CN116192264A - Optimization method of carrier suppression double-sideband modulation optical fiber wireless communication system - Google Patents
Optimization method of carrier suppression double-sideband modulation optical fiber wireless communication system Download PDFInfo
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- CN116192264A CN116192264A CN202211660251.3A CN202211660251A CN116192264A CN 116192264 A CN116192264 A CN 116192264A CN 202211660251 A CN202211660251 A CN 202211660251A CN 116192264 A CN116192264 A CN 116192264A
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
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- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/25—Arrangements specific to fibre transmission
- H04B10/2575—Radio-over-fibre, e.g. radio frequency signal modulated onto an optical carrier
- H04B10/25752—Optical arrangements for wireless networks
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/25—Arrangements specific to fibre transmission
- H04B10/2507—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion
- H04B10/2513—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion due to chromatic dispersion
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
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- Y02D—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN INFORMATION AND COMMUNICATION TECHNOLOGIES [ICT], I.E. INFORMATION AND COMMUNICATION TECHNOLOGIES AIMING AT THE REDUCTION OF THEIR OWN ENERGY USE
- Y02D30/00—Reducing energy consumption in communication networks
- Y02D30/70—Reducing energy consumption in communication networks in wireless communication networks
Abstract
The invention relates to the technical field of photoelectric signal processing, in particular to an optimization method of a carrier suppression double-sideband modulation optical fiber wireless communication system, which comprises the following steps: the invention can optimize the carrier suppression double-sideband CS-DSB modulation optical fiber wireless communication RoF system adopting the commercial MZM modulator, improve the optical sideband suppression ratio of the generated optical millimeter wave, and reduce the power jitter of the system output radio frequency target frequency multiplication signal caused by optical fiber dispersion.
Description
Technical Field
The invention relates to the technical field of photoelectric signal processing, in particular to an optimization method of a carrier suppression double-sideband modulation optical fiber wireless communication system.
Background
With the rapid development of information society, users have put higher demands on the bandwidth and rate of wireless communication access. Millimeter wave communication is a powerful competitor for broadband wireless communication due to its high bandwidth and high rate. The optical fiber wireless communication RoF (Radio over Fiber) technology combines the advantages of large bandwidth of optical fiber communication, flexible wireless communication access and the like, and can improve the transmission distance of millimeter wave communication. In recent years, rapid development of broadband wireless access networks requires RoF systems with higher microwave frequencies and longer transmission distances. However, in a radio frequency long-distance RoF system, the conventional DSB double-sideband modulation causes serious dispersion problem, and the electric signal power output by the system terminal photoelectric detector will show periodic oscillation fading along with the transmission distance and the radio frequency, which greatly reduces the system performance.
The output power of the carrier-suppressed double-sideband CS-DSB modulation RoF system is not affected by the periodic transmission oscillation fading effect, and the dispersion problem caused by DSB modulation is overcome. The reported method for realizing CS-DSB modulation RoF system is Mach-Zehnder MZM modulator method [ optical communication research 2013,15 (1): 53-56; journal of lasers, 2016,37 (10): 45-48], cascaded MZM modulator method [ experimental techniques and management, 2020,37 (6): 141-145], MZM modulator+filter method [ laser and infrared, 2020,50 (9): 1126-1130; radio engineering, 2013,43 (10): 8-9].
However, the above-described approach to implementing CS-DSB modulated RoF systems has some drawbacks. Although the Mach-Zehnder modulator method has a simple structure, the limited extinction ratio of the commercial MZM modulator can generate an unnecessary carrier component to cause the reduction of the optical sideband suppression ratio OSSR, so that a millimeter wave signal with high quality cannot be generated; although the cascade MZM modulator method can obtain higher Optical Sideband Suppression Ratio (OSSR) to generate high-quality millimeter waves, the method has complex structure, large insertion loss and higher cost; the MZM modulator + filter approach employs a filter (including a fiber bragg grating FBG) to filter out unwanted optical sidebands, which can prevent the approach from being applied in a WDM-RoF system, as well as increasing the complexity of the system and limiting its tunability.
Disclosure of Invention
The invention aims to solve the defects in the prior art, and provides an optimization method of a carrier suppression double-sideband modulation optical fiber wireless communication system.
