Hello friends,
We've been adjusting a laser collimating light path, and we found an interesting phenomenon. We move the collimator along the light axis, we can get a clear and round spot, and we continue moving the collimator, and we get a weird and swirl like spot. Did anyone have the similar experience and know why?
This sounds reasonable to me. I suppose it depends on what you mean by “weird swirl” but the basic idea of what you are describing seems very normal.
First, a few things, I presume the simple diagram is not very representative. I presume the receiver is at some large distance and you are truly collimating, not imaging. I couldn’t say for sure, but can I assume your laser divergence is large and your collimator is more complicated than a simple spherical thin lens? Maybe a sphere involved? Finally, where do you see this funny shape? I presume on a screen or paper in the near field not long after the collimator?
There are several things which cause funny beam shapes. First is the character of the laser light itself. At the output the light has an intensity profile (transverse mode) and a wave front. You can think of the wavefront as the angular content. At each point in the wavefront the light is propagating perpendicular to that front, and any variation from a plane wave means different parts of the beam are going different directions. When perfectly collimated A lens conjugates those properties. The angular profile becomes the spatial profile, and the spatial profile becomes the angular profile. Often lasers have very nice wavefront. The optically flat output coupler assures the beam is a plane wave at the output. When you collimate the beam, that nice plane wave becomes a diffraction limited Airy disc: the nice round spot you saw when properly collimated. This isn’t always the case. The output isn’t necessarily a plane wave, but more importantly, the near field intensity profile can also add angular content due to diffraction. Any misalignment of the laser cavity might cause high intensity with a sharp edge near one side of the aperture. Also, the shape of the aperture may not be round, so the diffraction might not be a circle. For example square apertures make sort of four fold symmetric stars. Many shapes are possible. Anyhow, you saw a nice circle. Note that the divergence profile is related to the near field intensity profile through diffraction, but they aren’t the same thing. A nice circular divergence can be produced by a beam with a fairly ugly transverse mode. Now, if you move the lens from f to 2f and look at the beam at 2f past the lens, now you are reimaging the near field beam. You’ve conjugated back. You get the transverse mode which can be completely unlike the far field pattern. But, what happens when the lens is between these two extremes? You get a mixture of the two pictures. Actually, a convolution of the two patterns at different size scales. Depending on how different the two patterns are, (and usually the near field mode is the weird one) this can go through funny shapes as you move the lens.
But that isn’t the only reason. In fact, with your pristine divergence pattern, your near field intensity pattern probably isn’t too strange. Another contributor is the aberrations of the lens. This is particularly important for fast collimators. Aspheric lenses are particularly bad. Basically the lens is a good collimator only A) at the working distance, and B) on axis. If you move the lens away from focus or especially if you tilt the lens or translate the lens sideways so the beam is off axis, all hell breaks loose. Fast lenses have large coma and astigmatism off axis.
So what are you seeing? Try reimaging the beam (thin lens in 2 f 2f configuration, or opposed collimator). Do you have an interesting transverse mode? Or I see your diagram indicates an array. Do they make a nice symmetric pattern? At the lens position where you see the odd swirl, try translating the lens across the beam and also try tilting the lens. Does the weirdness change significantly? If so it is caused by the aberrations of the lens. Is there a best most symmetric position? That’s “on axis” If not, then the weirdness is the mixing of the near field and far field pattern.
- Badges
- Science topic
- Similar topics
- Phytochemistry
- Extracts