Patent Publication Number: US-6212151-B1

Title: Optical switch with coarse and fine deflectors

Description:
PRIORITY INFORMATION 
     The present application is related to commonly assigned U.S. patent application Ser. No. 08/851,379, filed on May 5, 1997 and is incorporated herein by reference. 
     The present application claims priority from U.S. Provisional Application 60/065,580, filed on Nov. 12, 1997 and is incorporated herein by reference. 
    
    
     BACKGROUND ART 
     A number of optical switch technologies are currently used for controlling the optical passage of light. With one technology, electric current is applied to a polymer to create a thermal effect that changes a refractive index of a polymer. As the refractive index changes, a light beam passing through the polymer is selectively routed from an input to an output. Although faster than a comparable mechanical optical switch, the switching time of polymer optical switches is limited significantly by the thermal characteristics of the polymer. Additionally, the optical properties of the light transmitted through the polymer are undesirably affected by the optical characteristics of the polymer. 
     Another optical switch is disclosed by Leslie A. Field et al., in “The 8 th  International Conference on Solid-State Sensors and Actuators, and Eurosensors IX, Stockholm, Sweden, Jun. 25-29, 1995.” The optical switch is micro-machined in silicon and uses a thermally activated actuator to mechanically move a single send optical fiber relative to two receive optical fibers. Field et al. exhibits relatively slow mechanical movement due to inherent thermal effects. Additionally, Field et al. provides only one degree of optical alignment, resulting in inefficient transfer of light between optical fibers due to slight misalignments. 
     Another micro-machined optical switch is disclosed by Levinson in U.S. Pat. No. 4,626,066. Levinson uses a cantilevered micro-machined mirror that is electrostatically positioned between a stopped and unstopped position. While Levinson&#39;s mirror may deflect light between two optical fibers, as with the aforementioned switch designs, it also is capable of optical alignment in only one dimension. 
     What is needed is an optical switch that provides fast and precise switching of light between one input and a plurality of outputs, or vice versa. 
     SUMMARY OF THE INVENTION 
     The present invention includes an optical switch for deflecting a beam of light between an input and an output. The optical switch comprises a first actuator disposed in an optical path of the beam of light between the input and the output and a second actuator disposed in the optical path of the beam of light between the input and the output. In an exemplary embodiment, the first actuator comprises a low-bandwidth voice coil motor. The first actuator further comprises a rotary arm and a reflector. The reflector is coupled to the arm such that the beam of light is deflected by the mirror towards the second actuator. The optical switch further comprises a position sensing detector. In the preferred embodiment, a position of the beam of light is sensed by the position sensing detector, and the deflection of the beam of light by the first actuator is a function of the sensed position of the beam of light. In an exemplary embodiment, the second actuator comprises a hi-bandwidth two-stage voice coil motor and a directing lens, which two dimensionally deflect the beam of light. The optical switch further comprises a quad detector array. In the preferred embodiment, a position of the beam of light is sensed by the quad detector array, and the two dimensional deflection of the beam of light by the second actuator is a function of the sensed position of the beam of light. In the preferred embodiment, the first and the second actuators individually or in combination selectively deflect the beam of light towards the array of lenses. In the preferred embodiment the second actuator is disposed in the optical path between the array of lenses and the first actuator. In one embodiment, the optical switch is used in an optical storage drive to direct the beam of light towards an optical storage location. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 and 2 are block diagrams of an optical switch of a storage and retrieval system; 
     FIGS. 3 a - 3   e , illustrate the optical switch in further detail; 
     FIGS. 4 a-b  illustrate an imaging assembly coupled to a set of optical fibers; 
     FIG. 5 illustrates the optical switch as used in an exemplary optical path; 
     FIGS. 6 a-b  illustrate a directing optics in further detail; 
     FIGS. 7 a-e  illustrate a first actuator of the present invention in further detail; 
     FIG. 8 illustrates an exemplary geometry of a second detector; 
     FIG. 9 illustrates a servo circuit; 
     FIG. 10 illustrates outputs of A-D photo-detectors; 
     FIG. 11 illustrates connections between the A-D photo-detectors and the servo circuit; 
     FIG. 12, illustrates an exemplary embodiment of pre/post-calibration of the optical switch; and 
     FIGS. 13 a-b  illustrate two embodiments of a magneto-optical disk drive. 
