How do fiber lasers work?
Wednesday, April 13, 2022
If I say fiber optics, chances are you’ll think of high-speed internet. And indeed, by using small strands of optical fiber to transmit light, internet service providers can achieve fast and reliable data transmission.
However, fiber lasers are a very special kind of optical fiber. They do not just transport preexisting laser light. They generate it.
Let’s find out how fiber lasers work.
At the center of all optical fibers is the core. This is where the light is contained thanks to total internal reflection.
In the case of fiber lasers, the core (usually silica glass) contains atoms of rare-earth elements, called dopants. These dopants, usually ytterbium or erbium are responsible for generating the desired laser light.
But before they can generate laser light, the dopants must first accumulate energy. This is where optical pumping comes into play.
If you are familiar with CO2 lasers or diode lasers, then you will know that those types of lasers are electrically pumped, meaning that a current is run through the lasing medium to excite the atoms.
Optical pumping is the equivalent of that, except that the energy source is not electricity, but light. Laser light, to be precise. In a way, this means fiber lasers are laser‑powered.
Once the rare-earth atoms are excited by the pumping laser, they are primed for stimulated emission and ready for some light amplification.
It used to be that using fiber lasers meant choosing between beam quality and high power, one coming at the expense of the other.
This is because of the way single-mode and multi-mode fibers work. Modes are the different ways that light can propagate through the fiber, analogous to the different –and beautifully hypnotic – ways a metal plate can vibrate when exposed to sound as seen in this video.
Single-mode fiber lasers are small enough that only one mode of light can travel. This is great for signal quality, but imposes such stringent modal and alignment requirements on the pump light that high powers cannot be attained.
Multi-mode fibers are much the opposite. They can be readily coupled with higher-powered pump lasers, but the resulting intensity profile is composed of a multitude of modes, which muddles the intensity profile.
The key to attaining high powers without sacrificing beam quality comes in combining the two types of fiber in a double-clad fiber design.
In this design, a (multi-mode) passive fiber cladding surrounds a (single-mode) rare‑earth doped core. The cladding can transport high-power pump light in different modes, which will overlap with the mode in the core, enabling a transfer of energy to the core. The best of both worlds!
Now that you have a better understanding of how fiber lasers work, let’s see some of their advantages. Thanks to the hard work of many academic and industrial researchers and engineers, fiber lasers have come a long way and now offer some interesting advantages. Notably:
At Gentec-EO, we offer a wide range of laser power measurement solutions for all laser types, fiber lasers included. Browse our laser power measurement products or try our product finder to find the best solution for your needs.
We are also always happy to help if you need help, just get in touch with your local representative.