Real-time fourier transform method and device based on bidirectional circulating dispersion

Abstract

The application discloses a real-time Fourier transform method based on bidirectional cyclic dispersion. The method comprises the following steps: S1, using a bidirectional dispersion module to perform forward N-time cycle expansion on an optical pulse signal to obtain a cycle expansion optical pulse signal; N is an integer greater than or equal to 2; S2, using a to-be-detected microwave signal to modulate the cycle expansion optical pulse signal to obtain a modulated optical pulse signal; S3, using the bidirectional dispersion module to perform reverse N-time cycle compression on the modulated optical pulse signal and then outputting, so that the real-time Fourier transform of the to-be-detected microwave signal is realized. The application further discloses a real-time Fourier transform device based on bidirectional cyclic dispersion. Compared with the prior art, the application amplifies the dispersion amount through the multiplexing of a bidirectional dispersion module to perform the expansion and compression of the optical pulse signal, and through the cycle mode, only one dispersion module is needed to realize the real-time Fourier transform, so that higher resolution can be realized at a lower cost.

Description

TECHNICAL FIELD

[0001] The present application relates to a real-time Fourier transform method and device, belonging to the field of radio frequency cognitive technology. BACKGROUND

[0002] The spectrum cognitive system is an important part of the radio frequency cognitive system, and plays a more and more important role in the modern electromagnetic environment. For example, in the modern battlefield environment, the party obtaining the right to control the electromagnetic spectrum will dominate the entire battlefield situation. In order to cope with the future electromagnetic environment, people put forward the real-time and wideband spectrum sensing demand for the cognitive system. The traditional spectrum sensing method is realized based on electronic devices, and the low observation bandwidth and high loss of the electronic devices limit the performance of the spectrum sensing system. The traditional spectrum sensing method obtains spectrum information through sampling, quantization and digital operation. With the increase of signal bandwidth, the data acquisition amount is increasing, and the large load data processing makes the real-time performance worse. In recent years, due to the advantages of low loss, low delay and large bandwidth, the spectrum sensing technology based on microwave photons has gradually attracted people's attention.

[0003] According to the sensing mode, microwave photonic spectrum sensing techniques can be classified into five categories: instantaneous frequency measurement, photonic channelization, photonic sweeping, photonic compressive sensing and optical Fourier transform. Zou X H et al. proposed and studied a microwave frequency measurement method based on optical power monitoring. In the proposed system, the microwave signal is applied to a Mach-Zehnder modulator, which is biased at the minimum transmission point to generate a carrier-suppressed optical signal, and then sent to a complementary optical filter pair, the optical filter output power is measured by two optical power meters, and the mathematical expression between microwave frequency and optical power is derived to calculate the corresponding microwave frequency [Zou X H, Chi H, Yao J P. Microwave frequency measurement based on optical power monitoring using a complementary optical filter pair. IEEE Transactions on Microwave Theory and Techniques, 2009]. Xie X J et al. proposed and demonstrated a new photonic channelization scheme for wideband radio frequency signals based on optical frequency comb (OFC), comb filter and optical demultiplexer. In the proposed channelizer, the input wideband radio frequency signal is multicast by OFC, frequency-sliced by a Fabry-Perot filter, and then channelized by a conventional optical demultiplexer, and the instantaneous multi-frequency measurement is realized by directly detecting the channelized radio frequency signal [Xie X J, Dai Y T, Ji Y, et al. Broadband photonic radio-frequency channelization based on a 39-ghz optical frequency comb. IEEE Photonics Technology Letters, 2012]. Li R Y et al. proposed a serial photonic channelization radio frequency measurement scheme. This scheme is based on high-speed wavelength scanning, and the photonic radio frequency channelizer works in series in time domain, and each wavelength marks a specific radio frequency channel. With only one low-bandwidth photodetector, multi-channel radio frequency frequency measurement can be realized, which can be used for instantaneous multi-frequency measurement of linear frequency modulation signals and capture of key parameters [Li R Y, Chen H W, Yu Y, et al. Multiple-frequency measurement based on serial photonic channelization using optical wavelength scanning. Optics Letters, 2013].Liang Yunhua proposed a system scheme based on multi-path optical delay PRBS pulse, the system uses relatively low speed PRBS optical pulse to realize the compression sampling of ultra-wideband RF spectrum, and recovers the multi-subband signal at any but fixed position from the compressed spectrum of information rate bandwidth without signal-to-noise ratio difference [Liang Yunhua. Research on compression sampling technology based on photon assistance, 2014]. Dai Y T et al. proposed frequency-to-time mapping (FTM) by spectrally discrete chromatic dispersion to greatly improve the frequency sensitivity. By a new type of medium with a periodic on / off intensity frequency response, the quadratic phase distribution along the off channel matches the optical input with the finite output of repeated Fourier transform, and then the real-time FTM is obtained in each cycle [Dai Y T, Li J L, Zhang Z P, et al. Real-time frequency-to-time mapping based on spectrally-discrete chromatic dispersion. Optics Express, 2017].

