Abstract:
The apparatus for sensing plural gases is substantially a gas sensor adopting planar lightwave circuit for constructing reference optical path and sensing optical path, which is a flat structure with abilities of high accuracy, long-term stability, and short response time. The gas sensor can be widely applied for monitoring the safety of a working environment, securing the safety of workers, alerting potential hazard in a factory, inspecting harmful materials in a specific area, testing leakage of a pipeline, inspecting waste gas exhausted from automobile/motorcycle, and monitoring the living quality of household environment.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a gas sensor, and more particularly, to a sensing apparatus capable of detecting and monitoring at least a gas by the use of reference optical paths and sensing optical paths constructed from a planar lightwave circuit of the sensing apparatus.  
       BACKGROUND OF THE INVENTION  
       [0002]     The quantitative and qualitative analysis of gases and their mixtures has been found to be vastly applied in the fields of global environment monitoring, household safety inspecting, greenhouse environmental control, chemical concentration control, and certain applications relating to aerospace industry, etc. Nowadays, it is common to use gas sensors for performing the quantitative and qualitative analysis of gases and their mixtures, since not only the cost of monitoring the gases and their mixtures and the testing cycles required to be perform in the monitoring can be reduce, but also a real-time monitoring of the gases and their mixtures can be achieved thereby. However, cross sensitivity problem is common to those currently available gas sensors, such as semiconductor oxide gas sensors, metal oxide gas sensors, electrochemical gas sensors and solid electrolyte gas sensors, which can cause the reliability and repeatability of a monitoring result perform by such gas sensor to be adversely affected, i.e. the aforesaid gas sensors will fail to measure the individual concentration of each target gases of the monitoring accurately. Although, a gas sensor consists of an array of sensors being sensible to different gases can be used for detecting and measuring a plurality of gases, the cross sensitivity problem still can not be eliminated. For solving the foregoing cross sensitivity problem, the ability of certain gases to absorb infrared radiation has been successfully utilized in developing optical instruments for gas sensing, that is, gases can be selectively detected by the utilization of an infrared sensor via their specific absorption in the infrared spectral range. Despite their functional superiority, the optical gas sensors were not initially popular due to their structural complexity and high manufacturing cost, especially as the size of the optical gas sensor is increasing with the increasing of the amount of optical parts and relating elements of the optical gas sensor needed for detecting and measuring a plurality of gases. Therefore, the optical gas sensor currently available can only be used to detect and measure a gas of the specific infrared spectral range of the gas sensor, that the optical gas sensor can not be adaptively controlled for detecting and measuring various harmful gases coexisted in a same environment.  
         [0003]     Please refer to  FIG. 1 , which is a schematic illustration of a conventional optical gas sensor used for detecting a specific gas. The optical gas sensor  1  of  FIG. 1  is comprised of an infrared radiation source  10 , a reference light source  11 , a chamber  12 , a narrow-band optical filter  13  and a photodiode  14 , wherein the reference light source  11  is disposed in the chamber  12  intermediate to the first and second ends of the chamber  12 , that is, at a distance from the photodiode  14  less than the distance between infrared radiation source  10  and photodiode  14 , and the narrow band optical filter  13  selected for a specific wavelength with respect to a gas to be sensed is mounted between the reference light source  11  and the photodiode  14 . As the infrared radiation source  10  is emitting light of a defined wavelength range to be transmitted and reflected in the chamber  12 , the gas to be sensed in the chamber  12  will absorb the emitted light while enabling the absorbed light of the specific wavelength to pass through the narrow band optical filter  13  to be received by the photodiode  14 . Since the light of the specific wavelength emitted by the reference light source  11  is received by the photodiode  14  without having to travel across the chamber  12  filled of gas to be sensed and thus it is not subject to the absorption of the gas to be sensed, the gas to be sensed can be detected and the concentration of the same can be measured by comparing of the intensity of the light emitted form the reference light source  11 , which is used as a reference value or initial value, with that of the light emitted from the infrared radiation source  10  after passing through the chamber  12 . However, the use of the reference light source in this basic optical gas sensor configuration is to compensate for changes and deterioration of optical components with time and temperature. In practice, the reference light source is added to the sensor to correct for these potential problems.  
