1.25-Gb/s Millimeter-Wave Band Wired/Wireless Radio-over-Fiber System based on RSOA using an Injection-Locked FP-Laser

Transcription

1 1.25-Gb/s Millimeter-Wave Band Wired/Wireless Radio-over-Fiber System based on RSOA using an Injection-Locked FP-Laser Yong-Yuk Won*, Hyun-Seung Kim, and Sang-Kook Han Department of Electrical and Electronic Engineering, Yonsei University 134 Shinchon-dong, Seodaemun-gu, Seoul, Korea ABSTRACT A new architecture for bidirectional millimeter-wave band radio-over-fiber system is proposed; both optical carrier suppression and injection locking effect are used to simultaneously generate a 1.25-Gb/s wired signal and a 63-GHz wireless one. Error free transmissions (bit error rate of ) of downlink and uplink data are achieved to verify the proposed scheme. No impact of a downlink data transmission on an uplink one due to the wavelength reuse is observed. It is checked that there is only the 2-dB power penalty of uplink due to the Rayleigh backscattering noise. Keywords: Fabry-Pérot laser, injection-locking, millimeter-wave, radio-over-fiber (RoF), reflective semiconductor optical amplifier (RSOA), wired and wireless transmission 1. INTRODUCTION Next-generation networks, which are capable of simultaneously transmitting both wired data and wireless one, have been regarded as the most promising systems for the fulfillment of various services required by several customers. It becomes important what techniques can be focused on the implementation of this system depending on the IT (Information technology) environments of each country. Among these technologies, a radio over fiber (RoF) has been considered as an attractive solution for effectively supporting the fixed and mobile services to various subscribers [1]-[4]. It is important for us to deal with the following technical issues in order to lead to the successful development of real networks. Firstly, the simultaneous generation of both wired and wireless signal using simple and cost-effective methods should be implemented in a central station (CS). Secondly, a base station (BS) should be a simple and cost-effective one because a conventional RoF system requires many BS to cover a number of cells. Finally, it can be very helpful that the architecture of a BS is implemented the same way of making an optical network unit in the conventional passive optical network (PON) because a RoF system is more likely to connect with a PON rather than to be operated independently. The cost of installing a BS can be also reduced considerably because the ONU of a conventional PON system can play a role of a BS in a RoF system. Broadband Access Communication Technologies III, edited by Benjamin B. Dingel, Raj Jain, Katsutoshi Tsukamoto, Proc. of SPIE Vol. 7234, 72340J 2009 SPIE CCC code: X/09/$18 doi: / Proc. of SPIE Vol J-1

2 Recently, various architectures of a RoF system have been proposed based on the above mentioned technical points. The schemes proposed in [5]-[6] use an electro absorption modulator (EAM), interleaver, and various optical devices in order to simultaneously generate a baseband and millimeter-wave band signal. However, these methods can increase the cost of installing a total system due to the complexity of a CS. The architectures of [7]-[8] produce both a wired and a wireless signal using the optical carrier suppression (OCS) effect of a Mach-Zehnder modulator (MZM); they should utilize a dual-armed MZM to improve an OCS ratio and control stably the phase of RF signal applied to each arm. Also, it is difficult to connect with a conventional PON system because additional devices such as a fiber grating filter (FBG) and Mach-Zehnder Interferometer (MZI) are required to separate a wired signal from a wireless one at a BS. The scheme proposed in [9] proposes the simultaneous modulation of both baseband and millimeter-wave band signal using a dualarm MZM; it can provide a RoF system with a simple and cost-effective CS. However, this technique requires inevitably both local oscillator and RF mixer for millimeter-wave band in order to generate an analogue signal corresponding to wireless one. Also, this architecture can not only have a trouble to connect with a colorless PON system but can increase the cost of installing a BS because of an interleaver used in there. In this paper, a new architecture for a bidirectional millimeter-wave band RoF system is proposed. It uses both a singlearmed MZM and a relatively cheap Fabry-Perot (FP)-laser to simultaneously produce wired and wireless signal. A BS is implemented using only reflective semiconductor optical amplifier (RSOA) and optical receiver in order to easily connect with the existing access networks; it can be more meaningful to make a BS like this in case of implementing a colorless PON system. This scheme is demonstrated and verified by measuring the bit error rate (BER) of both upstream and downstream after an optical transmission through the implementation of a small scale test-bed. 2. BIDIRECTIONAL MILLIMETER-WAVE BAND ROF SYSTEM USING A SINGLE-ARMED MZM AND INJECTION-LOCKED FP-LASER The proposed bidirectional millimeter-wave band RoF system architecture is shown Fig. 1. A light from a continuous wave (CW) optical source is transformed into two optical sidebands with a suppressed optical carrier using a Mach- Zehnder modulator (MZM) modulated by RF signal. The converted light is injected into a FP-laser, which is directly modulated by a downlink baseband data, and then one of two sidebands is locked at a certain FP-mode. A light from an injection-locked FP-laser is consisted of a locked mode with a baseband data and an unlocked one (unmodulated optical carrier). These two modes are transmitted to a BS through single mode fiber (SMF). They are split into two parts by a power splitter (PS); one is detected at an optical receiver (Rx1) and then both a wired and a wireless data are generated, the other is injected into a RSOA and modulated by an uplink data (a wireless signal or a wired one). They are retransmitted back to a CS and then an uplink data is detected by the technique of baseband detection at an optical receiver (Rx2). Proc. of SPIE Vol J-2

