JPH0545516A - Multilayered film mirror for laser gyro - Google Patents

Abstract

PURPOSE:To obtain the multilayered film mirror for a laser gyro constituted to eliminate mode coupling. CONSTITUTION:This multilayered film for the laser gyro is constituted by alternately laminating dielectrics TiO2 having a thickness d1 and a high refractive index n1 and dielectrics SiO2 12 having a thickness d2 and a low refractive index n2 and has the wavelength of the laser to be used at lambdan. The incident angle of light on such mirror, designated as thetai and the refractive angles of the above-mentioned dielectrics 11 and 12, respectively designated as theta1 and theta2 are set at n1sintheta1=n2sintheta2=sinthetai0. In addition, the number of the total layers is determined as an even number and the uppermost layer 13 thereof is formed of the dielectric SiO2 having the low refractive index n2. The thickness d3 of the dielectrics of the uppermost layer 13 is so set as to attain theta2'=(2k+1/4) (lambdan/2)k=0, 1, 2, 3.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multi-layer mirror for a laser gyro used in a laser gyro using a ring laser.

[0002]

2. Description of the Related Art Conventionally, a laser using a ring laser
As the gyro, for example, an internal mirror type laser gyro as shown in FIG. 4 is known. Since such a laser gyro is well known, its details are omitted, but in FIG. 4, 1 is a substantially triangular glass block made of low expansion and transparent glass, and a mirror 2-1 is provided at each apex of the glass block. , 2-2 and 2-3 are fixed respectively. In the glass block 1, three pores 3 that are in communication with each other and serve as a laser optical path and form a discharge path 3-
1, 3-2 and 3-3 are formed along each side of the triangle, and these pores 3-1, 3-2 and 3-3 form these three mirrors 2-1 and 2-2. , 2-3 form a resonator. 3-1 ', 3-2' and 3-3 'are the pores 3-1 and
The central axes of 3-2 and 3-3 are shown respectively.

An anode 4 is provided on each side of the glass block 1.
-1, 4-2 and the cathode 5 are pores 3-1 and 3-, respectively.
It is attached facing 2, 3-3. Pores 3-1 and 3-
The internal spaces 2 and 3-3 are made completely airtight, and a gas (laser medium such as helium or neon) for oscillating laser light is enclosed inside.

Reference numeral 6 denotes a composite prism provided outside the mirror 2-3, for example, and is used to combine the clockwise and counterclockwise laser lights in the resonator to extract the angular velocity output.

Now, when the angular velocity is actually measured by a ring laser gyro, the characteristics shown in FIG. 5 are obtained. Figure 5
The dotted line shows the characteristics of an ideal ring laser gyro with no error. On the other hand, as shown by the solid line, in the measured value, the angular velocity output is usually lost when the input angular velocity Ω is very small. This is a phenomenon that has long been known as the "lock-in phenomenon" of the ring laser gyro. On the other hand, when the input angular velocity exceeds a certain value, the actual output becomes larger than the ideal value. This is called a positive error. In the ring laser gyro, the following means is usually used to eliminate such an error.

That is, in FIG. 4, reference numeral 7 denotes a hinge, which elastically supports the whole body including the glass block 1 about an axis (0) perpendicular to the paper surface of FIG.
Cooperating with an electric circuit such as an amplifier connected to this, the glass block 1 has an alternating angular vibration about the axis (0),
That is, the dither movement is given.

However, even if such a method is used, it is impossible to eliminate the error altogether. If the error shown in FIG. 5 is small, the final error is always reduced. Therefore, as shown in FIG. It is no exaggeration to say that "improvement of basic characteristics" is always the greatest research theme of the ring laser gyro.

