Suppression of Raman-induced pilot tone crosstalk using optical allpass filters

Abstract

We propose a new type of optical allpass filter useful for mitigation of Raman-induced crosstalk among pilot tones and similar low-frequency labels. Crosstalk rejection was characterized in a 40-wavelength lab test system, and shown to agree well with theory. Pilot Tones in Photonic NetworksLow-frequency tones overmodulated onto a digital data stream have been widely studied as a means of tracing lightpaths and monitoring signal quality in photonic networks [1][2][3][4][5]. Typically, each laser transmitter is assigned a unique tone frequency that allows tracing of the lightpath it originates. At each monitoring point, a small fraction of the light in the transmission fiber is tapped off and fed to a low-speed, all-wavelength receiver. Frequency lines in the demodulated electrical spectrum serve as labels to determine which lightpaths are present, and at what strength. However, nonlinearities in the optical transmission system give rise to crosstalk among these labels that degrades the effectiveness of the system. That is, a 10-kHz tone originally present on one wavelength can be transferred to other wavelengths sharing the same fiber or optical amplifier, so that a 10-kHz line will appear in the monitor receiver even after the original wavelength has been blocked or dropped. This phenomenon is sometimes referred to as a 'ghost tone'. As noted by Chung et al. [5], the most severe ghost tones arise from cross-gain modulation (XGM) in erbium-doped fiber amplifiers (EDFAs) and from stimulated Raman scattering (SRS) in transmission fibers. The effects of XGM can be mitigated either by fast gain stabilization in the EDFAs or by raising the frequency of the pilot tone above 1 MHz [5]. SRS-induced crosstalk, on the other hand, remains serious for tone frequencies up to 100 MHz or more, and may limit the number of spans over which pilot tones can be used effectively. Like the closely-related effect of Raman-induced gain tilt, the crosstalk becomes more severe as wavelength count, channel spacing, and launch power increase.In this paper, we introduce a new type of all-pass optical filter which suppresses SRS-induced crosstalk, enabling the use of pilot tones in systems with longer reach. Theoretical predictions of the filter's effectiveness are confirmed by a 40-wavelength lab experiment. Principle of Operation and TheoryA theoretical model of SRS-induced crosstalk in WDM links, based on a triangular Raman gain profile, was developed by Phillips et al. [6]. The Raman interaction between two wavelengths causes a transfer of energy from the shorter wavelength to the longer one, at a rate proportional to the product of the optical powers of the two wavelengths. Thus, modulation on the shorter wavelength will be transferred to the longer one, and vice versa. In