JPH04361582A - Q-switched two-frequency co2 laser device - Google Patents

Abstract

PURPOSE:To obtain a Q switched CO2 laser device which is able to stably output laser rays of two wavelengths for a long time from the same excitation space on the same optical axis using a single CO2 laser medium. CONSTITUTION:In a Q switched CO2 laser device, laser rays of two wavelengths are oscillated on the same axis through a composite resonator which uses a single partial transmission output mirror 2 and the primary diffracted light of two diffraction gratings 4 and 5 whose rotation axes cross each other at a right angle and a beam polarized beam splitter 6, and laser rays of one wavelength is oscillated as constantly synchronized with laser rays of the other wavelength through a loss control element 6 and the monitoring of zero-order diffracted light, whereby laser rays of two wavelengths can be obtained from a single laser medium on the same optical axis at the same time.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention is a pulsed CO2 laser that coaxially and stably oscillates at two wavelengths from the same excitation space, which is required in photochemical reactions using lasers such as laser isotope separation. Q switch CO to get light
2 regarding a laser device.

0002

[Prior Art] Laser photochemical reactions that utilize the monochromaticity and high brightness of laser light are a new field of application for lasers because they enable the synthesis of organic substances and the separation of isotopes that could not be obtained using conventional reaction techniques. Currently, extensive research is being conducted. Among these, the separation and enrichment of uranium-235 and carbon-13 isotopes using a CO2 laser has increased expectations for the realization of this industrial process due to the remarkable progress of CO2 laser technology in recent years. A laser photochemical reaction using a CO2 laser utilizes resonance absorption excitation and dissociation of infrared multiphotons of molecules, and the laser characteristics require pulsed light having a high peak output. Further, the interval between the excited levels of a molecule becomes narrower as the excited level becomes higher, that is, the resonance absorption wavelength shifts to the longer wavelength side. Therefore, by simultaneously using multiple wavelength light that excites the ground level to the intermediate excited level with short wavelength light and further dissociates with long wavelength light, the dissociation reaction efficiency is significantly improved compared to the case of single wavelength light. It is known.

In order to meet such demands, transversely excited atmospheric pressure operation (Transversely E
xcited at Atomosphere pr
A CO2 laser (hereinafter referred to as TEA) is used. In a TEA laser, since the laser medium is at near atmospheric pressure, the probability of collision between the laser media is high, and the time constant is 1 ns or less. On the other hand, TEA
The pulse time width of a laser is 50 to 100 ns, so if you try to obtain two or more wavelengths from the same space in the laser medium, competition will occur between the oscillation wavelengths within the pulse time, and only the wavelength with the highest gain coefficient will oscillate. Other wavelengths have problems in that they do not oscillate or their output is unstable.

Therefore, as a method for obtaining multiple wavelengths from a single TEA CO2 laser device, Sov. J. Quant
um Electron. Vol. 15 No. 5
P689-691 (1985) proposes a method in which the laser medium space is divided into three locations and three separate wavelengths are extracted from each space using a diffraction grating and an output mirror. However, this method has problems in that the oscillation efficiency is low because the entire laser medium space cannot be used effectively, and the oscillation optical axes are not the same.

[0005] JP-A-1-96977 discloses a multi-wavelength synchronous TEA CO2 laser oscillation method. In this method, a plurality of laser media having different laser gas compositions are excited by a single switching element, and laser beams of different wavelengths are simultaneously obtained from each laser medium. Although this method is effective in obtaining laser beams of multiple wavelengths simultaneously, it requires multiple laser media, which increases the size of the device, and as in the previous example, there is a problem that the oscillation optical axes are not the same. .

[0006]J. Appl. Phys. Vol. 57
No. 7 p. 2654-2655 (1985) proposes a method of obtaining two wavelength laser beams coaxially and synchronously from a single laser medium. The laser device uses a diffraction grating to separate the optical axis of each wavelength outside the laser medium, and is configured to have a separate laser resonator for each wavelength. This method suppresses oscillation by using a loss element such as a supersaturated absorber for the wavelength placed in a laser resonator for the wavelength with a high gain coefficient, and suppresses competition between wavelengths to prevent simultaneous oscillation of one wavelength from the other. It is an encouragement. In this method, the wavelength of CO2 laser is 10.6μ as two wavelengths.
Simultaneous oscillation can be obtained between relatively distant wavelengths in the m band and 9.6 μm band, but with the TEA CO2 laser, it is impossible to sufficiently suppress competition between two relatively close wavelengths within each oscillation wavelength band. Therefore, there was a problem that simultaneous oscillation could not be obtained. Furthermore, since the oscillation timing fluctuates in response to minute changes in laser gain, stable simultaneous oscillation during long-term continuous operation is difficult with a loss element having a fixed amount of loss.

