Symmetric gradient-index media reconstruction.

Ray tracing in gradient-index (GRIN) media has been thoroughly studied and several ray tracing methods have been proposed. Methods are based on finding the ray path given a known GRIN. In recent decades, the inverse problem, which consists of finding the GRIN distribution for a given light ray path, has been gaining attention. Given that it is not an easy task, the methods proposed in the literature vary in degrees of difficulty. In this work, an alternative method is presented to derive symmetric GRIN distributions whose implementation can be considered the simplest to date. Since it is based on invariants, which result from the symmetries of the system as stated by Fermat's principle, it is an exact numerical method, i.e., the physical system is not approximated. The robustness of the method permits the reconstruction of the GRIN distribution from a ray propagating in three-dimensions. In order to demonstrate its operation, different known symmetric GRIN media are reconstructed using rays that propagate in two and three dimensions.

Experimental evolution of the temporal and spectral profiles of noise-like pulses within the mode-locked regions of a figure-eight fiber laser.

We present an experimental analysis of the pulse profile variability within the mode-locked regions of an erbium-doped figure-eight fiber laser (EDFEFL). The tuning of the mode-locked regions was carried out by varying and recording the values of the angle of the polarization controllers in the ring section and in the nonlinear optical loop mirror (NOLM). Within the mode-locked regions, we obtained a large variability of the temporal profile, specifically amplitude and width of the noise-like pulses (NLPs). Subsequently, we recorded and studied the changes in the spectral domain. We identified the mode-locked regions where the temporal profile of the pulse remains constant (stationary state), and where it expels sub-packets (non-stationary state). Finally, a theoretical analysis of the power transmission through the polarizing in the ring section and in the NOLM switching characteristic as a function of wave plate angles is also performed, which allows an understanding of the existence of the multiple mode-locked regions and pulse profile adjustability. We analyze NLPs with a carrier wavelength of 1560 nm with duration of the order of nanoseconds and a repetition rate of 0.9 MHz.

Linear multimode interference fiber temperature sensor using the liquid in glass thermometer principle.

A temperature sensor based on a multimode interference thermometer is designed and fabricated. The operation mechanism is based on the thermal expansion of a specific volume of ethylene glycol contained in a glass bulb that is connected to a capillary of the same material, with a no-core fiber (NCF) inserted and centered into the capillary tube. As the temperature is increased, the liquid is expanded, and the NCF is gradually covered by the liquid, resulting in a peak wavelength shift that is correlated to the temperature variations. A sensitivity of 0.4447 nm/°C and highly linear response with an R2 of 0.99962 are obtained. The advantage of this configuration is that the sensing temperature range can be adjusted by changing either the inner diameter of the capillary tube or the bulb volume. We can also measure negative temperatures by simply modifying the freezing point of the liquid, which demonstrates the viability of the sensor for many applications.