CN115396036B - Broadband optical frequency comb generation method based on DPMZM and two IM cascading - Google Patents

Abstract

The invention discloses a broadband optical frequency comb generation method based on DPMZM and two IM cascades, which solves the problems of small spectrum bandwidth and low out-of-band suppression ratio of optical frequency combs generated by the existing technology. The steps implemented by the invention are: generating radio frequency signals; setting the DC bias voltage of the modulator; inputting optical carrier signals; generating modulated optical signals; offsetting the ±1-order sideband signals generated by the two sub-modulators; and suppressing high-order sideband signals. ; Generate 10-line broadband optical frequency comb; Generate 50-line broadband optical frequency comb. The present invention uses the main modulator of DPMZM to offset the ±1-order sideband signals generated by the two sub-modulators. The DPMZM outputs ±3-order sidebands, increases the spectrum bandwidth of the optical frequency comb, and adds a bandpass after the DPMZM. Optical filter improves the out-of-band suppression ratio of the optical frequency comb.

Description

Broadband optical frequency comb generation method based on DPMZM and two IM cascades

Technical field

The invention belongs to the field of communication technology, and further relates to a cascade based on a dual parallel Mach-Zehnder modulator DPMZM (Dual Parallel Mach-Zehnder Modulator) and two intensity modulators IM (Intensity Modulator) in the field of microwave photonic technology. Broadband optical frequency comb generation method. The optical frequency comb generated by the invention can be used as a multi-carrier light source and applied to optical communication systems.

Background technique

Microwave photonic technology is a technology about the interaction and impact of microwave signals and optical signals, including the use of optical methods to process, control and transmit microwaves. In communications, microwave wireless communication has flexible characteristics, and optical fiber communication has bandwidth advantages. Microwave photonic technology combines the two, which has huge application potential for high-speed broadband information transmission. An optical frequency comb refers to a series of evenly spaced and phase-coherent frequency components in the spectrum. With the rapid development of optical communication technology, optical frequency combs are widely used in fields such as dense wavelength division multiplexing, optical arbitrary waveform generation, and multi-wavelength ultrashort pulse generation. In order to meet the application requirements in the field of optical communications, it is urgent to produce optical frequency combs with large frequency bandwidth, large number of optical comb lines, variable spectral line spacing, good flatness and high out-of-band suppression ratio. At present, the generation methods of optical frequency comb include mode-locked laser method, nonlinear fiber method, electro-optic modulator method, etc. Among them, the mode-locked laser method requires relatively strict mode-locking conditions, and the spectral line spacing of the generated optical frequency comb is difficult to change. The optical frequency comb generated by the nonlinear effect of optical fiber has large power fluctuations and poor flatness. The method of generating optical frequency combs based on electro-optical modulators has a simple system structure, variable spectral line spacing and good flatness, so the electro-optical modulator method is widely used. However, due to the bandwidth limitation of the electro-optical modulator, the current optical frequency comb produced by the electro-optical modulator method often has a small bandwidth and a limited number of optical comb lines.

In their paper "Highly flexible optical Nyquist pulsesgeneration based on dual-parallel Mach-Zehnder modulator and intensitymodulator" (Photonic Network Communications, 2018, 36(3): 361-368), Shibao Wu et al. disclosed a method based on dual-parallel Mach-Zehnder modulator and intensity modulator. Optical frequency comb generation methods for parallel Mach-Zehnder modulators and intensity modulators. The specific steps to implement this method are that the laser generates a continuous optical signal through a cascade structure of a dual parallel Mach-Zehnder modulator DPMZM and an intensity modulator IM. The frequency of the radio frequency signal loaded on the DPMZM is set to 18.75GHz. By adjusting the power of the radio frequency signal and the DC bias of the modulator, the DPMZM generates a 5-line optical comb with a spectral line interval of 37.5GHz. Then an intensity modulator IM is cascaded, and by setting the power of the RF signal and the DC bias of the modulator, the IM modulates the frequency component generated by each DPMZM into a 3-line optical comb. This method ultimately produces a 15-line optical frequency comb. The spectrum bandwidth is 187.5GHz. The shortcomings of this method are that the spectral line spacing of the 5-line optical comb generated by DPMZM is twice the loaded RF frequency, the spectrum bandwidth is small, and the final optical frequency comb has fewer lines and out-of-band suppression. The ratio is not high, resulting in insufficient communication capacity when the generated optical frequency comb is applied to optical communication systems.