3 Central Station h Downl,nk asebnd data Base Station RF signal, fo cw MZM 2f FP-Iaser ii SMF (0 PS U R$OA LNA LO Upllnk Wir1s signal Up/ink WircI signal Uplink baseband detection Wired/Wireiss signal detection Fig. 1. Proposed bidirectional millimeter-wave band RoF system 3. EXPERIMENTAL SETUP Fig. 2 shows the experimental setup for the proposed bidirectional 60-GHz band RoF system. Two insets of each point (A and B) were measured optical spectra with resolution bandwidth of nm and video bandwidth of 1 khz; they were measured before and after an injection-locked FP-laser, respectively. A light from a tunable light source (TLS) was converted into two sidebands with a double-sideband suppressed carrier (DSB-SC) modulation using a single-armed MZM modulated by a 31.5-GHz RF signal. A transformed light was injected into a FP-laser with 1.1-nm spaced FPmodes from 1530 nm to 1560nm. A FP-laser was directly modulated by a 1.25-Gb/s baseband data with pseudorandom binary sequence (PRBS) and 2-Vp-p swing depth. Both an erbium doped optical amplifier (EDFA) and optical attenuator (ATT) were used to optimize an input optical injection power for the highest signal to noise ratio (SNR) of a 1.25-Gb/s data over a 63-GHz RF signal. The locking range of a directly modulated FP-laser is about 50 GHz at the condition of showing the BER of The output power of FP-laser was 7.5 dbm (threshold current of FP-laser is 8 ma). Optical spectra before and after an injection-locked FP-laser were measured and shown in insets (A) and (B) of Fig. 2. A center mode of each optical spectrum is reduced incompletely because of the imperfect optical carrier suppression. The small peaks of each spectrum located at both the left-hand edge and the right-hand edge are from harmonics of a 31.5-GHz RF signal. The propagation loss of this system was about 10 db, including two circulators, a 23-km SMF, and a power splitter. An input optical power injected into a RSOA was -2.5 dbm; it gave a 10-dB optical gain, considerably lower than the small signal gain of 20 db at an input power of -20 dbm, and had a polarization dependent gain of 2 db at this optical power. This tells us that a RSOA was operated under a gain saturation region. The polarization controller (PC3) was used to maximize the optical gain of an input light injected into a RSOA. The light modulated by a 1.25-Gb/s uplink data at a RSOA was retransmitted and then detected by a 1.25-GHz band PIN photodetector (Rx2). The BER curves of both downlink (1.25-Gb/s baseband and 63-GHz RF signal) and uplink data were Proc. of SPIE Vol J-3

