JP2007094065A - Method for manufacturing swnt saturable absorption optical material, pulse laser apparatus and all-optical type optical switch device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To realize an optical saturable absorber excellent in low loss and optical damage resistance or the non-linear optical waveguide structure of the absorber and to realize a super-short optical pulse laser and a small-sized optical switch capable of performing stable pulse oscillation and low-cost optical communication. <P>SOLUTION: In the method for manufacturing an SWNT (Single Walled Carbon Nanotube) saturable absorption optical material, a chlorobenzene or tetrahydrofuran (THF) is used as a dispersion solvent. At first, ultrasonic processing is applied to an SWNT solution completely dissolved in the solvent or an SWNT solution to which surface processing is applied by radical addition. Then one of transparent polymers is added to the dispersion solvent and the dispersion solvent is surely agitated while applying ultrasonic waves so that the SWNT is uniformly dispersed into plastic. Finally, the dispersion solution is slowly and sufficiently dried in atmosphere heated to several tens °C. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

This patent relates to a method for producing an optical material for stably exhibiting the saturable absorption characteristic of light of SWNT, and a laser and an optical switch device using the optical material, and several tens to hundreds of femtoseconds (1) from the laser. Femtosecond = 1 × 10 ⁻¹⁵ seconds) is used to generate an ultrashort pulse. In addition, an inexpensive ultrafast optical-optical switch can be realized.

  As a method for generating an ultrashort optical pulse of several picoseconds or less, “mode synchronization” has been widely used in the past, and a method using a saturable absorber is a simple method for realizing this. It is used a lot. This method is called passive mode synchronization. However, there is only a compound semiconductor multiple quantum well system (Non-Patent Document 1) as a saturable absorber operating in the vicinity of 1.55 μm, which is a wavelength band of conventional optical communication. This saturable absorber is expensive to manufacture and has low optical damage resistance, which makes an ultrashort optical pulse laser for optical communication expensive and fragile. Therefore, it has been desired to develop a saturable absorber having a low production cost and excellent light damage resistance.

  Single walled carbon nanotubes (SWNT) are known to exhibit properties as metals or semiconductors depending on how the graphene sheet is wound. In particular, with regard to the optical properties of SWNT, which shows the properties of semiconductors, the saturable absorption effect of SWNT (the light absorptivity decreases as the light intensity increases) is reported in the near-infrared region, which is the wavelength band of optical communications. In addition, it has been confirmed that the recovery time of the saturable absorption is an ultra-high speed of 1 ps or less (Non-Patent Document 2). Passive mode-locked fiber lasers that generate ultrashort light pulses by utilizing the optical characteristics of these carbon nanotubes have already been reported (Non-Patent Document 3). In addition, all-optical switches, saturable absorption mirrors of passive mode-locked lasers, ultra-short pulse waveform shapers, etc. have been tried using the saturable absorption effect of SWNT in the near infrared wavelength region. SWNT has been developed with its manufacturing technology, and cost reduction, high quality, and mass production are being promoted. SWNT is a very powerful material as a next generation saturable absorber because of its high environmental resistance. .

JP2003-121892A “Femtosecond pulses from a continuously self-starting passively mode-locked Ti: sapphire laser,” Opt. Lett., Vol. 16, pp. 1022-1024 (1991). “Ultrafast optical switching properties of single-wall carbon nanotube polymer composites at 1.55μm”, Appl. Phys. Lett. Vol, 81, pp. 975-977 (2002). “Mode-locked Fiber Lasers based on a Saturable Absorber Incorporating Carbon Nanotubes”, OFC2003, Post-deadline Paper PD44, March 2003. “A Noise Suppressing Saturable Absorber at 1550nm Based on Carbon Nanotube Technology”, OFC2003, paper FL2, March 2003. “Carbon nanotube-polymer composite saturable absorber for ASE noise suppression in optical amplifiers”, ECOC2003, Post-deadline Paper Th4.2.5, Sep. 2003.

