[Joshua, Tanaka, Disha, Fujimoto, Saito]

To perform the measurement proposed in klog:37169 using the beat signal at the OMC REFL, a new RFPD was installed at OMC REFL. After injecting the sub-laser into PRX and engaging the PLL, the beat signal was successfully observed with the newly installed RFPD. The amplitude and frequency of the beat signal both fluctuated, making it difficult to finely adjust the LO frequency to maximize the beat signal. However, since both the resonance and anti-resonance points were successfully identified, we plan to reduce the effect of these fluctuations by increasing the number of averaging frames on the spectrum analyzer. The beat frequency will then be determined by measuring the minimum and maximum frequencies at which the beat-signal amplitude begins to decrease.

• The reflected beam from the BS in front of the OMC REFL camera had previously been dumped, so this beam was utilized for the measurement. First, the beam dump was removed, and the beam power was measured to be approximately 3 mW using a power meter. A mirror, a lens with a focal length of 50 mm, an OD = 0.5 ND filter, and an RFPD were then installed (Fig. 1). The lens was inserted to focus the beam onto the RFPD, while the ND filter was used because the maximum allowable input power to the RFPD is 1 mW. The optical power measured immediately before the RFPD was approximately 0.95 mW. The alignment was then adjusted to maximize the DC output of the RFPD.
   
• Next, the sub-laser was injected into PRX, the PLL was engaged, and the beat signal was observed using the RFPD installed at OMC REFL. The beat signal was successfully detected. However, it was difficult to finely adjust the LO frequency used for the PLL to maximize the beat signal because both its amplitude and frequency fluctuated. Therefore, the LO frequency was swept by ±3 MHz around 160.4 MHz at a rate of 10 mHz. Under these conditions, both the resonance point (Fig. 2) and the anti-resonance point (Fig. 3) were observed. The spacing between adjacent resonance points was approximately 2.2 MHz. Based on these results, we plan to increase the number of averaging frames on the spectrum analyzer to suppress the influence of the fluctuations and determine the beat frequency by measuring the minimum and maximum frequencies at which the beat-signal amplitude begins to decrease.

Not " EYC P-55", "IYC P-55" is correct. 

[Tanaka, Hirose, Fujimoto, Saito]

As the PLL UGF was increased, the beat signal became progressively broader. It is therefore likely that the reason why no fringes were observed on the OMC REFL PD at a UGF of 10 kHz in the previous experiment (klog:37170) was that the broadened beat signal prevented the PLL from accurately detecting changes in the frequency difference between the LO and the beat signal, resulting in insufficient modulation of the PZT. Next, an offset was added to the error signal while operating at a UGF of 1 kHz, where the temporal fluctuation of the beat frequency was relatively small. However, the change in the beat frequency was nonlinear with respect to the applied offset, and it was difficult to measure the frequency shift accurately because the beat frequency continuously fluctuated by several MHz.
 

• First, since different behavior was observed between PLL operation at UGFs of 10 kHz and 1 kHz in the previous experiment (klog:37170), the dependence of the beat signal on the UGF was investigated to determine the most suitable UGF for evaluating the frequency shift caused by adding an offset to the error signal. The measured UGFs and FWHM of the beat signal were as follows:

  UGF: 100 Hz (SR560 gain = 20), FWHM: 75 kHz (Fig. 1)
  UGF: 1 kHz (SR560 gain = 200), FWHM: 536 kHz (Fig. 2)
  UGF: 10 kHz (SR560 gain = 2000), FWHM: 1.9 MHz (Fig. 3)

  These results show that the beat signal became broader as the UGF increased, presumably because the control noise was being fed back through the PLL. Therefore, it is likely that the absence of fringes on the OMC REFL PD at a UGF of 10 kHz in the previous experiment (klog:37170) was caused by the broadened beat signal, which prevented the PLL from accurately sensing changes in the frequency difference between the LO and the beat signal, resulting in insufficient PZT modulation.
   

