林 鵬，王天樞*，馬萬卓，陳俊達，姜會林

2.07 μm光纖激光在弱湍流條件下的傳輸特性研究

林 鵬1,2，王天樞1,2*，馬萬卓1,2，陳俊達1,2，姜會林1,2

1長春理工大學空間光電技術國家與地方聯合工程研究中心，吉林 長春 130022；2長春理工大學光電工程學院，吉林 長春 130022

本文報道了一種2.07 μm波段可調諧主動鎖模光纖激光并在室內模擬大氣湍流條件下進行傳輸特性研究。增益介質為1.5 m長的摻鈥光纖，主動鎖模通過LiNbO3強度調制器在腔內引入周期強度調制實現。腔內引入非線性偏振旋轉效應實現波長2058.4 nm~2078.6 nm可調諧。實驗獲得了穩(wěn)定的基頻鎖模脈沖和10階，24階，48階諧波鎖模脈沖，對應頻譜信噪比為66.79 dB、61.37 dB、54.82 dB和49.66 dB。鎖模脈沖經過數字調制后在實驗室內大氣湍流模擬池中進行傳輸，分別獲得了Δ為70 ℃，140 ℃和210 ℃時三種湍流強度和背對背條件下的眼圖；與背對背條件相比，在Δ=210 ℃時光信噪比降低了9.14 dB。

光纖激光器；主動鎖模；空間激光通信；大氣湍流

1 引 言

近年來，鎖模光纖激光器由于其具有線寬窄、波長可調諧、溫度特性好等特點而被廣泛研究[1-4]。由于2 μm波段覆蓋幾個強OH-吸收峰，并且處于大氣窗口，因此2 μm光纖激光器可被應用于激光醫(yī)療、激光雷達、空間激光通信等領域[5-6]。常用的2 μm波段增益光纖有摻銩光纖和摻鈥光纖，摻銩光纖的增益譜范圍為1700 nm~2000 nm，摻鈥光纖的增益譜范圍為2000 nm~2200 nm。根據米氏散射條件，光在大氣中傳輸的散射強度與波長的平方成反比，隨著激光波長的紅移，受到散射的影響減小。此外，波長大于2 μm的激光避開了CO2，CH4等幾種常見氣體分子吸收峰[7]，因此摻鈥鎖模光纖激光更適用于空間激光傳輸系統。

產生GHz量級的鎖模脈沖主要有主動鎖模和被動鎖模兩種方式[8-9]，相比于被動鎖模，主動鎖?？梢援a生重復頻率可調的高重頻、高穩(wěn)定性的脈沖序列，更適用于作為激光通信載波光源。主動鎖模產生的原理是通過在諧振腔內加入電光晶體調制器引入周期強度調制，當強度調制的頻率是諧振腔基頻的整數倍時，可以產生重復頻率與調制頻率相同的鎖模脈沖。這種方法的缺點是在實現鎖模的同時會在腔內產生超模噪聲，常用的抑制超模噪聲的方法主要包括：腔內濾波法、復合腔結構法和非線性效應法[10-12]。其中，腔內濾波法結構簡單，可以在抑制超模噪聲的同時實現窄線寬、波長可調諧鎖模激光輸出。近年來，研究人員對2 μm 波段高重頻鎖模脈沖的產生做了大量研究。2016年，Sergei等[13]報道了基于非線性偏振旋轉效應的摻鈥被動鎖模光纖激光器，實驗獲得了中心波長為2.9 μm，脈沖寬度為180 fs，單脈沖能量達7.6 nJ。2017年，Qin等[14]報道了一個主動鎖模皮秒脈沖源，重復頻率在1 GHz~6 GHz可調，脈沖寬度60 ps，中心波長為1958.5 nm。2018年，Zeng等[15]使用可飽和布拉格反射器實現了重復頻率1.25 GHz，脈沖寬度426 fs，中心波長1941 nm。然而，對于波長大于2 μm的研究主要集中于產生高能量窄脈寬的飛秒脈沖。2018年，Maria等[16]使用可飽和吸收體搭建了全光纖被動鎖模摻鈥光纖激光器，研究了展寬區(qū)鎖模脈沖的光譜與諧振腔總色散的關系，輸出脈沖寬度為190 fs，脈沖能量為2.55 nJ。目前，有關波長超過2 μm的高重頻鎖模脈沖的報道相對較少，而針對空間激光通信系統的2 μm鎖模光源還未見報道。

