JPH10268112A - Anamorphic prism - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an anamorphic prism having a less degree of frequency dependency. SOLUTION: Two prisms 1 and 2 are formed out of different glass materials, and the magnifications thereof are also optimized at values different from each other. As a result, what is called achromatic type anamorphic prism having the characteristic that an outgoing beam direction is constant regardless of a wavelength fluctuation can be obtained. When the anamorphic prism so formed is used for the optical system of an optical disc system, the positional fluctuation of a beam spot due to the fluctuation of laser wavelength can be prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an anamorphic prism which compresses or expands an incident light beam in a specific direction of a light beam cross section and emits it.

[0002]

2. Description of the Related Art Generally, in an optical disk system or the like, a magnification of an outgoing light beam with respect to an incident light beam is changed (that is, compressed or expanded) in a specific direction of a light beam cross section, and then the light is emitted in a direction parallel to the incident light beam. A prism called a so-called anamorphic prism is often used as an optical element. Conventionally, this type of anamorphic prism has been configured by combining two prisms having the same shape and the same glass material (glass material).

FIG. 3 shows a configuration example of a conventional anamorphic prism. This anamorphic prism is configured by combining two prisms 101 and 102 having the same glass material and the same apex angle. Here, the refractive index of both prisms is n, and the apex angle is t. The prism 101 is arranged such that when an incident light beam enters the first surface at an incident angle θ, the outgoing light beam exits perpendicularly to the second surface. Similarly, the prism 102
The incident light beam (the light beam emitted from the prism 101) is the first light beam.
It is arranged such that when it enters the surface at an incident angle θ, the outgoing light beam exits perpendicularly from the second surface. That is,
As shown in the figure, the prism 102 is arranged such that the first surface forms an angle θ with the second surface of the prism 101.

[0004] As shown in FIG.
When the emitted light beam is emitted from the second surface perpendicularly to the first surface when the light beam enters the first surface at an incident angle θ, the refraction angle on the first surface becomes equal to the apex angle t. Similarly, the refraction angle on the first surface of the prism 102 becomes equal to the apex angle t. In such an arrangement, the outgoing light beam is parallel to the incident light beam.

At this time, the relationship between the incident angle θ and the apex angle t is expressed by the following equation (1) according to Snell's law. sinθ = nsint (1)

At this time, the combined magnification β in the direction parallel to the paper surface in the cross section of the light beam is expressed by the following equation (2). β = W ₂ / W ₁ (2) Here, W ₁ indicates an incident light beam width in a direction parallel to the paper surface, and W ₂ indicates an outgoing light beam width in the same direction.

Since the prisms 101 and 102 are made of the same glass material and have the same apex angle, the respective magnifications are equal, and the product of both is the combined magnification β.

[0008]

In the anamorphic prism having such a configuration, consider a case where the wavelength of the incident light beam is slightly shifted and the refractive index of the prism is changed by Δn. The path in this case is as shown in FIG. 4, for example. As shown in this figure, the refraction angle on the first surface of the prism 102 is shifted from the original refraction angle t by Δt ₁ to t.
−Δt ₁ , the incident angle on the second surface is Δt ₁
Becomes Assuming that the refraction angle on the second surface is Δt ₂ , the incident angle on the first surface of the prism 102 is θ + Δt ₂ . Therefore, assuming that the refraction angle on the first surface of the prism 102 is Δt ₃ , refraction on each surface is represented by the following equations (3) to (5). sinθ = (n + Δn) sin (t-Δt 1) ...... (3) (n + Δn) sinΔt 1 = sinΔt 2 ...... (4) sin (θ + Δt 2) = (n + Δn) sin (t + Δt 3) ...... (5)

By rearranging the above equations (1), (3) to (5), the following equation (6) is obtained. Δt ₃ = [Δt ₁ / ((n + Δn) cost)] × [(n + Δn) cosθ−ncost] (6)

In FIG. 4, the condition for the output light beam to be parallel to the incident light beam even if the wavelength of the incident light beam shifts, that is, the condition for this prism system to be an achromatic prism is Δt ₃ = 0. That is. Therefore, the following equation (7) holds. (N + Δn) cos θ = n cost (7) Accordingly, the composite magnification β of the anamorphic prism is expressed by the following equation (8). β = (cost / cos θ) ² = [(n + Δn) / n] ² (8) Here, since n is sufficiently larger than Δn, the magnification β is substantially 1.

