The authors have presented in this paper the fabrication and characterization of double line written type waveguides in c-cut periodically poled potassium titanyl phosphate crystals. The waveguides were fabricated by using a femtosecond laser, and were utilized for second harmonic generation at 1064 nm. Our experiments have shown that single mode propagation was observed at optimal waveguide width of 14.5 µm. And a conversion efficiency of 39.6% can be achieved.
©2008 Optical Society of America
Waveguide fabrication by femtosecond laser pulses, as initially demonstrated in glasses in 1996 , has the unique ability to write the guides at a considerable depth below the surface of the sample, which are immune to environmental contamination and damage. Two types of waveguides have been identified. For multi-scan type waveguide, in which the guiding region is located at the trace of the femtosecond laser focus, and the thermal stability of such waveguide has to be involved, especially when irradiated with high launched input powers. This disadvantage can be overcome in double line written type waveguide, which is created between double writing lines. Much more interest has arisen on femtosecond laser-written lithium niobate (LN) waveguides [2-6], and the extension to periodically poled lithium niobate (PPLN) was also recently demonstrated [7-10]. However, high operating temperature is needed in order to avoid photorefractive effects, and conversion efficiency is limited by the low damage threshold.
Of a number of QPM materials, periodically poled potassium titanyl phosphate (PPKTP) represents an attractive material due to increased resistance to photorefractive damage, high effective nonlinearity and wide transparency. Generally, KTP waveguide was fabricated by ion diffusion , however, high visible degradation has to be involved, which may be enhanced in waveguides written with femtosecond laser pulses. Frequency doubling has been achieved in multi-scan type PPKTP waveguide operated at 980 nm and 800 nm, and the obtained relative low conversion efficiency was possibly due to the disruption of the periodically-inverted domain , which can be preserved in double line written type waveguide process.
In this paper, we describe the fabrication of thermally stable double line written type waveguides in z-cut flux-grown PPKTP crystal utilizing tightly focused femtosecond laser pulses, and the efficient SHG of a low repetition rate Q-switched Nd:YAG laser using quasi-phase matching (QPM) condition. Single mode propagation was observed at optimal waveguide width of 14.5 µm. And the asymmetric spectrum is induced by the nonlinear effect during the pulse propagation along the waveguide. A conversion efficiency of 39.6% has been achieved at a QPM wavelength of 1064 nm at room temperature.
2. Waveguide fabrication and experimental configuration
We used a Ti:sapphire femtosecond laser (HP-Spitfire, Spectr-Physics Inc.) to write the modification into the PPKTP crystal. The laser system operates at a central wavelength of 800 nm with a repetition of 1 kHz, and with the pulses of 50 fs full width at half maximum (FWHM). The maximum pulse energy from the system was 2 mJ. The laser beam was focused into the sample with a 25×microscope objective (NA=0.4) at a certain depth (~325 µm in our work) beneath the surface, and the laser spot size was about 2.5 µm. A CCD (KA-320) detector was used to monitor the focusing condition. The light was linearly polarized and incident along the z axis of the poled sample. Under the fixed focal spot, the sample mounted on a motorized stage was translated with a velocity of 200 µm/s perpendicular to z axis direction using a computer-controlled positioner. A 10-mm-long 1-mm-thick z-cut PPKTP sample was fabricated by the external pulse field poling technique, incorporated a single grating with a period of 9.0 µm for the first-order QPM SHG at 1064 nm. KCl liquid electrode was used to provide a contact with the external poling circuit, and the external poling electrical pulse was 2.0 kV/mm. In order to produce a thermally stable double line written type waveguide , we consecutively wrote pairs of straight lines separated by 14.5 µm in the x direction of the sample. Accordingly, through the variable attenuator, the single-pulse energy was adjusted to about 100 µJ, which is the optimum pulse energy for the waveguide fabrication. The paralleled double lines are written with a separation such that their lateral modification regions overlap in between, therefore symmetric mode profile can be obtained.
