Chromatic Dispersion in 60 GHz Radio-Over-Fiber ... - IEEE Xplore

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JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 29, NO. 24, DECEMBER 15, 2011

Chromatic Dispersion in 60 GHz Radio-Over-Fiber Networks Based on Mode-Locked Lasers Friederike Brendel, Julien Poëtte, Béatrice Cabon, Member, IEEE, Thomas Zwick, Senior Member, IEEE, Frédéric van Dijk, François Lelarge, and Alain Accard

Abstract—Chromatic dispersion is a limiting factor in intensitymodulated radio-over-fiber links with direct detection. This paper investigates its influence on single-mode fiber links based on modelocked laser sources through simulation and experiment. A timebased approach is used to describe the impact of chromatic dispersion after transmission across a single-mode fiber link. The simulations presented herein give hints for the design of dedicated modelocked lasers that can later serve in dispersion-tolerant radio-overfiber networks. The findings of the simulations are experimentally validated for a laser delivering a 60 GHz radio carrier, and for a link of 400 m length which represents a realistic link length for in-house distribution. Furthermore, the impact of chromatic dispersion on signal quality is demonstrated for WLAN transmission employing direct and external link modulation. Index Terms—Chromatic dispersion, mode-locked laser (MLL), radio over fiber (RoF).

I. INTRODUCTION

M

ODE-LOCKED lasers (MLL) exhibit a wide spectrum of equally spaced optical modes which are inherently phase-locked. For passive mode-locking, self-oscillation is possible under dc supply conditions only (see [1] and [2]). Upon direct detection, an electrical carrier signal can be recovered at a custom RF frequency corresponding to the free-spectral range (FSR) of the laser. This type of optical carrier generation is of particular interest for communication networks operating in the millimeter wave (mmw) range around 60 GHz, where electrical generators are still rather costly. MLL thus allow for the

Manuscript received February 09, 2011; revised July 08, 2011, October 04, 2011; accepted October 18, 2011. Date of publication October 28, 2011; date of current version December 16, 2011. F. Brendel is with the Institut de Microélectronique, Electromagnétisme et Photonique, 38016 Grenoble Cedex 1, France (e-mail: [email protected]. fr). J. Poëtte is with the Institut de Microélectronique, Electromagnétisme et Photonique, 38016 Grenoble Cedex 1, France, and also with the Grenoble Institute of Technology, 38031 Grenoble, France (e-mail: [email protected]. fr). B. Cabon was with the Institut de Microélectronique, Electromagnétisme et Photonique, 38016 Grenoble Cedex 1, France. She is now with the Grenoble Institute of Technology, 38031 Grenoble, France (e-mail: [email protected]. fr). T. Zwick is with the Institut für Hochfrequenztechnik und Elektronik, Karlsruhe Institut of Technology, 76131 Karlsruhe, Germany (e-mail: [email protected]). F. van Dijk, F. Lelarge, and A. Accard are with Alcatel-Thales III-V Lab, Joint Lab of Bell Labs and Thales Research and Technology, 91767 Palaiseau, France (e-mail: [email protected]; [email protected]; alain. [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JLT.2011.2173902

low-cost generation of mmw carriers in radio-over-fiber (RoF) links, as has been demonstrated in [3] and [4]. They are thus considered suitable to serve in transmitter modules for in-door RoF networks, where relatively short link lengths of several hundreds of meters have to be covered, and numerous picocells have to be reached. Dispersion effects come into play as the laser signal originating from the system’s central station (CS) is transmitted across the optical fiber to the antenna modules at the cell base stations (BS) whose relative distance to the CS is variable. Understanding the influence of dispersion across a certain link length is, therefore, of utmost importance for system-level network design. In this paper, we show that chromatic dispersion management must also be considered in the design of MLL sources, as the physical properties such as the number of modes, the shape of the optical spectrum as well as the chirp of the laser can have a strong impact on the transmission performances. To the best of our knowledge, these aspects of MLL-based carrier generation have not yet been studied systematically. This paper is organized into six sections. Following Section I, we present in Section II the dispersion model that serves for the simulations later shown in Section III. Section IV is dedicated to the experimental validation of the model, where we describe our experimental setup and introduce the reader to the different modulation techniques used for WLAN transmission. We then study the relationship between link length and RF carrier power and compare the measurements to our simulations. Finally, we investigate transmission quality related to chromatic dispersion in terms of error vector magnitude (EVM) and consider the bandwidth-distance performance of the system. The implications of our results are discussed in Section V, and we will conclude this paper in Section VI.

