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Continuous-wave and mode-locked laser operation of
epitaxially grown Yb:KLu(WO4)2 composites
U. Griebner, S. Rivier, J. Liu, R. Grunwald, V. Petrov,
Max-Born-Institute for Nonlinear Optics and Ultrafast Spectroscopy, 2A Max-Born-Str., D-12489 Berlin, Germany,
griebner@mbi-berlin.de
A. Aznar, R. Sole, M. Aguilo, F. Diaz
Universitat Rovira i Virgili, Marcel·lí Domingo, E-43007 Tarragona, Spain
Abstract: Epitaxial layers of Yb:KLu(WO4)2 were grown on KLu(WO4)2 substrates and
highly efficient cw (66% slope efficiency and 415 mW output power) and passively mode-
locked (114 fs pulse duration) operation at 1030 nm is demonstrated.
©2005 Optical Society of America
OCIS codes: (140.5680) Rare earth and transition metal solid-state lasers; (140.4050) Mode-locked lasers;
(160.5690) Rare earth doped materials
1. Introduction
Yb3+ is a very promising activating ion possessing a number of advantages over Nd3+ for laser operation in the
1 µm spectral region which are related to its simple two-level energy scheme. Yb-doped monoclinic
KRE3+(WO4)2 single crystals are host-dopant combinations interesting for highly efficient laser operation by
pumping with InGaAs laser diodes near 980 nm [1]. The doping level can reach the stoichiometric structure
KYb(WO4)2, or KYbW [2], but thermo-mechanical limitations do not allow the fabrication and use of active
elements with a thickness less than 100 µm corresponding to the absorption length. The latter, when operating in
the absorption maximum at 981 nm, reaches 13.3 µm for KYbW. In fact, layers of KYbW on KGd(WO4)2
(KGdW) or KY(WO4)2 (KYW) substrates seem feasible for thin-disk lasers [3]. Very recently, we demonstrated
for the first time laser operation based on epitaxial double tungstate structures by using a 25-µm thin, 20 at%
Yb-doped KYW layer on a KYW substrate crystal [4]. Continuous-wave (cw) lasing at 1030 nm with 40 mW of
output power could be achieved. However, the crystal lattice mismatch seems to be the basic limitation on the
achievable interface quality even in the case of KYW and KYbW. The closer ionic radii of Lu and Yb make the
low-temperature monoclinic phase of potassium lutetium tungstate KLu(WO4)2 (KLuW) potentially interesting
as a passive host due to the possibility not only for doping with very high concentrations of Yb3+ but also for the
growth of KYbW/KLuW epitaxies. The maximum absorption cross-section of the 981 nm line of Yb:KLuW
amounts to 1.18×10-19 cm2 (linewidth of 3.6 nm) and the emission cross section has a maximum of 1.47×10-19
cm2, also at 981 nm, for light polarization parallel to the Nm-crystallo-optic axis [5]. Both values and many other
relevant laser properties are very close to those reported for Yb-doped KGdW and KYW [1]. The closer unit cell
parameters of KYbW and KLuW, listed in Table 1, with differences of 0.12…0.74% against 0.39…1.01%
between KYbW and KYW can be seen as a prerequisite for the growth of high quality epitaxial structures. This
fact which was the main motivation for our recent investigation of the laser performance of Yb:KLuW bulk
crystals [5] permitted now the epitaxial growth of Yb:KLuW layers on KLuW substrates, which is demonstrated
for the first time in the present work.
Table 1: Unit cell parameters (C2/c space group, room temperature) of KLuW and KYW in comparison to the
stoichiometric KYbW as an ideal epitaxial layer. For comparison the values for YAG and the other known
stoichiometric Yb-crystal YbAG are also included.
lattice parameters a [Å] b [Å] c [Å] β average
KLuW (ref. [5]) 10.576(7) 10.214(7) 7.487(2) 130.68(4)°
KYbW (ref. [2]) 10.590(4) 10.290(6) 7.478(2) 130.70(2)°
difference 0.13% 0.74% 0.12% 0.02% 0.33%
YAG (ref. [6]) 12.0116(3)
YbAG (ref. [6]) 11.9380(5)
difference 0.61% 0.61%
KYW (ref. [2]) 10.6313(4) 10.3452(6) 7.5547(3) 130.752(2)°
KYbW (ref. [2]) 10.590(4) 10.290(6) 7.478(2) 130.70(2)°
difference 0.39% 0.53% 1.01% 0.04% 0.64%
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2. Epitaxial crystal growth
Yb-doped KLuW layers have been grown with high crystalline quality by the Liquid Phase Epitaxy (LPE)
method. The LPE experiments were performed in a vertical furnace with practically no axial gradient to obtain
homogeneous epitaxial layers. It is important to note that the epitaxial growth takes place on all natural faces of
the crystals used as substrates. The thickness of the Yb:KLuW layer with an Yb-doping concentration of 10%,
grown on the (010) face, amounted to 130 µm. For the laser experiments the (010) faces of the epitaxial crystal
were additionally polished resulting in a layer thickness of 100 µm. The substrate thickness amounted to
1.1 mm.
