Here is a more detailed version of the setup, so that we can gather the parts we will need.
Parts list:
- Optics, etc.:
- 1 NPRO
- 2 QWP
- 3 HWP
- 2 PBS
- 2 EOM (at least one broadband)
- 2 RFPD (at least one very-high-bandwidth for TRANS, e.g., 1611)
- 1 CCD camera
- OMC curved mirrors to be tested
- 1 low-loss flat reference mirror with appropriate transmission (e.g., G&H, ATF, etc.)
- ~3 long-ish lenses for MMT, EOM focusing
- ~2 short lenses for PD focusing
- 1 R ~ 80% power splitter for TRANS (can be more or less)
- ~7 steering mirrors
- ~3 beam dumps
- Mounts, bases, clamps, hardware
- Electronics:
- 1 fixed RF oscillator (e.g., DS345, etc.)
- 1 VCO (e.g., Marconi, Tektronix, etc.)
- 2 Minicircuits RF mixers
- 2 Minicircuits RF splitters
- 2 SMA inline LPFs
- Locking servo (SR560? uPDH? PDH2?)
- Some digital acquisition/FG system
- Power supplies, wiring and cabling.
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Here is the proposed RoC measurement setup. Koji tells me that this is referred to as "Anderson's method".
We would like to use a linear cavity to measure the RoC of the curved mirrors independently (before forming the ring cavity), since the degeneracy of HOMs will make the fitting easier.
- An NPRO is PDH locked to a linear cavity formed of a high-quality flat mirror on one end, and the OMC curved optic on the other.
- A second, broadband EOM is placed after the first one, and its frequency is swept with a VCO to generate symmetric sidebands about the carrier
- A TRANS RFPD's signal is demodulated at the secondary EOM frequency, to give a DC signal proportional to HOM transmission
- This HOM scan is fit to a model, with RoC the free parameter. Since there are two sidebands, the HOM spectrum of the model must be folded about the carrier frequency.
- To get a good signal, we should slightly misalign the input beam, allowing for higher overlap with HOMs.
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