Abstract:
A system and method for managing optical power for controlling thermal alteration of a sample undergoing spectroscopic analysis is provided. The system includes a moveable laser beam generator for irradiating the sample and a beam shaping device for moving and shaping the laser beam to prevent thermal overload or build up in the sample. The moveable laser beam generator includes at least one beam shaping device selected from the group consisting of at least one optical lens, at least one optical diffractor, at least one optical path difference modulator, at least one moveable mirror, at least one Micro-Electro-Mechanical Systems (MEMS) integrated circuit (IC), and/or a liquid droplet. The system also includes an at least two degree of freedom (2 DOF) moveable substrate platform and a controller for controlling the laser beam generator and the substrate platform, and for analyzing light reflected from the sample.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The teachings herein relate to limiting power density induced by a laser and a corresponding temperature increase in a sample interrogated by the laser. 
         [0003]    The invention generally relates to spectral analysis systems, more particularly, the invention particularly relates to improvements in Raman systems to permit effective and rapid sample identification. 
         [0004]    2. Description of the Related Art 
         [0005]    Due to the strong optical absorption in some solids, most of the signal needed to perform a spectroscopic identification is absorbed by the sample and thus unavailable for detection. At the same time, absorption may also lead to a significant thermal change such as a rapid heating, melting and even burning of the sample during the identification process. Absorption may also lead to detonation of some explosive samples. 
         [0006]    Thus, not only are the signal levels from the samples very small, but also these weak signals, particularly Raman signals, may be further obscured by large interfering backgrounds due to the fluorescence from thermally induced changes in the sample. 
         [0007]    For example, white plastics can be easily and rapidly identified in 0.1 seconds with a Raman spectrometer, such as that disclosed in International Publication WO 99/01750, using a 1 Watt diode laser power, while black plastics cannot be identified under the same conditions due to laser induced detrimental changes. 
         [0008]    In order to avoid laser induced detrimental changes in the plastic, it is necessary to decrease the laser power density on the surface of the sample. One way to reduce laser power density is to reduce total laser power that illuminates the surface of the black plastic. But at the same time, to accumulate enough signal for identification the signal collection time has to be increased proportionally. Obviously, this is not acceptable for rapid identification. 
         [0009]    Another way to reduce the power density of the laser beam is to increase the size of the laser spot that illuminates the surface of the plastic, while still maintaining a sufficiently high laser power of 1 Watt to allow rapid identification. Experiments have shown that to avoid laser induced detrimental changes in black plastic samples, in the case of 1 Watt total laser power at wavelength 800 nm, the size of the laser spot illuminating the surface of a black plastic sample needs to be increased 40 times, to a size that is greater than 3 mm in diameter to avoid adverse impact on the sample. As a consequence, the signal acceptance area of the collection fiber bundle and the acceptance area of the spectrograph (slit-height times slit-width) must also be increased 40 times. 
         [0010]    It will be appreciated that increasing the signal acceptance area of a collection fiber bundle by a factor of 40 is difficult, if not impossible, to achieve from a technical point of view. Enlarging the laser spot size without changing the optical train and components would cause the signal from the sample to overfill the collection fiber bundle and thus decrease the collected signal intensity. 
         [0011]    Thus, there exists a need for a quick yet effective method to identify materials using spectral analysis, particularly Raman spectroscopy without damaging the samples. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0012]    In accordance with one embodiment of the invention a system for managing optical power for controlling thermal alteration of a sample undergoing spectroscopic analysis is provided. The system includes a moveable laser beam generator for irradiating the sample and a beam shaping device for moving and shaping the laser beam to prevent thermal overload or build up in the sample. The system also includes a moveable substrate platform and a controller/analyzer for controlling the laser beam generator, the substrate platform, and for analyzing light reflected from the sample. 
         [0013]    In accordance with another embodiment of the invention a method for managing optical power for controlling thermal alteration of a sample undergoing spectroscopic analysis is provided. The method includes selecting a predetermined substrate movement pattern and selecting a predetermined beam movement pattern. The method also includes controlling laser beam dynamics by determining beam power duty cycle and selecting beam diameter change rate. In addition the method, after selecting the substrate material, irradiates and analyzes electromagnetic energy reflected from the sample. 
