JP2005064387A - Laser light source and optical pickup - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a laser light source which projects a plurality of wavelengths and reduces return optical noise, and also to provide an optical pickup. <P>SOLUTION: The difference (n1 L1-n2 L2) of the product of first and second effective refractive indexes n1, n2 in each of first and second light emitting parts and first and second resonator lengths L1, L2 is made larger than 0.065mm corresponding to thickness difference or the like of each protection layer of a DVD and a CD. Thereby, the relation between the distance to an optical disc (external resonator length) and a resonator length (inner resonator length) is kept in the state of reduced return noise as for both first and second light emitting parts by setting the distance of a laser light source properly to a DVD and a CD. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a laser light source and an optical pickup.

In an optical disc apparatus, recording and reproduction on an optical disc are performed by irradiating an optical disc (optical recording medium) with laser light emitted from a semiconductor laser.
At this time, if the laser light reflected by the optical disk returns to the semiconductor laser, the state of the semiconductor laser becomes unstable due to longitudinal mode competition, mode hops, etc., and noise may be generated from the semiconductor laser (so-called return). Generation of optical noise).
In order to reduce such return light noise, a technique for appropriately setting the distance between the semiconductor laser and the optical disk in a reading optical pickup is disclosed (see Patent Document 1).
Japanese Patent Application Laid-Open No. 11-16188.

By the way, in an optical disk apparatus, a semiconductor laser that emits light of a plurality of wavelengths in order to enable recording and reproduction of a plurality of types of optical disks (optical recording media) such as a CD (Compact Disc) and a DVD (Digital Versatile Disc). There is something with. As this semiconductor laser, a laser diode in which a plurality of regions emitting light having different wavelengths from each other are formed close to each other can be used. In this case, it is necessary to reduce the return light noise for the semiconductor laser emitting a plurality of wavelengths.
Further, since laser light having a higher intensity than that used during reproduction is used when recording on an optical disc, the influence of return light is likely to be greater than during reproduction.
In view of the above, an object of the present invention is to provide a laser light source and an optical pickup that can emit a plurality of wavelengths and reduce return light noise.

  A. A laser light source according to the present invention emits a first optical resonator having a first effective refractive index n1 and a first resonator length L1, and a first laser beam oscillated by the first optical resonator. A first light-emitting portion having a first light-emitting point, a second optical resonator having a second effective refractive index n2 and a second resonator length L2, and oscillation by the second optical resonator And a second light emitting point that emits the second laser beam, and a product of the first effective refractive index and the first resonator length and the second light emitting point. The difference between the effective refractive index and the product of the second resonator length (n1 · L1−n2 · L2) is larger than 0.065 mm.

  In order to reduce the return light noise of the laser light source, it is necessary to appropriately set the relationship between the resonator length (internal resonator length) and the distance between the laser light source and the optical disk (external resonator length). Here, the first and second effective refractive indexes n1 and n2 and the first and second effective refractive indexes in the first and second light-emitting sections correspond to the difference in thickness of the protective layer between the DVD and the CD, respectively. The difference (n1 · L1−n2 · L2) in the product of the resonator lengths L1 and L2 is greater than 0.065 mm. For this reason, the distance of the laser light source is appropriately set with respect to the DVD and the CD, the relationship between the internal resonator length and the external resonator length is returned for both the first and second light emitting units, and the optical noise is reduced. Can keep. As a result, recording and reproduction on DVDs and CDs can be performed with little return light noise.

Here, the first resonator length L1 and the second resonator L2 may be substantially equal (L1≈L2).
By making the resonator lengths of the first and second resonators the same, it becomes easy to integrally form the first and second resonators.

  B. An optical pickup according to the present invention emits a first optical resonator having a first effective refractive index n1 and a first resonator length L1, and a first laser beam oscillated by the first optical resonator. A first light-emitting portion having a first light-emitting point, a second optical resonator having a second effective refractive index n2 and a second resonator length L2, and oscillation by the second optical resonator And a second light emitting point that emits the second laser beam, and a product of the first effective refractive index and the first resonator length and the second light emitting point. The difference between the effective refractive index and the product of the second resonator length (n1 · L1−n2 · L2) is larger than 0.065 mm.

  For both the first and second light emitting units corresponding to the DVD and the CD, the relationship between the internal resonator length and the external resonator length can be maintained in a state where the return optical noise is low. As a result, recording and reproduction on DVDs and CDs can be performed with little return light noise.

