T.A. Strasser

Rayleigh scattering limitations in distributed Raman pre-amplifiers

The properties and fiber dependence of distributed Raman pre-amplifiers and, in particular, the effects of Rayleigh backscattering, which limits the sensitivity improvements that can be realized, is discussed. Bit-error-rate (BER) measurements show improvements In the effective receiver sensitivity of 6.2 and 7.0 dB for dispersion shifted fiber (DSF) and silica-core fiber (SCF), respectively. Theoretical predictions of the effective noise figure based on measured fiber parameters indicate improvements of 6.6 dB for DSF and 7.4 dB for SCF in good agreement with the measured optical SNR. The optimum receiver sensitivity is obtained in DSF with less than 600 mW of pump power, whereas the 0.8-dB superior performance of the SCF requires approximately 1 W.

In-line polarimeter using blazed fiber gratings

We fabricate highly blazed, polarization-sensitive fiber grating taps and show how these may be used in combination with a UV-induced fiber waveplate to form a compact, in-fiber polarimeter. We show how the polarimeter may be employed as a feedback element to control polarization and use the feedback loop to demonstrate the stable, broadband (>70 nm) operation of the fiber polarimeter.

Capacity upgrades of transmission systems by Raman amplification

Raman amplification is added to a transmission system to provide an increase in power budget allowing for a four-fold upgrade in capacity either by TDM or WDM. Here, a TDM upgrade from 2.5 to 10 Gb/s and a WDM upgrade from a single 10-Gb/s channel to four channels of 10 Gb/s each were experimentally verified with an improvement in the power budget of 7.4 dB. Raman gain is an attractive upgrade method for installed systems requiring no changes to the fiber span.

Cladding-mode resonances in hybrid polymer-silica microstructured optical fiber gratings

We examine the cladding-mode resonances of fiber gratings UV written into the core of a novel microstructured fiber whose air regions are subsequently infused with polymer. The cladding spectrum changes with temperature because of the strong temperature dependence of the polymer refractive index and we show that these changes can be understood qualitatively using a simple model of the polymer-silica waveguide. Our results imply wavelength and amplitude tuning regimes for long period gratings written into this fiber with tunability enhanced over conventional long period gratings because of the small effective inner cladding diameter of our fiber.

Concatenated fiber grating optical monitor

A new optical wavelength-division-multiplexed (WDM) wavelength monitor comprised of concatenated fiber Bragg gratings and an InGaAsP electroabsorption optical modulator is described. Input WDM signals were modulated at 10 kHz with 15-ns-width pulses and the signals reflected from the gratings were detected with an InGaAs p-i-n photodiode and displayed on an oscilloscope as a series of pulses corresponding to each wavelength component.

Access PON using downstream 1550-nm WDM routing and upstream 1300-nm SCMA combining through a fiber-grating router

We describe an 8-channel access passive optical network using a fiber grating router at the remote node, 50 Mb/s 1550-nm WDM downstream transmission from an integrated multifrequency laser, and 3 Mb/s 1300 nm SCMA upstream transmission. This approach combines the advantages of a downstream WDM PON and an upstream low-cost power-combining PON.

Advances in fiber optics

This paper describes the development of fiber optics technology — an essential element of the telecommunications revolution — and reviews the remarkable progress in fiber design and fabrication, optical amplifiers, UV-induced fiber gratings, novel microstructured optical fibers, and integrated optics. The paper also explores the limits of optical fiber channel capacity and considers the future of optical communications.

Raman amplifier with improved system performance

Summary form only given. We describe the predominant sources of noise for various amplifier configurations. In addition, we present the results of a successful 2.5 Gbit/s systems test of a two-stage Raman amplifier with net small signal gains of 29 dB and output powers exceeding + 15 dBm. Although Raman amplifiers do not suffer from increased noise figure due to incomplete population inversion, the passive loss of the amplifier does contribute to the noise figure through increased signal-spontaneous beating. Nonetheless, we have constructed a Raman amplifier with a noise figure contribution from signal-spontaneous beating that is less than 5 dB. A second source of noise in Raman amplifiers is the coupling of intensity fluctuations in the pump light to the signal. The fundamental cause of this noise is the lack of a long upper-state lifetime to buffer the Raman gain from fluctuations in the pump intensity. We present results demonstrating that the transit time through the amplifier can be used to average gain fluctuations if a strictly counterpropagating-pump geometry is employed. Lastly, double Rayleigh backscattering also contributes to the noise figure of the amplifier but can be reduced by decreasing fiber lengths and by constructing multistage amplifiers. Based on the above considerations, the two stage Raman amplifier with a counterpropagating pump was constructed. The requisite pump light was generated by a cascaded Raman laser operating at 1240 nm that was pumped by a cladding-pumped neodymium laser operating at 1060 nm. A 2.5 Gbit/s bit-error-rate test of this amplifier was performed.

ULTRAVIOLET LASER FABRICATION OF STRONG, NEARLY POLARIZATION-INDEPENDENT BRAGG REFLECTORS IN GERMANIUM-DOPED SILICA WAVEGUIDES ON SILICA SUBSTRATES

Strong Bragg reflectors (≳4 nm width) have been fabricated in Ge‐doped silica waveguides on silica substrates by ultraviolet (UV)‐induced refractive index change. Index changes in excess of 3×10−3 were obtained by deuterium sensitization. The passband quality, polarization dependence, and loss of the reflectors are reported.

Reflective-mode conversion with UV-induced phase gratings in two-mode fiber

In future fiber communications systems, efficient mode conversion has the potential to enable improved devices for dispersion compensation and wavelength routing. Previous reports of symmetric-asymmetric mode conversion in circular, multimode optical fibers have shown nonideal spectral characteristics due to polarization dependence and/or mode splitting of nearly degenerate components of the higher order LP/sub 11/ mode. This work avoids these problems through use of nominally polarization-independent UV-induced phase gratings while exploiting a reflective phase-matching condition that is much less sensitive to the mode splitting of the LP/sub 11/ mode than prior work.