Today, We tried to use smaller value of beam radius. I found that this can make coupling better. From 0.1mV to 0.2mV. The input laser is 5mW.

The fiber detector(DET01CFC/M) is 3.5V(5.5mW), that means we have 0.0003mW comes out from fiber now. This is definitly smaller than the case of last week. The coupling is only 0.01% now.

The procedure I did is like this:

1. Without inserting fiber, check the laser is almost going through the center of collimator.

2. Insert the fiber to collamitor, then adjust the x and y of collimator.

3. At the same time, use multi-meter to monitor the fiber detector(without using 50 Om). Until the voltage goes beyond the range of multi-meter, we start to use 50 Om.

4.Now you will 0.1mV on the multi-meter. Usually, this is the time to use two sterring mirrors to maximaze the coupling.

I did this procedure many times, but still the best result is 0.2mV. It seems something maybe wrong. 

(By the way, I spoke a wrong thing during today's optics meeting. When we can read 0.1mV on multi-meter, the output of fiber is very weak! I am so sorry that I remember something wrong!)