For various reasons, I had to switch NPROs (from the LightWave 126 to the Innolight Prometheus).
I installed the laser, realigned the polarization and modulation optics, and then began launching the beam into the fiber, though I have not coupled any light yet.
A diagram is below. Since I do not yet have the AOM, I have shown that future path with a dotted line. Since we will not need to make AMTFs and have a subcarrier at the same time, I have chosen to overload the function of the PBS using the HWP after the AEOM. We will operate in one of two modes:
- AMTF mode: The AOM path is used as a beam dump for the amplitude modulation setup. A razor dump should physically be placed somewhere in the AOM path.
- Subcarrier mode: The AEOM is turned off and the HWP after it is used to adjust the carrier/subcarrier power ratio. I chose a 70T / 30R beamsplitter for the recombining, since we want to be able to provide ~100 mW with the carrier for transmission testing, and we don't need a particularly strong subcarrier beam for probing.
One thing that concerns me slightly: the Prometheus is a dual-output (1064nm/532nm) laser, with separate ports for each. I have blocked and locked out the green path physically, but there is some residual green light visible in the IR output. Since we are planning to do the OMC transmission testing with a Si-based Thorlabs power meter---which is more sensitive to green than IR---I am somewhat worried about the ensuing systematics. I *think* we can minimize the effect by detuning the doubling crystal temperature, but this remains to be verified.
EDIT (ZK): Valera says there should be a dichroic beam splitter in the lab that I can borrow. This should be enough to selectively suppress the green.
EDIT (ZK): Koji points out that (1 - Ti) should really be the non-resonant reflectivity of the aligned cavity, which is much closer to 1. However, it should *actually* be the non-resonant reflectivity of the entire OMC assembly, including the steering mirror (see bottom of post). The steering mirror has T ~ 0.3%, so the true results are somewhere between my numbers and those with (1 - Ti) -> 1. In practice, though, these effects are swamped by the other errors.
More information about the power-dependent visibility measurement:
As a blanket statement, this measurement was done by exact analogy to those made by Sam and Sheon during S6 (c.f. LHO iLog 11/7/2011 and technical note T1100562), since it was supposed to be a verification that this effect still remains. There are absolutely better ways to do (i.e., ways that should give lower measurement error), and these should be investigated for our characterization. Obviously, I volunteer.
All measurements were made by reading the output voltages produced by photodetectors at the REFL and TRANS ports. The REFL PD is a BBPD (DC output), and the TRANS is a PDA255. Both these PDs were calibrated using a Thorlabs power meter (Controller: PM100D; Head: S12XC series photodiode-based---not sure if X = 0,2... Si or Ge) at the lowest and highest power settings, and these results agreed to the few-percent level. This can be a major source of error.
The power was adjusted using the HWP/PBS combination towards the beginning of the experiment. For reference, an early layout of the test setup can be seen in LLO:5978 (though, as mentioned above, the REFL and TRANS PDs have been replaced since then---see LLO:5994). This may or may not be a "clean" way to change the power, but the analysis should take the effect of junk light into account.
Below is an explanation of the three traces in the plot. First:
- TRANS: TRANS signal calibrated to W
- REFL_UL: REFL signal while cavity is unlocked, calibrated to W
- REFL_L: REFL signal while cavity is locked, calibrated to W
- Psb: Sideband power (relative to carrier)
- Ti: Input mirror transmission (in power)
Now, the traces
- Raw transmission: This measurement is simple. It is just the raw throughput of the cavity, corrected for the power in the sidebands which should not get through. I had the "AM_REF" PD, which could serve as an input power monitor, but I thought it was better to just use REFL_UL as the input power monitor and not introduce the error of another PD. This means I must also correct for the reduction in the apparent input power as measured at the REFL PD due to the finite transmission of the input coupler. This was not reported by Sam and Sheon, but can be directly inferred from their data.
