Real-time PAM4 fiber-IVLLC and fiber-wireless hybrid system with a parallel/orthogonally polarized dual-wavelength scheme

Chung-Yi Li; Hai-Han Lu; Chung-Wei Su; Hwan-Wei Chen; Yong-Nian Chen; You-Ruei Wu; Chung-Wei Hung

doi:10.1364/OSAC.1.000320

1. Introduction

Broadband heterogeneous networks are expected to boost present and future technologies, such as virtual reality, augmented reality, broadband Internet, self-driving cars, high-definition audio-visual broadcasting systems, and 4G/5G mobile communications [1]. They can be attained through the incorporation of optical fiber communications and optical/RF wireless communications to implement fiber-invisible laser light communication (IVLLC) and fiber-wireless hybrid systems. The features of fiber-IVLLC and fiber-wireless hybrid systems are that they can utilize the benefits of optical fiber and optical/RF wireless technologies, such as the innately remarkable bandwidth of the optical fiber, the unlicensed part of electromagnetic spectrum of the optical wireless link, and the large and unused bandwidth of RF wireless transmission. By incorporating the large bandwidth of optical fiber backhaul with the mobility of optical/RF wireless extender, fiber-IVLLC and fiber-wireless hybrid systems will have a stronger platform to support developed and emerging applications [2,3]. In comparison with other transmission systems, fiber-IVLLC and fiber-wireless hybrid systems hold the merits of high transmission rate and sufficient flexibility.

Previous work demonstrated the achievability of constructing a fiber-wireless and fiber-IVLLC hybrid system with dual-polarization modulation scheme and Mach-Zehnder modulator (MZM)-optoelectronic oscillator-based broadband light source (BLS) [4]. However, this fiber-wireless and fiber-IVLLC hybrid system is not competitive because a complicated and sophisticated central office (CO), including one MZM-OEO-based BLS, two wavelength-dependent optical interleavers (OILs), three optical band-pass filters (OBPFs), and four MZMs, is required. Moreover, establishing a fiber-wireless and fiber-IVLLC hybrid system with polarization-orthogonal modulation scheme was shown attainable [5]. Nevertheless, a costly and elaborate polarization rotator (PR) at the transmitting side is required, and the distortions owing to the beating among multiple optical sidebands (y-polarization) degenerate the performance of the hybrid systems. Another work demonstrated the possibility of building a fiber-IVLLC and fiber-wireless hybrid system with two orthogonally polarized optical sidebands [6]. Nonetheless, a sophisticated injection locking technique at the transmitting side and an expensive PR at the receiving side are necessary. Other works showed the feasibility of setting up multi-service radio-over-fiber (RoF) links with a phase-coherent orthogonal lightwave generator [7,8]. Whereas the performance of RoF links are degenerated by distortions produced by the parallel polarized optical sidebands. In addition, a former work demonstrated a millimeter-wave (MMW) RoF link with a parallel/orthogonally polarized dual-wavelength injection-locked scheme for converging wireline and wireless transmissions [9]. Yet, an elaborate colorless laser diode (LD) and a sophisticated dual-wavelength injection-locked scheme at the CO are demanded. In addition, a polarization-division-multiplexing direct-detection optical orthogonal frequency-division multiplexing (OFDM) RoF system with self-polarization diversity mechanism was presented in other prior work [10]. Nonetheless, an expensive and elaborate Faraday rotator mirror (FRM) at the remote node is needed. Besides, the OFDM signal must be calculated offline by MATLAB for the analysis of bit error rate (BER) performance. This offline calculation increases the complication of systems.

For a practical consideration of fiber-IVLLC and fiber-wireless hybrid systems, low complexity and cost-effective characteristics, and satisfactory performance are the key concerns of systems. In this work, we propose and practically demonstrate a real-time four-level pulse amplitude modulation (PAM4) fiber-IVLLC and fiber-wireless hybrid system with parallel/orthogonally polarized dual-wavelength scheme over the 25-km single-mode fiber (SMF) transmission with 100-m free-space link/5-m RF wireless transmission. A low-cost real-time PAM4 BER measurement is operated to calculate the total BER. The measurement is attractive because it prevents the offline calculation by MATLAB. This proposed system does not require complicated and sophisticated CO [4], costly and elaborate PR [5], sophisticated injection locking technique [6], elaborate colorless LD and sophisticated dual-wavelength injection-locked scheme [9], and expensive and elaborate FRM [10]. Thus, it exhibits a noteworthy one with low complexity and cost-effective advantages.

