CN115396036B - Broadband optical frequency comb generation method based on DPMZM and two IM cascading - Google Patents
Broadband optical frequency comb generation method based on DPMZM and two IM cascading Download PDFInfo
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Abstract
The invention discloses a method for generating a broadband optical frequency comb based on DPMZM and two IM cascading, which solves the problems of small spectrum bandwidth and low out-of-band rejection ratio of an optical frequency comb generated by the prior art. The method comprises the following steps of: generating a radio frequency signal; setting a DC bias voltage of the modulator; inputting an optical carrier signal; generating a modulated optical signal; canceling + -1-order sideband signals generated by the two sub-modulators; suppressing the high-order sideband signal; generating a 10 linewidth broadband optical frequency comb; a 50 linewidth broadband optical frequency comb is produced. The invention counteracts the + -1-order sideband signals generated by the two sub-modulators through the main modulator of the DPMZM, the DPMZM outputs + -3-order sidebands, the spectrum bandwidth of the optical frequency comb is increased, and a band-pass optical filter is added after the DPMZM, so that the out-of-band rejection ratio of the optical frequency comb is improved.
Description
Technical Field
The invention belongs to the technical field of communication, and further relates to a broadband optical frequency comb generating method based on cascade connection of a double parallel Mach-Zehnder modulator DPMZM (Dual Parallel Mach-Zehnder Modulator) and two intensity modulators IM (Intensity Modulator) in the technical field of microwave photons. The optical frequency comb generated by the invention can be used as a multi-carrier light source and applied to an optical communication system.
Background
Microwave photon technology is a technology related to the interaction and influence of microwave signals and optical signals, and comprises the steps of processing, controlling and transmitting microwaves by optical methods. In communication, microwave wireless communication has flexible characteristics, optical fiber communication has bandwidth advantages, and microwave photon technology combines the two, so that the method has great application potential for high-speed broadband information transmission. An optical frequency comb refers to a series of evenly spaced and phase coherent frequency components across the spectrum. With the rapid development of optical communication technology, the optical frequency comb is widely applied to the fields of dense wavelength division multiplexing, optical arbitrary waveform generation, multi-wavelength ultrashort pulse generation and the like. In order to meet the application requirements in these optical communication fields, it is needed to generate an optical frequency comb having a large frequency bandwidth, a large number of optical comb lines, a variable line spacing, good flatness and a high out-of-band rejection ratio. The current methods for generating optical frequency combs include a mode-locked laser method, a nonlinear optical fiber method, an electro-optic modulator method and the like. Among them, the mode locking laser method requires a severe mode locking condition, and the spectral line interval of the generated optical frequency comb is hardly changed. The nonlinear effect of the optical fiber is adopted to generate larger fluctuation of optical frequency comb power and poorer flatness. The method for generating the optical frequency comb based on the electro-optic modulator has the advantages of simple system structure, variable spectral line interval and better flatness, so the electro-optic modulator method is widely adopted. However, due to bandwidth limitations of the electro-optic modulator, the frequency bands of the optical frequency combs generated by the current electro-optic modulator method are often smaller, and the number of optical comb lines is limited.
Shibao Wu et al disclose in its published paper "Highly flexible optical Nyquist pulses generation based on dual-parallel Mach-Zehnder modulator and intensity modulator" (Photonic Network Communications,2018, 36 (3): 361-368) a method of optical frequency comb generation based on dual parallel Mach-Zehnder modulators and intensity modulators. The method comprises the specific steps that a laser generates continuous optical signals, and the continuous optical signals pass through a dual parallel Mach-Zehnder modulator DPMZM and intensity modulator IM cascading structure. The frequency of the radio frequency signal loaded on the DPMZM is set to be 18.75GHz, and the DPMZM generates a 5-line optical comb by adjusting the power of the radio frequency signal and the direct current bias voltage of a modulator, wherein the line interval is 37.5GHz. An intensity modulator IM is then cascaded, and the method finally generates a 15-wire optical frequency comb with a spectral bandwidth of 187.5GHz by setting the power of the radio frequency signal and the dc bias of the modulator such that the IM modulates the frequency component generated by each DPMZM into a 3-wire optical comb. The method has the defects that the spectral line interval of the 5-wire optical comb generated by the DPMZM is 2 times of the loaded radio frequency, the spectrum bandwidth is smaller, the number of wires of the finally generated optical comb is smaller, the out-of-band rejection ratio is not high, and the communication capacity is insufficient when the generated optical comb is applied to an optical communication system.
