DE4140545A1 - Optical pick=up with hologram unit for use with optical CD's - has output of laser diode split into three components passed through hologram plate and focussed onto disc. - Google Patents

Abstract

The optical pick-up for obtaining signals from the surface of a compact disc (7) transmits the output of a laser diode (1) by a beam splitter as 3 beams. The beams are passed through a hologram unit (200) and are focussed on the disc by an objective lens (6). The signals are reflected back to be received by a photo detector (10) and provide data and control signals. The hologram structure provides signal diffraction to generate phase differences in the signals. The differences in the control signals are used to control focussing and tracking. ADVANTAGE - Improved diffraction efficiency.

Description

The present invention relates to an optical Signal pickup device for picking up recorded Signals using a laser diode, and in particular with an optical signal pickup device a hologram device in which the laser diode beams throws on a plate after the rays pass through the First order hologram device diffracted and the rays reflected from the plate from the hologram device again in rays first Order were diffracted to increase the diffraction efficiency improve.

An optical signal pickup device which is widely used in compact disc players is shown in FIG. 1, in which the beams emitted by a laser diode 1 are diffracted by a diffraction grating 2 and focused by a converging lens 3 .

The beams focused by the converging lens 3 are passed through a beam splitter 4 and thrown onto a quarter-wave plate 5 for the 45 ° rotation of their polarization angle. Then the beams pass through an objective lens 6 and are focused on a plate 7 in order to detect the signals recorded on the plate 7 . The rays are reflected by the plate 7 and again focused on the objective lens 6 , from which they are rotated again by the quarter-wave plate 5 by 45 ° of their polarization angle.

The rays rotated through 90 ° of their polarization angle are reflected by the beam splitter 4 and projected through a focusing lens 8 and a cylindrical lens 9 onto a photodetector 10 , where the focal points of two perpendicular regions of the rays thrown onto the photodetector 10 differ by using the cylindrical lens can be adjusted to correct astigmatism.

The photodetector 10 detects focusing error signals in order to decide whether the beams have precisely focused the pits on the plate 7 ; and detects tracking error signals to decide whether the signal tracking has been performed accurately.

The conventional optical signal pickup device with classic lens, as described above, is because of the Lens as heavy as it is expensive, making it for the Mass production is unsuitable.  

To overcome these drawbacks, the present optical signal pickup device with hologram device created. The optical signal pickup device with Hologram device based on that in the pits and Slits occurring diffraction phenomenon met complicated functions in small dimensions, making it the optical signal pickup device that the classic lens used to use the reflection phenomenon in the Mass production at low cost is superior. It however, there is still a disadvantage that the Diffraction efficiency is too low.

It is therefore an object of the present invention to provide a optical signal pickup device with hologram device too create the through the hologram device First order diffracted rays are thrown onto the plate and from the signals recorded on the disc are reflected, whereupon a photodetector by the Hologram device again diffracted rays first Order detected, so that the optical diffraction efficiency in Compared to the conventional device is improved with the Peculiarity that diffracted by the hologram device Zero order rays thrown onto the plate and from this will be reflected, and that then again from the First order hologram diffracted beams from Photo detector can be detected.

Another object of the present invention is to Creation of an optical pickup device with Hologram device in which the hologram device first order diffracted beams have two focal points, so the astigmatism of the laser diode is just as accurate can be corrected as the difference between the both focal points, and without attempting to  Reducing the astigmatism of the laser diode's housing to tilt the laser diode.

In order to achieve the stated goals, the optical signal pickup device with a hologram device without the conventional classic converging lens and cylindrical lens has a laser diode 1 for emitting laser beams, a beam splitter 100 for splitting the beams emitted by the laser diode into a main beam of zero order and secondary beams ± first Order to project them onto the plate 7 for scanning. The device further has a hologram device 200 , which serves between the beam splitter 100 and an objective lens 6 for diffracting the beams emitted by the beam splitter 100 and the objective lens 6 in first-order beams, and it has a photodetector 10 for detecting those recorded on the plate 1 Signals as well as the focus and tracking error signals by measuring the rays reflected from the disk to control the precise focusing of the rays onto the pits of the disk.

The stated goals as well as further advantages of the present Invention go from the detailed below Description of the preferred embodiment of the invention with reference to the accompanying drawings forth.

Fig. 1 illustrates a perspective view of an optical signal pickup device with hologram apparatus according to the present invention;

FIG. 2 shows a schematic view of the diffraction phenomenon when using the hologram device according to FIG. 1;

Fig. 3 is a sectional view of the hologram device shown in Fig. 1;

FIG. 4 shows a curve diagram to illustrate the efficiency of the hologram device of FIG. 1, and

Fig. 5 shows a sectional view through a conventional optical signal pickup device.

Fig. 1 illustrates a perspective view of an optical signal pickup device according to the present invention.

