IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 28, NO. 21, NOVEMBER 1, 2016 2363

Generation of Millimeter-Wave Ultra-WidebandPulses Free of Strong Local Oscillation

and BackgroundYuan Yu, Member, IEEE, Fan Jiang, Haitao Tang, Lu Xu, Xiaolong Liu, Jianji Dong, Member, IEEE,

and Xinliang Zhang, Member, IEEE

Abstract— A novel scheme of generating millimeter-wave(MMW) ultra-wideband (UWB) pulses free of strong localoscillation (LO) and background is proposed and demonstrated.The frequency of the optical Gaussian pulses is first up-convertedby a Mach–Zehnder modulator, which is biased at its minimumtransmission point to achieve carrier suppression modulation.Then, the up-converted Gaussian pulses are equally split into twoparts with a relative time delay. At last, the two parts are injectedinto the two arms of a balanced photodetector (BPD) respectively.After the BPD, the low-frequency components are effectivelysuppressed and MMW-UWB pulses free of background andstrong LO are generated. The polarity of generated MMW-UWBcan be converted by adjusting the relative time delay betweenthe two parts. In the experiment, a pair of polarity-reversedbackground-free UWB pulses centered at 25 GHz is successfullygenerated. The 10-dB bandwidth of the MMW-UWB spectra is4.14 and 4.05 GHz, respectively.

Index Terms— Millimeter-wave ultra-wideband (MMW-UWB),background free, balanced photo-detection.

I. INTRODUCTION

ULTRA-WIDEBAND (UWB) is considered to be apromising technology for short-range and high capa-

bility wireless communications and sensor networks due toits intrinsic advantages, such as low power consumption,low cost, and high data date [1], [2]. The U.S. FederalCommunication Committee (FCC) has defined the UWB asany signal that occupies a spectral bandwidth over 500 MHz orfractional bandwidth over 20% with a power spectral density

Manuscript received May 17, 2016; revised July 4, 2016; acceptedJuly 21, 2016. Date of publication July 26, 2016; date of current versionOctober 6, 2016. This work was supported in part by the National NaturalScience Foundation of China under Grant 61501194 and Grant 61475052,in part by the National Science Fund for Distinguished Young Scholarsunder Grant 61125501, in part by the NSFC Major International JointResearch Project under Grant 61320106016, in part by the Foundationfor Innovative Research Groups within the Natural Science Foundation ofHubei Province under Grant 2014CFA004, in part by the Hubei ProvincialNatural Science Foundation of China under Grant 2015CFB231, in partby the Fundamental Research Funds for the Central Universities underGrant HUST 2016YXMS025, and in part by the Director Fund of WNLO.(Corresponding author: Xinliang Zhang.)

Y. Yu, J. Dong, and X. Zhang are with the Wuhan National Laboratoryfor Optoelectronics, School of Optical and Electronic Information, HuazhongUniversity of Science and Technology, Wuhan 430074, China (e-mail:yuan_yu@hust.edu.cn; jjdong@mail.hust.edu.cn; xlzhang@mail.hust.edu.cn).

F. Jiang, H. Tang, L. Xu, and X. Liu are with the Wuhan National Labora-tory for Optoelectronics, Huazhong University of Science and Technology,Wuhan 430074, China (e-mail: fjiang@hust.edu.cn; 2838340058@qq.com;yiquanx@163.com; 303805750@qq.com).

