[Komori, Smith, Fujimoto, Saito]

When the OMC REFL PD signal was observed without injecting the sub-laser light, the fringe amplitude was approximately 4 counts. After injecting the sub-laser light, the fringe amplitude increased to approximately 180 counts, corresponding to an SNR of about 45. As in the previous measurement (klog:37138), the LO frequency was frequency-modulated with a coefficient of ±15 MHz/V. As the external signal for the frequency modulation, a 300 Hz, 800 mVpp triangular or sinusoidal waveform generated by a function generator was applied, and scans of SRX and SRY were performed to acquire data. In addition, to obtain data with a higher sampling frequency, the channel "K1:IOP-OMC0_MADC0_TP_CH12" was used to monitor the OMC REFL PD signal, and the channel "K1:IOP-LSC0_MADC0_TP_CH22" was used to monitor the external signal used for frequency modulation. An attempt was made to determine the lengths of SRX and SRY from these data using curve fitting. However, the fitting procedure developed previously (klog:37145) depended strongly on the initial fitting parameters and did not produce reliable results. Therefore, the fitting method will be improved so that stable fitting can be achieved.
 

• First, the OMC REFL PD signal was observed without injecting the sub-laser light, and the fringe amplitude was found to be approximately 4 counts. Next, the sub-laser light was injected and the sub-laser PZT was driven, resulting in a fringe amplitude of approximately 180 counts. Therefore, the SNR was approximately 45. Since the signal amplitude was sufficiently large, attention was shifted to the PLL system. The beat signal amplitude was approximately 5 dBm, which was considered sufficient, and measurements proceeded to scanning SRX.
   
• As in the previous measurement (klog:37138), the LO frequency was frequency-modulated with a coefficient of ±15 MHz/V. A 300 Hz, 800 mVpp triangular waveform generated by a function generator was applied as the external signal for the frequency modulation. In addition, to obtain data with a higher sampling frequency, the channel "K1:IOP-OMC0_MADC0_TP_CH12" was used to monitor the OMC REFL PD signal, and the channel "K1:IOP-LSC0_MADC0_TP_CH22" was used to monitor the external signal for frequency modulation. The signals from these two channels were recorded, and after changing the external modulation signal from a triangular wave to a sinusoidal wave, another set of data was acquired.
   
• After acquiring the SRX scan data, measurements proceeded to SRY scanning. As in the SRX measurements, data were acquired both when the external signal for frequency modulation was a triangular waveform and when it was a sinusoidal waveform (Fig. 1). An attempt was then made to determine the lengths of SRX and SRY from these data using curve fitting. However, the fitting method developed previously (klog:37145) depended strongly on the initial fitting parameters and did not provide satisfactory results. Therefore, the fitting procedure will be improved to achieve more reliable and stable fitting results.

[Saito, Yokozawa, Smith, Komori, Fujimoto]

Abstract

We succeeded in reducing the OMC REFL intensity noise for PRX and PRY to a level comparable to that obtained in the SRY, by tuning the PRCL OLTF and suppressing the control noise.
As the next step, we plan to perform the PRX/PRY length measurements using the OLTF tuned in this work.

Details

In the SRY length measurement, we were able to reduce the main laser intensity noise observed with the OMC REFL PD by locking the OMC to the carrier and suppressing the control noise from the SRCL control. This significantly improved the SNR of the length measurement.

Therefore, as preparation for the next PRX/PRY length measurements, we tuned the PRCL OLTF so that the OMC REFL intensity noise for PRX and PRY becomes comparable to the level achieved in the SRY/SRX cases.

Tuning of the PRCL OLTF

First, we requested PRX_1F_LOCKED in the VERTEX guardian to lock PRX. At this point, excluding the simple gain filters, the filters used for the control were as follows, and no roll-off filter was applied:

- UGF20: z3p300, gain = 94.6
- DC8: 8 Hz integrator

We then created and applied the following filter in the PRCL1 filter bank:

- tmp260701:
  - gain = 1.2
  - z1p8 (to change the cutoff frequency of the 8 Hz integrator to 1 Hz)
  - ELP80

By adding tmp260701, the OLTF became UGF ~17 Hz, PM ~36 deg, roll-off at 80 Hz, and an integrator cutoff at 1 Hz, which is almost same as the one used for SRY.
The measured PRCL OLTF is shown in Fig. 1.

