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RETRACTED — Photonic generation of UWB pulses with multiple modulation formats
Optics & Laser Technology 45 (2013) 342–347
Contents lists available at SciVerse ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Photonic generation of UWB pulses with multiple modulation formats Yang Chen, Aijun Wen n, Lei Shang, Yong Wang
TE D
State Key Laboratory of Integrated Services Networks, Xidian University, Xi’an 710071, PR China
a b s t r a c t
Article history: Received 6 April 2012 Received in revised form 4 June 2012 Accepted 20 June 2012 Available online 18 July 2012
A new photonic approach to generate ultra-wideband (UWB) signals with on–off keying, pulse position and shape modulation is proposed and proved by simulation. The proposed system consists of two parallel similar subsystems, which are both made up of an intensity modulator (IM) and a dual-drive Mach–Zehnder modulator (DD-MZM). The optical signal is injected into the two subsystems with equal intensity and phase. The IMs are driven by the data signal, while the DD-MZMs are driven by the Gaussian pulse train. By properly adjusting the bias points of the IMs and DD-MZMs, the amplitudes of the data signals, and the time delays introduced by electrical delay lines, a position-modulated UWB monocycle, a shape-modulated UWB monocycle and doublet, and an on–off keying UWB monocycle can be generated. The fractional bandwidth of the generated UWB monocycle and UWB doublet are 171% and 150%, respectively. & 2012 Elsevier Ltd. All rights reserved.
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Keywords: Ultra-wideband (UWB) Monocycle and doublet pulse Modulation formats
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a r t i c l e i n f o
1. Introduction
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UWB, which is regulated by the Federal Communications Commission (FCC) for applications such as short-range broadband wireless communications and broadband sensor networks operating in the frequency range from 3.1 to 10.6 GHz, has been a topic of interest recently due to its inherent properties such as high data rate, low power spectral density, extremely short time duration, and immunity to multi-path fading [1,2]. Based on the FCC definition, UWB signals should have a fractional bandwidth more than 20% or a 10 dB bandwidth of at least 500 MHz [3]. The UWB frequency mask defined by the FCC is shown in Fig. 1. As can be seen, the frequency band assigned to UWB indoor communications systems extends from 3.1 GHz to 10.6 GHz, with a bandwidth of 7.5 GHz centered around 7 GHz. The power density is limited to 41.3 dBm/MHz. To offer availability of undisrupted service across different networks and to eventually achieve high-data-rate access at any time and from any place, UWB combined with fiber transmission, i.e., a technology called UWB over fiber, can provide an effective solution [4,5]. In general, there are two types of UWB: the direct sequence UWB [6] and the multiband UWB [7]. Direct sequence impulse radio is one of the most attractive techniques for UWB communications as it is carrier free, so there is no need for complicated frequency mixers and local oscillators to convert the carrier frequency. Conventionally, direct sequence UWB signals, usually
n
Corresponding author. Tel./fax: þ 86 29 88204468. E-mail address: [email protected] (A. Wen).
0030-3992/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlastec.2012.06.026
Gaussian monocycle or doublet pulse, are generated using electronic circuits. Recently, the generation of UWB signals in the optical domain has attracted a lot of interests, which is a better choice since there is no need for extra electrical-to-optical conversion. In addition, the generation of UWB signals in the optical domain provides other features such as light weight, small size, large tunability, and immunity to electromagnetic interference. In the past few years, various techniques for UWB pulse generation were reported. In Ref. [8], UWB pulses were generated using a high-speed electro-optic phase modulator and a fiberBragg grating-based frequency discriminator. In Refs. [9,10], UWB pulses were generated based on spectral-shaping and dispersioninduced frequency-to-time conversion. In Ref. [11], the nonlinear effect in a semiconductor optical amplifier was used for UWB pulse generation. In Refs. [12,13], UWB pulses were generated using external modulators. UWB signals in the optical domain based on SLEDs using active multi-mode interferometer were proposed in Refs. [14,15]. However, for a practical UWB over fiber communication system, the information must be encoded, which can be realized by using different pulse modulation schemes, such as on–off keying (OOK), pulse-position modulation (PPM), bi-phase modulation (BPM), pulse-amplitude modulation (PAM), and pulseshape modulation (PSM). All these modulation schemes can be implemented in the optical domain. In Ref. [16], all these modulation formats were realized based on a phase modulator (PM) and an electrical reconfigurable asymmetric Mach–Zehnder interferometer (AMZI). Ref. [17] proposed a novel method to generate UWB pulses with PPM. In Ref. [18], a DD-MZM and an AMZI were reconfigured to implement PPM, BPM, and PSM. In Ref. [19], UWB pulses with BPM and PSM were generated in the
Y. Chen et al. / Optics & Laser Technology 45 (2013) 342–347
