Ultra-Wideband Tunable Microwave Photonic Filter Based on Thin Film Lithium Niobate

Abstract

In signal processing of the growing semaphore, a microwave photonic filter (MPF) is capable of dealing with high-frequency signals, offering the advantages of high bandwidth, easy tuning, and more. This paper presents an efficient tunable microwave photonic filter that features a wideband tuning capability and narrow-band filtering effect based on the lithium niobate on insulator (LNOI) material platform. A multi-mode waveguide race-track type microring resonator has been designed, along with a thermal electrode that utilizes the thermo-optical effect of lithium niobate to adjust the microring resonator. The packaged device has been tested, with a wideband tunable range of 4.7~38.2 GHz achieved. This allows for cross-band continuous tuning across the C-band to K_a-band range. When supplied with 29.1 mW of electric power, the thermal tuning efficiency reaches 9.2 pm/mW, enabling high-frequency tuning of up to 38.2 GHz. The filter possesses a high resolution, exhibiting a 3 dB bandwidth of 662 MHz.

1. Introduction

In recent years, with the increasing demand for heterogeneous wireless networks, multi-service systems, and multi-function devices, multi-band communication can be found in various radio frequency systems. The challenge lies in the dynamic operation of multi-band channels to select the appropriate channel, optimize the system’s performance, and adapt to environmental alterations. This necessitates the filter to be cross-functional and capable of executing multi-band functions. Traditional electrical filters face challenges in achieving simultaneous tunable and multi-band spectral filtering. This presents a deficiency in meeting current demands. Additionally, the increased communication capacity requires a higher instantaneous working bandwidth than what traditional electrical microwave filters can provide. Microwave photon filters have the ability to convert radio frequency signals into optical domains for information processing. They offer several advantages, such as high frequency, large bandwidth, and tunability, and have attracted wide attention in recent years.

With the advancement of microwave photons, several material systems have been extensively researched, including silicon, silicon nitride, indium phosphide, and others. Among these, thin film lithium niobate (LiNbO₃) material has gained widespread attention as a top-performing material. In contrast to typical silicon-based materials, it boasts excellent electro-optical features and has made significant progress in the development of electro-optical modulators [1,2,3]. As a passive waveguide, LNOI has extremely low transmission loss. The transmission loss of the waveguide has been achieved as low as 0.027 dB/cm [4]. Additionally, the microring resonator has reached a Q-factor of 10⁷ [5]. Numerous LNOI-based devices are continuously emerging, including a waveguide-coupled beam splitter, a reconfigurable multi-functional chip [6], a photon delay line [7], a micro-cavity laser [8], a waveguide amplifier [9], a microwave photon chip [10], as well as a laser [11], electro-optical modulator [12], filter [13], etc., which are mixed and integrated on LNOI platform. Among these optical communication devices, a microwave photonic filter is important for information processing in optical fiber communication. The rapid development of optical communication has put forward higher filter requirements, such as high resolution, reconfiguration, tunable center wavelength, high side mode rejection ratio, small volume, etc. Tunable microwave photon filters developed thus far have a restricted range of tunable bandwidth. Huaqing Qiu et al. improved the Q-factor of microrings and achieved the tuning of the center frequency from 2 to 18.4 GHz, reducing the influence of adjacent resonant peaks [14]. In a different study, Shijie Song’s research group at the University of Sydney compensated for the residual phase influence by cascading double microrings, achieving a center frequency tuning range of 6–17 GHz [15]. Furthermore, Weifeng Zhang and colleagues at the University of Ottawa have successfully achieved a range of 7–25 GHz [16]. The microwave photonic filter, which is based on the microring resonator, primarily utilizes the resonant wavelength of the tuned microring in order to achieve the tuning of the working wavelength of the filter. The refractive index of the waveguide can be modulated by the thermo-optical or electro-optical impact of the material to alter the resonant conditions of the microring. Lithium niobate possesses exceptional electro-optical and thermo-optical properties. Studies have noted microwave photonic filters, which are founded on electro-optical and thermo-optical tuning. Electro-optical tuning typically necessitates significant voltage or larger device sizes, and the electro-optical effect often upsets phase stability, leading to easy drifting of control [17]. At present, most electro-optical tuning implementations operate at ultra-high voltage [18,19,20]. Devices operating at ultra-high voltage exhibit significant instability, potentially limiting their practical application. Conversely, thermal-optical tuning demonstrates strong stability, with the thermal-optical effect of lithium niobate significantly surpassing the electro-optical effect. Therefore, thermal-optical tuning achieves higher efficiency and shorter phase shift length [21].

