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• Abstract

SECTION 1

## Introduction

Ultrawideband (UWB) technology has been attracted great interest because it is a promising solution of short-range broadband wireless communication and sensor network for its intrinsic advantages, such as low power consumption, immunity to multipath fading, high data-rate, low required signal-to-noise ratio, and huge bandwidth [1], [2], [3]. According to the regulation of the U.S. Federal Communications Commission (FCC) Part 15, a UWB signal has a 10-dB bandwidth larger than 500 MHz or a fractional bandwidth greater than 20% with a power spectral density no more than −41.3 dBm/MHz. The FCC has regulated the 7.5-GHz frequency band from 3.1 GHz to 10.6 GHz in the centimeter wave (CMW) band for unlicensed use of UWB. A plenty of schemes have been reported to generate CMW-UWB signal in optical domain so that the UWB signal can be distributed over low-loss optical fiber for a long distance [4], [5], [6], [7], [8], [9]. The FCC also allocates several millimeter-wave (MMW) bands (around 24 GHz and 60 GHz) for UWB applications in MMW bands [10]. The generation of MMW-UWB signals in the optical domain can reduce the high cost of the MMW electrical circuits or devises [11]. Additionally, the “bottleneck” in the electrical domain can limit the generation of MMW UWB with higher frequency. There is also great demand to integrate the UWB (or MMW-UWB) systems into fixed or wireless networks without distance limitations. Thus, many experiments of photonic MMW-UWB signal generation based on frequency up conversion have been reported, such as using a highly nonlinear fiber (HNLF) [12], using polarization rotation effect in a semiconductor optical amplifier (SOA) [13], using cascaded Mach–Zehnder modulators (MZMs) [14], and using cascaded an SOA and an MZM [15]. Among these schemes, [12] demonstrated an all-optical MMW-carrier generation, which can be upgraded to generate 60-GHz band UWB easily. However, the power consumption is very high, since the HNLF is a passive device and high pump power is needed. In [13], [14], [15], relevant lower power-consuming solutions at the cost of bulky and complex system is presented.

In this paper, a novel scheme to realize 24-GHz MMW-UWB generation by using SOA-based nonlinear optical loop mirror (NOLM) is proposed and demonstrated. A 20-GHz optical clock as the probe light and a Gaussian pulse as the pump light are launched into the NOLM for cross-phase modulation (XPM). At the reflection port and transmission port of the NOLM, a pair of polarity-reversed MMW-Gaussian pulses is generated due to constructive and destructive interferences, and then it is mixed with proper time delay and orthogonal polarization states. Hence, a photonic MMW-UWB monocycle is obtained. By adjusting the time delay, bipolar MMW-UWB pulses are implemented. Compared with [12], [13], [14], [15], our scheme exhibits the advantages of compact structure and all-optical operation. And the MMW-UWB waveforms can be easily altered by changing the pulsewidth of input Gaussian pulses or time delay between the two output ports of the NOLM.

SECTION 2

## Operation Principle

Fig. 1 shows the schematic diagram of the MMW-UWB signal generation. Considering the NOLM shown in Fig. 1, a clock signal as the probe light is taken into the NOLM, whose intensity can be written as TeX Source $$P_{\rm in} = \hbox{DC} + P_{0}\cos(2\pi f_{0}t + \varphi_{0})\eqno{\hbox{(1)}}$$ where DC is the power of the direct current component and $P_{0}$ is the power of the alternating current component. $f_{0}$ denotes the frequency of the clock signal and $\varphi_{0}$ is the initial phase of the clock. Then the probe light is split to two halves, clockwise (CW) and counterclockwise (CCW), by an optical coupler (OC1). A pump light is launched into the loop simultaneously through another coupler (OC2) and co-propagates in the nonlinear element (NLE) with the CW light for XPM. Thus a phase shift is induced to the CW light by the pump power. The phase shift is demodulated to intensity information when CW light and CCW light interfere at OC1. Then the power of the probe light is separated two components, one is output from the pass port (up-arm) of NOLM and the other is from the reflection port (down arm).

Fig. 1. Schematic diagram of the MMW-UWB signal generation using an NOLM.

