A receiver device for an optical fiber terahertz communication system

Abstract

The application discloses a kind of receiver devices for optical fiber terahertz communication system, belong to optical fiber terahertz communication technical field.The application is combined by electro-optic modulator nonlinear modulation and beat frequency technology, generates high-quality terahertz radio frequency source and mixes with terahertz receiving signal, realizes the down-conversion of ultra-low noise, reduces the complexity of subsequent digital signal processing algorithm.At the same time, since the application does not need to generate high flatness optical frequency comb, only two frequency intervals near terahertz high-order sideband, without complex equipment, simple structure.In addition, the laser source in the application is used to generate the radio frequency source required for mixing, and also used to convert the intermediate frequency electrical signal after mixing into optical baseband signal by electro-optic modulator, has certain cost benefit.

Description

Technical Field

[0001] This invention belongs to the field of fiber optic terahertz communication technology, and more specifically, relates to a low phase noise, low cost receiver in a fiber optic terahertz communication system. Background Technology

[0002] With the large-scale commercial deployment of fifth-generation mobile networks (5G), research and exploration of next-generation mobile communications (6G) have begun. The "6G Overall Vision and Potential Key Technologies White Paper" released by the IMT-2030 6G Promotion Group in June 2021 points out that 6G is envisioned to bring various new communication services, including holographic communication, high-quality online video conferencing, and augmented reality / virtual reality, placing higher demands on network KPIs such as data rate, latency, and connection count. In particular, to achieve terahertz-level communication rates, terahertz band communication (0.1THz~10THz) is considered an important air interface technology solution for future 6G networks and has received widespread attention in recent years.

[0003] In September of the same year, IMT-2030 released the "Terahertz Communication Technology Research Report," which introduced the domestic and international research progress on the basic principles, application scenarios, core devices, and key technologies of terahertz communication. It clearly pointed out that terahertz communication can serve as a beneficial supplement to existing air interface transmission methods and may be applied to various 6G high-capacity and ultra-high-speed transmission scenarios. However, many problems still need to be solved, including the design of receiving schemes in terahertz communication systems.

[0004] In communication systems, for subsequent digital signal processing, the high-frequency received signal must first be down-converted to baseband at the receiving end. Existing terahertz communication system receiver down-conversion technologies can be divided into three categories: all-electric down-conversion, all-optical down-conversion, and optoelectronic hybrid down-conversion. Patent application CN113890629A discloses a terahertz signal receiving device, method, and signal transmission system, which achieves terahertz signal down-conversion by mixing a low-frequency electro-RF source with the received terahertz signal after multiple frequency multiplications and filtering out the high-frequency signal. However, this approach introduces significant phase noise after multiple frequency multiplications of the RF source. The literature "THz-to-optical conversion in wireless communications using an ultra-broadbandplasmonic modulator. Nature Photonics, 2019, pp. 519-524" describes directly modulating the terahertz received signal onto an optical carrier using an ultra-high-speed electro-optic modulator, and then filtering out sideband signals to obtain the optical baseband signal. However, this approach is not suitable due to the lack of mature terahertz electro-optic modulator fabrication technology. To overcome the shortcomings of the above two schemes, a hybrid optoelectronic down-conversion technique has been proposed. The literature "Fiber-THz-Fiber Link for THz Signal Transmission. IEEE Photonics Journal, 2018, pp. 1-6" first down-converts the terahertz received signal to an intermediate frequency (IF) via electro-mixing, then modulates the IF signal onto an optical carrier using an electro-optic modulator, and finally uses an optical filter to filter out the sideband signal to obtain the optical baseband signal. This approach relies on relatively few harmonics, resulting in relatively low phase noise, and the IF electro-optic modulator is relatively mature and low-cost, making it an alternative technology for terahertz communication system receivers.

[0005] However, as mentioned earlier, the optoelectronic hybrid downconversion relying on a small amount of electrical frequency multiplication still introduces some phase noise. Meanwhile, the high-order modulation formats used in ultra-high-speed transmission are highly sensitive to phase noise. To achieve the ultra-high-speed and ultra-reliable transmission requirements of terahertz communication, it is necessary to further reduce the phase noise generated by downconversion in terahertz receivers.

