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A Stable 2012.1 nm Single-Longitudinal-Mode Tm:YAG Ceramic Laser with Volume Bragg

Grating and Fabry—Perot Etalon

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2014 Chinese Phys. Lett. 31 084203

(http://iopscience.iop.org/0256-307X/31/8/084203)

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CHIN.PHYS. LETT. Vol. 31, No. 8 (2014) 084203

A Stable 2012.1 nm Single-Longitudinal-Mode Tm:YAG Ceramic Laser withVolume Bragg Grating and Fabry–Perot Etalon *

DAI Tong-Yu(戴通宇), DENG Yun(邓云), JU You-Lun(鞠有伦), DUAN Xiao-Ming(段小明),YAO Bao-Quan(姚宝权)**, WANG Yue-Zhu(王月珠)

National Key Laboratory of Tunable Laser Technology, Harbin Institute of Technology, Harbin 150001

(Received 3 June 2014)We demonstrate a stable single longitudinal mode Tm:YAG ceramic laser operating at a single wavelength of2012.1 nm. The single-longitudinal-mode Tm:YAG ceramic laser is obtained by employing volume Bragg gratingas one cavity mirror and inserting a Fabry–Perot etalon into the laser cavity. A maximum single longitudinalmode output power of 165mW with a slope efficiency of 6.6% is achieved under the pump power of 5.02W. Thelaser has a beam quality of 𝑀2 = 1.18 at the maximum single longitudinal mode output power.

PACS: 42.55.Xi, 42.60.Pk DOI: 10.1088/0256-307X/31/8/084203

The 2µm single longitudinal mode (SLM) lasersare very attractive for laser radar systems, high res-olution molecular spectroscopy, atmospheric remotesensing, and optical frequency standards.[1−3] Due tothe fact that the single thulium doped crystals havethe advantages of a long fluorescence lifetime, highquantum efficiency and the absorption peak (790 nm)of Tm3+ well matches the commercial laser diodes,[4]thus it is an excellent gain medium to generate a2µm laser. Several techniques have been used toachieve the SLM laser such as the monolithic non-planar ring oscillator,[5] the twisted-mode cavity,[6]the intra-cavity spectral filter,[7] and the extremelyshort cavity methods.[8] Up to now, the diode-pumped2µm SLM single Tm-doped lasers based on YAG,[9]YAP,[10] and YLF[11] have been widely demonstrated.

With the development of polycrystalline transpar-ent ceramics, various rare-earth-ion-doped ceramics asgain media become available. Transparent ceramicmaterials have the advantages of good optical andthermal properties, the same as single crystals, alsothe ceramic can be fabricated with high doped concen-tration and large sizes. The transparent laser ceramicshave become a more and more attractive alternativecompared with single crystals.[12−14] The SLM singleTm-doped ceramic lasers have been obtained with avolume Bragg grating (VBG) and double Fabry–Perot(F-P) etalons. Yao et al. reported an SLM Tm:YAGceramic laser characteristic with two F-P etalons.[15]Long et al. demonstrated a 1999.7 nm SLM Tm:YAGceramic laser with a VBG.[16] In fact, the stability ofthe SLM laser output utilizing VBG was guaranteedcompared with the double F-P etalons method dueto less inserted elements in the laser cavity. However,the relatively wide reflection bandwidth of the possibleVBG result to the potential longitudinal mode gener-ation and stable SLM laser are difficult to obtain dueto the fluctuations of the pump power, pump light

wavelength, and temperature of the gain material.In this Letter, we demonstrate an SLM Tm:YAG

ceramics laser operating at 2012.1 nm. A stable SLMTm:YAG ceramics laser was achieved by applying anF-P etalon combined with a VBG. The highest SLMoutput power of the Tm:YAG ceramics laser was upto 165 mW with a slope efficiency of 6.6% under thepump power of 5.02 W.

