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.

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/µɛ. 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.

No 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.

<p>Correct modeling of light emitting diode (LED) nonlinearity is vital for evaluating the performance of advanced modulation formats and nonlinear distortion mitigation methods in visible light communication systems. In this paper, we review the existing simplified nonlinear LED models: static polynomial, Wiener and Hammerstein and provide strong experimental evidence against their even coarse correctness. Instead, we propose an evenly uncomplicated model of LED, which is based on rigorous factorization of the Volterra 2nd order kernel stemming from the recombination rate equations of the LED. We further experimentally validate this model in four different LEDs by: (i) finding that linear transmitter pre-emphasis, against the results of the communications theory in a linear, bandlimited channel, can at very high input powers improve the transmission perfromance compared to linear equalization at the receiver, which is explained in our model by a reduction of the nonlinear distortion by the pre-emphasis; (ii) by comparing the measured 2nd order quadratic kernels of LEDs in regular form and with the constraints of our model. Finally, we propose a two-parameter calibration method of our model, which requires measurement of the LED bandwidth and 2nd order harmonic distortion at a single frequency. Excellent convergence of the signal transmitted in the calibrated model to the rate equation model and to the measurement is observed.</p>

<p>Correct modeling of light emitting diode (LED) nonlinearity is vital for evaluating the performance of advanced modulation formats and nonlinear distortion mitigation methods in visible light communication systems. In this paper, we review the existing simplified nonlinear LED models: static polynomial, Wiener and Hammerstein and provide strong experimental evidence against their even coarse correctness. Instead, we propose an evenly uncomplicated model of LED, which is based on rigorous factorization of the Volterra 2nd order kernel stemming from the recombination rate equations of the LED. We further experimentally validate this model in four different LEDs by: (i) finding that linear transmitter pre-emphasis, against the results of the communications theory in a linear, bandlimited channel, can at very high input powers improve the transmission perfromance compared to linear equalization at the receiver, which is explained in our model by a reduction of the nonlinear distortion by the pre-emphasis; (ii) by comparing the measured 2nd order quadratic kernels of LEDs in regular form and with the constraints of our model. Finally, we propose a two-parameter calibration method of our model, which requires measurement of the LED bandwidth and 2nd order harmonic distortion at a single frequency. Excellent convergence of the signal transmitted in the calibrated model to the rate equation model and to the measurement is observed.</p>

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.

No abstract