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The fluctuating intensity at the exit end of the fiber!

jwdwh

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Dec 23, 2013
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Hi gang,I decide to focuse laser into the SMF ( single mode fiber ),but found that the following system would produce a fluctuating intensity at the exit end of the fiber(Fig 1):

Helium-Neon laser (633 nm, 35 mW, DA(Divergence Angle)=0.66mrad, BD(Beam diameter)=1.23mm) --> lens(20X)--> single mode FC fiber (NA=0.14, 125 micron cladding, 3.6m length)-->power meter. The lens and SMF are all fixed on two separate 5-dimensional positioner(from Newport: http://search.newport.com/?q=*&x2=sku&q2=LP-05A) And all the system are fixed on the optical table.

Here is the problem:
The intensity reaching the power meter is about 26mW with fluctuations as large as 1mW with a frequency of 0.5-1Hz. Since the He-Ne laser has been on for hours and is rather stable at the time of measurement (as measured directly at the laser output), this fluctuation could be caused by the fiber coupling system described above? It is strange that when I vibrate the floor, the intensity doesn’t seem to change much. I suspect it is cause by mechanical deformation of the 5-dimensional positioned, while placed the whole system over a night, the fluctuations is still there. Furthermore, the intensity seems swing with the central point——26mW, and the swing trend seems no rules to follow——sometimes larger than 1mW, sometimes smaller than 0.5mW.
Since the laser is used for scientific experiments, this kind of fluctuating is unbearable. I am curious to know where the disturbance come from and how to eliminate it.

Thanks in advance!
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What power meter are you using? Is this noise in the beam still present between the laser and the lens?
 

jwdwh

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Thanks for reply!
The power meter is a cheap one made in China. While measure directly at the laser output, the reading is quite stable.
I have noticed the "beam" now, so I remove it and retest the output from the fiber, while it seems nothing change, the fluctuating is still there.
 
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I don't see how a fiber would cause this. Sounds like something else in the environment is interfering with the sensor.
 
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The fiber is mounted when taking a measurement from it, or is held in the hand in front of power meter? Probably the former, based on your pictures, and no offense meant, just to be clear. I wonder if even in a mount, a broken fiber might fluctuate transmission in that manner?
 

jwdwh

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The fiber is mounted when taking a measurement from it, or is held in the hand in front of power meter? Probably the former, based on your pictures, and no offense meant, just to be clear. I wonder if even in a mount, a broken fiber might fluctuate transmission in that manner?

Yes, the fiber is well mounted by a holder when taking measurement. I don't think the fluctuation is cause by a broken fiber.The fiber was bought several days ago and well protected when operated. Even if it is broken,the intensity reaching the power meter may decrease but should be stable. I can't figure out why it will fluctuate such random.
Anyway, thanks for your reply!
 

WizardG

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Try taking a measurement of the laser itself using a beam expander so the beam isn't just making a dot on the LPM sensor. If that is stable then try a different fiber. My $0.02
 




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