Abstract:
A scanner and an attenuated total reflection (ATR) objective for use in such scanners are disclosed. The ATR objective includes first and second optical elements and an input port. The input port receives an input collimated light beam that is focused to a point on a planar face of the first optical element by the second optical element such that substantially all of that portion is reflected by the planar face and no portion of the input beam strikes the planar face at an angle less than the critical angle. The second optical element also generates an output collimated light beam from light reflected from the planar thce that is characterized by a central ray that is coincident with the central ray of the input collimated light beam. A light beam converter receives the first collimated light beam and generates the input collimated light beam therefrom.

Description:
BACKGROUND 
     Quantum cascade lasers provide a tunable mid-infrared (MIR) light source that can be used for spectroscopic measurements and images. Many chemical components of interest have molecular vibrations that are excited in the MIR region of the optical spectrum, which spans wavelengths between 5 to 25 microns. Hence, measuring the absorption of MIR light at various locations on a sample can provide useful information about the chemistry of the sample as a function of position on the sample. 
     One class of imaging spectrometers measures the light directly reflected from the sample as a function of position on the sample and wavelength of the illuminating MIR light. The amount of light that is reflected depends on both the chemical and physical attributes of the sample. Hence, comparing spectra generated with direct reflection to absorption with known chemical absorption spectra that are available in libraries presents significant challengers. 
     SUMMARY 
     The present invention includes a scanner and an attenuated total reflection (ATR) objective for use in such scanners. The scanner includes a light source that generates a first collimated light beam and an ATR objective. The ATR objective includes first and second optical elements and an input port. The first optical element includes a planar face, characterized by a critical angle. The input port is adapted to receive a first input collimated light beam characterized by a central ray, the input port being characterized by a pivot point through which the central ray passes and an orientation direction that passes through the pivot point. The second optical element focuses a portion of the first input collimated light beam to a point on the planar surface such that substantially all of that portion is reflected by the planar face and no portion of the input beam strikes the planar face at an angle less than the critical angle. The second optical element generates a first output collimated light beam from light reflected. from the planar face. The first output optical beam is characterized by a central ray that is coincident with the central ray of the first input collimated light beam. A first detector measures an intensity of light in the first output collimated light beam. The scanner also includes a light beam converter that receives the first collimated light beam and generates the first input collimated light beam therefrom in response to an orientation signal that determines an orientation between the orientation direction and the central ray of the input collimated light beam. 
     In one aspect of the invention, the scanner includes a controller that generates the orientation signal and causes the light beam converter to sequence through a predetermined set of different orientations between the orientation direction and the central ray of the input collimated light beam. 
     In one aspect of the invention, the light beam converter includes first and second parabolic reflectors and a beam deflector. The beam deflector receives the first collimated light beam and deflects that light beam to a point on the first parabolic reflector, the point being determined by the orientation signal. The second parabolic reflector is positioned to receive light reflected from the first parabolic reflector and collimate the received light to generate the first input collimated light beam. 
     In another aspect of the invention, the scanner also includes a reflective objective having an input port, an optical element, and a third parabolic reflector. The input port is configured to receive a second input collimated light beam characterized by a central ray. The optical element focuses the second input collimated light beam to a spot at a predetermined point. The optical element receives light reflected from the predetermined point and forms a second output collimated beam therefrom. The second output collimated beam is characterized by a central ray that is coincident with the central ray of the second input collimated light beam. The third parabolic mirror intercepts light reflected from the first parabolic mirror and generates the second input collimated light beam therefrom when the third parabolic minor is in a first position. The third parabolic mirror does not intercept light from the first parabolic mirror when the third parabolic mirror is in a second position. The position of the parabolic material is determined by an actuator that is responsive to a mode signal. 
     In another aspect of the invention, the scanner also includes an actuator that moves the reflective objective in relationship to a specimen stage such that the spot moves in a line parallel to the stage. The controller causes the beam deflector to move the point on the first parabolic reflector such that the spot moves in a direction orthogonal to the line. 
