[Tanaka, Komori]

Abstract:

We evaluated several important parameters of the IMC, including MCI transmissivity, round trip loss, and throughput.
Here, we assume that the transmissivity of MCI and MCO is identical.

• MCI transmissivity: (0.64 ± 0.01)%
• Roundtrip loss: (2000 ± 100) ppm = (0.20 ± 0.01)%
• Throughput: (74 ± 1)%

Details:

We analyzed data from one FSR cavity scan.
Based on the MCE transmission (Fig.1) and the PDH signal (Fig.2 magenta), we determined the mode matching ratio at this time to be (93 ± 1)% and the modulation index of the RF sideband for the IMC control to be 0.09 ± 0.01.

From the IMC reflection (Fig.2 red), we estimated how closely the IMC approaches critical coupling.
By measuring the reflection depth, we obtained the MCI (MCO) transmissivity, IMC round trip loss, and cavity throughput assuming perfect mode matching, as summarized above.
Important information for this calculation includes the mode matching ratio, dark offset of the PD, and cavity finesse, estimated to be 420 ± 10 from the cavity pole (klog:29071) and the IMC absolute value (klog:29084).

The estimated MCI (MCO) transmissivity is surprisingly consistent with the LMA measurement (0.63%) referenced in klog:618.
However, due to the non-negligible round trip loss, the finesse is lower than the designed value, and the estimated throughput is lower than expected.
This throughput is not consistent with the ratio of the input and output power of IMC in the IMC medm screen (roughly 80% now), indicating a potential additional 5% error in either measurement or both.