Converting Spectra Physics ZLM Laser Head to 532NM (Part 1, ruined my FAP edition)

[brianpe]

First, the good news: I successfully designed a dual constant current power supply to drive two FAP800s for this laser head. It is small and can run off a LiPo RC car battery. It can deliver 50A to each FAP and can be controlled by a microprocessor:

Power Supply

• brianpe
• May 31, 2020

Home-designed constant current switching supply. Two channels each capable of up to 50A and...

I was able to easily take the two FAP800's up to 30W output. I didn't try to push them harder - this was about 40A.

Tonight I connected the fiber optic cables to them in preparation for connecting to the cavity. On low outputs, things went great. But things went horribly wrong when I turned the power up. I first noticed some yellow light visible through my goggles coming through the fiber shining on my LPM, then the power dropped. I shut everything down but the damage is done. The surface of the fiber coupler on the FAP is burnt and it won't do more than about 2 watts. Here is a photo of the end of the fiber:

Burned Fiber

• brianpe
• May 31, 2020

I ruined this fiber when I turned the power up.

The fiber I'm using is a custom build from Thor Labs using FT800EMT 800um fiber. I'm coupling the fiber to the FAP with a fiber adapter which is basically a threaded tube the diameter of the fiber ferrule. Does anyone know what I've done wrong with the coupling? Here's a close-up of the coupling:

Fiber Coupling

• brianpe
• May 31, 2020

A close up of the fiber connection to the FAP800s.

My thoughts: I think the two fibers are too far apart inside the fiber adapter and the output of the FAP spread past the diameter of the fiber. The resulting back reflection from the shiny ferrule caused a hot spot and damaged both the fiber and the FAP coupling. I suspect this has also damaged the FAP permanently.

Anyway, long sad story. I'd like to learn from it, so if anyone knows what I did and how to prevent it from happening again, I'd love to hear it.

 

[Encap]

Could be just what you think the problem is.
Best advice is to ask Thor Labs for their advice--email them same details and photos as appear here

 

[Cyparagon]

Sorry for the laser equivalent of "did you remember to plug it in", but did you thoroughly clean the fiber ends? A few specs of dust is enough to ruin your day at these power levels.

 

[Snecho]

Google it like me do as same the same same here as same same with as same and same

 

[brianpe]

The fiber ends were clean. I took Encap's advice and talked to both Thor Labs and Coherent. The problem is that Coherent's SMA connectors go in deeper than standard SMA 905, so there was too much air gap.

 

[Encap]

The fiber ends were clean. I took Encap's advice and talked to both Thor Labs and Coherent. The problem is that Coherent's SMA connectors go in deeper than standard SMA 905, so there was too much air gap.

Nice to hear you got the problem sorted out

 

[Cyparagon]

Is there a reason for that design, or is it just the apple approach of "make everything else incompatible"?

 

[brianpe]

Is there a reason for that design, or is it just the apple approach of "make everything else incompatible"?

A new FAP just arrived and I was careful to find one with the original SMA to SMA coupler on it . I also purchased official Coherent fibers so I can compare the differences. The fibers do look to be standard SMA905 -- same ferrule length. Although they look like they have an AR coating on the ferrule tip that the Thor Labs fiber didn't have. The difference is in the SMA coupler. The one I bought is 23.5mm long. The one that came with the FAP is 19.5mm long. So there is a 4mm longer air gap inside the Amphenol coupler I bought. Coherent doesn't sell just the coupler so I'm hunting for some more.