Light Meters, Fiber Optics and Optogenetics – Part 5 Little Mistake, Big Problem

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A customer was getting much lower power than expected from the fiber optics of his Prizmatix Optogenetics LED. His entire system including his power meter was shipped to our lab for debugging.

Turns out the whole problem was the FC adapter on the detector of his power meter.

The purpose of this FC fiberoptic connector/adapter is to have the fiber at the right spot at the detector, but in fact it was blocking most of the light from going into the detector. That is why, in our previous post the photos show the integrating sphere without any fiber optic connector.

Here is the description of the problem in the report written by our engineer.

The system was tested with the customer supplied power meter and the FC adaptor.
It appears the problem is with the FC adaptor used with the power meter.
Our fibers are very high NA fibers, with large diameter cores. The light coming out of the fiber is shining at 39 degree angles from the optical axis. The FC adaptor supplied with the integrating sphere has a small output aperture and stops the fiber a little distance from the edge of the adaptor:
When using a core larger than the aperture, a lot of the light is blocked leading to a reduction in power. When using the 500um core, a lot of the light is blocked because of the distance from the edge of the adaptor: C:\Users\Arie\AppData\Local\Microsoft\Windows\INetCache\Content.Word\Adaptor final.jpg
When testing the fibers (as received) without the adaptor the power was within normal parameters. When testing with the adaptor I received the same results you’ve (i.e. our customer) sent us.
Fiber LED Power w/ Adaptor Power W/O adaptor
1500um #1 Blue 350 [mW] 900 [mW]
1500um #1+2X500um Blue 40 [mW] 63 [mW]
1500um #2 Blue 380 [mW] 995 [mW]
1500um #2+2X500um Blue 44 [mW] 83 [mW]
1500um #3 Blue 375 [mW] 986 [mW]
1500um #3+2X500um Blue 43 [mW] 78 [mW]
1500um #1 Green 140 [mW] 360 [mW]
1500um #1+2X500um Green 18 [mW] 28 [mW]
1500um #2 Green 147 [mW] 391 [mW]
1500um #2+2X500um Green 18 [mW] 34 [mW]
1500um #3 Green 137 [mW] 390 [mW]
1500um #3+2X500um Green 18 [mW] 31 [mW]
Cannula (250um) power was tested; when using the blue LED the power was 20mW without the adaptor, and about 9 with the adaptor.
When using the green LED the power was 9mW without the adaptor and 3.8mW with the adaptor.

Bottom Line: When measuring power using an integrating sphere detector, make sure the FC adapter is not blocking light. Better follow the instructions in my last blog post and use the meter without the adapter.

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