• Next, an offset was applied to the error signal while operating at a UGF of 1 kHz, where the temporal fluctuation of the beat frequency was relatively small, although occasional frequency excursions of several MHz were still observed. When a 10 mV offset was applied, no measurable change in the beat frequency was observed during stable periods. Increasing the offset to 100 mV produced a frequency shift of approximately 0.1 MHz during stable periods. When the offset was increased to 200 mV, the beat frequency shifted by approximately 10 MHz, while fluctuating by about 5 MHz around this value. Finally, applying a 300 mV offset produced a frequency shift of approximately 70 MHz, accompanied by even larger fluctuations. These results indicate that the beat-frequency shift is nonlinear with respect to the applied error-signal offset. Moreover, because the beat frequency continuously fluctuated by several MHz, accurately quantifying the frequency shift was difficult.

[Fujimoto, Komori]

Abstract:

We discuss this estimation more concretely and quantitatively.
We should be able to measure the PRC and SRC lengths with a resolution of 0.1%, corresponding to an error of 6–7 cm.
Whether we can reach the 0.01% level, corresponding to 6–7 mm, will depend on how we determine the beat frequency.

Details:

The designed value of the SRY length, for instance, is 64.926 m, and the FSR of the SRY cavity is 2.3088 MHz.
On the other hand, using the method described in the original post, we should be able to measure the beat frequency with a resolution comparable to the cavity linewidth, which is approximately 0.2 MHz.
If we lock the beat frequency at 100 times the FSR, approximately 230 MHz, and measure it with a resolution of 0.2 MHz, the relative error of the measurement is 0.1%.
Therefore, the FSR can be estimated with the same relative error.

Let us simulate a realistic situation.
Suppose that we measure the beat frequency to be 221.3 ± 0.2 MHz.
The possible solutions are 95 × (2.330 ± 0.002) MHz and 96 × (2.305 ± 0.002) MHz, corresponding to absolute lengths of 65.03 ± 0.06 m and 64.33 ± 0.06 m, respectively.
Since the designed value is 64.926 m, and we can probably assume that the deviation from the designed value is less than 60 cm, we can select the solution of 65.03 ± 0.06 m.

However, an error of 6 cm is too large.
We have to measure the beat frequency with a relative error of 0.01% in order to estimate the absolute length with a resolution below 1 cm.

In addition to the method described in the original post, we propose another method to determine the beat frequency with the auxiliary laser frequency locked.
By dithering the auxiliary laser frequency at an audio frequency, for example 1 kHz and tuning the offset, we can minimize the peak height in the noise spectrum measured by the OMC REFL PD.
This method may be better than the original proposal because, generally speaking, minimizing a peak in a noise spectrum is easier than maximizing it.

The error of this method will be determined by RMS of the residual displacement of the SRC length.
If the RMS displacement with the SRC locked is less than 10% of the cavity linewidth, corresponding to 0.02 MHz, we should be able to achieve a relative error of 0.01% in the absolute length estimation.

[Kimura and Yasui]
 On July 6, as part of maintenance work on the cryogenic cooling units, we set up two valve units for the radiation shield cryo-coolers (IYC P-53 and EYC P-55).
 The remaining tasks are filling the system with G-1 class helium gas up to 15 bar and performing leak tests on all connections.

[Tanaka, Fujimoto, Saito]

The cutoff frequency of the high-pass filter in the SR560 used for the OMC REFL PD signal was reduced from 300 Hz to 30 Hz, allowing frequency sweeps to be performed at a slower rate. When the PLL UGF was increased to 10 kHz and the LO frequency was modulated to scan PRX, no fringes were observed on the OMC REFL PD. However, both the feedback signal and the beat signal appeared to be modulated properly. Therefore, the reason why no fringes were observed on the OMC REFL PD at a UGF of 10 kHz remains unclear. An alternative approach was also tested by scanning PRX through the addition of an offset signal to the error signal. As in the case of LO frequency modulation, no fringes were observed on the OMC REFL PD when the UGF was increased to 10 kHz. In addition, the feedback signal became distorted and no longer followed the waveform of the injected signal.
 

