510 Laser Phys. Lett. 5, No. 7, 510–513 (2008) / DOI 10.1002/lapl.200810024

Abstract: Lasing characteristics of a CW Tm:LuAG with a setof double cavity is presented in this letter. As the air gap etalonkeeping at 342 µm, the highest output power reaches 107.8 mWwhen the totally input pump power is 1.95 W. The output cen-ter wavelength is 2027.0 nm. A single longitude mode outputpower of 17.1 mW is obtained when incident pump power is855.1 mW. With the changing of DC voltage on the Piezoelec-tric Transducer (PZT) holding the output coupler, the reflectionof the effective mirror changes correspondingly. The oscillatingwavelength shifts from 2022 nm to 2031 nm. To our knowledge,this work is the first time to investigate the double cavity designin Tm:LuAG to achieve single longitude mode and tuning oscil-lation wavelength output laser.

T

FSR = 3.75 GHz

Vol

tage

Output pow

er, a.u.

Scan time, s

Single-longitude mode of Tm:LuAG

c© 2008 by Astro Ltd.Published exclusively by WILEY-VCH Verlag GmbH & Co. KGaA

Lasing characteristics of a CW Tm:LuAG laser with a setof double cavityC.T. Wu, ∗ Y.L. Ju, Z.G. Wang, Y.F. Li, H.Y. Ma, and Y.Z. Wang

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

Received: 3 March 2008, Revised: 11 March 2008, Accepted: 14 March 2008Published online: 27 March 2008

Key words: solid-state lasers; diode pumping; Tm:LuAG; double cavity

PACS: 42.55.Xi, 42.60.Pk

1. Introduction

2-µm laser has many important applications, such asgas sensing, wind measurement, and iatrical applica-tions. As the source of the LIDAR, the high power Q-switched solid-state laser must be seeded by a continu-ous wave single mode master oscillator [1–5]. Thulium-doped Lu3Al5O12 (Tm:LuAG) crystal is isomorphic toYAG, and the LuAG crystal has the advantage of high heatconductivity similar to the YAG host [6]. As a quasi-three-level laser material, Tm:LuAG is more promising for it haslower population density of the lower laser level. In addi-tion, the emission wavelength of Tm:LuAG (2.023 µm) isshifted slightly to longer wavelength in comparison withYAG (2.015 µm) crystal. A multiple resonator is one ofthe wonderful methods to achieve single longitude mode.

The fewer elements in the cavity make sure the stabilityof the output laser. Now, there’re some reports about thedouble cavity. In 1995, Christian Pedersen et al. [7] re-ported 350-mW of single-frequency power from a diode-pumped solid-state Nd:YVO4 laser by using of a 0.5-mmlaser rod and a coupled resonator. In 1998, Yokozawa et al.[8] used diode-pumped Tm:YLF with a multiple resonatorto obtain 1.3-mW single mode oscillation. Izawa et al. [9]reported 27-mW of single mode operation from a diode-pumped Tm,Ho:YLF microchip laser with an external gasetalon in 2000. In 2004, Nagasawa et al. [10] made useof a double cavity Tm,Ho:YLF microchip to obtain singlelongitude mode laser and different oscillating wavelengthoutput(from 2050 nm to 2060 nm).

This letter focuses on the lasing characteristics of aCW Tm:LuAG using a double cavity at room tempera-

∗ Corresponding author: e-mail: bigsnow1@126.com

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Laser Phys. Lett. 5, No. 7 (2008) 511

C12 C23

M1 M2 M3

Figure 1 Double resonator of Tm:LuAG laser resonator

ture. As the gas etalon keeping at 342 µm, the highestoutput power reaches 107.8 mW when the totally inputpump power is 1.95 W. The output center wavelengthis 2027.0 nm. A single-mode output power of 17.1 mWis obtained when incident pump power is 855.1 mW.With the changing of DC voltage on the PZT holdingthe output coupler, the reflection of the effective mirrorchanges correspondingly. The oscillation wavelength of2022.0 nm, 2022.5 nm, 2025.5 nm, 2027.0 nm, 2028.0 nm,and 2031.0 nm are achieved.