Xi'an University of Electronic Science and Technology disclosed a broadband optical frequency comb in its patent document "A device and method for generating a broadband optical frequency comb" (application number CN201310750920.0, application date 2013.12.30, application publication number CN 103744249 A) Methods for generating optical frequency combs. The method includes: laser, dual parallel Mach-Zehnder modulator DPMZM, two intensity modulators IM, radio frequency signal source, and spectrum analyzer. Set the frequency of the RF signal loaded on the DPMZM to 25GHz. By adjusting the power of the RF signal and the DC bias of the modulator, the DPMZM outputs ±2nd-order sidebands with equal power. Then cascade two IMs with the same working status, and set the RF signal power and DC bias loaded on the IM so that when each frequency component passes through the IM, it will be modulated to generate a 5-line optical comb, so this method ultimately generates 50 Line optical frequency comb, spectral line spacing is 4GHz, and spectrum bandwidth is 196GHz. The disadvantage of this method is that it relies heavily on the modulator bandwidth. The frequency interval of the ±2nd-order sidebands generated by DPMZM is 4 times the loaded RF frequency, and the spectrum bandwidth is small, resulting in the generated optical frequency comb. When applied to optical communication systems, the communication capacity is insufficient.

Contents of the invention

The purpose of the present invention is to propose a broadband optical frequency comb generation method based on a dual parallel Mach-Zehnder modulator DPMZM and two intensity modulators IM cascaded to solve the existing problems in the existing technology. The optical frequency comb generation method of electro-optical modulators has technical problems such as small spectrum bandwidth, small number of optical comb lines, and low out-of-band suppression ratio.

To achieve the above purpose, the technical idea of the present invention is: the present invention is implemented in a link consisting of a single-frequency laser, four RF signal sources, a dual parallel Mach-Zehnder modulator DPMZM including two sub-modulators, a bandpass optical filter, two intensity modulators IM and a spectrum analyzer. RF signals are loaded on the two sub-modulators on the DPMZM, and the frequency of the RF signal is 25GHz. The sub-modulators are operated in push-pull mode and biased at the minimum point so that both sub-modulators output odd-order optical sidebands, and then the main modulator is used to offset the ±1-order sidebands generated by the two sub-modulators. Finally, the DPMZM outputs ±3-order sidebands, and the frequency interval is 6 times the frequency of the loaded RF signal. Adding a bandpass optical filter after the DPMZM can effectively suppress high-order sidebands and improve the out-of-band suppression ratio of the optical frequency comb to be generated. Then, two IMs with the same working state are cascaded, and by adjusting the RF signal power and DC bias voltage loaded on the IM, the IM can generate a 5-line optical frequency comb, that is, the power of the 0th order, ±1st order sidebands and ±2nd order sideband signals is equal. In this scheme, the latter stage modulator modulates the optical sidebands generated by the previous stage modulator as light sources, and finally generates a 50-line broadband optical frequency comb.

The specific steps of the present invention are as follows:

Step 1, the RF signal source generates RF signals;

Step 2, set the DC bias voltage of the modulator:

The upper arm DC bias voltage of each sub-modulator in the dual parallel Mach-Zehnder modulator DPMZM is set to V _π /2, and the lower arm DC bias voltage is set to -V _π /2; the DC bias voltage of the main modulator of the dual parallel Mach-Zehnder modulator DPMZM is set to V _π , where V _π represents the half-wave voltage of the modulator;

Step 3, input optical carrier signal:

Input the optical carrier signal generated by the single-frequency laser into the dual parallel Mach-Zehnder modulator DPMZM after setting the DC bias voltage;

Step 4, generate modulated light signal:

The radio frequency signal RF1 and the radio frequency signal RF2 are respectively input into the sub-modulators of the respectively connected dual parallel Mach-Zehnder modulator DPMZM for optical carrier modulation, and two ±1-order sideband signals generated by the sub-modulators are output, one ± Third-order sideband signals, six sideband signals in total;

Step 5, cancel the ±1st order sideband signals generated by the two sub-modulators:

After making the power of the ±1-order sideband signals generated by the two sub-modulators equal, then input the ±1-order sideband signals generated by the two sub-modulators into the main modulator and cancel each other to obtain the ±3-order sideband signal;

Step 6, suppress high-order sideband signals:

The ±3rd-order sideband signal output by the dual parallel Mach-Zehnder modulator DPMZM is input into a bandpass optical filter connected in series after the dual-parallel Mach-Zehnder modulator DPMZM, and the ±3rd-order sideband signal after suppressing the high-order sidebands is output. sideband signals;

Step 7, generate a broadband optical frequency comb:

The radio frequency signal and the ±3-order sideband signal after suppressing the high-order sideband are respectively input into n serially connected intensity modulators in the same working state after the bandpass optical filter for optical signal modulation; each intensity modulator outputs 0 The power of first-order, ±1st-order, and ±2nd-order sideband signals are all equal. Each intensity modulator modulates 1 optical comb into 5 optical combs; when the following intensity modulator is satisfied, the frequency of the RF signal loaded by the latter intensity modulator is the previous intensity. When the frequency of the RF signal loaded by the modulator is 1/5, the nth intensity modulator will generate a 2×5 ⁿ -line broadband optical frequency comb.

