In response to the increase in Internet traffic, researchers at NEC Corp. in Shiga, Japan, are investigating low-cost, high-performance distributed feedback lasers for use in metropolitan area and access networks. They have demonstrated that the phase shift in distributed feedback corrugations affects the directly modulated lasers' chirping characteristics and external optical feedback resistance as a result of feedback.Researchers at NEC Corp. have investigated the relationship among phase shift, chirp and external optical feedback in distributed feedback lasers. They fabricated the devices in InGaAsP. Courtesy of Yidong Huang. Directly modulated distributed feedback lasers are inexpensive, but they display chirp that restricts the distance a signal can travel with an optical power supply. External optical feedback is another option, but it changes the diodes' threshold current, efficiency, linewidth and intensity noise. It also deteriorates the modulated diagram and produces bit-error-rate saturation. Moreover, the optical isolators that shut off external optical feedback make the laser modules large and expensive.Comparison resultsThe researchers compared λ/8, λ/4 and 3λ/8 phase-shifted InGaAsP distributed feedback lasers in the new work. They were able to verify that adjusting the phase shift diodes resulted in less chirp, and it improved external optical feedback resistance. The scientists also found that reducing the phase shift in the distributed feedback structure improved the transmission characteristics and external optical feedback resistance of the directly modulated laser diodes.Based on their findings, the scientists demonstrated a low-power penalty of less than 1 dB in a 100-km span at 2.5 Gb/s using a directly modulated λ/8 phase-shifted distributed feedback laser. The devices also enabled isolator-free transmission over 80 km under an external optical feedback of -14 dB.