All-optical binary phase-coded UWB signal ... - OSA Publishing

All-optical binary phase-coded UWB signal generation for multi-user UWB communications Jianji Dong,1, * Yuan Yu,1 Yin Zhang,1 Xiang Li, 1 Dexiu Huang, 1 and Xinliang Zhang 1 1

Wuhan National Lab for Optoelectronics, College of Optoelectronic Science and Engineering, Huazhong University of Science and Technology, Wuhan 43007, China *[email protected]

Abstract: An all-optical incoherent scheme for generation of binary phasecoded ultra-wideband (UWB) signals is proposed and experimentally demonstrated. The binary phase coding is performed based on all-optical phase modulation in a semiconductor optical amplifier (SOA) and phase modulation to intensity modulation (PM-IM) conversion in a fiber delay interferometer (DI) that serves as a multichannel frequency discriminator. By locating the phase-modulated light waves at the positive and negative slopes of the DI transmission spectra, binary phase encoded UWB codes (0 and π) are generated. We also experimentally demonstrate a bipolar UWB coding system with a code length of 4, operating at 1.25 Gb/s. And the decoding is analyzed as well. Our proposed system has potential application in future high-speed UWB impulse radio over optical fiber access networks. ©2011 Optical Society of America OCIS codes: (060.2330) Fiber optics communications; (060.4080) Modulation; (250.5980) Semiconductor optical amplifiers.

References and links 1.FCC, Revision of part 15 of the commission’s rules regarding ultra-wideband transmission systems, (2002). 2. M. Ran, B. I. Lembrikov, and Y. Ben Ezra, “Ultra-wideband Radio-Over-Optical fiber concepts, technologies and applications,” IEEE Photon. J. 2(1), 36–48 (2010). 3. J. Yao, “Photonics for Ultrawideband communications,” IEEE Microw. Mag. 10(4), 82–95 (2009). 4. J.-Y. Zheng, M.-J. Zhang, A.-B. Wang, and Y.-C. Wang, “Photonic generation of ultrawideband pulse using semiconductor laser with optical feedback,” Opt. Lett. 35(11), 1734–1736 (2010). 5. S. Pan, and J. Yao, “UWB-Over-Fiber Communications: Modulation and Transmission,” J. Lightwave Technol. 28(16), 2445–2455 (2010). 6. X. Yu, T. B. Gibbon, and I. T. Monroy, “Experimental Demonstration of All-Optical 781.25-Mb/s Binary PhaseCoded UWB Signal Generation and Transmission,” IEEE Photon. Technol. Lett. 21(17), 1235–1237 (2009). 7. X. Yu, T. Braidwood Gibbon, M. Pawlik, S. Blaaberg, and I. Tafur Monroy, “A photonic ultra-wideband pulse generator based on relaxation oscillations of a semiconductor laser,” Opt. Express 17(12), 9680–9687 (2009). 8. H. W. Wang, T. T. Le, and J. X. Cheng, “Label-free Imaging of Arterial Cells and Extracellular Matrix Using a Multimodal CARS Microscope,” Opt. Commun. 281(7), 1813–1822 (2008). 9. F. Wang, J. Dong, E. Xu, and X. Zhang, “All-optical UWB generation and modulation using SOA-XPM effect and DWDM-based multi-channel frequency discrimination,” Opt. Express 18(24), 24588–24594 (2010). 10. J. Dong, X. Zhang, J. Xu, D. Huang, S. Fu, and P. Shum, “Ultrawideband monocycle generation using crossphase modulation in a semiconductor optical amplifier,” Opt. Lett. 32(10), 1223–1225 (2007). 11. Y. Dai and J. Yao, “Multi-User UWB-over-Fiber System Based on High-Chip-Count Phase Coding,” in Proceedings of OFC/NFOEC, (2008). 12. S. Wang, H. Chen, M. Xin, M. Chen, and S. Xie, “Optical ultra-wide-band pulse bipolar and shape modulation based on a symmetric PM-IM conversion architecture,” Opt. Lett. 34(20), 3092–3094 (2009). 13. Q. Wang, and J. Yao, “Approach to all-optical bipolar direct-sequence ultrawideband coding,” Opt. Lett. 33(9), 1017–1019 (2008). 14. Y. Dai, and J. Yao, “High-chip-count UWB bi-phase coding for multi-user UWB-over-fiber system,” J. Lightwave Technol. 27(11), 1448–1453 (2009). 15. P. Ou, Y. Zhang, and C.-X. Zhang, “Optical generation of binary-phase-coded, direct-sequence ultra-wideband signals by polarization modulation and FBG-based multi-channel frequency discriminator,” Opt. Express 16(7), 5130–5135 (2008). 16. E. Hamidi, and A. M. Weiner, “Phase-Only Matched Filtering of Ultrawideband Arbitrary Microwave Waveforms via Optical Pulse Shaping,” J. Lightwave Technol. 26(15), 2355–2363 (2008). 17. I. S. Lin, and A. M. Weiner, “Selective Correlation Detection of Photonically Generated Ultrawideband RF Signals,” J. Lightwave Technol. 26(15), 2692–2699 (2008).

