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.

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.

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.

A method is proposed for predicting the evolution of the power distribution along graded-index plastic optical fibers. This method is verified against our recently reported measurements. The strong influence of mode coupling on the power distribution is demonstrated on a specific graded-index plastic fiber to illustrate the influence of mode coupling in applications, such as data transmission, power delivery, and sensing systems.

This paper presents data on wafer-fused 1550-nm vertical-cavity surface-emitting lasers (VCSELs) based on the active region and distributed Bragg reflectors (DBRs) grown by molecular beam epitaxy. VCSELs with a tunnel junction aperture diameter of 8 μm show lasing at a threshold current density j_th <; 3 kA/cm², an output optical power of ~4 mW, and a -3 dB bandwidth of approximately 7 GHz at a 10-mA bias current. The devices demonstrate single-mode continuous wave operation with the transverse side-mode suppression ratio (SMSR) varying in the range of 40-45 dB up to roll-over currents. The increase in mirror losses due to the etching of the top DBR makes the output optical power increase to 6 mW and causes the wallplug efficiency value to reach 20%, but SMSR remains in the range of 40-45 dB. This also makes it possible to reduce both the photon lifetime and, as a result, the effect of damping and increase the modulation bandwidth to 9 GHz. The observed open and clear eye diagrams indicate that non-return-to-zero operation is possible at bit rates of up to 30 Gbps without equalization or forward error correction. The high-output optical power and modulation performance pave the way for the dense wavelength division multiplexing application of wafer-fused 1550-nm VCSELs.

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.