JP2018142001A - Saturable absorbing element, and laser apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a saturable absorbing element having a wide absorption band, a high absorbance and a high modulation depth.SOLUTION: A method for producing a saturable absorbing element includes: a step of forming a carbon nanowall 12 on a substrate 10; a step of embedding the substrate 10 on which the carbon nanowall 12 are formed with resin 14; a step of releasing the substrate 10 from the resin 14 with which the substrate 10 is embedded after embedding the carbon nanowall 12 on the substrate; and a step of removing graphite 11 on bottoms of the carbon nanowall formed when forming the carbon nanowall on the substrate 10.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a saturable absorbing element and a laser device used in the optical field.

  Saturable absorbers are widely used in the optical field. The saturable absorber is used in a laser oscillator that generates short pulse light such as a passive Q-switched laser or a passive mode-locked laser. Conventionally, a saturable absorber using a semiconductor or a saturable absorber using an optical nonlinear effect has been used for such an oscillator.

  A saturable absorber using a semiconductor has a slow response speed of about picoseconds, and it is difficult to generate a short pulse with a pulse width of less than 100 femtoseconds. In addition, a saturable absorption element using a semiconductor is a complicated structure that creates a quantum well structure for artificially generating an absorption band in accordance with the oscillation wavelength of a laser device, and thus can be easily manufactured. Can not.

  A saturable absorber using a nonlinear effect has a faster response speed than a saturable absorber using a semiconductor, and can generate a short pulse. On the other hand, saturable absorbers using nonlinear effects are susceptible to environmental influences such as temperature and vibration, and may not be able to self-start depending on the environment.

  On the other hand, in recent years, carbon-based materials such as carbon nanotubes and graphene have attracted attention as new materials for saturable absorber elements (see, for example, Patent Document 1, Non-Patent Documents 1 and 2).

  Non-Patent Document 1 describes the absorbance (absorption characteristics) with respect to the wavelength of light of a saturable absorber using carbon nanotubes with reference to FIG. A saturable absorption element using carbon nanotubes has an absorption characteristic due to a band structure determined by the diameter of the carbon nanotubes, and thus has a feature that the wavelength of light to be absorbed is limited.

  On the other hand, the absorption characteristic of the saturable absorber using graphene has no problem depending on the wavelength. However, there is a problem that the saturable absorption element of graphene has low absorbance (for example, about 2.3% per layer). Therefore, it is also considered to use graphene in multiple layers. However, in Non-Patent Document 2, the modulation depth of the saturable absorbing element may decrease as the number of graphene layers increases using FIG. 3c). Explained.

JP 2007-94065 A

Norihiko Nishizawa, “Development and application of highly functional passively mode-locked ultrashort pulse fiber laser light source”, IEICE Journal, Vol94 (9), September 1, 2011, p.801-806 Qiaoliang Bao and 7 others, “Atomic-Layer Graphene as a Saturable Absorber for Ultrafast Pulsed Lasers”, Advanced Functional Materials Volume 19 Issue 19, October 9, 2009, p.3077-3083

  As described above, it is difficult to obtain a wide absorption band, high absorbance, and high modulation depth even in a saturable absorber using carbon nanotubes or graphene.

  In view of the above problems, an object of the present invention is to provide a saturable absorption element and a laser device having a wide absorption band, high absorbance, and high modulation depth.

  In order to achieve the above object, according to the first invention, the saturable absorbing element includes carbon nanowalls.

  According to the second invention, the carbon nanowall is doped with an impurity having a different number of valence electrons from carbon.

  According to the third invention, the carbon nanowall has a plurality of graphite pieces or graphene arranged in parallel in the same direction.

  According to a fourth invention, in the method for producing a saturable absorber, a step of forming a carbon nanowall on a substrate, a step of embedding the substrate on which the carbon nanowall has been formed with a resin or glass, After the carbon nanowall on the substrate is embedded, the step of peeling the substrate from the embedded resin or glass, and removing the graphite on the bottom formed when the carbon nanowall is formed on the substrate. And a process.

  According to the fifth invention, in the step of removing the graphite on the bottom surface, the graphite on the bottom surface is removed by polishing.

  According to the sixth invention, in the step of removing the graphite on the bottom surface, the graphite on the bottom surface is removed by ion etching or plasma etching.

  According to a seventh aspect of the present invention, the apparatus includes: a light source that outputs pumping light; an oscillator that includes an optical gain medium and a saturable absorbing element, and that oscillates laser light using light output from the pumping light; The saturable absorber element includes carbon nanowalls.

  According to the present invention, high absorbance and high modulation depth can be obtained in a wide absorption band.

