阅读说明：本技术 一种基于dpmzm的qpsk转bpsk全光调制格式转换系统 (DPMZM-based QPSK-BPSK all-optical modulation format conversion system ) 是由 张琦涵 巩小雪 郭磊 于 2020-05-26 设计创作，主要内容包括：本发明涉及一种基于DPMZM的QPSK转BPSK全光调制格式转换系统,属于通信技术领域。该系统包括：适用于基于FWM进行QPSK向BPSK格式转换的相位映射方式,适用于基于FWM进行QPSK向BPSK格式转换的DPMZM偏置点设置与电信号输入方式,基于FWM进行QPSK向BPSK格式转换的信号处理方式和基于FWM进行QPSK向BPSK格式转换之后的信号提取方式。所述系统只要所用器件的极限参数允许,不改变系统结构就可以直接提高输入信号的符号率；可以同时转化I路和Q路信号并直接解调,避免使用相干接收机和锁相回路,系统结构简单。(The invention relates to a DPMZM-based QPSK-BPSK all-optical modulation format conversion system, and belongs to the technical field of communication. The system comprises: the phase mapping method is suitable for a phase mapping mode for converting QPSK to BPSK based on FWM, a DPMZM bias point setting and electric signal input mode for converting QPSK to BPSK based on FWM, a signal processing mode for converting QPSK to BPSK based on FWM and a signal extraction mode after converting QPSK to BPSK based on FWM. The system can directly improve the symbol rate of input signals without changing the system structure as long as the limit parameters of the used devices allow; the signal of the I path and the signal of the Q path can be converted and directly demodulated, a coherent receiver and a phase-locked loop are avoided, and the system structure is simple.)

1. A QPSK-to-BPSK all-optical modulation format conversion system based on DPMZM is characterized in that: the system comprises a continuous wave laser source LD, 2 electrical amplifiers EA, a double parallel Mach-Zehnder modulator DPMZM, a polarization controller PC, a polarization beam splitter PBS, an erbium-doped fiber amplifier EDFA, a high non-linear fiber HNLF, 2 optical filters OF and 2 photodetectors PD.

2. The system of claim 1, wherein the system comprises a DPMZM-based QPSK-to-BPSK all-optical modulation format conversion method, which comprises: the method comprises the following steps:

s1: before the QPSK I path electric signal is input into the DPMZM, the QPSK I path electric signal is consistent with the I path data source signal bit;

s2: before the QPSK Q path electric signal is input into the DPMZM, the QPSK Q path electric signal is converted into an XNOR operation result of the I path data source signal bit and the Q path data source signal bit;

s3: IQ signals need to be amplified by the electric amplifier EA and then input into the DPMZM.

3. The method according to claim 2, wherein the DPMZM-based QPSK-to-BPSK all-optical modulation format conversion method comprises: the DPMZM bias point setting comprises:

the two MZMs in the X polarization state are both provided with a minimum bias point, and the phase control voltage is a maximum bias point;

the two MZMs in the Y polarization state are both provided with minimum bias points, and the phase control voltage is an orthogonal bias point;

the DPMZM electrical signal input comprises:

the radio frequency source generates radio frequency electric signals RF with equal power with the IQ two-path signals and simultaneously inputs the radio frequency electric signals RF into two MZMs in the DPMZM X polarization state;

the QPSKI path electric signal is input into an I path MZM of the DPMZMY polarization state;

and the QPSK Q path electric signal is input into a Q path MZM of the DPMZM Y polarization state.

4. The method according to claim 3, wherein the DPMZM-based QPSK-to-BPSK all-optical modulation format conversion method comprises: the signal processing for the QPSK to BPSK format conversion includes:

the output signal of the DPMZM passes through the PC and the PBS, and any output of the PBS is amplified to a power level capable of exciting the FWM by the EDFA;

the amplified signal passes through the HNLF excitation FWM.

