基于頻移反饋腔的全光纖射頻調(diào)制脈沖激光研究?

楊宏志 趙長明 張海洋 楊蘇輝 李晨

(北京理工大學(xué)光電學(xué)院,北京 100081)

基于頻移反饋腔的全光纖射頻調(diào)制脈沖激光研究?

楊宏志 趙長明 張海洋?楊蘇輝 李晨

(北京理工大學(xué)光電學(xué)院,北京 100081)

激光射頻調(diào)制,脈沖激光,頻移反饋腔,調(diào)制深度

1 引 言

光載微波激光雷達(dá)是指利用微波調(diào)制的激光作為探測載波,對目標(biāo)進(jìn)行測距、測速和其他特性探測的一種激光雷達(dá).與傳統(tǒng)的激光雷達(dá)、微波雷達(dá)不同,其以激光作為探測載波,具有激光雷達(dá)空間分辨率高的特點(diǎn),同時(shí)又利用微波信號進(jìn)行探測,具有微波雷達(dá)較強(qiáng)的抗大氣擾動的能力,是一種結(jié)合了激光雷達(dá)和微波雷達(dá)各自的優(yōu)點(diǎn)、又在一定程度上克服了二者不足的一種新型激光雷達(dá),具有廣泛的應(yīng)用前景[1?6].同時(shí),該體制借鑒新型的微波光子雷達(dá),在光纖上實(shí)現(xiàn)微波信號的產(chǎn)生、處理以及轉(zhuǎn)換,并將其作為探測載波用于目標(biāo)探測[7].在光載微波激光雷達(dá)系統(tǒng)中,激光源作為其中的核心部件,在一定程度上決定了雷達(dá)的工作模式、作用距離、探測精度等.當(dāng)前,遠(yuǎn)距離目標(biāo)的激光探測廣泛地使用脈沖激光作為發(fā)射機(jī).但受限于單脈沖工作體制,脈沖上升沿較窄,這對后端的脈沖激光放大、回波接收等提出了較高的要求.為了解決上述問題,部分學(xué)者提出了采用具有射頻調(diào)制的脈沖激光,即利用脈沖激光高峰值功率和射頻信號高精度的特性實(shí)現(xiàn)遠(yuǎn)距離、高精度的目標(biāo)探測[1,8,9].其中,Brunel等[9?11]結(jié)合雙頻脈沖激光和頻移反饋腔實(shí)現(xiàn)了雙頻脈沖激光射頻信號的相位鎖定,并將其用于目標(biāo)的速度測量.Zhang等[12]將調(diào)Q脈沖激光注入頻移反饋環(huán)中,通過脈沖疊加,實(shí)現(xiàn)了脈沖的射頻調(diào)制.

頻移反饋激光是在常規(guī)的Fabry-Perot腔(或者環(huán)形腔)內(nèi)插入移頻器,使得激光每次通過移頻器時(shí)頻率都發(fā)生變化.頻移反饋激光在鎖模脈沖產(chǎn)生、光域的實(shí)時(shí)傅里葉變換、頻率梳產(chǎn)生、啁啾光源與寬譜光源產(chǎn)生等方面有特殊的應(yīng)用[13?18].本文根據(jù)聲光斬波器的調(diào)制特性設(shè)計(jì)了基于頻移反饋腔的全光纖射頻調(diào)制脈沖激光.理論方面,建立了基于頻移反饋腔的激光外差相干理論模型,數(shù)值仿真了頻移反饋環(huán)路長度和斬波器觸發(fā)信號周期對射頻調(diào)制脈沖的影響,驗(yàn)證了光纖放大器增益系數(shù)對射頻調(diào)制深度的影響.實(shí)驗(yàn)上,利用聲光斬波器構(gòu)建頻移反饋腔,通過精確控制斬波周期和頻移反饋環(huán)路的長度,使得脈沖激光反復(fù)通過聲光斬波器,獲得了具有脈內(nèi)射頻調(diào)制的脈沖激光.同時(shí),在反饋環(huán)路中加入光纖放大器,通過改變光纖放大器的輸出功率實(shí)現(xiàn)了射頻信號調(diào)制深度的調(diào)節(jié).與之前的研究相比,該方案具有以下特點(diǎn):1)通過連續(xù)匹配斬波實(shí)現(xiàn)對納秒級短脈沖的射頻調(diào)制;2)通過改變光纖放大器的輸出功率可以實(shí)現(xiàn)射頻調(diào)制深度的連續(xù)可調(diào);3)采用全光纖器件,可靠性、穩(wěn)定性以及體積方面具有一定的優(yōu)勢.基于頻移反饋腔的全光纖射頻調(diào)制脈沖激光的研究為新體制光載微波雷達(dá)的研究奠定了理論和實(shí)驗(yàn)基礎(chǔ).

