CN100568779C - Downlink device of millimeter wave optical fiber transmission system and millimeter wave generation and information modulation method therein - Google Patents

Abstract

The invention relates to a downlink structure of a millimeter wave optical fiber transmission system and a millimeter wave generation and information modulation method therein. The downlink structure of this millimeter-wave optical fiber transmission system divides the single-frequency laser into two channels at the central station, performs phase modulation on one of the light waves with a low-frequency periodic microwave signal, and performs optical field amplitude modulation on the other light wave, and then transmits the The two-way light waves are superimposed and interfered, and transmitted to the base station through an optical fiber to be detected by a photodetector to generate a photocurrent. The current signal contains various harmonics of the low-frequency microwave signal, and a band-pass filter is used to extract the high-order harmonics into the desired millimeter wave. signal, and the millimeter wave signal has been modulated with information, so as to realize the downlink transmission of information from the central station to the base station. The link is simple in structure, easy to realize, stable in performance, low in cost and suitable for application.

Description

Downlink device of millimeter wave optical fiber transmission system and millimeter wave generation and information modulation method therein

technical field

The present invention relates to a downlink device of a millimeter-wave optical fiber transmission system and a millimeter-wave generation and information modulation method therein. A new downlink device of a millimeter-wave optical fiber transmission system is proposed here, in which a single-frequency laser is divided into two After different processing, the two signals are superimposed, and the required millimeter-wave signal can be output after optical fiber transmission to the base station after photoelectric conversion, and the millimeter-wave signal has been modulated by information, so that the information can be sent from the central station Downlink transmission to the base station.

Background technique

In recent years, radio frequency optical fiber transmission technology (RoF, Radio on Fiber) combines the advantages of optical fiber transmission frequency bandwidth, low loss, strong anti-interference and flexible wireless communication coverage. Fields of study such as communications become a very attractive solution.

In order to meet the increasing bandwidth requirements of end users, the radio frequency used in the radio frequency optical fiber transmission system must develop towards the high end of the spectrum. For example, the research on the application of the millimeter wave frequency band has attracted widespread attention. How to realize millimeter-wave transmission in optical fiber links is an important research topic of millimeter-wave optical fiber transmission technology (mmRoF, millimeter-waveRoF). One of the research focuses is how to realize information while generating millimeter-wave in the downlink. Transmission, so far, the international community has proposed a variety of schemes, mainly in the following categories:

1) Lightwave sideband modulation (modulation-side-band technique): The baseband signal modulates the millimeter wave, and then uses the millimeter wave to directly modulate the light wave. After reaching the base station, the difference frequency between the two sides of the light wave is used to obtain the millimeter wave. This method itself requires Millimeter wave oscillators, so millimeter waves are generated by electrical methods.

2) Dual-mode laser: The laser generates two frequencies, the difference between which is exactly the millimeter wave frequency.

3) Optical frequency modulation (phase modulation) combined with fiber dispersion (FM-modulated laser in conjunction with fiber dispersion): Use low-frequency microwave signals to modulate the frequency of the laser or modulate the phase of the light wave, and convert the phase change into an amplitude change through the dispersion fiber (FM-IM effect ), the amplitude changes include microwave signals and their high-order components, and the high-order harmonics are extracted as millimeter-wave signals. If the dispersion fiber is replaced by a comb filter, it is an optical frequency doubling method.

4) Harmonic upconversion in nonlinear lasers (harmonic upconversion in nonlinear lasers): Add low-frequency microwave signals to nonlinear lasers to excite high-order harmonics, and obtain the required millimeter waves at the difference frequency of the photodetector, and 3) Somewhat similar, but there are differences in principle.

5) Multiple optical source technique: The difference between the frequencies of the two light sources is the millimeter frequency, and the difference between the frequencies of the optical detectors is the millimeter wave.

Various technologies have their own advantages and disadvantages, and have their own unique application occasions and advantages, but most of them are costly and complex, which is not conducive to large-scale promotion and application.

Contents of the invention

The purpose of the present invention is to solve the problems of complex downlink system, high cost and difficult maintenance of the existing millimeter-wave optical fiber transmission system, and propose a millimeter-wave optical fiber transmission system and downlink device and a millimeter-wave generation and information modulation method therein. The link device is simple, easy to implement, stable in performance and low in cost, and is suitable for development and promotion of practical products.

