JPH0624275B2 - Optical resonator of laser device - Google Patents

Description

TECHNICAL FIELD The present invention relates to an optical resonator of a laser device.

In particular, the present invention relates to a technique capable of extracting a high quality laser beam from a laser device having a relatively small geometric aspect ratio and a relatively small beam round trip gain.

(Prior Art) A technique for extracting a high-quality laser beam from a laser device having a relatively small geometric aspect ratio and a relatively small beam round-trip gain is necessary, for example, in a lateral gas flow type CO ₂ laser device. .

In that type of laser device, a short and thick discharge region is used to smooth the gas flow, as opposed to the long and thin discharge region used in the axial flow type gas laser device. From such low output to medium output (about 0.5-5k
In the lateral flow CO ₂ gas laser device of W), it is difficult to use an output mirror having a high output coupling rate (hereinafter referred to as coupling rate) because the beam round-trip gain of the unstable resonator is very small.

Therefore, when a stable resonator that operates in a lower mode such as a TEM ₀₀ mode is used in such a laser device, the laser beam is repeatedly folded in the resonator so that the laser medium is sufficiently used. There is a need to. However, if such multiple folding is performed, the optical resonator becomes sensitive to the misalignment of the optical alignment, and the parasitic mode is likely to occur.

Since the stable resonator has such drawbacks, there is a demand for using an unstable resonator when a short and thick discharge region is required.

The main problem when using the conventional unstable resonator is that the coupling ratio of the output mirror is very small in order to generate a sufficient laser beam in the resonator and extract sufficient energy from the medium. That's what you have to do. However, in this case, the final output beam becomes an extremely thin ring, and the far-field focusing property is fundamentally deteriorated.

For example, in an unstable resonator used in a gas laser device, a main reflector made of a concave mirror is provided at one end of a resonant cavity, and a feedback reflector made of a convex mirror is provided at an opposite end of the cavity. Here, the main reflector and the feedback reflector are aligned with their respective centers of curvature on the longitudinal axis of the cavity. In such a configuration, the centers of curvature of both reflectors define the optical axis of the resonant cavity, and the center of curvature of the main reflector is located near the reflective surface near the output end of the cavity.

In this configuration, all laser beams along the optical axis are
It is repeatedly reflected between the main reflector and the feedback reflector and moves away from the optical axis in sequence. As a result, the laser beam eventually crosses the outer end of the feedback reflector and exits the resonant cavity as an output beam. This type of optical cavity is called an "unstable" resonator, or "unstable" resonant cavity, because every optical axis that travels along the optical axis always leaves the resonant cavity at the end.

The total amount of radiation that can be output from a gas laser device having a cavity is a function of the total beam in the optical cavity, the beam round-trip gain across the laser medium, and the coupling factor of the feedback reflector. Therefore, the gas medium, pressure, temperature,
Given the beam path length and the input, the output is a function of the coupling factor (transmittance) of the feedback mirror.

The output of this laser device makes a peak at a certain coupling rate,
It is known to gradually decay to zero on both sides of the peak. Also, this peak moves toward a position with a smaller coupling rate as the beam round-trip gain across the laser medium becomes smaller. Therefore, when trying to obtain the maximum output from a laser device having an unstable resonator, even if an output mirror having a high coupling ratio is used, if the beam round-trip gain obtained by the laser medium in the optical cavity is small, rather The output becomes small.

Therefore, in such an optical resonator, there is a problem that it is very difficult to obtain a laser beam having a high output and a good far-field focusing property.

Next, the optical resonator of the conventional laser device will be described in detail.

FIG. 1 is a conceptual diagram of an unstable cavity used in a gas laser device. This resonator comprises a main mirror 1 having a reflecting concave surface, and a feedback mirror 2 having a reflecting convex surface facing the concave mirror. Here, the centers of curvature of the two opposite slopes and the focal points of both are on the axis 3. Therefore, the axis 3 is the vertical axis of the cavity and also the optical axis of the optical system of the resonator.

