High-power diode-pumped cryogenicallycooled Yb:CaF2 laser

with extremely low quantum defectS. Ricaud,1,4,* D. N. Papadopoulos,2 A. Pellegrina,2 F. Balembois,1 P. Georges,1 A. Courjaud,4

P. Camy,3 J. L. Doualan,3 R. Moncorgé,3 and F. Druon1

1Laboratoire Charles Fabry de l’Institut d’Optique, UMR 8501 CNRS, Université Paris Sud, 91127 Palaiseau, France2Institut de la Lumière Extrême, CNRS, Ecole Polytechnique, ENSTA Paristech Institut d’Optique,

Université Paris Sud, Palaiseau Cedex, France3Centre de Recherche sur les Ions, les Matériaux et la Photonique (CIMAP),UMR 6252 CEA-CNRS-ENSICaen, Université de Caen, 14050 Caen, France

4Amplitude Systèmes, 6 allée du doyen Georges Brus, 33600 Pessac, France*Corresponding author: sandrine.ricaud@institutoptique.fr

Received March 3, 2011; accepted March 18, 2011;posted March 30, 2011 (Doc. ID 143643); published April 27, 2011

High-power diode-pumped laser operation at 992–993nm under a pumping wavelength of 981 of 986nm is demon-strated with Yb:CaF2 operating at cryogenic temperature (77K), leading to extremely low quantum defects of 1.2%and 0.7%, respectively. An average output power of 33W has been produced with an optical efficiency of 35%. Thisrepresents, to the best of our knowledge, the best laser performance ever obtained at such low quantum defects onintense laser lines. © 2011 Optical Society of AmericaOCIS codes: 140.3615, 140.3380, 140.3480, 140.3580.

Doped fluoride crystals have been known and identifiedas attractive laser media since the very beginning of thelasers [1]. In the field of ultrahigh-peak power lasers withhigh repetition rates, very interesting laser developmentsat cryogenic temperatures has been achieved withYb:YLF [2,3] and Yb:CaF2 [4,5]. In the case of Yb:CaF2,the crystallographic and luminescence properties havebeen known for a long time [6]. However, the character-istics of these materials have not been fully exploited,especially at high dopant concentrations [7–10]: a broademission band, a long emission lifetime of 2:3ms, withgood thermal conductivity [11,12], and, last but not least,a well-mastered growing process for high-quality andlarge single crystals. Thus, Yb doping has allowed cal-cium fluoride to make a comeback for high-power laserapplications; they have indeed, become, in the past fewyears, among the most promising laser materials for high-energy/high-power diode-pumped laser systems [13–16].In this Letter, we present another typical property of

Yb:CaF2 that allows for efficient laser emission ina “quasi-two-level” laser scheme without any highlywavelength-selective or narrow-linewidth intracavityelements. This could represent a breakthrough forultrahigh-power lasers since it could lead both to efficientdiode-pumped laser operation and to a minimal thermalload [5,17–19]. Moreover, as this quasi-two-level lasersystem operates at cryogenic temperatures, it brings an-other positive advantage for high-power laser devices,which is improvement of the thermal properties of thelaser element, such as its thermal conductivity.Yb3þ has a simple electron-level structure based ononly

two manifolds (2F7=2 and 2F5=2), which split into differentcrystal-field Stark sublevels whose number and energyseparation depend on the symmetry and the strength ofthe local crystal-field environment. Because of chargecompensation and the minimum-energy arrangementsof the ions in this system [6], the case of heavily doped

Yb:CaF2 is very particular. Indeed, the luminescent cen-ters responsible for the laser properties of the materialgive rise to a typical relatively weak crystal field andreduce crystal-field splitting of the Yb3þ energy levels[see in Fig. 1(a)].

Consequently, in addition to the broadband vibronicstructure, which extends from about 1000 to 1060 nm,and to the zero-phonon lines (corresponding to the so-called “zero-line” around 981 nm), there is another setof clear lines at 985.2 and 991:5 nm with substantial emis-sion/absorption cross sections. They correspond to zero-phonon transitions from the lowest 2F5=2 (emitting level atabout 10200 cm−1) to the second and third energy levels ofthe 2F7=2 ground multiplet (around 50 and 110 cm−1),respectively.

Thus, laser operation at a short wavelength and an “ul-tralow quantum defect” is possible by cooling the lasercrystal down to liquid nitrogen (LN2) temperature, aspresented in this Letter.

