Using a femtosecond Ti:Sapphire laser, micro-tracks of material damage were written into Yb:YAG crystals. Waveguiding was achieved in a channel between pairs of tracks with guiding losses of 1.3 dB/cm at a wavelength of 1063 nm, due to a stress induced change of the refractive index. Pumped at a wavelength of 941 nm, highly efficient laser oscillation in a Yb:YAG channel waveguide at a wavelength of 1030 nm was demonstrated. An output power of 0.8 W at 1.2 W of launched pump power was achieved, resulting in a record slope efficiency of 75%.
©2010 Optical Society of America
Due to nonlinear absorption processes , femtosecond lasers are suitable for the fabrication of three dimensional micro structures in dielectric materials. By focusing the laser pulses inside a glass sample, which is moved transversally, the fabrication of channel waveguides and more complex passive devices has been reported [2, 3, 4, 5]. Using rare-earth doped glasses, laser-oscillation with an output power up to 102mW and a maximum slope efficiency of 17% was obtained [6, 7].
The fabrication of waveguides with the same technique was also successful in different crystalline materials [8, 9, 10, 11, 12]. Using fs-laser inscription efficient waveguide-lasers were realized in Neodymium doped Y3Al5O12-crystals (Nd:YAG) and ceramics with output powers up to 1.3W and slope efficiencies as high as 60% [13, 14, 15].
Yb-doped YAG crystals are interesting for realizing ultra compact, and highly efficient waveguide lasers due to the relatively high emission cross-sections, long fluorescence lifetime of the upper laser level, low quantum defect, good thermal conductivity, and high mechanical stability. Recently Bain et al. demonstrated ultrafast laser inscribed diode-pumped Yb:KGd(WO4)2 and Yb:KY(WO4)2 channel waveguide lasers with a maximum slope efficiency of 14% and a maximum output power of 19mW .
In this paper we report on the fabrication of fs-laser written waveguides in Yb:YAG and present to the best of our knowledge the first fs-written waveguide laser in this material. With a slope efficiency of 75% and a maximum output power of nearly 0.8W this is the most efficient fs-written waveguide laser demonstrated so far.
2. Waveguide fabrication
Channel waveguides were produced in undoped YAG and Yb:YAG (7% Yb doping with respect to the Y-sites) crystals with typical dimensions of (10×15×3)mm3. Laser pulses delivered by a femtosecond laser system (Clark-MRX CPA-2010) with a wavelength of 775 nm, a pulse duration of 150 fs, pulse energies up to 1 mJ, and a repetition rate of 1 kHz were focused 300 µm below the polished surface of the crystals using a 50× microscope objective with a numerical aperture (NA) of 0.65.
During the writing process, the crystals were moved transversally with respect to the incident fs-laser pulses by a motorized translation stage (miCos HPS-170) with a velocity of 10 µm/s. The threshold for a material modification is around 1 µJ for this setup. Different pairs of parallel tracks were inscribed with an optimal pulse energy of about 1.3 µJ. The distance between two adjacent tracks was between 15 µm and 34 µm. A more detailed discussion about the optimal writing parameters can be found in . The polarization of the laser pulses had neither influence on the properties of the tracks nor on the waveguiding. The diameter of the focal spot in air was measured to be 2 µm (at 1/e2) by imaging the focus onto the sensor of a CCD-camera.
The tracks had a length of 9.25mm after polishing the end faces of the crystals.
3. Characterization of the fabricated structures
Figure 1(a) shows a bright-field microscope image of a pair of tracks with a distance of 26 µm written in a Yb:YAG crystal. The end facet of the crystal is shown in Fig. 1(c). Each track is around 25 µm in height and 5 µm in width. In both images the formation of cracks during the writing process can be observed. In Fig. 1(b) a microscope image obtained between crossed polarizers of the same pair of tracks as presented in Fig. 1(a) is shown. The tracks themselves exhibit strong birefringence as indicated by the white-blue color in the image. To obtain information about birefringence between two adjacent tracks a thin sample was cut from an undoped structured YAG-crystal and polished mechanically to a thickness of 45 µm. Figure 1(d) shows a microscope image of this sample obtained between crossed polarizers. Strong birefringence can be observed between the double-track structure.
The formation of cracks and the occurrence of strong birefringence are the result of stress caused by the written tracks. Due to the elasto-optical effect, local changes of the refractive index in the stress affected area occur. This results in regions with a local maximum of the refractive index, where waveguiding is possible. A detailed description of the underlying guiding effects can be found in .
Channel-waveguide experiments were performed by coupling a linear polarized Nd:YVO laser operating at a wavelength of 1063 nm with a lens (f = 25 mm) into a waveguide of the Yb:YAG crystal. The backside of the crystal was imaged by a 40× microscope objective (NA = 0.65) onto the sensor of a CCD camera. The near field intensity profile of the guided mode between two adjacent tracks is shown in Fig. 2. The nearly circular mode with a diameter of 15.8 µm (at 1/e2) is Gaussian as a fit to the intensity profile prooves. By rotating the polarization direction of the Nd:YVO laser with a λ/2-waveplate and measuring the transmitted light intensity using a CCD-camera, it was revealed, that only light polarized parallel to the y-axis (as defined in Fig. 2) was guided. The extinction ratio was higher than 256:1.
By determining the angle of aperture Θm of the guided mode in the far-field and assuming a step-index profile, the refractive index change was estimated using the equation
with the refractive index n of the bulk material. For a measured far-field angular aperture of Θm = 3.4° the refractive index change was around Δn ≈ 1 · 10−3.