2. Theory

The design of the cavity is double resonators, as shown inFig. 1. The resonator consists of two optical regions, C12

and C23. The resonator C12 is considered plane paralleland is formed by the crystal (Tm:LuAG) itself. One sideof the crystal acted as the M1 is coated total reflection.The other side (uncoated crystal surface of Tm:LuAG) isthe M2. The other resonator C23 is composed of M2 and acoupling mirror M3. Cavity C12 is active and cavity C23 ispassive. The plano output mirror M3 with a partially trans-mitted (T = 2%) coating at 2 µm is mounted in a PZT ring-actuator and placed a distance (342 µm in this work) fromthe output side of the crystal. This distance between thecrystal and the output coupler plays a role of an air gapetalon. The distance is varied easily by applying DC volt-age directly to PZT.

The spatial and the temporal coupling of the two cavi-ties must be considered to a set of coupled resonators. Toproduce low-loss coupling, the spatial modes of cavitiesC12 and C23 should be mode matched. The temporal cou-pling, which is associated with destructive or constructiveinterference that arises from multiple reflections at M2, interms of an effective mirror with reflection coefficient R23

and phase shift α are described as,

R23 =R2 − 2

√R2R3 cos ϕ + R3

1 − 2√

R2R3 cos ϕ + R2R3

, (1)

tan (α − φ) = (2)

=√

R3 (1 − R2) sin ϕ√R2 (1 + R3) −

√R3 (1 + R2) cos ϕ

.

LD

Fiber Couplermirrors

Tm:LuAG Outputcoupler

Laseroutput

Air gap PZT Si-plate

Figure 2 Schematic of the experimental setup

Where φ is the round-trip phase shift of cavity C23, R1,R2, and R3 are the reflection coefficients of M1, M2, andM3. The equations are derived from the Fabry-Perot the-ory under the assumption that loss-less multilayer mirrorswith arbitrary phase shifts are involved. The overall effectcan be described as that of a frequency dependent out-put coupler, which in turn leads to a loss discriminationbetween the longitudinal eigenmodes. The details can befound elsewhere [7].

3. Experimental setup

The Tm:LuAG laser cavity is a set of double resonatorswith end-pumped geometry, and shown in the Fig. 2. Com-pared with the single resonator method usually used, dou-ble resonators method is superior to the laser system hav-ing much broader fluorescence spectra which cannot pro-duce single longitude mode oscillation easily, such as Tmor Ho laser.

The dimension of Tm:LuAG crystal is 3×3×2 mm3.The doped concentration of Tm is 4%. The pumping sideof the crystal is coated with a high transmission at 788 nmand high reflectivity at 2 µm. The opposite surface of thelaser crystal is coated anti-reflection at 788 nm. The tem-perature of the crystal is measured on the surface of thecopper, which is the holder of the crystal cooled by a TE-cooler. The pump source is a 790-nm 2-W laser diode cou-pled by a fiber with core-diameter of 200 µm and numeri-cal aperture of 0.22. The wavelength of LD can be turnedto 788 nm by changing its operate temperature, which isone of the absorption to Tm:LuAG crystal. The LD out-put is shaped and focused by a series of convex lenses.Pump diode light is focused into the Tm crystal with abeam diameter of 240 µm.The mode matching betweenpump mode and laser mode is optimized by changing thepump beam waist radius and its location.

4. Experimental results

Fig. 3 shows the output characteristics of Tm:LuAG laser.The lasing threshold is 539 mW. Under pump power of1.95 W available from the laser diode, the maximumpower of 107.8 mW is achieved with the crystal temper-ature at 281 K. A linear fit to the data yields a slope

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512 C.T. Wu, Y.L. Ju, et al.: Lasing characteristics of a CW Tm:LuAG laserO

utpu

t pow

er, m

W

120

100

80

60

40

20

0400 600 800 1000 1200 1400 1600 1800 2000

Input pump power, mW

measured pointlinear fit

Figure 3 (online color at www.lphys.org) Output power ofTm:LuAG versus input power under free oscillating

T

FSR = 3.75 GHz

Vol

tage

Output pow

er, a.u.

