Abstract:
A holographic recording and reproduction system includes a servo optical path, which is used to provide a servo mechanism, so that holographic interference fringes can be stored continuously into a holographic recording medium, and when the reproduction signals are desired, they can be fetched and obtained swiftly and accurately by making use of the servo mechanism. In addition, the servo light spot of the servo optical path is located on the optical axis of an object lens, thus reducing the image aberrations produced and raising the quality of the signals read for the servo track searching. Moreover, the light intensity distribution of the reference beams reflected by the holographic recording medium is monitored and controlled, as such realizing the analysis and adjustment of the relative distance and inclination angle between the holographic recording and reproduction system and the holographic recording medium.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This non-provisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No(s). 096105518 filed in Taiwan, R.O.C. on Feb. 14, 2007, the entire contents of which are hereby incorporated by reference. 
       BACKGROUND 
       [0002]    1. Field of Invention 
         [0003]    The invention relates to a holographic recording and reproduction system, in particular to a holographic recording and reproduction system having servo optical path. 
         [0004]    2. Related Art 
         [0005]    Presently, in the market of optical storage medium, since the capacity of the commercialized blu-ray disc can hardly exceed the threshold of 100 GBytes, thus various kinds of potential ultra-high capacity data recording technologies are under intensive research and development, and among them, the holographic disc is the most promising choice. The research and development of holographic recording technology has had a long history, however, due to various reasons, it still has not been utilized in the consumer optical storage products. Taking it for an example, the holographic experiments conducted in the early days must be carried out by utilizing an enormously voluminous high power laser light source of several hundredths of milliwatts as well as a complicated optical system, and moreover, they must be used in cooperation with a heavy shockproof table. Furthermore, the Photo-Refractive Crystal utilized as the holographic recording medium is even more expensive than that of the ordinary medium. However, along with the rapid progress and development of this technology, the restrictions originally imposed on the holographic storage and recording technology have already solved and removed one-by-one. For example, the following systems and devices have made tremendous progress in their technologies, such as miniaturized high power laser, high photo-sensitive recording materials, and miniaturized data access optical system having position servo functions, such that the conventional thinking that the recording medium must be capable of being rewriteable is changed due to the consumer&#39;s behavior in the CD-R market. However, up to the present day, it is still a very difficult task for the recording medium of Photo-Refractive Crystal capable of being rewriteable and also able to satisfy the requirement of excellent material characteristics, high data stability and cheap prices. In recent years, due to the popularity and widespread utilization of ordinary-priced write-once optical disk CD-R/DVD-R, thus the recognition that the holographic recording does not have to strive for the medium material capable of being rewriteable has gained widespread acceptance. If the functional requirement of rewriteable is not taken into consideration, then there are plenty of cheap organic materials having high photo sensitivity, which can be chosen and utilized as data recording layer for the holographic CD, for example, a photo polymer is one of them. Under strong irradiation of recording light, the photopolymer may produce molecular chain like chemical reactions, thus the change of optical properties caused by the characteristics of sparsity and density of the molecular chains can be used to record and regenerate data related to 3-dimensional holographic interference fringes. 
         [0006]    The concept of the afore-mentioned miniaturized data access optical system having position servo functions is originated from the servo mechanism of CD/DVD player, and that is the key point in realization the implementation of holographic discs. 
         [0007]    With regard to the technology of holographic storage, as disclosed in U.S. Patent Gazette publication No 20040212859 and also U.S. Pat. No. 6,700,686, wherein, a transmissive holographic recording medium is utilized. Due to the transmissive type design, an image sensor is placed on the other side of the holographic recording medium, thus making the volume of the overall system enormously large. Moreover, in this transmissive type system framework design, usually, the optical axis of object lens to be passed by the signal beam is designed perpendicular to the holographic recording medium, while a reference beam is incident obliquely upon the holographic recording medium. As such, it may produce deviations related to the relative positions and directions of the reference beam and the holographic recording medium. Once the deviation occurs, and when the reference beam can not be incident upon the holographic recording medium along the original path, then no reproduced signal beams will be produced, thus there is no way of catching the reproduced signal beams through adjusting the signal optical path. Therefore, the image sensor used to receive the reproduced light signal will not receive any signals of the reproduced light, thus it can not restore the correct reproduced data by making use of the image processing technology. Though for the stationary holographic recording medium, the signal of the reproduced light may still be obtained, if the framework may enable the reference beam to make small scale scanning of its direction and position, thus being able to obtain the reproduced light signals. However, for a holographic recording medium in continuous motion, it is rather difficult to obtain the reproduced light signals. 
