A broadband signal generation method based on electro-optical modulator and resistant to dispersion fading

Abstract

This invention proposes a method for generating broadband signals that is resistant to dispersion fading based on an electro-optical modulator. By rationally changing the phase of the driving signal, a chirp signal with opposite chirp signs can be generated during the beat frequency, thereby switching between single chirp and double chirp. Furthermore, this scheme proposes carrier-suppressed single-sideband modulation of the chirp signal, which effectively avoids amplitude fading caused by dispersion in optical fiber transmission while eliminating interference signals that fall within the chirp signal band after the beat frequency. The proposed scheme based on the DP-DPMZM integrated device is easy to integrate, has a simple structure, and is low-cost. The generated dual-band chirp signal can be flexibly switched between up-chirp, down-chirp, and double chirp, greatly improving the flexibility of the system. Furthermore, it effectively eliminates amplitude fading caused by dispersion in optical fiber transmission, significantly increasing the transmission distance of traditional systems. This scheme is expected to provide technical support for the development of future high-performance radar technology.

Description

Technical Field

[0001] The present invention relates to the field of optical communications, and in particular to the design of a chirp signal generation scheme and the elimination of the amplitude fading effect caused by dispersion during transmission. The invention provides a broadband signal generation method based on an electro-optical modulator that is resistant to dispersion fading and has high system performance, low structural complexity, and low cost. Background Art

[0002] Radar, short for Radio Detection and Ranging, operates on the principle of using a modulated signal and a specific antenna to emit electromagnetic waves into a specific area of ​​space to capture a target. The detected target returns a portion of its energy to the radar system, where it is processed by a receiver, providing information about the target's spatial location. Radar is currently widely used in military and modern life, playing a vital role. For example, radar is often used for military target tracking, ground-based hazardous material detection, resource exploration, traffic speed measurement, and storm warning. Modern radar systems primarily consist of modules such as transmitters, transceiver antennas, and receivers and processors. The signal source is a core component of the radar's transmitter module, and the parameters of the transmitted signal directly impact the radar system's detection performance. Generally speaking, a radar waveform must provide sufficient energy to meet the radar system's transmit power, possess a large time-bandwidth product to enhance detection performance, and suppress interference echoes.

[0003] Linear chirp signals possess powerful pulse compression capabilities, breaking the constraints of pulse signal bandwidth and duration. This allows them to simultaneously improve the detection range and resolution of radar systems, leading to their widespread application in modern radar systems. However, if the target is in motion and has a high velocity, the Doppler frequency shift can cause significant ranging errors, a phenomenon known as range-Doppler coupling. To eliminate these measurement errors, modern engineering often employs transmitters that emit a pair of single-chirp signals with opposite chirp rates, or directly transmit a radar waveform containing complementary chirp rates, known as a dual-chirp signal. Therefore, high-quality dual-chirp signals with large TBWP not only possess powerful pulse compression capabilities but can also further reduce ranging errors caused by Doppler coupling, leading to their widespread application in radar systems. Early dual-chirp signals were generated directly in the electrical domain, using techniques such as quartz crystals, LC oscillator circuits, and digital synthesis. However, due to the speed limitation of electronic devices, the center frequency and bandwidth of the generated double-chirped microwave signal are generally small, which will directly affect the detection performance of modern radar systems.

[0004] In order to overcome the bottleneck problem of electronic devices, in addition to developing new materials, new technologies and new devices, people have taken a different approach and explored the combination of photonics technology and microwave technology to generate and process microwave and millimeter wave signals, so as to better solve various key problems in communication and information science. As a result, an emerging interdisciplinary subject has gradually emerged that combines the advantages of photonics and microwave technology.

[0005] Microwave photonics. At present, many methods for generating linear chirp signals based on photonics technology have been reported, mainly including the following three: (1) Linear chirp signal generation based on direct space-time domain pulse shaping. This is a method for generating linear chirp based on direct space-time (DST) mapping. The input optical signal is converted into a spatial mode, processed in the spatial domain, and then converted back to the time domain by the DST pulse shaper. The generated signal has a large adjustable range, but the system is large and has poor stability. (2) Linear chirp signal generation based on spectrum shaping and frequency-time mapping. The system of this method is mainly composed of a pulse light source, an optical spectrum filter, a dispersion device and a photodiode. The pulse light source is used to generate a time-domain narrow pulse signal, the optical spectrum filter is used to shape the spectrum of the optical pulse, and the dispersion element is used to generate a frequency-time mapping effect. It is usually implemented using a section of single-mode fiber or a chirped fiber grating. The time length of the generated waveform can only be limited to a few nanoseconds at most, which will limit its application in radar systems. (3) Linear chirp signal generation based on external phase modulation. The bandwidth of the signal currently generated by this method is limited by the modulation index, and the TBWP is small and the frequency is low.

