Deep UV-induced near-infrared photodarkening of Er/Yb-doped and undoped phosphate fibers

Abstract

Photodarkening (PD) of Er/Yb-doped and undoped phosphate fibers caused by pulsed 193 nm irradiation from an ArF excimer laser to form Bragg grating mirrors is investigated. Doped and undoped phosphate fibers exhibit the same level of significant UV-induced PD loss, which is associated with the formation of a color center band at 467 nm. The UV-induced absorption extends into the NIR and creates a loss on the order of 1 dB∕cm across the C-band. Photo-bleaching by a high-power supercontinuum source, and thermal-bleaching processes are performed on photodarkened samples. Both bleaching processes are found effective to fully erase the UV-induced PD loss. Rare-earth-doped phosphate glass (without silica) is a popular medium for high-power amplifiers and compact fiber lasers because of its higher solubility of rare-earth elements, relative to silica glass [1,2]. An additional benefit of this material is the absence of pump-induced photodarkening (PD) at near-infrared (NIR) wavelengths [3]. In this Letter, detailed results are presented about significant NIR PD caused by short wavelength ultraviolet (UV) irradiation at 193 nm, used to form Bragg grating laser mirrors in phosphate fibers, and about ways to eliminate this PD. The PD effect, referring to light irradiation-induced absorption, has been studied in many kinds of rareearth-doped optical fibers [4,5], mostly in Yb-doped silica and phospho-silica optical fibers, because the NIR darkening loss induced by high-power pumping severely degrades the output power and long-term reliability of high-power fiber lasers [6][7][8][9]. In addition to PD induced by pump light at 980 nm, PD was also observed to occur under irradiation at 193 and 488 nm [10,11]. Although the exact mechanism of the PD in Yb-doped silica fiber is still under debate, light-induced loss is commonly attributed to the formation of color centers by photoionization via either single-or multi-photon absorption processes, depending on the irradiation wavelength [6,[11][12][13][14]. The peak absorption wavelength observed in different glasses depends on the species of color centers generated and ranges from the visible (VIS) to the UV. Invariably, all color center absorption bands show a long tail extending into the NIR, and hence, a noticeable loss is generated at the pump and signal wavelengths of Yb-doped fiber lasers. In all these silica glasses, however, the absorption decreases gradually further into the NIR spectrum and becomes negligible in the 1550 nm telecommunication window. Hence, the PD effect has not been considered detrimental to Er-doped fiber lasers operating around 1550 nm, even with Yb codopants in the fiber.For higher power, shorter length fiber lasers made from phosphate glass with integrated Bragg grating mirrors, however, the situation is different. Since phosphate fibers lack photosensitivity in the 244 nm spectral range, commonly used to form fiber Bragg gratings, irradiation with intense 193 nm light from excimer lasers is required [15].In this Letter, it is shown that such irradi...

Radiation effects on phosphate glasses: Review

“…Reduced PD loss was reported in phosphate fibers doped with Yb 3+ compared to silica fiber when pumped at 980 nm . Similar photodarkening was observed when using an irradiation at 193 nm . Luminescence quenching in the visible range was also reported after irradiating using nanosecond UV laser of Ag and Cu containing phosphate glasses, which contain molecular clusters of copper, Cu + ions and/or Ag n Cu m neutral hybrid molecular clusters .…”

Photosensitivity and thermal stability of UV-induced fiber Bragg gratings in phosphate glass fibers

“…In our case, the result is a glass densification which occurs from the collapse of the long chain structure into a shorter-range structure. Color centers are also created in the bright fringes, with a strong absorption at 630 nm that has been assigned to phosphorus oxygen hole center as well as shorter wavelength bands assigned to other P centers [18,19]. Both of these effects increase the refractive index in the bright fringes but while the color center contribution to the index increase occurs faster, it saturates at a low level, and is thermally unstable [20].…”

Photosensitivity and Photodarkening of UV-induced Phosphate Glass Fiber Bragg Gratings and Application in Short Fiber Lasers

“…Silica fibers doped with rare earth ions (e.g. Er 3+ and Yt 3+ ) are routinely used as the gain medium of fiber lasers [1]. However, the low solubility of silica fiber as a host medium for rare earth ions places a limit on the potential doping concentration of these ions (< 0.1 mol%).…”

“…Color centers are also created in the bright fringes, with a strong absorption at 467 nm that has been assigned to phosphorus oxygen hole center as well as shorter wavelength bands assigned to other P centers [54,65]. Both of these effects increase the refractive index in the bright fringes but while the color center contribution to the index increase occurs faster, it saturates at a low level, and is thermally unstable [66].…”

“…Photo-bleaching and thermobleaching processes are demonstrated as both being effective for the complete erasure of PD loss while not diminishing the reflectivity of UV-induced FBGs. The work in thischapter has been published in one paper[65].…”