We propose and demonstrate a novel approach to optically generate ultrawideband (UWB) monocycle pulses by exploiting the parametric attenuation effect of sum-frequency generation (SFG) in a periodically poled lithium niobate (PPLN) waveguide. The SFG process changes the continuous-wave pump into dark optical pulse pump with undershoot, resulting in the generation of UWB monocycle through the combination of input signal and output pump with proper relative time advance/delay. Pairs of polarity-inverted UWB monocycle pulses meeting the UWB definition of U. S. Federal Communications Commission (FCC, part 15) are successfully obtained in the experiment.
© 2009 Optical Society of America
Ultrawideband (UWB) technology has attracted considerable interests for short-range large-capacity wireless communication systems and sensor networks due to many advantages, such as high data-rate, huge bandwidth, low power consumption, low spectral density, and immunity to multipath fading . The U. S. Federal Communications Commission (FCC) has regulated the 7.5 GHz spectral band from 3.1 to 10.6 GHz for unlicensed use of UWB with a 10 dB spectral bandwidth larger than 500 MHz or a fractional bandwidth greater than 20% . By wireless transmission, UWB communications systems can only support a very short propagation distance from a few meters to tens of meters. Such short-range wireless networks can operate mainly in an indoor environment [3, 4].
Recently, a robust technology called UWB-over-fiber has emerged to take the full advantages offered by both UWB and optical fiber, which provides an effective solution to overcome the UWB short-range wireless transmission limitation. UWB-over-fiber offers availability of undisrupted service across different networks and eventually enables high-data-rate access at any time and from any place. Note that there is a strong demand for UWB-over-fiber to generate, modulate and distribute UWB signals directly in the optical domain . Several approaches have been reported to optically generate UWB pulses showing impressive operation performance, such as UWB monocycle and doublet generation based on phase modulation to intensity modulation (PM-IM) conversion using an optical frequency discriminator [5–7], UWB monocycle generation based on cross-gain modulation (XGM) in a semiconductor optical amplifier (SOA) , all-fiber UWB pulse generation based on spectral shaping and dispersion-induced frequency-to-time mapping , switchable UWB pulse generation using a reconfigurable photonic microwave delay-line filter , and UWB pulse generation utilizing a Sagnac-interferometer-based intensity modulator .
In this paper, we propose and demonstrate for the first time to our best knowledge, another novel scheme to generate all-optical UWB monocycle pulses by exploiting the parametric attenuation effect of sum-frequency generation (SFG) in a periodically poled lithium niobate (PPLN) waveguide. PPLN is a promising candidate widely applied to various optical signal processing applications , including wavelength conversion , format conversion , and logic gate . However, so far PPLN-based UWB monocycle generation has not yet been investigated. Here we report the experimental demonstration on PPLN-based all-optical generation of pairs of polarity-inverted UWB monocycle pulses.
2. Experimental setup and operation principle
Figure 1 shows the experimental setup and principle of operation for PPLN-based all-optical UWB monocycle generation. The input data signal has a fixed pattern “10000000” at 10 Gbit/s with non-return-to-zero (NRZ) modulation format, which is equivalent to a 10 GHz optical pulse train with a pulse width of 100 ps. The data signal is then divided into two paths. The first path input data signal, together with a continuous-wave (CW) pump (point A in Fig. 1) generated from an external cavity laser (ECL), are combined by an optical coupler (OC2), amplified using a high-power erbium-doped fiber amplifier (HP-EDFA) with a small-signal gain of 40 dB and a saturation output power of 30 dBm , polarization state adjusted through a polarization controller (PC), and finally launched into PPLN to participate in the SFG nonlinear interaction. For the SFG process (ħvS + ħvP = ħvSF), signal (ħvS) and pump (ħvP) photons are annihilated to generate sum-frequency (ħvSF) photons. Thus the pump is consumed when the signal is present due to the parametric attenuation effect of SFG. As a result, the CW pump is evolved into a dark optical pulse with undershoot present at the output of PPLN after experiencing the SFG process (point B in Fig. 1). The output pulsed pump from PPLN is selected using a tunable filter (TF), and then combined with the second path input data signal to generate optical UWB monocycle pulse. Note that it is possible to produce pairs of polarity-inverted optical UWB monocycle pulses by properly controlling the optical power of the second path data signal with a variable optical attenuator (VOA) and appropriately adjusting the relative time advance/delay between the second path data signal and pump using an optical tunable delay line (ODL). As illustrated in Fig. 1, we can obtain positive UWB monocycle pulse as the signal is advanced in comparison to the pump, while achieve negative UWB monocycle pulse when the signal is delayed compared to the pump. The photograph of PPLN waveguide is also depicted in Fig. 1. It is fabricated by the electric-field poling method and annealing proton-exchanged (APE) technique. It has a length of 50 mm, a waveguide width of 12 μm, an initial proton exchange depth of 0.8 μm, a microdomain period of 14.7 μm, and a quasi-phase matching (QPM) wavelength of about 1542 nm at room temperature during the experiment. The optical spectra are observed by an optical spectrum analyzer (OSA, Anritsu MS9710C) with the highest spectral resolution of 0.05 nm, and the temporal waveforms are monitored by a communications signal analyzer (CSA, Tektronix 8000B).
