1. Introduction

Ultra-wide band, known as one of the most promising technologies in next generation short-range broadband wireless communications and sensor networks, has attracted considerable research interest because of its advantages such as immunity to multipath fading, low power consumption, and high data rate [1]. The U.S. Federal Communication Commission (FCC) has regulated the 7.5 GHz spectral band from 3.1 GHz to 10.6 GHz for the unlicensed use of UWB with a 10 dB bandwidth larger than 500 MHz or a fractional bandwidth greater than 20% [2].

The generation of UWB signals in the electrical domain has been extensively implemented. However, the restriction on the maximum transmission power spectrum density limits the operating range to less than tens of meters. In order to offer undisrupted service across different networks and to eventually achieve high-data-rate access at any time and from any place, UWB-over-fiber (UWBoF), in which UWB signals are distributed over a long distance using a fiber link, is an attractive solution. As a result, it is highly desirable to generate UWB signals directly in the optical domain without the need for E/O conversion.

Many photonic approaches have been reported for generating UWB pulses. These include, amongst others, the conversion of phase modulation to intensity modulation (PM-IM), photonic microwave delay line filters, optical spectrum shaping followed by frequency-to-time mapping, spectral line-by-line shaping, and nonlinear approaches; an overview of many of these techniques is provided in [3–5] and the references therein. However, most of the previously proposed UWB signal generation techniques focus on point-to-point communications. On the other hand, multiple access communications employing pulsed UWB technologies has drawn significant interest [6]. For UWBoF systems, multicasting using all-optical wavelength conversion followed by a wavelength selective device can be used for emerging bandwidth-intensive applications (such as video conferencing and video-on-demand service) over high-speed optical networks [7]. Therefore, point-to-multipoint multicasting using all-optical approaches will be an important function for future UWBoF networks.

Several approaches for multicasting UWB signals have been demonstrated, for example, utilizing cross-gain and cross-phase modulation (XGM/XPM) in semiconductor optical amplifiers as well as four wave mixing (FWM) in highly nonlinear photonic crystal fibers (HNL-PCF) [7–9]. Recently, we demonstrated a novel approach to generate UWB doublet pulses based on a nonlinear optical loop mirror (NOLM) [10]. In this paper, we extend the application of our method to generate monocycle pulses, as well as to provide multicasting capability.

2. Principle and experiment setup

Consider the NOLM shown in Fig. 1(a) . We take advantage of being able to shift the transfer function of the NOLM for pulse shaping in order to generate a pulse doublet from a Gaussian input pulse. In particular, Fig. 1(b) illustrates how the transfer function can be shifted for different values of φ,which represents the static phase shift between the two orthogonal polarization directions of the circulating waves, by controlling the polarization controller (PC) [11]. Therefore, by using the modified transfer function of the NOLM, a pulse doublet can be generated with proper pump power, as shown in Fig. 2 . On the other hand, if we inject a second weak probe signal to co-propagate with the pump through the highly nonlinear fiber (HNLF) only (i.e., not through the NOLM), then the probe will acquire a nonlinear phase shift which can be converted to a monocycle via phase-to-intensity conversion using an optical filter for frequency discrimination [3]. Note that the generation both monocycle and doublet pulses are based on XPM, which can be efficient over a relatively broad range of wavelengths. In [12], multicasting of 8 × 10 Gb/s RZ signals was demonstrated using a NOLM with very small crosstalk among each channel and power penalty compared to back-to-back operation. Therefore, multicasting operation for UWB generation can also be realized in our approach.

 

Fig. 1 (a) Schematic of a nonlinear optical fiber loop mirror. (b) Simulation of the NOLM transfer function for different values of φ.

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Fig. 2 Conventional transfer curve and modified transfer curve of NOLM.

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Figure 3(a) shows our experimental setup for the generation and multicasting of UWB pulses. An external cavity laser (ECL) at 1550.07 nm is modulated using a Mach-Zehnder modulator (MZM1) driven by an impulse generator to create a Gaussian pulse train at a repetition rate of 2.5 GHz. A second modulator, MZM2, driven by a PRBS from a pulse pattern generator (PPG) is used for amplitude (OOK) modulation. The modulated Gaussian pulse train is then amplified by an EDFA and launched into the NOLM as the pump signal through a WDM coupler (WDM1) together with a CW probe signal from ECL2 (1553.85 nm or 1554.06 nm). The length of HNLF in the NOLM is 1 km long with a zero-dispersion wavelength at 1552 nm, a dispersion slope of 0.02 ps/(nm²⋅km), and a nonlinear coefficient of 10 W⁻¹⋅km⁻¹. A second WDM coupler (WDM2) is used to remove the probe from the NOLM. A tunable bandpass filter (BPF) located at the output of WDM2 is used as frequency discriminator to generate monocycle pulses. For generating and multicasting doublet pulses, four CW probe DFB lasers at 1536.61 nm, 1537.41 nm, 1538.16 nm, and 1538.97 nm are combined by an arrayed waveguide grating (AWG) and launched into the NOLM via an optical circulator and a 3 dB coupler. The circulator is used to retrieve the reflected signal from the NOLM. A second EDFA (EDFA2) is used to compensate the insertion loss of the system, estimated to be 6 dB. A second AWG is used to separate the wavelength multicast doublet pulses. Temporal waveforms and eye diagrams are measured in the optical domain using an optical sampling module with 65 GHz bandwidth connected to a digital communications analyzer (DCA). For RF spectrum and BER measurements, the optical signals are first converted to an electrical signal using a high bandwidth photodiode (PD). The BER is measured based on the photonically generated UWB signals and does not involve wireless transmission.

