题目7单选题
在弱导波光纤中,光纤的相对折射指数差可近似式表示为( )A. <img src="https://tihai-oss-cloud.itihey.com/img/b7169d8ea9d220f60ab66b15d85a50e8.jpg">B. <img src="https://tihai-oss-cloud.itihey.com/img/af916185c6af91d7961645b385f830c2.jpg">C. <img src="https://tihai-oss-cloud.itihey.com/img/888c2bb25512d0e13bb60b2836646d0d.jpg">D. <img src="https://tihai-oss-cloud.itihey.com/img/95e4faed74d14ea83fd3b74d5c6e3359.jpg">