1. Introduction

The interest in Yb-doped laser materials and their utilization in solid-state lasers is unbroken. Due to their low quantum defect, absence of excited-state absorption at the pump and laser wavelengths, or quenching effects, Yb-doped gain media are generally well suited for high-power laser operation. Furthermore, a lot of these materials exhibit a broad emission bandwidth, a long excited-state lifetime, and a strong absorption band around 980 nm, making them ideal candidates for the development of ultrashort pulse, high-energy diode pumped lasers [1, 2]. Various Yb-doped crystals and glasses have been investigated for ultrashort pulse generation in the past [3–5]. But scaling the pulse energy of short-pulse lasers requires an increase in aperture size due to the finite damage fluence of the active elements. Despite their superior thermo-mechanical properties, most crystals still cannot be utilized in high-energy lasers due to the limited available crystal dimensions or insufficient optical quality at large apertures. On the contrary, laser glasses usually show a higher volume scalability potential and constitute a genuine alternative in large scale, high energy systems. So far actively doped phosphate [6, 7] as well as fluoride-phosphate glasses [8] were investigated for use in large scale lasers, showing relatively low thermo-mechanical stability and damage resistance. In contrast, fused silica is comparable or even superior to laser crystals with regard to thermal stress resistance [1]. But Yb-doped silica glasses are traditionally confined to utilization in fiber lasers due to the limited active volumes contrivable through deposition techniques as modified chemical vapor deposition or direct nanoparticle deposition. Recently, different approaches have been investigated to overcome this limitation. An Yb-doped multicomponent glass composition based on lanthanum-alumino-silicate glass has been developed, which could be produced directly out of a glass melt with large active volume and high optical quality [9]. In addition, a powder sinter technology (so called REPUSIL-technology) has been developed to produce Yb-doped fused silica with scalable size [10, 11]. REPUSIL-based fused silica already shows an excellent performance in fiber laser applications [12, 13] and even permits the use as bulk gain material for diode-pumped high-energy lasers. In this paper, we investigate the optical properties and lasing characteristics of highly Yb-doped (up to 0.4 mol%) fused silica in bulk volume manufactured by the REPUSIL method. To our knowledge this is the first laser study of bulk Yb-doped fused silica.

2. Fabrication and characteristics of Yb-doped fused bulk silica glass

The doped glass samples used in the experiments were developed by Heraeus Quarzglas GmbH & Co. KG, Hanau together with the Institute of Photonic Technology, Jena. The starting point for the production of Yb-doped fused bulk silica is a rare earth and aluminum doped suspension of SiO₂ particles. After granulation the material is sintered into rods with a typical size of 15 mm in diameter and 150 mm in length. This approach has the advantage that batch size limitations, which are typical for most of the other doping techniques, can be overcome. Furthermore, the dopant concentration and the refractive index of the doped silica can be set precisely and homogeneously. An additional homogenization step is performed to further smoothen the refractive index distribution of the rod and achieve laser glass with high optical quality. Figure 1 shows the uniform distribution of the oxides across the rod diameter resulting from this fabrication method. There is no certain size limitation for the utilized manufacturing process of Yb-doped bulk silica and therefore, it is possible to upscale the process and batch size to manufacture larger apertures and geometries. Furthermore, these rare earth doped rods may be reshaped mechanically or by a heat forming process. Reshaping of silica is a common process, which makes the production of large aperture slab elements required in high energy laser systems straightforward.

 

Fig. 1 Distribution of Ytterbia and Alumina oxides across the fused silica rod diameter measured by wavelength-dispersive X-ray spectroscopy (WDX).

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Fused silica rods with two different Yb doping levels have been fabricated, containing 0.39 mol% Yb₂O₃ (corresponding to an Yb³⁺ ion density of 1.7 ·10²⁰ cm⁻³) with 1.58 mol% Al₂O₃ and 0.27 mol% Yb₂O₃ (≡ 1.2·10²⁰ cm⁻³) with 1.31 mol% Al₂O₃, respectively, and exhibiting a rod diameter of 14.4 mm and 20 mm, respectively. The refractive index distribution maps (Fig. 2) show a high optical quality with variations in the 10 ppm range inside the 90 % clear aperture confidence region of the interferometric measurement. The residual ring shaped index structures are due to the homogenization process which can be further optimized. For comparison, the obtained optical quality is not far from what can be achieved for established large aperture Yb-doped crystals like CaF₂ or YAG exhibiting refractive index variations down to below 5ppm. The background attenuation of the host material in the laser wavelength region was determined to be lower than 50 dB/km for both specimens, confirming the possibility of obtaining very pure doped fused silica in bulk volume with extremely low scattering losses compared to e. g. Yb:CaF₂ or Yb:YAG crystals.

