Optics & Laser Technology 44 (2012) 960–962

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Optics & Laser Technology

0030-39

doi:10.1

n Corr

E-m

chinaw

journal homepage: www.elsevier.com/locate/optlastec

2 mm passive Q-switched mode-locked Tm3þ:YAP laser with single-walledcarbon nanotube absorber

Jie Liu a,n, Yonggang Wang b,n, Zunshi Qu a, Xiuwei Fan a

a College of Physics and Electronics, Shandong Normal University, Jinan 250014, Chinab Research Center for Applied Sciences, Academia Sinica, Taiwan, 30010, China

a r t i c l e i n f o

Article history:

Received 24 July 2011

Received in revised form

7 October 2011

Accepted 1 November 2011Available online 17 November 2011

Keywords:

Diode-pump 2 mm laser

Single-walled carbon nanotube saturable

absorber

Tm3þ:YAP crystal

92/$ - see front matter & 2011 Elsevier Ltd. A

016/j.optlastec.2011.11.001

esponding authors.

ail addresses: jieliu@sdnu.edu.cn, liujie-jn@so

ygxjw@hotmail.com (Y. Wang).

a b s t r a c t

We report the first demonstration, to our knowledge, of passive Q-switched mode-locking in a

Tm3þ:YAP laser, operating in the 2 mm broadly spectral region formed with a compact Z-flod cavity.

A transmission-type single-walled carbon nanotube saturable absorber (SWCNT–SA) is used for the

initiation of the pulse generation. The repetition rate of the Q-switched envelope was 60 kHz at the

pump power of 8.6 W. The mode-locked pulses inside the Q-switched pulse envelope had a repetition

rate of �92 MHz. A maximum average output power of 761 mW was obtained. The dependence of the

operational parameters on the pump power was also investigated experimentally.

& 2011 Elsevier Ltd. All rights reserved.

1. Introduction

There are many applications for high repetition rateQ-switched mode-locked (QML) 2 mm lasers with a broad spectralregion. Some examples are eye-safe lidar, medicine, spectroscopy,remote sensing, and generation of mid-IR light source.Q-switched mode-locked pulses can have peak powers manytimes higher than that of mode-locked pulses for similar currentinjection. Diode-pumped passively Q-switched and mode-lockedlasers can provide this kind of pulses and have the advantages ofsimplicity, compactness, low cost and high efficiency. So far, avariety of solid-state saturable absorption materials for Q-switched and mode-locked 2 mm laser have been investigated,such as InGaAs/GaAs and semiconductor saturable absorptionmirrorðSESAMÞ[1–6]. Single-walled carbon nanotube saturableabsorber (SWCNT–SA) can easily be produced in the form of thinfilms including three steps of nanotube growth, dispersion, anddeposition processes. It exhibit fast recovery time, chemicalstability, and broad spectral range, roughly between 1 and 2 mm[7–11]. More recently, vertical evaporation method was used tofabricate single-walled carbon nanotubes absorber for 1 mm modelocking solid-state lasers and the output power of the pulse laseris as high as 3.6 W [12,13]. But so far, researches on the

ll rights reserved.

hu.com (J. Liu),

Q-switched mode-locking 2 mm laser with SWCNT absorber havebeen hardly reported.

As a promising laser crystal, Tm3þ:YAlO3 (Tm3þ:YAP) crystal isan efficient diode-pumped solid-state �2 mm laser material[14,15]. Diode-pumped passively Q-switched Tm3þ:YAP withInGaAs/GaAs as a saturable absorber have been successfullydemonstrated [6]. However, it was not until recently thatQ-switched mode-locked Tm3þ:YAP laser with SWCNT absorberwere reported. In this paper, we firstly report, as far as we know,the performance of a diode-pumped passively Q-switched mode-locked Tm3þ:YAP laser at �2 mm with high repetition rate, usingSWCNT as saturable absorber, formed with a compact Z-typecavity. The spectrum of the Q-switched mode-locked laser iscentered at 2011 nm with a broadly spectral region (FWHM) of�31 nm. Several aspects of the passively Q-switched mode-lockedTm3þ:YAP lasers, have all been investigated in some details.