- trans_raw = TRANS ./ ( REFL_UL * (1 - Psb) * (1 - Ti) )
- Equivalently, trans_raw = (transmitted power) ./ (input power in carrier mode)
- Coupling: This is how much of the power incident on the cavity gets coupled into the cavity (whether it ends up in transmission or at a loss port). Sheon plots something like (1 - coupling) in his reply to the above-linked iLog post on 11/8/2011.
- coupling = ( REFL_UL * (1 - Ti) - REFL_L ) ./ ( REFL_UL * (1 - Psb) * (1 - Ti) )
- Equivalently, coupling = [ (total input power) - (total reflected power on resonance) ] ./ (input power in carrier mode)
- Visibility: How much of the light that is coupled into the cavity is emerging from the transmitted port? This is what Sam and Sheon call "throughput" or "transmission" and is what is reported in the majority of their plots.
- visibility = TRANS ./ ( REFL_UL * (1 - Ti) - REFL_L )
- Equivalently, visibility = (transmitted power) ./ [ (total input power) - (total reflected power on resonance) ]
- Also equivalently, visibility = trans_raw ./ coupling
The error bars in the measurement were dominated, roughly equally, by 1) systematic error from calibration of the PDs with the power meter, and 2) error from noise in the REFL_L measurement (since the absolute AC noise level in TRANS and REFL_L is the same, and TRANS >> REFL_L, the SNR of the latter is worse).
(1) can be helped by making ALL measurements with a single device. I recommend using something precise and portable like the power meter to make measurements at all the necessary ports. For REFL_L/UL, we can place a beam splitter before the REFL PD, and---after calibrating for the T of this splitter very well using the same power meter---both states can be measured at this port.
(2) can probably be helped by taking longer averaging, though at some point we run into the stability of the power setting itself. Something like 30-60s should be enough to remove the effects of the REFL_L noise, which is concentrated in the few-Hz region in the LLO setup.
One more thing I forgot was the finite transmission of the steering mirror at the OMC input (the transmission of this mirror goes to the QPDs). This will add a fixed error of 0.3%, and I will take it into account in the future.
I found that, in fact, I had lowered the modulation depth since when I measured it to be 0.45 rads --> Psb = 0.1.
Here is the sweep measurement:
This is Psb = 0.06 --> gamma = 0.35 rads.
This changes the "raw transmission" and "coupling", but not the inferred visibility:
I also measured the cavity AMTF at three powers today: 0.5 mW, 10 mW, and 45 mW input.
They look about the same. If anything, the cavity pole seems slightly lower with the higher power, which is counterintuitive. The expected shift is very small (~10%), since the decay rate is still totally dominated by the mirror transmissions even for the supposed high-loss state (Sam and Sheon estimated the roundtrip loss at high power to be ~1400 ppm, while the combined coupling mirrors' T is 1.6%). I have not been able to fit the cavity poles consistently to within this kind of error.
Tonight, we locked the "fauxMC". We obtained a visibility of >99%.
Koji had aligned it roughly last night, but we wanted to have a couple steering mirrors in the path for this practice cavity (the periscope mirrors will serve this function in the real setup), so we marked the alignment with irises and installed two extra mirrors.
After obtaining flashes with the WinCam placed at the output coupler, we removed the WinCam and put a CCD camera at one of the curved mirror transmissions and used this to get a strong TEM00 flash. Then, we installed the REFL PD/CCD, swept the laser PZT and optimized the alignment by minimizing the REFL dips. Finally, we connected the RF electronics and locked the cavity with the LB box. We used whatever cables we had around to trim the RF phase, and then Koji made some nice SMA cables at the 40m.
One thing we noticed was that we don't have enough actuation range to keep the cavity locked for very long---even with the HV amp (100V). We are going to offload to the NPRO temperature using an SR560 or pomona box circuit. We may also make an enclosure for the cavity to protect it from the HEPA blasting.
Tomorrow, after we do the above things, we will practice measuring the transmission, length (FSR) and mode spectrum of the cavity before moving on to the real McCoy.