Patent Publication Number: US-2016231550-A1

Title: Illuminator for multi-focus confocal imaging and optimized filling of a spatial light modulator for microscopy

Description:
RELATED APPLICATION DATA 
     This application claims the benefit of and priority under 35 U.S.C. §119(e) to U.S. Patent Application No. 62/113,083, filed Feb. 6, 2015, entitled “Improved Illuminator for Multi-focus Confocal Imaging/Optimized Filling Of A Spatial Light Modulator For Microscopy,” which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     Confocal microscopy is a popular technique in biology and medicine for generating optically sectioned images. A spinning disk confocal imager uses a multitude of pinholes which are focused onto the sample and then scanned over the sample to generate a complete image. Spinning disk confocal imagers are fast, robust and a vital tool for much of microscopy. 
     As the pinholes are scanned across the sample, they move in and out of the area that is being imaged. This area is projected to the detector (usually a digital camera) where the individual sweeps of the pinholes are integrated until the entire image is formed. As the illumination light must also pass through the pinholes, a field of excitation light is generated at the pinholes. In the case of multi-disc systems, the field can be at an array of microlenses which project the field through the pinholes. In either case, the illumination field is then projected to the sample. It is desirable to have the field be flat or uniform across the field and to have it not extend beyond the area being imaged. 
     U.S. Pat. No. 9,134,519 to Berman, which is incorporated herein by reference in its entirety, discloses a multi-mode fiber optically coupling a radiation source module to a multi-focal confocal microscope. A multi-mode optical fiber delivers light from a radiation source to a multi-focal confocal microscope with reasonable efficiency. A core diameter of the multi-mode fiber is selected such that an etendue of light emitted from the fiber is not substantially greater than a total etendue of light passing through a plurality of pinholes in a pinhole array of the multi-focal confocal microscope. The core diameter may be selected taking into account a specific optical geometry of the multi-focal confocal microscope, including pinhole diameter and focal lengths of relevant optical elements. For coherent radiation sources, phase randomization may be included. A multi-mode fiber enables the use of a variety of radiation sources and wavelengths in a multi-focal confocal microscope, since the coupling of the radiation source to the multi-mode fiber is less sensitive to mechanical and temperature influences than coupling the radiation source to a single mode fiber. 
     U.S. Pat. No. 8,922,887 to Cooper, which is incorporated herein by reference in its entirety, discloses imaging a distal end of a multimode fiber. Where a multimode fiber is used for light delivery in a microscope system and a transverse distribution of light exiting a distal end of the fiber is substantially uniform, the distal end is imaged onto a plane of a sample to be probed by the microscope system, or at a conjugate plane. Alternatively, the distal end is imaged onto a plane sufficiently close to the sample plane or the conjugate plane such that a radiant intensity of light at the sample plane or the conjugate plane is substantially uniform. In the case of a multi-focal confocal microscope system, the distal end of the multimode fiber is imaged onto a plane of a segmented focusing array. Alternatively the distal end is imaged onto a plane sufficiently close to the segmented focusing array plane such that a radiant intensity of the light at the segmented focusing array plane is substantially uniform. 
     EP Publication 1538470, which is incorporated herein by reference in its entirety, is entitled confocal microscope and relates to improvement in light-using efficiency in a confocal microscope incorporating a confocal laser scanner which rotates a Nipkow disk (3) at high speed together with microlenses. In an embodiment of the present invention, a beam splitter (4,12) is inserted and placed between two integrated disks (2,3), in each of which a plurality of microlenses and minute openings are arranged with the same pattern making an array respectively. This beam splitter must be of a plate type. In addition, the axis of the incident light is tilted by a significant angle to the vertical incident axis of the microlens. This cancels the light axis shift generated by a plate beam splitter and enables the incident light to the relevant microlens to be focused to the corresponding minute opening. 
     Recently spatial light modulators (SLMs) have been introduced as a tool for microscopy for selective photo-manipulation. This is important for studies of protein trafficking, drug delivery, and protein association. SLMs have become very important in the field of optogenetics where selective simultaneous stimulation is desired for studies of live brain activity. SLMs have the advantage of other technologies in that they can deliver large amounts of optical power simultaneously to several regions even at different depths. 
     A SLM works by changing a beam of light such that the phase of each part of the beam of light is digitally altered. That is, a SLM has an array of pixels that can be used to change the relative phase of the light that hits that pixel as opposed to its neighbors. After the change, an analyzer can be used to convert the beam of light into an image as formed on the SLM, but even more powerfully, the beam can be focused to create a real image that is the transform of the image on the SLM. This digital hologram can be used to generate a 3D pattern of choice on the sample. 
     The resolution, effective area, and accuracy of the 3D pattern generated are dependent on the number of pixels. Ideally the coherent light source impinges upon the entire array of pixels uniformly, but in practice a Gaussian beam is usually expanded to cover the array. This has two drawbacks: the illumination is not uniform over the pixel array, and illumination light is lost that hits outside of the pixel array. 
     The technology disclosed herein can be viewed in relation to the following patents (both of which are incorporated by reference in their entirety):
         1) U.S. Pat. No. 4,818,983—A Optical image generator having a spatial light modulator and a display device   2) U.S. 2014/0295413—Systems, methods, and workflows for optogenetics analysis       