According to FIG. 1, a beam splitter 100 , which divides the emitted beams into three groups of beams, is arranged above a laser diode 1 for emitting the beams. A hologram device 200 , which diffuses the beams emitted by the beam splitter 100 in first-order beams and throws them onto an objective lens 6 , is arranged above the beam splitter 100 . The objective lens 6 is in turn arranged above the hologram device 200 in order to focus the beam exactly on the pits of the plate 7 .

A photodetector 10 is disposed under the reflective beam splitter 100 to detect the signals recorded on the plate 7 as well as the focus and tracking error signals by measuring the beams reflected from the plate 7 .

The hologram device 200 diffracts the beams according to the phase differences generated on the surface of the pit structure by etching a glass plate.

The hologram device 200 is manufactured by the photolithographic step of pattern overwriting and by an etching step to produce the pit structure. This process is in common use as part of the semiconductor manufacturing process.

The reflective beam splitter 100 can be gold plated to increase the reflection rate to 97%, the main manufacturing process being essentially the same as the hologram device.

The optical signal pickup device comprises the beam splitter 100 and the hologram device 200 , the beams emitted by the laser diode 1 being thrown onto the beam splitter 100 and divided there into three groups of beams. These three beam groups are projected onto the hologram device 200 and diffracted in first order beams. The beams diffracted by the hologram device 200 then pass through the objective lens 6 to the pits of the plate 7 ; and then the signals taken from the plate 7 are directed to the hologram device 200 through the objective lens 6 .

The rays reflected by the plate 7 are diffracted by the hologram device 200 after passing through the objective lens 6 in first order rays.

The diffraction efficiency of the hologram device is discussed in connection with FIG. 2. The beams focused by the objective lens 6 are thus diffracted by the hologram device 200 , which is inclined by the angle Φ relative to the optical axis.

In this context, the diffraction angle α is determined by the optical and structural specification of the optical head, while the hologram device 200 has the combined functions of a round lens and a cylindrical lens in order to produce a certain size of astigmatism on the surface of the photodetector 10 .

For a zero order beam that has passed through the hologram device 200 , the phase function Φr (x, y) can be determined from the phase difference that exists at one point between the beam that has passed through the optical axis and the beam that has passed through the hologram device 200 , which deviates from the optical axis by the value X.

That is, the phase difference (δ d) is as follows can be formulated:

Assuming that the z-axis is perpendicular to the XY plane of the hologram device 200 and that (x ² + y ² ) / dd ^{2 is} 2x · sin0 / dd «1, the phase function Φr (x, y) des Zero order beam as follows:

Φr (x, y) = exp [jk · {(x² + y²) / 2dd + x · sin R}] (2)

Here j ² = -1, while k is the wave vector, λ is the wavelength and k = 2 π / 2.

In addition, the phase function for the first-order beam, whose diffraction angle has the value, can be determined from the distance gg to the first focal length by the function of the round lens of the hologram device 200 , the distance bb representing the second focal length, which is given by the cylindrical lens of the hologram device 200 is produced. The frequency fc is defined as:

1 / fc = 1 / gg -1 / bb (3)

For example, the phase function Φ (x, y) of the first order beam in the plane immediately behind the hologram device 200 can be formulated as follows:

Φ (x, y) = exp [jk {(x² + y²) / 2gg + x² / 2fc}] (4)

where fc <0.

The phase function determined by the formula (4) becomes by a nonlinear delimiter (not shown) in Binary numbers consisting of 1 and 0 implemented. So if the entered value is greater than a reference value c the value of the output from the non-linear limiter  Function h (x, y) size 1. However, if the entered one Value is smaller than the reference value c, has the Output function h (x, y) has the value 0 Function value h (x, y) can be written as follows:

h (x, y) = [{sin (∎πq)} / ∎π · exp {j∎ · Φ (x, y)}] (5)

If the pattern is now produced with the output signal "0" and a transmission ratio of 100%, while the output signal "1" has the transmission ratio of 0%, the pit pattern produced on the surface is formed in accordance with the output signals "0" and "1". That is, the thickness of the hologram device 200 is determined by the reference value c. If the requirement q = 0.5 is further met, the power ratio reaches 50%, while if q = 0 is met, the width of the pattern approaches the value 0 so that a line is formed.

The condition after which the output function h (x, y) is not has the value 0 reads as follows:

cosΦ (x, y) <cos (xq) (6)

It can also be expressed in the following form:

-q / 2 Φ (x, y) / 2π + n q / 2 (7)

where n is an arbitrarily selected integer. Furthermore, the value of q determines the width of the pattern and the efficiency of the beam + 1-order. So if this  Value for (7) is inserted into formulas (2), (4), (5), will get the following relationship:

-q / 2 (x² + y²) / 2λ · (1 / dd-1 / gg) + sin R · x / λ -x² / (2λfc) + n q / 2 (8)

That is, the area satisfying the formula (8) on the XY planes is the pattern on the hologram device 200 .