         [0014]    Embodiments of the invention are also directed towards a system for managing optical power for controlling thermal alteration of a sample undergoing Raman spectroscopic analysis. The system includes at least two degrees of freedom (2 DOF) moveable laser beam generator for irradiating the sample. The moveable laser beam generator includes a beam shaping device selected from the group consisting of at least one optical lens, at least one optical diffractor, at least one optical path difference modulator, at least one moveable mirror, at least one Micro-Electro-Mechanical Systems (MEMS) integrated circuit (IC), and/or a liquid droplet. The system also includes at least two degrees of freedom (2 DOF) moveable substrate platform and a controller for controlling the laser beam generator and the substrate platform. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a high level system architecture of the optical power management system in accordance with an embodiment of the invention. 
           [0016]      FIG. 2  is a flow chart showing one method for open loop analysis of a sample in accordance with the optical power management system shown in  FIG. 1 . 
           [0017]      FIG. 3  is a flow chart showing a method for closed loop analysis of a sample in accordance with the optical power management system shown in  FIG. 1 . 
           [0018]      FIG. 4  is a block diagram of a Micro-Electro-Mechanical Systems (MEMS) chip illustrating controlling beam shape in accordance with the optical power management system shown in  FIG. 1 . 
           [0019]      FIG. 5  illustrates placing a thin layer of heat conductive material on a sample in accordance with the optical power management system shown in  FIG. 1  and the method shown in  FIG. 2 . 
           [0020]      FIG. 6  illustrates placing a thin layer of sample on a heat conductive material in accordance with the optical power management system shown in  FIG. 1  and the method shown in  FIG.2 . 
           [0021]      FIG. 7  illustrates a magnetic stirrer for mixing a liquid sample in accordance with the optical power management system shown in  FIG. 1  and the method shown in  FIG. 2 . 
           [0022]      FIG. 8  illustrates snap freezing the sample for optical power management in accordance with the optical power management system shown in  FIG. 1  and the method shown in  FIG. 2 . 
           [0023]      FIG. 9  illustrates a piezo stirrer for moving a liquid or solid sample in accordance with the optical power management system shown in  FIG. 1  and the method shown in  FIG. 2 . 
           [0024]      FIG. 10  illustrates placing a water droplet on the sample to cool it and use it as an optical means to focus Raman radiation in accordance with the optical power management system shown in  FIG. 1  and the method shown in  FIG. 2 . 
           [0025]      FIG. 11  illustrates compressing a sample to increase the sample density, and thus the signal and thermal diffusivity in accordance with the optical power management system shown in  FIG. 1  and the method shown in  FIG. 2 . 
           [0026]      FIGS. 12A-12D  are diagrams showing use of a deformable lens to change beam focal length in accordance with the optical power management system shown in  FIG. 1  and the method shown in  FIG. 2 . 
           [0027]      FIG. 13  illustrates the use of electronic chopping for optical power management in accordance with the optical power management system shown in  FIG. 1  and the method shown in  FIG. 2 . 
           [0028]      FIG. 14  illustrates the use of mechanical chopping for optical power management in accordance with the optical power management system shown in  FIG. 1  and the method shown in  FIG. 2 . 
           [0029]      FIGS. 15A-15B  illustrate use of an optical grating with variable slit dimension to change the shape of the beam pattern in accordance with the optical power management system shown in  FIG. 1  and the method shown in  FIG. 2 . 
           [0030]      FIGS. 16A-16B  illustrate using a split beam and constructive/destructive interference to move beam in accordance with the optical power management systems shown in  FIG. 1  and  FIG. 2.FIG .  17  shows a waveguide and sample for optical power management in accordance with the optical power management system shown in  FIG. 1  and the method shown in  FIG. 2 . 
           [0031]      FIGS. 18A-18B  illustrate using piezo controlled mirror to move beam in accordance with the optical power management system shown in  FIG. 1  and the method shown in  FIG. 2 . 
           [0032]      FIGS. 19A-19B  illustrate using beam steering using heated glass or polymer to move light or change temperature gradient for induced refractive index gradient in accordance with the optical power management system shown in  FIG. 1  and the method shown in  FIG. 2 . 
           [0033]      FIG. 20A  and  FIG. 20B , collectively referred to herein as  FIG. 20 , shows changing temperature gradient and resulting beam steering in accordance with the optical power management system. 
           [0034]      FIG. 21  shows the use of moveable fiber bundles to change beam location for irradiation and signal collection in accordance with the optical power management system shown in  FIG. 1  and the method shown in  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0035]    Referring to  FIG. 1  there is shown a high level system architecture of the optical power management system  100  in accordance with an embodiment of the invention. The optical power management system includes a controller/analyzer  101  for generating beam control signals: beam power (bp); beam x-position (bx); beam y-position (by); and beam diameter (bd). The beam control signals are provided to beam generator  102 . Using the beam control signals the beam generator  102  initiates a laser beam  103  incident on sample  104 . Various beam control mechanisms in accordance with embodiments of the invention will be discussed herein. It will be understood that the sample  104  may be any suitable sample, such as a solid or liquid sample; or any suitable sample in a condensed phase, for example, gels, pastes, and other forms that may be construed as neither a solid nor a liquid. 
