Open Access
Add to BibSonomyAbstract
A sequential optical frequency-division multiplexing technique using cross-phase modulation in fibers with exactly frequency-controlled optical subcarrier signals is proposed and demonstrated. 12 channels of 10-Gb/s ASK/DPSK signals with 20-GHz exact channel spacing are successfully multiplexed all-optically at 12 stages with 1-km intervals.
©2011 Optical Society of America
1. Introduction
In the past years a lot of research has focused on data multiplexing technologies as well as advanced modulation technologies including multi-level PSK, QAM, OFDM, etc., to achieve large capacity and high spectral efficiency in transmission on a single wavelength optical carrier [1,2]. Intense research has also been conducted in the field of wireless communications for developing data transmission at carrier frequencies up to 100 GHz and above. As an example of optical/wireless interfaces, the radio-over-fiber (ROF) technology has been investigated along with ultra-wide band optical devices and high-speed electronics to effectively convert the base-band data signal to an RF signal of over 100 GHz [3,4]. Making optical networks more flexible so that arbitrary information can be freely/directly accessed and communicated across networks requires ultra-broadband data multiplexing technology that can cover a THz-class bandwidth.
We have proposed an all-optical frequency-division multiplexing (FDM) scheme, in which the data signals are sequentially multiplexed on a single optical carrier using cross-phase modulation (XPM) in fibers with locally provided optical subcarrier signals, as shown in Fig. 1 [5,6]. The subcarrier signals with different subcarrier frequencies were multiplexed on a single optical carrier by frequency assignment among each multiplexing nodes. In this bus-topology optical link, one can multiplex one’s own data onto the fiber link directly from where one would like to gain access [6]. The method was demonstrated in various experiments [5,6] such as 12-channel FDM of 100-Mb/s ASK signals by using directly modulated laser diodes, 16-channel FDM of 1-Gb/s ASK/DPSK signals over THz range using the optical beat signals generated by combining wavelength detuned optical signals, and 16-channel FDM of 10.75-Gb/s QPSK signals with 10.75-GHz channel spacing using beat signals extracted from an optical comb. Since the insertion loss of this scheme is very small, a lower number of amplifiers are required for the multi-stage cascading. Therefore a large number of cascaded stages will be realized in this scheme. For ensuring dense optical FDM effectively utilizing the THz-range broad bandwidth [7] by the proposed multiplexing scheme, a precise optical frequency control with fine spectral granularity is required.
In this paper we present cascaded FDM over 12 multiplexing stages with 1-km intervals in a loop configuration. At each stage, 10-Gb/s ASK/DPSK signals with exact 20-GHz spacing were multiplexed on a single optical carrier by XPM in highly-nonlinear fibers (HNLFs). The precise control of the optical subcarrier frequency of each multiplexed data signal at 20-GHz spacing was realized by using an optical frequency shifter consisting of an IQ-modulator (IQM) [8], which was driven with stable electric sinusoidal wave at 20-GHz. A THz-range FDM of 12x10-Gb/s signals, densely and exactly multiplexed with 20-GHz channel spacing, was successfully demonstrated with error free transmission through a 50-km single-mode fiber (SMF) link.
2. Experiment
We tested the 12-channel 10-Gb/s all-optical multiplexing using the setup shown in Fig. 2 . Successive FDM by XPM was simulated by using a circulating loop configuration. The optical carrier at a wavelength of λc = 1542.0 nm (νc = 194.4 THz) was input to a circulating loop with an optical power of + 13 dBm. Two optical multiplexers, XPM-1 and XPM-2, were inserted in the loop with a 1-km distance, and 500-m HNLFs were used for generating XPM. The total loop length was about 3 km. After multiplexing, the FDM signal was transmitted through a 50-km SMF link, and then a single channel was extracted by using an optical filter (0.1 nm bandwidth) before being received by direct detection.
Fig. 2 Experimental setup. The inset shows the configuration of the multiplexing loop and the two control signal generators (Ctrl-1 and Ctrl-2). SW: optical switch, Comb: optical comb generator, LOPS: looped optical frequency shifter, LNM: lithium niobate intensity/phase modulator, RX: receiver, SMF: Single-mode fiber.
