Method for adjusting a resonant cavity and cavity ring-down spectroscopy system

Abstract

A method for tuning a resonant cavity and a cavity ring-down spectroscopy system using the method are provided. A first mirror is actuated at a first end of the resonant cavity to move it in a direction between a first position relative to a second mirror at a second end of the resonant cavity, where the cavity length between the first and second mirrors is smaller than the resonance length for the laser beam, and a second position relative to the second mirror, where the cavity length is larger than the resonance length. An event is triggered when the cavity length approaches the resonance length. During the event, the first mirror is actuated continuously in a direction between the first and second positions.

Description

Technical Field

[0001] Cross - Reference to Related Applications This application claims the benefit of U.S. Provisional Patent Application No. 62 / 828,750, filed on April 3, 2019, the content of which is incorporated herein by reference in its entirety.

[0002] This specification generally relates to resonant cavities, and more particularly, to methods for tuning a resonant cavity and cavity ring - down spectroscopy systems.

Background Art

[0003] Cavity ring - down spectroscopy (「CRDS」) is generally a technique used to analyze gas samples via an absorption spectrum. A typical CRDS system uses a laser that generates a beam directed into a cavity of a chamber having two highly reflective mirrors. The beam is typically in the visible light spectrum or the near - infrared (「IR」) spectrum and is tuned to a single wavelength. The beam is then repeatedly reflected between the mirrors, which allows a portion of the light to escape from the ring - down cavity.

[0004] To 「fill」 the ring - down cavity, the cavity length needs to be harmonic with the laser wavelength. This is generally done by adjusting the position of one of the two mirrors. When the laser is resonant with the cavity mode, the intensity increases in the cavity due to constructive interference. When the light entering the cavity is extinguished, the intensity of the light in the ring - down cavity decays at a predetermined rate when empty. A small portion of the light is not reflected by the mirror and escapes from the ring - down cavity. The intensity of the escaping light is measured by a sensor component for determining the decay rate.

[0005] When a gas sample is placed in the ring-down cavity, the analyte present in the gas sample absorbs a portion of the light, thereby accelerating the attenuation of the light intensity in the ring-down cavity. The absorption spectrum is generated by measuring the light decay time when the gas sample is present at these wavelengths relative to the light decay time when there is no gas sample at a specific wavelength. The identification and quantification of individual analytes in the gas sample can be achieved through several methods, such as the performance of linear regression of the absorption spectrum measured for the gas sample against the known absorption spectra of various analytes.

[0006] To "fill" the ring-down cavity through constructive interference, the length of the ring-down cavity needs to be harmonized with the laser wavelength. This is achieved by adjusting the cavity length by moving one of the mirrors relative to the other. The mirrors are typically moved by one or more piezoelectric (referred to herein as "piezo") actuators driven by a piezoelectric driver. However, the piezoelectric driver generates discrete output voltages, resulting in discrete positions at which the mirrors can be positioned, thereby providing discrete cavity lengths between the mirrors. If neither the discrete positions nor, by extension, the discrete cavity lengths result in constructive interference of the laser light at a specific wavelength, the ring-down cavity can be difficult to fill for performing ring-down events. More complex / expensive piezoelectric drivers can have finer resolution, but their cost can be prohibitively high. Summary of the Invention Means for Solving the Problems

[0007] In a first aspect, a method of adjusting a resonant cavity is provided that includes operating a first mirror at a first end of the resonant cavity to move it in a direction between a first position relative to a second mirror at a second end of the resonant cavity, where the cavity length between the first and second mirrors is less than the resonant length for a laser beam, and a second position relative to the second mirror, where the cavity length is greater than the resonant length; causing an event when the cavity length is close to the resonant length; and continuing to operate the first mirror in the direction between the first and second positions during the event.

[0008] The event can be a first event, the direction can be a first direction, and the method can further include, after the continuing step, operating the first mirror to move it in a second direction opposite the first direction toward the first position; causing a second event when the cavity length is close to the resonant length; and continuing to operate the first mirror in the second direction between the second and first positions during the second event.

[0009] The method can further include repeatedly operating the first mirror to move it in the first and second directions; causing an event when the cavity length is close to the resonant length; and continuing to operate the first mirror during the event.

[0010] The method can further include applying a voltage waveform to at least one piezoelectric actuator coupled to the first mirror to operate the first mirror between the first and second positions.

[0011] The waveform can be a sine wave.

[0012] The method can further include adding a base voltage to the voltage of the waveform applied to the at least one piezoelectric actuator.

[0013] The method may further include a step of controlling a base voltage to find the position of an optical intensity peak through a photodetector connected to a resonant cavity, wherein the peak optical intensity occurs at the resonant length.

[0014] The method may further include a step of selecting an amplitude for a waveform of a voltage applied to at least one piezoelectric actuator that actuates a first mirror to be smaller than a wavelength of a laser beam illuminating the resonant cavity, and a step of controlling a base voltage such that two optical intensity peaks are detected during each period of the waveform of the voltage.

[0015] The method may further include a step of controlling a base voltage such that adjacent optical intensity peaks are separated by half of a period of the waveform of the voltage.

[0016] The method may further include a step of causing an event to occur when the detected optical intensity reaches a threshold intensity.

[0017] The event may be a ring-down event.

[0018] The photodetector can be connected to a timing circuit, and the timing circuit can be connected to one of an optical modulator and a laser to extinguish a laser beam from the laser or to detune a laser for the resonant cavity.

[0019] The method may further include a step of determining a predicted recurrence time for the threshold intensity and a step of causing extinction of the laser beam or detuning of the laser beam for the resonant cavity at the predicted recurrence time for the threshold intensity.

[0020] The method can further include a step of detecting the intensity of light in the resonant cavity through the photodetector, and the causing step includes causing an event when the intensity of light in the resonant cavity detected by the photodetector reaches the threshold intensity.

[0021] The photodetector can be connected to a timing circuit, which can be connected to either the optical modulator and the laser to extinguish the laser beam from the laser or to detune the laser.

[0022] In another aspect, a method of adjusting a resonant cavity is provided that includes varying the cavity length between a first mirror at a first end of the resonant cavity and a second mirror at a second end of the resonant cavity between a first cavity length that is less than the resonant length for the laser beam and a second cavity length that is greater than the resonant length for the laser beam, causing an event when the cavity length is close to the resonant length, and continuing to vary the cavity length toward the second cavity length during the event.

[0023] In a further aspect, a method of adjusting a resonant cavity is provided that includes actuating a first mirror at a first end of the resonant cavity to move in a direction between a first position relative to a second mirror at a second end of the resonant cavity where the cavity length between the first and second mirrors is less than the resonant length for the laser beam and a second position relative to the second mirror where the cavity length is greater than the resonant length, causing extinction of the laser beam illuminating the resonant cavity or detuning of the laser beam for the resonant cavity when the cavity length is close to the resonant length, and continuing to actuate the first mirror in the direction between the first and second positions while a photodetector registers the light intensity in the resonant cavity.

[0024] The direction can be a first direction, and the method can further include, after a subsequent step, causing a laser beam to illuminate or readjust a laser beam for a resonant cavity, operating a first mirror to move it towards a first position in a second direction opposite to the first direction, causing the disappearance of the laser beam or the detuning of the laser beam for the resonant cavity, and continuing to operate the first mirror in the second direction between a second position and the first position while a photodetector registers the light intensity in the resonant cavity.

[0025] The method can further include applying a sinusoidal waveform to at least one piezoelectric actuator coupled to the first mirror to operate the first mirror between the first position and the second position.

[0026] The method can further include adding a base voltage to the voltage of the waveform applied to the at least one piezoelectric actuator.

[0027] The method can further include controlling the base voltage to find the position of the light intensity peak via a photodetector coupled to the resonant cavity, wherein the peak light intensity occurs at the resonant length.

[0028] The method can further include selecting an amplitude for the waveform of the voltage applied to at least one piezoelectric actuator that operates the first mirror by less than one wavelength of the laser beam illuminating the resonant cavity, and controlling the base voltage such that two light intensity peaks are detected during each period of the voltage waveform.

[0029] The method can further include controlling the base voltage such that adjacent light intensity peaks are separated by half the period of the voltage waveform.

[0030] The cavity length close to the resonant length can be detected by the detected light intensity achieving a threshold intensity.

[0031] The photodetector can be connected to a timing circuit, which can be connected to one of the optical modulator and the laser to extinguish the laser beam from the laser or to detune the laser for the resonant cavity.

[0032] The method may further include determining a predicted recurrence time for achieving the threshold intensity as a proxy for when the cavity length is close to the resonant length.

