Michael J. Yadlowsky

Optical fibers and amplifiers for WDM systems

The development of optical-fiber amplifiers allowed a dramatic increase in the capacity of optical transmission systems while reducing system costs. Capacity increases are possible because the high output powers afforded by optical-fiber amplifiers support higher bit rates, while their broad bandwidth and slow gain dynamics allow multichannel operation. This benefit comes at the expense of having to manage signal-to-noise ratio degradations due to the accumulation of amplifier noise and dispersion distortions accumulated over the total system link. Furthermore, nonlinear optical effects become significant with the use of high power signals over long lengths of fiber, causing cross talk among the different channels and increasing signal distortions. To fully exploit the potential capacity of wavelength division multiplexing systems, the optical characteristics of the fibers and optical-fiber amplifiers must be optimized. The optical amplifiers should have low noise and flat gain, which can be achieved by using 980-nm pump lasers, optimized fiber glass composition, and gain-flattening filters. The optical fibers should have a small nonzero dispersion and large effective area. Both features can be achieved by optimizing the fiber index profile. This paper summarizes the state of the art in these components and points to directions for future exploration.

Pump wavelength-dependent spectral-hole burning in EDFAs

Gain-difference spectra are used to show that 980-nm band pump wavelength changes can significantly effect spectral-hole burning (SHB) in erbium-doped fiber amplifiers (EDFAs). Gain measurements made using a number of different signal and pump wavelengths help to characterize the underlying inhomogeneous spectral properties of EDFAs and a framework for modeling these effects is presented. Despite the difficulty of obtaining the needed subpopulation specific cross section spectra, this model can be used to explain seemingly anomalous changes in the gain spectra of EDFA as a result of changes in signal or pump wavelength.

980 nm band pump wavelength tuning of the gain spectrum of EDFAs

We report 2-3 dB changes in the gain spectra of EDFAs in response to nanometer changes in pump wavelength within the 980 nm pump band. The pump wavelength induced gain changes are characterized and several techniques are presented to use them to improve the gain flatness of EDFAs.

Independent control of EDFA gain shape and magnitude using excited-state trapping

Controlled use of excited-states of erbium are used to decouple the magnitude and shape of the gain spectrum of an erbium-doped fiber amplifier (EDFA) containing a low phonon energy glass. This makes it possible to operate a single EDFA at multiple gains without the gain flatness degradation referred to as dynamic gain-tilt. Dynamic gain-tilt free operation is demonstrated in the long-wavelength portion of the erbium gain spectrum using an erbium-doped ZBLAN fiber.

Experimental comparison of the effect of discrete and distributed path inband crosstalk on system performance: application to predicting system performance penalties

The system performance degradations of inband crosstalk produced by distributed Rayleigh scattering and a single discrete time delayed path have been measured in the same system. Rayleigh scatter crosstalk degrades performance more than equal amounts of discrete crosstalk. By parameterizing the system bit error rate (BER) and received power, we have separated the optical and electrical noise contributions to the BER based on their intensity and crosstalk scaling. We observed behavior consistent with earlier models, but found an unexpected increase in the total optical noise. This excess noise had a very regular linear scaling with crosstalk power. Because of its well defined crosstalk and intensity scaling, this easily measured noise term can be used as a correction to established models to more accurately estimate system performance at high optical powers and low crosstalk levels.

Pump-mediated inhomogeneous effects in EDFAs and their impact on gain spectral modeling

Summary form only given. Recently it has been shown that small changes in pump wavelength within the 980-nm pump band change the shape of the gain spectrum of erbium-doped fiber amplifiers (EDFAs). These gain variations can be as large as 3 dB in a typical line amplifier and cannot be explained by the well known homogeneous model for EDFAs. Pump-induced gain changes must be accounted for in multiwavelength amplifiers and systems operating over the pump-wavelength-sensitive short-wavelength portion of the erbium gain band. In this paper, we compare the impact of pump-mediated inhomogeneities on the gain spectra of EDFAs pumped in the 1480- and 980-nm pump bands. We find that the shape of the EDFA gain spectrum is less affected by changes in pump wavelength in the 1480-nm band, and show that in both cases the dynamic gain tilt function (DGTF) is relatively insensitive to changes in pump wavelength. The latter indicates that inversion-induced gain changes can be predicted from a (measured) DGTF despite pump band and some signal band inhomogeneities. We present fluorescence line narrowing measurements that explain these results and suggest a physical mechanism for 980-nm band pump-mediated inhomogeneities.

Experimental comparison of the effect of discrete and distributed inband cross talk on system performance

We present the first direct comparison of the impairments due to inband cross talk arising from distributed Rayleigh scattering and a single discrete interference path measured on the same system. The understanding of these cases is important because they represent the extremes of the cross talk statistics that are likely to occur in optical networks due to multipath interference (MPI), component cross talk, and four-wave mixing. We find that the commonly used Gaussian model is pessimistic for the single discrete path case. Furthermore, we show that when other intensity dependent noises are accounted for, the worst case two-beam interference model can be corrected. Our analysis provides an empirical correction with well-defined intensity and cross talk scaling, which automatically accounts for system features such as threshold optimization.

Optical Cables for Consumer Applications

As communication data rates increase, the maximum propagation distance in copper cables decreases. This trend is driving the use of fiber-optic links at shorter and shorter distances. Now, well established in the telecom and datacom market segments, optical fiber is poised to find applications in the consumer market segment. Consumer protocols such as USB and Thunderbolt are achieving data rates of 10 Gb/s and higher, and as a result, the reach of traditional copper interconnects becomes limited to a few meters. Optics can remove the distance limitation and enable longer-reach applications. However, to be successful in the consumer market, optical cables need to be robust--both optically and mechanically-and low cost. In this paper, we describe how the various elements of the optical link, including the fiber, the cable, and the coupling optics, can be engineered to work together on a system level in order to meet those requirements.

Ultra broadband optical amplifiers

Multiple optical amplifier technologies now exist that can be used to provide bandwidths exceeding 40 nm. Performance trade-offs as well as the economics of transmission systems and optical networks will determine the choice among these options.