JPH04307979A - Laser resonator - Google Patents

Abstract

PURPOSE:To make small an angle between both surfaces of a partial reflection mirror, which has a non-reflection coat applied on its one face, of a laser resonator and to obtain the laser resonator, whose alignment or the like is facilitated. CONSTITUTION:Such a partial reflecting mirror 5 as that the angle theta its non- reflection coat surface (a surface A) makes with a partial reflection coat surface (a surface B) satisfies the following conditions, is provided (Provided that, the surface B and an optical axis have a vertical relation to each other.) ¦theta¦>tan<-1> (x/8L). Provided that, X(cm) is a beam diameter in a direction, in which the optical axis is slant to the normal of the surface A, and L(cm) is a resonator length.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a laser resonator that creates an oscillation state by reciprocating light.

0002

2. Description of the Related Art FIG. 8 is a block diagram showing a conventional laser resonator disclosed in, for example, Japanese Patent Laid-Open No. 1-173680. In the figure, 1 is a laser medium, 2 and 3 are windows for guiding the laser beam from the laser medium 1, 4 is a total reflection mirror that totally reflects the laser beam, 52 is a partial reflection mirror that partially transmits the laser beam, 6 is a wavelength selection element for limiting the wavelength of laser light to a specific range.

Next, the operation will be explained. When a laser medium 1 such as an excimer is excited by a discharge, strong light is emitted through the window 2.
, 3 to the outside. A total reflection mirror 4 and a partial reflection mirror 52 are installed on both sides of the laser medium 1, respectively. Therefore, while the light travels back and forth between them, the light is intensified and becomes laser light. A portion of the laser light is then guided to the outside of the partial reflecting mirror 52. Note that the wavelength of the laser beam is limited to a specific range by the wavelength selection element 6.

Incidentally, in the wavelength distribution of laser light emitted from such a laser, a concavity as shown in FIG. 9 may occur. In the example shown in FIG. 9, 2.2pm to 2.3p
A dent occurs every m. These recesses are partially reflective mirror 5.
This is caused by the action of 2.

Therefore, in the device disclosed in the above-mentioned publication, the shape of the partial reflecting mirror 52 is made into a wedge shape as shown in FIG. ing.

[0006] Also, IEE Journal
of Quantum Electronics (IEEE
Journal of Quantum E
electronics) Volume QE-16 No. 2 (19
February 1980) P. 235~P. 241 discloses applying a non-reflective coating to the A side, and Optics Letters Vol. 8 No. 5 (19
May 1983) P. 274~P. No. 276 discloses that the partially reflecting mirror 52 is wedge-shaped.

[0007]

[Problems to be Solved by the Invention] Since the conventional laser resonator is constructed as described above, when the partial reflecting mirror 52 is formed into a wedge shape, alignment is required to bend the direction of light like a prism. (Adjusting the total reflection mirror 4 and the partial reflection mirror 52 so that they are perpendicular to the optical axis of the resonator) is difficult.

In particular, when using a visible light laser to confirm the alignment and beam emission direction of the total reflection mirror 4 and the partial reflection mirror 52, which are applied to an invisible light laser, the wavelengths of both laser beams are Because of the difference, the refraction angle in the partial reflecting mirror 52 is different, and even if alignment or the like is performed using visible laser light, there will be poor adjustment with respect to invisible laser light.

[0009] Furthermore, when a non-reflective coating is applied to one side (side A), it is difficult to create a completely non-reflective state (in reality, about 0.5% reflection occurs).
However, a problem with high gain lasers such as excimer lasers and dye lasers is that they oscillate due to small reflections and cannot exhibit sufficient effects.

As a countermeasure for the above-mentioned problem, a resonator is not formed by the A-plane coated with an anti-reflection coating, that is, so that the reflected light from the A-plane does not return to the laser medium 1.
The idea of tilting the A plane is shown in the above publication.

However, in order to prevent the reflected light from the A surface from entering the window 2, the distance between the partial reflecting mirror 52 and the window 2 should be d, the inclination angle of the partially reflecting mirror 52 should be θ, and the beam diameter should be When x, it is necessary to satisfy the relationship θ>x/2d. For example, when typical values of x=2cm and d=5cm are adopted, θ>0.
It becomes a large value of 2 rad. In this case, the partial reflecting mirror 52 has a shape almost like a prism, and the light is greatly refracted, resulting in a serious problem in practical use.

The present invention was made to solve the above-mentioned problems, and provides a laser resonator that can be easily aligned using a separate light source while preventing the wavelength distribution of the laser beam from becoming concave. The purpose is to obtain. Another object of the present invention is to obtain a laser resonator that can emit laser light with a narrower wavelength width by actively utilizing the depressions that occur in the wavelength distribution.

