We propose and demonstrate a scheme for optical ultrawideband (UWB) pulse generation by exploiting a half-carrier-suppressed Mach–Zehnder modulator (MZM) and a delay-interferometer- and wavelength-division-multiplexer-based, reconfigurable and multi-channel differentiator (DWRMD). Multi-wavelength, polarity- and shape-switchable UWB pulses of monocycle, doublet, triplet, and quadruplet are experimentally generated simply by tuning two bias voltages to modify the carrier-suppression ratio of MZM and the differential order of DWRMD respectively. The pulse position modulation, pulse shape modulation, pulse amplitude modulation and binary phase-shift keying modulation of UWB pulses can also be conveniently realized with the same scheme structure, which indicates that the hybrid modulation of those four formats can be achieved. Consequently, the proposed approach has potential applications in multi-shape, multi-modulation and multi-access UWB-over-fiber communication systems.
©2012 Optical Society of America
Ultra-wideband (UWB) is a promising technology in many applications, especially for short-range high-data-rate wireless communication systems and broadband sensor networks, since it has distinct advantages of high data capacity, huge bandwidth, low power spectral density (PSD), and immunity to multipath fading. The Federal Communications Commission (FCC) has allocated 7.5-GHz spectral band from 3.1 to 10.6 GHz for unlicensed use of UWB with a PSD of less than −41.3 dBm/MHz and a 10-dB spectral bandwidth larger than 500 MHz or a fractional bandwidth greater than 20% .
In order to take advantage of low transmission loss and extremely broad bandwidth offered by optical communication systems, the generation and modulation of UWB pulses directly in the optical domain have been considered to be of importance for UWB-over-fiber (UWBoF) technology . Hence, promising solutions for UWB pulse generation and modulation in the optical domain for single- or multi-user UWBoF communication systems have acquired special attention [3–5]. Zeng et al. proposed an approach to UWB doublet pulse generation and distribution over optical fiber by combining an electro-optic phase modulator, a length of single-mode fiber (SMF), and a photo-detector (PD) to form an all-optical microwave bandpass filter . Dong et al. reported a UWB monocycle generation scheme utilizing gain saturation of dark return-to-zero signal in an SOA . Bolea et al. presented a photonic structure for arbitrary waveform generation applicable to multiband UWB communications, which was based on a dispersive element and a balanced PD . Huang et al. suggested UWB pulse generation by use of a nonlinear-optical-loop-mirror-based optical switch . Li et al. demonstrated an approach to UWB monocycle or doublet pulse generation using cross-gain modulation in highly-nonlinear-fiber-based optical parametric amplifier . Wang et al. proposed all-optical UWB pulse generation based on the quadratic nonlinear interactions and sum-frequency generation in periodically poled lithium niobate waveguides [11–13]. A scheme for UWB pulse generation exploiting a Sagnac-interferometer-based intensity modulator was presented, which can provide flexible pulse shape and polarity control of monocycles and doublets . Switchable UWB pulse generation of monocycle and doublet by using a polarization maintaining fiber Bragg grating as frequency discriminator was reported . Photonic generation of UWB signals by direct current modulation of semiconductor lasers was proposed [16, 17]. Some promising schemes have been presented for generating power-efficient UWB waveforms, utilizing the techniques of arbitrary waveform generation or the combination of a number of low-order derivatives of Gaussian pulses [18, 19]. Moreover, some optical UWB generation techniques supporting various modulation formats have been demonstrated, which are capable to achieve two or three modulation formats with the same scheme structure, such as on-off keying (OOK) and binary phase-shift keying (BPSK) [20, 21]. Besides, the effects of fiber dispersion on UWB waveforms have also been studied, which can be compensated in some schemes by using special elements, such as dispersion compensation fiber, or by modifying the parameters of UWB generation system in order to reduce the unwanted dispersion-induced distortion [22–24]. In addition, since information is transmitted independently and concurrently over a shared physical channel in a multi-access UWBoF communication systems, extensive research appears in a multiple access UWB system using time division multiple access or code division multiple access techniques in order to separate the multiple users and avoid the crosstalk from other channels [25, 26]. Phase-coded UWB sequence generator for multiple access UWB communications based on a polarization modulator or phase modulator and a fiber-Bragg-grating (FBG) -based multi-channel frequency discriminator has been demonstrated [27, 28], avoiding using bulky FBG array with low stability. Recently, Abraha et al. have proposed, for the first time, service multicasting by all-optical routing of 1 Gb/s impulse-radio (IR) UWB using cross-gain modulation of SOA, which has potential applications for integrating IR-UWB with wavelength-division-multiplexed passive optical networks and in-building networks .
