Hi, Elena,

If possible, it is better to share the image which you observed the smallest spot with the ring(s). It is not easy to understand your situation accurately without the image.

But, I suppose that the ring besides the central spot is the Airy Disk.

Example of Airy Disk

https://www.edmundoptics.com/resources/application-notes/imaging/limitations-on-resolution-and-contrast-the-airy-disk/

How large lens are you using for focusing the beam? If your lens diameter is not enough large, you may have limitation in the minimum spot size at the focus.

It is recommended to use large NA (Numerical Aperture) lens.

In addition, in case of spherical lens, you can have a spherical aberration. One point you can possibly improve is replacement of the spherical lens with an asherical lens if you are using spherical lens.

Hi Elena,

How are you defining or measuring the beam waist of the fundamental? Is 9.14 μm the radius or the diameter?

For a 25 μm step index core diameter, I would expect the 1/e2 intensity of the fundamental mode to be approximately 18 μm diameter. If you need a smaller mode field diameter, Thorlabs offer https://www.thorlabs.com/thorproduct.cfm?partnumber=FG010LDA

What is the 1/e2 diameter of the laser beam incident on the 70 mm lens? Is the intensity profile close to Gaussian?

Is the 13.6 μm waist the diameter of the spot formed by the lens, or of the output from the fibre?

If you are launching a smaller diameter spot than 18 μm, then I would expect to excite the LP02 (HE12) mode in addition to the fundamental LP01 (HE11) mode. The LP02 mode has a central spot and a single annular ring.

It seems likely you need a better match between the incident beam waist and the fibre's fundamental mode field diameter, but I can't be sure from the information you have presented. Even if the 1/e2 diameters match, the intensity profiles will differ somewhat in shape. You may not suppress the higher modes entirely, but you can reduce the proportion of the higher order mode, and minimise power in the outer ring.

Are you able to use a smaller core fibre such as Thorlab's FG010LDA which has a shorter cut-off wavelength, and should not support the LP02 mode? The fibre will probably still support the LP11 modes (HE21, TM01, TE01), but you can suppress these by accurate transverse and angular alignment of the launch spot with the fibre axis.

Hi Elena, if I understand your issue correctly, you are trying to to clean up your signal to improve the fundamental modes. Try wrapping your fiber loosely around an object, not tighter than the bend radius, but enough to introduce some curvature. This will eject light from your higher order modes traveling down the cladding.

Alan Robinson I was giving the 1/e2 radii of the waists.

Marley Kunzler I saw a work where M2 of the 'fundamental' mode beam degraded from bending. I tried something like this with no improvement.

Here is a small report that I created. Go to part 2 where I used a more appropriate lens.

Elena,

As I understand from your Nov 20 report:

In part 1 you imaged the fibre exit facet with magnification 250/97 = 2.58. The 1/e2 radius at the camera was 34 μm, corresponding to 13.2 μm radius at the fibre. In the vertical scan, there is a secondary peak at radius ~50 μm, corresponding to 19 μm radius in the fibre.

If these magnification factors are correct, the secondary peak is in the cladding of the fibre, and cannot be from a core-guided mode (assuming you have a uniform refractive index in the cladding). How long is the fibre? Have you taken steps to remove cladding light?

In part 2 you imaged the fibre at magnification 210/108 = 1.94. The 1/e2 radius at the camera was 26 μm, corresponding to 13.4 μm radius at the fibre. This is broadly consistent with the 1/e2 radius measured in part 1, but shows a cleaner profile. However, there is still significant intensity at radii 30-55 μm (15 - 28 μm fibre radius).

This suggest that there is still appreciable optical power in the fibre cladding. Alternatively, could the broadening of the image spot be due to aberrations in the imaging lens? What kind of lens are you using?

You state that the optimum launch focal length is 67.4 mm to create a waist radius of 9.14 μm. Are you able to confirmed that the input beam 1/e2 diameter at the lens is 5 mm?

Hello everyone, sorry for the long absence.

Alan Robinson I am using a 1 inch 3 mm thick lens, plano-convex, so I doubt that it introduces significant aberrations. In fact, I remember it being hard to find a condenser with EFL>50 mm, probably because spherical surfaces are ok at those curvature radii. What I haven't done, though, is imaging of the beam's waist after the lens! This might be a good thing to try. However, I am repeatedly getting a symmetrical beam out of the fiber, with a great M^2~1.1 (though measured using only 5 points), and I decided that I can live with it for now, since it simplifies my model.

In general, it looks like with increasing V, it will be harder and harder to get a clean TEM00. Also, I noted that sharper bends degrade the beam quality. I now only have 2-3 wide bends, and the fiber is only 30 cm long, compared to the previous 60 cm.

Again, than you everyone for the advice and tips!