Frequency-selective filtering and analysis of radio-over-fiber using stimulated Brillouin scattering

Yonatan Stern, Avi Zadok

Faculty of Engineering, Bar-Ilan University

Ramat-Gan 52900, Israel Avinoam.Zadok@biu.ac.il

Kun Zhong, Ru Zhang School of Science

Beijing University of Posts and Telecommunications Beijing 100876, China

Thomas Schneider Institut für Hochfrequenztechnik

Hochschule für Telekommunikation D-04277 Leipzig, Germany

Moshe Tur School of Electrical Engineering, Faculty of Engineering

Tel-Aviv University Tel-Aviv 69978, Israel

Yossef Ben-Ezra Optiway Integrated Solutions Ltd.

11 Haavoda St., Rosh Haayin, Israel

Abstract—A method for tunable microwave-photonic and optical filtering of broadband waveforms is proposed and demonstrated. The principle relies on stimulated Brillouin scattering in standard, birefringent optical fibers, and the associated effect of polarization pulling. The method is employed in the implementation of tunable microwave-photonic band-pass filters with 45 dB selectivity, and in optical spectral analysis of sub-carrier-multiplexed data transmission with tones separation of only 10 MHz.

Keywords— Microwave Photonics (MWP); Radio-over-Fiber (RoF); Stimulated Brillouin Scattering (SBS); Optical Spectrum Analyzer (OSA).

I. INTRODUCTION Over the past three decades, microwave photonics (MWP)

has been an area of major academic and industrial interest. In MWP, a radio-frequency (RF) signal modulates an optical carrier and is processed using optical means. At the output end of the system, the optical signal is detected to recover a modified RF waveform [1]. The transmission of RF signals over long distances using optical fibers, or radio-over-fiber (RoF), is a current important application of MWP [2]. Many RoF implementations involve RF filtering. Since electrical-to-optical (O/E) conversion is inherently incorporated in RoF transmission, filtering in the optical-domain is often considered. In one common architecture of MWP filters [3, 4], several RF-modulated optical signals propagate along paths of different delay and attenuation, and recombine upon detection. Such delay-and-sum filters are characterized by a finite impulse response (FIR). Delay elements within the filters may be fixed or tunable, and they make use of fiber Bragg gratings (FBGs), arrayed waveguide gratings (AWGs) and other configurations [4].

Most delay-and-sum MWP filters are restricted to an incoherent summation of intensities, in order to avoid environmental phase drifts. The FIR coefficients of a sum-of-intensities filter are real and positive, and their spectral transfer function is therefore restricted. For example, the response has a resonance at zero frequency [5], and provides relatively modest selectivity [1]. Although several configurations successfully implement filters with negative and complex coefficients [6-10], the realization of sharp MWP band-pass filters using delay-and-sum architectures requires a large number of paths and remains challenging. In addition, the frequency response of delay-and-sum filters is inherently periodic, with multiple pass-bands. The realization of only a single pass-band would be advantageous in many RoF receivers.

Alternatively, quite few MWP systems rely on nonlinear optical propagation effects such as stimulated Brillouin scattering (SBS). In SBS, a relatively intense pump wave amplifies a counter-propagating signal wave that is detuned in frequency by the Brillouin shift [11]. SBS amplification is simply implemented in standard fibers, has a low power threshold of several mW [12], and is highly frequency-selective: its gain bandwidth is on the order of 30 MHz only [11]. SBS is therefore very attractive for all-optical signal processing and sensing applications. Examples include optical [13] and microwave-photonic [6] filtering, distributed fiber-optic sensing of temperature and strain variations [12, 14], SBS-induced slow light [15] and more.

Herein we present a significant enhancement of MWP filters that are based on SBS. The input RF signal modulates an optical carrier which propagates along an optical fiber. A counter-propagating pump, whose spectrum is broadened to replicate the shape of the desired RF filter, generates a SBS gain window which overlaps part of the spectrum of one RF-modulated sideband. A modified RF waveform is recovered by

This work was supported in part by the German-Israeli Foundation (GIF),under grant no. I-2219-1978.10/2009, and by the Chief Scientist Office,Israeli Ministry of Industry, Trade and Labor, within the TERA SANTAconsortium.

