We propose a novel approach to generating millimeter-wave (MMW) ultrawideband (UWB) signal based on nonlinear polarization rotation (NPR) in a highly nonlinear fiber (HNLF). The MMW UWB signal is background-free by eliminating the baseband frequency components using an optical filter. The proposed scheme is theoretically analyzed and experimentally verified. The generated MMW UWB signal centered at 25.5 GHz has a 10-dB bandwidth of 7 GHz from 22 to 29 GHz, which fully satisfies the spectral mask regulated by the Federal Communications Commission (FCC).
© 2014 Optical Society of America
Ultrawideband (UWB) is considered as a highly promising solution for future high-capacity wireless personal-area networks and sensor networks due to its various benefits, such as low power consumption, high data rate, and immunity to multipath fading . In 2002, the Federal Communications Commission (FCC) defined the UWB signal as any radio frequency (RF) signal that has spectral bandwidth of more than 500 MHz or larger than 20% fractional bandwidth and power density lower than –41.3 dBm/MHz. The main limitation associated with the UWB technology is the limited transmission distance (~10 m). In this context, UWB-over-fiber has been proposed to overcome this limitation [2,3]. Thus, it is highly desirable to generate the UWB signal directly in the optical domain. Many photonic approaches have been proposed to generate UWB signals in the centimeter wave (CMW) band from 3.1 to 10.6 GHz for indoor communications [2–6] and in the millimeter-wave (MMW) band from 22 to 29 GHz for outdoor communications [7–17].
For MMW UWB signal generation, direct frequency upconversion is widely used. It has been reported that the baseband signal can be upconverted to the MMW band based on fiber optical parametric amplifier , a single-drive Mach-Zehnder modulator (MZM) [8,9], a dual-parallel MZM (DPMZM) , or four-wave mixing in a highly nonlinear photonic crystal fiber . However, these schemes always suffer from two limitations. One is the strong residual local oscillator (LO) signal which derives from the beating between two distinguish optical carriers. The residual LO signal has to be suppressed because it will degrade the dynamic range of the UWB receiver and interfere with other wireless standards. The other limitation is that the intensity-modulated baseband frequency components which is the so called “background signals” are also recovered in the photodetector (PD). In the time domain, the background signals generate a large pedestal under the desired pulses . These background signals violate the noninterference with other narrowband signals and have to be fully eliminated. Recently, a few solutions have been proposed to overcome these limitations [12–18]. In , background-free arbitrary waveform was generated based on frequency-to-time mapping and polarization pulse shaping. The background signals were suppressed by more than 25 dB using balanced photodetection. F. zheng and S. Pan proposed a scheme using a DPMZM and an optical bandpass filter . However, the generated MMW UWB signal is not fully background-free since the optical carrier is intensity-modulated. In , a polarization modulator (PolM) was used to switch the state-of-polarization (SOP) of an optical carrier between an intensity-modulated state and an unmodulated state. This method suffers from the stability problem due to the use of Mach-Zehnder structure. Proposals based on the similar principle have also been reported using cascaded PolMs [16,17]. We have also reported an approach to generating background-free MMW UWB signal using a dual-drive MZM (DDMZM) . However, the power of the LO signal has to be precisely controlled to be 0.76Vπ where Vπ is the half-wave voltage of the DDMZM. Note that the FCC spectral mask for the MMW UWB signal has a rectangular shape. In most literatures, the generated MMW UWB signals have such as monocycle, Sinc, or Gaussian electrical spectra which only occupy a part of the FCC spectral mask. Thus, the generated MMW UWB signal is not power-efficient.
In this paper, we propose a novel approach to generating MMW UWB signal based on nonlinear polarization rotation (NPR) in a highly nonlinear fiber (HNLF). It is noted that the NPR effect has been used in semiconductor optical amplifier (SOA) to upconvert a monocycle pulse to the MMW band . However, the generated MMW UWB signal has strong residual LO signal and undesired background frequency components which violate the FCC spectral mask significantly. In our scheme, the operational principle is totally different from that in . The probe beam is polarization-modulated by a PolM driven by a LO signal. The SOPs of the optical carrier and two sidebands are therefore inherently orthogonal. Inside the HNLF, the optical carrier and sidebands undergo different phase shifts due to the NPR generated by the control beam. Finally, a polarizer (Pol) is used to realize polarization-to-intensity conversion. The MMW UWB signal is background-free thanks to the removing of the intensity-modulated control beam using a tunable optical filter (TOF). Moreover, the MMW UWB signal is fully FCC-compliant due to the excellent suppression of the residual LO component. The proposed scheme is theoretically analyzed and experimentally verified. The experimentally generated power-efficient MMW UWB signal has a 10-dB bandwidth of 7 GHz from 22 to 29 GHz and fully fills the FCC spectral mask.
