Open Access
Add to BibSonomyAbstract
A 64 Gb/s four-level pulse amplitude modulation (PAM4) vertical-cavity surface-emitting laser (VCSEL)-based free-space optical (FSO) link with an external light injection scheme is proposed and successfully demonstrated. Experimental results show that the 11.2 GHz VCSEL with an external light injection scheme is sufficiently powerful for 64 Gb/s PAM4 FSO links. This study is the first one that adopts a 1550-nm VCSEL transmitter with an external light injection scheme in a 64 Gb/s PAM4 FSO link. The link performances of the proposed PAM4 VCSEL-based FSO links have been analyzed in real-time in terms of eye diagrams and offline processed by Matlab in terms of bit error rate (BER) performances. Good BER performance and clear eye diagrams are acquired over a 100-m free-space link. Such a proposed 64 Gb/s PAM4 VCSEL-based FSO link with an external light injection scheme is a promising one for providing high transmission rate and long transmission distance.
© 2017 Optical Society of America
1. Introduction
The exponential growth of data rate requires highly assembled bandwidth in modern communications. Engineers have operated on different data formats that can meet bandwidth and transmission rate demands. Non-return-to-zero (NRZ) and four-level pulse amplitude modulation (PAM4) are commonly utilized in different applications involving existing data formats. Transmission of an NRZ signal is relatively simple. However, as the transmission rate increases, the NRZ signal reaches the bandwidth limitation. The PAM4 signal is generated by combining two-channel NRZ signals and is thereby considered a good replacement due to effective bandwidth utilization. PAM4 cuts the bandwidth for a given data rate in half by delivering two bits in each symbol. The PAM4 signal is twice as bandwidth efficient as the NRZ signal, so its format is potentially favorable for high-speed data communications [1–6].
Vertical-cavity surface-emitting laser (VCSEL) technology has evolved significantly in recent years. For example, VCSEL can be designed to perform in a window with 1.55 μm wavelength under a single transverse mode. The new-generation VCSEL-based scheme is optimized for modulation bandwidth up to a few tens of GHz. One of the promising schemes is VCSEL with an external light injection scheme. VCSEL with an external light injection scheme exhibits a significantly enhanced frequency response [7, 8]. Therefore, external light injection scheme is anticipated to provide good transmission performances in PAM4 VCSEL-based free-space optical (FSO) links [9–11]. In this paper, a 64 Gb/s PAM4 VCSEL-based FSO link with an external light injection scheme is proposed and experimentally demonstrated. To the authors’ knowledge, this work is the first one that successfully constructs a 64 Gb/s PAM4 FSO link based on VCSEL with an external light injection scheme. A pair of doublet lenses is employed to adopt the free-space link in the 1550 nm operation. Propagating a laser beam through the free space between the doublet lenses enables an FSO link to work as if the fibers were connected seamlessly. The free-space link is thereby greatly increased by doublet lenses. In result, a maximum free-space link of 100 m with acceptable transmission performances is obtained. The link performances of such proposed PAM4 VCSEL-based FSO links have been evaluated in real-time in terms of eye diagrams and offline processed by Matlab in terms of bit error rate (BER) performances. Good BER performance and clear eye diagrams (three independent eye diagrams) are acquired at a 100-m free-space operation. In former researches [12–14], BER measurements were performed by auto-searching using a one-channel 28 Gb/s error detector (ED) and the PAM4 3-eye (upper/middle/lower) sampling approach. Nevertheless, a one-channel 32 Gb/s ED is expensive and thus will add the cost of a PAM4 VCSEL-based FSO link. The proposed BER measurement by offline process is attractive because a costly one-channel 32 Gb/s ED is not required.
Previous studies demonstrated PAM4 VCSEL-based transmissions with light injection and optoelectronic feedback techniques [12, 13]. Other study showed the feasibility of establishing a 56 Gb/s PAM4 VCSEL-based light-based WiFi (LiFi) transmission with two-stage injection-locked technique [14]. However, sophisticated light injection and optoelectronic feedback techniques as well as two-stage injection-locked technique are required. These techniques will increase the complexity of PAM4 VCSEL-based transmissions. For an actual implementation of a PAM4 VCSEL-based transmission, it is necessary to employ a performance improvement scheme with low complexity. VCSEL with an external light injection scheme is a feasible scheme by which just right one-stage injection locking is required. Compared with light injection and optoelectronic feedback techniques as well as two-stage injection locking, it is attractive because it avoids the need of complex light injection and optoelectronic feedback techniques as well as two-stage injection locking with elaborate wavelength detuning. In addition, the transmission rates of 51.56 Gb/s [12] and 45 Gb/s [13] are less than 64 Gb/s of our proposed PAM4 transmissions. And the transmission rate of 56 Gb/s and the free-space link of 20 m [14] are much less than 64 Gb/s and 100 m of our proposed PAM4 FSO links. This proposed 64 Gb/s PAM4 VCSEL-based FSO link with an external light injection scheme is demonstrated to be superior over the prior PAM4 VCSEL-based transmissions. It is a promising one for efficient bandwidth utilization to achieve high transmission rate and long free-space link.
