Integratable pulsed 2-μm laser with Tm,Mg:LiNbO3 crystaland single-walled carbon nanotube saturable absorber

Shuaiyi Zhang a,n, Hongqiang Li b, Qiuling Zhao a, Hao Lv a, Xia Wang a, Weijin Kong d,Peixiong Zhang c, Jianqiu Xu b,nn, Yin Hang c

a Institute of Photonic Information Technology, School of Mathematics and Physics, QingDao University of Science & Technology, QingDao 266061, Chinab Key Laboratory for Laser plasmas and Department of Physics, Shanghai Jiaotong University, Shanghai 200240, Chinac Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, Chinad College of Physics, Qingdao University, Qingdao 266071, China

a r t i c l e i n f o

Article history:Received 22 February 2014Received in revised form19 April 2014Accepted 9 June 2014

Keywords:Diode-pumped lasersFerroelectric materialQ-switched

a b s t r a c t

We demonstrated an integratable passively Q-switched (PQS) Tm,Mg:LiNbO3 laser with a single-walledcarbon nanotube saturable absorber (SWCNT-SA) for the first time. Its laser characteristics wereinvestigated with different output couplers. At the pump power of 9.5 W, the maximum output powerof 0.93 W around 1963 nm was obtained with the output transmission of 1%. The optical conversionefficiency is 9.79%, corresponding to a slope efficiency of 17.4%. For pulsed operation, a shortest pulsewidth attained was 640 ns, with the pulse repetition frequency of 44.6 kHz, and the single pulse energywas 12.6 μJ.

& 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Integratable lasers are key devices in the integrated optoelec-tronics, which can realize lasing and multi-function signal proces-sing in a miniature chip simultaneously. There has been interest inachieving laser emission in integrate material, e.g. rare-earth-doped ferroelectric material, with the aim to the integratablelasers [1–3]. Solid state thulium (Tm3þ) doped lasers around1.9 μm wavelength have remained a topic of particular interestfor a variety of applications such as remote sensing, coherent laserradar and mid-infrared optical parametric oscillators [4–10]. Untilnow, Tm3þ has been readily substituted into many crystal hostssuch as YLF, YAP, and YAG. Significant results have been obtained[11–15]. However, lasing from Tm3þ-doped ferroelectric materialhas been reported rarely.

Owing to its large nonlinear coefficient and wide transmissionspectra, the nonlinear crystal LiNbO3 whose nonlinear opticalproperties are crucially dependent on their intrinsic defects hasbeen widely used in integratable optoelectronics, especially inperiodic poled lithium niobate for double-frequency generation[16,17]. As we know, the ferroelectric material has low thresholdsof damage and the photorefractive effect, which limit its furtherutilization for the high power device. Usually, these drawbacks can

be improved by doping magnesium oxide MgO into LiNbO3 crystal[18]. After that, the damage threshold of LiNbO3 rises greatly andits excellent nonlinear optical properties were maintained.Recently, our group investigated a 2.62 W continuous-wave (CW)laser at 1856 nm of a Tm,Mg:LiNbO3 crystal successfully with theslope efficiency of 19.6%, and the results show that Tm,Mg:LiNbO3

crystal is a promising material for high power operation [19].However the pulsed laser performance of Tm,Mg:LiNbO3 crystalhas not been investigated until now. Meanwhile, the nanosecondpulsed laser are mainly based on the laser crystal and bulkQ-switching, which make it impossible to realize integration.

In this paper, we applied a SWCNT as saturable absorber toobtain the pulse operation of Tm,Mg:LiNbO3, in which the SWCNTcan be coated on the surface of the crystal directly to achieveintegration. At the incident pump power of 9.5 W, the minimumpulse width of 640 ns at 1950 nm under the repetition frequencyof 44.6 kHz was gained, with output coupler of transmissionT¼0.5%, resulting in a single pulse energy of 12.6 μJ. Meanwhile,with the output coupling of T¼1%, the maximum pulse energy of24.8 μJ with the repetition frequency of 11.5 kHz was obtained atthe incident pump power of 6.3 W.

