Photonics

Photonics is the science and technology that studies electromagnetic waves, with the aim of developing components for generating, transmitting or converting optical signals. Among all these technologies, fiber lasers have won over manufacturers in sectors as diverse as micromachining, automotive and aeronautics.

Lasers can be divided into two categories: continuous and quasi-monochromatic light sources, or pulsed light sources with a broad spectrum. To achieve these different specificities, it is necessary to introduce passive and active components to manipulate the intracavity optical signal. 

Unlike continuous sources, pulsed sources are characterized, in part, by the cadence of the light pulses that make up the pulse train of the output signal. This rate is defined by the time it takes for a pulse to travel around the cavity. One might think of this value as absolute, but this is not the case. This is a problem, as some pulse sources are installed in synchronous systems where rate fluctuations are not allowed. For this reason, it is possible to introduce active intracavity delay generators, which, coupled with a feedback loop system, will correct this variation in the duration between two consecutive pulses. This is an optoelectronic component called Fiber Phase Shifter (FPS), which reduces output frequency fluctuations by adjusting the optical phase of the signal.
A fiber laser is a complex technology consisting mainly of several fiber optic components. If this technology is so popular with industry today, it's because its robustness and compactness make it remarkably easy to integrate. The robustness of this laser architecture lies mainly in the fact that the various fiber optic components are soldered together. The mechanical and optical quality of these welds between the different components that make up the fiber laser has enabled manufacturers to develop increasingly high-performance and reliable laser sources. These splices are made using specialized splicing machines, such as the FITEL, capable of fusing silica strands over a few micrometers while maintaining signal transmission .

Because of the wide range of different optical fibers, there are a multitude of splicing machines of varying degrees of complexity, capable of splicing these optical fibers correctly. These range from splicing standard optical fibers (single-mode fibers) to more complex welds such as polarization-maintaining fibers, where the splicer must ensure that, in addition to the cores and claddings specific to each fiber, the X and Y propagation axes of each fiber are aligned to maintain this polarization-maintaining property (FITEL S185 PM)

There are also much more complex fibers, such as microstructured fibers, whose guiding properties result, in part, from the structure of the periodic lattice around the core. For example, the fiber shown above is composed of silica and air-hole inclusions. When splkicing this fiber to a fiber architecture, it is essential to control the fusion process as much as possible (FITEL S185 PM ROF). Indeed, if the fiber is overheated during splicing and the air inclusions deform or even close, the confinement of light in the fiber core will be degraded, resulting in high optical losses. Conversely, if the fusion is insufficient, the mechanical strength of the solder joint will be unusable, as it is far too fragile.

Finally, there are also multicore fibers, i.e. optical fibers in which there is not just one core in which the light signal propagates, but several spatially separated cores within which a light signal propagates. As with the polarization-maintaining fibers mentioned above, these multicore fibers need to be welded using a specific splicing machine (FITEL S185 PM ROF).)that takes these different cores into account, in order to take advantage of this very special architecture.

Splicing optical fibers is a complex process, requiring not only the right soldering machine, but also all the equipment needed to prepare the optical fibers prior to soldering. The fibers need to be stripped, cleaned and cleaved for effective soldering. First, it is imperative to strip the fiber from the polymer sheath that encases the silica before welding. This step must be carried out with care, as the bare fiber is extremely fragile. By using a thermal stripper (3SAE Thermal Stripper), this step is carried out without any mechanical stress on the fiber, which could weaken it and affect the cleavage process. Once the fiber has been cleaved, it is imperative to clean the bare fiber of any residue that may have settled on the surface. This can be done using an ultrasonic bath (3SAE Ultrasonic Cleaner). The deposit of a foreign body on the surface of the fiber is an additional factor in bad splicing. The goal here is to ensure that the fusion zone consists exclusively of silica. About cleaving (NorthLab ProCleave HS), if the surface of the cleave is not flat and smooth, an air bubble may form in the gap between the two optical fibers during soldering. This air bubble will impair optical signal transmission and weaken the mechanical strength of the splice. Finally, once welded, the bare welded fibers can be covered with a polymer sheath to protect the silica from external mechanical stress. Finally, once welding is complete, the bare welded fibers can be covered with a polymer coating in order to protect the silica from external mechanical stress, using a recoater (NorthLab ProCoater).