CN100568779C - The down link device of millimeter wave optical fibre transmission system reaches the wherein generation and the information modulator approach of millimeter wave - Google Patents

Abstract

The down link structure that the present invention relates to a kind of millimeter wave optical fibre transmission system reaches wherein, and millimeter wave produces and the information modulator approach.The down link structure of this millimeter wave optical fibre transmission system, at central station single-frequency laser is divided into two-way, one road light wave is wherein carried out phase modulated with low-frequency cycle property microwave signal, another road light wave is carried out the optical electric field amplitude modulation(PAM), and then two-way light wave stack interference, be transmitted through the fiber to the base station and detect the generation photoelectric current with photo-detector, the each harmonic that has comprised the low frequency microwave signal in this current signal, taking out high order harmonic component with band pass filter is desired millimeter-wave signal, and this millimeter-wave signal is the modulation of acquired information, thus the downlink transmission of realization information from the central station to the base station.This link structure is simple, is easy to realize, and stable performance, with low cost, be suitable for using.

Description

The down link device of millimeter wave optical fibre transmission system reaches the wherein generation and the information modulator approach of millimeter wave

Technical field

The down link device that the present invention relates to millimeter wave optical fibre transmission system reaches wherein, and millimeter wave produces and the information modulator approach, at this a kind of down link device of new millimeter wave optical fibre transmission system is proposed, at central station single-frequency laser is divided into two-way, it is carried out two paths of signals being superposeed after the different disposal again, be sent to exportable needed millimeter-wave signal after the opto-electronic conversion of base station by optical fiber, and this millimeter-wave signal has obtained the modulation of information, thus the downlink transmission of realization information from the central station to the base station.

Background technology

Recent years radio frequency optical fiber transmission technique (RoF, Radio on Fiber) merged low, the anti-interference strong and radio communication of Optical Fiber Transmission bandwidth, loss with it and covered advantage flexibly, become a solution that haves a great attraction in research fields such as broadband radio access network, next generation mobile communication, multimedia communications.

In order to adapt to terminal use's requirement growing to bandwidth, the inevitable high-end development towards frequency spectrum of rf frequency that radio frequency fiber optic transmission system is used has caused concern widely as the application study at millimeter wave frequency band.How to realize that the transmission problem of millimeter wave in optical fiber link is millimeter wave optical fiber transmission technology (mmRoF, millimeter-waveRoF) important research project, one of them research emphasis is exactly how to realize transmission of Information in the down link when producing millimeter wave, up to the present, proposed multiple scheme in the world, mainly contained following several:

1) light wave sideband modulation (modulation-side-band technique): baseband signal is modulated millimeter wave, directly light wave is modulated with this millimeter wave then, light wave two sideband difference frequencies obtain millimeter wave behind the base station, itself just needs millimetre-wave generator this method, so millimeter wave is to produce by electric method.

2) dual-mode laser device (dual-mode laser): laser produces two kinds of frequencies, and the difference of frequency just in time is a millimeter-wave frequency.

3) light frequency modulation (phase modulation) is in conjunction with optical fiber dispersion (FM-modulated laser in conjunction with fiberdispersion): with the low frequency microwave signal to laser frequency modulation or the outer phase modulation of light wave, pass through dispersive optical fiber, phase change is converted into changes in amplitude (FM-IM effect), changes in amplitude comprises amount in microwave signal and the high order branch thereof, and extracting high order harmonic component is the signal of millimeter wave.If dispersive optical fiber is changed into comb filter, be exactly optical frequency-doubling method.

4) non-linear laser device harmonic wave up-conversion (harmonic upconversion in nonlinear lasers): the low frequency microwave signal is added to the non-linear laser device, inspire high order harmonic component, obtain required millimeter wave to the photo-detector difference frequency, with 3) somewhat similar, but have any different on the principle.

5) multiple light courcess technology (multiple optical source technique): the difference of two light source frequencies is millimeter frequencies, promptly gets millimeter wave to the photo-detector difference frequency.

