JP5936110B2 - Laser light diffraction method and diffractive optical element device - Google Patents

Description

  The present invention relates to a laser beam diffraction method and a diffraction optical element device.

  Conventionally, in ultra-high-power laser generators that output ultra-short pulse lasers with high output, frequency of the light from the oscillator is chirped to reduce peak intensity in order to reduce unwanted nonlinear effects in the amplifier. So-called chirped pulse amplification is used.

In chirp pulse amplification, a low-energy optical pulse from an ultrashort pulse laser oscillator is converted into a linear positive frequency chirp whose frequency increases with time by a pulse expander composed of a diffraction grating pair or the like. This frequency chirped long pulse light is amplified to a saturation level by a broadband laser amplifier, and finally passed through a pulse compressor having a negative dispersion characteristic, that is, a diffraction grating pair etc. in which a low frequency component propagates more slowly. By superimposing all frequency components at the same time, an ultrashort pulse / high peak output laser beam is obtained.
In an ultrahigh power laser generator that obtains an ultrashort pulse laser with high output by chirped pulse amplification, pulse compression is performed using a post-amplification diffraction grating or the like. This diffraction grating has a lower optical damage threshold than a normal mirror, requires a large area, and limits the scale-down of the entire system. As the diffraction grating becomes larger, the manufacturing technique becomes difficult, and the cost becomes a big problem. In addition, a normal mirror can clean surface contamination and the like, but in the case of a diffraction grating, it is often difficult, and as a result of accumulation of surface contamination, damage may result.

  As a method of diffracting high-intensity pulsed laser light, a method using a transient diffraction grating composed of a periodic spatial density distribution of plasma generated using two-beam interference of laser light has been proposed (for example, a patent) Reference 1).

  In the method proposed in Patent Document 1, the periodic spatial density distribution of plasma generated by light beam interference by irradiating laser light so as to cross in gas, liquid or solid transparent medium is transient. In addition to this, a method has also been proposed in which a close-packed wave is generated by sound waves in a medium using Brillouin nonlinear scattering or the like (see, for example, Non-Patent Document 1).

JP 2008-52266 A

H. Yoshida, T. Hatae, H. Fujita, M. Nakatsuka, and S. Kitamura, A high-energy 160-ps pulse generation by stimulated Brillouin scattering from heavy fluorocarbon liquid at 1064 nm wavelength, Optics Express, Vol. 17, Issue 16, pp. 13654-13662 (2009)

  As described above, when a laser is incident on an existing solid-state diffraction grating, the incident intensity is limited due to optical damage. When an optical element such as a normal diffraction grating is used with a high-intensity laser, the usable laser power is determined by the optical damage intensity of the diffraction grating. In general, diffraction gratings have damage thresholds that are orders of magnitude weaker than other mirrors, and as a result, the entire laser system has become large and limited in intensity.

In addition, in the method proposed in Patent Document 1, in order to induce sufficient refractive index modulation in the gas, it is necessary to use a laser beam having a strong intensity of about 10 ¹⁶ W / cm ² or more. In this case, the intensity of the writing laser is about the same as or higher than that of the diffracting laser to be used, and it cannot be used as an effective element. In order to modulate the refractive index, it is necessary for the atoms and molecules to absorb the writing light, but at such high intensity, due to the influence of thermal expansion due to absorption, etc., the coarse and dense shape is maintained for only a short time of picoseconds or less. As a result, writing is possible only with ultra-short pulse light. In this case, for example, when laser compression is considered, writing light having an intensity equal to or higher than that of sub-picosecond compression is present, and diffracted light can be produced, but there is no substantial merit.

  Furthermore, as described in Non-Patent Document 1, in the method of generating a dense wave by sound waves in a medium using Brilliant nonlinear scattering or the like, excitation light and scattered light are generated with phase matching, so that a phase conjugate wave or the like is generated. Used, but cannot return any phase such as chirp amplification.

