JP2003067968A - Optical head - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical head which can always maintain excellent recording/reproducing characteristics in a small and inexpensive configuration by preventing the optical axis from shifting due to dispersion in the prism optical system, and avoiding return light to the semiconductor laser and offsets in the servo signals. SOLUTION: The optical head makes the beam from a semiconductor laser 21 parallel flux using a collimator lens 22, and irradiates the optical recording medium 24 after passing through an objective lens 23 and the prism optical system 30 having a beam forming and splitter functions. The return light is separated by the prism optical system 30 and received by the optical detector 26. The prism optical system 30 is made by cementing a first prism 31 and a second prism 32 of glass materials different each other with the first prism 31 located at the collimator lens 22 side. It is constituted so that the incident angle of the light flux is not 0 degree against any prism surface and the refraction angle of the light flux from the second prism 32 is always kept almost the same for changes in wavelength of the light.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical head used for recording and / or reproducing information on an optical recording medium, and in particular to a beam shaping function for shaping the shape of a light beam emitted from a semiconductor laser. The present invention relates to an optical head including a prism optical system having a beam splitter function for separating return light from an optical recording medium.

[0002]

2. Description of the Related Art A light beam emitted from a semiconductor laser has different divergence angles in the horizontal and vertical directions with respect to an active layer.
Usually, the intensity beam has a Gaussian distribution with an ellipticity of 2 times or more. Therefore, in an optical head that uses a semiconductor laser as a light source, generally, after collimating emitted light from the semiconductor laser with a collimator lens, the beam intensity is shaped into a state close to a perfect circle with a shaping prism such as a triangular prism. The objective lens irradiates the optical recording medium as a spot.

Further, the semiconductor laser has a characteristic that the wavelength of emitted light also changes when the emitted power fluctuates. For this reason, in the optical head using the shaping prism as described above, the moment when the recording or erasing operation of the information with the high emission power shifts to the reproducing operation of the information with the low emission power or the opposite moment. In addition, the refractive index of the shaping prism changes due to the change in the wavelength of the emitted light of the semiconductor laser, which causes an optical axis shift and shifts the spot position focused on the optical recording medium, which causes an error in recording and reproducing information. Is concerned.

In order to solve such a concern, for example, in Japanese Patent Laid-Open No. 60-234247, two triangular prisms made of different glass materials are bonded and arranged between a collimator lens and an objective lens. , An optical head that simultaneously performs beam shaping and compensation of an optical axis shift due to wavelength dispersion is disclosed. Further, Japanese Patent Publication No. 6-93044 discloses a concrete conditional expression in the case where two prisms made of different glass materials are bonded and arranged to simultaneously perform beam shaping and compensation of an optical axis shift due to wavelength dispersion. It is disclosed.

[0005]

However, in each of the optical heads disclosed in the above publications, a light beam is vertically incident on the exit surface of a prism group formed by bonding two prisms together. I am trying to let you. Therefore, even if the emission surface is coated with an antireflection film,
In reality, since the reflectance of the emitting surface is about 0.5%, the reflected light on the reflecting surface follows the reverse optical path to form an image at the chip position of the semiconductor laser, which causes return light noise. There is concern that the recording / reproducing characteristics will be adversely affected and that an offset will occur in the servo signal when the return light from the optical recording medium is received by the photodetector to detect the servo signal or the like.

Particularly, as in the optical head disclosed in Japanese Patent Laid-Open No. 60-234247, the bonding surface of two triangular prisms has a beam splitter function to separate the return light from the optical recording medium. However, when the separated return light is received by the photodetector to detect servo signals, etc., since the beam splitter surface is composed of the bonding surfaces of different glass materials, the wavefront aberration is caused by the difference in thermal expansion due to temperature change. Is deteriorated, which deteriorates the spot of the return light incident on the photodetector, which may increase the offset of the servo signal.

As a method of preventing the occurrence of the above-mentioned offset, for example, as disclosed in Japanese Patent No. 2832017, first and second prisms made of different glass materials are bonded together and the second prism is formed. A third prism made of the same glass material is bonded to form a prism optical system, and the first and second prisms made of different glass materials suppress the optical axis shift due to wavelength dispersion, and join the second and third prisms made of the same glass material. It is conceivable to give the surface a beam splitter function.