2. Principle
2.1. UWB pulse with PPM As shown in Fig. 3, IM1 is biased at the maximum transmission point, while IM2 is biased at the minimum transmission point. The amplitudes of both the data signals applied to IM1 and IM2 are Vp, where Vp is the half-wave voltage of the IM. In this case, the output of IM1 will be just opposite to that of IM2. When the data signal is ‘‘0’’, the intensity of IM1 output will be maximum, and the intensity of IM2 output will be minimum. When the data signal is ‘‘1’’, the intensity of IM1 output will be minimum, and the intensity of IM2 output will be maximum. The optical signal switches between the two parallel IM–DD-MZM branches according to the data sequence. If Gaussian monocycles are generated with different time delays in DD-MZM1 and DD-MZM2 respectively, a UWB monocycle with PPM can be generated at the output of the PD. Assume that the electrical drive signals applied to DD-MZM1 and DD-MZM2 are V1(t) ¼V0j(t), V2(t)¼V0j(t þ t1), and V3(t) ¼V0j(t þ t2), V4(t)¼V0j(tþ t1 þ t2), where j(t) is a normalized Gaussian pulse, V0 is the amplitude, and t1 and t2 are the time delays introduced by the electrical delay lines. With an optical field of Ei(t)¼Ei exp(joct) injected, where oc is the angular frequency of the optical carrier, Ei is the amplitude, and taking the transmission curve shown in Fig. 3 into consideration, the optical field at the output of optical coupler can be expressed as E ðtÞ V 1 ðtÞ p V 2 ðtÞ þexp jp þj Eo1 ðtÞ ¼ i sðtÞ exp jp 2 2 Vp Vp Ei ðtÞ V 3 ðtÞ p V 4 ðtÞ ð1Þ þ þexp jp þj sðtÞ exp jp 2 2 Vp Vp
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The schematic diagram of the proposed UWB pulse generator is similar to a nested MZM, as shown in Fig. 2. It includes a main MZM and two IM–DD-MZM branches, and a static phase control electrode of the main MZM is driven by a DC bias to keep the two branches of the structure having the same optical delay. A lightwave emitted from a LD is divided into two parts with equal intensity and phase by an optical power splitter. The two outputs of the optical power splitter are sent to one of the two parallel IM–DD-MZM branches, respectively. The data signals are applied to the IMs, and the Gaussian pulses are applied to the DD-MZMs. The outputs of the two DD-MZMs are recombined by
an optical coupler after being modulated by a DC bias applied to the static phase control electrode of the main MZM, and then sent to a PD. A position-modulated UWB monocycle, a shape-modulated UWB monocycle and doublet, or an on–off keying UWB monocycle can be generated at the output of the PD with specific bias points of the IMs, amplitudes of the data signals, and time delays introduced by electrical delay lines.
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optical domain using a symmetric PM–IM conversion architecture. The schemes in Refs. [16–18] needed to control the polarization of the optical signal precisely, while the scheme in Ref. [19] employed a fiber Bragg grating (FBG) to implement PM–IM conversion. These may be unstable in practical applications. In this paper, we propose a simple and new UWB generator using two IMs and two DD-MZMs that can be reconfigured to implement PPM, PSM and OOK. By properly adjusting the bias points of IMs and DD-MZMs, the amplitudes of the data signals, and the time delays introduced by electrical delay lines, a position-modulated UWB monocycle, a shape-modulated UWB monocycle and doublet, or an on–off keying UWB monocycle can be generated. Our method is insensitive to the polarization of the optical signal, and does not need a FBG, which may be a new choice for UWB pulse modulation.
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Fig. 1. UWB frequency mask defined by the FCC for ultra-wideband indoor communications.
Fig. 3. Bias points and data signal amplitudes of IM1 and IM2 for PPM.
Fig. 2. Schematic of the optical UWB generator. LD: laser diode; IM: intensity modulator; DD-MZM: dual parallel Mach–Zehnder modulator; PD: photodetector.
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Y. Chen et al. / Optics & Laser Technology 45 (2013) 342–347
where s(t) is the 0–1 data sequence, and sðtÞ is its opposite data sequence, so sðtÞsðtÞ ¼ 0 establishes. In writing Eq. (1), the DD-MZM1 and DD-MZM2 are both biased at the quadrature transmission point. If the optical signal expressed in Eq. (1) is sent to a PD for square-law detection, the ac term of the photocurrent is
ac term of the photocurrent is 2
io2 ðtÞpsinb½jðt þ t1 ÞjðtÞsðtÞ cosb½jðt þ t1 þ t2 Þjðt þ t2 Þ sðtÞcosb½jðt þ t2 ÞjðtÞ þ sðtÞcosb½jðt þ t1 þ t2 ÞjðtÞ þ sðtÞsinb½jðt þ t2 Þjðt þ t1 ÞsðtÞsinb½jðt þ t1 þ t2 Þjðt þ t1 Þ
ð5Þ
2
ð2Þ For small-signal modulation, b is small, so we have sinb b. Taking characteristics of 0–1 data sequence into consideration, Eq. (2) is approximated as ( b½jðt þ t1 ÞjðtÞ sðtÞ ¼ 0 ð3Þ io1 ðtÞp b½jðt þ t1 þ t2 Þjðt þ t2 Þ sðtÞ ¼ 1 As can be seen, the output current is proportional to the firstorder difference of the input Gaussian signal. If t1 and t2 are sufficiently small, the first-order difference can be approximated as the first-order derivative. The entire system is equivalent to a first-order differentiator and a Gaussian monocycle is generated. Furthermore, Eq. (3) suggests that the position of the generated monocycle is different when the data signal is ‘‘0’’ and ‘‘1’’, so the UWB monocycle is position-modulated.
