A bandpass-bandstop switchable integrated microwave photonic filter based on a thin-film lithium niobate platform

Abstract

This invention discloses a bandpass / bandstop switchable integrated microwave photonic filter based on a thin-film lithium niobate platform, comprising: a Mach-Zehnder modulator integrated on a thin-film lithium niobate chip, a composite structure for adjusting the phase relationship between the optical carrier and optical sidebands, and a microring resonator for filtering out sidebands. This invention features a simple structure; switching between bandstop and bandpass filtering can be achieved using two microring resonators and a phase shifter, reducing system complexity and offering flexibility and economy. Operation is simple; switching the filtering function only requires adjusting the voltage of the phase shifter, and it can be dynamically adjusted according to actual needs, meeting diverse application scenarios.

Description

Technical Field

[0001] This invention relates to the field of microwave photonics, and more specifically to a bandpass-bandstop switchable integrated microwave photonic filter based on a thin-film lithium niobate platform. Background Technology

[0002] With the development of society and the economy, people's demand for information has increased dramatically, and the amount of information is growing exponentially. Traditional microwave radio frequency technology faces bottlenecks in speed and bandwidth, and cannot meet the ever-increasing communication needs. On the other hand, optical communication has the advantages of high frequency, large bandwidth, low loss, and resistance to electromagnetic interference. Moreover, optical devices are small in size and light in weight, which can effectively solve the problems faced by traditional electrical technologies and has great potential. Therefore, microwave photonics (MWP), as a product of the mutual penetration of microwave radio frequency communication and optical communication, has become an important interdisciplinary field.

[0003] Microwave photonics can convert radio frequency signals to the optical frequency domain, fully leveraging the advantages of light—high bandwidth, low loss, and high resistance to electromagnetic interference—while also possessing the wide coverage, portability, and high flexibility of microwaves. It has broad application prospects in radar, wireless communication, sensing, imaging, and instrumentation. Among these applications, microwave photonic filters are a key component of microwave photonics, using photonic technology to filter microwave signals.

[0004] Traditional microwave filters are mostly based on electrical technology, but these filters are often limited by narrow bandwidth, large size, high loss, and poor flexibility. Microwave photonic filters use electro-optic modulators to load electrical signals onto light waves and use photonic devices to process the signals, overcoming the "electronic bottleneck" of traditional microwave filters. They have advantages such as low loss, high bandwidth, resistance to electromagnetic interference, tunability, and reconfigurability, meeting the high-performance requirements of modern communication systems for filters.

[0005] With increasingly scarce spectrum resources and increasingly complex signal environments in modern wireless communication, filters require greater adaptability. Fields such as wireless communication, audio processing, radar, and medical electronic devices frequently necessitate different filtering functions under varying operating modes. Traditional filters, typically offering only a single filtering function, struggle to meet diverse application demands. Bandwidth-bandstop switchable filters, however, can dynamically adjust to meet specific needs, avoiding the hassle and cost of using multiple filters in different scenarios, thus improving system flexibility and economy. Achieving both bandstop and bandpass functionality with a single filter reduces the number of components, simplifies link design, and facilitates device miniaturization.

[0006] Over the past decade, the rapid development of photonic integrated circuit (PIC) technology has driven advancements in integrated microwave photonics (IMWP). IMWP filters, based on advanced PIC technology, significantly reduce size, weight, and power consumption while enhancing system stability by integrating key optical components onto a chip-level platform. Compared to traditional fiber-optic based MWP filters, IMWP filters utilize centimeter-scale photonic chips, offering superior performance and application potential. Summary of the Invention

[0007] The purpose of this invention is to provide a bandpass and bandstop switchable integrated microwave photonic filter based on a thin-film lithium niobate platform. This filter can switch between bandpass and bandstop filtering functions, which can meet the filtering requirements of different application scenarios and has versatility.

