可調(diào)諧納秒脈沖鎖模光纖激光器

劉宇星，姜盼秋，汪平河

可調(diào)諧納秒脈沖鎖模光纖激光器

劉宇星，姜盼秋，汪平河*

電子薄膜與集成器件國(guó)家重點(diǎn)實(shí)驗(yàn)室，電子科技大學(xué)光電科學(xué)與工程學(xué)院，四川 成都 611731

采用非線性放大環(huán)形鏡的被動(dòng)鎖模機(jī)制，通過(guò)加入手動(dòng)可調(diào)濾波器及光纖光柵等實(shí)現(xiàn)單波長(zhǎng)光譜輸出，設(shè)計(jì)出一種可調(diào)諧納秒脈沖光纖激光器。納秒矩形脈沖由被動(dòng)鎖模摻鉺光纖激光器產(chǎn)生，激光器的腔長(zhǎng)為430 m，脈沖的重復(fù)頻率為465 kHz。被動(dòng)鎖模光纖激光器中實(shí)現(xiàn)可調(diào)諧脈沖輸出的關(guān)鍵器件包括寬帶鎖模器件和可調(diào)諧濾波器，其中寬帶鎖模器件是反射式的非線性放大環(huán)形鏡。結(jié)果表明，當(dāng)激光器工作在1560 nm，泵浦功率為400 mW時(shí)，脈沖寬度為10.58 ns，單脈沖能量為70.28 nJ。激光器的可調(diào)諧范圍為1523.4 nm～1575 nm，比此前報(bào)道的可調(diào)諧范圍要寬10 nm。

光纖激光器；可調(diào)諧；鎖模；納秒脈沖

1 引 言

可調(diào)諧被動(dòng)鎖模光纖激光器由于其廣泛的應(yīng)用領(lǐng)域，如生物醫(yī)學(xué)和光纖傳感等，引起了人們的廣泛關(guān)注。可調(diào)諧被動(dòng)鎖模光纖激光器將被動(dòng)鎖模技術(shù)和波長(zhǎng)選擇器件結(jié)合在一起實(shí)現(xiàn)可調(diào)諧的脈沖輸出。有許多種不同的波長(zhǎng)選擇器件被使用在可調(diào)諧鎖模光纖激光器中，如可調(diào)諧帶通濾波器(TBF)[1-2]、多模干涉(MMI)濾波器[3-4]、高雙折射(HiBi)濾波器[5-7]，光纖光柵濾波器[8-10]，也有人使用模式選擇耦合器來(lái)實(shí)現(xiàn)光纖高階模式在腔內(nèi)振蕩的鎖模光纖激光器[11]。被動(dòng)鎖模光纖激光器需要一個(gè)寬帶飽和吸收器以獲得寬波長(zhǎng)范圍的可調(diào)諧特性。傳統(tǒng)的可飽和吸收器件有半導(dǎo)體飽和吸收鏡(SESAM)[6,12]、非線性偏振旋轉(zhuǎn)(NPR)[3-4,13]、非線性光學(xué)環(huán)形鏡(NOLM)[14]、碳納米管[2,9-10]。二維材料在過(guò)去幾年中被證明是一種很有前途的寬帶飽和吸收體[1,8,15-16]。近年來(lái)，有人報(bào)道了工作在1.5 μm波段調(diào)諧范圍在70 nm以上的被動(dòng)鎖模光纖激光器，但脈沖能量很低[17-18]。

