Point-Spread Function Engineering in Upconversion Imaging

We demonstrate an upconversion based 4-fimaging system and investigate how its point-spread function can be altered by spatially manipulating the amplitude and/or phase profiles of the otherwise Gaussian mixing field. This is accomplished by imaging different amplitude and phase masks illuminated by the mixing field into the Fourier plane of the imaging system where nonlinear sum frequency mixing occurs. A bulk lithium niobate crystal is used as the nonlinear medium and an Er-Tm fiber laser is used to generate the mixing and the signal fields at 1550 nm and 1877 nm, respectively. Object information is probed by the 1877 nm signal beam and upconverted to form an image at 849 nm, which allows for image acquisitioning using an inexpensive room-temperature Si camera.