Thermal Lensing in Diode-Pumped Ytterbium Lasers ... - IEEE Xplore

IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 40, NO. 9, SEPTEMBER 2004

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Thermal Lensing in Diode-Pumped Ytterbium Lasers—Part II: Evaluation of Quantum Efficiencies and Thermo-Optic Coefficients Sébastien Chénais, François Balembois, Frédéric Druon, Gaëlle Lucas-Leclin, and Patrick Georges

Abstract—A theoretical and experimental study of thermal lensing in Yb-doped crystals is presented. This papers follows the presentation of theoretical considerations and experimental wavefront measurements, which have been the subject of Part I. In this paper, we study the evolution of thermal lensing versus absorbed pump power, and we derive two parameters valuable for laser design and power scaling. The quantum efficiency and the thermo-optic coefficient, in Yb-doped YAG, GGG, GdCOB, YCOB, KGW and YSO. The clear difference between thermal lensing under lasing and nonlasing conditions is the proof that nonradiative effects occur in all the crystals under investigation. An analytical model which takes into account the laser extraction efficiency enables to explain all the experimental features and allows to infer the fluorescence quantum efficiency of the samples (in the range 0.7–0.96). Under nonlasing conditions, the thermal lens dioptric power experiences a roll-off, for which we propose an explanation based on the theory presented in Part I. These results are then used to yield the thermo-optic coefficient of the crystals. At last, we propose a simple analytical formulation useful for a rough estimation of the focal length. Index Terms—Photothermal effects, solid lasers, thermooptic effects, ytterbium.

in [1]. The reader will find these notations summarized in [1, Table I]. In order to avoid confusing cross references, the equations of this part are numbered sequentially following Part I [1]. II. STUDY OF THERMAL LENSING UNDER LASING CONDITIONS AND MEASUREMENT OF THE QUANTUM EFFICIENCY In this section, we study the evolution of the thermal lensing (TL) dioptric power versus the absorbed pump power, under lasing conditions, before and after threshold, and we compare the results with the model presented in Part I [1]. The experimental setup has been extensively described in [1, Fig. 6]. The absorbed pump power is measured here with a thermal power meter set up behind the dichroic meniscus [1, Fig. 6, mirror M2] so that a fraction of the transmitted pump beam is detected. The measurement of the real absorbed pump power under lasing (and nonlasing) conditions is then possible, which allows to take into account the absorption saturation effect correctly. The characteristics and pumping conditions of all the crystals are gathered in Table I.

I. INTRODUCTION

W

E INVESTIGATED theoretically, in Part I, [1] some properties of Yb-doped (quasi-three-level) materials as far as thermal and thermo-optical issues are concerneds. Since Yb-doped materials have to be end-pumped to guarantee high pumping intensity all along the crystal, the thermal lensing measurement technique must be able to evaluate accurately subwavelength phase shifts on small pump spots. We showed that a technique based on a Shack–Hartmann wavefront sensor, correctly positioned with respect to the crystal, was suitable to end-pumping configurations owing to its high sensitivity, and was furthermore very simple to set up. The experimental wavefronts presented in Part I [1] have shown that no detectable aberrations were present for any of the investigated materials, for pump powers around 10 W. As a consequence the thermal lens could only be defined by its focal length, or dioptric power. All the notations used in this part are identical to these employed

Manuscript received July 21, 2003; revised April 22, 2004. This work was supported in part by the Groupement de Recherches (GdR) Matériaux Laser, CNRS and in part by Délégation Générale de l’Armement (DGA). The authors are with the Laboratoire Charles Fabry de l’Institut d’Optique, Centre scientifique, Orsay cedex 91403, France (e-mail: chenais@galilee. univ-paris13.fr; [email protected]). Digital Object Identifier 10.1109/JQE.2004.833203

A. Results and Analytical Model At first, we emphasize on the results obtained with the Yb:GdCOB crystal (Fig. 1), a crystal for which the features we want to highlight are the most visible. We compare the TL dioptric power before threshold and after threshold, under lasing action (the data obtained without laser action and after threshold will be commented with more details in the next section). When the pump power exceeds threshold, the TL dioptric power experiences a decrease and then increases following a nearly straight line. This unusual behavior has never been observed previously, to the best of our knowledge. The clear difference between the dioptric power measured under lasing and non lasing conditions (for an identical absorbed power), proves the existence of significant nonradiative effects, which turn out to be “short-circuited” by stimulated emission when laser oscillation starts. A simple qualitative illustration of this is provided in Fig. 2. The theoretical description of [1, Sec. II] allows to account for these phenomena. For the discussion, we assume that the absorption saturation, under lasing action, is negligible. Under these conditions, the TL dioptric power writes [1, eq. (28)]

