Photonic ultra-wideband (UWB) pulses are generated by direct current modulation of a semiconductor optical amplifier (SOA) section of an SOA-integrated sampled grating distributed Bragg reflector (SGDBR) laser. Modulation responses of the SOA section of the laser are first simulated with a microwave equivalent circuit model. Simulated results show a resonance behavior indicating the possibility to generate UWB signals with complex shapes in the time domain. The UWB pulse generation is then experimentally demonstrated for different selected wavelength channels with an SOA-integrated SGDBR laser.
© 2010 OSA
Ultra-wideband (UWB), which is regulated by the Federal Communications Commission (FCC), has recently been developed for short range wireless communication within the frequency range from 3.1 to 10.6 GHz . Compared to traditional wireless communication technologies, UWB technology has many advantages such as low power consumption, high bit rate, immunity to multipath fading and so on [2,3]. Based on the FCC definition, a UWB signal should have a spectral bandwidth that is greater than 500 MHz or a fractional bandwidth that is greater than 20% .
In order to overcome the limitations of propagation distance of UWB signals and cost of radio-frequency (RF) electrical circuits or devices, radio-over-fiber (ROF) systems have been adopted to use the advantages provided by optical fiber, where the UWB signals can be directly generated in the optical domain and distributed over the optical fiber without the need for an extra electrical-to-optical conversion. There are three main methods for the optical generation of UWB pulses . One is to generate the first- or second-order derivative of a Gaussian pulse by using a photonic microwave delay-line filter, with two or three taps for monocycle or doublet, both with one negative tap. The negative coefficient can be generated based on cross-gain modulation (XGM) in a semiconductor optical amplifier (SOA)  or cross-polarization modulation (XPolM) in a polarization modulator (PolM) . The second method is based on optical spectral shaping and dispersion-induced frequency-to-time mapping using all-fiber components. The frequency-to-time mapping can be realized using a dispersive device, such as a dispersive fiber . The third method is based on phase-modulation-to-intensity-modulation (PM-IM), where the PM–IM conversion can be implemented in the optical domain by using either a dispersive device  or an optical frequency discriminator . Recently the generation of UWB signals with complex shape has been reported , which is based on relaxation oscillations of a semiconductor laser.
As proposed in , a widely tunable laser can be employed as an optical transmitter at a central station in a ROF system to achieve addressing and routing of traffic by switching between the wavelengths assigned to different base stations. Here, we extend this concept to combine the wavelength flexibility of tunable lasers with photonic UWB generation. For this purpose, an SOA-integrated sampled grating distributed Bragg reflector (SGDBR) laser is used, where the SGDBR laser is for wavelength tuning while the integrated SOA section is for generation of UWB signals. Compared with the method in , we take the advantage of the monolithically integrated widely tunable laser module to introduce the wavelength tunability to the photonic UWB generation. This feature will make the ROF systems more flexible.
Gain dynamics , XGM [5,13] and nonlinear polarization rotation  in SOAs have been widely demonstrated to generate UWB pulses. In this paper, however, resonance behavior of SOAs is exploited. Generation of UWB signals by direct current modulation of the SOA section integrated in the front of the SGDBR laser is demonstrated. With a microwave equivalent circuit model, modulation response in the time domain for the SOA section of the laser is first studied to theoretically produce the UWB pulse shapes. Then the UWB generation is demonstrated experimentally. For different selected wavelength channels the generated UWB pulses agree well with the Federal Communications Commission (FCC) mask in the frequency domain.
2. Small-signal equivalent circuit model of SOA section
The direct current modulation of an SOA monolithically integrated with an SGDBR laser has been well studied by Coldren’s group from UCSB . Here, to predict the transient response of the SOA section of the SOA-integrated SGDBR laser, a small-signal equivalent circuit model of SOA, which can be derived from [16,17], is introduced for theoretical analysis. As shown in Fig. 1 , there are three parts in the total small-signal equivalent circuit model: source, parasitics and the intrinsic SOA. The small-signal current is(t) is set by the user or control system and Rin is the source resistance, which is usually regarded as 50 Ω. The parasitic effect of this model can be described with five parasitic elements, in which the three elements CP, LP and RP represent the package parasitics, and the two other elements Cs and Rs characterize the chip parasitics. C and R are derived from the linearization of the rate equation for SOA carrier density. The small-signal voltage v(t) can be regarded as the small-signal optical gain of SOA corresponding to is(t).