2 模型及仿真

2.1 頻移反饋激光理論模型

基于頻移反饋腔的射頻調(diào)制脈沖激光原理圖如圖1所示.單頻種子激光從光纖合束器的一端輸入,經(jīng)光纖放大器后被聲光斬波器調(diào)制,產(chǎn)生移頻脈沖光,再經(jīng)光纖分束器2,其中一路反饋進(jìn)入環(huán)形腔中,另外一路輸出觀測.設(shè)兩個(gè)耦合器的傳輸矩陣為[aij],種子激光電場為Eseed,輸出頻移反饋激光為EFSF,頻移反饋環(huán)路中的激光電場詳見圖1(a).

根據(jù)聲光斬波器調(diào)制特性,當(dāng)觸發(fā)信號周期為Trep時(shí),聲光斬波器產(chǎn)生重復(fù)頻率為1/Trep的激光脈沖;當(dāng)脈沖觸發(fā)信號的周期等于脈沖激光在環(huán)形腔內(nèi)的傳輸時(shí)間,之前被調(diào)制的脈沖激光經(jīng)光纖環(huán)路延遲后會被再次調(diào)制,即同一調(diào)制脈沖會多次通過聲光斬波器,產(chǎn)生射頻調(diào)制的脈沖激光.

聲光斬波器是利用聲光效應(yīng)同時(shí)實(shí)現(xiàn)強(qiáng)度調(diào)制和頻率調(diào)制的器件,如圖1(b)所示;強(qiáng)度調(diào)制為當(dāng)觸發(fā)信號上升沿到來時(shí),輸入的連續(xù)激光通過聲光晶體,產(chǎn)生脈沖激光;頻率調(diào)制為布拉格衍射使得激光頻率發(fā)生改變.其強(qiáng)度傳輸函數(shù)模型,如(1)式所示:

其中,Gauss(t)為聲光斬波器產(chǎn)生的脈沖包絡(luò);δ為狄拉克函數(shù),用來對脈沖包絡(luò)進(jìn)行周期拓展;Trep是觸發(fā)信號的周期.

合束器1輸出端的激光電場E1為頻移反饋環(huán)路內(nèi)分束器2的輸入端激光電場E2為

其中S(t)ejωft是聲光斬波器的調(diào)制函數(shù),G是光纖放大器增益,τ為激光在環(huán)形腔中的傳輸時(shí)間,γ為腔內(nèi)的其他損耗.分束器2的兩輸出端激光電場分別為Eloop和EFSF,

將(2)和(3)式代入(4)式中,可得到

調(diào)制脈沖激光在頻移反饋腔中多次循環(huán)調(diào)制后,得到頻移反饋環(huán)路內(nèi)的電場Eloop(t?nτ):

圖1 (a)基于頻移反饋腔的射頻調(diào)制脈沖激光原理圖;(b)聲光斬波器原理圖Fig.1.(a)Principle of radio frequency-modulated pulse based on frequency-shifted feedback(FSF)loop;(b)principle diagram of acousto-optic chopper.

