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Abstract
Future fronthaul wireless networks will be more competitive and attractive if they can use power- and cost-efficient transmitter systems to realize both extended reach and enhanced network capacity. Photonic generation of high-frequency RF carrier signals is currently considered an attractive technique. The laser gain-switching technique for photonic generation of high-frequency RF carrier signals has attracted great interest recently and is a cost-efficient scheme. Vertical cavity surface emitting laser (VCSEL) are power-efficient optical sources requiring bias currents below 10-mA. The use of VCSELs for gain-switched optical frequency comb generation has proven to be attractive in the term of power-efficiency. However, the data transmission performance of these gain-switched VCSEL optical frequency combs are yet to be demonstrated. In this paper, for the first time to our knowledge, we numerically demonstrated that RF carrier signals generated from a gain-switched VCSEL optical frequency comb can support up to16-Gbps error-free data transmission over fiber length beyond 20-km. A 56-GHz RF carrier signal was amplitude-modulated with 10- and 16.3-Gbps data before error-free transmission over 20.5-km of standard single mode fibers. Transmission penalties of 2- and 7-dB were recorded at 10- and 16.3-Gbps, respectively, at receiver sensitivities below -17-dBm.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Application scenarios such as augmented reality, artificial intelligence, virtual reality, robotics, vehicle-to-vehicle communication, and 4k/8k high-definition videos are being researched and developed. These applications will require huge spectrum in both the fronthaul and the radio access parts of future 5G wireless networks. Radio-over-fiber (RoF) techniques are being adopted in the fronthaul section of the wireless network to maximize both capacity and reach, while high-frequency RF generation using photonic techniques is being appreciated to assist with high-frequency radio frequency (RF) generation for more bandwidth in the radio access part of the networks [1]. Photonic generation of high-frequency RF carrier signals is attractive because it can be used to realize both long-distance remoting of RF carrier signals using optical fibers, and to relax the complexity associated with electronic generation of high-frequency RF signals [2].
There are several well-known photonic techniques for generation of high-frequency RF carriers. These techniques include opto-electronic schemes based on modulating a null-biased intensity modulator with an electronic oscillator [3], optical heterodyning using dual-mode lasers [4], or optical heterodyning using two independent laser sources [5]. Optical frequency comb generation techniques based on mode-locked lasers [6], phase modulators [7], nonlinear comb generation techniques [8], and gain-switching of semiconductor laser diodes [9], are more attractive photonic techniques for high-frequency RF carrier generation because they can be used to generate multi-wavelength optical sources with high phase correlation. Any pair of the produced wavelengths from the generated optical comb can be heterodyned to produce RF carriers with good phase-noise performance [8]. Among the above-mentioned comb generation techniques, the technique based on gain-switching of a semiconductor laser is an attractive technique both in its design and the phase-noise performance of the resultant RF carrier signals, attracting great attention recently.
The wireless transmission window from 30- to 60-GHz has been of great interest due to its unlicensed spectrum [10]. Several 60-GHz RoF systems using gain-switching technique have been demonstrated and reported. To date, all these previously reported studies used power-hungry semiconductor laser such as distributed feedback (DFB) laser [11–13] to generate gain-switched optical combs for photonic RF generation. By adopting standard edge-emitting lasers for gain-switched comb generation, the amount of direct modulation power needed for an optical frequency comb giving 8-10 comb lines is about 0.5- to 1-W, requiring the necessity of radio frequency power amplifiers. By choosing power-efficient semiconductor laser sources, these previously reported gain-switched photonic-assisted fronthaul systems could greatly be improved to realize power- and cost-efficient fronthaul wireless networks. Vertical cavity surface emitting lasers (VCSELs) are power-efficient optical sources, requiring low bias currents, normally under 10 mA, to operate [14]. Their integration capabilities as well as their capacity for on-chip testing allow for low-cost optical subsystems with an excellent energy efficiency. Recently, we have demonstrated a simple and cost-effective photonic-assisted wireless system at Q-band using VCSELs as our light sources [15]. The spectral efficiency was doubled by adopting a simple pulse amplitude modulation (PAM-4) scheme. However, using two free-running VCSEL lasers for photonic generation of RF signal results in an RF carrier signal with high phase noise [2], and the use of two VCSELs can jeopardize the effort of designing simple, power- and cost-efficient photonic-assisted fronthaul wireless networks. Recent studies on gain-switching of VCSELs to generate optical combs showed, both numerically and experimentally, that VCSELs have the potential of generating power-efficient optical frequency combs [16–18]. Despite such promising findings, the data transmission performance of RF carrier signals generated from a gain-switched VCSEL comb is yet to be demonstrated both numerically and experimentally.
