Can a continous wave laser be converted to a more powerful pulsed laser?

[FTM1000]

Can a continuous wave laser weapon be converted to a Ultrashort Pulsed Laser like the one mentioned in this article?:
https://www.forbes.com/sites/davidh...hinegun-firing-light-bullets/?sh=6555843768a3
Israel is also developing laser weapon for intercepting rockets and is probably stationary and plugged to the power grid, what obstacles can they have in making this future laser weapon into an Ultrashort Pulsed Laser like the one in the article?.

 

[FinBurger]

Yes, you can make some CW lasers into pulsed lasers, but the CW laser generally has to have a chirped output. If the laser tunes from blue to red every microsecond. Then put that output into an optical fiber where the red light travels faster then the blue light. After enough travel time in the fiber, the blue beginning of the chirp catches up to the red end of the chirp (and all the colors in between). You'll have a short pulse.

A common way to do this is to first start with a short pulse, and then put it through a dispersive fiber to smear out the pulses into the aforementioned chirped pulses. Then you can amplify the chirped light while not burning out your amplifier (the peak power is much less). Afterwards, you recompress the pulses in the opposite kind of dispersion, and you get powerful short pulses.

 

[f95toli]

Note, however, that it is not at all obvious that doing this will make the laser "more destructive".
Pulsed lasers can often output a LOT of power , but it the repetition rate is low (say a few Hz) the amount of energy you are imparting on your target is still going to be too low to make any difference (unless the goal is to confuse and/or "blind" some sort of sensor).

 

[FinBurger]

Note, however, that it is not at all obvious that doing this will make the laser "more destructive".
Pulsed lasers can often output a LOT of power , but it the repetition rate is low (say a few Hz) the amount of energy you are imparting on your target is still going to be too low to make any difference (unless the goal is to confuse and/or "blind" some sort of sensor).

Actually, I think one can say that if I have two laser beams of the same power, and one is CW and the other is pulsed, then the pulsed one can cause more damage. But in any case, the referenced article was highlighting a self-focusing aspect of a pulsed beam. If the pulses are instantaneously bright enough, it causes ionization in the air, and if the stars and moons are aligned, it can allow the beam to propagate without diffraction. And that is quite useful.

 

[Gordianus]

Ah...the laser weapon...
I heard about them 40 years ago and I'm still waiting.
President Reagan fell under Teller's spell and believed X-ray lasers would destroy incoming missiles. Have you see those lasers yet?
Lasers are outstanding things but, as long range weapons...

 

[sophiecentaur]

Ah...the laser weapon...
I heard about them 40 years ago and I'm still waiting.
President Reagan fell under Teller's spell and believed X-ray lasers would destroy incoming missiles. Have you see those lasers yet?
Lasers are outstanding things but, as long range weapons...

I remember, several years ago, going to a lecture about lasers with a bunch of 6th Formers. The guy introduced the idea of Chemical lasers which obtain a population inversion by chemical means and do not involve significant electrical power. I remember he described such a laser (US Military), carried on board a plane and the density for storing Energy for a 'portable' laser was much higher than could be obtained using fuel to generate electrical power.

What he described sounded like 'just the job' but we don't hear of the system being used in practice (do we?).

There are several links. Here's one.

 

[Gordianus]

I've read about the plane-carried laser "cannon" in several incarnations. Forty years ago it was a gasdynamic CO2 laser. The rather long wavelength (10.6 micrometers) made it a poor choice against shiny metallic surfaces. Later on, they moved to the HF laser (2.7 micrometers). A beauty that harvested population inversion from the highly exothermic mixing of hydrogen and fluorine. But they had another hidden beast: the iodine-oxygen laser (1.5 micrometers). To the best of my knowledge it was the most powerful gas laser. Meanwhile, the plane grew. Somehow I remember the first one was a KC-135 and eventually became a 747. You will always get more money to develop new weapons. In my previous post I mentioned the ill-fated projects thatTeller sold to Reagan.

 

[FTM1000]

Ah...the laser weapon...
I heard about them 40 years ago and I'm still waiting.
President Reagan fell under Teller's spell and believed X-ray lasers would destroy incoming missiles. Have you see those lasers yet?
Lasers are outstanding things but, as long range weapons...

From what I read the laser weapons in the past failed because they needed a lot of dangerous chemicals and had a problematic logistics and that why they weren't suitable for practical use. but today it isn't a problem at all because lasers today doesn't need those chemicals.

 

[sophiecentaur]

but today it isn't a problem at all because lasers today doesn't need those chemicals.

I imagine you mean that you can get away with less power, using this pulse conversion technique (the 'machine gun effect'). Have recent laser designs with electrical power suddenly become a lot more efficient? Is that likely?
I take your point about chemical lasers being a bit 'inconvenient'.