146978-1-4673-6071-5/13/$31.00 ©2013 IEEE

interference of the filtered sideband with the optical carrier on a broadband photo-detector (PD). The obtained RF band-pass filters are characterized by uniform transmission windows, strong out-of-band rejection, and reconfigurable bandwidth and central transmission frequencies. Their frequency response is inherently aperiodic. Out-of-band rejection of the MWP filters is enhanced based on the polarization attributes of SBS in standard, weakly birefringent fibers [16]. The method was previously implemented in the realization of sharp and tunable optical band-pass filters [17].

A similar principle is also used in the realization of a high-resolution optical spectrum analyzer (OSA) [18]. By sweeping a Brillouin gain line across the spectral extent of an optical signal under test (SUT), the power spectral density (PSD) of the SUT is reconstructed. The proposed OSA was employed in monitoring the fiber-optic transmission of ten multiplexed sub-carriers with tone separation of 10 MHz, each modulated by an on-off keying, pseudo-random sequence at 2.5 Mbit/s.

II. PRINCIPLE OF OPERATION SBS gain in standard, birefringent fibers is highly

polarization-dependent [16]. For a given state of polarization (SOP) of the input pump, two orthogonal input signal SOPs can be identified, which provide the maximum and minimum signal output powers. The unit Jones vectors of these two SOPs are denoted by maxˆine and minˆine , respectively. For sufficiently long, weakly and randomly birefringent fibers, and in the undepleted pump regime, the corresponding maximum and minimum complex amplitude gain values are given by [16]:

( ) ( )( )( ) ( )( )

1max 3

1min 6

exp

exps s

s s

G g L

G g L

=

=

ω ω

ω ω . (1)

Here sω is a signal frequency variable, ( )sg ω is the exponential gain coefficient, which scales with the pump power, and L is the length of the fiber. The line shape of

( )sg ω depends on the modulation of the pump wave. When the pump PSD is substantially broader than the Brillouin linewidth, the real part of ( )sg ω , which governs the probe power gain, is proportional to the pump PSD with an offset of the Brillouin shift [13].

The values of maximum and minimum amplification are also associated with two orthogonal SOPs of the signal at the output end of the fiber. Let us denote the unit Jones vectors of the these SOPs as maxˆoute , minˆoute . An arbitrarily polarized input signal can be represented in the basis of maxˆine and minˆine :

( ) ( ) ( )0 0 max 0 minˆ ˆ, 0 in ins sE z E e eω ω α β= = + . (2)

Here ( )0 sE ω is a frequency dependent, scalar complex

magnitude and 2 20 0 1+ =α β . The Jones vector of the output

signal, corresponding to the input of (2), can be expressed as [16, 17]:

( ) ( ) ( ) ( )0 0 max max 0 min minˆ ˆ, out outs s s sE z L E G e G eω ω α ω β ω⎡ ⎤= = +⎣ ⎦ .(3)

In most practical cases max minG G>> . Therefore, the first term in (3) becomes dominant, and unless 0α is negligible, the arbitrarily polarized input signal will be drawn towards the SOP of maxˆoute . This phenomenon is referred to as SBS polarization pulling [16-18]. Further discrimination between amplified and unamplified spectral components is therefore possible by placing a properly-aligned polarizer at the output end of the signal. Let us denote the unit Jones vector of the transmission axis of the polarizer by:

max max min minˆ ˆ ˆout outtrp p e p e= + , (4)

where max,minp are the projections of ˆ trp onto max,minˆoute , respectively. If ˆ trp is adjusted so that * *

0 max 0 minp p= −α β , out-of-band spectral components of the signal can be blocked off entirely by the polarizer. Subject to this constraint, the PSD of the signal wave at the polarizer output becomes [17, 18]:

( )

( ) ( ) ( )

2

22 2*0 0 max max min

,s

s s s

E z L

E p G G

ω

ω α ω ω

= =

− . (5)

Subject to the constraint of complete out-of-band rejection, the maximum value of

2*0 maxpα is 0.25 [17, 18]. At the high-

gain limit, the in-band power gain of the polarizer-assisted SBS process is 6 dB weaker than ( ) 2

max sG ω . However, the rejection of the unamplified spectral components of the signal at the output of the polarizer is, in principle, infinite.