• First, after reducing the OMC REFL intensity noise for PRX and PRY (klog:37150), it was noticed that the cutoff frequency of the high-pass filter in the SR560 used for the OMC REFL PD had not been updated. Therefore, it was changed from 300 Hz to 30 Hz, allowing the frequency sweep to be performed more slowly.
   
• Next, with the PLL UGF set to approximately 1 kHz, the LO frequency was frequency-modulated with a sensitivity of ±15 MHz/V. An 800 mVpp, 300 Hz sinusoidal waveform generated by a function generator was used as the external modulation signal. Under these conditions, PRX was scanned and fringes were successfully observed on the OMC REFL PD. However, when the UGF was increased to approximately 10 kHz by increasing the gain of the SR560 used in the PLL, the fringes disappeared. The UGF was then returned to approximately 1 kHz, and the frequency of the external modulation signal was increased above 300 Hz. As the modulation frequency increased, the number of peaks observed on the OMC REFL PD also increased. Conversely, when the UGF was reduced below 1 kHz by decreasing the SR560 gain, the number of peaks observed on the OMC REFL PD decreased. Furthermore, even when the external modulation frequency was increased above 300 Hz with the UGF maintained at approximately 10 kHz, no fringes were observed on the OMC REFL PD. Nevertheless, the feedback signal at a UGF of 10 kHz showed the same modulation behavior as that at a UGF of 1 kHz, and the beat signal observed with the spectrum analyzer also appeared to vary as expected. Therefore, it remains unclear why no fringes can be observed on the OMC REFL PD when the UGF is increased to approximately 10 kHz.
   
• Finally, an attempt was made to scan PRX by adding an offset signal directly to the error signal. As shown in Fig. 1, a 20 Hz, 300 mVpp sinusoidal signal was added to the error signal. In Fig. 1, the red trace represents the error signal, while the blue trace represents the injected signal. As in the case of LO frequency modulation, increasing the UGF from approximately 1 kHz to approximately 10 kHz caused the fringes to disappear from the OMC REFL PD. In addition, the feedback signal became distorted and no longer matched the waveform of the injected signal (Fig. 2). The degree of distortion varied with time and occasionally became more pronounced. Furthermore, while measuring the open-loop transfer function, the feedback signal was found to become distorted whenever a low-frequency excitation signal was injected.

Here, I would like to propose a possible method for the PRCL/SRCL length measurement without sweeping the LO frequency.

The procedure is as follows:

0. Place an RFPD in OMC REFL.
1. Lock the main laser to SRY.
2. Lock the PLL of the auxiliary laser and the main laser.
3. Manually adjust the LO frequency and bring the auxiliary laser to resonance in SRY. In this step, the resonance can be checked using the beat signal observed with the RFPD in OMC REFL.
4. Measure the beat frequency in POS or OMC REFL and calculate the FSR by dividing it by an appropriate integer.

The SNR of this method can be roughly estimated as follows.
On the RFPD currently placed on the POS table, the powers and SNR are:

- Main laser: ~30/2 uW
- Auxiliary laser: ~1 mW
- SNR, defined as the ratio between the noise floor and the peak height: ~40 dB

On the other hand, the expected powers in OMC REFL are:

- Main laser: ~10 mW
- Auxiliary laser: ~20 uW

Therefore, if we reduce the main laser power incident on the RFPD placed in OMC REFL to 1 mW, the SNR will become worse than that on the POS table by a factor of about sqrt(10). However, the beat signal is still expected to be clearly visible.

In addition, in this measurement, we may be able to reduce the error in estimating the FSR by using a large LO frequency offset and bringing the auxiliary laser to a resonance far from the carrier resonance.

[Takahashi.M, Sawada.H, Nakagaki]

We ran cables in preparation for monitoring the status of the gate valve between PR3 and PRM.
 

The OMMT-GV interlock has been set to monitoring mode.

