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MIF (General)
shun.saito - 22:14 Thursday 25 June 2026 (37123) Print this report
Attempt to observe SRC flash while operating the PLL

[Tanaka, Fujimoto, Saito]

When the LO frequency was adjusted relative to the beat frequency so that the error signal became close to 0 V, it was found that the beat frequency and LO frequency matched at higher frequencies, whereas an offset appeared at lower frequencies. Next, in order to achieve locking using only the SR560, the SR560 was configured as a first-order low-pass filter with a cutoff frequency of 1 Hz and a gain of 200. Under this condition, locking was successfully achieved. Measurement of the open-loop transfer function showed that the UGF was approximately 17 Hz. Next, we attempted to use the FM modulation function of the LO source (Keysight E8663D) in order to sweep the LO frequency and scan the SRY. However, the FM modulation option was not installed. Therefore, the function generator of the Moku:Lab was used instead. When the frequency modulation was performed at a rate of 1 Hz, no SRY flashes were observed on the OMC REFL PD. This is likely because the frequency modulation was carried out at 1 Hz, whereas the cutoff frequency of the high-pass filter in the SR560 used for the OMC REFL PD signal was 300 Hz. Therefore, the cutoff frequency was lowered to 100 Hz, but the main-laser noise became comparable to the amplitude of the sub-laser flashes, so the cutoff frequency was restored to 300 Hz. In addition, increasing the frequency modulation rate of the LO caused the beat signal waveform to become distorted.
 

  • First, the LO frequency was adjusted relative to the beat frequency so that the error signal became close to 0 V. The resulting frequencies were as follows:

    Beat frequency    LO frequency
    33 MHz                    54 MHz
    78 MHz                    85 MHz
    139 MHz                 139 MHz
    179 MHz                 179 MHz

    Therefore, at higher frequencies there appears to be no offset, and the beat frequency matches the LO frequency. Furthermore, at lower frequencies, the beat signal and LO signal were directly observed using the Moku:Lab oscilloscope. The measured beat frequency agreed with the frequency observed on the Moku:Lab spectrum analyzer after the signal had been split by the power splitter, and the LO frequency agreed with the set value. Therefore, the observed offset appears to originate from the PFD.
     

  • Next, a DC voltage was applied to the SR560 to determine its allowable input and output voltage ranges before overload occurred. The maximum allowable input voltage was found to be 2 V, while the maximum output voltage was 5 V. When the error signal was near 0 V, observation of the error signal with a 100 kHz low-pass filter showed fluctuations of approximately 4 Vpp. Therefore, a 20 dB attenuator was inserted to ensure that the signal could be safely input to the SR560. Furthermore, when the error signal was 150 mV, the beat frequency and LO frequency differed by approximately 10 MHz, corresponding to a sensing efficiency of approximately 15 nV/Hz. To achieve locking using only the SR560, the SR560 was configured as a first-order low-pass filter with a cutoff frequency of 1 Hz and a gain of 200, and stable locking was achieved. The current control loop from the RFPD output to the sub-laser PZT is as follows:

    RFPD→ 12 MHz high-pass filter→ 20 dB RF amplifier→ 45 dB RF amplifier→ 10 dB attenuator→ PFD→ 20 dB attenuator→ 100 kHz low-pass filter→ SR560 (1 Hz cutoff frequency, gain ×200, first-order low-pass filter)→ Sub-laser PZT

    The open-loop transfer function was then measured, yielding a UGF of approximately 17 Hz (Fig. 1).
     

  • Next, in order to scan the SRY by sweeping the LO frequency, we attempted to use the FM modulation function of the LO source (Keysight E8663D). However, this option was not installed. Therefore, a 1 Vpp sinusoidal signal was generated using the Moku:Lab function generator and used as the LO signal. The LO frequency was first matched to the beat frequency and then frequency-modulated by ±2 MHz at a modulation rate of 1 Hz (Fig. 2). However, no SRY flashes were observed on the OMC REFL PD. This is likely because the modulation rate of 1 Hz is below the 300 Hz cutoff frequency of the high-pass filter in the SR560 used for the OMC REFL PD signal. The cutoff frequency was therefore reduced to 100 Hz, but the main-laser noise became comparable in magnitude to the sub-laser flash signal, so the cutoff frequency was returned to 300 Hz. The dominant main-laser noise frequency was approximately 60 Hz. In addition, the frequency modulation rate of the LO was increased, but the shape of the beat signal became distorted.

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Comments to this report:
Hiroki Fujimoto - 2:47 Friday 26 June 2026 (37124) Print this report

[Saito, Hirose, Tanaka, Fujimoto]

Abstract

We found that the intensity noise observed with the OMC REFL PD, which will be used for the SRCL/PRCL measurement, can be improved by aligning SRY.
This result suggests that we may be able to lower the cut-off frequency of the SR560 (high-pass) connected to the OMC REFL PD output from the current value of 300 Hz.
This may allow us to reduce the scan speed of the auxiliary laser.

Details

Improvement of the OMC REFL intensity noise by SRY alignment

In the evening, we measured the intensity fluctuation of the OMC REFL PD while the main laser was locked to SRY.
We found that the spectrum had become worse than the one measured yesterday.

In Fig. 1, the blue trace shows the spectrum measured in the evening, and the green trace shows the spectrum measured yesterday.
In particular, the 60 Hz line noise became prominent.

When we lowered the cutoff frequency of the SR560 (high-pass) attached to the OMC REFL PD from 300 Hz to 100 Hz today, a noise comparable in size to the auxiliary laser flashes appeared.
This noise was likely coming from the 60 Hz line noise, since its frequency was roughly around 70 Hz when we checked on the oscilloscope.

We then realigned SRY so as to maximize the AS DC power with ADS and manual alignment of ITMY.
As a result, the OMC REFL spectrum was reduced, as shown by the brown trace in Fig. 1, and became quieter than yesterday’s measurement.

In addition, the 60 Hz power-line noise was significantly reduced, and the noise appears to have moved to harmonics such as 120 Hz.

Fig. 2 shows the evolution of the OMC REFL spectrum during the process of improving the alignment. It can be seen that the intensity noise was gradually reduced.


Investigation of the origin of the 60 Hz power-line noise

Since the 60 Hz power-line noise was improved by the SRY alignment, mainly in the pitch direction, and the component moved to harmonics, it is suspected that one of the suspensions related to SRY is moving at 60 Hz in pitch.

Therefore, I measured the pitch and yaw spectra of the related suspensions and looked for suspensions showing a 60 Hz line.

The result is shown in Fig. 3. A 60 Hz component can be seen in the pitch spectrum of SR3. Since this component became invisible in the OMC REFL signal after improving the SRY pitch alignment, SR3 pitch motion is likely the cause of the 60 Hz noise seen in OMC REFL.

Plans for tomorrow

We plan to reduce the high-pass cutoff frequency of the SR560 attached to the OMC REFL PD as much as possible while keeping the OMC REFL intensity noise low by using the improved SRY alignment. This is expected to allow us to reduce the scan speed of the auxiliary laser.

And the current UGF of the PLL is about 20 Hz. We will try to increase the control bandwidth, for example by removing the 20 dB attenuator placed after the PFD, so that the auxiliary laser can follow a faster LO scan.

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