長(zhǎng)腔鎖模光纖激光器可以產(chǎn)生高能量的脈沖序列。被動(dòng)鎖模摻鉺光纖激光器中有多種鎖模機(jī)制，如傳統(tǒng)孤子[19]、色散管理(DM)脈沖[20]、自相似脈沖[21]、耗散孤子(DS)[22]。由于孤子能量的量子化效應(yīng)，傳統(tǒng)孤子的脈沖能量被限制在～0.1 nJ。色散管理脈沖的單脈沖能量可達(dá)到～1 nJ，自相似脈沖的單脈沖能量可達(dá)到～10 nJ。在全正常色散光纖激光器中，耗散孤子的脈沖能量可達(dá)～100 nJ。在負(fù)色散區(qū)，工作在耗散孤子共振(DSR)區(qū)的光纖激光器可以產(chǎn)生高能脈沖[23-24]。提高腔長(zhǎng)是提高脈沖能量的有效方法。最近，有人報(bào)道了腔長(zhǎng)為1536 m單脈沖能量10 μJ的長(zhǎng)腔鎖模光纖激光器[25]。

本文報(bào)道了一種基于反射式非線性放大環(huán)形鏡的可調(diào)諧長(zhǎng)腔被動(dòng)鎖模光纖激光器。采用3 dB帶寬約1 nm的可調(diào)諧帶通濾波器獲得可調(diào)諧脈沖輸出。激光器的可調(diào)諧范圍大于50 nm。本文首先對(duì)固定波長(zhǎng)激光器的性能進(jìn)行了詳細(xì)的研究，然后探討了激光器的可調(diào)諧特性。在整個(gè)調(diào)諧范圍內(nèi)，系統(tǒng)的信噪比、脈沖寬度、輸出功率和單脈沖能量隨中心波長(zhǎng)的變化有小范圍的波動(dòng)。這種可調(diào)諧納秒脈沖光纖激光器在生物醫(yī)學(xué)和光纖傳感等領(lǐng)域有著廣闊的應(yīng)用前景。

2 實(shí)驗(yàn)和原理

激光器結(jié)構(gòu)如圖1所示。鎖模器件是一個(gè)反射式非線性放大環(huán)形鏡，由50:50光耦合器(實(shí)測(cè)耦合比為48.6:51.4)、增益模塊、404 m單模光纖(SMF)和兩個(gè)偏振控制器(PC)組成。增益模塊由980/1550波分復(fù)用(WDM)、980 nm激光二極管和12 m摻鉺光纖(Nufern，EDFC-980-HP)組成。摻鉺光纖在1530 nm處的吸收系數(shù)為(6.0±1.0) dB/m。摻鉺光纖在1550 nm處的群速度色散為-12.2 ps/(nm×km)。SMF在1550 nm處的群速度色散為17 ps/(nm×km)。激光器的總腔長(zhǎng)為430 m，除摻鉺光纖外，所有的器件均由SMF組成。腔內(nèi)的凈色散為-8.87 ps2。將手動(dòng)可調(diào)諧帶通濾波器(TBF，Agiltron FOTF-02512123)插入腔中，TBF的兩個(gè)端口分別連接到環(huán)形器的端口1和端口3。TBF的3 dB帶寬約1 nm，可調(diào)諧范圍從1510 nm到1580 nm。環(huán)形器的端口2連接到反射式非線性放大環(huán)鏡上。光纖耦合器端口為2輸出端口，輸出光經(jīng)10:90耦合器分光后分別輸出到光譜儀和示波器。

圖1 實(shí)驗(yàn)裝置示意圖。

WDM：波分復(fù)用器；EDF：摻鉺光纖；OC：光耦；SMF：?jiǎn)文９饫w；PC：偏振控制器

Fig. 1 Experimental device diagram.

