Miyakawa, Tanaka
This morning, we found that the GRY absolute frequency appeared to be in a mode-hop region, because the Y-arm cavity transmitted power showed two distinct states.
Figure 1 shows the trend of the GRY flash and the oplev signals related to the Y-arm. Although no suspension seemed to move, the normalized GRY flash level frequently switched between 0.8 and 1.0.
Moreover, during today’s initial alignment, we noticed glitches in both GR transmitted powers (Fig. 2). Therefore, the current GR frequencies at their respective temperatures are not optimal for the arm cavity. We decided to adjust the temperatures of the IR and both GR lasers.
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### Adjusting the IR main laser temperature
Before adjustment, the IR main laser temperature was 36.27 °C (Fig. 3). Due to the work from two days ago, both GR laser temperatures had been reduced.
To raise them, we increased the IR laser temperature to 36.7 °C.
According to Endo-kun’s slide (JGWdoc), when the IR laser temperature is ~36.7 °C, both GR temperatures should exceed 30 °C.
Before adjustment, the GRX and GRY temperatures were 28.23 °C and 25.64 °C, respectively (Fig. 4). So we tried to raise both above 30 °C.
However, we could not find any PLL error signals when both GRs were around 30 °C. When we decreased the temperatures again, we recovered the PLL error signals near their original values. This behavior seems strange and suggests that the GR frequencies may not have shifted significantly.
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### Suspected temperature–frequency relation shift
We suspected that the relation between temperature and absolute frequency had shifted to lower values compared to Endo-kun’s slide. One possible reason could be the change in the IR master laser drive current from 2.0 A to 1.55 A.
In this case, to push both GR temperatures above 30 °C, the IR temperature would need to exceed 40 °C.
We tested this and increased the IR temperature to 41 °C (Fig. 5).
However, even then, we only observed the PLL error signals when GRX and GRY were at 29.31 °C and 26.35 °C, respectively (Fig. 6). These values represent at most ~1 °C increases from their previous values.
This indicates that the current relation between IR absolute frequency and temperature is completely different from the earlier result shown in Endo-kun’s slide.
The reason is unclear, and we will need to remeasure the relation.
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### ALS stability check
In this state, we engaged ALS_CALM to check the stability of ALS lock for both arms.
There was no significant drop in transmitted power for either arm, except when SR3 fluctuated strongly or when the actuators saturated (Fig. 7).
On the other hand, the PLLY occasionally became noisy (Fig. 8).
The interval between noisy moments was several hours, and each noisy moment lasted only a few minutes.
Therefore, while the current GR frequencies appear to be acceptable with respect to mode-hop, the PLLY stability is somewhat degraded. Still, it may be acceptable for lock acquisition.
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Tonight, we left the Y-arm locked with both IR and GR to check long-term stability.
