((Okay, so here's a little novel I wrote about what makes up the FEL. As far as I know PW okayed the general principle of the weapon, the only questions are those of exact ingame functions and balancing.
I'll be naming the weapon series unimaginatively this time - "Spectrum". Or "Spektr". Better names welcome. Bifrost?
The FEL is a Free-Electron Laser, as opposed to the solid state lasers that our regular laser rifles are. Its defining quality is its ability to adjust its output wavelength without any mechanical alterations to the device, simply by altering power inputs. The weapon, as reimagined into the ER-verse by me, consists of a 4-TPU generator, a cyclotron accelerator and a linear accelerator for electrons, a magnetic oscillator - "wiggler" - chamber that makes up the bulk of the weapon's barrel, and a focusing array. The laser is formed by the oscillation of an accelerated electron stream within the wiggler chamber.
The operating principle, as imagined here, is that once the weapon is turned on, the generator is constantly keeping the cyclotron accelerator running, maintaining a ready stream of electrons pre-accelerated to near the required velocity, so that when the trigger is pulled the weapon does not waste time spooling up. When the trigger is pulled, the electron stream from the cyclotron is passed into the linear accelerator, bringing the electron stream to required near-lightspeed velocity. The accelerated electron stream is directed into the wiggler chamber. The wiggler chamber contains vacuum, and the electromagnets along its length make the stream oscillate via complex-ish physical effects. At every "peak" of the oscillation, an electron generates a photon. Due to the nature of electrons, they do not all oscillate at the same rate or in the same phase, initially, but gradually fall into phase and start generating a more coherent beam. As more and more electrons fall into phase - a process called "microbunching" - the output beam gets stronger. To expedite this process, the output beam is often reflected back into the wiggler chamber, causing electrons to microbunch into a frequency matching the laser a lot faster. Once a required nominal power is reached - determined by the weapon's capacity to contain excess heat more than anything - the beam stops looping in the weapon and is released through the focusing array. If more power is needed, the user can take it on themselves to manually "overcook" the laser. The resulting output beam can be much stronger - in ideal conditions and with maximum microbunching rate, a FEL is said to be capable of reaching several times the energy of the electron stream itself. The heat from the resulting laser beam can easily cause damage to the focusing array or the wiggler chamber, however, so the high-power laser beam can only be maintained safely for a short time. Due to the design of the weapon (refer to the picture I've drawn earlier), excessive overcooking is likely to cause the laser to burn through the reflector on the end of the wiggler chamber - and, by extension, through the back of the weapon and the shooter. The maximum power is limited by the durability of the weapon.
Regarding the weapon's power: I'd speculate that due to efficiency and the high thermal capacity of the design, the "nominal" power level of the FEL rifle would be about 30-50% higher than that of the regular laser rifle. This is the level at which it would be able to fire continuously without sustaining damage - the Spectrum is intended to be useful as a squad support weapon, providing continuous supporting or suppressing fire. Its "maximum safe" power, at which it can dish out maybe a 1-second pulse without sustaining permanent damage, could probably be as high as twice of its nominal power. "Maximum theoretical" power, where it will only be good for one shot and melt afterwards, could be another 50% over that, with a nonzero chance of the weapon firing backwards through itself instead. All of these could probably be extended via some form of advanced cooling.
Which is effectively what the battlesuit-portable "heavy" version of the FEL would provide. Its main differences from the rifle-size FEL, aside from double the input power and generally larger components, is a large active cooling array that will make up a lot of the weapon's mass. Its "nominal" power would be higher than that of a cutting laser by about the same 50%, but its 200% markup "maximum safe" power would cause the active cooling to actively kick in, and give its at most the standard 5 seconds of firing time. "Maximum safe" shots are not supposed to cause weapon damage unless the shots are made consecutively without giving the weapon time to cool down.
In game terms, I think the "cooked" shots should be fired as one would use a dynamic bonus - i.e. announcing the charging a turn early - but at the same time, realistically, for the regular frequencies (unlike, i.e., the X-Ray entry below) the charging time for "maximum safe" power would be short enough to fire the weapon within the 5 seconds that a turn normally lasts, albeit one would have to exercise some precise timing - i.e. an Uncon check to actually fire the weapon precisely on time, in addition to a Con check to hit the target. For "maximum theoretical" power shots, you would really have to announce it beforehand.
Regarding the variable wavelength: The main feature of the FEL, as I see it, is the enhanced utility of the multi-wavelength emission. The focusing array, to this end, should probably work as an anti-focusing array too, instead allowing the beam to be shot as wide as a 90-degree cone out the barrel, allowing actual wide-area application.
The FEL would allow fine-tuning the frequencies to any specific wavelength, but for simplification purposes, I see it as mainly having 5 primary modes. Microwave, infrared, visible-light, ultraviolet, and X-ray.
Microwaves offer the best power transmission ratio through atmosphere, but they are easily blocked by any sort of obstacle. On a low-power setting they can be used to disperse crowds, cook food, and destroy exposed electronics. On high power they are probably the best way to deal with unarmored flesh-based lifeforms, as with a wide beam they will quickly cause severe burns, without unduly damaging the surroundings.
Infrared is mostly just heat. Most useful in industrial processes, its application in a weapon are scarce, and its only advantage over microwaves is ability to bypass meshlike layers of armor. This is the least useful mode unless you want to warm up a room without causing undue damage to anyone, or encounter an enemy somehow simultaneously shielded against microwaves and UV.
Visible light has the advantage of being visible. Besides optional use as the biggest flashlight ever, this has an advantage when you are trying to use the weapon for suppressive fire, as the highly visible beam will deter sane people from approaching it. Only slightly more useful than IR, although a small alteration to the focusing array may add another use. A prism will diffract visible light at a different angle depending on its spectrum, so rapid fluctuation of the output wavelength within the narrow visible light band, when directed through a prism, will create a "spread attack" effect, shotgun-like with no reduction in penetrating power from high diffusion, and without any physically moving parts within the weapon.
UV light has served everyone well so far in standard laser rifles. This should be the default mode for the weapon. If no special function is needed, this is probably what the weapon should stay at.
X-rays have the best penetration capability, and deal the most collateral damage to biological targets. They are highly damaging, but are also very hard to generate, or rather focus and reflect, meaning they are slow to gain power by the looping-beam seeding method. In game terms, this could mean that if you fire X-Rays, you can deliver extra damage at the cost of having to "cook" the weapon before every shot, like you would when overcharge shooting a different wavelength.
The practical upshots of most modes are affecting maximim range, and effectiveness against specific forms of armor and types of target. Synthflesh in particular could be more vulnerable to some frequencies than others, as well as conventional means of anti-laser defense.
((Wall of text warning. No pictures this time, sorry. I was/am busy. Tell me if you need further clarifications.))