Non-linear frequency conversion is the go-to method for generating colors not available from conventional laser systems. Recent work from Tel Aviv University has shown that, whereas crystals with uniform poling periods convert over only a narrow frequency band, computer generated holograms (CGHs) can be used to define a poling pattern that phase matches over a wide spectrum of wavelengths of one's choosing. They alternatively showed that, by appropriately tiling uniform quasi-phase-matched poling periods on top of one another, one can generate several discrete, widely spaced wavelengths simultaneously. Both these methods have been demonstrated in PP-KTP and PPLN to upconvert C-band light via SFG or SHG processes.
In this paper, a collaboration between the National Optical Institute in Florence, Italy, and the Israeli group, the authors experimentally show that both the CGH and quasi-periodic poling methods work for DFG in MgO:CLN to generate a variety of mid-IR spectra over <4.3 µm to 4.7 µm (a fingerprint region for several greenhouse gas isotopes). Using a CW Ti:sapphire pump, a 1 µm frequency comb signal (generated from a Ti:sapphire pumped PCF, spectrally spliced and amplified in Yb-doped fiber), and one of six poling patterns generated by either CGH or quasi-periodic tiling, the authors generate a variety of mid-IR spectra (broad continuum or pairs of discrete lines, both widely and closely spaced) that roughly match their designs. Preliminary measurements indicate that, when generating discrete lines, their beat note is narrow band, and so they maintain their phase relationship and are well-suited for cavity-enhanced spectroscopy. The efficiency of the process is low (they use W’s of pump and signal to generate µW’s of idler, which can be improved), but their method shows a promising route to generating phase- and amplitude-tailored mid-IR spectra.
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