CN116667939A - A receiver device for optical fiber terahertz communication system - Google Patents

Abstract

The invention discloses a receiver device for an optical fiber terahertz communication system, belonging to the technical field of optical fiber terahertz communication. The present invention combines the nonlinear modulation of the electro-optical modulator and the beat frequency technology to generate a high-quality terahertz radio frequency source and terahertz receiving signal for frequency mixing to realize ultra-low noise down-conversion and reduce the complexity of subsequent digital signal processing algorithms . At the same time, since the present invention does not need to generate an optical frequency comb with high flatness, it only needs two high-order sidebands with a frequency interval of nearly terahertz, no complex equipment is required, and the structure is simple. In addition, the laser source in the present invention is used not only to generate the radio frequency source required for frequency mixing, but also to convert the mixed intermediate frequency electrical signal into an optical baseband signal through the electro-optical modulator, which has certain cost-effectiveness.

Description

A receiver device for optical fiber terahertz communication system

technical field

The invention belongs to the technical field of optical fiber terahertz communication, and more specifically relates to a low-phase-noise low-cost receiver in an optical fiber terahertz communication system.

Background technique

With the large-scale commercial deployment of the fifth-generation mobile network (5G), the exploration and research on the next-generation mobile communication (6G) has begun. The "6G Overall Vision and Potential Key Technologies White Paper" issued by the IMT-2030 6G Promotion Group in June 2021 points out that 6G is envisaged to bring various new types of communication services, including holographic communication, high-quality video online conferencing, augmented reality/ Virtual reality, etc., put forward higher requirements for network KPIs such as data rate, delay, and number of connections. In particular, in order to achieve a communication rate of terabits per second, communication in the terahertz frequency band (0.1THz-10THz) is considered to be an important air interface technical solution for future 6G networks, and has received extensive attention in recent years.

In September of the same year, IMT-2030 released the "Research Report on Terahertz Communication Technology", which introduced the basic principles, application scenarios, core devices and key technologies of terahertz communication at home and abroad, and clearly pointed out that terahertz communication can be As a beneficial supplement to the existing air interface transmission method, it may be applied to various 6G large-capacity and ultra-high-speed transmission scenarios. However, there are still many problems to be solved urgently, including the design of the receiving scheme in the terahertz communication system.

In a communication system, in order to perform subsequent digital signal processing, it is first necessary to down-convert the high-frequency received signal to baseband at the receiving end. The existing terahertz communication system receiving end down-conversion technology can be divided into three categories, one is all-electric down-conversion technology, the other is all-optical down-conversion technology, and the third is photoelectric down-conversion technology. The patent application with the publication number "CN113890629A" discloses a terahertz signal receiving device, method and signal transmission system, which mixes the low-frequency electric radio frequency source with the terahertz received signal after multiple frequency multiplication and filters out the high frequency signal. frequency signal to realize down-conversion of terahertz signal. However, in this technical route, the RF source will introduce large phase noise after multiple frequency multiplication. The document "THz-to-optical conversion in wireless communications using an ultra broadband plasmonic modulator. Nature Photonics, 2019, pp.519-524" uses an ultra-high-speed electro-optical modulator to directly modulate the terahertz received signal onto the optical carrier, and then optically filter it The filter filters out the sideband signal to obtain the optical baseband signal. However, since there is currently no mature preparation technology for electro-optic modulators in the terahertz frequency band, this technical route is not applicable. In order to overcome the shortcomings of the above two solutions, the down-conversion technology of photoelectric hybrid mode is proposed. The document "Fiber-THz-Fiber Link for THz Signal Transmission. IEEE Photonics Journal, 2018, pp.1-6" first down-converts the received THz signal to an intermediate frequency by means of electrical mixing, and then modulates the intermediate frequency signal through an electro-optic modulator to the optical carrier, and finally use an optical filter to filter out the sideband signal to obtain an optical baseband signal. This technical route relies on relatively little frequency multiplication, which brings relatively small phase noise, and the intermediate frequency electro-optic modulator is relatively mature and low in cost, so it is an alternative technology for the receiver of the terahertz communication system plan.

However, as mentioned above, the photoelectric hybrid down-conversion that relies on a small amount of electrical frequency multiplication will still bring a certain amount of phase noise. At the same time, the high-order modulation formats used for ultra-high-speed transmission are sensitive to phase noise. In order 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 down-conversion in terahertz receivers.

