We have developed an approach to multiple-access lasercom that adopts the commercial paradigm of sharing the most
expensive terminal resources among all users. Space-time division multiple access (STDMA), analogous to an optical
space-time switch, hops the transmit beam and receive direction among multiple users and exchanges data while the
beam dwells on a user. A key enabler of STDMA is electronic beam steering using liquid crystal optical phased arrays,
which provides fast, precise, and agile beam re-pointing. We have built the first optical STDMA terminal, combining
beam hopping between remote terminals with coherent combining of both transmit and receive apertures, which is an
effective means for increasing antenna gain in systems for which large aperture components are impractical. Coherent
beam combining provided the expected increase in antenna gain, and the terminal was found to re-point the beam among
users quickly and precisely enough to suffer only minor throughput degradation. Communications test were performed
using 10 Gb/s Ethernet for a single-aperture configuration. Performance is presented as a function of angle scan speed
and STDMA dwell time per remote terminal. The results suggest that STDMA is a viable technology for supporting
multiple-access space-based 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.
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.
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.
Excited-state-absorption and stimulated-emission cross-section spectra have been measured in the 1300-nm region for a wide variety of Nd3+-doped glass compositions. The results indicate that fluoroberyllates are the best choice for fiber amplifiers in the 1300-nm optical communications window. Model calculations predict a gain spectrum peaked at 1314 nm with useful gain extending from 1304 to 1370 nm when amplified spontaneous emission is neglected.
Pr3+- and Nd3+-doped glasses are the most promising candidates for fiber amplifiers in the 1300-nm telecommunication window, but each has serious drawbacks. Using a quantitative numerical model and measured cross sections, we have examined the practical limits of performance for Pr3+-doped fiber amplifiers (PDFAs) in small-signal and power amplifier applications. A comparison has been made between power PDFAs and Nd3+-doped fiber amplifiers (NDFAs). In general, only PDFAs provide gain over the complete wavelength range of interest. NDFAs are expected to have higher efficiencies if the short wavelength amplified spontaneous emission is suppressed and operation is limited to the long wavelength side of the 1300-nm window. Even when potential improvements are considered, the performance of PDFAs and NDFAs does not approach that of Er3+- doped fiber amplifiers.
A technique for significantly increasing the pump power in an erbium doped fiber amplifier by multiplexing pump lasers at different wavelengths is demonstrated. Two pump lasers with a wavelength separation of at least 10 nm are combined with an insertion loss of less than 0.5 dB. The performance tradeoffs as a result of multiwavelength pumping are discussed.
The quantum conversion efficiency (QCE) and noise figure of erbium-doped fiber power amplifiers have been investigated experimentally. Under 980 nm co-directional pumping, efficiencies up to 89% were achieved. Very good agreement of the measured QCE, gain and noise figure with those predicted by a numerical model using only measured input parameters was obtained. The model was utilized to analyze the influence of fiber waveguide design, erbium confinement, signal wavelength and the signal input level on the QCE and noise figure for power amplifiers pumped at 980 nm or 1480 nm. Increasing the NA from 0.15 to 0.25 was found to increase the QCE by up to 52%, and decreasing the confinement factor from 1.0 to 0.5 produced up to an 18% increase of QCE. The noise figure is predicted to be up to 3 dB higher for pumping at 1480 nm than for pumping at 980 nm, and up to 2 dB higher for a signal wavelength of 1532 nm than for one at 1555 nm. As the signal input level increases, the QCE saturates and the noise figure begins to increase. This leads to a pump-power dependent optimum input signal level if both QCE and noise figure are a consideration.?
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 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.
Resonance-fluorescence-line-narrowing studies have been performed on a representative group of Er3+ -doped glasses for excitation in both the 1530 and 980-nm bands at 4.2, 77, 200, and 300 K. Fluorozirconate, fluorophosphate, phosphate, silicate, Ge/P-doped and A1/P-doped silica preforms, and Al/P-doped and Ge-doped fibers were examined. At 77 K the homogeneous linewidths were found to be relatively insensitive to composition. In particular, they vary from 1.4 to 1.8 cm-1 for the different hosts using excitation at 1531 nm. At 300 K weak resonant features were observed in most samples, consistent with a small inhomogeneous contribution to a predominantly homogeneous fluorescence emission.
The performance and efficiency of erbium-doped fibers pumped in the 800-nm band has been analyzed using a quantitative amplifier model. Both a silica and a fluorophosphate host were investigated. The analysis showed that under optimized conditions the fluorophosphate is superior to the silica with higher gain and up to 60 % higher quantum conversion efficiency. Only with respect to noise figure is the fluorophosphate poorer, because of the shorter lifetime of the metastable state.
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.
To explore the fundamental limits on Nd3-doped fiber amplifier performance at 1 . 3 m an analysis of the smallsignal pump efficiency saturation properties and signal/noise ratio has been performed. Ignoring ESA the pump efficiency (dB/mW) is found to be only moderately sensitive to the choice of glass host indicating that ESA is the critical parameter distinguishing experimental results reported for different materials. For fundamental reasons the pump efficiency for a linear Nd3 amplifier without ESA is an order of magnitude less than for an Er amplifier and degrades still further if ESA is included. Despite the general expectation of quantum-limited performance for a four-level amplifier the noise figure is found to degrade significantly if ESA is introduced. From this analysis we conclude that applications as power amplifiers are more promising because high power conversion efficiencies can be obtained and there is less sensitivity to noise figure. Nevertheless acceptable performance in any application requires operation at wavelengths where the ESA cross section is only a small fraction of the stimulated emission cross section.