[Enomoto, Sugimoto, Yokogawa]
We tried to couple the green beam for Y arm to the optical fiber last week and this week.
What we did:
(on Dec. 21st)
- aligned the steering mirrors and the colimeter lens position of the fiber coupler so that the beam is coupled to the test optical fiber with length of 5m. However, the transmittance of the fiber reached only 25%.
(on Dec. 26th)
- measured the beam profile just before the fiber coupler, but the measured beam profile was dirty. We thought that the beam was clipped on the Iris.
(on Dec. 27th)
- opend the iris slightly and measured the beam profile again. Then we found that the beam radius was smaller than that obtained from my mode match calculation. This seemed to be the effect of swapping the laser for the new one.
- recalculated the mode match.
(on Dec. 28th)
- moved the mode matching lens (f=250mm) about 40mm upstream based on the mode match clculation.
- tried to couple the beam to the test fiber. The transmittance reached 68% (input power: 53mW, transmitted power: 36mW)
- placed PDA100A on the POS table in order to measured the transmitted power of the fiber.
- set the PD gain to 0dB.
- measured the transmitted power of the long fiber. The measured value was about 2600cnt. This is equivalent to 3.31mW when converted to power and the transmittance of the fiber is expected to about 33mW considering that the PD receives the transmitted beam of the BS with transmittance of 10%.