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Segmented growth of monoclinic Yb:KY(WO4)2 on KY(WO4)2 substrates was successfully implemented and its excellent laser performance demonstrated. High slope efficiencies up to 80% and an output power of 375 mW were achieved under Ti:sapphire laser pumping in the continuous-wave regime. In the passively mode-locked regime, pulses as short as 99 fs with an average output power of 69 mW were obtained.
©2007 Optical Society of America
1. Introduction
Yb-doped materials are well suited for building simple and robust diode-pumped solid-state lasers in the 1-µm spectral range. The low temperature monoclinic phases of the potassium double tungstates KY(WO4)2 (KYW), KGd(WO4)2 (KGdW), and KLu(WO4)2 (KLuW) are well known host materials for doping with active rare-earth ions [1–2]. Their main advantage, partly due to the strong anisotropy of these biaxial crystals, is the very high values of the absorption and emission cross sections. With ytterbium doping, they exhibit an absorption maximum near 981 nm with a cross section about 15 times larger than that of Yb:YAG. Furthermore, the relatively broad linewidths reduce the requirements placed on the pump source and in particular on the laser diodes, and offer a high potential to achieve tunable generation and ultrashort pulses.
These characteristics distinguish the monoclinic double tungstates as exceptional candidates for power scaling in continuous-wave (CW) laser operation and in particular for ultrashort pulse generation. In recent years, these materials were intensively studied for Ybdoping, see [3] for review of the results. Most popular appears to be KYW as a crystal with more mature technology for which raw materials can be obtained with highest purity and at lower price. The highest powers both in the CW and in the mode-locked regime were obtained with this material, using the thin disk concept: an output power of 72 W in the CW mode was reported for an optical-to-optical efficiency of 60% [4] and an average power of 22 W was reached for a pulse duration of 240 fs with a mode-locked Yb:KYW laser [5].
The above results were achieved with single crystals. It is, however, well known that composite materials can significantly improve the laser performance at high power levels [6]. This is in particular true for Yb-doped laser media whose behavior is temperature dependent due to their quasi-three-level laser character. The addition of an undoped layer to the active crystal is mainly used for management of the thermal effects. This undoped cap acts as a heat sink for the active crystal, reducing the peak temperature at the input face (with respect to the pump beam) and thus the thermal lensing effect. Such structures also help to reduce the bulging of the surface of the thermally loaded layer; a compressive strain is maintained on the heated surface reducing the risk of fracture which is also important for the thin disk laser concept [5]. An additional advantage of an undoped cap is the isolation of any optical coating from the doped part of the crystal which is particularly relevant to dichroic coatings that often exhibit temperature sensitivity degrading their performance. Finally, undoped layers can act as guides for the pump light resulting in efficient and uniform absorption into the gain medium.
Two different approaches, diffusion bonding [7] and liquid phase epitaxy (LPE) [8], have been investigated to produce such composite structures. Diffusion bonded composites have already been demonstrated with various laser materials such as YAG or YVO4. The strong anisotropy of the monoclinic double tungstates, however, prevented so far the successful manufacture of such diffusion bonded crystals. On the contrary, liquid phase epitaxial layers were successfully produced based on both KYW and KLuW monoclinic double tungstates. The first CW laser operation for Yb-doping was demonstrated with a 25-µm thick 20 at. % doped KYW layer on a KYW substrate [9], with an output power of 40 mW. The thickness of such epitaxial layers is, however, limited to few hundred micrometers. Here we report on an alternative method, the segmented growth, a highly flexible technique to manufacture thick composite elements. The growth of high quality Yb:KYW/KYW segments and the laser performance of such structures, both in the CW and mode-locked regimes will be presented.
2. Segmented growth
Segmented crystal growth is an alternative method developed to obtain composite single crystals [10] in which two or more segments of bulk single crystalline material are grown on top of each other. In the simplest case of two segments, the resulting crystal can be used for the fabrication of composite optical elements with two parts of different concentration of dopants if these elements are core drilled across the interface between the segments.
Segmented growth of Yb:KYW on undoped KYW was performed in a standard top-seeded solution growth setup. The first-grown segment could be either doped or undoped. The seed orientation was along the monoclinic b-axis, thus yielding a flat growth interface. This fact is important for the growth of the second segment which had to start on this interface. The two segments were grown from solutions with different Yb concentration (in this case 0 and 13 at. %). The 13 mol% Yb concentration in the solution (with respect to the total Y+Yb amount), should amount to ≈10 mol% Yb in the crystal, if one takes into account an empirically determined segregation coefficient of 0.8 for Yb in KYW. The starting materials used for the crystals grown at FEE for the present study were all of purity grades equal to or better than 99.99%. The pulling rates were between 0.05 and 0.1 mm/h. The resulting crystals had typical lengths of 3 to 6 mm for the undoped and 5 to 10 mm for the doped parts, Fig. 1. Crack-free boules with 10×10 mm2 cross-sections were obtained. At present, the interface still shows some inclusions, the bulk material is of comparable quality to standard top-seeded grown samples. The observed densities of inclusions are in the range 500–1000 cm-2, a value much higher than in comparable bonded parts made of more conventional (and isotropic) materials such as YAG. Their origin in KYW is at present still unknown. Segmented growth is thus capable to provide composites with segment length of several millimeters.
