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A bidirectional hybrid phase modulation (PM)-based radio-over-fiber (RoF) and vertical cavity surface emitting laser (VCSEL)-based visible laser light communication (VLLC) systems employing injection-locked VCSEL-based PM-to-intensity modulation (IM) converters and optical interleavers (ILs) is proposed and demonstrated. To be the first one of using injection-locked VCSEL-based PM-to-IM converters and optical ILs in such bidirectional hybrid RoF and VLLC systems, the downstream light is successfully phase-remodulated with RoF signal for up-link transmission. Through a serious investigation in systems, bit error rate (BER) and eye diagram perform brilliantly over a 40-km single-mode fiber (SMF) transport and a 12-m free-space transmission. Such a bidirectional hybrid RoF and VLLC system would be very attractive for the integration of fiber backbone and in-door networks to provide broadband integrated services, including Internet and telecommunication services.
© 2014 Optical Society of America
1. Introduction
Recently, fiber-to-the-home (FTTH) to support high-speed broadband integrated services has been spreading widely. There are high expectations that optical broadband access networks and FTTH will become even faster and handle larger amount of data [1–3]. Optical fiber with its high capacity and low attenuation features offers an ideal way to distribute high quality optical signal. Single-mode fiber (SMF) has already established a status cannot be swayed in the fiber backbone network. However, as the SMF is deployed at in-door network, installation simplicity and convenience are practical issues needed to be solved. It needs a trained person with high-precision devices to mount SMF inside a house, so its construction is relatively complicated. As a result, the in-door conjunction becomes a crucial bottleneck to successfully transport high quality optical signal from the central office to the subscribers. In order to conquer the challenge, a new kind of in-door transmission medium is required to boost up the speed and capacity. Recently, visible laser light communication (VLLC) has been developed with its excellent optical characteristics [4, 5]. This promising scheme is the potential candidate to solve the last-mile end-user problem. Clear advantages are provided by the using of VLLC for in-door network. VLLC can provide communication link in particular areas in which wire and RF wireless communications are forbidden. Thereby, VLLC is an ideal scheme to integrate the fiber backbone and the in-door networks. As the light source is deployed at VLLC systems, data rate and free-space transmission distance are beyond disputed issues needed to be overcome. In order to overcome the limitations, vertical cavity surface emitting laser (VCSEL) is deployed at VLLC systems. VCSEL has evolved as the emitter for high-speed data communication, due to its narrow coherent beam and high power efficiency. The new generation of VCSEL is optimized for modulation data rate up to few Gbps. Therefore, it opens an innovative and promising way for high-speed operation in in-door network [6, 7].
With the rapid development of lightwave transmission technologies, the increasing requirements raise the demands for high-speed and high-volume applications; not only for the SMF-based backbone network, but also for the VLLC-based in-door one. In this paper, a bidirectional hybrid phase modulation (PM)-based radio-over-fiber (RoF) and VCSEL-based VLLC system is proposed and experimentally demonstrated. At the receiving site, a PM-to- intensity modulation (IM) converter based on injection-locked VCSEL scheme is employed to transform the phase-modulated optical signal into the intensity-modulated one [8]. It is not necessary to use an expensive delay interferometer (DI) or a sophisticated fiber Bragg grating (FBG) tilt filter to make a PM-to-IM conversion [9, 10]. Furthermore, an optical interleaver (IL) is developed to separate off the optical carrier and the optical sideband [11]. The optical IL has two outputs, one of the outputs has the optical signal only with the optical sideband (without optical carrier), and another output has the optical signal only with the optical carrier (without optical sideband). The optical sideband is used for down-link transmission, and the optical signal can be detected directly by a photodiode (PD). To be the first one of employing injection-locked VCSEL-based PM-to-IM converters and optical ILs in such bidirectional hybrid RoF and VLLC systems, the downstream light is successfully phase-remodulated with RoF signal for up-link transmission. The performances of downstream and upstream data signals are measured and analyzed by parameters such as bit error rate (BER) and eye diagram. Through a serious investigation in systems, BER and eye diagram perform brilliantly over a 40-km SMF transmission and a 12-m free-space VLLC link. Such a bidirectional hybrid PM-based RoF and VCSEL-based VLLC system reveals a prominent alternative not only to present its advancement in integration of fiber backbone and in-door networks, but also to reveal its convenience in last-mile end-user applications.
