Method for generating high-purity 32-frequency-multiplication millimeter wave signal based on four cascaded polarization modulators

Abstract

The invention relates to the technical field of signal modulation, in particular to a method for generating high-purity 32 frequency multiplication millimeter wave signals based on four cascaded polarization modulators. Firstly, a system architecture based on four cascaded polarization modulators and used for generating high-purity 32-frequency-multiplication millimeter wave signals is constructed, and the process of generating the high-purity 32-frequency-multiplication millimeter wave signals comprises the following steps: suppressing even-order optical sidebands except + / -(8n) order; suppressing a + / -8-order optical sideband; and suppressing a 0-order optical carrier to obtain a high-purity 32-frequency-multiplication millimeter wave signal. According to the design of the invention, millimeter waves with 32 times of frequency are generated by only using four polarization modulators instead of any filter; according to the scheme, the structure is simple, the cost is low, and the obtained signal quality is good; and compared with a scheme realized based on a Mach-Zehnder modulator, the Mach-Zehnder modulator has a simpler structure and a purer frequency spectrum, and meanwhile, the problem of bias point drift caused by excessive dependence of the Mach-Zehnder modulator on bias voltage is avoided.

Description

Technical Field

[0001] The invention relates to the technical field of signal modulation, in particular to a method for generating high-purity 32-fold frequency millimeter wave signals based on four cascaded polarization modulators. Background Art

[0002] With the advent of the digital age, the amount of data in the world is exploding. This growth trend has put forward stronger demands on the transmission capacity and transmission quality of communication networks. Therefore, if the signal frequency band of the wireless communication system can be widened to the millimeter wave band, it can not only solve the problem of tight spectrum resources, but also obtain faster transmission rates and greater transmission capacity. However, due to the limitations of its electronic design and process, it is more difficult to generate high-frequency millimeter wave signals in the electronic field. In contrast, photons are a better choice to generate high-frequency millimeter wave signals.

[0003] Optical signal processing has many advantages over electrical signal processing. First, optical signal processing can improve the processing speed of digital signals. Second, optical signal processing can realize parallel operation. In addition, optical signal processing is insensitive to electromagnetic interference and electrical short circuits. Based on the above advantages, optical signal processing technology has received extensive attention and research. At present, the schemes for generating photon millimeter waves include the use of optical heterodyne, optoelectronic oscillator, four-wave mixing, up-conversion modulation and external modulation technology. Among various modulation methods, external modulation technology has been widely studied and applied. External modulation technology mainly uses modulators to modulate signals, such as phase modulator (PM), Mach-Zehnder modulator (MZM), polarization modulator (PolM), etc.

[0004] In past studies, millimeter wave signals have been generated using many different schemes, mainly using Mach-Zehnder modulators. Although there are various schemes for generating millimeter wave signals using Mach-Zehnder modulators, since Mach-Zehnder modulators require the introduction of DC bias voltage, which causes bias drift, a simpler polarization modulator can be used to achieve the desired signal. In view of this, we propose a method for generating high-purity 32x frequency millimeter wave signals based on four cascaded polarization modulators. Summary of the invention

[0005] The object of the present invention is to provide a method for generating a high-purity 32-fold frequency millimeter wave signal based on four cascaded polarization modulators, so as to solve the problems raised in the above background technology.

[0006] In order to solve the above technical problems, the purpose of the present invention is to provide a method for generating a high-purity 32-fold frequency millimeter-wave signal based on four cascaded polarization modulators. First, a system architecture based on four cascaded polarization modulators for generating a high-purity 32-fold frequency millimeter-wave signal is constructed, including a continuous wave laser CW, a polarization beam splitter PBS, a polarization beam combiner PBC, a polarizer Pol and a photodetector PD electrically connected in sequence, the polarization beam splitter PBS and the polarization beam combiner PBC are respectively separated into an upper branch and a lower branch into two orthogonal branches; the upper branch is composed of four subsystems cascaded together, respectively represented as subsystem A, subsystem B, subsystem C and subsystem D, each subsystem includes a polarization controller PC, a polarization modulator PolM and a polarizer Pol; the four subsystems are simultaneously connected to the same radio frequency RF, and electrical phase shifters FPS are connected between subsystem B, subsystem C and subsystem D and the radio frequency RF; the polarized light beam of the lower branch does not need to be modulated;

[0007] The process of generating a high-purity 32x frequency millimeter wave signal using the above system architecture includes the following steps:

[0008] S1. Suppress even-order optical sidebands except ±(8n) order: The optical carrier signal emitted by the continuous wave laser CW is divided into two polarized light beam signals, upper and lower, by the polarization beam splitter PBS set to π / 4. The angles of the polarization controller PC and the polarizer Pol in each subsystem are set to π / 4 to suppress odd-order optical sidebands. The signal transmitted in the upper branch passes through subsystem A, subsystem B, subsystem C and subsystem D in turn, and outputs a millimeter wave signal in which even-order optical sidebands except ±(8n) order are suppressed.

