Optical demultiplexing of millimeter-wave subcarriers for wireless channel distribution employing dual wavelength FBGs

Optics Communications 275 (2007) 335–343 www.elsevier.com/locate/optcom

Optical demultiplexing of millimeter-wave subcarriers for wireless channel distribution employing dual wavelength FBGs Pere Pe´rez-Milla´n a,*, Andreas Wiberg b, Per Olof Hedekvist b, Peter A. Andrekson a, Miguel V. Andre´s a b

a Departamento de Fı´sica Aplicada y Electromagnetismo-ICMUV, Universitat de Vale`ncia, Dr. Moliner 50, 46100 Burjassot, Spain Department of Microtechnology and Nanoscience, Photonics Laboratory, Chalmers University of Technology, SE-412 81 Go¨teborg, Sweden

Received 4 September 2006; received in revised form 18 February 2007; accepted 16 March 2007

Abstract An optical mm-wave demultiplexer is presented. Double sideband modulation with suppressed optical carrier and filtering properties of dual overwritten fiber Bragg gratings are the fundamentals for optical demultiplexing of mm-wave radio-on-fiber signals: using a single optical carrier, Millimeter-wave signals of 20 and 40 GHz frequencies carrying independent data are created, transmitted over fiber, demultiplexed and wireless distributed to be detected and data recovered in a mobile unit. Double sideband modulation with suppressed optical carrier yields no power penalty due to chromatic dispersion, while the filtering properties of the dual overwritten fiber Bragg gratings allow less than 40 dB electrical power interchannel leakage. Independent 2.5 Gb/s On–Off-keyed data have been successfully transmitted through the 20 and 40 GHz channels. Ó 2007 Elsevier B.V. All rights reserved.

1. Introduction Research on broadband and cost-effective distribution mm-wave networks is becoming a key issue to reliably and efficiently satisfy the demand of wireless communications services that is expected for the near future. In this frame, radio-on-fiber systems arise as a promising technology, since their fiber-based access between networks nodes allow high bandwidth, low-loss and electric interferencefree transmission. The current trend of radio-on-fiber networking points to concentrate signal processing and optical subcarrier generation in the central station and to simplify base station architectures [1], so that the small area cell sizes at which mm-wave air transmissions have to be reduced (several meters in radius due to the high losses suffered at millimetric frequencies) can be covered at lower cost. Several methods to generate and transmit the mmwave signals from the central station to the base stations *

Corresponding author. Tel.: +34 963 543 432; fax: +34 963 543 146. E-mail address: [email protected] (P. Pe´rez-Milla´n).

0030-4018/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2007.03.029

through a backbone fiber have been proposed. They are based on different modulation formats to superimpose the mm-wave signals on optical waves. In order to avoid power fading due to chromatic dispersion, modulation formats like single sideband (SSB) [2] or double sideband with suppressed optical carrier [3] are needed, although the length of the optical link is still limited by the phase noise induced by the accumulated dispersion [4]. To distribute the mm-wave signals to their corresponding base stations, optical demultiplexing of the radio-on-fiber mm-wave signals has to be used. The concepts and techniques of wavelength demultiplexing in optical communications are being adapted for this purpose. The principle of these mm-wave optical demultiplexing techniques lays on extracting from the multiplexed signal the pair of optical harmonics that configure the optical subcarrier of a given mm-wave signal. When the pair of harmonics beat in the photodetector of the base station, the electric mm-wave is created and emitted by the base station antenna. The filtering technique used for the demultiplexing process will determine the optical spectral efficiency

