Abstract:
The present invention relates generally to generating and controlling optical trap arrays for manipulating particles. In particular, the invention relates to a dual function optical element able to both diffract laser light into beamlets and converge the beamlets (acting as a virtual lens for laser light), thereby eliminating the need for multiple physical lenses to transfer the diffracted laser beams to a focusing lens. The invention also relates to improved monitoring or optical traps by limiting the amount of noise reflected and scattered resulting from un-diffracted, laser light.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    Throughout this application various publications are referenced. The disclosures of these publications in their entireties are hereby incorporated by reference in this application in order to more fully describe the state of the art to which this invention pertains.  
           [0002]    1. Field of the Invention  
           [0003]    The present invention relates generally to optical traps. In particular, the invention relates to an apparatus, system and method for applying optical gradient forces to form a plurality of optical traps to manipulate small particles.  
           [0004]    2. Discussion of the Related Arts  
           [0005]    An optical tweezer is an optical tool which utilizes the gradient forces of a focused beam of light to manipulate particles with dielectric constants higher than the surrounding media. To minimize its energy such particles will move to the region where the electric field is the highest. Stated in terms of momentum, the focused beam of light produces radiation pressure, creating small forces by absorption, reflection, diffraction or refraction of the light by a particle. The forces generated by radiation pressure are almost negligible—a light source, such as a diode-pumped Nd:YAG laser operating at 10 mW, will only produce a few picoNewtons. However, a few picoNewtons of force is sufficient to manipulate small particles.  
           [0006]    Other optical tools which can be used to manipulate small particles include, but are not limited to, optical vortices, optical bottles, optical rotators and light cages. An optical vortex, although similar in use to an optical tweezer, operates on an opposite principle. An optical vortex produces a gradient surrounding an area of zero electric field which is useful to manipulate particles with dielectric constants lower than the surrounding media or which are reflective, or other types of particles which are repelled by an optical tweezer. To minimize its energy such a particle will move to the region where the electric field is the lowest, namely the zero electric field area at the focal point of an appropriately shaped laser beam. The optical vortex provides an area of zero electric field much like the hole in a doughnut (toroid). The optical gradient is radial with the highest electric field at the circumference of the doughnut. The optical vortex detains a small particle within the hole of the doughnut. The detention is accomplished by slipping the vortex over the small particle along the line of zero electric field.  
           [0007]    The optical bottle differs from an optical vortex in that it has a zero electric field only at the focus and a non-zero electric field at an end of the vortex. An optical bottle may be useful in trapping atoms and nanoclusters which may be too small or too absorptive to trap with an optical vortex or optical tweezers. J. Arlt and M. J. Padgett. “Generation of a beam with a dark focus surrounded by regions of higher intensity: The optical bottle beam,” Opt. Lett. 25, 191-193, 2000.  
           [0008]    The optical rotator is a recently described optical tool which provides a pattern of spiral arms which trap objects. Changing the pattern causes the trapped objects to rotate. L. Paterson, M. P. MacDonald, J. Arlt, W. Sibbett, P. E. Bryant, and K. Dholakia, “Controlled rotation of optically trapped microscopic particles,” Science  292 ,  912 - 914 ,  2001 . This class of tool may be useful for manipulating non-spherical particles and driving MEMs devices or nano-machinery.  
           [0009]    The light cage, described by Neal in U.S. Pat. No. 5,939,716, is loosely, a macroscopic cousin of the optical vortex. A light cage forms a ring of optical vortices to surround a particle too large, too reflective, or with dielectric constants lower than the surrounding media. If the optical vortex is like a doughnut, the light cage is like a jelly-filled doughnut. While the doughnut hole (for the vortex) is an area of zero electric field, the jelly-fill is an area of lowered electric field. In a gross sense, the gradient forces of the plurality of optical tweezers forming the doughnut “push” a particle, with a dielectric constant lower than the surrounding medium, towards the jelly-fill which may also be thought of as the less bright region which lies between the plurality of optical tweezers. However, unlike a vortex, no-zero electric field area is created.  
