[Ultra-Tech] What Are Rainbow Lasers?

[lexington]

The more I think about them the stranger they seem. Isn't one of the defining characteristics of lasers that they emit light of all one frequency? It seems like a superscience-esque piece of technology but isn't marked as such.

I know this isn't technically a GURPS specific question but I've never even seen the idea of polychromatic lasers mentioned elsewhere.

[IrishRover]

I don't have my books, but IIRC, a rainbow laser shoots only one frequency at a time. The fancy aspect is that it can change frequencies so it can, for example, work as a blue-green laser under water, or a gama ray laser in space, or an IR laser so it can heat things up, or the right frequency to optimize its use in exotic atmospheres.

[lexington]

Originally Posted by IrishRover

I don't have my books, but IIRC, a rainbow laser shoots only one frequency at a time. The fancy aspect is that it can change frequencies so it can, for example, work as a blue-green laser under water, or a gama ray laser in space, or an IR laser so it can heat things up, or the right frequency to optimize its use in exotic atmospheres.

I considered that (perfect frequency matching could explain an improved armor divisor) but it defocuses in vacuum and can't go under water, neither would be a problem if it just jumped frequencies.

[DaosusLeghki]

A laser has two key characteristics: it's light is all one frequency, and it's light is all in phase. You could potentially have a beam of more than one wavelength, where each color is in phase with itself. This would still be a laser, or more accurately, several lasers. This would have attenuation and diffraction much as a beam of regular light (with only the spectrum lines for the different frequencies present in the beam). Unlike a regular beam of light, it would still have a lot of energy because the various components are in phase. Because of the multispectral nature of the beam, you could also expect a beat frequency, though I don't know if that's significant or not. Such a beam would have less energy efficiency (ie, lower power) than a single-spectrum beam. However, it would likely deal better against armor, as your opponent no longer has to defend against just one predictable frequency.


PS -- You can get this by using mirrors/prisms to direct several laser beams into one physical location and vector, or you can use "high tech magic" to explain it.

[lwcamp]

Originally Posted by lexington

The more I think about them the stranger they seem. Isn't one of the defining characteristics of lasers that they emit light of all one frequency? It seems like a superscience-esque piece of technology but isn't marked as such.

I know this isn't technically a GURPS specific question but I've never even seen the idea of polychromatic lasers mentioned elsewhere.

A laser is a device which uses a process called stimulated emission to amplify light. All early lasers, and a good number of lasers these days, use materials that can only undergo stimulated emission over a very narrow band of frequencies - for example, a given ion might transition between a metastable exited state and a ground state, and will only support stimulated emission for photons with an energy that match that transition. Even these lasers are not truly monochromatic - any physical process has a finite linewidth, given by the Heisenberg uncertainty relation if nothing else (in practice, Doppler and pressure broadening increase the spread of energies well beyond the Heisenberg limit).

Some modern lasers are able to amplify light through stimulated emission over a wide range of frequencies. Titanium:sapphire lasers are one example, which can emit light between 650 and 1100 nm wavelength. In fact, Ti:sapphire lasers are the perfect laser to bring up here, since they seem to be the model for the GURPS rainbow laser. By the Heisenberg uncertainty relation, the shorter in time something occurs, the less well specified its energy is. In light (as with anything, actually, so says quantum mechanics) the energy is directly proportional to frequency, so very short pulses of light must by necessity have a broad frequency spread. Ti:sapphire lasers are thus commonly used to produce ultra-short laser pulses because they can amplify all the frequency components of the ultra-short pulse simultaneously. Ultra-short light pulses compress the energy of the pulse into a very short period of time, thus giving the pulse extremely high power (power is energy divided by time, so keeping the energy the same but decreasing the time gives more power).

When very high powered pulses pass through matter, they can have non-linear effects. In particular, they can increase the index of refraction where the light is most intense - in the center of the beam. This makes the matter act as a converging lens, as it refracts the laser toward the center. This is called self focusing.

Taken to its extreme, self focused light would contract catastrophically, causing intense ionization at a spot and becoming absorbed in the resulting plasma. However, this is not what is observed. Rather, when the light gets intense enough, the air becomes very sparsely ionized. This slightly decreases the index of refraction, causing a feedback effect which keeps the light very intense but not too intense. You end up with a plasma core surrounded by a nimbus of propagating light, going in the direction of the original beam. This process is called filamentation. Self focused light filaments can propagate quite some distance through air, but eventually they lose enough energy through ionization that they cease to self-focus and disperse.

