CN115547366A

CN115547366A - Data writing method for multi-layer recording medium and its read-write device

Info

Publication number: CN115547366A
Application number: CN202110653929.4A
Authority: CN
Inventors: 邹宇豪; 马建设; 徐君; 邢田; 刘明源
Original assignee: Huawei Technologies Co Ltd; Shenzhen International Graduate School of Tsinghua University
Current assignee: Huawei Technologies Co Ltd; Shenzhen International Graduate School of Tsinghua University
Priority date: 2021-06-11
Filing date: 2021-06-11
Publication date: 2022-12-30
Also published as: WO2022258062A1

Abstract

Embodiments of the present disclosure provide a data writing method of a multi-layered recording medium and a reading and writing apparatus thereof, wherein the multi-layered recording medium includes at least a first recording layer and a second recording layer. The method comprises the following steps: data is written in the second recording layer using data writing light, in which light incident on the first recording layer on which data is written is used as servo light of the data writing light. The data writing method of the present disclosure provides a concept of using light incident on a previously written recording layer as servo light, which helps simplify a servo scheme, and can achieve fast reading and writing of a recording medium having more storage layers.

Description

Data writing method for multi-layer recording medium and its read-write device

Technical Field

Embodiments of the present disclosure relate to the field of optical storage, and more particularly, to a data writing method for a multi-layered recording medium and a read/write apparatus thereof.

Background

The advent of the information age has led to a surge in data storage. By the time of 2025, global data would reach 175ZB, as predicted by international data company IDC. However, a large amount of data cannot be stored for a long time, for example, a monitoring video in a public place is generally stored for only 3 months and then deleted and overwritten. The main reason for this is the high price of the storage device.

Existing storage technologies, such as solid state disks, hard disks, and the like, cannot store data for a long time (> 5 years); the storage time of the magnetic tape can reach 20 years, but the requirement on the storage environment is high. Temperature, humidity, electromagnetic interference, etc. all affect the storage life of the tape. Optical storage media such as blu-ray media can store data for a long time at low cost, but their storage density cannot meet the storage demand for large amounts of data.

The development trend of optical storage to increase storage capacity is multilayer storage. Although the multilayer memory can effectively improve the memory density, the multilayer structure increases the difficulty of read-write operation and servo control.

Disclosure of Invention

An object of the present disclosure is to provide an improved data writing method of a multi-layered recording medium and a reading and writing apparatus thereof, which can contribute to an increase in the number of recording layers and storage capacity of a single disc. In addition, the writing/servo scheme of the present disclosure can also ensure high-speed reading and writing on the basis of disk capacity improvement.

According to a first aspect of the present disclosure, a data writing method of a multi-layered recording medium is provided. The multi-layered recording medium includes at least a first recording layer and a second recording layer, the method including: data is written in the second recording layer using data writing light, with light incident on the first recording layer where data is written as servo light of the data writing light.

Accordingly, the data writing method of the present disclosure provides a concept of using light incident to a previous recording layer on which data is written as servo light, which helps to simplify a servo scheme and make servo of a disc more free. Since the light incident on the recording layer can be used as servo light for writing data into another recording layer, high-speed reading and writing can be realized even if the number of recording layers of the recording medium is increased, which contributes to an increase in the number of recording layers and storage capacity of a single disc.

In some embodiments, the multi-layer recording medium further comprises a servo layer, the method further comprising: data is written in the first recording layer using a first write light, wherein light incident on the servo layer is used as the servo light of the first write light.

In some embodiments, the first recording layer and the second recording layer are adjacent to each other, and the second recording layer is located above the first recording layer.

In some embodiments, the data write light and its corresponding servo light are the same wavelength.

In some embodiments, the first recording layer is a recording layer adjacent to the servo layer, and the data writing order of the plurality of recording layers is layer-by-layer writing from the first recording layer.

In some embodiments, the motion trajectories of the servo light and the corresponding data write light remain consistent during writing.

In some embodiments, the servo layer comprises a wobbled groove track, and the writing data on the first recording layer using a first writing light comprises: and controlling the servo light corresponding to the first writing light to move along the swinging groove track.

In some embodiments, the writing data on the second recording layer using the data writing light includes: and controlling servo light corresponding to the data writing light to move along the data track of the first recording layer.

In some embodiments, the wavelengths are all 405nm.

According to a second aspect of the present disclosure, a read-write apparatus is provided. The read-write device includes: a servo optical path member for irradiating servo incident light to a first recording layer of a multi-layer recording medium having data written thereon, and receiving servo reflected light reflected via the first recording layer, wherein the multi-layer recording medium includes at least the first recording layer and a second recording layer; a main light path unit for irradiating data write light to the second recording layer; and a control device for controlling the data writing light to write data to the second recording layer with the servo reflected light as servo light.

In some embodiments, the control device is further configured to control the motion trajectories of both the data writing light and the corresponding servo light to be consistent during the writing process.

In some embodiments, the main optical path component is further configured to irradiate the first write light to the first recording layer, the servo optical path component is further configured to irradiate another servo incident light to a servo layer of the multi-layer recording medium, and receive another servo reflected light reflected via the servo layer; the control device is also used for controlling the first writing light to write data into the first recording layer by taking the other servo reflected light as servo light.

In some embodiments, the main light path assembly includes a first objective lens adapted to focus the data write light to a second recording layer on the multi-layer recording medium; the servo optical path assembly includes a second objective lens adapted to focus the servo incident beam to a first recording layer on the multi-layer recording medium.

In some embodiments, the first objective lens and the second objective lens are disposed on the same torquer, and both are adapted to move synchronously with movement of the torquer.

In some embodiments, the read-write apparatus further comprises: a laser source for emitting an initial laser beam; a first beam splitter for splitting the initial laser beam into a main incident beam and a servo incident beam; a first polarizing beam splitter located between the first beam splitter and the first objective lens for transmitting the main incident beam having a first polarization as the data write light.

In some embodiments, the read-write apparatus further comprises: a first compensation lens group positioned between the first polarizing beam splitter and the first objective lens and operable to compensate for a refractive index mismatch caused by a depth variation of a focal point of the main incident light beam within the multi-layered recording medium.

According to a third aspect of the present disclosure, there is provided a read-write apparatus for a multilayer recording medium. The read-write device includes: a laser source for emitting an initial laser beam; a first beam splitter for splitting the initial laser beam into a main incident beam and a servo incident beam; a first objective lens for focusing the main incident beam to a first position on the multi-layer recording medium; and a second objective lens for focusing the servo incident beam to a second position on the multi-layer recording medium and collecting a servo reflected beam from the second position, the second position being different from the first position; wherein the first objective lens and the second objective lens are disposed on the same torquer and both are adapted to move synchronously with movement of the torquer.