Most of those approaches are efficient for UWB pulse generation of monocycles and doublets applied to two or three modulation formats with the same scheme structure, while there appears less research, especially the experimental demonstration, on higher order derivatives of Gaussian pulses that are capable for multi-modulation formats, which would be better for matching the FCC mask in theory and more flexible for future UWBoF systems . Besides, limited by FCC mask from 3.1 to 10.6 GHz, the width of UWB pulses cannot be as narrow as required to further increase the transmission rate in a single channel. Especially, when pulse position modulation (PPM) is applied, the transmission rate of UWB pulses will be further restricted to a lower value since there should remain certain time spacing between pulses for PPM.
In this paper, a scheme for photonic generation of multi-wavelength, polarity- and shape-switchable UWB pulses, which utilizes a half-carrier-suppressed Mach–Zehnder modulator (MZM) and a delay-interferometer (DI) and wavelength-division-multiplexer-based, reconfigurable and multi-channel differentiator (DWRMD), is proposed and demonstrated. The major differences between this approach and those mentioned above are that the UWB pulses generated in the proposed scheme can vary from monocycles to quadruplets with switchable polarity at multi-wavelengths and that the modulation formats of PPM, pulse shape modulation (PSM), pulse amplitude modulation (PAM) and BPSK for UWB pulses can be achieved flexibly and independently by the same scheme structure with good tuning characteristics. Hence, based on the generation scheme, we propose, for the first time to the best of our knowledge, a method for UWB hybrid modulation of those four formats, which may break the transmission rate barrier formed by FCC mask and make the transmission bit rate increase to four times the original. Moreover, the long-distance and dispersion-compensation-free transmission of 75 km for OOK of UWB signals performs well both in temporal waveforms and corresponding electrical spectra, presenting that the UWB pulses have a good tolerance to chromatic dispersion over fiber. In addition, we demonstrate the wavelength division multiplexing of eight wavelengths of UWB pulses generated by the proposed scheme as eight different communication channels for multi-access, which propagates over 25-km SMF link with good performance. Consequently, the proposed approach is potential for future multi-shape, multi-modulation, multi-access, high-speed, long-distance and dispersion-compensation-free transmission UWBoF communication systems.
2. Operating principle
Figure 1 shows the schematic diagram of the proposal for UWB pulse generation with switchable pulse shape and polarity.
A laser beam of continuous-wave (CW) at wavelength is injected into an MZM in order to generate the signal pulse sequence. At the output of MZM, the field amplitude of the signal light can be approximately expressed by Eq. (2) into Eq. (1), the electric field amplitude of the output optical signal can be expanded in terms of the first kind Bessel functions asEquation (3) shows that the power of the input optical carrier will be expanded among the first-order, second-order, and higher order optical sidebands, of which the amplitude distribution can be optimized by modifying the parameters β and . For example, when is set as odd multiples of by tuning Vbias1, the optical carrier and all even-order sidebands are suppressed by transferring their power to the odd-order ones.
Under small signal modulation condition, only the optical carrier and the first order sidebands need to be considered, hence Eq. (3) can be simplified to
When the MZM is driven by a Gaussian pulse sequence, each sideband would be broadened due to the Fourier transform relation between the time and frequency domains. Similarly, by tuning to set close to odd multiples of , partial suppression of the frequency components near the optical carrier, which could be called the half-carrier-suppressed Mach–Zehnder modulation, would be achieved, while the generated optical spectrum is expected for the UWB pulses according to the relation between the optical spectra of UWB pulses and the FCC spectral mask. We have designed an experiment to substantiate the deduction above and truly got pulses in the shape of doublets at the output of the MZM, as shown in Fig. 1. The corresponding electrical spectrum of generated doublets is presented by an inset at the lower left of Fig. 1. Therefore, with the optimization of determined by , the output signals of the MZM can be shaped as Gaussian pulses or doublets alternatively.
Coupled with continuous probe light at wavelength , the signal pulses at from MZM are then fed into a highly nonlinear fiber (HNLF), arousing the cross-phase modulation (XPM) of the continuous probe light. Thus, the modulated probe light () after HNLF obtains a phase variation () that is approximately proportional to the optical power of input Gaussian pulses or doublets ().
Then the two light beams are sent into the DWRMD formed by connecting a 40GHz DI with an 8-channel 200GHz wavelength-division multiplexer (WDM), which is served as a filter to get rid of the unnecessary signal pulses at and a first-order or second-order multi-channel differentiator when the bias voltage () of DI and wavelength of probe light are properly set.