         [0008]    In addition, another related prior art is disclosed in U.S. Patent Gazette Publication No. 6721076 and U.S. Pat. No. 6,909,529, wherein, an optical framework used for reflective type holographic recording medium is disclosed in detail. 
       SUMMARY OF THE INVENTION 
       [0009]    In view of the above-mentioned problems and drawbacks of the prior art, the invention discloses a holographic recording and reproduction system having servo optical path and a method for implementing the same, thus developing and providing a corresponding optical framework in facilitating a speedy and convenient data access and storage. 
         [0010]    Therefore, to achieve the above-mentioned objects, the invention provides a holographic recording and reproduction system having servo optical path, including: a holographic recording medium, a light source, a Spatial Light Modulator (SLM), a servo light source, and a servo beam guidance portion. Wherein, the light source is used to generate a signal beam and a reference beam, and the reference beam is used to be incident upon a holographic recording medium along a first direction; the SLM is located on the optical path of the signal beam, thus the signal beam is incident upon the holographic recording medium along a second direction, after it enters and exits the SLM, hereby interfering with the reference beam in producing a holographic interference fringe in the holographic recording medium, wherein, when the reference beam is incident again upon the holographic interference fringe along the first direction, a reproduction beam is produced, and it is incident upon an image sensor along the direction opposite to that of the optical path of the signal beam; a servo light source, that is used to generate a servo beam; and a servo beam guidance portion, wherein the servo light is incident upon a servo track of a holographic recording medium along the first direction by the servo beam guidance portion, and reflects a servo beam from the servo track, thus enabling the holograms to be recorded along the servo track and be recorded in the holographic recording medium, thus the beam formed by the servo beam includes that formed by the reference beam. 
         [0011]    In addition, the invention also discloses a servo optical path, and that is used in a holographic recording and reproduction system. Wherein, the holographic recording and reproduction system includes a light source, which can be divided into a reference beam and a signal beam, and each of which is incident upon a holographic recording medium respectively along a first direction and a second direction, thus generating a holographic interference fringe. In the above description, the servo optical path includes a servo light source and a servo beam guidance portion. Wherein, the servo light source is used to generate a servo beam; and the servo beam guidance portion guides the servo beam and reference beam to be incident upon a holographic recording medium along the same direction; the servo beam includes a reference beam. In addition, both the servo beam and the reference beam converge on a holographic recording medium by passing through an object lens. Wherein, the converged reference beam is incident upon a reflection layer of the holographic recording medium, while the converged servo beam is incident upon a reflection layer of the holographic recording medium and a servo track. As such, the servo beam is modulated by the servo track, and the servo beam is reflected by the servo track. 
         [0012]    In conclusion, the invention proposes a holographic recording and reproduction system having servo optical path, and that is utilized to enable the reference beam to keep incident upon the holographic recording medium along the first direction in cooperation with a servo mechanism, so that the holograms are recorded continuously along the servo track, and is recorded continuously in a holographic recording medium. For this purpose, a plurality of optical frameworks is proposed. In addition, several holographic multiplexing mechanisms can be utilized in each of these frameworks, hereby further raising the capacity of the holographic recording medium. 
