FR2598521A1 - Optical system for a photoelectromagnetic recording and reproduction device - Google Patents

Abstract

The invention relates to a photoelectromagnetic reproduction device in which the reflected light signal coming from a photoelectromagnetic disc 24 is split by an astigmatic analyser 28. The analyser splits the light signal as a function of the linear polarisation, with the purpose of detecting the data signal, but furthermore introduces some astigmatism into one of the split beams in order to permit a focusing detection. … …

Description

This invention relates to a device for recording and

  photo-electro-magnetic reproduction. In particular, it relates to an optical system of a device which is so designed that it makes it possible to obtain a reproduction data signal and an autofocus control signal.

  A conventional optical system of a photo-electro-magnetic recording and reproducing device has the disadvantage of having a large number of components and a complex arrangement. Therefore, it is difficult to miniaturize the optical system, and the manufacturing cost is relatively high.

  An object of this invention is to eliminate the difficulties indicated above which accompany the conventional optical system with a photo-electro-magnetic recording and reproduction device. More specifically, an object of the invention is to provide an optical system intended for a photo-electromagnetic recording and reproduction device which is made simpler to manufacture by reducing the number of components. The above presented objects of the invention are achieved by an optical system for a photo-electro-magnetic recording and reproducing device for reading signals magneto-optically. The inventive system includes an astigmatism causing analyzer which has both an astigmatism producing function for the purpose of detecting autofocus control signals and a function of analyzer, used to detect

magneto-optical signals.

  In the optical system, the direction of polarization of a laser beam source for reading the magneto-optical signals and the direction of the recording tracks can be parallel or perpendicular to each other. The meridian surface and the incident surface of the astigmatism analyzer can be approximately 45 with the direction of polarization of the

  laser beam and direction of the recording tracks.

  The following description, intended to illustrate the invention, aims to give a better understanding of its characteristics and advantages; she draws on the drawings

  annexes, among which: FIG. 1 is an explanatory diagram showing a first embodiment of the optical system of a photo-electro-magnetic recording and reproduction device according to the invention;

15 20

  FIG. 1A is an explanatory diagram showing an example of the directions taken by the plane of polarization of the light beam for various positions in the optical system of FIG. 1; FIG. 1B is an explanatory diagram showing the detector 29B of the assembly of FIG. 1; FIG. 2 is an explanatory diagram showing a second embodiment of the optical system according to the invention; FIG. 2A is an explanatory diagram showing the direction taken, according to an example, by the plane of polarization of the light beam for various positions in the optical system of FIG. 2; Figure 3 is a side view showing another example of the astigmatism causing analyzer of Figures 1 and 2; and

  Figure 4 is a side view showing another example of an astigmatism causing analyzer.

  In FIG. 1, there is an explanatory diagram presenting a first embodiment of the optical system of a photo-electro-magnetic recording and reproduction device.

seton the invention.

  In FIG. 1, a laser beam source 21, for example a laser diode, produces a laser beam. A collimator 22 transforms the laser beam produced by the laser beam source 21 into a parallel beam. A semi-mirror 23 separates the outward and return beams from one another. The data is recorded on a photo-electromagnetic disk 24 by vertical magnetization. An objective lens 25 focuses the laser beam on the recording surface of the disc 24. A half-wave plate 26 changes the direction of the plane of polarization of the

  beam. A lens 27 concentrates the light.

  An astigmatism analyzer 28 has both an astigmatism function which is used to obtain an autofocus control (error) signal and an analyzer function which detects a signal magnetooptical. The astigmatism-causing analyzer 28 is formed by the formation of a polarization film 28a on a first side of a flat blade, the two sides of which are parallel to each other. The polarization film 28a, often called a polarizer, allows the linearly polarized component of the

  light which is aligned with its main axis of transmission.

  In the same way as in the case of a separator of

  polarization beam, the analyzer causing astigmatism, that is to say the blade 28, transmits substantially 100% of a polarized light component p and reflects substantially 100% of a polarized light component s. Thus, the plate 28 acts as an analyzer serving to detect the magneto-optical signal.