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE PRESENT INVENTION 
     Referring in detail to the drawings wherein similar parts of the invention are identified by like reference numerals, there is seen in FIGS. 1 and 2 a block diagram of an optical switch  104 . The optical switch  104  includes an input port  181  and N output ports  182 . In the preferred embodiment, an outgoing laser beam  191  from a laser source  131  is directed along an optical path through the input port  181  towards an actuator assembly  251 . A portion of the outgoing laser beam  191 , which is indicated as a first beam  191   a , is routed by the actuator assembly  251  towards an imaging assembly  252  and, subsequently, towards one of the N output ports  182 . FIGS. 1 and 2 illustrate the first beam  191   a  as it is directed by the actuator assembly  251  towards different ones of the N output ports  182 . In an exemplary embodiment N equals 12; however, other values for N are understood to be within the scope of the present invention. It is also understood that the while the present invention is described in the context of directing a laser beam between the input port  181  and the output port  182 , the optical switch  104  described herein could also be used to direct a laser beam between the output port  182  and the input port  181 . 
     Referring now to FIGS. 3 a - 3   e , the optical switch  104  is illustrated in further detail. In the preferred embodiment, the optical switch  104  further includes a first detector  358  and a second detector  359  (FIG. 3 d ), and the actuator assembly  251  includes a first actuator  333  and a second actuator  334 . The first and second actuators  333 ,  334  are both disposed along an optical path between the laser source  131  and a set of optical fibers  302 . Referring briefly to FIG. 3 e , the first actuator  333  includes a reflector  373  that is coupled to an actuator arm  371 . The arm  371  rotates about a pivot axis generally illustrated as P. In the preferred embodiment, the outgoing laser beam  191  is directed by the reflector  373  (FIG. 3 c ) towards the second actuator  334 . In FIG. 3 d , the second actuator  334  of the optical switch  104  is illustrated in a side sectional representation. The second actuator  334  includes a directing optics  375  and a redirection lens  311 . The outgoing laser beam  191  passes through the directing optics  375  and is optically separated by the directing optics  375  into first, second, and third beams  191   a-c , which are respectively directed towards the redirection lens  311 , the first detector  358 , and the second detector  359 . 
     Referring now to FIGS. 4 a  and  4   b , there is seen an imaging assembly coupled to a set of optical fibers. The imaging assembly  252  includes an array of lenses  476  and a set of V-grooves  444 . The first beam  191   a  is directed and focused by the redirection lens  311  onto a particular one of the lenses  476 , and the particular lens  476  directs the first beam  191   a  towards a respective proximal end of one of a set of optical fibers  302 . The proximal ends of each of the optical fibers  302  are disposed within the set of V grooves  444 . The set of V-grooves  444  and the optical axes of the lenses  476  are aligned such that each of the lenses  476  is focused onto the respective proximal ends of the set of optical fibers  302 . In an exemplary embodiment, the lenses  476  are molded plastic lenses that each have a 0.50 mm diameter and are disposed along a linear axis with a 0.50 mm center to center spacing. In the exemplary embodiment, the optical fibers  302  are 4.0 um diameter single-mode polarization maintaining optical fibers, and the polarization axes of the optical fibers  302  are all aligned with respect to each other. 
     Referring now to FIG. 5, there is seen the optical switch as used in an exemplary optical path. In an exemplary embodiment, a magneto-optical (MO) data storage and retrieval system  500  includes a set of Winchester-type flying heads  506  that are adapted for use with a set of double-sided first surface MO disks  507  (only one flying head shown flying over one MO disk surface in FIG.  5 ). The set of flying heads  506  are coupled to a rotary actuator magnet and coil assembly (not shown) by a respective suspension  530  and actuator arm  505  so as to be positioned over the surfaces of the set of MO disks  507 . In operation, the set of MO disks  507  are rotated to generate aerodynamic lift forces, which maintain the set of flying MO heads  506  in a flying condition approximately 15 micro-inches above the upper and lower surfaces of the set of MO disks  507 . The lift forces are opposed by equal and opposite spring forces applied by the set of suspensions  530 . During non-operation, the set of flying MO heads  506  are maintained statically in a storage condition away from the surfaces of the set of MO disks  507 . System  500  further includes: the laser source  131 , the optical switch  104 , and the set of optical fibers  302 . Each of the optical fibers  302  is preferably coupled through a respective one of the set of actuator arms  505  and set of suspensions  530  to a respective one of the set of flying MO heads  506 . 