In some embodiments, the read-write apparatus further comprises a first polarizing beam splitter positioned between the first beam splitter and the first objective lens for transmitting the main incident beam having a first polarization and reflecting the main reflected beam having a second polarization.

In some embodiments, the read-write apparatus further comprises a second polarizing beam splitter, located between the first beam splitter and the second objective lens, for reflecting the servo incident beam having a first polarization and transmitting the servo reflected beam having a second polarization.

In some embodiments, the read-write apparatus further comprises: a first detector for receiving the reflected main reflected light beam reflected from the first polarizing beam splitter and outputting a first detection result indicating a focal position and/or shape of the main incident light beam on the multilayer recording medium.

In some embodiments, the read-write apparatus further comprises: a second detector arranged to receive the servo reflected beam transmitted from the second PBS and output a second detection result indicating a focal position and/or shape of the servo incident beam on the multi-layered recording medium.

In some embodiments, the read-write apparatus further comprises: a first 1/4 wave plate located between the first polarizing beamsplitter and the first objective lens to generate the main reflected light beam having a second polarization toward the first polarizing beamsplitter.

In some embodiments, the read-write apparatus further comprises: a second 1/4 wave plate located between the second PBS and the second objective lens to generate the servo reflected light beam having a second polarization toward the second PBS.

In some embodiments, the read-write apparatus further comprises: a second beam splitter located between the first beam splitter and the first polarizing beam splitter for splitting a portion of the light from the primary incident beam from the first beam splitter; and a third detector for detecting the portion of the light from the second beam splitter and outputting a light intensity detection result indicating the laser intensity of the initial laser beam.

In some embodiments, the read-write apparatus further comprises: a second compensation lens group located between the second polarizing beam splitter and the second objective lens and operable to compensate for refractive index mismatch caused by depth variation of a focal point of the servo incident beam within the multilayer recording medium.

It should also be understood that the statements described in this summary are not intended to limit the key or critical features of the embodiments of the disclosure, nor are they intended to limit the scope of the disclosure. Other features of the embodiments of the present disclosure will become readily apparent from the following description.

Drawings

The above and other features, advantages and aspects of various embodiments of the present disclosure will become more apparent by referring to the following detailed description when taken in conjunction with the accompanying drawings. In the drawings, like or similar reference characters designate like or similar elements, and wherein:

fig. 1 shows a schematic structural diagram of an exemplary multi-layered recording medium.

Fig. 2 shows a schematic view of the structure of another exemplary multi-layered recording medium.

Fig. 3 illustrates an exemplary read-write optical path apparatus for the multi-layer recording medium shown in fig. 2.

Fig. 4 illustrates a read-write system framework diagram for a multi-layered recording medium according to an example embodiment of the present disclosure.

FIG. 5 illustrates a flowchart of a method embodying the writing and servo concepts of a multi-layer recording medium, according to an example embodiment of the present disclosure.

FIG. 6 illustrates a write/servo process schematic for a multi-layer recording medium according to an example embodiment of the present disclosure.

Fig. 7 schematically depicts an exemplary read-write flow of the exemplary embodiment of fig. 6 using a laser read-write module OPU.

FIG. 8 shows a schematic diagram of a first exemplary read-write optical path for a multi-layer recording medium, according to an example embodiment of the present disclosure.

FIG. 9 shows a schematic diagram of a second exemplary read-write optical path for a multi-layer recording medium, according to an example embodiment of the present disclosure.

FIG. 10 shows a schematic diagram of a third exemplary read-write optical path for a multi-layer recording medium, according to an example embodiment of the present disclosure.

Detailed Description

Embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. While certain embodiments of the present disclosure are shown in the drawings, it is to be understood that the present disclosure may be embodied in various forms and should not be construed as limited to the embodiments set forth herein, but rather are provided for a more thorough and complete understanding of the present disclosure. It should be understood that the drawings and embodiments of the disclosure are for illustration purposes only and are not intended to limit the scope of the disclosure.

The schematic structure of a conventional multilayer recording medium and its read and write optical paths will be described first with reference to fig. 1 to 4 to provide an understanding of the prior art. It is noted here that these descriptions are not meant to be an admission or default that the presented prior art is prior art in this field.

Fig. 1 shows a block diagram of an exemplary multi-layered recording medium. As shown in fig. 1, an exemplary multi-layer recording medium, such as an optical disc, may include two recording layers having a wobble (wobble) structure, each of which includes a plurality of medium layers. The multilayer dielectric layer may include, for example, a phase change material layer such as GeSbTe and a material layer such as ZnS-SiO ₂ The reflective layer of (2). When laser light is irradiated to the phase change material layer, the phase change material layer may be converted from a crystalline state to an amorphous state, which is accompanied by a change in refractive index, thereby recording data. Such as ZnS-SiO ₂ The reflective layer of (a) will also reflect signals carrying information that may include addresses, clocks, etc. modulated in the wobble structure.

For the structure shown in FIG. 1, tracking servo can be accomplished using an auxiliary drive, but in each recording layer such as ZnS-SiO ₂ The reflective layer(s) will reflect a portion of the laser energy. If the storage capacity is to be increased, the number of recording layers can be increased. However, the lower the energy reflected back to the detector by the lower recording layer, the lower the number of recording layers is, which results in an increase in the number of recording layers being limited.

In order to further increase the number of recording layers in order to increase the storage capacity, a recording storage medium of another structure as shown in fig. 2 has been proposed. As shown in fig. 2, only the lowermost layer of the disc has a wobble structure, and the remaining layers (e.g., up to 16 layers) are data layers for recording data. The relief structure of the wobble structure does not need to be prepared layer by layer in these recording layers, which simplifies the manufacturing process of the disc. In some embodiments, the recording layers a and B have reflectivities of only 0.7% and 1.5%, respectively, which enables the number of recording layers to be increased from 4 layers of a blu-ray recording medium to as many as, for example, 16 layers, which greatly increases the storage capacity of a single disc.

Fig. 3 is an exemplary read and write optical path for implementing the multi-layer optical disc structure shown in fig. 2. Among them, laser light emitted from a laser LD1 such as a 405nm blue light wavelength may be used as writing light. The signal obtained by the photodiode 1 can be used as a feedback signal to control the moment instrument to move along the z-axis, so as to complete tasks such as layer selection, focusing, servo and the like. During writing, tracking servo may be performed by red light from laser LD2, such as a 655nm red wavelength. By adjusting the position of the adjusting lens RL2, the focus of the red light can be kept at the swing structure layer of the disk all the time, so that the focus of the red light is not changed due to the up-down movement of the torquer. During reading, the laser LD1 with wavelength of 655nm can be stopped, and the tracking servo signal can be used as the signal feedback source to control the servo of the torquer in the radial direction of the disk, so as to read out the data.

