elogkeywd keyword1 keyword2 keyword3
>kermit (I used kermit command for serial communication.)
>set modem type none
>set line /dev/ttyS0 (ttyS0 should be the device name of your serial port)
>set speed 9600
>set parity none
>set stop-bits 1
>set flow-control none
AuthName "LVC password"
Today, I worked with Kakeru on ISS.
The problem is sort of elusive. Some time, the laser power looks fine, but after a while you may see many sharp drops in the power. Some times, the power drops happen so often that they look almost like an oscillation.
We made several measurements today and Kakeru is now putting the data together. Meanwhile, I will put my speculations on the ISS problem here.
The other day, Kakeru took the transfer function of the ISS feedback filter (he is supposed to post it soon). The filter shape itself has a large phase margin ( more than 50deg ?) at the lower UGF (~3Hz) if we assume the response of the current shunt to be flat. However, when we took the whole open loop transfer function of the ISS loop, the phase margin was only 20deg. This leads to the amplification of the intensity noise around the UGF. The attached plot is the spectrum of the ISS monitor PD. You can see a broad peak around 2.7Hz. In time series, this amplified intensity noise looks like semi-oscillation around this frequency.
Since it is very unlikely that the PD has a large phase advance at low frequencies, the additional phase advance has to be in the current shunt. We measured the response of the current shunt (see Kakeru's coming post). It had a slight high-pass shape below 100Hz (a few dB/dec). This high-pass response produces additional phase advance in the loop.
There seems to be no element to produce such a high-pass response in the current shunt circuit ( http://www.ligo.caltech.edu/docs/D/D040542-A1.pdf )
This Jamie's document shows a similar high-pass response of the current ( http://www.ligo.caltech.edu/docs/G/G030476-00.pdf page 7 )
Now the question is what causes this high-pass response. Here is my very fishy hypothesis :-)
The PA output depends not only on the pump diode current but also on the mode matching with the NPRO beam, which can be changed by the thermal lensing. If the thermal lensing is in such a condition that an increase in the temperature would reduce the mode matching, then the temperature increase associated with a pump current increase could cancel the power increase. This thermal effect would be bigger at lower frequencies. Therefore, the intensity modulation efficiency decreases at lower frequencies (high-pass behavior). If this model is true, this could explain the elusiveness of the problem, as the cancellation amount depends on the operation point of the PA.
To test this hypothesis, we can change the pump current level to see if the current shunt response changes. However, the PA current slider on the MEDM screen does not work (Rob told me it's been like this for a while). Also the front panel of the MOPA power supply does not work (Steve told me it's been like this for a while). We tried to connect to the MOPA power supply from a PC through RS-232C port, which did not work neither. We will try to fix the MEDM slider tomorrow.
I fixed the broken slider to change the current of the PA.
The problem was that the EPICS database assigned a wrong channel of the DAC to the slider.
I found that the PA current adjustment signal lines are connected to the CH3 &CH4 of VMIC4116 #1. However in the database file (/cvs/cds/caltech/target/c1psl/psl.db), the slider channel (C1:PSL-126MOPA_DCAMP) was assigned to CH2. I fixed the database file and rebooted c1psl. Then the PA current started to follow the slider value.
I moved the slider back and forth by +/-0.3V while the ISS loop was on. I observed that the amount of the low frequency fluctuation of the MOPA power changed with the slider position. At some current levels, the ISS instability problem went away.
Kakeru is now taking open-loop TFs and current shunt responses at different slider settings.
After the ISS work, I aligned the IFO and confirmed that DRMI locks with good SPOB and AS166 values.
During the cleanup of the lab. Steve found a box with two BNCs going to the ICS DAQ interface and an unconnected D-SUB on the floor under the AP table. It seemed like a temperature sensor.
The BNCs were connected to C1:PEM-OSA_APTEMP and C1:PEM-OSA_SPTEMP.
Steve removed the box from the floor. These channels can be now used as spare DAQ channels. I labeled those cables.