Old 01-14-2017, 01:21 AM #1
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Default best wavelenght to ionize air

hello everybody,I want to build laser,focus the beam and turn air into plasma.

My problem is,I dont know what is the best wavelenght to do it with.I want a wavelenght that is most efficiently absorbed by air molecules.Please dont show me that chart of atmosferic absorption,its useless becose its about the light from sun that have to travell through all the atmospheric layers,spectral absorption from heigh of 100km to ground is something completly different to laser being used at sea level at distance of 10 cm.

What I learned so far ( correct me if I am wrong )

1. every type of atom ( nitrogen,oxygen,ozone,... ) have its own unique spectral lines,if you hit the atom with laser that have wavelenght like one of its spectral lines,then its going to be absorbed more,the laser is going to travell less distance,it is going to turn into heat in shorter distance

2. pressure and humidity affect the absorption at given freqency,something something about spectral line broadening due to water vapour,I am not sure


I was thinking,since nitrogen makes up the biggest part of air,hitting one of its spectral lines would be the best? I tried to google Nitrogen infrared spectral lines but I didnt find anything


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Old 01-14-2017, 01:30 AM #2
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Default Re: best wavelenght to ionize air

Do you have any previous experience with lasers?
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Old 01-14-2017, 01:34 AM #3
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Default Re: best wavelenght to ionize air

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Originally Posted by Benm View Post
This might actually pose a problem if you could achieve it.

If you get a kW of light and focus it down to a spot the width of a hair in free air, i think you'll reach the point of air breakdown. This basically causes sparks to appear in mid air.

I have seen this effect with pulsed lasers (1064 nm) and it looks quite spectacular, but i wonder what would happen if a continous laser was to cause this... perhaps a permanent arc discharging away somewhere around the focal point?
Sounds like 1064nm should work?
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Old 01-14-2017, 01:36 AM #4
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Default Re: best wavelenght to ionize air

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Originally Posted by Crazlaser View Post
Sounds like 1064nm should work?
1064nm does work. But you need stupid amounts of power to do it, Q-switched YAG with peak powers well into the 1000s of watts. If OP doesn't have extensive experience with lasers I would advise against pursuing this.
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Old 01-14-2017, 01:42 AM #5
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Default Re: best wavelenght to ionize air

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Originally Posted by diachi View Post
1064nm does work. But you need stupid amounts of power to do it, Q-switched YAG with peak powers well into the 1000s of watts. If OP doesn't have extensive experience with lasers I would advise against pursuing this.
wait,so you want to tell me I need "1000s of watts" to do this?

https://www.youtube.com/watch?v=AoWi10YVmfE


https://www.youtube.com/watch?v=KfVS-npfVuY


the first video there is small plasma,the second is alot bigger but it doesnt seem like 1000s of watts.I want to make something like in the first video,is looks like it doesnt need too many watts

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Old 01-14-2017, 01:47 AM #6
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Default Re: best wavelenght to ionize air

1064nm is one of worst possible choices for ionizing air becose the absorption is very poor at this wavelenght,no wonder you need 1000s of watts.And thats exactly why I want to use wavelenght that is much more efficient so I dont need such powerfull laser
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Old 01-14-2017, 03:16 AM #7
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Default Re: best wavelenght to ionize air

I'm not sure about air, but these higher wavelengths don't seem to do well with absorption so I do understand. Most lasers won't cause the air to ionize mid beam though.
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Old 01-14-2017, 03:41 AM #8
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Default Re: best wavelenght to ionize air

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Originally Posted by Fonograph View Post
I want to build laser,focus the beam and turn air into plasma.
For what purpose? To what end? Why?
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Old 01-14-2017, 03:45 AM #9
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Default Re: best wavelenght to ionize air

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Originally Posted by Fonograph View Post
1064nm is one of worst possible choices for ionizing air becose the absorption is very poor at this wavelenght,no wonder you need 1000s of watts.And thats exactly why I want to use wavelenght that is much more efficient so I dont need such powerfull laser
Yes and no... You're missing a lot of the physics you'd need to understand to make this work well.