In this paper, for the first time to our knowledge, we have numerically investigated the data transmission performance of RF carrier signals generated from a gain-switched VCSEL optical comb. We applied the study in the frequency band around 60-GHz, frequency of interest for future millimeter wave wireless networks. The amplitude-noise of the generated RF carrier signals was numerically investigated in time-domain over a period of 200 ns. Moreover, a 56-GHz RF carrier signal was generated and amplitude-modulated with 10- and 16.3-Gbps data and transmitted over 20.5-km of standard single mode fibers (SSMF). Results showed that RF carrier signals generated from a gain-switched VCSEL comb are sensitive to the phase-correlation among the selected comb lines, calling for careful time- and frequency-domain characterization of these signals. Nevertheless, once the generated RF carrier signal is carefully characterized, it can be used for error-free data transmission over longer fiber distances exceeding 20-km, at a receiver sensitivity below -19 dBm.
2. Fundamental theory for envelop detection
Demodulation of the photonically-generated RF carrier signal was achieved using the direct detection scheme. Direct detection is mostly adopted in wireless communication systems due to its simplicity [19,20], improving the system in term of less-complexity. Figure 1 shows a direct detection scheme implemented in this work. A low-noise electrical amplifier (EA) with 26-dB gain was used to amplify the wireless RF carrier signal after its reception by the RF antenna.
The transmitted data signal was encoded as amplitude variation of the RF carrier signal, but not its phase, removing the necessity of an electrical local oscillator for down-conversion to intermediate frequency for RF carrier phase recovery. To recover the transmitted baseband signal, a low-pass filter with a cut-off frequency equal to the baseband frequency acted as an envelope detector circuit, removing the high-frequency RF carrier signal. The remaining low-frequency signal at baseband is then analyzed to determine the system performance. The absence of a local oscillator makes the direct detection scheme simple, phase-noise insensitive, and low cost [14]. Envelope detectors are based on the square law. The amplitude-modulated RF carrier signal received at the envelop detector input can be expressed as [21]
where A represents the DC-bias current, m is the intensity of the modulating baseband signal, $s(t )$ denotes the time-varying modulating baseband signal, and ${f_c}$ represents the frequency of the RF carrier signal. The resultant current at the input of the low-pass filter-based envelope detector can be described as3. Design setup
The experimental procedure was divided into two subsections. The first subsection will discuss the configuration of a gain-switched VCSEL comb generator. The second subsection discusses the full schematic used to realize the reported VCSEL-based fronthaul network.
3.1 Gain-switched VCSEL for optical frequency comb generation
Features of VCSELs discussed in section 1 make them attractive optical sources for use in cost- and power-sensitive fronthaul networks. A RayCan VCSEL with a direct-modulation frequency of 10-GHz and a saturation current around 9 mA was first characterized to extract the intrinsic parameters necessary for numerical analysis. These parameters were calculated and are given in Table 1. Using these parameters, we numerically generated and evaluated the characteristic of the comb generated by the system. The numerical study was carried out using the latest version of Optiwave software (Optiwave 17.0). For exclusively generating the optical comb and RF carrier signals, and for studying the dynamics of a gain-switched VCSEL optical comb, the schematic in Fig. 2 was modified as follows. The polarization controller (PC), Mach Zehnder modulator (MZM), pulse pattern generator (PPG), optical fiber, and the bit error rate (BERT) tester were initially not connected. The switching frequency of the sine generator was 14-GHz, slightly above the 10-GHz relaxation oscillation frequency of the VCSEL. The switching current of the sine generator was 10-mA, also just slightly above the saturation current of the VCSEL. It is worth mentioning that these operating points of the system should be determined. A rule of thumb, confirmed by experimental results, suggested that the electrical switching frequency to the VCSEL should be close to the relaxation oscillation frequency of the VCSEL, and the modulation current should be determined empirically [17]. Increasing the modulation current from the sine generator will need to be compensated by reducing the bias current of the VCSEL below the saturation current and above the threshold current. The wavelength-selective switch (WSS) was used to filter out two wavelengths, separated by the desired RF carrier frequency. These two filtered wavelengths were then combined at the optical coupler and allowed to beat at the photodetector (PD). This laser gain-switching system can generate high-frequency RF carrier signals, limited by the maximum span of the resultant optical frequency comb.
