Frequency doubling has been achieved in femtosecond-laserinscribed single-mode waveguides written in two periodically-poled potassium titanyl phosphate crystals. A conversion efficiency of 0.22 %W-1 was obtained for first-order quasi-phase matching at 980 nm and an efficiency of 0.02 %W-1 for third-order quasi-phase matching at 800 nm.
©2007 Optical Society of America
Femtosecond laser inscription has been applied successfully to create waveguides in a variety of optical materials including glasses  and, more recently, active laser media such as Nd:YAG  and Ti:sapphire . The technique also has the potential to create novel and high-efficiency waveguide devices in nonlinear crystals, and in this paper we describe progress towards producing high quality waveguides for second-harmonic generation in periodically-poled potassium titanyl phosphate (PPKTP). Recent studies in lithium niobate (LN) have demonstrated tight confinement of a pump beam along the entire length of a crystal , resulting in very efficient 49 % (0.52 %W-1) birefringent second harmonic generation (SHG) with nanosecond-pulses at a wavelength of 1064nm . The extension to periodicallypoled lithium niobate (PPLN) was also recently demonstrated with a reported SHG efficiency of 0.006 %W-1 from a low power continuous-wave (CW) 1563 nm laser . Here we present significantly improved results for a quasi-phasematched (QPM) interaction by fabricating waveguides in PPKTP using a femtosecond laser.
Waveguide creation in KTP was first demonstrated in 1987 by ion exchange with univalent ions . By incorporation of Ba2+ ions in the exchange melt domain reversal could be obtained when the waveguide is fabricated with segmentation at the c-side . With these waveguides it was possible to obtain quasi-phasematched frequency conversion [8, 9], and 107 mW of frequency doubled blue light was demonstrated only limited by the available pump power [10, 11]. However, the waveguides degraded with time at high visible generation which was attributed to diffusion of the ions forming the waveguide . In contrast to the ion exchange fabrication process femtosecond-waveguide inscription has the unique ability to write the guides at a considerable depth below the surface of the sample, thus producing guides that are immune to environmental contamination and damage, removing the necessity for a clean room for fabrication. Furthermore it is reasonable to believe that these waveguides could stand higher optical power as they are not created by ion diffusion.
The waveguides examined in the present study were created in two PPKTP samples fabricated from a 1mm thick c-cut flux-grown KTP wafer and poled for frequency doubling light at wavelengths of 800 nm and 980 nm. The crystal c-face was patterned with a 6.84 µm photoresist grating for the 980 nm (first-order phasematched) sample and a 10.07 µm grating for the 800 nm (third-order phasematched) sample. To avoid any damage to the crystal surface that might degrade the incoming femtosecond laser beam, no metal was deposited on any of the crystal faces. KCl liquid electrodes were used to provide a contact with the external poling circuit. The samples were poled by applying electrical pulses of 2.7 kVmm-1.
2. Waveguide inscription and characterisation
To write the waveguides we used a 5 kHz regenerative amplifier (Spectra Physics Hurricane), a single unit comprising CW and Q-switched pumps for an oscillator and regenerative amplifier respectively. The entire system was Ti:sapphire-based and capable of producing pulses with durations of 130 fs and maximum energies of 200 µJ at a central wavelength of 800 nm. The beam from the amplifier was linearly polarised and was perpendicular to the waveguide writing direction. The beam was focused into the KTP sample surface using a near-infrared-corrected infinite-conjugate long-working-distance microscope objective with a numerical aperture of 0.42. The sample was mounted on computer controlled 3-axis motion control stages, each with a 0.1 µm spatial resolution.
The waveguides were fabricated using a transverse writing geometry 180 µm below the crystal surface by translating the substrate material through the focus of the laser beam perpendicular to both the direction of propagation and polarisation of the laser beam. To control the cross sectional shape of the fabricated waveguide, we used the well known multi-scan fabrication technique [12–14], whereby the desired waveguide cross section is constructed by creating many lines of modified material, each shifted slightly relative to each other. Processing was carried out using a range of pulse energies form 0.6 µJ–2.2 µJ in steps of 0.2 µJ. Waveguides were fabricated using between 1 and 10 scans with an individual scan separation of 0.8 µm. The threshold for altering the material was found to be 1.0 µJ, below which no modification of the crystal structure could be detected. In KTP we found that there was a small energy range between 1.0 µJ to 2.0 µJ which induced the desired positive refractive index change without incurring substantial collateral damage to the area of the material where the waveguides were produced. Previous work using an almost identical NA lens and the multi scan method indicated that a sample translation speed of 10 µm s-1 was ideal for creating waveguides using a 1 kHz repetition frequency femtosecond laser , consequently, all of the waveguides described in this letter were manufactured at a sample translation speed of 50 µm s-1, achieving an equivalent spatial overlap between successive pulses for our 5 kHz repetition frequency laser. It was found that the optimal writing parameters in our case were a pulse energy of 1.4 µJ inscribed with 6 passes of the laser beam. All the results reported in this paper were measured from waveguides inscribed using these parameters. Preliminary investigations indicated that the loss of the waveguides produced in this way was in the region of 1 dB cm-1. A thorough investigation of the coupling and waveguide losses is currently underway.