It has been conventionally said that the mirror is a key part of the ring laser gyro. In particular, regarding the lock-in phenomenon, which is an obstacle to improving the performance, it is considered that the backscattering of the mirror is the main cause, and many researchers have been working to reduce this scattering. However, according to a recent research, a clue for improving the performance has been found by integrating and analyzing the lock-in phenomenon and the positive error by the concept of mode coupling. According to this idea, if the wavelength of the laser light currently used is λn,
Exactly half of this wavelength, ie

[0009]

[Equation 1]

Mode coupling occurs under the influence of the "electric susceptibility" of the substance on the optical path with respect to the wavelength of, and as a result, the characteristics of the gyro include lock-in phenomenon and positive error. It has become clear that is caused.

The electrical susceptibility is theoretically treated as a complex number, but its real part x'is the wavelength λ of the substance.
_When the refractive index for _h is n _h ,

[0012]

[Equation 2]

There is a relationship of

However, the inventors of the present invention have made various studies, but even if it is called a substance on the optical path, the refractive index of helium / neon gas close to vacuum is sufficiently close to "1", and the "electric susceptibility" of the internal gas. Is almost zero, and it turns out that it is the mirrors that dominate this phenomenon.

The conventional multi-layer film mirrors 2-1, 2-2, 2-3 for laser gyros will be described below. FIG. 6 shows a cross section of a multilayer mirror for a laser gyro which is currently used in a normal laser gyro. The substrate 10 is a glass substrate made of the same material as the glass block 1 in FIG. When dielectrics 11 having a refractive index of n ₁ and dielectrics 12 having a refractive index of n ₂ are alternately stacked on this substrate 10 to stack P pairs of layers, the layers up to the second P layer as shown in the drawing are provided. Become a mirror.

In the ring laser, the central axis 3 is used as a resonator.
Since it is necessary for the optical path matching −1 ′, 3-2 ′, 3-3 ′ to have an extremely good resonance property, it is possible to have almost perfect 100% reflectance for the wavelength λn of the oscillation laser light. It is known that, in such a case, the thicknesses d ₁ and d ₂ of each layer need to have the following relationship except for the final layer 13.

[0017]

[Equation 3]

Where θ ₁ and θ ₂ are dielectrics 11 and 1
The respective refraction angles of 2 are obtained by the following Snell's law.

[0019]

[Equation 4]

Here, the refractive index of the laser medium on the optical path is "1", and the incident angle of the laser light on the mirrors 2-1, 2-2, 2-3 is θ _i . The final layer 13 is generally doubled in size in the case of FIG. 6 to 2d ₂ .

As a specific example, the odd-numbered layer dielectric 11
Is TiO ₂ (titanium dioxide) and the even-numbered dielectric layer 12 is S
A material having iO ₂ (silicon dioxide) is a typical one, and in this case, a material having P of 10 or more, that is, having a total of 20 layers or more, can obtain a reflectance close to 100%.

In FIG. 7, TiO ₂ having a thickness of d ₁ and d having a thickness of d ₁
SiO ₂ in 13 pairs of _2, satisfies the number of three conditions to 26 layers, the example in which the thickness of the top layer 13 is 2d _2, showing the measured values of reflectivity for the wavelength of the incident light. The wavelength of the laser light of the helium and neon lasers is 632.8 nanometers (n
m), and the reflectance of the mirror as shown in FIG. 6 for this wavelength λn is 100% as planned.

[0023]

[Problems to be Solved by the Invention]
Λn / 2 = λ _h 316.4 nm involved in coupling
The reflectance corresponding to is around 20 to 30% as shown in FIG. The insides of the pores 3-1, 3-2 and 3-3 have, as already mentioned, only a slight amount of gas in the vacuum,
Since this refractive index is "1", to say that there is reflection, the equivalent refractive index n _m of the mirror is shown that not "1", the n _m the number 2 instead of n _h As you can see by substituting, it shows that at least the real part of the electric susceptibility exists. Therefore, if this conventional multi-layer mirror for laser gyro is used, mode coupling always exists.

Therefore, in the conventional laser gyro using the mirror as shown in FIG. 6, there is a disadvantage that a lock-in phenomenon and a positive error as shown in FIG. 5 occur.

The present invention has been made in view of the above problems, and an object thereof is to propose a multilayer film mirror for a laser gyro which eliminates mode coupling.