After injected into DI, the phased-modulated probe light is divided equally into two parts by a 3dB coupler, with a time delay difference of induced by a different path length in one arm and a phase shift of induced by applied on the other. Through another 3dB-and 2 × 2 coupler, the signals from two arms are recombined together. Thus, the signals at the two output ports can be expressed as
Deduced from Eq. (6), the transmission spectrum of DI is periodic. When is 25 ps for 40GHz DI and is 1550 nm, the transmission spectrum period of 40GHz DI () is 0.32 nm. Considering the channel spacing of the 200GHz WDM () is 1.6 nm, the whole DWRMD is multi-channel with a channel spacing of 1.6 nm since .
When detected by PD that works in the linear response region, the probe light at the output ports converts to photocurrent, the AC terms of which can be approximately written as
By choosing appropriate though tuning to let equal (N should be an integer) and considering that is small enough due to relatively low cross-phase modulation index and small , Eq. (7) can be simplified to
Considering that the output signals are proportional to the first-order difference of the signal pulses generated by MZM and is sufficiently small, the signals at the output of DI can be approximated as the first-order differential of the pulses generated from MZM. Moreover, the polarity of the output pulses from each port can be changed conveniently by setting N as an odd or even, which means that pairs of UWB pulses with inverted polarity can be simultaneously obtained from corresponding ports and the polarity of pulses from each port can be easily changed by tuning . Comparing the short time shift (mostly less than 3 fs) induced by a phase shift of the DI at the frequency of the optical carrier (lager than 190 THz) with the bit period of a UWB pulse (mostly more than 1 ns), the polarity of UWB pulses can be switched nearly without time shift or pulse position shift. Hence, when the hybrid modulation format of PPM and BPSK is applied, the influence of pulse position shift on PPM, which is caused by BPSK, is small enough to be ignored.
When the wavelength of the probe light () is located on the top flat of super-Gaussian spectral transmittance curve of one WDM channel, the WDM only acts as a filter and DI performs as a first-order differentiator, forming a first-order multi-channel differentiator with conveniently tuning characteristics. Moreover, the large slope variation range of super-Gaussian transfer function of each WDM channel is beneficial for optimizing the spectral response of DI, which indicates that it is possible to achieve desired positive and negative quadrature slopes of the whole DWRMD spectral transmittance curve by adjusting . Therefore, the DWRMD could also be reconfigured to be a second-order differentiator when the optical carrier of the phase-modulated probe light is located at those positions with quadrature slopes.
Considering that the device properties and the experimental conditions cannot be ideal and that there are some parasitic effects in the whole UWB pulse generation system, there would be slight differences between the pulse shapes predicted by the developed theory and the experimental results. Firstly, the MZM may not provide ideal transformation from electrical signals to optical ones, while the may jitter or not be properly set. Secondly, when probe lights and signal light propagate in the HNLF, there also exist some other nonlinear effects besides XPM, such as stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS) and four-wave mixing (FWM). Since the wavelength spacing of about 1.6 nm is relatively large and the power of probe light in each channel is set to be relatively low, the crosstalk between the adjacent channels which is caused by SBS, SRS and FWM is very small but still has a little influence on the waveforms at the output of the HNLF. Thirdly, the DWRMD may also induce some distortion when spectral transmittance curve of WDM is not ideal or jitters or is not properly set.
As a consequence, by tuning of the MZM to generate Gaussian pulses and of the DI to reconfigure the DWRMD as a first-order or second-order multi-channel differentiator, monocycles or doublets with inverted polarity at different wavelengths are obtained, as shown by the four insets from A to D at the upper right of Fig. 1. Similarly, by tuning of the MZM to generate doublets and of the DI to reconfigure the DWRMD as a first-order or second-order multi-channel differentiator, triplets or quadruplets with inverted polarity at different wavelengths are also achieved, as shown by the four insets from E to H at the lower right of Fig. 1. Moreover, since the DWRMD is multi-channel with a channel spacing of 1.6 nm, more CW probes could be added as communication channels for more users in order to establish a multi-access UWB systems.
3. Experimental demonstration and discussion
The experimental setup is shown in Fig. 2 . Emitted by a tunable laser source, the CW at 1542 nm () with an output power of 13 dBm is sent into MZM through a polarization controller, which is used for adjusting the polarization of the CW to be optimal for MZM. The MZM is driven by a bit pattern generator (BPG) and biased at different voltages () to generate Gaussian pulses or doublets. The BPG is used for generating electrical Gaussian pulse train with a fixed pattern “0000 0000 1000 0000” (one “1” per 16 bits) at a bit rate of 10 Gbit/s, corresponding to the pulse repetition rate of 0.625 GHz.
