Femto-second laser writing was used to fabricate waveguides in a z-cut KTP sample with losses below 0.8 dB/cm. They were used for efficient, broad bandwidth, Type II birefringent second harmonic generation to the green. The temperature and wavelength bandwidth were, 28⁰C∙cm and 0.85 nm∙cm, respectively.
©2012 Optical Society of America
Potassium titanyl phosphate KTiOPO4 (KTP) is a primary material for nonlinear optics thanks to a number of favorable properties like large nonlinear optical coefficients, wide transparency, good mechanical properties and strong resistance to visible light . Its extraordinary wide phasematching bandwidth for noncritical Type II frequency doubling with fundamental wavelengths around 1 μm has made it the material of choice for most green laser. Waveguides of high quality have been fabricated in KTP by ion-exchange in nitrate melts containing Rb+-, Sr+- or Cs+-ions . With an additional divalent ion, like Ba2+, in the melt spontaneous domain reversal of the ion-exchanged region can be obtained, which has enabled fabrication of segmented waveguides which are very efficient quasi-phase matched frequency converters . However, there is a reproducibility problem with ion-exchanged KTP waveguides as the diffusion coefficients tend to vary between samples, a property closely related to the stoichiometry of the crystal . Another problem for these waveguides has been that under high power excitation the optical field could drive the ion-exchange, i.e. modify the mode profile during operation . These two properties, reproducibility and stability, has limited the more frequent use of ion-exchanged KTP waveguides. An alternative ways of fabricating waveguides in KTP is by ion-implantation, but it is a rather cumbersome method and the losses have been much higher than for the ion-exchanged ones, particularly in the case of channel waveguides . On the other hand laser-written waveguides are stable, provide circular modes and have potentially low loss. This type of waveguide has been fabricated in periodically poled KTP (PPKTP) and used for efficient frequency doubling using fs- and ps-laser excitation [6,7]. However, in both cases the losses were significantly higher than for the ion-exchange waveguides, the phasematching bandwidth was narrow and the normalized conversion efficiency was far from what should be theoretically possible.
In this paper we use a two line laser waveguide writing procedure, which provide low loss, circular single-mode waveguides in KTP. These were evaluated by Type II frequency doubling and provided broadband phasematching and close to the theoretically expected efficiency, i.e. with an undamaged crystal structure in the guiding region and a maintained nonlinearity.
2. Laser-written waveguides
Three dimensional micro-structures can be fabricated in several dielectrics utilizing nonlinear absorption processes with focused femtosecond lasers . Stress-induced changes of the refractive index can be used for waveguiding and channel waveguides can simply be made by focusing the laser radiation inside the sample, while it is translated. Straight waveguides as well as more complex passive structures as splitters and couplers have been made this way [9–12]. In crystalline materials, like KTP, the refractive index change occurs due to stress-induced birefringence and/or lattice modification close to the focus region. Waveguiding can then, depending on the material and the writing condition, either occur in the modified region (Type I waveguides) or adjacent to it (Type II waveguides) . In the latter case writing of two closely placed tracks leads to good confinement as well as low loss .
Active devices like lasers and amplifiers have been obtained in rare-earth doped glasses , and also in different crystalline laser hosts like Neodymium doped Y3Al5O12-crystals (Nd:YAG) and Nd:YAG ceramics. Utilizing the two trace writing technique obtaining low loss waveguides, has made it possible to obtain laser emission with high slope efficiencies (> 60%) and output power (>1.3 W) . Recently even better results were obtained with Yb:YAG fs-written waveguides where a slope efficiency of 75% and an output power of nearly 0.8W was demonstrated [16,17]. For nonlinear optical application laser written waveguides have, besides in KTP, been made in lithium niobate (LN), periodically poled lithium niobate (PPLN) [18,19] and yttrium aluminum borate .
3. Waveguide fabrication
A set of Type II, x-propagating, channel waveguides were produced in a z-cut flux grown KTP sample with the dimensions (10 × 5 × 1) mm3. Laser pulses from an amplified 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 150 μm below the polished surface of the crystals using an aspheric lens (NA = 0.55, f = 4.5 mm). During the writing process, the crystals were translated transversally with respect to the incident fs-laser pulses by a motorized stage (Aerotech ABL 1000) with a velocity of 25 μm/s. The pulse energy was varied between 0.4 μJ and 4 μJ, and the threshold for material modification in this setup was approximately 0.6 μJ. 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.
Several different pairs of parallel tracks were inscribed in the sample and the distance between two adjacent tracks was changed between 16 μm and 25 μm to obtain waveguides with different width. A pulse energy of 2.5 μJ gave both good confinement and low loss, which was then used for the waveguides described below. After track writing the end faces were polished and the final length, l, of the sample became 9.5 mm.
4. Waveguide characterization
The laser written KTP waveguides provided guiding next to the damaged region, caused by a stress induced refractive index change. When two tracks of damage were written into the crystal, as can be seen in Fig. 1 , guiding could be obtained in six positions around the tracks where the strongest confinement was obtained in the two regions between the tracks. The damaged region was in this case approximately 21 μm in height and 3 μm in width. Due to the elasto-optic effect, a local change of the refractive index occurred in the stress affected area, and in the regions with an increased refractive index wave-guiding was possible. A detailed description of the underlying guiding effects can be found in .
The modal properties depend on the track separation, as well as the amount of induced damage, i.e. refractive index modification. Micro-cracks were in some cases seen below the tracks. Therefore our work concentrated on the upper central waveguide for each pair of tracks. With a track separation of 22 μm, or less, we obtained a fundamental transverse mode operation for both TE and TM polarization and an almost circular intensity distribution. Light from a HeNe laser at 633 nm was focused into the waveguides using an f = 25 mm lens, and the output was imaged onto a CCD camera with a microscope objective. In Fig. 2 the near field pattern for two different waveguides can be seen. In the left picture the track separation was 18 μm and the mode profile very close to circular. The numerical aperture of the waveguide was, N.A. = 0.03. In the right picture the track separation was 27 μm and the mode confinement poorer with deeper penetration of the field into the bulk.