[0013]

[Means for Solving the Problems] In the laser resonator according to the invention as set forth in claim 1, the angle θ between the normal to the anti-reflection coated surface of the partially reflecting mirror and the optical axis of the resonator is |θ|t
an-1(x/8L) (x is the beam diameter in the direction in which the optical axis is tilted with respect to the normal line,
L is the length of the resonator).

Further, in the laser resonator according to the second aspect of the invention, one surface of the partially reflecting mirror is perpendicular to the optical axis of the resonator, and the angle θ between the normal of the other surface and the optical axis is | θ｜〈
tan-1(x/8L), and further, the thickness t of the partially reflecting mirror is t>λ2/2n
λw (λ is the center wavelength of the laser, n is the refractive index of the substrate of the partially reflecting mirror, and λw is the intended wavelength width of the laser).

[0015]

[Operation] In the partial reflecting mirror according to the invention as claimed in claim 1, the angle forming the wedge shape is made smaller, and the difference between the refraction angle of the laser beam and the refraction angle of the light from another light source used for alignment is made smaller. Enables alignment using light from a separate light source.

[0016] Furthermore, the thickness of the partially reflecting mirror according to the second aspect of the invention is adjusted, and the partially reflecting mirror acts to form two resonators between one surface and the other surface thereof and the total reflecting mirror, so that the wavelength of the laser Make the width narrower.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. In FIG. 1, reference numeral 5 denotes a partial reflecting mirror having a shape and structure as described below, and the other parts are denoted by the same reference numerals and are the same as those shown in FIG. 8.

Next, the operation will be explained. Here, a case will be described in which an excimer excited by discharge is used as the laser medium 1. When the laser medium 1 is discharged, strong light is emitted from the windows 2 and 3. Then, the light is resonated and intensified by the total reflection mirror 4 and the partial reflection mirror 5, and becomes a laser beam. At this time, the wavelength width of the laser beam is spread by about 400 pm, but the wavelength selection element 6 narrows the wavelength width to 1/100 or less.

At this time, in order to prevent concavities in the wavelength distribution, it is necessary to apply an anti-reflection coating to one side (side A) of the partial reflection mirror 5 and to make the partial reflection mirror 5 wedge-shaped as already described. As stated above. Also, the angle formed by the wedge shape (
As already mentioned, if the wedge angle is too large, there is a practical problem.

[0020] Here, the minimum angle required as the wedge angle is determined. Therefore, when we investigated the cause of the dent in the wavelength distribution, we found that in FIG. , the resulting laser beam is shown as C), a resonator consisting of the B surface of the partial reflecting mirror 5, and the total reflecting mirror 4 (this resonator is called resonator D, and in FIG. It has become clear that this is caused by interference between the lights emitted by the laser beams (denoted by D) and the laser beams.

FIG. 2 schematically shows the oscillation frequencies of two resonators C and D, with frequency plotted on the horizontal axis. As shown in the figure, a large number of oscillation modes appear at frequency intervals determined by the lengths of the resonators C and D, respectively. Each mode produced by the two resonators C and D has a different frequency due to the influence of the thickness t of the partially reflecting mirror 5. Then, starting from the lowest frequency, both resonators C and D
The interval Δν from when the frequencies match once until they match again next time can be expressed as c/2nt. If we look at this interval with respect to the wavelength Δλ, we get the following equation. Δλ=λ2/2nt (λ: laser wavelength, n: refractive index of the substrate of the partially reflecting mirror 5)

[0022] In the above equation, the numerical value for the actual laser (λ=2
48nm, n=1.51, t=8mm), we get
Δλ=2.5pm. This corresponds to the interval between the locations where the depressions appear in FIG. That is, the dent in the wavelength distribution is a beat caused by interference between multiple modes generated in the two resonators C and D.

Therefore, the oscillation output when the wedge-shaped partial reflection mirror 5 is coated with a non-reflection coating (actually has a reflectance of 0.5%) and the oscillation output when the wedge-shaped partial reflection mirror 5 is coated with a partial reflection coating (reflectance of 10%) are compared. A comparison was made with the oscillation output when The results are shown in FIG. From this result, it can be seen that even the resonator C configured with the anti-reflection coated surface oscillates sufficiently (see the black circles and squares in the figure). It can also be seen that the oscillation output decreases as the partial reflecting mirror 5 is oscillated. Therefore, when using the numerical values in the example shown in FIG. The normal to the coated surface is 0.6 to 1
．． It can be seen that if the angle is tilted by about 2 mrad or more, oscillation caused by the non-reflection coated surface can be avoided. That is, it is possible to eliminate dents in the wavelength distribution.

