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Entry  Sun Sep 16 16:42:16 2012, tara, DailyProgress, fiber optic, setup for fiber optic distribution. 2012_09_16_fiber_setup.jpg
    Reply  Mon Sep 17 20:59:15 2012, tara, DailyProgress, fiber optic, setup for fiber optic distribution. 2012_09_18_fiber.jpgIMG_1761.jpg
Message ID: 1049     Entry time: Mon Sep 17 20:59:15 2012     In reply to: 1047
Author: tara 
Type: DailyProgress 
Category: fiber optic 
Subject: setup for fiber optic distribution. 

 I adjusted the mode matching a bit ( changing lenses positions and rotate the lens on the collimator). The coupling efficiency was up to 66%. This should be enough for now.

The power input can go up to ~20mW, so the output is ~12mW which should be enough for gyro.  I also adjusted the polarization, so that the polarization of the input beam matched the fast axis of the cable. I tested the polarization of the output beam with a PBS and got the extinction ratio of ~ 670.

The fiber is polarization maintaining fiber, nufern pm980.IMG_1761.jpg

It has fast and slow axes, and we have to match the polarization of the input beam to the fast axis. To do that

  1. rotate the angle of the output state (B) to find the minimum and maximum transmitted beam behind the PBS. The extinction ratio is max/min.
  2. rotate the input stage (A) and repeat (1) until you find maximum extinct ratio

If the beam polarization matched the fiber axis, the output beam will have linear polarization which gives the maximum extinct ratio. Meanwhile if the beam polarization does not match the fiber axis, the output beam will have elliptic polarization and the extinction ratio will be lower, since certain amount of power will be transmitted and reflected no matter how you rotate the beam.

2012_09_18_fiber.jpg

==Note==

  • The output polarization is quite sensitive to the fiber motion/position. I could not really clamp it down because I had to rotate the fiber, and need to give it some slack. So the measured power changed a bit during the measurement. It could moved from 5 uW to ~ 30 uW for the minimum transmission. This increased the error by a factor of 6.
  • The reason for low extinction ratio may also come from the fact that the output beam angle changes with the rotation at B. So the beam did not always hit the PBS perpendicularly. I kept the PBS and PD close to output as much as possible to minimize any effects due to the beam position. This can be repeat by keeping the beam output stable, and rotate the PBS with a rotatable mount instead. I think Dmass mentioned that he had the stage. I'll ask him later.

 

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