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We propose and demonstrate, by proof of concept, a novel method of ultra-wide band (UWB) photonic generation using photodetection and cross-absorption modulation (XAM) of multiple quantum wells (MQW) in a single short-terminated electroabsorption modulator (SEAM). As an optical pump pulse excite the MQWs of SEAM waveguide, the probe light pulse with the same polarity can be generated through XAM, simultaneously creating photocurrent pulse propagating along the waveguide. Using the short termination of SEAM accompanied by the delayed microwave line, the photocurrent pulse can be reversed in polarity and re-modulated the waveguide, forming a monocycle UWB optical pulse. An 89ps cycle of monocycle pulse with 114% fractional bandwidth is obtained, where the electrical power spectrum centered at 4GHz of frequency ranges from 0.1GHz to 8GHz for −10dB drops. Meanwhile, the generation processing is also confirmed by observing the same cycle of monocycle electrical pulse from the photodetection of SEAM. The whole optical processing is performed inside a compact semiconductor device, suggesting the optoelectronic integration template has a potential for the application of UWB photonic generation.
©2010 Optical Society of America
1. Introduction
According to the Federal Communications Commission (FCC) in year of 2002, the bandwidth from 3.1GHz to 10.6GHz has been issued for the unlicensed usage of ultra-wideband (UWB) communications [1]. Since then, UWB communications based on the form of impulsive sequence have recently attracted much attention in wireless mobile systems due to its ultra-wideband nature, high data rate, better immunity to multipath propagation, and availability of low-cost and carrier-free transceivers [2,3]. In the UWB communications based on impulsive electrical pulse sequence, the waveform of monocycle or doublet pulses have been demonstrated as a good candidate in radio system [4], intriguing the research interests on generating such waveforms. However, the property of short-distance transmission in UWB communications systems restricts the applications only in indoor or inside building. Alternatively, photonic generation of UWB using optical fiber distribution techniques has become a quite potential to offer the availability of undisrupted service across different networks and eventually achieve high-data-rate access at any time and from any place. It is thus important to attain the photonic impulsive waveform of UWB for the application of optical fiber communications.
There have been several techniques developed in generating monocycle or doublet UWB optical impulse [5–9]. Using a gain switched Fabry-Perot laser diode as light source, a microwave differentiator can be used to generate UWB monocycle pulses [5]. A UWB pulse can be attained by an optical beating between two spectral components of the timely frequency-shift-keying modulation [6]. Based on the cross-gain modulation in semiconductor optical amplifier, two different pulses with different polarities can generate an UWB signal through two cascaded fiber-Bragg-gratings [7]. Using a chromatically dispersive fiber to control the delay time, an optical phase-modulated signal can be transferred to a UWB signal [8]. Using nonlinear polarization in semiconductor-optical-amplifier, up-conversion of optical monocycle UWB has been demonstrated, realizing the application of high-speed photonic UWB generation in radio-on-fiber field [9]. As seeing all the schemes mentioned above, either fiber components or complicated modules are utilized, resulting in the problems of large volume, difficulty to control, and cost. As a result, developing a compact scheme suitable for generating UWB optical pulses thus becomes quite essential for the purpose of involving optical fiber communication fields. In pursuing all compact optoelectronic devices or modules for such applications, electroabsorption modulator (EAM) can be a good candidate due to its high speed, compactness, capability of easily integrating with other optoelectronic devices. Furthermore, since EAM can exhibit the versatile functions, such as modulator, optical cross-absorption modulation (XAM), and photodetection, the optical processing can thus be realized to be endorsed into a compact module. In this work, a novel scheme based on a single short-terminated electroabsorption modulator (SEAM) is proposed and developed to aim for creating UWB optical pulses. Using the properties of photodetection and XAM, SEAM can create two reversed-polarities and time-shifted optical pulses by the help of short resistor, combining into an UWB optical monocycle pulse. The optical processing of EAM thus has a high potential to enable photonic UWB generation into not only the technologies of optical fiber communications, like, wavelength division multiplexing (WDM), but also the photonic integration circuit template.
