Patent Publication Number: US-2010124158-A1

Title: Holography with interference revival

Description:
TECHNICAL FIELD 
     The technical field relates generally to optical systems and, more particularly, to a hologram recording apparatus. 
     BACKGROUND 
     An optical system such as a hologram recording apparatus can include a laser source for emitting an object beam and reference beam, which are radiated onto a hologram-recording media at the same position. The object beam and the reference beam interfere with each other inside the media to form a diffraction pattern or so called microhologram at the irradiation point for recording data on the media. 
     When the media on which the data is already recorded is irradiated with the reference light, diffracted light (reproducing light) is generated by the microhologram formed in the recording process. The reproducing light includes data superimposed on the object beam in the recording process. Therefore, the recorded signal can be reproduced by receiving the reproducing light with a photo-sensitive element such as a photodiode. 
     High interference between the signal light and the reference beam, or generally two or more light beams, is important for generating holography. The level of interference between the two or more light beams will depend upon the coherence of the two or more light beams. 
     In one exemplary configuration, the laser source can be a semiconductor laser diode for emitting the laser beam and reference beam. However, the semiconductor laser diode has the drawback of low coherence due to the multimode associated with the laser beam, and thereby lower interference. 
     In another exemplary configuration, the laser source of the optical system can be an external cavity laser including a laser diode emitting a laser beam which is collimated by a collimating lens, and radiated onto a reflective diffraction grating so that the laser beam is a single mode beam, thereby having higher coherence. However, a change in the current supplied to the laser diode or generally a change in the diode temperature can result in mode hop, which is a transition between single modes of the laser beam. A conventional external cavity can include an interferometer and detectors for periodically monitoring for the occurrence of mode hop. However, such objects can increase the manufacturing costs of the external cavity laser as well as enlarging the structure. 
     Hologram recording apparatus can write data on the entire volume of a media such as a disc of 1 mm in thickness. Taking advantage of the bit-by-bit holography method where each single microhologram represents a single data bit and can be made as small in size as the data bits in a Blu-ray disc technology, data can possibly be stored in, for example, 100 layers of a disc. In comparison, data is only stored in two layers of a Blu-ray disc. Despite this potential improvement in storage capacity, hologram recording apparatus have been limited to niche markets such as data storage because of, for example, the high costs associated with the external cavity laser and the complexity of the optical system as discussed above. That is, an optical system including a compact, low cost laser source that could feasibly be made for the general consumer market has not yet been realized. 
     SUMMARY 
     Accordingly, an optical system according to various novel embodiments includes a laser source for generating a laser beam; an optical subsystem for splitting the laser beam into a reference beam and an object beam; and first and second objective lens for receiving the reference beam and object beam, respectively, and focusing the reference and object beams at a focal point on media at which the reference beam and object beam interfere with each other. The reference beam and the object beam have first and second optical path lengths defined from the laser source to the focal point of the media. The optical subsystem is positioned so that an optical path length difference between the first and second optical path lengths is substantially equal to an integer multiple of an interference revival period associated with the reference beam and the object beam. The interference revival period is associated with peak values of visibility of the interference versus the optical path difference. 
     The laser source can be merely a semiconductor laser diode generating a multimode laser beam. Alternatively, the laser source can be an external cavity laser including a multimode laser diode source for generating a multimode laser beam and a grating for outputting a single mode of the light associated with the laser beam. 
     An optical system according to the various novel embodiments can implement a hologram recording apparatus which can write data on the entire volume of a media at a lower cost due to, for example, use of a semiconductor laser diode as the laser source, or a lower cost external cavity laser. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an exemplary Michelson interferometer for generating two light beams. 
         FIGS. 2A-2B  are illustrations of exemplary interference fringes for different angles and period created by interference of the two light beams in the sensor plane. 
         FIG. 2C  is a schematic diagram of overlapping beams. 
         FIG. 3  is a graph representing the visibility of the interference fringes versus path difference of the travel of two light beams. 
         FIG. 4  is an illustration of interference revival. 
         FIG. 5  is a schematic diagram illustrating an exemplary laser source of an optical system according to an exemplary embodiment. 
         FIG. 6  is a schematic diagram illustrating an optical system according to the exemplary embodiment. 
