Energy transfer in Tm,Ho:KYW crystal and 
diode-pumped microchip laser operation
Sergey Kurilehik/’^ ’*, Natali Gusakova/ Maxim Demesh/ Anatol Yasukevieh/ Viktor 
bflsel/ Anatoly Pavlyuk/ and Nikolai Kuleshov^
‘Center for Optical Materials and Technologies, Belarusian National Technical University, 65/17 Nezavisimosti
Ave., Minsk 220013, Belarus
‘Kazan (Volga region) Federal University, 18 Kremlevskaya Str., Kazan 420008, Russia 
‘Nikolaev Institute o f Inorganic Chemistry, Siberian Branch o f Russian Academy o f Sciences, 3 Lavrentyev Ave.,
Novosibirsk 630090, Russia 
kurilchik. svifOgmail. com
Abstraet: An investigation of Tm-Ho energy transfer in
Tm(5at.%),Ho(0.4at.%):KYW single crystal by two independent techiqnes 
was performed. Based on flnorescence dynamics measnrements, energy 
transfer parameters Рц and P28 for direct (Tm—>Ho) and back (Ho—>Tm) 
transfers, respectively, as well as eqnilibrinm constant 0  were evalnated. 
The obtained resnlts were snpported by calcnlation of microscopic 
interaction parameters according to the Forster-Dexter theory for a dipole- 
dipole interaction. Diode-pnmped continnons-wave operation of 
Tm,Ho:KYW microchip laser was demonstrated, for the first time to onr 
knowledge. Maximnm ontpnt power of 77 mW at 2070 mn was achieved at 
the fundamental TEMqo mode.
©2016 Optical Society of America
Lasers, solid-state;OCIS codes: (140.3580) 
Microcavity devices.
(260.2160) Energy transfer; (140.3948)
References and links
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6. G. L. Bourdet and R. A. Muller, “Tm,Ho:YLF microchip laser under Ti:sapphire and diode pumping,” Appl. 
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7. В. Q. Yao, F. Chen, C. T. Wu, Q. Wang, G. Li, C. H. Zhang, Y. Z. Wang, and Y. L. Ju, “Diode-end-pumped 
Tm,Ho:YV04 Microchip Laser at Room Temperature,” Laser Phys. 21(4), 663-666 (2011).
8. R. L. Zhou, Y. L. Ju, C. T. Wu, Z. G. Wang, and Y. Z. Wang, “A single-longitudinal-mode CW 0.25 mm 
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9. B. Q. Yao, F. Chen, P. B. Meng, C. H. Zhang, and Y. Z. Wang, “Diode Pumped Operation of Tm,Ho:YAP 
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P. Loiko, J. M. Serres, X. Mateos, K. Yumashev, N. Kuleshov, N. Petrov, U. Griebner, M. Aguiló, and F. Diaz, 
“Microchip laser operation of Tm,Ho:KLu(W04)2 crystal,” Opt. Express 22(23), 27976-27984 (2014).
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A. Brown, and W. Sibbett, “Optical spectroscopy and efficient continuous-wave operation near 2 pm for a 
Tm,Ho:KYW laser crystal,” Appl. Phys. В 97(2), 321-326 (2009).
A. A. Lagatsky, F. Fusari, S. Calvez, S. V. Kurilchik, V. E. Kisel, N. V. Kuleshov, M. D. Dawson, C. T. Brown, 
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172-174 (2010).
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nanifolds in Tm-sensitized Ho luminescence,” J. Lumin. 75(2), 89-98 (1997).
I 14. IB. M. Walsh, N. P. Bames, and B. Di Bartolo, “The temperature dependence of energy transfer between the Tm 
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<U1Uvv.
^ 3 5 (2 ) , : 
/  13. IB. M. t 
I
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Received 2 Nov 2015; accepted 10 Dec 2015; published 15 Mar 2016
21 Mar 2016 I  Vol. 24, No. 6 I  DOL10.1364/OE.24.645100 I  OPTICS EXPRESS 6451
15. N. Р. Bames. Е. D. Filer. С. A. Morrison, and C. J. Lee. “Ho:Tm lasers. I. Theoretical.” IEEE J.Quantum 
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spectral properties of Cr. Tm. Ho:YAG at 2.1 nm.” IEEE J. Quantum Electron. 27(9). 2142-2149 (1991).