         [0004]     There are many optical gas sensor currently available, such as those discloses in U.S. Pat. No. 6,067,840, U.S. Pat. No. 6,469,303, U.S. Pat. No. 6,392,234, U.S. Pat. No. 5,610,400, and U.S. Pat. No. 5,550,375. It is noted that those shown in U.S. Pat. No. 6,067,840, U.S. Pat. No. 6,469,303, U.S. Pat. No. 6,392,234, U.S. Pat. No. 5,610,400, and U.S. Pat. No. 5,550,375 are only suitable for detecting a specific gas while the reference light source for emitting reference light and the infrared radiation source for emitting testing light used in the device shown in U.S. Pat. No. 6,067,840 are two different light sources.  
         [0005]     From the above description, there are four major shortcomings can be summed up as following: 
        (1) By having reference light and testing light to be emitted from two different light sources as those used in prior-art sensors, it is possible that one might not be able to distinct the initial value, being obtained from the reference light representing no target gas sensed, from a response value, being obtained from the testing light representing the existence of the target gas, since the two light sources might begin to deteriorate at different times. Therefore, it is preferred to have the reference light and the testing light to be emitted from a same light source so that the time of deterioration of the two is identical and thus the distinction between the initial value and the response value is ease to identify.     (2) It is known that the reflection index of a material/atmosphere is varying along the change of ambient temperature, pressure or the properties of the material, and the change of reflection index will consequently cause the corresponding optical path to change. Hence, since the length of the optical path of the reference light is different from that of the testing light as those used in prior-art sensors while the initial value is subject to the influence of ambient temperature, pressure and the properties of the material, the accuracy and long-term stability of the gas sensor are reduced.     (3) The prior-art gas sensors can not be adapted for multi-gas testing.     (4) The structure of the prior-art gas sensor can not be flattened.        
 
         [0010]     Therefore, it is in great need to have an apparatus for sensing plural gases that is capable of overcoming the foregoing problems.  
       SUMMARY OF THE INVENTION  
       [0011]     The primary object of the invention is to provide an apparatus for sensing plural gases, which is free from the cross sensitivity problem while it is being used to sense plural gases, and is free from the influences of ambient temperature change, ambient pressure change, wave-guide material property change and the deterioration of light sources so as to increase the accuracy and long-term stability of the aforesaid apparatus, and is a flat gas sensor by the adoption of planar lightwave circuit.  
         [0012]     To achieve the above objects, the present invention provides an apparatus for sensing plural gases, which comprises: 
        a photogenerator, for emitting a signal light;     a planar lightwave circuit having a sensing pathway and a reference pathway, being coupled to the photogenerator by an input port thereof for enabling the same to receive the signal light and thus generate a sensing signal and a reference signal in respective; wherein the sensing pathway and the reference pathway respectively has at least an optic gap and at least an output port; and     at least a photodetector, each being disposed at one of the output ports selected form the group consisting of the output ports of the reference pathway and the output ports of the sensing pathway, for converting the sensing/reference signal into an electric signal.        
 
         [0016]     Preferably, the photogenerator is one device selected from the group consisting of an edge-emitting laser diode, a surface-emitting laser diode, and a light emitting diode.  
         [0017]     Preferably, the interval of the optic gap formed in the sensing pathway is the same as that in the reference pathway, and the length of the sensing pathway is the same as that of the reference pathway.  
         [0018]     Preferably, each optic gap of the reference pathway is sealed by an isolating element.  
         [0019]     Preferably, there can be a filter being disposed between each photodetector and the output port corresponding thereto.  
         [0020]     Moreover, the apparatus for sensing plural gases further comprises a substrate for carrying the photogenerator, the planar lightwave circuit and the photodetectors, wherein the planar lightwave circuit is formed directly on the surface of the substrate, and is made of a material selected from the group consisting of a semiconductor material, a polymer, and a metal.  
