Figure 3

Frequency-dependent reflection characteristics of the external-cavity circuit calculated with the parameters extracted from through-measurement (GC 1 → GC 3), see Supplementary Section 2. (a) Drop-port transmission of the individual rings. (b) Overall reflection spectrum of the external-cavity circuit. The main peak height indicates a minimum on-chip reflection loss of 1.7 dB, see Inset. (c) Exemplary gain spectrum of the RSOA according to Fig. 2b for an injection current of I = 20 mA (blue line) with schematic resonance peaks (black lines) as in (b). The blue solid line corresponds to measured values of the RSOA gain, whereas the blue dashed line is an estimated extrapolation towards higher frequencies. The solid black lines indicate the strong main reflection peak at the target frequency f₀, which, for the depicted case, corresponds to the center frequency (f_c = 193.2 THz; λ_c = 1550 nm) of the gain spectrum, along with the most prominent side peaks that are most prone to unwanted lasing. The dotted black lines indicate the corresponding peaks after a detuning by \(\Delta f > \Delta f_{\min } /2\) where \(\Delta f_{\min } \approx [T] \times {\text{FSR}}_{2}\), indicated by green arrows.