The power of these two components can be written as [16] TeX Source \eqalignno{P_{\rm pass} = &\, {1 \over 4}P_{\rm in}\left\{1 - \cos\left[\varphi\left(P_{\rm pump}(t)\right)\right]\right\}&\hbox{(2)}\cr P_{\rm refl} = &\, {1 \over 4}P_{\rm in}\left\{1 + \cos\left[\varphi\left(P_{\rm pump}(t)\right)\right]\right\}&\hbox{(3)}} where $P_{\rm pump}$ is the temporal power of the pump light. $\varphi (P_{\rm pump})$ is the phase shift of probe light induced by the XPM effect in the SOA and it is proportional to the pump power if the NLE is an HNLF [16]. The polarization states of these two components are tuned to be orthogonal by polarization controllers (PCs). They are then coupled by a polarization beam coupler (PBC) while the reflection component is time delayed by $\Delta t$ which can be tuned by an optical tunable delay line (OTDL). In this way, the power summation of these two components can be expressed as TeX Source $$P_{\rm out} = P_{\rm pass}(t) + P_{\rm refl}(t + \Delta t)\eqno{\hbox{(4)}}$$ when $\Delta t = NT_{0}$, where $T_{0} = 1/f_{0}$ and $N$ is an integer, (4) can be simplified as TeX Source $$P_{\rm out} = {1 \over 4}\left[\hbox{DC} + P_{1}\cos(2\pi f_{0}t + \varphi_{0})\right]\bullet\left\{2 - \cos\left[\varphi\left(P_{\rm pump}(t)\right)\right] + \cos\left[\varphi \left(P_{\rm pump}(t + NT_{0})\right)\right]\right\}.\eqno{\hbox{(5)}}$$ From (5), the output power is a product of two terms. The first term is a clock signal obviously. And the second term is approximate differentiation of input pump power if the phase shift linearly changes with the pump power. Hence, an MMW-UWB pulse can be generated. Assume that the pump light is a Gaussian pulse with a pulsewidth of 100 ps and adequate intensity, $\hbox{DC} = \hbox{P}_{1} =<$> <$>1\ \hbox{mW}$, $f_{0} = 20\ \hbox{GHz}$, $\varphi_{0} = 0$, and $N = 3$, a positive MMW-UWB monocycle can be achieved, as shown in Fig. 2(a). Its electrical spectrum is also calculated as shown in Fig. 2(b). When we change the value of $N$ to −3, a negative monocycle is obtained as well. Its temporal waveform and electrical spectrum are shown in Fig. 2(c) and (d), respectively. Therefore, the bipolar MMW-UWB monocycles have been generated in optical domain.

Fig. 2. Simulated results. (a) and (c) are the waveforms of the positive and negative MMW-UWB monocycles, respectively, and (b) and (d) are the corresponding electrical spectra, respectively.
SECTION 3

## Experimental Demonstration

The experimental setup of MMW-UWB signal generation is shown in Fig. 3. An SOA is used as the NLE to induce a phase shift to the probe light based on the XPM effect. The 1/e gain recovery time of the SOA is about 25 ps. The time interval from the SOA to the middle point of the loop is $\Delta T$. A probe light is emitted from a Laser diode (LD1) with a central wavelength of 1564.12 nm. Then, the probe light is modulated by MZM1, which is driven by a 10-GHz radio frequency (RF) signal. MZM1 is biased at transmission null of its modulation curve. Due to the carrier-suppression modulation (CSM), a 20-GHz optical clock signal is achieved as shown in Fig. 4(a). The average power of the probe light is about 8.7 dBm. The pump light at 1551.41 nm emitted from LD2 is modulated by another modulator MZM2, which is driven by a bit pattern generator (BPG) at a repetition rate of 20 Gb/s with a fix pattern of one “1” per 64 bits. The output of the MZM2 is a sequence of super-Gaussian pulses with a pulsewidth of 65 ps, as shown in Fig. 4(b). The peak pump light is amplified to 6.8 dBm by an erbium-doped fiber amplifier (EDFA), and then fed into the NOLM. According to the analysis of NOLM, the probe light is separated into two components, one is output from the pass port of the loop and the other is from the reflection port. The waveforms of the pass component and the reflection component are shown in Fig. 4(c) and (d), respectively. The envelopes of these two components show a pair of polarity-reversed MMW-Gaussian pulses. The ripple of the waveforms is generated because of the imperfect coherent interference of CW light and CCW light in OC1. The polarizations of these two components are carefully tuned to be orthogonal by two PCs, as shown in Fig. 3, and the reflection component is delayed by the OTDL nearly 150 ps, compared with the pass component. These two components are recoupled by a PBC. Then, the probe light is filtered out using a bandpass filter whose central wavelength is 1564 nm, and 3-dB bandwidth is 3.2 nm. Finally, the light is detected by a photodetector (PD). The output signal shows a positive MMW-UWB monocycle shape as shown in Fig. 5(a), and its electrical spectrum is measured by an electrical spectrum analyzer (ESA), as shown in Fig. 5(b). The measured central frequency and 10-dB bandwidth are 22.82 GHz and 4.26 GHz (from 21.05 to 25.31 GHz), respectively. When the time delay of the reflection component is adjusted at -150 ps, the negative MMW-UWB monocycle is generated as shown in Fig. 5(c), and its electrical spectrum is shown as Fig 5(d). The central frequency and 10-dB bandwidth are 22.24 GHz and 4.35 GHz (from 20.96 to 25.31 GHz), respectively. Comparing Figs. 2 and 5, it is known that the experimental results match well with the simulated results. Since the SOA has a larger gain dynamic bandwidth than pump modulation speed, there are no pattern effects.