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of the system. Multiple uniform fiber Bragg gratings, working as selective reflectors, have been used to successfully filter different subcarriers [5]. In this configuration, the mm-wave signals are carried by different laser sources. The optical sidebands of a given subcarrier, but also the remaining power from the optical carrier and the amplified spontaneous emission are reflected and directed to the detector, hence power fading due to dispersion is not fully avoided and the signal to noise ratio is limited by the reflected amplified spontaneous emission. The channel spacing achieved in [5] is limited to 35 GHz by the bandwidth of the fiber Bragg gratings. Better spectral efficiency and potential reconfigurability can be achieved by wavelength interleaving solutions, in which every optical component (optical carrier and sidebands) of the mm-wave subcarrier is selected individually through a given frequency space of the spectrum of the filter. The use of array-waveguide gratings [6] has been proposed for this purpose and experimental 25 GHz interleaved channel spacing, defined as the frequency difference between analogous components of adjacent channels, has been demonstrated. The uncontrolled phase mismatching induced in the demultiplexing process in [6] between the optical components of the subcarrier can be overcome if phase-shifted fiber Bragg gratings are used instead of array-waveguide gratings. An interleaved channel spacing of around 24 GHz has been demonstrated using phase-shifted fiber Bragg gratings [7]. Furthermore, fiber Bragg gratings are less expensive than array-waveguide gratings. The use of a Bragg grating designed for conventional WDM systems with 50 GHz spacing, which reflects the optical carrier together with one of the sidebands of a given channel, has been proposed also as a wavelength interleaving technique [8]. However, this solution does not efficiently remove adjacent channel sidebands and the bandwidth of the Bragg grating limits the channel spacing to 25 GHz. In this paper, a new optical demultiplexing system for optical mm-wave subcarriers, based on the filtering properties of dual overwritten fiber Bragg gratings is proposed. The two resonant reflection peaks of each dual overwritten fiber Bragg grating [9] are used to select the two optical components of the desired subcarrier from a wavelength-interleaved spectrum: either optical carrier and sideband or both sidebands whether single sideband modulation or double sideband modulation with suppressed optical carrier is used, respectively. Since the optical frequencies are reflected in the same position of the fiber phase mismatching is avoided. Compared to phaseshifted fiber Bragg gratings (which are difficult to manufacture), longer gratings with narrower dual spectra can be easily achieved by the overwriting technique (down to 2.5 GHz full-width half-maximum (FWHM) in our case, with a 5 cm grating). A 10 GHz interleaved channel spacing with less than 40 dB electrical power interchannel leakage is demonstrated. This filtering scheme, based in dual overwritten fiber Bragg gratings, could be straight forwardly adapted for

other purposes like add/drop [10] and label swapping [11] applications for optical communications or as an alternative to photonic mm-wave filters [12]. Also, it could be a substitute of pure electrical demultiplexers in cases in which avoiding propagation losses is of prior interest and the application can do without the higher spectral efficiency of electrical demultiplexers [13]. The performance of the demultiplexer (DEMUX) is tested for a two-channel (20 and 40 GHz) wireless link, whose transmit/receive radio-on-fiber architecture was recently proposed for a one channel performance [3]. In such proposal, an optical mm-wave subcarrier is created by double sideband modulation with suppressed optical carrier of a laser source and only one of its optical components is modulated with the data to be transmitted. After transmission of the subcarrier over a fiber link, the corresponding electrical mm-wave signal is created in a photodetector and received by self-heterodyning mixing. Unlike previous proposals for optical demultiplexing of mm-wave subcarriers [5–8] (in which each subcarrier is generated with a corresponding laser source), in the present work both subcarriers are generated by double sideband modulation with suppressed optical carrier of a single laser source, which increases the wavelength interleaving potentiality of the system. Data of up to 2.5 Gb/s are added independently to each of the subcarriers as explained in [3]. The multiplexed signal is launched through a backbone fiber to the demultiplexer, which distributes the subcarriers. These are detected and emitted by the base station antennas. Received after a 1.5 m free-space propagation, a self-heterodyning mixing setup is used to recover the data. The optical and electrical performance as well as the linearity of the link and BER measurements for the different channels are evaluated. 2. Principle of the dual overwritten gratings-based demultiplexer Fig. 1 shows the principle of operation of the demultiplexing system, which applies for any arbitrary wavelength-interleaved signal, configured by a given number of mm-wave subcarriers which are composed by two optical harmonics each, whatever the method they have been generated with. In Fig. 1, the input signal to the demultiplexer consists of two mm-wave optical subcarriers (carrying the information of channels 1 and 2 of the mm-wave fiber-optic link). Each of them is configured by a non-modulated harmonic and its corresponding data-modulated harmonic, with frequency separations of f1 and f2, respectively. The signal is launched first into a circulator, which directs the light towards a dual overwritten fiber Bragg grating (DFBG1), where the harmonics of channel 1 (separated a frequency f1 are reflected and redirected by the circulator to one of the output ports of the demultiplexer. After passing through DFBG1, the harmonics configuring the mm-wave optical subcarrier of frequency f2 (channel 2) are directed to the second output port using again a circu-