When this laser device is activated, CO with a population inversion
A gas medium G such as a mixed gas of N ₂ , N ₂ and He flows through the laser region 4. Then, within this laser region 4,
A laser beam oscillates by the laser action. This laser beam 5 moves away from the optical axis 3 while being reflected back and forth between the reflecting surfaces of the two mirrors 1 and 2. Each time the laser beam passes through the gas medium, it is amplified by the laser action of the medium. Since this ray gradually leaves the optical axis, after being repeatedly reflected, it passes over the edge of the feedback mirror 2 and is output from the cavity as an output beam.

The output coupling ratio of the optical resonator, which is determined by the ratio of the curvatures of the two mirrors, is determined so that the output beam has a good far-field focusing property. But,
The feedback amount of laser light provided by these mirrors is difficult to amplify and sustain laser oscillation having a large output coupling rate by itself.

To provide the feedback needed to sustain such oscillations, as well as to provide a laser beam with high power and good optical properties, Sziklas in US Pat. No. 4,123,150 The optical resonator shown in FIG. 2 is disclosed. In this optical resonator, the main mirror 1
Besides the feedback mirror 2, a third mirror 6 is used. The third mirror 6 stabilizes the system by giving a predetermined feedback by returning the light beam, which normally becomes the output radiation, to the main mirror 1.

As another example, Zikra discloses an optical resonator as shown in FIG. In this optical resonator, length L and thickness W
An annular gain region 26 of is disposed between the end mirror 20 and the unstable resonator 11. Here, the gas medium G passes through the gain region 26 radially from the vertical axis 24, as shown in FIG. On the other hand, the output laser beam 30 is output to the outside through the central through hole 28 of the main mirror 12.

Zikla will effectively identify higher transverse modes by providing suitable unstable resonators 11 or providing suitable continuous cross-sections for the output beam in such optical resonators. ,It has said.

In discussing the background of the invention, Mr. Zikra cites one optical resonator. In this optical resonator, the laser medium is arranged in a relatively simple unstable resonance and has the shape of a cylindrical sheath. The relatively simple unstable resonator comprises an annular convex surface arranged at one end of the cylindrical sheath and an annular concave mirror arranged at the other end of the sheath. According to Zikla, the annular laser beam output from this optical resonator has poor optical characteristics. Since the resonator has a high Fresnel number, the mode discrimination is extremely poor, and a large azimuth mode tends to occur.

In any case, the optical resonator of the conventional laser device including the optical resonator disclosed by Mr. Zikra outputs a high-quality laser beam and is not preferable in high-order transverse mode, parasitic mode, etc. There is a problem that it is not possible to reliably prohibit the generation of modes.

(Object of the Invention) A main object of the present invention is to extract a high-quality laser beam from a laser device having an unstable resonant cavity having a relatively small geometric aspect ratio and a relatively small beam round trip gain. Another object of the present invention is to provide an optical resonator of a laser device capable of achieving the above.

Another object of the present invention is an optical resonator having an unstable resonance cavity, which has a small output coupling ratio, but can realize high brightness in the far field by narrowing the dark portion. To provide an optical resonator.

Still another object of the present invention is to provide an optical resonator of a gas laser device capable of preventing oscillation of a parasitic mode in the optical resonator.

(Summary of the Invention) In order to achieve the above object, in the present invention, an unstable resonator having a main reflector and a feedback reflector respectively disposed at both ends of a resonant cavity and an outside of the unstable resonator are provided. Disposed between the auxiliary feedback reflector and the main reflector, the auxiliary feedback reflector being arranged to reflect a part of the output radiation emitted from the unstable resonator again toward the unstable resonator. And an obstacle means for inhibiting the oscillation of the parasitic mode.

(Embodiment) FIG. 4 shows a preferred embodiment of the present invention. In this embodiment, the laser region is provided in the unstable resonator and mode discrimination is provided by the spokes arranged in the feedback optical path between the auxiliary feedback mirror and the main mirror.

In the configuration of FIG. 4, the main mirror 40 and the feedback mirror 41 are arranged at both ends of the unstable resonance cavity.
Both mirrors are confocal and both the center of curvature and the focal point of both are on axis 42. The feedback light diffracted by the feedback mirror 41 forms a diffraction nucleus 43 around the axis 42. This nucleus 43 promotes resonance in the unstable cavity. An auxiliary feedback mirror 44 is placed behind the Fido back mirror 41 to sustain the resonance.

