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FAQ on beam distortion, power, divergence, and mode
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Gadgeteer
Wicked Lasers God
Joined: 30 Jan 2006
Posts: 2304
Location: North Texas
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 Posted: 5/16/06, 4:06 AM Post subject: FAQ on bea |
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(Preliminary)
Many issues about the laser output continue to be asked and I would like to address some of the possible causes.
The questions primarily involve beam distortion, output power, beam divergence, and multi-mode operation (multiple dots). Many of these effects are related and cause similar issues.
Beam distortion-
such as speckles, lines, rings and such are normally due to contamination or defects on the output lens. Typically, it is a contamination issue and proper cleaning techniques of the output surface will resolve the issue. Contaminated cleaning supplies will only make matters worse. Clean canned/compressed air is advised in moderation and at low pressure but can result in further issues if particles are pushed past the lens and land on the NIR (near IR) cut filter, input side of the output lens, or on the diverging lens. This condition is difficult to correct and requires removing the components for proper cleaning. Micro-fiber cloths and cleaning solution has been recommended but keep in mind that if a contaminated surface (finger, etc.) contacts an area of the cloth used to clean the lens then adverse effects may result, cross-contamination.
Output Power, Beam Divergence, and Multi-mode Operation-
The three issues need to be addressed as one, as they are mostly all related to one another. Conditions at the factory during testing can be assumed at fair room temp of 70-75F (whatever C that is) and end user temperature may not be at the same as the test temp.
Thermal variation during system warm-up is one of the reasons for power, divergence and mode variation. Manufacturing tolerance and physical limits/characteristics of the components is the other.
The laser cavity is of very short length and the small crystal size will be affected more with thermal lensing. That in itself does play a part in divergence and mode structure. Many users have noticed the beam is not TEM00 and simply diverging the beam through a lens will show this. Multi-mode is fine and it is the raw power you are after typically for these laser systems.
If you want the smallest spot size at a distance and TEM00 mode then you are better off with a 5mw or a high priced lab quality laser. Low input pump power is easier to regulate and control output variations on the 5mw. Lab quality greens use temp control of the pump diode and the doubler crystal to minimize the output power and mode variations.
Think of it this way, positive temperature coefficient.
Take a flat set of glued plates of glass with parallel sides at one temperature (i.e.- YVO4/KTP crystal). Apply heat and the sides will no longer be parallel but form a lens. Longer cavity forms a thick lens. A short cavity makes this effect more apparent.
I would not worry so much about the divergence and there are ways around it with add-on collimators or changing the output lens. If you don’t mind voiding the warranty or it is already out then changing the output lens to one of longer focal length can greatly help in reducing the far-field spot size but near field effects will be reduced unless adjustable focus is also added. That's assuming you have the skills and tools for the rework. It does require careful attention to detail and knowledge of optical alignment. Search for "collimators" on the main "index" page for more info.
Normally, and on high-powered systems, apertures are installed somewhere between the front and rear mirror to eliminate all other possible modes. This still depends on the input power to the laser and higher input can still result in multi-mode operation even with the aperture. I have seen where the divergence can increase during multi-mode operation and cause the beam size to increase.
Problem is, the aperture trick can't be used on a hybrid crystal but a form of it is accomplished by focusing the pump beam onto the crystal as in the case of the typical 5mw.
To my knowledge, all higher powered Wicked’s are designed for raw output power and do not use focusing lens between the pump diode and the crystal. The result is multi-mode output and some variation in power will be, and is, seen.
This is not a down side and part of the design to achieve higher outputs without crystal damage. Focusing the high power pump onto the crystal would VERY likely cause coating damage and possible thermal fracture.
Moral to the story, 1.2mrad is variable based on the design and will only be achieved under certain conditions. If the beam is 3+mrad then I would consider sending it back but every laser is going to have different optimal operating conditions based on temperature and battery condition.
Many factors are to be considered regarding observed issues. If the system is of low power then first make sure the batteries are good and lithium’s preferred for greatest output, NiMH for general use, and alkaline if that is all that’s available.
Hope this helps and look forward to further input and suggestions.
JD/Gadgeteer
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Last edited by Gadgeteer on 10/29/06, 4:01 PM; edited 1 time in total |
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stymfalm
Elite Laser
Joined: 05 Jan 2006
Posts: 214
Movie(s): 2
Location: Arizona
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| Posted: 5/16/06, 4:22 AM Post subject: |
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lots of good info there. i dont think many of the people here will understand some of the jargon, however. also, it might help if it were more Q&A based so that the average user could browse for their answer.
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Gadgeteer
Wicked Lasers God
Joined: 30 Jan 2006
Posts: 2304
Location: North Texas
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| Posted: 5/16/06, 4:28 AM Post subject: |
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(Preliminary)
and YES Sir, that's the point of asking for input!! Bud, you may be suprised at some of the knowledge and depressed at other. That's why I'm here, I'm here to assist and help when and where possible and enlighten the public.
Many thanks to ya Stymfalm bud! and I'd like to see another thread in collimation and focusing if and when you get the chance. Yourself and Steve0000 have far more focus on that issue than I do and the input from both of you would be very much appreiciated.