本文報道了一種可用于空間激光通信的主動鎖模摻鈥光纖激光器，諧振腔內通過加入非線性偏振旋轉效應濾除超模噪聲，提高鎖模脈沖的穩(wěn)定性，同時實現波長可調諧。波長可調諧范圍為2058.4 nm~2078.6 nm，最高重頻可達1.008 GHz，對應的頻譜信噪比為49.66 dB。鎖模脈沖被速率為1.008 Gb/s的數字信號調制后在三種不同的湍流條件下進行傳輸，解調后的眼圖信噪比分別為9.35 dB，6.83 dB和4.58 dB。

2 實驗結構與工作原理

2.07 μm光纖激光在模擬大氣湍流條件中傳輸的實驗結構如圖1(a)所示，增益介質為一段1.5 m長的摻鈥光纖(Nufern SM-HDF-10/130)。泵浦源為一個實驗室自制的摻銩光纖激光器(thulium-doped fiber laser, TDFL)和一個實驗室自制的摻銩光纖放大器(thulium-doped fiber amplifier, TDFA)。調制器為帶寬10 GHz的商用2 μm波段馬赫-曾德爾強度調制器(IXblue MX2000-LN-10)，微波信號源(Hittite HMC-T2220)可以產生頻率范圍10 MHz~20 GHz的正弦信號，最大輸出功率為30 dBm。諧振腔內由兩個偏振控制器(polarization controller，PC)和一個偏振相關隔離器(polarization dependent isolator，PD-ISO)組成非線性偏振旋轉結構，在實現波長可調諧、保證諧振腔內激光單項傳輸同時進行光譜濾波，抑制超模噪聲[17]。1×2光耦合器(optical coupler，OC)的80%端提供腔內反饋，20%端輸出后經過第二個馬赫-曾德爾強度調制器對輸出的鎖模脈沖加載數字信號，經過調制的脈沖信號由摻鈥光纖放大器(advalue photonics, AP-AMP1) 放大后在模擬大氣湍流裝置中傳輸，模擬大氣湍流池的兩端設置兩個光纖準直器進行光路準直。接收端脈沖信號由2 μm波段光電探測器探測，帶寬為12 GHz，脈沖時域波形圖可以從帶寬2.5 GHz的示波器(Agilent, DSO 9254A)觀測，光譜由光譜分析儀(YOKOGAWA, YQ6375)觀測，光譜觀測范圍為1200 nm~2400 nm，最小分辨率為0.05 nm，頻譜信號由頻譜分析儀(Agilent, N1996A)同步觀測，頻率范圍為100 kHz~3 GHz。圖1(b)為實驗室自制的摻銩光纖激光器結構圖，泵浦是中心波長為1565 nm的窄線寬代替激光器，經由鉺鐿共摻光纖放大器(erbium ytterbium co-doped fiber amplifier，EYDFA)放大到1 W后泵浦一段2.5 m長的摻銩光纖，腔內采用可調諧光濾波器(agilrron FOTF)控制諧振腔輸出激光波長，輸出激光中心波長為1900 nm，輸出功率為10 mW。圖1(c)為自制的摻銩光纖放大器結構圖，摻鉺光纖放大器(connet MFAS)最高輸出功率為5 W，增益光纖采用一段長度為4 m的銩鈥共摻光纖(INO TH550)，摻銩光纖放大器的可將1900 nm的激光功率放大至1.2 W。