2. Theory and Experiment Setup
The schematic diagram of generating UWB optical pulse using SEAM is shown in Fig. 1 , where the EAM fabrication is processed by following the work published in [10]. The optical waveguide is formed by a p-i-n hetero-layer structure, serving as EAM as well as photodetector. The active region of waveguide (i-layer) is grown by InGaAsP multiple quantum wells (MQWs, 1550nm) sandwiched by p- and n- InP semiconductor layers. Due to thin i-layer (300nm thick) in this p-i-n material, the highly confined electric field by reversely biasing p-i-n can lead to high efficient modulation. At both ends of optical waveguide, two microwave coplanar waveguides (CPW) are connected as microwave receiving port (“A” to “B”) and load port (“C” to “D”). The basic principle is based on the combination of two time-shifted and opposite-polarity pulses generated in optical waveguide. The scenario is described as following: once a high-power pump pulse (wavelength λp) excites the optical waveguide, the MQWs can be operated at saturation regime to modulate a CW probe light (wavelength λs) due to the XAM effect, simultaneously creating two opposite directions of electrical pulses along the optical waveguide. Such two electrical pulses are denoted as forward-electrical wave (FEW, toward “C”) and backward-electrical wave (BEW, toward “B”) by following the incident optical direction. As soon as the load line (“C” to “D”) is terminated by a short resistor, the FEW will be reversed in polarity and then reflected back to the optical waveguide with delayed-time of CPW load line. As XAM processing leave the probe light pulse with non-zero level, the reversed FEW could further modulate the probe light, which has the inversed polarity in comparison with XAM-generated pulse. Therefore, by adjusting the length CPW, two reversed-polarity and timely-shifted optical pulses can be superposed to generate a monocycle UWB optical pulse.
Fig. 1 The schematic diagram is used for generating photonic UWB signal by SEAM. As the pump light excites the waveguide, an optical probe pulse is generated through XAM, simultaneously creating photocurrent pulses. The paths of “C” to “D” and “A” to “B” are the CPW load line and CPW feed line, where a short-resistor is used to reversely reflect the forward electrical wave (FEW).
The optical measurement setup is following the schematic drawing in Fig. 2 . The optical pump pulse is generated through a CW 1556nm light modulated by a high-speed Mach-Zehnder modulator (MZM, LiNbO3), where the electrical driving power is set as 400MHz with 4% duty cycle periodic from a pulse-pattern-generator (P.P.G.) system. A 1560nm CW optical probe light of 4mW power level is utilized as the converted power. The fiber insertion loss of EAM is −14 dB at 1560nm. After amplified by an Erbium-doped-fiber amplifier (EDFA) to 13mW of average power, the pump pulse combing with CW probe light by an optical coupler is used to excite the device, performing optical wavelength conversion. The modulated probe light (λs) is then filtered out by an optical filter and detected by a high-speed photodetector. A high-speed sampling scope is finally connected with the device for analyzing the electrical signal from microwave receiving (“A” to “F”) and also from high-speed photodetector (point “E”).
Fig. 2 The schematic plot of measurement setup using SEAM. The abbreviations of P.P.G., high-speed P.D., L.D., and M.Z.M stand for pulse-pattern-generator, high-speed photodetector, laser-diode, laser diode, and Mach-Zender modulator.
3. Experimental results and discussion
Figure 3(a) shows the experimental results of the wavelength-conversion light pulse (probe) from SEAM. A monocycle UWB optical pulse with cycle of 89ps is obtained, where the input pump pulse (shown in insert) is set as 80ps of full-width at half-maximum (FWHM). Also, after detection by a high-speed P.D. and then sent to microwave spectrum analyzer, the right of Fig. 3(b) shows the corresponding microwave power spectrum of monocycle pulse. The center frequency is at 4GHz, ranging from 0.1GHz to 8GHz for −10dB drops. As seeing in the shape of monocycle pulse, the long tail of the pulse exhibits a decay time of below 400ps, faster than the requirement of 100Mb/s from UWB signal. By employing such SEAM for generating UWB optical pulse, high-speed with 1Gb/s data transmission could be expected.
Fig. 3 (a) The measured monocycle UWB optical pulse (probe) by converting the pump pulse in SEAM and the input optical-pump pulse (insert) from MZM modulator, (b) the corresponding microwave spectrum of the monocycle pulse after photodetection.
As seeing the pulse shape of UWB optical pulse in Fig. 3(a), a long decay time with lower amplitude exhibits in the reversed-polarity part, affecting the monocycle property and the high-speed data modulation. In order to further investigate the optical processing during the monocycle optical generation, the wavelength conversion of short optical pulse in the waveguide is tested. As to eliminate the re-modulation of optical waveguide during the reflection of electrical pulse in the ends of CPW load line and receiving line, a 50Ω resistor and a 50Ω high-speed sampling score are terminated in this measurement. The pump and probe light conditions are kept the same with SEAM experiment (Fig. 2). Figure 4 shows the normalized pulse shapes, where the solid curve is the wavelength-converted probe pulse and the dash curve is the corresponding photocurrent generated by pump pulse. As seen, two identical curves are found, however, as comparing with optical pump pulse shown in the insert of Fig. 3(a), a long decay time behavior is observed in both the wavelength-converted probe pulse and photocurrent pulse, indicating the escaping processing of photocarrier during XAM in M.Q.W.s of the waveguide is the main issue lowering and slowing the pulse [11-12].