     In an exemplary embodiment, the laser source  131  operates at a single wavelength, preferably at 635-85 nm within a red region of the visible light spectrum; however, it is understood that laser sources operating at other wavelengths may be used. In the preferred embodiment, the laser source  131  is a distributed feedback (DFB) diode laser source. A DFB laser source  131 , unlike an RF-modulated Fabry-Perot diode laser, produces a very narrowband single-frequency output due to the use of a wavelength selective grating element inside the laser cavity. Linearly polarized light from a DFB laser source  131  that is launched into a selected one of the polarization maintaining optical fibers  302  exits the optical fiber with a polarization state that depends on the relative orientation between the fiber axes and the incident polarization. The output polarization state is stable in time as long as external perturbations which alter the fiber birefringence are negligible. This behavior contrasts to that observed when using prior art RF-modulated Fabry-Perot diode laser sources. Fabry-Perot laser diodes are characterized by high-frequency fluctuations in their spectral output; therefore, when linearly polarized light is launched into a polarization maintaining optical fiber  302 , fluctuations in the laser wavelength lead to corresponding polarization fluctuations in the laser light exiting the output of the optical fiber. The resulting polarization noise is larger than the corresponding DFB diode laser source case owing to wavelength dependent mode coupling. Mode coupling in fibers is a phenomenon whereby a small portion of the light that is being guided along one polarization axis is coupled into the orthogonal axis by intrinsic or stress-induced defects. In MO recording it is important that the polarization noise be kept to a minimum, such that a signal to noise ration (SNR) in the range of 20-25 dB can be achieved. By using a DFB laser source  131  it is possible to achieve the aforementioned level of SNR in the magneto-optical (MO) data storage and retrieval system  500  when utilizing polarization maintaining optical fiber  302  for the delivery and return of the signal light to and from the MO disks  507 . 
     The representative optical path of FIG. 5 includes: the laser source  131 , the optical switch  104 , one of the set of optical fibers  302 , and one of the set of flying MO heads  506 . As described previously, the first beam  191   a  is directed towards a proximal end of a selected one of the optical fibers  302 . The linear polarization of the first beam  191   a  is preferably aligned in the optical path so as to enter the proximal end of the selected polarization maintaining optical fiber  302  at a 45 degree angle relative to the polarization axis of the optical fiber  302 . The first beam  191   a  is directed by the selected optical fiber  302  to exit a respective distal end of the optical fiber and is further directed by a set of optical elements (as described in commonly assigned U.S. patent application Ser. No. 08/851,379, herein incorporated by reference) located on the flying head  506  towards a surface recording layer  549  of a respective MO disk  507 . 
     During writing of information, the first beam  191   a  is selectively routed by the optical switch  104  towards a particular MO disk  507  so as to lower a coercivity of the surface recording layer  549  by heating a selected spot of interest  540  to approximately the Curie point of the MO recording layer  549 . The optical intensity of the first beam  191   a  is held constant at a power in a range of 30-40 mw, while a time varying vertical bias magnetic field is used to define a pattern of “up” or “down” magnetic domains perpendicular to the MO disk  507 . This technique is known as magnetic field modulation (MFM). Subsequently, as the selected spot of interest  540  cools at the surface layer  549 , information is encoded at the surface of the respective spinning disk  507 . 
     During readout of information, the first beam  191   a  (at a lower power compared to writing) is selectively routed to the MO disk  507  such that at any given spot of interest  540  the Kerr effect causes a reflected laser beam  192  (a reflection of the first beam  191   a  from the surface layer  549 ) to have a rotated polarization of either clockwise or counter clockwise sense  563  that depends on the magnetic domain polarity at the spot of interest  540 . The reflected laser beam  192  is received by the optical elements on the MO head  506  and is directed by the set of optical elements on the flying head  506  for subsequent electronic conversion and readout. 
     Referring now to FIGS. 6 a - 6   b , the directing optics are illustrated in further detail. The directing optics  375  (FIG. 3 d ) includes: an upper portion  675   a  and a lower portion  675   b ; both are coupled to a mounting portion  675   c . In the preferred embodiment, the outgoing laser beam  191  is directed by the first actuator  333  through the lower portion  675   b  and is optically separated by the lower portion  675   b  into the first beam  191   a  and a fourth beam  191   d . The lower portion  675   b  directs the first beam  191   a  towards the redirection lens  311  generally along or about a central optical axis of the redirection lens  311 . The lower portion  675   b  also directs the fourth beam  191   d  towards the upper portion  675   a . The fourth beam  191   d  is optically separated by the upper portion  675   a  into the second beam  191   b  and the third beam  191   c . The second beam  191   b  is directed by the upper portion  675   a  towards the redirection lens  311  along an optical axis that is generally parallel to but not co-extensive with the aforementioned central optical axis of the redirection lens  311 . The upper portion  675   a  also directs the third beam  191   c  towards the first detector  358 . Those skilled in the art will recognize that the upper portion  675   a  and lower portion  675   b  may comprise beam-splitters of a variety well known in the optical arts. Those skilled in the art will also recognize that in the present invention optical displacement of the outgoing laser beam  191  will result in analogous displacements of the first, second, and third beams  191   a-c . The displacements of the first, second, and third beams  191   a-c  will be utilized to effect an optical switching function as is described in further detail below. 
