Microwave photon filter based on stimulated Brillouin scattering effect

Abstract

The invention belongs to the technical field of microwave photonics, and particularly provides a microwave photonic filter based on a stimulated Brillouin scattering effect, which is used for solving the problems that the bandwidth is difficult to flexibly control, the relative intensity of each piece of pump light is difficult to control and the like in the existing microwave photonic filter based on the stimulated Brillouin scattering effect. The BPSK is used for modulating the pump light, flexible reconstruction of a filter passband can be achieved by adjusting the pulse modulation rate B or changing the P value, and meanwhile good tunability is achieved; besides, the lower light sideband generated by phase modulation is processed by utilizing the Brillouin loss spectrum in the MPF, so that the saturation effect generated by the Brillouin gain spectrum can be effectively avoided, and the input dynamic range of the MPF is expanded.

Description

Technical Field

[0001] This invention belongs to the field of microwave photonics technology, specifically providing a microwave photonic filter based on stimulated Brillouin scattering effect. Background Technology

[0002] Microwave photonics (MWP) is a novel interdisciplinary field that integrates microwave and photonic technologies. Current research primarily focuses on the information domain. Generally, microwave photonics can be divided into two main categories: microwave photonic devices and microwave photonic signal processing. The former emphasizes fundamental research on the generation and manipulation of physical quantities such as electromagnetic fields, photons, and electrons within devices; the latter focuses on converting microwave signals into the optical domain, using optical methods to achieve functions such as microwave signal generation, transmission, conversion, and control. Microwave photonics overcomes the electronic bottlenecks of traditional microwave technology in terms of processing speed and transmission bandwidth, offering advantages such as wide operating frequency bands, large transmission bandwidth, and resistance to electromagnetic interference.

[0003] Filters are fundamental to signal processing systems and are an indispensable component of modern mobile communications, relay communications, satellite communications, and radar systems. The basic purpose of a filter is to select or eliminate a specific frequency range in the input signal spectrum. However, traditional filters, due to their microwave signal processing methods, are limited by electronic bottlenecks, making it difficult to achieve tuning and passband reconstruction at high frequencies. Microwave photonic filters (MPFs), as a branch of MWP development, are optical subsystems used in microwave (Radio Frequency, RF) links and systems to perform the same tasks as traditional filters. Unlike traditional filters, MPFs first modulate the electrical signal onto an optical carrier, then process the signal in the optical domain using optoelectronic devices such as optical filters, and finally output the filtered electrical signal through photoelectric conversion. Compared to traditional filters, MPFs have advantages such as large bandwidth, low transmission loss, and resistance to electromagnetic interference, overcoming the electronic bottlenecks faced by traditional filters and achieving a large tuning range and passband reconstruction at high frequencies.

[0004] In existing MPF (Multi-Purpose Photonic Filter) technologies, MPFs based on the Stimulated Brillouin Scattering (SBS) effect utilize the narrowband characteristics of the SBS gain. Their complex coefficients are easy to control, and their threshold is low, resulting in high suppression ratios when modulating the amplitude and phase of the carrier signal. Furthermore, by designing the SBS effect and pump light, broadband tunability of the filter can be achieved, and the MPF passband shape and bandwidth can be reconfigured, making it one of the more mainstream technologies. However, existing reconfigurable SBS-based MPFs struggle to flexibly control the filter bandwidth by the number of pump lights, and controlling the relative intensity of individual pump lights becomes difficult when the number of pump lights is large. Additionally, most existing technologies are affected by the SBS gain saturation effect, resulting in a low input dynamic range for the MPF. To address these issues, this invention provides a novel microwave photonic filter based on the Stimulated Brillouin Scattering effect. Summary of the Invention

[0005] The purpose of this invention is to provide a microwave photonic filter based on stimulated Brillouin scattering (SBS) to solve the problems of difficult bandwidth control and difficulty in controlling the relative intensity of each pump light in existing microwave photonic filters based on SBS. The microwave photonic filter of this invention uses binary phase shift keying (BPSK) modulation technology to control the power spectral density of the pump light, thereby achieving flexible control of the filter passband. At the same time, it uses Brillouin loss spectrum to process the sideband of the output light of the phase modulator, avoiding the influence of SBS gain saturation effect.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A microwave photonic filter based on stimulated Brillouin scattering effect includes: a first laser source 1 (TLS1), a pulse pattern generator 2 (PPG), a Mach-Zehnder modulator 3 (MZM), an erbium-doped fiber amplifier 4 (EDFA), an optical circulator 5 (CIR), a second laser source 6 (TLS2), a phase modulator 7 (PM), a signal source 8 (SG), an optical isolator 9 (ISO), a highly nonlinear fiber 10 (HNLF), and a photodetector 11 (PD); characterized in that:

