This paper focuses on automatic locking of tracking filters used in optical frequency transfer systems. General concept of such a system is briefly described and the problems with its automatic startup, originating in the use of the analog phase locked loop to filter weak, received signal, are discussed. A supervisory circuitry and algorithm to solve these problems is proposed. The frequency of the signal to be filtered is measured indirectly and the output frequency of the tracking filter is monitored. In the case of lack of synchronism (i:e: after the startup) a significant difference of these frequencies is measured and the supervisory algorithm forces the filter to tune into the right frequency and then allows it to synchronize. A system with the proposed solution was implemented and tested experimentally on a fiber optic link with high attenuation and multiple optical connectors. Transient signals during locking were recorded to investigate the system’s behavior in real environment. The system was evaluated in the link causing synchronization losses every 17 min on average. During measurements over 3 days, the whole system was synchronized for over 99.98% of time despite these difficult conditions.
The aim of this work was to induce permanent birefringence both in typical liquid crystal cells and photonic crystal fibers (PCFs) by photo-polymerization. For this purpose three different liquid crystalline materials, namely E7, 5CB, and 6CHBT were combined with a mixture of RM257 monomer and a UV sensitive initiator with the percentage weight less than 10%. Due to the photo-polymerization process it was possible to achieve polymer-stabilized liquid crystal orientation inside LC cells and micro-sized cylindrical glass tubes. In particular, periodic change in spatial molecular orientation was achieved by selective photo-polymerization. Successful results obtained in these simple geometries allowed for the experimental procedure to be repeated in PCFs leading to locally-induced permanent birefringence in PCFs.
This paper deals with an issue of a rotational motion impact on a construction and presents civil engineering applications of a fiber optic rotational seismograph named Fiber-Optic System for Rotational Events & Phenomena Monitoring. It has been designed for a long- term building monitoring and structural rotations’ recording. It is based on the Sagnac effect which enables to detect one-axis rotational motion in a direct way and without any reference system. It enables to detect a rotation component in the wide range of a signal amplitude from 10-8 rad/s to 10 rad/s, as well as a frequency from DC to 1000 Hz. Data presented in this paper show the behavior of a reinforced concrete frame construction on different floors. Several measurements were carried out by placing the applied sensor on different floor levels of a building. The laboratory and in-situ measurements confirmed that Fiber-Optic System for Rotational Events & Phenomena Monitoring is an accurate and suitable device for applications in civil engineering.
An interferometric structure based on a Dual-Resonance Long-Period Grating (DRLPG) within a Fiber Loop Mirror (FLM) is presented in this paper. Its purpose is to measure the refractive index (RI) of liquid analytes. The grating is the RI sensing probe, while the FLM serves as a band-pass filter. Due to the high extinction ratio of the FLM, amplitude measurements can be obtained, allowing implementation of the differential interrogation method to establish the sensitivity of the device. The use of a polarization controller makes it possible to fine-tune the interferometric peaks with respect to the two notches of the DRLPG. Precisely aligned configuration produces a maximum sensitivity of 3871.5 dB/RIU within the RI range of 1.3333 up to 1.3419 with linear sensor response.