JPS6256576B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is applied to a device that optically reproduces or records information, such as a video disc device.
The present invention relates to a focus detection device having means for aligning a recording surface with a convergence point of a radiation beam for recording or reproduction.

In optical recording and reproducing devices, it is necessary to precisely align the record carrier with the focal point, and for this purpose, the lens is movable, and the lens is moved according to the surface fluctuation of the record carrier, so that the focus is always on the recording surface. A focus servo (focus control device) is required to do this. The focus servo error signal can be obtained using an optical system in which the position and shape of the reflected light from the recording surface changes depending on the position of the reflecting surface, and a photodetector that detects it. It has been known.

In order to apply focus servo, as a prerequisite, it is necessary to adjust the optical system or servo circuit so that the error signal becomes zero when the recording surface coincides with the focus. With conventional playback devices,
The photodetector that detects the focus error signal is adjusted so that the recorded disc is rotated to maximize the reproduced signal. There are various methods for recording devices, but one example is to consider that the state is in focus when the incident light and the reflected light on the recording lens are completely parallel, and adjust the photodetector so that the reflected light is parallel. are doing.

The present invention rotates the disk as described above,
A detection device that can accurately place the reflective surface at the focal point in order to eliminate the need to adjust the optical system and servo functions while applying focus servo, and complete all optical system adjustments at the optical system assembly stage. This is what we propose. Theoretically, if the reflective surface is exactly at the focal point, the reflected light will take a focused optical path, so the focus error signal is detected so that this position becomes the zero-crossing point of the focus error signal. It becomes possible to adjust the optical system of

The present invention is particularly useful when used in an optical head having a structure in which a lens barrel that integrates an optical system such as a light source, a photodetector, and a lens moves in accordance with the wobbling of the recording surface or the eccentricity of the recording track. becomes important. This is because in this case the focus detection optical system is also integrated and moves, so it is impossible to adjust the moving optical system from the outside.

The principle of the present invention and an embodiment that realizes the same will be explained based on the drawings. In Fig. 1, the spot narrowed down by lens 1 is shown by optical system 3, which produces an enlarged image.
(for example, a microscope can be used) and observed with the naked eye 4 or with a projection observation system. The projection observation system is capable of projecting a magnified real image of a spot by the lens 1 onto a vidicon surface and displaying it on a CRT screen, or in the case of visible light, projecting an enlarged image onto a simple screen. 2 is a diffraction grating placed near the spot. If the position of the diffraction grating 2 is before or after the spot position, the spot will be diffracted at the observation position, and the 0th-order and 1st-order diffracted light will form three or more points, as shown in Figure 2 b and c. Looks separated. The separation distance of the spots is proportional to the deviation of the diffraction grating position from the focal position, and since the spots converge at one point when they reach the focal position, it is possible to align the diffraction grating surface with the focal position.

By depositing a semi-transparent film on the diffraction grating surface,
It is possible to adjust the transmitted light so that they overlap at one point, place it at the focal position, and use the reflected light to adjust the optical system to the optimal position. It is also possible to make the diffraction grating surface a total reflection surface, separate the reflected light passing through the lens 1 with a beam splitter, and observe it with an enlarged imaging system.

The focal position accuracy that can be determined by the diffraction grating 2 will be described. If the wavelength of light is λ and the lens numerical aperture is NA, then the lens separation is λ/NA. If the number of diffraction gratings 2 is n/mm (if the pitch is P, then n=1/P), then the separation angle between the 0th-order and 1st-order diffracted light is nλ radians, so if the defocus amount is Δf, then , the spot separation width is Δfnλ. Therefore, Δfnλ=λ/NA is the separation limit. For example, if λ=0.8μm and NA=0.5, then n
When n=50, Δf=40 μm, and when n=1000, Δf=2 μm.

The depth of focus of the lens is ±2 μm, so n=
If you use a 1000 grating, you can place the grating within the depth of focus. However, if n is less than 1000, for example, n = 400, but this does not pose much of a problem in practice. This is because the focus error signal takes the form as shown in Figure 3 a and b, and the range in which the error signal voltage increases approximately in proportion to the focus error is generally several tens of micrometers, which is sufficiently large compared to the focal depth of the lens. . Therefore, the error signal is obtained from the difference between the outputs of two or more photodetectors, but by making a difference in the output sensitivity of each detector, the error signal can change the zero cross position within a range several times the depth of focus. Can be done. Therefore, even if the reflective surface cannot be completely brought within the depth of focus using the diffraction grating, it can be adjusted to be within the depth of focus using the servo circuit system.