         [0021]     Preferably, the apparatus for sensing plural gases further comprises a separation film for isolating dust and dirt while allowing target gases to pass through.  
         [0022]     Preferably, the apparatus for sensing plural gases further comprises a control circuit, coupled respectively to the photogenerator and the photodetectors.  
         [0023]     In a preferred embodiment of the invention, the present invention provides an apparatus for sensing plural gases, which comprises: 
        a plurality of photogenerators, each capable of emitting a signal light;     a plurality of planar lightwave circuits, each having a sensing pathway and a reference pathway and being coupled to one of the photogenerators by an input port thereof for enabling the same to receive the signal light and thus generate a sensing signal and a reference signal in respective; wherein the sensing pathway and the reference pathway respectively has at least an optic gap and at least an output port; and     a plurality of photodetectors, each being disposed at one of the output ports selected form the group consisting of the output ports of the reference pathway and the output ports of the sensing pathway, for converting the corresponding sensing/reference signal into an electric signal.        
 
         [0027]     Preferably, the two lights emitted from any two neighboring photogenerators of the plural photogenerators can be specified to a condition selected from the group consisting of: the condition of the same wavelength and the condition of different wavelengths.  
         [0028]     Preferably, the apparatus for sensing plural gases further comprises at least an intermittent photogenerator, each being disposed between any of the two neighboring photogenerators; wherein a planar lightwave circuit coupled to each intermittent photogenerator is connected to the planar lightwave circuits coupled to the two neighboring photogenerators, and the lights emitted from the intermittent photogenerator and those of the two photogenerators neighboring thereto can be specified to a condition selected from the group consisting of: the condition of the same wavelength and the condition of different wavelengths.  
         [0029]     Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the present invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]      FIG. 1  is a schematic illustration of a conventional optical gas sensor used for detecting a specific gas  
         [0031]      FIG. 2A  is a schematic illustration of an apparatus for sensing plural gases according to a preferred embodiment of the invention.  
         [0032]      FIG. 2B  is a schematic illustration of an isolating element used in the apparatus for sensing plural gases of the invention.  
         [0033]      FIG. 3A  is a schematic diagram showing optic gaps being formed in the sensing pathway according to a preferred embodiment of the invention.  
         [0034]      FIG. 3B  is a schematic diagram showing optic gaps being formed in the reference pathway according to a preferred embodiment of the invention.  
         [0035]      FIG. 4  is a schematic illustration of an apparatus for sensing plural gases according to another preferred embodiment of the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0036]     For your esteemed members of reviewing committee to further understand and recognize the fulfilled functions and structural characteristics of the invention, several preferable embodiments cooperating with detailed description are presented as the follows.  
         [0037]     Please refer to  FIG. 2A , which is a schematic illustration of an apparatus for sensing plural gases according to a preferred embodiment of the invention. The apparatus for sensing plural gases  2  of  FIG. 2A  is formed on a substrate  20 , which can be made of a semiconductor material, a polymer, a metal or a flexible material. The apparatus  2  comprises a photogenerator  21 , a planar lightwave circuit  22 , a filter  23  and at least a photodetectors  24 . In this preferred embodiment, there are two photodetectors, however, the number of the photodetectors is not limited thereby. The planar lightwave circuit  22  is coupled to the photogenerator  21  by an input port  223  thereof for enabling the same to receive the signal light emitted from the photogenerator  21 . It is noted that photogenerator  21  can be an edge-emitting laser diode, a surface-emitting laser diode, or a light emitting diode, that is chosen with respect to the type of gas to be sensed.  
