A method for quadrupling the frequency of linear frequency modulation signals based on cascade modulators

Abstract

The invention discloses a method for quadrupling a linear frequency modulation signal based on a cascade modulator, and the method relates to the fields of radar, optical communication and microwave technology. As shown in Figure 1 of the specification, the method comprises sending an optical carrier generated by an LD into IM1 driven by an IF-LFM signal, working at the MITP point, realizing CS-DSB modulation, and then amplifying the output signal of IM1 through an EDFA, and then sending it into IM2 working at the MITP for secondary modulation, driven by the same IF-LFM signal. At this time, a sideband and carrier frequency with twice the bandwidth can be obtained. Then, the chirp frequency component is filtered out by an FBG, and finally the optical signal is sent to a PD to complete the photoelectric conversion. All signal processing operations of the invention are completed in the analog domain, and the proposed scheme does not contain a halo, so that the system can complete the frequency doubling operation of the IF-LFM signal within a larger time span. The signal generator provides a simple and effective method for obtaining a large TBWP IF-LFM signal, greatly improving the detection accuracy of the radar system, and has potential application prospects in modern radar systems.

Description

Technical Field

[0001] The invention relates to the fields of radar technology, optical communication technology and microwave technology, and mainly to the use of photonics technology to generate a linear frequency modulation signal with quadruple frequency and bandwidth. Background Art

[0002] As the detection environment becomes increasingly complex, the requirements for radar detection are also getting higher and higher. When the radar system detects a target, the detection distance and distance resolution are indicators that need to be considered at the same time. To increase the detection distance, the average transmission power needs to be increased, and the pulse width of the detection signal needs to be relatively large; to improve the distance resolution, the detection signal needs to have a large bandwidth, which is contradictory to general signals, so pulse compression signals came into being. Linear frequency modulation signals play an important role in pulse compression in modern radar systems and have attracted more and more attention over the past few decades. In order to meet the growing requirements for long-distance and high-resolution detection, linear frequency modulation signals will have higher carrier frequencies, larger bandwidths and time-bandwidth products (TBWP). Traditional electrical devices usually face the problems of narrow working bandwidth, low carrier frequency, and poor sampling rate, so it is difficult to generate linear frequency modulation signals with large TBWP in the electrical field. The use of photon-assisted linear frequency modulation signals to generate linear frequency modulation signals with large bandwidth, low loss, and strong anti-electromagnetic interference capabilities is a signal generation technology with great development prospects.

[0003] Microwave photons have the advantages of large bandwidth, high sensitivity, high resolution, resistance to electromagnetic interference, and the ability to process multiple information points in parallel. Therefore, the optical generation of linear frequency modulation signals has gradually become a research hotspot.

[0004] In order to obtain chirp signals with large TBWP, various photon-assisted chirp signal generation schemes have emerged. A free-space optical system based on frequency-time mapping was established to obtain chirp signals with large TBWP, but the system has the disadvantages of large size and high loss. In addition, there are several methods to generate large chirp signals using linear chirp fiber Bragg gratings. However, the frequency tunability of the obtained chirp signals is poor, which is limited by the bandwidth of the linear chirp fiber Bragg grating. It should be noted that the scheme using high-order optical sidebands can only use narrow-band chirp signals, and the power spectral density of chirp signals with large bandwidth is often low. It is difficult for large-bandwidth chirp signals to excite high-order optical sidebands through the modulator. When the modulator is biased at the maximum transmission point, even-order sidebands can be obtained. However, since the optical carrier occupies most of the optical power, low-power high-order sidebands are caused. When the modulator is biased at the minimum transmission point, odd-order sidebands can be obtained, and the high-order sidebands have low power due to the low modulation index. A linear frequency modulation signal generator is a pressing requirement for radar systems. Summary of the invention

[0005] In order to solve the problems in the technical background, the present invention proposes a method for generating a linear frequency modulation signal with quadruple bandwidth and carrier frequency based on photons. The method uses two cascaded intensity modulators, both working at the minimum transmission point, and finally achieves double bandwidth and carrier frequency with a slight offset in the sideband.

[0006] The technical solution adopted by the present invention to solve the technical problem is: the device includes 1 laser LD, 2 two cascade intensity modulators IM1 and IM2, 3 arbitrary waveform generator AWG, 4 erbium-doped fiber amplifier EDFA, 5 fiber Bragg grating FBG, and 6 photodetector PD. It is characterized in that: the optical carrier generated by the laser LD is sent to IM1 driven by an intermediate frequency linear frequency modulation (IF-LFM) signal, IM1 works at the minimum transmission point MITP to realize carrier suppressed double sideband (CS-DSB) modulation, and then the output signal of IM1 is amplified by the erbium-doped fiber amplifier EDFA, and then sent to IM2 for secondary modulation, IM2 also works at MITP and is driven by the same intermediate frequency linear frequency modulation signal. At this time, the sideband and carrier frequency with twice the bandwidth can be obtained. It should be noted that due to the time delay of the two modulators, all sidebands have a slight frequency shift. Then the output signal of IM2 is injected into FBG to filter out the frequency component of chirp, and finally the optical signal after passing through FBG is sent to PD to complete the photoelectric conversion.