III. EXPERIMENTAL SETUP AND PRELIMINARY RESULTS Fig. 1 shows the experimental setup for the demonstration

of polarization-enhanced, SBS-based MWP band-pass filters. The output of a tunable laser source is split in two branches using a 90/10 directional coupler. Light at the 10% arm (probe branch, blue) is modulated by a RF sine-wave in suppressed-carrier (SC) format, and serves as a probe wave. The modulation frequency is generated by a vector network analyzer (VNA) and swept to obtain the frequency response of the filter. The output of the 90% arm is split again using a 50/50 coupler. The un-modulated optical carrier is retained in the middle branch (reconstruction branch, gray) for subsequent coherent, polarization-sensitive detection of the probe wave.

Light in the upper branch (pump branch, green) is used as a SBS pump wave. The optical spectrum of the pump is broadened through external modulation by a RF linear frequency modulated (LFM) waveform, in a SC format. LFM waveforms are easy to generate [19] and their PSDs are uniform. Consequently, a broadened SBS pump with a uniform PSD is obtained. LFM waveforms with a central frequency of 1 GHz are generated using an arbitrary waveform generator (AWG), mixed with a local-oscillator (LO) of frequency 6.75 GHz and up-converted to a central frequency to 7.75 GHz. A narrowband FBG is then used to retain only one sideband, which serves as a broadened pump. The broadened pump wave is then amplified using an erbium-doped fiber amplifier (EDFA). The pump wave and the RF-modulated probe wave are launched into the opposite ends of a 1 km-long highly-

147

Fig. 1. Experimental setup for the demonstration of stimulated Brillouinscattering polarization-enhanced microwave-photonic band-pass filter. TLS: tunable laser source. PC: polarization controller. AWG: arbitrary waveformgenerator. LO: local oscillator. MZM: Mach-Zehnder modulator. EDFA: erbium-doped fiber amplifier. HNLF: highly-nonlinear fiber. VNA: RF vector network analyzer. PD: photo-detector.

Fig. 2. Experimentally-obtained frequency response of a 500 MHz-wide polarization-enhanced stimulated Brillouin scattering-based microwave-photonic band-pass filter (black), and the corresponding simulated response (red). The latter is based on measurements of the broadened pump power spectral density.

1.2 1.4 1.6 1.8 2 2.2

−60

−50

−40

−30

−20

−10

0

Radio Frequency [GHz]

Nor

mal

ized

pow

er [d

B]

ExperimentalSimulated

Fig. 3. Experimental setup for stimulated Brillouin scattering-based optical spectrum analyzer. PC: polarization controller. MZM: Mach-Zehnder modulator. AWG: arbitrary waveform generator. VOA: variable optical attenuator. FBG: fiber Bragg grating. PBS: polarization beam splitter.

nonlinear fiber (HNLF). The HNLF was shorter than the coherence length of the light source. Following propagation in the HNLF, the modulation sidebands of the probe wave are mixed together with the optical carrier on a broadband PD. The reconstructed RF tone is analyzed by the VNA.

The reconstruction of the RF tone upon detection depends on the polarization alignment of the processed probe sideband in the lower branch (blue path) with respect to the carrier replica (gray path). The role of the interference between the two is therefore analogous to that of a polarizer at the probe output end (see (4)). The multiple polarization-controllers (PCs) in the setup were carefully aligned, in accord with the considerations of the previous section. The launch SOP of the pump wave was chosen arbitrarily, and held fixed for the entire experiment. For initial calibration, the input probe wave was disconnected, and the amplified spontaneous emission (ASE) that is associated with SBS was observed at the signal output end. The SOP of SBS-ASE is known to be aligned with maxˆoute [16, 20]. First maxˆoute was identified by setting PC 3 to maximum interference of SBS-ASE and the carrier wave, and the detected RF power was noted. Then, PC 3 was readjusted until the power of the interference term between SBS-ASE and carrier was reduced by 50%, a condition equivalent to

2*max 1 2p = . Lastly, an out-of-band probe wave was

reintroduced, and PC 2 was used to adjust its input SOP until the interference between unamplified probe waves and carrier was completely eliminated. This final adjustment is analogous to * *

0 max 0 minp pα β= − , as required.

