Thermal load in Nd3+ doped vanadate crystals with and without laser action at 1.34 µm is investigated. Excited state absorption contributes significantly to a fractional thermal loading as well as quantum defect.
© 2005 Optical Society of America
Diode-pumped solid-state lasers in the 1.3 µm region have received a lot of interest for use in various applications such as laser display, medical diagnosis and so on [1,2]. Neodymium-doped orthovanadates (Nd:REVO4) are promising crystals for 1.3 µm lasers because of their relatively large stimulated emission cross-section as well as strong absorption of pumping diode frequencies [3,4]. So far, several researchers have demonstrated 1.3 µm lasers based on vanadate crystals. The slope efficiency obtained using 1.3µm vanadate lasers is limited to only 30–40 %. This is considerably lower than that obtained with 1.06 µm lasers [5,6].
In the case of 1.3 µm lasers, a large quantum defect (the energy difference between pump and laser photon) induces significant energy dissipation through heat generation in the crystal. Development of efficient 1.3µm lasers requires careful thermal management.
Recently, Fornasiero et.al. mentioned that excited state absorption (ESA) further increased a fractional thermal loading and prevented efficient laser oscillation in 1.3µm vanadate lasers . They estimated the ESA cross-section for 1.3 µm photons by using a pump-probe method, however there was little discussion about the fractional thermal loading in 1.3 µm lasers.
In this paper, we present a quantitative estimate of the fractional thermal loading in 1.3 µm lasers for Nd doped vanadate crystals such as Nd:YVO4 and Nd:GdVO4 by thermal lens measurements with and without laser action.
2.1 laser system
The experimental setup is shown as Fig. 1. The cavity was composed of a simple plane-parallel resonator with an end mirror and a 95 % reflective output coupler. The cavity length was about 35 mm. The Crystals used were 0.3 at.% Nd3+ doped, a-cut Nd:YVO4 and Nd:GdVO4 with dimensions of 4×4×7 mm3. The 4 mm×4 mm faces of the crystal were AR-coated for the pump wavelength of around 808 nm. The crystal was longitudinally pumped by a fiber-coupled 15 W CW 808 nm diode laser array. The wavelength of the diode was tuned, thereby maximising absorption. The diode output was focused to a spot with a radius of 350 µm on the crystal face. Figure 2 shows the experimental output power as a function of the absorbed power. The Nd:YVO4 exhibited a slightly better performance than the Nd:GdVO4. In the case of Nd:YVO4 a slope efficiency of 22 % was obtained and the laser threshold was 1.9 W. The output power reached 1.4 W at a pump power of 8.1 W.
2.2 Thermal lens measurement
To measure thermal lenses in the crystals, holographic lateral-shearing interferometry  was performed as shown in Fig. 3. The laser cavity was composed of an end mirror, a totally reflective 45 degree turning mirror, and an output coupler. A mechanical shutter placed inside the cavity was used to control the laser action. A green laser was used as a probe laser. The collimated probe beam made a double pass through the crystal, and the presense of thermal lens caused the wavefront of the probe beam to be distorted. After passing through the imaging optics, the distorted probe beam was directed toward the holographic shear plate. The shear plate enabled the distorted probe beam to interfere with a laterally sheared copy of itself, forming a series of fringes. By analyzing the distortion of the fringes , the spatial phase shift distribution on Δϕ(r)(=a 0+a 1 r+a 2 r 2+…) is estimated. And then the thermal lens power D is given by
where k is the wave number for 1.3 µm laser. Figure 4 shows the fringes observed with and without 1.3 µm laser actions. It is found that the distortion of fringes with laser action is significantly stronger than that without laser action.
The measured thermal lens power as a function of the absorbed pump power is plotted in Fig. 5. In the case of Nd:YVO4, the thermal lens power without laser action was proportional to the absorbed pump power with a slope of 0.9 m-1/W. Once the laser turned on, the slope increased to 2.1 m-1/W. This value was 2 times larger than the slope without laser action. In order to confirm the measured value of the thermal lens power, we also investigated the cavity stability. When the cavity length was longer than 13cm, saturation of output power occurred at a higher pump power of 7 W (Fig. 6). This phenomenon means that the focal length of thermal lens at a pump power of 7 W is shorter than 13 cm. This criterion for the focal length of thermal lens is consistent with the value measured by the interferometric technique.
