KR20120122310A - Method For Manufacturing SWNT-Ceramics Composite, and Its Application to Optically Nonlinear Devices - Google Patents

Abstract

PURPOSE: A method for manufacturing a SWNT(single wall nanotube-ceramic composite) and an optical nonlinear element manufactured by the same are provided to obtain the uniformly dispersed SWNT by dispersing the SWNT to silica. CONSTITUTION: A mixture of a SWNT and silica powder is prepared using ball milling. The mixture of the SWNT and the silica powder are repetitively injected to an end surface of an optical fiber ferrule as powder using a nozzle. The mixture is uniformly injected to the end surface of the ferrule by reciprocating the optical fiber ferrule. The mixture is coated on the end surface of the ferrule as a predetermined thickness of a film form by controlling reciprocating speed and the number of repetitions.

Description

Method for manufacturing SWNT-Ceramics Composite, and Its Application to Optically Nonlinear Devices

The present invention relates to a nonlinear optical device manufactured by dispersing single-walled carbon nanotubes in a ceramic host and a pulse laser using the same.

The pulse laser refers to a laser capable of obtaining a pulsed output as compared to a continuous wave laser.

Among pulsed lasers, the development of ultrafast pulsed lasers has contributed greatly to many fields such as large-capacity optical signal processing, ultrafast sensing, microfabrication, and non-destructive inspection.

In order to generate pulse trains with short pulse width and high repetition rate in pulsed lasers, it is impossible to insert a mechanical switch that is on / off in the laser resonator and to achieve this mode-locking technique. This was developed.

 Mode locking techniques are divided into active mode locking and passive mode locking.

The active mode locking method includes a forced mode locking in which a modulator is placed in the laser to modulate at the same frequency as an external modulated signal, and a self-mode locking caused by a nonlinearity of the laser medium. In the passive mode locking method, a saturable absorber Absorber, SA) is generally used.

In passive mode locking, the phases of different modes with different periods coincide (i.e. lock) simultaneously with each other, causing constructive interference in certain areas, creating large pulses, and after some progress the phases are off. Therefore, the pulse is suddenly weakened due to the offset interference, and after a certain distance, the phases of the various modes are simultaneously matched, and thus the pulse is generated again.

One of the typical methods of the passive mode locking method is a method using a saturable absorber (SA). The saturable absorber serves to match the starting points of the various modes. This is a method of generating ultrashort pulses as the phases start to fit together.

As described above, the mode locking technology has been developed into an active mode locking method and a passive mode locking method, respectively. Recently, as the passive mode locking technology using semiconductors and nanomaterials has been developed, the small size, superior optical nonlinearity, and efficient economic efficiency, etc. It is attracting much attention based on its advantages.

On the other hand, the method using the semiconductor was a method using a semiconductor thin film (Semiconductor Saturable Absorber mirror; SESAM), but a complex and inexpensive semiconductor process must be used, in addition to the thermally weak and many problems in compatibility with the optical fiber Will have

Therefore, the introduction of new nanomaterials in the development of pulse lasers, attempts to increase the effect and the degree of integration of the pulse has been made through carbon nanotubes.

Specifically, studies on inducing mode locking in a laser resonator and passively forming high-quality pulses using optical nonlinear saturation absorption characteristics of carbon nanomaterials have been made rapidly.

In particular, many studies have been made on laser pulse generators based on single-walled carbon nanotubes (SWNTs). Due to the remarkable optical nonlinearity of saturable absorption of carbon nanostructures in one dimension, high quality laser pulses can be generated simply by inserting materials into the laser cavity without fabricating a separate optical device. Could form.

This SWNT has the following advantages over other devices. Examples include (i) small size in nano units, (ii) ultra fast recovery time of 500 fs or less, (iii) simple sample fabrication process, and the like.

However, these SWNTs also have some problems, such as: (i) high specific surface area of SWNTs, which coagulate well with each other, leading to a decrease in nonlinearity, and (ii) weak heat due to interaction with the laser. There is a limitation in the application, and (iii) its own mechanical strength is weak, so durability and stability are inferior.

In order to make up for this drawback, methods for dispersing SWNTs in various host materials have been studied.

