Wideband Chaos with Time-Delay Concealment in Vertical-Cavity Surface-Emitting Lasers

Description of impact

For the applications of optical chaos, both the time-delay (TD) signature and the bandwidth of chaos are important parameters. These two parameters have been investigated in the different schemes of chaos generation in vertical-cavity surface-emitting lasers (VCSELs). This comparison of schemes for the generation of TD signature suppression and enhanced bandwidth chaos offers opportunities for designing secure optical chaos communication systems and random number generation systems using chaotic VCSELs. The work would significantly benefit the chaos research community.

Generation of chaotic optical signal has attracted considerable research attention because of the
potential applications of chaos, such as, chaos-based optical communications, physical random number generators, chaotic logic gates, time domain reflectometry and chaotic lidar. Both the time-delay (TD) signature and the bandwidth of chaos are important parameters for chaos applications. Narrow chaos bandwidths will limit message transmission rates in chaos-based communication, the bit rate of random number generation and the spatial resolution of chaotic lidar and time domain reflectometry. The TD signature may provide the opportunity for an eavesdropper to extract a key parameter, which may compromise the security of chaotic optical communications. The TD signature reduces the randomness of the chaotic optical signal and affects the symmetrical distribution of random bits. So achieving high bandwidth with TD suppression chaos is very important for its applications. Different schemes of chaos generation based on low-cost, off-the-shelf vertical-cavity surface-emitting lasers (VCSELs) have been investigated. The results show that suppression of the TD signature and the enhanced bandwidth of chaos can be obtained simultaneously in two schemes of a chaotic beam unidirectionally injected into a CW VCSEL and a chaotic VCSEL mutual coupling with a CW VCSEL. These two schemes provide cost-effective, low power consumption, low complexity of generation of wideband chaos with TD Concealment. The work has a direct impact within the academic research community, especially benefit the chaos research community. The longer term impact of the work is seen to offer significant benefits to the applications of chaos in chaos-based optical communications, chaotic random number generators and time domain reflectometry.

Description of the underpinning research

In the autumn of year 2013, Dr Yanhua Hong (Lecturer), Prof Paul Spencer (Professor) and Prof K. Alan Shore (Professor) investigate the possibility of generating broad bandwidth chaos with time-delay (TD) signature concealment. Four schemes of generation chaos in VCSELs: (I) a CW optical beam unidirectionally injected into a chaotic VCSEL; (II) a chaotic beam unidirectionally injected into a CW VCSEL; (III) a chaotic VCSEL mutual coupling with a CW VCSEL; (IV) two mutual coupling CW VCSELs, have been studied experimentally. The results show that TD signature concealment and broad bandwidth chaos can be obtained simultaneously in a wide frequency detuning range in the second scheme. The chaos-pass filtering effect accounts for the suppression of the TD signature in the injection locking regime, where the bandwidth of chaos is enhanced. The VCSEL without optical feedback in the third scheme can also produce chaos with enhanced bandwidth and suppressed TD signature in some frequency detuning regimes. But suppressed TD signature and broad bandwidth of chaos cannot be achieved simultaneously in the first and fourth schemes. In November 2013, Prof Hong Lin from Bates College, USA, visited Bangor University. Prof Hong Lin, Dr Yanhua Hong (Lecturer) and Prof K. Alan Shore (Professor) studied the suppression of TD signature in a VCSEL subject to double-cavity polarization-rotated optical feedback. The results show that TD signatures with double-cavity feedback is weaker than those with single cavity feedback for the same experimental parameters. The result is in good agreement with the theoretical prediction. In March-June 2014, Dr Yanhua Hong (Lecturer), Dr Xianfeng Chen (Lecturer) Prof Paul Spencer (Professor) and Prof K. Alan Shore (Professor) explored the generation flat broadband chaos by using Low-cost VCSEL and semiconductor optical amplifier with a fibre ring resonator. The result shows that the flat bandwidth of chaos with the fibre ring resonator is >10 times higher than that with conventional mirror feedback. In January-April 2015, Dr Yanhua Hong (Lecturer) investigated the flatness of chaos generated by mutually coupled VCSELs. The experimental results show that the power spectra of chaos can easily be flattened by combining two uneven chaos. Flat bandwidth of the combined output can be 15 times higher than that of chaos from a single VCSEL at the optimum frequency detuning.

Beneficiaries and reach of impact

Potential long term benefits and applications in optical communications?

Impact status	Potential