JPS63197902A - Beam forming prism - Google Patents

Abstract

PURPOSE:To prevent the generation of a relative misalignment of a write erasion beam and a read-out beam on a disk, by joining a first prism and a second prism consisting of different kinds of materials, so that the specific conditions are satisfied. CONSTITUTION:The titled prism is formed by joining the first prism 1 and the second prism 2 consisting of different kinds of materials, whose refractive indexes are n1 and n2, respectively. In this state, when a light beam is made incident at an incidence angle theta from a medium whose refractive index is n0, such as air, etc., to the first prism, refracted at a refraction angle phi, and made incident at an incidence angle alpha from the first prism 1 to the second prism 2, refracted at a refraction angle beta, and emitted at an incidence angle 0 deg. from the second prism 2 to other medium, conditions of an equation I are roughly satisfied. In such a way, even if a variation of wavelength and a variation of a refractive index of the prism are generated due to a temperature change, etc., of a laser light source, an outgoing angle of a beam from the prism is not varied, and a relative shift of a write/read-out beam and an erasion beam in an optical disk device is not generated.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a beam shaping prism for a semiconductor laser used in an optical disk device or the like, particularly a two-beam system.

[Conventional technology]

Generally, the optical system of a two-beam type optical disk device using two light sources has a configuration as shown in FIG. 6) is a view of the prisoner in FIG. 6 as seen in the direction of arrow B. In this figure, 11 is a semiconductor laser for writing and reading beams, 12 is a semiconductor laser for erasing beams, 13 and 14 are collimating lenses, respectively. 15 is a four-division deflector,
16 is a beam shaping prism, 17 is a polarizing beam splitter, 18 is a 7-wave plate, 19 is an objective lens, 20 is a dichroic mirror, 21 is a condensing lens, and 22 is a cylindrical lens.

In such an optical disk device, a writing/reading beam from one light source 11 is transmitted through a collimating lens 13.
The S
The polarized light component becomes linearly polarized light and becomes circularly polarized light through the seven-wavelength plate 18. Thereafter, the light is focused onto the disk 23 by the objective lens 19. The beam reflected by the disk 23 is
The objective lens 19 returns the beam as parallel light, and the 7-wave plate 18 converts it into linearly polarized light.Polarizing beam splitter 17
is incident on the P-polarized light component as linearly polarized light. The beam that has passed through the polarizing beam splitter 17 is reflected by a dichroic mirror 20 and focused onto a four-division detifter 15 by a condenser lens z1 and a cylindrical lens 22. Based on the output of the deflector 15, the focus and track of the objective lens 19 are adjusted by the known astigmatism method or push-pull method.

On the other hand, the light beam from the semiconductor laser 12 for erasing beam is collimated by the collimating lens 14, passes through the dichroic mirror 20, and passes through the polarizing beam splitter 17.
The light beam enters as P-polarized light and is focused on the disk 23 in the same way as the light beam from the semiconductor laser 11.

In this optical disk device, the light beam emitted from the semiconductor laser has an elliptical pattern with a ratio of 2 to 3:1. Therefore, the semiconductor laser 11 for writing and reading has an axially symmetrical emission pattern (
It needs to be circular. A beam shaping prism 16 is used for this purpose.

However, the wavelength of semiconductor lasers changes depending on temperature and emission output. Furthermore, the refractive index of glass, which is the material of the prism, varies depending on the wavelength and changes depending on the temperature. Therefore, when the temperature changes, the refraction angle of the beam shaping prism 16 changes due to a change in the wavelength of the beam from the semiconductor laser and a change in the refractive index of the glass. For example, the change due to temperature of a semiconductor laser is about 0.25"%eg, and the wavelength dependence of the refractive index of glass is about -2X 10-"/nm (however, λ=
7oo~9o.

nm). Therefore, the change in refractive index caused by the wavelength change due to temperature change of the semiconductor laser is 5 x 1
0-'/deg. In addition, the refractive index change of the glass itself due to temperature increase is +2.7X1 when the glass is BK7.
There is 02°J. Therefore, the change in refractive index that occurs due to temperature changes is approximately -2,3X 10-'/deg.