 

[FTM1000]

I imagine you mean that you can get away with less power, using this pulse conversion technique (the 'machine gun effect'). Have recent laser designs with electrical power suddenly become a lot more efficient? Is that likely?
I take your point about chemical lasers being a bit 'inconvenient'.

It looks like we are already going to have lasers capable of destroying cruise missiles by 2022 https://breakingdefense.com/2019/12...ruise-missiles-pentagon-to-test-rival-lasers/
Israel also announced a breakthrough in laser technology last year https://www.defensenews.com/opinion...istic-is-israels-tactical-laser-breakthrough/

 

[DaveE]

Can a continuous wave laser weapon be converted to a Ultrashort Pulsed Laser like the one mentioned in this article?:
https://www.forbes.com/sites/davidh...hinegun-firing-light-bullets/?sh=6555843768a3
Israel is also developing laser weapon for intercepting rockets and is probably stationary and plugged to the power grid, what obstacles can they have in making this future laser weapon into an Ultrashort Pulsed Laser like the one in the article?.

I suppose you can, theoretically. But for high power lasers there are way, way, too many practical issues to solve. Things like damage of optical coatings, fitting a Q-switch into the resonator, etc. You'll get a pulsed laser, perhaps, but it won't be a good one. High power lasers are highly optimized to achieve their performance. You'll probably be better off saving some of the useful bits and starting over.

 

[Gordianus]

We already know light carries very little momentum. Thus, we forget about punching a surface the way a bullet does. We're left with melting/vaporizing the shiny surface of our target. We also want to do this at a large distance, many, many kilometers. The beam, after traveling such a long distance, has suffered several misventures and barely resembles the textbook definition of a laser beam. We end up relying in raw power, lots. If memory serves, the iodine laser reached 2-3 MW thanks to the highly exothermic reaction with oxygen. Try to get this power with electric means.

 

[FTM1000]

We already know light carries very little momentum. Thus, we forget about punching a surface the way a bullet does. We're left with melting/vaporizing the shiny surface of our target. We also want to do this at a large distance, many, many kilometers. The beam, after traveling such a long distance, has suffered several misventures and barely resembles the textbook definition of a laser beam. We end up relying in raw power, lots. If memory serves, the iodine laser reached 2-3 MW thanks to the highly exothermic reaction with oxygen. Try to get this power with electric means.

Actually from what I read(like the article in my original post) pulsed laser is different in that it cause ablation of the surface and this have a kinetic impact that is actually used to punch holes into steel and is even able to propel things in Ablative laser propulsion. Pulsed laser even have better range and is less affected by the atmosphere. I don't know what that iodine laser was supposed to intercept but according to articles about actual military plans 300 kw lasers are capable of destroying cruise missiles and weaker lasers can still destroy smaller rockets and artillery.

I suppose you can, theoretically. But for high power lasers there are way, way, too many practical issues to solve. Things like damage of optical coatings, fitting a Q-switch into the resonator, etc. You'll get a pulsed laser, perhaps, but it won't be a good one. High power lasers are highly optimized to achieve their performance. You'll probably be better off saving some of the useful bits and starting over.

Being able to produce strong CW laser means that you are also able to produce strong pulsed laser that use similar levels of energy?.

 

[DaveE]

Being able to produce strong CW laser means that you are also able to produce strong pulsed laser that use similar levels of energy?.

Do you mean can an engineer that designs and builds a high power CW laser also design and build a high power pulsed laser? Yes. But, at the very large laser manufacturer I know, those are different teams, with different expertise. This is especially true when you start dealing with ultra-fast lasers, those are especially different.

If you mean can a high power CW laser also produce high power pulses? Then no, not without modification. Even then it probably won't be a very good one. I do recall a CW YAG/YLF laser from the late 1980's (https://www.ebay.com/i/370601736579) that had an option to insert a mode-locker in the resonator to (relatively) easily convert it to pulsed operation. It was a good laser for it's time, but the industry has moved way beyond that now. Most all high power lasers are built to excel at a particular application. CW, pulsed, and ultra-fast lasers are typically used by different people to do different things, so they have no interest in conversions.

Perhaps you should review the distinction between energy and power. CW lasers aren't specified with energy, individual laser pulses are. Also consider what the peak power is in a pulsed laser compared to a CW laser, that is a big difference when you consider the reliability of your optics.

 

[Gordianus]

Let's say we'll resort to laser ablation to punch a hole on the skin of the missile. We'll use a pulsed laser and its pulsewidth should be as short as possible. The Ti:Sa laser brings the shortest pulses of a few femto seconds but, since pulse energy isn't too large, we´'ll need many, many pulses. Even at the typical pulse repetition rate in a mode-locked laser (hundreds of MHz) it'll take a very large number of pulses unless the energy per pulse grows to very large levels. I´'m not that stupid as to say the military won't spend 1T$ in building such a monster, but let me doubt about the feasibility. I can't forget Teller's project. It cost a large amount of money to no avail.

