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# Stupid Optics Question - Laser line divergence

##### Well-known member
LPF Site Supporter
I have a stupid question, if using a plano-convex cylinder lens or a glass rod with a laser beam which has been focused to infinity (to produce a thin wide line), what is the divergence of the line?

Some might look at this question as invalid; how is divergence related to a laser line which spreads into a thin plane many degrees wide? I mean in the opposite plane, or the thickness of the line. What determines how much thicker it gets as the line travels forward?

My assumption is the thickness of the line diverges at the same rate as the beam had prior to shooting into the line lens, correct? Yes, no? More info?

EDIT:

One more maybe not so stupid question, I want to bore a hole through a PBS cube, anyone know where I can send a cube to get it done? Needs to be accurate without damaging the cube. This would be to coaxially combine three beams, the beam going through the hole a much smaller diameter beam than the other two going through and being combined, of course. I'd rather have a cube which simply has an absence of a polarised layer in a very small area in the center, but haven't seen anything like that.

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#### paul1598419

##### Well-known member
If you look at the data sheets of these diodes you'll see divergent full angles for parallel and perpendicular with respect to the diode's band gap. They are independent of each other. Hope that helps.

As for your other question.... I have no idea.

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#### RedCowboy

##### Well-known member
Yes, both the vertical and horizontal axis diverge, one just diverges faster than the other, so if using a cylinder lens you only reshape 1 axis, the other axis continues to diverge unaffected.

If you make your beam look square at the aperture it does not mean it will look square in the far field because each axis although at the moment look the same will diverge differently.

Is that what you meant?

#### paul1598419

##### Well-known member
Yes, the uneffected axis will continue to diverge as if nothing had been done. The line is just a line and will continue to diverge very aggressively.

#### RedCowboy

##### Well-known member
All beams are either diverging or converging by some amount even when perfectly collimated, remember it's 3 dimensional, but you know this.

A perfect beam only exist in our minds, to have a truly perfect beam it would have to be a single string of photons, as in 1 photon wide by 1 photon tall and in a vacuum.

#### RedCowboy

##### Well-known member
Our moisture rich air is also littered with gases and dust, a laser beam catches hell over a long distance, not the 100 feet that I play with but even at 100 feet I see some strange things, like the boarder edge of the spot twitching, now I know when growing the substrate on the silicon wafer they introduce a line of trash at the edge to define it so that could be what I am seeing at distance where I am looking at a exploded view of the emitter.

But through our air a visible wavelength gets influenced my humidity.

What was the best wavelength, microwaves work well but wasn't it in the near IR that travels the best through air?

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#### steve001

##### Well-known member
I have a stupid question, if using a plano-convex cylinder lens or a glass rod with a laser beam which has been focused to infinity (to produce a thin wide line), what is the divergence of the line?

Some might look at this question as invalid; how is divergence related to a laser line which spreads into a thin plane many degrees wide? I mean in the opposite plane, or the thickness of the line. What determines how much thicker it gets as the line travels forward?

My assumption is the thickness of the line diverges at the same rate as the beam had prior to shooting into the line lens, correct? Yes, no? More info?

EDIT:
It depends on the focal length of the pcx lens.

A rod lens is similar to a ball lens both having very short focal lengths. In the use of a rod if placed perpendicular one axis will expand at a rate independent of its collimation mrd. The other axis is unchanged. If place parallel to the beam the same happens.

#### Benm

##### Well-known member
I guess is this is where the prism-pair corrective optics come in: Trade a narrow beam with high divergence to a wider one with smaller divergence, but only in one direction.

Something similar could possibly done with half-cylindrincal/rod lenses as well.

It can vastly improve the far-field shape of a laser dot, but in practical use do not expect any miracles. With a multimode dode the 'dot' will be a rectangle regardless of what you do. It may just become more of a rectangle than a line, which could be good enough for some applications.