[0033] In still other aspects, a resonant cavity having a first mirror at a first end of the resonant cavity and a second mirror at a second end of the resonant cavity, a photodetector connected to the resonant cavity for measuring the light intensity in the resonant cavity, at least one piezoelectric actuator connected to the first mirror for actuating the first mirror, and controlling the at least one piezoelectric actuator to actuate the first mirror to a first position relative to the second mirror, wherein the cavity length between the first mirror and the second mirror is less than the resonant length for the laser beam, and a second position relative to the second mirror, wherein the cavity length is greater than the resonant length, and moving in a direction between the first position and the second position, and when the cavity length is close to the resonant length, causing extinction of the laser beam illuminating the resonant cavity or detuning of the laser beam for the resonant cavity, and continuing to actuate the first mirror in the direction between the first position and the second position while the photodetector registers the light intensity in the resonant cavity, and a control module connected to the at least one piezoelectric actuator are provided with a cavity ring-down spectroscopy system.

[0034] The direction can be the first direction, and the control module causes the laser beam to illuminate or readjust the laser beam for the resonant cavity, activates the first mirror, and moves it towards the first position in the second direction opposite to the first direction, causing the disappearance of the laser beam or the detuning of the laser beam for the resonant cavity. While the photodetector registers the light intensity in the resonant cavity, the first mirror can continue to be activated in the second direction between the second position and the first position.

[0035] The control module can apply a sinusoidal waveform to at least one piezoelectric actuator connected to the first mirror to activate the first mirror between the first position and the second position.

[0036] The control module can add a base voltage to the voltage of the waveform applied to at least one piezoelectric actuator.

[0037] The control module can control the base voltage to find the position of the light intensity peak through a photodetector connected to the resonant cavity, and the peak light intensity occurs at the resonant length.

[0038] The control module can select the amplitude of the voltage waveform applied to at least one piezoelectric actuator that activates the first mirror by less than one wavelength of the laser beam illuminating the resonant cavity, and can control the base voltage so that two light intensity peaks are detected during each period of the voltage waveform.

[0039] The control module can control the base voltage so that adjacent light intensity peaks are separated by half the period of the voltage waveform.

[0040] The control module can determine that the cavity length is close to the resonant length when the detected light intensity reaches the threshold intensity.

[0041] The photodetector can be connected to a timing circuit, which can be connected to one of the optical modulator and the laser to extinguish the laser beam from the laser or to detune the laser for the resonant cavity.

[0042] Other technical advantages may become readily apparent to those skilled in the art after consideration of the following figures and description.

[0043] For a better understanding of the embodiments described herein and to more clearly show how the embodiments may be put into practice, reference is now made, by way of example only, to the accompanying drawings.

Brief Description of the Drawings

[0044]

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DETAILED DESCRIPTION OF THE INVENTION

[0045] Unless otherwise explicitly stated, the articles depicted in the drawings are not necessarily drawn to a specific scale.

[0046] For the sake of simplicity and clarity of illustration, where appropriate, reference numerals may be repeated among the figures to indicate corresponding or similar elements. Also, numerous specific details are set forth in order to provide a thorough understanding of the embodiments described herein. However, it will be understood by those of ordinary skill in the art that the embodiments described herein may be practiced without these specific details. In other instances, well-known methods, procedures, and components have not been described in detail so as not to obscure the embodiments described herein. Exemplary embodiments are shown in the figures and described below, but it should first be understood that the principles of the present disclosure may be implemented using any number of techniques, whether currently known or not. The present disclosure should not be limited in any way to the exemplary implementations and techniques shown in the drawings and described below.

[0047] As used throughout this description, various terms may be read and understood as follows, unless the context indicates otherwise. As used throughout, "or" is inclusive as though written "and / or," and singular articles and pronouns as used throughout include their plural forms and vice versa, and similarly, gendered pronouns include their opposite genders, such that pronouns should not be understood as limiting the things described herein to use, performance, or implementation by one gender, "exemplary" should be understood as "illustrative" or "using an example," and not necessarily "preferred" over other embodiments. Further definitions of terms may be set forth herein, and these may apply to the prior and subsequent examples of those terms as understood from reading this description.

[0048] Changes, additions, or omissions may be made to the systems, apparatuses, and methods described herein without departing from the scope of the disclosure. For example, the components of the systems and apparatuses may be integrated or separated. Further, the operations of the systems and apparatuses disclosed herein may be performed by more components, fewer components, or other components, and the methods described may include more steps, fewer steps, or other steps. Also, the steps may be performed in any suitable order. As used herein, "each" refers to each member of a set, or each member of a subset of a set.

[0049] Any module, unit, component, server, computer, terminal, engine, or device that executes the commands may include, or may have access to, a computer-readable medium such as a storage medium, a computer storage medium, or a data storage device (removable and / or non-removable), such as a magnetic disk, an optical disk, or a tape. A computer storage medium may include volatile, non-volatile, removable, and non-removable media implemented in any method or technology for the storage of information, such as computer-readable instructions, data structures, program modules, or other data. Examples of computer storage media include RAM, ROM, EEPROM, flash memory, other memory technologies, CD-ROM, digital versatile disk (DVD), other optical storage devices, magnetic cassettes, magnetic tape, magnetic disk storage devices, other magnetic storage devices, or any other medium that can be used to store the desired information and that can be accessed by an application, a module, or both. Any such computer storage medium may be part of the device or may be accessible or connectable to the device. Further, unless the context clearly dictates otherwise, any processing device or control device described herein may be implemented as a single processing device or as a plurality of processing devices. The plurality of processing devices may be arranged or distributed, and any processing function referred to herein may be executed by one or more processing devices, even if a single processing device is illustrated. Any method, application, or module described herein may be stored or otherwise held by such a computer-readable medium and may be implemented using computer-readable / executable instructions executable by one or more processing devices.

[0050] Various components of the CRDS system 20 according to a particular embodiment are shown in FIG. 1. A CO2 laser 24 and a carbon-13 O2 laser 28 are provided. The CO2 laser 24 and the carbon-13 O2 laser 28 are gas-filled tube lasers that emit at a well-known series of frequencies that are pseudo-equally spaced and can be quickly selected using an adjustable diffraction grating device. Gas-filled tube laser technology has a long history and is a stable and robust way to generate infrared radiation at accurately known frequencies. Both the CO2 laser 24 and the carbon-13 O2 laser 28 emit light in the mid-infrared spectrum.

[0051] Each of the CO2 laser 24 and the carbon-13 O2 laser 28 has an actuator that allows adjustment of the length of the laser cavity and an output coupler, and an actuator for changing the pitch of the grating to adjust which wavelength the grating reflects by changing the angle of the grating behind the cavity. By both adjusting the length of the laser cavity and changing the angle of the grating, the laser can be adjusted very precisely to a specific wavelength and a desired mode quality.

[0052] The CO2 laser 24 generates a first laser beam 32, and the carbon-13 O2 laser 28 generates a second laser beam 36. Depending on the desired optical frequency, either the CO2 laser 24 is adjusted to generate the first laser beam 32 while the carbon-13 O2 laser 28 is detuned, or the carbon-13 O2 laser 28 is adjusted to generate the second laser beam 36 while the CO2 laser 24 is detuned. In this approach, at most only one of the CO2 laser 24 and the carbon-13 O2 laser 28 outputs a beam at any given time, so the first beam 32 and the second beam 36 cannot be combined simultaneously. Mid-infrared, specifically infrared with a long wavelength, was selected as the type of light because the most volatile organic compounds absorb light in this range. As a result, multiple volatile organic compounds can be measured by one system. The CO2 laser operates in this range and has sufficient power and linewidth narrowing for ring-down spectroscopy. Using two lasers increases the range and number of available wavelengths that the CRDS system 20 can use to analyze a gas sample.

[0053] The first laser beam 32 is redirected through a mirror 40 in the optical mounting section towards a beam splitter 44. The beam splitter 44 is somewhat reflective and somewhat transmissive, splitting each of the first laser beam 32 and the second laser beam 36 into two beams: a sampling beam 48 and an operating beam 52 that has the same characteristics as the sampling beam 48 and can have a similar intensity to the sampling beam 48.

[0054] The sampling beam 48 is received by the high-speed infrared detector 56. The high-speed infrared detector 56 measures the amplitude and beat frequency of the sampling beam 48 using an oscilloscope. The beat frequency may indicate the presence of higher-order modes resulting from sub-optimal alignment of the CO2 laser 24 or the carbon-13 O2 laser 28. In response to the detection of an undesirable beat frequency, the corresponding laser 24 or 28 is adjusted until the amplitude of the beat frequency is minimized or eliminated while maximizing the intensity. If the amplitude of the beat frequency cannot be reduced below an acceptable level, the laser may be adjusted to a different wavelength.

