Patent Description:
The subject matter described herein relates to spectroscopic analyzers in which power and/or a shape of beam emitted by a light source is selectively variable.

Spectrometers use light sources for the detection and quantification of emission or absorption of radiation by matter (e.g. individual molecules in analysis of gas or liquid phase compounds). The radiation from such light sources is absorbed or emitted with a particular energy determined by transitions occurring to the molecules of an analyte. For example, in infrared spectroscopy, discrete energy quanta are absorbed by molecules due to excitation of vibrational or rotational transitions of the intra-molecular bonds.

Variations in environmental conditions as well as aging can cause transmitted and/or detected power and/or beam shape of a light source within a spectrometer to change over time. A change in transmitted and/or detected power and beam shape in a spectrometer can alter the spectrometer calibration and cause spectrometer reading offsets. Such reading offsets can cause higher operating costs for a controlled process, costly replacement of equipment, including but not limited to catalytic converters, or harmful conditions to humans or the environment if harmful concentration limits of a measured analyte are being exceeded. In some cases, spectrometers suffering such issues require manual calibration or even replacement by a technician. Such service calls are costly and result in downtime for the spectrometer while such repairs are being performed.

<CIT> discloses an automated acousto-optic analyzer system with an acousto-optic tunable filter coupled with a source of radiation to produce pulsed light at predetermined wavelengths. The acousto-optic filter is positioned in between the source of radiation and a detector.

<CIT> discloses a hand-held portable modular spectrometer unit. The unit includes a detachable head containing a light source and optical components for detecting spectral information from light reflected from or transmitted through a target and a processor for converting the detected spectral information into digital information. <CIT> discloses a filter wheel to select a wavelength or a wavelength band.

<CIT> discloses an optical sensor for detecting a chemical in a sample region includes an emitter for producing light, and for directing the light through the sample region. The sensor also includes a detector for receiving the light after the light passes through the sample region, and for producing a signal corresponding to the light the detector receives. The sensor further includes a thermo-optic filter disposed between the emitter and the detector.

<CIT> discloses a spectrophotometer that irradiates a sample with measuring light and analyzes the light absorbed by the sample. Such a spectrophotometer is used as a detector for a liquid chromatograph.

The present disclosure is directed to an optical absorption spectrometer according to claim <NUM>. At least one light source is configured to emit a beam into a sample volume of an absorbing medium. In addition, at least one detector array is positioned to detect at least a portion of the beam emitted by the light source. Further, a beam modification element is positioned between the detector array and the light source to selectively change a shape of the beam emitted by the light source as detected by the at least one detector array.

A control circuit is coupled to the beam modification element. The present disclosure is also directed to a method of using an optical absorption spectrometer according to claim <NUM>.

Such computing systems can be connected and can exchange data and/or commands or other instructions or the like via one or more connections, including but not limited to a connection over a network (e.g. the Internet, a wireless wide area network, a local area network, a wide area network, a wired network, or the like), via a direct connection between one or more of the multiple computing systems, etc..

The subject matter described herein provides many technical advantages. For example, spectrometer sensitivity to accuracy offsets due to age and environmental factors can be greatly reduced by selectively changing a shape of beam(s) emitted by a light source to ensure optimal spectrometer performance and calibration fidelity. In particular, the current subject matter can address spatial sensitivity variations across the active surface of the detectors, the limited dynamic range of the detectors to linearly respond to power / intensity of an incident beam, and varying response to DC and AC light power. Further, the current subject matter is advantageous in that it can be used to address a light beam being received at different locations across the detector surface or being received with different spot sizes which causes a different electronic loop gain and resulting calibration offsets.

Further advantages of the current subject matter include being able to address AC and DC gain changing as a function of incident DC power / DC intensity of the beam spot on the detector surface. In one example, a very high intensity DC beam spot on the detector surface can, casually speaking, bleach the detector area to some extent, altering the AC gain for a very low power 2f signal. The amount of "bleaching" of carriers has to do with the carrier mobility, the detector bias voltage and the proximity of the beam spot to the electrodes mounted on the back of a typical detector, including but not limited to a semiconductor detector. The smaller the spot, the higher the intensity, the more rapidly, the AC and DC gain will vary spatially.

