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Ultrahigh-capacity access network architecture for mobile data backhaul using integrated W-band wireless and free-space optical links with OAM multiplexing Yuan Fang,1 Jianjun Yu,1,* Junwen Zhang,1,2 Nan Chi,1 Jiangnan Xiao,1 and Gee-Kung Chang2 1
Key Laboratory for Information Science of Electromagnetic Waves (MoE), Fudan University, Shanghai 200433, China 2 School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, USA *Corresponding author: [email protected] Received May 2, 2014; revised May 29, 2014; accepted June 6, 2014; posted June 10, 2014 (Doc. ID 211270); published July 9, 2014 In this Letter, we propose and experimentally demonstrate a novel access network architecture using hybrid integrated W-band wireless and free-space optical (FSO) links with orbital angular momentum (OAM) multiplexing. The transmission of a 20 GBd quadrature phase-shift keying signal modulated over 10 OAM modes has been demonstrated over a 0.6 m FSO link and a 0.4 m W-band wireless link at 100 GHz. The experimental results show that the architecture can support future ultrahigh-capacity, converged optical–wireless access networks that require extra bandwidth and system flexibility in mobile data networks. © 2014 Optical Society of America OCIS codes: (060.5625) Radio frequency photonics; (060.2605) Free-space optical communication; (060.4230) Multiplexing. http://dx.doi.org/10.1364/OL.39.004168
The ever-increasing demand for emerging applications from mobile data users requires a strong improvement in existing broadband optical and wireless access networks in terms of data rate, capacity, and flexibility [1]. Recently, optical wireless systems operating in the W band (75–110 GHz), which has the advantage of a higher bandwidth, have attracted increasing interest by researchers as a promising candidate technique for providing multigigabit wireless access for 4G LTE Advanced and 5G mobile data networks. Moreover, by utilizing photonic millimeter-wave signal generation, processing, and detection techniques, the W-band optical wireless link features the integration of wireless and optical fiber networks, which has been widely studied in the research community [2–4]. It has been reported that a high-speed optical baseband signal generated by an externally modulating continuous-wavelength light wave will beat with another CW light wave at a different wavelength to generate the required high-speed wireless W-band signal [3]. Using these techniques, various optical wireless links in the W band with a spectral-efficient modulation format and digital coherent detection to improve performance have been proposed and experimentally demonstrated [3,4]. Meanwhile, orbital angular momentum (OAM) multiplexing has been considered as an emerging multiplexing technique to further improve spectral efficiency and capacity [5,6]. Orthogonal with existing multiplexing techniques, such as time division multiplexing (TDM), wavelength division multiplexing (WDM), polarization division multiplexing (PDM), spatial division multiplexing (SDM), and mode division multiplexing (MDM), OAM multiplexing offers an additional degree of freedom (DOF) for multiplexing with a theoretically infinite number of orthogonal eigenstates. The property of OAM carried by a Laguerre–Gaussian (LG) beam was studied by Allen [7] and then introduced to an optical communication system [8]. OAM multiplexing provides improved 0146-9592/14/144168-04$15.00/0
system capacity and spectral efficiency of optical systems, and many experiments utilizing OAM multiplexing based on free-space optical (FSO) links have been reported [5,6,8,9]. Moreover, OAM multiplexing is transparent to modulation formats while maintaining compatibility with other multiplexing techniques. In this Letter, we propose and experimentally demonstrate a novel optical–wireless access architecture for the integration of W-band wireless and FSO links with OAM multiplexing. The architecture we propose is capable of realizing high-capacity optical signal transmission and flexible wireless access with high spectral efficiency. With OAM multiplexing, multiple users will be able to independently and simultaneously utilize high bandwidth in the W band rather than sharing. We propose that to realize OAM multiplexing with FSO and avoid deploying additional specialty fiber to take advantage of the mature OAM-generation method using a spatial light modulator (SLM). We set up the experimental demonstration for the architecture we are proposing. 10 OAM modes, which are generated by a specially designed hologram pattern on a SLM, each carrying a 20 GBd quadrature phase-shift keying (QPSK) signal, are transmitted over a 0.6 m FSO link. A 100 GHz W-band wireless signal is generated through a remote heterodyne beating technique and delivered over a 0.4 m wireless link. The bit error ratio (BER) performance is under a soft-decision forward error correction (SD-FEC) of 2 × 10−2 with −17 dBm received optical power. The experimental results show that the architecture we propose is a promising option for supporting future high-capacity optical–wireless hybrid access networks with extra bandwidth and system flexibility in mobile data transport. The LG beam carrying the OAM mode features a helical phase front, which can be described by the azimuthal phase term expilφ. The value l is known as a topological charge, which stands for different orthogonal © 2014 Optical Society of America