Measured OMC REFL intensity noise with PRX/PRY

Using the OLTF described above, we measured the main laser intensity noise with the OMC REFL PD.
During the measurement, the ISS was turned on, and the OMC control was operated with gain = 3 and dither amplitude =1000, which realizes UGF of 30 Hz.

The red trace in Fig. 2 shows the intensity noise measured with the OMC REFL PD for PRX. It reaches a level comparable to the SRY intensity noise measured yesterday, shown by the black trace, above 10 Hz.

Fig. 3 shows the result for PRY. A similar noise level was also obtained in this case.

One point to note is the peak around 2–3 Hz. Since this peak was not observed in the SRY/SRX cases, it may be related to the motion of the Type-Bp suspensions. However, since its contribution to the RMS seems to be small, we decided to leave it as it is for now.

Plan for tomorrow

Tomorrow, we plan to perform the PRX/PRY length measurements using the OLTF tuned in this work.

However, since we have not resonated the auxiliary laser in PRX or PRY before, we will first check whether we can make the auxiliary laser resonate in the same way as in the SRY case, and whether we can lock the PLL.

The SRY scan data acquired in the previous experiment (klog:37138) were divided into segments according to the time intervals between changes in the slope of the triangular waveform used for frequency modulation, and each segment was fitted individually. When the same initial values for the fitting parameters were used for all segments, some of the fits failed to converge properly. However, these problematic cases could be fitted successfully by changing the initial parameter values, suggesting that individual initial values should be assigned for each data segment. Using identical initial values for all segments, the resulting SRY length was found to be 64.94 ± 0.02 m.
 

• First, the OMC REFL PD signal data acquired in the previous experiment (klog:37138) were normalized. The signal used for frequency modulation was converted from counts to frequency using the conversion factors of 610 μV/count and 15 MHz/V. Next, the data were divided according to the intervals between changes in the slope of the triangular waveform used for frequency modulation, and each segment was fitted using scipy.optimize.curve_fit. The fitting function was

  Φ=4πL(f+f_offset)/c
  P_t=A/(1+B(sin(Φ/2))^2)

  where f is the modulated frequency and c is the speed of light. The fitting parameters were L, f_offset, A, and B. The initial values of the fitting parameters were set as follows:

  L=65 m (the expected SRY length),
  f_offset=282 THz (the laser frequency),
  A = the maximum value of the OMC REFL PD signal within the fitting interval,
  B=6.48, corresponding to 4×finesse/π^2 for an assumed SRY finesse of 4.

  In addition, near the times at which the slope of the triangular modulation waveform changed, the frequency derived from the recorded data might differ from the actual frequency variation. Therefore, for each interval between successive slope changes, the first one-sixth and the last one-sixth of the data were excluded from the fitting procedure. The fit corresponding to the smallest uncertainty in the SRY length is shown in Fig. 1, while the fit corresponding to the largest uncertainty is shown in Fig. 2. The fit in Fig. 1 appears satisfactory, whereas the fit in Fig. 2 appears poor. Although the fit in Fig. 2 can be improved by changing the initial parameter values, doing so causes some other datasets to fit poorly. Therefore, it is likely necessary to choose the initial parameter values individually for each dataset. The SRY lengths obtained from all fits are shown in Fig. 3. Here, up sweep refers to the intervals in which the slope of the triangular modulation waveform was positive, while down sweep refers to the intervals in which the slope was negative. The mean values and standard errors calculated from all fitted SRY lengths are:

  Up sweep: 64.89±0.03 m
  Down sweep: 64.99±0.02 m
  →Overall: 64.94±0.02 m

[Fujimoto, Yokozawa, Smith, Saito, Komori]

Abstract:

We successfully reduced the intensity noise on the OMC REFL PD.
The noise sources were found to be control noise from the SRY length and OMC length control loops, as well as residual 9 Hz noise in the OMC length due to insufficient gain.

