Optically switchable Ultra-Wideband (UWB) monocycle and doublet pulse generation using an optically reconfigurable photonic microwave delay-line filter is proposed and demonstrated. The microwave filter can be reconfigured as a two- or three-tap microwave filter with coefficients of (1, -1) or (1, -2, 1). The function of the two- or three-tap microwave filter is equivalent to an operation of a first- or second-order difference, which can be approximated as a first- or second-order derivative. When a Gaussian pulse is inputted to the two- or three-tap microwave delay-line filter, a Gaussian monocycle or doublet pulse is generated. The proposed photonic microwave delay-line filter is implemented using a polarization modulator (PolM), a length of polarization maintaining fiber (PMF), and a balanced photo-detector (BPD). In the experiment, Gaussian monocycle and doublet pulses with a fractional bandwidth of about 170% and 130% are generated. The switchability of the proposed UWB pulse generator in pulse shape and polarity is also experimentally demonstrated.
©2007 Optical Society of America
Ultra-Wideband (UWB) is considered as a promising technology for short-range high data-rate indoor wireless communications and high data-rate sensor networks, thanks to the many advantages, such as low power consumption, low spectral density, high immunity to multipath fading, enhanced capability to penetrate through obstacles, and coexistence with other conventional radio systems –. In 2002, the US Federal Communications Commission (FCC) approved the unlicensed use of the UWB spectrum from 3.1 to 10.6 GHz, with a power spectral density (PSD) lower than -41.3 dBm/MHz . Based on the definition of FCC, a UWB signal should have a spectral bandwidth greater than 500 MHz or a fractional bandwidth greater than 20% .
For UWB communications, one of the key challenges is the generation of UWB pulses that satisfy the FCC-specified spectrum mask. Several UWB pulse generation techniques have been proposed recently –. Among these techniques, the implementation of the first- or the second-order derivatives of a Gaussian pulse, to generate a Gaussian monocycle or a Gaussian doublet, is considered as a simple and efficient technique for UWB pulse generation . UWB pulses can be generated in the electrical domain using electronic circuitry . Recently, the generation of UWB pulses in the optical domain has been a topic of interest. The generation of UWB pulses in the optical domain provides a higher flexibility, which enables the generation of UWB pulses with switchable pulse shapes and polarities. In addition, the huge bandwidth offered by photonics enables the generation of UWB pulses to fully occupy the spectrum range specified by the FCC. Different approaches have been recently proposed and demonstrated –. The major limitation of the approaches in – is that each scheme can only generate one type of UWB pulse (Gaussian monocycle or doublet). For some applications, such as pulse shape modulation (PSM), it is desirable that both Gaussian monocycle and doublet can be generated in a single system. In , different waveforms can be obtained, but the switching speed between the waveforms is limited by the speed of the liquid crystal modulator. Very recently, a design was proposed to generate UWB monocycle and doublet pulses in one system , in which a fiber Bragg grating (FBG) was used to serve as a frequency discriminator, to perform phase modulation to intensity modulation (PM-IM) conversion. By locating the optical carrier at the linear or the quadrature region of the FBG reflection spectrum, UWB monocycle or doublet pulses were generated . The main drawback of this scheme is the requirement for a high-speed tunable laser source (TLS) to realize the waveform switchability. In addition, the high sensitivity of the FBG to environmental changes would affect the stability of the system.
It is known that a Gaussian monocycle or doublet pulse can be generated by implementing the first- or the second-order derivative of a Gaussian pulse. The first- or the second-order derivative can be approximated by the first- or the second-order difference. It is known that the first-order difference can be realized by using a two-tap microwave delay-line filter with coefficients of (1, -1) and a second-order difference can be realized using a three-tap microwave delay-line filter with coefficients of (1, -2, 1) . In this paper, we propose an optically switchable UWB monocycle and doublet generation system based on a photonic microwave delay-line filter that can be reconfigured optically as a two- or three-tap microwave filter. A brief theoretical explanation of using a two- or three-tap microwave delay-line filter for UWB pulse generation is provided, and then the realization of the reconfigurable microwave delay-line filter with coefficients of (1, -1) or (1, -2, 1) is proposed. The microwave delay-line filter consists of a polarization modulator (PolM), a length of polarization maintaining fiber (PMF), and a balanced photo-detector (BPD). The use of the reconfigurable microwave delay-line filter for UWB monocycle or doublet generation is demonstrated. Gaussian monocycles or doublets with fractional bandwidths of about 170% or 130% are experimentally obtained. The switchability of the proposed UWB generator in pulse shape and polarity is investigated and experimentally demonstrated.
