IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 13, NO. 8, AUGUST 2001
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36-GHz 140-Mb/s Radio-Over-Fiber Transmission Using an Optical Injection Phase-Lock Loop Source L.A. Johansson and A. J. Seeds, Fellow, IEEE
Abstract—We report the first millimeter-wave radio-over-fiber transmission demonstration using optical phase-lock loop techniques. A 36-GHz 140-Mb/s ASK modulated carrier was transmitted through 65 km of standard single-mode fiber with a bit-error rate (BER) of 10 9 , using an optical injection phase-lock loop based on fiber-pigtailed commercially available components. Index Terms—Analog systems, optical fiber communications, optical modulation, optical phase-locked loops, millimeter-wave communications.
I. INTRODUCTION
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ROAD-BAND millimeter-wave over fiber transmission is an attractive technology for local multipoint distribution services (LMDS) and new generation broad-band wireless access systems, since it allows much of the system complexity to be remoted from the necessarily numerous antenna sites [1]. A key challenge is the generation of the required millimeter-wave modulated optical signals. Laser diodes generally cannot be directly modulated at millimeter-wave frequencies and chromatic dispersion limits the transmission distance of double sideband modulated 1550-nm signals to a few hundred meters in standard single-mode (SSM) fiber [2]. Previously, millimeter-wave over fiber transmission has been demonstrated using external modulators by either double sideband modulation with dispersion compensation [3], modulation with suppressed sideband [4] or suppressed carrier [5]. Optical heterodyning, i.e., the beating of two optical frequency components spaced by the required millimeter-wave frequency on a photodetector generates single sideband modulated signals with high dispersion tolerance. High power is available, as all optical power contributes to the generated signal and high frequencies are attainable, limited only by the photodetector bandwidth. Successful heterodyne-based transmission experiments using optical injection locking (OIL) techniques [6] and optical phase-lock loop (OPLL) techniques at 9 GHz [7] have been reported. However, these need milli-Kelvin precision laser temperature control, which adds complexity and cost. We have shown previously how an efficient optical millimeter-wave generator, the optical injection phase-lock loop (OIPLL), can be constructed using standard DFB laser diodes Manuscript received December 18, 2000; revised April 26, 2001. This work was supported by the U.K. Engineering and Physical Sciences Research Council under the Optical Systems Integration Program in collaboration with Nortel Networks and BT Advanced Communication Technology Centre. The authors are with the Department of Electronic and Electrical Engineering, University College London, London WC1E 7JE, U.K. (e-mail:
[email protected]). Publisher Item Identifier S 1041-1135(01)06447-3.
Fig. 1. Experimental arrangement for the fiber based OIPLL system. ML: Master laser. SL: Slave laser. PD: Photodetector. LF: Loop filter. AD: Adjustable delay line. Thick line indicates the optical path.
without the need for precision temperature control [8]. In this letter, we demonstrate how the OIPLL can be applied to generate a 36-GHz 140-Mb/s ASK single sideband modulated optical signal, using baseband external optical modulation. II. MILLIMETER-WAVE GENERATION SCHEME The millimeter-wave modulated optical signal generation technique used here [8] is similar to the optical phase-lock loop [7], [9] but adds an optical injection locking path to allow low-cost wide-linewidth semiconductor lasers to be used. This new technique overcomes the disadvantages of the OPLL; stringent loop delay or laser linewidth requirements, together with the necessity for special lasers with uniform FM response. It also overcomes the main disadvantage of OIL (narrow locking range) in a scheme that combines power efficiency, dispersion immunity, and wide locking range. Furthermore, it is simple to construct from commercially available fiber optic components. Fig. 1. shows the layout of the OIPLL. The system is based on two fiber-pigtailed 1540-nm DFB lasers with heterodyne linewidth of 200 MHz. The master laser is modulated at 12 GHz, generating a series of modulation sidebands. The wavelength of the slave laser is such that the beat with the central master laser line generates the desired millimeter-wave frequency 36 GHz by optical heterodyne. The slave mode is injection locked by one of the master laser harmonic sidebands to give broad-band phase noise suppression. The phase-lock loop has a second-order type 530 ns and 160 ns. It II filter with time constants provides close to carrier phase noise suppression and temperature tracking. An adjustable delay line provides phase matching between the OPLL and the OIL. Single sideband phase noise of
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 13, NO. 8, AUGUST 2001
Fig. 2. Phase error variance in a 100-MHz noise bandwidth as a function of slave laser detuning for the OIPLL and corresponding OIL system. Fig. 4. Optical spectra for the modulated master laser and the injection locked slave laser. Resolution bandwidth: 0.08 nm.