July 15, 2014 / Vol. 39, No. 14 / OPTICS LETTERS
eigenstates corresponding to different OAM modes, and φ is the azimuthal phase. The OAM is lh per photon, where h is the Dirac constant and l must be an integer l 0; 1; 2…. By employing a reflective SLM with a precalculated hologram pattern, the signal can be converted onto a specific OAM mode to realize multiplexing or be converted back to a fundamental Gaussian beam l 0 to realize demultiplexing. Figure 1 illustrates the architecture for the integration of W-band wireless and FSO links with OAM multiplexing. The architecture is composed of a central office with a transmitter, FSO link with OAM multiplexing, baseband unit for W-band signal generation, wireless link at the W band for access, and end user side to receive the signal. The central office generates a high-speed multichannel optical signal with multiple lasers and modulators. For each data channel, the CW from one laser is modulated with a multilevel modulation format and then combined with another idle CW. The wavelength spacing between the two lasers corresponds to the W-band carrier frequency. By using this photonic millimeter-wave technique, additional CW is necessary in the central office, but the W-band signal generation will be simplified at the baseband unit. The optical signal is then coupled into an FSO link. In the FSO link, a multichannel optical signal is converted from fundamental Gaussian mode to an LG beam with orthogonal OAM modes. The high-capacity optical transmission is realized by multiplexing all these OAM modes, which already carry data of different channels. Then the signal carried on the LG beam with orthogonal OAM modes is converted back to fundamental Gaussian mode independently. Thus, OAM demultiplexing and detection is realized at the same time. Then the beam with the signal is again coupled into the optical fiber. In the baseband unit, the signal undergoes optical-to-electrical (O/E) conversion and the W-band signal is generated through heterodyne beating of pretuned wavelengths. The resulting wireless link on the W band functions as flexible wireless access for multiple users. Depending on application demands, different configurations of W-band links can be adopted: delivering different data or services carried on different OAM modes to realize multiple access, or multicasting the same data carried on a certain OAM mode to users. Finally, the signal is received and processed to obtain
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the data at the end user side. Compared with ordinary optical–wireless links on the W band, the architecture we propose has two unique features: OAM multiplexing in an FSO link and flexible configurations in a wireless link. OAM multiplexing in an FSO link not only improves the bandwidth and spectral efficiency, but also avoids the deployment of specialty fiber. As for flexible configurations in the wireless link, multiple access and multicasting are both supported. In the former, the data or services delivered to users can be easily switched at the baseband unit. The experimental setup of the integration of W-band wireless and FSO links with OAM multiplexing is shown in Fig. 2. In the central office, two external cavity lasers (ECLs) with less than 100 kHz linewidth are employed. The operating wavelengths of ECL1 and ECL2 are set at 1549.62 nm and 1550.42 nm, respectively, to ensure the frequency spacing of 100 GHz. An inphase/quadrature (I/Q) modulator is used to modulate CW from ECL1 to generate a 40 Gbit∕s optical QPSK signal. With a polarization-maintaining optical couple (PM-OC), the modulated signal is combined with idle CW from ECL2. The following erbium-doped fiber amplifier (EDFA1) is used to compensate the insertion loss of the I/Q modulator. One polarization controller (PC) is then used to control the polarization. The PC is necessary for good performance since the SLM we employed to generate the LG beam with OAM is polarization sensitive. One collimator is used to couple the optical signal into the FSO link. OAM generation, multiplexing, and demultiplexing are all realized in the FSO link. One reflective liquidcrystal-on-silicon SLM is used for OAM generation. A precalculated phase-only hologram pattern is implemented to SLM1 to generate a specific OAM mode. It should be noted that we designed a special hologram pattern with an angular mask to generate 5 OAM modes simultaneously by one SLM instead of multiple SLMs just to simplify the experimental setup. The principle and experimental report about the special hologram pattern to 20GBaud
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Fig. 1. Architecture for the integration of W-band wireless and FSO links with OAM multiplexing.