  Conventionally, in order to form a dense refractive index in a gas, it has been generally considered that plasma is formed. However, when it is turned into plasma, free electrons are emitted, and the refractive index of the medium changes greatly. This wavelength dependence is a gradual change with the frequency = 0 as the maximum, and light and diffraction for writing. It was difficult to produce a difference in absorptance with the light to be generated, and the intensity required to generate the refractive index modulation and the maximum diffraction intensity were almost the same, and an efficient optical element could not be obtained.

  Accordingly, an object of the present invention is to provide a laser light diffraction method and a diffractive optical element device capable of diffracting even high-intensity laser light in view of the above-described conventional situation.

  Other objects of the present invention and specific advantages obtained by the present invention will become more apparent from the embodiments described below with reference to the drawings.

  In the present invention, a gas that is resonantly absorbed by laser light is formed by discharge, and a periodic spatial refractive index distribution is generated thereon, which is used as a transient diffraction element, so that even high-intensity laser light can be diffracted. Make it possible.

That is, the present invention is a method for diffracting laser light, in which a sheet-like diffraction grating recording region is formed by a gas that absorbs light resonantly, and the excitation laser light having a wavelength in the absorption band of the gas is diffracted. A periodic spatial refractive index due to interference between the two excitation laser beams by photoexciting the gas by irradiating it in the grating recording region so as to be absorbed by the gas forming the diffraction grating recording region. A distribution is generated in the diffraction grating recording area, and the periodic spatial refractive index distribution in the diffraction grating recording area is used as a transient diffraction grating, which is determined by the optical damage intensity incident on the transient diffraction grating. The diffracted laser beam having a wavelength in the non-absorption band of the gas up to the intensity is diffracted.

  In the laser beam diffraction method according to the present invention, the gas that absorbs light in a resonant manner is, for example, a gas containing ozone, and is irradiated with excitation laser light in the vicinity of 250 nm that is the absorption peak wavelength of the gas. A periodic spatial refractive index distribution can be generated in the diffraction grating recording region formed of gas, and the periodic spatial refractive index distribution in the diffraction grating recording region can be used as a transient diffraction grating.

The present invention is a diffractive optical element device, comprising: a gas generating means for forming a diffraction grating recording region by a gas that absorbs light resonatively; and excitation laser light having a wavelength in the absorption band of the gas. The gas forming the diffraction grating recording region is irradiated so as to cross within the region and absorbed by the gas to photoexcite the gas, and the periodic spatial refractive index distribution due to the interference of the two excitation laser beams is obtained. An optical excitation means for generating in the diffraction grating recording region, and using the periodic spatial refractive index distribution in the diffraction grating recording region as a transient diffraction grating , determined by the optical damage intensity incident on the transient diffraction grating. The diffracted laser beam having a wavelength in the non-absorption band of the gas up to a given intensity is diffracted.

  In the diffractive optical element device according to the present invention, the gas generating means is, for example, a gas generating means for forming the diffraction grating recording region with a gas containing ozone flowing out through a slit-like opening, and the light excitation means Irradiates two excitation laser beams in the vicinity of 250 nm, which is the absorption peak wavelength of the gas, so as to intersect within the diffraction grating recording region, and absorbs the gas in the diffraction grating recording region. The gas can be photoexcited to generate a periodic spatial refractive index distribution in the diffraction grating recording region.

  Further, in the diffractive optical element device according to the present invention, the gas generating means may be, for example, ozone by passing air or oxygen through silent discharge generated by applying a high frequency high voltage between electrodes sandwiching an insulator. Can be generated.

  In the diffractive optical element device according to the present invention, the gas generating means may include control means for controlling a flow rate of air or oxygen passing through the silent discharge.

  In the present invention, a periodic spatial refractive index distribution is generated in a gas that is resonantly absorbed by the laser, and is used as a transient diffraction grating. Therefore, even a high-intensity laser can be diffracted. It is possible to make a diffractive optical system that can be used for laser light. This makes it possible to reduce the size of the chirp amplification system that has been large because of the optical damage threshold.

