Since the formation of the Federation in 2161, Starfleet had been using beam weapons -- phase cannons and high-density particle beam weapons -- that had been derived from Vulcan and Andorian technologies. Phase cannons did not use much energy, but lacked effective firepower, and although high-density particle beam weapons were capable of much more damage, they also consumed far more energy and had slower refire rates. So Starfleet was at a significant disadvantage compared to the Klingons, whose disruptors had similar firepower to particle beams, but with the added advantage of higher refire rates and lower energy consumption.
Starfleet urgently needed a new weapon, since relations between the two powers had begun to degenerate in 2218. Within Starfleet Command there were initially two distinctly different views on which type of weapon technology should be developed. One proposal was for the development of a Federation version of the Klingon disruptors -- but this path was eventually rejected mainly because disruptors were seen as being too aggressive, in that their destructive firepower could not be regulated, whereas Starfleet required weapons which could deliver controllable amounts of damage, in order to provide the option of disabling a target when appropriate, rather than destroying it outright.
The second proposal was to develop a new phased beam weapon based on the then-aging phase cannon technology, but with greater firepower and lower energy consumption. This path was the one finally adopted. During the next ten years, ending in 2228, the Starfleet Corps of Engineers developed the first working prototype of the phased laser. However, the early prototypes suffered from the serious problem of their resonance chambers disintegrating during testing. This was not resolved until 2235, when the first fully functional phased lasers were finally mounted on the Legend-class cruiser USS Yorktown (NX-1531) for field trials.
- Per Ship Recognition Manual, Volume 4: Starships of the Original Series Era, the Enterprise's onboard laser weaponry was replaced with more powerful phaser technology in 2257. Cf., in the Orion Press continuity, phasers were introduced c. 2259. (Orion Press: "Incident on Xantharus")
The new weapons used the same principle as the phase cannons, but instead of having an independent laser device to collimate the light beam to be phased, the Engineers Corps incorporated the phaser chamber inside the resonance chamber of the laser device, creating the "phased laser".
This increased the weapon's damage to 30% higher than that of the disruptors and high-density particle beam weapons of the time and with similar energy consumption, but at the same time enabling the weapon to be made more compact.
In day-to-day usage, the new weapons were often called "lasers", which created some confusion with the original LASER (Light Amplification by the Stimulated Emission of Radiation) technology, which was never developed into a true weapon.