Abstract:
The invention provides a light scanning device for scanning light from a light source, the light scanning device having: a pivotably mounted mirror for receiving light from the light source; a counterbalance; and a drive for oscillatorily pivoting the mirror and the counterbalance simultaneously in opposite directions to reduce uncoupled forces.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a light scanning device, of particular but by no means exclusive application in the fields of scanning microscopy and scanning endoscopy.  
         [0002]     One existing scanning microscope is disclosed in International Patent Application No. WO 99/04301. This microscope employs a miniature tuning fork to one tine of which is attached an optical fiber light source. The tuning fork is driven by electromagnets at a frequency of approximately 1000 Hz so that the output of the optical fiber is scanned in the direction of vibration of the tuning fork.  
         [0003]     Another technique is disclosed in related U.S. Pat. Nos. 6,172,789 and 6,057,952. The system disclosed in these documents includes a moveable mirror, a fixed mirror and a converging lens. The moveable mirror has an opening at its centre and is supported to be swingable about at least one axis. The fixed mirror is fixedly supported by an optically transparent plate, with its reflection surface opposed to that of the moveable mirror. Light is admitted from the end of an optical fiber, through the opening in the centre of the moveable mirror and reflected from the fixed mirror towards the moveable mirror. Light is then reflected from the moveable mirror towards the converging lens, which focuses the light on to an object surface. The moveable mirror is swung in an oscillatory manner about a central axis by means of an electrostatic drive; input light reflected from the fixed mirror onto the moveable mirror is thereby scanned by the moveable mirror upon reflection therefrom.  
         [0004]     In many applications, however, it is important to minimize the loss of energy of vibration from the fast scanning elements. Some prior art devices, being small or in miniature form, have light and the vibrating elements that make up a substantial proportion of their mass. Thus a considerable amount of reactive motion of the shell or case of such devices can occur. While operating, contact between the case of such devices and biological tissue can cause a high and variable degree of mechanical damping. In engineering terms the “Q” of these systems will be substantially reduced. This damping may also make the oscillation of a fast scan mirror difficult to maintain or may cause substantial changes in the amplitude of vibration.  
         [0005]     As a separate effect, the coupling of vibrational energy from a fast resonant scan mirror may also cause unwanted vibration of other components in a scanning head. Such vibration may have deleterious effects.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention provides, therefore, a light scanning device for scanning light from a light source, said light scanning device having: 
        a pivotably mounted mirror for receiving light from said light source;     a counterbalance; and     a drive for oscillatorily pivoting said mirror and said counterbalance simultaneously in opposite directions to reduce uncoupled forces.        
 
         [0010]     Thus, in applications such as endoscopy where such a device would be located in an optical head, vibration or other uncoupled forces resulting from pivoting the mirror can be reduced, so that the transmission of vibration to the optical head can be reduced. The drive is used to pivot the mirror, so that light reflected from the mirror can be scanned. Such applications commonly have two scanning directions, that is, a slow y-axis or vertical scan and a fast x-axis or horizontal scan. It is envisaged that the invention would generally be employed to provide the latter (viz. fast) scan, but there is no reason why, in principal, it could not be used for slower scans. The benefits of the invention would generally diminish, however, with the scan rate.  
         [0011]     Preferably said device includes a torsion bar for supporting said mirror. More preferably said torsion bar comprises a filament.  
         [0012]     Preferably said counterbalance is mechanically coupled to said mirror. More preferably said device includes a torsion bar for supporting said mirror and said counterbalance, wherein said drive drives said counterbalance by driving said mirror and thereby said mechanically coupled counterbalance.  
         [0013]     The counterbalance may comprise a plurality of counterbalancing elements. In one embodiment said counterbalance comprises two counterbalancing elements, located at opposite sides of said mirror. In another embodiment, said counterbalance comprises an annular structure locatable around said mirror.  
         [0014]     In one embodiment, said mirror and said counterbalance are coupled electrostatically or electromagnetically.  
         [0015]     Preferably said drive is an electrostatic drive. In another embodiment said drive is an electromagnetic drive.  
         [0016]     In another embodiment said mirror and said counterbalance are located, respectively, on first and second supports, wherein said counterbalance is located behind said mirror and thereby does not obscure the light receiving face of said mirror.  
         [0017]     Thus, in this configuration the device can be narrower as the counterbalance is not located beside the mirror.  
