KAGRA Logbook

2019-01-16 With Enzo.

As Simon advised, we changed the optical layout of the optical lever such that the TILT QPD is placed 150 mm behind the lens which has a 150 mm focal length (ignoring the length sensing QPD at the moment). We switched the lens to with the 150 mm focal length because we did not want to bother with folding the beam path to fit every components in.

With the 1.1 m and 2500 mm collimator, we estimated the linear range of the tilt output. See calibration data of the QPD below:

SR2 TILT QPD calibration results (QPD behind lens, 1.1 m collimator)
YAW_OUTMON (cnt)	Horizontal Displacement (mm)	PIT_OUTMON	Vertical Displacement
2.55	9.08	-2.10	11.73
-18.2	9.09	-30.4	11.74
-39.7	9.10	-55.0	11.75
-53.6	9.11	-69.7	11.76
-65.7	9.12	-79.1	11.77
-75.1	9.13	-87.0	11.78
-83.2	9.14	-91.7	11.79
-87.8	9.15	-94.9	11.80
-91.2	9.16	14.5	11.72
-93.7	9.17	38.7	11.71
19.6	9.07	53.7	11.70
38.2	9.06	68.0	11.69
54.0	9.05	78.9	11.68
66.9	9.04	86.5	11.67
78.2	9.03	91.9	11.66
85.1	9.02	94.7	11.65
89.9	9.01
93.4	9.00
95.6	8.99

SR2 TILT QPD calibration results (QPD behind lens, 2500 mm focuser)
YAW_OUTMON (cnt)	Horizontal displacement (mm)	PIT_OUTMON (cnt)	Vertical displacement (mm)
5	9.82	-6.7	11.30
-9.7	9.83	-22.9	11.31
-23.7	9.84	-33.7	11.32
-36.9	9.85	-49.2	11.33
-49.3	9.86	-59.9	11.34
-59.2	9.87	-69.8	11.35
-70.2	9.88	-77.7	11.36
-77.8	9.89	-84.4	11.37
-83.8	9.90	-89.4	11.38
-89.3	9.91	-93.6	11.39
-92.5	9.92	-95.7	11.40
-93.8	9.93	6.3	11.29
17.2	9.81	17.7	11.28
30.7	9.80	33.1	11.27
43.8	9.79	44.6	11.26
56.9	9.78	55.9	11.25
67.0	9.77	65.0	11.24
75.2	9.76	73.1	11.23
82.1	9.75	79.3	11.22
87.0	9.74	84.3	11.21
90.7	9.73	87.8	11.20
92.7	9.72	90.5	11.19

2019-01-21

SR2 TILT QPD calibration results (QPD behind lens, 3500 mm focuser)
YAW_OUTMON (cnt)	Horizontal Displacement (mm)	PIT_OUTMON	Vertical Displacement
95	11.25	-90.6	11.40
94.5	11.26	-86.7	11.39
93.7	11.27	-82.4	11.38
92	11.28	-75.2	11.37
89.1	11.29	-62.4	11.36
84.1	11.30	-52.7	11.35
77.5	11.31	-38.7	11.34
68.5	11.32	-22.2	11.33
57.7	11.33	-3.66	11.32
42.8	11.34	16.8	11.31
28.0	11.35	32.6	11.30
7.44	11.36	47.3	11.29
-12.2	11.37	61.1	11.28
-28.3	11.38	70.5	11.27
-45.4	11.39	77.0	11.26
-56.8	11.40	83.6	11.25
-68.8	11.41	87.8	11.24
-79.3	11.42	91.4	11.23
-87.4	11.43	93.2	11.22
-91.3	11.44
-94.7	11.45

With the 1.1 m collimator, when the QPD is put behind the lens, the linear range in terms of QPD displacement is around ± 0.025 mm, which, according to the ray transfer matrix, coverts to ~ ±83 µrad tilt angle for TM. (See sr2tiltoplevcal1120190116.png)

With the 2500 mm focuser, under the same situation, the linear range in terms of QPD displacement is around ± 0.045 mm, which coverts to ~ ±150 µrad tilt angle for TM. (See sr2tiltoplevcal25000190116.png)

And, for the 3500 mm focuser, the linear range in terms of QPD displacement is around ± 0.035 mm, which coverts to ~ ±150 µrad tilt angle for TM. (See sr2tiltoplevcal3500190121.png)

At last, I switched back the optical lever receive side to the original layout with 1.1 m collimator but wasn't able to calibrate it yet. I intentionally block the beam path by setting the folding mirror (top right in the photo) such that the beam is hitting the lens holder because I wasn't sure where to put the length sensing QPD yet. I will see if there is any further changes to the optical layout. The lens in the holder is still with a 150 mm focal length, as opposed to 300 mm one, which is the original planned one. (see sr2oplevlayout20190121.jpeg)