All the parameters in the SOA model shown in Fig. 1 can be extracted by fitting the circuit model with measured microwave reflection coefficient S11 and transmission coefficient S21 of the SOA section. The fitting, performed with Advanced Design System (Agilent commercial software) for linear and non-linear simulations, utilizes a solving method based on a random optimizer algorithm at first, and then by a gradient one . The measurement setup is illustrated in Fig. 2 . Gain section and tuning sections are driven by low noise and high stability current drivers to keep the SGDBR laser working at a certain wavelength. The SOA section is biased through a wide bandwidth Bias-Tee, while a thermoelectric cooler (TEC) controller is introduced to obtain a precise setting and stabilization of the lasing wavelength. The output modulated optical signal of the SGDBR laser is routed to the wide bandwidth optical receiver unit into the Network Analyzer (NA), which can provide both the S11 reflection response (when configured for electrical measurement) and the S21 transmission response (when configured for electro-optical measurement). Figure 3 and Fig. 4 give the measured and fitted results of S11 and S21, respectively. The optimized results of all the elements for the SOA circuit model are shown in Table 1 .
3. Simulation for generation of UWB signals by direct current modulation of the SOA section
From the measured S21 in Fig. 4, we can find there are main resonance peaks of the SOA section around 3.2 GHz. Additionally, minor resonance peaks around 6.5 GHz can be observed. Resonance behavior of SOAs was first predicated in , which is similar to the well-known relaxation oscillation found in semiconductor lasers, but of a different physical origin. When the SOA section is set around the resonance point, the relaxation oscillation of SOA gain will appear, which could be helpful for producing UWB pulses with complex shapes (close to the shapes of high-order Gaussian pulses) at the output of the SOA-integrated SGDBR laser.
For demonstration of UWB pulse generation, the modulation response of the SOA section is simulated by the equivalent circuit model shown in Fig. 1. The parameters used in the simulation are listed in Table 1. Furthermore, the input electrical modulating 32-bit 10Gbit/s pattern sequence ‘0111 1111 1111 1111 1111 1111 1111 1111’ is chosen for is(t) and the DC bias current of the SOA section is set at 90 mA. Figure 5 presents the simulation results of the SOA section. As seen in Fig. 5(a), the peak-to-peak amplitude of modulation current is 20mA and the duration of one electrical bit is 100 ps. Figure 5(b) illustrates the variation of the output voltage of the circuit model of the SOA section corresponding to the modulating pattern sequence of is(t), which is proportional to the gain variation of the SOA section. Due to the resonance behavior of the SOA section, v(t) with a complex shape is obtained and thus the output pulses with the same shape can be generated, which is similar to the shape of higher-order Gaussian pulses. The resonance response of the SOA section is vital in the UWB pulse generation process. It will be difficult to observe this phenomenon when the resonance response is too low. Moreover, the SOA section has some differences in the resonance properties for different operating wavelengths of the SGDBR laser, as shown in Fig. 4. For a determined operating wavelength, the circuit parameters in our model are dependent on the structure parameters of the device, DC component of the electron and photon density, etc . For example, the resistance R and capacitance C in Fig. 1 are related with the length, width and thickness of the active region. At the same time, R is also related with the DC optical gain of the SOA section, which is determined by the DC bias current and input optical power of the SOA section . In this simulation, the input optical power of the SOA section, i.e. the output optical power of the SGDBR laser can be considered stable for a determined wavelength since the bias current of the gain section of the SGDBR laser is kept constant. The SOA bias of 90mA is selected to obtain a good resonance behavior of the SOA section.