         [0036]    Controller/analyzer  101  also generates substrate control signals: substrate-x position and substrate-y position. The substrate control signals are provided to substrate  105 . Substrate  105  may be any suitable stationary or moveable substrate for holding the sample  104  to be analyzed. For example, in one embodiment the substrate  105  may be a heat conductive material and/or a cooled substrate in order to reduce heat buildup in the sample  104  resulting from laser beam  103 . In another embodiment the substrate  105  may be a spinning substrate bringing the sample  104  within the laser beam  103  according to a fixed periodic rate. It will also be understood that the substrate  105  revolutions may be controlled (increased or decreased) by the controller/analyzer  101  as necessary to prevent excessive temperature build up in sample  104 . It will be further understood that substrate  105  may have at least two or more degrees of freedom. For example, the substrate  105  may be able to move in an x, y, or z direction in a Cartesian coordinate system. It will also be understood that the substrate  105  may be moved to effectively move the sample  104  in or out of the focal plane of the laser beam  103 . 
         [0037]    Controller/analyzer  101  also receives input from Beam dx/dt differentiator  106  which determines velocity of the laser beam  103  in the x-direction. Controller/analyzer  101  also receives input from Beam dy/dt differentiator  107  for determining velocity of the laser beam  103  in the y-direction. 
         [0038]    Similarly controller/analyzer  101  receives input from Substrate dx/dt differentiator  108  which determines velocity of the substrate  105  in the x-direction. Controller/analyzer  101  also receives input from Substrate dy/dt differentiator  109  for determining velocity of the substrate  105  in the y-direction. 
         [0039]    Controller/analyzer  101  also receives input from temperature sensors  110  for determining temperatures of the sample  104  and/or the temperatures of the substrate  105 . It will be appreciated that temperatures of the sample  104  and/or temperatures of the substrate  105  may be temperature gradient profiles of either the sample  104  or the substrate  105 . It will be further appreciated that temperature profiles may be used by the controller/analyzer  101  to optimize repositioning of the substrate  105  holding the sample  104  and the laser beam  103  positioning (bx,by) and laser beam  103  diameter (bd) irradiating the sample  104 . It will also be appreciated that temperature sensors  110  may also provide characteristic sample data prior to resulting from a low level laser probing beam generated by the beam generator  102 . 
         [0040]    Controller/analyzer  101  receives and analyzes Raman signals  111  from the sample  104  resulting from beam generator  102  generating laser beam  103  onto sample  104 . It will be understood that the controller/analyzer  101  may include any suitable spectrometer system. It will also be understood that controller/analyzer  101  and beam generator  102  may be collocated and may include any suitable combination of lens and/or fiber bundles. 
         [0041]    Referring also to  FIG. 2  there is shown a flow chart showing one method for open loop analysis of a sample in accordance with the embodiment shown in  FIG. 1 . Controller/analyzer  101  selects a predetermined substrate movement plan  201  for minimizing the amount of time laser beam  103  is incident on any one spot on the sample  104 . For example, the predetermined substrate movement plan  201  could be a zig-zag pattern or a circular movement plan. 
         [0042]    Next, controller/analyzer  101  determines  202  the laser beam  103  duty cycle. In other words, the ratio of laser beam  103  on-sample-time to on-sample-time plus off-sample-time is the laser beam  103  duty cycle. Referring briefly to  FIG. 13  and  FIG. 14  there are shown two methods of controlling the laser beam  103  duty cycle. 
         [0043]      FIG. 13  illustrates chopping for optical power management in accordance with the embodiments shown in  FIG. 1  and  FIG. 2 . In an electronic chopper an electronic signal to the beam generator  102  is electronically modulated (chopped), thus modulating the light intensity as shown in  FIG. 13 . It will be understood that very short pulses will generate ultrasound in the sample and less energy will be converted into heat, thus resulting in a lower temperature. 
         [0044]      FIG. 14  illustrates the use of mechanical chopping for optical power management in accordance with the embodiments shown in  FIG. 1  and  FIG. 2 . The laser beam  103  is modulated (chopped) by mechanical means. Use of a mechanical/optical chopping means, (e.g., with micro-fluidic cooling) is one example. The result is a modulated light beam  1402  reaching the sample  104  and, similar to the electronic chopping described above, short pulses generate ultrasound in sample  104  and less energy will be converted into heat. 
