Photon-photon resonance (PPR) has been researched to achieve high modulation frequency bandwidth in direct modulation laser diode (LD). The phenomena of PPR, which has the potential of having a resonance peak at a higher frequency rather than carrier-photon resonance (CPR, namely, regular resonance), is based on different internal oscillation in a laser chip, therefore, it is essential to involve plural different internal optical paths in a laser diode chip. One biggest issue is to achieve flat frequency response as the frequency response normally deteriorate at a higher frequency region than CPR. To mitigate this issue, we have demonstrated multiple PPR by using active-MMI configuration, and have proved more than 40 GHz band-width so far successfully. In this paper, we discuss about the recent progress of the active-MMI laser diode which enables to realize this multiple PPR phenomena, and the recent results. We hope and believe that this approach will contribute to realize over Tbps direct modulation laser diode, which will be integrated in the high-speed interconnection on board in the future.
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