William J. Miniscalco

Erbium-doped glasses for fiber amplifiers at 1500 nm

Material-dependent properties influencing the performance of fiber amplifiers are reviewed together with the available data for Er/sup 3+/. The major glass types potentially useful in this application are considered and compared to silica. The topics addressed include quenching processes and the solubility of rare-earth ions, transition strengths and bandwidths at the 1500-nm gain transition, and the characteristics at the 800-, 980-, and 1480-nm pump bands. Aluminum is shown to be an extremely useful codopant for silica, improving its ability to dissolve rare-earth ions and providing desirable spectroscopic properties for Er/sup 3+/. For some of the attributes considered, other glasses have advantages over Al silica, but only with respect to gain bandwidth and pumping performance at 800 nm is significantly better than expected from other glass compositions. >

Excited-state absorption at 980 nm in erbium-doped glass

Excited state absorption (ESA) spectra were measured for the 4I11/2? 4F7/2 transition at 980 nm in Er-doped fluoride and fluorophosphate glass, by varying the pump wavelength from a tunable Ti:sapphire laser and monitoring the relative strength of the green upconverted fluorescence. The ESA cross section spectra are the same order of magnitude in strength as the 980 nm ground state absorption, but shifted to shorter wavelength by 4-6 nm. The 980 nm ESA may limit the efficiency of high power erbium-doped fiber amplifiers, and provides a possible pump mechanism for IR upconversion pumped fiber lasers operating in the visible.

Excited state cross sections for Er-doped glasses

Excited-state-absorption (ESA) cross sections were determined for the region between 760 and 900 nm for Er-doped fluorophosphate phosphate and silicate glasses. Measurements were performed on multimode fibers pumping at 647 nm with powers 1 . 5 Wto invert the population into the saturation regime. Over much of the 800-nm band ground-state-absorption (GSA) cross sections are equal to or greater than ESA cross sections. For comparison ESA was also measured for singlemode Al/P-doped silica fiber. The cross sections were incorporated into an amplifier model and the phosphate and fluorophosphate glasses were found to provide higher gain than silica for pumping in the 800-nm band. Photoexcited fluorozirconates were found to have substantial populations in the first four excited states and ESA transitions originating from these states are identified.

Upconversion and 980-nm excited-state absorption in erbium-doped glass

Excited-state absorption measurements performed at 980 nm in erbium-doped silica fibers provide evidence that Er3+ ions undergo clustering to varying degrees, depending on glass composition. The technique employed allows a quantitative characterization of the degree of clustering. It is found that the smallest degree of clustering occurs in silica fibers co- doped with aluminum, and those prepared by the sol-gel method.

Optical Space-Time Division Multiple Access

This work presents an approach to multiple access for free-space laser communication (lasercom) called space-time division multiple access, which aggregates traffic from multiple users at the network edge. The objective is to share resources to lower the cost, size, weight, and power consumption per user, thereby making lasercom feasible for users that require only moderate average information rates. This concept relies on fast, agile electronic beam steering, which was implemented in this investigation using liquid crystal optical phased arrays. We designed and built an experimental terminal incorporating a bidirectional communication aperture that was shared among the users, and two independently operated acquisition and tracking apertures. Using two remote user terminals, experiments were conducted to measure access node performance for a variety of operating conditions traceable to anticipated applications. The transmit and receive directions of the downlink and uplink communications channels were rapidly hopped between the two users, and data were exchanged between the access node and a user while the optical channel dwelled on the latter. Results showed that the measured information throughput efficiency correlated well with model predictions and was high enough to realize the expected advantages in applications with many users. Throughput efficiencies, defined as the actual data throughput as a percentage of the throughput without multiple access, exceeded 85% for dwell times of 100 ms and greater. This translates into an average information rate of 400 Mb/s for as many as 20 simultaneous users. Current optical phased arrays are capable of providing fast transitions between remote users, with values measured in the range 10-18 ms. The use of persistent tracking links was a key factor in achieving fast transitions, and it was found that motion of the remote terminals had no significant impact on performance.

Radiation testing of liquid crystal optical devices for space laser communication

Liquid crystal optical phased arrays are an enabling technology for a variety of photonic and electronic beam manipulation functions, including steering, control of polarization, and amplitude and phase modulation. For applications in the emerging field of space laser communications, such devices would need to survive in the space environment for 10 to 15 years. To assess suitability and identify potential issues, a series of experiments were conducted in which nematic liquid crystal devices were subjected to three radiation environments: total dose (gamma), prompt dose (x rays), and fast neutrons. Tests were conducted using simple phase retarder devices, which served as surrogates for beamsteering devices. Impacts to optical and electrical characteristics of the devices at 1.55 µm were measured after incremental exposure trials. Modest effects were observed, but none were deemed significant enough to impact performance of the devices for space communication beamsteering applications.

Evaluation of the 800 nm pump band for erbium-doped fiber amplifiers

Performs a comprehensive experimental and theoretical investigation of methods for overcoming the excited-state absorption (ESA), which is the main obstacle to efficient pumping of erbium-doped fiber amplifiers (EDFAs) at 800 nm. The effects of ESA on gain can be reduced at the cost of an additional noise penalty by adopting bidirectional pumping or by pumping in the long-wavelength tail of the ground-state absorption (GSA) band. The GSA and ESA cross-section spectra on the glass host material. One of the most promising hosts, fluorophosphate, is compared to Al/P silica in a detailed analysis based on a quantitative numerical model. It is predicted that 2-3 dB less pump power is required for the fluorophosphate EDFA. For Al/P-silica EDFAs, it is found that approximately 7-dB-higher power is required when pumping in the 800 nm band than for pumping at 980 and 1480 nm. >

Quantitative characterization of clustering in erbium-doped silica glass fibers

The current evidence for clustering of erbium ions in silica glass is reviewed, including experiments on fiber amplifier efficiency, excited state decay, and nonsaturable absorption in Er3+ pumped at 980 nm. Experiments performed by the authors on nonsaturable excited state absorption in Er3+ pumped at 980 nm add further evidence for clustering. It is found that the degree of clustering is much reduced with the addition of Aluminum to the glass. However, it is also found that ion-ion interactions can potentially limit amplifier efficiency even when no clustering is present.

Design and analysis of conformal ku-band microstrip patch antenna arrays

A novel approach for the simulation of conformal microstrip patch antenna arrays is presented. 1×2 and 1×4 microstrip arrays operating at 16 GHz were designed and their radiation characteristics were simulated and analyzed as a function of bending. The performance characteristics modeled include S-parameters, radiation patterns, 3-dB beamwidths, side lobe levels, VSWR and radiation efficiency. For the 1×2 array, the degradation in performance in observed for the bent arrays compared to the planar case. However, there is not a significant change in the parameters as there is an increase in the bending radius. As we increase the number of elements in the array, interelement coupling becomes more important. Although a better performance in gain and directivity is obtained for the 1×4 planar array compared to the 1×2 planar array, the performance parameters degrade more with respect to the bending of the array. A maximum gain of 13.65 dB with 16.7° 3-dB beamwidth was achieved at Ku-band. Furthermore, it is shown that the gain, resonance frequency, 3-dB beamwidth, radiation efficiency decreases as bending of the array structure increases. The proposed design can be used in many applications including telemetry, satellites, aircraft and medical operations. This modeling effort is a prelude to development of printed antennas on flexible substrates.

Measurement and analysis of cross sections for rare-earth-doped glasses

Excited state lifetimes, ground state absorption, excited state absorption, and stimulated emission cross sections govern the performance of luminescent sources and amplifiers. The role of host glass in determining the shape and magnitude of these quantities is outlined and useful relationships among cross sections are presented. Using the theory of McCumber it is possible to obtain the shape of a cross section spectrum from a measurement of the inverse process and to set limits on the noise figure of resonantly pumped amplifiers. A simple analysis can be used together with easily obtained information to identify glass compositions most promising as hosts for Nd3+ amplifiers at 1300 nm.