[0004] At present, the real-time Fourier transform technology based on dispersion has great advantages in frequency measurement. The main parameters of such real-time Fourier transform system include the dispersion amount of the dispersion medium of the system, the time interval between the reference pulse and the signal pulse output by the system, and the time domain pulse width of the signal pulse. The three parameters affect the working bandwidth and frequency resolution of the system. When the pulse width of the system output signal pulse is fixed, the larger the dispersion amount of the system is, the smaller the theoretical frequency resolution of the system is. In order to realize higher resolution, the existing real-time Fourier transform system based on dispersion needs to use a dispersion module with larger dispersion amount or multiple dispersion modules. Since the dispersion module is high in cost and complex in manufacturing, it will cause the problems of high implementation cost and large volume of the system. SUMMARY

[0005] The technical problem to be solved by the present application is to overcome the contradiction between cost, volume and frequency resolution of the existing real-time Fourier transform system based on dispersion. A real-time Fourier transform method and device based on bidirectional cyclic dispersion are provided, which multiplexes bidirectional dispersion modules for expansion and compression, and amplifies the limited dispersion amount through a cyclic mode, so that higher resolution can be realized with lower cost and smaller volume.

[0006] The present application specifically adopts the following technical solutions to solve the above technical problems:

[0007] The real-time Fourier transform method based on bidirectional cyclic dispersion comprises the following steps:

[0008] S1, using a bidirectional dispersion module to perform forward N-time loop spreading on the optical pulse signal, to obtain a loop spreading optical pulse signal; N is an integer greater than or equal to 2;

[0009] S2, using a to-be-tested microwave signal to modulate the loop spreading optical pulse signal, to obtain a modulated optical pulse signal; S3, using the bidirectional dispersion module to perform reverse N-time loop compression on the modulated optical pulse signal and output, to realize real-time Fourier transform on the to-be-tested microwave signal.

[0010] Preferably, the bidirectional dispersion module is a bidirectional chirped fiber grating.

[0011] Further, the same bidirectional optical amplifier is used to amplify the optical pulse signal in the N-time loop spreading process and the modulated optical pulse signal in the N-time loop compression process.

[0012] Based on the same inventive concept, the following technical solutions can also be obtained:

[0013] A real-time Fourier transform device based on bidirectional loop dispersion comprises:

[0014] A loop spreading loop is configured to use a bidirectional dispersion module to perform forward N-time loop spreading on an optical pulse signal, to obtain a loop spreading optical pulse signal; N is an integer greater than or equal to 2.

[0015] A modulation module is configured to use a to-be-tested microwave signal to modulate the loop spreading optical pulse signal, to obtain a modulated optical pulse signal.

[0016] A loop compression loop is configured to use the bidirectional dispersion module to perform reverse N-time loop compression on the modulated optical pulse signal and output, to realize real-time Fourier transform on the to-be-tested microwave signal.

[0017] Preferably, the bidirectional dispersion module is a bidirectional chirped fiber grating.

[0018] Further, the loop spreading loop and the loop compression loop use the same bidirectional optical amplifier to amplify the optical pulse signal in the N-time loop spreading process and the modulated optical pulse signal in the N-time loop compression process.

[0019] Compared with the prior art, the technical solutions of the present application have the following beneficial effects:

[0020] The present application uses a bidirectional dispersion module to perform loop spreading and compression on an optical pulse signal, and amplifies the limited dispersion amount of the bidirectional dispersion module in a loop manner, so that real-time Fourier transform can be realized only by using one dispersion module, thereby realizing higher resolution at a lower cost and smaller size. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 A structural schematic diagram of a specific embodiment of the real-time Fourier transform device based on bidirectional cyclic dispersion of the application;

[0022] Figure 2 A principle schematic diagram of the specific embodiment. DETAILED DESCRIPTION

[0023] In view of the problem that the existing dispersion-based real-time Fourier transform system is difficult to balance the contradiction between cost, volume and frequency resolution, the solution idea of the application is to expand and compress the optical pulse signal by multiplexing bidirectional dispersion modules, and to amplify the limited dispersion amount of the bidirectional dispersion modules in a cyclic manner, so that a real-time Fourier transform can be realized by using only one dispersion module, thereby realizing higher resolution at a lower cost and smaller volume.