     In another aspect of the invention, the scanner includes a second detector that measures an intensity of light in the first collimated light beam. 
     An ATR objective according to the present invention includes first and second optical elements, an input port and a mask. The first optical element includes a planar face. The input port is adapted to receive a collimated beam of light characterized by a central ray, the input port being characterized by a pivot point through which the central ray passes. The mask divides the collimated beam of light into first and second portions, the mask preventing light in the first portion from reaching the planar face. The second optical element focuses the second portion on a point on the planar face such that substantially all of the second portion is reflected from the planar face, the second optical element collecting light reflected from the planar face and collimating the collected light into an output beam that leaves the input port in a collimated beam having a central ray coincident with the central ray of the input collimated light beam. 
     In one aspect of the invention, the mask absorbs light in the first portion. 
     In another aspect of the invention, the first optical element is transparent to light having a wavelength between 3 and 20 microns. In another aspect, the first optical element is transparent to light having a wavelength between 5 and 12.5 microns. 
     In another aspect of the invention, the first optical element includes a crystalline material, and the planar face is a facet of a crystal of the crystalline material. 
     In another aspect of the invention, the first optical element includes a glass that is transparent to light having a wavelength between 3 and 20 microns. In another aspect, the first optical element is a glass that is transparent to light having a wavelength between 5 and 12.5 microns. 
     In another aspect of the invention, the second optical element is a refractive element. In another aspect, the second optical element is a reflective element. 
     In another aspect of the invention, the input port is characterized by a direction that passes through the pivot point and wherein the point on the planar face depends on an orientation of the central ray of the collimated beam relative to the direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates one embodiment of a direct MIR imaging system. 
         FIG. 2  is a cross-sectional view of an interface crystal that can facilitate the measurement of the absorption of light by a sample in the reflective geometry mode. 
         FIGS. 3A-3D  illustrate a scanning ATR objective for a MIR microscope. 
         FIG. 4  illustrates an ATR objective that utilizes a reflective optical element to image the input collimated light beam. 
         FIG. 5  illustrates the collimated beam motion used to scan the focus point over the reflective face of an ATR objective. 
         FIG. 7  illustrates a parabolic mirror that utilizes two moving mirrors to cause a collimated light beam to scan in two dimensions. 
         FIG. 8  illustrates one embodiment of a dual mode MIR spectrometer according to the present invention. 
         FIG. 9  illustrates a scanning reflective objective that can be used in the spectrometer shown in  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION 
     The manner in which the present invention provides its advantages can be more easily understood with reference to  FIG. 1  which illustrates one embodiment of a direct MIR imaging system. Imaging system  10  includes a quantum cascade laser  11  that generates a collimated light beam  18  having a narrow band of wavelengths in the MIR. In one aspect of the invention, quantum cascade laser  11  is a quantum cascade laser having a tunable wavelength that is under the control of controller  19 . The intensity of light from quantum cascade laser  11  is controlled by a variable attenuator  11   a  that is also under the control of controller  19 . Collimated light beam  18  is split into two beams by a partially reflecting mirror  12 . Light beam  18   a  is directed to a lens  15  that focuses that beam onto a specimen  16  that is mounted on xyz-stage  17  that can position specimen  16  relative to the focal point of lens  15 . Light that is reflected back from specimen  16  is collimated into a second beam that has a diameter determined by the aperture of lens  15  and returns to partially reflecting mirror  12  along the same path as light beam  18   a . While the first and second beams are shown as having the same cross-section in  FIG. 1 , it is to be understood that the second beam could have a different cross-section than the first beam. A portion of the second beam is transmitted through partially reflecting mirror  12  and impinges on a first light detector  13  as shown at  18   b . Light detector  13  generates a signal related to the intensity of light in beam  18   b . Controller  19  computes an image as a function of position on specimen  16  by moving specimen  16  relative to the focal point of lens  15  using xyz-stage  17 . 