 

Fig. 2 Refractive index distribution maps of Yb-doped fused bulk silica with 0.39 mol% (left) and 0.27 mol% (right) doping. Below horizontal cross sections are shown, with peak-to-valley and root mean square values given inside 90 % clear aperture.

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Fig. 3 Experimental setup: M1, dichroic flat mirror (HR 1030–1200 nm; AR 800–990 nm); M2, concave mirror (RoC 300 mm); OC, flat output coupler (R=98.6 %); P, Brewster cut prism (SF10); BP, Brewster plate (fused silica); L1/L2, spherical lenses (f₁/f₂=50/75 mm).

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For a general classification different parameters relevant for high energy laser systems have been summarized in Table 1 for Yb-doped fused silica in comparison to other established bulk materials. The upper-state lifetime of the samples was measured to be in the range of 0.82–0.84 ms, corresponding well to values of doped fused silica found in the literature [14]. Regarding the emission cross section, Yb-doped fused silica exhibits a value comparable to that of other low gain materials with a broad amplification bandwidth (indicated by a large tuning range) like CaF₂, FP glass or CAlGO, requiring a large number of extraction passes. On the other hand, e. g. Yb:YAG as a high gain material does not possess the bandwidth necessary for direct ultrashort pulse amplification. Other promising materials (e. g. Yb:KGW) cannot be produced reliably at large apertures. Naturally, the thermal conductivity of glasses is lower than of most crystals, leading to a longer thermal relaxation time. Also, the negative temperature dependence of the refractive index of some materials may lead to a low thermo-optic coefficient reducing the effect of thermal lensing. But increasing dimensions in large scale laser systems ultimately require sophisticated cooling schemes and adaptive amplification setups independent of the actual gain material. On the other hand, fused silica possesses an extremely low thermal expansion coefficient (≈ 1 · 10⁻⁶K⁻¹) leading to a high thermal stress resistance [1], which is favorable for pulsed operation in high energy laser systems.

Table 1. Properties of different Yb-doped materials relevant for high energy laser systems: Δλ - tuning range (95 % power content); Ø - typical aperture available; τ_flou - fluorescence lifetime; σ_em - emission cross section (at 1030 nm); κ - thermal conductivity; dn/dT -temperature coefficient of refractive index; R_T - thermal stress resistance

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3. Laser cavity setup and experiments

Figure 3 illustrates the experimental setup of the V-shaped laser cavity. Various glass samples of the two specimens were prepared, featuring a thickness ranging from 3.5 mm up to 10 mm in order to achieve varied levels of pump light absorption of 75 %, 90 %, and 95 %, respectively. The samples were polished in laser quality and did not exhibit an anti-reflection coating. The fiber coupled laser diode (LUMICS) offered an optical power of up to 6 W at 975 nm out of a 105 μm core and 0.15 numerical aperture fiber. The pump beam was focused into the active material to a spot size (FWHM) of 150 μm. Mirror M1 exhibits a dichroic coating with a high transmission for the pump wavelength at 976 nm and a high reflectivity for the laser wavelength. The laser medium was placed close to M1 and was not actively cooled. To avoid thermally induced distortions the laser medium was pumped in quasi-cw (qcw) operation with a duty cycle of 10 %. The samples were carefully aligned with the facet perpendicular to the optical axis to keep the Fresnel reflections in the cavity in order to minimize losses. Mirror M2 exhibited a radius of curvature of 300 mm. The flat output coupler possessed a reflectivity of 98.4 % (PR 1040±50nm). The two cavity arm lengths were set to 160 mm and 400 mm respectively, exhibiting a seperation angle of 8° on M2. This resulted in a stable confocal resonator with a beam waist of about 95 μm on M1 and 500 μm on the output coupler. In order to achieve a polarized output a fused silica Brewster plate was placed inside the resonator close to the output coupler. Tuning of the laser wavelength could be achieved by inserting a Brewster cut prism (SF10) into the cavity, as indicated by the dashed line. The average optical power was measured with a thermopile detector (Gentec), the spectral traces were measured with an Ocean Optics spectrometer (MayaPro).