[0007] All signal processing operations of the present invention are completed in the analog domain, and the proposed scheme does not contain light rings, so that the system can complete the frequency doubling operation of the linear frequency modulation signal in a larger time span. In addition, by avoiding the stimulation of high-order optical sidebands and not using electronic equipment, the proposed scheme is easy to complete the frequency doubling operation of the linear frequency modulation signal with a larger bandwidth.

[0008] The present invention comprises the following steps when working:

[0009] (1) The IF-linear frequency modulation signal provided by the arbitrary waveform generator (AWG) can be expressed as:

[0010] V(t)=V LFM sin(ω IF t+πkt 2 )

[0011] Where V LFM is the amplitude of the linear frequency modulation signal, ω IF is the angular frequency of the intermediate frequency signal, and k is a parameter provided by the AWG.

[0012] (2) The optical carrier generated by the laser diode (LD) is fed into IM1 driven by the IF-LFM signal. IM1 works in MITP to implement carrier suppressed double sideband (CS-DSB) modulation. The optical signal after CS-DSB modulation can be expressed as:

[0013]

[0014] Where E0 and ω0 are the amplitude and angular frequency of the optical carrier respectively, m = πV LFM / V π is the modulation index of IF-LFM, V π is the half-wave voltage of the modulator, J n (·) is the nth-order Bessel function of the first kind.

[0015] (3) The output signal of IM1 is then amplified by an erbium-doped fiber amplifier (EDFA) and sent to IM2 for secondary modulation. IM2 also works in MITP and is driven by the same IF-LFM signal. The output signal of IM2 can be described as:

[0016]

[0017] Where G is the gain of the EDFA. After secondary modulation, two sidebands are generated at frequencies kτ and -kτ, respectively, and τ is the time delay between the two modulators. At this point, sidebands with twice the bandwidth and carrier frequency can be obtained. It should be noted that all sidebands have a slight frequency shift due to the time delay between the two modulators.

[0018] (4) Then the output signal of IM2 is injected into FBG to suppress the de-chirped frequency component. The optical signal after the fiber Bragg grating is expressed as:

[0019]

[0020] (5) Finally, the optical signal after FBG is sent to PD to complete the photoelectric conversion, and the output electrical signal is expressed as:

[0021]

[0022] The present invention proposes a method for generating a linear frequency modulation signal with four times the frequency and bandwidth based on photons. After two modulations, a single tone signal close to the carrier frequency is generated. At the same time, through photoelectric conversion, a sideband with twice the bandwidth and carrier frequency can also be obtained to obtain a linear frequency modulation signal with four times the carrier frequency and bandwidth.

[0023] The linear frequency modulation signal generator proposed by the present invention has a simple structure and good stability. The system does not contain any electrical devices, which will greatly improve the frequency tuning performance of the system. In addition, the entire optical link does not contain a loop, so that the system can complete the frequency multiplication operation of the linear frequency modulation signal within a longer duration. Two bias controllers can also be applied in this scheme to make the bias voltage of the modulator more stable. In addition, due to the simple link structure, the present invention is more suitable for integration. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Schematic diagram of a linear frequency modulation signal generator with four times the bandwidth and carrier frequency.

[0025] FIG2(a) is a spectrum diagram of the signal output from IM1, and FIG2(b) is a spectrum diagram of the signal output from IM2.

[0026] Figure 3 The dotted line is the spectrum diagram of the IM1 output signal, the dashed line is the spectrum diagram of the IM2 output signal, the solid line is the spectrum diagram of the FBG output signal, and the dashed line is the frequency response curve of the FBG.

[0027] Figure 4 The signal spectrum of the quadrupled carrier and broadband of the IF-LFM signal with a bandwidth of 0.5 GHz and a carrier frequency of 3 GHz output from the PD.

[0028] Figure 5(a) is a waveform diagram of a 0.5 GHz bandwidth, 3 GHz carrier frequency IF-LFM signal generating a four-fold frequency carrier and a wideband linear frequency modulation signal, Figure 5(b) is a time-frequency diagram of a 0.5 GHz bandwidth, 3 GHz carrier frequency IF-LFM signal generating a four-fold frequency carrier and a wideband linear frequency modulation signal, and Figure 5(c) is an autocorrelation result diagram of a 0.5 GHz bandwidth, 3 GHz carrier frequency IF-LFM signal generating a four-fold frequency carrier and a wideband linear frequency modulation signal.