To achieve the above object, the present invention adopts the following technical solutions:

A downlink device of a millimeter wave optical fiber transmission system, comprising a central station 1, a base station 2 and an optical fiber 3, the central station 1 and the base station 2 are interconnected through the optical fiber 3, and it is characterized in that: in the central station 1, a The laser 1-1 is connected to a polarizer 1-2 through a pigtail, and the polarizer 1-2 is connected to an input end of an optical splitter 1-3 through a polarization-maintaining fiber 1-9, and the optical The output end of the splitter 1-3 is connected to the input end of an optical phase modulator 1-4 through a polarization maintaining optical fiber 1-9, and the periodic signal source 1-5 is connected to the electrical input end of the optical phase modulator 1-4 , the output end of the optical phase modulator 1-4 is connected to an input end of a polarization maintaining coupler 1-6; the other output end of the optical splitter 1-3 is connected to an input end of an optical field amplitude modulator 1-7 Connected through polarization maintaining optical fiber 1-9, modulated signal 1-8 is connected from the electrical input end of optical field amplitude modulator 1-7, and the output end of optical field amplitude modulator 1-7 is connected to the polarization maintaining coupler 1- 6. The other input end is connected with a polarization maintaining optical fiber 1-9; the output end of the polarization maintaining coupler 1-6 is connected to the optical input end of the optical detector 2-1 of the base station 2 through an optical fiber 3; Among them, the electrical output end of the photodetector 2-1 is connected to the input end of a band-pass filter 2-2, and the output end of the band-pass filter 2-2 is connected to the input end of a millimeter wave amplifier 2-3, and the millimeter wave The output end of the amplifier 2-3 is connected with a millimeter wave transmitting antenna 3-4.

A millimeter-wave generation and information modulation method in the downlink of a millimeter-wave optical fiber transmission system, using the downlink device of the millimeter-wave optical fiber transmission system according to claim 1, characterized in that at the central station 1, the laser The laser output from 1-1 is divided into two paths, one path of light waves passes through the optical phase modulator 1-4, and the periodic signal source 1-5 is connected to the optical phase modulator 1-4; the other path of light waves passes through the optical field amplitude modulator 1-7 , the modulation signal 1-8 is connected to the electrical input terminal of the optical field amplitude modulator 1-7; then the output light wave of the optical phase modulator 1-4 and the output light wave of the optical field amplitude modulator 1-7 are superimposed and interfered, and then the synthesized wave is passed through The optical fiber 3 is sent to the photodetector 2-1, and the electrical output of the photodetector 2-1 is sent to the millimeter-wave bandpass filter 2-2. The present invention is described further below:

This scheme belongs to the optical phase (frequency) heterodyne technology, and the specific realization is as follows:

As shown in the figure, in the central station 1, the laser 1-1 sends a single-frequency laser to the polarizer 1-2 to generate a single polarization direction, and then it is divided into two light waves by the optical splitter, and the first route is a polarization-maintaining optical fiber. 1-9 to the optical phase modulator 1-4, the periodic signal source 1-5 generates a periodic signal (can be a sine-cosine wave, a sawtooth wave, a square wave, etc.) to modulate the phase of the optical phase modulator 1-4, and output to the polarization maintaining Coupler 1-6, the electric field expression of this phase modulation light wave is:

E ₁ (t)＝E _c exp[jω _c t+jβφ(t)+jΦ _PN ] (1)

Among them, E _c is the amplitude of the electric field of the light wave, ω _c is the central angular frequency of the light wave, β is the phase modulation index, φ(t) is the periodic signal generated by the periodic signal source 1-5, and Φ _PN is the random noise of the laser itself.

The second light wave is modulated (AM) by the optical field amplitude modulator 1-7, from the polarization maintaining fiber 1-9 to the polarization maintaining coupler 1-6. At this time, the electric field expression of the light wave is:

E ₂ (t)＝E _c kM(t)exp[jω _c (t-Δτ)+jΦ _PN ] (2)

Among them, E _c is the amplitude of the electric field of the light wave, k is the modulation index, M(t) is the modulation signal, this signal can be a baseband signal, or a microwave signal that has been modulated with baseband information, _ωc is the central angular frequency of the light wave, Δτ is the time delay difference after the light wave is divided into two channels, and Φ _PN is the random noise of the laser itself.