Table 1. Measured VCSEL Intrinsic Parameters
3.2 VCSEL-based photonic RF signal modulation and fiber transmission
We proposed an RoF network operating at 56-GHz carrier frequency for enhanced bandwidth in the radio access part. The proposed network is based on the same configuration of Fig. 2. To realize this, the polarization controller, Mach Zehnder modulator, pulse pattern generator, optical fiber, and the bit error rate tester were now connected. Two optical channels at 1552.2048-nm and 1552.6549-nm, separated to give an RF carrier frequency of 56-GHz, were selected by the WSS. The modulated channel was connected to a PC to match its polarization to that of the MZM biased at quadrature point [22]. To emulate a PPG, a pseudo random bit sequence of 10- and 16.3-Gbps was generated and passed to the pulse generator, giving out 10- and 16.3-Gbps electrical data to the MZM. The modulated wavelength was then combined with the unmodulated wavelength acting as a local oscillator, at the optical coupler. These two combined wavelengths were then transmitted over a 20.5-km of SSMF to a 60-GHz PIN photodiode having a responsivity of 0.8 A/W. The optical spectrum analyzer (OSA) was used to monitor the two optical channels. The variable optical attenuator (VOA) was used to vary and keep the power to the PD constant. The modulated RF carrier signal at the output of the PD was monitored on an electrical spectrum analyzer (ESA) before amplification with an EA of 26-dB gain. As described in section 2, the low-pass filter (LPF) acted as an envelope detector and was then used to recover the transmitted baseband data. System performance analysis was performed based on bit error rate calculations using the BERT.
4. Results and discussion
In this section, we present and discuss the results. The dynamics of the generated gain-switched VCSEL optical comb will be presented in section 4.1, and the transmission performance of the 56-GHz RF carrier signal generated from a gain-switched VCSEL frequency comb will be given and discussed in section 4.2.
4.1 Dynamic behaviors of a gain-switched VCSEL optical comb
Figure 3 shows the powers vs. bias current characteristics of the used VCSEL. Th VCSEL had a threshold bias current of 1.23 mA and a saturation current of 9.6 mA. Figure 4 shows the continuous wave (CW) spectrum generated by the VCSEL using the extracted intrinsic parameters of Table 1. The spectrum has a peak power of -0.53 dBm, centered at 1552.43 nm, one of the operational wavelengths of the VCSEL as given by the manufacture’s datasheet. In Fig. 5, we show the generated optical frequency comb. The results of Fig. 5 show a well-spaced optical frequency comb with clearly resolved spectral lines. Remarkably, at such low bias current and low modulation current, the system was able to give a 3-dB bandwidth (the number of optical lines when the power is reduced by 3-dB) of 11 comb lines. We have also investigated how the frequency comb evolved when both the electrical switching frequency and the modulation current were varied above or below their optimum values. Figures 6 and 7 shows how the comb behaved when the modulation current was reduced above and below the optimum modulation current, respectively.
Fig. 5. The generated optical comb of the gain-switched VCSEL laser at optimum switching current and frequency
When the modulation current was above the optimum value, the comb kept its shape, but the extinction ratio for each comb line was reduced, and the side modes were consumed as can be seen on Fig. 6. This was because at high modulation current from the sine generator, the VCSEL received power above its maximum saturation bias current, driving it into a highly-nonlinear regime. In this region, the dynamics in the cavity of the VCSEL modified its gain bandwidth, resulting into new optical lines being generated between each pair of the comb lines. Below the optimum modulation peak power, the combs lines remained clearly visible, but the number of comb lines were reduced as can be seen on Fig. 7. These behaviors were well-reported in a recent numerical study of a gain-switched VCSEL optical comb [16], and we reported them here for completeness.