Compared with the prior art, the present invention has the following advantages:

First, the present invention loads radio frequency signals on the two sub-modulators of the DPMZM. By setting the power of the radio frequency signal and the DC bias voltage of the modulator, the ±1-order sideband signals generated by the two sub-modulators are input to the main modulation of the DPMZM. After canceling each other out, the DPMZM outputs ±3rd-order sidebands, and the frequency interval is 6 times the frequency of the loaded radio frequency signal, which overcomes the shortcoming of the small bandwidth of the optical frequency comb in the existing technology, making the optical frequency comb produced by the present invention The spectrum bandwidth increases, and when applied to optical communication systems, the communication capacity increases.

Second, because the present invention adds a bandpass optical filter after the DPMZM, it can effectively suppress high-order sideband signals other than ±3rd-order sideband signals, making the optical frequency comb generated by the present invention applied to optical communication systems. , the out-of-band suppression ratio of the optical frequency comb is improved, and the signal-to-noise ratio of the signal source is increased.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a microwave photonic link diagram according to an embodiment of the present invention;

Figure 2 is a flow chart of the present invention;

Figure 3 is a simulation diagram of simulation experiment 1 of the present invention;

Figure 4 is an output spectrum diagram of simulation experiment 2 of the present invention.

Detailed ways

The present invention will be described in further detail below with reference to the accompanying drawings and examples.

The invention is implemented in a microwave photonic link.

Referring to Figure 1, the microwave photonic link used in the embodiment of the present invention is described in further detail.

The microwave photonic link in the embodiment of the present invention includes: a single-frequency laser Laser, four radio frequency signal sources, a dual parallel Mach-Zehnder modulator DPMZM including two sub-modulators MZ-a and MZ-b, and a bandpass optical Filter, two intensity modulators IM1 and IM2 and an optical spectrum analyzer OSA. The optical input port of DPMZM is connected to the single-frequency laser Laser. The RF ports of the two sub-modulators MZ-a and MZ-b of DPMZM are connected to the first RF signal source and the second RF signal source respectively. The two intensity modulators IM1 and IM2 The radio frequency ports are respectively connected to the third radio frequency signal source and the fourth radio frequency signal source. The DC bias voltage loaded on the main modulator of the DPMZM is on the lower arm of the DPMZM.

With reference to Figure 2 and the embodiment, the implementation steps of the present invention will be described in further detail.

Step 1. Generate radio frequency signal.

The two sub-modulators of the dual parallel Mach-Zehnder modulator DPMZM and the RF signal sources connected to the RF ports of the two intensity modulators respectively generate four RF signals: RF1, RF2, RF3 and RF4. The frequencies of these four RF signals Corresponding to: f ₁ , f ₂ , f ₃ and f ₄ respectively, where f ₁ =f ₂ .

Step 2, set the DC bias voltage of the modulator.

The upper arm DC bias voltage of each sub-modulator in the dual parallel Mach-Zehnder modulator DPMZM is set to V _π /2, and the lower arm DC bias voltage is set to -V _π /2, thereby ensuring that both sub-modulators work in push-pull mode and work in the minimum transmission point state. Among them, V _π represents the half-wave voltage of the sub-modulator, and the half-wave voltages of the two sub-modulators are equal.

Since the DC bias voltage of the main modulator in the dual parallel Mach-Zehnder modulator DPMZM is loaded on the lower arm of the dual-parallel Mach-Zehnder modulator DPMZM, the main modulator of the dual-parallel Mach-Zehnder modulator DPMZM The DC bias voltage is set to V _π , thereby ensuring that the phase difference of the output signals of the two sub-modulators is π to ensure that the phases of the output signals of the two sub-modulators are opposite.

Step 3: Input an optical carrier signal.

The optical carrier signal generated by the single-frequency laser Laser is input into the dual parallel Mach-Zehnder modulator DPMZM after setting the DC bias voltage.

Step 4: Generate a modulated optical signal.

