GI-POF Transmission At Different Bit Rates, Fiber Lengths and Wavelengths: 1310 and 1550 Nm

The paper demonstrates the transmission of variable bit rates NRZ streams over different lengths of perfluorinated graded-index plastic optical fiber GI-POF at wavelengths of 1550 and 1310 nm. Specifically, at <inline-formula><tex-math notation="LaTeX">$\lambda$</tex-math></inline-formula> = 1550 nm, 16, 14 and 12 Gbps of NRZ transmissions are demonstrated for 50 m GI-POF. Furthermore, at <inline-formula><tex-math notation="LaTeX">$\lambda$</tex-math></inline-formula> = 1310 nm, 10 and 8 Gbps NRZ transmissions are demonstrated for distances of 50, 100 and 200 m. The performance is evaluated for realized transmissions in terms of received bit-error rate versus transmitted optical power and is compared to back to back performance.

number of propagating optical modes, high inter-modal coupling and high inter-modal dispersion.Generally, step-index POF (SI-POF) is manufactured from polymer of methyl methacrylate PMMA, which has an attenuation much higher than Silica glass (i.e., 90-200 dB/km), with several minima in the visible region (520-670 nm) [5].Perfluorinated graded index (GI-POF) has emerged as a potential alternative capable to reduce the attenuation of SI-POFs, as indicated in Fig. 1 [6].This POF has a relatively small attenuation at the O transmission band, i.e., 60 dB/km in the 1290 and 1320 nm window.However, it has large attenuation i.e., > 200 dB/km in S, C, and L bands, which in turn limits the use of the existing dense and coarse wavelength division multiplexing (DWDM and CWDM, respectively) transceivers in POF communications.
GI-POFs have much lower dispersion than standard SI-POFs [7]- [9].Through the inter-playing between high mode coupling, low material dispersion, and differential mode attenuation by controlling the refractive-index, the GI-POFs reach higher capacity than their POF counterparts.The used GI-POF has 62.5 µm core diameter, 490 µm cladding diameter and 0.014 refractive index difference between the center of the core and the cladding [6].However, the intrinsic polymer nature of the POF along with its large attenuation, limits the maximum input optical power to a few tens of milliWatts.
Laser-based POF systems provide high capacity and offer a much better performance than that of LED-based ones.Recently, many laser transmission experiments were carried to compensate the linear and nonlinear transmission impairments of POF and to demonstrate the increase of its transmission capacities and distances.Among these experiments, four-channel wavelength division multiplexing (WDM) transmission over 50 m SI-POF at 14.77 Gb/s and bit error rate (BER) of 10 −3 using discrete multitone DMT modulation was demonstrated in [10].Also, a 4 × 1 coupler with low insertion loss (IL) was developed to combine the signals of 405, 450, 515 and 639 nm laser diodes.In [11], a capacity of 21.4 Gb/s over 50 m link based on SI-POF employing WDM with six channels (405, 442, 459, 490, 515 and 655) was demonstrated.Also, a record of 10.7 Gb/s WDM data transmission over 100 m SI-POF employing offline-processed DMT modulation was demonstrated in [12].
Inter-modal dispersion significantly limits the transmission capacity of POF-based systems.Therefore, adapting digital equalization presents appealing solution.In [13], a 10 Gb/s transmission over 10 m SI-POF with M-PAM and multi-layer perceptron (MLP) equalizer was demonstrated.The equalizer was used to mitigate an inter-symbol interference and non-linearity in the system.Also, 1.7 Gbps visible light transmission over 100 m POF employing a 638 nm laser and MLP post-equalizer is demonstrated in [14].Furthermore, a visible WDM system for real-time multi-Gb/s bidirectional transmission over 50 m SI-POF is demonstrated in [15].
For large core PMMA GI-POFs, 1 Gbps NRZ data transmission is achieved for 75 m with a BER below 10 −6 [9].Recently, POF was widely used to carry optical signals for optical wireless communications.A 5.2 Gbps transmission over two spatial multiplexed channels at 520 nm and 658 nm using passive optical front end and WDM-over-POF is demonstrated in [16].In [17], a transmission link comprising wavelength division multiplexing over 1 mm core size SI-POF is demonstrated for indoor optical wireless communication.A throughput of more than 2 Gbps has been achieved using 2 wavelengths and DMT modulation.
In this paper, we demonstrate the transmission of high data rates up to 16 Gbps at 1550 nm (in C band) for 50 m GI-POF.Also, we demonstrate the transmission of 10 and 8 Gbps for variable transmission distances of GI-POF (50 m, 100 m and 200 m) at 1310 nm (in O band).The paper is organized as follow: Section II describes the experimental setup for 50 m GI-POF transmission at 1550 nm.Section III presents the results and discussion.Section IV describes the experimental setup for GI-POF transmission at 1310 nm with variable transmission distances.Section V presents the transmission results at 1310 nm.In Section VI, we present the main conclusions.

