LU CHAO (AU)
MINASIAN ROBERT (AU)
TONG CHEN (AU)
UNIV HONG KONG POLYTECHNIC
WO2004113887A2 | 2004-12-29 |
GB1237547A | 1971-06-30 | |||
CA2108961A1 | 1995-04-22 | |||
US3734631A | 1973-05-22 | |||
UA82080C2 | 2008-03-11 |
CLAIMS 1 . An optical sampling system including: an electromagnetic radiation source for emitting electromagnetic radiation of at least a first and second wavelength; a sampling volume for projecting the electromagnetic radiation through; an optical sensing unit for sensing the intensity level of said first and second wavelength after transmission through said sampling volume. 2. An optical sampling system as claimed in claim 1 further including: a differencing unit for determining the difference in the intensity level of the sensed first and second wavelengths. 3. An optical sampling system as claimed in any previous claim further comprising an RF modulating unit for modulating the electromagnetic radiation of the electromagnetic radiation source before projection through said sampling volume. 4. An optical sampling system as claimed in claim 3 wherein said RF modulating unit produces an out of phase difference between the modulated electromagnetic radiation. 5. A method of sampling to determine the presence or absence of an absorbing medium, the method including the steps of: (a) projecting at least a first and second wavelength signals thorough the absorbing medium with the first and second wavelengths having differeing medium absorption characteristics; (b) determining a difference in the relative absoption of the two wavelength signals within the medium; and (c) utilising the difference as a measure of the degree of absorbing medium present. 6. A method as claimed in claim 5wherein said first and second wavelength signals are RF modulated. 7. A method as claimed in claim 5 wherein said first and second wavelength signals are projected through the absorbing medium multiple times. 8. An optical sampling system including: an electromagnetic radiation source for emitting electromagnetic radiation of at least a first and second wavelength having a predetermined phase relationship; with said first wavelength having a first absorption characteristic to a sample medium and said second wavelength having a second absorption characteristic to said sample medium; an electro-optic modulator for combining and modulating the first and second wavelengths to produce modulated wavelength radiation; a sampling volume for projecting the modulated wavelength radiation through a sample medium to produce sample attenuated radiation; and an optical sensing unit for sensing the relative attenuation of at least the first wavelength relative to the second wavelength. 9. An optical sampling system as claimed in claim 8 wherein said optical sensing unit includes a photodiode converting said sample attenuated radiation to a corresponding modulation level for modulating said electro-optic modulator. 1 0. An optical sampling system as claimed in claim 8 wherein said modulated wavelength radiation is projected multiple times through said sampling medium to amplify the relative differences in the first and second absorption characteristics. 1 1 . A method of optically sampling, the method including the steps of: (a) creating an optical signal including a first and second wavelength components having a predetermined phase relationship; (b) mixing the first and second wavelength components and applying an RF frequency modulation to the mixed components to produce a first output signal; (c) projecting the first output signal through a sampling chamber, wherein said first and second wavelength components have different absorption characteristics to a desired sample material; (d) detecting the sampling chamber optical output of the sampling chamber; and (e) determining a resultant intensity level profile of the sampling chamber optical output; and (f) determining the relative absorption intensities of said first and said wavelength components from the RF modulated nature of the intensity level profile. 1 2. A method as claimed in claim 11 wherein the first optical signal is looped through said sampling chamber multiple times. |
Field of the Invention
[0001 ] The present invention relates to optical sensing of gases and liquids and, in particular, discloses a rapid and sensitive optical sensor capable of detecting different gaseous or liquid substances.
Background
[0002] Any discussion of the background art throughout the specification should in no way be considered as an admission that such art is widely known or forms part of common general knowledge in the field.
[0003] There is a general need to accurately sense chemical substances in gaseous, liquid or solid form. Often, the sensing is done optically, with a light source probe which probes an atmosphere for the presence or absence of particular gases or fluids. For example, United States Patent 5,905,270 to McCaughey et al. discloses a gas sensing device relying on infra red absorption by a sensed gas.
[0004] There is a general need for accurate and rapid sensing of gases and liquids.
Summary of the invention
[0005] It is an object of the present invention to provide an improved form of optical sensor providing rapid and sensitive sensing of target materials.
[0006] In accordance with a first aspect of the present invention, there is provided and optical sampling system including: an electromagnetic radiation source for emitting electromagnetic radiation of at least a first and second wavelength; a sampling volume for projecting the electromagnetic radiation through; and an optical sensing unit for sensing the intensity level of the first and second wavelength after transmission through the sampling volume.
In some embodiments, the RF modulating unit produces an out of phase difference between the modulated electromagnetic radiation.