As described above, since the refractive index of the glass material differs depending on the wavelength of the light used, if the actual wavelength deviates from the design value, the exit angle from the prism will not be parallel to the incident light beam. Therefore, if an attempt is made to design so that the exit angle from the prism is parallel to the incident light beam, the resultant magnification β becomes almost 1 as shown in the above conclusion, and it becomes meaningless as an anamorphic prism. I will.

This may cause a serious problem in, for example, an optical modulation type optical disk system. That is, in this type of optical disk system, a laser is normally turned on and off to record a signal on a disk medium, but the wavelength of the laser often fluctuates when the laser rises and falls. In such a case, since the emission angle from the anamorphic prism changes, the beam spot moves on the disk when condensed by the objective lens. For example, in the case where a laser having a wavelength of 640 nm, an anamorphic prism having a synthetic magnification β of 2.5 times and a glass type of SF11, and an objective lens having a focal length of 3.63 mm are used,
Assuming that the wavelength changes by 3 nm, the beam spot moves by 0.25 μm on the disk recording surface. Usually, since the crack pitch is about 0.8 μm, this beam spot movement amount is a considerably large amount. Therefore, when the anamorphic prism is used in the linear density direction of the optical disc, it causes jitter, and when it is used in the track direction, it causes the amount of crosstalk to fluctuate.

The present invention has been made in view of such a problem, and an object of the present invention is to provide an anamorphic prism having little wavelength dependence.

[0014]

An anamorphic prism according to the present invention comprises: a first prism which converts an incident light beam at a predetermined magnification in a specific direction in a cross section of the light beam and emits the light; A second prism formed of a glass material different from the first prism, and performing a conversion at a magnification different from the magnification in the specific direction of a cross section of the light beam on the light beam emitted from the first prism and emitting the light beam. ing. Here, the first prism is, for example, its first prism.
An incident light beam having a predetermined wavelength incident on the surface at a first incident angle is refracted and emitted perpendicularly to the second surface, and the second prism is incident on the first surface at a second incident angle. The light beam emitted from the first prism is refracted and emitted perpendicularly from the second surface, so that the light beam incident on the first prism and the light beam emitted from the second prism are parallel. It is possible. Specifically, the refractive indices n ₁ and n _{2 of} the first and second prisms and the first with respect to the shift of the incident wavelength
And the change amount Δn ₁ , Δ of the refractive index of the second prism
n ₂ , the magnifications β ₁ and β ₂ of the first and second prisms, and the combined magnification β of the first and second prisms can satisfy the following expressions (9) to (13). . Further, the first incident angle θ ₁ and the second incident angle θ ₂ are values given by equations (14) and (15) described later, and the apex angles t ₁ and t of the first and second prisms.
₂ can be a value given by the following equations (16) and (17). Such an anamorphic prism is applicable to, for example, an optical disk system.

In the anamorphic prism of the present invention,
The two prisms are formed of different glass materials having different refractive indices and dispersions, and their respective magnifications are also set differently. Therefore, by appropriately setting the apex angle and incident angle of each prism and optimizing each magnification, it is possible to keep the exit angle of the exit light beam substantially constant even when the wavelength of the incident light beam fluctuates. Becomes For this reason, when this anamorphic prism is applied to an optical disk system, it is possible to prevent a change in the position of the beam spot due to a change in the wavelength of the laser.

[0016]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 shows a configuration of an anamorphic prism according to an embodiment of the present invention. The anamorphic prism includes two prisms 1 and 2 made of different glass materials and a base 3 for fixing these prisms. Prism 1
Are arranged such that when an incident light beam having a predetermined wavelength (hereinafter, referred to as a design wavelength) enters the first surface at an incident angle θ ₁ , the outgoing light beam exits perpendicularly to the second surface. ing. Similarly, the prism 2 is configured such that an incident light beam of a design wavelength (a light beam emitted from the prism 1) is incident on its first surface at an incident angle θ.
_The light is emitted from the second surface so as to be emitted perpendicular to the second surface when the light enters at ₂ . That is, as shown in the figure, the prism 2 is arranged such that the first surface forms an angle θ ₂ with the second surface of the prism 11. Then, the light beam incident on the prism 1 and the light beam emitted from the prism 2 are parallel.