However, in this case, since three prisms are required, there is a concern that the cost of the entire apparatus will increase and the size of the apparatus will increase.

Therefore, an object of the present invention made in view of the above point is that the optical axis shift due to the dispersion of the prism optical system can be effectively prevented with a compact and inexpensive structure, and the generation of return light to the semiconductor laser and An object of the present invention is to provide an optical head which can effectively prevent the occurrence of offset in a servo signal and can always maintain good recording and reproducing characteristics.

[0010]

In order to achieve the above object, the invention according to claim 1 is directed to a semiconductor laser, a collimator lens for collimating a luminous flux from the semiconductor laser, and a shape of the collimated luminous flux from the collimator lens. A prism optical system having a beam shaping function for shaping the light and a beam splitter function for extracting the return light from the optical recording medium, an objective lens for irradiating the optical recording medium with the light flux from the prism optical system, and the prism optical system In the optical head having a photodetector for receiving the returned return light, the prism optical system includes a first prism and a second prism made of different glass materials, and the first prism is located on the collimator lens side. If the incident angle of the light beam on any of the prism surfaces is not 0 degree, the objective lens from the second prism is bonded. Refraction angle beta ₂ of the beam emitted toward the figure, always to be substantially constant with respect to wavelength variation of the light, by being configured so as to substantially satisfy the equation (1) and (2) below It is a feature.

[0011]

[Equation 4] θ ₁ , θ ₂ ; incident angle from air to first prism, refraction angles α ₁ , α ₂ ; incident angle from first prism to second prism, refraction angles β ₁ , β ₂ ; from second prism to air Angle of incidence, refraction angle n ₁ , n ₂ ; refractive index Δn ₁ , Δn ₂ of glass material of the first and second prisms; refractive index change M due to wavelength change of glass material of the first and second prisms; beam shaping ratio

The invention according to claim 2 is a semiconductor laser,
A collimator lens for collimating the light flux from the semiconductor laser into a parallel light flux; a prism optical system having a beam shaping function for shaping the shape of the parallel light flux from the collimator lens and a beam splitter function for extracting return light from the optical recording medium; In an optical head having an objective lens for irradiating the optical recording medium with the light flux from the prism optical system and a photodetector for receiving the return light extracted by the prism optical system, the prism optical system includes a first prism. And a second prism are arranged independently so that the first prism is located on the collimator lens side, and the angle of incidence of the light flux on any prism surface is not 0 degree, the second prism from refraction angle beta ₂ of the beam emitted toward the objective lens is always to be substantially constant with respect to wavelength variation of the light, the lower Of (3), (4) and (5)
It is characterized in that it is configured so as to substantially satisfy the equation.

[0013]

[Equation 5] θ ₁ , θ ₂ ; incident angle from air to first prism, refraction angles γ ₁ , γ ₂ ; incident angle from first prism to air, refraction angles α ₁ , α ₂ ; incidence from air to second prism Angle, refraction angle β ₁ , β ₂ ; incident angle from second prism to air, refraction angle n ₁ , n ₂ ; refractive index Δn ₁ , Δn ₂ of glass material of first and second prisms; first and second Refractive index change due to wavelength change of prism glass material; beam shaping ratio

The invention according to claim 3 is the invention according to claim 1 or 2.
In the optical head described in (1) above, the prism surface of the first prism on the semiconductor laser side has a beam splitter function for extracting the return light from the optical recording medium.

According to a fourth aspect of the present invention, in the optical head according to the third aspect, of the light flux incident on the prism surface having the beam splitter function from the semiconductor laser through the collimator lens, the light is reflected by the prism surface. A front monitor detector for receiving the generated light flux is provided, and the emission power of the semiconductor laser is controlled based on the output of the front monitor detector.

According to a fifth aspect of the present invention, in the optical head according to any one of the first to fourth aspects, the first and second aspects are provided.
The refractive index change due to the temperature change of the prism is _expressed as Δn _τ1 , Δn
It is characterized in that the prism optical system is configured so as to substantially satisfy the following expression (6) when _τ2 is set.

