  This paper reports the results of a study into highly efficient sum frequency
generation from 792 and 1556 nm wavelength light to 525 nm wavelength light
using either a single or double resonant ring cavity based on a periodically
poled potassium titanyl phosphate crystal (PPKTP). By optimizing the cavity$'$s
parameters, the maximum power achieved for the resultant 525 nm laser was 263
and 373 mW for the single and double resonant cavity, respectively. The
corresponding quantum conversion efficiencies were 8 and 77\% for converting
1556 nm photons to 525 nm photons with the single and double resonant cavity,
respectively. The measured intra-cavity single pass conversion efficiency for
both configurations was about 5\%. The performances of the sum frequency
generation in these two configurations were studied and compared in detail.
This work will provide guidelines for optimizing the generation of sum
frequency generated laser light for a variety of configurations. The high
conversion efficiency achieved in this work will help pave the way for
frequency up-conversion of non-classical quantum states, such as the squeezed
vacuum and single photon states. The proposed green laser source will be used
in our future experiments, which includes a plan to generate two-colour
entangled photon pairs and achieve the frequency down-conversion of single
photons carrying orbital angular momentum.
