Chih-Hao Cheng

Chaos time delay signature suppression and bandwidth enhancement by electrical heterodyning.

We numerically investigated the chaos time delay signature (TDS) suppression and bandwidth enhancement by electrical heterodyning. Chaos signals generated with a semiconductor laser subject to optical feedback typically have distinct loop frequency peaks in their power spectra corresponding to the reciprocals of the time delays, which deteriorates the performance in applications including chaos radar/lidar and fast random bit generation. By electrically heterodyning the chaos signal with a single frequency local oscillator, we show that the power in the chaos spectrum can be redistributed and a smoother spectrum with a broader effective bandwidth can be obtained. Compared with the chaos directly generated from a semiconductor laser subject to optical feedback, the amplitudes of the TDS (ρ(TDS)) measured under different feedback strengths can be suppressed up to 63% and the effective bandwidths can be enhanced up to 46% in average after the electrical heterodyning is applied.

Dual-frequency laser Doppler velocimeter for speckle noise reduction and coherence enhancement.

We develop a dual-frequency laser Doppler velocimeter based on an optically-injected semiconductor laser. Spectral broadening suffered from the speckle noise is much suppressed benefitted by the longer effective wavelength.

Self-mixing dual-frequency laser Doppler velocimeter

In this study, we demonstrate a self-mixing dual-frequency LDV (SM DF-LDV) for integrating both the advantages of the SM-LDV and DF-LDV based on the hybrid dynamics of a diode laser subject to both optical injection and optical feedback.

Speckle noise reduction of a dual-frequency laser Doppler velocimeter based on an optically injected semiconductor laser

We develop and investigate a dual-frequency Laser Doppler Velocimeter (DF-LDV) based on an optically injected semiconductor laser. By operating the laser in a period-one oscillation (P1) state, the laser can emit light with two coherent frequency components separated by about 11.25 GHz. Through optical heterodyning, the velocity of the target can be determined from the Doppler shift of the beat signal of the dual-frequency light. While the DF-LDV has the same advantages of good directionality and high intensity as in the conventional singlefrequency LDV (SF-LDV), having an effective wavelength in the range of microwave in the beat signal greatly reduces the speckle noise caused by the random phase modulation from the rough surface of the moving target. To demonstrate the speckle noise reduction, the Doppler shifted signals from a moving target covered by the plain paper are measured both from the SF-LDV and the DF-LDV. The target is rotated to provide a transverse velocity, where the speckle noise increases as the transverse velocity increases. The bandwidth of the Doppler signal obtained from the SF-LDV is increased from 4.7 kHz to 9.4 kHz as the transverse velocity increases from 0 m/s to 5 m/s. In contrast, the bandwidth obtained from the DF-LDV maintains at 0.09 Hz with or without the rotation limited by the linewidth of the P1 state used. By phase-locking the laser with a RF current modulation, the linewidth of the P1 state can be much reduced to further improve the velocity resolution and extend the detection range.

Generation of Uncorrelated Multichannel Chaos by Electrical Heterodyning for Multiple-Input–Multiple-Output Chaos Radar Application

Multiple-input-multiple-output (MIMO) radar has received much attention in recent years for its great ability in imaging. However, the essence of MIMO radar has not been fully implemented due to the lack of proper transmission waveforms. In MIMO radar, the transmission waveform of each channel has to be uncorrelated with one another to avoid cross-interference between channels. To achieve this, we investigate the generation of uncorrelated multichannel chaos using electrical heterodyning for MIMO chaos radar (MIMO CRADAR) application. By electrically heterodyning a seed chaos source with multiple single-frequency local oscillators, chaos with different heterodyned spectra can be extracted and converted into multiple chaos channels. In this paper, the correlations between different channels of chaos generated are analyzed both numerically and experimentally. The minimal frequency spacing of the local oscillators for generating the largest amount of uncorrelated chaos channels is discussed. In our analysis, thousands of uncorrelated chaos channels can be simultaneously generated with a correlation time of several microseconds. Moreover, compared with those conventional waveform-designing methods that require complicated optimization and digital-to-analog conversion (DAC), the proposed heterodyned technique shows, for the first time, that multiple uncorrelated channels can be generated in real-time while breaking the bandwidth limitation of the DAC devices. A proof-of-concept experiment is successfully demonstrated to show the feasibility of using multichannel heterodyned chaos in the MIMO CRADAR application.

Single-sideband photonic microwave generation with an optically injected quantum-dot semiconductor laser

We studied single-sideband (SSB) photonic microwave generation with a high sideband rejection ratio (SRR) based on the period-one dynamical states of an optically injected quantum-dot (QD) semiconductor laser and demonstrated that the SSB signals have SRRs of approximately 15 dB higher than those generated with a conventional quantum-well semiconductor laser under conditions of optimal microwave power. The enhancement of SRR in the QD laser, which is important in mitigating the power penalty effect in applications such as radio-over-fiber optical communications, could be primarily attributed to a lower carrier decay rate in the dots, smaller linewidth enhancement factor, and reduced photon decay rate.

Dual-frequency laser Doppler velocimeter for speckle noise reduction

We develop a dual-frequency laser Doppler velocimeter based on an optically-injected semiconductor laser. Spectral broadening suffered from the speckle noise is much suppressed benefitted by the longer effective wavelength.