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lensing and microchip laser performance of Ng-cut Tm^*:KY(W04)2 crystal,” Appl. Phys. В 108(3), 603-607 
( 2012)._________________________________________________________________________________________________________________________________________
1. Introduction
Tm-sensitized Ho materials are considered to be among the most attractive solntions for lasers 
operating at wavelength slightly above 2 pm particnlarly when compact cavity design is 
reqnired [1]. Tm^  ^ions possess strong absorption near 800 nm where commercially available 
AlGaAs laser diodes operate. Two-for-one qnantnm efficiency cansed by cross-relaxation 
process in thnlinm, and snbseqnent non-radiative energy transfer to Ho^  ^ions enable efficient 
popnlation of holminm manifold [2]. When in addition microchip cavity design is 
implemented the final laser sonrce looks particnlarly compact and attractive for applications. 
Laser generation in microchip confignration has been achieved earlier with several Tm,Ho- 
codoped crystals snch as: YAG [3], YLF [4-6], YVO4 [7], GdV04 [8], YAP [9] and KLnW 
[10]. It was shown that Tm,Ho materials can be snccessfully nsed for obtaining laser radiation 
on holminm transition in snch compact laser design. Recently efficient continnons-wave [11] 
and femtosecond pnlse [12] laser operation has been reported nsing Tm,Ho:KY(W04)2 
(Tm,Ho:KYW) crystal nnder Ti-sapphire laser pnmping. These resnlts showed KYW crystal 
as an attractive host material for 2 pm lasers. There a description of the crystal growth, 
stractnre, spectroscopic properties and first resnlts of evalnation of energy transfer parameters 
were presented. However energy transfer processes were stndied incompletely and laser 
operation nnder diode-pnmping was not obtained. Thns in this paper an investigation of Tm- 
Ho energy transfer parameters in KYW crystal by two independent techniqnes was 
nndertaken and, for the first time to onr knowledge, continnons-wave laser operation nsing 
this crystal in a microchip cavity configmation nnder diode laser pnmping was demonstrated.
2. Study of energy transfer by fluoreseenee dynamies measurement
The study of energy transfer processes in Tm(5%),Ho(0.4%):KYW single crystal was 
implemented with an approach earlier described by B.M. Walsh in application to Tm,Ho- 
codoped YAG and YLF crystals [13,14]. According to this approach energy transfer 
parameters could be easily evaluated by fitting experimental data on fluorescence dynamics 
from ^F4 energy level of Tm^  ^ ions and from level of Ho^  ^ ions to the solutions of rate 
equations set describing rates of change of energy levels population. Inspite the fact that this 
approach doesn’t take into consideration up-conversion processes in the ions and can be used 
only at low excitation densities it was important to find the parameters earlier calculated for 
other Tm,Ho-codoped crystals to compare them and to understand place of KYW crystal 
among other host materials.
In our experiment OPO based on [3-BaB204 crystal and pumped by the third harmonic of 
actively Q-switched Nd:YAG laser was used as an excitation source of Tm and Ho 
fluorescence. The pulses had duration of 20 ns and repetition rate of 10 Hz. The fluorescence 
was collected by wide-aperture objective on entrance slit of monochromator MDR-12. The
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Received 2 Nov 2015; accepted 10 Dec 2015; published 15 Mar 2016
21 Mar 2016 I  Vol. 24, No. 6 I  DOI:10.1364/OE.24.645100 I  OPTICS EXPRESS 6452
signal was detected by InGaAs photodetecor and processed by a digital oscilloscope with 500 
MHz bandwidth. To eliminate inflnence of reabsorption on flnorescence dynamics a small 
piece of the crystal was grinded to crystalline powder and dilnted by liqnid silicone fonning 
homogeneons snspension which then was nsed as the sample. Pnlsed radiation at 1670 mn 
was nsed to selectively excite Tm^  ^ ions to ^p4 energy level, while the wavelength was 
changed to 1960 mn for excitation of Ho^  ^ ions to energy manifold. The flnorescence 
dynamics were measnred separately for Tm^  ^ions at 1860 mn and for Ho^  ^ions at 2056 mn. 
All the measnrements were carried ont at room temperatnre. The energy level transitions 
corresponding to absorption and emission wavelengths nsed in the experiment are shown in 
Fig. 1.
"F4 (2)
Direct transfer
% ( T )
(8)
Fig. 1. Energy level transitions in and ions corresponding to absorption and
fluorescence wavelengths used in the experiment.