[0007] The system preferably also includes a differencing unit for determining the difference in the intensity level of the sensed first and second wavelengths. [0008] In some embodiments, the system further includes an RF modulating unit for modulating the electromagnetic radiation of the electromagnetic radiation source before projection through the sampling volume.
[0009] In accordance with a further aspect of the present invention, there is provided a method of sampling to determine the presence or absence of an absorbing medium, the method including the steps of: projecting at least a first and second wavelength signals thorough the absorbing medium with the first and second wavelengths having differing medium absorption characteristics; determining a difference in the relative absorption of the two wavelength signals within the medium; and utilizing the difference as a measure of the degree of absorbing medium present.
[001 0] In some embodiments, the first and second wavelength signals are RF modulated and the first and second wavelength signals are projected through the absorbing medium multiple times.
[001 1 ] In accordance with a further aspect of the present invention, there is provided an optical sampling system including: an electromagnetic radiation source for emitting electromagnetic radiation of at least a first and second wavelength having a predetermined phase relationship; with the first wavelength having a first absorption characteristic to a sample medium and the second wavelength having a second absorption characteristic to the sample medium; a direction detection or an electro-optic modulator for combining and modulating the first and second wavelengths to produce modulated wavelength radiation; a sampling volume for projecting the modulated wavelength radiation through a sample medium to produce sample attenuated radiation; and an optical sensing unit for sensing the relative attenuation of at least the first wavelength relative to the second wavelength.
[001 2] The optical sensing unit preferably can include a photodiode converting the sample attenuated radiation to a corresponding modulation level for modulating the electro-optic modulator.
[001 3] The modulated wavelength radiation can be projected multiple times through the sampling medium to amplify the relative differences in the first and second absorption characteristics.
[0014] In accordance with a further aspect of the present invention, there is provided a method of optically sampling, the method including the steps of: creating an optical signal including a first and second wavelength components having a predetermined phase relationship; mixing the first and second wavelength components and applying an RF frequency modulation to the mixed components to produce a first output signal; projecting the first output signal through a sampling chamber, wherein said first and second wavelength components have different absorption characteristics to a desired sample material; detecting the sampling chamber optical output of the sampling chamber; and determining a resultant intensity level profile of the sampling chamber optical output; and determining the relative absorption intensities of said first and said wavelength components from the RF modulated nature of the intensity level profile.
[001 5] Preferably, the first optical signal is looped through said sampling chamber multiple times. Brief Description of the Drawings
[001 6] Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:
[001 7] Fig. 1 illustrates schematically a first optical sensor design of the preferred embodiments;
[001 8] Fig. 2 illustrates schematically a second modified optical sensor design of the present invention;
[001 9] Fig. 3 illustrates the RF responses for absorption at different wavelengths; and
[0020] Fig. 4 illustrates the overall differential response for the arrangement of Fig. 3.
[0021 ] Fig. 5 illustrates schematically a third modified optical sensor design of the present invention;
[0022] Fig. 6 illustrates schematically a further arrangement of a modified sensor design;
[0023] Fig. 7 illustrates schematically a further alternative arrangement having a multi wavelength source arrangement;
[0024] Fig. 8 illustrates schematically a further alternative arrangement having a multi wavelength source arrangement;
[0025] Fig. 9 illustrates schematically a further alternative arrangement having a broadband source arrangement;
[0026] Fig. 10 illustrates schematically a further alternative arrangement having a broadband source arrangement; [0027] Fig. 11 illustrates schematically a three source wavelength embodiment; and [0028] Fig. 12 illustrates schematically a three source wavelength embodiment. Detailed Description
[0029] In the preferred embodiment shown in Fig. l, there is provided an optical sensor for the rapid and accurate sensing of gases or fluids within an atmosphere or volume. The preferred employment utilizes the interference between first and second lasers emitting selected wavelengths λΐ and λ2. The first wavelength λΐ refers to an absorption wavelength with a second wavelength λ2 utilized as a reference wavelength. The two wavelengths are projected through a gas sampling tube 7 and the resultant differences are analyzed.
[0030] Turning initially to Fig. 1, there is illustrated a schematic diagram 1 of the preferred embodiment. In the arrangement 1, two lasers 2, 3 emit wavelengths λΐ, λ2. The first wavelength λΐ has a predetermined wavelength that is absorbent by the detectable medium. The second wavelength λ2 is used as a non-absorbing reference wavelength. The two wavelengths are input to an optical modulation scheme which provides RF signal modulation in an out of phase manner (i.e. with a 180° phase difference). This can be achieved via a dual-input electro optical modulator 4 as shown in Figure 1 which introduces 180° RF phase difference between the modulated light at its dual input ports and which is also currently commercial available. For example the electrooptic modulators available from
EOSPACE Inc. The out of phase modulation can also be achieved via other alternative ways such as using positive and negative slopes of two electro-optic intensity modulators or using cross gain modulations in semiconductor optical amplifiers.