Various technology are each has something to recommend him, and own unique application occasion and advantage are arranged, but cost is higher mostly, and system complex is unfavorable for large-scale promotion application.

Summary of the invention

The objective of the invention is to solve existing millimeter wave optical fibre transmission system downlink system complexity, problem with high costs, difficult in maintenance, propose a kind of millimeter wave optical fibre transmission system and down link device and wherein millimeter wave produce and the information modulator approach.This chain circuit device is simple, is easy to realization, and stable performance is with low cost, is suitable for the develop and spread of practical product.

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

A kind of down link device of millimeter wave optical fibre transmission system, comprise central station 1, base station 2 and optical fiber 3, central station 1 and base station 2 are by optical fiber 3 interconnection, it is characterized in that: in described central station 1, a laser 1-1 links to each other by tail optical fiber with a polarizer 1-2, described polarizer 1-2 is connected with an optical branching device 1-3 input by polarization maintaining optical fibre 1-9, described optical branching device 1-3 one output links to each other by polarization maintaining optical fibre 1-9 with an optical phase modulator 1-4 input, periodic signal source 1-5 is arranged from inserting with optical phase modulator 1-4 electrical input, optical phase modulator 1-4 output links to each other with polarization-maintaining coupler 1-6 one input; Described another output of optical branching device 1-3 links to each other by polarization maintaining optical fibre 1-9 with a light field amplitude modulator 1-7 input, have modulation signal 1-8 to insert from light field amplitude modulator 1-7 electrical input, light field amplitude modulator 1-7 output links to each other with polarization maintaining optical fibre 1-9 with described another input of polarization-maintaining coupler 1-6; Described polarization-maintaining coupler 1-6 output is connected to the photo-detector 2-1 light input end of base station 2 by optical fiber 3; In described base station 2, described photo-detector 2-1 electricity output links to each other with a band pass filter 2-2 input, band pass filter 2-2 output is connected with a millimeter wave amplifier 2-3 input, and millimeter wave amplifier 2-3 output links to each other with a millimeter wave transmitting antenna 3-4.

The generation of millimeter wave and information modulator approach in a kind of down link of millimeter wave optical fibre transmission system, adopt the down link device of millimeter wave optical fibre transmission system according to claim 1, it is characterized in that at central station 1, the laser of laser 1-1 output is divided into two-way, one road light wave is by optical phase modulator 1-4, and periodic signal source 1-5 links to each other with optical phase modulator 1-4; Another road light wave then passes through light field amplitude modulator 1-7, and modulation signal 1-8 links to each other with light field amplitude modulator 1-7 electrical input; Optical phase modulator 1-4 output light-wave and the stack of light field amplitude modulator 1-7 output light-wave are interfered again, then this composite wave is transported to photo-detector 2-1 by optical fiber 3, millimeter wave band pass filter 2-2 is arrived in the output of photo-detector 2-1 electricity again.Below the present invention is further illustrated:

This programme belongs to light phase modulation (frequently) heterodyne technology, and specific implementation is:

As shown in drawings, at central station 1, laser 1-1 sends single-frequency laser to polarizer 1-2, produce single polarization direction, be divided into the two-way light wave to optical branching device then, to optical phase modulator 1-4, periodic signal source 1-5 produces periodic signal (can be sine and cosine ripple, sawtooth waveforms, square wave etc.) optical phase modulator 1-4 is carried out phase modulation the first via by polarization maintaining optical fibre 1-9, output to polarization-maintaining coupler 1-6, this phase modulation light wave electric field expression formula is:

E ₁(t)＝E _cexp[jω _ct+jβφ(t)+jΦ _PN] (1)

Wherein, E _cBe light wave electric field amplitude, ω _cBe the central angle frequency of light wave, β is a phase-modulation index, and φ (t) is the periodic signal that periodic signal source 1-5 produces, Φ _PNIt is the random noise of laser itself.

