JPH03119773A - Waveguide-type gas laser - Google Patents

Abstract

PURPOSE:To execute a high-output oscillation whose waveguide loss is small and whose efficiency is high by a method wherein film thicknesses of coatings on wall faces of two pairs of opposite metal bodoes of a discharge path are made specific at each pair. CONSTITUTION:Out of opposite metal bodies 11, 12 which form a hollow region 10 to be used as a discharge path, one pair of metal bodies 11 are connected to a high-frequency power supply RF via a matching circuit and also function as electrodes for a high-frequency electric discharge in a transverse direction. A thickness d1 of thin films 13 which are coated on wall faces of the metal bodies and whose absorption loss is small at an oscillation wavelength band of a laser beam is set to nearly d1=lambdaq/(n -1)<1/2>. Wall faces of the other pair of metal bodies 12 are used as they are or are coated with thin films. When the films are coated, their thickness d2 is set to nearly d2=lambda(q-1)(n -1)<1/2>. In the formulas, lambda represents a wavelength of a laser beam, q represents a positive odd number and nf represents a refractive index of the coating film. Thereby, a high-output oscillation whose waveguide loss is small and whose efficiency is high can be executed.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a laser light source useful in fields such as laser processing, medical care, coherent optical application systems, detection of atmospheric pollutants, and inter-satellite optical communication. This invention relates to highly efficient waveguide gas lasers.

[Prior Art] A kW class high output gas laser has been developed for the purpose of laser processing such as welding and cutting, and attempts have been made to utilize laser light as thermal energy in production technology.

On the other hand, apart from these high-power gas lasers, small and highly efficient waveguide gas lasers are used not only for laser processing but also for medical treatment, coherent optical application systems, detection of atmospheric pollutants,
It is being considered in various fields such as inter-satellite optical communications.

A waveguide type gas laser is a laser whose discharge path is a hollow waveguide that guides laser light, and has the following characteristics compared to a normal Mira resonator type laser.

■It is compact because the discharge path can be made narrower and the laser gas can be easily sealed.

■Since the structure is simple and the discharge path can be formed from a material with good thermal conductivity, air-water cooling provides high cooling efficiency.

■Since lateral high-frequency excitation is easy, it is highly efficient and does not require high voltage.

■Since the laser gas pressure can be increased, the oscillation wavelength tuning range is wide.

■Easy to maintain and low cost.

Because of the various features described above, waveguide gas lasers with various structures have been proposed and are commercially available.

FIG. 4 shows a metal body surrounded by a pair of opposing metal bodies 41, which serve as mold edges for laterally exciting high frequencies, and a pair of opposing dielectric bodies 42, such as glass or ceramic, that insulate them. This is a waveguide type gas laser with a dielectric composite structure (US Pat. No. 4,169,251).

In addition, FIG. 5 shows that the hollow waveguide serving as the discharge path is made of a metal body 51.5.
2, these metal walls do not contact each other, and high-frequency waves are excited using a pair of opposing metal bodies 51.51 and 52.52 as electrodes ( U.S. Patent Specifications 4,805,18
No. 2).

[Problems to be Solved by the Invention] Generally, in a waveguide laser, the laser output P is expressed as follows. Here, T is the average transmittance of the mirrors attached to both ends of the waveguide, and Lc is the coupling loss in the mirrors at both ends of the waveguide. In a general waveguide gas laser,
A flat totally reflecting mirror and a flat partially transmitting mirror are used. 1 is the waveguide length, and A is the cross-sectional area of the transverse mode of the oscillated laser beam, which is given by A=(0,49a)2yr L21 when the waveguide cross-section is a square with side 2a. Is is the saturation intensity and go is the small signal gain. In the case of a waveguide type carbon dioxide gas laser, 2a=1.
511111, Is=25kw/cal.

The value go =O101cra-' has been reported.

(R1[. Abrans and w, 8.8ri
dges, IEEE J, QuanturaEl
ectron,,GE-9,940f1973)),
Lw is a waveguide loss, which increases as the waveguide width becomes smaller or as the waveguide length becomes longer. Generally, the longer the length of the discharge path, the larger the laser output P becomes in proportion to the length of the discharge path. However, in a waveguide type gas laser, the waveguide loss Lv cannot be ignored, and there is a limit to the laser output. Therefore, in order to obtain the laser output 11 with high efficiency and high power, the waveguide loss LW must be made as small as possible to avoid ζf.

