CN116626953A - Microwave photon filter based on stimulated Brillouin scattering effect - Google Patents

Abstract

The invention belongs to the technical field of microwave photonics, and provides a microwave photon filter based on the stimulated Brillouin scattering effect, which is used to solve the problem that the existing microwave photon filter is affected by the stimulated Brillouin scattering gain saturation effect and the input dynamics lower range issues. The invention includes laser source CW, arbitrary waveform generator AWG, dual parallel Mach-Zehnder modulator DPMZM, erbium-doped fiber amplifier EDFA, optical circulator OC, tunable laser source TLS, phase modulator PM, signal source SG, optical isolation ISO, highly nonlinear fiber HNLF, and photodetector PD, in the highly nonlinear fiber, the lower optical sidebands generated by the phase modulation are processed by the stimulated Brillouin loss spectrum excited by the pump light instead of the gain spectrum , avoiding the influence of the stimulated Brillouin gain saturation effect, so that the microwave photonic filter has a large input dynamic range; in addition, the filter also has good tuning and passband reconstruction characteristics.

Description

A Microwave Photonic Filter Based on Stimulated Brillouin Scattering Effect

technical field

The invention belongs to the technical field of microwave photonics, and specifically provides a microwave photon filter based on the stimulated Brillouin scattering effect.

Background technique

In a communication system, the quality of a transmission signal is usually affected by various noises and spurious signals in the communication system; in order to obtain a high-quality signal, it is necessary to eliminate or suppress the noise and spurious signals in the communication system, thereby improving the signal quality. Spectral purity; to achieve this, the signal is usually processed using filters. However, traditional microwave filters are limited by electronic bottlenecks, making it difficult to achieve tuning and passband reconstruction at high frequencies.

Microwave photonics is an emerging interdisciplinary subject integrating microwave technology and photonics technology. It mainly studies the interaction between microwave and light, and specifically studies how to generate, transmit and process microwave signals through optical methods; microwave photonics can overcome the traditional microwave The electronic bottleneck of technology in terms of processing speed and transmission bandwidth has the advantages of wide working frequency band, large transmission bandwidth, small transmission loss, and no electromagnetic interference. Microwave photonic filters are one of the important products in the development of microwave photonics. They are used to undertake the same tasks as traditional microwave filters in RF links and systems, and to solve the electronic bottlenecks faced by traditional microwave filters. In traditional microwave filters, RF signals from RF sources or antennas are injected into an RF circuit for processing; while in microwave photonic filters, RF signals are first loaded onto optical carriers and then passed through optical means of filtering.

It can be seen that, as a microwave photonic system, in addition to the above-mentioned advantages of microwave photonics, the microwave photonic filter can also overcome the electronic bottleneck faced by the traditional microwave filter, and can achieve a large tuning range and passband in the high frequency band. refactor. Among many microwave photonic filters, the microwave photonic filter based on the stimulated Brillouin scattering effect has become a research hotspot because of its more flexibility in tuning and passband reconstruction; while the existing stimulated Brillouin scattering based Most of the microwave photonic filters are affected by the gain saturation effect of stimulated Brillouin scattering, which makes the input dynamic range of the microwave photonic filter low, and when facing a signal with a large dynamic range, the signal will be distorted.

Contents of the invention

The object of the present invention is to solve the problem that the existing microwave photon filter based on the stimulated Brillouin scattering effect is affected by the saturated effect of the stimulated Brillouin scattering gain and has a low input dynamic range, and provides a kind of microwave photon filter based on the stimulated Brillouin scattering effect. The microwave photon filter of deep scattering effect; Microwave photon filter uses Brillouin loss spectrum (rather than gain spectrum) to process the optical sideband of phase modulator output among the present invention, has avoided stimulated Brillouin scattering gain saturation effect, thereby effectively improving the input dynamic range of the device.

To achieve the above object, the technical scheme adopted in the present invention is as follows:

A microwave photon filter based on stimulated Brillouin scattering effect, including: laser source CW (1), arbitrary waveform generator AWG (2), dual parallel Mach-Zehnder modulator DPMZM (3), erbium-doped fiber amplifier EDFA(4), optical circulator OC(5), tunable laser source TLS(6), phase modulator PM(7), signal source SG(8), optical isolator ISO(9), high nonlinear fiber HNLF (10), photodetector PD (11); It is characterized in that:

The light emitted by the laser source CW(1) enters the dual-parallel Mach-Zehnder modulator DPMZM(3), and the dual-parallel Mach-Zehnder modulator DPMZM(3) outputs under the electric frequency comb modulation from the arbitrary waveform generator AWG(2) Optical frequency comb, the output optical frequency comb is amplified by the erbium-doped fiber amplifier EDFA (4) and then enters the highly nonlinear optical fiber HNLF (10) as pump light to generate stimulated Brillouin scattering effect and excite the Brillouin loss spectrum Attenuating the reverse propagating light in the highly nonlinear fiber HNLF (10);

The tunable laser source TLS (6) emits an optical carrier into the phase modulator PM (7), and the signal source SG (8) emits a microwave signal to modulate the optical carrier in the phase modulator PM (7). The upper and lower optical sidebands will be generated; the optical carrier and the two optical sidebands are used as reverse transmission light, and undergo stimulated Brillouin attenuation in the highly nonlinear optical fiber HNLF (10) through the optical isolator ISO (9) Afterwards, the optical circulator OC (5) reaches the photodetector PD (11) for photoelectric conversion, and the photodetector PD (11) outputs an electrical signal.

Further, the frequency response function of the microwave photon filter is:

Among them, H(f _e ) represents the frequency response function of the microwave photonic filter, f _e is the frequency of the microwave signal output by the signal source SG, f _c is the center frequency of the optical carrier output by the tunable laser source TLS, and V _π is the half-wave voltage , R is the responsivity of the photodetector PD, and _HSBS is the transfer function of the Brillouin loss spectrum.

Furthermore, by using the Brillouin loss spectrum instead of the gain spectrum to attenuate the phase modulation optical sidebands, this processing method can avoid the influence of the gain saturation effect of the stimulated Brillouin scattering, thereby increasing the input of the filter Dynamic Range.

Further, the optical isolator ISO (9) is used to eliminate the pump light output from the highly nonlinear fiber HNLF (10), so as to avoid the influence on the tunable laser source TLS (6).

Furthermore, the highly nonlinear optical fiber HNLF (10) is used as a medium to stimulate the stimulated Brillouin scattering effect, and its length can be selected from 1 to 10 km. Properly increasing its length can reduce the threshold of the stimulated Brillouin scattering effect .

Further, by adjusting the output signal of the arbitrary waveform generator AWG (2), the pumping optical frequency comb is changed, thereby changing the Brillouin loss spectrum, and finally realizing the reconstruction of the passband of the filter.

Further, by adjusting the optical carrier frequency output by the tunable laser source TLS (6), the tuning of the central frequency of the passband of the filter is realized.

In terms of working principle:

The present invention provides a microwave photon filter based on the stimulated Brillouin scattering effect, and the light field E _0-in (t) output by the laser source CW can be expressed as:

E _0-in (t)＝E ₀ exp(j2πf ₀ t)

In the formula, E ₀ is the amplitude of the output light field of the laser source CW, f ₀ is the center frequency of the output light field of the laser source CW, and t is the time variable;

The dual-parallel Mach-Zehnder modulator consists of three Mach-Zehnder modulators, and there are two sub-MZM-α and MZM-β respectively on the two interference arms of a mother Mach-Zehnder modulator MZM-γ , the transfer function of the dual parallel Mach-Zehnder modulator is:

In the formula, V _π is the half-wave voltage, V _RF-α and V _RF-β are the RF signals loaded on MZM-α and MZM-β respectively, V _bias-α , V _bias-β and V _bias-γ are Bias voltage on MZM-α, MZM-β and MZM-γ;

When MZM-α and MZM-β are biased at the lowest power transmission point, and load RF signals with the same frequency and amplitude and a phase difference of π/2, that is, V _RF-α = V _RF cos(2πf _m t) and V _RF-β = V _RF sin(2πf _m t), V _RF is the amplitude of the radio frequency signal, f _m is the frequency of the radio frequency signal; at the same time, the bias voltage on the MZM-γ is introduced between the two sub-Mach-Zehnder modulators When the phase shift is π/2, the transfer function can be expressed as follows:

When V _RF is small, after the light emitted by the laser source CW passes through the dual-parallel Mach-Zehnder modulator, the light field becomes:

It can be seen from the above formula that after the light output by the laser source CW passes through the dual-parallel Mach-Zehnder modulator, the frequency of the light shifts down, and the frequency shift is equal to the frequency of the loaded radio frequency signal.

