In 81863 we looked a the RIN of the CO2 lasers on vs off. Today we repeated with PWM (from Synrad UC-2000), the noise level is considerably higher when PWM is on. Note that this isn't really the RIN as we're not dividing by the DC power.
Data saved to camilla.compton/Documents/tcs/templates/dtt/20250107_CO2_RIN.xml and 20250107_CO2Y_RIN.xml
- Refs 0-3: Laser on CW
- Refs 4-7: Laser off.
- Refs 8-11: 50% PWM (CO2X laser locked, CO2Y unlocked)
- Refs 12-15: 25% PWM (CO2X laser unlocked)
- Refs 16-19: 95% PWM (CO2X laser relocking)
- Refs 20-23: Laser on CW
The CO2Y plots also show the dark noise as there is no photo detector plugged into CO2Y_ISS_IN.
- CO2Y plots: AC only, DC only, all.
- CO2X plots: AC only, DC only
- It's strange that there more CO2X DC noise with the laser off and that the 25%, 50%, 95% PWM's are the opposite way around for the AC and DC outputs.
For some of the CO2X data, the CO2 guardian was relocking (sweeping PZT), noted above.
These channels are calibrated in Volts as the detector would output, before the gain of PD's amplifier D1201111 .
DC levels of power on the ISS PDs at the different powers are:
- CO2Y ISS_OUT_DC:
- 100% CW power = 4.86mV
- 50% PWM power = 3.95mV
- CO2X ISS_IN_DC:
- 100% CW power = 7.06mV
- 50% PWM power = 4.94mV
- 25% PWM power = 2.39mV
- 95% PWM power = 6.83mV
- CO2X ISS_OUT_DC:
- 100% CW power = 8.00mV
- 50% PWM power = 5.45mV
- 25% PWM power = 2.60mV
- 95% PWM power = 7.56mV
With PWM on, the noise level increases by around a factor of 10. Unsure why the AC channel sees this increase less than the DC channels.
Gabriele, Camilla
- DC value:
- Expect ~250mW on each the ISS PD (50W out of laser x 99/1 BS x 50/50 BS) but DC levels show 30-50mW on PDs:
- Measured 5 to 8mV / 66Ohm (measured 565) / 0.0025A/W (spec sheet) = 30-50mW. Factor of 5-10 below expected but the PD could be saturating.
- At 25% laser power, 2.60mV measured / 66Ohm (measured 565) / 0.0025A/W (spec sheet) = 16mW. Expect ~62mW.
- This a factor of ~5-10 from what we expect but we need to measure the power on the PD with a power meter and check for edge clipping with an iris to confirm what power is incident on the PDs.
- Expect ~250mW on each the ISS PD (50W out of laser x 99/1 BS x 50/50 BS) but DC levels show 30-50mW on PDs:
- Electronics noise (labeled as "dark noise" in plot):
- <10e-10 for AC, <10e-8 for DC. This noise dominates the dark noise and 100% CW signals for the DC channel.
- Dark noise (labeled as "laser off" in plots):
- Around 10-9 for AC output.
- 10-9V / 66Ohm (measured 565) / 0.0025A/W (spec sheet) = 6e-9W/rtHz This is similar to 541 measurement of 4e-7 V/rHz /66Ohm /0.0025A/W / 255 DC gain = 9.5e-9W/rtHz
- RIN with laser on 50% PWM:
- Looking at AC channel (CO2Y): 7e-9V/4e-3V(DC level) = 2e-6/rtHz. Want 10-7/rtHz so factor of 20 too high.
- Looking at DC channel (CO2Y): 2e-7V/4e-3V(DC level) = 5e-5/rtHz. Want 10-7/rtHz so factor of 500 too high.
- Similar results when using CO2X data.
- We don't understand why these is a difference between the AC and DC channels, both channels should be showing the same signal: 82181. We can try to sample channels at a higher rate to see if something strange is happening.
Erik showed me that we can look to higher frequencies using H1:IOP-OAF_L0_MADC{2,3}_TP_CH{10-13} channels at 65kHz. Can repeat 50% PWM next week.
Note that these are before the filtering (listed in 81868), we could filters back in with python... but it's mainly gains and some shaping of the AC channel below 20Hz and above 10kHz, a DC roll of is expected around 100kHz as in D1201111. DC values:
- CO2X IN DC: 5800
- CO2X OUT DC: 6600
- CO2Y IN DC: Unplugged
- CO2Y OUT DC: 3900
Matt Todd and I measured the powers before the CO2X ISS PDs today:
- Before BS: 425mW
- ISS Out path: 230mW (7.7e-3 on ITMX_CO2_ISS_IN_DC)
- (Inferred) ISS In path: 195mW (8.5e-3 on ITMX_CO2_ISS_OUT_DC)
In 82182 using specs of PDs we think we are measuring 30-50mW. We didn't have time to check for clipping with an iris or alignment, both beams looked small but the PD's are also small ~1mmx1mm.