Single-loop parity-time symmetric tunable optoelectronic oscillator based on sbs

Abstract

The application discloses a single-loop parity-time symmetry tunable optoelectronic oscillator based on SBS, which comprises a laser for generating an optical carrier signal, the optical signal is coupled into a z-cut lithium niobate phase modulator through a polarization controller, the z-cut lithium niobate phase modulator outputs two-mode modulation signals, the modulation optical signal and pump light occur stimulated Brillouin scattering (SBS) in a high nonlinear optical fiber, the positive first-order sideband of the modulation signal is amplified by SBS gain, and finally two microwave signals after photoelectric conversion are combined into one, and then a part is connected to a phase modulator through an electrical amplifier and a power divider to form a closed loop, and another part is connected to a spectrum analyzer to observe the output signal quality. The application improves the tuning accuracy of the system through a microwave photon filter, realizes broadband tunable output signals by adjusting the wavelength of pump light, realizes a parity-time symmetry structure under a single loop, and simplifies the system structure.

Description

Technical Field

[0001] This invention belongs to the field of microwave photonics technology, and particularly relates to a single-loop parity-time symmetric tunable opto-oscillator based on SBS. Background Technology

[0002] High-frequency, low-phase-noise tunable microwave signal sources play a crucial role in systems such as wireless communication, optical radio frequency transmission, and radar, and have become a research hotspot in the field of microwave photonics. Opto-oscillators, as a type of optically generated microwave signal source, possess advantages such as low phase noise and frequency-independent phase noise characteristics, thus attracting widespread attention.

[0003] Increasing fiber length can effectively reduce phase noise in the output signal, but long-distance fibers result in a large number of closely packed longitudinal modes in the output signal, with the mode spacing inversely proportional to the fiber length. In recent years, parity-time symmetry technology has become a simple and effective way to solve mode selection in oscillators. However, traditional parity-time symmetric optoelectronic oscillators generally use a physical dual-loop structure to achieve parity-time symmetry, which is relatively complex and difficult to implement. Summary of the Invention

[0004] This invention proposes a single-loop parity-time symmetric tunable optoelectronic oscillator based on SBS to solve the technical problems existing in the prior art.

[0005] To achieve the above objectives, the present invention provides a single-loop parity-time symmetric tunable photoelectric oscillator based on SBS, comprising: a laser source (1), wherein the laser source (1) is sequentially connected to the left port of a polarization controller (2), a z-cut lithium niobate phase modulator (3), an optical isolator (4), a highly nonlinear optical fiber (5), and an optical circulator (8); a tunable laser (6) is sequentially connected to the upper port of a polarization controller (7) and an optical circulator (8); and the right port of the optical circulator (8) is connected to a photodetector (9), thereby constituting a microwave photonic filter based on stimulated Brillouin scattering (SBS), wherein the microwave photonic filter is used to generate a finely tunable filter passband;

[0006] The microwave photonic filter is connected in sequence to the microwave amplifier (10) and the microwave power divider (11). One output terminal of the microwave power divider (11) is connected to the radio frequency input terminal of the z-cut lithium niobate phase modulator (3), and the other output terminal of the microwave power divider (11) is the microwave signal output terminal of the optoelectronic oscillator, thus forming a single-loop parity-time symmetric oscillation loop. The single-loop parity-time symmetric oscillation loop is used to generate microwave signals.

[0007] Preferably, the single-loop parity-time symmetric oscillating loop includes a single-loop parity-time symmetric structure, including a laser source (1), which is sequentially connected to a polarization controller (2), a z-cut lithium niobate phase modulator (3), an optical isolator (4), a highly nonlinear optical fiber (5), and the left port of an optical circulator (8), and the right port of the optical circulator (8) is connected to a photodetector (9).

[0008] Preferably, the z-cut lithium niobate phase modulator (3) outputs modulation signals of different modes. Based on adjusting the polarization angle θ between the incident light and the z-axis, the power splitting ratio of the modulation signal is adjusted, and based on the power splitting ratio, the parity-time symmetric phase condition is satisfied.

[0009] Preferably, when the system is in a parity-time symmetric state, the coupled mode equations of the nth eigenmode are:

[0010]

[0011] Solve the coupled mode equations:

[0012]

[0013] If g an =-g bn =g n ,but

[0014]

[0015] Among them, a n and b n These represent the nth eigenmode in each of the two loops; ω n g represents the angular frequency of the nth eigenmode. an and g bn Let k represent the gain and loss coefficients of the nth eigenmode in the two components, respectively; n This represents the coupling coefficient between two components; when the gain coefficient is greater than the coupling coefficient, g n >k n The conjugate oscillating mode and the decaying mode form a pair of conjugate modes. The mode is in a parity-time broken state and satisfies the parity-time symmetric phase condition.

