Abstract:
Robust terahertz time-domain spectrometer has a reflective surface arrangement that renders the sensor insensitive to x or y displacement. The apparatus includes: (a) first scanner head; (b) a first reflective surface; (c) emitter; (d) beam splitter to yield reference radiation pulses and sample radiation pulses; (e) first reflector to reflect sample radiation pulses that have been transmitted through the sample to generate reflected sample radiation pulses that are directed towards a web; (f) second reflector that reflects the reference radiation pulses to generate reflected reference radiation pulses that are directed towards the beam splitter which in turn transmits a portion of the reflected references radiation pulses towards the web; and (g) a detector that receives (i) the reflected sample radiation pulses that have interacted with the sample a plurality of times and (ii) reflected reference radiation pulses that have interacted with the sample a plurality of times.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to time-domain spectrometers that enable the sample pulses&#39; phase and amplitude to be tracked and corrected by means of reference pulses thereby significantly increasing the measurement precision of the spectrometers and more particularly to techniques to minimized the effects of scanner head misalignment especially when applied to dual head terahertz spectrometers. 
     BACKGROUND OF THE INVENTION 
     Time-domain systems are important analytical tools for measuring properties of an object. Recently, terahertz systems known as terahertz time-domain spectrometers (THz-TDS) have been developed. These systems often use visible to near-infrared laser pulses each lasting only 10 to several hundred femtoseconds to electromagnetic pulses (“T-rays”) that each last for about a picosecond. T-rays can be transmitted through various objects, using an imaging system of lenses and mirrors to focus or collimate the T-rays. As the T-rays pass through the object under test, they are typically distorted. These changes in the T-ray signals can be analyzed to determine properties of the object. Materials can be characterized by measuring the amounts of distortion—from absorption, dispersion and reflection—of the T-rays passing through to a detector. A digital signal processing system takes the digitized data from the THz detector and analyzes the data in either the spectral or temporal domain. 
     Because many compounds change T-rays in characteristic ways (e.g., absorption or dispersion), molecules and chemical compounds show strong absorption lines that can serve as “fingerprints” of the molecules. T-ray spectroscopy can distinguish between different chemical compositions inside a material even when the object looks uniform in visible light. A terahertz sensor for instance can be employed to measure caliper, moisture, and basis weight of paper whose thickness is similar to the wavelengths of T-Rays. 
     The precision of amplitude and phase measurements in time-domain (terahertz) spectroscopy (THz-TDS) is often limited by noise in the system. It has been demonstrated that the dominant types of noise present in THz-TDS are often time base and amplitude jitter characterized by pulses traveling through the same material (or air) which reach the detector at slightly different times and with slightly different amplitudes due to fluctuations in environmental parameters (e.g., temperature fluctuations or vibrations) or hardware errors (e.g., encoder errors in the delay line). In some specific THz-TDS systems, jitter makes a significant contribution to the noise and therefore limits the measurement precision of the system. In other THz-TDS systems, it is the multiplicative noise (i.e., amplitude noise), which comes primarily from the laser that is the main source of imprecision. 
     U.S. Pat. No. 8,378,304 to Mousavi et al. discloses an apparatus for implementation of time-domain spectroscopy that creates a continuous set of reference pulses whereby a sample pulses&#39; phase and amplitude can be tracked and corrected. The apparatus can be readily adopted into existing time-domain spectrometers where both amplitude and phase are of interest. A feature of the apparatus is that when it is employed in a THz-TDS, the effect of jitter can be significantly reduced. 
     U.S. Pat. No. 8,638,443 to Haran and Savard discloses a method, for compensating for errors in spectrometers, that includes measuring at least a portion of a path length for a signal traveling through the spectrometer during a measuring scan of a material. The detector signal corresponding to the measurement scanner is generated. Compensation for errors in the detector and signal is provided based on the measurement path length. 