Dispersion of SWNTs is an essential condition for high optical nonlinearity in that individual shapes of nanostructures are not deformed by cohesion with neighboring nanostructures, and the higher the dispersion degree, the higher the desired optical nonlinearity. Will be higher.

Basically, it is most effective to disperse the SWNT by dispersing it in glass, which is the same material as the optical fiber, and to improve compatibility with the optical fiber.However, the high temperature process is inevitable for the dispersion of nanomaterials in the glass. Has become a problem of burning.

Therefore, the method of dispersing SWNT in polymer material and the method of dispersing in soft glass have been studied in a circumferential way, but the protection of SWNT from thermal damage during high power pulse laser operation and the compatibility of silica optical fiber The problem remains in this respect.

On the other hand, the method of dispersing SWNT in the silica host at room temperature has been studied through the sol-gel method, but in this method, the solubility of the limited nanotubes generated in the liquid phase process, long process time, etc. It still remains a problem.

Accordingly, the present invention aims to solve the above problems. Specifically, an object of the present invention is to be able to manufacture a nonlinear optical device by dispersing SWNT in a ceramic host, in particular glass at room temperature, and to be able to produce a pulse laser using the same.

In order to achieve the above object, the present invention by dispersing the SWNT in silica having a number of advantages over the polymer or soft glass which is the host material studied previously, there is no thermal damage of the SWNT at room temperature, as well as by a dry method The device is fabricated at high process speeds without any solubility limitations, and in the process maintains high quality optical nonlinearity of SWNTs.

Specifically, in order to achieve the above object, an embodiment of the present invention provides a method for producing a saturated absorber, which is the core of the passive pulsed laser using the SWNT-silica composite through the aerosol deposition (AD) method to provide. The method for producing a saturated absorbent may include preparing a mixture of silica powder and SWNT using ball milling; Repeatedly spraying the mixture of the silica powder and the SWNT on the end face of the optical fiber ferrule through the nozzle as the exit powder; Reciprocating the optical fiber ferrule so that it is evenly sprayed on the end face of the ferrule; Adjusting the number of repetitions and the speed of the reciprocating motion so that the mixture is coated in the form of a film of a constant thickness on the end surface of the ferrule. The thickness of the film can be controlled by the density of the mixture and the speed of movement of the substrate.

The spraying may be performed at room temperature, and may be coated in the form of a film on the end surface of the optical fiber ferrule by the kinetic energy in which the mixture is sprayed at the room temperature.

On the other hand, in order to achieve the above object, an embodiment of the present invention provides a pulse laser using a SWNT-silica composite. In the resonator of the pulsed laser, a polarization controller for controlling the polarization of the laser proceeds; An isolator that shows the direction of the laser propagation, an erbium-doped fiber amplifier (EDFA), a medium that provides optical gain in the resonator, and an output stage is formed by dividing a certain output of the laser. And a single mode fiber (SMF) for connecting the devices and for controlling chromatic dispersion. Here, the ferrule end surface of the optical fiber is coated with a composite film of silica and SWNT, and the composite film acts as a saturated absorber, which is an optical nonlinear element, inducing a mode lock of the laser and thereby outputting a pulsed laser.

The interaction may be using a direct penetration scheme in which the non-linear effects of the composite film depend on the peak power of the laser while the laser penetrates the layer of the composite film. Alternatively, the interaction is carried out by the majority of the laser into the optical fiber core avoiding the transmission of the composite film layer, and the remaining portion of the laser is to interact with the composite film in the form of an evanescent field by saturation absorption It may be to use the way to generate the same pulse.

The composite film may be coated on the end surface of the ferrule through an aerosol deposition process.

 The present invention disperses SWNTs in silica having many advantages over polymers or soft glasses, which have been studied in the past, not only to prevent thermal damage of SWNTs, but also based on a dry method, without limiting solubility, The device can be manufactured by the process speed.

In addition, the present invention by dispersing the SWNT in the silica, it is possible to obtain a uniformly dispersed SWNT composite, thereby operating more stably, to prevent the damage of the SWNT due to rapid heat discharge to produce a high-power optical fiber laser Make it possible.