5. The method according to claim 4, wherein the DPMZM-based QPSK-to-BPSK all-optical modulation format conversion method comprises: the signal extraction after the QPSK to BPSK format conversion includes:

when the OF central frequency is set as sum or difference OF LD transmitting frequency and 2 times RF frequency, extracting BPSK optical signal bit-modulated by I path data source signal;

setting OF center frequency as difference or sum OF LD transmitting frequency and RF frequency, extracting BPSK optical signal bit-modulated by Q data source signal;

and the two BPSK signals are directly detected by the PD and then are demodulated into IQ two-path data source signal bits respectively.

Technical Field

The invention belongs to the technical field of communication, and relates to a DPMZM-based QPSK-BPSK all-optical modulation format conversion system.

Background

When multi-domain networks are interconnected, optical signals from different domains may have different modulation formats, or different modulation formats are required in different transmission scenarios, so that an all-optical network is inevitably required to have a basic capability of performing all-optical format conversion. For long-range transmission scenarios, Phase Shift Keying (PSK) has gained wide attention due to its higher spectral efficiency and ability to withstand greater dispersion and non-linear effects. Several existing techniques attempt to convert Quadrature Phase Shift Keying (QPSK) format into Binary Phase Shift Keying (BPSK) format more suitable for long-distance communication by using the four-wave mixing (FWM) effect of optical fiber, but these conversion techniques can only convert quadrature (Q) path signals of QPSK; some signals can be converted and demodulated only by using a complex coherent receiver at a receiving end or performing phase locking with a transmitting end by using a feedback circuit, so that the conversion speed is limited; also, to convert the in-phase (I) path, the introduction of gratings and optical circulators further increases the complexity of the system.

Disclosure of Invention

In view of the above, the present invention aims to provide a QPSK to BPSK all-optical modulation format conversion system based on a dual parallel mach-zehnder modulator (DPMZM).

In order to achieve the purpose, the invention provides the following technical scheme:

a QPSK-to-BPSK all-optical modulation format conversion system based on DPMZM comprises LD, 2 electrical amplifiers EA, DPMZM, PC, PBS, EDFA, HNLF, 2 OF and 2 PD.

The DPMZM-based QPSK-to-BPSK all-optical modulation format conversion method based on the system comprises the following steps:

s1: before the QPSKI path of electric signals are input into the DPMZM, the QPSKI path of electric signals are consistent with the I path of data source signal bits;

s2: before the QPSKQ path of electric signals are input into the DPMZM, the QPSKQ path of electric signals are converted into XNOR operation results of I path of data source signal bits and Q path of data source signal bits;

s3: IQ signals need to be amplified by the electric amplifier EA and then input into the DPMZM.

Optionally, the setting of the DPMZM bias point includes:

the two MZMs in the X polarization state are both provided with a minimum bias point, and the phase control voltage is a maximum bias point;

the two MZMs in the Y polarization state are both provided with minimum bias points, and the phase control voltage is an orthogonal bias point;

the DPMZM electrical signal input comprises:

the radio frequency source generates radio frequency electric signals RF with equal power with the IQ two-path signals and simultaneously inputs the radio frequency electric signals RF into two MZMs in the DPMZMX polarization state;

the QPSKI path electric signal is input into an I path MZM of the DPMZMY polarization state;

and the QPSKQ path electric signal is input into a Q path MZM of the DPMZMY polarization state.

Optionally, the signal processing for QPSK to BPSK format conversion includes:

the output signal of the DPMZM passes through the PC and the PBS, and any output of the PBS is amplified to a power level capable of exciting the FWM by the EDFA;

the amplified signal passes through the HNLF excitation FWM.

Optionally, the signal extraction after the QPSK to BPSK format conversion includes:

when the OF central frequency is set as sum or difference OF LD transmitting frequency and 2 times RF frequency, extracting BPSK optical signal bit-modulated by I path data source signal;

setting OF center frequency as difference or sum OF LD transmitting frequency and RF frequency, extracting BPSK optical signal bit-modulated by Q data source signal;

and the two BPSK signals are directly detected by the PD and then are demodulated into IQ two-path data source signal bits respectively.