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Fig. 2. Experimental setup for integrated W-band wireless and FSO links with OAM multiplexing.
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generate multiple OAM modes can be found in [9,10]. Meanwhile, benefiting from the mirror image relationship introduced by odd-times reflections [6], the OAM mode with an opposite charge can be generated and multiplexed by only two beam splitters (BSs) and odd mirrors. In this experimental setup, we generate 5 OAM modes (4, 8, 12, 16, and 20) by SLM1 and then split the beam into two optical paths by BS1. One of the paths contains three additional mirrors to generate OAM modes with the opposite sign (−4, −8, −12, −16, and −20). Although the 5 OAM modes generated by SLM1 are with identical data, the data carried on the OAM modes generated by reflection are not identical to the data carried on the original 5 modes. This results from the different optical path, which actually decorrelates the data. Two optical paths are combined by BS2. After a lens to adjust the beam size as well as collimation, SLM2 is employed to demultiplex and detect the OAM mode. By switching the hologram pattern on SLM2, the specific OAM mode with the data needed is converted back to fundamental Gaussian mode. An infrared camera is used to obtain the optical intensity profile. Then the Gaussian beam in free space is again coupled to the optical fiber. The FSO link between two SLMs is 0.6 m. In the baseband unit, EDFA2 is employed to compensate the transmission and coupling loss. One photodiode (PD) with a 90 GHz 3 dB bandwidth is used for O/E conversion. The PD also functions as a remote heterodyne beating since we utilized the photonics millimeter-wave technique to generate a W-band carrier. After the heterodyne beating, a 100 GHz W-band carrier with a 40 Gbit∕s QPSK signal is obtained. After that, one 100 GHz narrowband electrical amplifier (EA1) is used to amplify the electrical signal. A pair of horn antennas (HAs) with 25 decibels-isotropic (dBi) gain is used to deliver the W-band wireless signal. The wireless link between the two antennas is 0.4 m. At the receiver in the end user side, the received signal is mixed with a 75 GHz radio frequency (RF) signal to realize analog downconversion to a 25 GHz intermediate frequency (IF). Then the analog-to-digital conversion (ADC) is realized in the real-time oscilloscope (OSC) with a 120 GSa∕s sampling rate and 45 GHz electrical bandwidth. Finally, the sampled data is offline processed by digital signal processing (DSP). The main steps in DSP include: clock recovery, digital IF downconversion, CMA equalization, frequency-offset estimation (FOE), carrier phase estimation (CPE), differential decoding, and BER counting [11]. In CMA equalization, two complex-valued, 19-tap, T/2-spaced adaptive finite impulse response (IFR) filters based on the classic CMA are used to realize polarization demultiplexing. Figure 3 shows the optical spectrum before and after the FSO link. The peaks on the left and right are the QPSK signal modulated on CW from ECL1 and idle CW from ECL2, respectively. We found that the spacing between the two peaks is just 100 GHz to generate the W-band carrier. The transmission and coupling loss of the FSO link is about 15 dB, which can be observed in Fig. 3 by comparing the black and red lines. Figure 4 shows the hologram patterns implemented on an SLM for generation and detection of OAM modes. Figure 4(a) is the hologram pattern we implemented on SLM1 to generate 5 OAM modes (4, 8, 12, 16, and
Fig. 3.
Optical spectrum before and after the FSO link.