1 is a schematic diagram of an ultrafast laser pulse generated by SWNT dispersed in silica.
Figure 2 shows a photograph and an electron micrograph of the SWNT-silica composite formed on the ends of the optical fiber ferrule.
Figure 3 shows the results of the Raman spectrum analysis of the SWNT-silica complex membrane and the evaluation of the permeation characteristics of the complex.
4 shows an example in which an optical fiber ring laser is constructed using a composite film.
FIG. 5 shows optical spectra measured with an optical spectrum analyzer (OSA) for the optical fiber ring laser shown in FIG. 4.
FIG. 6 shows an oscilloscope measurement result for an optical fiber ring laser shown in FIG. 4 and an autocorrelator measurement result for knowing an actual pulse width.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings, wherein like reference numerals refer to like or similar elements throughout the several views, and redundant description thereof will be omitted. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail. It is to be noted that the accompanying drawings are only for the purpose of facilitating understanding of the present invention, and should not be construed as limiting the scope of the present invention with reference to the accompanying drawings.

Hereinafter, the concept of the present invention will be briefly described before explaining with reference to the drawings.

With very high optical nonlinearity in response to ultrafast photon operation, SWNTs can enable a variety of next generation nonlinear photonic devices and systems.

In particular, SWNT-based fiber-mode locked lasers have been actively studied and have yielded surprising results that can successfully form femto second laser pulses.

The pulsed methods demonstrated so far include: (i) the light traveling through the SWNT layers located in free space and the SWNTs interact directly, and (ii) the light traveling with the SWNT dispersed in the polymer host. This interaction includes (iii) non-blocking interactions between the evanescent field and SWNTs.

Very recently, it has been demonstrated that the scheme (iii) can overcome the low 'optical damage threshold' while maintaining the same pulse formation process by controlling the interaction with some of the light that proceeds. .

However, since nonlinear operation depends mainly on the peak power of light, for sophisticated applications of the same SWNT based nonlinear element, direct interaction is still considered very attractive.

Unlike SWNTs in free space for direct interaction, SWNTs in host materials have the important advantages of high stability, good heat dissipation, good formability on any substrate, and good compatibility with other devices. .

In particular, in view of compatibility with the optical fiber, a ceramic host material which is a material such as an optical fiber is very useful.

For example, to maximize the nonlinear effect, SWNTs may be doped in an optical fiber core to work with the peaks of light that the SWNTs travel through. In addition, not only harmful oxidation but also mechanical breakdown, the ceramic host can give SWNT the effect of a strong protective layer.

Unfortunately, so far, only some ways of bypassing have been studied. For example, at room temperature, since SWNTs do not dissolve into the ceramic host, only bypass methods using a host of polymer, soft glass, and sol-gel silica have been studied.

In addition, low dispersibility, low solubility of SWNTs in the host material, low reliability, limited heat dissipation of the polymer, and compatibility with soft glass and existing silica fibers remain challenges.

Accordingly, the present invention is characterized by using silica as a host material capable of dispersing SWNT at room temperature and using an aerosol deposition (AD) process.

Importantly, since the AD process uses a dried SWNT-SiO2 mixture as starting material, solubility and dispersion of SWNTs are not limited.

Accordingly, the present invention provides a method for physically accelerating silica powder and SWNT simultaneously at room temperature toward a substrate corresponding to the end face of an optical fiber ferrule to form a composite film with pure kinetic energy, thereby providing optical nonlinearity. Fabricate the device.

That is, the present invention provides an optical fiber pulse laser using a composite film of SWNT-silica at the end of an optical fiber ferrule by AD, which is a dry method at room temperature, and using this as a saturated absorber which is a nonlinear optical device.

More specifically, the present invention is directed to forming a composite film of SWNT-silica by using an AD method of mixing single-wall SWNTs with silica powder to accelerate these composite powders dry and impinge upon the substrate, wherein the formation of the film is an optical fiber ferrule. To be done at the end. This composite coated optical fiber ferrule exhibits optical nonlinear characteristics, and the optical fiber ferrule is used to construct a laser resonator to generate a pulsed laser.

1 is a schematic diagram of an ultrafast laser pulse generated by SWNT dispersed in silica.

In order to realize ultra-fast pulse formation by passive mode locking, a composite film of SiO2 and SWNT is coated on the ends of the fiber optic ferrule.

When a continuous wave (CW) is input as shown in FIG. 1 by the composite film coated on the end of the optical fiber ferrule, it may be output as an ultra-fast pulse.