Here, the first resonator length L1 and the second resonator L2 may be substantially equal (L1≈L2).
By making the resonator lengths of the first and second resonators the same, it becomes easy to integrally form the first and second resonators.

  According to the present invention, it is possible to provide a laser light source and an optical pickup capable of emitting a plurality of wavelengths and reducing return light noise.

(First embodiment)
FIG. 1 is a schematic diagram showing an optical disk reproducing apparatus 10 according to the first embodiment of the present invention.
The optical disc reproducing apparatus 10 includes an optical pickup 20 and an optical pickup driving unit 30, and records and reproduces information on a plurality of optical discs D (CD (Compact Disc), DVD (Digital Versatile Disc), etc.) having different standards (writing). Read).
The optical pickup 20 includes a laser diode LD, a grating 21, a beam splitter BS, a collimator lens 22, a mirror M, an objective lens 23, an actuator 24, an optical axis correction element 25, a detection lens 26, and a photodiode PD. The information is read out.
The optical pickup driving unit 30 is an actuator for moving (seeking or the like) the entire optical pickup 20.

  The laser diode LD is a laser light source that emits a first laser beam having a first wavelength (λ1) and a second laser beam having a second wavelength (λ2). Examples of the first and second wavelengths include a wavelength of 650 nm for reproducing a DVD and a wavelength of 780 nm for reproducing a CD.

2A and 2B are cross-sectional views showing a cross section in the end surface and side surface direction of the semiconductor chip 40 which is the main body of the laser diode LD. Here, the cross section in the side surface direction of FIG. 2B represents a state in which the laser diode LD is cut along AA in FIG.
The semiconductor chip 40 is a monolithic laser in which a laser diode LD1 (emission wavelength 650 nm) as a first light emitting unit for DVD and a laser diode LD2 (emission wavelength 780 nm) as a second light emitting unit for CD are mounted on one chip. It is a diode.
The laser diodes LD1 and LD2 are formed on the n-type substrate 41 and the n-type buffer layer 42, respectively. Note that an n-electrode 43 is formed on the lower surface of the n-type substrate 41.

  In the laser diode LD1, an n-type buffer layer 51, an n-type clad layer 52, an active layer 53, and a p-type clad layer 54 are stacked. The intermediate layer 57, the p-type contact layer 58, and the p-electrode 59 are laminated. On the other hand, the laser diode LD2 is formed by laminating an n-type cladding layer 61, an active layer 62, a p-type cladding layer 63, an insulating layer 64, a p-type cap layer 65, and a p-electrode 66.

The laser diodes LD1 and LD2 perform laser oscillation in the respective resonators having the reflection surfaces of the both end faces S11, 12, S21, and 22 and emit light points on the end faces S11 and S21 (end points of the active layers 53 and 62, respectively). ) To emit laser beams of the first and second wavelengths. The effective refractive index in each resonator is n1 and n2, and the resonator length is the same (L).
Here, the effective refractive indexes n1 and n2 and the resonator length L are set as in the following equation (1).
(N1-n2) · L> 0.065 [mm] (1)
Expression (1) is a condition that allows both the laser diodes LD1 and LD2 to be kept in a state of low return optical noise by appropriately setting the distance Ld between the laser diode LD and the optical disk D. As a result, recording and reproduction on the optical disk D can be performed in a state where the return light noise is small. The process of deriving equation (1) will be described later.

  The grating 21 is a diffraction grating for diffracting each of the incident first and second laser beams and dividing them into a main beam (0th order diffracted light) and two sub beams (± 1st order diffracted light). By dividing the beam into three, a differential push-pull signal (DPP signal) is generated in the photodiode PD. Using this as a tracking error signal, tracking control can be performed.

The beam splitter BS is a polarizing element that passes light having a predetermined polarization direction and reflects light having a polarization direction orthogonal to the polarization direction, and reflects the first and second laser beams incident from the laser diode LD. The first and second laser beams reflected by the optical disc D are set to pass through.
The collimator lens 22 is an optical element that converts the first and second laser beams emitted from the beam splitter BS into parallel beams and converts the first and second laser beams reflected from the optical disc D into convergent beams. is there.
The mirror M is an optical element that changes the directions of the first and second laser beams.

The objective lens 23 is an optical element for condensing the first and second laser beams onto the optical disc D and converting the laser beam reflected from the optical disc D into parallel light.
The actuator 24 moves the objective lens 23 in the front-rear direction and the radial direction of the optical disc D, and performs focusing (focusing) and spot position adjustment (tracking) of the first and second laser beams.