[0006] After researching linear chirp signal generation technologies, we found that dual-chirp signal generation based on external modulation is the most promising development direction. This method has a simple structure, and the radar signal generated is compressed after passing through a matched filter, producing a high autocorrelation peak. This effectively overcomes the influence of noise during radar signal reception and improves radar resolution. Summary of the Invention

[0007] This paper proposes a method for generating broadband signals based on an electro-optical modulator that is resistant to dispersion fading. By properly varying the phase of the driving signal, chirp signals with opposite chirp signs can be generated at the time of the beat frequency, enabling switching between single and double chirps. Furthermore, this scheme employs carrier-suppressed single-sideband modulation of the chirp signal, effectively avoiding amplitude fading caused by dispersion in optical fiber transmission while eliminating interfering signals that fall within the chirp signal band after the beat frequency. These research findings significantly enhance the system's flexibility and are expected to provide technical support for the future development of high-performance radar technology.

[0008] Light waves from LD After being injected into DP-DPMZM, it is divided into two paths. The upper branch is injected into DPMZM1 (composed of MZM1 and MZM2), where MZM1 and MZM2 are both biased at the minimum bias point, and MZM1 is biased by the RF signal V1cos(ω m t) is driven, and then this RF signal is driven by the phase shifter to generate a 90° phase shift and then drive MZM2. MZM1 and MZM2 use push-pull mode. At this time, by changing the bias voltage of the main MZM, different orders of single-sideband modulation or the generation of a four-foot optical frequency comb can be achieved, which lays the foundation for switching chirp symbols after the beat frequency. Another beam of light is injected into the DPMZM2 (composed of MZM3 and MZM4) in the lower branch. MZM3 is biased at the orthogonal bias point and is driven by V2cos(kt 2 ) and V2cos(kt 2 +π / 2) with MZM4 unloaded and the main MZM bias voltage adjusted to introduce a 180° phase difference. Combining MZM3 and MZM4 produces a carrier-suppressed single-sideband chirp signal. Combining the upper and lower branches in phase and detecting them with a photodiode generates a switchable chirp signal that is resistant to dispersion fading.

[0009] In the upper branch, MZM1 and MZM2 are biased at the minimum bias point (MITP), where MZM1 is biased by the RF signal V1cos(ω m t) drive, and then the RF signal is phase-shifted 90° by the phase shifter and drives MZM2. At this time, changing the bias voltage of the main MZM can produce different sideband conditions.

[0010] (1) Bias voltage is V π / 2 ,DPMZM1 produces negative first-order sideband and positive third-order sideband

[0011]

[0012] (2) Bias voltage is -V π / 2 ,DPMZM1 produces negative third-order sideband and positive first-order sideband

[0013]

[0014] (3) When the bias voltage is 0, positive and negative first-order sidebands and positive and negative third-order sidebands are generated

[0015]

[0016] Where V π / 2 is the half-wave voltage of MZM.

[0017] In the lower branch, MZM3 is biased at the quadrature bias point (QTP), and V2cos(kt 2) is driven to perform single-sideband chirp signaling, MZM4 is unloaded, and the main MZM is introduced with a 180° phase difference by the bias voltage. At this time, the output of DPMZM2 can be expressed as:

[0018]

[0019] After investigation, we know that the transmission function of optical fiber is:

[0020] H(ω,z)=e -j[β(ω)+γ]z (5)

[0021] Where z, β(ω) and γ are the length, propagation constant and amplitude attenuation coefficient, and Taylor expansion of β(ω) yields:

[0022]

[0023] where β(ω0), β′(ω0), and β″(ω0) are the propagation constant and the first-order and second-order derivatives at ω0, respectively.

[0024] Finally, after the light waves from DPMZM1 and DPMZM2 are combined and transmitted through an optical fiber of length z, there are three possible situations corresponding to the input of DPMZM1:

[0025] (1) When the main-MZM is biased at V π / 2 When , the output signal through the optical fiber with a length of z is:

[0026]

[0027] The transmitted signal is sent to the photodetector for beat frequency analysis to obtain the photocurrent:

[0028]

[0029] (2) When the main-MZM is biased at -V π / 2 When , the output signal through the optical fiber with a length of z is:

[0030]

[0031] The transmitted signal is sent to the photodetector for beat frequency analysis to obtain the photocurrent:

[0032]

[0033] (3) When the main-MZM bias is at 0, the output signal through the optical fiber with a length of z is:

[0034]

[0035] The transmitted signal is sent to the photodetector for beat frequency analysis to obtain the photocurrent:

[0036]

[0037] In (7)-(12), F{·} and F -1 {·} denote the Fourier transform and inverse Fourier transform, respectively. These results show that, in addition to the constant time delay introduced by optical fiber transmission, fiber dispersion also introduces corresponding phase delays in different frequency components, but it does not affect the amplitude of the optical wave.

