrome film deposited by electron-beam evaporation (EBE) process. Then the patterned chrome is used as a hard mask to etch LN by an inductively coupled plasma (ICP, Oxford Instruments Plasmalab System 100) etching process. Then, a 1-µm-thick silicon dioxide cladding layer is deposited onto the LN layer by plasma enhanced chemical vapor deposition (PECVD). A thin film of Ti (150 nm) is then deposited on the cladding as the heater, which is defined by EBL, EBE and lift-off processes. Finally, Au electrical connecting traces and Au electrode probing pads with thickness of 100nm are sequentially patterned by EBL, EBE and lift-off processes. Detailed fabrication process is discussed in Supplement 1.
Figure 4 shows optical micrographs and scanning electron microscope (SEM) images of the fabricated device. The whole structure of LNOI TO tunable filter can be seen in Fig. 4(a). Figure 4(b) shows the microscope image of a Ti micro-heater, an Au connecting trace and an Au electrode probing pad. The size of the micro-heater above the waveguide is 2.5 × 300 µm. The size of Au connecting trace is 5 × 50 µm and the size of the pad is 150 × 150 µm. Figure 4(c) show the SEM images of the grating coupler. The grating couplers are used for fiber-to-chip light coupling, which are designed for TE-polarizations. Each grating coupler has a coupling loss of about −6.5 dB at 1550nm. The period and duty cycle of the grating couplers used here are 0.96 µm and 0.44, respectively. The design details of the grating couplers can be found in our previous works [25]. The SEM image of cross section of ridge waveguide is shown in Fig. 4(d). Figure 4(e) shows the coupling area of MRRs and bus waveguide.
Fig. 4. (a) Microscope images of the whole fabricated device. (b)Microscope images of a Ti micro-heater, an Au connecting trace and an Au electrode probing pad. (c-e) SEM images of the grating coupler, cross section of ridge waveguide and the coupling region of MRR and bus waveguide.
The filter is characterized using the setup schematically illustrated in Fig. 5. A continuous tunable laser (Santec TSL-510), an optical power meter (YOKOGAWA AQ2211) and a programming computer constitutes the wavelength scanning component to measure the transmission spectrum. A precision power supply (B2901A, from KEYSIGHT company) and DC probes are used to apply voltages on the Au pads to provide heating power for micro-heaters.
Fig. 5. The schematic of the experimental setup for device characterization. The inset is the microscope image of the filter.
The through-port and drop-port response during thermo-optic tuning process is shown in Fig. 6(a) and 6(b). Before the thermo-optic tuning, the drop-port of the filter shows a resonant wavelength mismatch, which is similar to the simulation results shown in Fig. 3(e) and (f). It can be seen that the resonant wavelength mismatch can be compensated when applying suitable heating power. For the device we measure here, the drop-port spectra with a maximally flat top and a maximum extinction ratio is obtained when the heating power is about 3.91 mW. Detailed discussion about the thermo-optic tuning efficiency can be seen in Supplement 1. As is shown in Fig. 6(c), the ripples of the flat-top passband are less than 0.3 dB. We suppose that the ripples are caused by fabrication errors [24]. From Fig. 6(d), it can be observed that the filter has a 3 dB bandwidth (BW) of about 4.8 GHz. The on-chip loss of drop port is about −0.84 dB. The out-of-band rejection is about 34.73 dB, which is measured at about 30 GHz frequency shift from the central frequency. The 30 dB BW is 30 GHz and the calculated rectangular factor is 6.25. As shown in Fig. 6(d), the filtering curves obtained by calculation and measurement are in good agreement.
Fig. 6. (a)-(b)Measured through-port and drop-port response of the filter when applying different heating power. (c) Normalized transmission of drop port of the filter and a close-up picture of the passband. (d) Comparison of measured (red curves) and calculated (green curves) filtering curves of the filter.
Fig. 7. Measured center wavelength tunability of the LNOI TO MRRs filter with a tuning efficiency of 89.4 mW per FSR.
We further investigate the center wavelength tunability of the filter using another device on the same chip. As shown in Fig. 7, a flat-top band pass filter with a maximum extinction ratio is achieved before thermo-optic tuning. The voltage is applied on both pads to heat two MRRs simultaneously to maintain the best filter shape response. In practice, the tuning range of one FSR is sufficient to compensate the resonance frequencies mismatch caused by fabrication errors or temperature variations. Moreover, it is reasonable to compare the heating power per FSR shift because the tuning efficiency highly depends on the size of MRRs. As is shown in Fig. 7, the total power required to tune an entire FSR is about 89.4 mW. The heating power applied to the individual MRR is about 44.7 mW. The ripples in the passband are less than 0.3 dB during the tuning process. As demonstrated in [20,26,27], the TO tuning efficiency can be significantly improved with thermal isolation trenches. Therefore, it is expected to further reduce the power to tune a full FSR of our device using the structure of thermal isolation trenches.
4. Conclusion
In conclusion, we demonstrated a TO tunable optical filter based on LNOI platform. The GHz-bandwidth and flat top passband filter response is obtained after optimizing the design and post-fabrication tuning. The filter shows intra band ripple less than 0.3 dB, on-chip loss of 0.84 dB, 3 dB bandwidth of 4.8 GHz, out-of-band extinction ratio of 34.73 dB and rectangular factor of 6.25. The center wavelength tunability of the filter is also demonstrated with a tuning efficiency of 89.4 mW per FSR. The demonstrated filter is a supplement for photonic devices on LNOI. The filter can find its applications in optical information processing and can be integrated monolithically with modulators on LNOI platform for more functional application in microwave photonics.
Funding
National Key Research and Development Program of China (2019YFB2203501); National Natural Science Foundation of China (61835008, 61905079, 61905084, 62175079).
Acknowledgments
We thank the Center of Micro-Fabrication and Characterization (CMFC) of WNLO and the Center for Nanoscale Characterization & Devices (CNCD), WNLO of HUST for the facility support.
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.
Supplemental document
See Supplement 1 for supporting content.
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