Since the two paths of light waves have undergone polarization-maintaining processing, the polarization directions are consistent, and the electric field of the synthesized light waves after being superimposed by the polarization-maintaining coupler 1-6 is:

E _out (t)＝E ₁ (t)+E ₂ (t)＝E _c exp[jω _c t+jβφ(t)+jΦ _PN ]+E _c M(t)exp[jω _c (t-Δτ) +jΦ _PN ] (3)

The synthesized light wave is transmitted to the base station 2 by the optical fiber 3, and the light detector 2-1 detects the light intensity to form a photocurrent I _d (t), the expression (where R, K, and K ₁ are proportional coefficients) is as follows:

${\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; RE</span>}}_{\mathrm{<span\; class="notranslate">\; out</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; out</span>}}{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span>}^{<span\; class="notranslate">\; *</span>}$

$<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; RE</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; \{</span>\mathrm{<span\; class="notranslate">\; exp</span>}<span\; class="notranslate">\; [</span>\mathrm{<span\; class="notranslate">\; j</span>}{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; j\&beta;\&phi;</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; j</span>}{\mathrm{<span\; class="notranslate">\; \&Phi;</span>}}_{\mathrm{<span\; class="notranslate">\; PN</span>}}<span\; class="notranslate">\; ]</span><span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; kM</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; exp</span>}<span\; class="notranslate">\; [</span>\mathrm{<span\; class="notranslate">\; j</span>}{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; \&Delta;\&tau;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; j</span>}{\mathrm{<span\; class="notranslate">\; \&Phi;</span>}}_{\mathrm{<span\; class="notranslate">\; PN</span>}}<span\; class="notranslate">\; ]</span><span\; class="notranslate">\; \}</span>$

$<span\; class="notranslate">\; \&times;</span>{\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; \{</span>\mathrm{<span\; class="notranslate">\; exp</span>}<span\; class="notranslate">\; [</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; j</span>}{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; j\&beta;\&phi;</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; j</span>}{\mathrm{<span\; class="notranslate">\; \&Phi;</span>}}_{\mathrm{<span\; class="notranslate">\; PN</span>}}<span\; class="notranslate">\; ]</span><span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; kM</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; exp</span>}<span\; class="notranslate">\; [</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; j</span>}{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; \&Delta;\&tau;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; j</span>}{\mathrm{<span\; class="notranslate">\; \&Phi;</span>}}_{\mathrm{<span\; class="notranslate">\; PN</span>}}<span\; class="notranslate">\; ]</span><span\; class="notranslate">\; \}</span>$

$<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{{\mathrm{<span\; class="notranslate">\; RE</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; \{</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; +</span>{<span\; class="notranslate">\; [</span>\mathrm{<span\; class="notranslate">\; kM</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ]</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; kM</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; cos</span>}<span\; class="notranslate">\; [</span>\mathrm{<span\; class="notranslate">\; \&beta;\&phi;</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}\mathrm{<span\; class="notranslate">\; \&Delta;\&tau;</span>}<span\; class="notranslate">\; ]</span><span\; class="notranslate">\; \}</span>$

$<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; K</span>}<span\; class="notranslate">\; \{</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; +</span>{<span\; class="notranslate">\; [</span>\mathrm{<span\; class="notranslate">\; kM</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ]</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; kM</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; cos</span>}<span\; class="notranslate">\; [</span>\mathrm{<span\; class="notranslate">\; \&beta;\&phi;</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}\mathrm{<span\; class="notranslate">\; \&Delta;\&tau;</span>}<span\; class="notranslate">\; ]</span><span\; class="notranslate">\; \}</span>$

$<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; K</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; K</span>}{<span\; class="notranslate">\; [</span>\mathrm{<span\; class="notranslate">\; kM</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ]</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \{</span>\mathrm{<span\; class="notranslate">\; cos</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}\mathrm{<span\; class="notranslate">\; \&Delta;\&tau;</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; cos</span>}<span\; class="notranslate">\; [</span>\mathrm{<span\; class="notranslate">\; \&beta;\&phi;</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ]</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}\mathrm{<span\; class="notranslate">\; \&Delta;\&tau;</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; [</span>\mathrm{<span\; class="notranslate">\; \&beta;\&phi;</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ]</span><span\; class="notranslate">\; \}</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; )</span>$

It can be seen that there is no optical phase noise Φ _PN in the photocurrent, and the two channels of light have the same source, and the phase noise has been canceled after the frequency difference, thus avoiding the influence of optical phase noise on the signal. φ(t) can be various periodic signal waveforms such as sine, cosine, square wave, and sawtooth wave, which can be used to generate millimeter waves. For the convenience of explaining the problem, we choose cosine wave for formula derivation here, that is, φ(t)=cos (ω _s t), ω _s is the angular frequency.

