Abstract:
Irradiance control systems (“ICSs”) that control the irradiance of a beam of light are disclosed. ICSs include in a beam translator and a beam launch. The beam translator translates the beam substantially perpendicular to the propagating direction of the beam with a desired displacement so that the beam launch can remove a portion of the translated beam and the beam can be output with a desired irradiance. The beam launch attenuates the irradiance of the beam based on the amount by which the beam is translated. ISCs can be incorporated into fluorescent microscopy instruments to provide high-speed, fine-tune control over the irradiance of excitation beams.

Description:
CROSS-REFERENCE TO A RELATED APPLICATION 
       [0001]    This application claims the benefit of Provisional Application No. 61/447,711; filed Mar. 1, 2011. 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure relates to external systems for laser beam irradiance adjustment and control. 
       BACKGROUND 
       [0003]    Laser beam irradiance adjustment and control can be difficult to achieve over an irradiance range of one order of magnitude or more. Even within this range, the accuracy and stability in the irradiance of the light output from a typical laser is often suboptimal for certain applications. Typical solutions for controlling the irradiance of a laser include controlling the current applied to the source or placing neutral density filters in the laser beam path to reduce the beam irradiance. In recent years, laser shutters have been optimized for speed by reducing the size of the shutters and by increasing the electrical power used to the control the shutters. As a result, laser shutters can be placed in the laser beam path to turn the laser beam “on” and “off.” 
         [0004]    However, current control, density filtering, and use of shutters to adjust and control the irradiance of a laser beam is not optimal, especially when adjusting the laser on the sub-millisecond time scale is desired. For instance, the response time of a laser to a linearly controlled power source is typically non-linear, which limits the range of adjustability to about one order of magnitude. In addition, the temperature of a typical laser may fluctuate during operation, resulting in further irradiance instability. Neutral density filters may improve the irradiance range by several orders of magnitude, but filters provide only coarse irradiance adjustment, and typical high speed shutters are not capable of achieving sub-millisecond open and close times despite the reduced size of the aperture and higher driving voltages. For the above described reasons, engineers and scientists who develop and work with instruments that relay on high-speed control of laser light irradiance continue to seek mechanisms for laser beam irradiance adjustment and control on the sub-millisecond time scale. 
       SUMMARY 
       [0005]    Irradiance control systems (“ICSs”) that control the irradiance of a beam of light are disclosed. ICSs include in a beam translator and a beam launch. The beam translator translates the beam substantially perpendicular to the propagating direction of the beam with a desired displacement so that the beam launch can remove a portion of the translated beam and the beam can be output with a desired irradiance. The beam launch attenuates the irradiance of the beam based on the amount by which the beam is translated. ISCs can be incorporated into fluorescent microscopy instruments to provide high-speed, fine-tune control over the irradiance of excitation beams. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  shows a schematic representation of an example irradiance control system. 
           [0007]      FIGS. 2A-2B  shows a schematic representation of an example implementation of a beam launch of an irradiance control system and an isometric view of a single-mode optical fiber and associated acceptance cone. 
           [0008]      FIGS. 3A-3C  show an example demonstration of a beam translator and beam launch of an irradiance control system. 
           [0009]      FIGS. 4A-4B  show irradiance profiles for light input and output from a single-mode optical fiber. 
           [0010]      FIG. 5  shows a schematic representation of an example implementation of a beam launch of an irradiance control system. 
           [0011]      FIGS. 6A-6B  show a top-plan view and an isometric view of an example beam translator. 
           [0012]      FIG. 7  shows as example demonstration of the beam translator shown in  FIG. 6 . 
           [0013]      FIGS. 8A-8C  show snapshots of internal paths associated with a beam of light traveling through the beam translator shown in  FIG. 6 . 
           [0014]      FIG. 9A  shows an isometric view of an example beam translator. 
           [0015]      FIG. 9B  shows as example demonstration of the beam translator shown in  FIG. 9A . 
           [0016]      FIG. 10A  shows an isometric view of an example beam translator. 
           [0017]      FIG. 10A  shows as example demonstration of the beam translator shown in  FIG. 10A . 