  However, the problem here is that SWNTs tend to aggregate due to the strong intermolecular attractive force between individual SWNTs to form a bundle. For this reason, light loss called Rayleigh scattering having a scattering coefficient inversely proportional to the fourth power of the wavelength is likely to occur. Currently, when SWNT is used as a saturable absorber, SWNT is dispersed in an organic solvent such as dimethylformamide or ethanol, and sprayed onto a glass substrate to form a thin film (Patent Document 1, Non-Patent Document 4). Mainstream. However, in this method, since individual SWNTs aggregate each other, Rayleigh scattering becomes remarkable. For application to a mode-locked laser, amplification is superior to loss due to scattering, so that laser oscillation is possible, but an unstable laser tends to be obtained. In addition, since SWNT is constantly exposed to air and moisture, there has been a problem that long-term deterioration of characteristics cannot be avoided.

  In order to solve such a Rayleigh scattering problem, it is very important that the SWNTs are uniformly dispersed in a transparent dispersive medium at a wavelength used for optical communication so that the SWNTs do not aggregate. As the dispersive medium, a polymer or a solvent can be considered. As an example, Sugawara et al. Achieved a reduction in scattering loss by using polyvinyl alcohol (PVA) for this polymer (Non-Patent Document 5). However, PVA can only produce a film film, and water is used as a solvent, so the absorption of light in the 1.55 μm band by the O—H group is observed. For this reason, it was not optimal as a polymer for dispersing SWNT.

  In view of the above problems, an object of the present invention is to provide a saturable absorbing plastic material using SWNT that is stable and easy to handle, and a laser and an optical switch using the same.

  In order to achieve this object, in the present invention, as a transparent polymer used in the field of optical communication, polyacrylate (PAc) such as PMMA, polystyrene (PS), polycarbonate (PC), polyester (PEs), polyurethane ( PUNT, polynorbornene (PNB), etc. are used to uniformly disperse SWNTs in these plastics, thereby realizing a SWNT saturable absorber material with little absorption or its waveguide structure. Further, these are inserted into the laser or used as an optical switch to control the optical pulse. For this purpose, SWNT surface treatment is important for improving compatibility between the polymer, the solvent and SWNT, and improving compatibility.

  Since the SWNT-containing plastic material obtained by the present invention does not aggregate SWNTs, an optically saturable absorber having excellent loss resistance and optical damage resistance or a nonlinear optical waveguide structure thereof can be realized. By inserting the saturable absorber into a laser, it is possible to realize an ultra-short optical pulse laser or a small optical switch for optical communication that is capable of stable pulse oscillation and low cost.

  In the production of the SWNT-containing plastic material of the present invention, chlorobenzene or tetrahydrofuran (THF) is used as a dispersion solvent. First, an ultrasonic treatment is applied to a SWNT solution that is well-familiar with the solvent or a SWNT solution that has been surface-treated by radical addition. Next, one of the above transparent polymers is added to the dispersion solution, and ultrasonic waves are applied and stirred well so that SWNTs are uniformly dispersed in the plastic. Finally, the dispersion is slowly dried sufficiently in an atmosphere heated to several tens of degrees C. Thereby, a SWNT saturable optical material can be produced.

PAc and PS such as PMMA are polymers that are often used in the field of optical communications, such as plastic optical fibers (POF) and optical waveguides. The refractive indexes of PMMA and PS are 1.48 to 1.50 and 1.59 to 1.60, respectively, and the thermal deformation temperatures when a bending load of 18.6 kg / cm ² is applied are 102 ° C. and 104 ° C., respectively. PAc is soluble in ketone / ester chlorinated solvents such as aqueous ethanol, acetone, chloroform, methyl ethyl ketone, chlorobenzene, and tetrahydrofuran. On the other hand, PS is soluble in aromatic and chlorinated solvents such as chloroform, methyl ethyl ketone, ethyl acetate, and tetrahydrofuran.