In order to achieve the above purpose, the present invention adopts the following technical scheme:
an optimization method of a carrier-suppressed double-sideband modulation optical fiber wireless communication system comprises the following steps: the light wave emitted by the laser LD is modulated by radio frequency signals through an MZM modulator, the MZM modulator is biased at a minimum output point, the phase difference between the radio frequency driving signals of the upper arm and the lower arm is pi, and the phase difference is realized by a phase shifter;
the extinction ratio of the practical commercial MZM modulator is limited, when the modulation indexes of the upper arm and the lower arm are equal and the phase difference between driving signals is pi, the MZM modulator outputs +1 order, -1 order sidebands and a small amount of unnecessary carrier components under the condition of small signal modulation, the unnecessary carrier components can be restrained by optimally setting the asymmetric modulation indexes of the upper arm and the lower arm of the MZM modulator, the power jitter of an output target frequency doubling signal is reduced, and the optical band restraining ratio of CS-DSB optical carrier millimeter waves is improved.
Preferably, the MZM modulator is biased at a minimum output point V bias ＝V π ，V π Is the modulator half-wave voltage.
Preferably, the output light wave of the laser LD
Modulated by a radio frequency signal of frequency through an MZM modulator with bias voltage V π The phase difference between the RF driving signals of the upper and lower arms is pi.
Preferably, the driving voltages of the two arms of the MZM modulator can be respectively expressed as V 1 (t)＝V m1 sinω m t and V 2 (t)＝V m2 sin(ω m t + pi), the MZM modulator output is:
wherein m is 1,2 ＝πV m1,2 /V π Is the modulation index of the upper and lower arms of the MZM modulator, V m1,2 Is the amplitude of the radio frequency signals of the upper arm and the lower arm, V π Is half-wave voltage, MZM modulationThe two-arm shunt ratio gamma can be expressed as the extinction ratio epsilon of the MZM modulator
J n (m) is a first class of n-order Bessel functions, and the Bessel expansion of 2 nd order and above can be ignored under the condition of small signal modulation.
Preferably, the commercial MZM modulator has a finite extinction ratio (ε<Infinity, γ+.1), the modulation index of the upper and lower arms are equal (m 1 ＝m 2 When=m), there is a residual carrier component (γj) 0 (m 2 )-J 0 (m 1 ) Not equal to 0), the modulator outputs +1 order, -1 order sidebands and a small amount of unwanted carrier components, the MZM modulator (5) has upper and lower arm modulation indexes of m respectively 1 =m (1+a) and m 2 =m (1-a), the following condition needs to be satisfied to cancel the carrier component modulation index:
J 0 (m 1 )＝γJ 0 (m 2 ) I.e.
Under the condition of small signal modulation, m=1 can be set, when the extinction ratio epsilon of the MZM modulator is given, the optimal a can be determined by the numerical calculation, and then the modulation coefficients m of the upper arm and the lower arm can be determined 1 And m 2 。
Preferably, the asymmetric modulation indexes of the upper arm and the lower arm of the MZM modulator can inhibit or even eliminate unnecessary carrier components, so that the optical sideband suppression ratio OSSR of the optical millimeter waves is improved, and the power jitter of an output target frequency multiplication signal is reduced.
The beneficial effects of the invention are as follows:
1. the invention can optimize the carrier suppression double-sideband CS-DSB modulation optical fiber wireless communication RoF system adopting the commercial MZM modulator, improves the optical sideband suppression ratio of the generated optical millimeter waves, and can also reduce the power jitter of the system output radio frequency target frequency multiplication signal caused by optical fiber dispersion;
2. the invention adopts a common MZM modulator, can generate high-quality CS-DSB modulated optical millimeter waves without cascading the MZM modulator or an optical filter, has simple structure, stable system and easy realization, and can be applied to WDM-RoF wavelength division multiplexing optical fiber wireless communication systems and up-conversion systems.
Drawings
Fig. 1 is a system block diagram of an optimization method of a carrier-suppressed double-sideband modulated optical fiber wireless communication system.
In the figure: 1. a laser; 2. a radio frequency signal generator; 3. a phase shifter; 4. an electrical gain; 5. MZM modulator; 6. a single mode optical fiber; 7. a photodetector.
Detailed Description
The following description of the embodiments of the present invention will be made clearly and completely with reference to the accompanying drawings, in which it is apparent that the embodiments described are only some embodiments of the present invention, but not all embodiments.
Referring to fig. 1, an optimization method of a carrier rejection double-sideband modulation optical fiber wireless communication system comprises a laser 1, a radio frequency signal generator 2, a phase shifter 3, an electric gain 4, an MZM modulator 5, a single-mode optical fiber 6 and a photoelectric detector 7, wherein an optical spectrum analyzer can be used for measuring the optical sideband rejection ratio of generated optical millimeter waves, and a power meter is used for measuring the power jitter of a radio frequency target frequency multiplication signal.