The western electronic technology university discloses a method for generating a broadband optical frequency comb in the patent literature of the western electronic technology university (application number CN201310750920.0, application date 2013.12.30, application publication number CN 103744249A) applied thereto. The method comprises the following steps: a laser, a double parallel Mach-Zehnder modulator DPMZM, two intensity modulators IM, a radio frequency signal source and a spectrum analyzer. The frequency of the radio frequency signal loaded on the DPMZM is set to be 25GHz, and the power of the radio frequency signal and the direct current bias voltage of the modulator are adjusted to enable the output power of the DPMZM to be equal to + -2-order sidebands. Then cascading two IM with the same working state, setting the power of the radio frequency signal and the DC bias voltage loaded on the IM, so that each frequency component can be modulated to generate a 5-line optical comb when passing through the IM, and the method finally generates a 50-line optical frequency comb with 4GHz spectral line interval and 196GHz spectral bandwidth. The method has the defects that the bandwidth dependence on a modulator is large, the frequency interval of + -2-order sidebands generated by the DPMZM is 4 times of the loaded radio frequency, the frequency spectrum bandwidth is small, and the communication capacity is insufficient when the generated optical frequency comb is applied to an optical communication system.
Disclosure of Invention
The invention aims to solve the technical problems of smaller spectrum bandwidth, fewer optical comb lines and low out-of-band rejection ratio in the existing optical frequency comb generation method based on the electro-optic modulator by providing a broadband optical frequency comb generation method based on the cascade connection of a DPMZM (dual parallel Mach-Zehnder modulator) and two IM (intensity modulators) aiming at the defects of the prior art.
In order to achieve the above purpose, the technical idea of the invention is as follows: the invention is implemented in a link consisting of a single frequency laser, four sources of radio frequency signals, a dual parallel mach-zehnder modulator DPMZM comprising two sub-modulators, a bandpass optical filter, two intensity modulators IM and a spectrum analyzer. The two sub-modulators on the DPMZM are loaded with a radio frequency signal having a frequency of 25GHz. The sub-modulators are enabled to work in a push-pull mode and biased at a minimum point, so that the two sub-modulators output odd-order optical sidebands, then the main modulator is utilized to offset +/-1-order sidebands generated by the two sub-modulators, and finally the DPMZM outputs +/-3-order sidebands, wherein the frequency interval is 6 times of the frequency of the loaded radio frequency signal. A band-pass optical filter is added after the DPMZM, so that high-order sidebands can be effectively restrained, and the out-of-band rejection ratio of an optical frequency comb to be generated is improved. And two IM with the same working state are cascaded, and the power of a radio frequency signal and a direct current bias voltage loaded on the IM are regulated, so that the IM can generate a 5-line optical frequency comb, namely, the power of 0-order, +/-1-order sideband and +/-2-order sideband signals is equal. In the scheme, the light edge bands generated by the previous-stage modulator are respectively used as light sources for modulation by the next-stage modulator, and finally, 50-linewidth band optical frequency combs are generated.
The specific steps of the invention are as follows:
step 1, a radio frequency signal source generates a radio frequency signal;
step 2, setting the DC bias voltage of the modulator:
setting the upper arm DC bias voltage of each sub-modulator in the DPMZM of the double parallel Mach-Zehnder modulator to V π 2, the lower arm DC bias voltage is set to be-V π 2; setting the DC bias voltage of the main modulator of the DPMZM of the double parallel Mach-Zehnder modulator to V π Wherein V is π Representing the half-wave voltage of the modulator;
step 3, inputting an optical carrier signal:
inputting an optical carrier signal generated by a single-frequency laser into a double-parallel Mach-Zehnder modulator (DPMZM) with a set direct-current bias voltage;
step 4, generating a modulated optical signal:
the method comprises the steps of respectively inputting a radio frequency signal RF1 and a radio frequency signal RF2 into each connected sub-modulator of a double parallel Mach-Zehnder modulator DPMZM for optical carrier modulation, and outputting two + -1-order sideband signals, one + -3-order sideband signal and six sideband signals;
step 5, canceling the + -1-order sideband signals generated by the two sub-modulators:
after the power of the + -1-order sideband signals generated by the two sub-modulators is equal, the + -1-order sideband signals generated by the two sub-modulators are input into the main modulator and then mutually offset to obtain + -3-order sideband signals;
step 6, suppressing the high-order sideband signal:
inputting the + -3-order sideband signal output by the DPMZM of the double-parallel Mach-Zehnder modulator into a band-pass optical filter connected in series after the DPMZM of the double-parallel Mach-Zehnder modulator, and outputting the + -3-order sideband signal after restraining the high-order sideband;
step 7, generating a broadband optical frequency comb:
the radio frequency signal and the + -3-order sideband signal after the suppression of the high-order sidebands are respectively input into n modulators with the same working state in series after the band-pass optical filter to modulate the optical signalThe method comprises the steps of carrying out a first treatment on the surface of the The power of the 0-order, the +/-1-order and the +/-2-order sideband signals are equal to each other, and each intensity modulator modulates 1 optical comb into 5 optical combs; when the frequency of the RF signal loaded by the next intensity modulator is 1/5 of the frequency of the RF signal loaded by the previous intensity modulator, the nth intensity modulator generates 2×5 n Broadband optical frequency combs of the wire.