[0029] Figure 6 The signal spectrum of the quadrupled carrier and broadband of the IF-LFM signal with a bandwidth of 1 GHz and a carrier frequency of 5 GHz output from the PD.

[0030] Figure 7(a) is a waveform diagram of a 1GHz bandwidth, 5GHz carrier frequency IF-LFM signal generating a quadruple frequency carrier and a wideband linear frequency modulation signal, Figure 7(b) is a time-frequency diagram of a 1GHz bandwidth, 5GHz carrier frequency IF-LFM signal generating a quadruple frequency carrier and a wideband linear frequency modulation signal, and Figure 7(c) is an autocorrelation result diagram of a 1GHz bandwidth, 5GHz carrier frequency IF-LFM signal generating a quadruple frequency carrier and a wideband linear frequency modulation signal. DETAILED DESCRIPTION

[0031] The following is a detailed description of an embodiment of the present invention in conjunction with the accompanying drawings: This embodiment is implemented on the premise of the technical solution of the present invention, and a detailed implementation method and a specific operation process are given, but the protection scope of the present invention is not limited to the following embodiments.

[0032] Figure 1 This is the schematic diagram of a linear frequency modulation signal generator with four times the bandwidth and carrier frequency. The device includes 1 arbitrary waveform generator AWG, 2 laser diode LD, 3 intensity modulator IM, 4 erbium-doped fiber amplifier EDFA, and 5 photodetector PD. The optical carrier generated by the laser LD is sent to IM1 driven by an intermediate frequency linear frequency modulation (IF-LFM) signal. IM1 works at the minimum transmission point MITP to achieve carrier suppressed double sideband (CS-DSB) modulation. The output signal of IM1 is then amplified by the erbium-doped fiber amplifier EDFA and sent to IM2 for secondary modulation. IM2 also works at MITP and is driven by the same intermediate frequency linear frequency modulation signal. It should be noted that due to the time delay of the two modulators, all sidebands have a slight frequency shift. The output signal of IM2 is then injected into the FBG to filter out the frequency component of the chirp. Figure 3 The optical signal after passing through FBG is sent to PD to complete the photoelectric conversion, and finally it can be connected to an oscilloscope or spectrum analyzer for analysis.

[0033] In this example, the following steps are specifically included:

[0034] Step 1: The IF-LFM signal is generated by an AWG analog signal generator with a bandwidth of 0.5 GHz, a carrier frequency of 3 GHz, and a duration of 0.9 μs;

[0035] Step 2: A light source is used to provide a continuous light wave of 1549.6 nm and a power of 15 dBm. The continuous light wave is input as a carrier to the first IM modulator. The continuous light wave input realizes DS-DSB modulation in IM1 of the MITP.

[0036] Step 3: Use EDFA to amplify the output signal of IM1 to increase the gain of the optical link;

[0037] Step 4: Inject the amplified signal into IM2 for secondary modulation to generate a double frequency carrier and bandwidth;

[0038] Step 5: After two modulations, FBG is introduced into the optical link to filter out the frequency component of the chirp;

[0039] Step 6: The output optical signal is converted into photoelectricity through a high-speed PD with a response rate of 0.7A / W.

[0040] Step 7: To evaluate the quality of the obtained quadrupled carrier and broadband chirp signal, repeat steps 1 to 6, changing the AWG bandwidth to 1 GHz and the carrier to 5 GHz.

