Modal noise reduction in analog fiber optic links by superposition of high-frequency modulation

Abstract

ABSTRACT In this paper modal noise reduction by superposition of high frequency modulation is analyzed. Detailedanalysis revealed the dependence of the modal noise reduction factor on the modulation frequency and fiberlength, for both direct and external modulation. Experimental results confirmed that for multimode fiber,the reduction of the modal noise depends almost only on the modulation depth. 1.0 INTRODUCTION Although multimode fibers lost their battle with single-mode fibers in long-haul applications, they arestill first choice for such applications, like local area networks, or airplane industry. Main reason for this istheir insensitivity to alignment constraints (opposite to single mode fibers). Other advantages are their lowcost and the use of cheap lasers with low coherence time, as well as the use of GaAs OEIC technology at 800nm. However, because of the waveguide properties of the multimode fibers, together with the source powerspectrum (especially if it is high-coherence LD), the level of the modal noise may easily become very high,which leads to high BER, or poor signal/noise ratio.The modal noise is result of the interference among mode patterns of the different fiber modes, whenthe source is highly coherent. On the other side, phases are constantly fluctuating, which causes the specklepattern at each connector to fluctuate. For these reasons, the transmission through misaligned connectorsis dependent on the portion of speckles captured by the fiber. Speckle pattern is very sensitive to externaldisturbances, like temperature changes, vibrations etc.