Fig. 2 shows the normalized frequency response of the MWP band-pass filter (black). The power of the broadened pump was +21.5 dBm, and its PSD was nearly uniform over a bandwidth of about 500 MHz. The simulated filter response is shown as well (red), calculated based on measurements of the pump PSD. The power of the probe modulation sideband at the HNLF input was set to -37 dBm, to obtain large SBS gain without depletion. The central transmission frequency of the filter was arbitrarily chosen as 1.65 GHz. The response of the filter is characterized by a 500 MHz-wide pass-band with transmission uniformity of ±1.5 dB. The rejection of out-of-

band RF components is 44 dB. A rejection ratio of over 35 dB is obtained over a transition bandwidth of 120 MHz. The central frequency of the filter can be tuned through adjustment of LO frequency. The bandwidth and shape of the filter can be tuned arbitrarily and independently by changing LFM waveform properties.

Narrow-band SBS filters with polarization-enhanced out-of-band rejection were also used in a high-resolution OSA. Fig. 3 shows the experimental setup. The output of a tunable laser is split in two branches. Light in the upper branch (probe branch, green) is modulated by a RF waveform, generated by an AWG, and serves as a SUT. The optical SUT is then modulated again by a sine wave of frequency Lf = 200 kHz, to allow for lock-in power measurement at the output. Light in the lower branch (pump branch, blue) is single-sideband modulated by a sine wave of frequency ν , and amplified to serve as SBS pump. The pump and SUT counter-propagate in a 25 km-long section of standard single-mode fiber, which constitutes the SBS gain medium.

Following the principle outlined in section II, the signal SUT at the output of the gain medium is split using a polarization beam-splitter (PBS), and the output of the fast axis is detected. The frequency offset ν is swept across the Brillouin gain spectrum of the fiber, (analysis block, purple), and the SUT power at the polarizer output is noted. Locked-in measurements of the photo-current RF power at the frequency

148

Fig. 4. Experimental reconstruction of the PSD of a signal under test,using a polarization-enhanced SBS-OSA setup. The signal consisted of tensub-carriers, separated by 10 MHz. Each sub-carrier was independentlymodulated by a pseudo-random, on-off keying bit sequence at 2.5 Mbit/s(black). The second trace (red) shows the reconstruction of the same signal asabove, with sub-carrier no. 7 intentionally removed.

Lf block the SBS-ASE almost entirely, and allow for a more accurate reconstruction of the SUT PSD.

As in the previous experiment, the launch SOP of the pump was chosen arbitrarily, and held fixed. Initially the input signal was disconnected, and SBS-ASE was observed at the PBS output in order to identify maxˆoute [16, 20]. Next, PC 3 was readjusted until the power of the SBS-ASE was reduced by 50%, as before. Lastly, an out-of-band SUT was reintroduced, and PC 2 was adjusted until the output SUT, in the absence of SBS amplification, was entirely blocked by the PBS.

The SUT consisted of 10 sub-carrier multiplexed tones with 10 MHz spacing, each carrying an on-off keying pseudo-random bit sequence at 2.5 Mbit/s. The overall optical power of the SUT was -23 dBm, and the SBS pump power was +13 dBm. Fig. 4 shows the experimental reconstruction of the PSD SUT (black). Next, one of the sub-carrier tones was intentionally removed, representing a potential failure. SBS-OSA setup effectively recognized the missing sub-carrier (red).

IV. CONCLUSIONS SBS processes in weakly birefringent fibers were employed in highly-selective filtering of optical and RF waveforms. The out-of-band rejection of the filters was enhanced by the polarization pulling that is associated with SBS amplification. Sharp and reconfigurable MWP filters were demonstrated, with a single pass-band, an in-band transmission uniformity of 1.5 dB, and a selectivity of 44 dB. A SBS-based OSA which is able to resolve sub-carrier multiplexed RoF transmission with 10 MHz tone spacing directly in the optical domain, was demonstrated as well. The proposed selective filtering is applicable to the monitoring of advanced optical communication transmission formats, such as optical-orthogonal frequency domain multiplexing (O-OFDM) [21, 22], and in filtering and monitoring modules in RoF setups.

REFERENCES [1] J. Capmany, B. Ortega, and D. Pastor, “A tutorial on microwave

photonic filters,” Journal of Lightwave Technology, vol. 24, no. 1, pp. 201–229, 2006.

[2] J. Capmany and D. Novak, “Microwave photonics combines two worlds,” Nature Photonics, vol. 1, no. 6, pp. 319–330, 2007.

[3] D. E. Dudgeon, “Fundamentals of digital array processing,” Proceedings of the IEEE, vol. 65, no. 6, pp. 898–904, 1977.

