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Generation of coherent frequency combs in optical microresonators at normal GVD is a challenging task. It is well known that they can be generated in the form of the flat-top solitonic pulses, platicons, via controllable mode interaction or modulated pump. However, such methods are rather complicated, requiring either sophisticated mode interactions, complex two-cavity systems or high-frequency modulators. Recent investigations have shown that the self-injection locking effect provides interesting possibilities for frequency comb generation. It has been shown that this effect not only provides laser stabilization due to the resonant backscattering of laser radiation from the high-quality-factor microresonator but also leads to the nontrivial nonlinear dynamics in the same microresonator. First, this has been demonstrated for bright solitons with an ordinary laser diode as a pump source. Recently, it has been shown experimentally that such approach is also applicable for platicon generation and does not require additional equipment. In our work we study this process in detail and identify different generation regimes depending on the combination of the pump power and the backscattering coefficient providing the self-injection locking effect. The range of parameters necessary for the efficient platicon generation is found. We also report a novel mechanism of platicon generation based on the thermal effects inevitable in real-life systems. We show that it is possible if thermal effects are negative (the direction of the thermal shift of the microresonator resonance is opposite to the direction of the nonlinear shift) and the ratio of the thermal relaxation time to photon lifetime is small enough. Different generation regimes are found, and the possibility of the turn-key operation regime is demonstrated.
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