  In addition, the blade 28 is arranged obliquely to the optical axis of the incident light. In other words, the angle G between the incident surface of the blade 28 and the optical axis of the light beam is not 90. Although the angle e is shown as being about 45 in Figure 1, it does not necessarily have this value. In addition, the beam transmitted to the blade 28 has already been transformed into a convergent beam by the lens 27. Since the blade arranged obliquely in the light beam constitutes an astigmatic element for the convergent beam, the blade 28 also acts as a element causing astigmatism.

  As shown in FIG. 1, detectors 29A and 29B are provided for detecting data recorded by conversion of the light beam which has entered the detectors to be transformed into electrical signals. The detector 29b also detects

\ 35

  ment The focusing state of the beam and delivers a focus error signal. Differential amplifiers 30 and 31 process the output signals from detectors 29A and 29B.

  The detector 29B is for example a detector with four segments, comprising four elements 29B1, 29B2, 29B3 and 29B4, as shown in FIG. 1B. The detector 29B is arranged in such a way that the Lines dividing the Light receiving surface 298 to form the four elements must form angles of 450 respectively with respect to the Line of focus f located along the sagitable direction of the incident beam going s to detector 296 and the Line of focus f located in a m meridian direction. The light flow enters the detector 29B in such a way that it is distributed over the light receiving surface 29Ba, as indicated by the oblique lines in FIG. 1B. The output signals from elements 29B 1 and 29B3 are added by an adder to form a focus signal A1. The output signals from elements 29B2 and 29B4 are added by the other adder to form the other focusing signal A2. On the other hand, a data signal A3 is the sum of the four

  output signals 29B1, 29B2, 29B3 and 29B4. The data signal A3 is for example formed by adding the focusing signals A1 and A2 by an adder.

  The two focusing signals A1 and A2 of the detector 29B are applied to a differential amplifier 31 which supplies the difference between the two signals A1 and A2 as the focusing error signal Sf. The data signal A3 is applied to the differential amplifier 30, together with the data signal A4 which is delivered by the detector 29A, in order to form the reproduction data signal S.

  In addition, by modifying the angle e formed between the optical axis of the incident light and the incident surface of the blade 28, it is possible to adjust the sensitivity of the detection of the focusing error.

  We will now describe the operation of the optical system thus designed.

- - r-

  t A laser beam produced by the laser beam source 21 is transformed into a parallel beam by the collimator 22. The parallel beam is focused on the recording surface of the photo-electro-magnetic disc 24 through 05 of a semi-mirror 23 and an objective lens 25. The beam reflected by the recording surface of the disc 24 is separated by the semi-mirror 23, and its plane of rotation rotates under the action of the half-wave plate 26 so as to form an azimuth angle of 45 with the incident surface of the blade 28. As a result, the light beam 10 reaches the incident surface of the blade 28 so that the plane of polarization of the beam forms 45 with the transmission axis principal of the polarization film 28a being on the slide 28. Thus, the amplitude of the light (polarized light component p) transmitted through the polarization film 15a is equal to the amplitude of the light (light component

  polarized s) reflected by the polarization film 28a.

  FIG. 1A shows the direction of the plane of polarization E of the light for various positions in the optical system of FIG. 1, in the case where the main transmission axis of the polarization film 28a is located in

  a direction perpendicular to the drawing surface for example.

  FIG. 1A shows the direction of the plane of polarization E of the light beam for the respective positions taken along the plane perpendicular to the optical axis of the transmitted light beam. FIG. 1A also shows the direction of the lines dividing the light receiving surface 29Ba of the detector 29 into its four elements. The data was recorded on disk 24 with alternating changes in the directions of magnetization. Due to the 30 Kerr effect, the plane of polarization of the light beam reflected on the disc 24 turns slightly, depending on the state of magnetization of the part of the disc 24 where the beam is reflected. Thus, the direction of the plane of polarization of the light beam reflected on the part o is recorded the data on the disc 24 turns slightly and is different from the direction of the plane of polarization of the t i beam reflected at the level of the part not recorded. As a result, the light beam reflected at the level of the part o is recorded a datum reaches the plate 28 in such a way that the magnitude of the azimuthal angle of its plane of polarization relative to the incident surface of the blade 28 is slightly shifted by with respect to the 45 which constitute the azimuthal angle of the plane of polarization of the beam reflected by the unregistered part. Thus, the direction of the beam polarization plane is slightly different from the direction which makes an angle of 45 relative to the main transmission axis of the polarization film 28a located on the plate 28. Thus, the amplitudes , that is to say the intensity of the light transmitted through the plate 28 (polarized light component p) and that of the light reflected on the plate 28 (light component 15 polarized s) are modified relative to those obtained for the

  light beam reflected at the unregistered part.