2.2. UWB pulse with PSM
For small-signal modulation, b is small, so we have sinb b and cosb 1. Taking characteristics of 0–1 data sequence into consideration, Eq. (5) is approximated as ( b½jðt þ t1 þ t2 Þjðt þ t1 Þjðt þ t2 Þ þ jðtÞ sðtÞ ¼ 0 io2 ðtÞp b½jðt þ t1 ÞjðtÞ sðtÞ ¼ 1 ð6Þ As can be seen, the output current is proportional to the firstorder difference of the input Gaussian signal when s(t)¼1, and to the second-order difference of the input Gaussian signal when s(t) ¼0. Again, if t1 and t2 are sufficiently small, the first- and second-order differences can be approximated as the first- and second-order derivatives. The entire system is equivalent to a first- and second-order differentiator when s(t) equals to 1 and 0 respectively. In this case, Gaussian monocycle and doublet can be generated according to the 0–1 sequence, so the UWB pulse is shape-modulated.
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io1 ðtÞpsðtÞ sinb½jðt þ t1 ÞjðtÞ þ sðtÞ2 sinb½jðt þ t1 þ t2 Þjðt þ t2 Þ
2.3. UWB pulse with OOK
Furthermore, the proposed scheme can be reconfigured to implement OOK. As discussed in Section 2.2, a shape-modulated UWB monocycle and doublet can be generated according to Eq. (6), where a UWB monocycle represents ‘‘1’’ and a UWB doublet represents ‘‘0’’. If the time delay t2 in Eq. (6) is changed to 0, Eq. (6) can be simplified as ( 0 sðtÞ ¼ 0 ð7Þ io3 ðtÞp b½jðt þ t1 ÞjðtÞ sðtÞ ¼ 1
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As shown in Fig. 4, IM1 and IM2 are both biased at the quadrature transmission point. The amplitude of the data signal applied to IM1 is Vp, while that applied to IM2 is Vp/2. In this case, the output of IM1 is a constant (half the maximum value). The output of IM2 is the same value when the data signal is ‘‘0’’, and the output of IM2 will be minimum when the data signal is ‘‘1’’. So the output of the optical coupler will be the sum of the two parallel IM–DD-MZM branches when the data signal is ‘‘0’’, and be only the output of IM1–DD-MZM branch when the data signal is ‘‘1’’. If these two cases have different waveforms, UWB pulses with PSM can be generated at the output of the PD. Taking the transmission curve shown in Fig. 4 into consideration, the optical field at the output of optical coupler can be expressed as pffiffiffi V 1 ðtÞ p V 2 ðtÞ 2Ei ðtÞ þexp jp þj exp jp Eo2 ðtÞ ¼ 2 4 Vp Vp pffiffiffi V 3 ðtÞ 3p V 4 ðtÞ p 2Ei ðtÞ þ þ exp jp sðtÞ exp jp þj þj 2 2 4 Vp Vp ð4Þ
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To simplify the analysis, we assume the extinction ratios of the DD-MZMs are infinite. In writing Eq. (4), the DD-MZM1 is biased at the quadrature transmission point, while the DD-MZM2 is biased at the minimum transmission point. If the optical signal expressed in Eq. (4) is sent to a PD for square-law detection, the
Fig. 4. Bias points and data signal amplitudes of IM1 and IM2 for PSM.
Again, if t1 is sufficiently small, the first-order difference can be approximated as the first-order derivative. The entire system is equivalent to a first-order differentiator. Eq. (7) suggests that when the data signal is ‘‘0’’, there is no electrical output signal, and when the data signal is ‘‘1’’, a UWB monocycle will be generated. So, OOK UWB pulses are generated.
3. Simulation verification and discussion Simulations are performed using VPItransmissionMaker 8.3 based on the setup shown in Fig. 2 to verify the UWB pulse generation and the implementation of PPM, PSM and OOK schemes. All the components shown in Fig. 2 can be found in the simulation software, and parameters of these components should be configured according to the descriptions in Section 2. Waveform, electrical and optical spectra can be observed by using the oscilloscope and spectrum analyzer of the simulation software. A lightwave with 10 MHz linewidth and 13 dBm power from a LD is sent to a 50/50 optical power splitter. The two outputs of the optical power splitter are sent to one of the two parallel IM–DDMZM branches, respectively. A data signal is split into two channels, which are used to drive the two IMs. A Gaussian pulse train, which has a repetition rate of 625 MHz and a full-width at half-maximum (FWHM) of about 85 ps, is split into four channels, and then, applied to the two DD-MZMs with different time delays via the RF ports. The time delays t1 and t2 are both 40 ps in this simulation for PPM and PSM, while for OOK, t1 and t2 are 40 ps and 0, respectively. The IMs and DD-MZMs used in this simulation have a half-wave voltage of 5 V, an insertion loss of 6 dB, and an
Y. Chen et al. / Optics & Laser Technology 45 (2013) 342–347