The second road light wave then carries out light field amplitude modulation(PAM) (AM) by light field amplitude modulator 1-7, by polarization maintaining optical fibre 1-9 to polarization-maintaining coupler 1-6, this moment light wave electric field expression formula:

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

Wherein, E _cBe the light wave electric field amplitude, k is a modulation index, and M (t) is a modulation signal, and this signal can be a baseband signal, also can be the microwave signal of having modulated base-band information, ω _cBe the central angle frequency of light wave, Δ τ is the delay inequality after light wave divides the two-way transmission, Φ _PNIt is the random noise of laser itself.

Handle because the two-way light wave has all carried out protecting partially, the polarization direction is consistent, by the synthetic light wave electric field in polarization-maintaining coupler 1-6 stack back is:

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

Synthetic light wave is transferred to base station 2 by optical fiber 3, and photo-detector 2-1 detects luminous intensity, forms photoelectric current I _d(t), expression formula (R, K, K in the formula ₁Be proportionality coefficient) as follows:

$I_d\left(t\right)=\frac{1}{2}{\mathrm{RE}}_{\mathrm{out}}\left(t\right)E_{\mathrm{out}}{\left(t\right)}^{*}$

$=\frac{1}{2}{\mathrm{RE}}_c\left\{\exp \right[j{\mathrm{\ω}}_{c}t+\mathrm{j\β\φ}\left(t\right)+j{\mathrm{\Φ}}_{\mathrm{PN}}]+\mathrm{kM}\left(t\right)\exp [j{\mathrm{\ω}}_c(t-\mathrm{\Δ\τ})+j{\mathrm{\Φ}}_{\mathrm{PN}}\left]\right\}$

$\×E_c\left\{\exp \right[-j{\mathrm{\ω}}_{c}t-\mathrm{j\β\φ}\left(t\right)-j{\mathrm{\Φ}}_{\mathrm{PN}}]+\mathrm{kM}\left(t\right)\exp [-j{\mathrm{\ω}}_c(t-\mathrm{\Δ\τ})-j{\mathrm{\Φ}}_{\mathrm{PN}}\left]\right\}$

$=\frac{1}{2}{{\mathrm{RE}}_c}^2\{1+{\left[\mathrm{kM}\left(t\right)\right]}^2+2\mathrm{kM}\left(t\right)\cos [\mathrm{\β\φ}\left(t\right)+{\mathrm{\ω}}_c\mathrm{\Δ\τ}\left]\right\}$

$=K\{1/2+{\left[\mathrm{kM}\left(t\right)\right]}^2/2+\mathrm{kM}\left(t\right)\cos [\mathrm{\β\φ}\left(t\right)+{\mathrm{\ω}}_c\mathrm{\Δ\τ}\left]\right\}$

$=K/2+K{\left[\mathrm{kM}\left(t\right)\right]}^2/2+K_{1}M\left(t\right)\left\{\cos \left({\mathrm{\ω}}_c\mathrm{\Δ\τ}\right)\cos \right[\mathrm{\β\φ}\left(t\right)]-\sin \left({\mathrm{\ω}}_c\mathrm{\Δ\τ}\right)\sin [\mathrm{\β\φ}\left(t\right)\left]\right\}---\left(4\right)$

There is not light phase noise Φ in the visible light electric current _PN, two-way light homology, phase noise is offset behind the difference frequency, has avoided the influence of light phase noise to signal thus.φ (t) can be various cyclical signal waveforms such as sine and cosine, square wave, sawtooth waveforms, may be used to produce millimeter wave, and in order to say something conveniently, select cosine wave to carry out the derivation of equation here, i.e. φ (t)=cos (ω _sT), ω _sBe angular frequency.