While 1064nm isn't the best absorbed by the atmosphere, that's not really the game.

For a gas, you basically have to insert the full ionization energy into all of the particles in your target area before they can move away. This is a decent amount of energy that needs to be output very fast. A lot of energy over a little bit of time will always be a lot of power.

Now, dealing with extremely short times and high powers, absorption might not even be that big a concern since the particles may be ionized before they can reemit.

Basically, you'd need a stupendously powerful CW laser or a q-switched laser. That's a bigger issue than the wavelength.

As for the wavelengths, you need to look at a chart of atmospheric absorptivity. Most of them are NOT as you say. They factor mostly for nitrogen, oxygen, and CO2 and give a very good representation for what they absorb. Those charts were figured in a lab through spectroscopic equipment.
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Old 01-14-2017, 05:55 AM #10
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Exclamation Re: best wavelenght to ionize air

Quote:
Originally Posted by Fonograph View Post
wait,so you want to tell me I need "1000s of watts" to do this?

https://www.youtube.com/watch?v=AoWi10YVmfE


https://www.youtube.com/watch?v=KfVS-npfVuY


the first video there is small plasma,the second is alot bigger but it doesnt seem like 1000s of watts.I want to make something like in the first video,is looks like it doesnt need too many watts
As other are probably already explaining lasers come in many different types and varieties. Mostly we deal with CW (continuous wave lasers) which have a continuous output beam. Lasers that are able to ionize air are
pulsed output (speed of pulse in the uS, nS and pS) measured more accurately not in Watts but in Milli-Joules -Joules of energy, they usually employ large capacitor banks as in (TEA or transversely-excited atmospheric-pressure) CO2 (gas) or YAG/Ruby solid state medium.

A CW (diode) laser does not have the impulse power needed to generate the extreme temperatures to create air breakdown.

hope this answers your question.
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Old 01-14-2017, 07:30 AM #11
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Default Re: best wavelenght to ionize air

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Originally Posted by Fonograph View Post
wait,so you want to tell me I need "1000s of watts" to do this?

https://www.youtube.com/watch?v=AoWi10YVmfE


https://www.youtube.com/watch?v=KfVS-npfVuY


the first video there is small plasma,the second is alot bigger but it doesnt seem like 1000s of watts.I want to make something like in the first video,is looks like it doesnt need too many watts
See paper here that explains the details of how they created the 3D images in your YouTube links: https://arxiv.org/ftp/arxiv/papers/1506/1506.06668.pdf

also see: http://physicscentral.com/explore/ac...d-hologram.cfm

Here is a good one they use a 45GW to 100GW mid IR laser pulses to create plasma filaments in mid-air--they use https://phys.org/news/2016-06-laser-...ments-air.html
"One key to this team's success is the use of a high-power femtosecond laser with pulses just 30 femtoseconds, or millionths of a billionth of a second, long. At these mid-IR wavelengths, Hong says, this device produces "one of the highest peak-power levels in the world," producing 100 gigawatts (GW, or billion watts) of peak power."

What's your budget for this device?

Use the LPF Seach function and search "laser air ionization" there are several threads about the subject

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Old 01-14-2017, 12:53 PM #12
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Default Re: best wavelenght to ionize air

Quote:
Originally Posted by Fonograph View Post
hello everybody,I want to build laser,focus the beam and turn air into plasma.

My problem is,I dont know what is the best wavelenght to do it with.I want a wavelenght that is most efficiently absorbed by air molecules.Please dont show me that chart of atmosferic absorption,its useless becose its about the light from sun that have to travell through all the atmospheric layers,spectral absorption from heigh of 100km to ground is something completly different to laser being used at sea level at distance of 10 cm.

