Mulham Khoder

Wavelength Switching Speed in Semiconductor Ring Lasers With On-Chip Filtered Optical Feedback

We experimentally and numerically characterize the wavelength switching speed of a tunable semiconductor ring laser using filtered optical feedback. The feedback is realized employing two arrayed-waveguide gratings to split/recombine light into different wavelength channels. The wavelength tuning and switching is controlled by changing the currents injected in semiconductor optical amplifiers in the feedback section. A wavelength switching speed of a few nanoseconds is achieved. We investigate also the effect of the feedback parameters and noise strength on the wavelength switching speed.

Controlled multiwavelength emission using semiconductor ring lasers with on-chip filtered optical feedback

We report on an integrated approach to obtain multiwavelength emission from semiconductor ring lasers with filtered optical feedback. The filtered feedback is realized on-chip employing two arrayed-waveguide gratings to split/recombine light into different wavelength channels. Through experimental observations and numerical simulations, we find that the effective gain of the different modes is the key parameter which has to be balanced in order to achieve multiwavelength emission. This can be achieved by tuning the injection current in each amplifier.

Digitally tunable dual wavelength emission from semiconductor ring lasers with filtered optical feedback

We report on a novel integrated approach to obtain dual wavelength emission from a semiconductor laser based on on-chip filtered optical feedback. Using this approach, we show experiments and numerical simulations of dual wavelength emission of a semiconductor ring laser. The filtered optical feedback is realized on-chip by employing two arrayed waveguide gratings to split/recombine light into different wavelength channels. Semiconductor optical amplifiers are placed in the feedback loop in order to control the feedback strength of each wavelength channel independently. By tuning the current injected into each of the amplifiers, we can effectively cancel the gain difference between the wavelength channels due to fabrication and material dichroism, thus resulting in stable dual wavelength emission. We also explore the accuracy needed in the operational parameters to maintain this dual wavelength emission.

Fast phase response and chaos bandwidth enhancement in semiconductor lasers subject to optical feedback and injection.

We numerically show the quantitative relation between the chaos bandwidth enhancement and fast phase dynamics in semiconductor lasers with optical feedback and optical injection. The injection increases the coupling between the intensity and the phase leading to a competition between the relaxation oscillation (RO) frequency and the intrinsic response frequency of the phase. For large feedback strengths, it is found that the chaos bandwidth is determined by the intrinsic phase response frequency. For smaller feedback strengths, the system is not chaotic and its bandwidth is determined by the RO frequency.

Effect of External Optical Feedback on Tunable Micro-Ring Lasers Using On-Chip Filtered Feedback

We experimentally and numerically study the influence of external optical feedback on semiconductor ring lasers, which are wavelength tunable using on-chip filtered optical feedback. We investigate the dynamics in the lasers intensity due to the conventional optical feedback and how this dynamics is changed by adding filtered optical feedback. Experimental observations show that the filtered optical feedback shifts the onset of the feedback-induced dynamics to larger values of the feedback rate. We are able to numerically reproduce the experimental observations using a rate equations model.

Random number generator based on an integrated laser with on-chip optical feedback

We discuss the design and testing of a laser integrated with a long on-chip optical feedback section. The device and feedback section have been fabricated on a generic photonic integration platform using only standard building blocks. We have been able to integrate a 10 cm feedback length on a footprint of 5.5 mm2. By controlling the amount of feedback, we achieve chaotic dynamics in the long-cavity regime and show that the resulting dynamics is sufficiently complex in order to generate random bits based on the chaotic intensity fluctuation at a rate of 500 Mbits/s.