[0007] Proc.
eedings of international
Symposium on Isotope Sepa
ration and Chemical Excha
nge Uranium Enrichment. L-
1. (1990) discloses an apparatus in which optical axes of two wavelengths are separated using the wavelength dependence of the diffraction angle by a diffraction grating, and each laser resonator is constructed independently. This method is effective for obtaining coaxial two-wavelength laser light from a Q-switched CO2 laser. However, when the two wavelengths are close to each other, the difference in diffraction angle becomes small, and therefore the separation angle between the optical axes of the two wavelengths becomes small. Therefore, in order to configure two resonators independently without interfering with each other, it is necessary to increase the propagation distance of the laser beam until the two optical axes are separated by the diameter of the total reflection mirror of the laser resonator. Inevitably, the resonator length must be increased. As a result, the equipment becomes larger, the laser pulse time width increases,
Problems arise in that resonator loss increases and resonator alignment becomes unstable and complicated.

[0008]

[Problems to be Solved by the Invention] An object of the present invention is to provide two wavelength pulsed CO2 laser beams with a high peak output required in the field of laser photochemistry such as laser isotope separation using a simple configuration. Highly reliable Q
An object of the present invention is to provide a switched CO2 laser device.

[0009]

[Means for Solving the Problems] The present invention provides an apparatus for generating pulsed CO2 laser light using a Q-switch, in which a single partially transmitting output mirror and the primary order of two diffraction gratings having rotation axes orthogonal to each other are used. First and second wavelength laser beams whose polarization directions are orthogonal to each other are obtained independently by a composite resonator using folded light, and the two wavelength laser beams are combined by a polarizing beam splitter placed in the resonator, Two wavelength pulsed laser beams are generated on the same optical axis from a single laser medium, and two
The oscillation timing of the laser beam of the wavelength is set by a controllable loss element for the wavelength placed in the laser resonator of either wavelength, and simultaneous oscillation of two wavelengths is realized. By constantly monitoring the oscillation timing of each wavelength using the 0th order diffracted light of the diffraction grating, and dynamically controlling the amount of loss in response to the error from the simultaneous oscillation timing, the Q This is a switched CO2 laser device.

0010

[Operation] The present invention will be explained in detail below. Figure 2 shows CO2
FIG. 2 is a simplified schematic diagram showing only the upper and lower levels of the laser in order to explain the energy levels of the laser. CO2 molecules have symmetric stretching mode n1, bending mode n2
, has three vibrational modes, an asymmetric mode n3, and its vibrational energy level is expressed as (n1 n2m n3 ). 10.6μ, the typical oscillation wavelength of CO2 laser
For oscillation in the m band and 9.6 μm band, the rotation level in the (0 00 1) level is the upper laser level, and the (1 0
0 0) and (0 20 0) levels to the rotational level, laser oscillation is obtained. Since each vibrational level has a large number of rotational levels and a large number of oscillation lines, it is possible to perform laser oscillation by selecting a wavelength using a wavelength selection element such as a diffraction grating. Here, the relaxation rate between rotational levels increases in proportion to the pressure. Therefore, in a laser such as a TEA laser that operates at a laser gas pressure of about atmospheric pressure, relaxation occurs within the rise time of the laser pulse, so efficient laser oscillation can be obtained when oscillating at a single wavelength. However, when attempting to obtain oscillation of multiple wavelengths from the same laser medium, competition occurs between the transitions of each wavelength due to relaxation between the respective rotational levels, making it impossible to obtain stable multi-wavelength oscillation. on the other hand,
In the Q-switched CO2 laser used in the present invention, the laser gas pressure used for oscillation is about several tens of torr. At this level of gas pressure, the rotational level relaxation does not end within the time of the Q-switch laser pulse width (100 to 300 ns).
Simultaneous oscillation of two wavelengths is possible not only between different oscillation wavelength bands such as the μm band and 9.6 μm band, but also between relatively close wavelengths in each oscillation wavelength band.