           [0010]    Using a single beam of laser light with a diffractive optical element to form a plurality of diffracted laser beams focused to form an array of optical traps is known in the arL U.S. Pat. No. 6,055,106 issued to Grier and Dufresne describes arrays of optical traps. The patent teaches the use of physical transfer lenses to direct a diffracted laser beam to the back aperture of a focusing lens. Multiple physical lenses are employed to direct and overlay the laser beams at the back aperture of a focusing lens with sufficient overlap to achieve an effective numerical aperture (NA) of at least about 0.8, which, as taught in U.S. Pat. No. 5,079,169 issued to Chu and Kronis, is considered the minimum NA necessary to trap and manipulate particles in three dimensions. It is a drawback of the apparatus described in U.S. Pat. No. 6,055,106 that each lens requires a relatively large amount of physical space to operate within and each lens must be maintained, cleaned and aligned. Those familiar with transfer lens systems will recognize that the greater the number of lenses in the system the more opportunity for misalignment and other maintenance problems. Accordingly, there has existed a need to reduce the number of lenses in a transfer lens system used for forming an array of optical traps. The present invention satisfies this need.  
           [0011]    A common modality for monitoring the activity of an optical trap described in U.S. Pat. No. 6,055,106 is to place a beam splitter in the pathway of the laser beam and thereby yield an optical data-stream. One limitation of this modality is the detrimental effect on the optical data stream of noise. In the context of optical traps, noise refers to the interference with the imaging, measuring and/or viewing of the optical traps, their contents, or the surrounding region resulting from the presence in the system of un-diffracted focused beam of light or energy, light emanating from the optical traps, and light reflected or diffracted off a lens in a physical transfer lens system either due to imperfections in the lens, dust, dirt or due to misalignment. As taught in U.S. Pat. No. 6,055,106, one way to reduce noise is to direct the laser beam at an oblique angle relative to the diffractive element thereby urging the un-diffracted beam away from the objective lens. While useful for its intended purposes, the other sources of noise remain. There exists a need for reduction or elimination of the noise caused by the un-diffracted laser beam, scattered and reflected laser light off the components of a system producing an array of optical traps. The present invention also satisfies this need and other needs and provides related advantages.  
         SUMMARY OF THE INVENTION  
         [0012]    The present invention provides a novel and improved method, system and apparatus for generating, monitoring and controlling optical trap arrays employing but a single physical transfer lens. The within invention also improves monitoring and control of optical traps by filtering out, or shutting off, the “noise” caused by scattered, un-diffracted and reflected light within the system.  
           [0013]    Multiple transfer lenses are eliminated or reduced, in a system producing a plurality of optical traps, by encoding a lens function into a diffractive optical element. The diffractive optical element may also alter the phase of any of the beams. By encoding the diffractive optical element to converge the plurality of beams, the invention creates conditions favorable to use a single physical lens to transfer and overlay the plurality of beams at the back aperture of the objective lens and to form optical traps there-through.  
           [0014]    In a basic form, the invention (FIG. 1A) is a focused beam of light or energy, such as single laser beam diffracted by a diffractive optical element which has an encoded lens function. The laser beam is diffracted into a plurality of beams and each beam is also converged by the diffractive optical element then directed to a single transfer lens which in turn directs and overlaps the plurality of beams at the back aperture of a focusing lens (such as the objective lens of a microscope) thereby forming a plurality of optical traps. To alter the position of a given optical trap, the beam forming that trap may be steered to a new position, via the diffractive optical element, thereby altering the position of the optical trap resulting therefrom. A moving mirror may be added (FIGS. 1D, 1E,  2  and  4 ) to simultaneously alter the position of all the optical traps as a unit. In some cases, movement of the single transfer lens may also be desirable to alter the position of the given optical trap.  