  In the present invention, a periodic spatial refractive index distribution is generated in a photoexcited gas, and a reproducible transient diffraction grating is created based on the periodic spatial refractive index distribution. Reproduction is possible each time, and a diffractive optical system that can handle a high-intensity laser can be made. Therefore, even if light having a breaking strength or higher is incident, the light is regenerated every time, so that the light can be used without a safety factor at the use limit intensity.

  Further, in the present invention, by using resonance absorption of gas in the deep ultraviolet region, it is possible to make an absorption difference of 4 to 5 digits between the laser beam to be written and the laser beam to be used, and periodic spatial refraction generated with a small energy. A high-power laser can be controlled by a transient diffraction grating with a rate distribution.

  Furthermore, according to the present invention, the shape, modulation degree, and pitch of the grating are not fixed as in the case of the solid-state optical diffraction element, and the two excitation laser beams can be changed, so that various diffracted waves can be generated.

It is a perspective view which shows typically the structural example of the diffractive optical element apparatus to which this invention is applied. It is a figure which shows the observation result of the periodic spatial refractive index distribution produced | generated in the diffraction grating recording area in the said diffractive optical element apparatus. It is a figure which shows the result of having produced the periodic spatial refractive index distribution in the diffraction grating recording area | region in the said diffractive optical element apparatus, writing the diffraction grating, and having performed the diffraction experiment. It is a schematic diagram which shows the principle of operation of the said diffractive optical element apparatus.

  Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the drawings.

  The present invention is applied to a diffractive optical element device 100 configured as shown in FIG. 1, for example.

  The diffractive optical element device 100 includes an ozone-containing gas generator 10 that forms a uniform and continuous sheet-like diffraction grating recording area that flows out through the slit-shaped opening 11A, and two diffraction grating recording areas. A photoexcitation unit 20 is provided that irradiates the excitation laser beams L1 and L2 to photoexcite the ozone-containing gas Goo forming the diffraction grating recording region.

  In the diffractive optical element device 100, the ozone gas generation unit 10 functions as a gas generating unit that forms a sheet-like diffraction grating recording region by a gas that absorbs light in a resonant manner, and absorbs the light in a resonant manner. As a gas to be generated, an ozone-containing gas Goo having an absorption peak wavelength near 250 nm is generated, and a sheet-like diffraction grating recording region is formed by the ozone-containing gas Goo.

The ozone-containing gas generation unit 10 supplies a raw material gas Go containing oxygen O ₂ into a space gap 11 for generating silent discharge, and generates ozone-containing gas Goo by the silent discharge. And a pair of dielectrics 13A and 13B having a length of about 30 mm installed between the metal electrodes 12A and 12B, and a source gas source such as an oxygen cylinder or an air tank A raw material gas (high-concentration oxygen, dehumidified air, etc.) Go containing oxygen O ₂ is sent from 14 to the space gap 11 between the pair of dielectrics 13A, 13B by the diaphragm pump 15 and high-frequency high voltage by the high-frequency power source 16. Is applied to the pair of metal electrodes 12A and 12B. For the high-frequency power supply 16, for example, a power supply of 13 MHz · 100 W is used.

In the ozone gas generation unit 10, a high frequency high voltage is applied to the pair of metal electrodes 12A and 12B while passing the source gas Go containing oxygen O ₂ through the space gap 11 between the pair of dielectrics 13A and 13B. Ozone-containing gas Goo is generated by generating silent discharge in the space gap 11 and dissociating oxygen O ₂ by silent discharge to generate ozone O ₃ . Then, the ozone-containing gas Goo generated in the space gap 11 is caused to flow out through the slit-shaped opening 11A, thereby forming a diffraction grating recording region by the uniform and continuous sheet-like ozone-containing gas Goo. The space gap 11 is a gap of about 1 mm, and the pair of dielectrics 13A and 13B is made of glass or ceramic having high dielectric strength.

The ozone-containing gas Goo that has flowed out through the slit-shaped opening 11A is recovered by the recovery hood 17, and is supplied to the supply line of the source gas Go containing oxygen supplied from the source gas supply source 14 through the recirculation line 18. The diaphragm pump 15 is fed back into the space gap 11 between the pair of dielectrics 13A and 13B. A flow meter 19 for measuring the flow rate of the source gas Go containing oxygen O ₂ fed into the space gap 11 between the pair of dielectrics 13A and 13B is installed on the upstream side of the diaphragm pump 15.

