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A Resonantly Pumped Q-Switched Er:Lu2SiO5 Laser

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2013 Chinese Phys. Lett. 30 034207

(http://iopscience.iop.org/0256-307X/30/3/034207)

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CHIN.PHYS. LETT. Vol. 30, No. 3 (2013) 034207

A Resonantly Pumped Q-Switched Er:Lu2SiO5 Laser *

YAO Bao-Quan(姚宝权)**, YU Xiao(于潇), LIU Xiao-Lei(刘晓磊), DUAN Xiao-Ming(段小明),JU You-Lun(鞠有伦), WANG Yue-Zhu(王月珠)

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

(Received 17 July 2012)We describe the cw and Q-switched actions of a novel erbium-doped crystal Er:Lu2SiO5 pumped by a MgO:PPLN-OPO at 1536 nm. An efficient 1580.9 nm cw laser with the output power of 608mW is obtained when the incidentpower is 4.5 W, corresponding to a slope efficiency of 11.1%. In the Q-switched operation, the pulse energy upto 1.2 mJ is obtained at a repetition rate of 200Hz.

PACS: 42.55.Rz, 42.60.Gd, 42.65.Yj DOI: 10.1088/0256-307X/30/3/034207

Solid-state lasers emitting in the eye safe band1500–1700 nm have important applications in manyaspects such as range finding, spectroscopy, andDoppler wind lidar.[1,2] Crystals with Er3+ dopingare attractive active materials for such developments.High power and good optical efficiency have beenachieved in Er:YAG lasers. Up to date, the high-est reported power and slope efficiency operatingat 1645 nm are 60W and 80%, respectively.[3] ForQ-switched operation, a great deal of work havebeen carried out and good performances have beenachieved.[4−6] Other crystal hosts are also employedfor resonantly pumped Er laser action in the 1500–1700 nm emission wavelength band. In 1995, Spar-iosu et al.[7] reported an Er:YSGG laser operated at1643 nm at 300K, and the Er:YSGG laser exhibitedan 18 mJ threshold and a 10% slope efficiency. In2011, an efficient Er:YVO4 laser was demonstrated,resulting in a maximum slope efficiency of 57.9% withrespect to the absorbed pump power and 2.3W ofthe maximum output power.[8] Šulc et al.[9] reportedEr:YVO4 and Er:YVO4+CaO microchip lasers. Thelaser emission for an Er:YVO4 microchip was ob-served in detail in the range from 1593 nm to 1604 nm,while for Er:YVO4+CaO crystal, only 1604 nm wasgenerated. Setzler et al.[10] reported a high-average-power, near-diffraction-limited Er:LuAG laser withthe output power of 5W at 1648 nm in 2003. Ter-Gabrielyan et al.[11] recently reported an efficientroom-temperature Er:GdVO4 laser at 1598.5 nm, themaximum continuous wave output power of 3.5 W wasachieved with resonant pumping by an Er-fiber laserat 1538.6 nm. Harun et al.[12] recently demonstrateda simple, compact and low cost Q-switched erbium-doped fiber laser (EDFL) with pulse energy of 90.3 nJand pulse width of 11.6µs at 120 mW pump power.However, as we know, a Q-switched Er:LSO laser hasnot yet been reported.

In this Letter, for the first time to our knowl-edge, we demonstrate a new Q-switched Er:LSO laseroperating at 1580 nm. The measured pump absorp-

tion efficiency increases from 70% at 300 K to 97% at77 K. Thus we choose 77K as operating temperaturein order to increase the absorbed pump power of theEr:LSO crystal. The Er:LSO laser is pumped by aMgO:PPLN-optical parametric oscillator (OPO) witha wavelength of 1536 nm. The highest output power is608 mW with an incident pump power of 4.5W in cwoperation, representing a 11.1% slope efficiency. In Q-switched operation, the output pulse energy of 1.2mJis achieved at 200 Hz repetition rate.