         [0018]     Preferably said device has a housing containing a reduced atmosphere so that said mirror and said counterbalance operate in said reduced atmosphere.  
         [0019]     The present invention also provides a method of scanning light, comprising: 
        reflecting said light from a mirror;     driving said mirror so as to oscillatorily pivot said mirror and thereby scan said light reflected from said mirror; and     driving a counterbalance to act as a counterbalance to said mirror;     whereby uncoupled forces due to said pivoting of said mirror are reduced by means of said counterbalance.        
 
         [0024]     According to another aspect, the present invention provides a scanning microscope or endoscope including the light scanning device described above.  
         [0025]     Preferably the microscope or endoscope includes one or more optical fibers for transmitting incident light to said microscope or endoscope, for transmitting return light from a sample, or for both transmitting incident light to said microscope or endoscope and for transmitting return light from a sample.  
         [0026]     The microscope or endoscope may be a confocal microscope or endoscope.  
         [0027]     According to another aspect of the invention, there is provided a method of balancing a light scanning device as described above, comprising: 
        mounting said mirror and counterbalance on one or more sensors for detecting vibration in said device in one or more dimensions;     operating said device and monitoring by means of said sensors any vibration therein; and     adjusting said device so as to reduce any vibration therein.        
 
         [0031]     Preferably adjusting said device comprises ablating a portion of said counterbalance or a portion of said mirror, or ablating portions of both said counterbalance and said mirror.  
         [0032]     Preferably said ablating is laser ablating.  
         [0033]     Preferably said ablating is performed in a reduced atmosphere. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]     In order that the present invention may be more clearly ascertained, the preferred embodiments will now be described, by way of example, by reference to the accompanying drawings, in which:  
         [0035]      FIG. 1A  is a schematic view of a light scanning device according to a first embodiment of the present invention;  
         [0036]      FIG. 1B  is a schematic view of a variation of the light scanning device of  FIG. 1A ;  
         [0037]      FIG. 1C  is a schematic view of a further variation of the light scanning device of  FIG. 1A ;  
         [0038]      FIG. 2  is a view of the scanning element of the light scanning device of  FIG. 1A ;  
         [0039]      FIG. 3A  is a cross-sectional view of the scanning element of  FIGS. 1A and 2 ;  
         [0040]      FIG. 3B  is a cross-sectional view of the scanning element of  FIGS. 1C and 2 ;  
         [0041]      FIG. 4  is a view, similar to that of  FIG. 2 , of a light scanning element according to a second embodiment of the present invention;  
         [0042]      FIG. 5  is a view, similar to that of  FIG. 2 , of a light scanning element according to a third embodiment of the present invention;  
         [0043]      FIG. 6  is a partial cross-sectional view of the scanning element of  FIG. 5 ; and  
         [0044]      FIG. 7  is a partial cross-sectional view of the scanning element of  FIG. 5  in use. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0045]     A light scanning device in accordance with the first preferred embodiment of the present invention is shown generally at  2  in  FIG. 1A , in use with an optical fiber  8 . The light scanning device  2  includes a scanning element  14 , a fixed, plane mirror  16  supported on a transparent plate  18  at a converging lens  20 .  
         [0046]     The scanning element  14  includes a plane, pivotable mirror  22 , with a reflecting surface facing the reflecting surface of fixed mirror  16 . Pivotable mirror  22  is flanked by a counterbalance comprising twin counterbalancing elements  24   a ,  24   b , one each side of pivotable mirror  22 . Pivotable mirror  22  and counterbalancing elements  24   a ,  24   b  are mounted on silicon torsion bar  26 .  
         [0047]     Pivotable mirror  22  has a central, circular aperture  28 , which coincides with tip  30  of optical fiber  8 , so that light emitted from optical fiber  8  can pass unimpeded through aperture  28  in pivotable mirror  22 . The diameter of circular aperture  28  is thus greater than that of optical fiber  8 , so that the central aperture  28  does not act a spatial filter for outgoing or returning light. Similarly, return light can be received by tip  30  of optical fiber  8  after passing through aperture  28  in pivotable mirror  22 .  