With the above described experimental setup we investigated the qcw laser properties of the Yb-doped fused bulk silica glass (Fig. 4). The left plot exemplary shows the lasing performance of a sample with 0.39 mol% doping and 5.3 mm thickness (88 % measured pump absorption), both for unpolarized and polarized output. We could achieve a slope efficiency of more than 63 % for the unpolarized case, resulting in an and resulting optical-to-optical conversion efficiency of 58 % at the highest pump power. For the polarized case the slope efficiency was slightly lower due to additional losses caused by the inserted Brewster plate, but the output was highly linearly polarized with an polarization extinction ratio (PER) of 99.8 % (27 dB). On the right plot the summarized performance in terms of slope efficiency for the different samples is shown. Naturally, the slope efficiency increases with sample thickness as more pump light is absorbed, whereas both doping specimen show comparable performance (black stars). With respect to absorbed pump power the resulting slope efficiencies of all samples slightly spread around 70 % (blue stars). This demonstrates both the uniformity of material quality throughout the volume, and a highly efficient lasing capability comparable to that of fused silica fiber lasers.

 

Fig. 4 qcw laser performance of Yb-doped fused bulk silica glass with 0.39 mol% doping and 5.3 mm thickness (left); summary of laser performance for all samples (right).

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The center wavelength of the free running laser cavity was located around 1070 nm with a spectral width (FWHM) of 8 nm. By inserting a Brewster cut prism (SF10) into the laser cavity and thus introducing chromatic dispersion, wavelength tunability could be achieved by tilting the output coupler. At the highest pump power level the laser had a wide tuning range from 1010 nm to about 1110 nm (Fig. 5, left), originating from the broad spectral emission of Yb-doped fused silica [14] together with low intracavity losses.

 

Fig. 5 Wavelength tuning range of the fused silica laser cavity (left) and corresponding output beam profile (right).

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Fig. 6 Measured spectral gain distribution of 0.39 mol% doped sample with 5.3 mm thickness (blue dots) and numerically calculated gain curves corresponding to different inversion levels (dotted/dashed lines).

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The spatial beam profile of the free running laser output is close to a TEM₀₀ mode with a slight astigmatism due to the folded cavity design (Fig. 5, right). To equally ensure fundamental mode operation over the whole spectral tuning range, a pinhole was inserted into the cavity. This enables the suppression of higher order mode build-up when forcing the laser to the edges of the tuning range inherently comprising a very low overall gain.

4. Gain measurements

To demonstrate the intrinsic amplification bandwidth of Yb-doped fused silica without the influence of additive effects inside a resonant cavity (e. g. wavelength dependent inversion level and response of optical components), we measured the spectral gain distribution by setting up a pulsed double-pass amplifier stage. As pump source we utilized a diode array operating at 976 nm wavelength with up to 11 J pulse energy after homogenization. The square pump beam profile is imaged into the gain medium to a spot size of 6×6 mm² with a maximum pump intensity of 30 kW/cm². As signal source we used the collimated output of a fiber coupled, wavelength-tunable, narrow linewidth laser diode (Toptica DL pro), which was double-passed through the pump region of the fused silica sample. The amplifier output was spectrally separated from residual pump light and imaged onto a photodiode. We set the pump pulse duration to 1 ms in order to achieve a steady-state inversion level. The temporal trace of signal amplification was recorded via an oscilloscope. Figure 6 shows the measured spectral gain distribution for the 0.39 mol% doped sample with 5.3 mm thickness (blue dots) and numerically calculated gain curves corresponding to different inversion levels β = N_excited/N_doped (dotted/dashed lines), utilizing the well-known absorption and emission cross-sections of Yb-doped fused silica [14]. The inversion level of 0.34 corresponds to the estimated value resulting from the actual pump intensity used in the experiment, the traces for 0.1, 0.25 and 0.5 are shown for comparison (dotted lines). The error bars of the measured data characterize the fluctuation in size and position of the signal spot imaged onto the photodiode. This effect occurred due to the development of thermal distortions induced by the intra-pulse heat load in the gain medium during pumping. The calculated thermal relaxation time for this setup is in the range of 2 ms, which is comparable to the pump pulse duration. The influence may be reduced with an improved measurement setup less sensitive to signal pointing instabilities. Nevertheless, the measured data trend fits well to the 0.34 inversion level (dashed line), confirming the fused silica characteristic of broadband gain necessary for ultrashort pulse amplification.

5. Conclusion

In summary, the potential of bulk Yb-doped fused silica for high energy ultrashort pulse systems has been demonstrated. This is due to the combination of ultra-broad emission bandwidth, high optical quality in large volumes and efficient laser operation. In detail, we could show quasi-cw lasing with a maximum optical to optical efficiency of 60 % and slope efficiencies of about 70 % with respect to absorbed pump power. A highly polarized laser output could be achieved without significant loss. The laser cavity could be tuned in a wavelength range of 100 nm. The large amplification bandwidth of fused silica was verified by gain distribution measurements in a double-pass amplifier configuration. Thus, the various physical parameters that are relevant for high energy short pulse operation clearly confirm the attractiveness of this material. Further investigations will be carried out in a regenerative amplifier setup for ultrashort-pulse amplification.