The schematic diagram and the operational principle of the proposed scheme are shown in Fig. 1(a) and 1(b), respectively. The control beam from a laser diode (LD1) is launched to an intensity modulator (IM) which is driven by a baseband electrical signal, b(t), from an arbitrary waveform generator (AWG). The power of the control beam is given by5]. The SOP of the probe beam is rotated by the control beam, which is the so called NPR effect. If the self-phase modulation is neglected, the refractive index of the HNLF is given by
On the other hand, the probe beam from LD2 is sent to a PolM which is driven by a sinusoidal LO signal. The SOP of the probe beam is aligned at 45° to one principal axis (e.g. Ex) of the PolM. The optical field at the output of the PolM is given byEq. (3), we haveEq. (4) can be simplified as20], as shown in Fig. 1(b). Thus, Eq. (5) can be rewritten as5]. By tuning the PC1 and PC2, the fast axis of the HNLF (induced by the NPR) is aligned with the Ex’ polarization axis of the PolM. Therefore, the optical carrier and sidebands of the probe beam undergo different phase shifts in the HNLF due to the NPR induced by the control beam, as shown in Fig. 1(b). The optical field of the probe beam at the output of the HNLF can be expressed asEq. (9), the phase difference between the optical carrier and sidebands is given byEq. (11) can be simplified asEq. (11), the photocurrent consists of DC and AC terms. The DC term is independent of the power of the control beam (or the driving signal b(t)). It means that the baseband frequency components are completely eliminated. It should be noted that the baseband frequency components can be detected if the intensity-modulated control beam is also injected into the PD. To generate a background-free waveform, the intensity-modulated control beam is removed using the TOF. For the AC term, the RF signal at ωLO is generated, whose envelope is modulated by b(t). The FCC mask for the MMW UWB signal has rectangular shape in frequency domain. To fully fill the FCC mask, the b(t) basband signal should be a Sinc-shaped pulse in the time domain.
Physically, the proposed approach can be explained as: the phase difference between the optical carrier and sidebands θ is modulated by the b(t). If b(t) = 0, we have θ = π/2. In this case, the LO signal cannot be recovered in the PD because the beat component between the optical carrier and the lower sideband cancels that between the upper sideband and the optical carrier out perfectly (see Fig. 1(b)). The residual LO component can be fully suppressed as the LO signal is turned off when θ = π/2. Otherwise, the LO signal can be detected if b(t)≠0 (i.e. θ≠π/2), as shown in Fig. 1(b). Therefore, MMW UWB signal is generated by truncating a continuous wave (CW) sinusoidal microwave signal into pulses.
An experiment based on the setup shown in Fig. 1(a) was carried out to verify the proposed scheme. The control beam from the LD1 was fiber-coupled to an IM. The wavelength of the control beam was 1553.87 nm. The electrical signal driven to the IM was provided by an AWG. The waveform of the AWG was user-defined to be a Sinc function pulse which is given by h(t) = sin(πBRFt)/(πBRFt) where BRF is the RF bandwidth. In our experiment, BRF is chosen to be 7 GHz to match the FCC mask for MMW UWB signal. The Sinc-shaped pulse has time duration of 2 ns and duty cycle of 1/4. The probe beam provided by the LD2 was launched to the PolM. The wavelength of the probe beam was 1550.01 nm. The PolM has an integrated polarizer at the input of the PolM which was aligned with 45° to the Ex polarization state of the PolM. The frequency of the sinusoidal LO signal driven to the PolM was set at 25.5 GHz, which is the center frequency of the FCC mask for the MMW UWB signal. The power of the control and probe beams launched to the 3-dB optical coupler was 9 and 7.5 dBm, respectively. Two PCs (PC1 and PC2) were added to adjust the SOPs of the control and probe beams. The optical spectrum measured at the output of the 3-dB optical coupler is shown in Fig. 2(a).