Compared with the prior art, the invention has the following advantages:
firstly, the invention loads radio frequency signals on two sub-modulators of the DPMZM, and the power of the radio frequency signals and the direct current bias voltage of the modulators are set to ensure that + -1-order sideband signals generated by the two sub-modulators are mutually counteracted after being input into a main modulator of the DPMZM, the DPMZM outputs + -3-order sidebands, the frequency interval is 6 times of the frequency of the loaded radio frequency signals, the defect of smaller frequency bandwidth of an optical frequency comb in the prior art is overcome, the bandwidth of the optical frequency comb spectrum generated by the invention is increased, and the communication capacity is increased when the invention is applied to an optical communication system.
Second, because the invention adds a band-pass optical filter after DPMZM, can inhibit the higher order sideband signal except + -3 order sideband signal effectively, make the optical frequency comb that the invention produces apply to the optical communication system, have improved the out-of-band rejection ratio of the optical frequency comb, the signal to noise ratio of the signal source increases.
Drawings
FIG. 1 is a link diagram of microwave photons in an embodiment of the invention;
FIG. 2 is a flow chart of the present invention;
FIG. 3 is a simulation diagram of simulation experiment 1 of the present invention;
fig. 4 is an output spectrum of the simulation experiment 2 of the present invention.
Detailed Description
The present invention will be described in further detail with reference to the accompanying drawings and examples.
The invention is implemented in a microwave photonic link.
A microwave photon link used in an embodiment of the invention is described in further detail with reference to fig. 1.
The microwave photon link of the embodiment of the invention comprises: a single frequency Laser, four sources of radio frequency signals, a dual parallel mach-zehnder modulator DPMZM comprising two sub-modulators MZ-a and MZ-b, a bandpass optical Filter, two intensity modulators IM1 and IM2 and a spectrum analyzer OSA. The optical input port of the DPMZM is connected with a single-frequency Laser, the radio frequency ports of two sub-modulators MZ-a and MZ-b of the DPMZM are respectively connected with a first radio frequency signal source and a second radio frequency signal source, and the radio frequency ports of two intensity modulators IM1 and IM2 are respectively connected with a third radio frequency signal source and a fourth radio frequency signal source. The dc bias voltage applied to the main modulator of the DPMZM is on the lower arm of the DPMZM.
The implementation steps of the present invention will be described in further detail with reference to fig. 2 and the embodiment.
Step 1, generating a radio frequency signal.
Two sub-modulators of the dual parallel mach-zehnder modulator DPMZM and a radio frequency signal source connected to radio frequency ports of the two intensity modulators respectively generate four radio frequency signals: RF1, RF2, RF3 and RF4, the frequencies of these four radio frequency signals correspond respectively: f (f) 1 、f 2 、f 3 And f 4 Wherein f 1 ＝f 2 。
And 2, setting the direct-current bias voltage of the modulator.
Setting the upper arm DC bias voltage of each sub-modulator in the DPMZM of the double parallel Mach-Zehnder modulator to V π 2, the lower arm DC bias voltage is set to be-V π And/2, thereby ensuring that both sub-modulators operate in push-pull mode and in minimum transmission point state. Wherein V is π Representing the half-wave voltages of the sub-modulators, the half-wave voltages of the two sub-modulators being equal.