[0009] S2. Suppressing ±8th order optical sidebands: Eliminating ±8th order optical sidebands by setting appropriate modulation index to satisfy the condition that the amplitude of ±8th order optical sidebands is 0;

[0010] S3. Suppress the 0th-order optical carrier: Use the polarization beam combiner PBC to combine the signals output from the upper branch and the unmodulated lower branch. The signal output from the polarization beam combiner PBC then enters the last polarizer Pol. The angle of the polarizer Pol is adjusted to make the amplitude of the carrier signal equal to zero to eliminate the 0th-order optical carrier signal.

[0011] As a further improvement of the technical solution, a spectrometer OSA is connected between the polarizer Pol and the photodetector PD, and an output end of the photodetector PD is connected to an electric spectrometer ESA.

[0012] As a further improvement of the technical solution, in step S1, the optical carrier signal emitted by the continuous wave laser CW is divided into two signals, an upper signal and an lower signal, by a polarization beam splitter PBS set to π / 4. The signal transmitted in the upper branch first passes through subsystem A, and the signal E output from subsystem A isA (t) can be expressed as:

[0013]

[0014] Among them, E in (t) is the input light field of continuous light, which can also be written as E c and ω c are the amplitude and angular frequency of the optical carrier respectively; θ1 is the azimuth angle of the polarization controller; θ2 is the polarization angle of the polarizer; m is the modulation index of the polarization modulator, ω m is the angular frequency of the polarization modulator and j is the conventional coefficient.

[0015] As a further improvement of the technical solution, in step S1, in order to suppress odd-order optical sidebands, the angles of the polarization controller PolM and the polarizer Pol in each subsystem are set to π / 4; in this case, equation (1) can be simplified to:

[0016]

[0017] Expanding equation (2) using the first-kind Bessel function yields:

[0018]

[0019] Among them, J n (·) represents the first kind n-order Bessel function;

[0020] It can be seen from equation (3) that the output signal only contains even-order sidebands such as 0, ±2, ±4, and ±6. In order to eliminate even-order sidebands other than ±(8n), such as ±2, ±4, ±6, and ±10, the angles of the three phase shifters EPS need to be set to π / 4, π / 2, and 3π / 4, respectively.

[0021] As a further improvement of the technical solution, in step S1, the signal is transmitted to subsystem B, and the signal E output from subsystem B is B (t) can be written as:

[0022]

[0023] Similarly, the signal E output from subsystem C C (t) is expressed as:

[0024]

[0025] Signal E output from subsystem D D (t) is expressed as:

[0026]

[0027] By expanding, merging, and simplifying equation (6), we can obtain equation (7), which is expressed as follows:

[0028] Wherein, P, Q, R and S represent the amplitudes of the 0th order, ±8th order, ±16th order and ±24th order optical sidebands respectively;

[0029] It can be seen from equation (7) that at this time there are only 0th order, ±8th order, ±16th order and ±24th order optical sidebands, which means that all even-order optical sidebands except ±(8n) order are suppressed; and the signal output from subsystem D is the signal output from the upper branch.

[0030] As a further improvement of the technical solution, in step S3, the polarization beam combiner PBC is used to combine the signals output from the upper branch and the unmodulated lower branch, and the signal E output from the polarization beam combiner PBC is PBC (t) can be expressed as:

[0031]

[0032] Among them, E LB (t) is the light field of the lower branch.

[0033] As a further improvement of the present technical solution, in step S3, the signal output from the polarization beam combiner PBC then enters the last polarizer Pol, and the output signal E Pol (t) is:

[0034]

[0035] in, is the polarization angle of the last polarizer Pol.

[0036] As a further improvement of the technical solution, in step S3, in order to eliminate the 0th order optical carrier signal, it is necessary to adjust the angle of the last polarizer Pol so that the amplitude of the carrier signal is equal to zero; therefore, equation (10) can be obtained:

[0037]

[0038] Through calculation, we can get:

[0039]

[0040] Therefore, according to equation (11), it can be concluded that when the polarization angle is set to The 0th order optical carrier signal can be completely eliminated.