Waveguiding was also possible in different channels surrounding single tracks and outside the pair structure, but in contrast to the waveguide between a pair of tracks, the intensity profile of the guided mode was not circular and Gaussian anymore.
Waveguide losses were measured by coupling the Nd:YVO laser at a wavelength of 1063 nm into a single-mode, polarization maintaining, large mode-area photonic crystal fiber with a NA of 0.06 and a core diameter of d = 16 µm. At this wavelength absorption of light in the Yb:YAG waveguide is negligible. When butt-coupling this fiber to the waveguide, the transmitted power could be determined. To achieve minimal transmission losses of the fs-laser written waveguides, the guided light at the exit face of the fiber was polarized parallel to the y-axis. The coupling efficiency was calculated using the overlap integral
where Ψ1 and Ψ2 present the electrical field distribution of the fiber mode and the waveguide mode, respectively. A coupling efficiency of ηk = 92% was calculated. Considering the Fresnel reflections of 8.4% at each end facet of the crystal, waveguide losses were found to be about 1.3 dB/cm at 1063 nm. This values are in the same order of magnitude of the waveguide losses for a fs-laser written Cr4+:YAG waveguide (1.5 dB/cm)  as well as a channel waveguide fabricated in a Nd:YAG ceramic (0.6 dB/cm) . Minimal losses for fs-laser written waveguides in glasses are around 0.05 dB/cm .
Due to the perfect overlap between pump- and laser-mode and the strong confinement of the laser-mode, high gain is obtained in the active waveguide. Hence, the relatively high internal losses of 1.3 dB/cm do not have a strong influence on the laser characteristics and the waveguide laser is even operating without any mirrors, as shown in the next section.
4. Laser experiments
Laser experiments were performed by coupling the light of a cw-Ti:Sapphire laser operating at a wavelength of 941 nm into the Yb:YAG waveguide with a f = 25mm lens. The coupling efficiency for this setup was calculated using Eq. 2. By measuring the intensity profile of the focal spot of the pump laser and assuming that the profile of the guided mode at a wavelength of 941 nm was similar to the profile of the guided laser mode at 1030 nm (see Fig. 5(b)), a coupling efficiency of ηk = 92%±7% was determined. A λ/2-waveplate and a polarizer were used to control the polarization and the power of the pump laser beam. Additionally a mirror high-reflective (HR) at a wavelength of 1030 nm and anti-reflective (AR) for the pump laser was inserted between polarizer and the coupling lens to separate laser and pump beam. Thus, the power of the waveguide laser at both sides could be measured (Fig. 3).
Laser oscillation at a center wavelength of 1030 nm was achieved without mirrors, but with the Fresnel reflection of about 9% at each facet of the waveguide. Thus, the total output coupling was about 99%. Best laser performance was achieved for a pair of tracks with a separation of 26 µm. The laser delivered a maximum combined output power of 765mW at 1240mW±90mW of launched pump power. The slope efficiency was 75%±5% with respect to launched pump power (Fig. 4) and 68% with respect to incident pump power. The maximum output power was limited only by the maximum available pump power. The Yb:YAG-crystal was not actively cooled, but due to the high thermal conductivity and the low quantum defect, no thermal effects occurred at maximum pump power.
The laser power emitted at each side of the waveguide was not equal. Depending on the coupling conditions, the ratio of the output powers P out,2/P out,1 varied from 2.85 to 1.1 at nearly constant combined output power. Further detailed investigations of this effect are necessary.
In Fig. 5(a) a representative spectrum of the laser emission at an output power of 435mW is shown. The laser oscillated in multiple longitudinal modes. The spectral width of the emission was 0.37 nm (FWHM).
The guided laser mode had a nearly Gaussian intensity profile (Fig. 5(b)) with a diameter of 16 µm (at 1/e2), which is very similar to the guided mode in Fig. 2. The light of the laser was polarized parallel to the y-axis, as expected from the waveguide experiments.
We determined the beam quality factor M 2 of the waveguide laser by measuring the beam radius (at 1/e2) of the focused laser beam at different positions around the beam waist using a CCD-camera. At a combined output power of the waveguide laser of 86mW, a fit to the measured beam radii resulted in an nearly diffraction limited beam with an M 2 of 1.1±0.1 and 1.2±0.1 in the x- and y-axis, respectively.
5. Summary and conclusion
A fs-laser has been used to write pairs of tracks in undoped and Yb-doped YAG crystals. The tracks are the result of a destruction of the crystalline lattice in the focus of the fs-pulses. Stress-induced birefringence was observed between and in the surroundings of the written pairs of tracks. Due to the resulting change of the refractive index, waveguiding could be demonstrated with guiding losses of 1.3 dB/cm at a wavelength of 1063 nm. These losses could be reduced by optimizing the writing parameters, i.e. pulse duration, repetition rate, the depth of the focal spot below the surface or by writing more complex three-dimensional structures. The near-field of the guided mode is nearly Gaussian with a diameter of 15.8 µm. Laser oscillation at a wavelength of 1030 nm with an outcoupling rate of 99% was achieved with a maximum output of 765mW at 1240mW of launched pump power. The slope efficiency was 75%. Experiments with mirrors coated directly to the end facets of the crystal to realize different outcoupling transmissions are in progress. For a miniaturization of the waveguide laser-system the Ti:Sapphire-laser as pump source will be replaced by laser diodes. Furthermore, single mode operation might be realized in future using direct fs-laser written Bragg gratings .
The Yb:YAG crystals were provided by FEE GmbH, Germany. This work was supported by the Deutsche Forschungsgemeinschaft (Graduate School 1355) and the Joachim Herz Stiftung.
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