     Controller  19  also monitors the beam intensity of the light in collimated light beam  18  using a second light detector  14  that receives a portion of the light generated by quantum cascade laser  11  through partially reflecting mirror  12 . Quantum cascade laser  11  is typically a pulsed source. The intensity of light from pulse to pulse can vary significantly, and hence, the pixels of the image are corrected for the variation in intensity by dividing the intensity measured by light detector  13  by the intensity measured by light detector  14 . In addition, since the light intensity from quantum cascade laser  11  is zero between pulses, controller  19  only sums the ratio of intensities from light detectors  13  and  14  during those times at which the output of light detector  14  is greater than some predetermined threshold. This aspect of the present invention improves the signal-to-noise ratio of the resultant image, since measurements between pulses contribute only noise, which is removed by not using measurements between pulses. 
     Ideally, the input wavelength could be varied over an appropriate range of wavelengths and the light absorbed by the sample determined from the reflected light signal. That absorption spectrum could then be compared to standard absorption spectra from a library to provide information about the chemical composition of the sample at the point being illuminated. The difference in light intensity between the input beam that strikes the specimen and the light that is reflected from the specimen depends on the light that is absorbed by the specimen. Unfortunately, part of the light striking the sample is scattered. A significant fraction of the scattered light does not reach light detector  13 . The scattered light depends on the surface properties of the specimen. For example, a specimen having crystals embedded in its surface will specularly reflect the incoming light in a direction that depends on the angles of the crystal facets with respect to the incoming light. To compare the light losses as a function of wavelength with standard libraries, the contribution of the scattered light must be known or an arrangement in which the scattered light intensity is minimal must be used. 
     One type of reflection spectroscopy that does not suffer from the problems associated with scattered light is referred to as ATR spectroscopy. ATR functions can be more easily understood with reference to  FIG. 2 , which is a cross-sectional view of an interface crystal that can facilitate the measurement of the absorption of light by a sample in the reflective geometry mode. Crystal  21  has a high index of refraction. Light beam  26  enters crystal  21  thorough port  22  and strikes facet  23  at an angle that is greater than the critical angle. The light beam is totally reflected from facet  23  and exits the crystal through port  24 . At the point at which the light beam is reflected from facet  23 , the electric field associated with the light beam extends outside the crystal as shown at  25 . If the medium under facet  23  absorbs light at the wavelength of light beam  26 , the evanescent field will interact with the medium and energy will be transferred from the light beam to the medium. In this case, the energy in the beam leaving crystal  21  will be reduced. The difference in intensity between the input and output beams as a function of wavelength is a spectrum that matches a high-quality transmission spectrum and can easily be used for matching conventional spectra for various chemical compounds. 
     While an interface crystal of the type discussed above is useful in measuring a MIR spectrum of a point on a sample, it presents challenges if an image of an area on the specimen is needed, particularly if the surface of the specimen is not smooth. To form an image, the interface must be moved relative to the specimen. To prevent the interface crystal from damaging the specimen, the specimen must be moved vertically to allow the crystal to be located at the next point of interest. The time for such point to point measurements makes a combination imaging and spectrometer instrument impractical unless very long times are available to generate a spectrum at each point on a specimen in high resolution. 
     The present invention reduces the scanning time for ATR measurements by utilizing an ATR objective and a scanning MIR beam that allows small areas on the sample to be measured in ATR mode without moving the ATR objective of the specimen stage. Refer now to  FIGS. 3A-3D , which illustrate a scanning ATR objective for a MIR microscope. Refer first to  FIGS. 3A and 3B .  FIG. 3A  is a cross-sectional view through an ATR objective  30 , and  FIG. 3B  is a top view of ATR objective  30  as “seen” by a collimated light beam  39  entering ATR objective  30  at a non-normal angle to the top surface of ATR objective  30 . ATR objective  30  includes a crystal  32  having a high index of refraction for light in the MIR. Crystal  32  has a facet  34  that is parallel to the plane of specimen  16 . ATR objective  30  includes an optical component  31  which focuses the collimated light onto facet  34 . A beam blocker  33  prevents light from the center of collimated light beam  39  from striking facet  34  at an angle less than the critical angle, and hence, entering specimen  16 . The light reflected from facet  34  is collimated by optical component  31  and leaves ATR objective  30  along the same beam path as collimated light beam  39 . When light reflects from facet  34 , the evanescent field extends into the specimen. The energy absorbed by specimen  16  reduces the intensity of the light reflected from facet  34 . 
