IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 1, JANUARY 2005
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Optical Beat Noise Reduction in Subcarrier Multiplexed Optical Link Using Dual Current Modulation of Master and Slave Lasers Yong-Yuk Won, Hyun-Do Jung, Sang-Kook Han, Eui-Suk Jung, and Byung-Whi Kim
Abstract—A novel method to reduce optical beat noise (OBN) which is critical in subcarrier multiplexed-based optical access link is suggested. The chirped optical signal was generated by using dual current modulation through master and slave lasers. The sidelobes of the slave laser decreased up to 90 dBm close to the noise floor. The OBN was reduced by 20 dB because of the spectral broadening of the slave laser resulted from the increased chirp. Index Terms—Dual current modulation, master laser, optical beat interference (OBI), optical beat noise (OBN), slave laser, subcarrier multiplexing (SCM), wavelength-division multiplexing.
I. INTRODUCTION
O
PTICAL BEAT interference (OBI) and its effects on transmission of optical signals in a passive network infrastructure remains as one of the major issues for such networks. The effect of OBI occurs when the wavelength difference among the optical sources exists into both the bandwidth of the optical receiver and subcarrier frequency range. It impacts adversely on the system performance indicators such as the carrier-to-noise ratio (CNR) and bit-error ratio (BER). It is important for us to broaden the optical spectrum in order to decrease the effect of OBI because the optical beat noise (OBN) is inversely proportional to the optical spectrum width. Woodward et al. used the high modulation index of the clipping tone, which broadens optical spectrum and reduces the noise [1]. Feldman et al. took advantage of a light-emitting diode (LED) as an optical source of which the spectrum is much wider than that of the laser diode [2]. However, the use of additional clipping tone limits the usable subcarrier frequency range due to harmonics and the low output power of LED restricts the split ratio and transmission distance in the subcarrier multiplexing (SCM)-based optical link. In this letter, we propose a novel scheme for the OBN suppression. The dual pump current modulation has been applied to the two lasers to generate the chirped optical continuouswave (CW) signal with broad linewidth. The proposed scheme has been verified through experimental demonstration of OBN reduction. Manuscript received May 4, 2004; revised August 2, 2004. This work was supported by Korean Ministry of Information and Communication through ETRI. Y.-Y. Won, H.-D. Jung, and S.-K. Han are with the Department of Electrical and Electronic Engineering, Yonsei University, Seoul 120-749, Korea (e-mail:
[email protected]). E.-S. Jung and B.-W. Kim are with the Electronics and Telecommunication Research Institute, Daejeon 305-350, Korea. Digital Object Identifier 10.1109/LPT.2004.837745
Fig. 1. Schematic of the chirped optical signal generation using dual pump current modulation by master and slave lasers.
II. PRINCIPLE OF OPERATION Fig. 1 shows the diagram for the chirped optical CW signal generation using dual pump current modulation by master and slave lasers. The basic idea is as follows. The injection currents of the master and slave lasers are modulated at the same time so that the sidelobes generated by current modulation of the slave laser are exactly cancelled by the ones induced optically in the master laser. Compared with the slave laser, the phase of the sidelobes has been reversed and their amplitude remains the same. Also, the chirp effect caused by the slave laser exists regardless of the suppressed sidelobes. The perfect elimination of the sidelobes can be achieved by the radio frequency (RF) phase control and RF attenuation. In other words, theoretically, we can obtain the chirped optical signal without sidelobes by adjusting the phase and intensity of RF signal of master laser. Since the laser used in the proposed scheme is operated at normal current, the output power is much higher than the one of low biased laser for chirped signal generation. The problems such as the deterioration of BER and CNR owing to low output power would not occur in transmission. Also, the chirp effect increases more than the conventional laser with the use of a single RF signal because both RF signal and subcarrier signal with baseband data are simultaneously applied to the slave laser. The method using the clipping tone with high modulation index is limited in usable frequency range because of the clipping tone occupying the usable bandwidth [1]. The technique of reducing the OBN using the over-modulated lasers limits the
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 1, JANUARY 2005
Fig. 3. Measured spectrum of the RF modulated signal.
