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The negative impact of cascaded wavelength conversions has been largely ignored in optical burst switching performance evaluations. When optical bursts are transmitted all optically from source to destination, each wavelength conversion performed along the lightpath causes some signal-to-noise deterioration. If the distortion of the signal quality becomes significant enough, the receiver would not be able to recover the original data. In this paper we examine the performance degradation when an upper bound on the number of wavelength conversions that a signal can go through is enforced. We refer to this constraint as conversion cascading constraint. We propose a novel proactive routing scheme that considers the instantaneous link congestion at the moment when bursts arrive. It has three major advantages: 1) utilize the same offset times for the same node pairs while providing dynamic routing without using any fiber delay lines; 2) decrease burst loss probability to a great extent; 3) mitigate unfairness among bursts with different hop counts, which is even worse under conversion cascading constraint if without any special treatment.