Kohroh Kobayashi

Bio: Kohroh Kobayashi is an academic researcher from NEC. The author has contributed to research in topics: Laser & Semiconductor laser theory. The author has an hindex of 29, co-authored 100 publications receiving 5001 citations.

External optical feedback effects on semiconductor injection laser properties

Abstract: Influences on the semiconductor laser properties of external optical feedback, i.e., return of a portion of the laser output from a reflector external to the laser cavity, have been examined. Experimental observations with a single mode laser is presented with analysis based on a compound cavity laser model, which has been found to explain essential features of the experimental results. In particular, it has been demonstrated that a laser with external feedback can be multistable and show hysteresis phenomena, analogous to those of non-linear Fabry-Perot resonator. It has also been shown that the dynamic properties of injection lasers are significantly affected by external feedback, depending on interference conditions between returned light and the field inside the laser diode.

A GaAs-AlxGa1-xAs Double Heterostructure Planar Stripe Laser

Abstract: A stripe laser structure called a planar stripe was developed. The edge blurring of the current path is improved by the fact that the current spreads only in the thin p-AlxGa1-xAs layer with relatively high resistance. The planar stripe laser has a small threshold current resulting from the small current spreading effect and a good thermal contact. It also shows finely controlled transverse modes, compared with a usual contact stripe laser, and relatively high external differential quantum efficiency. The threshold current density is comparable to that of the proton bombarded stripe laser. The transverse mode shows an approximate Hermite-Gaussian distribution.

Room-temperature CW operation of AlGaInP double-heterostructure visible lasers

Abstract: Room-temperature continuous-wave (CW) operation of (Al0.4Ga0.6)0.5In0.5P/Ga0.5In0.5P/(Al0.4Ga0.6)0.5In0.5P double-heterostructure visible semiconductor lasers has been achieved for the first time. CW operation is obtained up to 50° C. The threshold current at 25° C is mA (4.1 kA/cm2). The lasing wavelength is 689.7 nm.

Single frequency and tunable laser diodes

Abstract: The state-of-the-art technologies for single-frequency and frequency-tunable laser diodes are reviewed. Spectral linewidth characteristics for distributed-feedback laser diodes are discussed, based on experimentally observed discrepancies from the theory and improvements obtained with quantum-well active regions. Frequency-tunable mechanisms are reviewed mainly for monolithic tunable laser diodes with distributed-Bragg-reflector (DBR) or distributed-feedback configurations. >

The Kuramoto model: A simple paradigm for synchronization phenomena

Abstract: Synchronization phenomena in large populations of interacting elements are the subject of intense research efforts in physical, biological, chemical, and social systems. A successful approach to the problem of synchronization consists of modeling each member of the population as a phase oscillator. In this review, synchronization is analyzed in one of the most representative models of coupled phase oscillators, the Kuramoto model. A rigorous mathematical treatment, specific numerical methods, and many variations and extensions of the original model that have appeared in the last few years are presented. Relevant applications of the model in different contexts are also included.

Chaos-based communications at high bit rates using commercial fibre-optic links

Abstract: Chaos is good, if you are looking to send encrypted information across a broadband optical network. The idea that the transmission of light-based signals embedded in chaos can provide privacy in data transmission has been demonstrated over short distances in the laboratory. Now it has been shown to work for real, across a commercial fibre-optic channel in the metropolitan area network of Athens, Greece. The results show that the technology is robust to perturbations and channel disturbances unavoidable under real-world conditions. Chaotic signals have been proposed as broadband information carriers with the potential of providing a high level of robustness and privacy in data transmission1,2. Laboratory demonstrations of chaos-based optical communications have already shown the potential of this technology3,4,5, but a field experiment using commercial optical networks has not been undertaken so far. Here we demonstrate high-speed long-distance communication based on chaos synchronization over a commercial fibre-optic channel. An optical carrier wave generated by a chaotic laser is used to encode a message for transmission over 120 km of optical fibre in the metropolitan area network of Athens, Greece. The message is decoded using an appropriate second laser which, by synchronizing with the chaotic carrier, allows for the separation of the carrier and the message. Transmission rates in the gigabit per second range are achieved, with corresponding bit-error rates below 10-7. The system uses matched pairs of semiconductor lasers as chaotic emitters and receivers, and off-the-shelf fibre-optic telecommunication components. Our results show that information can be transmitted at high bit rates using deterministic chaos in a manner that is robust to perturbations and channel disturbances unavoidable under real-world conditions.

Imaging with terahertz radiation

Abstract: Within the last several years, the field of terahertz science and technology has changed dramatically. Many new advances in the technology for generation, manipulation, and detection of terahertz radiation have revolutionized the field. Much of this interest has been inspired by the promise of valuable new applications for terahertz imaging and sensing. Among a long list of proposed uses, one finds compelling needs such as security screening and quality control, as well as whimsical notions such as counting the almonds in a bar of chocolate. This list has grown in parallel with the development of new technologies and new paradigms for imaging and sensing. Many of these proposed applications exploit the unique capabilities of terahertz radiation to penetrate common packaging materials and provide spectroscopic information about the materials within. Several of the techniques used for terahertz imaging have been borrowed from other, more well established fields such as x-ray computed tomography and synthetic aperture radar. Others have been developed exclusively for the terahertz field, and have no analogies in other portions of the spectrum. This review provides a comprehensive description of the various techniques which have been employed for terahertz image formation, as well as discussing numerous examples which illustrate the many exciting potential uses for these emerging technologies.

Fast physical random bit generation with chaotic semiconductor lasers

Abstract: Random number generators in digital information systems make use of physical entropy sources such as electronic and photonic noise to add unpredictability to deterministically generated pseudo-random sequences1,2. However, there is a large gap between the generation rates achieved with existing physical sources and the high data rates of many computation and communication systems; this is a fundamental weakness of these systems. Here we show that good quality random bit sequences can be generated at very fast bit rates using physical chaos in semiconductor lasers. Streams of bits that pass standard statistical tests for randomness have been generated at rates of up to 1.7 Gbps by sampling the fluctuating optical output of two chaotic lasers. This rate is an order of magnitude faster than that of previously reported devices for physical random bit generators with verified randomness. This means that the performance of random number generators can be greatly improved by using chaotic laser devices as physical entropy sources. Random-number generators are important in digital information systems. However, the speed at which current sources operate is much slower than the typical data rates used in communication and computing. Chaos in semiconductor lasers might help to bridge the gap.

Regimes of feedback effects in 1.5-µm distributed feedback lasers

Abstract: We have measured the effects of feedback on the spectra of 1.5-μm DFB lasers from feedback power ratios as low as -80 dB up to -8 dB. Five distinct regimes of effects are observed with well defined transitions between them. The dependence of these effects on the distance to the reflection is also investigated.