Fig. 2.
Experimental setup for suppressing the OBN.
available subcarrier frequency range due to harmonics [3]. On the contrary, the proposed scheme overcomes the above-mentioned limitations. When the proposed scheme of Fig. 1 is used as an optical network unit in the SCM-based optical access link, the OBN, which is the main obstacle of SCM-based access network, can be remarkably reduced due to the spectral broadening effects caused by chirp. III. EXPERIMENTALS Fig. 2 shows the experimental setup. The RF modulated signal with a frequency of 1.8 GHz was divided into two lasers of master (1543.54 nm) and slave laser (1545.52 nm). The threshold current of the two lasers was 10 mA. The injection current of the master laser was 34.08 mA and that of the slave laser was 19.68 mA. The directly modulated output signal of the master laser was injected into the slave laser. We used an RF phase controller and RF attenuator to perfectly suppress the sidelobes of the slave laser. We fixed the feedback loop on the optical table so that it gained enough stability to block up variation of the optical polarization due to external vibration. In addition, we shortened the length of the feedback path in order that the phase of sidelobes maintains 180 . Also, the polarization controller was used to get the high coupling efficiency. We found that the chirp effect on the spectrum caused by the current modulation of the slave laser exists away from the diminished sidelobes. We used the optical filter to have only the output light from the slave laser. For OBN generation, we applied the subcarrier signal1 (SC1) with a frequency of 1.0 GHz to the slave laser and the subcarrier signal2 (SC2) with a frequency of 1.1 GHz to the output of tunable laser source (TLS). After the optical signal of the slave laser was divided to 3 dB by the optical coupler, the spectrum of the RF modulated signal and the optical spectrum to check the chirp effect was monitored in one arm. To produce the OBN, the modulated optical signal of TLS, which was modulated by Mach–Zehnder modulator, and the output light from the slave laser was simultaneously received at the optical receiver2 via 3-dB coupler. We controlled the wavelength difference between TLS and slave laser such that it belongs to subcarrier frequency range by changing the wavelength of TLS.
Fig. 4. Measured optical spectrum of both the proposed and conventional schemes.
IV. RESULTS AND DISCUSSION Fig. 3 shows the measured spectrum of the RF modulated signal in slave laser. The gray line is the spectrum of the unsuppressed RF modulated signal. The black line is the spectrum of the suppressed one. The measured RF modulated signals corresponding to the sidelobes of the slave laser were approximately 90 dBm, which was the level of noise floor. We verified that the optical signal without sidelobes was generated using this scheme. As shown in Fig. 4, the optical spectrum was measured to check the chirp effect. The optical spectrum of output signal from slave laser in the proposed scheme was broader than the one of the conventional laser output by 0.15 nm at 20-dB bandwidth. This result tells us that the spectral broadening due to the chirp effect exists regardless of the sidelobes suppression. We verified that the chirped optical CW signal was obtained through experiment. This can be explained as follows. When directly modulated light from the master laser is injected into the active layer of the slave laser, the carrier consumption by this signal induces the variation of refractive index. Also, the output light from the slave laser changes the refractive index due to the carrier consumption by itself. Consequently, the output light from the slave laser is frequency modulated by the above-mentioned effects while the sidelobes, amplitude components in the output from the slave laser, are suppressed by that of the master laser through cross-gain modulation effect. The OBN can be reduced using the obtained optical signal from the slave laser. We measured the SC1, the SC2, and OBN when the OBN existed from 0 GHz to about 1 GHz. The RF spectra measured at the optical receiver2 are shown in Fig. 5(a) when the master laser was excluded from the experimental setup. This figure shows that the intensity of OBN was larger than the one of the subcarrier signals. We can expect that both CNR and BER will be very
WON et al.: OBN REDUCTION IN SCM OPTICAL LINK USING DUAL CURRENT MODULATION OF MASTER AND SLAVE LASERS
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Fig. 6. Measured RF spectra when the input powers of the RF modulated signal are 10 and 10 dBm.