[0008] The optical signal emitted by the first laser source enters the Mach-Zehnder modulator. The Mach-Zehnder modulator outputs a broadened optical signal under BPSK modulation generated by the pulse code generator. The broadened optical signal is amplified by the erbium-doped fiber amplifier and used as pump light. It is transmitted into the nonlinear fiber to generate stimulated Brillouin scattering effect and excite the Brillouin loss spectrum.

[0009] The optical signal emitted by the second laser source enters the phase modulator. The signal source emits a microwave signal to be filtered to modulate the optical carrier in the phase modulator. The optical carrier generates upper and lower optical sidebands under the modulation of the microwave signal to be filtered. The optical carrier and the two optical sidebands serve as signal light, which enters the highly nonlinear optical fiber through the optical isolator. The lower sideband of the signal light falls into the Brillouin loss spectrum and is attenuated. After attenuation, the signal light reaches the photodetector through the optical circulator for photoelectric conversion. The photodetector outputs the filtered electrical signal.

[0010] Furthermore, the frequency response function of the microwave photonic filter is:

[0011]

[0012] Where H′ represents the frequency response function of the microwave photonic filter, f RF V is the frequency of the radio frequency signal. π Let R be the half-wave voltage, R be the responsivity of the photodetector PD, and G be the transfer function of the Brillouin loss spectrum.

[0013] Furthermore, the center frequency of the Brillouin loss spectrum is the source frequency f of the first laser source. P0 With Brillouin frequency shift v B RF signal frequency f RF sum.

[0014] Furthermore, the Mach-Zehnder modulator operates in a carrier-suppressed double-sideband modulation mode.

[0015] Furthermore, the length of the highly nonlinear optical fiber used as the medium for exciting the stimulated Brillouin scattering effect is preferably 1km to 10km. Appropriately increasing its length can reduce the threshold for generating the stimulated Brillouin scattering effect.

[0016] In terms of working principle:

[0017] This invention provides a microwave photonic filter based on stimulated Brillouin scattering, wherein the carrier signal output from the laser source TLS1 is expressed as:

[0018]

[0019] In the formula, ω c =2πf c Let be the angular frequency of the optical carrier wave, and t be the time variable. The initial phase is BPSK modulation; it does not change the amplitude of the optical carrier, but instead loads the binary phase between 0 and π onto the pump light. When "0" is transmitted, the phase... When a "1" is sent, the phase The expression for the BPSK signal can then be written as:

[0020]

[0021] In the formula, A and -A represent the amplitude of the BPSK signal;

[0022] The power spectrum of light modulated by BPSK depends entirely on the characteristics of the modulation data mode; therefore, the P value is defined as the flip probability of 0 or 1. For example, when P = 1 / 2, it is in a completely random mode, while when P = 1, it is in a fixed mode such as 010101... The normalized autocorrelation function expression of the electric field modulated by BPSK is:

[0023] c(τ)=(1-2P) n [1-2P(B|τ|-n)]

[0024] n≤B|τ| <n+1

[0025] In the formula, B is the pulse modulation rate, n is an integer, and τ is the time delay; by performing a Fourier transform on the above formula, the normalized power spectral density of BPSK modulation is obtained as follows:

[0026]

[0027] BPSK modulation is used to broaden the power spectral density of the pump light. By changing the input pulse modulation rate, the effective Brillouin scattering loss spectrum linewidth is adjusted, thereby achieving the goal of broadening the filter passband.