This paper demonstrates a wideband tunable high-resolution microwave photonic filter realized using a multimode waveguide race-track type microring resonator cavity. Compared with the conventional ring resonator cavity, the race-track type microring resonator cavity has a longer coupling region, and this structure enables the multimode waveguide-based microring resonator cavity to have higher coupling coefficients so that the microring is in the critical coupling state as much as possible. In order to avoid the excitation of higher-order modes in multimode waveguides and the bending loss caused by the bending part of the microring, the conventional bending part of the microring is redesigned as a Eulerian bending so that the bending loss caused by the bending part is basically negligible, and the excitation of higher-order modes in the multimode waveguide is effectively suppressed. The fabricated microring has a high Q of 10⁵, and a high-resolution filter with a 3 dB bandwidth of 622 MHz is realized in order to realize the tunable function of the MPF, so that the MPF has a wider range of application scenarios, in the microring resonator on the design of the hot electrode on the center frequency of the MPF tuning, compared to the need for and the high voltage of the electro-optical tuning. The LN has a high thermo-optic coefficient of ($\frac{d{n}_e}{dT}$ = 3.34 × 10⁻⁵ K⁻¹ at 1523 nm and 300 K [22]). The thermo-optic tuning is more efficient and it does not need to be added to the difficulty to achieve a very high voltage. Additionally, electro-optical tuning control is easy to drift. Compared to thermo-optic tuning, it is more stable and more suitable for practical applications. In this work, an efficiently tuned thermal electrode is designed. By using the thermo-optical effect of LNOI, when the heating power is 29.1 mW, the ultra-wide wavelength tuning range of 33.5 GHz is realized, covering the communication band from the C-band to the K_a-band. It will play an important role in multi-band and cross-band communication application scenarios. The LNOI tunable microwave photon filter will have important applications in microwave photon systems.

2. Design and Fabrication

The central wavelength of the microring is tuned using the thermal-optical effect of lithium niobate. When a voltage is applied to the hot electrode, the temperature near the resistance increases, and the refractive index of lithium niobate is sensitive to temperature, thus changing. The change in the material’s refractive index will cause a change in the effective refractive index during light transmission, which will change the central wavelength of the microring resonator. When designing the thermal electrode, it is necessary to make the lithium niobate waveguide in the thermal field generated by the electrode as close as possible to ensure efficient thermal regulation. However, it cannot be too close in order to prevent the metal from absorbing light. In the design of the size of the resistance, in order to ensure that the thermal field generated by the resistance is large enough, the width of the nickel–chromium resistance is designed to be 4 μm. Figure 1 shows the temperature distribution of the waveguide cross-section at a power of 20 mW.

The design schematic diagram of the microring resonator cavity used to realize the filter function is shown in Figure 2. In order to increase the coupling efficiency, selecting the racetrack microring resonator and the length of the straight waveguide coupling region is increased to increase the optical coupling efficiency. Meanwhile, the microring waveguide chooses the ridge multi-mode waveguide [23]. When the light transmits in the waveguide, the main source of the transmission loss is the scattering loss caused by the roughness of the side wall of the waveguide. When the multi-mode waveguide is selected, the optical mode field has no contact with the side wall, which greatly reduces the scattering loss caused by the rough side wall of the waveguide, reducing the transmission loss of the waveguide. The width of the multi-mode waveguide is selected as 2.8 μm, which can ensure that the optical mode field does not contact the side wall of the waveguide and that there are not too many other high-order modes excited in the waveguide. Conventional circular bending will cause a bending loss in the light field. Euler bending is chosen in the bending part of the microring [24], which can avoid the excitation of higher-order modes and has no bending loss in the bending part. The heating electrode is designed in the region of the straight waveguide, and the thermal resistance material is nickel–chromium (NiCr). The design length is 200 μm, the width is 6 μm, and the thickness is 150 nm. The higher efficiency of heat modulation can be realized right above the straight waveguide. The gold-connected electrode is used, the design size is 600 × 20 μm, and the design size of the gold pad is 100 × 100 μm. After thermal power is added to the resistance, the resistance temperature increases. The effective refractive index of lithium niobate changes due to the thermal-optical effect of lithium niobate material, thus changing the resonant wavelength of the microring resonator. The transmission spectrum of this microring is shown in Figure 3, and it can be seen that it has a uniform resonant wavelength. The FSR of this microring is 87.5 GHz, there is no high-order mode excitation in the microring, and the Q value of the microring is up to 3.2 × 10⁵, which proves that this microring structure can effectively suppress the high-order mode.

