Fiber bundle switch

An actuator assembly directs a first beam of light towards an optical assembly. A particular angular displacement of the beam of light relative to the optical assembly by the actuator assembly directs the beam of light to exit the optical assembly at a particular output port.

FIELD OF THE INVENTION
 The present invention relates generally to the optical switching of light,
 and more particularly to the optical switching of light in an optical data
 storage and retrieval system.
 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.sup.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 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'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 an input and a plurality of outputs, or vice
 versa.
 SUMMARY OF THE INVENTION
 The present invention permits fast and precise optical switching of light
 between an input port and a large number of output ports in a volumetric
 space that is very compact.
 The present invention directs a first beam of light along an optical path
 between an input and an output. The present invention includes an optical
 assembly comprising a front surface, a back surface, and an optical axis
 disposed between the front surface and the back surface;
 The present invention further includes an actuator assembly. The actuator
 assembly selectively directs the first beam of light between the front
 surface and the actuator assembly with a selected angular orientation
 relative to the optical axis. The optical assembly directs the first beam
 of light between the front surface and the back surface. The back surface
 comprises a plurality of output locations, and individual ones of the
 plurality of output locations correspond to individual ones of the
 plurality of angular orientations.
 The present invention further comprises a plurality of optical fibers. The
 plurality of optical fibers each comprise a proximal end and a distal end.
 The proximal ends of the optical fibers are aligned in the optical path to
 direct the first beam of light between a particular one of the plurality
 of output locations and a particular one of the distal ends. In one
 embodiment, the proximal ends are disposed within a housing. In another
 embodiment, the proximal ends are disposed in a closely packed pattern. In
 one embodiment, the optical assembly comprises a GRIN lens.
 In the present invention, the actuator assembly comprises a first actuator
 and a second actuator. The first actuator comprises a first arm and a
 first reflector coupled to the first arm. The first reflector reflects the
 first beam of light at a first reflection point and the first arm rotates
 about a first rotation axis. The second actuator comprises a second arm
 and a second reflector coupled to the second arm. The second reflector
 reflects the first beam of light at a second reflection point and the
 second arm rotates about a second rotation axis. In one embodiment, the
 first and second actuators comprise voice coil motors. The first rotation
 axis falls along a horizontal plane passing a vertical distance above the
 point of reflection of the beam of light from the second reflector. The
 vertical distance is defined by approximately a distance from the
 reflection point of the first beam of light from the second reflector to
 the front surface of the optical assembly. The second rotation axis falls
 along a vertical plane approximately coextensive with the front surface of
 the optical assembly. The second reflection point is disposed along the
 optical path between the first reflection point and the front surface of
 the optical assembly. A portion of the first beam of light is directed
 towards a beam splitting element to exit the beam splitting element as a
 second beam of light. The present invention further comprises a detector.
 An optical position of the second beam of light is detected the detector,
 and the actuator assembly directs the first beam of light based on the
 optical position of the second beam of light. The first reflector reflects
 the second beam of light at a third reflection point and the second
 reflector reflects the second beam of light at a fourth reflection point.
 The optical position of the second beam is detected by the detector after
 the second beam is reflected from the third and fourth reflection points.
 The present invention may be used in an optical disk drive to selectively
 direct the first beam of light towards a particular storage location.

DETAILED DESCRIPTION OF THE INVENTION
 Referring in detail to the drawings wherein similar parts of the invention
 are identified by like reference numerals, there is seen in FIG. 1 a block
 diagram of an optical switch 104 of the present invention. The optical
 switch 104 includes an input port 181 and N output ports 182. In a
 preferred embodiment, an outgoing laser beam 191 from a laser source 131
 is directed towards the input port 181. The outgoing laser beam 191 is
 routed by the optical switch 104 towards one of the N output ports 182.
 Alternatively, the optical switch 104 routes a reflected laser beam 192
 (described below) from a particular one of the N output ports 182 towards
 the input port 181. In an exemplary embodiment N equals 19; however, it is
 understood that other values for N are within the scope of the present
 invention.
 Referring now to FIG. 2a and FIG. 2b, there is seen a displacement of the
 outgoing laser beam by an actuator assembly. In the preferred embodiment,
 the optical switch 104 includes an actuator assembly 251 and a focusing
 assembly 252. The laser source 131 transmits the outgoing laser beam 191
 towards the actuator assembly 251. The outgoing laser beam 191 is
 displaced by a rotational movement of the actuator assembly 251 about axes
 254a and/or 254b (only one shown) so as to be incident onto approximately
 the same central location of a front surface 276 of the focusing assembly
 252 and (depending on the rotational state of the actuator assembly 251)
 with a particular angular orientation relative to a central optical axis
 of the focusing assembly 252.
