High Power SESAM Mode-Locked Laser Based on Yb:YAlO3 Bulk Crystal

Yttrium aluminium perovskite YAlO3 (YAP) crystal, doped with rare-earth ions, has been extensively studied as a diode-pumped laser host material. The wide interest to rare-earth ions doped YAP crystals is explained by its good thermal and mechanical properties, high natural birefringence, widely used Czochralski growth method. The aim of this work was to study the Yb:YAlO3 crystal as an active medium for high power mode-locked laser. Yb-doped perovskite-like aluminate crystals have unique spectroscopic and thermooptical properties that allowed using these crystals as an active medium of high power continuous wave (CW) and modelocked (ML) bulk lasers with diode pumping. In our work spectroscopic properties of Yb:YAP crystal and laser characteristics in CW and ML regimes are investigated. Maximum output power of 4 W with optical-to-optical efficiency of 16.3 % and 140 fs pulse duration have been obtained for Yb:YAP E //c-polarization with 10 % output coupler transmittance. Tunability range as wide as 67 nm confirms high promise of using Yb:YAP crystal for lasers working in wide spectral range.

The wide interest to rare-earth ions doped YAP crystals is explained by its good thermal and mechanical properties similar to those of YAG, but growing faster and anisotropic [9]. Previously thermal conductivity of undoped YAP crystal was reported to be close to 11 W/[m·K] [9,10]. Lately more modest values of 7-8 W/[m·K] were published [11]. Nevertheless they remain two times higher then those of tungstate crystals [12]. YAlO 3 is a biaxial crystal and belongs to orthorhombic space group [9]. Its high natural birefringence dominates thermally induced one in lasers and leads to overcoming depolarization losses at high average powers [13]. It results in a very high polarization degree of the laser emission under different levels of pump power which is advantageous for non-linear frequency conversion [14], efficient modulation loss in Q-switch lasers [6] and other applications where linearly polarized light is necessary.
In this work we present the experimental study results of high power passively mode-locked laser based on yttrium aluminium perovskite crystal doped with Yb 3+ ions.

Crystal growth
Single crystals of Yb:YAP can be grown by several growth techniques [15-20] among which Czochralski is the most usable for practical applications.
For this study Yb:YAP crystals were grown by Czochralski method using Y 2 O 3 , Yb 2 O 3 and crystalline sapphire as starting oxides of at least 99.99 % purity and seeds oriented along the b axis. The melts were corresponding to stoichiometric compositions Y 1-x Yb x AlO 3 (x = 0-0.03). The growth melts (x = 0-0.03) were held in iridium crucibles (40 × 5 × 40 mm 3 ) and inductively heated under a pure Ar atmosphere. The pulling and rotation rates were 2.5 mm/h and 35 rpm. The grown boules are 15 mm in diameter and 30-40 mm long, transparent but some of them of a yellow-brown shade.

Spectroscopy
Polarized absorption spectra of Yb 3+ (2 at.%):YAP (corresponding ytterbium concentration was 4.02 × 10 20 cm −3 ) at room temperature were registered by a Varian CARY-5000 spectrophotometer. Absor-ption cross-section spectra for three light polarizations parallel to the a, b and c crystallographic axes are shown in Figure 1. It is well known that radiation trapping strongly affects the measured lifetime of Yb-doped materials because of significant overlap of the absorption and emission bands [21,22]. The comparatively high index of refraction of YAP (n c = 1.914 for λ = 1040 nm) also increases the probability of reabsorption even in optically thin samples because of the total internal reflection. Thus the special methods discussed in the literature [21,22] should be used to determine the  Figure 2). Starting from certain powder content, the lifetime remained constant despite further dilution (Figure 3), thus indicating that reabsorption effects became negligible. Emission lifetime for 8, 3, 2 and 1.5 at.% Yb-doped crystals was measured to be about 510 ± 20 µs that indicates a weak influence of the luminescence concentration quenching. Presented values are in good agreement with the previously obtained data [23].
The stimulated-emission (SE) cross sections were calculated by use of the modified reciprocity method in which it is not necessary to know the Stark level structure of the Yb 3+ manifolds ( 2 F 5/2 and 2 F 7/2 ) [24]: where τ rad is the radiation lifetime of an active center; c is the light velocity; h and k are Planck and Boltzmann constants, respectively; T is the crystal temperature; n is the refractive index of a crystal; α and β denote the polarization state; and σ ABS is the ground-state absorption cross section. The SE cross section spectra calculated with this method are presented in Figure 4.  The most intensive SE cross-section band at 999.2 nm has peak value of about 3.13 × 10 −20 cm 2 for E//c-polarization. Such a high value is very suitable for mode-locked and actively Q-switched laser operation.

