JPH10270780A - Beam splitter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a beam splitter that exhibits high reflectance to a certain wavelength and very high transmittance to a wavelength to be transmitted highly, and has a small light loss. SOLUTION: A coating film Fb that selectively exhibits high reflectance to a certain wavelength (2 ω wave) is formed on one surface 11b of a parallel flat plate 111, and a coating film Fa which reflects a wavelength (ω wave) to be transmitted highly is formed on the other surface 11a of the plate 111. Then, very high transmittance is realized by setting the reflectance of the surface 11a to a value which is nearly equal to that of the surface 11b and constituting the plate 111 carrying the coating films Fb and Fa to function as an etalon to the wavelength (ω wave) to be transmitted highly.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a beam splitter used for, for example, a mirror of a laser resonator.

[0002]

2. Description of the Related Art Conventionally, as a beam splitter for separating light of two wavelengths, both surfaces of a parallel flat plate made of quartz or optical glass are polished, and one of the surfaces is provided with a high transmission / different to a certain wavelength. In general, a coating film having a high reflectance with respect to a wavelength is formed, and a coating film having a high transmittance with respect to a wavelength at which the coating film on one surface has a high transmittance is formed on the other surface. I have.

[0003]

According to the above-described beam splitter, the coating film formed on one surface of the plane-parallel plate has a high reflection for a certain wavelength and a high transmission for another wavelength. Because it is a film with such two functions, it is difficult to obtain sufficient performance for both high reflection and high transmission specifications compared to a single-function coating film, and the other side of the parallel plane plate Even if a highly transparent film is formed, there is some loss that cannot be ignored, for example, as a mirror for a laser resonator because some reflection remains.

The present invention has been made in view of such circumstances, and has a low light loss, a beam having a high reflectivity at a certain wavelength and a very high transmittance at a wavelength to be transmitted at a high wavelength. The purpose is to provide a splitter.

[0005]

First, the reason why a conventional beam splitter cannot realize a high transmittance for a transmission wavelength will be described.

In the conventional beam splitter, when coating one surface (the surface 11b in FIG. 1) of the parallel plane plate, the wavelength (ω) to be highly transmitted is based on a high reflectance for a certain wavelength (2ω wave). (Wave) is designed so as to obtain as high a transmittance as possible within a thickness error considered during actual film formation, and the other surface (11a surface) has a high transmittance with respect to the ω wave. Although a technique for realizing the transmittance is adopted, such a technique generally has a natural limit in realizing a high reflectance for 2ω on the 11b surface.

[0007] In other words, when the reflection film is considered as a reflection film for 2ω waves, it is possible to form a reflection film with as small an overall film thickness as possible to obtain a high quality coating in terms of absorption and scattering. The requirements are constraints,
Increasing the total film thickness, for example, by increasing the total number, can also be a factor that limits the reflectance in theoretical design.

In order to realize a high transmittance for the ω wave, a high reflectance requirement for the 2ω wave is a constraint, and a certain degree of compromise is required from the design point of view (for example, a transmittance of 99% or less in design). I have to do it).

Therefore, the present invention focuses on the fact that the element functions as an etalon when coating is applied to both sides of a transparent parallel plane plate, and makes it possible to obtain a very high transmittance by utilizing the etalon effect. ing.

That is, in the beam splitter of the present invention, as illustrated in FIG.
A coating film Fb that selectively shows a high reflectance with respect to a certain wavelength band (2ω wave) is formed on one surface (a surface on the light incident side) 11b of the substrate 11, and the other surface 11a of the parallel flat plate 111 is formed. Then, a coating film Fa is formed to reflect a wavelength (ω wave) to be highly transmitted through the beam splitter, and the reflectivity of the other surface 11a is such that light (ω wave) having a wavelength to be highly transmitted is parallel flat. The reflectance is set to a value substantially equal to the reflectance when the light enters the one surface 11b from the face plate 111 side, and the wavelength (ω) at which the parallel flat plate 111 on which the two coating films Fb and Fa are formed should be highly transmitted.
), It has a high reflectivity for a certain wavelength (2ω wave), and is very versatile for the wavelength (ω wave) to be highly transmitted as a whole element. High transmittance is realized.

The specific method of the coating will be described below. A common coating design for etalons is to apply the same coating to two faces of a plane-parallel plate. However, in the case of the beam splitter of the present invention, it is necessary to avoid that the effect as the etalon extends to 2ω waves.

