Oct 15, 2007 - Hai-Han Lu, Member, IEEE, Ardhendu Sekhar Patra, Wen-Jeng Ho, Po-Chou Lai, and Ming-Huei Shiu. AbstractâA full-duplex radio-over-fiber ...
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 20, OCTOBER 15, 2007
A Full-Duplex Radio-Over-Fiber Transport System Based on FP Laser Diode With OBPF and Optical Circulator With Fiber Bragg Grating Hai-Han Lu, Member, IEEE, Ardhendu Sekhar Patra, Wen-Jeng Ho, Po-Chou Lai, and Ming-Huei Shiu
Abstract—A full-duplex radio-over-fiber transport system based on an Fabry–Pérot laser diode (FP LD) with an optical bandpass filter at the central station, as well as employing an optical circulator with a fiber Bragg grating at the base station, is proposed and demonstrated. Good transmission performances estimated by bit-error rate and eye diagram are obtained in our proposed systems. Since our proposed systems use only an FP LD as a light source for both down-link and up-link transmissions, it reveals a prominent alternative with advantages in simplicity and cost. Index Terms—Fabry–Pérot laser diode (FP LD), full-duplex, optical bandpass filter (OBPF), radio-over-fiber (ROF).
I. INTRODUCTION ADIO-OVER-FIBER (ROF), the integration of broadband wireless communication and optical systems, is a promising technology to realize high-speed access network with increasing the capacity as well as decreasing the cost. In ROF transport systems, the microwave signal is converted into the optical signal at a central station (CS), and then distributed to the remote base stations (BSs) by optical fiber that provides broad bandwidth and low attenuation characteristics [1]–[3]. Many BSs are necessary to cover the operational area due to high atmospheric attenuation, thereby, it is important to minimize the cost of BS. The design of microwave signal generation for down-link and up-link transmissions will be the vital one for the successful deployment of ROF transport systems [4]. In this letter, an architecture of full-duplex ROF transport systems based on a Fabry–Pérot laser diode (FP LD) with an optical bandpass filter (OBPF) at the CS, as well as employed an optical circulator (OC) with a fiber Bragg grating (FBG) at the BS, is proposed and demonstrated. Two longitudinal modes of FP LD are filtered out; one is used as the down-link light source, and the other is used as the up-link one. Such architecture is attractive because it enables the BS to share the light source remotely located at the CS, thus removing the need for an additional light source at the BS. Performances over a 40-km standard single-mode fiber (SMF) transmission both for up-link and down-link were investigated. Good performances of bit error rate (BER) and clear eye diagram were achieved in our proposed full-duplex ROF transport systems. To compare with the earlier proposed schemes [5], [6], our proposed system
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Manuscript received May 16, 2007; revised June 6, 2007. This work was supported by the National Taipei University of Technology and National Science Council of the Republic of China under Grant NSC 95-2221-E-027-095-MY3. The authors are with the National Taipei University of Technology, Institute of Electro-Optical Engineering, Taipei 10608, Taiwan, R.O.C. (e-mail: hhlu@ntut. edu.tw). Digital Object Identifier 10.1109/LPT.2007.905077
reveals a prominent one with simpler and more economic advantages. II. EXPERIMENTAL SETUP The experimental configuration of our proposed full-duplex ROF transport systems is depicted in Fig. 1. For down-link transmission, the CS is composed of one microwave signal generator, one FP LD, one OBPF, and one erbium-doped fiber amplifier (EDFA). The FP LD is directly modulated at 11-Mb/s data stream mixed with 2.4-GHz microwave carrier for local area network (LAN) application. The output power level of FP LD was 0 dBm, at a bias current of 18 mA. To prevent the wavelength drift of the FP LD induced by the temperature variation, a temperature controller is used in the directly modulated transmitter. The wavelength variation of the FP LD with the temperature controller is 0.003 nm/ C. The optical output power of FP LD was coupled into an OBPF to filter out two longitudinal modes. The optical power levels of these two modes were enhanced by an EDFA. For down-link transmission, the generated signal at CS is distributed to the BS over a 40-km SMF transport. At the BS, the received signal is fed into a three-port OC. The optical signal is coupled into Port 1 of OC, Port 2 of OC is connected with an FBG, and Port 3 of OC is connected with a Mach–Zehnder modulator (MZM). OC in combination with FBG are used to take on two roles: one is to filter out the optical wavelength for down-link light source, and the other is to reflect the optical wavelength for up-link one. A maximum central wavelength shift of 0.003 nm/ C is needed to avoid the wavelength misalignment problem. It is important to delicately control the temperatures of FP LD and FBG. If without temperature controller, performance results will be fluctuated in room temperature due to wavelength shift 0.003 nm/ C, resulting in large performance degradation. The down-link data signal is detected by a broadband photodiode (PD), and provided to a demodulator. An 11-Mb/s down-link data signal is demodulated, separated off by a 1 2 splitter, fed into a BER tester for BER analysis and digital oscilloscope for eye-diagram evaluation. As to the up-link transmission, the optical signal is remodulated by 11-Mb/s/2.4-GHz microwave data signal through an MZM, and amplified by an EDFA. The data rate of up-link and down-link transmission can be different; in order to setup a full-duplex LAN application, we let up-link and down-link transmission with the same data rate. The up-link signal is transmitted to the CS over a 40-km SMF transport, detected by a PD, demodulated, separated off by a 1 2 splitter, and fed into BER tester for BER analysis and digital oscilloscope for eye diagram evaluation.