The experimental configuration of the SLMTm:YAG ceramics laser is depicted in Fig. 1. Theresonator of the Tm:YAG ceramic laser has a plane-concave geometry with a physical cavity length of45 mm. The output coupler is coated for 2% trans-mission at 2µm with a 100 mm radius of curvature.The VBG (OptiGrate Corp.) was customized with adiffraction efficiency larger than 99.5% at the wave-length of 2012.6 nm and a full width at half-maximum(FWHM) of 1 nm. An uncoated YAG etalon witha thickness of 0.3 mm was inserted in the cavity toachieve the longitudinal mode of the Tm:YAG ceram-ics laser. The gain medium with a size of 3mm indiameter and 3mm in length has a thulium ion dop-ing concentration of 4 at.%, which is anti-reflection(AR) coated at the pump wavelength of 785 nm andthe laser wavelength of 2µm on both ends. The lasercrystal was wrapped with indium foil and mountedwith a water-cooled copper heat sink maintained at atemperature of 278 K. The pumping source is a 10Wfiber-coupled laser diode with a 100µm core diameterand a numerical aperture (NA) of 0.22. The outputwavelength of the laser diodes can be tuned to coin-cide with the Tm absorption band by changing theirworking temperature. The pumping light was focusedinto the crystal by a 1:1 coupling system.

Firstly, we investigated the output characteristicsof the free-running Tm:YAG ceramic laser without us-ing VBG and etalon in the cavity. A plane reflectionmirror coated with a high transmission for 785 nm

*Supported by the Fundamental Research Funds for the Central Universities under Grant No HIT.NSRIF.2015042, and theScience Fund for Outstanding Youths of Heilongjiang Province (JQ201310).

**Corresponding author. Email: yaobq08@163.com© 2014 Chinese Physical Society and IOP Publishing Ltd

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CHIN.PHYS. LETT. Vol. 31, No. 8 (2014) 084203

and high reflection for 2µm was used as the cavitymirror. For the free-running operation, the outputpower of the Tm:YAG ceramic laser versus the inci-dent pump power is shown in Fig. 2. A maximum out-put power of 747mW with a slope efficiency of 13.2% isobtained under the pump power of 6.74W, and the fit-ted threshold is 1.31 W. The output wavelength of theTm:YAG ceramic laser in the free-running mode wasmeasured, as shown in Fig. 3(a). The peak laser out-put is located at 2015.2 nm with plenty of other wave-lengths. To investigate the longitudinal mode struc-ture of the free-running Tm:YAG ceramic laser, theoutput of the laser was interrogated by using a scan-ning F-P interferometer with a free spectral range of3.75 GHz. The typical output signal of the scanningF-P interferometer is shown in Fig. 3(b). We can notethat the free oscillation laser is typically operated ona lot of longitudinal modes, and the competition be-tween the different modes is quite intense.

FiberOutput coupler

LD Tm:YAG ceramic

Coupling system

VBG or HR mirror

Laser output

etalon

Fig. 1. Experimental configuration of the Tm:YAG ce-ramic laser.

1000 2000 3000 4000 5000 6000 70000

100

200

300

400

500

600

700

800

Experiment data Slope efficiency=13.2%

Outp

ut

pow

er

(mW

)

Pump power (mW)

Fig. 2. The output power of the free-running Tm:YAGceramic laser.

To reduce longitudinal modes, a VBG instead ofthe plane reflection mirror was employed as the cav-ity mirror. We investigated the output characteristicsof the Tm:YAG ceramic laser with VBG only. Theoutput wavelength of the Tm:YAG ceramic laser withVBG only is shown in Fig. 4(a). Due to the effectof the VBG, the laser wavelength is forced to changefrom the peak wavelength of 2015 nm to the wave-length of 2012.5 nm. The scanning F-P spectrum ofthe Tm:YAG ceramic laser with VBG is shown inFig. 4(b). Compared with the free oscillation mode,the longitudinal mode of the Tm:YAG ceramic laserwith VBG is clearly less. However, extra modes stillexist on the observation in the scanning F-P spectrum.As can be found clearly, the laser is running on a mul-tiple mode.

The output power of the Tm:YAG ceramic laserwith VBG versus the pump power is depicted in Fig. 5.The highest output power and slope efficiency of theTm:YAG ceramic laser with VBG are 531 mW and9.3%, respectively. Compared with the free oscillationmode, the highest output power and slope efficiencyare decreased due to the fact that the emission crosssection of the gain medium at the shorter wavelengthis smaller.

2010 2014 2018

Wavelength (nm)

2015.2 nm

Scan time(ms)

Voltage (

V)

Inte

nsi

ty (

arb

. units)

Inte

nsi

ty (

arb

. units)

FSR3.75 GHz

10 V/div2 ms/div

(a) (b)

26

10

14

18

Fig. 3. The output wavelength and F-P scanning spec-trum of the free-running Tm:YAG ceramic laser.