         FIG. 7  is a schematic diagram illustrating an optical system according to another exemplary embodiment. 
         FIG. 8  is a schematic diagram illustrating an optical system according to another exemplary embodiment. 
         FIGS. 9A-9C  are schematic diagrams illustrating the variable path difference caused by movement of an optical element of the optical system according to the exemplary embodiments. 
         FIGS. 9D-9I  are schematic diagrams illustrating the optical system according to the exemplary embodiments. 
         FIG. 10  is a schematic diagram illustrating an optical system according to another exemplary embodiment. 
         FIG. 11  is a schematic diagram of an external cavity laser of the optical system of  FIG. 10 . 
         FIG. 12  is an illustration of the interference revival for the external cavity laser. 
     
    
    
     DETAILED DESCRIPTION 
     Various embodiments of an optical system utilizing holography to read and/or record data on a media will be discussed. The optical system reads and/or records data by generating two or more laser beams which generate interference at the media. The optical system can utilize an interference revival effect so that data is successively read from the media and/or recorded on the media even if a path difference between the two or more laser beams is greater than a coherence length of the two or more laser beams. The terms coherence and interference revival will be introduced below. 
     The concept of coherence is linked to a phase relationship between two points of an optical field separated either in time or space. The optical field is generated by an optical source of some finite dimension. That is, the optical field generally includes a large amount of atoms, each of which can be considered as a photon source. Inside a laser source there is always a high degree of phase correlation between the photons emitted by two atoms of the active region because the emission is mainly stimulated by other photons. Nonetheless, the correlation decreases with distance and time. The coherence length is the extent in space over which the electric field remains correlated or over which the field phase can be predicted. The coherence time is the time needed by a wavetrain to travel a distance equal to the coherence length. 
     The coherence length is proportional to the coherence time by the speed of light in a vacuum. On the other hand the coherence of a source is tightly linked to the source frequency bandwidth. For example, when the electric field emitted from the source is envisioned as the superposition of wavetrains with a certain duration, the very finite nature in time of these wavetrains is responsible for their infinite spectrum. The bandwidth of this spectrum is inversely proportional to the coherence time, which is the duration of the wavetrain. The proportionality constant depends on the shape of the wavetrain and on the criterion chosen to define the bandwidth, but the above is a general principle that applies to any source. Accordingly, the coherence of a laser source is an indication of the laser bandwidth. 
     The coherence of a laser source can be measured by a Michelson interferometer such as the exemplary Michelson interferometer  100  shown in  FIG. 1 . The Michelson interferometer  100  includes a laser source  102  for generating a laser beam  104  and a beam separator  106  for separating the laser beam into two beams  108 ,  110  that walk different paths. The interferometer  100  also includes first and second mirrors  112 ,  114  for reflecting the two beams  108 ,  110  back to the beam separator  106  to be rejoined. The rejoined beams are reflected by the separator  106  and the reflected beam  116  is received at a sensor  118 . The phase difference between the two beams  108 ,  110  is proportional to the path difference if it is smaller or comparable to the coherence length. Otherwise the phase of the two beams  108 ,  110  becomes uncorrelated.  FIGS. 2A-2B  show the interferences in the plane of the sensor  118 . The interference creates interference fringes that become very visible when the phases of the two beams are correlated, that is, when the path difference is smaller than the coherence length. The interference fringes on the sensor plane have an angle and period linked to the mutual position of the two beams  108 ,  110 . These beams  108 ,  110  can be slightly misaligned in order to form fringes clearly visible. The angle of the fringes is normal to the axis joining the two beams  108 ,  110  and the fringe period is inversely proportional to the beam separation. That is, the more the two beams  108 ,  110  overlap the larger the fringes will be until only one large fringe is visible. An example of overlapping beams is shown in  FIG. 2C . 
     A quantitative measure of the quality of the fringes in an interference pattern can be expressed by visibility, which is a measure of the degree of coherence of the two beams in the Michelson interferometer  100 . The visibility function can be measured by moving one arm (or one mirror) of the interferometer  100 . The resulting visibility function is the bell shaped curve shown in  FIG. 3 . The visibility is greatest when the path difference between the two beams is zero and degrades afterwards. Thus, a measure of the coherence length, which will be referred to as Full Width at Half Maximum (FWHM), can be defined as the distance traveled to reduce the visibility by 50% from its greatest value. 
