Experimental Study On The Narrow Linewidth Fiber Ring Laser Based On Stimulate Rayleigh Scanttering

Narrow linewidth fiber laser could be widely used in novel sensing system thatrequires super long distance, super precise and highly sensible detection, highlypreciseoptical spectrum, and fiber optic communication owing to its excellent coherence(with coherent length up to hundreds of kilometers). In recent years,researchersgradually realized that through traditional methods like short cavity method, saturatureabsorption method, we can hardly further narrow the laser linewidth. To overcome thisobstacle, a new mechanism for further narrowing the linewidth is essential. Boththeoretical and experimental studies are taken in this thesis based on the linewidthnarrowing mechanithem, approaches to mesurement and other related phenomenon inEr-doped fiber ring laser due to stimulated Rayleigh scattering, combined with thetechniques to restrain the stimulated Brillouin scattering in fiber.The main contents of this thesis are as following:1. The theories on Rayleigh and Brillouin are described briefly in this thesis.Rayleigh scattering is analyzed from the aspects of traditional and quantum optics,respectively. The former gave the Rayleigh gain caused by absorption andelectrostriction, while the latter gave the value of intrinsic energy by solving the Rabimodel. The Brillouin scattering which is induced by acoustic phonon is also discussedin this thesis and the method to increase the threshold of Brillouin scattering in opticalfiber is discussed according to the waveguide structure analysis.2. Four typical methods for measuring the laser linewidth are described in thisthesis. And major attention is paid to delayed self-heterodyne method andloss-compensated recirculating delayed self–heterodyne method. The error analysis,frequency broadening of optoelectronic current power spectrum and its ripple in delayheterodyne are also analysed. The simulation results show that the optoelectroniccurrent power spectrum would be broadened and have sideband ripple,when the timedelay dis less than the coherent time c. At this point, the measured linewidth iswider than the actual value and the error of Lorentz fitting is larger. The optoelectroniccurrent power spectrum converges to the lineshape of Lorentzian as the length of thedelayed fiber increase, and the measured results tend to approach the actual laserlinewidth. Theoretically, the delayed self-heterodyne method is adequate for measuringthe laser linewidth when d5c. In terms of loss-compensated recirculating delayed self–heterodynemethod, only when, there would be no power spectrummodulation and therefore can be used directly in mesurment.3. The experiment on1550nm fiber ring laser is conducted. A110m tapered fiberwith high stimulated Brillouin threshold is fabricated by using the taper rig. To selectmodes of the laser and further narrow the laser’s linewidth, Fiber Bragg grating withwavelength of1550nm is used as a wavelength selection component and the110mtapered fiber, variable optical attenuator and Farady reflector together are applied toprovide the Rayleigh scattering signal. The experiments show that the maximum outputpower is larger than1mW and the side mode superior ratio is up to50dB. Owing to thehigh cavity loss, the pump efficiency is lower than1%. Using the delayedself-heterodyne method in which the length of the delayed fiber is50km, the FWHM ofthe laser is measured to be800Hz. The laser linewidth can be further suppressed byincreasing the weight of Rayleigh signal in the fiber ring through tuning the value ofvariable optical attenuator. The FWHM has been measured by delayed self-homodynemethod using50km and60km delayed fiber, the result showing no significantdifference and that the minimum linewidth is200Hz. The result reliability is guaranteedby repeated measurements and analyzation of the polarization in delayed self-homodyne method.