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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 9, SEPTEMBER 2005
A Photonic Up-Converter for a WDM Radio-Over-Fiber System Using Cross-Absorption Modulation in an EAM Chul Soo Park, Student Member, IEEE, Choong Keun Oh, Student Member, IEEE, Chung Ghiu Lee, Member, IEEE, Dong-Hwan Kim, and Chang-Soo Park, Member, IEEE
Abstract—A photonic frequency up-converter with a wide wavelength bandwidth is demonstrated. The up-converter is based on wavelength conversion by cross-absorption modulation in an electroabsorption modulator. A two-tone measurement is performed to investigate the linearity of the proposed up-converter. Moreover, its dependence on the wavelength and bias voltage are found to be suitable for wavelength-division-multiplexing-based radio-over-fiber application. The proposed up-converter has a spurious free dynamic range larger than 73.8 dB Hz 2 3 throughout the 30-nm range of the local oscillator wavelength. Also, the up-converted signal at 26 GHz has a phase noise of 76.8 dBc/Hz at a 10-kHz offset. Index Terms—Cross-absorption modulation (XAM), electroabsorption modulator (EAM), photonic frequency up-conversion, radio-over-fiber (RoF) system.
I. INTRODUCTION
T
HE radio-over-fiber (RoF) system is believed to be a solution to future broad-band wireless services such as vehicular communications and video distribution systems since it inherently provides a broad bandwidth, low transmission loss, and electromagnetic interference immunity. For the implementation of the broad-band RoF system, the scheme that optically generates the radio frequency (RF) carrier at the central station (CS) and distributes it to several base stations (BSs) has been reported [1]–[3]. Furthermore, to increase the bandwidth and the spectrum utilization of the RoF system, several trials have been performed to merge it with the wavelength-division-multiplexing (WDM) networks such as the metro WDM ring network [1] or the star network incorporating WDM/subcarrier multiplexing network [2]. Also, the CS is positioned in the middle of the access network, being connected to the optical network units in the passive optical network for the application of video distribution through the enhancement band recommended by ITU-T G. 983.3. Accordingly, to be connected with a different network layer for versatile applications, the RF signal generation technique at the Manuscript received February 18, 2005; revised May 19, 2005. C. S. Park, C. K. Oh, and C.-S. Park are with the Department of Information and Communications, Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, South Korea (e-mail:
[email protected];
[email protected];
[email protected]). C. G. Lee and D.-H. Kim are with the Photonics System Laboratory, Korea Photonics Technology Institute (KOPTI), Gwangju 500-460, South Korea (e-mail:
[email protected];
[email protected]). Digital Object Identifier 10.1109/LPT.2005.853015
Fig. 1. Experimental setup (DSB-SC: Double sideband-suppressed carrier; ISO: Optical isolator; MOD: Optical modulator; OC: Optical circulator; PC: Polarization controller; PD: Photodiode; TLS: Tunable laser source).
CS must be able to provide the characteristics of a wide wavelength range with proper analog performances, and avoid chromatic dispersions when the RF signal is distributed through the fiber medium. Several techniques have been proposed for generating RF carriers using the optical method, which prevents chromatic dispersion during the fiber transmission [3]–[8]. These techniques include methods using an external intensity modulator [3], [4], optical phase locking methods [5], remote up-conversion [6], and the wavelength conversion technique that employs the cross-gain modulation and the cross-phase modulation effects in a semiconductor optical amplifier (SOA) [7] and an SOA Mach–Zehnder interferometer [8]. These methods dispense with the requirements for a mixer and a local signal source at the BSs. Here, other methods besides the SOA-based up-conversion spilt an electrical signal to drive a number of laser diodes at the CS for WDM-based RoF application. Instead, the SOA-based up-conversion makes it possible to distribute an optical signal to a number of BSs in the optical domain without power amplification. This letter describes a photonic frequency up-converter incorporating cross-absorption modulation (XAM) in an electroabsorption modulator (EAM), which provides a wide wavelength bandwidth. Its analog performance parameters, such as phase noise and spurious free dynamic range (SFDR), are investigated, and multiwavelength capability is also described for its application to the WDM-based RoF system. II. EXPERIMENTAL SETUP Fig. 1 shows the experimental setup used to investigate the SFDR of an EAM photonic up-converter. The commercial InPbased EAM with a counterpropagating scheme is adopted using an optical circulator and an isolator, which are connected with
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PARK et al.: PHOTONIC UP-CONVERTER FOR A WDM RoF SYSTEM USING XAM IN AN EAM
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Fig. 2. (a) Measured RF spectra of the up-converted signal and (b) its single sideband phase noise characteristics.