Wavelength (nm)

2012.5 nm

Scan time(ms)

Voltage(V

)

Inte

nsi

ty (

arb

. units)

Inte

nsi

ty (

arb

. units)

10 V/div2 ms/div

FSR3.75 GHz

26

10

14

18

2011 2013 2015

(a) (b)

Fig. 4. The output wavelength and F-P scanning spec-trum of the Tm:YAG ceramic laser with VBG.

1000 2000 3000 4000 5000 6000 70000

100

200

300

400

500

600

Experiment data Slope efficiency=9.3%

Outp

ut

pow

er

(mW

)

Pmup power (mW)

Fig. 5. The output power of Tm:YAG ceramics laser withVBG.

To achieve SLM operation of the Tm:YAG ceramiclaser, an uncoated YAG etalon with a thickness of0.3 mm was inserted into the cavity, beyond that aVBG was used as a cavity mirror instead of the planereflection mirror. Figure 6 shows the combined effectsof the VBG and F-P etalon in the range of 2011–2016 nm. As can be noted clearly, the wavelengthwith the highest diffraction efficiency of VBG is from2012.1 nm to 2013.1 nm, which means that only thisrange can oscillate in the cavity. By tuning the an-gle of the F-P etalon, the wavelength with the highesttransmission of the F-P etalon can be shifted to the

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CHIN.PHYS. LETT. Vol. 31, No. 8 (2014) 084203

edge of the range with the highest diffraction efficiencyof the VBG. By using the VBG and F-P etalon, thetotal losses of the laser mode we selected is less thanthe other modes and thus we achieve the single modeoscillate. The scanning F-P spectrum of laser outputis shown in Fig. 7(a). As can be seen, the laser is run-ning on a single longitudinal mode. The SLM outputlaser wavelength is depicted in Fig. 7(b), the emissionlines of the SLM Tm:YAG ceramic laser are locatedat 2102.1 nm.

2011 2012 2013 2014 2015 20160.0

0.2

0.4

0.6

0.8

1.0

VBG Efficiency F-P Transmission

Wavelength (nm)

Diffr

acti

on e

ffic

iency

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Tra

nsm

issi

on

Fig. 6. The transmission of VBG and F-P.

Scan time(ms)

Voltage(V

)

Inte

nsi

ty (

arb

. units)

10 V/div2 ms/div

FSR3.75 GHz

Wavelength (nm)

Inte

nsi

ty (

arb

. units)

2012.1 nm

26

10

14

18

(a) (b)

2010 2012 2014

Fig. 7. The F-P scanning spectrum and output wave-length of the SLM Tm:YAG ceramic laser.

2800 3200 3600 4000 4400 4800 520020

40

60

80

100

120

140

160

180 Experiment data Slope efficiency=6.6%

Outp

ut

pow

er

(mW

)

Pump power (mW)

Fig. 8. The output power of SLM Tm:YAG ceramicslaser.

The recorded output power of the SLM Tm:YAGceramic laser as a relation with the incident pumppower is shown in Fig. 8. The highest SLM outputpower of 165mW with a slope efficiency of 6.6% is ac-quired with the pump power of 5.02W, and the fittedthreshold is 2.53W. The highest output power andslope efficiency are clearly decreased due to the mode

select effect of the F-P etalon. The SLM operationcould be kept up over almost the whole output powerrange (39–165 mW) and mode hopping is not observed.

The beam qualities of the Tm:YAG ceramic laserwere recorded by applying a knife-edge technique atthe highest SLM output power. The laser beam qual-ity was improved for higher-order modes discriminateddue to the fact that the angle of incident light affectsthe diffraction efficiency of the VBG. The experimen-tal results are shown in Fig. 9. By fitting the exper-imental data, it can be found that the 𝑀2 factor ofthe laser is 1.18.

0 50 100 150 200 250

0.2

0.3

0.4

0.5

0.6

Experimental data

2=1.18

Beam

radiu

s (m

m)

Position (mm)

Fig. 9. Beam quality of the highest SLM output power.

In summary, a diode-pumped single longitudinalmode Tm:YAG ceramic laser has been achieved by us-ing an F-P etalon and a VBG as a longitudinal modeselect element. The central wavelength of the singlelongitudinal mode laser is 2012.1 nm by tuning theangle of the 0.3 mm F-P etalon. A maximum SLMoutput power of 165 mW is obtained under the pumppower of 5.02W, corresponding to a slope efficiency of6.6%. The laser has an 𝑀2 factor of 1.18 at the high-est SLM output power. This SLM Tm:YAG ceramiclaser is encouraging since it can be used as a seed laserfor radar systems instead of a Tm:YAG single crystallaser.

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