Beam shaping prism 1 in conventional optical disk device
6, the correction factor when converting an elliptical pattern into a circular pattern is 2.5, and the refractive index of the beam shaping prism 16 is 1.5.
1, the change in refraction angle due to the above temperature change is 7.
lx10deg and n when ejecting from the prism
(n is the refractive index of the prism), approximately 1. IX1
It is 0-'deg. This sets the focal length of the objective lens to 4
In the case of a seagull, the spot on the disk will move approximately 0.0077 μm with a change of 1°C.

In addition, since the emission pattern of the erasing light beam enters the objective lens 19 as a long ellipse in the disk radial direction, the actual NA of the objective lens is large in the disk radial direction and small in the circumferential direction. is an oval shape that is elongated in the circumferential direction of the disk.

The spots on the disk are adjusted so that the erase beam E precedes the write and read beam WR on the same group as shown in FIG.

At this time, the position of the erasing beam is strict in the radial direction, and even if it deviates by 0.1 μm in this direction, erasing will not be complete.

However, as described above, if the write/read beam shifts in the disk radial direction, the write/read beam will be servoed on the group, resulting in a relative shift of the erase beam.

If the entire device rises by 15 degrees, the beam will shift by about 0.1 μm, making it impossible to perform satisfactory erasure.

[Problem that the invention seeks to solve]

The problem to be solved by the present invention is a beam shaping prism used in an optical disk device, and even if the wavelength changes or the refractive index of the prism changes due to changes in the temperature of the laser light source, the beam shaping prism of the prism It is an object of the present invention to provide a beam shaping prism which prevents the emission angle from changing and prevents relative deviation between a write/read beam and an erase beam in an optical disk device.

[Means for solving problems]

In order to solve the above-mentioned problems, the beam shaping prism of the present invention is made by joining a first prism and a second prism made of different materials each having a refractive index of nl + n2. is incident on the first prism at an incident angle θ from a medium with a refractive index of no, and is refracted at a refraction angle φ.
The structure is such that the following conditions are approximately satisfied when the light enters the second prism from the prism at an incident angle α, is refracted at a refraction angle β, and exits from the second prism to another medium at an incident angle O0. This is what I did.

It shows the change in the refractive index of the medium, including the change in wavelength due to change in wavelength. In other words, the total change in the refractive index of the medium due to the wavelength fluctuation caused by temperature change and the refractive index change of the medium itself due to temperature change, assuming that the change in wavelength of the semiconductor laser due to temperature change is constant. It shows.

Therefore, it is.

The beam shaping prism of the present invention is configured as described above to correct the beam cross-sectional shape of a semiconductor laser beam or the like to make it approximately circular, and the exit angle is changed by changing the refractive index of the prism due to temperature changes. This is to ensure that there is no fluctuation.

Next, the present invention will be described in detail in order to clarify that the beam shaping prism of the present invention having the above-mentioned configuration achieves the second objective of the invention.

FIG. 1 is a diagram showing the principle of the present invention. As shown in this figure, consider the case where a beam passes through four media whose refractive indexes are no + nt l n2 r ns and whose boundaries are surfaces S+, S2, and Ss, respectively. That is, the beam is refracted at each interface and becomes 4, t2, As,
4, and the angle of incidence and angle of refraction at each interface at that time are set as θ, φ; α, β; γ, ε, respectively, as shown in the figure.

First, when the beam t is refracted at the boundary surface S1 and travels as t, the following relationship holds from Snell's law.

nomθ=nl sinφ From this relationship, if n+' = ", the following equation (1) is obtained.

It will be done.

House θdθ” sinφ・dn+'+1' partφdφ
・・・・・・・・・(1) Similarly, the boundary surface S2,
For refraction at Ss, respectively n+sinα=nzsi
The following equations (2) and (3) can be obtained from the relationships nβ and n2sinγ=n3s'mε.

sinα* dn+' + nl'cosα* dα”
sinβdnz' + n2' cosβ・dβ
-, ... (2) Liao γdn2' + n2'
cos70dγ= sinεdns' + ns'
ayst d t---(3) However, nt r n3-1.

no n.

Here, if the angle of incidence θ is constant and the angle of refraction ε is constant, then dθ=dε=0 and dφ=−dα, so equation (1) becomes the following equation (4). become.

dn+' dα= tanφ・=°°°゛°°゛°(4)nl Bunmu (3) is as follows from dγ=-dβ, and formulas (4) and (5) are By substituting into (2), the following conditional expression (6) is derived.