WDM: wave division multiplexer; EDF: erbium-doped fiber; OC: optocoupled; SMF: single-mode fiber; PC: polarization controller

激光的輸出光譜由光譜分析儀(AQ6370)測(cè)量，分辨率為0.02 nm。脈沖序列由10 G的光電探測(cè)器(KG-PR-10G-A，北京康冠技術(shù)有限公司)和數(shù)字示波器(TDS2024C，泰克公司)測(cè)量。激光器的頻譜由3 GHz頻譜分析儀(安捷倫E4402B)測(cè)量。

3 結(jié)果分析

對(duì)于工作在DSR區(qū)的光纖激光器，腔中的增益、損耗、色散、非線性和光譜濾波等因素影響會(huì)鎖模狀態(tài)[26-27]。本文首先研究了工作在單一固定波長(zhǎng)時(shí)激光器的性能，如圖2至圖5所示。

通過(guò)調(diào)節(jié)腔內(nèi)的PC可以實(shí)現(xiàn)鎖模狀態(tài)。鎖模的閾值泵浦功率約為80 mW。圖2顯示了不同泵浦功率下的輸出光譜。當(dāng)泵浦功率從100 mW增加到400 mW，激光器輸出的中心波長(zhǎng)為1560 nm，3 dB帶寬為0.84 nm，范圍從1559.68 nm到1560.42 nm。隨著泵浦功率的增加，光譜強(qiáng)度增加，中心波長(zhǎng)保持不變。

圖3是泵浦功率為400 mW時(shí)輸出脈沖的波形圖。脈沖序列的周期約為2.15 μs，重復(fù)頻率為465 kHz。激光器腔長(zhǎng)430 m，對(duì)應(yīng)的腔頻為465 kHz，因此激光器工作在基頻鎖模狀態(tài)。當(dāng)泵浦功率從100 mW增加到400 mW時(shí)，重復(fù)頻率保持不變。插入小圖顯示了輸出脈沖的頻譜圖，信噪比(SNR)為50.69 dB。波形圖和頻譜圖表明激光器工作在穩(wěn)定的鎖模狀態(tài)。

為了揭示脈沖演化的更多細(xì)節(jié)，我們研究了不同泵浦功率下的脈沖演化。圖4顯示了不同泵浦功率下的脈沖變化圖。當(dāng)泵浦功率以50 mW間隔從100 mW增加到400 mW時(shí)，脈沖寬度分別為3.88 ns、4.64 ns、6.36 ns、7.72 ns、8.76 ns、9.34 ns和10.58 ns。脈沖幅度保持不變，但脈沖寬度隨泵浦功率的增加而增加，這符合DSR鎖模脈沖的特性。

圖2 不同泵浦功率下的激光器單波長(zhǎng)光學(xué)光譜

圖3 基本頻率模式下DSR區(qū)域的示波器微量脈沖

圖4 單波長(zhǎng)模式鎖定脈沖在不同的泵功率演變

長(zhǎng)腔被動(dòng)鎖模光纖激光器的優(yōu)點(diǎn)是可以提高激光輸出脈沖的單脈沖能量。圖5分別顯示了不同泵浦功率下的輸出功率和單脈沖能量。當(dāng)泵浦功率以50 mW間隔從100 mW增加到400 mW時(shí)，輸出功率分別為10.02 mW、13.83 mW、20.21 mW、25.15 mW、27.31 mW、28.91 mW和32.68 mW，對(duì)應(yīng)的單脈沖能量分別為21.55 nJ、29.74 nJ、43.46 nJ、54.09 nJ、58.73 nJ、62.17 nJ和70.28 nJ (文中單脈沖能量均為功率與重復(fù)頻率的比值)。

在長(zhǎng)腔被動(dòng)鎖模光纖激光器中，鎖模脈沖的形成與腔內(nèi)的色散、非線性、增益和損耗有關(guān)。理論和實(shí)驗(yàn)表明，具有反常色散的長(zhǎng)腔鎖模光纖激光器可以在DSR區(qū)工作，產(chǎn)生矩形脈沖。DSR鎖模產(chǎn)生的矩形脈沖隨泵浦功率而增加，且脈沖寬度增加而振幅保持不變[26]。2014年，Zheng等[28]人報(bào)道了被動(dòng)鎖模八字形光纖激光器中的高能類噪聲矩形脈沖。最近，有理論提出類噪聲脈沖的產(chǎn)生與腔中濾波器的帶寬有關(guān)[27]。在本論文中使用的濾波器帶寬在1 nm左右，濾波器的帶寬太窄而不利于類噪聲脈沖的產(chǎn)生，因此激光器工作在DSR區(qū)。