As shown in the drawing, the width of the light beam incident on the first surface of the prism 1 (in the direction of the paper surface in the cross section of the light beam) is a, and the magnification of the prisms 1 and 2 (in the direction of the paper surface in the cross section of the light beam) is β _1, if the beta _2, light emitted flux width from the second surface of the prism 1 beta ₁ a, and the emitted light flux width of the second surface of the prism 2 becomes beta ₁ beta ₂ a. That is, the incident light flux is a combined magnification β = β of the two prisms 1 and 2.
_The light is enlarged and emitted by ₁ β ₂ .

Here, it is assumed that the wavelength of the incident light beam is slightly shifted from the design light beam. Since the refractive index of the glass material varies depending on the wavelength of light, in a conventional anamorphic prism having the same glass material and the same magnification as in the related art, the final exit angle is not parallel to the incident light beam in such a case. Therefore, if an attempt is made to design the outgoing light beam from the anamorphic prism so as to be parallel to the incident light beam, the resultant magnification β becomes almost 1 this time, and it becomes meaningless as an anamorphic prism. This has already been described in the section of the prior art.

On the other hand, in the anamorphic prism according to the present embodiment, the two prisms 1 and 2 are formed of different glass materials, and the two prisms are optimized so that the magnifications thereof are different from each other. That is, the refractive index, dispersion, and apex angle of the two prisms are set to different values. Specifically, the following (9) to (13)
Set each parameter to satisfy the formula. Where n
₁ and n ₂ indicate the refractive indices of the prisms 1 and 2, and Δn ₁ and Δn
n ₂ indicates the amount of change in the refractive index of each prism with respect to the shift of the incident wavelength, and β ₁ and β ₂ indicate the magnification of each prism.

Δn ₁ / Δn ₂ = (β−β ₁ n ₁ ) / (β ₁ −n ₂ ) (9) β ₁ = 2 ^−1/2 [(β ² + 1−A) − ((A − (Β−1) ² ) ^1/2 × (A− (β + 1) ² ) ^1/2 ] ^1/2 : n ₁ > n ₂ (10) β ₁ = 2 ^−1/2 [(β ² +1) −A) + ((A− (β−1) ² ) ^1/2 × (A− (β + 1) ² ) ^1/2 ] ^1/2 : n ₁ <n ₂ (11) A = [(n _{1 ^{2 -1) (n 2 2 -1}} ) / (n 1 n 2 -1) 2 ] ^{(β-1) 2 ...... (} 12) β = β 1 β 2 ...... (13)

Further, the angles of incidence θ ₁ and θ on the prisms 1 and 2
₂ is a value represented by the following equations (14) and (15), respectively, and the vertex angles t ₁ and t ₂ of the prisms ₁ and ₂ are represented by the following equations (16) and (17), respectively. Value.

[0022] θ ₁ = tan ^{-1 _{[n 1 2 (β 1 2 -1}} ) / (n 1 2 -1) ] _{...... (14) θ 2 = tan} -1 [n _{² 2} (β _{2 2} -1 ) / (n _{² 2} -1)] _{...... (15) t 1 = sin} -1 (sinθ 1 / n 1) ...... (16) t 2 = sin -1 (sinθ 2 / n 2) ...... (17 )

By setting the parameters of the prisms 1 and 2 so as to satisfy the expressions (9) to (17), even if the wavelength of the incident light beam is slightly shifted, the direction of the final outgoing light beam is incident. It can be maintained parallel to the direction of the light beam, and the synthesis magnification β can be set to a value of 1 or more.

