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

Douglas Hobbs,Sean Mcdaniel,Georgiy Tsoi,Ernest Sabatino,Michael Mirov,Gary Cook,Sergey Mirov,Dmitri Martyshkin,Bruce Macleod

doi:10.1364/ome.7.003377

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/cm2, 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/cm2 for the largest spot size, to 2.1 MW/cm2 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.

Highlights

Progress toward higher power, widely tunable solid-state lasers [1,2] operating over middleinfrared wavelengths in the 2-5μm range, has been slowed by the inconsistent performance, limited choice of materials, and poor reliability of thin-film anti-reflection (AR) coatings (TFARC) [3]
Similar results were found for Cr:zinc selenide (ZnSe) crystals tested at 2940nm, but mixed results were found for pulse testing at 2095nm
In accumulated power continuous wave (CW) damage testing at a wavelength of 1908nm, neither untreated nor AR microstructures (ARMs) treated ZnSe crystals could be damaged after long duration exposures at a maximum system intensity of 28.6 MW/cm2, a value many times higher than typically found with thin-film AR coated ZnSe

Summary

Introduction

Widely tunable solid-state lasers [1,2] operating over middleinfrared (mid-IR) wavelengths in the 2-5μm range, has been slowed by the inconsistent performance, limited choice of materials, and poor reliability of thin-film anti-reflection (AR) coatings (TFARC) [3]. In a previous study [5], the pulsed LiDT of ARMs textures in ZnSe and Cr:ZnSe was evaluated at a wavelength of 2095nm and found to be comparable to well polished material with no AR treatment This level was expected to be many times higher than that achievable with thin-film AR coatings. The CW LiDT of ARMs textures in Cr:ZnSe showed a damage resistance up to the 0.5 MW/cm level at a wavelength of 1940nm In this expanded study, the pulsed and CW LiDT of ARMs textures in ZnSe and Cr:ZnSe was directly compared with the LiDT of broad-band thin-film AR coated ZnSe and Cr:ZnSe at three laser wavelengths in the mid-IR

ARMs textures

Laser system testing

Conclusion