We investigate the dependence of Brillouin gain spectra on large strain of > 20% in a perfluorinated graded-index polymer optical fiber, and prove, for the first time, that the dependence of Brillouin frequency shift (BFS) is highly non-monotonic. We predict that temperature sensors even with zero strain sensitivity can be implemented by use of this non-monotonic nature. Meanwhile, the Stokes power decreases rapidly when the applied strain is > ~10%. This behavior seems to originate from the propagation loss dependence on large strain. By exploiting the Stokes power dependence, we can probably solve the problem of how to identify the applied strain, when the identification is difficult only by BFS because of its non-monotonic nature.

We develop a new noise suppression technique to perform distributed strain and temperature sensing based on a higher-speed configuration of Brillouin optical correlation-domain reflectometry even when a polymer optical fiber (POF) is used as a sensing fiber. We acquire the spectral difference between with and without reference light, leading to selective observation of the beat signal of Brillouin-scattered light and reference light, which is effective for distributed sensing. After experimentally showing the usefulness of this technique, we demonstrate POF-based distributed temperature sensing and dynamic strain sensing.

The basic properties of a thermal memory effect of polymer optical fibers (POFs) are experimentally investigated. We measure the thermally induced loss as a function of time at several high temperatures, and find that the loss becomes almost constant after heating for ~200 s. The loss remains unchanged even after the heated section is cooled to room temperature. We subsequently measure the optical time-domain reflectometry traces under three different conditions: 1) before a POF section is heated; 2) shortly after the POF section is heated at high temperature; and 3) after the heated section is cooled to room temperature. The traces measured under 2) and 3) are moderately identical, which indicates that the thermal memory effect can be exploited in developing excess-heat detecting system in future.

Forward Brillouin scattering (FBS) has been regarded as a key tool for distributed sensing of strain, temperature, acoustic impedance, etc. To date, FBS-based distributed sensing has been experimentally reported only by time-domain techniques. In this paper, we demonstrate correlation-domain distributed temperature sensing using cascaded FBS seeded by backward stimulated Brillouin scattering (SBS). By localizing the backward SBS using synthesized optical coherence function, cascaded FBS can also be localized. In the proof-of-concept experiment, we detect a 104 m long heated section in a 397 m long highly nonlinear fiber.

We observe stimulated Brillouin scattering (SBS) in polymer optical fibers (POFs) using a pump-probe technique without lock-in detection, and fully investigate the dependences of the Brillouin gain spectrum on the POF length, pump power, probe power, and temperature. Since the POF has relatively high propagation loss of 250 dB/km at 1.55 μm, an optimal POF length exists for SBS observation, which is found to be approximately 3.8 m with 21.1-dBm pump and 22.2-dBm probe waves. As the probe and pump powers are increased, the Stokes power is also raised but nonlinearly. The temperature dependence of the Brillouin frequency shift is -4.02 MHz/K, which agrees well with the previous report. These results indicate that the Brillouin signal in POFs observed with this technique can be directly applied to the development of POF-based Brillouin optical time-domain analysis systems for high-precision temperature sensing.