Figure 1 shows a schematic diagram of the experi-mental setup. The pump source of MgO:PPLN-OPOis an Yb:fiber laser, which is made in Germany byIPG. The Yb:fiber laser delivers up to 50 W of ra-diation at 1064 nm with an 𝑀2 factor of 1.05. Thehalf-wave plate is used to control the pump polar-ization for the phase matching. The Yb:fiber laserbeam is focused to a waist radius of 65µm at thecenter of the crystal using a lens, 𝐿1 (𝑓 = 500mm),𝐿2 (𝑓 = 1000mm). The OPO based on a 50 mmlong grating period Λ = 30µm MgO:PPLN crystal isconfigured in a linear cavity consisting of two plano-concave mirrors, M1 and M2 (𝑅 = 75mm) and twoplane mirrors M3, M4. All mirrors have 𝑅>99.8% at1.4–1.7µm, 𝑇>95% at 1064 nm, and 𝑇>95% at 3–5µm. As shown in Fig. 1, the mirror M4 is replacedby an output coupler with 𝑇 = 3.5% at 1.4–1.7µmin OPO operation. The total length of the resonantcavity is 310mm.

The Er:LSO crystal is 4×4mm2 in the cross sectionand 20mm in the length doped with 0.5at.% of Er3+.The laser crystal cooled by the liquid nitrogen works atthe cryogenic temperature of 77K. The pump sourceis the signal of OPO operating at 1536 nm. The di-ameter of the pump-beam is focused to approximately500µm. A plano-concave geometry is approximately160 mm comprising of a plane pump input couplerwith high transmission (>95%) at the pump wave-length (1536 nm) and high reflectivity (>99%) at thelasing wavelength (1600 nm) is used. The output cou-pler is coated with 10% transmission at 1600 nm with

*Supported by the Program for New Century Excellent Talents in University (NCET-10-0067).**Corresponding author. Email: Yaobq08@hit.edu.cn© 2013 Chinese Physical Society and IOP Publishing Ltd

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CHIN.PHYS. LETT. Vol. 30, No. 3 (2013) 034207

the 200 mm radius of curvature. The acousto-opticmodulator (AOM) is mounted in a copper heat sinkmaintained at a temperature of 20∘C by a thermoelec-tric cooler.

O

L1

M1 M2

M3

M4

(T=3.5%))

L2

M5

Pump

Yb FiberLaser

MgO:PPLN

Idler

Q-switchSignal1536 nm

Lens Inputcoupler

OutputcouplerEr:LS

λ/2

Fig. 1. Experimental setup of the Er:LSO laser.

12 13 14 15 16 17 18 191.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Yb:fiber laser

OP

O s

ignal outp

ut

pow

er

(W)

Incident pump power (W)

Fig. 2. The OPO signal output power versus the incidentpump power.

Wavelength (nm)

Inte

nsi

ty (

arb

.unit)

0.0

0.4

0.2

0.8

1.0

0.6

1534.11 1535.11 1536.11 1537.11 1538.11 1539.11

Fig. 3. The output wavelength of the MgO:PPLN-OPO.

The MgO:PPLN-OPO output power as a functionof the Yb:fiber laser is shown in Fig. 2, correspond-ing to a slope efficiency of 40% by the linear fitting.The output wavelength is recorded by a spectrum an-alyzer (WA-650, EXFO) combined with a wave me-ter (WA-1500, EXFO). The spectrum of MgO:PPLN-OPO shown in Fig. 3 is centered at 1536.3 nm.

For the cw operation of the Er:LSO laser, the AOMis removed. The maximum output power is 608 mWwith an incident pump power of 4.5W, which is mea-sured by a Coherent PM2 power meter , representing aslope efficiency of 11.1% and an optical-to-optical con-

version efficiency of 13.5%. The spectrum of Er:LSOlaser centered at 1580.9 nm is shown in Fig. 4. Itcan be seen clearly that the laser operates at multi-wavelengths.

