Optical generation of binary-phase-coded, direct ... - OSA Publishing

Optical generation of binary-phase-coded, direct-sequence ultra-wideband signals by polarization modulation and FBG-based multichannel frequency discriminator Pan Ou 1*, Ye Zhang 2, and Chun-Xi Zhang 1 1

School of Instrument Science and Opto-electronics Engineering, Beihang University, Beijing, 100083, P.R.China 2 Department of Electronic Engineering, Tsinghua University, Beijing 100084, P.R. China * Corresponding author: [email protected]

Abstract: In this paper a novel optical generation approach for binaryphase-coded, direct-sequence ultra-wideband (UWB) signals is experimentally demonstrated. Our system consists of a laser array, a polarization modulator (PolM), a fiber Bragg grating (FBG), a length of single mode fiber, and a photo detector (PD). The FBG, designed based on the superimposed, chirped grating, is used as the multi-channel frequency discriminator. The input electronic Gaussian pulse is modulated on the optical carrier by the PolM and then converted into UWB monocycle or doublet pulses sequence by the multi-channel frequency discriminator. The PolM is used so that the desired binary phase code pattern could be simply selected by adjusting the polarization state of each laser, rather than tuning the laser wavelengths. The desired UWB shape, monocycle or doublet, could be selected by tuning the FBG. Based on our proposed approach, four-chip, binary-phase-coded, DS-UWB sequences with different pulse shapes and code patterns are experimentally demonstrated. The impact of the fiber dispersion on the generated UWB pulses is also discussed in our paper. ©2008 Optical Society of America OCIS codes: (350.4010) Microwaves; (060.0060) Fiber optics and optical communications

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G. R. Aiello and G. D. Rogerson, “Ultra-wideband wireless systems,” IEEE Microw. Mag. 4, 36-47 (2003). H. Arslan, Z. N. Chen, and M. Benedetto, Ultra Wideband Wireless Communication. (Hoboken, NJ: Wiley, 2006). W. P. Lin and J. Y. Chen, “Implementation of a new ultrawide-band impulse system,” IEEE Photon. Technol. Lett. 17, 2418-2420 (2005). Y. Kim, S. Kim, H. Jang, S. Hur, J. Lee, and J. Jeong, “Performance evaluation for UWB signal transmissions in the distributed multi-cell environment using ROF technology,” in Proceedings of IEEE Conference on International Topical Meeting on Microwave Photonics (2005), pp.173-176. H. Chen, M. Chen, Z. Jian, and S. Xie, “UWB monocycle and doublet pulses generation in optical domain,” in Proceedings of IEEE Conference on International Topical Meeting on Microwave Photonics (2007), pp.145-148. H. Chen, M. Chen, C. Qiu, and S. Xie, “A novel composite method for ultra-wideband doublet pulses generation,” IEEE Photon. Technol. Lett. 19, 2021-2023 (2007). J. D. McKinney and A. M. Weiner, “Compensation of the effects of antenna dispersion on UWB waveforms via optical pulse-shaping techniques,” IEEE Trans. Microwave Theory Tech. 54, 1681-1686 (2006). S. Xiao and A. M. Weiner, “Coherent Fourier transform electrical pulse shaping,” Opt. Express 14, 30733082 (2006), http://www.opticsinfobase.org/abstract.cfm?URI=oe-14-7-3073 C. Wang, F. Zeng, and J. Yao, “All-Fiber Ultrawideband pulse generation based on spectral shaping and dispersion-induced frequency-to-time conversion,” IEEE Photon. Technol. Lett. 19, 137-139 (2007). F. Zeng and J. Yao, “An approach to ultrawideband pulse generation and distribution over optical fiber,” IEEE Photon. Technol. Lett. 18, 823-825 (2006).

