Parameter values used to obtain Figure 4.
We investigate resonant interactions in a specific electrical lattice that supports left-handed (LH) waves. The impact of LH waves on the three-wave mixing process, which is the most fundamental resonant interaction, is illustrated. In contrast to the ordinary right-handed (RH) waves, the phase of the LH wave moves to the different direction from its power. This exotic property together with the lattice’s dispersive features results in the resonant phenomena that are effectively utilized for practical electrical engineering, including the significant harmonic wave generation via head-on collisions, harmonic resonance, and short pulse generation driven by soliton decay. These resonances are quantified by the asymptotic expansion and characterized by numerical and/or experimental methods, together with several design criteria for their practical utilization. To cope with dissipation, a field-effect transistor (FET) is introduced in each cell. In particular, we characterize the stationary pulse resulting from the balance between dissipation and FET gain.
- three-wave mixing
- soliton decay
- harmonic resonance
- electrical lattices
- composite right- and left-handed transmission lines
- traveling-wave field-effect transistors
- coherent structures
Resonances have been utilized as the powerful tool to achieve harmonic wave generation in electrical engineering. This chapter introduces left-handedness to the interacting waves and discusses its impact in that field. In ordinary, that is, right-handed (RH) media, the wave vector directs to the same direction as the Poynting vector, so that the phase and power move to a common direction. In left-handed (LH) media, the situation is reversed.
To achieve strong resonant interactions, frequencies and wave numbers must be preserved. For example, when a wave of frequency
To investigate resonances involving LH waves, we introduce nonlinearity to composite right- and left-handed (CRLH) transmission lines. CRLH transmission lines have been investigated in electrical engineering community as the practical and broadband platform to support LH waves [1–4]. The line has noteworthy dispersive property that the propagating wave exhibits LH (RH) properties when its carrier frequency is greater (less) than the line’s characteristic frequencies. Furthermore, several activities have clarified the wave dynamics in CRLH lines with nonlinearity introduced by voltage-controlled devices [5–13]. In our case, the shunt capacitor each cell of a CRLH line contains is replaced with the Schottky varactor [10, 14]. The three-wave resonant interaction (3WRI) equations have been derived from the transmission equations of that nonlinear CRLH line via the derivative expansion method and is used to characterize the head-on collision of LH waves.
Similar spontaneous resonant interaction is expected in nonlinear CRLH lines. The soliton decay is realized for three waves having different group velocities. It requires the situation where the wave having the middle group velocity is incident to the line. Then, a soliton contained in the incident wave decays into the fast and slow solitons spontaneously. Inevitably, the slow soliton(s) occupies the LH branch for the nonlinear CRLH line; therefore, it starts to travel to the opposite direction to the incident and fast solitons, leading to the shortening of the fast soliton. By solving the eigenvalue problem of the Zakharov-Shabat (ZS) equation relating with the 3WRI equation, it is found that the fast soliton can become shorter for longer incident wave. Through these observations, we can utilize the soliton decay in the nonlinear CRLH line for generating broadband envelope pulses.
The use of nonlinear CRLH lines is sometimes limited because of wave attenuation caused by finite electrode resistance and substrate current leakage. In order to achieve loss compensation, a traveling-wave field-effect transistor (TWFET) is considered . For the voltage waves traveling over FET electrodes, two CRLH lattices are required, which are, respectively, loaded with the gate and drain in each cell. The unit-cell FET can be biased via the LH inductors. In addition, the inter-cell direct current flow is cut off by the LH capacitors. The device introduces LC resonant pairs in each cell, which can operate as nonlinear oscillators with the aid of FET gain; therefore, the device can be considered as a kind of spatially extended oscillator systems. Hereafter, we call the device as the CRLH-TWFET. In the case of supercritical Andronov-Hopf bifurcation, the oscillation amplitude gradually increases when the bifurcation parameter passes a critical value. Then, the relaxation time needed to initiate autonomous oscillation becomes sufficiently large; therefore, it succeeds in effectively suppressing autonomous oscillation to guarantee the loss-compensated propagation of LH pulse waves. On the other hand, the amplitude grows to become discontinuously finite in subcritical cases, where the system affords the coexistence of an oscillatory region with a nonoscillatory region in addition to the homogeneous oscillatory state . The resulting coherent structures function as the building blocks of the spatiotemporal patterns appearing in the system. When both boundaries at the ends of the oscillatory region preserve their relative positions, the oscillatory region preserving this envelope is called a pulse. Possibly, the boundary velocity vanishes, so that the pulse becomes localized and stationary [17, 18]. From the scientific viewpoint, a convenient electronic system to support such solitary waves is valuable for clarifying their interacting dynamics using either numerical or experimental method.
After describing the structure and dispersive properties of the nonlinear CRLH line, the head-on collision of envelope pulses is characterized numerically on that line to illustrate significant generation of harmonic waves through resonances. Next, the process is quantified by the 3WRI equations derived by applying the derivative expansion method to the transmission equations of a nonlinear CRLH line. Subsequently, two spontaneous resonant interactions: harmonic resonance and soliton decay are characterized, where the same 3WRI equations are used to model the wave dynamics. Finally, the development of a stationary pulse in a CRLH-TWFET is discussed.