  The point of the present invention is that chlorobenzene or tetrahydrofuran (THF), which is a solvent for the host polymer and also serves as a dispersion solvent for SWNT, is used as the dispersion solvent. The linear absorption characteristics of these solvents are shown in FIG. The figure also shows the linear absorption of dimethylformamide (DMF) and purified water. The absorption characteristics of each solvent were measured in a 5 mm long optical cell surrounded by a 1 mm thick quartz glass. Looking at the transmittance at 1550 nm, which is the wavelength band of optical communication, THF is 91.78%, chlorobenzene is 99.62%, and DMF is 94.87%. Purified water absorbs OH groups, so the transmittance at 1550 nm is almost 0%. This shows that THF or chlorobenzene is promising for low loss.

  Compared with the SWNT saturable absorbing optical material (spray method, PVA method) that has been studied in the past, the spray method has a problem in the long-term reliability of SWNT because the SWNT thin film is easily peeled off and exposed to the outside air. In addition, SWNT is dispersed in PVA using water. However, as shown in FIG. 1, there is a disadvantage that optical loss due to OH groups is large and only a film film can be produced. On the other hand, SWNT-dispersed PAc, PS, PC, PEs, and PNB have excellent characteristics in any of heat resistance, SWNT dispersibility, low light loss in the 1.55 μm band, and long-term reliability of SWNT. Furthermore, not only a thin film but also a waveguide structure can be formed, and it is extremely effective as a saturable optical material including various optical applications such as optical switches and modulators.

Here, the reason why the polymer and SWNT are mixed uniformly will be described first by using PMMA as an example of PAc. The reason why SWNT is uniformly dispersed in PMMA is thought to be due to the fact that the terminal part of SWNT or the defective part of the six-membered ring attracts the carbonyl group of PMMA. Figure 2 shows how intermolecular forces act on SWNT and PMMA to understand this. For example, it is known that a carboxyl group is bonded to a terminal end of SWNT or a defective portion of a six-membered ring by immersing in a mixed acid of SWNT, 98% sulfuric acid and 70% nitric acid and sonicating. The structure of the carbonyl group of PMMA is almost the same except that the hydrogen atom of the carboxyl group is replaced with a methyl group (—CH ₃ ). For this reason, an intermolecular force acts between the carbonyl group of PMMA and SWNT, although it is weak, and it is considered that uniform dispersion of SWNT becomes possible. For the same reason, PC, PEs, PU, and PNB having a carbonyl group can also be used as host polymers.

  On the other hand, the intermolecular force works between SWNT and PS because the benzene ring of PS and the six-membered ring on the surface of SWNT attract each other by van der Waals force and are closely bonded to each other, just like the structure of graphite. It is thought that it is because. At this time, the benzene ring and the six-membered ring are overlapped in a slightly deviated form without overlapping completely. Fig. 3 (a) shows how the benzene ring of PS and the six-membered ring of SWNT are bonded by overlapping π electron orbitals. FIG. 3 (b) schematically shows a state in which the PS is in close contact with the spiral around the SWNT surface.

If the concentration of SWNTs dispersed in the polymer is too high or the molecular weight of the polymer is too low, SWNTs will aggregate during the drying process. Therefore, in order to hold SWNT in a sufficiently dispersed state in the host polymer and complete it as a SWNT-containing plastic,
(1) The mass ratio of polymer (plastic) to SWNT is sufficient,
(2) the molecular weight of the polymer molecule is sufficient,
The following two conditions are necessary. (1), a specific value of (2) may vary depending the purity and quality of the SWNT, taking the SWNT made by HiPCO (Hi gh- p ressure c arbon mon o xide) method as an example, It is appropriate that the mass ratio of plastic is about 1000 times or more (SWNT concentration is 1000 ppm or less) with respect to SWNT, and the molecular weight of plastic molecules is about 20,000 or more. However, if the molecular weight of the plastic is too high, the viscosity of the SWNT / polymer solution increases and the uniformity of the material and the surface accuracy / parallelism deteriorate when dried as a bulk material.

  Examples 1, 2 and 3 shown below are examples of methods for dispersing SWNTs in PMMA using tetrahydrofuran (THF) and chlorobenzene as solvents. Example 4 is an example of a method of dispersing SWNT in PS using chlorobenzene as a dispersion solvent. Conditions such as the volume of the dispersion solvent, the mass of SWNT and PMMA, and sonication described below are typical examples, and are not particularly limited as long as dispersibility is obtained, and the type of SWNT, solvent, and plastic to be used It can be determined as appropriate. Example 5 is a pulse laser device in which the SWNT saturable absorbing optical material obtained by any one of Examples 1 to 4 is inserted into a laser resonator and used as a saturable absorber. Example 6 is an optical switch using a SWNT saturable absorbing optical material.

