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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 14, NO. 7, JULY 2002
Nonlinear Distortion Suppression in Directly Modulated DFB-LD by Dual-Parallel Modulation Hyun-Do Jung, Student Member, IEEE and Sang-Kook Han, Member, IEEE
Abstract—A novel method to suppress nonlinear distortion of directly modulated laser diode using dual parallel modulation scheme has been proposed. This scheme exploits that the frequency chirping influences the phase of RF signals. Two properly controlled undesired intermodulation signals with out-of-phase difference are cancelled via coherent RF recombination at the photodetector. The validity of this scheme was confirmed through experiments. From experiments, the 20-dB reduction of the third-order intermodulation distortion was achieved and the tunability of suppression region was observed. Index Terms—Analog links, dual parallel modulation, frequency chirping, laser diode, nonlinear distortion.
I. INTRODUCTION
T
HE CAPACITY of existing copper based telecommunication networks does not follow the increasing speed of information and subscriber of mobile communication and Internet. So, the optical fiber links with low loss and large bandwidth are very attractive as a substitute for current network. To transmit RF signal through the optical fiber, the modulation process is necessary. Modulation methods are divided into direct and indirect modulation. The indirect modulation has an advantage of low chirping but needs additional components. Therefore, the cost of the system is higher than that of directly modulated one. So, general optical communication systems adopt the direct modulation of semiconductor laser diode (LD). These systems require that the laser light intensity be a linear function of the bias current under large-signal modulation. However, directly modulated LD introduces a nonlinear distortion which is due to the inherent nonlinear characteristic of the LD. When the LD is directly modulated by RF signals, the nonlinear fluctuation of the electron and photon density in the active region causes the nonlinear products. These nonlinear products act as a limitation on the use of the direct modulation system in high frequency region [1]. So far, several techniques such as feedforward compensation [2], electronic predistortion [3], optical injection locking method [4], have been proposed to linearized LD. However, these are somewhat complicated to build system. We report in this letter that the nonlinear distortion of DFB-LD can be reduced significantly by using the dual-parallel modulation scheme [5]. This scheme is configurationally simple and uses the bias current as the only control parameter Manuscript received August 23, 2001; revised February 20, 2002. This work was supported in part by the University Research Program (2001-059-3) and by Ministry of Information and Communication in South Korea. The authors are with the Department of Electrical and Electronic Engineering, Yonsei University, Shinchon-Dong, 120-749 Seoul, Korea (e-mail:
[email protected]). Publisher Item Identifier S 1041-1135(02)04613-X.
Fig. 1.
Proposed dual parallel modulation scheme.
for suppressing the nonlinearity of DFB-LD. Through experiments, the third-order intermodulation distortion (IMD3) was suppressed by 20 dB and the tunability of nonlinear suppression RF power region was observed in narrow band operation at 2.2 GHz. II. DUAL PARALLEL MODULATION SCHEME The proposed dual-parallel modulation is shown in Fig. 1. This scheme consists of two DFB-LDs and one optical coupler. To suppress the nonlinear products, this scheme exploits that the frequency chirping influences the phase of RF signals. When LD is directly modulated by RF signal, the modulated current density is accompanied by the change of refractive index that makes the phase or frequency modulation (frequency chirping) of the optical signal. This frequency chirping affects the amplitude and phase of the RF signal [6]. However, the frequency chirping is dependent on dc bias current. Therefore, if we controlled the dc bias current, the amplitude and phase of the RF signal would change. In the proposed scheme, the undesired IMD3 signal is cancelled by that of the other DFB-LD with phase difference. The properly controlled dc bias current of two LDs makes the phase difference of IMD3 out-of-phase. The photodetector acts as a broadband in-phase microwave combiner, coherently adding the detected RF signals [7]. The undesired distortion signals add destructively whenever they are out-of-phase. To verify the concept of dual parallel modulation scheme, we examined the phase variations of detected RF signals of a single LD for various dc bias current. We have measured the phase variation of the fundamental RF carrier signal for the dc bias current. The phase variation of IMD3 signal was simulated through the large-signal rate equation model [8] with the extracted LD parameter from the measurements. Fig. 2 depicts these results.
1041-1135/02$17.00 © 2002 IEEE
JUNG AND HAN: NONLINEAR DISTORTION SUPPRESSION IN DIRECTLY MODULATED DFB-LD BY DUAL-PARALLEL MODULATION
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Fig. 3. Experimental setup.
Fig. 2. Phase variation of detected RF signal. (a) Measured fundamental signal. (b) Simulated IMD3 signal.
As shown in the figure, the phase of RF signal varies with dc bias current. In case of IMD3 signal, we could find two bias positions where they are phase difference. At the same bias positions, the fundamental signal shows little phase change. Therefore, when two LDs are operated at these bias positions, IMD3 signals could be suppressed. III. EXPERIMENT AND DISCUSSION The experimental setup to suppress the intermodulation distortion is illustrated in Fig. 3. We used two DFB-LDs with the wavelength of 1541 and 1554 nm. Two RF signals with the power level of 0 dBm were applied to each DFB-LD. The output from two DFB-LDs were combined to 3-dB optical coupler and detected at high-speed photodetector. These detected signals were measured by the RF spectrum analyzer. We carried out experiments for proving the proposed scheme. A single LD link and dual LD link were compared in experiments. The DFB-LD was directly modulated by two-tone RF signal of 2.2 and 2.21 GHz. The power level of both RF signals was fixed as 4 dBm. In case of a single LD link, DFB-LD was ). The generated IMD3 signal due to biased at 12 mA ( nonlinear characteristic of the DFB-LD is shown in Fig. 4(a). As shown in the figure, the RF power difference between fundamental and IMD3 signal was 14 dBc. To reduce this non-
Fig. 4.