II. DISPERSION MODEL In order to isolate chromatic dispersion effects from other possible propagation effects, such as modal dispersion, the following considerations will be based on single-mode fiber (SMF) transmission. The influence of chromatic dispersion has been described previously by means of a transfer function, considering the SMF a linear time-invariant system (see [5] and [6]). Here, another equally accessible, time-based representation is used that nevertheless allows varying mode intensities and phases according to the respective laser device. Note that while a similar approach has been published by Ohno et al. [7] on a four-mode device, this paper generalizes the model for an

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BRENDEL et al.: 60 GHZ RADIO-OVER-FIBER NETWORKS

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arbitrary number of modes. The optical comb spectrum of an MLL consisting of M modes can be described as follows:

(1) the intensity of mode and where is the mode number, and are the frequency and the related phase of mode 0. The beat frequency is the FSR of the laser and represents the phase of mode . When the optical field is transported across SMF, it will suffer from attenuation, associated with the fiber’s attenuation coefficient , and the phases of the comb will be modified across the fiber length according to a wavelength-dependent propagation coefficient : (2) We recall that the optical field can be factored out. The effect of fiber dispersion can thus be observed, without any loss of generality, on the remaining RF comb spectrum. In SMF, the second derivative of the propagation coefficient accounts for the influence of chromatic dispersion. can directly be related to the fiber’s dispersion parameter . Inserting (1) into (2) and factoring out the optical field and all contributions of which are not linked to chromatic dispersion gives (3) for the remaining RF field after propagation through the dispersive fiber. The electrical field is recovered by the photodetector. The electrical photocurrent is proportional to the incident light , yielding (4), where and represent at each time the numbers of two arbitrary interfering modes. The phase difference between two neighboring modes is given by constant. In order to recover a single RF carrier, the photodetector is required to be band-limited such that its bandwidth (BW) fulfills the condition . In this case, only the beat signal of each two neighboring modes will be present in the electrical spectrum.

(3)

(4)

shape of the optical spectrum, and the phase noise on the optical spectrum. The chirp phenomenon can be demonstrated for three modes, while the others are better highlighted looking at a broad spectrum of M modes. All simulations described herein were obtained using The MathWork’s MATLAB software. The simulation parameters were chosen as follows: is calculated for a wavelength of m and a dispersion coefficient ps/km/nm. The recovered RF power was evaluated at 60 GHz, holding thus the contribution of the beating of each pair of neighboring modes. The attenuation factor was set to 0.2 dB/km. Power values on the Y-axes of Fig. 1(a), (b), (d), and (f) indicate the order of magnitude of the recovered RF power and were adjusted during the simulation in order to match the measured power levels (see Section IV). A. Three-Mode Approach The limitation of the optical spectrum to three modes only results in the well-known problem of double-sideband transmission [5], as illustrated in Fig. 1(a): The interference of two optical modes results in a single beat note whose frequency corresponds to the separation in wavelength is given by of the two optical modes, and whose phase the difference of their phases. Adding a third, equally spaced optical mode will make appear a second beat note at the same frequency and phase , as well as a third one at the double frequency. The chirp phenomenon relates the phases of the optical modes to the phase-amplitude coupling parameter appearing in the laser’s rate equations. In the case of a quantum-dot MLL (as used in this study), is a complex relationship due to the interaction of several nonlinear processes between neighboring modes, and to the gain curve associated with quantum dots of different sizes [8]. The chirp induces a deterministic phase shift between the modes so that . The phases and will be modified by chromatic dispersion, giving rise to the periodic cancellation of the sum beat signal at . For a specific distance of optical fiber, the chromatic dispersion compensates the laser chirp effect, zeroing the phase difference between modes and giving a power maximum at a length of . In Fig. 1(a), the dashed line represents the recovered RF power for GHz, where all three modes are initially in phase. The periodicity corresponds to 2040 m for a 60 GHz signal. The solid line in the same figure shows the recovered RF power when the three modes carry an initial phase shift due to the chirp phenomenon . It can be observed that the signal power reaches its first maximum only after several hundreds of meters of transmission. This initial phase shift is precisely the effect of laser chirp which first counter-acts dispersion until being fully compensated at the power maximum. B. M-Mode Approach