The microring resonator is realized on the X-cut lithium niobate thin film (NanoLN, Jinan Jingzheng Electronics Co., Ltd., Jinan, China). The substrate of the lithium niobate thin film is silicon dioxide and silicon substrate, and the thickness of the lithium niobate thin film is 600 nm. Electron beam lithography (EBL) is used to expose the photoresist, and the patterned photoresist is used as the mask to deposit metal Cr. At the same time, the mask is easy to remove after the end of etching. When Cr is used as the mask, the etching ratio can reach 6:1, and the dry etching method is adopted to etch the lithium niobate film with an etching depth of 300 nm. An 800 nm thick SiO₂ coating was deposited on the top of the etched waveguide, and a metal nickel–chromium (NiCr) was grown on the coating by electron beam evaporation and then combined with the lift-off to form a thermal electrode. Figure 4 shows a microscopic diagram of the thermal electrode.

The fabrication process of the chip is shown in Figure 5. First, a layer of Cr with a thickness of 200 nm is steamed by an electron beam on the cleaned LNOI substrate as an etching mask. Dirt resulting from the Cr evaporation is cleared away before a 700 nm-thick negative adhesive (HSQ) applied to cover the Cr surface. The photoresist functions as a mask of the Cr. Subsequently, the use of electron beam lithography (EBL) serves to transfer the pattern before Cr etching is executed via ICP reactive ion. The Cr etch depth is examined using the focused ion beam (FIB), followed by HSQ removal. Subsequently, LNOI is etched using the argon ion-assisted inductively coupled plasma-enhanced reactive ion etching (ICP-RIE) method. An 800 nm thick PECVD silicon dioxide is deposited as the upper cladding layer of the lithium niobate waveguide. Finally, the film is uniformly coated with a layer of electron beam positive adhesive polymethyl methacrylate (PMMA). Layer alignment is achieved through the use of nested marks on the layout, followed by Electron beam lithography to pattern PMMA. This technique involves using PMMA as a mask, which is then subject to electron beam evaporation of a 150 nm thick nickel–chromium metal layer. Afterwards, the PMMA is removed from the NiCr layer using a lift-off process to form a thermal resistance. Finally, the same methodology is applied to create the gold electrode.

3. Measurement and Result

There are two approaches to achieving a tunable central wavelength of the filter. The first involves adjusting the central wavelength of the carrier to alter the central frequency of the filter, which subsequently leads to a change in signal frequency after the filter beat. This method depends on a tunable light source and necessitates the source to possess a wide band tuning range and high precision, making it unsuitable for practical applications. Another method for tuning the filter frequency is to adjust the center frequency of the optical filter. In the microring filter employed in this study, the voltage was applied on the heating electrode to modify the temperature of the hot electrode. As lithium niobate exhibits the thermo-optic effect, temperature alteration results in effective refractive index change, thereby impacting the light transmission in the waveguide. Therefore, the resonant conditions of the microring can be altered, leading to the ability to continuously adjust the resonant frequency by varying the applied voltage power. Therefore, the resonant conditions of the microring can be altered, leading to the ability to continuously adjust the resonant frequency by varying the applied voltage power. Consequently, the signal frequency can be tuned when the detector beats with the optical carrier.

