IOP PUBLISHING LASER PHYSICS
Laser Phys. 23 (2013) 125005 (4pp) doi:10.1088/1054-660X/23/12/125005
In-band pumped room-temperature
Er:KY(WO4)2 laser emitting around
1.6 µm
K N Gorbachenya1, V E Kisel1, A S Yasukevich1, A A Pavlyuk2 and
N V Kuleshov1
1 Center for Optical Materials and Technologies, Belarusian National Technical University,
65/17 Nezavisimosti Avenue, Minsk, 220013, Belarus
2 A V Nikolaev Institute of Inorganic Chemistry, Siberian Branch of Russian Academy of Sciences,
3 Lavrentyev Avenue, Novosibirsk, 630090, Russia
E-mail: gorby@bntu.by
Received 29 August 2013, in final form 9 October 2013
Accepted for publication 24 October 2013
Published 18 November 2013
Online at stacks.iop.org/LP/23/125005
Abstract
We present efficient continuous-wave operation of an Er:KY(WO4)2 crystal under in-band
pumping by a compact diode-pumped Er, Yb:GdAl3(BO3)4 laser. Maximum slope efficiency
of 27% and output power of 35 mW at 1609.5 nm were obtained with beam propagation factor
M2 < 1.2. Absorption and stimulated emission cross-section spectra, as well as the radiative
lifetime of the 4I13/2 energy level, were determined.
(Some figures may appear in colour only in the online journal)
1. Introduction
Erbium lasers emitting in the eye-safe spectral range around
1.6 µm are attractive for applications in laser range
finding, ophthalmology, fiber-optic communication systems
and optical location.
Continuous-wave (CW) room-temperature laser opera-
tion was demonstrated for several Er-, Yb-codoped crystals:
garnets [1], vanadates [2], oxoborates [3–6] and tungstates
[7, 8]. Here the ytterbium absorption band at 0.9–1.0 µm
was used for pumping and energy transfer to Er ions was
utilized. However, the intrinsic slope efficiency of lasers
based on these materials is limited to 60% due to the large
quantum defect. The most efficient diode-pumped laser action
to the best of our knowledge has been demonstrated for Er,
Yb:YAl3(BO3)4 crystal [6]. The slope efficiency of 35% with
output power up to 1 W was obtained. Lasers based on Er,
Yb:KY(WO4)2 crystal have also been realized. However, the
low energy transfer efficiency of tungstate crystals limited
the maximum slope efficiency to only several percent either
with Ti:sapphire [7] or laser diode pumping [8]. Additional
co-doping with Ce3+ was used in order to increase the slope
efficiency of tungstate lasers to 17% in a quasi-CW regime
[9, 10].
Direct in-band pumping of Er-doped materials in the
4I13/2 energy level with radiation at 1.5–1.6 µm wavelength
significantly reduces the quantum defect and thermal load. It
enables us to significantly increase the slope efficiency of the
lasers. CW room-temperature laser action with in-band pump-
ing was demonstrated for Er-doped bulk garnets [11, 12],
vanadates [13, 14], sesquioxides [15] and fluorides [16]
as well as fibers [17, 18]. However, due to significant
up-conversion losses the erbium concentration in the media
was limited to low values that oblige us to use crystals of
several centimeters in length. This fact imposes restrictions
on the quality of the pump beam. Among tungstate crystals
quasi-CW laser action on structurally disordered Er-doped
NaY(WO4)2 was reported at room temperature [19]. A
wide-band InGaAsP/InP laser diode emitting at 1.5 µm was
used as a pump source. The slope efficiency of 33% with
output power of 1.05 W at the wavelength of 1609 nm was
reported.
In this paper, we present, for the first time to the
best of our knowledge, efficient CW laser operation of
11054-660X/13/125005+04$33.00 c© 2013 Astro Ltd Printed in the UK & the USA
Laser Phys. 23 (2013) 125005 K N Gorbachenya et al
Figure 1. Er-doped KY(WO4)2 single crystal grown by the TSSG
method.
an Er:KY(WO4)2 crystal in-band pumped by a compact
diode-pumped Er, Yb:GdAl3(BO3)4 laser [20].