The specific method and the steps for optimizing the CS-DSB modulation optical fiber wireless communication RoF system are as follows:
the light wave emitted by the laser 1LD is modulated by a radio frequency signal via an MZM modulator 5, the MZM modulator 5 being biased at a minimum output point (V bias ＝V π ，V π Is half-wave voltage of the modulator), the phase difference between the radio frequency driving signals of the upper arm and the lower arm is pi, and the phase difference is realized by the phase shifter 3;
the extinction ratio of the ideal MZM modulator 5 is infinite (ε= infinity), when the modulation indexes of the upper and lower arms are equal (m 1 ＝m 2 When the phase difference between driving signals is pi, the modulator output is +1 order and-1 order sidebands under the condition of small signal modulation;
the extinction ratio of the commercial MZM modulator 5 is limited (epsilon < +), the modulator output is +1 order, -1 order sideband and residual carrier component, namely the carrier component which is not needed is generated, and the CS-DSB modulation optical millimeter wave performance is reduced;
the +1 order and-1 order sidebands generate a 2-order radio frequency signal (target signal) at the beat frequency of the photodetector 7 at the system terminal, and the carrier component generated by the limited extinction ratio of the commercial modulator can increase the power jitter of the radio frequency target signal and reduce the optical sideband suppression ratio thereof.
For example, epsilon=20 dB, the optical band rejection ratio OSSR of the millimeter wave is only 10dB, and the power jitter of the 2-order rf target signal is 1.2dB. By setting the asymmetric modulation index (m 1 ≠m 2 ) Unnecessary carrier components can be restrained, OSSR of CS-DSB modulated optical millimeter waves can be improved, and power jitter of output radio frequency target signals can be reduced.
The invention optimizes the principle of the carrier suppression double-sideband modulation optical fiber wireless communication system:
output light wave of laser 1LD
Is frequency omega through MZM modulator 5 m Is modulated by a radio frequency signal, which is interpreted by a radio frequency signal generator 2, and the bias voltage of the MZM modulator 5 is V π The phase difference between the RF driving signals of the upper and lower arms is pi, and the driving voltages of the two arms can be respectively expressed as V 1 (t)＝V m1 sinω m t and V 2 (t)＝V m2 sin(ω m t + pi), MZM modulator 5 outputs:
wherein m is 1,2 ＝πV m1,2 /V π Is the modulation index of the upper and lower arms of MZM modulator 5, V m1,2 Is the amplitude of the radio frequency signals of the upper arm and the lower arm, V π Is the half-wave voltage of MZM modulator 5. MZM modulator 5 double arm shunt ratio gamma can be expressed as MZM modulator 5 extinction ratio epsilon
J n (m) is a first class of n-order Bessel functions, and the Bessel expansion of 2 nd order and above can be ignored under the condition of small signal modulation.
The extinction ratio of an ideal MZM modulator is infinitely large (epsilon= infinity, gamma=1), and the modulation indexes of the upper arm and the lower arm are equal (m 1 ＝m 2 When=m), the carrier component (γj) is eliminated 0 (m 2 )-J 0 (m 1 ) =0), the ideal MZM modulator 5 outputs +1-order and-1-order sidebands, thereby achieving carrier-suppressed double-sideband CS-DSB modulation.
Commercial MZM modulator 5 has a finite extinction ratio (ε)<Infinity, γ+.1), the modulation index (m) of the upper and lower arms needs to be optimally set 1 ≠m 2 ) To satisfy the equation gamma J 0 (m 2 )-J 0 (m 1 ) =0, and further the carrier component is eliminated to realize CS-DSB modulation.
Assuming that the modulation indexes of the upper arm and the lower arm of the MZM modulator 5 are m respectively 1 =m (1+a) and m 2 =m (1-a), the following condition needs to be satisfied to cancel the carrier component modulation index:
J 0 (m 1 )＝γJ 0 (m 2 ) I.e.
Under the condition of small signal modulation, m=1 can be set, when the extinction ratio epsilon of the commercial MZM modulator 5 is given, the optimal a can be determined by the above formula through numerical calculation, and then the modulation coefficients m of the upper arm and the lower arm can be determined 1 And m 2 。
For example, epsilon=20 dB, a=0.17 can be obtained, and m is the case 1 =m (1+a) =1.17 and m 2 ＝m(1-a)＝0.83。
According to the analysis, when the extinction ratio epsilon of the commercial MZM modulator 5 is 20dB, the modulation indexes of the upper arm and the lower arm of the MZM modulator 5 can be optimally set to be m respectively 1 =1.17 and m 2 =0.83. And before optimization (m 1 ＝m 2 Compared with the reference number of 1), the optical band rejection ratio of CS-DSB modulated optical millimeter waves can be measured to be increased from 10dB to 17.5dB, and the power jitter of the 2-order radio frequency target signal output by the photodetector 7 is increased from 1.2dB is reduced to 0.1dB.