After amplified by the erbium-doped fiber amplifier (EDFA) that is pumped by two 1480-nm power-tunable lasers, the signal pulse train is combined with a group of continuous probe light at eight wavelengths () via an arrayed waveguide grating (AWG) and an optical coupler. Then the signal pulses and the continuous probe light group of 8 wavelengths, i.e. 1549.56 nm (ch1), 1551.12 nm (ch2), 1552.74 nm (ch3), 1554.36 nm (ch4), 1555.98 nm (ch5), 1557.60 nm (ch6), 1559.22 nm (ch7), 1560.84 nm (ch8), are simultaneously launched into an HNLF with a highly nonlinear coefficient of about 20 /W/km. After arousing XPM in the HNLF, the signal light and probe light are injected into the DWRMD, which is formed by collecting a 40GHz voltage-controlled DI with a voltage tuning range of 0-4V and an eight-channel WDM with a channel spacing of 200 GHz. The eight pairs of UWB pulses with inverted polarity at eight wavelengths are generated simultaneously from the corresponding output ports of ch to ch. As an example, the polarity-inverted quadruplets from ch8 and ch is shown by the insets in Fig. 2. The generated UWB pulses are then distributed over fiber links of different lengths. Then by injecting the optical signals into PD in order to convert optical signals to electrical signals, the temporal waveforms and corresponding electrical spectra of the UWB pulses are measured by a digital communication analyzer (Agilent DCA86100C) and an electrical spectrum analyzer (Anritsu MS2668C), respectively.
Firstly, we would like to take the signals from ch5 of the WDM as an example to observe the waveforms and spectra of generated UWB pulses. When the MZM is reconfigured properly to generate Gaussian pulses, polarity- and shape-switchable monocycles and doublets can be obtained by tuning the bias voltage () of the DWRMD. The monocycles with positive and negative polarity are shown by the red dashed curve and blue solid curve respectively in Fig. 3(a) . The upper full-width at half-maximum (FWHM) and lower FWHM of the positive monocycle are about 52 and 42 ps, respectively, while the monocycle with negative polarity has the upper and lower FWHM of approximately 50 and 42 ps respectively. Given by Fig. 3(b), the corresponding electrical spectrum shows a central frequency of about 5.4 GHz and a 10-dB bandwidth of about 9.25 GHz, indicating that the fractional bandwidth is about 171%. Displayed in Fig. 4(a) with red dashed curve and blue solid curve, the positive and negative doublet possesses the FWHMs of ~48 and ~40 ps, while the central frequency and 10-dB bandwidth of doublets, shown in Fig. 4(b), are ~8.69 and ~11.22 GHz, corresponding to a fractional bandwidth of ~129%.
When the MZM is reconfigured to produce doublets, triplets and quadruplets can be generated by adjusting . As depicted in Fig. 5(a) , the upper and lower FWHM of the positive triplet is about 36 and 40 ps respectively, while the negative triplet is illustrated with the upper and lower FWHM of approximately 30 and 42 ps respectively. The corresponding electrical spectrum, as shown in Fig. 5(b), has a central frequency of ~8.93 GHz, a 10-dB bandwidth of ~9.26 GHz and a fractional bandwidth of ~104%. Finally, the positive and negative quadruplets are displayed in Fig. 6(a) with the red dashed curve and blue solid curve, the FWHMs of which are ~40 and ~30 ps, respectively. As depicted in Fig. 6(b), the central frequency and 10-dB bandwidth are ~10.43 and ~7.14 GHz, presenting a fractional bandwidth of ~68%.
As shown in the part (b) of Fig. 3-6, the lower spectral components are suppressed and the central frequency increases as the order of Gaussian derivatives increases, showing that the higher order Gaussian derivatives are potential for better fitting the FCC mask. It is worth noting that the central frequency of quadruplets demonstrated in the experiments is not the optimal one, while by broadening the FWHM of input Gaussian pulses, the central frequency will decrease for generating quadruplets with corresponding electrical spectra better fitting the FCC spectral mask. Moreover, the proposed scheme can be easily improved to generate the fifth-order derivative of Gaussian pulses, i.e. quintuplet, which is promising to be FCC-mask compliant. Simply by implementing eight couplers and eight tuneable optical delay lines for eight wavelengths at the output port of DWRMD in order to combine the corresponding polarity-inverted quadruplets and add proper time delay between them, the quintuplets at 8 wavelengths will be obtained for multi-access, while the four modulation formats of PSM, PAM, PPM and BPSK and the hybrid modulation of the four that are demonstrated in the next part can also be similarly realized for quintuplets with the relative reconfiguring technologies.
