Uncooled 40Gb/s 4-level directly modulated laser source for datacoms applications

There is currently interest in using novel modulation formats for high bit-rate datacoms systems. 4-level modulation is an attractive method of halving the line-rate required for 40Gb/s systems. This 20GBaud line rate enables reduced bandwidth direct modulation of semiconductor lasers, thus reducing laser chirp, increasing transmission distances and also enabling simplified drive electronics to be used. In this experiment the 4-level signal is generated by electrically combining 2 de-correlated 20Gb/s data streams of differing amplitude from a pattern generator and then used to modulate a DFB laser. The directly modulated source is a DFB laser, emitting at 1310nm with a 3dB frequency response of 20GHz. This laser also has a very linear modulation response, with a spurious free dynamic range of over 100dBHz ^2/3 at 25°C and over 90 dBHz ^2/3 at 85°C. This highly linear behaviour is necessary to allow direct 4-level modulation source even at high temperature. The 40Gb/s 4-level signal is then transmitted along standard fibre and detected with an electrical receiver. In order to overcome the attenuation limited transmission distance of 20km a semiconductor optical amplifier, with a saturation power of 11dBm and fibre to fibre gain of 20dB, is used. The addition of an SOA enables transmission distances of 40km to be achieved with transmission penalties of as low as 2.6dB, even with the laser operating at 70°C. The robustness of the 4-level modulation is compared to NRZ and the impairments to both signals upon optical amplification are examined.