The composite film of SiO 2 and SWNT can be produced by an AD process, and the nonlinearity of the SWNT in the composite film is maintained as it is.

In addition, in a high power laser operating environment, the high thermal durability of the hosted SWNTs was measured.

On the other hand, the operation principle of the AD is based on the impact rod solidification base for impacting the surface of the substrate with ultrafine ceramic particles.

Since the AD process is carried out at room temperature and in a dry environment, the ceramic film produced by the AD process not only provides mechanical toughness but also dissipates heat generated in the interaction between the laser and SWNT, in theory, 95% or more. Can be achieved, and as a result, good compactness is ensured.

In addition, since the AD process has a unique deposition mechanism that can be explained by the “solidificaiton” of ceramic particles having a size of less than 1 micron, the deposition rate can be higher than 1 mm / min.

Moreover, the AD process makes it possible to overcome the solubility limitations of SWNTs that have not been overcome in sol-gel processes and polymer hosting due to their performance in a dry environment.

The apparatus for the AD process largely comprises two vacuum chambers. Each of the chambers is for aerosol formation and deposition, each connected via a tube.

 The aerosol chamber has a gas system for transporting the powder and a vibration system for mixing the gas and the starting powder. The aerosol produced in the aerosol chamber is delivered to the dispersion chamber by the pressure difference between the two chambers. The dispersion chamber forms and shapes the film. The dispersion chamber includes a nozzle, an aerosol inlet system and a substrate support for supporting the optical fiber ferrule. The rotary vacuum pump coupled to the mechanical booster pump of the aerosol inlet system is adjusted to ensure a vacuum level of about 100-5,000 Pascals during dispersion. To prepare a device for satisfactorily incorporating SiO 2 and SWNTs by the AD process, avoiding organic dispersants directly correlated to the nonlinear properties of SWNTs, and using ball milling to achieve uniform dispersion Prepare a mixture of SiO2 powder. The nozzle disperses the starting powder on the end face of the optical fiber ferrule so that the nanotubes provide direct interaction with the continuous wave (CW). The distance between the nozzle and the substrate corresponding to the end face of the Guam fiber ferrule is adjusted to about 3 mm, and the substrate is reciprocated to ensure uniform dispersion of the entire target area. Dispersion thickness can be controlled based on aerosol density, reciprocation speed and number of iterations.

Pulse formation is due to the saturation absorption phenomenon due to nonlinearity of SWNTs preserved even in ceramics.

Figure 2 shows a photograph and an electron micrograph of the SWNT-silica composite coated on the ends of the optical fiber ferrule.

In the electron micrograph shown in Figure 2 can be seen the densified microstructure of the silica, it can be seen that the SWNT exists between the microstructures.

Figure 2 (a) shows the coating of the SWNT-SiO2 composite film dispersed on the end surface of the optical fiber ferrule. Dispersion rate was 2 mm / sec and repeated 10 times.

As shown in the scanning electron microscope (SEM) image as shown in FIG. 2 (b), it is confirmed that the high-density microstructure of the SWNT-SiO 2 composite film can be obtained through high deposition rate AD at room temperature. That is, as can be seen with reference to Figure 2 (b), it is possible to check the SWNT surrounded by SiO2 in the composite film. Although the high impact energy of the starting powder used a dense structure that crushed SiO 2 particles, the form of SWNTs was preserved in the film.

In order to confirm the presence of SWNT on the synthetic film, a Raman analysis and a transmission characteristic analysis were performed on the film dispersed on the glass substrate as shown in FIG. 3.

3 shows Raman spectral analysis and transmission characteristic analysis results for SWNT-silica composite membrane.

As can be seen with reference to Figure 3 (a), it can be seen the Raman spectrum analysis results of the composite film, it is confirmed that the shape of the SWNT is preserved in the host due to the confirmation of the radial breathing mode (RBM).

As can be seen in FIG. 3 (a), the RBM peak is an indicator peak corresponding to the coherent vibration of carbon atoms in the radial direction of the SWNTs. There is a part where the peak falls somewhat, but it can be ignored because SWNT in the host can maintain the morphological characteristics without significant damage.

3 (b) shows the transmission characteristic curves for the reference SWNT sprayed on the glass substrate and the SWNT-SiO 2 composite film dispersed on the glass substrate by the AD process.