Next, we report on novel results regarding the time-domain characterization of the coherence of the generated optical comb lines and how they affected the generated RF carrier signal over some time period. These results are worth reporting here since previous studies on gain-switched VCSEL combs were focused on the flatness and the number lines of the generated frequency comb [16–18]. Here, we report the time-domain coherence of spectral lines from a gain- switched VCSEL optical frequency comb. It was found that two different comb lines pairs, each pair separated by a certain RF carrier frequency, can give identical spectra purity when the results are analyzed sorely in the frequency domain. We observed that, if the generated optical comb had well-spaced comb lines, the resultant RF signal will depict a pure RF spectrum with good phase noise performance when observed in the spectral domain. However, the time-domain results of RF carriers generated by a certain pairs of comb lines will not necessarily have similar results when investigated over a certain time period. Figure 8 and 10 shows two photonically-generated RF carrier signals, one at 50-GHz and the other at 56-GHz. These signals were both capture with the same radio frequency spectrum analyzer. The frequency domain results indicated that both these RF carrier signals had the same spectral purity and phase noise performance, and that both can be fit to be used as RF carriers for coherence wireless data transmission.
However, their respective time-domain results shown in Figs. 9 and 11 indicated that each of these generated RF carriers had amplitude variation different from the other. If these two RF carrier signals were to be used for QAM modulation where data is encoded both in the phase and intensity of the RF carrier signal, it can be safe to say that the 56-GHz RF carrier will have a better performance over the 50-GHz RF carrier. Even though the 50-GHz RF carrier signal might support purely phase modulation, it might not be fit as an RF carrier for amplitude modulation, especially when no digital signal processing and no forward error correction techniques are used. We therefore suggest that for gain-switched VCSEL optical combs, the resultant RF carrier signals could first be characterized both in time- and frequency-domain before being applied for coherence wireless data transmission. The amplitude variations in the generated RF carrier at 50-GHz can be attributed to a phase delay between the two filtered comb lines. This is so because, for the RF carrier signal to be generated by the PD, both comb lines should appear simultaneously at the input of the PD [23]. If one of the filtered comb lines is delayed, the PD only sees a single optical carrier, and therefore, no photonic down conversion will be established, resulting in the absence of the RF signal. The repetition behavior in the time-domain amplitude of the 50-GHz RF carrier showed that these two filtered comb lines were varying in phase with respect to each other at a defined frequency.
4.2 Transmission of RF carriers generated by a gain-switched VCSEL comb
For the first time to our knowledge, we report on the performance of a fronthaul network using a 56-GHz RF carrier signal generated from a gain-switched VCSEL optical frequency comb. The two filtered wavelengths with one wavelength modulated with data is shown in Fig. 12. The resultant 56-GHz RF carrier signal is shown in Fig. 13. The same data carried in one of the filtered wavelengths is also transferred to the generated 56-GHz RF carrier. Data transmission performance of the system was determined using BER calculations at both bit rates. The BER measurements were carried out at back-to-back (B2B) and over 20.5-km SSMF. The BER results for 10-Gbps data rate at B2B and over 20.5-km SSMF are shown in Fig. 14. As can be seen in Fig. 14, considering a BER of 10−9, a fiber transmission penalty of 2-dB was obtained with a receiver sensitivity of -22.4 dBm. These results indicate that RF carrier signals generated from a gain-switched VCSEL comb are competitive and capable for error-free data transmission at 10-Gbps over fiber lengths beyond 20 km. The eye diagram at 10-Gbps after 20.5-km fiber transmission is given in Fig. 15. The eye was clearly open even after 20.5-km fiber transmission, with an extinction ratio (ER) of 2.65-dB, qualitatively supporting the BER results.