Continuous wave laser experiment
For laser operation the most interesting polarization states in the crystal are E//c and E//b (с and b are crystallographic axes) due to high stimulated-emission cross sections values.
For a continuous wave laser experiments a set up with X-folded cavity design was used (see Figure 5). It consisted of two curved mirrors M1 and M2 and two plane mirrors: OC and HR. The calculated TEM 00 mode diameter in the crystal was about 100 μm. As a pump source, a multiple single emitter InGaAs fiber-coupled laser diode (Ø105 μm, NA = 0.15) with a maximum output power of about 25 W was used. An "off-axis" pump layout was used for longitudinal pumping of the active element (see Figure 5). This pump arrangement was successfully tested in our previous work [25][26] and the main advantage of such a pump scheme is that all the cavity mirrors have highly reflecting coating at 900-1100 nm. The pump light was formed by a set of lenses into the spot with a diameter of about 100 μm (1∕e 2 ). A 2 mm long Yb 3+ (2 at.%):YAlO 3 crystal was used as a gain medium. The crystal was a-cut to provide E//b and E//c polarized laser output. It was a slab with dimensions 2(a) × 5(b) × 1.5(c) mm 3 ; both 5 × 2 mm 2 lateral faces were maintained at 15 °C by means of copper plates (indium foil was used to improve thermal contact) and thermo-electrical cooling elements with water-cooled heat sink, while 1.5 × 5 mm 2 working faces were antireflection coated for pump and laser radiation.
The dependencies of the laser output power on the absorbed pump power for E//b-and E//cpolarized outputs and different OCs are shown in Figure 6. Absorbed pump power was real-time measured during the laser action. The maximum CW output power of 7.6 W at absorbed pump power of 13.6 W with slope efficiency of 64.2 % was demonstrated for E//c polarization with 5 % OC transmittance (Figure 6a). With output coupler transmission of 10 % and 20 % the laser output power slightly decreased to 7.3 W and 6.0 W, respectively, while the corresponding slope efficiencies increased to 76.7 % and 75.3 %. Similar output powers were demonstrated for E//b laser output (Figure 6b). With 10 % output coupler transmittance 5.9 W of output power was obtained at 11.7 W of absorbed pump power with 60.5 %

Mode-locked laser experiment
For the mode-locked laser experiment the same crystal was used as for CW one. Schematic of the experimental setup is shown in Figure 8. InGaAs-based SESAM with modulation depth of about 4.0 % was used in the experiments. The SESAM based on quantum wells separated by nano-structured barriers was grown by molecular beam epitaxy (MBE) technique over the semi-insulating GaAs substrate of (001) orientation. The crystallinity of each layer was controlled via reflection of high energy electrons diffraction (RHEED technique). The number of quantum wells, their thickness and the concentration of the ternary alloy were chosen to match the requirement on the saturable absorption modulation depth. The recovery time shortening was performed by the barriers separation into the thinner layers via the insertion narrow band gap material. The design of the SESAM described in [27]. The measured reflectivity spectrum of the SESAM is presented in Figure 9. Used SESAM enabled to support modelocking in the spectral range from 1000 nm to about 1050 nm. The result of the pump-probe testing of the SESAM with modulation depth of 4 % is shown in Figure 10. The saturation energy fluence of the SESAM was measured to be about 70-120 µJ/cm 2 . Pulses with 8 nm (see Figure 11) full width at half maximum (FWHM) obtained at 1009.7 nm central wavelength resulting in 140 fs pulse duration (see Figure 12) with time-bandwidth product of about 0.32 assuming Sech 2 pulse shape.

Conclusion
In conclusion, Yb:YAP bulk crystal as a gain medium for high power mode-locked lasers was investigated in our work. Maximum output power of 4 W with optical-to-optical efficiency of 16.3 % and 140 fs pulse duration have been obtained for Yb:YAP E//c-polarization with 10 % output coupler transmittance. Tunability range as wide as 67 nm confirms high promise of using Yb:YAP crystal for lasers working in wide spectral range.