Two-way lightwave transmission system with a centralized-light-source and VCSEL-based upstream wavelength selector

Hai-Han Lu; Chung-Wei Su; Chung-Yi Li; Hsiao-Wen Wu; Zhen-Han Wang; Chung-Wei Hung; You-Ruei Wu

doi:10.1364/OSAC.1.001195

1. Introduction

Two-way lightwave transmission systems with broadband heterogeneous services are anticipated to promote present and future technologies, such as cable television (CATV) signal transmission, 4G/5G mobile communication, as well as radio-over-fiber and high-speed Internet applications. They can be achieved via the integration of optical fiber communications and optical/RF wireless transmissions to realize the wired and wireless applications simultaneously. By integrating the large bandwidth of fiber backbone with the mobility of optical/RF wireless reach extender, two-way lightwave transmission systems based on fiber-free-space optical (FSO)/fiber-wireless/fiber-wired convergences hold a powerful platform for providing developed and developing applications [1–6]. In comparison with other transmission systems, two-way lightwave transmission systems with fiber-FSO/fiber-wireless/fiber-wired convergences possess the merits of high transmission capacity and sufficient mobility.

For practical deploying two-way lightwave transmission systems with fiber-FSO/fiber-wireless/fiber-wired convergences, the receiver side requires an optical carrier for uplink transmission. Wavelength reuse scheme is generally deployed in two-way lightwave transmission systems for upstream. Reflective semiconductor optical amplifier (RSOA), injection-locked Fabry-Perot laser diode (FP LD), and phase modulator (PM) have been utilized to implement wavelength reuse in conventional two-way lightwave transmission systems [7–9]. However, since the bandwidths of RSOA and FP LD are constraint, it is quite a challenge for RSOA and FP LD to deliver high-speed data stream for uplink transmission. As for the PM, PM affords a scheme to modulate the high-speed data stream for uplink transmission. Nevertheless, the intensity-modulated downlink signal will induce performance degradation on the phase-modulated uplink signal due to the effect of intensity modulation (IM)-to-phase modulation (PM) conversion [10]. In this study, we propose and experimentally demonstrate a two-way lightwave transmission system with centralized-light-source and vertical-cavity surface-emitting laser (VCSEL)-based upstream wavelength selector [11]. With the support of centralized-light-source and VCSEL-based upstream wavelength selector, the downstream light is directly transmitted as the upstream carrier. Additional optical components (such as RSOA, FP LD and PM) and sophisticated injection-locked technique are not needed for upstream modulation, which greatly cut down cost and complexity of two-way lightwave transmission systems. Impressive performances of carrier-to-noise ratio (CNR)/composite second-order (CSO)/composite triple-beat (CTB) for CATV signal transmission and bit error rate (BER) for millimeter-wave (MMW) and baseband (BB) data signal transmissions are attained over 25-km single-mode fiber (SMF) transmission with 100-m free-space link/5-m RF wireless transmission. This proposed two-way lightwave transmission system is presented to be a prominent architecture for providing broadband heterogeneous services with simple and economic advantages.

2. Experimental setup

Figure 1

Fig. 1 The architecture of the proposed two-way lightwave transmission systems with centralized-light-source and VCSEL-based upstream wavelength selector.

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illustrates the architecture of the proposed two-way lightwave transmission systems with centralized-light-source and VCSEL-based upstream wavelength selector. The output of the distributed feedback (DFB) LD, with wavelength of 1540.16 (1550.36) nm, is split into two parts. To minimize the interaction between the two-way transmissions, two DFB LDs with large channel spacing of 10.2 nm are adopted at both transmitter sides. One of the optical signal is fed into a dual-arm Mach-Zehnder modulator (MZM), and the other optical signal is fed into a MZM. The CATV signal (CH2-78; 50-550 MHz) is directly sent to one arm of the dual-arm MZM, and a 10 Gbps/30 GHz MMW data signal is sent to the other arm of the dual-arm MZM. By operating the dual-arm MZM at V_π, the 10 Gbps/30 GHz MMW data signal modulated on the zero-order term can be reduced to lower interference to the CATV signal. The optical spectrum after the dual-arm MZM is presented in Fig. 2(a)

Fig. 2 (a) The optical spectrum after the dual-arm MZM. (b) An injection locking behavior enhances the intensity of the second-order sideband and generates the optical spectrum. (c) Two optical signals are recombined to generate the optical spectrum. (d) The downstream signal is optically promoted to a 10 Gbps/60 GHz MMW data signal and generates the optical spectrum. (e) The optical spectrum after the OBPF.