Since the read-write optical path of fig. 3 requires the simultaneous use of two different wavelengths of light, such as 405nm and 655nm, this results in a perfect focus for only one wavelength (e.g., red wavelength) when designing the objective lens. For another wavelength, a refractive index mismatch problem may occur when selecting layers. For example, when reading and writing are performed using blue light as writing light focused on different layers, the blue light focus may vary at different depths of the disc, and the objective lens may not be able to perform perfect focusing for the blue light focus at any depth in the disc, which results in a larger focused spot, a stretched z-axis, and a reduced energy density of the spot. This is a refractive index mismatch problem. To solve this problem, the reading and writing optical path of fig. 3 compensates for the refractive index mismatch due to the change in depth of focus by adjusting the position of the adjustment lens RL 1.

Although the read/write optical path of fig. 3 can still guarantee the feasibility of read/write operations for multi-layer recording media based on the read/write servo principle, the read/write optical path of fig. 3 still has the following disadvantages.

1. To distinguish between servo light and write light, the scheme of FIG. 3 requires the use of red light as the servo light and blue light as the write light. However, the airy spot for red is larger than for blue, and accordingly, only the wobble track of the DVD standard can be selected as the servo layer. The track pitch of DVDs is larger compared to the blu-ray standard, and therefore, the areal density is reduced, thereby reducing the storage capacity.

2. The wavelengths of the servo light and the write light are different, which results in the monolithic objective lens not being able to compensate for chromatic aberration. Although the problems of chromatic aberration and refractive index mismatch can be compensated by adjusting the position of RL1, the design and processing requirements for the compensation mirror RL1 and the objective lens are high, which increases the difficulty and cost of producing the read-write optical path of fig. 3.

3. Due to the above-mentioned defects, it is difficult for the read/write optical path of fig. 3 to read/write from/to the disc with 16 or more recording layers, so the design of fig. 3 has limitations and cannot support servo read/write from/to more recording layers.

To this end, it is an object of the present disclosure to provide an improved read/write/servo scheme for multi-layer recording media and corresponding optical paths that can efficiently support not only an increase in the storage capacity of a single disc, but also high-speed reading and writing of multi-layer recording media (even recording media exceeding 16 layers). In addition, the difficulty of the optical path design and/or the processing difficulty/cost of the included optical path device can be reduced.

It is first noted here that the read/write/servo scheme of the present disclosure is primarily developed around a structure of a multi-layer recording medium similar to that of fig. 2, which requires: the bottom layer of the disk is a servo layer, and a plurality of recording layers are prepared on the servo layer. By way of example only, the servo layer may be formed, for example, by stamping or molding a wobble (wobble) structure on the substrate, and then sputtering a reflective layer on the wobble structure, and the plurality of recording layers may be formed by further sputtering or depositing a recording medium on the servo layer in sequence. In the embodiments of the present disclosure, the plurality of recording layers may be a structural layer during the disc manufacturing process or a logical layer of an overall structure but performed after writing data. It will be appreciated that the manner of preparation of the disc structures described herein is well known in the art and therefore their preparation is not described in detail herein.

As used in this disclosure, the terms "plurality of recording layers", "multi-layer recording medium" or "multi-layer" each refer to or include more than 1 recording layer. The present disclosure is not limited to a specific number of multilayers greater than 1 layer, and any multilayers of recording layers that can use the read-write schemes of the present disclosure are within the scope of the present disclosure. In some embodiments, the multiple layers of the present disclosure may include multiple layers greater than 16 layers. In addition, it will also be understood from the following description that, in the present disclosure, the servo/write manner is not affected by a change in the number of recording layers.

The implementation of the multi-layer recording medium-based read/write/servo scheme of the present disclosure and the associated optical path will be described below with reference to the disk structure defined above.

FIG. 4 depicts a read-write system framework diagram according to an example embodiment of the present disclosure, applicable to the disk structure defined above. As shown in fig. 4, the disc to be read/written here may be a multilayer recording medium with the structure defined above, and the laser read/write module OPU is an execution module for reading/writing the multilayer recording medium; the spindle motor drives the disc to rotate; the servo module controls the motor drive, a torquer and a spindle motor in the OPU, so that the OPU can be ensured to execute read-write operation at an accurate position; the upper computer sends a read-write instruction to the read-write system. Such frame structures are known to those skilled in the art and will not be described in detail here.

Fig. 5 illustrates a schematic diagram of a method embodying a writing and/or servo concept of a multi-layer recording medium, i.e., a multi-layer recording medium of the above-defined structure, which may include at least 2 recording layers, according to an example embodiment of the present disclosure.

The steps of the method 500 may include: in block 510, data is written on the second recording layer using data writing light, in which light incident to the first recording layer on which data is written is used as servo light of the data writing light. In some embodiments, the first recording layer and the second recording layer may be adjacent to each other, and the second recording layer is positioned above the first recording layer. However, this is not a limitation, and in other embodiments, it is also possible that the second recording layer is located above the first recording layer. The method 500 is advantageous in that it is possible to avoid using light incident on the servo layer as servo light when writing data to the second recording layer, which makes the servo of writing to the second recording layer more free.

In a further embodiment, the multi-layer recording medium may further include a servo layer. For example only, the servo layer may include a wobbled groove track. The method 500 may further include: data is written in the first recording layer using a first writing light, wherein a light incident to the servo layer is used as a servo light of the first writing light. In these embodiments, the aforementioned at least 2 recording layers may be located above the servo layer, and the first recording layer may be adjacent to the servo layer, and the data writing order of the plurality of recording layers may be layer-by-layer writing starting from the first recording layer.

In embodiments where the plurality of recording layers includes more than 2 recording layers, such as a third recording layer, a fourth recording layer, a fifth recording layer, and so forth, where the third recording layer may be adjacent to and above the second recording layer, the fourth recording layer may be adjacent to and above the third recording layer, the fifth recording layer may be adjacent to and above the fourth recording layer, and so forth.

Still further, the writing and/or servo concept of the present disclosure may also be such that for writing of a first recording layer, it uses light incident to the servo layer as servo light, while for writing of any subsequent other recording layer, it uses light incident to a preceding recording layer adjacent thereto on which data is written as servo light.