[0017]

[Equation 6]

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of an optical head according to the present invention will be described below with reference to the drawings.

First, prior to the overall structure, a prism optical system used in an embodiment of an optical head according to the present invention will be described with reference to FIGS.

The prism optical system 10 shown in FIGS.
The first prism 11 and the second prism 12 made of different glass materials are bonded to each other at the joint surface 13, and FIG. 1 shows the incident angle and the refraction angle of the light beam at each prism.
FIG. 3 shows a partially enlarged view of FIG. 3, and FIG. 3 shows the diameters of the incoming and outgoing light beams at each prism.

As shown in FIG. 1, incident angles and refraction angles of the first prism 11, the cemented surface 13, and the second prism 12 are θ ₁ , θ ₂ ; α ₁ , α ₂ ; β ₁ , β ₂ , respectively. The refractive index of the glass material of the first prism 11 is n ₁ , and the refractive index of the glass material of the second prism 12 is n ₂ . Further, the changes in the above angles θ ₂ , α ₁ , α ₂ , β ₁ , β ₂ with respect to the wavelength change of the light beam are respectively Δθ ₂ , Δα ₁ , Δα ₂ , Δβ ₁ , Δβ ₂ , and the refractive indices n ₁ , n The changes of ₂ are Δn ₁ and Δn ₂ , respectively. Note that θ ₁ is unchanged.

In the prism optical system 10, in order to prevent the optical axis deviation from occurring with respect to the wavelength change,
It suffices that β ₂ does not change with respect to the wavelength change.

Here, when the wavelength does not change, the following equation is obtained according to Snell's law.

[0024]

[Equation 7]

Further, when the wavelength changes, from equation (7),

[Equation 8] From equation (8),

[Formula 9] Δn ₁ · sinα ₁ + n ₁ · cosα ₁ · Δα ₁ −Δn ₂ · sinα ₂ = n ₂ · cosα ₂ · Δα ₂ (11) is obtained, and from the formula (9),

## EQU10 ## (n ₂ + Δn ₂ ) .sin (β ₁ + Δβ ₁ ) = sin (β ₂ + Δβ ₂ ) ... (12) is obtained, but Δβ ₂ = 0 with respect to the wavelength change. Therefore, the equation (12) is

[Equation 11] (n ₂ + Δn ₂ ) · sin (β ₁ + Δβ ₁ ) = sin β ₂ (13) Thus, from this equation (13),

[Equation 12] Is obtained.

Further, from FIG.

## EQU13 ## Δθ ₂ = −Δα ₁ (15), and similarly,

[Expression 14] Δβ ₁ = −Δα ₂ (16)

A conditional expression for the prism in which the refraction angle β ₂ does not change even if the wavelength changes will be obtained using these expressions. First, substituting equation (15) into equation (10),

[Equation 15] And the equation (16) is substituted into the equation (14),

[Equation 16] Is obtained.

Therefore, substituting the equations (17) and (18) into the equation (11),

[Equation 17]   Δn₁・ Sinα₁+ Δn₁・ Cos α₁・ Tan θ_Two-Δn_Two・ Sinα_Two       = Δn_Two・ Cos α_Two・ Tanβ₁  ... (19) Becomes In equation (19), sin α obtained from equation (8)_Two
= (N₁/ n_Two) ・ Sinα ₁And substitute 1 / cos on both sides
α₁Multiply by

[Formula 18] Δn ₁ · tanα ₁ + Δn ₁ · tanθ ₂ − (n ₁ / n ₂ ) · Δn ₂ · tanα ₁ = (cosα ₂ / cosα ₁ ) · Δn ₂ · tanβ ₁ (20) .

According to the equation (20), even if θ ₂ is determined, α ₁ cannot be obtained because α ₂ remains.