Color versions of one or more of the figures in this letter are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LPT.2016.2594045

no more than −41.3 dBm/MHz [3]. The FCC also approvedthe unlicensed use of the spectral band from 3.1-10.6 GHzfor indoor communicati- ons and 22-29 GHz for vehicularradar systems, respectively [3]. However, in free space, thetransmission of UWB is limited to only a few meters or tensof meters due to its low power spectral density (PSD) [4].The optical fiber can effectively increase the UWB coveragethanks to its ultra-low loss [5]. In UWB over fiber, it isdesired to generate and modulate UWB pulses in the opticaldomain and many approaches have been proposed to generatecentimeter wave (CMW) UWB pulse, which occupies thespectral band from 3.1-10.6 GHz [6]–[8]. Meanwhile, photonicgeneration of millimeter-wave (MMW) UWB signal, whichoccupies the spectral band from 22-29 GHz, has also beenproposed by up-converting the CMW-UWB signals, such asusing the all-optical mixing based on a Mach-Zehnder modu-lator (MZM) [9], [10], optical parameter amplifier (OPA) [11],four wave mixing (FWM) [12], nonlinear polarization rotation(NPR) [13], and cross phase modulation (XPM) [14]. How-ever, there are two main restrictions in these up-convertedMMW-UWB signals. The first is the strong local oscilla-tion (LO) signal always coexisting with the UWB, which willreduce the dynamic range of the receiver and power efficiencyof the UWB pulses. The second is that the low-frequencyspectral components, which are called as the “background”,still exists after frequency up conversion. The backgroundsignals will interfere with existing narrow-band wireless com-munications.

In order to generate MMW-UWB pulses free of LO andbackground, several approaches have been proposed anddemonstrated. It has been proposed to use two cascadedpolarization modulators (PolMs) and a polarizer to truncatea sinusoidal MMW into MMW-UWB pulses [15]. The NPRin a span of highly nonlinear fiber (HNLF) has also beenproposed to generate MMW-UWB pulses [16]. However,besides the electro-optic modulators (EOMs), the state ofpolarization (SOP) in these systems also needs to be preciselycontrolled in order to be aligned at a certain angle to theprinciple axis of the polarizer [15], [16] and aligned with thefast axis of HNLF which is pumped by a control light [16].Moreover, the employed optical bandpass filter (OBF) alsoincreases the system complexity. It is also proposed to generateMMW-UWB pulses by using a dual-parallel Mach-Zehndermodulator (DPMZM) and an OBF [17]. However, the powerof the optical carrier must be sufficiently small to effectively

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2364 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 28, NO. 21, NOVEMBER 1, 2016

Fig. 1. The operation principle of the proposed MMW-UWB generation.(a) is the waveform of an optical Gaussian pulse; (b) shows the superim-position of a pair of polarity-reversed waveforms after up-conversion whenthe upper waveform is ahead of the lower waveform with a relative timedelay of, and (c) shows the waveform after superimposition; (d) shows thesuperimposition of a pair of polarity-reversed waveforms after up-conversionwhen the upper waveform is behind of the lower waveform with a relativetime delay of, and (e) shows the waveform after superimposition.

suppress the low-frequency components. Thus, the generatedMMW-UWB is with a low power and low signal to noiseratio (SNR). It can be noted that a balanced photodetec-tor (BPD) was employed to subtract two monocycle pulseswith a relative time delay to generate a doublet pulse [18], oreliminate the pedestal in the optical pulse [19]. The amplitudeof generated waveform can be further increased by subtractingtwo complementarily modulated waveforms [20]. However,the polarization drift restricts the suppression ratio of thebackground signal.

In this letter, we propose and demonstrate a novel schemeto generate MMW-UWB pulses free of LO and backgroundsignals. In the proposed scheme, an electrical Gaussian pulsesequence is applied to the radio frequency (RF) port of aMZM (MZM1) to generate optical Gaussian pulses. Thenthe optical Gaussian pulses are launched to a second MZM(MZM2), which is biased at its minimum transmission point toachieve carrier suppression modulation (CSM). A sinusoidalMMW is applied to the RF port of MZM2. Therefore, thegenerated Gaussian pulses are frequency up-converted to twicethe frequency of the sinusoidal signal applied to MZM2.The frequency up-converted optical Gaussian pulse is equallydivided into two parts and a relative time delay is introducedbetween the two parts. Then the two parts are launched intothe two arms of a BPD and MMW-UWB pulses are generated.By adjusting the time delay, the polarity of the MMW-UWBcan be converted. In the experiment, a pair of polarity-reversedMMW-UWB pulses centered at 25 GHz is successfully gen-erated. The 10-dB bandwidths of the polarity-reversed MMW-UWB spectra are 4.14 and 4.05 GHz, respectively.