FIG. 2 shows the relationship between the shift amount of the incident light beam wavelength with respect to the design wavelength and the angle shift amount of the output light beam with respect to the incident light beam. In this figure, the horizontal axis represents the wavelength shift amount (nm), and the vertical axis represents the shift amount (milliradian) of the emission angle. It should be noted that the data shown here are obtained when the synthesis magnification β is 2.5 and the design wavelength is 640 nm. The data on the conventional anamorphic prism (indicated by a square) is obtained when both of the prisms are made of the glass material SF11. The data on the anamorphic prism according to the present embodiment (○) ), Prisms 1 and 2 are made of glass material SF, respectively.
11, LF6.

As shown in this figure, in the conventional anamorphic prism, the shift amount of the emission angle increases as the wavelength shift amount increases.
At nm, the deviation of the emission angle reaches as much as 1 milliradian. On the other hand, in the anamorphic prism according to the present embodiment, the emission angle hardly shifts even if the wavelength shift amount increases. That is, the anamorphic prism functions as an achromatic type anamorphic prism in which the direction of the emitted light beam is constant irrespective of wavelength fluctuation.

Next, a procedure for actually setting each parameter will be briefly described.

First, the synthesis magnification β is determined, and a glass material for the prism 1 is selected. Thus, β, n ₁ , Δ
n ₁ is determined. Next, a glass material for the prism 2 is selected. This allows
n ₂ and Δn ₂ are determined. Specifically, this step
This is performed by searching for a glass material having n ₂ and Δn ₂ satisfying the above equations (9) to (13) with respect to the design wavelength from a material list of an optical glass manufacturer. From the expressions (10) to (12), the magnification β _{1 of the} prism 1 is obtained. It is checked whether the obtained β ₁ satisfies the expression (9). As a result, the process proceeds to the case where the condition is satisfied, and returns to the process if the condition is not satisfied. From the equation (13), the magnification β _{2 of the} prism 2 is obtained. The incident angles θ ₁ and θ ₂ are obtained from the equations (14) and (15). The apex angles t ₁ and t ₂ are obtained from the equations (16) and (17).

[0028]

Next, an embodiment of the parameters obtained by the above procedure will be described. Here, the design wavelength is 635.
nm, the wavelength change amount is set to 3 nm, and the synthesis magnification β is set to 2.5.

In this embodiment, the prism 1 is made of a glass material SF.
11, and a glass material LF6 is used as the prism 2. Each parameter at this time is set as follows.

N ₁ = 1.777835 n ₂ = 1.56484 Δn ₁ = 0.00036 Δn ₂ = 0.00016 θ ₁ = 53.40 ° θ ₂ = 60.28 ° t ₁ = 26.84 ° t ₂ = 33.72 °

It should be noted that the refractive index change Δn _{1 with} respect to the refractive index n ₁ , n ₂ and the wavelength shift amount (here, 3 nm),
Δn ₂ can be obtained by using the dispersion equation shown in the following equation (18). n = A ₀ + A ₁ λ ² + A ₂ λ ^-2 + A ₃ λ ^-4 + A ₄ λ ^-6 + A ₅ λ ^-8 (18) where A _{0 to} A ₅ are constants specific to each glass material, The following values are respectively taken. [SF11] / [LF6] _{A 0 = 3.0800518 / 2.4053230 A 1} = -2.3511160 × 10 -2 / -8.7462006 × 10 -3 A 2 = 1.9460876 × 10 -2 / 1. 6897999 × 10 ⁻² A ₃ = 9.610658 × 10 ⁻³ /6.8383475×10 ⁻⁴ A ₄ = −1.247765 × 10 ⁻³ /−2.9236246×10 ⁻⁵ A ₅ = 8.7773942 × 10 ^-5 /3.1434256×10 ^-6

As described above, in the present embodiment, the two prisms 1 and 2 are made of different glass materials and the magnifications of the two prisms are optimized so as to be different from each other. It is possible to obtain a so-called achromatized anamorphic prism having the characteristic that the direction of the emitted light beam is constant irrespective of wavelength fluctuation while ensuring β.

Further, in the optical disk system, when the optical system is configured by using the anamorphic prism according to the present embodiment, it is possible to prevent the position change of the beam spot due to the wavelength change of the laser. Therefore, when the anamorphic prism is used in the linear density direction of the optical disk, the generation of jitter can be prevented, and when the anamorphic prism is used in the track direction, the fluctuation of the crosstalk amount can be prevented.