將(7)式多次迭代并簡化:

根據(jù)(4),(5)和(8)式,得到頻移反饋激光電場為

根據(jù)光電探測器的光強(qiáng)響應(yīng)特性,頻移反饋激光的光強(qiáng)I(t)為

圖2 聲光斬波器觸發(fā)信號周期對射頻調(diào)制脈沖的影響,其中激光在環(huán)形腔內(nèi)的單次傳輸時(shí)間為50μs,光纖放大器增益為6Fig.2.The in fluence of trigger period on radio frequency-modulated pulse by simulation.The roundtrip time is 50μs and optically ampli fier gain coefficient is 6.

2.2 仿 真

根據(jù)聲光斬波器的傳遞函數(shù)(1)式和頻移反饋激光的光強(qiáng)表達(dá)式(10),對輸出的頻移反饋脈沖激光進(jìn)行數(shù)值仿真.設(shè)耦合器的電場傳輸矩陣聲光頻移量ωf=100 MHz;激光在環(huán)形腔內(nèi)的傳輸時(shí)間為50μs;斬波器觸發(fā)信號周期可調(diào);同時(shí)考慮到連接損耗、長光纖引入的損耗等,設(shè)γ=0.5.

設(shè)激光在頻移反饋腔內(nèi)的傳輸時(shí)間τ=50μs,調(diào)節(jié)聲光斬波器觸發(fā)信號周期,可以得到不同調(diào)制形式的脈沖,如圖2所示.

由圖2可知:聲光斬波器產(chǎn)生脈沖寬度為110 ns的激光;當(dāng)斬波器的觸發(fā)信號周期(反饋腔對應(yīng)的固定延遲為50μs)從49.7μs逐漸增大到50μs時(shí)(如圖2左側(cè)一列),脈沖激光經(jīng)過無調(diào)制-右側(cè)調(diào)制-對稱調(diào)制過程;當(dāng)斬波器的斬波周期從50μs逐漸增大至50.3μs時(shí)(如圖2右側(cè)一列),脈沖激光經(jīng)過了與之前完全相反的過程,即對稱調(diào)制-左側(cè)調(diào)制-無調(diào)制.通過調(diào)節(jié)斬波周期與頻移反饋腔的長度,可以實(shí)現(xiàn):1)射頻調(diào)制的脈沖激光;2)調(diào)制形式的多樣化(脈沖前、后沿調(diào)制).

當(dāng)調(diào)節(jié)聲光斬波器的斬波周期等于頻移反饋腔內(nèi)激光的傳輸時(shí)間時(shí),仿真增益系數(shù)對輸出脈沖光的影響,如圖3所示.當(dāng)增益系數(shù)G=1時(shí),脈沖內(nèi)射頻信號的調(diào)制深度η=0.1隨著增益系數(shù)的提高,調(diào)制深度逐漸增大;當(dāng)放大器增益系數(shù)G=6時(shí),調(diào)制深度達(dá)到η=0.67.仿真結(jié)果表明,通過調(diào)節(jié)光纖放大器的增益系數(shù)可以調(diào)節(jié)脈沖內(nèi)射頻的調(diào)制深度,增益系數(shù)越大,調(diào)制深度越大.

圖3 不同光纖放大器增益系數(shù)的射頻調(diào)制脈沖仿真圖,其中觸發(fā)信號周期為50μs,頻移反饋環(huán)路長為10 kmFig.3.The simulation diagrams of radio frequency-modulated pulse with different gain coefficients.Trigger period is 50μs and loop length is 10 km.

