CN102662290A - Self-phase modulation effect based transient signal light modulus conversion system - Google Patents

Abstract

A self-phase modulation effect based transient signal light modulus conversion system comprises a laser. An output end of the laser is connected with an input end of a first circulator through an adjustable attenuator. A first output end of the first circulator is connected with a first faraday rotating mirror through a first dispersive medium, and a second output end thereof is connected with an input end of a double-arm single-end-output electric light modulator through a polarization controller. Microwave signals are divided into two channels to be connected with the double-arm single-end-output electric light modulator through a hybrid coupler. An output end of the double-arm single-end-output electric light modulator is connected with an input end of a second circulator, a first output end of the second circulator is connected with the second faraday rotating mirror through a second dispersive medium, and a second output end of the second circulator is sequentially connected with an electric modulus converter through an optical fiber amplifier and a photoelectric detector. The self-phase modulation effect based transient signal light modulus conversion system can reduce length and loss of the dispersive media and cost.

Description

Transient signal optical mode analog-to-digital converting system based on self phase modulation

Technical field

The present invention relates to optical mode number conversion technical field, what be specifically related to is a kind of transient signal optical mode analog-to-digital converting system based on self phase modulation.

Background technology

Along with the continuous lifting of digital signal processing capability, to performance of analog-to-digital convertor require increasingly high, nowadays for high sampling rate; High resolving power; The demand of high bandwidth analog to digital converter is increasing, is mainly used in advanced experimental apparatus, military system; The biomedical imaging system, key areas such as radar system and communication system.The Jalali of UCLA professor experimental group has at first proposed to utilize the method for time-stretching to improve the sampling rate of A/D conversion system in 1999; It is less that it has many advantages such as light pulse to shake with respect to traditional electric A/D conversion system, and measurable microwave signal bandwidth range is bigger or the like.Jalali professor experimental group has also proposed the high-speed transient signal A/D conversion system of 10TSa/s in 2007.

The principle of work of traditional high-speed transient signal optical mode analog-to-digital converting system that only utilizes the chromatic dispersion drawing process is that first and second section dispersive medium is successively passed through in light pulse, and the bandwidth of supposing light pulse is that (the bound wavelength is respectively λ to Δ λ ₁, λ ₂), the abbe number of dispersive medium is D (λ), then (length is L through first section dispersive medium ₁) after, pulse width becomes:

t ₁＝L ₁×τ ₁(λ)(1)

Wherein

is the optical pulse broadening on the unit length dispersive medium.(length is L through second section dispersive medium ₂) after, the burst length width becomes:

t ₂＝L ₁×τ ₁(λ)+L ₂×τ ₂(λ) (2)

If the front and back dispersive medium has identical dispersion characteristics, i.e. τ ₁(λ)=τ ₂(λ), then before and after the ratio (t of burst length width ₂/ t ₁) determined time-stretching multiple (being RF bandwidth reduction multiple):

M＝(L ₁+L ₂)/L ₂ (3)

This method bigger dispersion measure of needs in high-speed transient signal optical mode analog-to-digital converting system that only relies on chromatic dispersion that light pulse is stretched; The dispersive medium certain for abbe number often needs very long fiber lengths; Loss was bigger when light pulse was transmitted in longer dispersive medium, and system signal noise ratio and effective bit are not very high.

Summary of the invention

The objective of the invention is to deficiency to above-mentioned prior art; A kind of transient signal optical mode analog-to-digital converting system based on self phase modulation is proposed; The peak power of the light pulse of sending through adjustable attenuator adjustment passive mode-locking fiber laser; Thereby make through the light pulse of first section dispersive medium and under the acting in conjunction of self phase modulation and effect of dispersion, quicken broadening, and then reduce required dispersive medium length greatly.

Technical solution of the present invention is following:

A kind of transient signal optical mode analog-to-digital converting system based on self phase modulation; Its characteristics are that the formation of this system comprises laser instrument; The output terminal of this laser instrument is connected through the input end of adjustable attenuator with first circulator; First output terminal of first circulator is connected with first faraday rotation mirror through first dispersive medium; Second output terminal of first circulator is connected through the input end of the single-ended output electrooptic modulator of Polarization Controller and both arms, and microwave signal is divided into two-way through hybrid coupler, and one road signal is input on the arm of the single-ended output electrooptic modulator of both arms; Another road signal carries out through phase shifter being input on another arm of the single-ended output electrooptic modulator of both arms after the phase shift of 90 degree; The input end of output termination second circulator of the single-ended output electrooptic modulator of both arms, first output terminal of second circulator is connected with second faraday rotation mirror through second dispersive medium, and second output terminal of second circulator links to each other through fiber amplifier, photodetector and electric analog to digital converter successively.