[0041] As a further improvement of the technical solution, in step S3, the final output signal mainly includes ±16-order and ±24-order optical sidebands, wherein harmonics greater than 24 can be ignored; the output signal E out (t) can be written as:

[0042]

[0043] The photocurrent I generated by the photodetector PD PD (t) can be expressed as:

[0044]

[0045] in, and G are the responsivity and amplification gain of the photodetector, respectively.

[0046] As a further improvement of the technical solution, the suppression ratio of the optical band edge and radio frequency spurious of the final output signal is calculated, specifically:

[0047] According to the definition of optical sideband suppression ratio (OSSR), it can be concluded that:

[0048]

[0049] Similarly, according to the definition of radio frequency spurious suppression ratio (RFSSR), we can get:

[0050]

[0051] Where R and S represent the amplitudes of the ±16th and ±24th order optical sidebands, respectively.

[0052] Compared with the prior art, the present invention has the following beneficial effects:

[0053] 1. In the method for generating a high-purity 32-fold frequency millimeter-wave signal based on four cascaded polarization modulators, a polarization beam splitter is used to separate an optical carrier into two orthogonal branches, the polarized light beam of the upper branch is used as an input signal of a subsystem based on a polarization modulator, and an even-order optical sideband is generated through four cascaded subsystems. The polarized light beam of the lower branch does not need to be modulated and is transmitted through the lower branch to eliminate the optical carrier signal, and four cascaded polarization modulators are used to obtain even-order optical sidebands, and the phase difference is adjusted by an electrical phase shifter to eliminate optical sidebands except for ±(8n) order, so that the system only contains 0th order, ±8th order, ±16th order, etc. optical sidebands, and by setting the modulation index and the angle of the last polarizer, the ±8th order and 0th order optical sidebands are eliminated in turn, thereby generating a high-purity millimeter-wave signal, and finally the signal is combined at the PBC and passed through the polarizer to obtain the required 32-fold frequency signal;

[0054] 2. In the method of generating high-purity 32-fold frequency millimeter-wave signals based on four cascaded polarization modulators, only four polarization modulators PolM are used without any filters to generate millimeter waves with a frequency of 32 times; compared with existing solutions, the 32-fold frequency millimeter-wave signal generated by this scheme has a simpler structure, lower cost, and better signal quality; and compared with the scheme based on Mach-Zehnder modulator, it has a simpler structure and a purer spectrum, while avoiding the problem of bias point drift caused by excessive reliance on bias voltage of Mach-Zehnder modulator. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] Figure 1 This is an exemplary principle block diagram of generating a millimeter wave signal with a frequency 32 times that of the present invention;

[0056] Figure 2 Spectrum diagram obtained for an exemplary upper branch in the present invention;

[0057] Figure 3 Spectrum obtained for the exemplary lower branch of the present invention;

[0058] Figure 4 is an exemplary final output spectrum of the present invention;

[0059] Figure 5 is an exemplary final output spectrum diagram of the present invention;

[0060] Figure 6 This is a spectrum diagram when the modulation index deviates from ±0.005 in the present invention;

[0061] Figure 7 This is a spectrum diagram when the modulation index deviates from ±0.005 in the present invention;

[0062] Figure 8This is an exemplary spectrum diagram when the modulation index deviates from ±0.01 in the present invention;

[0063] Fig. 9 This is a spectrum diagram when the modulation index deviates from ±0.01 in the present invention;

[0064] Fig.10 One of the exemplary line graphs comparing the effects of modulation index on OSSR and RFSSR in the present invention;

[0065] Fig.11 This is a spectrum diagram when the angle of the polarizer in the present invention deviates from ±0.05°;

[0066] Fig.12 This is a spectrum diagram when the angle of the polarizer in the present invention deviates from ±0.05°;

[0067] Fig.13 This is a spectrum diagram when the polarizer angle deviates from ±0.1° in the exemplary embodiment of the present invention;

[0068] Fig.14 This is a spectrum diagram when the angle of the polarizer in the present invention deviates from ±0.1°;

[0069] Fig.15 The second line graph comparing the influence of the modulation index on OSSR and RFSSR in the present invention;

[0070] Fig.16 is a spectrum diagram when three phase shifters in the present invention deviate from ±0.5° at the same time;

[0071] Fig.17 is a spectrum diagram when three phase shifters in the present invention deviate from ±0.5° at the same time;

[0072] Fig.18 is a spectrum diagram when three phase shifters in the present invention deviate from ±1° at the same time;

