Wakao Sasaki

An adaptive optical circuit with self-organizing link

We have realized optical self-organizing nodes in which nonlinear weighting is enabled by an opt-electronic hybrid circuit based on optical bistability attained simply by coupling a light emitting diode (LED) and a photo diode (PD), which have their own nonlinearity. By connecting each nodes, we have demonstrated a self-organizing link which can build up a signal propagation path autonomously by itself, applying our newly considered adaptive algorithm. This algorithm is based on the fact that the nonlinearity attained by our coupled LED and PD devices can be regarded to be available for a well established nonlinear function which appears in typical adaptive control theory. We have also demonstrated our first fabrication of self-organizing optical network system with seven adaptive nodes. In this scheme, we have intentionally generated a disconnection or a breaking of wire link between certain nodes in the network, and then we have also confirmed that the optimum alternative route can be made autonomously and the transmission recovered. As a consequence, the present optical self-organizing network system can be a step toward a new method for peer-to-peer communications in optical transmission network.

Chaos synchronization and encrypted communication systems as an application of VCSELs

In this work we have proposed chaotic synchronization system using two identical electro-optical nonlinear delayed feedback systems (NDFS) utilizing VCSEL. This first time proposal has high potential to perform more sensitive chaotic dynamics for improvement of encrypted communications quality. In this system we have demonstrated the reduction of robustness to prevent tapping by someone else attaining higher correlation but only if the feedback gain is the same value. We have also demonstrated the variations of correlation if feedback gain has the slight difference. Moreover, we have demonstrated applications to encrypted communications using VCSEL.

A new technique for speckle noise reduction of laser projection displays using waveplates

In laser projection displays, countermeasure technique against glares on the screen called speckle noise is one of the focuses. In the experiments, a quarter-waveplate with slow axis rotated by 45 degrees against the polarization axis of the incident green laser source was inserted into the optical path of our laser projection display system. Only in such a setup, an enough coherence alleviation was attained. We have also carried out quantitative evaluations of the speckle noise by overlaying the projected images using a CCD camera on the histogram-based estimations of speckle noise intensities. Noise reduction of about 20% was accomplished.

Linewidth control for optical heterodyne beat of 850-nm vertical cavity surface emitting lasers

We have achieved a simple and precise frequency stabilization technique for commercially available 1mW, 850nm vertical cavity surface emitting laser (VCSEL) based on optical heterodyne beat frequency control by introducing two sets of frequency stabilized VCSELs with Fabry-Perot resonator (FPR) as frequency discriminator. The stabilized VCSELs were quite similarly fabricated with each other, in which the electrical negative feedback was supplied for stabilization. We have also detected optical heterodyne beat between these two VCSELs by adjusting the locking frequency of each VCSEL. Thus, we have further reduced the frequency fluctuations from these stabilized VCSELs by controlling their feedback current so that the variation in the optical beat frequency should be minimized. As a result, we have successfully suppressed the amount of optical beat frequency fluctuations within 2MHz at measuring time of 1 sec. That is, the attained Allan variance is within the order of 10-9. In this work, we have achieved simple and inexpensive and precise frequency stabilization for 850nm VCSEL by optical heterodyne beat frequency control, which is quite applicable to nanomanufacturing.

Compact and inexpensive frequency stabilization technique for 850-nm vertical cavity surface emitting lasers based on Fabry-Perot resonator

We have demonstrated a compact and inexpensive frequency stabilization technique for commercially available 1mW, 850nm vertical cavity surface emitting laser (VCSEL) using a Fabry-Perrot cavity as frequency standard. Recently VCSEL has been widely prevailed for uses of low cost and small sized sensors, since it may afford low power operations and manufacturing costs in comparison with edge emitting type Fabry-Perrot laser diodes. Therefore, a highly versatile and inexpensive frequency stabilized coherent light source which can be mass producible will be available if the frequency stabilization for this type of VCSELs is carried out. Generally, it has been commonly accepted that a satisfactory degree of coherence may be easily obtained from VCSELs without any additional frequency stabilization technique since highly reflective coatings are to be put on their laser cavity edges. Nevertheless, some VCSEL devices, especially inexpensive type commercial products show multi-mode behaviors along with polarization instabilities. In the present work, as a simple and inexpensive approach to commercially available VCSEL devices, we have demonstrated a frequency stabilization scheme using a Fabry-Perrot cavity. The error signal was derived by phase sensitive detection for the transmitted light from the Fabry-Perrot resonator. Thus, the lasing frequency of the VCSEL was locked to the zero-crossing of the error signal by negative feedback for injection current via a PID controller. As a result, we have successfully suppressed the amount of frequency fluctuations in the free-running VCSEL of as much as 2GHz to be within 500MHz at measuring time of 30sec, that is, the attained Allan variance is 4.1×10E-8.

