Temperature of the pumped volume of an ytterbium doped material has been measured while laser action is taking place. This is achieved by recording green emissions at 530 and 550 nm from Er3+ impurities. These emissions result from energy transfer upconversion processes between Yb3+ and Er3+. Experiments performed on a Yb3+:CaGdAlO4 crystal show the effect of pump power and laser wavelength on the sample internal temperature. Temperature variation along the sample length has also been measured. This method can complement data obtained by thermal cameras which can only access surface temperatures in most laser materials.
© 2011 OSA
Ytterbium doped materials have recently attracted much interest as amplifying medium in high power solid state lasers [1, 2]. In the latter, it is crucial to minimize thermal effects which can lead to material failure or beam quality decrease [3–5]. This can be achieved by reducing heat generation, which is usually produced by non radiative relaxations within the active ion. The amount of heat generated is proportional to the quantum defect ηlaser = 1 − λpump/λlaser where λpump and λlaser are the pump and laser wavelengths. The quantum defect reaches the lowest values in ytterbium doped materials where ηlaser can be as low as 3% in Yb:GdVO4  and even 0.8 % in Yb:CaGdAlO4 (CALGO)  compared to 25% in neodymium doped crystals. Other solutions to reduce thermal effects include high surface to volume geometries like fibers  and thin disks , and/or high thermal conductivity and low thermal expansion materials. To precisely assess the efficiency of these different approaches, it is necessary to measure temperature gradients in the material volume and during laser action to take into account pump absorption saturation effects for example. This is a difficult task as the temperature inside the material normally does not exceed 200 °C which corresponds to an infrared thermal emission peaking at 5.8 μm. Many materials, including oxide crystals are not transparent at these wavelengths and only surface temperatures can be measured by thermal cameras [10, 11]. This prevents direct measurements of the pumped volume of the material, where the highest temperatures are reached. In this paper, we demonstrate, for the first time to our knowledge, temperature measurements of the pumped region inside an Yb3+ doped crystal (Yb3+:CALGO) during laser action. This is obtained by recording the green emission from erbium impurities which is known as an accurate temperature monitor [12–15]. Er3+ visible luminescence results from efficient successive energy transfers between Yb3+ and Er3+ ions, a process called APTE (Addition de Photons par Transfert d’Energie) or ETU (Energy Transfer Upconversion) [16, 17]. The use of Er3+ upconverted emission in the green spectral region, instead of directly recording Yb3+ fluorescence , allows one to completely eliminate diffused light from pump and stimulated emission which may strongly perturb Yb3+ spontaneous emission spectrum. It is also a general method for Yb3+ doped materials since Er3+ is a very common impurity of Yb3+, these rare earth ions having close electronic configurations (respectively 4f11 and 4f13).
The experimental set-up for measurements during laser action is shown in Fig. 1. Orthogonally to a plano-concave laser cavity containing a 2 at%Yb3+:CALGO single crystal, two lenses collected visible emission on the entrance slit of a 25 cm Jobin-Yvon spectrometer equipped with a photomultiplier. A Schott BG 18 filter blocked infrared light around 1 μm. With that set of lenses, an image of the crystal was obtained on the input of the spectrometer with a magnification of 1.5. Translation of the first lens translated the crystal image on the spectrometer slit. Thus, the sample could be scanned in order to analyze the temperature profile along the pumped zone. With slits 500 μm wide, the temperature was averaged over a pumped zone approximately described by a cylinder of 100 μm radius and 330 μm length. The pump source was a Coherent 890 titanium sapphire (Ti-Sa) laser tuned to 980 nm. For Er3+ luminescence calibration, the same set-up (without the laser cavity) was used with a thin sample glued with thermal grease to a regulated heating plate allowing us to set sample temperature between 40 °C and 250 °C. This sample was only 1 mm thick, with a 1 cm2 surface in contact with the heating plate to improve heat transfer. Moreover, to ensure low thermal loading by the Ti-Sa excitation, the pump power was limited to 500 mW. In these conditions, temperature measurements by erbium luminescence (see below) showed no dependence on laser power. We then concluded that the sample temperature was that of the regulated plate, within the measurement accuracy of +/− 2°C (see below).
3. Results and discussion
3.1. Visible emissions in Yb:CALGO and temperature calibration
Visible emissions recorded in Yb3+:CALGO in the 440–600 nm range under 980 nm excitation are presented in Fig. 2 and are similar to those found in other Yb3+ doped materials . A broad line centered at 500 nm corresponds to the cooperative emission of ytterbium ions (2F5/2,2F5/2) → (2F7/2,2F7/2)  as confirmed by the calculation of the infrared emission (2F5/2 → 2F7/2) convolution (Fig. 4, inset). The emission peaking at 470 nm is attributed to thulium ions (1G4 → 3H6 transition), which are also a common impurity of Yb3+ ions. Tm3+ ions are excited from Yb3+ ions through a three step upconversion process [17,21]. Erbium ion emissions (2H11/2,4S3/2 → 4I15/2 transitions) are observed between 520 and 570 nm.
