*Article* **Liquid-Filled Highly Asymmetric Photonic Crystal Fiber Sagnac Interferometer Temperature Sensor**

#### **Yashar E. Monfared 1,\*, Amir Ahmadian 2, Vigneswaran Dhasarathan 3,4 and Chunhao Liang 5,\***


Received: 26 March 2020; Accepted: 15 May 2020; Published: 19 May 2020

**Abstract:** In this paper, we theoretically designed and numerically studied a high-resolution and ultrasensitive photonic crystal fiber temperature sensor by selective filling of a liquid with high thermo-optic coe fficient in one of the airholes of the fiber. The finite element method was utilized to study the propagation characteristics and the modal birefringence of the fiber under di fferent ambient temperatures. A large base birefringence value of 7.7 × 10−<sup>4</sup> as well as a large birefringence sensitivity of almost 29% to a 10 ◦C temperature variation was achieved for the optimized fiber design with liquid chloroform between 15 ◦C and 35 ◦C. We also studied the performance of the proposed optical fiber in a temperature sensing Sagnac interferometer. An average linear temperature sensitivity of 17.53 nm/ ◦C with an average resolution of 5.7 × 10−<sup>4</sup> ◦C was achieved over a temperature range of 20 ◦C (15 ◦C to 35 ◦C).

**Keywords:** fiber-optic temperature sensor; chloroform fiber; Sagnac interferometer