I _d (t)=K/2+K[kM(t)] ² /2

+K ₁ M(t){cos(ω _c Δτ)cos[βcos(ω _s t)]-kM(t)sin(ω _c Δτ)sin[βcos(ω _s t)]} (5)

Expanding it to Bezier has

${\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; K</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; K</span>}{<span\; class="notranslate">\; [</span>\mathrm{<span\; class="notranslate">\; kM</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ]</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}$

$<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \{</span>\mathrm{<span\; class="notranslate">\; cos</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}\mathrm{<span\; class="notranslate">\; \&Delta;\&tau;</span>}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; J</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&beta;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; cos</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}<span\; class="notranslate">\; ~</span>}\mathrm{<span\; class="notranslate">\; \&Delta;\&tau;</span>}<span\; class="notranslate">\; )</span>\underset{\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{<span\; class="notranslate">\; \&infin;</span>}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}{<span\; class="notranslate">\; (</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; no</span>}}{\mathrm{<span\; class="notranslate">\; J</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; no</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&beta;</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; cos</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; no</span>}{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span>$

$<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}\mathrm{<span\; class="notranslate">\; \&Delta;\&tau;</span>}<span\; class="notranslate">\; )</span>\underset{\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{<span\; class="notranslate">\; \&infin;</span>}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}{<span\; class="notranslate">\; (</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; J</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&beta;</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; cos</span>}<span\; class="notranslate">\; [</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; ]</span><span\; class="notranslate">\; \}</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 6</span>}<span\; class="notranslate">\; )</span>$

It can be seen from formula (6) that the output photocurrent signal is composed of a series of harmonic components, and the frequency interval of these components is ω _s , including odd harmonics and even harmonics:

$K_{1}m\left(t\right)\left\{2\cos \left({\mathrm{\&omega;}}_{c~}\mathrm{\&Delta;\&tau;}\right)\underset{\mathrm{no}=1}{\overset{\&infin;}{\mathrm{\&Sigma;}}}{(-1)}^{\mathrm{no}}{J}_{2\mathrm{no}}\left(\mathrm{\&beta;}\right)\cos \left(2\mathrm{no}{\mathrm{\&omega;}}_{\mathrm{the\; s}}t\right)\right\}$ is an even component

$K_{1}m\left(t\right)\{-2\sin \left({\mathrm{\&omega;}}_c\mathrm{\&Delta;\&tau;}\right)\underset{\mathrm{no}=1}{\overset{\&infin;}{\mathrm{\&Sigma;}}}{(-1)}^{\mathrm{no}-1}{J}_{2\mathrm{no}-1}\left(\mathrm{\&beta;}\right)\cos [(2\mathrm{no}-1){\mathrm{\&omega;}}_{\mathrm{the\; s}}t\left]\right\}$ is an odd component

As long as the appropriate high-order harmonic components are taken out, the millimeter wave we need can be obtained. For example, when the output frequency f _s = 5GHZ (ω _s = 2πf _s = 2π × 5GHZ) of the periodic signal source 1-5, it is necessary to generate a millimeter wave of 60GHZ wave, as long as the band-pass filter 2-2 is used to take out the 12th harmonic.

We can control the optical path of the two lights so that their delay difference is 0 or very close to 0, that is, Δτ→0, sin(ω _c Δτ)→0 and cos(ω _c Δτ)→1, so that the odd Secondary components, only even-order components and make the even-order components the largest (cos(ω _c Δτ)=1), so that the energy is more concentrated; if we want 2N harmonics, take out this item as follows:

F _2N (t)＝2K ₁ M(t)J _2N (β)cos(2Nω _s t) (7)

We can also select an appropriate phase modulation index β to maximize J _2N (β), even if the required millimeter wave amplitude is maximum. The resulting millimeter wave has modulated signal M(t).

In addition, when implementing the system, it is necessary to ensure the consistency of the polarization directions of the two paths of light, so that the best interference can be formed after superimposition, otherwise the useful signal output of the photodetector 2-1 photocurrent will attenuate or even become zero.

Therefore, this solution realizes the generation of millimeter waves and the modulation and transmission of information in the downlink.