Here, it should be noted that: although the first recording layer is described here as having the light incident to the servo layer as the servo light, this does not mean that the first recording layer is immediately adjacent to the servo layer. In other embodiments it is also possible that the first recording layer is at a distance from the servo layer, for example with a spacer layer between them, and in addition it is possible that the second recording layer is closer to the servo layer than the first recording layer.

In an embodiment where the plurality of recording layers further includes a third recording layer, the method 500 may further include: and writing data on a third recording layer using a third write light, wherein light incident on the second recording layer on which data is written is used as servo light of the third write light. In an embodiment where the plurality of recording layers further includes a fourth recording layer, the method 500 may further include: data is written on a fourth recording layer using a fourth write light, wherein light incident to the third recording layer on which data is written is used as servo light of the fourth write light. And so on.

Here, it should be noted that: generally, the first recording layer is closer to the servo layer, the second recording layer is farther from the servo layer, and the first recording layer is adjacent to the servo layer and adjacent to the first recording layer. This is not necessary and in some embodiments the first numbered recording layer may be further from the servo layer than the next numbered recording layer. In addition, the term "adjacent recording layers" or the like does not mean that there is necessarily no interlayer of the non-recording layer between the adjacent recording layers, and the term "adjacent to the servo layer" or the like does not mean that there is necessarily no interlayer of the non-recording layer between the servo layers.

In some embodiments, the writing light and the corresponding servo light require the same wavelength as each other, which may help to reduce the design difficulty of the objective lens and alleviate the problem of refractive index mismatch. For example only, the write light and the servo light may each be blue light having a wavelength of 405nm.

In still other embodiments, the first recording layer is a recording layer adjacent to the servo layer, and the data writing order of the plurality of recording layers is layer-by-layer writing starting from the first recording layer. In this way, writing and servo of a plurality of recording layers can be facilitated. This also indicates that writing of multiple recording layers will start from the deepest recording layer of the disc layer by layer until full.

In order to achieve data writing to the recording layer, in embodiments of the present disclosure, it is required that the motion trajectories of the servo light and the corresponding write light are kept consistent. Further, in an embodiment where the servo layer includes a wobbled groove track, writing data on the first recording layer may include: the servo light corresponding to the first write light is moved along the wobbled groove track. Further, writing data on the recording layer of the next number may include: the light incident on the adjacent previous-numbered recording layer on which data is written may be used as the servo light of the writing light for writing data on the next-numbered recording layer. For example, when writing data on the second recording layer using the second writing light, the servo light corresponding to the second writing light may be moved along the data track of the first recording layer. Note: for convenience of description, the second write light here may be the data write light in the embodiment shown in fig. 5.

To more intuitively understand the writing and servo processes for a multi-layer recording medium of the present disclosure, fig. 6 illustrates a writing/servo process schematic for a multi-layer recording medium according to an example embodiment of the present disclosure.

For simplicity, the multi-layer recording medium shown in fig. 6 shows only L0-L6 layers, wherein the L0 layer is a servo layer, which is, for example, engraved with a wobble track, and the L0 layer is located at a deeper layer of the disc than the recording layer, and thus can be defined as a bottom layer of the disc. L1 to L6 are recording layers for data recording, and a recording layer of the latter number is located above and adjacent to a recording layer of the former number. It will be understood that the present disclosure is not limited to the number of layers, and in other embodiments, there may be more or fewer recording layers. In addition, as mentioned above, the recording layers may be artificially prepared, e.g. different layers separated by different media, or logical layers of bulk recording material, which form the actual data recording layer after writing data. As shown in fig. 6, when writing is performed for the first recording layer L1 using the write light, the servo light can be focused to the servo layer L0; when writing with respect to the second recording layer L2, the servo light may be focused to the first recording layer L1 on which data is written; when writing to the third recording layer L3, the servo light can be focused to the second recording layer L2 where data is written, and so on, writing layer by layer until full.

First, in block 710, OPU positioning is performed: the control moment moves the objective lens to focus the servo light to the servo layer L0, and simultaneously adjusts the compensation mirror to focus the main beam to the data recording layer L1. In some embodiments, the servo layer L0 is provided with a wobble track structure.

Then, at block 720, a write operation is performed: the servo light is controlled by the torquer to move along the track of the servo layer to ensure that the main beam moves along the fixed track and writes data until the L1 layer is fully written.

At block 730, OPU layer movement is performed: the actuator is controlled to move downwards to focus the servo light onto the data recording layer Ln-1 just written, and simultaneously, the compensation mirror is adjusted to focus the main beam onto the recording layer Ln.

At block 740, a write operation is performed: moving servo light along a data track of the Ln-1 layer, performing servo control by using a signal fed back by the servo light, and controlling the motion attitude of the torquer; at the same time, the main beam is caused to write data on the Ln layer.

At block 750, a determination is made as to whether the disk is full. If not, a return to block 730 may be made to continue writing on the next recording layer.

At block 760, a read operation may be performed: instead of using the signal of the servo beam, the signal of the main beam on the detector can be used as a servo control signal while reading the data information in the layer.

As can be seen from the above description of fig. 6 and 7, which present an example embodiment of layer-by-layer writing, it is required that each time a layer is full to write the next layer. For example, when writing to a completely new disk, the servo light is first focused on the L0 layer. The L0 layer may be prepared with, for example, a wobble track, the servo light is reflected by the wobble track to a detector such as a four-quadrant detector, and after a signal is calculated and fed back to a control system, the state of the torquer is adjusted to ensure that the servo light is always focused on the wobble track of the servo layer, and the light spot can always move along the track when the disc rotates. Subsequently, data writing of the recording layer of the subsequent number can be performed, and the servo light of the recording layer of the subsequent number can be focused on the recording layer of the previous adjacent number at this time.

In some embodiments, the objective lenses for the main beam as the read/write light and the auxiliary beam as the servo light may be fixed on the same torquer as described later with reference to the example read and write optical paths shown in fig. 8-10. In this case, the two objective lenses are rigidly connected, and the movement trajectories of the main beam as write light and the servo light coincide with each other during writing. When the servo light moves along the wobble track, the main beam also moves along the same track to ensure that the tracks written by the main beam are not staggered/overlapped.

Although the motion track of the main beam and the auxiliary beam are consistent, the focal points of the main beam and the auxiliary beam are not in the same layer. For example, when the servo light is focused on the L0 layer, the main beam is focused on the L1 layer. A slight deviation in the depth of focus (e.g., about 10 μm) can be adjusted by a compensating mirror in the main beam path to ensure perfect focusing of the main beam on the L1 layer. And after the completion of the servo work is ensured, data writing can be started to the current layer.

