Krdytkyu

In my sci-fi universe, spaceship shields are generated by capturing plasma between two electromagnetic fields. Projectiles are evaporated by the heat of the plasma, and charged particles are deflected by the magnetic fields of the particles and the containment fields.

The problem arises when laser weaponry is considered. In theory, a dense plasma field should be capable of deflecting EM radiation (as it happens in the ionosphere). The denser the field, the higher frequencies it can deflect. But the problem is that enemies can use a wide variety of rays, from microwaves to gamma rays. Therefore, there has to be a way to detect the lasers coming in from afar, and dynamically adjust the shield's density.

Is there any way to detect an incoming laser without actually having it touch the ship, because that would defeat the whole purpose of the shield?

Don’t block the shot: block where they’re aiming.

Basically: you can’t block the laser once it’s been fired, as the moment you know about the laser is when you’re hit by it. What you can do is monitor the emissions/light bouncing off of your enemy prior to the laser being fired.

For this to work you need two things on your ship and one further thing to be true:

1: You need stupidly good sensor packages that can track nearby threats and their emissions/any light bouncing off them. Using LIDAR (like radar but with lasers) to actively paint potential threats would be useful, but kinda defeats the point here...

2: You need stupidly good threat analysis software that can identify ‘they are pointing their guns at me’ and ‘their gun is about to fire x kind of laser’, and bring up the appropriate shielding faster than they can aim and fire at you.

3: The enemy weapons need to be visible from your ship so you know where the laser will hit you. The exactness of these measurements will depend on how precise your lasers/shields need to be. More precision means better info on laser gun position is needed, but if your shields cover whole arcs of the ship then you can get away with knowing ‘my enemy is that way’.

Once you have those things you can bring up shields in the instant you get the ‘ they're Aiming at me and firing’ em radiation from your enemy, thus blocking the laser that will arrive shortly thereafter.

This method can be blocked/jammed/messed with in any number of wonderful plot-hooky ways, from your own shielding temporarily blinding you to your enemy deploying drones that spoof active laser signatures so your defences can’t be focussed, but if you can see your enemy you can use this as a pre-emptive defence method.

A laser beam is made of photons, and photons travel at the speed of light. Therefore whatever they emit will reach the target together with the laser, and there is no much use for it.

The only way is to detect the fingerprint of the device used to emit the laser, which will be necessarily working before the emission of the laser.

If the X-ray laser has a different fingerprint than the IR laser then the target can adjust the shield accordingly. Mind that the fingerprint can precede the shot of just few millisecond or even less, therefore the shield has to be able to quickly adjust itself.

However I also assume that the attacker will also try to shield or alter the fingerprints, so that the target is lured into using the wrong shield.

If you don't detect enemy ships and their laser-capabilities you just do not have any time for detecting the laser-shots as they go at the speed of light. For your ship moment of you see the shot is the same moment it hits you.

But there are ways to combat lasers.

1. 
   

   Every ship would want to have good detection capabilities and some measure of stealth to be able to detect enemy ships first or push range from where you can be sniped at far enough . Long distance would allow to employ an evasive patterns in the movement of your ship. For example, at the distance of Earth-Moon enemy ship would need to "guess" where your ship will be in 1.3 seconds. They try to predict you or get closer and both ships detect each other.

   
   
   

   Lasers may look like they are a lines, but they are more cones, so extra distance spreads it's energy over more area.

   
   



2. Passive defense. Your hull can be covered in layers of reflective surfaces with a gap with a metal foils, that when hit by a high-energy laser beam evaporate and produce plasma clouds, that absorb even more of the energy from the lasers.




3. Hybrid one. Use floating sheet drones around your ship, that cover up the ship if there is a danger of laser assault or any other methods that will give you time to detect enemy ships and return fire.

For more effective shield you may use them in layers. Each layer is optimal for different type of laser. After analysis of attack or scan of detected attacker you convert most of your layers to optimal version. Methods above may be used to give you more time for the preparations.

You look at what weapon is pointed at you.

Lasers are actually not very good as ranged weapons in space combat. The laser will spread out too much and lose the energy it has over toi much surface area to be effective. Also firing a laser generates a lot of heat on the ship itself, which is very hard to get rid off on a space ship. This means lasers are mostly thr CIWS of future space ships. Great for point defense against fighters and incoming missiles but useless against long-range targets.

Now imagine that because you have shields, your fights happen at much closer range. Now that laser is effective right? Unfortunately you have a shield that can deflect laaers. This means you need to alter your shield so that your lasers have as little effect on your own shield or you would lose both firepower and desabilize your own shield a little. But since your enemy can simply measure which wavelengths penetrate through your shield they can simply make their shield opaque to those wavelengths, neutralizing the effect of those lasers.

Ergo: you can react to something that moves at the speed of light by knowing what is necessary to fire something at the speed of light.