  In the diffractive optical element device 100, the optical excitation unit 20 includes a laser light source 21 that generates a laser light L in the vicinity of 250 nm, which is an absorption peak wavelength of the ozone-containing gas Goo, and two laser lights emitted from the laser light source 21. The diffraction gratings are branched into the laser beams L1 and L2 and crossed in the diffraction grating recording region of the uniform and continuous sheet-like ozone-containing gas Goo formed by the ozone-containing gas generator 10. The optical system 22 irradiates two excitation laser beams L1 and L2 to the ozone-containing gas Goo forming the recording area.

  The optical excitation unit 20 irradiates two excitation laser beams L1 and L2 in the vicinity of 250 nm, which is the absorption peak wavelength of the ozone-containing gas Goo, so as to intersect within the diffraction grating recording area. Is absorbed in the ozone-containing Goo, whereby the ozone-containing gas Goo is photoexcited to generate a periodic spatial refractive index distribution in the diffraction grating recording region.

  As described above, in the diffractive optical element device 100, the raw material gas Go is sent by the diaphragm pump 15 by applying a high frequency high voltage to the pair of metal electrodes 12 </ b> A and 12 </ b> B by the high frequency power supply 16 in the ozone-containing gas generation unit 10. Silent discharge is generated in the space gap 11 between the pair of dielectrics 13A and 13B, that is, the flow path through which the raw material gas Go flows, and oxygen is dissociated by the silent discharge to generate the ozone-containing gas Goo. In addition, a sheet-like diffraction grating recording region is formed by a continuous sheet-like ozone-containing gas Goo.

  Here, when the ozone-containing gas Goo is generated in the ozone-containing gas generation unit 10, the discharge becomes unstable when the oxygen concentration is high, and conversely when the oxygen concentration is low, the diffraction grating enters the diffraction grating recording region by the photoexcitation unit 20. Refractive index change during writing is reduced. Further, if the flow rate is slow, the ozone movement to the diffraction grating recording region is reduced, and if it is early, the ozone concentration is lowered by discharge and the refractive index is disturbed by the flow rate when writing the diffraction grating. Accordingly, the ozone amount is adjusted so that the refractive index modulation optimum for writing the diffraction grating into the diffraction grating recording area by the optical excitation unit 20 is obtained by adjusting the partial pressure of oxygen and the flow rate of the source gas. The photoexcitation unit 20 uses an ultraviolet laser beam in the vicinity of 250 mm, which is the absorption peak of the ozone-containing gas Goo, causes the ultraviolet laser beam to interfere with two light beams, and uses the fringe pattern to modulate the refractive index in the jet gas. By writing, the diffraction grating is written. If the intensity of the laser is too strong, the temperature of the grating portion increases and the speed of sound increases, so the life of the grating is shortened. If it is too weak, the degree of refractive index modulation becomes small.

  In this diffractive optical element device 100, two-beam interference of excitation laser beams L 1 and L 2 using ultraviolet laser beams in the vicinity of 250 mm in the diffraction grating recording region by the ozone-containing gas Goo formed by the ozone-containing gas generator 10. FIG. 2 shows the observation result obtained by generating a periodic spatial refractive index distribution by and applying the probe light.

  FIG. 3 shows the result of performing a diffraction experiment by generating a periodic spatial refractive index distribution in the diffraction grating recording region formed by the ozone-containing gas Goo and writing the diffraction grating. In FIG. 3, the first peak indicates the writing pulse for writing the diffraction grating, that is, the intensity of the excitation laser beams L1 and L2, and the second pulse indicates the diffraction intensity. In FIG. 3, the lower characteristic line F1 is a case where a grating is formed, and the upper characteristic line F2 is a case where a grating is not formed.