[0006] In recent years, with the rapid development of microwave photonics, the integration of optical and wireless technologies has become a research hotspot. Microwave photonics combines the advantages of high bandwidth, large transmission capacity, and low loss in optical fiber communication with the greater flexibility in wireless communication networking, making it of considerable research value for high-speed, high-bandwidth information transmission. In particular, optical frequency combs, one of the key technologies in microwave photonics, have become an important technology in terahertz communication. With the help of optical frequency combs, not only can high-quality terahertz signals be generated at the transmitting end, but ultra-low phase noise down-conversion can also be achieved at the receiving end. However, generating high-quality optical frequency combs usually requires complex system design and is costly. Therefore, it is necessary to simplify the acquisition of optical comb lines, simplify the structure of fiber optic terahertz receivers, and reduce costs. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of the prior art and provide a receiver device for fiber optic terahertz communication systems, reducing the complexity of signal processing algorithms and the complexity of receiver structure.

[0008] To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows:

[0009] A receiver device for fiber optic terahertz communication systems includes: a laser source, a microwave photonic downconversion circuit, and an intermediate frequency electro-optic conversion circuit;

[0010] The laser source is used to generate local oscillator light, which is split into two paths by a beam splitter. One path is input to a microwave photonic downconversion circuit as a modulation signal, which is used by an electro-optic modulator to generate high-order sidebands through nonlinear modulation. The other path is input to an intermediate frequency electro-optic conversion circuit, which is used to convert the downconverted to intermediate frequency received signal into an optical baseband signal.

[0011] The microwave photonic downconversion circuit includes a radio frequency source, an electro-optic modulator, two optical filters, a wavelength division multiplexing signal coupler, a photodetector, a low-noise power amplifier, a mixer, and two electrical filters.

[0012] In this invention, the electro-optic modulator, under the control of the modulation signal, generates multi-order sideband signals with frequency intervals equal to the modulation signal frequency according to a nonlinear modulation method. These signals are then filtered out by two optical bandpass filters to extract the two required sidebands, coupled into a single signal by a wavelength division multiplexing coupler, and input to a photodetector for beat frequency analysis. The signal is then filtered out by an electrical filter to obtain the desired frequency, resulting in a terahertz radio frequency (RF) source. A low-noise power amplifier amplifies the RF source, which is then mixed with the terahertz received signal via a mixer. After mixing, an electrical filter extracts the desired intermediate frequency (IF) signal, completing the down-conversion of the RF received signal to obtain an electrical IF signal. Compared to traditional electrical frequency multiplication, the RF source generated by this invention has lower phase noise. Furthermore, because the output signal of the electro-optic modulator can be flexibly controlled by the modulation signal, this invention is tunable and can achieve flexible and variable mixing based on the characteristics of the actual received signal.

[0013] The intermediate frequency electro-optic converter, under the action of the local oscillator light generated by the laser, converts the electrical intermediate frequency signal generated during the microwave photonic down-conversion process into an optical baseband signal, which is then transmitted via optical fiber to the terminal for photoelectric conversion and digital signal processing. Compared with traditional terahertz signal down-conversion methods, due to the ultra-low noise characteristics of microwave photonic down-conversion, this invention requires lower standards for digital signal processing algorithms to achieve the same bit error rate performance, which is beneficial for achieving key indicators such as 6G ultra-high speed and ultra-low latency.

[0014] Furthermore, the intermediate frequency electro-optic conversion circuit includes a phase modulator and an optical filter. Under the action of the local oscillator light generated by the laser, the phase modulator converts the intermediate frequency signal into an optical baseband signal, which is then transmitted to the terminal via optical fiber for photoelectric conversion and digital signal processing.

[0015] Furthermore, the output optical signal of the phase modulator of the intermediate frequency electro-optic conversion circuit contains a series of spectral lines. The interval of this series of spectral lines is equal to the frequency of the modulation signal and they are symmetrically distributed on both sides of the optical carrier. There are two spectral lines whose interval is equal to the frequency of the required terahertz radio frequency source.

[0016] Furthermore, the beat frequency of the photodetector in the microwave photonic downconversion circuit refers to the conversion of optical signals into electrical signals, the frequency of which contains the frequency of the desired terahertz radio frequency source.

[0017] The technical solution provided by this invention brings at least the following beneficial effects:

[0018] (1) Compared with the electric frequency doubling method, the radio frequency source generated by microwave photonics in this invention has extremely low phase noise and can flexibly adjust the frequency according to the actual reception situation.