Therefore, in the present invention, a general etalon design method is extended to obtain a high reflectivity for the 2ω wave on one surface 11 b of the parallel flat plate 111,
The coating film is designed to stably achieve the specified reflectance regardless of the manufacturing error in the region of the ω wave, and the other 11a surface has the same reflectance as the ω wave reflectance of the 11b surface. A technique is used in which a coating film is designed so as to achieve a reflectance in the region of waves and not so high for 2ω waves.

According to such a method, (1) the constraint conditions when designing the coating on the 11b surface of the parallel plane plate 111 are relaxed compared to the conventional beam splitter design method, and a good quality coating can be designed. .

(2) If the difference in reflectance between the two surfaces is up to about 2%, the resulting transmission loss is only about 0.5%, and there is a high possibility that a high peak transmittance can be obtained. There is an advantage.

Here, even if the above-described coating method is adopted, the design of the coating on the 11b surface still has restrictions, which lowers the efficiency of the second resonator.
The problem remains.

In order to solve such a problem, it is sufficient to form a coating film on the surface 11b purely to realize a high reflectivity purely for the 2ω wave. Can vary significantly within manufacturing tolerances.

As a countermeasure against this, in the present invention, after actually coating, the reflectivity of the surface 11b with respect to the ω wave is actually measured, and a coating film capable of stably achieving the reflectivity is designed, and the film is formed on the surface 11a. The coating method of forming is adopted.

In this method, the time for actual measurement and design is possible in less than two hours in view of the current technical ability, and it can be sufficiently put into practical use. In addition, such a coating method has an advantage that the setting of the reflectance of the coating film on the surface 11a with respect to the ω wave is simplified and the peak transmittance is increased as compared with the above method.

[0019]

FIG. 1 is a block diagram showing an embodiment of a beam splitter according to the present invention. Parallel plane plate (made of quartz) 1
Both surfaces of 11 are polished, and one surface (the surface on the light incident side) 11b is selectively provided with respect to a second harmonic (2ω wave) of a ring resonator type fourth harmonic generator described later. The coating film Fb showing a high reflectance is formed. On the other surface 11a of the plane-parallel plate 111, a coating film F that reflects a wavelength to be transmitted through the beam splitter at a high level, that is, a fundamental wave (ω-wave) of the fourth harmonic generator.
a is formed.

In this embodiment, the reflectance when the fundamental wave (ω-wave) to be highly transmitted is incident on the one surface 11b from the parallel plane plate 111 side is measured, and the measured reflectance is measured. Based on this, the reflectance of the other surface 11a for the fundamental wave (ω wave) of the coating film Fa is changed to the surface 11b.
The parallel flat plate 111 on which the two coating films Fb and Fa are formed so as to have the same value as the reflectance (actual measurement value) on the side thereof functions as an etalon for a fundamental wave (ω wave) to be highly transmitted. The feature is that it is configured to do so.

Next, the beam splitter having the above structure is
FIG. 2 shows an example in which the present invention is applied to a ring resonator type fourth harmonic generator.
Shown in In FIG. 2, the output light from the semiconductor laser 1 includes a collimator lens 2a and a focus lens 2b.
Irradiates the laser medium 4 via. Between the laser medium 4 and the focus lens 2b, it transmits the output wavelength of the semiconductor laser 1 and has a high reflectance with respect to a fundamental wave stimulated and emitted from the laser medium 4 and its second harmonic. A mirror 3 is arranged. Further, a second harmonic output mirror 6 is arranged to face the laser medium 4, and an optical resonator (first resonator) 21 is configured between the output mirror 6 and the mirror 3. .

The wavelength conversion element 5 is provided inside the optical resonator 21.
The wavelength conversion element 5 is irradiated with a fundamental wave stimulated and emitted by exciting the laser medium 4 with output light from the semiconductor laser 1 to generate a second harmonic.

In this example, the laser medium 4 of the optical resonator 21 for generating the second harmonic and the wavelength conversion element 5
The beam splitter 11 having the above-described structure is arranged such that one of the beams 11b faces the wavelength conversion element 5 and is arranged at an angle such that it functions as an etalon with respect to the fundamental wave (ω). The second harmonic ring resonator (second resonator) 22 is formed by the beam splitter 11, the three resonator mirrors 9, 10, 12, the optical diode 7, and the wavelength conversion element 8 for generating the fourth harmonic. Make up, fourth
It is configured to generate a harmonic (4ω).

