Band-pass and band-stop switchable integrated microwave photonic filter based on thin film lithium niobate platform

Abstract

The invention discloses a band-pass and band-stop switchable integrated microwave photonic filter based on a film lithium niobate platform. The band-pass and band-stop switchable integrated microwave photonic filter comprises a Mach-Zehnder modulator, a composite structure and a micro-ring resonator, wherein the Mach-Zehnder modulator is integrally manufactured on a film lithium niobate chip; the composite structure is used for adjusting the phase relation between an optical carrier and an optical sideband; according to the invention, the structure is simple, the switching between band-stop filtering and band-pass filtering can be realized through the two micro-ring resonators and the phase shifter, the system complexity is reduced, and flexibility and economical efficiency are realized. According to the invention, the operation is simple, the switching of the filtering function can be realized only by adjusting the voltage of the phase shifter, the dynamic adjustment can be realized according to the actual demand, and the variable application scenarios can be satisfied.

Description

Technical Field

[0001] The present invention relates to the field of microwave photons, and in particular to a bandpass and bandstop switchable integrated microwave photon filter based on a thin-film lithium niobate platform. Background Art

[0002] With the development of social economy, people's demand for information has increased dramatically, and the amount of information has increased exponentially. Traditional microwave radio frequency technology faces bottlenecks in speed and bandwidth and cannot meet the growing communication needs. On the other hand, optical communication has the advantages of high frequency, large bandwidth, low loss, and anti-electromagnetic interference. In addition, optical devices are small in size and light in weight, which can effectively solve the problems faced by traditional electrical technology 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 subject.

[0003] Microwave photonics can convert radio frequency signals into the optical frequency domain, which not only fully utilizes the advantages of large bandwidth, low loss, and high resistance to electromagnetic interference of light, but also has the wide coverage, mobility, and high flexibility of microwaves. It has broad application prospects in radar, wireless communication, sensing, imaging, instruments, etc. Among them, microwave photon filters are key components of microwave photonics, which realize the filtering operation of microwave signals through photon technology.

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

[0005] Modern wireless communication spectrum resources are increasingly scarce, and the signal environment is becoming more and more complex, which requires filters to have higher adaptability. Wireless communications, audio processing, radar, and medical electronic equipment often require different filtering functions in different operating modes, while traditional filters usually have a single filtering function and are difficult to meet the changing application requirements. Bandwidth-band-stop switchable filters can be dynamically adjusted according to actual needs, avoiding the trouble and cost of using multiple filters in different scenarios, and improving the flexibility and economy of the system. Using one filter to achieve both band-stop and band-pass functions reduces the number of components, simplifies link design, and helps to achieve miniaturization of equipment.

[0006] In the past decade, the rapid development of photonic integrated circuit (PIC) technology has promoted the progress of integrated microwave photonics (IMWP). Based on advanced PIC technology, IMWP filters integrate key optical components into a chip-level platform, significantly reducing size, weight and power consumption while enhancing system stability. Compared with traditional fiber-based MWP filters, IMWP filters use centimeter-level photonic chips and have better performance and application potential. Summary of the invention

[0007] The purpose of the present invention is to provide a bandpass and bandstop switchable integrated microwave photonic filter based on a thin film lithium niobate platform. The filtering function of this filter can be switched between bandpass and bandstop, which can meet the filtering requirements in different application scenarios and has universality.

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

[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 an optical carrier and an optical sideband, and a micro-ring resonator;

[0013] The electro-optic modulator comprises:

[0014] a first optical waveguide for transmitting an optical signal;

[0015] a first optical coupler connected to the output end of the optical waveguide;

[0016] a first phase-shift waveguide and a second phase-shift waveguide connected to the optical coupler;

[0017] A first traveling wave electrode group for modulating an optical signal in the optical waveguide;

[0018] a second optical coupler connected to the phase-shifting waveguide;

[0019] The composite structure for adjusting the phase relationship between the optical carrier and the optical sideband comprises:

[0020] a second optical waveguide connected to the second optical coupler;

[0021] a first ring resonator connected to the output end of the second optical waveguide;

[0022] A third optical waveguide coupled to the first ring resonator;

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

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

[0025] a third optical coupler connected to the second optical waveguide output end and the third optical waveguide output end;

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

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

[0028] A second ring resonator coupled to the fourth waveguide;

[0029] The thermoelectrode 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, which is made of lithium niobate material, and the cross-section of the annular waveguide is convex. Further preferably, the first annular resonant cavity is a racetrack style, with a waveguide width of 3.6μm, including a straight waveguide modulation arm area of ​​900μm on both sides and a 180-degree Euler bending waveguide connecting the straight waveguide. The 180-degree Euler bending waveguide is composed of two completely identical 90-degree Euler bends, with a maximum bending radius of 4000μm and a minimum bending radius of 100μm. The total circumference of the first annular resonant cavity is 2413μm. The annular resonant cavity waveguide is made of lithium niobate material, with a convex cross-section, and the height of the waveguide is 400nm, wherein the height of the top protrusion is 200nm, the coupling length of the cascade waveguide and the microring waveguide is 850μm, and the gap is 500nm.