     Thus, to ensure the interference of the two or more beams inside the media, the coherence length should be larger than an optical path difference in all operating conditions. However, for an optical system using the semiconductor laser diode, the coherence length will be very short. For example, the laser diode currently used in Blu-ray or DVD recording has a short coherence length that is usually less than 600 micrometers, which is not sufficiently long enough to provide a feasible optical system. 
     An effect referred to here as interference revival observed when the path difference is much higher than the coherence length and the laser is a multimode source will be introduced. In a Michelson interferometer setup, increasing the path difference steadily decreases the visibility of the fringes until no fringes are visible. However, after a travel of a few mm there will be again fringes forming on the sensor plane. This effect is the interference revival. As shown in  FIG. 4 , the fringe visibility, which is a measure of the degree of coherence, of a laser diode used for Blu-ray media undergoes periodic revivals with a constant step. The constant step will be referred to here as the interference revival period. 
     The coherence region is the width of the peak at the minimum allowed visibility. The size of the coherence region in any revival peak will be referred to here as the Peak Coherence Length (PCL). For example, the PCL of the laser diode shown in  FIG. 4  is as large as approximately 0.212 mm. In comparison, the PCL of the external cavity revival shown in  FIG. 12  is approximately 3.3 mm. Generally, the PCL of the semiconductor laser diode is always much smaller than the PCL of an external cavity laser. 
     An optical system, according to various embodiments, utilizes the periodic revival to permit the optical path difference to be greater than the short coherence length of, for example, Blu-ray or DVD laser light. Generally, the optical path difference between the two beams when focused in the mid layer can be set at substantially the interference revival period (or at an integer multiplier). 
     The holographic media should allow permanent refraction index changes depending on the intensity of the electric field of the light so that the holograms can be recorded. Data reading can be performed using the same light at lower intensity to avoid overwriting and observing the reflected light. 
     Referring to  FIGS. 5-6 , an optical system for reading and recording data on a media according to an exemplary embodiment will be discussed. A bit-by-bit recording media associated with a holographic architecture in which the beams travel in opposite directions, or volume holography in which the beams travel in the same direction or at a certain angle can be used. 
     Referring to  FIG. 5 , an exemplary laser source for the optical system will be discussed. In this embodiment, the laser source is an external cavity laser  500  which includes a laser diode  502  such as, for example, a semiconductor blue laser diode for generating a multimode laser beam  504 , a collimator lens  506  which converts the divergent light of the laser beam  504  into collimated light  508 , and a grating  510  capable of diffracting the incoming light over a finite number of discrete angles and partially reflecting back only the selected laser line thus producing single mode light  512 . Alternatively, the laser source can be merely the laser diode  502  and collimator lens  506  for generating the multimode light. The laser source is not limited to the semiconductor blue laser diode. Alternatively, the laser source can include semiconductor laser diodes of any wavelength. 
     Referring to  FIG. 6 , an optical subsystem  600  for the optical system according to the exemplary embodiment will be discussed. Incoming laser light  602 , which may be multimode or single mode, is split by a beam splitter  604  into a reference beam  606  and an object beam  608 . The beam splitter  604  may be, for example a polarization insensitive 50:50 beam splitter. A first mirror  610  reflects the reference beam  606  towards a second mirror  612 , which reflects the reference beam  606  to a first objective lens  614 . A third mirror  616  reflects the object beam  608  to a second objective lens  618 . The first and second objective lens  614 ,  618  respectively focus the reference and object beams at a focal point on the media  620  at which the reference beam  606  and object beam  608  interfere with each other. The first and second lens  614 ,  618  can be coupled to first and second actuators  622 ,  624 , which are configured to independently move the lens to focus on a correct layer inside the media  620  by, for example, a controlling device such as a processor. 