2. Experimental setup
The experimental configuration of our proposed bidirectional hybrid PM-based RoF and VCSEL-based VLLC systems over a 40-km SMF transport and a 12-m free-space transmission is shown in Fig. 1.The distributed feedback laser diode (DFB LD), with a central wavelength of 1545.32 nm, is fed into a phase modulator for phase-modulated RoF signal. A 5-Gbps data stream mixed with a 20-GHz RF carrier to generate the 5Gbps/20GHz RoF signal, and the resulting RoF signal is supplied to the phase modulator. A suitable RoF signal power level is provided to drive the phase modulator, in which resulting in the generation of optical carrier and the first-order sidebands (−1 and + 1 sidebands). The peak of the first-order sidebands is 20 GHz away from the optical carrier. The phase-modulated optical signal is amplified by an erbium-doped filter amplifier (EDFA), and delivered through a 40-km SMF transport. Over a 40-km SMF transport, the phase-modulated optical signal is went through a PM-to-IM converter. Such a PM-to-IM converter consists of an optical circulator (OC) and a VCSEL. As the VCSEL is injection-locked, the upper sideband ( + 1 sideband) of the phase-modulated optical signal will be amplified, while the lower sideband (−1 sideband) of the phase-modulated optical signal will be unchanged [12]. The optical spectrum before the VCSEL-based PM-to-IM converter is present in Fig. 2(a) [Fig. 1 insert (i)]. An injection locking enhances the intensity of the upper sideband and produces the optical spectrum as shown in Fig. 2(b) [Fig. 1 insert (ii)]. Moreover, small insertion loss is observed between Figs. 2(a) and 2(b). If the insertion loss is too large [8], the function of injection-locked VCSEL will be largely diminished. The PM optical signal needs to propagate through a SMF link of 40 km before injection into the PM-to-IM converter, so that the injection between the upper sideband of PM optical signal and the VCSEL lasing wavelength would be weak. To avoid weak injection, strong injection is deployed in systems by higher EDFA output power to compensate the coupling efficiency between the fiber and VCSEL and the insertion loss between the port 1 [Fig. 1 insert (i)] and port 3 [Fig. 1 insert (ii)] of OC. Following with the PM-to-IM converter output, the optical signal is passed through an optical IL. The optical IL has two outputs, one of the outputs has the optical signal only with the optical sideband (without optical carrier), and another output has the optical signal only with the optical carrier (without optical sideband). The optical spectrum for the optical IL output only with the optical sideband is given in Fig. 2(c) [Fig. 1 insert (iii)], and the optical spectrum for the optical IL output only with the optical carrier is given in Fig. 2(d) [Fig. 1 insert (v)]. Following with the optical IL output only with the optical sideband, the optical signal is detected directly by a PD to get the 5-Gbps data stream. Subsequently, the 5-Gbps data stream is amplified by a low noise amplifier (LNA), data regenerated by a data recovery scheme, and split by a 1 × 2 RF splitter. One of the RF splitter outputs is fed into a BER tester (BERT) for BER performance evaluation, and another RF splitter output is supplied to the VLLC systems.
Fig. 1 The experimental configuration of our proposed bidirectional hybrid PM-based RoF and VCSEL-based VLLC systems over a 40-km SMF transport and a 12-m free-space transmission.
Fig. 2 (a) The optical spectrum before the VCSEL-based PM-to-IM converter [Fig. 1 insert (i)]. (b) The optical spectrum after the VCSEL-based PM-to-IM converter [Fig. 1 insert (ii)]. (c) The optical spectrum for the optical IL output only with the optical sideband [Fig. 1 insert (iii)]. (d) The optical spectrum for the optical IL output only with the optical carrier [Fig. 1 insert (v)].
For the up-link transmission, the optical IL output only with the optical carrier is launched into another phase modulator for phase-remodulated RoF signal. The 5Gbps/20GHz RoF signal is fed into a phase modulator, amplified by an EDFA, and transmitted by a 40-km SMF link. The upstream signal can be sent back either by the same fiber or by another fiber. In order to avoid the signal interferences, two optical fibers are deployed for down-link and up-link transmissions to optimize the transmission performances. Over a 40-km SMF transmission, the optical signal is passed through a PM-to-IM converter and an optical IL, and detected directly by a PD to obtain the 5-Gbps data stream. The 5-Gbps data stream is boosted by a LNA, data recovered by a data recovery scheme, and separated off by a 1 × 2 RF splitter. One of the RF splitter outputs is fed into a BERT for BER analysis, and another RF splitter output is supplied to the VLLC systems.