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Fig. 1. Optical demultiplexer of mm-wave subcarriers.

lator and a dual overwritten fiber Bragg grating (DFBG2). Since the selected harmonics are reflected in the same position of the fiber, phase mismatching between harmonics is avoided. Given that the demultiplexing configuration is based on reflection filtering, the amplified spontaneous emission originated in previous amplification stages is efficiently removed. Fig. 2 shows the spectra of the dual overwritten fiber Bragg gratings used for the experiment. They were written on a conventional germanium–boron codoped photo-sensitive fiber by UV exposure using a doubled argon laser and a uniform period phase mask [14]. Each dual grating was designed to present two resonant wavelengths by overwriting two different period gratings on the same 5-cm-long piece of fiber. The two peak wavelengths of the grating DFBG1 are 0.16 nm (20 GHz) detuned, while the designed detuning for the peaks of the grating DFBG2 is of 0.32 nm

(40 GHz), being 10 GHz the interleaved channel spacing of the DEMUX. The reflectivities and FWHM bandwidths of all four peaks, designed to be equal, are of 98.3 ± 0.4% and of 2.6 ± 0.2 GHz. Both DFBGs accomplish the condition DKL P K2/p (where DK is the period difference between the overwritten gratings, L is the grating length and K is the mean of the overwritten gratings periods), for which the resulting reflection spectrum is equivalent to that of two independent uniform gratings, each centered at a different wavelength [9]. 3. Experiment Fig. 3 shows a scheme of the two channels fiber-optic wireless link. A detailed explanation of the performance of the link is stated in the following sections. 3.1. Generation of multiplexed subcarrier signals

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3.1.1. Generation of optical subcarriers The optical source of the link is a single external cavity laser (ECL in Fig. 3). The linewidth of the external cavity laser is approximately 100 kHz, the optical power is 6 dBm and the operating wavelength is 1544.575 nm. The output from the laser (see label A of Figs. 3 and 4) is modulated via a Mach Zender modulator (MZM in Fig. 3) driven with two sinusoidal electrical signals, previously combined, with frequencies f1/2 = 10 GHz and f2/2 = 20 GHz. The modulator is biased to achieve carrier suppression of the optical signal (double sideband modulation with suppressed optical carrier) with a DC voltage source. In order to get maximum suppression of the optical carrier, as well as maximum suppression of undesired harmonics, the modulator bias is optimized and the input polarization to the MZM is controlled with a polarization controller (PC in Fig. 3). The suppression of the optical carrier is improved by a uniform fiber Bragg grating, FBG1, with a 3-dB bandwidth of 5 GHz, centered at the operating optical frequency of the laser source, m0. An alternative to FBG1 for enhanced carrier suppression would be an extra

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Fig. 3. Two channels fiber-optic wireless link. ECL, external cavity laser; PC, polarization controllers; FBG, fiber Bragg grating; MZM, Mach Zender modulator; EDFA, erbium-doped fiber amplifier; PD, photodetector; BERT, bit error rate tester; PRBS, pseudo-random binary sequence.

polarizer after the MZM, which would suppress the outgoing non-modulated light from the MZM due to the limited polarizing efficiency of the inbuilt polarizer of the MZM. The optical components of the resulting signal (position B of Fig. 3), lower and upper sidebands of the modulation (LSB and USB), configure the optical subcarriers of frequencies f1 = 20 GHz and f2 = 40 GHz. They are represented in Fig. 4, label B.

optical subcarriers of frequencies f1 = 20 GHz and f2 = 40 GHz, which will be the output from the central station to the backbone fiber (label I of Figs. 3 and 4). The insertion losses in the MZMs suffered by the lower sidebands are compensated by the previous amplification performed by an erbium-doped fiber amplifier (EDFA1 in Fig. 3) in order to equalize the power of the upper sidebands.

