Monolithic fabrication of quasi phase-matched waveguides by femtosecond laser structuring the χ(2) nonlinearity

Abstract

We demonstrate second harmonic generation in quasi phase-matched waveguide structures fabricated by direct laser writing. Circular waveguides are inscribed in z-cut lithium niobate that provide well confined guiding of the fundamental and second harmonic wave. In contrast to classic schemes that employ periodically poled crystals, quasi phase-matching is realized by a laser-induced modulation of the nonlinearity inside the waveguide core. The proposed design allows monolithic integration of buried frequency conversion devices with tailored nonlinear response and excellent compatibility to on-chip optical elements. Second harmonic generation of 1064 nm radiation is demonstrated for different grating periods and associated matching temperatures. A maximum conversion efficiency of 5.72% is obtained for a 6 mm long, laser-induced quasi phase-matching grating.

Local domain inversion in MgO-doped lithium niobate by pyroelectric field-assisted femtosecond laser lithography

“…6 We have used this technique recently to fabricate integrated quasi-phase matched waveguides in LiNbO 3 . 7 The advantage of this method is that the nonlinearity can be modulated in three dimensions (3D). 8 However, due to the weak modulation of the nonlinearity, the conversion efficiency is smaller compared with electrically inverted ferroelectric domain structures.…”

Fabricating waveguide Bragg gratings (WBGs) in bulk materials using ultrashort laser pulses

“…Recently, Kroesen et al demonstrated quasi-phase-matched (QPM) waveguides using the modulation of the χ (2) nonlinearity within the coherence length rather than typical domain inversion of periodically poled lithium niobate [83]. By writing a depressed cladding waveguide and an internal 6.7 mm pitch multiscan grating (Figure 24), the authors were able to demonstrate 5% second harmonic conversion from 1064 to 532 nm.…”

High-repetition-rate femtosecond-laser micromachining of low-loss optical-lattice-like waveguides in lithium niobate

“…In general, fs-laser written waveguides in LN can be categorized into three groups depending on the mechanism of optical guiding: positive-refractive-index(RI)-induced [9][10][11][12][13] , stress-induced [14][15][16][17][18] , and depressed-cladding waveguides 14,[19][20][21][22][23][24] . The focus of this study is on the fabrication and optimization of waveguides of the third type, which present a number of advantages over the first two types, including higher thermal stability, output-profile controllability and low input-polarization dependence.…”