Optics & Laser Technology 44 (2012) 2149–2153

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Optics & Laser Technology

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

LD-end-pumped passively Q-switched Nd:YAG ceramic laser with single wallcarbon nanotube saturable absorber

C.Y. Li a,b, Y. Bo b,n, N. Zong b, Y.G. Wang c, B.X. Jiang d, Y.B. Pan d, G. Niu a, Z.W. Fan a,Q.J. Peng b, D.F. Cui b, Z.Y. Xu b

a Beijing GK Laser Technology Co., Ltd., Chinese Academy of Sciences, Beijing 100192, Chinab RCLPT, Key Lab of Functional Crystal and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, Chinac Department of Applied Physics and Materials Research Center, Hong Kong Polytechnic University, Hong Kong, Chinad Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201800, China

a r t i c l e i n f o

Article history:

Received 16 February 2012

Accepted 7 March 2012Available online 28 March 2012

Keywords:

Passively Q-switched

SWCNT-SA

Nd:YAG ceramic

92/$ - see front matter & 2012 Elsevier Ltd. A

x.doi.org/10.1016/j.optlastec.2012.03.010

esponding author.

ail addresses: zhaoyang2050@163.com (C.Y. L

@tsinghua.org.cn (Y. Bo).

a b s t r a c t

We report on a LD-end-pumped passively Q-switched Nd:YAG ceramic laser by using a novel single

wall carbon nanotube saturable absorber (SWCNT-SA). The SWCNT wafer was fabricated by electric Arc

discharge method on quartz substrate with absorption wavelength of 1064 nm. We firstly investigated

the continuous wave (CW) laser performance and scattering properties of Nd:YAG ceramic sample. For

the case of passively Q-switched operation, a maximum output power of 376 mW was obtained at an

incident pump power of 8.68 W at 808 nm, corresponding to an optical–optical conversion efficiency of

4.3%. The repetition rate as the increase of pump power varied from 14 to 95 kHz. The minimum pulse

duration of 1.2 ms and maximum pulse energy of 4.5 mJ was generated at a repetition rate of 31.8 kHz.

& 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Passive Q-switching of diode-pumped solid-state lasers have wideapplications in fields of remote sensing, scientific research, medicinedue to their advantages in terms of miniature, device simplicity,compactness, high efficiency and low cost, etc. [1]. In general, thegeneration of passively Q-switched laser pulses depends strongly onthe availability of suitable saturable absorbers. Historically, severalintra-cavity solid-state passive elements have been used as saturableabsorbers, such as bleachable dyes [2], color centers [3], Cr4þ:YAGcrystal [4], and semiconductor saturable absorbers (SESAM) [5].Among them, Cr4þ:YAG crystal is still primitive passive Q-switchingelement originated from its excellent optical and mechanical proper-ties, high thermal and radiation stability. However, its absorptionband is located around 1 mm. Hence Cr4þ:YAG is only convenient forpassively Q-switched Nd:YAG lasers at 1 mm.

Recently, carbon nanotube based saturable absorbers (CNTs-SA) have attracted considerable attention due to their outstandingproperties, such as sub-picosecond recovery time, low saturationpower, broad operation range, polarization insensitivity, easyfabrication, and mechanical and environmental robustness, etc.The significant progress of CNTs are focused on their mode

ll rights reserved.

i),

locking applications in all solid state lasers [6–9]. Besides thewell known application as mode locking absorber devices, theCNTs have been proved to be an effective Q switcher after properadjustment of preparation parameters. The CNTs using as satur-able absorbers can be attributed to their electronic propertieswhich are determined by the CNTs’ diameter and chirality [10].CNTs have a band gap varying inversely with their diameter. Theelectronic transitions between the valence bands and conductionbands of CNTs result in a wide spectral-range optical absorption.Absorption at a given wavelength creates electron–hole pairs.This causes band filling and the absorption saturates. A furtherpower increase results in a reduced overall absorption or ableaching of the sample, the intra-cavity loss modulation are thusimplemented. More recently, Cheng et al. reported a diode-pumped passively Q-switched Nd:LuYGdVO4 laser with aSWCNT-SA, they obtained a minimum pulse width of 52 ns andmaximum peak power 66.5 W [11]. Zhou et al., reported aQ-switched erbium doped fiber laser where a 7 ms width and14.1 nJ pulse energy was obtained [12]. Qin et al., reported aQ-switched Nd:YVO4 laser with a CNT saturable absorber, themaximum average output power was 477 mW with pulse widthof 323 ns and pulse energy of 326 nJ [13].

In addition, all solid-state lasers using polycrystalline ceramicgain materials in particular the Nd:YAG ceramic are extensivelystudied for generating high output power and high energy[14–18]. It is regarded as a promising candidate of crystal owingto its low cost, composition control, ease of fabrication, large size,

C.Y. Li et al. / Optics & Laser Technology 44 (2012) 2149–21532150

multi-functional, and mass production, etc. One can expect thatshort pulse ceramic lasers may find potential applications besidestheir traditional high power domains. However although pas-sively Q-switched Nd:YAG lasers have been widely addressed,there is few report on passively Q-switched ceramic laser. Herewe present a passively Q-switched Nd:YAG ceramic laser by useof a single wall carbon nanotube absorber. A stable laser pulse ofduration of 1.2 ms and pulse energy of 4.5 mJ were generated at arepetition rate of 31.8 kHz. The maximum output power was376 mW.