The foregoing is only a preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art, who is within the scope of the present invention, should make equivalent substitutions or modifications according to the technical scheme of the present invention and the inventive concept thereof, and should be covered by the scope of the present invention.
Claims (6)
1. An optimization method of a carrier suppression double-sideband modulation optical fiber wireless communication system is characterized in that light waves emitted by an LD of a laser (1) are modulated by radio frequency signals through an MZM modulator (5), the MZM modulator (5) is biased at a minimum output point, the phase difference between radio frequency driving signals of an upper arm and a lower arm is pi, and the phase difference is realized by a phase shifter (3);
the extinction ratio of the MZM modulator (5) is limited, when the modulation indexes of the upper arm and the lower arm are equal and the phase difference between driving signals is pi, the output of the MZM modulator (5) under the condition of small signal modulation is +1 order, -1 order sidebands and a small amount of unnecessary carrier components, the unnecessary carrier components can be restrained by optimally setting the asymmetric modulation indexes of the upper arm and the lower arm of the MZM modulator (5), the power jitter of an output target frequency multiplication signal is reduced, and the optical sideband restraining ratio of CS-DSB optical carrier millimeter waves is improved.
2. The optimization method of a carrier-suppressed double sideband modulated optical fiber radio communication system according to claim 1, characterized in that the MZM modulator (5) is biased at a minimum output point V bias ＝V π ，V π Is the modulator half-wave voltage.
3. The method for optimizing a carrier-suppressed double-sideband modulated optical fiber radio communication system according to claim 1, wherein said laser (1) LD outputs an optical wave
Modulated by a radio frequency signal with a frequency through an MZM modulator (5), the bias voltage of the MZM modulator (5) is V π The upper and lower parts thereofThe phase difference between the arm rf drive signals is pi.
4. A method of optimizing a carrier-suppressed double sideband modulated optical fiber radio communication system according to claim 3 wherein said MZM modulator (5) two arm drive voltages can be represented as V respectively 1 (t)＝V m1 sinω m t and V 2 (t)＝V m2 sin(ω m t+pi), the MZM modulator (5) output is:
wherein m is 1,2 ＝πV m1,2 /V π Is the modulation index of the upper and lower arms of the MZM modulator (5), V m1,2 Is the amplitude of the radio frequency signals of the upper arm and the lower arm, V π Is half-wave voltage, and the double-arm shunt ratio gamma of the MZM modulator (5) can be expressed as extinction ratio epsilon of the MZM modulator (5)
J n (m) is a first class of n-order Bessel functions, and the Bessel expansion of 2 nd order and above can be ignored under the condition of small signal modulation.
5. The method for optimizing a carrier-suppressed double sideband modulated optical fiber radio communication system according to claim 1 wherein said MZM modulator (5) has a finite extinction ratio (epsilon)<Infinity, γ+.1), the modulation index of the upper and lower arms are equal (m 1 ＝m 2 When=m), there is a residual carrier component (γj) 0 (m 2 )-J 0 (m 1 ) Not equal to 0), the modulator outputs +1 order, -1 order sidebands and a small amount of unwanted carrier components, the MZM modulator (5) has upper and lower arm modulation indexes of m respectively 1 =m (1+a) and m 2 =m (1-a), the following condition needs to be satisfied to cancel the carrier component modulation index:
J 0 (m 1 )＝γJ 0 (m 2 ) I.e.
Under the condition of small signal modulation, m=1 can be set, when the extinction ratio epsilon of the MZM modulator (5) is given, the optimal a can be determined by the above formula through numerical calculation, and then the modulation coefficients m of the upper arm and the lower arm can be determined 1 And m 2 。
6. The optimization method of a carrier-suppressed double-sideband modulated optical fiber wireless communication system according to claim 1, wherein the asymmetric modulation index (m1+.m2) of the upper and lower arms of the MZM modulator (5) can suppress or even eliminate the unwanted carrier component, thereby improving the optical sideband suppression ratio OSSR of the millimeter wave of the optical carrier and reducing the power jitter of the output target frequency multiplication signal.
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