Experimental Investigation of External Optical Injection and its Application in Gain-Switched Wavelength Tunable Optical Frequency Comb Generation

A detailed experimental investigation of the effects of external optical injection on a Fabry Perot laser when in continuous wave (CW) mode and in gain switched mode is presented. Such detailed characterisation can help in optimising operating condition of the transmitters to significantly improve the system performance for dedicated applications. In CW mode, we characterised various performance indicators of a laser such as side mode suppression ratio (SMSR), wavelength, peak power, modulation bandwidth, resonance frequency, optical linewidth, and phase noise under external optical injection with the use of tuning maps. We also present in brief the theoretical origin of these parameters using rate equations. We then investigated the performance of the externally injected Fabry Perot laser under gain switching for optical frequency comb (OFC) generation. We demonstrated the generation of an OFC with a free spectral range of 6.25 GHz to 25 GHz and a quasi-continuous wavelength tunability of 31 nm. The phase noise of the tones of the generated OFC is measured and is found to be similar to that of the externally injected light and the measured optical linewidth was ∼40 kHz. We also presented experimentally the trade-off that exists between bandwidth of the generated OFC and the phase noise of individual comb tones under external optical injection and their optimisation for the application at hand.