November 1, 2009 / Vol. 34, No. 21 / OPTICS LETTERS
Direct-detection full-duplex radio-over-fiber transport systems Wen-I Lin, Hai-Han Lu,* Hsiang-Chun Peng, and Ching-Hsiu Huang Institute of Electro-Optical Engineering, National Taipei University of Technology, Taipei, 10608, Taiwan, China *Corresponding author: [email protected]
Received June 23, 2009; revised September 24, 2009; accepted September 25, 2009; posted October 5, 2009 (Doc. ID 113183); published October 26, 2009 A full-duplex radio-over-fiber transport system based on a direct-detection scheme is proposed and demonstrated. In this approach, the rf power degradation induced by fiber dispersion can be avoided for downlink transmission within the base station zone and for uplink transmission, even as the optical carrier and two sidebands are transmitted. A data signal of 70 Mbps/ 10 GHz transmitted over an 80 km single-mode fiber transport for both downlink and uplink with a low bit error rate 共 ⬍ 10−9兲 and high spurious-free dynamic range 共 ⬎ 100 dB/ Hz2/3兲 values were achieved. © 2009 Optical Society of America OCIS codes: 250.0040, 060.2360, 140.3520, 350.4010.
The microwave/millimeter-wave radio-over-fiber (ROF) transport systems, which integrates the advantages of wireless radio and fiber optical communications, have been developed with high expectations for future communications [1,2]. In ROF transport systems, nevertheless, the fiber dispersion effect can cause intolerable amounts of nonlinear distortions. Several ways have been proposed to improve the performance of systems, such as using an optical singlesideband (SSB) transmitter at the transmitting site [3,4] or employing a dispersion compensation device (DCD) within the fiber transmission . However, a sophisticated and expensive optical SSB transmitter is required for the former. Moreover, poor receiver sensitivity performance is obtained for the latter owing to high power insertion loss of DCD. Furthermore, in traditional ROF transport systems, the optical signal at the receiving site is first detected by a broadband photodiode (PD) to convert it into an rf signal, and then the rf signal is demodulated by a group of high-bandwidth rf devices [6,7]. In this way, the bandwidth of systems suffers from the limitation of rf devices’ characteristics, and expensive rf devices increase the cost of systems. In this Letter, a fullduplex ROF transport system based on a directdetection scheme is proposed and demonstrated. With the help of the light-injection technique at the transmitting site and an optical bandpass filter (OBPF) at the receiving site, the optical carrier and
one of the sidebands are eliminated before detecting, resulting in the avoidance of rf power degradation for downlink/uplink transmission. A data signal of 70 Mbps/ 10 GHz transmitted over an 80 km singlemode fiber (SMF) transport for both downlink and uplink with a low bit error rate (BER) and high spurious-free dynamic range (SFDR) values were obtained. Figure 1 shows the schematic architecture of our proposed direct-detection full-duplex ROF transport systems. At the central station (CS), a 70 Mbps data stream is mixed with a10 GHz microwave carrier to generate the data signal. The resulting data signal is split into two parts by a 1 ⫻ 2 rf splitter and directly modulated into two distributed-feedback laser diodes (DFB LD1 and DFB LD3). The central wavelengths of these two DFB LDs are 1551.55 nm 共1兲 and 1556.35 nm共3兲, respectively. As to the light injection part, two DFB LDs (DFB LD2 and DFB LD4), with central wavelengths of 1551.62 nm 共2兲 and 1556.42 nm 共4兲, respectively, are coupled into port 1 of optical circulators (OCs). Both optical signals are combined by a 2 ⫻ 1 optical coupler, amplified by erbiumdoped fiber amplifier (EDFA)-I, and fed into the fiber backbone. For downlink transmission, the optical signal is generated and then distributed through a standard SMF to the remote base stations (BSs) by using cascaded EDFAs and optical add-drop multiplexers
Fig. 1. Schematic architecture of our proposed direct-detection full-duplex ROF transport systems. 0146-9592/09/213319-3/$15.00
© 2009 Optical Society of America
OPTICS LETTERS / Vol. 34, No. 21 / November 1, 2009
(OADMs). The appropriate wavelength is dropped and added by the OADM in the BS, as shown in Fig. 2. The OADM, with ⬎40 dB add-drop channel isolation, consists of one fiber Bragg grating (FBG) located between two OCs. The downlink data signal is adjusted by a variable optical attenuator (VOA) and separated off by an optical splitter. One of the outputs is passed through an OBPF to pick up the upper sideband, directly detected by a low-bandwidth PD, and fed into a BER tester for BER analysis. The other output is detected by a high-bandwidth PD, and it is applied to an antenna for wireless transmission. The SFDR is calculated for its corresponding noise floor by a two-tone signal at 10 GHz with a 40 MHz separation. For uplink transmission, the 70 Mbps/ 10 GHz data signal is transmitted through the SMF from the BS to the CS, passed through a tunable OBPF, attenuated by a VOA, directly detected by a broadband PD, and also fed into a BER tester for BER analysis. The SFDR is calculated without tunable OBPF at the receiving site. At the CS, the optical spectrum of directly modulated DFB LD3 with optical carrier and two sidebands is present in Fig. 3(a). Figure 3(b) shows the optical spectrum of injection-locking DFB LD3 locked at 4. The dynamics of injection-locked behavior can be described as  dA共t兲 dt d共t兲 dt
1 = g关N共t兲 − Nth兴A共t兲 − kAinj cos 共t兲, 2
g关N共t兲 − Nth兴 − k
sin 共t兲 − 2⌬f, 共2兲
= J − ␥NN共t兲 − 兵␥p + g关N共t兲 − Nth兴其A2共t兲, 共3兲
where A共t兲 is the field amplitude [A2共t兲 = S共t兲, where S共t兲 is the photon number], 共t兲 is the phase difference between the temporal laser field of the slave laser and the master laser, N共t兲 is the carrier number, g is the gain coefficient, Nth is the threshold carrier number, Ainj is the field amplitude injected into the slave laser, k is the coupling coefficient between the injected field and the laser field, ␣ is the linewidth enhancement factor of the slave laser, ⌬f is the lasing frequency difference between the master and the slave laser in the free-running state, J is the injec-
Fig. 3. (a) Optical spectrum of directly modulated DFB LD3 (DSB format). (b) Optical spectrum of injection-locking DFB LD3 locked at 4. (c) Optical signal with only one optical sideband (upper sideband, 4) for direct detection.