2. Experimental setup
Figure 1 shows the experimental configuration of the proposed 64 Gb/s PAM4 VCSEL-based FSO links with an external light injection scheme. Two binary pseudorandom bit sequence (PRBS) data streams with a length of 215−1 at 32 Gb/s are generated from a two-channel PRBS generator. The amplitudes of the binary data streams (NRZ signals) are 900 and 450 mV. These two 32 Gb/s NRZ signals are fed into a PAM4 converter to create a 64 Gb/s PAM4 signal with four levels and three independent eye diagrams. The VCSEL, with output power/3 dB bandwidth/central wavelength of 0 dBm/11.2 GHz/1550.42 nm, is directly modulated by a 64 Gb/s PAM4 signal. A distributed feedback (DFB) laser diode (LD), with optical power/3 dB bandwidth/central wavelength of 10 dBm/12 GHz/1550.45 nm, is employed as the master laser. The optical output of the DFB LD is injected into the VCSEL via a three-port optical circulator (OC) and a polarization controller (PC). The PC is employed to match the master polarization to the VCSEL preferred polarization to ensure the stability of injection locking. The 64 Gb/s PAM4 signal is then amplified by an erbium-doped filter amplifier (EDFA) with an input power of 0 dBm and an output power of 23 dBm, and attenuated by a variable optical attenuator (VOA). A VOA is introduced at the start of the free-space link to optimize the optical power launched into the free space and to obtain the optimum transmission performances. A pair of doublet lenses (doublet lenses 1 and 2), as shown in Fig. 2, is adopted to emit light from an optical fiber into the free space and to couple light from the free space into an optical fiber. The doublet lenses connected to single-mode fibers (SMFs) play vital roles in transmitting light through the free space between the two sides (transmitter and receiver sides). The light emitted from the ferrule of SMF (transmitter side) is launched into doublet lens 1, transmitted in the free space, inputted into doublet lens 2, and concentrated on the ferrule of SMF (receiver side). FSO links have different free-space links in the range of 0–150 m. The laser light reaches a 30 GHz photodiode (PD) with a trans-impedance amplifier (TIA) to convert the laser light into a 64 Gb/s PAM4 signal. The PD exhibits responsivity of approximately 0.75 A/W (at 1550 nm). The link performances of the proposed 64 Gb/s PAM4 VCSEL-based FSO links are analyzed in real-time in terms of eye diagrams and offline processed by Matlab in terms of BER performances. The eye diagrams of the transmitted 64 Gb/s PAM4 signal are seized by a digital storage oscilloscope (DSO) at the receiver side. The BER performances of the transmitted 64 Gb/s PAM4 signal are offline processed through synchronization, equalization, and hard decision processes.
Fig. 1 The experimental configuration of the proposed 64 Gb/s PAM4 VCSEL-based FSO links with an external light injection scheme.