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

Contents of the invention

The purpose of the present invention is to overcome the deficiencies of the prior art, provide a receiver device for an optical fiber terahertz communication system, reduce the complexity of the signal processing algorithm, and reduce the complexity of the receiver structure.

For realizing above-mentioned purpose of the invention, the technical scheme that the present invention adopts is:

A receiver device for an optical fiber terahertz communication system, comprising: a laser source, a microwave photon down conversion circuit and an intermediate frequency electro-optic conversion circuit;

The laser source is used to generate local oscillator light, which is divided into two paths by an optical splitter, one path is input into a microwave photon down-conversion circuit as a modulation signal, which is used for nonlinear modulation of an electro-optical modulator to generate high-order sidebands, and one path is input into an intermediate frequency electro-optical conversion circuit. A circuit for converting a received signal converted from down-converted to an intermediate frequency into an optical baseband signal;

The microwave photon down-conversion circuit includes a radio frequency source, an electro-optical modulator, two optical filters, a wavelength division multiplexing signal coupler, a photodetector, a low-noise power amplifier, a mixer and two electric filter;

Among them, under the control of the modulation signal, the electro-optic modulator generates a multi-order sideband signal with a frequency interval equal to the frequency of the modulation signal according to the nonlinear modulation method, and then filters out the required two sidebands through two optical bandpass filters After that, it is coupled into a signal by a wavelength division multiplexing coupler and input to a photodetector for beating frequency, and then the required frequency is filtered out by an electric filter to obtain a terahertz radio frequency source; the terahertz radio frequency source is obtained by using a low-noise power amplifier. The radio frequency source is amplified, and then mixed with the terahertz receiving signal through a mixer, and then the required intermediate frequency signal is filtered out by an electric filter, and the down conversion of the terahertz signal receiving signal is completed to obtain an electrical intermediate frequency signal; Compared with the traditional electrical frequency doubling method, the phase noise of the terahertz radio frequency source generated by the present invention is lower, and since the output signal of the electro-optical modulator can be flexibly adjusted through the modulation signal, the present invention has tunability and can be adjusted according to the actual received signal The characteristics of flexible and variable mixing;

The intermediate frequency electro-optic converter converts the electrical intermediate frequency signal generated by the microwave photon down-conversion process into an optical baseband signal under the action of the local oscillator light generated by the laser, and transmits it to the terminal through optical fiber for photoelectric conversion and digital signal processing. Compared with the traditional terahertz signal down-conversion method, due to the ultra-low noise characteristics of microwave photon down-conversion, in order to achieve the same bit error rate performance, the present invention has lower requirements for digital signal processing algorithms, which is very important for the realization of 6G super Key indicators such as high speed and ultra-low latency are beneficial.

Further, the intermediate frequency electro-optical 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 electrical intermediate frequency signal into an optical baseband signal, and transmits it to the terminal through an optical fiber for further processing. Photoelectric conversion and digital signal processing.

Further, 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 symmetrically distributed on both sides of the optical carrier, and there is an interval of two spectral lines Equal to the frequency of the desired terahertz RF source.

Further, the photodetector beat frequency of the microwave photon down-conversion circuit refers to converting an optical signal into an electrical signal, and the frequency of the electrical signal includes the frequency of the desired terahertz radio frequency source.

The technical solution provided by the present invention brings at least the following beneficial effects:

(1) Compared with the electric frequency doubling method, the radio frequency source generated by microwave photonics in the present invention has extremely low phase noise, and the frequency can be flexibly adjusted according to the actual receiving situation;

(2) Compared with the all-optical down-conversion method, the present invention has lower requirements for the preparation of the electro-optic modulator and lower cost;

(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 sidebands with suitable intervals;

(4) The microwave photon down-conversion system only pays attention to the sideband interval generated by the electro-optical modulator, and does not care about the center frequency of the optical carrier. Therefore, the photoelectric conversion and electro-optical conversion can share a light source, thereby reducing the power consumption and weight of the receiver.

Description of drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings that need to be used in the description of the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present invention. For those skilled in the art, other drawings can also be obtained based on these drawings without creative effort.

