Navy's New Laser Weapon: Hype Or Reality? 185
Lasrick writes: MIT's Subrata Ghoshroy deconstructs the Navy's recent claim of successful testing with the Laser Weapon System. It seems the test videos released to the press in December were nothing more than a dog-and-pony show with scaled-down expectations so as to appear successful: "When they couldn't get a laser lightweight enough to fit on a ship while still being powerful enough to burn through the metal skin of an incoming nuclear missile, they simply changed their goal to something akin to puncturing the side of an Iranian rubber dinghy." Ghoshroy is an entertaining writer and an old hand in the laser research industry. He gives a explanation here of the history of laser weapons, and how the search for combat-ready tech continues: 'At the end of the day, good beam quality and good SWAP—size, weight and power—still determine the success or failure of a given laser weapon, and we're just not anywhere near meeting all those requirements simultaneously.'
Sabotage? (Score:4, Funny)
The goal hasn't changed. (Score:5, Insightful)
Re:The goal hasn't changed. (Score:5, Interesting)
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The author also misses what happens when you shine a 5kw+ laser back down the lens-path of an opponent's very expensive Electro-Optical system- you turn it into immediate junk. FLIR or TV or laser spot tracker on a surface ship, drone, helo or jet fighter, some schmuck looking through binoculars, IR/Heatseeking sensor on a missile, all become very permanently blind. And that in of itself is pretty useful. Not to mention since the tracking is so good, you could do fun things like setting a pilot
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I'll add that maybe what is most impressive is not the laser power, but the control system required to keep the beam on a moving target at a mile away. The author seemed to miss that part of the technology.
We've had that technology [wikipedia.org] for decades now; it's not new or impressive.
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In WW2 we had analog computers that could aim guns at moving targets from moving platforms. This is actually a harder proposition than aiming a laser; bullets don't move at the speed of light and you've got to compute lead. They did it without electronic computers.
They also didn't do very well
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Actually, they did quite well. Consider the Prince of Wales, sunk by the Bismark. The Bismark was using just such electromechanical analog computers for fire control.
Likewise every other modern (for the period) crui
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Consider the Prince of Wales, sunk by the Bismark. The Bismark was using just such electromechanical analog computers for fire control.
Oh, I thought you were talking about anti-aircraft guns, not anti-ship weapons.
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The optical sight / analog computer fire control system on the Iowa class battleships was reputed to be very effective against aircraft.
sPh
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(and through a turbulent atmosphere)
Line of sight targeting is (relatively) easy (Score:3)
I'll add that maybe what is most impressive is not the laser power, but the control system required to keep the beam on a moving target at a mile away. The author seemed to miss that part of the technology.
That's actually rather easy. The navy solved that problem decades ago for ballistic trajectories which substantially more complicated computationally. Line of sight targeting is FAR easier with modern computers. That is why they use lasers to paint targets for missiles to home in on a target. It's much easier to target something with a laser which is not meaningfully affected by gravity or wind or time to intercept.
I think the really impressive bit would be how they could keep the laser operational in a
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In this latest test, which was the first against a maritime target, the disabling of a military-grade boat by puncturing its multiple-layer rubber hull required a sustained laser burst for 30 seconds. It demonstrated the ability of the ADAM system to lock on to a single point of a weaving, bobbing target at a distance of approximately 1.6 km (1 mi) with super-accuracy for sustained periods of time.
I know there are systems that can do trajectory tracking, and systems that can do laser targeting of slower moving objects from a stationary base, but to keep a beam with the accuracy required on a particular spot of the target, from a moving base, to object without a straight trajectory, seems to be something a bit more challenging. You don't have to be very accurate for a missile target.
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Interesting it is, but certainly nothing new.
It's your standard Fire Control problem. It's pretty much the same systems in place to keep ships guns pointed where they need to be regardless of how the ship is moving about in all axis. The only difference is this is a zero time of flight vs a ballistic solution. If you think about it, it's probably EASIER to keep a laser on target than it is to drop a projectile on one. ( You don't have to dial in spotter offsets for time of flight issues, wind, etc )
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The author also vastly underestimates the Iranian navy. They have some fairly advanced subs that would cause major problems for any attacking navy. They also have some of the fastest and deadliest torpedoes in the world, and some effective anti-ship missiles. Dingies would be the least of the US navy's worries.
