LD beam pattern collimated by short focal length aspherical lens

[imyu37]

In my work, we used a LD with fast axis divergency 18 degree. And we collimated it with C710TME-B which is an an aspherical lens bought from THORLABS. The following picture is what the collimated beam looks like about 10 meters away.
Is it normal that the collimated beam with heavy stray light shown on the picture? If it's not, how could we reduce it?Any suggestion is appreciated,thank you for advance.

[LaNeK779]

Silly question but can you check that the lens is all clean and shiny and that it is inserted the right way into the beam? (i mean these things usually have a front and back side i.e. a right and wrong side....)

Seeing that this is an f = 1.49 mm lens, the diode will need precise centering to the lens (mechanical tolerance of the diode mount and lens holder can offset the centering and cause artifacts such as this noise you see). Centering means that the beam hits the lens exactly in the middle and at 90degrees angle. However, I feel judging by the spot shape that this is probably not the only cause of noise, at least the centering does not look off to me

I do feel that this is mainly down to the lens being a glass molded type. Can you forward this pic to thorlabs and ask them too?

The easy way to filter this "noise" is use a masking aperture and block the stray light (i.e. a 3mm hole some distance away frm the diode), or if your application uses scanning galvanometers leave the galvo mirrors to act as appertures and cut out the stray light (at least significant portions of it)

The best and more precise way it to use a two identical lenses to form a 1:1 telescope and place a sub 0,5mm hole right into the focus point in the middle of the telescope. This is called a spatial filter and can be quite expensive. Its precise hole size can be calculated to fit your beam properties but it generally is in the order of 100-200microns (0.1-0.2mm). This will yield the best results, but also require precise micro-positioning in order to make it work and not cut your beam to almost zero light coming out...

So, it all depends on your application and the amount of beam quality you need. Just my two cents, I am no expert in lenses myself

Something like a spatial filter can be seen here (no precision equipment used, it was just a quick and dirty setup as proof of concept, the losses can be seen just by looking at this picture

By the way, what diode are you using? The spot size at 10meters using an f1,49 lens is very nice. Most probably single mode, low power diode?

[imyu37]

Thanks for the reply byLaNeK779.And the suggestion given is helpful to us.

I'm sad that i found the window glass got spotted occasionally.Now,after i clear it, the pattern of collimated beam is more better than before.

The LD is optically single-mode and its power is about 120mw.