CN105007120A - Eightfold-frequency optical millimeter wave generation method and system based on polarization modulator cascading - Google Patents

Abstract

The invention provides an eightfold-frequency optical millimeter wave generation method and system based on polarization modulator cascading. The schematic diagram is shown in an accompanying drawing. Through combination of polarization modulators with a polarizer, suppression of an odd-order sideband can be realized. Two polarization modulators are connected in series to realize twice suppression of the odd-order sideband. Meanwhile, only a pi/2 phase difference exists between radio-frequency signals loaded on the two polarization modulators, and the modulation indexes of the polarization modulators are controlled, so that other sidebands other than positive and negative four-order sidebands are suppressed in order to generate an eightfold-frequency optical millimeter wave signal. Through the frequency beat function of a photo-electric detector, an electric domain millimeter wave signal is obtained, and the frequency of the signal is eight times that of a radio frequency local oscillator. According to the method, direct-current offset does not need to be controlled, and no optical filter is needed, so that generated millimeter waves have the advantages of high stability, high frequency spectrum purity, adjustable frequency, wide adjustable range and the like.

Description

Based on 8 times frequency optical millimeter waves production method and systems of light polarization modulator cascade

Technical field

The present invention relates to optical communication and millimetre-wave attenuator field, particularly relate to the light that light carries radio frequency (RoF) system centre station and carry millimeter wave generation technology, provide that a kind of structure is simple, system stability, the method for the millimeter wave of frequency-tunable can be produced.

Background technology

It is a kind of Novel Communication technology combining optical fiber communication and microwave communication that light carries radio frequency (Radio overFiber, ROF) technology.On the one hand, for optical fiber communication, due to low-loss and the high bandwidth characteristic of optical fiber, optical fiber communication is gradually from the application development of core net to the Access Network close to user.Along with the appearance of increasing broadband multimedia services and universal, at a high speed, jumbo Optical Access Network become the developing direction of optical communication field.On the other hand, for radio communication, due to convenient, individualized and ubiquitous characteristic, wireless access technology has become another important directions of global communications network development.And both being combined the developing direction of future communications beyond doubt, RoF technology has also been born thus.

In numerous key technologies of ROF system, the generation of high-quality millimeter wave improves light carrier radio communication RoF systematic function especially and reduces the key technology of system cost, proposes the generation scheme of millimeter wave so far in existing many documents.Along with the development of microwave photon technology, utilize Microwave photonics to produce high frequency, the millimeter wave of high stability has significant advantage.The study hotspot in this field is the photoproduction millimeter-wave signal how obtaining high-quality, high band at present, the generation comprising high-purity millimeter-wave signal and the millimeter-wave signal overcoming fiber dispersion effects transmission.Recent years, academia reports the millimeter-wave signal optics generation technique of various ways, such as light heterodyne method, and investigation mission outside the city or town method for making, based on nonlinear effect four-wave mixing effect method and stimulated Brillouin scattering method etc.In the method for reported generation millimeter-wave signal, light investigation mission outside the city or town method for making because of its have that system configuration is simple, stable operation and the advantage of frequency-tunable be widely used.Some research institutions have also all utilized light investigation mission outside the city or town method for making to have devised various microwave multiple-frequency system with organizing, Clock Multiplier Factor be two frequencys multiplication, quadruple, six frequencys multiplication, octonary, twelvefold frequently not etc., the modulator of use also has intensity modulator, light polarization modulator and phase-modulator etc.In the middle of these external modulators, light polarization modulator (PolM) can to realize transverse electric mode and the transverse magnetic mode of opposite phase modulation index because of it as a special phase-modulator, relative usage is more flexible, it is under different driving powers, suppressed carrier can be realized respectively, suppress even-order harmonics and suppress the advantages such as odd-order harmonics, paid close attention to more and more widely and apply.

At present, the research be applied to by PolM in RoF system has the main results of the following aspects: 1, obtain frequency multiplication photoelectric oscillator (OEO) based on light polarization modulator, 2, based on light polarization modulator generated frequency comb spectrum, 3, the millimeter wave frequency multiplication based on light polarization modulator produces, but these frequency multiplication achievements are mostly confined to quadruple.