         [0038]     The planar lightwave circuit  22  further has a sensing pathway  221  and a reference pathway  222 . As seen in  FIG. 2A , the sensing pathway  221  is split into two waveguide branches  2212 ,  2213 , each having an optic gap  2210  formed thereon, and then the two waveguide branches  2212 ,  2213  are merged into a pathway. Moreover, the sensing pathway  221  has at least an output port  2211 , where a filter  23  and a photodetector  24  is disposed by arranging the filter  23  at a position between the output port  2211  and the photodetector  24 . Similarly, the reference pathway  222  also is split into two waveguide branches  2224 ,  2225 , each having an optic gap  2220  formed thereon, and then the two waveguide branches  2224 ,  2225  are merged into a pathway. Moreover, the reference pathway  222  also has at least an output port  2223 , where a filter  23  and a photodetector  24  is disposed by arranging the filter  23  at a position between the output port  2223  and the photodetector  24 . It is noted that the number of waveguide branch, such as the waveguide branches  2212 ,  2213  of the sensing pathway  221  and the waveguide branches  2224 ,  225  of the reference pathway  222 , is not limited by two as that shown in the embodiment of  FIG. 2A , that the number of waveguide branch can be three for both the sensing pathway  221  and the reference pathway  222  as those shown in  FIG. 3A  and  FIG. 3B . The purpose of arranging waveguide branches in a pathway is to increase the contact between the signal light and the gases to be sensed so that the accuracy of a measurement using the apparatus can be improved.  
         [0039]     In this preferred embodiment of the invention, the interval of the optic gap  2210  formed in the sensing pathway  221  is the same as that in the reference pathway  222 , and the length of the sensing pathway  221  is the same as that of the reference pathway  222 .  
         [0040]     Please refer to  FIG. 2B , which is a schematic illustration of an isolating element used in the apparatus for sensing plural gases of the invention. The arrangement of the reference pathway  222  in the planar lightwave circuit  22  is to provide a reference for a measurement, whereas gas used to obtain the reference is air. In order to prevent the gases to be sensed from mixing with air, an isolating element is used to seal each optic gap  2210  on each waveguide branches  2224 ,  2225  of the reference pathway  222 , that the isolating element is comprised of two isolating block  2222 , used to filled the two sides of an optic gap, and an isolating plate  2221 , used to cover the top of the optic gap. The apparatus of this embodiment further comprises a control circuit coupled respectively to the photogenerator  21  and the photodetectors  24 . The control circuit is used to control the signal light to be emitted by the photogenerator  21  and to process the electric signals generated by the photodetectors  24 .  
         [0041]     Operationally, the light emitted by the photogenerator  21  will be fed into the planar light wave circuit  22  through the input port  223  thereof, and then the light is split and guided by the operation of the planar light wave circuit  22  to be fed into the sensing pathway  221  and the reference pathway  222 . The light entering the sensing pathway  221  will contact the gases to be senses at the optic gaps  2210  thereof where the intensity of the light is varied by the absorption of the gases acting on the light, and the intensity-varied light pass the filter  23  and enter the photodetector  24  for enabling the photodetector  24  to issue a response signal accordingly. On the other hand, the light entering the reference pathway  222  will be block from contacting the gases to be sensed since the optic gaps thereof is sealed by the isolating element  2221  sop that the intensity of the light is maintained unchanged, and then the intensity-unchanged light pass the filter  23  and enter the photodetector  24  for enabling the photodetector  24  to issue a reference signal accordingly. The filter  23  is used to isolate lights other than the intended light emitted from the photogenerator  21  from entering the photodetector  24 , and the photodetector  24  is used to convert the received response/reference signal into a corresponding electric signal. In addition, in order to prevent the accuracy of the apparatus  2  of the invention to be adversely affected by the pollution of dust or dirt depositing in the optic gaps, a separation film is provided for isolating dust and dirt from entering the apparatus  2  while allowing the plural gases to pass through.  