 Referring now to FIG. 3a, there is seen the focusing assembly coupled to an
 optical fiber bundle assembly. In the preferred embodiment, the optical
 switch 104 further includes an optical fiber bundle assembly 353. The
 optical fiber bundle assembly 353 comprises a housing 354 and a set of
 optical fibers 302. In an exemplary embodiment, the housing 354 is a
 borosilicate glass capillary tube, which has a 1.8 mm outer diameter and a
 0.410 mm inner diameter 344. The optical fibers 302 each comprise an outer
 sheathing 333, which is preferably removed from a proximal end of each
 fiber 302. In an exemplary embodiment, the unsheathed proximal end of each
 optical fiber 302 comprises 80 um in diameter. In the exemplary
 embodiment, the unsheathed ends of the optical fibers 302 are arranged as
 a closely packed hexagonal structure (as viewed in a cross-section in FIG.
 3b), which is centrally disposed within the inner diameter 344 to extend
 through a front face 378 of the housing 354. The unsheathed proximal ends
 of the optical fibers 302 and the front face 378 of the housing 354 are
 subsequently polished as a unit to provide a planar surface for subsequent
 coupling of the optical fiber bundle assembly 353 to a back surface 277
 (FIGS. 2a-b) of the focusing assembly 252. The polarization axes of all
 the optical fibers 302 are aligned relative to one another, and the
 unsheathed proximal ends of the optical fibers 302 are secured within the
 inner diameter 344 of the housing 354 using low shrinkage epoxy. Other
 dimensions and other geometries for the housing 354 and the optical fibers
 302 are within the scope of the invention, for example, as shown in FIG.
 3c. In the exemplary embodiment, the set of optical fibers 302 comprises a
 set of 19 single-mode polarization maintaining (SMPM) optical fibers. In
 the preferred embodiment, the minimum number of optical fibers 302 is a
 function of the number of desired output ports 182, wherein the number of
 optical fibers (M) is greater than or equal to the number of output ports
 (N).
 In the preferred embodiment, the focusing assembly 252 comprises a GRIN
 lens, which in an exemplary embodiment is a 1.8 mm diameter,
 quarter-pitch, piano-piano GRIN lens (model SLW-1.8) rod manufactured by
 NSG America Inc., Somerset N.J. The GRIN lens of the present invention
 utilizes a radial index of refraction. The index of refraction is highest
 in the center of the lens and decreases with radial distance from its
 longitudinal axis. The following equation describes the refractive index
 distribution of the GRIN lens of the present invention N(r)=N.sub.0
 (1-(A/2)(r.sup.2)). The equation illustrates that the index falls
 quadratically as a function of a radial (r) distance of the lens. The
 resulting parabolic index distribution has a steepness that is a function
 of the value of the gradient index constant SQRT(A) and is a
 characterization of the lens' optical performance. In a GRIN lens, optical
 rays follow a sinusoidal path until reaching a back surface. A light ray
 that has traversed one pitch (P) traverses one cycle of the sinusoidal
 wave that characterizes a particular lens. The pitch (P) is the spatial
 frequency of the ray trajectory. The following equation relates the pitch
 (P) to the mechanical length (L) and the gradient index constant SQRT(A),
 2.pi.P=SQRT(A)(L). For appropriate values of the pitch, a focused spot may
 be formed on the back surface of the GRIN lens. The present invention
 identifies that a change in the angular orientation of the outgoing laser
 beam 191 relative to a central optical axis 280 of the focusing assembly
 252 (i.e., GRIN lens) results in translation of the laser beam 191 at an
 exit point along the back surface 277 of the focusing assembly 252 (i.e.,
 GRIN lens). In an exemplary embodiment, a change of 3 degrees in angular
 orientation of the outgoing laser beam 191 results in 0.187 mm linear
 translation of the focused outgoing laser beam 191 along the back surface
 277. For each of N unique angular orientations of the actuator assembly
 251, the outgoing laser beam 191 exits the back surface 277 of the
 focusing assembly 252 at a point corresponding to one of the N output
 ports 182. The present invention provides the ability to place a large
 number of output ports 182 as a very closely packed structure with minimal
 spacing between the output ports and, therefore to provide a minimal
 distance between the output ports 182 such that switching of the outgoing
 laser beam 191 between the output ports may be effectuated with a reduced
 speed. In an exemplary embodiment a maximal switching speed between any
 two output ports 182 is less than 1 ms. In the present invention a closely
 packed structure of the output ports 182 also makes possible an optical
 switch 104 in a reduced volumetric. Use of a focusing assembly 252
 comprising a single optical element (i.e., GRIN lens) also simplifies the
 design, alignment, and manufacturing of the focusing assembly 252. It is
 understood that the present invention should not be limited by the
 aforementioned lens design as other designs for the focusing assembly 252
 fall within the scope of the invention, albeit with an introduction in
 increased design complexity.