Yeah, they used them in the past, because they didn't need any more. They were not trying to start a war, and lasers would have been a huge escalation.
Integration. (Score:3)
I would also expect that there is quite a few details to work out when it comes to mounting any new weapon on a ship and integrating it into the CIC. There is no reason to wait until you have a planet buster to get all that going and in place.
drones (Score:2)
There never was a mission for the navy to shoot down nuclear missiles. there may have been a mission to shoot down anti-ship missiles. But they already had the Phalax and it is probably as effective as laser would ever be for that mission. But the drone situation changed everything. There wasn't a good way to deal with these, and the pinpoint accuracy of lasers combined with the low power requirements needed makes lasers the ideal weapon for this. Similarly, non-lethal weapons to fend off small craft bo
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phallax ammo needs replenishing, lands somewhere perhaps in the costal town you are shooting over, and can't be run continuously. It cannot deal with non-lethal modes of attack (rubber dingy). it's very expensive. it has the problems of toxicity from DU. And most of all it's short range.
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It cannot deal with non-lethal modes of attack (rubber dingy)
If the guys in the dingy are trying to kill you why would you limit your response to the non-lethal? You can defend against that shit with something [wikipedia.org] that's nearly as cheap as the laser, which has more than a century of proven effectiveness in combat.
We don't need to spend millions (billions?) of dollars on laser technology to deal with small boat attacks. Some people like to talk a big game about swarm attacks but there's no where to hide on the open ocean; going after any modern warship on the high seas
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I would find it amusing to watch a boat load of terrorists, who probably can't swim, suddenly finding themselves in a sinking raft, filled with guns and ammo.
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Her Majesty's Royal Army.
The Army that has no qualms about grabbing civilians,put them into camps, and has an official, formal policy of starving the inmates to death. Her Majesty's Royal Army has never rescinded the official order to starve civilians placed in concentration camps.
The army that decided that the best way to help wounded, captured enemy, was to execute them on the spot. Another order that has never been rescinded.
other uses for lasers weapons as well. (Score:2)
there are other uses for lasers that projectile weapons don't satisfy easily.
http://www.army.mil/article/82... [army.mil]
there a high peak power, low total energy, laser ionizes a trail from the laser to the target device. then you send a bolt of lightning down that air column, which continues to ionize it while it electrically destroys the target. This can be used to disable vehicles non-lethally from remote distances. It can even be used to destroy roadside IEDs.
Another use, in fact the one it was originally res
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laser ionizes a trail from the laser to the target device. then you send a bolt of lightning down that air column
That only works if the target itself is grounded, and the channel isn't too long.
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There never was a mission for the navy to shoot down nuclear missiles. there may have been a mission to shoot down anti-ship missiles. But they already had the Phalax and it is probably as effective as laser would ever be for that mission.
Phalanx and other gun based CIWS are being depreciated in favor of missiles like the Rolling Airframe Missile [wikipedia.org]. Guns can't deal as effectively with supersonic missiles and/or those that undertake terminal evasive maneuvers. They've also got a stopping power problem; breaking apart an incoming missile doesn't negate its kinetic energy and the inbound pieces retain the ability to do significant damage to modern warships even without a warhead detonation. The British lost at least one warship -- HMS Sheffiel [wikipedia.org]
you know nothing john snow. (Score:2)
Odd then that they already use Spinning missiles beacuse they help solve the re-entry problems
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I think so far they have only invested 20 or 30 million on the system- and the advantages of having one that works is obvious. Not sure where the 'billions' from the article comes from if we are talking about fiber laser systems.
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First, funding for basic research will continue. The military is probably the most fiscally irresponsible entities on the planet, and so labs claim military application for snail poop. The research money is not significant compared to the entire budget, and some of these things might work, so funding continues and labs continue to do work that will prod
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I would say the goal hasn't changed but they found applications short of the goal. The author seems to suggest that this is dishonest--it's not dishonest at all. Missile defense is/was a multi-billion dollar boondoggle but the current anti-mortar systems employed in the middle east are functional and effective. Just because we can't have megawatt class lasers yet doesn't mean that the current demonstrations of useful applications for lightweight lasers are smoke and mirror deception. His premise is
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That was my impression also. I don't think the purpose of the specific system being designed and tested right now is to allow a ship to destroy an ICBM. Maybe start with one that can destroy Iranian dinghies, then one that can shoot down incoming mortars and drones, then one that can acquire and destroy incoming tank or artillery rounds, then missiles launched from planes, then larger boats and planes, before you can scale the size down while keeping the same power required to track and destroy an ICBM.