Scan time, s

Figure 4 (online color at www.lphys.org) Multi mode laser op-eration

efficiency of 7.2%. The optical-to-optical conversion ef-ficiency at the maximum power level is approximately5.5%. The output laser is introduced into a Fabry-Perot in-terferometer, whose free spectral range (FSR) is 3.75 GHz,for the measurement of the mode measurement and itssmall shift. Fig. 4 shows the multi mode laser operationunder free oscillating. The green curve indicates the volt-age on the F-P interferometer and the blue curve shows therelative intensity of the output laser. The voltage differen-tial relay on the F-P interferometer is about 200 V, whichcan scan three to four FSR of the laser.

As the location of the pump beam changes, the single-frequency operation is obtained and the output powerchanges correspondingly. A maximum single frequencyoutput power of 17.1 mW is obtained when incident pump

Out

put p

ower

of s

ingl

e m

ode,

mW

18

16

14

12

10

8

6

4

2

01000950900850800750700650

Pump power, mW

Figure 5 Output power of single-longitude mode versus inputpower

T

FSR = 3.75 GHz

Vol

tage

Output pow

er, a.u.

Scan time, s

Figure 6 (online color at www.lphys.org) Single-longitudemode of Tm:LuAG

power is 855.1 mW. Fig. 5 gives the single-frequency out-put versus the input power. As the maximum output powerof single-mode is obtained, there’s a progress of descend-ing and ascending about the single-mode power. Maybethere’re some disturbances in cavity, resulting in the shiftof the mode. Fig. 6 shows the single-frequency output ofTm:LuAG laser.

The wavelength of Tm:LuAG laser is measured with amonochrometer. The chopped input laser is detected by aPbS detector connected with a TDS-3032B digital oscillo-scope. Increase the DC voltage on the PZT holding the M3

mirror, the distance of the air gap etalon between M2 andM3 changes. The power and the angle of the pump beamare adjusted by monitoring the Fabry-Perot interferome-ter to obtain single longitudinal mode oscillation. Then

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Laser Phys. Lett. 5, No. 7 (2008) 513

DC voltage in PZT, V Oscillation wavelength, nm0 2027.040 2028.050 2027.060 2031.070 2022.580 2022.090 2025.5

Table 1 Shift of the oscillation wavelength as a function of thechange of the DC voltage in PZT

an increase of the DC voltage leads to the changes of theoscillation mode. The oscillation wavelength change cor-respondingly with the single longitude mode is obtainedagain.

Table 1 shows the oscillation wavelength shifts as thelength of C23 changes. The adjustable range of PZT is 0–5 µm as the DC voltage on the PZT changes from 0 V to200 V, the linear relation between them is nearly 25 nm/V.The shift range of the oscillation wavelength is 9 nm bychanging the DC voltage from 40 V to 90 V. The outputwavelength under other DC voltage is also calculated andsimulated. The result of experiment is quite accordant withtheory, though there’s some error.

5. Conclusion

In summary, we have reported a diode end-pumpedTm:LuAG laser with maximal CW output power of107.8 mW. Lasing characteristics of a CW Tm:LuAG withthe double-resonator structure is presented. As the gasetalon keeping at 342 µm, a single-mode output power of17.1 mW is obtained and the output center wavelength is

2027.0 nm. With the changing of DC voltage on thePZT holding the output coupler, the oscillation wavelengthshifts correspondingly. The oscillating wavelength shiftsfrom 2022.0 nm to 2031.0 nm. In our future work, the re-lation between the DC voltage which will be controlledmore accurate and the thickness of the gas etalon, then theconnection to the effective reflection of the output couplerwould be investigated.

Acknowledgements This work is supported by the program ofExcellent Team in Harbin Institute of Technology.

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