an angled physical contact, to avoid the requirement for optical filtering and the reflections. Two tunable laser sources are used for both optical local oscillator (LO) and intermediate frequency (IF) wavelengths. Erbium-doped fiber amplifiers (EDFAs) are employed to provide sufficient power for absorption saturation in the EAM. The optical LO signal is generated through the double sideband-suppressed carrier modulation technique using a signal source of 12.5 GHz, with the electrooptic modulator biased at a null point. The two strong sidebands separated about 25 GHz with the suppressed carrier have been obtained with an electrooptic modulator that has 8-GHz bandwidth. The optical IF signal is injected into the right part of the EAM through an optical circulator. Then, the converted signals are made to pass through the second and third ports of an optical circulator and are finally detected by the photodiode (Discovery DSC-10H) that has an electrical bandwidth of 45 GHz. The SFDR measurement is carried out with a two-tone IF and 1.01 GHz , fed to the EAM by an signal of 1 electrooptic modulator that has 4-GHz bandwidth. Finally, the up-converted electrical spectrum is measured using a spectrum analyzer (HP 8565EC). During the SFDR measurement, a linear low-noise amplifier (LNA) with 23-dB RF gain and 6-dB noise figure is used to enhance the noise floor of the spectrum analyzer. And, the dc-block capacitor (Agilent 11 742A) before the spectrum analyzer is used. III. RESULTS AND DISCUSSION The performance of the proposed photonic up-conversion was verified by replacing the two-tone IF with a single IF of 1 GHz in the experimental setup shown in Fig. 1. The EAM was biased at 1.5 V, considering different absorption coefficient. The average optical power at the up-converter was measured to be 10 and 13 dBm for the optical LO and IF signals, while the wavelength of the optical LO and IF signals were 1560 and 1550 nm, respectively. Fig. 2(a) shows the measured spectrum of the up-converted signal without the LNA. The phase noise of the up-converted signal at 26 GHz was measured to be 76.8 dBc/Hz at a 10-kHz offset, as shown in Fig. 2(b). The conversion efficiency was measured from 6 to 27 dB over the range of 1565 to 1535 nm in the LO wavelength because of the different absorption coefficient. Here, the conversion efficiency is defined as the ratio of the up-converted sideband power to the IF power measured before the EDFA.
Fig. 3. Measured SFDR for the EAM biased at
01.5 V.
Fig. 4. Dependence of the SFDR and the carrier-to-sideband power ratio on the EAM bias voltage.
During the SFDR measurement, the signals appeared and 25.99 GHz at the frequency of 26.02 GHz , as shown in Fig. 3. The fundamental and signal powers had a slope of 1 and 3 with respect to the electrical IF power, respectively. The SFDR of the proposed up-converter was measured to be 85.9 dB Hz at the bias voltage of 1.5 V. As shown in Fig. 4, the SFDR decreased from 87.5 to as the bias voltage decreased. This decrease is 74.6 dB Hz due to the large absorption coefficient and the nonlinearity of the absorption slope. The SFDR of the proposed up-converter were dependent on its bias voltage, although, it appeared to satisfy the minimum requirement of 72 dB Hz [9]. The variations of the carrier-to-sideband power ratio for several bias voltages represent the modulation index changes by XAM in the EAM itself, since the optical modulation depth of the optical LO and IF signals remained constant throughout the measurement. It is attributed to the steeper absorption slope (defined as the ratio of the change in the absorption to that of the input power) [10] for the deeper bias voltages. The dependence of SFDR on the wavelength was investigated to apply the proposed scheme to the WDM-based RoF system.
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 9, SEPTEMBER 2005
Fig. 6. Dependence of the SFDR and the carrier-to-sideband power ratio on the LO signal wavelength.
at the bias voltage of 1.5 V. was found to be 85.9 dB Hz The phase noise of the up-converted signal was measured to be 76.8 dBc/Hz at 10 kHz. Over the bias voltages of 1.0 to 3.0 V and for the LO wavelength range of 30 nm from 1535 to 1565 nm, the SFDR was larger than 73.8 dB Hz . Due to its wide operating wavelength range with its proper analog characteristics, the proposed photonic frequency up-converter is expected to be useful for WDM-based RoF systems.
Fig. 5. Converted RF power dependence on (a) the IF signal wavelength and (b) the LO signal wavelength for the fixed SNR.
During the measurement, average optical powers for the optical LO (10 dBm) and IF (13 dBm) signals were maintained with a constant optical modulation depth. As shown in Fig. 5(a), measured by changing IF wavelengths for the fixed LO wavelength of 1557.5 nm, RF signal at 25 GHz was not varied since the converted RF power was measured for the wavelength converted LO signal. And, the amplitude of the sidebands was increased for the shorter wavelength due to the steeper slope. For the fixed IF wavelength at 1552.5 nm, however, the RF electrical powers from the up-converted signals were increased and saturated for the longer LO wavelength, as shown in Fig. 5(b). This is due to the lower absorption coefficient of the longer wavelength in the EAM [11]. Fig. 6 shows the variation of SFDR from 73.8 to 87.0 dB Hz for the LO wavelength change from 1535 to 1565 nm. This is due to the changes in the absorption coefficient as well as the nonlinearity of the absorption slope. For the practical applications, the amplifier with auto-gain control should be used to compensate the power differences originated from the wavelength dependent absorption coefficient. In our experiment, the measured wavelength bandwidth was limited to about 30 nm due to the gain-bandwidth of the EDFAs used. IV. CONCLUSION We have proposed the photonic up-converter using XAM in an EAM, and investigated its analog characteristics. The SFDR
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