It is a figure explaining the production | generation method of the saturable absorber which concerns on embodiment. It is a figure explaining the other example of the saturable absorption element which concerns on embodiment. It is a graph showing the wavelength dependence of the light absorbency of carbon nanowall. It is a figure explaining the laser apparatus which concerns on embodiment. It is a graph showing the experimental result of the laser apparatus which concerns on embodiment. It is a figure explaining the usage example of the saturable absorption element which concerns on a modification.

  Hereinafter, a saturable absorber, a method for generating a saturable absorber, and a laser device according to embodiments will be described. The saturable absorbing element is an element that absorbs light with low intensity and transmits light with high intensity. The saturable absorber according to the embodiment includes carbon nanowalls. Specifically, the saturable absorber according to the embodiment includes carbon nanowalls embedded in resin or glass. Here, a plurality of graphite pieces or graphene arranged perpendicular to one surface is defined as a carbon nanowall. Note that the graphite piece or graphene is perpendicular to one surface but need not be completely perpendicular. In the following description, a carbon nanowall having a graphite piece will be used. Further, the direction perpendicular to the one surface is defined as the height of the graphite piece and the carbon nanowall.

<Method for generating saturable absorber element>
A method for generating the saturable absorber according to the embodiment will be described with reference to FIG. In the first step, as shown in FIG. 1A, carbon nanowalls 12 are formed on the substrate 10 by sputtering, plasma CVD, or the like.

  Specifically, first, a thin graphite surface 11 is formed on the substrate 10. Thereafter, carbon nanowalls 12 having a plurality of perpendicular graphite pieces 12 a are formed on the graphite surface 11. At this time, the plurality of graphite pieces 12a are formed on the graphite surface 11 in a substantially uniformly dispersed state.

  In the second step, as shown in FIG. 1B, the carbon nanowalls 12 formed on the substrate 10 are embedded with a resin. Specifically, the gap between the graphite pieces 12a constituting the carbon nanowall 12 is filled with resin. Here, an example in which the polyimide precursor 13 is embedded as the resin will be described. However, the resin that embeds the carbon nanowall has transparency and heat resistance to laser light, and has a strength that is not destroyed during use. If it is a material, a kind will not be limited. Moreover, it is the same even if the carbon nanowall 12 is embedded with glass in addition to the resin. Note that the laser light here is not limited to visible light, but also includes invisible light.

  In the third step, as shown in FIG. 1C, the substrate 10 having the carbon nanowalls 12 embedded with the polyimide precursor 13 is solidified to form a polyimide 14.

  In the fourth step, as shown in FIG. 1 (d), the substrate 10 is peeled from the polyimide 14 embedding the carbon nanowalls 12. For example, the substrate 10 is peeled from the polyimide 14 by mechanical peeling.

  In the fifth step, the graphite surface 11 is removed from the polyimide 14 embedding the carbon nanowall 12 as shown in FIG. For example, the graphite surface 11 may be polished by argon ion etching or plasma etching, or the graphite surface 11 may be polished using a polishing apparatus or the like.

  In the sixth step, as shown in FIG. 1 (f), the polyimide 14 embedding the carbon nanowalls 12 is cut to an appropriate size to obtain the saturable absorber 1. Although the cutting method is not limited, for example, it is desirable that the size is 10 micrometers or more in which laser light can be used. In addition, the ratio of the magnitude | size of the board | substrate 10, the graphite piece 12a, and the saturable absorption element 1 which are shown in FIG. 1 differs from actual. Actually, the saturable absorber 1 that has been cut has carbon nanowalls 12 including a plurality of graphite pieces 12a.

  As described above, by forming the saturable absorber 1 using the carbon nanowall 12 formed on the substrate 10, the saturable absorber 1 in which the graphite pieces 12a are uniformly dispersed can be easily formed. it can. That is, in the case of a saturable absorption element using carbon nanotubes, it is difficult to uniformly disperse the carbon nanotube powder on the substrate when the carbon nanotubes are coated on the substrate. On the other hand, since the carbon nanowall 12 is formed by dispersing the graphite pieces 12a on the substrate 10, it is not necessary to use this as a powder to coat the substrate. Therefore, the saturable absorber 1 described above can be easily generated as compared with the saturable absorber using carbon nanotubes.

  The saturable absorbing element 1 has different absorbance depending on the height of the carbon nanowall 12. For example, when the height of the carbon nanowall 12 is increased, the absorbance of the saturable absorbing element to be generated is increased. Moreover, when the height of the carbon nanowall 12 is lowered, the absorbance of the saturable absorbing element to be produced is lowered. Therefore, when the saturable absorption element 1 is produced, the height of the carbon nanowall 12 can be adjusted to adjust the absorbance.