The hologram device 200 will now be described in detail with reference to FIG. 3.

In Fig. 3, d denotes the depth of the pits, Λ denotes the period of the pits, and ξ denotes the power ratio with the value 0 <ξ <1.

In the event that the beam from the laser diode 1 sweeps over the pits, the optical path difference Φ, if the refractive index of the medium of the hologram device 200 is denoted by n and the wavelength of the laser beam is denoted by λ, is described as follows:

Φ (x) = 2πd / λ · (n-1). . . (0 <x <ξΛ)

0. . . (ξΛ <x <Λ) (9)

Furthermore, in the case of the phase hologram device 200, the transmittance T (x) can be defined as follows:

T (x) = exp (-jΦ) (10)

In this formula is a period function of the period Λ while T (x) can be expressed in the form of a Fourier series consisting of the radio frequency period Λ. Taking into account the fact that the luminous intensity is inversely proportional to T, the respective diffraction efficiencies, when the transmittance efficiency of the zero-order beam is denoted by n ₀ and the diffraction efficiency of the first-order beam by n ₁ , can be based on the Fourier transform as can be calculated as follows:

n₀ (x) = [1 / Λ · ∫ T (x) · dx] ²

= (1 - ξ + ξ cos Φ) ² + ξ² · sin² Φ (11)

n₁ (x) = [1 / Λ · ∫ T (x) · exp (jKx) · dx] ²

= 1 / (π²) · [1-cos (2πξ)] · (1-cos Φ) (12)

where K = 2π / Λ.

In the case of an optical signal pickup device which uses the principle of the existing hologram based on the formulas (11) and (12), the final optical efficiency detected by the photodetector 10 reaches 50% (i.e.:: = 0.5), the wavelength λ of the laser beam 0.78 μm and the refractive index n _{0 of} the medium of the hologram device 200 have the value 1.5, the value n ₀ × n ₁ , the maximum efficiency being about 10%.

The efficiencies mentioned have been determined using the above formulas (11) and (12) and are shown in FIG. 4.

In the optical signal pickup device according to the present invention, however, the first-order beams diffracted by the hologram device 200 on their way from the beam splitter 100 to the objective lens 6 , and after reflection on the plate 7 , from the objective lens 6 to the beam splitter 100 . As a result, the efficiency of the rays detected by the photodetector 10 corresponds to the square of the first order diffraction efficiency n ₁ , as shown in FIG .

If the power ratio is 50%, that is, if ξ = 0.5 and Φ = π, the final efficiency n ₁ xn _{1 reaches} the value 0.16 because only first-order beams are used, so that the efficiency is reduced by 1.6 times better than the 10% of conventional technology.

As described above, the beams emitted from the laser diode 1 are reflected by the reflecting beam splitter 100 and then diffracted by the hologram device 200 into first-order beams. The diffracted rays are focused by the objective lens 6 onto the pits of the plate 7 . The rays reflected from the plate 7 pass through the objective lens 6 and are then again diffracted into first-order rays by the hologram device 200 . The diffracted rays are thrown onto the reflecting beam splitter 100 and detected by the photodetector 10 . That is, the photodetector 10 detects both the signals recorded on the disk and the focus and tracking error signals.

As described above, the optical Signal pickup device according to the present invention Beam splitter for splitting that emitted by the laser diode Beam; a hologram device for diffracting the Beam splitter and beams emanating from the objective lens in first order rays; an objective lens, and one Plate on, said components in the cited order are arranged. The plate receives the incident rays in the form of through the Hologram device diffracted first order rays that after reflection on the plate again from the First order hologram device diffracted will. The diffraction efficiency is accordingly around 1.6 times that of the conventional technology improved, because of the use of zeros and rays first order improved. Further the optical reaches Sufficient customer device the desired Astigmatism on the surface of the photo detector. Because of the Using the hologram device instead of the conventional converging lens and cylindrical lens can be one high yield in mass production, with low Unit price, can be achieved.

Claims (2)

1. Optical signal pickup device with a hologram device, but without the conventional classic converging lens and cylindrical lens, characterized in that it has the following components:
a laser diode for emitting laser beams;
a beam splitter for splitting the beams emitted from the laser diode into a zero-order main beam and ± 1-order beams for scanning a plate;
a hologram device which is arranged between the beam splitter and an objective lens and serves to diffract the rays coming from the beam splitter in first-order rays before they are reflected on the objective lens, and to diffract the rays coming from the objective lens in first-order rays before it be directed at the beam splitter; and
a photodetector for detecting the signals recorded on the disc as well as the focus and tracking error signals by measuring the rays reflected from the disc. 2. Optical signal pickup device according to claim 1, characterized characterized in that the reflective beam splitter is plated with gold to increase the reflection rate increase.