When an arbitrary waveform generator emits an electrical frequency comb After inputting to the dual parallel Mach-Zehnder modulator, the output light field is:

The optical field expressed in the above formula is an optical frequency comb, and the optical frequency comb is amplified by the erbium-doped fiber amplifier and enters the high nonlinear fiber as pump light to excite the Brillouin loss spectrum; the transmission of the loss spectrum under the action of the optical frequency comb The function can be expressed as:

In the formula, Γ _B /2π represents the intrinsic bandwidth of the Brillouin loss spectrum, L represents the length of the highly nonlinear fiber, g ₀ represents the Brillouin peak gain coefficient, f _B represents the Brillouin frequency shift, and I _m represents the optical frequency The intensity of each tooth in the comb; the shape of the visible loss spectrum can be reconstructed by changing the tooth spacing, number and intensity of the optical frequency comb.

The optical carrier emitted by the tunable laser source TLS is input into the phase modulator, and the low-power radio frequency signal V _e cos (2πf _e t) output by the signal source modulates the optical carrier in the phase modulator. Under small signal modulation, The output optical field of the phase modulator only includes the optical carrier and the upper and lower optical sidebands; if the input optical carrier E _c-in (t) is:

E _c-in (t)＝E _c exp(j2πf _c t)

In the formula, _Ec is the amplitude of the output light field of the tunable laser source TLS, and _fc is the center frequency of the output light field of the tunable laser source TLS;

Then the output light field can be expressed as:

In the formula, J ₀ (β) and J ₁ (β) are Bessel functions of order 0 and order 1, respectively, V _e is the output signal amplitude of the signal source;

The optical carrier and the upper and lower optical sidebands are input into a highly nonlinear optical fiber. In the highly nonlinear light, the intensity of the lower optical sideband is affected by the Brillouin loss effect, and the optical field detected on the photodetector for:

On the photodetector, the optical carrier and the upper and lower optical sidebands undergo photoelectric conversion, and the output radio frequency signal is:

In the formula, R represents the responsivity of the photodetector;

Therefore, the frequency response function of the microwave photonic filter is:

From the frequency response function of the microwave photonic filter, when f _c -f _e is in the loss spectrum, H _SBS (f _c -f _e )<1, H(f _e )≠0, which means that the filter allows the frequency The signal passes through to form a passband; when f _c -f _e is outside the loss spectrum, H _SBS (f _e )=1, H(f _c -f _e )=0, which means the filter does not allow the signal of this frequency pass, thereby forming a stop band.

As can be seen from the previous analysis, the shape of the passband of the microwave photonic filter in the present invention depends on the shape of the Brillouin loss spectrum, and the Brillouin loss spectrum can be changed by adjusting the output of the arbitrary waveform generator, so it can be obtained by Changing the output of the arbitrary waveform generator realizes the reconstruction of the passband of the filter. From the frequency response function of the microwave photonic filter, it can be seen that the center frequency of the passband depends on the size of f _c -f _e , so by adjusting the output optical carrier frequency f _c of the tunable laser source TLS, the filter pass can be realized Tuning with center frequency. In addition, the microwave photonic filter avoids the gain saturation effect and improves the dynamic range of the input signal by using the Brillouin loss spectrum instead of the gain spectrum to process the optical sidebands output by the phase modulator.

Compared with prior art, the beneficial effect of the present invention is:

The invention provides a microwave photon filter based on the stimulated Brillouin scattering effect. In a highly nonlinear optical fiber, the lower optical sideband generated by the phase modulation is carried out through the stimulated Brillouin loss spectrum excited by the pump light. Processing, rather than gain spectrum, avoids the influence of stimulated Brillouin gain saturation effect, so that the microwave photonic filter has a large input dynamic range; in addition, the filter also has good tuning and passband reconstruction characteristics.

Description of drawings

Fig. 1 is a schematic structural diagram of a microwave photon filter based on the stimulated Brillouin scattering effect provided by the present invention.

Fig. 2 is a schematic diagram of the phase-intensity modulation conversion principle in the microwave photon filter based on the stimulated Brillouin scattering effect provided by the present invention.

Fig. 3 is a schematic diagram of an optical frequency comb generating structure in a microwave photon filter based on the stimulated Brillouin scattering effect provided by the present invention.

Fig. 4 is a spectrum diagram of an optical frequency comb generated in the microwave photon filter based on the stimulated Brillouin scattering effect provided by the present invention.

Fig. 5 is the frequency response curves of the lower optical sideband generated by the phase modulation in the microwave photon filter based on the stimulated Brillouin scattering effect provided by the present invention and the traditional microwave photon filter at various powers.

Detailed ways

In order to make the purpose, technical solutions and beneficial effects of the present invention more clear, the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments.

