Optical pickup for high density recording/reproduction and method to detect a reproduction signal

An optical pickup for high density recording and reproduction, and a reproduction signal detection method, which prevent degradation of a reproduction signal due to shot noise which occurs when a low optical power light is used to form a small light spot for high density reproduction. The optical pickup includes: a light source unit to emit a plurality of light beams, such that a plurality of light spots are formed on a same track of a recording medium; a photodetector to receive and photoelectrically convert the light beams incident through an objective lens and an optical path changer after having been reflected from the recording medium; and a reproduction signal detecting circuit to delay at least one of the detection signals output from the photodetector to remove time gaps between the detection signals, and to sum the delayed detection signal and the remaining detection signals, to detect a reproduction signal. As a result, the reproduction signal with a reduced noise component is detected.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Korean Application No. 2000-33577, filed Jun. 19, 2000, in the Korean Industrial Property Office, the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical pickup for high density recording/reproduction, and a method to detect a reproduction signal, and more particularly, to an optical pickup for high density recording/reproduction, and a method to detect a reproduction signal, which are capable of preventing degradation of a reproduction signal caused by shot noise, which occurs when a low optical power light is used to form a small light spot for high density recording/reproduction.

2. Description of the Related Art

In general, for high density optical recording/reproduction, a light spot focused on a recording medium must be small. The diameter of the light spot, which has a Gaussian distribution, is larger than a track pitch, so a portion of the light spot focused on a main track extends into neighboring tracks.

Thus, if an intensity of the light spot is strong, an information signal can be recorded on and/or erased from the neighboring tracks, and a reproduction signal of the main track is adversely affected by the neighboring tracks. To avoid these problems, as a recording density increases, a light source with a lower optical power is needed.

FIG. 1illustrates an example of a conventional optical pickup100. As shown inFIG. 1, the conventional optical pickup100includes a light source1, an objective lens5for focusing light emitted from the light source1to form a light spot on an information recording surface of an optical disc10, a beam splitter3disposed on an optical path between the light source1and the objective lens5, to change the traveling path of incident light, and a photodetector7to receive the light beam passed through the objective lens5after having been reflected by the optical disc10, and directed toward the photodetector7by the beam splitter3, to detect an information signal and/or an error signal.

The conventional optical pickup100forms a single light spot on a main track of the optical disc10to record the information signal on or to reproduce the information signal from the information recording surface of the optical disc10. However, the conventional optical pickup100having the above configuration for high density recording and reproduction needs a low optical power light in reproducing information. The use of the low optical power light during reproduction causes shot noise, thereby sharply degrading the quality of a reproduction signal.

The reason that the quality of the reproduction signal degrades due to shot noise caused by the low optical power light is rooted in the quantal properties of light. In other words, it is probable that a photon is absorbed and photoelectrically converted by the photodetector7, so that a signal output from the photodetector7always contains shot noise ripples. If the optical power of the light source1is high, a large number of photons are received by the photodetector7, so that a high power signal is output from the photodetector7. In this case, a shot noise level of the photodetector7is small relative to an amplitude of the output signal, and thus the problem of shot noise is not significant. In contrast, if the optical power of the light source1is low, the number of photons absorbed by the photodetector7decreases. As a result, a low power signal is detected by the photodetector7and the problem of shot noise is significant. For example, to increase recording density by four times, the size of the light spot must be reduced by one fourth, and the shot noise level increases by about three times. In other words, when a low optical power light is used for reproduction, the quality of the reproduction signal degrades due to a low signal-to-noise ratio.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide an optical pickup for high density recording and reproduction, and a method to detect a reproduction signal, which prevent deterioration of the reproduction signal due to shot noise, which is caused when a low optical power light is utilized for high density reproduction.

Additional objects and advantages of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may learned by practice of the invention.

The foregoing and other objects of the present invention are achieved by providing an optical pickup for high density recording and reproduction, comprising: a light source unit to emit a plurality of light beams, such that a plurality of light spots are formed on a same track of a recording medium; an objective lens to focus the light beams incident from the light source unit to form light spots on an information recording surface of the recording medium; an optical path changer disposed on an optical path between the light source unit and the objective lens, to alter a traveling path of the incident light beams; a photodetector to receive and photoelectrically convert the incident light beams through the objective lens and the optical path changer after having been reflected from the recording medium; and a reproduction signal detecting circuit to delay a detection signal output from the photodetector to remove time gaps between detection signals, and to sum the delayed detection signal and the other detection signals output from the photodetector, to detect a reproduction signal.

The light source unit includes a plurality of semiconductor laser devices, and a diffraction element to split the light beam emitted from the light source into the plurality of beams by diffraction, such that the plurality of light spots are formed on the same track of the recording medium.