#144121 - $15.00 USD Received 16 Mar 2011; revised 20 Apr 2011; accepted 23 Apr 2011; published 13 May 2011

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23 May 2011 / Vol. 19, No. 11 / OPTICS EXPRESS 10587

18. F. G. David, S. Reza, A. F. Mark, and L. G. Alex, “All-Optical Correlator for High-Speed OOK and DPSK Signals,” in Proceedings of COTA/ICQI, CMC3, (2008).

1. Introduction Ultra-wideband (UWB) systems specified by the Federal Communications Commission (FCC) can provide large bandwidth and high data rate under unlicensed spectrum from 3.1 to 10.6GHz [1]. However, UWB signals can only transmit for short distance due to the extremely low radiation power less than 41.3dBm/MHz. In order to increase the area of coverage, it is desirable that the local UWB environments would be integrated into wired or wireless wide-range networks with UWB-over-fiber technology [2,3]. Therefore photonic manipulation (including generation, transmission, modulation, and encoding) of UWB signals is very attractive since it can be easily incorporated into UWB-over-fiber networks and eventually simplify the entire network [4–14]. Generally, UWB signals can be modulated by on-off keying (OOK), pulse position modulation, or binary phase-shift keying (BPSK). However BPSK signals give extra 3 dB in signal-noise ratio for any given noise level, and are of special interest for UWB-over-fiber systems. To date, several schemes have been proposed for generating BPSK by using cascaded fiber Bragg gratings and multi-laser sources [13,15], optical bandpass filters [12], polarization maintaining fibers associated with a polarizer [14], and relaxation oscillations of an optically injected distributed feedback laser [6]. Especially, Ref [14]. presented a highchip-count phase coding using only one laser source with low costs. These schemes showed promising applications in code division multiple access (CDMA) technology for multiple user communications. However, all the phase-coded UWB signals were controlled by electrical data patterns, not by optical pulses. In this paper, we present an incoherent approach to all-optical generation of BPSK UWB signals. The binary phase coding is performed based on all-optical phase modulation in a semiconductor optical amplifier (SOA) and phase modulation to intensity modulation (PMIM) conversion in a fiber delay interferometer (DI) that serves as a multichannel frequency discriminator. By locating multiple phase-modulated light waves at the opposite slopes of the DI transmission spectra, a monocycle sequence with opposite polarities would be generated. We also experimentally demonstrate a CDMA-UWB coding system with a code length of 4, operating at 1.25 Gb/s. And a decoding approach is proposed and demonstrated as well. Different from the schemes mentioned above, the optical CDMA coding is controlled by optical data streams rather than electrical patterns, revealing better compatibility to all-optical networks. 2. Operation principle The schematic of the proposed CDMA-UWB encoding system is shown in Fig. 1. The continuous waves (CW) from a laser array with four wavelengths (λ1~λ4) are combined by an arrayed waveguide grating (AWG) and injected into a nonlinear SOA. Another optical Gaussian signal s(t ) with its wavelength λs, as a control light, is fed into the SOA to arouse cross phase modulation (XPM). The SOA serves as an all-optical phase modulator. Then the phase-modulated light waves are sent to the DI with each wavelength being located at the positive or negative slope of the DI periodical spectra. The DI serves as a multichannel frequency discriminator to achieve PM-IM conversion. Consequently, a pair of AWGs is used to separate each wavelength, delay for each channel, and recombine each channel to form binary phase-coded pulses.


