I am trying to optimally couple the light from a pulsed Nd:YAG laser source into a 2mm core fibre. The wavelength range is between 680nm - 1050nm.
The output beam from the laser is large so currently we have been focusing the laser onto the fibre using a plano-convex lens, however I don't think this is the most optimal way of coupling the light. I have been looking at aspheric lens (such as https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=3811) and also FibrePort Couplers (such as https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=2940), however I am unsure if these will work with our system since we are focusing the light to a smaller spot size before coupling to the fibre. Is it possible to focus the light from the laser onto a FibrePort coupler, or does the light need to be collimated when entering the fibreport?
Any help/tips/info on this will be greatly appreciated!
Laser: Surelite OPO Plus (http://www.continuumlasers.com/index.php?option=com_content&view=article&id=652&Itemid=581)
Fibre: CeramOptec 2mm core, 0.22 NA (yes, the core is really 2mm in diameter - http://www.ceramoptec.com/products/fiber-cables.html)
Thanks!
Dear Oscar,
A few notes to start with:
(1) A 2-mm core fiber is a multimode fiber for your frequency-doubled 532 nm YAG OPO, and in general fiber coupling for MM fibers are quite straight forward - I don't believe you need aspheric lenses. Aspheric lenses are most useful when coupling single mode (Gaussian) beams into single mode fibers. So an aspheric lens in this case is likely an overkill.
(2) I noticed your OPO produces up to at least a few hundred mJ of pulse energy with short nano-second durations. The peak power is high and you should avoid focusing your beam INTO the fiber. We will discuss how below.
(3) I am not sure why you are coupling nanosecond OPO pulses into such a large core fiber. Just note that because of intermodal dispersion, you could end up with stretched pulse at the distal/output end. This depends on whether you intend to scramble the modes and how long your 2-mm fiber is.
Now .......
To launch the laser beam into the fiber, you will have to match the numerical aperture (NA) of your focusing lens to that of the 2-mm fiber for efficient energy coupling.
The NA of your coupling lens system is NA(lens)= n * sin (theta). n in this case is the refractive index of air, so NA(lens) = sin (theta). Theta is the half angle of the extent of your lens aperture diameter, D, to the focus, f; theta ~ (D/2)/f.
The fiber input NA is NA(fiber) = sqrt (n_core^2 - n_cladding^2). In this case, NA(fiber) = 0.22
Preferably, the lens system NA is slightly smaller than the fiber input NA, so that if there is slight angular misalignment between your lens system and the fiber, all the launched energy would still be coupled into the fiber. So build a coupling lens system with NA slightly below 0.22 (e.g. NA(lens) = 0.20 is okay, but never bigger than 0.22)
Since you are launching high peak power laser pulses, you should avoid having the beam waist (focal point) aligned too close to or into the fiber end. You could potentially damage the fiber. Rather, you should keep the beam waist in air away from the fiber end, but close enough that the expanding beam cross section is still underfilling the fiber core cross sectional area.
Underfilling launch condition will result in excitation of the lower-order modes, but you can 'homogenize' mode-mixing or scramble the beam to have more even mode distribution by bending/looping the fiber in between the input and output ends.
Useful References:
https://en.wikipedia.org/wiki/Numerical_aperture
http://www.tpub.com/neets/tm/109-3.htm
Happy coupling and best,
Kin
Thank you both for your inputs.
In response to Kin, I am coupling the light into a 2mm core fibre so that the light can be delivered to varying experimental setups relating to photoacoustic and photothermal therapy applications. I'm currently using a lens with an NA of 0.2 to couple the light into the fibre, so you think this is good enough? I was just unsure as to whether collimating the beam before it enters the fibre will result in a higher transmission of energy and a more Gaussian beam shape.
Dear Oscar,
If you are conducting photoacoustic (especially photoacoustic more so than photothermal) studies, it is more important for you to maximize the range of irradiance (W/cm2). Hence, my suggestion is you try to use as small a fiber as possible while ensuring you have optimal energy coupling. How small depends on the beam quality of the originating laser source, and you should ask the manufacturer (please ask for the M^2 value, as in "M-square" value). For example, if your laser source produced a Gaussian beam, the beam quality should be very good and M^2 ~ 1. If the cavity produced a bunch of higher order modes, M^2 >> 1. If you start out with a Gaussian beam (M^2 ~ 1) and use a large core fiber like the 2-mm fiber, you will lose the beam quality because once entering a large core fiber, higher order modes would be excited, and you will end up with mixed mode beam profiles at the output end, and potentially even stretched pulse which is typically not desirable.
By saying NA = 0.2, I assume the incoming laser beam is relatively well collimated at incidence to the coupling lens. If so, then yes NA=0.2 is good enough. In designing any optical system, please remember that the product of F/# and NA is conserved.
Best,
Kin
They sell optical coupling gel for exactly that use.
It basically eliminates the reflective losses.
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