This embodiment provides a microwave photon filter based on the stimulated Brillouin scattering effect, its structure is shown in Figure 1, specifically includes: laser source CW (1), arbitrary waveform generator AWG (2), double parallel Mach Zehnder modulator DPMZM(3), erbium-doped fiber amplifier EDFA(4), optical circulator OC(5), tunable laser source TLS(6), phase modulator PM(7), signal source SG(8), Optical isolator ISO (9), high nonlinear fiber HNLF (10), photodetector PD (11).

in particular:

The light emitted by the laser source CW(1) enters the dual-parallel Mach-Zehnder modulator DPMZM(3), and the dual-parallel Mach-Zehnder modulator DPMZM(3) outputs under the electric frequency comb modulation from the arbitrary waveform generator AWG(2) Optical frequency comb; the optical frequency comb output by the dual parallel Mach-Zehnder modulator DPMZM (3) is amplified by the erbium-doped fiber amplifier EDFA (4), and then enters the highly nonlinear optical fiber HNLF (10) as pump light to generate stimulated Bri The deep scattering effect excites the Brillouin loss spectrum, which is used to attenuate the reversely transmitted pump light in the highly nonlinear fiber HNLF (10);

The tunable laser source TLS (6) emits an optical carrier into the phase modulator PM (7), and the signal source SG (8) emits a microwave signal to modulate the optical carrier in the phase modulator PM (7). The upper and lower optical sidebands will be generated; the optical carrier and the two optical sidebands pass through the optical isolator ISO (9) in the high nonlinear fiber HNLF (10) and then pass through the optical circulator OC after being affected by the Brillouin loss spectrum (5) Arriving at the photodetector PD (11) for photoelectric conversion; the filter uses the phase-intensity modulation conversion principle to map the Brillouin optical frequency response characteristics to the electrical domain to realize the construction of the filter frequency response curve.

The principle of phase-intensity modulation conversion in the microwave photon filter based on the stimulated Brillouin scattering effect is shown in Figure 2, specifically:

When the laser source CW output frequency is f ₀ , when the light source enters the high nonlinear fiber directly through the erbium-doped fiber amplifier without any modulation in the dual-parallel Mach-Zehnder modulator, it will generate Bri Deep loss spectrum, the shape of the loss spectrum is shown by the dotted line in Figure 2, and its center frequency is the sum of the laser frequency f ₀ and the Brillouin frequency shift f _B ;

The tunable laser source TLS emits an optical carrier with frequency f _c , and after being modulated by a radio frequency signal with frequency f _e in the phase modulator, the phase modulator outputs the optical carrier and frequency as f _c + f _e and f _c - The upper and lower optical sidebands of f _e ; because the two optical sidebands have the same amplitude and initial phase difference of π, the RF signal loaded on the phase modulator cannot be restored when the photoelectric conversion is performed on the photodetector; When the sidebands fall into the Brillouin loss spectrum, the amplitudes of the two sidebands are no longer the same. When the photoelectric conversion is performed on the photodetector, the RF signal loaded on the phase modulator can be recovered, and the phase-intensity modulation is realized. conversion;

Due to the effect of phase-intensity conversion, the passband shape of the filter is determined by the shape of the Brillouin loss spectrum. Therefore, the reconstruction of the filter passband can be realized by changing the Brillouin loss spectrum.

The optical frequency comb generation structure in the microwave photonic filter based on the stimulated Brillouin scattering effect is shown in Figure 3, in which the dual parallel Mach-Zehnder modulator DPMZM consists of two sub-Mach-Zehnder modulators MZM-α, MZM- β and a female Mach-Zehnder modulator MZM-γ; the light emitted by the laser source CW enters the dual-parallel Mach-Zehnder modulator, and the electric frequency comb is emitted by the arbitrary waveform generator Light is modulated in dual parallel Mach-Zehnder modulators, where the two sub-Mach-Zehnder modulators are biased at the lowest power transfer point and the female Mach-Zehnder modulator is loaded with a bias voltage V _bias-γ such that one of the sub-Mach-Zehnder modulators The output light of the Mach-Zehnder modulator produces a phase shift of π/2, that is, the phase difference between the electric frequency combs loaded on the two sub-Mach-Zehnder modulators is π/2; if the input ports of the two parallel Mach-Zehnder modulators The input light field at in is E _in (t)=E ₀ exp(j2πf ₀ t), then the light field at the output port out is:

The optical field described by the above formula is an optical frequency comb. In order to make the microwave photon filter based on the stimulated Brillouin scattering effect have a relatively flat passband, the comb tooth spacing of the optical frequency comb should be selected within the 3dB bandwidth of the Brillouin spectral line About half of that, the Brillouin linewidth in the present invention is 30MHz, so the comb-tooth spacing of optical frequency comb is 15MHz; Because the comb-tooth spacing of optical frequency comb is narrower, in order to make optical frequency comb have better spectral characteristic, The selected laser source has a linewidth of 100 kHz. The spectrum of the optical frequency comb is shown in Figure 4, wherein the suppressed optical carrier is marked by the circle; the left side of the suppressed optical carrier is the generated optical frequency comb, including 5 comb teeth, and the spacing between the comb teeth is 15MHz, the right side of the suppressed optical carrier is the suppressed optical sideband.