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(37)

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TABLE I CHARACTERISTICS AND PUMPING CONDITIONS OF THE INVESTIGATED CRYSTALS

Fig. 2. Qualitative interpretation of the behavior observed in all the Yb-doped materials. The arrows thickness stands for the corresponding rate.

mission ) and finally, we neglect the influence of spatial hole burning (since the laser is not singlemode). With these assumptions, the intracavity laser intensity in a linear cavity is given by [2] (39)

Fig. 1. TL dioptric power (left) and laser power (right) in Yb:GdCOB.

where is a constant which depends on the thermo-optic coefficient and on the pump diameter. We remind that is given by (38) The laser extraction efficiency is a function of the intracavity laser intensity , as stated by (5). We make here complementary assumptions. We consider that the laser beam has a simplified top-hat profile, that passive losses are negligible compared to output coupling losses (accounted by the output coupler trans-

where is the measured output laser power in watts. At last, we consider that is very close to 1. Indeed, considering a less-than-unity does not change the conclusion on the existence of nonradiative processes, but yields underestimated values of . Furthermore, the dual-wavelength experiment reported in the following in Yb:YSO is a confirmation (for this crystal) of such an assumption. in Yb:GdCOB is illustrated in The procedure to derive Fig. 3, where the dioptric power and the laser extraction efficiency are both represented versus the absorbed pump power.

CHÉNAIS et al.: THERMAL LENSING IN DIODE-PUMPED YTTERBIUM LASERS—PART II

Fig. 4.

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TL dioptric power (left) and laser power (right) in Yb:YCOB.

threshold. This behavior is very well accounted by the theory. It can be attributed to the sharp variation of the laser extraction efficiency, represented in Fig. 3(a). The nonradiative relax) and become ations are significant just before threshold ( negligible (because short-circuited by laser emission) when tends toward 1. After became stationary (here roughly after 4 W of absorbed pump power), then the dioptric power grows linearly according to the approximate relation Fig. 3. Procedure applied to derive the fluorescence quantum efficiency (here in Yb:GdCOB). From the TL dioptric power measurements before and after threshold (lower figure), and from the laser power measurements (upper figure), one can trace the TL dioptric power as a function of laser power. Only two adjustable parameters remain: A and .

From (38) and (5), the expressions of the TL dioptric power before and after threshold are given, respectively, in (40), shown at the bottom of the next page. In this set of two equations two and , since both the absorbed parameters are unknown: power and the output laser power (and then ) are experimentally accessible. and that best fit the We then seek the values of experimental data. We obtain the theoretical curves plotted in Fig. 3(b). It is important to point out that there are no and . For Yb:GdCOB we other fitting parameters but , which proves the existence of significant obtain nonradiative effects. This low value is also at the origin of the unusual decrease of the dioptric power noticed just after

(41) (here ), which means that the unique heat source, after this point, is the quantum defect, as discussed in [1, Sec. II-A]. The same procedure has been applied to fit the data obtained with Yb:YCOB, Yb:YAG, Yb:GGG and Yb:KGW and (Figs. 4–7, respectively). The numerical values of are summarized in Table II. Nonradiative effects occur in all these crystals, but among them the Yb:KGW crystal exhibits a different, and apparently opposite, behavior. Indeed, the TL dioptric power is here higher under lasing action than under nonlasing conditions. It can be interpreted as illustrated in Fig. 8. The average fluorescence wavelength is particularly low in this material (993 nm, which has justified its interest for radiation-balanced lasers in particular [3]). The quantum defect with laser oscillation at 1030 nm is thus higher than without oscillation. This explanation implies of course that the quantum efficiency is high in this sample. Indeed, the fitting procedure . described above yields

(40)

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Fig. 7.

TL dioptric power (left) and laser power (right) in Yb:KGW.

Fig. 5. TL dioptric power (left) and laser power (right) in Yb:YAG.

Fig. 8. Qualitative interpretation of the behavior observed in Yb:KGW. The nonradiative relaxations are considered negligible here for simplification.

Fig. 6. TL dioptric power (left) and laser power (right) in Yb:GGG.