         [0013]    Further scope of applicability of the invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The invention will become more fully understood from the detailed description given hereinbelow for illustration only, and thus is not limitative of the invention, and wherein: 
           [0015]      FIGS. 1A to 1D  are schematic diagrams of a holographic recording and reproduction system having servo optical path according to a first embodiment of the invention; 
           [0016]      FIG. 2A  is a perspective view of the track searching servo optical path of the invention; 
           [0017]      FIG. 2B  is a side view of the track searching servo optical path of the invention; 
           [0018]      FIG. 2C  is a top view of the track searching servo optical path of the invention; 
           [0019]      FIG. 3  is a schematic diagram of a holographic recording and reproduction system having servo optical path according to a second embodiment of the invention; 
           [0020]      FIG. 4  is a schematic diagram of a holographic recording and reproduction system having servo optical path according to a third embodiment of the invention; 
           [0021]      FIG. 5A  is a schematic diagram of a holographic recording and reproduction system having servo optical path according to a fourth embodiment of the invention; and 
           [0022]      FIG. 5B  is a schematic diagram of a holographic recording medium utilized in a holographic recording and reproduction system having servo optical path according to the fourth embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    The purpose, construction, features, and functions of the invention can be appreciated and understood more thoroughly through the following detailed description with reference to the attached drawings. 
         [0024]    In the following, the preferred embodiments of the invention will be described in detail together with the attached drawings. 
         [0025]    Firstly, refer to  FIGS. 1A to 1D  for a schematic diagram of a holographic recording and reproduction system having servo optical path according to a first embodiment of the invention. As shown in  FIGS. 1A to 1D , a light source  100  is used to emit coherence light, which is split into a reference beam  101  and a signal beam  103  upon passing through a first light guidance portion  200 , and that is used to guide the reference beam  101  to be incident upon a holographic recording medium  900  along a first direction. In the embodiment, the first direction is a direction perpendicular to the holographic recording medium  900 ; while the signal beam  103  is incident onto a spatial light modulator (SLM)  500  after splitting by the first light guidance portion  200 , so that upon subjecting to the modulation of the SLM  500 , the signal beam  103  is then guided by a second light guidance portion  300  and is incident upon the holographic recording medium  900  along a second direction. Furthermore, since the polarization state of the signal beam  103  is the same as that of the reference beam  101 , so that the reference beam  101  and the signal beam  103  interfere with each other in a recording layer  920  of the holographic recording medium  900 , thus producing a holographic interference fringe  800 , and it is recorded in the recording layer  920  of the holographic recording medium  900 . 
         [0026]    Moreover, when the reference beam  101  is incident again onto the holographic interference fringe  800  along the first direction, a reproduction beam  105  will be produced, and this reproduction beam  105  will be incident onto an image sensor  350  along the same optical path of the signal beam  103  yet in reverse direction, and it is interpreted by the image sensor. 
         [0027]    Furthermore, the holographic storage media  900  includes a first substrate  910 , a second substrate  930 , and a recording layer  920 . The recording layer  920  is formed between the first substrate  910  and the second substrate  930 , and is used to record light signal, such as a holographic interference fringe  800  and the like. The second substrate  930  is composed of a reflection layer  936 , a protection layer  932 , and a servo track  934 . Wherein, the bottom of the second substrate is the protection layer  932 , and is used to protect the bottom of the holographic recording medium from being damaged. The servo track  934  is formed on the second substrate  930 , so that upon being incident onto the servo track  934 , the light beam will be modulated by the servo track  934 , and the shape of the servo track will be described more clearly later. The reflection layer  936  is covered on the surface of the servo track  934 , so as to reflect the incident signal beam  103  and the reference beam  101 . 
         [0028]    In addition, a servo light source  400  is provided and is used to generate a servo beam  410 , which is incident onto the servo track  934  of the holographic recording medium  900  through a servo beam guidance portion  420  along a first direction. Thus, the servo beam  410  is modulated and reflected by the servo track  934 , and is received by a sensor portion  600 , so that the reference beam  101  may be incident onto the holographic recording medium  900  along the first direction, hereby recording sequentially the holographic interference fringe  800  in the recording layer  920  of the holographic recording medium  900  along the servo track  934 . 