By the way, in general, the waveguide loss of a rectangular hollow waveguide can be evaluated by the loss of 'f' E mode and T' M mode of a parallel plate waveguide.

First, the waveguide loss of the waveguide type laser having the metal-dielectric composite structure shown in FIG. 4 will be considered.

In FIG. 4, the electric field of the laser beam is parallel to the wall surface of the metal body 111 and the low-order mode is the E mode, and the electric field is parallel to the wall surface of the dielectric material 42 and the low-order mode is the E mode. I'll call you.

The waveguide loss α (E) in the E ffl mode is 3 (rrn) a' (111111 (dB/fO) (3) expressed above, where λ is the wavelength of the oscillated laser light and in the case of a carbon dioxide laser. Considering the following, λ = 10.6.μm. n-a J and C are the complex refractive index of the metal body-11, and the second iron is aluminum, nMj /c, 5 = 20.5
- j 5g, 6. na-jKd is metal body 4
The complex refractive index of the dielectric material 42 that insulates 1 is quartz glass, and d=2.0- j 0.07 for na j.
It is said that 2a and 2b are respectively between the dielectrics 42;
This is the waveguide width between the metal bodies 41. Further, Re represents the real part of a complex number.

Equation (3) The first term n corresponds to the waveguide loss of the TEo mode of the aluminum parallel plate type slab waveguide, and the second term corresponds to the waveguide loss of the TM mode of the silica glass parallel plate type waveguide.

On the other hand, the waveguide loss α(E;':) in EX mode is expressed as follows. The first term in equation (4) corresponds to the T Mo mode waveguide loss of the aluminum parallel plate waveguide, and the second term corresponds to the TE mode waveguide loss of the silica glass parallel plate waveguide.

From equations (3) and (4), it can be seen that in the waveguide gas lasers with the metal-dielectric composite structure of 2a and 2b, the intensity distribution of the output light of the laser is close to circular, the E ffl mode with lower loss, that is, the laser light The electric field oscillates in a mode parallel to the wall surface of the metal body 41, and the waveguide loss is mainly due to the waveguide loss of the 7M mode of the silica glass parallel plate waveguide (the second term in equation (3)). is evaluated.

Next, the waveguide loss of a waveguide type laser having an all-metal structure as shown in FIG. 5 will be considered. As in the case of Figure 4,
The lowest order mode in which the electric field of the laser beam is parallel to the wall surface of one metal body 51 will be referred to as E ffl mode, and the lowest order mode in which the electric field is parallel to the wall surface of the other metal body 52 will be referred to as E r mode.

The waveguide loss α(En) of E n mode is expressed as follows. n lI, J , , n, , -jK
, l are the composite refractive indexes of the metal body 51 and the metal body 52, which both carry aluminum here. 2a and 2b are the waveguide widths between the metal bodies 52 and between the metal bodies 51, respectively. Similarly, the ExX mode waveguide loss α(E2) is expressed as follows. In a metal parallel plate waveguide, the loss is extremely low in the TE mode where the electric field of the laser beam is parallel to the metal wall surface, but the loss is extremely high in the 7M mode where the electric field has a component perpendicular to the metal wall surface. become. Therefore, the waveguide loss of the all-metal structure waveguide laser shown in FIG.
It is mainly evaluated by mode waveguide loss.

In the waveguide type laser with the metal-dielectric composite structure shown in Fig. 4, E
Since the difference in waveguide loss between n mode and Eπ mode is large, it is easy to obtain output light in a straight line polarized in a certain direction. On the other hand, in the all-metal structure waveguide laser shown in FIG. 5, the metal bodies 51 and 52 are made of the same material, and 2a=2
In the case of b, the waveguide loss of Eπ mode and EX mode is the same, so in order to obtain linearly polarized light with an end wave in a certain direction, it is necessary to change the material of the metal bodies 51 and 52. Either a different material, a rectangular waveguide cross-section, or a Brewster window must be used. Furthermore, when comparing the waveguide loss between a waveguide laser with a metal-dielectric composite structure and a waveguide laser with an all-metal structure, the former has smaller waveguide loss if the waveguide width is the same. However, in an all-metal waveguide laser, the discharge path can be constructed entirely of metal bodies with good thermal conductivity, so high cooling efficiency can be obtained. Furthermore, since the discharge path can be made of the same material with the same coefficient of thermal expansion, it has the characteristic that it is less susceptible to thermal distortion and can provide a strong, reliable, and stable output.