B. Simple Example of Thermal Lensing Measurements at Two Laser Wavelengths in Yb:YSO The fractional thermal load (38) is dependant on the operating wavelength of the laser. However, this dependence is very often hidden by the fact that the laser extraction efficiency also depends a lot on the laser wavelength, since it is linked to the emission cross section. The Yb:YSO crystal exhibits two bumps of comparable amplitude in its emission spectrum, at 1042 and 1058 nm, respectively [4]. In addition the output is naturally linearly polarized along the crystallophysic axis for both wavelengths. It is thus a good candidate to put into evidence clearly the influence of laser wavelength on thermal lensing. We added a SF6 dispersive prism cut at Brewster angle in the collimated arm of the cavity [1, Fig. 6, between mirrors M2 and M3], so that identical laser efficiencies were achieved at both wavelengths (2.1 W were obtained for 8.5 W of absorbed pump power). The results are shown in Fig. 9. It appears clearly that the thermal

Fig. 9. TL dioptric power at 1042 and 1058 nm (left) and laser power (right) in Yb:YSO.

lens is weaker when the laser oscillates at 1042 nm, as expected since quantum defect is lower at this wavelength. From the experimental data obtained before and after threshold at 1042 nm, we infer the values of and , as explained in the previous subsection. The theoretical curve derived for the 1058-nm laser


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TABLE II SUMMARY OF THE EXPERIMENTAL RESULTS

oscillation was obtained from the 1042-nm curve by just modifying the wavelength and the emission cross section in the fitting algorithm, without any adjustable parameter. Not only it provides a further proof of the validity of the model, but it also proves that the assumption we made in Section I-A, which con, is perfectly valid for this sisted in considering that material. This simple experiment shows the interest of multiwavelength TL measurements in broadband materials. Indeed, we have considered here a simple formulation of the fractional thermal loading [given by (2)] which gives here satisfactory results; but it is possible, from the work of Patel et al. [5] for instance, to derive a more accurate expression of the fractional thermal load, which takes into account the probability of excitation transfer to a neighboring ion [6]. If TL measurements are performed versus absorbed power at different (more than 2) laser wavelengths, for which the laser extraction efficiency is also known, this means that we have the possibility to infer other spectroscopic parameters (such as the transfer probability to a neighboring ion) involved in the expression of the thermal load. C. Discussion on the Obtained Quantum Efficiencies In all the ytterbium-doped crystals under investigation in this study, nonradiative relaxations have been put into evidence. Up to now, to the best of our knowledge, less-than-unity quantum efficiencies have been reported in Yb:YAB by Blows et al. [7], in Yb:MgO:LiNbO by Ramirez et al. [8] and finally by several authors in Yb:YAG crystals. As far as this latter crystal is concerned, Barnes et al. [9] used a photometric and a calorimetric method and measured a quantum efficiency of 0.898 and 0.932 (respectively) with the two methods in a 1-at. % doped

YAG sample. Patel et al. [5] used a lifetime method and obtained radiative quantum efficiencies of 0.97 for YAG crystals with doping rates of 5 and 10 at. %. Ramirez et al. [8] obtained 0.85 for a 6-at. % YAG sample using a simple temperature meafor an 8-at.% surement. Adding our own results ( doped YAG crystal), a large dispersion of results is noticed. This dispersion tends to assess the conjecture of concentration quenching as the nonradiative source in Yb-doped materials. This conjecture has been very recently alleviated in highly-doped Yb:YAG samples and was attributed to cooperative processes between two Yb ions toward Yb impurities [10]. Owing to the intrinsic nature of concentration quenching and the major role played by impurities, it is clear that the radiative quantum efficiency is a parameter that pertains to a single given sample, characterized by its doping concentration, the growth technique and its associated environment (in particular the nature of the crucible), and of course the degree of purity of the compounds. The quantum efficiencies given in Table II have then to be considered as the values for the particular samples we had and not as a general property of the crystals. The unusually low quantum efficiency measured in our (non commercial) YAG sample is certainly due to the presence of impurities involved during the growth process. In the Yb:KGW crystal (from Eksma Inc.), Lituania, the high radiative quantum efficiency is consistent with the very low Yb concentration. This is due to the high peak absorption cm at 981 nm comcross section of this material, 12 10 pared to 0.7 10 cm at 968 nm for Yb:YAG, and possibly on the growth method itself (KGW is grown by the flux method) which is well known to carry less impurities than the Czochralski process.

