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Message ID: 2015     Entry time: Wed Feb 10 00:17:31 2016
Author: KojiN 
Type: Misc 
Category: PD QE 
Subject: Alignment of the single mode fiber and re-measurement of the beam profile 

The beam measued yesterday was very elliptic.

Thus, for obtaining a round beam, we decided to use a single mode fiber.

The single mode fiber was aligned with KojiA's instruction.

(There is the specific method in the last of this log)

As a result, the obtained mode matching ratio was 64.5%. (Input power for the fiber was 18.7 mW, and output power of the fiber was 12.07 mW.)

Then, the setup as shown in Fig. 1 was prepared.

In this setup, the beam profile after the single mode fiber was measured as shown in Fig. 2.

When Fig. 2 is observed, there is still a kind of lines in the y-direction and we consider it comes from the problem of camera.

The measured data and fitted curves are shown in Fig. 3.(Zero point of the distance was set at the position of the collimator #2)


Specific method to align the single mode fiber

1. place the collimator #1 at the focal point of the laser

2. align the laser light (IR light) to the collimator #1 roughly with steering mirror

3. unset the collimator #2 and set a fiber illuminator instead of the collimator #2

4. at just after the Iris #2, align the laser from the fiber illuminator (Red light) to the IR light with knobs of the collimator #1 holder.

5. at just before the collimator #1, align the IR light to the Red light with the steering mirror #1

6. repeat step 4. and 5. a few times

7. unset the fiber illuminator and confirm the IR laser is ouput from the collimator #2. (If the IR light cannot be observed, go back to step 3.)

8. set a power meter after the collimator #2

9. maximize the IR light power using yaw knobs of the collimator #1 holder and the steering mirror #1

10. maximize the IR light power using pitch knobs of the collimator #1 holder and the steering mirror #1

11. repeat step 9. and 10.

12. If a good mode matching ratio can be obtained, this fiber alignment is finished. If the good mode matching ratio cannot be obtained, change the Lens #1 and re-start from step 1.

Fig. 1 current setup.
Fig. 2 Screen of the beam profiler.
Fig. 3 Measured data and fitted curves of the beam profile.


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