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UWB pulses, such as Ref. [16]. In general, the spectrum in Fig. 5(d) is more consistent with the FCC-specified indoor spectral mask compared with that in Fig. 5(b). However, a small irregularity does not influence the feasibility of the results, because in practical applications, the generated UWB pulses will pass a filter before they are sent to the UWB transmitting antenna to remove the unnecessary parts of the spectrum. The fractional bandwidth of the generated UWB pulses may decrease after passing the filter, but it is still much bigger than the FCC definition—20%. To verify the multiple modulation formats, a data signal with 625 Mbit/s data rate and a fixed pattern ‘‘10010110’’ is applied to the IMs. By adjusting the amplitudes of the data signal, the bias point of the IMs and DD-MZMs, and the time delays introduced by electrical delay lines as discussed in Section 2, PPM, PSM and OOK are realized. The waveforms of the modulated UWB signals are shown in Fig. 6. Fig. 6(a), (b) and (c) represents PPM, PSM and OOK respectively. As shown in Fig. 6(a), the time difference between ‘‘1 0’’ is about 1.55 to 1.56 ns, the time difference between ‘‘0 1’’ is about 1.64 to 1.65 ns, and the time differences between ‘‘0 0’’ and ‘‘1 1’’ are around 1.60 ns. These values are very consistent with Eq. (3) and the 40 ps time delay in the simulation. Fig. 6(b) shows that a UWB monocycle represents ‘‘1’’ and a UWB doublet represents ‘‘0’’, which is consistent with the results obtained from Eq. (6). Fig. 6(c) shows the OOK UWB monocycle, in which, a monocycle represents ‘‘1’’, and no signal represents ‘‘0’’. Compared with the waveforms in Refs. [16–19], the waveforms in Fig. 6 are very standard and do not have some parasitic pulses. This is because that in the simulation, some influence factors in practical
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extinction ratio of 30 dB. The outputs of the two DD-MZMs are recombined by an optical coupler, and then sent to a PD with the waveforms observed by an oscilloscope and the spectra measured by an electrical spectrum analyzer. The responsivity of the PD is 0.6 A/W. In order to verify the generated Gaussian monocycle and doublet pulses, the data signal with all ‘‘0’’ or all ‘‘1’’ is applied to the two IMs in the first simulation. Fig. 5 shows the waveforms and the spectra of the generated UWB Gaussian monocycle and doublet pulses. In Fig. 5(a) and (c), we compare the calculated results with the simulated results. The calculated results are obtained using Eqs. (2) and (5). The simulated results are very consistent with the calculated results. In Fig. 5(b) and (d), the thick black lines represent the FCC-specified indoor spectral mask. As can be seen from Fig. 5(a) and (b), the monocycle pulse has a FWHM of 64.3 ps, and the spectrum has a central frequency of 4.375 GHz and a – 10 dB bandwidth of 7.5 GHz, which indicates a fractional bandwidth of about 171%. Fig. 5(c) shows the waveform of the generated UWB Gaussian doublet, which has a FWHM of about 34.9 ps. Fig. 5(d) shows the spectrum of the generated UWB doublet, which has a central frequency of 6.25 GHz and a 10dB bandwidth of 9.375 GHz, corresponding to a fractional bandwidth of about 150%. The resolution bandwidth in the simulation is 4 kHz. In the two spectra, there are some spectral lines which are little higher than the FCC-specified indoor spectral mask, such as the 1.25 GHz, 1.875 GHz and 2.5 GHz spectral lines in Fig. 5(b), and the 1.25 GHz, 11.25 GHz and 11.875 GHz spectral lines in Fig. 5(d). This also happens in some published papers, which give the FCC-specified indoor spectral mask in their spectra of the generated
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Fig. 5. Waveforms and spectra of the generated UWB pulses: (a) generated UWB monocycle, (b) corresponding electrical spectrum, (c) generated UWB doublet, and (d) corresponding electrical spectrum.
Y. Chen et al. / Optics & Laser Technology 45 (2013) 342–347
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Fig. 6. Waveforms of the (a) pulse-position-modulated, (b) pulse-shape-modulated, and (c) on–off keying UWB signals.
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applications may be ignored, so our results are more ideal. The simulation results proved the viability of our scheme.
4. Conclusion
A simple and flexible optical modulation scheme based on a parallel IM–DD-MZM structure is proposed for the generation of UWB pulses. The proposed scheme features a reconfigurable and stable setup. Simulation results show that a UWB monocycle or doublet pulse with a fractional bandwidth of 171% or 150% can be generated. The proposed scheme is insensitive to the polarization of optical signal and does not use unstable device, it appears to be a good choice for flexible UWB pulse modulation.
Acknowledgment This work was partially supported by National Basic Research Program of China (973 Program) under Grant no. 2010CB328300,
and by 111 project under Grant no.D08038, and by the Fundamental Research Funds for the Central Universities (JY10000901005).
References [1] Porcino D, Hirt W. Ultra-wideband radio technology: potential and challenges ahead. IEEE Communications Magazine 2003;41(7):66–74. [2] Aiello G, Rogerson G. Ultra-wideband wireless systems. IEEE Microwave Magazine 2003;4(2):36–47. [3] Federal Communications Commission. Revision of Part 15 of the Commission’s rules regarding ultra-wideband transmission systems. Technical report. ET-Docket 98-153, FCC02-48; April 2002. [4] Yao J, Zeng F, Wang Q. Photonic generation of ultrawideband signals. Journal of Lightwave Technology 2007;25(11):3219–35. [5] Yao J. Photonics for ultrawideband communications. IEEE Microwave Magazine 2009;10(4):82–95. [6] Nassar CR, Zhu F, Wu Z. Direct sequence spreading UWB systems: frequency domain processing for enhanced performance and throughput. In: Proceedings of the IEEE international conference on communication. vol. 3; May 2003. p. 2180–6. [7] Balakrishnan J, Batra A, Dabak A. A multi-band OFDM system for UWB communication. In: Proceedings of the IEEE conference on ultra wideband systems and technology; November 16–19, 2003. p. 354–8.
