High power long fiber optical microwave link structure and design method thereof

Abstract

The application relates to the field of microwave photon technology, and relates to a high-power long optical fiber optical carrier microwave link structure design method based on phase modulation. The application utilizes phase modulation to realize an electro-optical conversion process of a radio frequency signal in an externally modulated optical carrier microwave link, utilizes an optical filter to realize a demodulation process of a phase modulation signal into an intensity modulation signal in the link, and controls the demodulation process of the phase modulation signal by controlling parameters such as a passband frequency range and a center frequency of the optical filter, so that an output signal of the externally modulated optical carrier microwave link through the phase modulation method has smaller scattering distortion.

Description

Technical Field

[0001] This invention relates to the field of microwave photonics technology, specifically to a high-power long fiber optic microwave link structure and its design method. Background Technology

[0002] Traditional electronic-based microwave and millimeter-wave signal generation, transmission, and detection systems face significant challenges. These systems are not only complex in structure, bulky, and expensive, but also suffer from electronic rate limitations in generating and processing high-frequency microwave signals. To achieve high-speed, high-capacity data transmission, microwave technology is combined with photonics. Photonics-assisted generation, transmission, and processing of microwave and millimeter-wave signals in the optical domain is termed Microwave Photonics (MWP). Radio over Fiber (RoF) links are photonics-assisted microwave and millimeter-wave signal transmission links. The main components of an externally modulated RoF link include an externally modulated laser (EML), an optical modulator, and a photodetector. A constant-intensity optical carrier output from the laser is modulated by the microwave / millimeter-wave signal via the optical modulator, altering the laser's amplitude, phase, frequency, and intensity to modulate the light wave. The generated RoF signal is then transmitted through optical fiber, and at the receiving end, a photodetector performs photoelectric conversion to recover the microwave / millimeter-wave signal.

[0003] Currently, research on externally modulated optical-on-microwave links generally focuses on intensity modulation links based on Mach-Zehnder modulators (MZM). In 2012, experiments by C. Lethien et al. in the UK achieved a novel multi-service hybrid fiber optic transmission method and remote power supply. This method, based on WDM, utilizes the 850nm band to transmit IEEE 802.11g signals, uses the 850nm band for remote power supply, and applies the 1300nm band to transmit 10Gbit / s Ethernet signals. From 2016 to the present, a research group at Beijing University of Posts and Telecommunications has utilized polarization characteristics to achieve a 60GHz millimeter-wave generation and wavelength reuse optical-on-microwave system without optical filtering. These studies have improved the spurious-free dynamics and transmission frequency of externally modulated optical-on-microwave links.

[0004] In summary, currently reported externally modulated optical microwave links employ intensity modulation schemes. However, for these links, because the microwave signal is modulated into the intensity information of the optical carrier, the intensity of the modulated optical signal varies over time. During transmission through the optical fiber, this variation in signal intensity leads to nonlinear distortions such as self-phase modulation due to fiber nonlinearity. Furthermore, these distortions become more severe as the optical signal power and fiber length increase. Therefore, the following problems exist: when the transmission fiber is long and the transmitted optical signal power is high, the scattering and nonlinear distortion caused by the nonlinear effects of the fiber become more pronounced. Consequently, this approach cannot meet the increasingly complex transmission requirements for broadband microwave signals in radio frequency environments. Summary of the Invention

[0005] This paper addresses the shortcomings of existing externally modulated optical-on-microwave link technologies by proposing a design method for a high-power long fiber optical-on-microwave link structure based on phase modulation.

[0006] A design method for a high-power long fiber-to-microwave optical link structure based on phase modulation includes: a microwave signal source, a continuous wave laser, a polarization controller, a phase modulator, an optical amplifier, a single-mode fiber, an optical filter, a photodetector, and an antenna.

[0007] The components are connected in the following order: the output of the continuous-wave laser is connected to the input of the polarization controller; the input of the polarization controller is connected to the optical signal input of the phase modulator; the optical signal output of the phase modulator is connected to the input of the optical amplifier; the output of the optical amplifier is connected to the input of the single-mode fiber; the output of the single-mode fiber is connected to the input of the optical filter; the output of the optical filter is connected to the optical signal input of the photodetector; the electrical signal output of the photodetector is connected to the input of the antenna; and the output of the microwave signal source is connected to the electrical signal input of the phase modulator.