The schematic configuration of the VLLC systems employing a VCSEL modulated by a 5-Gbps data stream over a 12-m free-space transmission is shown in Fig. 3.The optical beam with divergent angle of VCSEL and convergent angle of PD are also present in Fig. 3. The VCSEL, with 3-dB modulation bandwidth/wavelength range/color of 5.2GHz/678-680nm/red, is directly modulated by a 5-Gbps data stream. After emitted from the VCSEL, the light is diverged, launched into the first convex lens, transmitted in the free-space, launched into the second convex lens, and focused on the PD. The first convex lens is utilized to transform the divergent beam into the parallel beam, and the second convex lens is utilized to concentrate the parallel beam into a point. The PD has a detection wavelength range of 320-1000 nm, an active area diameter of 0.08 mm, and a responsivity of 0.44 mA/mW (at 680 nm). The received data signal is then amplified by a LNA and recovered by a data recovery scheme. Eventually, the data signal is supplied to a BERT for BER performance evaluation.
Fig. 3 The schematic configuration of the VLLC systems employing a VCSEL modulated by a 5-Gbps data stream over a 12-m free-space transmission.
3. Experimental results and discussions
Wavelength stabilization is important and critical for VCSEL under injection locking. The outer boundary of the locking range for laser under light injection is given by [13]
where and denote coupling coefficient between injected field and laser field, and are injected field and laser mode field intensity, and is the locking range. If the injection locking behavior doesn’t happen, then the conversion effect will not work. However, the main mode of the VCSEL can be adjusted to match with the upper sideband of the phase-modulated optical signal by adjusting the driving current of VCSEL. As the VCSEL driving current increase from 3.2 mA to 9.3 mA, the central wavelength of VCSEL shifts from 1544.32 nm to 1547.32 nm. A 3-nm tuning range is obtained to dynamically match with the upper sideband of the phase-modulated optical signal. Furthermore, to prevent the wavelength drift of the VCSEL induced by the temperature variation, temperature controller is used in the directly modulated transmitter. The wavelength variation of the VCSEL with temperature controller is ~0.003 nm/°C.To consider only the first-order upper and lower sidebands of a PM signal, the PM signal, , can be expressed as [8, 14]
where e0 and w0 are the amplitude and the angular frequency of the optical carrier, respectively. denotes the nth-order Bessel function of the first kind; is the angular frequency of the modulating signal, respectively. As indicated in Eq. (2), both sidebands of a PM signal are out-of-phase by 180 degrees. If the 5Gbps/20GHz PM-based RoF signal is directly detected by a PD, only a DC component will be generated unless one of its sideband is suppressed. By using such a VCSEL-based PM-to-IM conversion scheme, the upper sideband will be amplified, and the lower sideband with different phase will be unchanged. Thereby, an optical PM-to-IM conversion is achieved successfully. As to the function of the optical IL, the optical IL is utilized to separate the optical signal into odd channel and even channel. The optical IL has two outputs, one of the outputs has the optical signal only with one optical sideband (upper sideband) for down-link transmission, and another output has the optical signal only with the optical carrier for up-link transmission.The PM-to-IM converter can also be composed of an optical IL and an optical band-pass filter (OBPF). The function of the OBPF is to pick up one of the sidebands from the optical IL output port with two sidebands (lower sideband and upper sideband). However, to consider the flexibility, the combination of optical IL and OBPF-based PM-to-IM converter is not flexible, since the free spectral range (FSR) of an optical IL is fixed. In contrast, the PM-to-IM conversion process achieved by a VCSEL-based converter can be flexibly adjusted to match with the injected PM optical signal. The proposed VCSEL-based PM-to-IM converter reveals its flexibility and alternative to meet the demand of PM-to-IM conversion.
For down-link transmission, the measured BER curves at a data signal of 5Gbps/20GHz are presented in Fig. 4(a) [measured at point (A) of Fig. 1]. At a BER of 10−9, there exists a small power penalty of 0.4 dB between back-to-back (BTB) and 40-km SMF transmission cases due to the use of PM scheme and the avoidance of RF power degradation induced by fiber dispersion. Since PM scheme is deployed at systems, yet noise and distortion induced by systems will be suppressed automatically, in which leading to great BER performance improvement. Furthermore, over a 40-km SMF transport, fiber dispersion deteriorates the performance of systems due to the double sideband (DSB) nature of the optical signal. In only one optical sideband (upper sideband) systems, since optical carrier and lower sideband are eliminated before detection, the RF power degradation induced by fiber dispersion can be avoided [15]. Only one optical sideband is processed by optical devices, the baseband data of 5 Gbps is obtained directly from the optical sideband. Moreover, the measured BER curves of downstream VCSEL channel as a function of the free-space transmission distance are presented in the Fig. 4(b). At a free-space transmission distance of 12 m; without employing LNA and data recovery, the BER is about 10−5; with employing LNA and data recovery, the BER is reached to 10−9. To show a more direct association with LNA and data recovery, we remove one of the schemes and measure the BER values. It is obvious that the BER performance improvement is restricted as only one improvement scheme is employed. With employing LNA only, the BER is about 10−6; with employing data recovery only, the BER is about 10−7. Such results indicate that LNA and data recovery play key roles for error corrections on BER performance improvements.