Figure 6a shows the schematic diagram of the filter’s test link. The experiments were conducted at room temperature (25 °C) using a tunable laser as the light source. This miniaturized module is much easier to use compared to traditional benchtop tunable light sources. The location of the optical carrier in relation to the microring notch impacts the frequency tuning range. Before conducting the test, the wavelength of the optical carrier is fine-tuned to a suitable position to ensure that the frequency response of the filter operates at a lower frequency. The modulation sideband is generated after the optical carrier emitted by the laser module passes through the lithium niobate phase modulator. After modulation, both the positive and negative sidebands have equal intensity, and the phase difference is π. The modulated light wave subsequently enters the filter, resulting in a notch at the resonant wavelength of the microring. This causes an asymmetry between the two modulated sidebands. The signal analysis is performed using a vector network analyzer (ZNB40: NETWORK ANALYZER), and the test setup is depicted in Figure 6b. Due to the intermediate device’s plug loss, the signal power is significantly reduced before it reaches the detector. To address this issue, it is recommended to employ an optical amplifier to enhance the signal before detection.

Testing the response of microwave photon filtering on the coupling platform to meet practical application requirements proves challenging, with the metal pad being prone to damage from the application of a heat-tuned voltage through the pressure probe. To address this issue, we encapsulated the lithium niobate microring coupling in a tube shell featuring DC pins and connected the pad to the tube shell pins via gold wire bonding. Figure 7 shows the photos of the packaged object and microscope. The pin can be thermally tuned by applying a voltage directly. Generally speaking, packaging will reduce the performance of the device, but this test is conducted directly on the packaged device. It can still obtain a wideband tuning range, proving our device has excellent performance. The packaged device is convenient for testing and use.

When the chip is encapsulated into a device, the initial stage is to choose the suitable height of the heat sink based on the chip’s design. Subsequently, the heat sink is fixed in the shell with conductive silver paste, followed by fixing the lithium niobate microring chip in the heat sink with conductive silver paste. The shell is then placed in an oven for two hours to secure the heat sink and the microring chip. The conductive silver paste ensures indirect contact of the chip with the shell, thus enhancing conductivity and thermal conductivity. Afterward, the chip coupling is carried out using metalized lens fiber on a high-precision six-axis coupling table (10 nm, AP-AASS006-3X-CUSTOM). The position of the xyz-axis is adjusted to minimize coupling loss. Once the optimal coupling position has been determined, the lens fiber’s position is fixed with UV adhesive. It should be noted that during curing, the UV glue causes deformation of the lens fiber, leading to a reduction in coupling efficiency. Therefore, it is imperative to reserve a portion of the pulling space in line with empirical parameters and make real-time adjustments to minimize coupling loss. The coupling loss of each end face of the encapsulated device is approximately 4 dB.

Prior to the formal test, the laser module depicted in Figure 6b is replaced with a tunable light source (santec, TSL-550). Subsequently, the most effective central wavelength at which the microwave photonic filter laser source, based on this microring, operates is evaluated to be 1552.6 nm, thus enabling the tuning of the filter’s central frequency over a broad range. The small laser module is connected to the upper computer, and its working current and temperature are adjusted to stabilize the output wavelength at 1552.6 nm. At the start of the experiment, the laser module and the optical amplifier receive power. The output power of the VNA is set to 0 dBm, while the RF signals are scanned within the interval of 1–40 GHz, and the frequency scanning interval is set to 40 MHz. A DC voltage is loaded onto the shell’s pins, and the voltage value is altered. After each voltage is varied and maintained for five minutes, the data is recorded once.

The relationship between the electric power and the tuning frequency is shown in Figure 8. Due to the limitation of the tunable bandwidth of the microwave photonic filter and the bandwidth of the modulator in the range of the vector network analyzer (40 GHz), the test results show that the tuning frequency ranges from 4.7 GHz to 38.2 GHz, and the low frequency is tuned from 4.7 GHz. When the low-frequency signal is too low, the filter response is not ideal, which is due to the 3 dB bandwidth of the microring notch. When the maximum power is 29.1 mW and the corresponding voltage is 2.08 V, the central frequency of the filter is 38.2 GHz. Compared with the high voltage required for tuning using the electro-optical effect, the thermo-optical effect can achieve a wide tuning range without high voltage. The difficulty of microwave tuning is greatly reduced. The bandwidth of this electrical power filtering is less than 1 GHz, which is reduced by eight times compared with the bandwidth of [25] and improves the filtering fineness and filter resolution. The test results are shown in Figure 9. It can be seen from the test figure that when the tuning frequency is lower than 30 GHz, the frequency response is relatively consistent. The signal power decreases somewhat when the frequency moves to the high-frequency direction. We can see that when the filtering frequency is in the high-frequency region of 38.2 GHz, the power significantly decreases. At this time, due to the limitation of the phase modulator at 40 GHz, when the RF signal is very close to the operating limit of the modulator, the unstable modulation state will make the filtering state unstable, leading to a decrease in power. When the center frequency of the filter is 20.7 GHz, the 3 dB bandwidth of the filter is 622 MHz.

