Raman Amplification Using High-Power Incoherent Semiconductor Pump Sources
Source/Type: White Papers/Technical Papers

Author: Ahura Corporation

March 31, 2003...

Abstract: We have demonstrated 7 dB of C-band amplification from a DRA; pumped using novel incoherent high-power (>450 mW) semiconductor sources with a broad spectral bandwidth(>35 nm). The low-frequency RIN of these sources can be suppressed below -140 dB/Hz.

1. Introduction
Distributed Raman amplifiers (DRAs) have attracted much attention as they can be used to extend the reach of longhaul DWDM communication systems. DRAs have many inherent advantages over their erbium doped fiber amplifier (EDFA) counterparts. Broader and flatter gain spectrums with lower effective noise figure can be achieved at center wavelengths covering not only the C-Band but S-Band, L-Band, and 1300 nm bands. Despite these advantages, DRA performance suffers from limitations imposed by today’s high-power pump laser technology.

We have demonstrated DRA amplification using high-power and spectrally incoherent semiconductor pump sources rather than traditional laser sources. The ability to create modules with more than 450 mW of broadband pump light allows us to address many of these DRA limitations in a fundamental manner.

In particular, the noise performance of high-power pump lasers is a critical challenge. Counter-pumped DRA configurations are the most widely used as noise from the laser pumps is effectively averaged along the length of the amplifier such that only noise spectra ~ <10 kHz efficiently transfers to the signal [1]. Fabry-Perot semiconductor lasers and fiber lasers with relative-intensity-noise (RIN) as high as -110 dB/Hz can thus successfully be used [2]. Co-pumped, or forward, DRA configurations are desirable to a system architect as it allows the launched signal power to be reduced which intern reduces quality (Q) penalties from optical non-linearities [3]. However, for forward DRAs the fast gain dynamics of the amplifier allows efficient RIN transfer between the co-propagating pump light and signal light to frequencies beyond 10 MHz; the cutoff frequency is dictated by the fiber’s dispersion [1]. This wide bandwidth of efficient RIN transfer limits the maximum allowable low-frequency RIN which yields acceptable system Q.

In addition to noise performance challenges, wide gain bandwidth and flatness are traditionally achieved using a multitude of high power pump lasers. To reduce the polarization dependent gain, each pump wavelength requires light to be launched at two orthogonal linear polarizations. For the case of semiconductor pump lasers, this requires the multiplexing of generally 2 to 16 lasers in polarization and wavelength. Narrow linewidth pump lasers are therefore very desirable.

Fiber-Bragg-grating stabilized Fabry-Perot lasers have a stable center linewidth, however, their low-frequency noise performance is questionable. Further, the dimensional requirement of having the grating’s location ~1 meter away from the semiconductor facet precludes construction of a single butterfly package containing all of the needed pump sources and WDM optics in a compact footprint with efficient electrical power consumption.

Distributed feedback lasers (DFB) are more compatible with this higher level of package integration. There have also been many progresses towards achieving reliable high power devices [4]. The RIN of a DFB can be better than -150 dB/Hz. However, their inherent narrow linewidth leads to stimulated Brillouin scattering (SBS) at pump power of only ~5-10 mW. The frequency modulation for suppression of SBS, in the case of forward DRAs, can lead to an amplitude modulation of the pump that is transferred to the signal and degrades system Q. Furthermore, at least two DFB chips per wavelength for each polarization and several wavelengths for gain flatness are required. The long coherence length of the laser also makes a small and compact “de-polarizer” impractical.

As we will subsequently describe, the nature of broadband light allows us to address many of these challenges in a fundamental manner. View full article in PDF format (600K)

Authors: D. Vakhshoori, M. Azimi, P. Chen, B. Han, M. Jiang, K.J. Knopp, C.C. Lu, C.J. Pinzone, Y. Shen, G. Vander Rhodes, S. Vote, P.D. Wang, X. Zhu

Ahura Corporation, 46 Jonspin Road, Wilmington, MA 01887 Tel: 978-657-5555 x102 Fax: 978-657-5921 Email: kjknopp@ahuracorp.com

 Return to Search Results
Start a New Search

*****