通過(guò)在1510 nm到1580 nm的波長(zhǎng)范圍內(nèi)調(diào)節(jié)TBF的輸出波長(zhǎng)，被動(dòng)鎖模光纖激光器實(shí)現(xiàn)了可調(diào)諧的脈沖輸出。該激光器能在1523.4 nm到1575 nm范圍內(nèi)保持穩(wěn)定的鎖模狀態(tài)。閾值泵浦功率在80 mW左右，有一定的波動(dòng)。為了分析該激光器的可調(diào)諧特性，詳細(xì)研究了該激光器在1523.4 nm到1575 nm范圍內(nèi)六個(gè)不同中心波長(zhǎng)下的激光器的輸出特性，包括輸出脈沖的頻譜、脈沖演變、激光輸出功率和單脈沖能量。

當(dāng)泵浦功率為400 mW時(shí)，激光器的可調(diào)諧特性如圖6至圖9所示。

圖6顯示了中心波長(zhǎng)在1523.4 nm、1527.0 nm、1540.0 nm、1553.0 nm、1565.0 nm和1575.0 nm處的輸出光譜。這些光譜的3 dB帶寬分別為1.05 nm、1.11 nm、1.01 nm、1.02 nm、0.82 nm和0.79 nm。

圖5 (紅色)不同泵功率的平均輸出功率和(藍(lán)色)單脈沖能量

圖6 泵功率為400 mW激光輸出的不同單波長(zhǎng)光學(xué)光譜

圖7 泵功率為400 mW時(shí)對(duì)應(yīng)的SNR射頻

圖8 單波長(zhǎng)模式鎖定脈沖在不同中心波長(zhǎng)的演化

六個(gè)不同波長(zhǎng)處激光器輸出脈沖的頻譜圖如圖7所示，相應(yīng)頻譜的信噪比分別為50.25 dB、50.03 dB、48.50 dB、48.53 dB、50.69 dB和50.75 dB，基頻為465 kHz。這表明在這些波長(zhǎng)處，激光器都工作在基頻狀態(tài)，而且鎖模狀態(tài)都很穩(wěn)定。圖8顯示了在六個(gè)不同波長(zhǎng)處激光器輸出脈沖的波形圖。在六個(gè)波長(zhǎng)處對(duì)應(yīng)的輸出脈沖的寬度分別為11.52 ns、10.16 ns、11.16 ns、15.2 ns、9.48 ns和10.84 ns。圖9顯示了在六個(gè)不同波長(zhǎng)處激光器的輸出功率和單脈沖能量。輸出功率分別為28.60 mW、29.28 mW、30.59 mW、35.51 mW、31.76 mW和32.52 mW，單脈沖能量分別為61.51 nJ、62.97 nJ、65.78 nJ、76.37 nJ、68.30 nJ和69.94 nJ。

在整個(gè)調(diào)諧范圍內(nèi)，激光器輸出脈沖的頻譜圖的信噪比一直保持50 dB左右，這說(shuō)明激光器在調(diào)諧過(guò)程中始終保持穩(wěn)定的鎖模狀態(tài)。由于摻鉺光纖的增益隨波長(zhǎng)有一定的變化，因此激光器的脈沖寬度、輸出功率和單脈沖能量都有一定的波動(dòng)。