[0024] The application specifically adopts the following technical solutions to solve the above technical problems:

[0025] The real-time Fourier transform method based on bidirectional cyclic dispersion comprises the following steps:

[0026] S1, using a bidirectional dispersion module to perform forward N-cycle expansion on an optical pulse signal to obtain a cyclically expanded optical pulse signal; N is an integer greater than or equal to 2;

[0027] S2, modulating the cyclically expanded optical pulse signal with a microwave signal to be measured to obtain a modulated optical pulse signal; S3, using the bidirectional dispersion module to perform reverse N-cycle compression on the modulated optical pulse signal and outputting, thereby realizing real-time Fourier transform on the microwave signal to be measured.

[0028] The real-time Fourier transform device based on bidirectional cyclic dispersion comprises:

[0029] A cyclic expansion loop for using a bidirectional dispersion module to perform forward N-cycle expansion on an optical pulse signal to obtain a cyclically expanded optical pulse signal; N is an integer greater than or equal to 2;

[0030] A modulation module for modulating the cyclically expanded optical pulse signal with a microwave signal to be measured to obtain a modulated optical pulse signal;

[0031] A cyclic compression loop for using the bidirectional dispersion module to perform reverse N-cycle compression on the modulated optical pulse signal and outputting, thereby realizing real-time Fourier transform on the microwave signal to be measured.

[0032] In order to facilitate the public to understand, the technical solutions of the application will be described in detail below through a specific embodiment and in combination with the drawings:

[0033] The structure of the real-time Fourier transform device of the embodiment is as shown in Figure 1The light source is a pulse light source, generating light pulse 1 with a period of T, which is input into a dispersion module through optical circulator 1. The dispersion module is a bidirectional dispersion module, with a positive dispersion coefficient when the optical signal is input in the forward direction and a negative dispersion coefficient when the optical signal is input in the reverse direction. In this embodiment, a bidirectional chirped fiber grating is used as the dispersion module, which is positively dispersive when input in the forward direction and negatively dispersive when input in the reverse direction. After the light pulse 1 is expanded by the dispersion module and amplified by the bidirectional amplifiable optical amplifier, the output light pulse 2 is output, passes through optical circulator 2, and is then divided into two paths through optical coupler 2. One of the two paths is input into acousto-optic modulator 1, which is used as an optical switch and is driven by square wave signal 1 with a period of T. The output of acousto-optic modulator 1 is input into the dispersion module through optical coupler 1 and optical circulator 1 for secondary expansion, amplified by the optical amplifier, and then input into acousto-optic modulator 2 through optical circulator 2, to obtain light pulse 3, i.e., to achieve twice the cycle pulse expansion. Acousto-optic modulator 2 is also driven by square wave signal 2 with a period of T. Light pulse 3 is input into intensity modulator, which modulates the microwave signal to be measured onto the light pulse. The output of the intensity modulator is light pulse 4, which is input into the optical amplifier through optical coupler 3 and optical circulator 2 for amplification, compressed by the dispersion module, and then input into acousto-optic modulator 3 through optical circulator 1 and optical coupler 4, to obtain light pulse 5. Acousto-optic modulator 3 is driven by square wave signal 3 with a period of T. The output of acousto-optic modulator 3 is input into the optical amplifier through optical coupler 3 and optical circulator 2 for amplification, and then input into acousto-optic modulator 4 through the dispersion module for secondary compression, to obtain light pulse 5. Acousto-optic modulator 4 is driven by square wave signal 4 with a period of T, so as to complete real-time Fourier transform.

[0034] In this embodiment, twice the cycle expansion and twice the cycle compression are performed. Acousto-optic modulators 1-4 with large extinction ratios are used as optical switches. The duty cycles and phases of square wave signals 1, 2, 3, and 4 are adjusted as shown in Figure 2 The time-domain waveforms of light pulses 1, 2, 3, 4, 5, and 6 are shown in Figure 2 By changing the period of the square wave signal and adjusting the appropriate duty cycle and phase, the number of cycles can be increased, thereby increasing the amount of dispersion.