II. EXPERIMENTAL SETUP FOR 1550 NM TRANSMISSION
The block diagram of the experimental setup used for NRZ transmission at 1550 nm for 50 m GI-POF is indicated in Fig. 2. At the transmitter side, a distributed feedback laser diode (DFB-LD) with λ = 1550.12nm and maximum output power of 12 dBm is used.The transmitted data is generated using bit pattern generator (BPG SHF12100a) in NRZ format using pseudo random bit streams (PRBS) with length of 2 15 − 1 .The transmission data rate is controlled using an external clock generated by a swept frequency synthesizer.The polarization of the laser beam is controlled using a manual polarization controller to get the optimal polarization for the optical modulator.The output of the PC is connected to a Mach-Zehnder intensity modulator (MZM) contained in 50 Gbps optical transmitter (SHF46210).The RF signal of BPG is connected to the MZM to modulate the optical signal.The modulated optical signal is then boosted using an erbium doped fiber amplifier (EDFA), to compensate the large POF attenuation.The amplified optical signal is filtered using an add-drop multiplexer with 100 GHz bandwidth and 1550.12nm center wavelength, to remove the ASE noise generated in EDFA.To control the optical power level, the optical signal is attenuated using variable optical attenuator VOA.The output of VOA is connected to a 1:99 optical coupler to measure the launched optical signal power.The 99% coupler output is a single mode transmission using standard single mode glass optical fiber GOF with 9/125 µm diameters.This output is directly connected to a 50m 62.5 µm core GI-POF using normal FC-FC fiber optics adaptor (connects two FC connectors).This adaptor introduces a low insertion loss less than 0.3dB.The used GI-POF is GigaPOF-62SR, produced by Chromis Fiberoptics, Inc.
At the receiver side, the output of the GI-POF is connected to 25 GHz multimode PIN photo-diode (PD) with trans-impedance amplifier (TIA) produced by VITEX.The receiver module provides two differential RF outputs.One output is connected to a digital sample oscilloscope (Tektronix DSA8200) to display Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.The performance of the back to back (B2B) NRZ transmission at λ = 1550.12nm is indicated in Fig. 3.The performance is evaluated in terms of the received BER versus the transmitted optical power for three different transmission rates: 12, 14 and 16 Gbps.Clearly, at the same level of input optical power, increasing the transmission rate increases the received BER.Also, the BER gap between 12 and 14Gbps transmissions is larger than the one between the 14 and 16 Gbps transmissions, due to the limited bandwidth of the used PIN photo-detector (25 GHz).
The transmission performance for the 50 m GI-POF is indicated in Fig. 4. The performance is evaluated at transmission rates of 12, 14 and 16 Gbps.From this figure, the required input power levels to achieve a BER of 10 −6 at transmission rates of 12, 14 and 16 Gbps are −0.5, 0.6 and 1 dBm, respectively.These relatively high input power levels are required to compensate for the large POF attenuation at λ = 1550.12nm.Moreover, as the POF has a much lower maximum input power (less than 15 dBm due to its polymer nature) compared to that of the glass fiber, transmissions at 100 m or longer distances can't be achieved at λ = 1550.12nm.
Another drawback and limiting transmission factor of POF is the large number of propagating optical modes.In the tested fiber, the propagating optical signal is guided by more than 140 optical modes.This introduces two linear impairments which are inter-modal dispersion and mode coupling [18].Consequently, at the transmission rate of 12 Gbps, the same BER value is achieved at a much lower transmitted optical power level than that of the ones required for 14 and 16 Gbps transmissions.Precisely, at BER of 10 −6 , the 12 Gbps transmission requires less launching power of 1.1 and 1.5 dBm than that of the ones required for 14 and 16 Gbps transmissions, respectively.Obviously, decreasing the bit duration will increase the impact of inter-modal dispersion on the detection process.
Generally, random mode coupling occurs due to cable vibrations and random cable bends.This causes random variations in the received optical signal which increases the BER levels.However, the experiment is carried without deploying an equalizer at the receiver side and the BER is evaluated in real time basis.Although, this simplifies the receiver architecture, it limits the transmission data rate to maximum reliable value (16 Gbps in our case) due to the excessive inter-modal dispersion and inter-modal coupling.Increasing the transmission rate above this value will lead to unreliable communications.In other words, the effect of inter-modal dispersion will have large impacts with shorter bit durations in unequalized transmissions.