3. Laser experiments
3.1 Continuous-wave operation
The polished (010)-faces of the epitaxial crystal were normal to the Np-principal optical axis and the sample was
oriented for polarization parallel to the Nm-optical axis and propagation approximately along the Np-optical axis.
The composite crystal was positioned between two focusing mirrors (an RC=-5 cm end mirror and an R=-10 cm
folding mirror) under Brewster angle to minimize the Fresnel losses in an astigmatically compensated three-
mirror cavity. The laser was longitudinally pumped through the folding mirror using an f=6.28 cm lens by two
pump sources: a tunable cw Ti:sapphire laser, delivering more than 3 W near 980 nm focused to a beam waist of
about 30 µm in the position of the crystal and a tapered InGaAs diode laser (TDL) delivering up to 2 W of
output power near 978 nm which could be tuned by temperature. The optimum pump wavelength, at the
relatively low doping level, corresponds to the absorption peak at 981 nm. The measured single pass low-signal
absorption of the 10% Yb/Lu-site KLuW layer at 981.5 nm amounted to 64% in good agreement with the
calculated value of 64.7% for a 100 µm thickness under Brewster angle.
0 100 200 300 400 500 600
0
20
40
60
80
100
120
(a)
output power [mW]
absorbed pump power [mW]
TOC= 1.1%, η=20.3%
TOC= 3%, η=31.2%
TOC= 5%, η=37.1%
0 200 400 600 800
0
100
200
300
400
500
(b)
output power [mW]
absorbed pump power [mW]
TOC=1.1%, η=46.8%, λL=1040nm
TOC=3%, η=62.8%, λL=1032nm
TOC=5%, η=65.7%, λL=1029nm
TOC=10%, η=66%, λL=1026nm
Fig. 1: Continuous-wave laser performance of the Yb:KLuW/KLuW epitaxial composite for the diode-pumped (a) and the
Ti:sapphire laser pumped (b) configurations. The lasing wavelength in (a) was λL=1030 nm in all cases.
In the experiments with TDL pumping, cw laser operation could be obtained for transmissions (TOC) of the
plane output coupler between 1.1% and 10%. The output power versus the absorbed pump power (Pabs),
measured in the lasing state, for three different TOC is shown in Fig. 1a. The laser threshold achieved with the
100-µm-thin Yb:KLuW layer was as low as Pabs ≈120 mW for TOC=1.1%. The thresholds we measured were 3
to 5 times lower than with the bulk Yb:KLuW samples [5] where reabsorption forced the laser to oscillate at
longer wavelengths λL. At the maximum applied pump power the output power amounted to 105 mW and the
pump efficiency with respect to Pabs reached 20%. The highest slope efficiency with respect to Pabs, η=37.1%,
was achieved for TOC=5%. The actual absorption depends on the depletion effect and can be substantially lower
than the small signal value. It depends, however, also on the output coupler transmission since the different
intracavity power produces a different recycling effect which counteracts the depletion [7].
With the Ti:sapphire laser used as a pump source in the same cavity configuration, a much better
performance was achieved due to the improved matching between pump and laser modes as compared to the
TDL pumping. The cw output characteristics are presented in Fig. 1b. The maximum measured output power of
415 mW corresponds to a maximum pump efficiency of 55% with respect to Pabs. The slope efficiency increases
with TOC reaching a maximum value of η=66% for TOC=10%. Both the pump and the slope efficiencies exceed
those we recently reported for a 2.2-mm thick 10 at% Yb-doped bulk KLuW in a similar pump and laser
configuration [5]. This is attributed to the strongly reduced reabsorption which leads to about 4 times lower
thresholds in the case of the epitaxial sample and to shorter laser wavelengths λL. Note that the dependence of
λL on TOC in Fig. 1b can be explained by stronger absorption depletion and more homogeneous pumping along
the beam path. It is interesting to note that even without cooling no damage of the epitaxial crystal occurred
regardless of the high power levels (intracavity intensity exceeding 1 MW/cm2) applied.
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3.2 Mode-locked operation
Here we demonstrate for the first time to our knowledge mode-locked operation of an epitaxially grown
composite crystal and present results obtained both in the picosecond and the femtosecond regimes. We studied
a Z-shaped astigmatically compensated resonator with two RC=-10 cm folding mirrors in the middle. The
Ti:sapphire pump beam was launched into the cavity through one of these mirrors. The cavity waist was formed
at the position of the Yb:KLuW/KLuW crystal, inserted under Brewster angle. One arm contained an additional
focusing mirror (RC=-10 cm) to increase the intensity on the semiconductor saturable absorber mirror (SAM)
which was used for passive mode locking. Two dispersion prisms could be inserted in the other arm containing
the plane output coupler.