Described first dispersive medium and second dispersive medium are the dispersive medium with higher chromatic dispersion transfer ratio.

Described first dispersive medium and second dispersive medium are dispersion compensating fiber.

Described first faraday rotation mirror and second faraday rotation mirror have lower insertion loss for protecting inclined to one side faraday rotation mirror.

Described laser instrument is the passive mode-locking fiber laser of 1540-1560nm.

Know-why of the present invention is following:

1. from phase modulation technique

When the ultrashort light pulse that passive mode-locking fiber laser sends passed through first section dispersive medium, its transmission equation satisfied non-linear Schrodinger equation:

$i\frac{\∂U}{\∂\mathrm{\ξ}}=\mathrm{sgn}\left({\mathrm{\β}}_2\right)\frac{1}{2}\frac{{\∂}^{2}U}{{\∂\mathrm{\τ}}^2}-N^2{e}^{-\∂L}{\left|U\right|}^{2}U---\left(4\right)$

Wherein, ξ=L/L _DRepresent normalized apart from variable, τ=T/T ₀Represent normalized time variable, L _DExpression chromatic dispersion length, T ₀3dB width for light pulse.Parameter N is defined as:

${\mathrm{<figure-callout\; id="2"\; label="N"\; filenames="HDA00001709330300031.png,HDA00001709330300041.png"\; state="\{\{state\}\}">N</figure-callout>}}^2=\frac{L_D}{L_{\mathrm{NL}}}=\frac{\mathrm{\&gamma;}{P}_0{\mathrm{<figure-callout\; id="0"\; label="T"\; filenames="HDA00001709330300022.png,HDA00001709330300031.png"\; state="\{\{state\}\}">T</figure-callout>}}_0^2}{2.77\left|{\mathrm{\&beta;}}_2\right|}---\left(5\right)$

Wherein, L _NLBe non-linear length, γ is the nonlinear factor of dispersive medium, β ₂Can be expressed as:

${\mathrm{\&beta;}}_2=-\frac{{\mathrm{D\&lambda;}}^2}{2\mathrm{\&pi;c}}---\left(6\right)$

Wherein, D is the abbe number of dispersive medium, and λ is an optical wavelength, and c is an optical pulse propagation speed.

Parameter N is determining that light pulse is by from phase modulation (PM) or effect of dispersion plays a major role when in dispersive medium, transmitting.< < 1 the time, effect of dispersion plays a major role as N; As N>> 1 the time, self phase modulation plays an important role; And when N=1, chromatic dispersion and self phase modulation play no less important.Can know by (5); Light pulse for fixed pulse width; N is relevant with the light pulse peak power in getting into dispersive medium, therefore can adjust the peak power of light pulse through adjustable attenuator, and then the size of self phase modulation when controlling optical pulse propagation.

In traditional chromatic dispersion drawing process, only utilize effect of dispersion to come the broadening light pulse, the pulse strenching multiple in this method is shown with the relation table of transmission range:

$m=\sqrt{1+{(L/L_D)}^2}---\left(7\right)$

Among the present invention, adjust the peak power of light pulse, and then utilize self phase modulation to promote the broadening degree of chromatic dispersion light pulse through adjustable attenuator

Through numerical simulation is carried out in (4), can obtain transmitting (L=3L under the same distance condition _D), the degree of optical pulse broadening is with the different situation of change of N (peak power of corresponding light pulse).Fig. 1 is under transmission same distance condition, only have effect of dispersion and self phase modulation (N value be 1 and the N value be under 3 situation) during the promotion chromatic dispersion, the comparison diagram of pulse strenching.Can obviously find out when self phase modulation promotes chromatic dispersion pulse strenching more.Because that has opened up when pulse gets into second section dispersive medium is very wide; And because the loss of modulator, its peak power is fallen very lowly, and no longer producing nonlinear effect is self phase modulation; Therefore system's draw ratio with only have under the dispersion situation identically, still be M=1+L ₂/ L ₁(L ₁Be the length of first dispersive medium, L ₂Be the length of second dispersive medium).