Because the radio frequency ports of the two sub-modulators of the dual parallel Mach-Zehnder modulator DPMZM are respectively connected to the first radio frequency signal source and the second radio frequency signal source. Therefore, the two radio frequency signals RF1 and RF2 are input into the respective connected sub-modulators of the dual parallel Mach-Zehnder modulator DPMZM for optical carrier modulation. The six sideband signals output are as follows:

Among them, E _a,±1 (t) represents the ±1-order sideband signal output from the output end of the sub-modulator MZ-a at the t time, and E _a,±3 (t) represents the sub-modulator MZ-a at the t time. The ±3-order sideband signal output by the output terminal a, E _b,±1 (t) represents the ±1-order sideband signal output by the output terminal MZ-b of the sub-modulator at the t time, E ₀ represents the output of the single-frequency laser Laser The electric field amplitude of the optical carrier signal, exp(·) represents the exponential operation with the natural constant e as the base, j represents the imaginary unit symbol, ω _c represents the central angular frequency of the optical carrier signal output by the single-frequency laser Laser, and ω ₁ represents The central angular frequency of the radio frequency signal RF1 loaded on the sub-modulator MZ-a, t represents the sequence number of the moment when the sub-modulator output terminal outputs the sideband signal, J ₁ (·) represents the first-order Bessel function of the first kind, m ₁ represents the modulation index of the radio frequency signal RF1 loaded on the sub-modulator MZ-a, m ₁ =πV _RF1 /V _π , π represents the pi, V _RF1 represents the amplitude of the radio frequency signal RF1 loaded on the sub-modulator MZ-a, V _π represents the half-wave voltage of the sub-modulator, represents the sinusoidal function, φ ₁ represents the phase difference caused by the DC bias voltage of the upper arm of the sub-modulator MZ-a, φ ₁ =π/2, J ₃ (·) represents the third-order Bessel function of the first kind, ω ₂ represents the sub- The central angular frequency of the radio frequency signal RF2 loaded on the modulator MZ-b, m ₂ represents the modulation index of the radio frequency signal RF2 loaded on the sub-modulator MZ-b, m ₂ =πV _RF2 /V _π , V _RF2 represents the sub-modulator The amplitude of the radio frequency signal RF2 loaded on MZ-b, φ ₂ , represents the phase difference caused by the DC bias voltage of the upper arm of the sub-modulator MZ-b, φ ₂ =π/2.

Step 5, cancel the ±1-order sideband signals generated by the two sub-modulators.

In order to cancel the ±1-order sideband signals output by the two sub-modulators, make the power of the ±1-order sideband signals output by the two sub-modulators obtained in step 4 equal, we can get J ₁ (m ₁ ) = J ₁ (m ₂ ). Therefore, the ±1-order sideband signals generated by the two sub-modulators cancel each other after being input to the main modulator. The dual parallel Mach-Zehnder modulator DPMZM outputs a ±3-order sideband signal. The frequency interval of the ±3-order sideband signal is The radio frequency signal RF1 is 6 times the frequency f ₁ .

Step 6: Suppress high-order sideband signals.

A bandpass optical filter Filter is connected in series after the dual parallel Mach-Zehnder modulator DPMZM, the center frequency of which is equal to the frequency of the optical carrier signal, and the 3dB bandwidth is the frequency interval between the ±3rd order sidebands. When the output signal of the dual parallel Mach-Zehnder modulator DPMZM is input into the bandpass optical filter Filter, the high-order sideband signals other than the ±3rd order sideband signals can be effectively suppressed, thereby improving the out-of-band suppression ratio of the optical frequency comb to be generated.

Step 7: Generate a 10-line broadband optical frequency comb.

The bandpass optical filter Filter is cascaded with the intensity modulator IM1, and the ±3rd-order sideband signal output by the bandpass optical filter Filter after suppressing the high-order sidebands is input to the intensity modulator IM1. Set the DC bias voltage loaded by the intensity modulator IM1 to V _DC3 . Since the RF port of the intensity modulator IM1 is connected to the third RF signal source, the RF signal RF3 is input to the intensity modulator IM1 for optical signal modulation. The RF signal RF3 The frequency f ₃ and the frequency f ₁ of the radio frequency signal RF1 need to satisfy 6f ₁ =5f ₃ . The 0th-order, ±1st-order, and ±2nd-order sideband signals output by the intensity modulator IM1 are