Figure 16 shows the system performance at 16.3-Gbps data transmission at B2B and over 20.5-km SSMF. Considering the same BER of 10−9, a slightly high transmission penalty of 7-dB was obtained with a receiver sensitivity of -18 dBm. Nevertheless, the system still demonstrated an attractive data transmission performance even at such a high bit rate. This further demonstrated the potential of VCSEL-based optical frequency combs for high-frequency RF carrier signal generation and for high-speed data transmission. Corresponding eye diagrams at 16.3-Gbps after 20.5-km fiber transmission is given in Fig. 17. The eye is still well open and had an ER of 2.49-dB. The results reported here have shown that RF carrier signals generated from gain-switching of VCSEL for optical frequency comb generation can support error-free data transmission above 16-Gbps and could therefore be an attractive solution for designing future 5G fronthaul networks requiring low design cost and enhanced network capacity.
5. Conclusions
We have reported on the use of a gain switched VCSEL optical frequency comb for generation of high-frequency RF carrier signals and for error-free data transmission above 16-Gbps over fiber distance beyond 20-km. A 56-GHz RF carrier signal generated from two filtered comb lines of the resultant gain-switched VCSEL optical frequency comb was modulated with 10- and 16.3-Gbps data before being transmitted over 20.5-km of SSMF. Error-free transmission penalties of 2- and 7-dB were achieved at 10- and 16.3-Gbps data rate, respectively. The receiver sensitivity at both data rates was below -17 dBm. First time to our knowledge, we have simultaneously reported on optical frequency comb generation, time-domain coherence characterization, and data transmission performance of a gain-switched, low-power VCSEL comb frequency comb. These results are attractive as they demonstrated and motivated the possibility of using gain-switched VCSELs in future fronthaul 5G wireless systems to realize both simple and power-efficient wireless networks.
Acknowledgments
This research is funded by Telkom, Dartcom, ATC South Africa, Lambda Test Equipment, the NRF and SARAO, to whom the authors would like to express their gratitude.
Disclosures
The authors declare no conflicts of interest.
References
1. K. Liu, S. Jia, S. Wang, X. Pang, W. Li, S. Zheng, H. Chi, and X. Jin, “100 Gbit/s THz Photonic Wireless Transmission in the 350-GHz Band With Extended Reach,” IEEE Photonics Technol. Lett. 30(11), 1064–1067 (2018). [CrossRef]
2. J. Yao, “Microwave photonics,” J. Lightwave Technol. 27(3), 314–335 (2009). [CrossRef]
3. T. Sakamoto, T. Kawanishi, and M. Izutsu, “Optoelectronic oscillator using push-pull Mach-Zehnder modulator biased at null point for optical two-tone signal generation,” presented at the 2005 Conference on Lasers and Electro-Optics, Baltimore, pp. 877–879.
4. D. Novak, A. Nirmalathas, R. B. Waterhouse, C. Lim, P. A. Gamage, T. R. Clark Jr., M. L. Dennis, and J. A. Nanzer, “Radio-Over-Fiber Technologies for Emerging Wireless Systems,” IEEE J. Quantum Electron. 52(1), 1–11 (2016). [CrossRef]
5. R Karembera, S Wassin, G Isoe, K Nfanyana, and TB Gibbon, “All-optical flexible 5G signal generation and transmission for spectrum resource optimization,” J. Opt. Soc. Am. B 37(11), A324–A330 (2020). [CrossRef]
6. J. Jin, “Dimensional metrology using the optical comb of a mode-locked laser,” Meas. Sci. Technol. 27(2), 022001 (2016). [CrossRef]
7. S. Ozharar, F. Quinlan, I. Ozdur, S. Gee, and P. J. Delfyett, “Ultraflat Optical Comb Generation by Phase-Only Modulation of Continuous-Wave Light,” IEEE Photonics Technol. Lett. 20(1), 36–38 (2008). [CrossRef]
8. L. Maleki, A. B. Matsko, A. Savchenkov, V. Ilchenko, and D. Seidel. “Low-noise RF oscillation and optical comb generation based on nonlinear optical resonator,” U.S. patent 1,117,22 B1 (2009).