When the drive completes writing the first recording layer, the actuator will jump layer to move the focus of servo light to the L1 layer where data has just been written. Correspondingly, the main beam is focused to the L2 layer. At this time, the data of the L1 layer will be used as servo information to guide the servo light to move along the data track, so as to ensure that the main light beam does not shift during the writing process. The data writing process of the subsequent layer is similar to that of the previous layer, the servo light is focused on the Ln-1 layer auxiliary servo, and the main light beam is focused on the Ln layer for data writing until the whole optical disc is fully written.

It will be appreciated that the above-described write/servo scheme can eliminate the need for layer-by-layer fabrication of the reflective layer and the wobble track, reducing the cost of disk fabrication. The part of the recording layer without the reflection layer can improve the transmissivity of a single recording layer, thereby increasing the number of recording layers and the storage capacity of a single disc. In addition, the writing/servo scheme of the present disclosure can also ensure high-speed reading and writing on the basis of disk capacity improvement.

In addition, under the read/write and servo schemes of the present disclosure, the main beam as the read/write light and the auxiliary beam as the servo light may employ the same wavelength. This means that a single light source can be used to generate the main and auxiliary beams. As an example, the wavelength of blue light may be selected to be 405nm, which may effectively improve the linear density of the memory. Compared to the scheme of fig. 2 using red light as the servo light, the spot using blue light may be smaller than the spot of red light. In this case, the track width of the servo layer, e.g., the wobble track, can be reduced from about 750nm to 320nm, which can effectively improve the storage density of a single layer.

Different exemplary embodiments of read and write optical paths that can implement the read and write operations of fig. 5-7 described above will be described below with reference to fig. 8-10.

As shown in fig. 8, which shows a first exemplary read-write optical path 100 for a multi-layer recording medium 1, the first exemplary read-write optical path 100 may include at least a main beam optical path a and a servo beam optical path B.

In some embodiments, the main beam path A may include at least a laser source 18, a first beam splitter 13, a first polarizing beam splitter 9, a first objective lens 3-1, and a first detector 12. The servo beam path B may include at least a laser source 18, a first beam splitter 13, a second polarizing beam splitter 14, a second objective lens 3-2, and a second detector 21. That is, the main beam optical path a and the servo beam optical path B may share part of the optical path, which may contribute to a more compact and less costly optical path. However, it will be appreciated that in other embodiments, the main beam path a and the servo beam path B are independent of each other, and it is also possible that there is no shared optical path between them.

The laser source 18 functions to generate an initial laser beam. In some embodiments, the laser source 18 may be, for example, a semiconductor laser. In still other embodiments, the emission wavelength of the laser source may be a blue wavelength, such as 405nm.

The first beam splitter 13 may receive the initial laser beam from the laser source 18 and split it into a main incident beam and a servo incident beam. The main incident beam and the servo incident beam may be incident on the multilayer recording medium 1 along a main beam optical path a and a servo beam optical path B, respectively. In some embodiments, the splitting ratio between the main incident beam and the servo incident beam can be determined by the ratio of the light intensities required for actual writing and servo. In some embodiments, the first beam splitter 13 may be a beam splitting prism.

In some embodiments, a beam shaping device 15 may be provided between the laser source 18 and the first beam splitter 13, which may be used, for example, to collimate, spot-shape, the initial laser beam from the laser source 18. By way of example only, the beam shaping device 15 may be, for example, a combination of a collimating lens 16 and a modified prism pair 17. For example, the collimator lens 16 is arranged to be operatively moved so that the focal point of the collimator prism coincides with the exit position of the laser light, thereby ensuring that the laser light is parallel light after passing through the collimator lens 16. The angle of the anamorphic prism pair 17 is then operatively adjustable such that the shape of the spot is adjusted to a circular shape after the laser light passes through the anamorphic prism pair 17.

The first objective lens 3-1 and the second objective lens 3-2 may be respectively disposed at positions adjacent to the multi-layered recording medium 1, and may focus the main incident beam from the main beam optical path a and the servo incident beam from the servo beam optical path B to different positions of the multi-layered recording medium 1, respectively. Meanwhile, the first objective lens 3-1 and the second objective lens 3-2 may also collect a main reflected beam and a servo reflected beam from different positions of the multilayer recording medium 1, respectively, wherein the main reflected beam and the main incident beam travel in opposite directions, and the servo incident beam and the servo reflected beam travel in opposite directions.

It should be noted here that the main incident light beam and the main reflected light beam may be collectively referred to as a main light beam, which may be used as writing light or reading light, and the servo incident light beam and the servo reflected light beam may be collectively referred to as a servo light beam or an auxiliary light beam.

To facilitate control of the main and servo beams in accordance with the writing and servo control scheme described above, in some embodiments the first objective lens 3-1 and the second objective lens 3-2 may be arranged on the same torquer 2 and adapted to move synchronously with the movement of the torquer 2. In this way, the movement of both the main and auxiliary beams can be conveniently coordinated so that the trajectories of the main and servo beams remain consistent. In some embodiments, the torquer 2 may be a three-dimensional torquer. Note that: during assembly, the assembly can be carried out first such that the focal points of the two objective lenses are guaranteed to be approximately at the same horizontal position.

During reading and writing, the position deviation of the two objective lenses in the three dimensions of focusing, tracking and tilting can be controlled by the moment arm 2. By adjusting the position of the objective lens in the vertical direction, the depth of the focal spot within the disc can be controlled, ensuring that the focal spot moves over the data recording layer. By adjusting the position of the objective lens in the radial direction of the disc, it is possible to ensure that the focal point writes data on a given track. By adjusting the relative angle between the objective lens and the disc, the laser can be ensured to be vertically incident into the disc.

A first polarizing beamsplitter 9 may be arranged between the first beam splitter 13 and the first objective lens 3-1 for transmitting the main incident beam having the first polarization and reflecting the main reflected beam having the second polarization. Further, the main reflected light beam of the second polarization reflected via the first polarizing beam splitter 9 may be directed to a first detector 12, and the first detector 12 may output a first detection result indicating a focal position and/or shape of the main incident light beam on the multilayer recording medium 1 for feedback control of the main light beam. In some embodiments, the first detector 12 may be a four quadrant detector. To achieve detection by means of the astigmatic method, the reflected primary reflected light beam having the second polarization from the first polarization beam splitter 9 may be incident on the four-quadrant detector via a combination of a collimator lens 10 and a cylindrical lens 11. In some embodiments, the optical path of the main reflected beam may share part of the optics with the optical path of the main incident beam, such as the first polarizing beamsplitter 9 and the objective lens 3-1, but it will be appreciated that this is not required.