圖1 實驗結構。(a) 2.07 μm主動鎖模光纖激光器及室內湍流模擬傳輸實驗結構圖；(b) 摻銩光纖激光器泵浦結構圖；(c) 摻銩光纖放大器結構圖

3 結果與討論

主動鎖模通過調制器在腔內引入周期強度調制實現，當調制頻率為腔基頻的整數倍時可獲得穩(wěn)定的鎖模脈沖輸出。實驗中，諧振腔總長度為9.52 m，對應基頻為21 MHz。摻銩光纖激光器經放大后功率固定在1.2 W，獲得摻鈥主動鎖模光纖激光器的光譜如圖2(a)所示，中心波長為2066.4 nm，3 dB線寬為0.09 nm，邊模抑制比為42.05 dB。由于諧振腔內引入非線性偏振旋轉效應限制主動鎖模激光線寬，在鎖模過程中產生的高階諧波可以被有效抑制，鎖模脈沖的穩(wěn)定性得到提高。另一方面，非線性偏振旋轉效應還可以實現波長可調諧，實驗獲得波長可調諧范圍為2058.4 nm~2078.6 nm，邊模抑制比大于40 dB。

將調制器的偏置電壓設置在p/2處(p是調制器的半波電壓)，正弦信號源的頻率調至21.013 MHz，可以獲得基頻模式下的鎖模脈沖激光，如圖3(a)所示。脈沖間隔為47.6 ns，脈沖寬度為3.74 ns，平均輸出功率為13.3 dBm，鎖模脈沖可以穩(wěn)定運行幾個小時，峰峰值抖動較小。增加正弦調制信號頻率至210.118 MHz，504.212 MHz和1.008 GHz，分別獲得了10階，24階和48階諧波鎖模脈沖波形，如圖3(b)~3(d)所示。插圖反映了小范圍時域波形圖，可以觀測到脈沖間隔分別下降到4.76 ns，1.98 ns和992.06 ps。隨著脈沖重復頻率的增加，諧振腔內產生更多的超模噪聲導致脈沖的噪底增加，脈沖強度抖動增強，穩(wěn)定性下降。

圖2 摻鈥主動鎖模激光光譜。(a) 單波長激光輸出特性，邊模抑制比大于42 dB；(b) 可調諧激光光譜，2058 nm~2078 nm

圖3 摻鈥鎖模激光脈沖序列。(a) 重復頻率21.013 MHz；(b) 重復頻率210.118 MHz；(c) 重復頻率504.212 MHz；(d) 重復頻率1.008 GHz

為了衡量主動鎖模脈沖穩(wěn)定性與脈沖重復頻率的關系，分別測量了21.013 MHz，210.118 MHz，504.212 MHz和1.008 GHz四種重復頻率下的頻譜，如圖4(a)~4(d)所示。頻譜分辨率為100 Hz，頻域掃描范圍分別為2 MHz，4 MHz，8 MHz和16 MHz?；l狀態(tài)下，鎖模脈沖的頻譜信噪比可達66.79 dB，三種諧波對應的頻譜信噪比分別為61.37 dB，54.82 dB和49.66 dB。四種鎖模狀態(tài)在頻譜掃描范圍內無其他邊模，說明腔內超模噪聲被有效抑制，且鎖模脈沖工作在一個穩(wěn)定的狀態(tài)。

實驗獲得的高重頻摻鈥鎖模激光具有線寬窄，穩(wěn)定性高，位于大氣傳輸窗口等優(yōu)勢，可應用于2 μm空間激光通信系統。將主動鎖模脈沖調制信號的一半作為外部時鐘參考發(fā)送至任意波形發(fā)生器，任意波形發(fā)生器根據時鐘同步產生一組相同速率的二進制偽隨機碼(pseudo-random binary sequence)，經過微波信號放大器放大后驅動第二個馬赫-曾德爾調制器對鎖模脈沖進行強度調制，脈沖信號的眼圖由帶寬為10 GHz的光眼圖儀(Agilent, 86100C)測量獲得。由圖5(a)中可以得出背對背傳輸條件下的光信噪比為13.72 dB，表明系統噪聲容限高，調制深度深。調制后的光脈沖信號分別在三種湍流強度條件下傳輸，接收端通過任意波形發(fā)生器發(fā)送一組時鐘信號進行同步解調，解調后的光眼圖如圖5(b)~5(d)所示。隨著池體兩端的溫差升高至70 ℃，140 ℃和210 ℃，信道的湍流強度逐漸上升，導致接收光眼圖逐漸惡化，眼高變小，噪聲容限降低，對應的光信噪比減小至9.35 dB，6.83 dB和4.58 dB。