         [0029]    Wherein, the first light guidance portion  200  splits the light beam coming from the light source  100  into the signal beam  103  and the reference beam  101 , and guides the reference beam  101  to be incident onto the holographic recording medium  900  along the first direction. As such, firstly, a first polarizing plate  210  is placed in front of the light source  100 , so as to gather the light of the light source  100  and obtain a polarized light having a specific polarization, namely, a linearly polarized light. In case that the polarization direction of the linearly polarized light is parallel to the surface of the page, then it is referred to as in a P polarization state, and in case that the polarization direction of the linearly polarized light is perpendicular to the surface of the page, then it is referred to as in an S polarization state. Thus, upon passing through the first polarizing plate  210 , the light beam coming from the light source  100  is converted into a linearly polarized light in P polarization state, and that is split by a light splitter  220  into the signal beam  103  and the reference beam  101 , and both are in a P polarization state. 
         [0030]    A second light guidance portion  300  is provided, and is used to guide the signal beam  103  modulated through the SLM  500  to be incident onto the holographic recording medium  900  along the second direction. Through the application of a lens and a set of reflection mirrors  310 , the direction of transmission of the signal beam  103  is changed after passing through the lens and the set of reflection mirrors  310 , and thus it is transmitted and incident onto the holographic recording medium  900  along the second direction. A pin hole  320  is placed between the lens and the set of reflection mirrors  310 , and that is used to filter out the miscellaneous light beams other than the signal beam  103  and the reproduction beam  105 . As such, the reference beam  101  and the signal beam  103  are guided through the first light guidance portion  200  and the second light guidance portion  300  to be incident onto the recording layer  920  of the holographic recording medium  900  respectively along the first and second directions. Since the polarization states of the reference beam  101  and the signal beam  103  to be incident onto the holographic recording medium  900  are identical, thus the reference beam  101  and the signal beam  103  will interfere with each other to produce a holographic interference fringe  800  and store it onto the recording layer  920 . 
         [0031]    Moreover, an object lens  230  is provided in the first light guidance portion  200 , and that is used to converge and focus the reference beams  101  onto the reflection layer  936  of the holographic recording medium  900 , so that the reference beam  101  may be reflected from the reflection layer  936  along the original route. 
         [0032]    In implementing the reproduction system of the holographic recording and reproduction system, the direction of the transmission route of the reference beam  101  is opposite to that of the reference beam  101  while performing the data recording, so that in the reproduction process, when the reference beam  101  passing through the holographic interference fringe  800  of the recording layer  920 , a conjugate reproduction beam  105  is produced, which is transmitted in the same route as that of the original signal beam  103  yet in an opposite direction. Therefore, in order to obtain the conjugate reproduction beam  105 , a first phase delay sheet  250  is added to the first light guidance portion  200 , and a second phase delay sheet  330  is added to the second light guidance portion  300 . The first phase delay sheet  250  is placed at one side of the beam splitter  220 , so that when a part of the reference beam  101  is incident upon the first phase delay sheet  250 , its polarization state is changed to S polarization state and is referred to as the right reference beam  101 ; while the other part of the reference beam  101  which has not passed the first phase delay sheet  250  still maintains its P polarization state, and is referred to as the left reference beam  101 . The second phase delay sheet  330  of the second light guidance portion  300  is disposed in the optical path of the signal beam  103 , so that the signal beam  103  will be first incident upon and exits the SLM  500 , and then be incident upon the second phase delay sheet  330 , such as a half wave plate (½λ wave plate), thus its polarization is changed from a P polarization state to an S polarization state. 
         [0033]    The right reference beam  101  obtained by passing through the first phase delay sheet  250  having S polarization state as its polarization state, and upon to be incident onto the reflection layer  936  of the holographic recording medium  900 , it will be reflected to the opposite direction and exit the holographic recording medium  900 ; while the signal beam  103  is incident upon the holographic recording medium  900  along the second direction. When the signal beam  103  having S polarization and the right reference beam  101  having S polarization intercept each other, an interference phenomenon would occur to produce a holographic interference fringe  800 , and it is stored in a recording layer  920  of the holographic recording medium  900 , as shown in  FIG. 1B . 