3.1.2. Data insertion A circulator and a pair of chirped fiber Bragg gratings (FBG2 and FBG3 in Fig. 3) with 30-dB bandwidths of 16 GHz are used to split the upper and lower sidebands. FBG2 reflects the lower sidebands and lets the upper sidebands pass through (label C of Figs. 3 and 4). FBG3 is used to remove residual power of the upper sidebands reflected by side lobes of FBG2. Free of undesired harmonics and amplified by an erbium-doped fiber amplifier (EDFA in Fig. 3), the lower sidebands are launched to the data-insertion setup (label D of Figs. 3 and 4). A circulator and a uniform grating (FBG4 in Fig. 3) with a 30-dB bandwidth of 9 GHz are used to split the lower sidebands, which are independently modulated with On–Off-keyed data in the Mach Zender modulators MZM1 (channel 1) and MZM2 (channel 2), respectively (see labels E to H in Figs. 3 and 4). The On–Off-keyed signals are pseudo-random binary sequence data with a word length of 231 1. Combined with a 3:1 coupler, the upper sidebands (unmodulated) and the lower sidebands (modulated with data) configure the interleaved multiplexed signal of two

3.2. Demultiplexing Once the multiplexed signal from the backbone fiber reaches the point from which it is wanted to be distributed to the corresponding small area cells base stations, it is demultiplexed as explained in Section 2. Extra uniform fiber Bragg gratings are used after the DEMUX to enhance the filtering process (see Section 4). Each optical subcarrier is then distributed to its corresponding base station (labels J and K of Figs. 3 and 4). In order to determine the interchannel cross-talk, optical and electrical spectral analysis of the demultiplexer performance was carried out. 3.3. Wireless transmission, reception and recovering In the base station, the optical subcarrier is detected by a 50-GHz bandwidth photodetector (PD in Fig. 3). Since its optical components are separated a frequency f, a datamodulated mm-wave carrier of frequency f is created by heterodyne detection. The electrical signal is then amplified and emitted by a horn antenna over 1.5 m of free-space transmission. By optically filtering prior to photodetection

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was tested only for channel 2. The mm-wave signal is received in the mobile unit by a horn antenna of same characteristics as the emitting antenna. The data are recovered by down-converting the 40 GHz signal to base band, utilizing a simple self-heterodyning setup [3]. The performance of the complete link was tested for channel 2 (40 GHz), measuring the bit error rates (BER) of 1.25 and 2.5 Gb/s On–Off-keyed pseudo-random binary sequence data. 4. Results 4.1. Optical and electrical DEMUX performance

Fig. 4. Scheme of the optical spectra through the fiber-optic wireless link. USB, upper sidebands; LSB, lower sidebands.

of the separate f1 and f2 signals, the useful amplitude in the detector is magnified. If the full signal were detected first, and then electrically filtered, the maximum output of the photodetector would be shared in the two electrical signals. The horn antennas available for the experiment operated at a 33–50 GHz bandwidth, so wireless transmission

In order to test the quality of the DEMUX, optical and electrical spectra of a two-frequencies double sideband modulated optical signal with carrier suppressed have been measured through the demultiplexing process of the individual optical mm-wave subcarriers of 20 and 40 GHz (channels 1 and 2, respectively). Fig. 5a shows the input optical signal to the DEMUX, measured with 10 pm resolution. The central peak corresponds to the optical carrier, which has suffered a suppression of 25 dB, of which, 10 dB are due to the optimally biased double sideband modulation and 15 dB are due to FBG1. The inner and outer peaks are the components of the optical mm-wave subcarriers of 20 and 40 GHz, respectively. The noise floor is due to previous optical amplification performed via an erbiumdoped fiber amplifier. The optical output from the DEMUX for channels 1 and 2 are depicted in Fig. 5b.1 and b.2. As illustrated in Fig. 1 and explained in Section 2, the subcarrier of channel 2 suffers a double filtering process with respect to the subcarrier of channel 1: The reflection suffered by the components of channel 2 in sidelobes of DFBG1 yields an interchannel relative power leakage of 17 dB of components of channel 2 to the output of channel 1, as can be seen in Fig. 5b.1. On the other hand, according to the reflectance of DFBG1, only around 2% of the power of the incoming components of channel 1 pass through DFBG1, of which, only the power reflected by the sidelobes of DFBG2 is detected in the output of channel 2. This cumulative filtering process is the reason for which the interchannel relative power leakage of contiguous channel components is of 33 dB for channel 2 (as can be seen in Fig. 5b.2), while it is of 17 dB for channel 1. These values are consistent with those expected from the peak-to-sidelobe reflectance ratios of the dual overwritten gratings at the wavelengths of the corresponding contiguous channel components, which values are between 16 and 18 dB. Figures of the interchannel relative power leakage could be improved by using apodized dual wavelength gratings, which would reduce the reflectance of the sidelobes. In the present configuration, the dual overwritten gratings show peak-to-sidelobe reflectance ratios below 16 dB at peak-to-sidelobe frequency distances above 8 GHz, thus 8 GHz would be the minimum interleaved channel spacing achievable by the system with the performance demonstrated in the experiment.