     Patent 1) contains a description of a spatial light modulator and Patent Application 2) describes an important use of a spatial light modulator for optogenetics. 
     FIELD 
     An exemplary embodiment generally relates to confocal imaging in optical microscopes. More specifically, an exemplary embodiment relates to the illumination optics in a confocal scanning unit. Even more specifically, an exemplary embodiment relates to a high-efficiency flat-field illuminator for a spinning disc confocal imager. 
     Another exemplary embodiment generally relates to photo-manipulation in microscopes. More specifically, an exemplary embodiment relates to using a spatial light modulator (SLM) as a photo-manipulation device. Even more specifically, an exemplary embodiment relates to a high-efficiency flat-field illuminator for a SLM based photo-manipulation device. 
     SUMMARY 
     An exemplary illumination system for a multi-focus confocal unit would have one or more of the following exemplary and non-limiting goals or ideals:
         Flat or uniform illumination across the field.   No stray illumination beyond the field being imaged.   Maximum efficiency of the input light into the field.   The shape of the field is the shape of the detector.   The illuminator maintains these ideals for many different wavelengths of illumination light.       

     Typically an illuminator comprises an expanded beam that illuminates the field. The profile of the beam is Gaussian, so it is necessary to over-expand the beam and then crop it to match the shape of the sensor. This results in a large loss of light and a field that is never quite uniform. 
     With current technology an aspherical element can be added to the beam path to change it from a Gaussian profile to something more uniform (flat-top). The simplest of these elements would be an aspheric lens that shapes the beam to a circular beam with a flat top. This in general improves the illuminator but the illumination field must still be cropped to match the shape of the sensor. 
     Some of these elements can also shape the round beam to something rectilinear to match the sensor. A holographic element could be added which changes the phase of different parts of the beam such that when the real image is formed, it is a uniform rectilinear shape. Most confocal units would require a collimated beam at the illumination field and so the holographic element would instead be required to generate the transform of the desired beam profile. This can be problematic. Also many holographic elements are currently wavelength dependent and so there would be difficulty using them in a multiple wavelength system. Holographic units also typically suffer from bright spots or speckle in the image they produce. 
     Recent technology allows the creation of an aspherical optical element that will generate a uniform rectilinear beam. This element uses a complex shape to redirect the light beam. These units can be made achromatic, so they will work well with several wavelengths or a wavelength range. By using one of these optical elements, one can create a near-ideal imager. All of the input light is redirected to generate a field of illumination that is uniform and has the right shape without cropping. Diffractive optical elements also can be made to be achromatic and so are useful for making a confocal illuminator. 
     Unfortunately, the phase profile of the now rectilinear, uniform beam is no longer uniform across the beam after use of such a device. A second diffractive optical element(s) can be needed to fix the phase uniformity. This is particularly important for a spinning-disc confocal unit, as phase changes will change the efficiency of the illumination through the field of pinholes, making the final illumination non-uniform. 
     Accordingly, one exemplary embodiment is directed toward an illuminator for a multi-focus confocal imager that uses one of these aspherical beam shapers. 
     The exemplary apparatus can comprise:
     aspheric optics to shape the beam to match the shape of the detector and make the field uniform; and   one or more other optics for one or more of beam expansion, magnification, and alignment.   