Fig. 1 is a schematic structural diagram of a receiver device for an optical fiber terahertz communication system provided by an embodiment of the present invention;

Fig. 2 is the schematic diagram of Bessel function of the first kind;

Fig. 3 is the photoelectric signal spectrum diagram in the structure shown in Fig. 1 and the schematic diagram of the principle of the embodiment of the present invention;

Figure 4 is a schematic diagram of a terahertz radio frequency source generated by electrical frequency doubling, where 4(a) is a 25GHz pure radio frequency source, 4(b) is a frequency multiplied by 2 to 50GHz, 4(c) is a frequency multiplied by 4 to 100GHz, and 4 (d) is 8 multiplied to 200GHz;

Fig. 5 is a schematic diagram of a terahertz radio frequency source generated by the present invention, wherein 5(a) is a time-domain waveform of a 250GHz radio frequency source, and 5(b) is a power spectrum of a 250GHz radio frequency source.

Detailed ways

In order to make the purpose, technical solution and advantages of the present invention clearer, the following will further describe in detail the embodiments of the present invention in conjunction with the accompanying drawings.

For the convenience of description, first, the relevant technical terms appearing in the embodiments of the present invention are explained:

OS (Optical Splitter): optical splitter;

PM (Phase Modulator): phase modulator;

PD (Photoelectric Detector): photoelectric detector;

OF(Optical Filter): optical filter;

EF (Electric Filter): electric filter;

WDM (Wavelength Division Multiplexing): wavelength division multiplexing;

WDMC (WDM Coupler): WDM coupler;

LNA (Low Noise Amplifier): Low Noise Amplifier.

The embodiment of the present invention aims at the deficiencies of the prior art, and provides a low-phase-noise low-cost receiver in an optical fiber terahertz communication system, which generates high-quality terahertz radio frequency through the combination of electro-optical modulator nonlinear modulation and beat frequency technology The source and the terahertz receiving signal are mixed to realize ultra-low noise down-conversion and reduce the complexity of subsequent digital signal processing algorithms. At the same time, since the present invention does not need to generate an optical frequency comb with high flatness, it only needs two high-order sidebands with a frequency interval of nearly terahertz, no complex equipment is required, and the structure is simple. In addition, the laser source in the present invention is used not only to generate the radio frequency source required for frequency mixing, but also to convert the mixed intermediate frequency electrical signal into an optical baseband signal through the electro-optical modulator, which has certain cost-effectiveness.

As shown in Figure 1, a receiver device oriented to an optical fiber terahertz communication system provided by an embodiment of the present invention includes: a laser source, a microwave photon down-conversion circuit and an intermediate frequency electro-optical conversion circuit; wherein the laser source is used to generate The local oscillator is divided into two channels by the optical splitter OS, one is used for the nonlinear modulation of the electro-optical modulator to generate high-order sidebands, and the other is used to convert the received signal from the down-converted frequency to the intermediate frequency into an optical baseband signal, that is, the other input microwave photons The down-conversion circuit, and the other input intermediate frequency electro-optical conversion circuit. The microwave photonic down-conversion circuit includes a radio frequency source, an electro-optical modulator PD, two optical filters OF, a WDM signal coupler, a photodetector PD, a low-noise power amplifier LNA, a mixer, two electric filter EF; wherein, under the control of a pure radio frequency source (modulation signal), the electro-optical modulator generates n (preset value) order sidebands according to the principle of nonlinear modulation, and the two required two optical bandpass filters After the two sidebands are filtered out, the WDM coupler is coupled into a signal and input to the photodetector to realize the beat frequency, and then the required frequency is filtered out by the electric filter, that is, the terahertz radio frequency source. Use a low-noise power amplifier to amplify the terahertz radio frequency source, and then mix it with the terahertz receiving signal (the terahertz receiving signal received by the THz receiving antenna) through a mixer, and then use an electric filter to convert the required The intermediate frequency signal is filtered out, and finally the down-conversion of the terahertz received signal is realized. The intermediate frequency electro-optical 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 photon down-conversion process into an optical baseband signal and transmits it through the optical fiber It is transmitted to the terminal for photoelectric conversion and digital signal processing.