 Referring now to FIGS. 4a-4d, there is seen the actuator assembly in
 further detail. In the preferred embodiment, the actuator assembly
 includes a first actuator 433 and a second actuator 434. In an exemplary
 embodiment the first 433 and second 434 actuators comprise voice coil
 motor assemblies(VCMs). In the preferred embodiment, the actuator assembly
 251 is disposed in an optical path between the laser source 131 and the
 focusing assembly 252 (visible in FIG. 4d, wherein only the VCM 434 is
 shown). The actuator assembly 251 includes: a first right angle reflector
 455, a second right angle reflector 456, a leaky-beam splitter 450, a
 polarizing beam splitter 479, and a set of polarization detectors 458 and
 459. A quarter-wave plate (not shown) is disposed between the polarizing
 beam splitter 479 and the leaky-beam splitter 450. In the preferred
 embodiment, the laser source 131 directs the outgoing laser beam 191
 towards the leaky-beam splitter 450. A portion of the outgoing laser beam
 191 is directed by the leaky-beam splitter 450 towards a first mirror 451a
 and is directed by the first mirror 451a towards a second mirror 451b. In
 an exemplary embodiment, approximately 92% of the outgoing laser beam 191
 is directed by the leaky-beam splitter 450 towards the first mirror 451a
 and is reflected by the first mirror 451a towards the second mirror 451b.
 A remaining portion of the outgoing laser beam 191 (hereinafter referenced
 as laser beam 191a) is directed by the leaky-beam splitter 450 towards the
 first right angle reflector 455, by the right angle reflector 455 towards
 the first mirror 451a, and by the first mirror 451a towards the second
 mirror 451b. The first mirror 451a and the second mirror 451b are coupled
 to respective VCMs 433 and 434 by displaceable arms 469 and 470 located
 therebetween. In the preferred embodiment, the first and second mirrors
 451a and 451b are positioned by the arms 469 and 470 about respective axes
 254a and 254b such that the laser beam 191 is displaced by a unique
 angular orientation towards a central point (to a first order
 approximation) on the front surface 276 (FIG. 3a) of the focusing assembly
 252.
 As described in further detail below, the unique angular orientation
 defines through which of the particular optical fibers 302 convey the
 laser beam 191 towards a destination, or vice versa. Although in the
 preferred embodiment the optical path traversed by the outgoing laser beam
 191 is altered by the actuator assembly 251 that comprises VCMs 433 and
 434, it is understood that alternative actuator assembly 251 designs for
 altering the optical path of the outgoing laser beam 191 are feasible. For
 example, in a first alternative embodiment shown in FIG. 2c, an optical
 path traversed by the outgoing laser beam 191 may be redirected by a
 reflector 261 that is movable in two dimensions wherein the reflector 261
 directs the outgoing laser beam 191 towards the focusing assembly 252
 through a fixed imaging optics 262. In a second alternative embodiment
 shown in FIG. 2d, an optical path of the outgoing laser beam 191 may be
 directed directly or by a fixed reflector 266 towards an optical element
 267 that is movable in two dimensions and by the optical element 267
 towards the focusing assembly 252.
 Referring now to FIG. 5, there is seen the optical switch as used in a
 magneto-optical (MO) data storage and retrieval system. 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 (one flying head
 for each MO disk surface). 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 (only one shown). In operation, the
 set of MO disks 507 are rotated so as to generate aerodynamic lift forces
 and to 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. In the preferred embodiment, light is directed
 by the optical fibers 302 along 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 635-685 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 one of the
 single-mode 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 very 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 one of the
 polarization maintaining optical fibers 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 and dispersion of the phase
 difference between orthogonal polarization modes. Mode coupling in
 polarization maintaining optical 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 ratio (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
 fibers for the delivery and return of the signal light to and from the MO
 disks 507.
 Referring back to FIG. 5, the optical switch 104 directs the laser beam 191
 to selectively exit one of the N output ports 182 and as described below
 in further detail, to enter a respective proximal end of one of the set of
 M optical fibers 302. The linearly polarized outgoing laser beam 191 is
 preferably aligned in the optical path so as to enter the proximal end of
 a particular single-mode polarization maintaining optical fiber 302 at a
 45 degree angle relative to the polarization axis of the optical fiber
 302. The outgoing laser beam 191 is directed by the optical fiber 302 to
 exit a respective distal end. As used in the system 500, the outgoing
 laser beam 191 is further directed by optical elements disclosed on the
 head 506 towards a surface recording layer 549 of a respective MO disk
 507.