A second polarizing beamsplitter 14 may be arranged between the first beamsplitter 13 and the second objective lens 3-2 for reflecting the servo incident light beam with the first polarization and transmitting the servo reflected light beam with the second polarization. The servo reflected beam transmitted via the second pbs 14 may be directed to a second detector 21, and the second detector 21 may output a second detection result indicating a focal position and/or shape of the servo incident beam on the multilayer recording medium for feedback control of the servo light. In some embodiments, the second detector 21 may be a four-quadrant detector. To achieve detection by means of astigmatism, the servo reflected light beam with the second polarization transmitted from the second pbs 14 may be incident on the four quadrant detector via a combination of a collimator lens 19 and a cylindrical lens 20. In some embodiments, the optical path of the servo reflected beam may share part of the optics with the optical path of the servo incident beam, such as the second polarizing beamsplitter 9 and the objective lens 3-2, but it will be appreciated that this is not required.

In some embodiments, the first polarization and the second polarization may be orthogonal to each other. For example, the main incident light beam and the servo incident light beam having the first polarization may be s-light, and the main reflected light beam and the servo reflected light beam having the second polarization may be p-light.

To achieve the above-described main incident light beam having the second polarization and servo reflected light beam, in some embodiments, a first 1/4 wave plate 4-1 may be arranged between the first polarizing beam splitter 9 and the first objective lens 3-1 to generate a main reflected light beam having the second polarization towards the first polarizing beam splitter 9. A second 1/4 wave plate 4-2 may be arranged between the second polarizing beam splitter 14 and the second objective lens 3-2 to generate a servo reflected beam having the second polarization towards the second polarizing beam splitter 11. In the example shown in FIG. 8, the first 1/4 wave plate 4-1 and the second 1/4 wave plate 4-2 are shown disposed adjacent to the first objective lens 3-1 and the second objective lens 3-2, respectively. However, it should be understood that this is merely an example, and the first 1/4 wave plate 4-1 may be disposed anywhere between the first polarizing beamsplitter 9 and the first objective lens 3-1, and the second 1/4 wave plate 4-2 may be disposed anywhere between the second polarizing beamsplitter 14 and the second objective lens 3-2. It will also be appreciated that after the laser beam has been transmitted through the 1/4 wave plates 4-1, 4-2, it is reflected from the disc and passes through the 1/4 wave plates 4-1, 4-2 again, in which case the polarization direction of the laser light passing through the 1/4 wave plates twice is rotated by 90 °, which thereby allows the main reflected beam having the second polarization to be reflected from the first polarizing beam splitter 9 and the servo reflected beam having the second polarization to be transmitted from the second polarizing beam splitter 14.

Further, in some embodiments, the reading and writing optical path 100 may further include a first compensation lens group 30, which may be disposed between the first polarization beam splitter 9 and the first objective lens 3-1, and may be operable to compensate for refractive index mismatch caused by depth variation of a focal point of the main incident optical beam within the multilayer recording medium. As an example, the first compensation lens group 30 may for example comprise a combination of a movable compensation mirror 7 and a fixed lens 6, wherein the compensation mirror 7 may be arranged on a (one-dimensional) stepping motor 8, thereby enabling an operational movement of the compensation mirror. In some embodiments, the direction of movement of the stepper motor 8 may be parallel to the direction of propagation of the light, with the laser light passing through the center position of the compensator mirror 7. It will be appreciated that by shifting the position of the compensating mirror 7, for example, the problem of refractive index mismatch can be compensated for, ensuring that the focal spot shape of the disc is close to the diffraction limit, for example, meeting the requirement that the spot energy RMS <0.07 lambda.

In still other embodiments, mirrors 5-1 and 5-2 may be further disposed after the first and

second PBS

9 and 11, respectively, to change the propagation direction of the light beam, for example, to change the light beam from the original horizontal direction to the vertical direction, for more convenient incidence to the multi-layered recording medium.

The traveling optical paths of the main beam and the servo beam and the control logic for reading and writing are described in brief with reference to fig. 8.

A laser beam with a first polarization is emitted from the laser source 18, the laser beam forms a main incident beam with the first polarization and a servo incident beam after passing through the beam splitter of the first beam splitter 13, the main incident beam is transmitted through the first polarization beam splitter 9, the first compensation lens group 30, and the first 1/4 wave plate, and then is incident to a first position (for example, a position on the first recording layer L1) of the multilayer recording medium 1 via the first objective lens 3-1, the first objective lens 3-1 collects reflected light from the first position to form a main reflected light beam, and the main reflected light beam is incident to the first polarization beam splitter 9 after passing through the first 1/4 wave plate 4-1 and the first compensation lens group 30 again in a reverse direction. Since the light beam passes through the first 1/4 wave plate 4-1 twice, the main reflected light beam may have a second polarization direction orthogonal to the first polarization direction. The first polarizing beamsplitter 9 reflects the main reflected beam with the second polarization towards the first detector 12. At the same time, the servo incident beam is incident from the first beam splitter 13 to a second polarizing beam splitter 14, which may be designed to reflect the servo incident beam with the first polarization. The servo incident light beam reflected by the second PBS 14 passes through the second 1/4 wave plate 4-2 and is incident on a second position (e.g., a position on the servo layer L0) of the multi-layered recording medium 1 via the second objective lens 3-2, the second objective lens 3-2 collects the reflected light from the second position to form a servo reflected light beam, and the servo reflected light beam again passes through the second 1/4 wave plate 4-2 in the opposite direction and is incident on the second PBS 14. Since the light beam passes the second 1/4 wave plate 4-2 twice, the servo reflected light beam may have a second polarization direction orthogonal to the first polarization direction. The second pbs 14 transmits the servo reflected beam having the second polarization to the second detector 21.

During writing, the signal of a second detector 21, such as a four quadrant detector, may be used to control the focusing, tracking and angular tilting of the actuator 2. After the focus of the servo light is positioned to the adjacent layer Ln-1 of the recording layer Ln to be written, the first detector 12, such as a four-quadrant detector, can adjust the first compensation lens group 30 (e.g., adjust the position of the movable compensation mirror 7 in the first compensation lens group 30 by the one-dimensional stepping motor 8), focus the focus of the main incident light beam to the recording layer Ln, and correct the problem of refractive index mismatch. When writing is started, the forcer 2 will keep the focus of the servo beam moving along the wobble track or data track on the Ln-1 layer. Since the two objective lenses 3-1 and 3-2 are rigidly fixed on the torquer, the focus of the main beam will also move along a fixed track, ensuring that the written data will not be subjected to track crosstalk.

During reading the second detector 21 may not be active and the signals obtained by the first detector 12 alone may be used to control the torquer 2 for focus, tracking and tilt servo. And simultaneously, the stepping motor 8 is controlled to adjust the first compensation lens group 30 to complete the compensation of refractive index mismatch.