           [0018]      FIG. 11  shows a schematic representation of an example fluorescence microscopy instrument with an incorporated irradiance control system. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]      FIG. 1  shows a schematic representation of an example irradiance control system (“ICS”)  100 . The ICS  100  includes a beam translator  104 , a beam launch  106 , a splitter  108  and a feedback control  110 . Directional arrow  112  represents a beam of collimated light output from a light source  102  to the beam translator  104 . Directional arrow  114  represents the beam output from the translator  104  which propagates in the z-direction and is input to the launch  106 . The translator  104  translates or shifts the beam  114  substantially perpendicular to the propagation direction of the beam  114 , such as in the x-direction as indicated by directional arrows  116  and  118 . Directional arrows  120  represent the beam output from the launch  106  which attenuates the irradiance of the input beam  114  based on the amount by which the beam  114  is translated by the translator  104 . For example, dashed line  122  represents the optical axis of the launch  106 . The farther the input beam  114  is translated away from the optical axis  122 , the smaller the irradiance of the output beam  120 .  FIG. 1  includes an example plot  124  of the irradiance of the output beam  120  versus the distance from the center of the input beam  114  to the optical axis  122 . Curve  126  represents how the irradiance of the output beam  120  decreases as the input beam  114  is translated away from the optical axis  122 . In this example plot, the irradiance of the output beam  120  is greatest when the center of the input beam  114  is coincident with the optical axis  122  and the irradiance of the output beam  120  approaches zero, or is turned “off,” as the input beam  114  is translated away from the optical axis  122 . 
         [0020]    As shown in  FIG. 1 , the splitter  108  is located in the path of the output beam  120  to reflect a portion  128  of the output beam  120  to the feedback control  110  and transmits the bulk of the irradiance in the output beam  130 . The splitter  108  can be a beamsplitter cube or partially silvered mirror. The feedback control  110  is an electronic device that controls the operation of the translator based on the irradiance of the portion  128 . The feedback control  110  includes a photodetector, such as a photodiode, a processor and memory. The feedback control  110  continuously monitors the irradiance of the portion  128  to determine whether or not the irradiance in the output beam  120  has changed and adjusts the irradiance of the beam  120  accordingly. When the irradiance of the portion  128  is outside selected minimum and maximum irradiance thresholds, the control  110  sends electronic signals to the translator  104  to translate the input beam  114  so that the irradiance of the portion  128  is within the minimum and maximum irradiance thresholds. For example, when the irradiance of the portion  128  falls below the minimum threshold, the feedback control  110  sends signals to the translator  104  to translate the beam toward the optical axis  122 . On the other hand, when the irradiance of the portion  128  exceeds the maximum threshold, the feedback control  110  sends signals to the translator  104  to translate the beam away from the optical axis  122 . Alternatively, the feedback control  110  can direct the beam translator  104  to vary the irradiance in the beam  120 . For example, the feedback control  110  can direct the translator  104  to shift the beam  114  back and forth to produce a desired modulated irradiance pattern in the beam  120 . For example, the beam  120  may have a sinusoidal wave pattern or can be modulated to encode information. 
         [0021]      FIG. 2A  shows a schematic representation of an example implementation of the beam launch  106 . The launch  106  includes a plate  202  with a circular aperture  204 , a lens  206 , and a single-mode optical fiber  208 . As shown in  FIG. 2A , the center of the aperture  206 , optical axis of the lens  206 , and optical axis of the fiber  208  are coincident to form the optical axis  122  of the launch  106  described above. In the example of  FIG. 2A , the lens  206  is positioned along the optical axis  122  so that the focal point  210  of the lens  206  lies along the optical axis  122  and the diameter d′ of the acceptance cone  212  of the fiber  208  at the lens  206  is approximately equal to the diameter of the aperture  204  (i.e., d≈d′). The diameter of the acceptance cone  212  is determined by the acceptance angle θ with the optical axis  122 , which is correlated with numerical aperture of the fiber  208 .  FIG. 2B  shows an isometric view of the fiber  208  and the acceptance cone  212 . Light focused onto the focal point  210  lies within the acceptance cone  212  and is confined to the core  212 . In other words, the lens  206  shapes the beam of light output from the aperture  204  so that the light lies within the acceptance cone  212 . 