[0073] Fig.19 This is a spectrum diagram of the three exemplary phase shifters in the present invention when they deviate from ±1° at the same time. DETAILED DESCRIPTION

[0074] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0075] Example 1

[0076] like Figure 1 As shown, this embodiment provides a system architecture based on four cascaded polarization modulators for generating high-purity 32-fold frequency millimeter-wave signals, including a continuous wave laser CW, a polarization beam splitter PBS, a polarization beam combiner PBC, a polarizer Pol5 and a photodetector PD electrically connected in sequence, the polarization beam splitter PBS and the polarization beam combiner PBC are respectively separated into an upper branch and a lower branch into two orthogonal branches; the upper branch is composed of four subsystems cascaded together, respectively represented as subsystem A, subsystem B, subsystem C and subsystem D, each subsystem includes a polarization controller PC, a polarization modulator PolM and a polarizer Pol; wherein: subsystem A includes polarization controller PC1, polarization modulator PolM1 and polarizer Pol1; subsystem B includes polarization controller PC2, polarization modulator PolM2 and polarizer Pol2; subsystem C includes polarization controller PC3, polarization modulator PolM3 and polarizer Pol3; subsystem D includes polarization controller PC4, polarization modulator PolM4 and polarizer Pol4;

[0077] Four subsystems are simultaneously connected to the same radio frequency RF, wherein a sinusoidal signal with a frequency of 10 GHz can be selected as the input radio frequency signal to drive four subsystems based on polarization modulators, and electric phase shifters FPS are connected between subsystem B, subsystem C and subsystem D and radio frequency RF (subsystem B is connected to electric phase shifter FPS1, subsystem C is connected to electric phase shifter FPS2, and subsystem D is connected to electric phase shifter FPS3), and the phase differences of the radio frequency signals of the four polarization modulators (PolM1, PolM2, PolM3 and PolM4) are 0, π / 4, π / 2 and 3π / 4 rad respectively; the polarized light beam of the lower branch does not need to be modulated;

[0078] Furthermore, a spectrometer OSA is connected between the polarizer Pol5 and the photodetector PD, and an output end of the photodetector PD is connected to an electric spectrometer ESA.

[0079] Specifically, the operating principle of the system architecture is as follows: a continuous wave laser with a frequency of 193.1THz is used to emit light waves, and a polarization beam splitter PBS with an angle adjusted to +45° is used to separate the optical carrier into two orthogonal branches; the polarized light beam of the upper branch is used as the input signal of the subsystem based on the polarization modulator; four cascaded polarization modulators PolM are used to obtain even-order optical sidebands, and the phase difference and modulation index are adjusted to eliminate unwanted optical sidebands; finally, only ±16-order sidebands and optical carrier sidebands are left; the polarized light beam of the lower branch does not need to be modulated and is transmitted through the lower branch; the two light beams are combined into one light beam using a polarization beam combiner PBC, and then the 0th-order optical sideband after the merging of the upper and lower branches is eliminated by adjusting the angle of the last polarizer Pol5 to obtain the required frequency 32 times signal.

[0080] This embodiment further provides a method for generating a high-purity 32-fold frequency millimeter wave signal based on four cascaded polarization modulators, based on the above-mentioned system architecture based on four cascaded polarization modulators for generating a high-purity 32-fold frequency millimeter wave signal, comprising the following steps:

[0081] S1. Suppress even-order optical sidebands except ±(8n) order: The optical carrier signal emitted by the continuous wave laser CW is divided into two polarized light beam signals, upper and lower, by the polarization beam splitter PBS set to π / 4. The angles of the polarization controller PC and the polarizer Pol in each subsystem are set to π / 4 to suppress odd-order optical sidebands. The signal transmitted in the upper branch passes through subsystem A, subsystem B, subsystem C and subsystem D in turn, and outputs a millimeter wave signal in which even-order optical sidebands except ±(8n) order are suppressed.

[0082] In this step, the optical carrier signal emitted by the continuous wave laser CW is divided into two signals, upper and lower, by the polarization beam splitter PBS set to π / 4. The signal transmitted in the upper branch first passes through subsystem A, and the signal E output from subsystem A A (t) can be expressed as:

[0083]

[0084] Among them, E in (t) is the input light field of continuous light, which can also be written as E c and ω c are the amplitude and angular frequency of the optical carrier respectively; θ1 is the azimuth angle of the polarization controller; θ2 is the polarization angle of the polarizer; m is the modulation index of the polarization modulator, ω m is the angular frequency of the polarization modulator and j is the conventional coefficient.