 During writing of information, the outgoing laser beam 191 is selectively
 routed by the optical switch 104 towards a particular MO disk 507 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. Preferably, the optical intensity of outgoing laser beam 191 is held
 constant at a power on 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). Alternatively, outgoing laser beam 191
 may be modulated in synchronization with the time varying vertical bias
 magnetic field at the spot of interest 540 in order to better control
 domain wall locations and reduce domain edge jitter. 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 outgoing laser beam 191 (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 outgoing laser beam 191 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 aforementioned optical path is
 bidirectional in nature; accordingly, the reflected laser beam 192 is
 received by the optical elements located on the flying MO head 506 and is
 directed by the optical elements towards the distal end of the optical
 fiber 302.
 Referring back to FIGS. 4a-4d, the reflected laser beam 192 propagates
 along the optical fiber 302 and is directed towards the back surface 277
 of the focusing assembly 252. The reflected laser beam 192 exits the front
 surface of the focusing assembly 252 and is reflected by the second mirror
 451b and the first mirror 451a towards the leaky beam splitter 450. The
 reflected laser beam 192 is routed by the leaky beam splitter 450 through
 the quarter-wave plate (not shown) towards the polarizing beam splitter
 479. A portion of the reflected laser beam is directed towards the
 polarization detector 458 and a remaining portion is directed by the
 second right angle reflector 456 towards the polarization detector 459. As
 is well established in the art, this type of differential detection scheme
 measures the optical power in two orthogonal polarization components of
 the reflected laser beam 192, with a differential signal being a sensitive
 measure of polarization rotation induced by the Kerr effect at the surface
 of one of the set of MO disks 507. After conversion by the set of
 polarization detectors 458 and 459, the differential signal is processed
 by a differential amplifier (not shown) for output as a representation of
 encoded information stored on the MO disks 507. The optical switch 104
 further comprises: a beam splitter 473, a third right angle reflector 471,
 a first position sensing detector (PSD) 474, and a second position sensing
 detector (PSD) 475. In the preferred embodiment the PSDs 474 and 475 are 3
 mm.times.3 mm one dimensional PSDs manufactured by Hamamatsu Corp.,
 Bridgeport N.J. Each position sensing detector 474 and 475 comprises a
 measuring surface of sufficient dimension to provide an output signal over
 the desired range of motion of the laser beam 191a. In the preferred
 embodiment, the laser beam 191a is reflected by the first right angle
 reflector 455 and subsequently by the first and second mirrors 451a and
 451b towards the beam splitter 473. The beam splitter 473 directs a
 portion of the laser beam 191a towards the measuring surface of the first
 position sensing detector 474. The remaining portion of the laser beam
 191a is directed by the beam splitter 473 towards the third right angle
 reflector 471 and (by the third right angle reflector 471) towards the
 measuring surface of the second position sensing detector 475.
 Referring now to FIG. 6a, the optical paths traversed by the laser beams
 191, 191a, and 192 are shown. In the preferred embodiment, the axis 254b
 falls along a vertical plane approximately coextensive with the front
 surface 276 of the focusing assembly 252, and the axis 254a falls along a
 horizontal plane passing a vertical distance Z above the point of
 reflection of the outgoing laser beam 191 from the second mirror 451b; the
 vertical distance Z being defined by approximately the distance from the
 reflection point of the outgoing laser beam 191 from the second mirror
 451b to the front surface of the focusing assembly 252 (also illustrated
 in FIG. 6b). As shown in FIG. 6a, the displacements experienced by the
 outgoing laser beam 191 and the laser beam 191a resulting from
 displacement of the first mirror 451a and/or the second mirror 451b are
 generally the same. In the preferred embodiment, the measurement surface
 of the first position sensing detector 474 is disposed to detect
 displacement of the laser beam 191a that results from the angular movement
 of the arm holding second mirror 451b, and the measurement surface of the
 second position sensing detector 475 is disposed to detect displacement of
 the laser beam 191a that results from the angular movement of the arm
 holding the first mirror 451a. The first and second position sensing
 detectors 474 and 475 each provide a signal that may be used to represent
 the angular position of the outgoing laser beam 191 as a set of
 coordinates in two dimensional free space. The signal provided by the
 first detector 474 may represent a X coordinate, and the signal provided
 by the second detector 475 may represent a Y coordinate. The two
 dimensional displacements experienced by the laser beams 191 and 191a are
 generally the same, thus, the X-Y coordinates measured by the first
 detector 474 and the second detector 475 preferably correlate to the two
 dimensional angular orientation of the laser beam 191 incident on the
 front surface 276 of the focusing assembly 252. Because each unique
 angular orientation of the outgoing laser beam 191 corresponds to a
 displacement of the focused laser beam 191 across the back surface 277,
 the X-Y coordinates that correspond to the displacement of the laser beam
 191a across the measurement surfaces of the first and second position
 sensing detectors 474 and 475 may be used to selectively direct the
 outgoing laser beam 191 towards a particular exit point along the back
 surface 277 of the focusing assembly 252 and, consequently, towards a
 particular one of the N output ports 182. In an exemplary embodiment of
 operation, the first and second mirrors 451a and 451b are rotated by
 respective VCMs about axes 254a and 254b to scan the focused laser beam
 191 progressively from top to bottom across the back surface 277 of the
 focusing assembly 252. Signals from the first and second polarization
 detectors 458 and 459 are summed and are monitored by detection circuitry
 (not shown) for a first peak indication in the reflected laser beam 192.