We also demonstrate the UWB transmission over fiber links. The negative triplets from the ch5 of the WDM are analyzed as an example, of which the waveforms and corresponding electrical spectra are measured after propagating over 5-, 10-, 25-, 35-, 50-, 75-km-length SMF without dispersion compensation. Figure 7 shows the pulse width, central frequency and 10-dB bandwidth as a function of the fiber length. The waveforms of the triplets corresponding to different fiber lengths are shown by the insets in Fig. 7(a). Both the upper and lower pulse widths of the signals rise slightly with the increase of transmission distance, while the central frequency and 10-dB bandwidth decrease, mostly due to the chromatic dispersion in SMF. Especially, Fig. 7(a) presents a faster increase of upper FWHM of the triplets than the lower FWHM that might be caused by a slight frequency chirp generated in the scheme, which has little influence overall on the temporal waveforms and corresponding electrical spectra shown in the experimental demonstration. Considering that the 10-dB bandwidth decreases faster than the central frequency as shown in Fig. 7(b), the lower-frequency components of the UWB pulses decrease faster than the upper ones as the length of fiber link increases. Since the lower-order derivatives of Gaussian pulses often possess high lower-frequency components, the presented phenomenon is beneficial for better UWB pulses for matching the FCC mask in the access points, which is mostly due to the slight frequency chirp of the UWB pulses generated after the DWRMD. Especially, the waveform and corresponding electrical spectrum of triplet train are shown by Fig. 8(c) and (d) , presenting a better electrical spectrum for matching the FCC mask. In general, the UWB pulses generated by the proposed scheme have no significant distortion when propagating over long fiber link, indicating a good tolerance to chromatic dispersion over fiber. Additionally, mainly due to the slight frequency chirp that is potential to give positive impact on UWB pulses propagating over fiber, the dispersion over fiber is conversely beneficial for optimizing UWB pulses generated by the proposal for better electrical spectra according to the FCC mask, which is demonstrated by the experimental results [4,6].
In order to achieve a higher transmission rate for OOK of generated UWB pulses, we use the fixed pattern of “0000 1000” (one “1” per 8 bits) for BPG at a bit rate of 10 Gbit/s to represent a “1” and all “0” per 8 bits to represent a “0”, which is equivalent to a pulse repetition rate of 1.25 GHz. When the pattern representing “1” is applied to BPG, each probe light is all-optical phase-modulated due to the XPM effect, which is then injected into DWRMD to generate a UWB pulse appearing in the corresponding time slot. Conversely, when the pattern representing “0” is applied, the probe light keeps constant and no UWB pulse is generated. The temporal waveforms of OOK for UWB pulses propagating over a fiber link of 75 km with data sequences of “101011” and “111111”are shown in Fig. 8(a) and (c). The corresponding electrical spectrum of the data sequence of “101011” is displayed in Fig. 8(b) with a central frequency of ~7.67 GHz and a 10-dB bandwidth of ~6.94 GHz, presenting a fractional bandwidth of ~91%. As shown in Fig. 8(d), the corresponding electrical spectrum of the data sequence “111111” has a central frequency and a 10-dB bandwidth of ~8.52 and ~8.14 GHz respectively, indicating a fractional bandwidth of ~96%. The measured results present that the OOK modulated UWB pulses perform well both in waveforms and corresponding electrical spectra.
After propagating over 75-km SMF, the generated triplets are analyzed as the bias voltage of DWRMD () tunes continuously in the range of 0-4V. Figure 9 reveals the central frequency and 10-dB bandwidth as a function of . As shown in Fig. 9, there is no UWB pulses generated when is tuned in the range of 2.4-3.4V due to the coherent cancellation in DI influenced by the corresponding , while the left and right parts are UWB regions. The central frequency and 10-dB bandwidth have the overall growth of different levels when is tuned from 0 to 1.6V. Meanwhile, they slightly decreases when is set in the range of 1.6-2.4V and 3.4-4.0V. Thus, to some extent, the DWRMD might also be reconfigured to slightly optimize the corresponding electrical spectra obtained in base station in order to better meet the FCC’s definition by appropriately tuning in a small range.
