Diode Pumped Ho:YAG Thin Disk cw Laser at 2.09 µm
J. Speiser1, G. Renz, D. Oberbeckmann, and A. Giesen
Institute of Technical Physics, German Aerospace Center,
Pfaffenwaldring 38, 70569 Stuttgart, Germany
Introduction
Rare-earth ions, which operate in the ‘eye-safe’ wavelength region above 1.5 µm are attractive
for use in remote sensing, laser material processing, laser surgery as well as military
applications. In this report, a Ho:YAG thin disk cw laser will be pumped by a fiber coupled
narrowband (2 nm, FWHM) InP diode laser stack with an on-chip grating technology, emitting
near 1.908 µm. A multi-pass pumping scheme with 24 pump passes1 will be used. For
optimization, three different Ho-concentrations (1%, 2% and 3%) will be investigated. Using
the concept of thin disk pumping with its inherent good thermal management, scaling diode
pumped Ho:YAG thin disk laser systems into a power range of multiple 10 W are feasible.
Experiments
The thin disks are contacted onto water-cooled copper-tungsten heat sinks with a water
temperature of about 5°C. The layout of the thin disk laser is shown schematically in figure 1
(left). The resonator type is a stable, linear or V-shaped setup. The output mirror has a radius
of curvature of 100 cm and the resonator length is approximately 20 cm (linear resonator) or 2
times 20 cm (V-shaped resonator). The output coupling of the output mirror is close to 2%.
2.09 µm thin disk laser power (W)
8
2.5% Ho:YAG, 300 µm disk
1.6% Ho:YAG, 240 µm disk
3.0% Ho:YAG, 300 µm disk
6
4
2
0
0
5
10
15
20
25
30
35
40
diode stack pump power (W)
Fig. 1. Schematic layout of the Ho:YAG thin disk cw laser system (left) and laser output power for three different Ho-concentrations (right).
The output power of the Ho:YAG thin disk cw laser is shown in figure 1 (right) for the three
different Ho-concentrations. The 2.5% Ho:YAG leads to the highest output power. Reducing
the cooling temperature of the diode stack below 0°C resulted in an output power of 8.5 W
with an almost linear power scaling, that is due to the small wavelength shift of the diode stack
of 0.1nm/W. It is known, that Holmium in YAG shows an up-conversion process2 which can
reduce drastically the output power, seen in the 3%-Ho:YAG thin disk experiment. A
measurement of the transmission spectrum of Ho:YAG depending on the temperature is
depicted in figure 2 (left). For higher temperatures, the minimum of the transmission shifts
toward higher wavelengths while the absorption is reduced. Therefore, both wavelength shifts
have to be considered in an optimized layout of the grating of the diode stack.
1
Corresp. author: [email protected], Phone: +49 711 6862 451, Fax: +49 711 6862 715
1.0
50
0.5% Ho:YAG
crystal length 32 mm
300°C
250°C
200°C
150°C
100°C
50°C
0.6
45
40
35
Efficiency [ % ]
transmission
0.8
0.4
30
25
20
Experiment
15
 = 0, Lint= 1%
10
0.2
5
 = 3*10
-18
cm s , Lint= 0.5%
 = 3*10
-18
cm s , Lint= 1%
 = 6*10
-18
cm s , Lint= 0.5%
3 -1
3 -1
3 -1
0
0.0
1.904
1.906
1.908
1.910
1.912
0
1.914
10
20
30
40
50
60
Pump power [ W ]
wavelength (µm)
Fig. 2. Transmission spectrum of a 32 mm long 0.5% Ho:YAG rod (left) and results of numerical simulations in
comparison to an experimentally achieved efficiency for 2% Ho:YAG (right).
Numerical Simulations
In the past, a successful numerical model for thin disk lasers – focused on Yb:YAG – was
developed with a first publication dated 19973. More details and comparison with experimental
results were published in the following years4. In contrast to Ytterbium, Holmium has some
additional non-radiative decay channels of the upper laser level. Starting from the 5I7 manifold,
two up-conversion channels are possible, either to the 5I5 or the 5I6 manifold. Assuming a
quadratic nature of the up-conversion process, the well-known rate equation for the density of
excited ions, N 2 , has to be modified slightly to:
N
N 2  Q  2   laser r  k N 22

with Q the absorbed pump photons per volume and time,  the fluorescence lifetime,  laser
gain coefficient,  r the laser photon flux density, and k the coefficient of the additional decay
channel (i.e. the sum of the up-conversion probabilities). Simulations were done for several
values of kΣ, whereas the internal losses Lint were varied, too. Comparing the results with the
optical-optical efficiencies achieved in the experiments (cf. Figure 2, right), an up-conversion
coefficient of kΣ = 3·10-18 cm3s-1 seems reasonable compared to published values for the upconversion probabilities into the 5I5 and 5I6 manifolds2.
Summary
In conclusion, Holmium doped YAG thin disk laser concepts are scalable laser systems that
can be pumped by InP diode laser stacks with an on-chip grating technology. Compared to Tm
fiber pumping, diode stack pumping shows a three times higher wall-plug efficiency. With a
single diode laser stack of 40 W an output power of 8.5 W has been realized. In a next step
multiple diode stacks will be coupled through their 600 µm transfer fibers into a single SMA
lock in a tightest packing configuration.
References
Giesen, A. et al., “Scalable concept for diode-pumped high power solid-state lasers”, Appl. Phys. B58, 365
(1994).
2
Shaw, L.B., Chang. R.S., and Djeu, N., “Measurement of up-conversion energy-transfer probability in Ho:YAG
and Tm:YAG”, Physical Review B, Vol.50, Nr. 10, 6609-19 (1994).
3
Contag, K. et al. “Simulations of the lasing properties of a thin disk laser combining high output powers with
good beam quality” Modeling and Simulation of Higher-Power Laser Systems IV. Bellingham (WA): SPIE,
1997, p. 23 (SPIE Proc. Vol. 2989).
4
Contag, K., Karszewski, M., Stewen, C., Giesen, A., and Hügel, H., “Theoretical modeling and experimental
investigations of the diode-pumped thin-disc Yb:YAG laser”. Quantum Electronics 29(8), 697 (1999).
1