Wavelength (nm)

Inte

nsi

ty (

arb

.unit)

0

5

4

3

2

1

8

9

6

7

1580

.21

1580

.41

1580

.61

1580

.81

1581

.01

1581

.21

1581

.41

1581

.61

Fig. 4. The free running spectrum of Er:LSO laser at1580.9 nm.

160 180 200 220 240 260

0.2

0.4

0.6

0.8

1.0

Distance (mm)

Radiu

s (m

m)

Experimental data Fit curve

Fig. 5. Measurement of 𝑀2 factor with the highest out-put power.

2.5 3.0 3.5 4.0 4.5100

200

300

400

500

600

Incident pump power (W)

200 Hz300 Hz400 Hzcw

Avera

ge o

utp

ut

pow

er

(mW

)

Fig. 6. Average output power versus the incident pumppower with different PRFs.

As shown in Fig. 5, we measure the output beampropagation by measuring the beam radius with aknife-edge technique at several positions. The dataare fitted by the least-squares analysis to standardmix-mode Gaussian beam propagation equations todetermine the beam quality, or 𝑀2 parameter. Byfitting the measured data the 𝑀2 factor is calculatedto be 2.47 for 𝑥-direction.

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CHIN.PHYS. LETT. Vol. 30, No. 3 (2013) 034207

Pulse width=30.8 ns

Fig. 7. The pulse shape at 1.2mJ output energy.

4I9/2

4I11/2

4I13/2

4I15/2

Pump Laser

UP-conversion

Fig. 8. Simplified energy-level diagram of Er3+ ion inLSO crystal showing the excitation and emission transi-tions in up-conversion processes.

The maximum pulse energy is 1.2 mJ at the rep-etition rate of 200 Hz. The pulse shape at 1.2 mJoutput energy is demonstrated in the inset of Fig. 7,which exhibits a pulse width of about 30.8 ns. Figure6 shows the average output power for both cw andQ-switched operations. Obviously, the output powerof Q-switched operation is lower than that of cw op-eration. The lower the PRF is, the lower the averageoutput power is achieved. The expected dependenceof the average power on PRF for a cw pumped Q-switched laser is given by

𝑃av(PRF)/𝑃av(cw) = 𝜏s/𝜏q[1− exp(−𝜏q/𝜏s)], (1)

where 𝑃av(PRF) is the average power in the Q-switched operation, 𝑃av(cw) is the average power inthe cw operation, 𝜏s is the effective lifetime of the up-per state and 𝜏q = 1/PRF.[13] Using Eq. (1), we calcu-late the upper state lifetimes versus PRF at the pumppower of 4.5W, which are 2.24 ms for 200 Hz, 3.06msfor 300Hz, and 4.12 ms for 400 Hz. The lifetimes de-crease with the reduced repetition rate. The energyloss in Fig. 6 can be explained by the much shorterupper state lifetime than the Er radiative lifetime of6.9 ms reported by Payne et al. The energy level of

Er3+ ion in LSO crystal shown in Fig. 8 describes theexcitation and emission transitions in up-conversionprocesses. The energy transfer up conversion (ETU)is mainly caused by the cooperation on conversion be-tween Er ions,[14] leading to a further increase in thethermal loading as well as a significant reduction in theenergy storage time, particularly in the Q-switchedmode.

In summary, we have presented the experimen-tal results of a Q-switched Er:LSO laser pumped bya MgO:PPLN OPO. In cw operation, output powerof 608 mW is achieved, corresponding to a slope ef-ficiency of 11.1%. The cw laser has a beam qualityof 𝑀2 = 2.47 at the maximum output power. In Q-switched operation, up to 1.2 mJ pulses with a pulsewidth of 30.8 ns are obtained at 200 Hz PRF. Maybelower doped Er:LSO can get higher pulse energy, be-cause it can decrease the ETU effect. The higher pulseenergy will be a goal for future studies.

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