Broadband quasi-phase-matched wavelength converters

This thesis proposes new concepts in broadband optical frequency conversion in quasi-phase matched guided-wave devices. Quasi-phase matching (QPM) using domain-inverted gratings in nonlinear materials such as lithium niobate (LN) by reversing the sign of the second-order nonlinear coefficient, has been successfully applied in recent years to take advantage of the highest nonlinear coefficient for second harmonic generation (SHG), sum frequency generation (SFG) and difference frequency generation (DFG). Nevertheless, the periodic quasi-phase matched structure has a negative impact by limiting the bandwidth of frequency up-conversion.;The modified single-pass and double-pass configurations are also proposed in this dissertation lead to bandwidth broadening and efficiency enhancement of waveband wavelength converters based on quasi-phase matched cascaded sum frequency generation and difference frequency generation (SFG + DFG) in LN waveguides. It has been shown that increasing the pump wavelength difference in cascaded SFG + DFG devices, enhances the bandwidth, but with noticeable variation in efficiency response for larger pump wavelength differences, leading to uneven equal-input-power signal channel conversion.;Two different solutions have been suggested in this thesis to solve the problem in both single-pass and double-pass configurations, namely, detuning of a pump wavelength, and using a few-section step-chirped grating (SCG). It is shown that, if a pump wavelength is increasingly detuned, the SFG and DFG phase-mismatch decreases differently which leads to flattening of the conversion efficiency response. Alternatively, the period of a few-section SCG can also be increased slightly to arrive at a similar result.;The role of these schemes is very important as wide waveband converters can be used for the wavelength division multiplexing (WDM) systems, in which the 3-dB bandwidth of the device should be broad enough to cover the 1.55 mum optical window. For this purpose, the pumps are set at wavelengths out of the optical window which is 75-nm wide, whilst we obtain flat responses with high conversion efficiency. For different single-pass and double-pass configurations using pump detuning or the SCG, design diagrams for the choice of low-loss waveguide length, and the assignment of pump powers of to achieve the desired efficiency, ripple and bandwidth are presented. (Abstract shortened by UMI.);In this dissertation, it has been demonstrated for the first time that using the technique of apodization with chirped gratings, the bandwidth of frequency doublers based on SHG in lithium niobate waveguides can be broadened at will, and the ripple damped out (< +/-0.05dB) to be low enough so as to be considered almost flat. A consequence of this technique is that the guided wave device may be used as a source over a wide unregulated temperature range, for the first time. It has been shown that using the apodized step-chirped gratings (ASCG) approach, the wide spectral conversion efficiency profile (> 50 nm) is suitably controllable. The large domain-widths and chirp-steps ease device manufacture. The impact of this design is clearly very high as firstly, this broadband wavelength converter can be used for variable waveband wavelength conversion in future all-optical networks, as the device based on the cascaded second harmonic generation and difference frequency generation (SHG + DFG), should be a broadband frequency doubler. Secondly, for displays it may lead to the realization of high-efficiency three-color frequency converters that do not change in intensity, even without the use of temperature control. However, the SHG efficiency of an ASCG device is lower than that of the uniform grating with the same length. It is shown for the first time that it is possible to increase the efficiency of an ASCG device for broadband frequency doubling, by using a tight confinement in small-effective-cross-section low-loss waveguides, and with the resonance of fundamental harmonics in a singly pump-resonance waveguide.