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Received 13 Dec 2007; revised 11 Mar 2008; accepted 17 Mar 2008; published 28 Mar 2008

31 March 2008 / Vol. 16, No. 7 / OPTICS EXPRESS 5130

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F. Zeng and J. Yao, “Ultrawideband impulse radio signal generation using a high-speed electrooptic phase modulator and a fiber-Bragg-grating-based frequency discriminator,” IEEE Photon. Technol. Lett. 18, 2062-2064 (2006). Q. Wang and J. P. Yao, “Switchable optical UWB monocycle and doublet generation using a reconfigurable photonic microwave delay-line filter,” Opt. Express 15, 14667-14672 (2007), http://www.opticsinfobase.org/abstract.cfm?URI=oe-15-22-14667 Y. Dai and J. Yao, “An Approach to Optical Generation and Distribution of Binary Phase Coded Direct Sequence Ultra-Wideband Signals,” in Proceedings of IEEE Conference on International Topical Meeting on Microwave Photonics (2007), pp.173-176. J. Capmany, B. Ortega, D. Pastor, and S. Sales, “Discrete-time optical processing of microwave signals,” J. Lightwave Technol. 23, 702-723 (2005). R. Slavik, I. Castonguay, S. La Rochelle, and S. Doucet, “Short multiwavelength fiber laser made of a large-band distributed Fabry-Perot structure,” IEEE Photon. Technol. Lett. 16, 1017-1019 (2004).

1. Introduction Ultra-wideband (UWB) technology has attracted considerable interests in recent years, especially for future wideband personal access networks [1, 2]. UWB is approved by the Federal Communications Commission (FCC) for unlicensed use for a spectrum range from 3.1 GHz to 10.6 GHz with a spectral power density less than -42 dBm/MHz. The impulse UWB communications have some unique advantages, such as low power consumption, high data-rate, and immunity to multi-path fading. However, the traditional UWB technology has some disadvantages for wide and low-cost uses in the near future. First, the traditional UWB signals are confined within a short range because of the nature of the UWB transmission property. Then there are difficulties when one want to integrate the local UWB environments into a wired or wireless wide-rang network. Second, with the current stage of technology, it is rather hard and expensive to generate a UWB pulse with a fractional bandwidth even greater that 100% at the central frequency of about 7GHz [3]. Recently the UWB-over-fiber technology has been developed because the novel technology is considered to have the ability to solve the above problems [4]. In the UWBover-fiber technology, the UWB signals are desired to be generated directly in the optical domain, and then be transmitted along the fiber and sent to the receiver front-end. Many optical generation approaches for UWB signals have been successfully demonstrated. For example, in [5, 6] the electrical Gaussian pulses are converted to the UWB pulses by a differential group delay device as well as a semiconductor optical amplifier (SOA) due to the gain saturation and recovery mechanism in SOA. In [7, 8] the UWB pulses are generated by optical waveform synthesis setups where the Fourier transform optical pulse shaping technologies are used. The optical spectral shaping and frequency-to-time conversion technique were also used for UWB all-optical generation [9]. Authors in [10-12] demonstrated a relatively simpler approach, which is named phase modulation to intensity modulation (PMIM) conversion technology, for the UWB monocycle and doublet pulse generation. However, all the above approaches focus on the technology for single user communications. For practical applications, the UWB-over-fiber technology has to consider the multiple users communications. In [13] a novel UWB-over-fiber technology, involving the direct-sequence code division multiple access (DS-CDMA) which has been considered as a potential technology to solve the wireless UWB multiple users communications, is proposed and demonstrated. A binaryphase-coded, DS-UWB sequence can be optically generated by a tunable laser array and a multi-channel UWB pulse shaper. However, in the demonstrated approach a tunable laser array is used, which may be expensive for practical applications. In this paper, we will propose and demonstrate a new optical generation approach for binary-phase-coded, DSUWB sequence to solve the multiple users communications problems in current UWB-overfiber technology. In our approach, the proposed DS-UWB sequence generator consists of a laser array with fix wavelengths, a polarization modulator (PolM), a fiber Bragg grating (FBG), a length of fiber, and a photo detector (PD). Because of the use of the PolM, the phase #90849 - $15.00 USD