FIG. 9 illustrates a second exemplary read-write optical path 200, which differs from the first exemplary read-write optical path 100 of FIG. 8 in that: an energy feedback system 40 for the laser source is added to the second exemplary read-write optical path 200.

By way of example only, the energy feedback system 40 may include at least a second beam splitter 22 and a third detector 24, wherein the second beam splitter 22 is disposed between the first beam splitter 13 and the first polarizing beamsplitter 9 for splitting a portion of the light from the primary incident beam from the first beam splitter 13 and directing it to the third detector 24. The third detector 16 receives and detects the portion of the light split from the second beam splitter 22 and outputs a third detection result indicative of the laser intensity of the initial laser beam, which can be used as a feedback result to control the laser source 18 to stabilize the intensity of the laser light emitted therefrom. As an example, the third detector 16 may be a light intensity detector. In some embodiments, a lens 15 may also be disposed between the second beam splitter 22 and the third detector 24 to achieve convergence of the light spots to facilitate detection of the light intensity by the third detector 24.

It will be appreciated that the exemplary read and write optical paths of fig. 8 and 9 may be particularly suitable for multilayer recording media that are fabricated and written layer-by-layer (i.e., layer fabrication and writing are not separated). This is because: when writing, each writing light can be focused on the most superficial Ln layer, and the distance from the adjacent recording layer Ln-1 for servo to the above most surface can be kept constant. Thus, the servo light may not need to overcome the refractive index mismatch problem. Depending on the servo beam, the perfect focus of the objective lens can also be designed on the sub-surface data layer. At the same time, the position of the compensating mirror 7 can be adjusted to ensure that the main beam has a perfect focus on the disc surface.

FIG. 10 shows a third exemplary read-write optical path 300, which differs from the second exemplary read-write optical path 200 of FIG. 9 in that a second compensation lens group 50 for the servo beam is newly added.

For example only, the second compensation lens group 50 may include a movable compensation mirror 22 and a fixed mirror 24, wherein the compensation mirror 22 is disposed between the second polarization beam splitter 14 and the second objective lens 3-2 so as to be operable to compensate for refractive index mismatch caused by depth variation of the focus of the servo incident beam within the multilayer recording medium. In some embodiments, the movable compensation mirror 22 may be arranged on a (e.g., one-dimensional) stepper motor 23, and the movement of the movable compensation mirror 22 may be controlled by operating the stepper motor 23. Like the stepping motor 8, the moving direction of the stepping motor 23 may be parallel to the traveling direction of the light. In some embodiments, the control of the stepper motor 23 may be determined by the signal of the second detector 21 and the applied voltage in the focus servo direction of the torquer 2.

It will be appreciated that in the example of fig. 10, the main beam and the servo beam each have a compensating lens group, which allows both beams to correct or compensate for aberration problems that occur during reading and writing, respectively, and thus correct for refractive index mismatch problems, ensuring that the two objective lenses focus in the disc meets diffraction limit requirements. It will be appreciated that the exemplary read and write optical paths of FIG. 10 are well suited for multi-layer recording media having a variety of configurations, and may be particularly suited for disks that are not separated by layer data writing and disk manufacturing processes, or for disks that are pre-prepared, separated by writing and disk manufacturing processes.

Exemplary optical paths in a read/write apparatus according to various exemplary embodiments of the present disclosure have been described above in detail. It should be understood that the read/write apparatus of the present disclosure may not be limited to the various exemplary read/write optical paths described above, but may encompass a wider range of read/write apparatuses that may implement the above-described data writing/servo methods of the present disclosure.

For example, in some embodiments, the read/write device may include: a servo optical path member for irradiating servo incident light to a first recording layer of a multi-layer recording medium having data written thereon, and receiving servo reflected light reflected via the first recording layer, wherein the multi-layer recording medium includes at least the first recording layer and a second recording layer; a main light path unit for irradiating data write light to the second recording layer; and a control device configured to control the data writing light to write data to the second recording layer with the servo reflected light as servo light.

In still other embodiments, the main light path assembly may be further configured to irradiate the first writing light to the first recording layer; the above-mentioned servo light path assembly may further irradiate another servo incident light to a servo layer of the multilayer recording medium and receive another servo reflected light reflected via the servo layer; the control device can also control the first writing light to write data into the first recording layer by using the other servo reflected light as servo light.

As an example of the implementation of the main optical path assembly, the servo optical path assembly and the control device described above. By way of example only, the servo optical path assembly may comprise, for example, a servo beam path as in fig. 8-10 and associated devices, which may comprise, for example, at least the second objective lens 3-2, the second polarizing beamsplitter 14, the second detector 21, the second 1/4 wave plate 4-2, and the second compensation lens group 50. The primary light path assembly may, for example, include at least a primary light beam path as in fig. 8-10 and associated devices, which may, for example, include at least a first objective lens 3-1, a first polarizing beamsplitter 9, a first 1/4 wave plate 4-1, a first compensation lens group 30. The control device can be realized by one or more of a digital servo algorithm realization module, a precision mechanical platform module, an analog front-end placing drive module and an upper PC control module in FIG. 4.

It should be understood that suitable additional designs or variations or combinations of features in the various embodiments or examples of the above described example read/write/servo processes or read/write apparatus (including the optical devices therein) may be made to take into account different application scenarios (e.g., different thickness of multi-layer discs, different structure of discs), and such additional designs or variations or combinations still fall within the scope of the present application. The new added privilege for FIG. 10 is explained

For example, to increase the range over which the reading and writing system can read and write, the objective lens and the actuator may be designed such that the focal point of the objective lens is in the center of the writable area of the disc when the actuator is in a static state. For another example, for a disc in which the total thickness of the read/write area of the disc and the surface protective layer is 600 μm, the focal point of the objective lens can be designed to be 300 μm away from the surface without applying a current to the torque. Also for example, when designing an objective lens, it is considered that the perfect focus of the objective lens is inside the disc, not in air.

