High-efficiency synchronization method and device based on double optical frequency comb broadband spread spectrum communication

Abstract

The application discloses a high-efficiency synchronization method and device based on double optical frequency comb broadband spread spectrum communication, wherein at a transmitting end, an optical frequency comb 1 of a double optical frequency comb generation module 1 is loaded with a communication signal, a wideband spread spectrum signal is obtained through heterodyne beat after the optical frequency comb 2 is combined, and the signal reaches a receiving end after transmission; at the receiving end, an adjustable optical time delay line is inserted after parallel optical frequency comb 3 or optical frequency comb 4 branches of a double optical frequency comb generation module 2, then the optical signals of the two branches are combined, a local oscillation electric frequency comb is obtained through heterodyne beat, and high-efficiency synchronization is implemented between the received wideband spread spectrum signal and the local oscillation electric frequency comb by using the adjustable optical time delay line of the double optical frequency comb generation module 2 at the receiving end. The application greatly improves synchronization tuning efficiency, significantly reduces the hard requirement of large optical delay, and has important application significance in fast synchronization demand scenes such as wideband spread spectrum communication and wideband covert communication.

Description

Technical Field

[0001] This invention pertains to spread spectrum communication technology, and particularly relates to an efficient synchronization method and apparatus based on dual-optical-frequency-comb broadband spread spectrum communication. Background Technology

[0002] In the digital age where information security demands are unprecedentedly high, covert communication, or low-probability-of-detection communication, has become a core technology for military command, government confidential transmission, and medical privacy protection. Its core objective is not only to protect the content of the communication (e.g., through encryption), but also to conceal the existence of the communication itself, making it undetectable to eavesdroppers ("Lowprobability of detection communication: opportunities and challenges," IEEE Wireless Communications, pp. 19-25, 2019). However, covert communication also faces numerous challenges and limitations, such as various noise interferences in increasingly complex electromagnetic environments, and limitations on communication frequency bands and power ("Hiding information in noise: fundamental limits of covert wireless communication," IEEE Communications Magazine, pp. 26-31, 2015).

[0003] Spread spectrum communication, such as direct sequence spread spectrum communication, uses the same spreading code time-domain waveform to achieve spectrum spreading and autocorrelation demodulation at both the transmitting and despreading ends, recovering the transmitted information from noisy environments. However, due to the electronic bottlenecks of purely electrical methods, communication speed, transmission bandwidth, and anti-interference performance are significantly reduced. Especially in the case of acquiring and tracking broadband spread spectrum signals under strong noise interference, traditional methods have high hardware complexity and consume large amounts of digital resources ("A Review of Spread Spectrum Communication Interference Suppression Technology", Journal of Ordnance Equipment Engineering, pp. 301-308, 2024). Utilizing the advantages of microwave photonics technology, such as its large bandwidth and resistance to electromagnetic interference, photonic synchronization has become a new method for efficient synchronization in broadband spread spectrum communication. Summary of the Invention

[0004] The purpose of this invention is to provide an efficient synchronization method and device based on dual-optical-frequency-comb broadband spread spectrum communication, which has the characteristics of wide bandwidth, low interception, and high-efficiency synchronization.

[0005] This invention discloses an efficient synchronization method for broadband spread spectrum communication based on dual optical frequency combs. At the transmitting end, a continuous single-frequency laser generates an optical carrier, which is split into two by an optical coupler and injected into a dual optical frequency comb generation module. One optical path directly generates optical frequency comb 1 and modulates the communication signal using a single-sideband modulator. The other optical path first modulates the optical carrier to the +1 order sideband using a single-sideband modulator and then generates optical frequency comb 2. A spectral shaper performs amplitude modulation and phase encoding on each tooth of optical frequency comb 2. After the dual optical frequency combs are coupled, each pair of optical frequency comb components beats the signal in a heterodyne manner through a photoelectric balanced detector to obtain a synthesized broadband radio frequency signal, which is then transmitted by an antenna, propagates in free space, and reaches the receiving end.

[0006] At the receiving end, the generation method of the dual optical frequency comb generation module 2 is consistent with that of the dual optical frequency comb generation module 1, generating optical frequency comb 3 and optical frequency comb 4. After the branch of optical frequency comb 3 or optical frequency comb 4, an adjustable optical delay line is inserted. Then, each pair of optical frequency comb components is beat-frequency generated by the heterodyne method of the photoelectric balanced detector 2 to generate a local oscillator frequency comb with amplitude and phase coding. The local oscillator laser is split into two by the optical coupler and injected into the broadband spread spectrum signal modulator and the local oscillator frequency comb modulator respectively. After passing through the photoelectric balanced detector 3, coherent despreading is performed in the analog spectrum convolution method. The bandpass filter is applied at the gain intermediate frequency point to finally obtain the despread communication signal, which is then subjected to offline digital signal processing.

[0007] Furthermore, there is a difference between the free spectrum region ω1 of optical comb 1 and the free spectrum region ω2 of optical comb 2 in the dual optical comb generation module one; there is the same difference between the free spectrum region ω3 of optical comb 3 and the free spectrum region ω4 of optical comb 4 in the dual optical comb generation module two.

[0008] Furthermore, the equivalent synchronization delay constructed by the inserted tunable delay line is amplified by M = ω3 / (ω3-ω4) or M = ω4 / (ω3-ω4) times compared to the initially set delay. The amplified equivalent delay is used to implement synchronization at both ends of the transmitting and receiving ends of the broadband spread spectrum communication system based on dual optical frequency combs, thereby improving the synchronization efficiency by M times.

[0009] This invention provides an efficient synchronization device based on dual-optical-frequency-comb broadband spread spectrum communication, which implements the above-mentioned efficient synchronization method based on dual-optical-frequency-comb broadband spread spectrum communication, and includes a transmitter and a receiver.

[0010] The transmitting end includes a continuous single-frequency laser, an optical frequency comb, a single-sideband modulator, a single-sideband modulator, an optical frequency comb, a spectrum shaper, a phase-locked crystal oscillator, a photoelectric balance detector, and a transmitting antenna.

[0011] The receiving end includes a continuous single-frequency laser, an optical frequency comb, a tunable optical delay line, a single-sideband modulator, an optical frequency comb, a spectrum shaper, a phase-locked crystal oscillator, a photoelectric balance detector, a receiving antenna, a local oscillator laser, a broadband spread spectrum signal modulator, a local oscillator frequency comb modulator, a photoelectric balance detector, and an intermediate frequency filter.

[0012] The beneficial technical effects of this invention compared to the prior art are as follows:

[0013] 1) This invention uses dual optical frequency combs to synthesize broadband spread spectrum signals and despread communication signals at both the transmitting and receiving ends, making full use of the advantages of microwave photonics technology, such as large bandwidth and resistance to electromagnetic interference, resulting in a compact and efficient link.

[0014] 2) Based on the dual optical frequency comb spread spectrum despreading architecture, the present invention constructs an amplified equivalent synchronization delay by cascading adjustable optical delay lines in the optical frequency comb generation branches, which greatly improves the delay matching efficiency without increasing the system link complexity. Attached Figure Description

[0015] Figure 1 This is a block diagram illustrating the principle of the efficient synchronization method based on dual-optical-frequency-comb broadband spread spectrum communication of the present invention.

[0016] Figure 2 For a broadband spread spectrum communication system based on dual optical frequency combs: (a) spread spectrum signal generation; (b) despreading process.

[0017] Figure 3 (a) The spectrum of the broadband spread spectrum signal synthesized at the transmitter, (b) The spectrum of the local oscillator frequency comb generated at the receiver, (c) The time-domain waveform of the local oscillator frequency comb with zero-phase phase difference coding or pseudo-random phase difference coding, (d) The time-domain waveforms of the frequency comb with adjustable delay lines that control the delay at 0ps, 8ps, and 16ps respectively, with delay amounts of 0ns, 1.66ns, and 3.32ns, respectively, (e) The despread coherent gain peak after strict synchronization, and (f) The constellation diagram of the communication signal recovered from additive white Gaussian noise with a negative signal-to-noise ratio. Detailed Implementation

[0018] The present invention will be further described in detail below with reference to the accompanying drawings and specific implementation methods.

[0019] This invention proposes an efficient synchronization method and device based on dual-optical-comb broadband spread spectrum communication. The dual-optical-comb spread spectrum method employed (“Electro-optic frequency combs,” Advances in Optics and Photonics, pp. 223-287, 2020.) can significantly increase the bandwidth of the spread spectrum signal, reduce the signal transmission power, and significantly improve the signal-to-noise ratio and anti-interception performance (“Covert wireless communication using massive optical comb channels for deep denoising,” Photonics Research, pp. 1124-1133, 2021.). In synchronization, traditional delay matching methods typically involve a one-to-one correspondence between the adjusted delay and the applied delay. This results in a large rigid length requirement and high cost of polarization-maintaining fiber (or ordinary single-mode fiber), which is not conducive to on-chip integration. Therefore, this invention provides an efficient delay matching method. This method utilizes the vernier effect of heterodyne beat frequency of dual optical frequency combs ("Opticallymagnified dispersion of microwave signals with a wide and flexible tunable range," Optics Letters, pp. 1057-1060, 2022.), which not only avoids complicating the link but also significantly improves the efficiency of delay synchronization tuning.

[0020] The present invention provides an efficient synchronization method based on dual-optical-frequency-comb broadband spread spectrum communication, as follows: Figure 1 As shown, at the transmitter 10, a continuous single-frequency laser 101 generates an optical carrier, which is split into two by an optical coupler and injected into a dual optical frequency comb generation module 1. One optical path directly generates optical frequency comb 1 (102) and modulates the communication signal using a single-sideband modulator 103. The other optical path first modulates the optical carrier to the +1 order sideband using a single-sideband modulator 104, and then generates optical frequency comb 2 (105). A difference Δω is set between the free spectral ranges of the two optical frequency combs.

[0021] Δω=ω1-ω2

[0022] Where ω1 and ω2 are the free frequency regions of optical frequency comb 1 (102) and optical frequency comb 2 (105), respectively. Furthermore, each pair of optical frequency comb components is paired, and the frequency interval Δf of the nth pair of optical frequency comb components... n for:

[0023] Δf n =Ω+n×Δω

[0024] Where Ω is the optical carrier frequency shift frequency of the single-frequency laser generated by the single-sideband modulator II 104, 0≤n≤N, and N is the total number of comb teeth pairs of the dual-frequency comb.

[0025] Simultaneously, the spectral shaper 106 performs amplitude modulation and phase encoding on each tooth of the optical frequency comb 2 (105), such as zero phase difference encoding, 0 or π phase difference encoding, and pseudo-random phase difference encoding. Figure 2 As shown in (a), after dual optical frequency comb coupling, each pair of optical frequency comb components is beat-frequencyd through a photoelectric balanced detector (108 heterodyne mode) to obtain a synthesized broadband radio frequency signal. Its spectrum is a periodic repetition of the communication signal spectrum, such as... Figure 3 As shown in (a), the broadband spread spectrum signal is transmitted by the antenna, propagates in free space, and reaches the receiving end.

[0026] At the receiving end, the generation method of the dual optical frequency comb generation module two is consistent with that of the dual optical frequency comb generation module one, generating optical frequency comb 3 (202) and optical frequency comb 4 (205). An adjustable optical delay line 203 is inserted after the branch of optical frequency comb 3 (202) or optical frequency comb 4 (205). Then, each pair of optical frequency comb components is beat-frequency generated by the heterodyne method of the photoelectric balanced detector two 208 to generate a local oscillator frequency comb with amplitude and phase encoding, such as... Figure 3 As shown in (b). Figure 3 (c) shows the time-domain waveforms of near-flat local oscillator frequency comb zero-phase difference coding and pseudo-random phase difference coding.

[0027] The local oscillator laser 210 is split into two by an optical coupler and injected into a broadband spread spectrum signal modulator 211 and a local oscillator frequency comb modulator 212, respectively. Coherent despreading is then performed by a photoelectric balanced detector 213 using analog spectral convolution. Analog spectral convolution refers to the process of beating the broadband spread spectrum signal and the local oscillator frequency comb using heterodyne to obtain an intermediate frequency signal in the electrical domain. For example... Figure 2 As shown in (b), this intermediate frequency signal is composed of the coherent and in-phase superposition of multiple replicated communication signal spectra, which significantly improves the signal-to-noise ratio and anti-interference performance at the intermediate frequency point. The experimental results are as follows. Figure 3 As shown in (e).

[0028] Finally, a bandpass filter 214 is applied at the gain mid-frequency point to obtain the despread communication signal, which is then subjected to offline digital signal processing. Furthermore, the system can recover the communication signal from a low signal-to-noise ratio (SNR) additive white Gaussian noise channel. Since the noise superposition is incoherent while the communication signal spectrum in this method is a multi-coherent in-phase superposition, the received SNR and carrier-to-noise ratio (CNR) can be significantly improved, thereby enabling RF wireless communication with large bandwidth and low power spectral density, or ensuring safe and reliable communication even when encountering high-power noise interference. For example, in an environment where the broadband spread spectrum signal has a negative SNR, the QPSK constellation diagram recovered by the receiver after coherent despreading and gain is as follows: Figure 3 As shown in (f), its hard decision bit error rate is less than 3.8 × 10⁻⁶. -3 .

[0029] It is worth emphasizing that strict synchronization between the broadband spread spectrum signal and the local oscillator frequency comb is an important condition for ensuring the despreading coherence gain. There are two main uncertainties in synchronization: firstly, the RF drive frequencies of the dual optical frequency combs at both the transmitting and receiving ends must be completely consistent, which can be achieved by external phase-locked crystal oscillator 107 and external phase-locked crystal oscillator 207; secondly, the arrival time interval between the broadband spread spectrum signal and the local oscillator frequency comb must satisfy an integer multiple of the local oscillator frequency comb repetition period, which can be achieved by the adjustable optical delay line 203. Traditional delay matching schemes typically involve adding an adjustable optical delay line after the local oscillator frequency comb modulator 212, with the adjusted delay amount corresponding one-to-one with the equivalent delay amount. The innovation of this invention lies in adding an adjustable optical delay line 203 after optical frequency comb 3 (202) or optical frequency comb 4 (205) to efficiently achieve synchronous delay matching between the transmitting and receiving ends. Assuming the expression for the optical field E1(t) after the adjustable optical delay line 203 loaded after optical frequency comb 3 (202) is:

[0030]

[0031] Where ω0 and ω1 and ω2 are the center angular frequency and initial phase of laser 201, respectively; ω3 is the free spectrum region of optical frequency comb 3 (202); τ is the time delay controlled by tunable optical delay line 203; and j is the imaginary unit. The expression for the amplitude-flat, zero-phase-difference encoded optical field E2(t) after shaping by spectral shaper 206 is:

[0032]

[0033] Where ω4 is the free frequency spectrum region of the optical frequency comb 4(205). Then, the electric field e(t) after the dual optical frequency combs are coupled through the photoelectric balanced detector 208 is:

[0034]

[0035] Where R is a constant, It is the phase after the delay. Based on the Fourier transform delay property, it is easy to see that the slope of the phase change between the comb teeth in the frequency domain of the local oscillator is the delay of its time domain waveform; that is, there is a linear amplification relationship between the initial delay of the control and the effective delay.

[0036]

[0037] This indicates that by utilizing the free spectrum region of the dual optical frequency comb and the difference between the two free spectrum regions (similar to the principle of vernier calipers), an amplified equivalent synchronization delay is constructed, and the amplification factor M is the quotient of the free spectrum region and the difference between the two free spectrum regions. Figure 3 (d) The experiment visually demonstrates the delay matching effect of the zero-phase-difference encoded local oscillator frequency comb. The free spectrum regions of optical frequency comb 3 (202) and optical frequency comb 4 (205) are 33.16 GHz and 33 GHz, respectively. The initial delay values ​​of 0 ps, ​​8 ps, and 16 ps respectively affect the equivalent delay values ​​of the local oscillator frequency comb of 0 ns, 1.66 ns, and 3.32 ns, which are consistent with the amplification factor M = 207.25.

[0038] The present invention provides an efficient synchronization device based on dual-optical-frequency-comb broadband spread spectrum communication, which implements the above-mentioned efficient synchronization method based on dual-optical-frequency-comb broadband spread spectrum communication, and includes a transmitter 10 and a receiver 20.

[0039] The transmitter 10 includes a continuous single-frequency laser 101, an optical frequency comb 1 (102), a single-sideband modulator 103, a single-sideband modulator 2 104, an optical frequency comb 2 (105), a spectrum shaper 106, a phase-locked crystal oscillator 107, a photoelectric balance detector 108, and a transmitting antenna 109.

[0040] The receiver 20 includes a continuous single-frequency laser 201, an optical frequency comb 3 (202), a tunable optical delay line 203, a single-sideband modulator 3 204, an optical frequency comb 4 (205), a spectrum shaper 206, a phase-locked crystal oscillator 207, a photoelectric balance detector 208, a receiving antenna 209, a local oscillator laser 210, a broadband spread spectrum signal modulator 211, a local oscillator frequency comb modulator 212, a photoelectric balance detector 3 213, and an intermediate frequency filter 214.

[0041] The above descriptions are merely preferred embodiments of the present invention. It should be noted that, without departing from the essence of the method and core device of the present invention, several modifications and refinements may be made in actual implementation, which should also be included within the scope of protection of the present invention.

Claims

1. A highly efficient synchronization method based on dual-optical-frequency-comb broadband spread spectrum communication, characterized in that: At the transmitter (10), a continuous single-frequency laser (101) generates an optical carrier, which is split into two by an optical coupler and injected into a dual optical frequency comb generation module. One optical path directly generates an optical frequency comb 1 (102) and modulates the communication signal using a single-sideband modulator (103). The other optical path first modulates the optical carrier to the +1 order sideband using a single-sideband modulator (104) and then generates an optical frequency comb 2 (105). A spectrum shaper (106) performs amplitude modulation and phase encoding on each tooth of the optical frequency comb 2 (105). After the dual optical frequency combs are coupled, each pair of optical frequency comb components beats the signal in a heterodyne manner through a photoelectric balance detector (108) to obtain a synthesized broadband radio frequency signal, which is then transmitted by the antenna, propagates in free space, and reaches the receiver. At the receiving end, the generation method of the dual optical frequency comb generation module 2 is consistent with that of the dual optical frequency comb generation module 1, generating optical frequency comb 3 (202) and optical frequency comb 4 (205). After the branch of optical frequency comb 3 (202) or optical frequency comb 4 (205), an adjustable optical delay line (203) is inserted. Then, each pair of optical frequency comb components is beat-frequency generated by the heterodyne method of the photoelectric balanced detector 2 (208) to generate a local oscillator frequency comb with amplitude and phase coding. The local oscillator laser (210) is split into two by the optical coupler and injected into two carrier-suppressed double-sideband modulators respectively. After passing through the photoelectric balanced detector 3 (213), coherent despreading is performed in the analog spectrum convolution method. The bandpass filter (214) is used to filter at the gain mid-frequency point to finally obtain the despread communication signal, which is then processed offline digitally.

2. The efficient synchronization method based on dual-optical-frequency-comb broadband spread spectrum communication according to claim 1, characterized in that: There is a difference between the free spectrum region ω1 of the optical frequency comb 1 (102) and the free spectrum region ω2 of the optical frequency comb 2 (105) in the dual optical frequency comb generation module one; there is the same difference between the free spectrum region ω3 of the optical frequency comb 3 (202) and the free spectrum region ω4 of the optical frequency comb 4 (205) in the dual optical frequency comb generation module two.

3. The efficient synchronization method based on dual-optical-frequency-comb broadband spread spectrum communication according to claim 2, characterized in that: The equivalent synchronization delay constructed by the inserted tunable optical delay line (203) is amplified by M = ω3 / (ω3-ω4) or M = ω4 / (ω3-ω4) times compared to the initially set delay. The amplified equivalent delay is used to implement synchronization at both ends of the transmitting and receiving ends of the broadband spread spectrum communication system based on dual optical frequency comb, thereby improving the synchronization efficiency by M times.

4. A high-efficiency synchronization device based on dual-optical-frequency-comb broadband spread spectrum communication, characterized in that: The efficient synchronization method based on dual-optical-frequency-comb broadband spread spectrum communication as described in claim 1 includes a transmitter (10) and a receiver (20); The transmitter (10) includes a continuous single-frequency laser (101), an optical frequency comb (102), a single-sideband modulator (103), a single-sideband modulator (2) (104), an optical frequency comb (2) (105), a spectrum shaper (106), a phase-locked crystal oscillator (107), a photoelectric balance detector (108), and a transmitting antenna (109). The receiver (20) includes a continuous single-frequency laser (201), an optical frequency comb (202), a tunable optical delay line (203), a single-sideband modulator (204), an optical frequency comb (205), a spectrum shaper (206), a phase-locked crystal oscillator (207), a photoelectric balance detector (208), a receiving antenna (209), a local oscillator laser (210), a broadband spread spectrum signal modulator (211), a local oscillator frequency comb modulator (212), a photoelectric balance detector (213), and an intermediate frequency filter (214).

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