A method for preparing SWNT-dispersed PMMA using THF as a solvent is as follows.
(1) Add 0.5 mg of SWNT (28 ppm) to 20 ml of THF, and perform ultrasonic treatment with an ultrasonic homogenizer for about 30 minutes. An absolute amount is not ask | required if a ratio is this grade.
(2) While adding ultrasonic waves to the dispersion solution of (1), add 2 g of PMMA little by little, and perform ultrasonic treatment for about 3 hours after charging. The amount of PMMA corresponds to (1). If the amount of (1) is large, the amount is increased accordingly.
(3) The dispersion solution of (2) is placed on a petri dish or a quartz substrate and allowed to air dry for about 2 days.

  Since THF is highly volatile, when PMMA dries rapidly, the surface is fine and bubbles are generated at the wavy edges. Care must be taken to prevent air from entering in order to produce a bulk of PMMA with a thickness of 1 mm or more.

Another method for preparing SWNT-dispersed PMMA using THF as a solvent is as follows.
(1) Add SWNT surface-treated with a silane treatment agent having a methacrylic group and radical polymerization initiator to methyl methacrylate (MMA), which is a monomer of PMMA, and perform ultrasonic treatment for about 30 minutes with an ultrasonic homogenizer.
(2) Seal the sonicated solution in an ampoule, deaerate, and heat polymerize. 2 g of SWNT-containing PMMA thus obtained is dissolved in 20 ml of THF and subjected to ultrasonic treatment for about 3 hours with an ultrasonic homogenizer.
(3) The dispersion solution of (2) is placed on a petri dish or a quartz substrate and allowed to air dry for about 2 days.

  Agglomeration is not observed even when 2 mg of SWNT is dispersed in 2.0 g of PMMA in the polymerized material, but since THF is highly volatile, the surface of PMMA is dried rapidly as in Example 1. Bubbles are generated finely at the wavy edge. Care must be taken to prevent air from entering in order to produce a bulk of PMMA with a thickness of 1 mm or more.

A method for producing SWNT-dispersed PMMA using chlorobenzene as a solvent is as follows.
(1) Add 1 mg (49 ppm) of SWNT to 18 ml of chlorobenzene, and mix SWNT and chlorobenzene using an agate mortar. An absolute amount is not ask | required if a ratio is this grade.
(2) The solution of (1) is sonicated with an ultrasonic homogenizer for about 30 minutes.
(3) While adding ultrasonic waves to the dispersion solution of (2), add 2 g of PMMA little by little, and perform ultrasonic treatment for about 1 hour after addition. The amount of PMMA corresponds to (1). If the amount of (1) is large, the amount is increased accordingly.
(4) The dispersion solution of (3) is placed in a fluororesin petri dish and dried for about 3 days in an atmosphere heated to about 40 ° C.

  Chlorobenzene has low volatility, and the dispersion solution dries slowly, so even if a solution with a depth of 1 cm or more is placed in a petri dish and dried, the inside does not become hollow as in the case of THF, and the thickness is 1 mm. The above-described flat SWNT-mixed PMMA can be produced. In addition, chlorobenzene shows better dispersibility in SWNT than THF, so even if 1 mg of SWNT is dispersed in 2.0 g of PMMA, no aggregates are observed, and uniformly dark SWNTs are observed. PMMA mixed uniformly can be produced. In the same manner as in Example 2, after SWNT surface-treated with a silane treatment agent was dispersed and bonded to MMA, the polymerized material could be dispersed in 2 mg of SWNT per 2.0 g of PMMA. Aggregates are not observed, and it is also possible to produce PMMA in which uniformly dark SWSW is mixed.