  By using the periodic spatial refractive index distribution in the diffraction grating recording region as a transient diffraction grating, the diffracted laser light having a wavelength in the non-absorption band of the ozone-containing gas Goo incident on the transient diffraction grating. Lin can be diffracted.

  Here, the incident directions of the two excitation laser beams L1 and L2 and the diffracted laser beam Lin are set to the Y-axis direction orthogonal to the thickness direction (Z-axis direction) of the diffraction grating recording region. May be repeatedly inserted from the Z direction, and the diffraction grating may be recorded in the thickness direction (Z direction). In addition, a volumetric diffraction grating can be formed by a desired diffraction wave.

That is, in this diffractive optical element device 100, as shown in FIG. 4, in the ozone-containing gas generator 10, the source gas Go containing oxygen O ₂ is introduced into the space gap 11 between the pair of dielectrics 13 </ _b > A and 13 </ _b > B. a pair of metal electrodes 12A by the high-frequency power source 16 while passing, 12B to generate a silent discharge in the space gap 11 by applying a high-frequency high voltage, generate ozone O ₃ by dissociating oxygen O ₂ by silent discharge By doing so, a sheet-like diffraction grating recording region is formed by the ozone-containing gas Goo. Further, the ozone that forms the diffraction grating recording region by irradiating two excitation laser beams L1 and L2 in the vicinity of 250 nm, which is the absorption peak wavelength of the ozone-containing gas Goo, within the diffraction grating recording region. The ozone containing gas Goo is photoexcited by being absorbed by the containing gas Goo, and a periodic spatial refractive index distribution due to the interference of the two excitation laser lights L1 and L2 is generated in the diffraction grating recording region. Then, by using the periodic spatial refractive index distribution in the diffraction grating recording region as a transient diffraction grating, the diffracted light having a wavelength in the non-absorption band of the ozone-containing gas Goo incident on the transient diffraction grating is used. The diffracted laser beam Lo can be emitted by diffracting the laser beam Lin.

  In the diffractive optical element device 100, the ozone-containing gas generating unit 10 adopting the silent discharge method forms a sheet-like diffraction grating recording region by the ozone-containing gas Goo. However, the ozone-containing gas is obtained by an electrolytic method or an ultraviolet lamp method. And a sheet-like diffraction grating recording region by the ozone-containing gas Goo may be formed. The ozone-containing gas Goo may be a high-concentration ozone gas obtained by evaporating liquid ozone, for example.

  In the diffractive optical element device 100, a diffraction grating recording region is formed by the ozone-containing gas Goo, and two excitation laser beams L1 and L2 near the absorption peak wavelength of the ozone-containing gas Goo are set to excitation light. However, the present invention is not limited to the diffractive optical element device 100 alone, and the gas forming the diffraction grating recording region is not limited to the ozone-containing gas Goo, and absorbs light in a resonant manner. Any gas can be used.

  That is, in the present invention, a sheet-like diffraction grating recording region is formed by a gas that absorbs light in a resonant manner, and two excitation laser beams having wavelengths in the gas absorption band are crossed in the diffraction grating recording region. The gas forming the region of the diffraction grating recording region is irradiated with the gas so that the gas is photoexcited, and the periodic spatial refractive index distribution due to the interference of the two excitation laser beams is obtained. Generated in the recording area, using the periodic spatial refractive index distribution in the diffraction grating recording area as a transient diffraction grating, and diffracted in the wavelength of the non-absorption band of the gas incident on the transient diffraction grating Laser light can be diffracted.

  Thus, in the present invention, a periodic spatial refractive index distribution is generated in a gas that is highly absorbed in resonance with the laser, and it is used as a transient diffraction grating, so that even a high-intensity laser can be diffracted. A small diffractive optical system that can be used for high-intensity laser light can be produced. This makes it possible to reduce the size of the chirp amplification system that has been large because of the optical damage threshold. Also, in the diffractive optical element device to which the present invention is applied, since the absorption at the used wavelength is small, the density of the refractive index in the propagation direction is changed in the propagation direction so that the reflection point changes depending on the writing wavelength as in the fiber grating. Can be used.

