Continuous-Wave Broadband Generation In Filterless Erbium-Doped Fiber Laser

Broadban light drew a lot of attention because they have potential applications in wavelength-division multiplexing(WDM), sensing, optical component, biomedical imaging, and spectroscopy. A prominent medical application is optical coherence tomography(OCT) system, since a broadband source can improve the axial resolution efficiently. Most sensor application based on low-coherence interferometry require a broadband source with a high degree of spatial coherence.The generation of super-continuum(SC) light source needs a ultra-short pulse pump laser, which is hard to achieve. However, the filterless erbium doped fiber laser(EDFL), which there is no filter in the cavity, can generate continuous-wave(CW) spectrum when highly nonlinear fiber(HNLF) is inserted into the cavity. In this paper, we propose two filterless EDFLs, one is the linear cavity fiber laser in which the spatial-hole burning(SHB) effect extends the band width. The SHB effect in the linear cativty fiber laser is important for the CW broadband generation. The other one is the CW broadband generated in an erbium-doped fiber ring laser(EDFRL) with single mode fiber(SMF) in Sagnac loop structure.(1) The influence of the SHB effect in the output bandwidth. In this paper, we investigate two filterless EDFL with 4m EDF in L-wave band and 1km HNLF,unidirectional amplification and bidirectional amplification. For comparison, we demonstrate the spectral characteristics of the two different structure fiber lasers at300 mW pump power in detail. The performance of the laser with unidirectional amplification is the following: the central wavelength is at 1564.8nm, the bandwidth is18.14nm; the performance of the laser with the SHB effect is the following: the central wavelength is at 1571.1nm, the bandwidth is 51.77 nm. The spectrum broadening is mainly due to the SHB effect in the cavity. Simultaneously, addopting the bidirectional amplification EDFL, we alter 9m EDF in L-band and 12 m EDF in C-band into the EDFA to discuss the effect of the EDF gain in the output spectral bandwidth.(2) CW filterless EDFRL based on SMF. In order to get the CW generation in a filterless EDFRL with SMF, We investigate three experimental schemes. One is the filterless EDFRL, which output spectrum is narrow and chaotic. The second one is the filterless EDFRL with 5km SMF. The bandwidth of output spectrum is 2.52 nm whenthe pump power is 300 mW and the central wavelength is at 1566.2nm. The third one is the filterless EDFRL with 5km SMF in the Sagnac loop structure. At 300 mW pump power, the bandwidth of output spectrum is 33.98 nm and the central wavelength is at1565.6nm. Finally, the laser gets broadband output because of the enhancement of the nonlinear effect, such as self-phase modulation(SPM) and cross-phase modulation(XPM).