     Referring back to FIGS. 3 a-e , in an exemplary embodiment, the first actuator  333  comprises a flat voice coil motor assembly (VCM), described in further detail below. In the preferred embodiment, rotation of the arm  371  by the first actuator  333  positions the reflector  373  such that the optical path traversed by the outgoing laser beam  191  is deflected in a plane that is parallel to a linear axis defined by the array of lenses  476 . The outgoing laser beam  191  is directed by the reflector  373  through the directing optics  375 , towards the redirection lens  311  and the array of lenses  476 , and through a particular lens  476  toward a particular proximal end of the optical fibers  302 . In an exemplary embodiment, the first actuator  333  operates with an open loop compensated crossover frequency of approximately 0.4 Khz and is capable of approximately 200 g&#39;s of acceleration. In the exemplary embodiment, the first actuator  333  rotates the arm  371  about pivot axis P over a +/−3.5 degree range of motion. The following performance characteristics are exhibited by the optical switch  104  using the first actuator  333 : 829 um deflection of the first beam  191   a  across the front surface of a particular lens  476  and 55 um deflection of the first beam  191   a  across a proximal end of the optical fibers  302  per degree of rotation of the arm  371 ; and 0.1 um alignment accuracy of the first beam  191   a  onto a proximal end of the optical fibers  302  per 1.5 um of deflection of the first beam  191   a  across the array of lenses  476 . 
     Referring now to FIGS. 3 e  and  7   a-e , the first actuator  333  of the present invention is illustrated in further detail. The first actuator  333  includes a set of generally planar assemblies comprising cross-hinges  9  and  22  that are disposed between a set of extended arms  41 ,  42  and a rigid mounting base  300 . The set of hinges  9  and  22  of the present invention replace the pivot and bearing assembly of prior art motors. In an exemplary embodiment the hinges are used in conjunction with a voice-coil motor (“VCM”). 
     Referring now to FIGS. 3 e  and  7   a , there are seen a perspective view of the first actuator  333  including: a first hinge  10 , a second hinge  11 , a third hinge  23 , and a fourth hinge  24 . In the preferred embodiment, the hinges  10 ,  11 ,  23 , and  24  are shaped as generally planar rectangles, however, as will be understood below, depending on the performance characteristics desired, the hinges  10 ,  11 ,  23 , and  24  may comprise other shapes, for example, generally planar squares, etc. 
     Referring now to FIG. 7 b , there is seen a view of the first hinge  10  and the second hinge  11  attached to respective fixed support  30  and movable support  31 . First hinge  10  includes a first end  13  that is attached to a notched upper portion of the movable support  31  and a second end  12  that is attached to a notched (notch not visible) upper portion of the fixed support  30 . Second hinge  11  includes a first end  15  that is attached to a notched lower portion of the movable support  31  and a second end  14  that is attached to a notched lower portion of the fixed support  30 . In the preferred embodiment, attachment of the ends of the hinges  10  and  11  to the supports  30  and  31  may be performed utilizing epoxy or a suitable adhesive; it is understood, however, that other methods of attachment are also within the scope of the present invention, for example, screws, rivets, etc. Hinges  23  and  24  are attached to fixed support  50  and movable support  51  in a similar manner. 
     Referring now to FIG. 7 c , the first hinge  10  and the second hinge  11  are viewed to comprise an upper pair of cross-hinges  9 , which in a top view cross each other in an “X” shaped manner. 
     Referring now to FIG. 7 d , the third hinge  23  and the fourth hinge  24  are similarly attached to fixed supports  50  and  51  and form a lower pair of cross-hinges  22 . 
     Referring now to both FIG. 3 e  and FIGS. 7 a-d , the movable supports  31  and  51 , and the fixed supports  30  and  50  each include eight faces A-H, including a top and bottom face G and H, respectively. The faces A-H of the movable supports  31  and  51  correspond to the faces A-H of the fixed supports  50  and  51 , respectively. The face A of the fixed support  30  is attached to an upper side of the rigid mounting base  300 . The bottom face H of the movable support  31  is attached to a top surface of an upper extended arm  41 . The face A of the fixed support  50  is attached to a lower side of the rigid mounting base  300 . The upper face G of the fixed support  51  is attached to an area of a bottom surface of a lower extended arm  42 . 
