1. Introduction

In 1996 Davis et al. created refractive index changes in dielectric materials by focusing femtosecond laser pulses into glass samples [1]. Due to the nonlinearity of the underlying absorption process [2] the material can be modified in three dimensions on a micrometer scale. Using this technique active and passive micro-optical devices were realized in various glasses in the last 15 years [3].

In different single-crystals and ceramics highly efficient waveguide lasers were fabricated by the direct writing technique [4–8]. For these lasers a Ti:Sapphire laser system was used for pumping. However, to achieve further miniaturization of the waveguide laser system, pumping with laser diodes is beneficial. Up to now diode pumped waveguide lasers, fabricated in Er,Yb- and Yb-doped glasses, are limited in slope efficiency to 21% and do not exceed a maximum output power of 100 mW [9, 10]. Recently, very efficient Tm:ZBLAN waveguide lasers in the 1.88 μm spectral range were demonstrated with a slope efficiency of 50%. However, the maximum output power was limited to 47 mW [11].

Also diode pumped crystalline waveguide lasers are limited in output power and slope efficiency. The highest slope efficiency of 14% was demonstrated by Bain et al. [12] in an Yb-doped tungstate waveguide laser. Furthermore, about 170 mW of output power were achieved with a fs-laser written Nd:YAG waveguide laser [13].

In this paper we report on an Yb:YAG waveguide laser pumped with different laser sources. By pumping with a distributed bragg reflector tapered diode laser (DBR-TPL) an output power of 2.35 W was demonstrated. This is to the best of our knowledge the highest output power of a fs-laser written waveguide laser. Furthermore, with an optically pumped semiconductor laser as pump source 1.76 W of output power was measured at a slope efficiency of 51%.

2. Fabrication of the waveguide lasers

For the fabrication of channel waveguides 150 fs long pulses with a central wavelength of 775 nm of a 1 kHz repetition rate CPA-laser system (Clark-MRX CPA-2010) were focused about 300 μm below the polished surface of the Yb:YAG crystal, doped with 7% Yb with respect to the Yttrium sites. For focusing a 50× microscope objective (NA = 0.65), whose aperture was completely illuminated, was used. The sample was moved transversally to the incident laser pulses by a motorized translation stage (miCos HPS 170) with a velocity of 10 μm/s. During the writing process tracks of modified material were formed.

In the vicinity of these tracks stress induced birefringence can be observed. The resulting refractive index change in the order of 10⁻³ supports waveguiding in the area surrounding the tracks. Additionally, a reduction of the refractive index within the tracks of modified material was observed in Nd:YAG ceramics and Nd:YAG single crystals [13–15]. By writing pairs of parallel tracks with distances between 15 μm and 34 μm an overlap of the stress distribution of both tracks is achieved [16]. The combination of refractive index reduction within the tracks and stress induced positive refractive index change results in an index distribution, which supports guiding and excellent confinement of the fundamental mode at a wavelength of about 1 μm. Further details about the structuring process and characterization of the waveguides can be found elsewhere [6, 16]. For the laser experiments discussed in this paper a waveguide was used, which was fabricated with a pulse energy of 1.3 μJ. The two adjacent tracks of the pair structure were separated by 26 μm. After polishing the end-facets of the crystal the waveguides had a length of 9.25 mm.

3. Laser experiments

Figure 1 shows a schematic of the setup for the laser experiments. For pumping the waveguide the light of different laser sources was coupled into the waveguide with various lenses/microscope objectives. Due to the properties of each particular pump source different incoupling optics had to be used to achieve a good coupling efficiency. With a λ/2-waveplate and an optical isolator the incident pump power was adjusted. Incident pump power is defined in our setup as the power measured in front of the waveguide corrected by the Fresnel reflection losses at the surface of the crystal. Furthermore, the optical isolator protected the pump source from back reflections. Since in these waveguides only one polarization direction is guided [6], a second λ/2-waveplate had to be used to control the polarization direction of the pump beam for optimal waveguiding conditions.