Therefore, the beam shaping ratio will be considered in order to eliminate α ₂ . As shown in FIG. 3, the diameters of the incoming and outgoing luminous fluxes are D ₁ , D ₂ , D ₃ and D ₄ , respectively, and the beam shaping ratio M is defined as M = D ₄ / D ₁ , and M ₁ and
M ₂ and M ₃ are replaced by M ₁ = D ₂ / D ₁ and M ₂ = D ₃ / D ₂ ,
When defined as M ₃ = D ₄ / D ₃ , M ₁ = cos θ ₂ / cos θ
₁ , M ₂ = cos α ₂ / cos α ₁ , M ₃ = cos β ₂ / cos β ₁
So

[Formula 19] Becomes From this equation (21), when cosα ₂ / cosα ₁ is represented by a beam shaping ratio,

[Equation 20] Therefore, by substituting the equation (22) into the equation (20), the following equation (23) is obtained, and α ₂ can be eliminated.

[Formula 21] Δn ₁ · tan α ₁ + Δn ₁ · tan θ ₂ − (n ₁ / n ₂ ) · Δn ₂ · tan α ₁ = M ₂ · Δn ₂ · tan β ₁ (23)

By further rearranging this equation (23),

[Equation 22] Is obtained.

The expression (1) is a condition that does not cause the optical axis shift with respect to the wavelength change. However, M ₂ is premised on satisfying the expression (22), and β ₂ ≠ 0 is a condition that the reflected light on the prism surface of the second prism 12 does not return to the light source.

Next, the case where the temperature changes will be described.
It The bending of the first and second prisms 11 and 12 due to temperature changes
Folding rate change is Δn_τ1, Δn _τ2Then, in equation (1)
Δn₁, Δn_TwoΔn_τ1, Δn_τ2If you replace
Equation (1) does not change the optical axis with respect to temperature changes.
It will be a condition.

That is, the equation (1) is

[Equation 23] Therefore, the condition that the optical axis shift does not occur with respect to both the wavelength change of the light flux and the temperature change of the prism optical system 10 is as follows.

[Equation 24] Becomes

As a result, as the first prism 11 and the second prism 12 which compose the prism optical system 10, a combination of materials in which the ratio of change in refractive index due to wavelength change and the ratio of change in refractive index due to temperature change are substantially equal to each other. You can see that you can choose.

Next, the specific structure of the prism optical system 10 will be described by taking the case of using a semiconductor laser that emits light having a central wavelength of 405 nm as an example. In an optical head that uses a semiconductor laser that emits light with a short wavelength as described above, the dispersion of the glass material increases as the wavelength of the light used decreases, so in particular in the prism optical system 10, the optical axis shift due to dispersion is prevented. It becomes important to do. In addition, as a glass material selection condition in this case, the transmittance at a thickness of 10 mm needs to be 95% or more.

FIG. 4 shows a first specific example. In this prism optical system 10, the beam shaping ratio M is set to M = 2.7, and the refraction angle β ₂ at the second prism 12 is β ₂ = 3 °.
Is set to. The first prism 11 is formed of S-BSL7 (manufactured by OHARA CORPORATION) (refractive index n ₁ = 1.529724) which is a very popular glass material, and the second prism 12 is BaC4 (manufactured by HOYA CORPORATION). (Refractive index n ₂ = 1.58605). The incident angle and the refraction angle of the light beam on the first prism 11, the cemented surface 13 and the second prism 12 are as shown in FIG.

According to the prism optical system 10 shown in FIG.
For example, even if the emission power of the semiconductor laser changes from the reproducing power to the erasing power and the wavelength of the light entering the prism optical system 10 changes from 405 nm to, for example, 407 nm by about 2 nm, Δn ₁ = −0.000254, Δn ₂ =-
Because it is 0.000333, the optical axis shift of the light beam emitted from the second prism 12, 2.0 × and ^{10 -6} °, it is possible to zero to five decimal digits, the information recording and reproducing exactly the problems Can be no level. Further, since the refraction angle β ₂ of the second prism 12 is 3 °, it is more effective that the light reflected by the prism surface returns to the semiconductor laser, as compared with the case where β ₂ = 0 ° as in the conventional case. Can be prevented. Furthermore, when β ₂ = 0 °, the types of glass materials that exist are limited, and as described above, glass materials that have good light transmittance at short wavelengths often have poor chemical properties, etc. However, β ₂ ≠
By setting the angle to 0 °, the optical axis deviation can be easily minimized from a limited glass material.