[0031] The second annular resonant cavity is composed of an annular waveguide, which is made of lithium niobate material, and the cross-section of the annular waveguide is convex. Further preferably, the second annular resonant cavity is a racetrack style, with a waveguide width of 3.6μm, including a straight waveguide modulation arm area of ​​1000μm on both sides and a 180-degree Euler bending waveguide connecting the straight waveguide. The 180-degree Euler bending waveguide is composed of two identical 90-degree Euler bends, with a maximum bending radius of 4000μm and a minimum bending radius of 100μm. The total circumference of the first annular resonant cavity is 2613μm. The annular resonant cavity waveguide is made of lithium niobate material, with a convex cross-section, and the height of the waveguide is 400nm, wherein the height of the top protrusion is 200nm, the coupling length of the cascade waveguide and the microring waveguide is 850μm, and the gap is 500nm.

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

[0033] The present invention is based on a thin-film lithium niobate (LNOI) platform. The chip input end is connected to a laser input optical carrier. The on-chip electro-optical modulator realizes the conversion of electrical signals to optical signals. The converted optical signal adjusts the phase relationship between the optical carrier and the optical sideband through a composite structure, thereby realizing the switching of bandpass filtering and bandstop filtering. After the sideband is filtered out by a micro-ring, the beat frequency is restored to an electrical signal through a photodetector. This integrated microwave photon filter with adjustable bandpass and bandstop can meet the changing application requirements. By using PIC technology, the weight, power consumption and size of the system are reduced, making the system have a wide range of application prospects in various communication and electronic equipment.

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

[0035] 1. The present invention innovatively proposes a new simple structure, which can realize the switching of passband and stopband functions by applying different voltages to the phase shifter, and has a simple structure. The system has no requirements on the modulator function, and the modulator can realize the switching of the filtering function by using a phase modulator and an intensity modulator.

[0036] Second, LN material has a high electro-optic coefficient, and thus has an excellent electro-optic effect, which enables the modulator to operate at a lower driving voltage. This feature not only effectively reduces power consumption, but also reduces the impact of thermal effects on device performance, thereby improving system efficiency and stability.

[0037] 3. Compared with traditional materials, thin-film lithium niobate modulators can support higher modulation bandwidth, so the microwave photon filtering system has a higher upper limit of operating frequency.

[0038] Fourth, the operation is easy. The passband and band-stop switching can be achieved by simply adjusting the DC bias voltage of the phase shifter. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 It is a schematic diagram of the structure of the integrated microwave photon filter in the present invention;

[0040] Figure 2 This is a simulation result diagram of the microwave photon filter in the present invention; Figure 2 (a) is the spectrum of the input electrical signal; Figure 2 (b) is the spectrum diagram 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 diagram of the through end of the first ring resonator; Figure 2 (e) The spectrum of the through-end output and the drop-end output of the first ring resonator after phase shift and superposition by 2×1 MMI; Figure 2 (f) is the spectrum of the signal after the sideband is filtered out by the second ring resonator; Figure 2 (g) is the filter simulation result diagram when a voltage is applied to the phase shifter to make its phase shift zero. Figure 2 (h) is the filter simulation result diagram when a voltage is applied to the phase shifter to shift its phase by π / 2. DETAILED DESCRIPTION

[0041] like Figure 1 As shown, a bandpass and bandstop switchable integrated microwave photon 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 output end of the chip. The thin film lithium niobate chip includes: a Mach-Zehnder modulator; a composite structure for adjusting the phase relationship between an optical carrier and an optical sideband; a micro-ring structure for filtering the optical sideband; a silicon substrate 19; a SiO2 buried oxide layer 18 disposed on the silicon substrate 19 and a lithium niobate film 17 (with a thickness of 400nm) disposed on the SiO2 buried oxide layer 18.

[0042] The Mach-Zehnder modulator comprises: a first optical waveguide 2 for transmitting an optical signal; a first optical coupler 3 connected to an output end of the first optical waveguide 2; a first phase-shift waveguide 5 and a second phase-shift waveguide 6 connected to the first optical coupler 3; a traveling wave electrode group 4 for modulating the optical signal in the first phase-shift waveguide 5 and the second phase-shift waveguide 6; and a second optical coupler 7 connected to the first phase-shift waveguide 5 and the second phase-shift 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 thermal electrode resistor 11 located directly above the third optical waveguide 10; an electrode group 12 connected to the first thermal 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 the optical sideband 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 resonator 9 is a racetrack-shaped pattern, with a waveguide width of 3.6 μm, including a straight waveguide modulation arm region of 900 μm on both sides and a 180-degree Euler bending waveguide connecting the straight waveguide, the 180-degree Euler bending 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 resonator is 2413 μm. The ring resonator waveguide is made of lithium niobate material, with a convex cross-section, a waveguide height of 400 nm, wherein the height of the top protrusion is 200 nm, the coupling length of the cascade waveguide and the micro-ring waveguide is 850 μm, and the gap is 500 nm.