         [0048]     In use, light from a suitable source (not shown), generally a laser source, is transmitted along optical fiber  8  towards tip  30 , and emitted from tip  30  and through circular, central aperture  28  of pivotable mirror  22  towards fixed mirror  16 . This light is reflected from fixed mirror  16  towards pivotable mirror  22 , and reflected by pivotable mirror  22  towards converging lens  20 . In this process, as will be appreciated, some light may be lost through reflection from fixed mirror  16  back into central aperture  28  or otherwise.  
         [0049]     The light that reaches converging lens  20  is converged towards point  18 , at which will be located a sample. Similarly, light returned by that sample (whether by reflection or fluorescence), which will be collected by converging lens  20  and returned along the same optical path to tip  30  of optical fiber  8 . By suitable beam splitting techniques, this return light—or a portion thereof—can then be directed to a detector (not shown).  
         [0050]     Though not shown in  FIG. 1A , scanning element  14  includes an electrostatic drive for pivoting pivotable mirror  22  about torsion bar  26  in an oscillatory fashion, so that light reflected from pivotable mirror  22  is scanned (in the view shown in  FIG. 1A ) in and out of the plane of the figure. Counterbalancing elements  24   a  and  24   b  are also pivoted, but 180° out of phase with the motion of pivotable mirror  22 , to thereby provide a counterbalancing effect.  
         [0051]     A variation of the light scanning device  2  is shown generally at  4  in  FIG. 1B , again in use with optical fiber  8 . Though in most respects identical with scanning device  2  of  FIG. 1A , scanning device  4  includes a beamsplitter  10  to divert return light through 90° into return fiber  12  (connected to a suitable light detector, not shown).  
         [0052]     Another variation of the light scanning device  2  is shown generally at  6  in  FIG. 1C , again in use with optical fiber  8 . In this variation, the tip  30  of optical fiber  8  is located within (or, optionally, marginally forward of) central aperture  28  of the scanning device  6 . The central aperture  28  has, in this variation, a slightly larger diameter than in the variations shown in  FIGS. 1A and 1B , so that the fiber  8  can be accommodated—including when the mirror  22  is, in use, pivoting—without interfering with the motion of the mirror  22 .  
         [0053]     In still another variation of the scanning device  2 , a second (return) fiber is located adjacent to optical fiber  8  and the scanning device  2  includes an additional optical element located either between scanning element  14  and plate  18  or between plate  18  and converging lens  20  for diverging light returning from the sample by a small amount. This returning light is therefore collected by the second fiber rather than by fiber  8 , thereby avoiding the need provide fiber  8  with a beamsplitter for directing return light out of fiber  8  and towards a detector.  
         [0054]     The scanning element  14  is shown in greater detail in  FIG. 2 . Also visible in this figure are silicon microfabricated support pillars  30   a  and  30   b  extending from the rear wall  32  of frame  34  of scanning element  14  to torsion bar  26 , and located between mirror  22  and, respectively, counterbalancing element  24   a  and counterbalancing element  24   b . Support pillars  30   a  and  30   b  are provided to inhibit waves from being induced in torsion bar  26  by the motion of mirror  22  and counterbalancing elements  24   a  and  24   b.    
         [0055]     Light from optical fiber  8  (not shown) is emitted through central aperture  28  in direction  36 .  
         [0056]     The electromagnetic drive can assume any suitable form, including that taught in U.S. Pat. Nos. 6,057,952 and 6,172,789. That being the case, the reflective surface  38  of mirror  22  may be in the form of an applied conducting and reflective material to act both as an electrode and a reflector.  
         [0057]     Another suitable, alternative drive comprises an electromagnetic drive, comparable to that disclosed in WO 99/04301.  
         [0058]     As will be appreciated, counterbalancing elements  24   a  and  24   b  are designed to precisely counterbalance the mirror  22  to minimize the coupling of uncoupled forces being transmitted to frame  34  and from there to whatever optical head contains the device  2 . Counterbalancing elements  24   a  and  24   b  can be driven out of phase with mirror  22  in at least two ways. Firstly, they can be driven by the electrostatic drive that drives mirror  22 , but out of phase with mirror  22 . Alternatively, the electrostatic drive can be used to drive mirror  22  and, through the mechanical coupling of mirror  22  and counterbalancing elements  24   a  and  24   b  via torsion bar  26 , also to drive counterbalancing elements  24   a  and  24   b . In either case, however, the drive drives both the mirror  22  and the counterbalancing elements  24   a  and  24   b.    
