Fan-out periodically poled structures in Rb-doped KTiOPO4 for continuously tunable QPM devices

Abstract

Since the first demonstration of electric field poling in 1993, the use of quasi-phase matching (QPM) technique has gained wide adoption in a multitude of applications. The QPM field today is dominated mainly by the ferroelectric oxide materials from LiNbO3 (LN) and KTiOPO4 (KTP) families, where QPM structures are implemented by the electric field poling technique. While typical QPM devices have a fixed-period, one-dimensional domain grating design, which is the most straightforward to implement, numerous applications require the ability to continuously tune the wavelength over a wider spectral range. For applications where temperature tuning is not desired, a fan-out QPM grating design may be advantageous. The tuning here is performed by transverse translation of the structure in respect to the pump beam, while keeping the crystal temperature constant. While the implementation of fan-out gratings is reasonably well researched in LN, there is a lack of reliable data for KTP isomorphs. Taking into account the high domain growth anisotropy in KTP, an important factor becomes the angle between the domain walls and the b-axis of the crystal. This angle directly affects the quality and dimensions of the QPM device. However, its upper boundary has not been determined to date. In this work we discuss the prospects and limitations of PPKTP devices with fan-out grating designs. We present a fan-out PPRKTP device, where the transverse fan-out rate is 0.5 μm/mm. In an OPO configuration pumped by 532 nm such PPRKTP crystal is able to provide continuously tunable radiation between 0.7 – 2.2 μm.