III. SIMULATION We consider the transmission of an optical field across a link of variable length , the field exhibiting a spectrum with an FSR corresponding to 60 GHz, and a variable number of modes. The phenomena studied in this section include the laser chirp, the

In the case of an optical spectrum of M modes, the beat signal of each two neighboring modes contributes to the recovered signal at . The M-mode approach can be considered a superposition of the three-mode case. The chirp phenomenon shown in the previous section exerts the same influence as before on

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Fig. 1. Simulations for GHz. (a) Influence of initial phase shift. (b) Apparition of local maxima for additional modes. (c) dB. (d) Influence of the shape of the optical spectrum. (e) Spectra used in Fig. 1(d). (f) Influence of phase noise. power maximum at

M optical modes. Subsequently adding more modes of equal intensity makes appear additional local maxima, while the previously observed principal maximum stays the same and the period remains fixed at 2040 m, as shown in Fig. 1(b) for 5 and 30 modes, respectively. All modes are assumed to have equal intensity and the phase relation between the modes is constant . However, the maximum lobe is narrowed as more modes are added. The relationship between the number of modes and the usable fiber length around a power maximum , i.e., the fiber length around a power maximum for a power drop-off of dB, is displayed in Fig. 1(c)(*). Alongside this curve, we have plotted the RF power penalty induced when limiting an initial flat spectrum of 30 modes to an arbitrary number of modes by means of a filter technique. 1) Influence of the Shape of the Optical Spectrum: Another point to observe is the influence of the shape of the laser’s optical spectrum. MLL can exhibit rather flat spectra [9], but even so, the gain curve and the repartition of cavity losses cause the spectrum roll off at its edges, and the outer modes do not significantly contribute to the beat signal power. It is therefore

around

worthwhile to consider the influence of the shape of the optical spectrum on the recovered RF power at a given link length. In Fig. 1(d), two curves are represented for 30 modes each. This value was chosen with respect to the spectrum measurements carried out on the laser device used in Section IV. While the solid line represents the RF power recovered from a field whose 30 modes exhibit equal intensities, the dashed curve represents the results from a measured spectrum whose modes vary with the laser’s optical spectrum. Fig. 1(e) displays the measured optical spectrum with a flatness of 3.1 dB/10 nm; the normalized intensities of the 30 inner modes within the dash-dotted line were used. These modes contain % of the total power in the optical spectrum. When comparing the two curves in Fig. 1(d), it is observed that maxima and minima are considerably damped and, thus, reduced in power. Signal cancellation is consequently mitigated such that the local nulls disappear completely. 2) Phase Noise on the Optical Modes: So far, the phase relation between the optical modes was considered constant. This is the ideal mode-locking condition, but it might eventually be violated due to cavity imperfections or the like. We therefore


study the influence of random phase noise on the optical modes. In Fig. 1(f), a curve for 30 modes, equal intensities, and constant phase relation (solid line) is represented against various other curves (dashed lines), the modes exhibiting an additional phase error. The phase error was modeled to follow a Gaussian distribution with a mean of and a variance of . Exemplary curves are shown for , and . It is observed that the introduction of a random phase error tends to dampen the effect of signal cancellation already for small values of , while for higher values of , this effect becomes even more pronounced and the minima are shifted toward lower lengths. Note that the phase errors on different modes were assumed to be perfectly uncorrelated, while for a real device, the correlation between inner modes and outer modes might vary. Among the phenomena studied in this section, the chirp and the shape of the optical spectrum of the laser were found to be of particular importance for chromatic dispersion (see Section IV-B). While the effect of phase noise on chromatic dispersion might seem secondary with respect to the delivery of system power, it should be kept in mind that the phase noise of the optical modes remains a crucial parameter with respect to the linewidth of the recovered RF signal. It can thus nevertheless influence the system performance in terms of carrier stability. IV. EXPERIMENTAL VALIDATION A. Experimental Setup Fig. 2 displays the setup we used for the experimental validation of the model. An InAs/InP-based quantum dash laser diode (previously presented in [9]) served as MLL source at a wavelength of 1.55 m. Its self-oscillation frequency is 58.4 GHz. At the CS, the laser chip was placed on a copper submount and coupled to a lensed fiber by positioning the fiber at the edge of the chip. The laser received its current signal via a coplanar transmission line using a ground-signal-ground RF probe. In order to study the unmodulated RF carrier, the laser was dc biased, and its output signal was transmitted over an SMF link of variable length. The setup allows for a length variation of up to 400 m and a step resolution of 12.5 m. At the BS, a photodiode with a bandwidth of 70 GHz was used to detect the RF beat signal. For WLAN transmission, a vector signal generator (VSG) served as modulation source, modulating either the laser injection current, as in Fig. 2(a), or the bias voltage of an electro-optic Mach–Zehnder modulator (MZM), as in Fig. 2(b). The power of the modulating waveform was fixed to 10 dB m. The WLAN sidebands were centered at 1 GHz, resulting in an electrical double sideband spectrum at 57.4 and 59.4 GHz, respectively. For the externally modulated link, an optical amplifier (erbium-doped fiber amplifier, 22 dB) followed by a variable attenuator was needed to compensate for the losses of the MZM. An additional RF amplifier (24 dB) was inserted after the photodiode. Due to the bandwidth of the analyzer (6 GHz VXI and VXI-based vector signal analyzer software (VSA) package), a mixing stage (LO: 55 GHz/12 dB m) was necessary. B. Dispersion Effects on RF Carrier Generation The relationship between link length and carrier power for different laser bias conditions is examined here. The optical