[0016] Preferably, the tunable laser source (6) emits pump light of different wavelengths, and the center frequency of the microwave photonic filter is obtained based on the wavelength of the pump light, and the scattering frequency matching condition of stimulated Brillouin scattering is satisfied based on the center frequency.

[0017] Preferably, the single-mode oscillation of the optoelectronic oscillator is achieved based on the parity-time symmetric phase condition and the stimulated Brillouin scattering frequency matching condition.

[0018] Preferably, the output signal of the photoelectric oscillator is:

[0019]

[0020] Among them, H SBS-MPF (ω) is the transfer function of SBS-MPF;

[0021] The power spectrum of the radio frequency signal generated by the cyclic oscillation of the initial single-frequency noise signal in the photoelectric oscillator is as follows:

[0022]

[0023] Where G(ω) is the effective loop gain, τ is the loop delay, τ=nL / c, n is the effective refractive index of the medium, L is the length of the HNLF, and the frequencies of the eigenmodes that satisfy the phase matching condition must satisfy:

[0024]

[0025] A frequency signal that is 2mπ out of phase with the fundamental frequency signal and simultaneously satisfies the frequency matching condition of stimulated Brillouin scattering and the phase condition of parity-time violation is used to complete the single-mode oscillation of the optoelectronic oscillator.

[0026] Compared with the prior art, the present invention has the following advantages and technical effects:

[0027] This invention provides a single-loop parity-time symmetric tunable opto-oscillator based on Stimulated Brillouin Scattering (SBS). The modulated signal passes through a microwave photonic filter composed of a laser, a phase modulator, a highly nonlinear fiber, a tunable laser, and a photodetector. By adjusting the pump light wavelength, the center frequency of the microwave photonic filter changes accordingly, ultimately achieving fine-tunable output frequency. While ensuring fine-tunability of the stimulated Brillouin scattering-based opto-oscillator, a z-cut lithium niobate phase modulator supporting both ordinary and extraordinary light modes is used, outputting modulated signals in both modes to form a parity-time symmetric structure within a single loop, simplifying the system structure. Attached Figure Description

[0028] The accompanying drawings, which form part of this application, are used to provide a further understanding of this application. The illustrative embodiments and descriptions of this application are used to explain this application and do not constitute an undue limitation of this application. In the drawings:

[0029] Figure 1 This is a schematic diagram of a single-loop parity-time symmetric tunable opto-oscillator based on SBS according to an embodiment of the present invention.

[0030] Figure 2The frequency response curve of the microwave photonic filter according to an embodiment of the present invention is shown.

[0031] Figure 3 This is the output spectrum diagram of an embodiment of the present invention that does not satisfy the PT broken state;

[0032] Figure 4 This is an output spectrum diagram of the PT broken state according to an embodiment of the present invention;

[0033] Figure 5 This is a tunable output spectrum diagram near the 18GHz frequency of an embodiment of the present invention;

[0034] Among them, 1-laser source, 2-polarization controller, 3-z-cut lithium niobate phase modulator, 4-optical isolator, 5-highly nonlinear fiber, 6-tunable laser, 7-polarization controller, 8-optical circulator, 9-photodetector, 10-microwave amplifier, 11-microwave power divider, 12-spectrum analyzer. Detailed Implementation

[0035] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. This application will now be described in detail with reference to the accompanying drawings and embodiments.

[0036] It should be noted that the steps shown in the flowchart in the accompanying drawings can be executed in a computer system such as a set of computer-executable instructions, and although a logical order is shown in the flowchart, in some cases the steps shown or described may be executed in a different order than that shown here.

[0037] Example 1

[0038] like Figure 1 As shown, this embodiment provides a single-loop parity-time symmetric tunable optoelectronic oscillator based on SBS, comprising: a microwave photonic filter based on stimulated Brillouin scattering (SBS) and a single-loop parity-time symmetric oscillation loop.

[0039] Microwave photonic filters based on stimulated Brillouin scattering are used to generate finely tunable filter passbands.

[0040] A single-loop parity-time symmetric oscillating loop is used to generate microwave signals.