20). The phase-only hologram pattern is precalculated and converted to a gray level image. It is the superimposition of two separate phase-only hologram patterns (4, 8, 12 and 12, 16, 20) with a complementary angular mask. It should be noted that the multiple OAM modes generated by the angular mask in the phase-only hologram pattern do not show equal power. In fact, the power of the side mode (e.g., 4 and 20 in our setup) will be lower than the OAM mode in the middle. This helps to explain the different BER performances between these modes. Figures 4(b)–4(f) are hologram patterns implemented on SLM2 to demux OAM modes −4, −8, 12, 16, and 20, respectively. The number of the topological charge determines the number of parts into which 2π is sliced, and the sign of the topological charge determines the varying direction of the gray level image. Figure 5 gives the optical intensity profile obtained by an infrared camera. Figure 5(a) is the intensity profile after generating 10 OAM modes. Different from the ring profile in single OAM mode generation, the intensity profile with 5 OAM modes looks like a rectangle. The different sizes and angles in “rectangles” in Fig. 5(a) is caused by different optical paths introduced by additional reflections. Figures 5(b)–5(f) are intensity profiles of demultiplexing OAM modes −4, −8, 12, 16, and 20, respectively. The round points in the centers correspond to the Gaussian mode after demultiplexing. Compared to OAM modes −8, 12, and 16, the demultiplexed Gaussian mode in the centers for −4 and 20 are blurry and small, because these two OAM modes at the sides will have lower power, as explained above. The BER performance versus received optical power is shown in Fig. 6. Comparing the red and black curves,
Fig. 4. Hologram patterns for (a) 5 OAM modes (4, 8, 12, 16, 20), (b) −4, (c) −8, (d) 12, (e) 16, and (f) 20.
July 15, 2014 / Vol. 39, No. 14 / OPTICS LETTERS
Fig. 5. Optical intensity profile of (a) 10 OAM modes, (b) demux −4, (c) demux −8, (d) demux 12, (e) demux 16, and (f) demux 20.
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with OAM multiplexing. The architecture we propose is capable of realizing high-capacity optical signal transmission and flexible wireless access with high spectral efficiency, especially when fiber deployment is over difficult terrain or not cost-efficient. In our experimental test bed, 10 OAM modes, each carrying a 20 GBd QPSK signal, are generated by a specially designed hologram pattern on an SLM and are transmitted over a 0.6 m FSO link. A wireless signal carried by a 100 GHz W band is generated through the remote heterodyne beating technique and delivered over a 0.4 m wireless link. The BER performance is under SD-FEC of 2 × 10−2 with −17 dBm received optical power. The experimental results show that the architecture we propose is promising for supporting future high-capacity optical–wireless hybrid access networks with extra bandwidth and attaining system flexibility in mobile data transport. This work was supported in part by the NHTRDP (973 Program of China Grant No. 2010CB328300), NNSF of China (No. 61325002, 61250018), The National Key Technology R&D Program (2012BAH18B00), and Key Program of Shanghai Science and Technology Association (12dz1143000, 12510705600, and 13JC1400700).
Fig. 6. BER versus received power for integrated FSO and W-band wireless transmission links.
we find the penalty of QPSK over the 0.4 m W-band wireless link is less than 1 dB. The curves for 16, −4, 20, and −8 show the performance of OAM modes in different cases, namely the mode at the middle, original path; the mode at the side, reflection path; the mode at the side, original path; and the mode at the middle, reflection path. By comparing these four modes, we found the penalty introduced by the power imbalance in the middle and side modes is about 3 dB, while the penalty caused by the reflection path is about 1–2 dB. BER performance is under the SD-FEC limit of 2 × 10−2 , with −19 dBm received optical power for OAM −4 as the worst performance. Comparing -8 with the FSO link and with both the FSO link and W-band wireless link, we find the integration of the FSO link and W-band wireless link brings only little penalty. In this Letter, we propose and experimentally demonstrate a novel optical wireless access network architecture for the integration of W-band wireless and FSO links
References 1. J. Yu, G. K. Chang, Z. Jia, A. Chowdhury, M. F. Huang, H. C. Chien, Y. T. Hsueh, W. Jian, C. Liu, and Z. Dong, J. Lightwave Technol. 28, 2376 (2010). 2. J. Capmany and D. Novak, Nat. Photonics 1, 319 (2007). 3. X. Li, Z. Dong, J. Yu, N. Chi, Y. Shao, and G. K. Chang, Opt. Lett. 37, 5106 (2012). 4. J. Zhang, J. Yu, N. Chi, Z. Dong, X. Li, and G. K. Chang, IEEE Photon. Technol. Lett. 25, 780 (2013). 5. A. E. Willner, “Orbital angular momentum transmission,” in Proceedings of the European Conference and Exhibition on Optical Communications, London, UK (2013), paper Mo.4.A.1. 6. J. Wang, J. Yang, I. M. Fazall, Ni. Ahmed, Y. Yan, H. Huang, Y. Ren, Y. Yue, S. Dolinar, M. Tur, and A. E. Willner, Nat. Photonics 6, 488 (2012). 7. L. Allen, M. W. Beijersbergen, R. J. C. Spreeuw, and J. P. Woerdman, Phys. Rev. A 45, 8185 (1992). 8. G. Gibson, J. Courtial, and M. J. Padgett, Opt. Express 12, 5448 (2004). 9. Y. Yan, Y. Yue, H. Huang, Y. Ren, N. Ahmed, M. Tur, S. Dolinar, and A. E. Willner, Opt. Lett. 38, 3930 (2013). 10. B. Jack, M. J. Padgett, and S. Franke-Arnold, New J. Phys. 10, 103013 (2008). 11. J. Zhang, Z. Dong, J. Yu, N. Chi, L. Tao, X. Li, and Y. Shao, Opt. Lett. 37, 4050 (2012).