The embodiment of the present invention provides a receiver device for an optical fiber terahertz communication system, which uses the linear electro-optical effect (Pockles effect) of the phase modulator to reflect the voltage change of the radio frequency driving signal to the change of the optical carrier phase. By changing the frequency of the modulation signal, the frequency interval of the sidebands output by the electro-optic modulator is adjusted, so as to obtain two sidebands whose interval is close to terahertz. Two optical filters are used to filter out the above two sidebands respectively, and after being coupled into one signal by a WDM coupler, it is input to a photodetector to realize optical heterodyne beat frequency. In theory, the frequency of the output electrical signal includes the difference and sum of the above two sideband frequencies, and the difference and sum of the two sideband frequencies and itself. Since the photodetector has a certain bandwidth limitation, it is assumed that it can support a signal output with a bandwidth of several hundred GHz, such as based on a Schottky photodetector. Therefore, the spectrum of the output electrical signal only contains a zero frequency and a terahertz frequency, and the electrical filter is used to filter out the terahertz frequency to realize the conversion of the optical signal to a low-phase-noise terahertz radio frequency source. The pure radio frequency source is mixed with the terahertz received signal and the intermediate frequency signal is filtered out to complete the down-conversion with low phase noise. At the same time, 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.

Example

In this embodiment, as shown in FIG. 1 , at the receiving end of the optical fiber terahertz communication system, low-phase-noise and low-cost terahertz signal reception is realized. Specifically, if the frequency of the terahertz signal received by the receiving antenna is f _R =260 GHz, the ultimate goal is to down-convert the received signal to an intermediate frequency f _IF =10 GHz, and realize the down-conversion by frequency mixing, then the microwave photonic down-conversion system produces The frequency of the terahertz radio frequency source should be f _THz =250GHz.

In order to generate an accurate 250GHz RF source, it is necessary to design in advance the order of the sidebands to be filtered out and the corresponding modulation signal frequency. According to the mathematical principle of the phase modulator, when the optical field of the incident optical carrier is E _in =E _c cos(ω _c t), and the electric field of the external monotone modulation signal is V=V _m sin(ω _m t), the output The light field can be expressed as

E _out ＝E _c cos(ω _c t+Qsin(ω _m t)) (1)

Among them, E _c represents the maximum electric field strength of the incident optical carrier, ω _c represents the angular frequency of the incident optical carrier, ω _m represents the angular frequency of the modulation signal, t represents the time, Q=π V _m /V _π is the modulation of the modulation signal The index, V _m represents the maximum voltage of the modulating signal, and V _π is the half-wave voltage of the phase modulator. Using the Bessel function to expand the above formula, we can get

Among them, J _n (Q) represents the function value of the first nth-order Bessel function when the independent variable is equal to the frequency modulation index Q, and its specific value can be found in the Bessel function table, as shown in Figure 2. Perform Fourier transform on the above formula to get

It can be seen that the frequency spectrum of the output signal of the phase modulator contains a series of components. When n=0, it is the optical carrier frequency f _c , and side frequencies with an interval of f _m are symmetrically distributed on both sides of the optical carrier frequency. If the phase modulator can generate at least k-order sidebands, select the k-order sidebands f ₁ and f ₂ (f ₂ >f ₁ ) on both sides of the optical carrier frequency so that f ₂ -f ₁ =f _THz , then the frequency of the modulated signal It should be f _m =(f ₂ -f ₁ )/(2k). Since it is relatively easy to generate a radio frequency source of tens of GHz, and the value of f _m should be kept as simple as possible, the 5th order sideband can be selected to obtain the aforementioned 250 GHz terahertz radio frequency source, and f _m = 25 GHz at this time.

After the phase modulator produces the required two sidebands, use two optical bandpass filters to filter out _f1 and _f2 respectively, and then input the WDM coupler to couple the two frequencies into an optical signal. After the optical signal is input into the photodetector to realize the beating frequency, the frequency of the output electrical signal will contain f ₂ -f ₁ =f _THz , f ₂ -f ₂ =f ₁ -f ₁ =0, etc., use a band-pass filter to convert f The frequency of _THz can be filtered out. Since the power of the terahertz radio frequency source obtained through the preceding steps is low, it can be amplified by a low-noise amplifier. Mix the obtained high-quality terahertz radio frequency source with the terahertz signal received from the antenna, the frequency of the output signal will contain f _IF =f _R –f _THz =10GHz, etc., and the intermediate frequency f _IF is filtered out to complete the 260GHz Down-conversion of the received signal. Then, using the same laser source as the optical carrier, the intermediate frequency electrical signal is modulated onto the optical carrier by an electro-optical modulator, and then transmitted to the terminal through optical fiber for digital signal processing. The specific implementation principle of the above steps is shown in FIG. 3 .