Details:

As a continuation of the work reported in klog:37137, we investigated the origin of the intensity noise measured on the OMC REFL PD.

First, we measured the noise with the MCE feedback, as described in klog:37137.
We found that we had already tried this configuration previously (brown vs. red line in klog:37137), and that using only frequency feedback without mass feedback gives better performance.

Next, we tried to reduce the intensity noise by locking the OMC and allowing the carrier intensity noise to escape to the OMC transmission port, as proposed in klog:37139.
Although the intensity noise was reduced at high frequencies, the noise bump around 100 Hz did not change.
This suggests that the noise originates from intensity fluctuations of the RF sidebands.

The next step was to identify the origin of the RF sideband intensity noise.
In principle, the coupling from SRC length noise to intensity noise should be quadratic, but asymmetry can introduce linear coupling.

We turned on the whitening filter of the RF PD currently used to measure the SRY length signal (POP17-I), since sensing noise can introduce additional noise through the feedback control.
However, this did not change the noise, which means that the sensing noise is not dominated by ADC noise, but by other sources such as PD dark noise.

The hypothesis that sensing noise causes control noise in the SRY length loop seemed to be correct, because the shape of the noise spectrum reflects the transfer function of the SRCL filter, including the phase compensation and roll-off elliptic filters, as shown by the green line in Fig. 1.
When we changed the cutoff frequency of the elliptic filter from 300 Hz to 100 Hz, we immediately obtained an improved spectrum without the OMC locked, shown in orange, and a further improved spectrum with the OMC locked, shown in blue.

We also observed a 9 Hz peak, which is known to be a problematic resonance around the OMC, along with many peaks at high frequencies.
We successfully reduced these peaks by increasing the UGF of the OMC length control from 7 Hz to 30 Hz, as shown in brown.

In addition, we adjusted the filters to use a lower gain in order to further reduce the control noise.
We also found that the control noise of the OMC length loop was another dominant noise source around 100 Hz, so we reduced the filter gain while maintaining the same UGF by increasing the dither amplitude.

Finally, we obtained the lowest noise spectrum, shown in red, which is close to the dark noise level, shown in purple.
For reference, the coherence between the OMC REFL signal and the ISS/SRCL noise is shown in Fig. 2.

Using this configuration, we will perform the cavity scan with a better SNR.

• Recovery procedure was simplified thanks to the new status bit served by pylon-camera-server.
• A new function to record current gain/exposure as the default values (see Fig.1). Please record them if you want to keep these values after rebooting camera process (e.g. in the case of server maintenance).

With Takaba-san
We checked the items for the Ncal installation at X-end. It seems that the parts needed to set up and fix the pylons are available. The details are summarized below.

• Around the sill plate (for laser hazard area) area:
  • M10 bolts: 8 × 2
  • M10 nuts: 8 × 2
  • M16 bolts: 12 × 2
  • M16 washers: 12 × 2
  • M10 spring washers: 8
  • Pylons: 2
• Behind EXA, under the pipe:
  • Laser sensor sets: 2
  • Magnetic fluid seals: 2
  • Stractures for bearing?: 1
  • Belt: 1
  • O-rings: 3?
  • Rotor shafts: 2
  • Motors: 2
  • Small vacuum chambers: 2
  • Pylons: 2

In the recent SRY length measurements, the intensity noise of the main laser observed with the OMC REFL PD has been limiting the improvement of the SNR.

I came up with an idea to lock the OMC to the main laser during the measurement.
With this configuration, the main laser, which is the source of the intensity noise, will be transmitted through the OMC, while the auxiliary laser, whose frequency is shifted from the main laser, will be reflected by the OMC. Therefore, this method may improve the SNR.

I would like to discuss this method at tomorrow morning' meeting.

[Fujimoto, Komori]

Abstract:

We will try to reduce the intensity noise on the OMC REFL PD in order to perform cavity scans with a better signal-to-noise ratio.
We measured the intensity noise of the injected light and confirmed that it is sufficiently small.