0
the output signal magnitude of the laser, the one of slave laser become broad [4]. Also, the chirp amount of the slave laser increases more than that of the ordinary laser when RF signal is applied to each laser. Fig. 5(b) shows that the OBN decreased by about 5 dB more than the one of Fig. 5(c). This is because the carrier depletion grows larger due to RF modulated signal and chirp effect become more intense when the output signal of master laser with RF modulated signal is injected into the slave laser. To compare the OBN variation for different RF power, the RF spectra in Fig. 6 were measured for RF powers of 10 and 10 dBm. This shows that the OBN decreases due to the increased spectral linewidth for the increased RF power. In other words, the broader linewidth of laser with relatively high RF modulation power would be more effective in reduction of OBN in our scheme. V. CONCLUSION
Fig. 5. (a) Measured RF spectra at the optical receiver2 without the master laser. (b) Measured RF spectra at the optical receiver2 using the proposed scheme. (c) Measured RF spectra at the optical receiver2 when the optical signal from the master laser without RF modulated signal was injected into the slave laser.
low. Fig. 5(b) shows the RF spectra when the proposed scheme was applied. The OBN became much smaller than the one of Fig. 5(a) by 20 dB. This is because of the linewidth broadening caused by the chirp effect due to the RF modulated and SC1 signal. Based on these measured results, we could confirm that OBN can be suppressed with the proposed scheme. Fig. 5(c) shows the RF spectra when the optical signal from the master laser without RF modulation was injected into the slave laser. As shown in this figure, the OBN became lower than the case of Fig. 5(a) by about 15 dB. The OBN reduction even without the RF modulation of the master laser can be explained as follows. The directly modulated signal from the master laser depletes carriers in the active medium of the slave laser. The slave laser is operated in the gain-saturated region because of the carrier depletion by the master laser. Accordingly, the output power of the slave laser decreases due to the gain reduction. Because the linewidth of the conventional laser is inversely proportional to
We have proposed a novel method for the OBN reduction by using the dual pump current modulation of master and slave laser and the OBN reduction was experimentally demonstrated. The RF-modulated signal, which was used to broaden the optical spectrum and obtain the CW optical signal, became low to 90 dBm and the spectrum of output signal from slave laser was much broader than that of the conventional laser by about 0.1 nm. The SC1, SC2, and OBN were measured when the OBN existed from 0 to 1 GHz. The noise due to OBI in our scheme was reduced by 20 dB more than that generated between conventional lasers. Accordingly, we may say that our proposed scheme can be free from the OBN generated in wavelengthdivision-multiplexing SCM-based optical link. REFERENCES [1] S. L. Woodward, X. Lu, T. E. Darcie, and G. E. Bodeep, “Reduction of optical-beat interference in subcarrier networks,” IEEE Photon. Technol. Lett., vol. 8, no. 5, pp. 694–696, May 1996. [2] R. D. Feldman, K.-y. Liou, G. Raybon, and R. F. Austin, “Reduction of optical beat interference in a subcarrier multiple-access passive optical network through the use of an amplified light-emitting diode ,” IEEE Photon. Technol. Lett., vol. 8, no. 1, pp. 116–118, Jan. 1996. [3] I. Seto, T. Tomioka, and S. Ohshima, “Error-free transmission of radio QPSK signals in an optical subcarrier multiple access system suppressing optical beat interference with over-modulation,” in Int. Topical Meeting Microwave Photonics 2000, Sep. 2000, pp. 43–46. [4] C. Henry, “Theory of the linewidth of semiconductor lasers,” IEEE J. Quantum Electron., vol. 18, no. 2, pp. 259–264, Feb. 1982.