PM or shoot me here on the thread of what jargon you think would be an issue to clean it up please. I would have PM you back but I need to know the general knowledge and requirements. Others do please respond if you are unaware of the terminology used so that I can correct and clarify. Again, pm's are fine or just reply here. We want to keep this clean and make a final revision to assist all.
Great many thanks to ALL inputs!
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Athoul
Wicked Lasers God
Joined: 13 Sep 2005
Posts: 7511
Location: Canada
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Morepower!
Wicked Lasers God
Joined: 26 Apr 2006
Posts: 1072
Location: Australia
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| Posted: 6/25/06, 12:12 AM Post subject: |
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I have a beam question.How is it we are able to see the beam??? By that i mean to be able to see it does it have to be reflecting off something,i.e. particals in the air?? If not would it still be visible if it passed through a vacuum??
If the Mods think this post is not applicable here could they please edit or move it.Thanks
BTW Gadgeteer you never cease to amaze me with your willingness to help others,good one. |
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Entity
Wicked Lasers God
Joined: 14 Mar 2006
Posts: 1426
Location: Ca
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| Posted: 6/25/06, 12:43 AM Post subject: |
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Thanks gadgeteer.
Morepower: Yes, the laser beam is visible as the light collides with dust particles, water vapor, and even air molecules. In a vacuum space the beam would not be visible.
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Gadgeteer
Wicked Lasers God
Joined: 30 Jan 2006
Posts: 2304
Location: North Texas
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| Posted: 6/25/06, 3:45 AM Post subject: |
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Awww, shucks guys... Seriously, MorePower, I'm glad you asked the question here since this is where it belongs and Thank you Entity for the prompt response.
Technically, the reason for the effect is due to RATS:
Reflection
Absorbtion
Transmission &
Scattering
Anytime light, whether in a coherent beam of stimulated emmision (lasers) or even spontaneous emmision from a candle flame, light bulb, or even something hot and emitting in the Infrared, and even UV.... The principle of RATS applies.
Specifically to your question, the effects of seeing the beam is due to reflection and scattering and I am sure you are now aware that beam visibility increases as you approach an increased angle from the emitted source up to 180 degrees. Anything beyond basically looking directly into the beam will look weaker. Point of reference and (not you MP, you know better!!) but PLEASE dont look into the light (beam)!!!! This condition is due to the small particles in the air scattering and sometimes reflecting the light back to your eyes. This is why you see the beam as the light is directed back from an almost point source but along a line.
Scattering at smaller angles occurs more frequently than reflection at greater angles. That is really more due to the airborne particles for general components. I would like to say that shining the beam in a cloud of aluminum dust would produce other effects, and it would, but that would go back to questioning the size and shape of the particles to determine the reflectance and scattering effects. Particle size, wavelength of the input beam, and dispersion of the particles also come into play (and so do SO many other things...).
Basic water vapor and dust in the air are the main factors in permiting one to see the beam. As Entity mentioned that you would not see the beam in a vacuum, this is correct, entirely, depending on the nature and level of the vacuum for one (but this is not a vacuum science forum), the intensity of the beam, the mean free space (going back to vacuum science), and if there is an existing plasma within that area (plasma discharge and plasmas I've been taught are totally reflective, though...there is more discussion with that one). Techincally, even each of those are beyond meaningful discussion on my part but I know variations can occur within those envirioments.
I think the term "mean free space" best sums it up and a very common factor in the air you breath right now and how it reacts with an intense beam of laser radiation. Water vapor is the primary constituent of the air you breath typically, unless you've been to Phoenix lately. It reflects and scatters a great portion of visible light and primarily at an angle less than the directed source. Dust acts the same way and I would not say more so due to not factoring in the reflectance of the dust particles in question. As a science and to offer real figures, measurements would need to be made, and have been of course, but I cannot provide them without further research.
Typically and generally, water vapor and dust particles will scatter more light to the sides and generally at angles of 90-180 degrees (180 degrees being line of sight) from the incident light input regardless of laser or spontaneous emitted emission.
I like Entity's response better!! Far less time suming it all up. On the other hand, this is something that I've been wanting to offer for general knowledge to all.
JD
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Morepower!
Wicked Lasers God
Joined: 26 Apr 2006
Posts: 1072
Location: Australia
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| Posted: 6/25/06, 11:28 PM Post subject: |
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Thanks Gadgeteer.
One other thing i personally would like to see put up here is the math's and method for measuring divergence/mrad,as hopefully i will need it soon!!
Thanks. |
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gtfan
4.99mW Green Laser Toy
Joined: 14 Nov 2006
Posts: 6
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| Posted: 11/14/06, 2:45 PM Post subject: |
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| Morepower! wrote: |
Thanks Gadgeteer.
One other thing i personally would like to see put up here is the math's and method for measuring divergence/mrad,as hopefully i will need it soon!!
Thanks. |
Divergence is defined as twice the angle between the beam edges and the beam center. Beam edges are defined as where the power is approximently 37% (1/e) of the maximum power. Simple trig should allow you to calculate the divergence with a few measurements. |
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I made it a sticky, though at some point I'd like to combine them all into one to make al the faq's easier to locate.