2. Experimental details

Fig. 1 depicted the laser experimental scheme of Tm,Mg:LiNbO3

crystal, which was pumped by a fiber-coupled LD with the central

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Optics & Laser Technology

http://dx.doi.org/10.1016/j.optlastec.2014.06.0150030-3992/& 2014 Elsevier Ltd. All rights reserved.

n Corresponding author.nn Corresponding author.E-mail addresses: shuaiyi163@163.com (S. Zhang), jqxu09@sjtu.edu.cn (J. Xu).

Optics & Laser Technology 65 (2015) 36–38

wavelength of 792 nm at room temperature. The core diameter ofthe LD employed in our experiment was 100 μm and its numericalaperture was 0.22. Through the focusing optics system, the pumpbeam was reshaped into the laser crystal with the diameter of100 μm. The employed Tm,Mg:LiNbO3 crystals were grown by theCzochralski technique along the ⟨001⟩ direction. And it had a Tm3þ

concentration of 2 wt % and 5 wt% MgO doping, with a3 mm�3 mm cross section, and a 10 mm length. Both surfacesof the Tm,Mg:LiNbO3 crystals were antireflection (AR) coated at790715 nm and 1950750 nm. To dissipate the heat effectively,the crystal was wrapped in indium foil and water-cooled by acopper micro-channel heat-sink. The temperature of the coolingwater was kept to be 16 1C all the time. The laser cavity consistedof two mirrors and its total length was 25 mm. And the rear mirrorM1 was a flat mirror with AR coated at 790715 nm and1950750 nm. The output couplers M2 were also flat mirrors withdifferent transmissions of 0.5%, 1% and 2% at 1950750 nm.

A beam splitter was arranged behind M2, making it possible tomonitor output power and pulses simultaneously. After the CWexperiment of Tm,Mg:LiNbO3 crystal was finished, we continuedto measure its pulsed characteristics by applying a SWCNT-SAattached to the Tm,Mg:LiNbO3 crystal into the cavity, whosetransmission spectrum at 1950 nm was �86% (see Fig. 1). Thesaturable absorber was fabricated by the rapid vertical evaporationtechnique, and its outside surfaces were coated with 150 nm ZnOprotection film by Atomic Layer Deposition [20]. Actually, theSWCNT can be evaporated on the surface directly to form inte-grated devices.

3. Results and discussion

The CW laser operation of Tm,Mg:LiNbO3 crystal was investi-gated without SWCNT-SA firstly. The power performance andoutput spectra characteristics of CW Tm,Mg:LiNbO3 lasers opera-tion with different output mirrors are shown in Fig. 2. In order toprevent the damage of the laser medium, we limit the incidentpump power not exceeding 10 W. At the same time, thermo-electric cooler was used to adjust the central wavelength to satisfythe absorption peak of Tm,Mg:LiNbO3 crystal at 794 nm [19].At incident pump power of 9.5 W, a maximum output powerof 0.93 W at 1963 nm was obtained with the output coupling ofT¼1%, giving a slope efficiency of 17.4% and optical efficiency ofFig. 1. The schematic diagram of Tm,Mg:LiNbO3 PQS laser.

Fig. 3. (a) PQS output power of Tm,Mg:LiNbO3 crystal with different output couplers and (b) PQS laser spectrum of Tm,Mg:LiNbO3 crystal.

Fig. 2. (a) CW output power of Tm,Mg:LiNbO3 crystal with different output couplers and (b) CW laser spectrum of Tm,Mg:LiNbO3 crystal.