[0031 ] The modulation output is forwarded to fast-speed switch 5. Switch 5 acts in a clocked manner, initially switching its input A to output D. Subsequently, input B is switched to output D and then input B is switched to output C. Optical switches with fast response time are commercially available. For example, the fiber optic switch from Agiltron has a response time (rise, fall) of 300ns. Recent research shows that the transition time of optical switches can be reduced to 30 to 50ns by using liquid-crystal cells (M. W. Geis, R. J. Molnar, G. W. Turner, T. M. Lyszczarz, R. M. Osgood, and B. R. Kimball (2010). 30 to 50 ns liquid-crystal optical switches. Proc. SPIE 7618: 76180J/1-5). In addition, an optical delay line in the loop provides optical time control which releases the switching time requirement of the optical switch. [0032] The output D is forwarded via linear optical amplifier (LOA) 6 to a gas sampling tube 7, where the absorbent wavelength undergoes a degree of absorption depending on the density of absorbing gas within the chamber. The non absorbent wavelength also traverses the gas sampling tube. The linear optical amplifier 6 is inserted in the loop to compensate for component insertion loss and to keep loop gain at unity for, at least, the non-absorbent wavelength. An optical delay line 8 can also be inserted in the loop to provide optical time control which releases the switching time requirement of the optical switch.
[0033] Since the RF signals modulated on the two wavelengths has a 180° phase difference at the output of the modulator, the detected frequency response of the two wavelengths has opposite RF phase after O/E conversion. This results in a subtraction or differential output at the photo-diode 9.
[0034] A target gas sample is presented in sampling tube 7. When the target gas is presented, the absorbed wavelength λΐ experiences an extra attenuation due to gas absorption. The reference wavelengths λ2 remains unaffected. The loss offset between the two wavelengths is presented at the output photo diode 9 as a subtraction and a differential output is obtained, which is proportional to the amount of the gas present.
[0035] The dual wavelength parallel processing scheme of the preferred embodiment provides for reduce calibration requirements and is robust against environmental fluctuations. When no absorbing gas is present in the gas sampling tube, both wavelengths of light are equally affected by the normal ambient changes, such as temperature, humidity or dust particles. This induces a small insertion loss. There will be minimal differential output obtained and the result from the subtraction remains stable. This allows for self compensation for any zero drift.
[0036] The switch 5 allows for the two wavelengths to continually circulate in the recirculating loop 6, 7, 8. The switch 5 is used to control the number of loops that light travels inside the recirculating loop before of it is switched to the photo diode 9 via output port C. The sensitivity of the sensor 1 is enhanced by repeatedly sampling in a short time period equivalent to the number of loops travelled inside the recirculating loop. This sampling can occur in a clocked manner. As an extra attenuation will be added to the absorbing wavelength λΐ every time the light travels through the sampling tube inside the loop, the amount of attenuation will accumulate and enlarge the loss difference between λΐ and λ2 before detection occurs by photodiode 9. In other words, this attenuation is amplified throughout the repeated sampling process. This provides for enhanced low concentration levels sensing within the gas sampling tube 7. [0037] In the alternative embodiment shown in Fig.2, an optical coupler 14 is used to replace the optical switch in the loop. Two optical light sources at optical wavelengths λ 1 , λ2 11, 12 that carry RF signals with opposite phases are sent to an optical loop structure composed of an optical linear amplifier 15, a gas sampling tube 16, an optical delay line 17 and an optical coupler 14. The first wavelength λΐ has a predetermined wavelength that is absorbent by the detectable medium. The second wavelength λ2 is used as a non-absorbing reference wavelength. These wavelengths are then sent to an all-optical loop structure that provides appropriate delays and weights for the positive taps and the negative taps. When the target gas is presented, the absorbed wavelength λΐ experiences an extra attenuation due to gas absorption. The reference wavelengths λ2 remains unaffected. Since a loss offset between the two wavelengths is presented, the two wavelengths in the amplified loop see slightly different loop gains. Therefore two independent bandpass responses with different filter coefficients are realized by circulating the two different wavelengths in the loop.