         [0034]    The left reference beam  101  not passing through the first phase delay sheet  250  still keeps its P polarization state as its polarization state, and when it is similarly incident upon the reflection layer  936  of the holographic recording medium  900 , it will be reflected and exits the holographic recording medium  900  in an opposite direction, meanwhile, the signal beam  103  is still incident upon the holographic recording medium  900  along the second direction. As such, the signal beam  103  and the left reference beam  101  intercept each other. However, since the left reference beam  101  is in the P polarization state and the signal beam  103  is in the S polarization state, the respective two polarizations are perpendicular to each other, thus the interference phenomenon would not occur, as shown in  FIG. 1C . 
         [0035]    Therefore, when it is desired to generate light beam in the holographic recording and reproduction system, and in case that the left reference beam  101  not having passed the first phase delay sheet  250  is incident upon the holographic recording medium  900 , it will pass through the holographic interference fringe  800  and the reflection layer  936 , and it will then be reflected by the reflection layer  936  to the opposite direction and again pass through the holographic interference fringe  800  and exits the holographic recording medium  900 . Since the transmission route of the left reference beam  101  is opposite to that of the right reference beam  101  while recording signal, thus a conjugate reproduction beam  105  is produced, and it will return along the original route of the signal beam  103 . Due to the fact that the conjugate reproduction beam  105  is produced when the left reference beam  101  is incident upon the holographic interference fringe  800 , the conjugate reproduction beam  105  is also in a P polarization state. In addition, when the conjugate reproduction beam  105  returns along the original route of the signal beam  103 , it will pass through the second phase delay sheet  330 , thus its polarization is changed to the S polarization, and when it is incident upon a first polarized beam splitter  340 , it will be reflected by the first polarized beam splitter  340  because of the S polarization state of the conjugate reproduction beam  105 . A two dimensional image sensor  350  is placed on one side of the first polarized beam splitter  340  and in the reflection direction of the conjugate reproduction beam  105 , and it is used to receive and interpret the conjugate reproduction beam  105 , as shown in  FIG. 1D . 
         [0036]    Moreover, in the first embodiment of the invention, the holographic recording and reproduction system further includes a servo optical path, that is used to enable the holographic recording and reproduction system to achieve the speedy track search servo function. Wherein, the optical path includes a servo light source  400  and a servo beam guidance portion  420 . The servo light source  400  is used to generate a servo beam  410 , and the wavelength of the servo beam  410  is different from that of the signal beam  103  and the reference beam  101 . 
         [0037]    The servo beam guidance portion  420  includes a first Dichroic Prism  440 , a second polarizing plate  470 , and a second polarized beam splitter  430 . Wherein, the second polarized beam splitter  430  is disposed between the beam splitter  220  and the first phase delay sheet  250 . The first Dichroic Prism  440  is used to separate light beams of different wavelengths. Therefore, the reference beam  101  may transmit through the first Dichroic Prism  440  unaffected. However, when the servo beam  410  is incident upon this first Dichroic Prism  440 , it can not transmit through and is totally reflected. Thus, the first Dichroic Prism  440  is placed between the beam splitter  220  and the second polarized beam splitter  430 , and the servo light source  400  is disposed on one side of the first Dichroic Prism  440 . When the servo beam  410  generated by the servo light source  400  is first incident upon the second polarizing plate  470 , so that the servo beam  410  is made into a servo beam having a specific polarization state, such as P polarization state, and thus when it is incident again onto the first Dichroic Prism  440 , it will be reflected and change its direction and is incident onto a servo track  934  of the holographic recording medium  900 . Yet before the servo beam  410  is incident upon the holographic recording medium  900 , it must first pass through the second polarized beam splitter  430 , so that only the servo beam  410  having P polarization state can get through, and a part of the servo beam  410  having P polarization will pass through the first phase delay sheet  250  and changes its polarization to S polarization, and is referred to as the right servo beam  410 ; while the other part of the servo beam  410  not passing through the first phase delay sheet  250  will maintain its P polarization, and is referred to as the left servo beam  410 . When the right servo beam  410  is incident upon and is reflected by the servo track  934  of the holographic recording medium  900 , it will exit the holographic recording medium  900  and then it is incident upon a second polarized beam splitter  430 , thus the right servo beam  410  becomes a right servo beam of S polarization, and it will be reflected by the second polarized beam splitter  430  and change its direction and is incident upon a servo beam sensor  630  of a sensor portion  600 . On the other hand, the left servo beam  410  not passing through the phase delay sheet  250  will be incident upon the servo track  934  of the holographic recording medium  900 , and it will likewise be reflected by the servo track  934  and exits the holographic recording medium  900 , and upon passing through the first phase delay sheet  250 , the left servo beam  410  will change its polarization to S polarization. Thus, when the left servo beam  410  is incident upon the second polarized beam splitter  430 , it will be reflected by the second polarized beam splitter  430 , change its direction, and be incident upon a servo beam sensor  630  of a sensor portion  600 . As such, both the left servo beam and the right servo beam  410  will be reflected by the second polarized beam splitter  430  and incident upon the servo beam sensor  630  of the sensor portion  600 , and that is utilized to detect and examine the servo beam  410  modulated by the servo track  934 , and then the servo beam  410  is converted into an electric signal for transmitting to a control device (not shown). This control device is used to move an optical framework or a holographic recording medium  900 , so that the holographic interference fringe  800  can be read from and/or recorded into a recording layer  920  of the holographic recording medium  900  sequentially along a servo track  960 , as shown in  FIGS. 1A and 1B . 