2. Fundamental properties of nonlinear CRLH TLs
Because the nonlinear electrical lattice we investigate is based on CRLH lines, we first describe their fundamental properties. The unit-cell structure is shown at the top of Figure 1(a), where
To introduce nonlinearity, we employ the Schottky varactor in place of
3. Head-on collision of LH waves
It is well known that the efficiency of resonant interactions between two waves is maximized, when the phase-matching condition:
The resonance is briefly discussed for the two colliding pulses having different carrier frequencies . Let the carrier frequency of the left (right)-moving pulse denote as
In the next section, the evolution equations of the envelope functions of the incident and collision-induced pulses are obtained by the application of the derivative expansion method to the transmission equation of a nonlinear CRLH line . In particular, the generation efficiency of the second-harmonic wave is formulated for the case when the left- and right-moving pulses have a common frequency and wavelength.
4. Three-wave mixing of LH waves
In the present study, we consider the case where the pulse spreads over many cells, and the lattice is regarded as being homogeneous, such that the discrete spatial coordinate
whose denominator becomes zero only at so that
In summary, the value of
Based on this
5. Harmonic resonance
In this section, we investigate harmonic resonance in a nonlinear CRLH line . As discussed in Section 1, the harmonic resonance becomes significant when the phase velocities of the fundamental and second harmonic waves are coincident. Figure 5(a) shows the typical dispersion of a CRLH line, where
6. Soliton decay
To describe the soliton decay in a nonlinear CRLH line, we again consider the 3WRI equations of a nonlinear CRLH line. By introducing , Eq. (12) is transformed into the standard 3WRI equation, that is,
for the spatial waveform of the incident
where defines the eigenvalue corresponding to the soliton in the
We validate the analysis with the numerical integration of Eqs. (6) and (7). The line is designed to be balanced by setting
As a broadband pulse generator, it suffices for a nonlinear CRLH line to succeed in the emission of the first pair of solitons. To output the short envelope pulse uniquely, we only set up a band-pass filter extracting frequencies around
Figure 9(a) shows the structure of a CRLH-TWFET. Two coupled transmission lines are periodically loaded with FETs in such a way that one of the lines is connected to the gate and the other to the drain . The gate line consists of the series inductor, series capacitor, shunt inductor, and shunt varactor, whose values are respectively denoted as
As in the case of nonlinear CRLH lines, the device can generate long wavelength harmonic wave via head-on collision of LH waves. Interestingly, such collision-induced wave evolves to a stationary pulse. Figure 9(b) demonstrates that, for the varactor,
In practice, the line parameter values fluctuate, such that finite disorder is introduced to the lattice dynamics, which effectively serves the Pieres-Nabarro potential to the wave dynamics. When the pulse cannot overcome the potential, it is partially reflected to become a stationary pulse via resonance. Thus, the stationary pulse is expected to develop more frequently on the line when the fluctuation increases. To examine the property of the practical line, we fabricated a test line on print circuit board. Actually, the parameter values used to obtain Figure 9(b) simulate those of the test line. Figure 10(a) shows the measured spatiotemporal voltage profile. A sech-shaped envelope pulse was inputted only at the near end. The pulse moving to the far end was significantly reflected near the 300th cell and two different stationary pulses developed after reflection. Figure 10(b) shows the calculated voltage profile to simulate the measured result, where the fluctuation has 7% standard deviation. The device fluctuation cannot be modeled exactly. However, it successfully demonstrates both the reflection and the development of a stationary pulse. With the balance between the dissipation and FET gain in a disordered lattice, resonant interactions lead to this interesting wave dynamics.
We first describe the three-wave mixing process in nonlinear CRLH lines. The head-on collision of LH waves results in a significant amount of harmonic waves, whose efficiency is accurately predicted by the asymptotic method.
The CRLH dispersion allows us two spontaneous resonant processes to generate harmonic waves: the harmonic resonance and soliton decay. The harmonic resonance in a nonlinear CRLH line succeeds in generating second-harmonic waves even under the presence of finite line resistance, when the line is designed for the second-harmonic waves to cause cavity resonance. The left-handedness of the fundamental wave guarantees that both the fundamental and second harmonic waves can gain amplitude as phase advances. The soliton decay in a nonlinear CRLH line gives the effective way for generating broadband envelope pulses. The incident envelope spontaneously emits several pairs of the fast and slow solitons. In general, slow solitons exhibit left-handedness to travel backward and their fast counterparts become shorter than the incident pulse. In addition, the wider the incident pulse, the narrower the fast solitons.
A CRLH-TWFET is shown to support stationary nonlinear oscillatory pulse waves, which is generated by the collision of two counter-moving waves through resonance. The presence of disorder helps the development of stationary pulses. The bias voltage of varactor in each cell can be set independently and control the position and number of such stationary pulses.
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