Fig. 4 The plots of the converted probe light (λs, solid) and photocurrent (dash) pulses for 50Ω termination at CPW load and receiving ends. Both curves are normalized for comparison.
One factor affecting the monocycle shape of the UWB optical pulse is from the time-delayed reflection by the short terminator. To look into such effect, the received photocurrent pulse from CPW feed line of SEAM (Fig. 2) is measured for analysis. Figure 5(a) shows the received photocurrent response. A monocycle waveform with cycle of 84ps is found, quite consistent with the optical generation shown in Fig. 3(a). As comparison with the 50Ω-terminated photocurrent response shown in Fig. 4, a negative polarity of waveform is found after the main pulse, suggesting the short terminator can reverse the polarity of the reflected FEW. In order to further exam the time-delay of the CPW line, a superposition of two inversed-polarity and time-delay 50Ω-terminated photocurrent responses, shown in the insert of Fig. 3(a), could construct the measured electrical monocycle pulse. As shown in Fig. 5(b), the superposed wave (solid curve) with cycle of 89ps is plotted through adding two inverse polarities of photocurrent responses (dash curves). Here, the delayed time between reflected-FEW and BEW is mainly attributed to the round-trip traveling time on CPW load line. As given by a 50Ω CPW (Au metallization) on an intrinsic InP substrate, the calculated microwave refractive index is 2.6 [14], corresponding to an 8.6ps round trip time in 500μm long CPW line. The shape of the superposed wave (solid curve in Fig. 5(b)) exhibits a quite consistent shape with the measured results (Fig. 5(a)), further confirming the effect of short resistor on reversing photocurrent pulse polarity. Regarding the different amplitudes on the positive and negative portions of optical monocycle pulse, the reason is mainly from the supposition between the slow decay time of XAM pulse and the delayed re-modulation from the reversed photocurrent under high-power excitation. At this point, it can also be confirmed by comparing the pulse tail of Fig. 3(a) and Fig. 4, where two similar decay rates are found. The monocycle waveform can be improved by increasing bias voltage to fast remove the photocarrier, using M.Q.W.s with fast carrier sweeping processing [11,12], and the CPW length for the delayed time. The recovery time of XAM have been shown to be in the order of tens ps [11–13], which is shorter than the cycle of monocycle pulse. Therefore, combining two short-tail and reversed-polarity optical pulses, the monocycle UWB optical pulse fitting to FCC spec can be expected.
Fig. 5 (a) The photogenerated current waveform of SEAM. (b) The superposed waveform by two polarity-reversed and delayed-time 50Ω-terminated photocurrent pulses (Fig. 4).
4. Conclusion
A novel scheme of generating monocycle UWB optical pulse through the cross-absorption modulation (XAM) and photodetection of quantum well in a short-terminated electroabsorption modulator (SEAM) is demonstrated. Using XAM, a pump light pulse can be converted to the probe light with the same polarity, simultaneously creating an electrical pulse propagating the waveguide of SEAM. By the help of short termination of SEAM, the reversed-polarity electrical pulse with the delay time is reflected back to the waveguide, re-modulating the waveguide and thus generating reversed-polarity optical probe pulse. As such two probe light pulses are superposed, a monocycle pulse generation can be realized. A cycle of 89ps is found in such monocycle optical pulse. The power spectrum shows the center frequency is 4GHz, ranging from 0.1GHz to 8GHz for −10dB drops. All the optical processing inside SEAM is confirmed from the XAM processing of waveguide and the delayed electrical pulse from short termination. Above 1Gb/s data transmission through this optical UWB can thus be expected. In addition, the whole all-optical processing is finished inside a compact EAM waveguide and circuits, indicating that the advantages of EAM, such as high-speed, low-driving voltage, integration capability with other devices, are allowed to endorse photonic integration circuit template into UWB photonic generation.
Acknowledgments
The authors would like to thank the financial supports from the National Science Council, Taiwan (NSC96-2221-E-110-097-MY3), Technology Development Program for Academia (96-EC-17-A-07-S1-025), and “Aim for the Top University Plan Taiwan” (97C030133). The wafer supporting from LandMark Optoelectronics Corporation is also of great help for this project.
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