The measnred flnorescence dynamics of Tm(5at.%),Ho(0.4at.%):KYW single crystal are 
shown in Fig. 2.
(a) (b)
(C) ( d )
Fig. 2. Fluorescence dynamics of Tm(5at.%),Ho(0.4at.%):KY(WO4)2 crystal from ^p4 manifold 
of Tm^* ions (a) and from l^y manifold of Ho^ * ions (b) under excitation of thulium at 1670 nm 
as well as from l^y manifold of Ho^ * ions (c) and ^p4 manifold of Tm^* ions (d) under excitation 
of holmium at 1960 nm.
After Tm^  ^ions excitation at 1670 mn fast decay of thnlinm flnorescence at 1860 mn is 
observed at early times (Fig. 2(a)) with simnltaneons growth of flnorescence of holminm at
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21 Mar 2016 I Vol. 24, No. 6 | DOI:10.1364/OE.24.645100 | OPTICS EXPRESS 6453
2056 nm (Fig. 2(b)). This behaviour results froiu direct energy transfer from to Ho^  ^
ions, where fast decrease of ^p4 energy level population of accompaiued by an increase 
of І^7 level population of Ho^ .^ At later times fluorescence from both ions starts to decay 
exponentially with the same time constant, which was estimated to be 2.4 ms. This can be 
explained by the fact that growth of holmium population intensifies back energy transfer 
from Ho^  ^to Tm^  ^ions and quickly brings the ions to the state of thermodynamic equilibrium 
when populations of ^p4 level of thulium and of holmium are detennined by Boltzmarm 
statistics as for a coupled system. It should be mentiond that this behavior is typical for Tm- 
Ho co-doped media [13]. Similar behaviour is observed when Ho^  ^ ions are excited at 1960 
mn. Fast decay of holmium fluorescence at 2056 mn at early times (Fig. 2(c)) goes along with 
simultaneous growth of thulium fluorescence at 1860 mn (Fig. 2(d)), that is a consequence of 
back energy transfer from Ho^  ^ to Tm^  ^ ions. At later times fluorescence from both ions in 
similar marmer exponentially decays with the same time constant, which in this case was 
estimated to be 2.7 ms. This behaviour denotes setting of thermodynamic equilibrium 
between ions.
The experimental curves were then fitted by the solutions of rate equations set governing 
the rate of change of populations in Tm^  ^^p4 and Ho marufolds [14] in case of thulium 
excitation:
«2(0 f P
n^ {Q) y a + P
■exp(-(«+P)t), (1)
n,{t) a
n^(0) {^a+j3
and in case of holmium excitation:
•ex p l-1
«7(0)
a
a + p
P.expf-lV,a + P )  \,oc+p
ex p (-(« + p)t).
■exp(-(«+p)t).
(2)
(3)
npt) P •exp — P ■exp(-(cir+p)t). (4)
npO) \^a+P) ) \Gc+Py
Here the subscripts 1, 2, 7, and 8 denote the Tm ^He, Tm ^p4. Ho ^k, and Ho ^k marufolds, 
respectively (see Fig. 1). This indexing of the levels earlier was introduced by Bames [15] and 
is used through this paper to get an agreement with the results obtained with other hosts, и, is 
a population of / level, where / = 2, 7; x is the time constant of exponential decay, a, fi are the 
parameters determining direct and back energy transfer, respectively, which intrinsically are 
energy transfer probabilities with units of s ' . These parameters are concentration dependent. 
However if they are devided to concentration of corresponding ions the obtained values will 
be energy transfer parameters [13]: P28 = o/Nho and P71 = p/Nim, which are concentration 
independent and characterize host material itself. Nho and Nim are the holmium and the 
thulium concentrations. P28 and P71 are the energy transfer parameters for direct and back 
energy transfer, respectively, with uiuts of cm /^s.
The best fitting for all experimental data both in case of thulium and holmium excitation 
was obtained with the same values of energy transfer probabilities: a = 7000 s ' and p = 6100 
s '. The energy transfer parameters P28 and P71 were calculated to be 2.74 x ю '® cm^/s and 
0.19 X 1 0 '® cm^/s, respectively. We aslo found the ratio P71/P28, called equilibrium constant 0  
[13]. In our case this value was estimated to be 0.069. The obtained results were tabulated in 
Table 1 in comparison with the data reported for other Tm,Ho-codoped laser crystals.
