Table of Contents
Narrow-Linewidth Fiber Lasers for Optical Fiber Sensing
Narrow-linewidth fiber lasers with fiber output connectors are widely used as laser sources in high-sensitivity optical fiber sensing fields, such as submarine hydroacoustic detection and distributed acoustic sensing.
Multi-Wavelength Requirements for Fiber-Optic Hydrophone Arrays
Currently, the practical optical fiber hydrophone system is based on the optical fiber interferometer technology. A multi-wavelength, narrow-linewidth laser or a laser array composed of multiple single-wavelength, narrow-linewidth lasers, where the multiple wavelengths should be in the International Telecommunication Union (ITU) grid, is needed for the large-scale hydrophone array.
The laser array, composed of multiple single-wavelength laser modules, is always cumbersome and complicated in its configuration. Consequently, a laser with a multi-wavelength output is attractive for its compact structure and usability. In recent years, the narrow linewidth fiber laser with a compact structure has been realized in a distributed feedback (DFB) fiber laser where a short phase-shifted fiber grating in erbium-doped fiber as the distributed feedback laser cavity assures the robust single longitudinal mode operation. This distributed feedback fiber laser can be used as a sensing element in the highly sensitive acoustic detection field and has been extensively studied previously. However, since the linewidth of the DFB fiber laser is very sensitive to the ambient noise, the precise temperature control for each cavity, the sound isolation packages for each cavity, and the whole laser configuration are necessary to make it stable and practical.
Power Uniformity and Scalability Challenges
As for the wavelength number that this configuration can realize, it depends on the pump threshold of each laser cavity, the loss in the array, and the required power uniformity of the laser output. If the wavelength number increases, the power uniformity is more difficult to control. Usually, the pump threshold of the cavity is very low and much smaller than 5 mW, normally ~1mW.
Currently, the eighth-wavelength laser with 120-mW total power and 1.2-dB power difference within the wavelengths is obtained. Scientists expect that more wavelengths should be realized through reducing the loss in the array and adjusting the structural parameters of the laser cavities to meet the requirements in the larger scaled optical fiber sensing. The prototype based on this laser configuration is just in progress in the lab.