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Under proper injection of an incoming optical signal to be format-converted, a semiconductor laser can be driven at period-one dynamics due to the dynamical competition between the injection-imposed laser oscillation and the injection-shifted cavity resonance. Equally separated spectral components therefore emerge, the intensity and frequency of which strongly depend on the intensity and frequency of the incoming optical signal. Optical modulation format conversion between amplitude-shift keying (ASK) and frequency-shift keying (FSK) can thus be achieved by applying such a mechanism. The conversion depends solely on the property of the incoming optical signal for a given laser and therefore only a typical semiconductor laser is necessary as the key conversion unit. Due to the unique underlying mechanism, both ASK-to-FSK and FSK-to-ASK conversions can be achieved using the same system. The bit-error ratio at 10 Gb/s is observed down to 10-12 with a slight power penalty for both conversions. Simultaneous frequency conversion of the incoming optical carrier is also possible. By adopting different spectral components or different injection conditions, different output modulation indices can be obtained.