Table 1. Energy transfer parameters in Tm,Ho-codoped KYW crystal compared to other 
host materials at room temperatnre
Crystal P28xl0~‘° cm7s P7ixl0~‘° cm7s 
X 10"“ cm7sec 0 = P 7 l / P 2 !
Ref.
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21 Mar 2016 I Vol. 24, No. 6 | D01:10.1364/OE.24.645100 | OPTICS EXPRESS 6454
Tm,Ho:KYW 2.74 0.19 0.069 [This work]
Tm,Ho:YAG 1.3 0.15 0.12 [14]
Tm,Ho:YAG 1.3 0.10 0.08 [161
Tm,Ho:YAG 2.0 0.26 0.13 І1Д
Tm,Ho:YLE 1.2-2.3 0.09-0.19 0.075 ______И ___
The Table 1 demostrates that Tm,Ho:KYW crystal possesses the lowest equilibrium 
constant among other host materials with a significantly higher value of direct energy transfer 
parameter P28. This shows KYW host material as a very promising candidate for Tm,Ho- 
codoping, which provides favourable conditions for direct energy transfer with regards to 
back transfer.
Assuming that all the exitation resides in the ^p4 energy level of thulium and level of 
holimium we have also calculated a fraction of Ho^  ^ ion residing in the level at thermal 
equilibrium,/но = o/(a + (3) [14]. The obtained value was to be 53.4%, which shows that more 
than a half of excitation energy in Tm(5at.%),Ho(0.4at.%):KYW crystal at low excitation 
densities transfers to Ho^  ^ ions. Additionaly we found the value of /но after increasing of 
holmium concentration up to 1 at. % with unchanged thulium concentration. The result was to 
be 74.2%. Further increase of holmium content up to 2 at.% leads to /ко = 85.2%. So 
Tm,Ho:KYW crystals with higher doping level of holmium could be of interest for future 
investigation.
Fluorescence dynamics of Tm,Ho:KYW single crystal were also measured under 
excitation of thulium ions at 802 mn to higher laying ^H4 energy manifold The obtained 
results are shown in Fig. 3.
(b)
Fig. 3. Fluorescence dynamics of Tm and Ho ions in KYW single crystal under excitation of 
thulium at 802 nm at early times (a) and at later times (b).
Fast growth of thulium fluorescence notable at early times is attributed to cross-relaxation 
process in Tm^  ^ions, leading to population of ^p4 energy manifold. The time constant of this 
process was evaluated to be 6.5 ps. The further behavior of fluorescence is similar to the case 
of thulium ^p4 excitation with setting of thermodynamic equilibrium at later times (Fig. 3(b)).
3. Energy transfer mieroparameters aeeording to the Forster-Dexter theory
To support the results obtained from fluorescence dynamics measurement we calculated 
microscopic interaction parameters for our Tm,Ho:KYW single crystal according to Forster- 
Dexter theory of resonant energy transfer [18]. According to the theory mieroparameters of 
energy transfer from donor (D) to acceptor (A) ions can be calculated with the
expression:
-|гту(2)<(2)гіА (5)D — Л л  4 2\6k  n '
Here c is the velocity of light in vacuum, n -  refractive index of the crystal, <ry -  
emission cross-section of donor ion, o f '  -  absorption cross-section of acceptor ion, is a 
factor describing the relative orientation in space of the transition dipoles of the donor and
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21 Mar 2016 I  Vol. 24, No. 6 I  D01:10.1364/OE.24.645100 I  OPTICS EXPRESS 6455
acceptor. When the relative orientation of donors and acceptors in a medinm is random, bnt 
fixed and do not change dnring excited state lifetime of the ions, as it is in case of crystalline 
matrix, the orientation factor can be taken as 0.476 [19,201 •
So the valne of microparameter according to Forster-Dexter theory is basically 
detennined by the overlap of absorption and emission spectra of donor and acceptor ions. To 
calcnlate direct energy transfer microparameter стт^ш, one mnst have emission spectram of 
Tm^  ^ions and absorption spectram of Ho^  ^ions in KYW crystal. Whereas to calcnlate back 
energy transfer microparameter сно^тт, one mnst have emission spectram of Ho^  ^ ions and 
absorption spectram of Tm^  ^ions in the crystal. The absorption spectra of the ions in KYW 
host where measnred for singly doped crystals nsing spectrophotometer Cary 5000 at room 
temperatnre for polarization of light along principal axes N^, TVp and TVg of the crystals. The 
emission cross-section spectra where calcnlated for each polarization by reciprocity method.