FIG. 5 shows a second specific example. The prism optical system 10 includes a second prism 12 and a BaF11.
(Manufactured by HOYA Co., Ltd.) (refractive index n ₂ = 1.69068
9), and other configurations are the same as those in FIG. The incident angles and refraction angles of the light rays on the first prism 11, the cemented surface 13, and the second prism 12 in this case are as shown in FIG.

In the prism optical system 10 shown in FIG. 5, as in the case of FIG. 4, for example, the emission power of the semiconductor laser changes from the reproducing power to the erasing power, and the wavelength of the light incident on the prism optical system 10 changes. Even if it changes about 2 nm from 405 nm to 407 nm, Δn ₁ = −0.0.
Since 00254 and Δn ₂ = −0.000474,
The deviation of the optical axis of the light beam emitted from the second prism 12 is 1.
It can be set to 3 × 10 ⁻⁶ ° and zero to the fifth digit of the decimal point, and it can be set to a level at which there is no problem in recording and reproducing information.

In this case, the first and second prisms 1
When the glass material temperature changes from 25 ° C. to 55 ° C., the first prism 1
1 is Δn _τ1 = −0.000113, and the second prism 12 is Δn _τ2 = −0.000212. Since the above equation (6) is almost satisfied, the optical axis shift due to the wavelength change is canceled, The total optical axis deviation can be suppressed to 0.07 seconds, which is a level that does not affect the performance at all.

FIG. 6 shows the overall schematic construction of the first embodiment of the optical head according to the present invention. In this embodiment, the prism optical system 10 shown in FIG. 5 is used. Figure 6
In the first prism 1 of the prism optical system 10, the divergent light flux having the elliptic intensity distribution emitted from the semiconductor laser 21 is converted into a parallel light flux by the collimator lens 22.
Angle of incidence θ ₁ = 71.44808 on the prism surface 11a of No. ₁
A light beam that is incident at 5 ° and is refracted and transmitted through the prism surface 11a is joined to the joint surface 13 and the prism surface 1 of the second prism 12.
By sequentially refracting and transmitting 2a, the beam shaping ratio M
= 2.7, the light beam shaped into a beam is emitted at a refraction angle β ₂ = 3 °, and the emitted light is condensed on the optical recording medium 24 by the objective lens 23.

Further, the light beam reflected by the optical recording medium 24 follows the same path as the outward path, is incident on the prism surface 11a of the prism optical system 10, and is returned from the optical recording medium 24 reflected by the prism surface 11a. The light is passed through the first prism 11
The detection system lens 2 by refracting and transmitting from the prism surface 11b of
After passing 5, the light is received by the photodetector 26 which is arranged to be inclined with respect to the optical axis of the return light, and the reproduction signal and the servo signal are detected based on the light reception output.

On the other hand, in the outward path, the prism optical system 10
The light flux from the semiconductor laser 21 reflected by the prism surface 11a of the semiconductor laser 21 is received by the monitor photodetector 27 arranged to be inclined with respect to the optical axis, and the emission power of the semiconductor laser 21 is controlled based on the received light output. To do.

According to this embodiment, in addition to the effect described with reference to FIG. That is, assuming that the focal length of the collimator lens 22 is, for example, 9 mm, the reflected light reflected by the prism surface 12 a of the second prism 12 in the forward path is not focused on the chip position of the semiconductor laser 21, and is 9 mm from the light emitting point. tan (2.7 × 3 × 2) ° ≒
The light will be condensed at a position laterally separated by 2.6 mm. Therefore, the reflected light on the prism surface 12a in this outward path hardly adversely affects the semiconductor laser 21, so that the deterioration of the recording / reproducing characteristics can be effectively prevented.

Since the beam shaping ratio in the prism optical system 10 with respect to the return light from the optical recording medium 24 is almost 1, the focal length of the detection system lens 25 is, for example, 20 mm.
Then, the reflected light on the prism surface 12a in the forward path is 20 mm × tan (3 × 2) ° ≈ from the photodetector 26.
The light will be condensed at a position laterally separated by 2.1 mm. Therefore, if the light receiving portion of the photodetector 26 is, for example, 0.5 mm square, it largely deviates from the photodetector 26, so that there is no offset in the servo signal.