The foregoing and other objects of the present invention may also be achieved by providing a reproduction signal detection method comprising: forming a plurality of light spots separated by a predetermined distance on a same track of a recording medium; separately receiving and photoelectrically converting light beams reflected from the recording medium, to output electrical signals; delaying at least one of the electrical signals to remove time gaps between the electrical signals; and summing the electrical signals from which the time gaps have been removed, to detect a reproduction signal from a same place of the recording medium.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring toFIG. 2, an optical pickup for high density recording and reproduction200according to an embodiment of the present invention includes a light source unit30to emit first and second light beams31aand31b; an objective lens35to focus the first and second light beams31aand31bincident from the light source unit30to form first and second light spots32aand32bon an information recording surface of a recording medium50, for example, an optical disc; an optical path changer33placed on an optical path between the light source unit30and the objective lens35, to change traveling paths of the first and second light beams31aand31b; first and second photodetectors37and39to receive the first and second light beams31aand31b, respectively, passed through the objective lens35after being reflected by the recording medium50, and directed by the optical path changer33toward the first and second photodetectors37and39, and to perform photoelectric conversion of the received first and second light beams31aand31b; and a reproduction signal detecting circuit40to detect a reproduction signal from electrical signals output from the first and second photodetectors37and39.

Although, as shown inFIG. 2, a beam splitter to reflect and to transmit the incident light beam has been adopted as the optical path changer33, any element capable of changing the traveling path of light, for example, a hologram optical element (HOE), or a combination of a polarization beam splitter or a polarization HOE and a wave plate can be adopted as the optical path changer33.

The light source unit30emits the first and second light beams31aand31b, such that first and second light spots32aand32bare separated by a predetermined distance on a same track of the recording medium50. In the present embodiment, the light source unit30may be comprised of two semiconductor laser devices. For example, the light source unit30may have a hybrid structure or a monolithic structure. The hybrid structure is manufactured by mounting two separate semiconductor laser devices on the same substrate, such that the semiconductor laser devices are electrically insulated from each other, and then by wire bonding each of the semiconductor laser devices. The monolithic structure is manufactured by combining two semiconductor laser devices through the same manufacturing process, and then by wire bonding each of the semiconductor laser devices. Alternately, the light source unit30can be constructed of two separate semiconductor laser devices. The semiconductor laser device may be either an edge emitting laser device or a vertical cavity surface emitting laser (VCSEL) device.

As shown inFIG. 3, the reproduction signal detecting circuit40includes a delay41to delay one of the electrical signals output from the first and second photodetectors37and39, which precedes the other electrical signal, and an adder45to sum the signal delayed by the delay41and the other electrical signal, to thereby output a noise-free reproduction signal.

For example, given that the recording medium50rotates counterclockwise and the first light spot32ais formed preceding the second light spot32b, as shown inFIG. 2, the first photodetector37outputs an electrical signal S(t)+n′ by receiving the first beam31areflected from the recording medium50. The output electrical signal S(t)+n′ precedes the electrical signal S(t−τ)+n output from the second photodetector39, which receives the second beam31b. The time gap τ between the signals S(t)+n′ and S(t−τ)+n is the quotient obtained by dividing the distance between the first and second light spots32aand32bby the linear velocity ν of the recording medium50. Here, the terms s(t) and S(t−τ) indicate an information signal component reproduced from the recording medium50, and n′ and n indicate noise components including shot noise, which are included in the electrical signals output from the first and second photodetectors37and39.

In this way, the delay41is connected to the output end of the first photodetector37. The delay41delays the electrical signal S(t)+n′ from the first photodetector37by the time τ, and outputs the delayed signal S(t−τ)+n′. As a result, the adder45receives signals from a same place of a track, which have no time gap, and outputs a sum signal 2s(t−τ)+n√2 as a reproduction signal. In other words, as for the sum of the signals from the first and second photodetectors37and39, the reproduction signal component becomes two times each reproduction component, i.e. 2s(t−τ), and the noise component become n√2. Accordingly, the reproduction signal output from the adder45has an improved signal-to-noise ratio. Because there is no correlation between the noise components n and n′, the root mean square (RMS) of each noise component is summed, so that the sum of the noise components n and n′ becomes equal to n√2.

Although, in the present embodiment, the reproduction signal detecting circuit40is designed to be suitable for an inventive optical pickup which has been constructed to form the first and second light spots32aand32bon the recording medium50with the same intensity, the configuration of the reproduction signal detecting circuit40can be varied if the intensities of the first and second light spots32aand32bare not the same. For example, an amplifier (not shown) to correct the intensity difference may be installed at an output end of the first and second photodetectors37and39.

In the detection of the reproduction signal having such a high signal-to-noise ratio, the optical pickup for high density recording and reproduction200operates as follows. First, the light source unit30emits the first and second light beams31aand31bwhose optical power is low enough to perform high density reproduction. The first and second light beams31aand31bpass through the optical path changer33, and are focused by the objective lens35, such that the first and second light spots32aand32bare formed on the same track of the recording medium50. After having been reflected from the recording medium50, the first and second beams31aand31bpass through the objective lens35and the optical path changer33, and then are received by the first and second photodetectors37and39, respectively. Thus, the first and second photodetectors37and39can detect reproduction information signals from the same place of the recording medium50, which are separated by a predetermined time gap.

