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Linearity Enhancement of an Electroabsorption Modulated Laser by Dual-Parallel Modulation Hyun-Do Jung, Duk-Ho Jeon, and Sang-Kook Han
Abstract—A novel linearization process in electroabsorptionmodulated laser (EML) is proposed and experimentally demonstrated. Dual-parallel modulation scheme is used to compensate the nonlinear component of the EML by controlling the dc bias voltages of the each EML separately. The validity of the proposed modulation scheme is confirmed through the simulations and experiments. From a dual-parallel modulation experiment at 8 GHz, a reduction of 23 dB in IMD3 and the following increase of linear dynamic range of 19.6 dB are achieved compared to those of a single EML operation. Index Terms—Dual-parallel modulation, electroabsorption modulated laser (EML), harmonic/intermodulation distortion (HMD/IMD), linear dynamic range (LDR), linearization, subcarrier multiplexing (SCM).
I. INTRODUCTION
A
N ELECTROABSORPTION modulator (EAM) takes advantages of a low driving voltage, a small size, and monolithic integration capability with other optical components. But the transfer function of an EAM shows a complicated nonlinear transfer characteristic, which limit the dynamic range of an analog optical link by the production of harmonic distortion (HMD) and the intermodulation distortion (IMD). The previous reported linearization schemes of an EAM have utilized a feed-forward and a predistortion technique those rely either partly or completely upon the electrical components [1], [2]. These methods require high frequency operating electrical devices and add complexity to reduce the nonlinear products for SCM optical transmissions. In this letter, we propose a novel linearization scheme consists of two multiple-quantum-well (MQW) electroabsorption modulated laser (EML). Two EMLs are connected in parallel so as to compensate the nonlinearity of the main EML by using a phase-reversed nonlinearity of the sub-EML. The validity of the proposed linearization process has been simulated and confirmed by experimental demonstration of IMD products for a single and dual EML cases. II. PRINCIPLE OF OPERATION The proposed dual-parallel modulation scheme [3], [5] consists of two different EMLs and a 3-dB optical coupler. A continuous-wave (CW) optical wave is emitted from the distributedfeedback laser diode (DFB-LD) and modulated by the external Manuscript received June 8, 2001; revised August 15, 2001. This work was supported by the Korean Ministry of Information and Communication and Brain Korea 21 program. The authors are with the Department of Electrical and Electronic Engineering, Yonsei University, Seoul 120-749, Korea (e-mail:
[email protected]). Publisher Item Identifier S 1041-1135(02)01869-4.
microwave signal injected in each EAM. The 3-dB optical coupler combines the modulated optical signals and the modulated RF signal on an optical carrier is recovered by a photodetector (PD). For a mathematical treatment, we define an input optical power from DFB-LD as , and the corresponding field ampli. The optical fields at the outputs of the EMLs tude as are defined as [4] (1a) (1b) and describe the transfer function of where EMLs. Since we have used two different EML modules with a different lasing wavelength, two light sources are not coherent each other. So, the electrical current produced in a square-law PD becomes in proportion to the output power and can be expressed as below (2) To analyze the distortion characteristics, we can expand and as a Taylor series for the modulation , near the dc bias point of interest. voltage, In a suboctave application, the IMD3 is the dominant nonlinear product and is related to the cubic term of the Taylor series expansion of (2). To eliminate the cubic term, it is required 0, or that (6) Since the Taylor series coefficients strongly depend on the dc bias voltage [3], the nonlinear distortion compensation can be achieved by controlling the dc bias of two EMLs separately. Therefore, the spurious distortion RF signals imposed on the sub EML have a 180 phase difference with respect to those of the main EML by controlling the dc bias of the main and sub EML differently. The PD acts as an inphase microwave combiner such that it coherently adding the detected RF signals. Two of the third-order distortion signals with 180 phase difference are added destructuvely and canceled each other. Thus, the nonlinear distortion can be compensated. From the simulation using the above formulation, we have obtained 14.5-dB reduction of IMD3 product and 9-dB improvement of LDR. III. EXPERIMENT AND RESULTS To verify the proposed linearization process, we have performed experiments by using commercially available 10-Gb/s
1041-1135/02$17.00 © 2002 IEEE
JUNG et al.: LINEARITY ENHANCEMENT OF ELECTROABSORPTION MODULATED LASER BY DUAL-PARALLEL MODULATION
Fig. 1.
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Experimental setup.