The waveguide losses were quite low for all the waveguides. A conservative upper limit for the loss for the single-mode waveguides was estimated by coupling the HeNe laser beam through the waveguide and comparing the transmitted and the launched power. Taking the Fresnel losses at the two sample facets under consideration, but not the incoupling losses, gave a power loss below 0.8 dB/cm for both the TE and the TM mode.
5. Second harmonic generation experiments
A waveguide aligned along the x-axis in KTP can be used for Type II second harmonic generation (SHG). The condition for birefringent phasematching is obtained as,21]. This gives a bulk phasematching fundamental wavelength, λF, of just above 1080 nm at room temperature. The index increase for these stress-induced waveguides is small, below 10−3 as estimated from the numerical aperture, which means that the waveguide phasematching should be obtained close to this wavelength. It is a wavelength region where the Ti-sapphire laser has quite low power, and therefore we constructed a polarized, tunable, Yb-fiber laser to be used as the source at the fundamental wavelength. The laser spectrum was stabilized with a volume Bragg grating in a configuration similar to what Jelger et al. previously used . It was tunable from 1030 nm – 1100 nm, with a beam quality, M2 < 1.35, and a linewidth, Δλ ≈0.2 nm. For the SHG experiments the power was controlled with a polarizer and a waveplate and launched with a 45 degree polarization angle to the z-axis for phasematching, utilizing the d24 coefficient of KTP. First the properties for bulk noncritical Type II phasematching were determined. A f = 50 mm lens was used to get close to confocal focusing in the crystal and the fundamental and SH radiation were separated with a dichroic mirror on the output before the power was measured using calibrated power meters. The sample was temperature controlled with a Peltier element. At room temperature the phasematching was obtained at 1079.26 nm with temperature and wavelength bandwidths (FWHM) of 29.8 ⁰C and 0.91 nm, respectively. For 1.0 W of launched fundamental power 3.3 mW of SHG was generated, corresponding to a conversion efficiency of η = 0.33%, or normalized ηnorm = 0.35%/Wcm. The phasematching wavelength could be temperature tuned with 0.032 nm/⁰C. This results are in very close agreement with previous published data on phasematching temperature and bandwidth . We deduce a nonlinear coefficient, d24 = 2.35 pm/V, which also agrees with previously published data .
In the following the waveguides were investigated, and in particular those that were guiding single-mode, i.e. with track distance of 22 μm or less. A 10x microscope objective was used to simultaneously excite the TE00 and TM00 modes. The radiation on the output was collected with a 20x objective, filtered and imaged on a calibrated power meter. The SH power and phasematching wavelength did not change much between the different waveguides which indicates that the laser writing was reproducible and left a homogeneous and undamaged waveguide region. The highest SH output power was 1.31 mW, obtained in a waveguide where the tracks were separated by 22 μm, for a launched IR power of 126 mW at 1080.82 nm. It corresponds to normalized conversion efficiencies of, η = 8.25%/W or η = 9.1%/Wcm2. This is a 25 times increase in efficiency over the bulk SHG. The temperature and wavelength tuning curves were almost identical to the bulk tuning curves with bandwidths of 29.7 ⁰C and 0.90 nm, respectively, which within measurement errors are the same as for the bulk. A comparison of normalized efficiency and bandwidth with previous demonstrations of SHG in laser written waveguides in LN , PPLN  and PPKTP  is shown in Table 1 . Normally quasi-phase matched (QPM) SHG in PPLN and PPKTP exploiting the d33 nonlinearity gives considerably higher efficiency than birefringent phasematching. However, here we obtained higher efficiency with birefringent phasematching than previously have been obtained with QPM, and in fact both the highest efficiency and the widest bandwidth for SHG so far obtained in laser written waveguides. The high efficiency is attributed to the fine quality of the waveguide and unperturbed nonlinearity, while the wide bandwidth is a material property of KTP when used in noncritical type II phasematching.
6. Conclusions and outlook
Femto-second laser writing was used to fabricate single-mode waveguides along the x-axis in a 9.5 mm long z-cut KTP sample with losses below 0.8 dB/cm at 633 nm. The waveguides were used for type II birefringent SHG and 1.31 mW of green light was generated with 126 mW of fundamental power at 1080.82 nm, corresponding to a normalized conversion efficiency of 9.1%/Wcm2. The temperature and wavelength bandwidth were, 29.7 ⁰C and 0.90 nm, respectively. This is practically identically with the numbers for bulk SHG which demonstrates that the waveguide quality is excellent. The SHG efficiency is 50 times higher than previous results on laser written waveguides in KTP. The broad SHG bandwidth is another attractive feature of these waveguides, which also was larger than in previous work.
Future work should include attempts to get higher confinement by inducing larger refractive index change through laser writing. That would result in a smaller overlap area, Aovl, between the fundamental and SH modes, which will increase the efficiency, as the waveguide over bulk conversion efficiency scales as ;25]. Finally, by using this laser writing technique for PPKTP would enable even more efficient frequency conversion, flexible phasematching throughout the transparence region of KTP and other attractive features of quasi-phase matching.
The authors would like acknowledge technical support from Hanna Al-Maawali and financial support from the Linnaeus Center ADOPT financed by the Swedish Science Council, the Deutsche Forschungsgemeinschaft (Graduate School 1355) and the Joachim Herz Stiftung.
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