     This exemplary apparatus when combined with a confocal scanning imager, a microscope, and a detector would provide a way to acquire confocal images. 
     This device has one exemplary advantage over currently available illuminators in that it has superior flatness and much superior light efficiency. 
     Aspects are thus directed toward confocal imaging in optical microscopes. 
     Still further aspects are directed toward an illumination system for a confocal imager. 
     Even further aspects are directed toward an improved illuminator with beam shaping optics to uniformly illuminate a field the shape of the detector. 
     Still further aspects are directed toward an achromatic, aspherical beam shaper for use in a confocal imager. 
     Still further aspects relate to an apparatus for an illuminator for a multifocus confocal imaging device including:
     one or more optical elements for shaping the input beam,   means for uniformly illuminating the pinholes in the area to be imaged, and   means for shaping the illumination field to match the sensor.   

     The aspect above, where an optical element is an asphere lens. 
     The aspect above, where an optical element is a holographic diffuser. 
     The aspect above, where an optical element is an aspherical beam shaper. 
     The aspect above, where the holographic diffuser creates a shape that is the transform of the desired sensor shape. 
     The aspect above, where the beam shaper shapes the illumination field to match the sensor. 
     The aspect above, where cropping of the illumination field is not needed. 
     The aspect above, where a second holographic element is used to correct for phase non-uniformity introduced by the first element. 
     The aspect above, where the optics can be used at multiple wavelengths. 
     The aspect above, where the apparatus is combined with a confocal imager. 
     The aspect above, where the apparatus is combined with an electronic imaging device such as a camera. 
     The aspect above, where the apparatus is combined with a microscope. 
     In accordance with yet another exemplary embodiment, as the illumination of the pixel array of a spatial light modulator (SLM) becomes less uniform, the resolution and accuracy of the pattern generated degrades. Specifically, using a Gaussian illumination pattern results in the outer pixels contributing less to the hologram. These pixels are on the outside of the back aperture of the objective, and so effectively the numerical aperture (NA) of the hologram is reduced. 
     Typically, the Gaussian beam is over-expanded to make the field more uniform. This results in loss of illumination light which for many applications is not a concern. However, in the field of optogenetics, increasingly there is a need for multi-photon effect stimulation into deeper tissue and this increases the need for power. More power can be obtained from bigger lasers with shorter pulses, but this can be expensive. There is a need for conserving the power as much as possible. 
     As discussed, with current technology an aspherical element can be added to the beam path to change it from a Gaussian profile to something more uniform (flat-top). The simplest of these elements would be an aspheric lens that shapes the beam to a circular beam with a flat top. This in general improves the illuminator but the illumination field still extends beyond the pixel array and light is lost. 
     Some of the optical elements can also shape the round beam to something rectilinear to match the sensor. A holographic element could be added which changes the phase of different parts of the beam such that when the real image is formed, it is a uniform rectilinear shape. Most SLMs would require a collimated beam at the illumination field and so the holographic element would instead be required to generate the transform of the desired beam profile. This can be problematic. Also many holographic elements are currently wavelength dependent and so there would be difficulty using them in a multiple wavelength system. Holographic units also typically suffer from bright spots or speckle in the image they produce. 
     Recent technology allows the creation of an aspherical diffractive optical element (DOE) that will generate a uniform rectilinear beam. This element uses a complex shape to redirect the light beam. These units can be made achromatic, so they will work well with several wavelengths or a wavelength range. By using one of these optical elements, one can create a near-ideal imager. All of the input light is redirected to generate a field of illumination that is uniform and has the right shape without cropping. Nearly 100% of the illumination light can be used and all of the pixels in the array are illuminated equally. 
     Use of any of these optics can change the phase profile of the illumination beam and so will disturb the resultant hologram. This phase profile is constant and so can be corrected for on the SLM. No second holographic element is needed as in the case with a confocal imager. 
     Accordingly, one exemplary embodiment is directed toward an illuminator for a SLM photo-manipulation device one of these DOE beam shapers. 
     The exemplary apparatus can comprise:
     aspheric optics to shape the beam to match the shape of the SLM pixel array and make the field uniform; and   one or more other optics for one or more of beam expansion, magnification, and alignment.   