FIG. 1 is a block diagram of the present invention. The laser resonator includes a first resonator that oscillates a laser beam 10 of a first wavelength, which is composed of a partially transmitting output mirror 2 and a diffraction grating 4, and a second cavity that is composed of a partially transmitting output mirror 2, a polarizing beam splitter 6, and a diffraction grating 5. It is composed of a second resonator that oscillates a laser beam 12 of the same wavelength.

First, coaxial two-wavelength laser beam oscillation from the first laser medium will be explained. Diffraction gratings 4 and 5 perform wavelength selection by rotating around axes parallel to the grating grooves, and their respective rotation axes are perpendicular to each other. A diffraction grating is an element that diffracts light in wavelength-dependent directions ranging from 0th order to higher orders. In the present invention, laser oscillation is obtained by resonance of first-order diffracted light. Further, laser light obtained from a laser resonator using diffraction by a diffraction grating is polarized in a direction perpendicular to the grating grooves. Therefore, the laser beams 10 and 12 resonated by the first-order diffracted lights of the diffraction gratings 4 and 5 become linearly polarized beams orthogonal to each other. Here, in FIG. 1, the diffraction gratings 4 and 5 have rotation axes perpendicular and parallel to the plane of the paper, respectively, so that 10 and 1
The polarization directions of 2 are parallel (P wave) and perpendicular (to the plane of the paper), respectively.
S wave).

The polarizing beam splitter 6 is an element that separates light of a specific wavelength using polarization.
Thin Film Polarizer
, hereinafter referred to as TFP), and polarizing prisms such as Glan-Thompson type and Wollaston type. TFPs for CO2 lasers are made of a ZnSe substrate coated with a thin film. Figure 3 shows the transmittance TP for CO2 lasers and the transmittance TP for P waves and S wave It shows the wavelength dependence of the reflectance RS. As is clear from the figure, the wavelength is 0.4 μm
It can be seen that by shifting to the longer wavelength side, TP decreases to about 50%, whereas RS has an almost constant high reflectance. Therefore, by using this TFP and two diffraction gratings, it is possible to obtain laser oscillation by independently selecting the laser beam 10 in the vicinity of 9.5 μm and the laser beam 12 within the range of approximately 0.4 μm. Furthermore, since the two wavelength laser beams are combined on the same optical axis within the laser medium 1, the two wavelength laser beams can be obtained coaxially from a single laser medium. Here, TP and RS are about 98%, and the laser beam 1
There will be a loss of about 2% for the 0 and 12 resonators, but the transmittance of the partially transmitting output mirror 2 (30 to 7
The loss is sufficiently small compared to 0%) and does not interfere with laser oscillation. Furthermore, the separation angle of the optical axes of the two wavelengths separated by TFP is 180, where n is the refractive index of the TFP substrate.
.degree.-2 tan-1 (n), and for ZnSe substrates used in CO2 lasers, the separation angle is approximately 50.degree., or this angle is wavelength independent. Therefore, compared to a method that uses wavelength-dependent diffraction angles to separate the optical axes, there is no need to lengthen the laser resonator and separate the optical axes even when selecting two wavelengths that are close to each other, and the device can be made more compact. It is possible.

Next, simultaneous oscillation of two wavelength laser beams will be explained using FIG. 4. In a two-wavelength laser, the laser gain coefficient for each wavelength is generally not uniform; for example, CO2
9.55μ for laser oscillation wavelength group in the 9.6μm band
The gain coefficient is highest near m(9p(20)) and decreases as the wavelength is longer or shorter. Figure 4 (
As shown in A), the delay time from Q switching to the start of oscillation is short for wavelengths with large gain coefficients (G1), and long for wavelengths with small gain coefficients (G2).
Simultaneous oscillation of two wavelengths cannot be obtained. Therefore, as shown in Fig. 4(B), the loss coefficient L for the oscillation wavelength with a large gain coefficient is
It is possible to obtain two-wavelength simultaneous oscillation by adding a loss element that gives . According to the present invention, as shown in FIG.
The variable loss element 7 easily gives the loss L to the first resonator without affecting the second resonator.
can be added, and simultaneous oscillation of two wavelength laser beams can be obtained.