The schematic diagram of a general N-tap photonic microwave delay-line filter is shown in Fig. 1. It consists of an optical source, an optical modulator, a delay tine device, and a photodetector (PD). The microwave signal to be filtered is modulated onto the lightwave generated from the optical source via the optical modulator. The modulated lightwave is then sent to an N-tap delay-line device to introduce different time delays with an identical time delay difference between two adjacent taps. The time-delayed signals are then applied to the PD. The time delay difference determines the free spectral range (FSR) and the coefficients determine the shape of the filter response. Mathematically, the frequency response of an N-tap microwave delay-line filter is given
where τ is the time delay difference and ak is the coefficient of the kth tap. For a two-tap filter with coefficients of (1, -1), the frequency response is given
For a three-tap filter with coefficients of (1, -2, 1), the frequency response is given
As can be seen, Eq. 4 and Eq. 5 have the same frequency responses as a first- and second-order differentiator except that a scaling factor and a linear phase shift are added, which will not affect the shape of the output microwave signal. The condition of small ωτ /2 can be easily satisfied in the system by controlling the time-delay difference to be small. Therefore, we can conclude that a two- or a three-tap microwave delay-line filter with coefficients of (1, -1) or (1, -2, 1) and a small time-delay difference can be used to realize the first- or second-order derivative. If the input microwave signal to the two- or three-tap microwave delay-line filter is a Gaussian pulse, a Gaussian monocycle pulse or doublet can be generated.
3. Experiment and results
The microwave delay-line filter that can be reconfigured as a two- or three-tap photonic microwave delay-line filter with coefficients of (1, -1) or (1, -2, 1) is experimentally realized. The experiment setup is shown in Fig. 2. The filter consists of a PolM, a length of PMF and a BPD. The key components in the system are the PolM and the BPD. The PolM is a high-speed electrically tunable arbitrarily retarding, multiple-order wave plate , which can be used to generate the negative coefficient. When a linearly polarized incident light is oriented with an angle of 45° to one principal axis of the PolM, the polarization state of the output lightwave would change from linear polarization to orthogonal linear polarization, passing through elliptical and circular polarization states as the modulation voltage is varied by a half-wave voltage Vπ , polarization modulation is thus achieved. The BPD (Discovery Semiconductors) consists of two reversely-biased photo-detectors, with a tunable time-delay line incorporated in one branch of the BPD; therefore, it can realize time-delayed differential detection.
In Fig. 2, the output lightwave from a laser diode (LD) with a power of about 8 dBm is sent to the PolM at an incident polarization angle of 45° for polarization modulation. Two complementary microwave signals modulated onto two orthogonally polarized optical carriers are obtained at the output of the PolM. The total optical power at the output of the PolM keeps constant. The two orthogonally polarized optical microwave signals are then sent to the PMF with their polarization directions aligned with the fast and the slow axes of the PMF. A time-delay difference τ is introduced to the two signals due to the birefringence of PMF. The output optical microwave signals from the PMF is then equally split by a 50:50 fiber coupler and sent to the BPD. Two polarization controllers, PC1 and PC2, are incorporated at the input and the output of the PolM, with PC1 being used to adjust the polarization angles of the incident lightwave to have an angle of 45° to one principal axis of the PolM, and PC2 being used to align the polarization directions of the two complementary optical microwave signals with the fast and the slow axes of the PMF.
The whole system in Fig. 2 is an optically reconfigurable photonic microwave delay-line filter. It is configured as a two-tap delay-line filter if one arm of the coupler is connected to only one input port of the BPD. In this case, only one PD in the BDP is used. The time delay difference between the two taps is generated due to the birefringence of the PMF. Since the two orthogonal polarization modes in the PMF are complementary in amplitude, the two-tap microwave filter has opposite coefficients of (1, -1). The system in Fig. 2 can also be configured as a three-tap microwave delay-line filter with coefficients of (1, -2, 1) if the second arm of the coupler is connected to the other input port of the BPD, with an additional time-delay difference τ introduced between the two branches by adjusting the internal optical delay line in the BPD, as shown in Fig. 2. Note that the proposed reconfigurable photonic microwave delay-line filter is an incoherent filter as no optical interference occurs between the two orthogonally polarized lightwaves.