The frequency response curves of the lower optical sidebands produced by phase modulation in the microwave photon filter based on the stimulated Brillouin scattering effect and the traditional microwave photon filter in the present invention are as shown in Figure 5 at various powers, wherein, (a) Represent the frequency response curve of the lower optical sideband under the effect of Brillouin gain in the traditional microwave photonic filter, (b) represents the frequency response curve of the lower optical sideband in the present invention under the effect of Brillouin loss; From Fig. 5 (a) It can be seen that when the power of the downlight sideband is -10dBm, -20dBm and -30dBm respectively, the maximum gain on the frequency response curve is 15dB, 20dB and 22dB respectively, which means that in the Brillouin gain effect Next, the gain obtained by the lower optical sideband will be affected by the power of the lower optical sideband, and the power of the lower optical sideband is directly related to the power of the RF signal loaded on the phase modulator, so the microwave based on the Brillouin gain effect The photonic filter is a nonlinear system for RF signals with a large dynamic range (that is, large power changes), and the RF signal with a large dynamic range will be distorted after passing through this filter; it can be seen from (b) in Figure 5 It is found that when the power of the lower optical sideband is -10dBm, -20dBm and -30dBm respectively, the maximum loss on the frequency response curve is 26dB, which means that under the action of Brillouin loss, the lower optical sideband It has almost the same frequency response curve, which means that the microwave photon filter based on the Brillouin loss effect is a linear system, so compared with the microwave photon filter based on the Brillouin gain effect, the microwave photon filter based on the Brillouin loss effect Photonic filters are capable of processing RF signals with a large dynamic range and have a large input dynamic range.

The above is only a specific embodiment of the present invention. Any feature disclosed in this specification, unless specifically stated, can be replaced by other equivalent or alternative features with similar purposes; all the disclosed features, or All method or process steps may be combined in any way, except for mutually exclusive features and/or steps.

Claims (2)

1. A microwave photon filter based on stimulated Brillouin scattering effect, including: laser source CW (1), arbitrary waveform generator AWG (2), dual parallel Mach-Zehnder modulator DPMZM (3), erbium-doped Optical fiber amplifier EDFA(4), optical circulator OC(5), tunable laser source TLS(6), phase modulator PM(7), signal source SG(8), optical isolator ISO(9), high nonlinearity Optical fiber HNLF (10), photodetector PD (11); It is characterized in that: The light emitted by the laser source CW(1) enters the dual-parallel Mach-Zehnder modulator DPMZM(3), and the dual-parallel Mach-Zehnder modulator DPMZM(3) outputs under the electric frequency comb modulation from the arbitrary waveform generator AWG(2) Optical frequency comb, the output optical frequency comb is amplified by the erbium-doped fiber amplifier EDFA (4) and then enters the highly nonlinear optical fiber HNLF (10) as pump light to generate stimulated Brillouin scattering effect and excite the Brillouin loss spectrum Attenuating the reverse propagating light in the highly nonlinear fiber HNLF (10); The tunable laser source TLS (6) emits an optical carrier into the phase modulator PM (7), and the signal source SG (8) emits a microwave signal to modulate the optical carrier in the phase modulator PM (7). The upper and lower optical sidebands will be generated; the optical carrier and the two optical sidebands are used as reverse transmission light, and undergo stimulated Brillouin attenuation in the highly nonlinear optical fiber HNLF (10) through the optical isolator ISO (9) Afterwards, the optical circulator OC (5) reaches the photodetector PD (11) for photoelectric conversion, and the photodetector PD (11) outputs an electrical signal. 2. by the described microwave photon filter based on stimulated Brillouin scattering effect of claim 1, it is characterized in that, the frequency response function of described microwave photon filter is: Among them, H(f _e ) represents the frequency response function of the microwave photonic filter, f _e is the frequency of the microwave signal output by the signal source SG, f _c is the center frequency of the optical carrier output by the tunable laser source TLS, and V _π is the half-wave voltage , R is the responsivity of the photodetector PD, and _HSBS is the transfer function of the Brillouin loss spectrum.