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>Now give me that "phat venture capital check" is what they're (i.e. the Navy's contractors) advertising.
Back around 2000, there were a couple of firms that claimed to outfit laser weapons on private yachts. Weapon systems that could destroy the boats that the pirates were using.
These systems were not cheap, and once installed, did limit which countries one could visit.
I dreamed of warp travel since childhood (Score:3)
But I will settle for space lasers.
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It's sickening to see the mindset of the Space Nutter. If this planet is a "dirt rock", then what's so important about going to see other "dirt rocks"?
Because every square centimeter of this rock is owned by someone? And no one owns any square centimeters of the other rocks in the solar system? It's about the frontier, and its opportunities.
They're sterile, barren, lifeless, radioactive hells!
Nobody said it had to be an easy frontier.
You or I might not be interested in making the attempt, because sterile, barren, bathed in radiation, etc. but other people do, and why do you care if other people run off and get themselves killed in an attempt to acquire unclaimed land?
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>OK, here's a challenge, a leisure society for all with minimum livelihood and medical care guaranteed for all? I mean since we'll *obviously* have these incredible levels of energy and resources and technology, right????
With a couple of changes in government rules, regulations, statutes, and laws, that could be implemented today.
(By today, I mean that if the laws were literally changed today, the transition could start on 1 June 2015, and the entire process be complete by 1 September 2015.)
The major iss
Laser gun.... who knows. Railgun though (Score:2)
Laser guns, meh. Limited to line-of-sight. Railguns on the other-hand are a whole 'nother ball of wax. Designs for Navy vessels now have to focus more and more on supplying power (as in electricity).
-Matt
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Designs for Navy vessels now have to focus more and more on supplying power (as in electricity).
I believe the DDG-1000 series was supposed to address that, I remember reading about the power system and how it was modular enough to allow virtually all power to be directed to any particular system. "All power to weapons."
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The real advantage of lasers is speed of light - perfect for anti-missile weapons. Shoot them when you see them and move out of the way.
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A railgun projectile that travels slow enough that the target can evade before the projectile reaches it? Meh.
Shotgun railguns!!!
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Non line of sight attack. A rail gun can hit a target 100 miles out(impossible for surface level lasers). In seconds. Dodging isn't practical.
When ou have long line of sight you are correct. But lasers are limited to 10 miles or so.
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At 5000mph, a projectile 100 miles out will have 66 seconds to respond. Closer in, far less. Still, for an enemy craft width of, say, 300 feet would need to move 150' to avoid collision. At 0.1g lateral acceleration, fairly trivial, you could move an H-4 Hercules out of the line of fire in just over 3 seconds. That's a 5 mile range for the largest (wingspan) plane in the world. And from 6 miles out, the EM signature from a rail gun would be pretty obvious. That's very practical if the target is equipped wi
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Energy weapon that travels at the speed of light?
requires line of sight and fails in heavy rain snow and thick fog, sandstorms.
A railgun projectile that travels slow enough that the target can evade before the projectile reaches it? Meh.
only consider slow when your comparing it to speed of light, when conventional weapon (missiles rockets bullets) it fucking super fast,
and things you are likely to target with a rail gun like many like fortification, buildings bases, cities don't move at all or don't move fast enough like aircraft carriers battle ships.
they are both good weapons for different things. You use your laser to stop missiles and planes and use y
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Only if that target is the Flash.
The posted advantage of a railgun is it's range. At the extremes, time of flight for the projectile is probably long enough that it can be evaded. Of course, those extremes are beyond what you could even fire the laser at in the first place. If i've learned anything from Eve though, it doesn't hurt to have railguns AND lasers.
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I think currently demonstrated ship-mountable railguns can emit a 7+ pound projectile at Mach 7. More to the point, these can be kinetic projectiles, meaning no explosives required, and there's more room for other things like, oh guidance systems.
Good luck evading that.
-Matt
The Power to Destroy (Score:4, Funny)
The power to destroy an Iranian rubber dinghy is insignificant next to the power of the Force.
oy (Score:5, Interesting)
Too heavy to fit on a ship? (Score:2)
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Oh come on, it is easy:
The number of slaves need to row the ship becomes too great.