     Typically, on-line spectrometer sensor devices are operated to periodically traverse, or “scan,” traveling webs of sheet material during manufacture. Scanning usually is done in the cross direction, i.e., in the direction perpendicular to the direction of sheet travel. These sensors typically employ single or double sided packages which traverse the width of the sheet, guided on rail systems affixed to stiff beam structures. The accuracy of the sensor system is related to the relative x, y, and z displacement alignment between upper and lower sensor halves. The scanner heads can become misaligned up to a few millimeters or more between forward and backward scanning directions. Even small displacements can adversely affect the detected THz signal. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a robust terahertz spectrometer that incorporates reflective surface arrangement that renders the sensor insensitive to x or y displacement especially when the spectrometer has dual scanner heads. 
     In one aspect, the invention is directed to a time-domain spectroscopy apparatus, that has a continuous reference for measuring at least one property of a sample that is a continuous web having a first side and a second side which travels in a downstream direction, that includes: 
     (a) a first scanner head disposed adjacent to the first side of the web, the first scanner head having a first operative surface facing the first side of the web; 
     (b) a first reflective surface facing the second side of the web; 
     (c) an emitter positioned in the first scanner head that generates pulses of radiation; 
     (d) means for splitting the pulses of radiation to yield reference radiation pulses and sample radiation pulses wherein the sample radiation pulses are directed to the first side of the web; 
     (e) a first reflector positioned in the first scanner head to reflect sample radiation pulses that have been transmitted through the sample to generate reflected sample radiation pulses that are directed towards the web; 
     (f) a second reflector positioned in the first scanner head that reflects the reference radiation pulses to generate reflected reference radiation pulses that are directed towards the means for splitting the pulses of radiation which transmits a portion of the reflected references radiation pulses towards the web; and 
     (g) a detector positioned in the first scanner head to receive (i) the reflected sample radiation pulses that have interacted with the sample a plurality of times before being detected and (ii) reflected reference radiation pulses that have interacted with the sample a plurality of times before being detected. 
     In one embodiment of the apparatus that has dual scanner heads, a second scanner head is disposed adjacent to the second side of the web. The second scanning head has a second operative surface facing the second side of the web and includes the first reflective surface. The first and second scanner heads move in a synchronized fashion along a cross direction. In another embodiment of the apparatus, the first reflective surface is formed on a metallic roll or on an elongated beam, that is positioned parallel to movement of the first scanner head. 
     In a further aspect, the invention is directed to a method of improving the precision of a time-domain spectroscopy apparatus that includes an emitter, positioned in a first scanner head that generates pulses of radiation, and a detector, positioned in the first mounting head that receives pulses of radiation that interacts with a sample that is a continuous web, the method including the steps of: 
     positioning a first reflective surface; 
     positioning a sample reflector in the first scanner head; 
     positioning a reference reflector in the first scanner head; 
     positioning a planar beam splitter along an optical path between the emitter and the detector to branch pulses of radiation from the emitter into sample radiation pulses and reference radiation pulses wherein the planar beam splitter and the first reflective surface are parallel and define a portion of the measurement gap through which the continuous web travels; and 
     directing the pulses of radiation from the emitter at the planar beam splitter at an incident angle between 0 and 90 degrees such that the sample radiation pulses interact with the sample a plurality of times along a first optical path between the sample reflector and the detector and such that the reference radiation pulses interact with the sample a plurality of times along a second optical path between the reference reflector and the detector. 
     With the present invention, by orienting the incident beam, such as THz radiation, from the emitter at a sufficiently small angle relative to the beam splitter, the displacement at the converging lens at the detector will be small even with misalignment. In addition, the z displacement that is between the scanner heads can be corrected by measurement of a reference beam. Finally, with the novel configuration both the sample beam and the reference beam goes through the sheet a plurality of times thereby improving sensor precision. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 and 3  depict dual scanner head devices for generating continuous referencing in time-domain spectroscopy; 
         FIGS. 2 and 4  depict devices with for generating continuous referencing in time-domain spectroscopy wherein the lower reflective surface is formed on a metallic roll or beam; and 
         FIGS. 5 and 6  show sheetmaking systems implementing the robust spectrometer configurations. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention is directed to techniques for enhancing the precision of time-domain spectroscopy systems, which can be implemented by modifying standard time-domain spectrometers. While the invention will be described in relationship with a terahertz time-domain spectrometer, it is understood that the invention improves the precision of any time-domain spectrometer wherein pulses of radiation are applied to a sample and the time resolved characteristics of transmitted pulses that emerge from the sample are analyzed. 