[0085] In order to suppress odd-order optical sidebands, the angles of the polarization controller PolM (PolM1~PolM4) and the polarizer Pol (Pol1~Pol4) in each subsystem are set to π / 4; in this case, equation (1) can be simplified to:

[0086]

[0087] Expanding equation (2) using the first-kind Bessel function yields:

[0088]

[0089] Among them, J n (·) represents the first kind n-order Bessel function;

[0090] It can be seen from equation (3) that the output signal only contains even-order sidebands such as 0, ±2, ±4, and ±6. In order to eliminate even-order sidebands other than ±(8n), such as ±2, ±4, ±6, and ±10, the angles of the three phase shifters EPS (EPS1, EPS2, and EPS3) need to be set to π / 4, π / 2, and 3π / 4, respectively.

[0091] The signal is then transmitted to subsystem B, and the output signal E from subsystem B B (t) can be written as:

[0092]

[0093] Similarly, the signal E output from subsystem C C (t) is expressed as:

[0094]

[0095] Signal E output from subsystem D D (t) is expressed as:

[0096]

[0097] By expanding, merging, and simplifying equation (6), we can obtain equation (7), which is expressed as follows:

[0098] Wherein, P, Q, R and S represent the amplitudes of the 0th order, ±8th order, ±16th order and ±24th order optical sidebands respectively;

[0099] It can be seen from equation (7) that at this time there are only 0th order, ±8th order, ±16th order and ±24th order optical sidebands, which means that all even-order optical sidebands except ±(8n) order are suppressed; and the signal output from subsystem D is the signal output from the upper branch.

[0100] S2. Suppressing ±8th order optical sidebands: Eliminating ±8th order optical sidebands by setting appropriate modulation index to satisfy the condition that the amplitude of ±8th order optical sidebands is 0;

[0101] The principle is that the ±8th order optical sidebands can be eliminated by making the amplitude of the ±8th order optical sidebands 0, that is, this condition can be met by setting a suitable modulation index; in this solution, the modulation index is preferably set to 5.961.

[0102] S3, suppressing the 0th order optical carrier: using the polarization beam combiner PBC to combine the signals output from the upper branch and the unmodulated lower branch, the signal output from the polarization beam combiner PBC then enters the last polarizer Pol5, and the angle of the polarizer Pol5 is adjusted to make the amplitude of the carrier signal equal to zero to eliminate the 0th order optical carrier signal;

[0103] In this step, the polarization beam combiner PBC is used to combine the signals output from the upper branch and the unmodulated lower branch. The signal E output from the polarization beam combiner PBC is PBC (t) can be expressed as:

[0104]

[0105] Among them, E LB (t) is the light field of the lower branch.

[0106] The signal output from the polarization beam combiner PBC then enters the last polarizer Pol5, and the output signal E Pol (t) is:

[0107]

[0108] in, is the polarization angle of the last polarizer Pol5.

[0109] In order to eliminate the 0th order optical carrier signal, it is necessary to adjust the angle of the last polarizer Pol5 so that the amplitude of the carrier signal is equal to zero; therefore, equation (10) can be obtained:

[0110]

[0111] Through calculation, we can get:

[0112]

[0113] Therefore, according to equation (11), it can be concluded that when the polarization angle is set to 3.74°, the 0th-order optical carrier signal can be completely eliminated.

[0114] Therefore, the final output signal mainly includes ±16-order and ±24-order optical sidebands, among which harmonics greater than 24 can be ignored; the output signal E out (t) can be written as:

[0115]

[0116] The photocurrent I generated by the photodetector PD PD (t) can be expressed as:

[0117]

[0118] in, and G are the responsivity and amplification gain of the photodetector, respectively.

[0119] In addition, the method also includes calculation of the suppression ratio of the optical band edge and radio frequency spurious of the final output signal, specifically:

[0120] According to the definition of optical sideband suppression ratio (OSSR), it can be concluded that:

[0121]

[0122] Similarly, according to the definition of radio frequency spurious suppression ratio (RFSSR), we can get:

[0123]

[0124] Where R and S represent the amplitudes of the ±16th and ±24th order optical sidebands, respectively.

