The generation of on-chip optical frequency combs in silicon-based charge injection modulators through strong phase modulation has been investigated as a prospective source for on-chip wavelength division multiplexing (WDM). Being CMOS-fabrication compatible, these modulators are easily integrated with other silicon photonic components. Resonant structure based implementations using PN-doped silicon micro-ring modulators have been used to generate frequency combs. However, in contrast to linear modulators, the use of such resonant structures leads to resonance shifts resulting from microwave induced thermal effects as well as causing a degradation in the quality of the modulator resonance. In this work, we report and investigate interesting effects observed as the optical and microwave power driving the comb generator is changed. We utilize a comb generator operating at a repetition rate of 10GHz and driven by optical powers in the range of 3mW to 30mW and microwave power in the range of 6dBm to 25dBm. We observe novel effects wherein the skewness of the comb envelope, the number of comb lines generated varies in a non-monotonic function with changes in optical power and DC bias. Abrupt transitions are reported from a broadband comb to no comb generation conditions with slight changes to drive RF powers. We attribute the variations to an interplay between the thermo-optic effects in the ring and its impact in moving the resonance of the ring with changing power.
Measuring the profile of a laser beam is of critical importance, especially for high power laser systems. Although different techniques exist to measure the beam profile, owing to the use of optoelectronic detectors or cameras, they primarily work at lower powers and require tapping and attenuating the beam. In this process, there is potential for the diagnostic system affecting the beam quality. In this work, we propose a simple technique which can measure the beam profile at full power using a thermal imager without the need for additional optical components. The method involves taking a thermal image of the beam while it is incident on an absorptive surface such as a thermopile head which is used to measure optical power. In addition, a second image is taken using a focused incidence on the surface at low powers. The second image which is reused provides the point spread function. We then make use of the linearity of the heat equation which allows the deconvolution of the point spread function from the original image to obtain the actual beam profile. In this work, we utilized the technique to directly analyze the beam profile at full power of a 100 W class fiber laser and analyzed deviations from single-modedness. In addition, we utilized offset splices to few-mode fibers to launch higher order modes at the 100W level and demonstrate their direct characterization of multimode nature of the profile. This technique provides a simple alternative, using instruments present in most laser labs for direct, high power laser beam profiling.