E _IM1,0 (t)＝ _ED (t)J ₀ (m ₃ )cosφ ₃

E _IM1,±1 (t)＝±j·E _D (t)exp(±jω ₃ t)J ₁ (m ₃ )sinφ ₃

E _IM1,±2 (t)＝E _D (t)exp(±j2ω ₃ t)J ₂ (m ₃ )cosφ ₃

Among them, E _IM1,0 (t) represents the 0th-order sideband signal output by the output terminal of the intensity modulator IM1 at the t time, and E _IM1,±1 (t) represents the ± output terminal of the intensity modulator IM1 at the t time. The 1st-order sideband signal, E _IM1,±2 (t) represents the ±2nd-order sideband signal output from the output terminal of the intensity modulator IM1 at the tth moment, E _D (t) represents the dual parallel Mach-Zehnder modulation at the tth moment The signal output from the output terminal of the DPMZM device, J ₀ (·) represents the 0th order Bessel function of the first kind, m ₃ represents the modulation index of the radio frequency signal RF3 loaded on the intensity modulator IM1, m ₃ =πV _RF3 /V _π , V _RF3 represents the amplitude of the radio frequency signal RF3 loaded on the intensity modulator IM1, V _π represents the half-wave voltage of the intensity modulator IM1, cos() represents the cosine function, φ ₃ represents the DC bias voltage on the upper arm of the intensity modulator IM1. Phase difference, φ ₃ = πV _DC3 /2V _π , V _DC3 represents the DC bias voltage loaded on the intensity modulator IM1, ω ₃ represents the central angular frequency of the radio frequency signal RF3 loaded on the intensity modulator IM1, t represents the intensity modulator IM1 The serial number of the moment when the output terminal outputs the sideband signal, J ₂ (·) represents the second-order Bessel function of the first kind.

Assuming the power of the 0th order, ±1st order, and ±2nd order sideband signals output by the intensity modulator IM1 is equal, we can get m ₃ =1.84, φ ₃ ≈0.5. From the formula m ₃ =πV _RF3 /V _π , we can get the amplitude V _RF3 of the RF signal RF3, and from the formula φ ₃ =πV _DC3 /2V _π , we can get the DC bias voltage V _DC3 of the intensity modulator IM1. Therefore, by setting the amplitude V _RF3 of the RF signal RF3 and the DC bias voltage V _DC3 of the intensity modulator IM1, the intensity modulator IM1 modulates 1 optical comb into 5 optical combs. When the signal output by the bandpass optical filter Filter is input to the intensity modulator IM1, it will be modulated as a light source respectively, and finally the intensity modulator IM1 will generate a 10-line broadband optical frequency comb, the spectral line interval of which is the frequency f ₃ of the RF signal RF3.

Step 8: Generate a 50-line broadband optical frequency comb.

The intensity modulator IM1 that generates a 10-line optical frequency comb is cascaded with the intensity modulator IM2, and the optical signal generated by the intensity modulator IM1 is input to the intensity modulator IM2. The DC bias voltage V _DC4 of the intensity modulator IM2 is equal to the DC bias voltage V _DC3 of the intensity modulator IM1. Since the radio frequency port of the intensity modulator IM2 is connected to the fourth radio frequency signal source, the radio frequency signal RF4 is input to the intensity modulator IM2 for optical signal modulation. The amplitude V _RF4 of the radio frequency signal RF4 is equal to the amplitude V _RF3 of the radio frequency signal RF4. The frequency f ₄ of the radio frequency signal RF4 and the frequency f ₃ of the radio frequency signal RF3 need to satisfy f ₃ =5f ₄ . Therefore, the working state of the intensity modulator IM2 is the same as that of the intensity modulator IM1. When the 10-line optical frequency comb generated by the intensity modulator IM1 is input to the intensity modulator IM2, it will be separately treated as a light source for modulation. Finally, the intensity modulator IM2 A 50-line broadband optical frequency comb will be generated, and the spectral line spacing of the optical frequency comb is the frequency f ₄ of the radio frequency signal RF4.

The effect of the present invention can be further proved through the following simulation experiments:

1. Simulation experimental conditions.

The hardware platform of the simulation experiment of the present invention is: a HP notebook computer with an Intel i5-1035G1 CPU as the processor and an 8GB memory.

The software platform of the simulation experiment of the present invention is: Windows 10 operating system and OptiSystem 15.

2. Analysis of simulation content and results.

There are two simulation experiments of the present invention.

Simulation experiment 1 uses the method of the present invention, uses 20 groups of different modulation indexes m ₁ of RF1 and m ₂ of RF2, and simulates the output of DPMZM 20 times, and the results are shown in Figure 3. From the simulation results, find out m 1 and m ₂ that make the ±3rd order sideband signal power and out-of-band suppression ratio of DPMZM output larger, and provide parameter settings of _{m 1 and} m ₂ for simulation experiment 2.