3. Experimental results and discussions
The injection-locking range () for laser under light injection can be stated as [15]:
where is the line width enhancement factor, is the injection ratio, and k is the coupling coefficient. Within the locking range, the wavelength of the injection-locked laser (slave laser) is locked nearly to that of the injection source laser (master laser). The injection locking behavior takes place when a master laser (DFB LD) is detuned to a wavelength longer than that of the slave laser (VCSEL) by a small amount, that is, positive wavelength detuning is adopted to achieve the injection locking. Optimum injection locking can be achieved if the wavelength of the injection source laser (master laser) is longer than that of the injection-locked laser (slave laser) by a small amount. Here, the optimum injection locking condition is observed when the wavelength detuning between DFB LD and VCSEL is 0.03 nm (λDFB LD - λVCSEL = 0.03). Figure 3(a) shows that the optical spectrum of DFB LD, and Fig. 3(b) shows the optical spectrum of VCSEL for the scenarios of free-running and injection-locked. Obviously, as VCSEL is injection-locked, its optical spectrum shifts to a longer wavelength by a small amount. One key trait of injection locking is that the injection-locked laser is compelled to oscillate at the injection wavelength instead of its original free-running wavelength. The resonance frequency of the injection-locked laser () can be given by [15]:whereis the relaxation oscillation frequency of the free-running slave laser, and is the steady-state phase difference between the injection-locked slave laser and the master laser. Clearly, the resonance frequency of the injection-locked laser is proportional to the square of the injection ratio. Increasing the injection ratio will enhance the resonance frequency. On the other hand, if negative wavelength detuning (when a master laser is detuned to a wavelength shorter than that of the slave laser by a small amount) with a high injection ratio or positive wavelength detuning with a low injection ratio are adopted to achieve the injection locking, then the increment for the frequency response of the PAM4 VCSEL-based FSO links is limited. Thereby, positive wavelength detuning with a high injection ratio should be adopted to achieve the injection locking with a significantly enhanced frequency response. The frequency responses of the PAM4 VCSEL-based FSO links for the scenarios of free-running and injection-locked are presented in Fig. 4. For the free-running scenario, the 3-dB bandwidth is 11.2 GHz. For the injection-locked scenario, the 3-dB bandwidth is increased up to 22.8 GHz. Injection-locked scheme enhances the frequency response of the VCSEL to about 2.04 times (22.8/11.2 ~2.04). This result shows that the 1550-nm VCSEL with an external light injection scheme is sufficiently strong for a 64 Gb/s (22.82 = 64.5 > 64) PAM4 VCSEL-based FSO link.Fig. 3 (a) The optical spectrum of DFB LD and (b) the optical spectrum of VCSEL for the scenarios of free-running and injection-locked.
Fig. 4 The frequency responses of the PAM4 VCSEL-based FSO links for the scenarios of free-running and injection-locked.
Since the optical characteristics of DFB LD are better than those of VCSEL, a DFB LD with a high 3-dB bandwidth of 22.7 GHz (22.72 = 64) could be employed to replace the 11.2 GHz VCSEL with an external light injection scheme to establish a 64 Gb/s PAM4 FSO link. Nevertheless, it will increase the cost of a PAM4 FSO link. For a real implementation of a PAM4 FSO link, a cost-effective light source is the key issue for system designer. In terms of cost, a low 3-dB bandwidth VCSEL with external light injection scheme is a promising approach. Moreover, for a 64 Gb/s PAM4 DFB LD-based FSO link, a sophisticated and costly 22.7 GHz linear driver is needed to drive the DFB LD. However, for a 64 Gb/s PAM4 VCSEL-based FSO link, a sophisticated and costly linear driver is not needed to drive the VCSEL. Thereby, our proposal presents an outstanding one than that of PAM4 DFB LD-based FSO link.
Figure 5 presents the block diagram of the decision-feedback equalizer (DFE). For the DFE, x is the input, cn is the feedforward coefficient, bn is the feedback coefficient, y is the sum of the weighted taps, d is the decision output, and e is the error. We sample the signal at instant t0 + kT, the sum of the weighted taps y is given by [16]:
