JPS62122186A - Waveguide laser - Google Patents

Abstract

PURPOSE:To permit a highly efficient oscillation as well as to improve the cooling effect by forming a thin film having a small absorption loss at an oscillation wavelength on the surfaces of a pair of the opposing metal electrodes. CONSTITUTION:Zinc selenide (ZnSe), for example, is used as the material of thin films 3 and the thin films can be easily formed by sputtering and vacuum evaporation. When germanium (Ge) is used, the films 3 can be easily formed by plating as well. A waveguide path 4 to be used as the laser discharge path is encircled with metal electrodes 1 coated with the thin films 3 and a dielectric 2 and its section is formed into a configuration close to a square configuration for making the output strength distribution approach a circular distribution. Mixed gas of He, CO2 and N2 gasses and so on having a gas pressure of about 100-200torr, for example, is encapsulated in the waveguide path 4. A flat or concave totally reflecting mirror and a partially transmitting mirror are attached on both ends of the waveguide path 4 and the laser beam is outputted through the partially transmitting mirror. Thus, as the waveguide loss is small, a high- output oscillation is possible and furthermore, as the waveguide path wall is constituted of a metal having a high-heat conductivity, the cooling effect can be increased.

Description

Detailed Description of the Invention [Industrial Application Fields] The present invention relates to a laser that is useful in the field of laser processing for welding, cutting, etc., coherent optical communication, and detection of atmospheric pollutants, particularly in the field of small and high-speed laser processing. This invention relates to efficient waveguide gas lasers.

[Prior Art] A waveguide laser consists of a rectangular hollow waveguide surrounded by a pair of opposing metal electrodes for exciting high-frequency discharge in the lateral direction and a pair of opposing dielectric materials such as glass or alumina. Gods Suggested (U.S. Pat. No. 4,169,25
1 Specification, U.S. Patent No. 4,152.188''
ms＞.

Such a horizontal RF discharge pumped waveguide laser has the following features compared to a vertical □C discharge pumped waveguide laser.

■ It is small.

■Wide oscillation wavelength tuning range.

■ High efficiency (positive resistance discharge, no stabilizing resistance required).

■ Does not require high voltage.

■ Expected to extend the life of the seal.

On the other hand, apart from waveguide lasers with a metal-dielectric composite waveguide structure in which the metal electrode forms part of the waveguide wall, the waveguide is constructed entirely of dielectric material such as alumina or glass, and the metal electrode A method is also being considered in which RF discharge is performed without contact with the encapsulated scum (C, P).

Christenser, F. X., Powell, an.
dN, Djeu, IEEE, 1. Quantum
EIectronf, 11E-16,949 (198
0) ).

[Problems to be solved by the invention] In a waveguide laser in which a metal electrode forms part of the waveguide, the smaller the waveguide width or the longer the waveguide length, the more the waveguide loss becomes negligible. , high efficiency laser output cannot be obtained. In order to minimize waveguide loss, attempts have been made to construct a flat rectangular waveguide by separating the spacing between opposing electrodes, so that the electrodes do not contribute to the waveguide characteristics. Problems arise such as oscillation and the Doba beam becoming elliptical.

Waveguide type lasers, in which all of the waveguides are made of dielectric material, are characterized by more stable discharge and no deterioration of the metal electrodes due to sputtering or oxidation, but the cooling effect is is not advantageous.

Alumina is a dielectric material with relatively good thermal conductivity, but metals such as aluminum and copper have even higher thermal conductivity.
A structure in which the metal electrode forms part of the waveguide is expected to have a greater cooling effect. The thermal conductivities of glass, alumina, aluminum, and copper are 3 and 2 x 10-3, respectively.
cal/cm sec”c, 0.0G cal/cm
sec'C10, 487cal/cm sec℃, 0
, 923 cal/crn Scc°C. )\
Lilia has a thermal conductivity of 0.5cal/cm Sec℃
ζ is a Funara material that constitutes the waveguide instead of alumino or glass, but it is a toxic substance and poses problems in laser production, so it is avoided as a material for waveguide lasers. There is.

The present invention was devised to solve the problems of the prior art as described below, and aims to provide a waveguide laser that is capable of highly efficient oscillation and has an excellent cooling effect. There is something to do-C.

[Means for Solving the Problems] In the present invention, in a waveguide laser in which a hollow waveguide is formed by a pair of opposing metal electrodes and a pair of opposing dielectrics, the surface of the pair of opposing metal electrodes is By forming a thin film with low absorption loss at the oscillation wavelength, we have been able to reduce waveguide loss and achieve highly efficient oscillation even if the waveguide width is small.

The material used for the metal electrode must be one in which the absolute value of the complex refractive index is seven times larger than that of the dielectric, or the imaginary part of the abdominal refractive index is sufficiently larger than the real part. Examples include CIJ, Ag, Au, AI, etc.