[0019] (2) Compared with the all-optical downconversion method, the present invention has lower requirements for the fabrication of electro-optic modulator and lower cost;

[0020] (3) Compared with the high flatness optical frequency comb generation technology, the present invention does not require complex system design, and only needs to adjust the frequency of the modulation signal to generate two appropriately spaced sidebands;

[0021] (4) The microwave photonic downconversion system only focuses on the sideband spacing generated by the electro-optic modulator and does not care about the center frequency of the optical carrier. Therefore, photoelectric conversion and electro-optic conversion can share a single light source, thereby reducing the power consumption and weight of the receiver. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 This is a schematic diagram of the structure of a receiver device for an optical fiber terahertz communication system provided in an embodiment of the present invention;

[0024] Figure 2 This is a schematic diagram of a Bessel function of the first kind;

[0025] Figure 3 yes Figure 1 The photoelectric signal spectrum diagram of the structure shown and the schematic diagram of the principle of the embodiment of the present invention;

[0026] Figure 4 This is a schematic diagram of a terahertz radio frequency source generated by electrical frequency multiplication. In this diagram, 4(a) is a pure radio frequency source at 25 GHz, 4(b) is a frequency multiplied by 2 to 50 GHz, 4(c) is a frequency multiplied by 4 to 100 GHz, and 4(d) is a frequency multiplied by 8 to 200 GHz.

[0027] Figure 5 This is a schematic diagram of a terahertz radio frequency source generated by the present invention, wherein 5(a) is the time-domain waveform of the 250GHz radio frequency source and 5(b) is the power spectrum of the 250GHz radio frequency source. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.

[0029] For ease of description, the relevant technical terms appearing in the embodiments of this invention will be explained first:

[0030] OS (Optical Splitter): Optical Splitter;

[0031] PM (Phase Modulator): Phase modulator;

[0032] PD (Photoelectric Detector): A photoelectric detector;

[0033] OF (Optical Filter): Optical filter;

[0034] EF (Electric Filter): Electrical filter;

[0035] WDM (Wavelength Division Multiplexing): Wavelength division multiplexing;

[0036] WDMC (WDM Coupler): WDM coupler;

[0037] LNA (Low Noise Amplifier): Low noise amplifier.

[0038] This invention addresses the shortcomings of existing technologies by providing a low-phase-noise, low-cost receiver in a fiber optic terahertz communication system. It combines electro-optic modulator nonlinear modulation and beat frequency technology to generate a high-quality terahertz radio frequency source, which is then mixed with the received terahertz signal to achieve ultra-low noise down-conversion, reducing the complexity of subsequent digital signal processing algorithms. Furthermore, since this invention eliminates the need for a high-flatness optical frequency comb, requiring only two high-order sidebands with a near-terahertz frequency spacing, it eliminates the need for complex equipment and boasts a simple structure. In addition, the laser source in this invention serves both to generate the radio frequency source required for mixing and to convert the mixed intermediate frequency electrical signal into an optical baseband signal via an electro-optic modulator, offering cost-effectiveness.

[0039] like Figure 1As shown in the figure, a receiver device for an optical fiber terahertz communication system provided by an embodiment of the present invention includes: a laser source, a microwave photonic downconversion circuit, and an intermediate frequency electro-optic conversion circuit; wherein, the laser source is used to generate local oscillator light, which is split into two paths by a beam splitter OS, one path is used for nonlinear modulation by an electro-optic modulator to generate higher-order sidebands, and the other path is used to convert the downconverted to intermediate frequency received signal into an optical baseband signal, that is, one path is input to the microwave photonic downconversion circuit, and the other path is input to the intermediate frequency electro-optic conversion circuit. The microwave photonic down-conversion circuit includes a radio frequency (RF) source, an electro-optic modulator (PD), two optical filters (OF), a WDM signal coupler, a photodetector (PD), a low-noise power amplifier (LNA), a mixer, and two electrical filters (EF). The electro-optic modulator, under the control of the pure RF source (modulation signal), generates n (preset)-order sidebands according to the nonlinear modulation principle. The two desired sidebands are filtered out by two optical bandpass filters, coupled into a single signal by the WDM coupler, and input to the photodetector for beat frequency generation. The desired frequency, i.e., the terahertz RF source, is then filtered out by the electrical filters. The terahertz RF source is amplified by the LNA and then mixed with the received terahertz signal (received via a THz receiving antenna) by the mixer. After mixing, the desired intermediate frequency (IF) signal is filtered out by the electrical filters, ultimately achieving down-conversion of the terahertz received signal. The intermediate frequency electro-optic conversion circuit includes a phase modulator (PM) and an optical filter (OF). Under the action of the local oscillator light generated by the laser, the phase modulator converts the electrical intermediate frequency signal generated by the microwave photonic down-conversion process into an optical baseband signal and transmits it to the terminal for photoelectric conversion and digital signal processing via optical fiber.

[0040] This invention provides a receiver device for fiber optic terahertz communication systems. It utilizes the linear electro-optic effect (Pockles effect) of a phase modulator to reflect voltage changes in the radio frequency drive signal onto changes in the optical carrier phase. By changing the frequency of the modulation signal, the sideband frequency spacing of the electro-optic modulator output is adjusted, resulting in two sidebands with a spacing close to terahertz. Two optical filters are used to filter out these two sidebands, which are then coupled into a single signal via a WDM coupler and input to a photodetector to achieve optical heterodyne beat frequency. Theoretically, the frequency of the output electrical signal includes the difference and sum of the frequencies of the two sidebands, as well as the difference and sum of the frequencies of the two sidebands and themselves. Since the photodetector has a certain bandwidth limitation, it is assumed to support signal output with a bandwidth of several hundred GHz, such as a Schottky photodetector. Therefore, the spectrum of the output electrical signal contains only a zero frequency and a terahertz frequency. An electrical filter is used to filter out this terahertz frequency, thereby achieving the conversion of the optical signal to a low-phase-noise terahertz radio frequency source. This pure radio frequency source is mixed with the terahertz received signal, and the intermediate frequency signal is filtered out to complete the low-phase-noise down-conversion. Meanwhile, using the same laser source as the optical carrier, another phase modulator is used to convert the intermediate frequency signal into an optical baseband signal for optical fiber transmission.

[0041] Example

[0042] In this embodiment, as Figure 1 As shown, low-phase-noise and low-cost terahertz signal reception is achieved at the receiver end of a fiber optic terahertz communication system. Specifically, if the frequency of the terahertz signal received by the receiving antenna is f... R =260GHz, the ultimate goal is to downconvert the received signal to an intermediate frequency f IF =10GHz, and downconversion is achieved through mixing, then the frequency of the terahertz radio frequency source generated by the microwave photonic downconversion system should be f. THz =250GHz.

[0043] To generate a precise 250GHz RF source, the order of the sidebands to be filtered out and the corresponding modulation signal frequency need to be designed in advance. According to the mathematical principles of phase modulators, when the incident optical carrier's optical field is E... in =E c cos(ω c t), the electric field of the applied single-tone modulated signal is V = V m sin(ω m If t), then the output light field can be expressed as

[0044] E out =E c cos(ω c t+Qsin(ω m t)) (1)

[0045] Among them, E c ω represents the maximum electric field intensity of the incident optical carrier. c ω represents the angular frequency of the incident optical carrier. m Let Q represent the angular frequency of the modulating signal, t represent time, and Q = π·V. m / V π It is the modulation index of the modulated signal, V m V represents the maximum voltage of the modulated signal. π This is the half-wave voltage of the phase modulator. Expanding the above equation using Bessel functions yields...

[0046]

[0047] Among them, J n (Q) represents the value of the first-order n-th order Bessel function when the independent variable is equal to the frequency modulation exponent Q. For specific values, please refer to the Bessel function table, such as... Figure 2 As shown. Taking the Fourier transform of the above equation yields...

[0048]

[0049] Therefore, the spectrum of the phase modulator output signal contains a series of components, where the optical carrier frequency f is the component when n = 0. c Symmetrically distributed on both sides of the optical carrier frequency are spaced f. m The sidebands. If the phase modulator can generate at least k-order sidebands, select k-order sidebands f1 and f2 (f2 > f1) on both sides of the optical carrier frequency such that f2 - f1 = f THz Then the frequency of the modulating signal should be f. m = (f2-f1) / (2k). Since generating radio frequency sources in the tens of GHz range is relatively easy to achieve, f should be minimized as much as possible. m The numerical values ​​are simple, therefore, to obtain the aforementioned 250GHz terahertz RF source, a 5th-order sideband can be selected, at which point f... m =25GHz.

[0050] After the phase modulator generates the two required sidebands, two optical bandpass filters are used to filter out f1 and f2 respectively. These are then input to a WDM coupler to couple the two frequencies into a single optical signal. This optical signal is then input to a photodetector to achieve beat frequency, and the output electrical signal frequency will contain f2 - f1 = f2. THz f2–f2=f1–f1=0, etc., using a bandpass filter to reduce f THz The frequency can be filtered out. Since the terahertz radio frequency source obtained through the aforementioned steps has low power, a low-noise amplifier can be used for amplification. The resulting high-quality terahertz radio frequency source is then mixed with the terahertz signal received from the antenna; the frequency of the output signal will contain f. IF =f R -f THz =10GHz, etc., will the intermediate frequency f IF Filtering completes the down-conversion of the 260GHz received signal. Then, using the same laser source as the optical carrier, an electro-optic modulator modulates the intermediate frequency electrical signal onto the optical carrier, allowing it to be transmitted via optical fiber to the terminal for digital signal processing. The specific implementation principle of the above steps is as follows: Figure 3 As shown.