The invention has the beneficial effects that: the system can directly improve the symbol rate of input signals without changing the system structure as long as the limit parameters of the used devices allow; the signal of the I path and the signal of the Q path can be converted and directly demodulated, a coherent receiver and a phase-locked loop are avoided, and the system structure is simple.

Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the means of the instrumentalities and combinations particularly pointed out hereinafter.

Drawings

For the purposes of promoting a better understanding of the objects, aspects and advantages of the invention, reference will now be made to the following detailed description taken in conjunction with the accompanying drawings in which:

fig. 1 is a structural diagram of a QPSK to BPSK all-optical modulation format conversion system based on a DPMZM according to an embodiment of the present invention;

FIG. 2 is a diagram of a PD detection signal E of an anti-Stokes zone (Stokes) according to an embodiment of the present invention_Idle1PD(t) introducing a phase shift with HNLFA variation graph of (2);

FIG. 3 is a PD detection signal E of Stokes band in accordance with an embodiment of the present invention_Idle2PD(t) introducing a phase shift with HNLFA variation graph of (2);

FIG. 4 is a diagram of simulation results of a data source signal bit signal according to an embodiment of the present invention;

FIG. 5 is a diagram illustrating simulation results of format converted output signals according to an embodiment of the present invention;

FIG. 6 is a Q-way conversion result chart of an actual experiment according to an embodiment of the present invention.

Detailed Description

The embodiments of the present invention are described below with reference to specific embodiments, and other advantages and effects of the present invention will be easily understood by those skilled in the art from the disclosure of the present specification. The invention is capable of other and different embodiments and of being practiced or of being carried out in various ways, and its several details are capable of modification in various respects, all without departing from the spirit and scope of the present invention. It should be noted that the drawings provided in the following embodiments are only for illustrating the basic idea of the present invention in a schematic way, and the features in the following embodiments and examples may be combined with each other without conflict.

Wherein the showings are for the purpose of illustrating the invention only and not for the purpose of limiting the same, and in which there is shown by way of illustration only and not in the drawings in which there is no intention to limit the invention thereto; to better illustrate the embodiments of the present invention, some parts of the drawings may be omitted, enlarged or reduced, and do not represent the size of an actual product; it will be understood by those skilled in the art that certain well-known structures in the drawings and descriptions thereof may be omitted.

The same or similar reference numerals in the drawings of the embodiments of the present invention correspond to the same or similar components; in the description of the present invention, it should be understood that if there is an orientation or positional relationship indicated by terms such as "upper", "lower", "left", "right", "front", "rear", etc., based on the orientation or positional relationship shown in the drawings, it is only for convenience of description and simplification of description, but it is not an indication or suggestion that the referred device or element must have a specific orientation, be constructed in a specific orientation, and be operated, and therefore, the terms describing the positional relationship in the drawings are only used for illustrative purposes, and are not to be construed as limiting the present invention, and the specific meaning of the terms may be understood by those skilled in the art according to specific situations.

A QPSK to BPSK all-optical modulation format conversion system based on DPMZM, as shown in fig. 1, comprising:

the following main common devices: LD, 2 EAs (EA1 and EA2), DPMZM, PC, PBS, EDFA, HNLF, 2 OF (OF1 and OF2), 2 PD (PD1 and PD 2);

in fig. 1, which is more detailed in the following description, a specific structure of the DPMZM is shown as a dashed box;

an Arbitrary Waveform Generator (AWG) and an Oscilloscope (OSC) are shown in fig. 1, showing the two devices to provide the system with an input of electrical signals and an output of electrical signals processed by the system, simulating the input and output of real signals when the system is in use;

also shown in fig. 1 are a Radio Frequency (RF) source and a direct current voltage source (DC1 and DC2), both of which are shown to provide an operating voltage input and an RF input to the system. Simulating real operating voltage settings and RF inputs when using the system;

it should be noted that the AWG, OSC, RF, and DC are shown only to illustrate the present embodiment, and are not part of the system since they may have different instrument models and implementations in actual use.