Although the present invention has been described with reference to the embodiment, the present invention is not limited to this embodiment, and can be variously modified within an equivalent range. For example, in the above embodiment, the prisms 1 and 2 are SF11 and LF6.
The parameters are set to the values shown above using the above glass material. However, if the parameters are within the range satisfying the above equations (9) to (17), other glass materials and parameter values are appropriately changed. It is possible.

[0035]

As described above, according to the anamorphic prism of the present invention, the two prisms are formed of different glass materials having different refractive indexes and dispersions, and the magnification of each prism is also different from each other. By setting the apex angle and incident angle of each prism appropriately and optimizing each magnification, the output angle of the output light beam is almost constant even if the wavelength of the input light beam fluctuates. , And the combined magnification of both prisms can be set to a value larger than 1. That is, an achromatized anamorphic prism can be realized while securing a synthesis magnification larger than 1. Further, when this anamorphic prism is applied to an optical disk system, it is possible to prevent a change in the position of a beam spot due to a change in laser wavelength.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a configuration of an anamorphic prism of the present invention.

FIG. 2 is a diagram illustrating a relationship between a wavelength shift amount of an incident light beam and a shift amount of an emission angle.

FIG. 3 is a diagram illustrating a configuration of a conventional anamorphic prism.

FIG. 4 is an explanatory diagram for describing a problem in the anamorphic prism of FIG. 3;

[Explanation of symbols]

1 ... prism (first prism), 2 ... prism (second prism)
3) Base.

Claims (4)

[Claims] A first prism that converts an incident light beam at a predetermined magnification in a specific direction in a cross section of the light beam and emits the light; and a first prism formed of a glass material different from the first prism;
An anamorphic prism comprising: a second prism that converts a light beam emitted from the prism described above into a light beam cross-section at a magnification different from the magnification in the specific direction and emits the converted light beam. 2. The first prism refracts an incident light beam having a predetermined wavelength incident on a first surface thereof at a first incident angle and emits the light beam perpendicularly to the second surface from the second surface. Refracts the luminous flux from the first prism that has entered the first surface at a second angle of incidence and emits the luminous flux perpendicularly to the second surface. The luminous flux incident on the first prism and the 2. The light beam emitted from the second prism is parallel.
Anamorphic prism as described. 3. The first and second prisms have a refractive index of n ₁ and n ₂ , respectively, and the first and second prisms have a first index with respect to a shift of an incident wavelength.
And the amount of change in the refractive index of the second prism is Δn
₁ and Δn ₂ , the magnifications of the first and second prisms are respectively β ₁ and β _2, and the composite magnification of the first and second prisms is β, these parameters are represented by Δn ₁ / Δn ₂ = (Β-β ₁ n ₁ ) / (β ₁ -n ₂ ) β ₁ = 2 ^-1/2 [(β ² +1 -A)-((A- (β-1) ² ) ^1/2 × ( A− (β + 1) ² ) ^1/2 ] ^1/2 : n ₁ > n ₂ β ₁ = 2 ^−1/2 [(β ² + 1−A) + ((A− (β−1) ² ) ^{1 / 2} × (A− (β + 1) ² ) ^1/2 ] ^1/2 : n ₁ <n ₂ A = [(n ₁ ² −1) (n ₂ ² −1) / (n ₁ n ₂ −1) ^{2 3. The} anamorphic prism according to claim ^{2, wherein} (β-1) ² β = β ₁ β ₂ is satisfied. Wherein said first and second prism, the theta ₁ is first incident angle and the second angle of incidence, theta ₂ is, theta ₁ = tan ^-1 [n ₁ ² (beta ₁ ^{2 _{-1) / (n 1 2 -1}} ) ] θ ₂ = tan ^{-1 _{[n 2 2 (β 2 2 -1}} ) / (n 2 2 -1) ] are arranged so as to satisfy the said first and second The apex angles t ₁ and t ₂ of the _two prisms are given by t ₁ = sin ⁻¹ (sin θ ₁ / n ₁ ) t ₂ = sin ⁻¹ (sin θ ₂ / n ₂ ). An anamorphic prism according to 3.