The experimental setup of the frequency doubling process from such type of waveguide in PPKTP sample is shown in Fig. 1. A Q-switch Nd:YAG laser (Continuum Surelite II-10) was used to generate 5-ns pulses with a repetition rate of 10 Hz at 1064 nm. The spatial profiles of optical pulses were nearly Gaussian obtained by spatial filtering. The z axis polarized light was coupled into the waveguide by a 10×microscope objective (NA=0.25). Both end faces of the waveguide were polished and coated with high transmissibility films at 532 nm. The second harmonic energy was detected by placement of a glass filter after the collimating lens (f=10 mm) to stop 99.5% of the 1064 nm laser light and allow 90% of the 532 nm laser through.
3. Waveguide performance and discussion
To evaluate the guiding properties of the fabricated waveguide, light from a Nd:YAG 1064 nm was coupled into one end of the waveguide, using a 10×microscope objective (NA=0.25), and the output facet was then imaged onto a visible camera. An optical microscope image of the end facet of the sample is shown in Fig. 1(b). The observed facet shape was formed from the double line written technique. The inset in Fig. 2 is the corresponding near field image of the guided 532 nm mode. From our investigation of the guiding properties it was found that the mode profile was circular, which indicates the desired positive refractive index change was created between the double written lines. Figure 2 shows the spectrum of the harmonic light measured using a high-resolution spectrometer (Ocean Optics). And the full width half maximum (FWHM) is about 1.3 nm. The asymmetric profile is caused by dispersion and nonlinear effects during the pulse trains propagate along the waveguide.
Although a microscope objective was used for realizing mode matching with a waveguide, only partly matching pump power could be coupled into the waveguide, and the rest of the power dissipated. When the filter was removed, spectra of second harmonic wave and pump wave after the waveguide were obtained simultaneously, as shown in Fig. 3. The remaining pump signal is less than one-third that of the second harmonic; i.e., the pump power coupling into the waveguide was efficiently converted into the second harmonic wave. The spectral widths of fundamental and second harmonic wave are 1.4 nm and 1.3 nm respectively when the waveguide was pumped at the pulse energy of 3.18 µJ.
Following the collimation and separation from residual fundamental light, the waveguide SHG was fed into a spectrometer and an optical power meter. At the QPM wavelength of 1064 nm, the pulse energy of fundamental wave was varied and SHG energy was measured as a function of fundamental energy, which is shown in Fig. 4. With the fundamental wave energy increases, the second harmonic energy increases nearly linear. At the pump energy of 3.18 µJ in the waveguide, a second harmonic energy of 1.26 µJ is achieved, yielding maximum conversion efficiency of 39.6%. It is much higher than that of bulk conversion efficiency of 26.1% under the same experimental parameters. The Fresnel reflection loss about 4% of both facets in all calculations has been taken into account.
The conversion efficiencies obtained experimentally are still significantly below the values that have been achieved in PPLN waveguides using double line written method , but further advance should be possible in optimal writing geometry and better confinement mode propagation . For multi-scan type waveguide , the guiding region was produced within the stress field which is situated close to the damaging, and may disrupt the periodically inverted domain structure as a result of lower conversion efficiency, and also introduce optical loss. However, for double line written type waveguide, the guiding location was not directly structured, and the nonlinearity is preserved in this type of waveguide. Therefore, waveguides created on double line written type modifications are promising candidates for three-dimensional nonlinear photonic devices, since they can stand high temperatures and fluences. A thorough investigation of minimizing the effective cross section and reducing waveguide losses in order to achieve higher conversion efficiency are currently underway.
In conclusion, we have demonstrated the maskless non-lithographic fabrication of thermally stable double line written type waveguides in periodically poled potassium titanyl phosphate with tightly focused femtosecond laser pulses. Efficient second harmonic generation has been achieved in such waveguides by using a low repetition rate Q-switch Nd:YAG laser. At the QPM wavelength of 1064 nm, a conversion efficiency of 39.6% has been achieved at room temperature. And single mode propagation was observed at optimal waveguide width of 14.5 µm. The waveguides written with femtosecond laser pulses are promising candidates for three-dimensional nonlinear photonic devices.
This work was supported by the National Natural Science Foundation of China (60677013), the Specialized Research Fund for the Doctoral Program Foundation of Institute of Higher Education of China (20060055021), and the Chinese National Key Basic Research Special Fund (2006CB921703).
References and links
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