OPTICS LETTERS / Vol. 39, No. 14 / July 15, 2014
Ultrahigh-capacity access network architecture for mobile data backhaul using integrated W-band wireless and free-space optical links with OAM multiplexing Yuan Fang,1 Jianjun Yu,1,* Junwen Zhang,1,2 Nan Chi,1 Jiangnan Xiao,1 and Gee-Kung Chang2 1
Key Laboratory for Information Science of Electromagnetic Waves (MoE), Fudan University, Shanghai 200433, China 2 School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, USA *Corresponding author: [email protected] Received May 2, 2014; revised May 29, 2014; accepted June 6, 2014; posted June 10, 2014 (Doc. ID 211270); published July 9, 2014 In this Letter, we propose and experimentally demonstrate a novel access network architecture using hybrid integrated W-band wireless and free-space optical (FSO) links with orbital angular momentum (OAM) multiplexing. The transmission of a 20 GBd quadrature phase-shift keying signal modulated over 10 OAM modes has been demonstrated over a 0.6 m FSO link and a 0.4 m W-band wireless link at 100 GHz. The experimental results show that the architecture can support future ultrahigh-capacity, converged optical–wireless access networks that require extra bandwidth and system flexibility in mobile data networks. © 2014 Optical Society of America OCIS codes: (060.5625) Radio frequency photonics; (060.2605) Free-space optical communication; (060.4230) Multiplexing. http://dx.doi.org/10.1364/OL.39.004168
The ever-increasing demand for emerging applications from mobile data users requires a strong improvement in existing broadband optical and wireless access networks in terms of data rate, capacity, and flexibility [1]. Recently, optical wireless systems operating in the W band (75–110 GHz), which has the advantage of a higher bandwidth, have attracted increasing interest by researchers as a promising candidate technique for providing multigigabit wireless access for 4G LTE Advanced and 5G mobile data networks. Moreover, by utilizing photonic millimeter-wave signal generation, processing, and detection techniques, the W-band optical wireless link features the integration of wireless and optical fiber networks, which has been widely studied in the research community [2–4]. It has been reported that a high-speed optical baseband signal generated by an externally modulating continuous-wavelength light wave will beat with another CW light wave at a different wavelength to generate the required high-speed wireless W-band signal [3]. Using these techniques, various optical wireless links in the W band with a spectral-efficient modulation format and digital coherent detection to improve performance have been proposed and experimentally demonstrated [3,4]. Meanwhile, orbital angular momentum (OAM) multiplexing has been considered as an emerging multiplexing technique to further improve spectral efficiency and capacity [5,6]. Orthogonal with existing multiplexing techniques, such as time division multiplexing (TDM), wavelength division multiplexing (WDM), polarization division multiplexing (PDM), spatial division multiplexing (SDM), and mode division multiplexing (MDM), OAM multiplexing offers an additional degree of freedom (DOF) for multiplexing with a theoretically infinite number of orthogonal eigenstates. The property of OAM carried by a Laguerre–Gaussian (LG) beam was studied by Allen [7] and then introduced to an optical communication system [8]. OAM multiplexing provides improved 0146-9592/14/144168-04$15.00/0
system capacity and spectral efficiency of optical systems, and many experiments utilizing OAM multiplexing based on free-space optical (FSO) links have been reported [5,6,8,9]. Moreover, OAM multiplexing is transparent to modulation formats while maintaining compatibility with other multiplexing techniques. In this Letter, we propose and experimentally demonstrate a novel optical–wireless access architecture for the integration of W-band wireless and FSO links with OAM multiplexing. The architecture we propose is capable of realizing high-capacity optical signal transmission and flexible wireless access with high spectral efficiency. With OAM multiplexing, multiple users will be able to independently and simultaneously utilize high bandwidth in the W band rather than sharing. We propose that to realize OAM multiplexing with FSO and avoid deploying additional specialty fiber to take advantage of the mature OAM-generation method using a spatial light modulator (SLM). We set up the experimental demonstration for the architecture we are proposing. 10 OAM modes, which are generated by a specially designed hologram pattern on a SLM, each carrying a 20 GBd quadrature phase-shift keying (QPSK) signal, are transmitted over a 0.6 m FSO link. A 100 GHz W-band wireless signal is generated through a remote heterodyne beating technique and delivered over a 0.4 m wireless link. The bit error ratio (BER) performance is under a soft-decision forward error correction (SD-FEC) of 2 × 10−2 with −17 dBm received optical power. The experimental results show that the architecture we propose is a promising option for supporting future high-capacity optical–wireless hybrid access networks with extra bandwidth and system flexibility in mobile data transport. The LG beam carrying the OAM mode features a helical phase front, which can be described by the azimuthal phase term expilφ. The value l is known as a topological charge, which stands for different orthogonal © 2014 Optical Society of America