[0055] The operating beam 52 continues to the first optical modulator 60, which deflects the operating beam 52 towards the mirror 64 in the optical mounting section. The mirror 64 redirects the light towards the second optical modulator 68, which further deflects the operating beam 52 towards the focusing lens 72. Optical modulators are used to control the intensity of the light beam generated by the laser. In the present embodiment, the first optical modulator 60 and the second optical modulator 68 are acousto-optic modulators (AOMs), also referred to as Bragg cells. An AOM is a type of optical modulator that uses a piezoelectric transducer coupled to a material such as germanium or glass. In the described embodiment, the material is germanium. When an oscillating electrical signal is applied to the piezoelectric transducer, the piezoelectric transducer vibrates and creates sound waves in the material. These sound waves expand and compress the material, thereby creating a periodic oscillation in the refractive index and enabling Bragg diffraction. Light entering the AOM at the first Bragg angle with respect to a plane perpendicular to the axis of propagation of the sound waves will be deflected by an amount equal to twice the Bragg angle at maximum efficiency. Eliminating the electrical signal removes the Bragg diffraction characteristics of the material, allows the light to pass through without deflection, and effectively attenuates the light along the deflected optical path. A by-product of the AOM is that the frequency of the deflected light is shifted.

[0056] In other embodiments, the optical modulator may alternatively be an electro-optic modulator. An electro-optic modulator is another type of optical modulator that applies a DC or low-frequency electric field to a material to distort the position, orientation, and / or shape of the molecules of the material. As a result, the refractive index is changed, and the phase of the outgoing beam is varied as a function of the applied field. By sending the beam through a polarizer, the phase modulation is converted into intensity modulation. In other ways, a phase modulator can act as an intensity modulator when placed in a branch of an interferometer.

[0057] Furthermore, although the CRDS system 20 has been described as having two optical modulators, in other embodiments, the CRDS system may have fewer or more optical modulators.

[0058] The first optical modulator 60 and the second optical modulator 68 act as attenuators to adjust the intensity of the operating beam 52 and extinguish the beam at the start of the ring-down event. The ring-down event includes the extinction of the operating beam 52 illuminating the ring-down cavity, or the detuning of the laser for the ring-down chamber and the collection of optical intensity data from the ring-down chamber. Since the first optical modulator 60 and the second optical modulator 68 are AOMs, the acousto-optic effect is used to diffract light using acoustic waves (usually acoustic waves at high frequencies). In each of the first and second optical modulators, a piezoelectric transducer is coupled to a material such as germanium or glass, and an oscillating electrical signal is used to vibrate the piezoelectric transducer. The vibrating piezoelectric transducer expands and compresses the material, thereby creating a periodic vibration in the refractive index and creating an acoustic wave in the material that enables Bragg diffraction. Light entering the AOM at the Bragg angle with respect to a plane perpendicular to the axis of propagation of the acoustic wave will be deflected by an amount equal to twice the Bragg angle at maximum efficiency. When the electrical signal is extinguished, the Bragg diffraction characteristics of the material are removed, allowing the light to pass through without being deflected, effectively extinguishing the light along the deflected optical path. Thus, the intensity of the sound can be used to modulate the intensity of the light in the deflected beam.

[0059] The intensity of the light deflected by each of the first optical modulator 60 and the second optical modulator 68 can be between about 85% representing the maximum deflection efficiency of the optical modulators 60, 68 and the attenuation limit of each of the first optical modulator 60 and the second optical modulator 68 of about 0.1% of the input light intensity. When the acoustic wave applied to germanium is turned off, the deflected beam loses about 30 dB or 99.9% of its previous intensity. The attenuation limit means the upper limit of how much the input light intensity can be reduced by the optical modulator.

[0060] As a side effect, when the input light is received at the first end, the optical modulator Doppler-shifts the frequency of the light in the first mode, and when the input light is received at the second end and the attenuation power is the same, the optical modulator is asymmetric in that it Doppler-shifts the frequency of the light in the second mode opposite to the first mode. However, the Doppler shift of the frequency of the light is in the same direction regardless of whether the light enters at the first end or the second end.

[0061] Conventional CRDS systems use a single optical modulator and, as a result, have an operating beam with a frequency-shifted frequency. These frequency shifts are generally small relative to the frequency of the light and can change the method by which the light is absorbed by the substance in the cavity, but this frequency shift can be corrected during the analysis. When the diffraction is towards the acoustic wave source of the AOM, the frequency shift is downward, and when the diffraction is away from the acoustic wave source, the frequency shift is upward. As previously discussed, the effect is minimal.

[0062] The operating beam 52 deflected by the second optical modulator 68 is focused through the focusing lens 72. The laser beam, and thus the operating beam 52, continues to branch as it proceeds from the CO2 laser 24 or the carbon-13 O2 laser 28. The focusing lens 72 is adjusted to return the operating beam 52 to its original focus.

[0063] Thereafter, the mirror 76 of the optical mounting portion redirects the operating beam 52 towards the ring-down chamber 80. The two mirrors 64, 76 extend along the length of the path of the operating beam 52.

[0064] Referring now to FIGS. 1 and 2, the ring-down chamber 80 is an elongated tube that defines a resonant cavity called the ring-down cavity 84. A front cavity mirror 88a and a rear cavity mirror 88b (alternatively, referred to herein as cavity mirror 88) are positioned at the longitudinal ends of the ring-down cavity 84. The cavity mirror 88 is highly reflective to both light directed from outside the ring-down cavity 84 toward the cavity mirror 88 and light directed within the ring-down cavity 84 toward the cavity mirror 88. As a result, a portion of the actuation beam 52 is directed toward the front cavity mirror 88a, approximately 0.1% passes through the front cavity mirror 88a, enters the ring-down cavity 84, and most of the actuation beam 52, approximately 99.9%, is reflected back toward the mirror 76.

[0065] The cavity mirror 88 is mounted on a mirror mount 92 that is operable to adjust the positioning and orientation of the cavity mirror 88. Specifically, the front cavity mirror 88a facing forward of the ring-down cavity 84 is mounted on the mirror mount 92 that is operable via three mechanized micrometers 96a. The rear cavity mirror 88b facing rearward of the ring-down cavity 84 is mounted on the mirror mount 92 that is operable via three piezoelectric micrometers 96b that can be adjusted manually for optical alignment, or on the mirror mount 92 that is operable with a piezoelectric body that can be further adjusted with a piezoelectric actuator.

[0066] Each angle of the cavity mirrors 88 can be varied so that the optical beam is sufficiently aligned such that it does not deviate when the optical beam enters the ring-down cavity 84. If one of the cavity mirrors 88 is angled, a portion of the light is reflected off the side of the ring-down cavity 84, the intensity of the light is lost, and particularly higher-order modes occur. The micrometer 96 can be adjusted simultaneously to vary the length of the ring-down cavity 84 without affecting the angular alignment. This enables adjustment of the ring-down cavity 84 such that it resonates at the frequency of the light entering the ring-down cavity 84.

[0067] The focusing lens 72 aligns the focus of the laser light to the optical mode of the ring-down cavity 84, whereby the minimum waist of the beam is positioned at the same location as the minimum beam waist of the ring-down cavity 84. The position of the focusing lens 72 can be adjusted to match the optical mode in the range of the laser wavelength.

[0068] An optical sensor in the form of a detector 100 cooled by liquid nitrogen is positioned behind the rear cavity mirror 88b to receive the light escaping through the rear cavity mirror 88b. The liquid nitrogen cooled detector 100 measures the intensity of the light escaping from the ring-down cavity 84. Other types of sensors for measuring the intensity of the escaping light may be used instead of the liquid nitrogen cooled detector 100.

[0069] A gas sample is filled from a thermal desorption tube 104 used to collect the gas sample for testing into the ring-down cavity 84. The thermal desorption tube is generally made of stainless steel and contains various types of solid adsorption materials. The solid adsorbent is selected to sample a particular compound to capture and hold the compound of interest even in the presence of other compounds, and the recovered compound can be easily desorbed or extracted for analysis. Also, the selected solid adsorbent does not react with the compound of interest.

[0070] In a specific example, the gas sample is a sample of human breath recovered from a patient. The receiving end 108 of the thermal desorption tube 104 receives the human breath recovered from a person for testing. As a result, the compounds of interest are concentrated more towards the receiving end 108 of the thermal desorption tube 104.

[0071] The pneumatic system 112 is used to fill the thermal desorption tube 104 with the gas sample into the ring-down cavity 84 and to evacuate the interior of the pneumatic system 112 including the ring-down cavity 84. During the filling of the gas sample, the pneumatic system 112 fills the ring-down cavity 84 with the recovered gas sample (i.e., desorbs the gas sample from the thermal desorption tube 104 and places the gas sample into the ring-down cavity 84 without introducing contaminants), sets the pressure in the ring-down cavity to 1 atmosphere, sets the temperature to 50 degrees Celsius, and seals the ring-down cavity 84. In this embodiment, the absorption spectra for the set of samples for which the measured absorption spectra are compared are determined at this pressure and temperature to ensure consistency among these parameters that can affect the results. However, in other embodiments, the pressure and temperature may be fixed at other levels for the known absorption spectra and the measured absorption spectra. During the evacuation of the gas sample, the pneumatic system 112 sweeps out the previously provided gas sample from the ring-down cavity 84 and the various conduits for guiding the gas sample from the thermal desorption tube 104 to the ring-down cavity 84.