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III. INVESTIGATION OF THERMAL LENSING WITHOUT LASER ACTION In this section we comment the results obtained without laser action (after threshold), and we use the model described in [1, Sec. II] to propose an explanation of the observed behavior. The experimental data are featured in Figs. 1, 4, 5, 7, and 10. It is clearly observable that in every crystal under investigation, the TL dioptric power experiences a roll-off versus the absorbed power. This cannot be attributed to a simple phenomenon of pump absorption saturation, since the absorbed power has been measured under “real” (that is nonlasing) conditions. Nonlinear variations of dioptric power have been already reported in Chromium-doped colquirites [11] and Neodymium-doped YLF [12], and were attributed in both cases to energy-transfer upconversion (which is absent in ytterbium-doped materials). It resulted in an overheating of the crystal at increasing pump powers, which lead to an opposite behavior of what is observed here, since a roll-off indicates that the heating is reduced at high pumping intensities. In order to account for this unusual behavior, we rely on the theory presented in [1, Sec. II] which takes into consideration the absorption saturation as well as the divergence of the pump beam inside the crystal. It has been established at the end of this section that an analytical formulation was possible in this case ) was the only contribution provided that the index term ( to the thermal lens. We make here the additional assumption is replaced by . The TL that (29) remains valid when dioptric power under non lasing conditions is then given by (42) is calculated numerically, in each slice of inwhere finitesimal thickness inside the crystal, according to (14). The experimental data and the theoretical plot are presented in Fig. 10, in the case of the Yb:YSO crystal. The thermo-optic coefficient is the only adjustable parameter of this model (we discuss more about the obtained values for in the next section). This model allows to explain the experimental roll-off, provided that both the saturation of the absorption and the pump divergence into the crystal are considered. As expected, when the di), vergence of the pump beam is set equal to zero ( no roll-off appears. Physically, it can be understood due to the calculated temperature profile featured in [1, Fig. 3]. In presence of saturation [Fig. 3(b)], there is a mismatch between the heated zone and the fundamental cavity mode volume (restrained to the central portion of the rod, approximately along a cylinder of radius ). The entrance and output regions of the crystal thus represent heat sources that are not “seen” by the probe beam since it does not pass through them. This simple model accounts for the roll-off in most crystals under investigation, as can be seen from Figs. 1–7, where the theoretical plot has been superimposed to the experimental data. However, the agreement is not perfect and the predicted theoretical roll-off clearly underestimates the experimentally observed roll-off at the highest pump powers. Several explanations may be proposed, exposed here by degree of presumed importance.

1) When the absorbed power increases, this is in reality the diode operating current that is increased, at fixed diode temperature. When the diode current is set from zero to maximum, the diode wavelength shifts, resulting in a significant change in the absorption cross section at pump wavelength (the zero-line absorption peak being sharp). As a consequence, the pump saturation intensity, given by equation (15), decreases with the absorbed power. This effect permits to understand why the real roll-off should be more pronounced that the one given by the model. This effect could be important and should be confirmed by measurements using high quality nonpolarizing attenuators set on the pump beam path. 2) Cooperative luminescence [13], [14] could play an important role in some materials. It results in a quenching of the excited population with no heat generation, and then represents a possible explanation for an additional roll-off. As far as our samples are concerned, this effect is clearly detectable by a green fluorescence in Yb:GdCOB, Yb:YCOB, Yb:KGW, Yb:YAG, Yb:GGG, and is present also in Yb:YSO but is, in this case, hidden by the red-blue fluorescence provided by thulium impurities. The measurement of the green luminescence spectrum provides evidence of a cooperative process in Yb:YCOB [6], since the obtained spectrum approximately superimposes the autocorrelation of the infrared spectrum. However the intensity of cooperative luminescence is very small (commonly 10 to 10 lower than the infrared fluorescence [14]) so that additional investigations are required to determine whether such an effect is likely to play a major role. 3) In an Yb-doped material the absorption cross sections depend on temperature since they are linked to the fractional thermal populations of the electronic levels. Since the center of the rod is hotter than the edge, this can contribute to “dig a hole” in the thermal load profile, even if the pump profile remains perfectly “top hat”. Since it leads to a decrease of the temperature at the center of the rod, it also allows explaining a more prominent roll-off than predicted. IV. ESTIMATION OF THE THERMO-OPTIC COEFFICIENT It is of great practical interest to evaluate the thermo-optic coefficient in the purpose of power scaling, above all in crystals for which most of the parameters involved in the expression of (see eq. 26) are still unknown. The interest of the model presented in the previous section [see (46)] is its ability to yield a value of this coefficient. However, it is clear that, after all the restrictions we just commented, the value of that we can infer in this way is only a rough estimation; but it is still more accurate compared to a simple linear fit performed on the TL dioptric power curve without lasing action (see Fig. 10). The values of derived from (46) are summarized in Table II. We obtain for YAG a value (10 10 K ) which is compaparameter measured by Wynne rable in magnitude to the et al. [15] (9 10 K ); this is also consistent with the fact that photoelastic effect is negligible in this material (after [16]).