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[inset (a) of Fig. 1]. The resolution bandwidth employed to measure such optical spectrum is 0.02 nm. Furthermore, appropriate 38 GHz MMW signal is supplied to the MZM. Only the first-order and second-order sidebands are created. Intensity-modulated optical signal is sent to a VCSEL-based upstream wavelength selector to flexibly select one of the optical sidebands. This VCSEL-based upstream wavelength selector comprises a 3-port optical circulator (OC), a polarization controller (PC), and a VCSEL. As the optical sideband is flexibly chosen by the VCSEL-based upstream wavelength selector, an injection locking behavior enhances the intensity of the second-order sideband and generates the optical spectrum as shown in Fig. 2(b) [inset (b) of Fig. 1]. It is necessary to use some devices, including a DFB LD, a MZM, an OC, a PC and an injection-locked VCSEL, for upstream carrier generation and transmission. However, they are worth employing due to excellent characteristics including flexible selection for upstream wavelength, providing high-speed data stream for uplink transmission, and no requirement of additional optical components at the upstream transmitter side for upstream modulation. Subsequently, two optical signals are recombined by a 2 × 1 optical combiner to generate the optical spectrum as illustrated in Fig. 2(c) [inset (c) of Fig. 1]. The combined optical signals are amplified by an erbium-doped fiber amplifier (EDFA) and transmitted through a 25-km SMF via two OCs.

At the receiver side, the downstream signals are split by a 1 × 3 optical splitter. For upper path, the downstream signal is inputted into a 100-m free-space link with a pair of doublet lenses. A pair of convex lenses could be employed to build a free-space link. However, it is difficult and challenging to build a 100-m free-space link by utilizing a pair of convex lenses. Afterward, the downstream signal is applied to a CATV receiver. Following the CATV receiver, the CATV signal is supplied to a push-pull scheme for distortion elimination [12], and inputted into a spectrum analyzer for CNR, CSO, and CTB performance analysis. Since CATV receiver only responses to 50-550 MHz CATV band, yet only CATV signal will be detected by CATV receiver. Thereby, no extra optical band-pass filter (OBPF) is required to pick up the optical carrier modulated with CATV signal. For middle path, the downstream signal is applied to a 30G/60G optical interleaver (OIL) to optically promote a 10 Gbps/30 GHz MMW data signal to a 10 Gbps/60 GHz one and generate an optical spectrum as presented in Fig. 2(d) [inset (d) of Fig. 1]. The 10 Gbps/60 GHz optical signal is detected by a high-bandwidth photodiode (PD), amplified by a power amplifier (PA) with frequency range of 58–64 GHz, and wirelessly transmitted by a horn antenna (HA) working at V-band (40–75 GHz). Over a 5-m RF wireless transmission, the 10 Gbps/60 GHz MMW data signal is received by a HA, boosted by a 60-GHz low-noise amplifier (LNA), down-converted by an envelope detector with a frequency range of 1–10 GHz, and sent to a BER tester (BERT) for BER performance evaluation. For lower path, an OBPF with a 3-dB bandwidth of 0.1 nm and a filter slope of 500 dB/nm is utilized to filter out the desired optical carrier selected by the VCSEL-based upstream wavelength selector. The optical spectrum after the OBPF is given in Fig. 2(e) [inset (e) of Fig. 1]. The filtered optical carrier is modulated with a 10-Gbps uplink BB data stream by a MZM. Subsequently, the modulated optical signal is amplified by an EDFA, transmitted through a 25-km SMF via two OCs, detected by a low-bandwidth PD, boosted by a 10-GHz LNA, and sent to a BERT for BER value calculation. It is worth noting that two SMFs are deployed for downlink and uplink transmissions to avoid the crosstalk produced by downstream (upstream) optical signal.

3. Experimental results and discussions

As injection locking occurs, the main mode of VCSEL will be attuned to match one of the optical sidebands of the intensity-modulated optical signal by adapting VCSEL driving current. Table 1

Table 1. The central wavelength of the VCSEL under different driving currents.

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presents the central wavelength of the VCSEL under different driving currents. As the VCSEL driving current changes from 2.2 mA to 8.2 mA, the central wavelength of VCSEL changes from 1539.16 nm to 1542.36 nm. A 3.2-nm tuning range is attained to elastically match one of the optical sidebands of the intensity-modulated optical signal. The output of VCSEL-based upstream wavelength selector can be tuned to select one of the optical sidebands by adapting VCSEL driving current. In this experiment, the second-order sideband of the 38 GHz MMW signal (76 GHz/0.61 nm away from the central carrier) is picked up to minimize the interference, rather than the first-order sideband (38 GHz away from the central carrier).