The control signals were generated as optical beat signals (Ctrl-1 and Ctrl-2) by combining a CW light and a data signal. The provided control signals in each optical carrier circulation had different subcarrier frequencies with an exact channel spacing of 20 GHz which was realized by the looped optical frequency shifter (LOFS) presented in Fig. 3 . The optical frequency shift is provided by applying a sinusoidal wave to the balanced child MZs of the IQM at 90° out of phase with respect to each other [8]. The output optical frequency, νk (k is the number of circulation), is shifted by the control frequency, f = 20 GHz, with respect to the optical frequency in the former lap, νk-1, i.e., νk = νk-1 ± f. The optical frequency shifter in the circulating loop [9] simulates the sequential FDM with different optical subcarriers with exactly the same frequency spacing of 20 GHz. The generated shifted optical frequency sequence is shown in Fig. 4 . The initial optical frequency was 192.45 THz and the optical frequencies of each lap were down-converted by 20 GHz. It achieved a frequency shift well over 100 GHz. The unwanted modes, especially the third-order mode on the opposite side, were suppressed to less than −40 dB by optimizing the driving condition of the IQM. The data signal was generated by modulating the shifted optical frequency sequence using a LN intensity/phase modulator with a 10-Gb/s ASK/DPSK data signal (PN: 27-1).
Fig. 4 Output optical spectrum of LOFS in each sequence (Res.: 0.01 nm) with the initial optical frequency of 192.45 THz and the shifted frequency of −20 GHz.
The first control signal (Ctrl-1) consists of a CW light (CW-1) at λCW1 = 1555.8 nm (νCW1 = 192.69 THz) and a sequentially provided data signal at λS1 ~1557.8 nm with shifted optical frequencies (νS1~192.45, 192.43, 192.41, 192.39, 192.37 and 192.35 THz). The second control signal (Ctrl-2) consists of a CW light (CW-2) at λCW2 = 1554.8 nm (νCW2 = 192.81 THz) and a data signal at λS2 ~1557.8 nm (νS2~192.45, 192.43, 192.41, 192.39, 192.37, and 192.35 THz). The CW-2 was generated with optical frequency up-conversion by extracting a certain mode of the optical comb [10]. The CW-2 was 120-GHz up-converted light with respect to the CW-1, which was extracted a fifth higher frequency mode from a 24 GHz frequency spacing optical comb derived from CW-1. The state of polarization (SOP) of the local CWs and data signals was matched for optimum generation of XPM.
The HNLFs had a nonlinear coefficient of γ ~20 W−1km−1 and a zero-dispersion wavelength of λ0 ~1550 nm. WDM couplers were used for combining and separating the optical carrier and control signals. The optical carrier and control signal wavelengths were approximately symmetrically allocated with respect to λ0 to achieve the optimum phase matching. The input power of the control signal to the HNLF was set to + 15 dBm. The SOP of Ctrl-1 and Ctrl-2 was controlled to match the carrier signal. The circulating loops for both multiplexing and optical frequency shift were synchronized by adjusting the gating time of the optical switch and the loop length. The gating time was about 15 μs.
Figure 5 shows the frequency dependence of the modulation efficiency in XPM-1 and XPM-2. The carrier light νc and the control signal of νCWj and νCWj-Δν were launched into XPM-j (j = 1, 2). The generated XPM components at the lower frequency (longer wavelength) against the center carrier were measured. A flat response with the 3-dB bandwidth of about 1 THz was obtained for both XPM-1 and XPM-2. The observed decay of the efficiency was mainly due to a residual phase mismatch between the carrier and the control signal.
Figure 6 shows the optical spectra of the optical carrier before the receiver. The data signals were sequentially frequency-division multiplexed, two channels in each loop, on the optical carrier. Then six data signals were multiplexed after 3 laps (Fig. 6(a)) and twelve data signals were multiplexed after 6 laps (Fig. 6(b)). Every data signal was multiplexed in the range from 0.24 THz to 0.48 THz with 20-GHz channel spacing from the center carrier at almost the same efficiency as shown in Fig. 6. Figure 7 shows the BER characteristics of channels 1, 3, 6, 7, 9, and 12. For detecting the DPSK signal, a 1-bit delay demodulator was additionally used and a single port output was detected. In the measurement each channel was separately selected by using an optical filter (0.1-nm bandwidth) as depicted in the inset of Fig. 7 (c), which shows the optical spectrum of the extracted 3rd channel. Even after transmission over a 50-km SMF, we could achieve error free operation within a sensitivity variation of less than 1 dB (at BER of 10−9). The chromatic dispersion through SMF transmission was compensated by a dispersion compensating fiber inserted before the receiver. Received waveforms after the transmission are shown in the insets of Figs. 7(a) and 7(b). A dense FDM of 12x10-Gb/s signals with a spectral efficiency of 0.5 b/s/Hz in 240-GHz bandwidth was achieved. Almost the same performance was obtained for both ASK and DPSK signals.
