REVIEW The most important feature of an optical fiber waveguide is its bandwidth, which defines its information-carrying capacity. A major limitation on the bandwidth of multimode glass and plastic optical fibers is modal dispersion, in which different optical modes propagate at different velocities and the dispersion grows linearly with length. However, in plastic optical fibers, experimental and theoretical results indicate that the modes are not independent but are highly coupled, which leads to a characteristic square-root length dependence and an unanticipated large enhancement of the bandwidth to gigahertz levels. The ever increasing demands for low-cost, high-bandwidth communications media for voice, video, and data transmission in short- and medium-distance applications are generating a new assessment of multimode optical fibers to serve as high-speed fiber links.
The observed dissociation of tetrakis rare-earth chromophores in rare-earth-doped organic polymers is reported, and a time-resolved spectroscopic technique is presented to determine quantitatively the fractional concentrations of different chromophore species in various solvents. Based on this technique, the equilibrium constants of tetrakis rare-earth chromophores in rare-earth-doped polymers are determined. Thus, the equilibrium constants for tetrakis rare-earth compounds of benzoyltrifluoroacetonate (BTF), for example, Sm(BTF)(4)P and Eu(BTF)(4)P in methyl methacrylate monomer, are K = 4.7 ? 0.5 x 10(-4) M and K = 1.2 ? 0.5 x 10(-4) M, respectively. In contrast, the tetrakis rare-earth compounds of hexafluoroacetylacetonate (HFA), for example, Sm(HFA)(4)Net(4) and Eu(HFA)(4)Net(4), are quite stable and show no evidence of dissociation. We further characterize the dissociation of several chromophore systems and discuss the influence such dissociation has on overall optical performance. The results enable determination of optimal doping concentrations as well as preferred rare-earth chromophore preparation and doped polymer fabrication procedures.
Single-mode optical waveguides based on planar silica have found increasing application in passive optical components such as arrayed waveguide gratings (AWG), couplers, and splitters. Key aspects ofthese devices are their low insertion losses and relative insensitivity to temperature. Planar poiymer waveguides present a complementary technology that is finding deployment in thermally activated components such as thenno-optic switches, variable attenuators and tunable filters. This results from the large thermo-optic effects and low thermal conductivities in polymers that lead to low power, compact and rapid thermal activation. However, the widespread deployment of planar polymer waveguides has been slowed by inability of single-mode polymer waveguides to achieve the low waveguide losses that have been attained in planar silica. In this paper we look at the sources of loss in polymer optical waveguides, assess approaches to reducing losses, and discuss several important loss measurement techniques valuable for evaluation ofnew polymer materials.
Email: ayeniay@photon-rner, gao@photon-rnrt 1 . introduction L-band erbium doped fiber amplifiers (EDFAs) have recently gained interest as potential candidates for increasing transmission capacity beyond C-band in dense wavelength division multiplexing (DWDM) systems. L-band amplification (1565-1605 nm) can be obtained from a conventional EDFA with a relatively long gain medium using high power pump SOUIC~S.'-' In principle, L-band amplifiers take advantage ofthe fact that erbium doped fibers (EDFs) have higher emission cross-sections than absorption cms~sectiom at long wavelengths. Therefore, for long EDFs, amplified spontaneous emission (ASE) be-Comes more pronounced at long wavelengths, resulting in amplification in the L-band region. However, optimized, efficient L-band amplifiers have been difficult to achieve due to the long EDF length and the off-emission-peak nature of erbium in the EDF. Thus, L-band amplifiers suffer from excessive pump power consumption by backward propagating ASE. To ove~come this effect in L-band EDFAs, several designs have been implemented, relying on multiple stage configurations and ASE end-reflection, resulting in increased performance.&' More recently, we reported an efficient L-band EDFA design based on a co-propagating C-band seed technique by usinga IOW power external c witylaser andafibergrating (FG) forpre-amplifier applications! In the present paper, we propose improved designs for higher power and broader bandwidth L-band amplification. Here, we utilize various bandwidths of the backward and forward propagating C-band ASE as CO-and counterpropagating pump seeds that provide more efficient ASE suppression along the EDF. In comparison to conventional and single seed single pump designs. we obtained 11 dB and 6 dB gain enhancements, respectively, and over 34 dB gain with over 20 dBm saturated output power within 34 nm bandwidth with a similar noise figure. Figure I(A) shows a schematic of the L-band EDFA with a co-propagating C-band seed. The amplifier is comprised of two sections: The first section consists of a narrow bandwidth FGs, which i s used to provide small seed power in the C-band to suppress the saturated backward ASE and also to provide a secondary pump for L~band amplification. The second section is the amplification stage,whichcontainsanEDFpumped bya Amplifier Deslgn %2 z2ThJZ Fig. 1. L-band amplifier designs with A) aca-propagatingandB) multiplecoandcaunterpropagating C-band seed technique.180 mW 980 nm laser. For high power applications, we used a single stage double-pump design with CO-and counter-propagating seeds as shown in Figure I(B). In order to find the optimal Lband amplification using C-band seed wavelength(~), we simulated the amplifier for L-band gain and noise figure in terms of the EDF length and the FG wavelengths, which will be discussed in detail in the next section. Results and DiscussionFigurezshowsthesimulationrerultsfor 1600nm s m d signal gain evolution with a single co~propagating I mW seed of different wavelengths. It is shown that with...
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