Yet, when utilizing self phase modulation, can make light pulse produce certain phase shift, can be expressed as:

Wherein, Δ λ is the spectrum width of light pulse, and M is the draw ratio of system.This phase shift meeting that produces from phase modulation (PM) makes the microwave signal frequency of input receive bigger restriction, utilizes single-sideband modulation can eliminate this restriction, introduces in the face of the method down.

2. single-sideband modulation is technological

Microwave signal is input in the hybrid coupler, and is divided into two-way, wherein one road a port output from hybrid coupler, and another road makes it produce the phase shift of 90 degree through phase shifter after another port output of hybrid coupler.Two road microwave signals are loaded on two arms of the single-ended output electrooptic modulator of both arms again, utilize single-sideband modulation that it is modulated in the light pulse again, behind second section dispersive medium, the detected output marking current of photodetector is expressed as:

Wherein, a is the index of modulation,

The phase shift that causes for chromatic dispersion,

Be the phase shift that causes from phase modulation (PM), f _mThe frequency of microwave signal for input.From (9), can significantly find out and utilize the single-sideband modulation can be with chromatic dispersion with among the phase shift that phase modulation (PM) is introduced is transformed into the output current phase place; So avoided the problem of output microwave signal power cycle property decline to produce, also just removed the phase shift introduced from phase modulation (PM) and chromatic dispersion importing the restriction of microwave signal frequency.

3. from the steepening effect

When light pulse is transmitted in dispersive medium, if produce self phase modulation, then also can produce the high-order nonlinear effect accordingly through its peak power of adjustment, in this system, this high-order nonlinear effect mainly is that pulse is from the steepening effect.When light impulse length one timing, luminous power can not unconfinedly increase, and its upper limit should receive the restriction of generation threshold limit value when the steepening effect.If consider the high-order nonlinear effect in the optical fiber; The optical pulse propagation equation should satisfy the general non-linear schrodinger equation, if only study from the steepening effect, establishes from phase modulation (PM); The influence of chromatic dispersion does not exist and the fibre loss value is zero, and then the general non-linear schrodinger equation of this abbreviation is:

$\frac{\&PartialD;U}{\&PartialD;Z}+s\frac{\&PartialD;}{\&PartialD;T}\left({\left|U\right|}^{2}U\right)=i{\left|U\right|}^{2}U---\left(10\right)$

Wherein U is the envelope function of light pulse;

is in order to describe from the steepening effect; (10) are found the solution, can obtain in the pulse shape expression formula at transmission range Z place be:

I(Z,T)＝exp[-(T-3sI(Z,T)Z) ²]（11）

Non-linear length when wherein

describes optical pulse propagation; When pulsewidth one timing; S is a fixed value; Therefore when Z correspondence different value, pulse different from the steepening degree.Through numerical simulation is carried out in (11), can observe its steepening degree.

Fig. 2 is same pulse, (corresponding different Z values) its analogous diagram in various degree from steepening when the transmission different distance.Visible less from the steepening degree when s*Z=0.025 from figure, the corresponding optical power value of the Z value of this moment should become the threshold value of light pulse peak power.

4. circulator cooperates the faraday rotation mirror structure

System design is become circulator connect dispersive medium; The structure of dispersive medium output termination faraday rotation mirror; Can make light pulse at first enter into dispersive medium through circulator; Through the faraday rotation mirror reflection, light pulse gets into once more carries out the chromatic dispersion stretching in the dispersive medium, finally turn back to circulator then.Thisly make light pulse accomplish one to remove one time transmission structure, it is half that required fiber lengths is reduced, thereby save cost greatly.

Compared with prior art, beneficial effect of the present invention is following:

1. when light pulse is transmitted in first section dispersive medium; Produce self phase modulation through adjusting its peak power; It can impel chromatic dispersion to quicken the broadening of light pulse, can obviously reduce first section required dispersive medium length and reach the multiple that pulse needs broadening.

2. because when second section dispersive medium, self phase modulation a little less than, only colouredly distribute the effect of waving, system's draw ratio still is expressed as M=1+L ₂/ L ₁, therefore can significantly reduce fiber lengths, reduce cost, and loss reducing in the dispersive medium, system signal noise ratio and effective bit get a promotion.

