The Dynamic Measurement Of Linewidth Of Wavelength-sweep Laser

ABSTRACT:Tunable fiber ring laser based on SOA plays an important role in fiber sensing,fiber communication, optical information processing, non-linear optics and other fields, especially in medical instrument equipment, and it is also the key component for optical fiber sensing and optical coherence tomography(OCT). Its performance has a direct impact on the whole OCT system and its output spectral characteristics determine the resolution of the sensing and imaging systems. Therefore, it has great significance on the measurement of spectral characteristics of its dynamic output spectral linewidth.This paper theoretically analyses the working mechanisms of fiber ring laser based on SOA as well as output spectral characteristics and discusses the feasibility of measuring dynamic output spectral linewidth. An improved delayed heterodyne method based on MZI structure are proposed to dynamically measure the output spectral characteristics in two working state of a self-made prototype of a tunable fiber ring laser. The spectral linewidth, hence the coherence length of the laser source, are obtained to show its increasing dependence on the wavelength-swept frequency. The paper also represents a novel and direct measurement method by fast synchronous time gating with an electro-optic modulator (EOM) to measure the output spectral linewidth in any wavelength of spectrum of FDML. Besides this.the characteristic in time-domain of pulses from EOM which has influence on output spectrum of FDML is discussed. The results of experiments show that:there’s a relationship between two series of results of two methods based on various working mechanisms while they are different. The linewidth measured by the EOM method is greater than that by the heterodyne method due to the modulation, and the respective linewidth are37pm and4.9pm while the wavelength-swept frequency is60kHz. The results of experiments shows the advantages and limitations of two methods which make for understanding more deeply the working mechanisms of tunable fiber ring laser and provide a good foundation of realizing the real-time and direct measurement of spectral characteristics of FDML.