Acousto-Optic Q-Switched Operation Ho:YAP Laser Pumped by a Tm-Doped Fiber Laser

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2011 Chinese Phys. Lett. 28 074210

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CHIN. PHYS. LETT. Vol. 28, No. 7 (2011) 074210

Acousto-Optic Q-Switched Operation Ho:YAP Laser Pumped by a Tm-DopedFiber Laser

ZHOU Ren-Lai(周仁来)∗∗, JU You-Lun(鞠有伦), WANG Wei(王巍), ZHU Guo-Li(朱国立),WANG Yue-Zhu(王月珠)

National Key Laboratory of Tunable Laser Technology, Harbin Institute of Technology, Harbin 150080

(Received 14 March 2011)We report on the acousto-optic Q-switched operation of a Ho:YAP laser double-passed pumped by an all-fiberTm-doped fiber laser. The output characteristics of the Ho:YAP laser are studied at pulse repetition frequenciesof 1, 2, 5 and 10 kHz. The shortest pulse width, the maximum pulse energy and the highest peak power aremeasured to be 17 ns, 1.71mJ and 71.25 kW, respectively. The output wavelength is centered at 2115.96 nm witha bandwidth of about 1.5 nm. The beam quality factor M2 = 2.41 ± 0.03 at the output power of 3.54W wasmeasured by using the traveling knife-edge method.

PACS: 42.60.Gd, 42.55.Rz,42.55.Wd DOI:10.1088/0256-307X/28/7/074210

Lasers emitting in the 2µm region lie in the eye-safe region and have potential applications in vari-ous fields such as Doppler lidar wind sensing, wa-ter vapor profiling by differential absorption lidars,range finding or water vapor profiling.[1,2] In addition,high-power quasi-continuous wave 2µm lasers withhigh peak powers are effective pump sources of opticparametric oscillators (OPOs) for frequency conver-sion in the 3–12µm range, which belongs to the spec-tral region containing numerous fundamental rota-tional and vibrational features, highly specific for eachmolecule.[3] Rare-earth-ion Tm3+ and Ho3+ co-dopingmaterials are commonly used in 2µm lasers. How-ever, the output power in room-temperature Tm:Holasers are usually limited because co-doped materi-als suffer from severe upconversion losses, which leadsto a reduction in the effective upper lifetime and toan increase in thermal loading.[4,5] Diode-pumped Tmlasers and resonantly pumped Ho lasers have been in-vestigated in recent years. Using them, thermal load-ing can be alleviated due to the low quantum de-fect of less than 10% between pump and laser, andthe Ho upconversion losses are also reduced at roomtemperature.[6,7]

Among rare-earth doped host materials, yttriumaluminium oxide (YAlO3, YAP) is an attractive laserhost for holmium because of its good thermal, nat-ural birefringence and mechanical properties (similarto those of YAG).[8] In addition, thermally inducedbirefringence does not degrade the laser performanceat high power levels due to the birefringence charac-teristics of the YAP. Compared to cubic YAG, theYAP has low symmetry and, as an important conse-quence of that, the absorption and fluorescence prop-erties of rare-earth ion-doped YAP are dependent onpolarization.[9]

Compared to solid state Tm:YLF lasers at thewavelength of approximately 1.9µm, which werestudied in our pervious work, clad-pumped all-fiberthulium doped fiber lasers have many advantages in-cluding high reliability, good beam quality, compactfabric, wide tunable range, heat dissipation, high con-version efficiency and low operational cost. Further-more, they can be operated without the need for activecooling or optic alignment.[10,11] To our knowledge,2µm lasing has been realized to date for Ho3+ singlydoped YAG and YLF crystals resonantly pumped bya 1.9-µm Tm-doped fiber laser.[12,13] However, thereare fewer reports on a room-temperature Ho3+ singlydoped YAP laser pumped by 1.9-µm Tm-doped fiberlasers. In this Letter, we report the acousto-optic(AO) Q-switched operation of a Ho:YAP laser double-passed pumped by an all-fiber Tm-doped fiber laser.We study the output characteristics of the averageoutput power Pout, pulse energy E, pulse duration tpand peak power Pp at PFRs of 1, 2, 5 and 10 kHz. Inaddition, the output wavelength and the beam qualityfactor M2 are also measured