Still further, the current subject matter can address differences in gain for the AC component (including but not limited to a harmonic 2f signal resulting from a frequency modulation of the light source) and the DC component (total incident light power) of the light beam whether due to detector temperature changes or otherwise. As the concentration reading of a 2f TDL spectrometer is being derived from ratioing the 2f signal with the DC signal, a changing AC to DC gain relationship on the detector, with respect to conditions at time of calibration, will alter the concentration reading and cause calibration offsets (as a function of incident power and power distribution on the detector surface).

It should be noted that the current subject matter contemplates both a closed sample cell and an open path system for detecting trace gases and/or liquids. The terms "sample gas volume", "gas volume", "sample liquid volume" and "liquid volume" as used herein therefore refers to either a flowing volume or a static, batch volume of gas or liquid (as the case may be).

To address the aforementioned and other potential issues due to beam sensitivity with spectroscopic measurements, implementations of the current subject matter can provide a spectrometer having a light source and an optical assembly with the ability to change power and/or shape of a detected beam or a portion thereof. Gas and/or liquid sampled from a source can include absorbing media (e.g., one or more analyte compounds, etc.). Detection and/or quantification of the concentration of such absorbing media can be performed by spectroscopic analysis. The spectrometer can include the at least one beam modification element that causes the power and/or shape of a transmitted and/or detected beam emitted by the light source to change as specified by a controller. In some variations, the system can include spatial detectors / detector arrays so that the control unit can determine a position and/or shape and/or power of the beam and cause the beam modification element to make any required changes.

Analyte compounds with which implementations of the current subject matter can be used include, all gas, liquid and solid phase atoms, molecules and ions, which absorb light, but are not limited to, hydrogen sulfide (H2S); hydrogen chloride (HCl); water vapor (H2O); hydrogen fluoride (HF); hydrogen cyanide (HCN); hydrogen bromide (HBr); ammonia (NH3); arsine (AsH3); phosphine (PH3); oxygen (O2); carbon monoxide (CO); carbon dioxide (CO2); chlorine (Cl2),; nitrogen (N2), hydrogen (H2); hydrocarbons, including but not limited to methane (CH4), ethane (C2H6), ethylene (C2H4), acetylene(C2H2), etc.; fluorocarbons; chlorocarbons; alcohols; ketons; aldehydes; acids, bases and the like.

<FIG> is a process flow diagram <NUM> in which, at <NUM>, at least one light source emits a beam into a sample volume comprising an absorbing medium. Thereafter, at <NUM>, at least one detector detects at least a portion of the beam emitted by the light source. It is later determined, at <NUM>, based on the detected at least a portion of the beam and by a controller that at least one of (i) a power intensity, or (ii) a shape of the beam should be changed. The beam emitted by the light source is then, at <NUM>, selectively changed by a beam modification element under control of the controller. In addition, a concentration of the absorbing media can be quantified or otherwise calculated (using the controller or optionally a different processor that can be local or remote).

<FIG> are diagrams <NUM>-<NUM> that show example spectrometers for implementing the current subject matter. While the following is described in connection with detecting absorbing media within gas, it will be appreciated that the current subject matter can also be applied to detecting absorbing media within liquid. A light source <NUM> provides a continuous or pulsed light that is directed to a detector <NUM> via a path length <NUM>. The light source <NUM> can include, for example, one or more of a tunable diode laser, a tunable semiconductor laser, a quantum cascade laser, an intra-band cascade laser (ICL), a vertical cavity surface emitting laser (VCSEL), a horizontal cavity surface emitting laser (HCSEL), a distributed feedback laser, a light emitting diode (LED), a super-luminescent diode, an amplified spontaneous emission (ASE) source, a gas discharge laser, a liquid laser, a solid state laser, a fiber laser, a color center laser, an incandescent lamp, a discharge lamp, a thermal emitter, and the like. The detector <NUM> can include, for example, one or more of an indium gallium arsenide (InGaAs) detector, an indium arsenide (InAs) detector, an indium phosphide (InP) detector, a silicon (Si) detector, a silicon germanium (SiGe) detector, a germanium (Ge) detector, a mercury cadmium telluride detector (HgCdTe or MCT), a lead sulfide (PbS) detector, a lead selenide (PbSe) detector, a thermopile detector, a multi-element array detector, a single element detector, a photo-multiplier, a CMOS (complementary metal oxide semiconductor) detector, a CCD (charge coupled device detector) detector and the like.