     This apparatus when combined with a SLM photo-manipulation device, a microscope, and a detector could provide a way to simultaneously stimulate multiple areas distinct in three dimensions. 
     This exemplary device has one exemplary advantage over currently available illuminators in that it has superior flatness and much superior light efficiency. 
     Aspects of are thus directed toward photo-manipulation in microscopy. 
     Still further aspects are directed toward an illumination system for a SLM photo-manipulation device. 
     Even further aspects are directed toward an improved illuminator with beam shaping optics to uniformly illuminate the pixel array of the SLM. 
     Still further aspects are directed toward an achromatic, DOE beam shaper for use in a SLM device. 
     Still further aspects relate to an apparatus for an illuminator for a SLM based photo-manipulation device comprising:
     one or more optical elements for shaping the input beam;   means for uniformly illuminating the pixel array of the SLM; and   means for shaping the illumination field to match the pixel array.   

     The aspect above, where an optical element is an asphere lens. 
     The aspect above, where an optical element is a holographic diffuser. 
     The aspect above, where an optical element is an aspherical beam shaper. 
     The aspect above, where an optical element is a DOE. 
     The aspect above, where the holographic diffuser creates a shape that is the transform of the desired sensor shape. 
     The aspect above, where the beam shaper shapes the illumination field to match the pixel array. 
     The aspect above, where cropping of the illumination field is not needed. 
     The aspect above, where the optics can be used at multiple wavelengths. 
     The aspect above, where the apparatus is combined with a SLM. 
     The aspect above, where the apparatus is combined with a microscope. 
     These and other features and advantages are described and, or are apparent from, the following detailed description of the exemplary embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The exemplary embodiments of the invention will be described in detail, with reference to the following figures wherein: 
         FIG. 1  illustrates an exemplary optical system using an aspherical optic to shape the beam. 
         FIG. 2  illustrates the beam profile from  FIG. 1  before use of the aspherical optics. 
         FIG. 3  illustrates the beam profile from  FIG. 1  at the pinhole array after the beam shaping optics. 
         FIG. 4  illustrates an exemplary optical system where the diffractive optic generates the transform of the desired beam shape. 
         FIG. 5  illustrates an exemplary optical system for typical illumination of the SLM. 
         FIG. 6  illustrates an exemplary optical system for illumination of the SLM including beam shaping optics. 
         FIG. 7  illustrates the beam profile from  FIG. 2  at the SLM. 
     
    
    