[0072] The pneumatic system 112 has an intake portion that includes a nitrogen gas source 116. The nitrogen gas source 116 is a supply of very clean nitrogen gas that can be pressurized or that can pressurize nitrogen gas to at least above atmospheric pressure. In this embodiment, the nitrogen gas source 116 is pressurized above ambient pressure by 5 psi, but can be varied so long as it is sufficient to compress the ring-down cavity 84 to atmospheric pressure or to another selected atmospheric pressure at which analysis is to be performed. In the illustrated embodiment, the nitrogen gas source 116 is nitrogen gas that evaporates from a liquid nitrogen container. The nitrogen gas source 116 is connected to a gas inlet valve 124a via a conduit 120. An auxiliary gas inlet valve 124b allows for the connection of other gases, but is not normally used. The gas inlet valve 124a and the auxiliary gas inlet valve 124b communicate with a gas intake pipe 120a. A pressure gauge 128 is positioned along the gas intake pipe 120a, and a gas intake pipe valve 124c is also positioned. A filter 130a is positioned along the gas intake pipe 120a in front of a cavity inlet valve 124d that seals the gas intake pipe 120a from the ring-down cavity 84. The filter 130a suppresses the entry of contaminants into the ring-down cavity 84 where contaminants could deposit on the cavity mirror 88 and interfere with reflections.

[0073] The gas inlet valve 124a and the auxiliary gas inlet valve 124b communicate with a path valve 124e. The path valve 124e enables or disables direct access to the desorption tube piping 120b and the sample outlet piping 120c.

[0074] The desorption tube piping 120b includes a forward valve 124f and a reverse valve 124g. The thermal desorption tube 104 is positioned between the forward valve 124f and the reverse valve 124g with the receiving end 108 of the thermal desorption tube 104 positioned toward the reverse valve 124g. The thermal desorption tube 104 is positioned within a heater 132.

[0075] The sample outlet piping 120c includes a sample outlet valve 124h and a mass flow controller 136.

[0076] The pneumatic system 112 also has an outlet portion that includes a cavity outlet valve 124i that communicates with the ring-down cavity 84. An outlet pipe 140 communicates with the cavity outlet valve 124i. A pressure gauge 144 is positioned along the outlet pipe 140. A vacuum shut-off valve 124j is positioned between the pressure gauge 144 and the vacuum pump 148. A vacuum suction valve 124k communicates with the vacuum pump 148 and draws air through a pump suction pipe 150. A filter 130b is positioned in the pump suction pipe 150 to suppress the intrusion of contaminants that could interfere with the operation of the vacuum pump 148.

[0077] Valves 124a - 124k may also be referred to herein as valve 124 for the sake of simplicity.

[0078] The cavity inlet valve 124d and the cavity outlet valve 124i are shown connected to the ring-down cavity 84 at a specific location for convenience, but it is understood that the location where valves 124d, 124i are connected to the ring-down cavity 84 may vary. In a preferred configuration, the cavity inlet valve 124d communicates with the ring-down cavity 84 towards the end of the ring-down cavity 84 adjacent to the front cavity mirror 88a, and the cavity outlet valve 124i communicates with the ring-down cavity 84 towards the end of the ring-down cavity 84 adjacent to the rear cavity mirror 88b.

[0079] When a new gas sample is filled into the ring-down cavity 84, the thermal desorption tube 104 containing the new gas sample is connected to the pneumatic system 112 as shown in FIG. 1.

[0080] During the evacuation phase, vacuum suction valve 124k is opened and vacuum pump 148 is started. Next, vacuum suction valve 124k is closed, and vacuum isolation valve 124j, cavity outlet valve 124i, cavity inlet valve 124d, gas suction pipe valve 124c, and path valve 124e are successively opened. The contents of this path and the piping along ringdown cavity 84 are evacuated from CRDS system 20 by vacuum pump 148. Pressure gauge 144 enables determination when the system is sufficiently evacuated, particularly when pressure gauge 128 is isolated from vacuum pump 148. When it is determined that the system is sufficiently evacuated, these same open valves 124j, 124i, 124d, 124c, and 124e are closed in reverse order. Thereafter, during the nitrogen filling phase, valves 124a, 124c, 124d, 124i, and 124j are opened to allow nitrogen gas from nitrogen gas source 116 to fill piping 120a and 140. Next, the nitrogen gas is purged using other evacuation phases. The nitrogen filling and evacuation phases can be repeated when desired to sweep the piping. Thus, the previously tested gas sample is evacuated from CRDS system 20.

[0081] During filling of a new sample, thermal desorption tube 104 is flushed to remove carbon dioxide and water from thermal desorption tube 104 such that the amount of carbon dioxide and water filling into ringdown cavity 84 is minimized. To flush thermal desorption tube 104, gas inlet valve 124a, gas suction pipe valve 124c, and check valve 124g are opened to provide a path to nitrogen gas to flush thermal desorption tube 104 in the forward direction. Thermal desorption tube 104 is selected to suppress the collection of carbon dioxide and water with the gas sample, but generally there is still some carbon dioxide and water in thermal desorption tube 104.

[0082] 500 ml of nitrogen gas is passed through the thermal desorption tube 104 to expel the carbon dioxide and water remaining in the thermal desorption tube from the original sample. Next, the forward valve 124f and the sample outlet valve 124h are opened to provide a path to the mass flow controller 136. The mass flow controller 136 can release nitrogen gas and the contained carbon dioxide and water at a specific flow rate. In the configuration here, this flow rate is 500 ml / min. Next, all valves 124 are closed.

[0083] Once the carbon dioxide and water are removed from the thermal desorption tube 104, the pneumatic system 112 is again discharged using the same process as previously considered to remove the nitrogen gas that was just introduced in the pneumatic system 112 piping. Next, the heater 132 surrounding the thermal desorption tube 104 heats the thermal desorption tube 104 to a desired temperature to thermally desorb the new sample in the thermal desorption tube 104. The gas inlet valve 124a, the path valve 124e, the forward valve 124f, the reverse valve 124g, and the cavity inlet valve 124d provide a direct path for nitrogen gas from the nitrogen gas source 116 through the thermal desorption tube 104 having the desorbed compound of interest to the ring-down cavity 84.

[0084] Achieving a pressure of 1 atmosphere inside the ring-down cavity 84 is desirable because all of the collected and analyzed reference data is at this pressure level, thereby ensuring that the results are reproducible.

[0085] The gas inlet valve 124a is toggled open and closed by the system, so the system waits for the pressure reading at the pressure gauge 128 to stabilize and achieve 1 atmosphere. If the pressure reading is still less than 1 atmosphere during the stabilization of the pressure gauge 128, the gas inlet valve 124a is toggled again to repeat the process until the pressure reading reaches 1 atmosphere. When the pressure gauge 128 indicates that the pressure level in the ring-down cavity 84 is 1 atmosphere, all valves are closed.

[0086] If desorption at multiple temperatures is desired, vacuum pump 148 is started and cavity outlet valve 124i and vacuum shut-off valve 124j are opened to evacuate the contents of ring-down cavity 84. Next, cavity outlet valve 124i is closed before the desorption process is repeated.

[0087] Complete evacuation is generally not performed between multiple desorptions because some gas sample that would otherwise be lost is still between check valve 124g and cavity inlet valve 124d.

[0088] By pressurizing a fixed-volume ring-down cavity containing a gas sample to a desired pressure level in this manner, the surface area within the ring-down cavity to which the compound adheres can be made smaller compared to a variable-volume ring-down cavity that can be used to raise the pressure within the cavity to the desired level.

[0089] Furthermore, pressure gauge 128 is upstream from the path of the gas sample from thermal desorption tube 104 to ring-down cavity 84, thereby preventing its contamination by the sample.

[0090] FIG. 3 is a schematic diagram of an electronic control subsystem 200 for various components of the also-illustrated CRDS system 20. All lines represent electrical or electronic signals, the arrows represent one-way communication, voltage setting, etc., and the lines without arrows represent two-way communication.

[0091] A computer 204 including one or more processing devices acts as a control module that controls the functions of the various components shown in FIGS. 1 and 2. Computer 204 has one or more processing devices 205 and a storage device 206 that stores computer-executable instructions that, when executed by processing device 205 as described herein, cause processing device 205 to direct other components of CRDS system 20.