PAM4 can reduce the bandwidth requirement for optical and electrical devices, which is suitable for high-speed transmissions. For a fixed bandwidth, PAM4 enables twice the transmission capacity in comparison with none-return-to-zero (NRZ) signal. Furthermore, the polarization domain has been investigated considerably for high-speed optical communications. Parallel/orthogonally polarized dual-wavelength scheme is highly compatible with the optical backhaul of fiber-IVLLC and fiber-wireless hybrid systems. It can split the optical signal into two branches and transform one of the branches into orthogonal polarization, resulting in two orthogonal polarizations. With the use of parallel/orthogonally polarized dual-wavelength scheme, the transmission capacity can be increased to two times. Moreover, if the transmission capacity of fiber-IVLLC convergence is equal to that of fiber-wireless integration, then the transmission capacity of fiber-IVLLC and fiber-wireless hybrid systems is two times faster than that of fiber-IVLLC convergence or fiber-wireless integration. Therefore, the throughput of the proposed PAM4 fiber-IVLLC and fiber-wireless hybrid systems with parallel/orthogonally polarized dual-wavelength scheme can be totally enhanced by increasing the transmission capacity to eight times [2 (due to PAM4 modulation) × 2 (due to parallel/orthogonally polarized dual-wavelength scheme) × 2 (due to fiber-IVLLC and fiber-wireless hybrid systems) = 8]. Our proposed system shows a promising alternative not only because of its development in the incorporation of fiber-based backhaul and optical/RF wireless-based extender, but also because of its benefit in the communication link for providing high transmission capacities.

2. Experimental setup

Figure 1

Fig. 1 The architecture of offered real-time PAM4 fiber-IVLLC and fiber-wireless hybrid systems with parallel/orthogonally polarized dual-wavelength scheme.

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presents the architecture of offered real-time PAM4 fiber-IVLLC and fiber-wireless hybrid systems with parallel/orthogonally polarized dual-wavelength scheme. A multiple wavelengths generator modulated with 15 GHz RF signal is utilized to obtain multiple wavelengths with coherent characteristics [insert (a)]. The output of multiple wavelengths generator is supplied to a 15G/30G OIL to split odd and even wavelengths. For OIL output with even wavelengths, two optical wavelengths spaced by 60 GHz [insert (b)] are inputted into a MZM with 40 GHz. The MZM is driven by a 45 Gb/s PAM4 signal with a pseudorandom binary sequence (PRBS) length of 2¹⁵-1 (PRBS 15). Given that PAM4 linearity is an important parameter, a modulator driver with high linearity is used to drive the PAM4 electrical signal with a driving signal swing of 1.8 Vpp. The output of the MZM is split into two branches along the two orthogonal polarization states (x-polarization and y-polarization) by using a polarization beam splitter (PBS). The x-polarization [insert (c)] and y-polarization [inset (d)] lights are then recombined by a polarization beam combiner (PBC) [insert (e)]. An optical delay line is employed in the lower path to compensate for the phase mismatch between two paths. The lights are boosted by an erbium-doped fiber amplifier (EDFA) and controlled by a variable optical attenuator (VOA) at the beginning of the 25 km SMF. Over a span of 25-km SMF, the lights travel through an OBPF to filter out the outer distortions. Subsequently, the lights are separated by a polarization controller (PC) and a PBS. The PC is employed to adjust the state of polarization. For an actual realization, an automated PC with feedback function can be adopted to maintain the state of polarization [11].