         [0059]     Mirror  22  and counterbalancing elements  24   a  and  24   b  are driven with a resonant oscillatory motion, as will be understood by those in the art. The system has a high Q value, so that as little energy as possible must be input to sustain the oscillation. The scanning device  2  is provided with a lock-in sensor (not shown) which, in conjunction with the drive, enables the mirror  22  and counterbalancing elements  24   a  and  24   b  to be driven and maintained at the resonant frequency.  
         [0060]     The scanning device  2  is constructed within a case or optical head (not shown) such that mirror  22  is contained within a reduced atmosphere. This reduces the resistance of the atmosphere to the motion of the mirror  22  and the counterbalancing elements  24   a  and  24   b , but more generally the sealed optical head makes the elements contained therein less vulnerable to contamination from moisture, oil or dust. Indeed, in one embodiment a transparent seal is located over the converging lens  20 ; this seal can be cleaned without the risk of damaging the focusing optics provided by converging lens  20 .  
         [0061]      FIG. 3A  is a cross-sectional plan view of scanning element  14 , in which may be seen mirror  22 , counterbalancing elements  24   a  and  24   b , torsion bar  26 , support pillars  30   a  and  30   b , and optical fiber  8 . As is apparent from this view, optical fiber  8  is secured within an aperture  40  in rear wall  32  of the frame  34  of scanning element  14 . Aperture  40  is aligned with central aperture  28  of mirror  22 . Optical fiber  8  may, optionally, be additionally supported if necessary, such as with a collar extending from rear wall  32  towards central aperture  28 . Optical fiber  8  extends as far towards central aperture  28  as possible, without interfering with the pivoting motion of mirror  22 .  
         [0062]      FIG. 3B  is similar to  FIG. 3A , but illustrates scanning element  14  according to the variation shown in  FIG. 1C , that is, with a somewhat larger central aperture  28  to accommodate fiber  8 .  
         [0063]     Referring to  FIG. 4 , according to a second preferred embodiment of the present invention the scanning element is substantially identical to that shown in  FIG. 2 , but with an essentially circular mirror  122  and with a counterbalance in the form of a single counterbalancing element  124  comprising an annular element located so as to surround mirror  122 , co-centered with the mirror  122 .  
         [0064]     This arrangement has a number of benefits: the counterbalancing element  124 , as it surrounds mirror  122 , has a significant portion of its mass located further from torsion bar  126  than does mirror  122  itself. Consequently, the moment of inertia of counterbalancing element  124  is relatively high for its mass, compared with that of counterbalancing elements  24   a  and  24   b  of FIGS.  1  to  3 . Consequently, the same degree of counterbalancing can be provided by counterbalancing element  124  for a relatively lesser mass, so that the overall scanning element can be less massive.  
         [0065]     In this embodiment, silicon microfabricated support pillars may also be provided behind (in the view of  FIG. 4 ) torsion bar  126 , between mirror  122  and counterbalancing element  124 .  
         [0066]     The scanning element of the second embodiment is also provided with an electrostatic drive, shown schematically in  FIG. 4 . The electrostatic drive comprises two alternating power supplies  142   a  and  142   b , each connected to electrodes  144   a  and  144   b  respectively and attached to the mirror  122  and counterbalancing element  124  in the following manner.  
         [0067]     Electrode  144   a  extends from power supply  142   a , proceeds along torsion bar  126  to counterbalancing element  124 , then around counterbalancing element  124  in approximately a semicircle until it again reaches torsion bar  126 , follows torsion bar  126  to mirror  122 , and passes around the periphery of mirror  122  in approximately a semicircle remote from its path around counterbalancing element  124  until it reaches torsion bar  126 . By means of power supply  142   a , therefore, the upper (in the view of  FIG. 4 ) portion of counterbalancing element  124  and the lower portion of mirror  122  can be simultaneously charged.  
         [0068]     Electrode  144   b  of power supply  142   b  is arranged in a complementary fashion so that the lower (in the view of  FIG. 4 ) portion of counterbalancing element  124  and upper portion of mirror  122  can be simultaneously charged by means of power supply  142   b.    
         [0069]     In use, power supply  142   a  and power supply  142   b  have outputs that are 180° out of phase. The output of power supply  142   a  is essentially sinusoidal between a maximum negative value and 0, while that of power supply  142   b  is positive and sinusoidal, between a (lesser) maximum positive value and 0.  