1041-1135/$25.00 © 2007 IEEE
LU et al.: FULL-DUPLEX ROF TRANSPORT SYSTEM BASED ON FP LD WITH OBPF AND OC WITH FBG
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Fig. 1. Experimental configuration of our proposed full-duplex ROF transport systems.
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Fig. 2. (a) Optical spectrum of FP LD before the OBPF. (b) Optical spectrum of FP LD after the OBPF. (c) Optical spectrum transmitted by the FBG. (d) Optical spectrum reflected by the FBG.
III. EXPERIMENTAL RESULTS AND DISCUSSION The optical spectra of FP LD before [Fig. 1(A)] and after [Fig. 1(B)] the OBPF are shown in Fig. 2(a) and (b), respectively. It can be seen that FP LD inherently possesses a wider spectra envelope (1520–1565 nm). After it was passed through an OBPF, the multiple longitudinal modes of FP LD were changed into two longitudinal modes, with a sidemode suppression ratio (SMSR) of 28 dB. Two wavelengths of (1542.2 nm) and (1543.3 nm) were chosen for down-link and up-link light sources, respectively. Mode partition noise (MPN) and mode hopping noise (MHN) are observed in an FP LD with multiple longitudinal modes, such that MPN and MHN cause significant noise in the system. However, the use of two longitudinal modes of FP LD will eliminate noise which varies with mode distribution [7]. At the BS, the optical spectra transmitted [Fig. 1(C)] and reflected [Fig. 1(D)] by the FBG are present in Fig. 2(c) and (d), with SMSR of 22 and 21 dB, respectively. The FBG filter has a narrow 3-dB bandwidth of
Fig. 3. Measured down-link ( ) and up-link ( ) BER curves as a function of the received optical power level.
0.3 nm to guarantee only one wavelength is transmitted for . And further, to ensure only the down-link light source , one wavelength is reflected for the up-link light source
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 20, OCTOBER 15, 2007
result in receiver sensitivity degradation. Fig. 4(a) and (b) displays the eye diagrams for down-link and up-link transmission, respectively. Good and clear eye diagrams are achieved for both down-link and up-link transmission. In Fig. 4(a), the corresponding jitter and SNR are 3.9-ps rms and 30.5 dB, respectively. In Fig. 4(b), the corresponding jitter and SNR are 5.7-ps rms and 28.3 dB, respectively. More fluctuation is observed in up-link transmission owing to optical signal remodulation-induced phase error and longer fiber-link-induced dispersion.
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IV. CONCLUSION We proposed and demonstrated a full-duplex ROF transport system based on an FP LD with an OBPF at the CS, as well as employed an OC with an FBG at the BS. Good performances of BER and clear eye diagram were achieved in our proposed systems. This demonstrated that such a full-duplex ROF transport system is attractive for bidirectional LAN (11 Mb/s/2.4 GHz) application. ROF transport systems are used to provide broadband services, like LAN and an intelligence transport system (20 Mb/s/5.8 GHz). The proposed scheme can operate at a higher data rate and carrier frequency with the same performance if only the light source FP LD with higher 3-dB frequency response.
(b) Fig. 4. Eye diagrams for (a) down-link transmission and (b) up-link transmission.
the FBG exhibits sharp cutoff and minimum ripples in the reflection spectrum. and up-link BER curves The measured down-link as a function of the received optical power level are given in , for down-link transmission, the Fig. 3. At a BER of the received optical power level is 23.5 dBm; for up-link transmission, the received optical power level is 22.1 dBm. Because of optical signal remodulation and longer transmission distance, the up-link transmission exhibits a power penalty of 1.4 dB compared to the down-link one. Since the optical signal is modulated twice in up-link transmission, the transmission performance of up-link is worse than that of down-link due to a phase error between the down-link and remodulation data signal. Over longer fiber link, fiber dispersion may cause RF power degradation in the system. This RF power degradation will degrade the signal-to-noise ratio (SNR) value of the , system. Since BER performance is proportional to SNR value decreases will lead to bad BER performance, and
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