圖9 (紅色)不同波長(zhǎng)的平均輸出功率和(藍(lán)色)單脈沖能量

對(duì)于長(zhǎng)腔鎖模光纖激光器而言，激光器的性能受到多種因素的影響。比如可調(diào)諧濾波器在腔內(nèi)的位置對(duì)激光器的可調(diào)諧鎖模范圍有明顯的影響，在實(shí)驗(yàn)過(guò)程中可以看到當(dāng)可調(diào)諧濾波器位于非線性環(huán)形鏡內(nèi)時(shí)，可調(diào)諧鎖模范圍會(huì)減小，這可能與濾波器帶來(lái)的損耗有關(guān)系。當(dāng)調(diào)節(jié)腔內(nèi)的偏振控制器時(shí)，除了可以得到基頻的鎖模以外，還可以得到高階的諧波鎖模。

長(zhǎng)腔鎖模光纖激光器可以產(chǎn)生高能量的脈沖序列。當(dāng)激光器工作在DSR區(qū)時(shí)，通過(guò)增加泵浦功率，輸出脈沖的能量會(huì)更高，脈沖寬度也會(huì)更大。寬帶可飽和吸收體是可調(diào)諧被動(dòng)鎖模光纖激光器的關(guān)鍵器件之一。與碳納米管、石墨烯等新型可飽和吸收體相比，反射式非線性放大環(huán)形鏡具有成本低、制作方便、可調(diào)諧范圍寬等優(yōu)點(diǎn)。

激光器的可調(diào)諧范圍受到了濾波器的可調(diào)諧帶寬、增益介質(zhì)的增益帶寬和鎖模器件性能的影響。在本論文中，增益介質(zhì)的增益峰值在1553 nm附近，通過(guò)采用具有更寬增益帶寬的新型增益介質(zhì)，有可能提高可調(diào)諧范圍[29]。本論文中所使用的鎖模器件為非線性環(huán)形鏡，非線性環(huán)形鏡的性能與光耦合器的耦合比有直接的關(guān)系，而光耦合器的耦合比是和波長(zhǎng)有關(guān)的，因此它的鎖模帶寬也是有限的，這也是限制激光器可調(diào)諧范圍的因素之一。

4 結(jié) 論

在本文中我們報(bào)道了一種長(zhǎng)腔被動(dòng)鎖模光纖激光器。當(dāng)在激光器腔中插入可調(diào)諧濾波器時(shí)，激光器可以實(shí)現(xiàn)可調(diào)諧鎖模脈沖輸出。當(dāng)濾波器的輸出波長(zhǎng)為1560 nm，泵浦功率為400 mW時(shí)，激光器工作在DSR區(qū)，產(chǎn)生重復(fù)頻率為465 kHz、脈寬為10.58 ns、單脈沖能量為70.28 nJ的矩形脈沖。當(dāng)泵浦功率為400 mW時(shí)，鎖模激光器的可調(diào)諧范圍為1523.4 nm～1575 nm。在整個(gè)調(diào)諧范圍內(nèi)，激光器的信噪比、脈沖寬度、輸出功率和單脈沖能量隨中心波長(zhǎng)的變化有較小的波動(dòng)。可調(diào)諧納秒脈沖光纖激光器在許多領(lǐng)域有著潛在的應(yīng)用。

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A tunable nanosecond pulse mode-locking fiber laser

Liu Yuxing, Jiang Panqiu, Wang Pinghe*

State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Optoelectronic Science andEngineering, University of Electronic Science and Technology of China, Chengdu, Sichuan 611731, China

Single-wavelength mode locking pulse evolution at di?erent pump power

Overview:The tunable passive mode locked fiber laser has attracted a lot of interest because of its wide field applications, such as biomedical research and fiber optical sensing. The tunable passive mode locked fiber laser incorporate the passive mode locking technique and wavelength-selective devices in the cavity. Recently, the passive mode locked fiber lasers at 1.5 μm with a tuning range over 70 nm are demonstrated, but the pulse energy is low. The long cavity mode locked fiber laser can produce the pulse train with high pulse energy. A mode locked fiber laser with 1536 m cavity length and 10 μJ pulse energy was reported.

