Rectangular-wave Modulation Research Articles

Unidirectional invisibility of an acoustic multilayered medium with parity-time-symmetric impedance modulation

Non-Hermitian scattering systems respecting parity-time symmetry exhibit unidirectional invisibility or reflectionlessness at an exceptional point of the scattering matrix. In this study, we investigate the scattering properties of a one-dimensional acoustic parity-time-symmetric multilayered medium. Parity-time symmetry is defined by a spatial small-amplitude square-wave modulation of the medium parameters in the complex plane. Such a multilayered medium has been demonstrated to reach an exceptional point and support unidirectional invisibility for balanced real and imaginary part modulations of the refractive index. Through the analysis of interference within the medium, we show that the complex modulation of acoustic impedance instead of the refractive index governs the exceptional point of the scattering matrix and determines the occurrence of the unidirectional invisibility. This unidirectional invisibility is further investigated for more general cases of complex parameter distribution including unequal modulation amplitudes and rectangular wave modulation. Our study provides an intuitive physical picture of the occurrence of unidirectional invisibility induced by an exceptional point in acoustic wave systems. Moreover, this study could facilitate the design and practical realization of acoustic parity-time-symmetric structures.

Step-Pulse Modulation of Gain-Switched Semiconductor Pulsed Laser

To improve the peak power and extinction ratio and produce ultra-short pulses, a novel approach is presented in this paper offers a highly effective modulated method for a gain-switched semiconductor laser by using step-pulse signal modulation. For the purpose of single pulse output, then the effects on the output from the gain-switched semiconductor laser are studied by simulating single mode rate equation when changing the amplitude and width of the modulated signal. The results show that the proposed method can effectively accelerate the accumulation speed of the population inversion and we can acquire the output pulse with higher peak power and shorter width. Compared with the traditional rectangular wave modulation, this method is advantageous to obtain a high gain switching effect by increasing the second modulation current and reduce the pulse width to saturation at the best working point. It can be incorporated as a practical and cost-effective approach for many fields which need high extinction ratio short pulse, such as the optical time domain reflectometry.

Optimization of solid atomic magnetometer with frequency modulation method on ensembles of nitrogen vacancy color centers in diamond

In this paper, we explored the optimal rate and amplitude parameters of microwave-modulated signal applied on ensembles of NV centers where the linear slope of the derivation of optically detected magnetic resonance reached the maximum both theoretically and experimentally. Further, we compared the linear slope of the derivation of the optically detected magnetic resonance and frequency response induced by rectangular modulation, which was 1.38 and approximately 10 times higher than that of sine-wave modulation. Besides, the noise levels when two-signal modulation was used were compared, which showed that the magnetic field sensitivity of the magnetometer with rectangular-wave modulation approached as low as 9.4 nT √Hz−1.

Initial result of collective Thomson scattering using 77 GHz gyrotron for bulk and tail ion diagnostics in the Large Helical Device

The collective Thomson scattering (CTS) technique has been utilized with the backscattering configuration in the collective scattering regime to diagnose the velocity distribution functions in the Large Helical Device (LHD). The receiver was equipped with 16 channels and the first test has been carried out using the eight channels for scattered radiation and these channels cover a few GHz frequency shift from the 76.95 GHz probe beam. During the discharge, the electron density and temperature at the central region of the LHD are 1×1019m−3, and 1.0 keV, respectively. The probing beam with rectangular wave modulation is injected by 50 Hz in order to be distinct from the background electron cyclotron emission (ECE). The scattered radiation is resolved successfully at each channel of CTS receiver system. The detected signals of bulk ion and electron components are by a factor of 10 ~ 102 larger than the background ECE signal. We found that the measured spectra are in reasonably agreement with the theoretical spectra calculated by using the reliable measured electron temperature and density for input parameters. The CTS receiver system will be improved to obtain more accurate velocity distributions in high temperature plasmas.

The cavitation valley phenomenon of rectangular wave modulation ultrasound

To study the effect of a low-frequency pulse envelope on cavitation, the modulation depth was changed gradually, keeping the ON/OFF unchanged, and the ‘cavitation valley’ phenomenon was discovered. The results show that the cavitation effect generated by amplitude modulation ultrasound appears to be minimized by an appropriate choice of the modulation depth.

Rectangular-Wave Modulation

Rectangular-wave modulated pulse lengths are expressed as a function of the modulation index. The theoretical expressions are experimentally verified. An experimental measurement of signal-to-noise ratio for rectangular-wave modulation confirms the theoretical analysis. Jelonek's signal-to-noise ratio formula for pulse-length modulation is also found applicable to rectangular-wave modulation. It is concluded that rectangular-wave modulation is closed-loop pulse-length modulation.

Measurement of Signal-To-Noise Ratio for Rectangular-Wave Modulation

An experimental measurement of signal-to-noise ratio for rectangular-wave modulation confirms the theoretical signal-to-noise ratio expression. Jelonek's pulse-length modulation signal-to-noise ratio formula is also applicable to rectangular modulation.

TDM Using a Boxcar Circuit for Crosstalk Reduction

A method of time division multiplexing using a boxcar circuit in place of the ordinary low- pass filter has been devised. The method has been applied for multi-channel transmission through pulse interval modulation and rectangular wave modulation using indigenous solid state devices. The multiplexed output obtained from the boxcar circuit has been processed in pulse interval modulator and rectangular wave modulator, and the receiver circuit uses complimentary circuits with synchronized clock pulses. The technique of multiplexing developed in this article can also be used in Δ—, M, Δ—ΣM, HUPCM and other pulse modulation systems.

Signal/noise ratio in rectangular-wave modulation

It is shown here that the earlier formula for the calculation of the signal/channel-noise ratio (s.c.n.r.) in rectangular-wave modulation is not correct. According to the new result, the s.c.n.r. is not proportional to the cube of the channel bandwidth; instead it closely approximates to the s.c.n.r. expression for pulse-length modulation.