In the conventional waveguide laser shown in Figures 4 and 5, a pair of metal bodies are used as electrodes to excite high frequency waves.
The metal wall surface is exposed to the laser gas in a plasma state.

Therefore, the metal wall surface may deteriorate due to sputtering. In a sealed CO2 laser, if the discharge path is made of a metal material that is easily oxidized, the CO□ in the laser gas will be split into CO and 0, making it impossible to create a long-life laser. Although copper has good thermal conductivity, it is susceptible to sputtering and oxidation, so it is not used in sealed CO□ lasers.

The present invention solves the problems of conventional waveguide gas lasers, and realizes a waveguide gas laser that has both the features of waveguide lasers with a metal-dielectric composite structure and an all-metal structure. Moreover, it is an object of the present invention to provide a waveguide f/I type gas laser with higher efficiency and higher output, which can further reduce waveguide loss than any conventional laser structure.

[Means for Solving the Problems] The gist of the present invention is to provide a waveguide type gas laser in which a discharge path is a space surrounded by the walls of two pairs of opposing metal bodies. is coated with a thin film that has low absorption loss in the oscillation wavelength band of the laser beam, and the wall surfaces of one pair of metal bodies and the wall surfaces of another pair of metal bodies are either non-contact or in contact with each other with an insulator interposed. , and the thickness of the film coated on the wall surfaces of the pair of metal bodies, d, satisfies the following, and the wall surfaces of the other pair of metal bodies are coated with a thin film as is or are coated with a thin film, and the thickness of the thin film is d2 is [where λ is the oscillation wavelength of the laser beam, and q is a positive odd number (
1, 3, 5...), nf is the refractive index of the thin film coated on the wall surface of the metal body].

5. Effect 7. According to the above configuration, at least one pair of metal wall surfaces are coated with a thin film that has low absorption loss in the laser oscillation wavelength band, so that waveguide loss is small and high efficiency and high output oscillation is possible.

Furthermore, by coating with a thin film, the metal wall surface is no longer exposed to the active laser gas in the plasma state, so a long-life sealed laser that is less susceptible to sputtering and oxidation can be realized. Furthermore, since the metal body with good thermal conductivity efficiently releases heat to the outside, cooling efficiency is high and stable output light can be obtained.

Generally, when a closed metal flat plate waveguide is coated with a thin film with low absorption loss, depending on the thickness of the thin film, the TE mode
The waveguide loss in the TM mode changes periodically as shown in FIG. Figure 4 shows the waveguide loss in the TE mode and TM mode with respect to the thickness of the germanium thin film when the aluminum wall surface is coated with a germanium thin film in an aluminum parallel plate waveguide with a waveguide width of 2 rmrn. .

As you can see from this figure, in a hollow waveguide surrounded by two pairs of opposing metals, if a thin film with low absorption loss is coated on the opposing fully curved wall surfaces to an appropriate thickness,
A specific mode polarized in a certain direction can have extremely low loss.

[Examples] Preferred embodiments of the present invention will be described below based on the attached drawings.

In Fig. 1, 11.12 is a hollow area 1 that becomes a discharge path.
These are opposing metal bodies forming 0.

Reference numeral 13 indicates 3WA, which has a small absorption loss in the oscillation wavelength band of the laser beam coated on the wall surface of the metal body 11. The metal body 11 is connected to a high frequency power source R via a matching circuit (not shown).
It also serves as an electrode that is connected to F and performs high frequency discharge in the direction of the groove.

The film thickness d· of the thin film 13 coated on the wall surface of the metal body 11 shall be satisfied. Here, 9 is a positive odd number (1°3
), nf is the refractive index of the thin film. At this time-
The lowest order mode in which the electric field of the light is parallel to the wall surface of the metal body 11 is called the EH mode, and the lowest order mode in which the electric field is parallel to the wall surface of the metal body 12 is called the E'S mode.
Waveguiding loss α(
The waveguiding loss α(E) of the En) and Eπ modes are respectively expressed as . Here, it is set to λ-10.6 μmn. In addition, 11fi+-JAl'm+ and IIn+tJ have 2
are the complex refractive indexes of the metal bodies 11 and 12, respectively, and for comparison with a conventional waveguide laser, aluminum is chosen for both here. Further, germanium is selected here for thin 1]u13 and nt=4. Gel manila 11 is a material with small absorption loss in the oscillation wavelength band of the carbon dioxide laser. Moreover, 2a and 2b are the waveguide widths between the metal bodies 12 and between the metal bodies 11, respectively.

