LASER PROCESSING METHOD AND LASER PROCESSING SYSTEM

BACKGROUND OF THE INVENTION
1. Field of the Invention

The present invention relates to a laser processing method and a laser processing system.

2. Description of Related Art

It is well known that a laser beam can propagate a long distance without spreading compared with natural light. Therefore, a laser can make a hole by focusing it, in a metal sheet or the like which is placed more than 10km away. But to make this hole, a high-power laser which has a good air transmissivity, without the laser being absorbed by nitrogen, oxygen and water vapor, is necessary. Such a laser is a solid state laser (i.e. an Nd:YAG laser, a fiber laser or the like) operating at an IR region or an iodine laser.

Regarding the above two types of lasers, the iodine laser, which is sometimes called COIL (Chemical Oxygen Iodine Laser), is well known to be able to operate at a high power CW (continuous wave) mode with a wavelength of 1.315 μm. In order to operate the COIL, singlet oxygen molecule (O₂(¹Δ_g)) is generated from the chemical reaction of chlorine gas with a BHP solution which is a mixed solution of hydrogen peroxide solution (H₂O₂) and potassium hydroxide (KOH) or sodium hydroxide (NaOH). By transferring the energy of O₂(¹Δ_g) to a basic iodine atom (I) (i.e. by producing I(²P_3/2) in an exited state from I(²P_1/2) in a ground state), the laser can be operated. “Stephen C. Hurlick, et al., “COIL technology development at Boeing,” Proceedings of SPIE Vol. 4631, 101-115 (2002)”, “Masamori Endo, “History of COIL development in Japan: 1982-2002,” Proceedings of SPIE Vol. 4631, 116-127 (2002)”, “Edward A. Duff and Keith A. Truesdell, “Chemical oxygen iodine laser (COIL) technology and development,” Proceedings of SPIE Vol. 5414, 52-68 (2004)” and “Jarmila Kodymova, “COIL-Chemical Oxygen Iodine Laser: advances in development and applications,” Proceedings of SPIE Vol. 5958, 595818 (2005)” explain about the iodine laser.

Conventionally, it is difficult to have the laser propagate to a target placed more than 10 km away even with a laser having good air propagation characteristics if there is a cloud or a fog in the air. In other words, as a cloud or a fog is a cluster of water molecules which accumulate together and become enormous, a laser beam is scattered by the cluster. When there is a fine particle which becomes a core, the molecules of water are clustered. The core of the cluster is sometimes referred to as aerosol. Alternatively, the cluster of water molecules may be referred to as aerosol

Therefore, research was performed to improve the air transmissivity by using a laser which can vaporize a cloud or a fog. According to “REPORT SRL 02-F-1989, “LASER PULSE FORMATTING TO REDUCE THERMAL BLOOMING BY AEROSOL VAPORIZATION,” FINAL TECHNICAL REPORT, 17 Jan. 1989”, a KrF excimer laser can vaporize a cloud or a fog since its beam has a good absorption in regard to aerosol. A laser which can vaporize aerosol is referred to as LAV (Laser for Aerosol Vaporization).

SUMMARY OF THE INVENTION

However, in a case where the cloud or fog appears in the air, even if the laser which has good absorption in water is used (the laser is referred to as a vaporization laser) in order to vaporize a fog or a cloud in the air, the air always fluctuates. Especially on a windy day, a cloud or a fog flows at a speed of several tens of meters per second. The vaporization laser needs to be irradiated during the laser process because the air moves all the time. Therefore, a very high power CW (continuous wave) laser is required for the vaporization, which is a problem.

It is considered that a light cloud contains 0.05 g of water in a cubic meter, and a dark cloud contains 5 g of water. Therefore, if a cloud contains 1 g of water in a cubic meter, a total of 20 g of water is contained in a 100 m beam path with an average diameter of 50 cm during passing of the beam path through the cloud with a thickness of 100 m. The reason for assuming that the beam path has such a large diameter is that a laser beam having a diameter of around 1 m is necessary for the initial beam to focus the laser beam on a target several kilometers away. Considering the fact that approximately 2560 J of heat is necessary to evaporate 1 g of 25 degrees C. water, 51 kJ of laser energy is needed to evaporate the 20 g of water. This can be derived by adding the heating-up energy of 20 g of water from 25 to 100 degrees C. with the water vaporization energy of 2250 J/g. But this laser energy is required only for an instant. If a cloud flows 10 meters per second, it takes 0.05 s for water to travel 50 cm across the beam diameter. This means that if the water is continuously vaporized for 1 s, laser energy 20 times larger than that of the above laser energy is necessary. Consequently, approximately 1 MW average power is required for the vaporization laser if a CW laser or a high repetition laser is used. Since it is quite difficult to develop such a high power laser, it is unrealistic to use the high power laser as the vaporization laser. As described above, it was practically impossible to achieve the laser processing when a cloud or a fog appears.

In order to solve the above mentioned problem, the present invention employs a MOPA system. The MOPA system includes a flashlamp-pumped pulsed iodine laser oscillator that generates a laser pulse and a chemical oxygen-iodine laser amplifier that amplifies a double pulse. The flashlamp pumped pulsed iodine laser oscillator is a master oscillator of the MOPA system. The chemical oxygen-iodine laser amplifier is a power amplifier of the MOPA system. The first pulse of the double pulse is used as vaporization laser. The second pulse is used as the processing laser.