[0038] Fig. 3 shows the filter responses at different absorption at the two wavelengths which introduces the different loop gains. Since the frequency responses formed by the two modulated optical light sources have opposite phase, therefore they subtract each other at the photodetection. The overall response is shown in Fig.4 where a high stopband attenuation is realized due to the subtraction of the two RF responses. The extinction ratio of the overall RF response is defined as the RF power difference between the passband and the stopband which indicates the loss difference of the two optical wavelengths. The extinction ratio is a parameter to monitor the gas concentration level in the sampling tube. The extinction ratio can be obtained by using different methods such as measuring the overall RF response via a network analyzer and then calculate the value difference of the peak and notch of the RF response, or modulating two RF signals locating at the bandpass and stopband frequencies on the two optical wavelengths and then measuring the RF power difference of the two RF signals after photodetection via a spectrum analyzer or RF power meter.
[0039] In alternative embodiments, direction modulation and balanced detection schemes can be used to modulate the signals on the two wavelengths and to achieve differential output after photodection. This allows for no optical phase shift being required.
[0040] In the embodiment shown in Fig. 3, RF modulated signals are carried by two optical light sources at wavelengths (λΐ and λ2) via direction modulation. The two lightwave signals are combined via an optical coupler and then forwarded to an optical loop structure including an optical switch 23, an optical linear amplifier 24, a gas sampling tube 25, and an optical delay line 26. [0041 ] The optical loop performs the same optical function as shown in Fig. 1 and as described above. The two optical sources continually circulate in the recirculation loop where the number of circulations within the loops can be controlled by using the optical switch 23. An optical demultiplexer 27 is used at the output of the optical switch to separate the two optical signals into its frequency components at wavelength λΐ and λ2, which are coupled to two separate photodiodes 28, 29. A subtraction or a differential output is achieved in the electrical domain after optical to electrical conversion, which is proportional to the amount of gas presented.
[0042] Similarly the embodiment shown in Fig.2 can also be modified via introducing direction modulation and differential detection schemes as illustrated in Fig.5.
[0043] Other embodiments of the present invention are possible. For example, Fig. 7 and Fig. 8 illustrate schematically a multi wavelength source arrangement. In the arrangement of Fig. 7, a multi- wavelength source consists two laser emission wavelengths 71, 72 which are fed into single input EOM 74 via an optical combiner 73, then the light at the each port of modulator output contains both wavelength λΐ and 12. Due to the inverse polarity of the modulator output, the modulated light from two ports will have 180 degree phase difference.
[0044] By being separated using two optical filters 75, 76 centred at λΐ and λ2 via optical filters, and combined together using a second optical combiner 77, the different wavelength of light centred at λΐ and 12 will be 180 degree out of phase.
[0045] While circulating in the loop via a switch/coupler 78, the two wavelengths will experience different attenuation effect due to the gas absorption in sampling tube 80. Then the RF response generated by two wavelengths will be subtracted after photo-detection 81 , and only the difference part will be remaining, which corresponds to the amount of relative loss due to gas absorption.
[0046] Fig. 8 illustrates a similar arrangement to Fig. 7 wherein the optical coupler 78 is replaced by a switch 98, with the switch periodically switching between outputs C and D.
[0047] Besides optical lasers, spectrum sliced optical source can also be used in the design. In Fig. 9, a broadband spectrum sliced light source 111 is modulated 112 and then two optical narrowband filters centred at λΐ and 12 are used 113, 114 at the dual outputs to generate required optical spectrum. [0048] In Fig. 1, the broadband source 121 is spectrum sliced by using two optical slicing filters 122, 123, centred at λΐ and 12. The spectrum sliced source is modulated by using a dual input EOM 124 to introduce 180° RF phase difference between the modulated light.
[0049] After circulating in the loop simultaneously, the two wavelengths generate two RF responses after photodetection. Due to the gas absorption effect, the two generated RF responses will be subtracted after photo-detection, and only the difference part will be remaining, which corresponds to the amount of loss due to gas absorption.