           [0015]    The selective generation and control of the array of optical traps with a single lens transfer system may be useful in a variety of commercial applications, such as, optical circuit design and manufacturing, nanocomposite material construction, fabrication of electronic components, opto-electronics, chemical and biological sensor arrays, assembly of holographic data storage matrices, energy source or optical motor to drive MEMS, facilitation of combinatorial chemistry, promotion of colloidal self-assembly, manipulation of biological materials, interrogating biological material, concentrating selected biological material, investigating the nature of biological material, and examining biological material.  
           [0016]    In some embodiments of the invention (FIGS. 2 and 4) real time viewing of the activity of the optical trap array is enabled by placing a beam splitter in the path of the beams prior to the focusing lens and then introducing a filter to limit the passage of un-diffracted, scattered or reflected light along the optical data stream thus reducing this noise which can disrupt video or other monitoring of the optical data stream. A moving mirror, useful to adjust the position of the whole array of optical traps, may also be combined with the beam splitter (FIGS. 2 and 3) or added to the system (FIG. 4).  
           [0017]    Noise reduction may also be achieved by periodically shuttering off the laser light (FIG. 3) and monitoring the optical data stream and/or by shuttering off the optical data stream when the laser is on.  
           [0018]    Other features and advantages of the present invention will be set forth, in part, in the descriptions which follow and the accompanying drawings, wherein the preferred embodiments of the present invention are described and shown, and in part will become apparent to those skilled in the art upon examination of the following detailed description taken in conjunction with the accompanying drawings, or may be learned by practice of the present invention. The advantages of the present invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appendent claims. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0019]    [0019]FIG. 1A illustrates a system for manipulating an array of small particles.  
         [0020]    [0020]FIG. 1B illustrates a first alternate system for manipulating an array of small particles.  
         [0021]    [0021]FIG. 1C illustrates a second alternate system for manipulating an array of small particles with a reflective diffractive optical element.  
         [0022]    [0022]FIG. 1D illustrates a third alternate system for manipulating an array of small particles with a movable mirror.  
         [0023]    [0023]FIG. 1E illustrates a fourth alternate system for manipulating an array of small particles with a movable mirror.  
         [0024]    [0024]FIG. 2 illustrates a fifth alternate system for manipulating an array of small particles adapted for real time and noise free viewing.  
         [0025]    [0025]FIG. 3 illustrates a sixth alternate system for manipulating an array of small particles adapted for real time and noise free viewing.  
         [0026]    [0026]FIG. 4 illustrates a seventh alternate system for manipulating an array of small particles adapted for real time viewing.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]    Certain terminology will be used in the following specification, for convenience and reference and not as a limitation, brief definitions are provided below:  
         [0028]    A. “Beamlet” refers to a sub-beam of focused light or other source of energy that is generated by directing a focused beam of light or other source of energy, such as that produced by a laser or collimated output from a light emitting diode, through a media which diffracts it into two or more sub-beams. An example of a beamlet would be a higher order laser beam diffracted off of a grating.  
         [0029]    B. “Phase profile” refers to the phase of light or other source of energy in a cross-section of a beam.  
         [0030]    C. “Phase patterning” refers to imparting a patterned phase shift to a focused beam of light, other source of energy or beamlet which alters its phase profile, including, but not limited to, phase modulation, mode forming, splitting, converging, diverging, shaping and otherwise steering a focused beam of light, other source of energy or a beamlet.  
         [0031]    Various embodiments of the inventive apparatus for forming a plurality of movable optical traps, generally designated as  8 , are shown in FIGS. 1A, 1B and  1 C. In the embodiment shown in FIG. 1A, a movable array of optical traps is formed by generating a focused beam of energy, such as electromagnetic wave energy. In preferred embodiments, the electromagnetic waves are light waves, preferably having a wavelength of from about 400 nm to about 1060 nm, and more preferably having a wavelength in the green spectrum. The beam is formed of a collimated light, such as the collimated output from a light emitting electrode or, preferably, a laser beam  10 , as shown in FIGS.  1 A-E.  
         [0032]    The focused beam of light is directed along optical axis  500  through a phase patterning optical element having a variable optical surface, such as a diffractive optical element  12  having a variable optical surface disposed substantially in a plane conjugate to a planar surface  15  of a back aperture  16  of a focusing lens, such as an objective lens  18 , to produce a plurality of beamlets  32  and  33  (two shown) having a selected phase profile. Varying the optical surface of the diffractive optical element alters the beamlets.  
