Eye safe emission in Tm³⁺/Ho³⁺ and Yb³⁺/Tm³⁺ co-doped optical fibers fabricated using MCVD-CDS system

Highlights

• Fabrication of Tm³⁺/Ho³⁺ and Yb³⁺/Tm³⁺co-doped optical fibre preforms is possible using MCVD Chelate Doping Technology.

• Wide emission spectra for Tm³⁺/Ho³⁺ and Yb³⁺/Tm³ can be observed in the co-doped silica optical fibers.

• Amplified Spontaneous Emission (ASE) in Yb³⁺/Tm³⁺ co-doped optical fibre is presented.

Abstract

The RE-co-doped silica optical fibers emitting in the eye-safe spectral range are still attractive for new applications like LIDAR, pollution monitoring, navigation, and free-space optical communication. High gain and beam quality of fiber sources based on large mode area (LMA) active fibers can be fabricated by the well-known Modified Chemical Vapor Deposition (MCVD) technology improved by the chelate doping system. Considering silica matrix, the most interesting co-dopants in the region above 1400 nm are thulium and holmium. In the paper Tm³⁺/Ho³⁺ and Yb³⁺/Tm³⁺ co-doped silica optical fibers (refractive index and rare earth distribution profile, luminescence) are presented. Their respective emission spectra has been measured over the range 1550–2150 nm for Tm³⁺/Ho³⁺ (exc. at 796 nm) and 1600–2100 nm for Yb³⁺/Tm³⁺ (exc. at 976 nm). A wide luminescent spectrum has been recorded from RE co-doping of silica glass during the MCVD process and we were able to achieve the appropriate and uniform concentration for getting the conditions of energy transfer. The fabrication and characterization of Tm³⁺/Ho³⁺ and Yb³⁺/Tm³⁺ co-doped optical fibres are presented. The analysis of luminescence spectrum changes vs. fibre length has shown that there is Amplified Spontaneous Emission (ASE) at 1960 nm in Yb³⁺/Tm³⁺ co-doped optical fibre. The possibilities of obtaining novel constructions of RE co-doped optical fibers for eye-safe emission region will be also shown.

Introduction

Low attenuation silica-based optical fibre technology has been developed in the 70's of XX century by using chemical vapor deposition process MCVD to form ultra-transparent glass preforms. The MCVD process is based on the oxidation reactions by heats chemical vapors and oxygen in the pure silica tube. Among different chemical glass forming processes, the MCVD technique is commonly used for the production of doped optical fibers [[1], [2], [3]]. The fully automated technology with closed reaction chamber (silica tube), ultra-pure chemicals and carrying gasses results in the precisely controlled refractive index profile. Moreover, due to the low OH group contamination, the low attenuation is obtained in optical fibres produced with this method [4]. Well known advantages of silica are: mechanical and chemical stability, thermal robustness, low attenuation and high damage threshold. The numerous applications of doped silica based fibers e.g. Amplified Spontaneous Emission (ASE) and fibre lasers have been investigated [[5], [6], [7]]. Unfortunately, the high phonon energy of silica (c.a. 1100 cm⁻¹) limits the infrared applications above 2.2 μm [8]. The solution doping technique is commonly used for the incorporation of rare-earth (RE) ions into the silica preforms. In this process forming a thick and uniform layer of soot is difficult. Moreover, in most cases, the rare earth salt solution impregnation process (e.g. acetone, ethanol) is performed outside the MCVD system which is a way to incorporate the water molecules and impurities into the fibre preforms. In fact, the solution doping method has some disadvantages like small active core diameter and relatively low maximum concentration of lanthanides [9,10]. To overcome these problems, the Chelate Doping Technology (CDT) has been developed and applied recently [[11], [12], [13], [14], [15]]. The organometallic complexes of lanthanides can be sublimated directly from solid-state thanks to the organic ligand and decomposed directly in the reaction zone inside the silica tube. The main advantages of this technique are a single step preform fabrication process, good control of the process parameters, possibility of fabricating large core fibers and much better reproducibility over the solution doping method. In fact, there are also some challenges using MCVD-CDT: controlling of condensation of organic materials in delivery lines and deposition area in the silica tube since organic part of complexes breakdown over c.a. 300ᵒC and in the consequence variation of dopant distribution inside the substrate tube can occur [15]. The incorporation of lanthanides in the optical fibre core gives an opportunity for obtaining the emission in optical fibers by using electronic transitions in the RE ions structure. This process was successfully used for telecommunication amplifiers (Erbium Doped Fibre Amplifier – EDFA), optical fibre radiation sources and fibre lasers [16,17]. The new opportunity of luminescence profile tuning in lanthanides co-doped fibres is based on the emission of rare earth ions and energy transfer/conversion phenomena (donor-acceptor energy transfer, cross-relaxation). The near infrared radiation over 1.4 μm, due to the low absorption of human tissue, is called eye-safe emission and can be used in the free space laser operation (environmental monitoring, laser scanners technology, free space telecommunication, medicine, material processing) [18]. There are several groups of researchers working in this area that use MCVD solution doping system for the incorporation of rare-earth ions. The thulium-holmium wideband forward and backward combined amplified spontaneous emission in the range of 1.6–2.3 μm was described in Ref. [19]. The laser action was also observed in the Yb³⁺, Tm³⁺, Ho³⁺ doped fibre for 2.1 μm, 4.3W [20]. Moreover, the tunable emission was presented in the isolator-free widely tunable (~1.89–2.05 μm) thulium/holmium fiber laser [6]. The MCVD-CDT technology was also reported in the literature [[11], [12], [13], [14], [15]]. The proposition of co-doped Tm³⁺/Ho³⁺ and Yb³⁺/Tm³⁺ optical fibers can extend the above applications field.