 The first peak indication preferably corresponds to an alignment of the
 laser beam 191 onto a core of a first of the M optical fibers 302. In the
 exemplary embodiment, the X-Y coordinate corresponding to the first peak
 indication is stored in a calibration look-up table. The alignment process
 is repeated in a similar manner until peak indications are measured and
 corresponding X-Y coordinates are stored in the look-up table for the
 remaining N of the M optical fibers 302. Accordingly, the laser beam 191
 may be selectively directed towards any one of the optical fibers 302 by
 displacing the laser beam 191a until an X-Y signal corresponding to an
 particular output port 182 is detected at the outputs of the first and
 second position sensing detectors 474 and 475. In the preferred
 embodiment, the PSDs 474 and 475 resolve movement of the laser beam 191a
 across respective measurement surfaces such that the laser beam 191 may be
 directed with sufficient accuracy towards a particular one of the optical
 fibers 302.
 The calibration table may be updated on a periodic basis, for example, when
 relative alignments of the components comprising the optical switch 104
 are altered by changes in temperature. In rapidly changing environments,
 for example environments subject or shock or vibration, the optical
 positioning of the laser beam 191 onto a core of a particular fiber 302
 may be maintained using closed loop servo techniques that are well known
 in the art. Because in the exemplary embodiment M equals 19 and N equals
 12, those skilled in the art will recognize that not all of the optical
 fibers 302 are necessarily used. In those embodiments where M is greater
 than N, any one of the unused M-N optical fibers 302 may be used in place
 of any one of the N optical fibers 302, for example, when one of the N
 optical fibers 302 is found to defective.
 In the preferred embodiment, the PSDs 474 and 475 may also be used to
 provide a signal that is proportional to the intensity of the laser source
 131, which may be used as a feedback signal for control of the intensity
 of the laser source 131.
 Referring now to FIG. 7a, 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 first surface MO disks 507 and at least twelve flying MO
 heads 506. The twelve flying MO heads 506 each include a respective
 optical fiber 302 as part of a very small mass and low profile high NA
 optical system. Use of optical fibers permits 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 and high speed optical switching of light
 between the laser source 131 and the MO disks 507 is provided by the
 optical switch 104.
 In an alternative embodiment shown in FIG. 7b, 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.
 The low profile optical paths disclosed by the present invention may be
 used to convey information to and from a storage location without
 requiring objective optics on the MO head 506 (e.g., using a tapered
 optical fiber or an optical fiber with a lens formed on an end); and/or
 reflective substrates (e.g., using a curved optical fiber to convey
 information along surfaces of the magneto-optical head 506). The optical
 switch 104 described herein may also be used in many different
 environments and in many different embodiments, for example: in other form
 factors (e.g., full height), with other optical sources of light (e.g.,
 Fabrey-Perot laser diodes), with other optical fibers (e.g.,
 non-polarization maintaining optical fibers), and/or with other types of
 optical elements. The present invention is also applicable to transfer of
 information using other storage and retrieval technologies, for example,
 compact disks (CD) and digital video disks (DVD). The present invention
 may be also used for optical switching of light in optical communications
 systems.
 Therefore, although the present invention has been described herein with
 reference to particular embodiments, a latitude of modification, various
 changes, and substitutions are intended in the foregoing disclosure. It
 will be appreciated that in some instances some features of the invention
 may be employed without a corresponding use of other features without
 departure from the scope of the invention as set forth.