 

[sophiecentaur]

We already know light carries very little momentum. Thus, we forget about punching a surface the way a bullet does.

Damage doesn't just rely on momentum transfer. There are many different designs of conventional projectile, some are fast and light; others are slower and more massive. I'm thinking about the circular shape of Moon craters, for all angles of arrival. In that case, it's the Energy and not the Momentum that dominates.

Delivering the damage quickly can be the most important factor.

 

[rc1]

I've seen articles about laser welding pencils used on the shuttle. Could be used for metal 3d printing but thermite gell is probably better in many use cases. Uses fibre and q-switching from memory. Divergence seems to be the issue for offence- for defence the range could work in some cases but again there are better ways.

 

[Shane Kennedy]

Can a continuous wave laser weapon be converted to a Ultrashort Pulsed Laser like the one mentioned in this article?:
https://www.forbes.com/sites/davidh...hinegun-firing-light-bullets/?sh=6555843768a3
Israel is also developing laser weapon for intercepting rockets and is probably stationary and plugged to the power grid, what obstacles can they have in making this future laser weapon into an Ultrashort Pulsed Laser like the one in the article?.

In my limited experience, pulse power can be a lot higher, but average power considerably lower.

 

[sophiecentaur]

In my limited experience, pulse power can be a lot higher, but average power considerably lower.

It would be down to the efficiency of the system. But a highly reflective surface would make things very difficult. A missile could go a long way towards its target if the short -term power (Peak Pulse Power times rate of pulses) were not pretty high.
Without getting political, I have to say that the 'Iron Dome' system is pretty impressive. But at what cost, I wonder?

 

[FTM1000]

It would be down to the efficiency of the system. But a highly reflective surface would make things very difficult. A missile could go a long way towards its target if the short -term power (Peak Pulse Power times rate of pulses) were not pretty high.
Without getting political, I have to say that the 'Iron Dome' system is pretty impressive. But at what cost, I wonder?

About the highly reflective surface thing, from what I understood different materials have different reflectivity in different wavelengths and some of the laser weapons are actually several laser beams focused into one spot(this what I read about the Israeli laser weapon) so can't the laser beams work on different wavelengths that at least one of them the reflective surface wouldn't be able to reflect efficiently without heating instantly?.
And heat doesn't affect the ability of materials to reflect light?.

 

[sophiecentaur]

different materials have different reflectivity in different wavelengths

Yes - it's a bit like the inverse to the Stealth coating. Problem is that the available Laser wavelengths would probably soon be known. Possibly even dealt with by an interference filter coating but not under non-linear field conditions and a short enough pulse.

 

[FTM1000]

Yes - it's a bit like the inverse to the Stealth coating. Problem is that the available Laser wavelengths would probably soon be known. Possibly even dealt with by an interference filter coating but not under non-linear field conditions and a short enough pulse.

There is a material that can be extremely reflective at every wavelength possible for a laser?.

 

[Gordianus]

Not even gold can withstand a laser pulse in the femto second range. The pulsewidth is so short that the atom becomes decoupled from its neighbors and it can't relax into the bulk. The material is vaporized. An enticing idea for a weapon, isn't it?. However, the devil lurks in the details. The "vaporizing" process I mentioned above is achievable in a lab (though expensive) but scaling it up to the needs in a battlefield sounds still far fetched.
Lasers are wonderful toys and I've worked with them for a very long time. Lasers can't be the answer to every problem. I'm still waiting for the laser driven fusion that gives a net gain.

 

[DaveE]

Not even gold can withstand a laser pulse in the femto second range. The pulsewidth is so short that the atom becomes decoupled from its neighbors and it can't relax into the bulk. The material is vaporized. An enticing idea for a weapon, isn't it?. However, the devil lurks in the details. The "vaporizing" process I mentioned above is achievable in a lab (though expensive) but scaling it up to the needs in a battlefield sounds still far fetched.
Lasers are wonderful toys and I've worked with them for a very long time. Lasers can't be the answer to every problem. I'm still waiting for the laser driven fusion that gives a net gain.

Yes, this is the point of ultra-fast (pico-second or less) material processing. It's not just a lab trick anymore, but it is expensive and only makes sense for high precision applications (turbine blades, medical devices, etc.)

https://www.coherent.com/assets/pdf/IndustrialUltrafast_Whitepaper.pdf

OTOH, there are so many problems with using them in weapons. Things like dispersion, lensing, or attenuation in the atmosphere (why people often restrict the discussion to space), reliability, ruggedness, cost. Do you really want an expensive finicky weapon that you can't use on a cloudy day? I also think there is an issue with low energy; yes, the peak powers are huge, but the pulse energy isn't, so the beam may need to be very accurately targeted for a while to cause significant damage.

So, yes, IMO, very far fetched. More in the realm of science fiction when you start to think of the practical issues.