         [0038]    Moreover, in the first embodiment of the invention, the left reference beam  101  not passing through the first phase delay sheet  250  is reflected by the reflection layer  936  of the holographic recording medium  900  and the left reference beam  101  is incident upon the first phase delay sheet  250  with its polarization being changed into S polarization. Thus, when the left reference beam  101  is incident again upon the second polarized beam splitter  430 , it will be reflected, change its direction, and incident upon the sensor portion  600 . In order that the reference beam  101  will not be incident upon the servo beam sensor  630  together with the servo beam  410  thus affecting the reading and analysis of the servo beam  410 , a wavelength filter  620  is provided in the sensor portion  600 , so that the beams having the wavelength of servo beam  410  are allowed to pass through, and the beams having the wavelength of the reference beam  101  are filtered out and can not pass through. 
         [0039]    In addition, refer to  FIGS. 2A to 2C , which show respectively the perspective view, the side view, and the top view of the track searching servo optical path of the invention. When the reference beam  101  is incident onto the holographic recording medium  900  via the first light guidance portion  200  along the first direction, the servo beam  410  is also incident onto the holographic recording medium  900  along the first direction via the servo beam guidance portion  420 . Wherein, the servo beam  410  includes the reference beam  101 . Therefore, when the servo beam  410  and the reference beam  101  both pass and are converged through the object lens  230 , the light spot formed by the converged servo beam  410  is larger than the light spot as formed by the converged reference beam  101 , and that the light spot of the servo beam  410  includes the light spot of the reference beam  101 . In this embodiment, as shown in  FIGS. 2A to 2C , the reference beam  101  and the servo beam  410  can be coaxial beams, that are incident onto object lens  230  and are converged and focused on its optical axis, thus forming coaxial light beams, and are realized as the concentric light spots on the holographic recording medium  900 . 
         [0040]    Furthermore, the light spot of the reference beam  101  is converged on the even area  9341  of the groove of the holographic recording medium, thus it will not be affected by the servo track  934  to cause scattering. However, the light spot of servo beam  410  may irradiate on the areas outside the even area  9341  of the groove, and it may even irradiate on the groove edge  9342  having a specific shape. As such, it may be modulated by the servo track  934 , and thus enabling the holographic interference fringe  800  be recorded onto the holographic recording medium  900  sequentially along a servo track  934 . 
         [0041]    However, in this embodiment, the groove having wobbling edges is taken as an example of the servo track  934 , as shown in  FIG. 2C . The grooves having specific edge shape are formed in a second substrate  930  of the holographic recording medium  900 : The groove is provided with an even area  9341  and two side groove edges  9342 . Wherein, the groove edge  9342  is of a shape, for example, a wobbling shape of a groove edge or a pre-pit shape of groove edge, thus forming the servo track  934  of the holographic recording medium  900 , and wherein includes the signal for modulating the servo beam  410 . As to the method of encoding or modulating the servo beam  410  by making use of the above-mentioned servo track  934 , usually, the coding is realized as based on the variations and combinations of the various edge shapes by making use of the wobbling shape groove edge or pre-pit shape groove edge that can be irradiated by the servo beam  410 . As such, the signals of various frequencies can be obtained such as high and low frequencies, thus enabling the modulation of the servo beam  410  and achieving the speedy track search in cooperation of the recording position of the holographic interference fringe  800 . 