[4] J. Capmany, B. Ortega, D. Pastor, and S. Sales, “Discrete-time optical processing of microwave signals,” Journal of lightwave technology, vol. 23, no. 2, pp. 702–723, 2005.

[5] B. Moslehi, J. W. Goodman, M. Tur, and H. J. Shaw, “Fiber-optic lattice signal processing,” Proceedings of the IEEE, vol. 72, no. 7, pp. 909–930, 1984.

[6] A. Loayssa, J. Capmany, M. Sagues, and J. Mora, “Demonstration of incoherent microwave photonic filters with all-optical complex coefficients,“ Photonics Technology Letters, IEEE, vol. 18, no. 16, pp. 1744–1746, 2006.

[7] Y. Zhang and S. Pan, “A Complex Coefficient Microwave Photonic Filter Using a Polarization-Modulator-Based Phase Shifter,” Photonics Technology Letters, IEEE, vol. 25, no. 2, pp. 187–189, 2013.

[8] T. X. Huang, X. Yi, and T. &. M. R. A. Chen, “Phase-Modulation-Based Microwave Photonic Bandpass Filter Using a Programmable-Induced Frequency Response,” Photonics Technology Letters, IEEE, vol. 24, no. 14, pp. 1197–1199, 2012.

[9] J. Capmany, D. Pastor, A. Martinez, S. Sales, and B. Ortega, “Using phase inversion in an electro-optic modulator to implement RF incoherent photonic filters with negative coefficients,” in Microwave Photonics, 2003. MWP 2003 Proceedings. International Topical Meeting on, 2003.

[10] J. Mora, J. Capmany, A. Loayssa, and D. Pastor, “Novel technique for implementing incoherent microwave photonic filters with negative coefficients using phase modulation and single sideband selection,” Photonics Technology Letters, IEEE, vol. 18, no. 18, pp. 1943–1945, 2006.

[11] R. W. Boyd, Nonlinear optics, Academic press, 2003. [12] T. Horiguchi, T. Kurashima, and M. Tateda, “A technique to measure

distributed strain in optical fibers,” Photonics Technology Letters, IEEE, Vol. 2, no. 2, pp. 352–354, 1990

[13] A. Zadok, A. Eyal, and M. Tur, “Gigahertz-wide optically reconfigurable filters using stimulated Brillouin scattering,” Journal of lightwave technology, vol. 25, no. 8, pp. 2168–2174, 2007

[14] X. Bao and L. Chen, “Recent progress in Brillouin scattering based fiber sensors,” Sensors, vol. 11, no. 4, pp. 4152–4187, 2011.

[15] A. Zadok, A. Eyal, and M. Tur, “Stimulated Brillouin scattering slow light in optical fibers [Invited],” Applied Optics, vol. 50, no. 25, pp. E38–E49, 2011.

[16] A. Zadok, E. Zilka, A. Eyal, L. Thévenaz, and M. Tur, “Vector analysis of stimulated Brillouin scattering amplification in standard single-mode fibers,” Optics Express, vol. 16, no. 26, pp. 21692–21707, 2008.

[17] A. Wise, M. Tur, and A. Zadok, “Sharp tunable optical filters based on the polarization attributes of stimulated Brillouin scattering,” Optics Express, vol. 19, no. 22, pp. 21945–21955, 2011.

[18] S. Preussler, A. Zadok, A. Wiatrek, M. Tur, and T. Schneider, “Enhancement of spectral resolution and optical rejection ratio of Brillouin optical spectral analysis using polarization pulling,” Optics Express, vol. 20, no. 13, pp. 14734–14745, 2012.

[19] R. J. Mailloux, Phased array antenna handbook, Artech House Boston, MA, 2005.

[20] M. O. Van Deventer and A. J. Boot, “Polarization properties of stimulated Brillouin scattering in single-mode fibers,” Journal of lightwave technology, vol. 12, no. 4, pp. 585–590, 1994.

[21] J. Armstrong, “OFDM for optical communications,” Journal of lightwave technology, vol. 27, no. 3, pp. 189–204, 2009.

[22] A. J. Lowery, L. B. Du and J. Armstrong, “Performance of optical OFDM in ultralong-haul WDM lightwave systems,” Journal of Lightwave Technology, vol. 25, no. 1, pp. 131–138, 2007.

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