Figure 1 schematically shows the Ho:YAP laserpumped by an all-fiber Tm-doped silica fiber laser.The Tm-doped fiber was pumped by a 40 W laserdiode at 793 nm (2.5 nm bandwidth) with a 200µmpigtail fiber coupling output. The pigtail fiber wastapered and spliced in a 25/250µm 0.1/0.48 NA dou-ble clad photosensitive fiber. The fiber Bragg grat-ings (R ≈ 85%) was written in a sensitive fiber by800 nm 50 fs laser pulses in our laboratory and thecentral wavelength was located at 1947.8 nm. TheTm-doped silica fiber used in the experiment was a3.5-m-long 25/250µm 0.1/0.48NA LMA Tm-dopedfiber (Nufern) and the Tm3+ doped concentration was2.2wt%. The splice spot between the photosensitive

∗∗Email: zrlhit@126.com© 2011 Chinese Physical Society and IOP Publishing Ltd

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fiber and the Tm-doped fiber was mounted on a con-duction cooled spool.

793 nmPump diode

Splice spot

Cool fiber spool

Cool fiber plate

Thulium Doped fiber

Diaphragm 1

L1

L2

45O mirror

45O mirror

HR@1900 nm

HT@793 nm

HR@1900-2200nm

Ho:YAP

45O dichromatic

mirror

AO Q swich

2 mm laser output

HR@1900 nm

HT@793 nm

M1

M2

M3M4

FBGs

Fig. 1. Experimental setup of the AO Q-switchedHo:YAP laser pumped by an all fiber Tm-doped fiber laserat room temperature.

5 10 15 20 25 30 35 40 450

2

4

6

8

10

12

1945 1946 1947 1948 1949 1950

0

2

4

6

8

10

Outp

ut

pow

er

(W) Data

Linear fit of data

Launched pump power (W)

Inte

nsity

(arb

. unit

s)

Wavelength (nm)

1947.8407 nm78.1 pm

Fig. 2. (a) Output power of cw Tm-doped fiber laser de-pendence on incident LD power. (b) Output spectrum ofTm-doped fiber laser at power of 10W. Inset: the am-plificatory part of the laser spectrum with the FWHMmeasured to be about 78.1 pm.

The Ho:YAP laser resonator geometry was plano-concave with a 150-mm-radius curvature concave out-put coupler and the physical cavity length of the res-onator was about 135mm. L1 was a coupling lens witha 25 mm focal length. The mode matching lens (L2)had a 150 mm focal length. M3 was a 45◦ dichro-matic mirror with reflectivity of 99.5% at 2.12µmand transmission of 88% at 1.94µm. M4 was a flatmirror with 99.5% high reflectivity in the 1.9–2.2µmwavelength range. M5 was an output-coupling mirrorwith transmission of 20% at 2.12µm. The Ho:YAPcrystal for the experiment was a-cut with dimensions4×4×44 mm3, and the Ho3+ dopant concentrationwas 0.5 at.%. Both end faces of the crystal were an-tireflection coated for laser wavelengths around 2.1µm

and the pump wavelength was around 1.91µm. TheHo:YAP crystal was placed in the vicinity of lens fo-cus (L2), and the confocal spot of the thulium dopedsilica fiber laser, inside the Ho:YAP crystal surface,was measured to to approximately 0.42mm. The lasercrystal was wrapped in an indium foil and clampedin a copper heat-sink held at a temperature of 15◦Cwith a thermal electric cooler (TEC) for precise tem-perature control. The Q-switching experiment wasachieved with a 46 mm long fused silica acousto-opticQ-switch. Its maximum RF power was 50 W and therepetition rate could be tuned continuously from 1Hzto 100 kHz.