[0092] Pairs of the RF drive unit 208 send signals of approximately 40 MHz to supply power to the CO2 laser 24 and the carbon-13 O2 laser 28. Each of the lasers 24, 28 is adjusted using an output coupler and a diffraction grating. The grating actuator 212 actuates (rotates) the diffraction grating. Other actuators actuate (translate) the output coupler. Each output coupler is driven by an output coupler piezoelectric body 216 of 1000 V. The two-channel high-voltage amplifier 220 that supplies power to the output coupler piezoelectric body 216 is adjustable between 0 V and 1000 V. The high-voltage amplifier 220 is set by an analog output signal from a data acquisition (“DAQ: Data Acquisition”) card 224 in the computer 204. The DAQ generates an output between 0 V and 10 V, and the high-voltage amplifier 220 amplifies the signal by 100 times to generate a signal between 0 V and 1000 V to supply power to the output coupler piezoelectric body 216. Each grating actuator 212 that changes the angle for the grating is driven by an actuator driver 228 commanded by the computer 204 via RS-232. Each grating actuator 212 is moved by several millimeters, and this movement is converted into the pitch angles of the lasers 24, 28.

[0093] Data signals from the pressure gauges 128, 144 of the pneumatic system 112 are received through RS-232.

[0094] The high-speed infrared detector 56 is connected to a small amplifier 232 and an oscilloscope 236 that can be used to read the amplitude and frequency of the beat signals used to adjust the lasers 24, 28.

[0095] The temperature control device 240 for the heater 132 of the thermal desorption tube is controlled by the computer 204 via RS-232. The tube heater 132 includes a temperature sensor and an aluminum piece having a heating tape wound around the temperature sensor. Both the heating tape and the temperature sensor are connected to the temperature control device 240 which is a PID (Proportional-Integral-Derivative) control device. The control device sets and repeats the temperature to the main computer 204 via RS-232.

[0096] The relay board 244 is connected to the computer 204 and is used to turn all of the solenoid valve 124 and the vacuum pump 148 on and off.

[0097] The three-channel piezoelectric driver 248 drives a piezoelectric actuator 252 that actuates a micrometer 96b to adjust the length of the ring-down cavity 84. Each channel has two components: a communication section to the piezoelectric driver via RS-232 and an analog input section from the DAQ card 224. In other embodiments, more than two piezoelectric drivers may be used.

[0098] Each optical modulator 60, 68 is driven by an RF drive unit 256 that sends a signal of approximately 40 MHz. By changing the frequency of the RF drive unit 256, the Bragg angle for a given optical wavelength is changed, or the optical wavelength for a given or fixed Bragg angle is changed. When the RF drive unit 256 is adjusted to a specific frequency and set to full power, most (about 85%) of the operating beam 52 passes through. When adjusted to 80%, 70%, the optical modulators 60, 68 will attenuate. When the RF drive unit 256 is set to zero, the optical modulators 60, 68 stop completely. The frequency of the RF drive unit is set via RS232 through the components. Analog and digital components can set the amplitude and on / off state of the RF drive unit 256. Specifically, the DAQ card 224 sends a signal to a timing circuit 260, and the timing circuit 260 further generates four required signals necessary to enable and set the amplitude of the RF drive unit. The timing circuit 260 can operate under steady-state conditions or under conditions where the timing circuit 260 sets four voltages to zero and causes a ring-down that returns to the previous voltage level after a predetermined length of time.

[0099] There is a digital output ("DO") from the DAQ card 224 that controls the timing circuit 260.

[0100] Referring now to FIGS. 1 and 3, when a gas sample is filled into the ring-down cavity 84, one of the lasers 24, 28 is tuned to a specific wavelength, and the light is directed to pass through the first optical modulator 60, reflected by the mirror 64, passed through the second optical modulator 68, and reflected by the mirror 76 into the ring-down chamber 80. The optical modulators 60, 68 attenuate the actuation beam 52 somewhat to modulate the intensity of the actuation beam 52.

[0101] When the actuation beam 52 reaches the front cavity mirror 88a, about 0.1% of a portion passes through the front cavity mirror 88a and enters the ring-down cavity 84. Most of the actuation beam, about 99.9%, is first reflected to return along the same path to the actuation laser 24 or 28.

[0102] Initially, the ring-down cavity 84 is not illuminated. As light enters the ring-down cavity 84 and most of the light in the ring-down cavity 84 is reflected between the two cavity mirrors 88, the amount or power of the light in the ring-down cavity 84 begins to increase as more light is introduced from the outside via the actuation beam 52. A specific portion of the light leaks past the cavity mirror 88. It takes a period of time to "fill" the ring-down cavity 84 with light, which can occur when the cavity length CL is equal to the adjacent resonance length of the ring-down cavity 84 for the tuned laser. At this point, there is an equilibrium between the incoming light and the leakage. When this equilibrium is achieved, the lasers 24, 28 are extinguished or stopped from entering the ring-down cavity 84 via the optical modulators 60, 68. In other embodiments, the lasers can be detuned so as not to resonate for the configured cavity length.

[0103] Referring now to FIGS. 2 and 4A, one technique for finding the positions of the adjacent resonance lengths of the ring-down cavity 84 for a selected laser wavelength is shown. In the illustrated technique, the piezoelectric driver 248 starts at 0 volts, gradually increases to 100 volts, which results in a movement of the rear cavity mirror 88b of approximately 7.5 microns in the configuration described herein, and then gradually decreases back to 0 volts, and the output is sent to each of the piezoelectric actuators 252 under the control of the computer 204. The corresponding triangular wave is shown in FIG. 4A. In other embodiments, the voltage range can be varied. When the voltage provided to the piezoelectric actuator 252 is increased or decreased, the piezoelectric micrometer 96b moves the rear cavity mirror 88b in a direction towards or away from the front cavity mirror 88a, thereby changing the cavity length CL of the ring-down cavity 84. The voltage that needs to be applied to the piezoelectric actuator 252 to operate the rear cavity mirror 88b such that the cavity length matches the adjacent resonance length is shown as the adjacent resonance length RL and may alternatively be referred to hereinafter as the adjacent resonance length RL.

[0104] As a result of the selected amplitude of this triangular wave and the configured wavelength of the laser light, the cavity length CL becomes equal to either twice or four times the resonance length RL, as shown in FIGS. 4A and 4C respectively, depending on the difference between the initial cavity length CL when zero voltage is applied to the piezoelectric actuator 252 and the first resonance length RL1 at the initial voltage.

[0105] The ring-down cavity 84 is in resonance, and when approaching equilibrium (i.e., when the amount of light leaking out through the cavity mirror 88 is equal to the amount of light entering from the operating beam 52), there is a weakening interference with the incoming laser light so that the incoming laser light is not reflected at all or is reflected only very slightly by the front cavity mirror 88a. As a result, when the ring-down cavity 84 reaches equilibrium, the reflection of a portion of the operating beam 52 within the bandwidth of the ring-down cavity directed towards the front cavity mirror 88a is substantially eliminated.

[0106] As shown in FIG. 4A, when the voltage is increased, the intensity of the light detected by the liquid nitrogen cooled detector 100 peaks at P1, and when the voltage is decreased, the intensity of the light detected by the liquid nitrogen cooled detector 100 peaks at P2. Of interest is that the voltage at which P1 occurs is generally higher than the voltage at which P2 occurs. For example, when the voltage is increasing, the light intensity peak may occur at 70 volts, and when the voltage is decreasing, the light intensity peak may occur at 63 volts. This can be a result of the hysteresis of the piezoelectric actuator 252 of the piezoelectric micrometer 96b. The resulting light intensity detected by the liquid nitrogen cooled detector 100 is shown in FIG. 4B.

[0107] Similarly, as shown in FIG. 4C, when the voltage is increased, the intensity of the light detected by the liquid nitrogen cooled detector 100 peaks at P1 and P2, and when the voltage is decreased, the intensity of the light detected by the liquid nitrogen cooled detector 100 peaks at P3 and P4. The voltages at which P1 and P2 occur are generally higher than the voltages at which P3 and P4 occur. The resulting light intensity detected by the liquid nitrogen cooled detector 100 is shown in FIG. 4D.

[0108] Different methods have been developed to adjust the ring-down cavity 84 and recover cavity ring-down, generally indicated at 300 in FIG. 5. Method 300 begins with the generation of a sinusoidal waveform that is sent to the piezoelectric driver 248 (310). The computer 204 instructs the DAQ 224, via the digitizer 264, to generate a sinusoidal waveform and send it to the piezoelectric driver 248. The generated sinusoidal waveform voltage is an analog waveform generated from 30,000 points in this embodiment, although this number can vary considerably. The sinusoidal waveform in this embodiment has a frequency of about 200 Hz, and the sinusoidal waveform voltage has an amplitude of about 2 volts and an offset of 2 volts, although these can vary in other embodiments. A sinusoidal waveform generated digitally at 500,000 samples per second is used to instruct the piezoelectric driver 248 how to apply the voltage to the piezoelectric actuator 252. Thus, the piezoelectric driver 248 applies the sinusoidal waveform voltage to the piezoelectric actuator 252. Actuation 224 of the cavity mirror 88b after the sinusoidal waveform voltage reduces the generation of vibrations that can occur around sharp changes in velocity, such as can occur at the peaks and valleys of the triangular waveform.