[0041] The spectrum of the IM1 output signal is as follows: Figure 3 As shown, represented by the red line. It can be found that the signal has achieved CS-DSB modulation. After secondary modulation in IM2, optical sidebands with twice the bandwidth and carrier frequency can be obtained. At the same time, two single-tone signals are generated near the optical carrier. The spectrum of the IM2 output signal is represented by the blue line. The spectrum of the signal after FBG is drawn with a purple line, and the frequency response curve of FBG is represented by a green line. As shown by the magenta line, when the optical signal passes through the FBG, only the sidebands of the quadruple carrier frequency and bandwidth remain, and the de-chirped frequency components are filtered out. After the FBG filters out the chirp frequency component, a high-speed PD is used to complete the photoelectric conversion, and a spectrum analyzer is used to monitor the electrical spectrum of the obtained signal, as shown Figure 4 As shown in Figure 5, it is the electrical spectrum of the linear frequency modulation signal with four times the bandwidth and carrier frequency. It can be found that two broadband signals can be observed in the electrical spectrum of 3-3.5 GHz and 6-7 GHz, which is mainly due to the limited extinction ratio of the two IMs and the limited notch depth of the FBG. In order to evaluate the quality of the linear frequency modulation signal with four times the bandwidth and carrier frequency, we use an oscilloscope to observe the waveform of the linear frequency modulation signal, as shown in Figure 5(a). It contains 4 cycles, each cycle lasts for 1.2 μs, and the duration of the LFM signal is 0.9 μs. Using short-time Fourier transform, the signal quality can be analyzed in the time-frequency dimension. As shown in Figure 5(b), the generated signal bandwidth is 2 GHz and the TBWP is 1800. In addition, the pulse compression capability of the obtained signal is also evaluated, and the autocorrelation result is shown in Figure 5(c). In Figure 5(a), the obtained signal waveform has periodic changes. The time-frequency diagram of the signal is clearly visible, and there are no redundant frequency components, as shown in Figure 5(b). As can be seen from Figure 5(c), the peak sideband suppression ratio (PSR) of the autocorrelation peak is 7.05dB, the full width at half maximum (FWHM) is 0.625ns, and the corresponding pulse compression ratio (PCR) is 1440. In order to verify the system's frequency multiplication capability for LFM signals with larger bandwidth and carrier frequency, the carrier frequency and bandwidth of the IF-LFM signal are changed to 5GHz and 1GHz respectively. Then the electrical spectrum of the signal is obtained as shown in Figure 6As shown. Due to the limitations of the extinction ratio of the two IMs and the notch depth of the FBG, two broadband signals can be observed in the electrical spectrum of 5-6 GHz and 10-12 GHz. However, it should be noted that the amplitudes of the two broadband signals obtained are very low and far away from the desired signal. A LFM signal with a four-fold carrier frequency and bandwidth is generated from 20 GHz to 24 GHz, and there is no unnecessary interference near the signal we generated. We also evaluated the quality of the LFM signal obtained after changing the carrier frequency and bandwidth. The waveform, time-frequency diagram and autocorrelation results are detected, as shown in Figures 7(a) to (c), respectively. The waveform of the signal is shown in Figure 7(a), and the time-frequency diagram is shown in Figure 7(b). It can be found that the bandwidth of the generated LFM signal has increased by 4 times to 4 GHz, and the corresponding TBWP is 3600. In order to evaluate the pulse compression capability of the obtained signal, the autocorrelation result is shown in Figure 7(c). The PSR of the generated signal is 6.79 dB, the half-width modulation is 0.3 ns, and the corresponding pulse compression ratio (PCR) is 3000.

[0042] The linear frequency modulation signal generator proposed in the present invention has a simple structure and good stability. The system does not contain any electrical devices, which will greatly improve the frequency tuning performance of the system. And the entire optical link does not contain a loop, so that the system can complete the frequency doubling operation of the linear frequency modulation signal in a longer time. Due to the simple link structure, the scheme proposed by us is more suitable for integration. In addition, the signal generator proposed in the present invention provides a simple and effective method for obtaining a large TBWP linear frequency modulation signal, which will greatly improve the detection accuracy of the radar system and has potential application prospects in modern radar systems.

[0043] In summary, the above-described embodiments are only examples of the present invention and are not intended to limit the scope of protection of the present invention. It should be noted that for those skilled in the art, several equivalent variations and substitutions can be made based on the content disclosed in the present invention. For example, the final experimental results still contain some undesirable frequency components, which can be eliminated by using a modulator with a higher extinction ratio and an FBG with a higher notch ratio. In addition, two bias controllers can be applied in the system to make the bias voltage of the modulator more stable.

Claims

1. A method for quadrupling a linear frequency modulation signal based on a cascade modulator, comprising a laser LD, two cascaded intensity modulators IM, an arbitrary waveform generator AWG, an erbium-doped fiber amplifier EDFA, a fiber Bragg grating FBG and a photodetector PD; characterized in that: The optical carrier generated by the laser LD is sent to the first intensity modulator IM1 driven by the intermediate frequency linear frequency modulation IF-LFM signal. IM1 works at the minimum transmission point MITP to realize carrier suppressed double sideband CS-DSB modulation. Then the output signal of IM1 is amplified by the erbium-doped fiber amplifier EDFA and sent to the second intensity modulator IM2 for secondary modulation. IM2 also works at MITP and is driven by the same intermediate frequency linear frequency modulation signal to obtain the sideband and carrier frequency with twice the bandwidth. Due to the time delay of the two modulators, all sidebands have a slight frequency shift. Then the output signal of IM2 is injected into the FBG to filter out the chirp frequency component. Finally, the optical signal after passing through the FBG is sent to the PD to complete the photoelectric conversion to obtain a linear frequency modulation signal with a bandwidth and carrier frequency of four times the frequency.

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