  In the present invention, a periodic spatial refractive index distribution is generated in a photoexcited gas, and a reproducible transient diffraction grating is created based on the periodic spatial refractive index distribution. Reproduction is possible each time, and a diffractive optical system that can handle a high-intensity laser can be made. Therefore, even if light having a breaking strength or higher is incident, the light is regenerated every time, so that the light can be used without a safety factor at the use limit intensity.

  Further, in the present invention, it is possible to provide an absorption difference of 4 to 5 digits by using resonance absorption of gas in the deep ultraviolet region, and a transient diffraction grating by a periodic spatial refractive index distribution generated with small energy. Can control a large output laser.

  Furthermore, according to the present invention, the shape, modulation degree, and pitch of the grating are not fixed as in the case of the solid-state optical diffraction element, and the two excitation laser beams can be changed, so that various diffracted waves can be generated.

  DESCRIPTION OF SYMBOLS 10 Ozone containing gas generation part, 11 Spatial gap, 11A opening, 12A, 12B Metal electrode, 13A, 13B Dielectric, 14 Source gas source, 15 Diaphragm pump, 16 High frequency power supply, 17 Recovery hood, 18 Recirculation line, 19 Flow rate Total, 20 Light excitation unit, 21 Laser light source, 22 Optical system, 100 Diffractive optical element device, Go source gas, Goo ozone-containing gas, L1, L2 excitation laser light, Lin diffracted laser light, Lo diffracted laser light

Claims (6)

A sheet-like diffraction grating recording area is formed by a gas that absorbs light resonantly,
The gas is photoexcited by irradiating an excitation laser beam having a wavelength in the absorption band of the gas so as to cross within the diffraction grating recording region and absorbing the gas in the diffraction grating recording region, Generating a periodic spatial refractive index distribution in the diffraction grating recording area by the interference of the two excitation laser beams;
Using the periodic spatial refractive index distribution in the diffraction grating recording region as a transient diffraction grating, the wavelength of the non-absorption band of the gas up to an intensity determined by the optical damage intensity incident on the transient diffraction grating. A method for diffracting laser light, characterized by diffracting the diffracted laser light. The gas that absorbs light resonantly is a gas containing ozone,
Irradiation with two excitation laser beams in the vicinity of 250 nm, which is the absorption peak wavelength of the gas, generates a periodic spatial refractive index distribution in the diffraction grating recording region formed by the gas. 2. The method of diffracting laser light according to claim 1, wherein the periodic spatial refractive index distribution in is used as a transient diffraction grating. A gas generating means for forming a sheet-like diffraction grating recording region by a gas that absorbs light resonantly;
The gas is photoexcited by irradiating an excitation laser beam having a wavelength in the absorption band of the gas so as to cross within the diffraction grating recording region and absorbing the gas in the diffraction grating recording region, Optical excitation means for generating a periodic spatial refractive index distribution in the diffraction grating recording region by interference of the two excitation laser beams,
The periodic spatial refractive index distribution in the diffraction grating recording region is used as a transient diffraction grating, and the diffracted light of the gas in the non-absorption band of the gas up to the intensity determined by the optical damage intensity incident on the transient diffraction grating. A diffractive optical element device characterized by diffracting laser light. The gas generating means is a gas generating means for forming the diffraction grating recording region with a gas containing ozone flowing out through a slit-shaped opening,
The optical excitation means irradiates two excitation laser beams in the vicinity of 250 nm, which is the absorption peak wavelength of the gas, so as to intersect within the diffraction grating recording area, and to the gas forming the diffraction grating recording area. 4. The diffractive optical element device according to claim 3, wherein the gas is photoexcited by absorption to generate a periodic spatial refractive index distribution in the diffraction grating recording region. The gas generating means generates a gas containing ozone by passing pure oxygen or a mixture of air and oxygen during silent discharge generated by applying a high frequency high voltage between electrodes sandwiching an insulator. The diffractive optical element device according to claim 4. 6. The diffractive optical element device according to claim 5 , wherein the gas generating means includes control means for controlling a flow rate of air or oxygen passing through the silent discharge.

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