Fig. 1. Segmented grown Yb:KYW/KYW crystal. The top, undoped KYW segment is grown first. The second KYW segment is 13 at. % Yb-doped. Scale grid: 10 mm.
For the laser experiments, the (010) face (i.e. the face normal to the crystallographic b axis which coincides with the principal optical axis Np) of the grown Yb:KYW crystal segment was polished down to 200 µm with high optical quality. The undoped segment was subsequently polished parallel to give a total composite structure thickness of 1.4 mm.
3. Laser experiments
For the laser investigations, the crystal was placed under Brewster angle between the two curved mirrors (radius of curvature RC=-10 cm) of an astigmatically compensated Z-type cavity similar to that described in previous work [2]. A Ti:sapphire laser which emitted up to 1.8 W at 981 nm was used as a pump source. A 62.8-mm lens was applied to focus the pump beam through one of the folding mirrors onto the active sample. The pump spot had a Gaussian waist of 35-µm fitting well the parameters of the resonator. Such beam sizes allow avoiding the inclusions in the interface. The Yb:KYW/KYW sample was mounted on a copper holder without active cooling. It was subsequently oriented for pump and laser polarization parallel to the Nm principal optical axis.
3.1 Continuous-wave operation
It was possible to obtain CW laser operation for output coupler transmission (TOC) between 1% and 10% with laser wavelengths (λL) between 1031 and 1024 nm, respectively. The laser performance of the segmented grown Yb:KYW/KYW composite crystal is presented in Fig. 2(a) and summarized in Table 1. A maximum output power of 375 mW was achieved for an absorbed power of 591 mW and TOC=3%. As no saturation in the output power was observed, further power scaling should be possible. The maximum slope efficiency (η=80%) with respect to the absorbed power was obtained for TOC=5%. The corresponding optical-to-optical efficiency with respect to the absorbed power was 68%. The thresholds for the different output couplers ranged from 139 mW down to only 52 mW of absorbed power for the 1% output coupler.
Fig. 2. CW output power versus absorbed pump power of the segmented grown Yb:KYW/KYW laser for different output coupler transmission TOC (a), and single pass absorption versus incident pump power (b).
The high slope efficiency together with the very low thresholds obtained with the segmented grown Yb:KYW/KYW composite crystal is an evidence of its high quality as well as the excellent quality of the interface between the two segments, at least those parts which are free of inclusions. Moreover, these results were achieved without active cooling which points out the improved thermal management due to the undoped KYW segment.
Table 1. Threshold (Pth) and slope efficiency (η) with respect to the absorbed power, optical-tooptical efficiency (ηopt), maximum output power (Pout), and laser wavelength (λL) for different output coupler transmissions (TOC) of the Yb:KYW/KYW laser.
The absorption was measured in the lasing and non-lasing state, as shown in Fig. 2(b). The measurement reveals strong bleaching effect in the non-lasing state as expected from the quasi-three-level nature of ytterbium lasers. The absorption decreases from 57% for low input power down to 18% for maximum incident power. Under lasing conditions, however, the strong bleaching is counterbalanced by the recycling effect, which leads to a decrease in the absorption from 57% to only 50% for the 1% output coupler.
The tunability of the Yb:KYW/KYW laser was investigated by inserting a Lyot filter inside the cavity. At an incident pump power of 1.4 W, laser oscillation with an output coupler of 1% was obtained for wavelengths from 1007 to 1063 nm, as shown in Fig. 3. The corresponding FWHM was 39 nm. The broad tunability is an indication of the high potential of segmented grown Yb:KYW/KYW crystals for short pulse generation.
3.2 Mode-locked operation
Good thermal management due to the undoped segment is also favorable for the mode-locking regime as this supports higher inversion in the active layer and therefore the generation of shorter pulse durations. A composite structure with two segments having almost identical refractive index is also advantageous to avoid any parasitic reflections or birefringence effects that could strongly affect the femtosecond regime [11].
