Abstract:
Preferential optical splitters are used in a multichannel wavelength measurement device. The optical splitters preferentially provide light at a certain wavelength to a detector. Preferentially providing light to the detectors allows for increased optical efficiencies.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to optical spectroscopic detectors and especially to multichannel spectroscopic measurement units. 
   The optical energy detection at a particular optical wavelength is used for a number of different purposes. Often, detected light intensity at different wavelengths is used to produce a ratio that gives information about a process, such a paper making processes. Typically, one of the wavelengths is related to a process variation such as water level and another is a reference wavelength related to process conditions. 
     FIG. 1A  shows a typical prior art system. In this system, light from optical path  102  is sent to an optical splitter  104 , filter  106 , focusing optics  108 , to the optical energy detector  110 . The optical energy detector  110  is adapted to detect light at the wavelength D 1 . Some of the light passes through the splitter  104 ; this light goes to the second splitter  112 . Half of the light is sent to the optical energy detector  114 . The other portion of the light is sent to the optical splitter  116 . The optical splitter  116  sends half of the light to the optical energy detector  118  and half of the light to the optical mirror  120 . The mirror  120  sends the light to the optical energy detector  122 . In this example, more light is sent to the optical energy detector  110  than is sent to the other optical energy detectors  114 ,  118  and  122 . 
     FIG. 1B  illustrates a prior art system in which light from a fiber optic cable is split. In this example, the optical fiber  128  is split into three branches that equally distribute the light to the optical energy detectors  130 ,  132 ,  134  and  136  after passing through filters  138 ,  140 ,  142  and  143 . 
   SUMMARY OF THE PRESENT INVENTION 
   The present invention uses wavelength preferential optical wavelength splitters in an optical path in order to be more efficient in optical wave energy wavelength separation than conventional optical splitters. 
   One embodiment of the present invention is a multichannel wavelength measuring device including multiple optical detectors. Each detector is adapted to detect light at a different wavelength. The multichannel wavelength device also includes a sequence of optical wavelength splitters in an optical path. Each of the optical splitters is adapted to preferentially provide light to a least one of the detectors at the desired detecting wavelength of the detector. 
   One embodiment of the present invention uses optical splitters to preferentially provide light from an optical path to detectors at the desired detected wavelength of the detectors. The method also uses the detector to detect light at the desired detected wavelength. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  is a diagram that illustrates a prior art system using optical splitters. 
       FIG. 1B  is a diagram that illustrates a prior art fiber optical splitter. 
       FIG. 2  is a diagram that illustrates an embodiment of the present invention using filter that preferentially provides light at certain wavelengths to optical detectors. 
       FIG. 3A-3C  is a diagram that illustrates an exemplanary embodiment of the optical transmission characteristics of the optical splitters of  FIG. 2 . 
       FIG. 4  is a diagram that illustrates an alternate embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
     FIG. 2  shows a multichannel wavelength measurement device  200 . Multiple optical detectors are provided to detect light at different wavelengths. In this example, detectors  202 ,  204 ,  206  and  207  shown. These detectors can be of conventional design used to detect light at certain wavelengths. For the purposes of this patent application, the term “light” includes both visible light and other forms of optical energy, such as infrared light. A sequence of optical splitters  207 ,  210  and  212  are placed in the optical path  214 . The optical splitters are adapted to preferentially provide enough light to one of the detectors at the desired detected wavelength of the detector. For example, in  FIG. 2 , the optical splitter  208  preferentially reflects light at wavelength D 1  to the optical detector. In one embodiment, the optical splitter  208  preferably transmits light at the wavelengths D 2 , D 3 , and D 4  to detectors  207 ,  204  and  206 . Also shown in  FIG. 2  is an optical mirror  216 . The optics  218 ,  220 ,  222  and  224  can be used to focus the light to the detectors  202 ,  204 ,  206  and  207 . Looking again at  FIG. 2  note that this embodiment does not show filters associated with the detectors  202 ,  204 ,  206  and  207 . However, additional optical band pass filters may be used to further filter the light energy going to the detectors. 
   The efficiency of the optical energy detected is improved over the prior art systems. By preferentially providing light using the optical splitters rather than using a traditional optical splitter, more of the optical energy at the wavelengths of interest are sent to the detectors. Preferentially providing light means reflecting or transmitting light at a desired wavelength more than at other wavelengths.  FIG. 2  illustrates an example where 80% of the optical energy of interest is removed from the optical path by each optical splitter. The system of the present invention can be calibrated so that it can compensate for splitter transmission variations. In one embodiment, this calibration is done by sending a test signal through the system and measuring the output from the detectors. Such a calibration is typically already required due to variations in the detector and optics alignment. 
     FIG. 2  shows light of specific wavelengths being extracted from a single optical path section. The optical path can also be fanned out with both reflective and transmitted light from an optical filter  208  sent to additional optical splitters. 
     FIG. 3A  illustrates exemplary optical transmission characteristics of the optical filter  208  of  FIG. 2 . Note that the energy at wavelength D 1  is reflected while energy wavelengths of D 2 , D 3  and D 4  are transmitted. The advantage of optical splitter of  FIG. 2  is that a large percentage of the energy at D 1  is transmitted to the optical detector  202  while allowing the energy of other wavelengths to be transmitted to the other detectors.  FIG. 3B  shows an optical splitter in which a large percentage of the energy at wavelength D 1  is reflected to the detector  204  while energy at wavelengths, D 3  and D 4  are transmitted to the other detectors.  FIG. 3C  shows an optical splitter for which energy at wavelength D 3  is sent to detector  206  while energy at wavelength D 4  is transmitted to be sent to the detector  207 . 
   The selective optical splitters can be produced in a number of different fashions. For example, dialectic material can be deposited upon a glass surface to form a stack that preferentially transmit light at certain frequencies. Such optical high or low pass filters for use as splitters are commercially available. In one embodiment, the light enters the optical splitter at an angle and this can effect the transmission characteristics. In the example of  FIGS. 3A–3C  each of the optical splitters operates as a high pass filter. 
     FIG. 4  shows a system  400  with an optical splitter  402  that operates as a low pass filter passing light at the wavelength D 1  to the wavelength detector  404  while reflecting light at wavelengths D 2 , D 3  and D 4  to the remainder of the system. 
   Multi-wavelength optical detectors are used in the pulp and paper industry for example in moisture and coat weight sensors. A detected signal at a wavelength that indicates the presence of water can be divided by detected signal at a reference wavelength. The use of the reference wavelength removes the dependency of the system to source or path variations. 
   The foregoing description of preferred embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many embodiments were chosen and described in order to best explain the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention for various embodiments and with various modifications that are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims and their equivalence.