Fig. 10. TL dioptric power under nonlasing conditions in Yb:YSO. Experimental data (markers) and theoretical model (solid line) taking into account the pump beam divergence and the absorption saturation. To better illustrate the presence of a roll-off and the necessity of a refined model, a linear fit performed on the first experimental points is featured (dotted line).

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Fig. 11. “Complete model” (solid line. See Sections I and II) in Yb:YAG; Comparison with 1) “linear approximation” where an effective pump radius is considered (bold dotted line: see Section IV) and 2) “linear approximation” without consideration of pump beam divergence (narrow dotted line). The error is only 5% in the first case, but reaches 40% in the second case.

V. USEFUL FORMULATION FOR TL FOCAL LENGTH ESTIMATION The evaluation of the end effect in this special configuration is not easy and would require a FEA calculation or a TL measurement based on the reflection of the probe beam, as performed by Peng et al. [17]. But recently, Andrade et al. [18] have measured the thermo-optic coefficient in end-pumped Nd:YAG and found 13.7 10 K , which is also consistent with our measurements. Yb:KGW beneficiates of a low thermo-optic coefficient together with a relatively high thermal conductivity, which explains the low thermal lensing compared to other materials. The discrepancy noticed between YAG and GGG, although they are very close materials in terms of structure, is is far higher in quite consistent with the fact that the GGG (17.5 10 K versus 9 10 K for YAG). The same discrepancy is noticed for GdCOB compared to YCOB, at our disposal to but in this case we have no value of further comment the difference. Finally, let us remind that the values of reported here apply to end-pumped crystals only, and are somewhat dependant on the cooling scheme; in particular they do not apply to composite crystals, where the bulging of end faces is vanishing. In the purpose of comparing different materials between then, as far as thermal lensing is concerned, a good figure of merit is (we do not include the fractional thermal loading the ratio in this figure of merit since it depends on the laser wavelength and operating regime). To make the comparison relevant, we consider the thermal conductivity for undoped crystals. This ratio is displayed in Table II. YAG and KGW appear to be the best materials, although thermal lensing is two times stronger in KGW. GdCOB suffers of a low thermal conductivity but also beneficiates of a low thermo-optic coefficient, which finally gives it a figure of merit comparable to that of YSO, which in contrast exhibits both high thermal conductivity and high .

When the thermo-optic coefficient and the fractional thermal loading are known for a given crystal, it may be useful to have a straightforward expression of the TL dioptric power, as a function of easily accessible parameters such as the absorbed pump power or the pump beam spot size. In most practical cases, taking into account the roll-off, as described previously, is not useful. This is especially true when the crystal is thin (this is probably the reason why no roll-off could be clearly measured in the experiments related by Blows et al. [7]). This is all the more true when the thermal lens under lasing conditions (and in the CW regime) is considered, because in this case, as seen in Section II, the effect of pump saturation can be neglected. In this last subsection we address a less rigorous and simpler calculation which assumes a linear evolution of the TL dioptric power with absorbed pump power. However, if we apply directly the classical relation (28) with the values of and calculated before, large errors are greatly depends on the pump spot radius: encountered, since the divergence of the pump beam cannot be ignored. In this purpose we introduce an effective pump radius defined so that the TL dioptric power writes under the form (43) where

is defined by (44)

Here

is the (nonsaturated) absorption function (45)

In Fig. 11 are shown the experimental points obtained with Yb:YAG; the solid line is the fit obtained with the previous