[0026]

A multilayer mirror for a laser gyroscope according to the present invention comprises a first dielectric 11 having a thickness d ₁ and a high refractive index n _1, as shown in FIGS. become a thin film of the second dielectric 12 having a lower refractive index n ₂ at d ₂ are laminated alternately, the wavelength of light used λn
In the multi-layer mirror for laser gyro, the total number of layers is set to an even number and the uppermost layer 13 is set to the low refractive index n ₂
When the second dielectric has a refractive index of n ₂ ′ and the refraction angle is θ ₂ ′ for light having a wavelength half that of the light, the thickness d ₃ of the dielectric of the uppermost layer 13 is To It was made to do so.

The multilayer mirror for a laser gyroscope according to the present invention has, for example, as shown in FIGS. 2 and 3, a first dielectric 11 having a high thickness d ₁ and a high refractive index n _1, and a low thickness d ₂ . A multilayer film mirror for a laser gyro, which is formed by alternately stacking thin films with the second dielectric 12 having a refractive index n ₂ and has a wavelength of light to be used as λn, and the total number of layers is an odd number. When the uppermost layer 13 is a dielectric having a low refractive index n _{2 and} the refractive index of the second dielectric is n ₂ ′ and the refraction angle is θ ₂ ′ for light having a wavelength half the wavelength of the light,
The thickness d ₄ of the dielectric of this uppermost layer 13 In addition, the reflectance for a wavelength of approximately ½ of the wavelength λn of the used light is remarkably lowered.

[0028]

According to the present invention, the thickness d ₁ and the high refractive index n ₁
Of the first dielectric material such as TiO ₂ 11 and a second dielectric material such as SiO ₂ 12 having a thickness d ₂ and a low refractive index n ₂ are alternately laminated. The upper layer 13 is a dielectric having a low refractive index n ₂ , and the thickness d ₃ of the uppermost layer 13 is Or the total number of layers is an odd number, the uppermost layer 13 is a dielectric having a low refractive index n ₂ , and the thickness d ₄ of the dielectric of the uppermost layer 13 is Since the reflectance for the wavelength of about ½ of the wavelength λn of the used light is remarkably lowered, the multi-layer film mirror for the laser gyro which constitutes this resonator has a wavelength λn of the laser light as shown in FIG. Can be set to 100%, and the reflectance at a wavelength λ _h , which is approximately ½ of the wavelength λ n of the laser light, can be made extremely low.

Therefore, when the multi-layer film mirror for laser gyro of the present invention is used, a laser gyro which is less affected by mode coupling can be obtained.

[0030]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a multilayer film mirror for a laser gyro according to the present invention will be described below with reference to FIGS. In FIG. 1, parts corresponding to those in FIG. 6 are designated by the same reference numerals, and detailed description thereof will be omitted.

The laser gyro in which the multilayer film mirror for the laser gyro of this embodiment is used is a glass block 1 having a substantially triangular shape and made of transparent glass having a low expansion as shown in FIG.
Multi-layer film mirror 2 for laser gyro of this example at each vertex of
Fix -1, 2-2 and 2-3 respectively. In the glass block 1, three pores 3-1, 3-2, 3-3 which serve as a laser optical path and communicate with each other to form a discharge path are provided.
Are formed along each side of the triangle, and these pores 3-1 and
3-2 and 3-3 form a resonator together with these three laser gyro multilayer film mirrors 2-1, 2-2 and 2-3. Anodes 4-1 and 4-on each side of the glass block 1.
2 and the cathode 5 are attached so as to face the pores 3-1, 3-2 and 3-3, respectively. The inner spaces of the pores 3-1, 3-2, 3-3 are made completely airtight, and a gas (laser medium such as helium or neon) for laser light oscillation is sealed inside.

Further, a multi-layer film mirror for laser gyro 2
A composite prism 6 is provided on the outer side of 3, and the composite prism 6
At, the clockwise and counterclockwise laser lights in the resonator are combined to obtain the angular velocity output.

