Narrow Bandwidth Tunable Watt Level Tm:YAP Laser using Two
Etalons
Uzziel Sheintop
1,2
, Eytan Perez
1
and Salman Noach
1
1
Department of Applied Physics, Electro-optics Engineering Faculty, Jerusalem College of Technology, Jerusalem, Israel
2
Department of Applied Physics, The Hebrew University of Jerusalem, Jerusalem, Israel
Keywords: Lasers, Solid-state, Mid Infrared Lasers, Tunable Laser, Narrow Bandwidth Laser.
Abstract: Narrow band, tunable, end pump Tm:YAP laser is demonstrated in this paper. The 35 nm wavelength
tunability, ranges continuously from 1917 to 1951 nm, having a spectral linewidth of 0.15 nm FWHM. The
tuning and spectral band narrowing was obtained using a pair of YAG Fabry-Perot Etalons with thicknesses
of 25 and 500 μm. Watt level output power were measured along the laser tunable range. Maximum output
power of 3.84 W was achieved at 1934 nm. Slope efficiency of 43.2% was calculated for an absorbed pump
power of 12.1 W. The combination of the narrow bandwidth with tunability at those levels of output power,
makes this laser a promising tool for bio-medical, sensing and material processing applications.
1 INTRODUCTION
Diode-pumped Tm
3+
-doped solid state lasers
emitting in the 2 μm region, have many applications
in a variety of fields, such as medical surgery,
material processing, gas spectroscopy (Eremeikin,
2010), Doppler LIDAR and as a pumping source for
the mid infra-red region lasers e.g. Cr:ZnSe and
Cr:ZnS lasers. Some of the applications require the
source to deliver both tunable and narrow bandwidth
emissions. (Godard, 2007, Scholle et al., 2010,
Sorokina, 2003)
Tunable lasers in the 2 μm area are a key
technology for environmental applications, such as
sensitive detection of atmospheric gases and
molecules, as well as for remote chemical sensing,
in terms of safety, quality control and regulatory
enforcement.
As a pump source for other lasers in the IR
region, the ability of tuning the pump source to the
maximum absorption peaks of the laser gain
medium, can improve significantly the system
performance. Narrowing the pump source bandwidth
contributes to the efficiency of the pump source in
the case of narrow band absorption peaks.
For medical application, the high absorption
coefficient of water in the 2 μm region allows
limited penetration depth in biological tissues. Since
the absorption of liquid water changes significantly
as a function of wavelength, spectral tunable source
allows to achieve varying and precise penetration
depth, while the laser radiation interacts with the
tissue. This property is considered to be very
applicable in micro surgery.
Tm-doped lasers have a broad spectral tunability
range, due to the relative large Stark splitting of the
laser low energy level
3
H
6
. The exact Stark splitting
changes from one host to another, depending on the
host symmetry and host crystal field intensity at the
Tm site. This causes changes in the amplification
curve from host to host.
Numerous techniques are used for wavelength
laser tuning such as Prisms, Lyot filter, Volume
bragg grating and Etalon plates.
Despite its limited spectral range, comparing to
other methods, tuning by Etalon has its advantages:
avoiding the necessity of polarized beam, simplicity
of operation as well as being relatively low cost.
It can be mentioned, that tunable Tm-doped
lasers were widely implemented. Works on this
topic that were performed up to 2007, were reviewed
in (Godard, 2007). Since then, tunability was also
achieved with the following Tm-doped crystals:
Tm:LuAG (2018-2029 nm) (Wang et al., 2013),
Tm:LuYAG (1935-1995 nm) (Sun et al., 2012),
Tm:LiLuF (1817-2056 nm) (Coluccelli et al., 2007),
and Tm:LSO (1959-2070 nm) (Feng et al.,2013,
Feng et al., 2014).
Sheintop, U., Perez, E. and Noach, S.
Narrow Bandwidth Tunable Watt Level Tm:YAP Laser using Two Etalons.
DOI: 10.5220/0006721403030307
In Proceedings of the 6th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2018), pages 303-307
ISBN: 978-989-758-286-8
Copyright © 2018 by SCITEPRESS Science and Technology Publications, Lda. All rights reserved
303
In addition to the spectral tunability the Etalon
allows, it can also be exploited as a bandwidth
narrowing device, due to its Fabry Perot design.
