Figure 6
From: Hybrid external-cavity lasers (ECL) using photonic wire bonds as coupling elements
FM-noise spectrum and linewidth measurement. The FM-noise spectrum of the ECL is obtained through heterodyne detection with a narrow-band reference LO laser (Keysight N7714A) and subsequent digital signal processing (red trace). The sampled time-domain waveform comprises 9 × 105 points, recorded at a sampling rate of 256 GSa/s. The spectrum is first smoothed by taking a moving average over 50 neighboring points, and then fitted by a model function of the form \(S_{{\text{F}}} (f) = S_{0} + S_{1} f^{ - 1}\) (solid blue line). An instantaneous linewidth of \(\delta f = \pi S_{0} \approx 105\;{\text{kHz}}\) is extracted from the spectrally white part of the FM noise spectrum (dashed blue line). In an independent experiment, we also superimpose the tones of two nominally identical LO lasers and extract the FM noise spectrum for each of these lasers (black trace). This measurement confirms that the FM noise level of the LO laser line is at least one order of magnitude below that of the ECL-LO beat note, thereby supporting the assumption that the extracted Lorentzian linewidth of 105 kHz can indeed be attributed to the phase noise of the ECL for the frequency range considered here.