Light filaments naturally compress further in time and spread out further in frequency. This makes the pulse a broadband white light - a process called supercontinuum generation. Light filaments glow white from their white light supercontinuum and from the plasma they produce. Since supercontinuum light is preferentially scattered in the forward and reverse direction, the filament appears brightest in front of and behind the laser, and less bright off to the sides.

This page has some photos of filamenting laser pulses
http://www.teramobile.org/teramobile.html

Here is a web page I wrote of applying filamentation to laser death rays
http://panoptesv.com/SciFi/SelfFocus.html

Luke

[lexington]

Originally Posted by lwcamp

A laser is a device which uses a process called stimulated emission to amplify light. All early lasers, and a good number of lasers these days, use materials that can only undergo stimulated emission over a very narrow band of frequencies - for example, a given ion might transition between a metastable exited state and a ground state, and will only support stimulated emission for photons with an energy that match that transition. Even these lasers are not truly monochromatic - any physical process has a finite linewidth, given by the Heisenberg uncertainty relation if nothing else (in practice, Doppler and pressure broadening increase the spread of energies well beyond the Heisenberg limit).

Some modern lasers are able to amplify light through stimulated emission over a wide range of frequencies. Titanium:sapphire lasers are one example, which can emit light between 650 and 1100 nm wavelength. In fact, Ti:sapphire lasers are the perfect laser to bring up here, since they seem to be the model for the GURPS rainbow laser. By the Heisenberg uncertainty relation, the shorter in time something occurs, the less well specified its energy is. In light (as with anything, actually, so says quantum mechanics) the energy is directly proportional to frequency, so very short pulses of light must by necessity have a broad frequency spread. Ti:sapphire lasers are thus commonly used to produce ultra-short laser pulses because they can amplify all the frequency components of the ultra-short pulse simultaneously. Ultra-short light pulses compress the energy of the pulse into a very short period of time, thus giving the pulse extremely high power (power is energy divided by time, so keeping the energy the same but decreasing the time gives more power).

When very high powered pulses pass through matter, they can have non-linear effects. In particular, they can increase the index of refraction where the light is most intense - in the center of the beam. This makes the matter act as a converging lens, as it refracts the laser toward the center. This is called self focusing.

Taken to its extreme, self focused light would contract catastrophically, causing intense ionization at a spot and becoming absorbed in the resulting plasma. However, this is not what is observed. Rather, when the light gets intense enough, the air becomes very sparsely ionized. This slightly decreases the index of refraction, causing a feedback effect which keeps the light very intense but not too intense. You end up with a plasma core surrounded by a nimbus of propagating light, going in the direction of the original beam. This process is called filamentation. Self focused light filaments can propagate quite some distance through air, but eventually they lose enough energy through ionization that they cease to self-focus and disperse.

Light filaments naturally compress further in time and spread out further in frequency. This makes the pulse a broadband white light - a process called supercontinuum generation. Light filaments glow white from their white light supercontinuum and from the plasma they produce. Since supercontinuum light is preferentially scattered in the forward and reverse direction, the filament appears brightest in front of and behind the laser, and less bright off to the sides.

This page has some photos of filamenting laser pulses
http://www.teramobile.org/teramobile.html

Here is a web page I wrote of applying filamentation to laser death rays
http://panoptesv.com/SciFi/SelfFocus.html

Luke

Thanks! Admittedly I don't follow the whole thing but I think I understand the concept.

[Fred Brackin]

Originally Posted by IrishRover

I don't have my books, but IIRC, a rainbow laser shoots only one frequency at a time. The fancy aspect is that it can change frequencies so it can, for example, work as a blue-green laser under water, or a gama ray laser in space, or an IR laser so it can heat things up, or the right frequency to optimize its use in exotic atmospheres.

This was how "rainbow lasers" worked in 3e.

They kept the term in 4e but used it for the new gimmick described by LW.

[swampthing]

No, no.

Rainbow lasers are what you pack when hunting unicorns.

[Stoob]

A rainbow laser hurls huge pots of gold at incredible speeds upon its target. It's an extremely effective, albeit expensive, weapons system.

[Dwarf99]

Originally Posted by Stoob

A rainbow laser hurls huge pots of gold at incredible speeds upon its target. It's an extremely effective, albeit expensive, weapons system.

I disrupted a whole computer lab laughing!