JP2699986B2 - Apparatus for detecting focus error of optical head - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a means for detecting a focus error of an optical head adapted for reading and writing data on an optical recording medium, and more particularly, to a focus error. An improved means for generating a signal and using it as an input to a servo system that operates to hold a light beam at a focal point on a surface.

B. Prior Art The focus error detection methods previously used for optical storage generally employ a knife edge, astigmatism correction lens, or critical angle prism. These technologies are
A very critical alignment of these light elements and segmented photodetectors is required.

EP-A-0164687 teaches that a laser beam reflected from an optical disc is sent through an objective lens to a prism, where the beam width is reduced to 1 / M in one direction, and the elliptical beam is focused on a knife-edge focus. A detection technique is disclosed for sending to an error detection system. The above specification states that following equation (25 '), the use of a prism increases the focus error signal by a factor of M compared to the normal knife-edge method without a prism.

Outline of the conference on optical data storage held in Washington, USA, October 15-17, 1985 (The Digest of
The Topical Meeting on Optical Data Storage) includes Yamamoto et al., “Design Consideration of Optical Pr
egroove Dimensions) ”has been published. The above article discloses a six-element photodetector for detecting far-field spot size variations related to focus errors.

C. Problems to be Solved by the Invention With a relatively small number of components, a large beam beam which significantly improves the quality of the focus error signal and requires only non-critical alignment of the unidirectional segmented photodetector. It is an object to provide a very sensitive focus error detection technique that provides size.

D. Means for Solving the Problem For this purpose, according to the present invention, there is provided an apparatus for detecting a focus error of an optical head by placing a prism in an optical path of a return light beam reflected from an optical recording medium. Provided. Prism reduces the beam width in the one direction to 1 / M, further increase the angle of divergence or convergence regarding focus error of the beam M times in the direction of the, thereby ^doubling M quality as desired focus error signal To improve. The focus error is detected by a segmented photodetector having inner and outer photosensitive regions as shown in Yamamoto et al.
The photodetector generates an electric signal indicating a focus error from a difference in light intensity between the inner region and the outer region. Also, the photodetector is preferably segmented in such a way as to provide a tracking error signal.

E. Embodiment The principles of the present invention are best illustrated in FIG.
The output of the diode laser 10 passes through a circularizer / collimator system 11 into a parallel beam 12.
The beam 12 becomes a beam 14 when reflected from the beam splitter 13. The beam 14 is focused on a surface 16 of the optical recording medium by a lens 15. When the surface 16 is at the focal point of the lens 15, the collimated beam 14 sent from the beam splitter 13 to the lens 15 overlaps itself and is retroreflected, and is sent by the splitter 13 to the prism 17 as a return beam 14. The beam 14 is refracted by the prism 17 into a beam 18 and travels to the photodetector 19 without interruption. The full cross-section beam pattern of beam 18 impinges on photodetector 19. Beam 18
Has an elliptical intensity pattern with a major axis equal to D and a minor axis equal to A. Prism 17 reduces the width of beam 14 to 1 / M of the width of refracted beam 18 (defined as M = D / A).

Now assume that surface 16 is displaced from the focal point of lens 15 by an amount δz. Under this condition, the beam 14 'reflected from the surface 16 diverges at an angle δθ after passing through the lens 15.

δθ = Dδz / f ² (1) where f is the focal length of the lens 15.

If δz is negative, δθ will be negative and beam 14 'will converge. In the following analysis, consider the case where δz is positive,
When is small, the same analysis is valid for both the divergent beam and the convergent beam 14 '.

The refracted beam 18 'emerging from the prism 17 has a divergence angle Mδθ
And the width W of the light beam on the detector 19 is given by:

W ≒ A + 2lMδθ (2) where l is the distance from the prism 17 to the photodetector 19.

 Using equation (1), it can be obtained by the following equation.

W ≒ A + 2lMDδz / f ² ≒ A + 2lM ² Aδz / f ² or δW / δz = (WA) / δz = 2lM ² A / f ² (3) Therefore, the prism in the path of the return beam 14 ′ from the surface 16 by inserting the 17, the change in beam width δW showing a focus error δz is improved to ^twice M. M is determined by the vertex angle of the prism 17 and the refractive index of the prism material.
M can be any size. However, the alignment tolerances of the system become increasingly tight as M increases. A practical maximum for a single prism 17 is approximately M = 5. However, several prisms can be used in series, in which case the synthesis coefficient M is the product of the values M of each prism.