Simulation experiment 2 uses the method of the present invention and prior art 1 to simulate the microwave photonic link applicable to the present invention as shown in Figure 1. The obtained output spectrum diagram is shown in Figure 4. The resulting 50 lines are analyzed. Spectrum bandwidth, flatness, out-of-band suppression ratio and other properties of broadband optical frequency comb.

Existing technology 1 refers to the cascading intensity modulation-based waveform generation by optical frequency comb based on cascading intensity modulation proposed by Xin Zhou et al. in "Optics Communications, 284(15):3706-3710, 2011" Optical frequency comb generation method.

The parameters in the microwave photonic link are set as follows: the frequency of the optical carrier signal generated by the single-frequency laser is 281.95THz, the line width is 10MHz, the power is 20dBm, the half-wave voltage of the DPMZM is 3.5V, the extinction ratio is 35dB, and the insertion loss is 5dB, the DC bias voltage V _DC1 of the sub-modulator MZ-a is 3.5V, the DC bias voltage V _DC2 of the sub-modulator MZ-b is 3.5V, and the DC bias voltage V _DC of the main modulator is 3.5V, The frequencies of the radio frequency signals RF1 and RF2 are both 25GHz, the center frequency of the bandpass optical filter is 281.95THz, the 3dB bandwidth is 150GHz, the insertion loss is 1dB, the half-wave voltages of the intensity modulators IM1 and IM2 are both 3.5V, and the extinction ratio is 35dB, the insertion loss is 5dB, the DC bias voltage V _DC3 of IM1 is 3.5V, the DC bias voltage V _DC4 of IM2 is 3.5V, the frequency of the radio frequency signal RF3 is 30GHz, the modulation index _m3 of RF3 is 1.84, the radio frequency The frequency of signal RF4 is 6GHz, and the modulation index m ₄ of RF4 is 1.84.

The effects of the present invention will be further described below with reference to Figures 3 and 4.

Figure 3(a) is a graph of the first-order and third-order Bessel functions of the first kind. The abscissa represents the size of the modulation index m, the ordinate represents the value of the first-order Bessel function, and the solid line curve is the first-order Bessel function. Bessel function curve of the first kind, the dotted curve is a third-order Bessel function curve of the first kind. MZ-a mainly generates ±1-order and ±3-order sideband signals, MZ-b mainly generates ±1-order sideband signals. In order to offset the ±1-order sideband signals generated by the two sub-modulators of DPMZM, the two radio frequency signals RF1 and The modulation indices m ₁ and m ₂ of RF2 need to satisfy J ₁ (m ₁ )=J ₁ (m ₂ ) and m ₁ >m ₂ . As can be seen from Figure 3(a), the first maximum value point of J ₁ (m) is (1.85, 0.582), and the second zero point is 3.83, so the value range of m ₁ is set to (1.85, 3.83), the value range of m ₂ is (0,1.85), there are countless groups of m ₁ and m ₂ that meet the conditions, and they correspond one to one. In simulation experiment 1, 20 m ₂ with an interval of 0.05 are selected in the range [0.05,1], and the corresponding value of m ₁ is calculated through the formula J ₁ (m ₁ )=J ₁ (m ₂ ). From these 20 sets of m ₁ and m ₂ values, through the formulas m ₁ =πV _RF1 /V _π and m ₁ =πV _RF1 /V _π , the amplitudes V _RF1 and V RF2 of the radio frequency signals RF1 and _RF2 are obtained, respectively. In 20 simulation experiments, the output signal of DPMZM was obtained.

Figure 3(b) is a diagram showing the relationship between the ±3rd-order sideband signal power output by DPMZM and the out-of-band suppression ratio and its changes with m _1. The abscissa represents the size of the modulation index m ₁ , and the ordinate on the left represents the out-of-band suppression ratio. The size, the unit is dB, the ordinate on the right represents the size of the ±3rd order sideband signal power, the unit is dBm. In Figure 3(b), the curve marked with "○" represents the change relationship curve of the out-of-band suppression ratio with m ₁ , and the curve marked with "*" represents the change relationship curve of the ±3rd order sideband signal power with m ₁ . It can be seen from Figure 3(b) that as the modulation index m ₁ increases, the ±3rd-order sideband signal power increases, but the out-of-band suppression ratio decreases. Connecting a bandpass optical filter in series after the DPMZM can improve the out-of-band suppression ratio of the optical frequency comb, so the present invention selects a larger m ₁ to make the ±3rd order sideband signal power larger. According to the change diagram in Figure 3(b), the out-of-band suppression ratio basically decreases in proportion to m ₁ , but the ±3-order sideband signal power does not. When m ₁ >3.53, the ±3-order sideband The signal power value basically does not increase, so m ₁ =3.53 and m ₂ =0.25 are selected. In simulation experiment 2, set m ₁ =3.53 and m ₂ =0.25.