S. Zhang et al. / Optics & Laser Technology 65 (2015) 36–38 37

9.79%. In the same condition, the M2 is measured to be about1.5 with the scanning-knife-edge method. And its threshold pumppower was 4.14 W. No saturation phenomenon appeared even atthe maximum pump power, which indicates the excellent heatdispersion. Instead of other output couplers, the maximum aver-age output power of 0.85 W at 1967 nm and 0.76 W at 1945 nmwere also achieved for T¼0.5% and T¼2%, leading to a slopeefficiency of 15.2% and 15.1% and the threshold pump power of3.9 W and 4.46 W, respectively.

By placing SWCNT-SA closely behind the laser crystal, the Tm,Mg:LiNbO3 laser could easily run into Q-switched regime afteraligning the cavity. Stable Q-switched operation was achieved assoon as the Tm,Mg:LiNbO3 lasers exceed their thresholds. Fig. 3describes the average output power and output spectra character-istics of PQS Tm,Mg:LiNbO3 lasers. Under the pump power of9.5 W, the maximum output power of 0.65 W at 1945 nm wasobtained at the transmission of 1%, resulting in an optical effi-ciency of 6.84% and a slope efficiency of 14.3%, corresponding to aQ-switched efficiency of 69.9%. The threshold pump powers were4.86 W for T¼1%, 4.7 W for T¼0.5%, and 5.5 W for T¼2%. Themaximum average output power of 0.56 W at 1950 nm and 0.49 Wat 1934 nm were obtained instead of output coupler of T¼0.5%and T¼2%, corresponding to slope efficiencies of 10% and 12.3%,respectively.

The pulse repetition rate were recorded by a digital oscillo-scope and a fast InGaAs photodetector as shown in upper set ofFig. 4(a). As can be seen, the repetition rate almost increasedlinearly, with the increasing of pump power. The highest repetitionrate was 44.6 kHz, 35.4 kHz and 27.1 kHz, corresponding to T¼0.5%,

1% and 2% of the output couplers. Thus, it greatly affected theincrease of pulse energy, leading to its insensitivity towards theincident pump power (see Fig. 4(c)). It can be also found that, thepulse width decreased gradually with the increase of incidentpump power. In the condition of output mirror of T¼0.5% andincident power of 9.5 W, the minimum laser width of 640 ns withrepetition rate of 44.6 kHz was obtained, leading to a single pulseenergy of 12.6 μJ. Moreover, the shortest pulse duration of 755 nsunder repetition rate of 35.4 kHz and 815 ns under repetition rateof 27 kHz were also achieved with T¼1% and 2%, corresponding tosingle pulse energies of 18.6 μJ and 18.0 μJ, respectively. Thecorresponding pulsed profiles are illustrated in Fig. 5, whichdemonstrates a nice Q-switching stability. And the ratio of thepeak power between different pulses was more than 90%. Inaddition, a maximum single pulse energy of 24.8 μJ was obtainedat incident pump power of 6.3 W, when the transmission of theoutput coupler was 1%. The results indicate that Tm,Mg:LiNbO3

PQS laser was a potential candidate for integrated optoelectronics.

4. Conclusions

In conclusion, We demonstrated a passively Q-switched laseroperation of the Tm,Mg:LiNbO3 crystal with SWCNT-SA. Theminimum pulse width of 640-ns was realized, with the pulserepetition frequency of 44.6 kHz and the single pulse energy of12.6 μJ. Considering Tm,Mg:LiNbO3 crystal and SWCNT can beintegrated together, the results indicate the passively Q-switchedtechnique is a promising routine to the pulsed 2-μm integratablelaser devices.

Acknowledgments

This work is supported by the Doctoral Foundation of ShandongProvince (No. BS2012DX010), the Science & Technology Project ofQingdao (No. 11-2-4-3-(1)-jch), NSFC (Nos. 11274189 and 11274188),and the State Key Program for Basic Research of China (Grant no.2010CB630703).

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Fig. 4. PQS laser output characteristics of Tm,Mg:LiNbO3 crystal.

Fig. 5. Typical pulse profile of the shortest pulse width under different transmissions.

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