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It's a stepping stone needed before having sharks with freaking lasers on their heads.
screw it: use small rail guns. (Score:2)
Re:cover everything with mirrors (Score:5, Informative)
Your mirror would cease to be a mirror in very short order by either sheer ablation or the formation of oxides, reducing its ability to reflect, causing the absorption of more energy, at which point your mirror ablates. HTH.
Is that even correct ? (Score:5, Interesting)
At such a 50kW Laser at 95% reflectance would mean 5% absorbance or only 2.5 kW. That means to give the same amount energy at the same distance for the same surface you need 20 time the same time. Or put in another way if you need to give 10.000 Joule to ablate that surface , you would need 4 seconds exposition rather than 1/4 of a seconds for a non reflective surface.
So where do I make an error ? Where do you see that the mirror would quickly lose the ability to reflect compared to exposure time ? Keep in mind that in the case of a balistic projectile, you only need to make sure the laser do not pierce the skin long enough that targeting would be hard. I do not see why you keep telling reflectance has no impact on such laser. It certainly has an impact on how much kW will the target absorb.
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The mirror does not reflect 95% up until it fails and then starts absorbing 95% of the laser energy. Your (let's say) silver mirror would absorb a tiny bit of energy (say 2.5kW), which is enough to start heating up. This accelerates the formation of oxides, and the oxides are nowhere near 95% reflective. Those oxides rapidly absorb the laser energy, ablate, and so on. Obviously having shiny mirrored surfaces help, but not nearly to the extent one would hope.
dom
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Where do you see that the mirror would quickly lose the ability to reflect compared to exposure time ?
Because, even assuming your surface remains polished and dirt free so it can have such reflectance, it doesn't take much energy to ablate a microscopic amount of material and greatly reduce the reflectance of the surface. Even if it is solid metal, laser damage tends to produce very absorbing surfaces as opposed to just exposing more reflective surfaces. The amount of energy to degrade a surface is much less than needed to do bulk damage or even bulk heating, and hence tends to be a small fraction of the
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From your link:
Saturation attacks. Since a laser can attack only one target at a time, requires several seconds to disable it, and several more seconds to be redirected to the next target, a laser can disable only so many targets within a given period of time. This places an upper limit on the ability of an individual laser to deal with saturation attacks—attacks by multiple weapons that approach the ship simultaneously or within a few seconds of one another. This limitation can be mitigated by installing more than one laser on the ship, similar to how the Navy installs multiple CIWS systems on certain ships.
Hardened targets and countermeasures. Less-powerful lasers—that is, lasers with beam powers measured in kilowatts (kW) rather than megawatts (MW) 10 — can have less effectiveness against targets that incorporate shielding, ablative material, or highly reflective surfaces, or that rotate rapidly (so that the laser spot does not remain continuously on a single location on the target’s surface) or tumble.
Now, another reference [arxiv.org]:
In order to facilitate comparison with the findings of other authors we chose to express the threshold fluence in units of pulse energy per unit area . The multipulse damage threshold for molybdenum at 1064 nm reported by Zhou [29] of 1 J/cm^2 for 10 ns pulses is rather higher than the value of the order of 0.3 J/cm^2 we find. Similarly, we find that stainless steel gets damaged at about 0.2 J/cm^2, whereas the value of 2.3 J/cm^2 for 120 ns pulses at 1064 nm found by Leontyev [30] would lead us to expect a threshold of around 0.4 J/cm^2 for 5 ns pulses.
Also, you're severely overestimating the reflectivity of materials likely to be exposed to the atmosphere, especially in battle conditions. 70% would be nice. They also discuss that, as well as how much short pulses at short wavelengths reduce the reflectivity (up to around ~25% in nitrogen, which air is largely composed of).
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stainless steel gets damaged at about 0.2 J/cm^2
To produce a 0.2J pulse in 10 ns requires a peak power of 20MW, plus a beam that stays within the cm^2 over a long distance.
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Lockheed has
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And after their $10k system nullifies a $10T system, and the $10T system is replaced with a $100T system, they'll install cooling lines within the rocket that better distribute the heat, nullifying the next step.
When they use internally-cooled mirrored rotating targets,
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We're not going to be seeing gently-rolling guided cruise missiles any time soon (and certainly not for $10k) and we're not going to see long-range guided rolling rockets.