Fig. 6 Optical spectrum of multiplexed carrier light: (a) 6-channel multiplexing after 3 laps and (b) 12-channel multiplexing after 6 laps. Res.: 0.1 nm.
Fig. 7 BER characteristics. The insets show (a) the waveform after transmission of an ASK signal (H: 20 ps/div.), (b) the waveform after transmission of a DPSK demodulated signal (H: 20 ps/div.), and (c) the optical spectrum of extracted 3rd channel. (H: 0.2 nm/div., V: 10 dB/div., Res.: 0.01 nm).
3. Conclusion
We demonstrated highly cascaded all-optical frequency-division multiplexing of 10-Gb/s data signals on a single optical carrier using cross-phase modulation (XPM) in fiber. 12-stage sequential optical FDM was demonstrated using a circulating loop and an up-converted signal sequence precisely controlled by an optical frequency shifter. 12 × 10-Gb/s ASK/DPSK signals were successfully frequency-division multiplexed with an exact 20-GHz channel spacing. Error-free transmission of the FDM signal over a 50-km single-mode fiber link was achieved. Investigations on increasing the multiplexing number and the bit-rate and applying advanced modulation formats for the enhancement of capacity are the future issue.
References and links
1. P. J. Winzer and R.-J. Essiambre, Optical Fiber Communications V (Academic Press, 2008), Vol. B, Chap. 2.
2. T. Richter, E. Palushani, C. Schmidt-Langhorst, M. Nölle, R. Ludwig, and C. Schubert, “Single wavelength channel 10.2 Tb/s TDM-data capacity using 16-QAM and coherent detection,” in Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2011), paper PDPA9. http://www.opticsinfobase.org/abstract.cfm?URI=OFC-2011-PDPA9
3. A. Hirata, M. Harada, and T. Nagatsuma, “120-GHz wireless link using photonic techniques for generation, modulation, and emission of millimeter-wave signals,” J. Lightwave Technol. 21(10), 2145–2153 (2003), http://www.opticsinfobase.org/jlt/abstract.cfm?URI=jlt-21-10-2145. [CrossRef]
4. T. Nagatsuma, H.-J. Song, Y. Fujimoto, K. Miyake, A. Hirata, K. Ajito, A. Wakatsuki, T. Furuta, N. Kukutsu, and Y. Kado, “Giga-bit wireless link using 300-400 GHz bands,” in Proceeding of International Topical Meeting on Microwave Photonics, 2009, paper Th2.3 (2009).
5. T. Kato, R. Okabe, R. Ludwig, C. Schmidt-Langhorst, C. Schubert, and S. Watanabe, “All-optical THz-band frequency multiplexing on a single optical carrier using fiber cross-phase modulation,” in Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2011), paper OThC5. http://www.opticsinfobase.org/abstract.cfm?URI=OFC-2011-OThC5
6. S. Watanabe, T. Kato, R. Okabe, R. Elschner, R. Ludwig, and C. Schubert, “All-optical data frequency multiplexing on single-wavelength carrier light by sequentially provided cross-phase modulation in fiber,” IEEE J. Sel. Top. Quantum Electron. ID 2111358 (to be published).
7. G. P. Agrawal, Nonlinear Fiber Optics (Academic Press, 2000).
8. M. Izutsu, S. Shikama, and T. Sueta, “Integrated optical SSB modulator/frequency shifter,” IEEE J. Quantum Electron. 17(11), 2225–2227 (1981). [CrossRef]
9. H. Takesue, T. Horiguchi, and T. Kobayashi, “Numerical simulation of a lightwave synthesized frequency sweeper incorporating an optical SSB modulator composed of four optical phase modulators,” J. Lightwave Technol. 20(11), 1908–1917 (2002), http://www.opticsinfobase.org/jlt/abstract.cfm?URI=jlt-20-11-1908. [CrossRef]
10. K. Kitayama, “Highly stabilized millimeter-wave generation by using fiber-optic frequency-tunable comb generator,” J. Lightwave Technol. 15(5), 883–893 (1997). [CrossRef]