     The reference beam  606  and the object beam  608  have first and second optical path lengths defined from the laser source to the focal point of the media. For example, excluding the path between the laser source and the beam splitter  604 , the path of the object beam  608  is A+D 0  and the path of the reference beam  606  is B+C+D 1 . Generally, the optical subsystem  600  is positioned so that an optical path length difference between the first and second optical path lengths is substantially equal to an integer multiple of an interference revival period associated with the reference beam  606  and the object beam  608 . As discussed above, the interference revival period is associated with the constant steps of peak values of visibility of the interference versus the optical path difference. 
     Assuming that the laser source is merely the laser diode  502  and collimator lens  506  for generating the multi-mode light, the interference revival period will be equal to 2nL, wherein n is the refraction index of the diode material and L is the length of the diode. For a Blu-ray laser diode, the interference revival period is approximately 4260 micrometers as shown in  FIG. 4 . In this case, the path difference should be approximately either 4260 micrometers, an integer multiple of 4260 micrometers or zero. 
     Assuming that the laser source is the external cavity laser  500  such as the external cavity laser  500  shown in  FIG. 5 , the interference revival period will equal n 1 *L 1 +L 2 +n 2 *L 3 +L 4 , wherein n 1  is the refraction index of the diode material, n 2  is the refraction index of the collimator lens, L 1  is the length of the diode material, L 2  is the distance between the laser facet and the lens first surface, L 3  is the thickness of the collimator lens  506 , and L 4  is the distance between the lens second surface and the grating  510 . For an exemplary external cavity laser, the interference revival period is 36.55 mm as shown in  FIG. 12 . 
     Referring to  FIG. 7 , an optical subsystem  700  for the optical system according to another exemplary embodiment will be discussed. Incoming laser light  702 , which may be multimode or single mode, is reflected by a first mirror  704 . The reflected light from the first mirror  704  can be the reference beam  706 . The reference beam  706  passes through a first lens  712  and is focused at a focal point on the media  716 . The reference beam  706  passes through the media  716  and a second lens  714  to a second mirror  708 , and is reflected back by the second mirror  708  to form the object beam  710 . The first and second lens  712 ,  714  respectively focus the reference and object beams at a focal point on the media  716  at which the reference and object beams interfere with each other to create microholograms. The microholograms previously written can be read by, for example, directing only the reference beam  706  at the focal point while blocking the object beam  710 . The first and second lens  712 ,  714  can be coupled to first and second actuators  718 ,  720 , which are configured to independently move the lens to focus on a correct layer inside the media  716  by, for example, a controlling device such as a processor. 
     Similarly to the optical system  600 , the reference beam  706  and the object beam  710  have first and second optical path lengths defined from the laser source to the focal point of the media. The path lengths of the reference and object beams  706 ,  710  are common up to the focal point inside the media  716 . However, a path difference between the two beams is present from the focal point of the media  716  to the second mirror  708  and back to the focal point of the media  716 . Thus, the optical path length difference is equal to 2D, wherein D is the distance between the second mirror  708  and the focal point of the media  716 . The second mirror  708  is preferably disposed a predetermined distance D from the focal point so that the optical path length difference between the first and second optical path lengths is substantially equal to the integer multiple of the interference revival period. For example, if the distance D from the focal point to the surface of the second mirror  708  is an integer multiplier of the interference revival half-period, preferably one half of the revival period, the optical path difference will be an integer multiple of the interference revival period. This will allow even in this architecture with an intrinsic unavoidable path difference the use of a system with short coherence. 
     Referring to  FIG. 8 , an optical subsystem  800  for the optical system according to another exemplary embodiment will be discussed. Incoming laser light  802 , which may be multimode or single mode light is reflected by a first mirror  804 . The reflected light from the first mirror  804  can be the reference beam  806 . The reference beam  806  is reflected back by a retroreflector  808  to form the object beam  810 . First and second lens  812 ,  814  respectively focus the reference and object beams at a focal point on the media  816  at which the reference and object beams interfere with each other. The first and second lens  812 ,  814  are coupled to first and second actuators  818 ,  820 . The actuators  818 ,  820  can be configured to independently move the lens to focus on a correct layer inside the media  816  by, for example, a controlling device such as a processor. 
