CN100568777C - A kind of millimeter wave carrier forming device of radio frequency fiber optic transmission system and method - Google Patents

Abstract

The present invention relates to millimeter wave carrier forming device and the method for a kind of radio frequency fiber optic transmission system (RoF, Radio on Fiber).This device is made up of periodicity optical filter, photo-detector, the millimeter wave band pass filter of semiconductor laser, phase-modulator, sine wave drive and the base station of central station.The present invention utilizes phase-modulator at central station, adopts the output light-wave of sinusoidal wave noise spectra of semiconductor lasers to carry out the optics frequency sweep; Utilize the transmission response fluctuation characteristic of Fabry-Perot optical filter in the base station, the light wave of frequency change is transformed into the light wave of Strength Changes, behind photo-detector and millimeter wave band pass filter, obtain millimeter wave carrier.The millimeter wave that the present invention proposes generates method and apparatus can generate millimeter wave with passive method in the base station, guarantee very low system cost, also have simultaneously temperature stability good, can select advantage such as output frequency flexibly.

Description

A kind of millimeter wave carrier forming device of radio frequency fiber optic transmission system and method

Technical field

The present invention relates to a kind of radio frequency fiber optic transmission system (RoF, Radio on Fiber) millimeter wave carrier production method and device, the sinusoidal wave scan light wave phase of particularly a kind of central station electricity consumption, base station add the method and apparatus of the optics frequency sweep method generation millimeter wave carrier of photo-detector with the Fabry-Perot optical filter.

Technical background

Developing direction that has competitiveness as ultrahigh speed wireless communication networks (high-speed mobile communications of future generation and broadband radio access network), millimeter wave optical fibre transmission system has merged the advantage of millimeter wave and optical fiber, has solved the connectivity problem of broadband wireless communications net base station and central station.It can provide miscellaneous services such as WLAN (wireless local area network), broadband access and mobile multimedia to a large amount of users, have plurality of advantages such as message capacity is big, portable, communications security is strong.

The millimeter wave optical fibre transmission system of ultrahigh speed wireless communication networks is made up of three parts, and a part is a central station, and every function of signal processing that it is integrated is as modulation, demodulation, driving, the amplification of high speed baseband signal, the transmission of light wave and detection etc.Second portion is a fiber transmission link, finishes the transmitted in both directions of light wave between central station and the base station.Third part is the base station, finishes the conversion of light wave and millimeter wave, and is transparent to baseband signal.

Because the transmission range of millimeter wave is limited, it is very little to require coverage with a base station to obtain in a ultrahigh speed cordless communication network, is called Pico cell.Because sharply increasing of number of base stations, the equipment of each base station must very simply and inexpensive just have the value of applying.Thereby in millimeter-wave communication system, play the millimeter wave generator of emission source and local vibration source effect as the heart of communication system, the realization for communication system aspect two of cost and systematic functions of millimeter wave generation technique that it is related and millimeter wave optical fiber transmission technology plays crucial effects.

Find that by literature search Ken-Ichi Kitayama is at A Taylor﹠amp; Francis Journal:Fiber andIntegrated Optics, 19:P167-P186, introduced four kinds of methods that produce millimeter wave in the RoF system among the Architectural Considerations of Fiber-RadioMillimeter-Wave Wireless Access Systems that delivers on 2000: investigation mission outside the city or town method for making, optical self-heterodyne method, light is frequency variation method and electric absorption transceiver method up and down.But these methods or need the electrooptic modulator of precision offset or accurate coherent laser or millimeter wave local vibration source and frequency mixer cause architecture of base station numerous and jumbled, and cost is very high; Architecture of base station has been simplified, but the influence of optical fiber dispersion is more serious, and modulation depth is not high, and nonlinear effect is arranged, or stronger RF-reflective is arranged, and diplex operation is crosstalked.

Summary of the invention

The object of the present invention is to provide the millimeter wave carrier forming device and the method for a radio frequency fiber optic transmission system, these apparatus and method can guarantee very low system cost, to satisfy the needs of mm wave RF fiber optic transmission system.

