Optical: systems and elements – Optical modulator – Light wave directional modulation
Reexamination Certificate
2003-07-18
2004-10-19
Dang, Hung Xuan (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave directional modulation
C359S299000, C385S016000
Reexamination Certificate
active
06806996
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART STATEMENT
The present invention relates in general to an optical switching system which is capable of distributing and controlling a signal light composed of light pulses in a communication wavelength band with a pulse-like control light. In particular, the invention relates to an optical switching system utilizing characteristics of an absorption change of a light which a carbon nanotube has.
A carbon nanotube (CNT) is a tubular carbon crystal into which a graphene sheet is rounded, and may take any state from metal to semiconductor depending on a rounding state of the graphene sheet. The carbon nanotubes are classified into a single wall carbon nanotube (SWNT) composed of a graphene sheet of a single layer, and a multi-wall carbon nanotube (MWNT) in which graphene sheets are made into a multi-layer structure. These carbon nanotubes are being studied as a material for a fuel cell, a hydrogen occulusion, a field emission source or the like in the various fields due to a unique property thereof (refer to an article of “The Foundation of Carbon Nanotube”, by Yahachi Saitoh and Shunji Bandoh, CORONA PUBLISHING CO., LTD., 1998, or the like). However, the study for carbon nanotubes till now mainly aims at the application to electric and electronic materials, and hence it may not safely be said that they have been sufficiently examined for optical application.
In case of the optical application, an access to a single carbon nanotube with a fine probe as in case of the application to electric and electronic devices is difficult to be made. Hence, an access to an aggregate of a carbon nanotube with a beam condensed so as to have a diameter of several hundreds nm to several tens &mgr;m is chiefly carried out. As the chief reasons that the examination for the optical application gets behind the examination for the application to the electric and electronic materials, it is conceivable that it is difficult to obtain highly pure carbon nanotube samples, especially, SWNT samples, in a scale required for the optical evaluation, and any of carbon nanotubes is difficult to be dissolved into solvent so that it is difficult to obtain an optically uniform films, and so forth.
With respect to the optical applications, the SWNT which has a single absorption band, and which is advantageous in comparison of the optical characteristics thereof with calculated values is chiefly being examined. In an example in which non-linear optical constants of the SWNT in a state of being dispersed in solution are evaluated in 1,064 nm, 532 nm and 820 nm as a non-resonance region, such large linearity as to expect the practicability has not yet been reported (refer to an article of X. Liu et al.: Applied Physics Letters, 74(1999), pp. 164 to 166; Z. Shi et al.: Chemical Communications, (2000), pp. 461 to 462).
On the other hand, the calculation result reveals that the SWNT has an absorption band in the range of 1.2 to 1.6 &mgr;m as a communication wavelength region depending on a diameter of a tube (refer to an article of H. Kataura et al.: Synth. Met., 103(1999). pp. 2555 to 2558). In addition, it is reported that a tube diameter can be controlled by a temperature in manufacturing the SWNT (refer to an article of O. Jost et al.: Applied Physics Letters, 75(1999), pp. 2217 to 2219). These results suggest the possibility that the resonance effect of the SWNT can be utilized in the communication wavelength region.
We, on the basis of the foregoing notion, made examination with respect to the non-linear optical effect under the condition of the resonance in the communication wavelength region of the SWNT. As a result, we found out that an SWNT thin film shows absorption saturation of high efficiency in that wavelength region, and by utilizing this phenomenon have completed optical elements, each using an SWNT thin film, each of which is operated in the communication wavelength region (refer to Japanese Patent Application No. 2001-320383).
The optical elements thus found out are an optical switch, a saturable absorption mirror, a waveform shaping unit and the like each of which utilizes high efficient absorption saturation in the communication wavelength region of the SWNT. Though these elements provide non-linear optical elements which are respectively low in cost and can be operated at high efficiency, the performance evaluation thereof is carried out only with respect to the static performance such as driving energy, and hence the dynamic performance such as time response characteristics for a control light has not yet been evaluated.
For constructing an optical switching system which can be operated in the communication wavelength region and at a very high speed, it is necessary to evaluate the dynamic performance, more specifically, the time response characteristics for a control light. That is to say, for constructing a very high speed optical switching system using the carbon nanotubes, it is required that a speed of recovery of an absorption change induced by application of the control light is sufficiently high (a time constant of recovery of absorption saturation is very small). Hence, in a stage in which there is found out the above-mentioned optical elements which are not yet evaluated with the dynamic performance thereof, the construction of an optical switching system which can be operated at a very high speed belongs to an unknown province.
The characteristics required for an optical switching system, in addition to high efficiency and high speed response as described above, are excellent processability, high productivity, a low cost, possibility of promotion of an increased area, a high S/N ratio and the like. Hence, there has been desired a material meeting these characteristics at a high grade.
SUMMARY OF THE INVENTION
In the light of the foregoing, the present invention has been made in order to solve the above-mentioned problems associated with the prior art, and provides an optical switching system which is capable of being operated at a very high speed in a communication wavelength region and of meeting the various requests at a high grade under a condition in which carbon nanotubes are submitted for the optical applications.
The present inventors made evaluation of the dynamic performance of a thin film made of carbon nanotubes (hereinafter referred to as “a carbon nanotube thin film” for short when applicable), more specifically, the time response characteristics of an absorption change of the carbon nanotube thin film in the communication wavelength region. As a result, we confirmed that the absorption change induced by application of a control light recovers at a very high speed within a period of time of 1 ps (ps=10
−12
sec). Then, we have completed an optical switching system which has a novel construction using a thin film made of carbon nanotubes, and which can be operated at a very high speed on the basis of the important knowledge concerning the function of the thin film made of the carbon nanotubes.
That is to say, according to the present invention, there is provided an optical switching system including: a light control portion to be irradiated with a signal light composed of a light pulse train; a control light applying unit that applies a pulse-like control light synchronized with the signal light to the light control portion to selectively transmit a light pulse train within the signal light to form an output signal light; a clock extracting unit that synchronizes the control light applied by the control light applying unit with the signal light; and a signal detecting unit that receives the output signal light, in which the light control portion is composed of a thin film made of carbon nanotubes.
According to the present invention, the light control portion is composed of a thin film made of carbon nanotubes and hence an absorption change induced by application of the control light recovers at a very high speed. Thus, a pulse-like light is used for each of the signal light and the control light, whereby a signal light pulse t
Furuki Makoto
Iwasa Izumi
Pu Lyong Sun
Sato Yasuhiro
Tatsuura Satoshi
Fuji 'Xerox Co., Ltd.
Martinez Joseph
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