         [0022]      FIGS. 3A-3C  show an example demonstration of the beam translator  104  and launch  106  operated to adjust the irradiance of a beam of light  302  output from the source  102 . As shown in  FIGS. 3A-3C , the translator  104  redirects the collimated beam  302  toward the launch  106  parallel to the optical axis  122 . In order to achieve maximum coupling efficiency of the light into the fiber  208 , the diameter of the beam  302  is tuned to approximately match the diameter of acceptance cone  212  of the fiber  208 . In the example of  FIG. 3A , the translator  104  translates the beam so that the center of the beam  302  is coincident with the optical axis  122 . As a result, outer portions of the beam  302  are cut off or clipped by the plate  202  around the edge of the aperture  204 , such as at aperture edge points  306  and  308 , to produce a slightly narrower beam  304 . The lens  206  focuses the beam  304  into a shape that lies substantially within the acceptance cone  212  of the fiber  208  so that the beam  304  enters the core  214 .  FIG. 3A  includes an irradiance plot  310  associated with the beams  302  and  304 . Vertical axis  312  represents the irradiance, and horizontal axis  314  represents the distance through the center of the beams  302  and  304  in the x-direction. Curve  316  represents a Gaussian-shaped irradiance distribution profile through the center of the beam  302  in the x-direction. In the example of  FIG. 3A , the beam  302  is output from the translator  104  so that the highest irradiance portion of the beam is coincident with the optical axis  122 , which is represented by peak  318  coincident with the optical axis  122 . The total area under the curve  316  represents the irradiance of the beam  302  along the x-axis. Shaded area  320  under the curve  316  represents the irradiance of the beam  304  along the x-axis. Unshaded tails  324  and  326  represent the irradiance of the beam  302  that is cut off or clipped by the plate  202  around the aperture  204 . As a result, the irradiance of the beam  304  entering the core  214  is less than the irradiance of the beam  302 . 
         [0023]    In  FIG. 3B , the translator  104  translates the beam  302  in the direction  328  so that the center of the beam  302  is off the optical axis  122 . In this example, a large outer portion of the beam  302  is cut off by the plate  202  around the edge of the aperture  204  to produce a narrower beam  330 . The lens  206  focuses the beam  330  into a shape that lies substantially within the acceptance cone  212  of the fiber  208  so that the beam  330  enters the core  214 .  FIG. 3B  includes an irradiance plot  332  with the curve  316  shifted away from the optical axis  122  to represent the irradiance profile of the beams  328  and  330 . Shaded area  334  under the curve  316  represents the irradiance of the beam  330  along the x-axis. Shaded area  336  represents the irradiance portion of the beam  302  cut off by the plate  202 . Plot  332  reveals that the highest irradiance portion of the beam  330  is located away from the optical axis  122  and the irradiance of the beam  330  entering the core  214  is significantly less than the irradiance of the beam  302 . 
         [0024]    In  FIG. 3C , the translator  104  translates the beam  302  still farther in the direction  328  resulting in a significant portion of the beam  302  being cut off by the plate  202  to produce a very narrow beam  338 . The lens  206  focuses the beam  338  into a shape that lies substantially within the acceptance cone  212  of the fiber  208  so that the beam  338  enters the core  214 .  FIG. 3B  includes an irradiance plot  340  with the curve  316  shifted away from the optical axis  122  to represent the irradiance profile of the beam  338 . Shaded area  342  under the curve  314  represents the irradiance of the beam  334  along the x-axis. Shaded area  344  represents the irradiance portion of the beam  302  cut off by the plate  202 . Plot  340  reveals that the beam  302  is output from the translator  104  with much of the irradiance of the beam  302  cut off by the plate  202  leaving the beam  338  to enter the core  214  with a much smaller irradiance than the beam  302 . 