Fig. 3. Detected RF spectra after 25-km fiber transmission, res. b/w 1 MHz.
Fig. 5. Experimental layout for data transmission experiment. Thick line indicates optical path.
90 dBc/Hz at 10 kHz offset was measured and a phase error variance of less than 0.005 rad in a 100-MHz bandwidth. The locking bandwidth is greater than 30 GHz, which relaxes the differential temperature control requirements between master and slave lasers to 3.5 K, thus ensuring good long-term operating stability. This compares to a locking bandwidth of around 3 GHz for the OIL system alone, as illustrated by Fig. 2. As a result, it would be possible to construct the OIPLL around two uncooled DFB lasers, if they have a common heat sink. Fig. 3 shows the output spectra of the OIPLL with 140-Mb/s ASK modulation after transmission through 25 km of fiber, detection and downconversion to 2 GHz, and Fig. 4 shows the optical spectra for both the modulated master and the injection-locked slave laser. III. TRANSMISSION EXPERIMENT Fig. 5 shows the experimental layout of the data transmisPRBS was used to switch sion experiment. A 140-Mb/s 2 an electroabsorption modulator connected to the output of the OIPLL, generating a 36-GHz ASK-modulated signal with an extinction ratio of 12.2 dB. The launched power was 8 dBm and the optical signal was transmitted through 0–65 km of SSM fiber. For transmission distances higher than 25 km, a receiver optical preamplifier was needed with a 1-nm bandwidth amplified spontaneous emission (ASE) filter. An optical attenuator was used to regulate the detected optical power in the photodetector. The received 36-GHz signal was downconverted to 2 GHz in a triple balanced mixer. The ASK modulated carrier was then demodulated in an envelope detector, a postdetection 300-MHz low-pass filter was used and the recovered baseband signal was either observed on a digital oscilloscope, or connected to a bit-error rate (BER) detector.
Fig. 6. BER as a function of received optical power for 0, 25, 40, and 65 km of SSM fiber.
Fig. 6 shows the BER as a function of received optical power for different fiber spans. A BER lower than 10 was obtainable for all fiber lengths. For 25-km fiber path, a BER of 10 was observed for 17 dBm received optical power, corresponding to 63 dBm received millimeter-wave power. The efficiency of the OIPLL millimeter-wave modulated optical signal generation is indicated by the low optical power needed to achieve 10 ). A more quantitative estierror-free detection (BER mate can be obtained from a comparison with ideal two line optical heterodyne detection, where 19.5 dBm optical power is needed to generate 63 dBm received millimeter-wave power for a photodetector responsivity of 0.4 A/W. The 2.5-dB difference can be attributed to the unwanted power contained in the master laser modulation sidebands, shown in Fig. 4. Beats between these sidebands also add to the wanted 36-GHz heterodyne beat either constructively or destructively, depending on
JOHANSSON AND SEEDS: 36-GHz 140-Mb/s RADIO-OVER-FIBER TRANSMISSION
the relative phase of the master-slave laser at the OIPLL output and the additional phase induced by fiber dispersion. This explains the 1.5-dB higher optical power required to achieve a given BER at 0- and 65-km fiber spans than for 25 and 40 km, as illustrated in Fig. 6. The explanation is confirmed by the received millimeter-wave power after the photodetector, 65.7 dBm at 0 km, 63.0 dBm at 25 km, 63.4 dBm at 40 km, and 65.5 dBm at 65 km for a constant detected optical power of 17 dBm. The receiver performance is limited by thermal noise. With an amplifier noise figure of 3.5 dB and using a double-sided noise bandwidth of 2 300 MHz, and taking the lower amplifier and mixer gain (5 dB) at the image frequency ( 40 GHz) into consideration, the