     Similarly to the second mirror  708  of the above embodiment, the retroreflector  808  is disposed a predetermined distance from the focal point so that the optical path length difference is substantially equal to the integer multiple of the interference revival period. For example, the distance D from the focal point to the retroreflector  808  can exactly match an integer multiplier of the interference revival half-period, preferably one half of the revival period. 
     Generally, as discussed above, the optical subsystem according to the above embodiments is positioned so that an optical path length difference between the first and second optical path lengths is substantially equal to an integer multiple of an interference revival period associated with the reference and object beams. However, a certain coherence length may still be needed to compensate for a variable optical path difference (Pvar) relevant to the many writing layers of the media. The variable optical path difference which occurs along with movement of the lens and optical element of the optical system is illustrated in  FIGS. 9A-9C . First and second lens respectively focus a reference beam and an object beam reflected from a mirror or retroreflector (optical element) at a focal point on the media.  FIGS. 9A-9C  illustrate the focal point on a lower layer, middle layer and upper layer of the media. In a most extreme case ( FIG. 9C ), the variable optical path difference (Pvar) will be equal to 2nT, wherein n is the refraction index of the media and T is the thickness of the media. 
     The optical system can sufficiently read or write to all layers of the media as long as the PCL of the revival peak is greater than Pvar for the most extreme case. Such an optical system is illustrated in  FIGS. 9D-9E . The optical system  900  includes a first objective lens  902  for focusing the reference beam  904  on a focal point of the media  906 . An optical element  908 , which is preferably a mirror or retroreflector, reflects an object beam  910  onto a second objective lens  912 , which focus the object beam  910  on a focal point of the media  906 . As shown in  FIG. 9E , the first and second objective lens  902 ,  912  move to focus the object beam  910  on a different layer of the media  906 . The maximum travel (Δ) of the lenses is linked to the numerical aperture (NA) of the lenses themselves, the thickness (Y) of the media  906 , and the refraction index (n) of the holographic media  906  by the following formula: Formula 1 
     
       
         
           
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     First and second actuators (not shown) can be configured to control the motion of the first and second objective lens  902 ,  912 . The Pvar for the most extreme case will be equal to 2nT. In this case, the revival peak of the beams  904 ,  910  can be greater than Pvar for the most extreme case. 
     According to another embodiment of an optical system illustrated in  FIGS. 9F-9G , the variable optical path difference Pvar for the most extreme case can be reduced. In the optical system  900 ′, an actuator  914  is coupled to the optical element  908 . The actuator  914  can be configured to move the optical element  908  together with the second objective lens  912  up to a certain distance Δ. In this case, Pvar is decreased according to Formula 2: P var =2[nT−Δ], wherein Δ is the max travel distance of the second objective lens  912 . Preferably, the actuator  914  can be the same actuator used to move the objective lens  912 . Motion of the object lens  912  and the optical element  908  is shown in  FIG. 9G . 
     However, moving together the optical element  908  and the second objective lens may not be sufficient for reducing Pvar to be less than the PCL when, for example, the laser source is the bare laser diode due to its small PCL. However, according to another embodiment illustrated in  FIGS. 9H-9I , the optical system  900 ″ includes first and second actuators  916 ,  918  configured to independently move the second objective lens  912  and the optical element  908  to achieve a variable optical path difference of zero. For example, the second actuator  918  can be configured to move the optical element  908  up to a distance 2nT depending on the focus layer inside the media to achieve zero Pvar. 
     Generally, an optical system including the external cavity laser as the laser source may provide laser light with a PCL sufficient so that the configuration of optical system  900  or  900 ′ can be used. If a bare laser diode is the laser source, then the configuration of optical system  900 ″ may be needed. 
     Referring to  FIGS. 10-11 , an optical system  1000  according to another exemplary embodiment for reading and writing data to a holographic media  1001  will be discussed. The optical system  1000  includes an external cavity laser  1002  as a laser source for generating a laser beam. The external cavity laser  1002  includes a laser diode  1004  for generating multimode light, a laminated prism  1006  and a visibility monitor  1008 . As shown in  FIG. 11 , the laminated prism  1006  includes an integrated grating  1102  for generating single mode light, and an interferometer  1106  for generating interfering beams  1108  which are received at the visibility monitor  1008 . The interferometer  1106  can be, for example, a Mach Zender interferometer which includes three half mirrors and regions with different refraction indexes. The refraction index of these regions can be designed in order to create an optical path difference between the two arms of the interferometer equal to the max optical path difference between object and reading beams inside the media (the optical path difference in the worst case). The visibility monitor  1008  will thus measure the worst case visibility. The interferometer  1106  also reflects the laser light to generate the external cavity laser output. 