Concerning oscillation timing of the processing laser and the vaporization laser, the processing laser oscillates within 1 ms after the oscillation of the vaporization laser. This enables to propagate the beam of the processing laser in a high transmission path formed by the propagation of the vaporization laser before the path is made to fly away by wind.

Assuming that a wind flows toward the beam crossing direction at 10 m/s, the high transmission path moves only 10 mm if the processing laser oscillates at 1 ms after the vaporization laser oscillates. Therefore, the beam radius of the vaporization laser has to be adjusted to be only more than 10 mm larger than that of the processing laser under such a windy condition.

Since the chemical oxygen-iodine laser amplifier can produce a giant-pulse laser with a high power. This enables a hole to be made in a metal sheet or the like by a single shot. Therefore, using a pulsed vaporization laser, a high transmission path is formed by only a single shot. This enables a required energy for the pulsed vaporization laser to be reduced to a small value. “M. Endo, K. Shiroki, and T. Uchiyama, “Chemically pumped atomic iodine pulse laser,” Appl. Phys. Lett. Vol. 59, 891-892 (1991)”, “Kenji Suzuki, Kozo Minoshima, Daichi Sugimoto, Kazuyoku Tei, Masamori Endo, Taro Uchiyama, Kenzo Nanri, Shuzaburo Takeda, and Tomoo Fujioka, “High pressure pulsed COIL assisted with an instantaneous production of atomic iodine,” Proc. SPIE 4184, 124-127 (2001)” and “Masamori Endo, Kozo Minoshima, Koichi Murata, Oleg Vyskubenko, Kenzo Nanri, Shuzaburo Takeda, and Tomoo Fujioka, “High pressure pulsed COIL assisted with an instantaneous production of atomic iodine II,” Proc. SPIE 5120, 397-404 (2003)” explain about the pulsed iodine laser. “K. Takehisa “New concepts for realizing an oxygen molecule laser,” Proc. SPIE 9251 (2014)” explains about the oxygen molecule laser.

In the case where the pulsed iodine laser or the oxygen molecule laser is used as the processing laser, in order to automatically oscillate the vaporization laser immediately before the oscillation of the processing laser, a flashlamp-pumped solid-state laser can be used as the vaporization laser. The flashlamp would be triggered using a signal which controls an open/close valve of the chlorine gas tank used for a single oxygen generator of the pulsed iodine laser or the oxygen molecule laser. This enables the pulsed iodine laser or the oxygen molecule laser to be oscillated less than 1 ms after the vaporization laser oscillates. Therefore, the processing laser can propagate through the high transmission path even in a strong wind.

The present invention provides a laser processing method and a laser processing system which can process a target placed a long distance away even in cloudy or foggy air.

The above and other objects, features and advantages of the present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional drawing of a laser processing system 1 according to an embodiment of the invention;

FIG. 2 is a cross-sectional side-view drawing of a processing laser apparatus as a pulsed iodine laser oscillator;

FIG. 3 is a cross sectional drawing, perpendicular to the optical axis, of the pulsed iodine laser oscillator;

FIG. 4 is a cross sectional drawing of a vaporization laser 200 according to an embodiment of the invention;

FIG. 5 is a graph showing wavelength dependency of a water absorption coefficient;

FIG. 6 is a graph explaining the operation timing of the pulsed iodine laser and the pulsed vaporization;

FIG. 7 is a graph showing radii of the beam of the processing laser and that of the vaporization laser;

FIG. 8 is a graph showing a subtraction of the beam radius of the processing laser from that of the evaporation laser;

FIG. 9 is a graph showing radii of the beam of the processing laser and that of the vaporization laser with wavelength of 0.248 um;

FIG. 10 is a graph showing a subtraction of the beam radius of the processing laser from that of the evaporation laser with wavelength of 0.248 um;

FIG. 11 is a graph showing radii of the beam of the processing laser and that of the vaporization laser with wavelength of 10.6 um;

FIG. 12 is a graph showing a subtraction of the beam radius of the processing laser from that of the evaporation laser with wavelength of 10.6 um;

FIG. 13 is a cross sectional drawing of a laser processing system according to a second embodiment of the invention;

FIG. 14 is a cross-sectional side-view drawing of a pulsed iodine laser of the laser processing system according to the second embodiment;

FIG. 15 is a graph showing timings of the laser pulses and the oxygen pressure and the iodine pressure in the amplifier chamber;

FIG. 16 is a cross sectional drawing of the pulsed iodine laser oscillator 401 of a laser processing system 400 according to the third embodiment;

FIG. 17 is a graph showing a relative intensity of the water absorption line as a function of wavenumber;

FIG. 18 is a graph showing a transmissivity curve of an intracavity etalon 417a; and

FIG. 19 is a graph showing a transmissivity curve of an intracavity etalon 417b.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Exemplary embodiments of the present invention are explained with reference to the attached drawings. The exemplary embodiments explained below are only examples of the present invention, and the present invention is not limited to these exemplary embodiments. Note that components denoted by the same reference numerals in the specification and drawings indicate the same components.