2. Experimental

The SWCNT used in this experiment was grown by electric arcdischarge technique and was identical to our previous work [19].The mean diameter of the SWCNTs is about 1.5 nm. First, 0.15 mgSWCNTs powder was poured into 10 ml 0.1% sodium dodecylsulfate (SDS) aqueous solution to disperse SWCNTs in aqueoussolution. The SWCNT solution was then ultrasonically agitated for12 h. Subsequently, the dispersed solution of SWCNTs was cen-trifuged to remove sedimentation of larger SWCNTs bundles.After decanting the upper portion of the centrifuged solution,the SWCNTs dispersion was poured into a polystyrene cell.Finally, we inserted vertically a hydrophilic quartz substrate intothe polystyrene cell and kept it steady for gradual evaporation atatmosphere. It took about two weeks for complete evaporation onthe substrate. Now the substrate coated with SWCNTs is readyfor using as a saturable absorber. Fig. 1 shows the transmissionspectrum of the prepared SWCNT-SA. The transmission rate isabout 84% at the wavelength of 1060 nm.

Fig. 1. Transmission spectrum of the SWCNT-SA.

M1

Nd:YAGceramic

LD pump

Fig. 2. Schematic diagram of the SWCNT pas

We adopted a four mirror Z fold laser resonator as shown inFig. 2. The laser gain medium is a 1 at% Nd doped YAG ceramic(prepared by the Institute of Ceramics, Chinese Academy ofSciences) and has dimensions of +3�40 mm. Both surfaces ofthe Nd:YAG ceramic are AR (anti reflection) coated at 808 nm and1064 nm. It was wrapped with indium foil and mounted in awater cooled copper heat sink at 20 1C. The pump source was afiber coupled laser diode with water cooled to center wavelengthof 808 nm. The fiber of the LD had a core diameter of 400 mm andnumerical aperture of 0.22. The fiber’s output face was imaged to2 times the diameter spot in the ceramic. M1 is a diachronicplano–plano mirror coated at HR (high refection) at 1064 nm andAR at 808 nm. The folding mirror M3 with a curvature radius of150 mm was HR coated for 1064 nm. The rear mirror M4 was aflat mirror also HR coated at 1064 nm. M2 is the output mirrorwith 5% transmittance at 1064 nm. The cavity was carefullydesigned with the ABCD matrix formalism to maintain a relativelylarge laser mode in the gain medium simultaneously keeping theQ switch stability through the available pump power range. Thearms L1, L2 and L3 were chosen to be 160, 170 and 70 mm,respectively. The whole optical cavity length was approximately440 mm. The laser beam radii were calculated to be about 350 mmat the center of the laser medium and 90 mm on the SWCNT,respectively. The folded angle of M3 was very small in order toreduce astigmatism. The SWCNT sample was placed close to M4.

3. Results and discussion

3.1. Laser properties of the Nd:YAG ceramic sample

It is well known the attenuation loss coefficient of a lasermaterial has enormous influence on the laser performance. Manyresearch have shown the scattering is the critical loss mechanism intransparent laser ceramic materials. We firstly measured the scat-tering coefficient and absorption coefficient of the ceramic samplewith an integrating sphere technology [20]. For comparison, aNd:YAG crystal was also used. The measured scattering and absorp-tion coefficient at 1064 nm for ceramic and crystal were (0.005,0.002 cm�1) and (0.002, 0.001 cm�1), respectively. The wholeattenuation loss coefficient at 1064 nm was the sum of the scatter-ing and absorption coefficient that were 0.007 cm�1 for ceramic,and 0.003 cm�1 for crystal, which were very close in value.

Following, we investigated the CW laser performance for theceramic sample as compared with the Nd:YAG crystal. The lasersystem adopted LD side-pumped by 808.5 nm and was configuredin a plano-plano short cavity. The transmittance ratio of outputcoupler at 1064 nm was 20%. The output power on LD pumppower for the case of both ceramic and crystal was shownin Fig. 3. The maximum output power of 25 W for ceramicrod was obtained at a pump power of 69 W, corresponding to

L1

M2

M3L3

SWCNT

M4

L2

sively Q-switched ceramic Nd:YAG laser.

Fig. 3. Comparative results of the output power for ceramic and crystal rod laser.

Fig. 4. Typical oscilloscope trace and single pulse image: (a) 31.8 kHz pulse trains,

and (b) extended shape of a single pulse.

Fig. 5. Average output power as a function of absorbed pump power for the

Q-switched laser.

C.Y. Li et al. / Optics & Laser Technology 44 (2012) 2149–2153 2151

optical-to-optical conversion efficiency of 36.2%. For the case ofNd:YAG crystal, the maximum output power was 26.7 W. It canbe seen that the output power for ceramic laser has only 6.3%lower than that of the crystal. This result shows though themeasured scattering coefficient and absorption coefficient at1064 nm have slight difference for Nd:YAG ceramic and crystal,they have little influence on the laser performance.