Anti-ship missiles are getting faster and faster, and CIWS is getting less and less likely to work. Aegis cruisers can already take out most ballistics that would threaten a carrier group. CIWS needs repla
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Off the shelf, benchtop lasers can easily produce 2.5 J pulses in 10 ns
So where is the disconnect ? If such a benchtop laser can produce 2.5J pulses, and we only need 0.2J to damage steel, how come the article says we need lasers the size of football fields ?
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"X% reflectance" is a linear assumption. It suggests that you reflect X watt out of every 100 watt, and also X kilowatt out of every 100 kilowatt.
However, reflecting an electromagnetic wave is caused by currents in/near the reflecting surface. Metals reflect well because such currents encounter little resistance. But there's a limit to the current carrying capacity of metals. There are only a few free electrons per atom at best. If the incoming pulse is strong enough, the wave simply cannot be reflected. Th
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"X% reflectance" is a linear assumption. It suggests that you reflect X watt out of every 100 watt, and also X kilowatt out of every 100 kilowatt.
However, reflecting an electromagnetic wave is caused by currents in/near the reflecting surface. Metals reflect well because such currents encounter little resistance. But there's a limit to the current carrying capacity of metals. There are only a few free electrons per atom at best. If the incoming pulse is strong enough, the wave simply cannot be reflected. The current simply exceeds the current carrying capacity. Instead, the atoms of the metal ionize. That means the surface turns into a plasma, and a heated plasma at that. Plasma's absorb all that light instead of reflecting it. You just created a plasma torch on the metal surface.
This AC is informative. MW lasers are different in kind from normal light - think "column of exploding plasma" not "bright light".
The problem the Navy faces with these lasers is that they're several KW, not several MW. That can still be effective against some important kinds of targets, however. Targets made of rocket fuel in a metal can, for example: you don't have to dump in that much energy to convince a rocket that it's not going to hit its target today.
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So where do I make an error ?
This part:
At such a 50kW Laser at 95% reflectance would mean 5% absorbance or only 2.5 kW. That means to give the same amount energy at the same distance for the same surface you need 20 time the same time.
You're assuming that the entire time while the laser is striking the mirror, and the mirror is absorbing some amount of energy, that the reflection stays constant.
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Because a 2.5kW laser is still sufficient to burn things, and once the mirror starts to lose reflectance it will start down a path of exponentially reducing reflectance.
That is,
0.0s - 0.1s: 95% reflectance
0.1s - 0.2s: 90% reflectance
0.2s - 0.3s: 80% reflectance
0.3s - 0.4s: 60%
0.4s - 0.5s: 20%
Etc. Source: Numbers are made up and hypothetical.
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Here's a demonstration of the effects of a 10 kW laser on a black rubber boat.
https://www.youtube.com/watch?... [youtube.com]
This is a joke. Takes them half a minute to burn through black rubber, and the boat wasn't even moving. Now imagine it is twisting and turning and splashing water on the hot spots.
I never pretended it would help for a long time (Score:2)
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I am jsut stating that it should help.
Covering your weapons in mirrors is stupid and impractical. The "just put mirrors on everything LOLZ" trope your kind always trots out doesn't become more feasible just because you're naive enough to keep repeating it.
For a much cheaper price than such a lser system itself.
Defense is always more expensive that offence. It's easier to break things. That's why being wealthy is important to self preservation.
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Spinning does jack shit if I can focus right on the tip of the missile.
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You spin and cool your weapons, and make them as reflective as practical and this system won't do any damage
I've never seen anyone agree with this assessment who has professed knowledge of the subject. The issue is that lasers still dump a lot of energy, which starts to mar the mirror really incredibly fast. 2.5kW of energy is like applying a soldering iron tip to the mirror, which rapidly (1s) causes it to mar and become useless. Not only that, atmospheric pollutants, water droplets, grease-- anything-- will make that part of the mirror useless. Applying mirrors to your turrets? Good luck keeping them clean
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Starting from your good estimate, 2.5 kW is about about two or three hair dryers of heat concentrated at a small unit volume (not sure of the area, but the mirror won't be that deep). Depending on the thermal conductivity of the mirror and the materials behind it, it is likely that that surface will get hot enough to oxidize and then the absorption will be greater than 5%, which will then get hotter and lead to failure.
Re:Is that even correct ? (Score:4, Funny)
So you are going to cover every surface of every target with a mirror? Let me know how that goes for you.