Further, the prism surface 1 of the first prism 11
By providing 1a with a beam separation function, it is possible to suppress the influence of distortion due to a difference in linear expansion between the first prism 11 and the second prism 12 due to a temperature change.
It is possible to ensure the quality of the reproduction signal and the servo signal detected based on the light reception output at 6.

Further, the prism surface 11 of the first prism 11
Since the light beam from the semiconductor laser 21 separated by the prism surface 11a can be received by the monitor photodetector 27 and the power of the semiconductor laser 21 can be controlled by using the beam separation function of a, the number of parts is reduced. It is possible to reduce the size of the device and reduce the size of the device, and to reduce the influence of the adhesive on the bonding surface as compared with the conventional case where a light beam is separated by a beam splitter having a bonding surface to obtain monitor light. Since it is not received, stable power control becomes possible. Further, the semiconductor laser 21, the prism optical system 10, the detector 2
It is possible to easily take an arrangement configuration in which the reflected lights of 6 and 27 are completely avoided from each other.

Next, a prism optical system used in another embodiment of the optical head according to the present invention will be described with reference to FIGS.
Will be described with reference to.

The prism optical system 30 shown in FIGS. 7 and 8.
Is a first prism 31 of a different glass material that is separately arranged.
7 shows the incident angle and refraction angle of the light beam in each prism, and FIG. 8 shows the diameters of the incoming and outgoing light beams in each prism.

As shown in FIG. 7, from the air the first prism
The incident angle to 31 and the refraction angle are θ₁, Θ _Two, The first prism 3
The incident angle and the refraction angle from 1 to air are γ₁, Γ_TwoFrom the air
The incident angle and the refraction angle to the second prism 32 are α₁, Α_Two, First
2 The incident angle and the refraction angle from the prism 32 to the air are β₁, Β
_TwoAnd the refractive index of the glass material of the first prism 31 is n.₁, Second
The refractive index of the glass material of the prism 32 is n_TwoAnd Also, the pre
The beam shaping ratio by the optical system 30 is M and the wavelength of the light beam is changed.
The above refractive index n with respect to₁, N_TwoChange of Δ
n₁, Δn_TwoAnd Note that θ₁Is immutable.

Here, the beam shaping ratio M is M = D₅/ D
₁And M₁, M_Two, M _Three, M_FourTo M₁=
D_Two/ D₁, M_Two= D_Three/ D_Two, M_Three= D_Four/ D_Three, M
_Four= D₅/ D_FourIs defined as M₁= Cos θ_Two/ Cos
θ₁, M_Two= Cosγ_Two/ Cosγ₁, M_Three= Cos α_Two/ Cos α
₁, M_Four= Cosβ_Two/ Cos β₁So

[Equation 25] Becomes

In the prism optical system 30, in order to prevent the optical axis deviation from occurring with respect to the wavelength change,
It suffices that β ₂ does not change with respect to the wavelength change.

Therefore, the prism optical system 30 is constructed so as to satisfy the following expressions (3) to (5).

[0055]

[Equation 26]

FIG. 9 shows a second embodiment of the optical head according to the present invention.
It shows a schematic configuration of the entire form. This embodiment
Then, using the prism optical system 30 shown in FIG.
1 prism 31 is LaC13 (manufactured by HOYA Co., Ltd.)
(Refractive index n₁= 1.71548), the second prism
S-BSL7 (manufactured by OHARA CORPORATION) (refractive index n
_Two= 1.529724). Also, prism light
The beam shaping ratio M = 2.7 according to the academic system 30,
Incident angle of each prism face of the two prisms 31 and 32
The refraction angle is set as shown.

In FIG. 9, a divergent light beam having an elliptic intensity distribution emitted from the semiconductor laser 41 is converted into a parallel light beam by the collimator lens 42, and then the prism optical system 30.
Incident angle θ ₁ on the prism surface 31a of the first prism 31 of
The first prism 31 is made to enter at 59.333671 °.
From the prism surface 31b of No. _{2 with a} refraction angle γ ₂ = 3 °,
The emitted light is incident at an incident angle α1 = -58.6374 ° prism surface 32a of the second prism 32, is emitted at angle of refraction beta ₂ = 3 ° from the prism surface 32b of the second prism 32, the output The emitted light is focused on the optical recording medium 44 by the objective lens 43.

