Stimulated Brillouin Scattering Effect And Its Application In Multi-wavelength Fiber Laser

Stimulated Brillouin scattering?SBS?is a kind of very important nonlinear effect in fibers.The SBS effect is very easy to be generated in optical fibers because of its low threshold,which is harmful for optical fiber communication system.The SBS effect will cause the signal light to produce new wavelengths of light in the medium,which will cause cross talk and higher noise in the communication process,and lead to greatly increased signal transmission loss.However,the application prospect of SBS effect in other aspects is very wide.For example,it has many practical applications in fiber sensing,optic fiber gyroscope,optic fiber laser and so on.Because of the potential application value of SBS,studies on it have been going on since it was discovered.The research on SBS mainly focuses on two aspects.One is how to suppress the generation of SBS,because the suppression of SBS will reduce the damage to the optical signal transmission system.Another aspect focuses on how to use the SBS effect to generate new wavelengths of light.In this paper,the backward SBS in optical fibers has been investigated numerically and experimentally.The main contents of this paper are as follows:Firstly,the backward SBS in step-index fiber and photonic crystal fiber are analyzed by using finite element method.The effective refractive indices and effective mode-field areas of the fundamental mode in step-index fiber and photonic crystal fiber are calculated,respectively.The backward SBS in two different optical fibers is simulated,and the interaction with the acoustical fields is investigated.The properties of Brillouin gain and Brillouin frequency shift are simulated.It can be concluded that the maximum gain of Brillouin gain in different gain fibers is different,and the Brillouin frequency shift is also different when the maximum Brillouin gain is obtained.We can conclude from the simulation that in the step-index fiber,at the Brillouin frequency shift of 1.10296?10¹⁰Hz,the backward Brillouin gain reaches the maximum value of 1.1435?10^-11m/W.And we can also conclude from the simulation that in the photonic crystal fiber,at the Brillouin frequency shift of 1.0839?10¹⁰Hz,the backward Brillouin gain reaches the maximum value of 1.4592?10^-9m/W,and the maximum Brillouin gain is two orders of magnitude higher than the maximum value in the step-index fiber.Secondly,a multi-wavelength fiber laser is designed based on the SBS effect.At first,no tunable filter is added to the laser.In this case,the free oscillations in the laser cavity under different pump powers were tested.Two output peaks were observed from the output spectra,around 1569 and 1605 nm,respectively,which mainly came from the free oscillation and birefringence in the cavity.In addition,the output characteristics of the laser are measured by changing the wavelength of the pump light.When the wavelength of the pump light is near the first free oscillation peak,a larger intracavity linear gain can be obtained,and then a higher nonlinear gain can be generated to output the Brillouin stokes optical signal of first order or even higher order.When the pumping wavelength is at 1565 or 1570 nm,the pumping wavelength is close to the first free oscillation peak,where the laser can produce first-order stokes light.Finally,on the basis of above experiments,a tunable filter is inserted into the appropriate position of the laser to suppress the free oscillation in the cavity,improve the laser performance and achieve the output of Brillouin stokes light with higher orders.The affects of pump power,pump wavelength,amplifier power and Brillouin gain medium length on the output characteristics of the laser are studied.According to the analysis of the experimental data,when the 2 km dispersion-shifted fiber is used as the gain medium,the maximum output power of the laser can reach 88.7m W,and the maximum can generate 95 channel optical signals.The stability of the laser was measured,too.The output spectrum and output power of the laser were measured every5 minutes within 30 minutes.The fluctuation of total output power is within 0.05 d B.The stability of the laser is excellent.