To sum up consider, in order to produce the millimeter wave of high frequency, high stability, and to reach that system configuration is simple, the object of stable operation and frequency-tunable, the octonary light that this patent proposes based on cascade two PolM carries millimeter wave generating method.System configuration designed by the method is simple and easy-to-use, because the modulator used is PolM, so do not need direct current biasing, thus the impact not having offset drift to bring, the stability of a system is high; In addition owing to not needing the optical filter needed for traditional external modulation method, the frequency tuning speed producing millimeter wave is like this high, and adjustable extent is large, and transmission performance is good.

Summary of the invention

The invention provides a kind of 8 times frequency optical millimeter waves production methods based on two light polarization modulator cascades.

Utilize the PolM of two cascades, realize the suppression of twice pair of odd-order sideband, and finally only remain positive and negative quadravalence sideband by the control of radio frequency drive singal and PolM modulation index, thus reach octonary effect.

First, the linearly polarized light sent by laser is injected into a light polarization modulator PolM by a Polarization Controller PC, PC is regulated to make the polarization direction of linearly polarized light become 45 degree with two main shaft angles of PolM, realize the component of linearly polarized light in two orthogonal polarisation state of PolM equal, PolM is f in frequency _rFradio frequency signals drive under, two crossed polarized lights create two contrary phase-modulations respectively, and the light signal expression formula that PolM exports is as follows:

$\left[\begin{array}{c}{E}_{ax}\\ E_{ay}\end{array}\right]=\frac{\sqrt{2}}{2}{E}_0\left[\begin{array}{c}\exp \left({\mathrm{j\ω}}_{o}t+{\mathrm{j\β}}_1{\mathrm{cos\ω}}_{RF}t\right)\\ \exp \left({\mathrm{j\ω}}_{o}t-{\mathrm{j\β}}_1{\mathrm{cos\ω}}_{RF}t\right)\end{array}\right]---\left(1\right)$

Wherein E ₀and ω ₀=2 π f ₀amplitude and the angular frequency of the light carrier of input respectively, β ₁the modulation index of first PolM, ω _rF=2 π f _rFit is the angular frequency of radio-frequency (RF) driving signal.Two crossed polarized lights shown in formula (1) make two crossed polarized lights synthesize a linearly polarized light by a polarizer pol, the angle regulating pol main shaft and two crossed polarized lights is 45 degree, then the expression formula of the linearly polarized light of polarizer output is

Formula (2) is launched based on the primal Bessel function,

$\begin{array}{c}{E}_c=\frac{1}{2}{E}_0\exp \left({\mathrm{j\ω}}_{o}t\right)\[\underset{n=-\mathrm{\∞}}{\overset{+\mathrm{\∞}}{\Σ}}{j}^n{J}_n\left({\mathrm{\β}}_1\right)\exp \left({\mathrm{jn\ω}}_{RF}t\right)+\underset{n=-\mathrm{\∞}}{\overset{+\mathrm{\∞}}{\Σ}}{j}^n{J}_n\left(-{\mathrm{\β}}_1\right)\exp \left({\mathrm{jn\ω}}_{RF}t\right)\]\\ =\frac{1}{2}{E}_0\exp \left({\mathrm{j\ω}}_{o}t\right)\left\{\underset{n=-\mathrm{\∞}}{\overset{+\mathrm{\∞}}{\Σ}}\[1+{\left(-1\right)}^n\]j^n{J}_n\left({\mathrm{\β}}_1\right)\exp \left({\mathrm{jn\ω}}_{RF}t\right)\right\}\end{array}---\left(3\right)$

Wherein, J _nx () represents n rank the primal Bessel function, as can be seen from formula (3), the odd-order sideband of output optical signal is suppressed.Then, light signal shown in formula (3) incides in second PolM, its effect is identical with first PolM, but the radio-frequency (RF) driving signal of second PolM and first have the phase shift of pi/2, afterwards, synthesize through second polarizer pol, second pol main shaft angle that is same and two crossed polarized lights is 45 degree, then its output can be expressed as again