In the SiO2-SWNT composite film, a basic S1 absorption peak caused by band-edge transition was found at the center wavelength of about 1250 nm. However, there was a slight shift in the slightly shorter wavelength direction of the SWNT hosted with silica, and the transmission level was significantly reduced as the wavelength shifted to shorter wavelengths.

It can be analyzed that the peak shift is caused by the compressive stress remaining in the SiO2 host, and the wavelength-dependent transmission level indicates that the surface of the carbon nanostructure is caused by the surrounding host material during the contamination and dynamic dispersion process. It may be caused by (point defect). As a result, it was confirmed that the nonlinear absorption level of the hosted SWNT can be maintained at about 5% in the region of 1550 nm.

Experimentally, since SWNTs hosted by SiO2 by the AD process exhibited nonlinear saturation absorption, it was demonstrated that passive mode locking can be used to form high quality pulses of fiber lasers. That is, saturation absorption was achieved by coating the SWNT-SiO 2 film on the optical fiber ferrule through the AD process.

Figure 4 shows an example of the optical fiber ring laser using a composite film produced according to the present invention.

As shown in FIG. 4, the resonator of the pulse laser includes: a polarization controller for controlling polarization of the laser that proceeds; An isolator that shows the direction of the laser progression, an erbium-doped fiber amplifier (EDFA), a medium that provides optical gain in the resonator, and an output stage by dividing a certain output of the laser to form an output stage. And a single mode fiber (SMF) for connecting the devices and for controlling chromatic dispersion. Here, the ferrule end surface of the optical fiber is coated with a composite film of silica and SWNT, and the composite film acts as a saturable absorber which is an optical nonlinear element, inducing mode-locking of the laser and thereby outputting a pulsed laser. do.

The composite film may be located at an end surface of the ferrule of the optical fiber and may interact with a laser guided by the optical fiber.

In this case, the interaction may use a direct penetration scheme in which the non-linear effect of the composite film depends on the peak power of the laser while the laser penetrates the layer of the composite film. . Alternatively, the interaction is carried out by the majority of the laser into the optical fiber core avoiding the transmission of the composite film layer, and the remaining portion of the laser is to interact with the composite film in the form of an evanescent field by saturation absorption It may be to use the way to generate the same pulse.

As a result, femtosecond pulses are implemented from a single resonator by a saturated absorber based on SWNTs dispersed in silica, and this stable dispersion of the SWNTs in silica is made possible by the AD process.

When the input current at the EDFA reaches 150 mA and the polarization of the advancing light is adjusted, the mode lock begins with an output power of 3.02 dBm. Once the mode lock has been achieved, the mode lock initiates itself each time the laser is turned off and on repeatedly.

5 shows the optical spectrum measured by an optical spectrum analyzer (OSA) with a resolution level on the order of 0.05 nm.

The center wavelength and full width at half maximum (FWHM) are on the order of 1599.2 nm and 3.31 nm, respectively. The extinction ratio read in the OSA spectrum is over 30 dB as can be seen in FIG. 5. Some of the continuous wave components shown at the top of the spectrum can be controlled according to all resonator internal conditions as well as power levels, polarization states, and dispersion levels.

FIG. 6 shows oscilloscope and autocorrelator measurement results for the optical fiber ring laser shown in FIG. 4.

As can be seen with reference to FIG. 6. The width of each pulse measured by the autocorrelator was measured to confirm that the width was 890 femtoseconds.

Fig. 6 (a) shows the result measured by the 10 Gbit / s light sensor for the output pulse train formed by the SiO2 hosted SWNT. After limiting the harmonic mode lockout, we achieved a repetition rate of 9.52 MHz or more, corresponding to a cavity length of 21.0 m, and showed that the pulses formed operate in the basic mode.

The transient pulse persistence of the individual pulses was measured by the autocorrelator shown in FIG. 5 (b). By inferring the data as sech curves, we found that the measured FWHM is 890 fs. As the time-frequency product was 0.363, it proved that the generated pulses were of high quality, approaching the transform-limited value of 0.315.

Considering the pulse factors, the pulse energy and peak power were calculated assuming sech pulse formation, yielding 0.2 nJ and 0.21 ㎾.