Comparing equations (8) and (9), it can be seen that the E mode has lower loss, and the waveguide laser of the present invention oscillates in the E3 mode.6 Assuming that the waveguide widths 2a and 2b are equal, Comparing the waveguide losses in the conventional waveguide laser and the waveguide laser of the present invention, 2 (3), (5)
, (9) The ratio of the waveguide loss of the laser with the metal-dielectric composite structure shown in FIG. 5, the laser with the all-metal structure shown in FIG. 6, and the laser according to the present invention shown in FIG. 1 is 1.41x 10-': 1
．． 25: 5.54x 10-', which shows that the loss of the laser according to the present invention is the smallest.

In the above calculation, in order to compare the conventional waveguide laser and the waveguide laser of the present invention, the material of the metal body is
All made of aluminum. Even when copper is used as a material, there is almost no difference in the magnitude of loss between the conventional waveguide laser and the copper of the waveguide laser of the present invention. In the waveguide laser of the present invention, the wall surface of the metal body 11 coated with a thin film is not directly exposed to the laser gas, so copper having good thermal conductivity can be used. In particular, the @oxygen side has extremely good thermal conductivity, high cooling efficiency, and is resistant to distortion due to heat, so it is optimal as a material for the metal body in the waveguide laser of the present invention.

Further, it is possible to coat the wall surface of the metal body 12 with a thin film similar to the thin film 13. However, the film thickness d2 of the thin film coated on the wall surface of the metal body 12 at this time is
As shown in Fig. 4, the thin film coated on the wall surface of the metal body 12 has no effect on the loss, but the thin film coating has no effect on the loss. By this, the metal body 12
Since the wall surface of is no longer in direct contact with the laser gas,
The metal body 12 may also be made of copper, which has good thermal conductivity.

In addition to germanium, zinc selenide,
Examples include calcium fluoride, KH2-5, and chalcogenate glass. These materials can be easily coated by vacuum deposition, sputtering, or the like. In the case of germanium, electroplating can also produce cotissig on the wall surface of the metal body.

FIG. 2 shows another embodiment of the invention. In the embodiment shown in FIG. 1, the pair of metal bodies 11 and the outer pair of metal bodies 12 are not in contact with each other, but in the embodiment shown in FIG.
and the other pair of metal bodies 22 are in contact with each other via an insulator 24. This makes the waveguide a closed hollow region 20, making it easier to seal in the laser gas. Furthermore, a waveguide laser which is mechanically stronger than the waveguide laser shown in FIG. 1 can be obtained. Suitable materials for the insulator 24 include fluoride resins, polymer resins such as polyimide resins, ceramic materials such as alumina, and glass materials such as quartz because they have good insulation properties and are resistant to heat.

By the way, since the waveguide type laser according to the present invention has small waveguide loss, it is possible to increase the discharge path length for high output. A waveguide laser has a feature that the discharge path can be made small in diameter, but a long and thin laser is not necessarily a small laser and is short, but according to the present invention, as shown in FIG. 3,
The discharge path can be bent for miniaturization.

In FIG. 3, the metal body 31 coated with a thin film 33 that satisfies equation (7) is a flat plate, but the wall surface of the metal body 32 that is not coated with the thin film has a bent surface, which causes discharge. Hollow region 30 forming a channel
It has a structure in which it is folded back into a 0-shape. Also, the metal body 32 and thin TI! Metal body 31 coated with A33
is in contact with via an insulating material 34.

Consider the case where the discharge paths of the conventional waveguide laser having a metal-dielectric composite structure shown in FIG. 4 and the waveguide laser according to the present invention are bent at right angles. The oscillation mode of a conventional waveguide laser with a metal-dielectric composite structure is an E n mode in which the electric field of the laser light is parallel to the wall surface of the metal body 41 and has a component perpendicular to the wall surface of the dielectric body 42. be. In order to bend the discharge path at right angles, the wall surface of the dielectric 42 is tilted by 45° as in FIG. 3. The reflectance on the wall surface of the dielectric 42 tilted by 45° is:
It can be evaluated based on the reflectance of the TM wave incident on the dielectric wall surface from an angle of 45°.

On the other hand, in the oscillation mode of the waveguide laser according to the present invention, the electric field of the laser beam has a component perpendicular to the wall surface of the metal body 31 coated with the thin film 33 that satisfies equation (7), and This is EX mode, which is parallel to the wall surface. In order to bend the discharge path at right angles, the wall surface 45 of the metal body 32
° Tilt. The reflectance on the wall surface of the metal body 32 tilted at 45 degrees can be evaluated by the reflectance of a TE wave incident on the metal body wall surface from a direction obliquely at 45 degrees.