The WDM transmission system for the multi-access of UWB signal is shown in Fig. 10 . A pair of AWGs is applied to multiplex and demultiplex the generated UWB pulses at eight different wavelengths as different communication channels for different users, while the polarity of the UWB pulses at each wavelength can be changed by choosing corresponding output ports for multiplexing. The optical spectra of the eight-wavelength probe lights after multiplexing are shown by an inset in Fig. 10. Since the pulse repetition rate for every channel is 1.25 GHz, the total transmission rate of UWB pulses in the SMF after multiplexing reaches 10 Gbit/s. Figure 11 depicts the pulse width, the central frequency and 10-dB bandwidth of the triplets propagating over 25-km SMF as a function of the eight wavelengths of the probe light, corresponding to the eight communication channels. Since the wavelength spacing of adjacent probe lights may not be precisely equal to the integral multiples of the period of DI (), the upper and lower FWHM fluctuates a little as shown in Fig. 11(a), which influences little on the temporal shape as shown by the insets for the waveforms of ch1, ch4 and ch8. Meanwhile, for the same reason, there appears very slight fluctuation of the central frequency and the 10-dB bandwidth in different channels, as displayed in Fig. 11(b). By well tuning the wavelength for each channel, the fluctuations can be minimized and a better performance for multi-access communication can be obtained. Though the conversion bandwidth of XPM in HNLF is very wide, the features of devices limit the available bandwidth for wavelength-division multiplexing. Mainly considering the 3-dB bandwidth of the gain spectrum of EDFA implemented for pre-amplifying the probe lights before transmission and the wavelength spacing of 1.6 nm, the number of efficient wavelengths for multiplexing may reach about 25 in the proposal when the number of WDM channels increases synchronously.
Additionally, most approaches for UWB generation exploiting nonlinearities will be involved in the estimation of power budget and system efficiency, mainly due to the relatively low nonlinearity efficiency [7, 9, 11–13]. However, considering that large numbers of probe lights can be added for providing additional channels without changing the signal light power according to the principle of XPM, the system efficiency in the proposed scheme will increase a lot compared with conventional cases. Moreover, when the materials or structures with high nonlinearity are implemented, the system efficiency will be further improved. For example, the chalcogenide rib waveguide or fiber taper can be applied for arousing the XPM, which provides the nonlinearity parameter of several hundred or thousand times that in HNLF .
Finally, it is worth noting that the proposal may show some features for the potentiality to be robust to some effects of the antenna and wireless channel. For example, considering that the pulse shape can vary a lot from monocycle to quadruplet by tuning and , the two bias voltages are beneficial to modify the UWB pulse shape for pre-distortion correction in order to compensate the shape distortion introduced from the antenna or wireless channel.
4. Demonstration and discussion on hybrid modulation
The above experiments demonstrate that the proposed scheme is efficient for multi-wavelength, polarity- and shape-switchable UWB pulse generation and transmission. According to the analysis of reconfiguring methods in principle, the modulation formats of PPM, PSM, PAM and BPSK can also be accomplished independently with the same structure.
One way for realizing the PPM is to change the position of “1” in the fixed pattern for generating UWB pulses, but it is an electrical method for multicasting, only providing discrete pulse positions for PPM. Considering , there are, in every channel, at least three wavelengths of probe light that can be chosen to generate the same UWB pulses, which can be used in all-optical PPM applied for wavelength division multiplexing together with chromatic dispersion in fiber since the UWB pulses are tolerant to chromatic dispersion as substantiated in the experiments. Assuming the chromatic dispersion of the SMF for transmission at wavelength is 17 ps/nm/km, the differences of group delay between those three wavelengths of probe light per kilometer in SMF are about ps/km and ps/km. Hence, when the SMF length is 20 km, the time shift of 108.8 ps and 217.6 ps is obtained only by changing the wavelength of probe light. Moreover, when the length and chromatic dispersion of SMF for transmission is fixed, with the tuned continuously together with in order to let remain constant or have even multiples of shift, the all-optical continuous change of pulse position in the range of more than 200 ps could be easily achieved without changing the shape, polarity or amplitude of UWB pulses.
To implement PSM of UWB pulses from monocycle to quadruplet for multicasting, and should be modified simultaneously for tuning the carrier-suppression ratio and the differential order of the DWRMD respectively. Here we take the PSM of monocycles and triplets as an example. Considering that those two kinds of UWB pulses can be regarded as the first order differential of Gaussian pulses and doublets, we can firstly tune to let equal (N is an integer) so that the DWRMD performs as a first order differentiator. Then, by tuning to adjust the carrier-suppression ratio for alternatively generating the Gaussian pulses or doublets, the PSM of monocycles and triplets can be obtained without changing other characteristics of UWB pulses for other three modulation formats.