[0041] The single-loop parity-time symmetric oscillation loop includes a laser source 1, which is sequentially connected to a polarization controller 2, a z-cut lithium niobate phase modulator 3, an optical isolator 4, a highly nonlinear fiber 5, and the left port of an optical circulator 8. A tunable laser 6 is sequentially connected to a polarization controller 7 and the upper port of the optical circulator 8. The right port of the optical circulator 8 is connected to a photodetector 9. The photodetector 9 is sequentially connected to a microwave amplifier 10 and a microwave power divider 11. One output terminal of the microwave power divider 11 is connected to the radio frequency input terminal of the z-cut lithium niobate phase modulator 3, and the other output terminal of the microwave power divider 11 is the microwave signal output terminal of the photoelectric oscillator.

[0042] The microwave photonic filter based on stimulated Brillouin scattering includes a laser source 1, which is sequentially connected to a polarization controller 2, a z-cut lithium niobate phase modulator 3, an optical isolator 4, a highly nonlinear optical fiber 5, and the left port of an optical circulator 8. A tunable laser 6 is sequentially connected to a polarization controller 7 and the upper port of the optical circulator 8. The right port of the optical circulator 8 is connected to a photodetector 9 to complete photoelectric conversion.

[0043] The single-loop parity-time symmetric structure includes a laser source 1, which is sequentially connected to a polarization controller 2, a z-cut lithium niobate phase modulator 3, an optical isolator 4, a highly nonlinear optical fiber 5, and the left port of an optical circulator 8. The right port of the optical circulator 8 is connected to a photodetector 9. The main function of the single-loop parity-time symmetric structure is to utilize physical properties to prevent more side modes from oscillating, thereby improving the side-mode suppression ratio.

[0044] Further optimization of the scheme: the SBS-based microwave photonic filter includes a laser source 1 and a tunable laser source 6. The center operating wavelength of the laser source 1 is 1550nm, the center operating wavelength of the tunable laser source 6 is 1550nm, and the maximum tuning accuracy is 0.0001nm.

[0045] Further optimization of the scheme: in the SBS-based microwave photonic filter, fiber 5 is a highly nonlinear fiber.

[0046] Further optimization of the scheme involves using a z-cut lithium niobate phase modulator 3 in the single-loop parity-time symmetric fine-tunable optoelectronic oscillator. This modulator supports both ordinary and extraordinary light modes, resulting in the actual output being modulated signals in both modes. By adjusting the polarization angle θ between the incident light and the z-axis, the power splitting ratio of the two modes can be adjusted, thereby satisfying the parity-time symmetric phase condition and ultimately achieving single-mode oscillation.

[0047] This embodiment provides the working principle of a single-loop parity-time symmetric finely tunable optoelectronic oscillator as follows:

[0048] After passing through the z-cut lithium niobate phase modulator 3, the two modes of output optical signals can be represented as follows:

[0049]

[0050]

[0051] In the above formula, θ is the polarization angle formed by the incident light and the z-axis. By adjusting the polarization angle θ, the optical power of the two modes can be changed. RF (t) is the microwave signal loaded onto the z-cut lithium niobate phase modulator 3, V RF (t)=V RF cos(ω m )t.

[0052] When the phase-modulated signal enters the highly nonlinear fiber 5, after being amplified by the gain spectrum generated by the SBS effect, one of the first-order sidebands is amplified. The amplified sideband is a positive first-order sideband. Therefore, the forward-propagating signal light can be expressed as:

[0053]

[0054]

[0055] The signal exhibits inconsistent amplitude due to amplification by the +1st order sideband. Therefore, after passing through photodetector 9, it can be used to output a microwave signal. Ignoring the DC and higher-order components in the output microwave signal, the resulting photocurrent is expressed as:

[0056]

[0057] Where, f = f p -f c -v B R is the PD responsiveness, f c with f p These are the signal light frequency and the pump light frequency, respectively. G(f) represents the amplitude gain due to the SBS effect in the highly nonlinear fiber 5. This indicates the phase change introduced by the SBS effect;

[0058] According to formula (5), the transfer function of the MPF can be derived as follows:

[0059]

[0060] Based on the above analysis, the optical filter can be converted into a microwave photonic bandpass filter through mapping. According to the theoretical analysis, the fine-tuning of the system's frequency is mainly achieved through the microwave photonic filter; therefore, the design of the relevant parameters of the microwave photonic filter is particularly important. The center frequency of the MPF is changed by altering the frequency interval between the pump light and the signal light. Assuming it is near a wavelength of 1550 nm, when f... RF A change of 1 GHz results in a change of Δλ = 0.008 nm, meaning that a 0.008 nm change in wavelength alters the filter's passband frequency by 1 GHz. Therefore, the filter's passband center frequency can be changed by altering the pump light wavelength, achieving tunable filtering. The tunable range depends on the adjustment range of the pump light wavelength and the measurable range of the photodetector. The minimum tunable resolution depends on the tunability accuracy of the pump light. Current lasers can achieve a tuning accuracy of 0.0001 nm; a 0.0001 nm change in pump light wavelength alters the filter's passband center frequency by 12.5 MHz. Figure 2 As shown, when the pump light wavelength changes from 1449.9755nm to 1449.9765nm, the center frequency of the MPF changes from 17.94GHz to 18.06GHz in steps of 12.5MHz.