The combined optical signal was sent to a HNLF to generate the NPR. The HNLF has a length of 1 km, a nonlinear coefficient of 10 W–1·km–1, and a zero dispersion wavelength of 1552 nm. Following the HNLF, a Pol was used to realize polarization-to-intensity conversion. The PC1, PC2, and PC3 were adjusted as described in Section 2. The measured optical spectrum at the output of the Pol is shown in Fig. 2(b). It can be seen that the optical spectrum is modulated due to the NPR effect. A TOF with a bandwidth of 80 GHz is attached after the Pol to select the desired probe beam. As can be seen from Fig. 2(c), the control beam and the undesired higher-order sidebands of the probe beam are removed by the TOF. A PD with a bandwidth of 40 GHz was used to convert the optical signal to electrical signal. The generated MMW UWB signal was measured using an oscilloscope (OSC) and an electrical spectrum analyzer (ESA).
Figure 3 shows the measured MMW UWB waveform (blue line). It can be seen that the CW LO signal at 25.5 GHz is truncated into UWB pulses. The normal baseband Sinc function pulse (red line) from the AWG and the inverted one (green line) are also shown in Fig. 3. The inverted baseband signal was obtained by inverting the measured data of the normal one. As can be seen, the envelope of the MMW UWB pulse matches the original baseband pulse very well. It can be expected that the baseband electrical components in the electrical domain can be eliminated since there is no pedestal observed for the MMW UWB pulse.
Figure 4(a) shows the electrical spectrum of the baseband Sinc function pulse from the AWG. The single-sideband electrical spectrum has a rectangular shape and a 10-dB bandwidth of 3.5 GHz. The measured electrical spectrum of the MMW UWB signal is shown in Fig. 4(b). As can be seen, the baseband signal is successfully upconverted to the MMW band. The generated MMW UWB signal centered at 25.5 GHz has a 10-dB bandwidth of 7 GHz from 22 to 29 GHz as regulated by the FCC. Moreover, the baseband frequency components are fully eliminated. It is worth noting that the LO signal is also well suppressed. The FCC spectral mask for the MMW UWB is shown in Fig. 4(b) (red line). It can be seen that the generated MMW UWB signal fits the FCC mask well and is thus power-efficient.
Actually, the MMW UWB signal can be generated by simply upconverting a basedband signal to the LO band using an electrical MMW mixer. In order to show the advantages of our approach over the electrical mixing method, we also carried out an experiment to generate MMW UWB signal using an electrical mixer. The mixer (CMB26260807H from Cernex, Lnc.) available in our lab has an intermediate frequency (IF) bandwidth from DC to 8 GHz, a LO bandwidth from 14 to 26 GHz, and an RF bandwidth from 14 to 26 GHz. Since the RF bandwidth of the mixer cannot cover the frequency range from 22 to 29 GHz, we tried to upconvert the baseband signal to the LO band from 19 to 26 GHz, which is 3 GHz lower than the requirement of the FCC standard. To do so, a LO microwave signal at 22.5 GHz was mixed with the baseband signal using the MMW mixer. The measured electrical spectrum of the upconverted signal at the RF port of the mixer is shown in Fig. 5(a). As can be seen, the baseband signal was successfully upconverted to the LO band from 19 to 26 GHz. The revised FCC mask which is 3 GHz lower than the original one is also shown in Fig. 5(a). The generated electrical signal fits the revised FCC mask at most frequencies. However, there is a strong residual LO signal at 22.5 GHz due to the limited isolation of the LO signal. Figure 5(b) shows the corresponding waveform. The residual LO signal generates strong sinusoidal microwave signal at both sides of the Sinc-shaped waveform. The strong residual LO signal violates the FCC mask significantly. In order to avoid the interference with other wireless standards, the power of the electrical signal has to be attenuated by 26 dB, which significantly weakens the generated UWB signal and makes the UWB signal power-inefficient. However, the generated MMW UWB signal based on the proposed approach can overcome this problem as shown in Figs. 3 and 4(b). The residual LO signal is well suppressed and the generated MMW UWB signal fits the FCC mask very well. Moreover, the proposed method also benefits from the advantages brought by the microwave photonics techniques [1–3]. In addition, the direct generation of MMW UWB signals in the optical domain is highly desirable for the UWB-over-fiber applications.
We have theoretically and experimentally demonstrated a novel approach to generating MMW UWB signal based on NPR effect in a HNLF. The generated MMW UWB signal is background-free thanks to the removing of the intensity-modulated control beam using a TOF. The power-efficient MMW UWB signal centered at 25.5 GHz occupies a 10-dB bandwidth of 7 GHz from 22 to 29 GHz, which is fully FCC-compliant. The proposed scheme can be upgraded to the 60-GHz band for frequency up-conversion thanks to the ultra-fast response of the HNLF.
This work was supported by the National Natural Science Foundation of China under 61377069, 61335005, 61321063, and 61090391.
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