     Referring back to FIG. 3 e  and FIG. 7 a , the first actuator  333  includes a coil  900  disposed between a first magnet  70  and a first cold-rolled steel block  60 , and a second magnet  80  and second cold rolled steel block  90 . The coil  900  is attached to the actuator arm  371 . The actuator arm  371  connects to two extended arms  41  and  42 . The reflector  373  is attached to the coil support  900  between the extended arms  41  and  42 . The coil  900  includes an input and an output whereat a current may be applied by a current source (not shown). In an embodiment where supports  30 ,  31 ,  50 , and  51  are not conductive, the input and output may attach to different ones of the hinges  10 ,  11 ,  23 ,  24 . In other embodiments, the input and output may attach to different ones of the supports  30  and  50 , or  31  and  51 . The input and output may also be attached other points, for example, to a point somewhere in space. 
     Referring now to FIG. 7 e , there is seen a representative top view of a movement of the hinges  10  and  11 . In the preferred embodiment, application of a current to the input of the coil  900  creates a magnetic field opposite to the magnetic field of the magnets  70 ,  80 . The opposing magnetic fields create a repulsion force that moves the arm  371  away from the magnets  70 ,  80  (shown as an exemplary direction A). As discussed below, because the arm  371  is coupled through the extended arm  41  to the movable support  31 , when current is applied to the coil  900 , the movable support  31  will also move (shown as an exemplary direction W). 
     With reference to cross hinges  9 , because the first ends  13  and  15  of the hinges  10  and  11  are rigidly attached to the fixed support  30  along respective surfaces C and E, any independent movement of the second ends  12  and  14  of the hinges  10  and  11  may occur along an extent of the second ends and with an angle theta. Those skilled in the art will recognize that in an embodiment in which the hinges  10  and  11  comprise rigid material, attachment of the second ends  12  and  14  to the movable support  31  would fixidly constrain the movable support  31  in free space. In the present invention, however, the hinges  10  and  11  comprise a sufficiently flexible material such that application of a force to the movable support  31  will result in a constrained rotation of the second ends  12  and  14  and, thus, the movable support  31  about a pivot axis passing generally through a region P. The pivot axis is generally defined by a flexure point of the hinges  10  and  11  and corresponds to the center of the “X” shape described in FIG. 7 c . In an exemplary embodiment, the hinges  10 ,  11 ,  23 , and  24  are rigid enough such that the arm  371  rotates without excessive up, down, or torsional motion and yet are flexible enough that the arm  371  rotates without excessive disturbances through its +/−3.5 degree range of angular motion. In the preferred embodiment, the hinges  10 ,  11 ,  23 , and  24  comprise a single layer 0.0015 inch thick stainless steel material. In an alternative embodiment, the hinges  10 ,  11 ,  23 , and  24  may comprise a 0.001 inch layer stainless steel layer, a 0.001 inch viscoelastic layer, and a 0.001 inch plastic layer. Also, the hinges  10 ,  11 ,  23 , and  24  preferably do not exhibit undesirable resonance over the desired operating bandwidth of the first actuator  333 . The aforementioned discussion of FIG. 3 is understood to also apply to hinges  23  and  24 . 
     When current is applied to coil  900 , the combination of upper cross-hinges  9  and lower cross-hinges  22  act to center the rotational movement of the arm  371  (i.e., direction A) and thus the movable support  31  (i.e., direction W) parallel to a plane passing between the magnets  70  and  80 . In the present invention, although the movable support  31  rotates about the pivot axis within the region P with the angle theta, the flexure of the hinges  10  and  11  causes a translation of the pivot axis (i.e., in a direction T). Thus, the region P through which the pivot axis passes is defined by the flexure point of the cross hinges  9  and  22  as well as the extent of translational motion T. 
     Those skilled in the art will recognize that the cross-hinges  9  and  22  of the first actuator  333  are advantageous over prior art bearings as no frictional forces exist between bearing surfaces. In the present invention, the hinges  10 ,  11 ,  23 , and  24  exhibit hysteresis effects. However, in the present invention, the hysteresis effects are preferably smaller than the frictional forces of the prior art. The reduced frictional forces of the present invention result in a reduced current as compared to the prior art current required to impart movement with a given force to the arm  371 . Furthermore, for a required given force, the coil  900  may be made smaller than the prior art. A smaller coil reduces the mass such that rotation may occur about the pivot axis with an increased speed and improved operating bandwidth. Also, in the present invention, the cross hinges  9  and  22  are not limited by a minimum size as are commercially available bearings, and, therefore, the first actuator  333  of the present invention may be made with a smaller form factor than the prior art. While implementation of a set of hinges  10 ,  11 ,  23 , and  24  has been described in a voice coil motor embodiment, it is understood that the hinges  10 ,  11 ,  23 , and  24  could also be used in other types of actuators to effect other types of displacements, for example mechanical displacements and the like and, therefore, the hinges  10 ,  11 ,  23 , and  24  of the present invention should be limited only by the scope of the following claims. 
