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godfrey
Wicked Lasers Master
Joined: 23 Feb 2006
Posts: 957
Location: St. Louis, MO
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 Posted: 6/02/06, 2:20 PM Post subject: Beam Modes |
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I don't know if these images were ever posted on this site, however - I used them (w/o permission) from Shanghai Laser & Optics Century Co., Ltd.
It probably gives the clearest description of beam modes.
The URL to this page can be found at http://www.lasercentury.com/techLasers.asp (under "Laser Modes")
The fundamental TEM00 mode is only one of many transverse modes that satisfy the round-trip propagation criteria. The figure below shows examples of the primary lower-order Hermite-Gaussian (rectangular) solutions to the propagation equation.
Note that the subscripts n and m in the Eigenmode TEM nm are correlated to the number of nodes in the x and y directions. In each case, adjacent lobes of the mode are 180° out of phase.
The propagation equation can also be written in cylindrical form in terms of radius (r) and angle (f). The eigenmodes (Erf) for this equation are a series of axially symmetric modes, which, for stable resonators, are closely approximated by Laguerre-Gaussian functions, denoted by TEMrf. For the lowest order mode, TEM00, the Hermite-Gaussian and Laguerre-Gaussian functions are identical, but for higher order modes, they differ significantly, as shown in the figure below.
Low-order axisymetric resonator modes
The mode, TEM01*, also known as the "bagel" or "doughnut" mode, is considered to be a superposition of the Hermite-Gaussian TEM10 and TEM01 modes, locked in phase quadrature.
In real-world lasers, the Hermite-Gaussian modes predominate since strain, slight misalignment, or contamination on the optics tends to drive the system toward rectangular coordinates. Nonetheless, the Laguerre-Gaussian TEM10 "target" or "bulls-eye" mode is clearly observed in well-aligned gas-ion and helium neon lasers with the appropriate limiting apertures.
Phil |
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lhnova
Fusion Laser
Joined: 30 Apr 2006
Posts: 413
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| Posted: 6/02/06, 2:52 PM Post subject: Re: Beam M |
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dear god I think I only recognised about a quarter of the words you used in that post...lol... ...must read into this mode business a bit...hehe,
but thanks for the post and those diagrams are very handy,
when I shine my laser through some binoculars or some other lense that can diverge the beam greatly I can see the laser switching between modes as it warms up, initially when it is cold it will be in tem00 and then start switching rather fast through a number of them, I think I have seen up to 4 dots (though only visible when the beam is diverged like this) and sometimes it stays on tem01 for a bit before it warms up further...interesting:)
Anyway
cheers
Wicked
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Shroom
Elite Laser
Joined: 19 May 2006
Posts: 155
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| Posted: 6/02/06, 2:53 PM Post subject: |
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Wow.....Too many big words! I didn't understand a thing about that. but the first pics u posted, are those the modes taht seem to happen on most WL products? If so, that helped me alot.
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godfrey
Wicked Lasers Master
Joined: 23 Feb 2006
Posts: 957
Location: St. Louis, MO
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| Posted: 6/02/06, 3:17 PM Post subject: |
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| Shroom wrote: |
| Wow.....Too many big words! I didn't understand a thing about that. but the first pics u posted, are those the modes taht seem to happen on most WL products? If so, that helped me alot. |
Mode issues can occur with any laser - gas or diode pumped solid state (DPSS).
There have been many posts in the recent months since I joined the forum asking for assistance to identify a specific problem. Some have been mode related. I thought I'd post this handy reference so you can narrow down your problem, should you have one.
Thus far, mode issues are common - but are not the only problems that can occur. I am not an 'expert'. I've owned various lasers over the years but never became this involved with them until the past year and a half. I do not have any training in lasers, laser repair or optics. More than likely, my guess would be as good as yours. I spend most of the time researching this on the internet. If you know what to look for, and it's not hard...you'll find it.