Since the DC bias voltage of the main modulator in the dual parallel Mach-Zehnder modulator DPMZM is applied to the lower arm of the dual parallel Mach-Zehnder modulator DPMZM, the DC bias voltage of the main modulator of the dual parallel Mach-Zehnder modulator DPMZM is set to V π Thereby ensuring that the phase difference of the output signals of the two sub-modulators is pi so as to ensure the two sub-modulatorsThe phase of the output signals of the controllers are opposite.
And step 3, inputting an optical carrier signal.
And an optical carrier signal generated by a single-frequency Laser is input into a double-parallel Mach-Zehnder modulator DPMZM with a set direct-current bias voltage.
And 4, generating a modulated optical signal.
The radio frequency ports of the two sub-modulators of the dual parallel mach-zehnder modulator DPMZM are connected to the first radio frequency signal source and the second radio frequency signal source, respectively. The two radio frequency signals RF1 and RF2 are respectively input into the sub-modulators of the respectively connected dual parallel mach-zehnder modulators DPMZM for optical carrier modulation, and the six output sideband signals are as follows:
wherein E is a,±1 (t) represents the + -1-order sideband signal, E, output from the output terminal of the t-th time sub-modulator MZ-a a,±3 (t) represents the + -3-order sideband signal, E, output from the output terminal of the t-th time sub-modulator MZ-a b,±1 (t) represents the + -1-order sideband signal, E, output from the output terminal of the t-th time sub-modulator MZ-b 0 Representing the electric field amplitude of the optical carrier signal output by the single-frequency Laser, exp (·) representing an exponential operation based on a natural constant e, j representing an imaginary unit symbol, ω c Represents the central angular frequency omega of the optical carrier signal output by the single-frequency Laser 1 Represents the center angular frequency of the RF signal RF1 loaded on the sub-modulator MZ-a, t represents the sequence number of the moment when the output end of the sub-modulator outputs the sideband signal, J 1 (. Cndot.) represents a first order Bessel classFunction, m 1 Representing the modulation index, m, of the RF signal RF1 loaded on the sub-modulator MZ-a 1 ＝πV RF1 /V π Pi represents the circumference ratio, V RF1 Representing the amplitude, V, of the RF signal RF1 loaded on the sub-modulator MZ-a π Representing the half-wave voltage of the sub-modulator,representing a sine function, phi 1 Indicating the phase difference, phi, caused by the DC bias voltage of the upper arm of the sub-modulator MZ-a 1 ＝π/2，J 3 (. Cndot.) represents a third-order Bessel function, ω 2 Represents the center angular frequency, m, of the radio frequency signal RF2 loaded on the sub-modulator MZ-b 2 Representing the modulation index, m, of the radio frequency signal RF2 loaded on the sub-modulator MZ-b 2 ＝πV RF2 /V π ，V RF2 Representing the amplitude, phi, of the radio frequency signal RF2 loaded on the sub-modulator MZ-b 2 Indicating the phase difference, phi, caused by the DC bias voltage of the upper arm of the sub-modulator MZ-b 2 ＝π/2。
And 5, canceling 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, the power of the + -1-order sideband signals output by the two sub-modulators obtained in the step 4 is equalized to obtain J 1 (m 1 )＝J 1 (m 2 ). Thus, the + -1-order sideband signals generated by the two sub-modulators are mutually cancelled after being input to the main modulator, and the double parallel Mach-Zehnder modulator DPMZM outputs + -3-order sideband signals, the frequency interval of which + -3-order sideband signals is the RF1 frequency f of the radio frequency signal 1 Is 6 times as large as that of the above.
And 6, suppressing the high-order sideband signal.
A band-pass optical Filter is connected in series behind the DPMZM of the double parallel Mach-Zehnder modulator, the center frequency of the band-pass optical Filter is equal to the frequency of the optical carrier signal, and the 3dB bandwidth is the frequency interval between + -3-order sidebands. When the output signal of the double parallel Mach-Zehnder modulator DPMZM is input into the band-pass optical Filter, high-order sideband signals except for + -3-order sideband signals can be effectively restrained, and the out-of-band rejection ratio of an optical frequency comb to be generated is improved.
And 7, generating the 10-linewidth broadband optical frequency comb.