  As explained above, the beam which has passed through the blade 28 and the beam reflected by the blade 28 see their intensity vary as a function of the data recorded on the

  disc 24 or directions of magnetization on disc 24.

  Thus, recorded data signals A4 and A3 are respectively delivered by the detectors 29A and 29B, as a function of the magnetization directions present on the disc 24. The variations in intensity of the beams respectively transmitted and reflected by the analyzer 28 are mutually in phase opposition. Thus, when the plane of polarization of the beam has rotated and the amplitude of the beam transmitted through the analyzer 28 has increased, the amplitude of the beam reflected on the analyzer 28 has decreased. And, if the plane of polarization of the beam has rotated so as to decrease the amplitude of the beam transmitted through

  the analyzer 28, the amplitude of the beam reflected on the analyzer 28 has increased. Thus, the difference between the recorded data signals A4 and A3 supplied by the detectors 29A and 29B is obtained and is supplied as the reproduction data signal S 35 by the differential amplifier 30.

  As described above, since the recorded data signal S is obtained from the difference between the two data signals, it is possible to cancel and ignore the noise of the variation of the optical intensity of the laser beam 05 produced by the laser beam source. Such an effect is based on the fact that, since the plane of polarization is located at approximately 45 relative to the main transmission axis of the polarization film lying on the blade, the noise components for the variation of optical intensity are equal in phase and in amplitude, so that they cancel each other out in the differential amplifier 30. The beam passing through the blade 28 reaches a degree of astigmatism suitable for detecting a focusing error. The detector 29B receives the astigmatic light beam and delivers the focusing signals A1 and A2 at the same time as the recorded data signal A3. The focus signals A1 and A2 from the detector 29B are applied to the differential amplifier 31, which produces the difference between the focus signals A1

  and A2 as the focus error signal Sf.

  As described above, the detector 29B delivers to the

  both the data signal and the focus error signal.

  Thus, in the case where it is not necessary to obtain the signal of the recorded data from the difference between the two data signals, the detector 29A is not a necessary element of the device.

  In the first embodiment of the optical system of the photo-electromagnetic recording and reproducing device, as described above, the half-wave plate 26 is disposed in front of the lens 27. However, the half-wave blade 26 if the blade 28 is arranged in such a way

  that the light beam reaches the incident surface of the blade 28 while the beam polarization plane makes an azimuth angle of 45 relative to the incident surface of the blade 28, in the same way as for FIG. 1A, it is that is, the plane of polarization of the beam makes an angle of 45 with the axis of trans-

  t, main mission of the polarization film 28a located on the blade 28. To arrange the blade 28 in the manner described above, for example as indicated in FIG. 2, the polarization direction indicated 05 by arrow E for the laser beam produced by the source of

  laser beam 21 and the direction of the recording track existing on the recording medium, indicated by the arrow T, these directions being located in the plane of the drawing in FIG. 2.

  In addition, the blade 28 is arranged so that the direction of polarization E and the direction of the track T make an angle of with the incident surface and the meridian plane of the blade 28. The meridian plane is perpendicular to the incident surface and contains the main transmission axis. For example, the above-mentioned assembly is carried out for the blade 28 by rotating the blade 28 as shown in FIG. 1A where the main transmission axis of the polarization film is perpendicular to the surface of the drawing in the figure. 1A, at an angle of 45 on the optical axis of the light beam which reaches the blade 28. The assembly of the blade 28 is presented in FIG. 2A, which, by way of example, shows the direction T of the track, the direction E of the plane of polarization of the beam, the direction of the main axis of transmission of the polarization film and the directions of the lines dividing the light receiving surface of the detector

29B in four parts.