First Embodiment

Hereinafter, the first embodiment according to the present invention is described based on FIG. 1. FIG. 1 is a cross sectional drawing of a laser processing system 1 for a long distance process according to an embodiment of the invention. In the laser processing system 1, a pulsed iodine laser is used as the processing laser 100, and a flashlamp-pumped Er:YAG laser oscillator is used as the vaporization laser 200. The Er:YAG laser is a solid-state laser oscillating at 2.94 μm wavelength, which has a high absorption rate in water as shown in FIG. 5. “Janez Diaci and Boris Gaspirc, “Comparison of Er:YAG and Er,Cr:YSGG lasers used in dentistry,” Journal of the Laser and Health Academy, Vol. 2012, No. 138 explains about the Er:YAG laser.

A pulsed laser L2 is extracted from the vaporization laser 200 of the laser processing system 1 and is reflected at a mirror 4. The laser L2 reflected at the mirror 4 enters on a dichroic mirror 5 where the laser L2 is transmitted, while the pulsed iodine laser, as the processing laser 100, oscillates immediately after the vaporization laser 200 oscillates. Then a pulsed laser L1 with the wavelength of 1.315 um is extracted, and enters on the dichroic mirror 5 where the laser L1 is reflected. The laser L3, propagating from the dichroic mirror 5, is spatial superimposition of the laser L1 and the laser L2. But the laser L2 propagates temporally earlier than the laser L1.

The laser L3 is reflected at a deformable mirror 6, and propagates through a center hole of a large focusing mirror 8 which has a diameter of approximately 1 meter. Then the laser L3 which passes thought the center hole is reflected at the convex mirror 7, and propagates toward the reflecting surface of the focusing mirror 8 (which is illustrated as a concave surface at a right side in FIG. 1). The laser L3 enters the whole surface of the focusing mirror 8. The reflected laser L3 propagates and is focused on the target 10 which is an Aluminum plate. Since the propagation length is long, the beam profile is illustrated as separated lines. The deformable mirror 6 corrects the wavefront of the laser L3 in order to compensate for the effect of the turbulence during the propagation.

In FIG. 1, an envelope line of the 1.315 μm laser L4 and L6 is indicated by solid lines, while an envelope line of the 2.94 μm laser L5 and L7 is indicated by dotted lines. Both laser beams are in a single transverse mode. Since a longer wavelength laser has a larger diffraction angle, the profile of the 2.94 μm laser beam is illustrated as shown to be as wide as the laser L5 and the laser L7.

Concerning the focused sizes of the lasers at the target 10, it is approximately 1.7 mm for the 1.315 μm-wavelength laser L6 which is extracted from the processing laser 100, while it is approximately 4 mm for the 2.94 μm-wavelength laser L7 which is extracted from the vaporization laser 200. Therefore, the beam path of the processing laser 100 is contained in the beam path of the vaporization laser 200. This is the reason why the processing laser beam is not scattered during the propagation, and the processing laser beam can be delivered to the target 10. This is one of the advantageous effects of the present invention, which is realized by using a wavelength for the vaporization laser equal to or longer than that of the processing laser.

On the contrary, if an excimer laser is used for the vaporization laser 200, the beam path of the excimer laser is narrower than that of the processing laser near the target 10 since the excimer laser has a shorter wavelength than that of the processing laser. Consequently a part of the beam path of the processing laser is outside of the beam path of the vaporization laser (this is opposite to the illustration in FIG. 1). This is explained later with reference to FIG. 9 and FIG. 10.

The pulsed iodine laser, as the processing laser 100, is controlled to oscillate approximately 1 ms after the oscillation of the Er:YAG laser, as the vaporization laser 200, by a controller 3. The controller 3 outputs a signal S1 to the processing laser 100, and outputs a signal S2 to the vaporization laser 200. The signal S1 controls the oscillation timing of the processing laser 100, while the signal S2 controls the oscillation timing of the vaporization laser 200. This causes the oscillation of the processing laser 100 to be immediately after the oscillation of the vaporization 200.

Therefore, if a cloud or a fog is made to fly by wind blowing at 10 m/s, the cloud or the fog moves around only 10 mm in 1 ms. So a part of the beam path of the processing laser 100, which gets outside of the beam path of the vaporization laser 200, is negligibly small.

Here the details of the pulsed iodine laser, as the processing laser 100, are explained using FIG. 2 and FIG. 3. FIG. 2 is a cross-sectional side-view drawing of the processing laser 100. FIG. 3 is a cross-sectional drawing of the processing laser 100 perpendicular to the optical axis.

As shown in FIG. 2, the processing laser 100 has a laser cavity which is placed in a vacuum-tight housing 101. The laser cavity has a total reflector 102 and an output mirror 103. A singlet oxygen generator 104 is provided under the laser cavity. The singlet oxygen generator 104 stores BHP solution 105. Many discs 106 are placed side by side above the BHP solution 105. The common axis 107 of the discs 106 can rotate in the direction as shown by the arrow 107A. Namely, the singlet oxygen generator 104 is a rotating disc type. Kevin B. Hewett, “Singlet oxygen generators—the heart of chemical oxygen iodine lasers: past, present and future,” Proceedings of SPIE Vol. 7131 (2009) explains about the rotating disc type singlet oxygen generator.

In order to oscillate the processing laser 100, the inside of the housing 101 is vacuumed beforehand. At first the valve 109 is opened to vacuum the housing 101 with a vacuum pump (not shown in FIG. 2) in the direction shown by the arrow 108A. Then a large amount of chlorine gas is instantaneously supplied into the housing 101 and contacts the BHP solution 105. This generates singlet oxygen molecules. The details of this are explained later using FIG. 3.