For achieving the above object, the present invention adopts following technical proposals:

The millimeter wave carrier forming device of a kind of radio frequency fiber optic transmission system (RoF, Radio on Fiber) by central station (1), optical fiber link (2) and base station (3) formation, is characterized in that:

(1) described central station (1) is connected to form successively by semiconductor laser (1-1), phase-modulator (1-2) and intensity modulator (1-3), adopt sinusoidal wave (1-4) to drive phase-modulator (1-2), baseband signal (1-5) is modulated on the light wave by intensity modulator (1-3);

(2) base station (3) are connected to form successively by periodicity optical filter (3-1), photo-detector (3-2), millimeter wave band pass filter (3-3), amplifier (3-4), antenna (3-5);

(3) described periodicity optical filter (3-1) is the Fabry-Perot optical filter.

A kind of millimeter wave carrier generation method of radio frequency fiber optic transmission system, device in the employing is characterized in that:

(1) in central station (1), adopt sinusoidal wave (1-4) to drive phase-modulator (1-2), the phase place of light wave is changed according to sinusoidal rule in effective range, the frequency of light wave changes according to the cosine rule in effective range, realizes the frequency scanning of light wave;

(2) in base station (3), adopt the Fabry-Perot optical filter, utilize its transmission response fluctuation characteristic, the frequency sweep light wave is carried out filtering, the light wave of frequency change is transformed into the light wave of Strength Changes;

(3) in base station (3), adopt photo-detector (3-2) and millimeter wave band pass filter (3-3) that the light wave by Fabry-Perot optical filter (3-1) is carried out opto-electronic conversion and filtering, generate millimeter wave carrier;

(4) in central station (1), baseband signal (1-5) is modulated on the light wave by intensity modulator (1-3); In base station (3), baseband signal is transferred on the millimeter wave carrier, launches through amplifier (3-4) and antenna (3-5).

The principle of this hair is as follows:

In central station (1), adopt semiconductor laser (1-1) to make light source, make phase modulated with the light wave of phase-modulator (1-2) noise spectra of semiconductor lasers (1-1) output, select angular speed to equal ω _SwSine wave (1-4) as the driving voltage of phase-modulator (1-2), output light-wave is made periodic scan in effective wavelength range Δ λ (respective frequencies range delta f), keep laser optical power constant simultaneously.Baseband signal (1-5) is modulated on the light wave by intensity modulator (1-3), and what transmit in the optical fiber link (2) is the light wave that intensity is scanned by baseband signal (1-5) modulation and frequency within the specific limits.In base station (3), the input light wave is after the light-to-current inversion of the filtering of periodicity optical filter (3-1) and photo-detector (3-2), baseband signal (1-5) has been transferred on the amplitude of millimeter wave carrier, launches through amplifier (3-4) and antenna (3-5) again.Because baseband signal (1-5) does not influence the millimeter-wave frequency of generation, so in the production process of explaining millimeter wave carrier, do not consider baseband signal (1-5).Under the situation that does not transmit baseband signal (1-5), the light wave electric field of the last output of central station is:

E(t)＝E _c?exp[jω _ct+jβsinω _swt)] (1)

ω wherein _cBe the central angle frequency of light wave, E _cBe the light wave electric field amplitude, β is a phase-modulation index.

In base station (3), enter before the optical receiver from the lightwave signal of optical fiber link, at first pass through periodically optical filter (Fabry-Perot optical filter) (3-1), its impulse response is:

$h_{\mathrm{fp}}\left(t\right)=t^2[\mathrm{\&delta;}(t-\frac{{\mathrm{\&tau;}}_{\mathrm{fp}}}{2})+{\mathrm{<figure-callout\; id="2"\; label="r"\; filenames="C2006100292410009H1.png"\; state="\{\{state\}\}">r</figure-callout>}}^2\mathrm{\&delta;}(t-\frac{3{\mathrm{\&tau;}}_{\mathrm{fp}}}{2})+...+r^{2n}\mathrm{\&delta;}(t-\frac{(2n+1){\mathrm{\&tau;}}_{\mathrm{fp}}}{2})+...]---\left(2\right)$