Further, the light beam reflected by the optical recording medium 44 follows the same path as the outward path, is incident on the prism surface 31a of the first prism 31, and is returned from the optical recording medium 44 reflected by the prism surface 31a. The light is refracted and transmitted from the prism surface 31c of the first prism 31, and the detection system lens 45
After that, the photodetector 46 arranged to be inclined with respect to the optical axis of the return light receives the light, and the reproduction signal and the servo signal are detected based on the light reception output.

On the other hand, in the forward path, the light flux from the semiconductor laser 41 reflected by the prism surface 31a of the first prism 31 passes through the shaping prism 47 and the condenser lens 48 and is arranged for the monitor to be inclined with respect to the optical axis. Light is received by the photodetector 49, and the emission power of the semiconductor laser 41 is controlled based on the received light output.

According to the present embodiment, for example, the emission power of the semiconductor laser 41 changes from the reproducing power to the erasing power, and the wavelength of light incident on the prism optical system 30 is 405.
Even if it changes about 2 nm from nm to 407 nm, Δ
n ₁ = -0.000423, Δn ₂ = -0.00025
Therefore, the deviation of the optical axis of the light beam emitted from the second prism 32 can be set to -2.3 × 10 ⁻⁶ °, which is zero up to the fifth digit of the decimal point, and there is no problem in recording and reproducing information. Can be level.

Further, since the first prism 31 and the second prism 32 are separately arranged without being adhered by the adhesive, the absorption of light by the adhesive and the chemical change of the adhesive do not occur. It is possible to prevent deterioration of the recording / reproducing characteristics due to initial and secular changes. further,
By increasing the incident angle from the first prism 31 to the air as a parameter of the optical axis shift, the degree of freedom in design is increased, and the degree of freedom in the configuration of the signal detection system can be increased.

In the third embodiment of the present invention, in the configuration shown in FIG. 9, the first prism 31 and the second prism 32 constituting the prism optical system 30 are respectively S-BSL.
And the beam shaping ratio M = 2.7. Also, γ
₂ = β ₂ = -6 °, θ ₁ = 67.2132 °, θ ₂ = 3
7.06293 °, α ₁ = −42.7989 °, α ₂ =
-26.36900 °.

According to this structure, even if the wavelength of the emitted light of the semiconductor laser 41 changes from 405 nm to, for example, about 2 nm, Δn ₁ = Δn ₂ = −0.000254, so the light flux emitted from the second prism 32. The optical axis deviation of
It is possible to set zero to −1.1 × 10 ⁻⁶ degrees up to five decimal places, and it is possible to attain a level at which there is no problem in recording and reproducing information as in the above embodiment.

Further, since the prism optical system 30 is composed of S-BSL7 which is inexpensive, has almost no absorption, and is excellent in chemical resistance, it is possible to realize an inexpensive and highly reliable optical head and Since the glass materials of the first and second prisms 31 and 32 are the same, there is an advantage that the refractive index changes due to temperature changes are the same, and therefore the optical axis shift due to temperature does not occur in principle.

[0065]

As described above, according to the present invention, the optical axis shift due to the dispersion of the prism optical system due to the wavelength change of the semiconductor laser can be effectively prevented with a compact and inexpensive structure, and the semiconductor laser can be effectively prevented. It is possible to effectively prevent the generation of return light and the offset to the servo signal, and it is possible to always maintain good recording and reproducing characteristics.

[Brief description of drawings]

FIG. 1 is a diagram showing a prism optical system used in an embodiment of an optical head according to the present invention.

FIG. 2 is a partially enlarged view of FIG.

3 is a diagram showing the diameters of incoming and outgoing light beams in the prism optical system shown in FIG.

FIG. 4 is a diagram showing a first specific example of the prism optical system shown in FIG.

FIG. 5 is likewise a diagram showing a second specific example.

FIG. 6 is a diagram showing an overall schematic configuration of an optical head according to a first embodiment of the invention.

FIG. 7 is a diagram showing a prism optical system used in another embodiment of the optical head according to the present invention.