Polarization averaged absorption and emission cross-section spectra of Tm^  ^ and Ho^  ^
ions in KYW crystal are shown in Fig. 4.
Fig. 4. Thulium emission and holmium absorption (upper) and holmium emission and thulium 
absorption (lower) cross-section spectra in KYW single crystal.
With Eq. (5) we calcnlated the valnes of interation microparameters for direct and back 
energy transfers: стт^ш = 35.1 x cm®-s“' and сно^тт = 2.16 x cm®-s“\
respectively. The energy transfer probabilities for a dipole-dipole interaction can be fonnd by 
dividing microparameter to the six power of the distance between interacting ions. So the ratio 
сно^тт/стт^но will Ьо oqnal to the ratio of energy transfer probabilities, which is analogons 
parameter to eqnilibrinm constant &, that was calcnlated for this crystal from flnorescence 
dynamics measnrements. After calcnlations сно^тт/стт^ш was fonnd to be 0.061. This valne 
is in a good agreement with the valne of eqnilibrinm constant (0  = 0.069), that coirfirms the 
resnls of flnorescence dynamics analysis. The obtained resnlts were tabnlated in Table 2 in 
comparison with the data reported for other Tm,Ho-codoped hosts.
Table 2. Energy transfer nilcroparameters in Tm,Ho-codoped KYW crystal according to 
Forster-Dexter analysis compared to other host materials
Crystal Стт-»Но Сно-»Тт Сно->Тт/
Стт-»Но
Ref.
X 10^“" cm‘-sec“‘
Tm,Ho:KYW 35.1 2.16 0.061 [Tills work]
Tm,Ho:BNN 36.5 4.1 0.112 [21]
Tm,Ho:CaSGG 24 1.2 0.05 [22]
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Tm,Ho:YAG 10.9 1.14 0.1049 [13]
Tm,Ho:YLF 16.9 1.24 0.0735 [13]
T m, Ho: GdsGasOi 2 10.5 0.3 0.0286 [23]
One can see from the table that the energy transfer coefficient Стт^ш for Tm,Ho:KYW 
crystal is higher than that for the most of the other laser hosts. Also KYW shows a low valne 
of the ratio Сно^тт/стт^ш that is lower than that observed in BNN (Ba2NaNb50i5), YAG 
(Y3AI5O12) and YLF (LiYp4), and slightly higher than for CaSGG (Ca3Sc2Gc30i2) and 
Gd3Ga50i2(Ca,Zr). However in these latter hosts, the valne of the direct energy transfer 
process is mnch smaller than that of KYW.
4. Microchip laser experiment
To demonstrate a potential of Tm,Ho:KYW crystal for nsing in microchip laser devices we 
have carried ont laser experiment with a laser diode (LD) as a pnmp somce. Ng-cnt Tm(5 
at.%),Ho(0.4 at.%):KYW crystal with thickness of 2.98 mm was nsed as an active element. It 
was earlier shown that snch orientation of the crystal is favorable for arising of positive 
thermal lens in the crystal that enables stability of plane-plane microchip cavity confignration 
[24]. The experimental setnp of Tm,Ho:KYW laser is shown in Fig. 5.
Tm(5%),Ho(0.4%):KYW
TEMoo 
2070 nm 
2058 nm
Aluminum
heat-sink
Fig. 5. Experimental setup of CW microchip laser.
Fiber-conpled (0 = 105 pm, N.A. = 0.15) AlGaAs laser diode with maximnm available 
ontpnt power of 3 W at 802 mn and = 20 was nsed as a pnmping sonrce. The diode 
wavelength was shifted to the absorption peak of the ^H4 level (Tm^^) by temperatnre tniung 
of LD. The laser diode radiation was collimated and focnsed into the active element to a spot 
of 120 pm diameter with two spherical lenses (fi = 70mm, f2 = 80mm). The laser resonator 
was formed by two plane mirrors which were positioned in close proximity to the ends of the 
active element. The HR plane inpnt mirror Ml was AR coated for pnmp radiation (802 mn). 