Graphene’s nonlinear-optical physics revealed through exponentially growing self-phase modulation

Graphene is considered a record-performance nonlinear-optical material on the basis of numerous experiments. The observed strong nonlinear response ascribed to the refractive part of graphene’s electronic third-order susceptibility χ(3) cannot, however, be explained using the relatively modest χ(3) value theoretically predicted for the 2D material. Here we solve this long-standing paradox and demonstrate that, rather than χ(3)-based refraction, a complex phenomenon which we call saturable photoexcited-carrier refraction is at the heart of nonlinear-optical interactions in graphene such as self-phase modulation. Saturable photoexcited-carrier refraction is found to enable self-phase modulation of picosecond optical pulses with exponential-like bandwidth growth along graphene-covered waveguides. Our theory allows explanation of these extraordinary experimental results both qualitatively and quantitatively. It also supports the graphene nonlinearities measured in previous self-phase modulation and self-(de)focusing (Z-scan) experiments. This work signifies a paradigm shift in the understanding of 2D-material nonlinearities and finally enables their full exploitation in next-generation nonlinear-optical devices.Graphene enables extraordinary nonlinear-optical refraction, far exceeding predictions based on conventional nonlinear-susceptibility theory. Here, Vermeulen et al. show that rather than the nonlinear susceptibility, a complex saturable refraction process is central to graphene’s unusual behavior.

Stability of steady and periodic states through the bifurcation bridge mechanism in semiconductor ring lasers subject to optical feedback

With the development of new applications using semiconductor ring lasers (SRLs) subject to optical feedback, the stability properties of their outputs becomes a crucial issue. We propose a systematic bifurcation analysis in order to properly identify the best parameter ranges for either steady or self-pulsating periodic regimes. Unlike conventional semiconductor lasers, we show that SRLs exhibit both types of outputs for large and well defined ranges of the feedback strength. We determine the stability domains in terms of the pump parameter and the feedback phase. We find that the feedback phase is a key parameter to achieve a stable steady output. We demonstrate that the self-pulsating regime results from a particular Hopf bifurcation mechanism referred to as bifurcation bridges. These bridges connect two distinct external cavity modes and are fully stable, a scenario that was not possible for diode lasers under the same conditions.

Dynamics of semiconductor microring lasers subject to on-chip filtered optical feedback

Tunable laser diodes are needed in a range of applications including wavelength division multiplexing, optical instrument testing, optical sensing and tera hertz generation. In this work, we investigate the stability of lasers which use filtered optical feedback for wavelength tuning. We investigate experimentally the dynamics induced by this on-chip filtered optical feedback. In this study, we choose to use a compact device which combines a semiconductor ring laser with on-chip filtered optical feedback to achieve wavelength tunability. The filtered optical feedback is realized by employing two arrayed waveguide gratings to split/recombine light into different wavelength channels. Semiconductor optical amplifiers are placed in the feedback loop in order to control the feedback of each wavelength channel independently. Experimental observations show that the stability of the clockwise and counterclockwise propagation modes depends on the feedback strength. Experiments also show that for a specific range of the feedback strength, anti-phase oscillations in the intensity of the clockwise and counterclockwise propagating modes can be induced. These oscillations could not be seen in the same semiconductor ring laser without filtered optical feedback. We investigate how the frequency and the amplitude of these oscillations change under the effect of filtered optical feedback. We also discuss how these anti-phase oscillations can be suppressed by properly choosing the feedback strength.

Design and characterization of a laser integrated with long on-chip optical feedback usable as compact random number generator

We discuss the design and the dynamics of a semiconductor laser integrated with a long on-chip optical feedback (see also [Verschaffelt et al., Chaos: An Interdisciplinary Journal of Nonlinear Science, vol. 27, pp. 114310, 2017]1). Such lasers with optical feedback are interesting for several applications that make use of their rich dynamical behavior. Moreover, they are ideal test-beds to experimentally study delay induced dynamics, because the systems parameters (such as the laser injection current and the optical feedback strength) can be easily accessed and accurately controlled. The system discussed here uses only standard building blocks of the generic Jeppix platform for photonic integrated lasers. The design is based on a DBR-laser with a spiral delay waveguide. We have included several control pads with which we can tune the fabricated lasers emission wavelength, the feedback strength and phase in order to compensate for fabrication tolerances. We have been able to integrate a 10 cm feedback length on a footprint of 5.5 mm2. We illustrate that this delay is sufficiently long to drive the laser into a chaotic regime, and we analyze the chaotic dynamics based on the spectrum, autocorrelation and permutation entropy. We show - using the NIST statistical test suite for random number generators - that the observed delay dynamics is sufficiently complex for random number generation at a rate of 500 Mbits/s.