Realization and simulation of high-power holmium doped fiber lasers for long-range transmission

Abstract

We report on our realization of a high-power holmium doped fiber laser, together with the validation of our numerical simulation of the laser. We first present the measurements of the physical parameters that are mandatory to model accurately the laser-holmium interactions in our silica fiber. We then describe the realization of the clad-pumped laser, based on a triple-clad large mode area holmium (Ho) doped silica fiber. The output signal power is 90 W at 2120 nm, with an efficiency of about 50% with respect to the coupled pump power. This efficiency corresponds to the state of the art for clad-pumped Ho-doped fiber lasers in the 100 W power class. By comparing the experimental results to our simulation, we demonstrate its validity and use it to show that the efficiency is limited, for our fiber, by the non-saturable absorption caused by pair-induced quenching between adjacent holmium ions.

High efficiency holmium-doped triple-clad fiber laser at 2120 nm

“…Given the active core diameter D core = 20 µm, we thus find an overlap mode/doping Γ(λ peak ) = 80%. The resulting value is σ abs (λ peak ) = 4.1 10 −25 m 2 , in good agreement with our previous measurement in a similar host [11].…”

“…Despite our rare-earth doping process by aqueous solution, we manage to reduce drastically the concentration of hydroxyl groups OH in the fiber with several drying steps (including proper desiccating gaz), to remove the water from the porous layer. In the previous fiber, we estimated from the clad attenuation that the OH concentration was approximately 0.04 ppm [11], and we reach such low values for all rare earth based fibers. Since we did not modify the fabrication process for the core, we assume that the hydroxyl concentration is of the same order of magnitude in our new fiber, i.e.…”

“…In these experiments, the holmium-doped triple-clad fiber had a silica clad diameter of 118 µm [10]. We demonstrated a similar efficiency, reaching a laser signal of 90 W with a fiber with two-fold larger clad diameter, corresponding to a four-fold lower overlap between pump beam and active core [11]. This result illustrated the high optical quality of the glass, with low holmium clustering and hydroxyl (OH − ) concentration, which are the main contributions to the propagation losses in this type of fiber [12], [13].…”

“…As shown on Figure 2, the average holmium concentration ρ Ho in the fiber core is close to 3 10 25 /m 3 . On a single-clad Ho-doped fiber with similar core composition, we measured a fraction of clustered ions 2k = 15 ± 1% for a holmium concentration of about 4 10 25 /m 3 [11]. Assuming that the fraction 2k is proportional to ρ Ho in this range, we estimate that 2k 11% in our new Ho-3CF.…”

“…The holmium-doped fiber has a triple-clad structure, very similar to the fiber described in [11]. Its main novelty lies in the new geometry, with a reduced diameter of the silica clad, now at 105 µm flat-to-flat instead of 250 µm and an outer diameter of 145 µm for the fluorinated clad.…”