Polystyrene (PS) is also dispersed in good condition when the same chlorobenzene as in Example 3 is used as a dispersion solvent. The production method of SWNT-dispersed PS is as follows.
(1) Add 1 mg of SWNT to 18 ml of chlorobenzene and mix SWNT and chlorobenzene using an agate mortar. An absolute amount is not ask | required if a ratio is this grade.
(2) The solution of (1) is sonicated for 30 minutes with an ultrasonic homogenizer.
(3) While adding ultrasonic waves to the dispersion solution of (2), add 2 g of PS little by little, and perform ultrasonic treatment for 2 hours after charging. The amount of PS corresponds to (1). If the amount of (1) is large, the amount is increased accordingly.
(4) The dispersion solution of (3) is placed in a fluororesin petri dish and dried for 3 days in an atmosphere heated to about 40 ° C.
In this way, a PS with a thickness of 1 mm or more and SWNT mixed uniformly can be produced.

  The configuration of a passively mode-locked fiber laser using a SWNT-containing plastic as a saturable absorber is shown in FIG. This laser includes a gain medium 1, a polarization maintaining fiber 2, a polarization maintaining optical isolator 3, a CNT optical system 4, an optical filter 5, a polarization control element 6, and a polarization maintaining optical coupler 7. Furthermore, the CNT optical system 4 includes a collimator 41, a lens 42, a SWNT-containing plastic 43 whose both end surfaces are optically polished, and a Z-axis variable stage 44.

As the gain medium 1, an erbium-doped optical fiber or a semiconductor optical amplifier having a gain in the 1.55 μm band is used. In order to oscillate femtosecond pulses stably, the resonator is composed of the polarization maintaining fiber 2, and the polarization maintaining optical isolator 3 and the optical filter 5 are inserted. Since SWNT-containing plastic does not maintain polarization, the polarization control element 6 is inserted into the resonator. The optical pulse is output through a polarization maintaining optical coupler. In the CNT optical system 4, the optimum saturable absorption effect is produced by adjusting the incident light intensity to the SWNT by the Z-axis variable stage 44 in the range of several MW / cm ² to 1 GW / cm ² . In the conventionally reported spray method, PVA vinyl content method, or method of growing SWNTs on the optical connector end face, optical polishing of any thickness was impossible, but this plastic material is easily A significant difference is that high-precision optical polishing of thickness can be realized. As a result, the range of application to optical devices is overwhelmingly widened. Also, a mechanism (Z-axis variable stage 44) that can adjust the light-receiving lens 42 in the Z-axis direction is provided to minimize the connection loss of light when moving samples or inserting samples with different refractive indexes and thicknesses. Is desirable.

  By changing the dispersion concentration of SWNTs in the solvent from 100 to 10,000 ppm, the rate at which saturation occurs can be adjusted. If the dispersion state of SWNT is the same, the performance as a saturable absorber is determined by the product of the dispersion concentration and the thickness.

  For example, when SWNT is dispersed in PMMA at 1000 ppm and the thickness of SWNT-containing PMMA is 0.5 mm, the pulse oscillation characteristics are the same as when the concentration is 500 ppm and the thickness is 1 mm. However, if the PMMA containing SWNT with a concentration of 1000 ppm is made as thick as 1 mm, light cannot oscillate in the resonator because of the large absorption by SWNT. When the dispersion concentration is further increased, the limit of dispersibility of PMMA is exceeded, so SWNTs aggregate and Rayleigh scattering increases. Actually, when the oscillation characteristics of the laser are measured using this SWNT-containing PMMA as a saturable absorber, the state in which the light pulse oscillates can last only for 2 to 3 seconds and becomes very unstable. On the other hand, if the dispersion concentration is reduced to 200 ppm when the thickness is 1 mm, the absorption of SWNT is too small and the saturable absorption effect necessary for pulse oscillation cannot be obtained. As a result, although CW oscillation occurs, pulse oscillation does not occur. As a result, the optimum dispersion concentration and thickness conditions for pulse oscillation are given by the following equation (1).

  The output waveform of the optical pulse obtained in such a situation is shown in FIG. A stable pulse with a narrow pulse width of 375 fs is obtained.

