[Paco, Radhika]
Beatnote recovery
Restarted ECDL characterization last Friday. After some lab cleanup, and beatnote amplitude optimization we borrowed Moku Lab from Cryo lab to fasttrack phase noise measurements. Attachment #1 shows a sketch of our delayed selfheterodyne interferometer. The Marconi 2023A feeds +7 dBm to a ZHA3A amplfier which shifts the frequency of the laser in one of the arms using a free space AOM. The first order is coupled back into a fiber beamsplitter to interfere with a 10 m delay line beam.
Improved beatnote detection
The 38.5 MHz beatnote was barely detectable before when using PDA20CS2 because at unity (lowest) gain stage, the bandwidth was only 11 MHz... We instead switched to an FPD310FCNIR type which has a more adequate highfrequency response. Attachment #2 shows the beatnote power spectrum taken with Moku Lab spectrum analyzer. The two vertical lines indicate (1) the heterodyne beatnote frequency and (2) the "free spectral range" indicating the actual delay in the MZ arms, which is calibrated to = 9.73 m (using 1.46 for n, the fused silica fiber index).
Phase meter and freq noise calibration
We then tried using the phase meter application on the Moku. The internal PLL automatically detected the 38.499 MHz center frequency and produced an unwrapped RF phase timeseries (e.g. shown in Attachment #3). The MZ interferometer gives an AC signal
oscillating at , i.e. the angular beatnote frequency. The delay (calibrated above) characterizes the response of the MZ relating the RF phase noise spectrum to the optical phase noise spectrum. The RF phase obtained through the phase meter has a fourier transform
So the optical phase spectral density is related to the rf phase spectral density by a transfer function Then, the RF & optical phase power spectral densities are related by or
Then, because the instantaneous laser frequency is , in fourier domain the frequency and phase PSDs are related by the magnitude square of this transfer function like
Following this prescription, we compute an estimate for the frequency noise ASD (square root of the PSD) shown in Attachment #4. The frequency noise estimated by this method has several contributions and *does not* necessarily represent the freerunning ECDL frequency noise.
Next steps
 Noise budgeting (experiment)
 Control loop (open/closed) models
