**6.4 WDM transmission employing NRZ modulation and FFE equalization**

This subchapter shows the improved experimental setup and gives the measurement results of the simultaneous four-channel NRZ transmission over 50 m SI-POF. To mitigate the effects of ISI, the FFE equalization was implemented at the receiver side. The experimental setup is shown in **Figure 16**. It comprised a fourchannel Agilent M8190A arbitrary waveform generator (AWG), four butt-coupled edge-emitting laser diodes, four-legged multiplexing POF bundle, SI-POF link of two different lengths, interference-based POF demultiplexer, Graviton SPD-2 receiver, and four-channel Agilent DSA91604A real-time oscilloscope with built-in software for digital signal processing. A photo of the general setup for investigating four-channel high-speed POF WDM transmission is shown in **Figure 17**.

To multiplex the signals from four laser diodes onto the SI-POF link, a fourlegged POF bundle was used. A multiplexing interface is formed by positioning the fiber bundle against 1 mm SI-POF.


**Table 6.**

*Transmission parameters for 10 m SI-POF link at an aggregate bit rate of 5 Gb/s.*

**Figure 16.**

*Experimental setup for the measurements employing NRZ modulation and offline-processed FFE: ATT, attenuator; AMP, amplifier; DC, direct current.*

#### **Figure 17.**

*Experimental setup for investigating four-channel high-speed POF WDM transmission: (1) Agilent M8190A AWG; (2) attenuator and MERA-556+ wideband amplifier; (3) bias tee; (4) Thorlabs TCLDM temperaturecontrolled laser diode mount, (5) Thorlabs ITC8022 module; (6) Thorlabs PRO8000 modular chassis; (7) fourlegged multiplexing POF bundle; (8) SI-POF link; (9) four-channel interference filter-based demultiplexer; (10) graviton SPD-2 receiver; (11) Agilent DSA91604A real-time oscilloscope; (12) Melles Griot universal optical power meter with 13 PDH 005 integrating sphere; (13) Thorlabs PM100D power meter with S140C integrating sphere.*

For realization of the bundle, an Asahi KASEI DB-400 PMMA SI-POF with 400 μm cladding diameter and NA = 0.5 was used. Four 60-cm-long fibers were terminated at the input side with 400 μm FC connectors. The opposite ends of the fibers were joined together and glued inside 970 μm FC connector to form the fiber bundle (**Figure 18**). As illustrated in **Figure 18**, an FC connector-mating sleeve was used to bring together and align the bundle and the input of the SI-POF link, thereby forming the multiplexing interface. An index-matching gel was applied between the connectors to reduce the losses.

*Optoelectronic Key Elements for Polymeric Fiber Transmission Systems DOI: http://dx.doi.org/10.5772/intechopen.86423*

#### **Figure 18.**

*Four-legged multiplexing POF bundle: (a) cross sections of four 400 μm cladding diameter fibers arranged within a circle with 970 μm diameter (left) and of 980/1000 μm SI-POF (right); (b) principle of operation of the POF bundle as a multiplexer; (c) four 400 μm fibers glued within 970 μm FC connector; (d) formation of a multiplexing interface with the POF bundle aligned against the SI-POF link using an FC connector-mating sleeve.*

#### **Figure 19.**

*Eye diagrams for 50 m SI-POF link at an aggregate bit rate of 7.8 Gb/s.*

The described multiplexing solution was first shown in [29]. Shortly before, the patent application for an optical POF multiplexer based on a multi-legged POF bundle, which referred to arbitrary channel counts and fiber diameters, was submitted to the German Patent and Trade Mark Office (DPMA) under number DE 102013 020236.1. A similar approach was later adopted in [30, 31] to realize the low loss seven-legged and three-legged multiplexers, respectively.

The AWG simultaneously generated four independent NRZ data streams (**Figures 16** and **19**) based on 27-1 PRBS with the maximum sampling rate.


#### **Table 7.**

*Optical power loss at the transmitter side when using four-legged POF bundle.*

The received electrical signals were acquired by the real-time oscilloscope with 8-bit vertical resolution and oversampling. The digital receiver equalization was carried out in the offline mode. For that purpose the oscilloscope's built-in Serial Data Equalization software was used [32].

To prevent the equalizer from amplifying the noise components at higher frequencies where the energy content of useful signal was low, the bandwidth of the oscilloscope was set to the value in GHz corresponding to one half of the data rate in Gb/s. A phase-locked loop was used to extract the clock from the equalized data.

The eye diagram of an equalized waveform was displayed on the oscilloscope's screen for further analysis. The oscilloscope's built-in software EZJIT Complete was used to estimate the corresponding Q-factor [33]. Thereby, only a small time window (2% of the unit interval) in the middle of the equalized eye diagram was taken into consideration. The BER value was then calculated using Eq. (5). **Table 7** shows the optical power losses of the four used WDM channels.