[0008] A bandpass-bandstop switchable integrated microwave photonic filter based on a thin-film lithium niobate platform includes:

[0009] 1550nm wavelength tunable laser;

[0010] Thin-film lithium niobate chip;

[0011] A photodetector connected to a thin-film lithium niobate chip;

[0012] The thin-film lithium niobate chip includes: an electro-optic modulator, a composite structure for adjusting the phase relationship between the optical carrier and the optical sideband, and a micro-ring resonator;

[0013] The electro-optic modulator includes:

[0014] The first optical waveguide used for transmitting optical signals;

[0015] The first optical coupler is connected to the output terminal of the optical waveguide;

[0016] The first phase-shifting waveguide and the second phase-shifting waveguide are connected to the optical coupler;

[0017] The first traveling wave electrode group is used to modulate the optical signal within the optical waveguide;

[0018] A second optical coupler connected to the phase-shift waveguide;

[0019] The composite structure used to adjust the phase relationship between the optical carrier and the optical sideband includes:

[0020] The second optical waveguide is connected to the second optical coupler;

[0021] A first annular resonant cavity connected to the output end of the second optical waveguide;

[0022] A third optical waveguide coupled to the first annular resonant cavity;

[0023] The first thermal resistor is located directly above the third optical waveguide;

[0024] The first electrode group connected to the first thermal resistor;

[0025] A third optical coupler connected to the output terminals of the second and third optical waveguides;

[0026] The micro-ring structure for filtering optical sidebands includes:

[0027] The fourth optical waveguide is connected to the third optical coupler;

[0028] The second annular resonant cavity is coupled to the fourth waveguide;

[0029] The thermoelectric electrode material is TiN, with a length of 200 μm and a height of 600 μm.

[0030] The first annular resonant cavity is composed of an annular waveguide made of lithium niobate material, and the cross-section of the annular waveguide is convex. More preferably, the first annular resonant cavity is racetrack-shaped, with a waveguide width of 3.6 μm, including two 900 μm long straight waveguide modulation arm regions on both sides and a 180-degree Euler-bent waveguide connecting the straight waveguides. The 180-degree Euler-bent waveguide is composed of two identical 90-degree Euler bends, with a maximum bending radius of 4000 μm and a minimum bending radius of 100 μm. The total circumference of the first annular resonant cavity is 2413 μm. This annular resonant cavity waveguide is made of lithium niobate material, has a convex cross-section, and a waveguide height of 400 nm, of which the height of the top convex protrusion is 200 nm. The coupling length between the cascaded waveguide and the micro-ring waveguide is 850 μm, and the gap is 500 nm.

[0031] The second annular resonant cavity is constructed from an annular waveguide made of lithium niobate material, and the cross-section of the annular waveguide is convex. More preferably, the second annular resonant cavity is racetrack-shaped, with a waveguide width of 3.6 μm, including two 1000 μm long straight waveguide modulation arm regions on both sides and a 180-degree Euler-bent waveguide connecting the straight waveguide. The 180-degree Euler-bent waveguide is composed of two identical 90-degree Euler bends, with a maximum bending radius of 4000 μm and a minimum bending radius of 100 μm. The total circumference of the first annular resonant cavity is 2613 μm. This annular resonant cavity waveguide is made of lithium niobate material, has a convex cross-section, and a waveguide height of 400 nm, of which the height of the top convex protrusion is 200 nm. The coupling length between the cascaded waveguide and the micro-ring waveguide is 850 μm, and the gap is 500 nm.

[0032] The microwave photonic filter structure is based on an X-cut thin-film lithium niobate platform, comprising a silicon substrate, a SiO2 buried oxide layer disposed on the silicon substrate, and a lithium niobate thin film (thickness of 350–450 nm, more preferably 400 nm) disposed on the SiO2 buried oxide layer. Light propagates along the y-direction, and the electric field is directed in the z-direction. All waveguides used are ridge waveguides.

[0033] This invention is based on a thin-film lithium niobate (LNOI) platform. The chip input is connected to a laser input optical carrier. An on-chip electro-optic modulator converts the electrical signal to an optical signal. The converted optical signal is then processed by a composite structure to adjust the phase relationship between the optical carrier and the optical sidebands, thereby switching between bandpass and bandstop filtering. After sideband removal via a micro-ring, the signal is recovered as an electrical signal through a photodetector. This integrated microwave photonic filter with adjustable bandpass and bandstop can meet diverse application requirements. Utilizing PIC technology, it reduces the system's weight, power consumption, and size, making it widely applicable in various communication and electronic devices.

[0034] Compared with the prior art, the present invention has the following advantages:

[0035] I. This invention innovatively proposes a novel and simple structure that enables bandpass and bandstop switching simply by applying different voltages to the phase shifter, resulting in a simple structure. The system has no specific requirements for the modulator's function; both phase and intensity modulators can be used to achieve filtering function switching.