equivalent input noise level to the amplifier is 81.8 dBm. The theoretically predicted required signal-tonoise ratio (SNR) for asynchronous heterodyne demodulation of ASK signals can be derived by using the following approximate relation [10]: SNR For a BER of 10 , a SNR of 16.1 dB is required. With 81.8 dBm equivalent input noise level to the amplifier, the theoretically required signal power would be 65.7 dBm. IV. CONCLUSION In this letter, the use of an optical injection phase-lock loop as a source for millimeter-wave radio-over-fiber transmission systems has been demonstrated for the first time. This demonstration shows the advantages of using millimeter-wave generation by optical heterodyning; high efficiency and dispersion resistant single sideband modulation. The OIPLL compares favorably with alternative methods of optical heterodyning, OPLL, and OIL, in that it can be constructed around two standard DFB laser diodes without any need for precision temperature control. It also provides a highly phase stable signal, ensuring bandwidth efficiency in the radio path for any wireless application. This technique has been shown to be capable of transmitting a 36-GHz 140-Mb/s ASK signal through 65 km of SSM fiber with
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a BER lower than 10 . Furthermore, no optical amplification is required for error-free transmission at fiber spans up to 25 km, sufficient to meet most envisaged system requirements. The dispersion penalty for transmission through the fiber is small, 1.5 dB in optical power, which shows that this system is resistant to fiber dispersion. ACKNOWLEDGMENT The authors would like to acknowledge Dr. D. Wake for the supply of lasers, fiber, and optical modulator. REFERENCES [1] H. Ogawa, D. Polifko, and S. Banba, “Millimeter-wave fiber optics systems for personal radio communication,” IEEE Trans. Microwave Theory Tech., vol. 40, pp. 2285–2292, Dec. 1992. [2] U. Gliese, S. Norskov, and T. N. Nielsen, “Chromatic dispersion in fiberoptic microwave and millimeter-wave links,” IEEE Trans. Microwave Theory Tech., vol. 45, pp. 1716–1724, Oct. 1997. [3] K. Kitayama, “Fading-free transport of 60 GHz-optical DSB signal in nondispersion shifted fiber using chirped fiber grating,” in Int. Topical Meeting on Microwave Photonics, Tech. Dig., Princeton, NJ, 1998, pp. 223–226. [4] G. H. Smith and D. Novak, “Broadband millimeter-wave (38 GHz) fiberwireless transmission system using electrical and optical SSB modulation to overcome dispersion effects,” IEEE Photon. Technol. Lett., vol. 10, pp. 141–143, Jan. 1998. [5] J. J. O’Reilly, P. M. Lane, R. Heidemann, and R. Hofstetter, “Optical generation of very narrow linewidth millimeter wave signals,” Electron. Lett., vol. 28, pp. 2309–2310, Dec. 1992. [6] C. G. Schäffer, F. H. Lübeck, R.-P. Braun, G. Grosskopf, F. Schmidt, and M. Rohde, “Microwave multichannel system with a sideband injection locking scheme in the 60 GHz-band,” in Int. Topical Meeting on Microwave Photonics, 1998 Tech. Dig., 1998, pp. 67–69. [7] U. Gliese, T. N. Nielson, S. Norskov, and K. E. Stubkjaer, “Multifunctional fiber-optic microwave links based on remote heterodyne detection,” IEEE Trans. Microwave Theory Tech., vol. 46, pp. 458–468, May 1998. [8] L. A. Johansson and A. J. Seeds, “Millimeter-wave modulated optical signal generation with high spectral purity and wide locking bandwidth using a fiber-integrated optical phase-lock loop,” IEEE Photon. Technol. Lett., vol. 12, pp. 690–693, June 2000. [9] R. T. Ramos and A. J. Seeds, “Fast heterodyne optical phase-lock loop using double quantum well laser diodes,” Electron. Lett., vol. 28, pp. 82–83, Jan. 1992. [10] R. E. Ziemer and W. H. Tranter, Principles of Communications: Systems, Modulation and Noise. New York: Wiley, 1995.
