Narrow Bandwidth Tunable Watt Level Tm:YAP Laser using Two

Etalons

Uzziel Sheintop

1,2

, Eytan Perez

and Salman Noach

Department of Applied Physics, Electro-optics Engineering Faculty, Jerusalem College of Technology, Jerusalem, Israel

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

-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

. 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

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

= 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

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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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