As shown in FIG. 2A, photodetector 19 includes three photosensitive stripe regions 1, 2, 3. The dashed ellipse is the beam
18 represents the full cross-section beam pattern created on the photodetector 19. The electrical signal generated by light incident on these photosensitive areas 1,2,3 and I _1, I _2, I _3, respectively.

Signals I ₁ and I ₃ is amplified summed in amplifier 20 provides an output signals S _1, but the signal I ₂ is amplified by the amplifier 21 provides an output signal S _2. If the beam 14 is in focus, S ₁ is equal to S _2. Signals S ₁ and S ₂ are supplied to the differential amplifier 22 and the summing amplifier 26, whose outputs are fed to the divider circuit 28. The bandwidth of the amplifiers 20, 21, 22 and 26 is well above the focus servo bandwidth (typically less than 10kHz), so that
The phase difference between the outputs of amplifiers 22 and 26 is negligible. The output of the divider 28 is a focus error signal FES.

This focus error signal is applied to the focus servo control system 23 (FIG. 1). As shown, the system 23 comprises a conventional system for adjusting the current in the coil 24 of the voice coil motor, such as a voice coil driver, thereby adjusting the relative position of the lens 15 with respect to the surface 16. Means are provided. A typical servo system is shown in the above-mentioned EP-A-0164687.

In a diffraction limited optical system, the intensity profile of the beam 18 has a Gaussian distribution. That is, The width 2a of the area 2 of the detector 19 is such that the disk surface 16 is
At the focal point, half of the light incident on
And select the other half to fall on regions 1 and 3.

 The total power of beam 18 is given by:

The output of the detector 19 on the area 2 is given by the following equation.

This is equal to P / 2 when δz = 0 and W = A, if a = 0.169A.

To assess the sensitivity of the FES for the change in the focal position dz of the lens 15, it is necessary to determine the relative change in S ₂ due to dz. S ₂ is proportional to I _2, since I ₂ is proportional to P _2, the amount of problems, Becomes Using equations (3) and (7), The value of this derivative is:

When δZ = 0 and W = A, the above amount is equal to -0.269A. Therefore, It is.

Typical l in the prior art (see the above-mentioned paper by Yamamoto et al.)
(50mm) and f (4mm) without prism (M = 1)
For, this amount is equal to 5.4 / mm. Focus error δz is 0.00
If large as 1 mm, the change in P _2, thus changing the S ₂ 0.5
Only 4%. Since the change in S ₁ is equal to the change in S ₂ and the sign is opposite, from equation (4), FES with a range of ± 1
Becomes only 0.0108.

Incorporating an M = 5 prism according to the principles of the present invention increases the FES to 0.264 and increases the sensitivity of the focus servo loop.
25 times.

A preferred configuration of the photodetector is shown in FIG. 2B. Photodetector 25
Each generates an electrical signal I ₁ ~I _6, includes six photosensitive areas. The focus error signal is given by the following equation.

The track error signal is given by the following equation.

Where S ₁ = k (I ₁ + I ₃ + I ₄ + I ₆ ) S ₂ = k (I ₂ + I ₅ ) S ₃ = k (I ₁ + I ₂ + I ₃ ) S ₄ = k (I ₄ + I ₅ + I ₆ ) Here, k is the gain of the amplifiers 20 and 21.

The method of adjusting the track error using the track error signal is conventional and is not part of the present invention.
FIG. 2B shows that the inventor's means for detecting and correcting the focus error is compatible with a photodetector such as 25 which produces both a focus error signal and a track error signal, and with it as desired. It is only added to show that it can be used.

3A and 3B illustrate an embodiment of an optical head for use with a read-only, write-once, or phase change disk. According to this embodiment, the general direction of the head is parallel to the tracks on the disk 50. Therefore, this head is very easily adapted to a disk drive mechanism using a swing arm type actuator. The beam emanating from laser 52 (FIG. 3A) is collimated by lens 54 and travels through PBS 56 without refraction or reflection. The beam 58 emitted from the PBS 56 has an elliptical cross-section having a width D in one direction and A in a direction perpendicular thereto. This beam 58 passes through a quarter wave plate 60 to a prism assembly 62. At the first side 64 of the prism assembly 62, the beam 58 is made circular and then the second side
From 66, the light is reflected by total internal reflection (TIR) and strikes a reflecting surface 68 from which a combined beam 72 is focused onto a disk 50 through an objective lens 70. Beam 72 is a reflected beam 58 '
As reflected from the disk. In this embodiment, the surface
The refraction at 64 serves two functions: to make the beam 58 towards the disc circular and to improve the quality of the focus error signal. The beam 58 ', whose cross section is also elliptical, is
Is reflected by the surface 74 of the Photodetector 76 is preferably segmented as shown in FIG. 2A or 2B. The complete unobstructed beam pattern of beam 58 'strikes photodetector 76.

