Athermal design for Solc-type f ilter based on PPKTP

Abstract

An athermal design for Solc-type filter based on periodically poled KTP (PPKTP) is proposed. The athermal static phase retardation (ASPR) direction in KTP along which the quasi-phase-matched (QPM) condition is insensitive to the temperature is found. The optimal design for Solc-type filter along ASPR direction is obtained. The coupling theory of QPM linear electro-optic effect is employed to study the polarization coupling in PPKTP. The study results demonstrate that the central passing wavelength of Solc-type filter keep unchanged with an almost 100% output when the temperature varies from 268 to 328 K. OCIS codes: 190.4360, 230.2090, 250.4390. doi: 10.3788/COL201210.S21901. A Solc-type filter based on periodically domain-inverted ferroelectric crystal was extensively studied for its compactness and fine tuning control. In such a Solc-type filter, the central passing wavelength is governed by the condition for quasi-phase-matching (QPM) in linear electro-optic effect, which is related to the poling period and the refractive indices of ordinary- and extraordinaryray [1] . Generally, the central passing wavelength of the filter will shift with the changing of temperature, since the refractive indices of the crystals are sensitive to the temperature. Taking advantage of this property, researchers proposed a tunable Solc-type filter by adjusting the working temperature of the crystal [2−3] . However, in some special applications in which the passing wavelength of the filter is required to be unchanged, a thermostatic container is necessary for such a Solc-type filter. Potassium titanyl phosphate (KTP) is extensively used in optical frequency converters due to a relatively high nonlinearity, excellent mechanical and optical properties, high damage threshold and thermally stable phasematching properties. The KTP crystal is of great important application in second-harmonic generation (SHG) of Nd 3+ based lasers emitting near 1 � m [4] . The available spectrum of KTP has dramatically broaden with the development of material growth and poling technologies, since a desired wavelength light over the whole transparency region of KTP can be achieved using noncritical phase matching in periodically poled KTP (PPKTP) [5−7] . In the mean time, its large linear electrooptic r-coefficients and low dielectric constants make KTP attractive for various electro-optic applications [8,9] . In addition, the higher damage threshold of KTP is advantageous for the output of high power laser. However, one of the primary limitations in the application of KTP as an electro-optical device is its naturalbirefringence, which is sensitive to the temperature. In our previous work, we have found that the athermal static phase retardation (ASPR) direction in KTP, along which the Static Phase Retardation caused by naturalbirefringence is insensitive to the temperature [10] . In periodically domain-inverted ferroelectric crystals, the nonlinear optical coefficients including the electro-optic (EO) coefficients, are also modulated periodically because of the periodically reversed ferroelectric domains. In this letter, we present a Solc-type filter based on PPKTP with the propagating light along the ASPR direction, and the theoretical study demonstrates that the central passing wavelength of Solc-type filter keep unchanged with an almost 100% output when the temperature varies from 298 to 328 K . As well known, KTP is a birefringent crystal, and the static phase retardation between the two independent polarized waves is = 2π