2. Experiment

2.1. Fabrication of single-walled carbon nanotube

saturable absorber

The SWCNT–SA used in this experiment were grown byelectric arc discharge technique. The mean diameter of theSWCNT is about 1.5 nm. At the first step of preparing SWCNTfilms, several mg of SWCNT powder was poured into 10 ml 0.1%sodium dodecyl sulfate (SDS) aqueous solution. SDS was used as asurfactant to disperse SWCNT in aqueous solution. The SWCNT

J. Liu et al. / Optics & Laser Technology 44 (2012) 960–962 961

aqueous solution was then ultrasonically agitated for 12 h. Afterthe ultrasonic process, the dispersed solution of SWCNT wascentrifuged to remove sedimentation of larger SWCNT bundles.Then the SWCNT dispersion was diluted and poured into apolystyrene cell. Then we inserted vertically a hydrophilic quartzor glass substrate into the polystyrene cell and put on steady forbeing gradually evaporated at atmosphere. It took about twoweeks for complete evaporation on the substrate. Finally, a100 nm thick ZnO film was coated both side of the SWCNT/Quartz/SWCNT material to isolate the SWCNT from the air. Thenthe substrate coated with SWCNT is ready for using as a saturableabsorber. An UV-Visible-NIR spectrophotometer was employed tomeasure the linear optical transmission of the SWCNT absorbersof different concentrations, as shown in Fig. 1.

2.2. Experimental setup

To generate a Q-switched and mode-locked pulse, the inten-sity fluctuation must be sufficiently strong. Therefore a cavity isrequired that has a small beam area in the SWCNT–SA. In order toacquire high intracavity intensity, we adopted folded cavity.A schematic setup of the passively Q-switched mode-lockedTm3þ:YAP laser is shown in Fig. 2. A Z-type cavity was used toensure a small spot size on the SWCNTs. The pump source used inthe experiment is a fiber-coupled diode laser whose core size is400 mm in diameter and the numerical aperture is 0.22. Theemitting wavelength of the laser diode is 795 nm with themaximum available output power of 30 W. The pump laser isfocused onto the Tm3þ:YAP crystal with a radius of 100 mm by1:0.5 focus lens. The dimensions of the Tm3þ:YAP crystal is3�3�5 mm3 with a Tm-doped concentration of 5 at.%. It waswrapped with indium foil and mounted in a water-cooled copperblock with stable temperature maintained at 17 1C. The crystal

Fig. 1. Absorption spectrum of SWCNT–SA.

LD

M1 M2

M3M4

Tm : YAP

SWCNT

Fig. 2. Schematic of the Tm:YAP Q-switched mode-locked laser.

had antireflection coating at the pump and laser wavelengths onboth side of its surfaces. The cavity mirror M1 was a flat mirror,which was antireflection coated at 790–810 nm and high-reflec-tion at 1.9–2.1 mm. The length of the folded cavity is 1.63 m. Thecavity mirrors M2 (R2¼50 cm) and M3 (R3¼30 cm) were HR-coated at 1.9–2.1 mm. The output coupler mirror (M4) was a flatmirror and had the transmittance of 5%. The transmission-typeSWCNT–SA used in the experiment was put next to M4. Wecalculated the TEM00 Gaussian modes for the resonators used inthe experiments by applying the ABCD-matrix formalism, andassuming that the pumped crystal can be modeled as a paraxiallens-like medium. The mode radius on the SWCNT–SA is about50 mm.