II. OPERATION PRINCIPLE AND SIMULATIONS

The operation principle is illustrated in Fig. 1. An opticalGaussian pulse is generated at first, as shown in Fig. 1 (a).Then the optical Gaussian pulse is frequency up-convertedto the desired frequency by carving the optical pulse witha sinusoidal MMW. The up-converted optical pulse is thenequally split into two parts and a relative time delay of �tis introduced between the two optical pulses. If the twooptical pulses are polarity reversed, as shown in Fig. 1 (b),a MMW-UWB pulse can be generated by superimposing thetwo counter-phase waveforms, as shown in Fig. 1 (c). It should

Fig. 2. Simulation results. (a) and (b) are the Gaussian pulse and itscorresponding spectrum respectively; (c) shows the generated two waveformsat the BPD when the frequency up-converted waveforms are injected into thetwo arms of the BPD; (d) shows the electrical spectrum of the frequencyup-converted waveform; (e) and (f) show the generated waveform at the BPDand its corresponding electrical spectrum.

be noted that adjusting the time delay can convert the polarityof generated MMW-UWB pulse. In Fig. 1 (b), the upperpulse is ahead of the lower pulse with �t . If the upper pulseis behind of the lower pulse with �t , a polarity reversedMMW-UWB pulse can be generated, as shown in Fig. 1 (e).Thus, a pair of polarity-reversed MMW- UWB pulses can begenerated.

Based on the principle illustrated in Fig. 1, a simulation iscarried out and the simulation results are shown in Fig. 2.

Figure 2 (a) and (b) show the Gaussian pulse with full widthat half maximum (FWHM) of 230 ps and its correspondingelectrical spectrum, respectively. It can be observed that theelectrical power is mainly concentrated on the low frequencycomponent. Then the frequency of the Gaussian pulse isup converted to 25 GHz, which is realized by carving theoptical Gaussian pulse using carrier suppression modulation(CSM) in a MZM driven by a 12.5 GHz sinusoidal wave.The generated waveforms are shown in Fig. 2 (c). The redsolid curve and black dotted curve represent the frequencyup-converted Gaussian pulse and polarity-reversed pulse witha time delay of 10 ps, respectively. The electrical spectrum ofthe frequency up-converted Gaussian pulse is shown in Fig. 2(d). It can be observed that a strong frequency component islocated at 25 GHz. However, the low-frequency componentsare also strong enough to interfere the UWB spectrum. Aftersuperimposing the two waveforms shown in Fig. 2 (c), thegenerated waveform is shown in Fig. 2 (e). It can be observedthat an MMW-UWB pulse is generated. The correspondingelectrical spectrum is shown in Fig. 2 (f). It can be observedthat the electrical power is mainly concentrated on the spectralband of around 25 GHz, and the low frequency componentis suppressed 22 dB below the 25 GHz component. Thereis also no strong LO signals in the generated MMW-UWB.Thus, using the delayed and balanced detection structure caneffectively suppress the low-frequency and LO components.

YU et al.: GENERATION OF MMW UWB PULSES FREE OF STRONG LO AND BACKGROUND 2365

Fig. 3. Experimental setup. LD: laser diode; PC: polarization controller;MZM: Mach-Zehnder modulator; BPG: bit pattern generator; LPF: low passfilter; OC: optical coupler; OVDL: variable optical delay line; BPD: balancedphotodetector; DCA: digital communication analyzer; ESA: electrical spec-trum analyzer.

Fig. 4. Measured results after MZM1 and MZM2. (a) and (b) are thegenerated Gaussian pulse and its optical spectrum respectively; (c) and (d) arefrequency up-converted Gaussian pulse and its optical spectrum respectively;(e) is the electrical spectrum of frequency up-converted Gaussian pulse.