It will be appreciated that the solution of the present disclosure may have several advantages over the read-write systems of existing multi-layer discs:

a. the write/servo scheme can omit the processes of preparing a reflecting layer, a swing track and the like layer by layer, and reduce the preparation cost of the disk. The part of the recording layer without the reflection layer can improve the transmissivity of a single recording layer, thereby increasing the number of recording layers and the storage capacity of a single disc. In addition, the writing/servo scheme of the present disclosure can also ensure high-speed reading and writing on the basis of disk capacity improvement

b. Only 1 light source is required. The servo light and the wavelength form of the main light beam, so that the quality reduction of a light spot caused by chromatic aberration can be not considered in the design of an objective lens and other optical devices, and the difficulty of design and processing is simplified.

c. The servo light used in the method can also be blue light with the wavelength of 405nm, and compared with the existing scheme of using red light as the servo light, the focused airy disk is smaller, which is beneficial to improving the surface density of the disk, so that the storage capacity of a single disk can be increased.

d. Because the objective lens for focusing the servo light and the main light beam is rigidly connected to the torquer, the motion tracks of the two light beams are kept consistent, and therefore, the problem of track crossing of data written by the main light beam cannot occur. In addition, the torquer for clamping the double objective lenses ensures that the two objective lenses are rigidly connected, and can meet the requirement of high-speed response of the driver.

e. The reading and writing optical path can compensate aberration possibly generated during focusing and layer selection, so that the problem of refractive index mismatch is solved.

f. The servo scheme and optical path of the present disclosure can support more layers of optical discs for read and write operations. Besides reading and writing the disk prepared layer by layer, the block material without the reading and writing layer can also be read and written. The disk manufacturing mode can reduce the preparation cost of the disk, and the read-write optical path and the servo scheme ensure high-speed read-write of the disk.

It will be appreciated that the above described methods and apparatus are merely examples. Although the steps of a method are described in a particular order in the specification, this does not require or imply that the operations must be performed in that particular order, or that all of the illustrated operations must be performed, to achieve desirable results, but rather that the steps depicted may be performed in an order that varies. Additionally or alternatively, certain steps may be omitted, multiple steps combined into one step execution, and/or one step broken down into multiple step executions.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

In the claims, the word "comprising" does not exclude other elements, and the indefinite article "a" or "an" does not exclude a plurality. A single element or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain features are recited in mutually different embodiments or in dependent claims does not indicate that a combination of these features cannot be used to advantage. The scope of protection of the present application covers any possible combination of features recited in the various embodiments or in the dependent claims, without departing from the spirit and scope of the application.

Furthermore, any reference signs in the claims shall not be construed as limiting the scope of the invention.

Claims

1. A data writing method for a multi-layered recording medium, the multi-layered recording medium including at least a first recording layer and a second recording layer, the method comprising:

data is written in the second recording layer using data writing light, in which light incident on the first recording layer on which data is written is used as servo light of the data writing light.

2. The data writing method of claim 1, wherein the multi-layer recording medium further includes a servo layer, the method further comprising:

data is written in the first recording layer using a first writing light, wherein light incident on the servo layer is used as a servo light of the first writing light.

3. The data writing method according to claim 1, wherein the first recording layer and the second recording layer are adjacent to each other, and the second recording layer is located above the first recording layer.

4. The data writing method according to any one of claims 1 to 3, wherein the data writing light and its corresponding servo light wavelength are the same.

5. The data writing method according to claim 2, wherein the first recording layer is a recording layer adjacent to the servo layer, and the data writing sequence of the plurality of recording layers is layer-by-layer writing from the first recording layer.

6. A data writing method according to claim 1 or 2, wherein the servo light and the corresponding data writing light have a motion trajectory that is kept consistent during writing.

7. The data writing method according to claim 2, wherein the servo layer includes a wobbled groove track, and the writing data on the first recording layer using the first writing light includes:

and controlling the servo light corresponding to the first writing light to move along the swinging groove track.

8. The data writing method according to claim 1 or 2, wherein the writing of data on the second recording layer using the data writing light includes:

and controlling servo light corresponding to the data writing light to move along the data track of the first recording layer.

9. The data writing method according to claim 4, wherein the wavelengths are all 405nm.

10. A read-write apparatus, comprising:

a servo optical path member for irradiating servo incident light to a first recording layer of a multi-layer recording medium having data written thereon, and receiving servo reflected light reflected via the first recording layer, wherein the multi-layer recording medium includes at least the first recording layer and a second recording layer;

a main light path unit for irradiating data write light to the second recording layer; and

and a control device for controlling the data writing light to write data to the second recording layer by using the servo reflected light as servo light.

11. Read-write device according to claim 10,

the control device is also used for controlling the motion tracks of the data writing light and the corresponding servo light to be consistent in the writing process.

12. Read-write apparatus according to claim 10 or 11,

the main optical path assembly is further used for irradiating first writing light to the first recording layer, the servo optical path assembly is further used for irradiating another servo incident light to a servo layer of the multi-layer recording medium, and receiving another servo reflected light reflected by the servo layer;

the control device is also used for controlling the first writing light to write data into the first recording layer by taking the other servo reflected light as servo light.

13. Read-write apparatus according to claim 10 or 11,

the main optical path assembly comprises a first objective lens (3-1) adapted to focus the data writing light to a second recording layer on the multi-layer recording medium;

the servo optical path assembly comprises a second objective lens (3-2) adapted to focus the servo incident light beam onto a first recording layer on the multi-layer recording medium.

14. Read-write device according to claim 13,

wherein the first objective lens (3-1) and the second objective lens (3-2) are arranged on the same torquer (2) and both are adapted to move synchronously with the movement of the torquer.

15. The reader/writer apparatus according to claim 13, further comprising:

a laser source (18) for emitting an initial laser beam;

a first beam splitter (13) for splitting said initial laser beam into a main incident beam and a servo incident beam;

a first polarizing beam splitter (9) located between the first beam splitter and the first objective lens for transmitting the main incident beam with a first polarization as the data write light.

16. The reader/writer apparatus according to claim 15, further comprising:

a first compensation lens group (30) located between the first polarizing beam splitter and the first objective lens and operable to compensate for refractive index mismatch caused by depth variation of a focal point of the main incident light beam within the multilayer recording medium.

17. A read-write apparatus for a multilayer recording medium, comprising:

a laser source (18) for emitting an initial laser beam;

a first objective lens (3-1) for focusing the main incident light beam to a first position on the multilayer recording medium; and

a second objective lens (3-2) for focusing the servo incident light beam to a second position on the multilayer recording medium and collecting a servo reflected light beam from the second position, the second position being different from the first position;

18. The reader/writer apparatus according to claim 17, further comprising:

a first polarizing beamsplitter (9) located between the first beamsplitter and the first objective lens for transmitting the primary incident light beam having a first polarization and reflecting the primary reflected light beam having a second polarization.

19. The reading/writing apparatus according to claim 17 or 18, further comprising:

a second polarizing beam splitter (14) located between the first beam splitter and the second objective lens for reflecting the servo incident beam having a first polarization and transmitting the servo reflected beam having a second polarization.

20. The reading/writing apparatus according to claim 18, further comprising: