A radio frequency arbitrary waveform generator and method based on optical pulse time-frequency shaping

Abstract

This invention discloses a radio frequency arbitrary waveform generator and method based on optical pulse time-frequency shaping. The radio frequency arbitrary waveform generator includes a mode-locked laser, the output of which is connected to the input of a first dispersive fiber. The output of the first dispersive fiber is connected to the input of a dual parallel Mach-Zehnder modulator. The output of the radio frequency signal generator array is connected to an electrical coupler via a radio frequency line. One output of the electrical coupler is connected to the radio frequency input of the upper arm sub-modulator of the dual parallel Mach-Zehnder modulator via a radio frequency line, and the other output is connected to a 90° phase shifter via a radio frequency line and then to the radio frequency input of the lower arm sub-modulator of the dual parallel Mach-Zehnder modulator. The output of the dual parallel Mach-Zehnder modulator is connected to the input of a second dispersive fiber via an optical fiber. The output of the second dispersive fiber is connected to the input of a photodetector via an optical fiber. The output of the photodetector is connected to the input of a low-pass filter via a radio frequency line.

Description

Technical Field

[0001] This invention belongs to the field of optical communication technology, specifically relating to a radio frequency arbitrary waveform generator and a method for generating radio frequency arbitrary waveforms based on optical pulse time-frequency shaping. Background Technology

[0002] Broadband radio frequency (RF) arbitrary waveform generation is crucial in many applications, such as capacity enhancement and noise cancellation in future 5G / 6G RF systems; multipath storage capabilities and stealth in communication and radar systems; material penetration and multipath interference mitigation in ultra-wideband wireless systems; and multipath immunity and high-precision ranging capabilities provided by the generated large-bandwidth waveforms. However, limited by the relatively low sampling rate and resolution of electronic digital-to-analog converters (ADCs), it is difficult to achieve RF arbitrary waveforms with bandwidths exceeding 10 GHz without sacrificing bit resolution using current commercial electronic RF arbitrary waveform generators. To overcome this electronic bottleneck, optically assisted high-frequency and ultra-wideband RF arbitrary waveform generation methods have attracted researchers' attention.

[0003] (Chi H, Yao J. Symmetrical waveform generation based on temporal pulseshaping using amplitude-only modulator[J]. Electronics Letters, 2007, 43(7): 415-417.) The time-domain pulse shaping (TPS) system mentioned in the literature shapes the pulse in the time domain using a mode-locked laser, two conjugate dispersion elements with matched dispersion, and an electro-optic modulator. Its essence is the Fourier transform relationship between the input and output waveform pairs. Due to the nature of the Fourier transform, only amplitude modulation is performed, and the generated arbitrary waveform is always symmetrical. (Jeonghyun, Azana, Jose. In-fiber reconfigurable generation of arbitrary (asymmetric) picosecond temporal intensity waveforms by time-domain optical pulse shaping.) The literature mentions a method that combines phase modulation-based TPS with a genetic algorithm to generate asymmetric waveforms. However, the iteration of this algorithm consumes a lot of time. Therefore, this method cannot generate the required arbitrary waveform in real time.

[0004] Meanwhile, the carrier amplitude of arbitrary waveforms generated by traditional TPS is too high, which greatly reduces the system's sensitivity and is not conducive to arbitrary waveform generation. Therefore, designing an arbitrary waveform generator that is simple to operate and can generate signals in real time has become a research hotspot in recent years. Summary of the Invention

[0005] To address the aforementioned shortcomings of existing technologies, this invention provides a radio frequency (RF) arbitrary waveform generator and a method for generating RF arbitrary waveforms based on optical pulse time-frequency shaping. This invention utilizes an RF signal generator array to generate RF signals with different center frequencies. After being multiplexed by an electrical coupler, these signals are split into two paths and fed into a dual parallel Mach-Zehnder modulator to achieve suppressed carrier single-sideband modulation. This ensures that each RF signal corresponds to a sampling point of the output waveform. By changing the amplitude of the generated RF signal, user-defined arbitrary waveforms can be generated in real time.

[0006] To achieve the above-mentioned objectives, the present invention adopts the following technical solution:

[0007] A radio frequency arbitrary waveform generator based on optical pulse time-frequency shaping includes a mode-locked laser, a first dispersive fiber, a dual parallel Mach-Zehnder modulator, a radio frequency signal generator array, an electrical coupler, a 90° phase shifter, a second dispersive fiber, a photodetector, and a low-pass filter. The output of the mode-locked laser is connected to the input of the first dispersive fiber. The output of the first dispersive fiber is connected to the input of the dual parallel Mach-Zehnder modulator. The output of the radio frequency signal generator array is connected to the electrical coupler via a radio frequency line. One output of the electrical coupler is connected via a radio frequency line to the input of the upper arm sub-modulator of the dual parallel Mach-Zehnder modulator, and the other output is connected via a radio frequency line to the 90° phase shifter and then to the input of the lower arm sub-modulator of the dual parallel Mach-Zehnder modulator. The output of the dual parallel Mach-Zehnder modulator is connected to the input of the second dispersive fiber via an optical fiber. The output of the second dispersive fiber is connected to the input of the photodetector via an optical fiber. The output of the photodetector is connected to the input of the low-pass filter via a radio frequency line.

[0008] The present invention also discloses a method for generating arbitrary radio frequency waveforms based on the above-mentioned generator, comprising the following steps:

[0009] S1. A mode-locked laser generates a transformation-restricted ultrashort optical pulse, which is propagated through a first segment of dispersive fiber for spectral chirping and time-domain broadening before entering a dual parallel Mach-Zehnder modulator.

[0010] S2. The radio frequency signal generator array generates radio frequency signals with different center frequencies. The generated radio frequency signals with different center frequencies are multiplexed by an electrical coupler. One of the multiplexed radio frequency signals enters the upper arm sub-modulator of the dual parallel Mach-Zehnder modulator, and the other enters the lower arm sub-modulator after passing through a 90° phase shifter.

[0011] S3. The output radio frequency signals with different center frequencies are used in a dual parallel Mach-Zehnder modulator to perform suppressed carrier single-sideband modulation on the broadened optical signal.

[0012] S4. The modulated signal output from the dual parallel Mach-Zehnder modulator is propagated through the second dispersive fiber and subjected to a time-domain Fourier transform to output a discrete pulse train.

[0013] S5. The output light pulse train is detected by a photodetector using square law, and then passed through a low-pass filter to generate any desired radio frequency waveform.

[0014] Furthermore, in step S1, the repetition period of the generated ultrashort optical pulse is T0, and the time domain representation of the ultrashort optical pulse is as follows:

[0015] g1(t)=exp(-t 2 / τ0 2 )

[0016] Where τ0 represents the pulse width with a peak intensity of 1 / e.

[0017] Furthermore, in step S1, the frequency response of the first segment of the dispersive fiber can be expressed as:

[0018]

[0019] in, This represents the dispersion of the first segment of the dispersive fiber.

[0020] Furthermore, in step S1, when the far-field condition is met, i.e. At that time, the frequency domain expression of the broadened optical signal output from the first dispersive fiber segment is:

[0021]

[0022] Wherein, G1(ω) is the frequency domain expression of the ultrashort optical pulse output by the mode-locked laser.

[0023] Further, in step S2, the radio frequency signal generated by the radio frequency signal generator array after multiplexing the radio frequency signals with different center frequencies through the electrical coupler can be expressed as:

[0024]

[0025] Where N is the number of signals generated, A n For the amplitude value of each signal, f n This represents the center frequency of the corresponding signal.

[0026] Further, in step S2, the multiplexed radio frequency signal after passing through a 90° phase shifter can be represented as:

[0027]

[0028] Furthermore, in step S3, both the upper and lower arm sub-modulators of the dual parallel Mach-Zehnder modulator operate in push-pull mode. The optical signal after suppressing carrier single-sideband modulation can be expressed as:

[0029]

[0030] Where g2(t) is the time-domain expression of the signal output from the first dispersive fiber segment.

[0031] Furthermore, in step S4, the dispersion of the second segment of the dispersive fiber is the same as that of the first segment, but in the opposite direction, and the expression for the output discrete pulse train is:

[0032]

[0033] Where, ω n =2πf n This corresponds to the angular frequency of the radio frequency signal.

[0034] Furthermore, in step S5, the photocurrent after square-law detection by the photodetector is:

[0035]

[0036] i(t) is the photocurrent after passing through the photodetector.

[0037] The advantages of this invention are:

[0038] Compared with traditional arbitrary waveform generation methods based on time-domain pulse shaping, the technical solution of this invention employs suppressed carrier single-sideband modulation, solving the problem of sensitivity degradation caused by excessively high carrier amplitude in traditional TPS systems. The technical solution of this invention has a simple structure, using only one integrated dual parallel Mach-Zehnder modulator, with one RF signal corresponding to one sampling point of the output signal. This greatly improves the waveform freedom, fidelity, and reconstruction speed of the RF arbitrary waveform generator, allowing for real-time generation of user-defined arbitrary waveforms by controlling the amplitude and center frequency of the input RF signal. Attached Figure Description

[0039] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0040] Figure 1 This is a flowchart of an arbitrary radio frequency waveform generator based on optical pulse time-frequency shaping;

[0041] Figure 2 This is a schematic diagram of a radio frequency arbitrary waveform generator based on optical pulse time-frequency shaping;

[0042] Figure 3 The image shows the result of the sinc function generated by an arbitrary RF waveform generator based on optical pulse time-frequency shaping.

[0043] Figure 4 The image shows a sawtooth wave generated by an arbitrary radio frequency waveform generator based on optical pulse time-frequency shaping.

[0044] Figure 5 This is a diagram of a triangular wave generated by an arbitrary radio frequency waveform generator based on optical pulse time-frequency shaping.

[0045] Figure 2 In the middle: 1. Mode-locked laser; 2. First dispersion fiber; 3. Dual parallel Mach-Zehnder modulator; 4. Radio frequency signal generator array; 5. Electrical coupler; 6. 90° phase shifter; 7. Second dispersion fiber; 8. Photodetector; 9. Low-pass filter. Detailed Implementation

[0046] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that, unless otherwise specified, the following embodiments and features described therein can be combined with each other.

[0047] Based on existing time-domain pulse shaping techniques, this invention utilizes a dual parallel Mach-Zehnder modulator to generate arbitrary waveforms by suppressing the carrier single sideband.

[0048] like Figure 2As shown, this embodiment of the radio frequency arbitrary waveform generator based on optical pulse time-frequency shaping includes: a mode-locked laser 1, a first dispersive fiber 2, a dual parallel Mach-Zehnder modulator 3, a radio frequency signal generator array 4, an electrical coupler 5, a 90° phase shifter 6, a second dispersive fiber 7, a photodetector 8, and a low-pass filter 9. The specific connection relationships are as follows:

[0049] The output of mode-locked laser 1 is connected to the input of first dispersive fiber 2 via optical fiber; the output of first dispersive fiber 2 is connected to the input of dual parallel Mach-Zehnder modulator 3 via optical fiber; the output of RF signal generator array 4 is connected to electrical coupler 5 via RF line; one output of electrical coupler 5 is connected to the RF input of the upper arm sub-modulator of dual parallel Mach-Zehnder modulator 3 via RF line, and the other output is connected to 90° phase shifter 6 via RF line and then to the RF input of the lower arm sub-modulator of dual parallel Mach-Zehnder modulator 3; the output of dual parallel Mach-Zehnder modulator 3 is connected to the input of second dispersive fiber 7 via optical fiber; the output of second dispersive fiber 7 is connected to the input of photodetector 8 via optical fiber; the output of photodetector 8 is connected to the input of low-pass filter 9 via RF line.

[0050] In this embodiment, a mode-locked laser is used to generate transform-restricted ultrashort optical pulses; two dispersive fibers with the same dispersion but different signs are used for frequency-time mapping; a dual parallel Mach-Zehnder modulator is used for suppressed-carrier single-sideband modulation; an RF signal generator array is used to generate modulated signals with different center frequencies; an electrical coupler is used to multiplex RF signals with different center frequencies; a 90° phase shifter is used to change the phase information of the RF signals with different center frequencies generated by the signal generator array; a photodetector is used to convert the optical signal into an electrical signal; and a low-pass filter is used to smooth the electrical signal using a low-pass filter.

[0051] like Figure 1 As shown in the figure, this embodiment provides a method for generating an arbitrary radio frequency waveform based on the above-mentioned optical pulse time-frequency shaping. The specific steps are as follows:

[0052] Step S1. The mode-locked laser generates a transformation-restricted ultrashort optical pulse, the time-domain and frequency-domain expressions of which are as follows: The generated optical pulse propagates through the first segment of dispersive fiber, undergoes spectral chirping and time-domain broadening, and then enters the dual parallel Mach-Zehnder modulator. The frequency response of the first segment of dispersive fiber is: The frequency domain expression after the first segment of dispersive fiber is:

[0053]

[0054] Step S2: The radio frequency signal generator array generates radio frequency signals with different center frequencies. The generated radio frequency signals with different center frequencies are multiplexed by an electrical coupler. The multiplexed radio frequency signal is: The multiplexed radio frequency signal enters the upper arm sub-modulator of the dual parallel Mach-Zehnder modulator in one path, and then enters the lower arm modulator after passing through a 90° phase shifter. The radio frequency signal after passing through the 90° phase shifter is:

[0055] Step S3: The output radio frequency signals with different center frequencies are subjected to suppressed carrier single-sideband modulation on the broadened optical signal in a dual parallel Mach-Zehnder modulator. The output signal after modulation is: Its frequency domain expression is:

[0056]

[0057] Where * represents the convolution operator;

[0058] Step S4: The output modulated signal propagates through the second segment of dispersive fiber and undergoes a time-domain Fourier transform to output a discrete pulse train. The frequency domain expression of the output discrete pulse train is:

[0059] The time-domain expression of the output discrete pulse train can be obtained by Fourier transform as follows:

[0060]

[0061] Step S5: The output light pulse train undergoes square-law detection by a photodetector. The electrical signal after passing through the photodetector is: Finally, the waveform is smoothed by a low-pass filter to generate any desired RF waveform.

[0062] In summary, the radio frequency arbitrary waveform generator based on optical pulse time-frequency shaping of this invention includes a mode-locked laser, two conjugate dispersive fibers with the same dispersion value but opposite signs, a dual parallel Mach-Zehnder modulator, an electrical coupler, a radio frequency signal generator array, a 90° phase shifter, a photodetector, and a low-pass filter. This invention utilizes the mode-locked laser to generate transform-restricted ultrashort optical pulses, which undergo frequency domain chirping and time domain broadening after passing through the first dispersive fiber. The pulses then enter the dual parallel Mach-Zehnder modulator. Radio frequency signals with different center frequencies generated by the radio frequency signal generator array are multiplexed by the electrical coupler and then subjected to suppressed-carrier single-sideband modulation. The modulated optical signals undergo photoelectric conversion by the photodetector and are then smoothed by the low-pass filter, thus generating arbitrary waveforms. The technical solution of this invention utilizes the dual parallel Mach-Zehnder modulator to achieve suppressed-carrier single-sideband modulation. Each input radio frequency signal corresponds to a sampling point of the output waveform. By changing the amplitude of the generated radio frequency signal, user-defined arbitrary waveforms can be generated in real time. This system greatly improves the waveform freedom, fidelity, and reconstruction speed of the RF arbitrary waveform generator. It has a simple structure and is easy to operate and integrate.

[0063] This invention solves the problems of excessive carrier energy and waveform symmetry in arbitrary waveform generation systems based on time-domain pulse shaping. It can generate user-defined arbitrary waveforms in real time, greatly improving the waveform freedom, fidelity, and reconstruction speed of RF arbitrary waveform generators. At the same time, it has a simple structure and is easy to implement.

[0064] Note that the above description is merely a preferred embodiment of the present invention and the technical principles employed. Those skilled in the art will understand that the present invention is not limited to the specific embodiments described herein, and various obvious changes, readjustments, and substitutions can be made without departing from the scope of protection of the present invention. Therefore, although the present invention has been described in detail through the above embodiments, the present invention is not limited to the above embodiments, and may include many other equivalent embodiments without departing from the concept of the present invention, the scope of which is determined by the scope of the appended claims.

Claims

1. A radio frequency arbitrary waveform generation method of a radio frequency arbitrary waveform generator based on optical pulse time-frequency shaping, characterized by, The radio frequency arbitrary waveform generator includes a mode-locked laser, a first dispersive fiber, a dual parallel Mach-Zehnder modulator, a radio frequency signal generator array, an electrical coupler, a 90° phase shifter, a second dispersive fiber, a photodetector, and a low-pass filter. The output of the mode-locked laser is connected to the input of the first dispersive fiber. The output of the first dispersive fiber is connected to the input of the dual parallel Mach-Zehnder modulator. The output of the radio frequency signal generator array is connected to the electrical coupler via a radio frequency line. One output of the electrical coupler is connected via a radio frequency line to the radio frequency input of the upper arm sub-modulator of the dual parallel Mach-Zehnder modulator, and the other output is connected via a radio frequency line to the 90° phase shifter and then to the radio frequency input of the lower arm sub-modulator of the dual parallel Mach-Zehnder modulator. The output of the dual parallel Mach-Zehnder modulator is connected to the input of the second dispersive fiber via an optical fiber. The output of the second dispersive fiber is connected to the input of the photodetector via an optical fiber. The output of the photodetector is connected to the input of the low-pass filter via a radio frequency line. The radio frequency arbitrary waveform generation method includes the following steps: S1. The mode-locked laser generates a transformation-restricted ultrashort optical pulse. The optical pulse propagates through the first dispersive fiber, undergoes spectral chirping and time-domain broadening, and then enters the dual parallel Mach-Zehnder modulator. S2. The radio frequency signal generator array generates radio frequency signals with different center frequencies. After being multiplexed by an electrical coupler, the first path of the radio frequency signals enters the upper arm sub-modulator of the dual parallel Mach-Zehnder modulator, and the second path enters the lower arm sub-modulator after passing through a 90° phase shifter. S3. Radio frequency signals with different center frequencies are used in a dual parallel Mach-Zehnder modulator to perform suppressed carrier single-sideband modulation on the broadened optical signal; S4. The modulated signal output from the dual parallel Mach-Zehnder modulator is propagated through the second dispersive fiber and subjected to a time-domain Fourier transform to output a discrete pulse train. S5. The output pulse train is detected by a photodetector using square law, and then passed through a low-pass filter to generate any desired radio frequency waveform.

2. The method according to claim 1, characterized in that, In step S1, the repetition period of the generated ultra-short light pulse is The time-domain representation of the ultra-short light pulse is ； where t is a time variable, represents the pulse width of the pulse having a peak intensity of e is the natural logarithm; In step S1, the frequency response of the first dispersive fiber is expressed as: ； Where j is the imaginary sign, The dispersion measure of the first-dispersion fiber. It represents the optical angular frequency.

3. The method according to claim 2, characterized in that, In step S1, when the far-field condition is met, i.e. At that time, the frequency domain expression of the broadened optical signal output from the first dispersive fiber is: ； in, This is the frequency domain expression for the ultrashort optical pulse output by a mode-locked laser.

4. The method according to claim 3, characterized in that, In step S2, the radio frequency signals with different center frequencies generated by the radio frequency signal generator array are multiplexed by the electrical coupler to produce the following radio frequency signal: ； Where N is the number of signals generated. For the amplitude value of each signal, This represents the center frequency of the corresponding signal.

5. The method according to claim 4, characterized in that, In step S2, the multiplexed RF signal is represented by a 90° phase shifter as follows: 。 6. The method according to any one of claims 1-5, characterized in that, In step S3, both the upper and lower arm sub-modulators of the dual parallel Mach-Zehnder modulator operate in push-pull mode.

7. The method according to claim 6, characterized in that, In step S3, the optical signal after single-sideband modulation with suppressed carrier is represented as follows: ； in, This is the time-domain expression of the signal output from the first segment of the dispersive fiber.

8. The method according to claim 7, characterized in that, In step S4, the dispersion of the second dispersive fiber is the same as that of the first dispersive fiber but opposite in direction, and the expression for the output discrete pulse train is: ； in, The lower sideband after single-sideband modulation with suppressed carrier. This corresponds to the angular frequency of the radio frequency signal.

9. The method according to claim 8, characterized in that, In step S5, the photocurrent after square-law detection by the photodetector is: 。

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