A novel tunable multiband ultra-wide band (MB-UWB) pulses generation method using synchronously-apodized polarization modulation and birefringence time delay is proposed and demonstrated in this letter. Proper apodization profile is used to get MB-UWB pulses with spectra sidelobes suppression over 20dB. Five bands of MB-UWB pulses are generated by tuning the modulation frequency and transmitted over wireless channel. The central frequencies are from 4GHz to 6GHz and bandwidths larger than 500MHz. This method can be used in multiband modulation UWB over fiber systems.
© 2008 Optical Society of America
Ultra-Wide-Band (UWB) is recently emerging as a solution for future wideband personal access networks (PANs) –. It has many advantages compared to traditional wireless communication technologies, such as low power consumption, high bit rate, immunity to multipath fading and so on –. With the radio-over-fiber technology improvement, UWB over fiber can be a candidate solution for future wideband access networks with the advantage of low loss, high linearity, etc . So it’s greatly helpful to generate UWB pulses in optical domain. In UWB system, there are two main signaling formats, single-band and multi-band UWB (MB-UWB) waveforms. Compared to single-band UWB, multi-band UWB waveforms utilize overlapping groups of UWB signals such that each has a bandwidth of approximately 500MHz. This format ensures adherence to the FCC (The Federal Communications Commission) minimum bandwidth requirements and allows efficient utilization of the available spectrum . Recently, there are many excellent schemes to be carried out to optically generate and distribute the monocycle and doublet pulses in single band –, which show a good prospect for future UWB over fiber applications. However, up to now, there have been few attempts in generating MB-UWB pulse in optical domain.
In this letter, a simple method to optically generate MB-UWB pulses based on synchronous apodization and birefringence time delay is proposed and demonstrated. Electrical pulses are used to modulate optical signal so as to get polarity-reverse optical pulses pattern in orthogonal polarization orientation. The optical signal is then modulated with another apodization pulse and fed into a section of polarization-maintaining fiber (PMF). When the two polarity-reverse optical pulses are launched along two principle axes respectively, the birefringence time delay will help to generate the desired pulses. In experiment, we get five bands of MB-UWB pulses with different central frequency from 4GHz to 6GHz and a bandwidth larger than 500MHz.
The proposed method is based on cross-polarization modulation and the main principle is shown in Fig. 1. The LiNbO3 phase modulator (PM) can be used as a polarization modulator as long as the input continuous-wave (CW) is launched at 45° relative to the principle axis. The main idea of polarization modulation and differential group delay (DGD) method is to generate a positive pulse in one polarization and a negative pulse in the other polarization by polarization modulation via RF pulses. The DGD stage redistributes this energy in time domain resulting in a time delay between the positive and negative pulses resulting in a monocycle pulse after O/E conversion since the photo detector only integrates the energy in time independent of polarization .
The signal after first PM driven by a pulse train can be expressed as:
where M is the bit length, Bk(t) is binary code of k-th bit, P(t) is one pulse shape and êout is the unit vector along the output electric field polarization. When P(t)=0, êout·êin=1, and êout·êin=0 when P(t)=Vπ. At this time, every pulse has identical amplitude. The output signals of PM1 are fed into second PM with same polarization orientation. PM2 is driven by an apodization pulse Ap(t) synchronized with driver signal of PM1. The input orientation of PM2 is kept the same as the output of PM1. When the cross polarization pulse train is fed into PM2, the apodization RF pulses are used to modulate them again. Due to the same principle as PM1, the output signal of PM2 can be expressed as:
Thus, after two cascaded phase modulators, the optical carrier is cross polarization modulated with apodized pulse train. If the driving bit pattern is “01” interleaved and after DGD component with time delay of one bit duration, the received signal can be expressed as:
where D 0 is direct current component, Ap(t) is apodization pulse shape, N is bit number of generated pulse pattern, T is bit duration and P(t) is one pulse shape in the pulse train. In Fig. 1, the final received signal can be seen as apodized monocycle pulse pattern with N=5. This kind of pulse has narrow bandwidth than single monocycle or doublet pulses and can be used in MB-UWB systems. The apodization pulse determines the pulse duration and shape of generated pulses. And this scheme has simple structure to realize.