     Referring back to FIGS. 3 a-e , in the preferred embodiment, the first detector  358  is disposed in the optical path traversed by the third beam  191   c  (in direction  356 ) such that positional output signal provided by the first detector  358  corresponds to positional displacement of the outgoing laser beam  191  by the arm  371 . The first detector  358  comprises a position sensing detector (PSD) of a variety well known in the art. In an exemplary embodiment, the first detector  358  comprises a one-dimensional 1×7 mm PSD manufactured by Hamamatsu Photonics K.K, Hamamatsu City, Japan that exhibits operating characteristics that are similar to Hamamatsu PSD model no. S3931. As described in an exemplary method of use in further detail below, displacement of the first beam  191   a  across the array of lenses  476  also corresponds to positional displacement of the outgoing laser beam  191  by the arm  371 . Positional output signals provided by the first detector  358  can, thus, be used to ascertain the position of first beam  191   a  with respect to a particular lens  476 . In the preferred embodiment, the positional output signals provided by the first detector  358  are used with a feedback servo circuit (e.g., illustrated in FIG. 9 as an AD880 manufactured by Analog Devices, Norwood, Mass.) to provide an input signal to the coil  900  of the first actuator  333  and, thus, to controllably direct the first beam  191   a  (in one-dimension) towards a desired optical fiber  302 . 
     Referring back to FIGS. 3 a-e  again, in the preferred embodiment, the second actuator  334  comprises a two stage voice-coil motor (VCM) of a variety well known in the art. In an exemplary embodiment, the redirection lens  311  is coupled to the second actuator  334  using well known optical mounting techniques such that 0-350 um of motion can be imparted by the second actuator  334  to the redirection lens  311  in one or both of the indicated directions  356 ,  379 . 
     In the preferred embodiment, the redirection lens  311  receives the first beam  191   a , and the second actuator  334  controllably directs the first beam  191   a  (in two dimensions) towards a particular lens  476  and, thus, towards a desired optical fiber  302 . In the preferred embodiment, the redirection lens  311  also receives the second beam  191   b  and directs the second beam  191   b  towards the second detector  359 . As described in an exemplary method of use in further detail below, displacement of the first beam  191   a  across the array of lenses  476  also corresponds to positional displacement of the second beam  191   b  by the second actuator  334 . Positional output signals provided by the second detector  359  can, thus, also be used to ascertain the position of first beam  191   a  with respect to a particular lens  476 . In the preferred embodiment, the positional output signals provided by the second detector  359  are used with a feedback servo circuit (e.g., illustrated in FIG. 11 as an AD880 manufactured by Analog Devices, Norwood, Mass.) to provide an input signal to the second actuator  334  and, thus, to controllably direct the first beam  191   a  (in two-dimension) towards a desired optical fiber  302 . 
     In the exemplary embodiment, the second actuator  334  operates with an open loop compensated crossover frequency of approximately 1.5 Khz and is capable of 100 g&#39;s of acceleration. In the aforementioned embodiment, the resulting performance characteristics are desired: 59 um of displacement of the first beam  191   a  across a particular lens  476 , 56 um of displacement of the first beam  191   a  across a proximal end of the optical fibers  302 , and 150 um displacement of the second beam  191   b  across the second detector  359 , per 1000 um of linear displacement of the redirection lens  311  by the second actuator  334 . One benefit derived from use of the redirection lens  311  with the second actuator  334  in the optical path of the outgoing beam  191  is that reduced mechanical tolerances are possible for achieving alignment of the outgoing laser beam  191  onto a particular lens  476 . 
     Referring now to FIG. 8, there is seen an exemplary geometry of the second detector. In the preferred embodiment, the second detector  359  comprises a quad photo-detector array. As illustrated in dimensional detail in FIG. 8, the second detector  359  comprises a plurality of photo-detector pairs that are alternatively and adjacently disposed along a generally semicircular arc. Those skilled in the art will recognize that the measurement surfaces of adjacent photo-detector pairs can be used with well known quad array detection techniques to detect and measure the position of the second beam  191   b  across the measurement surfaces. 