In this paper, we present a tunable long-cavity passive mode-locked fiber laser based on reflective nonlinear amplifying loop mirror (NALM). The reflective NALM serves as the mode locker, which is made up of a 50: 50 optical coupler (the measured ratio is 48.6: 51.4), an erbium-doped fiber amplifier, 404 m single mode fiber and two polarization controllers. The net dispersion in the cavity is -8.87 ps2. A manual tunable bandpass filter (TBF) is inserted into the cavity. The tunable range of the TBF is from 1510 nm to1580 nm.

At first, the laser at 1560 nm is investigated in detail. The laser operates in DSR region and generates rectangular pulses with 465 kHz repetition rate. Figure shows the pulse evolution at different pump powers. When the pump power increases from 100 mW to 400 mW at 50 mW interval, the pulse durations are 3.88 ns, 4.64 ns, 6.36 ns, 7.72 ns, 8.76 ns, 9.34 ns and 10.58 ns, respectively. The single-pulse energy is 70.28 nJ when the pump power is 400 mW. The tuning characteristics of the passive mode locked fiber laser is investigated by adjusting TBF’s transmission wavelength. When the pump power is 400 mW, the laser can keep stable mode-locking status at the range from 1523.4 nm to 1575 nm. The threshold pump power has a little fluctuation around 80 mW. During the whole tuning range, the SNR of the laser is ～50 dB, which proves that the laser keeps stable mode-locking status during the tuning course. The pulse duration, the output power and single pulse energy have a little fluctuation because that the gain of the EDF changes with the wavelength. The tunable nanosecond pulse fiber laser has a lot of potential applications in many fields.

Liu Y X, Jiang P Q, Wang P HA tunable nanosecond pulse mode-locking fiber laser[J]., 2021, 48(9): 210195; DOI:10.12086/oee.2021.210195

A tunable nanosecond pulse mode-locking fiber laser

Liu Yuxing, Jiang Panqiu, Wang Pinghe*

State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Optoelectronic Science andEngineering, University of Electronic Science and Technology of China, Chengdu, Sichuan 611731, China

A tunable nanosecond pulse fiber laser is demonstrated in the paper. The laser adopts the passive mode locking mechanism of the nonlinear amplifying loop mirror and a manually adjustable filter and fiber grating are added to achieve single-wavelength spectral output. The passive mode locked erbium-doped fiber laser with 430 m cavity length generates the nanosecond rectangle pulse at 465 kHz repetition rate. The tunable passive mode locked fiber laser incorporates a broad bandwidth mode locking device and a tunable filter in the cavity. The broad bandwidth mode locker is the key device for the tunable pulse output, which is based on a reflective nonlinear amplifying loop mirror. The result shows that the pulse duration and the single-pulse energy are 10.58 ns and 70.28 nJ respectively when the laser works at 1560 nm and has 400 mW pump power. The tunable range is from 1523.4 nm to 1575 nm.Keywords: fiber laser; tunable; mode-locking; nanosecond pulse

劉宇星，姜盼秋，汪平河. 可調(diào)諧納秒脈沖鎖模光纖激光器[J]. 光電工程，2021，48(9): 210195

Liu Y X, Jiang P Q, Wang P HA tunable nanosecond pulse mode-locking fiber laser[J]., 2021, 48(9): 210195

TN248；TN713

A

10.12086/oee.2021.210195

2021-06-07；

2021-07-13

國(guó)家重點(diǎn)研發(fā)計(jì)劃項(xiàng)目(2016YFF0102003，2016YFF0102000)

劉宇星(1993-)，男，碩士，主要從事光學(xué)相干層析成像技術(shù)和光纖激光器方面的研究。E-mail：1427759032@qq.com

汪平河(1976-)，男，博士，教授，主要從事光學(xué)相干層析成像技術(shù)和光纖激光器的研究。E-mail：wphsci@uestc.edu.cn

the National Key R&D Program of China (2016YFF0102003, 2016YFF0102000)

* E-mail: wphsci@uestc.edu.cn