ABSTRACT
The stable one-time tuning of silicon-photonic directional couplers, over a broad range of coupling ratios, is achieved through the selective photo-removal of an upper cladding layer of chalcogenide glass. Analysis shows that the coupling coefficient per unit length between two parallel fully-etched silicon waveguides may be changed by 45%. The power coupling ratio of a 50 µm-long directional coupler between two such waveguides may be tuned arbitrarily between 0 and 1, with weak residual wavelength dependence. Smaller modifications in the coupling coefficient per unit length are obtained between two partially-etched ridge waveguides, on the order of 10%. The proposed procedure is demonstrated in the post-fabrication tuning of transmission notches of a race-track resonator, from over-coupling through critical coupling to weak coupling. The extinction ratio of specific resonances is varied between 4 and 40 dB. The coupling ratio of a tuned device remains stable following three months of storage.
ABSTRACT
The postfabrication modification of the group delay in silicon-photonic waveguides is proposed, simulated and demonstrated experimentally. Group delay variations of 2% are achieved through photo-induced changes to an upper cladding layer of photosensitive As10Se90 chalcogenide glass. The illumination of the cladding layer by intense green light for a few seconds leads to mass transfer and removal of material, away from irradiated regions. The phenomenon is employed in the localized removal of the cladding layer from above the core region of a silicon-on-insulator waveguide, thereby modifying its phase and group delays. Using the proposed method, the free spectral range of a chalcogenide-on-silicon Mach-Zehnder interferometer was modified by 1%. The technique is applicable to the postfabrication adjustment of the frequency response of silicon-photonic filters, comprised of several cascaded elements.
ABSTRACT
The synchronization of chaotic lasers and the optical phase synchronization of light originating in multiple coupled lasers have both been extensively studied. However, the interplay between these two phenomena, especially at the network level, is unexplored. Here, we experimentally compare these phenomena by controlling the heterogeneity of the coupling delay times of two lasers. While chaotic lasers exhibit deterioration in synchronization as the time delay heterogeneity increases, phase synchronization is found to be independent of heterogeneity. The experimental results are found to be in agreement with numerical simulations for semiconductor lasers.
Subject(s)
Lasers , Nonlinear DynamicsABSTRACT
Topologies of two, three and four time-delay-coupled chaotic semiconductor lasers are experimentally and theoretically found to show new types of synchronization. Generalized zero-lag synchronization is observed for two lasers separated by long distances even when their self-feedback delays are not equal. Generalized sub-lattice synchronization is observed for quadrilateral geometries while the equilateral triangle is zero-lag synchronized. Generalized zero-lag synchronization, without the limitation of precisely matched delays, opens possibilities for advanced multi-user communication protocols.
ABSTRACT
Zero-lag synchronization (ZLS) between chaotic units, which do not have self-feedback or a relay unit connecting them, is experimentally demonstrated for two mutually coupled chaotic semiconductor lasers. The mechanism is based on two mutual coupling delay times with certain allowed integer ratios, whereas for a single mutual delay time ZLS cannot be achieved. This mechanism is also found numerically for mutually coupled chaotic maps where its stability is analyzed using the Schur-Cohn theorem for the roots of polynomials. The symmetry of the polynomials allows only specific integer ratios for ZLS. In addition, we present a general argument for ZLS when several mutual coupling delay times are present.
ABSTRACT
The fluctuating intensity of a chaotic semiconductor laser is used for generating random sequences at rates up to 12.5 Gbits/s. The conversion of the fluctuating intensity to a random bit sequence can be implemented in either software or hardware and the overall rate of generation is much faster than any previously reported random number generator based on a physical mechanism. The generator's simplicity, robustness, and insensitivity to control parameters should enable its application to tasks of secure communication and calculation procedures requiring ultrahigh-speed generation of random bit sequences.
ABSTRACT
Semiconductor lasers with optical feedback have chaotically pulsating output behavior. When two similar chaotic lasers are optically coupled, they can become synchronized in their optical fluctuations. Here we show that the synchronization is not only in the amplitude and in the timing of the pulses but that the short pulses are also phase coherent with each other. This is true even when the lasers are separated by distances much larger than their coherence length.
ABSTRACT
We measure the propagation properties of a highly nonlinear photonic crystal fiber (PCF). The spatial, temporal and frequency dependent properties of the propagating modes are measured under conditions of high power, seven picosecond excitation, white light continuum generation. The experimentally determined multi-mode nature of the white light continuum is found to be in good agreement with numerical simulations.
ABSTRACT
We have implemented a simple method for generating an "amplified" phase-coherent light pulse in which a pulse train of phase-coherent, equidistant input light pulses from a mode-locked Ti:sapphire laser is coupled into a ring cavity resonator whose length is matched to the mode-locked pulse repetition frequency at 82 MHz. Pulses are thus coherently superimposed and added inside the buildup cavity and form an intense pulse that is switched out from the cavity via a fast acousto-optic modulator. The method thus provides a pulse train at a reduced and controlled repetition frequency and with higher pulse energies than the original mode-locked pulses.
ABSTRACT
The scattering of light from a slightly rough surface overlying a reflecting surface is investigated. It is shown that the long-scale component of the roughness spectrum plays a critical role in the scattering patterns obtained. The scattered interference patterns are critically dependent on small variation of the rms height of the long-scale component of the roughness. Conventional perturbation theory is found to be invalid in cases in which interference phenomena in the scattering are of importance. A model is proposed that quantitatively describes the measured angular intensity distributions.