4. Experimental results and discussion
The generation of UWB pulses has been demonstrated with a monolithic tunable laser and the experimental setup is shown in Fig. 6 . The device is a ridge waveguide SOA-integrated SGDBR laser, fabricated by Accelink Technologies Co., Ltd. for our research projects. We previously reported our SGDBR lasers with quasi-continuous wavelength coverage over 35nm, side mode suppression ratio (SMSR) of more than 30 dB for all selected channels  and our SOA-integrated devices with rapid shuttering function . Here, a Pulse Pattern Generator (PPG) is introduced to generate a 32-bit 10Gbit/s pattern sequence ‘0111 1111 1111 1111 1111 1111 1111 1111’ to modulate the driving current of the SOA section. At the receiver side a sampling oscilloscope is used to measure the generated pulses in the time domain, and the signals in frequency domain are also observed on an Electrical Spectrum Analyzer (ESA) after an O/E module.
In our experiments, the gain section of the SGDBR laser is biased at 110 mA and the three tuning sections are driven by different current combinations for different output wavelengths. Figure 7 provides the measured results of UWB signal generation for three different wavelength channels. The shapes of the generated UWB pulses are directly dependent on the waveforms of the input pulses to the SOA section. Figure 7(a) gives the pulses train out from the Pulse Pattern Generator, the 10%~90% fall and rise time are 39 and 41 ps, respectively. The spectra of the selected wavelengths are measured by Optical Spectrum Analyzer (OSA), shown in Fig. 7(b). Figure 7(c) gives the generated UWB pulses in the time domain for different wavelengths. We can see clearly the relaxation phenomenon when the “1” level falls down to the “0” level. The shapes of the measured UWB pulses in the time domain agree well with the simulated one as shown in Fig. 5(b) qualitatively. The corresponding electrical spectra of the generated pulses are displayed in Fig. 7(d). There are some differences among the three generated pulse shapes in Fig. 7(c) and the corresponding electrical spectra in Fig. 7(d). This is due to the differences in resonance property of the SOA section for different operating wavelengths of the SGDBR laser in Fig. 4. Resonance behaviors of the integrated SOA are also related with the output power from the SGDBR laser. In our experiments, the output power from the SGDBR laser is slightly different for the selected three different wavelengths, which can be derived from Fig. 7(b). The minor resonance peak may affect the shape of the generated UWB pulses as well, however, it is not included in our model. It would be expected to improve our model and parameter extraction method for the optimization of the simulation in the future. Owing to the length limitation of the modulating bit pattern, Fig. 7(d) only shows some discrete frequency components with a repetition rate of 312.5 MHz (10 Gbit/s / 32 bit). From the spectral envelopes shown in Fig. 7(d), we can obtain fractional bandwidths of 51%, 60% and 42% for the operating wavelengths of 1535.06 nm, 1538.85 nm and 1552.20 nm respectively, which meet the FCC minimum requirement of 20%. Due to the different shapes of generated UWB signals for different wavelengths, in practical applications, the time-domain waveforms and RF spectra of the generated UWB signals can be tailored to meet the requirement of the user by choosing the appropriate operating wavelength.
Tunable UWB pulse generation was demonstrated by direct current modulation of the SOA section of an SOA-integrated SGDBR laser. A microwave equivalent circuit model was used to study modulation response of the SOA section. Numerical simulation was performed to show the generation of UWB signals which have complex shapes in the time domain. The pattern of 10Gbit/s 32-bit sequence “0111 1111 1111 1111 1111 1111 1111 1111” was then applied on the SOA section of the SOA-integrated SGDBR laser. The generated UWB pulses agreed well with the simulation results in the time domain and the corresponding spectral envelopes in frequency domain were compliant with the FCC requirement. Experimental results demonstrated that tunable UWB pulse generation can be achieved by control of the integrated tunable laser. This could be attractive for applications of tunable lasers in microwave photonics.
This work has been supported in part by the National High Technology Developing Program of China under Grant No. 2009AA03Z418, in part by the National Natural Science Foundation of China under Grant No. 60677024, and in part by Ministry of Education 111 Project of China under Grant No.B07038. The authors would like to thank technical supports from Wuhan Office, Anritsu Company with Pulse Pattern Generator (MP1800A) and Network Analyzer (37369D).
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