The measurement of branching ratios, cross sections and radiative lifetimes for rare earth ions in solids is considered. The methods are applied to Tm and Ho in YLF as a test case. De-activation rates for electric dipole and magnetic dipole emission are calculated for many of the lower lying manifolds in Tm:YLF and Ho:YLF in the context of the Judd-Ofelt theory to determine radiative lifetimes. Measured values for the branching ratios as well as the absorption and emission cross sections are also presented for many of the excited state manifolds. From these measurements, a methodology is developed to extract measured values for the radiative lifetimes. These results are compared with the Judd-Ofelt theory as a guide for consistency and for determining the accuracy of the Judd-Ofelt theory in predicting branching ratios and radiative lifetimes. The parameters generated by the methods covered here have potential applications for more accurate modeling of Tm:Ho laser systems.

Lanthanide series ions are considered in the context of acquiring spectroscopic parameters and their application to modelling of quasifour-level lasers. Tm:Ho codoped crystals of YLiF4 (YLF) and the isomorphs LuLiF4 (LuLF) and GdLiF4 (GdLF) as 2.0 μm lasers are used for illustration of the experimental and theoretical techniques presented here. While these materials have similar physical properties, they differ in the strength of the crystal field at the site of optically active lanthanide dopant ions such as Tm3+ and Ho3+. This is due in part to the size of the Lu3+, Y3+, and Gd3+ ions, which comprise part of the host lattice, but ionicity plays a role as well. This selection of lanthanide: host materials provides a useful basis on which to assess laser materials with regards to changes in the strength of the crystal field at the dopant ion site. It is demonstrated that Tm:Ho:LuLF has a larger crystal field splitting than Tm:Ho:YLF and Tm:Ho:GdLF, leading to smaller thermal populations in the Ho lower laser level. To assess this effect quantitatively, the energy levels of the first ten manifolds in Ho:LuLF have been determined. Measurement of Ho:XLiF4 (X=Y,Lu,Gd) emission cross sections at 2.0 μm, Tm:XLiF4 pump absorption cross sections around 0.78 μm, manifold to manifold decay times and energy transfer parameters in Tm:Ho:XLiF4 systems are also determined to provide a consistent set of parameters to use in laser modeling. The techniques presented here are applicable to any lanthanide series ion in a crystalline host. A theoretical laser model has been developed that is easily adapted to any lanthanide ion in a crystal host. The model is used to predict diode side-pumped laser performance of Tm:Ho:LuLF and Tm:Ho:YLF using input parameters determined from the spectroscopy presented here. An explanation is presented for the improved performance of Tm:Ho:LuLF over Tm:Ho:YLF by modeling the laser. A demonstration that small changes in lower laser thermal population can substantially alter laser performance is noted, an effect that has not been fully appreciated previously.

No abstract

A high-power double-clad Tm-doped silica fiber laser, pumped by two beam-shaped and polarization-coupled diode bars at 787 nm, was wavelength tuned by use of an external cavity containing a diffraction grating. The Tm fiber laser produced a maximum output power of 7 W at 1940 nm for 40 W of incident diode power and was tuned over a wavelength range of 230 nm from 1860 to 2090 nm, with .5-W output power over the range 1870-2040 nm. The prospects for further improvement in performance and extension of the tuning range are discussed. © 2002 Optical Society of America OCIS codes: 140.0140, 140.3510, 140.3480, 140.3600. Solid-state laser sources operating in the eye-safe 2-mm spectral region have seen rapid development over the past decade owing to their numerous applications in areas such as lidar and medicine. Conventional, bulk solid-state lasers based on crystals doped with thulium 1 or with thulium and holmium 2 have been the focus of the most attention because of their compatibility with high-power GaAlAs diode-bar pumping and a fortuitous two-for-one cross-relaxation process 3 that can lead to pumping quantum eff iciencies approaching 2. However, progress in scaling of the output power of these devices to meet the needs of many applications has been hindered by the need for high pump intensities. This requirement is dictated by the low gain cross sections and quasi-four-level character of these sources and also brings the consequence of the strong thermal effects that result from the high thermal loading density and degrade both efficiency and beam quality at high power levels. A further drawback of these devices is that their range of operating wavelengths is typically rather narrow, which limits their applicability.An alternative strategy for power scaling in the 2-mm spectral region is to employ a cladding-pumped Tm-doped fiber laser. 4 This approach has the attraction that the heat generated as a result of the laser pumping cycle is distributed over a long length of fiber, reducing the risk of damage caused by melting or fracture. In addition, the output beam quality is dictated by the waveguiding properties of the Tm-doped core and can easily be designed to ensure a singlespatial-mode output beam. Recently the present authors and others reported a cladding-pumped Tm-doped silica fiber laser that produces 14 W of single-mode output at 2 mm-the highest to our knowledge reported so far for a f iber laser operating in the 2-mm spectral region. 5A further advantage that fiber lasers offer is the potential for broad wavelength tunability owing to the broad transition linewidths in glass hosts.Here we describe a cladding-pumped Tm-doped silica fiber laser with an operating wavelength tunable over a 230-nm range from 1860 to 2090 nm, at multiwatt power levels. To the best of our knowledge, this result represents the broadest tuning range for a cladding-pumped fiber laser reported to date.The Tm-doped f iber used in our experiments was pulled from a perform fabricated in house by the standard chemical-vapor de...

While the energy levels in Ho:YLF have been measured previously, they have not been as thoroughly investigated in the isomorphs, Ho:LuLF and Ho:GdLF. We report here the measurement of the energy levels of the trivalent lanthanide Ho3+ in GdLiF4 (GdLF), YLiF4 (YLF), and LuLiF4 (LuLF). The measurement of the energy levels of Ho:YLF, although they have been measured before, are repeated here for self-consistent comparison to Ho:LuLF and Ho:GdLF. The Stark split levels for the first ten Ho manifolds in these materials have been measured, and the results have been fitted to a free-ion plus crystal-field Hamiltonian to generate a theoretical set of energy levels. Crystal-field parameters were varied to determine the best fit between experimental and theoretical energy levels. The energy levels of Ho:GdLF and Ho:LuLF are seen to be very similar to those in Ho:YLF. However, subtle changes resulting from replacing Y3+ with Gd3+ or Lu3+ in the fluoride crystal YLiF4 result in shorter transition wavelengths in GdLF and longer transition wavelengths in LuLF. This has implications for Ho lasers operating at ∼2.0 µm. The energy levels for Ho:GdLF and Ho:LuLF determined here indicate that Ho:GdLF will have a larger lower laser level thermal population than Ho:YLF, while Ho:LuLF lasers will have a smaller lower laser level thermal population than Ho:YLF. This is consistent with the larger Stark splitting associated with the larger host ions that Ho substitutes for in these lithium fluoride materials. The intensity parameters are also determined from a Judd–Ofelt analysis and used to calculate radiative lifetimes and branching ratios for the first ten manifolds in Ho:GdLF, Ho:YLF and Ho:LuLF.