     The details of the optical system that directs light into ATR objective  30  will be discussed in more detail below. For the purposes of the present discussion, it is sufficient to note that the position of spot  38   a  is determined by the angles at which the collimated light beam  39  enters port  35 . The direction of collimated light beam  39  relative to ATR objective  30  can be specified by the two angles shown at  36  and  37 . Consider the XYZ coordinate system shown in the figure. Angle  36  is the angle between the normal to port  35  and the direction of collimated light beam  39 . Angle  37  is the angle between the x-axis and the projection of the direction of collimated light beam  39  on the xy plane. By changing these two angles, the point at which the light beam is focused on facet  34  can be varied. 
     Refer now to  FIGS. 3C-3D , which illustrate ATR objective  30  for a different input direction for collimated light beam  39 .  FIG. 3C  is the same cross-sectional view of ATR objective  30  shown in  FIG. 3A , and  FIG. 3D  is the same top view of ATR objective  30  as shown in  FIG. 3B . In this case, the direction of collimated light beam  39  has changed such that angle  37  is 180 degrees larger than angle  37  shown in  FIG. 3B . The point,  38   b , at which the collimated light is focused on facet  34  has now moved as shown in  FIG. 3C . 
     An ATR objective according to the present invention is defined to be an optical subsystem having an optical element with a reflection face that internally reflects an input collimated light beam from the reflection face. The reflection face is parallel to the plane of the surface of a specimen being imaged. The collimated input beam is focused to a point on the reflection face at a location that is determined by the angular orientation of the collimated input beam relative to the orientation direction that characterizes an input port to the ATR objective. By changing the angular orientation of the input collimated light beam while maintaining the central ray of the input collimated light beam such that the central ray passes through a pivot point associated with the ATR objective, the point on the reflection face at which the light is focused is changed. In addition, the ATR objective also includes a mask that prevents light from the input beam from striking the reflection face at an angle less than the critical angle for the material from which the optical element is constructed, and thus prevents light from the input beam from directly entering the specimen. Finally, the optical subsystem collects light reflected from the reflection face, collimates that reflected light, and causes that collimated light to exit the ATR objective on a path that is coincident with the path at which the input collimated light beam entered the ATR objective. 
     The output optical beam is collimated; however, the central portion of that beam is devoid of light, since the light that would have filled that portion of the output optical beam was removed by the mask. To increase the signal-to-noise in the detector that measures the intensity of light in the output optical beam, the detector can be configured as an annular detector to match the cross-section of the output optical beam with a central region that is insensitive to light. 
     The optical element must be transparent to the MIR light. In one aspect of the invention, the optical element is transparent to light from 3 to 20 microns. In another aspect, the optical element is transparent to light between 5 and 12.5 microns. The later range is sufficient for many chemical identification applications while reducing the cost of the optical element. 
     In addition, a material with a large index of refraction is preferred to minimize the amount of light that must be blocked to prevent the focused light beam from directly passing into the specimen. in one aspect of the present invention, the preferred optical element is a crystal of a material that is transparent to light in the desired scanning range and which has a planar facet that can be utilized as the face. However, a crystalline material for the optical element is not required. For example, a glass that was transparent to the MIR light could be utilized. In one aspect of the invention, the crystal is chosen from the group consisting of ZnS2, Diamond, ZnS, Ge, Thallium bromide, and Si. Chalcogenide glasses which are transparent to light over a broad range of infrared wavelengths are available commercially. 