We found in our experiments that such a Z-flod cavity couldsatisfy the Q-switched and mode-locking condition. We obtainedQML, which is near to CW mode locking. The intracavity pulseenergy (power) might be smaller than the critical intracavitypulse energy, which is required for obtaining stable CW modelocking [16]. The output power was measured by the power meter(30A –SH–V1, made in Israel). Fig. 3 showed the average outputpower of the passively Q-switched mode-locking state and con-tinuous wave (CW) state (without SWCNT–SA) as a function ofthe incident pump power. The QML Tm3þ:YAP laser appearedwhen the pump power exceed 3.0 W. At an incident pump powerof 8.6 W, the output power of passively QML state and CW stateare 761 mW and 1.1 W, respectively, which is stable for severalhours. No damage to the crystal was observed over hours ofoperation, and the laser performance was reproducible. In orderto protect the SWCNT–SA and Tm3þ:YAP crystal from damage, wedidn’t increase the pump power any more.

The pulse’s temporal behavior was recorded by a TektronixTDS 5104 Digital Phosphor Oscilloscope (1 GHz bandwidth) and afast photodiode detector (ET-3500) with a rising time of 250 ps.The maximum repetition rate of the Q-switched envelopincreased to 60 kHz when the incident pump power increasedto 8.6 W. The pulse-to-pulse amplitude fluctuation of theQ-switched pulse train was found to be less than 715%. Themaximum energy of single Q-switched pulse was 12.7 mJ.Fig. 4(a) displayed the temporal traces of the QML laser pulses,nearly CW mode locking obtained under the incident pumppower of 8.6 W. The mode-locked pulses inside the Q-switchedpulse envelope had a repetition rate of 92 MHz, which corre-sponds to the roundtrip time of light traveling in the laser cavity.The Fig. 4(b) presents the expanded oscilloscope traces of a trainof mode-locked pulses obtained. The mode-locked single pulseenergy is about 8.3 nJ. Caused the restriction of the lab condition,we didn’t measure the microwave spectrum and the profile of

20.0

0.2

0.4

0.6

0.8

1.0

1.2

QML CW

Ave

rage

Out

put P

ower

/W

Incident Pump Power/W3 4 5 6 7 8 9

Fig. 3. Average output power versus incident pump power in CW and Q-switch

mode locking (QML) regimes.

Fig. 4. Q-switched mode-locked pulse trains recorded in (a) 1 ms and (b) 10 ns per

division (div) time scales.

19000.0

0.2

0.4

0.6

0.8

1.0

inte

nsity

(a.u

.)

wavelength/nm

31.78nm

1950 2000 2050 2100

Fig. 5. Optical spectrum for the Tm:YAP QML laser at 2011 nm.

J. Liu et al. / Optics & Laser Technology 44 (2012) 960–962962

autocorrelation signal. With the rise time of the probe and theoscilloscope employed [17], the duration of the mode-lockedpulse is estimated to be about sub-100 ps.

The QML pulse spectrum was measured by an optical spec-trum analyzer (AvaSpec–NIR256–2.5). It is shown in Fig. 5. Thecentral wavelength was 2011 nm. The spectral bandwidth(FWHM) was 31.78 nm. It can be seen that broadly spectralregion 2 mm pulse laser was obtained. With further optimizationof the modulation depth in the SWCNT–SA, the CW mode-locking2 mm laser can be realized.

3. Conclusions

In summary, broadband SWCNT absorber was fabricated byvertical evaporation method. For the first time to our knowledgewe demonstrated a QML diode-pump Tm3þ:YAP broadly spectralregion 2 mm laser using the SWCNT as the saturable absorber.A 92 MHz mode-locked pulse train was enveloped in Q-switchedlaser pulse. At an incident pump power of 8.6 W, the maximumaverage output power was 761 mW with the Q-switched enveloprepetition rate of 60 kHz. The maximum Q-switched envelop

pulse energy was 12.7 mJ. The pulse width of the mode-lockedpulse inside the Q-switched envelope was estimated to be aboutsub-100 ps. Our experimental results have shown that theSWCNT–SA is promising for the potential absorber in the diode-pumped Tm-doped crystal �2 mm pulse laser.

Acknowledgments

The authors acknowledge support from the National NaturalScience Foundation of China (no. 61078032) and Science andTechnology on Solid-State Laser Laboratory of China (no.9140C0403011106).

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