The complex refractive index of each of these materials at a wavelength of 10.6 μm is Cu: 14.1-, i64.5, Ag: 1:
3.5-j 75.2, Au: 17.2-j56゜0
, AI: 20.5-. It is 158.6. These materials also have high thermal conductivity and can provide a large cooling effect.

Materials used for dielectrics include glass with a smooth surface, ceramics such as alumina with relatively good thermal conductivity,
Polymer resins such as fluororesins are used, which are suitable for gas containment.

The thin film formed on the surface of the metal electrode is a material with low absorption in which the imaginary part of the complex refractive index is sufficiently negligible compared to the real part in the oscillation wavelength band. For example, at a wavelength of 10.6 μm, ZnSe, Ge, NaCI, KCI, etc.
, KRS-5, CdTe, Si
, Z n S , P b F 2 and chalcogenide glass. All of these materials, which are important transmission media for electromagnetic waves in the light wave band, behave as dielectrics, but even if they are formed on a metal surface, they are RF discharges, so the discharge is stable.

In this way, by coating the metal electrode with a thin film of C that has low absorption, it not only functions as an electrode, but also forms a waveguide wall with low loss, and also prevents deterioration of the electrode surface while maintaining a high cooling effect. However, a smaller and more efficient waveguide laser can be obtained.

[Embodiment] FIG. 1 is an explanatory diagram of an embodiment of the present invention, and shows an outline of a waveguide cross section of a waveguide type laser.

1 is a metal electrode, 2 is a dielectric, 3 is a thin film formed by coating on the surface of the metal electrode l, and 4 is a hollow waveguide.

The metal electrode 1 is connected to a constant RF power source through a matching circuit to perform RF discharge in the lateral direction. The metal electrode 1 is made of copper, which has good thermal conductivity, for example.

As the dielectric 2, for example, glass with a smooth surface is used.

The thin film 3 is made of, for example, zinc selenide (ZnSe), and can be easily formed by sputtering or vacuum deposition. When germanium (Ge) is used, it can also be easily formed by plating.

The waveguide 4 as a laser discharge path has a metal electrode 1 coated with a thin film 3 and a dielectric body 2 surrounded by a body 2, and has a nearly square cross section (2ax2b) in order to make the output intensity distribution close to a circular distribution. It has a shape. In the waveguide 4, for example, there is a gas pressure of about 100 to 200 torr+ l(e, CO
, N The mixed dregs are made into Tsukijin. A flat or concave total reflection mirror and a partial transmission mirror are attached to both ends of the waveguide 40, and the laser beam is outputted through the partial transmission mirror.

In a waveguide laser, the laser output P is expressed as:

Here, T and T are the transmission in the mirrors at both ends of the waveguide,
1 2 is the pass rate, and when one mirror is a total reflection mirror (T, = 0), T
=2T. L L is both ends of the waveguide 2
cl' c2 is the coupling loss in the mirror, and if the plane mirror is placed sufficiently close to the end of the waveguide, 1. c≦1.5%. 1 is the waveguide length, A is the cross-sectional area of the moat, and when the waveguide cross-section is square (2a=2b), A= (0,49;L) 2π (4
) is given by g is the small signal gain, and according to the law of similarity in the case of pressure spread, g does not depend on the waveguide width. I
s is the saturation strength, which is proportional to the square of the pressure p and 2 of the waveguide width.
is inversely proportional to the power. Lw is waveguide loss. In the waveguide type laser shown in FIG. 1, this L- can be made smaller than in the conventional type, so a small and highly efficient laser can be obtained. The waveguide loss Lw in the rectangular waveguide is 2
The sum of the waveguide losses of the TE mode and the TM mote in the dimensional hollow slab waveguide gives an X+ value.

Figure 2 shows copper, glass, and zinc selenide (ZnS).
e) of various hollow slab waveguides constructed of copper coated with 'I'' Mo7-t (A, (:.

E) and TEoMoF'()J, r), shows the calculated values of transmission loss for F). In this way, the appropriate I
In a metal hollow waveguide ([mi, F) coated with a small absorption /iI film with a CI thickness, the ′rE moat and ′
It is shown that the transmission loss is reversed with that of the 1'M moat, or that both of the dielectric hollow waveguides (C, D) have a lower loss than the 'rE and TM motes (M, Miya)<+
,A,l1onHo,and S,Kawakam+
, 1EEE J, Quantum, 1(?ct
1.136 (1983)).

First, the waveguide loss of a conventional waveguide laser (in the case where the four films 3 are not present in FIG. 1) will be considered.

