The laser beam that enters LIGO's interferometers begins inside a laser diode, which uses electricity to generate a 4-watt (W) 808 nm beam of near-infrared laser light. While laser diodes are the kind of devices that one finds in everyday laser pointers, at 4 watts, LIGO's is 800 times more powerful than most off-the-shelf laser pointers. This 4 W beam is first sent into a fingernail-sized crystal of man-made garnet, within which it circulates and stimulates the emission of a 2 watt beam with a wavelength of 1064 nanometers (nm). This crystal lasing device is called a Non-Planar Ring Oscillator (NPRO) and the resulting 2 W beam is called a “seed” beam because it will eventually grow into a much more powerful laser.
While 1064 nm is the target wavelength for LIGO’s laser, it needs 100 times more power (or 100 times more photons emitted per unit of time) before it can enter the interferometer. To get there, the 2 watt seed beam undergoes two additional amplification stages that can boost its power up to nearly 200 W.
In the multi-faceted first step of amplification, the seed beam passes through a device called a Master Oscillator-Power Amplifier (MOPA). The MOPA contains four thin laser amplifier rods that begin the power-boosting process. These rods, composed of a glass-like material made of neodymium, yttrium, lithium, and fluoride, are about the size of the graphite inside a pencil: just 3 mm in diameter and 5 cm long.
To amplify the seed beam, the molecules in each rod are first energized by shining a separate 808 nm laser into each one. When the seed beam travels through the first rod, the rod's molecules respond by emitting 1064 nm photons with the same properties (phase, wavelength) as the incoming seed beam. These new 1064 nm photons join those from the seed beam traveling in the same direction, and the power boosting begins (remember, more photons equals more power). This more-powerful beam then travels to the second rod where the amplification process occurs again, and then again in the third, and again in the fourth rod. Like tributaries to a river, at each stage of this amplification process, more and more photons of the same wavelength join the seed beam, gradually increasing its power.
By the time the seed beam has passed through all four rods, its power has increased from 2 watts to about 35 watts all while maintaining a wavelength of 1064 nm. The path that the beam takes from the NPRO through the four rods is shown in the figure at right (click on the image for a larger version).
The final stage of power amplification is achieved in another four-rod device called a high power oscillator (HPO). The HPO uses four more amplifier rods, about the same size as the MOPA's rods but made of a different material. As the beam passes through these rods, just like in the MOPA, it gets an additional power boost from laser light funneled through bundles of fiber-optic cables arranged like a flower some 3 mm across.
Each fiber carries 45 watts of laser power, so each bundle delivers 315 W (7 fibers x 45 W each) into each HPO rod to prime it to emit more and more laser light. By the time the beam exits the HPO it has finally achieved its desired power of 200 W. The complexity of this device is revealed in the image below left (click on the image for a larger version).
Frequency and Power Stability
Despite the purity and stability of the initial beam, upon exiting the HPO it is still not stable enough for use. LIGO's laser needs to be 100-million times more stable than it is intrinsically. To achieve this unprecedented level of stability, the beam’s natural frequency variations (caused by its inability to continually radiate a single, discrete color of light) and power fluctuations are mechanically reduced by about a factor of 100-million through a series of feedback mechanisms before the laser is used in the interferometer. This whole process is akin to tuning the world’s most complex piano.
Power fluctuations are also reduced through feedback control loops, which use photodetector data to sense power fluctuations. To learn more about LIGO's feedback and control systems, visit Feedback and Control Systems.