Thin gas cell with GRIN fiber lens for intra-cavity fiber laser gas sensors

Abstract

Fiber laser gas sensors based on the intra-cavity absorption spectroscopy require the use of gas cells. We propose a simple and reliable gas cell using graded-index fiber lens (GFL) based all-fiber collimator. Conventional gas cells usually utilize direct fiber-to-fiber coupling without collimators or graded-index (GRIN) lens as collimators. Direct fiberto- fiber gas cell has simple configuration, but it suffers from high coupling loss and stray light interference. Gas cells applying fiber pigtailed GRIN lens are advantageous to achieve low coupling loss. However, fiber pigtailed GRIN lens requires accurate and complicated alignment and glue packaging which could compromise long term reliability and thermal stability. The proposed technique fabricates all-fiber collimators by simply splicing a short section of gradedindex fiber to single mode fiber which is both compact and durable. With that collimator, the gas cell can be fabricated very thin and are suitable for extreme environments with high temperature and vibration. In this paper, we have carried out experiment and analysis to evaluate the proposed technique. The coupling efficiency is studied versus different GFL gradient parameter profiles using ray matrix transformation of the complex beam parameter. Experiments are also done to prove the practical feasibility of the collimator. The analysis indicates that gas cell using GFLs can overcome the disadvantages of traditional design; it may replace the conventional gas cells in practical applications.