IV. EXPERIMENTAL SETUP FOR 1310 NM TRANSMISSION
The experimental setup for variable bit rate NRZ transmission at 1310 nm for different lengths of GI-POF is indicated in Fig. 5.At the transmitter side, the clock generator is connected to BPG to control the transmission rate.The RF output of the BPG is amplified using a 12.5 Gbps RF amplifier (Picosecond Pulse Labs Model 5865) in order to match the input requirements of the optical modulator.The amplified RF signal is connected to a 10 Gbps modulator integrated distributed feedback (DFB) laser diode module (Eudyna FLD3F10NP).This module has an electro-absorption modulator monolithically integrated with a conventional DFB laser.The output of the optical modulator is amplified using semiconductor optical amplifier (SOA QLightÂ SAO29p) to compensate for the large attenuation in POF transmission.The SOA amplifies signals at wavelengths not accessible to commercial EDFA.The output of the SOA is connected to a VOA to control the amount of transmitted optical power.Also, a 1:99 coupler is used to monitor the amount of the launched optical power to the POF cable.The optical signal generated from laser source and reach the optical coupler is propagating through single mode GOF.The 99% output of the optical coupler is connected to the GI-POF through FC-FC adapater which introduces an insertion loss less than 0.3dB.
At the receiver side, the same setup used for 1550 nm transmission is implemented for 1310 nm transmission.In other words, the multimode PIN PD with TIA receiver, digital sampling oscilloscope and SHF11100A error analyzer are used to receive the 1310 nm transmission.The B2B NRZ transmission performance at λ = 1310 nm is evaluated in terms of received BER versus launched optical power as indicated in Fig. 6.The evaluations are carried for 8 and 10 Gbps transmission rates due to the bandwidth limitations of electro-absorption modulator and RF amplifier.Clearly, the BER level is slightly increased by increasing the transmission rate from 8 to 10 Gbps due to the frequency response and bandwidth of the used PIN PD.Also, the BER gap is slightly increased at higher levels of input optical power.
The BER performances at 8 and 10 Gbps NRZ transmissions for 50, 100, and 200 m are indicated in Figs. 7, 8, and 9, respectively.GI-POF has lower attenuation (60 dB/km) at λ = 1310 nm compared to the one at λ = 1550 nm (220 dB/km).This enables longer transmission distances at λ = 1310 nm.Furthermore, short transmission distances require a small amount of optical power to realize efficient communications.Particularly, as indicated in Fig. 7, at λ = 1310 nm with transmission distance of 50 m, input powers of −9.1 and −8.8 dBm are needed to obtain BER of 10 −8 for transmission rates of 8 and 10 Gbps, respectively.On the other hand, as indicated in Fig. 9, increasing the transmission distance to 200m requires launching optical power of 2.5 and 3 dBm to achieve BER of 10 −8 for bit rates of 8 and 10 Gbps, respectively.Generally, at the same transmission distance and same launching optical power, increasing the transmission rate degrades the performance.Clearly, as indicated in Fig. 7 at transmission distance of 50 m and launching power of −10.3 dBm, increasing the transmission rate from 8 to 10 Gbps increases the BER from 10 −5 to 5 × 10 −4 .The reasons behind that are decreasing the PD performance with higher frequencies along with increasing the impact of inter-modal dispersion with lower bit durations.
The BER gap between 8 and 10 Gbps transmissions is increased by increasing the transmission distance.This is due to the intensive increase of the inter-modal dispersion in long Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.transmission distances [19].At transmission distance of 50m, the BER gab between 8 and 10 Gbps transmissions is about one third order of magnitude while for 200m transmission is about an order of magnitude.Also, this performance gap is observed in the eye diagrams for distances of 50 and 100m as indicated in Fig. 10.At both 10 and 8 Gbps, the eye amplitudes for 50m transmission are larger than that of 100m transmission due to the increased attenuation.Moreover, increasing the transmission distance decreases the eye width due to the intensive increase of inter-modal dispersion as indicated in Fig. 10(a) and (c) for 8 Gbps transmission.Furthermore, the figure shows excess optical noise in transmitted one bits "high optical power" due to the generated ASE noise in the SOA.

VI. CONCLUSION
In this paper, 16 Gbps NRZ transmission is demonstrated for 50 m GI-POF at a wavelength of 1550 nm.However, increasing transmission rate above this value requires the implementation of equalization techniques to overcome strong inter-modal dispersion and mode coupling.Also, the paper demonstrates the transmission of 10 Gbps for 200 m GI-POF at wavelength of 1310 nm.The results reveal that much lower launched optical power levels are required for the transmission at 1310 nm compared to the ones needed at 1550 nm.Also, as a future work, coarse wavelength division multiplexing CWDM could be realized in both S and C bands for 50 m POF transmission to significantly increase its capacity.In this case, the maximum number of deployed CWDM channels depends on the maximum input power of the used POF.Our results indicate that 8 CWDM channels could be safely realized in 50 m POF transmission with a total transmission capacity of 108 Gbps.

Fig. 5 .
Fig. 5. Block diagram of the system setup used to demonstrate the NRZ transmission at λ = 1310 nm for GI-POF with 62.5 µm core diameter.In the diagram, we see several building blocks, among which: semiconductor optical amplifier (SOA), variable optical attenuator (VOA), optical coupler (OC) with splitting ratio of 1:99.