3 實(shí) 驗(yàn)

3.1 實(shí)驗(yàn)裝置

實(shí)驗(yàn)裝置如圖4所示.窄線寬連續(xù)激光(波長1080 nm,線寬<2 kHz,最大輸出功率10 mW)通過光纖耦合器1注入頻移反饋腔中,摻Y(jié)b3+光纖放大器(最大輸出功率500 mW)提供腔內(nèi)增益,被放大后的激光經(jīng)過長約10 km的光纖延時(shí)后通過光纖聲光斬波器,產(chǎn)生脈沖激光,光纖耦合器2的一個(gè)端口作為輸出端與帶寬為3.5 GHz高速光電探測器相連;另一輸出端接耦合器1的輸入端,脈沖激光再次進(jìn)入頻移反饋腔內(nèi)重復(fù)上述過程.光纖放大器輸出的輸出功率可以通過改變激光二極管(LD),抽運(yùn)光功率實(shí)現(xiàn),觸發(fā)信號由任意波形發(fā)生器提供,其方波周期可調(diào),其余實(shí)驗(yàn)參數(shù)同仿真參數(shù)設(shè)置.

圖4 實(shí)驗(yàn)裝置圖Fig.4.Schematic diagram of experiment setup.

圖5 基于頻移反饋腔的射頻調(diào)制脈沖時(shí)域圖 (a)聲光斬波器的觸發(fā)信號;(b)射頻調(diào)制脈沖激光;(c)射頻調(diào)制脈沖激光局部放大圖Fig.5.The experiment investigation of radio frequency-modulated pulse based on FSF loop in time domain:(a)Trigger signal of acousto-optic chopper;(b)radio frequency-modulated pulse train;(c)a larger version of radio frequency-modulated pulse train.

圖6 基于頻移反饋腔的射頻調(diào)制脈沖頻譜圖 (a)輸出脈沖激光射頻調(diào)制頻譜(分辨率帶寬為1 kHz);(b)第一階調(diào)制頻譜圖(中心頻率100 MHz,分辨率帶寬為1 kHz)Fig.6.Experimental spectral power of radio frequency-modulated pulse:(a)Output radio frequencymodulated pulse spectral power(resolution bandwidth is equal to 1 kHz);(b) first order radio frequencymodulated spectrum(central frequency 100 MHz,and resolution bandwidth is equal to 1 kHz).

3.2 實(shí)驗(yàn)結(jié)果

激光脈沖在聲光斬波器觸發(fā)信號上升沿處產(chǎn)生,如圖5(a)和圖5(b)所示:調(diào)節(jié)聲光斬波器的周期,使得Trep=τ,聲光斬波器產(chǎn)生的脈沖激光經(jīng)過環(huán)形腔后可以再次被調(diào)制,此時(shí)聲光斬波器周期為49.135μs.調(diào)節(jié)光纖放大器的輸出功率為80 mW,此時(shí)輸出的射頻調(diào)制激光脈沖的脈沖寬度為110 ns,調(diào)制深度為0.67,如圖5(c)所示.同時(shí),測量該脈沖的功率譜密度,如圖6(a)所示.射頻調(diào)制的最高調(diào)制頻率達(dá)到了700 MHz(7ωf).基頻的頻譜寬度約為9.5 MHz(圖6(b))對應(yīng)于110 ns的脈沖寬度,符合脈沖信號時(shí)間帶寬積基本定律.

圖7 實(shí)驗(yàn)探究光纖聲光斬波器周期對射頻調(diào)制脈沖的影響Fig.7.The in fluence of trigger signal period on radio frequency-modulation pulse in experiment.