Another important factor influencing the laser performance isthe absorption character of laser medium to pump power, inparticular for an end-pumped laser system. We invoke thepreviously measured absorption result of used ceramic sampleat 808 nm pumping [21]. The peak absorption coefficient was9.1 cm�1. According to the absorption law and taking into theabsorption coefficient, the calculated length for full absorption ofpump power was 6 mm. In this region more than 99% pumppower was absorbed.

3.2. Passively Q-switched operation of the Nd:YAG ceramic laser

With an appropriate alignment of the laser cavity, stablepassive Q-switching operation was achieved above the laserthreshold. We used a high speed photon-diode detector (ThorlabsInc., DET200) and a 4 GHz oscilloscope (Tektronix DPO70404) tomonitor the 1.064 mm pulse temporal behaviors in the wholeincident pump power range. The typical pulse oscilloscope wave-form of the Q-switched laser system was shown in Fig. 4. Herein,Fig. 4(a) shows the recorded pulse trains with pulse repetitionfrequency of 31.8 kHz; Fig. 4(b) shows the extended shape of asingle laser pulse where the pulse width is 1.2 ms.

The average output power of the laser was measured by anOphir F3A power meter, as shown in Fig. 5. One can see that theoutput power was increased approximate linearly with theincident pump power. The threshold power was about 3.2 W.The maximum output power of 376 mW was obtained at thepump power of 8.68 W, corresponding to an optical–opticalconversion efficiency of 4.3%, slope efficiency of 7.6%. We mea-sured the power stability at the highest output power versus timein every 5 min. The laser power instability was less than 73%.

Here one point we should mention was the ceramic Nd:YAGsample is 40 mm in length and has 1 at% dopant. As depictedabove, the absorption length was only 6 mm. The un-absorbedregion could increase the cavity loss when the laser operatesabove threshold. For the LD end-pumped laser, too long crystalcaused serious intra-cavity loss due to the re-absorption and LDcoupling system design. This induced the difficulties of cavity

configuration and higher output power. Thus, the long ceramicsample was adverse to the augment of Q-switched output power.One alternative way was to adopt a 885 nm LD pump source sinceit has lower peak absorption coefficient and high quantumefficiency than 808 nm pumping. Further, the coupling imagingsystem exerts 800 mm spot to match the laser mode. A large spot

C.Y. Li et al. / Optics & Laser Technology 44 (2012) 2149–21532152

size reduces the pump power density that is either not beneficialfor increasing output power.

We measured the repetition frequency with the incident pumppower. In Q-switched laser, the repetition rate depends stronglyon the pump power. As the pump power increases, more gain isprovided to saturate the SA. Since pulse generation relies onsaturation, the repetition rate increases consequently. Accordingto the repetition rate and the average output power, the pulseenergy of the single Q-switched envelope can be calculated. Fig. 6shows the pulse repetition rate and the pulse energy as thefunction of the incident pump power, the repetition rate variesfrom 14 kHz to 95 kHz. As it can be seen from Fig. 6, themaximum pulse energy appears at repetition of 31.8 kHz wherethe correlative single pulse energy was 4.5 mJ. The decrease ofpulse energy with the augment of pump power may be caused bythe accumulated thermal effects at higher pump level. Here onething should be mentioned that, we observed the damage on theCNT when the pump power was above 9 W. The Q-switchingoperation was broken due to the failure of the CNT. We attributedthis to the oxidation of the carbon nanotubes at a high powerdensity situation. This indicates the damage threshold of theCNTSA should be further improved. A more robust passivelyQ-switched laser can be expected after suitable coating on theCNTSA to isolate the oxygen.

Fig. 6. Pulse repetition range and the pulse energy versus the incident

pump power.

Fig. 7. Intensity distribution of Q-switched Nd:YAG ceramic laser at the maximum

output power.

We tested the beam quality of this SWCNT passivelyQ-switched laser in current cavity configuration at the maximumoutput power of 376 mW. The M2 value was measured by aSpiricon M2-200 laser beam analyzer using the second momentmethod. The measured M2 factor was 1.7 that is a near diffractionlimited beam output. Fig. 7 shows the typical 2-D beam profile,where a near Gaussian-like spatial distribution of laser intensitycould be deduced from the beam spatial intensity distribution.

4. Conclusions

In summary, we presented a LD-end-pumped passivelyQ-switched Nd:YAG ceramic laser with a single wall carbonnanotube (SWCNT) saturable absorber. At an incident pumppower of 8.68 W at 808 nm, stable Q-switched pulse trains withduration of 1.2 ms, single pulse energy of 4.5 mJ were obtained.The approaches for obtaining higher Q-switching output powerwere proposed via investigating the absorption and scatteringproperties of ceramic samples.

Acknowledgments

This work is financially supported by the Major Program of theNational Natural Science Foundation of China with no. 50990304and Natural Science Foundation of Shanghai, China with no.09ZR1435600.

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