Well at least that way there won't be any trouble tracking it.
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Your mirror would cease to be a mirror in very short order
You can extend the length of that "short order" by spinning the target. That way the laser will not be focused on the same spot. Of course, the mirror will still fail, but if you can extend the time, it makes it more difficult to use a laser to shoot down hundreds of incoming warheads, plus ten thousand dummy balloons. If you can double the time to kill each one, you double the number that get through.
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to shoot down hundreds of incoming warheads, plus ten thousand dummy balloons.
I don't think anyone is realistically planning to use an ABM system against China or Russia, it's much more likely to be used against the one or two missile boats that North Korea or Iran manage to outfit with a handful of missiles.
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Mirrors would be a highly impractical defense (Score:2)
You can extend the length of that "short order" by spinning the target.
Even if that would work you would have to:
1) spin the target in a useful rotation (difficult when target is being hit head on by laser)
2) have enough surface area to allow energy dissipation between rotations.
3) be able to dissipate energy fast enough or absorb sufficient energy for any rotation to actually matter
4) hope that the laser cannot maintain it's ability to target a single point for a length of time that matters
5) be able to practically coat the target with a mirrored coating
6) that the mirror cou
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No mirror is 100% reflective; a fraction of light energy is absorbed by the material making up the mirror. The reflective portion of a mirror is a metal film. A high-energy laser will overcome its reflective properties and burn the metal film. There are metals (like beryllium) that will reflect up to 98% of light energy, but the cost to cover an entire vehicle or structure in thick enough beryllium to negate the effects of the laser would be stupendous (and you would have to cover everything; lasers are int
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Sure, but the laser beam itself is less than ideal too, as it its targeting. We're talking about hitting a moving target from another moving target with a less than perfect beam dispersed through whatever's in the atmosphere between them. Adding reflected waste energy to that equation and mirroring might not be perfect protection, but I'd bet it could make the attacker's job a lot tougher.
I have no doubt that at short range under laboratory conditions lasers can burn through any mirror conceived by the mi
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Puffing out some smoke or fog should present a pretty effective scattering defense.
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Put a cover on it. The laser vaporizes the cover, and then hits the clean reflective layer underneath.
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Use a material that burns without leaving a residue.
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Lasers can burn through that, or depending upon the wavelength, not be attenuated to any serious degree.
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Use a thin film of beryllium deposited on a thicker layer of copper to spread the heat.
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Thermal conductivity isn't exactly THAT simple.
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Too expensive and difficult.
Simply engineer a heat shield for the warhead that is ablative and outgasses enough to provide a protective layer around the object. The advantage of this over a mirror is that the laser's heat is carried away by the emitted gas.
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Simply engineer a heat shield for the warhead that is ablative and outgasses enough to provide a protective layer around the object. The advantage of this over a mirror is that the laser's heat is carried away by the emitted gas.
For countries with MIVR'ed ICBM's, the dummy vehicles can be replaced with some sort of defensive weapon. In the case of Russia, you could have two warheads with four defensive vehicles per warhead on each ICBM, or the US could have 3 warheads with 3 defensive vehicles each per ICBM. Since they can be independently targeted, the defensive vehicles could arrive ahead of the warhead between the line of site of the laser and the trajectory of the active warhead. A nice thick cloud of opaque smoke could do the
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A nice thick cloud of opaque smoke could do the trick.
How do you lay a nice thick cloud of opaque smoke at hypersonic reentry velocities?
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Dump unoxidized carbon out the sides. Also, at hypersonic velocities, you're heating and dissociating the air by definition,which reduces the efficiency of the laser.
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Re:cover everything with mirrors (Score:5, Funny)
Cover everything with mirrors?
Liberace called, he wants his mirror encrusted missile cruiser back.
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Let's take a 100kw laser system like being demonstrated. Let's say your mirror is a 90% mirror (very good mirror) free of dirt. That's still 10kw on something maybe 2" across. If you've got a polycarbonate cover for your mirror surface it's now black. If you're using polished chrome it's already been burned off down to the metal underneath which is also probably charred. Also the lasers are often in the near-infrared range not the optical range.
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If it doesn't kill the crystal (assuming solid-state laser,) the next pulse will have the added power of the returned photons (minus losses due to the mirror and thermodynamics, etc.) or if no follow-up pulse, it will just re-emit as a very weak beam until it can no longer lase.
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