Wherein r and t are respectively the electric field reflection coefficient and the transmission coefficient of optical filter, and t ²=1-r ², τ _FpBe light signal in time of delay back and forth of Fabry-Perot optical cavity internal reflection.The output light-wave electric field of Fabry-Perot optical filter (3-1) can be expressed as:

$E\left(t\right)=\frac{E_c(1-R)}{1-{\mathrm{<figure-callout\; id="2"\; label="R"\; filenames="C2006100292410009H1.png"\; state="\{\{state\}\}">R</figure-callout>}}^2\exp (-\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="C2006100292410009H1.png"\; state="\{\{state\}\}">j</figure-callout>}2{\mathrm{\&omega;}}_c{\mathrm{\&tau;}}_{\mathrm{fp}})}\left\{\exp \right[j{\mathrm{\&omega;}}_c(t-\frac{{\mathrm{\&tau;}}_{\mathrm{fp}}}{2})+\mathrm{j\&beta;}\sin {\mathrm{\&omega;}}_{\mathrm{sw}}(t-\frac{{\mathrm{\&tau;}}_{\mathrm{fp}}}{2})]$

$+\mathrm{Rexp}[j{\mathrm{\&omega;}}_c(t-\frac{3{\mathrm{\&tau;}}_{\mathrm{fp}}}{2})+\mathrm{j\&beta;}\sin {\mathrm{\&omega;}}_{\mathrm{sw}}(t-\frac{3{\mathrm{\&tau;}}_{\mathrm{fp}}}{2})]\}---\left(3\right)$

Wherein R is a reflection coefficient of power, R=r ²Therefore the photoelectric current of photo-detector (3-2) output should be:

$i_d\left(t\right)=\frac{i_0{(1-R)}^2}{1+R^4-2{\mathrm{<figure-callout\; id="2"\; label="R"\; filenames="C2006100292410009H1.png"\; state="\{\{state\}\}">R</figure-callout>}}^2\mathrm{<figure-callout\; id="2"\; label="cos"\; filenames="C2006100292410009H1.png"\; state="\{\{state\}\}">cos</figure-callout>}2{\mathrm{\&omega;}}_c{\mathrm{\&tau;}}_{\mathrm{fp}}}\{1+{\mathrm{<figure-callout\; id="2"\; label="R"\; filenames="C2006100292410009H1.png"\; state="\{\{state\}\}">R</figure-callout>}}^2+2R\cos [{\mathrm{\&omega;}}_c{\mathrm{\&tau;}}_{\mathrm{fp}}+2\mathrm{\&beta;}\sin \left(\frac{{\mathrm{\&omega;}}_{\mathrm{sw}}{\mathrm{\&tau;}}_{\mathrm{fp}}}{2}\right)\cos ({\mathrm{\&omega;}}_{\mathrm{sw}}t-{\mathrm{\&omega;}}_{\mathrm{sw}}{\mathrm{\&tau;}}_{\mathrm{fp}})\left]\right\}$

I wherein ₀Be the average light electric current, and i ₀=FP ₀, P ₀, F is respectively the input optical power and the responsiveness of photo-detector (3-2).

Get τ _Fp=0.5/f _Sw, f _SwBe scanning frequency, following formula expanded into Bessel function:

$i_d\left(t\right)=\frac{i_0{(1-R)}^2}{1+R^4-2{\mathrm{<figure-callout\; id="2"\; label="R"\; filenames="C2006100292410009H1.png"\; state="\{\{state\}\}">R</figure-callout>}}^2\mathrm{<figure-callout\; id="2"\; label="cos"\; filenames="C2006100292410009H1.png"\; state="\{\{state\}\}">cos</figure-callout>}2{\mathrm{\&omega;}}_c{\mathrm{\&tau;}}_{\mathrm{fp}}}\{1+{\mathrm{<figure-callout\; id="2"\; label="R"\; filenames="C2006100292410009H1.png"\; state="\{\{state\}\}">R</figure-callout>}}^2+2R\cos {\mathrm{\&omega;}}_c{\mathrm{\&tau;}}_{\mathrm{fp}}{J}_0\left(2\mathrm{\&beta;}\right)+$