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models (already displayed in Fig. 5, leading to the quantum efficiency and the thermo-optic coefficient reported in Table II); the bold dotted line represents the analytical expression (43), or equivalently the relation (40) where the coefficient has been substituted by (46) This constitutes a fully analytical formulation, very easy to implement. The difference between the two approaches turns out to be below 5% (Fig. 11), which shows that taking into account the pump divergence by means of (44) leads to very acceptable results. In contrast, if the divergence is purely ignored (that is if the pump radius is set equal to the waist radius along the whole crystal thickness), the error reaches 40%, as shown by the thin dotted line in Fig. 11. Identically, for crystals whose length is far higher than the Rayleigh distance of the pump beam (here around leads to larger errors. For 0.5 mm), the expression (44) for the 3-mm long Yb:GdCOB crystal for instance, the error is about 20%. In these latter cases, the rigorous treatment presented above is necessary.

We then presented a simple and useful analytical approach to infer the TL dioptric power in materials for which the fractional thermal load and thermo-optic coefficient are known or can be evaluated. We have shown that the divergence of the pump beam has to be taken into account in all cases, but that it can be done by just considering an “effective” pump spot radius, provided that the crystals are not too long.

ACKNOWLEDGMENT The authors would like to thank Ecole Supérieure d’Optique, Orsay, France for loaning them the Shack–Hartmann wavefront sensor. They would also like to thank B. Viana, G. Aka, and D. Vivien from the Laboratoire de Chimie Appliquée de l’Etat Solide (Ecole Nationale Supérieure de Chimie Paris, France), A. Brenier and G. Boulon from the Laboratoire de Physico-Chimie des Materiaux Luminescents, Lyon, France, and B. Ferrand from CEA-LETI, Grenoble, France, for growing the crystals investigated here, and providing very helpful advice for the interpretation of the results. Finally, this paper has greatly benefited from innumerable and fruitful discussions with R. Gaumé.

VI. CONCLUSION In Part II of this series, we have investigated the TL dioptric power dependence with absorbed power. At first the comparison between TL before and after threshold has supported evidence of significant nonradiative effects in all investigated crystals. Furthermore, we observed for the first time (and more evidently in Yb:GdCOB) a decrease of the TL dioptric power just after threshold. By taking into consideration the evolution of laser extraction efficiency with the absorbed pump power we were able to explain all the observed features. Furthermore, the comparison between experience and model yields the radiative quantum efficiency of the studied samples. They are in the range 0.7–0.96. Nonradiative effects are supposed to be linked to concentration quenching effects, as alleviated by recent papers. An original experiment of dual-wavelength TL measurement has been presented in Yb:YSO. The thermal load dependence on laser wavelength has been clearly put into evidence, and measurements have brought the proof that the pump quantum efficiency was equal to unity in this material. At last, we focused on the evolution of dioptric power versus pump power under nonlasing conditions. For the first time to our knowledge a significant roll-off is demonstrated in all materials. By applying the results of the theoretical modeling developed in Part I [1], we saw that the pump saturation absorption, along with pump beam divergence inside the crystal provided a correct explanation of the observed roll-off. However, in all cases the experimental roll-off is more pronounced than predicted by theory for high pump powers. We have discussed three possible origins of these deviations. However, taking this roll-off into consideration allows to derive a more accurate estimation of the thermo-optic coefficient than what can be done if we assume a simple linear fit. The obtained values of thermo-optic coefficient are consistent with published results in YAG, and represent useful parameters for laser design since they represent a global information on thermal effects.

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[12] P. Hardman, W. Clarkson, G. Friel, M. Pollnau, and D. Hanna, “Energy-transfer upconversion and thermal lensing in high-power end-pumped Nd:YLF laser crystals,” IEEE J. Quantum Electron, vol. 35, pp. 647–655, Apr. 1999. [13] E. Nakazawa and S. Shionoya, “Cooperative luminescence in YbPO ,” Phys. Rev. Lett, vol. 25, p. 1710, 1970. [14] S. Magne, Y. Ouerdane, M. Druetta, J. P. Goure, P. Ferdinand, and G. Monnom, “Cooperative luminescence in an ytterbium-doped silica fiber,” Opt. Comm., vol. 111, pp. 310–316, 1994. [15] R. Wynne, J. Daneu, and T. Y. Fan, “Thermal coefficients of the expansion and refractive index in YAG,” Appl. Opt., vol. 38, no. 15, pp. 3282–3284, 1999. [16] W. Koechner, Solid State Laser Engineering, 5th ed. Berlin, Germany: Springer–Verlag, 1999. [17] X. Peng, A. Asundi, Y. Chen, and Z. Xiong, “Study of the mechanical properties of Nd:YVO crystal by use of laser interferometry and finiteelement analysis,” Appl. Opt., vol. 40, no. 9, pp. 1396–1403, 2001. [18] A. Andrade, T. Catunda, and M. Baersso, “Fluorescence quantum efficiency and ds/dT measurements in Nd:YAG,” in Conf. Lasers and Electro-Optics Europe (CLEO Europe 2003), Tech. Dig., Conf. Ed., Münich, Germany, 2003, CG1M. [19] S. Bowman and C. Mungan, “New materials for optical cooling,” Appl. Phys. B, vol. 71, pp. 807–811, 2000.