       FIG. 1  illustrates one embodiment of the continuous real time referencing generating device  10  that include scanner sensor heads  2  and  4 . The upper scanner head  2  houses emitter  16 , retro-reflectors or mirrors  18 ,  20 , detector  14  and beam splitter  22 . The lower head  4  includes a reflective surface  24 . The operative surfaces  70  and  72  on the upper and lower scanner heads  2  and  4 , respectively, define a measurement gap through which a web of material  68 , such as paper, moves in the machine direction (MD). Lateral openings  92  and  94  of the measurement gap allow the scanner to move in a perpendicular cross direction (CD) as the paper travels in the MD. Openings  92  and  94  serve as the web entry and web exit, respectively. Movement of the dual scanner heads  2 ,  4  is synchronized with respect to speed and direction so that they are aligned with each other. 
     The planar beam splitter  22  is parallel with the planar specular reflective surface  24  which can consist of a mirror. For terahertz radiation, a preferred beam splitter comprises a thick piece of high resistivity (&gt;10,000 O-cm) silicon slab. The thickness of the beam splitter is selected to be large enough so that reflections from the back surface thereof lie outside the measurement window. If a thinner beam splitter is used, the pulse shape will look different since multiple reflections from the backside are included in the window. In principal either configuration can be employed with the present invention. A thicker beam splitter is preferred because it is less prone to vibrate. The high resistivity silicon slab is particularly suited for use as a beam splitter as it has a high refractive index with acceptable absorption in terahertz frequencies. Alternatively, sapphire or polyethylene could be used as the beam splitter although they have higher absorption than that of silicon. For terahertz spectrometers, the emitter and detector can be, for example, photoconductive antennae. 
     Emitter  16  and associated focusing lens  66  generate incident light  30  that is incident on the beam splitter at an incident angle that ranges from greater than 0 to 90 degrees and preferably from 15 to 75 degrees. Retro-reflector  18  is oriented to reflect light that is branched from beam splitter  22 , retro-reflector  20  is oriented to reflect light that is reflected from reflective surface  24 , and detector  14  and associated converging lens  64  is oriented to capture light reflected from reflective surface  24 . Sources of dry air  12  are positioned to prevent debris from interfering with detector  14 , emitter  16  and retro-reflectors  18  and  20 . Dry air can also be used to purge debris along the optical paths. Tilting between the two scanner heads could introduce errors to the measurement. Low-resolution z-sensors  26  and  28  can be employed to continuously account for this displacement by measuring the z gap distance between the scanner heads. 
     In operation, laser pulses  6  and  8  are directed to emitter  16  and detector  14 , respectively. Initially, a terahertz radiation pulse that is generated by emitter  16  is incident on beam splitter  22  at a small incident angle resulting in two pulses traveling in perpendicular directions. Pulse  40  is transmitted through sample  68  whereas the reflected pulse  32  is used to track the fluctuations in time and amplitude. Since the two pulses are generated at the same time and position, their phases and amplitudes correlate very strongly and this correlation is used to correct measurement errors. A pulsed laser source (such as a femtosecond (fs) laser) can be employed to generate laser pulses  6  and  8 . Typically, the fs laser emits a beam of optical pulses and a beam splitter splits the optical pulses into two beams, a reflected beam and a transmitted beam. The transmitted beam comprising laser pulses  6  is directed to the emitter  16  whereas the reflected beam comprising laser pulses  8  is delayed a specified amount of time before being directed to detector  14 . 
     Transmitted light  42  is reflected by reflective surface  24  into retro-reflector  20  and the returned light  44  is reflected by reflective surface  24  towards beam splitter  22 . Light  44  is partially reflected by beam splitter  22  towards reflective surface  24  and partially transmitted into lens  66  and emitter  16 . The reflected light  46  is thereafter reflected by reflective surface  24  to generated sample light  38  that is collected by converging lens  64  into detector  14 . Pulses of light  40  from emitter  16  travel through and interact with different parts of the sample web  68 . In this configuration, the sample light passes through web  68  six times as the light propagates through the measurement gap. 