The band-pass optical Filter is cascaded with the intensity modulator IM1, and the + -3-order sideband signal which is output by the band-pass optical Filter and is used for restraining the high-order sideband is input into the intensity modulator IM 1. Setting the direct-current 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, and the frequency f of the RF signal RF3 3 And the frequency f of the radio frequency signal RF1 1 Needs to satisfy 6f 1 ＝5f 3 . The 0 order, ±1 order, ±2 order sideband signals output by the intensity modulator IM1 are
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
Wherein E is IM1,0 (t) represents the 0-order sideband signal output by the output end of the intensity modulator IM1 at the t-th moment, E IM1,±1 (t) represents the + -1 st order sideband signal outputted from the output terminal of the intensity modulator IM1 at the t-th moment, E IM1,±2 (t) represents the + -2 nd order sideband signal outputted from the output terminal of the intensity modulator IM1 at the t-th moment, E D (t) represents the signal output by the output end of the DPMZM of the double parallel Mach-Zehnder modulator at the t-th moment, J 0 (. Cndot.) represents the 0 th order Bessel function of the first class, m 3 Representing the modulation index, m, of the radio frequency signal RF3 loaded on the intensity modulator IM1 3 ＝πV RF3 /V π ，V RF3 Representing the amplitude, V, of the RF signal RF3 applied to the intensity modulator IM1 π Representing the half-wave voltage of the intensity modulator IM1, cos () representing the cosine function, phi 3 Indicating the phase difference, phi, caused by the DC bias voltage of the upper arm of the intensity modulator IM1 3 ＝πV DC3 /2V π ，V DC3 Representing the DC bias voltage, ω, applied by the intensity modulator IM1 3 Represents the center angular frequency of the radio frequency signal RF3 loaded on the intensity modulator IM1, t represents the sequence number of the moment when the sideband signal is output at the output of the intensity modulator IM1, J 2 (. Cndot.) represents a second order first class Bessel function.
Let the power of the 0 order, ±1 order, ±2 order sideband signal output by the intensity modulator IM1 be equal to obtain m 3 ＝1.84，φ 3 And approximately 0.5. From formula m 3 ＝πV RF3 /V π The amplitude V of the radio frequency signal RF3 can be obtained RF3 From the formula phi 3 ＝πV DC3 /2V π The DC bias voltage V of the intensity modulator IM1 can be obtained DC3 . Thus, by setting the amplitude V of the radio frequency signal RF3 RF3 And the DC bias voltage V of the intensity modulator IM1 DC3 To ensure that the intensity modulator IM1 modulates 1 comb into 5 combs. When the signals output by the band-pass optical Filter are input to the intensity modulator IM1, the signals are respectively used as light sources for modulation, and finally the intensity modulator IM1 generates 10 linewidth optical frequency combs with spectral line intervals of the frequency f of the radio frequency signal RF3 3 。
And 8, generating a 50-linewidth broadband optical frequency comb.
An intensity modulator IM1 generating a 10-line optical frequency comb is cascaded with an intensity modulator IM2, and an optical signal generated by the intensity modulator IM1 is input to the intensity modulator IM 2. DC bias voltage V of intensity modulator IM2 DC4 With the DC bias voltage V of the intensity modulator IM1 DC3 Equal. Since the RF port of the intensity modulator IM2 is connected to the fourth RF signal source, the RF signal RF4 is input to the intensity modulator IM2 for optical signal modulation, and the amplitude V of the RF signal RF4 RF4 Amplitude V of RF4 signal RF3 Equal, frequency f of RF signal RF4 4 And the frequency f of the radio frequency signal RF3 3 Needs to satisfy f 3 ＝5f 4 . Thus, the working state of the intensity modulator IM2 is the same as that of the intensity modulator IM1, and when the 10-line optical frequency comb generated by the intensity modulator IM1 is input into the intensity modulator IM2, the 10-line optical frequency comb will be respectively regarded asThe final intensity modulator IM2 generates a 50 linewidth optical frequency comb whose spectral line spacing is the frequency f of the RF signal RF4 4 。
The effect of the invention can be further demonstrated by the following simulation experiment:
1. and (5) simulating experimental conditions.
The hardware platform of the simulation experiment of the invention is: the processor is an Intel i5-1035G1 CPU and the memory is an 8GB Hewlett-packard notebook computer.
The software platform of the simulation experiment of the invention is: windows 10 operating system and OptiSystem15.
2. Simulation content and analysis of results thereof.
The simulation experiment of the invention has two.