Two ontpnt conplers with transmission of 0.8 and 1.8% were nsed. The crystal faces were 
AR-coated for the pnmp (802 mn) and laser (2.07 pm) radiations as well. The lateral sides of 
the laser crystal were in thermal contact with the alnminnm heat sink whose temperatnre was 
precisely maintained with a thermoelectric cooler which temperatnre was to be 16°C.
CW laser operation was relized at the fundamental TEMqo mode and lasing radiation was 
polarized along Np principal axis of the crystal. The laser performance characteristics are 
demonstrated in Fig. 6.
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21 Mar 2016 I  Vol. 24, No. 6 I  D01:10.1364/OE.24.645100 I  OPTICS EXPRESS 6457
Wavelength, nm
Fig. 6. Input-output characteristics of Tm,Ho:KYW laser (a) and gain spectra of Ho^ * ions in
KYW (b).
The highest output power of 77 mW was obtained with 0.8% output eoupler. The laser 
emission speetrum was eentered at 2070 nm. This matehes with a loeal maximum in the gain 
speetrum of Ho:KYW, see Fig. 6, dashed line represents losses (Tqc = 0.8%). The 
eorresponding slope effieieney of the laser with respeet to ineident pump power was estimated 
to be 8.5%. The laser threshold was about 0.8 W of ineident pump power. The slope 
effieieney for output eoupler Tqc = 1.8% at low pump power was higher than 9%, however 
maximum output power was limited by 46 mW. In the last ease the laser wavelength shifted 
to 2058 nm that is attributed to higher level of eavity losses. Nonlinear dependenee of the 
output power with respeet to ineident pump power and visible fluoreseenee was observed 
during lasing. Roll-over in input-output eharaeteristie with higher output eoupler transmission 
value (1.8%) was evident at 1.4 W of ineident pump power. Sueh behavior ean be eaused by 
the higher up-eonversion losses whieh inerease heat release in the erystal. It’s evident that 
higher transmission of the output eoupler requires greater population inversion of the Ho^  ^
upper laser level l^y and this leads to inerease in up-eonversion losses in the Tm, Ho:KYW. 
Similar behavior of Tm,Ho-laser was observed in [11].
5. Conclusion
Energy transfer in Tm(5at.%),Ho(0.4at.%):KYW single erystal has been investigated by two 
independen teehniques. With an analysis of fluoreseenee dynamies of the erystal 
eoneentration independent energy transfer parameters for direet P71 and baek transfer P28 
proeesses were determined, whieh were 2.74 x 10“'® emVs and 0.19 x 10“'® emVs, 
respeetively. Equilibrium eonstant 0  = P28/P71 was ealeulated to be 0.069. These results 
demonstrate domination of direet energy transfer in the erystal and in eomparison with other 
host materials provides favourable eonditions for population of ®І7 energy level of holmium. 
A fraetion of Ho^  ^ ions residing at ®І7 energy manifold in the erystal at equilibrium eonstant 
was ealeulated to be 53,4%. An inerease of this fraetion was predieted with further growth of 
holmium eontent. The results obtained from fluoreseenee dynamies measurement were 
eonfirmed by independent ealeulation of interaetion mieroparameters in aeeordanee with 
Forster-Dexter theory. The mieroparameters were ealeulated to be Схт^но = 35.1 x 10“""' 
em®-s“' and Сно^тт = 2.16 x 10“""' em®-s“'. The ration Сно^тт/ Wm^ Ho was to be 0.061, that is 
in a good agreement with the equilibrium eonstant obtained from fluoreseenee dynamies. CW 
laser operation with Tm,Ho:KYW in mieroehip eonfiguration with LD pumping was realized 
for the first time to our knowledge. Maximum output power of 77 mW at 2070 nm was 
obtained with slope effieieney of 8.5% with respeet to ineident pump power. The laser was 
operating at the fundamental TEMqo mode.
Acknowledgments
In a part of energy transfer this work was funded by the subsidy of the Russian Government 
(agreement No.02.A03.21.0002) to support the Program of Competitive Growth of Kazan 
Federal University among World’s Leading Aeademie Centers. Mieroehip laser experiments 
were supported by Russian Seienee Foundation grant (Projeet # 15-12-10026).
#252708
© 2016OSA
Received 2 Nov 2015; accepted 10 Dec 2015; published 15 Mar 2016
21 Mar 2016 I  Vol. 24, No. 6 I  DOI:10.1364/OE.24.645100 I  OPTICS EXPRESS 6458