[0036] Second, LN materials have a high electro-optic coefficient, thus exhibiting excellent electro-optic effects, enabling the modulator to operate at lower drive voltages. This characteristic not only effectively reduces power consumption but also minimizes the impact of thermal effects on device performance, thereby improving system efficiency and stability.

[0037] Third, compared with traditional materials, thin-film lithium niobate modulators can support higher modulation bandwidth, thus giving the microwave photonic filter system a higher upper limit on operating frequency.

[0038] Fourth, it is easy to operate; the switching between bandpass and bandstop can be achieved simply by adjusting the DC bias voltage of the phase shifter. Attached Figure Description

[0039] Figure 1 This is a schematic diagram of the integrated microwave photonic filter structure in this invention;

[0040] Figure 2 The following is a simulation result diagram of the microwave photonic filter in this invention; Figure 2 (a) is the spectrum of the input electrical signal; Figure 2 (b) is the spectrum of the signal after modulation; Figure 2 (c) is the output spectrum of the drop end of the first ring resonator; Figure 2 (d) is the output spectrum of the through terminal of the first ring resonator; Figure 2 (e) The spectrum of the output at the through end and the output at the drop end of the first ring resonator after phase shifting and superimposed by 2×1MMI; Figure 2 (f) is the spectrum of the signal after the sidebands are filtered out by the second ring resonator; Figure 2 (g) is the simulation result of the filter when a voltage is applied to the phase shifter so that its phase shift is 0. Figure 2 (h) shows the simulation results of the filter when a voltage is applied to the phase shifter to shift its phase by π / 2. Detailed Implementation

[0041] like Figure 1 As shown, a bandpass-bandstop switchable integrated microwave photonic filter based on a thin-film lithium niobate platform includes: a 1550nm wavelength tunable laser 1; a thin-film lithium niobate chip; and a photodetector 16 connected to the chip's output. The thin-film lithium niobate chip includes: a Mach-Zehnder modulator; a composite structure for adjusting the phase relationship between the optical carrier and optical sidebands; a micro-ring structure for filtering the optical sidebands; a silicon substrate 19; a SiO2 buried oxide layer 18 disposed on the silicon substrate 19; and a lithium niobate thin film 17 (400nm thick) disposed on the SiO2 buried oxide layer 18.

[0042] The Mach-Zehnder modulator includes: a first optical waveguide 2 for transmitting optical signals; a first optical coupler 3 connected to the output of the first optical waveguide 2; a first phase-shifting waveguide 5 and a second phase-shifting waveguide 6 connected to the first optical coupler 3; a traveling-wave electrode group 4 for modulating the optical signals in the first phase-shifting waveguide 5 and the second phase-shifting waveguide 6; and a second optical coupler 7 connected to the first phase-shifting waveguide 5 and the second phase-shifting waveguide 6.

[0043] The composite structure for adjusting the phase relationship between the optical carrier and the optical sideband includes: a second optical waveguide 8 connected to the second optical coupler 7; a first ring resonant cavity 9 connected to the output end of the second optical waveguide 8; a third optical waveguide 10 coupled to the first ring resonant cavity 9; a first thermoelectric resistor 11 located directly above the third optical waveguide 10; an electrode group 12 connected to the first thermoelectric resistor; and a third optical coupler 13 connected to the output ends of the second optical waveguide 8 and the third optical waveguide 10.

[0044] The micro-ring structure for filtering optical sidebands includes: a fourth optical waveguide 14 connected to the third optical coupler 13; and a second ring resonant cavity 15 coupled to the fourth optical waveguide 14.

[0045] The first ring resonant cavity 9 is racetrack-shaped with a waveguide width of 3.6 μm. It includes straight waveguide modulation arm regions of 900 μm in length on both sides and a 180-degree Euler-bent waveguide connecting the straight waveguides. The 180-degree Euler-bent waveguide is composed of two identical 90-degree Euler bends, with a maximum bending radius of 4000 μm and a minimum bending radius of 100 μm. The total circumference of the first ring resonant cavity is 2413 μm. The ring resonant cavity waveguide is made of lithium niobate material, with a convex cross-section and a waveguide height of 400 nm, of which the height of the top convex part is 200 nm. The coupling length between the cascaded waveguide and the micro-ring waveguide is 850 μm, and the gap is 500 nm.