The x-polarization light is split using a 1 × 2 optical splitter. Two wavelengths spaced by 60 GHz generate a 45 Gb/s/60 GHz PAM4 MMW signal [inset (f)]. For upper path, the upper wavelength is selected by an optical circulator (OC) combined with a fiber Bragg grating (FBG, λ_c = 1540.84 nm) [inset (g)], and then inputted into a 100-m free-space link with a pair of doublet lenses. A doublet lens comprises a concave lens and a convex lens. The light sent out from the ferrule of SMF (transmitting side) is fed into an IVLLC subsystem, and concentrated on the ferrule of SMF (receiving side). At the receiving side, the optical signal is received by an x-polarized receiver. Afterwards, the detected and boosted 45 Gb/s PAM4 signal is equalized electrically by an equalizer. The performance of delivered 45 Gb/s PAM4 signal is calculated and analyzed by BER values and eye diagrams in real-time. BER measurement is calculated through an automatic search that adopts one-channel error detector (ED) and the PAM4 three-eye sampling way. Furthermore, a digital storage oscilloscope (DSO) is adopted to take the eye diagrams of 45 Gb/s PAM4 signal. For lower path, the optical signal is supplied to a 5-m RF wireless transmission with a couple of horn antenna (HA). The 45 Gb/s/60 GHz PAM4 MMW signal [inset (f)] is detected in x-polarization by a single-end photodiode (PD), amplified by a power amplifier (PA) with frequency range of 58-64 GHz, and wirelessly transmitted by a HA working at V-band (40-75 GHz). Over a 5-m RF wireless transmission, the 45 Gb/s/60 GHz PAM4 MMW signal is received by a HA, boosted by a 60-GHz low-noise amplifier (LNA) with a noise figure of approximately 4 dB, and down-converted by an envelope detector with a frequency range of 1-10 GHz. After electrical equalization by an equalizer, the performances of transmitted 45 Gb/s PAM4 signal are calculated and analyzed in real-time in view of BER values and eye diagrams. Furthermore, a DSO is utilized to capture the eye diagrams of 45 Gb/s PAM4 signal.

In the same manner, the light in y-polarization is also separated using an optical splitter. Two wavelengths spaced by 60 GHz produce a 45 Gb/s/60 GHz PAM4 MMW signal [inset (h)]. For upper path, the upper wavelength is picked by an OC integrated with a FBG [inset (i)], and then fed into a 100-m free-space transmission. At the receiving side, the optical signal is received by a y-polarized receiver. Thereafter, the detected and amplified 45 Gb/s PAM4 signal is equalized by an electrical equalizer. The performance of the transmitted 45 Gb/s PAM4 signal is computed and investigated in real-time by BER values and eye diagrams. Additionally, a DSO is utilized to seize the eye diagrams of 45 Gb/s PAM4 signal. For lower path, the optical signal is sent to a 5-m RF wireless transmission with a couple of HA. Two wavelengths with channel spacing of 60 GHz generate a 45 Gb/s/60 GHz PAM4 MMW signal. The 45 Gb/s/60 GHz PAM4 MMW signal [inset (h)] is detected in y-polarization by a single-end PD, amplified by a PA, and transmitted wirelessly by a HA. Over a 5-m RF wireless transmission, the 45 Gb/s/60 GHz PAM4 MMW signal is received by a HA, boosted by a LNA, and down-converted by an envelope detector. After equalization by an electrical equalizer, the performance of the delivered 45 Gb/s PAM4 signal is computed and investigated in real-time by BER and eye diagrams. And further, a DSO is employed to seize the eye diagrams of 45 Gb/s PAM4 signal.

As schematic in Fig. 2

Fig. 2 Configuration of multiple wavelengths generator.