  Specifically, in order to optimize the absorbance, the height of the carbon nanowall 12 of the saturable absorber 1 is preferably about 1 μm.

  For example, when the carbon nanowall 12 is formed in the first step described above with reference to FIG. 1A, the thickness of the graphite piece 12a and the graphite surface 11 are adjusted by adjusting the formation time (for example, sputtering time). The height etc. from can be adjusted. Alternatively, for example, when the graphite surface 11 is polished in the fifth step described above with reference to FIG. 1E, a part of the carbon nanowall 12 is also polished together with the graphite surface 11, thereby increasing the height of the carbon nanowall 12. Can be adjusted.

  Moreover, the carbon nanowall 12 of the saturable absorber 1 can be doped with impurities to adjust the modulation depth and response speed. At this time, a substance having a valence electron number different from that of carbon (valence electron number: 4) is used as an impurity doped into the carbon nanowall 12. For example, doping with boron (number of valence electrons: 3), nitrogen (number of valence electrons: 5), phosphorus (number of electrons: 5), or the like as an impurity can be considered. By doping a substance having a valence electron number different from that of carbon, the absorbance, modulation depth, response speed, and the like of the saturable absorber 1 can be adjusted.

  For example, the carbon nanowall 12 can be formed while doping impurities in the first step described above with reference to FIG. Alternatively, an impurity doping step is added between the first step of forming the carbon nanowall 12 described above with reference to FIG. 1 and the second step of embedding the carbon nanowall 12 with resin or glass. May be.

  Furthermore, as shown in FIG. 2, a plurality of graphite pieces 12 a may be arranged in parallel in the same direction, embedded in resin or glass, and cut to form the saturable absorber 1. In the saturable absorber 1 generated in this way, it is possible to adjust so that all the graphite pieces 12a are irradiated with light having the same polarization direction, and the saturable absorption performance can be improved. .

  The saturable absorber 1 according to the above-described embodiment can improve performance as compared with the saturable absorber formed of carbon nanotubes or graphene. Specifically, the carbon nanotube has absorbance due to the band structure and does not correspond to light in a wide range of wavelengths. On the other hand, as shown in FIG. 3, the saturable absorption element 1 has a substantially constant absorbance with respect to the wavelength, and can correspond to a wide range of wavelengths.

  In addition, graphene has low absorbance when used in one layer, and the modulation depth decreases as the number of layers increases. On the other hand, the saturable absorber 1 has a sufficient absorbance and a high modulation depth with respect to incident light.

  For example, the saturable absorption element 1 is used for generation of terahertz light, such as a non-thermal processing apparatus using an ultrashort pulse laser using the saturable absorption element 1, a processing apparatus for transparent materials, and a processing apparatus for difficult-to-process materials. Can do. In addition, for example, the saturable absorption element 1 can be used for large-capacity optical fiber communication using ultrashort pulses.

<Laser device>
Below, the laser apparatus which concerns on embodiment which has the saturable absorption element 1 mentioned above is demonstrated. As shown in FIG. 4A, the laser apparatus 2 according to the embodiment includes a light source 20 that outputs excitation light, and an oscillator 21 that oscillates laser light using light output from the excitation light. Yes. The oscillator 21 includes an optical gain medium 22 and a saturable absorber 1. This laser apparatus 2 is a laser apparatus that outputs pulsed light, and is an ultrashort pulse laser apparatus in which the pulse width of the output pulsed light is at a femtosecond or picosecond level.

  The optical gain medium 22 includes an optical fiber cable 221, an optical coupler 222 that couples the light of the optical fiber cable 221 and the light of the light source 20, an optical gain medium 223 that amplifies the light, and a collimator lens that adjusts the light to parallel light. 224, wave plates 225 a and 225 b that adjust the polarization of light, and a beam splitter 226 that outputs a part of the light as an ultrashort pulse and returns a part to the optical fiber cable 221. Here, the wave plate 225a is a quarter wave plate, and the wave plate 225b is a half wave plate.

  Further, the saturable absorbing element 1 is a saturable absorbing element having the carbon nanowall 12 described above, and is connected in the optical fiber cable 221 by a connector 23 as shown in FIG. In FIG. 4B, the saturable absorbing element 1 is not perpendicular to the optical fiber cable 221 but is inclined, but the angle of the saturable absorbing element 1 with respect to the optical fiber cable 221 is not limited. .

  The oscillator 21 transmits and absorbs light input from the optical fiber cable 221 by the saturable absorption element 1, thereby realizing mode locking and outputting an ultrashort pulse.