Where n is an integer from 0 to N-1 (n = 0, 1, 2, ……, N-1), t0 is the initial time, and T is the signaling interval. The error e is decided by the sum of weighted taps y and the decision output d:The channel responses gradually update the feedforward coefficient cn and the feedback coefficient bn: where u is the step size. The error e continually updates the cn and bn coefficients and thus adaptively compensates for the received PAM4 signal as the output of the DFE. The hard decision after the DFE (as illustrated in Fig. 1) are conducted to the equalized signal. Both DFE and hard decision are made for maintaining good BER performance.The BER curves of the 64 Gb/s PAM4 VCSEL-based FSO links over different free-space links in the range of 0–150 m are presented in Fig. 6. The BER value increases as the free-space link increases. As the free-space link is larger than 100 m, the BER value is higher than 10−9. At a 10−9 BER operation, a power penalty of 4 dB exists between the scenarios of back-to-back (BTB) and over a 100-m free-space link. As the free-space link is longer than 100 m, the BER value is higher than 10−9. Over a 125-m free-space link, the BER value deteriorates to 10−5 due to the decline of optical signal-to-noise ratio (OSNR) and divergent focal spot size between the doublet lens 2 and the ferrule of optical fiber (receiver side). Over a 150-m free-space link, however, an error floor exists clearly because of the significant decline of OSNR and large divergent focal spot size between the doublet lens 2 and the ferrule of optical fiber. As the free-space link increases, however, the received OSNR decreases because the received optical power includes additional distortion lights from the environment. Such an OSNR decrease results in the BER performance degradation. The focal spot size between the doublet lens 2 and the ferrule of the optical fiber (receiver side) is crucial for the transmission performances of 64 Gb/s PAM4 VCSEL-based FSO links. A long free-space link causes a divergent focal spot size and leads to a poor transmission performance. System designers must address the maximum focal spot size (the acceptable divergent focal spot size) to guarantee a feasible implementation of 64 Gb/s PAM4 VCSEL-based FSO link. It can be concluded that as the free-space link is shorter than or equal to 100 m, the BER performance degradation due to the decline of OSNR is tolerable. The maximum free-space link by which 10−9 BER operation can be reached is around 100 m. Since the 1550 nm invisible laser light has an attenuation of around 0.04 dB/m in the free space, yet the power budget is about 4 dB (0.04 dB/m × 100 m = 4 dB). Over a 100-m free-space link, the launch optical power at the transmitter side is 8.2 dBm and the received optical power at the receiver side is 4.2 dBm (8.2 – 4.2 = 4), by which 10−9 BER operation is obtained. Nevertheless, as the free-space link is longer than 100 m, the BER performance degradation due to the decline of OSNR and divergent focal spot size is intolerable. Over a 150-m free-space link, the received optical power at the receiver side is around 10 dBm to compensate the significant decline of OSNR and large divergent focal spot size. However, it can be seen that the compensation effect is limited by which just only 10−2 BER operation is obtained.
Fig. 6 The measured BER curves of the 64 Gb/s PAM4 VCSEL-based FSO links over different free-space links in the range of 0–150 m.
The eye diagrams of the 64 Gb/s PAM4 signal over different free-space links in the range of 0–150 m are displayed in Figs. 7(a)–7(d), respectively. The qualities of the 64 Gb/s PAM4 signal for the scenarios of BTB and over a 100-m free-space link are shown in Figs. 7(a) and 7(b). However, amplitude and phase fluctuations exist by a large amount for the scenario of over a 125-m free-space link [Fig. 7(c)]. In addition, close eye diagrams exist obviously for the scenario of over a 150-m free-space link [Fig. 7(d)].
Fig. 7 The eye diagrams of the 64 Gb/s PAM4 signal (a) for BTB, (b) over a 100-m free-space link, (c) over a 125-m free-space link, and (d) over a 150-m free-space link scenarios.
4. Conclusions
A 64 Gb/s PAM4 VCSEL-based FSO link with an external light injection scheme is proposed and experimentally demonstrated. Results show that a 11.2-GHz VCSEL with an external light injection scheme is powerful for a 64 Gb/s PAM4 FSO link. To the authors’ knowledge, it is the first one that employs a 1550-nm VCSEL transmitter with an external light injection scheme in a 64 Gb/s PAM4 FSO link. Good BER performance of 10−9 (offline processed by Matlab) and clear eye diagrams (measured in real-time) are acquired over a 100-m free-space link. Such a proposed 64 Gb/s PAM4 VCSEL-based FSO link provides the advantages of optical wireless communications at high transmission rate and long transmission distance.
Funding
Ministry of Science and Technology of the Republic of China (MOST 104-2221-E-027-072-MY3 and MOST 105-2633-E-027-001).