  The optical axis correction element 25 corrects the optical paths of the first and second laser beams emitted from the laser diode LD with respect to the optical axis of the optical system of the optical pickup 20 so that the first and second laser beams are photon-exposed. This is an optical element for condensing light at substantially the same location of the diode PD. Since the emission points of the first and second laser beams are misaligned, for example, when the emission point of the first laser beam is made coincident with the optical axis, the emission point of the second laser beam is offset from the optical axis. Shift. Accordingly, the first and second laser beams are condensed at different positions on the photodiode PD. For this reason, the optical path of the emitted light is adjusted by the optical axis correction element 25 at each of the first and second wavelengths so that the first and second laser beams are condensed at substantially the same position on the photodiode PD. ing.

FIG. 3 is a side view showing a hologram 70 as an example of the optical axis correction element 25.
The hologram 70 has a substrate 71 (refractive index: n) and a plurality of diffraction portions 72 formed on the substrate 71. The diffraction part 72 has a six-step staircase shape with a level difference t.
By appropriately setting the level difference t of the diffractive portion 72, the first and second wavelengths of light that pass through the hologram 70 can be made into 0th-order diffracted light and 1st-order diffracted light, respectively. For example, the refractive index n of the substrate 71 is 1.5, and the step t is 1.3 μm. At this time, phase differences φ1 and φ2 (φ1 = 2π (n−1) t / λ1, φ2 = 2π (n−1) t / λ2) generated at the respective stages of the first and second wavelengths λ1 and λ2. Are 2π and 10π / 6, respectively. As a result, the first and second wavelengths of light passing through the hologram 70 are substantially 0th order diffracted light and 1st order diffracted light.

  The 0th-order diffracted light passes through the hologram 70 without being diffracted. On the other hand, the first-order diffracted light is diffracted, bent by the diffraction angle, and passes through the hologram 70. In this way, the hologram 70 can give different directions to the first and second wavelength laser beams (the first wavelength laser beam: going straight, the second wavelength laser beam: only the diffraction angle). Bend). As a result, the laser beams having the first and second wavelengths that have passed through the hologram 70 (optical axis correction element 25) are condensed at the same position on the photodiode PD.

The detection lens 26 is an optical element for condensing the first and second laser beams on the photodiode PD.
The photodiode PD as a light receiving element is an element for detecting the first and second laser beams reflected by the optical disc D and reading information from the optical disc D.
The photodiode PD has a detection area divided so that each of these three beams can be detected independently in response to the laser beam being divided into a main beam and two sub beams by the grating 21. By detecting and calculating each of the three beams, a tracking error signal (differential push-pull signal: DPP signal) is generated by the differential push-pull method (DPP method).

(Derivation method of equation (1) representing the relationship between effective refractive indexes n1 and n2 and resonator length L)
Hereinafter, the process of deriving Equation (1) representing the relationship between the effective refractive indexes n1 and n2 and the resonator length L will be described. Since the laser diodes LD1 and LD2 are integrally formed on a single substrate (monolithic), the respective resonator lengths are the same (L), but here the respective resonator lengths are L1 and L2. Will be described.
Generally, the return light noise of the laser light source is such that the optical path length (product of distance and refractive index) L0 from the light emitting point to the reflecting surface of the disk D is the product n · Lr of the effective refractive index n of the laser light source and the resonator length Lr. On the other hand, it appears greatly when the condition of the following equation (2) is satisfied.
L0 = i · n · Lr / 2 Equation (2)
Here, i is an integer.

In applying the formula (2), it is necessary to consider the following 1) to 2).
1) The optical path length L0 from the light emitting point to the reflecting surface of the disk D varies due to the surface shake of the disk D or the like. Therefore, it is necessary to secure a margin when setting the optical path length L0.
2) In the laser diode LD, the laser diodes LD1 and LD2 are concentrated on the same substrate (widely in the same package), so that the light emitting points of the first and second wavelengths are arranged close to each other. .
On the other hand, the thickness of the protective layer varies depending on the type of the optical disk. For example, the protective layer thickness Tdvd for DVD and the protective layer thickness Tcd for CD are 0.6 mm and 1.2 mm, respectively. Therefore, when the refractive indexes Ndvd and Ncd of the protective layer in DVD and CD are 1.55 and 1.56, respectively, the optical path length L0 from each light emitting point to the reflecting surface of the disk D is 0.942 mm in absolute value. There is a difference of (= | 1.55 · 0.6−1.56 · 1.2 |). In other words, when the optical path lengths of DVD and CD are L01 and L02, respectively, the relationship of the following formula (3) is established between them.
L01 −0.942 = L02 (3)

