Sazzad MS Imran
The performance of semiconductor injection lasers is limited by optical feedback noise. The laser intensity noise is greatly increased even by small amount of optical feedback. Although superposition of high-frequency current on the injection current is popularly used to solve the problem, proper modulation frequency and depth must be chosen empirically. In this paper, we numerically analyze this problem based on a set of multimode rate equations, modified to include OFB (optical feedback) and HFI (high frequency current). We concentrate only on low frequency type noise where intensity noise is maximum in the lower frequency region. The results show that the feedback noise was reduced when HFI released coupling effects and thereby suppressed mode hopping. The feedback noise was still remained high when the modulation frequency was coincided with a rational number of the round trip time period of optical feedback. A simple formula together with a tree is newly reported from experimental and numerical data to find the rational numbers at which the feedback noise remains high for a particular external cavity length.
Dependency of the rational numbers on external cavity lengths is also reported in this paper.
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