References and Links
1. C. Yang, R. Hu, M. Luo, Q. Yang, C. Li, H. Li, and S. Yu, “IM/DD-Based 112-Gb/s/lambda PAM-4 Transmission Using 18-Gbps DML,” IEEE Photonics J. 8(3), 7903907 (2016). [CrossRef]
2. S. Zhou, X. Li, L. Yi, Q. Yang, and S. Fu, “Transmission of 2 × 56 Gb/s PAM-4 signal over 100 km SSMF using 18 GHz DMLs,” Opt. Lett. 41(8), 1805–1808 (2016). [CrossRef] [PubMed]
3. R. Motaghiannezam, T. Pham, A. Chen, T. Du, C. Kocot, J. Xu, and B. Huebner, “52 Gbps PAM4 receiver sensitivity study for 400GBase-LR8 system using directly modulated laser,” Opt. Express 24(7), 7374–7380 (2016). [CrossRef] [PubMed]
4. R. Motaghiannezam, I. Lyubomirsky, H. Daghighian, C. Kocot, T. Gray, J. Tatum, A. Amezcua-Correa, M. Bigot-Astruc, D. Molin, F. Achten, and P. Sillard, “Four 45 Gbps PAM4 VCSEL based transmission through 300 m wideband OM4 fiber over SWDM4 wavelength grid,” Opt. Express 24(15), 17193–17199 (2016). [CrossRef] [PubMed]
5. D. Sadot, G. Dorman, A. Gorshtein, E. Sonkin, and O. Vidal, “Single channel 112Gbit/sec PAM4 at 56Gbaud with digital signal processing for data centers applications,” Opt. Express 23(2), 991–997 (2015). [CrossRef] [PubMed]
6. F. Karinou, C. Prodaniuc, N. Stojanovic, M. Ortsiefer, A. Daly, R. Hohenleitner, B. Kögel, and C. Neumeyr, “Directly PAM-4 modulated 1530-nm VCSEL enabling 56 Gb/s/λ data-center interconnects,” IEEE Photonics Technol. Lett. 27(17), 1872–1875 (2015). [CrossRef]
7. H. H. Lu, C. Y. Li, C. A. Chu, T. C. Lu, B. R. Chen, C. J. Wu, and D. H. Lin, “10 m/25 Gbps LiFi transmission system based on a two-stage injection-locked 680 nm VCSEL transmitter,” Opt. Lett. 40(19), 4563–4566 (2015). [CrossRef] [PubMed]
8. X. Zhao, D. Parekh, E. K. Lau, H. K. Sung, M. C. Wu, W. Hofmann, M. C. Amann, and C. J. Chang-Hasnain, “Novel cascaded injection-locked 1.55-μm VCSELs with 66 GHz modulation bandwidth,” Opt. Express 15(22), 14810–14816 (2007). [CrossRef] [PubMed]
9. C. Y. Li, H. H. Lu, T. C. Lu, C. J. Wu, C. A. Chu, H. H. Lin, and M.-T. Cheng, “A 100 m/320 Gbps SDM FSO link with a doublet lens scheme,” Laser Phys. Lett. 13(7), 075201 (2016). [CrossRef]
10. H. H. Lu, C. Y. Lin, T. C. Lu, C. A. Chu, H. H. Lin, B. R. Chen, C. J. Wu, and W. S. Tsai, “150 m/280 Gbps WDM/SDM FSO link based on OEO-based BLS and afocal telescopes,” Opt. Lett. 41(12), 2835–2838 (2016). [CrossRef] [PubMed]
11. M. A. Khalighi and M. Uysal, “Survey on free space optical communication: a communication theory perspective,” IEEE Commun. Surveys Tuts. 16(4), 2231–2258 (2014). [CrossRef]
12. H. H. Lu, C. Y. Li, C. K. Lu, C. M. Ho, H. W. Chen, M. T. Cheng, Z. Y. Yang, and S. J. Huang, “A 103.12 Gb/s WDM PAM4 VCSEL-based transmission with light injection and optoelectronic feedback techniques,” IEEE Photonics J. 9(1), 7900808 (2017).
13. H. H. Lu, C. Y. Li, H. W. Chen, Z. Y. Yang, X. Y. Lin, M. T. Cheng, C. K. Lu, and T. T. Shih, “45 Gb/s PAM4 transmission based on VCSEL with light injection and optoelectronic feedback techniques,” Opt. Lett. 41(21), 5023–5026 (2016). [CrossRef] [PubMed]
14. H. H. Lu, C. Y. Li, H. W. Chen, C. M. Ho, M. T. Cheng, Z. Y. Yang, and C. K. Lu, “A 56 Gb/s PAM4 VCSEL-based LiFi transmission with two-stage injection-locked technique,” IEEE Photonics J. 9(1), 7900208 (2017).
15. S. Mohrdiek, H. Burkhard, and H. Walter, “Chirp reduction of directly modulated semiconductor lasers at 10 Gb/s by strong CW light injection,” J. Lightwave Technol. 12(12), 418–424 (1994). [CrossRef]
16. S. U. H. Qureshi, “Adaptive equalization,” Proc. IEEE 73(9), 1349–1387 (1985). [CrossRef]