 

Fig. 1 Schematic of the setup for the laser experiments.

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As demonstrated before, this kind of waveguide laser works without external mirrors [5–8,16]. The Fresnel reflection of about 9% at each end-facet of the waveguide delivers enough feedback to start laser oscillation. Since no external mirrors were used the laser is emitting to both sides. Therefore a dichroitic mirror, which is highly reflective (HR) for the laser wavelength and anti reflective (AR) for the pump wavelength, was placed in front of the incoupling optics to separate pump and laser beam. In all input/output charcteristics the combined output power P _{out, total} = P _{out, 1} + P _{out, 2} is plotted against the incident pump power. Additionally, the near field profile of the waveguide laser mode was imaged onto the chip of a CCD-camera with a microscope objective.

Figure 2(a) shows a comparison of the input/output characteristics of the waveguide laser for two different pump sources. The characteristic obtained by pumping with a single mode laser diode (axcel photonics M9-940-0300), emitting 330 mW of output power at a wavelength of 940 nm is drawn in red. The incoupling optic was a 10× microscope objective with an NA of 0.22. Due to losses at the optical isolator and the other optical elements, a maximum incident pump power of 266 mW was available. The slope efficiency of the waveguide laser was 51% and a maximum output power of 43 mW could be achieved. The laser threshold was as low as 183 mW.

 

Fig. 2 (a) Laser output power as a function of incident pump power: Comparison between Ti:Sapphire (black squares) and single-mode laser diode (red dots) as pump source. (b) near field mode profile of the waveguide laser-mode. The tracks are indicated by the white dotted lines.

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The black curve shows the characteristics of the waveguide laser close to laser threshold by pumping with a Ti:Sapphire laser. The Ti:Sapphire laser was coupled into the waveguide with a lens, which had a focal distance of f = 25 mm. The waveguide laser shows nearly the same behavior as in the diode pumped case. A comparable slope efficiency and laser threshold were measured. This shows, that with both pump sources the same coupling efficiency could be achieved. However, as the inset of Fig. 2(a) shows, the Ti:Sapphire laser delivered a maximum incident pump power of 1.3 W. The slope efficiency, measured over the whole range of pump power, was 68%, which makes clear that the waveguide laser is much more efficient, if it is pumped well above threshold. As result, single mode laser diodes can be coupled into the waveguide with the same efficiency as a Ti:Sapphire laser. However, the waveguide laser was limited in output power and efficiency due to limited available pump power. The output power could be maximized by realizing optimized output coupling transmissions and consequently reducing the threshold.

Figure 2(b) shows the near field intensity profile of the laser mode. The mode is nearly circular with a diameter of about 19 μm and nearly Gaussian as a fit through the profile in x-and y-direction proves.

To achieve higher pump powers two further laser sources were used for pumping the Yb:YAG waveguide. A tunable optically pumped semiconductor laser (OPS) could deliver an incident pump power of up to 3.7 W at a wavelength of 969 nm. In these experiments the pump power was controlled electronically by adjusting the power of the OPS and the light was coupled into the waveguide with a f = 25 mm lens.

The second pump source was a high brightness distributed bragg reflector tapered diode laser (DBR-TPL) [17]. Such a diode laser combines a high optical output power (up to 12 W) with a narrow spectral emission bandwidth (< 12 pm FWHM) and a good beam quality (M ² < 2) in a compact monolithic device. Therefore, applications that require a high spectral brightness benefit from this compact pump source [18, 19]. The most important feature of the DBR-TPL in our experiment is the good beam quality which yields an efficient incoupling of pump light into the waveguide. The DBR-TPL used in our experiment emitted an output power up to 12 W at a wavelength of 970.5 nm with a spectral bandwidth of less than 12 pm (FWHM). However, to avoid surface damage of the crystal the incident pump power was limited to a maximum of about 5.5 W. To achieve this amount of incident pump power the laser diode had to be operated at 7.8 W, due to losses at the optical elements. At this operation point the DBR-TPL shows a nearly diffraction limited main-lobe with an $M_{1/{\text{e}}^2}^2\hspace{0.17em}=\hspace{0.17em}1.1$, containing more than 80% of the overall device power. The incoupling optic was an aspheric f = 18.4 mm lens.