[0028] Assume the power spectrum of the pump light is P p (f p The intensity of the pump light I p (f p ) can be represented as I p (f p )=f p (f p ) / A eff Then the intensity expression of the pump light can be obtained as:

[0029]

[0030] Therefore, the complete forms of the effective Brillouin gain function g(f) and effective Brillouin loss function α(f) caused by this pump light should be written as:

[0031]

[0032]

[0033] In the formula, I P0 v represents the intensity of the optical carrier emitted by the laser source TLS1. BRepresents the Brillouin frequency shift, Δv B f is the Brillouin line width. P f is the frequency of the pump light. p0 f is the frequency of the optical carrier emitted by the laser source TLS1. RF For the frequency of the radio frequency signal, g i (f)=g B / [1-i(f+v B -f P ] is the intrinsic Brillouin gain function, α i (f)=g B / [1-i(fv B -f P )] is the intrinsic Brillouin loss function.

[0034] As can be seen from the above formula, by using BPSK modulation to broaden the power spectral density of the pump light, different power waveforms of the pump light can be obtained by adjusting the effective Brillouin scattering gain / loss spectrum by changing the P value and the modulation rate B of the input pulse code. When the modulation rate B is fixed, the P value needs to be adjusted to make the top of the waveform flat, so that a filtered signal with better flatness can be obtained at the detector.

[0035] The optical carrier emitted by the tunable laser source TLS2(6) is input into the phase modulator. The expression for the input optical carrier can be expressed as E0exp(j2πf c Let V be the low-power microwave signal output by the signal source. m cos(2πf m t) The optical carrier in the phase modulator is modulated. Under small-signal modulation conditions, the output optical field can be expressed as:

[0036]

[0037] In the formula, J0(β) and J1(β) are the 0th and 1st order Bessel functions, respectively. V m V is the amplitude of the output signal from the signal source. π For half-wave voltage, f m The frequency of the signal to be filtered is denoted as .

[0038] An optical carrier and two optical sidebands are input into a highly nonlinear optical fiber. Within the highly nonlinear ray, the intensity of the lower optical sideband is affected by the Brillouin loss effect. Therefore, the optical field detected by the photodetector is:

[0039] E PD (t)=E0{J0(β)exp(j2πf c t)+J1(β)exp[j(2πf c t+2πfm t+π / 2)]-G(f)·J1(β)exp[j(2πf c t-2πf m t-π / 2)]}

[0040] In a photodetector, the optical carrier and the upper and lower optical sidebands undergo photoelectric conversion, and the output electrical signal is:

[0041]

[0042] In the formula, R represents the responsivity of the photodetector;

[0043] Therefore, the frequency response function of the MPF is:

[0044]

[0045] In the formula

[0046]

[0047] L is the length of the highly nonlinear optical fiber;

[0048] Based on the frequency response function of the MPF mentioned above, it can be seen that, theoretically, the intermediate frequency of the MPF passband should be:

[0049] f center =f c -f p -v B

[0050] As the above analysis shows, the passband shape of the MPF in this invention depends on the shape of the Brillouin loss spectrum of the mobile phone. The shape of the Brillouin loss spectrum is related to the pulse code modulation rate and the P value. Therefore, by changing the P value and the pulse code modulation rate B, the MPF passband width can be adjusted, thus achieving MPF passband reconstruction. According to the frequency response function of the MPF, the center frequency of the filter passband depends on f. c -f p -v B Therefore, by changing the pump light frequency f p The optical carrier frequency f of the second laser source TLS2 in the lower branch c It allows for adjustment of the MPF center frequency. Furthermore, this MPF utilizes the Brillouin loss spectrum to process the optical sidebands of the phase modulator output, effectively avoiding gain saturation and improving the dynamic range of the input signal.

[0051] In summary, the beneficial effects of the present invention are as follows:

[0052] This invention provides a microwave photonic filter based on stimulated Brillouin scattering (BPSK). By modulating the pump light using BPSK, the passband of the filter can be flexibly reconstructed by adjusting the pulse modulation rate B or changing the value of P, while also exhibiting good tunability. Furthermore, by processing the lower optical sideband generated by phase modulation using the Brillouin loss spectrum within the MPF, the saturation effect generated by the Brillouin gain spectrum can be effectively avoided, thereby improving the input dynamic range of the MPF. Attached Figure Description

[0053] Figure 1 A schematic diagram of the structure of a microwave photonic filter based on stimulated Brillouin scattering effect provided by the present invention.

[0054] Figure 2 This is a schematic diagram illustrating the principle of spectral processing in a microwave photonic filter based on stimulated Brillouin scattering provided by the present invention.

[0055] Figure 3 The power spectrum of binary phase shift keying modulation in a microwave photonic filter based on stimulated Brillouin scattering provided by this invention.