The path length <NUM> can traverse one or more volumes. In the example systems <NUM>-<NUM> shown in <FIG>, the path length <NUM> can twice traverse a volume <NUM> of an optical cell <NUM> that includes a window or other at least partially radiation transmissive surface <NUM> and a mirror or other at least partially radiation reflective surface <NUM> that at least partially define the volume <NUM>. Sample gas can, in some implementations, be obtained from a gas source, which in the examples of <FIG> and <FIG> is a pipeline <NUM>, for delivery to the volume <NUM>, for example via a sample extraction port or valve <NUM> that receives the sample gas from the source. Gas in the volume <NUM> can exit via the same valve <NUM> or a second outlet valve or port <NUM>.

As illustrated in <FIG> and <FIG>, in some variations, the volume <NUM> can be part of a housing that defines a sample cell that can be, for example, one or more of a Herriott Cell, an off-axis optical resonator, an on-axis optical resonator, an elliptical light collector, a parabolic light collector, a spherical light collector, a White cell, an optical cavity, a hollow core light guide, a multiple pass configuration in which the light beam is reflected at least once or a single pass configuration in which the light is not being reflected while the light traverses the sample cell. In other variations, as illustrated in <FIG> and <FIG>, the volume <NUM> can be part of an open path system that does not include a dedicated sample cell. Open path systems can be used for various applications including atmospheric pollutant studies, fence line monitoring, process line/tank leak detection, industrial gas-purity applications, and monitoring and control of combustion processes, especially on exhaust stacks.

A controller <NUM>, which can include one or more programmable processors or the like, can communicate with one or more of the light source <NUM> and the detector <NUM> for controlling the emission of the light <NUM> and receiving signals generated by the detector <NUM> that are representative of the intensity of light impinging on the detector <NUM> as a function of wavelength. In various implementations, the controller <NUM> can be a single unit that performs both of controlling the light source <NUM> and receiving signals from the detector <NUM>, or it can be more than one unit across which these functions are divided. Communications between the controller <NUM> or controllers and the light source <NUM> and detector <NUM> can be over wired communications links, wireless communications links, or any combination thereof. The controller <NUM> can also, in some cases, be used to quantify an amount of absorbing media using the signal generated by the detector <NUM>. In other variations, the quantification can be performed by at least one external data processor / computing device.

In some cases, the controller <NUM> can be configured so that the incident DC power on the detector <NUM> is maintained within the linear response range of the detector and the downstream electronic circuit, which in turn, can assure better concentration calibration fidelity.

In some implementations, the 2f signal can be normalized by DC in order to eliminate the impact of non-resonance laser intensity attenuation on 2f signal amplitude. The analyte concentration is calculated from the DC normalized 2f signal. It has been found that the opto-electrical gain of the detector GD, the electronic gain of 2f G2f, and the electronic gain of DC GDC, may not all be constants. For example, GD can change with the incident laser intensity on the detector ID due to detector local saturation or nonlinearity. As a result, the GD can be expanded into Fourier cosine/sine series. Then both the 2f and DC signals (in terms of # of terms and constituents of each term) will be changed, i.e. the DC normalized 2f signal will also change consequently.

As another example, the ratio of G2f /GDC can also change with different GDC due to circuit cross talk/nonlinearity. In such a case, both the 2f and DC signals still contain the same terms, but the DC normalized 2f signal amplitude changes due to the variation of the ratio of G2f /GDC.

A beam modification element <NUM> can be coupled to the light source <NUM> and the controller <NUM>. The controller <NUM> can send a signal to the beam modification element <NUM> to cause it to selectively change a shape and/or power of the beam emitted by the light source <NUM> as detected by the detector <NUM>. In some variations, the beam modification element <NUM> can be any controllable device that causes the power and/or shape of the beam to change (and as such the beam modification element <NUM> is not directly intermediate either of the beam path, on one hand, and the light source <NUM> and the detector <NUM>, on the other hand). For example, with this variation, the beam modification element <NUM> can be / include /or be coupled to at least one actuation element such as a least one piezo actuator element, an inch-worm, a mechanical actuator, a magnetic actuator, an electrostatic actuator, an inductive actuator, a rotary actuator, a heated actuator, a pressure actuator, a stress and strain actuator, an analog motor, a stepper motor, an electro-optical actuator, an acousto-optical actuator, a quantum well tuning element, and/or a micro-electro-mechanical systems (MEMS) actuation device which move the beam modification element <NUM> in at least one of the x-axis, y-axis, or z-axis or rotate it along any axis by some angles.