[0109] FIG. 6A shows the sinusoidal waveform voltage SV generated by the piezoelectric driver 248 in response to receiving a sinusoidal waveform from the DAQ 224. As can be seen, movement of the cavity mirror 88b after the sinusoidal waveform voltage does not result in adjustment of the cavity length CL via actuation of the rear cavity mirror 88b to match the adjacent resonance length RL of the ring-down cavity. The effective constant movement of the rear cavity mirror 88b can overcome the low resolution of the digital settings in the piezoelectric. Although the points are discrete, there are enough points to provide smooth movement of the piezoelectric actuator 252. In fact, the piezoelectric actuator 252 has momentum when driven between points and thus allows for a much greater effective resolution.

[0110] FIG. 6B shows the light intensity resulting from the movement of the post-cavity mirror 88b using the sine wave form SV of FIG. 6A as detected by the liquid nitrogen cooled detector 100. Since the cavity length CL does not approach the resonance length RL, the interfering attenuation does not increase the light intensity in the ring-down cavity 84.

[0111] Referring back to FIG. 5, next, the sine wave voltage SV is shifted (320) until the adjacent resonance lengths RL are centered in the sine wave voltage SV. The computer 204 adds a base voltage to the piezoelectric driver 248 via a digital signal sent via RS-232 until a laser light intensity peak is detected by the liquid nitrogen cooled detector 100, and instructs the piezoelectric driver 248 to increase the base voltage. The liquid nitrogen cooled detector 100 reports the detected light intensity level to the digitizer 264, and in turn to the computer 204, thereby creating a feedback loop. By adding a sine wave voltage that effectively mimics the digital base voltage with a continuous curve, much higher resolution is achieved at higher voltages.

[0112] FIG. 6C shows the base voltage BV added to the sine wave voltage SV to generate the total voltage AV. The total voltage AV moves the post-cavity mirror 88b through a position where the cavity length CL between the pre-cavity mirror 88a and the post-cavity mirror 88b equals the adjacent resonance length RL of the ring-down cavity 84, and then moves back through that position again.

[0113] FIG. 6D shows the corresponding light intensity detected by the liquid nitrogen cooled detector 100 and reported back when the total voltage AV is applied to the piezoelectric driver 248 as shown in FIG. 6C. The light intensity at the time when the cavity length CL matches the resonance length RL peak reaches a peak as shown at peaks P1, P2, and P3, and is generally considered noise between these events.

[0114] Computer 204 continues to increase the base voltage BV via RS-232 until the light intensity peaks detected by the liquid nitrogen cooled detector 100 are equally spaced with respect to time at half the period T / 2, as shown in FIG. 6E. When the light intensity peaks are equally spaced, the resonance length RL is matched by the middle of the cavity length CL between the minimum and maximum values of the total voltage AV. While the base voltage BV is being adjusted, the variation in the voltage at adjacent resonance lengths RL is monitored. During the initialization process, the voltage corresponding to adjacent resonance lengths RL may vary, at least in part, as a result of the "warming up" of the components as much as possible. This voltage generally stabilizes after a short time. The offset of the sinusoidal voltage SV to position the voltage corresponding to adjacent resonance lengths around the voltage corresponding to adjacent resonance lengths in the sinusoidal voltage SV is only completed when the voltage corresponding to adjacent resonance lengths RL is considered to be stable.

[0115] FIG. 6F shows the light intensity detected by the liquid nitrogen cooled detector 100 corresponding to the total voltage AV applied to the piezoelectric driver 248, as shown in FIG. 6E. As can be seen, the peaks P1 - P5 are equally spaced or separated by half of the period of the sinusoidal voltage SV, which is equal to 15,000 points of the digitizer 264, or T / 2.

[0116] FIGS. 7A - 7C show the interfering or reinforcing interference of the laser light in the ring - down cavity 84 at times t1, t2, and t3 respectively, shown in FIG. 6E. Specifically, in FIG. 7A, the cavity length CL is shortened through the increase in the voltage applied to the piezoelectric driver 264, and thus through the movement of the rear cavity mirror 88b towards the front cavity mirror 88a, but is longer than the adjacent resonance length RL of the ring - down cavity 84. As a result, the light reflected from the rear cavity mirror 88b is out of phase with the light approaching the rear cavity mirror 88b, resulting in interfering interference. When the light continues to be reflected between the front cavity mirror 88a and the rear cavity mirror 88b, the interfering interference cancels out the light intensity.

[0117] In FIG. 7B, the cavity length CL is still shortened through an increase in the voltage applied to the piezoelectric driver 264 and is equal to the resonance length RL of the ring-down cavity 84. As a result, the light reflected from the rear cavity mirror 88b is in phase with the light approaching the rear cavity mirror 88b, resulting in constructive interference, thereby increasing the light in the ring-down cavity 84. At this position, the rear cavity mirror 88b is continuously actuated toward the front cavity mirror 88a, and thus passes through the position where the cavity length is equal to the adjacent resonance length without effectively stopping.

[0118] In FIG. 7C, the cavity length CL is still shortened through an increase in the voltage applied to the piezoelectric driver 264 but is shorter than the adjacent resonance length RL of the ring-down cavity 84. Here, the rear cavity mirror 88b continuously advances toward the front cavity mirror 88a. As a result, the light reflected from the rear cavity mirror 88b is out of phase with the light accompanying the light approaching the rear cavity mirror 88b, resulting in destructive interference. When the light is continuously reflected between the front cavity mirror 88a and the rear cavity mirror 88b, the destructive interference cancels out the light intensity.

[0119] Referring again to FIG. 5, when the voltage corresponding to the resonance length RL is centered within the range of the total voltage AV, the computer determines the threshold light intensity (330). In this embodiment, the threshold light intensity is set experimentally to 90% of the peak light intensity in order to allow the time for the laser light to disappear because the electronic components have a delay. The light intensity is received from the liquid nitrogen cooled detector 100.

[0120] Next, computer 204 continues to operate the rear cavity mirror 88b using the total voltage AV shown in FIG. 6E. Specifically, computer 204 instructs piezoelectric driver 248 to generate a base voltage BV, controls DAQ 224 to provide a sine wave voltage SV to piezoelectric driver 248 that is used by piezoelectric driver 248 and added to base voltage BV to reach total voltage AV. The total voltage AV generated by piezoelectric driver 248 moves rear cavity mirror 88b in the first or second direction (340) through the position where cavity length CL equals resonance length RL.

[0121] When detecting the previously determined threshold intensity, digitizer 264 sends a start pulse to timing circuit 260 (350) to extinguish the laser light provided to ringdown cavity 84 by optical modulators 60, 68. Specifically, digitizer 264 activates timing circuit 260 to set the voltage of the RF drive unit to zero.

[0122] Timing circuit 260 simultaneously instructs first optical modulator 60 and second optical modulator 68 to attenuate the optical beam to or near the attenuation limit of optical modulators 60, 68 to reduce the intensity of the optical beam from first optical modulator 60. In CRDS system 20, by instructing both optical modulators 60, 68 to stop simultaneously, the amount of light deflected by first optical modulator 60 during a short time length is significantly reduced by second optical modulator 68 because second optical modulator 68 is stopped.

[0123] The second optical modulator 68 greatly increases the attenuation achieved only through the first optical modulator 60. In the embodiments described herein, if the first optical modulator 60 allows for 30 dB of attenuation and the second optical modulator 68 allows for an additional 30 dB of attenuation, the overall attenuation achieved through the optical modulators 60, 68 is the sum of their attenuations, i.e., 60 dB. While filling the ring-down cavity 84 with light, the optical modulators 60, 68 attenuate the operating beam 52 to modulate its intensity. In the configuration here, each of the optical modulators 60, 68 attenuates the operating beam 52 by 5 dB for an overall attenuation of 10 dB. As a result, each of the optical modulators 60, 68 can further attenuate the operating beam 52 by 25 dB for an additional overall attenuation of 50 dB during the disappearance of the operating beam 52. In a conventional setup, one optical modulator would need to attenuate the operating beam by 10 dB, making 20 dB of additional attenuation available to cause the operating beam to disappear. As can be understood, the operating beam 52 can be made to disappear much faster through an additional 50 dB of attenuation through the two optical modulators 60, 68 than through one optical modulator with 20 dB of additional attenuation. As a result, the amount of additional light introduced into the ring-down cavity 84 after the optical modulators 60, 68 are instructed to stop is a much smaller fraction of the light further introduced by the setting of a single optical modulator in a conventional CRDS system. By making the operating beam 52 disappear faster, the measured attenuation of light in the ring-down cavity 84 is less affected by additional light during the ramp-down time of the optical modulators 60, 68, and thus provides higher accuracy when matching the observed attenuation time to the known attenuation time.