After the singlet oxygen molecules are generated, iodine molecules are supplied through the iodine injection tube 110 as shown by the arrow 110A. Since the iodine molecule is solid at room temperature, the iodine molecules which are vaporized by heating are supplied together with argon gas or helium gas. There are many holes in the surface of the iodine injection tube 110, located above the singlet oxygen generator 104, in order to supply iodine molecules. When the iodine molecules react with singlet oxygen molecules, excited iodine is generated. The excited iodine can produce a laser action, generating the laser L1 which is extracted from the output mirror 103. The iodine injection tube 110 itself can be heated instead of heating the iodine molecules.

The details of the pulsed iodine laser, as the processing laser 100, are explained using FIG. 3. FIG. 3 is a cross sectional drawing, perpendicular to the optical axis, of the pulsed iodine laser oscillator 100. The main components of the pulsed iodine laser are a laser cavity 112 and the singlet oxygen generator 104. After the singlet oxygen molecules are generated from the surface of the discs 106, the singlet oxygen molecules go into the laser cavity 112 as shown by the arrow 111. The laser cavity 112 is placed above the singlet oxygen generator 104. The singlet oxygen molecules react with iodine molecules which come out from the holes of the injection tube 110. Since the lower halves of the discs 106 are soaked in the BHP solution 105, the surfaces of the upper halves of the discs 106 are wetted with the BHP solution 105 by being rotated around the rotation axis 107.

Although the chlorine gas to generate singlet oxygen molecules is supplied from a chlorine gas container 115, the chlorine gas is temporarily reserved in a chlorine gas tank 116 which has a large internal volume. This is because the chlorine gas needs to be supplied into the singlet oxygen generator 104 at a high flow rate. When a valve 117 opens, the chlorine gas is supplied into the singlet oxygen generator 104 through a chlorine supplying tube 118. Then the supplied chlorine gas immediately contacts the upper halves of the discs 106. Consequently, a large number of singlet oxygen molecules are generated, and the iodine laser gives a pulse oscillation. Therefore in order to start the pulse oscillation, the signal S1 is sent to open the valve 117.

Although as explained above, in the first embodiment, the pulsed iodine laser is used as a processing laser 100, a flashlamp-pumped Nd:YAG laser can be used instead. The reason for using a pulsed iodine laser is that it enables a high-quality beam to be obtained easily because it is a gas laser which can easily generate a near diffraction-limit beam with a single transverse mode. As shown in FIG. 2, this is because the mode volume can easily fowl a long geometry, in which laser oscillation becomes a single transverse mode easily.

The details of the vaporization laser 200 illustrated in FIG. 1 are explained using FIG. 4. The FIG. 4 is a cross sectional drawing of an Er:YAG laser oscillator as the vaporization laser 200 along the optical axis of the Er:YAG laser oscillator.

An Er:YAG crystal 201 used as the laser medium forms slab shape. The Er:YAG crystal 201 is located in the laser cavity which has a total reflector 202 and an output mirror 203. Near the upper surface of the Er:YAG crystal 201, a flashlamp 204A is located. Likewise, near the lower surface of the Er:YAG crystal 201, a flashlamp 204B is located. The flashlamps 204A and 204B are connected to an electric circuit 206 through power cables 205A1, 205A2, 205B1, and 205B2.

In order to oscillate the vaporization laser 200, a signal S2 is supplied to the electric circuit 206. Then the flashlamps 204A and 204B flash emit light, and the Er:YAG crystal 201 is excited. Consequently the vaporization laser 200 oscillates, and a pulsed laser L2 is extracted from the output mirror 203. Here the oscillation timing for both the processing laser 100 and the vaporization laser 200 is explained using FIG. 6. The horizontal-direction axis of FIG. 6 indicates time and the vertical axis of FIG. 6 indicates intensity. But the vertical-direction axis does not indicate a quantitative value. Time of 0 indicates the timing of the signal S1.

When the chlorine gas starts to be supplied, upon the signal S1 being given, into the singlet oxygen generator 104 of the processing laser 100, the pressure of the oxygen molecules in the housing 112 starts to increase linearly. But the iodine laser starts to oscillate after the oxygen pressure reaches some value. In this embodiment, the iodine laser starts to oscillate at approximately 4 ms after the start of supplying the chlorine gas, and then the laser L1 is extracted.

When the signal S2 is generated at approximately 3 ms after the signal S1, the flashlamps 204A and 204B, used for the vaporization laser 200, start to flash. Consequently the vaporization laser 200 oscillates at approximately 1 ms after the flashlamps 204A and 204B start to flash, and then laser L2 is extracted. Therefore laser L2 is extracted at approximately 1 ms before laser L1 is extracted. Needless to say, the vaporization laser 200 may oscillate 1 ms or less before the oscillation timing of the processing laser 100.

The following is an explanation of why the beam path of the processing laser 100 can become a high transparent path of the vaporization laser 200 used in the laser processing system 1. FIGS. 7 to 12 show the simulation results concerning the processing laser 100 and the vaporization laser 200 on the assumption that the two laser beams both have diffraction-limited high beam quality. In the simulation results shown in FIGS. 7 to 12, the processing laser and the vaporization laser are focusing at a target at a distance of 10 km. FIGS. 7, 9 and 11 show the beam radii of the processing laser and the vaporization laser. FIGS. 8, 10 and 12 show the values of the radius of the processing laser subtracted from that of the vaporization laser.