     The embodiment of an ATR objective shown in  FIGS. 3A-3D  utilizes a refractive optical component  31  to perform the imaging of the input collimated light beam onto the crystal facet. However, a reflective optical element could also be utilized. Refer now to  FIG. 4 , which illustrates an ATR objective that utilizes a reflective optical element to image the input collimated light beam. To simplify the drawing, the housing that supports the components discussed below and the specimen have been omitted from the drawing. ATR objective  40  includes a first reflector  41  having a parabolic reflective inner surface  42  that focuses light reflected from a second parabolic reflector  43 . Second parabolic reflector  43  also serves the masking function provided by beam blocker  33  described above. Light in the center portion of collimated light beam  39  is blocked from being imaged onto reflective face  45 . The area of the second parabolic reflector that does not reflect light that reaches point  46  is preferably coated with a light absorbing material to prevent light reflected by that area from being reflected back in the direction of collimated light beam  39 . 
     The light reflected from parabolic reflective inner surface  42  is focused to a point  46  on reflective face  45  of optical element  44 . The light reflected from reflective face  45  is collimated back into a beam that traverses the same path as collimated light beam  39 . The location of point  46  depends on the orientation of collimated light beam  39  relative to the aperture of input port  47 . 
     Refer now to  FIG. 5 , which illustrates the collimated beam motion used to scan the focus point over the reflective face of an ATR objective. In general, an ATR objective has an input port  52  that is characterized by a point  51  at which the central ray  53  of the collimated input beam intersects the plane of the input port. The scanning motion causes the input collimated light beam to change orientation with respect to input port  52  in a manner in which the intersection of central ray  53  with input port  52  maintains central ray  53  such that central ray  53  always passes through point  51 . That is, central ray  53  pivots about point  51 . The orientation of central ray  53  can be characterized by the angle  55  of central ray  53  with an axis that passes through point  51  and is normal to the input port plane, and by an angle  56  between an axis in the plane of input port  52  and the projection of central ray  53  onto the plane of input port  52 . Hence, to cause the focus point on the reflective surface to scan the reflective surface, an optical system that maintains the central ray of the input beam such that it pivots through point  51  but varies angles  55  and  56  is required. 
     Refer now to  FIG. 6 , which illustrates a scanning spectrometer according to one embodiment of the present invention. Light from laser  61  is split by beam splitter  62  into two beams. The first beam is directed to detector  63   a , which measures the intensity of the laser pulse. The second beam is directed to position modulator  64  which adjusts the point of illumination of the beam on an off-axis parabolic reflector  65 . The position of illumination determines the position at which the light from parabolic reflector  65  strikes a second off-axis parabolic reflector  66 . Parabolic reflector  66  re-collimates the beam and sets the diameter of the beam to match the input aperture of ATR objective  67 . The inclination of the beam entering ATR objective  67  is determined by the point of illumination on parabolic reflector  65 . The light reflected back by ATR objective  67  retraces the path of the incoming light and a portion of that light is directed by beam splitter  62  into detector  63   b . Controller  69  can then determine the amount of light that was lost in the reflection from ATR objective  67 , and hence, determine the amount of light absorbed by specimen  16 . To image another small area on specimen  16 , controller  69  operates a three axis stage  68 . 
     The above-described embodiments utilize a position to cause a collimated light beam to scan the surface of a parabolic mirror. In one aspect of the invention, a parabolic mirror is constructed from two moving mirrors. Refer now to  FIG. 7 , which illustrates a parabolic mirror that utilizes two moving minors to cause a collimated light beam to scan in two dimensions. Parabolic mirror  70  includes a first mirror  72  that causes the input beam  71  to scan in a first direction, X′, and a second mirror  73  that causes the output of the first mirror to scan in a second direction, Y′. Minors  72  and  73  are caused to rotate about axes  74  and  75 , respectively, by actuators  76  and  77 , respectively. The actuators can be constructed from galvanic actuators that cause a single mirror to rotate back and forth. The actuators can also be constructed from a polygon scanning mirror that rotates continuously. A MEMS resonator that deflects a single minor with respect to two axes can also be utilized. 