[0061] When the system is in a parity-time symmetric state, the nth eigenmode in the parity-time symmetric system can be described by the coupled mode equations shown below.

[0062]

[0063] Among them, a n and b n These represent the nth eigenmode in each of the two loops; ω n G represents the angular frequency of the nth eigenmode; an and g bn Let k represent the gain and loss coefficients of the nth eigenmode in the two components, respectively; n This represents the coupling coefficient between the two components. When the system is in a stable oscillating state, solving the coupled mode equations, the eigenfrequency of the nth eigenmode in the parity-time symmetric system is shown in the following equation:

[0064]

[0065] Under parity-time symmetry, g an =-g bn =g n Under the above conditions, the eigenfrequency of the nth eigenmode of the system in formula (8) is rewritten as follows:

[0066]

[0067] According to formula (9), when the gain coefficient equals the coupling coefficient, g n =k n The system exhibits a singularity. When the gain coefficient is greater than the coupling coefficient, g n >k n The conjugate oscillating mode and the decaying mode form a conjugate mode pair. This mode pair is frequency degenerate, i.e., a single-mode oscillation, and at this time, the mode is in a parity-time broken state. When the gain coefficient is less than the coupling coefficient, g n <k n Any pair of intrinsic modes will exhibit bounded neutral oscillations. This mode exhibits mode splitting, and at this time, the mode is in a parity-time unbroken state.

[0068] The amplitude of the output signal of the parity-time symmetric photoelectric oscillator with an added SBS-based microwave photonic filter is also equal to the superposition of the amplitudes of each electric field cycle, and the expression for the output signal can be obtained as follows:

[0069]

[0070] In the above formula, H SBS-MPF (ω) is the transfer function of a microwave photonic filter based on stimulated Brillouin scattering.

[0071] The power spectrum of the radio frequency signal generated by the cyclic oscillation of the initial single-frequency noise signal in the photoelectric oscillator, obtained from formula (10), is as follows:

[0072]

[0073] In the above formula, G(ω) is the effective loop gain, τ is the loop delay, τ=nL / c, n is the effective refractive index of the medium, and L is the length of the HNLF. The frequencies of the eigenmodes that satisfy the phase matching condition must meet the following:

[0074]

[0075] As can be seen from the above equation, only frequency signals that are 2mπ out of phase with the fundamental frequency signal and simultaneously satisfy the stimulated Brillouin scattering frequency matching condition and the phase condition of the parity-time broken state can oscillate. The phase condition of the parity-time broken state is mainly related to the polarization angle θ. Through theoretical analysis and calculation, when θ is 0.3π, the system satisfies the parity-time broken phase state. At this time, the output signal is a single-mode signal with the maximum side-mode suppression ratio, such as... Figure 4 As shown; when θ is not 0.3π, the system does not satisfy the parity-time broken phase state condition, the system experiences mode splitting, and no single-mode output signal is obtained, such as... Figure 3 As shown.

[0076] The frequency of the system's output signal is f = fp -f c -v B The Brillouin frequency shift (MPF) is deterministic when the fiber type is fixed and the wavelength change is small. The center frequency of the MPF can be altered by changing the frequency spacing between the pump light and the signal light. Assuming a wavelength around 1550 nm, when f... RF A change of 1 GHz results in a change of Δλ = 0.008 nm, meaning that a 0.008 nm change in wavelength alters the filter's passband frequency by 1 GHz. Therefore, the filter's passband center frequency can be changed by altering the pump light wavelength, achieving tunable filtering. The tunable range depends on the adjustment range of the pump light wavelength and the measurable range of the photodetector. The minimum tunable resolution depends on the tunability accuracy of the pump light. Current lasers can achieve a tuning accuracy of 0.0001 nm; a 0.0001 nm change in pump light wavelength results in a 12.5 MHz change in the filter's passband center frequency. Figure 5 As shown, when the pump light wavelength changes from 1449.9755nm to 1449.9765nm, the center frequency of the MPF changes from 17.94GHz to 18.06GHz in steps of 12.5MHz, and the corresponding output signal of the system also changes from 17.94GHz to 18.06GHz, with a tuning accuracy of 12.5MHz.