Measured RF spectrum. (a) Single LD link. (b) Dual LD link.
linear product, the proposed dual-parallel modulation link was applied. As mentioned previously, dc bias current of the second DFB-LD was controlled with observing the RF spectrum analyzer to find a proper bias position. When the bias position of the second LD was 12.9 mA, IMD3 signal was suppressed by 20-dB near noise floor as shown in Fig. 4(b). In this bias position, however, the IMD5 signals were not fully compensated. This seems that the phase variation of each frequency component of directly modulated signal by frequency chirping
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Fig. 5.
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 14, NO. 7, JULY 2002
RF power response [LD1 bias: 12 mA (fixed)].
In experiments, we focused on the verification of our scheme and all experiments were carried out under the back-to-back condition. To apply the proposed scheme to a practical optical system, however, some issues should be considered. The proposed technique exploits the facts that the frequency chirp influences the phase of RF signals. The phase variation of each frequency component of modulated signal by frequency chirping is slightly different. So, this scheme is somewhat sensitive to the variation of optical wavelength. Since we have used two cooled-type DFB-LDs in the experiments, this issue was not considered. In back-to-back measurements, it was not in serious problem that two LDs are not identical. However, due to the chromatic dispersion effects of two different optical wavelengths, it is expected that the suppression condition may not be maintained after transmission with the same bias condition. Nevertheless, the control of nonlinearity is still achievable by having different set of bias condition according to the transmission distance. Therefore, if the proposed technique were to be used in a practical optical transmission system, both the wavelength locking mechanism and the technique reducing the chromatic dispersion effects should be taken into account. IV. CONCLUSION
Fig. 6. Tunable RF suppression region [LD1: 10 mA. LD2: 17 mA (0 dBm). LD1: 12 mA. LD2: 12.9 mA (4 dBm). LD1: 11 mA. LD2: 25 mA (8 dBm)].
is slightly different. Therefore, when we control the bias current of DFB-LD, the IMD3 and IMD5 signals are differently affected by frequency chirping. So, the condition to suppress the IMD3 signals may not completely satisfy the out-of-phase condition of IMD5 signals. There was also a few decibel reduction of fundamental signals. This reduction was caused by the extra loss of the RF power divider, optical 3-dB coupler, and the phase mismatch of two fundamental signals. We also measured the RF power response for various input RF powers and the results are shown in Fig. 5. From the figure, it is known that the suppression of IMD3 is maintained at the RF power region about 4 dBm. It is thought that the frequency chirping is affected by input RF power. So, out-of-phase condition is not satisfied at other power region. As varying bias current of the second LD, the linear dynamic range was changed. This change also happened for the same reason stated above. Consequently, the parameters that influence the phase of RF signal are the modulation RF power and LD bias current. Therefore, to tune the suppressed RF power region, the new bias position set is required. Fig. 6 shows these results. The IMD3 signals were suppressed at the specific RF power regions. At these suppression regions, the bias current sets are different in each case to meet the out-of-phase condition.
We have proposed and experimentally demonstrated a novel dual-parallel modulation scheme for nonlinear distortion suppression of directly modulated DFB-LD. To suppress the nonlinear products, this scheme used the frequency chirping dependent on dc bias current. Two properly controlled undesired intermodulation signals with out-of-phase difference were cancelled via coherent RF recombination at the photodetector. In experiments, we focused on the verification of our scheme. For narrow-band applications, the 20-dB reduction of IMD3 was achieved and the tunability of suppression region was observed in experiments. Although some technical issues are still remained, the proposed method is simple and robust and it would be useful for an analog optical transmitter. REFERENCES [1] K. Y. Lau and A. Yariv, “Intermodulation distortion in a directly modulated semiconductor injection laser,” Appl. Phys. Lett., vol. 45, pp. 1034–1036, Nov. 1984. [2] L. S. Fock and R. S. Tucker, “Reduction of distortion in analog modulated semiconductor lasers by feedforward compensation,” Electron. Lett., vol. 27, pp. 669–671, Apr. 1991. [3] T. E. Darcie and G. E. Bodeep, “Lightwave subcarrier CATV transmission systems,” IEEE Trans. Microwave Theory Tech., vol. 38, pp. 524–533, May 1990. [4] G. Yabre and J. L. Bihan, “Reduction of nonlinear distortion in directly modulated semiconductor lasers by coherent light injection,” IEEE J. Quantum Electron., vol. 33, pp. 1132–1140, July 1997. [5] H. D. Jung and S. K. Han, “Dynamic nonlinearity reduction of DFB-LD by dual-parallel modulation,” in Proc. IEEE/LEOS 2000, Puerto Rico, Dec. 2000, pp. 74–75. [6] S. Kobayashi, Y. Yamamoto, M. Ito, and T. Kimura, “Direct frequency modulation in AlGaAs semiconductor lasers,” IEEE J. Quantum Electron., vol. QE-18, pp. 582–595, Apr. 1982. [7] K. K. Loi, J. H. Hodiak, X. B. Mei, C. W. Tu, and W. S. C. Chang, “Linearization of 1.3-m MQW electroabsorption modulations using an all-optical frequency-insensitive technique,” IEEE Photon. Technol. Lett., vol. 10, pp. 964–966, July 1998. [8] J. C. Cartledge and G. S. Burley, “The effect of laser chirping on lightwave system performance,” J. Lightwave Technol., vol. 7, pp. 568–573, Mar. 1989.