         [0070]     A reference electrode (not shown) is provided behind mirror  122  and counterbalancing element  124 , within the frame (also not shown) of the element of this embodiment and the reference electrode is maintained with a charge, either +ve or −ve.  
         [0071]     When power supplies  142   a  and  142   b  apply the above described voltages across respective electrodes  144   a  and  144   b  and the ground electrode, 180° out of phase, the resulting electrostatic forces between the ground electrode and electrodes  144   a  and  144   b  cause the mirror  122  and counterbalancing element  124  to pivot in an oscillatory fashion about torsion bar  126 , simultaneously but 180° out of phase, so that uncoupled forces are minimized.  
         [0072]      FIG. 5  is an exploded, schematic view of the scanning element  214  of a third embodiment of the present invention. The scanning element  214  includes a forward frame  234 , including a mirror  222  pivotably mounted on a torsion bar  226 . Mirror  222  includes a circular, central aperture  228 . Optical fiber  208  is arranged with its exit tip  230  behind (in the view of  FIG. 5 ) and aligned with central aperture  228 .  
         [0073]     Mirror  222  is driven in an oscillatory or swinging manner by means of an electrostatic or electromagnetic drive (see above).  
         [0074]     Scanning element  214  also includes a rear frame  250 , mechanically coupled to forward frame  234  by means of four corner pillars  252 .  
         [0075]     Rear frame  250  is, in most respects, similar with forward frame  234 . However, instead of having a pivotable mirror, rear frame  250  includes a similarly arranged pivotable circular counterbalance mounted on a torsion bar. The scanning element  214  is configured, however, so that the counterbalancing element of rear frame  250  is driven 180° out of phase with mirror  222 .  
         [0076]     The configuration of the pivotable elements (i.e. mirror  250  and counterbalance) is shown more clearly in partial cross section  FIG. 6 , in which it can be seen that, located behind pivotable mirror  222 , is pivotable counterbalance  254 . Counterbalance  254  has a circular, central aperture  256 , coaxial with central aperture  228  of mirror  222 . Central aperture  256  of counterbalance  254  has a greater diameter than does central aperture  228  of mirror  222 , because optical fiber  208  passes through central aperture  256  of counterbalance  254 , while merely terminating behind central aperture  228  of mirror  222 . Central aperture  256  of counterbalance  254  has a sufficiently large diameter that counterbalance  254  can pivot as required without making contact with optical fiber  208 .  
         [0077]     Referring to  FIG. 7 , which is a side cross sectional view similar to  FIG. 6 , in use, mirror  222  and counterbalance  254  are driven 180° out of phase, to minimize the transmission of uncoupled forces to other components.  
         [0078]     In this embodiment, the scanning element  214  is preferably provided with an electrostatic drive comparable to that shown in  FIG. 4 , but with the ground electrode located between mirror  222  and counterbalance  250 . The placement of the electrodes is adjusted accordingly.  
         [0079]     In each of the above embodiments, the preferred technique for manufacturing the counterbalance (comprising one or more counterbalancing elements), so that it as closely as possible balances the mirror includes the following steps.  
         [0080]     The counterbalance is initially manufactured heavier than necessary, and tuned by the progressive laser ablation of the counterbalance until it is found to accurately counterbalance the mirror. This is assessed by mounting the scanning element on three piezo-sensors, and driving the scanning element while measuring the signal from the piezo-sensors. Uncoupled forces in the scanning element can then be detected by the piezo-sensors, and the counterbalance progressively laser ablated until no (or negligible) output is detected from the piezo-sensors.  
         [0081]     This tuning process can also be performed in a reduced atmosphere, to more precisely simulate the ultimate, preferred operating conditions.  
         [0082]     If the counterbalance is metallic, a readily ablated coating can be applied so that tuning comprises the laser ablation of the coating, rather than the counterbalance itself. Alternatively, in such embodiments the coating could be applied to the mirror or other counterbalanced element, and that coating ablated.  
         [0083]     Modifications within the spirit and scope of the invention may be readily effected by those skilled in the art. It is to be understood, therefore, that this invention is not limited to the particular embodiments described by way of example hereinabove.  
         [0084]     Further, any reference herein to prior art is not intended to imply that that prior art forms or formed a part of the common general knowledge.