Research On Erbium-doped Fiber Laser With Random Distributed FBGs

Since the concept of random fiber laser(RFL)was proposed,more and more researchers have made extensive researches on it.Various new types of random fiber lasers have been reported successively.The Erbium doped random fiber laser based on disordered distributed Fiber Bragg Gratings(FBGs)is a coherent random fiber laser,which has the advantages of low lasing threshold,narrow linewidth,tunable wavelength and so on.When this kind of laser works,there are usually multiple modes excited,and they compete with each other,resulting in the output wavelength jump and time-domain instability.It is necessary to study the corresponding control ways to effectively regulate or control the output characteristics.Meanwhile,the disordered characteristics of this kind of laser structure also provide an important platform for the research and control of photon localization and disordered system luminescence,and there are many topics worthy of further exploration.Under the above background,we demonstrate the wavelength locking of Er-doped RFL,using the traditional injection locking technology.In the experiment,we find that the threshold of pump power is of 17.32 mW under the condition of injection locking,which is reduced by 44% compared with that of the system running freely.In addition,by scanning the wavelength of the seed light source,the intrinsic stochastic laser mode of the system can be excited in turn by resonance excitation,and mode competition can be restrained to achieve controllable lasing of specific wavelength.Finally,we give the threshold curve of the wavelength locking,which shows that with the increasing of pump power,more and more lasing modes are excited in the grating,and the average power of each mode increases.The system requires higher seed light power to obtain wavelength injection-locking lasers with specific random modes.Above all,we have realized a random laser source which can be used in speckle-free imaging,sensing and so on.This study also provides an efficient and universal way to control and utilize lasing of disordered systems that exist extensively in various photonic structures.In addition to wavelength locking,the output mode locking and the corresponding laser pulse mechanism are also studied.Aiming at multiple intrinsic resonance modes in the system,a mode locking method combining “mode stripping” and saturated absorption(i.e.,mode selection using nonlinear polarization rotation effect and pulse narrowing combined with saturated absorption effect)is proposed.Based on this method,the pulse laser output with adjustable repetition frequency is realized under the condition of “constant cavity length”.The experimental results show that the threshold of mode-locking is of 85 mW.By adjusting the non-linear polarization rotation effect,three repetition frequency laser pulses are obtained without changing the fiber length,and the output of harmonic mode-locked pulse with repetition frequency of 10.896 MHz can be also realized.It needs to be pointed out that the mode-locked laser pulse will become unstable when the pump power is changed under the same polarization state.At this time,the polarization controller needs to be fine-tuned to make it return to the mode-locked state.This work opens up a way of controlling output of disordered system,and provides a powerful source with flexible pulse generation,showing unique potential for applications such as laser coding and ultrafast spectroscopy.It is also different from the traditional mode-locked laser which uses external cavity modulation to produce adjustable repetition frequency.It is a completely new way of modulation.