Here, consider the case where the optical disk D performs recording and reproduction using the laser diodes LD1 and LD2 as laser light sources for DVD and CD, respectively.
Hereinafter, the conditions of the effective refractive indexes n1 and n2 and the resonator lengths L1 and L2 of the laser diodes LD1 and LD2 are obtained in consideration of the above 1) and 2).
Assuming that the margin of deviation of the surface of the DVD is ± 0.5 mm, from the equation (2), in order to avoid the return light noise of the laser diode LD1, the optical path length L01 from the light emitting point to the reflecting surface of the disk D (DVD) is The condition to be satisfied is expressed by the following formula (4) (j: integer).
j · n1 · L1 / 2 + 0.5 <L01 <(j + 1) · n1 · L1 / 2−0.5
...... Formula (4)
Assuming that the CD deflection margin is ± 0.7 mm, the optical path length L02 from the light emitting point to the reflecting surface of the disk D (CD) is calculated from the equation (2) in order to avoid the return light noise of the laser diode LD2. The condition to be satisfied is expressed by the following formula (5) (k: integer).
k · n2 · L2 / 2 + 0.7 <L02 <(k + 1) · n2 / L2 / 2−0.7
...... Formula (5)

Here, Formula (3) to Formula (4) can be transformed into the following Formula (6).
j · n1 · L1 / 2 + 0.5 <L02-0.942
<(J + 1) · n1 · L1 / 2−0.5 (6)

Since both the left side of Expression (5) and the right side of Expression (6) hold, the following Expression (7) holds (m: integer).
m · n2 · L2 / 2 + 0.7 <L02 <m · n1 · L1 / 2−0.5 + 0.942
...... Formula (7)
Expression (7) means that the following expression (8) holds.
m · n 2 · L 2/2 + 0.7 <m · n 1 · L 1/2 + 0.442 (8)

Hereinafter, a condition for determining an appropriate value range of m in Equation (8) will be considered.
First, the following expressions (9) and (10) are obtained from the expressions (4) and (5), respectively.
n1 · L1> 2.0 (9)
n2 · L2> 2.8 Equation (10)

  Considering the movement range (working distance) of the objective lens 23 when focusing on the optical disc D, the focal length of the objective lens 23 is considered to be about 1.5 mm at a minimum. In addition, when performing recording and reproduction on a DVD, it is considered that the focal length of the collimator lens 22 is required to be at least about 5 times the focal length of the objective lens 23 (1.5 · 5 = 7 at the minimum). .5 mm). Furthermore, a certain distance is required between the objective lens 23 and the mirror M and between the mirror M and the collimator lens 22. From the above, the optical path length L02 between the laser diode LD2 and the recording surface of the optical disc D (CD) needs to be about 12 mm at a minimum.

From the formulas (9) and (10), it can be seen that the minimum optical path length L02 is about 12 mm and that the minimum value of m in the formula (8) is about 8.
Therefore, by substituting 8 for m in the equation (8), the following equation (11) representing the relationship between the effective refractive indexes n1 and n2 and the resonator lengths L1 and L2 is obtained.
n1 · L1-n2 · L2> 0.065 [mm] (11)
As can be seen from the above, it is possible to suppress the occurrence of laser return light noise by using a laser light source that satisfies the condition of equation (11).

There are two methods for emitting laser light having two wavelengths from a semiconductor laser in one package. One of them is a hybrid system in which two semiconductor chips for a semiconductor laser are separately produced and mounted when they are packaged. The other is a monolithic method in which two light emitting portions are formed on one semiconductor chip.
In the hybrid system, it is easy to make the resonator lengths of the resonators corresponding to the two wavelengths different. For this reason, the return light noise of a semiconductor laser can be suppressed by satisfy | filling the relationship of Formula (11).
Since the semiconductor laser LD of this embodiment is a monolithic system, the resonator lengths of the two semiconductor lasers LD1 and LD2 have the same value L. Accordingly, by substituting L1 = L2 = L into equation (11), a relational expression that can suppress the return light noise of both the semiconductor lasers LD1 and LD2 is derived, and this corresponds to equation (1).

(Operation of optical pickup 20)
The reproduction operation of the optical pickup 20 will be described. Here, it is usual that only one of the first and second laser beams is emitted according to the type of the optical disk D, etc., but the first and second lasers are easy to understand. A description will be given in contrast to light.