tion current, ␥N is the carrier decay rate, and ␥p is the photon decay rate. As the upper sideband of DFB LD3 共1556.40 nm兲 is injection locked, its optical spectrum shifts a slightly longer wavelength 共1556.42 nm兲, and the intensity of upper sideband is
Fig. 2. Schematic diagram of OADM and BS configuration.
November 1, 2009 / Vol. 34, No. 21 / OPTICS LETTERS
Fig. 5. Fundamental, IMD3, and IMD5 powers (downlink: CS→ BS2) against the rf power.
Fig. 4. (a) Measured downlink BER curves of 70 Mbps/ 10 GHz data channel. (b) Measured uplink BER curves of 70 Mbps/ 10 GHz data channel.
enhanced. At BS2 共CS→ BS2兲, the optical signal with only one optical sideband (upper sideband, 4) for direct detection is shown in Fig. 3(c). The OBPF is employed to pick up the upper sideband. The measured downlink BER curves of the 70 Mbps/ 10 GHz data channel from CS to BS2 are present in Fig. 4(a). At a BER of 10−9, for a doublesideband (DSB) system, the received optical power is −2.9 dBm; for only one optical sideband scheme, the received optical power is −18.2 dBm. Improvement of 15.3 dB receiver sensitivity is achieved, as only one optical sideband scheme is employed. As to the eye diagram of the data signal, amplitude and jitter fluctuations are clearly observed in an optical DSB system. However, amplitude and jitter fluctuations are not clearly seen in only one optical sideband scheme. The DSB signal suffers from a fading problem because of fiber dispersion, resulting in the increment of amplitude and jitter of the eye diagram. The BER curve in the case of the injection-locking technique but without OBPF is also given in Fig. 4(a). We can see that at a BER of 10−9 a power penalty of 7.4 dB exists between the back-to-back case and this one. By
employing the injection-locking technique, the rf power degradation induced by fiber dispersion can be suppressed; nevertheless, it cannot be canceled. Furthermore, the measured uplink BER curves of 70 Mbps/ 10 GHz data channel from BS1 to CS are present in Fig. 4(b). At a BER of 10−9, for a DSB system, the received optical power is −2.8 dBm; for only one optical sideband scheme, the received optical power is −18 dBm. Improvement of 15.2 dB receiver sensitivity is achieved as only one optical sideband scheme is employed. Figure 5 shows the fundamental, third-order intermodulation distortion 共IMD3兲, and fifth-order intermodulation distortion 共IMD5兲 powers (downlink: CS→ BS2) plotted against the rf power. With the system noise floor at −150 dBm, good SFDR values for IMD3 共102 dB/ Hz2/3兲 and IMD5 共110 dB/ Hz2/3兲 are obtained. A full-duplex ROF transport system based on a direct-detection scheme is proposed. We have demonstrated the feasibility of systems and obtained good BER and SFDR performances over a long-haul fiber link. Since the optical carrier and the lower sideband are eliminated before detecting, the rf power degradation can be avoided for downlink/uplink transmission. In this way, the expensive rf devices are not involved, and thus systems can be more inexpensive. References 1. H. H. Lu, H. L. Ma, and A. S. Patra, IEEE Photonics Technol. Lett. 20, 1618 (2008). 2. Q. Chang, H. Fu, and Y. Su, IEEE Photonics Technol. Lett. 20, 181 (2008). 3. H. H. Lu, W. S. Tsai, H. C. Peng, and Y. J. Ji, IEEE Commun. Lett. 9, 649 (2005). 4. K. Yonenaga and N. Takachio, IEEE Photonics Technol. Lett. 5, 949 (1993). 5. D. Piehler, C. Y. Kuo, J. Kleefeld, and C. Gall, in 22nd European Conference on Optical Communication (ECOC’96) (IEEE, 1996), Vol. 3, pp. 217–220. 6. M. Bakaul, A. Nirmalathas, C. Lim, D. Novak, and R. Waterhouse, IEEE Photonics Technol. Lett. 17, 2718 (2005). 7. J. Capmany, B. Ortega, A. Martinez, D. Pastor, M. Popov, and P. Y. Fonjallaz, IEEE Photonics Technol. Lett. 17, 471 (2005). 8. F. Mogensen, H. Olesen, and G. Jacobsen, IEEE J. Quantum Electron. QE-21, 784 (1985).