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IOO (Laser Bench)

yutaro.enomoto - 20:45 Wednesday 20 June 2018 (5333)

Green Fiber Noise Measurement @UToyama

[Sugimoto, Enomoto] @UToyama

From last Friday we started the measurement of phase noise of a fiber, which will be used for Green Lock system.
Please refer to the document for overview of this measurement.
A top view of the current setup for this measurement can be found in the first attached figure.

Today we did two things:
One is to get the green beam through the fiber, by using a fiber checker.
We achieved this last Friday, but we mistakenly lost the beam alignment to the input fiber collimator, so we did this today again.

The other is to recombine two beams, one of which is output of the fiber, to construct a Mach-Zehnder interferometer.
After measuring the beam propagation profile of each beam and mode-matching two beams, we successfully observed the fringe of MZI. See the second attached figure.
I am kind of satisfied with this contrast.

Next thing to do:
Since we don't have a good BS for 532 nm (BS that will be used for recombine), the light power at the detection port was too small.
Once we get a BS, we will swap the BS and then try to lock the MZI.

Images attached to this report

Comments to this report:

yutaro.enomoto - 20:54 Thursday 21 June 2018 (5347)

Today we improved the mode matching between two beams. Here are the current beam profile (attachments).
Thanks to this the contrast of MZI improved to some extent.

Images attached to this comment

yutaro.enomoto - 20:30 Thursday 28 June 2018 (5417)

== Finished ==
[Sugimoto, Enomoto]

We measured the phase (frequency) noise of a 5 m fiber with a Mach-Zehnder interferomter (see the first picture).
The result can be found in the second attachment. This result shows that Δf ~ 2e-3 Hz/sqrt(Hz) in f > 10 Hz and 3e-2*(1Hz/f)^2 Hz/sqrt(Hz) below ~3 Hz.

= How we measured =
We locked the MZI at mid-fringe. The power of the interference light was measured by a PDA100A, and the output of the PD was low-passed after the mid-fringe offset was subtracted.
LPF and offset subtracting was realized by an SR560. The cutoff frequency of the low pass was 100 Hz.
The output of the SR560 was then sent to a PZT driver. The output of the PZT driver went to the laset PZT.
The measured OLTF is shown in the third attachment.
The actuator efficiency (input of the PZT driver to MZI phase) was also measured, by sweeping the input of the PZT driver with 10 Hz triangular wave (see the fourth attachment).
The measured efficiency is: 2π/0.992 V = 6.33 rad/V. The error of this should be within ~ 5%.
Since we are interested in low frequency region, we meaured the PSD of the control signal (input of the PZT driver) to estimate the frequency noise of the fiber.

During the measurement the clean air fan was turned off, and we put a few irises to eliminate nasty scattered light.

I also attach the pictures that show the contrast and the lock acquisition.

= Notes on the result =
-- Around 100-300 Hz, there is a structure probably due to acoustic noise.
-- In high frequency, the noise is limited by the electrical noise, more explicitly the noise of the spectrum analyzer.
I know this can be lowered by band-passing and amplifying the signal, but I didn't; I don't care much about high frequency.

= Next =
Assuming this result can be scalable to the KAGRA real 50m fiber, I am going to implement this into the ALS noise budget.
If the noise could be critical, we might need to consider measuring the noise of the 50m fiber after the installation.

Images attached to this comment