III. EXPERIMENTAL RESULTS

In order to verify the proposed scheme, an experimen-tal setup illustrated as Fig. 3 is performed. A continuouswave (CW) light emitted from a laser diode (LD, AlnairTLG-200) is injected into a Mach-Zehnder modulator (MZM1,Fujitsu FTM7937EZ611) via a polarization controller (PC1),which is used to adjust the optical polarization state alignedwith the polarization principle axis of MZM1. The electricalpulse generated in the bit pattern generator (BPG, SHF 44E)is applied to the RF port of the MZM1 via an electrical lowpass filter (LPF). The LPF with FWHM bandwidth of 2 GHzis used to reshape the electrical pulse from a super-Gaussianpulse to a Gaussian pulse. The data rate is 6.5 Gb/s and witha fixed pattern of one “1” per 16 bits, which indicates therepetition rate is 0.41 Gb/s and the duty cycle is 16. Thus, anoptical Gaussian pulse with FWHM of 240 ps can be generatedafter MZM1. The result measured by the digital commu-nication analyzer (DCA, Angilent Infiniium DCA-J86100C)is shown in Fig. 4 (a). Fig. 4 (b) shows the correspondingoptical spectrum measured by the optical spectrum analyzer(OSA, YOKOGAWA AQ6370C). The optical Gaussian pulseis then launched to the MZM2 (Fujitsu FTM7937EZ611)via PC2. An electrical sinusoidal wave with a frequency of12.5 GHz is generated by the signal generator (Agilent,E8247C) and applied to the RF port of MZM2. By adjustingthe bias voltage of MZM2, the CSM is realized. The generatedwaveform is shown in Fig. 4 (c). It can be observed thatthe Gaussian pulse is successfully frequency up converted to25 GHz. Fig. 4 (d) shows the corresponding optical spectrum.It should be noted that the optical carrier is suppressed onlyabout 5 dB lower than the first order sideband, which is smallerthan previously reported result which is achieved by usinga single-drive MZM [21]. This is because the MZM is adual-drive MZM and it is single driven in the experiment.Therefore, large chirp exists in the modulated optical pulseand the chirp leads to the generation of the optical carrier

Fig. 5. Measured MMW-UWB pulses and the corresponding electricalspectra. (a1), and (a2) are measured optical waveforms at the point A andB respectively when the pulse at point A is ahead of the pulse at point B with20 ps; (a3) and (b) are the measured waveform after BPD and the corre-sponding electrical spectrum respectively; (c1) and (c2) are measured opticalwaveforms at point A and B respectively when the pulse at point A is behindof the pulse at point B with 20 ps; (c3) and (d) are measured waveform afterBPD and the corresponding electrical spectrum respectively.

component in the optical spectrum. However, the extinctionratio (ER) of generated frequency up-converted pulse will notbe deteriorated by the chirp because the optical phase variationcannot affect the intensity waveform. In fact, large ER can stillbe observed in the frequency up-converted pulse, as shownin Fig. 4 (c). Therefore, the generated optical carrier cannotdeteriorate the experimental results. The electrical spectrum ofthe frequency up-converted signal is measured by the electricalspectrum analyzer (ESA, Keysight N9030A) and the result isshown in Fig. 4 (e). The FCC mask is also present in Fig. 4(e), shown as the red trace. Compared with the FCC mask,larger low-frequency components can be observed.

At the output of MZM2, the optical pulse is equallydivided into two parts by an optical coupler (OC). An opticalvariable delay line (OVDL) is used to introduce a relativetime delay between the two parts. Then the two parts areinjected into the two arms of a balanced photodetector (BPD,u2t BPDV2020R). After the BPD, the RF power is split intotwo parts by a RF splitter. The generated waveform and thecorresponding RF spectrum are shown in Fig. 5.