Fig. 4 (a) The measured BER curves at a downstream data signal of 5Gbps/20GHz. (b) The measured BER curves of downstream VCSEL channel as a function of the free-space transmission distance.
As to the up-link transmission, the measured BER curves at a data signal of 5Gbps/20GHz are shown in Fig. 5(a) [measured at point (B) of Fig. 1]. At a BER of 10−9, a small power penalty of 0.4 dB is existed between BTB and 40 km SMF transmission cases. Such a result can be attributed to the use of PM scheme and the cancellation of RF power degradation. In addition, the measured BER curves of upstream VCSEL channel as a function of the free-space transmission distance are shown in the Fig. 5(b). At a free-space transmission distance of 12 m; without employing LNA and data recovery, the BER is about 10−5; with employing LNA and data recovery, the BER is reached to 10−9. As LNA and data recovery are employed simultaneously, error free transmission is achieved to demonstrate the feasibility of establishing a bidirectional hybrid PM-based RoF and VCSEL-based VLLC system.
Fig. 5 (a) The measured BER curves at an upstream data signal of 5Gbps/20GHz. (b) The measured BER curves of upstream VCSEL channel as a function of the free-space transmission distance.
Figures 6(a) and 6(b) display the eye diagrams of 5-Gbps data stream over a 12-m free-space link under the conditions of without employing LNA and data recovery [Fig. 6(a)] and with employing LNA and data recovery [Fig. 6(b)], respectively. In Fig. 6(a), the corresponding jitter, signal-to-noise ratio (SNR), and rise/fall time are 12.5-ps rms, 21.5 dB, and 120 ps, respectively. In Fig. 6(b), the corresponding jitter, SNR, and rise/fall time are 3.4-ps rms, 31 dB, and 40 ps, respectively. The amplitude and phase fluctuations in the signal are obviously observed for the case of without employing LNA and data recovery [Fig. 6(a)]. However, a clear eye diagram is obtained due to amplitude and phase fluctuation suppressions [Fig. 6(b)]. Moreover, as the bandwidth is 0.7 times the data rate, the maximum transmission data rate for a 3-dB modulation bandwidth of 5.2-GHz VCSEL is 7.43 Gbps (). Figures 7(a) and 7(b) display the eye diagrams of 7-Gbps data stream over a 12-m free-space link under the conditions of without employing LNA and data recovery [Fig. 7(a)] and with employing LNA and data recovery [Fig. 7(b)], respectively. In Fig. 7(a), the corresponding jitter, SNR, and rise/fall time are 15.7-ps rms, 17.8 dB, and 138 ps, respectively. In Fig. 7(b), the corresponding jitter, SNR, and rise/fall time are 4.3-ps rms, 28.5 dB, and 51 ps, respectively. The amplitude and phase fluctuations in the signal are also obviously observed for the case of without employing LNA and data recovery [Fig. 7(a)]. However, somewhat clear eye diagram is obtained due to amplitude and phase fluctuation suppressions [Fig. 7(b)].
Fig. 6 The eye diagrams of 5-Gbps data stream over a 12-m free-space link: (a) without employing LNA and data recovery, (b) with employing LNA and data recovery.
Fig. 7 The eye diagrams of 7-Gbps data stream over a 12-m free-space link: (a) without employing LNA and data recovery, (b) with employing LNA and data recovery.
4. Conclusions
A bidirectional hybrid PM-based RoF and VCSEL-based VLLC systems over a 40-km SMF transport and a 12-m free-space transmission is proposed and experimentally demonstrated. To be the first one of employing injection-locked VCSEL-based PM-to-IM converters and optical ILs in such bidirectional hybrid RoF and VLLC systems, the downstream light is successfully phase-remodulated with RoF signal for up-link transmission. Good transmission performance of BER and clear eye diagram are obtained over a 40-km SMF transmission and a 12-m free-space VLLC link. Such a bidirectional hybrid RoF and VLLC system would be very attractive for fiber link applications in integration of fiber backbone and in-door networks to provide broadband integrated services, including Internet and telecommunication services.
Acknowledgment
The authors would like to thank the financial support from the National Science Council of the Republic of China under Grant NSC 100-2221-E-027-067-MY3, NSC 101-2221-E-027-040 -MY3, and NSC 103-2218-E-027-001.
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