         [0045]    Returning to  FIG. 1  and  FIG. 2 , the controller analyzer  101  selects a predetermined beam movement plan  203  for minimizing the amount of time laser beam  103  is incident on any one spot on the sample  104 . For example, similar to the substrate movement plan, the predetermined beam movement plan  203  could be a zig-zag pattern or a circular movement plan. 
         [0046]    It will be understood that laser beam  103  may be moved by any suitable method. For example, the laser beam  103  may be laterally moved through the use of a rotating glass plate or lens with suitable refraction characteristics.  FIGS. 12A-12D  are diagrams showing use of a deformable lens  1201  to change beam  103  focal length  103 A. In this way the beam  103  is moved away/towards the sample  104  or moved along the sample  104 . In addition, lens  1201  may be any suitable converging or schlieren lens. In alternate embodiments,  FIGS. 15A-15B  illustrate use of an optical grating  1501  with variable slit dimensions to move or change the shape of the beam pattern  1502 . It will be appreciated that the grating may be changed by any suitable method necessary to generate a diffractive pattern for moving or shaping the laser beam pattern incident upon the sample  104 . In addition, the grating may be generated by acoustic standing waves or by changing the refractive index of air. In another embodiment the beam  103  shape may be moved or changed by splitting the laser beam  103  and changing the optical path length of one of the split beams.  FIGS. 16A-16B  illustrate using a splitter  1601  to split beam  103  and an optical modulator  1602  for generating constructive/destructive interference patterns  1603  of laser beam  103  incident on sample  104 .  FIGS. 18A-18B  illustrate using piezo controlled mirror  1801  to move beam  103  incident on sample  104 .  FIGS. 19A-19B  illustrate using beam steeling by using heated glass  1901  or polymer to move laser beam  103  or changing the temperature gradient for induced refractive index gradient. This principle is illustrated in  FIG. 20  which shows changing temperature gradient and resulting beam steeling. 
         [0047]    In yet another embodiment,  FIG. 21  shows the use of moveable fiber bundles  2101  to change beam  103  location on sample  104  for irradiation and signal collection for subsequent analysis. It will be appreciated that any suitable lens or optical collector, e.g., a spherical mirror, telescope, or a zoom lens system may be used. 
         [0048]    Controller/analyzer  101  also selects, or predetermines, a beam diameter (bd)  204  change rate. For example, in conjunction with the beam and substrate movement plans the controller/analyzer  101  can also vary the diameter size of the laser beam  103  incident on the sample  104 . Referring also to  FIG. 4  there is shown a block diagram of a MEMS chip  401  illustrating controlling laser beam  103  shape. For example, in order to manage the optical power to excite a Raman signal from the sample  104  under test, the MEMS chip  401  with integrated optical fiber (fiber Bragg grating (FBG) or chirped FBG)  402  may be used. The laser beam  103  power distribution for a given area can be dynamically changed by MEMS chip  401 , including embedded micro-heater array  403 . The on-chip close-loop circuit  404  can control the laser beam  103  size while monitoring the sample  104  temperature in real time. It will be appreciated that any suitable method for controlling laser beam  103  size and shape may be used. For example a Digital Micro-mirror Device, or DMD chip, may also be used to shape and steer laser beam  103 . Referring again to  FIG. 2 , a suitable substrate  105  is selected according to predetermined characteristics  205 . As noted earlier, the substrate may be any suitable substrate such as a suitable heat dissipater or a previously cooled substrate.  FIG. 17  shows a waveguide  1701  and sample  104  for optical power management. The waveguide  1701  conducts the laser beam  103  and is also a suitable heat sink to dissipate heat transferred from the sample  104 . 
         [0049]    Controller/analyzer  101  generates command signals to the beam generator  102  to irradiate and analyze  206  the resulting signals returning from the sample  104 . 
         [0050]    Referring now to  FIG. 1  and  FIG. 3 .  FIG. 3  there is shown a flow chart illustrating a method for closed loop analysis of a sample in accordance with the embodiment shown in  FIG. 1 . A sample  104  is prepared  301  for analysis. As noted earlier, the sample  104  may be positioned or otherwise attached to a suitable substrate  105 . 