However, even with the above configuration, if the spot size is the same as or smaller than the groove size of the grating when placed at the focal position of the diffraction grating,
Since the diffracted intensity distribution of the reflected light differs depending on the position of the spot on the grating, an optical system is required to detect the focus error signal (including tracking error signal in some cases) using the reflected light. It becomes difficult to adjust to the optimal position. Therefore, a plane reflection section without a diffraction grating is made on the same plane as the diffraction grating surface, and after focusing on the diffraction grating surface, the plane reflection section is moved parallel to the spot position, and the focus error is determined from the reflected light. The signal detection system, tracking error signal detection optical system, etc. can be adjusted. Furthermore, by carving a coarse diffraction grating and a fine diffraction grating on the same surface, if the focus is significantly out of focus during adjustment, the rough diffraction grating can be used to roughly determine the position, and then the finer diffraction grating can be used to accurately position the grating. You can also focus on

In the above-mentioned integrated optical head, if the semiconductor laser serving as the light source deteriorates, it may be necessary to replace the light source. At this time, if the distance between the lens and the diffraction grating is positioned constant, the replaced light source can be brought to the optimal position by adjusting the light spots separated by the diffraction grating so that they match. Can be done.

In the case of the above-mentioned optical head, which integrates a semiconductor laser and an optical system for excitation, the optical system must be as small and light as possible, and must have a structure that is durable and does not resonate so as to withstand excitation up to several KHz.
Therefore, it is extremely difficult to provide a mechanism for finely adjusting the photodetector position, laser position, and other optical systems. Therefore, once adjusted, the structure is fixed with adhesive, etc., but in this case,
It is absolutely necessary to be able to correctly align the reflective surface with the focal position during the assembly stage. In addition, as mentioned above, semiconductor lasers can deteriorate, so it is desirable to have a structure that allows the semiconductor laser to be replaced. However, even if the laser is replaced, other optical systems cannot be adjusted, so the position of the semiconductor laser must be changed before replacement. It is necessary to attach it within 10 μm from the position. It is very difficult to achieve this level of precision mechanically, so in order to determine the position of the semiconductor laser, we use the above detection device to move the position of the semiconductor laser so that the separated spots by the diffraction grating coincide. It is possible to adjust the

For the same purpose as the present invention, it is also possible to consider a method of observing a far-field pattern of diffracted light by a diffraction grating placed near a focal point. The principle of this method is that when the focal point is before and after the diffraction grating surface, interference fringes are generated due to interference between the 0th-order light and the 1st-order diffracted light, and the distance between the fringes is inversely proportional to the amount of defocus, and when the focus is It takes advantage of the phenomenon of stripes disappearing. However, especially in the case of light with a wavelength of 700 Å to 8400 Å, which is close to infrared, such as from semiconductor lasers, the only way to observe the far-field pattern is to use a highly sensitive vidicon, as the light diverges and becomes weak. There's no way. If the focus detection device of the present invention is used, observation is performed using an imaging system, which means that the light is focused and observed, so the intensity is strong, and it is possible to perform visual observation, projection observation using an IR sphere, or visible observation. Projection observation onto a white screen is also possible if the light is close to the light.

In the above embodiment, the case where the present invention is applied to the optical system of an optical reproducing device has been described, but it goes without saying that the present invention is equally effective in the case of an optical recording device.

As described above, the focus detection device of the present invention is characterized by aligning magnified images of light spots separated by a diffraction grating to a single point, and the adjustment of the optical system and circuit system related to the focus servo is extremely difficult. This is particularly effective for optical heads with integrated light sources.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of an embodiment of the focus detection device of the present invention applied to an optical head, Fig. 2a,
FIGS. 3b and 3c are front views for explaining the spot separation state when the diffraction grating is at or before and after the focal position, and FIGS. 3a and 3b are waveform diagrams of the focus error signal. 1...Lens, 2...Diffraction grating, 3...Magnifying optical system, 7...Semiconductor laser, 9...Photodetector,
10... Differential amplifier, 11... Servo circuit, 12
...Drive coil.

Claims (1)

[Scope of Claims] 1. A radiation beam, a lens for condensing the radiation beam, a diffraction grating placed approximately perpendicular to the optical axis near the convergence point of the lens for the radiation beam, and a diffraction grating for the diffraction beam. an optical system and a light detection means for detecting the reflected light from the grating and detecting an error signal corresponding to the deviation of the diffraction grating surface from the focal point; A magnified image of the light spot separated by the diffraction grating, comprising a servo system for making the distance between the surface and the lens constant, and an optical system for making the light spot at the condensing point into an observable enlarged image. A focus detection device characterized in that the distance between the diffraction grating and the lens is adjusted so that the diffraction gratings and the lens overlap at one point. 2. The diffraction grating surface is a semi-transparent surface for light, and an optical system that makes the light point at the condensing point an observable enlarged image is disposed on the side of the transmitted light from the diffraction grating surface. A focus detection device according to scope 1. 3. The diffraction grating surface has a portion in which no diffraction grating is carved, or a portion in which diffraction gratings of different pitches are carved, and the diffraction grating surface is movable in a direction perpendicular to the optical axis. 2. The focus detection device according to claim 1, wherein different portions of the focus detection device are movable to the center position of the optical axis.