Details:

As reported in klog:37124, we are suffering from intensity noise on the OMC REFL PD, which degrades the signal-to-noise ratio of the cavity scan.
Figure 1 shows the recently measured intensity noise, where the red and orange lines represent the best-case measurements with good alignment and intensity stabilization turned on.
If we can reduce this noise, we will be able to estimate the FSR more precisely.

First of all, we suspected that the intensity stabilization might have degraded because its performance had not been checked for some time.
Figure 2 shows the intensity noise measured by the out-of-loop PD with the ISS turned off and on, shown in red and blue, respectively.
This result indicates that the ISS is working properly, and that the other noise sources should be investigated as possible origins of the intensity noise on the OMC REFL PD.

Other noise candidates are as follows:

• Frequency noise
  Currently, the IMC is locked using a combination of frequency feedback and MCE mass feedback.
  We do not know the crossover frequency at the moment, but if it is too high, the frequency noise may not be sufficiently suppressed.
• Suspension noise
  Some suspensions related to the SRY cavity may be noisy enough to imprint displacement and angular motion onto the intensity noise.
• Beam clipping
  Beam clipping may occur at the OMC REFL PD.

[Smith, Fujimoto, Saito]

The cutoff frequency of the high-pass filter in the SR560 used for the OMC REFL PD signal was set to 300 Hz. As in the previous experiment (klog:37130), the PLL was locked using the SR560 configured as a first-order low-pass filter with a cutoff frequency of 1 Hz and a gain of 2000, together with an additional integrator with a cutoff frequency of 100 Hz implemented in Moku:Lab. The LO frequency was frequency-modulated with a sensitivity of ±10 MHz/V. When a 300 Hz, 800 mVpp sinusoidal or triangular signal generated by a function generator was used as the external modulation signal, SRY flashes were successfully observed. Furthermore, data for an SRY scan were acquired using a frequency modulation sensitivity of ±15 MHz/V. The FSR and the length of the SRY will be determined from these data.
 

• Since the main-laser noise was smaller when the cutoff frequency of the SR560 high-pass filter used for the OMC REFL PD signal was set to 300 Hz than when it was set to 100 Hz, the cutoff frequency was set to 300 Hz.
   
• Next, only the sub-laser beam was injected, and the alignment was adjusted to maximize the fringe amplitude. The main-laser beam was then injected as well. The alignment of the PLL path was also optimized to maximize the beat signal amplitude.
   
• As in the previous experiment (klog:37130), the PLL was locked using the SR560 configured as a first-order low-pass filter with a cutoff frequency of 1 Hz and a gain of 2000, together with an additional integrator with a cutoff frequency of 100 Hz implemented in Moku:Lab. A 1 Vpp sinusoidal signal generated by the Moku:Lab function generator was used as the LO signal. The LO frequency was adjusted to approximately match the beat frequency and then finely tuned so that the feedback signal was close to 0 V. The LO frequency was then frequency-modulated with a sensitivity of ±10 MHz/V. When a 300 Hz, 800 mVpp sinusoidal signal generated by a function generator was used as the external modulation signal, the result shown in Fig. 1 was obtained. The upper plot shows the OMC REFL PD signal, while the lower plot shows the external modulation signal used for the frequency modulation. When the external signal was changed to a triangular waveform, the result shown in Fig. 2 was obtained. In both Fig. 1 and Fig. 2, the sub-laser PZT was swept over a range of ±4 MHz, and approximately seven peaks were observed during one period of the external modulation signal. Since the FSR is approximately 2.2 MHz, these peaks are believed to correspond to SRY flashes. Increasing the frequency of the external modulation signal resulted in cleaner peak shapes, but the number of observed peaks decreased. Therefore, a modulation frequency of approximately 300 Hz appears to be optimal.
   
• Finally, an SRY scan was performed using a frequency modulation sensitivity of ±15 MHz/V, and the OMC REFL PD signal together with the external modulation signal were recorded. The FSR and the corresponding SRY length will be determined from these data.

I found a frequency counter with the frequency range up to 200MHz in NAOJ.

It will be shipped to Kamioka tomorrow.