To investigate the guiding properties of the fabricated waveguides, light from a fibre-pigtailed 980 nm laser diode was coupled into one end of the waveguides using direct waveguide-fibre butt-coupling, the output facet was then imaged onto a CCD camera. An optical microscope image of the end facet of the crystal is shown in Fig. 1(a). The observed square shape of the darkened absorption region arises from the multi-scan inscription technique. Figure 1(b) is a corresponding near field image of the guided 980 nm mode. From our investigations of the guiding properties it was found that guiding was only observed when the absorption region in the material appeared to be damaged under optical microscope inspection. In this case, guiding took place in two locations, above and below the dark damage region (Fig. 1, y-direction). This observation is consistent with guiding that takes place in the highest density region of a local strain field which accompanies the damage.
Figure 2 shows the mode profiles of the 980 nm light carried by a waveguide above the damage region. The full-width half-maximum (FWHM) guided mode diameters in the x and y axis were 8.1 µm and 9.3 µm, respectively. Visually, the mode was circular, but elongated at one side in the y-direction as the stress field diminished further from the absorption region.
3. Waveguide performance and discussion
We investigated the second-harmonic conversion efficiency in the guides using two separate tunable CW Ti:sapphire lasers at 980 nm and 800 nm. The pump laser was butt-coupled into the waveguide via a SMF28 fibre with polarisation control. The unconverted pump radiation was blocked by a filter and the second harmonic light was measured on a power meter. The best efficiency was observed for an 11mm long first-order quasi-phasematched sample pumped at 979 nm with a CW Ti:sapphire laser whose linewidth was 0.06 nm. This waveguide produced 0.20 µW of 489.4 nm light for an average coupled power of 9.5 mW, implying a normalised conversion efficiency of 0.22 % W-1. The bandwidth of the second-harmonic light was 0.3 nm, and Fig. 3 shows the spectrum of the second harmonic light measured using a high resolution optical spectrum analyser. The temperature dependence of the SHG was investigated and found to have a FWHM acceptance bandwidth of 14 °C. We were unable to accurately measure the mode-field diameter of the second-harmonic light but it is reasonable to assume it is similar to the fundamental mode (approximately 8 µm diameter).
The second sample was third-order quasi-phasematched for frequency-doubling of 800 nm light and was 9.5 mm in length. The highest second-harmonic power generated was 51 µW, for a fundamental power of 500 mW coupled through the guide, implying a normalised conversion efficiency of 0.02 % W-1. A lower conversion efficiency is expected for third-order phasematching since the effective nonlinear coefficient for a mth order quasi-phasematched process is 2/mπ of the effective nonlinear coefficient for perfect phasematching . For a third-order interaction we therefore expect a ninefold reduction in conversion efficiency compared to an equivalent first-order quasi-phasematched process. Applying this correction factor to the 800 nm results implies a conversion efficiency of 0.18 % W-1 should be achievable in first-order at 800 nm. This predicted value is in close agreement with the independently measured value of 0.22 % W-1 obtained for first-order phasematching at 979 nm. In all calculations of loss the Fresnel reflections have been taken into account.
The conversion efficiencies obtained experimentally are still significantly below the values that have been achieved in waveguides manufactured within PPKTP by other methods [9–11], but further progress should be possible by optimising the writing geometry and procedure. In comparison with earlier work in PPLN  we have achieved far superior conversion efficiencies which we believe is due to the more circular shape of our guided modes, producing better confinement and input coupling of the fundamental light. Our waveguides were produced within the stress field created by damaging one point along the length of the crystal, and the resulting waveguide is situated close to the damaged region which may disrupt the periodically-inverted crystal domain structure and also introduce optical loss. In contrast, the dual-damage-point method of  produces a waveguiding region that is physically separated from both damaged regions, and this may result in lower losses and better conversion efficiency.
Frequency doubling in a femtosecond-inscribed PPKTP waveguide has been demonstrated for the first time. The waveguide performance indicates conversion efficiencies which are lower than those expected theoretically, implying that the periodically-inverted domain structure may have been disrupted by the waveguide writing process. Nevertheless, our results show a thirty six-fold efficiency increase compared with earlier work in PPLN and suggest that further improvement in conversion efficiency should be achievable by optimising the writing process.
References and links
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