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[14] Zang Z, Minato T, Navaretti P, Hinokuma Y, Duelk M, Velez C, et al. Highpower (4110 mW) superluminescent diodes by using active multimode interferometer. IEEE Photonics Technology Letters 2010;22(10):721–3. [15] Zang Z, Mukai K, Navaretti P, Duelk M, Velez C, Hamamoto K. Thermal resistance reduction in high power superluminescent diodes by using active multi-mode interferometer. Applied Physics Letters 2012;100(3). [16] Pan S, Yao J. UWB-over-fiber communications: modulation and transmission. Journal of Lightwave Technology 2010;28(16):2445–55. [17] Mu H, Yao J. Photonic generation of UWB pulses with pulse position modulation. Electronics Letters 2010;45(1):99–100. [18] Pan S, Yao J. A photonic UWB generator reconfigurable for multiple modulation formats. IEEE Photonics Technology Letters 2009;21(19):1381–3. [19] Wang S, Chen H, Xin M, Chen M, Xie S. Optical ultra-wide-band pulse bipolar and shape modulation based on a symmetric PM–IM conversion architecture. Optics Letters 2009;34(20):3092–4.
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[8] Zeng F, Yao JP. Ultrawideband impulse radio signal generation using a high-speed electrooptic phase modulator and a fiber-Bragg grating-based frequency discriminator. IEEE Photonics Technology Letters 2006;18(19):2062–4. [9] Abtahi M, Mirshafiei M, LaRochelle S, Rusch LA. All-optical 500-Mb/s UWB transceiver: an experimental demonstration. Journal of Lightwave Technology 2008;26(15):2795–802. [10] Wang C, Zeng F, Yao J. All-fiber ultrawideband pulse generation based on spectral-shaping and dispersion-induced frequency-to-time conversion. IEEE Photonics Technology Letters 2007;19(3):137–9. [11] Dong J, Zhang X, Xu J, Huang D, Fu S, Shum P. Ultrawideband monocycle generation using cross-phase modulation in a semiconductor optical amplifier. Optics Letters 2007;32(10):1223–5. [12] Li J, Fu S, Xu K, Wu J, Lin J, Tang M, et al. Photonic ultrawideband monocycle pulse generation using a single electro-optic modulator. Optics Letters 2008;33(3):288–90. [13] Pan S, Yao J. Switchable UWB pulse generation using a phase modulator and a reconfigurable asymmetric Mach–Zehnder interferometer. Optics Letters 2009;34(2):160–2.
347
Contents lists available at SciVerse ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Photonic generation of UWB pulses with multiple modulation formats Yang Chen, Aijun Wen n, Lei Shang, Yong Wang
TE D
State Key Laboratory of Integrated Services Networks, Xidian University, Xi’an 710071, PR China
a b s t r a c t
Article history: Received 6 April 2012 Received in revised form 4 June 2012 Accepted 20 June 2012 Available online 18 July 2012
A new photonic approach to generate ultra-wideband (UWB) signals with on–off keying, pulse position and shape modulation is proposed and proved by simulation. The proposed system consists of two parallel similar subsystems, which are both made up of an intensity modulator (IM) and a dual-drive Mach–Zehnder modulator (DD-MZM). The optical signal is injected into the two subsystems with equal intensity and phase. The IMs are driven by the data signal, while the DD-MZMs are driven by the Gaussian pulse train. By properly adjusting the bias points of the IMs and DD-MZMs, the amplitudes of the data signals, and the time delays introduced by electrical delay lines, a position-modulated UWB monocycle, a shape-modulated UWB monocycle and doublet, and an on–off keying UWB monocycle can be generated. The fractional bandwidth of the generated UWB monocycle and UWB doublet are 171% and 150%, respectively. & 2012 Elsevier Ltd. All rights reserved.
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Keywords: Ultra-wideband (UWB) Monocycle and doublet pulse Modulation formats
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a r t i c l e i n f o
1. Introduction
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UWB, which is regulated by the Federal Communications Commission (FCC) for applications such as short-range broadband wireless communications and broadband sensor networks operating in the frequency range from 3.1 to 10.6 GHz, has been a topic of interest recently due to its inherent properties such as high data rate, low power spectral density, extremely short time duration, and immunity to multi-path fading [1,2]. Based on the FCC definition, UWB signals should have a fractional bandwidth more than 20% or a 10 dB bandwidth of at least 500 MHz [3]. The UWB frequency mask defined by the FCC is shown in Fig. 1. As can be seen, the frequency band assigned to UWB indoor communications systems extends from 3.1 GHz to 10.6 GHz, with a bandwidth of 7.5 GHz centered around 7 GHz. The power density is limited to 41.3 dBm/MHz. To offer availability of undisrupted service across different networks and to eventually achieve high-data-rate access at any time and from any place, UWB combined with fiber transmission, i.e., a technology called UWB over fiber, can provide an effective solution [4,5]. In general, there are two types of UWB: the direct sequence UWB [6] and the multiband UWB [7]. Direct sequence impulse radio is one of the most attractive techniques for UWB communications as it is carrier free, so there is no need for complicated frequency mixers and local oscillators to convert the carrier frequency. Conventionally, direct sequence UWB signals, usually
n
Corresponding author. Tel./fax: þ 86 29 88204468. E-mail address: [email protected] (A. Wen).