$4R\cos {\mathrm{\&omega;}}_c{\mathrm{\&tau;}}_{\mathrm{fp}}\underset{n=1}{\overset{\&infin;}{\mathrm{\&Sigma;}}}{(-1)}^n{J}_{2n}\left(2\mathrm{\&beta;}\right)\cos \left[2n({\mathrm{\&omega;}}_{\mathrm{sw}}t-{\mathrm{\&omega;}}_{\mathrm{sw}}{\mathrm{\&tau;}}_{\mathrm{fp}})\right]$

$4R\sin {\mathrm{\&omega;}}_c{\mathrm{\&tau;}}_{\mathrm{fp}}\underset{n=1}{\overset{\&infin;}{\mathrm{\&Sigma;}}}{(-1)}^n{J}_{2n+1}\left(2\mathrm{\&beta;}\right)\cos \left[(2n+1)({\mathrm{\&omega;}}_{\mathrm{sw}}t-{\mathrm{\&omega;}}_{\mathrm{sw}}{\mathrm{\&tau;}}_{\mathrm{fp}})\right]\}---\left(4\right)$

As seen, the signal of telecommunication of photo-detector (3-2) output is made up of some harmonic components, by selecting suitable scan angle frequencies omega _Sw, design center frequency is 2nf _SwArrowband electrical filter (3-3), just can obtain needed millimeter wave after the filtering, its frequency is

f _m＝2nf _sw (5)

The expression formula of millimeter wave carrier is:

$i_{d2n}\left(t\right)=\frac{4i_{0}R{(1-R)}^2\cos \left({\mathrm{\&omega;}}_c{\mathrm{\&tau;}}_{\mathrm{fp}}\right)}{1+R^4-2{\mathrm{<figure-callout\; id="2"\; label="R"\; filenames="C2006100292410009H1.png"\; state="\{\{state\}\}">R</figure-callout>}}^2\cos \left(2{\mathrm{\&omega;}}_c{\mathrm{\&tau;}}_{\mathrm{fp}}\right)}\&CenterDot;J_{2n}\left(2\mathrm{\&beta;}\right)\cos \left(2n{\mathrm{\&omega;}}_{\mathrm{sw}}t\right)---\left(6\right)$

Can be by the long d in chamber (the control τ that optimizes Fabry-Perot filter (3-1) _Fp=0.5/f _Sw), reflection R and phase-modulation index β, thereby make needed millimeter wave component power maximum, concrete grammar is as follows:

1. in order to make needed harmonic component maximum, make Bessel function get maximum, promptly by Max{J _2n(2 β)) release the optimum value of Bessel function independent variable β; 2. consider to make even-order harmonic component maximum, eliminate the odd harmonic component, make ω _cτ _Fp=k π determines ω _cBest value; 3. by

Release the optimum value of reflection coefficient of power R.

The present invention has following outstanding feature and remarkable advantage compared with prior art: millimeter wave generating device that is proposed and method are a kind of passive ways, can make the parts of base station (3) simple and inexpensive, can guarantee that the cost of whole RoF system is very low; Base station (3) adopts the Fabry-Perot optical filter to make periodically optical filter (3-1), has good temperature stability, can solve the problem of millimeter-wave frequency and power fluctuation; The central station output light-wave is adopted sinusoidal wave scan mode, it doesn't matter with scanning sinusoidal wave harmonic frequency for the size of the best power reflection coefficient of Fabry-Perot optical filter (3-1) in the base station, as changing the output millimeter-wave frequency, only need to change phase-modulation index β so that required harmonic component maximum gets final product, need not to change Fabry-Perot optical filter (3-1), so system is very flexible in the selection of output frequency.In addition, also has an outstanding advantage, because scanning frequency is lower, and the optical fiber link of Pico cell RoF system is very short, needn't worry the influence of optical fiber dispersion, can use quartzy multimode fiber even plastic fiber, further reduce the material cost of system and lay expense as transmission medium.

Description of drawings

Fig. 1 is the structured flowchart of the millimeter wave carrier forming device of radio frequency fiber optic transmission system of the present invention.