In addition or in the alternative (as shown in <FIG> and <FIG>), the beam modification element <NUM> can be placed intermediate the light source <NUM> and the detector <NUM> and/or to intersect the beam path. With such an arrangement, the beam modification element <NUM> can be any device / element that optically causes the shape (distribution of power across the beam) and/or power of the beam emitted by the light source <NUM> to selectively change (in some cases without moving or changing the operation of the light source <NUM>).

The beam modification element <NUM> can be / include one or more different elements. The beam modification element can include one or more of: an optical transmission filter or an optical diffuser, a transmissive diffuser or a reflective diffuser, a reflector with a selectively adjustable surface, a liquid crystal optical element, a diffractive optical element, a refractive optical element, an adjustable aperture, a waveguide (e.g., an electrically controllable waveguide, etc.), an optical fiber, an optical element with at least one layer of a dielectric material, a tunable filter, a thermal optical tuner, a quantum well tuning element, a neutral density (ND) filter, an optical interference filter, a filter wheel having two or more different optical elements, which change beam power and/or shape, that are selectively movable within the beam path, or a filter wedge with changing optical transmission across its surface.

The beam modification element <NUM> can comprise saturable absorbers (e.g., photochromic lenses, etc.) in some variations. The beam modification element <NUM> can comprise spatial filters including, but not limited to, solid and hollow or photonic crystal optical lightguides which can be bent or strained to cause transmitted power loss or which can have an z- axis actuable focusing lens (which can reduce power throughput if moved from its ideal focal spot at the fiber entrance). The beam modification element <NUM> can include adjustable transmission elements such as liquid crystal optical element, electro-optic element, acousto-optic elements, waveguides with coupled grating structures (electrically, stress and strain driven and heat driven changes can alter the transmission of light of a spectral frequency and bandwidth). The beam modification element <NUM> can include films made from at least one layer of dielectric material, films made from organic material which can be rotated, and the like. The beam modification element <NUM> can include volume Bragg gratings (rotatable or heatable), fiber Bragg gratings, light valves, polarizers and/or other types of light power actuators.

The beam modification element <NUM> can include diffusers and optical elements which modify the beam profile (illuminators or "top hat converters" and the like). Such diffusers and optical elements can distribute laser power across a larger detector area and thus reduce localized high intensity regions where the laser beam impinges upon the surface area, including but not limited to (i) transmissive optical elements with diffractive optical structures which alter the beam profile, (ii) transmissive optical elements (flat surfaces or random curvature) with rough polishing, (iii) reflective optical elements with surface structure which causes scattering of an incident laser beam or alters the beam profile, (iv) diffractive optical elements, (v) Fresnel type optical elements, and/or (vi) films or optical elements made from hydrocarbon materials.

The beam modification element <NUM> can also be a beam splitting device, free space or fiber coupled to split the original beam and dump the extra optical power.