The experiments were performed with a 2.2%Yb-doped, 5-mm-long calcium fluoride crystal. The

Fig. 1. (Color online) (a) Spectroscopic lines of Yb:CaF2 at77K, (b) experimental measurements of pump and laser wave-lengths for pumping at 981(blue curve) or 986nm (red curve),and gain cross section of the Yb:CaF2 at 77K and for β ¼ 0:1(purple curve).

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experimental setup is displayed in Fig. 2. In order topump the crystal longitudinally and to allow simulta-neously an extremely short wavelength separation be-tween the pump and the laser, a broadband highlyreflective (HR) mirror of 2mm diameter glued onto a25mm diameter antireflective plate is implemented. Thispump-beam-occulting mirror uses the advantage of thelarge diameter pump beam (collimated fiber-coupled la-ser diode with NA ¼ 0:22) compared to the laser beaminside the Yb:CaF2 laser resonator, forming a so-calledmodal multiplexer. The corresponding losses observedon the pump beam do not exceed 4%. Moreover, the laseris free from any spectral selection and operates very effi-ciently at its maximum spectral gain without additionallosses.According to the emission and absorption spectra of

Yb:CaF2 registered at low temperature [5], the lasershould have naturally operated, without extra wave-length selector, at 992 nm for an inversion population(β) higher than 10%. However, this is not the case, sincelaser operation remains fixed at 1034 nm, even for anaverage inversion population higher than 40%. This isprobably due to uncertainty in the temperature elevationin the crystal. In order to favor the short wavelengthemission with a minimum of losses, we insert in the cav-ity (Fig. 2) a wavelength selector consisting of two high-pass dichroic mirrors HR between 980 and 1000 nm withlosses per bounce of <0:1% at 992 nm, <1% at 997 nm,and a high transmission (>95%) around 1030 nm. Conse-quently, the impact of this selector is acceptable andallows efficient laser at low wavelengths. In these condi-tions, laser operation occurs between 992 and 997 nm.Figure 3(a) displays the output power obtained for dif-

ferent output couplers. Thereby, we determine the spec-tral gain of the crystal at varying population inversionlevels, estimated by using the following equation:

ð1 − Toc − LÞ expððβσem þ ð1 − βÞσabsÞNlÞ ¼ 1; ð1Þ

where N is the Yb-dopant concentration, σem and σabs arethe emission and absorption cross sections at the laserwavelength (λL), l is the length of the crystal, Toc isthe output coupler transmission, and L is the losses(versus λL).At low inversion (β < 0:05, e.g., Toc ¼ 5%), only 997 nm

is observed, whereas, at intermediate levels(0:05 < β < 0:08), the gain is flat between 993 and997 nm, e.g., for a 10%output coupler (β > 0:065), the laseroperates simultaneously at 997.1, 994.2, and 993:0 nm. Athigher inversion levels (β > 0:08), the laser operatesaround 992 nm: for an output coupler of 15% (β ¼ 0:09)

or higher, the laser wavelength lies between 992.7and 992:0 nm.

The best cw laser performance at 992 nm has beenobtained with the 15% output coupler (β ¼ 0:11) with alaser emission of 33W for 93W absorbed pump power(under laser operation). The laser efficiency is then35% [Fig. 3(b)]. The measured small signal gain is foundto be equal to 2.7.

The laser and pump emission wavelengths were mea-sured simultaneously. Figure 1(b) reports these pumpand laser wavelengths at the maximum output power.On the same graph, the gain cross section is plottedfor the value β ¼ 0:1 corresponding to the optimal power.The predominance of the gain at 992:0 nm clearly ap-pears, corroborating the experimental results. The meanemission wavelength is 992:7nm and the mean pumpwavelength is 980:7 nm, which corresponds to a verylow laser quantum defect ηQD laser ¼ 1 − λp=λL ¼ 1:2%.Those results clearly indicate the strong potential ofYb:CaF2 used at cryogenic temperature for high-powerlaser developments where efficiency and heat load arean issue.

It is interesting to note that, for broadband amplifica-tion (992–997 nm), the optimal operation should beobtained at low inversion levels.