July 15, 2014 / Vol. 39, No. 14 / OPTICS LETTERS
eigenstates corresponding to different OAM modes, and φ is the azimuthal phase. The OAM is lh per photon, where h is the Dirac constant and l must be an integer l 0; 1; 2…. By employing a reflective SLM with a precalculated hologram pattern, the signal can be converted onto a specific OAM mode to realize multiplexing or be converted back to a fundamental Gaussian beam l 0 to realize demultiplexing. Figure 1 illustrates the architecture for the integration of W-band wireless and FSO links with OAM multiplexing. The architecture is composed of a central office with a transmitter, FSO link with OAM multiplexing, baseband unit for W-band signal generation, wireless link at the W band for access, and end user side to receive the signal. The central office generates a high-speed multichannel optical signal with multiple lasers and modulators. For each data channel, the CW from one laser is modulated with a multilevel modulation format and then combined with another idle CW. The wavelength spacing between the two lasers corresponds to the W-band carrier frequency. By using this photonic millimeter-wave technique, additional CW is necessary in the central office, but the W-band signal generation will be simplified at the baseband unit. The optical signal is then coupled into an FSO link. In the FSO link, a multichannel optical signal is converted from fundamental Gaussian mode to an LG beam with orthogonal OAM modes. The high-capacity optical transmission is realized by multiplexing all these OAM modes, which already carry data of different channels. Then the signal carried on the LG beam with orthogonal OAM modes is converted back to fundamental Gaussian mode independently. Thus, OAM demultiplexing and detection is realized at the same time. Then the beam with the signal is again coupled into the optical fiber. In the baseband unit, the signal undergoes optical-to-electrical (O/E) conversion and the W-band signal is generated through heterodyne beating of pretuned wavelengths. The resulting wireless link on the W band functions as flexible wireless access for multiple users. Depending on application demands, different configurations of W-band links can be adopted: delivering different data or services carried on different OAM modes to realize multiple access, or multicasting the same data carried on a certain OAM mode to users. Finally, the signal is received and processed to obtain
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the data at the end user side. Compared with ordinary optical–wireless links on the W band, the architecture we propose has two unique features: OAM multiplexing in an FSO link and flexible configurations in a wireless link. OAM multiplexing in an FSO link not only improves the bandwidth and spectral efficiency, but also avoids the deployment of specialty fiber. As for flexible configurations in the wireless link, multiple access and multicasting are both supported. In the former, the data or services delivered to users can be easily switched at the baseband unit. The experimental setup of the integration of W-band wireless and FSO links with OAM multiplexing is shown in Fig. 2. In the central office, two external cavity lasers (ECLs) with less than 100 kHz linewidth are employed. The operating wavelengths of ECL1 and ECL2 are set at 1549.62 nm and 1550.42 nm, respectively, to ensure the frequency spacing of 100 GHz. An inphase/quadrature (I/Q) modulator is used to modulate CW from ECL1 to generate a 40 Gbit∕s optical QPSK signal. With a polarization-maintaining optical couple (PM-OC), the modulated signal is combined with idle CW from ECL2. The following erbium-doped fiber amplifier (EDFA1) is used to compensate the insertion loss of the I/Q modulator. One polarization controller (PC) is then used to control the polarization. The PC is necessary for good performance since the SLM we employed to generate the LG beam with OAM is polarization sensitive. One collimator is used to couple the optical signal into the FSO link. OAM generation, multiplexing, and demultiplexing are all realized in the FSO link. One reflective liquidcrystal-on-silicon SLM is used for OAM generation. A precalculated phase-only hologram pattern is implemented to SLM1 to generate a specific OAM mode. It should be noted that we designed a special hologram pattern with an angular mask to generate 5 OAM modes simultaneously by one SLM instead of multiple SLMs just to simplify the experimental setup. The principle and experimental report about the special hologram pattern to 20GBaud
Central Office
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Fig. 1. Architecture for the integration of W-band wireless and FSO links with OAM multiplexing.