α(Ex
). Here, the input is the wavelength and the input is 10.6//m. n −jK is the determination of metal electrode 1 fTl1
Select copper based on Tl refractive index n - JK = 14.1 - j6m
m 4.5. n J K d is the complex refractive index of the dielectric material 2 that insulates the metal electrode l, and when glass is selected, n
-, 1Kd=2.1-, i 1.15.

2a and 2b are between the dielectrics 2 and 2, and the metal electrode 1
, 1. Further, Re represents the real part of a complex number. The first term in equation (5) is T of the copper slab waveguide.
The second term of the Eo moat waveguide loss corresponds to the TMo mote waveguide loss of the glass slab waveguide.

Y V On the other hand, the transmission loss α(E , ,) of Ell mote is α(
It is expressed as 1巳11). The first term of equation (6) is 1' of the copper slab waveguide.
The second term of the Mo mote waveguide loss corresponds to the Eo mote waveguide loss of the glass slab waveguide.

In a conventional waveguide laser with a transmission rate of 2 HL = 21', the E11 mode, which has a lower loss, becomes the propagation mode, and its loss is mainly evaluated by the loss of the 7M mode of the dielectric slab waveguide.

Next, the waveguide loss of a waveguide type laser coated with a thin film 3 as shown in FIG. 1 will be considered. As shown in Fig. 2, the transmission t@loss of the metal slab waveguide coated with a thin film with low absorption changes periodically depending on the film thickness of the oil film. The L is (q=I, 2. knee 3 ・ ・ ・)X
When satisfying X,
E Mote's transmission TFI loss α(+・:,,.

is expressed as α(1':,,). Here, nf is the refractive index of the thin film 3 coating the metal electrode l, and selenide 11'' lead is selected U'
n t ” 2.4. The first term in equation (8) is the waveguide loss of the TEo moat of the zinc selenide-included copper slab waveguide, and the second term is the waveguide loss of the ′I゛Mo moat of the glass slab waveguide. However, as can be seen from Fig. 2, the 11α changes sensitively to 1 momme of lI of 1114 lead selenide.

On the other hand, the transmission loss α(F, Y) of the EV mote <Ma, II
I+α(E,
, ). The first term in equation (9) corresponds to the waveguide loss of the TMo moat of the zinc selenide-included copper slab waveguide, and the second term corresponds to the waveguide loss of the TEo mote of the glass slab waveguide.

From the above, in the thin film coach ink waveguide type lasers of 2a and 2b, the lower loss El mode becomes the propagation mode, and the loss is mainly evaluated by the loss of the TE mode of the dielectric slab waveguide. In a thin film coated waveguide type laser, the waveguide loss with respect to the film thickness of the thin film 3 is around a minimum of 1 [11] and changes slowly, so the same holds true even if the film thickness deviates somewhat from equation (7).

The absolute value of the complex refractive index n −jK of the metal electrode is m
The larger m is, or the real part n of the complex refractive index is
The smaller the value is, the smaller the first step (1) in equation (9) becomes.In that sense, it is more advantageous to use silver (Ag) than copper.However, if the entire electrode is made of silver, Therefore, when silver is used, it is economical to interpose a silver thin film between the copper electrode 1 and the thin film 3.In addition to silver, the metal thin film material to be interposed may include silver. The use of chemically stable gold (Au) is also effective.

Moreover, on the surface of the metal electrode 1, two
By alternately stacking thin films with smaller absorption losses than those of different types, waveguide loss can be roughly reduced. In this case, when the thickness t of each thin film is selected to satisfy the following, as in the case of a single layer, the crystal can effectively reduce waveguide loss. Here, 011 is the refractive index of the thin film.

On the other hand, the larger the absolute value of the complex refractive index n-jKd of the dielectric, or the smaller the real part n of the complex refractive index is sufficiently smaller than the imaginary part K, the second term in equation (9) increases. becomes smaller. In Example 1, glass was selected as the material because of its smooth surface, but other dielectrics with smooth surfaces or dielectrics with relatively good thermal conductivity may have an absolute complex refractive index higher than that of these dielectrics. The surface of the dielectric body 2 may be coated with a thin film in which 11α is larger or the real part of the complex refractive index is sufficiently smaller than the imaginary part.

At a wavelength of 10.6 μm, the use of alumina or beryllia has been considered in order to reduce the second term in equation (5) in conventional waveguide lasers, but the thin film coated waveguide laser of the present invention In this case, there is no need to choose a material with a small r for n + , and there is a high degree of freedom in selecting the dielectric material for insulating between the electrodes, so glass or fluororesin can also be used.

Figure 3 shows the output power P (7
Show J calculation 1+ff. L-L-C52a:2b:1
．． 5mm, l=40cm, )? :1
1. 00ficm, I S =20 kw/
cTrI' (R,1,, Abrams an
d W, 1. lrldges, IEE:
J, (Juantum Eft・cLronf,
(lF,'-9,9/10 (1!373)). At this time, from equation (5), the waveguide I (9 lasers is 1.
According to equation (13), Lw=0.5% in the thin 11-tangle I-ring waveguide #I type laser of the present invention.