     With respect to reflected light  32  which is directed toward retro-reflector  18 , the returned light  34  is partially reflected by beam splitter into lens  66  and emitter  16  and the remaining portion consisting of light  36  is transmitted through beam splitter  22  towards reflective surface  24 . Reference light  48  is collected by lens  64  into detector  14 . The reference light passes through web  68  twice as the light propagates through the measurement gap. It is preferred that the positions of detector  14 , emitter  16 , and retro-reflectors  18  and  20  be aligned so that the sample and reference lights propagate along the machine direction. 
     The tolerance for this design in the x-y plane depends on the mirror dimensions and could be in the centimeter range, for instance, which is large enough for industrial applications, such as, for monitoring paper in a paper making machine. The tolerance it is difficult to assess in the z direction (the gap) because the tolerance depends on the two scanner heads&#39; mechanical spacing (gap), the optical path and components used. Thus if the system as a 5 cm diameter beam splitter ( 22 ), beam ( 30 ) width of 3 cm incident at 45 deg, retro-reflector ( 20 ) and lens ( 64 ) both having a 5 cm diameter and heads gap being 2.5 cm, then the system depicted in  FIG. 2  would be robust against ±300 micron variations which are typical z variations encountered in industrial scanners. 
       FIG. 2  illustrates another embodiment of the continuous real time referencing generating wherein like reference numbers refer to the same features described in  FIG. 1 . Spectrometer  110  employs a reflective surface  124  instead of the lower scanner head  4  ( FIG. 1 ). As shown in  FIG. 2 , the reflective surface  124  can be formed on a metallic roll, elongated beam member or any suitable substrate that presents a planar specular reflecting surface. 
       FIG. 3  illustrates another embodiment of the continuous real time referencing generating device  60  that include scanner sensor heads  2  and  4 . The upper scanner head  2  houses emitter  16 , retro-reflectors or mirrors  18 ,  20 , detector  14  and beam splitter  22 . The lower head  4  includes a reflective surface  24 . The operative surfaces  70  and  72  of the upper and lower scanner heads  2  and  4 , respectively, define a measurement gap through which a web of material  68 , such as paper, moves in the MD. Lateral openings  92  and  94  of the measurement gap. Movement of the dual scanner heads  2 ,  4  is synchronized with respect to speed and direction so that they are aligned with each other. 
     The planar beam splitter  22  is parallel with the planar specular reflective surface  24  which can consist of a mirror. Specular reflectors  50  and  52  are positioned along the downstream and upstream position relative to beam splitter  22 . Emitter  16  and associated focusing lens  66  generate incident light  30  that is incident on the beam splitter at an incident angle that ranges from greater than zero to 90 degrees and preferably from 15 to 75 degrees. Retro-reflector  18  is oriented to reflect light branched from beam splitter  22 , retro-reflector  20  is oriented to reflect light that is reflected from reflective surface  24 , and detector  14  is oriented to capture light that is reflected from reflective surface  24 . Sources of purging dry air  12  are strategically located in the spectrometer. Tilting between the two scanner heads is accounted for with low-resolution z-sensors  26  and  28 . 
     In operation, laser pulses  6  and  8  are directed to emitter  16  and detector  14 , respectively. Initially, a terahertz radiation pulse is generated by emitter  16  is incident on beam splitter  22  at a small angle resulting in two pulses traveling in perpendicular directions. Pulse  40  is transmitted through sample  68  whereas the reflected pulse  32  is used to track the fluctuations in time and amplitude. Since the two pulses are generated at the same time and position, their phases and amplitudes correlate very strongly and this correlation is used to correct measurement errors. 
     Transmitted light  142  is reflected by reflective surface  24  and reflector  50  as the light travels into retro-reflector  20 . Similarly, returned light  144  is reflected by reflective surface  24  and reflector as the light travels towards beam splitter  22 . Light  144  is partially reflected by beam splitter  22  towards reflective surface  24  and partially transmitted into lens  66  and emitter  16 . The reflected light  46  is thereafter reflected by reflective surface  24  to generated sample light  138  that is focused by lens  64  into detector  14 . Pulses of light  40  from emitter  16  travel through and interact with different parts of the sample web  68 . In this configuration, the sample light passes through web  68  eight times as the light propagates through the measurement gap. 