0030-3992/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlastec.2012.06.026
Gaussian monocycle or doublet pulse, are generated using electronic circuits. Recently, the generation of UWB signals in the optical domain has attracted a lot of interests, which is a better choice since there is no need for extra electrical-to-optical conversion. In addition, the generation of UWB signals in the optical domain provides other features such as light weight, small size, large tunability, and immunity to electromagnetic interference. In the past few years, various techniques for UWB pulse generation were reported. In Ref. [8], UWB pulses were generated using a high-speed electro-optic phase modulator and a fiberBragg grating-based frequency discriminator. In Refs. [9,10], UWB pulses were generated based on spectral-shaping and dispersioninduced frequency-to-time conversion. In Ref. [11], the nonlinear effect in a semiconductor optical amplifier was used for UWB pulse generation. In Refs. [12,13], UWB pulses were generated using external modulators. UWB signals in the optical domain based on SLEDs using active multi-mode interferometer were proposed in Refs. [14,15]. However, for a practical UWB over fiber communication system, the information must be encoded, which can be realized by using different pulse modulation schemes, such as on–off keying (OOK), pulse-position modulation (PPM), bi-phase modulation (BPM), pulse-amplitude modulation (PAM), and pulseshape modulation (PSM). All these modulation schemes can be implemented in the optical domain. In Ref. [16], all these modulation formats were realized based on a phase modulator (PM) and an electrical reconfigurable asymmetric Mach–Zehnder interferometer (AMZI). Ref. [17] proposed a novel method to generate UWB pulses with PPM. In Ref. [18], a DD-MZM and an AMZI were reconfigured to implement PPM, BPM, and PSM. In Ref. [19], UWB pulses with BPM and PSM were generated in the
Y. Chen et al. / Optics & Laser Technology 45 (2013) 342–347
2. Principle
2.1. UWB pulse with PPM As shown in Fig. 3, IM1 is biased at the maximum transmission point, while IM2 is biased at the minimum transmission point. The amplitudes of both the data signals applied to IM1 and IM2 are Vp, where Vp is the half-wave voltage of the IM. In this case, the output of IM1 will be just opposite to that of IM2. When the data signal is ‘‘0’’, the intensity of IM1 output will be maximum, and the intensity of IM2 output will be minimum. When the data signal is ‘‘1’’, the intensity of IM1 output will be minimum, and the intensity of IM2 output will be maximum. The optical signal switches between the two parallel IM–DD-MZM branches according to the data sequence. If Gaussian monocycles are generated with different time delays in DD-MZM1 and DD-MZM2 respectively, a UWB monocycle with PPM can be generated at the output of the PD. Assume that the electrical drive signals applied to DD-MZM1 and DD-MZM2 are V1(t) ¼V0j(t), V2(t)¼V0j(t þ t1), and V3(t) ¼V0j(t þ t2), V4(t)¼V0j(tþ t1 þ t2), where j(t) is a normalized Gaussian pulse, V0 is the amplitude, and t1 and t2 are the time delays introduced by the electrical delay lines. With an optical field of Ei(t)¼Ei exp(joct) injected, where oc is the angular frequency of the optical carrier, Ei is the amplitude, and taking the transmission curve shown in Fig. 3 into consideration, the optical field at the output of optical coupler can be expressed as E ðtÞ V 1 ðtÞ p V 2 ðtÞ þexp jp þj Eo1 ðtÞ ¼ i sðtÞ exp jp 2 2 Vp Vp Ei ðtÞ V 3 ðtÞ p V 4 ðtÞ ð1Þ þ þexp jp þj sðtÞ exp jp 2 2 Vp Vp
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The schematic diagram of the proposed UWB pulse generator is similar to a nested MZM, as shown in Fig. 2. It includes a main MZM and two IM–DD-MZM branches, and a static phase control electrode of the main MZM is driven by a DC bias to keep the two branches of the structure having the same optical delay. A lightwave emitted from a LD is divided into two parts with equal intensity and phase by an optical power splitter. The two outputs of the optical power splitter are sent to one of the two parallel IM–DD-MZM branches, respectively. The data signals are applied to the IMs, and the Gaussian pulses are applied to the DD-MZMs. The outputs of the two DD-MZMs are recombined by
an optical coupler after being modulated by a DC bias applied to the static phase control electrode of the main MZM, and then sent to a PD. A position-modulated UWB monocycle, a shape-modulated UWB monocycle and doublet, or an on–off keying UWB monocycle can be generated at the output of the PD with specific bias points of the IMs, amplitudes of the data signals, and time delays introduced by electrical delay lines.
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optical domain using a symmetric PM–IM conversion architecture. The schemes in Refs. [16–18] needed to control the polarization of the optical signal precisely, while the scheme in Ref. [19] employed a fiber Bragg grating (FBG) to implement PM–IM conversion. These may be unstable in practical applications. In this paper, we propose a simple and new UWB generator using two IMs and two DD-MZMs that can be reconfigured to implement PPM, PSM and OOK. By properly adjusting the bias points of IMs and DD-MZMs, the amplitudes of the data signals, and the time delays introduced by electrical delay lines, a position-modulated UWB monocycle, a shape-modulated UWB monocycle and doublet, or an on–off keying UWB monocycle can be generated. Our method is insensitive to the polarization of the optical signal, and does not need a FBG, which may be a new choice for UWB pulse modulation.
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Fig. 1. UWB frequency mask defined by the FCC for ultra-wideband indoor communications.
Fig. 3. Bias points and data signal amplitudes of IM1 and IM2 for PPM.
Fig. 2. Schematic of the optical UWB generator. LD: laser diode; IM: intensity modulator; DD-MZM: dual parallel Mach–Zehnder modulator; PD: photodetector.