In this embodiment, the AWG will generate two paths of electrical signals, QPSK-I and QPSK-Q, respectively, as shown in FIG. 1. QPSK-IQPSK I path signal; the QPSK-Q corresponds to the QPSK Q signal, and the phase mapping manner is specifically shown in table 1.

The resulting QPSK-I and QPSK-Q electrical signals are amplified by EA1 and EA2 and fed to the DPMZM as set. Specifically, the method comprises the following steps:

let the laser electric field generated by LD beP₀Is the average power of the laser, ω₀For the emission frequency, as can be seen from the fine structure of the DPMZM shown in fig. 1, if the power of the laser is equalized, the laser input power of each MZM branch should be 1/4 of the total power, i.e. the laser electric field input of each MZM branch is

TABLE 1 phase mapping scheme

I (t) denotes the QPSK-I signal normalized to between-1 and +1, Q (t) denotes the QPSK-Q signal normalized to between-1 and +1, A_QPSKIndicating their magnitude;

four MZMs in the DPMZMs are push-pull structures, in a single driving mode, the extinction ratios are assumed to be the same and are expressed as ER, and the attenuation and the temperature change caused when optical signals pass through the four MZMs are ignored;

the DC half-wave voltage of each MZM is V_πDCThe half-wave voltage of AC is V_πRFUnder the control of QPSK-I, the output electric field of MZM1 is:

under the control of QPSK-Q, the output electric field of MZM2 is:

wherein, V_DCNamely the direct current bias point of the MZM.

For QPSK modulation, which is a modulation mode for suppressing carrier, DC1 of MZM1 and MZM2 in the DPMZM should be set to a minimum bias point (Min), namely V_DC＝-V_πDCAnd/2, then:

QPSK modulation requires that the phase of the I-path modulation signal and the phase of the Q-path modulation signal be 90 degrees apart and added, so the phase control voltage for the DPMZM Y polarization state is set to the quadrature bias point (the DC for setting this voltage is not shown in FIG. 1), which will cause E to be_MZM2(t) optical carrier in the expressionIs replaced byThe final QPSK modulated signal obtained is:

in order to obtain a pump input for generating the FWM, the invention uses an amplitude A_RFFrequency of ω_RFIs set to A_RFsin(ω_RFt), producing ω₀A nearby symmetrical pulse, but undesirably introducing too much omega₀I.e. suppressed carriers. Therefore, DC2 of MZM3 and MZM4 in the DPMZM should be set to Min, and the output electric fields of MZM3 and MZM4 are as follows:

meanwhile, sidebands generated by MZM3 and MZM4 need to be added in the same direction to ensure the power of the pump input, so the phase control voltage of the DPMZMX polarization state should be the maximum bias point (the DC for setting this voltage is not shown in FIG. 1), E_MZM4The optical carrier in (t) remains unchanged, then the pump input generated using RF is:

the above formula uses Jacobi-Angel expansion (Jacobi-anger identity), where n is an integer, J_n(x) Is an nth order bessel function with respect to x.

When n is an even number, J_n(x) Is an even function, having J_n(-πA_RF/V_πRF)＝J_n(πA_RF/V_πRF). Ideally, ER ═ infinity, E_RF(t) the parenthesis part in the expression is 0, there will be only odd order pump sidebands; however, due to process effects, ER is typically only about 20dB (i.e., ER ═ 100), and the parts inside the braces still retain small values. This indicates that the sidebands used as FWM pumps contain in addition to the desired ω₀+ω_RF(or. omega.) of₀-ω_RF) Will also contain omega₀+2ω_RF、ω₀+3ω_RFEqual frequency component, but because of J_n(x) The power of these frequency components is lower and lower due to the influence of the coefficients.