3. electrooptic modulator is selected the single-ended output of both arms Mach-Ceng De (Mach-Zehnder) electrooptic modulator for use, utilizes single-sideband modulation, can remove because the problem of chromatic dispersion and the microwave signal power cycle property decline that causes from phase modulation (PM).

4. adopt circulator and faraday rotation mirror structure can make light pulse in dispersive medium, accomplish twice transmission course of contact, required chromatic dispersion total amount reduces by one times, so the minimizing of dispersive medium length is half the, thereby reaches the purpose that reduces cost.

Description of drawings

Fig. 1 a is original Gauss pulse, (L=3L under the identical traffic distance condition _D);

Fig. 1 b is at the broadening that only has the chromatic dispersion clock pulse to dash;

Fig. 1 c is under phase modulation (PM) (N=1) situation, promotes the broadening of chromatic dispersion paired pulses;

Fig. 1 d is under phase modulation (PM) (N=3) situation, promotes the broadening of chromatic dispersion paired pulses.

Fig. 2 is for the identical Gauss pulse of pulsewidth, exists when the steepening effect, and the shape of pulse is with the situation of change of Z value.

Fig. 3 is the structural representation that the present invention is based on the transient signal optical mode analog-to-digital converting system of self phase modulation.

Among the figure: the 1-laser instrument, the 2-adjustable attenuator, 3-first circulator, 4-first dispersive medium, 5-first connects faraday rotation mirror, 6-Polarization Controller, the single-ended output electrooptic modulator of 7-both arms.The 8-hybrid coupler, 9-second circulator, 10-second dispersive medium, 11-second faraday rotation mirror, 12-fiber amplifier, 13-photodetector, 14-electricity analog to digital converter, 15-microwave signal, 16-phase shifter.

Fig. 4 is the variation relation (left ordinate) of dispersive medium length with luminous power, and the raising value of system's effective bit is with the variation relation (right ordinate) of luminous power.

Embodiment

Below in conjunction with embodiment and accompanying drawing the present invention is described further, but should limit protection scope of the present invention with this.

Please consult Fig. 3 earlier, Fig. 3 is the structural representation that the present invention is based on the transient signal optical mode analog-to-digital converting system of self phase modulation.As shown in the figure; A kind of transient signal optical mode analog-to-digital converting system based on self phase modulation, the formation of this system comprises laser instrument 1, adopts passive mode-locking fiber laser in the present embodiment; The output terminal of laser instrument connects the input end of first circulator 3 through adjustable attenuator 2; First output terminal of first circulator 3 is through first dispersive medium 4, and second output terminal that connects first faraday rotation mirror, 5, the first circulators 3 connects the input end of the single-ended output electrooptic modulator 7 of both arms through Polarization Controller 6.Microwave RF signal 15 is divided into two-way behind hybrid coupler 8, the one tunnel connects an arm of the single-ended output electrooptic modulator 7 of both arms, and another road carries out through phase shifter 16 receiving another arm of the single-ended output electrooptic modulator 7 of both arms after the phase shift of 90 degree.The input end of output termination second circulator 9 of the single-ended output electrooptic modulator 7 of both arms; First output terminal of second circulator 9 is connected with second faraday rotation mirror 11 through second dispersive medium 10, and second output terminal of second circulator 9 is connected with electric analog to digital converter 14 through fiber amplifier 12, photodetector 13 successively.

First dispersive medium 4 has bigger nonlinear factor, and first dispersive medium 4 and second dispersive medium 10 have bigger abbe number.Present embodiment is selected dispersion compensating fiber for use.First faraday rotation mirror 5 and second faraday rotation mirror 11 are for protecting inclined to one side faraday rotation mirror and having lower insertion loss.

Fig. 4 for the dispersive medium abbe number is-120ps/nm.km nonlinear factor 4.5/w.km, loss 0.4dB/km.The input pulse pulsewidth is 1.5ps; Repetition frequency is 37MHz; Crossing the first dispersive optical fiber pulse stretcher is 450ps, and draw ratio is 50, if the sampling rate of rear end electricity analog to digital converter is 20GSample/s; The required dispersive medium length of hypervelocity transient signal optical mode analog-to-digital converting system that then total sampling rate is 1TSample/s concerns (left ordinate) with optical power change, and the raising value of system's effective bit is with the variation relation (right ordinate) of luminous power.