A compact and efficient hyper coherent light source of visible violet laser diode based on Pound-Drever-Hall technique

In the present work, we have developed an efficient and well stablized hyper coherent diode laser light source as compact as even portable using commercially available visible 400 nm band laser diodes. The attained coherence of the present system can always be controlled at the best condition indifferent to changes in its settled environmental conditions by applying Pound-Drever-Hall technique in which the frequency of a 160mW type 405nm GaN violet laser diode is locked to a reference Fabry-Perot cavity by negative electrical feedback for the injection current of the laser diode based on FM sideband technique. In addition to this frequency stabilization system, we have also realized a stability evaluation system that can measure the Allan variance of the frequency fluctuations of our frequency stabilized laser source in real-time basis by using simple devices of a portable computer and a digital signal processing unit. As a result, we have accomplished a compact and efficient hyper coherent laser system which can always perform its optimum conditions even if the environmental conditions around the laser are to be dynamically changed when used in a field basis. The attained values of power spectral density (PSD) of FM noise calculated from the error signals of our system under controlled condition were better by about 1~2 orders than typical values of free-running conditions in the fourier frequency domain from 100Hz to 300kHz. The best achieved value of PSD was about 2.56×107 [Hz2/Hz] in the fourier frequency domain from 100Hz to 1kHz, while as for the Allan variance as another measure of frequency stability, the achieved value of the minimum square root of Allan variance was 3.46×10-11 in a 400nm type violet laser diode at integration time of 10 ms, which has been well comparable to the hyper coherent condition for the laser diode light sources.

A new architecture of self-organizing network for optical peer-to-peer communications

We demonstrated our newly designed architecture of optical self-organizing network which enables concurrent peer-to-peer communications of more than two pairs in our previous model. We have realized this architecture by expanding the previous two dimensional lattice connection into three dimensional lattice connection. Two main features achieved by our present work are as follows. First, we have designed the tubular shaped architecture of equivalent peer-to-peer type network which enables two pairs to perform transmission from any open ports located at each end of the tubular connected nodes. Secondly, we have considered to layer the tubular shaped network structure described above in order to increase the limit of pairs by which concurrent communications are possible. In numerical simulation, we have confirmed the efficiency of our above mentioned two architectures of networks. The merit of these two networks is that they can be composed by our previously reported three-port node we have simply realized in optoelectronic hardware or by just adding the fourth port to it. As a consequence, we have proved that our present scheme of simple architectures of optical self-organizing network has potential utility value in actual use for peer-to-peer optical communications.

Controlling chaotic oscillation modes in visible semiconductor lasers by external light input and its applications to novel optical communications

We have demonstrated a simple method for controlling the nonlinear oscillations using only semiconductor lasers and photodetectors. An electro-optical NDFS (nonlinear delayed feedback system) has been composed by making use of this method. With this system, multi-stable oscillations and chaotic transitions with various patterns have been observed. In addition, some characteristic routing sequences form period-doubling bifurcation to chaos have been reproducibly observed when some external modulations with acoustic frequencies are applied to the NDFS. In this case, we have proved that the harmonic numbers of the multi- stable oscillation modes to appear as system output are to be controlled in terms of timing (phase) as well as frequencies of external input signals at the moment they are applied. And we have also confirmed such remarkable transition behaviors of the multi-stable oscillation modes by numerical analysis based on nonlinear delayed differential equations describing our NDFS. Consequently, we have demonstrated that the oscillation harmonic numbers can be directly chosen from the 1st order up to the 9th by setting the frequencies of the external input signals to be their characteristic values specifying the desired harmonic numbers to appear for output and also by setting the timing of input signals with respect to the phase of original waveform of the oscillating modes. Thus, our system is expected to be a potential for something novel, intelligent communication technique based on chaos.

Application of self-organized genetic algorithms to a novel color recognition system of optical neural network

We have demonstrated a color recognition system composed by a novel, self-organized optical neural network system that includes genetic algorithms along with back propagation schemes, which make it possible that the system avoids the local minimum problems and make the learning processes faster and better. Our system is composed of a color liquid crystal display panel (LCD), bistable semiconductor lasers, photo-diodes, and liquid crystal light projector (LCP). The LCD weighs the intensities of light that passes through it and works as synapses in the neural network. The optical bistable semiconductor lasers originate the optical sigmoid functions and serve as threshold processing units. Using these devices can simplify the configuration of the optical neural network system. The color of light emitted from the LCP will be recognized by the neural network system. A monochromatic light beam generated by the LCP is illuminated on all over the LCD surface displaying the colored boxes in the three primary components. Thus, the light beam is weighed when it passes through the boxes on the LCD. As a consequence, we have achieved a novel, simplified color recognition system using the genetic algorithms for self- organization of the optical neural network. The unique feature of this system is to make use of the genetic algorithms and the back propagation at the same time to derive selectively the merits from these two methods. By this system, more naturalized color recognition like human will be performed, being able to distinguish the colors under different conditions of environment, e.g. lightening conditions, surface conditions of colored material, etc.

Anomalous broadening spectra due to isotope effects in the high-power He-CdII white light lasers

It has been recognized that since the He-CdII lasers are usually operated under considerably high buffer-gas pressure, the oscillation modes in these lasers can automatically reduced by their homogeneous broadening mechanism due to the high buffer-gas pressure. Therefore, it has been expected that the oscillation scheme in these lasers can easily be single-moded when they are operated in high power. We have examined this fact with a most high-power (100mW) He-CdII white light laser which has just come to be commercially available in Japan. As a result, this is true only in the cases of the 537.8 nm and 636.Onm lines, but in the case of the 441.6nm lines an anomalous broadening of the spectral width, which amounts to as much as more than 2GHz has been measured. This degrades the coherence length of this blue line less than 15cm. Such a large broadening can be attributed to the isotopes involved in the natural cadmium metal used for the laser material.