     With respect to reflected light  32  which is directed toward retro-reflector  18 , the returned light  34  is partially reflected by beam splitter  22  into lens  66  and emitter  16  and the remaining portion consisting of light  36  is transmitted through beam splitter  22  towards reflective surface  24 . Reference light  148  is focused by lens  64  into detector  14 . The reference light, which is reflected by reflective surface  24  and reflector  52  passes through web  68  four times as the light propagates through the measurement gap. It is preferred that the positions of detector  14 , emitter  16 , and retro-reflectors  18  and  20  be aligned so that the sample and reference lights propagate along the machine direction. 
       FIG. 4  illustrates another embodiment of the continuous real time referencing generating wherein like reference numbers refer to the same features described in  FIG. 3 . Spectrometer  160  employs a reflective surface  124  instead of a lower scanner head  4  ( FIG. 3 ). As shown in  FIG. 4 , the reflective surface  124  can be formed on a metallic roll, elongated beam member or any suitable substrate that presents a planar specular reflecting surface. 
       FIG. 5  illustrates a particular implementation of the robust terahertz spectrometers that have dual scanner heads as depicted in  FIGS. 1 and 3 . In particular, the emitter, beam splitter, detector, retro-reflectors, and reflective surface are housed in a dual head scanner  88  of scanner system  80  which can be employed to measure properties in paper or materials. Upper scanner head  90  moves repeatedly back and forth in the CD across the width of the moving sheet  86 , which moves in the MD, so that the characteristics of the entire sheet are measured. Scanner  88  is supported by two transverse beams  82 ,  84  on which are mounted upper and lower scanning heads  90 ,  92 . The operative faces of the lower and upper scanner heads  90 ,  92  define a measurement window or cell that accommodates sheet  86 . The lower scanner head  92  may include a sheet stabilization system such as an air-bearing stabilizer (not shown) to maintain the sheet on a consistent plane as it passes through the measurement cell. The movement of the dual scanner heads  90 ,  92 , is synchronized with respect to speed and direction so that they are aligned with each other. 
       FIG. 6  illustrates a particular implementation of the robust terahertz spectrometers that employ a single scanner head as depicted in  FIGS. 2 and 4 . In particular, the emitter, beam splitter, detector and retro-reflectors, and reflective surface are housed in scanner  190  of scanner system  180 . Scanner system  188  is supported by two transverse beams  182 ,  184  with upper scanning head  190  being mounted to traverse along the axis of beam  182 . A reflective substrate  192  is positioned below scanner head  190  such that operative face of upper scanner head  190  and the reflective substrate  192  define a measurement gap that accommodates sheet  186 . Upper scanner head  90  moves repeatedly back and forth in the CD across the width of the moving sheet  186 . The reflective substrate  192  is stationary. 
     The present invention can be implemented in time-domain spectroscopy systems. Near THz or THz-TDS can be used in-situ to coincidentally obtain one or more parameters/properties of a sheet material including the water weight, physical thickness (caliper) and dry weight volume fraction. The sheet material can comprise paper or a plastic. From these parameters/properties in combination with one or more calibration parameters, caliper, basis weight and moisture content and other physical characteristics of the sheet material may be obtained. 
     An algorithm for using data entails conducting a calibration whereby reference measurements of pulses ( 36 ) and ( 40 ) is taken without any web product positioned between the scanner heads and the measurements are stored in memory. In operation, measurements of pulses ( 36 ) and ( 40 ) are taken repeatedly to extract product parameters. The product will induce a different shift in time and modify the shape or amplitude differently on both ( 36 ) and ( 40 ). In order to extract the product parameters, different transfer functions using Fresnel equations are applied on pulses ( 36 ) and ( 40 ) to reproduce the ones measured. Residuals are minimized to get accurate values of product composition. 
     The foregoing has described the principles, preferred embodiments and modes of operation of the present invention. However, the invention should not be construed as being limited to the particular embodiments discussed. Thus, the above-described embodiments should be regarded as illustrative rather than restrictive, and it should be appreciated that variations may be made in those embodiments by workers skilled in the art without departing from the scope of the present invention as defined by the following claims.