For PAM, we can dynamically set a proper power of 1480-nm pump light for EDFA (), which is used for amplifying the signal pulses generated from MZM. Considering the cross-phase modulation index is proportional to the amplitude of those signal pulses, the amplitude of the generated UWB pulses can be modified by continuously changing the power of the 1480-nm pump light, presenting that the all-optical PAM with continuous tuning characteristics can be realized for multicasting. Similar to PPM and PSM, the PAM also has no influence on the other three modulation formats.
Since two groups of output ports of DWRMD can simultaneously provide pairs of polarity-inverted UWB pulses ( and ), there are two methods for realizing BPSK. One method applied for wavelength division multiplexing is to apply certain optical switches between UWB pulse generator and AWG for multiplexing in order to alternatively choose the required polarity of UWB pulses from two groups of output ports of DWRMD, one group of which is from ch1 to ch8 and the other is from ch to ch. The other method for multicasting is to tune to let have a shift of odd multiples of . Thus, the influence on the other three modulation formats, which is caused by BPSK, is also negligible.
Therefore, as PSM, BPSK, PPM and PAM can be independently achieved for multicasting applications by tuning , , and which contribute four degrees of freedom for modulation, the hybrid modulation of those four formats can potentially be obtained for multicasting.
As there is only one BPG in our laboratory that is not enough to implement the four modulation formats together with the generation, the hybrid modulation format for UWB pulses is numerically simulated by using Optisystem 7.0.
The transmission rate of UWB pulses is 1.25 Gbit/s, while the length and chromatic dispersion of SMF for transmission is 25 km and 17 ps/nm/km respectively. Thus, when we choose the different wavelengths with the shift of one spectrum period of DI, the time shift for PPM between two differently modulated pulses is approximately 136 ps. As shown in Fig. 12 , the numerical results present that, with the hybrid modulation of four formats, the UWB signal sequence of 16 pulses can convey 16 states of information in Gray code from 0000 to 1000, which means that the signals of 16 pulses transmit totally the data of 64 bits over the fiber link. The PPM, PSM, PAM and BPSK represent the 4 bits from the highest to lowest respectively. For PPM, pulse position of approximate 450 and 314 ps, measured starting from the beginning of the pulse period, represent 0 and 1, which is technically achieved by setting the wavelength of probe light as and respectively. For PSM, monocycle and triplet represent 0 and 1, which is technically obtained by setting the bias voltage of MZM as and respectively. For PAM, the half amplitude and the full one represent 0 and 1, which is technically accomplished by setting the pump light power for EDFA as and respectively. For BPSK, the negative and positive polarity represent 0 and 1, which is technically realized by setting the bias voltage of DWRMD as and respectively. One of the methods for the technical implementation in a practical way is to use a dual-drive MZM, a 40GHz DI with RF input port for injecting high speed signals in order to induce rapid phase shifting between the two arms, a single-drive MZM and a 80GHz DI with RF input port as a wavelength switch for tuning , , and respectively, which is also the technical scheme chosen for the simulation .
The transient state between “0” and “1” or “1” and “0” in the data signal generates some undesirable parasitic pulses as shown in Fig. 12, which may deteriorate the performance of a UWB communication system. Methods to get rid of the parasitic pulses are under investigation.
Considering that a bit group of digital sequences for information communication is mostly coded from a corresponding sampling value of an analog signal that often changes continuously, the adjacent bit groups may often vary in only one bit with Gray code applied. Therefore, by using hybrid modulation formats and Gray coding, the adjacent UWB pulses may often vary in only one characteristic among position, shape, amplitude and polarity, which is relatively convenient for detection and demodulation in practical applications.
The above demonstration performs well when 4-bit groups code the analog signals. If more bits are used for coding and the number of bits in use is integer multiples of 4, which is common in actual coding process such as the 8-bit bytes for computing, we can divide one whole bit group that represents one sampling value of analog signals into several parts with every part consisting of 4 bits. Additionally, when the number of bits in bit group is not the integer multiples of 4, the division should start from the lowest bit to the highest one so that only the part including the highest bit do not consists of 4 bits. Then the part less than 4 bits can be represented by less than four corresponding formats for hybrid modulation, while the other formats are ignored in demodulation. Thus, the 4-bit Gray coding for hybrid modulation in proposed scheme can also function well. Each pair of corresponding 4-bit parts or shorter than 4-bit parts in adjacent bit groups remains the same or varies in only one bit at most times, indicating that each pair of corresponding UWB pulses in adjacent pulse groups also stays the same or varies in only one characteristic mostly.