[0046] The second annular resonant cavity 15 is racetrack-shaped with a waveguide width of 3.6 μm. It includes two 1000 μm long straight waveguide modulation arm regions on both sides and a 180-degree Euler-bent waveguide connecting the straight waveguides. The 180-degree Euler-bent waveguide is composed of two identical 90-degree Euler bends, with a maximum bending radius of 4000 μm and a minimum bending radius of 100 μm. The total circumference of the second annular resonant cavity 15 is 2613 μm. This annular resonant cavity waveguide is made of lithium niobate material, with a convex cross-section and a waveguide height of 400 nm, of which the height of the top convex part is 200 nm. The coupling length between the cascaded waveguide and the micro-ring waveguide is 850 μm, and the gap is 500 nm.

[0047] After the 1550nm tunable laser outputs light, it is modulated by a Mach-Zehnder modulator to achieve the conversion of electrical signals into optical signals. The complex amplitude of the light input to the modulator can be expressed as:

[0048]

[0049] Where E in Let E be the amplitude of the input light, and ω0 be the angular frequency of the input light. On the modulation arms, let the amplitudes of the light waves from the two modulation arms be E. A and E B Due to the change in refractive index, the phase of light changes. The phase change on modulation arm A caused by the change in refractive index is m. A sin(ω m t), where m A ω represents the amplitude of the phase change. m This represents the frequency of the modulating signal; similarly, the phase change on modulating arm B is... Where m B ω represents the amplitude of the phase change. m This represents the frequency of the modulating signal and the phase difference between the driving signals of the two modulating arms. Furthermore, there is a static phase bias between the two modulation arms. The complex amplitude of the light waves output from both arms is:

[0050]

[0051] Performing a Bessel expansion on equations (2) and (3), considering only the two sidebands closest to the optical carrier, we obtain:

[0052]

[0053] Where J1(m) represents the first-order Bessel function curve, J -1 J(m) represents the -1st order Bézier function curve, J0(m) represents the 0th order Bézier function curve, and J1(m) = -J -1 (m). Ideally... The light output from the modulator is the superposition of light interference from the two modulation arms, and the complex output amplitude is:

[0054]

[0055] The output signal can be viewed as a superposition of three frequencies of light: the upper sideband, the optical carrier, and the lower sideband, with corresponding frequencies ω and ω, respectively. m +ω0、ω m ω m -ω0. Let the amplitudes of the upper sideband signal, optical carrier, and lower sideband modulation signal be E. u E c E d The phases are respectively Equation 6 can be expressed as:

[0056]

[0057] The optical signal intensity can be obtained from equation (7):

[0058]

[0059] Generally there is E u <<E C E u <<E C Therefore, equation (8) can be simplified to:

[0060]

[0061] When the two arms are differentially driven and biased at the orthogonal operating point, The complex amplitude of the output light is:

[0062]

[0063] It can be deduced that:

[0064]

[0065] The two sidebands are in phase with respect to the optical carrier. By adjusting the laser input wavelength, the wavelength of the optical carrier is aligned with the resonant wavelength of the first ring resonator. The modulator output signal after filtering out the optical carrier is output from the through end of the first ring resonator. The phase of the optical carrier output from the drop end is adjusted by adjusting the phase shifter. At the third coupler, the output from the through end of the micro-ring and the phase-adjusted drop end output are superimposed, thus controlling the phase relationship between the two sidebands and the optical carrier.

[0066] The optical carrier is generated at the drop end of the first microring resonator. Phase shift, when the phase shift value of the phase shifter is... At that time, the phase relationship between the two sidebands and the optical carrier is:

[0067]

[0068] The upper and lower sidebands are in phase with respect to the optical carrier. After a portion of the sidebands are filtered out by the second micro-ring resonator, the optical signal recovers the electrical signal by detecting the light intensity through a photodetector. The filter system has band-stop filtering characteristics, and the filter center frequency is the frequency difference between the resonant frequency of the second micro-ring resonator and the frequency of the optical carrier.

