Randy W. Equall

Recent Progress in Developing New Rare Earth Materials for Hole Burning and Coherent Transient Applications

To develop new spectral hole burning materials and optimize known materials for applications such as optical correlator and memory devices, a broad range of experiments, from optical coherent transients to photoelectron spectroscopy, have been used to elucidate fundamental aspects of the rare-earth electronic structure. We report progress in the characterization of Er 3+ doped materials where we have measured an ultra-narrow line width of 50Hz in Er 3+ :Y 2 SiO 5 and a Γ inh /Γ h ratio as high as 10 8 in Er 3+ :LiNbO 3 . Progress is also reported for Nd 3+ :YVO 4 where the high oscillator strength is an advantage over other rare earth ions and excellent coherence properties can be achieved at modest magnetic fields. Finally, we report the advances in the pursuit of photon-gated hole burning materials through the study of the energies of the localized rare earth energy states relative to the host band states, providing the foundation for understanding photoionization in these materials.

Yb:YAG absorption at ambient and cryogenic temperatures

We have performed absorption measurements and generated absorption cross sections as a function of wavelength for the laser material YAG doped with ytterbium at 300, 175, and 75 K. This data was generated to enable a direct comparison of the absorption intensity and linewidths at room and cryogenic temperatures, and in particular near the temperature of liquid nitrogen at 77 K. The data have been used to compute universal absorption contour plots that display absorption as a function of the incident light center wavelength and optical thickness (doping density times penetration depth) for a number of bandwidths, and assuming that the spectrum of the incident light can be described as a Gaussian. Curves are presented for both 300 and 75 K, and may be used to optimize the absorption and laser efficiency.

Systematics of 4f electron energies relative to host bands by resonant photoemission of rare earth doped optical materials

Abstract Relative energies of 4f n electronic states and crystal band states are important for a fundamental understanding of rare-earth-doped optical materials and a practical understanding of each materials potential performance in specific applications. With this motivation, the 4f n ground state binding energies of rare earth ions have been studied in the gallium garnets using resonant photoemission spectroscopy and compared with the aluminum and iron garnets. The 4d–4f photoemission resonance was used to separate and identify the 4f n and valence band components of the spectra, and theoretical 4f photoemission spectra were fit to experimental results to accurately determine electron binding energies. A two-parameter empirical model was used to successfully describe the relative energies of the 4f n ground states in these materials. The success of this empirical model indicates that measurements on as few as two different rare earth ions in a host are sufficient to predict the energies of all rare earth ions in that host. This analysis shows that systematic shifts in the relative energies of 4f n states and crystal band states between different garnets arise entirely from shifts of the band states, while each rare earth ion maintains the same absolute binding energy for all garnets studied. These results suggest that further studies of additional host compounds using both photoemission and optical spectroscopy will rapidly lead to a broader picture of the host crystals effect on 4f electron binding energies.

Spatial-spectral holographic correlator at 1536 nm using 30-symbol quadriphase- and binary-phase-shift keyed codes.

Optical 30-symbol quadriphase-shift keyed (QPSK) and binary-phase-shift keyed (BPSK) codes were processed in a spatial-spectral holographic correlator with the Er(3+): Y(2)SiO(5) spectral hole-burning material operating at 1536 nm in the important 1550-nm communications band. The results demonstrate the ability of spatial-spectral holographic correlators to process QPSK codes and BPSK codes with the same apparatus. The high-fidelity correlations produced by this optical coherent transient device exhibit the low sidelobe characteristics expected for the codes used.

Measurement of photon echoes in Er:Y 2 SiO 5 at 1.5 µm with a diode laser and an amplifier

We have measured the optical dephasing time of Er3+ transitions near 1.5xa0µm in the two crystallographically inequivalent sites of Y2SiO5, using an external-cavity diode laser amplified by an erbium-doped fiber amplifier. Two-pulse photon echoes were observed at zero field and in magnetic fields up to 55u2009u2009kG, with dephasing times as long as 580xa0µs (corresponding to a linewidth of 550u2009u2009Hz). Stimulated echoes were also measured and showed evidence of spectral diffusion during the 13-ms lifetime of the xa04I13/2 level.