[0008] A method for designing a high-power long fiber optic microwave link structure includes the following steps:

[0009] a. The optical signal generated by the continuous wave laser is modulated onto the optical carrier by a phase modulator to form a microwave signal;

[0010] b. The optical signal obtained in step a is used to adjust the optical signal power through an optical amplifier;

[0011] c. The optical signal obtained in step b is transmitted into a single-mode optical fiber;

[0012] d. The optical signal obtained in step c is demodulated into an intensity-modulated signal by an optical filter;

[0013] e. Use a photodetector to perform photoelectric conversion on the optical signal obtained in step d to recover the microwave signal;

[0014] f. Perform subsequent processing operations on the microwave signal output in step e.

[0015] Specifically, in step a, the electro-optic modulation method is as follows:

[0016] In an externally modulated optical microwave link, a microwave signal is modulated onto a continuous-wave optical signal by a phase modulator. The expression for the modulated optical signal is given by [equation missing], where m is the power of the microwave signal. Specifically, in this patent, the microwave signal is modulated into the phase information portion of the optical signal, rather than the intensity information portion, through step a, before subsequent signal transmission.

[0017] The phase information demodulation method in step d is as follows:

[0018] Adjusting the center frequency and passband bandwidth of the optical filter allows it to filter out only the optical carrier and positive first-order sideband frequency components of the phase-modulated signal, while other frequencies are filtered out. The phase-modulated signal is then demodulated into intensity-modulated information.

[0019] This invention utilizes a phase modulator and an optical filter to achieve photoelectric modulation and demodulation of microwave signals, and uses phase modulation technology to suppress the scattering of optical signals by optical fibers and the effects of nonlinear distortion.

[0020] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are:

[0021] (1) When the optical power is low, the spurious-free dynamic range of the phase-modulated optical microwave link is basically the same as that of the intensity-modulated optical microwave link.

[0022] (2) Due to the phase modulation link used, the intensity of the optical signal transmitted in the optical fiber does not change with time, and the scattering and nonlinear distortion of the optical signal are also small. Attached Figure Description

[0023] Figure 1 A schematic diagram of a high-power long fiber optic microwave link structure based on phase modulation provided by the present invention.

[0024] Figure 2 The spurious-free dynamic range diagram of the link output results of the link structure provided by the present invention when the optical signal power is 25dBm and the single-mode fiber length is 10km.

[0025] Figure 3 The spectrum diagram of the link output signal of the link structure provided by the present invention when the optical signal power is 25dBm, 30dBm, and 35dBm, and the single-mode fiber length is 10km, 15km, and 20km, respectively.

[0026] Figure 4 The spectrum of the output signal of a traditional external modulation direct detection optical microwave link structure is shown when the optical signal power is 25dBm, 30dBm, and 35dBm, and the single-mode fiber length is 10km, 15km, and 20km. Detailed Implementation

[0027] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0028] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0029] like Figure 1 As shown, the device structure consists of a microwave signal source 1, a continuous wave laser 2, a polarization controller 3, a phase modulator 4, an optical amplifier 5, a single-mode fiber 6, an optical filter 7, a photodetector 8, and an antenna 9.

[0030] Specifically, the continuous wave optical signal generated by the continuous wave laser 2, after having its polarization direction adjusted by the polarization controller 3, is modulated onto the optical carrier by the microwave signal generated by the microwave signal source 1 in the phase modulator 4, achieving electro-optic modulation. The modulated optical signal is expressed as follows:

[0031] E(t)=E0exp(-jω0t)·exp(jmcos(ωt))(1)

[0032] Where m is the power of the microwave signal, E0 is the optical power of the optical carrier signal, and ω0 is the optical carrier frequency. The microwave signal is modulated into the phase information part of the optical signal, rather than the intensity information part, through step a. The optical signal after single-mode fiber transmission is expanded using Bessel functions, as shown in equation (2) below, which facilitates subsequent filtering.