Fig. 3. Experimental setup for the MMW-UWB signal generation.
Fig. 4. (a) Waveform of 20-GHz optical clock. (b) Waveform of the input pump pulse. (c) and (d) Waveforms of the pass and the reflection ports of the NOLM, respectively.
Fig. 5. Measured results. (a) and (c) are the waveforms of the positive and negative MMW-UWB monocycles, respectively. (b) and (d) are their corresponding electrical spectra, respectively.

It should be noted that using the CSM scheme, our system is difficult to generate 60 GHz band UWB signal without high-frequency electrical components. The same limitation lies in [13], [14], [15]. But in fact, the 60 GHz MMW carrier can be easily generated using a frequency-quadrupling technique from a low-frequency modulator [17], [18]. The modulator needs to be DC biased at the maximum transmission point. By suppressing the optical carrier, the beating frequency at the PD is frequency-quadrupling. In this way, our scheme can be easily upgraded to generate 60 GHz band UWB signal.

It should be noted that in our scheme, SOA has some distinct features for MMW-UWB monocycles generation, since the pump light phase modulates not only the CW light but the CCW light as well. That means one pump pulse injection will generate a pair of monocycles, one of whom is generated for the phase shift on the CW light, and the other is for the phase shift on the CCW light, as shown in Fig. 6(a). But this phenomenon will not appear when an HNLF is used as the NLE because the pump light will only modulate the CW light. Comparing these two monocycles in Fig. 6(a), one can see these two waveforms are not completely the same. The main reason lies in that the clock phases of CW light and CCW light are different when they interact with the pump light. However, after the MMW-UWB signal is frequency down converted using a low-pass filter with a pass-band from DC to 10 GHz, the waveforms become exactly identical, as shown in Fig. 6(b), and its electrical spectrum is shown in Fig. 6(c). Thus, the parameter $\varphi_{0}$ in (1) has a large tolerance for the system. In fact, the generation of a pair of monocycle pulses is not always harmful. Since the MMW-UWB pulses appear by pair, the output MMW-UWB bit sequence can has a doubled frequency compared to the original input Gaussian pulses. In such case, the time interval of the pair of MMW-UWB signal pulses should be adjusted to be the half of the time interval of the pump pulses.

Fig. 6. (a) Pair of MMW-UWB monocycles generation by one pump pulse. (b) Waveforms of these two monocycles after frequency down-conversion. (c) Electrical spectrum after frequency down-conversion.
SECTION 4

## Discussion

The time delay between the reflection component and pass component of the NOLM and pulsewidth of the pump light need to be optimized to characterize the UWB RF spectrum. First, we set the clock frequency at 20 GHz and its initial phase at zero. The pulsewidth of the pump light is fixed at 100 ps. When the time delay is changed from 50 to 500 ps, the central frequency and 10-dB bandwidth of each MMW-UWB signal have been calculated as shown in Fig. 7. One can see that the 10 dB-bandwidth become smaller and the central frequency shift to the low-frequency domain as the time delay increases.

Fig. 7. Central frequency and 10-dB bandwidth under different time delay.

Then, we fix the time delay of 50 ps and change the pulsewidth. In a practical experiment, the pulsewidth of the pump light can be controlled by editing the number of the successive code “1” of the BPG. Fig. 8 shows the central frequency and 10-dB bandwidth as a function of pulsewidth of pump pulse. The 10-dB bandwidth decreases quickly and the central frequency becomes lower when the pulsewidth increases. Increasing the time delay or the pulsewidth of pump light will make the pulsewidth of monocycle larger, showing a narrower 10-dB bandwidth of RF spectrum. Thus, whether increasing the time delay $\Delta T$ or the pulsewidth of the pump light, the 10-dB bandwidth of MMW-UWB signal will be narrower. Therefore, a high-quality MMW-UWB monocycle could be obtained using a shorter pulse as the pump light or making a short time delay between the two ports of NOLM.

Fig. 8. Central frequency and 10-dB bandwidth under different pulsewidth of pump pulses.
SECTION 5

## Conclusion

We have experimentally demonstrated the generation of bipolar MMW-UWB monocycle with an SOA-based NOLM. A 20-GHz optical clock generated by CSM is launched into the NOLM as a probe light. A Gaussian pulse as a pump light is fed into the loop to induce XPM effect in the SOA. At the reflection and transmission ports of the NOLM, a pair of polarity-reversed MMW-Gaussian pulses will be generated due to constructive and destructive interferences, which will be mixed with proper time delay and orthogonal polarization states to form a MMW-UWB monocycle, and the polarity can be reversed by simply adjusting the time delay between the two ports. The optimization of pulsewidth of pump light and the time delay between the two ports is discussed to characterize the UWB RF spectrum.

## Footnotes

This work was supported in part by the National Basic Research Program of China under Grant 2011CB301704, in part by the National Science Fund for Distinguished Young Scholars under Grant 61125501, and in part by the National Natural Science Foundation of China under Grant 60901006 and Grant 11174096. Corresponding author: J. Dong (e-mail: jjdong@mail.hust.edu.cn).

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