         [0042]    In this embodiment, the SLM  500  utilized is a transmissive type spatial light modulator, such as a transmissive type liquid crystal panel. 
         [0043]    Subsequently, refer to  FIG. 3  for a schematic diagram of a holographic recording and reproduction system having servo optical path according to a second embodiment of the invention. The structure of the second embodiment is similar to that of the first embodiment, thus it will not be repeated here for brevity. However, the major difference between the second embodiment and the first embodiment is lying in that, in the second embodiment, a second polarized beam splitter  430  of a servo beam guidance portion  420  is not placed between the beam splitter  220  and the first phase delay sheet  250  as it does in the first embodiment. Instead, a first Dichroic Prism  440  is disposed between the beam splitter  220  and the first phase delay sheet  250 , and a second polarized beam splitter  430  is placed on one side of the first Dichroic Prism  440 . 
         [0044]    Therefore, in the second embodiment of the invention, when a servo beam  410  is generated by a servo light source  400 , it is first incident into a second polarized beam splitter  430  and is polarized by the second polarized beam splitter  430  to form a servo beam  410  of P polarization, and then it is incident upon a first Dichroic Prism  440 , is reflected by it, and then changes the direction of the servo beam. Similarly, a part of the servo beam  410  will pass through the first phase delay sheet  250  and changes its polarization into S polarization, and is referred to as the right servo beam  410 ; while the part of servo beam  410  not passing the first phase delay sheet  250  will keep its P polarization, and is referred to as the left servo beam  410 . Then, when the right servo beam  410  is incident upon the servo track  934  of the holographic recording medium  900 , the right servo beam  410  is reflected by the servo track  934  and exits the holographic recording medium  900 , and then it is incident upon a first Dichroic Prism  440 , is reflected by it and changes direction and incidents again onto the second polarized beam splitter  430 . In addition, the right servo beam  410  changes its polarization into S polarization. Thus, the right servo beam  410  will be reflected by the second polarized beam splitter  430 , changes its direction and is incident onto the servo beam sensor  630  of a sensor portion  600 . On the other hand, the left servo beam  410  not passing through the first phase delay sheet  250  is incident onto the servo track  934  of the holographic recording medium  900 , and it will be reflected by the servo track  934  and exits the holographic recording medium  900 . After passing through the first phase delay sheet  250 , the left servo beam  410  changes its polarization to S polarization. Subsequently, the left servo beam  410  is incident upon a first Dichroic Prism  440  and is reflected by it and changes direction to be incident upon the second polarized beam splitter  430 , and then the left servo beam  410  is reflected and changes its direction, and is incident upon the servo beam sensor  630  of a sensor portion  600 . As such, the servo optical path of the embodiment is similar to that of the first embodiment in that, the servo beam  410  to be detected can be induced into the servo beam detector  630 , and the servo beam  410  modulated by the servo track  934  can be converted into an electric signal and is transmitted to a control device (not shown). This control device is used to move an optical framework or the holographic recording medium  900 , so that the holographic interference fringe  800  can be read from and/or recorded into a recording layer  920  of the holographic recording medium  900  sequentially along a servo track  960 . 