Simulation experiment 1A method of the present invention was used to obtain 20 sets of different modulation indices m of RF1 1 And modulation index m of RF2 2 The output of the DPMZM was simulated 20 times, and the result is shown in fig. 3. Finding m which enables the power and out-of-band rejection ratio of + -3-order sideband signal of DPMZM output to be large from simulation results 1 And m 2 Providing m for simulation experiment 2 1 And m 2 Is set by the parameter of the computer.
The simulation experiment 2 is to simulate the microwave photon link applied by the invention shown in fig. 1 by adopting the method of the invention and the prior art 1, the obtained output spectrum chart is shown in fig. 4, and the performances of spectrum bandwidth, flatness, out-of-band rejection ratio and the like of the finally generated 50-linewidth broadband optical frequency comb are analyzed.
Prior art 1 refers to a cascade intensity modulation based optical frequency comb generation method as proposed by Xin Zhou et al in "All optical arbitrary waveform generation by optical frequency comb based on cascading intensity modulation, optics Communications,284 (15): 3706-3710, 2011".
The parameters in the microwave photon 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, the insertion loss is 5dB, and the direct current bias of the sub-modulator MZ-aVoltage V DC1 DC bias voltage V of 3.5V for sub-modulator MZ-b DC2 At 3.5V, the DC bias voltage V of the main modulator DC The frequencies of the RF signals RF1 and RF2 are 25GHz at 3.5V, the center frequency of the band-pass 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 3.5V, the extinction ratio is 35dB, the insertion loss is 5dB, and the direct-current bias voltage V of the IM1 DC3 DC bias voltage V of 3.5V, IM2 DC4 At 3.5V, the frequency of the RF signal RF3 is 30GHz, and the modulation index m of the RF3 3 1.84, the frequency of the RF signal RF4 is 6GHz, and the modulation index m of the RF4 4 1.84.
The effects of the present invention are further described below with reference to fig. 3 and 4.
Fig. 3 (a) is a graph of first and third order first-class bezier functions, in which the abscissa represents the magnitude of the modulation index m, the ordinate represents the first-class bezier function value, the solid line curve is the first-order first-class bezier function curve, and the broken line curve is the third-order first-class bezier function curve. MZ-a generates mainly + -1-order and + -3-order sideband signals, MZ-b generates mainly + -1-order sideband signals, modulation indexes m of two radio frequency signals RF1 and RF2 are used for counteracting + -1-order sideband signals generated by two sub-modulators of DPMZM 1 And m 2 Needs to meet J 1 (m 1 )＝J 1 (m 2 ) And m is 1 ＞m 2 . As can be seen from FIG. 3 (a), J 1 (m) the first maximum point of (1.85,0.582) and the second zero point of 3.83, so m is set 1 The value range of (5) is 1.85,3.83), m 2 The value range of (5) is (0,1.85), and m satisfies the condition 1 And m 2 There are numerous groups and one-to-one correspondence. In simulation experiment 1, in the range [0.05,1 ]]20 m with internal selection interval of 0.05 2 By the formula J 1 (m 1 )＝J 1 (m 2 ) Calculate the corresponding m 1 Is a value of (2). From these 20 groups m 1 And m 2 Through the formula m 1 ＝πV RF1 /V π And m 1 ＝πV RF1 /V π Obtaining the amplitude V of the radio frequency signals RF1 and RF2 RF1 And V RF2 And respectively loading the signals into 20 simulation experiments to obtain the output signals of the DPMZM.
FIG. 3 (b) is a sum of the power and out-of-band rejection ratio of the + -3-order sideband signal of the DPMZM output and the dependence on m 1 Wherein the abscissa represents the modulation index m 1 The left ordinate indicates the magnitude of the out-of-band rejection ratio in dB, and the right ordinate indicates the magnitude of the ± 3-order sideband signal power in dBm. FIG. 3 (b) is a graph marked with ". Smallcircle" 1 Is represented by a curve marked with "#" representing the power of a 3 rd order sideband signal as a function of m 1 Is a function of the change in the relationship of (a). As can be seen from fig. 3 (b), with the modulation index m 1 Is increasing in the 3 rd order sideband signal power, but is decreasing in the out-of-band rejection ratio. The band-pass optical filter is connected in series behind the DPMZM to improve the out-of-band rejection ratio of the optical frequency comb, so the invention selects larger m 1 So that the + -3-order sideband signal power is large. According to the variation graph in FIG. 3 (b), the out-of-band rejection ratio is substantially m-dependent 1 Proportionally reduced, while the + -3-order sideband signal power is not, when m 1 At > 3.53, ±3-order sideband signal power values do not substantially increase, so m is selected 1 ＝3.53，m 2 =0.25. In simulation experiment 2, set m 1 ＝3.53，m 2 ＝0.25。
Fig. 4 is an 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.