  Using the optical system of the second embodiment. \ sation described above of the invention, which does not use the half-wave plate 26, it is possible to obtain a satisfactory reproduction data signal S. This is based on the fact that, since the plane of polarization makes an angle of about 45 with the main transmission axis of the polarization film on the blade, the components of the noise of variation of the optical intensity are almost equal in phase and amplitude, so that they

  cancel out in the differential amplifier 30.

  As shown in FIG. 2A, one of the lines 35 separating the four segments from each other is parallel to the direction of the track. Thus, it is possible to obtain a focus error signal Sf which is less affected by the transverse noise of the track. This is based on the fact that, since the axis of symmetry of a diffraction light configuration formed by the track 05 coincides with one of the lines which divides the light receiving surface of the detector 29B, the transverse noise of the track s' in

finds lessened.

  A similar result can be obtained when the direction of polarization E is perpendicular to the direction T of the track.

  In Figure 2, the differential amplifiers 30 and 31 are not shown.

  In the examples described above, the astigmatism causing analyzer 28 is in the form of a flat blade having two parallel sides, as described above.

  However, the invention is not limited to this and is not limited by this. Other examples of the analyzer 28 are such as

shown in Figures 3 and 4.

  The analyzer 28 of FIG. 3 is in the form of a wedge which has a polarization film 33 on one of its surfaces. The analyzer 28 in FIG. 4 is a polarization beam splitter in the form of a cube which is formed by the union of prismatic elements having different refractive indices (n1 n2) and having a polarization film 34 between them. The two analyzers can provide the

  same effects as the analyzer 28 of FIGS. 1 and 2.

  As described above, in the photo-electro-magnetic recording and reproducing device for reading signals magneto-optically, an astigmatism causing analyzer 28 has an astigmatism function for detecting the autofocus control signal and analyzer function for detecting the magneto-optical signal

according to the invention.

  In other words, the photo-electro-magnetic recording and reproducing device is thus designed that a single optical element has the function of astigmatism and the function of analyzer. Thus, the device can be made small and light in weight, and the number of components as well

  that the manufacturing cost is reduced.

  In general, with photo-electromagnetic reproduction with small optical variations as data signals, the losses in optical intensity that can be attributed to the optical elements strongly affect the signal-to-noise ratio. However, the reduction in the number of constituent elements 10 according to the invention contributes to reducing the losses of intensity.

  and improve the signal-to-noise ratio.

  Of course, those skilled in the art will be able to imagine, from the device whose description has just been given for illustrative purposes and in no way limitative, various variants and modifications that do not go beyond the ambit of invention.

Claims (5)

  1. Reproduction device for reading signals, characterized in that it comprises: means (23) for obtaining a reflected light signal from a photoelectromagnetic recording medium (24); an astigmatism-causing analyzer element (28) for receiving said reflected light beam and dividing it into a first light signal and a second light signal, said first and second light signals having different polarizations, said second light signal being more astigmatic than said first light signal; a first light detector (28A) for a first data signal (A4) which receives said first light signal; and   a second light detector (28B) for a focus signal which receives said second light signal.   2. Reproduction device according to claim 20 1, characterized in that said second light detector (28B) also produces a second data signal (A3) and further comprises means making it possible to combine said first and second data signals.   3. Reproduction device according to claim 1, characterized in that said means for obtaining comprises a source (21) of laser light linearly polarized in a first direction substantially parallel or perpendicular to the direction of a track of said support recording, and in that the incident surface and the meridian surface of said analyzer are substantially at 45 with respect to said first direction and said second direction.   4. Reproduction device according to claim 1, characterized in that said analyzer has an incident face which receives said light signal reflected at an inclination relative to the direction of propagation of said light signal   reflected and which is coated with a polarization film.   "-0X ;; f 5. Reproduction device according to claim 1, characterized in that said analyzer comprises two prisms having - 05 different refractive indices which are brought together at a surface inclined with respect to the direction of propagation of said reflected light signal.