     DETAILED DESCRIPTION 
     The exemplary embodiments of this invention will be described in relation to microscopes, imaging systems, and associated components. However, it should be appreciated that, in general, known components will not be described in detail and/or can be found in some of the related literature which was incorporated by reference. For purposes of explanation, numerous details are set forth in order to provide a thorough understanding of the present invention. It should be appreciated however that the present invention may be practiced in a variety of ways beyond the specific details set forth herein. 
       FIG. 1  illustrates an exemplary optical system that performs beam shaping on the illumination. An input beam of light (usually a laser)  2  is expanded with lens  11  and recollimated with lens  12 . The expanded beam  4  then passes through the beam shaping optics  13 . The beam is then sent to the pinhole array  14 . Before the optical axis at plane  15  the beam has a Gaussian profile. At the pinhole array the beam has a uniform rectilinear profile to match the sensor. 
       FIG. 2  illustrates the beam profile from  FIG. 1  before the beam shaping optics. The profile,  21 , is Gaussian and the shape,  22 , is isotropic or circular. 
       FIG. 3  illustrates the beam profile from  FIG. 1  after the beam shaping optics as the beam profile is shaped at the pinhole array. The profile has a flat uniform top,  31 , and the shape,  32 , is rectilinear to match the detector. 
       FIG. 4  illustrates an exemplary optical system  40  that performs beam shaping on the illumination. An input beam of light  4  is expended and recollimated with the optics  41 . This beam then passes through the beam shaping optics  42 . The beam shaping optics create a beam profile that is the transform of the desired sensor shape. After recollimating with a lens  43 , the beam is now the desired uniform profile that is the shape of the sensor at the pinhole array,  44 . Immediately after the beam shaping optics,  45 , the profile of the beam is the Fourier transform of the desired beam shape. 
     It is therefore apparent that there has been provided above an exemplary illuminator for a multi-focus confocal imager. While this embodiment has been described in conjunction with a number of embodiments, it is evident that many alternatives, modifications and variations would be or are apparent to those of ordinary skill in the applicable arts. Accordingly, it is intended to embrace all such alternatives, modifications, equivalents and variations that are within the spirit and scope of this invention. 
     Another exemplary embodiment will now be described which also relates to microscopes, imaging systems, and associated components.  FIG. 5  illustrates an exemplary optical system  5  for a typical illumination of the SLM. An input beam of light (usually a laser),  11 , is expanded with  12 . The expanded beam then impinges on the SLM  13 . The beam at the pixel array is an over-expanded Gaussian. This results in uneven illumination of the pixels and loss of light outside of the pixel array. 
       FIG. 6  illustrates an exemplary optical system for illumination of the SLM including beam shaping optics. Here, the beam  21  is expanded using element(s)  22  and then passes through the beam shaping optics  23 . The beam shaping optics  23  shape the beam so that at the SLM  24  the beam is the same shape as the pixel array and uniformly illuminates all of the pixels. 
       FIG. 7  illustrates the beam profile from  FIG. 6  at the SLM. The beam has a uniform (flat-top) intensity across the pixel array  31 . The beam also has the same shape  32  as the pixel array (i.e., rectilinear). 
     The exemplary techniques illustrated herein are not limited to the specifically illustrated embodiments but can also be utilized with the other exemplary embodiments and each described feature is individually and separately claimable. 
     The systems of this invention can cooperate and interface with a special purpose computer, a programmed microprocessor or microcontroller and peripheral integrated circuit element(s), an ASIC or other integrated circuit, a digital signal processor, a hard-wired electronic or logic circuit such as discrete element circuit, a programmable logic device such as PLD, PLA, FPGA, PAL, any comparable means, or the like. 
     Furthermore, the disclosed control methods and graphical user interfaces may be readily implemented in software using object or object-oriented software development environments that provide portable source code that can be used on a variety of computer or workstation platforms. Alternatively, the disclosed control methods may be implemented partially or fully in hardware using standard logic circuits or VLSI design. Whether software or hardware is used to implement the systems in accordance with this invention is dependent on the speed and/or efficiency requirements of the system, the particular function, and the particular software or hardware systems or microprocessor or microcomputer systems being utilized. 
     It is therefore apparent that there has been provided, in accordance with the current embodiment an improved illuminator for an SLM based photo-manipulation device. While this aspect has been described in conjunction with a number of embodiments, it is evident that many alternatives, modifications and variations would be or are apparent to those of ordinary skill in the applicable arts. Accordingly, it is intended to embrace all such alternatives, modifications, equivalents and variations that are within the spirit and scope of this invention.