         [0033]    Encoded in the diffractive optical element  12  is a virtual lens that converges the plurality of beamlets at a position between the encoded diffractive optical lens and a single transfer lens. The beamlets emanating from the encoded diffractive optical element, after converging, are directed thorough a transfer lens so as to overlap the beamlets at the back aperture of a focusing lens, such as an objective lens objective lens  18 . The beamlets are then converged by the focusing lens to form a plurality of optical traps  1002  and  1004  in working focal region  2000 . The working focal region  2000  is that area where a media containing particles  3000  or other material  3002  to be examined, measured or manipulated by the optical traps  1002  and  1004  is placed.  
         [0034]    Any suitable laser can be used as the source of the laser beam  10 . Useful lasers include solid state lasers, diode pumped lasers, gas lasers, dye lasers, alexanderite lasers, free electron lasers, VCSEL lasers, diode lasers, Ti-Sapphire lasers, doped YAG lasers, doped YLF lasers, diode pumped YAG lasers, and flash lamp-pumped YAG lasers. Diode-pumped Nd:YAG lasers operating between 10 mW and 5 W are preferred.  
         [0035]    When the focused beam of light  10  is directed through the encoded diffractive optical element  12 , the encoded diffractive optical element produces a plurality of diffracted beamlets  32  and  33  having an altered phase profile. Depending on the type of optical trap desired, the alteration may include wavefront shaping, phase shifting, steering, diverging and converging to form different classes of optical traps including optical tweezers, optical vortices, optical bottles, optical rotators, light cages, and combinations of the different classes. For clarity, only two diffracted beamlets and two corresponding optical tweezers  1002  and  1004  are shown, but it should be understood that an array of such beamlets are created by the encoded diffractive optical element.  
         [0036]    The location of each trap is selectively controlled by the encoded diffractive optical element. It is a significant feature of the invention that movement of each trap, be it rotation in a fixed position, rotation in a non-fixed position, two-dimensional and three dimensional, continuous and stepped is selectively controllable. The control is achieved by varying the surface of the diffractive optical element through which the beam passes, thereby altering the position of convergence of the beamlets emanating from the encoded diffractive optical element.  
         [0037]    Suitable diffractive optical elements are characterized as transmissive or reflective depending on how they direct the focused beam of light. Transmissive diffractive optical elements, as shown in FIGS. 1A and 1B, focus the beam of light, while reflective diffractive optical elements, as shown in FIG. 1C, reflect the beam.  
         [0038]    Within the two general groups, a diffractive optical element can be categorized as being formed from either static or dynamic media. Examples of suitable static diffractive optical elements include diffractive optical elements with a fixed surface, such as gratings, including diffraction gratings, reflective gratings, transmissive gratings, holograms, stencils, light shaping holographic filters, polychromatic holograms, lenses, mirrors, prisms, waveplates and the like.  
         [0039]    The static diffractive optical element may have different regions, each region configured to impart a different phase profile to the beamlets. In such embodiments, the surface of the static diffractive optical element can be varied by moving the surface relative to the laser beam  10  to select the appropriate region to change the desired characteristics imparted to the beamlets, i.e., to change the desired phase profile of at least one of the resulting beamlets. In some embodiments, the static surface contains two or more discreet non-homogeneous regions. In alternative embodiments, the static surface is substantially continuously varying.  
         [0040]    Examples of suitable dynamic diffractive optical elements having a time dependent aspect to their function include variable computer generated diffractive patterns, variable phase shifting materials, variable liquid crystal phase shifting arrays, micro-mirror arrays, piston mode micro-mirror arrays, spatial light modulators, electro-optic deflectors, accousto-optic modulators, deformable mirrors, reflective MEMS arrays and the like. With a dynamic diffractive optical element, the features of the encoded surface can be altered, for example, by a computer, to effect a change in the number of beamlets, the phase profile of at least one of the beamlets, and the location of at least one of the beamlets.  