3. Experiments and results
Experimental setup of our proposed scheme is shown in Fig. 2. The laser wavelength is 1550nm. Electrical pulses from pulse pattern generator (PPG: Advantest D3186) triggered with tunable microwave oscillator are used to drive an optical phase modulator (PM) and keep the peak-to-peak level to Vπ of PM. The linearly-polarized light is launched at 45° relative to the principal axis of PM. The driver electrical signal is fixed pattern of “1010101010101010101010101010101” (32 bits with “1” and “0” interleaved in 1024 bits period) with trigger frequency tunable from 8GHz to 12GHz. So the output of PM1 is orthogonal polarization pulse pattern with uniform amplitude. The second PM is driven by synchronous apodization pulse to adjust the amplitude profile of orthogonal pulse pattern. The trigger frequency of apodization pulse is 1/32 of microwave oscillator with bit pattern “01” interleaved. A low pass filter is placed after PPG2 with narrow bandwidth in order to reshape the apodization pulse. In fact, the signal out of PPG2 is square wave, and after low pass filter it will be reshaped to tailored pulse with gentle rising and falling edges. In the experiment, two filters are used with bandwidth of 300MHz and 200MHz to test performance of different apodization pulse shapes. The output orientation of PM2 is adjusted by a polarization controller (PC) and fed into a section of PMF with length of 90m after an EDFA. The beat length of PMF is 3.8mm and the time delay of two principle state of polarization (PSP) is about 120ps. After PMF, the optical signals are sent to the photo detector (PD) to become apodized monocycle pulse pattern. A wideband antenna is placed after PD without any electrical amplifier. And after wireless transmission, the UWB signal is received by another antenna and measured by a digital sampling oscillator (DSO, Tektronix TDS8200) and electrical spectrum analyzer (ESA, Agilent E4446A).
First, we measured spectra of different apodized monocycle pulse patterns with trigger frequency of 12GHz. From Fig. 3, one can see that the monocycle pulses pattern without apodization has a peak at 6GHz in the spectrum with obvious low frequency part (from 1 to 4GHz) shown by dotted line. When 300MHz low pass filter is used, the monocycle pulse pattern is trapezium apodized with low frequency part suppressed over 5dB shown by dash line. And if 200MHz low pass filter is used, the apodization pulse is triangle-like, the final spectrum is quite well with greatly suppressed low frequency components shown by solid line in Fig. 2. The -10dB and -20dB bandwidths are 680MHz and 2GHz respectively which will cause little interference with adjacent channels in MB-UWB systems.
Secondly, with tunable microwave trigger source, different bands of MB-UWB pulses are generated and transmitted. We changed the trigger frequency from 8GHz to 12GHz and the low pass filter bandwidth is 200MHz. Figure 4 shows the time profiles of different frequency MB-UWB pulses before antenna and Fig. 5 shows the corresponding spectra. The generated MB-UWB pulses are all with tailored apodization. There are obvious rising and falling edges in pulse duration. From Fig. 5, one can see that five bands of MB-UWB are generated in FCC mask by tuning the trigger frequency. Bandwidth of each sub-band is larger than 500MHz which is in line with FCC definition. And the bandwidth is increasing with trigger frequency which is mainly because the pulse duration is shorter with higher frequency. The sidelobes are suppressed over 20dB for all sub-bands.
The MB-UWB pulses after wireless link and receiver antenna are shown in Fig. 6 and the spectra are shown in Fig. 7. There are some distortions after wireless transmission. And the spectra also show the distortions. There are obvious lobes in the range between 1GHz to 2GHz which are mainly because the imperfection emission spectra of the lab-made taper-slot antennas used in our experiment. We believe that the spectral response will be better with commercial UWB antennas.
Finally, this method can give us some useful discussions. This method is simple in principle and can be realized by integrated devices. The pulse train can be easily apodized by an apodization pulse triggered with frequency divider and shaped by filter or other devices. The center frequency and bandwidth of generated pulse is determined by trigger frequency and pulse duration respectively, which are pulse duration of P(t) and the number N in formula 3 as long as the modulator bandwidth is large enough. The sidelobe suppression is the contribution of apodization shape, which is Ap(t) in formula 3. So it’s easy and possible to tune the center frequency and bandwidth of sub-bands.
Due to the low power density of UWB pulse, the optical signal can be just detected by a photo detector with a wideband antenna and there is no need for a microwave amplifier, which will greatly simplify the structure of remote node. The received signal quality is not only determined by optical pulse generation method but also the wireless channel and antenna couple performance. In our experiment, the wireless link is kept at 1m, and the received pulses get worse with the distance increasing.
The scheme proposed in this letter utilizes both synchronous apodized polarization modulation and birefringence time delay to generate MB-UWB pulses. The monocycle pulse pattern is properly apodized to generate MB-UWB pulse with high sidelobe suppression over 20dB. Five bands of MB-UWB pulses are generated by tuning the trigger frequency and transmitted over wireless link with central frequency from 4GHz to 6GHz and bandwidth larger than 500MHz. This method can be easily realized and used in future multiband modulation UWB over fiber systems.
The authors would like to thank Dr. W.H. Chen for his help in providing the taper-slot antennas.
This work is supported by Tsinghua Basic Research Fund under Grant JC2007020, NSFC under Contract 60736002, National 863 Program of China under Contract 2007AA01Z264, National Basic Research Program of China (973 Program) under Contract 2006CB302806 and Open Fund of Key Laboratory of Optical Communication and Lightwave Technologies (Beijing University of Posts and Telecommunications), Ministry of Education, P. R. China.
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