2. Experimental details
KY(WO4)2 (KYW) is a monoclinic crystal with C2/c space
group. The parameters of the unit cell are a = 10.64 A˚, b =
10.35 A˚, c = 7.54 A˚, β = 130.5(2)◦ and the weight density is
6.5 g cm−3 [21]. Er:KYW single crystals were grown by the
top-seeded solution growth (TSSG) method (see figure 1).
For investigation of the spectroscopic properties the
sample with an Er3+ concentration of 0.5 at.% was used. Po-
larized absorption spectra at room temperature were measured
with a spectrophotometer, Varian CARY 5000. The spectral
bandwidth was 0.4 nm. The lifetime measurements were
performed using an optical parametric oscillator emitting
near 1530 nm based on a β-Ba2B2O4 crystal and pumped
by the third harmonic of a Q-switched Nd:YAG laser. The
fluorescence from the sample was collected on the entrance
slit of a 0.3 m monochromator and registered by an InGaAs
photodiode coupled with a 500 MHz digital oscilloscope.
A plane–plane Np-cut Er(2 at.%):KYW crystal with a
length of 14.5 mm was used as an active medium. The facets
of the crystal were antireflection-coated for both pump and
laser wavelengths. The crystal was mounted on the copper
heatsink without any additional cooling. A diode-pumped Er,
Yb:GdAl3(BO3)4 laser with output power up to 650 mW
at 1531 nm was used as a pump source. The laser output
was linearly polarized with close to Gaussian intensity profile
(M2 < 1.2). The combination of two lenses (f1 = 25 mm, f2 =
60 mm) was used to focus the pump beam in the gain medium
into a spot of 35 µm radius (1/e2 intensity). The pump beam
polarization corresponded to the Nm optical axis of the crystal.
The laser experiments were performed in a three-mirror cavity
which is presented in figure 2.
3. Results and discussion
3.1. Spectroscopy
The absorption cross-section spectra of the Er:KYW
crystal in the spectral range of 1440–1640 nm (transition
Figure 2. Cavity setup for laser experiments.
Figure 3. Room-temperature polarized absorption cross-section
spectra of Er:KYW crystal.
4I15/2 → 4I13/2 of erbium ions) are presented in figure 3.
The highest absorption can be observed at 1534 nm for light
polarization E ‖ Nm, where the maximum cross-section is
2.4 × 10−20 cm2. This wavelength corresponds to emission
spectra of the InGaAsP/InP laser diode, which gives us the
opportunity to consider the Er:KYW crystal as a promising
laser medium under in-band pumping.
The stimulated emission cross-section spectra were
calculated by the reciprocity method [22], using the energy
level scheme of 4I13/2 and 4I15/2 manifolds reported in [23].
The spectra are shown in figure 4 and are similar to the
results of [23]. One can see a broad and smooth emission in
the spectral range of 1570–1630 nm that can be utilized in
mode-locked operation.
It is well known that radiation trapping strongly
influences the fluorescence dynamics of Er3+ ions in crystals
and glasses because of the significant overlap of the
absorption and emission bands. The high refractive index of
Er:KYW (ng = 2.05; nm = 2.01; np = 1.97) increases the
probability of reabsorption even in optically thin samples
because of the total internal reflection [24]. In our experiments
we used a fine powder of Er(0.5 at.%):KYW crystal
immersed in glycerin. The fluorescence lifetime decreased
with the decreasing of powder concentration in suspension.
Starting from a certain powder content, the lifetime remained
constant despite further dilution, which indicates negligible
reabsorption influence. The lifetime of the 4I13/2 level was
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Laser Phys. 23 (2013) 125005 K N Gorbachenya et al
Figure 4. Polarized emission cross-section spectra of Er:KYW
crystal.