In this embodiment, as shown in fig. 1, the input frequency ω of the light source LD of the DPMZM₀＝2π×193.4145，P₀10 mW; the symbol rate of QPSK-I and QPSK-Q is 1G/S; the RF frequency of the MZM3 and MZM4 inputs is 20 times the symbol rate of QPSK-I and QPSK-Q, i.e., ω_RF＝2π×20×1；ER＝100，V_πRF＝8V，V_πDC8V, amplified QPSK-I input MZM1, amplified QPSK-2 input MZM2, A_QPSK＝A_RF＝2.5V；

The center frequency omega can be obtained at the output end of the DPMZM₀QPSK modulation format signal and the distance between both sides of the signal is n omega_RFPump side ofThe polarization combined beam (PBC) signal, composed of band pulses, is represented by the Jones (Jones) vector as:

performing signal processing on the PBC signal, specifically:

for subsequent PD demodulation convenience, the PBC signal should be combined onto one polarization state without introducing an additional phase shift between the two polarization states. For this purpose, a signal is input to a PC to change the Jones vector representing the polarization state of a PBC signal, the PC is composed of two quarter-wave plates whose principal axes are rotated by 90 degrees with respect to each other, and a half-wave plate is sandwiched between the quarter-wave plates, and when the rotation angle of the quarter-wave plate is set to 135 degrees and the rotation angle of the half-wave plate is set to 0 degree, the change of the Jones vector is represented by using a Jones matrix:

this equation illustrates that the two polarization state signals of the PBC signal are added or subtracted without an additional phase shift between each other. The PBC signal after the change of the polarization state is separated by using the PBS, so that a combined signal along the same polarization state can be obtained:

wherein k is an integer.

Get E_QPSKPCω in (t)₀The corresponding frequency component being a signal wave, omega₀+ω_RFThe corresponding frequency component being a pump for exciting FWM, E_QPSKPC(t) before HNLF is input, it is amplified by EDFA to a level that can excite FWM effect. In this example, E_QPSKPCThe power of (t) is amplified to 20dBm by the EDFA. Omega₀The corresponding wavelength (1550nm) is just the zero dispersion wavelength of the HNLF, the signal wave and the pump work near the zero dispersion wavelength, the phase matching condition of FWM is easily met, the FWM effect can be excited by the signal wave and the pump, and the idler frequency on the anti-Stokes band is obtainedWave E_Idle1Idler E on (t) and Stokes bands_Idle2(t)：

Since the amplitudes of the signal wave and the pump affect only the power of the output frequency component, the amplitudes of the signal wave and the pump are omitted in the discussion of the generation of the idler wave so as to focus only on the frequency and phase conditions.

For E_Idle1(t) carrier frequency of ω₀+2ω_RFThe phase is the conjugate of the phase of the signal wave. At the same time from E_QPSKPC(t) As can be seen, the signal output from HNLF is other than E_Idle1(t) frequency component contained, E_QPSKPC(t) itself also carries the frequency componentThis is equivalent to providing the pair E_Idle1(t) optical carrier wave, E_Idle1(t) PD demodulation can be used directly in subsequent signal extraction;

for E_Idle2(t) carrier frequency of ω₀-ω_RFThe phase is twice the phase of the signal wave. And E_Idle1(t) As in the case of E_QPSKPC(t) having a frequency component of its ownThis provides the pair E_Idle2(t) optical carrier wave, E_Idle2(t) PD demodulation may also be used directly thereafter.