Here, when the laser medium is introduced into the laser region 45,
Laser light is excited in this region. When the laser medium is a gas, it is preferably flowed across the laser region 45. As a result, the laser region becomes short and thick, as opposed to the laser region of a long and thin axial laser device.

The auxiliary feedback mirror 44 is a semitransparent mirror. Therefore, a part of the incident light on this mirror passes through the mirror and becomes an output beam. Most of the incident light on the auxiliary mirror is reflected back toward the main mirror 40. As a result, a large beam luminous flux is held in the diffraction nucleus 43. The reflectance of the auxiliary feedback mirror 44 is set to a necessary and sufficient level to assist the feedback from the mirror 41.

Radially extending spokes 46 are arranged in the optical path between the main mirror 40 and the auxiliary feedback mirror 44 to prevent parasitic modes from being formed. It is empirically known that the width of the pork should be about (λL) ^1/2 . Here, L is the distance between the auxiliary mirror and the main mirror, and λ is the wavelength of the emitted light to be suppressed. The spokes 46 form obstacles in the optical path between the main mirror 40 and the auxiliary mirror 44, and inhibit the occurrence of parasitic modes.

It is convenient to support the feedback mirror 41 on the spokes 46. This is because the feed rearview mirror 41 must be supported by some method in any case. Therefore, in the above embodiment, as shown in FIG. 4, one end of the spoke 46 is fixed to the support base 47,
The other end of the spoke 46 supports the feedback mirror 41.

In this configuration, the spokes 46 do not interfere with the reflected light reflected between the feedback mirror 41 and the main mirror 40. However, the spokes 46 cast a shadow on the output beam in the configuration.

The cross section of the radiation beam output through the semi-transmissive auxiliary feedback mirror 44 is a thick annular shape centered on the optical axis 42, as shown in FIG. Therefore, this beam LB has a light output and a good far-field focusing property.

Another embodiment of the invention, shown schematically in FIG. 6, uses a total internal reflection auxiliary feedback mirror 60 in place of the semitransparent auxiliary feed mirror used in the embodiment of FIG.
In the embodiment of FIG. 6, the auxiliary feedback mirror 6
0 returns a part of the emitted light, which would otherwise be output from the periphery of the feedback mirror 62, into the resonant cavity 45 defined by the main mirror 61 and the feedback mirror 62.

A through hole 60a is formed in the auxiliary feedback mirror 60, and a part of radiation is transmitted through the through hole 60a.
Output as an output beam. An annular cross section of this beam is shown in FIG. The annular output beam obtained in this example does not have as good a far field luminance as the beam obtained in the example of FIG. This is because most of the beam cross section is taken up by the feedback mirror 62. Therefore,
The main advantage of the FIG. 6 embodiment is that it provides a preferred option for high power laser devices that do not use transmissive optical elements.

In the embodiment of FIG. 6, parasitic modes are prevented by the radially extending spokes 63, as in the embodiment of FIG.

In both of the above embodiments, the basic principle of the unstable resonator is retained. In other words, the laser region is filled with the emitted light flux, and the light flux moves radially outward from the diffraction nucleus surrounding the optical axis while being continuously reflected. In the optics of both embodiments, all emitted light is output from the diffraction nucleus as output radiation or returned to the diffraction nucleus as feedback radiation. The various transverse mode intensity shapes and phase states that may be present in the above embodiments are due to diffraction effects from small feedback mirrors and from other edges such as light path walls in the illuminated cavity and protruding electrodes. Determined by and.

Although both of the above embodiments have a confocal arrangement, those skilled in the art
It will be readily appreciated that the mirrors of the unstable resonator do not have to be in a confocal arrangement, as long as the auxiliary feedback mirrors have the proper curvature. Also, it will be apparent to those skilled in the art that a negative branch resonator can be used instead of the positive branch resonator used in the above two embodiments. In fact, negative branch resonators have the advantage of being insensitive to optical misalignment.