Compared with the prior art, the present invention has the following prominent substantive features and significant advantages: (1) the present invention uses a common low-frequency microwave source and photon technology to combine to generate millimeter waves, and directly uses a millimeter-wave local oscillator source to modulate the light of the RoF Compared with the transmission system, there is no need for expensive millimeter-wave oscillators and electro-absorption modulators (EAMs), and a real low-cost and practical mmRoF solution that uses microwave and optical communication technologies to generate millimeter waves is realized. (2) Compared with traditional optical multiplication, the comb filter (Mach-Zenhnder interferometer or Fabry-Perot filter, etc.) is omitted, and the core architecture only needs an optical phase modulator, an optical field amplitude modulator and a few The partial passive optical device greatly reduces the system cost and structure, and improves the stability. (3) Compared with the millimeter-wave generation technology using dual-mode lasers and dual light sources, the present invention only needs an ordinary single-frequency laser, which saves costs , avoiding the influence of optical phase noise (4) As long as the optical paths of the two optical waves are controlled to be equal so that the delay difference is 0, the odd harmonics will be completely eliminated regardless of the optical frequency, and the optical frequency can be used without adjusting the optical frequency. Harmonics are less, and the spectrum energy is more concentrated, which is conducive to improving the signal-to-noise ratio and system performance of the useful millimeter wave, making system maintenance easier (5) the modulation technology of the present invention is convenient and flexible, and is added to the modulation of the light intensity modulator 1-7 Signals 1-8 can be direct digital baseband signals, or several channels of microwave signals with modulated information (such as two QPSK signals with carrier frequencies of 2.4GHZ and 2.6GHZ), and then realize subcarrier multiplexing (6) The invention has an ingenious and simple structure and low cost, making the generation technology of millimeter wave further practical

Description of drawings

Fig. 1 is a structural diagram of a downlink device of a millimeter wave optical fiber transmission system.

Detailed ways

A preferred embodiment of the present invention is a downlink device applied to a 60GHz RoF system. In the central station 1, a laser 1-1 and a polarizer 1-2 are connected by a pigtail, and the polarizer 1-2 is connected by a pigtail. The polarizer 1-2 is connected to the input end of an optical splitter 1-3 through a polarization maintaining optical fiber 1-9, and the output end of the optical splitter 1-3 is connected to the input end of an optical phase modulator 1-4 through The polarization maintaining optical fiber 1-9 is connected, and the periodic signal source 1-5 is connected to the electrical input end of the optical phase modulator 1-4, and the output end of the optical phase modulator 1-4 is connected to a polarization maintaining coupler 1-6. The other output end of the optical splitter 1-3 is connected to the input end of an optical field amplitude modulator 1-7 through a polarization-maintaining optical fiber 1-9, and the modulated signal 1-8 is transmitted from the optical field amplitude modulator 1-7 is connected to the electrical input end, and the output end of the optical field amplitude modulator 1-7 is connected to the other input end of the polarization-maintaining coupler 1-6 with a polarization-maintaining optical fiber 1-9; the polarization-maintaining coupler The 1-6 output end is connected to the light input end of the photodetector 2-1 of the base station 2 through the optical fiber 3; -2 is connected to the input end, the output end of the bandpass filter 2-2 is connected to the input end of a millimeter wave amplifier 2-3, and the output end of the millimeter wave amplifier 2-3 is connected to a millimeter wave transmitting antenna 3-4. Referring to FIG. 1 , the downlink device of the millimeter-wave optical fiber transmission system consists of a central station 1 , a base station 2 and an optical fiber 3 connecting them. The downlink millimeter wave generation and information transmission process is as follows:

At the transmitting end of the central station 1, a semiconductor laser 1-1 used as a light source operates at a wavelength of 1550 nm, a line width of 10 MHz, and a power of 10 mW. The periodic signal source 1-5 generates a cosine wave with a frequency of f _s =5 GHZ to drive the optical phase modulator 1-4, and the phase modulation index β=13.8, then the first optical wave from the optical splitter 1-3 is phase-modulated to The electric field expression of the polarization maintaining coupler 1-6 is:

E ₁ (t)＝E _c exp[jω _c t+jβcosω _s t+jΦ _PN ]

Among them, E _c is the amplitude of the electric field of the light wave, ω _c is the central angular frequency of the light wave, β is the phase modulation index, ω _s is the angular frequency of the cosine signal generated by the periodic signal source 1-5, ω _s =2πf _s =2π×5GHZ , Φ _PN is the random noise of the laser itself.

The optical path difference of the two light waves has been controlled to be 0, then Δτ=0, and the second light wave is amplitude modulated, and the modulation signal 1-8 is a digital baseband signal M(t) with a rate of 100Mbps, which is sent to the polarization maintaining coupler The expression of the light wave electric field after:

E ₂ (t)＝E _c kM(t)exp[jω _c t+jΦ _PN ]

where k is the modulation index.