         [0025]    Allowing the beam  302  to strike the plate  202  to cut off a portion of the beam  302  irradiance, as described above with reference to  FIGS. 3A-3C , may result in unacceptable scattering. In alternative embodiments, the plate  202  may also include an angled reflective surface or mirror (not shown) located around a portion of the aperture  204  used to cut off the light. The mirror can be angled and configured to reflect the portion of the beam  302  to be cut off by the plate  202  to an optical beam dump (not shown) with high power handling capabilities. For example, the beam dump can be a cone of aluminum anodized to a black color and enclosed in a canister with a black, ribbed interior. Only the point of the cone is exposed to the reflected beam so that most of the reflected light grazes the cone at an angle. Any reflections from the black surface are then absorbed by the canister. 
         [0026]    The single-mode optical fiber  208  provides spatial filtering of the asymmetrical beams output from the lens  206 . For example, as described above with reference to  FIGS. 3B and 3C , the beams  330  and  338  have asymmetrical irradiance distributions when the beams enter the core  214  of the fiber  208 . Because the fiber  208  is a single-mode optical fiber, even though the beams enter the fiber  208  with asymmetrical irradiance distributions, the beams are output at the opposite end of the fiber  208  with symmetrical irradiance distributions.  FIGS. 4A-4B  show input and output ends, respectively, of the fiber  208 . In  FIG. 4A , the input end of the optical fiber is located along the optical axis  122  facing the lens  206  as shown in  FIGS. 2-3 . Shaded area  402  represents a region of the core  214  occupied by the largest irradance portion of an asymmetrical beam input to the fiber  208 .  FIG. 4A  includes a plot  404  of the irradiance profile associated with the beam input to the core. Vertical axis  406  represents the distance across the core  214  in the x-direction, horizontal axis  408  represents the irradiance, and curve  410  represents the irradiance of the asymmetrical beam that enters the input end of the fiber  208 . Plot  404  reveals that largest portion of the irradiance of the input beam is located away from the center of the core  214 . In  FIG. 4B , shaded area  412  represents a region of the core  214  occupied by the largest irradiance portion of the beam output from the fiber  208 .  FIG. 4B  includes a plot  414  of the irradiance profile associated with the beam output from the core  214 . Vertical axis  416  represents the distance across the core  214  in any direction, horizontal axis  418  represents the irradiance, and curve  420  represents the irradiance of the beam output from the fiber  208 . Plot  414  shows a symmetrical irradiance profile for the beam output from the fiber  208 . 
         [0027]    In alternative embodiments, the single-mode optical fiber of the beam launch can be replaced by a plate with a diffraction-limited pinhole aperture, also referred to as a spatial filter.  FIG. 5  shows a schematic representation of an example implementation of a beam launch  500 . The launch  500  is similar to the launch  106  described above but with the single-mode optical fiber  208  replaced by a plate  502  with a circular pinhole sized aperture  504 . As shown in  FIG. 5 , the centers of the apertures  204  and  504  are located along the optical axis  506  of the lens  206 , which is also the optical axis of the launch  500 . The aperture  504  is a diffraction-limited aperture with a diameter approximately that of a single-mode fiber core. For example, the diameter of the aperture  504  can range from approximately 3-4 microns. 
         [0028]      FIGS. 6A-6B  show a top-plan view and an isometric view of an example beam translator  600 . The translator  600  includes a scanning mirror  602 , a first flat stationary mirror  604  and a second flat stationary mirror  606 . The reflective surface of the first mirror  604  is angled toward the region between the scanning mirror  602  and fixed mirror  606 , and the reflective surface of the second mirror  606  is angled toward the region between the scanning mirror  602  and the first mirror  604 . The reflective surfaces of the mirrors  604 ,  606  and  608  are substantially perpendicular to the same plane with the mirror rotated about an axis that is perpendicular this plane. In the example of  FIGS. 6A-6B , the scanning mirror  602  is a galvanometer mirror that includes a flat pivot mirror  608  attached to a rotatable shaft of a motor  610 , which can be a galvanometer motor or a stepper motor. Alternatively, the scanning mirror can be a piezoelectric controlled mirror. As shown in  FIGS. 6A-6B , the mirror  608  is continuously rotated back and forth by the motor  610  through a range of angles. Because the beam translator  600  is composed of mirrors and not lenses, the translator  600  is essentially achromatic. In other words, the translator  600  operates the same for all wavelengths without chromatic aberrations. 