In this example, the multilayer film mirror for laser gyro 2 fixed to the apex of the glass block 1 is used.
1, 2, 2 and 2-3 are constructed as shown in FIG. In FIG. 1, reference numeral 10 denotes a glass substrate made of the same material as that of the glass block 1. On this glass substrate 10, a dielectric material having a thickness of d ₁ and a high refractive index n ₁ , such as TiO ₂ 11 and a thickness of d
₂ and a low-refractive-index n ₂ dielectric, for example, SiO ₂ 12 are alternately stacked up to (2P-1) layers, and the uppermost layer 13 thereon.
Is a dielectric having a thickness of d ₃ and a low refractive index n ₂ , such as Si
Stack O ₂ . That is, the even number of 2P layers is 26 in this example. The thicknesses d ₁ and d ₂ of the respective layers are set to the relationship of Equation 3, that is, n ₁ d ₁ cos θ ₁ = n ₂ d ₂ cos θ ₂ = λn / 4. However, θ ₁ and θ ₂ are the dielectrics 11 and 12, respectively.
Is the refraction angle of each and is obtained by Snell's law of equation 4. n ₁ sin θ ₁ = n ₂ sin θ ₂ = sin θi Here, the refractive index of the laser medium on the optical path is “1”, and the multilayer mirrors 2-1 and 2-2 for the laser gyro of the laser light.
The incident angle on 2, 2-3 is θi. The wavelength of the laser light used is λn. In this example, the thickness d ₃ of this uppermost layer 13 is

[0034]

[Equation 5]

The case of k = 2 in the relation of Here, n ₂ ′ is the wavelength λ _h = λn / 2 of the low refractive index dielectric 12
Is the index of refraction for the wavelength n
₂ is generally different. θ ₂ ′ also

[0036]

[Equation 6]

Is the refraction angle according to. As shown in FIG. 2, the reflection characteristics of the multi-layer mirror for a laser gyro having the structure shown in FIG. 1 have a wavelength λn63 of the laser light of a helium or neon laser.
The reflectance at 2.8 nm is 100%, and the wavelength λ _h 31 which is 1/2 of this wavelength λn involved in mode coupling.
The reflectance at 6.4 nm was about 0%.

It was confirmed that the value of d ₃ for which the characteristic as shown in FIG. 2 is obtained is not limited to the above-mentioned value and may be any value satisfying the expression 5.

Since the laser gyro described above uses the multilayer film mirror for laser gyro according to the present example as the mirrors 2-1, 2-2, 2-3 forming the resonator, its characteristics are as shown in FIG. The reflectance at the wavelength λn of the laser light used is set to 100%, and 1 /
Since the reflectance at wavelength λ _h of 2 is approximately zero, the electrical susceptibility at λn / 2 = λ _h of this optical path becomes zero, and the mode
_Eliminates the coupling, the basic characteristics according to斯Ru laser gyro is profit that can obtain an output angular velocity for such good input angular rate Ω is shown in dashed schematic 5.

FIG. 3 shows another example of the multilayer film mirror for laser gyro of the present invention. To describe FIG. 3, parts corresponding to those in FIGS. 1 and 6 are designated by the same reference numerals, and detailed description thereof will be omitted.

In the example of FIG. 3, the dielectric on the glass substrate 10
The film layers are odd layers (2P + 1) and the glass substrate 1
0 has a thickness of d₂And low refractive index n₂Dielectric, example
For example SiO₂12 and the thickness of the second layer is d₁With high refractive index
n₁Dielectric, eg TiO ₂11 and exchange these sequentially
Stack up to 2P layers on top of each other, and put the top layer 13 on top of it
d_FourAnd low refractive index n₂Dielectric material such as SiO₂Pile up
Overlap. In this example, the number of layers is 27. Thickness d of each layer₁，
d₂Is the relation of Equation 3, that is, n₁d₁cos θ₁= N₂d₂c
osθ₂= Λn / 4. Thickness d of this uppermost layer 13_Four
To

[0042]

[Equation 7]

The relationship is as follows. Such a mirror as shown in FIG. 3 also has a reflectance of 100 at the wavelength λn of the laser light as shown in FIG.
%, And it was found that the reflectance at the wavelength λ _h , which is ½ of the wavelength λ n, becomes substantially zero. Needless to say, when the multilayer film mirror for a laser gyro shown in FIG. 3 is used as the mirrors 2-1, 2-2, 2-3 of the laser gyro, the same effects as the above can be obtained. ..