6 (a) and 6 (b) show an optical switch using SWNT-containing plastic as a core material, and a waveguide and plastic fiber clad with a slightly low refractive index material to form an optical waveguide therearound. Respectively. These devices cannot be realized by conventional methods. Both switches are based on the relaxation of SWNT's fast saturable absorption. When a control light pulse λ _C synchronized with the signal light pulse λ _S is incident, λ _S cannot be transmitted due to absorption saturation when there is no control light. However, when λ _C is incident, λ _S is transmitted only when the light saturates at high speed and both overlap. λ _S and λ _C may have the same wavelength, but can be used as shown in FIG. 6B by setting different wavelengths. The wavelength selective filter is designed to transmit λ _S and reflect λ _C. Since these switches operate at high speeds even in the femtosecond region, they can be used as ultra-high-speed optical switches. In bulk materials, it is necessary to collect light with a lens having a short focal length in order to cause a nonlinear optical effect. However, the waveguide type optical device shown in FIG. A highly efficient optical switch can be realized.

  As explained in detail above, by uniformly dispersing SWNTs in a transparent plastic material with a wavelength of 1.55 μm, such as PAc, PS, PC, PEs, PU, and PNB, OH becomes a problem in the optical communication wavelength band. A low-loss saturable absorbing material that suppresses absorption and Rayleigh scattering by the base can be realized. In addition, the optical damage resistance can be improved by making the SWNT-containing plastic material into a waveguide structure. As a result, by using this material, it is possible to realize ultrashort optical pulse lasers for optical communications that have strong optical damage resistance and enable stable pulse oscillation, and ultrahigh-speed optical switches with high performance and functionality. It becomes. Unlike the previously reported SWNT saturable absorbing material, this plastic material is characterized in that high-precision optical polishing with an arbitrary thickness can be easily realized. As a result, this material can be widely applied to various optical devices.

Linear absorption characteristics of THF, chlorobenzene, DMF and purified water Intermolecular force acting on SWNT and PMMA Bonding of benzene ring of PS and 6-membered ring of SWNT due to overlap of π electron orbitals Configuration of passively mode-locked fiber laser using SWNT as saturable absorber Femtosecond optical pulse output from passively mode-locked laser Configuration of optical switch using SWNT-containing plastic as saturable absorbing optical waveguide and fiber. (a) Optical waveguide type, (b) Optical fiber type

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Gain medium 2 Polarization maintaining fiber 3 Polarization maintaining optical isolator 4 CNT optical system 5 Optical filter 6 Polarization control element 7 Polarization maintaining optical coupler 41 Collimator 42 Lens 43 SWNT containing plastic 44 Z axis variable stage

Claims (5)

  In a method for producing an optical material for stably expressing the saturable absorption characteristic of light possessed by a single-walled carbon nanotube (SWNT), the SWNT is made of transparent plastic by using chlorobenzene (CLB) or tetrahydrofuran (THF) as a solvent. A method for producing a SWNT saturable absorbing optical material, characterized by being uniformly dispersed in the material.   2. The method for producing a SWNT saturable absorbing optical material according to claim 1, wherein the ratio of saturable absorption is adjusted by setting the dispersion concentration of SWNT in chlorobenzene or tetrahydrofuran to 100 to 10,000 ppm.   In claim 1, ultrasonic waves are applied to the dispersion (solvent) so that SWNTs are uniformly mixed with the plastic, and the mixture is thoroughly stirred and then slowly dried sufficiently in an atmosphere heated to several tens of degrees C. SWNT saturable absorbing optical material manufacturing method.   The both ends of the SWNT saturable absorbing optical material obtained by claim 1 are optically polished, inserted into a laser resonator as an optical material, and used as a saturable absorber, thereby stably performing pulse oscillation. A pulse laser device characterized by that.   A saturable absorption optical waveguide is manufactured by using the SWNT-containing plastic of claim 1 as a core material and sandwiching the core with a clad material having a lower refractive index than that, and a signal light pulse and a synchronized control light pulse in the optical waveguide. All-optical type optical switch device that turns signal light pulses on and off at an ultra-high speed by injecting light.

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