IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 13, NO. 8, AUGUST 2001 899 Bidirectional DWDM Transmission Using a Beat-Frequency-Locking Method Yong-Sang Ahn, Sang-Yuep Kim, Sang-Hoon Han, Jae-Seung Lee, Sang-Soo Lee, and Wan-Seok Seo Abstract—In order to achieve the dense-wavelength-division multiplexing (DWDM) in bidirectional optical transmission systems, we present a beat-frequency-locking method to stabilize the relative channel frequency offset between two communicating nodes. Using the proposed method, we demonstrate a bidirectional DWDM transmission having 50-GHz channel spacing in both 2.5- and 10-Gb/s bit rates. Index Terms—Backscattering, Brillouin scattering, fiber optics, optical fiber communication, Rayleigh scattering, wavelength-di- vision multiplexing. I. INTRODUCTION W AVELENGTH-DIVISION-MULTIPLEXING (WDM) systems are mostly used for unidirectional transmis- sions using multiple fiber lines. As the Internet traffics glow exponentially, upgrading conventional systems to bidirec- tional systems become increasingly attractive to reduce the fiber-rental cost [1], [2]. Unfortunately, the Rayleigh scattering, the stimulated Brillouin scattering (SBS), and many kinds of device reflections produce backward optical noises in bidirectional optical transmission systems. Thus, in [1], the channel spacing was doubled compared with the corresponding unidirectional WDM system. In [2], different optical bands were used for counter propagating channels. Both approaches reduce the transmission capacity per direction after the upgrade. In this letter, we propose a beat-frequency-locking method to suppress foregoing backward optical noises stably even when counter propagating channels are very close in spectral domain. The beat-frequency-locking method detects counterpropagating bidirectional channels using a same detector. The beat signal, thus, obtained is used to stabilize the relative channel frequency offset defined as the minimum channel frequency difference between two nodes communicating through a single fiber line. If we choose the relative channel frequency offset sufficiently away from zero and from the SBS resonance frequency, we can suppress the penalties from foregoing backward optical noises. Manuscript received February 28, 2001; revised May 8, 2001. This work was supported by the Basic Research Program of the Korea Science and Engineering Foundation under Grant 2000-1-30200-004-3. Y.-S. Ahn, S.-Y. Kim, S.-H. Han, and J.-S. Lee are with the Department of Electronic Engineering, Kwangwoon University, Nowon-Gu, Seoul 139-701, Korea. S.-S. Lee and W.-S. Seo are with the Optical Multiplexing Technology Team, ETRI, Yusong, Taejon 305-600,Korea. Publisher Item Identifier S 1041-1135(01)06430-8. II. EXPERIMENT We tested our method experimentally using two nodes, node-A and node-B, connected by a 75-km conventional single mode fiber (SMF). The setup is shown schematically in Fig. 1. The transmitter-A in node-A and the transmitter-B in node-B had two channels with 50-GHz channel spacing. The transmitter-A had channel wavelengths at nm and at nm. The transmitter-B had channel wavelengths at and at with ,( , 2). The channel allocation is shown in Fig. 2 including the relative channel frequency offset which will be denoted as hereafter. All these channels were externally modulated in 2.5 Gb/s at first and then in 10 Gb/s. The modulation pattern was a 2 1 pseudorandom bit sequence. The total launched powers were about 5.4 dBm in both directions. In Fig. 1, the 3-dB couplers, C2 and C3, had 38-dB backre- flections from P1 to P3 ports. These reflected powers were not negligible compared with the received optical signal power at P1, 14.6 dBm. Thus, polarization controllers were used after EDFA1 and EDFA3 to minimize the polarization overlap. They were needed especially for the 10 Gb/s case where the channel spectrum spreads more broadly than the 2.5 Gb/s case. The po- larization controllers could be removed using a bidirectional line amplifier in the midst of the transmission fiber. In node-B, a polarization scrambler was used to randomize the polarizations of received channels tapped from a 9 : 1 cou- pler. The polarization scrambled received channels were com- bined with the transmitter-B output using 3-dB couplers, C4 and C5, and detected by a high-speed pin photodiode. The input powers to C5 were 1.5 dBm for both inputs. The photodiode output was amplified using two radio frequency (RF) amplifiers and detected by a 26-GHz RF detector. The second RF amplifier had bandpass characteristics centered around 7 GHz. was controlled using a 16-bit microprocessor within the wavelength control circuit that changed the laser diode temper- atures of transmitter-B. The wavelength control circuit had two operation modes, a feedback mode and a manual mode. In feed- back mode, the microprocessor adjusted and to maxi- mize the beat frequency signal after the RF detector, which au- tomatically made the channel spacing in node-B the same as that in node-A. In this mode, we obtained GHz at 2.5 Gb/s. At 10 Gb/s, the beat frequency spectrum became broad. Thus, we added an RF bandpass filter before the RF detector having a center frequency at 9 GHz with 3.1-GHz 3-dB band- width and obtained GHz. Other values were ob- tained within 0.2 GHz changing the laser diode temperatures in manual mode. 1041–1135/01$10.00 © 2001 IEEE