We also measured it for Nd:GdVO4 crystal. The slope ratio of thermal lens with and without laser action was 2.1. Nd:GdVO4 shows comparable thermal lens with Nd:YVO4, though Nd:GdVO4 has good thermal conductivity in comparison with Nd:YVO4. In general, thermal lens is proportional to the product of thermal conductivity and (dn/dT). The unexpected large thermal lens in Nd:GdVO4 is due to its large thermo-optic coefficient (dn/dT) [10,11].
The large increase in thermal loading with 1.3 µm laser action is due to a relatively large quantum energy defect between the pump and laser photons as well as strong ESA (a process in which the excited ions lying in the upper laser level transit to the higher excited level through 1.3 µm lasing photon absorption).
ESA is evidenced by visible emission from the pumped crystal due to visible fluorescence from higher lying levels such as 4G7/2. We observed the fluorescence by using a spectrometer and an intensified CCD camera. The fluorescence spectrum is shown in Fig. 7.
Above the threshold, three strong peaks appeared around 540 nm, 600 nm and 670 nm, while below the threshold these peaks were much weaker. These correspond to the 4G7/2→4I9/2, 4G7/2→4I11/2 and 4G7/2→4I13/2 transitions, respectively. Their intensities with laser action were much stronger than them without laser action. Figure 8 shows fluorescence intensity corresponding to radiative transition of 4G7/2→4I11/2 as a function of absorbed power. The fluorescence intensity with 1.3µm laser action was 3-times larger than that without laser action at the maximum pump level. These show ESA contributes significantly to the fractional thermal loading with 1.3µm laser action.
The fluorescence lifetime, τf, as a function of Nd ions concentration is presented in Table 1.
The radiative lifetime, τrad, was estimated by using Dexter’s theory . It was 120 µs for Nd:YVO4 and 122 µs for Nd:GdVO4. Then, we estimated the fluorescence lifetime of our crystals Nd ion concentration was 0.3 at. % in which. The value of β, the fraction of energy stored in the upper laser level that decays non-radiatively was calculated using the result,
The fractional thermal loading without laser action ηnon is given by,
where ηq is the pump quantum efficiency, and λp and λl are the pump and laser wavelengths respectively.
The fractional thermal loading (fraction of absorbed power deposited as heat) depends on laser action. The ratio of the thermal lens powers with and without laser action was near unity at threshold. Above threshold, this ratio increased to a value of 2 for both crystals. Above the threshold, the population of the Nd3+ ions in the upper laser level, Nu, described by the following expression,
where c is the speed of light, σe is the stimulated emission cross-section, σesa is ESA cross-section, ηq is the pump quantum efficiency, Ppump is the pump photon rate, and Nl is the laser photon density, respectively. Though a part of the ions lying in 4G7/2 decay to the ground level through a radiative process, most of them decay non-radiatively through the internal energy conversion. To simplify the model, we assumed that the radative decay rate of 4G7/2 level is negligible. And then, the fractional thermal loading with laser action is given by,
where λp is the pump wavelength and λl is the laser wavelength, respectively. Thus, the ratio of the fractional thermal loading between with and without laser action, α, is given by,
Substituting the physical parameters given in references [7,13,18,19], the ratios for Nd:YVO4 and Nd:GdVO4 crystals were estimated to be 2.0 and 2.2, respectively. These values are in good agreement with those measured from the thermal lens powers with and without laser action. These show that ESA for 1.3 µm laser photon should significantly increase the fractional thermal loading and as little as 40 % of the absorbed power can contribute effectively to the laser output. These results are in good agreement with the experimental slope efficiency obtained previously by 1.3 µm laser based on the Nd doped vanadates.
We have investigated the fractional thermal loading in Nd:YVO4 and Nd:GdVO4 crystals with the 1.3 µm laser action by thermal lens measurement using an interferometric method. Above threshold, the fractional thermal loading with laser action increased by a factor of 2 for both crystals. The spectroscopic measurement of the visible fluorescence from the pumped crystals confirms that the increase in the thermal loading is due to excited state absorption.
References and links
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