Enhancement of Tetrahedral Chromium (Cr⁴⁺) Concentration for High-Gain in Single-Mode Crystalline Core Fibers

Abstract

Tetrahedral chromium (Cr 4+)-ions is the main contributor to gain in the broadband single-mode Cr-doped crystalline core fibers (SCCFs). We have demonstrated a gross gain of 8-dB on a length of 27-cm for a 300-nm broadband SCCF using a thermal annealing technique. Gross gain and fiber length are the highest yet reported for the SCCF. The record gain is mainly due to both the longer fiber by aligned molten-zone growth and Cr 4+-ions enhancement by thermal annealing. The enhancement mechanism of the Cr 4+-ions in the SCCF is experimentally verified by the means of superconducting quantum interference device (SQUID) and electron probe X-ray micro-analysis (EPMA). As a practical application of the proposed 8-dB gain in the SCCFs in fiber transmissions, a 25-Gb/s error-floor free link is successfully demonstrated.

17-dB Net Gain of Broadband Single-Mode Cr-Doped Crystalline Core Fiber Fabricated by Small Core

“…Besides, the addition coating of CaO layer on the outer layer of Cr 2 O 3 layer prevents out-diffusion of Cr-ions from the core [12]. The Cr 4+ (tetra ) enhanced mechanism of annealing process of chemical shifts might be represented by the two equations as follow [10]:…”

“…While other fiber amplifiers, such as Tm-doped fiber (TDF) in the S-band [5], Pr-doped fiber (PDF) in the O-band [6], and Bi-doped fiber (BDF) in the U-band [7], have been developed, they cannot yet be used for a single fiber amplifier covering the entire 300-nm bandwidth. Recently, researchers have explored single-mode chromiumdoped crystalline core fibers (CCFs) as possible candidates for broadband amplifiers to cover the interval from 1.3 to 1.6-µm [8]- [10]. The core of the CCF is composed of chromium doped with yttrium aluminum garnet (Cr:YAG), and the fluorescence spectrum of the CCF emitted from Cr 3+ -ions of the octahedral site and Cr 4+ -ions of the tetrahedral site provides a wide usable bandwidth.…”

“…Maintaining a high concentration of Cr 4+ (tetra)-ions in the CCF core is difficult due to the volatility of chromium oxide during the high-temperature growth process. Previous research has shown that optimization of Cr 4+ -ions in CCFs resulted in a net gain of 7.1-dB [9], 8.0-dB [10], 6.7-dB [11], and 11-dB [12]. To increase the fluorescence intensity of Cr:YAG, annealing of the Cr:YAG crystal can be performed to transform Cr 3+ (octa)-ions into Cr 4+ (tetra)-ions [8].…”