(5irIa + ca; a・tanφ)dnl'...
・−・・・・・・(6) This conditional expression (6) means that when the refractive index changes, θ is constant and ε
This is a conditional expression for keeping constant.

If the fourth medium is the same as the first medium, n3=n
o, so dns' = O. Therefore, in this case, equation (6) becomes as follows.

(sinα+cmαtanφ) dn+' = (=β
On the other hand, the correction factor m when correcting a beam with an elliptical cross section to a beam with a circular cross section is expressed as follows.

Here, At is the incident angle on the i-plane, and Bi is the refraction angle on the i-plane.

Third medium (n2) to fourth medium (ns = no)
If the beam t3 is refracted at the boundary surface t3 and proceeds as a beam t4, generally 12> na
= no, in that case ε>γ and the correction factor will be reduced, so generally γ=ε=08
I often do this. Therefore, conditional expression (7) becomes as follows.

(mα+cosα・tanφ) dn,' = gtn
βdnz' From this, the following conditional expression (8) is derived.

Therefore, θ (therefore φ), α so that conditional expression (8) holds true for the refractive index change dnt' l tin2'
If you choose , the beam emitted from the prism will not change even if the refractive index changes due to changes in temperature or wavelength.

The oscillation wavelength of a semiconductor laser changes with temperature change, so this must be taken into consideration. Changes in the refractive index of the pick medium must be considered, including changes in wavelength due to changes in the temperature of the light source.

Generally, in a semiconductor laser, the seed dλ shifts toward longer wavelengths at a constant rate as the temperature increases. Therefore... is constant (
= C), the change in refractive index due to key temperature, including the change in wavelength, is given by the following equation (9).

Therefore, the change in refractive index dn,' in conditional expression (8)
In other words, θ (and therefore φ) is set so that the above formula is approximately satisfied.
, α, it is possible to avoid variations in the beam emitted from the correction prism, including variations in the wavelength of the light source due to temperature changes.

Now, consider a beam shaping prism consisting of a first prism 1 and a second prism 2 as shown in FIG.

Here, if BK4 is used as the glass material of the first prism 1 and BK7 is used as the glass material of the second prism 2, BK4. The characteristics of BK7 are as follows.

ntdn7. ,,dn7. λBK4 1
．． 494 3.74X10-' -1,88X10-
'BK7 1.510 2.52X10-' -1,
95
It is nm/deg. Therefore, if the temperature of the semiconductor laser and the glass material change evenly, the prism 2 (BK4
) and prism 3 (BK7) due to temperature changes must be considered including changes in wavelength, and the appearance is as follows.

BK4 Short History = -0,96X10=T BK7 And= -2,355X10-'T Substituting the above value into equation (8) and setting θ=72°, φ=
39.538° and α=30. '045°, β=2
It becomes 9.694°. The beam shaping correction factor m at this time is as follows.

The above explanation based on Fig. 2 is just an example; in general, two types of glass media are selected, and the apparent change is caused by the wavelength change due to temperature rise of the semiconductor laser and the refractive index change of the material itself. Above and below, θ, φ, α, and β, which are the necessary beam shaping correction factors, can be determined depending on n.

〔Example〕

Next, examples of the beam shaping prism of the present invention will be described.

Figure 3 shows a beam shaping prism with the same configuration as in Figure 2.
FIG. 2 is a diagram showing the configuration of an optical disc device when applied to the two-beam type optical disc device shown in the figure. In this figure, reference numeral 10 indicates a beam shaping prism according to the present invention, and a first prism 1 and a second prism 2 are made of different materials.
It is a combination of

In this embodiment, the writing/reading beam from the semiconductor laser 11 is shaped into a circular pattern by the beam shaping prism, and according to the above-mentioned principle, even if a temperature change occurs, the deviation in the emission angle is corrected and the pattern remains the same. is ejected in the direction. Therefore, there is no risk of relative positional deviation on the disk between the erasing beam of the semiconductor laser 12 and the like.

FIG. 4 shows another second embodiment of the beam shaping prism of the present invention, and FIG. 4 [F]) is a view of FIG. 4 taken in the direction of arrow B. In this second embodiment, a second prism 2 is joined to a polarizing beam splitter 17 and integrated, and the configuration of the beam shaping prism 10 in which the first prism 1 and the second prism 2 are joined is as follows. It is substantially the same as Figure 2.