I _d(t)＝K/2+K[kM(t)] ²/2

+K ₁M(t){cos(ω _cΔτ)cos[βcos(ω _st)]-kM(t)sin(ω _cΔτ)sin[βcos(ω _st)]} (5)

It is carried out Bezier launches to have

$I_d\left(t\right)=K/2+K{\left[\mathrm{kM}\left(t\right)\right]}^2/2$

$+K_{1}M\left(t\right)\{\cos \left({\mathrm{\ω}}_c\mathrm{\Δ\τ}\right)J_0\left(\mathrm{\β}\right)+2\cos \left({\mathrm{\ω}}_{c~}\mathrm{\Δ\τ}\right)\underset{n=1}{\overset{\∞}{\mathrm{\Σ}}}{(-1)}^n{J}_{2n}\left(\mathrm{\β}\right)\cos \left(2n{\mathrm{\ω}}_{s}t\right)$

$-2\sin \left({\mathrm{\ω}}_c\mathrm{\Δ\τ}\right)\underset{n=1}{\overset{\∞}{\mathrm{\Σ}}}{(-1)}^{n-1}{J}_{2n-1}\left(\mathrm{\β}\right)\cos \left[(2n-1){\mathrm{\ω}}_{s}t\right]\}---\left(6\right)$

Seen that by formula (6) the output photo-signal is made up of a series of harmonic components, these component frequency intervals are ω _s, include odd harmonic and even-order harmonic:

$K_{1}M\left(t\right)\left\{2\cos \left({\mathrm{\ω}}_{c~}\mathrm{\Δ\τ}\right)\underset{n=1}{\overset{\∞}{\mathrm{\Σ}}}{(-1)}^n{J}_{2n}\left(\mathrm{\β}\right)\cos \left(2n{\mathrm{\ω}}_{s}t\right)\right\}$ It is even order components

$K_{1}M\left(t\right)\{-2\sin \left({\mathrm{\ω}}_c\mathrm{\Δ\τ}\right)\underset{n=1}{\overset{\∞}{\mathrm{\Σ}}}{(-1)}^{n-1}{J}_{2n-1}\left(\mathrm{\β}\right)\cos [(2n-1){\mathrm{\ω}}_{s}t\left]\right\}$ It is the odd component

As long as take out suitable higher harmonic components, just can obtain the millimeter wave that we need, such as working as periodic signal source 1-5 output frequency f _s=5GHZ (ω _s=2 π f _s=2 π * 5GHZ), produce the millimeter wave of 60GHZ are as long as take out 12 subharmonic with band pass filter 2-2.

It is 0 or very near 0 that the light path that we can control two-way light makes their delay inequality, i.e. Δ τ → 0, sin (ω _cAnd cos (ω Δ τ) → 0 _cΔ τ) → 1, so just can eliminate the odd component, have only even order components and make even order components maximum (cos (ω _cΔ τ)=1), make energy more concentrated; If what we wanted is the 2N subharmonic, take out that this is as follows:

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

We also can choose suitable phase-modulation index β and make J _2NEven (β) maximum is the millimeter wave amplitude maximum that needs.The millimeter wave that obtains has been modulated signal M (t).

In addition, in the specific implementation system, must guarantee the consistency of two-way polarisation of light direction, can make stack back form best the interference like this, otherwise photo-detector 2-1 photoelectric current useful signal output attenuatoin even become 0.

This programme has been realized the generation of millimeter wave and the modulation transmissions of information in the down link thus.