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, the multiple wavelengths generator is implemented by a distributed feedback (DFB) LD with a central wavelength of 1540.36 nm and a narrow linewidth of 10 MHz, a 40-GHz MZM, a 3-port OC, a delay interferometer (DI), a fiber mirror, an EDFA, and an OBPF. An optical power of 5 mW (~7 dBm) emanated from a DFB LD is inputted into a MZM. The MZM is linearly driven by a 15-GHz RF signal, leading to the generation of multiple wavelengths with channel spacing of 15 GHz. Then, the generated multiple wavelengths are then sent to an optical signal-to-noise ratio (OSNR) improvement scheme to improve the OSNR values. The OSNR improvement scheme is realized by an OC, a DI with 15 GHz free spectral range (FSR), a fiber mirror with reflectance of 98%, and an EDFA. The OC is utilized to forward the multiple wavelengths into the DI. After passing through the DI, these multiple wavelengths are reflected by a fiber mirror. We set the multiple wavelengths periodically in accordance with the FSR of DI to improve the OSNR owing to noise reduction between every two wavelengths. Subsequently, an EDFA is utilized to amplify the wavelengths. As the wavelengths pass through an EDFA, they are amplified to enhance the OSNR with regard to carrier amplification for each wavelength. Finally, the multiple wavelengths are inputted into an OBPF, with a 3-dB bandwidth of 0.52 nm and a filter slope of approximately 500 dB/nm, to remove the outer wavelengths. Given that a close channel spacing of 15 GHz exists, an OBPF with a sharp filter response is required.

3. Experimental results and discussions

The optical spectra of the multiple wavelengths generator with/without DI and EDFA simultaneously are displayed in Fig. 3

Fig. 3 Optical spectra of the multiple wavelengths generator with/without DI and EDFA simultaneously.

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. As shown in the figure, approximately 7–12 dB OSNR value improvement is attained for each wavelength as DI and EDFA are employed concurrently. Given that the MZM is biased at its null point, the generated multiple wavelengths are operated at optical carrier suppression mode. For multiple wavelengths generator, the amount of wavelengths is basically decided by the amplitude of the modulating signal on the MZM. With a proper driving RF signal (10 dBm) on the MZM, multiple wavelengths are generated with a channel spacing of 15 GHz (0.12 nm). As −2 and + 2 sidebands (dual-wavelength) are selected, a MMW signal with a space of 60 GHz can be obtained. For the restriction of transmission rate, the numerical value of transmission rate should be lower than that of the MMW carrier frequency. For example, if the MMW carrier frequency is 60 GHz, then the maximum transmission rate should be lower than 60 Gbps. However, high transmission rate increases the configuration complexity and technique challenge. The transmission rate adopted in this work is 45 Gb/s PAM4 signal to have a balance between the transmission rate and configuration complexity/technique challenge. In comparison with NRZ modulation, PAM4 modulation is adopted to enhance the spectrum efficiency and increase the transmission rate.

With parallel/orthogonally polarized dual-wavelength scheme, the corresponding square law detection can be calculated as [12]:

\begin{array}{l} {| S_{x 1} + S_{x 2} + S_{y 1} + S_{y 2} |}^{2} = {| S_{x 1} |}^{2} + {| S_{x 2} |}^{2} + {| S_{y 1} |}^{2} + {| S_{y 2} |}^{2} \\ + 2 R e {S_{x 1} • S_{x 2}} + 2 R e {S_{y 1} • S_{y 2}} \\ + 2 R e {S_{x 1} • S_{y 1}} + 2 R e {S_{x 1} • S_{y 2}} \\ + 2 R e {S_{x 2} • S_{y 1}} + 2 R e {S_{x 2} • S_{y 2}} \end{array}

where S_x1 and S_x2 are optical signals (dual-wavelength) in x-polarization, and S_y1 and S_y2 are optical signals (dual-wavelength) in y-polarization. Considering the orthogonal characteristic of x-polarization and y-polarization lights, cross-beating terms (S_x1·S_y1, S_x1·S_y2, S_x2·S_y1, S_x2·S_y2) with orthogonal polarization will not exist. Thus, Eq. (1) can be changed into:

\begin{array}{l} {| S_{x 1} + S_{x 2} + S_{y 1} + S_{y 2} |}^{2} = {| S_{x 1} |}^{2} + {| S_{x 2} |}^{2} + {| S_{y 1} |}^{2} + {| S_{y 2} |}^{2} \\ + 2 R e {S_{x 1} • S_{x 2}} + 2 R e {S_{y 1} • S_{y 2}} \end{array}

Noticeably, self-beating terms (S_x1·S_x2, S_y1·S_y2) dominate the performance of systems. Distortions caused by self-beating lead to performance degradation. To have a better transmission performance, an OBPF with a 3-dB bandwidth of 0.52 nm and a filter slope of approximately 200 dB/nm is adopted after 25-km SMF transmission so as to filter out the distortions due to self-beating between dual wavelengths (as illustrated in Fig. 4

Fig. 4 Optical spectrum with signals and distortions due to self-beating between dual wavelengths.

