We proposed and demonstrated a novel scheme of photonic ultra-wide-band (UWB) doublet pulse based on monolithic integration of tapered optical-direction coupler (TODC) and multiple-quantum-well (MQW) waveguide. TODC is formed by a top tapered MQW waveguide vertically integrating with an underneath passive waveguide. Through simultaneous field-driven optical index- and absorption- change in MQW, the partial optical coupling in TODC can be used to get a valley-shaped of optical transmission against voltage. Therefore, doublet-enveloped optical pulse can be realized by high-speed and high-efficient conversion of input electrical pulse. By just adjusting bias through MQW, 1530nm photonic UWB doublet optical pulse with 75-ps pulse width, below −41.3dBm power, 125% fractional bandwidth, and 7.5 GHz of −10dB bandwidth has been demonstrated, fitted into FCC requirement (3.1GHz~10.6GHz). Doublet-pulse data transmission generated in optical fiber is also performed for further characterization, exhibiting a successful 1.25Gb/s error-free transmission. It suggests such optoelectronic integration template can be applied for photonic UWB generation in fiber-based communications.
©2012 Optical Society of America
Photonic ultra-wide band over fiber (UWBoF) [1–3] is an emerging field as a solution of short-range wireless access networks into widely spread- and broadband- optical fiber networks. High-capacity and high-speed wireless networks can be upgraded for future data information access like data, music, videos, and so forth. In consequence, it has drawn considerable attention due to its intrinsic properties such as the immunity to multipath fading, carrier-free transmission, broad occupied bandwidth, availability of low-cost transceivers, and low-power consumption. By taking both advantage of photonic ultra-wide band (UWB) transmission in electronic and optical fiber technologies, data capacitance and information mobility can be efficiently boosted for internet requirement. Nevertheless, the UWB radio signal communication is still restricted in indoor or inside building (in a short distance from a few to tens of meters) due to the restriction on power density from Federal Communications Commission (F.C.C.) regulation . Thus, UWBoF has been considered as a good solution for improving such problems. In order to achieve UWBoF, impulsive waveform transmission has been attracted with system demonstration due to carrier-free property, low cost, and high bit rate . It is thus quite important to efficiently attain impulsive waveform of UWB over the fiber domain for UWBoF access into optical fiber communications field.
Several schemes of UWBoF generation for either monocycle or doublet pulse waveform have been proposed. Using a laser diode with a microwave differentiator to extract UWB monocycle pulses . Photonic UWB pulse generation can be realized from electro-optical intensity modulator by optical beating from two timely frequency-shift-keying modulations or its nonlinear optical transmission [7,8], where long distance due to the fiber dispersion may lead to pulse distortion and also data transmission properties. Taking advantages of nonlinear gain switching mechanism of semiconductor optical amplifier or electroabsorption modulator, monocycle or doublet pulses have be shown [9,10]. However, lots of optical components are needed for adjustment of interaction between elements, leaving complex architecture. In addition, using phase modulation in an electro-optical modulator and the dispersion in long-distance fiber transmission, UWBoF can be operated . But, the wavelength selection and also data transmission in long fiber are difficult to achieve, leading to low tolerance in adjustment of individual element. Recently, electroabsorption modulators based on quantum confinement Stark effect (QCSE) have been proposed for UWB photonic generation due to the inherently low driving voltage operation, low chirp, and easy integration with other optoelectronic devices [12,13]. Through QCSE of multiple-quantum-well (MQW), strong exciton induces highly nonlinear property creates minimum optical transfer against voltage, thus leading to doublet pulse envelope creation and data transmission . But the intrinsic strong absorption near exciton regime, low optical transmission renders UWB generation unfit with low optical power operation.
In this paper, a new scheme based EAM integrated with tapered optical-direction coupler (TODC) for UWB photonic generation and data transmission is proposed and demonstrated. By taking advantage of photonics integration, both electro-optic and electro-absorption effects of multiple-quantum-well (MQW) can be used for efficiently creating valley type optical modulation for doublet waveform. Such novel scheme has lots of advantages such as simple architecture, high-tolerance design, and low driven power. With such technique, UWBoF application can have high potential with photonic integration template.
2. Principle of generating UWB doublet pulse
Figure 1 shows the schematic diagram of TODC-integrated EAM, where the TODC is used as input ends of optical transmission and also as partial power divider between top waveguide (active waveguide, AW) and bottom passive waveguide (PW). AW is defined by MQW sandwiched by p-InP (above MQW) and n-InP (below MQW) layer, where the width of MQW is laterally tapered through selective wet etching. Underneath the MQW, a passive waveguide (PW) is defined by depositing the interlaced InGaAsP/InP material. The device fabrication is following the previous work published in reference [14–16], except TODC in this work is set as partial optical power divider. Optical input and output received points are placed in PW (point “A”) and PW below point “C” respectively. The input feed line into the EAM and also TODC is fabricated by a coplanar waveguide (CPW) electrical line, so the high speed data and also DC bias can be connected.