The PolM (Versawave Technologies) used in the experiment can operate up to 40 GHz with a working wavelength range from 1530 to 1560 nm. The PMF (Corning PM1550) has a length of 42 m and a beat length of 3.75 mm. The time-delay difference τ between the fast and slow axes of the PMF is about 57 ps.
In the first experiment, the reconfigurability of the system in Fig. 2 as a two- and three-tap microwave filter is demonstrated. By setting the optical switch at the OFF and ON states, coefficients of (1, -1) and (1, -2, 1) are realized, respectively. The frequency responses, shown in Fig. 3(a) and 3(b), are measured using a vector network analyzer (VNA, Agilent 8364A). Comparing Fig. 3(a) and 3(b), we can find that the transmission lobe of the three-tap microwave filter is sharper than that of the two-tap microwave filter, this is because that the three-tap microwave filter has two degenerate zeros while the two-tap microwave filter has only one zero. The simulated frequency responses are also shown in Fig. 3(a) and 3(b). A good agreement is observed.
In the second experiment, the switchability of the proposed UWB generator is demonstrated. To do this, a non-return-to-zero pulse train generated by a bit-error-rate tester (BERT, Agilent N4901B) is applied to the PolM via its RF input port. The Gaussian pulse train has a data rate of 10 Gbit/s, a fixed pattern of “1000…” (one “1” in every 64 bits), which is equivalent to a pulse train with a repetition rate of 156 Mbit/s. The full-width at half-maximum (FWHM) of a Gaussian pulse is measured to be 80 ps. By configuring the system in Fig. 2 as a two- or three-tap microwave delay-line filter with coefficients of (1, -1) or (1, -2, 1), a Gaussian monocycle or doublet pulse is generated, with the waveforms shown in Fig. 4(a) and 4(b). Note that the time delay difference τ in the experiment is 57 ps, which is small enough to satisfy the condition at lower frequencies to get Eq. 4 and Eq. 5. For higher frequencies, the approximation error for Eq. 4 and Eq. 5 is larger. However, we realize that a larger time delay difference would bring a higher suppression at higher frequencies, which would make the generated UWB pulse spectrum better confined in the FCC mask.
The polarity of the UWB monocycle and doublet can also be inverted by adjusting PC2 to switch the slow and fast axes of the PMF. The pulse widths of the generated monocycle and doublet are measured to be about 200 ps and 258 ps. Their spectra are shown in Fig. 5(a) and 5(b). It is clearly seen that the spectrum of the Gaussian doublet is located at higher frequencies compared with that of the Gaussian monocycle, which is reasonable as the three-tap microwave filter has a sharper spectral response than that of the two-tap microwave filter. The -10 dB bandwidths of the monocycle and doublet pulses are measured to be 9.1 GHz and 8.0 GHz, with fractional bandwidths of about 170% and 130%.
An optically switchable UWB pulse generator using a reconfigurable photonic microwave delay-line filter was proposed and demonstrated. The filter could be easily reconfigured as a two- or three-tap delay-line filter. When the filter was configured as a two-tap microwave delay-line filter with coefficients of (1, -1), Gaussian monocycle pulses were generated. When the filter was configured as a three-tap delay-line filter with coefficients of (1, -2, 1), Gaussian doublet pulses were generated. The reconfigurability of the system could be easily realized by using an optical ON-OFF switch, which enables the system to operate at a very high speed. Therefore, the proposed system can be used to achieve high-speed pulse shape modulation (PSM). The system can also be used to achieve high-speed pulse polarity modulation (PPM) if PC2 in the system is replaced by a high-speed electrically tunable half-wave plate, which can be another PolM. Therefore, the proposed UWB generation system has the capability in implementing both PSM and PPM at high speed, which is essential for a UWB communication system with different data modulation schemes. The generation of UWB pulses using the proposed reconfigurable photonic microwave delay-line filter was experimentally demonstrated, Gaussian monocycle and doublet pulses with a fractional bandwidth as high as 170% and 130% were obtained.
The work was supported by The Natural Sciences and Engineering Research Council of Canada (NSERC).
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