[0051] To highlight the extremely low phase noise advantage of the down-conversion method in this invention, a comparison was made between a terahertz radio frequency source generated by electrical frequency multiplication and the terahertz radio frequency source generated in the present invention. Figure 4 and Figure 5 .in, Figure 4 (a) is a clean radio frequency source with a frequency of 25 GHz. Figure 4 (b)-4(d) show the radio frequency sources obtained by multiplying the frequency by 2, 4, and 8, respectively. Figure 4It can be seen that as the frequency multiplication factor increases, the jitter in the time-domain waveform and power spectrum of the RF source becomes increasingly pronounced. This indicates that frequency multiplication accumulates phase noise, and the higher the frequency multiplication factor, the greater the phase noise. Furthermore, electronic frequency multipliers composed of nonlinear resistors typically have high frequency multiplication noise, which becomes even greater after accumulation. Conversely, if... Figure 5 As shown, the time-domain waveform of the terahertz radio frequency source generated by this invention has almost no jitter, and its power spectrum shows that the noise is extremely small.

[0052] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

[0053] The above descriptions are merely some embodiments of the present invention. Those skilled in the art can make various modifications and improvements without departing from the inventive concept of the present invention, and these all fall within the scope of protection of the present invention.

Claims

1. A receiver device for fiber optic terahertz communication systems, characterized in that, include: One laser source, one microwave photonic downconversion circuit, and one intermediate frequency electro-optic conversion circuit; The laser source is used to generate local oscillator light, which is split into two paths by a beam splitter. One path is input to a microwave photonic downconversion circuit as a modulation signal, which is used by an electro-optic modulator to generate high-order sidebands through nonlinear modulation. The other path is input to an intermediate frequency electro-optic conversion circuit, which is used to convert the downconverted to intermediate frequency received signal into an optical baseband signal. The microwave photonic downconversion circuit includes a radio frequency source, an electro-optic modulator, two optical filters, a wavelength division multiplexing signal coupler, a photodetector, a low-noise power amplifier, a mixer, and two electrical filters. In this process, an electro-optic modulator uses a radio frequency (RF) source as the modulation signal. Under the control of the modulation signal, the electro-optic modulator generates multi-order sideband signals with frequency intervals equal to the modulation signal frequency according to a nonlinear modulation method. Then, after filtering out the two required sidebands through two optical filters, the signals are coupled into a single signal through a wavelength division multiplexing coupler and input into a photodetector for beat frequency. The desired frequency is then filtered out by an electrical filter to obtain the terahertz RF source. The terahertz RF source is amplified using a low-noise power amplifier and then mixed with the terahertz received signal through a mixer. After mixing, the desired intermediate frequency (IF) signal is filtered out by an electrical filter, completing the down-conversion of the terahertz received signal to obtain the electrical IF signal. The intermediate frequency electro-optic conversion circuit converts the electrical intermediate frequency signal generated during the microwave photonic down-conversion process into an optical baseband signal under the action of the local oscillator light generated by the laser source. The signal is then transmitted to the terminal via optical fiber for photoelectric conversion and digital signal processing.

2. The receiver device as claimed in claim 1, characterized in that, The intermediate frequency electro-optic conversion circuit includes a phase modulator and an optical filter. Under the action of the local oscillator light generated by the laser source, the phase modulator converts the intermediate frequency signal into an optical baseband signal, which is then transmitted to the terminal via optical fiber for photoelectric conversion and digital signal processing.

3. The receiver device as described in claim 2, characterized in that, The output optical signal of the phase modulator of the intermediate frequency electro-optic conversion circuit contains a series of spectral lines. The interval of the series of spectral lines is equal to the frequency of the modulation signal and they are symmetrically distributed on both sides of the optical carrier. There are two spectral lines whose interval is equal to the frequency of the required terahertz radio frequency source.

4. The receiver device as claimed in claim 1, characterized in that, The beat frequency of the photodetector in the microwave photonic downconversion circuit refers to the conversion of optical signals into electrical signals, the frequency of which contains the frequency of the desired terahertz radio frequency source.

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