Free space vs fiber for high power lasers

Dear all
I am going to build an epi fluorescence (TIRF) microscope for single molecule super res imaging. I want to work with 3 lines of about 1000 mW power lasers. Can anybody let me know the practical advantages and disadvantages of fiber vs free space laser?

Thanks

Could you specify what laser manufacturer(s) you are interested in? The question is a little unclear. Do you mean a fiber laser? Or a fiber coupled laser? The fiber lasers that I’m familiar with are also free space lasers in the sense that you have to couple the output of the laser yourself into a fiber, if that is what you want to do.

Thanks for your reply.
Yes, I mean coupling the output with the fiber vs using lens and mirrors on the optics table to guide it to microscope.
Lasers from Coherent, Oxxius, etc.

(Edited to correct laser powers)

Thanks for the clarification, my list of advantages / disadvantages.

Fiber coupled:

  1. Should be more stable with less pointing stability issues, though these aren’t usually a big problem anyway even for TIRF.
  2. The beam out of the fiber (assuming single mode) will have a very high quality.
  3. At least some manufacturers such as Nikon sell a TIRF module that at least in theory you could connect to with the fiber. In practice you probably need to have a fiber with a similar numerical aperture.
  4. You can put the lasers on a separate table or in another room.

Free space:

  1. You will probably have some free space optics already anyway in order to control the laser powers and couple the light into the fiber.
  2. Less power loss. It isn’t easy to efficiently couple into a single mode fiber and your lasers will have to have a pretty good mode. Also you mention 1W, which is a lot of power for a single mode fiber. I’ve been told that really high powers like that can cause Brillouin scattering in the fiber, drastically reducing the efficiency (I’ve never tried to couple that kind of power myself).
  3. Single mode fibers are usually specific to a certain range of wavelengths, though Newport does sell some that will work for multiple wavelengths.

Some other considerations:

  1. Focusing 1W on the back aperture of your microscope objective may also cause problems for the objective. I’ve seen objectives damaged with at powers levels like that, at least with shorter wavelengths (488 or less). This may have been made worse by a dust particles on the objective.
  2. My experience is that you usually lose about 1/2 of your laser power by the time the beam gets to the objective, even with free space. This is in systems that have multiple dichroics and lenses, as well as an AOTF.
  3. For a 50x50um field of view SMLM experiment you’ll ideally have about 50mW or more of laser power going into the objective.
3 Likes

Many thanks for your useful information.
Actually those are about 1W lasers (1000mW) and of course I will not work at full power all the time.
As far as I understand, generally you prefer the fiber type, am I right?

Heh, sorry I slipped 3 orders of magnitude there…, so what I meant was that 1W is a lot to send through a single mode fiber and may also damage the objective depending on how it is focused. If you are planning on using those kinds of powers then free space is probably a better choice.

My personal preference is to just do everything free space and not use a fiber. At least for what I am doing the advantages in terms of beam quality and being able to isolate possibly noisy lasers on another table are not worth the effort of dealing with fiber coupling.

1 Like