Fiber injection, spectrum and connection of 2nd stage amplifier, posted by Xinyi Lu at Optical room about lasers and optics 
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Today, Daniele and I injected the laser into the fiber, installed the telescope, connected the second stage of the amplifier, and obtained resonances.
- The output power of the menhir laser @ 216MHz is 150mW, after CVBG is 28mW , 9.6mW injected into the fiber, and 1.6mW via AOM and EOM. This
is not far from the minimum 1mW seed power required by the amplifier. |
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2 mirror cavity high power experiments, posted by Xinyi Lu at Optical room about lasers and optics
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Today, Ronic and I recorded some intracavity power and cavity mode size as shown in Fig. 1.
Coupling was calculated using the locking curve of this overcoupled cavity. Pr/Pi = 1-Cgeo*Cimp, Cimp = 1-|1-2T1/RTL|^2
We can see that the effective gain, coupling, and mode size decrease with increasing power. And the beam is constantly moving. |
high-power experiments of 2-mirror cavity, posted by Xinyi Lu at Optical room about lasers and optics
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Today, Ronic, Daniele and I redo the high-power 2-mirror cavity experiments, and the results are shown in the table (Figure 1 and Excel 2 ).
- The intracavity power ~500kW can be obtained at 47W injection, but we then have no increase or even a decrease in intracavity power when
increasing the injection power, and the coupling is decreasing. It looks like the saturation power of the current device. |
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high-power experiments of 2-mirror cavity, posted by Xinyi Lu at Optical room about lasers and optics
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here is a Matlab code to try to optimize the telescope for a hot cavity,
taking into account the thermal lens in the coupling mirror.
from that code, one can deduce using the "Gaussian Beam" software (using the attached xml file) an optimized telescope with 100% geometrical |
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high-power experiments of 2-mirror cavity, posted by Xinyi Lu at Optical room about lasers and optics 6x
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Here's a summary of our experiment last week:
The initial telescope position: 920 mm (f=+250mm) and 1148 mm (f=-150mm) from the amplifier output.
Mon May 6: We moved the concave lens 0.5mm closer to the cavity. |
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FSR and Finesse measurement, posted by Ronic Chiche at Optical room about lasers and optics
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Yesterday the cavity has been aligned and locked with the CW Koheras laser.
the FSR has been measured by modulation technique at 133.344MHz at 1mbar pressure in the cavity.
the polarization has to be optimized for the Finesse measurement otherwise, some "shoulders" appear beside the Airy peak and reduce |
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2nd stage amplifier issue, posted by Ronic Chiche at Optical room about lasers and optics
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the 2nd stage amplifier needed several hours (4-5h) to reach its nominal power (we look at photodiode level on a scope), instead of the awaited 30 minutes.
could it come from the probable spectrum shifting of the OneFive laser ?
(the power coming from the CVBG, coupled to the fiber, is lower than expected). |
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2nd stage amplifier issue, posted by Ronic Chiche at Optical room about lasers and optics
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Last time, we switched ON directly the 2nd stage at 6A without increasing/decrinsing slowly the current.
today, we switched ON the chiller, switched ON the 1st stage, switch ON the power supply of the 2nd stage at 0A and then we increased slowly the
current until 6A... and the problem disappeared. |
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2nd stage amplifier issue, posted by Ronic Chiche at Optical room about lasers and optics
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2nd stage output power was going down. We checked the pump diode technical data sheet and the operating temperature is [25°C:35°C].
We increased the chiller temperature setpoint from 19°C to 23°C.
Then the output power increased (93mW on 2nd stage photodiode). |
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first data with 3A on the 3rd stage, posted by Ronic Chiche at Optical room about lasers and optics
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Today, at 3A on the 3rd stage, we saw some HOM effects.
the transmissions is about 100mW which corresponds to 30kW inside cavity.
we tried to play with D shape motors but without success. |
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Beam size behind M2, posted by Ronic Chiche at Optical room about lasers and optics
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Beam diameter behind M2 :
- 2nd stage @ 6A - 1kW inside cavity
sx = 2120 µm |
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first data with 3A on the 3rd stage, posted by Ronic Chiche at Optical room about lasers and optics
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2 pictures :
typical beam with HOM
typical beam after moving the D-shape motor : no more HOM |
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Optical amplifier polarization state, posted by Ronic Chiche at Optical room about lasers and optics
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We placed a PBS + 2 photodiodes (PhD1, PhD2) at the output of the amplifier to check how the polarization of the amplifier changes with power.
example with 2nd stage @ 6A :
PhD1 = 24.7 mV |
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Optical amplifier polarization state, posted by Ronic Chiche at Optical room about lasers and optics
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The polarimeter was giving a strange 50% of DOP of the light coming from the cavity.
we had to calibrate (LONG calibration process with care) the polarimeter to get a proper 100% of DOP !
the polarimeter needs also a good alignment with 2 mirrors, a colimated beam and a max power on photodiode between 0.7 and 0.8 (use electronic gain |
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Beam size behind M2, posted by Ronic Chiche at Optical room about lasers and optics
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the telescope matchs the cold cavity beam, so it is normal to have a power decrease on the transmission photodiode when the cavity is heating at high
power.
we can try to adjust the telescope by moving lens, one by one, to increase the cavity power. |
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Optical amplifier polarization state, posted by Ronic Chiche at Optical room about lasers and optics
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polarization state of the cavity at higher power : 20kW, 30kW and 33kW (slight CEP and alignment optimization) :
the polarization state changes only a little to ~ 87° and is almost linear.
Ronic |
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Beam size behind M2, posted by Ronic Chiche at Optical room about lasers and optics
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Yesterday, we tried to better adapt positions of the telescope lenses, dynamically, during the lock, to improve the matching between input beam and cavity
mode.
it is a difficult task because it is quite sensitive to the alignment. we need to realign very often... and it is a long process. |
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Cavity beam axis moving with power, posted by Ronic Chiche at Optical room about lasers and optics 9x
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how the cavity beam axis is moving during a lock when the cavity is hot ?
could it explain a part of the Transmission / Coupling signal decay ?
we placed 2 Basler camera, one at (30+Z) cm and the other at (85+Z) cm (Z is about 15cm) from the M3 mirror, we recorded the video during a lock |
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analysis of stored and reflected power, posted by Ronic Chiche at Optical room about lasers and optics 6x
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the 10th of January, we increased the power of the amplifier to study the cavity transmitted and reflected power signals.
analyzing the noise transfer functions of transmitted and reflected power one could deduce the Finesse of the cavity.
the power of this technic (if it is confirmed) does not depend on the decay time of one signal which depends on the speed of the cut off but on |
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analysis of stored and reflected power, posted by Ronic Chiche at Optical room about lasers and optics
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A chart which summarizes the data we have or we can estimate.
in orange, the case 1, where we suppose the initial cold Finesse is the one measured by modulation technique in December 2019 (F=20.8k). |