1. Thermal lensing and the Gingin test 1
Jérôme Degallaix
Gingin international workshop
October 2005

2. Presentation of the first test
80 meters long cavity
6W input laser
5kW circulating power
Strong thermal lensing
Sapphire test mass
Diameter: 100 mm
Waist size: 8 mm
Abs = 50ppm/cm
Uniform absoprtion

3. Effect of thermal lensing
Change the apparent ITM Radius
Change cavity modes
Example: 6W input power, 5kW circulating power
Pabsorbed = 2.5 W
FITM = 472 m

4. Presentation of the first test
Pcirc
Pin
Gain = Pcirc / Pin

5. No thermal lensing

6. 1W input power

7. 2W input power

8. 3W input power

9. 4W input power

10. 5W input power

11. 6W input power

12. 7W input power

13. 8W input power

14. 9W input power

15. 10W input power

16. Do we need a FFT simulation ?
Thermal lensing deformations: non spherical!

17. Higher order modes comparison

18. Idea of compensation

19. HOM spectrum with CP CP works fine for TEM00
Make higher order mode worst!

20. Problem with the PRC Arm cavity PRC
PRC degenerate  Higher order mode resonant!
g factor PRC =
Frequency separation between HOM
f = 10 kHz
( FWHMPRC = 130 kHz )

21. Make the PRC non degenerate
Arm cavity
PRC
For example:
g =  f = 340 kHz (> 2 * FWHMPRC )
Do not compensate totally thermal lensing
fITM = 10 km

22. Compensation plate works (for TEM00)
Need to investigate the behavior of HOM
Does not any correct any inhomogeous absorption

24. Profile of the deformations
Thermal lensing deformations: non spherical!

25. Circulating power comparison

26. Reflected power

27. Higher order modes comparison

28. Higher order modes comparison

29. 4W circulating power profile