Furthermore, during long-time laser operation, the delay time changes from time to time due to fluctuations in the gain coefficient caused by minute changes in the discharge excitation density, etc. Therefore, in order to obtain stable simultaneous oscillation of two wavelengths, it is necessary to It is necessary to constantly monitor the oscillation delay time and dynamically control the amount of loss in response to changes in the delay time. According to the present invention, if the intensity of the 0th order diffracted light 13 from the diffraction gratings 4 and 5 is detected using a photodetecting element 9 such as CdHgTe,
It is possible to monitor changes in delay time without affecting laser resonance. Therefore, the delay time is monitored using a waveform analyzer 8 such as a digital oscilloscope, and the loss amount of the variable loss element 7 is adjusted at any time by the loss amount control device 14 in response to the error from the set delay time when two wavelengths are oscillated simultaneously. control, and always stable simultaneous oscillation of two wavelengths is possible. The amount of loss can be controlled by controlling the pressure of a gas that exhibits supersaturated absorption for light of a specific wavelength. For example, wavelength 9
．． Freon 21 (CFC) is used for laser light in the 6 μm band.
l2H) is suitable.

[0016] Here, the CO2 laser's 9.6 μm
Although the simultaneous oscillation of two wavelengths in the vicinity has been described, it goes without saying that wavelength selection in other CO2 laser wavelength ranges is possible as long as TFP coating is possible. As a loss control method, pressure control of the supersaturated absorbing gas has been mentioned, but for the purpose of the present invention, any element that can control the amount of loss for the CO2 laser wavelength used will suffice, so NaCl
The transmittance may be controlled by rotating a laser beam transmitting material such as a plate, or a loss element such as a Fabry-Perot etalon or a diaphragm may be used. In addition, in Q-switched CO2 lasers, a rotary chopper is the most preferable Q-switch device from the viewpoint of high-speed switching, high durability, and ensuring long-term stability of resonator alignment. It may be an element, a rotating mirror, or the like.

Furthermore, in the above explanation, a case was described in which two wavelength laser beams are oscillated simultaneously, but according to the present invention, since the oscillation timing can be freely set by loss control, the time interval between two wavelength laser pulses can be set arbitrarily. Needless to say, it is also possible to set up and operate stably.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an embodiment of a two-wavelength simultaneous oscillation Q-switched CO2 laser device according to the present invention. The first resonator consisting of a partially transmitting output mirror 2 and a diffraction grating 4 has a wavelength of 9.55 μm.
(9P(20)) so that the laser beam 10 oscillates as P-polarized light.
, the second resonator consisting of the diffraction grating 5 has a wavelength of 9.62 μm (
9P(28)) laser beams 12 are adjusted to oscillate as S-polarized light, and the resonator length is 150 cm. Further, in order to synchronize the oscillation timing, a container in which a supersaturated absorbing gas Freon 21 whose pressure can be controlled is sealed is placed as a variable loss control element 7 in the first resonator which oscillates the laser beam 10 having a large gain coefficient. Diffraction grating 4
, 5 are blazed at 150 lines/mm, and the rotation axes for wavelength selection are orthogonal to each other. In addition, the TFP as the polarizing beam splitter 6 has a wavelength of 9.55 μm.
The transmittance TP for P waves is 98.0%, and the reflectance RS for S waves is 98.5%. RS here
98 has almost the same value even if the wavelength is shifted by about 0.4 μm, so even at a wavelength of 9.62 μm, 98
．． It has a high reflectance of 5%. The partially transmitting output mirror 2 is a concave mirror with a radius of curvature of 10 m, and its transmittance is 3 at each wavelength.
It is 0%.

Laser medium 1 is CO2 in a glass tube:
A mixed laser gas of N2:He is sealed at a total pressure of 90 torr, and the laser medium length is 50 cm. The laser gas is supplied by a roots blower (not shown) at 20
It is circulated at high speed in the direction of the laser optical axis at a linear velocity of 0 m/s. Further, the laser medium and the outside air are shielded from each other by a ZnSe window coated with anti-reflection coating for the oscillation wavelength. High-frequency discharge is used as the excitation method, and pulse modulation is performed in synchronization with the timing of the Q switch. The maximum repetition frequency of the pulses is 10kHz.