The polarization directions of the two light waves are consistent after polarization control, so the synthetic light wave is:

E _out (t)＝E ₁ (t)+E ₂ (t)＝E _c exp[jω _c t+jβcosω _s t+jΦ _PN ]+E _c kM(t)exp[jω _c t+jΦ _PN ]

The optical signal is transmitted to the photodetector 2-1 of the base station through the optical fiber 3 to generate photocurrent I _d (t)

${\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; K</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; K</span>}{<span\; class="notranslate">\; [</span>\mathrm{<span\; class="notranslate">\; kM</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ]</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; J</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&beta;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 2</span>}{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span>\underset{\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{<span\; class="notranslate">\; \&infin;</span>}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}{<span\; class="notranslate">\; (</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; no</span>}}{\mathrm{<span\; class="notranslate">\; J</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; no</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&beta;</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; cos</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; no</span>}{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span>$

Among them, K and K ₁ are proportional coefficients.

The 12th harmonic is the desired 60GHZ millimeter wave. After passing through the bandpass filter 2-2 with a center frequency of 60GHZ and a passband of 400MHZ, the harmonic is taken out, and the obtained signal is as follows: (K ₂ is an amplitude constant)

K ₂ M(t)J ₁₂ (13.8)cos(12ω _s t)＝0.2858K ₂ M(t)cos(2π×60×10 ⁹ t)

It can be seen that not only the 60GHZ millimeter wave is generated, but also the AM modulation of the 100Mbps digital baseband modulation signal 1-8M(t) on the 60GHZ millimeter wave is realized, and then the millimeter wave signal is amplified by the millimeter wave amplifier 2-3, and transmitted by the millimeter wave Antennas 2-4 transmit out, thus completing the downlink information modulation and transmission function of the 60GHZ RoF transmission system from the central station to the base station.

Claims (2)

1. A downlink device of a millimeter-wave optical fiber transmission system, comprising a central station (1), a base station (2) and an optical fiber (3), and the central station (1) and the base station (2) are interconnected by an optical fiber (3), It is characterized in that: in the central station (1), a laser (1-1) and a polarizer (1-2) are connected through pigtails, and the polarizer (1-2) is protected by The polarization fiber (1-9) is connected to the input end of an optical splitter (1-3), and an output end of the optical splitter (1-3) is connected to an input end of an optical phase modulator (1-4) Connected through polarization maintaining optical fiber (1-9), a periodic signal source (1-5) is connected to the electrical input end of the optical phase modulator (1-4), and the output end of the optical phase modulator (1-4) is connected to an optical phase modulator (1-4) One input end of the polarization maintaining coupler (1-6) is connected; the other output end of the optical splitter (1-3) is connected to the input end of an optical field amplitude modulator (1-7) through the polarization maintaining optical fiber (1 -9) is connected, and the modulation signal (1-8) is connected from the electrical input end of the optical field amplitude modulator (1-7), and the output end of the optical field amplitude modulator (1-7) is connected to the polarization maintaining coupler (1-6) another input end links to each other with polarization-maintaining optical fiber (1-9); Described polarization-maintaining coupler (1-6) output end is connected to the photodetector ( 2-1) optical input terminal; in the base station (2), the electrical output terminal of the optical detector (2-1) is connected to the input terminal of a band-pass filter (2-2), and the band-pass filter The output end of the device (2-2) is connected with the input end of a millimeter wave amplifier (2-3), and the output end of the millimeter wave amplifier (2-3) is connected with a millimeter wave transmitting antenna (3-4). 2. the generation of millimeter wave in the downlink of millimeter wave optical fiber transmission system and information modulation method, adopt the downlink device of millimeter wave optical fiber transmission system according to claim 1, it is characterized in that in central station (1 ), the laser output from the laser (1-1) is divided into two paths, one path of light waves passes through the optical phase modulator (1-4), and the periodic signal source (1-5) is connected with the optical phase modulator (1-4); the other One path of light waves passes through the optical field amplitude modulator (1-7), and the modulation signal (1-8) is connected to the electrical input terminal of the optical field amplitude modulator (1-7); then the optical phase modulator (1-4) outputs The light wave and the light field amplitude modulator (1-7) output light wave superposition and interference, and then the synthesized wave is sent to the photodetector (2-1) through the optical fiber (3), and the photodetector (2-1) electric output is then to the mm wave bandpass filter (2-2).

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