[0069] When the phase shift value of the phase shifter is At that time, the phase relationship between the two sidebands relative to the optical carrier is as follows:

[0070]

[0071] The upper and lower sidebands are out of phase with respect to the optical carrier, similar to the sidebands generated by a phase modulator. After a portion of the sidebands are filtered out by the second micro-ring resonator, the equal amplitude and opposite phase relationship between the two sidebands is disrupted. The light wave is detected by a photodetector to recover the electrical signal. The filtering system exhibits bandpass filtering characteristics. The filter center frequency is the frequency difference between the resonant frequency of the second micro-ring resonator and the frequency of the optical carrier.

[0072] A simulation model of the microwave photonic filter was built using Ansys Lumerical INTERCONNECT software to verify its principle. In this model, the electrical input signal of the microwave photonic filter is as follows: Figure 2 As shown in (a), the laser input wavelength is 193.101 THz, corresponding to the resonance peak of the first microring resonator. The spectrum of the input signal after modulation is as follows. Figure 2 (b) After the optical signal passes through the first micro-ring resonant cavity, the output signal at the micro-ring drop end is as follows: Figure 2 As shown in (c), the output signal at the through terminal is as follows: Figure 2 As shown in (d), the spectrum of the drop output signal coupled and superimposed with the through output signal at the third optical coupler after passing through the phase shifter is as follows: Figure 2As shown in (e), the resonant frequencies of the second micro-ring resonator near the optical carrier are 193.074 THz and 193.127 THz. The extinction ratios at these two resonant frequencies are different, corresponding to a center frequency of approximately 26 GHz for the filter. The output spectrum after filtering out the sidebands is shown in [e]. Figure 2 (f).

[0073] When the optical carrier phase shift is 0, the filter is a band-stop filter, and the simulation results are as follows: Figure 2 As shown in (g), the center wavelength is 26 GHz and the bandwidth is 1 GHz. When the optical carrier phase shift is π / 2, the filter is a bandpass filter, and the simulation results are as follows. Figure 2 As shown in (h), the center wavelength is 26GHz and the bandwidth is 1GHz. By adjusting the applied voltage of the phase shifter to change the relative phase between the sideband and the optical carrier, the switching between band-stop filtering and band-pass filtering can be achieved.

Claims

1. A bandpass-bandstop switchable integrated microwave photonic filter based on a thin-film lithium niobate platform, comprising: A thin-film lithium niobate chip, characterized in that the thin-film lithium niobate chip includes an electro-optic modulator, a composite structure for adjusting the phase relationship between the optical carrier and the optical sideband, and a micro-ring resonator for filtering out the optical sideband, connected in sequence. The electro-optic modulator includes: The first optical waveguide used for transmitting optical signals; The first optical coupler is connected to the output terminal of the optical waveguide; The first phase-shifting waveguide and the second phase-shifting waveguide are connected to the optical coupler; The first traveling wave electrode group is used to modulate the optical signal within the optical waveguide; A second optical coupler connected to the phase-shift waveguide; The composite structure used to adjust the phase relationship between the optical carrier and the optical sideband includes: The second optical waveguide is connected to the second optical coupler of the electro-optic modulator; A first annular resonant cavity connected to the output end of the second optical waveguide; A third optical waveguide coupled to the first annular resonant cavity; The first thermal resistor is located directly above the third optical waveguide; The first electrode group connected to the first thermal resistor; A third optical coupler connected to the output terminals of the second and third optical waveguides.

2. The bandpass-bandstop switchable integrated microwave photonic filter based on a thin-film lithium niobate platform according to claim 1, characterized in that, The thin film thickness of the aforementioned thin-film lithium niobate chip is 350~450nm.

3. The bandpass-bandstop switchable integrated microwave photonic filter based on a thin-film lithium niobate platform according to claim 1, characterized in that, The microring resonator for filtering out optical sidebands includes: The fourth optical waveguide is connected to the third optical coupler; The second annular resonant cavity is coupled to the fourth optical waveguide.

4. The bandpass-bandstop switchable integrated microwave photonic filter based on a thin-film lithium niobate platform according to claim 1, characterized in that, The first annular resonant cavity is composed of an annular waveguide made of lithium niobate material, and the cross-section of the annular waveguide is convex.

5. The bandpass-bandstop switchable integrated microwave photonic filter based on a thin-film lithium niobate platform according to claim 3, characterized in that, The second annular resonant cavity is composed of an annular waveguide made of lithium niobate material, and the cross-section of the annular waveguide is convex.

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