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Fig. 2. Experimental setup for integrated W-band wireless and FSO links with OAM multiplexing.
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generate multiple OAM modes can be found in [9,10]. Meanwhile, benefiting from the mirror image relationship introduced by odd-times reflections [6], the OAM mode with an opposite charge can be generated and multiplexed by only two beam splitters (BSs) and odd mirrors. In this experimental setup, we generate 5 OAM modes (4, 8, 12, 16, and 20) by SLM1 and then split the beam into two optical paths by BS1. One of the paths contains three additional mirrors to generate OAM modes with the opposite sign (−4, −8, −12, −16, and −20). Although the 5 OAM modes generated by SLM1 are with identical data, the data carried on the OAM modes generated by reflection are not identical to the data carried on the original 5 modes. This results from the different optical path, which actually decorrelates the data. Two optical paths are combined by BS2. After a lens to adjust the beam size as well as collimation, SLM2 is employed to demultiplex and detect the OAM mode. By switching the hologram pattern on SLM2, the specific OAM mode with the data needed is converted back to fundamental Gaussian mode. An infrared camera is used to obtain the optical intensity profile. Then the Gaussian beam in free space is again coupled to the optical fiber. The FSO link between two SLMs is 0.6 m. In the baseband unit, EDFA2 is employed to compensate the transmission and coupling loss. One photodiode (PD) with a 90 GHz 3 dB bandwidth is used for O/E conversion. The PD also functions as a remote heterodyne beating since we utilized the photonics millimeter-wave technique to generate a W-band carrier. After the heterodyne beating, a 100 GHz W-band carrier with a 40 Gbit∕s QPSK signal is obtained. After that, one 100 GHz narrowband electrical amplifier (EA1) is used to amplify the electrical signal. A pair of horn antennas (HAs) with 25 decibels-isotropic (dBi) gain is used to deliver the W-band wireless signal. The wireless link between the two antennas is 0.4 m. At the receiver in the end user side, the received signal is mixed with a 75 GHz radio frequency (RF) signal to realize analog downconversion to a 25 GHz intermediate frequency (IF). Then the analog-to-digital conversion (ADC) is realized in the real-time oscilloscope (OSC) with a 120 GSa∕s sampling rate and 45 GHz electrical bandwidth. Finally, the sampled data is offline processed by digital signal processing (DSP). The main steps in DSP include: clock recovery, digital IF downconversion, CMA equalization, frequency-offset estimation (FOE), carrier phase estimation (CPE), differential decoding, and BER counting [11]. In CMA equalization, two complex-valued, 19-tap, T/2-spaced adaptive finite impulse response (IFR) filters based on the classic CMA are used to realize polarization demultiplexing. Figure 3 shows the optical spectrum before and after the FSO link. The peaks on the left and right are the QPSK signal modulated on CW from ECL1 and idle CW from ECL2, respectively. We found that the spacing between the two peaks is just 100 GHz to generate the W-band carrier. The transmission and coupling loss of the FSO link is about 15 dB, which can be observed in Fig. 3 by comparing the black and red lines. Figure 4 shows the hologram patterns implemented on an SLM for generation and detection of OAM modes. Figure 4(a) is the hologram pattern we implemented on SLM1 to generate 5 OAM modes (4, 8, 12, 16, and
Fig. 3.