The use of one Etalon, gives a tradeoff between
the free spectral range (FSR) determined by the
thickness and the spectral bandwidth. In comparison,
the use of two different thicknesses Etalons can
maximize the tuning range and the bandwidth
narrowing level. Bandwidth narrowing using this
method was also implemented in Tm:YAP (Yu-Feng
et al., 2007, Li et al., 2010).
None of the aforementioned works achieved a
combination of a narrow bandwidth, tunable and
Watt level source laser in Tm-doped crystals ions,
specifically for the Tm:YAP crystal being one of the
most promising lasers in the 2 μm area.
We report a CW Tm:YAP tunable laser with narrow
bandwidth using 2 Etalons, having a maximum
output power of 3.84 W at 1934 nm. The laser was
tuned between 1917 to 1951 nm with an output
power ranging from 2.46 to 3.84 W. The laser
bandwidth was narrowed to 0.15 nm full width at
half maximum (FWHM).
To the best of our knowledge, this is the first
Tm:YAP laser having such spectral narrow band-
width with a 35 nm tunable range in the watt level.
2 TM: YAP CRYSTAL
Tm-doped lasers have numerous and important
optical characteristics. The noticeable of those are,
long fluorescence lifetime, cross relaxation
mechanism allowing for high quantum yield, as well
as high power eye-safe lasing. Moreover, the
absorption spectrum of Tm ions near 790 nm matches
with the commercially available AlGaAs laser diodes.
Yttrium aluminum perovskite (YAP), has been a
well known laser host, and also being one of the
most attractive Tm-doped crystals. The Tm:YAP is
important especially in the medical field because of
his emission near 1940 nm, which is an absorption
peak of water (twice as strong than the 2 μm from
Tm:YAG and four time stronger than the 2.1 μm
from Ho:YAG) (see Figure 1).
Figure 1: Fluorescence spectrum of Tm:YAP crystals for
different doping concentrations (Godard, 2007).
Due to the quasi-three-level nature of the Tm
2μm transition, most of Tm laser are blue shifted as
the output coupler (OC) transmission or the cavity
losses increases. As an example, the Tm:YLF changes
its emitted wavelength from 1910 to 1880 nm, in
contrast to the Tm:YAP gain spectrum peak, which
barely changes for increased OC transmission,
making the Tm:YAP spectral emissions insensitive to
cavity losses shown in Figure 2.
The Tm:YAP has higher absorption and emission
cross section than the Tm:YLF. Additionally, the
YAP thermal conductivity is comparable with the
well-known YAG host (Koechner, 2006).
The Tm dopant has a wide absorption peak
around 795 nm, which allow for efficient pumping
with high brightness diodes. Moreover, the wide
fluorescence spectra allow for a broad range of
tunability.
Figure 2: Calculated gain spectrum for different output couplers. Left-Tm:YLF, Right-Tm:YAP. (Li, 2014).
PHOTOPTICS 2018 - 6th International Conference on Photonics, Optics and Laser Technology
304
Figure 3: Schematic of the experimental setup.
3 SETUP
The Tm:YAP laser setup is shown in Figure 3. The
pumping source is a fiber-coupled laser diode
emitting up to 15.8 W at 793 nm with a 105 μm core
diameter and a N.A of 0.22. The pump beam was
focused using a pair of antireflection (AR) coated at
650-1050 nm bi-convex lenses on the Tm:YAP
crystal, allowing a minimal spot size inside the
Tm:YAP crystal of about 260 μm. The pump was
delivered through a plano-plano rear cavity mirror,
having AR coating for the pump wavelength and
high reflectance (HR) coating for the 1850-2000 nm.
We used a plano-concave mirror with ROC of 200
mm as OC. The OC was partially reflecting (PR)
coated with 70% reflectance for the 1850-2000 nm.
The total cavity length was 220 mm.
The Tm:YAP gain crystal having a Tm
concentration of 3%, was 10 mm long while having
a cross-section of 3x3 mm. The laser crystal was AR
coated for both the pump and laser wavelengths, and
was wrapped within indium foil and fasten into an
aluminum holder cooled by a water chiller at a stable
18˚C.
Two uncoated YAG Etalon plates, with 500 µm
and 25 µm thickness were fixed on a rotating stage
inside the laser cavity in order to narrow the
emission spectral width. Additionally, by rotating
the 25 µm Etalon angle we were able to tune the
laser wavelength.