[Smith, Hirose, Fujimoto, Saito]

The 20 dB attenuator placed before the SR560 input was removed, and the SR560 gain was increased to 2000. In addition, a Moku:Lab was inserted before the SR560, and an integrator with a cutoff frequency of 100 Hz was added. Measurement of the open-loop transfer function showed that the UGF was approximately 2 kHz. With the cutoff frequency of the high-pass filter in the SR560 used for the OMC REFL PD signal set to 300 Hz, SRY flashes were observed when the LO frequency was modulated by ±2 MHz at a modulation rate of 300 Hz. Furthermore, SRY flashes were also observed when the cutoff frequency of the SR560 high-pass filter was changed to 100 Hz and the modulation rate was reduced to 100 Hz. However, the noise level was higher than in the 300 Hz cutoff-frequency case.
 

• First, as in the previous experiment (klog:37123), the PLL was locked using only the SR560 by configuring it as a first-order low-pass filter with a cutoff frequency of 1 Hz and a gain of 200. The open-loop transfer function was then measured, yielding a UGF of approximately 2.5 Hz (Fig. 1). The reason why the UGF decreased compared with the previous result (klog:37123) is unclear. Next, the 20 dB attenuator before the SR560 input was removed, and the SR560 gain was increased to 2000. Measurement of the open-loop transfer function under these conditions showed a UGF of approximately 2 kHz. The corresponding open-loop transfer function is shown by the light orange trace in Fig. 2. Subsequently, a Moku:Lab was inserted before the SR560, and an integrator with a cutoff frequency of 100 Hz was added. The measured open-loop transfer function again showed a UGF of approximately 2 kHz. The corresponding transfer function is shown by the orange trace in Fig. 2.
   
• Next, while attempting to scan the SRY, it was noticed that the DC signal from the OMC REFL PD was approximately -300 counts. When the light incident on the PD was blocked using a sensor card, the signal returned to approximately 0 counts (Fig. 3). The reason for the offset of approximately -300 counts remains unknown.
   
• Next, a 1 Vpp sinusoidal signal generated by the Moku:Lab function generator was used to modulate the LO frequency. The LO frequency was first set equal to the beat frequency and then frequency-modulated by ±2 MHz at a modulation rate of 300 Hz. Under these conditions, with the cutoff frequency of the SR560 high-pass filter used for the OMC REFL PD signal set to 300 Hz, four peaks were observed within one 300 Hz modulation cycle (Fig. 4). Since the FSR of the cavity is approximately 2.2 MHz, these peaks are believed to correspond to SRY flashes. Furthermore, when the modulation rate was reduced to 100 Hz and the cutoff frequency of the SR560 high-pass filter was also reduced to 100 Hz, four peaks were again observed within one 100 Hz modulation cycle (Fig. 5). However, the noise level was higher than in the case with the 300 Hz cutoff frequency.

Measured TFs from the IP actuator to the ACCs (or FLDACCs) showed increasing gain toward DC below 70mHz for L and 50mHz for T as reported in klog. It is due to the cradle effect. It is difficult to correct the gain using general servo filters. We are considering whether feedforwarding can reduce the gain at low frequencies. I tried to create ACC L signals compensated by the low-passed signals offline. Plot 1 shows the used low-pass filter. Plot 2 shows time-domain data of LVDT, ACC, and low-passed ACC signals. The IP L was excited with the IP actuator during 300~1500sec. Plot 3 shows the ratio of the LVDT TF and the ACC TF. The gain with compensated signals was smaller than the original, but the phase difference was larger.

[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.

[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.

[Ushiba, Smith, Tanaka, Fujimoto, Saito]