The present invention compared with prior art, have following outstanding substantive distinguishing features and remarkable advantage: (1) the present invention uses common low frequency microwave source and photon technology in conjunction with producing millimeter wave, the RoF transmission system of light modulation is compared with the millimeter wave local vibration source with direct, need not expensive millimetre-wave generator and electroabsorption modulator (EAM), realize that truly utilize microwave and optical communication technique produce the low-cost practical mmRoF scheme of millimeter wave.(2) compare with a traditional optical times multiplication, saved comb filter (Mach-Zenhnder interferometer or Fabry-Perot filter etc.), core architecture only needs an optical phase modulator and light field amplitude modulator and a little to protect inclined to one side Passive Optical Components, system cost and structure have been significantly reduced, and having improved stability (3) compares with two light sources generation millimeter-wave technologies with adopting the dual-mode laser device, the present invention only needs a common single frequency laser, saved cost, having avoided light phase The noise (4) as long as the equivalent optical path of control two-way light wave makes it produce delay inequality is 0 o'clock, odd harmonic will thoroughly be eliminated and have nothing to do with light frequency, need not to adjust light frequency and just can make harmonic wave still less, spectrum energy is more concentrated, help improving the signal to noise ratio and the systematic function of useful millimeter wave, make system maintenance easier (5) modulation technique of the present invention convenient, flexible, the modulation signal 1-8 that is added to light intensity modulator 1-7 can be direct digital baseband signal, also can be that (such as two carrier frequency is 2.4GHZ to several roads microwave signal of having modulated information, 2.6GHZ the QPSK signal), and then realize that subcarrier multiplex (6) structure of the present invention is ingenious simple, with low cost, make the generation technique of millimeter wave further move towards practicability

Description of drawings

Fig. 1 is the down link structure drawing of device of millimeter wave optical fibre transmission system.

Embodiment

A preferred embodiment of the present invention is that a down link device that is applied to 60GHz RoF system is in described central station 1, a laser 1-1 links to each other by tail optical fiber with a polarizer 1-2, described polarizer 1-2 is connected with an optical branching device 1-3 input by polarization maintaining optical fibre 1-9, described optical branching device 1-3 one output links to each other by polarization maintaining optical fibre 1-9 with an optical phase modulator 1-4 input, periodic signal source 1-5 is arranged from inserting with optical phase modulator 1-4 electrical input, optical phase modulator 1-4 output links to each other with polarization-maintaining coupler 1-6 one input; Described another output of optical branching device 1-3 links to each other by polarization maintaining optical fibre 1-9 with a light field amplitude modulator 1-7 input, have modulation signal 1-8 to insert from light field amplitude modulator 1-7 electrical input, light field amplitude modulator 1-7 output links to each other with polarization maintaining optical fibre 1-9 with described another input of polarization-maintaining coupler 1-6; Described polarization-maintaining coupler 1-6 output is connected to the photo-detector 2-1 light input end of base station 2 by optical fiber 3; In described base station 2, described photo-detector 2-1 electricity output links to each other with a band pass filter 2-2 input, band pass filter 2-2 output is connected with a millimeter wave amplifier 2-3 input, and millimeter wave amplifier 2-3 output links to each other with a millimeter wave transmitting antenna 3-4.Referring to Fig. 1, the down link device of this millimeter wave optical fibre transmission system is made up of central station 1, base station 2 and the optical fiber 3 that is connected them.Generation of down link millimeter wave and message transmitting procedure are as follows:

At the transmitting terminal of central station 1, the semiconductor laser 1-1 that is used as light source is operated in the 1550nm wavelength, live width 10MHz, power 10mW.It is f that periodic signal source 1-5 produces frequency _sThe cosine wave of=5GHZ drives optical phase modulator 1-4, phase-modulation index β=13.8, and then the first via light wave that comes out from optical splitter 1-3 after by phase modulation to the electric field expression formula of polarization-maintaining coupler 1-6 is:

E ₁(t)＝E _cexp[jω _ct+jβcosω _st+jΦ _PN]

Wherein, E _cBe light wave electric field amplitude, ω _cBe the central angle frequency of light wave, β is a phase-modulation index, ω _sBe the angular frequency of the cosine signal of periodic signal source 1-5 generation, ω _s=2 π f _s=2 π * 5GHZ, Φ _PNIt is the random noise of laser itself.

The optical path difference of two-way light wave controlled well is 0, Δ τ=0 then, and the second road light wave carries out amplitude modulation(PAM), and modulation signal 1-8 is the digital baseband signal M (t) of speed 100Mbps, the light wave electric field expression formula behind the polarization-maintaining coupler:

E ₂(t)＝E _ckM(t)exp[jω _ct+jΦ _PN]

Wherein, k is a modulation index.