[0024] This 0.6 mrad is an angle such that the light does not go out of the beam width (range where gain exists) while the light travels back and forth through the resonator four times (see FIG. 4(a)). Resonator length L is 130 cm, beam diameter x is 0.6 cm, 1.
2cm (gas flow direction, discharge direction), the corresponding angle is 0.6cm/(2 x 130cm) x 4 round trips = 0.577
×10-3 1.2cm/(2×130cm)×4 round trip=1.15×
10-3, 0.6 mrad, 1.2 for each direction
Become mrad.

From the above, the following equation can be adopted as a condition under which oscillation does not occur in the resonator constituted by the non-reflection coated surface of the partial reflecting mirror 5. |θ|〉tan-1(x/8) Here, θ is the angle between the normal line of the anti-reflection coating surface and the optical axis, and x (cm) is the direction in which the optical axis is tilted with respect to the normal line. This is the beam diameter (distance between p and q in FIG. 4(b)). Note that the number of reciprocations can be reduced as the gain is larger, but the laser discussed here has a gain of 15%/cm and a length of the laser medium 1 of about 40cm, and the laser oscillates during four reciprocations. It's a very average laser.

If θ is larger than the angle shown on the right side of the above equation, the purpose of eliminating the concavity in the wavelength distribution is achieved. However, if the angle θ is made too large, the light will be refracted as in the prior art, making alignment difficult. Therefore, assuming that a He--Ne laser with a wavelength of 633 nm is used for alignment, the refraction angle was investigated.

FIG. 5 shows how laser light is refracted by a He--Ne laser and an excimer laser using KrF as an excimer. Letting the wedge angle of the partial reflecting mirror 5 be θv, the refraction angle α can be expressed by the following equation. α=(n-1)θv (n is the refractive index) When using a laser beam for processing and transmitting the laser beam to a processing table, it is inconvenient if the refraction angle α1 is large. When the refraction angle α1 is large, consideration must be given to fixing the partial reflecting mirror 5 in an accurate direction, or installing the partial reflecting mirror 5 appropriately so that 2α1 is within a range that does not cause problems in use. It won't happen.

Furthermore, as already mentioned, when using a high-output laser such as an excimer laser, alignment is performed in advance using another laser. In that case, a He-Ne laser, which is a visible light laser, is often used. Since the He-Ne laser beam has a different refractive index from the excimer laser beam, the refraction angle is also different. For example, the refractive index for excimer laser light is 1.51, and the refractive index for He-Ne laser light is 1.475. For example, 5mr as θv
When ad is adopted, the refraction angle of excimer laser light is 2.5
5mrad, refraction angle of He-Ne laser beam is 2.38m
It becomes rad. The difference in refraction angles Δα is expressed by the following equation. Δα=Δn·θv Here, Δn is the difference in refractive index between the two laser beams, and in this case, it is 0.035.

If this difference Δα is larger than tan-1 (x/8L), a situation will occur in which the excimer laser does not oscillate after alignment is performed with the He-Ne laser. In this case, such a situation will occur if θv is larger than 17.1 mrad. Therefore, it is desirable that the wedge angle θv of the partial reflecting mirror 5 falls within the range of the following equation. θv<[tan-1(x/8L)]/Δn

[0030]
In the above embodiment, the wedge angle in the case where the light is almost perpendicularly incident on the partial reflecting mirror 5 has been explained, but the wedge angle in the case where the resonator is shown in FIG.
As shown in FIG. 2, a configuration further including a total reflection mirror 7 can be considered in the same manner. That is, if we consider the case where the light reflected by the total reflection mirror 7 is further reflected by the partial reflection mirror 5 and returns to the laser medium 1, the concept in the above embodiment can be applied as is.

In the above embodiment, an excimer laser was explained, but if it is a high gain laser (for example, a dye laser) that oscillates due to its surface even if it is coated with an anti-reflection coating, such a laser can be used. The same effect can be achieved by using .

FIG. 7 is a configuration diagram showing a laser resonator according to another embodiment of the present invention together with the laser medium 1 and the like. In the figure, 8 is a holder for the partial reflecting mirror 51, 9 is a heater,
10 is a temperature detector, 11 is a heater 9 and a temperature detector 1
A temperature control device connected to 0.

Next, the operation will be explained. In this case, one side of the partial reflecting mirror 51 is perpendicular to the optical axis, and the angle θ between the normal to the other side and the optical axis is θ<tan-1(x/8L). That is, a dent is actively created in the wavelength distribution. Therefore, no anti-reflection coating is applied. Then, that surface is coated using the reflectance of 4% of the substrate itself of the partial reflecting mirror 5, or such that a desired reflectance is obtained by adding the reflectances of both surfaces to that surface.

