Laser testing of anti-reflection micro-structures fabricated in ZnSe and chromium-ion doped ZnSe laser gain media

Abstract

An expanded study has been completed of the pulsed and continuous wave (CW) laser induced damage threshold (LiDT) of surface relief anti-reflection (AR) microstructures (ARMs) etched in zinc selenide (ZnSe) crystals and chromium-ion-doped (Cr:ZnSe) laser gain media. In multiple-sample per variant testing at wavelengths of 2095 and 2940nm, the pulsed laser damage resistance of ARMs textured ZnSe crystals was found to be equivalent to untreated, as-polished crystals, with an LiDT as much as 5 times that of thin-film AR coated ZnSe crystals. Similar results were found for Cr:ZnSe crystals tested at 2940nm, but mixed results were found for pulse testing at 2095nm. In accumulated power CW damage testing at a wavelength of 1908nm, neither untreated nor ARMs treated ZnSe crystals could be damaged after long duration exposures at a maximum system intensity of 28.6 MW/cm 2 , a value many times higher than typically found with thin-film AR coated ZnSe. For Cr:ZnSe gain media, the CW LiDT was observed to be dependent on the focused beam size at the sample surface, with thresholds for untreated and ARMs-treated Cr:ZnSe being nearly equivalent, ranging from 0.6 MW/cm 2 for the largest spot size, to 2.1 MW/cm 2 for a spot area 4 times smaller. An operational laser test was performed where an ARMs textured Cr:ZnSe crystal operated with higher slope efficiency and 1.5 times the output of a thin-film AR coated crystal in an identical resonator configuration.

Review of Surface Modification Technologies for Mid‐Infrared Antireflection Microstructures Fabrication

“…[ 50 ] The fabricated structures are highly resistant to contamination [ 35 ] and laser damage. [ 15,88 ]…”

“…Moreover, interference in layers of ARC enables low laser‐induced damage threshold (LIDT). [ 15–17 ] Hence, ARC usage in high‐power applications is limited. These shortcomings can only be mitigated by the special design of coating.…”

“…As the core physical principle behind ARM is completely different from that of ARC, ARM is free of the drawbacks that are peculiar to ARC, including susceptibility to angled light incidence [ 29–31 ] and low LIDT. [ 15,32,33 ] The main advantage of an ARM is that it can theoretically achieve almost 100% transmittance covering the entire mid‐IR range. [ 34 ] Other advantages include high durability and resistance to various damages [ 35 ] and contamination as well as self‐cleaning properties.…”

Feature issue introduction: shaping and patterning crystals for optics

“…Surface relief microstructures are efficient for reducing external reflections, as shown by Hobbs et al [5]. An expanded study has been completed of the pulsed and continuous wave (CW) laser induced damage threshold (LiDT) of surface relief anti-reflection (AR) microstructure (ARMs) etched in zinc selenide (ZnSe) crystals and chromium-ion-doped (Cr:ZnSe) laser gain media.…”

“…For Cr:ZnSe gain media, the CW LiDT was observed to be dependent on the focused beam size at the sample surface, with thresholds for untreated and ARMs-treated Cr:ZnSe being nearly equivalent, ranging from 0.6 MW/cm 2 for the largest spot size, to 2.1 MW/cm 2 for a spot area 4 times smaller. An operational laser test was performed where an ARMs textured Cr:ZnSe crystal operated with higher slope efficiency and 1.5 times the output of a thin-film AR coated crystal in an identical resonator configuration [5].…”