Since the present invention can be implemented in various forms, the scope of the present invention is not limited to the above embodiments. The scope of the invention is
It is defined by the claims and the range equivalent to the claims.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a conventional unstable resonance cavity,
A main mirror and a field back mirror are arranged at both ends of this resonant cavity, and the center and focus of both mirrors are on the vertical axis of the cavity, and the optical axis is aligned with the vertical axis of the cavity. Shown, FIG. 2, U.S. Pat. No. 4,12 to Zikura.
FIG. 3 is a schematic diagram showing a stable resonator disclosed in US Pat. No. 3,150, which shows that a conventional unstable resonator cavity is used as a constituent element of the stable resonator. FIG. It is the schematic which shows the other structure which is disclosed, and is a figure which shows that a stable resonator is used for a gas laser apparatus, and FIG. 4 is this invention which uses a confocal unstable resonator and a semitransparent mirror. FIG. 4 is a schematic diagram illustrating an embodiment of the present invention, in which an output beam is output through the semi-transmissive mirror and radial spokes are used to prevent oscillation of a parasitic mode; FIG. 5 is a view showing a sectional shape of an output beam outputted from the embodiment shown in FIG. 4, and FIG. 6 is another embodiment using a confocal unstable resonator and a total reflection auxiliary mirror. FIG. 4 is a schematic diagram showing the total internal reflection auxiliary mirror of the output radiation. Return part of it to the unstable resonator,
FIG. 7 shows the radial spokes arranged to prevent parasitic mode oscillations, and FIG. 7 shows a cross section of the beam obtained from the embodiment shown in FIG. 40, 61 ... Main mirror, 41, 62 ... Feedback mirror, 24, 42 ... Optical axis, 45 ... Laser region,
43 ... Diffraction nucleus, 44, 60 ... Auxiliary feedback mirror, 46, 63 ... Spoke

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Ethan Day. Hotsuto East Boston, Massachusetts, United States Boston Webster Street 177

Claims (9)

[Claims] 1. (I) An unstable resonator having a main reflector and a feedback reflector respectively disposed at both ends of a resonant cavity; and (II) an unstable resonator disposed outside the unstable resonator. An auxiliary feedback reflector that reflects a part of the output radiation emitted from the resonator toward the unstable resonator again, and (III) is arranged between the auxiliary feedback reflector and the main reflector, An optical resonator of a laser device, comprising: obstacle means for inhibiting oscillation of a parasitic mode. 2. The optical resonator of the laser device according to claim 1, wherein the auxiliary feedback reflector is a semi-transmissive mirror. vessel. 3. An optical resonator for a laser device as set forth in claim 1, wherein said auxiliary feedback reflector has an annular mirror whose inner diameter is large enough to pass an output beam. An optical resonator of a laser device characterized in that 4. An optical resonator for a laser device as set forth in claim 1, wherein the obstruction means is a spoke extended in a radial direction orthogonal to an optical axis of the optical resonator. An optical resonator for a laser device. 5. An optical resonator for a laser device as set forth in claim 4, wherein the width of the spokes is equal to that of the main reflector of the unstable resonator and the auxiliary feedback reflector. An optical resonator of a laser device, wherein when the distance is L and the radiation wavelength is λ, it is about (λL) ^1/2 . 6. An optical resonator for a laser device as set forth in claim 4, wherein said spokes are arranged in a radial direction orthogonal to said optical axis from a position on the outer circumference of said feedback reflector. An optical resonator for a laser device, which is to be stretched. 7. An optical resonator for a laser device as set forth in claim 6, wherein the spokes support the feedback reflector. 8. An optical resonator for a laser device as set forth in claim 7, wherein the width of the spokes is equal to that of the main reflector of the unstable resonator and the auxiliary feedback reflector. An optical resonator of a laser device, wherein when the distance is L and the radiation wavelength is λ, it is about (λL) ^1/2 . 9. An optical resonator for a laser device as set forth in claim 1, wherein the resonant cavity of the unstable resonator has a lateral flow of a gas laser medium inside the resonant cavity. An optical resonator of a laser device comprising a gas flow region for guiding.