The correlation techniques of receiver would be suitable to demodulate the signals, which have been reported for demodulating the four modulation formats separately [34, 35]. There are mainly two types of that receiver architecture, one of which is to use analog circuits with low cost and high-integrated level and the other is to exploit an analog-to-digital converter and digital signal processing (DSP) technology for good robustness to the variety of the wireless transmission link and convenient application of the adaptive detection technology [24, 36].
With the hybrid modulation format demonstrated above, one UWB pulse can convey four bits of binary information, presenting that the transmission bit rate will increase to four times the original one. For example, when repetition rate of UWB pulses in each channel is 2.5 GHz and hybrid modulation of four formats is applied, the actual data rate in transmission reaches 10 Gbit/s, while, with the hybrid modulation of PPM and BPSK applied to 8-channel WDM, the transmission data rate would increase to 40 Gbit/s in fiber links. Moreover, since the principle of the proposed scheme presents that there are more pulse positions suitable for PPM, more pulse amplitudes suitable for PAM and more pulse shapes for PSM, the number of bits of binary information conveyed by one UWB pulse can further increase to six or more, indicating that a higher bit rate for transmission can be achieved. Hence, the hybrid modulation may be one of the potential methods to further increase the bit rate in UWBoF systems. Furthermore, considering that the wireless link may provide another bottleneck for limiting the data rate, the time division multiple access techniques can be applied to divide the high-speed data into different users in the access points, so the high data rate in fiber link can be possible to keep balance with the bottleneck of limited transmission rate in wireless channel. Besides, since one UWB pulse can convey four-bit data or more, the data rate obtained in the receiver still increases to four times the original, presenting that the hybrid modulation may also be potential to further increase the data rate limited by the wireless link. In addition, the proposed hybrid modulation format is suitable for Gray coding, providing a relatively convenient demodulation process and a better compatibility with DSP since Gray code is commonly used for pulse code modulation in DSP for its better tolerance to hardware-induced errors of digital circuits .
We have theoretically and experimentally demonstrated a scheme for optical UWB pulse generation based on a half-carrier-suppressed MZM and a DWRMD. Eight pairs of UWB pulses with inverted polarity at eight wavelengths, i.e. 16 UWB pulses, are simultaneously generated from one signal pulse, while each of the 16 UWB pulses is switchable in shape of monocycle, doublet, triplet, and quadruplet with inverted polarity, presenting that the proposed generation scheme is efficient in multicasting UWBoF systems. By tuning two bias voltages, the wavelength of probe light and the pump light power for EDFA, i.e. , , and , to modify the carrier-suppression ratio of MZM, the differential order of DWRMD, the group delay of optical carrier in UWB transmission and the cross-phase modulation index, PSM, BPSK, PPM and PAM are all achieved for multicasting. The methods for PPM and PAM are all-optical with continuous tuning characteristics. Since each of the four modulation formats operates with no significant influence on the other, we have proposed, for the first time to the best of our knowledge, a UWB hybrid modulation of those four formats based on the proposed scheme. With Gray coding applied, the UWB hybrid modulation format increases the transmission bit rate to four times the original with a relatively convenient demodulation process and a better compatibility with DSP, showing a potential to break the transmission bit rate barrier formed by FCC mask. Moreover, considering that the PPM and BPSK of UWB pulses at eight wavelengths can be achieved independently by tuning the wavelength of each probe light and applying eight optical switches at the output of the proposed UWB generator to choose the required polarity of UWB pulses respectively, the PPM and BPSK can be obtained separately for eight different channel signals. Therefore, the PSM and PAM or the hybrid modulation of the two can be used for multicasting, while the PPM and BPSK or the hybrid modulation of them for wavelength division multiplexing in separate modulation of eight or more channel signals. Furthermore, the long distance and dispersion-compensation-free transmission over a SMF link of 75 km for the OOK and 25 km for 8-channel WDM of UWB pulses performs well both in temporal waveforms and corresponding electrical spectra, presenting that the generated UWB pulses have a good tolerance to dispersion over fiber and a possible application in wavelength division multiple access. In addition, by replacing the HNLF with another similar highly-nonlinear component that is easy for integration  and using the photonic integrated circuit (PIC) technology, the whole scheme structure might be integrated in a single chip with the advantages of smaller size, lower loss, better tuning characteristics, and better compatibility with other optical integrated processing units. Consequently, the proposed approach is potential for future multi-shape, multi-modulation, multi-casting, multi-user, high-speed, dispersion-compensation-free transmission and PIC-based UWBoF communication systems.
This work was supported by the National High Technology Research and Development Program of China (Grant No.2009AA03Z410) and the National Natural Science Foundation of China (Project number: 60977044 and 61077051).
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