What I learned so far ( correct me if I am wrong )

1. every type of atom ( nitrogen,oxygen,ozone,... ) have its own unique spectral lines,if you hit the atom with laser that have wavelenght like one of its spectral lines,then its going to be absorbed more,the laser is going to travell less distance,it is going to turn into heat in shorter distance

2. pressure and humidity affect the absorption at given freqency,something something about spectral line broadening due to water vapour,I am not sure


I was thinking,since nitrogen makes up the biggest part of air,hitting one of its spectral lines would be the best? I tried to google Nitrogen infrared spectral lines but I didnt find anything
Here's a link to an old thread asking the same question. Air ionization & lasers
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Old 01-14-2017, 10:07 PM #13
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Default Re: best wavelenght to ionize air

I have heard of a weapon which ionizes the air to the soldier with the help of a LASER and then strokes him with high voltage.
Is that your goal ?
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Old 01-14-2017, 11:52 PM #14
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Default Re: best wavelenght to ionize air

This little Er:Yag laser head is ideal for air breakdown. I believe it is from an Israeli tank range finder.
You're going to need some big capacitors for it though.
Optics are going to have to be specially designed as to not to absorb the output wavelength of 2.94uM. You've got a choice of either Sapphire, ZnSe or CaF2. Neither is cheap! (SiO2 is out of the question)



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Old 01-15-2017, 01:40 AM #15
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Default Re: best wavelenght to ionize air

Quote:
Originally Posted by diachi View Post
1064nm does work. But you need stupid amounts of power to do it, Q-switched YAG with peak powers well into the 1000s of watts. If OP doesn't have extensive experience with lasers I would advise against pursuing this.
The thing is exactly in those requirements. You need very large output power levels to do it, and q-switched YAG's provide that.

The peak power of a fairly small unit (say size of a shoebox) can be serveral kW when q-switched, and hence be enough to break down air. This is exactly what is used for demonstrating this effect: a fairly compact q-switched YAG that fires at about a pulse a second through a lens, collimating the light such that a spark is generated in mid air at the focal distance of that lens.

Averaged the power output of such lasers isn't even that large. A 10 kW pulse lasting 100 nS every second is on average only a watt. I don't see any practical way of doing something like that with diode lasers though.

As for practical application i don't know of any. There have been some -ideas- of using laser to form an ionized path and then to employ that as a 'wireless taser', but such systems have never been demonstrated. If possible they would probably be illegal to use in combat anyway as such a powerful laser could much more easily be employed as a weapon to blind the enemy.
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Old 01-15-2017, 03:15 AM #16
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Lightbulb Re: best wavelenght to ionize air

Quote:
Originally Posted by Benm View Post
The thing is exactly in those requirements. You need very large output power levels to do it, and q-switched YAG's provide that.

The peak power of a fairly small unit (say size of a shoebox) can be serveral kW when q-switched, and hence be enough to break down air. This is exactly what is used for demonstrating this effect: a fairly compact q-switched YAG that fires at about a pulse a second through a lens, collimating the light such that a spark is generated in mid air at the focal distance of that lens.

Averaged the power output of such lasers isn't even that large. A 10 kW pulse lasting 100 nS every second is on average only a watt. I don't see any practical way of doing something like that with diode lasers though.

As for practical application i don't know of any. There have been some -ideas- of using laser to form an ionized path and then to employ that as a 'wireless taser', but such systems have never been demonstrated. If possible they would probably be illegal to use in combat anyway as such a powerful laser could much more easily be employed as a weapon to blind the enemy.
Army range finder on an abrams tank.
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532nm 10mW-15mW DPSS Module IR filtered(S.Korea)RyanSoh3 Class IIIb
450nm CarbonFiber ElectricPlasma build 9mm +6W Class IV
445nm 3.49W pk ~3.15W av. Sinner Cypreus II +DTR 9mm 445nm Class IV
445nm RHD Saber Twins 2.7W ~2.35W pk Class IV
445nm A140 module x2 Maxsink Class IV
405nm Daguin Kryton Groove SO6J 700mW build. Class IV
405nm Olike 560mW-600mW pen sold Scopeguy20
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