[0038] The present invention provides a method for generating a broadband signal based on an electro-optical modulator and capable of resisting dispersion fading.

[0039] The solution consists of a dual-polarization dual-parallel Mach-Zehnder modulator (DP-DPMZM), a single-mode fiber transmission link, and a photodetector. The DP-DPMZM includes a polarization controller (PR), two parallel DPMZMs, and a polarization beam combiner (PBC). Each DPMZM consists of two sub-MZMs and a main MZM.

[0040] Compared with other chirp signal generation schemes, the advantages of the present invention are: (1) a relatively simple structure is proposed to simultaneously generate dual-band up-chirp signals and down-chirp signals or dual-band dual-chirp signals, and the switching between up-chirp, down-chirp and dual-chirp can be achieved by changing the bias voltage of the main-MZM in DPMZM1; (2) a carrier-suppressed single-sideband modulation is proposed, which can eliminate the single-frequency interference signal that falls within the chirp signal band after the beat frequency, and can effectively avoid the power fading caused by dispersion in optical fiber transmission. Based on the above two advantages, the scheme proposed by the present invention is easy to integrate based on the DP-DPMZM integrated device, has a simple structure and low cost, and the dual-band chirp signal generated can be flexibly switched between up-chirp, down-chirp and dual-chirp, which greatly improves the flexibility of the system. In addition, it effectively eliminates the amplitude fading caused by dispersion in optical fiber transmission, thereby greatly improving the transmission distance of the traditional system, which is of great significance to the research of high-performance radar. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 This is a schematic diagram of the principle of a broadband signal generation method based on an electro-optical modulator that is resistant to dispersion fading.

[0042] Figure 2 This is the spectrum of the -1st and +3rd order optical sidebands produced by DPMZM1 in the first case.

[0043] Figure 3 This is the spectrum of the -3rd and +1st order optical sidebands produced by the second case of DPMZM1.

[0044] Figure 4This is the spectrum of the four-foot optical frequency comb produced by the third case of DPMZM1.

[0045] Figure 5 The spectrum of the single-sideband modulation of the carrier suppressed chirp signal generated by DPMZM2 is

[0046] Figure 6 This is the spectrum of the first case after 30km of optical fiber transmission. The chirp occurs at 10GHz and the chirp occurs at 30GHz after the beat frequency.

[0047] Figure 7 This is the spectrum diagram of the first case after 30km of optical fiber transmission. The chirp is at 10GHz and the chirp is at 30GHz after the beat frequency.

[0048] Figure 8 This is the spectrum diagram of double chirp at 10 GHz and 30 GHz after 30 km of optical fiber transmission, corresponding to the first case.

[0049] Figure 9 This is a line graph showing the power difference of signals with different chirp rates after 30km of optical fiber transmission. DETAILED DESCRIPTION

[0050] The present invention proposes a method for generating broadband signals based on an electro-optical modulator that is resistant to dispersion fading. Figure 1 As shown, the specific implementation steps are as follows:

[0051] In the simulation, the laser diode (LD) emits light with a center frequency of 193.1 THz, an optical power of 10 dBm, and a linewidth of 10 MHz. The DP-DPMZM consists of four MZMs and three 1x2 optical isopower splitters. The four sub-MZMs have a half-voltage of 4 V and an extinction ratio of 30 dB. The PD has a responsivity of 1 A / W.

[0052] The light wave emitted by the laser diode (LD) is injected into the DP-DPMZM and is divided into two equal beams by the power divider. In its lower arm, the light wave is injected into DPMZM2 and then divided into two paths. The first path is injected into MZM3, and MZM3 is biased at QTP. The arbitrary waveform generator generates a chirp signal with an amplitude of 1V, a duration of 102.4ns, and a bandwidth from 0GHz to 2GHz, which drives the upper arm of MZM3. The chirp signal is then driven into the lower arm of MZM3 after a phase shift of π / 2, which will generate a single-sideband modulated chirp signal with a carrier. The other light wave enters MZM4, which is unloaded. After passing through MZM4, the light wave is attenuated by 9.7dB using an attenuator to match the carrier output by MZM3. The main modulator adds V π / 2 The bias voltage introduces a 180° phase shift. The two combined optical paths will produce a carrier-suppressed chirp signal, a single-sideband modulated signal such as Figure 5 shown.