FIG. 5 shows a third embodiment of the beam shaping prism of the present invention. This embodiment integrates a beam shaping prism and a polarizing beam splitter. That is, the second prism 2' in this third embodiment is provided with a semi-transparent surface 2'a that is inclined at 45 degrees with respect to the beam, and thereby serves both as a second prism and a beam splitter. This is what I did. Therefore, the beam incident on the beam shaping prism passes through the first prism 1 and the second prism 2 (transmits through the semi-transparent surface 2'a), is reflected by the dichroic mirror 20, and then passes through the semi-transparent surface 2'a. The light is reflected at the exit surface 2'b and exits from the exit surface 2'b. FIG. 1 shows this prism 2' and the first prism 1.

If the configuration is based on the principle explained in FIG. 2, the influence of temperature changes can be eliminated.

In the optical disk device using the beam shaping prism of the third embodiment of the present invention shown in FIG. Beam shaping prism 1o
The light is focused through the objective lens 19. The reflected light passes through a beam splitter (second prism) 2', passes through a dichroic mirror 24, and is detected by a four-division detifter 15 using a condenser lens 21 and a cylindrical 1/lens 22. On the other hand, the beam from the semiconductor laser 12 for readout is collimated into a circular pattern by a collimating lens 14 with a small NA, is reflected by a semi-transparent surface 2'a of a beam splitter 2', and is reflected by an objective lens 19 into a disk. The light is focused on 23. The light reflected here passes through the surface 2'a of the beam splitter 2', is reflected by the dichroic mirror 24, and then passes through the condenser lens 21' to the deflector 1.
The light is focused on 5' and detected.

A beam shaping prism that satisfies the above-mentioned requirements by bonding a first prism and a second prism made of different materials as in the above embodiment corrects the beam to a perfect circle and also prevents temperature changes. - There is no possibility of relative positional deviation between the arms.

〔Effect of the invention〕

By having the beam shaping prism for the semiconductor laser in the optical disk device of the present invention configured as described above, it is possible to eliminate a shift in the wavelength of the semiconductor laser due to a temperature change and a change in the emission angle due to a change in the refractive index of the prism. Therefore, the exit angle of the emitted beam does not change due to temperature changes.

Therefore, when this is used in a 2-beam type optical disk device, for example, there is no positional deviation between the writing/reading beam and the erasing beam, or only the positioning of the writing/erasing beam and the reading beam, and complete erasing and reading are possible. It has the following effects.

[Brief explanation of the drawing]

1 and 2 are diagrams showing the principle of the beam shaping prism of the present invention, and FIG. 3 is a diagram showing the configuration of an optical disk device using the beam shaping prism of the first embodiment of the present invention. 4
The figure shows the configuration of a second embodiment of the present invention, FIG. 5 shows the configuration of an optical disk device using a beam shaping prism of the third embodiment of the present invention, and FIG. Seventh diagram showing the configuration of an optical disk device using a beam shaping prism
The figure shows the relative positional relationship between a write/read beam and an erase beam on a disk in an optical disk device. 1...First prism, 2...Second prism,
10... Beam shaping prism.

Claims (1)

[Claims] (1) A prism that corrects the cross-sectional shape of a semiconductor laser beam, and is made by joining a first and second prism made of different materials with refractive indexes of n_1 and n_2, respectively, and a refractive index of n_0 such as air. The light enters the first prism from the medium at an incident angle θ, is refracted at a refraction angle φ, then enters the second prism at an incidence angle α, is refracted at a refraction angle β, and is transferred from the second prism to another medium. A beam shaping prism is characterized in that the following relationship is substantially satisfied when emitting light at an incident angle of 0°. (tanφ+tanα)/n_1'・dn_1'(t)
/dT=tanα/n_2'・dn_2'(t)/dT
However, T is the temperature, and dn_1'/dT・dn_2'(t
)/dT are respectively n_1'=n_1/n_0, n_2'=
When n_2/n_0, wavelength change due to temperature change of semiconductor laser dλ/dT=C, dn_1'(t)/dT=C×(dn_1'/dλ)+
(dn_1'/dT), dn_2(t)/dT=C×(
The value is expressed by dn_2'/dλ)+(dn_2'/dT).