We analyze in details experimental measurements of high frequency laser dynamics and chaos generated by a laser diode subjected to phase-conjugate feedback realized using nonlinear wave mixing in a SPS photorefractive crystal in a CAT configuration. In addition to the typical figure of merit, ie, chaos bandwidth, the corresponding spectral flatness and permutation entropy at delay is analyzed. The experiments reveal that chaos, with a bandwidth up to 30 GHz, a spectral flatness up to 0.75, and a permutation entropy at delay of up to 0.99 can be generated. These optimized performances are observed over a large range of parameters and have not been achieved in the conventional optical feedback configuration. Interestingly, when the pump current is reduced, the chaos bandwidth is also reduced while keeping the spectral flatness and the permutation entropy. Our experimental findings are in qualitative agreement with the presented numerical simulations produced using the Lang-Kobayashi model. Such chaotic laser diodes can be used in chaotic cryptography, high-rate random number generation and optical metrology with the enhancement of lidar resolution.
A semiconductor laser having an emission wavelength of 852 nm is subjected to phase-conjugated feedback. The phase conjugation is implemented using a Rh-doped BaTiO3 photorefractive crystal of the dimension 5mm × 5mm × 5mm. A combination of several phenomena including light fanning, total internal reflection and four-wave mixing occur in tandem inside the crystal to generate the phase conjugate of the light entering the crystal. Our recent works have established that such configuration exhibits significantly enhanced chaos bandwidth with high spatiotemporal complexity at varied feedback strengths, compared to its conventional optical feedback counterpart in a long external cavity setup. The presented work studies the systematic progression of spatiotemporal complexity in a long-length (≈1.5 m) external cavity setup as a function of the feedback strength. System outputs at varied operating conditions are found to be highly complex with PE upwards of 0.9 and chaos BW in the order of several GHz. Such complex outputs have relevance for applications such as security-based communication and random-bit generation. These system outputs are shown to be dynamically diverse ranging from wide-band chaos to time-periodic oscillations of the output signal corresponding to a higher multiple of the external cavity frequency. The analyses performed in the temporal and frequency domain use several diagnostic tools including permutation entropy, continuous wavelet transform, and statistically calculated chaos bandwidth to unveil and dissect the evolution of the system's characteristic temporal signatures. Thus, we comprehensively determine the true chaotic nature of the system outputs in the spatiotemporal domain.
In this manuscript, we study the influence of various parameters on the dynamics of a semiconductor laser with phase-conjugate feedback. We report the chaos bandwidth of its chaotic states, and simultaneously the frequency of its self-pulsing modes. We demonstrate that both indicators are unaffected by the delay length at any given feedback strength. A higher pumping current enhances both indicators, as does an increase of the carrier lifetime. We further investigate the influence of the linewidth enhancement factor (α-factor ): restabilization occurs for higher values of feedback strength, thereby enhancing the system's limit of attaining higher values of the chaos bandwidth. The effective bandwidth is observed to follow the general trend of the chaos bandwidth with an exception of transition dynamical solutions. We show that the PCF system is able to generate high-frequency and broadband chaos without any additional active optical element. Because of the high quality of chaos induced by the PCF system, delayed laser systems are strong candidate to generate high-performance chaotic signals.
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