Potassium titanyl phosphate (KTP) is a widely used frequency-doubling crystal for realizing high-efficiency laser sources from near infrared to visible wavelengths through phase matching (PM) and quasi-phase matching (QPM) configurations . Optical waveguides are basic photonic components which confine the light propagation in very small volumes with dimensions of micron or sub-micrometer scales. Swift heavy-ion irradiation (with energies not less than 1 MeV/amu) has emerged to be a very powerful technique for waveguide fabrication in various materials .
In this work, the KTP sample with size of 6.0×3.1×1.7mm3 was cut along the direction optimal for second harmonic generation (SHG) of 532 nm from the fundamental wavelength of 1064 nm (θ = 90°, φ = 23.5°) under the Type II phase matching configuration, i.e., eω + oω → e2ω. Firstly, the sample was irradiated by 17 MeV O5+ ions at fluence of 1.5×1015 ions/cm2 on one surface (6.0×1.7 mm2) through the 3MV tandem accelerator, forming a planar waveguide layer with thickness of ~8 μm beneath the sample surface. Afterwards, a diamond rotating blade on top of the irradiated planar waveguide surface moving in the direction parallel to the blade was used to construct air grooves with depth of 12 μm. The rotating speed and cutting velocity were set to 20.000 rpm and 0.2 mm/s, respectively. As a result, a ridge waveguide with 20-μm width and 8-μm depth was formed by controlling the distance of two adjacent grooves.
An end-face coupling system was arranged to realize SHG through KTP ridge waveguide. We utilized 1064 nm pulsed wave laser as the fundamental light source. The pulsed laser beam (80 µJ pulses with duration of 11.05 ns at a repetition of 5 kHz) was coupled into the KTP waveguide using a convex lens. The generated SH laser beam of 532 nm from the waveguide exit facet was captured by using a 20× microscope objective and characterized by a CCD camera, a spectrometer, and a power meter. We can find that the fundamental wave at 1064 nm was with TE polarization and the generated wave at 532 nm exhibited TM polarization, which was in agreement with the bulk Type II (eω+oω→e2ω) 1064 nm→532 nm of KTP crystal. The maximum SH peak power obtained was 17.6 W, when the 1064 nm pump power was 73 W, resulting in a conversion efficiency of ηpulsed=24.1%. Compared with the reported value of Nd:GdCOB ridge waveguide , the conversion efficiency of KTP ridge waveguide is significantly higher. These results imply potential applications for O5+ ion irradiated ridge KTP waveguide to be an efficient frequency-doubling device.
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