4 measured to verify the proposed scheme. The impact of downlink data on uplink one due to the wavelength reuse was also checked. Additionally, it was investigated that how much the BER performance of two kinds of downlink data (wired and wireless data) would be varied depending on the input optical power injected into a FP-laser. Cs 125-Gb/s downllnk data 31.5-GHz RF sina! PCI MZM - EDFA ATT FP-Iaserl PC2 23-km SMF (0 BS 125-Gb/s uplink data 125-GHz PIN-PD LPF LNA + Error Detector Wireless data (1.25-Gb/s) LPF Wireline data ( 25-Gb/s) LNA ASK Receiver Direct detection with schotiky diode LPF 4- LNA E cri 0 63-GHz (A) 4-. yii E0 (B) o 63-GHz a> 0 TT VVavelength (nm) a> tt 0-20 L0 1\fj S41 1S S44 Wavelength (nm) r Fig. 2. Experimental setup for simultaneous generation and bidirectional transmission of 1.25-Gb/s wired and 63-GHz wireless signal using a proposed; two insets are measured optical spectra of each point (A and B). 4. EXPERIMENTAL RESULTS AND DISCUSSION It is necessary to optimize an input optical power injected into a FP-laser in order to obtain the maximum SNR of 63- GHz wireless data. Fig. 3 shows the variation of receiver sensitivity for an error free transmission (BER of ) against an input optical power injected into a FP-laser. We could observe that receiver sensitivity was improved up to -3.5 dbm when an input optical power was 14 dbm and the output power of an injection-locked FP-laser was 7.5 dbm. It is because one of two sidebands, which are generated by DSB-SC method, is locked and then the locking effect keeps its Proc. of SPIE Vol J-4

5 intensity fixed, while the other for the generation of 63-GHz wireless signal is increased depending on the intensity of input optical power. Based on the operational conditions obtained in the Fig. 3, the measured RF spectra and BER curves of both a 1.25-Gb/s wired data and a 63-GHz wireless one are presented in Fig. 4. Fig. 4(a) and (b) show the measured RF spectra of 63-GHz wireless signal and 1.25-Gb/s baseband, respectively. The inset of Fig. 4(a) shows the demodulated 63-GHz data which is recovered at baseband due to the square-law detection characteristics of the Schottky diode. The BER curves were measured repeatedly for both a 1.25-Gb/s wired data and a demodulated 63-GHz wireless one. The electrical eye patterns after 23-km transmission are also shown in insets of Fig. 4(c). In the BER curve of a demodulated 63-GHz wireless data, it is checked that there is no power penalty at the BER of due to the chromatic dispersion after 23-km transmission. This result tells us that there will be no problem due to the optical transmission in case of the proposed RoF architecture being connected with the conventional PON system. Fig. 5 shows the measured BER curves of a 1.25-Gb/s uplink data; they were measured repeatedly for the presence of a downlink transmission in order to check the impact of downlink data on the performance of an uplink data due to the wavelength reuse. Two insets of Fig. 5 show each eye pattern after 23-km transmission. It is observed that the power penalty of an uplink data is negligible with a downlink transmission; this is because the gain saturation effect of a RSOA becomes more severe than that of the injection of a single optical carrier in case of the simultaneous injection of two optical sidebands like the proposed scheme. Additionally, there is a 2-dB power penalty in an uplink transmission after 23-km transmission. It is attributed to the Rayleigh backscattering noise due to the interference between a remodulated uplink light and an amplified Rayleigh backscattered signal from a RSOA. a a Received opticel power (dbrn) a Fig. 3. Variation of receiver sensitivity for error free transmission (BER of 10-11) against the input optical power injected into a FP-laser Proc. of SPIE Vol J-5

