Optimization of gain in traveling-wave optical parametric amplifiers by   tuning the offset between pump- and signal-waist locations
  We present experimental demonstration and modeling of the optimization of a phase-sensitive optical parametric amplifier by tuning the relative position between the pump- and signal-beam waists along the propagation direction. At the optimum position, the pump beam focuses after the signal beam, and this departure from co-located waists increases with increasing pump power. Such optimization leads to more than 3\,dB improvement in the measured de-amplification response of the amplifier. 
Gideon Alon, Oo-Kaw Lim, Amar Bhagwat, Chao-Hsiang Chen, Muthiah   Annamalai, Michael Vasilyev and Prem Kumar
physics.optics