-6 -4 -2 0 2 4 6 8
0.0
0.2
0.4
0.6
0.8
1.0
1020 1040
0.0
0.5
1.0
(a)
τ = 1.8 ps
τ∆ν = 1.78
intensity [a.u.]
delay [ps]
wavelength [nm]
3.5 nm
1000 1020 1040
0.0
0.5
1.0
-400 -200 0 200 400
0.0
0.2
0.4
0.6
0.8
1.0
wavelength [nm]
13 nm
(b)
τ = 114 fs
τ∆ν = 0.43
intensity [a.u.]
delay [fs]
Fig. 2: Mode-locked laser performance of the Yb:KLuW/KLuW epitaxial composite in the picosecond (a) and the femtosecond (b) regimes.
Without intracavity prisms the SAM mode-locked laser operated in the picosecond regime with a pulse
repetition frequency of 100 MHz and a maximum average output power of 117 mW (TOC=3%). Assuming a
sech2-pulse shape the deconvolved FWHM of the pulse, from the autocorrelation trace in Fig. 2a, is τ=1.8 ps.
The femtosecond regime was realized by adding two SF10 Brewster prisms with a separation of 31 cm into
the arm containing a TOC=1.1% output coupler, which resulted in a cavity round trip time of 101 MHz. Pulses as
short as 114 fs (Fig. 2b) at 1030 nm could be achieved with an average output power of 31 mW. The time-
bandwidth-product of 0.43 is slightly above the Fourier limit which is related to the asymmetric spectrum
generated (inset Fig. 2b). We believe, the lower limit for the pulse duration and the observed deviation from the
transform-limited pulse performance are related to the reflection characteristics of the folding mirrors which are
restricted by the close separation between the pump and lasing wavelengths.
4. Conclusion
In conclusion we studied for the first time to our knowledge lasing in the 1 µm spectral range of Yb3+-doped
epitaxial layers using the monoclinic KLuW as a substrate and host. Pulse durations as short as 114 fs could be
obtained in the initial mode-locking experiments with such a composite crystal. CW output powers exceeding
400 mW with pump efficiencies as high as 55% were achieved without active cooling the crystal. This is an
improvement of about one order of magnitude in comparison to the results reported with Yb:KYW/KYW
epitaxies [4].
References
[1] A. A. Lagatsky, N. V. Kuleshov, and V. P. Mikhailov, "Diode-pumped CW lasing of Yb:KYW and Yb:KGW", Opt. Commun. 165,
71-75 (1999).
[2] M. C. Pujol, M. A. Bursukova, F. Güell, X. Mateos, R. Sole, J. Gavalda, M. Aguilo, J. Massons, F. Diaz, P. Klopp, U. Griebner, and
V. Petrov, "Growth, optical characterization and laser operation of a stoichiometric crystal KYb(WO4)2", Phys. Rev. B 65, 165121:1-
11 (2002).
[3] S. Erhard, J. Gao, A. Giesen, K. Contag, A. A. Lagatsky, A. Abdolvand, N. V. Kuleshov, J. Aus der Au, G. J. Spühler, F. Brunner, R.
Paschotta, U. Keller, “High power Yb:KGW and Yb:KYW thin disk laser operation,“ Conference on Lasers and Electro-Optics
CLEO'01, Baltimore (MD), USA, May 6-11, 2001, paper CWF2, OSA Trends in Optics and Photonics (TOPS) Vol. 56, pp. 333-334.
[4] A. Aznar, R. Solé, M. Aguiló, F. Díaz, U. Griebner, R. Grunwald, and V. Petrov, „Growth, optical characterization and laser operation
of epitaxial Yb:KY(WO4)2/KY(WO4)2 composites with monoclinic structure“, Appl. Phys. Lett. 85, 4313-4315 (2004).
[5] X. Mateos, V. Petrov, M. Aguiló, R. M. Solé, J. Gavaldà, J. Massons, F. Díaz, and U. Griebner, “Continuous wave laser oscillation of
Yb3+ in monoclinic KLu(WO4)2“, IEEE J. Quantum Electron. 40, 1056-1059 (2004).
[6] X. Xu, Z. Zhao, P. Song, G. Zhou, J. Xu, P. Deng, “Structural, thermal, and luminescent properties of Yb-doped Y3Al5O12 crystals”, J.
Opt. Soc. Am. B 21, 543-547 (2004).
[7] C. Lim and Y. Izawa, “Modeling of end-pumped cw quasi-three-level lasers”, IEEE J. Quantum Electron. 38, 306-311, (2002).
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