Development of Trivalent Ytterbium Doped Fluorapatites for Diode-Pumped Laser Applications

Abstract

ABSTRACT:One of the major motivators of this work is the Mercury Project, which is a 1 kW scalable diode-pumped solid-state laser system under development at Lawrence Livermore National Laboratory (LLNL). Major goals include 100 J pulses, 10%wallplug efficiency, 10 Hz repetition rate, and a 5 times diffraction limited beam. To achieve these goals the Mercury laser incorporates ytterbium doped Sr 5 (PO 4 ) 3 F (S-FAP)as the amplifier gain medium. The primary focus of this thesis is a full understanding of the properties of this material which are necessary for proper design and modeling of the system.Ytterbium doped fluorapatites, which were previously investigated at LLNL, were found to be ideal candidate materials for a high power amplifier systems providing high absorption and emission cross sections, long radiative lifetimes, and high efficiency. A family of barium substituted S-FAP crystals were grown in an effort to modify the pump and emission bandwidths for application to broadband diode pumping and short pulse generation. Crystals of Yb 3+ :Sr 5-x Ba x (PO 4 ) 3 F where x < 1 showed homogeneous lines offering 8.4 nm (1.8 times enhancement) of absorption bandwidth and 6.9 nm (1.4 times enhancement) of emission bandwidth. The gain saturation fluence of Yb:S-FAP was measured to be 3.2 J/cm 2 using a pump-probe experiment where the probe laser was a high intensity Q-switched master oscillator power amplifier system. The extraction data was successfully fit to a homogeneous extraction model. The trivalent ytterbium ion 1-5 is attractive as a laser ion due to its simple electronic structure, long radiative lifetimes, and high quantum efficiency. The electronic structure of trivalent ytterbium can be derived from the outer orbital configuration of 4f 13 , or 1 unpaired electron in the 4f shell. This gives rise to only two energy levels in the free ion, since J = L + S = 3 ± ½, with Russell-Saunders notation 2 F 7/2 and 2 F 5/2 . Since the 4f shell is more than half filled, Hund's rules dictate that the 2 F 7/2 level will be the lower level (ground state) and the 2 F 5/2 level the upper one. Quantum mechanics gives 2J + 1 possible states for each of these levels, but since the number of electrons is odd (13 in the f orbital), Kramer's degeneracy states that there must be at least a two-fold degeneracy of these states. Therefore, the 2 F 5/2 will split into three levels, and the 2 F 7/2 into four levels.When Yb 3+ is doped into a crystalline or glass host medium, these levels are Stark split forming excited and ground state manifolds of three and four levels respectively (Fig. 1).The long radiative lifetimes of ytterbium doped materials make it possible to excite these materials to inversion at lower intensities, since there is a longer integration time. The quantum defect is defined as η q = ν laser /ν pump , where ν pump is the pump frequency, and ν laser is the laser frequency. Since the Yb 3+ ion has only two electronic levels, even the worst case quantum defect is only the deficit incurred from the combined ...

Linear optical properties and Raman spectroscopy of natural fluorapatite

“…Due to the low birefringence and the slope of the dispersion curves in the IR region no phase matched THG is possible in fluorapatite. However, the strong sharp total symmetric breathing mode ν 1 (A 1 ) of the [PO 4 ] group present in the Raman spectra of fluorapatite for all investigated excitation geometries allows to draw the analogy to synthetic Yb-doped Sr 5 (PO 4 ) 3 F [11], and thus efficient χ (3) -based SRS can be expected to occur in crystals of fluorapatite with the ν 1 (A 1 ) mode with a Raman shift of ß965 cm −1 being a SRS promoting vibration mode.…”

“…Being a crystal with a centrosymmetric structure fluorapatite cannot act as a nonlinear optical crystal for χ (2) ‐based processes, such as second harmonic generation, but allows in general χ (3) ‐based processes, such as third harmonic generation (THG) or stimulated Raman scattering (SRS). The latter effect can cause parasitic nonlinear losses in laser crystals and was studied for apatite‐type crystals of Yb‐doped Sr 5 (PO 4 ) 3 F in . Unfortunately, concerning Ca 5 (PO 4 ) 3 F no information is given in literature about nonlinear optical χ (3) ‐based processes, and even data of refractive indices and their dispersion in a suitable wavelength range for phase matching calculations for parametric χ (3) processes are missing.…”

Effect of La3+ incorporation on the spectroscopic properties of Yb3+ -doped Y1−x La x Ca4 O(BO3 )3 (x = 0.09 and 0.25) crystals

“…The advantages and disadvantages of Yb 3þ ion over Nd 3þ ion in laser hosts has been well described in Ref. [7]. The simple 4f 13 electronic structure of Yb 3þ , ensures the small quantum defect ($11%) and the absence of some detrimental de-excitation processes (e.g., concentration quenching, excited state absorption, and upconversion) [1].…”