Er3+ excitation proceeds by a first energy transfer from an excited ytterbium ion: (4I15/2(Er3+),2 F5/2(Yb3+)) → (4I11/2(Er3+),2 F7/2(Yb3+)) (see Fig. 3). This is much more efficient than direct Er3+ absorption at 980 nm, which is negligible compared to Yb3+ one since erbium concentration is much lower than ytterbium one (see below). In a second step, an energy transfer from an excited Yb3+ ion to an already excited Er3+ ion in the 4I11/2 multiplet can take place, resulting in a Er3+ ion in the 4F7/2 multiplet: (4I11/2(Er3+),2 F5/2(Yb3+)) → (4F7/2(Er3+),2 F7/2(Yb3+)). After a fast non radiative relaxation, the 4S3/2 and 2H11/2 multiplets are finally populated and emissions occur at about 550 nm (4S3/2 →4 I15/2) and 530 nm (2H11/2 →4 I15/2) (Fig. 3). Typical distances for efficient energy transfer between rare earth ions are on the order of a few nanometers, leading to energy diffusion lengths on the order of 1 μm [22, 23], which is negligible compared to the excited volumes in laser experiments. Erbium fluorescence may therefore be safely assumed to originate from the infrared excited volume. In laser experiments, this volume could be different from the laser mode one. In these two volumes, Yb3+ excited state populations and therefore Er3+ ones can be significantly different. Temperatures deduced from Er3+ fluorescence will therefore correspond to an average over these volumes.
Er3+ concentration is estimated to be ≈10 ppm (1.25 × 1017 ions/cm3), based on the purity of the starting materials. At this level, Er3+ has a negligible effect on Yb3+ excited state population, as can be deduced from data obtained in CaYAlO4, a close isostructural compound [24,25]. In this host, the lifetimes of erbium 4I11/2 and ytterbium 2F5/2 multiplets are very close (435 μs and 420 μs respectively), leading to a thermalisation between these levels in the codoped material . The corresponding relative decrease in 2F5/2 population is therefore approximately equal to the Er3+ to Yb3+ concentration ratio, i.e. 1/2000. The second energy transfer from Yb3+ to Er3+, which populates 2H11/2 and 4S3/2 multiplets (Fig. 3), has a rate of C = 2.5 × 10−16 cm3s−1 . For a given population N of the 4I11/2 multiplet, this transfer will add a term CN to Yb3+ relaxation rate (W = 2380 s−1). Setting N to half of erbium concentration, we find a maximum rate of CN = 16 s−1, so CN ≈ W/150. This also shows that a two-or three-fold increase in Er3+ concentration (e.g. by intentional doping) should be possible without significantly affecting Yb3+ excited state population. This may be useful to get a larger Er3+ green emission compared to Yb3+ cooperative one.
Populations N in the 4S3/2 and 2H11/2 multiplets (denoted by l and h respectively) obey Boltzmann thermal equilibrium:Fig. 2, where a strong decrease of the 550 nm emission relative to the 530 nm one is observed as temperature is increased, in agreement with Eq. (1). In the present case, the energy difference ΔE between the 4S3/2 and 2H11/2 multiplets is about 800 cm−1. Erbium green emissions are therefore an appropriate probe to observe local temperature variations from room temperature (kT ≈ 210 cm−1) to 1000 °C(kT ≈ 890 cm−1).
Since transition oscillator strengths are unknown, temperature measurements require a calibration, i.e. recording Er3+ emission spectra as a function of temperature. As seen in Fig. 2, Yb3+ cooperative emission partly overlaps Er3+ one in the 520–540 nm range. This was taken into account by modeling cooperative emission by a Gaussian lineshape. Indeed the latter is a very good fit to Yb3+ infrared emission convolution (see inset in Fig. 4). The absence of sharp features in the cooperative emission is due to Yb3+ very broad infrared emission in CALGO . In other materials, a more detailed modeling of cooperative emission may be necessary. In order to further minimize errors due to cooperative emission, temperature was then calibrated by measuring emission peak intensity ratios at 530 and 550 nm (A and B arrows, Fig. 4), subtracting cooperative emission contribution. Possible changes in cooperative emission shape and/or intensity as a function of temperature or laser excitation intensity was therefore taken into account.
A plot of the intensity ratio between 530 and 550 nm peak emission intensities versus crystal temperature is presented in Fig. 5, showing a good agreement with an exponential law (Eq. (1)), although the latter is in principle only valid for integrated intensities. This result is in agreement with the limited change in erbium emission shapes observed in this temperature range (Fig. 2). The fitted value for ΔE in Eq. 1 is 750 cm−1, corresponding to the value deduced from emission mean energies. The fitted exponential curve was then used to deduce temperatures from peak intensity ratios, with an accuracy of ±2°C. This value was limited by the pump laser intensity fluctuations and the spectrum acquisition time on the spectrometer. It could be improved by using a CCD detector and laser diode excitation.
