Good Mode Quality Research Articles

Ultracompact Graphene-Assisted Tunable Waveguide Couplers with High Directivity and Mode Selectivity

Graphene distinguishes itself as a promising candidate for realizing tunable integrated photonic devices with high flexibility. We propose a set of ultracompact tunable on-chip waveguide couplers with mode-selectivity and polarization sensitivity around the telecom wavelength of 1.55 μm, under the configuration of graphene-laminated silicon waveguides patterned with gold nanoantennas. Versatile couplings can be achieved in a widely tunable fashion within a deep-subwavelength area (210 × 210 nm2), by marrying the advantages of tight field confinement in plasmonic antennas and the largely tunable carrier density of graphene. Incident light signals can be selectively coupled into different fundamental modes with good mode quality and high directionality exceeding 25 dB. Design scenarios for asymmetric couplings are presented, where the operation wavelength can be tuned across a 107-nm range around 1.55 mm by altering the chemical potential of graphene from 0 to 1.8 eV. Furthermore, the proposed schemes can be leveraged as mode-sensitive on-chip directional waveguide signal detectors with an extinction ratio over 10 dB. Our results provide a new paradigm upon graphene-assisted tunable integrated photonic applications.

Hollow-core fibers for high power pulse delivery.

We investigate hollow-core fibers for fiber delivery of high power ultrashort laser pulses. We use numerical techniques to design an anti-resonant hollow-core fiber having one layer of non-touching tubes to determine which structures offer the best optical properties for the delivery of high power picosecond pulses. A novel fiber with 7 tubes and a core of 30µm was fabricated and it is here described and characterized, showing remarkable low loss, low bend loss, and good mode quality. Its optical properties are compared to both a 10µm and a 18µm core diameter photonic band gap hollow-core fiber. The three fibers are characterized experimentally for the delivery of 22 picosecond pulses at 1032nm. We demonstrate flexible, diffraction limited beam delivery with output average powers in excess of 70W.

Efficient diode-laser side-pumped TEM_00-mode Nd:YAG laser

A diode-laser side-pumped Nd:YAG laser that uses a simple, multipass pumping geometry has been investigated. This geometryallows efficient pumping of small-diameter laser rods, reduced dependence on pump wavelength, and efficient TEM(00)-mode operation. A 5.8-mJ output energy in a near TEM(00) mode has been obtained for 21.5-mJ pump energy, whichcorresponds to an optical efficiency of 27%. Corresponding results from numerical simulations show similar good mode quality and an optical efficiency of 40%.

Modulated submillimeter laser interferometer system for plasma density measurements

A high resolution submillimeter interferometer system for measurement of electron densities in the 10(14)-cm(-3) </= n(e) </= 2 x 10(15)-cm(-3) range has been developed for use in high density tokamaks. Phase modulation at ~1 MH(z) is accomplished by difference frequency mixing of two cavity tuned laser oscillators. The optically pumped CH(3)OH lasers, which operate on the 118.8-microtm line, feature a novel output coupling design that permits good mode quality and low beam divergence. The beat signals are detected using a newly developed Ge:Li photoconductor, and a direct measurement of the phase shift is obtained from the time lag between probe and reference signals. The sensitivity of the resulting phase measurement is independent of the instantaneous phase and unaffected by fluctuations in the amplitude or in the frequency of the modulation.