[0033]

[0034] The modulated optical signal is transmitted through single-mode fiber 6 after the power is controlled by optical amplifier 5. The optical signal transmitted to the destination enters optical filter 7, where the phase modulation signal is demodulated into an intensity modulation signal. By controlling the center frequency and passband bandwidth of the optical filter, the filter only filters out the carrier frequency component and the positive first-order sideband frequency component of the optical signal, as shown in the following expression (3):

[0035]

[0036] The demodulated signal then enters photodetector 8 and is converted back into a microwave signal via photoelectric conversion, as shown below:

[0037]

[0038] in The responsivity of the photodetector is used to obtain the input microwave signal after photoelectric conversion, which is then processed.

[0039] Example

[0040] This example uses OptiSystem software for simulation. Numerical simulation was performed on the high-power long fiber-to-microwave link structure design method based on phase modulation provided by this invention under the following specific parameters: the output optical signal wavelength of the continuous wave laser 2 is 1550nm, the optical power is 15.5dBm, the laser linewidth is 1kHz, the RIN noise is -160dB / Hz, the microwave signal frequency generated by the microwave signal source 1 is 3GHz, the fiber loss of the single-mode fiber 6 is 0.2dB / km, bata2=-3.8237ps^2 / km, bata3=0.063147ps^3 / km, the fiber transmission is configured to consider self-phase modulation, and the fiber length is set to 10km, 15km, and 20km; the filter bandwidth of the optical filter 7 is 5GHz, the suppression ratio is 25dB, and the optical power entering the single-mode fiber 6 is adjusted to 25dBm-35dBm by setting the amplification power of the optical amplifier 5. Based on the above parameters, the spurious-free dynamic range and output signal spectrum of a phase-modulated optical microwave link can be simulated when the optical signal power is between 25dBm and 35dBm and the fiber length is between 10km and 20km.

[0041] Figure 2 As can be seen, when the optical signal power entering the single-mode fiber is 25dBm and the single-mode fiber length is 10km, the spurious-free dynamic range of the link provided by this invention is shown in the figure. The calculated result is that the noise floor of the link is noise = -153.42dB / Hz. Then, by fitting the fundamental frequency term and IMD3 term data of the measured output signal, the curves of the fundamental frequency term and IMD3 term are obtained. Based on the curves and the noise floor, the spurious-free dynamic range of the link is calculated to be SFDR = 105.12dB·Hz. 2 / 3 This result is basically consistent with the simulation results of the current mainstream external modulation direct detection optical microwave link using MZM modulator.

[0042] Figure 3 and Figure 4The output microwave signal spectra of the phase-modulated optical microwave link provided in this invention and the traditional intensity-modulated externally modulated direct-detection optical microwave link are compared when the optical signal power entering the single-mode fiber is 25dBm, 30dBm, and 35dBm, and the fiber length is 10km, 15km, and 20km, respectively. It can be observed that the output signal spectrum quality of the phase-modulated optical microwave link does not deteriorate with increasing optical signal power and single-mode fiber length, and no stray or nonlinear distortion frequency components appear. However, in the intensity-modulated externally modulated direct-detection optical microwave link, the output signal spectrum deteriorates severely with increasing optical signal power and single-mode fiber length, and stray and nonlinear distortion frequency components appear in the spectrum. This phenomenon can be explained by the transmission equation of optical fiber. The basic transmission equation for optical signals propagating in optical fiber can be expressed as: Where A is the optical signal transmitted in the optical fiber, β0 is the transmission coefficient of the optical fiber, and α is the transmission loss coefficient of the optical fiber. From the above transmission equation, it can be seen that for the phase-modulated optical microwave link provided by this invention, since its intensity does not change with time, the partial derivative of A with respect to time t in the transmission equation can be simplified, becoming the following form. It can be observed that the optical signal transmitted through the optical fiber at this time only has the fiber loss portion compared to the original optical signal, and there is no nonlinear distortion. However, for the traditional external modulation direct detection intensity-modulated optical microwave link, because the intensity of the optical signal transmitted in the optical fiber changes with time, the partial derivative of A with respect to time t in the transmission equation cannot be simplified, and there is an imaginary term in the transmission equation. Therefore, after solving for the transmitted optical signal, nonlinear distortions such as self-phase modulation will occur.