         [0045]    Moreover, refer to  FIG. 4  for a schematic diagram of a holographic recording and reproduction system having servo optical path according to a third embodiment of the invention. The framework of this embodiment is similar to that of the first embodiment, thus it will not be repeated here for brevity. However, the major difference between this embodiment and the first embodiment is lying in that, in this second embodiment, the wavelength filter  620  is not disposed in the sensor portion  600  as it does in the first embodiment. Instead, a second Dichroic Prism  640  is provided. As such, when the reference beam  101  is incident upon the holographic recording medium  900 , the left reference beam  101  not passing through the first phase delay sheet  250  is reflected by a reflection layer  936  of the holographic recording medium  900 , and then it is incident upon the first phase delay sheet  250 , hereby changing its polarization to S polarization, thus the left reference beam  101  is incident on a second polarized beam splitter  430  and is reflected and changes its direction, and then is incident on the second Dichroic Prism  640  of the sensor portion  600 . As such, the reference beam  101  is unaffected by the second Dichroic Prism  640 , and may be incident directly onto a reference beam sensor  610 . Then, the left reference beam  101  is then converted into an electric signal and is transmitted to another control device (not shown). On the other hand, the right reference beam  101  passing through the first phase delay sheet  250  is incident on and is reflected by a reflection layer  936  of the holographic recording medium  900 , and it may also be incident directly onto the second polarized beam splitter  430 . Since the right reference beam  101  changes its polarization to S polarization after passing through the first phase delay sheet  250 , thus it may also reflected by the second polarized beam splitter  430 , and then changes its direction and is incident onto the second Dichroic Prism  640  of a sensor portion  600 , thus being able to incident directly onto the reference beam sensor  610 . Therefore, both the left reference beam and the right reference beam  101  may be reflected via the second polarized beam splitter  430 , be incident onto the reference beam sensor  610  of the sensor portion  600 , and then transmitted to a control device, so that when the reference beam  101  is on, the control device is able to conduct analyses based on the signal received, and make more accurate adjustment of the relative positions and inclination angles of the holographic recording and reproduction system and the holographic recording medium  900 . 
         [0046]    In the above-mentioned embodiments, the holographic recording and reproduction system having servo optical path can be realized in cooperation with various multiplex mechanisms, such as angle multiplexing, and peristrophic multiplexing, hereby raising the storage capacity of the invention. 
         [0047]    In addition, the system of the invention may be realized not only in the reflective type holographic recording medium, but also in transmissive type holographic recording medium, and the details of which are given as follows. 
         [0048]    Refer to  FIGS. 5A and 5B  for a schematic diagram of a holographic recording and reproduction system having servo optical path and a schematic diagram of holographic recording medium respectively according to a fourth embodiment of the invention. As shown in  FIGS. 5A and 5B , in the implementation of data storage of the holographic recording and reproduction system of this embodiment, a signal beam  103  and a reference beam  101  are incident onto a transmissive type holographic recording medium  900   a  in a first direction and a second direction respectively, as such the signal beam  103  and the reference beam  101  interfere with each other in a recording layer  920 , hereby producing a holographic interference fringe  800 , and then record it in the recording layer  920 . When the data reproduction is proceed, only the reference beam  101  is used to be incident onto the position of the holographic interference fringe  800 , then the reproduction beam will pass through the recording layer  920  and a lens located below the transmissive type holographic recording medium  900   a  along the first direction, and then is incident on a 2-dimensional image sensor  350  placed below the lens. 
         [0049]    In this embodiment, the transmissive type holographic recording medium  900   a  includes a first substrate  910 , a second substrate  930 , and a recording layer  920 . Wherein, the second substrate  930  is provided with a wavelength selection film  938 , in addition to a servo track  934  and a protection layer  932 . Similarly, the reference beam  101  and the signal beam  103  may be interference with each other, produce and record a holographic interference fringe  800  in the recording layer  920 . 
         [0050]    In the above description, a wavelength selection film  938  is placed on a groove and a servo track  934 . This wavelength selection film  938  is used to selectively transmit a light beam of a specific wavelength range, and light beams of other wavelength ranges are all reflected. Therefore, in this embodiment, when the reference beam is incident on the transmissive type holographic recording medium  900   a , it is able to transmit through the wavelength selection film  938  and the transmissive type holographic recording medium  900   a . However, the servo beam  410  will be reflected by the wavelength selection film  938  and exit the transmissive type holographic recording medium  900   a , and then is incident upon the servo beam sensor  630  through the servo beam guidance portion  420 , as mentioned in the second embodiment. 
         [0051]    The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.