     Other forms of optical deflectors could also be utilized to cause the beam to scan in two dimensions. For example, acoustic-optical deflectors and electro-optical scanners are also known to the art. In addition, deflectors based on piezo-actuator are known. 
     It would be advantageous to combine the ability to perform ATR imaging spectroscopy with that of a MIR reflective spectrometer such as imaging system  10  described above with respect to  FIG. 1 . In one aspect of the invention, the ATR imaging spectrometer  60  shown in  FIG. 6  is modified to include a second MIR objective to provide a dual mode scanning spectrometer. Refer now to  FIG. 8 , which illustrates one embodiment of a dual mode MIR spectrometer according to the present invention. Spectrometer  80  is similar to ATR imaging spectrometer  60  in operation when ATR imaging is utilized, and hence, the components of spectrometer  80  that serve the same functions as components of ATR imaging spectrometer  60  are given the same numerical labels and will not be discussed further here. Spectrometer  80  includes a moveable parabolic mirror  82  which is moved such that moveable parabolic mirror  82  intercepts the light from parabolic reflector  65 , collimates that light, and directs the collimated light beam to a MIR objective  81  that focuses the collimated beam on specimen  16 . Parabolic reflector  65  is moved via an actuator that is part of parabolic reflector  65  and controlled by controller  69 . To simplify the drawing, the actuator and connection to controller  69  have been omitted from the drawing. 
     When position modulator  64  modulates the position of the light beam striking parabolic reflector  65 , the resulting motion causes collimated beam  83  to alter its orientation relative to the input aperture of MIR objective  81 , and hence, scan a small area on specimen  16 . To scan a larger area, the sample must be repositioned relative to MIR objective  81 . The repositioning can be performed by stage  68  or another mechanism that enhances the speed with which MIR objective  81  moves with respect to specimen  16 . 
     To scan a large area on a sample using ATR mode, the large area must be divided into smaller areas that are scanned by positioning the ATR objective over the area of interest and then moving the ATR objective such that it touches the sample surface. This motion requires the stage to be moved in at least two directions between scan areas. In contrast, when scanning in the MIR reflective mode, the sample does not have to be in contact with the objective. Hence, when moving from one small area to the next, the sample and stage need only move in one direction with respect to one another. The mass of the objective is much less than the mass of the stage, and hence, it is advantageous to move the objective in one direction rather than moving the stage in that direction. 
     Refer now to  FIG. 9 , which illustrates a scanning reflective objective that can be used in the spectrometer shown in  FIG. 8 . Objective  91  is mounted on a rail  92  that moves in the Y-direction. Objective  91  includes a mirror  93  that directs a collimated beam of light to a optical system  94  that focuses that light to a point on specimen  16 . To simplify the drawing a single lens is shown in the drawing; however, it is to be understood that the optical system can involve a number of lens elements. The mass of objective  91  is much smaller than that of stage  98 , and hence, stage  98  can be moved at a much faster speed. Rail  92  includes a linear actuator  95  that couples to objective  91  to provide the required motion  97 . A mirror  96  moves collimated beam  83  shown in  FIG. 8  to objective  91 . In one aspect of the invention, position modulator  64  shown in  FIG. 8  can be operative during the scanning with objective  91  to provide a small scanning amplitude in the X-direction while objective  91  is moved in the Y-direction. This aspect of the invention allows the spectrometer to image in a spatial band, and hence, reduce the number of Y-positions that must be provided by stage  98  to scan a given area. 
     The above-described embodiments of the present invention have been provided to illustrate various aspects of the invention. However, it is to be understood that different aspects of the present invention that are shown in different specific embodiments can be combined to provide other embodiments of the present invention. In addition, various modifications to the present invention will become apparent from the foregoing description and accompanying drawings. Accordingly, the present invention is to be limited solely by the scope of the following claims.