         [0041]    The virtual lens encoded on the diffractive optical element alters the phase of light incident on the optical element. A representative virtual lens is a pattern similar to a Fresnel lens encoded, for example, in the orientation of a reflective grating or nematic liquid crystals. The virtual lens is distinguishable from a physical lens, which affects all the beamlets  32  and  33  as a whole, as the virtual element can alter the relative position of each beamlet  32  and  33  independently.  
         [0042]    The diffractive optical element is also useful to impart a particular topological mode to the laser light. Accordingly, one beamlet  32  may be formed in a Guass-Laguerre mode while another beamlet  33  is formed in a Guassian mode.  
         [0043]    Preferred virtual lens-encoded diffractive optical elements include phase-only spatial light modulators such as the “PAL-SLM series X7665, manufaetured by Hamanmatsu of Japan or the “SLM 512SA7” manufactured by Boulder Nonlinear Systems of Layfette, Colo. These encoded diffractive optical elements are computer controllable and multifunctional, so that they can generate the beamlets  32  and  33  by diffracting the laser beam  10  and selectively impart desired characteristic to the resulting beamlets.  
         [0044]    Each of the diffracted beamlets emanates from an area A on the front surface  13  of the encoded diffractive optical element and each must also pass through an area B, on the back aperture  16 , thereby the beamlets are overlapping at the back aperture  16  of the objective lens  18 . In the embodiment shown in FIG. 1, near precise overlapping is efficiently accomplished by combining the virtual lens encoded diffraction optical element with a single, movable downstream optical lens L 1 .  
         [0045]    The laser beam  10  preferably has a beam diameter w which substantially coincides with the diameter of the back aperture  16  and it is an advantage of the inventive system that there is little or no overfill of the back aperture  17  of the objective lens  18  both conserving the intensity of the laser beam  10  and preserving the strength of the electrical field gradient creating effective optical traps  1002  and  1004  in the desired pattern within the working focal region  2000 .  
         [0046]    Stated in mathematical terms: If the effective NA to establish gradient forces sufficient to form an optical trap is 0.8 and if the effective NA is calculated from the formula NA=n*Sin φ/ 2 , wherein n represents the index of refraction for the medium outside of the objective lens and φ is the angle of convergence for the diffracted beams and if an oil immersion objective lens having an index of refraction as high as 1.5 is used, then to maintain an effective NA of 0.8, φ should be maintained at least 66 degrees during movement of the optical trap in order to form and maintain an optical trap effective to manipulate particles in three dimensions.  
         [0047]    Alternatively, it is possible to form an effective optical trap without filling the back aperture  16 , if an oversized focusing lens having an oversized back aperture, relative to the laser beam  10 , is used. However, such a lens would require greater physical space and may be more costly.  
         [0048]    Turning to the alternative embodiment shown in FIG. 1B, the controllable array of optical traps is formed by passing the laser beam  10  through a diffractive optical element  12  encoded with a virtual lens that is disposed substantially in a plane  14 ′ forming an acute angle β relative to the optical axis  500 . In this embodiment, beamlets  32  and  33  emanating from area A n the front surface of the encoded diffractive optical element are directed by the diffractive optical element so as to pass through area B on the back aperture  16  of the objective lens  18  and form optical traps  1002  and  1004  in the working focal region  2000 . By altering the position of the laser beam  10  relative to the optical axis  500 , a portion of the un-diffracted light  34  is removed which in turn reduces the noise caused by the un-diffracted light  34  thereby improving efficiency and effectiveness of forming optical traps  1002  and  1004 . Additionally, non-movable optical traps (not shown) which may form from an un-diffracted portion of the laser beam when the laser beam is directed along the optical axis as shown in FIGS. 1A and 1C is eliminated.  
         [0049]    [0049]FIG. 1C illustrates an alternative embodiment where the controllable array of optical traps is formed by reflecting the laser beam  10  off a diffractive optical element  12 ″ having an encoded virtual lens.  