$\begin{array}{c}{E}_c=\frac{1}{4}{E}_0\exp \left({\mathrm{j\ω}}_{o}t\right)\[\exp \left({\mathrm{j\β}}_1{\mathrm{cos\ω}}_{RF}t\right)+\exp \left(-{\mathrm{j\β}}_1{\mathrm{cos\ω}}_{RF}t\right)\]\\ \·\[\exp \left({\mathrm{j\β}}_2{\mathrm{sin\ω}}_{RF}t\right)+\exp \left(-{\mathrm{j\β}}_2{\mathrm{sin\ω}}_{RF}t\right)\]\end{array}---\left(4\right)$

Wherein β ₂it is the modulation index of second PolM.Formula (4) is launched based on the primal Bessel function, obtains

$\begin{array}{c}{E}_c=\frac{1}{4}{E}_0\exp \left({\mathrm{j\ω}}_{o}t\right)\{\underset{n=-\mathrm{\∞}}{\overset{+\mathrm{\∞}}{\Σ}}\[1+{(-1)}^n\]j^n{J}_n\left({\mathrm{\β}}_1\right)\exp \left({\mathrm{jn\ω}}_{RF}t\right)\}\\ \·\{\underset{n=-\mathrm{\∞}}{\overset{+\mathrm{\∞}}{\Σ}}\[1+{(-1)}^m\]J_m\left({\mathrm{\β}}_2\right)\exp \left({\mathrm{jm\ω}}_{RF}t\right)\}\end{array}---\left(5\right)$

As can be seen from formula (5), the odd-order sideband of the light signal of output is suppressed, only remaining carrier wave and even-order sideband.When the value of modulation index is less, the Bessel function of quadravalence and more high-order can be ignored, so formula (5) is unfolded as follows by we

$\begin{array}{c}{E}_c=\frac{1}{4}{E}_0\exp \left({\mathrm{j\ω}}_{o}t\right)\[2J_0\left({\mathrm{\β}}_1\right)-2J_2\left({\mathrm{\β}}_1\right)\exp \left(-2{\mathrm{j\ω}}_{RF}t\right)-2J_2\left({\mathrm{\β}}_1\right)\exp \left(-2{\mathrm{j\ω}}_{RF}t\right)\]\·\\ \[2J_0\left({\mathrm{\β}}_2\right)+2J_2\left({\mathrm{\β}}_2\right)\exp \left(-2{\mathrm{j\ω}}_{RF}t\right)+2J_2\left({\mathrm{\β}}_2\right)\exp \left(2{\mathrm{j\ω}}_{RF}t\right)\]\\ =E_0\exp \left({\mathrm{j\ω}}_{o}t\right)\{\[J_0\left({\mathrm{\β}}_1\right)J_0\left({\mathrm{\β}}_2\right)-2J_2\left({\mathrm{\β}}_1\right)J_2\left({\mathrm{\β}}_2\right)\]+\\ \[J_0\left({\mathrm{\β}}_1\right)J_2\left({\mathrm{\β}}_2\right)-J_2\left({\mathrm{\β}}_1\right)J_0\left({\mathrm{\β}}_2\right)\]\[\exp \left(-2{\mathrm{j\ω}}_{RF}t\right)+\exp \left(2{\mathrm{j\ω}}_{RF}t\right)\]+\\ \[-J_2\left({\mathrm{\β}}_1\right)J_2\left({\mathrm{\β}}_2\right)\exp \left(-4{\mathrm{j\ω}}_{RF}t\right)\]+\[-J_2\left({\mathrm{\β}}_1\right)J_2\left({\mathrm{\β}}_2\right)\exp \left(4{\mathrm{j\ω}}_{RF}t\right)\]\}\end{array}---\left(6\right)$

Make the light signal of generation only containing positive and negative quadravalence sideband, the light carrier in formula (6) and positive and negative second order suppress sideband need fall by we, and therefore we need to ensure J ₀(β ₁) J ₀(β ₂)-2J ₂(β ₁) J ₂(β ₂)=0, simultaneously J ₀(β ₁) J ₂(β ₂)-J ₂(β ₁) J ₀(β ₂)=0, can obtain thus the modulation index of two PolM the relation that meets be β ₁=β ₂=1.7.

