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A birefringent fiber loop (BFL) is proposed to improve the signal from a directly modulated distributed feedback laser. The BFL is a simple and passive fiber device. The BFL reduces amplitude transients, frequency chirp, and pattern dependence. It also improves the extinction ratio of the directly modulated optical signal. We show that the BFL effectively extends the 10-Gb/s transmission range to 25 km at 1550-nm wavelength over conventional single-mode fiber.