Full length articlePerformance of a quantum defect minimized disk laser based on cryogenically cooled Yb:CaF2
Introduction
The heat generated in the active medium of an amplifier or oscillator often affects the laser operation. Temperature-dependent emission and absorption cross sections [1] and distorted beam profiles, e.g. due to stress induced birefringence limit the laser performance [2]. Furthermore, the quantum defect fundamentally limits the minimally achievable heat load in the material during the laser process. The quantum defect is the fraction of the pump photon's energy which is not transferred to the laser photon due to the latter's longer wavelength. It also limits the maximum efficiency of the laser process. Yb3+ is an interesting candidate for a laser ion with minimal quantum defect due to its simple electronic structure. There are only two energy levels, each broadened by the Stark-splitting of the host material. In the most common case of Yttrium-Aluminum-Garnet (YAG) as the host material, it is pumped around 940 nm and emits at 1030 nm or 1050 nm. In these quasi-3-level systems the transition between the lower laser level and the ground level is driven by phonon interactions. Due to the thermal population of the lower laser level at room temperature the pump threshold is rather high compared to real 4-level systems as, e.g. neodymium based lasers. As several experiments have shown, cryocooling significantly enhances the spectral and the thermo-optical properties of Yb-doped laser materials [3], [4].
V. Jambunathan et al. [5] investigated the possibility to pump Yb:YAG at the zero-phonon line (969 nm) at cryogenic temperatures, reducing the quantum defect to about 7%. While that could be realized for temperatures above 120 K, it was not possible at liquid-nitrogen temperature since the absorption cross section's spectral width is smaller than 0.2 nm (FWHM) at 80 K.
To further reduce the quantum defect below 9% which is common for Yb:YAG at room-temperature, calcium fluoride (CaF2) can be used as the host material. Its thermal conductivity at room temperature and 3 mol% doping is 4 W/(m K) [6] i.e. about a factor of two smaller than that of YAG, but its spectral properties are much better suited for operation with a low quantum defect. Fig. 1 shows the effective cross sections of Yb:CaF2 at 80 K temperature for different inversion levels. Here, are the emission and absorption cross sections respectively, and I is the inversion, i.e. the density of Yb-ions in the upper Stark-manifold divided by the doping density.
As it can be seen in Fig. 1, the zero-phonon line of Yb:CaF2 is at 980 nm with a FHWM width of 1.8 nm [7] which enables pumping with a laser diode stabilized by a volume-bragg grating (VBG). Additionally, suitable emission lines of Yb:CaF2 (991 nm, 996 nm and 1030 nm) are closer to the pump wavelength, hence reducing the quantum defect. While the applications for wavelengths in the 996 nm regime are the same as for 1064 nm lasers, the detection with silicon photodiodes is much easier [8]. Nevertheless, cryocooling of the laser material is necessary to suppress the reabsorption at such wavelengths by reducing the thermal population of the lower laser level. A similar approach to operate a Yb:CaF2 laser at low quantum defects was already described by S. Ricaud et al. [9]. An efficiency of 35% output power versus absorbed pump power was achieved with a bulk-laser setup. In [10] N. Ter-Gabrielyan et al. report on a similarly low quantum defect in Er:Sc2O3, but the efficiency was limited to about 44% output power versus absorbed power.
One of the main problems with such a small quantum defect is the separation of the pump and the laser beam because dichroic mirrors require a larger wavelength separation. This can be solved by using a disk laser setup in which pump and laser beams are separated by an angle. Furthermore, due to the quasi three-level behavior of laser ions with small quantum defects, the disk laser scheme is also better suited for these applications. Compared to a bulk laser setup, the higher number of pump passes increases the effective pump intensity and the smaller active volume increases the inversion (at constant absorption). K. S. Wentsch et al. [11] reported on a Yb:CaF2 thin-disk laser with an output power up to 250 W, 47% optical efficiency, and a 92 nm wide tuning range. However, no significant output power was achieved at 996 nm. As they describe, the preparation of disks thinner than 250 µm is very challenging because CaF2 is more brittle than YAG.
Section snippets
Experimental setup
Here, we report on the realization of a cryogenically cooled, Yb:CaF2-based disk laser which could be operated at different laser wavelengths. With the setup depicted in Fig. 2 we were able to reduce the laser's quantum defect down to 1.1% only. To allow for cryogenic cooling of the Yb:CaF2 disk, the whole optical system was placed inside a vacuum chamber to prevent intra-cavity losses from windows. Using a vacuum vessel with a diameter of 16 cm, six double passes of the pump radiation could be
Numerical calculations
We carried out numerical simulations using the COMSOL Multiphysics heat transfer module to calculate the temperature distribution for a given 3D-heat source and a fixed temperature of the cooling finger. The recorded pump-spot profile was used as the heat source and the integral heat generation was set to the measured value (see below). This simulation has shown that the average temperature of the emitting area is about 50 K higher than the heatsink-temperature.
Additionally we performed
Results and discussion
The most efficient operation was achieved using an OC with 1% transmission at 1032 nm. As Fig. 4 shows, an output power of 15.8 W was measured outside the vacuum chamber with a heatsink temperature of 30 K. This corresponds to an optical efficiency of 74% incident pump power to output power including all optics and windows between the OC and the detector. Before the thermal rollover an efficiency of 78% has been achieved. This is to the best of our knowledge the highest efficiency with regard to
Conclusion
We have demonstrated operation of a cryogenically cooled Yb:CaF2 disk laser with a record high optical efficiency of 78% versus incident pump power. The heat generation at the heatsink was as low as 15% of the output power. The different loss mechanisms were discussed. From the comparison with results from other groups [9], [10], [16] we conclude that a disk laser scheme is optimally suited for efficient operation at a small quantum defect. An optical efficiency of 61% was achieved at a quantum
Funding
This work was supported by the European commission's (EC) 7th Framework Programme (LASERLAB-EUROPE, grant number 228334), the Bundesministerium für Bildung und Forschung (BMBF) (grant numbers 03ZIK445 and 03Z1H531), and the European Social Fund (ESF) through Thuringian Ministry of Economy, Employment, and Technology (grant number 2011 FGR 0122).
Acknowledgment
We thank Dr. A. Herrmann from the Otto-Schott-Institute of Materials Research in Jena for the spectral measurements and expertise in searching possible impurities.
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