當(dāng)調(diào)節(jié)觸發(fā)信號周期,使得聲光斬波器觸發(fā)信號周期不嚴(yán)格地對應(yīng)頻移反饋腔的腔長時(shí),會產(chǎn)生偏移調(diào)制的激光脈沖,如圖7所示.這是由于聲光斬波器產(chǎn)生的脈沖經(jīng)過環(huán)形腔后不能完全對應(yīng)再次通過.當(dāng)斬波周期小于49.135μs(斬波周期小于脈沖激光在環(huán)形腔內(nèi)的傳輸時(shí)間)時(shí),聲光斬波器開啟觸發(fā)信號,前一脈沖還未到達(dá)聲光斬波器,因此脈沖前沿產(chǎn)生了無射頻調(diào)制的脈沖;隨著時(shí)間的增加,前一脈沖激光通過了聲光斬波器,而此時(shí)如果還處于聲光斬波器的開啟階段,那么脈沖后沿就具有了射頻調(diào)制.同樣當(dāng)斬波周期從49.135μs增加到49.3μs時(shí),調(diào)制信號實(shí)現(xiàn)了從對稱調(diào)制-前沿調(diào)制-無調(diào)制的過程.與仿真結(jié)果(圖2)相比,實(shí)驗(yàn)中反饋腔對應(yīng)的周期為49.135μs(仿真為50μs),這主要是由于:1)反饋腔的長度除包含了10 km長度的光纖外,還有光纖耦合器、聲光斬波器以及光纖放大器引入的長度;2)單模光纖的折射率不嚴(yán)格等于1.5.但對比可以發(fā)現(xiàn),仿真結(jié)果與實(shí)驗(yàn)的變化趨勢是相符合的.

如圖8所示,當(dāng)改變光纖放大器的輸出功率時(shí),脈沖內(nèi)射頻信號的調(diào)制深度會隨之發(fā)生變化,這是由于光纖放大器補(bǔ)償了腔內(nèi)移頻光的損耗,改變了移頻光和非移頻光的幅度比,最終表現(xiàn)為射頻信號調(diào)制深度的變化.當(dāng)光纖放大器輸出功率為80 mW時(shí),得到了調(diào)制深度η=0.67具有射頻調(diào)制的脈沖激光,此時(shí)輸出脈沖的峰值功率約為10 mW.與仿真結(jié)果(圖3)相比,實(shí)驗(yàn)中是通過控制光纖放大器的輸出功率,實(shí)現(xiàn)了射頻調(diào)制的改變,這主要是由于光纖放大器本身為輸出功率控制而非增益控制.但對比可以發(fā)現(xiàn),仿真結(jié)果與實(shí)驗(yàn)的變化趨勢相符合,即提高輸出功率(增益),調(diào)制深度也隨之增加.

圖8 實(shí)驗(yàn)上不同光纖放大器輸出功率對射頻信號調(diào)制深度的影響Fig.8.The in fluence of optically ampli fier output power on the modulation depth of radio frequencymodulated pulse in experiment.

4 結(jié) 論

射頻調(diào)制的脈沖激光是光載微波雷達(dá)的重要組成部分.根據(jù)頻移反饋腔和聲光斬波器的頻率和強(qiáng)度調(diào)制特性,建立了基于頻移反饋腔的激光外差相干理論模型,仿真了聲光斬波器觸發(fā)信號的周期、光纖放大器增益對射頻調(diào)制脈沖的影響,并進(jìn)行了實(shí)驗(yàn)研究.通過連續(xù)匹配斬波實(shí)現(xiàn)了對納秒級脈沖的射頻調(diào)制;通過改變光纖放大器的輸出功率可以實(shí)現(xiàn)射頻調(diào)制深度的連續(xù)可調(diào).基于頻移反饋腔的射頻調(diào)制脈沖激光是一種新體制下的脈沖激光,與傳統(tǒng)的脈沖激光相比,其獨(dú)特的反饋腔結(jié)構(gòu)使得脈沖內(nèi)產(chǎn)生了射頻調(diào)制,同時(shí)又利用脈沖內(nèi)射頻調(diào)制深度的可調(diào)節(jié),在窄脈寬激光調(diào)制以及水下探測等領(lǐng)域?qū)⒕哂袕V闊的應(yīng)用前景.

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All- fiber radio frequency-modulated pulsed laser based on frequency-shift feedback loop?