Sébastien Chénais was born in 1977. He received the degree of optical engineering from the Ecole Supérieure d’Optique, Orsay, France, in 1999 and the Ph.D. degree from the University of Paris XI, Paris, France, in 2002, for his work on thermal effects in solid state lasers and new laser materials. In September 2003, he became Assistant Professor at the University of Paris 13, Villetaneuse, France.

François Balembois was born in 1968 in Béthune, France. He received the Engineering degree from the École Supérieure d’Optique, Orsay, France, in 1991. He received the Ph.D. degree from the University of Paris, Paris, in 1994 for work on diode-pumped Cr:LiSAF lasers. In 1994, he was appointed Assistant Professor at the École Supérieure d’Optique, and he is currently Professor. His research topics cover diode-pumped solid-state lasers, picosecond and femtosecond pulse generation, and nonlinear optics.

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Frédéric Druon was born in Aubervilliers (Neuf Trois), France, in 1972. He received the degree from the Electrical Engineering School of Paris, Supelec, France, in 1996 and in parallel studied optics and photonics at the University of Paris XI, Paris, France. He received the Ph.D. degree in 2000 for his work at the Institut d’Optique, Orsay, France, on ultracompact and efficient picosecond UV sources based on microchip lasers and on femtosecond diode-pumped laser systems. In 1997, he completed his military service as a Research Visitor at the Center for Ultrafast Optical Science (CUOS), Ann Arbor, MI, where he worked on wave-front analysis and correction of terawatt laser chains. After a one-year postdoctoral position working on mode-locking processes involving fast saturable absorbers, he joined the French National Center for Scientific Research (CNRS) within the Laboratoire Charles Fabry de 1’Institut d’Optique in 2001. He is currently engaged in research toward new laser materials, femtosecond techniques, photonic crystal fibers and nonlinear optics. Dr. Druon is a member of the Optical Society of America.

Gaëlle Lucas-Leclin was born in Guérande, France. She received the engineering degree from the Ecole Supérieure d’Optique, Orsay, France, in 1994, and the Ph. D. degree from the University Paris XI, Paris, France in 1998 for her work on frequency stabilized laser diodes for Cs atomic frequency standards at the Laboratoire de l’Horloge Atomique, University Paris XI. In 1998, she became an Assistant Professor at the École Supérieure d’Optique, where she was involved in optical design, semiconductor physics, and nonlinear optics. She joined the Laboratoire Charles Fabry de l’Institut d’Optique in 1998. Her research interests are essentially on new diode-pumped solid-state lasers, including nonlinear optics and rare-earth doped fiber amplifiers. Dr. Lucas-Leclin is a member of the Optical Society of America.

Patrick Georges was born in Metz, France, on September 13, 1962. He received the engineering degree from the Ecole Supérieure d’Optique, Orsay, France, in 1985 and the Ph.D. degree in 1989 for his work on colliding pulses mode locked dye lasers at different wavelengths and pulses compression. He is currently Directeur de Recherche at the Centre National de la Recherche Scientiflque (CNRS) in 1991. Since then, he has been working at the Institut d’Optique, Orsay. He now leads the Solid State Lasers and Applications team (ELSA) within the Nonlinear Optics group, in the Laboratoire Charles Fabry de l’Institut d’Optique. His current research topics cover diode-pumped solid-state lasers, new laser materials, picosecond and femtosecond lasers, high brightness laser diodes, and fiber amplifier systems. He is also working on applications of picosecond or femtosecond lasers in biophotonics (time resolved fluorescence spectroscopy, two photons microscopy). Dr. Georges is a member of the Optical Society of America.