When meeting above-mentioned condition, the light of generation carries millimeter wave expression formula and is

$E_c=-E_0{J}_2^2\left(\mathrm{\β}\right)\{\exp \[j({\mathrm{\ω}}_0+4{\mathrm{\ω}}_{RF})t\]+\exp \[j({\mathrm{\ω}}_0-4{\mathrm{\ω}}_{RF})t\]\}---\left(7\right)$

This shows, in the light wave of generation, be only left positive and negative quadravalence sideband; Detect by photoelectric detector PD, generate electrical domain millimeter-wave signal, its frequency is the octuple of radio-frequency (RF) driving signal frequency, and its photoelectric current expression formula is

I＝μ|E _c| ²＝2J ₂ ⁴(β)μE ₀ ²[1+cos8ω _RFt] (8)

The invention provides a kind of 8 times frequency optical millimeter waves based on two light polarization modulator cascades and produce system.

System comprises: continuous laser source CW, Polarization Controller PC, radio frequency source RF, a phase-shifter PS, two light polarization modulator PolM, two polarizer pol and photoelectric detector PD.Continuous lasing light emitter CW: the frequency required for generation is f ₀continuous light wave; Polarization Controller PC: regulate PC to make the polarization direction of the linearly polarized light of input and PolM two main shafts in angle of 45 degrees, thus make the component of linearly polarized light in two orthogonal polarisation state of PolM equal; Radio frequency source RF: as the RF driving source of PolM, its frequency is f _rF, then the frequency of the octonary electrical domain millimeter-wave signal produced is 8f _rF; Phase-shifter PS: for the radiofrequency signal be modulated on two PolM provides the phase shift of a pi/2, thus the light carrier making twice modulation produce and positive and negative second order sideband have contrary amplitude, produce the effect of the superposition that disappears mutually, when modulation depth is 1.7, light carrier and the positive and negative second order sideband amplitude after superposing that disappears mutually is 0, and then curbs these sidebands; Light polarization modulator PolM: be f in frequency _rFradio frequency signals drive under, for two crossed polarized lights being input to modulator provide two contrary phase-modulations; Polarizer pol: two crossed polarized lights of input are synthesized a linearly polarized light, the size of the linearly polarized light of its synthesis is determined by angle between polarizer main shaft and two crossed polarized lights, in the present invention, this angle is set to 45 degree, then the linearly polarized light synthesized is the secondary coherent superposition such as grade of two crossed polarized lights; Photoelectric detector PD: be 8f for realizing frequency interval _rFthe beat frequency of two optical sidebands, being converted into frequency is 8f _rFthe millimeter wave signal of telecommunication.

Compared with the method and system of other frequencys multiplication, the advantage of provided by the invention times of frequency generating method and system is: modulator used is light polarization modulator, do not need to control its direct current biasing, so can not impact because of offset drift, it is high that the octonary light of generation carries millimeter-wave signal stability; This generation light carries the control mode of the method for millimeter wave very simply simultaneously, only needs between two radio-frequency (RF) driving signal, add that a phase-shifter is to provide the phase shift of pi/2, regulates the modulation index of PolM just can reach the object suppressing unnecessary sideband simultaneously.Consider that the method does not need optical filter, so the octonary light produced carries millimeter-wave signal frequency-adjustable and adjustable extent is wide.