[0056] Figure 4 The frequency response curves of the microwave photonic filter based on stimulated Brillouin scattering effect and the conventional microwave photonic filter provided by the present invention are shown under different input signal light power. Among them, (a) represents the frequency response of the lower optical sideband of the conventional microwave photonic filter under the action of Brillouin gain spectrum, and (b) represents the frequency response of the lower optical sideband of the present invention under the action of Brillouin loss. Detailed Implementation

[0057] To make the objectives, technical solutions, and beneficial effects of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0058] This example provides a microwave photonic filter based on stimulated Brillouin scattering, the structure of which is as follows: Figure 1 As shown, it specifically includes: a first laser source 1 (TLS1), a pulse pattern generator 2 (PPG), a Mach-Zehnder modulator 3 (MZM), an erbium-doped fiber amplifier 4 (EDFA), an optical circulator 5 (CIR), a second laser source 6 (TLS2), a phase modulator 7 (PM), a signal source 8 (SG), an optical isolator 9 (ISO), a highly nonlinear fiber 10 (HNLF), and a photodetector 11 (PD); specifically:

[0059] The optical signal emitted by the laser source TLS1 enters the Mach-Zehnder modulator MZM. The Mach-Zehnder modulator MZM outputs a broadened optical signal under the BPSK modulation generated by the pulse pattern generator PPG. The broadened optical signal is amplified by the erbium-doped fiber amplifier EDFA and used as pump light. It is transmitted into the nonlinear fiber HNLF to generate stimulated Brillouin scattering effect and excite the Brillouin loss spectrum.

[0060] The optical signal emitted by the laser source TLS2 enters the phase modulator PM. The signal source SG emits a microwave signal to be filtered, which modulates the optical carrier in the phase modulator PM. The optical carrier generates upper and lower optical sidebands under the modulation of the microwave signal to be filtered. The optical carrier and the two optical sidebands serve as signal light, which enters the highly nonlinear fiber HNLF through the optical isolator ISO. The lower sideband of the signal light falls into the Brillouin loss spectrum and is attenuated (stimulated Brillouin effect), which breaks the phase balance. Then the signal light reaches the photodetector PD through the optical circulator CIR for photoelectric conversion. The photodetector PD outputs the filtered electrical signal.

[0061] The above-described spectral processing procedure for a microwave photonic filter based on stimulated Brillouin scattering using BPSK modulation is as follows: Figure 2 As shown, specifically:

[0062] The laser source TLS1 outputs a frequency of f P0 The light source is modulated by BPSK from a pulse pattern generator within a Mach-Zehnder modulator, stabilizing the modulator in carrier-suppressed double-sideband modulation mode. The BPSK-modulated, bandwidth-broadened pump light is amplified by an erbium-doped fiber amplifier before entering a highly nonlinear fiber. This will generate a bandwidth-broadened Brillouin loss spectrum within the fiber, with the center frequency of the loss spectrum being the frequency f of the first laser source. P0 With radio frequency signal frequency f RF Brillouin frequency shift v B ;

[0063] The frequency output of the laser source TLS2 is f c The optical carrier enters the phase modulator PM and is modulated by a frequency f. m After the microwave signal to be filtered is modulated, the phase modulator outputs an optical carrier and two upper and lower optical sidebands, which are transmitted as signal light into a highly nonlinear optical fiber. However, since the two optical sidebands have the same amplitude and an initial phase difference of π, the microwave signal loaded on the phase modulator cannot be recovered when the photodetector performs photoelectric conversion. Therefore, it is necessary to use the stimulated Brillouin effect to break the phase balance.

[0064] When the lower sideband of the signal light falls into the Brillouin loss spectrum, the signal light sideband falling into the loss spectrum will be attenuated, while the amplitude of other signal light sidebands remains unchanged; when photoelectric conversion is performed on the photodetector, the microwave signal loaded on the phase modulator can be recovered, realizing phase-intensity modulation conversion;

[0065] Because of the phase-intensity modulation conversion, the passband shape of the filter is determined by the shape of the Brillouin loss spectrum. Therefore, the passband of the filter can be reconstructed by changing the shape of the Brillouin loss spectrum.