FIGS. 7 and 8 correspond to the above embodiment of the laser processing system 1, in which the processing laser is a 1.315 μm wavelength iodine laser and the vaporization laser is a 2.94 μm wavelength Er:YAG laser. The beam radius of laser L1 from the processing laser 100 is assumed to be 500 mm at the large focusing mirror 8, and the beam radius of laser L2 from the vaporization laser 200 is assumed to be 510 mm at the large focusing mirror 8.

As shown in FIG. 8, the subtraction of the beam radius is always plus, which means that the beam path of lasers L4 and L6 from the processing laser 100 is contained in a high transmission path made by lasers L5 and L7 from the vaporization laser 200. Therefore lasers L4 and L6 from the processing laser 100 can be efficiently propagated to the target 10.

The changing characteristics of beam radius are shown in FIGS. 9 and 10 in the case of using a KrF excimer laser oscillating at 0.248 μm as a vaporization laser 200. The laser at wavelength of 0.248 μm is relatively well-absorbed into water. In FIGS. 9 and 10, the processing laser 100 is the iodine laser with a wavelength of 1.315 μm as described above. The beam radius of laser L1 from the processing laser 100 is assumed to be 500 mm at the large focusing mirror 8. The beam radius of laser L2 from the vaporization laser 200 is assumed to be 500 mm at the large focusing mirror 8. As shown in FIG. 10, the subtraction becomes minus. This means the beam diameter of the laser from the KrF excimer laser is smaller than that of the laser from the processing laser 100 near the target 10.

Although FIGS. 9 and 10 show the changing characteristics of beam radius in the case of using a chemical iodine laser as the processing laser 100, if a Nd:YAG laser is used as a processing laser 100, both lasers have a longer wavelength than that of the KrF excimer laser. Therefore the radius of the beam from the KrF excimer laser becomes smaller near the target 10 if the beam radius at the large focusing mirror 8 from the KrF excimer laser is adjusted to be equal or larger than that of the processing laser. This is because the shorter wavelength beam has a smaller beam diffraction, and hence, has a smaller focusing size. Consequently the part of the beam from the processing laser gets outside of the high transparent path. This is the reason the wavelength of the vaporization laser 200 is equal to or longer than that of the processing laser 100 in the present invention of the laser processing system 1. For example, the wavelength of the vaporization laser 200 may be equal to or longer than 1.06 μm.

Additionally, the changing characteristics of the beam radius are shown in FIGS. 11 and 12 in the case of using a CO2 laser oscillating at 10.6 μm as the vaporization laser 200. In FIGS. 11 and 12, the processing laser 100 is the iodine laser with a wavelength of 1.315 μm as described above. The beam radius of laser L1 from the processing laser 100 is assumed to be 500 mm at the large focusing mirror 8, and the beam radius of laser L2 from the vaporization laser 200 is assumed to be 550 mm at the same large focusing mirror 8.

As shown in FIG. 12, when the vaporization laser 200 oscillates at 10.6 μm, the subtraction of the beam radius is always plus. Therefore, lasers L4 and L6 from the processing laser 100 can propagate in the high transmission path formed by lasers L5 and L7 from the vaporization laser 200. Therefore absorption of the lasers L4 and L6 by a cloud or a fog can be reduced.

In this embodiment, a pulsed laser such as a flashlamp-pumped solid-state laser oscillating at a 1.06 μm wavelength, a pulsed iodine laser, or an oxygen molecule laser can be used as the processing laser 100. And a pulsed laser oscillating at a wavelength equal to or longer than 1.06 μm can be used as a vaporization laser 200. Also the controller 3 controls the oscillation of the processing laser 100 just after the oscillation of the vaporization laser 200.

In this configuration, since the vaporization laser 200 is also a pulsed laser, the beam path of the processing laser 100 can become a high transmission path by a single pulse from the vaporization laser 200. Even in a strong wind, the lasers L4 and L6 from the processing laser 100 can propagate in the highly transparent path. Therefore, the target 10 placed at a far distance can be processed even if a cloud or a fog is present during the propagation in the air.

For the vaporization laser 200, using an Er:YAG laser or a CO₂laser is desirable, as it can effectively vaporize the cloud and the fog. For the processing laser 100, using a pulsed iodine laser or an oxygen molecule laser, and using the timing of supplying chlorine gas to the singlet oxygen generator used for the control of the oscillation timing of the iodine laser or the oxygen molecule laser, are desirable.

Second Embodiment

Hereinafter, the second embodiment according to the present invention is described based on FIG. 13. FIG. 13 is a cross sectional drawing of a laser processing system 300 for a long distance process according to the second embodiment of the invention. In the laser processing system 300, a single pulsed iodine laser 301 is used both as the processing laser and as the vaporization laser. The main difference between the pulsed iodine laser 301 in the second embodiment and the pulsed iodine laser 100 in the first embodiment is that the pulsed iodine laser 301 can produce a double pulse: two successive pulses. Therefore, the first pulse of the double pulse is used as the vaporization laser, and the second pulse of the double pulse is used as the processing laser.

Since a basic optical configuration of the processing system 300 is the same as that of the laser processing system 1, a detailed description is omitted. For example, the optical configuration between a deformable mirror 303 and a target 306 of the laser processing system 300 is the same as that between the deformable mirror 6 and the target 10 of the laser processing system 1 illustrated in FIG. 1. Therefore, a deformable mirror 303, a convex mirror 304, a focusing mirror 305 and a target 306 correspond to the deformable mirror 6, the convex mirror 7, the focusing mirror 8 and target 10 described in the first embodiment, respectively. In this embodiment, the dichroic mirror 5 described in the first embodiment is replaced with a reflection mirror 302.

The pulsed iodine laser 301 generates the double pulse. The double pulse propagates toward the target though the reflection mirror 302, the deformable mirror 303, the convex mirror 304 and focusing mirror 305.

The pulsed iodine laser 301 has a MOPA (Master Oscillator Power Amplifier) system as illustrated in FIG. 14. The pulsed iodine laser 301 includes a flashlamp-pumped iodine laser oscillator 310 and a COIL amplifier 320. The flashlamp-pumped iodine laser oscillator 310 is a master oscillator of the MOPA system. The COIL amplifier 320 is a power amplifier of the MOPA system.

The flashlamp pumped iodine laser oscillator 310 includes two Xe flashlamps 314a and 314b. The flashlamp pumped iodine laser oscillator 310 further includes a laser tube 313, a total reflector 312 and an output mirror 313. The laser tube is placed between the total reflector 312 and the output mirror 313. The laser tube 311 is made of transparent quartz glass. In the flashlamp-pumped iodine laser oscillator 310, the laser tube 311 is filled with vapor of n-C₃F₇I as an iodine compound. Near the laser tube 311, the two Xe flashlamps 314a and 314b are placed.

The Xe flashlamp 314a is a first flashlamp and the Xe flashlamp 314b is a second flashlamp. The Xe flashlamp 314a is connected to a power supply 316a through power cables 315a. The Xe flashlamp 314b is connected to a power supply 316b through power cables 315b. A controller 330 independently controls the power supply 316a and the power supply 316b. Therefore, the Xe flashlamp 314b flashes after the Xe flashlamp 314a flashes.

The pulsed iodine laser 301 generates a double pulse which includes a first laser pulse and a second laser pulse. The first laser pulse is generated by a flash of the Xe flashlamp 314a. The second laser pulse is generated by a flash of the Xe flashlamp 314b after the first laser pulse is generated.

When a trigger signal S304a is input to the power supply 316a, a large pulsed electric current flows through the power cables 315a. Then the Xe flashlamp 314a flashes. Likewise, when a trigger signal S304b is input to the power supply 316b, a large pulsed electric current flows through the power cables 315b. Then the Xe flashlamp 314b flashes. The trigger signals S304a and S304b are supplied from the controller 330. The controller 330 controls both the flashlamp-pumped iodine laser oscillator 310 and the COIL amplifier 320.

The first and second laser pulses of a laser L300 are extracted from the output mirror 313. The laser L300 enters the amplifier chamber 321 of the COIL amplifier 320 through the convex mirrors 318a and 318b.

The COIL amplifier includes the amplifier chamber 321, a SOG (Singlet Oxygen Generator) 325, a high-pressure chlorine tank 324, and an iodine molecule tank 326. The amplifier chamber 321 is filled with the amplified medium. As shown in FIG. 14, the SOG 325 is located under the amplifier chamber 321. The large SOG 325 is directly connected to the amplifier chamber 321. The high-pressure chlorine tank 324 supplies chlorine gas to the SOG 325. Specifically, supply tubes connect the high-pressure chlorine tank 324 to the SOG 325. The supply tubes are equipped with electromagnetic valves. All these electromagnetic valves mounted in the supply tubes are referred to as a valve V322. Opening or closing of the valve V322 is controlled by an open/close signal S302 from the controller 330. The high-pressure chlorine tank 324 supplies the chlorine gas to the SOG 325 by opening the valves V322.

The iodine molecule tank 326 is directly connected to the amplifier chamber 321 by supply tubes 332 having valves V323. Therefore the amplifier chamber 321 can be filled with excited oxygen and the iodine molecules. That is, the amplifier chamber 321 contains the singlet oxygen and the iodine molecules.

The iodine molecule tank 326 supplies iodine molecules and buffer gas to the amplifier chamber 321 by opening the valves V323. The controller 330 outputs an open/close signal S303 to the valves V323. Opening or closing of the valves V323 is controlled by the open/close signal 5303.

The COIL amplifier 320 also has an exhaust tube 323. The exhaust tube 323 is connected to a vacuum pump which is not shown in the fig. The vacuum pump pumps out the amplifier chamber 321 through the exhaust tube 323 before the laser operation. The exhaust tube 323 has a valve V321. The controller 330 outputs an open/close signal S301 to the valve V321. The opening/closing operation of the valve 321 is controlled by the open/close signal S301.

A pressure gauge 327 is attached to the amplifier chamber 321. The pressure gauge detects a pressure of the amplifier chamber 321. Specifically, the pressure gauge 327 monitors the pressure of the total oxygen which the amplifier chamber 321 is filled with. The pressure gauge 327 outputs a monitoring signal S305 indicating the total oxygen pressure to the controller 330.

After the amplifier chamber 321 is pumped out, the valve V321 is closed. Then, by opening the valves V322, the chlorine gas is injected into the SOG 325 from the high-pressure chlorine tank 324. Consequently, the singlet oxygen is generated in the SOG 325. Therefore, the amplifier chamber 321 is filled with the singlet oxygen. The controller 330 also controls the timing of the opening/closing of the valves V323 according to the pressure of the amplifier chamber 321 which is detected by a pressure gauge 327. The controller 330 outputs an open/close signal S303 based on the monitoring signal S305. Specifically, the controller 330 controls the valves V323 at the moment when the pressure reaches a predetermined value.