The output characteristics of the Tm-doped fiberlaser are shown in Fig. 2. Under the pumping powerof 40.5W available from the LD, the maximum powerof 11.2 W was obtained with the crystal temperatureheld at 18◦C. A linear fit to the data yielded a slopeefficiency of 37.5%, the corresponding optic-to-opticconversion efficiency was 27.7%, as shown in Fig. 2(a).The output spectrum of the Tm-doped fiber laser wasrecorded with a Burleigh WA-650 spectrum analyzercombined with a WA-1500 wavemeter (0.7 pm reso-lution). Figure 2(b) shows the output spectra of theTm-doped fiber laser at output power of 10 W. Theemission oscillates at the FBG resonant wavelengthof 1947.8407 nm with the full width at half maximum(FWHM) of 78.1 pm. The beam radius of the out-put laser was also measured by the 90/10 knife-edgemethod at the output power of 10W. By fitting theGaussian beam standard expression to these data, thebeam quality is estimated to be M2 = 1.42.

2 4 6 8 100

1

2

3

Outp

ut

Ho:Y

AP

pow

er

(W)

Incident Tm fiber power (W)

Repetition rate: 1 kHz 2 kHz 5 kHz 10 kHz

Fig. 3. The laser output power at PRFs of 1, 2, 5, and10 kHz versus incident Tm-doped fiber laser power.

Performance of the AO Q-switched Ho:YAP laserwas investigated at different pulse repetition frequen-cies (PRF). The output characteristics are depictedin Fig. 3. The maximum output powers are 1.71,3.4, 3.45 and 3.54W at the repetition frequenciesof 1, 2, 5 and 10 kHz, respectively. The change ofthe slope efficiency is not obvious at different rep-etition frequencies. At 10 kHz, a linear fit to thedata yields a slope efficiency of 41.3%, the correspond-ing optic-to-optic conversion efficiency is 31.89% and

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the diode-to-holmium optic conversion efficiency is8.7%. Compared with the output characteristics of theHo:YAP laser pumped by a Tm:YLF laser reportedpreviously,[14] the output power increases unsteadilyand the slope efficiency is low. It can be explainedthat the detrimental up-conversion process becomesstronger when the output wavelength of the Tm-dopedfiber laser locates at the absorption peaks of Ho:YAP.From the absorption spectrum of Ho:YAP,[15] a lowerconcentration of the Ho3+ ions doped in the Ho:YAPcrystals is necessary when the pump wavelength is lo-cated at the absorption peaks.

2 4 6 8 100.0

0.3

0.6

0.9

1.2

1.5

1.8 Repetition rate:

1 kHz 2 kHz 5 kHz 10 kHz

Puls

e e

nerg

y (

mJ)

Incident Tm fiber power (W)

Fig. 4. The pulse energy with four PRFs of 1, 2, 5 and10 kHz versus incident Tm-doped fiber laser power.

From the average laser output power Pout and thecorresponding PRF, the pulse energy generated at agiven incident Tm-doped fiber laser power Pin can bedetermined by E = Pout PRF. Figure 4 depicts thepulse energy versus Pin, generated with the differentPRFs of 1, 2, 5 and 10 kHz. A substantial enhance-ment in laser pulse energy was achieved in the case ofPRF=1 kHz, in comparison with the cases of PRF=2,5 and 10 kHz. The highest pulse energies obtainedwith PRFs of 1, 2, 5 and 10 kHz are, respectively, 1.71,1.7, 0.69 and 0.354 mJ.

3 4 5 6 7 8 9 10 11

0

100

200

300

400

500

Puls

e w

idth

(ns)

Incident Tm fiber power (W)

Repetition rate: 1 kHz 2 kHz 5 kHz 10 kHz

Fig. 5. The pulse width versus incident pump power atPRFs of 1, 2, 5 and 10 kHz.