[0124] The disappearance of the laser light provided to the ring-down cavity 84 can initiate a ring-down event. The resonant laser light provided to the ring-down cavity 84 may be extinguished by other means in alternative embodiments, for example, by detuning the laser. By initiating to cause a ring-down event through a threshold, the ring-down event can be time-adjusted to occur between peaks, while the ring-down cavity 84 resonates with the laser light and is not on one side of the peak. Further, since the resonance bandwidth is about 10 millivolts, the resolution of the piezoelectric actuator 248 is too coarse to properly follow the peak.

[0125] During the ring-down event, the computer 204 registers (360) the light intensity data reported by the liquid nitrogen-cooled detector 100 that exits from the rear end of the ring-down cavity 84. The ring-down event, in the configuration here, lasts for about 10 microseconds, but may last for a longer or shorter time in other embodiments. The light decay time is about 2 microseconds.

[0126] Approximately 100 microseconds after a ringdown event is triggered, the timing circuit 260 directs the optical modulators 60, 68 to again start passing the operating beam 52 into the ringdown cavity 84 (370). Next, it is determined whether sufficient ringdown data has been collected (380). The CRDS system 20 is configured in this embodiment to collect data from 500 ringdown events. If data from 500 ringdown events has been captured, the computer 204 stops the operation of the piezoelectric actuator 248 and then determines the decay rate from the ringdown event data. Instead, if at block 380 it is determined that more ringdown data is to be collected, the computer 204 continues to direct the piezoelectric actuator 248 to actuate the rear cavity mirror 88b. As shown in FIG. 6E, when the total voltage AV reaches a maximum or minimum, the total voltage AV begins to proceed in the opposite direction (390). That is, if the total voltage AV is increasing before reaching the maximum voltage in its range, the total voltage AV decreases back towards the voltage corresponding to the resonance length RL. Instead, if the total voltage AV is decreasing before reaching the minimum voltage in its range, the total voltage AV increases back towards the voltage corresponding to the resonance length RL. In this approach, ringdown events are triggered in both directions.

[0127] Ringdown event data (i.e., the optical intensity during a ringdown event) is collected as quickly as possible because various outputs can flow. For example, the piezoelectric actuator 252 may have a settling time, referred to as piezoelectric creep.

[0128] Ringdown event data regarding the optical intensity decay in the ringdown cavity collected in one direction may produce an error that is the opposite of an error that may occur in the ringdown event data collected in the other direction. Therefore, it has been found that averaging the ringdown event data can result in more accurate data.

[0129] The decay constant, defined as the length of time for the intensity to decrease to 1 / e of the initial intensity (approximately equal to 0.37), or other levels can be determined and compared to the reference decay time without the sample to determine how much light is absorbed by the gas sample. The acceleration in ringdown is due to the presence of the gas sample in the ringdown cavity 84. Using the measured decay time, the absorption coefficient can be calculated as a function of frequency / wavelength.

[0130] The method 300 above can allow the laser to be adjusted and set, and the ringdown cavity 84 to be adjusted and set relative to the laser, as opposed to constantly tracking the laser adjustment and the adjustment of the ringdown cavity 84. Further, by operating at least one of the cavity mirrors relative to each other through effective continuous operation, the need to use expensive piezoelectric actuators with microvolt resolution can be avoided. Still further, since the voltage generated by the piezoelectric actuator is swept over a small range, ringdown event data can be collected over a relatively short period of time (a few seconds in this embodiment).

[0131] The use of the method of positioning at the center allows for more accurate sampling of the peaks near the resonance point, giving a much stricter resolution and a smaller standard deviation.

[0132] To generate an absorption spectrum for the gas sample, the process is repeated for light at multiple frequencies. For example, the light generated by the CO2 laser 24 provides the absorption coefficient for a range of frequencies. Similarly, the absorption coefficient can be generated for a range of frequencies for the light from the carbon-13 O2 laser 28. In this way, an absorption spectrum can be developed for the sample.

[0133] In other alternative embodiments, the time-adjusted location of the resonance peak can be determined, and the time-adjusted location when the threshold intensity is achieved can be registered. These time-adjusted locations may be used as a proxy for the detection of the threshold intensity and the proximity of the cavity length to the resonance length.

[0134] In the embodiments described above, the light source is two lasers that generate light in the mid-infrared range, but it is understood that other light sources may be used. For example, a laser that generates light in the visible spectrum or a near-infrared laser may be used. Further, in some situations, the CRDS system may comprise only one laser or three or more lasers to generate the operating beam.

[0135] An electro-optic modulator may be used instead of the acousto-optic modulator.

[0136] The acousto-optic modulator can be configured such that the frequency of the operating beam is shifted up or down. As long as the frequency of the operating beam is shifted such that the net frequency shift introduced by the acousto-optic modulator is significantly away from the frequency of the operating beam generated by the laser, so that it is outside the bandwidth of the laser light from which the reflected light is generated, the magnitude of the interference between the reflected light and the generated operating beam can be minimized.

[0137] In other embodiments, three or more optical modulators may be used in the CRDS system to provide additional extinction capabilities and to more quickly extinguish the operating beam at the start of the ring-down event. Further, in additional embodiments, only one optical modulator may be used.

[0138] One or more focusing lenses may be used in other embodiments and may be translated to enable repositioning of the lenses to enable mode matching of each wavelength of the laser.

[0139] The same approach may be employed for other types of resonant cavities, particularly for optical resonant cavities.

[0140] Other types of events can be caused when the cavity length is close to the resonant length of the cavity for a specifically selected wavelength.

[0141] The analysis of the gas sample can be carried out at pressures other than 1 atmosphere in other embodiments. Accordingly, the width of the absorption spectrum may vary.

[0142] Certain advantages have been listed above, but various embodiments may include some, all, or none of the listed advantages.

[0143] In the embodiments described above, the resonant cavity is a ring-down cavity, but in other embodiments, other types of resonant cavities may be employed.

[0144] Those skilled in the art will understand that even more alternative implementations and modifications are possible and that the above examples are merely illustrative of one or more implementations. Therefore, the scope is limited only by the claims appended hereto.

Description of Reference Numerals

[0145] 20 CRDS System 24 CO2 Laser 28 Carbon-13 O2 Laser 32 First Laser Beam 36 Second Laser Beam 40 Mirror 44 Beam Splitter 48 Sampling Beam 52 Output Beam 56 High-Speed Infrared Detector 60 First Optical Modulator 64 Mirror 68 Second Optical Modulator 68 Focusing Lens 76 mirrors 80 ring-down chamber 84 ring-down cavity 88 cavity mirror 88a front cavity mirror 88b rear cavity mirror 92 mirror mounting part 96 micrometer 96a mechanized micrometer 96b piezoelectric micrometer 100 liquid nitrogen cooler detector 104 thermal desorption tube 108 receiving end 112 pneumatic system 116 nitrogen gas source 120 conduit 120a gas inlet pipe 120b desorption tube pipe 120c sample outlet pipe 124 solenoid valve 124a gas inlet valve 124b auxiliary gas inlet valve 124c gas inlet pipe valve 124d cavity inlet valve 124e path valve 124f forward valve 124g reverse valve 124h sample outlet valve 124i cavity outlet valve 124j vacuum shut-off valve 124k vacuum inlet valve 128 pressure gauge 130a, 130b filters 132 heater 136 mass flow controller 140 outlet pipe 144 pressure gauge 148 vacuum pump 150 pump inlet pipe 200 electronic control subsystem 204 computer 205 processing unit 206 storage device 208 RF Driving Unit 212 Lattice Actuator 216 Output Coupler Piezoelectric Body 220 High-Voltage Amplifier 224 DAQ Card 228 Actuator Driving Unit 232 Amplifier 236 Oscilloscope 240 Temperature Control Device 244 Relay Substrate 248 3-Channel Piezoelectric Driver 252 Piezoelectric Actuator 256 RF Driving Unit 260 Timing Circuit 264 Digitizer CL Cavity Length P1, P2, P3, P4, P5 Peaks RL, RL1, RL2 Resonance Lengths t Time V Voltage 300 Method 310 Generate a waveform and transmit it to the piezoelectric driver 320 Shift the waveform until the resonance length is centered 330 Determine the position of the threshold intensity 340 Actuate the mirror in the first or second direction through the resonance length 350 Extinguish the light at the position of the threshold intensity 360 Collect the ring-down data 370 Restart illuminating the cavity 380 Has sufficient data been collected? 390 Change to the other direction SV Sinusoidal Waveform Voltage T Period BV Base Voltage AV Total Voltage t1, t2, t3 Times

Claims

1. A method for adjusting a resonant cavity, comprising: operating a first mirror at a first end of the resonant cavity to a first position relative to a second mirror at a second end of the resonant cavity, wherein the cavity length between the first mirror and the second mirror is less than the resonant length for a laser beam, and moving in a first direction between the first position and a second position relative to the second mirror, wherein the cavity length is greater than the resonant length, and wherein the first mirror is actuated by applying a base voltage and a sinusoidal voltage to at least one piezoelectric actuator coupled to the first mirror for operating the first mirror between the first position and the second position, and wherein the base voltage is controlled such that adjacent optical intensity peaks whose positions are found via a photodetector coupled to the resonant cavity are separated by half the period of the sinusoidal voltage; causing an event when the cavity length is close to the resonant length; continuing to operate the first mirror in the first direction between the first position and the second position during the event; operating the first mirror to move it towards the first position in a second direction opposite to the first direction; causing a second event when the cavity length is close to the resonant length; continuing to operate the first mirror in the second direction between the second position and the first position during the second event; A method comprising the above steps.

