We investigate the performance of a magnesium-oxide-doped periodically poled lithium niobate crystal (MgO:PPLN) in an optical parametric oscillator (OPO) synchronouslypumped by 530nm, 20ps, 230MHz pulses with an average power of up to 2W from a frequency-doubled, gain-switched laser diode seed and a multi-stage Yb:fiber amplifier system. The OPO produces ~165mW (signal, 845nm) and ~107mW (idler, 1421nm) of average power for ~1W of pump power and can be tuned from ~800nm to 900nm (signal) and 1.28µm to 1.54µm (idler). Observations of photo-refraction and green-induced infrared absorption (GRIIRA) in different operational regimes of the MgO:PPLN OPO are described and the role of peak intensity and average power are investigated, both with the aim to find the optimal operating regime for pulsed systems.

A direct UV-written multimode interference device is constructed with a pair of Bragg mirrors that have adaptively manipulated period to minimise excess loss. Excess loss achieved is comparable to that of a regular MMI device.

In any focussed nonlinear interaction the focus induced phase shift, known as the Gouy phase shift, provides an imperfection in phase matching for any linearly invariant material. However, using an appropriately designed quasi-phase matched structure it is theoretically possible to completely compensate for the deleterious effects of the Gouy phase shift, allowing a symmetric frequency response and tighter focussing for higher conversion efficiencies.Nonlinear optics provide an essential source of laser light at wavelengths that are otherwise difficult to obtain. These wavelengths can be created with nonlinear effects such as second harmonic generation (SHG) [1, 2], sum and difference frequency generation and optical parametric oscillation (OPO) [3]. The conversion efficiency of such processes are highly dependent on the fundamental laser power, with harmonic output generally increasing quadratically with the fundamental intensity. The large electric fields required for efficient operation normally require focusing to a tight waist.The effects of focussing laser beams, in particular Gaussian beams, on nonlinear parametric interactions have been examined analytically by Boyd and Kleinman [4] (BK). By analysing the interactions of fundamental and harmonic beams in nonlinear materials under the assumption of no source depletion, they were able to calculate optimal focussing conditions of the laser to give maximal conversion efficiency. This can be interpreted as a balance between tight focusing to give a high intensity beam waist and the need to utilise as much interaction length as possible, with the optimal focussing condition given aswhere L is the nonlinear material length, b is the confocal parameter and z R = πω 2 o n/λ is the Rayleigh range. However, as we will show, this particular condition applies only to a linearly invariant crystal where crystal length and Gouy phase shift are both compensated for by compromises in the beam focus.Here, the Gouy phase shift [5,6] is an inevitable consequence of beam focusing and which limits nonlinear conversion efficiency by preventing perfect phase matching. Its influence can be seen experimentally as an alteration in phase matching conditions between the planewave and optimal focused case, and is most commonly seen as a small temperature or angle tuning shift and increasing asymmetry with tighter focus.In this paper we shall theoretically demonstrate that the domain reversal techniques of quasi-phase matching (QPM) can totally compensate for the phase error resulting from the Gouy phase shift. Our QPM grating designs, which are no longer linearly invariant, allow for higher efficiencies than can be obtained with standard QPM and furthermore require no shift in phase matching under focused conditions when compared to a plane wave. By carrying out an analysis based on the BK approach we are thus able to predict an optimal quasi-phase matching structure and calculate its performance.In our initial study we consider the Gouy phase shift as it relates to Gaussian beams. Un...

Abstract:We demonstrate liquid crystal-based integrated optical devices with >140GHz electrical tuning for potential applications in dynamic optical networks. Bragg wavelength tuning covering five 25GHz WDM channel spacings has been achieved with 170V (peak-to-peak) sinusoidal voltages applied across electro-patterned ITO-covered glass electrodes placed 60µm apart. This tunability range was limited only by the initial grating strength and supply voltage level. We also observed two distinct threshold behaviors that manifest during increase of supply voltage, resulting in a hysteresis in the tuning curve for both TE and TM input light.

Liquid crystal tunable Bragg Gratings defined in planar substrates via a laser patterning technique exhibit complex wavelength tuning. This tuning displays threshold points and hysteresis. These tuning features are shown to be a manifestation of physical processes occurring in the confined geometry of our tunable devices. Such physical processes include the formation and removal of line disclinations and an associated wall. We discuss the effect of walls in the liquid crystal with regards to voltage tuning characteristics and whether they may allow faster wavelength tuning.