Fiber lasers are the lasers developed with special doped optical fibers. They may be used as purely optical power sources in many applications or both power source and fiber sensing elements in FLS systems. On one hand, high-power fiber lasers can supply as much as tens of kilowatts currently making them ideal for laser processing and machining. Having said that, lower-power fiber lasers could be the sensing elements such those detecting the feeble sound signal this is certainly even weaker compared to the deep sea noise into the submarine sensing application fields.
Fiber lasers could be classified into many types relating to different criteria. With regards to cavity structures, fiber lasers may be classified into ring cavity fiber lasers, that is, ring fiber laser (RFL) and linear cavity fiber laser diode burn in, which include DBR fiber laser (DBRFL), DFB fiber laser (DFBFL), and composite cavity fiber laser (CCFL). Generally speaking, fiber lasers may emit different wavelengths with various doped rare-earth ions or any other active materials and under different pump and cavity conditions. There are many fiber lasers of special designs such as double-cladding fiber lasers, large-core fiber lasers, multimode fiber lasers, and photon crystal fiber lasers.
The selection of a specific laser diode structure depends mainly on specific application condition and requirement. The erbium-doped DBRFL and DFBFL emit 1.55 μm wavelength and relatively low output power; however, they can operate robustly in single longitudinal mode without special cooling control. Although they can provide high output power, the erbium–ytterbium co-doped fiber laser diode have a powerful heating effect inside fiber and need precise cooling control to help keep the stable operation. Hence, Er-doped DBR and DFBFLs are extensively investigated and widely used since the sensing elements in sensing applications such as fiber hydrophone and geophones.
The wavelength and intensity responses of fiber laser diode testing affect both the laser output as well as the sensing performance in FLS. So it is very important to analyze and optimize these responses for specific FLS applications.
For DBRFL and CCFL, simple approximation may be made according to multiple reflection phenomena, leading to simple analytical model. Distinguished intensity and wavelength responses may be observed between single cavity and composite cavity laser diode. However, for DFBFL, the optical field is spatially distributed in the grating, rendering it a bit more complex to investigate wavelength and intensity responses.
The cavity of DBRFL is in fact formed by two FBGs inscribed on a consistent EDF. DBRFL can also be a single-cavity laser diode LIV that has an identical type as DFBFL, but it has significantly longer effective cavity length. This results in a far more efficient pump-lasing power conversion. Previously, incorporation of an external reflector to DBR lasers happens to be studied for several purposes including linewidth narrowing decrease in low-frequency wavelength instability and continuous wavelength tuning. It’ll be demonstrated by inclusion of a dynamic feedback (hence composite cavity) that phase condition of laser diode burn-in can be significantly altered, that will be useful for wavelength-type sensing.