Optical spectrum before and after the FSO link.
20). The phase-only hologram pattern is precalculated and converted to a gray level image. It is the superimposition of two separate phase-only hologram patterns (4, 8, 12 and 12, 16, 20) with a complementary angular mask. It should be noted that the multiple OAM modes generated by the angular mask in the phase-only hologram pattern do not show equal power. In fact, the power of the side mode (e.g., 4 and 20 in our setup) will be lower than the OAM mode in the middle. This helps to explain the different BER performances between these modes. Figures 4(b)–4(f) are hologram patterns implemented on SLM2 to demux OAM modes −4, −8, 12, 16, and 20, respectively. The number of the topological charge determines the number of parts into which 2π is sliced, and the sign of the topological charge determines the varying direction of the gray level image. Figure 5 gives the optical intensity profile obtained by an infrared camera. Figure 5(a) is the intensity profile after generating 10 OAM modes. Different from the ring profile in single OAM mode generation, the intensity profile with 5 OAM modes looks like a rectangle. The different sizes and angles in “rectangles” in Fig. 5(a) is caused by different optical paths introduced by additional reflections. Figures 5(b)–5(f) are intensity profiles of demultiplexing OAM modes −4, −8, 12, 16, and 20, respectively. The round points in the centers correspond to the Gaussian mode after demultiplexing. Compared to OAM modes −8, 12, and 16, the demultiplexed Gaussian mode in the centers for −4 and 20 are blurry and small, because these two OAM modes at the sides will have lower power, as explained above. The BER performance versus received optical power is shown in Fig. 6. Comparing the red and black curves,
Fig. 4. Hologram patterns for (a) 5 OAM modes (4, 8, 12, 16, 20), (b) −4, (c) −8, (d) 12, (e) 16, and (f) 20.
July 15, 2014 / Vol. 39, No. 14 / OPTICS LETTERS
Fig. 5. Optical intensity profile of (a) 10 OAM modes, (b) demux −4, (c) demux −8, (d) demux 12, (e) demux 16, and (f) demux 20.
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with OAM multiplexing. The architecture we propose is capable of realizing high-capacity optical signal transmission and flexible wireless access with high spectral efficiency, especially when fiber deployment is over difficult terrain or not cost-efficient. In our experimental test bed, 10 OAM modes, each carrying a 20 GBd QPSK signal, are generated by a specially designed hologram pattern on an SLM and are transmitted over a 0.6 m FSO link. A wireless signal carried by a 100 GHz W band is generated through the remote heterodyne beating technique and delivered over a 0.4 m wireless link. The BER performance is under SD-FEC of 2 × 10−2 with −17 dBm received optical power. The experimental results show that the architecture we propose is promising for supporting future high-capacity optical–wireless hybrid access networks with extra bandwidth and attaining system flexibility in mobile data transport. This work was supported in part by the NHTRDP (973 Program of China Grant No. 2010CB328300), NNSF of China (No. 61325002, 61250018), The National Key Technology R&D Program (2012BAH18B00), and Key Program of Shanghai Science and Technology Association (12dz1143000, 12510705600, and 13JC1400700).
Fig. 6. BER versus received power for integrated FSO and W-band wireless transmission links.
we find the penalty of QPSK over the 0.4 m W-band wireless link is less than 1 dB. The curves for 16, −4, 20, and −8 show the performance of OAM modes in different cases, namely the mode at the middle, original path; the mode at the side, reflection path; the mode at the side, original path; and the mode at the middle, reflection path. By comparing these four modes, we found the penalty introduced by the power imbalance in the middle and side modes is about 3 dB, while the penalty caused by the reflection path is about 1–2 dB. BER performance is under the SD-FEC limit of 2 × 10−2 , with −19 dBm received optical power for OAM −4 as the worst performance. Comparing -8 with the FSO link and with both the FSO link and W-band wireless link, we find the integration of the FSO link and W-band wireless link brings only little penalty. In this Letter, we propose and experimentally demonstrate a novel optical wireless access network architecture for the integration of W-band wireless and FSO links
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