Figure 3

Fig. 3 The measured CNR/CSO/CTB values for the scenarios over 25-km SMF transmission and over 25-km SMF transmission with 100-m free-space link.

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presents the measured CNR/CSO/CTB values for the scenarios over 25-km SMF transmission and over 25-km SMF transmission with 100-m free-space link. Over 25-km SMF transmission, the CNR/CSO/CTB values of ≥50.5/61/61.6 dB are attained, in which satisfying the CNR/CSO/CTB requirements of ≥50/60/60 dB at optical node. Over 25-km SMF transmission with 100-m free-space link, the CNR/CSO/CTB values of ≥46.1/56.7/57.3 dB are acquired, in which meeting the CNR/CSO/CTB demands of ≥43/53/53 dB at subscriber. CNR, CSO, and CTB degradations of 4.4/4.3/4.3 dB exist between these two scenarios. Given that the CNR/CSO/CTB is defined as the ratio of the carrier power to the noise/second-order distortion/third-order distortion power, these CNR, CSO, and CTB degradations result from atmospheric attenuation (~3 dB/100 m) due to 100-m free-space link, noise and distortion increase (~0.8 dB) due to other lights form the environment, and carrier reduction (~0.5 dB) due to laser beam misalignment. A 100-m free-space link causes higher atmospheric attenuation, leading to lower received optical power and lower RF carrier level, and resulting in worse CNR/CSO/CTB. And further, the noise and distortion are increased as other lights form the environment are received by CATV receiver, conducing to poorer CNR/CSO/CTB. Moreover, laser beam alignment between the doublet lens 2 and the ferrule of SMF is crucial to the CATV transmission performances. CNR/CSO/CTB decreases as the laser beam misalignment increases. Because the receiving area of the ferrule is substantially small, the laser beam alignment between the doublet lens 2 and the ferrule of SMF is important to keep the free-space link feasibly. The ferrule of SMF must be precisely placed at the back focal point of doublet lens 2 to achieve a good laser beam alignment. Good pointing and alignment techniques are needed to achieve good CATV transmission performances.

To have a direct association with 10 Gbps/30 GHz MMW data signal and CATV transmission performances, the measured CNR/CSO/CTB values over 25-km SMF transmission with 100-m free-space link under the scenarios with/without 10 Gbps/30 GHz MMW data signal are shown in Fig. 4

Fig. 4 The measured CNR/CSO/CTB values over 25-km SMF transmission with 100-m free-space link under the scenarios with/without 10 Gbps/30 GHz MMW data signal.

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. The CNR/CSO/CTB values with 10 Gbps/30 GHz MMW data signal are degraded about 0.5, 0.4 and 0.3 dB, respectively, compared with systems without 10 Gbps/30 GHz MMW data signal. Small CNR, CSO, and CTB degradations are due to the use of CATV receiver and operating the dual-arm MZM at V_π. Because the CATV receiver only reacts to CATV band (50-550 MHz), thereby only CATV signal will be detected. Since MMW data signal will not be detected by CATV receiver, yet it will not induce CNR, CSO, and CTB degradations in CATV band. In addition, by operating the dual-arm MZM at V_π, the 10 Gbps/30 GHz MMW data signal modulated on the zero-order term will be minimized to reduce interaction to the CATV signal. No effect of interaction is observed between these two bands (CATV and MMW), conducing to small CNR, CSO, and CTB degradations.

The BER values of 10 Gbps/60 GHz MMW data signal for the conditions of back-to-back (BTB), over 25-km SMF transmission (with CATV signal), and over 25-km SMF transmission with 5-m RF wireless transmission (with/without CATV signal) are shown in Fig. 5

Fig. 5 The BER values of 10 Gbps/60 GHz MMW data signal for the conditions of BTB, over 25-km SMF transmission (with CATV signal), and over 25-km SMF transmission with 5-m RF wireless transmission (with/without CATV signal).

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. The figure shows the BER performances with/without CATV signal transmission are nearly equal, indicating that the influence by CATV signal transmission is very small. Due to HA only reacts to V-band (40-75 GHz), thereby only 60 GHz MMW signal will be wirelessly transmitted (50-550 MHz CATV signal will not be wirelessly transmitted). Moreover, the highest carrier frequency for CATV band is 550 MHz, far from the 60 GHz MMW carrier. The 110th harmonic distortion for 550 MHz is located at 60.5 GHz, in which it is close to the 60 GHz. However, the amplitude of distortion decreases with the increasing of the order number. Thus, the 110th harmonic distortion has very small amplitude, it will not induce distortion in MMW band. Thus, no effect of interaction is observed between these two bands. At a BER operation of 10⁻⁹, a power penalty of 5.6 dB exists between the conditions of BTB and over 25-km SMF transmission. A power penalty of 5.6 dB is contributed by fiber dispersion due to 25 km SMF transmission. In addition, at a BER operation of 10⁻⁹, a power penalty of 1.4 dB exists between the conditions of over 25-km SMF transmission and over 25-km SMF transmission with 5-m RF wireless transmission. A power penalty of 1.4 dB is ascribed to fading effect due to 5 m RF wireless transmission. And further, in parallel with verifying power penalty and SMF length, the received optical powers at a BER operation of 10⁻⁹ under different SMF lengths are given in Table 2

Table 2. The received optical powers at a BER operation of 10⁻⁹ under different SMF lengths.