[0050] Other alternatives are possible. The scheme can be upgraded to sense multiple detectable media by simply including more wavelengths. For example, Fig. 11 illustrates a three wavelength system 130 that can be used to sense two different detectable media in one sampling tube. In the arrangement, three lasers emit wavelengths λΐ, 12 and λ3. The first wavelength λΐ has a predetermined wavelength that is absorbent by the detectable medium 1. The second wavelength 12 has a
predetermined wavelength that is absorbent by the detectable medium 2. The third wavelength λ3 is used as a non-absorbing reference wavelength. The three wavelengths are input to an optical modulation scheme which provides RF signal modulation 132 in an out of phase manner (i.e. with a 180° phase difference). This can be achieved via various ways. As an example, a dual-input electro optical modulator is used in the design which introduces 180° RF phase difference between the modulated light at its dual input ports. When the target gas is presented, the absorbed wavelengths λΐ and 12 experience an extra attenuation due to gas absorption, while the reference wavelength λ3 remains unaffected. After photodection, the RF responses at these three wavelengths can be obtained. By subtracting the RF responses between the wavelength λΐ and λ3, the gas absorption of the detectable medium 1 can be obtained, which is proportional to the amount of the gas present. Similarly by subtracting the RF responses between the wavelength 12 and λ3, the gas absorption of the detectable medium 2 can be obtained, which is proportional to the amount of the gas present. This can be realized via various schemes. In Fig. 11, an optical switch 133 is used at the optical source part, to control the input wavelengths of the dual input EOM. This makes sure that only one absorbed- wavelength RF response presents at the photodetector along with the reference- wavelength RF response.
[0051 ] An alternative arrangement is shown 140 in Fig. 12. In this arrangement, the two detectable media can be measured at the same time. The two wavelengths λΐ and 12 are combined 14 before being differentially modulated 142 with a reference wavelength λ3.
[0052] The wavelengths are circulated via switch 144 in gas sampling tube 143. Subsequently they are split 144 and filtered 144, 145. The optical filters 144, 145, are used before the photodetection to separate the wavelengths into two groups, wherein one absorbed wavelength and one reference wavelength present in each group. Since the RF signals modulated on the two wavelengths (reference and absorbed wavelengths) has a 180° phase difference, the detected frequency response of the two wavelengths has opposite RF phase after photodection. This results in a differential output at the output of the photo-diode, which is inversely proportional to the amount of the gas present.
[0053] The arrangement of the preferred embodiments can undergo many enhancements. For example, the sampling tube can be replaced by immersion sensors so that the system is able to detect samples within compounds, such as liquids.
[0054] Further, the number of wavelengths can be extended. The extension can utilise different RF frequencies to drive each separate wavelength, and detect for multiple absorptions simultaneously. The absorption characteristics of particular gases to each wavelength being utilized to detect corresponding concentrations in a post processing step.
[0055] Other refinements are possible. For example, the length of the optical delay line will be substantially inversely proportional to the switching speed required by the optical switch, in that a longer optical delay will allow for slower switching speeds.
Interpretation
[0056] The description and figures make use of reference numerals to assist the addressee understand the structure and function of the embodiments. Like reference numerals are used in different embodiments to designate features having the same or similar function and/or structure.
[0057] The drawings need to be viewed as a whole and together with the associated text in this specification. In particular, some of the drawings selectively omit including all features in all instances to provide greater clarity about the specific features being described. While this is done to assist the reader, it should not be taken that those features are not disclosed or are not required for the operation of the relevant embodiment.
[0058] Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.
[0059] Similarly it should be appreciated that in the above description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, Fig., or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the Detailed
Description are hereby expressly incorporated into this Detailed Description, with each claim standing on its own as a separate embodiment of this invention.
[0060] Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those skilled in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.
[0061 ] Furthermore, some of the embodiments are described herein as a method or combination of elements of a method that can be implemented by a processor of a computer system or by other means of carrying out the function. Thus, a processor with the necessary instructions for carrying out such a method or element of a method forms a means for carrying out the method or element of a method. Furthermore, an element described herein of an apparatus embodiment is an example of a means for carrying out the function performed by the element for the purpose of carrying out the invention.
[0062] In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.
[0063] Similarly, it is to be noticed that the term coupled, when used in the claims, should not be interpreted as being limited to direct connections only. The terms "coupled" and "connected," along with their derivatives, may be used. It should be understood that these terms are not intended as synonyms for each other. Thus, the scope of the expression a device A coupled to a device B should not be limited to devices or systems wherein an output of device A is directly connected to an input of device B. It means that there exists a path between an output of A and an input of B which may be a path including other devices or means. "Coupled" may mean that two or more elements are either in direct physical or electrical contact, or that two or more elements are not in direct contact with each other but yet still co-operate or interact with each other.
[0064] Thus, while there has been described what are believed to be the preferred embodiments of the invention, those skilled in the art will recognize that other and further modifications may be made thereto without departing from the spirit of the invention, and it is intended to claim all such changes and modifications as fall within the scope of the invention. For example, any formulas given above are merely representative of procedures that may be used. Functionality may be added or deleted from the block diagrams and operations may be interchanged among functional blocks. Steps may be added or deleted to methods described within the scope of the present invention.
Next Patent: ANKLE SUPPORTS