We have demonstrated an operation and 65-GHz bandwidth simultaneously for three different designs of high-speed directly modulated lasers (DMLs), including distributed reflector (DR) laser, modified-grating DBR laser, and DFB laser with a reflection from an integrated passive waveguide. The DMLs maintained a RIN lower than -155 dBc/Hz under an optical reflection of as high as < -8 dB. The device physics behind the operation relates to the cavity design which has a strongly wavelength-dependent mirror loss. Modified-grating DBR laser was used to demonstrate an error-free 100 GBd PAM2 transmission, and also PAM4 transmission. This is the record fast data rate realized by DML as of today.
Period doubling is an irregular phenomenon exhibited by semiconductor lasers under high frequency modulation. When it occurs, the repetition frequency of the laser power becomes half of that of the modulation signals, which is undesirable to most applications. This paper reports the control of period doubling in modulated semiconductor lasers by external optical injection and demonstrates for the first time that such nonlinear dynamics can be used advantageously in realizing all-optical clock frequency division. We show that period doubling in modulated semiconductor lasers can be either suppressed or enhanced by providing external CW optical injection. The dependence on injection wavelength, and injection power has a been investigated systematically to establish an improved understanding towards the control of period doubling in modulated semiconductors lasers. The fact that period doubling can be enhanced by optical injection is further used to realize all-optical clock frequency division. To demonstrate this, an optical clock signal at 19.6 GHZ was injected into a semiconductor laser. By adjusting the resonance frequency of the laser to around 9.8 GHz through dc bias, strong period doubling was observed, which resulted in a frequency-halved optical clock signal at 9.8 GHz with a remarkably low level of phase noise. Our investigations have also shown that without changing the biasing condition of the laser this low level of phase nose can be maintained within 1-dB range over an input frequency range of 400 MHz, which is a distinct advantage over other techniques.