Figure 3(a) shows the OPS-pumped laser characteristics. The laser threshold was determined to be below 200 mW of incident pump power. The waveguide laser delivered a maximum output power of 1.76 W with a slope efficiency of 51%. However, at pump powers above 2.3 W the output power does not increase linearly anymore. This minor roll over can not be explained by thermal effects, since with the DBR-TPL a pump power of 5.5 W was applied without observing any roll over in output power (compare Fig. 3(b)). In fact, the reason for this effect can be found in the decreasing beam quality of the OPS laser with higher output powers. Consequently, less power can be coupled into the waveguide.

 

Fig. 3 (a) Laser output power as a function of incident pump power: Comparison between Ti:Sapphire (black squares) and OPS (blue dots) as pump source. (b) Laser output power as a function of incident pump power with an DBR-TPL as pump laser. Red dots with open aperture, black squares with closed aperture.

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For comparison the waveguide was pumped with a Ti:Sapphire laser at a wavelength of 969 nm. The curve, indicated by the black squares in Fig. 3(a), shows nearly the same behavior as in the case of pumping with the Ti:Sapphire laser at 940 nm (compare inset of Fig. 2(a)).

By pumping the waveguide with the DBR-TPL mentioned above the possibility of fabrication of an integrated miniaturized laser system was demonstrated. The input/output curve is shown in Fig. 3(b), indicated by red dots. A maximum output power of 2.35 W at an incident pump power of 5.52 W could be achieved, which is to the best of our knowledge the highest output power demonstrated for any ultrashort pulse written waveguide laser. However, the higher laser threshold of 0.32 W and the lower slope efficiency of 45% in comparision to the other pump schemes, especially in comparision to the Ti:Sapphire pumped case, show that the complete power of this diode can not be coupled into the waveguide.

To estimate an upper limit of the coupling efficiency an aperture was located in front of the incoupling lens and closed as far as no change in output power at P _{out, 1} and P _{out, 2} was measured. With this setup for spatial filtering of the pump diode side lobes the incident pump power was reduced by 31%. Consequently the coupling efficiency of the DBR-TPL into the waveguide can not be higher than 69%. With this lower values of incident pump power at closed aperture the slope efficiency is increased to 65%. The corresponding input output curve is also shown in Fig. 3(b) (black squares). The corrected slope efficiency is slightly lower than in the Ti:Sapphire pumped case. However, one has to consider the fact, that the wavelength of the DBR-TPL was not optimized for Yb:YAG, where the zero phonon line absorption is located at a wavelength of 968.8 nm. So, at the wavelength of the DBR-TPL of 970.5 nm not more than 95% of pump power was absorbed. Considering the lower absorption at the wavelength of the DBR-TPL one can conclude, that about 70% of the complete power from this laser diode can be coupled into the waveguide with the same efficiency as an Ti:Sapphire laser.

At maximum output power of the waveguide laser an excellent beam quality with an M ² factor of less than 1.3 was measured.

4. Summary and conclusion

Different laser sources were used for pumping a fs-laser written waveguide laser. The light of a single mode laser diode could be coupled into the waveguide as good as a Ti:Sapphire laser, but the waveguide laser was limited in output power and efficiency because of limited available pump power. The output power of this waveguide laser could be optimized by realizing different output coupling transmissions with direct coated end-facets of the waveguide laser.

By OPS pumping an output power of 1.76 W could be achieved at a slope efficiency of 51% regarding incident pump power.

A high power diode pumped waveguide laser device was demonstrated by coupling a DBR-TPL into the waveguide. The waveguide laser delivered a maximum output power of 2.35 W, which is the highest output power measured for any fs-laser written waveguide laser. A miniaturization and integration of this system might be achieved by coupling the light of the DBR-TPL with a micro lens, which is directly attached to the laser diode, into the waveguide.