Accompanying drawing explanation

Figure 1 shows that system link figure

Figure 2 shows that octonary light carries millimeter-wave signal spectrogram

The radio spectrum figure of the photoelectric current that Fig. 3 obtains through photodetector

The oscillogram of the photoelectric current that Fig. 4 obtains through photodetector

Embodiment

First, as shown in Figure 1, frequency is f ₀=193.1THz, live width are that the linearly polarized light that the continuous wave laser of 500MHz sends injects PC, adjustment PC makes linearly polarized light polarization direction and PolM two main shafts in angle of 45 degrees, realize the component of linearly polarized light in two orthogonal polarisation state of PolM equal, PolM is driven by 10GHz radiofrequency local oscillator, and to arrange modulation index be 1.7.Two crossed polarized lights exported from a PolM are collectively referred to as a linearly polarized light through a pol, pol main shaft is regulated to become miter angle with two crossed polarized lights, the linearly polarized light then synthesized is the secondary coherent superposition such as grade of two crossed polarized lights, thus suppress odd-order sideband, only remaining light carrier and even-order sideband.The light wave that pol exports is injected into the 2nd PolM, and its RF driving source is the 10GHz local oscillator identical with first, but has the phase shift of pi/2 with first radiofrequency signal, and this phase shift is provided by phase-shifter, and the modulation index of second PolM is also set to 1.7.Two crossed polarized lights exported from second PolM are combined into linearly polarized light by another pol, second pol major axes orientation still becomes miter angle with two crossed polarized lights, thus create positive and negative quadravalence sideband and eliminate light carrier and positive and negative second order sideband, the spectrogram generated as shown in Figure 2, can find out that the spectrum of generation mainly comprises two positive and negative quadravalence sidebands.After detecting through PD, two quadravalence sideband beat frequencies will produce octonary millimeter wave, because radio-frequency (RF) local oscillator is 10GHz, so will the electrical domain millimeter-wave signal of 80GHz be produced, the radio spectrum of the electrical domain millimeter-wave signal generated as shown in Figure 3, radio spectrum can be found out except comprising 80GHz millimeter-wave signal, also comprise the millimeter-wave signal of 40GHz and 120GHz, its reason is that the light carrier of also residual small part in the spectrum generated and positive and negative eight rank sidebands are not totally constrained, only residual in spectrogram Fig. 2 small part light carrier and positive and negative eight rank sidebands have been flooded by phase noise, so do not observe, because phase noise has good coherence, so can cancel out each other after beat frequency, then in the radio spectrum generated, there is the radiofrequency signal of 40GHz and 120GHz, but the amplitude of the radiofrequency signal of 40GHz and 120GHz generated is very little, the impact caused 80GHz millimeter-wave signal is also very little, the oscillogram of this point photoelectric current as shown in Figure 4 also can be found out.Owing to there is not optical filter in native system, so the millimeter-wave signal frequency-adjustable produced, by regulating the frequency of radio-frequency (RF) local oscillator, the frequency of the electrical domain millimeter-wave signal of generation can be changed, its frequency-adjustable wide range.

In sum, the present invention utilizes the mode of two PolM cascades to produce octonary millimeter wave, and creates the electrical domain millimeter wave of 80GHz by beat frequency.The method does not need optical filter, and structure is simple, and the millimeter wave of generation has that stability is high, spectral purity is high, frequency-adjustable and the wide advantage of adjustable extent.

The above; be only the specific embodiment of the present invention; but protection scope of the present invention is not limited thereto; anyly be familiar with those skilled in the art in the technical scope that the present invention discloses; change can be expected easily or replace; in addition, the frequency range of millimeter wave is also not limited to 80GHz, and the adjustment of these equivalent variations and replacement and frequency range also should be considered as protection scope of the present invention.

Claims (5)

1., based on 8 times frequency optical millimeter waves production methods of light polarization modulator cascade, its feature comprises:

First, the linearly polarized light sent by laser is injected into a light polarization modulator PolM by a Polarization Controller PC, regulate PC to make the polarization direction of linearly polarized light become 45 degree with two main shaft angles of PolM, realize the component of linearly polarized light in two orthogonal polarisation state of PolM equal, PolM is f in frequency _rFradio frequency signals drive under, two crossed polarized lights create two contrary phase-modulations respectively, two crossed polarized lights are made to synthesize a linearly polarized light through a polarizer pol, the angle regulating pol main shaft and two crossed polarized lights is 45 degree, then the odd-order sideband of output optical signal is suppressed, only remaining light carrier and even-order sideband, then, the light wave exported incides in second PolM, its effect is identical with first PolM, but the radio-frequency (RF) driving signal of second PolM and first have the phase shift of pi/2, afterwards, synthesize through second polarizer pol again, second pol main shaft angle that is same and two crossed polarized lights is 45 degree, light carrier and even-order sideband is still only comprised in the light signal then exported, radio-frequency driven due to two PolM has the phase shift of pi/2, and the modulation index of two PolM is 1.7, then light carrier, positive and negative second order sideband and positive and negative six rank sidebands suppressed, positive and negative eight rank sideband amplitude are very little, so positive and negative quadravalence sideband is occupied an leading position in the light wave generated, detect by photoelectric detector PD, generate electrical domain millimeter-wave signal, its frequency is the octuple of radio-frequency (RF) driving signal frequency.