(1) The first and second laser beams emitted from the laser diode LD are divided into three beams by the grating 21. The three beams are reflected by the beam splitter BS, enter the collimator lens 22, and are converted into parallel light.
(2) Thereafter, the first and second laser beams are reflected by the mirror M, enter the objective lens 23, and are condensed on the optical disc D. For example, the first laser beam is focused on the DVD and the second laser beam is focused on the CD.

(3) The first and second laser beams reflected by the optical disk D pass through the beam splitter BS through the objective lens 23, the mirror M, and the collimator lens 22.
At this time, the sub-beams of the first and second laser beams are not transmitted through the beam splitter BS, but part of the light amount is reflected by the beam splitter BS, passes through the grating 21, and passes through the laser. Return to diode LD. This is because the sub-beam is diffracted light (first-order diffracted light) and the polarization state is different from that of the main beam that is not diffracted light (zero-order diffracted light).

  Here, it is assumed that a part of the first and second laser beams emitted by the laser diodes LD1 and LD2 are re-entered as return light. Even in this case, it is possible to set the optical path lengths L01 and L02 so that the re-entered return light satisfies the condition of the expression (1) and thus both the expressions (4) and (5) are satisfied. For this reason, return light noise can be suppressed in both the laser diodes LD1 and LD2.

(4) The first and second laser beams that have passed through the beam splitter BS have their optical paths corrected by the optical axis correction element 25, enter the photodiode PD via the detection lens 26. The first and second laser beams are condensed at the same position on the photodiode PD by the optical axis correction element 25. Signals corresponding to the three beams are output from the photodiode PD, and a DPP signal is generated by calculating these three outputs, and tracking control of the optical pickup 20 can be performed.

  At this time, since the return light noise from the laser diodes LD1 and LD2 is reduced, the signal (reproduction signal or DPP signal) output from the photodiode PD is kept in a low noise state (high S / N ratio). be able to.

The reproduction operation of the optical pickup 20 has been described above.
Even when the optical pickup 20 performs a recording operation, the return light noise from the laser diodes LD1 and LD2 is reduced, so that recording on the optical disk D can be performed with a high S / N ratio. The reduction of the return light noise of the laser diodes LD1 and LD2 is substantially the same as in the case of performing reproduction, and thus detailed description thereof is omitted.

As described above, in this embodiment, a laser light source (laser diode LD) in which two light sources (laser diodes LD1 and LD2) are housed in one package can be used in a state in which return light noise is reduced.
For this reason, compared with the case where two separated laser light sources are used, (1) the number and size of the light sources can be reduced, and (2) a beam splitter for combining two wavelengths can be omitted. As described above, the number of parts can be reduced and the size of the optical pickup can be reduced.

1 is a schematic diagram showing an optical disc reproducing apparatus according to a first embodiment of the present invention. It is sectional drawing showing the end surface of the semiconductor chip which is a main body of a laser diode, and the cross section of a side surface direction. It is a side view showing the hologram as an example of an optical axis correction element.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Optical disk reproducing apparatus 20 Optical pick-up 21 Grating 22 Collimator lens 23 Objective lens 24 Actuator 25 Optical axis correction element 26 Detection lens 30 Optical pick-up drive part D Optical disk LD Laser diode M Mirror PD Photo diode

Claims (4)

A first optical resonator having a first effective refractive index n1 and a first resonator length L1, a first light emitting point for emitting a first laser beam oscillated by the first optical resonator, A first light emitting unit having:
A second optical resonator having a second effective refractive index n2 and a second resonator length L2, and a second light emitting point for emitting a second laser beam oscillated by the second optical resonator; A second light emitting part having
A difference (n1 · L1−n2 · L2) between the product of the first effective refractive index and the first resonator length and the product of the second effective refractive index and the second resonator length is 0.065 mm. Large laser light source. The first resonator length L1 and the second resonator length L2 are substantially equal (L1≈L2).
The laser light source according to claim 1. A first optical resonator having a first effective refractive index n1 and a first resonator length L1, a first light emitting point for emitting a first laser beam oscillated by the first optical resonator, A first light emitting unit having:
A second optical resonator having a second effective refractive index n2 and a second resonator length L2, and a second light emitting point for emitting a second laser beam oscillated by the second optical resonator; A second light emitting part having
A difference (n1 · L1−n2 · L2) between the product of the first effective refractive index and the first resonator length and the product of the second effective refractive index and the second resonator length is 0.065 mm. An optical pickup characterized by its large size. The first resonator length L1 and the second resonator length L2 are substantially equal (L1≈L2).
The optical pickup according to claim 3.

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