         [0051]    In alternate embodiments a solid sample may be prepared as shown in  FIG. 5 ,  FIG. 6 ,  FIG. 8 , or  FIG. 10 .  FIG. 5  illustrates placing a thin layer of heat conductive material  501  on the sample  104 .  FIG. 6  illustrates placing a thin layer of the sample  104  on a heat conductive material substrate  105 .  FIG. 8  illustrates snap freezing the sample  104  for optical power management (i.e., reduce sample heat).  FIG. 10  illustrates placing a droplet  1001  of a liquid transparent to the light source on the sample  104  to cool the sample  104 , and also use the droplet as an optical means to focus the laser beam  103 . By changing the droplet geometry using electro wetting or thermo wetting techniques the droplet behaves like a lens with a tunable focal length which enables us to vary the optical power density at the surface of the sample. In addition, the sample may be coated with optical dyes to enhance spectral analysis. 
         [0052]    In yet more alternate embodiments a liquid sample may be prepared as shown in  FIG. 7 ,  FIG. 8 ,  FIG. 9 , or  FIG. 11 .  FIG. 7  illustrates a magnetic stirrer  701  for mixing a liquid sample  702 . The liquid sample  702  is stirred with stirring bar  703  so that new fluid is exposed to the laser light and heating is reduced.  FIG. 8  again illustrates snap freezing the sample  104  for optical power management.  FIG. 9  illustrates a piezo stirrer  901  for moving a liquid or solid sample  902 .  FIG. 11  illustrates compressing sample  104 . Compressing the sample  104  volume by applying mechanical pressure which results in an increase in sample  104  density and a corresponding increased Raman cross-section and better heat conductivity. It will be understood that any of the sample preparations described herein may be used independently or in conjunction with each other or other suitable sample preparations that will allow the sample  104  to be analyzed without destructive irradiation. 
         [0053]    Referring again to  FIG. 3  and  FIG. 1 , the controller/analyzer  101  positions  302  the substrate  105  according to a predetermined x-y position. It will also be understood that the controller/analyzer  101  may dynamically position the substrate according to sample  104  size and selected beam movement plan discussed earlier. Similarly, controller/analyzer  101  positions  303  beam generator  102  according to a predetermined x-y position. It will again be understood that the controller/analyzer  101  may dynamically position the beam generator  102  according to sample  104  size and selected beam movement plan discussed earlier. The controller/analyzer  101  initializes  304  laser beam  103  diameter in accordance with a predetermined beam diameter plan  203  to reduce power density and resulting generated heat within the sample  104 . It will be further understood that beam generator  102  may have at least two or more degrees of freedom. For example, beam generator  102  may be able to move in an x, y, or z direction in a Cartesian coordinate system. It will also be understood that the beam generator  102  may be moved to effectively move the sample  104  in or out of the focal plane of the laser beam  103 . 
         [0054]    Beam power is initialized  305  by the beam generator  102  in accordance with predetermined data or dynamically derived data. For example, sample  104  may be irradiated with a low level laser beam  103  to determine sample characteristics such as sample melting point, reflectance qualities, or detect evaporating molecules as an early indicator for sample degradation. The sample  104  is illuminated for a predetermined time span and properties of the sample, e.g., temperature increase, reflectivity are detected. The material property information is then used by controller/analyzer  101  to maximize the output power for the laser beam  103  so that the sample  104  critical temperature is not exceeded. Still referring to  FIG. 3  and  FIG. 1 , the sample  104  is irradiated  306  by beam generator  102  and reflected signal is analyzed  307  by controller/analyzer  101 . If the analysis is complete  308  the method process stops  309 . Otherwise, the controller/analyzer  101  determines  310  if the sample  104  is exceeding, or will exceed, predetermined criteria, (e.g., a temperature threshold). If the sample  104  predetermined temperature threshold is exceeded, or will be exceeded, under the function parameter set, (e.g., beam position, beam velocity, and beam power), the controller/analyzer  101  optimizes  311  function parameters Bp, Bd, Bx, By, Sx, Sy to minimize sample  104  temperature and/or projected sample temperature. It will be understood that the function parameters may also be in other coordinates such as polar or rotational coordinates to accommodate rotational substrates. Similarly, the controller/analyzer  101  determines  312  if the substrate  105  is exceeding, or will exceed, predetermined criteria, e.g., a temperature threshold or temperature gradient. If the substrate  105  predetermined temperature threshold is exceeded, or will be exceeded, under the function parameter set, e.g., beam position, beam velocity, and beam power, the controller/analyzer  101  optimizes  311  function parameters Bp, Bd, Bx, By, Sx, Sy to minimize the substrate  105  temperature threshold and/or temperature gradient. The controller/analyzer  101  then continues to irradiate  306  the sample  104  until the sample  104  is analyzed or a predetermined time span has been exceeded (not shown). 
         [0055]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, and is generally described by the appended claims.