Magnetic-free optical nonreciprocity in the telecommunication band is of considerable interest for optical communication systems, where compact and integrable nonreciprocal devices are highly desirable. However, conventional nonreciprocal devices generally rely on magneto-optical effects and external magnetic fields, thereby posing challenges for miniaturization and integration. In this work, we investigate magnetic-free optical nonreciprocity in the telecommunication band using acetylene molecules confined within a hollow-core photonic crystal fiber (HC-PCF).
Rovibrational transitions of acetylene molecules in the telecom S and C bands are used to construct a Lambda-type three-level system for electromagnetically induced transparency (EIT). Owing to microscopic Doppler shifts arising from molecular thermal motion, the probe and control fields experience distinct two-photon detunings for opposite propagation directions. As a result, the forward-propagating probe field satisfies the EIT condition and experiences reduced absorption, while the backward-propagating probe field remains highly absorbed, thereby enabling direction-dependent optical transmission.
We perform numerical simulations to analyze how the nonreciprocal response depends on the control-field power and the ground-state decoherence. Within an appropriate parameter range, high forward transmission and strong backward absorption can be achieved. Experimentally, a 10-meter-long acetylene-filled HC-PCF at a gas pressure below 1 mbar is fabricated to enhance light-matter interaction. Nonreciprocal transmission of a 1517 nm probe field is observed under varying control-field powers. As the control-field power increases, the forward EIT transmission is enhanced and its bandwidth broadens, in good agreement with theoretical predictions. In contrast, the backward probe field consistently exhibits absorption. At a maximum control power of 517 mW, a nonreciprocal transmission with approximately 4 dB insertion loss and 40 MHz bandwidth is obtained.
These results present a feasible scheme for realizing magnetic-free optical nonreciprocity at room temperature and offer a promising route towards all-fiber compact nonreciprocal devices in the telecommunication band.