Then iodine molecules and buffer gas are supplied into the amplifier chamber 321. Immediately after opening the valves V323, the controller 330 outputs the trigger signal S304a to flash the flashlamp 314a. Then, the first pulse of the laser L300 oscillates, and the laser L300 is extracted from the output mirror 313. The controller 330 outputs the signal S304b shortly after sending the signal S304a. Then, the flashlamp 314b flashes, and the second pulse of the laser L300 is extracted from the output mirror 313. That is, the second pulse is generated by a flash of the flashlamp 314b after the first pulse is generated. Therefore, the laser L300 becomes a double pulse. The COIL amplifier 320 amplifies the double pulse.

The timings of the laser pulses and the oxygen pressure and the iodine pressure in the amplifier chamber 321 are explained with reference with FIG. 15. In FIG. 15, the first pulse of the laser L300 is shown as L300a, and the second pulse of the laser L300 is shown as L300b. In FIG. 15, a sum of a pressure of iodine molecules (I₂) and a pressure of iodine atoms (I) is indicated as I₂and I pressure. Since the amplified laser L302 also becomes a double pulse, the first pulse of laser L302 is shown as L302a, and the second pulse of the laser L302 is shown as L302b. Although the pulsed energy of the laser L300a and that of the laser L300b are almost equal, the amplified laser L302b has a larger energy than that of the laser L302a. This is because the iodine pressure in the amplifier chamber 321 when the laser L300b enters the amplifier chamber 321 is higher than the iodine pressure when the laser L300a enters the amplifier chamber 321. The laser L302a is generated immediately after iodine molecules are injected into the amplifier chamber 321.

The COIL amplifier 320 amplifies the second pulse of the double pulse after the COIL amplifier amplifies the first pulse of the double pulse. The laser L302a is used as the vaporization laser, and the laser L302b is used as the processing laser. The vaporization laser and the processing laser each have a wavelength of 1.315 um.

The feature of this embodiment is that the same laser (a pulsed iodine laser 301) is used for both the vaporization laser and the processing laser. Therefore, not only does the system become simple, but also the optical alignment between the vaporization laser beam and the processing laser beam is not necessary. Since the laser wavelength of the vaporization laser is the same as that of the processing laser, these beam sizes are also the same. The propagating beam paths of these lasers are completely the same. Therefore, the beam path of the processing laser can be completely cleared. Further, the laser having a 1.315 um wavelength has good absorption in water. Therefore, the first pulse can vaporize a cloud or a fog in the beam path. Since the second pulse is propagated through a dried area, the second pulse can be efficiently propagated to the target 306. The second pulse may be generated within 1 ms after the generation of the first pulse.

Third Embodiment

Hereinafter, the third embodiment according to the present invention is described based on FIGS. 16 to 19. FIG. 16 is a cross sectional drawing of the pulsed iodine laser oscillator 401 of a laser processing system 400 for a long distance process according to the third embodiment. Since the amplifier of the laser processing system 400 has the same configuration as the amplifier 320 of the laser processing system 300, only the pulsed iodine laser oscillator 401 is shown in FIG. 16. That is, the amplifier of the laser processing system 400 is not illustrated in FIG. 16.

The main difference between the pulsed iodine laser oscillator 401 according to the third embodiment and the pulsed iodine laser oscillator 310 according to the second embodiment is a configuration of the pulse iodine laser oscillator. In the third embodiment, the pulsed iodine laser oscillator 401 includes two flashlamp-pumped iodine laser oscillators. The pulsed iodine oscillator 401 has the flashlamp-pumped iodine laser oscillator 410a and the flashlamp-pumped laser oscillator 410b. The flashlamp-pumped iodine laser oscillator 410a produces a pulsed laser L400a. The flashlamp-pumped laser oscillator 410b produces a pulsed laser L400b. These two flashlamp-pumped iodine laser oscillators 410a and 410b oscillate at a slightly different timing from each other. Thus, a double-pulse laser L401 is generated by combining the pulsed laser L400a and the pulsed laser L400b by a beam splitter 419. The pulsed laser L400a is a first pulse of the double pulse, and the pulse laser L400b is a second pulse of the double pulse. The pulsed laser L401 is amplified by a COIL amplifier which is not shown in FIG. 16. The following is a detailed explanation about the flashlamp-pumped iodine laser oscillator 410a and the flashlamp-pumped laser oscillator 410b.

The flashlamp-pumped iodine laser oscillator 410a includes two Xe flashlamps 414a1 and 414a2. The flashlamp-pumped iodine laser oscillator 410a further includes a laser tube 411a, a total reflector 412a and an output mirror 413a. The laser tube 411a is placed between the total reflector 412a and the output mirror 413a. In the flashlamp-pumped iodine laser oscillator 410a, the laser tube 411a is filled with vapor of n-C₃F₇I as an iodine compound. Near the laser tube 411a, the two Xe flashlamps 414a1 and 414a2 are placed. The Xe flashlamps 414a1 and 414a2 are connected to a power supply 416a through power cables 415a1, 415a2. A controller 420 controls the power supply 416a. Therefore, the controller 420 controls the timing of the oscillation of the flashlamp-pumped iodine laser oscillator 410a. The controller 420 outputs a trigger signal S400a to the power supply 416a, and thereby the Xe flashlamps 414a1 and 414a2 flash. Then, the flashlamp-pumped iodine laser oscillator 410a oscillates, and a pulse laser L400a is extracted from the output mirror 413a.