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where s(t) is the 0–1 data sequence, and sðtÞ is its opposite data sequence, so sðtÞsðtÞ ¼ 0 establishes. In writing Eq. (1), the DD-MZM1 and DD-MZM2 are both biased at the quadrature transmission point. If the optical signal expressed in Eq. (1) is sent to a PD for square-law detection, the ac term of the photocurrent is
ac term of the photocurrent is 2
io2 ðtÞpsinb½jðt þ t1 ÞjðtÞsðtÞ cosb½jðt þ t1 þ t2 Þjðt þ t2 Þ sðtÞcosb½jðt þ t2 ÞjðtÞ þ sðtÞcosb½jðt þ t1 þ t2 ÞjðtÞ þ sðtÞsinb½jðt þ t2 Þjðt þ t1 ÞsðtÞsinb½jðt þ t1 þ t2 Þjðt þ t1 Þ
ð5Þ
2
ð2Þ For small-signal modulation, b is small, so we have sinb b. Taking characteristics of 0–1 data sequence into consideration, Eq. (2) is approximated as ( b½jðt þ t1 ÞjðtÞ sðtÞ ¼ 0 ð3Þ io1 ðtÞp b½jðt þ t1 þ t2 Þjðt þ t2 Þ sðtÞ ¼ 1 As can be seen, the output current is proportional to the firstorder difference of the input Gaussian signal. If t1 and t2 are sufficiently small, the first-order difference can be approximated as the first-order derivative. The entire system is equivalent to a first-order differentiator and a Gaussian monocycle is generated. Furthermore, Eq. (3) suggests that the position of the generated monocycle is different when the data signal is ‘‘0’’ and ‘‘1’’, so the UWB monocycle is position-modulated.
2.2. UWB pulse with PSM
For small-signal modulation, b is small, so we have sinb b and cosb 1. Taking characteristics of 0–1 data sequence into consideration, Eq. (5) is approximated as ( b½jðt þ t1 þ t2 Þjðt þ t1 Þjðt þ t2 Þ þ jðtÞ sðtÞ ¼ 0 io2 ðtÞp b½jðt þ t1 ÞjðtÞ sðtÞ ¼ 1 ð6Þ As can be seen, the output current is proportional to the firstorder difference of the input Gaussian signal when s(t)¼1, and to the second-order difference of the input Gaussian signal when s(t) ¼0. Again, if t1 and t2 are sufficiently small, the first- and second-order differences can be approximated as the first- and second-order derivatives. The entire system is equivalent to a first- and second-order differentiator when s(t) equals to 1 and 0 respectively. In this case, Gaussian monocycle and doublet can be generated according to the 0–1 sequence, so the UWB pulse is shape-modulated.
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io1 ðtÞpsðtÞ sinb½jðt þ t1 ÞjðtÞ þ sðtÞ2 sinb½jðt þ t1 þ t2 Þjðt þ t2 Þ
2.3. UWB pulse with OOK
Furthermore, the proposed scheme can be reconfigured to implement OOK. As discussed in Section 2.2, a shape-modulated UWB monocycle and doublet can be generated according to Eq. (6), where a UWB monocycle represents ‘‘1’’ and a UWB doublet represents ‘‘0’’. If the time delay t2 in Eq. (6) is changed to 0, Eq. (6) can be simplified as ( 0 sðtÞ ¼ 0 ð7Þ io3 ðtÞp b½jðt þ t1 ÞjðtÞ sðtÞ ¼ 1
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As shown in Fig. 4, IM1 and IM2 are both biased at the quadrature transmission point. The amplitude of the data signal applied to IM1 is Vp, while that applied to IM2 is Vp/2. In this case, the output of IM1 is a constant (half the maximum value). The output of IM2 is the same value when the data signal is ‘‘0’’, and the output of IM2 will be minimum when the data signal is ‘‘1’’. So the output of the optical coupler will be the sum of the two parallel IM–DD-MZM branches when the data signal is ‘‘0’’, and be only the output of IM1–DD-MZM branch when the data signal is ‘‘1’’. If these two cases have different waveforms, UWB pulses with PSM can be generated at the output of the PD. Taking the transmission curve shown in Fig. 4 into consideration, the optical field at the output of optical coupler can be expressed as pffiffiffi V 1 ðtÞ p V 2 ðtÞ 2Ei ðtÞ þexp jp þj exp jp Eo2 ðtÞ ¼ 2 4 Vp Vp pffiffiffi V 3 ðtÞ 3p V 4 ðtÞ p 2Ei ðtÞ þ þ exp jp sðtÞ exp jp þj þj 2 2 4 Vp Vp ð4Þ
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To simplify the analysis, we assume the extinction ratios of the DD-MZMs are infinite. In writing Eq. (4), the DD-MZM1 is biased at the quadrature transmission point, while the DD-MZM2 is biased at the minimum transmission point. If the optical signal expressed in Eq. (4) is sent to a PD for square-law detection, the
Fig. 4. Bias points and data signal amplitudes of IM1 and IM2 for PSM.
Again, if t1 is sufficiently small, the first-order difference can be approximated as the first-order derivative. The entire system is equivalent to a first-order differentiator. Eq. (7) suggests that when the data signal is ‘‘0’’, there is no electrical output signal, and when the data signal is ‘‘1’’, a UWB monocycle will be generated. So, OOK UWB pulses are generated.