Exploiting such a small quantum defect configurationmight be challenging and it is worth considering a numberof points. It is important to identify that thermal loads inytterbium-doped laser materials come from three types ofnonradiative relaxations: the laser quantum defect(ηQD laser ¼ 1:2%) between pump and laser photons, thefluorescence quantum defect (ηQD fluo ¼ 1 − λp=λF ¼3:1%) between the pump and fluorescence photons,and nonradiative de-excitations from 2F5=2 to 2F7=2 levels,evaluated in our case to 0.7% of the absorbed pumpphotons [12]. Then, 1:2% × 35% (35%¼ laser efficiency)of these absorbed pump photons heat the crystal by laserquantum defect and 3:1% × 64% by fluorescence quantumdefect. Consequently, for a total absorbed pump power of93W, this leads, respectively, to 0:65 W, 0:39W, and1:85W (total of 2:9W).

This clearly indicates that, in a small quantum defectlaser, the thermal loads due to the fluorescence quantumdefect cannot be neglected. Therefore, the laser effi-ciency directly impacts the thermal loads. In our experi-ment, the efficiency is limited by the losses providingfrom the not fully ideally coupled cavities of the uncoatedFig. 2. (Color online) Experimental setup.

Fig. 3. (Color online) (a) Experimental and theoretical emis-sion wavelengths and laser power obtained for different outputcouplers. (b) Corresponding laser power (at optimum) versusabsorbed pump power at 981nm obtained with a 15% outputcoupler and associated beam profiles at low and high powers.

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facet of the crystal and/or to residual pollution due to ournonperfect cryostat vacuum.The second issue to be considered, especially at high

pump power, is the thermal conductivity of the lasermaterial. From this point of view, Yb:CaF2 is particularlyinteresting since its thermal conductivity at LN2 tempera-ture rises up to 68W=m=K for an undoped material [11]and to 23W=m=K [5] for a 2.2% Yb-doped one. This meansthat the thermal loads can be efficiently evacuated andthat the thermal lensing effects should be greatly re-duced. This is clearly what we noticed in our experi-ments, since no thermal lensing effect was observedeven at full pump power [Fig. 3(b)].A last point that can be noticed by examining the gain

cross section for β ¼ 0:1 reported in Fig. 1 is the absorp-tion line at 986 nm that can be used to decrease furtherthe laser quantum defect. As plotted in Fig. 4, the theo-retical absorption of our crystal at 986 nm (for a singlepump pass) is only 30% at maximum (and without satura-tion) and becomes null for β ¼ 0:21 (or, equivalently, fora laser at 992 nm and with Toc ¼ 39%). On the otherhand, we have to remember that there is also a constraintfor β to emit at 992 nm. As a matter of fact, the optimalinversion population was found around 0.09, which cor-responds in our case to an output coupler of 15%. Theexperiment was performed and we obtained a laser out-put power of 4W for an absorbed pump power of 35W(efficiency of 11%). The laser wavelength [Figs. 1(b) and4] was 992:9 nm, leading to a low quantum defect of 0.7%.In conclusion, we have demonstrated, simultaneously

and for the first time, to the best of our knowledge, alow-quantum-defect, highly efficient, and high-power la-ser operation of an Yb:CaF2 laser crystal at cryogenic tem-perature. This represents an important step towardspractical lasers based on Yb:CaF2 operating at very highpower levels. As amatter of fact, by using the simple figureof merit given by the ratio (thermal conductivity)/(quan-tum defect), we find record values of 5700W=m=K forpumping around 981 nm and 9700W=m=K around986 nm [14]. Moreover, Yb:CaF2 really appears as a favor-able material for such a kind of low-quantum-defect laseroperation (more than any other material) because of theexistence of this sharp emission peak around 992 nmwitha substantially high emission cross section. Such a peakdoes not exist in a system like Yb:CALGO [15],

Yb:KGdLuðWO4Þ) [17], materials, which also gave riseto very low-quantum-defect laser operation, but with amuch lower laser efficiency. Such a peak exists in the caseof Yb:YLF [18]. However, the lowest quantum defect thatcouldbe (theoretically) obtainedwouldbearound2%, andthe one achieved so far, by pumping around 960 nm(which is not a common diode wavelength) was around3.5%. Yb:CaF2 has then this rare property of having clearpeaks very close to the zero-phonon line, which is veryauspicious to efficient ultralow-quantum-defect diode-pumped lasers. Moreover, concerning absorption im-provement, this can be done using pump recycling, suchas in thin disks.

The authors gratefully acknowledge financial supportfrom the Program “Femtocryble” of Agence Nationale dela Recherche and the contract ILE 07-CPER 017-01.

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Fig. 4. (Color online) Theoretical emission wavelength andtheoretical percentage of absorbed pump power at 986nmversus β. Experimental laser wavelength and laser efficiencyhave been plotted for the optimal value β ¼ 0:09 (15% outputcoupler).

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