The optical waveforms measured at point A and point Bare shown in Fig. 5 (a1) and Fig. 5 (a2), respectively. It canbe observed that the optical waveform measured at point Ais ahead of the optical waveform measured at point B with20 ps. When the two pulses are injected into the BPD, thegenerated MMW-UWB pulse is shown in Fig. 5 (a3). Thecorresponding electrical spectrum is shown in Fig. 5 (b). Itcan be observed that a MMW-UWB pulse is successfullygenerated. The central frequency is 25 GHz, and the10dB bandwidth is 4.14 GHz from 23.94 GHz to 28.08 GHz.It can be observed that the low-frequency components aresuppressed by over 30 dB below the 25 GHz componentsand no strong LO signals exist in the MMW-UWB spectrum.Thus, its electrical spectrum is fully complied with theFCC mask. Then we adjust the OVDL to change the timedelay. When the optical waveform measured at point A isbehind of the optical waveform at point B with 20 ps, themeasured optical waveforms at point A and B are shown

2366 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 28, NO. 21, NOVEMBER 1, 2016

in Fig. 5 (c1) and Fig. 5 (c2), respectively. The measuredMMW-UWB waveform and electrical spectrum after the BPDare shown in Fig. 5 (c3) and Fig. 5 (d), respectively. It canbe observed that a polarity-reversed MMW-UWB pulse issuccessfully generated. The central frequency is also 25 GHz.The 10 dB bandwidth is 4.05 GHz from 23.94 to 27.99GHz. There is also no strong LO signals in the MMW-UWBspectrum. The electrical spectrum also complies with the FCCmask very well. It can be observed that the low-frequencycomponents in the experiment is suppressed by 35 dBbelow the 25 GHz component, which is lower than thatin simulation. This is because there are more frequencycomponents around 25 GHz in the generated MMW-UWBpulse. In Fig. 5 (a3) and Fig. 5 (c3), it can be observed thatthere is oscillation around 25 GHz, which is caused by thenonideal Gaussian pulse. The oscillation contributes the 25spectral components around 25 GHz. Hence, the power of thefrequency components around 25 GHz is much larger thanthat of the low-frequency components.

Thus, a pair of polarity-reversed MMW-UWB pulses issuccessfully generated in the experiment. The backgroundis suppressed around the FCC mask. There is also no LOsignals around 25 GHz. It can be noted that the polarityof MMW-UWB is changed by adjusting the OVDL, whichindicates that the polarity-switching speed is low. However,if an optical switch with high speed is used [22], [23] toswitch the time delay between the two arms of BPD, thepolarity can be switched at a high speed and MMW-UWBbi-phase modulation can be realized. It should be noted thatthe pulse amplitude difference and time delay between thetwo arms of BPD can affect the power efficient and phase-shift ability of MMW- UWB signals respectively. A smalleramplitude difference will result in higher power efficient. If thebackground is required to be suppressed below the FCC mask,which means that the power spectral density (PSD) of thebackground is less than -75dBm/MHz, the amplitude variation,which is defined as the ratio of the amplitude variation andthe amplitude of the frequency up-converted Gaussian pulse,is required to be approximately less than 1%. Meanwhile, inorder to realize the bi-phase MMW-UWB pulses, the timedelay should be less than the cycle time of the MMW. Forexample, in the proposed scheme, the time imbalance shouldbe less than 40 ps.

IV. CONCLUSIONS

In conclusion, a novel approach to generating a pair ofpolarity-reversed MMW-UWB pulses free of background andstrong LO signals is proposed and experimentally demon-strated. Based on the proposed delayed and balanced detec-tion structure, a pair of polarity-reversed UWB impulses issuccessfully generated. The central frequency is 25 GHz, andthe 10 dB bandwidths are 4.14 GHz from 23.94 GHz to 28.08GHz and 4.05 GHz from 23.94 to 27.99 GHz, respectively. Inthe experiment, the background is successfully suppressed byover 30 dB below the 25 GHz component.

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