     FIG. 10 illustrates outputs of A-D photo-detectors and their connections to the servo circuit illustrated in FIG.  11 . The servo circuit utilizes A-D photodetector outputs to provide the second actuator  334  with vertical and horizontal error signals for positioning of the redirection lens  311 . In the preferred embodiment, the semicircular arc along which the A-D photo-detectors are disposed is a function of the location where the second beam  191   b  is deflected through the redirection lens  311  by the first and second actuators  333 ,  334 . Those skilled in the art will recognize that the shape of the semicircular arc depends on various parameters, including: the size of the redirection lens  311 , the distance between the lenses  476 , the distance between the redirection lens  311  and the second detector  359 , and the desired performance characteristics. In an exemplary embodiment, the second detector  359  is disposed in the optical path of the first beam  191   a  such that 100 um displacement of the second beam  191   b  across the second detector  359  corresponds to 33 um of displacement of the first beam  191   a  across a particular proximal end of the optical fibers  302 . 
     Those skilled in the art will recognize that at higher operating frequencies (i.e., higher switching speeds of the optical switch  104 ) the servo circuitry and, thus, the first and second detectors  358 ,  359  will exhibit a concomitant increase in output noise, which will act to decrease the positional accuracy with which the first beam  191   a  may be directed towards the optical fibers  302 . It is also understood that other sources of high frequency noise may also be present, for example, noise resulting from shock, vibration, optical, and/or thermal effects. The positional accuracy with which the first beam  191   a  may be directed is a function of the positional output signal SNR provided by the first and second detectors  358 ,  359 . Because the output signals provided by the first detector  358  are obtained across a wider measurement surface than from the second detector  359  and because a 0.5 um positional accuracy of the first beam  191   a  onto a particular lens  476  is desired from both the first and second actuators  333 ,  334 , a given amount of output noise will comprise a larger percentage of the positional output signal provided by the first detector  358  than the second detector  359 . In the preferred embodiment, the SNR of positional output signal from the first detector  358  may be increased by passing the signal through a lowpass filter to effectively reduce the high frequency noise components resulting from, for example: the servo circuit itself, shock, vibration, optical, and/or thermal effects. However, those skilled in the art will recognize that reduction in high frequency noise may result in a decrease in the operating bandwidth over which the first actuator  333  can be used to deflect the outgoing laser beam  191 . 
     The present invention overcomes this limitation by using the relatively low-bandwidth first actuator  333  for coarse optical positioning the first beam  191   a  over a wide surface area (i.e., the entire array of lenses  476 ) in conjunction with the relatively hi-bandwidth second actuator  334  for fine optical positioning of the first beam  191   a  over a relatively narrow surface area (i.e., a particular lens  476 ). The use of a hi/low bandwidth actuator combination preferably eliminates the need for very expensive low/noise/high-frequency servo electronics that would be required when using a single actuator for fast and precise optical switching of light between the input port  181  and the output ports  182 . 
     In an exemplary embodiment, switching the first beam  191   a  between the optical fibers is made up of a two stage process: 1) positioning the first beam  191   a  between lenses  476  using the first actuator  333  with a settling accuracy of about 1 um and within about 3 ms; and 2) fine positioning the first beam  191   a  to a new position over a particular optical fiber  302  within about 1 ms. In the aforementioned process, the open loop acceleration portion of the second actuator  334  preferably overlaps the motion of the first actuator  333  to reduce the amount of motion required by the second actuator  334 . 