The optical diode 7 shown in FIG. 2 is an optical element for regulating light in the ring resonator 22 in one direction (the direction of the arrow in the figure). The mirrors 9 and 10 of the resonator mirrors 9, 10, and 12 constituting the ring resonator 22 reflect both the second harmonic and the fourth harmonic with high efficiency. The mirror 12 is a fourth harmonic output mirror that reflects the second harmonic with high efficiency and transmits the fourth harmonic with high efficiency.

FIG. 3 is a diagram showing an example in which the beam splitter having the structure shown in FIG. 1 is applied to a standing wave type resonator type fourth harmonic generator. In this example, as in the example shown in FIG. 2, the semiconductor laser 1, the lenses 2a and 2b, the mirror 3, the laser medium 4, and the second harmonic output mirror 6
, And an optical resonator (first resonator) is configured between the output mirror 6 and the previous mirror 3. A wavelength conversion element 5 is housed inside the optical resonator, and a fundamental wave stimulated and emitted by exciting the laser medium 4 with output light from the semiconductor laser 1 irradiates the wavelength conversion element 5. As a result, a second harmonic is generated.

In this example, the beam splitter 11 having the above-described structure is provided between the laser medium 4 of the optical resonator 21 for generating the second harmonic and the wavelength conversion element 5, and one of the beams 11b is used for wavelength conversion. The beam splitter 11 and the resonator mirror 2 are arranged toward the element 5 and at an angle that functions as an etalon with respect to the fundamental wave (ω).
By the wavelength conversion element 8 that generates 0 and the fourth harmonic,
A second harmonic standing wave resonator (second resonator) is configured to generate a fourth harmonic (4ω).

The laser medium 4 used in the above two examples is, for example, Nd: YAG or Nd: YVO _{4. When} Nd: YAG is used, the fundamental wave induced and emitted from the solid-state laser medium is used. Is converted into a second harmonic by using a nonlinear optical crystal that is stable with respect to temperature, for example, LBO (LiB ₃ O ₅ ) or BBO (β
—BaB ₂ O ₄ ). When Nd: YVO ₄ is used as a solid-state laser medium, KPT (KTiOP) is used as a wavelength conversion element for converting a fundamental wave into a second harmonic.
O ₄ ) or the like.

Here, according to the two examples shown in FIGS. 2 and 3 described above, the arrangement of the beam splitter 11
Since an etalon is added in the resonator for generating harmonics (first resonator), there are advantages that both the wavelength selectivity and the coherence of the oscillation laser are improved, and that the output is more easily stabilized. In addition, only a minimum loss occurs in both the first resonator and the second resonator, and high output can be expected.

The beam splitter of the present invention can be used for laser devices other than the fourth harmonic generator as described above.

[0030]

As described above, according to the beam splitter of the present invention, a coating film having a high reflectivity for a certain wavelength is formed on one surface of a parallel plane plate, and On the other surface, a coating film that reflects a wavelength to be highly transmitted is formed, and the etalon is formed by setting the reflectance of the other surface to a value substantially equal to the reflectance of one surface. Due to the effect, a very high transmittance is realized for a wavelength to be highly transmitted, so that light loss can be reduced and wavelength selectivity can be increased.

Therefore, the beam splitter of the present invention can be effectively used for a mirror for a laser resonator such as a fourth harmonic generator.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing a structure according to an embodiment of the present invention.

FIG. 2 is a diagram showing an example in which the beam splitter of the present invention is applied to a ring resonator type fourth harmonic generator.

FIG. 3 is a diagram showing an example in which the beam splitter of the present invention is applied to a standing wave type resonator type fourth harmonic generator.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 semiconductor laser 3 mirror 4 laser medium 5 wavelength conversion element (for second harmonic generation) 6 second harmonic output mirror 7 optical diode 8 wavelength conversion element (for fourth harmonic generation) 9, 10 resonator mirror 11 beam Splitter 12 Resonator mirror (for fourth harmonic output) 21 Optical resonator 22 Ring resonator 111 Parallel plane plate Fa, Fb Coating film

Claims (1)

[Claims] 1. A beam splitter used for separating light of two wavelengths, comprising:
A coating film that selectively shows a high reflectance for a certain wavelength band is formed, and on the other surface of the parallel plane plate, a coating film that reflects a wavelength to be highly transmitted by the beam splitter is formed. The reflectance of the other surface is set to a value substantially equal to the reflectance when the light having the wavelength to be highly transmitted is incident on one surface from the parallel flat plate side, and the parallel light having the two coating films is formed. A beam splitter, wherein the plane plate is configured to function as an etalon for a wavelength to be highly transmitted.