[0043] The above experimental examples demonstrate that for high-power long fiber optic microwave links, the solution provided by this invention can effectively suppress nonlinear distortion and spurious emissions compared to traditional intensity-modulated externally modulated direct-detection optical microwave links. The simulation results are consistent with expectations. Specific examples show that this invention provides a structural design method for high-power long fiber optic microwave links based on phase modulation. It has the advantages of suppressing nonlinear distortion and spurious emissions in high-power, long-fiber transmission, and also has strong anti-electromagnetic interference capabilities.

[0044] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. A method for designing a high-power long-fiber optical-carrier microwave link structure, characterized in that, The high-power long optical fiber microwave carrier link structure comprises a microwave signal source (1), a continuous wave laser (2), a polarization controller (3), a phase modulator (4), an optical amplifier (5), a single-mode optical fiber (6), an optical filter (7), a photodetector (8) and an antenna (9); The various devices are connected in the following order: the output end of the continuous wave laser (2) is connected to the input end of the polarization controller (3), the input end of the polarization controller (3) is connected to the optical signal input end of the phase modulator (4), the optical signal output end of the phase modulator (4) is connected to the input end of the optical amplifier (5), and the output end of the optical amplifier (5) is connected to the input end of the single-mode optical fiber (6); the output end of the single-mode optical fiber (6) is connected to the input end of the optical filter (7), the output end of the optical filter (7) is connected to the optical signal input end of the photodetector (8), and the electrical signal output end of the photodetector (8) is connected to the input end of the antenna (9); the output end of the microwave signal source (1) is connected to the electrical signal input end of the phase modulator (4); The design method comprises the following steps: a. The optical signal generated by the continuous wave laser is modulated with the microwave signal on the optical carrier through the phase modulator; b. The optical signal obtained in step a is adjusted in power through the optical amplifier; c. The optical signal obtained in step b is transmitted through the single-mode optical fiber; d. The optical signal obtained in step c is demodulated into an intensity modulation signal through the optical filter; e. The optical signal obtained in step d is photoelectrically converted by the photodetector to recover the microwave signal; f. The microwave signal output in step e is subjected to subsequent processing operations; The method is specifically as follows: The continuous wave optical signal generated by the continuous wave laser is adjusted in polarization direction by the polarization controller, and the microwave signal generated by the microwave signal source is modulated on the optical carrier in the phase modulator to realize electro-optic modulation, and the modulated optical signal is expressed as follows: ； wherein m is the power of the microwave signal, is the optical power of the optical carrier signal, is the optical carrier frequency, the microwave signal is modulated into the phase information part of the optical signal by step a, and the non-intensity information part of the optical signal after transmission in the single-mode optical fiber is expanded by the Bessel function, as shown in the expression below, which facilitates subsequent filtering processing, ； The modulated optical signal is transmitted through the single-mode optical fiber after being controlled in power by the optical amplifier, and the optical signal transmitted to the terminal enters the optical filter to demodulate the phase modulation signal into an intensity modulation signal, and the center frequency and the passband bandwidth of the optical filter are controlled to make the filter only filter out the carrier frequency component and the positive first-order sideband frequency component of the phase modulation signal, and the expression is as follows: ； Then, the demodulated signal enters the photodetector to recover the microwave signal through photoelectric conversion, and the form is as follows: ； wherein is the responsivity of the photodetector, the input microwave signal after photoelectric conversion, and the subsequent processing operation.

2. The method of claim 1, wherein the high-power long-fiber optical microwave link structure design is characterized by, In step a, the specific method of electro-optic modulation is as follows: The microwave signal is modulated onto the continuous wave optical signal by a phase modulator in the externally modulated optical microwave link, and the modulated optical signal expression is where m is the power of the microwave signal.

3. The method of claim 2, wherein the high-power long-fiber optical microwave link structure design is characterized by, The phase information demodulation method in step d is as follows: The center frequency and the passband bandwidth of the optical filter are adjusted to make the optical filter only filter out the optical carrier and the positive first-order sideband frequency component of the phase modulation signal, and other frequencies are filtered out, and the phase modulation signal is demodulated into intensity modulation information.

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