[0077] The above description is merely a preferred embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A single-loop parity-time symmetric tunable opto-oscillator based on SBS, characterized in that, include: The laser source (1), the left port of the laser source (1), the first polarization controller (2), the z-cut lithium niobate phase modulator (3), the optical isolator (4), the highly nonlinear fiber (5), and the optical circulator (8) are connected in sequence, the upper port of the tunable laser (6), the second polarization controller (7), and the optical circulator (8) are connected in sequence, and the right port of the optical circulator (8) is connected to the photodetector (9). The structure composed of the laser source (1), the first polarization controller (2), the z-cut lithium niobate phase modulator (3), the optical isolator (4), the highly nonlinear fiber (5), the tunable laser (6), the second polarization controller (7), the optical circulator (8), and the photodetector (9) constitutes a microwave photonic filter based on stimulated Brillouin scattering. The microwave photonic filter is used to generate a finely tunable filter passband. The photodetector (9), microwave amplifier (10), and microwave power divider (11) in the microwave photonic filter are connected in sequence. One output terminal of the microwave power divider (11) is connected to the radio frequency input terminal of the z-cut lithium niobate phase modulator (3), and the other output terminal of the microwave power divider (11) is the microwave signal output terminal of the photoelectric oscillator. The structure composed of the microwave photonic filter, microwave amplifier (10), and microwave power divider (11) constitutes a single-loop parity-time symmetric oscillation loop, which is used to generate microwave signals. The z-cut lithium niobate phase modulator (3) outputs modulation signals in ordinary light mode and extraordinary light mode. By adjusting the polarization angle θ between the incident light and the z-axis, the power splitting ratio of the modulation signal is adjusted, thereby satisfying the parity-time symmetric phase condition. By changing the wavelength of the pump light emitted by the tunable laser (6), the center frequency of the microwave photonic filter is changed, and the center frequency satisfies the stimulated Brillouin scattering frequency matching condition.

2. The SBS-based single-loop parity-time symmetric tunable opto-oscillator according to claim 1, characterized in that, The single-loop parity-time symmetric oscillating loop includes a single-loop parity-time symmetric structure, which includes: the left port of the laser source (1), the first polarization controller (2), the z-cut lithium niobate phase modulator (3), the optical isolator (4), the highly nonlinear fiber (5), and the optical circulator (8) connected in sequence, and a photodetector (9) connected to the right port of the optical circulator (8).

3. The SBS-based single-loop parity-time symmetric tunable opto-oscillator according to claim 1, characterized in that, When a parity-time symmetric system is in a parity-time symmetric state, the coupled mode equations of the nth eigenmode are: ； Solve the coupled mode equations: ； like g an = -g bn = g n ,but ； in, a n and b n These represent the nth eigenmode in each of the two loops; ω n This represents the angular frequency of the nth eigenmode; g an and g bn These represent the gain coefficient and loss coefficient of the nth eigenmode in the two components, respectively. k n This represents the coupling coefficient between two components; when the gain coefficient is greater than the coupling coefficient, g n >k n The conjugate oscillating mode and the decaying mode form a pair of conjugate modes. The mode is in a parity-time broken state and satisfies the parity-time symmetric phase condition.

4. The SBS-based single-loop parity-time symmetric tunable opto-oscillator according to claim 3, characterized in that, When the parity-time symmetric phase condition and the stimulated Brillouin scattering frequency matching condition are satisfied, the single-mode oscillation of the photoelectric oscillator is completed.

5. The SBS-based single-loop parity-time symmetric tunable opto-oscillator according to claim 4, characterized in that, Output signal of the photoelectric oscillator: ； in, H SBS-MPF (ω) The transfer function for SBS-MPF; The power spectrum of the radio frequency signal generated by the cyclic oscillation in the optoelectronic oscillator is as follows: ； in, G (ω) is the effective loop gain, τ is the loop delay, τ=qL / c, q is the effective refractive index of the medium, L is the length of the HNLF, and the frequencies of the eigenmodes that satisfy the phase matching condition must meet the following: ； A frequency signal that is 2mπ out of phase with the fundamental frequency signal and simultaneously satisfies the stimulated Brillouin scattering frequency matching condition and the phase condition of the parity-time broken state enables the optoelectronic oscillator to complete single-mode oscillation.

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