Multi-mode 2 μm Tm-silica fibre lasers with 1.61 μm in-band pumping

Abstract

In-band pumping of multi-mode Tm-silica fibre lasers operating at 2 mm has strong potential due to higher quantum efficiency, increased damage threshold, increased nonlinear threshold and practical larger pumping area. Results are presented on the investigation of multi-mode Tm-silica fibre lasers operating near 1.85 mm, containing a high Tm doping concentration of 1.1 wt% (9 Â 10 25 m À3 ), when in-band pumped at 1.61 mm near an absorption peak in Tm-silica. The relationships between lasing performance and fibre length and cavity losses are reported. The measured highest slope efficiency and threshold achieved are about 42% and 0.4 W obtained with a short optimum fibre length of 23 cm and cavity reflectivities of (99%-4%), and on a small scale in this configuration a maximum output power of 370 mW was produced. The estimated maximum slope efficiency for the cavity reflectivities of (4%-4%) is up to 56%.

Tellurite glass lasers operating close to 2μm

“…9 compares the absorption and emission crosssections of the Tm 3+ : 3 F 4 energy level and is used to calculate the peak absorption to emission cross section ratio, σ a /σ e of 6 at 1.606 μm. This compares to a value of 8.6 at 1.61 μm, and ∼ 23 at 1.57 μm in silica fibre [11] suggesting that lower thresholds and higher slope efficiencies could possibly be expected in silica fibre when pumped at around 1.57-1.61 μm. In fluoride fibre the peak σ a /σ e value is around 4 at 1.63 μm and around 2.5 at 1.61 μm, which matches quite well with the optimum pump wavelength range of 1.63 -1.69 μm found by R.M.…”

“…The peak absorption and emission cross-section values are 5.2×10 −25 m 2 and 1.5×10 −24 m 2 , respectively. In silica glass the Tm 3+ : 3 F 4 level has an absorption crosssection of 4.5×10 −25 m 2 and an emission cross-section of 6.0×10 −25 m 2 [11], and in fluoride glass an absorption cross-section of 2.5×10 −25 m 2 and an emission crosssection of 7.5×10 −25 m 2 [10]. This enhancement in emission cross-section in tellurite glasses is beneficial for lasing on this level of Tm 3+ compared to silica and fluoride glasses.…”

“…Modelling and experiment has identified the optimum pump wavelength for this in-band pumping as 1.70 µm and 1.66 µm, respectively, for Tm 3+ -doped silica [9] and fluoride [10] fibre lasers. Because absorption of the pump light and emission of the laser light occurs to and from the same energy level of Tm 3+ , the pump wavelength which should yield the highest slope efficiency should be obtained for the pump wavelength at which the ratio of absorption and emission cross sections (σ a /σ e ) of 3 F 4 is the highest [11].…”

“…A disadvantage of this pumping scheme is the lack of convenient low cost and high power pump sources operating at close to these wavelengths. Two options, which have previously been demonstrated, are a colour centre laser [10] and an Er 3+ /Yb 3+ -codoped silica fibre laser [11]. Low power semiconductor diode lasers operating at close to 1.6 µm have also been used to pump a Tm 3+ -doped fluoride fibre laser with high efficiency [12].…”