[0053] In the upper arm, the light wave is injected into DPMZM1 and then split into two paths. Both paths are modulated by a 3.9V 10GHz RF signal with a phase difference of 90° through a push-pull mode sub-MZM. Both sub-MZMs are biased at the minimum bias point (MITP). In the first case, when the bias voltage V π / 2 When, such as Figure 2 , DPMZM2 can generate -1st and +3rd order sidebands. Although there is a residual -5th order interference sideband, its amplitude is much smaller than the -1st and +3rd order sidebands and can be ignored. At this time, the light waves generated by DPMZM1 and DPMZM2 are combined and transmitted through a 30km optical fiber. Then, the optical detection is performed by a PD to obtain a dual-band chirp signal with center frequencies of 10GHz and 30GHz. Figure 6 As shown in Figure 2, an up-chirp signal from 10 GHz to 12 GHz and a down-chirp signal from 30 GHz to 28 GHz are generated. π / 2 When, such as Figure 3 , DPMZM2 can generate +1st and -3rd order sidebands, and the +5th order interference sideband can be ignored. Figure 7 , generating a down-chirp signal from 10GHz to 8GHz and an up-chirp signal from 30GHz to 32GHz. In the third case, when the bias voltage of the main MZM is 0, as shown in Figure 4 , DPMZM2 can generate ±1st and ±3rd order sidebands, and similarly ±5th order sidebands can be ignored. After 30km of optical fiber transmission, Figure 8 , it can generate a double chirp signal with a center frequency of 10 GHz and a bandwidth of 30 GHz, which is twice that of the driving signal.

[0054] In summary, this paper proposes a method for generating broadband signals that is robust to dispersion fading and based on an electro-optical modulator. This scheme switches the sign of the chirp signal simply by changing the modulator's bias voltage. Theoretical derivation and simulation verification demonstrate excellent transmission performance, effectively avoiding amplitude fading due to fiber dispersion. It also boasts a simple structure, low cost, and ease of integration, improving radar flexibility and transmission performance, and is expected to provide technical support in radar research.

[0055] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any modifications or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in the present invention should be included in the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be based on the scope of protection of the claims.

Claims

1. A method for generating a broadband signal resistant to dispersion fading based on an electro-optical modulator, the characteristics of which include: The light wave emitted by the laser diode is injected into the DP-DPMZM and is divided into two beams by the equal power divider; in its lower arm, the light wave is injected into DPMZM2 and then divided into two paths. The first path is injected into MZM3 and the bias DC signal voltage V is adjusted. 3DC , biasing MZM3 at the quadrature bias point and modulating it with the chirped signal V3cos(kt2). At this point, MZM3 can output a chirped phase lightwave with an optical carrier and single-sideband modulation. Another signal is injected into MZM4, which is unloaded. The lightwave output by MZM4 is injected into the attenuator to match the amplitude of the optical carrier in the signal output by MZM3. The bias voltage of the main MZM in DPMZM2 is adjusted to create a 180° phase difference between the first and second lightwaves. The lightwaves are then combined in the same direction. Finally, DPMZM2 can generate a carrier-suppressed sideband signal modulated by the chirped signal. In the upper arm of the DP-DPMZM, the lightwave is also split into two paths after being injected into DPMZM1. The two lightwaves are injected into MZM1 and MZM2 respectively. The bias voltage is set to bias the two modulators at the minimum bias point. The RF signal V1cos(ωmt) generated by the RF source drives MZM1 on one path and drives MZM2 on the other path after a 90° phase shift. By adjusting the RF drive signal voltage, the modulation index β is guaranteed to be πV / V π =3.05, so that the amplitudes of the two smallest-order sidebands are equal and the amplitudes of the other high-order sidebands are as small as possible; at this time, adjust the bias voltage driving the main modulator, the bias voltage is V π / 2, DPMZM1 generates a negative first-order sideband and a positive third-order sideband, which are combined in the same direction with the light waves generated by DPMZM2. After transmitting a certain distance in the optical fiber, the beat frequency can be obtained at the center frequency ω m The positive chirp signal with the center frequency at 3ω m The negative chirp signal has the characteristics of resisting amplitude fading caused by dispersion; when the bias voltage is -V π / 2, DPMZM1 generates a negative third-order sideband and a positive first-order sideband. After being combined with the light wave generated by DPMZM2 in the same direction, it is transmitted for a distance in the optical fiber. The beat frequency can be obtained at the center frequency 3ω m The positive chirp signal with the center frequency at ω m The negative chirp signal has the characteristics of anti-dispersion-induced amplitude fading. When the bias voltage is 0, DPMZM1 generates positive and negative first-order sidebands and positive and negative third-order sidebands. After being combined with the light wave generated by DPMZM2 in the same direction, the beat frequency can be obtained after a certain distance of transmission in the optical fiber at the center frequency ω. m The double chirp signal with the center frequency at 3ω m The double-chirped signal has the characteristics of being resistant to amplitude fading caused by dispersion.

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