Demonstration of real-time address header decoding for optical data routing at 1536??nm

We have demonstrated real-time decoding of 20-bit biphase-coded address header pulses, using stimulated photon echoes in a phase-matched crossed-beam configuration. This decoding is one of the functions required for coherent transient optical data routing, packet switching, and processing. The active medium used was single-crystal Y(2)SiO(5) doped with Er(3+), which provides an operating wavelength of 1536 nm.

Optical dephasing mechanisms in Tm 3+ :Y 2 Si 2 O 7

Optical dephasing measurements are reported for Tm(3+):Y(2)Si(2)O(7) as a function of temperature and laser excitation intensity. By use of photon echoes, a coherence time of T(2) = 23 micros was measured at 1.24 K for the (3)H(6)(1) ? (3)H(4)(1) transition at 790.427 nm, corresponding to a homogeneous linewidth of 14 kHz. The relatively broad inhomogeneous linewidth of 100 GHz gives an inhomogeneous-to-homogeneous linewidth ratio of Gamma(inh)/Gammah = 7 x 10(6). This large ratio, a transition wavelength suitable for GaAlAs semiconductor lasers, and the absence of hyperfine structure hole burning make this material a good candidate for time-domain signal processing and transient optically addressed data storage.

Relating localized electronic states to host band structure in rare-earth-activated optical materials

crystal’s electronic band states relative to the 4f N or 4f N-1 5d 1 states responsible for the ion’s optical transitions is important for understanding the properties and performance of each material since energy and electron transfer between these states influences the material’s efficiency and stability. 1 Little is known about the relationships between these states, but there is growing motivation to explore these properties for developing ultraviolet laser materials, phosphors for applications including plasma displays and mercury-free lamps, scintillator materials for medical imaging, and optical data processing and storage technologies based on photorefractivity or photon-gated photoionization holeburning. Continued advances in optical technologies require knowledge of the systematic trends and behavior of rare-earth energies relative to crystal band states so that the properties of current materials may be fully understood and new materials may be logically developed. We have recently initiated a systematic study of the relative energies of the rare-earth ions’ electronic states and the host band states in optical materials using resonant electron photoemission spectroscopy (RPES). 2,3 RPES directly determines the energies of all occupied electronic states relative to a common energy reference and can unambiguously separate and assign spectral features to a particular electronic state. 4 Figure 1 presents results for yttrium aluminum garnet (YAG), the most important host crystal for solidstate lasers. Circles represent measured binding energies of the rare-earth 4f N ground state relative to the valence band maximum (the host’s highest energy occupied state). These results have led to an empirical model that successfully describes the rareearth binding energies in optical materials with two parameters: one describes a constant shift experienced by all rare-earth ions and the second describes a smaller dependence on the rare earth’s ionic radius. These empirical parameters may be determined from measurements on just two different rare-earth ions, or, in certain cases, simply from measurements on the host crystal itself. With parameters determined from our measured

A 100 mJ Q-switched Nd YAGxYSAG(1-x) laser at 944.1 nm

A new laser material, Nd:YAGxYSAG(1−x), has produced efficient Q-switched laser operation with energies exceeding 100 mJ per pulse at 944.1 nm. This represents one of the few lasers to be designed to operate at a specific, user preselected wavelength without the use of intraresonator tuning elements. Material design, Q-switched lasing and single pass amplifier results are presented.

A comparative study of Nd-doped mixed garnet laser materials for application to 0.94um lasers

A comparative study of Nd-doped mixed garnet laser materials for compositional tuning in the 0.94 µm region is presented. Of primary importance is a determination of the 0.94 µm line center with composition and the ratio of 0.94 µm cross section to the cross section at 1.06 µm. An assessment is made.regarding the best choice of material for 0.94 µm lasers in H2O DIAL applications.