Figure 4 is the output spectrum diagram of simulation experiment 2, in which the abscissa represents the frequency of the output signal in THz, and the ordinate represents the power of the output signal in dBm.

Figure 4(a) and (b) are the output spectrum diagrams of DPMZM sub-modulators MZ-a and MZ-b respectively. As can be seen from Figure 4(a), in the output of MZ-a, the ±3rd order sideband The power of the signal is the largest, followed by the ±1-order sideband signal. As can be seen from Figure 4(b), in the output of MZ-b, the power of the ±1-order sideband signal is the largest, and it is the same as the ±1-order sideband signal output by MZ-a. The sideband signal powers are equal.

Figure 4(c) is the output spectrum diagram of DPMZM. It can be seen from Figure 4(c) that DPMZM uses the main modulator to offset the ±1-order sideband signals generated by the two sub-modulators, and the final output ±3-order sideband The signal bandwidth is 150GHz, and the out-of-band suppression ratio of ±3rd-order sideband signals, that is, the suppression ratio of ±3rd-order sideband signals and ±5th-order sideband signals is 13.5dB.

Figure 4(d) is the output spectrum diagram of the bandpass optical filter. It can be seen from Figure 4(d) that the bandpass optical filter effectively suppresses high-order sidebands other than ±3rd-order sidebands. The out-of-band suppression ratio of sideband signals, that is, the suppression ratio of ±3rd-order sideband signals to 0th-order sideband signals, is 24.2dB.

Figures 4(e) and (f) are the output spectrum diagrams of IM1. As can be seen from Figure 4(e), IM1 modulates the ±3rd-order sideband signals into a 10-line optical frequency comb with a spectral line spacing of 30 GHz and an out-of-band suppression ratio of 14.8 dB. Figure 4(f) is an enlarged view of the 10-line optical frequency comb in Figure 4(e). It can be seen that the flatness of the 10-line optical frequency comb is 0.21 dB.

Figure 4(g) and (h) are the output spectrum diagrams of IM2. It can be seen from Figure 4(g) that IM2 modulates the 10-line optical frequency comb into a 50-line optical frequency comb. The spectral lines of the 50-line optical frequency comb The spacing is 6GHz, the out-of-band suppression ratio is 14.6dB, and the spectrum bandwidth is 294GHz. Figure 4(h) is an enlarged view of the 50-line optical frequency comb in Figure 4(g). It can be seen that the 50-line optical frequency comb is flat The degree is 0.37dB.

Claims (3)