Yang Hong-ZhiZhao Chang-Ming Zhang Hai-Yang?Yang Su-HuiLi Chen

(School of Opto-Electronics,Beijing Institute of Technology,Beijing 100081,China)

6 April 2017;revised manuscript

11 May 2017)

Lidar-radar by using an radio frequency modulated(RF-modulated)laser transmitter is a powerful technique for applications involving remote sensing.The method is based on the use of an optically carried RF signal in order to acquire the merits of both the directivity of the optical beam(lidar)and the accuracy of RF signal processing(radar).Compared with single-frequency coherent lidars,lidar-radars are less sensitive to atmospheric turbulence and the speckle noise induced by target roughness.For long range detection,pulsed operation is usually required because of the high peak power.In order to meet the requirement for long range detection,an RF-modulated pulse train based on an all- fiber frequency shifted feedback loop is proposed in this paper.A continuous-wave single-frequency fiber laser(seed laser)is coupled into a fiber link and an acousto-optic chopper is used as a frequency shifter and beam chopper.A Yb3+-doped f i ber ampli fier is used to compensate for the loss of the signal in the fiber loop.The pulse duration is determined by the open time of acousto-optic chopper,which is fixed at 110 ns.A square wave generated by an arbitrary waveform generator is used as a trigger signal of the acousto-optic chopper.The RF within the pulse results from the interference of frequency shifed pulse with the seed laser.By inserting a 10 km fiber in the loop and accurately controlling the trigger cycle of the acousto-optic chopper equal to the roundtrip time of the loop,the pulse train generated by acousto-optic chopper can circulate in the loop,leading to the generation of RF-modulated pulse with about 20 kHz repetition rate and 110 ns width.The gain provided by fiber ampli fier in the loop partially compensates for the loss.By adjusting the gain of f i ber ampli fier,the modulation depth of RF within the pulse can be continuously adjusted and the maximum modulation depth is 0.67.We also present an time-delayed scalar interference model which includes the loop length,trigger cycle,frequency-shift,and the gain.According to the theoretical model,the RF-modulated pulse affected by trigger cycle and fiber ampli fier is numerically simulated.The experimental results accord well with theoritical predictions.The RF-modulated pulse has the advantage of high pulse-to-pulse coherence,which provides potential applications in lidarradar detection.Besides,with an additional frequency doubling stage one can obtain a source for underwater detections and communications.Extension of the scheme to the 1.5μm telecommunication window is straightforwardfor various radio-over- fiber applications.

laser with radio frequency-modualtion,pulse laser,frequency shifted feedback loop,modulation depth

PACS:42.60.Fc,42.79.Jq,42.60.Rn,42.68.WtDOI:10.7498/aps.66.184201

*Project supported by the National Natural Science Foundation of China(Grant No.61308054).

?Corresponding author.E-mail:ocean@bit.edu.cn

(2017年4月6日收到;2017年5月11日收到修改稿)

射頻調(diào)制的脈沖激光是激光雷達(dá)探測領(lǐng)域內(nèi)的一項(xiàng)重要研究內(nèi)容.根據(jù)聲光斬波器的強(qiáng)度和頻率調(diào)制特性,設(shè)計(jì)了基于頻移反饋腔的全光纖射頻調(diào)制脈沖激光.理論上,建立了基于頻移反饋腔的激光外差相干理論模型,并進(jìn)行了數(shù)值仿真.根據(jù)理論模型,實(shí)驗(yàn)上嚴(yán)格控制頻移反饋腔的長度和聲光斬波器觸發(fā)信號的周期,在100 MHz的射頻信號驅(qū)動下,產(chǎn)生了脈沖寬度110 ns、重復(fù)頻率約20 kHz的具有最高700 MHz射頻調(diào)制的脈沖激光(脈內(nèi)調(diào)制激光);同時(shí)微調(diào)斬波周期可以實(shí)現(xiàn)脈沖前沿或后沿的多樣性射頻調(diào)制.通過改變反饋腔內(nèi)光纖放大器的輸出功率實(shí)現(xiàn)了射頻調(diào)制深度的連續(xù)可調(diào),最高達(dá)到了0.67.

10.7498/aps.66.184201

?國家自然科學(xué)基金(批準(zhǔn)號:61308054)資助的課題.

?通信作者.E-mail:ocean@bit.edu.cn