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Ytterbium offers a number of advantages as the active ion in solid-state laser crystals, but is hindered by the disadvantages of a three level lasing scheme. Yb3+-doped oxyorthosilicates have emerged in recent years as potentially quasi-four level laser materials. Two such crystals, Yb:GdYSiO5 and Yb:LuYSiO5, are investigated to determine the extent of four-level behavior. It is shown that these crystals demonstrate a significant reduction in the pump intensity required to reach threshold, but still exhibit three-level effects in terms of self-absorption, population inversion, and thermal sensitivity. The important material properties such as the coefficient of thermal expansion and the thermo-optic coefficient are measured.
©2009 Optical Society of America
1. Introduction
Ytterbium-doped laser crystals are of continued interest due to their simple electronic-level structure consisting of only two manifolds. The lack of higher energy parasitic levels reduces the extent of loss mechanisms such as excited-state absorption, upconversion, cross-relaxation and concentration quenching. Additionally the lower intrinsic quantum defect reduces deleterious thermal effects, while the broad absorption bands in the 940 nm to 980 nm range overlap well with high efficiency laser diodes. Also beneficial is the approximately 1 msec fluorescence lifetime of the material, which can increase energy storage for a given pump configuration as well as facilitate increased efficiency during Q-switched operation. However Yb doped gain media are typically three level laser systems with splitting of the fundamental manifold only a few hundreds of cm−1. As a result thermal population of the terminal laser level, especially at or beyond room temperature, causes strong re-absorption of the lasing emission and consequently higher threshold pump intensities [1].
In recent years a variety of Yb-doped oxyorthosilicate gain media have been presented in the literature. The primary advantage of the silicate host is increased splitting of the 2 F 7/2 fundamental manifold due to a strong field coupling characteristic of the Yb3+ ion to the large crystal field strength [2]. Splitting as large as 1067 cm−1 occurs, thereby allowing the material to operate in a quasi-four level laser scheme that reduces the threshold pump intensity as a result of decreased thermal population of the terminal laser level [2,3]. These crystals thereby offer the potential for efficient laser operation at higher temperatures than are typically obtained with Yb:YAG.
Oxyorthosilicate materials have been in use as laser crystal hosts since the 1970’s. Early publication discussed the laser performance of Nd doped Y2SiO5 (YSO) crystals [4]. Subsequent work explored shorter wavelength operation in this same crystal via a ground state depletion scheme [5]. Three level dopants were investigated in the YSO host, including Cr4+ in a cryo-cooled configuration as well as Tm3+ and Yb3+-Er3+ at room temperature [6–8]. In the latter work it was first suggested that the high energy components of the Stark level splitting in the oxyorthosilicate could lead to quasi-four level operation. However, in a direct comparison between Er:YSO and Er:glass, the laser threshold for the YSO crystal was significantly higher [8]. Subsequent work with the YSO crystal highlighted three level behavior, especially for output in the 1000 – 1010 nm spectral range [9], and this crystal continues to be of interest for ultrafast laser experiments [10].
A renewed interest in oxyorthosilicate laser crystals, with particular attention given to quasi-four level operation, began with the doping of Yb3+ into the well known scintillator material Gd2SiO5 (GSO) [9]. GSO offered advantages over YSO in terms of increased thermal conductivity and higher energy Stark splitting, but suffered from problems with cleavage due to its P21/c monoclinic structure. An alloyed Yb:GdYSiO5 (GYSO) crystal was then introduced to combine the benefits of the energy splitting from GSO with the more rigid and isotropic structure of YSO [11]. A second alloyed crystal, Yb:LuYSiO5 (LYSO), followed shortly thereafter that exhibited a larger and more continuous tuning of the output wavelength [12]. Most recently another variation, Yb:Sc2SiO5 (SSO), has been tested, demonstrating advantages for thin-disk applications [13]. In this work we present the results of an experimental analysis of Yb:GYSO and Yb:LYSO with respect to the extent of the four level nature of the lasing output when operated at the long wavelength emission near 1090 nm. This band offers a low pump threshold, large emission cross section (and lowest thermal population compared to the material’s various emission bands [14]. In particular we quantify the pump intensity required to achieve threshold for comparison with values reported for Yb:YAG. We also report a measured value for the temperature dependence of the index of refraction (dn/dT), and the coefficient of thermal expansion (CTE), two parameters necessary for modeling and packaging laser designs based on these materials.
2. Experiment
Samples with a nominal Yb3+ doping level of 5 at.% were obtained from the Shanghai Institute of Optics and Fine Mechanics at the Chinese Academy of Sciences. All samples were 6 mm x 6 mm cross section, with the GYSO crystals having lengths of 10.7 mm and 6.5 mm and the LYSO having lengths of 10.1 mm and 5.0 mm. Optical distortion through all four samples was no greater than 1/8 wave, although the LYSO samples exhibited some slight wedging. The end faces were polished flat and parallel, and were treated with a broadband AR coating centered at 1 µm. The manufacturer did not provide any details regarding the orientation of the crystal axes. A detailed study of the absorption and emission cross-sections, as well as the emission spectra over a temperature range of 35K – 350K, was performed in collaboration with CREOL at the University of Central Florida, (results published separately) [15]. In that effort it was shown that these materials exhibit their strongest emission lines in regions that are self-absorbed, with the effect being more prominent in LYSO (at 978 nm and 1004 nm) than GYSO (at 1031 nm). The observed self-absorption and radiation trapping is indicative of three-level behavior rather than quasi-four level. Both materials demonstrated emission regions where the stimulated emission cross section varies minimally with temperature, suggesting the possibility of improved output stability for applications requiring exposure to a wide range of ambient temperatures, the details of which are included in [15].