 

Fig. 3 (a) Schematic diagram of the experimental setup. (b) Optical spectrum measured at the output of the NOLM.

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3. Results

First, we disconnected MZM2 so that a periodic Gaussian pulse train was injected as the pump to the NOLM. By tuning the PCs at the output of each DFB laser as well as the one placed in the NOLM, doublet pulses at 4 wavelengths are obtained. By tuning the wavelength of ECL2 or the BPF, positive and negative monocycles can also be generated. The spectral output from the NOLM is shown in Fig. 3(b). The lower power of the pump signal and the monocycle is due to suppression by WDM2. Figure 4 shows the waveforms for the generated doublet and monocycle pulses, and the corresponding electrical spectra. Note that the negative doublet pulses are obtained from the reflection port of the NOLM due to the complementary relationship between the two ports. As we can see, the electrical spectra of doublet pulses are almost compliant with the requirements of FCC UWB mask (the pulses are not fully compliant owing to the inherent characteristics of monocycle and doublet pulses; more complex pulse shapes are needed to ensure full compliance with the FCC mask). In particular, the RF spectra show that the power spectral density around 1 GHz exceeds slightly the FCC mask. We believe this can be improved by introducing an electrical low pass filter after the signal. The monocycle pulses have a larger frequency component at 2.5 GHz, but we believe that better performance can be achieved by using a more properly designed filter such as one based on a fiber Bragg grating [3]. The fractional bandwidth of the monocycle pulses are around 100% while those of the doublet pulses range from 100% to 133%.

 

Fig. 4 Waveforms and corresponding electrical spectra (resolution bandwidth = 300 kHz) of each channel. (a) 1536.61 nm, (b) 1537.41 nm, (c) 1538.16 nm, (d) 1538.97 nm, and (e) 1553.85 nm or 1554.06 nm.

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Next, MZM2 was driven by a 2¹⁵ - 1 PRBS generated by the PPG to produce a 2.5 Gb/s OOK signal as the pump. As expected, five multicast UWB signals were successfully generated at the output of the NOLM and WDM2. Figure 5 shows the eye diagrams for the generated doublet and monocycle OOK signals, and the corresponding electrical spectra. The fractional bandwidth is still compliant with the requirement of the FCC. We believe more channels can be added in our system since the XPM effect in HNLF is efficient over a relatively broad range of wavelengths and crosstalk between adjacent channels should be low.

 

Fig. 5 Measured eye diagram and corresponding electrical spectra (resolution bandwidth = 300 kHz) of each channel. (a) 1536.61 nm, (b) 1537.41 nm, (c) 1538.16 nm, (d) 1538.97 nm, and (e) 1554.06 nm.

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The measured BER performance of each multicast UWB signal is illustrated in Fig. 6 . Error-free performance can be achieved for every channel. It should be mentioned that the BER measurement is based on the generated optical signals and we set the sampling times at the center of the doublet pulse or at the center of the positive cycle in the monocycle pulse and adjust the threshold accordingly. The purpose of these measurements is to demonstrate the similarity in performance of each multicast signal. The performance of the 4 doublet channels are very similar (less than 1 dB variation between the best and worst channels). The monocycle pulses have a stronger continuous wave component; therefore, higher power is required to achieve error-free performance.

 

Fig. 6 Measured BER of each multicast channel (doublet pulses and monocycle pulses).

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4. Conclusion

We have theoretically and experimentally demonstrated an all-optical point-to-multipoint UWB signal generation technique based on a NOLM. A PC is placed in the NOLM to change the transfer function so that doublet pulses can be generated. At the same time, a probe signal can be injected into the NOLM together with the pump and followed by filtering using a BPF to produce monocycle pulses. These additional functionalities represent significant advances compared to our preliminary work presented in [10]. Since both positive and negative monocycle and double pulses can be produced, it should be easy to realize multiple modulation formats for UWB signals such as OOK, pulse polarity modulation, and pulse shape modulation. On the other hand, there are some limitations of our proposed scheme such as high power consumption and increased complexity. These may be improved in part by using short lengths of HNLF or using silicon photonics-based devices. We believe that our scheme has significant potential applications in point-to-multipoint UWBoF systems.