4. Discussion

Tunable microwave photonic filters are of significant importance for communication systems application requirements, hence, extensive research has been conducted on them. Most of the tunable microwave photonic filters based on thin-film lithium niobate utilize the electro-optical effect of lithium niobate, i.e., the waveguide is in an electric field by adding a sufficiently high voltage to the electrode, and the effective refractive index of light transmitted in the waveguide changes as a result of the electric field, so as to achieve the purpose of tunability. However, there are some problems with electro-optical tuning. In order to increase the tuning range, electro-optical tuning usually requires extremely high voltage values. Ref. [26] utilizes the electro-optic effect to tune the microring, which shifts the resonance peak of the microring by 105 pm at applied voltages of up to 100 V. Ref. [27] achieves a shift of 0.2 nm of the resonance peak of the microring using applied voltages ranging from −300 to 300 V. In this paper, a more stable and easily achievable thermal tuning method is selected over electro-optic tuning, in order to better cater to practical applications of microwave photonic filter tuning. The maximum voltage applied using this method is 2.08 V, which is two orders of magnitude lower than that required for electro-optic tuning. Moreover, the stability of thermo-optic tuning can make microwave photonic filters more suitable for practical applications than the easy drift of electro-optic tuning.

As the main component in microwave photonic filters, this paper presents the innovative design of a rack-trace type microring resonating cavity. The design is based on a 2.8 µm wide waveguide on a thin-film lithium niobate platform. To mitigate the bending loss caused by the transmission of light through the curved portion of the microring, a Eulerian bending is introduced in the ring. The aforementioned design minimizes contact between the optical mode field and the rough sidewalls of the waveguide during transmission in the microring, resulting in a significant reduction in transmission loss, an enhancement in the Q factor of the microring resonance cavity, and an improvement in the resolution of the microwave photonic filter.

Table 1. Performance Comparison of Microwave Photonic Filters with Different Platform.

Ref.	Platform	Filter Type	Tuning Range (GHz)	FWHM (MHz)
[14]	SOI	Bandpass	2–18.4	17
[15]	SOI	Bandpass	6–17	/
[16]	SOI	Bandpass	7–25	2300
[28]	SiN	Notch	1–11	60
[29]	As2S3	Notch	1–30	33–88
[25]	LiNbO3	Bandpass	/	4800
This work	LiNbO3	Bandpass	4.7–38.2	622

compares previously reported microwave photonic filters of various types based on different material platforms. In contrast, our work demonstrates the realization of ultra-wideband tunable bandpass microwave photonic filters on the LNOI platform. This achievement provides a significant reference for future monolithic integration of all-optical devices on the LNOI platform.

5. Conclusions

In this paper, the thermal-optical effect of lithium niobate is used. The effective refractive index of the lithium niobate waveguide is changed by applying a voltage to the thermal electrode to change the temperature near the thermal electrode. The change of effective refractive index changes the central wavelength of the microring and changes the filtering frequency of the microwave photon filter. A wideband tunable microwave photonic filter can be easily and efficiently realized by adjusting the voltage on the thermal electrode. By utilizing a thin film lithium niobate platform, the ultra-wideband tunable microwave photonic filter is capable of achieving frequency tuning within the range of 4.7~38.2 GHz with a 3 dB bandwidth as narrow as 622 MHz, thus significantly enhancing the filter’s resolution. High-frequency tuning at 38.2 GHz is attainable with an electric power of 29.1 mW, resulting in a thermal tuning efficiency of 9.2 pm/mW. This feat holds immense potential for application in large-capacity communication, radar technology, and related fields. Nevertheless, excessive signal loss happens during link construction due to plug loss in each device. To reduce system loss, future work will involve the integration of the modulator with the filter.