In some variations, the controller <NUM> can make a determination that a power and/or shape of the beam should be changed based on an intensity level detected by the detector <NUM> without reference to spatial location of such beam. For example, the intensity level can indicate that a center of the beam has diverged and/or that there is some interference along the at least a portion of a beam path. In addition or in the alternative, the controller <NUM> can make a determination that the power and/or shape of the beam should be changed based on a position and/or angle of the beam as detected by the detector <NUM>. With such latter variations, an array of photoreceivers and/or a detector with an array of cells can be used. For example, the detector <NUM> can be a quad cell detector and/or a position sensing photodiode. With the latter example, the position of the center point of the emitted beam can be determined by a comparison of the detected signals from each cell. Horizontal position of the center point can be calculated by [(cell<NUM> + cell<NUM>) - (cell<NUM>+cell<NUM>)] / (cell<NUM>+cell<NUM>+cell<NUM>+cell<NUM>) and the vertical position of the center point can be calculated by [(cell<NUM> + cell<NUM>) - (cell<NUM>+cell<NUM>)] / (cell<NUM>+cell<NUM>+cell<NUM>+cell<NUM>). In another example, the position sensitive detector can be a detector which detects the x and y position as well as the x and y angles of the beam. Furthermore, a multi-element linear detector array can be used to determine the beam position. In another embodiment, a <NUM>-dimensional detector array can be used to determine the beam position. In another example, the position sensitive detector can be a detector which detects the x and y position as well as the x and y angles of the beam. Furthermore, a multi-element linear detector array can be used to determine the beam position. In another variation, a <NUM>-dimensional detector array can be used to determine the beam position. With such spatially sensitive detectors, a pre-defined position (along two or more dimensions) and/or pre-defined angle (as specified by two or more dimensions) can be maintained via the controller <NUM> and the beam modification element <NUM>. According to the invention, the photodetector is a photodetector array which detects a position and/or angle of the beam to determine that a shape of the beam should be changed.

The volume <NUM> can be maintained at a stable temperature and pressure. Alternatively, the volume <NUM> can include one or more temperature and/or pressure sensors to determine a current temperature and pressure within that volume for use in one or more calculations to compensate for temperature and/or pressure changes relative to a validation or calibration condition of the spectroscopic instrument. Furthermore, the volume <NUM> can be adjusted to preset temperature and pressure by heating elements and pressure control elements or mass flow controllers.

The controller <NUM>, or alternatively one or more other processors that are either collocated with the other components or in wireless, wired, etc. communication therewith, can perform the processing functions discussed above in reference to the method illustrated in <FIG>.

These computer programs, which can also be referred to programs, software, software applications, applications, components, or code, include machine instructions for a programmable processor, and can be implemented in a high-level procedural language, an object-oriented programming language, a functional programming language, a logical programming language, and/or in assembly/machine language. The machine-readable medium can alternatively or additionally store such machine instructions in a transient manner, such as for example as would a processor cache or other random-access memory associated with one or more physical processor cores.

To provide for interaction with a user, one or more aspects or features of the subject matter described herein can be implemented on a computer having a display device, such as for example a cathode ray tube (CRT) or a liquid crystal display (LCD) or a light emitting diode (LED) monitor for displaying information to the user and a keyboard and a pointing device, such as for example a mouse or a trackball, by which the user may provide input to the computer. For example, feedback provided to the user can be any form of sensory feedback, such as for example visual feedback, auditory feedback, or tactile feedback; and input from the user may be received in any form, including, but not limited to, acoustic, speech, or tactile input. Other possible input devices include, but are not limited to, touch screens or other touch-sensitive devices such as single or multi-point resistive or capacitive trackpads, voice recognition hardware and software, optical scanners, optical pointers, digital image capture devices and associated interpretation software, and the like.

In the descriptions above and in the claims, phrases such as "at least one of' or "one or more of" may occur followed by a conjunctive list of elements or features.

Claim 1:
An optical absorption spectrometer (<NUM>, <NUM>, <NUM>, <NUM>) comprising:
a sample volume;
at least one light source (<NUM>) configured to emit at least one beam into the sample volume comprising an absorbing medium, the light source comprising a laser;
at least one detector array (<NUM>) positioned to detect at least a portion of the beam emitted by the light source;
at least one beam modification element (<NUM>) positioned between the detector array (<NUM>) and the light source (<NUM>) to selectively change a shape of the beam emitted by the light source (<NUM>) as detected by the detector array (<NUM>); and
a control circuit coupled to the beam modification element (<NUM>), wherein the control circuit is configured to perform operations comprising:
making a determination that the shape of the beam should be changed based on a position and/or angle of the beam as detected by the detector array (<NUM>),
causing the beam modification element (<NUM>) to change the shape of the beam detected by the detector array (<NUM>) in response to the position and/or angle as detected by the detector array (<NUM>); and
selectively changing the shape of the beam by the beam modification element (<NUM>) to ensure spectrometer performance and calibration fidelity by addressing spatial sensitivity variations across the active surface of the detector array (<NUM>) such as to reduce the accuracy offsets due to age and environmental factors.