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Fig. 5. Optical and electrical performance of the demultiplexer (DEMUX). (a) Input optical signal to the DEMUX. (b.1 and b.2) Optical output from the DEMUX for channels 1 and 2, respectively. (c.1 and c.2.) Optical spectra of channel 1 and 2 after extra notch filtering. (d.1 and d.2) Corresponding electrical spectra of demultiplexed channels 1 and 2.

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Since the demultiplexing process is based in reflection filtering, the amplified spontaneous emission is efficiently removed for both channels. Once the subcarriers are demultiplexed (keeping the phase among subcarrier components properly matched due to the special structure of the dual overwritten fiber Bragg gratings), extra notch filtering of leaked components for high demultiplexing performance can be easily added by the use of uniform fiber Bragg gratings. Without them, 17 and 33 dB optical suppression of components of the adjacent channel are achieved. These values, although comparable to previously published all-optical DEMUX configurations [5–8] being the interleaved channel spacing narrower (10 GHz against 24–35 GHz of previous), must be improved if electric filters are to be replaced by all-fiber solutions. Extra notch FBG filtering is a direct way to do it within this DEMUX configuration. Fig. 5c.1 and c.2 illustrates the optical spectra of channels 1 and 2 after extra notch filtering. The corresponding electrical spectra of both channels after photodetection are shown in Fig. 5d.1 and d.2. The interchannel electrical power leakage is below the electrical spectrum analyzer sensitivity, 43 dB for channel 1 and 56 dB for channel 2. This high-performance configuration has been used for testing the complete 2 channel fiber-optic wireless link (see Fig. 3).

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Fig. 6. (a) Scheme of the two-tone intermediate frequency (IF) modulation for testing the linearity of the system in channel 2 (40 GHz). (b) Electrical spectrum of the channel after detection in the base station. IMD3, intermodulation distortion harmonics.

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In order to determine the spurious-free dynamic range (SFDR) of the system, both channels were subjected to two-tone linearity measurements, as illustrated in Fig. 6a for channel 2 (40 GHz). Instead of being driven by a pseudo-random binary sequence data signal, MZM1 (the same in Fig. 3) now was driven by two harmonic signals with equal amplitude and frequencies IF1 = 1 GHz and IF2 = 1.1 GHz (frequency difference DF = 100 MHz). The electrical spectrum of the channel (after being multiplexed together with the other channel, launched to the fiber backbone, distributed by the DEMUX to its corresponding base station, photodetected and electrically amplified) will be as represented in Fig. 6b. The fundamental components in Fig. 6b are due to the dual harmonic modulation (1 and 1.1 GHz) of the optical lower sidebands in MZM1 and the later beating with its corresponding unmodulated upper sidebands of channel 2. At a given electrical input power of the harmonic RF signals to the Mach Zender modulator MZM1, third order intermodulation distortion components (IMD3 in Fig. 6b) appear in the electrical spectrum, mainly due to the nonlinear behavior of the Mach Zender modulator MZM1, the photodetector and the electrical amplifier after photodetection. Fig. 7 represents the output power at the photodetector of channel 2 of the fundamental and intermodulation distortion components as well as the noise power (measured with a 1 Hz resolution bandwidth) as a function of the individual electrical power of each of the RF signals modulating the Mach Zender modulator MZM1.

