Large sapphire substrate absorption measurements

for LIGO

S.M.A.- VIRGOS.M.A.- VIRGOUniversité Claude Bernard LYON IUniversité Claude Bernard LYON I

IPNL - IN2P3 – CNRSIPNL - IN2P3 – CNRS22, Boulevard Niels Bohr22, Boulevard Niels Bohr 69622 - Villeurbanne Cedex - France69622 - Villeurbanne Cedex - France

Univ

ers

ité

Lyon I

SMASMASMASMA

April 2003

Sapphire substrate from Crystal Systems Inc. Boule N° AJ5, L46

1

• Orientation of the substrate The sapphire substrate is slightly damaged at the edge of one side. For all the measurements, the substrate is positioned so that the defects are in the front and at the bottom of the substrate (slide # 3).

• Accuracy of the absorption measurements The absolute value of the sapphire absorption is calculated thanks to the measurement of the absorption of a silica reference, the density and the specific heat capacity of sapphire and silica. The modulation frequency used for the measurement was high: 3000 Hz.The absorption of the silica reference is calculated thanks to the measurement of the absorption of ultra pure water (0.13 cm -1), the density and the specific heat capacity of silica and water. A cross calibration was made with the Laboratoire d’Optique at ESPCI Paris. The agreement is in the percent range.Moreover, we recorded during 3 hours and 20 minutes the absorption of the center part of the sapphire substrate in order to check the stability of the measurement. We got: 30.0 +/- 0.3 ppm.cm-1 (slide # 3).

• Elementary volume measured by our photothermal deflection bench The elementary volume scanned by the absorption measurement depends on the waists of the probe and the pump beams (100 µm x 100 µm) and on the interaction length of probe and pump beams (3 mm). The interaction lenght is set by the angle between probe and pump beams: 3.5° (slide # 3).

• Artifacts due to non-photothermal deflection of the probe One drawback of the photothermal deflection technique is that the measured absorption is deduced from the deflection of the probe beam.If the deflection is only due to a photothermal effect, than the absorption measurement is correct, but if the deflection is due to scratches, small inclusions, refractive index variations of the substrate, … than the absorption measurement is wrong. In order to estimate the artifacts due to non-photothermal deflection of the probe, we recorded an « absorption » map, without the pump laser. The maximum artifact is 1.4 ppm.cm -1 and the mean value is 0.4 ppm.cm-1

(slide # 4). These values are high compared to what we would have with a super polished substrate without scratches nor inclusions but they are negligible compared to the absorption of the sapphire which is always higher than 30 ppm.cm -1.

• Absorption vs depth At 3 different (X,Y) positions, we scanned the absorption versus the depth (Z). The ranges of absorption variation for each (X,Y) position are: [32-36 ppm.cm-1] , [45-74 ppm.cm-1] and [67-127 ppm.cm-1]. At Z=0 and Z=-131 mm, the absorption is influenced by the « inspection polished » surfaces (slide # 5).

• Absorption maps We recorded 6 maps at 6 different depths. The diameter of the maps is 200 mm and the step between the points is 2.5 mm. The general shape of all the maps doesn’t change very much (slide # 6), but for each map,the absorption varies from 30 to 130 ppm.cm-1 (slides # 7-12). We recorded two zooms with small steps of 0.1 mm on 2 pixels of the previous maps. The zooms show variation of absorption at the 0.1 mm scale. (slides # 9;10).

Large sapphire substrate absorption measurements for LIGO

2

• Accuracy of the absorption measurements : +/- 0.3 ppm.cm-1

Absorption recorded at X = 0 mmY = 1 mmZ = -57 mm

during 3h 20mn:

30.0 +/- 0.3 ppm.cm-1

Defects in the front at the bottom 131 mm thick

X

Y

Z

0

314 mm• Orientation of the substrate:

• Elementary volume measured by our photothermal deflection bench:

X 0.1 mmY 0.1 mmZ 3 mm

XZ

Y

3

Z = -74 mm

In this experiment, the YAG is switched off, in order to evaluate the influence of scratches, small inclusions,refractive index variations, … on the deflection of the probe laser.

Mean absorption: 0.45 ppm.cm-1 200 mm scan

4 mm steps

• Artifacts due to non-photothermal deflection of the probe are in the sub-ppm.cm-1 range.

4

X = 0 mmY = 0 mm

X = -50 mmY = -5 mm

0

20

40

60

80

100

120

-150 -125 -100 -75 -50 -25 0Z (mm)

Abs

orpt

ion

(ppm

/cm

)

0

20

40

60

80

100

-150 -125 -100 -75 -50 -25 0

Z (mm)

Abs

orpt

ion

(ppm

/cm

)

X = -42.5 mmY = -55 mm

0

20

40

60

80

100

120

140

-150 -125 -100 -75 -50 -25 0

Z (mm)

Abs

orpt

ion

(ppm

/cm

)

• Absorption vs depth

5

Z = -68 mm

Max = 36.0 ppm.cm-1

Min = 32.2 ppm.cm-1

Max = 74.4 ppm.cm-1

Min = 45.3 ppm.cm-1

Max = 127.3 ppm.cm-1

Min = 67.3 ppm.cm-1

X

Y

Z

0

Z = -63 mmZ = -57 mm Z = -68 mm Z = -74 mm

Z = -18 mm Z = -126 mm

6

• Absorption mapsat 6 different depths

Mean absorption: 57 ppm.cm-1 200 mm scan

2.5 mm steps

7

Absorption map at Z = -18 mm

Mean absorption: 67 ppm.cm-1 200 mm scan

2.5 mm steps

Mean absorption: 91 ppm.cm-1

30x30 mm² zoom1 mm steps

8

Absorption map at Z = -57 mm

Mean absorption: 67 ppm.cm-1 200 mm scan

2.5 mm steps

Mean absorption: 101 ppm.cm-1 3.5x3.5 mm² zoom

0.1 mm steps

9

Absorption map at Z = -63 mm

Mean absorption: 66 ppm.cm-1 200 mm scan

2.5 mm steps

Mean absorption: 113 ppm.cm-1

3.5x3.5 mm² zoom0.1 mm steps

10

Absorption map at Z = -68 mm

Mean absorption: 62 ppm.cm-1 200 mm scan

2.5 mm steps

11

Absorption map at Z = -74 mm

Mean absorption: 61 ppm.cm-1 200 mm scan

2.5 mm steps

12

Absorption map at Z = -126 mm