[0046] The second ring resonator 15 is a racetrack-shaped waveguide with a waveguide width of 3.6 μm, including a straight waveguide modulation arm region with a length of 1000 μm on both sides and a 180-degree Euler bending waveguide connecting the straight waveguide. The 180-degree Euler bending 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 ring resonator 15 is 2613 μm. The ring resonator waveguide is made of lithium niobate material, with a convex cross-section, a waveguide height of 400 nm, wherein the height of the top protrusion is 200 nm, the coupling length of the cascade waveguide and the micro-ring waveguide is 850 μm, and the gap is 500 nm.

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

[0048]

[0049] Where E in is the amplitude of the input light, and ω0 is the angular frequency of the input light. On the modulation arm, let the amplitude of the light waves of the two modulation arms be E A and E B , due to the change in refractive index, the phase of the light changes. The phase change caused by the change in refractive index on the modulation arm A is m A sin(ω m t), where m A represents the phase change amplitude, ω m represents the frequency of the modulated signal. Similarly, the phase change on the modulation arm B is Where m B represents the phase change amplitude, ω m Indicates the frequency of the modulation signal and the phase difference between the two modulation arm drive signals And there is a static phase offset between the two modulation arms Then the complex amplitude of the light waves output by the two arms is:

[0050]

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

[0052]

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

[0054]

[0055] The output signal can be regarded as the optical superposition of three frequencies, namely the upper sideband, optical carrier and lower sideband, and the corresponding frequencies are ω m +ω0、ω m ,ω m -ω0. Assume that the amplitude of the upper sideband signal, optical carrier, and lower sideband modulation signal is E u , E c , E d , the phases are Formula 6 can be expressed as:

[0056]

[0057] From formula (7), the optical signal strength can be obtained as:

[0058]

[0059] Generally there is E u <<E C , E u <<E C , so formula (8) can be simplified as:

[0060]

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

[0062]

[0063] Can launch:

[0064]

[0065] The two sidebands have the same phase relative 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 through end of the first ring resonator outputs the modulator output signal after filtering the optical carrier. The optical carrier output from the drop end adjusts the phase by adjusting the phase shifter. At the third coupler, the output of the through end of the micro-ring and the phase-adjusted drop end output are superimposed, so that the phase relationship of the two sidebands relative to the optical carrier can be controlled.

[0066] The optical carrier is generated at the output of the first microring resonator drop terminal Phase shift, when the phase shift value of the phase shifter is When , the phase relationship between the two sidebands and the optical carrier is:

[0067]

[0068] The upper and lower sidebands are equal in phase relative to the optical carrier. The optical signal after filtering out a part of the sidebands through the second microring resonator is detected by the photodetector to restore the electrical signal. The filter system has a band-stop filter characteristic, and the filter center frequency is the frequency difference between the resonant frequency of the second microring resonator and the optical carrier.

[0069] When the phase shift value of the phase shifter is When , the phase relationship between the two sidebands and the optical carrier is:

[0070]

[0071] The upper and lower sidebands are opposite in phase to the optical carrier, similar to the sidebands generated by the phase modulator. After filtering out a part of the sidebands through the second microring resonator, the equal amplitude and opposite phase relationship of the two sidebands is destroyed. The light wave is detected by the photodetector to restore the electrical signal. The filtering system has a bandpass filtering characteristic. The filtering center frequency is the frequency difference between the resonant frequency of the second microring resonator and the optical carrier.

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

[0073] When the optical carrier phase shift is 0, the filter is a band-stop filter. The simulation results are as follows: Figure 2 As shown in (g), the central 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. The simulation results are as follows Figure 2 As shown in (h), the central wavelength is 26 GHz and the bandwidth is 1 GHz. 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 and 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 comprises an electro-optic modulator, a composite structure for adjusting the phase relationship between an optical carrier and an optical sideband, and a micro-ring resonator for filtering out the optical sideband, which are connected in sequence; The composite structure for adjusting the phase relationship between the optical carrier and the optical sideband comprises: a second optical waveguide connected to a second optical coupler of the electro-optic modulator; a first ring resonator connected to the output end of the second optical waveguide; A third optical waveguide coupled to the first ring resonator; a first thermal resistor located directly above the third optical waveguide; a first electrode group connected to the first thermal resistor; A third optical coupler is connected to the second optical waveguide output end and the third optical waveguide output end.

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

3. The bandpass and bandstop switchable integrated microwave photonic filter based on a thin film lithium niobate platform according to claim 1, characterized in that: The electro-optic modulator comprises: a first optical waveguide for transmitting an optical signal; a first optical coupler connected to the output end of the optical waveguide; a first phase-shift waveguide and a second phase-shift waveguide connected to the optical coupler; A first traveling wave electrode group for modulating an optical signal in the optical waveguide; A second optical coupler is connected to the phase-shifting waveguide.

4. The bandpass and 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 comprises: a fourth optical waveguide connected to the third optical coupler; A second ring resonator coupled to the fourth optical waveguide.

5. The bandpass and 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 in a convex shape.

6. The bandpass and bandstop switchable integrated microwave photonic filter based on a thin film lithium niobate platform according to claim 1, 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 in a convex shape.

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