The operation principle is based on the bias-dependent coupling efficiency of TODC as well as the electro-absorption modulation in the top waveguide. As shown in Fig. 1, by the gradual effective index variation in the tapered AW of TODC, the coupling efficiency can be controlled by varying MQW refractive index through field-driven electro-optical (EO) effect, performing function of power divider. Simultaneously, QCSE in MQW is also applied to perform electroabsorption (EA) effect. In order to design TODC for V-shaped optical transmission by using EO- and EA- effects in MQW, a beam propagation method (BPM) software is used to simulate the optical coupling. The MQW and also the layer structure are based on the previous work . Figure 2(a) plots the calculated effective index of top tapered AW with width from 0.8μm to 3.7μm and also the bottom PW, while the higher bandgap material in PW gives a lower bulk index. The lengths of tapered AW and EAM are 300μm and 200μm. As increasing the active region width to increase the AW effective index, there will be a cross point with underneath PW, forming a resonant region between AW and PW and thus inducing strong optical coupling. Therefore, setting EO effect through the reversed bias of p-i-n diode, the location of resonant point can be controlled to change the coupled optical power into EAM waveguide. Figure 2(b) shows the optical absorption coefficiency of QW with bias at different wavelengths (1510, 1530, and 1550nm). The QCSE of QW leads to a red shift of QW transition levels, simultaneously inducing the index change due to EO effect. Using Kramers-Kronig relation, as shown in Fig. 2(c), QW index changes relative to 0V-biased point can be extracted. Here, the optical wavelength is chosen as 1530nm, where the optical index of AW monotonically increases with bias. Also, by designing AW structure (the function of width with propagation distance), there can exist an offset of index difference at 0V bias from the optimized optical transferring efficiency. Using the BPM and the eigenmode field overlapping, the extracted transferred efficiency into AW along propagation distance is plotted in Fig. 2(d). There are two extreme approaches at the optical transfer efficiency. At low bias regime, the transferred efficiency is increased due to the increased index of AW by following increased bias. After the long propagation, the optical power in AW and EAM will be depleted, resulting in negative slope relation for detected power (point “C”) with bias. Once the bias is increased over the maximum transferred efficiency point, the increased index in AW by bias will give a lower transferred efficiency, leading to a positive slope relation for detected power. Based on such two limitations, the valley-shape transfer function of received optical power with bias can be obtained.
3. Experiment for verification of concept and discussion
In order to verify the valley-shape optical transmission function generated from TODC-integrated EAM for photonic UWB doublet pulse, direct current (D.C.) and large-signal alternated current (A.C.) transmission experiment are performed. Figure 3 is the schematic plot of experiment setup. D.C. bias test and also A.C. signal are combined through a bias tee. Laser sources centered at wavelengths of 1525, 1530, and 1535nm with TE polarization is used as the input optical power, exciting device from PW side and collected at EAM side, where optical receiving point is located underneath EAM waveguide (point “C” of Fig. 1). Figure 4(a) plots the measured optical transmission against bias. Valley-shaped optical transmissions with bias are observed for all wavelength regimes, suggesting wavelength division multiplexer (WDM) technique could be applied by this structure. In order to further investigate such behavior, a single EAM waveguide without TODC is also processed in the same chip and used for characterizing active region material. As seen in the insert of Fig. 4(a), the transmission curves of active region against voltage are plotted. Obviously, in contrary to the results of Fig. 4(a), there are no significant valley shapes observed. Also, all valley curves in TODC-integrated EAM occur at regimes of below 8V bias. That is to say, exciton behavior in MQW of AW leading to abrupt optical absorption change is not the only factor responsible for forming valley-shaped transmission, suggesting that the mechanism of index change from field-driven MQW should be accounted for all over response. As receiving optical power below EAM waveguide (point “C” in Fig. 1), the minimum point occurs nearly at resonant condition, where the high bias is set to increase MQW index and then leads to high conversion in TODC as well as large optical absorption. Because of low loss in PW, it suggests that symmetric V-shape of transmission with deep valley against voltage can be made for high efficient UWB generation through long waveguide and bandgap engineering. The BPM simulation gives a −7dB drop of transmission at 1530nm. No anti-reflection (AR) coating is deposited. Considering the reflection loss of −3dB in fiber coupling, the measured −11dB (at 0V) of transmission from “A” to “C” of PW is consistent with simulation. As for checking the further information in Fig. 4(a), the minimum points of different wavelength excitations are located at different biases, where longer optical wavelength causes higher bias operation. Since active region material (below bandgap) experiences lower refractive index at longer wavelength, higher bias is needed to reach the resonant point, further confirming EO effect from field-driven AW is one of the main mechanisms attributing to valley-shaped curve.