1. A broadband optical frequency comb generation method based on DPMZM and two IM cascades, characterized by canceling the ±1-order sideband signals generated by the two sub-modulators and suppressing high-order sideband signals; the broadband optical frequency comb The specific steps of the production method include the following: Step 1, a radio frequency signal source generates a radio frequency signal; Step 2, set the DC bias voltage of the modulator: Set the upper arm DC bias voltage of each sub-modulator in the dual parallel Mach-Zehnder modulator DPMZM to V _π /2, and set the lower arm DC bias voltage to -V _π /2; set the dual parallel Mach-Zehnder modulator The DC bias voltage of the main modulator of DPMZM is set to V _π , where V _π represents the half-wave voltage of the modulator; Step 3, input optical carrier signal: The optical carrier signal generated by the single-frequency laser is input into the dual parallel Mach-Zehnder modulator DPMZM after setting the DC bias voltage; Step 4, generate modulated light signal: The radio frequency signal RF1 and the radio frequency signal RF2 are respectively input into the sub-modulators of the respectively connected dual parallel Mach-Zehnder modulator DPMZM for optical carrier modulation, and two ±1-order sideband signals generated by the sub-modulators are output, one ± Third-order sideband signals, six sideband signals in total; Step 5, cancel the ±1-order sideband signals generated by the two sub-modulators: After making the power of the ±1-order sideband signals generated by the two sub-modulators equal, then input the ±1-order sideband signals generated by the two sub-modulators into the main modulator and cancel each other to obtain the ±3-order sideband signal; Step 6, suppress high-order sideband signals: The ±3rd order sideband signals output by the dual parallel Mach-Zehnder modulator DPMZM are input into a bandpass optical filter connected in series after the dual parallel Mach-Zehnder modulator DPMZM, and the ±3rd order sideband signals after suppressing the high order sidebands are output; Step 7, generate a broadband optical frequency comb: The radio frequency signal and the ±3rd order sideband signals after suppressing the high-order sidebands are respectively input into n intensity modulators with the same working state connected in series after the bandpass optical filter for optical signal modulation; the power of the 0th order, ±1st order, and ±2nd order sideband signals output by each intensity modulator is equal, and each intensity modulator modulates one optical comb into five optical combs; when the frequency of the radio frequency signal loaded by the latter intensity modulator is 1/5 of the frequency of the radio frequency signal loaded by the previous intensity modulator, the nth intensity modulator will generate a 2×5 ^n- line broadband optical frequency comb. 2. The broadband optical frequency comb generation method based on DPMZM and two IM cascades according to claim 1, characterized in that the six sideband signals in step 4 are as follows: Among them, E _a,±1 (t) represents the ±1-order sideband signal output from the output end of the sub-modulator MZ-a at the t time, and E _a,±3 (t) represents the sub-modulator MZ-a at the t time. The ±3-order sideband signal output by the output terminal a, E _b,±1 (t) represents the ±1-order sideband signal output by the output terminal MZ-b of the sub-modulator at the t time, E ₀ represents the output of the single-frequency laser Laser The electric field amplitude of the optical carrier signal, exp(·) represents the exponential operation with the natural constant e as the base, j represents the imaginary unit symbol, ω _c represents the central angular frequency of the optical carrier signal output by the single-frequency laser Laser, and ω ₁ represents The central angular frequency of the radio frequency signal RF1 loaded on the sub-modulator MZ-a, t represents the sequence number of the moment when the sub-modulator output terminal outputs the sideband signal, J ₁ (·) represents the first-order Bessel function of the first kind, m ₁ represents the modulation index of the radio frequency signal RF1 loaded on the sub-modulator MZ-a, m ₁ =πV _RF1 /V _π , π represents the pi, V _RF1 represents the amplitude of the radio frequency signal RF1 loaded on the sub-modulator MZ-a, V _π represents the half-wave voltage of the sub-modulator, sin(·) represents the sine function, φ ₁ represents the phase difference caused by the DC bias voltage of the upper arm of the sub-modulator MZ-a, φ ₁ =π/2, J ₃ (·) represents Third-order Bessel function of the first kind, ω ₂ represents the central angular frequency of the radio frequency signal RF2 loaded on the sub-modulator MZ-b, m ₂ represents the modulation index of the radio frequency signal RF2 loaded on the sub-modulator MZ-b, m ₂ =πV _RF2 /V _π , V _RF2 represents the amplitude of the radio frequency signal RF2 loaded on the sub-modulator MZ-b, φ ₂ represents the phase difference caused by the DC bias voltage of the upper arm of the sub-modulator MZ-b, φ ₂ =π /2. 3. The broadband optical frequency comb generation method based on DPMZM and two IM cascades according to claim 1, characterized in that each intensity modulator described in step 7 outputs 0th order, ±1th order, ±2 The order sideband signals are as follows: E _IM1,0 (t)＝E _D (t)J ₀ (m ₃ )cosφ ₃ E _IM1,±1 (t)＝±j·E _D (t)exp(±jω ₃ t)J ₁ (m ₃ )sinφ ₃ E _IM1,±2 (t)＝E _D (t)exp(±j2ω ₃ t)J ₂ (m ₃ )cosφ ₃ Among them, E _IM1,0 (t) represents the 0th-order sideband signal output by the output terminal of the intensity modulator IM1 at the t time, and E _IM1,±1 (t) represents the ± output terminal of the intensity modulator IM1 at the t time. The 1st-order sideband signal, E _IM1,±2 (t) represents the ±2nd-order sideband signal output from the output terminal of the intensity modulator IM1 at the tth moment, E _D (t) represents the dual parallel Mach-Zehnder modulation at the tth moment The signal output from the output terminal of the DPMZM device, J ₀ (·) represents the 0th order Bessel function of the first kind, m ₃ represents the modulation index of the radio frequency signal RF3 loaded on the intensity modulator IM1, m ₃ =πV _RF3 /V _π , V _RF3 represents the amplitude of the radio frequency signal RF3 loaded on the intensity modulator IM1, V _π represents the half-wave voltage of the intensity modulator IM1, cos(·) represents the cosine function, φ ₃ represents the DC bias voltage on the upper arm of the intensity modulator IM1. _{The phase difference of ​} The serial number of the moment when the IM1 output terminal outputs the sideband signal, J ₂ (·) represents the second-order Bessel function of the first kind.