         [0029]      FIG. 7  shows a top-plan view of the beam translator  600  in operation. A beam of light  700 , such as a beam of light output from the light source  102  as described above, is directed toward the mirror  608 .  FIG. 7  shows the mirror  608  rotated in three positions  1 ,  2  and  3 , which represent just three of a continuum of rotational positions for the mirror  608 . Differently patterned lines  701 - 703  represent three different paths the beam  700  travels through the translator  600  when the pivot mirror  608  is rotated into one of the three positions  1 ,  2  and  3 , respectively. The reflections off of the mirrors  604 ,  606  and  608  are coplanar. As shown in the example of  FIG. 7 , the stationary mirrors  604  and  606  and the pivot mirror  608  are positioned so that the beam is output along one of three substantially parallel paths via four reflections. In other words, the mirror  608  can be rotated into any of one of a continuum of positions that result in the beam being output from the translator  600  after four reflections along one of a continuum of substantially parallel paths. The amount by which the paths  701 - 703  are separated is determined by the amount by which the mirrors  604 ,  606  and  608  are separated. In general, the farther the mirrors  604 ,  606  and  608  are spaced apart, the greater the translation. Ideally the output paths along which the output beam can travel are parallel or non-intersecting, but in practice, it is recognized that the paths may be only approximately parallel or intersect at very long distances away from the translator  600  due to slight variations in the relative placement and orientation of the mirrors. As a result, the paths along which the beam can be output from the translator  600  are referred to as approximately parallel. 
         [0030]    For each rotational position of the pivot mirror  308  that results in the beam  700  being placed on one of the parallel paths, the beam  400  is reflected off of the pivot mirror  308  two times, the first stationary mirror  304  one time, and the second stationary mirror  306  one time for a total of four reflections.  FIGS. 8A-8C  show snapshots of internal paths the beam  700  traveling through the translator  600  when the pivot mirror  608  is rotated into the three positions  1 ,  2  and  3 , respectively. In  FIG. 8A , the pivot mirror  608  is rotated into position  1 . The beam  700  strikes the pivot mirror  608  at the point  802  and undergoes four reflections off of the mirrors  604 ,  606  and  608  with the reflections numbered sequentially  1 ,  2 ,  3  and  4 . The  4   th  reflection off of the pivot mirror  608  at the point  804  places the beam on the path  701  also shown in  FIG. 7 . In  FIG. 8B , the pivot mirror  608  is rotated into position  2 . The beam  700  strikes the pivot mirror  608  at the point  806  and undergoes four reflections off of the mirrors  604 ,  606  and  608  with the reflections numbered sequentially 1′, 2′, 3′ and 4′. The 4′ th  reflection off of the pivot mirror  608  near the point  806  places the beam on the path  702  also shown in  FIG. 7 . In  FIG. 8C , the pivot mirror  608  is rotated into position  3 . The beam  700  strikes the pivot mirror  608  at the point  808  and undergoes four reflections off of the mirrors  604 ,  606  and  608  with the reflections numbered sequentially 1″, 2″, 3″ and 4″. The 4″ th  reflection off of the pivot mirror  608  at the point  810  places the beam on the path  703  also shown in  FIG. 4 . 
         [0031]    When the beam translator  600  is implemented with a galvanometer mirror for the scanning mirror  602  sub-millisecond translation of the output beam is attainable, while typical shuttering times are around 0.2 milliseconds. Additionally, the translator  600  provides an effective means of implementing power control and power stabilization when optical feedback is present, as described above with reference to  FIG. 1 . Precise power control with at least 2 orders of magnitude of dynamic range and 0.1 millisecond time or faster is also attainable. 
         [0032]      FIG. 9A  shows an isometric view of an example beam translator  900 . The translator  900  is a scanning mirror disposed on a track (not shown). The scanning mirror includes a fixed flat pivot mirror  902  attached to a motor  904  that translates the mirror  902  back and forth along the track, as indicated by directional arrow  906 . The mirror  902  is oriented so that the beam of light output from a light source strikes the mirror at a non-normal angle of incidence.  FIG. 9B  shows the translator  900  in operation with the mirror  902  fixed at a 45° angle with respect to of an incident beam  908 . In the example of  FIG. 9B , the mirror  902  is moved into three different positions  1 ,  2  and  3 , which represent just three of a continuum of positions. Differently patterned lines  911 - 913  represent different substantially parallel paths the beam  910  is reflected into when the mirror  902  is translated into the three positions  1 ,  2  and  3 , respectively. The beam translator  900  uses the mirror  902  to translate the beam  908  and, as a result, is also essentially achromatic. 