圖4 摻鈥主動鎖模激光頻譜。(a) 重復頻率21.013 MHz；(b) 重復頻率210.118 MHz；(c) 重復頻率504.212 MHz；(d) 重復頻率1.008 GHz

圖5 鎖模脈沖在湍流條件下傳輸眼圖。(a) 背對背，光信噪比13.72 dB；(b) ΔT=70 ℃，光信噪比9.35 dB；(c) ΔT=140 ℃，光信噪比6.83 dB；(d) ΔT=210 ℃，光信噪比4.58 dB

4 總 結

本文研究了一種主動鎖模摻鈥光纖激光器并在室內弱湍流條件下進行了傳輸特性分析。通過在諧振腔內引入非線性偏振旋轉效應來實現穩(wěn)定的高重頻、窄線寬鎖模脈沖輸出，同時實現了2058.4 nm~2078.6 nm波長可調諧激光輸出。分析了21.013 MHz，210.118 MHz，504.212 MHz和1.008 GHz四種重復頻率下的鎖模脈沖時域及頻域特性，頻譜信噪比為66.79 dB，61.37 dB，54.82 dB和49.66 dB，由于腔內濾波效應濾除了超模噪聲，增加了鎖模脈沖穩(wěn)定性。實驗獲得的摻鈥鎖模激光經過調制后在三種不同湍流條件下進行傳輸對比，當池體溫差分別增加至70 ℃，140 ℃和210 ℃時，調制光信號的信噪比相應地降低了4.37 dB，6.89 dB和9.14 dB。結果表明，主動鎖模摻鈥光纖激光器在2 μm空間激光通信上有潛在的應用價值。

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Propagation characteristics of 2.07 μm fiber laser in weak turbulence condition

Lin Peng1,2, Wang Tianshu1,2*, Ma Wanzhuo1,2, Chen Junda1,2, Jiang Huilin1,2

1National and Local Joint Engineering Research Center of Space Optoelectronics Technology, Changchun University of Science and Technology, Changchun, Jilin 130022, China;2College of Opto-Electronic Engineering, Changchun University of Science and Technology, Changchun, Jilin 130022, China

2.07 μm actively mode-locked fiber laser and transmission system

Overview:In recent years, 2 μm band fiber laser has attracted widespread attention with the advent of thulium-doped fiber and holmium-doped fiber because of its wide application in laser medicine, material processing and Lidar. In addition, the 2 μm laser works in atmospheric window, which lays the potential for free-space optical communication. However, the absorption peaks of many common gas molecules gather at 2 μm, such as H2O and CO2. The holmium-doped fiber can radiate laser with wavelength greater than 2 μm, which is more suitable as a gain fiber for optical communication laser source than thulium-doped fiber. There are several researches on holmium-doped fiber laser. In recent years, researchers have published a lot of research on the generation of high repetition frequency mode-locked pulse in 2 μm band. In 2017, Qin et al reported an actively mode-locked picosecond (ps) pulsed laser source with the repetition rate of 1 GHz~6 GHz, the pulse width is 60 ps, and the central wavelength is 1958.5 nm. In 2018, Zeng et al realized the repetition rate of 1.25 GHz, the pulse width is 426 fs with a central wavelength of 1941 nm. However, the study of wavelength greater than 2 μm mainly focuses on the generation of femtosecond pulse with high energy and narrow pulse width. In 2016, Sergei et al reported a passively mode-locked holmium-doped fiber laser based on nonlinear polarization rotation, the central wavelength is 2.9 μm, the pulse energy is 7.6 nJ, and a repetition rate of 43.1 MHz. In 2018, Maria et al built a dispersion-managed holmium-doped fiber laser with a graphene saturable absorber, the relationship between the spectrum of the mode-locked pulse and the total dispersion of the resonant cavity was studied, the output pulse width is 190 fs with a repetition rate of 21 MHz. It can be seen that the reports on mode-locked fiber laser with high repetition rate are still insufficient, and the 2 μm fiber laser for free-space optical communication system has not been reported.