The Q-switched laser pulse was detected by an In-GaAs photodiode and recorded with a 350 MHz digitaloscilloscope (wavejet332, Lecroy). The output pulse

width characteristics were studied at PRFs of 1, 2, 5and 10 kHz. Figure 5 depicts the variation of pulseduration tp versus the incident Tm-doped fiber laserpower measured for operation at PRFs of 1, 2, 5 and10 kHz. The pulse duration begins to narrow at thesame PRF with the laser output power increasing, andthe energy is over 1mJ (as compared to the energyunder 1mJ), the pulse duration changes unconspicu-ously, and the minimum pulse widths obtained withPRFs of 1, 2, 5 and 10 kHz are, respectively, 17 ns,21 ns, 58 ns and 110 ns.

4 6 8 10

0

20

40

60

80

Incident Tm fiber power (W)

Repetition rate: 1 kHz 2 kHz 5 kHz 10 kHz

Peak p

ow

er

(kW

)

Fig. 6. The peak power versus incident pump power atPRFs of 1, 2, 5 and 10 kHz.

2110 2112 2114 2116 2118 2120 2122

0

10

20

30

40

50

60

70

80

Inte

nsi

ty (

arb

. unit

s)

Wavelength (nm)

2115.96 nm

Fig. 7. Output spectrum of the acousto-optic (AO) Q-switched Ho:YAP laser.

From the measured tp and pulse energy E, thepeak power can be estimated by Pp = E/tp. Fig-ure 6 shows the peak power as a function of Pin forPRFs of 1, 2, 5 and 10 kHz. Clearly the peak power in-creases with Pin much more rapidly in the Q-switchedoperation at PRF=1, 2 kHz, reaching a maximum of71.25 kW and 60.24 kW at Pin=7.26W. In the caseof PRF=5, 10 kHz, the increase of peak power withPin becomes considerably slow, with the highest peakpower of only 11.89 kW and 3.22 kW, and the slowincrease can be attributed to the significant pulse du-ration increase.

The output wavelength of the acousto-opticQ-switched Ho:YAP laser was recorded with amonochrometer (300-mm focal length, 600 lines/mmgrating blazed at 2.0µm). The lasing radiation was

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CHIN. PHYS. LETT. Vol. 28, No. 7 (2011) 074210

monitored by an InGaAs detector with an SR830 lock-in amplifier for signal extraction. As shown in Fig. 7,the output laser wavelength was centered at 2.116µmwith the FWHM of about 1.5 nm. The relationship be-tween the output wavelength and the crystal coolingtemperatures at different pump power levels was in-vestigated, no visible wavelength shifts were observed.The output wavelength is not sensitive to the changeof crystal temperature and pump power levels.

100 120 140 160 180 200 220 240

2

3

4

5

6

7

Beam

radiu

s (m

m)

location (mm)

2=2.41

Fig. 8. Beam radius for the Ho:YAP laser as a functionof the distance from the focusing lens at the output powerof 3.4W.The beam radius for the acousto-optic (AO) Q-

switched Ho:YAP laser at the output power of 3.4 Wwas also measured by the 90/10 knife-edge method.The lens with f = 150 mm was located 200 mm awayfrom the output-coupling mirror. Figure 8 shows themeasured beam radius at different positions after thelens. By fitting the Gaussian beam standard expres-sion to these data, we estimate the beam quality tobe M2=2.41±0.03.

In summary, we have investigated AO Q-switchedoperation of a Ho:YAP laser double-passed pumpedby an all-fiber Tm-doped fiber laser. The output

power, pulse energy, pulse duration and peak powerare investigated at PRFs of 1, 2, 5 and 10 kHz. Theoutput spectrum of the Ho:YAP laser is centered at2115.96 nm with bandwidth of about 1.5 nm. Thebeam quality factor of M2 = 2.41 ± 0.03 at the out-put power of 3. 54 W is demonstrated by using thetraveling knife-edge method.

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