An Instantaneous-wavelength-variation Monitoring System Based on Mach-Zehnder Interferometer for the Wavelength-swept DFB Laser

We propose and demonstrate an instantaneous- wavelength-variation monitoring system for the wavelength- swept distributed feedback (DFB) laser, which consists of the wavelength-swept DFB laser, an unbalanced Mach-Zehnder interferometer (MZI), a 3-port optical circulator and a fiber Bragg grating (FBG). The unbalanced MZI contains two arms with different delay lengths to generate a beat signal when the DFB laser sweeps its wavelength. An acoustic optic modulator (AOM) is added to the delay arm to introduce an extra frequency shift to increase the beat signal. So the tuning direction of the DFB’s wavelength can be recognized via the change of the beat frequency. In-phase/Quadrature (IQ) demodulation algorithm is selected as the optimized method to demodulate the change of wavelength, and the FBG acts as a reference to confirm the absolute wavelength during wavelength tuning. In the experiment, the wavelength-tuning direction can be accurately determined when the laser wavelength sweeps by 1.6 nm within 30 ms. Furthermore, this rapid and precise wavelength-tuning monitoring system can be used to enhance the performance of the FBG interrogator to achieve real-time and accurate temperature and strain sensing in the future.

I. Introduction

With the development of optical fiber sensing technology and the wide application of wavelength selection devices such as fiber laser [1], dispersion compensator [2] and bandpass filter [3], the fiber Bragg grating (FBG) sensing system has made significant progress in both theories and experiments. The FBG demodulation system based on wavelength-swept laser has become one of the most popular demodulation techniques for its advantages of high signal-to- noise ratio and high measuring resolution. However, most current commercial FBG demodulation systems based on wavelength-swept laser lack a suitable tunable laser source with a compact structure and low cost. Furthermore, the methods of accurately and rapidly monitoring the instantaneous wavelength variations of laser with fast tuning speed is also a big challenge for FBG interrogation systems.