         [0042]     Please refer to  FIG. 4 , which is a schematic illustration of an apparatus for sensing plural gases according to another preferred embodiment of the invention. The apparatus for sensing plural gases  3  is formed on a substrate  30 , that is basically used for detecting three different gases. The structure of the apparatus  3  is similar to the apparatus  2  shown in  FIG. 2A  and the only difference between the two is that the apparatus  3  has three planar lightwave circuit and the devices corresponding thereto. The apparatus  3  has a first planar lightwave circuit  24 , a second planar lightwave circuit  35  and a third planar lightwave circuit  36 , wherein the input port of the first planar lightwave circuit  34  is coupled to a first photogenerator  31 , and the input port of the second planar lightwave circuit  35  is coupled to a third photogenerator  33 , and the input port of the third planar lightwave circuit  36  is coupled to a second photogenerator  32 . Furthermore, the substrate  30  can be made of a semiconductor material, a polymer, a metal or a flexible material; the wavelengths of the lights emitted from the first photogenerator  31 , the second photogenerator  32  and the third photogenerator  33  can be different from each other and each of the three photogenerator  31 ,  32 ,  33  can be a device selected from the group consisting of an edge-emitting laser diode, a surface-emitting laser diode, and a light emitting diode. It is noted that the wavelength of light emitted by the three respectively is chosen with respect to the type of gas to be sensed.  
         [0043]     The third planar lightwave circuit  36  is split into two waveguide branches  361 , which are connected respectively to the first planar lightwave circuit  34  and the second planar lightwave circuit  35 . As seen in  FIG. 4 , the configuration of the first planar lightwave circuit  34  is the same as that of the second planar lightwave circuit  35 , that the first planar lightwave circuit  34  has a sensing pathway  341  and a reference pathway  342  while the second planar lightwave circuit  35  has a sensing pathway  351  and a reference pathway  352 . Each of the two sensing pathways  341 ,  351  is split into two waveguide branches, each having an optic gap formed thereon, i.e. optic gap  3410  of the first planar lightwave circuit  34  and the optic gap  3510  for the second planar lightwave circuit  35 , and then the two waveguide branches are merged into a pathway. Moreover, each of the two sensing pathways  341 ,  351  has at least an output port, where a filter and a photodetector is disposed by arranging the filter at a position between the output port and the photodetector, i.e. the filter  371  and the photodetector  381  for the sensing pathway  341  and the filter  373  and the photodetector  383  for the sensing pathway  351 .  
         [0044]     Similarly, each of the two reference pathways  342 ,  352  also is split into two waveguide branches, each having an optic gap formed thereon, and then the two waveguide branches are merged into a pathway. Moreover, each of the reference pathways  342 ,  352  also has at least an output port, where a filter and a photodetector is disposed by arranging the filter at a position between the output port and the photodetector, i.e. the filter  372  and the photodetector  382  for the reference pathway  342  and the filter  374  and the photodetector  384  for the reference pathway  352 . In order to prevent the gases to be sensed from mixing with air, an isolating element, i.e. the two isolating elements  3420 ,  3520  shown in  FIG. 4 , is used to seal each optic gap on each waveguide branches of the reference pathways  342 ,  352 . It is noted that the principle of detection of the apparatus shown in  FIG. 4  is the same as that shown in  FIG. 2A , and thus is not described further herein.  
         [0045]     By the proper application of planar lightwave circuit, the apparatus of the invention has advantages list as following: (1) The structure of the apparatus can be flattened for enabling the same to be a thin gas sensor; (2) the apparatus of the invention can be adapted to detect and measure plural gases while the number of gases can be numerous; (3) the signal response time is shortened; (4) the apparatus of the invention can have high accuracy and better long-term stability.  
         [0046]     From the above description, it is noted that all the number of the device used in the apparatus for sensing plural gases of the present invention, such as the numbers of the input port of the planar lightwave circuit, the number of the reference pathway, the number of the sensing pathway, and so on, are only used for illustration and are not limited thereby. In summary that this invention has been disclosed and illustrated with reference to particular embodiments, the principles involved are susceptible for use in numerous other embodiments that will be apparent to persons skilled in the art. Consequently, the present invention has been examined to be progressive and has great potential in commercial applications.  
         [0047]     While the preferred embodiment of the invention has been set forth for the purpose of disclosure, modifications of the disclosed embodiment of the invention as well as other embodiments thereof may occur to those skilled in the art. Accordingly, the appended claims are intended to cover all embodiments which do not depart from the spirit and scope of the invention.