The reflectance of a TM wave incident on a dielectric wall surface at an angle of 45 degrees is only 4.2% when the dielectric material is quartz. On the other hand, the reflectance of a TE wave incident on the wall surface of a metal body from an oblique direction of 45 degrees is 98.5% when the metal body is aluminum.

The loss caused by bending the discharge path in this manner is much greater in conventional waveguide lasers having a metal-dielectric composite structure, and it is impossible to bend the discharge path for miniaturization. On the other hand, in the case of the waveguide type 8 laser according to the present invention, the loss caused by bending the discharge path is extremely small.

Especially with carbon dioxide laser, the small signal gain g. is large, so
In the waveguide type laser of the present invention, in which the waveguide loss Lw can be ignored, the effect of increasing the output by lengthening the discharge path is large, and the effect of loss due to bending this slanted discharge path is small, resulting in high output. Nevertheless, an extremely compact waveguide gas laser can be realized.

Note that, as described above, in order to prevent the influence of oxidation and sputtering on the wall surface of the metal body 32, it is possible to coat the wall surface of the metal body 32 with a material similar to the thin film 33. However, the thickness of the thin film at this time must satisfy equation (10).

Further, although FIG. 3 shows a case where the discharge path is bent by 90', the angle at which the discharge path is bent is not limited to 90 degrees.

[Effects of the Invention] As explained above, according to the present invention, the following remarkable effects are exhibited.

(1) Since waveguide loss is small, high efficiency and high output oscillation is possible.

(2) Since the discharge path can be made of a metal material with good thermal conductivity, a high cooling effect and stable output can be obtained.

(3) Since the discharge path can be bent, an extremely compact structure can be achieved even during high-output oscillation.

(4) Since the metal wall surface is coated with a thin film, there is little deterioration of the metal electrode, and dissociation of the laser gas can be suppressed, so a long-life sealed laser can be realized.

(5) Linearly polarized output light polarized in a fixed direction can be easily obtained without using a Brewster window.

[Brief explanation of drawings]

Fig. 1 is a schematic diagram showing a cross section of a discharge path of a waveguide type gas laser which is an embodiment of the present invention, and Fig. 2 is a schematic diagram showing a cross section of a discharge path of a waveguide type gas laser which is another embodiment of the present invention. FIG. 3 is a schematic perspective view of a waveguide type gas laser showing still another embodiment of the present invention, and FIG. 4 is an aluminum parallel plate waveguide with a waveguide width of 2n+11. A diagram showing the calculated values of waveguide loss in TE mode and 7M mode with respect to the thickness of the germanium thin film when coated with a germanium thin film. Figure 5 shows the discharge path of a conventional waveguide-type gas laser with a metal-dielectric composite structure. FIG. 6 is a schematic diagram showing a cross section of a discharge path of a conventional waveguide 1i'B type gas laser having an all-metal structure. In the figure, 10, 20, and 30 are hollow areas that serve as discharge paths, and 11
, 12, 21, 22, 31.32 are metal bodies, 13.23
．． 33 is a thin film, and 24 and 34 are insulators.

Claims (1)

[Claims] 1. In a waveguide gas laser whose discharge path is a space surrounded by two pairs of opposing metal body walls, the oscillation wavelength of the laser beam is located on at least one pair of metal body walls. The wall surfaces of one pair of metal bodies and the wall surfaces of another pair of metal bodies are either not in contact with each other or are in contact with each other with an insulator interposed therebetween; The thickness d_1 of a pair of films coated on the wall surfaces of the body satisfies d_1≒λq/(√n_f^2-1), and the wall surfaces of the other pair of metal bodies are coated with a thin film or are coated with a thin film. The thickness of the thin film d_2 is d_2≒λ(q-1)/√(n_f^2-1) [where, λ is the oscillation wavelength of the laser beam, and q is a positive odd number (1, 3
, 5...), n_f is the refractive index of a thin film coated on the wall surface of a metal body]. 2. The waveguide type gas laser according to claim 1, wherein the wall surfaces of the pair of metal bodies coated with the thin film having a thickness d_2 are curved surfaces. 3. The waveguide type gas laser according to claim 1, wherein the material of the metal body coated with the thin film is oxygen-free copper.