9. P. M. Anandarajah, SPÓ Dúill, R. Zhou, and L. P. Barry, “Enhanced Optical Comb Generation by Gain-Switching a Single-Mode Semiconductor Laser Close to Its Relaxation Oscillation Frequency,” IEEE J. Sel. Top. Quantum Electron. 21(6), 592–600 (2015). [CrossRef]
10. T. S. Rappaport, Y. Xing, G. R. MacCartney, A. F. Molisch, E. Mellios, and J. Zhang, “Overview of Millimeter Wave Communications for Fifth-Generation (5G) Wireless Networks—With a Focus on Propagation Models,” IEEE Trans. Antennas Propag. 65(12), 6213–6230 (2017). [CrossRef]
11. E. P. Martin, T. Shao, V. Vujicic, P. M. Anandarajah, C. Browning, R. Llorente, and L. P. Barry, “25-Gb/s OFDM 60-GHz Radio Over Fiber System Based on a Gain Switched Laser,” J. Lightwave Technol. 33(8), 1635–1643 (2015). [CrossRef]
12. T. Shao, E. Martin, A. M. Prince, and L. P. Barry, “DM-DD OFDM-RoF System With Adaptive Modulation Using a Gain-Switched Laser,” IEEE Photonics Technol. Lett. 27(8), 856–859 (2015). [CrossRef]
13. C. Browning, H. H. Alwan, E. P. Martin, S. O’Dull, and P. Shelda, “Gain-Switched Optical Frequency Combs for Future Mobile Radio-Over-Fiber Millimeter-Wave Systems,” J. Lightwave Technol. 36(19), 4602–4610 (2018). [CrossRef]
14. S. Wassin, G. M. Isoe, A. W. R. Leitch, and T. B. Gibbon, “All-optical wavelength reservation for flexible spectrum networks using amplifier saturation and VCSEL injection,” Opt. Eng. 58(4), 046110 (2019). [CrossRef]
15. G. M. Isoe, R. S. Karembera, and T. B. Gibbon, “Advanced VCSEL photonics: Multi-level PAM for spectral efficient 5G wireless transport network,” J. Opt. Commun. 461, 125273 (2020). [CrossRef]
16. A. R. C. B. Serrano, D. C. de Fernandez, E. P. Cano, M. Ortsiefer, P. Meissner, and P. Acedo, “VCSEL-Based Optical Frequency Combs: Toward Efficient Single-Device Comb Generation,” IEEE Photonics Technol. Lett. 25(20), 1981–1984 (2013). [CrossRef]
17. A. Quirce, C. de Dios, A. Valle, and P. Acedo, “VCSEL-Based Optical Frequency Combs Expansion Induced by Polarized Optical Injection,” presented at the 2019 21st International Conference on Transparent Optical Networks (ICTON), Angers, France, pp. 1–4.
18. E. Prior, C. de Dios, Á. R. Criado, M. Ortsiefer, P. Meissner, and P. Acedo, “Expansion of VCSEL-Based Optical Frequency Combs in the Sub-THz Span: Comparison of Non-Linear Techniques,” J. Lightwave Technol. 34(17), 4135–4142 (2016). [CrossRef]
19. C.-H. Li, M.-F. Wu, C.-H. Lin, and C.-T. Lin, “W-band OFDM RoF system with simple envelope detector down-conversion,” Presented at the 2015 Optical Fiber Communication Conference, Optical Society of America, p. W4G. 6.
20. S. Mikroulis, M.P. Thakur, and J.E. Mitchell, “Investigation of a robust remote heterodyne envelope detector scheme for cost-efficient E-PON/60 GHz wireless integration, pesented at the 2014 16th International Conference on Transparent Optical Networks, IEEE, 2014, pp. 1–4.
21. D. R. Hummels, C. Adams, and B. K. Harms, “Filter Selection for Receivers Using Square-Law Detection,” IEEE Trans. Aerosp. Electron. Syst. AES-19(6), 871–883 (1983). [CrossRef]
22. Kimmitt, et al., “Control of an Optical Modulator for Desired Basing of Data and Pulse Modulators,” M. Kimmitt, J. E. Kaufmann, Y. Shohet, K. Springer, T. Fjelde, P. V. Mamyshev, B. P. Mikkelsen, U.S. patent 7,394,992 B2 (July 1, 2008).
23. O. Buccafusca, J. L. A. Chilla, J. J. Rocca, C. Wilmsen, and S. Feld, “Ultrahigh frequency oscillations and multimode dynamics in vertical cavity surface emitting lasers,” Appl. Phys. Lett. 67(2), 185–187 (1995). [CrossRef]