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Fig. 2. Experimental setups for both link modulation techniques. (a) Direct Modulation. (b) External Modulation.

spectrum of the MLL is shifted toward higher wavelengths at higher bias current, while its flatness does not essentially change. The intensity on a certain mode is subject to vary slightly with the bias, determining the RF power that can be collected at a given length. Note that for this first series of measurements, no data signal was transmitted and only the variation of the unmodulated carrier was studied. Fig. 3(a) displays the RF power recovered over the link length for a dc bias of 200 mA and 260 mA (*). These two values were chosen as they result in stable mode-locking with a reduced RF linewidth of about 80 kHz. The effect of the laser chirp is compensated by chromatic dispersion at about 72 m for both scenarios, indicating that the chirp does not strongly depend on the dc-bias current. It can further be observed that the recovered RF power follows about the same curve shape for short links, but varies significantly for link lengths above 250 m. We attribute this result to the varied intensities of the optical spectrum and have confirmed this assumption by comparing the simulated curves for both bias conditions to the measurements. In order to represent the measured scenario as closely as possible in the simulations, the optical spectra at both 200 and 260 mA were measured. The mode intensities were extracted and fed into the simulations. While the general behavior of the curves can be found using 30 modes, optimum coincidence with the measured values is found for a spectrum of 58 modes where the power ratio between the center and the edges of the spectrum is 30 dB. is found to be approximately 70 m. Recalling Fig. 1(c), the value of 70 m corresponds indeed to the value of of about 30 modes. As discussed in Section III-B1, this circumstance can be attributed to the influence of the shape of the optical spectrum, indicating a strong contribution of 30 powerful central modes and a weak contribution of the outer modes due to the rolling-off of the optical spectrum at its edges. In general, it is observed that the simulations coincide very well with the measured values, reproducing the principal maxima and the bumps that follow. In particular,

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Fig. 3. Experimental Validation. (a) 400 m fiber length and varied bias conditions. (b) 5 km fiber length: Periodicity. (c) External Modulation. (d) Direct modulation.

the counter behavior of the two measurement series for a link length m is also recreated by the simulations. While the laser chirp and the shape of the optical spectrum are indeed found to be relevant to the effects of chromatic dispersion, it is remarkable that the measured curves are closely reproduced without modeling phase noise. We thus draw the conclusion that an eventual phase jitter on the optical modes does not exert a nameable influence compared to the variations of RF power induced by the variation of mode intensity. For that matter, the MLL is thus sufficiently phase-locked. Fig. 3(b) reveals a periodicity of 2.11 km. Optimum link lengths are, therefore, values of m, and with the second maximum at 2182 m. C. WLAN Transmission WLAN transmission was realized by modulating the entire optical spectrum at the CS and recovering the superimposed beat notes of the sidebands at the BS. A 24 Mpbs–16 QAM modulated baseband signal (IEEE 802.11a [10]) serves as test signal for the VSG. The link quality is evaluated by means of the EVM which represents the residual symbol error after an ideal version of the symbol has been stripped away. The EVM provides an easy-to-handle figure of merit that can be recorded by the VSA package. While the extension of the WLAN standard for the mmw range (IEEE 802.11ad) has not yet been published, we refer to [10] for standard EVM values associated to the transmit signal, which is % for 24 Mbps–16 QAM. Fig. 3(c) shows that the measured values for external modulation are slightly higher, reaching a floor around 17% (dashed line). Note that the noise figure of the mixer was not available and has, therefore, not been taken into account for EVM calculation. The equivalent EVM in the mmw range is, however, estimated to reach a standard-compliant floor at about