First, the appearance of the sub-laser beam on the OMC REFL camera was examined under the single-pass condition. The mode shape appeared elongated in the horizontal direction. Next, to avoid saturating the OMC REFL PD with the main-laser light, the PD gain was reduced from 40 dB to 0 dB. The PD output was connected to an SR560, which was configured as an AC-coupled second-order high-pass filter with a cutoff frequency of 300 Hz and a gain of 100. When the sub-laser PZT was driven with an 8 Vpp, 500 Hz triangular wave, the noise from the main laser and the amplitude of the sub-laser fringes were comparable, making it difficult to distinguish the fringes. However, when the ISS was turned on, the noise from the main laser was reduced, and by lowering the PZT drive frequency to 170 Hz, the sub-laser fringes became clearly visible. Therefore, it appears feasible to scan the SRY. Next, the ND filter in the sub-laser path of the PLL setup was changed from OD = 0.5 to OD = 0.6. After realigning the PLL path to maximize the beat signal, the beat signal amplitude reached approximately 4.82 dBm. When the LO frequency was tuned to the beat frequency, the PLL successfully locked. Furthermore, when the LO frequency was varied, the beat frequency followed accordingly, confirming that the lock was functioning. However, even while locked, the beat frequency sometimes drifted spontaneously by several MHz. Therefore, the loop filter parameters will need to be adjusted to suppress this motion, and the open-loop transfer function will also be measured. In addition, a piezo driver will be introduced to drive the PZT over a wider frequency range.
 

• First, the main laser was turned off and the SRM was misaligned to observe how the sub-laser beam appeared on the OMC REFL camera in the single-pass configuration. The mode shape appeared horizontally elongated (Fig. 1). This is likely because the beam was hitting the lower part of a lens that had originally been installed in the optical path. Next, when the SRM was aligned, the fringe amplitude was approximately 2 μW (Fig. 2). Since this was comparable to the result obtained previously (klog:37116), the alignment of the sub-laser was not further adjusted.
   

• Next, the main-laser power measured at the OMC REFL PD under the single-pass condition was approximately 1.95 mW, while it increased to approximately 6 mW when the SRM was aligned. To avoid saturating the OMC REFL PD with the main-laser light, the PD gain was reduced from 40 dB to 0 dB. Under this condition, when the main laser was resonant in the SRY, the PD output was approximately 3600 counts, corresponding to approximately 2.1 V, allowing the PD output to be connected directly to the SR560. The SR560 was configured in AC-coupled mode, and the cutoff frequency of the high-pass filter was chosen while observing the OMC REFL PD signal with only the main laser present so as to minimize signal fluctuations. The gain was then increased while ensuring that the SR560 did not overload. Next, only the sub-laser beam was injected and it was confirmed that fringes could be observed when the sub-laser PZT was driven. The main-laser beam was then reintroduced. With the SR560 configured as a second-order high-pass filter with a cutoff frequency of 300 Hz and a gain of 100, and with the sub-laser PZT driven by an 8 Vpp, 500 Hz triangular wave, the noise from the main laser and the sub-laser fringe amplitude were of comparable magnitude, making fringe identification difficult (Fig. 3). However, after turning on the ISS, the noise from the main laser was significantly reduced. Furthermore, by lowering the PZT drive frequency to 170 Hz, the sub-laser fringes became clearly visible (Fig. 4). The power spectra shown in Figs. 3 and 4 correspond to K1:OMC-REFL_DC_IN1. In Fig. 4, the blue trace was taken while the sub-laser PZT was being driven, while the red trace was taken without driving the PZT. Comparing Figs. 3 and 4 clearly shows that the ISS reduced the noise level. Therefore, scanning the SRY appears to be feasible.

• Next, the optical powers measured immediately before the RFPD in the PLL path were 45.5 μW for the main laser and 2.122 mW for the sub-laser. Since the sub-laser power might have been too high, the ND filter in the sub-laser path was changed from OD = 0.5 to OD = 0.6. The sub-laser power was then remeasured and found to be 1.349 mW. The PLL optical path was then realigned to maximize the beat signal (Fig. 5), resulting in a beat signal amplitude of approximately 4.82 dBm. Using a filter similar to that employed in klog:37052 and setting the LO frequency equal to the beat frequency, the PLL successfully locked. Furthermore, changing the LO frequency caused the beat frequency to follow accordingly, confirming proper PLL operation. Next, an attempt was made to scan the SRY by sweeping the LO frequency. However, clean fringes such as those shown in Fig. 4 were not observed. In addition, even while the PLL was locked, the beat frequency occasionally drifted spontaneously by several MHz, indicating that further adjustment of the loop filter is necessary to stabilize the lock. The open-loop transfer function will also need to be measured. Furthermore, a piezo driver will be introduced in order to drive the PZT over a wider frequency range.