For dispersion compensation, two SF10 prisms were inserted in the cavity arm containing the output coupler. In the other arm, an additional focusing mirror (RC=-15 cm) was added in order to increase the pulse fluence on the semiconductor saturable absorber mirror (SAM), which was used for passive mode-locking. The SAM was grown on a 2” GaAs substrate by metal-organic vapor phase epitaxy (MOVPE). The structure consists of a Bragg mirror comprising 25-AlAs/GaAs quarter-wave layer pairs. The high reflectivity band of the Bragg mirror with R>99% extends from 990 to 1080 nm. The absorbing part is a single InGaAs quantum well embedded in a GaAs layer. The top GaAs layer has a thickness of only 2 nm. The surface-SAM relies on the acceleration of the saturable absorber relaxation by the surface states without introducing internal defects. The measured pump-probe curve fitted by a double exponential reveals a very fast interband relaxation time of only 1.0 ps. These results indicate that enhanced tunneling into surface states occurs. The modulation depth of this surface-SAM was estimated to be less than 1%.
Fig. 4. Autocorrelation trace and spectrum (inset) of the shortest pulses obtained with the Yb:KYW/KYW laser (a), and output power of the laser below and above the mode-locking threshold versus incident pump power (b).
The shortest pulses obtained with the Yb:KYW/KYW laser using an output coupler of 1% had a FWHM of τ=99 fs with an average power of 69 mW at a repetition rate of 92 MHz, Fig. 4(a). The corresponding output spectrum was centered at 1029 nm and had a bandwidth (FWHM) of 12.6 nm. The related time-bandwidth product was 0.353, which is close to the Fourier limit for a sech2-pulse shape. The achieved pulse durations correspond to the shortest pulses for a SAM mode-locked Yb:KYW oscillator. Using Yb:KYW single crystals, shorter pulses of 71 fs were obtained only with Kerr-Lens mode-locking [12] which is usually not self-starting and requires critical alignment. For SAM mode-locked lasers, previous work reported pulse durations of 101 fs with bulk KYW [13] and sub-100 fs pulses (τ=81 fs) were achieved with Yb:KLuW single crystals [2]. Composite crystals were also investigated for short pulse generation. In particular, pulse durations of 114 fs were obtained with a 100 µm thick, 12 at. % epitaxial Yb:KLuW/KLuW laser [2]. Diode-pumped hetero-composite structures based on the uniaxial Yb:SYS/YAG produced 100 fs pulses for an average power of 70 mW [14]. Related to these works the pulse duration achieved here corresponds also to the shortest pulses obtained for composite structures based on Yb-doped laser media.
The slope efficiency with respect to the input power was investigated with an alignment approximately giving the shortest pulses, Fig. 4(b). CW operation was obtained for incident pump powers below 680 mW. Above this level, the laser switched to the mode-locked regime. Additional CW peaks were observed in the spectrum near the mode-locking threshold but then, when increasing the input power, mode-locked operation was stable and no tendencies for Q-switching were observed. In general, pulses tended to shorter durations when the incident power was increased reaching an optimum around 1.4 W, also for τ=99 fs. The slope efficiencies in the CW and mode-locked regimes calculated with respect to the incident pump power were comparable, Fig. 4(b). Changing to output coupling of 3%, the average output power increased to 164 mW but the pulse durations increased, τ=130 fs.
Fig. 5. Tunability of the mode-locked Yb:KYW/KYW laser with TOC=1% (a), and the corresponding pulse durations and average output powers for different central wavelengths (b).
The tunability was investigated using a slit placed between the second prism and the output coupler. Stable mode-locking was achieved for central wavelengths from 1023 to 1043 nm, with sub-200 fs pulse duration for almost the entire tuning range, Fig. 5.
4. Summary
Segmented growth, a promising method to grow thick composite crystals, was successfully demonstrated for the strongly anisotropic monoclinic double tungstates. The grown composite crystals had a typical thickness of several millimeters for the undoped and Yb-doped KYW segments. The use of a 1.4-mm thick composite containing a 200-µm thick 13 at. % Yb-doped KYW segment resulted in a very high laser slope efficiency of 80% and a low threshold of only 52 mW of absorbed power which is indicative of the very high quality of the composite crystal and the interface. The laser was tunable in the CW regime from 1007 to 1063 nm, and in the femtosecond regime, pulses as short as 99 fs were obtained with a SAM. The high performance of the segmented grown Yb:KYW/KYW is an evidence that there are no principle limitations originating from the composite nature of the active crystal even for monoclinic symmetry which makes us confident that superior performance can be expected in the near future from high-power short pulse laser systems based on this material.
Acknowledgment
This work was supported by the EU project DT-CRYS, NMP3-CT-2003-505580. We thank M. Zorn and M. Weyers (Ferdinand-Braun-Institut, Berlin, Germany) for providing the SAM.
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