Mesoporous Silica Coated Silica Fiber Directional Coupler for Ammonia Gas Sensing for Medical and Industrial Applications

摘要

In the present work, an optical fiber directional coupler based ammonia gas sensor is modeled and simulated. The sensor model comprises of two uncladded straight single mode silica fibers having coated with mesoporous silica material over a small sensing region, and an optoelectronic photodetector. In the model, when the sensor is exposed to ammonia gas, ammonia gas gets adsorbed in the mesoporous silica coating. Because of this, the refractive index of mesoporous silica changes. The change in refractive index is a measure of amount of gas adsorbed. The variation in refractive index affects optical power in the fibers. Power in the input fiber is fed to the photodetector. The photodetector produces proportional photocurrent. Thus, ammonia gas is detected. The sensor model is extensively simulated for gradual variation in refractive index due to adsorption of ammonia gas, for various fiber diameters, and optimization is done. It is observed that the sensor produces photocurrents of the order of few hundreds of μA. It is observed that 1600 nm-1800 nm diameters show linear variation in photocurrent with refractive index. Hence, these diameters are best suitable for constructing this sensor. The simulations show that the required sensing length is about few hundreds of μm. Thus, a small amount of mesoporous material will be required for the coating purpose. The sensor is capable of detecting the variation in refractive index, of the order of 10~(-5). This corresponds to 10 ppm of gas adsorbed. Ammonia gas gets selectively adsorbed in mesoporous silica. Hence, the sensor shows selectivity property. The response time of the sensor is equal to the response time of ammonia gas adsorption in mesoporous silica, which is a few minutes. The sensor can be recovered by rising temperatures or by cleaning. Due to this, the sensor shows stability. The proposed sensor is shown to be miniature, sensitive, selective and reversible. Because of high sensitivity, the sensor will be useful in early detection of gas leakage in industry, and for breath ammonia detection as an endoscopic tool in the medical field. The possibility of detection of ammonia gas by mesoporous silica coated fiber directional coupler has been proposed and explored for the first time in this work.