In Fig. 3, assuming that the coupling ti loss Lc is 0% and 1.5%, Lw + 1-, (: :0, 5.2.3.4.5
% (lα was omitted.

In formula (1), '3P1 T=, O is T
= C"-("L,-=-hand 2TAE-)-1(1w
+Lc) When (It), the output power P is the optimal output P
Then, pt P'=IsA[Fj--Ding-Fnuew+Lcr]2op
t.

It is expressed as

FIGS. 4 and 5 show the optimum output that is equal to the waveguide half width a (assuming 2a=2b) when the transmittance T of the output mirror is multiplied by ζ so as to satisfy equation (II). Figure 1 is 1. r
=O%, FIG. 5 shows lt =1. It is Fi%. Further, the saturation strength IS was set to Is = 11.25/a2kw/cyn' according to the law of similarity in pressure spread.

In FIGS. 4 and 5, the broken line indicates the optimum output of the conventional waveguide laser, and the solid line indicates the optimum output of the thin film coated waveguide laser according to the present invention.

The effects of the present invention become more pronounced as the waveguide width becomes narrower and the waveguide length becomes longer.

[Effects of the Invention] As described above, according to the present invention, the following remarkable effects are exhibited as compared to conventional waveguide lasers.

■ High output oscillation is possible because the waveguide loss is small.

■ The waveguide walls are made of metal with high thermal conductivity, which increases the cooling effect.

■ Deterioration of metal electrodes due to sputtering and oxidation can be suppressed.

■ Great flexibility in selecting the dielectric material that insulates between electrodes.

Another feature of the waveguide laser of the present invention is that it oscillates in a mode in which the electric field has a component perpendicular to the metal electrode.

[Brief explanation of drawings]

Fig. 1 is a cross-sectional explanatory diagram of one embodiment of the present invention, Fig. 2 is a graph showing calculated values of waveguide loss of TMo mode and TEo mode of various hollow slab waveguides, and Fig. 3 is transmission of output mirror. A graph showing the calculated value of output power versus rate, Figure 4 is L
A graph showing the calculated value of the optimum output power for the waveguide width when c=O%, and Figure 5 shows the calculated value for the optimum output power for the waveguide width when L(:=1.5%). This is a graph. 1: Metal electrode, 2: Dielectric, 3: FF film, 4: Hollow waveguide. Agent: Patent attorney Fujio Sato) <F -72+]v (A) (C) (E)
3 Illustration 4 +”D

Claims (8)

[Claims] (1) A hollow waveguide is formed surrounded by a pair of opposing metal electrodes for exciting high-frequency discharge and a pair of opposing dielectrics, and the surface of the pair of opposing metal electrodes has a waveguide at the oscillation wavelength. A waveguide laser characterized in that a thin film with small absorption loss is provided, and the thickness t of the thin film is set to satisfy t≈λq/4√(n^2_f-1). However, λ is the oscillation wavelength, and q is a positive odd number (1, 3, 5...
), n_f is the refractive index of the thin film. (2) The waveguide laser according to claim 1, wherein the width between the metal electrodes and the width between the dielectric materials are set to be substantially equal. (3) The waveguide laser according to claim 1, wherein the metal electrode is made of a material selected from Cu, Ag, Au, or Al. (4) The waveguide laser according to claim 1, wherein the dielectric is made of a material selected from glass, ceramic, or polymer resin. (5) The thin film is ZnSe, Ge, KCl, KRS-5
, NaCl, CdTe, Si, ZnS, PbF_2 or chalcogenide glass. (6) A metal thin film is interposed between the metal electrode and the thin film, and the absolute value of the complex refractive index is larger than that of the metal electrode, or the real part of the complex refractive index is sufficiently smaller than the imaginary part. A waveguide laser according to claim 1. (7) A thin film is provided on the surface of the dielectric material, and the absolute value of the complex refractive index is larger than that of the dielectric material, or the real part of the complex refractive index is sufficiently smaller than the imaginary part. The waveguide laser according to item 1. (8) A hollow waveguide is formed surrounded by a pair of opposing metal electrodes and a pair of opposing dielectrics for exciting high-frequency discharge, and the surfaces of the pair of opposing metal electrodes each have a different refraction. A multilayer thin film is provided with a small absorption loss at a certain oscillation wavelength, and the thickness t_i of each thin film is set to satisfy t_i≒λq/[4√(n^2_f_i−1)]. A waveguide type laser characterized by the following characteristics: However, λ is the oscillation wavelength, and q is a positive odd number (1, 3, 5...
), n_f_i is the refractive index of each thin film.