Do you know the specs of your attenuator? The two I have (HP 8156A & Tektronix AO5012) cannot take more power than the OSA itself. If it is a free space optic based attenuator like this setup that is off course something else: https://www.newport.com/f/manual-variable-laser-attenuator-kit
Yes I do!
It takes a 50um single mode fiber.
It's the Anritsu MN924A Optical Attenuator which is an optic based attenuator.. it's the only affordable one I could find.
I also had a friend who had a fiber coupler also for 50um single mode.. again not ideal but it's the only affordable fiber coupler I could find with the adjustements.
I did some more studying and some low wattage tests (not hooking it up to the OSA) and am aware coupling into single mode is hard.. I found videos that guided through the process very well.
The losses by trying to couple the beam into the fiber would actually be desirable in this case, along with a beam sampler from Thorlabs are kind of what I want to reduce the power to more acceptable levels. Then let the attenuator do the last hurdles to bring the power down in a more controlled manner.
I'd measure the output of the fiber at the end by using my handheld laser power meter before connecting it up to the OSA.
Thorlabs beam sampler can be found here:
https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=913&fbclid=IwAR0cwAQSyAm__ppzoYDHB-BdkK-xIJ8zThJk8qeVR4en4-PNeRumn5DWD10
It's the Anritsu MN924A Optical Attenuator which is an optic based attenuator.. it's the only affordable one I could find.
That is indeed similar to the attenuators I use. But these can typically only handle something around 20-25 dBm.
I also had a friend who had a fiber coupler also for 50um single mode.. again not ideal but it's the only affordable fiber coupler I could find with the adjustements.
50µm is multi-mode, 9µm is single mode. Your attenuator appears to be single mode,
The losses by trying to couple the beam into the fiber would actually be desirable in this case, along with a beam sampler from Thorlabs are kind of what I want to reduce the power to more acceptable levels. Then let the attenuator do the last hurdles to bring the power down in a more controlled manner.
Be careful counting on the losses due to the inefficient coupling. Typical problem is that at first to coupling is so bad you do not even see a signal on the OSA, and while adjusting the coupler (or by some vibrations of the table) you can suddenly hit the sweet spot having way too much power going to the OSA. Fiber couplers are VERY sensitive. If this would happen, I have no idea how your attenuator will then act, but I can imagine that if you severely overload it, it will start passing more power to your OSA, especially with the high laser powers you are talking about!
Don’t forget to calculate your power density. With a 40 Watt laser, it easy to destroy the beam sampler if the beam is too narrow.
It could be an option to place 2 of those beam samples in series so an attenuation of at least 100x is guaranteed independent of the polarization.