Multi-wavelength emission using compact semiconductor ring laser with filtered optical feedback

Laser diodes that emit multiple wavelengths simultaneously are needed in a range of applications including wavelength division multiplexing, high speed optical networks, optical instruments testing, optical sensing and tera-hertz generation. In this work we report on an integrated approach to obtain multi-wavelength emission from a semiconductor ring laser based on on-chip filtered optical feedback. Semiconductor ring laser have the advantage that they can be easily integrated with other optical components as they do not require mirrors to form the cavity. Moreover, no thermal control of the wavelength emission is needed and therefore the device can be in principle fast. Ring lasers can be used for switching as they support lasing in two directions (clockwise and counterclockwise). The device consists of a semiconductor ring laser, two arrayed waveguide gratings and four semiconductor optical amplifiers. The different components are connected by passive waveguides. Light at the semiconductor ring laser's output is coupled to the arrayed waveguide gratings by a directional coupler. The filtered optical feedback is realized by employing the two arrayed waveguide gratings to split/recombine light into different wavelength channels. Semiconductor optical amplifiers are placed in the feedback loop in order to control the feedback strength of each wavelength channel independently. Experimental observations have shown that the effective gain is the key parameter that has to be balanced using the feedback in order to achieve multi-wavelength emission. This can be achieved by tuning the injection current in each amplifier which will change the feedback phase and strength. We can select the number of emitted wavelengths by changing the number of gates being pumped. We can tune the emitted wavelengths by changing the currents injected in each of the gates. Numerical simulations using rate equation reproduce the experimental results and show the effects of feedback phase and strength on the multi-wavelength emission.