Phil |
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Entity
Wicked Lasers God
Joined: 14 Mar 2006
Posts: 1426
Location: Ca
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| Posted: 6/02/06, 3:21 PM Post subject: |
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Thanks for that informative post!
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pixar
Phoenix Laser
Joined: 12 May 2006
Posts: 85
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| Posted: 6/02/06, 3:38 PM Post subject: |
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| Yeah, thanks - I've wondered about these modes and have been meaning to search for them. A pity the originating site does not seem to work in FF (links off the many topics do nothing) so I had to blow the dust off IE |
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chargerman
Wicked Lasers God
Joined: 08 Sep 2005
Posts: 1721
Movie(s): 2
Location: Big Horn Mountains, Wyoming
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xenoz fate
Nexus Laser
Joined: 22 Jun 2006
Posts: 98
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| Posted: 6/23/06, 9:22 AM Post subject: |
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that post was very usefull
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jmui30
25mW Classic Wicked Laser
Joined: 22 Sep 2006
Posts: 48
Location: Winnipeg, Canada
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| Posted: 10/26/06, 8:18 PM Post subject: |
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| is ther anything wrong with my laser then? Recently my laser would be very low powered and go from 2 dots to only 1 dot (TEM 01 to TEM 00). Is this something i should tell steve or is this normal and fixable? |
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gtfan
4.99mW Green Laser Toy
Joined: 14 Nov 2006
Posts: 6
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| Posted: 11/14/06, 2:15 PM Post subject: |
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There arent very many ways an end user can force single mode operation. You could operate it at extremly cold temperatures to decrease doppler broadening.
The primary industry methods of "single moding" a laser are as follows:
First method: Reduce cavity size. Cavity mode seperation is defined as C/(2*L). If you reduce L to such a degree that cavity mode seperation is greater than the linewidth, then single mode operation will occur.
The problem with doing this is that it highly restricts your output power because your cavity is tiny.
Second Method: You increase the losses of all other modes except for the mode with the highest Q to such a degree that only the highest Q mode is over the gain threshold. The basic idea of this method is you precisly place a peice of glass into the cavity with parrallel faces. You will get about 75% of your multi-mode output power using this method. Q is defined as (resonant frequency(v))/(linewidth(delta_v)) |
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ixfd64
Wicked Lasers Master
Joined: 27 Jul 2006
Posts: 571
Location: California
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| Posted: 1/30/07, 11:51 PM Post subject: |
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Would "near TEM00" be something like an oval-ish beam?
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The Great Cornholio
Nexus Laser
Joined: 15 Jan 2007
Posts: 99
Location: California
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| Posted: 2/03/07, 12:44 PM Post subject: |
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 TEM modes aside, YOUR AVATAR RULES!!!
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thecheat
Wicked Lasers God
Joined: 13 Jan 2006
Posts: 3301
Picture(s): 213
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| Posted: 2/21/07, 4:43 PM Post subject: |
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here are some REAL TEM beam pictures...
TEM01:
TEM02:
TEM03:
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fnugget
Elite Laser
Joined: 21 Jul 2005
Posts: 131
Picture(s): 2
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| Posted: 5/27/07, 1:50 PM Post subject: modes |
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It takes some time for a laser to settle into a steady state. so it probably does switch modes, but TEM00 (it's in the specs, you can see for yourself) seems to be the DESIGNED steady state mode.
By the way, when you talk about lasers, "some time" can be less than 1ms.
You might ask "why do you need different modes?", but I'd just point you to a textbook for the answer. really, it's a long explanation. Just note that it's possible for the laser to switch modes, and a dot is what you want, and if it isnt a dot, you've got a problem. |
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Raul-Wesche
Fusion Laser
Joined: 06 Jul 2007
Posts: 330
Picture(s): 31
Movie(s): 22
Location: Martinez Ca
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| Posted: 7/30/07, 1:47 AM Post subject: |
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| so the lasers that are tem02+ would be like 2 lasers or more ? |
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