8 is a diagram showing the diameters of incoming and outgoing light beams in the prism optical system shown in FIG.

FIG. 9 is a diagram showing an overall schematic configuration of a second embodiment of an optical head according to the present invention.

[Explanation of symbols]

10 Prism optical system 11 First prism 11a, 11b Prism surface 12 Second prism 12a prism surface 13 Bonding surface 21 Semiconductor laser 22 Collimator lens 23 Objective lens 24 Optical recording medium 25 Detection system lens 26 Photodetector 27 Monitor photodetector 30 prism optical system 31 first prism 32 Second prism 41 Semiconductor laser 42 Collimator lens 31a, 31b, 31c Prism surface 32a, 32b Prism surface 43 Objective lens 44 Optical recording medium 45 Detection system lens 46 Photodetector 47 Shaped prism 48 condenser lens 49 Monitor photodetector

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Claims (5)

[Claims] 1. A semiconductor laser, a collimator lens for collimating the light flux from the semiconductor laser, a beam shaping function for shaping the shape of the collimated light flux from the collimator lens, and a beam for extracting return light from an optical recording medium. In an optical head having a prism optical system having a splitter function, an objective lens for irradiating an optical recording medium with a light flux from the prism optical system, and a photodetector for receiving return light extracted by the prism optical system, In the prism optical system, a first prism and a second prism made of different glass materials are attached so that the first prism is located on the collimator lens side, and an incident angle of a light beam on any prism surface is 0. At an angle that is not degrees,
In order that the refraction angle β ₂ of the light beam emitted from the second prism toward the objective lens is always substantially constant with respect to the change of the wavelength of the light, the following formulas (1) and (2) are almost calculated. An optical head characterized by being configured to satisfy. [Equation 1] θ ₁ , θ ₂ ; incident angle from air to first prism, refraction angles α ₁ , α ₂ ; incident angle from first prism to second prism, refraction angles β ₁ , β ₂ ; from second prism to air Angle of incidence, refraction angle n ₁ , n ₂ ; refractive index Δn ₁ , Δn ₂ of glass material of the first and second prisms; refractive index change M due to wavelength change of glass material of the first and second prisms; beam shaping ratio 2. A semiconductor laser, a collimator lens for collimating the light flux from the semiconductor laser, a beam shaping function for shaping the shape of the collimated light flux from the collimator lens, and a beam for extracting return light from the optical recording medium. In an optical head having a prism optical system having a splitter function, an objective lens for irradiating an optical recording medium with a light flux from the prism optical system, and a photodetector for receiving return light extracted by the prism optical system, In the prism optical system, a first prism and a second prism are independently arranged so that the first prism is located on the collimator lens side, and an incident angle of a light beam on any prism surface is 0. in not angular degrees, the refractive angle beta ₂ of the beam emitted toward the objective lens from the second prism, the wavelength change of light Always to be substantially constant, (3) below, (4) and (5) the optical head is characterized by being configured so that substantially satisfied equation. [Equation 2] θ ₁ , θ ₂ ; incident angle from air to first prism, refraction angles γ ₁ , γ ₂ ; incident angle from first prism to air, refraction angles α ₁ , α ₂ ; incidence from air to second prism Angle, refraction angle β ₁ , β ₂ ; incident angle from second prism to air, refraction angle n ₁ , n ₂ ; refractive index Δn ₁ , Δn ₂ of glass material of first and second prisms; first and second Refractive index change due to wavelength change of prism glass material; beam shaping ratio 3. The light according to claim 1, wherein the prism surface of the first prism on the semiconductor laser side is provided with a beam splitter function for extracting return light from the optical recording medium. head. 4. A front monitor detector for receiving a light beam reflected from the semiconductor laser from the semiconductor laser beam passing through the collimator lens and incident on the prism surface having the beam splitter function is provided. 4. The optical head according to claim 3, wherein the emission power of the semiconductor laser is controlled based on the output of the monitor detector. 5. When the refractive index changes due to temperature changes of the first and second prisms are Δn _τ1 and Δn _τ2 , the prism optical system is configured to substantially satisfy the following expression (6). The optical head according to any one of claims 1 to 4, which is characterized. [Equation 3]