3. Experimental results
Figure 2 depicts the typical measured optical spectra for UWB monocycle generation using SFG in a PPLN waveguide. The signal and pump in the 1.5 μm band are set at 1546.5 and 1537.6 nm respectively to satisfy the SFG QPM condition. The SFG process consumes signal and pump and produces a new sum-frequency (SF) wave in the 0.7 μm band at 771.0 nm as shown in the inset of Fig. 2.
Figure 3 displays the temporal waveforms of different optical waves for PPLN-based UWB monocycle generation. It is interesting to find that the input CW pump is changed into output dark optical pulse pump with undershoot owing to the parametric attenuation effect of SFG process. The UWB monocycle pulse can be obtained through the combination of input signal and output pump. As shown in Fig. 3, R1 represents the produced positive UWB monocycle pulse as the signal is ahead of the pump by 100 ps. R2 denotes the generated negative UWB monocycle pulse when the signal drops behind the pump by 100 ps. Thus a pair of polarity-inverted UWB monocycle pulses can be achieved based on SFG in a PPLN waveguide. Figure 4 plots the temporal waveforms for UWB monocycle pulses with the time advance/delay between input signal and output pump taking different values. R3-R5 respectively indicate the obtained positive UWB monocycle pulses with decreasing time advance (80 ps, 60 ps, 40 ps) between input signal and output pump. R6-R8 respectively correspond to the achieved negative UWB monocycle pulses with decreasing time delay (70 ps, 40 ps, 20 ps) between input signal and output pump. Therefore, pairs of polarity-inverted UWB monocycle pulses are successfully generated. The typical powers of UWB monocycle pulses shown in Figs. 3 and 4 are measured to be about 2 mW.
To further confirm the successful realization of PPLN-based optical UWB monocycle generation, Fig. 5 shows the envelopes of the radio-frequency (RF) spectra for measured temporal waveforms of UWB monocycle pulses. For the obtained a pair of polarity-inverted UWB monocycle pulses R1 and R2 shown in Fig. 3, the measured corresponding RF spectra shown in Figs. 5(a) and 5(b) have the respective center frequency of 3.32 and 3.39 GHz, 10 dB bandwidth of 5.53 and 6.16 GHz, and therefore fractional bandwidth of 166.57% and 181.71%. It is noted that the obtained RF spectra are in good agreement with the FCC UWB definition (FCC, part 15). For the generated pairs of polarity-inverted UWB monocycle pulses R3-R8 shown in Fig. 4, the corresponding RF spectra are shown in Figs. 5(c)–5(h). It is found that the RF spectra for R3-R8 also satisfy the FCC UWB definition (FCC, part 15). Figure 6 depicts the center frequency, 10 dB bandwidth, and factional bandwidth of UWB monocycle pulses as a function of the relative time advance/delay between input signal and output pump corresponding to Fig. 5. It can be seen that these key parameters of UWB monocycle pulses change slightly with the relative time advance/delay and all the obtained UWB monocycle pulses meet the FCC UWB definition (FCC, part 15). Therefore, the proposed PPLN-based UWB monocycle generation has good tolerance to the relative time advance/delay.
The proposed PPLN-based UWB monocycle generation using parametric attenuation effect of SFG process has shown several advantages and disadvantages.
PPLN has distinct features of ultrafast response and negligible spontaneous emission noise, thus ultrafast operation is available and negligible spontaneous emission noise is introduced for PPLN-based UWB monocycle generation. These are attractive for high data rate UWB communications.
Note that the suggested PPLN-based UWB monocycle generation can produce pairs of polarity-inverted UWB monocycle pulses, providing a potential to perform pulse polarity modulation (PPM) (positive-negative). However, in the current experiment, the switching speed of the positive and negative UWB monocycle pulses is low because of the use of mechanically tunable optical delay line, which causes great limitation to the practical highspeed PPM. Several potential solutions are found as follows. First, taking into account the electrically tunable operation for PPM , similarly, it can effectively improve the speed of PPM with the proposed PPLN-based scheme by using electrically tunable optical delay line. Second, we can employ two optical delay lines with different time delays, respectively corresponding to the generation of positive UWB monocycle and negative UWB monocycle pulses. Two optical delay lines are parallel-arranged and on-off-connection controlled using a ‘Switch’ with fast switching speed. Thus high-speed PPM maybe available. Third, the mechanically controlled optical delay line can be replaced by the optically controlled slow-light device [17, 18] which can offer ultrafast switching speed between different time delays. This can also help to achieve high-speed PPM.