[0125] In summary, firstly, the odd-order optical sidebands are suppressed by setting the angles of the polarization controller PC and the polarizer (Pol) before and after each polarization modulator, so that the output signal only contains even-order optical sidebands; next, the even-order optical sidebands except the ±(8n) order are suppressed by controlling the phase difference between the four polarization modulators, that is, only the 0th order, ±8th order, ±16th order and other optical sidebands are retained; then, by setting the modulation index, the amplitude of the ±8th order optical sideband is made equal to 0, thereby eliminating the ±8th order optical sidebands; finally, the orthogonally modulated signal of the upper branch and the unmodulated signal of the lower branch are combined and rotated using the polarizer to eliminate the 0th order optical carrier. After the polarizer is rotated, the two 0th order optical carrier components cancel each other out to obtain the required ±16th order optical sidebands.

[0126] In addition, this scheme can also be implemented by connecting multiple Mach-Zehnder modulators in series or in parallel. However, if Mach-Zehnder modulators are used, it is necessary to control the drift of the bias point, making the implementation scheme complicated and unstable, and further modulation and verification are required.

[0127] Simulation Verification Example

[0128] In order to verify the accuracy of the millimeter wave generation solution proposed in the above scheme, based on Figure 1 The principle shown in the figure is used to build a simulation model using a simulation experiment platform. The main processing process of this scheme is that in the four polarization modulators of the upper branch, the spectrum obtained after the output of the upper branch is as follows Figure 2 As shown in Figure 2. Since the lower branch is not modulated, the spectrum obtained is as follows: Figure 3 As shown. Finally, the spectrum of the optical carrier signal output after the polarizer is eliminated is as follows Figure 4 As shown in Figure 1, it contains ±16-order and ±24-order optical sidebands. Of course, the dominant ones are the ±16-order optical sidebands. The RF spectrum of the final output signal is shown in Figure 1. Figure 5 As shown. Figure 5 It can be seen that the signal frequency obtained by the required ±16th order harmonic beat frequency is 320 GHz, which is 32 times that of the 10 GHz RF signal.

[0129] Although the ideal values ​​for implementing the proposed scheme have been obtained through theoretical calculations and simulation results, in practical applications, errors may exist in each parameter. Once errors occur in these decisive parameters, the OSSR and RFSSR of the signal will be affected. Therefore, the parameters that affect OSSR and RFSSR will be analyzed one by one, including the modulation index, the angle drift of the phase difference between the four polarization modulators, and the angle drift of the last polarizer.

[0130] In this scheme, the ±8th order optical sidebands are eliminated by setting the modulation index value, that is, the amplitude of the ±8th order optical sidebands is equal to zero. When the modulation index is set to 5.961, the obtained signal is the most ideal. However, when the modulation index value deviates from 5.961, the obtained signal is not ideal because the ±8th order optical sidebands are not completely eliminated. The spectrum diagram and RF spectrum diagram when the modulation index deviates from ±0.005 are shown as follows: Figure 6 and Figure 7 When the modulation index deviates from ±0.01, the spectrum diagram and RF spectrum diagram are shown as follows. Figure 8 and Fig. 9 As shown in the two spectra, it is obvious that as the offset increases, the amplitude of the ±8th order optical sidebands also increases. Therefore, due to the remnants of the ±8th order harmonics, both the OSSR and RFSSR decrease.

[0131] Moreover, the effect of modulation index deviation on OSSR and RFSSR is as follows: Fig.10 As shown. It can be seen that as the deviation of the modulation index increases, the values ​​of OSSR and RFSSR become lower and lower. When the modulation index is 5.921 or 6.001 (drift ±0.04), the OSSR and RFSSR are as low as about 16dB and 10dB respectively. Therefore, it is the maximum acceptable range of modulation index deviation.

[0132] In this scheme, the two 0th order optical carriers are eliminated by rotating the angle of the polarizer after the polarization beam combiner, so that the amplitude of the 0th order optical carrier is zero. Theoretical calculations show that when the polarizer is adjusted to 3.74°, the 0th order optical carrier can be completely offset. Therefore, once the angle is offset, the 0th order optical carrier cannot be completely suppressed, resulting in residual 0th order optical carrier. For example, when the polarizer angle deviates from ±0.05°, the spectrum diagram and RF spectrum diagram are as follows: Fig.11 and Fig.12 When the polarizer angle deviates from ±0.1°, the spectrum and RF spectrum are shown as follows. Fig.13 and Fig.14 It can be seen that when the angle of the polarizer deviates from the calculated ideal value, the 0th order optical carrier remains and increases with the increase of the offset angle.