Monitoring start time: 15:30 (JST)
Automatic close threshold: 9.9e-04
Vacuum pressure at start of monitoring
 SRMGV : 2.5e-05
 OMMTGV : 3.8e-05

[Tanaka, Fujimoto, Saito]

The high-voltage amplifier tested in klog:37154 was inserted between the SR560 used in the PLL and the PZT of the sub-laser. By setting the SR560 gain to 1000 and the cutoff frequency of the Moku:Lab integrator to 1 kHz, and by adding a 10 kHz low-pass filter in Moku:Lab, the UGF increased to approximately 10 kHz. The 10 kHz low-pass filter was introduced because oscillations at around 85 kHz were observed in the error signal. However, under these conditions, no fringes could be observed on the OMC REFL PD during a PRX scan. Reducing the gain of the SR560 allowed the fringes to reappear, so the SR560 gain was changed to 200 and the cutoff frequency of the Moku:Lab integrator was reduced to 100 Hz. The LO frequency was then frequency-modulated with a sensitivity of ±30 MHz/V. As the external modulation signal, an 80 Hz, 800 mVpp sinusoidal waveform generated by a function generator was applied, and a PRX scan was performed to acquire data. Owing to an earthquake, it was not possible to obtain the current open-loop transfer function or data using a triangular-wave modulation signal, so these measurements will be carried out next time.

• First, the high-voltage amplifier tested in klog:37154 was inserted between the SR560 used in the PLL and the PZT of the sub-laser. Before introducing the amplifier, the SR560 gain had been set to 2000. Since the amplifier provided an additional gain of 10, the SR560 gain was reduced to 200, and it was confirmed that PLL operation was still possible. To increase the UGF beyond the previous value of approximately 1 kHz, the gain of the SR560 was gradually increased. However, oscillations appeared in the error signal at around 85 kHz. Therefore, a 10 kHz low-pass filter was added using Moku:Lab. With the SR560 gain set to 1000 and the cutoff frequency of the Moku:Lab integrator set to 1 kHz, the open-loop transfer function was measured, yielding a UGF of approximately 10 kHz (Fig. 1). An attempt was then made to perform a PRX scan, but no fringes were visible in the OMC REFL PD signal. When the gain of the SR560 was reduced, fringes became visible again in the OMC REFL PD signal. Therefore, the SR560 gain was set to 200 and the cutoff frequency of the Moku integrator was set to 100 Hz.
• Because the high-voltage amplifier enabled the PZT to be driven over a larger range, the frequency modulation sensitivity was increased to ±30 MHz/V. To drive the PZT as slowly as possible while maintaining a clean fringe shape, an 80 Hz, 800 mVpp sinusoidal waveform generated by a function generator was used as the external signal for frequency modulation. Data for both the OMC REFL PD signal and the external modulation signal were then acquired (Fig. 2).
   
• Due to an earthquake, it was not possible to obtain the current open-loop transfer function or the data corresponding to the case where a triangular-wave signal was used for frequency modulation. These measurements are planned for the next session.

Test of alignment-recording setup before installing a new laser source in Pcal-Y

With Misato Onishi and Seiya Matsuo.

We performed a test of the setup to record the current Pcal-Y alignment before installing a new laser source in the Pcal-Y Tx module.

We are planning to install an additional laser source inside the Pcal-Y Tx module. Since this work may change the present optical alignment by accident, we would like to record the alignment outside the vacuum chamber during the installation work as a backup reference.

Our current plan is to extract the two Pcal-Y beams from the Tx module, propagate them along the tunnel toward ETMY, and record their positions using irises. Today, we assembled a jig to extract the beams from inside the Tx module.

However, it took longer than expected to extract both beams from the Tx module at the same time. At the moment, one of the beams is slightly blocked by another structure. It should be possible to solve this by further adjusting the positions of the optical components, but we had to stop the work today due to time limitations.

We plan to continue this work next week.

Tanaka, YamaT

This noon, Large earthquake occured in Hirara, Okinawa. On the other hands, ITMX guardian seems to keep the LOCK_ACQUISITION state even though ITMX control seemed not to work due to this earthquake (fig.1).

We found that the SUS_KICKED threshold (=100), which is judged whether the suspension is kicked from RMS of MN_OLDAMP error, seems to be high (fig.2). So current Type A cannot escape to the emergency state in this threshold.

Then we decided to lower the threshold from 100 to 40. We hope this threshold works well.