In this paper, we demonstrated an actively mode-locked holmium-doped fiber laser, which can be used in free-space optical communication. By adding nonlinear polarization rotation effect in the cavity to filter out super-mode noise, the stability of mode-locked pulse was improved and the wavelength tunable can be realized. The wavelength tuning range is 2058.4 nm to 2078.6 nm, the repetition rate is 1.008 GHz and the corresponding radio frequency (RF) signal-to-noise ratio can reach 49.66 dB. Moreover, the mode-locked pulse sequence was modulated by the digital signal and transmitted under three different turbulent conditions. The optical signal-to-noise ratio of eye diagram after demodulation is 9.35 dB, 6.83 dB and 4.58 dB, respectively.

Citation: Lin P, Wang T S, Ma W Z,Propagation characteristics of 2.07 μm fiber laser in weak turbulence condition[J]., 2020, 47(3): 190588

Propagation characteristics of 2.07 μm fiber laser in weak turbulence condition

Lin Peng1,2, Wang Tianshu1,2*, Ma Wanzhuo1,2, Chen Junda1,2, Jiang Huilin1,2

1National and Local Joint Engineering Research Center of Space Optoelectronics Technology, Changchun University of Science and Technology, Changchun, Jilin 130022, China;2College of Opto-Electronic Engineering, Changchun University of Science and Technology, Changchun, Jilin 130022, China

We demonstrate an actively mode-locked holmium-doped fiber laser with a central wavelength of 2.07 μm, and the propagation characteristics under weak turbulent condition are analyzed. A segment of 1.5 m holmium-doped fiber is used as gain medium. Actively mode-locked can be realized by introducing periodic intensity modulation into cavity through LiNbO3intensity modulator. The nonlinear polarization rotation effect is introduced into the cavity to realize the tunable wavelength of 2058.4 nm~2078.6 nm. Stable mode-locked pulses with fundamental frequency and 10th, 24th, 48thorder harmonic operations can be obtained. The signal to noise ratio (SNR) of the corresponding radio frequency (RF) spectrum is 66.79 dB, 61.37 dB, 54.82 dB and 49.66 dB. The stable mode-locked pulse modulated by digital signal and is transmitted in a simulated atmospheric turbulence device. The eye patterns can be obtained at the condition of Δ=70 ℃，140 ℃, 210 ℃ and back-to-back (BTB). The SNR at Δ=210 ℃ decreased 9.14 dB compared with BTB condition.

fiber laser; actively mode-locked; free-space optical communication; atmospheric turbulence

TN248；TN929.12

A

10.12086/oee.2020.190588

: Lin P, Wang T S, Ma W Z,. Propagation characteristics of 2.07 μm fiber laser in weak turbulence condition [J]., 2020,47(3): 190588

2019-09-29；

2019-11-29基金項目：國家自然科學基金資助項目(61975021)

林鵬(1994-)，男，博士研究生，主要從事空間光學技術的研究。E-mail：2018200044@mails.cust.edu.cn

王天樞(1975-)，男，博士，教授，主要從事光纖激光器與空間激光通信技術的研究。E-mail：wangts@cust.edu.cn

林鵬，王天樞，馬萬卓，等. 2.07 μm光纖激光在弱湍流條件下的傳輸特性研究[J]. 光電工程，2020，47(3): 190588

Supported by National Natural Science Foundation of China (61975021)

* E-mail: wangts@cust.edu.cn