Figure 5. Fluorescence lifetime of the Er(0.5 at.%):KYW
crystalline powder immersed in glycerin.
determined to be 3.1 ms (see figure 5), which is slightly
different from the results of [7, 23].
The radiative lifetime of the 4I13/2 erbium level was
calculated by the use of the integral reciprocity method [25]:
τrad = 3
8pin2c
Z2
Z1
1∑
β
∫
σ
β
abs(λ)λ
−4 exp(−hc/(kTλ))dλ
× exp(−hc/(kTλ0)) (1)
where n is the average refractive index of a crystal, c is
the velocity of light, β denotes the polarization state, h and
k are the Planck and Boltzmann constants, respectively, T
is the host crystal temperature and σabs is the ground-state
absorption cross-section; Z1 and Z2 are partition functions of
the lower and upper multiplets, defined as
Zm =
∑
k
gmk exp(−Emk /(kT)) (2)
where m = 1, 2; gmk is the degeneration of the sublevel having
the number k and the energy Emk measured from the lower
sublevel of the corresponding multiplet; λ0 is the wavelength
corresponding to the energy EZL and EZL is the energy
Figure 6. Input–output characteristics of CW in-band pumped
Er:KYW laser.
distance between the lower sublevels of the multiplets for the
ground and excited electronic states.
The radiative lifetime of the 4I13/2 level was calculated
to be 3.06 ms and it is in excellent agreement with the value
obtained from the experiment. The luminescence quantum
yield was estimated to be close to 1.
3.2. Laser performance
Figure 6 shows input–output characteristics of the CW
in-band pumped Er:KYW laser. The absorbed pump power
was calculated from the measured absorption coefficient
at the laser threshold. The maximum slope efficiency of
27% with output power up to 35 mW was obtained for an
output coupler transmittance of 2.2%. The laser radiation was
linearly polarized along the Nm optical axis of the crystal. For
1% output coupler transmittance the slope efficiency reduced
to 16% and output power was 23 mW. The laser threshold
was 190 mW of absorbed pump power. A slope efficiency of
25% and maximum output power of 31 mW were obtained
for 2.7% output coupler transmittance. The laser wavelength
was measured to be 1609.5 nm and did not depend on the
output coupling (see figure 7). The spatial profile of the output
beam was close to Gaussian with M2 < 1.2 (see the inset in
figure 7). To find a correspondence between laser and spectral
properties of the Er:KYW crystal gain coefficient spectra were
calculated.
The gain coefficient g(λ) was calculated for different
values of inversion parameters β:
g(λ) = N(β · σse(λ)− (1− β) · σabs(λ)), (3)
where the inversion parameter β = Nex/N shows the ratio
of the volumetric density of excited Er ions Nex to the total
Er ions concentration N, σse is the stimulated emission cross
section and σabs is the absorption cross section. Figure 8
shows the gain coefficient spectra for polarization E ‖ Nm
with β = 0.21 and 0.22 as well as the loss coefficient
spectra estimated for output couplers with 1% and 2.7%
transmittance. It can be clearly seen that the laser wavelength
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Laser Phys. 23 (2013) 125005 K N Gorbachenya et al
Figure 7. Laser spectrum of the in-band pumped Er:KYW laser.
Inset: intensity profile of the laser output.
Figure 8. Gain coefficients (solid lines) calculated for β = 0.21
and 0.22; loss coefficients (dashed lines) estimated for output
coupler transmittance of 1% and 2.7%.
should not be shifted when the output coupler transmittance
increases from 1 to 2.7%.
During the laser experiments strong green fluorescence
caused by the up-conversion to the level 4S3/2 was observed.
To our mind, reducing the erbium concentration will
contribute to a decrease of energy transfer up-conversion and
will result in an increase of the laser slope efficiency and
output power.
4. Conclusions
In conclusion, a CW in-band pumped room-temperature
Er:KY(WO4)2 laser was demonstrated for the first time to
the best of our knowledge. Maximum slope efficiency of 27%
and output power of 35 mW at 1609.5 nm were obtained with
beam propagation factor M2 < 1.2.
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