[0133] In addition, the effect of the offset of the polarizer on OSSR and RFSSR is as follows Fig.15 As shown in the above two sets of spectrum diagrams and RF spectrum diagrams, it can be seen that the amplitude of the 0-order optical carrier has been increasing, causing the OSSR and RFSSR to decrease, as shown in Fig.15 As shown. It can also be seen that when the angle offset is ±0.05°, the OSSR drops from 53.74dB to 38.82dB, and the RFSSR drops from 47.59dB to 33.07dB. This shows that a drift of only 0.05° has a significant effect on both the OSSR and RFSSR. When the bias is within ±0.7°, the OSSR and RFSSR remain above 15dB and 10dB, respectively.

[0134] In this scheme, the phase difference between the four polarization modulators is generated by adjusting the angles of the three electrical phase shifters, thereby suppressing even-order optical sidebands except for the ±(8n) order. However, when the angles of the three phase shifters deviate, even-order optical sidebands such as ±2, ±4, ±6, and ±10 except for the ±(8n) order cannot be completely eliminated. For example, when the angles of the three phase shifters deviate from ±0.5° at the same time, the spectrum and RF spectrum obtained are as follows: Fig.16 and Fig.17 As shown in the figure, it can be clearly seen that the unwanted optical sidebands are not eliminated. The OSSR and RFSSR are only 24.09dB and 17.84dB respectively. When the angle offset is ±1°, the spectrum and RF spectrum are as follows: Fig.18 and Fig.19As shown. The OSSR and RFSSR obtained at this time are 18.55dB and 11.73dB respectively, and it can be seen that the values ​​of OSSR and RFSSR cannot be lower. Therefore, the angle allowed for the phase shifter to shift is ±1°. Beyond this range, the amplitude of other residual optical sidebands will be larger, and the ±16th order optical sidebands will not be the main optical sidebands.

[0135] Therefore, through the above analysis, it is found that these errors are inevitable in practice. However, through analysis, it can be seen that when the modulation index is set within the range of 5.921 to 6.001, the angle offset of the last polarizer is controlled within the range of ±0.7°, and the angle offset of the phase shifter is controlled within the range of ±1°, the signal results generated by this solution are acceptable.

[0136] Those skilled in the art will appreciate that the process of implementing all or part of the steps of the above embodiments may be completed by hardware, or may be completed by instructing related hardware through a program.

[0137] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited by the above embodiments. The above embodiments and descriptions are only preferred examples of the present invention and are not intended to limit the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. A method for generating a high-purity 32-fold frequency millimeter wave signal based on four cascaded polarization modulators, characterized in that: Firstly, a system architecture based on four cascaded polarization modulators for generating high-purity 32-fold frequency millimeter-wave signals is constructed, including a continuous wave laser CW, a polarization beam splitter PBS, a polarization beam combiner PBC, a polarizer Pol and a photodetector PD which are electrically connected in sequence, and the polarization beam splitter PBS and the polarization beam combiner PBC are respectively separated into an upper branch and a lower branch into two orthogonal branches; The upper branch is composed of four subsystems cascaded together, represented as subsystem A, subsystem B, subsystem C and subsystem D, each of which contains a polarization controller PC, a polarization modulator PolM and a polarizer Pol; the four subsystems are simultaneously connected to the same radio frequency RF, and an electric phase shifter FPS is connected between subsystem B, subsystem C and subsystem D and the radio frequency RF; the polarized light beam of the lower branch does not need to be modulated; The process of generating a high-purity 32x frequency millimeter wave signal using the above system architecture includes the following steps: S1. Suppress even-order optical sidebands except ±(8n) order: The optical carrier signal emitted by the continuous wave laser CW is divided into two polarized light beam signals, upper and lower, by the polarization beam splitter PBS set to π / 4. The angles of the polarization controller PC and the polarizer Pol in each subsystem are set to π / 4 to suppress odd-order optical sidebands. The signal transmitted in the upper branch passes through subsystem A, subsystem B, subsystem C and subsystem D in turn, and outputs a millimeter wave signal in which even-order optical sidebands except ±(8n) order are suppressed. S2. Suppressing ±8th order optical sidebands: Eliminating ±8th order optical sidebands by setting appropriate modulation index to satisfy the condition that the amplitude of ±8th order optical sidebands is 0; S3. Suppress the 0th-order optical carrier: Use the polarization beam combiner PBC to combine the signals output from the upper branch and the unmodulated lower branch. The signal output from the polarization beam combiner PBC then enters the last polarizer Pol. The angle of the polarizer Pol is adjusted to make the amplitude of the carrier signal equal to zero to eliminate the 0th-order optical carrier signal.