         [0033]      FIG. 10A  shows an isometric view of an example beam translator  1000 . The translator  1000  includes a transparent plate  1002  attached to a motor  1004  that rotates the mirror  1002  back and forth, as indicated by directional arrows  1006  and  1007 . The plate  1002  can be composed of glass or a transparent plastic with a desired index of refraction, and the motor  1004  can be a galvanometer motor or a stepper motor.  FIG. 10B  shows the translator  1000  in operation with the plate  1002  rotated to refract an incident beam of light  1008 . In the example of  FIG. 10B , the plate  1002  is moved into three different positions  1 ,  2  and  3 , which represent just three of a continuum of positions. Differently patterned lines  1011 - 1013  represent different substantially parallel paths the beam  1010  is refracted into when the plate  1002  is rotated into the three positions  1 ,  2  and  3 , respectively. 
         [0034]    The example beam translators  600 ,  900  and  1000  also preserve s- and p-polarization of the incident beam (i.e., s-polarization refers to light with electric field component direction perpendicular to the plane of the mirrors  604 ,  606  and  608 ). In other words, when a beam is input to the translators  600 ,  900  and  1000  with either s-polarization or p-polarization, the polarization of the beam is preserved as the beam is reflected off of the mirrors  604 ,  606  and  608 . 
         [0035]    ICSs can be incorporated into fluorescent microscopy instruments to control and adjust the irradiance of an excitation beam.  FIG. 11  shows a schematic representation of an example fluorescence microscopy instrument  1100 . There are many different types of fluorescent microscopy instruments and corresponding optical paths. The instrument  1100  is not intended to represent the optical paths within all the different, well-known variations of fluorescence microscopy instruments, but is instead intended to illustrate the general principals of a fluorescent microscopy instrument that includes an ICS. The instrument  1100  includes a light source  1102 , an ICS  1104 , a lens  1106 , a dichroic mirror  1108 , an objective lens  1110 , a stage  1112 , a lens  1114 , and a detector  1116 . The light source  1102  can be a laser that emits a collimated, high-intensity, substantially monochromatic beam of excitation light  1118  that stimulates emission of fluorescent light from fluorophores of fluorescent probes that bind to particular materials in a specimen  1120  disposed on the stage  1112 . The ICS  1104  is configured and operated as described above with reference to  FIG. 1  to receive the excitation beam  1118  and output the excitation beam with a desired and controlled irradiance toward the lens  1106 . 
         [0036]    The lens  1106  focuses the excitation beam and the dichroic mirror  206  reflects the excitation beam into the back of the objective lens  1110 . A portion of the fluorescent light emitted from fluorophores in the specimen  1120  are captured and collimated by the objective lens  1110  into a beam, represented by a shaded region  1122 , that passes through the dichroic mirror  1108 , and is focused onto the detector  1116  by the lens  1114 . The detector  1116  can be a photomultiplier, photodiode, or a solid-state charged coupled device (“CCD”). Alternatively, the dichroic mirror  1108  can be configured to transmit the excitation beam and reflect the fluorescent light, in which case the locations of the ICS  1104  and the light source  1102  are switched with the lens  1114  and the detector  1116 . 
         [0037]    The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the disclosure. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the systems and methods described herein. The foregoing descriptions of specific examples are presented for purposes of illustration and description. They are not intended to be exhaustive of or to limit this disclosure to the precise forms described. Obviously, many modifications and variations are possible in view of the above teachings. The examples are shown and described in order to best explain the principles of this disclosure and practical applications, to thereby enable others skilled in the art to best utilize this disclosure and various examples with various modifications as are suited to the particular use contemplated. It is intended that the scope of this disclosure be defined by the following claims and their equivalents: