The high speed frequency modulation (FM) capability of DFB lasers1 is attractive for Gbit/s direct optical modulation since it avoids the wavelength chirping of direct on/off intensity modulation and the additional systems complexity and loss of external modulation. There have been recent reports of frequency shift keying (FSK) modulation of a DFB laser at 5 Gbit/s2 and direct phase modulation at 4 Gbit/s3 using a related technique of FM with a return-to-zero (RZ) signal format. However, thermal FM which accompanies injection current modulation with non-return to zero (NRZ) or RZ formats can severely distort the demodulated signal.4 We report here direct optical DPSK modulation of a 1530-nm DFB laser at 5 Gbit/s without thermal FM using injection current modulation with a bipolar signal format.5

© 1989 Optical Society of America

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