One of the signals output from the first and second photodetectors37and39, which precedes the other signal, is delayed in the delay41by the predetermined time τ to remove the time gap between the two signals output from the first and second photodetectors37and39. Next, the delayed signal and the other signal, which have been read from the same place of the track without the time gap, are input to the adder45, and the sum of the two signals is output from the adder45as a reproduction signal with a higher signal-to-noise ratio.

Although the optical pickup for high density recording and reproduction200described in the present embodiment adopts the light source unit30having low optical power for high density reproduction, information recorded on the same place of the recording medium50is detected with the time gap using at least two light spots32aand32b, which are focused a predetermined distance apart from each other. The time gap between the detected signals is removed and then summed, so that the reproduction signal having an enhanced signal-to-noise ratio can be detected.

In particular, given that a single light spot is formed on the recording medium50by the conventional optical pickup100shown inFIG. 1, and each of the first and second light spots32aand32bfocused on the recording medium50by the inventive optical pickup200have the same intensity, and the photodetector7of the conventional optical pickup and each of the first and second photodetectors37and39receives the same amount of light, the conventional optical pickup100detects a reproduction signal S(t−τ)+n, whereas the inventive optical pickup200for high density recording and reproduction detects a reproduction signal 2S(t−τ)+n√2. In other words, the reproduction signal detected by the conventional optical pickup100has a signal-to-noise ratio of S(t−τ)/n, whereas the reproduction signal detected by the inventive optical pickup200has a signal-to-noise ratio of √2S(t−τ)/n.

Compared with the conventional optical pickup100, the optical pickup for high density recording and reproduction200according to the present invention can reproduce an information signal with a √2 times higher signal-to-noise ratio at the same recording density, which is equivalent to increasing the optical power of a light source for reproduction.

Although the present embodiment of the optical pickup200has been described as it detects the reproduction signal by focusing just two light spots32aand32bthe predetermined distance apart from each other on the same track of the recording medium50, three or more light spots can be focused separated by a predetermined distance on the same track of the recording medium50, and the reproduction signal can be detected using the three or more light spots, thereby further increasing the signal-to-noise ratio of the reproduction signal. In this way, the light source unit30is constructed such that it is able to emit three or more light beams to form the three or more light spots the predetermined distance apart from each other on the same track of the recording medium50, and three or more photodetectors, corresponding to the number of the light beams emitted from the light source unit30, are adopted. Furthermore, the reproduction signal detecting circuit40further includes one or more delays to eliminate time gaps between the signals output from the photodetectors. Then, the information signals read from the same place of the recording medium50are summed. For example, if the optical pickup200for high density recording and reproduction forms three light spots separated by a predetermined distance on the same track of the recording medium50, the signal-to-noise ratio of the reproduction signal increases by √3 times, compared with the conventional optical pickup100, which focuses a single light spot.

FIG. 4is a diagram illustrating another embodiment of an optical pickup300for high density recording and reproduction according to the present invention. The optical pickup300ofFIG. 4differs from the optical pickup200ofFIG. 2in that a light source130includes a single light source131and a diffraction element135. InFIG. 4, the same elements as those ofFIG. 2are denoted with the same reference numerals, and descriptions of these elements are omitted.

The single light source131may be an edge emitting laser or a VCSEL. The diffraction element135may be a HOE. The diffraction element135splits a single light beam incident from the light source130into a plurality of light beams by diffraction, such that a plurality of light beams are focused on the same track of the recording medium50with a predetermined interval. For example, as shown inFIG. 4, the diffraction element135diffracts the incident beam into 0th-order and +1st-order light, to thereby split the incident light beam into the first and second beams31aand31b.

The diffraction element135splits the incident light beam such that the split light beams have the same amount of light. If the intensity of the split light beams from the diffraction element135is not the same, at least one amplifier (not shown) with an appropriate gain factor can be installed in the reproduction signal detecting circuit40.

The detection of the reproduction signal having a higher signal-to-noise ratio by the inventive optical pickup300shown inFIG. 4is substantially the same as described with reference toFIGS. 2 and 3, and thus the operation of the optical pickup300ofFIG. 4will not be described here.

As previously described, the optical pickup for high density recording and reproduction according to the present invention can detect a reproduction signal with a higher signal-to-noise ratio, compared with a conventional optical pickup, by focusing a plurality of light spots32aand32b, separated by a predetermined distance on the same track of the recording medium50, in which the light spots32aand32bare separately received by a plurality of photodetectors37and39after having been reflected from the recording medium50, and the detection signals of the photodetectors37and39are summed after removal of the time gaps between the detection signals, so that the reproduction signal with a relatively reduced noise component is detected using a low optical power light for reproduction. In the present invention, the formation of a plurality of light spots32aand32bwith low optical power is equivalent to increasing the optical power of an information reproduction light.