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code pattern of the generated UWB sequence can be tuned by adjusting the polarization of each laser in the laser array, rather than by tuning the laser wavelength. Besides, by tuning the FBG, the generated UWB pulse shape could be monocycle or doublet. In our paper the theory of our approach is presented in detail. Four-chip, binary-phase-coded, DS-UWB sequences with different pulse shape and code patterns are experimentally demonstrated as an example. 2. Principle UWB sequence with desired phase code pattern could be generated from an input pulse using the following sequence generator illustrated in Fig. 1. The scheme is similar to a finite impulse response (FIR) filter, containing a multiple-tap structure, time delay for each tap, and signal summing process. Binary phase coding is introduced by positive and negative taps according to the desired phase pattern. Different from a regular FIR filter, a UWB pulse shaper is needed in the system to convert the combined phase coded pulse sequence into UWB pulses. Actually a UWB pulse shaper is a differentiator. If a Gaussian pulse is input to the system, monocycle or doublet sequence can be generated by use of a first or second order differentiator, respectively. Obviously the same code pattern as that of the taps can be obtained in the UWB pulse sequence after the differentiator. Based on the above principle, two techniques are needed in the UWB sequence generator. First is the generation of positive and negative taps, which has been deeply investigated in microwave photonics filters [14]. The other is the UWB pulse shaper, or differentiator.

Fig. 1. Phase coding DS-UWB sequence generator.

In this paper we propose a polarization modulator (PolM) based microwave photonics delay line structure for phase coding, and a fiber Bragg grating (FBG) based multi-channel frequency discriminator for UWB pulse shaper, which is illustrated in Fig. 2. N CW lasers are coupled and then modulated at a PolM by Gaussian pulses. The polarization state of each laser can be adjusted by a polarization controller (PC). The modulated lights are reflected by an FBG-based multi-channel filter which acts as a first or second order differentiator for the input Gaussian pulse. A length of fiber provides different time delays for each wavelength (tap). The output is received by a photo diode (PD). The PolM used in the generator is actually two phase modulators with opposite modulation index at two orthogonal principle states of polarization (PSPs), i.e., if the polarization state of the input light is parallel to one of the PSPs, the PolM acts just like a phase modulator and the sign (positive or negative) of the modulation index depends on the PSP, as shown in Fig. 1(a). Then the required FIR structure with positive and negative taps can be easily achieved by adjusting the polarization state of each laser. Since multiple lasers are used, a multi-channel filter is then required as a multi-channel frequency discriminator, which is an FBG in our system. The proposed grating is the superposition of two identical chirped FBG with however different position along the fiber. The two chirped FBG forms a distributed Fabry-Perot filter [15], and the channel spacing, Δλ, is determined by the distance of the two chirped FBG, ΔL

Δλ =

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λ2 2neff ΔL

(1)



where λ is the mean Bragg wavelength of the FBG, and neff is the effective refractive index of the fiber. For example, with ordinary fiber (neff=1.455) Δλ=0.4 nm (corresponding to 50 GHz) can be obtained if the distance of the two superimposed grating is 2 mm. The total bandwidth of the FBG is equal to one of the identical chirped FBGs, which can be large if the chirp of the grating is large or the grating length is long. Gratings fabricated with commercial chirped phase masks can have bandwidth larger than 30 nm. So such superimposed chirped FBG is applicable even if the desired UWB sequence is long and high channel count is required in the UWB sequence generator.

Fig. 2. (a) Phase coded UWB sequence generator based on PolM and FBG-based multi-channel filter. Phase coding is realized by adjusting the polarization state of each laser. The lasers are CW lasers and polarization-modulated at the PolM by Gaussian pulses. (b) Monocycle and doublet sequence can be generated by tuning the spectrum of the FBG.

3. Experiment

In our experiment the multi-channel FBG is fabricated by a frequency-doubled Argon laser and a chirped phase mask with chirp coefficient of 0.8 nm/cm. The grating length is about 5cm. ΔL is 2 mm corresponding to the channel spacing of 50 GHz. The fabricated FBG is measured and the result is plotted in Fig. 3. Four lasers are used in our experiment, and the wavelength spacing is set to 300 GHz. Then one of every six channels of the FBG is used as the UWB shaper. An electronic Gaussian pulse with full width at half maximum (FWHM) about 60 ps is generated by a bit error rate tester (BERT) and is input to the PolM. The length of the single mode fiber is 10 km. Then the time delay between two adjacent UWB pulses in one sequence is about 400ps. The output of the PD is measured by a high-speed sampling oscilloscope. By tuning the FBG, the wavelengths of the four lasers can be located at the linear or quadratic slopes of the multichannel filter, then monocycle and doublet sequences are generated respectively. By adjusting the PCs to parallel the polarization state of each laser to one of the PSPs of the PolM, different code patterns can be obtained. The measured results are plotted in Fig. 4. Code patterns with {0, 0, 0, 0}, {0, 0, π, 0}, and {0, π, π, 0} are obtained.