Two light wave polarization directions are consistent through Polarization Control, so synthetic light wave is:

E _out(t)＝E ₁(t)+E ₂(t)＝E _cexp[jω _ct+jβcosω _st+jΦ _PN]+E _ckM(t)exp[jω _ct+jΦ _PN]

Light signal process optical fiber 3 is transferred to the photo-detector 2-1 of base station, produces photoelectric current I _d(t)

$I_d\left(t\right)=K/2+K{\left[\mathrm{kM}\left(t\right)\right]}^2/2+K_{1}M\left(t\right)J_0\left(\mathrm{\β}\right)+2K_{1}M\left(t\right)\underset{n=1}{\overset{\∞}{\mathrm{\Σ}}}{(-1)}^n{J}_{2n}\left(\mathrm{\β}\right)\cos \left(2n{\mathrm{\ω}}_{s}t\right)$

Wherein, K, K ₁It is proportionality coefficient.

12 subharmonic are desired 60GHZ millimeter wave, are to take out this harmonic wave behind the band pass filter 2-2 of 400MHZ for the 60GHZ passband through centre frequency, and it is as follows to obtain signal: (K ₂Be the amplitude constant)

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

As seen, not only produced the 60GHZ millimeter wave, also realized the AM modulation of 100Mbps digital baseband modulation signal 1-8M (t) to the 60GHZ millimeter wave, again this millimeter-wave signal is amplified through millimeter wave amplifier 2-3,2-4 launches with the millimeter wave transmitting antenna, has so just finished the function of the downlink information modulation transmissions of 60GHZ RoF transmission system from the central station to the base station.

Claims (2)

1. the down link device of a millimeter wave optical fibre transmission system, comprise central station (1), base station (2) and optical fiber (3), central station (1) and base station (2) are by optical fiber (3) interconnection, it is characterized in that: in described central station (1), a laser (1-1) links to each other by tail optical fiber with a polarizer (1-2), the described polarizer (1-2) is connected with an optical branching device (1-3) input by polarization maintaining optical fibre (1-9), described optical branching device (1-3) output links to each other by polarization maintaining optical fibre (1-9) with an optical phase modulator (1-4) input, periodic signal source (1-5) is arranged from inserting with optical phase modulator (1-4) electrical input, optical phase modulator (1-4) output links to each other with a polarization-maintaining coupler (1-6) input; Another output of described optical branching device (1-3) links to each other by polarization maintaining optical fibre (1-9) with a light field amplitude modulator (1-7) input, have modulation signal (1-8) to insert from light field amplitude modulator (1-7) electrical input, light field amplitude modulator (1-7) output links to each other with polarization maintaining optical fibre (1-9) with another input of described polarization-maintaining coupler (1-6); Described polarization-maintaining coupler (1-6) output is connected to photo-detector (2-1) light input end of base station (2) by optical fiber (3); In described base station (2), the electric output of described photo-detector (2-1) links to each other with a band pass filter (2-2) input, band pass filter (2-2) output is connected with a millimeter wave amplifier (2-3) input, and millimeter wave amplifier (2-3) output links to each other with a millimeter wave transmitting antenna (3-4).

2. the generation of millimeter wave and information modulator approach in the down link of a millimeter wave optical fibre transmission system, adopt the down link device of millimeter wave optical fibre transmission system according to claim 1, it is characterized in that in central station (1), the laser of laser (1-1) output is divided into two-way, one road light wave is by optical phase modulator (1-4), and periodic signal source (1-5) links to each other with optical phase modulator (1-4); Another road light wave then passes through light field amplitude modulator (1-7), and modulation signal (1-8) links to each other with light field amplitude modulator (1-7) electrical input; Again optical phase modulator (1-4) output light-wave and the stack of light field amplitude modulator (1-7) output light-wave are interfered, then this composite wave is transported to photo-detector (2-1) by optical fiber (3), millimeter wave band pass filter (2-2) is arrived in the output of photo-detector (2-1) electricity again.

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