Although SiO ₂ is used as the dielectric material having a low refractive index n _{2 in} the above-mentioned embodiments, other dielectric materials having a low refractive index such as MgF ₂ can be used instead, and a high refractive index n 2 can be used.
_Although TiO ₂ was used as the dielectric of _1, the dielectric having a high refractive index such as Ta ₂ O ₅ can be used instead.

Further, the present invention is not limited to the above-mentioned embodiments, and it goes without saying that various other configurations can be adopted without departing from the gist of the present invention.

[0046]

According to the present invention, the reflectance of the laser light at the wavelength λn is set to 100%, and the reflectance for the wavelength λ _{h of} about ½ of the wavelength λn of the laser light is made extremely low and substantially zero. It is possible to obtain a multilayer film mirror for a laser gyro, and when the multilayer film mirror for a laser gyro according to the present invention is used in a laser gyro, there is little error in the basic characteristics with little influence of mode coupling. There is a benefit of being able to obtain an output for an input angular velocity.

[Brief description of drawings]

FIG. 1 is a sectional view showing an embodiment of a multilayer film mirror for a laser gyro according to the present invention.

FIG. 2 is a diagram showing a reflection characteristic of the mirror of FIG.

FIG. 3 is a cross-sectional view showing another embodiment of the multilayer film mirror for laser gyro of the present invention.

FIG. 4 is a configuration diagram showing an example of a laser gyro.

FIG. 5 is a diagram for explaining the present invention.

FIG. 6 is a cross-sectional view showing an example of a conventional multilayer film mirror for laser gyro.

FIG. 7 is a diagram showing a reflection characteristic of the mirror of FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Glass block 2-1, 2-2, 2-3 Mirror 3-1, 3-2, 3-3 Micropores 4-1, 4-2 Anode 5 Cathode 6 Synthetic prism 7 Hinge 10 Substrate 11, 12 Dielectric material 13 top layer

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Satoshi Ishikawa 2-16-46 Minami-Kamata, Ota-ku, Tokyo Within Tokimetsu Co., Ltd. (72) Inventor Kenmori Masushima 2-16-46 Minami-Kamata, Ota-ku, Tokyo No. Stock Company Tokimetsu

Claims (2)

[Claims] 1. Thin films of a first dielectric having a thickness d ₁ and a high refractive index n ₁ and a second dielectric having a thickness d ₂ and a low refractive index n ₂ are alternately laminated. In a multilayer mirror for a laser gyro having a wavelength of light to be used as λn, the total number of layers is an even number, and the uppermost layer is the above-mentioned low refractive index n ₂
When the second dielectric has a refractive index of n ₂ ′ and the angle of refraction is θ ₂ ′ for light having a wavelength half that of the light, the thickness d ₃ of the uppermost dielectric is To A multilayer mirror for a laser gyro, characterized in that 2. A thin film of a first dielectric having a thickness of d ₁ and a high refractive index n ₁ and a thin film of a second dielectric having a thickness of d ₂ and a low refractive index n ₂ are alternately laminated. In a multi-layer mirror for a laser gyroscope, in which the wavelength of light used is λn, the total number of layers is an odd number, and the uppermost layer is the above-mentioned low refractive index n.
A _second dielectric, a refractive index of the second dielectric for light with a wavelength half the wavelength of the light n ₂ when ', the refraction angle theta _2' and the thickness d of the dielectric of the uppermost layer ₄ A multilayer mirror for a laser gyro, characterized in that the reflectance for a wavelength of approximately ½ of the wavelength λn is remarkably lowered.