In order to highlight the advantages of extremely low phase noise of the down-conversion of the present invention, the terahertz radio frequency source generated by the electric frequency multiplication method is compared with the terahertz radio frequency source generated in the solution of the present invention, as shown in Fig. 4 and Fig. 5 . Among them, Figure 4(a) is a pure radio frequency source with a frequency of 25GHz, and Figures 4(b)-4(d) are the radio frequency sources obtained by multiplying the frequency by 2, 4 and 8 respectively. It can be seen from Fig. 4 that with the increase of frequency doubling times, the jitter of the time-domain waveform and power spectrum of the RF source becomes more and more obvious. This shows that frequency multiplication will accumulate phase noise, and the more times of frequency multiplication, the greater the phase noise. Moreover, the frequency multiplication noise of the electronic frequency multiplier composed of non-linear resistors is usually relatively large, and will be even greater after accumulation. On the contrary, as shown in FIG. 5 , the time-domain waveform of the terahertz radio frequency source generated by the present invention has almost no jitter, and it can be seen from its power spectrum that the noise is extremely small.

Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: it can still be Modifications are made to the technical solutions described in the foregoing embodiments, or equivalent replacements are made to some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the various embodiments of the present invention.

What have been described above are only some embodiments of the present invention. For those skilled in the art, without departing from the inventive concept of the present invention, several modifications and improvements can be made, and these all belong to the protection scope of the present invention.

Claims (4)

1. A receiver apparatus for an optical fiber terahertz communication system, comprising: a laser source, a microwave photon down-conversion circuit and an intermediate frequency electro-optic conversion circuit;

the laser source is used for generating local oscillation light, the local oscillation light is divided into two paths through the optical splitter, one path of the local oscillation light is input into the microwave photon down-conversion circuit to serve as a modulation signal, the local oscillation light is used for generating a high-order sideband through nonlinear modulation of the electro-optic modulator, and the other path of the local oscillation light is input into the intermediate frequency electro-optic conversion circuit to convert a received signal which is down-converted to an intermediate frequency into an optical baseband signal;

the microwave photon down-conversion circuit comprises a radio frequency source, an electro-optic modulator, two optical filters, a wavelength division multiplexing signal coupler, a photoelectric detector, a low-noise power amplifier, a mixer and two electric filters;

under the regulation and control of a modulation signal, the electro-optical modulator generates a multi-order sideband signal with a frequency interval being the frequency of the modulation signal according to a nonlinear modulation mode, then two required sidebands are filtered out through two optical band pass filters, coupled into a signal through a wavelength division multiplexing coupler and input into a photoelectric detector for beat frequency, and then the required frequency is filtered out through an electric filter, so that a terahertz radio frequency source is obtained; amplifying the terahertz radio frequency source by using a low-noise power amplifier, mixing the terahertz radio frequency source with a terahertz receiving signal through a mixer, filtering out a required intermediate frequency signal through an electric filter after mixing, and completing down-conversion of the terahertz receiving signal to obtain an electric intermediate frequency signal;

the intermediate frequency electro-optic converter converts an electric intermediate frequency signal generated in the microwave photon down-conversion process into an optical baseband signal under the action of local oscillation light generated by the laser, and transmits the optical baseband signal to the terminal through the optical fiber for photoelectric conversion and digital signal processing.

2. The receiver apparatus of claim 1, wherein the intermediate frequency electro-optic conversion circuit comprises a phase modulator and an optical filter, the phase modulator converts the electrical intermediate frequency signal into an optical baseband signal under the action of local oscillation light generated by the laser, and the optical baseband signal is transmitted to the terminal via the optical fiber for photoelectric conversion and digital signal processing.

3. The receiver device of claim 1, wherein the output optical signal of the phase modulator of the intermediate frequency electro-optic conversion circuit comprises a series of spectral lines spaced apart by a distance equal to the frequency of the modulated signal and symmetrically distributed on either side of the optical carrier, and wherein the two spectral lines are spaced apart by a distance equal to the frequency of the desired terahertz radio frequency source.

4. The receiver device of claim 1, wherein the photodetector beat frequency of the microwave photon down-conversion circuit is: the optical signal is converted into an electrical signal, and the frequency of the electrical signal comprises the frequency of the terahertz radio frequency source.