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). The reduction of distortions produced by self-beating is one of the problems to be addressed to construct a high-quality fiber-IVLLC and fiber-wireless hybrid system. The filter slope is decreased to approximately 300 dB/nm, in comparison with the OBPF shown in Fig. 2. Given that the spacing between the signal and distortion (60 GHz, Fig. 4) is far greater than that of the multiple wavelengths (15 GHz, Fig. 2), an OBPF with a low filter slope is sufficient.

A pair of doublet lenses (doublet lens 1 and doublet lens 2), as shown in Fig. 5

Fig. 5 A pair of doublet lenses (doublet lens 1 and doublet lens 2) is adopted to emanate light from the ferrule of SMF (transmitting side) to the free-space and to conduct light from the free-space into the ferrule of SMF (receiving side).

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, is adopted to emanate light from the ferrule of SMF (transmitting side) to the free-space and to conduct light from the free-space into the ferrule of SMF (receiving side). Doublet lens 1 and doublet lens 2 have a focal length of 100 mm, a diameter of 30 mm, and a back focal length of 87.8 mm. The numerical aperture of the SMF is 0.14, and thereby the diameter (d) of the laser light is:

d = 2 \times (100 \times 0.14) = 28 (mm)

Since the diameter of the laser beam (28 mm) is smaller than that of the doublet lens 1 (30 mm), the doublet lens 1 is practicable for a free-space link. Given that the lens has a spatial frequency cutoff (SFC) and a corresponding beam radius (r), the relation between the spatial frequency cutoff and the corresponding beam radius is given by:

r = 2.3 \times \frac{1}{S F C \times 2 π} = 3.6 (mm)

In addition, the divergence of the objective lens (

θ

) is:

θ = \frac{3.6 (μm)}{100 (mm)} = 36 \times 10^{- 6}

Over a 100-m (L) free-space link, the spot size (

ω_{o}

) has developed

ω_{o} = {[d^{2} + {(2 θ L)}^{2}]}^{1 / 2} = {[28^{2} + {(7.2)}^{2}]}^{1 / 2} = 28.9 (mm)

Since the spot size (28.9 mm) is smaller than that of the doublet lens 2 (30 mm), a 100-m free-space link is constructed. If a large spot size (> 30 mm) on the doublet lens 2 exists, it is difficult to construct a 100-m free-space link due to an apparent decrease in received optical power. As for the optical power level, a small coupling loss of 1.1 dB exists between the laser light and the doublet lens, and an atmospheric attenuation of 1.2 dB exists for a 100-m free-space link. Thereby, a link budget of 3.4 dB (1.1 × 2 + 1.2) exists between the doublet lens 1 and the doublet lens 2. The optical power sent out from the ferrule of SMF (transmitting side) is 3.9 dBm, indicating that the optical power concentrated on the ferrule of SMF (receiving side) is 0.5 dBm. Propagating a laser light through the free-space between the doublet lenses performs the free-space link to work as if the SMFs were connected. A pair of convex lenses could be deployed to replace a pair of doublet lenses for constructing a free-space link. Nevertheless, it is quite a challenge to construct a 100-m free-space link by using a pair of convex lenses.