In this case, the dent in the wavelength distribution is
It is preferable that the wavelength distribution be present on both sides of the expected center wavelength and have a simple shape such as a Lorentzian shape.

In order to arrange the recesses on both sides of the center wavelength, the thickness of the partial reflecting mirror 51 may be precisely processed or adjusted by etching or vapor deposition. Alternatively, the temperature of the partial reflecting mirror 51 may be changed. The following equation holds true between the wavelength shift amount Δλs and the temperature change ΔT. Δλs=χλΔT Here, χ is the coefficient of linear expansion of the substrate of the partially reflecting mirror 5 (
For example, 5 x 10-7 for quartz).

For example, when setting the wavelength width of the laser to 3 pm, if the concavity is located at the center wavelength in the wavelength distribution, it would be sufficient to shift the wavelength by 1.5 pm.
If the temperature is changed by approximately 10°, Δλs=1.5pm (λ=
300 nm). Furthermore, the temperature after the change is ±
If it is stabilized within 1°, the wavelength at which the dent appears will be 0.
Stabilizes with an accuracy of 15pm.

Here, the heater 9 changes the temperature of the partial reflecting mirror 51, and the temperature control device 11 may control the amount of heat generated by the heater 9 in accordance with the temperature detected by the temperature detector 10.

[0038] When the depressions can be placed on both sides of the center wavelength, in order to prevent the depression from occurring within the desired wavelength width,
It is sufficient that the interval between the recesses is larger than the wavelength width λw. That is, the thickness t(
What is necessary is to determine the thickness at the position where the beam center passes through. t>λ2/2nλw

[0039]

As described above, according to the invention as set forth in claim 1, the minimum angle of the non-reflection coated surface of the partially reflecting mirror of the laser resonator with the optical axis of the beam has been clarified.
A partial reflecting mirror with a smaller wedge angle can be used, and there is an effect that a laser resonator can be obtained in which alignment and the like can be easily performed while preventing the formation of a dent in the waveform distribution.

Further, according to the second aspect of the invention, two laser resonators are constructed between one side and the other side of the partially reflecting mirror whose thickness is adjusted and the total reflecting mirror. This has the effect of being able to extract laser light with a narrower wavelength width.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram showing a laser resonator according to an embodiment of the present invention together with a laser medium.

FIG. 2 is an explanatory diagram for explaining a concavity that occurs in the wavelength distribution.

FIG. 3 is an explanatory diagram showing a change in laser output with respect to a deflection angle of a partially reflecting mirror.

FIG. 4 is an explanatory diagram showing how light travels back and forth and the definition of a beam diameter.

FIG. 5 is an explanatory diagram for explaining the relationship between wedge angle and refraction angle.

FIG. 6 is a configuration diagram showing a laser resonator according to another embodiment of the present invention together with a laser medium and the like.

FIG. 7 is a configuration diagram showing a laser resonator according to still another embodiment of the present invention together with a laser medium and the like.

FIG. 8 is a configuration diagram showing a conventional laser resonator together with a laser medium and the like.

FIG. 9 is an explanatory diagram showing an example of wavelength distribution.

[Explanation of symbols]

1 Laser medium 2 Window 3. Window 4 Total reflection mirror 5 Partial reflector 6 Wavelength selection element 7 Total reflection mirror 8 Holder 9 Heater 10 Temperature detector 11 Temperature control device 51 Partial reflector 52 Partial reflector

Claims (2)

[Claims] 1. A laser resonator having a total reflection mirror and a partial reflection mirror installed to sandwich a laser medium, wherein the partial reflection mirror has a partial reflection coating on one side and a non-reflection coating on the other side. , and the angle between the normal to the partially reflective coated surface of the partially reflective mirror and the optical axis of the resonator is θ, the beam diameter in the direction in which the optical axis is inclined with respect to the normal is x, and the total A laser resonator characterized by having a relationship of |θ|>tan-1(x/8L), where L is a distance between a reflecting mirror and the partial reflecting mirror. 2. A laser resonator having a total reflection mirror and a partial reflection mirror installed to sandwich a laser medium, wherein the partial reflection mirror has one surface perpendicular to the optical axis of the resonator; The angle between the normal of the other surface and the optical axis of the resonator is θ, the beam diameter in the direction in which the optical axis is inclined with respect to the normal is x, and the angle between the total reflection mirror and the partial reflection mirror is When the distance between them is L, the relationship is |θ|<tan-1(x/8L), and the thickness of the partially reflecting mirror is t, the center wavelength of the laser is λ, and The refractive index of the substrate is n,
A laser resonator characterized by having a relationship of t>λ2/2nλw, where λw is the wavelength width of the laser.