After FWM, extracting a signal after format conversion, specifically:

obviously filter out E_Idle1(t) obtaining a path of signal after format conversion. From E_QPSKPC(t) includes the symmetry of the frequency component, and ω may be selected₀-ω_RFThe corresponding frequency component is the pump input of FWM, the result of which will be E_Idle1(t)With respect to ω₀A symmetric idler. Thus, OP1 is at ω₀+2ω_RFOr ω₀-2ω_RFFiltering for center frequency to extract E_Idle1(t) a switching signal. As can be seen from the sampling theorem, the filtering bandwidth is certainly at least twice the symbol rate, but not too wide, and E cannot be set_QPSKPC(t) the higher power pump component is included which results in the demodulated signal being modulated by the difference frequency of the central frequency of OP1 and the included pump component.

Although the signal input to HNLF contains its optical carrier, it is affected by other nonlinear effects in HNLF, the optical carrier after FWMWill inevitably introduce a phase shift related to the length of HNLF, the non-linear coefficient gamma and the input signal powerThis will result in the introduction of a phase shift in accordance with the signal detection performed

Same pair of cases E_Idle2(t) also holds, OP2 in ω₀-ω_RFOr ω₀+ω_RFFiltering for center frequency to extract E_Idle2(t) and introducing a phase shift

As with the discussion of FWM, E_Idle1PD(t) and E_Idle2PD(t) the power of the idler and optical carrier is omitted from the discussion, and they only affectAnddoes not affect E_Idle1PD(t) and E_Idle2PD(t) results.

FIGS. 2 and 3 show E in the present embodiment when the source bit is in the values given in Table 1_Idle1PD(t) and E_Idle2(t) followingAndchange from 0 to 2 pi.

In this embodiment, HNLF is selected to have a length of 200m, γ being 10W^-1And/km. The simulation principle of the professional optical communication system simulation software vpitranssmisionmaker for the embodiment is as follows: the data source bits transmit a 32-bit pseudo-random signal. Simulation result displayWill be in the range indicated by the dashed box in figure 2,in the range shown by the dashed box in fig. 3. As can be seen from FIG. 2, E_Idle1PD(t) the demodulation result is 0, 1, 0, namely the inverse I path signal of the data source signal bit; as can be seen from FIG. 3, E_Idle2PDThe (t) demodulation results are 1, 0, i.e. Q-path signals of data source signal bits, which is also given by simulation results of vpitranssmission maker in fig. 4 and 5. The principle and the simulation result clearly show that the invention successfully converts the QPSK signal into two BPSK signals based on the DPMZM.

It should also be noted that the HNLF length, γ, and input signal power are selected so long as they are usedAndthe range in the dashed box of FIGS. 2 and 3, E_Idle1PD(t) and E_Idle2PDThe demodulation result of (t) can be as described above, so the invention also has certain guarantee for the stability of HNLF.

This example also performed actual experiments using an instrument of the type described in table 2 with a 32-bit pseudo-random signal set by the vpitransnission maker software.

TABLE 2 Instrument models and Specifications for practical experiments

Limiting parameters (mainly increase of ω) limited to the instruments used_RF) In practice, only the Q-way conversion from QPSK to BPSK is obtained, as shown in FIG. 6. Although some noise is included, it can be seen that fig. 6 is consistent with the Q-path simulation results of fig. 5, and the problem has been explained. The limit parameters of the instrument are improved (mainly the bottom noise of OSC is reduced), and the result of converting the path I of QPSK into BPSK can be obtained by only setting the center frequency of OP under the condition of not changing the structure of the current experimental system; since FWM is almost instantaneous, the symbol rate of the data source bits can also continue to increase without changing the system architecture (only the output rate of AWG needs to be increased, but at the same time ω_RFAlso increased); the coherent receiver is not used, and other conversion links are constructed without adding complex optical devices, so that the system structure is simple, and the advantages of the invention are achieved.

Finally, the above embodiments are only intended to illustrate the technical solutions of the present invention and not to limit the present invention, and although the present invention has been described in detail with reference to the preferred embodiments, it will be understood by those skilled in the art that modifications or equivalent substitutions may be made on the technical solutions of the present invention without departing from the spirit and scope of the technical solutions, and all of them should be covered by the claims of the present invention.