For the PAM-M (where M is an integer) signal transmission, the relation between BER and symbol error rate (SER) is expressed as [13]:

B E R = d_{i j} \frac{S E R}{\log_{2} M}

where d_ij is the Hamming distance between the labels of symbols i and j. The total BER can be attained by measuring the SER of top, middle and bottom eyes of PAM4 signal:

B E R = \frac{1}{2} S E R_{t o p} + S E R_{m i d} + \frac{1}{2} S E R_{b o t}

Real-time BER is calculated by auto-searching employing a one-channel ED and the PAM4 three-eye (upper/middle/lower) sampling method [14]. Just sending the PAM4 signal to a one-channel ED, it is a simple and low-cost PAM4 BER measurement method for calculating the total BER. A PAM4 fiber-IVLLC and fiber-wireless hybrid system with BER real-time measurement is attractive because complicated offline calculation by MATLAB is not needed.

The S₂₁ characteristic of the equalizer is presented in Fig. 6

Fig. 6 The S₂₁ characteristic of the equalizer.

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. Equalization refers to an operation intended to increase the levels of high frequency. In this study, an equalizer compensates for the frequency response (especially for high frequencies) and enhances the transmission rate of PAM4 fiber-IVLLC and fiber-wireless hybrid systems.

Figure 7

Fig. 7 BER values of the 45 Gb/s PAM4 signal for the scenarios of BTB, over 25-km SMF transmission with 100-m free-space link (x-polarization), and over 25-km SMF transmission with 100-m free-space link (y-polarization).

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shows the BER values of the 45 Gb/s PAM4 signal for the scenarios of back-to-back (BTB), over 25-km SMF transmission with 100-m free-space link (x-polarization polarization) and over 25-km SMF transmission with 100-m free-space link (y-polarization). Notably, the BER performances of the x-polarization and y-polarization are nearly identical. Hence, the contribution for the transmission capacity in x-polarization and y-polarization is almost identical. At a BER value of 10⁻⁹, the power penalty is 6.5 dB between the scenarios of BTB and over 25-km SMF transmission with 100-m free-space link (x-polarization or y-polarization). Such a power penalty of 6.5 dB results from fiber dispersion produced by 25 km SMF transmission, residual distortion because of self-beating between the dual wavelengths, OSNR decrement because of the transmission over 100-m free-space link, and laser beam misalignment between the doublet lens 2 and the ferrule of SMF (receiving side). Over a span of 25-km SMF, RF power degradation induced by fiber dispersion reduces the performance because of the innate features of the dual wavelengths. An OBPF is utilized after 25-km SMF transmission to remove the distortions caused by self-beating between the dual wavelengths. However, the residual distortion due to self-beating degrades the BER performance. A 100-m free-space link causes higher atmospheric attenuation, resulting in lower OSNR and higher BER. Furthermore, the noise increases as the free-space link increases. The OSNR is decreased as stray light form the environment is received by optical receiver, resulting in worse BER. Moreover, laser beam alignment between the doublet lens 2 and the ferrule of SMF is important to the performances of fiber-IVLLC convergence. Because the receiving area of the ferrule is considerably small, the laser beam alignment between the doublet lens 2 and the ferrule of SMF is vital for the convergence of the fiber-IVLLC to maintain the free-space link feasibly. The ferrule of SMF (receiving side) should be precisely located at the back focal point of doublet lens 2 (as illustrated in Fig. 5) to have a good laser beam alignment. Given that BER increases with the increase in laser beam misalignment, good alignment technique is required to attain good transmission performance.

Figures 8(a) and 8(b)

Fig. 8 Eye diagrams of the 45 Gb/s PAM4 signal for (a) the BTB and (b) over 25-km SMF transmission with 100-m free-space link (x-polarization) scenarios.

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show the eye diagrams of the 45 Gb/s PAM4 signal for BTB and over 25-km SMF transmission with 100-m free-space link (x-polarization) scenarios, respectively. Considering that the BER performances of x-polarization and y-polarization are nearly identical, only the eye diagrams of 45 Gb/s PAM4 signal in x-polarization are observed. For BTB scenario, at a received optical power of −6.2 dBm and a BER value of 10⁻⁹, eye diagrams with good quality apparently exist [Fig. 8(a)]. For the scenario of over 25-km SMF transmission with 100-m free-space link, at a received optical power of 0.3 dBm and a BER value of 10⁻⁹, three independent slightly clear eye diagrams exist [Fig. 8(b)]. Results show that this proposed fiber-IVLLC convergence satisfies the high availability requirement of fiber backhaul and optical wireless extender.The BER values of the 45 Gb/s PAM4 signal for the conditions of BTB, over 25-km SMF transmission with 5-m RF wireless transmission (x-polarization), and over 25-km SMF transmission with 5-m RF wireless transmission (y-polarization) are shown in Fig. 9