When the disk 50 is in focus, as shown in FIGS. 3A and 3B, the return beam 58 'coincides with the beam 58 and the width of the beam at the photodetector 76 is equal to A. But,
If the data on disk 50 is out of focus, return beam 58 'will diverge from 58, causing the width of the beam toward photodetector 76 to be greater than A. This effect is similar to beam 1 shown in
Similar to 8 'divergence. Photodetector 76 is connected to a circuit similar to that shown in FIG. 2 to generate FES. When the disc needs to be in focus, the relative position of the lens 70 with respect to the surface of the disc 50 is adjusted in response to this FES by suitable servo means similar to that shown in FIG.

In this embodiment, the expansion / contraction ratio M of the prism 66 must match the emission pattern of the laser 52.

In the configuration shown in Figures 3A and Figure 3B, unlike those taught by the prior art, the quality of the focus error signal is improved as desired M ² times.

F. Effects of the Invention The present invention discloses a focusing system that makes full use of the prism's ability to enhance sensitivity. The present invention does not use a knife edge as in EP-A-0164687, but instead uses a three-zone detector with inner and outer zones. The focus error signal is derived by taking the difference between the outer and inner regions. By using this technique, an unobstructed complete return beam is available.
There is no knife edge, and the focus error signal is basically derived from a three-element detector that measures the size of the beam.
Accordingly, the present invention can achieve complete M ² times more sensitive by a photodetector.

In the above-mentioned European patent application, the photodetector for the focus error is provided separately from the photodetector for the tracking error, but the photodetector for the focus error of the present invention is a photodetector for the tracking error. Also used (shared) as a detector.

Another feature of the present invention is that the prism can be used to perform two functions of improving the quality of focus and bending the beam. See FIGS. 3A and 3B of the specification. A beam flexure is needed to direct light from the horizontal direction vertically to impinge on the media. The present invention uses a prism having a third reflecting surface to serve as a bending device. This eliminates the need for additional components and helps reduce the size of the optical head. This is very important for swing arm actuator applications.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing the principle of the present invention. 2A and 2B show two preferred configurations of a segmented photodetector. Both generate a focus error signal, and the detector of FIG. 2B also generates a track error signal. 3A and 3B are views showing an embodiment for use with an optical disk drive mechanism using a swing arm type actuator. 10 …… Diode laser, 11 …… Circularizer /
Collimator system, 13 …… Beam splitter, 1
5, 70 ... lens, 16 ... surface, 17, 66, 62 ... prism, 18, 18 '... refracted beam, 19, 25, 76 ... photodetector, 23 ... focus servo control system, 24 …… coils, 26
…… Additional amplifier, 28 …… Division circuit, 52 …… Laser, 50…
… Disk, 56… PBS, 60… quarter wave plate, 68…
… Reflective surface

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Daniel Rugar 228, Webster Street, Apartment Day, Palo Alto, California, United States of America Hei 3-17835 (JP, A)

Claims (1)

(57) [Claims] An apparatus for detecting a focus error of an optical head for reading and / or writing data on an optical recording medium, comprising: a light generating means for supplying a parallel light beam. A quarter-wave plate (60) for receiving parallel rays from the light generating means; a first surface (64) for making the parallel rays from the quarter-wave plate circular; A second surface (66) where the light is reflected by the complete internal reflection, and a third surface (68) that is a reflecting surface
A return light beam reflected from the optical recording medium when the return light beam exits the prism via the third surface, the second surface, and the first surface in this order. By reducing the width of the return light beam to 1 / M in a certain direction, and at the same time, increasing the angle of divergence or convergence of the return light beam by M times in the certain direction, so that the return light beam indicating the focus error of the optical head in the certain direction prism for enlarging the change in width to ^twice M (62)
A lens (70) for condensing the light reflected from the third surface onto the optical recording medium; a polarizing beam splitter (56) for receiving the returning light from the prism; and inner and outer photosensitive regions. Receiving an unobstructed return beam from the prism via a polarizing beam splitter,
A photodetector (76) for generating a focus error signal in response to a difference in light intensity between the inner and outer regions.