The crystals were wrapped in indium foil and mounted in a copper block, which was in turn mounted to a water cooled aluminum cold plate maintained at 20 C. Long pulse and cw lasing were tested using the experimental configuration shown in Fig. 1 . The pump light was a single peaked spectral band centered at 974 nm with a width of 3.5 nm FWHM, corresponding to an absorption cross section for both crystals of approximately 6.5 cm−1 [15]. Pump light was focused to a waist of 1 mm within the laser crystal while the diameter of the fundamental mode of the resonator was on the order of 600 µm. The ratio of the fundamental mode to the pump diameter was therefore less then the desired value of 0.7 – 0.8, causing the output to tend from the fundamental mode to the first order annular multimode when operating at maximum efficiency [16,17]. For pulsed operation the repetition frequency was limited to 2 Hz in order to avoid thermal lensing effects. The pulsed performance, using a nominally 480 µs pump pulse, and CW output is shown in Fig. 2 for both crystals. The per-pulse pump energy and pump power delivered from the fiber was a maximum of 29 mJ and 13 W. The maximum output was obtained with the longer crystals, solely as a consequence of increased absorption of the pump light. Otherwise there were no differences in the behavior between the two crystal lengths.
Fig. 1 Optical schematic of the experimental setup is shown; from right to left…fiber coupled pump light, collimating optic, focusing optic, flat HR mirror, laser crystal, and 0.5 m ROC output coupler. The laser cavity length measured 7.5 cm between mirror faces. Pump light delivered via a 400 µm core fiber.
When pumped, LYSO exhibited a faint blue-green fluorescence similar to that observed in other Yb-doped crystals and attributed to cooperative luminescence [18,19]. GYSO however exhibited a bright blue fluorescence under diode pumping as shown in Fig. 3 . The wavelength of this emission was measured to be centered at 290 nm with a FWHM of 5 nm. The output wavelength from the laser was centered at 1084 nm in LYSO and 1092 nm in GYSO, with a FWHM of 2 nm for both crystals. The longer wavelength output is expected in LYSO, due to the observation of strong self-absorption for the emission regions closer to 1 µm. The longer output from GYSO indicates that self-absorption is still relatively strong in this crystal as well, contrary to what was suggested by fluorescence testing [15], but consistent with previous work in GSO [14].
The threshold and efficiency data for pulsed and CW operation is presented in Table 1 . In both cases LYSO operated with less efficiency and larger threshold pump intensity than GYSO, consistent with similar comparisons between GSO and both LSO and YSO [20]. However both materials exhibited a noticeable bleaching effect during pulsed operation (shown in Fig. 4 ) that was present even with pump intensities increased to 3.2 J/cm2, indicative of a dominant three-level effect. It should be noted though that the threshold pump intensities were significantly lower than values reported for Yb:YAG [1,21].
Table 1. Efficiency and threshold data
The green trace represents the input current pulse to the pump diodes, and the yellow trace is the photodiode sampling the laser output. Three level behavior of the output is evident.
3. CTE and dn/dT
Knowledge of the crystal thermomechanical properties is essential for the development of high power laser systems. Therefore a Fizeau interferometric technique was used to determine the coefficient of thermal expansion by overlaying the reflections of a HeNe laser from both optical faces of the crystal, and heating the crystal to observe the corresponding fringe behavior [22]. Due to the biaxial nature of these materials, our experiment produces an average value of the material properties. Using this method, the number of fringes that move over a given temperature interval can be related to the physical properties of the materials through Eq. (1):
Because the probe beam transmits through the crystal, both the CTE and the dn/dT terms contribute to the total fringe count N that moves past a given reference over an experimental temperature range. Therefore it was necessary to determine dn/dT before a measurement of CTE could be attempted.The dn/dT value was estimated through an analysis of the thermal lensing of the crystal according to Eq. (2):
where ξ is the fractional heat load, Kc is the thermal conductivity, Pabs is the absorbed pump power, and ωpa is the pump spot size averaged over the length of the crystal [23]. Applying this method produced estimates for dn/dT of 6.5 x 10−6 /K and 8.0 x 10−6 /K for GYSO and LYSO, respectively. CTE was then calculated to be 3.3 x 10−6 /K and 2.4 x 10−6 /K, respectively. These parameters are reported for Yb:GYSO and Yb:LYSO for the first time to the best of our knowledge, and are in agreement with the dn/dT value of 7.2 x 10−6 /K reported for the non-alloyed oxyorthosilicate YSO [12].4. Summary
Yb:GYSO and Yb:LYSO offer attractive properties for use in compact diode-pumped lasers such as a long (1 msec) energy storage time, a CTE that compares favorably to Nd:YAG, and a reasonable thermo-optic coefficient. Additionally the broad and generally unstructured absorption bands relax wavelength and temperature control constraints for the pump sources. Combined with broad output bands containing spectral regions exhibiting minimal change in the stimulated emission cross section over temperature, these materials could offer advantages in the design of compact, athermal laser systems.
However the quasi-four level nature of the strong Stark splitting of the energy manifolds was observed only in the pump intensity required to reach the lasing threshold. Three level behavior was evident in the self-absorption suppression of the stronger emission lines near 1 µm as well as the bleaching observed during pulsed operation. High slope efficiency was obtained at room temperature for low duty cycles, but efficiency was significantly lower for CW operation. Therefore even though the threshold was significantly influenced by the quasi-four level effects of the larger splitting, optimum laser designs will still require consideration of the three-level nature of the materials.
Acknowledgements
This effort is based upon work supported by the US Army Night Vision and Electronic Systems Directorate under Contract No. W15P7T-06-D-R401. Any opinions, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the Night Vision and Electronic Systems Directorate.
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