2. method according to claim 1, is characterized in that, described light polarization modulator:

Light polarization modulator PolM is a special phase-modulator, index of modulation phase-modulation on the contrary can be carried out to the transverse electric mode of the light wave wherein transmitted and transverse magnetic mode, under the cooperation of suitable parameter configuration and polarizer pol, the suppression of dual numbers order harmonics or odd-order harmonics can be realized.

3. method according to claim 1, is characterized in that, described Polarization Controller:

The effect of Polarization Controller PC regulates the optical electric field vector direction being injected into the linearly polarized light of PolM, makes it to be miter angle with PolM waveguide transverse electric mode and transverse magnetic mode direction, and then the transverse electric mode excited in PolM and transverse magnetic mode have identical amplitude.

4. method according to claim 1, is characterized in that, described polarizer:

The effect of polarizer pol is that two crossed polarized lights of input are synthesized a linearly polarized light, and the size of the linearly polarized light of its synthesis is determined by angle between polarizer main shaft and two crossed polarized lights; In the present invention, best results when this angle is 45 degree, the linearly polarized light of synthesis is the secondary coherent superposition such as grade of two crossed polarized lights; In first pol, odd-order sideband disappears superposition mutually, and even-order sideband is long superposition mutually; In second pol, except odd-order sideband disappears except superposition mutually, light carrier, positive and negative second order sideband also can be made to disappear mutually due to other same light wave frequently superposition of modulation generation, and then suppress light carrier and positive and negative second order sideband, when modulation depth is 1.7, best results.

5. 8 times frequency optical millimeter waves based on two light polarization modulator cascades produce a system, it is characterized in that:

Described system comprises: continuous laser source CW, Polarization Controller PC, radio frequency source RF, phase-shifter PS, two light polarization modulator PolM, polarizer pol and photoelectric detector PD;

Continuous laser source CW in described system: the frequency required for generation is f ₀continuous light wave;

Polarization Controller PC in said system: regulate PC to make the polarization direction of the linearly polarized light of input and PolM two main shafts in angle of 45 degrees, thus make the component of linearly polarized light in two orthogonal polarisation state of PolM equal;

Radio frequency source RF in said system: as the RF driving source of PolM, its frequency is f _rF, then the frequency of the octonary electrical domain millimeter-wave signal produced is 8f _rF;

Phase-shifter PS in said system: for the radiofrequency signal be modulated on two PolM provides the phase shift of a pi/2, thus the light carrier making twice modulation produce and positive and negative second order sideband have contrary amplitude, produce the effect of the superposition that disappears mutually, when modulation depth is 1.7, light carrier and the positive and negative second order sideband amplitude after superposing that disappears mutually is 0, and then curbs these sidebands;

Light polarization modulator PolM in said system: be f in frequency _rFradio frequency signals drive under, for two crossed polarized lights being input to modulator provide contrary phase-modulation;

Polarizer pol in said system: two crossed polarized lights of input are synthesized a linearly polarized light, the size of the linearly polarized light of its synthesis is determined by angle between polarizer main shaft and two crossed polarized lights, be 45 degree in the present invention, the linearly polarized light of synthesis is the secondary coherent superposition such as grade of two crossed polarized lights;

Photoelectric detector PD in described system: be 8f for realizing frequency interval _rFthe beat frequency of two optical sidebands, being converted into frequency is 8f _rFthe millimeter wave signal of telecommunication.