The fast fine tuning can be performed by scanning the external cavity length and/or the injected current. To observe the fine tuning behavior, the resolution of our monochromator is not successive peaks for the used FPI is 0.05 cm 1 (1.5 GHz). The FPI peaks are obtained by scanning the ECL length by generating a sine wave at 0.5 kHz using a function generator (Tektronix, AFG3021B) and a piezo controller (Thorlabs, MDT693B) modulating the ECL piezoelectric transducer (Thorlabs, PE4). A fine tuning range is found to be 7 cm 1 (210 GHz) by counting the number of FPI peak intervals (140 peaks).