Brian M. Walsh

Branching ratios, cross sections, and radiative lifetimes of rare earth ions in solids: Application to Tm3+ and Ho3+ ions in LiYF4

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.

On the distribution of energy between the Tm 3F4 and Ho 5I7 manifolds in Tm-sensitized Ho luminescence

Abstract A theory on the distribution of energy between the Tm 3 F 4 and Ho 5 I 7 manifolds in Tm-sensitized Ho materials is presented. A rate equation approach is utilized to arrive at an expression for the temperature-dependent equilibrium constant, Θ. Experimental support for this calculation is obtained by two different experimental procedures. Approximations used in the theory and the limits of applicability are also discussed. In the course of the development of the theory, a distinction is made between low and high excitation regimes, and criterion for applicability are given. Equations for the fractional population of Tm and Ho ions of the total excited state population in the low excitation density limit are derived in the context of the present theory, which differ from those widely used in the literature.

Ho:Tm:YLF laser amplifiers

A series of models is developed that describes a Ho:Tm laser, and predictions from these models are compared with experimental measurements for a variety of Ho:Tm:YLF amplifiers. Modeling is complicated by the plethora of required parameters needed to describe the dynamics of the Ho:Tm laser system versus the paucity of measured parameters. To remedy this, calculations presented here begin with measured energy levels and a quantum-mechanical model to determine a set of crystal-field parameters that are then used to calculate the energy-transfer parameters. Energy-transfer parameters, which describe the dynamics of energy exchange in the Ho:Tm system, are subsequently used in a rate-equation model to describe the dynamics of the lowest four manifolds of both Ho and Tm. Next, predictions of the rate-equation model are used in an amplifier model, which, among other effects, includes the variation of the pump energy density with the position of the probe beam in the amplifier. Results of the amplifier model are then compared with small-signal gain measurements from a variety of Ho:Tm:YLF laser amplifiers. Finally, the models are used to investigate the ultimate performance of a Ho:Tm:YLF laser amplifier by varying the concentrations of Ho and Tm in addition to the length of the end-pumped device.

Spectroscopy and lasing characteristics of Nd-doped Y 3 Ga x Al (5–x) O 12 materials: application toward a compositionally tuned 0.94-μm laser

Nd-doped Y3GaxAl(5-x)O12 (YGAG) materials are investigated for their usefulness as compositionally tuned laser materials. An analysis of line-center wavelength, line broadening, and line strength with Ga concentration is conducted to assess the feasibility of constructing a compositionally tuned 0.94-μm laser. Spectroscopic results on this analysis are presented. A laser rod of Nd:Y3Ga1.35Al3.65O12 was made to lase at 0.94465 μm and its performance characterized. This represents the first such laser demonstration on the 4F3/2→4I9/2 laser transition in Nd:YGAG. Improved optical quality and the use of compositional variations of the YGAG structure is expected to improve the laser performance as these materials are more fully developed.

Compositionally tuned 0.94-/spl mu/m lasers: a comparative laser material study and demonstration of 100-mJ Q-switched lasing at 0.946 and 0.9441 /spl mu/m

A new and innovative composite laser material Nd:YAG/sub x/YSAG/sub 1-x/ has been developed with several objectives in mind; tunability, efficiency, and minimization of the deleterious effects of amplified spontaneous emission (ASE) in Q-switched operation. Wavelength tuning to the requisite wavelength 0.9441 /spl mu/m was achieved by using the technique referred to as compositional tuning; that is, using nonstoichiometric laser materials to shift the wavelength for precise tuning. Laser efficiency was achieved by studying the physics of 0.94-/spl mu/m transitions in nonstoichiometric materials; i.e., by examining the effects of the host on the linewidth and cross section of of 0.94 /spl mu/m neodymium (Nd) transitions, ASE was minimized by choosing materials with a small ratio of 1.06- to 0.94-/spl mu/m peak cross sections. A comparative study of six different Nd-doped mixed garnet laser material systems was performed to meet the objectives above. Within these six material systems, over 20 laser materials were spectroscopically analyzed. The optimal laser material was found to be Nd:YAG/sub x/YSAG/sub 1-x/, which has been demonstrated to lase at the preselected wavelength of 0.9441 /spl mu/m, an important wavelength for remote sensing of water vapor. Operating this laser on the /sup 4/F/sub 3/2//spl rarr//sup 4/I/sub 9/2/ transition in Nd:YAG/sub 0.18/YSAG/sub 0.82/ at 0.9441 /spl mu/m, has produced for the first time over 100 mT in the Q-switched mode. This represents one of the few lasers that have been designed to operate at a specific, user-preselected wavelength.