10%, following an approach based on the device’s conversion loss [11]. However, at this point, we consider even more important the relative degradation of EVM over link length, closely related to the power in the received sideband (solid line). For a link length of more than 100 m, the band power is quickly reduced, and the EVM curve consequently bends upward. A local maximum is observed at about 140 m, where the power rises again, improving EVM, and reduces once more. On the other hand, the measurements reveal that the EVM is quite constant over a length of about 100 m. The indicated floor might also originate from a system limitation other than the recovered RF power, such as carrier phase noise. The measurements for direct modulation are shown in Fig. 3(d). The same behavior as in the case for external modulation is observed across the link length. For link length m, EVM detection cannot be performed as the recovered band power is too low. D. Bandwidth-Distance Considerations When the MLL is modulated, an optical sideband appears on each side of a mode and the corresponding beat notes in the electrical spectrum (intermode, inter-sideband, or mode-sideband beating) will suffer a penalty induced by chromatic dispersion. With regard to the bandwidth-distance performance, it is crucial to remember that the fading effect on the generated RF beat notes does not depend on the frequencies generated by the beatings, but on the wavelength ( )-distance between the optical beatings which superimpose at the same electrical frequencies; namely, intermode beatings, or mode-sideband beatings labeled 1 to 4 in Fig. 4, which can easily be extended to M modes. The primary fading behavior is set by intermode beating where the -distance involved is laser’s FSR of 58.4 GHz, leading to a periodicity of 2.11 km. For the modulated spectrum, the -distance between a mode and a sideband corresponds to the subcarrier


Fig. 4. Principle of beating.

frequency . The associated fading periodicity is km for GHz [12]. The two types of fading will superimpose to give the overall fading behavior of the link: the fast-fading periodicity of 2.11 km will be modulated by a slow-fading periodicity induced by . The -distances involved are thus FSR , but not . This principle can be illustrated and using Fig. 4: The mode-sideband beatings 1 and 3 interfere and create an RF signal at GHz, but as these two beatings are separated by FSR in the optical spectrum, the fading periodicity will be associated to the FSR, independent from . The same is true for beatings 3 and 4. Therefore, we observe simultaneous fading of beatings (1,3) and beatings (2,4) with a periodicity of 2.11 km, and their respective extinction occurs at the same value of fiber length. As the link lengths considered for the target application are limited to a couple of hundreds of meters, the slow-fading periodicity related to can be neglected ( dB for km), and we do not expect a significant change in RF power for the transmission of signals of a bandwidth GHz. V. DISCUSSION OF RESULTS As has been shown in the previous sections, chromatic dispersion does not limit the absolute transmission distance of the system; however, the link length will be limited to discrete lengths as the relationship of RF power and transmission distance is periodic. We shall now identify the key issues to be taken into consideration for mmw RoF system design in order to best exploit MLL devices. The findings of the simulations in Fig. 1(c) suggest that the number of MLL modes should be limited to less than 30 in order to maintain a link length tolerance of at least 100 m. This leaves a realistic margin for the physical construction of such networks, e.g., in an in-house picocellular environment for high-speed access. This result can appear counterintuitive to device designers that might consider a wide optical spectrum a much sought after property for ultrashort pulse generation. If a limitation of the spectrum cannot already be introduced at the stage of laser design, it is conceivable to filter a certain number of modes, e.g., by means of a fiber Bragg grating that will, however, cause the RF power level to decrease [see Fig. 1(c)]. If optical filtering cannot be performed, the link length should be considered carefully when connecting an RoF cell to the system’s CS. In the probable case of dealing with a moderate link length of several tens or hundreds of meters, the designer can quite simply use a fiber of 72 m for all distances shorter than 72 m, or 2182 m for distances between 72 and 2182 m and store the surplus in the CS. The EVM measurements for external and direct modulation, presented in Fig. 3(c) and (d), respectively, suggest furthermore that the quality of transmission remains about constant over a width of about 100 m which