6 -20 E -60 cr b0 o -50- C D a, Frequency (GHz) (a) Frequency (GHz) (b) 2.0 w 0 J ObtS. 633I* no trn%mlinn o I 25-Obs, 63-GIt7 2)km rnmitwii I 25-Gb cthiri w1c d A t25-gbs O.'seb,ri wir 23kn flflt5jfi I c.j - * * Received optical power (dbm) (c) Fig. 4. (a), (b) Measured RF spectra and (c) BER curves of both 1.25-Gb/s wired data and a 63-GHz wireless one after 23- km transmission. 5. CONCLUSION A new bidirectional millimeter-wave band RoF architecture was proposed. Both optical carrier suppression and injection locking effect were used to simultaneously generate 1.25-Gb/s wired data and 63-GHz wireless one. A RSOA was employed for the implementation of a simple and cost-effective BS. Error free transmission (BER of ) of wired, wireless, and uplink data were accomplished after 23-km transmission. We observed that there is no impact of a downlink data transmission on an uplink one due to the wavelength reuse. Only the 2-dB power penalty of uplink data was observed due to the Rayleigh backscattering noise generated in an uplink system using a bidirectional single fiber. These experimental results tell us that the proposed architecture can be a good model for near future gigabit wired and wireless access networks based on a RoF technology. Proc. of SPIE Vol J-6

7 w With dotoillink rio transmission) Without downhlnk (notransnrasion) WOh downhink (23km Iransiassion) a Without doronhink (23kmoansmhsion) Recieved optical power (dbm) Fig. 5. Measured BER curves of a 1.25-Gb/s uplink data in the presence of a downlink transmission. REFERENCES [1] [2] [3] [4] [5] [6] D. Wake, M. Webster, G. Wimpenny, K. Beacham, and L. Crawford, Radio over fiber for mobile communications, in 2004 IEEE Int. Topical Meeting on Microwave Photonics, Oct. 2004, pp Y. Y. Won, H. C. Kwon, and S. K. Han, 1.25-Gb/s Wavelength-Division Multiplexed Single-Wavelength Colorless Radio-on-Fiber Systems Using Reflective Semiconductor Optical Amplifier, J. Light. Technol., vol. 25, no. 11, pp , Nov D. W. Lee, Y. Y. Won, and S. K. Han, Bidirectional Gigabit Millimeter-wave Wavelength Division Multiplexed- Radio over Fiber Link Using a Reflective Semiconductor Optical Amplifier, IEICE Transaction on Communications, vol. E91-B, no. 7, pp , Jul M. K. Hong, Y. Y. Won, and S. K. Han, Gigabit Optical Access Link for Simultaneous Wired and Wireless Signal Transmission Based on Dual Parallel Injection-Locked Fabry-Perot Laser Diodes, Journal of Lightwave Technology, vol.26, no.15, pp , Aug L. Chen, H. Wen, and S. Wen, A Radio-Over-Fiber System With a Novel Scheme for Millimeter-Wave Generation and Wavelength Reuse for Up-Link Connection, IEEE Photon. Technol. Lett., vol. 18, no. 19, pp , Oct C. S. Park, Y. Guo, Y. K. Yeo, Y. Wang, L. C. Ong, and S. Kato, Fiber-Optic 60-GHz Wireless Downlink Using Cross-Absorption Modulation in an EAM, IEEE Photon. Technol. Lett., vol. 20, no. 8, pp , Apr Proc. of SPIE Vol J-7

8 [7] [8] Z. Jia, J. Yu, D. Boivin, M. Harris, and G. K. Chang, Bidirectional ROF Links Using Optically Up-Converted DPSK for Downstream and Remodulated OOK for Upstream, IEEE Photon. Technol. Lett., vol. 19, no. 9, pp , May Z. Jia, J. Yu, and G. K. Chang, A Full-Duplex Radio-Over-Fiber System Based on Optical Carrier Suppression and Reuse, IEEE Photon. Technol. Lett., vol. 18, no. 16, pp , Aug [9] Z. Jia, J. Yu, A. Chowdhury, G. Ellinas, and G. K. Chang, Simultaneous Generation of Independent Wired and Wireless Services Using a Single Modulator in Millimeter-Wave-Band Radio-Over-Fiber Systems, IEEE Photon. Technol. Lett., vol. 19, no. 20, pp , Oct Proc. of SPIE Vol J-8