3.2. Temperature measurements during laser action
Temperatures of a 2 at%Yb:CALGO were measured from erbium green emissions ratio with or without laser action taking place in the crystal, using Fig. 1 setup. The sample had a section of 4 × 4mm2 and a thickness of 1.8 mm. Only one side of the crystal was in contact with a mount using a plastic layer for thermal insulation. The contact area between the layer and the crystal was small so that heat exchanges occurred mainly between the crystal and the surrounding air. This was necessary to generate high enough temperatures from the limited power (up to 2W at 980 nm) delivered by the Ti-Sa laser. Fig. 6 shows the temperature recorded at the center of the sample (with respect to thickness and averaged over a small volume, see Experimental section) as a function of pump power. In the investigated power range, 63 % of the incident pump power was absorbed. Thanks to thermal insulation, temperatures as high as 110 °C were reached. These were similar to those measured at the surface of the same material with 83 W of pump power but with the latter inserted in a copper mount which allows for high heat transfer .
By slightly misaligning the output mirror we could perform the same experiment without laser action. The temperatures obtained are higher than those observed under laser action, reaching 112 °C instead of 104 °C at the maximum pump power. The mean fluorescence wavelength (1004 nm) is significantly smaller than the laser emission one (1040 nm), which should result in a higher thermal load and therefore higher temperatures during laser action. However, non radiative relaxations (mainly energy transfers to traps) of the excited state decrease when laser action takes place because of stimulated emission . This has the effect of reducing the thermal load compared to the non lasing case. In our experiments, the latter effect seems to dominate over the fluorescence and laser wavelength difference. Another process which has to be taken into account at high pump intensities is Yb3+ absorption saturation . Since stimulated emission decreases saturation, absorption tends to be larger during laser action, which in turn increases the thermal load. In our experiments, transmitted pump power showed a linear dependence on input power, so that no saturation occurred. However, it may explain the higher temperature measured during laser action, compared to the case without laser action, in Ref. , where high pump powers were used.
In the limit of a high intracavity laser intensity, it can be considered that non radiative relaxations are negligible and that the thermal load is directly proportional to the quantum defect, ηlaser = 5.7% in our case . Without laser action, the thermal load due to non radiative relaxations adds to the one due to the difference between pump and mean fluorescence wavelengths. The latter corresponds to a quantum defect of ηfluo = 2%. Fig. 6 shows that the thermal loads with and without laser action are similar, which leads to a quantum efficiency of ≈ 96 % for Yb3+ 2F5/2 excited multiplet. This value is in reasonable agreement with Yb3+ lifetime measurements which gives a quantum efficiency of 93 ± 5 % . Yb3+ quantum efficiency includes the effect or Er3+ impurities, which therefore contribute to the thermal load. However, from estimations given in section 3.1, the corresponding decrease in Yb3+ quantum efficiency should not exceed 1 %, even taking into account the relatively small quantum efficiencies of the 4I11/2 and 4S3/2 multiplets, 17 % and 43 % respectively . The contribution of Yb3+ quantum defect to the thermal load during laser action was confirmed by changing the cavity output mirror, in order to obtain a shorter laser emission wavelength (1013 nm). This should decrease the thermal load and we indeed observed a 13 % decrease in crystal temperature at maximum pump power.
To check the validity of the temperature measurement, a simple simulation was run using a finite element program, assuming that the thermal load during laser action was only due to the quantum defect. Simulation parameters included the sample dimensions and quantum defect given above, as well as the absorption coefficient (0.99 cm−1 at 980 nm), the pump beam waist (100 μm) and the crystal mean thermal conductivity (k = 6.5 Wm−1K−1 ). Experimental temperatures could be reproduced with a heat transfer coefficient for all sample sides of 17 Wm−2K−1). Although we could not measure the latter value, it is consistent with published values .
Finally, temperature was measured along the cavity axis at 2 W pump power (Fig. 7) using a 5 mm long crystal. A significant difference (10 ±4 °C) can be observed between points located at 0.7 and 4.3 mm from the side where the pump beam entered the crystal. The decrease of the temperature with increasing distance from the input face simply reflects the corresponding decrease of the absorbed pump power. Since the Rayleigh length of the pump beam was 3 cm, we did not expect to see a temperature minimum inside the sample .
Temperature measurements of the pumped volume of an ytterbium doped material under laser action have been demonstrated. This was obtained by recording green emission from Er3+ impurities excited from Yb3+ ions by upconversion. Temperatures are deduced from the ratio between erbium emissions at 530 and 550 nm and a calibration performed between 40 and 250 °C. Experiments were performed on a Yb3+:CaGdAlO4 crystal with or without laser action and at different pump powers and laser emission wavelengths. These results can be explained by relating the thermal load to quantum defect, mean fluorescence wavelength and Yb3+ quantum efficiency. Measuring temperatures of the pumped volume of a material can complement those obtained by using thermal cameras which can only access surface temperatures in oxide materials. This method could be extended to other laser materials as long as Er3+ green upconverted emission is larger than Yb3+ cooperative emission. Small intentional Er3+ doping could be used for this purpose.
The authors would like to thank the reviewers for useful comments and suggestions.
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