The flashlamp-pumped iodine laser oscillator 410b includes two Xe flashlamps 414b1 and 414b2. The flashlamp-pumped iodine laser oscillator 410b further includes a laser tube 411b, a total reflector 412b and an output mirror 413b. The laser tube 411b is placed between the total reflector 412b and the output mirror 413b. In the flashlamp-pumped iodine laser oscillator 410b, the laser tube 411b is filled with vapor of n-C₃F₇I as an iodine compound. Near the laser tube 411b, the two Xe flashlamps 414b1 and 414b2 are placed. The Xe flashlamps 414b1 and 414b2 are connected to a power supply 416b through power cables 415b1, 415b2. A controller 420 controls the power supply 416b. Therefore, the controller 420 controls the timing of the oscillation of the flashlamp-pumped iodine laser oscillator 410b. The controller 420 transmits a trigger signal S400b to the power supply 416b, and thereby the Xe flashlamps 414b1 and 414b2 flash. Then, the flashlamp-pumped iodine laser oscillator 410b oscillates, and a pulse laser L400b is extracted from the output mirror 413b.

The pulsed laser L400a is reflected by a mirror 418, and then reflected by a beam splitter 419, while the pulsed laser L400b transmits the beam splitter 419. Therefore the pulsed laser L401 becomes a double pulse since the oscillation timing of the pulsed laser L400a is controlled, by the controller 420, to be a little before the oscillation timing of the pulsed laser L400b.

The flashlamp-pumped iodine laser oscillator 410a has an intracavity etalon 417a which functions as an oscillation line selector. In this embodiment, the oscillation lines of the flashlamp-pumped iodine laser oscillator 410a are adjusted to be three lines of the 2-1 transition, the 2-2 transition and the 2-3 transition by tilting the setting angle of the intracavity etalon 417a. The F′-F transition means that F′ is a total angular momentum quantum number of the upper level (²P_1/2) of the laser transition, and F is that of the lower level (²P_3/2) of the laser transition. Such a hyperfine structure is explained in, for example, “Hyperfine structure and collision parameters of the 1.315 um iodine laser transition studies by a frequency-controlled laser, J. Phys. D, Vol. 11, pp. 1303-1318 (1978)”. A typical iodine laser oscillator oscillates at 6 lines simultaneously.

In FIG. 17, the relative intensity of the water absorption line is shown as a function of wavenumber, which is stated in “Atmospheric propagation properties of various laser systems, Proceedings of SPIE Vol. 8380, 83800V, 2012.” The iodine laser oscillation lines are plotted in dotted lines in FIG. 17, which indicate that at the wavenumber of the 2-1 transition, the 2-2 transition or the 2-3 transition, the water absorption is much larger than the water absorption at the 3-4 transition, the 3-2 transition or the 3-2 transition. Therefore, the flashlamp-pumped iodine laser oscillator 410a is suitable to be used as the vaporization laser.

The transmissivity curve of the intracavity etalon 417a is shown in FIG. 18. Since the intracavity etalon 417a has a FSR (Free Spectral Range) of around 1 cm⁻¹, by adjusting the maximum transmissivity wavenumber to be near the 2-2 line, it is possible to suppress the oscillation of the lines of the 3-4 transition, the 3-3 transition and the 3-2 transition. The flashlamp-pumped iodine laser oscillator 410a selects the oscillation lines to be F′=2 (²P_1/2) for an upper level of iodine laser transitions. The amplifier 320 can effectively amplify the first pulse which has a large water absorption rate. Accordingly, the first pulse can effectively vaporize clouds or fog.

In this embodiment, the flashlamp-pumped iodine laser oscillator 410b can also have an intracavity etalon 417b (see FIG. 16). This enables the oscillation lines of the laser L400b to be the 3-2 transition, the 3-3 transition and the 3-4 transition by tilting the setting angle of the intracavity etalon 417b to have a high transmissivity near the 3-4 transition (see FIG. 19). The flashlamp-pumped iodine laser oscillator 410b selects the oscillation lines to be F′=3 (²P_1/2) for an upper level of the iodine laser transitions. Consequently, the laser L400b has a negligibly small water absorption (see FIG. 17). The amplifier 320 can effectively amplify the second pulse which has a small water absorption rate. The second pulse can effectively propagate through air. Therefore, the flashlamp-pumped iodine laser oscillator 410b is more suitable as the processing laser than the oscillator without using a wavelength selector such as the intracavity etalon 417b.

The present invention has the capability of making a hole in a target placed at a far distance away through a cloudy or foggy air. The present invention can make a hole in the body of an aircraft which may attack, and can force it to stop flying, or it can shoot it down.

While the invention has been particularly shown and described with reference to exemplary embodiments thereof, the invention includes various changes which do not negatively affect the purpose and benefits of the invention and is not limited to these exemplary embodiments.

From the invention thus described, it will be obvious that the embodiments of the invention may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended for inclusion within the scope of the following claims.

LASER PROCESSING METHOD AND LASER PROCESSING SYSTEM

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