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. Power penalties of 4.2 dB, 7 dB, and 10.7 dB exist between the scenarios of BTB and over 12.5-km SMF transmission with 5-m RF wireless transmission, BTB and over 25-km SMF transmission with 5-m RF wireless transmission, and BTB and over 40-km SMF transmission with 5-m RF wireless transmission, respectively. Obviously, power penalty increases with the increasing of SMF length. Longer SMF length causes larger fiber dispersion, in which resulting in larger power penalty. Besides, the eye diagram of 10 Gbps/60 GHz MMW data signal is also displayed in Fig. 5. Clear eye diagram exists for the condition of over 25-km SMF transmission with 5-m RF wireless transmission. However, it can be seen that the eye diagram exhibits somewhat overshoot and eye-crossing issues. At the downstream receiver side, the OIL is used mainly for the sake of the HA working bandwidth. In this way, detecting the data on both optical sidebands would introduce signal-signal beat interference, thus appearing signal distortion for the NRZ signal and generating overshoot and eye-crossing for the eye diagram.

For uplink transmission, the BER values of 10 Gbps BB data stream for the conditions of BTB and over 25-km SMF transmission (with/without MMW and CATV signals) are presented in Fig. 6

Fig. 6 For uplink transmission, the BER values of 10 Gbps BB data stream for the conditions of BTB and over 25-km SMF transmission (with/without MMW and CATV signals).

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. The figure shows the BER performances with/without MMW and CATV signals transmission are approximately the same, indicating that the influence by MMW and CATV signals transmission is almost none. Due to OBPF with a sharp filter slope of 500 dB/nm, thereby only upstream carrier selected by VCSEL-based upstream wavelength selector will be filtered out (downstream carriers modulated with MMW and CATV signals will not be filtered out). At a BER value of 10⁻⁹, a power penalty of 5.8 dB exists between the conditions of BTB and over 25-km SMF transmission. Power penalty of 5.8 dB is obtained as a result of the fiber dispersion induced by 25 km SMF transmission. Furthermore, to have a close relation between power penalty and SMF length, the received optical powers at a BER operation of 10⁻⁹ under different SMF lengths are given in Table 3

Table 3. The received optical powers at a BER operation of 10⁻⁹ under different SMF lengths.

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. Power penalties of 2.9 dB, 5.8 dB, and 9.6 dB exist between the scenarios of BTB and over 12.5-km SMF transmission, BTB and over 25-km SMF transmission, and BTB and over 40-km SMF transmission, respectively. Obviously, power penalty increases with the increasing of SMF length. Longer SMF length causes larger fiber dispersion, in which leading to larger power penalty. Besides, the eye diagram of 10 Gbps BB data stream is also displayed in Fig. 6. Apparently, clear eye diagram is observed for the condition of over 25-km SMF transmission. Nevertheless, it can be seen that the eye diagram exhibits somewhat overshoot and eye-crossing issues. At the downstream receiver side, the OBPF is used to filter out the desired optical carrier. In this way, the residual distortion after the OBPF will generate overshoot and eye-crossing for the eye diagram.

4. Conclusions

A two-way lightwave transmission system employing centralized-light-source and VCSEL-based upstream wavelength selector is proposed and practically demonstrated. With centralized-light-source and VCSEL-based upstream wavelength selector, the downstream light is directly transmitted as the upstream carrier. No additional optical components and sophisticated injection-locked technique are needed for upstream modulation. Impressive performances of CNR, CSO, CTB and BER are acquired over 25-km SMF transmission with 100-m free-space link/5-m RF wireless transmission. This proposed lightwave transmission system is shown to be a prominent one not only presents its advancement in two-way applications but also reveals its simplicity and economy for construction.

Funding

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

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Two-way lightwave transmission system with a centralized-light-source and VCSEL-based upstream wavelength selector

Abstract

1. Introduction

2. Experimental setup

3. Experimental results and discussions

4. Conclusions

Funding

References

Cited By

Figures (6)

Tables (3)

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