Cr:Er:Tm:Ho:yttrium aluminum garnet laser exhibiting dual wavelength lasing at 2.1 and 2.9 μm: Spectroscopy and laser performance

Over 1.0 J of 2.1 μm laser energy and over 0.5 J of 2.9 μm laser energy have been demonstrated in a single flashlamp pumped solid state laser material, specifically Cr:Er:Tm:Ho:YAG. Flashlamp pumped laser operation of Ho:YAG at 2.1 μm and Er:YAG at 2.9 μm in various host materials is well known. We have developed an innovative laser system that operates at each of these wavelengths independently or simultaneously in a single solid state laser material with performance comparable to single wavelength systems Er:YAG and Cr:Tm:Ho:YAG. Variation of the flashlamp pump pulse length provides a method to discriminate between lasing at 2.1 and 2.9 μm. This effect results from Er→Tm→Ho energy transfer, the short lifetime of the upper lasing manifold in Er, the 4I11/2 manifold, and the relatively long upper laser level lifetime in Ho, the 5I7 manifold. This simple tuning method of achieving two widely separated wavelengths without the use of optical tuning elements has potential applications in remote sensing and me...

Nd:LuLF operating on the 4 F 3/2 → 4 I 11/2 and 4 F 3/2 → 4 I 13/2 transitions

Nd:LuLF, that is, Nd:LuLiF4, was grown with a Czochralski technique and characterized spectroscopically to include absorption and emission data and lifetime. Evaluation of this laser material for operation on the 4F3/2→4I11/2 and the 4F3/2→4I13/2 transitions was performed. Normal-mode laser performance was achieved on both the π and the σ polarizations for both transitions by use of a simple polarization-selective resonator. Both normal-mode and Q-switched performance was characterized on the 4F3/2→4I11/2 transition.

A spectroscopic and Judd–Ofelt analysis of the relaxation dynamics of Tm3+ in the fluorapatites, FAP, S-FAP, and B-FAP

Abstract The spectroscopic analysis of the absorption properties of the Tm3+ ion in the three fluorapatite crystals has been performed. Using the room temperature absorption data, the spontaneous emission rates, cross-sections and cross-relaxation rates have been determined using Judd–Ofelt theory. Based on emission lifetimes and emission cross-sections for the 3 F 4 → 3 H 6 and 3 H 4 → 3 H 6 transition, as well as the rate of energy transfer by cross-relaxation, the probabilities of cross-relaxation and population inversion by cross-relaxation compare favorably to other laser hosts such as YLF and YAG.

High-energy diode-pumped Ho:Tm:LuLiF4 laser for lidar application

Two-micron lasers can be used in a variety of remote sensing and medical applications. In recent years, such lasers have been used for remote sensing of wind and CO2 to expand our understanding of the global weather system. The detection of clear air turbulence and wake vortex from aircraft has been proven to enhance air travel safety. In this paper, we present the design and performance of a high-energy diode pumped solid-state 2-micron laser transmitter. There has been a large body of work on 2 μm laser crystals using Tm and Ho ions doped in YLF and YAG hosts, but the use of LuLiF4 as a host is relatively recent. Studies comparing Ho:LuLiF4 and Ho:YLF show that both crystals have similar emission cross-sections for both 2.05 μm and 2.06 μm transitions. Tm:Ho:LuLiF4 has proven to produce 15%-20% more energy than Tm:Ho:YLF. This is primarily attributed to the variation of the thermal population distribution in the Ho: 5I7 and 5I8 energy levels. The laser crystal used for this experiment is grown in the crystalline a-axis. The resonator is a bow tie ring configuration with 3-m length. One of the mirrors in the resonator has a 3.5m curvature, which sets up a 1.8 mm TEMoo mode radius. The output mirror reflectivity is 72% and it is the dominant source of the resonator loss. An acousto-optic Q-Switch with Brewster angle switches the Q of the oscillator and defines the polarization of the laser output. This laser has a potential to produce a multi joule energy and replace the traditionally used Ho: Tm: YLF crystal.

300-mJ diode-pumped 1.9-μm Tm:YLF laser

We report on a diode pumped Tm: YLF laser generating 1.9 micrometers output. Recently, research is being pursued to produce laser wavelength around 2 micrometers by separating the Ho and Tm ions in different laser hosts. Compared to co-doped laser hosts; a higher efficiency performance can be achieved by directly pumping the Holmium with a 1.9 micrometers Tm laser due to the elimination of energy sharing between Tm and Ho as well as deleterious upconversion effects in co-doped systems. A 300-mJ Tm:YLF laser at room temperature has been demonstrated. The laser design and laser performance is described. To our knowledge, this is the highest energy ever reported for this laser material.