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From the two-tone measurements represented in Fig. 7, the SFDR of channel 2, defined as the ratio between the power of the fundamental and the intermodulation distorsion components at the input power for which the intermodulation distorsion components (IMD3 in Fig. 7) reach the noise level, is calculated to be 80 dB Hz2/3 as shown in Fig. 7, by fitting the experimental values for the fundamental and IMD3 components shown in Fig. 7 to linear curves of fixed slopes

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of 1 and 3, respectively, in correspondence to their first and third-order theoretical dependence on the input electric power [15,16]. The third-order input intercept point (IIP3 in Fig. 7), defined as the input power at which intermodulation distorsion components would reach the fundamental components power, corresponds to 14 dBm, while the output power of the third-order output intercept point (OIP3 in Fig. 7) is 10 dBm. The same measurements were performed for channel 1 (20 GHz), giving a SFDR of 72 dB Hz2/3, an input intercept point of 8 dBm and an output intercept point of 4 dBm. The 6 dB difference of all three values with respect to the corresponding ones of channel 2 are due to the fact that two cascaded amplifiers after photodetection were needed to perform the two-tone measurements of channel 1, inducing an extra 6 dB noise compared to the only one amplifier used for channel 2. 4.3. Data transmission The subcarrier of each channel was modulated with independent On–Off-keyed pseudo-random binary sequence data via the Mach Zender modulators MZM1 and MZM2, respectively, and bit error rates for the complete central station to mobile unit data transmission were measured for channel 2 (40 GHz). As illustrated in Fig. 8, 1.25 and 2.5 Gb/s modulation rates with word lengths of 231 1 were tested. For a given optical power at the base station, no significant BER difference was observed for the 40 GHz signal irrespective of whether channel 1 was carrying a simultaneous independent signal at 20 GHz. This is due, first, to the efficient separation of 3

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the lower sidebands in the data insertion process (see Fig. 4, labels D–H), which ensures independent data modulation of each channel, so that no data cross-talk between channels occurs. And second, due to the DEMUX performance, which guarantees that the optical power before detection comes solely from the 40 GHz optical subcarrier. The concept of the DEMUX does not limit its performance to On–Off-keyed modulation. Data transmission with other modulation formats as phase-shift keying (PSK), quaternary amplitude modulation (QAM) or wireless local area network (WLAN) signals can be performed. The system, despite the number of fiber Bragg gratings used, happens to be robust in the laboratory environment, the multiplexed signals being stable in amplitude and phase with time. Except for the dual overwritten gratings, the bandwidth of the fiber Bragg gratings used in the experiment is wide enough to not bias significant power changes of the filtered signals due to wavelength drifting of the laser source or by random drifting of the gratings spectra caused by temperature changes. The dual overwritten gratings, whose narrow bandwidths make their filtering performance very sensitive to environmental changes, were stabilized by a simple cover. Nevertheless, for a potential field application of the fiber-optic mmwave wireless link, temperature stabilization of the fiber Bragg gratings would be of major importance, which could be performed reliably and cost-effectively by passive athermal packaging of FBGs [17,18]. 5. Conclusion A new optical mm-wave demultiplexing system, based on the filtering properties of dual overwritten fiber Bragg gratings and in double sideband modulation with suppressed optical carrier, has been proposed and experimentally studied. In order to test its performance, the DEMUX has been implemented in a complete 2-channel optical wireless link, fed by a single optical laser source. The optical interleaving efficiency of the DEMUX was as low as 10 GHz and the link interchannel electrical power leakage was below 43 dB for the 20 GHz channel and below 56 dB for the 40 GHz channel. The linearity of the system was measured in terms of the spurious free dynamic range, which was found to be 72 and 80 dB Hz2/3, for the 20 and 40 GHz channels, respectively. The filtering efficiency of the DEMUX and the data insertion system yielded cross-talk free data transmission for 1.25 and 2.5 Gb/s modulation rates with word lengths of 231 1, the 2 channels being modulated with independent On–Off-keyed pseudo-random binary sequence data. Although robust in a laboratory environment, temperature stabilization of the dual overwritten gratings should be needed for a field application of the DEMUX. The proposed system has been demonstrated for two channels, but could easily be expanded to several channels.

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