Fig. 4 (a) and (b) are output spectra of sub-modulators MZ-a and MZ-b of the DPMZM, respectively, and it can be seen from fig. 4 (a) that the power of the + -3-order sideband signal is the largest among the outputs of MZ-a, and then the + -1-order sideband signal is the largest among the outputs of MZ-b, and is equal to the + -1-order sideband signal power outputted from MZ-a.
Fig. 4 (c) is an output spectrum diagram of the DPMZM, and as can be seen from fig. 4 (c), the DPMZM cancels the ±1-order sideband signals generated by the two sub-modulators by using the main modulator, the bandwidth of the finally output ±3-order sideband signal is 150GHz, and the out-of-band rejection ratio of the ±3-order sideband signal, that is, the rejection ratio of the ±3-order sideband signal to the ±5-order sideband signal is 13.5dB.
Fig. 4 (d) is an output spectrum diagram of the band-pass optical filter, and as can be seen from fig. 4 (d), the band-pass optical filter effectively suppresses the high-order sidebands except for the ±3-order sidebands, and the out-of-band suppression ratio of the ±3-order sideband signal, that is, the suppression ratio of the ±3-order sideband signal to the 0-order sideband signal is 24.2dB.
Fig. 4 (e) and (f) are output frequency charts of IM1, and as can be seen from fig. 4 (e), IM1 modulates the ±3-order sideband signal into a 10-line optical frequency comb, the line spacing of the 10-line optical frequency comb is 30GHz, the out-of-band rejection ratio is 14.8dB, and fig. 4 (f) is an enlarged view of the 10-line optical frequency comb in fig. 4 (e), and as can be seen, the flatness of the 10-line optical frequency comb is 0.21dB.
Fig. 4 (g) and (h) are output frequency charts of IM2, and as can be seen from fig. 4 (g), IM2 modulates a 10-line optical frequency comb into a 50-line optical frequency comb, the line spacing of the 50-line optical frequency comb is 6GHz, the out-of-band rejection ratio is 14.6dB, the spectral bandwidth is 294GHz, and fig. 4 (h) is an enlarged view of the 50-line optical frequency comb in fig. 4 (g), and as can be seen, the flatness of the 50-line optical frequency comb is 0.37dB.
Claims (3)
1. A broadband optical frequency comb generating method based on DPMZM and two IM cascading is characterized in that + -1-order sideband signals generated by two sub-modulators are counteracted, and high-order sideband signals are restrained; the method for generating the broadband optical frequency comb comprises the following specific steps:
step 1, a radio frequency signal source generates a radio frequency signal;
step 2, setting the DC bias voltage of the modulator:
setting the upper arm DC bias voltage of each sub-modulator in the DPMZM of the double parallel Mach-Zehnder modulator to V π 2, the lower arm DC bias voltage is set to be-V π 2; setting the DC bias voltage of the main modulator of the DPMZM of the double parallel Mach-Zehnder modulator to V π Wherein V is π Representing the half-wave voltage of the modulator;
step 3, inputting an optical carrier signal:
inputting an optical carrier signal generated by a single-frequency laser into a double-parallel Mach-Zehnder modulator (DPMZM) with a set direct-current bias voltage;
step 4, generating a modulated optical signal:
the method comprises the steps of respectively inputting a radio frequency signal RF1 and a radio frequency signal RF2 into each connected sub-modulator of a double parallel Mach-Zehnder modulator DPMZM for optical carrier modulation, and outputting two + -1-order sideband signals, one + -3-order sideband signal and six sideband signals;
step 5, canceling the + -1-order sideband signals generated by the two sub-modulators:
after the power of the + -1-order sideband signals generated by the two sub-modulators is equal, the + -1-order sideband signals generated by the two sub-modulators are input into the main modulator and then mutually offset to obtain + -3-order sideband signals;
step 6, suppressing the high-order sideband signal:
inputting the + -3-order sideband signal output by the DPMZM of the double-parallel Mach-Zehnder modulator into a band-pass optical filter connected in series after the DPMZM of the double-parallel Mach-Zehnder modulator, and outputting the + -3-order sideband signal after restraining the high-order sideband;
step 7, generating a broadband optical frequency comb:
the radio frequency signal and the + -3-order sideband signal after the high-order sideband is restrained are respectively input into n modulators with the same working state in series after the band-pass optical filter to modulate the optical signal; the power of the 0-order, the +/-1-order and the +/-2-order sideband signals are equal to each other, and each intensity modulator modulates 1 optical comb into 5 optical combs; when the frequency of the RF signal loaded by the next intensity modulator is 1/5 of the frequency of the RF signal loaded by the previous intensity modulator, the nth intensity modulator generates 2×5 n Broadband optical frequency combs of the wire.