[0066] The power spectrum of the BPSK modulation in the microwave photonic filter based on stimulated Brillouin scattering is as follows: Figure 3 As shown, it can be seen that even if the value of P changes very little, the shape of the power spectrum of BPSK modulation changes significantly; in order to make the microwave photonic filter based on stimulated Brillouin scattering effect have a relatively flat passband, the flip probability of adjacent bit logic states, i.e., the value of P, is taken as 9 / 16 in this invention.

[0067] The frequency response of the microwave photonic filter based on stimulated Brillouin scattering in this invention and the conventional microwave photonic filter under different input signal optical powers are as follows: Figure 4 As shown, (a) represents the frequency response of the lower optical sideband in a conventional microwave photonic filter under the influence of Brillouin gain spectrum, and (b) represents the frequency response of the lower optical sideband in this invention under the influence of Brillouin loss; from Figure 4 As can be seen from (a), the stimulated Brillouin gain response gradually decreases with the increase of the input signal optical power. This indicates that under the influence of the Brillouin gain spectrum, the gain obtained by the optical sideband will be affected by the optical sideband power, which will affect the dynamic range of the filter. When a microwave signal with a large dynamic range passes through this filter, distortion will occur. Figure 4 As can be seen in (b), as the signal optical power increases, the maximum value of the stimulated Brillouin loss remains at around 39dB. This indicates that under the action of Brillouin loss, the optical sideband has almost the same frequency response under different signal optical powers, which means that the microwave photonic filter based on the stimulated Brillouin loss spectrum is a linear system. Therefore, compared with the filter using the Brillouin gain spectrum, the microwave photonic filter based on the stimulated Brillouin loss spectrum can handle microwave signals with a larger dynamic range and has a larger dynamic range.

[0068] The above description is merely a specific embodiment of the present invention. Any feature disclosed in this specification may be replaced by other equivalent or similar features unless otherwise specified. All disclosed features, or steps in all methods or processes, may be combined in any way except for mutually exclusive features and / or steps.

Claims

1. A microwave photonic filter based on stimulated Brillouin scattering, comprising: The system comprises a first laser source (TLS1), a pulse pattern generator (PPG), a Mach-Zehnder modulator (MZM), an erbium-doped fiber amplifier (EDFA), an optical circulator (CIR), a second laser source (TLS2), a phase modulator (PM), a signal source (SG), an optical isolator (ISO), a high nonlinear fiber (HNLF), and a photodetector (PD); characterized in that: The optical signal emitted by the first laser source enters the Mach-Zehnder modulator. The Mach-Zehnder modulator outputs a broadened optical signal under BPSK modulation generated by the pulse code generator. The broadened optical signal is amplified by the erbium-doped fiber amplifier and used as pump light. It is transmitted into the nonlinear fiber to generate stimulated Brillouin scattering effect and excite the Brillouin loss spectrum. The optical signal emitted by the second laser source enters the phase modulator. The signal source emits a microwave signal to be filtered to modulate the optical carrier in the phase modulator. The optical carrier generates upper and lower optical sidebands under the modulation of the microwave signal to be filtered. The optical carrier and the two optical sidebands serve as signal light, which enters the highly nonlinear optical fiber through the optical isolator. The lower sideband of the signal light falls into the Brillouin loss spectrum and is attenuated. After attenuation, the signal light reaches the photodetector through the optical circulator for photoelectric conversion. The photodetector outputs the filtered electrical signal.

2. The microwave photonic filter based on stimulated Brillouin scattering effect according to claim 1, characterized in that, The frequency response function of the microwave photonic filter is: Where H′ represents the frequency response function of the microwave photonic filter, f RF V is the frequency of the radio frequency signal. π Let R be the half-wave voltage, R be the responsivity of the photodetector PD, and G be the transfer function of the Brillouin loss spectrum.

3. The microwave photonic filter based on stimulated Brillouin scattering effect according to claim 1, characterized in that, The center frequency of the Brillouin loss spectrum is the source frequency f of the first laser source. P0 With Brillouin frequency shift v B Radio frequency signal frequency f RF sum.

4. The microwave photonic filter based on stimulated Brillouin scattering effect according to claim 1, characterized in that, The Mach-Zehnder modulator operates in carrier-suppressed double-sideband modulation mode.

5. The microwave photonic filter based on stimulated Brillouin scattering effect according to claim 1, characterized in that, The length of highly nonlinear optical fibers ranges from 1 km to 10 km.

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