3. Simulation verification and discussion Simulations are performed using VPItransmissionMaker 8.3 based on the setup shown in Fig. 2 to verify the UWB pulse generation and the implementation of PPM, PSM and OOK schemes. All the components shown in Fig. 2 can be found in the simulation software, and parameters of these components should be configured according to the descriptions in Section 2. Waveform, electrical and optical spectra can be observed by using the oscilloscope and spectrum analyzer of the simulation software. A lightwave with 10 MHz linewidth and 13 dBm power from a LD is sent to a 50/50 optical power splitter. The two outputs of the optical power splitter are sent to one of the two parallel IM–DDMZM branches, respectively. A data signal is split into two channels, which are used to drive the two IMs. A Gaussian pulse train, which has a repetition rate of 625 MHz and a full-width at half-maximum (FWHM) of about 85 ps, is split into four channels, and then, applied to the two DD-MZMs with different time delays via the RF ports. The time delays t1 and t2 are both 40 ps in this simulation for PPM and PSM, while for OOK, t1 and t2 are 40 ps and 0, respectively. The IMs and DD-MZMs used in this simulation have a half-wave voltage of 5 V, an insertion loss of 6 dB, and an
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UWB pulses, such as Ref. [16]. In general, the spectrum in Fig. 5(d) is more consistent with the FCC-specified indoor spectral mask compared with that in Fig. 5(b). However, a small irregularity does not influence the feasibility of the results, because in practical applications, the generated UWB pulses will pass a filter before they are sent to the UWB transmitting antenna to remove the unnecessary parts of the spectrum. The fractional bandwidth of the generated UWB pulses may decrease after passing the filter, but it is still much bigger than the FCC definition—20%. To verify the multiple modulation formats, a data signal with 625 Mbit/s data rate and a fixed pattern ‘‘10010110’’ is applied to the IMs. By adjusting the amplitudes of the data signal, the bias point of the IMs and DD-MZMs, and the time delays introduced by electrical delay lines as discussed in Section 2, PPM, PSM and OOK are realized. The waveforms of the modulated UWB signals are shown in Fig. 6. Fig. 6(a), (b) and (c) represents PPM, PSM and OOK respectively. As shown in Fig. 6(a), the time difference between ‘‘1 0’’ is about 1.55 to 1.56 ns, the time difference between ‘‘0 1’’ is about 1.64 to 1.65 ns, and the time differences between ‘‘0 0’’ and ‘‘1 1’’ are around 1.60 ns. These values are very consistent with Eq. (3) and the 40 ps time delay in the simulation. Fig. 6(b) shows that a UWB monocycle represents ‘‘1’’ and a UWB doublet represents ‘‘0’’, which is consistent with the results obtained from Eq. (6). Fig. 6(c) shows the OOK UWB monocycle, in which, a monocycle represents ‘‘1’’, and no signal represents ‘‘0’’. Compared with the waveforms in Refs. [16–19], the waveforms in Fig. 6 are very standard and do not have some parasitic pulses. This is because that in the simulation, some influence factors in practical
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extinction ratio of 30 dB. The outputs of the two DD-MZMs are recombined by an optical coupler, and then sent to a PD with the waveforms observed by an oscilloscope and the spectra measured by an electrical spectrum analyzer. The responsivity of the PD is 0.6 A/W. In order to verify the generated Gaussian monocycle and doublet pulses, the data signal with all ‘‘0’’ or all ‘‘1’’ is applied to the two IMs in the first simulation. Fig. 5 shows the waveforms and the spectra of the generated UWB Gaussian monocycle and doublet pulses. In Fig. 5(a) and (c), we compare the calculated results with the simulated results. The calculated results are obtained using Eqs. (2) and (5). The simulated results are very consistent with the calculated results. In Fig. 5(b) and (d), the thick black lines represent the FCC-specified indoor spectral mask. As can be seen from Fig. 5(a) and (b), the monocycle pulse has a FWHM of 64.3 ps, and the spectrum has a central frequency of 4.375 GHz and a – 10 dB bandwidth of 7.5 GHz, which indicates a fractional bandwidth of about 171%. Fig. 5(c) shows the waveform of the generated UWB Gaussian doublet, which has a FWHM of about 34.9 ps. Fig. 5(d) shows the spectrum of the generated UWB doublet, which has a central frequency of 6.25 GHz and a 10dB bandwidth of 9.375 GHz, corresponding to a fractional bandwidth of about 150%. The resolution bandwidth in the simulation is 4 kHz. In the two spectra, there are some spectral lines which are little higher than the FCC-specified indoor spectral mask, such as the 1.25 GHz, 1.875 GHz and 2.5 GHz spectral lines in Fig. 5(b), and the 1.25 GHz, 11.25 GHz and 11.875 GHz spectral lines in Fig. 5(d). This also happens in some published papers, which give the FCC-specified indoor spectral mask in their spectra of the generated
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Fig. 5. Waveforms and spectra of the generated UWB pulses: (a) generated UWB monocycle, (b) corresponding electrical spectrum, (c) generated UWB doublet, and (d) corresponding electrical spectrum.
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Fig. 6. Waveforms of the (a) pulse-position-modulated, (b) pulse-shape-modulated, and (c) on–off keying UWB signals.
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applications may be ignored, so our results are more ideal. The simulation results proved the viability of our scheme.
4. Conclusion
A simple and flexible optical modulation scheme based on a parallel IM–DD-MZM structure is proposed for the generation of UWB pulses. The proposed scheme features a reconfigurable and stable setup. Simulation results show that a UWB monocycle or doublet pulse with a fractional bandwidth of 171% or 150% can be generated. The proposed scheme is insensitive to the polarization of optical signal and does not use unstable device, it appears to be a good choice for flexible UWB pulse modulation.
Acknowledgment This work was partially supported by National Basic Research Program of China (973 Program) under Grant no. 2010CB328300,
and by 111 project under Grant no.D08038, and by the Fundamental Research Funds for the Central Universities (JY10000901005).
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