[Izumi, Yokogawa]
Yesterday, it was confirmed that the broadband structure from 1 Hz to 100 Hz on the PLL feedback spectrum disappeared as we swiched off the KOACH air system (log6796), but there was flat noise after around 10 Hz.
We concluded that these noise is freerun frequency noise of the PROMETHEUS laser.
what we did:
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made a comparison of the relationship between the measured PLL feedback spectrum and the spectrum obtained from the noise budget.
- Here is the plot of these spectra. The pink line is the measured PLL feedback spectrum and the gray line is the spectrum obtained from the noise budget culculation. We assumed that the free-run frequency noise of the PROMETHEUS is 1e6/f as a typical value. These are matched at 1 to 10 Hz, but there are strange noises after 10 Hz.
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tried to do PLL by using the low noise VCO instead of the laser frequency and evalueted the sensing noise of PLLX loop.
- We saw that there is no relationship between the strange noises and the sensing noise of PLL.
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made a comparison of the relationship between the PLL feedback spectrum and the PDH feedback spectrum
- If the PLL works well, the free-run frequency noise of the PROMETHEUS laser and the sensing noise of PLL should be seen in the PDH feedback spectrum multiplied by 1/(1+G) and G/(1+G) respectivelly. (G is the olg of PLLX loop.) Here is the screenshot of these spectra. We indeed see that the PDH feedback spectrum is limited by the PLL sensing noise around 100 Hz and the free-run frequency noise of PROMEHTEUS laser is suppressed by the factor 1/(1+G) after 1 kHz. Therefore, the strange noise in the PLL feedback spectrum is the freerun frequency noise of the PROMETHEUS laser.
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