Polarized random fibre lasers based on the tilted fibre Bragg grating

Abstract

In the traditional random fibre lasers (RFLs), it is complicated to generate polarized random fibre lasing. In this thesis, I present a new method to obtain polarized random fibre lasers (RFLs). In my research, 45° tilted fibre Bragg grating (45° TFG) fabricated in single mode fibre (SMF) or in Erbium doped fibre (EDF) has been integrated with the fibre laser and the polarized random lasing was achieved using 45° TFG with high polarization extinction ratio (PER) characteristics. In this thesis, first of all, I present the review of random lasers (RLs), random fibre lasers (RFLs), and corresponding polarization characteristics. And then I present a detailed research work on the method, characteristics and applications of fibre gratings, including fibre Bragg gratings (FBGs), long period gratings (LPGs), and TFG with small and 45° titled structures. One of the major contributions of my work in this thesis is the investigation on phase-match conditions, transmission spectra, PER for different types of fibre gratings including the TFG, FBG, and LPG. The results show that 45o TFG has high PER with 14 dB and the 7o TFG has comb-like resonance transmission. And then I present the experimental research of the thermal sensing of different types of fibre grating. LPG with grating period of 480 m has the highest thermal sensitivity with 297.1 pm/°C, superior to the FBG, small angle TFG and LPG. Meanwhile, I also researched the strain sensing application of FBG. The strain sensitivity of the FBG is 0.71±0.01 pm/°C. The other important contribution from the work is that I researched the polarization random fibre lasing feature with 45° TFG in the two fibre laser systems, which are Raman distributed SMF system and EDF with Raman distributed SMF hybrid system. In the Raman distributed SMF with length of 20.1 km system, there are three kinds of emission spectra with the increase of pump power from 1.70 W to 3.00 W: Raman emission, non-stationary random lasing emission, and stationary random lasing emission. The random lasing threshold is measured as 2.10 W. When the 45°TFG is added in the system, the polarized random lasing with PER of 12.02 dB has been obtained. In the EDF (2 m length) and Raman distributed SMF (11 km length) hybrid system without 45° TFG, there are two kinds of emission with the increase of pump power from 2.60 W to 4.40 W: Raman emission, and non-stationary random lasing. The stationary random lasing has not been observed due to the pump power limit. When the 45° TFG has been integrated in the hybrid system, we observed two kinds of emission in a range of pump power of 1.00 W to 2.20 W: Raman and Er3+ emission, and stationary random lasing. Under the effect of weak cavity formed by 45° TFG reflection and SMF distributed feedback, there occurs stabilized random fibre lasing emission. The threshold has been determined as 1.44 W. In addition, under the effect of 45° TFG, the polarized random fibre lasing with PER of 15.3 dB has been obtained.