The method of work of transient signal optical mode analog-to-digital converting system that the present invention is based on self phase modulation is following:

1. the ultrashort light pulse that sends of laser instrument is regulated its peak power through adjustable attenuator, thus the size of control light pulse self phase modulation during through first dispersive medium.

2. the adjusted light pulse of peak power gets into first dispersive medium through first circulator; And generation self phase modulation; It can quicken the broadening of chromatic dispersion paired pulses, through first faraday rotation mirror, turns back to first circulator through first dispersive medium once more at last.

3. the chirped optical pulse of first circulator output guarantees that through its polarization state of Polarization Controller adjustment depth of modulation can reach higher value.

4. microwave signal is divided into two-way after being input to hybrid coupler, and wherein one tunnel microwave signal is from one of them port output, and another road microwave signal is carried out the phase shift of 90 degree from another port output back of hybrid coupler through phase shifter.Two way microwave signals is loaded on two arms of modulator, and then is modulated on the chirped optical pulse.

5. the lightwave signal after the modulation gets into second dispersive medium through second circulator because this moment, the light wave peak power was lower, so self phase modulation a little less than, dispersion interaction is only arranged, so the draw ratio of system still is M=1+L ₂/ L ₁, if M is constant, because L ₁Length reduces greatly, then L ₂Also reduce greatly.Lightwave signal returns second circulator again through second faraday rotation mirror.

6. the lightwave signal of second circulator output compensates the loss of light wave power through fiber amplifier, detects microwave signal through photodetector again, delivers to electric analog to digital converter and samples quantification treatment.

The results showed that the present invention utilizes self phase modulation in first dispersive medium, further strengthen the broadening effect of chromatic dispersion, than traditional method that only relies on chromatic dispersion paired pulses broadening, the length of saving dispersive medium greatly light pulse.Reduce the loss in the dispersive medium when reducing cost, and then significantly improve the signal to noise ratio (S/N ratio) and the effective bit of system.In addition, utilize circulator to cooperate the faraday rotation mirror structure, it is half that dispersive medium length is reduced, and further reduces system cost.Be expected to obtain widespread use in hypervelocity transient signal optical mode analog-to-digital converting system preparation field.

Claims (5)

1. transient signal optical mode analog-to-digital converting system based on self phase modulation; The formation that it is characterized in that this system comprises laser instrument (1); The output terminal of this laser instrument (1) is connected through the input end of adjustable attenuator (2) with first circulator (3); First output terminal of first circulator (3) is connected with first faraday rotation mirror (5) through first dispersive medium (4); Second output terminal of first circulator (3) is connected through the input end of the single-ended output electrooptic modulators of Polarization Controller (6) and both arms (7); Microwave signal (15) is divided into two paths of signals through hybrid coupler (8); One road signal is input on the arm of the single-ended output electrooptic modulators of both arms (7); Another road signal carries out through phase shifter (16) being input on another arm of the single-ended output electrooptic modulators of both arms (7) after the phase shift of 90 degree; The input end of output termination second circulator (9) of the single-ended output electrooptic modulators of both arms (7), first output terminal of second circulator (9) is connected with second faraday rotation mirror (11) through second dispersive medium (10), and second output terminal of second circulator (9) links to each other through fiber amplifier (12), photodetector (13) and electric analog to digital converter (14) successively.

2. the transient signal optical mode analog-to-digital converting system based on self phase modulation according to claim 1 is characterized in that, described first dispersive medium (4) and second dispersive medium (10) dispersive medium for having higher chromatic dispersion transfer ratio.

3. the transient signal optical mode analog-to-digital converting system based on self phase modulation according to claim 2 is characterized in that described first dispersive medium (4) and second dispersive medium (10) are dispersion compensating fiber.

4. the transient signal optical mode analog-to-digital converting system based on self phase modulation according to claim 1 is characterized in that, described first faraday rotation mirror (5) and second faraday rotation mirror (11) are for protecting inclined to one side faraday rotation mirror.

5. the transient signal optical mode analog-to-digital converting system based on self phase modulation according to claim 1 is characterized in that described laser instrument (1) is the passive mode-locking fiber laser of 1540-1560nm.

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