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is reassuring for the connection of neighboring cells. A final remark should be made on the well-known solution of optical single-sideband transmission to overcome chromatic dispersion in fiber-wireless links [13]. It consists in eliminating one of two optical sidebands before fiber transmission, e.g., by using a dephased push–pull configuration of an MZM. We point out that this technique is only applicable in the presence of a single carrier. In the case of an MLL-based system, the RF signals are constructed from various superimposed beat signals. In the case where the second sideband of a particular optical mode is eliminated, it will nevertheless appear in the RF spectrum through the beating of the remaining optical sideband with a higher neighboring mode. VI. CONCLUSION MLLs represent a low-cost and thus attractive solution for RF carrier generation in mmw RoF systems. However, their wide optical spectra represent a drawback with respect to chromatic dispersion. This periodic phenomenon can nevertheless be managed by a careful design of the MLL components. In this paper, we have analyzed and experimentally validated the propagation of MLL signals across SMF and identified the crucial laser parameters. We have shown that the optical MLL spectrum should be limited to only a few coherent modes whenever MLLs are used in the building of RoF networks tolerant to dispersion effects. In the case of a wide optical spectrum, it is all the more crucial to choose appropriate link lengths for all BS that are operated from one particular CS, taking into account the spectral characteristics of the laser, as the link length is limited to discrete values as a function of the laser’s FSR. Phase noise is considered a secondary concern in terms of chromatic dispersion, but will have an impact on system performance through RF linewidth. We have shown that for an MLL operating in the 60 GHz band, the recovered band power and the related EVM can be maintained at acceptable values over a length variation of more than 50 m from the maximum. ACKNOWLEDGMENT This work was accomplished in the framework of the French ANR Teldot Project. REFERENCES [1] J. Renaudier et al., “Phase correlation and linewidth reduction of 40 GHz self-pulsation in distributed Bragg reflector semiconductor lasers,” IEEE J. Quantum Electron, vol. 43, no. 2, pp. 147–156, Feb. 2007. [2] F. Van Dijk et al., “Quantum dash mode-locked laser for millimeterwave coupled opto-electronic oscillator,” in Proc. IEEE Int. Top. Meet. Microw. Photon., Victoria, BC, 2007, pp. 66–69. [3] R.-P. Braun et al., “Optical microwave generation and transmission experiments in the 12 and 60-GHz region for wireless communication,” IEEE Trans. Microw. Theory Tech., vol. 46, no. 4, pp. 320–330, Apr. 1998. [4] B. A. Khawaja and M. J. Cryan, “Wireless hybrid mode locked lasers for next generation radio-over-fiber systems,” J. Lightw. Technol., vol. 28, no. 16, pp. 2268–2276, Aug. 2010. [5] U. Gliese, S. Norskov, and T. Nielsen, “Chromatic dispersion in fiberoptic microwave and millimeter-wave links,” IEEE Trans. Microw. Theory Tech., vol. 44, no. 10, pp. 1716–1724, Oct. 1996. [6] Y. Le Guennec et al., “Improvement of dispersion resistance in analog radio on fiber upconversion links,” J. Lightw. Technol., vol. 21, no. 10, pp. 2211–2216, Oct. 2003.

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[7] T. Ohno et al., “Application of DBR mode-locked lasers in millimeterwave fiber-radio system,” J. Lightw. Technol., vol. 18, no. 2, pp. 44–49, Jan. 2000. [8] M. Gioannini and I. Montrosset, “Numerical analysis of the frequency chirp in quantum-dot semiconductor lasers,” IEEE J. Quantum Electron., vol. 43, no. 2, pp. 941–949, Feb. 2007. [9] F. Lelarge et al., “Recent advances on InAs/InP quantum dash based semiconductor lasers and optical amplifiers operating at 1.55 m,” IEEE J. Sel. Topics Quantum Electron., vol. 13, no. 1, pp. 111–124, Jan. 2007. [10] Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications: High-Speed Physical Layer in the 5 GHz Band, IEEE Std.802.11a-1999, 1999, IEEE. [11] G. H. Nguyen et al., “Generation of 60-GHz MB-OFDM signal-overfiber by up-conversion using cascaded external modulators,” J. Lightw. Technol., vol. 27, no. 11, pp. 1496–1502, Jun. 2009. [12] G. H. Nguyen et al., “Importance of chirp effect in millimeter wave optical upconversion systems,” J. Lightw. Technol., vol. 29, no. 12, pp. 1753–1758, Jun. 2011. [13] G. Smith et al., “Overcoming chromatic dispersion effects in fiberwireless systems incorporating external modulators,” IEEE Trans. Microw. Theory Tech., vol. 45, no. 8, pp. 1410–1415, Aug. 1997.

