ABSTRACT
Transmitter pre-emphasis is one of the techniques proposed to improve the relatively poor frequency responses of light-emitting diodes (LEDs) used in visible light communications (VLC) as light sources. However, according to the communications theory in linear channels, pre-emphasis may only be as good as the not particularly efficient feed-forward equalizer (FFE). Given the increase in peak-to-average power ratio (PAPR) caused by pre-emphasis, its use in place of receiver equalization can be considered counterproductive. In this study, we show that the performance of the transmitter pre-emphasis is indeed equivalent to the FFE at low input powers, where the LED is a linear system. However, at higher modulation voltages, the transmitter pre-emphasis has a clear advantage over FFE, as it mitigates the nonlinear distortion of the LED. This mechanism is for the first time to our knowledge explained in both theory and experiment.
ABSTRACT
In this article, a cost-effective and fast interrogating system for wide temperature measurement with Fiber Bragg Gratings is presented. The system consists of a Vertical Cavity Surface Emitting Laser (VCSEL) with a High Contrast Grating (HCG)-based cavity that allows for the fast tuning of the output wavelength. The work focuses on methods of bypassing the limitations of the used VCSEL laser, especially its relatively narrow tuning range. Moreover, an error analysis is provided by means of the VCSEL temperature instability and its influence on the system performance. A simple proof of concept of the measurement system is shown, where two femtosecond Bragg gratings were used to measure temperature in the range of 25 to 800 °C. In addition, an exemplary simulation of a system with sapphire Bragg gratings is provided, where we propose multiplexation in the wavelength and reflectance domains. The presented concept can be further used to measure a wide range of temperatures with scanning frequencies up to hundreds of kHz.
ABSTRACT
In this Letter, we propose the application of a Volterra predistorter to compensate for the second-order nonlinear distortion generated in light-emitting diode (LED) communication systems. We demonstrate that a predistorter dedicated specifically to the LED has linear (not quadratic) numerical processing complexity. Experimental investigation performed using pulse amplitude modulation (PAM-4) indicates that it achieves similar performance as the classical Volterra equalizer located at the receiver. The highest measured gain in electrical signal to noise and interference power ratio of predistortion over linear equalization was at 300 Mbaud and was approximately 3.5 dB. At the same baud rate, the unpredistorted signal had a received optical power penalty of up to 2.5 dBm compared with the predistorted signal.
ABSTRACT
This work presents a numerical spectral properities analysis of a chirped-Bragg-grating-based Fabry-Perot (F-P CTFBG) cavity written in tapered fiber together with its application for strain monitoring. The work focuses on analyzing the structure's sensing performance and spectral response for codirectionally and counter-directionally written reflectors for various manufacturing process parameters (reflector lengths, phase mask chirp ratios, and positions on the linear transition of tapered fiber). In turn, it is shown that by manipulating the Bragg wavelength distribution of the cavity's reflectors, it is possible to control strain sensitivity character (i.e., positive or negative). The discussion also focuses on signal processing of the acquired spectrum through analytical derivation of the digital filter parameters that allows for unambiguous extraction of the cavity length for a given axial force applied to the each sensor irrespectively. Finally, a sensing system consisting of two cavities with either co-directionally or counter-directionally written reflectors is discussed.
ABSTRACT
In this Letter, a novel, to the best of our knowledge, Fabry-Perot cavity, based on Bragg grating technology for temperature and strain monitoring, is presented. Such a structure consists of two linearly chirped fiber Bragg gratings of a significant length written in a thermally tapered optical fiber. The technological process for manufacturing such a grating allows for utilization of almost every tapered fiber, by means of its profile and also phase masks with various chirp ratios. For this type of structure, a method for strain discrimination based on monitoring of the cavity length is proposed, enabling potential multiplexation of the sensor of two structures, which have the similar reflection spectra, by means of their spectral position. The utilized sensing mechanism allowed for achieving strain sensitivity by means of the cavity length change as high as 5 µm/µÉ. Also, as it has been experimentally shown a structure can also be employed for measurements of temperature, with the sensitivity equal to 8.96 pm/°C.
