Grzegorz Stepniak

Binary-Phase Spatial Light Filters for Mode-Selective Excitation of Multimode Fibers

In this paper, spatial light modulation is proposed to increase the transmission capacity of graded-index multimode fibers. In the method, selected linearly polarized eigenmodes of the fiber are excited using simple binary-phase spatial filters. Numerical results indicate that the selectivity of the method is very high, also for fibers with perturbed profiles. The excess attenuation of the method is very low. In the experiment, a threefold increase of fiber bandwidth for several filters is obtained.

Compensation of a VLC Phosphorescent White LED Nonlinearity by Means of Volterra DFE

In this letter, a visible light communication link is analyzed that employs a white phosphorescent light emitting diode (LED) as transmitter and a p-i-n photodiode combined with optical filter as receiver. Four- and eight-level pulse amplitude modulation is used for data transmission. The link is not only limited by a rather small bandwidth of ~20 MHz of the LED, but also its nonlinearity becomes a problem when the modulation index exceeds ~60%. It is shown that a receiver with a decision feedback equalizer (DFE) with nonlinear Volterra feed-forward section up to the second order can efficiently compensate effects of nonlinearity of the transmitting LED and performs up to 5 dB better in terms of optical power than using a standard DFE.

Instability of the Multimode Fiber Frequency Response Beyond the Baseband for Coherent Sources

Fluctuations of the multimode fiber (MMF) frequency response beyond the baseband are observed experimentally when highly coherent light sources are used. This behavior is caused by the variation of the mode group powers caused by the relative modal phase fluctuation combined with spatial filtering at elements such as nonideal connectors. The observed effect may have vital influence on subcarrier multiplexing systems operation outside the baseband of the MMF. The effect is investigated both theoretically and experimentally.

Advanced Modulation Formats in Phosphorous LED VLC Links and the Impact of Blue Filtering

A theoretical model for transmission of advanced modulation schemes [pulse amplitude modulation (PAM), carrierless amplitude-phase (CAP), and discrete multitone (DMT)] in visible light communications (VLC) link using white phosphorescent LEDs is presented. The model extends and adapts the Randel et al., framework for advanced modulation format transmission in intensity-modulated direct-detection for the VLC channel. The main contributions are: CAP modulation, effect of transmit signal spectral shaping in PAM and CAP, analytical calculation of crest factor for CAP, and analysis of inter-symbol and cross-carrier interference due to an insufficient cyclic-prefix in DMT. Based on this model, the impact of blue filtering at the receiver is studied in the context of spectrally efficient modulation formats. The results indicate that blue filtering has a marginal effect on the performance for all the investigated modulation formats, while PAM with decision feedback equalizer shows the best performance. Without blue filtering, slightly higher data rates can be achieved, but at a price of extensive signal processing.

Single-Mode 850-nm VCSELs for 54-Gb/s ON–OFF Keying Transmission Over 1-km Multi-Mode Fiber

By combing Zn-diffusion and oxide-relief apertures with strong detuning (>20 nm) in our demonstrated short-cavity (λ/2) 850-nm vertical-cavity surface-emitting lasers (VCSELs), wide electrical-to-optical bandwidth (29-24 GHz), low-differential resistance (~100 Q), and (quasi) single-mode (SM) with reasonable output power (~1.4 mW) performances can be simultaneously achieved. Error-free ON-OFF keying transmission at 54-Gb/s data rate through 1-km OM4 multi-mode fiber can be achieved by using highly SM device with forward error correction and decision feedback equalization techniques. As compared with the reference device with a larger oxide-relief aperture and a multi-mode performance, the SM device exhibits lower bit-error rate (1 × 10-5 versus 1 × 10-2) at 54 Gb/s. This result indicates that modal dispersion plays more important role in transmission than that of output power does. We benchmark these results to an industrial 50-Gb/s SM VCSEL. It shows a higher bit-error-rate value ~3.5×10-3 versus ~1.4×10-4 under the same received optical power.

Comparison of Efficiency of N-Dimensional CAP Modulations

There is a growing interest in applying carrierless amplitude-phase (CAP) modulation in intensity modulated optical links. CAP as a multilevel modulation scheme can reach high spectral efficiency and be generated at low cost. An interesting feature of CAP is that its signal basis can be extended to three or more dimensions (D). This paper aims to complete the knowledge on 3-D CAP. Discussed are design of transceiver filters, spectral efficiency, and digital equalization in 3-D. Several problems accompanying 3-D CAP transmission are identified. The most significant are the peak to average power ratio, which is higher than in one or two dimensions and noise correlation at the output of a 3-D equalizer. It is found that the latter problem severely degrades bit error rates achieved but can be efficiently mitigated using noise whitening equalization. Finally, 1-D, 2-D, and 3-D CAP are compared experimentally in a step-index polymer optical fiber link.

Increasing multimode fiber transmission capacity by mode selective spatial light phase modulation

We propose spatial light modulation with binary phase filters to increase the transmission bandwidth of multimode fibers. In the experiment more than 300 % of bandwidth enhancement is obtained.

Experimental Comparison of PAM, CAP, and DMT Modulations in Phosphorescent White LED Transmission Link

In the paper, an experimental comparison of pulse amplitude, carrierless amplitude-phase, and discrete multitone modulations is carried in a visible light communications link employing white phosphorescent light-emitting diode (LED) as a transmitter. By changing the modulation index, the influence of LED nonlinearity on the performance is studied. The results indicate similar performance of pulse-amplitude modulation (PAM) and carrier-less amplitude-phase (CAP) (with a slight advantage of the former) and substantially worse performance of DMT.

A 2 and 3 channel mode group diversity multiplexing transmission over graded and step index multimode fibers

In this paper, operation of a cost-effective mode group diversity multiplexing system allowing 3 channel operation is demonstrated. Two different multiplexing/demultiplexing techniques are presented.

Transmission Beyond 100 Mbit/s Using LED Both as a Transmitter and Receiver

In this letter, the experimental operation of a wireless transmission link employing identical LEDs both as transmitter and receiver is demonstrated. The transmitted signal is a simple, two-level pulse amplitude modulation. Digital equalization at the receiver is applied to overcome the bandwidth limitation of the link. Transmission throughput exceeding 100 Mbit/s is reported, even if the receiving red LED operates in the photovoltaic mode.