         [0050]    [0050]FIGS. 1D and 1E show alternative embodiments having a movable mirror  41  for steering the beamlets emanating from the phase patterning optical element as a group prior to overlapping the beamlets at the back aperture of the focusing lens. The movable mirror  41  is disposed upstream of the transfer lens L 1  with its center of rotation at area C. The representative beamlet  32  passes from area A on the front surface  13  of the encoded diffractive optical element  12  through the transfer lens L 1  and on to area C which reflects it to area B at the back aperture  16 . Tilting the movable mirror  41  effects a change of the angle of incidence of the beamlet  32  relative to the mirror  41  and can be used to translate the array of optical traps  1002  and  1004 .  
         [0051]    This movable mirror is useful for both precisely aligning the optical trap array within a stationary substrate, to dynamically stiffen the optical traps through small-amplitude rapid oscillatory displacements, and to effectively increase the trapping action by precisely altering positions of the array of optical traps while pulsing the optical traps to form two or more alternating sets of optical traps from the same number of beamlets.  
         [0052]    The embodiment shown in FIG. 1D minimizes beamlet misalignments by including a conventional telescope system  42  disposed between the movable mirror  41  and the objective lens  18 . The telescope system is constructed of two lenses L 2  and L 3  placed between conjugate planes  43  and  45 , such that the beamlets pass from area A on the front surface  13  of the encoded diffractive optical element  12  on to the center of rotation of beam splitter  51  at area C in plane  43 , and then through area B on the back aperture  16  of the objective lens  18  in plane  44 . In the embodiment shown in FIG. 1E, the movable mirror  41  is placed in close proximity to the back aperture  16  in order to minimize beamlet misalignments.  
         [0053]    Using the embodiment of the invention shown in FIGS. 2 and 3, real time viewing of an optical data stream of the optical traps  1002  and  1004  interacting with small particles  3000  within the working focal region  2000  is possible. The movable array of optical traps  1002  and  1004  are formed using a single lens transfer optic L 1 . Only one beamlet  32  is shown for clarity, but it should be understood that a plurality of such beamlets are created by the optical element  12 .  
         [0054]    To produce the array of optical traps, the laser beam  10  passes through the diffractive optical element  12  to produce the beamlet  32  which emanates from area A on the front surface  13  of the encoded diffractive optical element  12  then passes on to area C. Area C is the center region on the surface of a beam splitter  51  located before the objective lens  18 . The beam splitter  51  is constructed of a static or a movable dichroic mirror, a static or a movable photonic band gap mirror, a static or a movable omnidirectional mirror, or other similar device. The beam splitter shown in FIG. 2 is movable and, therefore, serves the dual function of movable mirror and beam splitter. In the alternative embodiment shown in FIG. 3, the beam splitter  51  is fixed.  
         [0055]    The beam splitter  51  selectively reflects the wavelength of light used to form the optical traps and transmits other wavelengths to form two streams of beamlets. Thus, as seen in FIGS. 2 and 3, the first stream of beamlets proceeds from area C through area B at the back aperture  16  of the objective lens  18  thereby effectively overlapping all the beamlets at the back aperture and forming the optical traps  1002  and  1004 . The second stream of beamlets is reflected by the beam splitter  51  to a monitor and used to provide a real time optical data stream, aided by an imaging illumination source (not shown). The second stream of beamlets passes through the apparatus  8  to be visually inspected  64   a  by a human monitor  65 . The monitor  65  may also interface with a computer  66  and cause the computer to effect changes in the parameters of the system to alter the position of the position of one or all of the beamlets  32 .  
         [0056]    Alternatively, a spectrum  64   b  of the optical data stream can be obtained, then analyzed and/or the optical data stream can be converted into a video signal and monitored with a video monitor  64   c . In some embodiments, the optical data stream is passed through a spectrometer and the position of the convergence of at least one beamlet is then altered in response to an analysis of the spectrum or the video monitoring to change the location of the corresponding optical trap(s).  