2. The method for generating a broadband optical frequency comb based on DPMZM and two IM cascades according to claim 1, wherein the six sideband signals in step 4 are as follows:
wherein E is a,±1 (t) represents the + -1-order sideband signal, E, output from the output terminal of the t-th time sub-modulator MZ-a a,±3 (t) represents the + -3-order sideband signal, E, output from the output terminal of the t-th time sub-modulator MZ-a b,±1 (t) represents the + -1-order sideband signal, E, output from the output terminal of the t-th time sub-modulator MZ-b 0 Representing the electric field amplitude of the optical carrier signal output by the single-frequency Laser, exp (·) representing an exponential operation based on a natural constant e, j representing an imaginary unit symbol, ω c Represents the central angular frequency omega of the optical carrier signal output by the single-frequency Laser 1 Represents the center angular frequency of the RF signal RF1 loaded on the sub-modulator MZ-a, t represents the sequence number of the moment when the output end of the sub-modulator outputs the sideband signal, J 1 (. Cndot.) represents a first order Bessel function of the first class, m 1 Representing the modulation index, m, of the RF signal RF1 loaded on the sub-modulator MZ-a 1 ＝πV RF1 /V π Pi represents the circumference ratio, V RF1 Representing the amplitude, V, of the RF signal RF1 loaded on the sub-modulator MZ-a π Representing the half-wave voltage of the sub-modulator, sin (·) representing the sine function, phi 1 Indicating the phase difference, phi, caused by the DC bias voltage of the upper arm of the sub-modulator MZ-a 1 ＝π/2，J 3 (. Cndot.) represents a third-order Bessel function, ω 2 Represents the center angular frequency, m, of the radio frequency signal RF2 loaded on the sub-modulator MZ-b 2 Representing the modulation index, m, of the radio frequency signal RF2 loaded on the sub-modulator MZ-b 2 ＝πV RF2 /V π ，V RF2 Representing the amplitude of the radio frequency signal RF2 loaded on the sub-modulator MZ-b,φ 2 indicating the phase difference, phi, caused by the DC bias voltage of the upper arm of the sub-modulator MZ-b 2 ＝π/2。
3. The method of generating a broadband optical frequency comb based on DPMZM and two IM cascades according to claim 1, wherein each of the intensity modulators described in step 7 outputs 0 th order, ±1 st order, ±2 nd order sideband signals 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
wherein E is IM1,0 (t) represents the 0-order sideband signal output by the output end of the intensity modulator IM1 at the t-th moment, E IM1,±1 (t) represents the + -1 st order sideband signal outputted from the output terminal of the intensity modulator IM1 at the t-th moment, E IM1,±2 (t) represents the + -2 nd order sideband signal outputted from the output terminal of the intensity modulator IM1 at the t-th moment, E D (t) represents the signal output by the output end of the DPMZM of the double parallel Mach-Zehnder modulator at the t-th moment, J 0 (. Cndot.) represents the 0 th order Bessel function of the first class, m 3 Representing the modulation index, m, of the radio frequency signal RF3 loaded on the intensity modulator IM1 3 ＝πV RF3 /V π ，V RF3 Representing the amplitude, V, of the RF signal RF3 applied to the intensity modulator IM1 π Representing the half-wave voltage of the intensity modulator IM1, cos (. Cndot.) representing the cosine function, phi 3 Indicating the phase difference, phi, caused by the DC bias voltage of the upper arm of the intensity modulator IM1 3 ＝πV DC3 /2V π ，V DC3 Representing the DC bias voltage, ω, applied by the intensity modulator IM1 3 Represents the center angular frequency of the radio frequency signal RF3 loaded on the intensity modulator IM1, t represents the sequence number of the moment when the sideband signal is output at the output of the intensity modulator IM1, J 2 (. Cndot.) represents a second order first class Bessel function.
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