Remarkably, although the obtained UWB monocycle pulses in the experiment satisfy the FCC part 15 definition (center frequency: 3.1~10.6 GHz, fractional bandwidth: >20%), they are not exactly in the FCC defined UWB spectral mask. The spectra violate the mask in the frequencies less than 1.6 GHz as shown in Fig. 5. Some previous experimental demonstrations also suffered from the same deficiency [8, 9]. Several efforts are considered to make the UWB pulses conform to both the FCC part 15 definition and the FCC specified UWB spectral mask. First, to respect the FCC spectral mask for indoor communications, it is possible to properly reshape the spectrum by additional filtering technology . Second, the spectrum of the generated UWB pulses can be adjusted by carefully optimizing the pulse shape and pulse width of input data signal. Third, it is expected to generate higher-order UWB pulses such as UWB triplet which can exactly conform to FCC spectral mask .
Additionally, in the experiment, the input data signal has a fixed pattern “10000000” at 10 Gbit/s with NRZ modulation format. The NRZ format is similar to super-Gaussian pulse shape with sharp front and rear edges which might excite higher-frequency components as shown in Fig. 5. This may cause frequency violation against FCC specified spectral mask (indoor limit) and bring limitation to practical applications. As a potential improvement, we can use input data signal with return-to-zero (RZ) format to reduce the higher-frequency components. RZ format with Gaussian pulse shape has slow-varying and smooth front and rear edges. Thus negligible higher-frequency components are excited.
Furthermore, not only UWB monocycle, but also UWB doublet, UWB triplet and other higher-order UWB pulses can potentially be generated by appropriately choosing the power level and pulse composition of input data signal. UWB monocycle, UWB doublet, and higher-order UWB pulses correspond to different order derivatives of Gaussian pulse. In fact, all the derivatives of the Gaussian pulse can be regarded as the combination of ‘overshoots’ and ‘undershoots’. Input data signal providing different kinds of ‘overshoots’ can easily be obtained by dividing periodic pulse train into several branches which are then relatively delayed, attenuated, and finally recombined together. The CW pump is evolved into optical pulse with ‘undershoots’ after experiencing parametric attenuation by SFG in a PPLN waveguide. The combination of ‘overshoots’ and ‘undershoots’ with proper delay creates UWB pulses. Figs. 7 and 8 depict the schematic illustration of PPLN-based pairs of polarity-inverted UWB doublet and UWB triplet generation, respectively. Similarly, it is expected to further generate higher-order UWB pulses, such as UWB quadruplet, UWB quintuplet, etc.
Future detailed theoretical calculations and experimental demonstrations on PPLN-based higher-order UWB pulse generation are under way to be further investigated.
As a side consideration, the use of HP-EDFA, TF, ODL, and VOA more or less increases the cost and complexity of the proposed PPLN-based UWB monocycle generation scheme. With future improvement, it is possible to achieve a low-cost realization along several potential directions. First, the HP-EDFA can be replaced by a common EDFA by adopting a higher efficient PPLN. Moreover, the EDFA maybe further removed by using A-PPLN operating at ultralow-power level . Second, the TF and ODL can be removed simply by adding a piece of cost-effective single-mode fiber (SMF) after PPLN. The signal and output pump after PPLN can directly be delayed by chromatic dispersion of SMF. Thus it is possible to obtain UWB pulses at the output of SMF. In addition, VOA can also be removed by carefully adjusting the powers of input data signal and input CW pump.
We have reported a new approach to generate all-optical UWB monocycle pulses by using SFG in a PPLN waveguide. Due to the parametric attenuation effect of SFG process, the input CW pump is evolved into output dark optical pulse pump with undershoot present, and the combination of input signal and output pump with proper relative time advance/delay produces UWB monocycle pulses. PPLN-based all-optical generation of pairs of polarity-inverted UWB monocycle pulses are successfully demonstrated in the experiment. The obtained results of UWB pulses (pulse shape, center frequency, 10 dB bandwidth, and fractional bandwidth) conform to the FCC part 15 definition.
This work was supported by the Natural Science Foundation of Hubei Province of China under Grant 2008CDB313 and National Natural Science Foundation of China under Grant 60577006. The authors would like to sincerely thank M. M. Fejer and J. R. Kurz at Stanford University for fabricating the PPLN waveguide used in the experiments.
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