Central station 1, optical fiber link 2, base station 3, laser 1-1, phase-modulator 1-2, intensity modulator 1-3, sine-wave oscillator 1-4, baseband signal 1-5, periodicity optical filter 3-1, photo-detector 3-2, band pass filter 3-3, amplifier 3-4 and antenna 3-5

Embodiment

A preferred embodiment of the present invention is 60GHz millimeter wave optics generating apparatus and the method in a kind of RoF of the being applied to system, and in conjunction with the accompanying drawings, details are as follows:

Referring to Fig. 1, the millimeter wave carrier forming device of this radio frequency fiber optic transmission system is made of central station 1, optical fiber link 2 and base station 3, it is characterized in that:

(1) described central station 1 is connected to form successively by semiconductor laser 1-1, phase-modulator 1-2 and intensity modulator 1-3, adopts sinusoidal wave 1-4 to drive phase-modulator 1-2, and baseband signal 1-5 is modulated on the light wave by intensity modulator 1-3;

(2) base station 3 is connected to form successively by periodicity optical filter 3-1, photo-detector 3-2, millimeter wave band pass filter 3-3, amplifier 3-4, antenna 3-5.

The millimeter wave carrier generation method of this radio frequency fiber optic transmission system is to adopt above-mentioned device, works as follows:

At the transmitting terminal of central station 1, the semiconductor laser 1-1 that is used as light source is operated in wavelength 1550nm, live width 10MHz, power 10mW.Adopt LiNbO ₃Phase-modulator 1-2 uses f _SwThe sinusoidal wave 1-4 of=5GHz drives it, thereby makes the light wave of semiconductor laser 1-1 output have following expression behind phase-modulator 1-2:

E(t)＝E _c?exp[jω _ct+jβsinω _swt)]

ω wherein _cBe the central angle frequency of light wave, E _cBe the light wave electric field amplitude, β is a phase-modulation index.

The at first Fabry-Perot optical filter 3-1 through having the pectination transmission response of light wave is imported in order in the base station 3, the phase modulated light wave of central station is converted to the intensity modulated light wave, again after photo-detector 3-2 light-to-current inversion, 3-3 takes out needed harmonic component with the millimeter wave band pass filter, through launching behind amplifier 3-4 and the antenna 3-5.

Referring to relational expression (5) and expression formula (6), get f _m=60GHz, n equals 6 as can be known, and 2n equals 12, promptly needs to extract the 12nd subharmonic, and the expression formula of this harmonic wave is:

$i_{d12}\left(t\right)=\frac{4i_{0}R{(1-R)}^2\cos \left({\mathrm{\&omega;}}_c{\mathrm{\&tau;}}_{\mathrm{fp}}\right)}{1+R^4-2{\mathrm{<figure-callout\; id="2"\; label="R"\; filenames="C2006100292410009H1.png"\; state="\{\{state\}\}">R</figure-callout>}}^2\cos \left(2{\mathrm{\&omega;}}_c{\mathrm{\&tau;}}_{\mathrm{fp}}\right)}\&CenterDot;J_{12}\left(2\mathrm{\&beta;}\sin \frac{{\mathrm{\&omega;}}_{\mathrm{sw}}{\mathrm{\&tau;}}_{\mathrm{fp}}}{2}\right)\cos \left(12{\mathrm{\&omega;}}_{\mathrm{sw}}t\right)$

In order to make needed harmonic power maximum, system parameters should meet the following conditions: τ _Fp=0.5/f _Sw=0.1ns, ω _cτ _Fp=k π, β=6.3.Have only even-order harmonic in the output signal like this, and the power of 12 order harmonic components reaches the strongest.Can be reduced to the photoelectric current expression formula this moment:

$i_{d12}\left(t\right)=\frac{4i_{0}R}{{(1+R)}^2}\&CenterDot;J_{12}\left(2\mathrm{\&beta;}\right)\cos \left(12{\mathrm{\&omega;}}_{\mathrm{sw}}t\right)$

The value of reflection coefficient of power R when further exporting the millimeter wave power maximum with sening as an envoy to after the range coefficient differentiate:

$\frac{d\left[\frac{4i_{0}R}{{(1+R)}^2}\right]}{\mathrm{dR}}=0\&DoubleRightArrow;R=1$

The value of reflection coefficient of power R can not equal 1 in the actual fabrication, but it is not difficult near 1 Fabry-Perot optical filter 3-1 to make reflection coefficient of power, and R approaches 1 more, and the selecting frequency characteristic of optical filter is just sharp-pointed more.When transmission base band, base-band information need be loaded on the light wave by intensity modulator, the output spectrum of central station can broaden like this, if the reflection coefficient of Fabry-Perot optical filter 3-1 is too high, the passband of optical filter is just very narrow, can cause very big decay to the frequency spectrum of useful signal.Consider the size of baseband signal bandwidth and the degree of stability of laser frequency, the reflection coefficient of Fabry-Perot optical filter 3-1 is not the bigger the better in actual applications, the reflection coefficient that suitably reduces Fabry-Perot optical filter 3-1 not only can increase the width of Fabry-Perot optical filter passband, and can not cause too much influence to the power of millimeter wave.The baseband signal 1-5 of transmission 100Mbps, getting R=0.8 is advisable, at this moment compare with R=1.0, the system output millimeter wave power 0.1dB that only descends, and the distortion of output millimeter wave modulated wave is smaller, the rising edge and the trailing edge of the baseband signal that terminal is recovered all compare suddenly, and the requirement to timing signal when detecting judgement can reduce.

N value and system unit characteristic have restriction relation, promptly by LiNbO ₃The ratio decision of the Free Spectral Range FSR of the range of scanned frequencies Δ f of phase-modulator 1-2 output light-wave and Fabry-Perot optical filter 3-1, n=Δ f/FSR.

FSR=10GHz by Fabry-Perot optical filter 3-1 calculates LiNbO ₃The range of scanned frequencies Δ f=60GHz of phase-modulator 1-2, best phase-modulation index β=6.3.

The harmonic component power maximum of 60GHz in the millimeter wave frequency spectrum that produces during transmission base band 1-5 not, about the big 3dB of harmonic wave than 50GHz, about the big 5dB of harmonic wave than 70GHz, it is easy extracting 60GHz millimeter wave component with millimeter wave filter.

Claims (2)

1. the millimeter wave carrier forming device of a radio frequency fiber optic transmission system (RoF, Radio on Fiber) is made of central station (1), optical fiber link (2) and base station (3), it is characterized in that:

A) described central station (1) is connected to form successively by semiconductor laser (1-1), phase-modulator (1-2) and intensity modulator (1-3), adopt sinusoidal wave (1-4) to drive phase-modulator (1-2), baseband signal (1-5) is modulated on the light wave by intensity modulator (1-3);

B) base station (3) are connected to form successively by periodicity optical filter (3-1), photo-detector (3-2), millimeter wave band pass filter (3-3), amplifier (3-4), antenna (3-5);

C) described periodicity optical filter (3-1) is the Fabry-Perot optical filter.

2. the millimeter wave carrier generation method of a radio frequency fiber optic transmission system adopts the described millimeter wave carrier forming device of claim 1, it is characterized in that:

A) in central station (1), adopt sinusoidal wave (1-4) to drive phase-modulator (1-2), the phase place of light wave is changed according to sinusoidal rule in effective range, the frequency of light wave changes according to the cosine rule in effective range, realizes the frequency scanning of light wave;

B) in base station (3), adopt the Fabry-Perot optical filter, utilize its transmission response fluctuation characteristic, the frequency sweep light wave is carried out filtering, the light wave of frequency change is transformed into the light wave of Strength Changes;

C) in base station (3), adopt photo-detector (3-2) and millimeter wave band pass filter (3-3) that the light wave by Fabry-Perot optical filter (3-1) is carried out opto-electronic conversion and filtering, generate millimeter wave carrier;

D) in central station (1), baseband signal (1-5) is modulated on the light wave by intensity modulator (1-3); In base station (3), baseband signal is transferred on the millimeter wave carrier, launches through amplifier (3-4) and antenna (3-5).

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