EML devices. The experimental setup is shown in Fig. 1. Two-tone RF signals of 8.00 and 8.01 GHz were applied to both EMLs. Two EML consist of a MQW electroabsorption (EA) modulator with a DFB laser and operate at different wavelength of 1542.9 and 1533.1 nm. The modulated optical signals from each EML are coupled into the optic-to-electric (O/E) converter that consists of a 25-GHz PD with 0.6 A/W responsivity and a 20-dB gain RF amplifier. After the photodetection, the magnitudes of nonlinear products were measured compared to the fundamental RF carrier by the RF spectrum analyzer. The optical power of each EML was fixed with 0 dBm at zero bias voltage for both EMLs. In measurement, we used an EML2 emitting at 1533.1 nm as the main EML because of its relatively high slope efficiency. Then, in order to find the optimal bias condition for the reduction of the IMD3 at specific dc bias voltage of the main EML, we have measured the variations of the received RF power of the main and sub-EML. From the Fig. 2, we could expect that the third derivative value of the main EML in high bias region ( 1 V) has a reversed slope compared to that of sub EML operating in a low bias region ( 0.8 V). From this observation, we have decided that the bias positions of the main and the sub EML were selected as 1.2 and 0.6 V respectively, where the phases of IMD3 of the main and sub-EML became out of phase and the IMD3 component was reduced. The reduction of IMD3 in frequency domain is shown in Fig. 3. The IMD3 of 25 dBc was measured relative to the fundamental signal (8 GHz) in a single EML. The suppression of IMD3 was achieved by using dual EML schemes as shown in Fig. 3(b). From the measurement, the IMD3 for dual EML case was about 48 dBc and about 23-dB improvement of the IMD3 suppression has been obtained. Fig. 4 represents the RF output powers of the fundamental signals and the third-order intermodulation signals as a function of the RF input power for a single and dual EML schemes to estimate the dynamic range. A single EML without linearization scheme showed a LDR of 90.4 dB/Hz , which in dual EML scheme where an was increased to 110 dB/Hz equivalent noise floor of 170.6 dBm/Hz were used. In principle, the carrier power of dual EML case should be lower than that of single EML. In this figure, however, the carrier power of dual EML is higher because we had injected higher RF power in dual EML case to compensate the additional losses by the additional bias-tee, power combiner/splitter and optical 3-dB cou-
Fig. 2. Variation of received RF power (Fundamental, IMD3 and IMD5). (a) EML1_sub (RF power 0 dBm). (b) EML2_main (RF power 0 dBm).
=
=
Fig. 3. Measured IMD3 of (a) single EML and (b) dual EML. The dc bias is 1.2 and 0.6 V for main and sub EML respectively.
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 14, NO. 4, APRIL 2002
of radio frequency and the proposed scheme can be applied for a wide radio frequency of interest. Since we have dealt with a radio frequency of 8 GHz, the operating bandwidth was restricted into the narrow-band in this case. Both the IMD3 and IMD5 need to be suppressed simultaneously for a narrow-band operation. In our experiment, since the IMD5 was very small and close to the noise level for most of dc bias point, we could not treat the IMD5 as the dominant nonlinear product that we need to suppress in a narrow-band operation case. Instead, we have shown the suppression of the IMD3 and the same suppression scheme would be adopted for the IMD5. We believe that this experimental demonstration is good enough to prove the proposed linearization concept. This work was on the basis of back-to-back operation and the investigation of transmission characteristics of the proposed scheme incorporating the fiber chirp will be another communication. Fig. 4. Measured LDR for both single EML and dual EML. The predicted (solid and dashed lines) and measured (marks) results are represented.
IV. CONCLUSION We have proposed a novel dual-parallel EML modulation to improve the linearity of an analog optical modulator. This method is relatively simple and easily controllable compared to the previously reported linearization schemes. In addition, this method can suppress two nonlinear components simultaneously by controlling two bias points, while only one nonlinear component can be suppressed in a single modulation case. To verify the linearization process of the proposed scheme, two-tone measurements have been performed at 8 GHz. The IMD3 produced by the nonlinear transfer curve of the main EML was reduced as much as 23 dB and the following enhancement of the LDR of 19.6 dB was measured. Since the demonstrated linearization process is a simple and robust, we believe that it would be very useful for an efficient analog optical transmitter. REFERENCES
Fig. 5. Frequency dependence of the fundamental and IMD terms for fixed bias voltages.
pler. As a result, the improvement of the dynamic range of 19.6 dB has been obtained. These results are well matched to the calculated results though there are a slight discrepancy of bias conditions and a degree of suppression. To verify the effectiveness of the proposed scheme for the radio frequency range of interest, the distortion suppression measurements were performed from 1 to 8 GHz, while keeping two dc bias of EMLs the same. As we can see in Fig. 5, the distortion suppression is independent
[1] R. M. DeRidder and S. K. Korotky, “Feedforward compensation of integrated-optic modulator distortion,” in OFC, San Francisco, CA, 1990, Paper WH5. [2] R. B. Childs and V. A. O’Byrne, “Predistortion linearization of directly modulated DFB lasers and external modulators for AM video transmission,” in OFC, San Francisco, CA, 1990, Paper WH6. [3] G. W. Lee and S. K. Han, “Linearization of a narrowband analog optical link using integrated dual electroabsorption modulator,” in MWP ’99, Melbourne, Australia, 1999, pp. 21–24. [4] S. K. Korotky and M. Rene, “Dual parallel modulation schemes for lowdistortion analog optical transmission,” IEEE J. Select. Areas Commun., vol. 8, pp. 1377–1381, July 1990. [5] H. D. Jung and S. K. Han, “Dynamic nonlinearity reduction of DFB-LD by dual parallel modulation,” in IEEE/LEOS 2000, Puerto Rico, 2000, pp. 74–75.