  FIG. 4A shows an example having one saturable absorber element 1, but a plurality of saturable absorber elements 1 connected via different connectors 23 may be provided in parallel. As a result, even when only one saturable absorbing element 1 cannot obtain sufficient absorbance, it is possible to adjust the absorbance and output an ultrashort pulse.

  Further, the laser device 2 described above with reference to FIG. 4 has been described by taking a fiber laser using the optical fiber cable 221 as an example. However, the use of the saturable absorbing element 1 is possible even in other laser devices such as a solid-state laser. Thus, mode-locked oscillation can be obtained.

<Experimental result>
FIG. 5 shows an experimental result of the laser apparatus 2 according to the embodiment.

  FIG. 5A shows a pulse train waveform obtained by measuring the output of the laser device 2 with an oscilloscope. As a result, it can be seen that the repetition frequency of the output of the laser device 2 is 45 MHz.

  FIG. 5B shows an autocorrelation waveform obtained by measuring the output of the laser device 2 with an autocorrelator and a waveform (sech fit) fitted to a hyperbolic secant function. As a result, it can be seen that the pulse width is 321 femtoseconds (fs). Therefore, it can be seen that the laser device 2 is an ultrashort pulse laser.

  Further, FIG. 5C is a spectrum obtained by analyzing the wavelength of the output of the laser device 2 with an optical spectrum analyzer. As a result, it can be seen that the spectral width of the output of the laser device 2 is 12 nm and the center wavelength is 1564 nm.

  Moreover, when the output of the laser apparatus 2 was measured with the power meter, the output was 8 mW. Thus, as the performance of the laser apparatus 2, an output of 8 mW, a repetition frequency of 45 MHz, a pulse width of 321 fs, a spectrum width of 12 nm, and a center wavelength of 1564 nm were obtained.

<Modification>
In the above-described embodiment, the saturable absorption element 1 in which the carbon nanowall 12 is embedded in resin or glass has been described. On the other hand, the carbon nanowall 12 included in the saturable absorbing element 1A according to the modification is not embedded in resin or glass.

  When the saturable absorber element 1A according to the modification is generated, the step of embedding the carbon nanowall 12 with resin or glass, that is, the second step and the third step described above with reference to FIG. 1 are not performed. . Therefore, the carbon nanowall 12 cannot be peeled from the substrate 10 in the fourth step. Therefore, when producing the saturable absorber 1A according to the modification, the carbon nanowall 12 is formed on the substrate 10A that does not need to be peeled off. Specifically, a quartz substrate that transmits light or a mirror that reflects light is used as the substrate 10A, and a carbon nanowall 12 is formed on the substrate 10A to generate the saturable absorber 1A.

  Moreover, since the saturable absorbing element 1A according to the modification is not embedded with resin or glass, the strength by contact is weak. Therefore, as described above with reference to FIG. 4, the optical fiber cable 221 cannot be contacted. Therefore, the saturable absorbing element 1A is used in a spatial optical system using a lens 25, for example, as shown in FIG.

  Specifically, FIG. 6A is an example in which a quartz substrate is used as the substrate 10A, and light is condensed and transmitted to the saturable absorber 1A by the lens 25. FIG. 6B is an example in which a mirror is used as the substrate 10 </ b> A, and light is collected and reflected by the lens 25 onto the saturable absorber 1 </ b> A. The saturable absorber element 1A used in this way does not need to be embedded in resin or glass.

  As mentioned above, although this invention was demonstrated in detail using embodiment and a modification, this invention is not limited to embodiment described in this specification. The scope of the present invention is determined by the description of the claims and the scope equivalent to the description of the claims.

DESCRIPTION OF SYMBOLS 1 Saturable absorption element 10 Board | substrate 11 Graphite surface 12 Carbon nanowall 12a Graphite piece 13 Polyimide precursor 14 Polyimide 2 Laser apparatus 20 Light source 21 Oscillator 22 Optical gain medium 221 Optical fiber cable 222 Optical coupler 223 Optical gain medium 224 Collimator lens 225a, 225b Polarizing plate 226 Beam splitter 23 Connector

Claims (3)

Carbon nanowalls,
Resin or glass embedding the carbon nanowall;
A saturable absorption element comprising:   The saturable absorber according to claim 1, wherein the carbon nanowall is doped with a substance having a different number of valence electrons from carbon. A light source that outputs excitation light;
An optical gain medium and a saturable absorber, and an oscillator that oscillates laser light using light output from the pumping light,
The saturable absorber element is
Carbon nanowalls,
Resin or glass embedding the carbon nanowall;
A laser device comprising:

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