2. The method for generating a high-purity 32-fold frequency millimeter wave signal based on four cascaded polarization modulators according to claim 1, characterized in that: A spectrometer OSA is connected between the polarizer Pol and the photodetector PD, and an output end of the photodetector PD is connected to an electric spectrometer ESA.

3. The method for generating a high-purity 32-fold frequency millimeter wave signal based on four cascaded polarization modulators according to claim 2, characterized in that: In step S1, the optical carrier signal emitted by the continuous wave laser CW is divided into two signals, an upper signal and an lower signal, by a polarization beam splitter PBS set to π / 4. The signal transmitted in the upper branch first passes through subsystem A, and the signal E output from subsystem A is A (t) can be expressed as: Among them, E in (t) is the input light field of continuous light, which can also be written as E c and ω c are the amplitude and angular frequency of the optical carrier respectively; θ1 is the azimuth angle of the polarization controller; θ2 is the polarization angle of the polarizer; m is the modulation index of the polarization modulator, ω m is the angular frequency of the polarization modulator and j is the conventional coefficient.

4. The method for generating a high-purity 32-fold frequency millimeter wave signal based on four cascaded polarization modulators according to claim 3, characterized in that: In step S1, in order to suppress odd-order optical sidebands, the angles of the polarization controller PolM and the polarizer Pol in each subsystem are set to π / 4; in this case, equation (1) can be simplified to: Expanding equation (2) using the first-kind Bessel function yields: Among them, J n (·) represents the first kind n-order Bessel function; It can be seen from equation (3) that the output signal contains only even-order sidebands; in order to eliminate even-order sidebands except for ±(8n) orders, the angles of the three phase shifters EPS are set to π / 4, π / 2, and 3π / 4, respectively.

5. The method for generating a high-purity 32-fold frequency millimeter wave signal based on four cascaded polarization modulators according to claim 4, characterized in that: In step S1, the signal is transmitted to subsystem B, and the signal E output from subsystem B B (t) can be written as: Similarly, the signal E output from subsystem C C (t) is expressed as: Signal E output from subsystem D D (t) is expressed as: By expanding, merging, and simplifying equation (6), we can obtain equation (7), which is expressed as follows: Among them, P, Q, R and S represent the amplitudes of the 0th order, ±8th order, ±16th order and ±24th order optical sidebands respectively; it can be seen from equation (7) that only the 0th order, ±8th order, ±16th order and ±24th order optical sidebands exist at this time.

6. The method for generating a high-purity 32-fold frequency millimeter wave signal based on four cascaded polarization modulators according to claim 5, characterized in that: In step S3, the polarization beam combiner PBC is used to combine the signals output from the upper branch and the unmodulated lower branch, and the signal E output from the polarization beam combiner PBC is PBC (t) can be expressed as: Among them, E LB (t) is the light field of the lower branch.

7. The method for generating a high-purity 32-fold frequency millimeter wave signal based on four cascaded polarization modulators according to claim 6, characterized in that: In step S3, the signal output from the polarization beam combiner PBC then enters the last polarizer Pol, and the output signal E Pol (t) is: in, is the polarization angle of the last polarizer Pol.

8. The method for generating a high-purity 32-fold frequency millimeter wave signal based on four cascaded polarization modulators according to claim 7, characterized in that: In step S3, in order to eliminate the 0th order optical carrier signal, it is necessary to adjust the angle of the last polarizer Pol so that the amplitude of the carrier signal is equal to zero; therefore, equation (10) can be obtained: Through calculation, we can get: Therefore, according to equation (11), when the polarization angle is set to When , the 0th-order optical carrier signal can be completely eliminated.

9. The method for generating a high-purity 32-fold frequency millimeter wave signal based on four cascaded polarization modulators according to claim 8, characterized in that: In step S3, the final output signal mainly includes ±16-order and ±24-order optical sidebands, ignoring harmonics greater than 24; the output signal E out (t) can be written as: The photocurrent I generated by the photodetector PD PD (t) can be expressed as: in, and G are the responsivity and amplification gain of the photodetector, respectively.

10. The method for generating a high-purity 32-fold frequency millimeter wave signal based on four cascaded polarization modulators according to claim 9, characterized in that: It also includes the calculation of the suppression ratio of the optical band edge and RF spurious of the final output signal, specifically: According to the definition of optical sideband suppression ratio OSSR, it can be concluded that: Similarly, according to the definition of RF spurious suppression ratio RFSSR, we can get: Where R and S represent the amplitudes of the ±16th and ±24th order optical sidebands, respectively.

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