The Q-switch device 3 is a rotary chopper. The rotary chopper has 12 slits with a width of 2 mm on the circumference of an aluminum disk, and has a maximum rotation speed of 70,000 rpm.
It rotates at high speed. Further, in order to perform high-speed Q-switching, a confocal telescope 16 is provided, and Q-switching is performed by chopping the laser beam at its focal position.

The pulse time waveform of each wavelength is observed using a digital oscilloscope 8 using CdHgTe as the photodetector element 9 to detect the 0th order diffracted lights 11 and 13, and using a monitor signal from a rotating chopper as a sweep trigger. The loss amount control device 14 detects the delay time from Q switching to the start of oscillation based on the signal from the digital oscilloscope.
The variable loss element is controlled so that the laser beam oscillates at the same time as the laser beam 12 starts oscillating. The supersaturated absorbing gas freon 21 as the variable loss element 7 has a wavelength of 9.6
ZnSe coated with anti-reflection for μm band light
Pressure 10 to 3 in a glass container with a window (container length 5 mm)
It is sealed at 0 torr. Further, the pressure of the gas is controlled with an accuracy of 0.1 torr by a pressure regulating valve 15 controlled by an electric signal from the loss control device 14.

Using the above device, the wavelength of 9.55 μm, 9
．． When simultaneous oscillation of 62 μm two-wavelength laser light was performed,
Total energy 30mJ, each pulse time value width 200
ns two-wavelength laser light was stably obtained for more than 8 hours.

[0023]

As explained above, according to the present invention, in a Q-switched CO2 laser, two independently variable wavelengths can be generated on the same optical axis from a single laser medium using a compact device configuration. It has the advantage that laser light can be stably obtained for a long time at the same time.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram showing the present invention.

FIG. 2 is a schematic diagram of energy levels of a CO2 laser.

FIG. 3 shows the wavelength dependence of the transmittance TP and the reflectance RS for P waves and S waves of a TFP for a wavelength band of 9.6 μm.

FIG. 4 is an explanatory diagram of two-wavelength oscillation timing.

[Explanation of symbols]

1 CO2 laser medium 2 Partial transmission output mirror of laser resonator 3 Q switch device 4, 5 Diffraction grating 6 Polarization beam splitter (TFP) 7 Variable loss element 8 Laser pulse waveform analyzer 9 Photodetection element 10 Resonates with first-order diffracted light Laser light of first wavelength (P polarized light) 11 0th order diffracted light of laser light of first wavelength 12 1
Laser light of the second wavelength (S polarized light) that resonates with the diffracted light of the second wavelength 13 Zero-order diffracted light of the laser light of the second wavelength 14 Loss control device 15 Pressure control valve 16 Telescope

Claims (3)

[Claims] Claim 1: In a CO2 laser device that extracts pulsed laser light using a Q switch, a first laser resonator that oscillates at a first wavelength and includes a partially transmitting output mirror and a first diffraction grating; a second laser resonator that oscillates at a second wavelength, which is composed of an output mirror of the device and a second diffraction grating having a rotation axis perpendicular to the rotation axis of the first diffraction grating, and lasers at the first and second wavelengths. A polarizing beam splitter that coaxially combines light and one Q placed anywhere within the laser resonator that coaxially combines laser beams of the first and second wavelengths.
A two-wavelength oscillation Q-switched CO2 laser device comprising a switch device and coaxially obtaining two-wavelength laser light from a single excitation space. [Claim 2] A loss control element is provided in either the first or second laser resonator to provide a loss to the laser transition at the wavelength, and the amount of loss can be controlled.
2. The dual-wavelength oscillation Q-switch CO2 according to claim 1, wherein the delay time from when the switch is released to the start of laser oscillation for each of the first and second wavelengths is set to the same value to obtain two wavelength laser beams at the same time. laser equipment. 3. In the first and second laser resonators, laser oscillation is performed using the first-order diffracted light of the diffraction grating, and the delay time from the time the Q switch is released to the start of each laser oscillation is independently determined by the zero-order diffracted light. It is constantly detected by the photodetecting element of 2
A claim characterized in that two wavelength laser beams are always obtained simultaneously by changing the amount of loss of the loss control element according to claim 2 as needed in response to an error from a delay time setting value for obtaining wavelength simultaneous oscillation. 2. The two-wavelength laser oscillation Q-switched CO2 laser device according to item 1.