[Ushiba, Komori, Fujimoto, Saito]

As in the SRC scan (klog:37151), the LO frequency was frequency-modulated with a coefficient of ±15 MHz/V. As the external signal for the frequency modulation, 300 Hz, 800 mVpp triangular and sinusoidal waveforms generated by a function generator were applied, and scans of PRX and PRY were performed to acquire data. These data will be used to determine the lengths of PRX and PRY.
 

• First, only the sub-laser light was injected into PRX, and the sub-laser PZT was driven with an 8 Vpp triangular wave at 170 Hz. Fringes were observed on the OMC REFL PD, and the alignment was adjusted to maximize the fringe amplitude (Fig. 1). Next, the main laser was turned on, and the alignment of the PLL path was optimized to maximize the beat signal, resulting in a beat amplitude of approximately 5 dBm.
   
• The measurement then proceeded to the PRX scan. As in the SRC scan (klog:37151), the LO frequency was frequency-modulated with a coefficient of ±15 MHz/V. A function generator was used to provide external modulation signals consisting of 300 Hz, 800 mVpp triangular and sinusoidal waveforms. The OMC REFL PD signal was recorded using "K1:IOP-OMC0_MADC0_TP_CH12", while the external modulation signal was monitored using "K1:IOP-LSC0_MADC0_TP_CH22". Data from these two channels were acquired simultaneously (Fig. 2). The measurements then moved on to the PRY scan. As in the PRX measurement, data were acquired using both triangular-wave and sinusoidal-wave external modulation signals. These data will be analyzed to determine the lengths of PRX and PRY.

[Saito, Komori, Ushiba, Miyoki, Fujimoto]

Abstract

We borrowed a high-voltage amplifier from Miyoki-san and tested it for use in the PLL for the PRCL/SRCL length measurements.
The measured results are as follows:

- Gain: 20 dB
- Phase delay: ~1 deg at 10 kHz, and ~45 deg at 509 kHz
- Input-referred noise: ~1.4e-8 V/rtHz at 1 kHz

Tomorrow, we plan to implement this amplifier in the PLL control in order to extend the range of the PLL scan and  increase the UGF.

Details

The current PLL scan range in the PRCL/SRCL length measurement is limited by the output range of the SR560, which is ±5 V. This corresponds to a scan range of about ±10 MHz.
Therefore, in order to extend the PLL scan range, we decided to introduce a high-voltage amplifier made by Miyoki-san, which is the same type as the one used for the EOM in the CARM control.

Instruments

We borrowed a high-voltage amplifier (PA-85), shown in Fig. 1, and its power supply, shown in Fig. 2.
The gain is fixed at 20 dB, and there is no offset. A banana-plug-to-Hirose-4-pin cable is used for the power supply connection.
Since the PZT input range of the Mephisto is ±65 V, we will set the supply voltage to about ±60 V when using this amplifier.

Transfer function

Fig. 3 shows the transfer function measured with Moku:Lab. The results are as follows:

• Gain: 20 dB
• Phase delay: ~1 deg at 10 kHz, and ~45 deg at 509 kHz

The phase delay is quite small for a high-voltage amplifier. Therefore, we expect that a UGF of 10 kHz can be achieved even with this amplifier.

Input-referred noise

In Figs. 4 and 5, red lines show the output noise measured with Moku:Lab and the black lines correnspond to the Moku: Lab ADC noise.

Taking into account the ADC noise of Moku:Lab, the input-referred noise of the high-voltage amplifier is estimated to be as follows:

• ~7e-9 V/rtHz at 10 kHz–100 kHz
• ~1.4e-8 V/rtHz at 1 kHz

In the actual control system, this amplifier will be connected after the SR560, whose gain is expected to be 300–3000. Therefore, the input-referred noise levels above are considered to be negligibly small.

Plan for tomorrow

Tomorrow, we will implement this amplifier in the PLL control and optimize the OLTF by increasing the UGF, adjusting the integrator, and making other necessary changes.
In addition, since this amplifier will extend the PLL range, we plan to perform the PRCL/SRCL length measurements with a larger scan amplitude.