         [0057]    Spectroscopy  64   b  of a sample of biological material can be accomplished with an imaging illumination suitable for either inelastic spectroscopy or polarized light back scattering, the former being useful for assessing chemical structure and the later being suited for measuring nucleus size. The computer  66  analyzes the data to identify suspected cancerous, pre-cancerous and/or non-cancerous cells, and directs the optical array to segregate and concentrate a sample of the selected cell types. Those skilled in the art will recognize that the methodology used to concentrate cells based on parameters specific to cancerous cells may be altered without departing from the scope of the invention to identify and/or concentrate other types of cells based on other parameters. The wavelengths of the laser beam  10  used to form optical traps useful for manipulating biological material include the infrared, near infrared and visible wavelengths from about 400 nm to about 1060 nm.  
         [0058]    In still other embodiments, the optical data stream may be received by a computer  66  adapted to record the optical data stream, analyze the optical data stream, and/or precisely adjust the position of one or all of the beamlet(s)  32  via the diffractive optical element  13 , the position of the single transfer lens L 1 , and/or the position of the movable beam splitter  51 . Alternatively, the optical data stream may be processed by a photodectector to monitor intensity, or any suitable device to convert the optical data stream to a digital data stream adapted for use by a computer  66 .  
         [0059]    The real time optical data stream provides more useful information if the noise is controlled. As seen in FIG. 2, a filter element  53 , such as a polarizing element or band pass element, is placed within the pathway of the optical data stream to reduce the amount of reflected, scattered or un-diffracted laser light  10  passing along the axis of the optical data stream. The filter element  53  filters out one or more preselected wavelengths and, in some embodiments, all but a preselected wavelength of the optical data stream.  
         [0060]    Another method of limiting the noise in the optical data stream is to shutter, or pulse, the optical data stream. FIG. 3 illustrates a system of controllable shutters  62  and  63 . One advantage of shutters is to eliminate substantially all of the noise or interference from the optical data stream. The shutter  62  selectively blocks and unblocks the optical data stream from passing freely from the system by coordinating its opening with the turning on and off of the laser beam  10 . When the laser beam is not being generated, the optical data stream is blocked and when focused beam of light is being generated, the optical data stream is unblocked. To maintain control of the small particles (not shown) within the optical traps when shuttering the laser beam  10  (which in-turn will cause the beamlet(s)  32  and the resulting optical trap(s)  1002  to switch “on” and “off”) the pulse rate of the shutter is adjusted dependent on the nature of the particles being manipulated. However, too slow a pulse rate will allow the trapped particles to drift. For those situations in which drift is desired, the pulse rate may be adjusted to encourage drift.  
         [0061]    Alternatively, the shutter  63  blocks the laser beam (not shown) or the beamlets from passing freely into the objective lens. By coordinating the opening and closing of the shutter  63  with the turning “on” and “off” of the monitoring of the optical data stream noise is reduced. In some instances, dual shuttering may be desirable. One advantage of dual shuttering is that both the laser beam  10  and the monitoring equipment can remain “on” at all times. In such an arrangement only the activity of the shutters  62  and  63  need be coordinated. The computer  66  may be used to selectively control the shutter(s)  62  and  63 .  
         [0062]    Shown in FIG. 4 is an embodiment useful where the availability of equipment, physical space limitations or other performance parameters will benefit from using a traditional telescope transfer lens system  42  placed in the system after the single transfer lens L 1  combined with a moving mirror  41  and a beam splitter  51  to provide an optical data stream.  
         [0063]    The apparatus  8  is useful as part of a system for manipulating a plurality of small particles. In addition to the apparatus, the system includes a light source (not shown) for producing the focused beam of light, the focused beam of light  10 , and a plurality of small particles  3000  that are manipulated by the optical traps  1002  and  1004 .  
         [0064]    The plurality of optical traps created in accordance with the invention. In some embodiments, the optical traps form the gradient conditions necessary to manipulate biological material.  
         [0065]    Since certain changes may be made in the above apparatus without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description, as shown in the accompanying drawing, the specification, and the claims shall be interpreted in an illustrative, and not a limiting sense.