2. repeatedly operating the first mirror to move it in the first direction and the second direction; causing an event when the cavity length is close to the resonant length; continuing to operate the first mirror during the event; The method according to claim 1, further comprising the above steps.

3. The method according to claim 1, further comprising the step of controlling the base voltage to find the position of the light intensity peak through a photodetector connected to the resonance cavity, wherein the light intensity peak occurs at the resonance length.

4. Selecting an amplitude for the sine wave voltage applied to the at least one piezoelectric actuator to operate the first mirror smaller than a wavelength of a laser beam illuminating the resonance cavity; Controlling the base voltage so that two light intensity peaks are detected during each period of the sine wave voltage; The method according to claim 3, further comprising.

5. The method according to claim 1, further comprising the step of causing the event to occur when the detected light intensity reaches a threshold intensity.

6. The method according to claim 5, wherein the event is a ring-down event.

7. The method according to claim 6, wherein the photodetector is connected to a timing circuit, and the timing circuit is connected to one of an optical modulator and the laser to extinguish the laser beam from the laser or to detune the laser for the resonance cavity.

8. Determining a predicted recurrence time for the threshold intensity; Causing the extinction of the laser beam or the detuning of the laser beam for the resonance cavity at the predicted recurrence time for the threshold intensity; The method according to claim 7, further comprising.

9. The method further comprises detecting the intensity of light in the resonance cavity via a photodetector, The method according to claim 1, wherein the causing step includes causing when the light intensity in the resonant cavity detected by the photodetector reaches a threshold intensity.

10. The method according to claim 9, wherein the photodetector is connected to a timing circuit, and the timing circuit is connected to one of an optical modulator and the laser to extinguish the laser beam from the laser or to detune the laser.

11. A method of adjusting a resonant cavity, comprising: changing a cavity length between a first mirror at a first end of the resonant cavity and a second mirror at a second end of the resonant cavity between a first cavity length smaller than a resonant length for the laser beam and a second cavity length larger than the resonant length for the laser beam, wherein the first mirror is actuated relative to the second mirror by applying a base voltage and a sinusoidal voltage to at least one piezoelectric actuator connected to the first mirror to operate the first mirror between a first position and a second position, and the base voltage is controlled such that adjacent light intensity peaks whose positions are found via a photodetector connected to the resonant cavity are separated by half of a period of the sinusoidal voltage; causing an event when the cavity length approaches the resonant length; continuing to change the cavity length towards the second cavity length during the event; and a method comprising.

12. A method of adjusting a resonant cavity, comprising: Actuating a first mirror at a first end of a resonant cavity to a first position relative to a second mirror at a second end of the resonant cavity, wherein the cavity length between the first mirror and the second mirror is less than the resonant length for a laser beam, and moving the first mirror in a direction between the first position and a second position relative to the second mirror, wherein the cavity length is greater than the resonant length, wherein the first mirror is actuated by applying a base voltage and a sinusoidal voltage to at least one piezoelectric actuator coupled to the first mirror to move the first mirror between the first position and the second position, and wherein the base voltage is controlled such that adjacent light intensity peaks whose positions are found via a photodetector coupled to the resonant cavity are separated by half the period of the sinusoidal voltage. Causing extinction of a laser beam illuminating the resonant cavity, or detuning of the laser beam for the resonant cavity, when the cavity length is close to the resonant length. Continuing to actuate the first mirror in the direction between the first position and the second position while the photodetector registers the light intensity in the resonant cavity. A method comprising the above steps.

13. The direction is a first direction, and after the continuing step, the method further comprises: Causing illumination of the laser beam or readjustment of the laser beam for the resonant cavity; Actuating the first mirror to move it towards the first position in a second direction opposite to the first direction; Causing extinction of the laser beam or detuning of the laser beam for the resonant cavity; Continuing to actuate the first mirror in the second direction between the second position and the first position while the photodetector registers the light intensity in the resonant cavity. The method according to claim 12, further comprising the above steps.

14. A step of controlling the base voltage to find the position of the optical intensity peak through a photodetector connected to the resonance cavity, wherein the optical intensity peak occurs at the resonance length, further comprising the method according to claim 13.

15. Selecting an amplitude for the sine wave voltage applied to the at least one piezoelectric actuator to operate the first mirror smaller than a wavelength of a laser beam illuminating the resonance cavity; Controlling the base voltage such that two optical intensity peaks are detected during each period of the sine wave voltage; Further comprising the method according to claim 14.

16. Further comprising a step of controlling the base voltage such that adjacent optical intensity peaks are separated by half of the period of the sine wave voltage, according to the method of claim 15.

17. The cavity length proximate to the resonance length is detected by the detected optical intensity achieving a threshold intensity, according to the method of claim 13.

18. The photodetector is connected to a timing circuit, and the timing circuit is connected to one of an optical modulator and the laser to extinguish the laser beam from the laser or to detune the laser for the resonance cavity, according to the method of claim 17.

19. Further comprising a step of determining a predicted recurrence time for achieving the threshold intensity to predict when the cavity length is proximate to the resonance length, according to the method of claim 13.

20. A first mirror at a first end of the resonance cavity; A second mirror at a second end of the resonance cavity; At least one processing device; A storage device for storing computer-executable instructions, comprising; When the computer-executable instructions are executed by the at least one processing device, the at least one processing device is caused to actuate the first mirror to move in a first direction between a first position relative to the second mirror, where the cavity length between the first mirror and the second mirror is less than the resonance length for the laser beam, and a second position relative to the second mirror, where the cavity length is greater than the resonance length, the first mirror being actuated by applying a base voltage and a sinusoidal voltage to at least one piezoelectric actuator coupled to the first mirror to actuate the first mirror between the first position and the second position, the base voltage being controlled such that adjacent optical intensity peaks whose positions are found via a photodetector coupled to the resonant cavity are separated by half the period of the sinusoidal voltage; cause extinction of a laser beam illuminating the resonant cavity or an event of detuning of the laser beam for the resonant cavity when the cavity length approaches the resonance length; continue to actuate the first mirror in the first direction between the first position and the second position during the event; actuate the first mirror to move toward the first position in a second direction opposite the first direction; cause a second event when the cavity length approaches the resonance length; and continue to actuate the first mirror in the second direction between the second position and the first position during the second event, a resonant cavity system. **Claim 21**: When the computer-executable instructions are executed by the at least one processing device, the at least one processing device is caused to The step of controlling the base voltage to find the position of the optical intensity peak via a photodetector connected to the resonance cavity, wherein the optical intensity peak occurs at the resonance length, the step of causing to be performed, the resonance cavity system according to claim 20. **Claim 22** When the computer-executable instructions are executed by the at least one processing device, the at least one processing device selecting an amplitude for the sine wave voltage applied to the at least one piezoelectric actuator that actuates the first mirror to be less than a wavelength of the laser beam illuminating the resonance cavity; controlling the base voltage so that two optical intensity peaks are detected during each period of the sine wave voltage, the resonance cavity system according to claim 21, the step of causing to be performed. **Claim 23** The resonance cavity system according to claim 20, wherein the cavity length being close to the resonance length is detected by the detected optical intensity achieving a threshold intensity. **Claim 24** The photodetector is connected to a timing circuit, and the timing circuit is connected to one of an optical modulator and the laser to extinguish the laser beam from the laser or to detune the laser for the resonance cavity, the resonance cavity system according to claim 23. **Claim 25** When the computer-executable instructions are executed by the at least one processing device, the at least one processing device causing to perform the step of determining a predicted recurrence time for achieving the threshold intensity to predict when the cavity length is close to the resonance length, the resonance cavity system according to claim 20.

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