Friederike Brendel received the Dipl. Ing. (M.S.E.E.) degree in electrical engineering and information technology from the University of Karlsruhe, Karlsruhe, Germany, in 2009. Since October 2009, she has been working toward the Ph.D. degree at the Institute for Microelectronics, Electromagnetism and Photonics (IMEP-LAHC), Grenoble, France. Her research interests include optical RF generation and radio-over-fiber systems, in particular, in the 60 GHz region.

Julien Poëtte received the Engineer Diploma from the French Engineering School ENSSAT (National School for Applied Science and Technologies), Lannion, France, in 2002 (with a specialization in optroelectronics), and the Ph.D. degree in physics from Rennes 1 University, Rennes, France, in 2005, for his work in noise in laser dedicated to telecommunication. He is currently an Associate Professor at the Grenoble Institute of Technology, Grenoble, France. In 2008, he joined the Institute for Microelectronics, Electromagnetism and Photonics (IMEP-LAHC) Laboratory. His research interests include next-generation communication systems involving microwavephotonics techniques at 60 GHz.

Béatrice Cabon (S’93–M’95) received the Ph.D. degree in microelectronics from the Grenoble Institute of Technology (Grenoble-INP), Grenoble, France, in 1986. She has been a Professor at the Grenoble-INP since 1989. In 1993–2007, she was the Head of a research group on RF, microwaves, microwave-photonics techniques at the Institute for Microelectronics, Electromagnetism and Photonics (IMEP-LAHC) Laboratory, Grenoble. She was a Coordinator of the IST-


2001-32786 “NEFERTITI” (Network of Excellence on broadband Fiber Radio Techniques and its Integration Technologies), funded by the European Commission, and of the network of excellence FF6-IST-26592 “ISIS” (InfraStructures for broadband access in wireless/photonics and Integration of Strengths in Europe). Her research interests include microwave-photonics, photonic-microwave signal processing, and optical links for high bit rate signals. She has contributed to more than 230 technical publications and is the Editor of four books in these areas.

Thomas Zwick (S’95–M’00–SM’06) received the Dipl.-Ing. (M.S.E.E.) and the Dr.-Ing. (Ph.D.E.E.) degrees from the Universität Karlsruhe, Karlsruhe, Germany, in 1994 and 1999, respectively. From 1994 to 2001, he was a Research Assistant at the Institut für Höchstfrequenztechnik und Elektronik (IHE), Universität Karlsruhe. From 2001 to 2004, he was at the IBM T. J. Watson Research Center, Yorktown Heights, NY. From 2004 to 2007, he was with Siemens AG, Lindau, Germany. Since 2007, he has been the Director of the IHE, Karlsruhe Institute of Technology, Karlsruhe. He is the author or coauthor of more than 100 technical papers and more than ten patents. His research interests include wave propagation, millimeter wave antenna design, and wireless communication.

Frédéric van Dijk is currently with Alcatel-Thales III-V Laboratory, Palaiseau, France, where he is focused on design, fabrication, and characterization of laser sources for microwave applications. He is, in particular, involved in studies on directly modulated distributed feedback lasers, mode-locked lasers for frequency synthesis and clock recovery, dual wavelength lasers, and microwave photonic systems.

François Lelarge received the Diploma degree in material science in 1993 and the Ph.D. degree in 1996, both from the University of Pierre et Marie Curie, Paris, France. He is currently with Alcatel-Thales III-V Laboratory, Palaiseau, France, where he is involved in the research on InGaAsP/InP gas source molecular beam epitaxy growth for optoelectronic devices, in particular, using quantum dot (QD)-based active layers. He is the author or coauthor of more than 110 papers and communications in national and international conferences. He was in charge for III-Vlab of the technical management of the Zodiac Project (IST 2005–2008) on QD-based lasers. He is actually prime of an ANR Project on QD-based mode-locked laser (ANR-TELDOT).

Alain Accard was born in France in 1950. He received the M.S. engineer degree in physics from the Institut National des Sciences Appliquees, Lyon, France, in 1975. He is currently with Alcatel Thales III-V Laboratory, Palaiseau, France. His current research interests include Bragg grating design and fabrication of Fabry–Perot and distributed feedback lasers.