Holmium doping concentration influence
on Ho:YAG crystal spectroscopic properties
Jan Šulc1, Michal Němec1, David Vyhlı́dal1, Helena Jelı́nková1, Karel Nejezchleb2, Jan Polák2
1

FNSPE, Czech Technical University in Prague, Břehová 7, 115 19 Prague, Czech Republic (jan.sulc@fjfi.cvut.cz)
2

Crytur, Ltd. Turnov, Na Lukách 2283, 511 01 Turnov, Czech Republic (nejezchleb@crytur.cz)
SPIE Photonics West 2021 – LASE – Paper 11664-34

INTRODUCTION

MATERIALS AND METHODSRESULTS

Holmium based lasers, operating in the 2.1 µm spectral

Ho:YAG crystal samples were grown by Czochralski

Ho:YAG absorption for 2.1 µm bandrange, have a variety of scientific, medical, and technicalmethod from high purity raw materials

applications, including atmospheric remote sensing, highSample SN

13237 13256 13286 13304 13332 13355

harmonic generation, etc.

Ho3+ doping [ at.% ]

0.34

0.34

0.51

0.9

1.61

1.98

The holmium/thulium co-doped crystals, enabling theThickness

[cm]

0.48

0.25

0.25

0.25

0.30

0.29

direct 0.8 µm diode pumping, present some problems,

Diameter

[cm]

5.4

5.1

5.3

5.3

5.3

5.4

Ho:YAG absorption cross-sections & JOA

limiting the possibilities of this approach.
Reliable Ho-doped material for direct Ho3+ excitation is
Ho:YAG. The low quantum defect of Ho:YAG together
with the excellent thermo-mechanical properties of YAG,it makes the heat load small and gives the potential for
scaling to high output power.

Ho:YAG 2.1 µm fluorescence decay time

Methods of characterization
Spectroscopic properties of Ho:YAG samples wereinvestigated at room temperature.
The transmission spectra of the samples were measured

Ho:YAG 2.1 µm fluorescence spectrain the range from 185 up to 6500 nm.
The fluorescence spectra were measured in the range
Ho:YAG can be resonantly pumped in the absorption
band peaking at 1907 nm using high-brightness,
Tm-based fiber or bulk lasers (long, low-doped Ho:YAGcrystals are used).
Current status of 1.9 µm laser diode development willfrom 470 up to 6000 nm.
The dependence of 2 µm Ho3+ fluorescence decay timeon Ho3+ doping was measured using confocal imaging of
the fluorescence emitting spot to decrease the influence

CONCLUSION

of fluorescence reabsorption.
Absorption and emission cross-sections for 2.1 µm laser

allow high power operation of directly diode pumped

transition was calculated

thin-disc Ho:YAG laser in near future.

Absorption is linearly dependent on nominal Ho3+ doping

Low-brightness laser diode pumping or thin-disc

The 2.1 µm fluorescence decay is single-exponential

laser scheme require higher absorption (higherHo3+-doping) for the efficient laser operation.
To develop such system and to optimize Ho:YAG crystal

L1 – excitation focusing lens, LC – laser crystal, L2, L3 – fluorescence collimation & focusing lens, (L1,2,3: f = 75 mm, D = 50 mm),F – excitation blocking filter Thorlabs BF2250-500, A – pinhole
aperture (D = 100 µm), D – detector Thorlabs FD05D

Fibre coupled LD DPSSL Single emitter LD
Modelnecessary.
We present the influence of Ho3+ doping
concentration (from 0.3 up to 2 at.%) on Ho:YAG
spectroscopic properties.

Necsel BlueOPO

Tm:GdVO4 Dilas BA-1908 Ekspla NT252

Wavelength

448 nm1863 nm1907 nm

690 – 2600 nm

Pulse length

14 ms

10 ms

10 ms

3 ns

Pulse energy

20 mJ

10 mJ

10 mJ

0.2 – 1 mJ

Pulse rep-rate

5 Hz

5 Hz

5 Hz

5 Hz/1 kHz

Spot size

400 µmNo upconversion was observed under 1.9 µm excitation
The Ho-doping concentration significantly

Excitation sources

design, the detailed spectroscopic informations are

Fluorescence at 2.1 µm is concentration quenched

30 um

100 um

Acknowledgement: Research was supported by Technology Agency of the Czech Republic, project FW01010219.

40 µm

influences mainly the upper laser level lifetime,
which drops from 7 ms to 5.6 ms for the highly
doped Ho:YAG sample. However, for this sample,
the absorption coefficient is 3 cm−1

@ 1907 nm

which is promising for construction of diodepumped Ho:YAG laser.