By implementing two intra-cavity Etalon plates,
it is possible to achieve a narrow spectral bandwidth,
having the thinner Etalon responsible for the tuning
range according to his free spectral range (FSR),
while the thicker Etalon defines the spectral
bandwidth.
The output power was measured using power
meter (Ophir, L50(150)A-35), after filtering the
residual pump power. The laser spectrum was
acquired by an OSA (Thorlabs, OSA205C) having a
resolution of 0.13 nm.
Figure 4: Laser performance.
4 EXPERIMENTAL RESULTS
The Tm:YAP laser performance for the Free-
running operation, without the intra-cavity Etalons,
is shown in Figure 4 as a function of the absorbed
pump power. The Tm:YAP crystal absorbed 77% of
the pump power. The laser threshold was 3.73 W of
the absorbed pump power. A maximum output
power of 4.79 W was achieved at 12.1 W absorbed
pump power, corresponding to an optical-to optical
conversion of 39.3%, and a slope efficiency of
53.1%. The measured emission wavelength was of
1935 nm, for a spectral width of ~2 nm at the
FWHM shown in Figure 5 having a beam profile
quality M
2
= 1.4.
Narrow Bandwidth Tunable Watt Level Tm:YAP Laser using Two Etalons
305
Figure 5: Free running spectrum of the Tm:YAP laser.
By inserting the two Etalons plates intra-cavity,
we achieved a spectral bandwidth reduction down to
~0.15 nm (near the resolution limit of the OSA used)
at the FWHM shown in Figure 6. The laser
wavelength was tuned from 1917 to 1951 nm, giving
a tuning range of 35 nm. The tuning range achieved,
agrees closely to the 40 nm calculated FSR,
corresponding to the 25 μm thick Etalon. Along this
range, the output power measured did not fall from
2.46 W for a constant maximal pump power as
shown in Figure 7. For the narrowed bandwidth
operation of the laser, a maximum output power of
3.84 W was achieved at a wavelength of 1934 nm,
corresponding to an optical-to optical conversion of
31.7 % and a slope efficiency of 43.2 %. The data is
shown as function of the absorbed pump in Figure 4.
Figure 6: Narrowed band spectrum of Tm:YAP laser using
two etalons.
Figure 7: Laser tunability Performance.
5 DISCUSSION
In this work, a Tm:YAP laser was designed. Up to
3.84 W output power in the tunable configuration
using two Etalon plates for bandwidth narrowing
was observed for a maximum pump power of 15.8
W (corresponding to an absorbed pump power of
12.1 W). This relatively low pump power, in
combination with the small dimension of the overall
resonator (about 22 cm), opens a path toward the
realization of 2 μm solid state lasers in multiple field
of interest.
Concerning further developments of the present
laser system, several paths should be investigated.
Within the application point of view, the
Tm:YAP laser should be used to perform
preliminary experiments of laser ablation of
biological tissue to check parameter such as, heat
affected zone, ablation depth and collateral damage
in order to assess its potential as a surgical tool. At
the peak absorption of water near 1940 nm. Using
the tunability demonstrated in this paper, we could
expect precise control for ablation depths smaller
than 100 μm. Such a small penetration depth enables
precise ablation of tissue in microsurgical schemes.
Moreover, scaling up the pump power and
modifying the configuration to active or passive Q
switch, together with improved thermal management
efforts could enable higher average powers and
could be useful especially in the plastic material
processing field.
6 CONCLUSIONS
In summary, we demonstrate a tunable CW
Tm:YAP laser with a compact diode end pumped
architecture, reaching Watt level output power with
PHOTOPTICS 2018 - 6th International Conference on Photonics, Optics and Laser Technology
306
a tunability of 35 nm between 1917-1951 nm, with a
maximum power of 3.84 W at 1934 nm wavelength.
The use of Etalon plates allowed us to reach a
narrow bandwidth of ~0.15 nm.
Comparing to previous work done with Tm:YAP
lasers, we report a first Watt level tunable narrow
bandwidth laser. Tm:YAP with the b-cut orientation,
which emits around the water absorption peak at
1940 nm, could be applicable for tissues ablation
with a controlled penetration depth, besides the other
applications of tunable source in the 2 μm region.
ACKNOWLEDGEMENTS
The authors would like to thank Y. Sebbag and
Prof. U. Levy from the Hebrew University of
Jerusalem, for there willingness and support, and to
D. Sebbag and R. Nahir for his fruitful discussion
and collaboration.
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