[0035] Suppose that the light pulse 1 generated by the light source can be represented as u1(t) in the time domain, with a pulse width of δ τ and a repetition period of T. The frequency spectrum can be represented as U0(ω). When the dispersion module is input twice in the forward direction, the transfer function and the impulse response function can be represented as H1(ω) and h1(t), respectively.

[0036]

[0037] When the dispersion module is input twice in the reverse direction, the transfer function and the impulse response function can be represented as H2(ω) and h2(t), respectively.

[0038]

[0039] When the optical pulse 1 is input to the dispersion module, it is stretched in time domain to obtain the optical pulse 2. When the optical pulse 2 is input to the dispersion module again, the optical pulse 3 is obtained, which can be expressed as:

[0040]

[0041] It can be seen that the time domain stretched optical pulse 3 has a time domain performance of the spectrum of the optical pulse 1 with 2β0 (2) z is the result of the coefficient mapping to the time domain, which has a frequency domain performance of the spectrum of the optical pulse 1 with the phase introduced by the dispersion, and the amplitude envelope has no change. Therefore, after the optical pulse 1 is stretched twice by the dispersion, the frequency domain and the time domain are one-to-one mapped, becoming a swept light source with a larger chirp amount, the bandwidth of which is the spectrum width of the optical pulse 1, and the time width of which is the product of the spectrum width and the dispersion coefficient 2β0 (2) z.

[0042] Then, the optical pulse 3 is modulated by the microwave signal to be measured through an intensity modulator. It is assumed that the microwave signal to be measured can be expressed as V m s(t), V m is the amplitude of the modulation signal, and s(t) is the normalized modulation signal. The output signal optical pulse 4 of the intensity modulator can be expressed as:

[0043]

[0044] wherein β m = πV m / V π is the modulation coefficient, V π is the half-wave voltage of the intensity modulator, and a and b respectively represent the relative amplitudes of the optical carrier and the positive and negative first-order sidebands, which can be changed by the bias voltage of the intensity modulator. The output signal of the intensity modulator is input to the dispersion module in the reverse direction, and two pulse compressions are realized in the loop, i.e. experiencing the second-order dispersion of -2β0 (2) z, and the output signal optical pulse 6 can be expressed as:

[0045]

[0046] wherein S(ω) is the spectrum of the microwave signal to be measured s(t). It can be seen that through the real-time Fourier transform device of the application, the spectrum of the microwave signal to be measured can be directly mapped to the time domain. It is shown in equation (5) that the output signal contains two parts, one part is the optical pulse 1, and the other part is the signal pulse representing the convolution of the signal after the linear mapping of the spectrum of the microwave signal to be measured to the time domain and the optical pulse 1.

[0047] In particular, when the microwave signal s(t) to be measured is a single-frequency signal with a frequency f, the spectrum s'(t) in formula (4) can be expressed as S'(ω)=aδ(ω)+bβ m δ(ω+2πf) / 2+bβ m δ(ω-2πf) / 2. Therefore, the spectrum of the optical pulse 6 can be expressed as:

[0048]

[0049] The time-domain signal expression thereof is:

[0050]

[0051] As can be seen from formula (6) and formula (7), when the microwave signal to be measured is a single-frequency signal, the output signal of the system is three copies of the original optical pulse in the time domain, wherein the middle pulse is the reference pulse, and the two side pulses are signal pulses. The time interval Δt between the reference pulse and the signal pulse is related to the frequency f of the microwave signal to be measured and the twice system second-order dispersion amount 2β0 (2) z linearly, and can be expressed as Δt=4πβ0 (2) zf. Therefore, the frequency information of the microwave signal to be measured can be inversely deduced according to the pulse time interval of the output signal. By changing the period of the square wave signal and adjusting the appropriate duty ratio and phase, the number of cycles can be increased, and the dispersion amount can be increased.

[0052] In summary, the real-time Fourier transform device constructed by the application only needs to use one bidirectional dispersion module, realizes the real-time Fourier transform device of pulse cycle expansion and compression by controlling the timing of the optical switch, and has the advantage of being capable of improving the resolution of the system within limited cost.