The SWNT-SiO2 sample of the present invention proved to withstand operating power levels in the resonator of 20.8 dBm. Unlike SWNTs, which are thermally weak in free space, beyond the optical power level of 10 dBm (for direct interaction) in a typical single fiber, the sample still demonstrates the important role of ceramic hosting, and still remains after high power experience. Nonlinearity was shown.

As a result, the fabrication of silica-SWNT composites based on the AD process was realized under the dry-drying process conditions, aiming for effective direct interaction between SWNT and laser light. Experiments have demonstrated that the composite film deposition process, which is very effective, can overcome the limitations of SWNT solubility as well as thermal damage issues.

By demonstrating mode-locked optical fiber pulsed laser by SWNT-SiO2 composite film as saturable absorber, it has been verified that nonlinearity of SWNT in the host is preserved.

In the above described exemplary embodiments of the present invention by way of example, but the scope of the present invention is not limited only to these specific embodiments, the present invention is in various forms within the scope of the spirit and claims of the present invention Can be modified, changed, or improved.

Claims (10)

In preparing a pulsed laser using single-walled carbon nanotubes (SWNT), a saturated absorber is produced in the form of a SWNT-silica composite by aerosol deposition (AD) method,
Preparing a mixture of silica powder and SWNTs using ball milling;
Repeatedly spraying the mixture of the silica powder and the SWNT on the end face of the optical fiber ferrule through the nozzle as the exit powder;
Reciprocating the optical fiber ferrule so that it is evenly sprayed on the end face of the ferrule;
And adjusting the number of repetitions and the speed of the reciprocating motion so that the mixture is coated on the end face of the ferrule in the form of a film of a constant thickness. The method of claim 1, wherein the thickness of the film
Pulse laser manufacturing method using a saturated absorber, characterized in that controlled by the density of the mixture. The method of claim 1,
The spraying is carried out at room temperature,
The method of manufacturing a pulse laser using a saturated absorber, characterized in that the coating is coated in the form of a film on the surface of the optical fiber by the kinetic energy that the mixture is injected at room temperature. The method of claim 1,
The spraying proceeds in a dry manner without the need for a solvent, and thus a method for producing a saturated absorbent body, characterized in that it is possible to add other components such as ceramics, metals and polymers. As a pulse laser using carbon nanotubes,
An optical nonlinear saturable absorber coated with a composite film of silica and SWNT on the ferrule end surface of the optical fiber and inducing mode-locking of the laser by the composite film to output a pulsed laser;
In the resonator of the pulse laser, an Erbium-doped fiber amplifier (EDFA) doped with an erbium, a medium that provides an optical gain in the resonator;
A polarization controller for controlling the polarization of the laser to proceed;
An isolator presenting the direction of laser progression;
An optical coupler for dividing a certain output of the laser to form an output stage,
And a single mode fiber (SMF) for connecting the devices and for controlling chromatic dispersion. The method of claim 5, wherein the composite film is located on the end surface of the ferrule of the optical fiber, and interacts with the laser guided by the optical fiber,
The interaction is:
A direct penetration scheme in which the nonlinear effect of the composite film depends on the peak power of the laser while the laser penetrates the layer of the composite film, and most of the laser is the composite film layer Pulsed, characterized in that it proceeds into the optical fiber core avoiding transmission, and the remaining portion of the laser interacts with the composite film in the form of an evanescent field to produce the same pulse by saturation absorption. laser. The method of claim 5, wherein the laser is
Characterized in that the device is used to extend the electromagnetic field of the laser to be coated on the end surface of the ferrule, deposited on a transparent substrate and positioned in the laser path, or to act as part of the laser (missing field) through the AD process. Pulse laser. The method of claim 5, wherein the composite film
Pulsed laser characterized in that the AD process allows customized control of the SWNT concentration without limiting the solubility of the SWNT in the silica host. The method of claim 5, wherein the composite film
Pulsed laser, characterized in that the SWNT is not damaged by the high power laser operation of 10 dBm or more in the laser resonator. The method of claim 5, wherein the laser is
A pulse laser, characterized in that the repetition rate of the laser pulse is 100 kHz to 40 GHz, the pulse width is 10 ps to 10 fs, and the operating wavelength band operates at all wavelengths at which the SWNT is operated.

Images