Robert K. Hickernell

Y-branch waveguide glass laser and amplifier

A Y-branch channel waveguide laser operating near 1057 nm was fabricated by electric-field-assisted ion exchange in Nd-doped silicate glass. The overall length was 24 mm. Optical pumping was performed with a cw Ti:sapphire laser. Mirrors were bonded to the polished waveguide facets. The slope efficiency was 5.1% when a 4%-transmitting output coupler was used. Threshold was reached at 26-mW absorbed pump power. The device exhibited a single-pass small-signal gain of 0.034 dB/mW when operated as an amplifier. The 3-dB splitting loss of the Y-branch structure was overcome when the absorbed pump power was approximately 85 mW.

Pump-induced dispersion of erbium-doped fiber measured by Fourier-transform spectroscopy.

We report the measurement of group index and dispersion in an erbium-doped fiber amplifier by Fourier transformation of low-coherence interferograms. In a germania-codoped fiber whose background dispersion was - 14 ps/(km nm) we measured resonant gain-induced changes as high as 9 and -12 ps/(km nm) near 1.536 microm. The interferometric measurements agree with calculations based on a Kramers-Kronig transformation of absorption and emission spectra.

Characterization of vertical‐cavity semiconductor structures

Several analytical tools are applied to characterize vertical‐cavity surface‐emitting laser structures grown on GaAs wafers. These epitaxial structures are amenable to x‐ray, electron‐beam, and optical metrologies. Cross‐sectional scanning electron microscopy and transmission electron microscopy were used to measure layer thicknesses and uniformity. Photoluminescence wafer mapping was used to determine alloy composition uniformity across the wafer. Photoreflectance was also used to determine alloy composition. Cross‐sectional microphotoluminescence was used to measure average alloy compositions in the top and bottom mirrors. Reflectance spectroscopy was used to characterize the cavity resonances and mirror layers. Double‐crystal x‐ray diffractometry (DCXRD) was used to characterize mirror layer dimensions, uniformity, and average alloy composition. Excellent agreement was found among these measurement techniques and between simulations and measurements. The results demonstrate the accuracy of the device s...

Waveguide polarizers with hydrogenated amorphous silicon claddings.

We have fabricated TE- and TM-pass waveguide polarizers with polarization isolations of 42 and 35 dB, respectively. The devices were fabricated by the growth of hydrogenated amorphous silicon claddings on K(+)-Na(+) ionexchanged channel waveguides in glass. Cladding thicknesses were accurately tuned to permit optimum coupling of either a TE or a TM mode to the cladding. We have also demonstrated that a waveguide polarizer attenuation as high as 760 dB/cm can be measured by using a photothermal deflection technique.

Noise reduction in optical in situ measurements for molecular beam epitaxy by substrate wobble normalization

We demonstrate a normalization method for removing noise introduced into optical in situ measurements by sample rotation wobble during molecular beam epitaxy. The technique consists of measuring the angle of rotation of the sample through optical triggers attached to the sample manipulator rotation drive, acquiring a normalization curve at the various trigger points, then applying the normalization appropriate to each trigger to subsequent data. This cyclic normalization is demonstrated on normal-incidence optical reflection data and atomic absorption measurements in which the flux-monitor light beam is reflected from the sample to allow determination of layer thickness in addition to atomic flux. Noise reductions by factors of 3 to 30 were observed in both systems, with the larger improvements for samples with larger wobble angles, while preserving the original time resolution of the data. We achieve normalized optical reflectance data with a noise standard deviation of 1% over a period of one to two hou...

Compact solid-state waveguide lasers

Glass waveguide lasers will fill an important niche as optical sources in communication, RF photonics, and optical metrology. This is primarily because waveguide lasers benefit from compact size, low noise, relatively high output powers, long upper-state lifetimes, and simple integration with optical-fiber-based systems. Although we do not expect waveguide lasers and amplifiers to ever supplant fiber and semiconductor lasers and amplifiers in every possible communications application, waveguide lasers have a number of advantages over traditional lasers for these uses. Single-frequency waveguide lasers provide narrow linewidth and high output power in a compact, monolithic package. The narrow linewidth is an important advantage over standard semiconductor lasers, and the compact size makes single-frequency waveguide lasers better suited than fiber lasers or extended-cavity semiconductor lasers for many applications.

Evaluating epitaxial growth stability

Abstract We have investigated variations of epitaxial layer thicknesses from uniform periodicity in compound semiconductor Bragg-reflectors experimentally and theoretically. Specifically, we characterized the variation of individual layer thicknesses in the growth direction at a given point on the wafer, thereby assessing the growth stability in time. The characterization is based on the correlation of experimental reflectance spectroscopy and high resolution X-ray diffractometry measurements and precisely fitted simulations made on growth runs which include both random and systematic variations from perfect periodicity. We find good agreement between the measurement techniques and between the measurements and their simulations.

Determination of the complex refractive index of individual quantum wells from distributed reflectance

We investigate the measurement of the complex refractive index of individual quantum wells by reflectance spectroscopy. Placing the wells at half‐wavelength spacing to cause resonant feedback produces an order‐of‐magnitude increase in measurement sensitivity over that of nonresonant structures. Quantum well dispersive and absorptive effects on reflectance can be differentiated in certain spectral regions. Experimental data confirm a theoretical model of refractive index and absorption for quantum wells of GaAs in Al0.2Ga0.8As in the region of the well band gap.

Vertical-cavity semiconductor lasers: structural characterization, CAD, and DFB structures

One of the key technologies required for manufacturing vertical-cavity laser arrays at a production scale is rapid and nondestructive evaluation of the laser material. A brief review of methods for materials characterization of vertical-cavity semiconductor lasers is presented. Techniques based on reflectance spectroscopy, photoluminescence, photoreflectance, double crystal x-ray diffractometry, scanning electron microscopy, and transmission electron microscopy are used to determine alloy composition, cavity spacer thickness, and Bragg mirror layer thicknesses. Critical aspects of data gathering, analysis, interpretation, and simulation are highlighted. The optical simulation software used for computer aided device design and simulation of reflectance spectra is also briefly discussed.

Single-frequency and mode-locked Er/Yb co-doped waveguide lasers

Rare-earth-doped waveguide lasers have seen a significant increase in commercial and research interest over the last decade both in single-frequency and pulsed laser designs. This is due primarily to the higher doping concentrations of active ions in rare-earth-doped bulk glasses relative to rare-earth-doped fiber devices. Higher dopant concentrations allow for laser operation at decreased cavity length, which improves mode stability. We present results using Er,Yb co-doped waveguides in cw narrow-linewidth and mode-locked lasers.