     Referring now to FIG. 12, an exemplary embodiment of pre/post-calibration of the optical switch  104  is illustrated and is described as follows. Initially, the first actuator  333  (FIGS. 3 a - 3   e ) positions the reflector  373  towards one extreme of the rotation indicated as  355  to reflect the outgoing laser beam  191 , and the second actuator  334  (FIGS. 3 a - 3   e ) positions the redirection lens  311  towards one extreme in the direction indicated as  356  and towards a second extreme in the direction indicated as  359 . The first beam  191   a  is thus directed towards the array of lenses  476 , however, the first beam  191   a  is at this point not necessarily optimally focused by a particular lens  476  towards a particular optical fiber  302 . Next, the reflector  373  is scanned in the direction  355  until a first increase in the amplitude of the reflected laser beam  192  (FIG. 5) is detected. If after rotation in the direction  355  an increase in the amplitude of the reflected laser beam  192  is not detected, the reflector  373  is again positioned towards one extreme of the rotation  355  and the second actuator  334  is incremented in the vertical direction  356  and the reflector  373  is then again scanned in the direction  355 . The aforementioned process is repeated until a first increase in amplitude in the reflected laser beam  192  is detected (Point A). At point A, the second actuator  334  is alternatively incremented horizontally and vertically in the directions  356  and  379  until a maximum amplitude of the reflected laser beam  192  is obtained. A maximal value preferably corresponds to optimal alignment of the first beam  191   a  over the optical axis of a first of the array of lenses  476 . As previously described above, point A will thus correspond to values measured by the first and second detectors  358 ,  359  (FIG. 3 e ). These values are stored for subsequent positioning of the first beam  191   a  towards the first lens of the array of lenses  476  (only two lenses shown). The position over the lens is maintained utilizing the first and second actuators  333 ,  334  and their respective servocircuits. Those skilled in the art will recognize that, as previously described, the position of any remaining lenses  476  can also be determined through appropriate scanning in the directions  355 ,  356 , and  379  while monitoring the reflected laser beam  192  for respective increases in amplitudes. The output values measured by the first and second detectors  358  and  359  at the respective maximum amplitudes of the reflected laser beam  192  can, thus, be used to preferably position the first beam  191   a  towards any one of the array of lenses  476  and, thus, towards any one of the optical fibers  302 . The calibration sequence described above can be utilized to adjust for subsequent misalignments resulting from, for example, shocks and temperature. Those skilled in the art will also recognize that the method of use described above should not limit the present invention, as other methods of use are also within the scope of the invention, which should be limited only by the scope of the ensuing claims. 
     Referring now to FIG. 13 a , an embodiment of a magneto-optical disk drive is illustrated. In an exemplary embodiment, the magneto-optical (MO) data storage and retrieval system  500  comprises an industry standard 5.25 inch half-height form factor (1.625 inch) within which are disposed at least six double-sided MO disks  507  and at least twelve flying MO heads  506 . The flying MO heads  506  are manufactured to include  12  optical fibers  302  as part of a very small mass and low profile high NA optical system so as to enable utilization of multiple MO disks  507  at a very close spacing within the system  500  and; therefore, to comprise a higher areal and volumetric and storage capacity than is permitted in an equivalent volume of the prior art. In the preferred embodiment, a spacing between each of the at least six MO disks  507  is reduced to at least 0.182 inches. High speed optical switching between the laser source  131  and the MO disks  507  is provided by the optical switch  104  and the first actuator  333  contained therein. 
     In the present invention, when the outgoing laser beam  191  is aligned over a particular lens  476 , the output port  182  from which the outgoing laser beam  191  exits towards a particular MO disk  107  may be identified by a sensing a signal from outputs of the corresponding A-D photo-detectors of the second detector  359 , for example, a maximum in a summed signal of the outputs of a particular set of A-D photo-detectors. The signals from the sensing circuit may be used subsequently to identify which of the set of MO disks  107  is being read or written at any given time. This compares to the prior art that requires MO disks be identified by data marks written to the MO disks. 
     In an alternative embodiment shown in FIG. 13 b , the system  500  may include a removable MO disk cartridge portion  710  and two fixed internal MO disks  507 . By providing the removable MO disk cartridge portion  710 , the fixed internal and removable combination permits external information to be efficiently delivered to the system  500  for subsequent transfer to the internal MO disks  507 . The copied information may, subsequently, be recorded back onto the removable MO disk cartridge portion  710  for distribution to other computer systems. In addition, the removable MO disk cartridge portion  710  allows for very convenient and high speed back-up storage of the internal MO spinning disks  507 . The fixed internal and removable combination also permits storage of data files on the removable MO disk cartridge portion  710  and system files and software applications on the internal MO spinning disks  507 . In another alternative embodiment (not shown) system  500  may include: any number (including zero) of internal MO disks  507  and/or any number of MO disks  507  within any number of removable MO disk cartridge portions  710 . 
     While the present invention is described as being used in an MO disk drive system  500 , the optical switch  104  and the first actuator  333  contained therein may be used in many different environments and many different embodiments, for example, with first and second actuators other than voice-coil motor actuators, with other form factors, with other optical sources of light, with other types of optical fibers, and/or with other types of optical elements. The optical switch  104  is also applicable to information transfer using other head technologies, for example, optical heads in compact disks (CD) and digital video disks (DVD). The optical switch  104  of the present invention can also be used for optical switching of light in other optical communications applications, e.g., fiber optic communications. 
     Thus, the present invention has been described herein with reference to particular embodiments thereof, a latitude of modification, various changes and substitutions are intended in the foregoing disclosure and it will be appreciated that in some instances some features of the invention will be employed without a corresponding use of other features without departure from the scope of the invention as set forth.