Claims

1. A real-time Fourier transform method based on bidirectional cyclic dispersion, characterized in that, The method comprises the following steps: S1, using a bidirectional dispersion module to perform forward N-time loop expansion on the optical pulse signal to obtain a loop expanded optical pulse signal; N is an integer greater than or equal to 2; the forward N-time loop expansion is specifically as follows: a first optical pulse with a period of T is input into the bidirectional dispersion module through a first optical circulator to perform expansion, the obtained second optical pulse is input into a second optical circulator, and then is divided into two paths through a second optical coupler, one of the two paths is input into a first optical switch driven by a square wave signal with a period of T, the output of the first optical switch is input into the bidirectional dispersion module through a first optical coupler and a first optical circulator to perform secondary expansion, and the like is repeated; the optical pulse after N-time expansion is input into a second optical switch also driven by the square wave signal with the period of T through a second optical circulator to obtain the loop expanded optical pulse signal after N-time loop expansion; S2, using a to-be-tested microwave signal to modulate the loop expanded optical pulse signal to obtain a modulated optical pulse signal; S3, using the bidirectional dispersion module to perform reverse N-time loop compression on the modulated optical pulse signal to output, that is, real-time Fourier transform on the to-be-tested microwave signal is realized; the reverse N-time loop compression is specifically as follows: the modulated optical pulse signal is input into the bidirectional dispersion module through a third optical coupler and a second optical circulator to perform compression, and then is input into a third optical switch driven by the square wave signal with the period of T through a first optical circulator and a fourth optical coupler, the output of the third optical switch is input into the bidirectional dispersion module through a third optical coupler and a second optical circulator to perform secondary compression, and the like is repeated; the modulated optical pulse signal after N-time compression is input into a fourth optical switch also driven by the square wave signal with the period of T through the first optical circulator and the fourth optical coupler, and the output of the fourth optical switch is the modulated optical pulse signal after reverse N-time loop compression.

2. The method of claim 1, wherein the real-time Fourier transform is based on bidirectional circular dispersion. The bidirectional dispersion module is a bidirectional chirped fiber grating.

3. The method of claim 1, wherein the method is based on a two-way cyclic dispersion real-time Fourier transform. The same bidirectional optical amplifier is used to amplify the optical pulse signal in the N-time loop expansion process and the modulated optical pulse signal in the N-time loop compression process.

4. Real-time Fourier transform apparatus based on bidirectional circulating dispersion, characterized in that, The method comprises the following steps: a loop expansion loop, configured to use a bidirectional dispersion module to perform forward N-time loop expansion on an optical pulse signal to obtain a loop expanded optical pulse signal; N is an integer greater than or equal to 2; the forward N-time loop expansion is specifically as follows: a first optical pulse with a period of T is input into the bidirectional dispersion module through a first optical circulator to perform expansion, the obtained second optical pulse is input into a second optical circulator, and then is divided into two paths through a second optical coupler, one of the two paths is input into a first optical switch driven by a square wave signal with a period of T, the output of the first optical switch is input into the bidirectional dispersion module through a first optical coupler and a first optical circulator to perform secondary expansion, and the like is repeated; the optical pulse after N-time expansion is input into a second optical switch also driven by the square wave signal with the period of T through a second optical circulator to obtain the loop expanded optical pulse signal after N-time loop expansion; a modulation module, configured to use a to-be-tested microwave signal to modulate the loop expanded optical pulse signal to obtain a modulated optical pulse signal; The cyclic compression loop is used for outputting the modulated light pulse signal after reverse N times of cyclic compression by using the bidirectional dispersion module, that is, realizing real-time Fourier transform of the microwave signal to be measured; the reverse N times of cyclic compression is specifically as follows: the modulated light pulse signal is input to the bidirectional dispersion module for compression through a third optical coupler and a second optical circulator, and then is input to a third optical switch driven by a square wave signal with a period of T through a first optical circulator and a fourth optical coupler, the output of the third optical switch is input to the bidirectional dispersion module again for secondary compression through a third optical coupler and a second optical circulator; and so on; the modulated light pulse signal after N times of compression is input to a fourth optical switch also driven by a square wave signal with a period of T through a first optical circulator and a fourth optical coupler, and the output of the fourth optical switch is the modulated light pulse signal after reverse N times of cyclic compression.

5. The real-time Fourier transform apparatus based on bidirectional circulating dispersion according to claim 4, wherein, The bidirectional dispersion module is a bidirectional chirped fiber grating.

6. The real-time Fourier transform apparatus based on bidirectional circulating dispersion according to claim 4, wherein, The cyclic expansion loop and the cyclic compression loop use the same bidirectional optical amplifier to amplify the optical pulse signal in the N times of cyclic expansion and the modulated light pulse signal in the N times of cyclic compression.

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