In characterization of large-signal doublet optical signal conversion, the setup of test is shown in Fig. 3. An electrical super-Gaussian pulse train is created from a 10Gb/s pattern generator with electrical data format of “10000000”(1/8 fractional frequency divider), forming a 1.25GHz repetition rate return-to-zero (RZ) pulse train with pulse window of 100ps. To get doublet waveform, Fig. 4(b) schematically illustrates the pulse generation through EO conversion in TODC-integrated EAM, where the transfer function with bias is the linear scale plot of the measured transmission curve at 1530nm. By applying the bias at negative slope in transmission curve, the shape of doublet pulse can be obtained. Also, adjusting the bias, the amplitudes of upper waveform and double bottom waveform can be controlled, realizing the reconfiguration capability of signal processing through the bias in this structure. Due to the operation of QCSE in MQW, the valley-shaped optical transmission is defined inside an A.C. voltage regime of 2.5 Vpp, indicating efficient photonic UWB generation can be realized. Also, as from Fig. 4(a), based on EO effect of MQW, valley-shaped transmission from different wavelength excitation can just be adjusted by different bias, suggesting that such photonic UWB generation can be applied to WDM technology in optical fiber communication.
In order to further test the photonic UWB doublet pulse and also the bias point of operation, a single bit of “1” with 1.25 GHz RZ (pulse window 100ps) is used excite TODC-integrated EAM. After double pulse is generated, the optical signal is amplified by an Erbium-doped fiber amplifier (EDFA), and received by optical receiver for analysis. The pulse train is examined by sampling scope for extracting waveform and also sent into electrical spectrum analyzer. Figure 5 shows the doublet pulse waveform and its spectrum, where the bias of 5.2, 5.6, and 6V is applied to EAM and TODC waveguide. The spectrum of F.C.C. regulation is also plotted for checking the spectrum. It clearly shows that the doublet waveforms are sensitive to the bias. With higher bias, the upward waveform to the downward one is increased, consistent with the doublet generation shown in Fig. 4(b). With the bias of 5.6V, 75ps full width half maximum (FWHM) in the waveform is observed, while the whole waveform is located inside a 250ps wide regime, suggesting above 1Gb/s data transmission can be expected. Furthermore, the spectrum shown in Fig. 5 can be fitted to F.C.C. regulation (below −41.3dBm from 3.1 to 10.6GHz). Also the fractional bandwidth and −10dB bandwidth from spectrum of 5.6V are about 125% and 7.5 GHz. The second envelop of spectrum is due to super-Gaussian pulses originated from pulse pattern generator. In order to improve the data pattern, low pass microwave filter can be used to get electrical Gaussian-like pulse. As modulating data into TODC-integrated EAM, Fig. 6 plots the 1.25Gb/s eye diagram of doublet UWB pulses and its bit error rate (BER) of back-to-back data transmission. The clear eye diagram and error-free transmission are obtained, suggesting such integration devices can be potentially applied to photonic UWBoF with bit rate of above 1Gb/s.
A novel scheme of photonic UWB doublet pulse is demonstrated based on integration of MQW waveguide and TODC, where a top MQW waveguide vertically integrated underneath passive waveguide forms TODC. By field-driven index- and absorption- change in MQW, a valley-shaped of optical transmission with voltage can be realized from partical optical coupling of TODC. Thus, high-speed and high-efficient UWB conversion from electrical pulse to doublet-enveloped optical pulse can be made based on electro-optical properties of MQW. By just adjusting bias through MQW, photonic UWB doublet optical pulse with 75-ps pulse width, optical power below −41.3dBm, 125% fractional bandwidth, and 7.5 GHz of 10-dB bandwidth is obtained, fitted to F.C.C. requirement. Data transmission measurement is also used to exam such double UWB pulse, leading to 1.25 Gb/s with error-free transmission, suggesting such optoelectronic integration template can be endorsed to photonic UWB generation in fiber base.
The authors would like to thank the financial supports from the National Science Council, Taiwan (NSC99-2221-E-110-029-MY3 and NSC101-2622-E-110-004-CC3), and “Aim for the Top University Plan Taiwan” (98C030133). Also, the authors would like to thank the wafer growth from Land Mark Optoelectronic Corporation.
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