Direct frequency modulation of photonic crystal laser by thermal tuning with low-intensity modulation (Conference Presentation)

Abstract

The ever decreasing demand for bandwidth in optical communications has made silicon photonics one of the promising technologies as it can dramatically reduce energy consumption and footprint in photonic integrated circuits (PIC). Many research efforts have aimed to incorporate silicon into the PIC platform by using it as a resonant reflector in the form of a microdisk, racetrack resonator, ring resonator or photonic crystal (PhC) cavity. Tuning of these devices allow for modulation of the lasing frequency by means of the electro-optic or thermo-optic effect. Our solution utilises a III-V hybrid laser with a reflective semiconductor optical amplifier (RSOA) and a PhC cavity resonant reflector. Current research shows electro-optical modulation of a PN junction on the Si-reflector as a means of tuning the reflectance wavelength. This work focuses on the thermo-optical effect in silicon to achieve modulation of the lasing frequency. Modulation of the current to the PN junction on the Si-reflector of the external cavity laser will change the refractive index which will tune the reflectance wavelength and hence modulate the lasing frequency. PhC cavities are smaller in area than a typical ring resonator and have larger free spectral range that results in less severe mode competition effects. For trace gas detection a frequency modulated laser scanned across the absorption frequency of the target gas will result in change in the output power of the laser. The PhC laser we demonstrate shows to have a very small intensity modulation (IM) on the output offering it as an ideal candidate for this application. Experimental results show the laser to have a threshold current of 15 mA with output optical power of 300 µW. With an applied heating power of 25 mW, a frequency shift of 10 GHz was observed. At a modulation frequency of 10 kHz, a modulation depth of 2 GHz was observed.