     Returning to  FIG. 10 , the optical system  1000  includes a collimator lens  1010  for generating a collimated laser beam  1012 . A polarization beam splitter  1014  and quarter waveplate  1018  act as an optical isolator for changing vertically polarized light from the laser source into circular polarized light  1016  to be deflected toward the rest of the optical system and changing light  1022  reflected back from the media  1001  into horizontal polarization to be transmitted to the reading sensor  1020 , which receives the image of the microholograms in the media when the media is read. 
     A half transparent mirror  1023  reflects a first portion  1026  of the laser beam  1016 , which is the reference beam, towards a first objective lens  1028  and a second portion  1024  towards an optical element  1029 , which focuses the second portion  1024  of the laser beam into a power monitor  1032  for measuring the beam power level. 
     The reference beam  1026  passes through the first objective lens  1028 , which focuses it at a focal point on the media  1001 . The reference beam  1026  passes through the media  1001 , a second objective lens  1034  and a shutter  1036 , and is reflected back by a mirror  1038  to form an object beam  1040 . The first and second lens  1028 ,  1034  respectively focus the reference and object beams  1026 ,  1040  at the focal point on the media  1001  at which the beams interfere with each other to create microholograms. The microholograms previously written can be read by, for example, directing only the reference beam  1026  at the focal point while activating the shutter  1036  to block the object beam  1040 . The first and second lens  1028 ,  1034  can be coupled to actuators  1042 ,  1043 , which can be configured to independently move the lens to focus on a correct layer inside the media  1001 . Alternatively a single actuator can move both objective lenses together. A controlling device such as a processor (not shown) can control the actuators  1042 ,  1043 . Further, similarly to the various embodiments illustrated in  FIGS. 9D-9I , the actuator  1043  can be configured to move the mirror  1038  together with the lens  1034 , or the other actuator  1044  can independently control the mirror  1038  to reduce or eliminate the variable path difference. 
     The reference beam  1026  and the object beam  1040  have first and second optical path lengths defined from the external cavity laser  1002  to the focal point of the media  1001 . The optical subsystem is positioned so that an optical path length difference between the first and second optical path lengths is substantially equal to an integer multiple of an interference revival period associated with the reference beam  1026  and the object beam  1040 , the interference revival period associated with peak values of visibility of the interference versus the optical path difference. Generally, the path difference will be substantially equal to twice the path from the focal point of the media  1001  to the mirror  1038 . 
     As shown in  FIG. 12 , the exemplary external cavity laser  1002  of the optical system  1000  has an interference revival period of approximately 36.55 mm and a PCL of approximately 3.3 mm in dual mode operation ( 1204 ). Accordingly, the distance between the focal point of the media  1001  and the mirror  1038  can be an integer multiple of 36.55/2=18.275 mm for this particular exemplary configuration. The PCL is greater in single mode operation ( 1202 ). 
     Therefore, the present disclosure concerns an optical system for reading and recording data on a media. According to an embodiment, the system can include: a laser source for generating a laser beam; an optical subsystem for splitting the laser beam into a reference beam and an object beam; and first and second lens for receiving the reference and object beams, respectively, and focusing the reference and object beams at a focal point on the media at which the reference beam and object beam interfere with each other. The reference beam and object beam have first and second optical path lengths defined from the laser source to the focal point of the media. The optical subsystem is positioned so that an optical path length difference between the first and second optical path lengths is substantially equal to an integer multiple of an interference revival period associated with the reference beam and the object beam. 
     Other embodiments of the optical system will be apparent to those skilled in the art from consideration of the specification and practice of the optical system as disclosed herein. For example, the subsystem can include a tracking system to ensure the proper positioning of the objective lenses and the optical elements. Further, the system can include computer code for configuring the processor to adjust the lens and optical elements based upon data related to the interference revival period. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.