Investigation of mid-IR picosecond image upconversion

Abstract

Imaging and spectroscopy in the mid-infrared (Mid-IR) wavelength region have received considerable attention in recent years. The reason is the high Mid-IR spectral specificity of many gases and complex molecules. In this pilot study we focus on picosecond upconversion imaging exploiting the χ (2) nonlinearity of a bulk lithium niobate crystal as a means to convert the optical Mid-IR signal into the visible wavelength region, thus allowing the use of fast and sensitive silicon based CCD cameras. The picosecond upconversion system is synchronously pumped in order to increase the quantum efficiency, hence allowing for upconversion of faint pulsed Mid-IR light.

Parametric upconversion imaging and its applications

“…Thus, lowering the duty cycle can improve the SNR for upconversion detection as long as the arrival time of the IR pulses are well known [163]. Generally, synchronous pumping is achieved by incorporating a mechanical delay [164], however, recent publication shows that it can be done electronically [165], discarding the need for any complex mechanical/optical alignment.…”

“…Furthermore, the thickness of the ring, i.e. the ΔψIR, varies with its radius, hence the FoV changes with radial position [164].…”

“…Thus resolution (if the crystal size sets the PSF) also differs for these two directions [45], [95]. For a small FoV, going from a monochromatic to a polychromatic illumination, the resolution does not vary significantly [164], however with increasing FoV the image starts to appear blurred at the periphery of the FoV [175] as the spectral acceptance bandwidth increases significantly with the PM angle [69]. A list of reported resolution for upconversion imaging is given at the end of section 7.…”

“…This introduces a magnification error in the image, however, this can be corrected by post-processing or using a "correction plate" [23]. Without rotation, this kind of magnification error has been observed for broadband upconversion imaging [164]. For broadband operation, another type of distortion namely Barrel (or Pincushion) arises.…”