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BPG

ATT

λs

LD0

LD1 LD2 LD3 LD4

OSA

λ1 MZM

λ2

SOA ATT

ODL

DCA

λ3

OC

λ1 λ2 λ3 λ4

DI

λ4

λ3 λ4

EDFA

λ2

λ1

PD

AWG

AWG

ESA

AWG

ODL AWG

Time

Fig. 1. Experimental setup of the CDMA-UWB coding system

Due to cross phase modulation (XPM) effect in the SOA, the optical field of CW probe signals after modulation can be expressed as

Ei (t )  exp  ji t  j  s (t )   0  ,

(1)

where ωi (i = 1, 2, 3, 4) is the angular frequency of the CW light waves, and Φ0 is the initial phase. β is the phase-modulated index. Then the modulated light is fed to DI with an approximate linear frequency response H ( )  K (  0 ) , where ω is the optical frequency variable,  0 is the optical frequency notch, K denotes the filter slope. The output signal of the linear filter in frequency domain is given by Eout ( )  H ( ) Ei ( ) , where E(ω) is the Fourier transform of E(t). Applying the inverse Fourier transform, the output probe light in time domain can be described by

Eout (t )  [ K (i  0 )  K  s(t ) t ]Ei (t ). (2) The detected optical power is given by Pout (t ) | Eout (t ) |2 . From Eq. (2), we can deduce the output power in the form with (3) Pout (t )  K 2 (i  0 )2  K 2  2 (s(t ) t )2  2K 2  (i  0 ) s(t ) t . The first term on the right-hand side (RHS) in Eq. (3) is a DC term. If the optical carrier is located at the filter’s linear slope region with a small phase-modulation index, the third term on the RHS of Eq. (3) has much larger magnitude than the second term, so the second term can be neglected. Then the detected optical power is simplified by

Pout (t )  2 K 2 (i  0 )  s(t ) t   DC ,

(4)

where  DC is a DC term. One can see that the output power is the first order derivative of input signals. Since s(t ) is a Gaussian pulse train, the output power of each channel should be Gaussian monocycle pulse. Moreover, a pair of polarity-reversed UWB monocycle pulses can be obtained, dependent on whether i  0  0 or not. A Gaussian monocycle and its inverted version represent a “+1” and a “-1”, respectively in the UWB coding system. And each input Gaussian pulse is mapped to four Gaussian monocycle pulses with binary codes ( + 1 and 1). 3. Experimental demonstration and discussion Based on the theory of CDMA coding, the code length should be at least N to accommodate N users. As a demonstration of multi-user communications, an all-optical bipolar UWB encoding system with a code length of 4 is experimentally investigated. The experimental setup for CDMA-UWB encoding system is also shown in Fig. 1. Four tunable laser diodes (LD1~LD4) are used as the laser array, whose wavelengths are 1554.1nm (CH1), 1555.75nm


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(CH2), 1558.17nm (CH3), and 1559.56nm (CH4), respectively. The wavelengths can be fine tuning with a resolution of 1pm. This is important to be well aligned to the DI spectrum slopes. The optical Gaussian pulse train is generated from a Mach-Zehnder modulator (MZM) driven by a bit pattern generator (BPG). The signal wavelength is fixed at 1563.5 nm. The repetition rate and the pulsewidth of the Gaussian pulses are fixed at 1.25GHz and 60ps, respectively. A commercial SOA (manufactured by CIP) is used for nonlinear optical signal processing with low polarization dependence (