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Reflectivity (dB)

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Fig. 4. Phase coded monocycle (up) and doublet (down) sequences with different code patterns are measured in time domain. The code patterns from (a) to (c) are {0 0 0 0}, {0 π π 0}, and {0 0 π 0}, respectively. -40

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Fig. 5. Spectra of generated monocycle (left) and doublet (right) pulses.

To measure the spectra of the generated UWB pulses, we polarization modulate one of the four lasers by a 13.5 Gb/s pseudorandom binary sequence (PRBS) signal (generated by BERT) [10, 13]. The output of the PD is measured by an electrical spectrum analyzer (ESA). The measured monocycle and doublet spectra are plotted in Fig. 5. 4. Discussion

In our demonstration a length of single mode fiber is used to provide the required group delay for each UWB pulse. Though the group velocity dispersion provides the time delay for each tap, unwanted dispersion within each channel could show negative impact on the generated UWB sequence. We take the generation of monocycle sequence as an example. It has been #90849 - $15.00 USD

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analyzed in Ref. [10] that because of the PM-IM conversion, the dispersion of the fiber acts as a second order differentiator on the phase modulated signal, i.e. if there is no FBG, the output of the PD will be a doublet pulse if the PolM is derived by a Gaussian pulse. If the fiber is lengthened, more phase modulation will be converted to the intensity modulation and the generated doublet pulse will be stronger. When the phase modulated light passes through both the FBG and the fiber two kinds of PM-IM conversions (to monocycle and to doublet) will both occur, and the pulse received by the PD is then the superposition of monocycle and doublet. If the fiber is long, a doublet-like pulse will be generated instead of a monocycle. Fig. 6 shows our simulation.

Amplitude (a.u.)

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Time (ps) Fig. 6. The distortion of generated monocycle pulse because of the dispersion of fiber. The monocycle pulse is changed to a doublet-like pulse when the fiber is lengthened.

In simulation, the FWHM of the Gaussian pulse is 60 ps. The phase modulation depth is The superimposed FBG is simulated by transmission matrix method with the same parameters as our design for experiment. The output pulses from PD corresponding to fiber length of 0 to 80 km are calculated. Clearly a doublet pulse is added on the desired monocycle. When the fiber is 80 km, a doublet-like pulse is generated. When the doublet sequence is wanted, both PM-IM conversions generate doublet pulses, and the pulse received by the PD is then the superposition of two doublets. Since the two doublets are generated from the same phase-modulated pulse, such superposition results in one doublet. So the fiberdispersion-induced distortion is much smaller. In our experiment a 10 km fiber is used, the pulse distortion could be ignored, which has been proved by the experiment.

π/2.

5. Conclusion

In this letter a phase coded UWB sequence generator containing a PolM and an FBG-based multi-channel frequency discriminator was demonstrated. Monocycle or doublet sequence could be generated by tuning the FBG, and different code pattern could be achieved by adjusting the polarization state of each laser. Based on our proposed approach, four-chip, binary-phase-coded, DS-UWB sequences with different pulse shape and code patterns were experimentally demonstrated. The proposed approach has also the ability to generate longchip-count DS-UWB sequence. Compared with the previously demonstrated DS-UWB phase encoder in Ref. [13], our approach has the advantage that it requires no tunable laser array. In our paper the impact of the fiber dispersion on the generated monocycle UWB pulse was studied. We have shown that in our approach the impact of the dispersion could be ignored if the fiber length is less than 20 km, which is generally long enough to connect the base station and the central office.

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