Fig. 9 BER values of the 45 Gb/s PAM4 signal for the conditions of BTB, over 25-km SMF transmission with 5-m RF wireless transmission (x-polarization), and over 25-km SMF transmission with 5-m RF wireless transmission (y-polarization).

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. The figure shows the BER performance of x-polarization and y-polarization is nearly equal, thereby indicating that the influence for transmission capacity in x-polarization and y-polarization is almost equal. At a BER operation of 10⁻⁹, a power penalty of 8.7 dB exists between the conditions of BTB and over 25-km SMF transmission with 5-m RF wireless transmission (x-polarization or y-polarization). A large power penalty of 8.7 dB is contributed by fiber dispersion due to 25 km SMF transmission, residual distortion due to self-beating between the dual wavelengths, and fading effect due to 5 m RF wireless transmission. Over 5-m RF wireless transmission, fading effect causes amplitude and phase variations in the received signal, leading to poor BER performance.

The eye diagrams of 45 Gb/s PAM4 signal for the conditions of BTB and over 25-km SMF transmission with 5-m RF wireless transmission (x-polarization) are presented in Figs. 10(a) and 10(b)

Fig. 10 Eye diagrams of the 45 Gb/s PAM4 signal for the conditions of (a) BTB and (b) over 25-km SMF transmission with 5-m RF wireless transmission (x-polarization).

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, respectively. Considering that the BER performance of x-polarization and y-polarization is nearly equal, we only capture the eye diagrams of 45 Gb/s PAM4 signal in x-polarization. For BTB condition, at a received optical power of −6.5 dBm and a BER value of 10⁻⁹, three independent clear eye diagrams are attained [Fig. 10(a)]. For the condition of over 25-km SMF transmission with 5-m RF wireless transmission, at a received optical power of 2.2 dBm and a BER value of 10⁻⁹, eye diagrams with acceptable quality are achieved [Fig. 10(b)]. Results reveal this proposed fiber-wireless integration meets the high accessibility demand of fiber backhaul and RF wireless extender.

4. Conclusions

A real-time PAM4 fiber-IVLLC and fiber-wireless hybrid system with parallel/orthogonally polarized dual-wavelength scheme is proposed and successfully demonstrated. Real-time PAM4 BER measurement is executed for computing the total BER. It is attractive due to the avoidance of offline calculation by MATLAB. To the best of our knowledge, this work is the first one that utilizes parallel/orthogonally polarized dual-wavelength scheme in a real-time PAM4 fiber-IVLLC and fiber-wireless hybrid system. Through the PAM4 signal transmission, parallel/orthogonally polarized dual-wavelength scheme, and fiber-IVLLC and fiber-wireless convergences, the total transmission capacities are increased up to eight times. Impressive transmission performances of BER and eye diagrams are attained over 25-km SMF transmission with 100-m free-space link/5-m RF wireless transmission. In comparison with previously proposed fiber-IVLLC and fiber-wireless hybrid systems, it presents a prominent alternative with simplicity and cost-effective benefits. This illustrated PAM4 fiber-IVLLC and fiber-wireless hybrid system is notable for the incorporation of fiber-based backhaul and optical/RF wireless-based front-end.

Funding

Ministry of Science and Technology (MOST) of Taiwan (107-2636-E-027-002, 107-2221-E-027-077-MY3, 107-2221-E-027-078-MY3).

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Real-time PAM4 fiber-IVLLC and fiber-wireless hybrid system with a parallel/orthogonally polarized dual-wavelength scheme

Abstract

1. Introduction

2. Experimental setup

3. Experimental results and discussions

4. Conclusions

Funding

References

Cited By

Figures (10)

Equations (8)

OSA Continuum